Symposium EP03 : Beyond-Graphene 2D Materials---Synthesis, Properties and Device Applications

Two-dimensional (2D) transition-metal dichalcogenide semiconductor (TMD), MoS₂ has received extensive attention for decades due to their outstanding electrical and mechanical properties for next generation devices. One weakness of MoS₂, however, is that it only shows n-type conduction revealing its limitations for homogeneous PN diodes and complementary inverters. Here, we introduce a charge-transfer method to modify the conduction property of MoS₂ from n- to p-type. We initially deposited n-type InGaZnO (IGZO) film on top of MoS₂ flake, so that electron charges might be transferred from MoS₂ to IGZO during air ambient annealing. As a result, electron charges were depleted in MoS₂. Such charge depletion lowered MoS₂ Fermi level, which makes hole conduction favorable in MoS₂ when optimum source/drain electrodes with high work-function are selected. Our IGZO-supported MoS₂ flake field effect transistors (FETs) clearly display the channel type conversion from n- to p-channel in this way. Under short and long annealing conditions, n- and p-channel MoS₂ FETs are achieved, respectively and low voltage complementary inverter is demonstrated using both channels in a single MoS₂ flake.

Interlayer interactions in a heterostructure offer a powerful method to realize exotic quantum phenomena. The physics of interlayer interactions has received renewed momentum with the emergence of 2D materials where the reduced electrical screening provides an ideal platform to study many-body interactions of their quasiparticles. In this letter, we report the observation of a new quasiparticle, the ‘polaronic trion’, resulting from a strong interfacial coupling of MoS₂ directly grown on SrTiO₃ (STO), where the charged quasiparticle, trion in MoS₂ is dressed by the time-dependent dipolar electric field of a soft phonon mode in STO. We observe from temperature dependent photoluminescence emission that the binding energy of this quasiparticle can be tuned up to 90meV by varying the trion-phonon coupling as a function of temperature, electric field and substrate orientation. Our work provides a platform to realize unprecedented selective tunability of the quasiparticles in 2D semiconductors, driven by many body effects.

Monolayers of transition metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to their two-dimensional nature and weak dielectrical screening. Modifying the interaction via changing the dielectric environment or exciting charge carriers offers a possibility to control the interactions and as a consequence also the macroscopic optical properties. More generally, due to their many-body interactions, excited charge carriers directly influence the electronic and optical properties of monolayer TMDCs. This includes scenarios of electrical and optical excitation as well as charge carrier doping.
In this talk the strong many-particle renormalizations caused by the Coulomb interaction of the excited carriers will be investigated for MX₂ with M = (Mo, W) and X = (S, Se). To study the properties of these four TMDCs we solve the semiconductor Bloch equations on the full Brillouin zone using ab initio band structures and interaction matrix elements.
By increasing the density of excited carriers, we find a reduction of the exciton binding energy due to Coulomb screening and Pauli blocking. In addition to the excitation-induced band-gap shrinkage, these effects lead to redshifts of the excitonic resonances on the order of several hundred meV until the excitons dissociate into a fermionic electron-hole plasma. As excitons are charge-neutral bosonic particles, the degree of their ionization strongly influences many-body effects such as screening. We quantify the exciton ionization degree depending on excitation density and environmental screening.
The central finding of our investigation is a relative shift between the K- and the neighbouring Σ-valley in the conduction band induced by the renormalizations. All four TMDCs show a tendency to become more indirect as the Σ-valley shifts to energetically lower values than K. While monolayer TMDCs are usually celebrated for offering direct band gaps, we predict a transition from a direct to an indirect band gap for MoS₂ and WS₂. This transition should also lead to a drain of carriers from the K-valley and thus to a quenching of the photoluminescence, similar as in strained monolayers. Our findings are also relevant to transport and optical applications e.g. the emergence of superconductivity.

2D materials such as graphene have been investigated to be applied for biosensors due to their excellent electrical properties and high specific surface area. More recently, MoS₂ has exhibited higher sensitivity than graphene due to its semiconducting nature [1]. Designed peptides on the surface of 2D materials form uniform and ordered structures arise from self-assembly with non-covalent integrations. These self-assembled peptides are expected as a molecular scaffold for immobilizing probe molecules without degrading the electrical properties. It has been reported that under dry condition self-assembled peptides on the surface of the graphene and MoS₂ change the electrical properties [2]. Since the operation of the biosensor is carried out under wet condition, behavior of MoS₂ field-effect transistor (FET) in solution is important. However, the response of MoS₂ FET to the absorbed peptides has not been reported yet. In this work, we observed the influence of the self-assembled peptides in buffer solution on the electrical characteristics of MoS₂ FET.
MoS₂-FET were fabricated by transferring a single layer MoS₂ to an electrode prepared by a lithography technique. For the measurement, the source-drain current with respect to the gate voltage was measured with a platinum reference electrode in 10mM phosphate buffer. After the formation of the peptide self-assembled structures, measurements were carried out in the same manner. The morphology of the self-assembled peptides on MoS₂ surface was also measured by Atomic force microscopy (AFM). The peptides utilized in this work have “GAGAGA” amino acid sequence, which is inspired from silk protein, fibroin. Fibroin forms stable b-sheet structure based on the amino acid sequence of GAGAGS. We found that the GAGAGA peptides form uniform and ordered structures on MoS₂ surface. In the conductivity measurements, MoS₂-FET shows a shift of threshold voltage after forming peptide self-assembled structure on the surface, but no change in the transistor mobility. This result suggests that our peptides may be useful as a molecular scaffold for MoS₂ biosensor.
Reference
[1] Sarkar, D. et al. ACS Nano 8, 3992–4003 (2014).
[2] Hayamizu, Y. et al. Sci. Rep. 6, 33778 (2016).

The growth of lateral heterostructures of transition metal dichalcogenides (TMDs) was recently demonstrated, which has created the potential for fabricating semiconductor devices with novel electronic properties. Specifically, it would combine distinct properties of materials derived from different sources into one device. It has been shown that under well-controlled growth conditions, MoS₂-WS₂ lateral heterostructures with atomically sharp interfaces can be synthesized. While the growth of such materials can be challenging, the development of analytical methods with the capability of providing chemical information, in addition to morphological information, with nanometer-scale spatial resolution is equally challenging.
Raman spectroscopy is used to study chemical composition of materials with high specificity, but it lacks sensitivity due to the inherent weakness of the Raman scattering phenomenon. Besides, its spatial resolution is diffraction-limited to ~0.5λ. These drawbacks can be overcome by combining Raman spectroscopy with Scanning Probe Microscopy (SPM) in which certain metal particles placed at the end of the SPM tip act as plasmonic substrate. This technique is referred to as tip-enhanced Raman spectroscopy (TERS). This combination provides not only the benefits of both SPM and Raman microscopy at the same time, but also enables Raman mapping with spatial resolution proportional to the size of the coated SPM tip (well below the diffraction limit) due to plasmonic enhancement of Raman signal.
In this work, we present characterization of MoS₂-WS₂ lateral heterostructures based on morphology, electrical properties, photovoltaic properties, and chemical composition. Scanning Kelvin imaging was used to map the surface potential as well as electro-mechanical contrast proportional to capacitance from the heterostructures. Their surface potential and capacitance change dramatically in a reversible manner when the heterostructures are illuminated by a laser, highlighting their photovoltaic properties. Raman and photoluminescence (PL) maps were recorded with 532 nm excitation, which enabled collection of Raman and PL bands from both materials simultaneously with reasonable separation. In addition, tip-enhanced Raman and PL maps were collected across the interface, with sub-diffraction limited spatial resolution. In summary, a unique collection of characterization techniques were used based on AFM-Raman instrumentation to study morphological, electrical, photovoltaic properties, and chemical composition of MoS₂–WS₂ lateral heterostructures.

There is an ever-increasing need for portable and wearable electronics, because of the societal demand for active, efficient, and integrated devices. With the emerging Internet of Things, these devices require wireless communication devices that are lightweight and portable. Therefore, new fabrication techniques are needed to develop these unobtrusive wireless communication devices. Antennas for these novel devices are required to be conformal and compact, but retain good radio-frequency conductivity which is an essential property of antenna materials. Recently, nanomaterials such as graphene¹, carbon nanotubes², carbon onions³ and conductive polymers⁴ have come into play, but low conductivity is a limitation for their use. However, discovered in 2011, 2D titanium carbide MXene⁵ eliminates the problem of conductivity due to its metallic behavior. The conductivity of MXene Ti₃C₂ film reaches 10000 S/cm⁶ which makes it a promising candidate for portable wireless communication devices. Here we show a class of radio-frequency devices for wireless communication based on two-dimensional titanium carbide MXene prepared by a single step spray coating. We fabricated a transparent MXene antenna with less than 100 nm thickness with less than -10 dB reflection coefficient at a resonant frequency of 2.4 GHz. By increasing the antenna thickness to 8 µm, we achieved -70 dB of reflection coefficient. Additionally, we fabricated a 1-µm-thick MXene RFID tag in the 875 MHz band reaching a read range of 8 meters. Our finding shows that two-dimensional MXenes operate below the skin depth of copper or other metals as well as give an opportunity to produce transparent antennas. Being the most conductive among the solution processed two-dimensional materials, as well as water dispersible, MXenes opens new avenues for manufacturing various classes of radio-frequency devices. Using MXene as a conductor will thus be essential for the development of novel portable, flexible, and wearable electronic devices.

1. X. Huang et al., Graphene radio frequency and microwave passive components for low cost wearable electronics. 2D Mater., 3.2, 025021 (2016)
2. I. Puchades et al., Carbon Nanotube Thin-Film Antennas. ACS Appl. Mater. Interfaces, 8, 20986-92 (2016)
3. N. A. Vacirca et al., Onion-like Carbon and Carbon Nanotube Film Antennas, Applied Physics Letters, 103 , 073301 (2013)
4. N. J. Kirsch et al., Optically transparent conductive polymer RFID meandering dipole antenna, 2009 IEEE International Conference on RFID 278-282 (2009)
5. M. Naguib et al., Two-dimensional nanocrystals produced by exfoliation of Ti₃AlC₂. Advanced Materials 23.37, 4248-4253 (2011)
6. C. J. Zhang et al., Transparent, flexible, and conductive 2D titanium carbide (MXene) films with high volumetric capacitance. Adv. Mater., 29, 4848-4856, (2017)

Interest in transition metal dichalcogenides (TMDs) has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. Due to the strong electronic confinement and the reduced electrostatic screening in two dimensions the electronic and optical properties of 2D materials are generally more susceptible to strain, surface modifications or structural defects than those of their bulk counterparts. While structural defects in semiconductors are usually considered detrimental, the large tunability of 2D materials provide an effective way to create novel functional properties via defect engineering beyond the conventional concept of doping.
Here, we use a set of complementary imaging techniques - photoelectron spectroscopy, photoluminescence, Kelvin probe – to correlate locally the band structure, chemical state, and optical properties of 2D transition metal dichalcogenides.[1] In particular, we employ spatially resolved angle resolved photoemission spectroscopy (nano-ARPES) to map the variations in band alignment, effective mass and chemical composition of CVD-grown monolayer WS₂.[2] By correlating the spectroscopic information from nano-ARPES with hyperspectral photoluminescence data, we reveal the interplay between local material properties, such as defect density or chemical composition, and the formation of charged trions, defect-bound excitons and neutral excitons. Finally, we compare these results to combined atomic force and scanning tunneling microscopy, where we can unambiguously identify the occurring point defects and their electronic structure at the atomic level.
[1] Cross-correlating Excitons, Band Structure and Defects in Synthetic WS₂, C. Kastl et al., submitted, 2018.
[2] Multimodal Spectromicroscopy of Monolayer WS2 Enabled by Ultra-Clean van der Waals Epitaxy, C. Kastl et al., 2D Materials (accepted) 2018.

Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. S. U. acknowledges funding from VILLUM FONDEN (Grant no. 15375). R.J.K. acknowledges funding from the German Academic Exchange Service (DAAD). B.S. appreciates support from the Swiss National Science Foundation under project number P2SKP2_171770.

Layered materials that can be exfoliated into two dimensional materials are currently limited to van der Waals solids with weak interlayer interactions. One material, called a layered electride, defies this rule by being exfoliable despite its large binding energy. Electrides are similar to ionic materials, except the anion is instead an electron with no nucleus. In layered electrides, these electrons manifest as a 2D electron gas contained between cationic slabs. The electride [Sr2N]+e- contains one electron per formula unit, and the energy of these electrons is near the Fermi level, resulting in a low workfunction (3.2 eV). While calculations show that [Sr2N]+e- may be exfoliable, this has not been demonstrated experimentally. In this work, [Sr2N]+e- is exfoliated into thin flakes which retain the structural and electronic properties of the bulk, providing evidence for a 2D material with the lowest workfunction known among exfoliable materials.

The exfoliation potential of layered materials can be estimated by calculating the binding energy using density functional theory (DFT). The total energy of a bilayer is calculated at varying interlayer distances and the lowest energy confirmation is extracted. For the electride [Sr2N]+e-, the binding energy is only five times larger than graphite, a material which is well known to exfoliate in a variety of solvents. Expectedly, [Sr2N]+e- was able to be suspended in propylene carbonate upon sonication, producing thin flakes observable by transmission electron microscopy. The flakes with lateral sizes of 0.5-3 µm showed clear electron diffraction patterns which matched structurally to [Sr2N]+e-, confirmed also by X-ray diffraction. From these results, we can conclude that the structure of [Sr2N]+e- is retained after exfoliation.

In addition to comparing structural characteristics, it is also important to observe changes in the electronic properties. UV-Vis-NIR spectroscopy was performed on the exfoliated [Sr2N]+e- sample to visualize band transitions occurring within the material. A Drude-Lorentz response typical of metals was observed with two peaks at 375 nm and 625 nm and a linear tail in the infrared region, which was attributed to reflection by the electron gas. The band structure confirms the metallicity of [Sr2N]+e- since the electron gas band crosses the Fermi level, and the two peaks observed in the spectrum correlate to peaks in the joint density of states, which is an integration of the band transitions near the Fermi level. From these results, we can conclude that the electronic properties of [Sr2N]+e- are also retained in the electrene form. Given the low workfunction for bulk [Sr2N]+e-, the 2D form of this layered electride could act as a strong electron donor to other two-dimensional materials, defining a new kind of heterostructure not demonstrated previously. Future work for electrene [Sr2N]+e- will involve further quantifying the electron donation ability of this new 2D material.

Van-der-Waals crystals of metal thiophosphates can be seen as derivatives of transition metal dichalcogenides where 1/3 of metal atoms is replaced with diphosphorous, thereby stabilizing the remaining 2/3 of metal ions in low oxidation states. As a result, thiophosphates develop a panoply of desired properties, such as magnetic, dipolar and correlated electron orderings, all of which are rare or non-existent in the dichalcogenide family. Thiophosphates therefore enable ultrathin magnetic, ferroelectric and Mott insulating materials, while also presenting new opportunities for multifunctional interfaces with electronic 2d materials. We recently established giant out-of-plane piezoelectric coefficients, ferroelectric switching, and dielectric tunability in copper indium thiophosphate (CuInP₂S₆) [1]. Here, we reveal that CIPS additionally exhibits ionic conductivity that enables localized extraction of Cu ions from the lattice above the Curie temperature upon application of electric fields via a conductive scanning-probe-microscopy tip. Surprisingly, the extraction is fully reversible, dependent on the polarity of the applied electric field. Cu crystallites of up to 90 nm in height can be formed and erased on the surface utilizing ionic motion in a process that can be precisely controlled by the amplitude of the applied voltage, frequency, temperature, and position of the tip. The underlying resilience of CIPS to large-scale ionic displacements and Cu vacancies is further corroborated by density-functional-theory calculations. At room temperature, newly created vacancy-rich areas exhibit even higher electromechanical response than pristine CIPS areas, making it possible to tailor piezoelectric properties through ion transport. The tunable surface deformation provides interesting opportunities for applications as sensors and actuators, and control of van-der-Waals heterostructures.

Research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. The experimental work was supported by the Division of Materials Sciences and Engineering, Basic Energy Sciences, Department of Energy.

[1] Neumayer et al, ”Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric”, arXiv:1803.08142 (2018)

Recently, the family of van-der-Waals layered transition metal thiophosphates –exhibiting ferroelecric, antiferromagnetic and correlated electron ground states – have gained attention as possible control dielectrics for the rapidly growing family of 2D and quasi-2D electronic materials [1]. Being van-der-Waals crystals, the surfaces of these materials can be created without dangling bonds, unlike those of complex oxides. Yet, because of robust insulating properties, the structure of their surfaces, the role of disorder, the structure of the topological defects in the order parameter and many other properties directly relevant to their prospective interfaces is almost entirely unknown.
Here we present the first atomically resolved imaging of CuScP₂S₆ s carried out using cryogenic non-contact atomic force microscopy. The surface exhibits good crystalline ordering at the atomic scale, revealing contrast on sub-unit cell level. The most remarkable property is long-range commensurate modulation of the surface morphology, with a topographic amplitude of only 2-3 pm. Combined with XRD analysis of the bulk and Monte-Carlo simulation of the Ising model on triangular lattice, we propose that the modulation arises from antiferroelectric polarization domains, albeit with frustrated long-range order. The key structural ingredient for this state is centrosymmetric position of Sc³⁺ within the layer, which forces the surrounding displacing Cu⁺¹ ions to adopt a frustrated antiferroelectric state - in direct analogy frustrated magnetic systems. We will further discuss the peculiarities of nc-AFM imaging of this materials from the statistical analysis of the variation of images between scan, as well as the force-distance curve arrays. The possibility to directly visualize polar order opens broad opportunities to understand the atomistic aspect of ferroelectric, glassy and incommensurate phases in this material class, beginning with CuInP₂S₆ – which exhibits Curie temperature ~315K and giant negative electrostriction [2]. More generally, non contact atomic force microscopy promises to resolve structural and defect properties of 2D materials particularly those with large band-gaps or isolated from electrical contacts. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. Microscopy experiments were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
[1] Susner Michael A., Chyasnavichyus Marius, McGuire Michael A., Ganesh Panchapakesan, and Maksymovych Petro, Advanced Materials 29, 1602852 (2017).
[2] S. M. Neumayer, E. A. Eliseev, M. A. Susner, A. Tselev, B. J. Rodriguez, S. Jesse, S. V. Kalinin, M. A. McGuire, A. N. Morozovska, P. Maksymovych, and N. Balke, ArXiv:1803.08142 [Cond-Mat] (2018).

Ultrathin transition metal dichalcogenides (TMDs) have recently been under intense research towards their electronic device applications. Some of TMDs such as MoS₂ and WSe₂ have band-gaps and retain semiconductor properties even if they are atomically thin. The application of TMDs to field effect transistors (FETs) need understanding how carrier type and carrier concentration are controlled with electric field effects in a nanoscale. Therefore, a tool for the nano-scale investigation of the electric field effects are important for boosting the studies in this field. Scanning nonlinear dielectric microscopy (SNDM) [1] is a scanning probe microscopy method, which can determine dominant carrier types and give information on carrier concentration distribution in high resolution. In this method, the electronic properties are reflected in a dC/dV image through the measurement of depletion layer capacitance locally modulated under the tip with ac voltage. In fact, it has recently been demonstrated that SNDM is applicable to a few-layer semiconductor [2]. In addition, SNDM is possible to use for the investigation of electric field effects. In this presentation, we discuss the electric field effects in few-layer WSe₂ on SiO₂ substrates based on the SNDM imaging. It has been reported that WSe₂ FET can show ambipolar behaviour [3].
Our sample was prepared by mechanically exfoliating WSe₂ on a thermally oxidized Si substrate. The electric field effects were investigated by applying dc-bias voltages to the substrate relative to the tip. For no dc electric field, our few-layer WSe₂ sample including a single layer area showed very weak dC/dV contrast, which implies that the sample can be seen an almost intrinsic semiconductor. On the other hand, we basically obtained n-(p-)type contrast for positive(negative) dc-bias. The observed polarities were consistent with those expected from the electric field effects, while spatial inhomogeneity was also seen. Charge injection from the tip may also be a possible cause of the observed carrier polarities. However, we did not consider charge injection was dominant in our case, which was suggested by simultaneous electrostatic force microscopy imaging. In addition, we found slow transient variation of carrier concentration after switching the polarity of dc-bias. This probably came from the interface charge states. These results demonstrate that SNDM is useful for the investigation of electric field effects on ultrathin semiconductors.
This work was partly supported by a Grant-in-Aid for Scientific Research (Nos. 15K04673, 16H02330) from the Japan Society for the Promotion of Science and the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.
References:
[1] Y. Cho, A. Kirihara, and T. Saeki, Rev. Sci. Instrum. 6, 2297 (1996).
[2] K. Yamasue and Y. Cho, Appl. Phys. Lett. (2018, accepted).
[3] C. Zhou et al., Adv. Funct. Mater. 26, 4223(2016).

Monolayer transition metal dichalcogenides have been the primary materials of interest in the field of valleytronics for their potential in information storage, yet the limiting factor has been achieving long valley decoherence times. We explore the dynamics of four monolayer TMDCs (MoS₂ , MoSe₂ , WS₂ , WSe₂) by describing electron-electron and electron-phonon interactions from first principles. We isolate the crucial impact of spin-orbit coupling on transport properties by comparing calculations which both omit and include relativistic effects. Spin-orbit coupling is found to increase carrier lifetimes at the valence band edge by over an order of magnitude, with a corresponding rise in hole mobility. This drastic change is attributed to spin-valley coupling. At temperatures of 50 K, we find valley coherence times on the order of 100 ps, with a maximum value of ~140 ps in WSe₂. Our results capture the entangled relationship between spin and valley degrees of freedom, which is critical for valleytronic applications. Further, our work points towards interesting quantum properties on-demand in transition metal dichalcogenides that could be leveraged via driving spin, valley and phonon degrees of freedom.

Heterostructures formed by weak van der Waals interactions between 2D materials have revealed novel physics and device functionalities. However, it is challenging to precisely control the number of layer at wafer scale. The tape mechanical exfoliation method has been used as main method to obtain few monolayer 2D flakes from various bulk crystals, which allows stacking of multiple 2D materials. The method relies on trial-and-error based operation, which lead substantial time-consuming. In addition, the typical size of stacked heterostructures is only limited to hundreds of microns. An effort to avoid these issues focuses on direct growth of 2D materials on wafers. However, it has been noted that controlling nucleation of 2D layers via growth is even more challenging because of easy-additonal nulei formation on top of the initial nuclei. Accordingly, it has been required to develop alternative way.
Here we report a layer-resolved splitting (LRS) for 2D materials that permits precise control of the number of layer of 2D materials. It produces multiple monolayers of wafer-scale 2D materials from one multilayer 2D material growth. We grow thick 2D materials, such as WS₂, hBN, WSe₂, and MoSe₂, and precisely split them into multiple monolayers. We study the underlying mechanics of LRS for the 2D material multilayers into many individual monolayers. The wafer-scale monolayer of transition metal dichalcogenides after LRS exhibits substantial photoluminescence enhancement uniformly across a 2-inch wafer which maybe related to indirect-to-direct band gap transition. Through this LRS approach, we successfully demonstrate van der Waals heterostructures with single-atom resolution. We strongly believe LRS will open up new venue for 2D material research community.

Two-dimensional (2D) van der Waals layered materials possess unique advantages as integrable sensors, due to their thinness, flexibility and sensitivity. They can be seamlessly integrated onto surfaces with different geometries where detection of near-field signal is desired. In this study, we develop a device transfer technique to integrate 2D devices onto an arbitrary smooth surface. Such technique utilizes a sacrificial polymer under layer and achieves clean and non-destructive full device transfer. For demonstration, we transferred a complete 2D multilayer InSe photodetector onto a stripped optical fiber. Due to the extreme vicinity of the 2D photodetector with the fiber core, the device can efficiently couple with the evanescent-field and accurately detect information transmitted inside the optical fiber. In addition, these super thin flexible devices can be integrated onto the fibers themselves to non-invasively monitor the optical fiber performance. The demonstration of optically coupled, conformal 2D devices on substrates of different form factors can enable a variety of near-field optical and sensor applications.

Top-down exfoliation from bulk transition metal dichalcogenides like MoS₂ usually leads to micron-sized flakes. We have recently developed an alternative growth method of two-dimensional transition metal diselenides (TMDS) from multilayers down to a single layer based on the Van der Waals (VdW) epitaxy. In the VdW epitaxy, the TMDS is grown either on a passivated surface with a very low density of dangling bonds (it is then called quasi-VdW epitaxy) or on a layered VdW substrate. The basic concept of this growth method relies on the very weak interaction between the epilayer and the substrate in order to largely release the constraint of lattice matching. Therefore it leads to the formation of fully relaxed TMDS layers. Moreover, this technique allows for the growth of uniform layers over centimeter scale surfaces making it compatible with the development of a large scale 2D electronics based on these materials. Using VdW epitaxy, we have recently grown MoSe₂ and WSe₂ multi and monolayers on various substrates: SiO₂, epitaxial graphene [M.-T. Dau et al., ACS Nano 12, 2319 (2018)], mica and selenium passivated platinum crystal. Owing to our ability to deposit two transition metals simultaneously, we have explored electrical and magnetic doping of TMDS. In this presentation, we will show our recent results on three different topics relying on the VdW epitaxy: (i) the epitaxy of multi and monolayers of p-type doped (with Nb) WSe₂ and magnetically doped (with Mn) MoSe₂ on mica, graphene and platinum crystal, (ii) the full characterization of the layers using Raman and photoemission spectroscopies, x-ray diffraction, transmission electron microscopy and SQUID magnetometry and (iii) their transfer onto a SiO₂/Si substrate to study electrical and magnetotransport properties. In particular, we find a ferromagnetic order in Mn-doped MoSe₂ layers at low temperature adding a new member to the recently discovered 2D ferromagnets family [C. Gong et al., Nature 546, 265 (2017), B. Huang et al., Nature 546, 270 (2017), M. Bonilla et al., Nature Nanotech. 13, 289 (2018)].

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are currently under widespread scrutiny for a diverse set of optoelectronic applications. At the monolayer limit, TMD crystals offer a direct optical bandgap, enabling the light emission within a large spectral range covering a large portion of the visible spectrum and the near infra-red regime. In this line, the synthesis of ternary alloys has served as a powerful technique for changing the optical/electrical bandgap of monolayer TMD crystals beyond what a binary compound (i.e., MX₂, M: transition metal and X: chalcogen) may offer. We have previously shown that “ternary” alloys can be produced via doping a “binary” MX₂ crystal by an isoelectronic element (e.g., X’) to yield MoX’_2xX_2(1-x) monolayer alloys [1]. This approach has shown a great promise for the synthesis of lateral heterostructures with arbitrary shapes and dimensions [1, 2]. In this approach, the quality of the as-synthesized alloys depends on the fine details of the starting binary MX₂ crystal.
Here, we study the influence of native defects in starting MX₂ monolayers on the obtained properties of the MX’_2xX_2(1-x) alloys. For the demonstration purpose, we use monolayer MoSe₂ films as starting crystals in which replacing Se atoms by S atoms yields MoS_2xSe_2(1-x) ternary compounds. Our results show that native point defects (primarily chalcogen vacancies) in the starting MoSe₂ lattice serve as atomic sites from which S atoms incorporate into the lattice of MoSe₂ monolayer and form the MoS_2xSe_2(1-x) alloy. Thus, the abundance of the chalcogen vacancies in the starting MoSe₂ films promotes the alloying process and reduces the required temperature needed for this process. We believe that our findings shed light on the fundamental details of the alloying mechanism in 2D TMD crystals as well as opening a new avenue for synthesis of complex alloys via defect engineering.

[1] H. Taghinejad et. al, “Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys” npj 2D Materials and Applications volume 2, article number: 10 (2018).
[2] Mahjouri-Samani, M. et al. Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors. Nat. Commun. 6, 7749 (2015).

Two-dimensional (2D) transition metal carbides and/or carbonitrides (MXenes) have been developed as promising materials for a wide variety of applications due to their advantageous mechanical, electrochemical, electrical, and optical properties.¹ MXenes exhibit a general formula of M_n+1X_nT_x, where M represents a transition metal (Ti, Mo, Nb, V, Cr, etc.), X is either carbon and/or nitrogen, and T_x represents surface terminations.¹ Along with the high electronic conductivities (8,000-10,000 S/cm), the materials are hydrophilic and can be synthesized in large quantities (~100 g per batch) and high concentrations (>50 mg/mL), making them fundamentally feasible for solution processing.^2,3 However, the as-produced colloidal solutions contain flakes with widespread lateral sizes (ranging from 0.1 to ~5 µm), presenting a challenge for maintaining precise control over properties and performance.

Here, we will discuss recent progress on developing size selection methods to control and sort 2D MXene flake sizes after synthesis based on sonication and density gradient centrifugation, respectively.⁴ Furthermore, characterization of size-dependent properties will be discussed, including tuning the conductivity of free-standing films, changes in optical absorption, and differences in electrochemical behaviors. This size selection methodology allows for control of a diverse family of materials, optimizing the performance in applications for MXenes beyond energy storage, including but not limited to, gas sensors, conductive inks, coatings, fibers, and electronic, optic and biomedical devices.

References
1. Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y., Nature Reviews Materials 2017, 2, 16098.
2. Alhabeb, M.; Maleski, K.; Anasori, B.; Lelyukh, P.; Clark, L.; Sin, S.; Gogotsi, Y., Chemistry of Materials 2017, 29, 7633-7644.
3. Akuzum, B.; Maleski, K.; Anasori, B.; Lelyukh, P.; Alvarez, N. J.; Kumbur, E. C.; Gogotsi, Y., ACS Nano 2018, 12, 2685-2694.
4. Maleski, K.; Ren, C.E.; Zhao, M.-Q.; Anasori, B.; Gogotsi, Y.; ACS Applied Materials and Interfaces 2018, in press

Colloidal quasi-two-dimensional (quasi-2D) nanoplatelets and nanosheets have attracted a broad interest due to the novel properties resulted from their anisotropic nanostructures. As an infrared material, quasi-2D PbS has its own unique properties due to its small energy gap, large exciton Bohr radius, and strong spin-orbit coupling. Since a larger nanosheet has more surface states, the photoluminescence quantum efficiency of nanosheets is low. To improve their optical properties, reducing the lateral size of the nanosheets is one of the solutions since the number of surface trap states per sheet can be significantly reduced.
In our experiments, colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane co-solvents.¹ The few-atom-thick nanoribbons have a typical width 20 nm and a length of more than 50 nm. In contrast to nanosheets, the nanoribbons are much brighter. At room temperatures, well-passivated nanoribbons have achieved a typical 30% photoluminescence quantum yield in the infrared spectrum. While the best one has reached 60% photoluminescence quantum yield, exceeding the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.
The highly bright nanoribbons are advantageous to explore the intrinsic novel properties of the PbS caused by the anisotropic confinement in the quasi-2D nanostructure including enhanced optical radiative recombination and slow Auger recombination as well as to find the applications in infrared photonic and optoelectronic devices.

References
1. Antu, A. D.; Jiang, Z.; Premathilka, S. M.; Tang, Y.; Hu, J.; Roy, A.; Sun, L. Bright Colloidal PbS Nanoribbons. Chem. Mater. 2018, 30, 3697-3703.

To accelerate the commercialization of various innovative electronic and optical devices using molybdenum disulfide (MoS₂), it is required to develop a large-scale deposition method of MoS₂ with a high film quality and a low thermal budget. Among many deposition techniques, atomic layer deposition (ALD) is the most ideal approach due to its atomic-scale thickness controllability. Recently, several researchers have demonstrated initial feasibilities for the ALD of MoS₂ using several Mo precursors, such as Mo(CO)₆, Mo(NMe₂)₄, Mo(thd)₃ and MoCl₅ [1-6]. However, there are still many problems to be solved, such the requirement of high deposition temperature or high-temperature post-deposition annealing to improve film crystallinity. In addition, more in-depth understanding of the effects of various process parameters on the film growth and quality is required.
In this presentation, a few-layered MoS₂ films were synthesized via ALD using MoCl₅ and H₂S precursors at temperatures less than 450 °C. Particularly, the effects of various process parameters, such as a substrate temperature, a MoCl₅ canister temperature, a flow rate of H₂S, on the MoS₂ film characteristics were investigated. The nucleation density and final surface coverage of the MoS₂ films were examined using scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Raman and photoluminescence characteristics were also studied, and the electrical properties were evaluated by fabricating transistor devices.

[1] J. J. Pyeon, S. H. Kim, D. S. Jeong, S.-H. Baek, C.-Y. Kang, J.-S. Kim, and S. K. Kim, Nanoscale, 2016, 8, 10792.
[2] S. Oh, J. B. Kim, J. T. Song, J. Oh, and S.-H. Kim, J. Mater. Chem. A, 2017, 5, 3304.
[3] T. Jurca, M.J. Mody, A. Henning, J.D. Emery, B. Wang, J.M. Tan, T.L. Lohr, L.J. Lauhon, and T.J. Marks, Angew. Chem., 2017, 129, 5073.
[4] M. Mattinen, T. Hatanpaa, T. Sarnet, K. Mizohata, K. Meinander, P.J. King, L. Khriachtchev, J. Raisanen, M. Ritala, and M. Leskela, Adv. Mater. Interfaces, 2017, 4, 1700123.
[5] L. K. Tan, B. Liu, J. H. Teng, S. Guo, H. Y. Low, and K. P. Loh, Nanoscale, 2014, 6, 10584.
[6] L. Liu, Y. Huang, J. Sha, and Y. Chen, Nanotechnology, 2017, 28, 195605.

Monolayers of transition metal dichalcogenides (TMD) are promising components for flexible optoelectronic devices due to their direct band gap and atomically thin nature. The photoluminescence (PL) from these materials is strongly dependent on mid-gap defects which serve as non-radiative recombination sites for excitons. Improving the PL is a key step towards realizing high efficiency optoelectronics such as solar cells and light emitting diodes. Here, we demonstrate up to a 200x increase in PL intensity by exposing MoS₂ synthesized by chemical vapor deposition (CVD) to laser light in ambient. This spatially resolved passivation treatment is air and vacuum stable, which indicates strong bonding of moieties from ambient. Regions unexposed to laser light remain dark in fluorescence despite continuous impingement of ambient gas molecules. This suggests a large barrier towards adsorption and subsequent PL brightening. A wavelength dependent study confirms that the PL brightening is concomitant with exciton generation in the MoS₂; laser light below the optical band gap of MoS₂ fails to brighten the TMD. In contrast to previous laser modification studies that use mW's of laser power, we highlight the photo-sensitive nature of the process by successfully brightening with a broadband white light source (< 10 nW). Cryogenic studies show that the treatment results in an increase of both the bound and neutral exciton intensities. Additionally, we measure higher absorption from the laser brightened MoS₂ regions. We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement, which is independent of exciton generation, confirms that the laser brightening reduces non-radiative recombination sites in the MoS₂. We propose that isovalent O₂ passivates sulfur vacancies in the CVD-grown MoS₂ but requires photo-generated excitons to overcome the large adsorption barrier. This work represents an important step in understanding the passivation of CVD synthesized TMDs.

This research was performed while S.V.S and M.R.R held a National Research Council fellowship and H.-J.C. held an American Society for Engineering Education fellowship at NRL.

Exfoliating layered materials within liquids to create dispersions of nanosheets allows for a scalable method to both isolate and manipulate 2-dimensional materials. However, most methods for this so-called “liquid phase exfoliation” rely on aggressive chemical or physical processes to separate the layers, followed by ultracentrifugation to remove large aggregates, making the processes difficult to scale-up. Moreover, such routes typically result in metastable suspensions of fragmented, physically damaged, or chemically modified nanosheets. I will discuss recent work on the true, spontaneous dissolution of 2-dimensional materials from layered materials [1,2]. For the naturally uncharged layered materials, true dissolution into polar aprotic solvents is achieved by introducing charges onto the layers via intercalation of ions. This gentle process maintains the morphology of the starting material, is stable against reaggregation and can achieve solutions containing exclusively individualized, crystalline monolayers. The charge on the anionic nanosheets solutes is reversible, enables targeted deposition over large areas via electroplating and can initiate novel self-assembly upon drying. I will go on to present more recent work which takes advantage of the charge on the nanosheets for further processing. Specifically, a distribution of metal nanoparticles of optimal size and high uniformity has been achieved on 2-dimensional materials for the oxygen reduction reaction (ORR), resulting in a catalyst with remarkable stability.

[1] Ionic solutions of two-dimensional Materials, Cullen PL, Cox KM, Bin Subhan MK, Picco L, Payton OD, Buckley DJ, Miller TS, Hodge SA, Skipper NT, Tileli V, Howard CA 9, 244–249, Nature Chemistry (2017)
[2] Single crystal, luminescent carbon nitride nanosheets formed by spontaneous dissolution. Miller T, Suter T,Telford, A,Picco L, Russel-Pavier F, Cullen PL, Payton O, Sella A, Shaffer M, Nelson J, Tileli V, McMillan P, Howard CA Nano Letts (2017)

Molybdenum disulfide (MoS₂) and its related layered transition-metal dichalcogenides (TMDs) have attracted much attention as potential electrocatalysts for converting carbon dioxide to fuels due to their lower price compared with precious metals and their prominent catalytic features. MoS₂ and MoSe₂ have recently been shown to perform as excellent electrocatalysts in ionic-liquid-based systems for the CO₂ reduction reaction (CO₂RR)^1,2. However, achieving selectivity in the CO₂RR is challenging due to the numerous possible chemical reaction pathways and their very similar reduction potentials, often leading to a multitude of CO₂RR products. Theoretical calculations³ indicate that the conduction band (CB) edge position of TMD materials can be tuned by adjusting the layer thickness (i.e. monolayer vs. bilayer), as well as by chemical alloying (e.g. MoSSe). The TMDs therefore offer a suitable material system for adjusting the CB edge of the catalyst relative to a given reduction potential for CO₂RR. Herein, we report the thickness-controllable, large-area (1 cm²) synthesis of MoS_2-xSe_x thin-films for the CO₂RR via metal–organic chemical vapor deposition (MOCVD) followed by a post-selenization process. The post-treatment enables the Se incorporation into the MoS₂ films as well as tunability of the S/Se ratio in MoS_2-xSe_x alloys. As a first step, we evaluated bulk crystals of MoS₂, MoSSe, and MoSe₂ for electrochemical CO₂RR in aqueous K₂CO₃ solution (pH = 6.8) at -0.4 V vs. RHE. The results showed that both MoSSe and MoSe₂ produce 4 times more CO and CH₄ than MoS₂. To further explore the effect of the CB edge position relative to the CO₂RR, we examined the performance of thickness-controlled, MoS_2-xSe_x electrodes incorporated on glassy carbon substrates for electrocatalysis. We found that the MoS_2-xSe_x (x=0.46) films own the highest faradaic efficiencies about 16.6% , which is much higher than MoS₂ film. The CO₂RR product analysis as a function of film composition will be discussed and compared to the relevant CO₂RR reduction potentials.

Recently, two-dimensional transition metal chalcogenides (TMC) such as MX₂ and MX₃ (M= Mo, W, Nb, Ti; X= S, Se, Te) have gained considerable attention due to their unique physical and chemical properties. Among these TMCs, titanium trisulfide (TiS₃) has a unique semiconducting behaviour independent of its thickness with a direct band gap of ~1eV which could be ideal for future (opto) electronics applications¹. On the other hand, titanium disulfide (TiS₂) is a highly conductive semimetal which is suitable for energy storage², photovoltaics³, and catalysis⁴ applications. Synthesising both TiS₂ and TiS₃ at low temperature in a controlled manner over a large area would be beneficial for the aforementioned applications. Atomic layer deposition (ALD) provides advantages such as precise thickness control, low temperature processing environment, uniformity over large area, and conformality over a 3D substrate. Therefore, ALD could be a valuable method to grow layered titanium sulfides for a wide range of applications compared to other deposition techniques.
In this work, TiS₂ and TiS₃ thin films were deposited by ALD using a metalorganic precursor and H₂S at temperatures between 50 and 250°C. TiS₂ growth was achieved by both thermal and plasma-enhanced atomic layer deposition (PE-ALD) processes, whereas TiS₃ growth was only attained by PE-ALD process where H₂S plasma was used as the co-reactant. To the best of our knowledge, this is the first work to report on synthesising TiS₃ by ALD. In-situ spectroscopy ellipsometry was used to confirm the self-limiting ALD behaviour and also to measure the growth per cycle (GPC). For both TiS₂ and TiS₃ films, GPC between ~1.6 and 2.5Å was achieved with both ALD processes in the studied temperature range. In case of PE-ALD process, TiS₃ was only grown at lower temperature, whereas at higher temperature pyrolysis leads to formation of TiS₂. The synthesis of both TiS₂ and TiS₃ was elucidated by Raman spectroscopy studies where vibrational modes identical to two-dimensional TiS₂ and TiS₃ structure were observed. X-ray photoelectron spectroscopy and Rutherford back scattering measurements indicated sulphur deficient TiS₂ films, while for TiS₃ excess sulphur content was detected. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed control over film morphology was achieved by varying the processing conditions. Four point probe measurements yielded low resistivity for TiS₂ films in the order of 1x10^-3Ω-cm. To summarise, we show a novel low temperature process to synthesis both di- and tri- sulfides with precise thickness control over a large area.
¹ J.O. Island et. al, 2D Mater. 4, 022003 (2017).
² K. Sun et al, J. Electrochem. Soc. 164, A1291 (2017).
³ P. Huang et al, ACS Appl. Mater. Interfaces 10, 14796 (2018).
⁴ Y. Liu et al, Adv. Mater. Interfaces 5, 1700895 (2018).

Over the past decade, the class of 2D materials has been mined for substances with unique properties to serve as the foundation for a potential post-silicon or silicon-plus era. The effects of scaling which demands a miniaturization of the device size in all three dimensions, has led research into single or few-layer thick materials. Black phosphorus (BP) is highly attractive since it contains a direct band gap ranging from 0.3 (bulk) to 1.7-2.0 eV (monolayer) allowing for the electrical properties of the material to be tailored to specific devices, such as optoelectronics and transistors, while retaining a high carrier mobility. Mono and few-layer films of BP are called phosphorene.

A synthesis method to grow large area phosphorene films under easily obtained conditions is required in order for commercialization. A reasonable first step would be the growth of black phosphorus thin films directly onto substrates. Here we present a single-step growth of black phosphorus thin film crystals directly on silicon using a self-contained short-way transport technique under relatively low-pressure conditions (<1.5 MPa). The synthesis proceeds with SnI₄, Sn, and red phosphorus as the starting materials. The region of precursor concentrations and growth conditions required for thin film crystals are found to be substantially different from bulk crystal growth. Violet phosphorus has been suggested to be an important intermediate in the bulk growth process. We find that it is the preferred phosphorus phase at low SnI₄ concentrations, while black phosphorus is the preferred phase at higher SnI₄ concentrations. The effect of the starting molar ratio has been mapped to optimize direct growth of BP on silicon wafers. However, the successful direct growth of BP has also been achieved on silicon dioxide, silicon nitride, and sapphire wafers. The same phase forms on each substrate, however, silicon is found to produce slightly larger crystals. This may be due to the fact that the optimization was done on Si and suggests that substrate epitaxy is not a critical factor in growing BP thin films.

Raman spectroscopy of many of the BP deposits show no sign of degradation for A_g¹, B_2g, and A_g² phonon modes found at 360.5, 436.5, and 464 cm^-1, respectively. This is true up to several months of exposure to ambient conditions. Auger electron spectroscopy reveals an ultra-thin (~3 atoms) layer of Sn uniformly coating the surface of BP crystals which may act as a passivation. A BP hero crystal was formed with lateral dimensions of 10 x 85 mm and a thickness of 115 nm. Electronic properties of BP crystals are currently being characterized using back-gated transistors grown on a silicon wafer with a 60 nm thick thermal oxide. Lastly, contact resistance and sheet resistance will be determined by transfer length measurements (TLM) done using Au (5 nm)/Ti (50 nm) contacts.

Alloying and doping are considered versatile strategies for tuning charge and heat transport in nanostructures. Whether the resulting alloy structure is random or ordered can have a profound impact on the macroscale electronic, optoelectronic, vibrational, and transport properties of the material. In this talk, we present structural and chemical ordering as a mechanism to design anisotropy in the family of 2D transition metal dichalcogenides (TMDs) alloys. Leveraging recent advancements in atomic resolution scanning/transmission electron microscopy imaging and spectroscopy, we show the formation of chemically ordered states and vacancy/dopant coupling that leads to unusual relaxation effects around dopant-vacancy complexes leading to local strain and symmetry breaking around individual dopant sites. In addition, we will further uncover the defect structure, i.e. grain boundaries and anti-phase boundaries, and their stability and dynamics in monolayer TMD crystals and their alloys. This understanding can have a strong impact on the synthesis and functionality of novel nanostructure alloys and provides the key to properly design devices for heat dissipation applications, energy storage, electronics, optoelectronics, and thermoelectrics.

Manufacturing of paper, which started two thousand years ago, simplified all aspects of information technology: generation, processing, communication, delivery and storage. Similarly powerful changes have been seen in the past century through the development of integrated circuits based on silicon. In this talk, I will discuss how we can realize these integrated circuits thin and free-standing, just like paper, using two-dimensional materials and how they can impact the modern information technology.
In order to build these atomically thin circuits, we developed a series of chemistry-based approaches that are scalable and precise. They include wafer-scale synthesis of three atom thick semiconductors and heterojunctions (Nature, 2015; Science 2018), a wafer-scale patterning method for one-atom-thick lateral heterojunctions (Nature, 2012), and recently, atomically thin films and devices that are vertically stacked to form more complicated circuitry (Nature, 2017). Once realized, these atomically thin circuits will be foldable and actuatable, which will further increase the device density and functionality. The fact that these circuits could be realized and function without any substrate will allow them to be used tether-free (or wirelessly) in environments not previously accessible to conventional circuits, such as water, air or in space.

Monolayer transition metal dichalcogenides (TMDs, MoS₂, WSe₂, etc.) possess a range of intriguing optical and electronic properties including direct bandgap, large exciton binding energies, valley polarization, etc. Current research is typically carried out using flakes exfoliated from bulk crystals or monolayer domains grown by powder source chemical vapor deposition (CVD) which are challenging to scale to large area. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and other compound semiconductors, based on gas source CVD. This approach provides a high overpressure of chalcogen species needed to maintain stable growth at elevated temperature and excellent control of the precursor partial pressures to achieve monolayer growth over large area wafers.

Our initial studies focused on the epitaxial growth of binary TMD monolayers (MoS₂, WS₂, WSe₂ and MoSe₂) using metal hexacarbonyl and hydride chalcogen precursors on 2” sapphire substrates in a cold-wall CVD reactor. A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of monolayer domains on the substrate. This approach also enables measurement of metal-species surface diffusivity and lateral growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates at growth rates on the order of ~1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire with narrow X-ray full-width-at-half-maximum indicating minimal rotational misorientation of domains within the basal plane. Post-growth transmission electron microscopy carried out on monolayers transferred from the sapphire demonstrates that the films consist of large single crystal regions with anti-phase grain boundaries that result from a merging of 0^o and 180^o oriented domains that form on sapphire. Growth of (Mo,W)S₂ alloy mono- and few layers was also achieved over the entire composition range by controlling the inlet gas phase ratio of Mo and W hexacarbonyl precursors. Applications and challenges of this technique for the growth of vertical 2D heterostructures will also be discussed.

The layer-dependent material properties and emergent electronic properties of van der Waals heterostructures make 2D materials attractive for a wide range of electronic and energy applications. However, the biggest hurdle for realizing these applications lies in the lack of scalable synthesis of 2D materials with precision control over crystallinity, doping, and other parameters that greatly influence their electrical properties. In this talk, I will discuss our efforts in scalable synthesis of sulfides, tellurides, and their heterostructures and the effects of strain on the crystalline quality and morphology of the synthesized thin films [1-4]. Based on these results, an alternative synthetic route is being explored for large-area synthesis. I will also discuss the electrocatalytic activities of MoS₂ and WTe₂ for hydrogen evolution reaction, paying particular attention to how electron injection and transport within the catalyst can be a rate limiting step [5], irrespective of thermodynamic energy considerations of hydrogen adsorption on the catalytic site. By performing the electrochemical hydrogen evolution reaction on nanodevices of isolated flakes and heterostructures, we showed that the presence of a Schottky barrier between a catalyst and an electrode can significantly inhibit the catalytic reactions. The nanodevice approach based on a single-crystalline flake enables us to measure interfacial effects accurately, which is challenging in complex nanostructured catalysts. Our finding is broadly applicable to other semiconducting catalysts or catalysts with low electrical conductivity.

[1] ACS Nano 8, p.9550 (2014)
[2] Nano Lett. 14, p.6842 (2014)
[3] ACS Nano 10, p.2004 (2016)
[4] Adv. Func. Mater. 27, 1605028 (2017)
[5] Adv. Mater. 30, 1706076 (2018)

Liquid exfoliation has become an important production technique to give access to large quantities of two-dimensional nanosheets in colloidal dispersion.[1] Importantly, this is a highly versatile technique that can be applied to numerous layered materials beyond graphene such as transition metal dichalcogenides, III-VI semiconductors, black phosphorus, layered oxides to name just a few. These can be cast into films and composites and have proven useful in a number of application areas. Recent progress in size selection has enabled the production of high quality nanosheet dispersions with controlled thickness and lateral size.[2] This was made possible by spectroscopic size and thickness metrics that allow us to extract nanosheet lateral size, thickness and monolayer content from simple measurements such as extinction spectroscopy.[2,3,4]
We now use this basic understanding to systematically compare a range of 2D materials with respect to exfoliation, size selection and size-dependent properties to identify unifying principles. In all cases, we investigate chemical degradation/oxidation via the characteristic optical fingerprints.
In addition, we explore different ways for further functionalisation of TMD nanosheets by various methods. This includes noncovalent approaches, where we can track the functionalization in optical spectra due to the dielectric screening of the excitonic transitions. We also elaborated ways to chemically modify the basal plane [5,6] or edge sites [7] of TMDs. For example, we demonstrate that chemical functionalisation of vacancy defect sites can be used to protect otherwise instable materials from degradation while edge defects such as thiols can be converted to disulfides in redox reactions. These findings present an important step towards defect engineering in 2D materials.

[1] Adv. Mater. 2016, 28, 6136-6166.
[2] ACS Nano 2016, 10, 1589-1601.
[3] Nat. Commun. 2014, 5, 4576.
[4] PSS B 2017, 254, 1700443.
[5] Angew. Ch. Int. Ed. 2015, 54, 1-6.
[6] ACS Nano 2015, 9, 6018-6030.
[7] NPJ 2D Mat. 2018, 1, 43.

Doping plays a crucial role for tuning the functionalities of semiconductors. The ability to control the charge carrier density and polarity as well as their optical properties is the fundamental to our current optoelectronic technology. Atomically thin transition metal dichalcogenides (TMDs) have been the material of focus in recent years due to their exceptional physical properties and great potential as the platform for next generation optoelectronic devices. Here we present an air stable and scalable doping method based on metal-organic chemical vapor deposition (MOCVD) where the charge carrier polarity (electrons or holes) and carrier densities can be precisely controlled. Using gas phase precursors, we achieved 2 cm by 2 cm scale (limited by the size of the furnace) growth of Nb-doped and Re-doped MoS₂ which are p-type and n-type semiconductors respectively, that is homogeneous over the entire substrate. The amounts of dopants of each polarity can be precisely tuned from sub 1% to 25%. The resulting film is stable in ambient up to months evident by the intact conductivity of the as fabricated FETs. Furthermore, the resulting p-type and n-type films can be further detached from the substrate and stacked together generating more functional devices in large scale.

Single atom catalysts provide exceptional activity and turnover frequencies. However, single atom catalytic activity is often measured by dispersing atoms and nanoparticles on catalyst support. This makes is difficult to study the fundamental activity from a single atom in real electrochemical environments. Here, we report catalytic activity from precise number of sulfur vacancies on two dimensional molybdenum disulfide grown by chemical vapor deposition. The vacancies were introduced by precisely varying the dose of helium ions in a helium ion microscope (HIM). We then counted the number of vacancies per cm² using high resolution scanning transmission electron microscope (STEM). The catalytic activity was measured using micro-electrochemical cells previously developed in our group [1]. The devices allow precise control of the area over which the catalytic properties are measured to ensure that activity is only coming from the sulfur vacancies. Our results show that the catalytic properties of the MoS₂ improves with the number of vacancies. More importantly, we demonstrate that it is possible to extract the activity from a single sulfur vacancy. We have found that at a vacancy concentration of 5.4 x 10¹⁴ cm^-2, the intrinsic turn over frequency (TOF) and Tafel slope of a single atomic vacancy is ~ 6 s^-1 and 38 mV/dec, respectively. The TOF is amongst highest reported and the Tafel slope approached that of platinum.



1. Voiry et al. The role of electronic coupling between substrate and 2D MoS₂ nanosheets in electrocatalytic produc tionofhydr ogenTherole of ele ctroniccoupling, Nature Materials, 2016, 15, 1003

The strong absorption and visible spectrum energy bandgaps for the transition metal dichalcogenides (TMDCs) of molybdenum and tungsten render them as attractive candidates for photovoltaics (PV) and optoelectronics. Further, the atomically thin nature is favorable for efficient separation and collection of photo-excited charge carriers. Thus if the three major optoelectronic criteria, i.e., i) sunlight absorption, ii carrier collection and iii) operating voltage can be addressed for TMDC materials, they may be candidates for high efficiency photovoltaics and photocatalysis¹. We recently demonstrated near-unity broad-band absorption of above band-gap photons for < 15 nm TMDC layers², and have also achieved high external quantum efficiency in < 10 nm thick active layer photovoltaic devices in a pn junction of WSe₂/MoS₂with graphene contacts³. To date, achievement of high open-circuit voltage (Voc) has remained an outstanding challenge for achieving high photovoltaic efficiency. We report here high open circuit voltages (Voc) in TMDC absorbers based photovoltaic devices, achieved by tailoring the conduction and valence band-alignments between a single TMDC absorber layer and carrier-selective contact layers for electron collection (titanium oxide) and hole collection (nickel oxide), respectively. The band alignments measured using X-ray photoelectron spectroscopy indicate the asymmetric and selective nature of the metal oxide carrier-selective contacts, and we observe open circuit voltages exceeding 700 mV under AM 1.5 illumination at 1 Sun. We will also discuss TMDC passivation, and architectures for photocatalysts and photoelectrochemical devices that employ these materials as absorbers and catalysts.

References:
1. D. Jariwala, J. Wong, A. R. Davoyan, H. A. Atwater, ACS Photonics12, 2962-2970 (2017).
2. D. Jariwala, A. R. Davoyan, G.Tagliabue, M. C. Sherrott, J.Wong, H. A. Atwater,Nano Lett., 16,
pp. 5482-5487 (2016).
3. J. Wong, D. Jariwala, G. Tagliabue, K.Tat, A. R. Davoyan, M. C. Sherrott, H. A. Atwater, ACS Nano, 11, 7230–7240 (2017).

Intersubband transitions stem from quantum confinement of charge carriers to a nanoscale quantum well. The discovery of intersubband transitions in III-V semiconductor heterostructures had a huge impact on large parts of the condensed matter physics community and ultimately led to the development of quantum well infrared photodetectors [1] and quantum cascade lasers [2]. Highest quality quantum wells are typically grown by molecular beam epitaxy. However, this technology suffers two major limitations: There is a lattice-matching requirement, restricting the freedom in materials to choose from and the thermal growth leads to diffusive interfaces. Intersubband transitions in few-layer 2D materials hold the potential to overcome these limitations. 2D materials naturally form a quantum well with atomically sharp interfaces. Furthermore, no epitaxial growth on a matching substrate is needed, allowing to couple intersubband transitions to other electronic and optical systems such as Si CMOS and optical cavities, as well as any material of the large 2D family, which include semimetals, topological insulators, superconductors and ferromagnets [3, 4]. Surprisingly, intersubband transitions in few-layer 2D materials have never been studied before, neither experimentally nor theoretically.

Here we present the first experimental observation of intersubband transitions in quantum wells of few-layer semiconducting 2D materials (TMDs) [5]. We apply scattering scanning near-field optical microscopy (s-SNOM) as an innovative measurement approach allowing for spectral absorption measurements with a spatial resolution below 20 nm. Nano-imaging of intersubband transitions has never been performed before, not even in bulk semiconductors. Our exfoliated TMDs comprise terraces of different layer thicknesses over lateral sizes of about a few micrometers. We directly observe the intersubband resonances for these different quantum well thicknesses within a single device. Furthermore, we can electrostatically tune the charge carrier density and demonstrate intersubband absorption in both valence and conduction band. We support our findings with detailed theoretical calculations that are fully consistent with the observed results.

[1] B. F. Levine, Journal of Applied Physics 74, R1–R81 (1993).
[2] J. Faist et al., Science 264, 553–556 (1994).
[3] K. S. Novoselov et al., Science 353, 461 (2016).
[4] A. K. Geim et al., Nature 499, 419–425 (2015).
[5] P. Schmidt et al., under review.

The band structure of transition metal dichalcogenides (TMDCs) with valence band edges at different locations in the momentum space could be harnessed to build devices with operation relying on the valley degree of freedom. To realize such valleytronic devices, it is necessary to control and manipulate the charge density in these valleys. In addition, the long lifetime of interlayer excitons in TMDC heterostructures and the associated long diffusion length offer new opportunities. In my talk, I will present our recent efforts in this direction. In the first part, I will present our direct measurements of the conduction band splitting in MoS₂ quantum point contacts. In the second, I will present our recent results on the electrical manipulation of interlayer excitons in TMDC heterostructures.

Metal-semiconductor contacts are one of the most ubiquitous and fundamental types of junctions in all of optoelectronic devices, usually associated with the formation of a Schottky barrier and a depletion region. In the limit that the metal-induced depletion region is larger than the thickness of the semiconductor, an effective doping of the semiconductor occurs. Two-dimensional semiconductors such as MoS₂ present an attractive platform to examine this proximity effect because of their naturally passivated surfaces and interlayer van der Waals interactions, allowing them to be placed on arbitrary substrates. We explore this substrate interaction through angle resolved photoemission spectroscopy measurements of few-nm thick MoS₂ on Au, showing evidence for Fermi level shifts that are dependent on the thickness of MoS₂. This effect suggests the possibility of creating novel homojunction devices through thickness variation, even in the absence of bandstructure renormalization. Here, for the first time to the best of our knowledge, we measure the spatiotemporal dynamics of such junctions with ultrafast scanning electron microscopy. We observe ultrafast anisotropic carrier transport in time and in space. We further investigate the carrier dynamics in such a complex junction with laser-induced photocurrent mapping, which suggests counter-intuitive carrier dynamics. We conclude by analyzing and describing the time and length scales that are relevant for these junctions, which have far-reaching consequences for applications in atomically-thin semiconductor devices.

Excitons in two-dimensional (2D) semiconductors possess giant oscillator strength and play a fundamental role in mediating the strong light-matter interaction. Among others, monolayer transition metal dichalcogenides exhibit strong excitonic absorption due to band nesting [1] and allow exploration of hybrid quasi-particle states such as plexcitons [2] through strong dipole-dipole coupling [3]. The first part of this talk will focus on our approaches to realizing electrical generation, manipulation, and detection of excitons and their complexes based on various van der Waal heterostructures. Specifically, I will discuss how MIS-type heterostructures allow electrically tunable excitonic electroluminescence and electro-optic upconversion in linear optics regime. We demonstrate that hexagonal boron nitride can serve as a unipolar tunnel barrier that allows hot carrier injection and energy harvesting [4]. The second part will discuss our recent discovery of a novel monolayer MoS₂ growth mechanism based on vapor-liquid-solid conversion [5]. We show that alkali metal plays a key role in reducing the melting point of the precursors and triggering the vapor-liquid-solid mode, yielding expitaxial growth of monolayer nanoribbons.

[1] D. Kozawa et al. “Photocarrier relaxation pathway in two-dimensional semiconducting transition metal dichalcogenides” Nat. Comm. 5, 4543 (2014).
[2] W. Zhao et al. “Exciton-plasmon coupling and electromagnetically induced transparency in monolayer semiconductors hybridized with Ag nanoparticles” Adv. Mater. 28, 2709 (2016).
[3] D. Kozawa et al. "Efficient interlayer energy transfer via 2D dipole coupling in MoSe₂/WS₂ heterostructures" Nano Lett. 16, 4087 (2016).
[4] S. Wang et al. “Efficient carrier-to-exciton conversion in field emission tunnel diodes based on MIS-type van der Waals heterostack”Nano Lett. 17, 5156 (2017).
[5] S. Li, et al. “Vapor-liquid-solid growth of monolayer MoS₂ nanoribbons” Nat. Mater. 17, 535 (2018).

Layered hexagonal boron nitride (hBN) is a wide band gap semiconductor (> 6 eV), which attracts a growing interest for its strong UV photoluminescence properties [1]. The optical properties of bulk hBN as well as BN layers are governed by strong excitonic effects. They have been studied recently, but experiments are difficult because of the necessity to work in the far UV range [2].
In this contribution, we present a detailed theoretical study of the first excitonic levels, and characterize their energies and shape by combining ab initio calculations (GW plus Bethe-Salpeter equation) and a simple tight-binding model (as put forward by Wannier long time ago) [3].
In case of the monolayer, we analyse the first five excitons of single layer hBN, corresponding to the first three bright peaks and two dark states. Strong deviations from the usual hydrogenic model are evidenced due to both lattice effects and the 2D nature of the screening which can be approximated by a potential of the Keldysh type. Moreover, the analysis of the excitonic dispersion at finite q is also discussed. Then the effects of the number of layers on quasiparticle energies, absorption spectra and excitonic states is presented, placing particular focus on the Davydov splitting of the lowest bound excitons. We show that for N > 2 (N being the number of layers), one can distinguish between surface excitons mostly localized on the outer layers, and inner excitons, which leads to an asymmetry in the energy separation between split excitonic states. In particular, the bound surface excitons lie lower in energy than their inner counterparts. Additionally, this enables us to show how the layer thickness affects the shape of the absorption spectrum [4].

[1] L. Wirtz et al., Phys. Rev. Lett. 96 126104 (2006)
[2] L. Schué et al., Nanoscale 8 6986 (2016)
[3] T. Galvani et al., Phys. Rev. B. 94,125303 (2016)
[4] F. Paleari et al., arXiv:1803.00982 (2018)

As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin direct-gap semiconductors of transition metal dichalcogenides provide an appealing platform for studying exciton physics and for optically addressing the valley degree of freedom. In ultra-high quality WSe₂ monolayers, the photoluminescence (PL) emission peaks are sharp and can arise from excited exciton states at high energies and multi-particle bound states at low energies. We observe PL of the 1s, 2s, 3s and 4s Rydberg series [1]. Interestingly the 2s exciton exhibits much better valley polarization and coherence than the 1s exciton [2]. We also observe PL emission from correlated quantum states involving three, four and five particles [3]. Through a set of control experiments including charge doping, thermal activation, and magnetic-field tuning, we determine that the biexciton consists of a bright exciton and a dark exciton, while the exciton-trion is composed of a bright trion and a dark exciton, and that both of them are intervalley entities. Such unique spin-valley configuration gives rise to emissions with large, negative valley polarizations in contrast to that of the well-known two-particle excitons. Our experimental results provide new opportunities for building valleytronic quantum devices harnessing a variety of excitations in the system.

References
[1] S.-Y. Chen, Z. Lu, T. Goldstein, K. Watanabe, T. Taniguchi, D. Smirnov, and J. Yan, Bull. Am. Phys. Soc. March Meet. 2018, Los Angeles CA P07.00002 (2018).
[2] S.-Y. Chen, T. Goldstein, J. Tong, T. Taniguchi, K. Watanabe, and J. Yan, Phys. Rev. Lett. 120, 46402 (2018).
[3] S.-Y. Chen, T. Goldstein, T. Taniguchi, K. Watanabe, and J. Yan, https://arxiv.org/abs/1802.10247 (2018).

Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to identify the biexciton state in BN encapsulated single-layer WSe₂. The biexciton state only exists in charge neutral WSe₂, which is realized through the control of efficient electrostatic gating. In the lightly electron-doped WSe₂, one free electron binds to a biexciton and forms the trion-exciton complex. Improved understanding of the biexciton and trion-exciton complexes paves the way for exploiting the many-body physics in TMDs for novel optoelectronics applications.

The elemental two dimensional material, Black phosphorus (BP), exhibits layer-dependent tunable optical and electronic characteristics. [1] BP is known for its widely tunable and thickness dependent band gap which makes it a choice of material for various applications. [2] Each application demands an operational band gap and a material with a wide tuning band gap range will suit the purpose. [3] There are no simple and reliable technique to control the thickness of BP without introducing defects or in-gap states.This study demonstrates two ways of doping BP to tune the band gap by defect doping and by chemical doping. [4-6] Doping, a non destructive method, was used to to efficiently modulate the carrier concentration by extracting or injecting carriers into few layer pristine BP enabling a convenient pathway to control its optical and electronic properties. Here, we study the effects of the doping by engineering the defects and tuning the in-gap states introduced during the process. This opens up opportunities to expand the potential applications of few layered pristine BP. Effect of doping on few layer pristine Black phosphorus.
[1] Gusmão, R.,et al., M., 2017. Angewandte Chemie International Edition, 56(28), pp.8052-8072.
[2] Eswaraiah, V., et al., Z., 2016. Small, 12(26), pp.3480-3502.
[3] Cho, K., Yang, J., & Lu, Y. (2017). 32(15), 2839-2847.
[4] Utt, K.L., et al., 2015. ACS central science, 1(6), pp.320-327.
[5] Jing, Y., et al., 2015. Nanotechnology, 26(9), p.095201.
[6] Zhang, R., et al., 2015. The Journal of Physical Chemistry C, 119(5), pp.2871-2878.

The research on hexagonal boron nitride (hBN) has been intensified recently due to its application as a promising system of single photon emitters. As yet its single photon origin remains under debate even though a lot of experiments and theoretical calculations have been done. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperature by using diamond anvil cell (DAC) device. It is found that the absolute values of pressure coefficients of discrete PL emission lines are all below 15 meV/GPa which is much lower than 36 meV/GPa, the pressure-induced red-shift rate of hBN bandgap. These PL emission lines are originated from the atom-like localized defect levels confined within the band gap of hBN flakes. Interestingly, the experimental results of pressure-dependent PL emission lines present three different types of pressure response, corresponding to red-shift (negative pressure coefficient) and blue-shift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate that there is a competitive relation between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL-peak shift.

Catalysts that can reduce a kinetic overpotential, a potential barrier for charge transfer from a solid-state electrode to a liquid-phase electrolyte, are necessary for the realization of efficient photoelectrochemical (PEC) hydrogen generation. Recently, atomic layered transition metal dichalcogenides (TMDCs) such as MoS₂ have emerged as a promising candidate for non-precious and earth-abundant catalysts. Considerable research efforts have been devoted to identifying the active sites such as atomic edges and vacancies in those layered materials and activating (or maximizing) such catalytic sites for efficient hydrogen evolution reaction (HER). In addition to the optimization of catalytic active sites, the photo-excited charges must be efficiently separated and transported from the photocathode to the electrolyte for further enhancement on the PEC performances. To achieve such a goal, many previous studies have attempted to utilize the heterojunctions that form the cascade alignment between the band edge of a semiconductor and the hydrogen reduction potential. In this respect, the capability to build atomically thin heterojunctions with the designed energy alignment using various TMDCs with different band edges and work functions offers unexplored opportunities in optimizing the interfacial kinetics of photoelectrolysis. Nevertheless, it is difficult to exactly correlate PEC performance with the specific properties of the heterostructures and to study the corresponding HER mechanism due to ensemble averaging effects of various active sites. Here, we propose a novel strategy to reduce an overpotential by employing the atomically thin TMDC heterojunction as a HER catalyst. To prove our hypothesis, we newly develop the spatially-resolved PEC characterization platform using scanning photocurrent microscopy combined with a standard electrochemical measurement, allowing not only to visualize the enhanced PEC activity of the heterojunction catalyst but also to solely reveal their effects on HER without ensemble averaging effects of other extrinsic factors. Through the spatially-defined characterization under global illumination, we further confirm that the overpotential and charge transfer resistance at the interface can be significantly reduced by the atomically thin heterojunction catalyst. Our demonstration offers unprecedented opportunities not only to investigate the fundamental HER mechanism correlating with the tailored properties of a catalyst, but also to design highly efficient PEC cells.

Semiconducting two-dimensional transition metal dichalcogenide(TMDC) with wide visible range bandgaps of about 1-2 eV has been extensively studied for potential applications in electronics, optoelectronics, a memory device, and sensor. Furthermore, the combination of spin valley and valley degrees of freedom in semiconductor TMDC creates the possibility of manipulating them for future applications in spintronics and valleytronics devices. However, there are still challenges such as air stable and thermally stable Ohmic contacts with an order of magnitude lower contact resistance. To date, large gate doping is required for graphene contact and 1T-phase of TMDC is considered to be unstable in phase at cryogenic temperature. Here, we demonstrate low resistance Ohmic contact to single layer molybdenum disulfide (MoS₂) using highly n-type graphene, nitrogen-doped graphene, which has a work function similar to the electron affinity of single-layer MoS₂. FETs fabricated on hexagonal boron nitride (hBN) encapsulated MoS₂ has n-type behavior with an on/off ratio of about ~10⁶ and a field-effect mobility of 65 cm²/Vs at room temperature. The Vth of the transistors fabricated on pristine MoS₂ flakes falls in the negative regime (-75 ± 5 V) with an average on/off current ratio of ~10⁶. Transfer characteristics (I_SD versus V_G) for different source-drain voltages V_SD (10-50 mV) recorded in 2-point configuration shows pronounced increase of I_SD at positive V_G due to the formation of an n-type channel. By decreasing the temperature, the V_th of MoS₂ transistor can be left-shifted from the more n-type behavior of nitrogen-doped graphene to enhancement mode with the dominance of the pyridinic-N configuration in N-doped graphene. This work demonstrates the V_th tuning of the MoS₂ transistor and the newly developed contact engineering approach is applicable to a wide range of TMDC, compatible with conventional semiconductor processes. Also, this is expected to be a good electrode material for observing the band split in the conduction band of MoS₂ for future application in spintronics and valleytronics.

Two-dimensional transition-metal dichalcogenides (MX₂) have attracted considerable research interests in their electro-optical properties and potential application of high-performance and flexible electro-optical devices. Although 2D MX₂ can be directly grown on various substrates by vapor deposition synthesis, such synthesis can often be unreliable and recently people realize water vapor or uncontrolled environmental humidity can play important roles in the growth of MX₂, regardless if metal chlorides, metal oxides, or MX₂ bulk powder precursors are used. It was found that water vapor can work as a transport agent that react with bulk transition-metal dichalcogenide at high temperature and form volatile metal oxyhydroxide species (MO_x(OH)_y) and hydrogen chalcogenides (H₂X) that dramatically increase the effective vapor pressure. Here we report the controllable synthesis of WS₂, MoS₂ and their heterostructures using water-assisted chemical vapor transport (CVT). The amount of water vapor can be tunably controlled by utilizing the thermal decomposition of calcium sulfate dihydrate (CaSO₄●2H₂O), which as a solid state source of water vapor not only provides much lower vapor pressure baseline than liquid water, thus guarantees a wider tunable range, but can also be easily integrate into a chemical vapor deposition system without changing the setup. This allows precisely control of the nucleation and growth on the substrate, leading to the lateral epitaxial growth on the edge of transition-metal dichalcogenide and more reproducible growth of large area heterostructures. Raman spectral mappings confirm the heterostructure and electrical transport measurements revealed the p-type transport behavior for WS₂ and the n-type transport behavior for MoS₂. Our research provides chemical insight into the growth mechanism of transition-metal dichalcogenide and it can potentially be a general approach to controllable grow other metal chalcogenides.

Multilayer hexagonal boron nitride (h-BN) has been widely recognized as an ideal insulator to bring out the intrinsic properties of 2D materials [1,2]. Catalytic chemical vapor deposition (CVD) method is capable to produce large-area h-BN films suitable for practical applications [3-5]. However, Cu foils/films mainly give monolayer h-BN, which is not thick enough to screen out influences from substrate surface [3,4]. Recently, CVD growth of multilayer h-BN film was reported by using Fe foil, in which dissolution and segregation processes occur during the CVD due to relatively high B and N solubilities in Fe, but the multilayer h-BN film lacks uniformity [5].
Here, we demonstrate the successful synthesis of large-area, uniform multilayer h-BN on Ni-Fe alloy by suppressing the grain boundaries and tuning the solubilities of B and N [6]. To compare the crystallinity of Fe and Ni-Fe alloy catalysts, we measured their electron back-scatter diffraction (EBSD) after h-BN growth. Although Fe has polycrystalline structure with different crystal planes, Ni-Fe alloy is highly crystalized with a low density of grain boundaries. Moreover, the multilayer h-BN film transferred from Ni-Fe alloy to SiO₂/Si substrate shows a uniform optical contrast, while the one transferred from Fe indicates a large variation of thickness. Raman E_2g intensity collected from the h-BN film produced on Ni-Fe alloy is highly uniform, supporting the high uniformity of the h-BN thickness. To demonstrate the performance of our multilayer h-BN as a substrate for 2D materials, we have investigated the optical properties of WS₂ grown on the multilayer h-BN [6]. Intriguingly, we observed an intense and narrow PL peak from the WS₂ grown on h-BN as compared with that on SiO₂/Si_. The minimum PL line width of WS₂/h-BN (24 meV) is much narrower than that observed on SiO₂ substrate (67 meV) and is almost comparable to that of WS₂ on mechanically exfoliated h-BN (26 meV) [2]. This result demonstrates that our h-BN is effective as a substrate for 2D materials. Therefore, our CVD-grown multilayer h-BN is expected to offer an ideal platform for 2D materials, greatly contributing to their practical applications.

[1] C. R. Dean et al., Nat. Nanotechnol., 5, 722 (2010).
[2] M. Okada et al., ACS Nano, 8, 8273 (2014).
[3] K. Kim et al., Nano Lett., 12, 161 (2012).
[4] Y. Uchida et al., Phys. Chem. Chem. Phys., 19, 8230 (2017).
[5] S. M. Kim et al., Nat. Commun., 6, 8662 (2015).
[6] Y. Uchida et al., ACS Nano (2018) DOI: 10.1021/acsnano.8b03055

The 2D transition metal dichalcogenide (TMDC) WS₂ has attracted great interest due to its unique properties and prospects for future electronics. However, compared to MoS₂, the development of a reproducible and scalable deposition process for 2D WS₂ has not advanced very far yet. Here, we report on the systematic investigation of 2D WS₂ growth on sapphire (0001) substrates using a hydride-free metalorganic vapour phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9%) and di-tert-butyl sulfide (DTBS, 99.9999%) are used as MO sources. MO precursors allow a high-purity material synthesis as well as fine-tuning the H₂ partial pressure which is crucial for controlling nucleation, lateral growth and unwanted C deposition. Sapphire substrates enable a 3-on-2 superstructure of WS₂ and hence a clearly defined crystal alignment and orientation. All samples are characterized using Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical in-situ and ex-situ reflectometry.
MOVPE processes are started with a substrate prebake step at 1050 °C for 15 min in H₂. Our previous work on 2D MoS₂ has shown that such a step prior to growth can reduce the nucleation density and strongly inhibit the formation of a parasitic carbonaceous film directly on the sapphire surface. On the prebaked substrate, WS₂ is deposited at 30 hPa total pressure in N₂ atmosphere.
Comparably to MoS₂, the suitable growth temperatures for WS₂ lie between 750 °C and 845 °C (845 °C chosen as optimum here). Higher temperatures lead to an increasing parasitic C deposition, while lower temperatures result in a rapid decrease of the WS₂ domain size. Because nucleation and growth of TMDC are mainly controlled by the metal species, the influence of the WCO flow on lateral growth was investigated. Increasing the WCO flow systematically from 1 nmol/min to 10 nmol/min leads to an increase of the triangular domain size from 50 nm to 160 nm in 10 h processes. Simultaneously, the total surface coverage rises from 2.5 % to 12 %. Extending gradually the growth time from 10 h to 40 h leads to domain sizes of up to 400 nm and a surface coverage of about 60 %. However, Raman measurements show that longer growth times cause increasing C contaminations on and around the TMDC crystals, hampering further lateral growth. From previous experiments, it is also known that H₂ is able to etch C contaminations and to efficiently reduce WS₂ nucleation under process conditions. Adding minute amounts of H₂ to the growth atmosphere (100-200 sccm H₂ in approx. 20,000 sccm N₂) supports a competing metal etching reaction requiring the WCO supply to be increased to up to 20 nmol/min. First deposition experiments (10 h, 15 h, 20 h) in this process window allowed depositing fully coalesced WS₂ samples without C-related Raman peaks and with only sparse bilayer nucleation.

Atomically thin two-dimensional semiconductors have emerged as an interesting class of material systems, both for applications and fundamental studies. For opto-electronic applications like displays, light sources, and photovoltaics, transition-metal-dichalcogenides (TMDs) are an appealing system, as they combine great mechanical
strength with high carrier mobility and an direct optical band gap. In this rapidly developing field, attention has recently shifted towards the realization of nanostructures.

The generation of localized state, either induced via defects or via systematic confinement engineering, opens the possibility to deterministically generate single-photons or, more generally, provide sources of quantum light. For this purpose, flakes of TMDs have been placed on nanowires, over gold edges and over etched holes to form single-photon emitters. We focus on a different platform, which consists of TMD nano-bubbles that develop if air is enclosed during the stacking of layers.

The physics governing all of these examples has predominantly been discussed in terms of strain engineering. Due to the high bending rigidity, strain induces large variation of the band gap that can lead
to a transition from a direct to an indirect band gap. Another, much less discussed, mechanism is the change of the dielectric environment that also induces strong bandgap variations.

We report on results of atomistic tight-binding calculations of different sizes and height-to-diametercratios of these nanostructures and demonstrate that the formation of confined quantum-dot-like single-particle states is caused by an interplay of strain and dielectric screening. We show that the strain pockets are caused by a crumpling of the material due to its high bending rigidity and discuss the implications of the underlying physics to other TMD-based nanostructures.

Molybdenum disulfide (MoS₂) monolayer is a two-dimensional (2D) material, which is expected to provide the next generation of electronic devices together with graphene and other 2D materials. Due to its significance for future electronics applications, gaining a deep insight of the fundamental mechanisms upon MoS₂failure is crucial to prevent mechanical failure towards reliable applications. Here, we report atomic simulations to investigate how the mechanical properties are affected by various point defects and grain boundary. The mechanical properties (elasticity and strengths) of three different point vacancies (S₁, S₂, Mo) and three different point substitutions (MoS, MoS₂, Mo₂S₂) are systematically investigated from the defect ratios 0 to 30%. Also, the bi-crystals with different tilt angles (0~60°) are generated from our newly developed algorithm, where the defect shapes show good agreement with those from the previous experiments. The strengths of the bi-crystals reveal novel mechanistic insight into MoS₂with various defects and grain boundary.

2D material-based heterostructures have been intensively studied because of their unique device functionalities and novel physics. However, it is extremely challenging to have clean interface in the large area 2D heterostructures because a transfer method always accompanies residue issue on top of the 2D materials, which degrades the performance of the 2D heterostructures. For example, the most common approach for transfer of the 2D material at large scale is based on a polymer supporting layer such as poly(methyl methacrylate). It always remains the polymer residue on the 2D materials and wet-processing during the transfer is unavoidable, which can have extra molecular contamination involved at interface of 2D heterostructures during stacking. Accordingly, it has been required to develop an alternative way to produce extremely clean interface in 2D heterostructures
Here, we report that atomically smooth metal glue for 2D heterostructures. We studied the underlying mechanics to find a suitable material for 2D materials transfer. It was important to investigate and simulate the interfacial toughness at each interface and the internal stress in the material to induce driving force for exfoliation and transfer. Based on our experience, nickel is the best material which satisfies the crucial criteria for atomically smooth glue for 2D material stacking. Removing metal has been extensively studied and we could find the best way to get rid of the nickel glue from the 2D materials. In addition, this approach does not accompany wet-process during transfer and stacking, which preserves the cleanness at interface uniformly across wafer. We also confirmed that this discovery substantially improved uniformity in terms of electrical and optical properties of a wafer-scale heterostructure compared to one of 2D heterostructures made by wet-process. We believe this finding will lead a new opportunity for 2D material research since it can preserve ultra clean interface at 2D heterostructures at wafer-scale.

Oil spill pollution in marine environment is a major treat to the ecosystem, thus invokes immediate yet viable solutions. Amongst various technologies developed for oil-water separation, directly applying sorbent materials of high porosity to spilled oil presents advantages of low cost and operational simplicity. Efficient sorbent materials should combine high hydrophobicity and high porosity for simultaneous water repulsion and oil absorption, which has generally been achieved by their complicated surface modification. In this study, we report two-dimensional (2D) molybdenum disulfide (MoS₂) coated polydimethylsiloxane (PDMS) sponge and demonstrate their high proficiency in rapid and spontaneous oil-water separation and spilled oil detection. This novel sorbent material realized by the simple template-assisted fabrication of sponges and the dip-coating of 2D MoS₂ layers benefits from followings; 1) hydrophobic and semiconducting 2D MoS₂ layers for water repulsion and oil detection, respectively, and 2) intrinsic porosity in the sponge for oil absorption. The MoS₂-PDMS sponge presents the water contact angle of > 152° demonstrating excellent superhydrophobicity as well as achieving high oil absorption (> 100 wt%) for various tested oils including vegetable oil and fuel waste such as standard bilge mix (SBM). Also, the material can be recycled for > 10 times upon repetitive absorption/squeezing maintaining the excellent oil absorption capacity, enabled by its intrinsically excellent elasticity. The versatility of this sorbent material has been further extended for the spontaneous detection and identification of oils containing electrically conducting components, e.g., crude oil and SBM.

In recent years, 2D transition metal carbides, also known as MXenes, have attracted much attention and found applications in many different areas due to their unique physical structure and properties [1]. High electronic conductivity, good chemical and structural stability, and high surface area of MXenes allowed them to find applications ranging from energy storage to medicine to optoelectronics [1-2]. However, in spite of a clay-like behavior, not much is known about the rheological response of MXenes in solution. Understanding the rheological properties of two-dimensional (2D) materials such as MXenes in suspension is critical in the development of solution processing and manufacturing techniques. In this study [3], some of our most recent work on investigating the rheological response of single- and multi-layer Ti₃C₂ MXenes in aqueous environments will be presented. Viscosity and viscoelastic properties of these solutions at various loadings will be combined with “processability charts” to understand the suitability of different MXene morphologies for various fabrication techniques.

References
[1] Anasori, B., Lukatskaya, M. R., & Gogotsi, Y. (2017). Nature Reviews Materials, 2, 16098.
[2] Lipatov, A., Alhabeb, M., Lukatskaya, M. R., Boson, A., Gogotsi, Y., & Sinitskii, A. (2016). Advanced Electronic Materials, 2(12), 1600255.
[3] Akuzum, B., Maleski, K., Anasori, B., Lelyukh, P., Alvarez, N. J., Kumbur, E. C., & Gogotsi, Y. (2018), ACS nano, 12(3), 2685-2694.

Phosphorene is a promising semiconducting nanomaterial for electronic and optoelectronic applications. It exhibits tunable photoluminescence in the infrared, based on the number of phosphorene layers. However, phosphorene rapidly degrades in the presence of water and oxygen, which significantly limits its viability for real-world applications. In this work, we have developed a biomimetic strategy for encapsulation of phosphorene for degradation stabilization. To control oxygen diffusion, which results in degradation, phosphorene has been encapsulated in self-assembled natural and synthetic lipid bilayers. The kinetics of degradation have been examined by measuring changes in lipid bilayer vesicle size using dynamic light scattering, fluorescence, coupled quartz crystal microbalance analysis, and scanning probe microscopy. Additionally, the tunability of this biomimetic protective layer has been examined using vesicles composed of different lipid compositions and photo-crosslinkable lipids.

MXenes are a growing family of 2D transition metal carbides and nitrides, including 2D Ti₂C, Ti₃C₂, Mo₂TiC₂and W_1.33C. In recent years, since the discovery of Ti₄N₃, this family grew to include nitrides. 2D transition metal nitrides have attracted attention due to their potential applications in plasmonics and energy storage, due to their higher values of electrical conductivity. Although most MXene carbides are synthesized by the selective etching of bulk MAX phase precursors, new routes have been investigated for the synthesis of MXene nitrides. Recently reported 2D Mo₂N and V₂N/VN can be synthesized using one of several new routes, nitridation. With this technique, MXene carbides are used as precursors that are transformed into 2D metal nitrides via treatment under ammonia at elevated temperatures. While this procedure yields materials with higher electrical conductivities than their precursors with new potential applications, the mechanism of this transformation and the products that are formed are not well understood. In this presentation, a systematic investigation of the nitridation of several carbide MXene systems will be presented to determine how and why certain MXene systems transform. Changes in electrical conductivity and other improvements in performance will also be presented.

Over the past decade, comprehensive investigations have been conducted to develop two-dimensional (2D) materials to harness their excellent and unprecedented properties such as high electrical conductivity, optical transparency, mechanical strength, and flexibility. This necessitates developing means for their mass production. This study details a newly conceived compressible flow exfoliation method for producing 2D h-BN nano sheets using a multiphase flow of 2D layered materials suspended in a high pressure gas undergoing expansion. The expanded gas-solid mixture is sprayed in a suitable solvent, where a significant portion (up to 10%) of the initial hexagonal boron nitride (h-BN) material is found to be exfoliated with a mean thickness of 4.2 nm. Moreover, 43% of the exfoliated h-BN were found to be 10 layers or less thick while having a mean flake length of 276 nm and concentration of 0.22 mg/mL in Isopropyl Alcohol (IPA) after mild centrifugation. Shear-induced exfoliation occurs due to the high velocities that expanding and accelerating gases can achieve in small orifices coupled with viscous friction effects resulting in a high shear rate (γ>10⁵ s^-1) experienced by the bulk h-BN particles. Decoupling of exfoliation step from liquid suspension step leads to a very stable suspension of h-BN nano sheets into IPA for a long time (6 months). The CFE method has significant advantages over current 2D material exfoliation methods, such as chemical intercalation and exfoliation, as well as liquid phase shear exfoliation, with the most obvious benefit being the fast, continuous nature of the process. Other advantages include environmentally friendly processing, reduced occurrence of defects and versatility to be applied to any 2D layered material using gaseous medium. Scaling this process to industrial production has a strong possibility of reducing the cost of creating 2D nanomaterials.

To demonstrate the utility of our ultrafast CFE method, we considered improving the barrier property of polyethylene terephthalate (PET) by reinforcing it with exfoliated h-BN nano sheets. PET is commonly used for food and beverage packaging where the simultaneous requirements of high optical transparency and limiting oxygen transport have proven to be a technical challenge. The PET films remained transparent (>90%) when 0.017 and 0.15 vol% of CFE-BN were added. Addition of 0.15 vol% CFE-BN resulted in an improvement of the modulus of PET by 21%. Also this resulted in the dropping of oxygen permeation rate (OPR) of the PET film by 26%

Black phosphorus (BP) has recently attracted great attention due to its tunable, layer-dependent bandgap, high carrier mobility, good current on/off ratio, as well as unique in-plane anisotropy, which renders the material attractive for nanoelectronic, thermoelectric and photonic devices.^[1]
The major challenge for a successful application lies in the fabrication of few- or single layer nanosheets of BP. Several methods have been developed, while mechanical and liquid exfoliation of the bulk crystals figure amongst the most prominent ones.^[2] However, large-scale exfoliation leading to uniform and stable dispersions in high yield remains a challenge.
Herein, we present the electrochemical exfoliation of BP, which has already been successfully employed for graphene and other 2D materials, yielding nanosheets in good quality and high yield. Importantly, this technique is not limited to aqueous systems^[3] but has also been reported for various organic solvents and ionic liquids, which have an outstanding protective effect on BP.^[4] We present a screening of different electrolytes and anodic/cathodic conditions for the exfoliation process. Finally, Raman spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) have been applied to characterize the material, confirming the formation of BP nanosheets of good quality and in high yields.

[1] A. Hirsch, F. Hauke, Angew. Chem. Int. Ed. 2017, 57, 4338.
[2] D. Hanlon, C. Backes, E. Doherty, C. S. Cucinotta, N. C. Berner, C. Boland, K. Lee, A. Harvey, P. Lynch, Z. Gholamvand, S. Zhang, K. Wang, G. Moynihan, A. Pokle, Q. M. Ramasse, N. McEvoy, W. J. Blau, J. Wang, G. Abellan, F. Hauke, A. Hirsch, S. Sanvito, D. D. O'Regan, G. S. Duesberg, V. Nicolosi, J. N. Coleman, Nature communications 2015, 6, 8563.
[3] A. Ambrosi, Z. Sofer, M. Pumera, Angew. Chem. Int. Ed. 2017, 56, 10443.
[4] G. Abellán, S. Wild, V. Lloret, N. Scheuschner, R. Gillen, U. Mundloch, J. Maultzsch, M. Varela, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 2017, 139, 10432.

GeS belongs to a class of 2D group-IV monochalcogenides with a layered structure similar to that of black phospohorus. It has been theoretically predicted to combine robust room temperature ferroelectricity, high carrier mobility, and large excitonic effects [1-5]. We have recently demonstrated that photoexcitation of GeS nanoribbons results in an ultrafast shift current, suggesting possible applications in solar energy conversion devices based on the bulk photovoltaic effect [6]. Theory also predicts that extraordinary anisotropic electronic and optical properties of GeS can be engineered by strain and controlled by external fields [4]. Another attractive avenue for tailoring optoelectronic properties of GeS is intercalation of guest species at the van der Waals gap [7-10]. In particular, intercalation of zero-valent metals such as Cu and Sn into layered materials has been shown to result in new physical and chemical behaviors such as increased conductivity, enhanced conductivity and catalytic activity [11].
In this work, we have used time-resolved terahertz spectroscopy to investigate how intercalation of ~ 3 atomic percent of zero-valent Cu and Sn impacts transient photoconductivity of GeS nanoribbons. GeS, Cu-intercalated and Sn-intercalated GeS nanoribbons were excited with 1.55 eV, 100 fs pulses. We find that Cu-intercalated GeS nanoribbons have a shorter, ~ 30 ps vs 58 ps life time of the photoexcited free carriers, and a slightly lower, ~ 800 cm²/Vs vs ~ 1150 cm²/Vs photoexcited carrier mobility. The effect of Sn intercalation on the transient photoconductive response of GeS nanoribbons is negligible. These measurements demonstrate that zero-valent metal intercalation is a promising avenue for tuning the optoelectronic and chemical functionality of GeS nanostructures without sacrificing the trademark high carrier mobility of GeS. Furthermore, zero-valent Cu intercalation can also be used to modify the dynamics of photoconductivity for possible application of GeS nanoribbons and other nanostructures in high-speed electronic devices.

[1] A.M. Cook, M.F. B, F. de Juan, S. Coh, J.E. Moore, Nat Commun 8, 14176 (2017).
[2] Q. Hua Wang and Xiaofeng, 2D Materials 4, 015042 (2017).
[3] M. Wu, X.C. Zeng, Nano Lett. 16, 3236 (2016).
[4] F. Li, X. Liu, Y. Wang, Y. Li, J. Mater. Chem. C, 4, 2155 (2016).
[5] B.R. Tuttle, S.M. Alhassan, S.T. Pantelides, Phys. Rev.B 92, 235405 (2015).
[6] K. Kushnir, M. Wang, P.D. Fitzgerald, K.J. Koski, L.V. Titova, ACS Energy Lett. 4, 1434 (2017).
[7] Y. Jung, Y. Zhou, J.J. Cha Inorganic Chemistry Frontiers 3, 452 (2016).
[8] K.J. Koski, C.D. Wessells, B.W. Reed, J.J. Cha, D. Kong, Y. Cui, J. Am. Chem. Soc. 134, 13773 (2012).
[9] J.Y. Wan, S.D. Lacey, J.Q. Dai, W.Z. Bao, M.S. Fuhrer, L.B. Hu, Chem. Soc. Rev. 45 6742 (2016).
[10] M.J. Wang, K.J. Koski, ACS Nano 9, 3226 (2015).
[11] M.J. Wang, I. Al-Dhahir, J. Appiah, K.J. Koski, Chem. Mater.29, 1650 (2017).

Extensive reports have shown band-gap tuning in transition metal dichalcogenides (TMDs), from indirect band gap in the bulk material to a direct gap in single layer due to the absence of interlayer coupling. This unique property stems from the modified electronic states. The phononic properties are extremely critical in optimizing and understanding such electronic and optpelectronic behaviors. Several physical phenomena such as layered effect, quantum confinement effect, inhomogeneous heating effect, doping effect and disorder induce effect have strong influence on the phonon lineshape of TMDs. Using WS₂ as a prototype, we employ analytical studies to elucidate on how each of these process affect the phonon modes and provide a systematic route on how to clearly delineate these effects in the phonon lineshapes of TMDs.

Molybdenum disulfides (MoS₂), is composed of one layer of Mo atom covalently bonded with two S atoms on either site, had attracted much interest for the optoelectronic applications due to its significantly high photon absorption and electron-hole pair creation at even room temperature resulting from the strong interaction with the incident light. However, the absolute light absorption and the spectral selectivity of a monolayer MoS₂ are limited by its atomically thin layers and the electronic band gap, respectively. Recently, several methods, including the plasmonic surface and 3D nanostructure with the high surface area, have been developed with the aim to increase the light-matter interaction for light absorption enhancement and broaden detection ranges. Especially, the desirable dispersion relation of a monolayer MoS₂ allows readily controlling their plasmon resonance wavelength ranges than metals. According to the Drude model, a plasmon resonance within selective wavelength ranges can be observed by controlling the doping concentration in the semiconductors. Generally, the doping concentration should be larger than 10²¹ cm^-3 for observing the plasmonic peaks in the NIR and visible regions. Thus, developing the 3D plasmonic surface of a monolayer MoS₂ can be one of the crucial methods to effectively promote the detection ability of photodetectors by amplifying photoelectric gain and extending the detection range.
In this study, the spectral light absorption and plasmon resonance induced by the charged exciton (named as trion) in a monolayer MoS₂ with highly doped vertical nanoflakes are explored. First, we successfully synthesize the monolayer MoS₂ with vertically standing nanoflakes using one-step chemical vapor deposition (CVD) process. The size and density of the nanoflakes can be controlled by the CVD condition such as the Mo source/target substrate distance and the growth time. From the photoluminescence (PL) analysis, a higher contribution of the charged exciton in the nanoflake regions than the lateral MoS₂ layer is observed, which evidence that the structural defects from the edges of nanoflakes induce the excess charge carriers. Furthermore, the electron concentration of the nanoflakes can be tuned from 10²⁰ to 10²¹ cm^-3 by the oxygen chemical bonding. The absorption peak in the range of 800-1000 nm is observed that is evidence of the presence of the plasmon resonance in a monolayer MoS₂ with highly doped vertical nanoflakes. This result is consistent with the theoretical estimates of plasmonic peaks as a function of doping concentration. Finally, we demonstrate the enhancement of photodetection ability regarding broadening detection ranges toward NIR as well as the visible regions in a monolayer MoS₂ with vertical nanoflakes compared with a monolayer MoS₂.

The implementation of chemical vapor deposition (CVD)-grown two-dimensional (2D) transition metal dichalcogenides (TMDs) into practical applications largely relies on a robust and efficient transfer of TMDs onto target substrates. However, conventional transfer approaches such as the poly(methyl methacrylate) (PMMA)-based wet-transfer technique, suffer from surface contamination from polymer residues and structural damages induced by high-temperature etching process. In this work, using cellulose acetate (CA) as an alternative protection layer, we developed a facile CA-transfer technique for CVD-grown TMDs which can address the issues above. The structural and optical properties of the transferred TMD materials are well-preserved in the CA-transferred samples. Atomic force microscopy (AFM) in combination with scanning transmission electron microscopy (STEM) characterizations reveal largely improved micro- and nano-scale cleanliness and no observable wrinkles and/or cracks in TMDs transferred with CA-assisted method. The surface cleanliness and morphology improvement can be attributed to the low adhesion energy between CA and TMDs, as well as the usage of room-temperature etching process. Furthermore, we also demonstrated the capability of CA-transfer to be integrated with a deterministic transfer system for transferring flakes to selected locations on the substrate despite the substrate fragility. The CA-transfer technique developed in this work can be used as a clean and low-cost alternative to the PMMA-transfer method, thus paving the way to the high-resolution nanometer-scale characterizations and the implementations of novel applications of CVD-grown TMDs.

Molybdenum disulfide (MoS₂) has remarkable properties and is expected to be promising for next generation device applications. Fabrication of MoS₂ films with RF magnetron sputtering has various advantages, such as large area uniformity and reduction of carrier concentration. However, morphological properties of sputtered polycrystalline MoS₂ films are quite different from the single-crystal MoS₂ fabricated with mechanical exfoliation and chemical vapor deposition. For example, its grain size is greatly reduced due to fundamental problems of deposition environment, such as sulfur-deficient ambient originated from using Mo:S=1:2 target and high deposition rate. Moreover, such growth conditions produce vertically oriented MoS₂ grains in a thick region. Due to the morphological properties, Raman spectra of the sputtered MoS₂ films show unique behavior. In the case of single-crystal MoS₂, it is well known that frequency difference between E¹_2g and A_1g Raman modes (Dω) decreases with decreasing layer number. However, sputtered MoS₂ show a reverse trend. In addition, E¹_2g and A_1g Raman modes show shifting towards lower wavenumber in a thick region (> 20 nm). In order to reveal the origin of the unique phenomena and investigate the film characteristics, such as grain size, residual stress, and crystal orientation, we utilized thermal and strain influence introduced phonon conferment model (PCM) and simulated Raman spectra of horizontally and vertically oriented polycrystalline sputtered MoS₂ films. From the results of PCM simulation, the grain size of sputtered MoS₂ film deposited at 250°C was estimated to be 10 nm. The residual stress value increase with increasing film thickness, which was caused by crystal orientation transition and the difference of the linear expansion coefficient between a- and c-axis.
This work was partly supported by JSPS KAKENHI Grant Number 16J11377 and JST CREST JPMJCR16F4.

The resonance Raman spectra of monolayer MoS2 contains a rich variety of frequency-dependent features. Strong excitonic effects present in this 2D semiconductor call for further scrutiny on its Raman intensities. We present a first-principles computational framework to calculate resonance Raman intensities (beyond the Placzek approximation) based on perturbation theory, where the Bethe-Salpeter equation (BSE) is only solved once statically, offering a clear advantage compared with finite differences methods where the BSE is solved twice for each Raman mode. Comparisons with experimental Raman spectra for monolayer MoS2 using more than 30 laser excitaion lines are discussed.

Molybdenum disulfide (MoS₂) opens up new possibilities for 2D electronic devices in terms of applications for low standby and low operating power electronics coupled with further miniaturization beyond Moore’s Law. Unfortunately, previous synthetic approaches appear to be technically hampered for practical applications due to their inability to obtain large-area and continuous MoS₂ layers. Here, a facile methodology for the large-scale production of layer-controlled MoS₂ layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll-to-roll-based thermal decomposition was developed. The resulting 50 cm-long MoS₂ layers synthesized on Ni foils possessed excellent long-range uniformity and optimum stoichiometry. Moreover, this methodology was promising because it enables simple control of the number of MoS₂ layers by simply adjusting the concentration of (NH₄)₂MoS₄. For actual industrial applications, not only a method for large-scale production but also an efficient transfer method following the synthesis to locate the sample onto diverse substrates is a prerequisite. We thus transferred layer-controlled MoS₂ onto PET films using our custom-made roll-to-roll transfer machine. Based on these results, it is envisaged that the cost-effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor-based multifaceted applications.

Two-dimensional (2D) atomic layered crystals are believed to be the most promising candidates for optoelectronic applications, due to their unique properties such as their optimum thickness scalability, superior intrinsic strain limit and near ideal transparency. Large-bandgap transition metal dichalcogenides (TMDs) (For example, MoS₂ and WSe₂) offer experimental mobility approaching single-crystal silicon thin-film transistors (TFTs), with two orders of magnitude thinner profile and high strain limits (up to 20-30%).^[1] More importantly, the broad spectrum which MoS₂ can absorb light from visible to near-infrared spectral region (350-950 nm) is higher than GaAs and Si.^[2] Although the layered transition metal dichalcogenides (LTMDs) exhibit very strong light-matter interactions and very high light absorption coefficients, atomically thin p-n junctions from van der waals-bonded TMDs layers with superior photovoltaic (PV) performance have not been demonstrated. The junctions exhibit both rectifying electrical characteristics and PV response, but the maximum achievable photovoltage is limited by the smallest band offsets for electrons and for holes. For example, in the case of MoS₂/WSe₂, the band offset for electrons is approximately 0.7 eV.^[3]
In this work, I present the characterization of a MoS₂ vertical homostructure in the intermediate thickness regime (~15nm) between the monolayer and the bulk. The MoS₂ vertical homostructure is fabricated via an ‘In-Depth’ doping process by plasma surface treatment. To generate a p-n homojunction in multilayer (ML)-MoS₂, an effective doping depth control is a challenging due to their atomistically thin dimensions. Compared to surface functionalization and substitutional CVD doping, plasma treatment emerges as the most effective doping technique for layered MoS₂ due to a wide range of doping that can be achieved with good control and selectivity. Fluorine (F)- and oxygen (O) atoms are the critical dopants responsible for the p-doping in MoS₂, due to a surface charge transfer processes between the strong electronegativity dopants and MoS₂ layers. Additionally, such surface charge transfer processes induced by the dopant atoms can only affect the top few layers due to the screening of electric field in MoS₂. The ‘In-Depth’ plasma-assisted doping process can realize a vertical p-n homojunction in the ML-MoS₂. The MoS₂ vertical homostructure demonstrates a potential of doped MoS₂ for quasi-transparent optical components in light harvesting cells and nanoscale optoelectronics.

Acknowledgement:
This work was supported by JST in Japan, Research and Education Consortium for Innovation of Advanced Integrated Science (CIAiS).

References:
[1] D. Akinwande, N. Petrone, J. Hone, Nat. Commun. 5:5678, 2014.
[2] V. Dhyani, S. Das, Sci. Rep. 7:44243, 2017.
[3] J. Kang, S. Tongay, J. Zhou, J. Li and J. Wu, Appl. Phys. Lett., 102, 12111, 2013.

Tungsten disulfide (WS₂), a two-dimensional layered material, has attracted attention due to its unique properties such as the band structure change from an indirect transition type of 1.35 eV in bulk to a direct transition type of 1.79 eV in a single layer, and no dangling bond on the surface. Because of these favorable physical, chemical, and optical properties, researchers have been developing channel materials and various sensors. We have been fabricating WS₂ and other TMD thin films with sputtering deposition. In order to fabricate film with high quality, the substrate is heated to a high temperature. However, S atoms tend to desorb from the film at high substrate temperature. In this research, positive DC bias was applied for the purpose of suppressing desorption of S atoms in the sputtering process even with high substrate temperature. WS₂ thin films were prepared by RF magnetron sputtering. The deposition was carried out on SiO₂ substrate using WS₂ target. As sputtering conditions, the sputtering time was 46.7 s, the RF power was 100 W, the substrate temperature was 250 °C, and the Ar flow rate was 7.6 sscm. The bias voltage was varied from 0 to +60 V. The prepared samples were evaluated for chemical bonding state by X-ray photoelectron spectroscopy. The formation of the bond between W and S was confirmed from the peak positions of W 4f and S 2p spectra. The composition ratio of S and W was calculated from the peak area ratio. As a result, the S/W ratio increased as the bias voltage increased positively, and the desired value of 2.0 was achieved with a bias voltage of +40 V. From this result it is suggested that the positive bias voltage effectively attracts S^2-.
This work was partly supported by JSPS KAKENHI Grant Number 16J11377 and JST CREST JPMJCR16F4.

Atomic monolayer (ML) NbS₂ and NbSe₂ crystals provide interesting platforms to explore electronic phase transitions at the atomic thickness limit. Here, we report controlled growth of ML heterostructures, composed of NbS₂ and NbSe₂, via sequential chemical vapor depositions. We have achieved two kinds of heterostructures, such as lateral epitaxial NbS₂/NbSe₂ MLs and ML alloys, with growth kinetics controls. Initial observations of superconducting transitions at the ML interfaces, probed by scanning tunneling microscopy, are discussed around the critical roles of point and extended defects on coupled transitions.

Spontaneously area-selective deposition of van der Waals (vdW) semiconductor monolayers in large scales, i.e., transition-metal dichalcogenides (TMDCs) in wafer-scales, can provide a new 2D device platforms without conventional lithography for various applications. In fact, such self-assembled 2D structures may establish “clean” atomically thin crystals, free of defects and impurities from lithography and lift-off fabrication processes. In this work, we utilize a cost-effective spun-on polymer mask, which can be easily patternable and removable in large scales, as an inhibitor layer during crystallization of various TMDC monolayers. Thereby selective deposition of MoS₂ monolayers in wafer-scales is achieved by metal-organic chemical vapor deposition. We discuss the critical roles of such inhibitor layers within the framework of nucleation kinetics in vdW epitaxy.

Two-dimensional (2D) semiconductors have emerged as a promising channel material for next-generation electronics due to the ultrathin channel and the lack of short channel effect [1,2]. 2D-semiconductor-based field-effect transistors (FETs) incorporated with layered insulators for enabling low contact resistance has been reported [3]. However, a feasible method to grow large-area, high-quality tunneling barrier has not been realized. We demonstrate highly controlled surface oxidation in layered indium selenide (InSe) FETs to achieve enhanced transport properties. By utilizing the inherent layered structure of 2D materials, the surface oxidation can be controlled with atomic-scale precision as indicated by transmission electron microscopy. The characteristics of the oxide layer is further examined by a combination of Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The InSe FETs with oxide tunneling barrier exhibit distinct transport properties, including a field-effect mobility of 1240 cm²/Vs, ohmic contact at low temperature, low trap state density, and a contact barrier of 13 meV. With the ultrathin oxide barrier, we successfully achieve the suppression of the Fermi-level pinning effect, which severely limits the tunability of the electrical transport properties. Consequently, the contact barrier of the InSe FETs can be effectively modified by using contact metals with different work functions. The realization of high performance InSe FETs with low contact barrier via oxide tunneling barrier shows promising results in utilizing 2D semiconductor materials for practical applications.

References
[1] Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 7, 699-712, doi:Doi 10.1038/Nnano.2012.193 (2012)..
[2] Chhowalla, M., Jena, D. & Zhang, H. Two-dimensional semiconductors for transistors. Nat Rev Mater 1, doi:Artn 16052
[3] Cui, Xu, et al. "Low-temperature Ohmic contact to monolayer MoS2 by van der Waals bonded Co/h-BN electrodes." Nano Letters 17.8 (2017): 4781-4786.

Methods for synthesizing nanosheet materials are divided into two types, the top-down method and the bottom-up method. In the top-down method, the ultrasonic exfoliation method is known as a common process which is simple and highly productive process, but it is hard to synthesize few-layered solid solutions such as Mo(S,Se)₂ because a bulk solid-solution is required as a starting material. On the other hand, as a representative of the bottom-up method, the chemical vapor deposition method is widely known for the synthesis of nanosheets with high quality and large area, but its drawbacks include low yield, poor productivity, and a vacuum process entailing high process costs. Thus, each recent technology entails trade-offs between quality, productivity and cost.
Therefore, this study proposes a synthesis method of metal chalcogenide solid solutions with high productivity under supercritical reductive conditions. Moreover, this process realizes synthesizing Mo(S,Se)₂ and (Mo,W)S₂ nanosheets from MoO₃, WO₃, pure sulfur, selenium and non-toxic solvents at 400C for a shorter time than 60 min.
XRD patterns and Raman spectrum for Mo(S,Se)₂ and (Mo,W)S₂ were consistent with previous reports. The peak shift was observed by Se or W being incorporated in MoS₂. TEM images revealed that Mo(S,Se)₂ and (Mo,W)S₂ have edge-rich structures composed of nanosheets for both samples. The formation of this nanosheet structure should be caused by high affinity between supercritical fluid and the surface of TMDs nanosheets. The XPS analyses showed the almost stoichiometric composition (Metals : Chalcogens ≒ 1 : 2) among the both samples. The hexavalent Mo and W derived from MoO₃ and WO₃ were not detected, whereas only tetravalent Mo and W were detected; thus, we concluded that the reduction process had completely progressed. Furthermore, STEM images at an atomic level showed that elements were solid-dissolved without segregation.
We achieved the synthesis of Mo(S,Se)₂ and (Mo,W)S₂ using less-toxic starting materials for a short time. This synthesis process combining environmental adaptability and easiness can be also applied for synthesis of other various kinds of two-dimensional metal chalcogenides.

Two-dimensional layered Black phosphorus has shown great potential for next-generation electronics with tunable band gap and high carrier mobility. For the electronic applications, the thickness modulation of a BP flake is an essential due to its thickness-dependent electronic properties. However, controlling the precise thickness of few-layer BP is a challenge for the high-performance device applications. In this study, we demonstrate that thermal treatment under ambient condition precisely controls the thickness of BP flake. The thermal etching method utilizes the chemical reactivity of BP surface with oxygen and water molecules by the repeating formation and evaporation of phosphoric acid during thermal annealing. The thermally etched BP FETs shows a high hole mobility of ~576 cm²V⁻¹s⁻¹ and a high on-off ratio of ~10⁵. In spite of the thermal treatment under air ambient condition, the BP FETs showed long-term stability without significant degradation for one month resulting from the conservation of phosphorus layered structure under phosphorus oxide layers.

Fully-printable devices have attracted much interest in the field of large-scale printable electronics. Apart from high-performance inks for conductive patterns, a dielectric component is also very important in electronics, for example, in capacitors and electroluminescent devices. The current printable dielectric inks most commonly utilise barium titanate (BaTiO₃) as the main filler material, having the drawbacks of high cost, thick printed layer and demanding curing process. Here, we employed solution processed hexagonal boron nitride (h-BN), as a filler material incorporated into a commonly used polymer binder matrix, with enhanced dielectric properties, enabling a cost effective, single-pass printable, pin-hole free dielectric ink.

We exfoliate the h-BN directly into the solvent and polymer binder matrix by ultrasonication for 12 hours followed by centrifuge. This ‘one-pot’ process introduces exfoliated h-BN flakes directly into the ink system and does not require any additional chemical surfactant or post processing. A stable h-BN ink system is formulated by an optimised proportion of the h-BN filler material, polyurethane (PU) polymer binder and butyl cellosolve (BC) solvent. The ink shows excellent printability on polymer substrates (e.g. PET) with 15mPa.s viscosity at 10 s^-1 shear rate under 25°C, and is suitable for coating and printing. The deposited film has a high transparency with ~ 85% transmittance. Measurement on printed parallel-plate capacitor arrays with our inks containing 0.6wt% h-BN show a two-fold increase in relative permittivity (ε_r) over pristine PU. An average areal capacitance 336pF/cm² is achieved by averaging an array of capacitor samples with an electrode-overlap area of 20-50mm². We then incorporate our printed capacitors to construct a simple 4-bit dynamic random-access memory (DRAM), demonstrating the viability of our ink for printable electronics.

Atomically thin layered two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS₂) have been proposed to enable aggressive miniaturization of FETs We previously reported ultra-short channel MoS₂ FETs with channel length down to 15 nm and 7.5 nm using graphene and directed self-assembly pattern technique, respectively. However, the power scaling in such devices suffers from the same issues as in CMOS technology. Obtaining a subthreshold swing (SS) below the thermionic limit of 60 mV/dec by exploiting the negative capacitance (NC) effect in ferroelectric materials is a novel effective technique to allow for the reduction of the supply voltage and power consumption in field effect transistors (FETs). Conventional ferroelectric materials (i.e., lead zirconate titanate, bismuth ferrite, and polymer ferroelectric dielectrics such as P(VDF)-TRFE are not technologically compatible with standard CMOS fabrication processes. On the other hand, fluorite-type doped HfO₂ ferroelectric thin-films deposited by ALD offers the CMOS compatibility and scalability required for advanced electronic applications.
In this work, we demonstrate NC-MoS₂ FETs by incorporating a ferroelectric doped HfO₂ (Al:HfO₂ or Si: HfO₂ ) in the FET gate stack. Standard HfO₂ has monoclinic crystal structure which can be transformed into orthorhombic phase by temperature, pressure, or doping. The electrical properties of the doped HfO₂ thin-films can be tuned from dielectric to ferroelectric and even antiferroelectric by changing dopant type (Zr, Al, Si, Gd, Y, etc.), dopant fraction and/or capping layer. The ferroelectric nature of typical doped HfO₂ thin film can be confirmed by the polarization measurement . Here, Si:Hf composition is kept fixed by controlling the 3DMAS/TEMAH pulses during the ALD. We observe steep SS in FETs when used these FE in the gate stack with carefully matched FE/DE bilayer. The NC-MoS₂ FET built on a typical FE/DE bilayer showed a significant enhancement of the SS to 57 mV/dec at room temperature, compared with SS_min = 67 mV/dec for the MoS₂ FET with only HfO₂ as a gate dielectric.

Non-linear dynamical systems such as neuristors embody essential complexity in spike-timing behavior for hardware implementation of neuromorphic computing architectures. In order to experimentally realize a neuristor, the constituent materials need to possess attributes that enable concurrent functionality as a distributed energy source, an energy storage element, and an active channel. Towards this end, two-dimensional (2D) materials such as MoS₂ show promise for neuristors since they have demonstrated bipolar resistive switching at low fields, gate-tunable learning response, heterosynaptic plasticity in memtransistors, and multi-bit memory. Furthermore, solution-processed 2D material thin films possess versatile chemical tailoring, compositional tunability, and compatibility with flexible substrates for large-area printed electronics. In this talk, solution-processed thin-film MoS₂ memristors are shown to possess all-or-nothing spiking behavior in a geometry that is amenable to in situ optical, Raman, and photoluminescence microscopy. Furthermore, in situ thermal imaging reveals three kinds of switching behaviors (i.e., soft switching with negative differential negative resistance, hard switching with thermal runaway, and irreversible switching) that are correlated with distinct features of filaments and dendrites. This resistive switching is found to be a generalized phenomenon across a wide range of 2D materials such as MoS₂, WS₂, ReS₂, and InSe films since the morphology of composite thin-films with large curvature flakes plays a dominant role in field-emission transport between neighboring flakes. The net result is that curvature-induced large local fields achieve resistive switching at low electric fields (~4 kV cm^-1) due to electrical discharge. Overall, this work utilizes temperature as a new state-variable, which can be exploited for a range of applications including higher-order memristors, neuristors, random number generators, and chaotic circuits. In particular, the circuit modeling neuristors based on solution-processed MoS₂ thin-films are used to realize Pearson-Anson oscillators with qualitative features similar to the Hodgkin-Huxley axon model.

We have observed non-volatile resistance switching (NVRS) memory effect in semiconducting or insulating atomic sheets in a standard vertical device configuration. Results suggest a rich multi-physics effect persistent in both poly- and single- crystalline monolayer and few-layer sheets. NVRS is observed in several TMDs including MoS2, MoSe2, WSe2, and WS2 and also in h-BN. This alludes to a universal memory effect in non-metallic 2D materials. Our findings overturn the contemporary thinking that non-volatile memoryis not scalable below a few nanometers. Emerging concepts in non-volatile flexible memory fabrics, zero static power radio-frequency switches, and brain-inspired (neuromorphic) computing could benefit substantially from the pervasive NVRS effect in atomic sheets. Experimentally results for RF switching have been achieved at operating freqeuncies beyond 50GHz with the potential for THz bandwidths.

Nowadays, ultimate device scaling of the conventional metal-oxide field-effect transistors (MOSFETs) together with lowered threshold voltage makes the subthreshold current problem more significant than that in long channel MOSFETs in terms of energy consumption. Recently, as an alternative of this conventional MOSFETs, tunneling field-effect transistors (TFETs) have received attention due to its potential low-power operation with smaller subthreshold swing. Also, it is known that the heterojunctions in TFETs are able to have higher device performance over homojunction-based TFETs because of those flexibility to form a steep band edge at the junction and select the channel material with higher carrier mobility. Two-dimensional (2D) van der Waals (vdW) materials are good candidates to overcome the typical lattice mismatch at the heterojunctions due to their absence of dangling bonds on the surfaces. Furthermore, diversity of heterostructures based on 2D vdW materials gives rise to high flexibility to engineer device properties of TFETs. Previously, BP-SnSe₂ heterostructure diode with the NDR behavior has been realized elsewhere, but only bulk form of the diode was developed at that time.[Yan, R. et al., Nano Lett. 2015, 15 (9), 5791] The point is that gate-tunable p-n heterojunction and band-to-band tunneling are the key to operate as TFET.
Here, we demonstrated black phosphorus-tin diselenide (BP-SnSe₂) heterostructure tunneling field-effect transistors operating as both Esaki diode mode with negative differential resistance (NDR) region and backward diode mode at negative and positive gate bias, respectively. By introducing relatively high carrier concentration even in few-layer form of SnSe₂ sheets, only BP channel was able to be modulated by global electrostatic gating, making the device fabrication and operation much simpler. Gate-tunable type 2 staggered band alignment at the heterojunction was demonstrated by scanning photocurrent microscopy (SPCM). Temperature-dependent NDR behavior was unlikely with a conventional tunneling diode, but it was successfully explained by series resistance and thermal assisted tunneling mechanism. Also, the backward rectification behavior followed the thermionic emission theory, which confirmed that the another potential barrier had been created at the heterojunction due to the negative bias electrostatic gating. Besides, we pointed out that deposition conditions of the Al₂O₃ passivation on the device were important to observe these effects.

In this talk, I will discuss our recent work in studying the electronic, optical and ferromagnetic properties of emerging two-dimensional materials, and in developing them for novel electronic and photonic device applications. The first part of the talk will focus on discussing the basic properties of emerging 2D materials such as black phosphorus and its applications in mid-infrared optoelectronics. Our recent research on the phonon-spin and phonon-electron coupling in ferromagnetic monolayer materials will also be presented. In the second part of the talk, I will discuss our work on the origami and kirigami of 2D materials and the development of substrate-free foldable and adaptive membrane electronics based on these materials. A unique method for the deterministic folding of 2D materials and their heterostructures will be discussed. Reconfigurable electronic devices and circuits realized through the patterned folding of 2D material heterostructures will be reported. I will conclude with remarks on promising future research directions of two-dimensional material electronic and photonic devices, and how the newly developed 2D material origami and kirigami techniques may enable adaptive reconfigurable electronics and advanced sensor technologies.

Heterostructures built from various two-dimensional (2D) layered materials, including semimetallic graphene, semiconducting transition metal dichalcogenides, and insulating hexagonal boron nitride, are emerging material platforms for low-power and high-performance electronic devices because of their high-quality heterointerfaces with atomic precision as well as the exceptional properties from their atomically thin constituent materials. In addition, the competitive ability to electrostatically control the energy barrier (or band alignment) at the van der Waals (vdW) interfaces allows us to rationally design 2D functional heterostructures by band-structure engineering for a variety of gate-tunable electronic devices. Particularly, a vertical triode with a gate-controlled Schottky barrier, so-called the ‘barristor’, has been proposed as a new switching device with potential advantages in scaling and integrating highly-networked device functionality. Such unique capabilities of 2D heterostructured devices can also offer unexplored opportunities for realizing an energy-efficient artificial synapse with high controllability. Nevertheless, the artificial synapse based on 2D heterostructures has rarely been demonstrated, as appropriate materials with robust memristive switching characteristics and an adequately integrated device architecture are not available.
Here, we report a new class of artificial synaptic architecture, a three-terminal device consisting of a vertically integrated monolithic tungsten oxide memristor and a variable-barrier tungsten selenide/graphene Schottky diode, termed as a ‘synaptic barristor’. The device can implement essential synaptic characteristics, such as short-term plasticity, long-term plasticity, and paired-pulse facilitation. Owing to the electrostatically controlled barrier height in the ultrathin vdW heterostructure, the device exhibits gate-controlled memristive switching characteristics with tunable programming voltages of 0.2−0.5 V. Notably, by electrostatic tuning with a gate terminal, we can additionally regulate the degree and tuning rate of the synaptic weight independent of the programming impulses from source and drain terminals. These capabilities eventually enable the accelerated consolidation and conversion of synaptic plasticity, functionally analogous to the synapse with an additional neuromodulator in biological neural networks. Our demonstration represents an important step toward highly networked and energy-efficient neuromorphic circuits.

The ubiquity of a self-aligned gate in integrated circuits is due to processing simplicity, versatility, and compatibility with diverse technologies, allowing for significantly increased circuit complexity. Meanwhile, two-dimensional (2D) materials have recently shown tremendous potential for digital and analog electronics due to high mobility, superior scaling limits, and arbitrary permutations of materials in defect-free van der Waals heterojunctions (vdWHs). Therefore, the combination of 2D materials with self-aligned fabrication approaches has the potential to unite these characteristics in a platform with significant advantages for next-generation electronics. Toward this end, a general self-aligned fabrication scheme is reported here whereby control of lithography resist undercut profiles enables the realization of a diverse class of short-channel electronic devices based on van der Waals materials [1]. Self-alignment enables the fabrication of 135 nm channel source-gated transistors in monolayer MoS₂ with reduced short-channel effects and near-ideal current saturation characteristics. Additionally, self-alignment of van der Waals p-n heterojunction diodes based on black phosphorus and MoS₂ achieves complete electrostatic control of both the p-type and n-type constituent semiconductors in a dual-gated geometry, resulting in gate-tunable mean and variance of anti-ambipolar Gaussian transfer characteristics. Through finite-element device simulations, the operating principles of source-gated transistors and dual-gated anti-ambipolar devices are elucidated, thus providing design rules for additional devices that employ self-aligned geometries. In addition, this technique reduces the number of lithography steps for fabricating complicated geometries, is scalable to large areas via photolithography with sub-wavelength channel lengths, and can be generalized to mixed-dimensional organic and inorganic nanomaterials while minimizing electrical shorts through porosity, pinholes, or related defects. Overall, this self-aligned fabrication method represents an important step toward the scalable integration of van der Waals heterojunction devices into more sophisticated circuits and systems.

[1] V. K. Sangwan, M. E. Beck, A. Henning, J. Luo, H. Bergeron, J. Kang, I. Balla, H. Inbar, L. J. Lauhon, and M. C. Hersam, “Self-aligned van der Waals heterojunction diodes and transistors,” Nano Letters, 18, 1421-1427 (2018).

Two-dimensional (2D) layered materials have been actively explored for electronic device applications because of their ability to form van der Waals heterostructures with unique electronic properties. Vertical integration of atomically thin 2D materials can enable the design of three-dimensional (3D) circuit which is a promising pathway to continuously increase device density. In this study, we vertically stack 2D materials, such as graphene, MoS₂ and black phosphorus (BP) to build transistors, heterostructure p-n diodes and 3D logic circuits. The vertical transistors built from MoS₂ or BP semiconductor exhibit a good on-off ratio of up to 10³ and high current density of ~200 Acm^-2 at a very small V_DS of 50 mV. The graphene/BP/MoS₂ vertical heterostructure p-n diodes show a high gate-tunable rectification ratio of 10². Finally, we have demonstrated a 3D CMOS inverter by vertical integration of graphene, BP (p-channel), graphene, MoS₂ (n-channel) and a 50-nm-thick gold film in sequence. The ability to vertically stack 2D layered materials by van der Waals interactions offers an alternative way to design future 3D integrated circuits.

As the dimensions of semiconducting channels in field effect transistors (FETs) decrease, the contact resistance of metal-semiconductor interface at the source and drain electrodes dominates the performance. Two dimensional (2D) transitional metal dichalcogenides (TMD) such as molybdenum disulfide (MoS₂) have been demonstrated to be excellent semi-conductors for ultra-thin FETs. However, unusually high contact resistance has been observed across the metal-2D TMD interface. Recent studies have shown that van der Waals (vdW) contacts formed by graphene on 2D TMDs provide lowest contact resistance. However, vdW contacts between evaporated three-dimensional metal and 2D TMDs have yet to be demonstrated. Here, we report the realization of ultra-clean vdW contacts between indium metal electrodes and thin MoS₂. Using scanning transmission electron microscopy (STEM) imaging, we show that the indium-MoS₂ interface is atomically sharp with no detectable chemical interaction between the metal and 2D TMD, suggesting van-der-Waals-type bonding between the metal and MoS₂. We show that the contact resistance of indium electrodes is ~ 800 Ω-μm – amongst the lowest observed for metal electrodes on MoS₂ and is translated into high performance FETs with mobility in excess of 100 cm²-V-s^-1. We also demonstrate low contact resistance of 220 Ω-μm on NbS₂ and near ideal band offsets, indicative of defect free interfaces, in WS₂ and WSe₂ contacted with indium.

Gates can electrostatically control charges inside two-dimensional materials. However, integrating independent gates typically requires depositing and patterning suitable insulators and conductors. Moreover, after manufacturing, gates are unchangeable. Here we introduce tunnelling triboelectrification for localizing electric charges in very close proximity of two-dimensional materials. As representative materials, we use chemical vapour deposition graphene deposited on a SiO2/Si substrate. The triboelectric charges, generated by friction with a Pt-coated atomic force microscope tip and injected through defects, are trapped at the air–SiO₂ interface underneath graphene and act as ghost floating gates. Tunnelling triboelectrification uniquely permits to create, modify and destroy p and n regions at will with the spatial resolution of atomic force microscopes. As a proof of concept, we draw rewritable p/n⁺ and p/p⁺ junctions with resolutions as small as 200 nm. Our results open the way to time-variant two-dimensional electronics where conductors, p and n regions can be defined on demand.

Electrides are specific form of ionic crystals that contain interstitial anionic electrons (IAEs) serving as anions. IAEs occupying the structural cavities have been found as the form of solvated electrons in liquid or localized electrons in solid. Since the discovery of two-dimensional (2D) inorganic electride, [Ca₂N]⁺e^- with 2D electron gas at interlayer space, various 2D inorganic electrides have been studied, which are distinguished by the localization degree of IAEs in 2D interlayer space.¹ Recently, [Y₂C]²⁺2e^- electride demonstrates anisotropic magnetism along the direction originated from spin-alignment of IAEs in 2D interlayer.² Here, we report a new type of 2D electride with van der Waals layered structure with 2D electron gas at intralayer space. The experimental and theoretical investigation on the electrical and ferromagnetic properties of 2D van der Waals [YCl]²⁺2e^- and [LaCl]²⁺2e^- electrides will be discussed. Our findings provide an important insight into the design strategy of new 2D electrides with magnetism based on the IAEs.

We report our recent research findings about the impacts of strain on the emerging two-dimensional ferromagnetic chromium tri-iodide monolayer [1]. By combining first-principles density functional theory and Metropolis Monte Carlo methods, the strain-dependent magnetocrystalline anisotropy energy, Heisenberg isotropic symmetric exchange effects, anisotropic symmetric exchange effects, magnetic moment, and Curie temperature are quantitatively analyzed.

The Villari effect, i.e. the inverse magnetostrictive effect, and the Nagaoka-Honda effect, i.e. the inverse Barret effect are unraveled. It is shown that, though a small strain exerts small impacts on the Curie temperature, it can noticeably influence the hysteresis curve shape and significantly alter the coercive magnetic field. This offers one of the possible explanations of the large variation of coercive magnetic field measured on the strain-prone exfoliated CrI3 monolayers [2]. This also indicates the importance to vanish strain to reduce device-to-device variation of magnetic properties in the monolayer-based spintronics memory and logic devices. It is revealed that strain can induce changes on a series of key magnetic properties (e.g., the strain-induced magnetization direction flip, the strain-induced ferromagnetic/antiferro- magnetic transition, the strain-induced change of magnetic coercivity, etc.). The strain-induced properties unraveled here are of practical interests to implement next-generation monolayer-based devices and sensors.
Reference:
[1] Jie Liu, Pinghui Mo, Mengchao Shi, Dan Gao, and Jiwu Lu, "Multi-scale analysis of strain-dependent magnetocrystalline anisotropy and strain- induced Villari and Nagaoka-Honda effects in a two-dimensional ferromagnetic chromium tri-iodide monolayer", Journal of Applied Physics 124, 044303 (2018); doi: 10.1063/1.5036924
[2] Bevin Huang and Genevieve Clark and Efr{\'{e}}n Navarro-Moratalla and Dahlia R. Klein and Ran Cheng and Kyle L. Seyler and Ding Zhong and Emma Schmidgall and Michael A. McGuire and David H. Cobden and Wang Yao and Di Xiao and Pablo Jarillo-Herrero and Xiaodong Xu, "Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit", Nature, 2018. DOI: 10.1038/nature22391

Doping, as a fundamental technique to functionalize conventional semiconductors, holds unique promise to tune the properties of 2D materials. However, due to the ultra-low thickness and simple structure of 2D materials, the chemical environment (i.e. substrates) may drastically modify the properties of synthetic 2D materials, leading disagreements between the experiments and simulations. Indeed, realizing the in-situ substitutional doping of monolayer MoS₂ by chemical vapor deposition necessitates the in-depth understanding of the substrates’ properties. Here, we present successful in-situ doping studies achieved by fine-tuning the substrate properties. First, the substrate surface inertness directly impacts on the doping efficacy of manganese (Mn). Substitutional Mn doping of monolayer of MoS₂ can only be achieved on 2D substrates such as graphene due to the absence of polarized dangling bonds on the surface. On 3D substrates such as sapphire and SiO₂, Mn is adsorbed by the surface dangling bond instead of being bonded in the MoS₂ lattice (Nano Lett., 2015, 15 (10) 6586-6591). Second, the substrate surface termination guides the film/substrate charge transfer and limits carrier density tunability by foreign dopants. Aluminum terminated sapphire (c-sapphire) surface exhibits strong electron doping to the synthetic MoS₂ monolayers, leading the 100x higher electron concentration in MoS₂ than its counterpart grown on oxygen terminated sapphire (r-sapphire) surface, evident by field effect transistors (FETs) characterization. Meanwhile, the reduced electron doping and the interaction between Mo and surface oxygen on r-sapphire results in 100x enhancement of photoluminescence (PL) intensity and carrier lifetime (~1 ns) (Sci. Rep., 7 (1), 16938). Utilizing r- sapphire, nearly degenerately n-doped MoS₂ is realized by 1 at% Re substitutional doping, evident by various electronic measurements including conductive atomic force microcopy, scanning tunneling microscopy and FETs, agreeing well with density function theory (DFT) calculations. In contrast, 1 at% Re doping is unable to effectively tune the electron concentration due to the strong electron doping from Al terminated sapphire. Surprisingly, X-ray photoelectron spectroscopy (XPS) and low temperature PL indicates that Re doping is also able to reduce the density of sulfur vacancies by 25% (Adv. Funct. Mater., 2018, 28, 1706950), demonstrating the possibility of multifunctional doping of 2D materials.

Recent experimental success in the realization of two-dimensional magnetism has invigorated the search for low-dimensional material systems with tunable magnetic anisotropy that exhibit intrinsic long-range ferromagnetic order. Using a crystal field theory model and first-principles simulations, we demonstrate intrinsic ferromagnetism, high magnetic moments, high Curie temperatures, and intrinsic semiconducting and half-metallic transport behavior in nitride and ordered double-transition-metal MXenes. We report that modifying the surface termination and transition metal in monolayer M₂NT_x nitride MXenes gives rise to a rich diversity of noncollinear spin structures and finely tunable magnetocrystalline anisotropy. We predict that manipulating the strength of the spin-orbit interaction and electron localization via the chemical degrees of freedom can induce sufficient anisotropy to counteract thermal fluctuations that suppress long-range magnetic order. Ti₂NO₂ and Mn₂NF₂ MXenes have continuous O(3) and O(2) spin symmetries, respectively, that may be broken by an applied field, while Cr₂NO₂ and Mn₂NO₂ are intrinsic Ising ferromagnets with out-of-plane easy axes and magnetic anisotropy energies up to 63*10^-6 eV/atom. These systems also exhibit both gapped and gapless Dirac points near the Fermi level. Our work suggests that nitride MXenes offer a promising avenue for achieving both practical spintronic devices and investigating fundamental spin processes in two-dimensional materials.

With the discovery of graphene, there is a renewed interest in two-dimensional (2D) nanostructures due to their unique physical and chemical properties, as well their potential applications in different fields ranging from electronics to biomedical ones. Many inorganic (IGAs) structures similar to graphene have been synthesized and are being exploited for novel applications. Many of these structures are van der Waals 2D solids, non van der Waals solids are still rare. In this work we investigated a non van der Waals 2D material named hematene, obtained from natural iron ore hematite (α-Fe₂O₃) through liquid exfoliation [1]. The 2D morphology of hematene was confirmed by transmission electron microscopy (TEM). Magnetic measurements together with density functional theory (DFT) calculations confirm ferromagnetic (FM) order in hematene, while its 3D parent form (hematite) is antiferromagnetic (AF). Two structural hematene models were proposed, satisfying three experimentally observed characteristics: oxygen/iron ratio (=0.66), the thickness (~0.6 nm), and the crystal symmetries. These structures were investigated using full atomistic reactive molecular dynamics (FARMD). These structures remained stable after 200 ps (picoseconds) and [001] oriented plane is the most stable one [1].

[1] A. P. Balan et al., Nature Nanotechnology – in press.

Electride is a generalized form of interstitial anionic electrons (IAEs) confined in the positively charged cavities as found in many compounds and elements. Depending on the size and geometry of cavities, IAEs show different degrees of localization, exhibiting various exotic properties. The most strongly localized IAEs are theoretically conceptualized as quasi-atoms in elemental electrides under high pressure. However, an experimental evidence for quasi-atomic IAEs has yet been discovered in practical electrides. Furthermore, as the pressurized potassium can be stabilized by ferromagnetic ordering, a quasi-atomic IAE, which is the most simple magnet with a distinct magnetic moment, can behave as a ferromagnetic atom, but no ferromagnetic electride originating from quasi-atomic IAEs has been realized. In this talk, present the IAEs in layered [Gd₂C]^2+.2e^- electride behave as atoms of ferromagnetic elements in two-dimensional interlayer space. We verified that the inherent quasi-atomic IAEs have their own magnetic moments of ~0.5 Bohr magneton per one IAE, which are critical for the ferromagnetism in [Gd₂C]^2+.2e^- electride. It was found that the ferromagnetic IAEs facilitate the exchange interactions between not only intralayer gadolinium atoms but also interlayer gadolinium atoms across IAEs, forming ferromagnetic Gd-IAE-Gd layer separated by non-magnetic carbon atomic layer, which exhibits a characteristic anisotropy in magnetoresistance analogous to those of typical ferromagnetic/non-magnetic layered systems. The substitution of paramagnetic chlorine atoms for IAEs proves the existence of ferromagnetic quasi-atomic IAEs from a clear transition to non-electride antiferromagnetic Gd₂CCl caused by attenuating all interatomic exchange interactions of IAEs with gadoliniums, which are consistent with theoretical calculations. These results confirm that strongly localized quasi-atomic IAE at two-dimensional interlayer space acts itself as a single ferromagnetic element and triggers a spin alignment in antiferromagnetic [Gd₂C]²⁺ lattice framework. These results present a broad opportunity to tailor intriguing ferromagnetism originated from quasi-atomic interstitial electrons in low-dimensional materials.

Reference
1. Lee, K.; Kim, S. W.; Toda, Y.; Matsuishi, S.; Hosono, H., Nature 2013, 494, 336-340.
2. Park, J.; Lee, K.; Lee, S. Y.; Nandadasa, C. N.; Kim, S.; Lee, K. H.; Lee, Y. H.; Hosono, H.; Kim, S. G.; Kim, S. W., J. Am. Chem. Soc. 2017, 139, 615-618.
3. Seung Yong Lee, JaeYeol Hwang, Jongho Park, Chandani N. Nandadasa, Yonghak Kim, Kimoon Lee, Kyu Hyoung Lee, Yunwei Zhang, Yanming Ma, Hideo Hosono, Seong-Gon Kim, and Sung Wng Kim^{* _} Submitted

The bulk chromic halides (CrCl₃, CrBr₃ and CrI₃) have long been known as ferromagnetic materials with layered van der Waals structure. Recently, the monolayer CrI₃ has been isolated and confirmed to be two-dimensional (2D) ferromagnet. Those 2D magnets offer an exciting platform for studying the interplay between the light and magnetic ordering. Here, we present the Raman spectroscopy study of monolayer CrBr₃ from the chromic halides family, which has also possess ferromagnetic properties in its monolayer form. The phonon modes are identified by combining the polarized Raman spectroscopy and density functional perturbation theory (DFPT). The Raman frequencies show an abnormal redshift below the Curie temperature (CT) due to strong spin-phonon interaction. The CT of monolayer CrBr₃ is measured to be at 20 K, which is about 15 K lower than that in its bulk form. Moreover, the resonance Raman spectroscopy measurement is used to reveal the mechanism of vibronic (phonon-electron) coupling in the monolayer CrBr₃. The E_2g mode at 150 cm^-1 plays a key role in the Jahn–Teller distortion with the parity-forbidden d-d transition (⁴T₂), which is confirmed by DFPT calculations. This study paves the way to understand the magneto-optical interaction in the 2D transition metal halides.

Recent experimental observations of ferromagnetism in two-dimensional van der Waals crystals open new possibilities in the applications of atomic layer materials. One characteristic feature in atomic layer materials is the controllability of the electronic structures through the gate voltage or the electric field. In this study, we discuss the electric field dependence of the electronic structure in a two-dimensional ferromagnet, Cr₂Ge₂Te₆ from first-principles calculations. Using the spin-current method[1], we evaluate the Dzyaloshinskii-Moriya interaction induced by the electric field. The results indicate the possibility of controlling the magnetic behavior by the electric field.

[1] T. Kikuchi, T. Koretsune, R. Arita, G. Tatara, Phys. Rev. Lett. 116 247201 (2016).

Fe₃GeTe₂ is one of the most promising candidates for two-dimensional (2D) ferromagnetic material based spin devices due to its high Curie temperature (T_c = 220 K at bulk state). Long-range ferromagnetism in mechanically exfoliated Fe_3-xGeTe₂ flakes, which has a van der Waals layered structure, were investigated with varying thickness from bulk to a few layers. The Fe_3-xGeTe₂ crystal was synthesized with Fe deficiencies (x ~ 0.3) which yield significant variations in magnetic properties. The thickness dependence of the Curie temperature and magnetic coercivity were studied by observing the thickness dependent magnetic hysteresis loops using magneto optical Kerr effect (MOKE) and anomalous Hall effect measurements. Compared to Fe_3-xGeTe₂ with no Fe deficiencies, the Fe deficient Fe_3-xGeTe₂ shows much smaller coercivity, i.e. it is a magnetically softer material, at all thicknesses. Since soft ferromagnetism is advantageous for certain spin device applications such as current induced magnetic switching, Fe_3-xGeTe₂ is a 2D ferromagnetic material with high potential for spintronic applications.

Since the exfoliation of graphene, the magnetic orderings in two-dimensional (2D) materials have been intensively searched for the great potentials in nanostructured spintronics or valleytronics applications. However, the magnetism in 2D materials is once considered prohibited by Mermin-Wagner restriction and therefore the introduction of 2D magnetism may require external modulations, such as external electric field, defects, adatoms, and carrier doping. By density functional theory based first-principles calculations, we have predicted that substitutional doping of Co atoms at the edge of the most stable configuration of zigzag WSe₂ nanoribbons can induce long-range ferromagnetic ordering and robust half-metallicity that enables spin-selected transport along the nanoribbon edges.¹ The half-metallicity was also found quite robust against large size, low doing density, and large tensile strain. The 2D extended materials with intrinsic magnetic ordering is more advantageous for practical spintronics or vallytronics applications due to their facile fabrication and good controlability. Recently in experiments, various 2D ferromagnets, such as CrI₃, have been synthesized, breaking the Mermin-Wagner restriction by realizing strong magnetocrystalline anisotropy in 2D structures. These results have attracted considerable attention in searching more ideal ferromagnetic 2D materials with high stability, robust long-range ordering, high Curie temperature (T_C), and possible tuning of magnetism. Through first-principles investigation, we have investigated a series of transition metal thiophosphate TMPS₄ monolayers and predicted their stable long-range ferromagnetic orderings with room-temperature T_C.² Integrated with 2D transition metal dichalcogenides (TMDCs) as heterostructures, they allow spin-valley coupled carrier injection from TMPS₄ into TMDCs.

References
(1) Xu, R. et al., Half-Metallicity in Co-Doped WSe₂ Nanoribbons, ACS Appl. Mater. Interfaces 9, 38796-38801 (2017)
(2) Unpublished results

Intrinsic long-range ferromagnetic order has recently been discovered in two-dimensional (2D) atomic crystals. However, the prospect of 2D magnets remains largely hindered by the scarcity of 2D ferromagnets with limited diversity in magnetic attributes. In this context, creative ways to bring forth ferromagnetism from non-magnetic 2D materials are attractive and profitable for both fundamental physics and device applications. In this talk, I will show you the possibility of such creation based on our magneto-optical study of 2D heterostructures and discuss the underlying material physics. Our work paves the new path to harvest 2D ferromagnetism.

We developed a new way to enhance the photoresponsivity of a van der Waals heterojunction p-n diode using surface acoustic waves (SAWs). The diode was constructed on top of a piezoelectric LiNbO₃ substrate and composed of p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS₂) flakes that partly overlapped with each other. This layout facilitated the applied SAWs to rapidly drive carriers out of the depletion region. In this structural design, SAWs promoted the separation of photogenerated carriers, and thus greatly increased the photocurrent. The measured photocurrent for the device with SAWs was about 10³ times higher than that of the device without SAWs. The device using SAWs showed a photoresponsivity as high as 2.17 A/W at the wavelength of 532 nm.

This excellent performance was attributed to the SAWs suppressing electron–hole recombination in the device under light illumination. When light is incident on the surface of the material, the SAW piezoelectric field confines the photogenerated electrons and holes within the maxima and minima of the potentials in the conduction and valence bands, respectively. The potential along the confinement direction (z-direction) enables electrons and holes to rapidly shift their mean position and separates them spatially, which increasing carrier lifetime and lowering their recombination rate. Moreover, the BP and MoS₂ flakes form a V-shaped configuration that provides two misaligned paths that aid electrons and holes to rapidly drift into neutral regions. Our device exhibits promise as a high-performance photodetector and reveals new possibilities for acoustic devices in optoelectronics.

We investigate and compare changes in the optical response of 2H-MoTe₂ and 2H-Mo_1-xW_xTe₂ alloys as they undergo phase transition between semiconductor-to-semimetal phases in an all vdW device structure using an hBN gate dielectric and graphene contacts. Transition metal dichalcogenides (TMDs), such as Mo_1-xW_xTe₂, exhibit a wide range of electronic properties (semiconducting, semimetal and metallic phases) with unique polymorphs that depend on atomic stacking and coordination. Phase modulation in TMDs is of interest for applications in electronics and optoelectronics. However, reversible tunability between such phases is challenging. This is largely due to a combination of significant differences in the free energy between TMD polymorphs and phase transition kinetic barriers. Phase modulation in MoTe₂ has, however, gained recent interest due to the low barrier for transition (~40 meV) between its semiconducting and semimetal phases when compared to other TMDs. Evidently, dynamic control between the thermodynamically favorable semiconducting 2H phase and the metastable semimetal 1T’ phase in MoTe₂ is achieved with a variety of external stimuli, including strain, temperature and electrostatic doping. In the latter case, theory predicts that this phase transition occurs when the charge density exceeds ~10¹⁴ cm⁻², which has been experimentally verified in monolayer MoTe₂ using ionic gating. However, reversible switching between these phases with a solid-state electrostatic gate still remains elusive. To circumvent the charge density requirements, theory predicts that the barrier for phase transition can be reduced in MoTe₂ when alloyed with tungsten. Evidence of this was demonstrated by Zhang et al. (arXiv:1709.03835), where an all solid-state resistive random-access memory device fabricated with Mo_1-xW_xTe₂ showed reversible switching between high and low resistance states. In addition to changes in conductivity, another feature of the phase transition in MoTe₂ includes in-plane structural changes that should give rise to district optical anisotropic responses in monolayers. However, changes in the optical response in MoTe₂ and Mo_1-xW_xTe₂ alloys as they undergo phase transition remains highly unexplored.
In this study, both exfoliated and grown 2H-Mo_1-xW_xTe₂ monolayers with x=0 and x=0.09 are explored. The optical response of the vdW stack are investigated using angle- and polarization-dependent reflection measurements as a function of gate voltage. Initial results showed changes in the optical response of the vdW stack between (1.1–1.4 μm) with applied gate voltage in alloys with x=0.09. Furthermore, polarization- and temperature-dependent Raman measurements are also performed to map out structural changes in Mo_1-xW_xTe₂ as a function of applied gate voltage. These results will provide new insight into the optical response of these materials to enable new avenues for application in low-voltage and ultrafast modulators and other nanophotonic devices.

The elemental two dimensional (2D) material, Black-phosphorus (BP), is sought after for its unique properties, especially the tuneable wide range direct band gap.[1] The mechanically exfoliated few-layer BP has been explored for a variety of electrical and optoelectronic applications[1]. Plasma-assisted thinning has emerged as an exciting technique to achieve BP crystals of desired thickness [2,3,4]. It is critical to understand the effects of different plasma environments on the electrical and optoelectronic properties of the resultant material to fully realise the true potential of plasma-thinned BP and other emerging 2D materials. This study focuses on the effects of plasma-assisted BP thinning. It offers new insights into the influence of Ar and O2 plasma on the electrical and optoelectronic properties of plasma-thinned BP flakes. Our study reveals that by manipulating the environment under which BP is exposed to the plasma, it is possible to engineer defects introduced during the process, that leads to new photoluminescence (PL) emission peaks without compromising the switching ratios or carrier mobilities of BP-based field effect transistors (FETs). Overall, our study finds the use of O2 plasma as a more suitable approach to retain and enrich the intrinsic optoelectronic properties of BP. It forms an oxide layer during the process which gives controlled thinning of the bulk layers of BP and protects the underlying layers of BP. Additionally, our study, experimentally reveals the ability of BP to respond to UV excitation
[1] Eswaraiah, V., Zeng, Q., Long, Y. and Liu, Z., 2016. Black phosphorus nanosheets: synthesis, characterization and applications. Small, 12(26), pp.3480-3502.
[2] Lu, W., Nan, H., Hong, J., Chen, Y., Zhu, C., Liang, Z., Ma, X., Ni, Z., Jin, C. and Zhang, Z., 2014. Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization. Nano Research, 7(6), pp.853-859.
[3] Jia, J., Jang, S.K., Lai, S., Xu, J., Choi, Y.J., Park, J.H. and Lee, S., 2015. Plasma-treated thickness-controlled two-dimensional black phosphorus and its electronic transport properties. ACS nano, 9(9), pp.8729-8736.
[4] Pei, J., Gai, X., Yang, J., Wang, X., Yu, Z., Choi, D.Y., Luther-Davies, B. and Lu, Y., 2016. Producing air-stable monolayers of phosphorene and their defect engineering. Nature communications, 7, p.10450.

Tuning active sites through metal-support interactions (MSIs) has emerged as an effective strategy to design supported metal catalysts. The development and selection of supports are paramount for achieving the desired catalytic performance, but remain significant challenges. In this presentation, we will discuss using two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts. Combining atomic resolution electron microscopy, in-situ spectroscopies and DFT calculations, we demonstrate that the MXenes supports modulate the compositional and electronic structures of the active sites, making them promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures.

The conflict between the material quality and production scalability is one of the major challenges for future applications of two-dimensional (2D) materials. The typical size of monolayer transition metal dichalcogenides (TMDCs), such as MoS₂, achieved by mechanical exfoliation (which yields the highest quality materials) is currently limited to ca. 100 µm on insulating substrates [1]. Recently, direct exfoliation or synthesis of TMDCs on metallic substrates of much larger dimensions has increasingly gained attention with a focus on potential applications in optoelectronics and catalysis [2,3].
Herein, we report mechanical exfoliation of centimeter-sized monolayer MoS₂ on gold substrates, facilitated by strong physisorption between the two materials. The surface contamination and roughness of the Au substrates were found to be the key parameters for successful high-yield exfoliation. Microscopic and spectroscopic characterization of the MoS₂/Au heterostructures confirmed the existence of a strong van der Waals interaction (physisorption) between the two materials, resulting in a significant charge transfer without compromising the structural integrity of the monolayer MoS₂. Furthermore, electrochemical characterization revealed an efficient modulation of the monolayer MoS₂ density of electronic states by the underlying Au.
This simple and reproducible exfoliation technique is a useful practical guide for the production of TMDCs, enabling studies of fundamental phenomena at the atomically-flat semiconductor-metal interface. It is likely that these findings will be applied in research areas such as electrode modification, photovoltaics, and photocatalysis.

[1] Velický, M. et al., Nano Lett. 16 (2016), 2023-2032.
[2] Desai, S. B. et al., Adv. Mater. 28 (2016), 4035-4058.
[3] Bruix, A. et al., ACS Nano 9 (2015), 9322-9330.

Defects involved in the two-dimensional (2D) materials such as monolayer WS₂ during the chemical vapor deposition (CVD) process is always an inevitable and critical problem. Here, we report a simple and straightforward approach using Kelvin probe force microscopy (KPFM) to reveal the intrinsic structural defects distribution in monolayer WS2. By combining the KPFM surface potential, conductivity, Raman and PL mapping, we found a vigorous correlation among them, which are consistent with our DFT estimation and STEM results. This KPFM potential image contains rich information of material band structure, which brings us a new perspective to explore material physical properties and may stand as a new reference of the band structure change in near future research.

The ability to control dopant profiles in the WS₂ and other transition metal dichalcogenide films is essential for creating p-n junctions, and applications in a wide range of electronic and optical devices including but not limited to transistors, photodiodes, and light emitting diodes. We report successful Nb doping of few layered 2H WS₂ films grown by pulsed laser deposition, by controlling the Nb content of the ablation targets. The undoped controls were n-type, exhibited a Hall mobility of 0.4 cm²/V.s, and characterized by a Fermi level at 1.41 eV from the valence band edge. The latter was determined using ultraviolet photoelectron spectroscopy. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. With increased Nb doping, the hole concentrations increased from 2.7x10¹⁷ to 8.6x10¹⁸ cm^-3, while the mobility decreased from 7.2 to 2.6 cm²/V.s, presumably due to increased ionized impurity scattering. X-ray photoelectron spectroscopy indicates that Nb substitutes on W lattice sites. The approach demonstrates the potential of PLD for targeted doping of transition metal dichalcogenides.

Van der Waals (vdW) epitaxy is a special case of epitaxy, which may occur when a film/substrate interface consists of inert surfaces without dangling bonds – as is the case with 2D materials. In contrast to conventional epitaxy, an indispensable tool for high-quality film growth, in vdW epitaxy the absence of covalent bonding between the film and the substrate allows for epitaxial growth even in case of large lattice mismatch or different crystal structure.[1,2] Thus, vdW epitaxy offers unprecedented possibilities to grow high-quality 2D materials as well as 2D/2D and 2D/3D heterostructures.

Atomic layer deposition (ALD) is a gas-phase thin film deposition technique based on sequential surface-reactions of alternately pulsed precursors. ALD enables facile and accurate control and excellent uniformity of film thickness on wafer scale and complex 3D structures. Nevertheless, ALD also has certain challenges in the deposition of 2D materials stemming from the typically low reactivity of precursors on 2D surfaces: the growth rates may be low, films may become rough, and the grain size may be small due to large nucleation density and low deposition temperatures. These challenges, perhaps excluding the growth rate, may be overcome using suitable vdW epitaxy substrates.

We present results on ALD of various 2D materials, including semiconductors SnS₂,[3] ReS₂,[4] and PbI₂[5] on commonly used vdW epitaxy substrates, sapphire and muscovite mica. In contrast to most previous reports on vdW epitaxy based on CVD or MBE, the depositions are performed both at relatively low temperatures from 75 to 400 °C and in modest, mbar-level vacuum conditions. The epitaxial relations are studied by out-of-plane and in-plane X-ray diffraction. Different tested materials and substrates show varying degrees of registration to the substrate as well as different numbers of domains. In some cases, epitaxy is enabled or improved by mild post-deposition annealing. In general, we find muscovite mica more effective than sapphire in enabling low-temperature vdW epitaxy of large-scale 2D films by ALD with possible applications in (opto)electronics, photovoltaics, and catalysis, for example.

Soft bioelectronic devices provide new opportunities for next-generation implantable devices due in large to their minimal immune responses and tissue damages. However, a soft form of the optoelectronic device for optical sensing and retinal stimulation has not been developed yet mainly because of the rigidity and bulkiness of conventional imaging modules. In this study, we describe a human-eye-inspired soft optoelectronic device using a high-density curved image sensor (CurvIS) array that leverages the atomically thin MoS₂-graphene heterostructure and strain-releasing device designs. Unique advantages of the soft omnidirectional CurvIS array include the high-density array design, small optical aberration, and simplified optical. High photoresponsivity and infrared blindness are important benefits of the MoS₂-graphene-based ultrathin imager, and the CurvIS array successfully acquires pixelated optical signals. It is the first attempt to achieve high-quality imaging using the ultrathin MoS₂-based optoelectronic device in a hemispherically curved format with the single-lens optics. We corroborate the validity of the proposed soft materials and device designs through theoretical analysis based on mechanics and optics. The ultrathin CurvIS array is applied to the human-eye-inspired soft implantable optoelectronic device that can detect optical signals and apply programmed electrical stimulation to optic nerves with minimum mechanical side effects. The proposed human-eye-inspired soft optoelectronic device is a step forward to the next-generation soft bioelectronics and the soft imaging elements of the retinal prosthesis.

Reference
1. C. Choi et al. Nature Communications 8, 1664 (2017)

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) layers are currently being investigated intensely in anticipation that they may play a major role in future optoelectronic technology, their importance being mainly motivated by the wide range of band gap tunability, formation of strongly bounded excitons, and spin-valley coupling phenomena in the monolayer (ML) form [1]. Combining TMDC MLs with other 2D layers to form van der Waals heterostructures represents a promising strategy to further extend the span of control of the electronic and optical properties of TMDC layers [2] as charge redistribution and structural changes can generally occur within the neighbouring layers, depending on their relative orientation and separation [3]. Understanding the impact of interlayer interactions and/or structural defects on the electronic properties in TMDC- based van der Waals heterostructures is therefore a critical first step towards asserting their advantage for optoelectronic devices. Here we report the electronic band dispersion of MoS₂ single crystal ML on highly order pyrolytic graphite substrate (HOPG) [4], a model system for the study of 2D Van der Waals heterostructure interface, as investigated by angular resolved photoemission spectroscopy (ARPES). The role of the MoS₂-substrate interaction in determining the valence band dispersion of the TMDC layer has been investigated with the support of dedicated band structure calculations. The complex interplay of wave function spatial extension, substrate-layer interaction and interfacial defects in determining the quasiparticle dynamics in the MoS₂ layer will be discussed and clarified via detailed ARPES lineshape analysis. We discuss the relevance of our results for charge-transport in single layer devices as well as for the control of valley polarization in TMDC-based van der Waals heterostructures.

References
[1] S.Z. Butler, et al., ACS Nano 7, 2898 (2013).
[2] R. Frisenda, E. Navarro-Moratalla, P. Gant, D. Pérez De Lara, P. Jarillo-Herrero R. V. Gorbachev, and A. Castellanos-Gomez, Chem. Soc. Rev. 47, 53 (2018).
[3] J. Kim, C. Jin, B. Chen, H. Cai, T. Zhao, P. Lee, S. Kahn, K. Watanabe, T. Taniguchi, S. Tongay, M.F. Crommie, and F. Wang, Sci. Adv. 3, e1700518 (2017).
[4] Y.-F. Lim, K. Priyadarshi, F. Bussolotti, P.K. Gogoi, X. Cui, M. Yang, J. Pan, S.W. Tong, S. Wang, S.J. Pennycook, K.E.J. Goh, A.T.S. Wee, S.L. Wong, and D. Chi, ACS Nano 12, 1339 (2018).

Black phosphorus (BP) is a semiconducting material with a direct bandgap of ∼2.0 eV in its monolayer and has attention in the application of field effect transistors (FETs). It is known that BP has an n-type contact with Cu which is a high work function metal, representing the strong Fermi level pinning (FLP) at Cu-BP interface. However, such FLP may hinder the achievement of high performance of field effect devices. In this regard, it is crucial to understand the FLP which occurs at metal-semiconductor interface. So, the possibility to reduce the FLP at Cu-BP interface by passivating the Cu(111) surface was examined using first-principles calculations in this study. The passivation by chlorine, sulfur, fluorine, nitrogen, and hydrogen atoms has been considered. The calculated results illustrate that the passivated atoms can prevent the direct contact between BP and Cu(111), thus reducing FLP at Cu-BP interface. In particular, significant reduction in FLP can be achieved by chlorine- and sulfur-passivation. Furthermore, chlorine- and sulfur-passivated Cu(111) can form ohmic contact with BP, indicating almost zero Schottky barrier height (SBH). Our results suggest an effective surface passivation route towards the control of SBH for the BP-Cu system.

Hafnium disulfide (HfS₂) has attracted significant interest because of the predicted excellent electronic properties superior to group VIB transition metal dichalcogenides. For instance, the room-temperature acoustic-photon-limited mobility of HfS₂ was calculated to be above 1800 cm² V^-1 s^-1, which is much higher than that of widely studied MoS₂ (340 cm² V^-1 s^-1). These extraordinary properties make HfS₂ attractive for applications in logic and optoelectronic devices. On the other hand, hexagonal boron nitride (h-BN) has an atomically flat and dangling-bond-free surface and a wide band gap, which makes it an ideal dielectric substrate for optoelectronic applications of other 2D materials. Recent reports reveal that heterostructures vertically aligned by two different 2D materials exhibit novel properties, offering a promising approach to design and fabricate novel electronic devices. Thus, searching for a suitable 2D material as substrate to achieve high-quality 2D HfS₂ is highly desirable.

Herein, for the first time we report the synthesis of high-quality HfS₂ on h-BN transferred on SiO₂/Si by chemical vapor deposition. It is found that the HfS₂ layers are selectively grown on h-BN rather than on SiO₂/Si. Density functional theory calculations are performed to help understand the mechanism of selective growth of HfS₂. Furthermore, the photodetectors based on the HfS₂/h-BN heterostructures exhibit excellent visible-light sensing performance, such as a high on/off ratio of more than 10⁵, an ultrafast response rate of about 200 μs, a high responsivity of 26.5 mA W^-1 and a competitive detectivity exceeding 3×10¹¹ Jones, superior to the vast majority of the reported 2D materials based photodetectors. These results indicate that HfS₂/h-BN heterostructures, as well as the selective direct growth of HfS₂, are very promising for future applications in high performance nano-optoelectronics.

Hexagonal boron nitride (hBN) has emerged as an important substrate and dielectric for electronic, optoelectronics, and photonic devices based on graphene and other atomically thin two dimensional materials. Many of these devices operate at high temperature, thus it is important to establish how well hBN can protect the underlying material from oxidation under ambient conditions. Here we report on the initial oxidation of (0001) hBN single crystals in ambient air as a function of temperature and time, as determined by atomic force microscopy (AFM). Oxidation causes pits to form nonuniformly on the hBN surface, presumably where dislocations are located. The pits are formed by the reaction of hBN with moisture and oxygen in the air, leading to volatile boron oxides and hydroxides. For oxidation times of 20 minutes, the first evidence of oxidation appears at 900 °C, with the formation of shallow, hexagonal-, and irregular-shaped pits that are less than100 nm across and several nanometer deep. Oxidation for 60 minutes at 900 °C produced well-defined symmetric pits that were roughly 300 nm across, and about 20 nm deep, and asymmetric pits which were elongated along one direction in the plane of the crystal. Close inspection revealed steps within the pits. Oxidation at 1100 °C for 20 minutes produced 1.0-2.0-micron size pits with flat and pointed bottoms that were approximately hexagonal-shaped, but with rough and irregular edges, and multiple interior steps. Detailed results of AFM comparing the thermal oxidation at different temperatures will be presented along with direction of future work.

Efficient modulation of carriers in semiconducting materials is the fundament to implement various functional electronic and optoelectronic devices. As the thickness of semiconducting materials decreases to the atomic scale, the carrier modulation in transition metal dichalcogenides (TMDs) not only becomes more critical but also confronts numerous unprecedented challenges. Conventional doping technologies that are applicable to TMDs, such as substitution of transition metal by other elements, ion implantation, and plasma treatment, may alter and/or introduce abundant defects to the intrinsic lattice structure, which diminish the carrier mobility of TMDs. Other defect-free doping approaches reported recently are based on charge transfer between TMDs and gases, ions, nanoparticles, etc. These methods, however, usually suffer from long-term stability issue and involve enduring solution based processes, which are hardly compatible with standard CMOS processes.
On the other hand, photo-induced doping is potentially a promising way to realize such a goal for atomically thin nanomaterials in a rapid and defect-free manner. However, the wide applications of photo-induced doping in nanomaterials are severely constrained by the low doping concentration and poor stability that can be reached. Here, we propose a novel photo-induced doping mechanism based on external photoelectric effect of metal coating on TMDs to significantly enhance the achievable doping concentration and stability. We demonstrate this photo-induced doping effect on Au coated MoS₂ field effect transistors (FETs) annealed in the gas mixture of argon (Ar) and oxygen (O₂) (V:V = 98:2) . Under ultra-violet (UV) light illumination, the modified MoS₂ achieves degenerated n-type doping density of 10¹⁴ cm^-2 rapidly according to the experimentally observed >10⁴ times incensement in the channel current. The doping level persists after the removal of UV illumination with non-observable decrease over 1 day in vacuum (less than 23% over 7 days under ambient environment). In addition, we have applied the same approach to another two TMDs: ReS₂ and MoSe₂, both exhibit similar photo-induced doping effect. The achieved stable and high concentration doping on the annealed Au/MX₂ (M is Mo or Re, and X is S or Se) FETs by convenient UV treatment enables various photoelectronic applications such as photo-switch, photo-memory and photonic neuromorphic devices.

Layered crystals have revolutionized the field of nanoelectronics and optoelectronics due to their potential to be exfoliated (mechanically or chemically) into atomically-thin two-dimensional (2D) surfaces (for example graphene from graphite). Several 2D crystals and their complex van der Waals heterostructures are recently realized via micromechanical cleavage (peeling) technique. Phosphorene, a two-atom thick 2D material has found significant attention because of its unique physical properties including: sizable band gap (0.3-1.2 eV), in-plane anisotropy, and high charge carrier mobility. Further, the 2D phosphorene layers can be easily exfoliated from the bulk 3D black phosphorous crystals and can also be transferred onto any arbitrary substrates. However, the major problem associated with the technological applications of phosphorene is that these 2D material crystal structures can deteriorate in the ambient condition (in water or oxygen). Here, we stabilize the crystals of phosphorene by passivating with a 2D layer of hexagonal boron nitride (h-BN). Heterostructures of phosphorene and h-BN (h-BN/phosphorene/h-BN) were fabricated to remove lattice degradation of phosphorene from both the top and bottom surfaces. Confocal Raman vibrational spectroscopic results indicate that the h-BN/phosphorene/h-BN architecture (capsulated phosphorene) is more stable in contrast to the un-capsulated phosphorene (phosphorene/h-BN or phosphorene/SiO₂). Interestingly, we observe that the structural degradation nucleates from the basal surfaces rather than edges or steps.

Transition metal sulfides have been widely studied as electrocatalysts for the hydrogen evolution reaction (HER). Though elemental doping is an effective way to enhance sulfide activity for HER, most studies have only focused on the effect of doping sulfide edge sites. Few studies have investigated the effect of doping the basal plane or the effect of doping concentration on basal plane activity. Probing the dopant concentration dependence of HER activity is challenging due to experimental difficulties in controlling dopant incorporation. Here, we overcome this challenge by first synthesizing doped transition metal oxides and then sulfurizing the oxides to sulfides, yielding core/shell Co-doped WS₂/W₁₈O₄₉ nanotubes with a tunable amount of Co. Our combined density functional theory (DFT) calculations and experiments demonstrate that the HER activity of basal plane WS₂ changes non-monotonically with the concentration of Co due to local changes in the binding energy of H and formation energy of S-vacancies. At an optimal Co doping concentration, the overpotential to reach -10 mA/cm² is reduced by 210mV, and the Tafel slope is reduced from 122 to 49 mV per decade (mV/dec) compared to undoped WS₂ nanotubes.

MoS₂ is a layered transition metal dichalcogenide (TMDC) with crystallographic anisotropy featuring chemically active edge sites and relatively inert basal sites. Few-layered MoS₂ materials have chemical and electronic properties that can be tuned based on crystallographic orientation, making it attractive for (electro)catalytic applications for the hydrogen evolution reaction (HER) and for hydrodesulfurization (HDS). Doping MoS₂ crystals with transition metals has been shown to improve the HER and HDS activity due to the decoration of Mo edge sites with metals atoms. While the effect of additions of metal atoms within the MoS₂ lattice on physicochemical properties has been investigated, there is a lack of understanding of how the addition of other transition metals influences the synthesis and fabrication process of MoS₂ from the precursor stage. For the development of more efficient catalyst materials, improved knowledge of catalyst synthesis is essential to enable engineering of structure and properties. In this study, we use in situ transmission electron microscopy (TEM) to investigate the growth of MoS₂ crystals with and without the addition of a Ni metal film. Thermolysis of ammonium thiomolybdate is used to grow crystalline MoS₂ within the TEM. The pure MoS₂ precursor forms a polycrystalline film with predominantly small, vertically-oriented grains of MoS₂ at 400°C, with the grains growing until 700°C and transitioning to (100) or horizontal grains above 800°C. Using a similar thermolysis process in the presence of Ni, a polycrystalline film with a mixture of horizontal and vertical MoS₂ grains already form by 400°C and further heating causes the growth of large-area and horizontal single crystals with an average grain size of ~70 nm. Thermogravimetric experiments also suggest differences in the crystallization kinetics of MoS₂ upon addition of Ni. Finally, x-ray photoelectron spectroscopy (XPS) experiments are used as a complementary technique to probe the chemical interaction of metal films with edge sites and basal planes after synthesis. These findings have important implications for the controlled synthesis of MoS₂-based catalysts, which is necessary for engineering catalytic materials with improved activity.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and tunable band gap energy. However, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications.[1] Specifically, unnecessary procedures, such as a photoresist deposition or ultraviolet exposure, can degrade the electrical characteristics of TMDCs channel layers. Recently, the use of graphene electrodes or hexagonal boron nitride dielectric layers to introduce dangling bond free interfaces on TMDCs has been suggested.[2] However, these layers require complicated and time-consuming etching and transferring processes with additional supporting layers. To facilitate the advantages of 2D TMDCs in practical applications, strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Motivated by this challenge, we report fully printed transparent chemical vapor deposition (CVD)-synthesized monolayer molybdenum disulfide (MoS₂) phototransistor arrays on flexible polymer substrates.[3] All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing onto transferred monolayer MoS₂, and their annealing temperature of <180 °C allows the direct fabrication on commercial flexible substrates without additional assisted-structures. By integrating the soft organic components with ultrathin MoS₂, the fully printed MoS₂ phototransistors exhibit excellent transparency and mechanically stable operation under tensile stress. Furthermore, by conducting the carefully optimized printing processes, the fabricated fully printed phototransistors exhibited comparable photocharacteristics, including photoresponsivity and external quantum efficiency (EQE), to those of previously reported phototransistors with inorganic components fabricated by conventional photolithographic processes on rigid SiO₂/Si substrates.

[1] T.-Y. Kim, M. Amani, G. H. Ahn, Y. Song, A. Javey, S. Chung, and T. Lee, ACS Nano, 10, 2819 (2016).
[2] G.-H. Lee, Y.-J. Yu, X. Cui, N. Petrone, C.-H. Lee, M.S Choi, D.-Y. Lee, C. Lee, W. J. Yoo, K. Watanabe, T. Taniguchi, C. Nuckolls, P. Kim, J. Hone, ACS Nano, 7, 7931 (2013).
[3] T-Y. Kim, J. Ha, K. Cho, J. Pak, J. Seo, J. Park, J-K. Kim, S. Chung, Y. Hong, T. Lee, ACS Nano, 11, 10273 (2017).

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, the realization of such mechanically-driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer-scale in a controlled manner. In this work, we report chemically grown centimeter-scale corrugated 2D molybdenum disulfide (MoS₂) layers with tailored dimensions and study their strain-tunable multi-functionalities. For the precise structural engineering of 2D MoS₂ layers, we developed a water-assisted method to transfer them from the growth substrates onto the large area (> 2cm²) pre-strained three-dimensionally patterned elastomeric substrate. The transferred and integrated 2D layers maintain precisely defined and corrugated structures preserving their intrinsic material quality as well as exhibiting the excellent controllability of in-plane strain. We identified the well-retained electrical conductivity of these large-area corrugated 2D MoS₂ layers even upon significant (> 30 %) tensile stretch. In addition, these corrugated MoS₂ layers present periodically tunable photoresponsivity, optical absorbance and surface wettability which are concurrently actualized upon mechanical stretching. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors.

Atomically thin nanoporous membranes are versatile structures with applications in energy, sensing, and filtration platforms including reverse electrodialysis, DNA sequencing, and reverse osmosis. The functionality of these devices is derived from the chemistry and geometry of the nanopore edge. Current methods for nanopore formation and processing include electron beam drilling, electrochemical reaction, and oxidative annealing. Since each of these methods offers a set of trade-offs, no universal method exists to precisely control both pore nucleation (e.g. number and location) as well as functionality (e.g. size, shape, and edge chemistry). Here we report vacuum annealing as a route toward reproducible nanopore engineering in two-dimensional (2D) transition metal dichalcogenides (TMDs) with tunable pore sizes, shapes, and edge chemistries.

In order to study nanopore growth kinetics and edge reconstruction in 2D TMDs, nanopores were introduced and annealed in situ in an aberration-corrected scanning transmission electron microscope. We find that nanopore edges become more ordered at higher annealing temperatures, favoring metal-terminated edges along the zigzag orientation. Further, we observe that nanopore area increases quadratically with time with an Arrhenius temperature-dependent rate corresponding to an activation energy of approximately 0.6 eV for pore growth in a MoS₂ model system. Our findings are guided and corroborated by Monte Carlo simulations comparing the relative effects of time, temperature, and pore size on growth rate. The implementation of these findings in the fabrication of nanopore devices will aid in the rational design of targeted geometries and chemistries of nanopore edges to optimize device performance.

Securing the sufficient lifetime of accelerator technology-relevant photocathodes while maintaining their high quantum efficiency has been a decadal problem. This is due to the extremely high susceptibility of high quantum efficiency (QE) semiconductor photocathodes and lack of a suitable protection layer material. To address this issue, our approach is to coat state-of-the-art high QE antimonide photocathodes with atomically thin two-dimensional (2D) materials as protecting layers to enhance their lifetime. We report successful deposition of high QE bialkali antimonide K₂CsSb photocathodes on hexagonal boron nitride (hBN) and graphene films to advance our ultimate goal. A QE of 17 % at ~3.1 eV (405 nm) on graphene substrate is the highest value reported so far on a atomically thin film and is comparable to that obtained on stainless steel and nickel reference substrates. The spectral responses of the photocathodes on graphene exhibit signature features of K₂CsSb including the characteristic absorption at ~2.5 eV. Materials characterization based on X-ray fluorescence (XRF) and X-ray diffraction (XRD) reveals that the composition and crystal quality of these photocathodes deposited on graphene is comparable to those deposited on a reference substrate. Quantitative agreement between optical calculations and QE measurements for the K₂CsSb on free suspended graphene and a graphene coated metal substrate further confirms the high quality interface between the photocathodes and graphene. A correlation between the QE and graphene quality as characterized by Raman spectroscopy suggests that a lower density of atomistic defects in the graphene films leads to higher QE of the deposited K₂CsSb photocathodes. Deposition of K₂CsSb photocathodes on hBN substrates and evaluation of their performance are currently underway.

A lot of recent research efforts have been devoted to the creation of stacks of single sheets of 2D materials, due to their enticing possibilities in designing new materials. However, large scale production of such structures still poses a significant experimental challenge. The Modulated Elemental Reactants (MER) technique is a promising candidate for the synthesis of a large variety of new heterostructures, as it allows independent control of constituents and layering sequence. MER uses a two-step process, where first an amorphous precursor mimicking the appearance of the target material is deposited on a substrate, which then self-assembles upon annealing at relatively low temperatures. Three-dimensional [MX_1+δ]_m[TX₂]_n layered structures grown with MER are generally referred to as ferecrystals. They consist of nanocrystalline layers that show rotational disorder along the crystals’ c-axis. Their unique structure leads to emerging properties which make them interesting for fundamental materials science research as well as potential applications.

This contribution will give an overview over recent results obtained from surface science techniques on various ferecrystalline compounds. For example, X-ray photoelectron spectroscopy (XPS) is a powerful tool to investigate electronic interactions as well as structural modulations within and between the single layers, such as interlayer charge transfer in [SnSe_1+δ]_m[TiSe₂]₂, polytypism of MoSe₂ layers, and antiphase boundary formation in rocksalt-like BiSe.

Furthermore, we will present the synthesis and characterization of new van-der-Waals heterostructures consisting of monolayer MoSe₂ grown by MER on epitaxial graphene on SiC(0001), thus pushing the MER synthesis towards the 2D limit. Similar to 3D ferecrystals, these two-dimensional structures show nanocrystalline domains and rotational disorder. This fundamental study may provide the base of expanding this concept to create more sophisticated heterostructures and devices by combining large-area epitaxial graphene grown on SiC(0001) with products of the MER synthesis.

The ability of creating new materials with improved properties upon transformation processes applied to conventional materials is the keystone of materials science. Here, hexagonal boron nitride (h-BN), a large bandgap insulator, is transformed into a conductive two-dimensional (2D) material – bonitrol – that is stable at ambient conditions [1]. We report on the compression-induced modification of few-layered h-BN into such a conductive boron nitride 2D material via scanning probe microscopy (SPM) experiments. The full phenomenology of bonitrol's electronic properties is consistent with a proposed ab initio model of a hydroxylated, two-dimensional sp³-bonded BN material. In our calculations, pressure-induced sp²-sp³ re-hybridization between the two uppermost BN layers in the presence of hydroxyl chemical groups lead to formation of this new material. Both experiments and theory indicate that bonitrol is a conductive magnetic material with a large work function. Finally, the present work may also mark the consolidation of a new strategy in the discovery of exclusively man-made materials via SPM nanomanipulation, resulting in the rise of uncountable novel 2D materials with yet undescribed properties.
Reference:
[1] – A. P. M. Barboza et al., ACS Nano (2018). DOI: 10.1021/acsnano.8b01911

Recent advances in atomically thin two-dimensional transition metal dichalcogenides (2D TMDs) have led to a variety of promising technologies for nanoelectronics, photonics, sensing, energy storage, and opto-electronics, to name a few. The TMDs are finding niche applications for next-generation electronics and optoelectronics devices rely on ultimate atomic thicknesses [1]. Albeit several challenges in developing scalable and defect-free TMDs on desired substrates, new growth techniques compatible with traditional and unconventional substrates have been developed to meet the ever-increasing demand of high quality and controllability for practical applications. This talk will present two important subjects; (1) Synthesis of large scale 2D TMDs and their bandgap engineering for opto-electronics application - especially our recent development of uniform and scalable single-layer TMDs by CVD method followed by a laser thinning process will be presented. Excitons’ behavior based on composition and layer dependent photoluminescence analysiswill be highlighted [2-4]. (2) 2D MoS₂ protective layer for Li-metal anodes in Li-S batteries - we observe stable Li electrodeposition and the suppression of dendrite nucleation sites from the 2D MoS₂ coated Li-metal. The deposition and dissolution process of a symmetric MoS₂ coated Li-metal cell operates at a current density of 10 mA cm-² with low voltage hysteresis, and a three-fold improvement in cycle-life than using bare Li-metal. In a Li-S full cell configuration, we obtain a specific energy density of ~600Wh kg^-1 and a Coulombic efficiency of ~98% for over 1200 cycles at 0.5 C. Our approach can lead to the realization of high energy density and safe Li-metal based batteries [5].The large-scale synthesis of 2D TMDs and their tunable optical properties and atomic layer passivation of 2D MoS₂ on Li-metal could empower a great deal of flexibility in designing atomically thin optoelectronic devices and Li-metal batteries.
References
1. Recent development of 2D materials and their applications, W Choi, N Choudhary, J Park, D Akinwande, Y Lee, Materials Today, 116-130, 20, (2017).
2. Synthesis of uniform single layer WS₂ for tunable photoluminescence, Juhong Park, MinSu Kim, Eunho Cha, Jeongyong Kim & Wonbong Choi, Scientific Reports, 7, 16121 DOI:10.1038/s41598-017-16251-2 (2017).
3. Wafer Scale Patterned Growth of Vertically Stacked Few Layer 2D MoS₂/WS₂ van der Waals Heterostructures, N. Choudhar, J. Park, J. Hwang, H. Chung, K. Dumas, S. Khondaker, W. Choi, Y. Jung, Scientific Report 6, 25456 (2016).
4. Composition-tunable synthesis of large-scale Mo_1-xW_xS₂ alloys with enhanced photoluminescence, J. Park, M. Kim, B. Park, S. Oh, J. Kim, W. Choi, ACS Nano DOI:10.1021/acsnano.8b03408 (2018).
5. 2D MoS₂ as an efficient protective layer for lithium metal anodes in high performance Li-S batteries, Cha, E., Patel, M., Park, J., Hwang, J., Prasad, V., Cho, K., Choi, W., Nature Nanotechnology, 13, 337–344 (2018).

Identifying new 2D materials with both high carrier mobility and large electronic bandgap is a pivotal goal of fundamental research since the ultrathin limit cannot be reached for traditional semiconductors such as Si and GaAs. However, to date, air-stable ultrathin semiconducting materials with superior performances remain elusive. Here our recent studies on the controlled synthesis of high-mobility semiconducting 2D crystals such as layered bismuth oxychalcogenides (BOX, Bi₂O₂X: X = S, Se, Te), as well as their electronic and optoelectronic properties will be discussed. We report on chemical vapor deposition (CVD) growth of large-area layered Bi₂O₂Se single crystals with the thickness down to monolayer, exhibiting excellent air-stability, low electron effective mass of ~0.14 m₀, and tunable bandgap values of ~ 0.8 eV. 2D BOX crystals can be fabricated into high-performance field-effect transistors and NIR photodetectors, in which pronounced quantum oscillations were also observed.

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above 10¹³/cm²to below 10¹¹/cm². Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

Following the success of ambient-stable two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN), new classes of chemically reactive layered solids are being explored since their unique electronic and optical properties hold significant promise for improved device performance [1]. For example, chemically reactive 2D semiconductors such as black phosphorus (BP) and indium selenide (InSe) have shown significantly enhanced field-effect mobilities under controlled conditions that minimize ambient degradation [2]. In addition, 2D boron (i.e., borophene) is an anisotropic metal with a diverse range of theoretically predicted phenomena including confined plasmons, charge density waves, and superconductivity [3], although its high chemical reactivity has limited experimental studies to inert ultrahigh vacuum conditions [4,5]. Even the nominally ambient-stable transition metal dichalcogenides (e.g., MoS₂) are susceptible to chemically driven phase transformations under relatively benign conditions [6]. Therefore, to fully study and exploit the vast majority of 2D materials, methods for mitigating or exploiting their relatively high chemical reactivity are required. This talk will thus explore recent efforts to control and tailor the electronic properties of chemically reactive 2D materials. In particular, covalent organic functionalization of black phosphorus minimizes ambient degradation, provides charge transfer doping, and enhances field-effect transistor mobility and on/off ratio [7]. In contrast, noncovalent organic functionalization of borophene leads to the spontaneous formation of electronically abrupt lateral organic-borophene heterostructures [8]. Even in the absence of chemical reactants, field-driven bond breaking in polycrystalline MoS₂ enables reconfiguration of doping profiles and novel device functionality such as hybrid memristor and transistor responses [9], which can be exploited in neuromorphic memtransistors [10]. Emerging efforts with additional chemically reactive 2D materials (e.g., InSe) will also be discussed.

[1] A. J. Mannix, et al., Nature Reviews Chemistry, 1, 0014 (2017).
[2] D. Jariwala, et al., Nature Materials, 16, 170 (2017).
[3] A. J. Mannix, et al., Nature Nanotechnology, 13, 444 (2018).
[4] A. J. Mannix, et al., Science, 350, 1513 (2015).
[5] G. P. Campbell, et al., Nano Letters, 18, 2816 (2018).
[6] C. R. Ryder, et al., ACS Nano, 10, 3900 (2016).
[7] C. R. Ryder, et al., Nature Chemistry, 8, 597 (2016).
[8] X. Liu, et al., Science Advances, 3, e1602356 (2017).
[9] V. K. Sangwan, et al., Nature Nanotechnology, 10, 403 (2015).
[10] V. K. Sangwan, et al., Nature, 554, 500 (2018).

I will present an innovative series of wet chemical strategies to intercalate atomic species including heavy metals, semiconductors, and semimetals (Ag, Au, Bi, Cr, Cu, Ge, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru, Sb, and W) into 2D layered materials. The zero-valent nature of the intercalant allows super-stoichiometric intercalation of atoms and can be used to achieve interlayer alloys, metal-semiconductor heterostructures, and semiconductor-semiconductor heterostructures. This new intercalation chemistry is reversible, performed at low-temperatures, and is non-destructive. Atomic intercalation is general and works for chalcogenides, oxides, and graphitic materials. These intercalation reactions give access to unique physical properties including new phase behavior, polytypic superlattices, and chemically tunable transparency and color.

Graphene has highlighted the crucial role that dimensionality plays in the fundamental properties of materials. Graphene's evolution and the methodology developed in preparing ultrathin layers has led to the exploration of other 2D materials. In particular, single layers of transition metal dichalcogenides (TMDs) with lamellar structures similar to that of graphite have received significant attention because some of them are semiconductors with sizable bandgaps and are naturally abundant. This offers opportunities for fundamental and technological research in a variety of fields including catalysis, energy storage, sensing, and electronic devices. In order to TMDs fulfill their potential, a precise control i) of surface functionalization and ii) of the number of stacked TMD monolayers are mandatory. Surface functionalization was shown to play a key role in tuning photoluminescence properties of MoS₂, formation of controllable and low defect density dielectric/MoS₂ interfaces obtained by atomic layer deposition, and etching of MoS₂ layers. Halogenation is one of the most promising functionalization techniques of TMDs. MoS₂ etching with atomic layer control was already achieved by chlorine adsorption associated with Ar⁺ sputtering. Moreover, doping techniques to tune the conductivity and photoemission properties of MoS₂ are essential. Previous works have already shown that incorporation of F-containing species in MoS₂ leads to doping, as well as other interesting properties such as tunable ferromagnetic ordering. In this work, the incorporation of Cl and F on MoS₂ was investigated. Bulk exfoliated MoS₂ as well as CVD-grown monolayer MoS₂ samples were used. Chlorination was achieved by irradiating the samples with UV light in Cl₂ flux. Prior to chlorination, sputtering of the MoS₂ with Ar ions is performed to induce S removal. X-ray Photoemission Spectroscopy measurements and Rutherford Backscattering Spectrometry showed that S vacancies play a fundamental role in the chlorination process, with vacancy concentration dictating the balance between etching of the MoS₂ layer and Cl incorporation. Fluorination was performed by exposing the samples to pulses of XeF₂. Different degrees of fluorination are achieved by varying exposure time. S is removed and F is incorporated without any loss of Mo. Chemical displacement on the Mo 3d and S 2p XPS peaks was observed after both halogenations processes. These results can clarify the mechanisms of Cl and F incorporation. Finally, the halogenation techniques proposed can be simple and useful methods to adapt the MoS₂ properties for future applications.

MoTe₂ exists in two thermodynamically stable crystal forms: semiconducting 2H phase at ambient conditions, and semimetallic 1T’ phase at elevated temperatures. Reversibility of the 2H↔1T’ phase transition can be controlled by temperature or, as theoretically predicted, by other external stimuli [1], which makes this material attractive for advanced 2D electronics.

For this work, we have grown MoTe₂ single crystals by chemical vapor transport (CVT). Structural properties and composition of bulk MoTe₂ crystals and exfoliated layers were evaluated by TEM, SEM/EDS/EBSD, XRD, XPS, SIMS and Raman measurements. The CVT growth conditions were optimized to stabilize 2H phase or semimetallic 1T’phase. A particular crystal structure was controlled by the growth temperature as well as by the post-growth annealing: 1T’ phase was stabilized by quenching from ≥ 900 °C processing temperature, while the growth/annealing temperature below ≈ 800 °C resulted in 2H phase. For the semiconducting 2H phase, a choice of particular CVT transport agent dictated its intrinsic conductivity: iodine-assisted growth resulted in p-type, while TeCl₄-assisted growth led to n-type behavior, as determined from the FET transport measurement on thick (over ≈ 50 nm) MoTe₂ layers. A conductivity type was additionally tuned by scaling down the FET channel thickness: for TeCl₄-assisted growth, the polarity was switching sign from n-type for thick to p-type for thin (below ≈15 nm) layers, with an ambipolar behavior for the intermediate, 15 nm to 50 nm, thickness range. On the contrary, for the iodine-assisted CVT growth, p-type polarity remained unaffected by reducing the MoTe₂ layer thickness in the ≈ 50 nm to ≈ 5 nm range.

The talk will conclude by comparing 2H↔1T’ phase transition in bulk MoTe₂ induced by the growth/annealing temperature, with the electric-field-induced reversible phase change during formation of conducting filaments in 2H-MoTe₂ memristive devices [2].

References:
[1] Y. Li, K.-A. N. Duerloo, K. Wauson, and E.J. Reed, Nature Commun. 7, 10671 (2016).
[2] F. Zhang, S. Krylyuk, H. Zhang, C.A. Milligan, D.Y. Zemlyanov, L.A. Bendersky, A.V. Davydov, and J. Appenzeller, arXiv:1709.03835 (2017)

Biexcitons have been of keen interest for both fundamental studies of the remarkable many-body interactions and investigations of novel device applications, such as quantum logic gates, biexciton lasing devices, entangled photon sources, etc. [1] Recently, tightly bound biexcitons have been observed in monolayer TMDs, such as WSe₂, MoS₂ and WS₂. These biexcitons in monolayer TMDs show an ultra-large binding energy in the range of 50−70 meV, which is more than 1 order of magnitude higher than the values found in III−V quasi-2D quantum wells [2]. This strong binding necessitates the complete understanding of the structures of these biexcitons and their dynamics in 2D materials as well as characterization of their properties and full investigation of their potential functionalities. Hence, it is necessary to demonstrate a system that exhibits these biexcitons with high binding energy to make their study possible. Here we have successfully used PL spectroscopy to study biexcitons in free standing monolayer MoSe₂. We observed tightly bound biexcitons with a binding energy of ∼60 meV in atomically thin MoSe₂ [3]. The measured binding energy matches well with the theoretically predicted value of the excited state biexcitons in MoSe₂. We further probed the formation dynamics of these biexcitons and found that the density of biexcitons increases with increasing density of negative trions and decreases with increasing density of excitons. This finding suggests that the biexcitons observed here are excited state biexcitons instead of ground state biexcitons. More importantly, we successfully triggered the emission of excited state biexcitons at room temperature in a freestanding bilayer MoSe₂ by modulating three independent parameters: (1) dielectric screening, (2) density of trions, and (3) excitation power. The implications of the tightly bound biexcitons at room temperature in 2D materials are far reaching. It provides a room-temperature 2D platform to explore fundamental many-body interactions, which provides a route for quantum logical devices and entangled photon sources operating at room temperature.

Solid-state nanopores (SSN) have emerged as one of the most versatile tools for biomolecule detection and manipulation [1]. One of the most promising features of SSN is the DNA single base resolution and protein sequencing, at a low cost and faster than the current standards, with potential applications for detection and diagnosis of diseases. SSN sequencing experiments are based on the measurements of ionic current variations when a biomolecule emerged in an ionic solution translocates through the pore. As the biomolecule passes through it, it occupies the pore volume, blocking the passage of the ions. Therefore, ultrafast monitoring of ionic flow during the passage of the biomolecule yields information about its structure and chemical properties. Since the conductance of the ion flow through nanopores scales inversely with the membrane thickness, few atoms thick materials are ideal candidates with an expected high signal-to-noise ratio [2]. Beyond graphene, transition metal dichalcogenides such as molybdenum disulfide (MoS₂) are potentially advantageous due to their rich optoelectronic and mechanical properties [3]. In this study we report the results of all-atom molecular dynamics simulations that investigate the performances of MoS₂ nanopores for protein sequencing. First, we investigate the dynamics of KCl ions through sub-5 nanometer single layer MoS₂ pores [4]. We provide a new model to characterize the ionic conductivity through such nanochannels in order to finely describe translocation simulations/experiments [5]. Second, we investigate the translocation of model peptides such as poly(L-Lysine) and highly cationic peptide in the presence of an electric field in order to for protein sequencing applications [6].

References
[1] K. Liu et al. Atomically Thin Molybdenum Disulfide Nanopores with High Sensitivity for DNA Translocation. ACS Nano, 8, 2014.
[2] H.Arjmandi-Tashetal. Single molecule detection withgraphene andother two-dimensional materials: nanopores and beyond. Chem. Soc. Rev, 45, 2016.
[3] J. Priyanka et al. Angstrom-Size Defect Creation and Ionic Transport through Pores in Single-Layer MoS2. Nano Lett, 18, 2018.
[4] M. D. Barrios et al. Computational investigation of the ionic conductance through molybdenum disulfide (MoS2) nanopores. WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS, 16, 2017.
[5] M. D. Barrios et al. Molecular dynamics study of the ionic conductivity through solid-state MoS₂ nanopores from ions dynamical properties. In preparation.
[6] A. Nicolaï et al. Employing poly-Lysine tags for translocation of proteins through MoS₂ nanopores: Insights from Molecular Dynamics simulations. In preparation.

Graphite and transition-metal dichalcogenides are typical two-dimensional materials. Currently, mono-layer of each material is available, and the composites of those layers are producible in the laboratory. It has been theoretically proposed that a combination of the graphene and mono-layer MoS2 will admit intercalation of lithium ions. [1,2] If we stack such a combination vertically, the material is expected to be a lithium storage. The Li intercalation is known to occur in the dichalcogenides, such as, 2H-MoS₂. Those proposals suggest that the part of the dichalcogenide layers can be replaced by the graphene with smaller weight. In this study, by theoretical calculations, we propose that the three-dimensional stack of the graphene and the transition-metal dichalcogenide layers can store the lithium ions, and the planar density of lithium is three times of that of graphite. By selecting the chalcogenide compounds, the specific capacity, lithium content per weight, will be comparable to that of graphite. A candidate is Ti_0.5V_0.5S₂, which has lower density than MoS₂.
[1]R.H. Miwa and W.L. Scopel, J. Phys.: Condens. Matter 25, 445301 (2013). [2]X. Shao, et al. J. Phys. Chem. C, 119, 25860 (2015).

We are interested in exploring the rational design of materials and functionality at the atomic level to develop next-generation electronics, photonics, and catalysts with performance far beyond what can be achieved now. Our focus is on atomically thin two-dimensional (2D) materials, in particular, transition metal dichalcogenides (TMDC) like MoS₂, WS₂, MoSe₂, WSe₂, and h-BN.

In this talk, I will present our recent results in the synthesis of wafer-scale high-quality 2D TMDC materials with precisely controlled physical features. We may also perfectly transfer the synthesized wafer-scale films onto any arbitrary substrates without compromising the quality and surface smoothness. With the unique synthetic and transfer capabilities, we have systematically studied the electronic, optical, and catalytic properties of 2D TMDC materials, and developed a variety of novel devices.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are new members of the 2D materials family with wide tunability, highly dynamic structural features and excellent physical properties. The mechanical properties of such functional materials are both fundamentally and practically important to achieve both high performance and mechanically stable (flexible) devices. Here we report for the first time both the in-plane and out-of-plane mechanical properties of 2D lead iodide perovskites. We measured the out-of-plane mechanical properties of a series of 2D layered lead iodide HOIPs as a function of structural sub-units (e.g., variation of the length of the organic spacer molecules -R and the number of inorganic layer -n) by nanoindentation. We found 2D HOIPs have much lower elastic moduli and hardness than 3D HOIPs, and larger n number and shorter R result in stiffer material.¹ Furthermore, by AFM-based nanoindentation of suspended 2D HOIP membranes, we measured the in-plane Young’s modulus and breaking strength of ultrathin 2D HOIP membranes and their dependence on the membrane thickness. The in-plane Young’s moduli of 2D HOIPs are smaller than that of conventional covalently bonded 2D materials. Both the Young’s modulus and breaking strength first decrease and then plateau as the thickness increases from monolayer to 4 layers due to interlayer slippage during deformation. Our results show that ultrathin 2D HOIPs exhibit outstanding breaking strength/Young’s Modulus ratio compared to many other widely used engineering materials and polymeric flexible substrates, which renders them suitable for application into flexible electronic devices. We compared our findings with other 2D materials and shed light on routes to further tune the mechanical properties of 2D layered HOIPs.
References
1. Qing Tu, Ioannis Spanopoulos, Shiqiang Hao, Chris Wolverton, Mercouri G. Kanatzidis, Gajendra S. Shekhawat, and Vinayak P. Dravid, “Out-of-plane Mechanical Properties of 2D Hybrid Organic-Inorganic Perovskites by Nanoindentation”, ACS Applied Materials & Interfaces, doi: 10.1021/acsami.8b05138

A number of devices exploiting the remarkable optoelectronic properties of monolayer Transition Metal Di-chalcogenides (TMDs) have been demonstrated in recent years [1–3]. Most of these devices rely exclusively on the transport of unbound electrons or holes. However, purely excitonic devices leverage the high tunability of TMDs’ band structure and their large binding energies [4] to control the motion of excitons in room-temperature applications. Here, we experimentally demonstrate a visualization of exciton transport in a non-uniformly strained WSe₂ monolayer. The strain modulates the monolayer’s direct band gap giving rise to a built-in excitonic potential that results in the funneling of excitons in the direction of the strain gradient.

Our device consists of 400nm diameter, 300nm tall SiO₂ pillars on a Si substrate. The WSe₂ monolayer is deposited onto one of the pillars using a dry transfer technique [5]. We performed the optical characterization of the strain field by measuring the photoluminescence (PL) emission of the monolayer across the strained region and tracking the shift in the spectra with respect to an unstrained point. In addition, by scanning an avalanche photodiode detector across the PL beam we obtained a map of the time-dependent exciton density as a function of position [6]. This technique reveals essential excitonic transport parameters such as diffusivity and mobility. Our measurements indicate two regimes of diffusive transport [7]: a short, super-diffusive regime characterized by a time-varying diffusivity ranging between 1.6-1.9cm²/s, and a sub-diffusive regime with a time-varying diffusivity ranging between 0.5-1.5cm²/s. Similarly, our measurements indicate that the excitons drift in the direction of the strain gradient in two steps: a fast drift with an approximate strain mobility of 11.4cm²/s/%, and a slow drift with a strain mobility of 5.3cm²/s/%. These results will prove to be vital in the design of next-generation excitonic devices.

[1] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. Nanotechnol. 8, 497 (2013).

[2] J. S. Ross, P. Klement, A. M. Jones, N. J. Ghimire, J. Yan, D. G. Mandrus, T. Taniguchi, K. Watanabe, K. Kitamura, W. Yao, D. H. Cobden, and X. Xu, Nat. Nanotechnol. 9, 268 (2014).

[3] F. Withers, O. Del Pozo-Zamudio, A. Mishchenko, A. P. Rooney, A. Gholinia, K. Watanabe, T. Taniguchi, S. J. Haigh, A. K. Geim, A. I. Tartakovskii, and K. S. Novoselov, Nat Mater 14, 301 (2015).

[4] K. F. Mak and J. Shan, Nat. Photonics 10, 216 (2016).

[5] A. Castellanos-Gomez, M. Buscema, R. Molenaar, V. Singh, L. Janssen, H. S. J. van der Zant, and G. A. Steele, 2D Mater. 1, 011002 (2014).

[6] G. M. Akselrod, P. B. Deotare, N. J. Thompson, J. Lee, W. A. Tisdale, M. A. Baldo, V. M. Menon, and V. Bulović, Nat. Commun. 5, 3646 (2014).

[7] J. Wu and K. M. Berland, Biophys. J. 95, 2049 (2008).

Since the emergence of nanopores as promising biosensors for single-molecule detection, two classes of them have been developed: solid-state and biological (mostly protein-based). Solid-state nanopores have drawn attention during the past few years since they provide a more physically robust framework for hosting a nanopore, and further allow experiments under experimental conditions in which lipid membranes or protein channels may not be chemically compatible with. The quest to find an atomically thin membrane that can provide superior spatial resolution along with high mechanical stability has introduced two-dimensional (2D) materials such as Graphene, MoS₂, WS₂, and BN as second generation solid-state nanopores.

MXenes are an emerging family of 2D transition metal carbides and nitrides with a general formula of M_n+1X_nT_x (i.e. Ti₃C₂T_x), where M is a transition metal, X represents carbon or nitrogen (n=1, 2, and 3), and T_x indicates different functional groups (O, OH, F) present on surface of MXenes.¹ MXenes are produced by selective removal of A layer atoms from a group of layered ternary carbides and nitrides called MAX phases. Since their discovery in 2011, MXenes have been vastly studied for a variety of applications including energy storage and snesors, however, their applications in single molecule sensing and biophysics has remained unexplored. Herein, we investigate the application of a freestanding thin membrane of MXenes, in biomolecules detection and characterization. After fabrication of sub -10 nm diameter pore on the ultrathin MXene membranes, and immersion of the membrane in two buffer baths such that the pore is a sole liquid junction between the two chambers, application of a small voltage across the membrane results in an electric field that threads polymers such as DNA through the nanopore. This results in a transient drop in the ion current, which is used to probe electrically and/or optically structural features in the polymer.

Our preliminary data shows that MXenes provide high mechanical robustness, long-time stability, and low-noise ionic currents through the pore for single molecule detection. With more than 20 different MXenes synthesized, each having different properties, and their facile tunability, this study provides the basis knowledge for application of MXenes as potential materials for second-generation solid-state nanopores.

1. Anasori, B., Lukatskaya, M. R. & Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2, 16098 (2017).

Monolayer transition metal dichalcogenides (1L-TMDs) are promising two-dimensional semiconductors with direct bandgap that ranges the energy of visible and infrared light, suitable for atomically thin nanophotonic device applications. However the very low quantum yields of intrinsic 1L-TMDs is a major drawback for practical use. There have been extensive studies that were able to induce significant increase of light emission of 1L-TMDs, among which chemical treatment using bis(trifluoromethane) sulfonimide TFSI have been shown to be surprisingly effective giving rise to a few order of magnitude increase of photoluminescence. While the healing of sulfur vacancies of 1L-MoS₂ were pointed out as the origin of increased quantum yield, the mechanism of healing the defects is in veil, and hindering practical use of this technique. Here we provide direct observation of atomic healing of 1L-TMDs caused by TFSI treatment. We performed the optical characterization and STEM imaging on the same samples of CVD-grown 1L-MoS₂ and 1L-WS₂ before and after TFSI treatment and found that most (~90%) of sulfur vacancies observed in pristine 1L-MoS₂ were fully repaired with TFSI treatment. Such greatly reduced density of sulfur vacancies were not observed when other chemical treatment that caused moderate increase of PL was used, which indicates that sulfur atoms dissociated from TFSI molecules have filled in vacancies of pristine 1L-MoS₂. The direct filling of sulfur atoms was found to be energetically stable by density functional theory calculation. Our observation unveils the intriguing healing process of lattice defects of 1L-TMDs and suggests that 1L-TMDs can be made completely defect-free, widening and prompting the practical uses of 1L-TMDs in nanophotonics applications. This work was in part supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (NRF-2018R1D1A1B07042917)

Two-dimensional (2D) semiconductors are finding a renewed interest in recent years, due to their combination of physical properties, e.g., mobility, fluorescence and spin selectivity, with a potential implementation in new and emerging opto-electronic and spin-electronic devices. The current work discusses structural properties and various magneto-optical phenomena, found in three different 2D systems: The transition metal phosphorous trichalcogenides, indium sulfide and magnetically doped colloidal nanoplatelets. The magneto-optical properties are investigated by following optical polarization in the presence of an external magnetic field, as various temperatures, as well as implementing the use of optically detected magnetic resonance spectroscopy.
The transition metal phorphorous trichalcogenides resemble the most common layered dichalcogenides, but one-third of the metals are replaced with a P-P pair; hence, the chemical formula is written as M_2/3(P-P)_1/3X₂ or M₂P₂X₆. The dilution of metal site by non-magnetic atoms, endows a column like arrangement of the remaining metals, leading to a special magnetic properties, from a full antiferromagnetic Neel, through antiferromagnetic zigzag to ferromagnetic character. The various arrangements can be tuned by variation of the metal cations (among the first row of transitions metal atoms). The work focuses on the influence of the created magnetism on the magneto-optical properties of the M₂P₂X₆ semiconductors. The indium sulfide (In₂S₃) nanoplatelets crystallize in a few different morphological shapes, hexagon, dodecagon and truncated triangular structures, and are associated either with different crystallographic phases.
Transition metal dopant embedded in colloidal semiconductor nanoplatelets (NPLs) exhibit special magnetic properties, resemble the bulk diluted magnetic semiconductors. However, the NPLs confined thickness induces an extremely intense spin-exchange interaction between the resident photo-excited carriers and the guest magnetic spins. Such an interaction leads to a giant magnetization and g-factor, and consequently endows the materials with special magneto-optical properties. The work emphases the investigation of the spin-exchange interaction while varying the magnetic dopants, by following variation in the magneto-optical properties, when detecting either an ensemble of NPLs or a focus on a single platelet. **

** Magnetically doped NPLs project was carried out in collaboration with Prof. Volkan Hilmi Demir and his groups from Nanyang Technological University – NTU Singapore 639798, Singapore and from Bilkent University, Ankara 06800, Turkey.

The transition metal dichalcogenides are promising materials due to their unusual magnetic, optical and electrical properties. As it has been recently shown, Weyl semimetal including MoTe₂ can exhibit carrier mobility of 4000 cm²/Vs and giant magnetoresistance (MR) of 16 000% in a magnetic field of 14 T at 1.8 K [1]. Most of the transport results of the MoTe₂, a relatively unexplored transitional metal dichalcogenide, are obtained on mechanically exfoliated samples and concern only temperature dependence of resistance [2-3]. Nowadays, there is a substantial progress in obtaining MoTe₂ by thin-film epitaxy or deposition [4-8]. In this paper, we present the studies of the MoTe₂ layers grown by molecular beam epitaxy (MBE). As it is well known the substrate is of critical importance for the electronic properties of the thin 2D layer. We have studied the role of the substrate using many differernt substrates: Al₂O₃, GaAs [100], silicon and GaAs [111B] with and without ZnTe buffer. An appropriate choice of growth temperature allows us to switch between 2H and 1T’ politypes. It also influences the sample morphology, changing it from regular plane to nanodendrites. Magnetotransport properties of the layers will be presented and the impact of substrate will be discussed.

Transition metal di-chalcogenides (TMDs) form in 2H (trigonal prismatic) and 1T’ (distorted octahedral) polymorphs. For transition metals from the VIB and IVB columns, the 2H phase is semiconducting and the 1T’ phase is a narrow-gap semiconductor or semi-metal. Switching between the 2H and 1T’ phases could be the basis for useful phase-change technology, and the structural distortion between the phases – a martensitic translation of a single plane of chalcogen atoms in a van der Waals material – suggest that phase-change behavior could be relatively immune to fatigue.
We will present our work towards designing phase-change TMDs for application to infrared photonics. Our density functional theory (DFT) calculations predict that the refractive index contrast between the 2H and 1T’ phases is large and useful in the near-infrared (NIR) wavelength range 1 – 1.5 mm that is relevant to telecommunications. We study the (Mo,Cr,Ti)(S,Se)₂ alloy system to optimize the energy and accelerate the kinetics of switching. Our DFT calculations predict that the 2H – 1T’ transition occurs near 50 at. % Cr or Ti. To experimentally access this range of materials we use combinatorial metal sputtering, followed by sulfurization/selenization using H₂S and H₂Se gas, to make composition-graded TMD films across 2” wafers. We use Raman and X-ray diffraction mapping to distinguish the phase transition boundary and confirm our DFT calculations, and we use ellipsometry to measure the complex refractive index in the NIR. Our work helps build the foundation for rational design of thin film TMD phase-change materials that can be switched optically, electrically, and mechanically.

Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS2 with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems.
Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer α-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d33 piezoelectric coefficient of α-In2Se3 increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd-even effect. In
addition, we also demonstrate a type of α-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and
electronic skin. The out-of-plane and in-plane piezoelectricity in α-In2Se3 flakes offers an opportunity to enable both
directional and nondirectional piezoelectric devices to be applicable for self-powered systems and adaptive and straintunable electronics/optoelectronics.

Single or few-layer molybdenum disulfide (MoS₂) has demonstrated significant promise in a wide variety of applications including solar cells, transistors, supercapacitors, batteries and as photo/electrocatalysts. Chemical exfoliation of MoS₂ by the traditional n-butyllithium route remains one of most common and efficient exfoliation methods which provides relatively high yields of single layers. However, it also results in the partial conversion of the MoS₂ from the initially semiconducting 2H polymorph to the 1T polymorph which is a metastable metallic phase. Due to its metallic properties, 1T-MoS₂ is the preferred polymorph when used, for example, as an electrocatalyst for the hydrogen evolution reaction (HER), in photoelectrochemical (PEC) water splitting and as electrodes for batteries and supercapacitors. Due to its technological promise, the air and water stability of 1T- and 2H-MoS₂ single layers have drawn considerable attention. Herein, we report the discovery of a photo-induced degradation mechanism that can cause the rapid oxidation of MoS₂ in water. Ambient light and pH are found to play a significant role in rate of oxidation of a 1T polymorph-dominated aqueous dispersion. Under certain conditions, only one week is necessary to cause the near complete conversion of the MoS₂ into oxidation products and one month to convert an initially dark dispersion to transparent. The time evolution of the chemical and structural changes within the material and dispersion are monitored by a variety of advanced techniques such as Raman spectroscopy, Fourier transform infrared spectroscopy, UV/vis spectroscopy, atomic force microscopy and zeta-potential analysis. From these results, we discuss the nature of the decomposition product and the mechanism of the light-induced degradation. We probe conditions under which degradation is slow enough that the material remains stable and discuss the most suitable analytical techniques for detecting decomposition. We believe that the findings from this study will be of great value to those using MoS₂ monolayers in various applications, where the stability of the active material is of paramount importance.

Epitaxial single crystal thin films are very interesting which has drawn great scientific attention in recent time. Such thin films lack grain boundaries because of which they exhibit unique electric and transport properties leading a broad way for large scale applications in nanoelectronics, optoelectronics. Thin films over very large areas can be grown with controlled thicknesses, which paves the door for fabricating large-scale novel devices using MoTe₂. We have grown 2D-MoTe₂ thin film on Si/SiO₂ substrate by chemical vapor deposition. We will present the growth, structural and transport studies of these films. These films exhibit semimetallic behavior and has unsaturated quadratic magnetoresistance. Hall resistivity measurements confirm the majority charge carriers are holes and using single band model, the concentration of holes is 6×10²¹ cm^-3 at 10 K and increases with the increase in temperature.

Transition metal dichalcogenides (TMDs) are two-dimensional semiconducting analogues to graphene that have been widely explored for biomedical applications. However, cell-substrate interactions of TMDs are not well reported in literature, and TMD-incorporated cell culture platforms require further study to illuminate the influence of TMDs on the adhesive interactions of biological cells and the subsequent integration of these nanomaterials for potential biological applications involving biotic/abiotic interfaces.

We have explored cell-substrate interactions in which the influence of TMD geometric features on the resulting cellular morphology and proliferation is characterized quantitatively, offering insight into manipulating the adhesion of cells on TMD surfaces. We utilized WS₂ and MoS₂ on cytotoxic and cytocompatible substrates as cell culture platforms, and probed cell-substrate interactions via metrology and analysis of cellular morphometric features (i.e., cell area and eccentricity). Further, we performed analyses relating the influence of local TMD spatial parameters to the resulting cellular morphology, which provides a greater understanding on the causal relationship between TMD surfaces and cell attachment.

Our results show that TMD surfaces in arbitrary order can be used to enhance cell adhesion on substrates that are inertly undesirable to cell attachment, rendering otherwise cytotoxic substrates that are useful as 2D cell culture platforms. We also plotted and determined the significant correlation between TMD geometric variables and the resulting morphometric features of adherent cells, demonstrating that increased TMD island count and size parameters correlated to significantly larger and more elongated cells. These results indicate that by tailoring the geometric parameters of TMDs to a size scale comparable to the adhered cells, one can further guide the desirable adhesion of cells on diverse substrates (even those inherently adverse or cytotoxic to cells), which ultimately influences cellular survival, proliferation, differentiation and migration; coupled with TMD's unique electronic and chemical properties, these findings can be further explored and applied in fields that utilize biotic/abiotic interfaces including biosensing, pharmacological testing, and organ printing.

Integration of Group VI 2H-MX₂ Transition Metal Dichalcogenides (M = Mo, W; X = S, Se Te) into technologies ranging from chemical sensors to thermal-optical coatings and structural nanocomposites require direct chemical modification of the chalcogenide basal surface. Defect termination with elemental chalcogenide or organic ligands with terminal chalcogenides are routine, however the electronic structure of Group VI Transition Metal Dichalcogenides (TMDs) leads to an inert basal surface. Hybridization strategies therefore rely on extreme reduction processes to drive a transformation from the inert 2H to the reactive 1T phase. However, conversion back to the 2H phase is incomplete, resulting in heterogeneous materials. Here in, we demonstrate an alternative method for direct hybridization of Group VI 2H-MX₂ TMDs via alkyl organometallic chemistry. Using the recently development redox exfoliation method, single to few layer TMDs are stabilized in a range of polar organic solvents that are compatible with organometallic chemistries. X-ray Absorption spectroscopy demonstrate strong chalcogen-ligand interaction while X-ray diffraction resolve an expanded interlayer gallery consistent with basally conjugated short-chain organics. The functionalized Group VI 2H-MX₂ can be transferred to non-polar solvents (THF, CHCl_3, CH₂Cl₂), and show enhanced stability from oxidation and degradation. Finally, the organometallic chemistry is tolerant to –ene and –yne, facilitating post-functionalization modification via click chemistry, such as thiol-ene and thiol-yne conjugation. Tuning the optical, electrical and mechanical properties of MoX₂ and NbX₂ through integration into composites via surface functionalization is demonstrated.

Transition Metal Dichalcogenides (TMDs) have attracted considerable attention due to their extensive property suite, which compliments many other low dimensional nanomaterials (e.g. graphene, BN, aluminosilicates, phosphenes, etc.). However, energy intensive sonication, aggressive chemistries, and a small choice of stabilizing solvents limit scalable exfoliation strategies, and thus access to large quantities of well-defined layers. Mechanistically, exfoliation of low-dimensional nanoparticles from a large crystalline particle has two crucial steps: favorable modification of the nanoparticle surface, and subsequent nanoparticle separation to expose new surfaces. Therefore, the rate and efficiency of exfoliation is optimal for a combination of mechanical forces and surface chemistry; that is either alone is insufficient. Here in, we demonstrate that high-speed shear mixing rather than aggressive sonication balances the rates of chemical processes with nanoparticle separation for TMD exfoliation by the redox method. Chemically, this strategy generates in-situ polyoxometalates, which stabilize single-to-few layer Group IV – VII TMD sheets via coloumbic repulsion. Introduction of shear mixing during the formation of the polyoxometalates, rather than during the initial oxidation step or after the termination of chemical processing, substantially increases single layer exfoliation yields (> 10%) while drastically reducing reaction times (< 24 hrs). Stable dispersions of Group IV – VII TMD TMDs in polar organic solvents (acetonitrile, acetone, alcohols) at high concentration (10% w/w) are demonstrated for use in numerous bulk technologies, such as thermal/optical coatings, structural nanocomposites, and inks for flexible electronics.

Molybdenum disulfide (MoS₂) is an atomically thin semiconductor with direct bandgap in single layer (SL) which shows a great potential to wide range of applications in modern 2D electronics and optoelectronics. CVD based co-evaporation of molybdenum (Mo) and sulfur (S) precursors is becoming prevalent for the synthesis of SL MoS₂ since conventional exfoliation methods cannot produce large scale samples for scalable device fabrication. For certain optoelectronic applications, choice of transparent growth substrate is critical as it can avoid transfer methods that involve chemicals which can degrade optical and electrical quality of grown SL crystals. Sapphire being an excellent candidate to serve this purpose has the additional benefit of good control over the crystal orientation of MoS₂ grains during the growth avoiding high density of grain boundaries in large area growth. Even though molybdenum trioxide (MoO₃) is the commonly used Mo precursor for co-evaporation synthesis, we observed that it is extremely difficult to reproducibly grow MoS₂ on sapphire with it. We recently found that by using MoO₃ thin films as a precursor rather than MoO₃ powder it is possible to grow SL MoS₂ on Si/SiO₂ substrates with high reproducibility since the uniform evaporation rate of these thin films create much stable vapor pressure at the growth phase [1]. Here, we show that this thin film based technique also produce MoS₂ crystals with high reproducibility on sapphire without any special substrate treatments and we also studied the effects of parameters such as MoO₃ film thickness and S concentration on the crystal growth. These crystals are as large as 25µm confirmed to be high quality SL MoS₂ by atomic force microscopy measured thickness of 0.65 nm, 18.5 cm^-1 gap between A_1g and E¹_2g peaks of the Raman single spectrum and very high photoluminescence (PL) response at 1.85eV. High optical quality of these crystals attests their applications in next generation optoelectronic devices.

[1] Sajeevi S Withanage, Mike Lopez, Hirokjyoti Kalita, Hee-Suk Chung, Tania Roy, Yeonwoong Jung and Saiful I Khondaker, CVD Growth of Monolayer MoS₂ by using MoO₃ Thin Films as a Precursor for Co-Evaporation. submitted

Chemical vapor deposition (CVD) based co-evaporation is a well-established method for the single crystal growth of transition metal dichalcogenides (TMDs) including molybdenum disulfide (MoS₂) with excellent electrical and optical properties, still reproducibility of the results has been a challenge since the growth is very sensitive to small perturbations of the growth conditions. Poor understanding in the growth mechanism of this single crystal growth makes it even harder to control growth parameters for reproducible growth. Atomic scale control of the precursors in the vapor phase is the key for such growth. We found this is very hard to achieve with the solid precursors and other limited resources such as lacking independent temperature control of precursors, changes and turbulence of the flow due to internal and external conditions, etc. We evidenced that majority of the growth results in intermediate sub-oxide (MoO_3−x) crystals rather than single crystal MoS₂ on the substrate due to these less optimized conditions, especially excess molybdenum environment. Characterization of these microcrystals by Raman spectroscopy showed that they are a combination of MoO₂ and MoS₂. We were able to successfully sulfurize and recrystallize them into MoS₂ single crystals which are confirmed by Raman spectroscopy, photoluminescence spectroscopy and atomic force microscopy. We present systematic experimental results for MoO_3−x formation and resulfurization process elucidating the growth mechanism of this CVD process. This study shows that MoO_3−x formed at the intermediate state can be useful toward reproducible growth of MoS₂.

Transition metal dichalcogenides (TMDs) including molybdenum disulfide (MoS₂) is a new class of 2D materials which has a wide range of applications due to their unique optical and electronic properties. Weak inter-layer Van der Waal bonding, direct band gap in single layer, photo conductivity, high sensitivity and high mobility of these materials has promising applications in next generation electronics and optoelectronics. Significant amount of research has been done over the years to synthesize high quality large area monolayers of MoS₂; a solid method for large area single crystal growth with good reproducibility is yet to be achieved. We report novel method for chemical vapor deposition (CVD) based growth of monolayer MoS₂ by using thermally evaporated thin films of molybdenum trioxide (MoO₃) as the molybdenum (Mo) source for co-evaporation. Compared to the commonly used MoO₃ powder, thin films give much precise control over the amount of MoO₃ and uniform evaporation rate of these films creates a stable vapor pressure at the growth phase, hence facilitate much uniform, clean single crystal growth reproducibly. These high quality monolayer crystals are as large as 95µm and were characterized by Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and transmission electron microscopy. The bottom gated field effect transistors (FETs) fabricated on as grown single crystals shows n type transistor behavior with a good on/off ratio of 10⁶ and mobilities up to 4.5 cm²/Vs under ambient conditions approving the ability of these materials in future semiconductor electronics.

Transition metal dichalcogenides (TMDs), which are semiconductors of the type MX₂, where M is a transition metal atom such as Mo or W and X is a chalcogen atom such as S or Se are available as building blocks for novel optoelectronic and photonic devices. Among the various TMDs materials, Molybdenum disulfide (MoS₂) a representative n-type semiconductor, possesses a band gap ranging from an indirect bandgap (1.2 eV) in bulk form to a direct bandgap (1.8 eV) in the monolayer which enables its applications in photo-electronics such as photocatalyst and phototransistors devices. Monolayer MoS₂ demonstrated mobility of ~0.1-10 cm²V^-1s-¹ and on/off ratios of 10⁸ at room temperature. The reported monolayer MoS₂ photodetectors have obtained a responsivity of 880 AW^-1 at a wavelength of 561 nm and a photo-response in the 400-680 nm range. Also, a hybrid 2D materials and quantum dots phototransistor has been reported to show very high gain and optical sensitivity determined by the property of the quantum dots.
Here we present a quantum dots/MoS₂ photodetector consisting of graphene quantum dots and multilayer MoS₂ sheets. Graphene quantum dots and MoS₂ hybrid films are characterized by Raman/PL spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). We measured electrical property of GQDs/MoS₂ devices under varied light range at low temperature. Our results suggest that the MoS₂ and graphene quantum dots hybrid structure can be promising as efficient photo-detecting devices and optoelectronic circuits.

Two-dimensional (2D) materials have attracted much attention from the scientific community due to their unique properties, such as optical, magnetic and mechanical. For example, due to its high Young’s modulus (>270 GPa), 2D-MoS₂ is highlighted as a great candidate for nanofiller in polymeric composites. However, although 2D composite materials can be applied in many fields, the quality and the yield of their fabrication process are a constant concern. MoS₂ is the most studied transition metal dichalcogenide in the world and many techniques have been developed to obtain this material, such as chemical vapor deposition and mechanical exfoliation. Recently, liquid exfoliation using ultrasonication has been used to obtain MoS₂ sheets and functionalized MoS₂ sheets in a single step.¹ The exfoliation in the presence of the polymer polybutadiene (PB) increases the yield of the process, since the PB bonds in the edge of the MoS₂ sheets stabilize the 2D structure during the exfoliation. Accordingly, in this study we provide some fundamental insights into MoS₂ exfoliation process and yield increasing. For this, 2D-MoS₂ materials were prepared by exfoliation under ultrasonication by using a bath and a tip, with three different molar mass of PB (5k (PB_5k), 200k (PB_200k) and 200k-300k (PB_300k)) and two different solvents to disperse the polymers (toluene and tetrahydrofuran). When the exfoliation was performed by using PB_200k or PB_300k, MoS₂ sheets with very similar quality were obtained, inasmuch as Raman spectroscopy showed a frequency difference between A_1g and E¹_2g bands around 25 cm^-1 (value for bulk MoS₂ is 26.3 cm^-1), regardless of the solvent used. On the other hand, when the exfoliation was performed by using PB_5k, better results are obtained when no solvent was used to disperse the polymer, since this polymer is liquid at room temperature (i.e., its role occurs without the need of a new medium). Moreover, characterizations of the 2D-MoS₂ materials by X-ray and electron diffraction showed a higher degree of exfoliation in the presence of the polymer than in its absence. Besides, although the nature of the used solvents to disperse the polymer did not influence the quality of the sheets, the yield of the process was quite enhanced when toluene was used (11.26 % for the exfoliation when PB_200k was used). This methodology provides a promising avenue toward the producing 2D-MoS₂ composites with high degree of exfoliation and yield.

We thank FAPESP (CDMF, proc. 2013/07296-2) for the financial support; FAPESP (2016/14493-7) for the scholarship; CNPq and CAPES.

[1] GONÇALVES, R. H.; FIEL, R.; SOARES, M. R. S.; SCHREINER, W. H.; SILVA, C. M. P.; LEITE, E. R. Single-Step Exfoliation and Covalent Functionalization of MoS₂ Nanosheets by an Organosulfur Reaction. Chemistry – A European Journal, v. 21, p. 15583-15588, 2015.

MXenes are a new class of 2-D layered compounds that show promise as a highly-tunable material for next generation composites. In the last decade it has been demonstrated that these ceramic-derived materials can be easily processed into inks, aerosols, paints, and thin films via solution methods not available to many other 2-D layered materials. Furthermore, they are inherently hydrophilic which is favorable for chemical functionalization. Prior work has demonstrated favorable applications in energy storage and electromagnetic shielding. However, a fundamental understanding of the intrinsic properties of MXenes is lacking, and methods for surface functionalization are needed.

To the purpose of developing tailored MXene bulk films for aerospace applications, we present fundamental electronic, chemical, and structural studies of individual MXene flakes utilizing experimental and theoretical methods. We have size-selected ultrathin sheets and verified their thickness using AFM and TEM and developed anaerobic protocols to surface functionalize individual particles using chemistries of catechols and silanes. Additionally, we confirmed modifications to the crystal lattice and local functional groups using XPS and Raman spectroscopies, as well as used EELS and ultrafast transient optical spectroscopy to extract microscopic electronic properties which were corroborated by theoretical calculations.

Large-area, high-quality two-dimensional molybdenum disulfide crystals have commonly been grown through powder vaporization because of its simplicity and cost-effectiveness. However, the sulfurization process for molybdenum trioxide, through interactions with sulfur, still remains poorly understood and insufficiently investigated. Ab initio calculations and molecular dynamics simulations of the reaction of sulfur allotropes (S_2-8) with molybdenum trioxide and experimentally proposed molybdenum oxysulfide intermediates (MoO₂S, MoOS₂) have been used to elucidate the mechanistic steps in the sulfurization pathway. These simulations show that the sulfurization process takes place through the formation of molybdenum oxysulfide rings, which can decompose into more sulfurized molybdenum species, oxysulfur molecules and/or smaller sulfur allotropes. As the sulfur allotrope distribution in sulfur vapor is temperature dependent, our results not only provide critical mechanistic insight into the sulfurization process, but also identify how experimental growth parameters can be adjusted to maximize the efficiency of the powder vaporization process.

Emerging two-dimensional gallium chalcogenides, such as gallium telluride (GaTe), are considered promising layered semiconducting materials that can serve as vital building blocks towards the implementation of nanodevices in the fields of nanoelectronics, optoelectronics and quantum photonics. Studies on the physical behaviors of layered GaTe are still at early stages of fundamental research, and thus, are of significant interest from the scientific and technological point of view. However, oxidation-induced electronic, structural and optical changes observed in ambient-exposed gallium chalcogenides need to be further investigated and addressed. Herein, we report on the thickness-dependent effect of air and oxygen exposure on the Raman and photoluminescence (PL) properties of GaTe flakes, with thicknesses spanning in the range of a few layers to 65 nm. We have developed a novel chemical passivation that results in complete encapsulation of the as-exfoliated GaTe flakes in ultrathin hydrogen-silsesquioxane (HSQ) film. A combination of correlation and comparison of Raman and PL studies reveal that the HSQ-capped GaTe flakes are effectively protected from oxidation in air ambient over the studied-period of 7 months, and thus, preserving their structural and optical characteristics. This contrasts with the behavior of uncapped GaTe, where we observe a significant reduction of the GaTe-related PL (~100×) and Raman (~4×) peak intensities. Furthermore, the time-evolution of the Raman spectra in uncapped GaTe is accompanied with the appearance of two new prominent broad peaks, at ~130 cm^-1 and ~146 cm^-1, which are associated to photo-induced effects, such as oxidation and amorphization, in GaTe. The oxidation of the uncapped GaTe is consistent with our Auger spectroscopy findings. Moreover, our surface-passivation has a dual role. Along with effectively protecting the flakes, it offers the capability to simplify the integration process of fabricating GaTe-based nanodevices, given HSQ is the most commonly used resist for electron beam lithography. Our holistic approach can help further explore and reveal the physical properties of few-layer GaTe, towards the practical realization of nanophotonic devices with structural, and photoluminescence stability.

Transparency of two-dimensional (2D) materials to inter-molecular interactions has been an unresolved topic. It was found that water droplets interact with underlying substrates through graphene, as if the graphene is “transparent”. However, graphene’s transparency determined by droplet wetting angle has been controversial. Recently, precise atomic alignment between epitaxial films and substrates through monolayer graphene has been discovered in a GaAs/graphene/GaAs structure. This finding experimentally confirms the existence of remote atomic interaction through graphene. However, the mechanism of remote interaction through 2D materials at atomic-scale and its relationship with the bonding chemistry of 2D materials have not been fully understood.
Here, we report that remote atomic interaction through 2D materials is governed by the polarity of atomic bonds both in substrates and 2D materials. Our density functional theory (DFT) calculation reveals that polarization of atomic bonding in substrates enhances the strength of the electrostatic potential penetrated through 2D materials, preserving the information of atomic registry away from the substrate. Such trend is well observed by performing epitaxy of Si, GaAs, GaN, and LiF on their own substrates through graphene, whose ionic bonding character fractions are 0%, 31%, 50%, and 90%, respectively. The results show pure covalent-bonded Si loses its remote atomic interaction across monolayer graphene, leading to polycrystalline formation, while we obtained epitaxially aligned single-crystalline GaAs, GaN, and LiF through monolayer (1ML), bilayer (2ML), and trilayer (3ML) graphene, respectively. More interestingly, we discovered that such field penetration is substantially attenuated through hexagonal boron nitride (hBN) that contains polarity in its bonding. Thus, van der Waals epitaxy seeded from hBN and remote epitaxy seeded from graphene occur simultaneously when we perform epitaxy of GaN through 1ML hBN. Through 3ML hBN, complete transition from remote epitaxy to van der Waals epitaxy occurs. The excellent consistency between the theoretical and experimental findings unequivocally confirms that the ionic character of atomic bonding determines remote atomic interaction through 2D materials. Our work demonstrates that the transparency of 2D materials can be sensitively probed at atomic scale by performing epitaxy through them. This transparency highly depends on the nature of substrate materials, while it can also be tuned by modulating the thickness and bonding chemistry of the 2D materials.

MoS₂ transition metal chalcogenide (TMD) thin films are often made using MoO₃ as a precursor. Directly converting metallic Mo to MoS₂ would hold advantages for certain studies such as research on metal alloy TMDs, and for processes such as vacuum deposition in an oxygen-free environment. Recent work shows that although the thermodynamics of MoS₂ formation in sulfur-rich environments may be favorable, the outcome is highly dependent on the chemical pathway taken and in particular on the oxidation state of the Mo precursor [1,2].

Here we report a study of the role of oxygen as a catalyst for the sulfurization of metal Mo thin films in chemical vapor deposition (CVD). We study the formation of MoS₂ at high temperature from Mo and MoO₃ films in the presence of S and H₂S vapor and with different trace levels of oxygen. By systematically varying the reduction potential of the CVD furnace environment we demonstrate the essential role of oxygen in catalyzing MoS₂ formation at relatively low temperature. Our results help to interpret the sizable literature on the growth of MoS2 thin films by CVD, and provide useful guidance for developing processes to make wafer-scale MoS₂ films for applications ranging from photonics to catalysis.

[1] N. Salazar et al., Phys. Chem. Chem. Phys. 19, 14020-14029 (2017),

[2] S. Hong et al., J. Phys. Chem. C 122, 7494-7503 (2018).

Lattice defects such as chalcogen vacancies drastically affect the optoelectronic properties of monolayer transition metal dichalcogenides (TMDs) grown by chemical vapor deposition (CVD). They can be passivated through charge-transfer doping by laser irradiation in air. Here we perform a systematic in situ study to elucidate the passivation mechanism upon laser irradiation and show how to controllably n-dope monolayer MoS₂. By combining resonance Raman and photoluminescence (PL) spectroscopy we show that an increase in defect density correlates with a red-shifted PL emission and hence an increase in electron density. Density Functional Theory (DFT) calculations identify chalcogen vacancies to be facilitators (not the source) of n-doping, and population of mid-gap levels upon doping lowers the activation barrier for O₂ adsorption from 0.3 to 0.03 eV. Laser irradiation aids in the oxygen-passivation of chalcogen vacancies, manifested by an increase in PL intensity and blueshifted emission, and this blueshift is determined by the laser power density. The passivation occurs on two timescales, with the removal of surface adsorbates first, followed by oxygen adsorption at the sulfur vacancy sites. Our systematic study provides valuable insights into the defect passivation process, and also demonstrates a practical way to modulate the carrier concentration of monolayer MoS₂.

MXenes are a relatively new class of nanosheets, and they have gained significant interest due to their unique chemical, dielectric and transport properties. Since their discovery in 2011, they have shown promise in a wide range of applications such as electromagnetic shielding, supercapacitors, batteries and water desalination. Typically, MXenes are made from layered carbides and/or nitrides called MAX phases, by selectively etching the A layer using hydrofluoric acid. Here, we successfully demonstrate the electrochemical etching of Al from porous Ti₂AlC electrodes in dilute hydrochloric acid (no F ions) to form a Ti₂CTx MXene layer on Ti₂AlC. These MXenes possess chloride terminal groups, along with the common ones, such as –O and –OH. However, electrochemical etching can result in subsequent over-etching of parent MAX phases to carbide-derived carbon (CDC). We propose a core–shell model to explain electrochemical etching of Ti₂AlC to Ti₂CTx and CDC. The proposed model suggests that a careful balance in etching parameters is needed to produce MXenes while avoiding over etching. Our electrochemical approach expands the possible range of both etching techniques and resulting MXene compositions.
We also demonstrate that colloidal MXene nanosheets encapsulated within spray-dried droplets can be scrolled, bent, and folded into 3D crumpled structures by capillary forces during drying. This morphological change was observed to be reversible upon rehydration.
Finally, Ti₃C₂ MXenes are prone to oxidation, which causes them to chemically degrade to TiO₂ over time and become impractical for desired applications. This makes processing of Ti₃C₂ MXenes difficult. We demonstrate that processing Ti₃C₂ MXenes in organic solvents and polymer composites leads to lower TiO₂ content and higher conductivity compared to Ti₃C₂ MXenes processed in water. We monitored the oxidation content by measuring the conductivity of the processed films.

As the synthesis and material properties of 2D materials is better understood, new fronts of potential applications are opened, such as chemical and biochemical sensing. 2D materials offer many advantages in this field, including physical flexibility and large surface-to-volume ratio. While the mechanisms and factors affecting nanorods and nanotubes using conventional 3D materials have been well studied, such details in 2D transition metal dichalcogenide based sensors are not fully understood. In this work, we look at the role of the electronic contact in the sensing capabilities of directly synthesized MoS₂ sensors for trimethylamine (TEA). Comparing MoS₂ sensors with conventional metal contacts to graphene contacts, we find the band structure and corresponding charge transfer at the contact-channel interface plays a large role in the sensing characteristic of such devices. Devices with Ti/Au – MoS₂ contacts, experimentally shown to have Schottky barriers with approximately twice the height of barriers across epitaxial graphene (EG) – MoS₂ contacts), exhibit up to 5 orders of magnitude higher sensing amplitude (defined as a fractional change in the device conductivity, ΔG/G₀) compared to devices using EG contacts. In addition, effects of controlled defects, such as substitutional doping and plasma induced defects, on the MoS₂ channel is studied. By incorporating 2 atomic % of Nb, for example, the TEA sensitivity threshold is reduced by more than an order of magnitude.

Lattice defects formed in 2D hexagonal boron nitride (hBN) have emerged as unique nanopores. Their most useful property is photoluminescence (PL) from the defects in the form of single-photon emission even at room temperature. Even though the defects exhibit PL at multiple wavelengths in a visible range, which vary from defect to defect, the statistic characterization and analysis have been limited attention. To systematically characterize individual defects of hBN, we develop scanning probe spectroscopy system combined with an atomic force microscope and a time-correlated single photon counting module. This system allows us to automatically measure thousands of PL spectra at each point, and correlate with PL lifetime, PL excitation, and thickness from individual defects. The statistical analysis of these collected data provides us insight into physical and chemical properties of nanoscale defects in hBN and potential for ultrasensitive sensing and quantum photonics.

The growth of ¹⁰B and ¹¹B isotope-enriched hexagonal boron nitride (hBN) single crystals from molten metal solutions was carried out through a combined experimental and theoretical approach. Using elemental ¹⁰B or ¹¹B and nitrogen as the source, the hBN crystal size and quality obtained in molten iron (Fe), iron and chromium (Cr), and mixed Ni-Cr solutions were compared. In this work, the Fe-based solvent is shown to provide a less expensive growth environment than the more established nickel and chromium solvent.
hBN crystals were grown at 1,550 °C and atmospheric pressure, under continuously flowing nitrogen, in pure iron, 50 wt% Ni and 50 wt% Cr, and 80 wt % Fe and 20 wt% Cr. For the crystals grown using a pure Fe solvent, the lateral hBN crystal size and thickness were 0.5 mm and 30 microns, respectively. The predominant crystal domain shapes were equilateral triangles, with edge lengths up to 200 microns. This solvent produced the smallest crystals, presumably due to multiple nucleation sites due to the high solubility of nitrogen in Fe. The crystals grown from the Ni-Cr solution produced larger crystals sheets, up to 2.0 mm laterally and 80-100 microns thick. The surfaces were covered in parallel steps, suggesting the hBN grains had similar orientations. Some metallic inclusions were seen in the hBN produced from the Ni-Cr solvent. The domain shapes were equilateral triangles, truncated triangles, and hexagons. The hBN grown from 80 wt % Fe and 20 wt% Cr solvent had a similar lateral size and thickness with less surface steps, compared to crystal grown from Ni-Cr. Within the sheet were large individual grains, with lateral dimensions up to 400 microns. The main shape of domains were equilateral triangle and parallelogram. The crystals were colorless and free from any inclusions.
A critical requirement for successful hBN crystal growth is that the solvent needs to dissolve and boron nitrogen effectively. DFT was applied to determine the solubility of the nitrogen and boron in bulk Fe, a 50 at% Fe- 50 at % Cr mixture, and a 50 at% Ni-50 at% Cr mixture. At 1,550 °C, the calculated order of nitrogen solubility was: Fe>Fe-Cr>Ni-Cr. For boron, the order was: Fe-Cr>Fe >Ni-Cr. Thus, both Fe and Fe-Cr were predicted to be good solvents for hBN crystal growth. The DFT calculations and experimental results are consistent.

MXenes are a large family of 2D materials made by selective etching of the aluminum layers from M_n+1AlC_n MAX phases into open-structures with single layers of M_n+1C_n. These new highly-conductive materials present a remarkable capacitance (1500 F/cm³ in H₂SO₄). In 2015, the MXene family was extended to include double-transition metal (M) MXenes by introducing two different M types in separate atomic layers of a MXene single-flake, such as Mo₂TiC₂. The latter phase has already found use as electrode material in supercapacitor and battery-type devices. More than two dozen different compositions of the ordered double-M MXenes were predicted theoretically. Here, we present the synthesis and characterization of Mo_xV_4-xAlC₃ MAX, and its resulting Mo_xV_4-xC₃ MXene. Unlike Mo₂TiC₂, this phase is less ordered than previously reported double-M phases. Besides this, without having a dramatic change in its crystallographic structure, Mo_xV_4-xC₃ can be produced with various ratios of Mo and V. This tailoring modifies the electronic and electrochemical properties, and surface terminations, making Mo_xV_4-xC₃ a promising candidate for various applications. For example, in aqueous supercapacitors (1 M H₂SO₄), Mo_xV_4-xC₃ has demonstrated one of the largest positive electrochemical window so far (up to 0.6 V vs. RHE, 0.75 V voltage window) with capacitance of 850 F/cm³. And, because of this, Mo_xV_4-xC₃, positive electrode, can be coupled with Ti₃C₂, negative electrode, in a first full-MXene supercapacitor device producing 150 F/cm³ device capacitance with a 1 V window. Besides the application in energy storage, Mo_xV_4-xC₃ may be a promising electrocatalyst for hydrogen generation in water splitting devices.

To realize tremendous potential of two-dimensional (2D) monolayers of transition metal dichalcogenides (TMDs) requires high quality crystals with controllable morphologies and sizes, supported by various substrates, which in turn assumes deep understanding of the growth mechanisms. Moreover, these monolayers are particularly sensitive to surface contaminants associated with lithographical processes to create desired geometries. Here, we show that during the chemical vapor deposition growth of MoS₂ monolayer single crystals from MoO₂ and NaBr powder precursors, the presence of halide alters island growth (3D) mode of MoS₂ to layer-by-layer growth (2D) mode. We found that alkali metal halide salt additive behaves as a surfactant in the growth of TMDC monolayers by chemical passivation of the edges that facilitates the strain and thereby kinetically suppresses 3D islanding growth. This insight enables us to enhance the growth of MoS₂ monolayers on various substrates by preliminarily depositing a salt layer. Such a surfactant-mediated growth can also be applied to other 2D TMDs (e.g., WS₂, MoSe₂). Moreover, using various patterns of pre-deposited salt, we demonstrate direct growth of high quality MoS₂ monolayers with corresponding patterns, which can be ready for patterned 2D electronic devices. Our results open a perspective for direct patterned fabrication of pristine MoS₂ monolayers on various substrates without lithography or transferring.

Age-related macular degeneration (AMD), a retinal degenerative disease that results in a continuous degeneration of photoreceptors in the retina which eventually leads to complete blindness [1]. One approach to combat AMD is through the use of artificially implantable photodetectors that are physically placed on the retina. The photodetector pixels allow the implantable photodetectors to be in intimate contact to retinal pigment epithelium. Interestingly, 2D materials such as photosensitive and semiconducting molybdenum disulfide (MoS₂) and electrically conducting graphene have recently received tremendous promise due to their unique photonic and optoelectronic properties properties and their potential in various types of micro and nano devices [2,3]. In this study, we have tested the biocompatibility of various 2D materials, such as graphene and MoS₂ in several organic solvents. A highly biocompatible photodetector on a flexible polyimide substrate was designed, fabricated using inkjet printing to form photosensitive pixels and tested as a function of photo intensity and strain.
Inkjet printed graphene and MoS₂ inks were characterized using techniques such as Raman Spectroscopy, PL, SEM and AFM. The inkjet printed 2D heterostructure devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. This is 10³ times higher compared to prior reports, and detectivity D was calculated to be ~ 3.6 × 10¹⁰ Jones at room temperature. The strain-dependent measurements of photocurrent with bending showed a photocurrent of ~ 1.16 μA with strain levels for curvature up to ~ 0.262 cm^-1, indicating the feasibility of such devices for large format arrays printed on flexible substrate, unlike conventional Si implantable detectors that are rigid and nonconformable.
In conclusion, the inkjet printed, biocompatible 2D hetero-junction photodetector formed on flexible and conformable substrates was successfully shown to be photoresponsive to a wide range of light intensities and strain levels, making it a promising prospect for in vivo bio-sensing applications for AMD.

References:
[1] Age-Related Eye Disease Study Research Group. A randomized, placebo-controlled, clinical trial of high-dose supplementation with vitamins C and E, beta carotene, and zinc for age-related macular degeneration and vision loss.Archives of Ophthalmology, 119, 1417-1436 (2001).
[2] Hossain, R. F., Deaguero, I. G., Boland, T., & Kaul, A. B. Biocompatible, large-format, inkjet printed heterostructure MoS₂-graphene photodetectors on conformable substrates. npj 2D Materials and Applications, 1(1), 28 (2017).
[3] Fadil, D., Hossain, R. F., Saenz, G. A., & Kaul, A. B. On the chemically-assisted excitonic enhancement in environmentally friendly solution dispersions of two-dimensional MoS₂ and WS₂. Journal of Materials Chemistry C, DOI: 10.1039/C7TC01001J (2017).

The recently developed ability to manipulate one-dimensional edge states in conventional topological insulators (TIs) by selectively breaking time-reversal symmetry is a promising step towards developing TI-based devices. The same control of topological states is also possible in topological crystalline insulators (TCIs), such as SnTe, by selectively breaking crystalline symmetry. Large area quasi-two-dimensional TCI films have been difficult to achieve, however, making control of topological states challenging. This work presents structural and transport characterization of high quality ultrathin SnTe films grown by molecular beam epitaxy on single crystal SrTiO₃. Films thin enough to achieve edge state control are continuous and single domain in nature, and transport measurements show two-dimensional topological conduction through the protected SnTe/SrTiO₃ interface. Analysis of quantum effects observed in magnetotransport reveals the nature of topological states in the SnTe films, presenting a path to manipulating these states in devices.

The rapid development of device technologies based on 2D transition metal dichalcogenides (TMDs) causes increasing demand for synthesis of high quality large area monolayer and few layer films. Our previous work demonstrated epitaxial growth of large area monolayer WSe₂ films on c-plane sapphire using gas source chemical vapor deposition (CVD). However, the optical and electrical properties of coalesced monolayer films grown on sapphire are negatively impacted by the existence of anti-phase boundaries (APBs) resulting from a merging of an equal mixture of 0° and 180° oriented domains as well as non-uniformities arising from steps and charge-induced doping associated with the sapphire surface. Prior studies using powder source CVD demonstrated a preferred domain orientation for MoS₂ grown on hBN and first-principle calculations suggest this phenomenon originates from single atom vacancies on the hBN surface that act as nucleation sites. In this study, we further investigate the mechanism of defect-assisted domain alignment of 2D TMDs on hBN and demonstrate the growth of fully-coalesced WSe₂ films on hBN with a reduced density of APBs and improved optical and electrical properties compared to films grown on sapphire.
WSe₂ monolayer films were grown by gas source CVD at 800°C using W(CO)₆ and H₂Se in a H₂ carrier gas employing a multi-step process to separately control nucleation density and lateral growth and coalescence of domains. Single crystal hBN flakes exfoliated from bulk crystals and transferred onto c-plane sapphire were used as substrates. He plasma treatment and NH₃ annealing were used to modify the surface defect density of hBN. Detailed studies of WSe₂ deposition on hBN as a function of growth conditions and substrate pre-treatment confirm that domain nucleation is controlled by the surface defect density rather than the precursor concentration. Over 85% of WSe₂ domains have consistent orientation via the defect-assisted growth. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe₂ monolayer films on hBN were produced for subsequent characterization. High resolution scanning transmission electron microscopy (S/TEM) analysis demonstrates the absence of APBs in coalesced regions formed by the merging of 0° oriented domains. The resulting Moiré pattern formed by the WSe₂ on hBN indicates an epitaxial relationship of 3×3 WSe₂ on 4×4 hBN. Temperature-dependent photoluminescence measurements show sharp and enhanced exciton and trion emission peaks, with no defect-related bound exciton emission from monolayer WSe₂/hBN down to 80K. Backgated FET devices fabricated on WSe₂/hBN films transferred to SiO₂/Si substrates show an order of magnitude increase in room temperature carrier mobility (~5 cm²/V-s) compared to similar devices fabricated using monolayer WSe₂ films transferred from sapphire. The results demonstrate the potential of hBN as a substrate for epitaxial growth of high quality monolayer TMD films.

Substitutional doping has been demonstrated as an efficient way to tune the electronic and optoelectronic properties of 2D materials.^1,2 Beyond the singular functionalization of 2D materials, multifunctional doping of 2D materials provides opportunities to realize great potentials to 2D materials by one dopant, which can be highly compatible to advanced technologies.³ Here, we report the multifunctional doping of monolayer MoS₂ for electronic and chemical sensing applications. Niobium doped MoS₂ with various doping concentration (0.5 at%-57 at%) is synthesize by metal organic chemical vapor deposition on c-plane sapphire. The Fermi level is tuned from 2.0 eV above valence band maxima (VBM) to 0.3 eV below VBM (degenerate doping) as doping concentration increases. Interestingly, this Fermi level tuning exhibits a significant shift from theoretical prediction. Evident from x-ray photoelectron spectroscopy and conductive atomic force microscopy, 0.5 at% can only slightly reduce the electron concentration in monolayer MoS₂. ~5 at% Nb concentration is needed to push the Fermi level to 0.2 eV above valence band maxima (p-doped) and >20 at% Nb doped MoS₂ is degenerately p-doped, while <1 at% Nb for degenerate p-doping in theory. This unusual phenomenon is attributed to the strong electron doping from the sapphire substrate.⁴ Tunable Nb doping realizes multifunctionality of MoS₂. 5 at% Nb doped MoS₂ exhibits ~15x higher sensitivity (signal/noise ratio) to tetraethylammonium (TEA, a nerve agent) with <10ppb defection limit due to the enhanced conductivity and p-type conductance. Meanwhile, degenerate p-doped MoS₂ provides an Ohmic contact between metals (Pt/Ir etc.) and both 2D (MoS₂) and 3D (GaN) semiconductors, which may play an important role in contacting semiconductors. This work presents a promising route to cultivate the multifunctionality of doped 2D materials, enabling novel design of multifunctional electronic and sensing devices.
Reference
(1) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J. J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; et al. Recent Advances in Two-Dimensional Materials Beyond Graphene. ACS Nano 2015, 9, 11509–11539.
(2) Tedstone, A. A.; Lewis, D. J.; O’Brien, P. Synthesis, Properties, and Applications of Transition Metal-Doped Layered Transition Metal Dichalcogenides. Chem. Mater. 2016, 28, 1965–1974.
(3) Zhang, K.; Bersch, B. M.; Joshi, J.; Addou, R.; Cormier, C. R.; Zhang, C.; Xu, K.; Briggs, N. C.; Wang, K.; Subramanian, S.; et al. Tuning the Electronic and Photonic Properties of Monolayer MoS2 via In Situ Rhenium Substitutional Doping. Adv. Funct. Mater. 2018, 1706950.
(4) Zhang, K.; Borys, N. J.; Bersch, B. M.; Bhimanapati, G. R.; Xu, K.; Wang, B.; Wang, K.; Labella, M.; Williams, T. A.; Haque, M. A.; et al. Deconvoluting the Photonic and Electronic Response of 2D Materials: The Case of MoS2. Sci. Rep. 2017, 7, 16938.

Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process and studying contributions of exciton complexes to the PL emission are essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform single-layer Mo_1-xW_xS₂ alloys by a two-step CVD method followed by a laser thinning process and investigations on their excitonic behavior with compositional changes. The amount of W content (x) in the Mo_1-xW_xS₂ alloy is systemically controlled by the co-sputtering technique, and the post-laser process allows layer-by-layer thinning of the as-synthesized few-layer Mo_1-xW_xS₂ alloys down to a single-layer. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the Mo_1-xW_xS₂ alloys is a self-limiting process caused via heat dissipation to the substrate, resulting in spatially uniform single-layer Mo_1-xW_xS₂ alloy films. As W content (x) increases, the single-layer alloys reveal controlled optical band gaps ranging from 1.871 to 1.971 eV. Furthermore, we found that the number of excessive charge carriers decreases as x increases, resulting in the change in the predominant component of the PL emission from trions for single-layer MoS₂ to neutral excitons for single-layer WS₂. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMDs alloys and the design of versatile optoelectronic devices.

Superconducting van der Waals ultrathin crystals have recently emerged as an excellent platform for studying the role of disorder and quantum fluctuations in reduced dimensions. Here we report our observation of two types of QPT in phase-engineered nanosheets of Li_xMoS₂ having different contents of superconducting T/T’ and non-superconducting 2H phase. Our scaling analysis reveals that the as-synthesized samples with a predominantly T/T’ phase exhibit the ordinary QPT from superconducting to insulating state whereas the samples with a higher content of 2H phase display QPT with signatures of Griffiths phase evidenced by the diverging critical exponent. Our observations suggest that phase engineering allow formation of quenched disorder with a length scale suitable for the formation of rare superconducting puddles that lock the global superconducting phase above the mean upper critical field.

Since 2012, heterostructures formed by two-dimensional materials have drawn considerable attention due to their potentials of combining novel properties of the component materials. In developing these materials, one key issue is to understand and control interlayer charge and energy transfer. In this presentation, latest progress on experimental studies of ultrafast charge and energy transfer in van der Waals heterostructures will be discussed. After introducing the main experimental technique used in these studies – transient absorption microscopy with layer selectivity, previous results on ultrafast charge and energy transfer in heterostructures formed by two transition metal dichalcogenide monolayers and one transition metal dichalcogenide monolayer with a graphene layer will be reviewed. Using three different trilayer structures formed by transition metal dichalcogenide and graphene as examples, the role of the layer-coupled states on ultrafast charge transfer will be discussed. New measurements on the effect of electric field on interlayer charge transfer will be introduced. These discussions will be followed by two examples showing that band-alignment engineering can be used to control photocarrier lifetime in van der Waals multilayers. Finally, experiments on photocarrier transfer across lateral heterostructures formed by monolayer semiconductors will be discussed.

Heterostructures constructed from two-dimensional (2D) van der Waals materials such as transition metal dichalcogenides (TMD), graphene, and boron nitride, have sparked wide interest in both device physics and materials science.¹ Apart from these inorganic 2D materials, two-dimensional organic-inorganic hybrid lead halide perovskites (2D PVSKs) have recently emerged as promising materials for solar cells, with power conversion efficiencies over 12% and stability over 2000 hours, compared to 10 hours for traditional 3D PVSKs.² 2D PVSKs also show higher photoluminescence quantum yield (~26%) than their 3D counterparts (<1%), suggesting their intrinsic optoelectronic quality may be much higher.³

Fundamental photophysics of 2D PVSK materials can be further explored upon making their electronically active heterostructures with other van der Waals semiconductors. In this study we explore their heterostructures with semiconducting double layers of transition metal dichalcogenides built via exfoliation and dry viscoelastic stamping. We observed 116-fold enhancement in indirect transition occurring at 681-706 nm (1.76-1.82 eV) and 277-fold enhancement in trion emission peak at 658-666 nm (1.86-1.88 eV) of WS₂ photoluminescence in double layer WS₂ on (BA)₂(MA)₃Pb₄I₁₃ 2D-PVSK heterostructure. We have also observed ~60meV blue shift in indirect transition (from 1.76eV to 1.82eV) in heterostructure comparing to bare WS₂. The larger enhancement of photoluminescence intensity of trion emission in comparison to indirect transition and blueshift of indirection transition indicate increased carrier concentration in WS₂ in heterostructure, due to either charge transfer from or defect passivation by 2D PVSK. Emission enhancement in WS₂ by 2D PVSK shows promising features of the heterostructure, and may provoke further studies of Van der Waals heterostructures to understand fundamental photophysics of charge separation and novel, long lived excited states.

Reference
1. Rivera, P. et al. Observation of long-lived interlayer excitons in monolayer MoSe₂–WSe₂ heterostructures. Nat. Commun. 6, 6242 (2015).
2. Tsai, H. et al. High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells. Nature 536, 312–316 (2016).
3. Dou, L. et al. Atomically thin two-dimensional organic-inorganic hybrid perovskites. Science 349, 1518–1521 (2015).

Altering local properties (< 10 micron length scale) of 2D materials with light has the potential to enable new and exciting devices that were previously very difficult to make. Moreover, by precisely controlling the transient temperature within the material through light-matter interactions, the manipulation and transformation can be done directly on flexible substrates. In this talk, we discuss manipulating amorphous and polycrystalline 2D materials to create and improve 2D crystal quality, control local chemistry, and to demonstrate device performance on flexible substrates. Experiments reveal the successful phase transformation of amorphous transition metal dichalcogenides (TMD) including MoS₂ and WSe₂ deposited on stretchable polymer substrates to their crystalline van der Waals layered structures. Detailed kinetic studies of crystal formation were accomplished via high throughput in-situ Raman spectroscopy at different surface temperatures and environmental conditions. With this technique, heterostructures were formed incorporating multiple TMD layers that were annealed simultaneously, and insights into the role of surface diffusion and activation energy for crystallization will be discussed. Laser writing within MoS₂ exhibits controllable local heating and transformation to form various oxide phases with altering electronic properties. Additionally, large area, wafer-scale crystallization of stretchable 2D photodetectors with the use of a broadband pulsed lamp source demonstrate the feasibility of laser transformation as a means to create unique device constructs.

Hybrid (inorganic-organic) perovskites have demonstrated an extraordinary potential for clean
sustainable energy technologies and low-cost optoelectronic devices such as solar cells; light
emitting diodes, detectors, sensors, ionic conductors etc. In spite of the unprecedented progress in
the past six years, one of the key challenges that exists in the field today is the large degree of
processing dependent variability in the structural and physical properties. This has limited the
access to the intrinsic properties of hybrid perovskites and led to multiple interpretations of
experimental data. In addition to this, the stability and reliability of devices has also been strongly
affected and remains an open question, which might determine the fate of this remarkable material
despite excellent properties.
In this talk, I will describe our recent work on Ruddlesden-Popper halide perovskites as a potential
alternative to the bulk hybrid perovskites. I will describe the versatility and tunability of this novel
system through our efforts on achieving high-efficiency light emitting diodes with stability. I will
describe the design principles based on structure, grain-size and composition of phase-pure layered
2D perovskites and demonstrate proof-of-concept color tunable light emitting diodes and discuss
their stability.

Plasmon free surface enhanced Raman scattering (SERS) based on two dimensional (2D) materials is an emerging field in non-deductive analysis^[1-3]. However, the application of 2D materials as SERS substrates is still seriously impeded by their low SERS enhancement and inferior detection sensitivity. Here, we have demonstrated that metallic 2D materials such as niobium disulfide (NbS₂), 1T-molybdneum disulfide (1T-MoS₂), graphene, can be used as ultrasensitive Raman enhancement substrates for SERS. Metallic transition metal dichalcogenides (TMD) such as NbS₂ show substantial Raman enhancement effect, which has not been well studied. In this study, thickness-tunable NbS₂ synthesis was achieved by alkali assisted chemical vapor deposition (CVD) method. The electrical measurements show metallic nature of the CVD-grown NbS₂ flakes with the conductivities of up to ~1000 S/cm. The detection limit of methyl blue and methylene blue on NbS₂ was found to be as low as 10^-8 mol/L and 10^-14 mol/L, respectively, which is comparable to that of plasmon-driven noble metal SERS materials. The promising SERS effect due to charge transfer resonance is attributed to strong interactions between the analyte and the metallic NbS₂ which has an abundant density of states at the Fermi level. These metallic 2D materials-based SERS substrates have also been demonstrated to be effective in food safety and environmental pollution monitoring, as well as biological sensing.

Reference:
[1] Ling, X., et al. Can Graphene Be Used as a Substrate for Raman Enhancement? Nano Lett. 2010, 10, 553–561.
[2] Tan, Y., et al. Two-Dimensional Heterostructure as a Platform for Surface-Enhanced Raman Scattering. Nano Lett. 2017, 17, 2621–2626.
[3] Ling, X., et al. "Raman enhancement effect on two-dimensional layered materials: graphene, h-BN and MoS₂." Nano Lett. 2014, 14, 3033-3040.

Raman microscopy proved to be an extremely useful technique for characterization of 2D materials such as graphene, transition metal dichalcogenides (TMDs), black phosphorous, etc. Unfortunately, natural spatial resolution of confocal Raman microscopy, which is limited by the wavelength of the laser used (400-800 nm), is not sufficient for mapping heterogeneities and defects in these materials with characteristic dimensions of few – to few tens of nanometers.
Tip Enhanced Raman Spectroscopy (TERS) provides dramatically improved spatial resolution of Raman maps, down to few nanometers, and in addition provides dramatic enhancement of the Raman signal. Since TERS is a relatively new technique, peculiarities of the near-field Raman response of many 2D materials still remain to be discovered and explained.
We report an interesting observation we made during TERS imaging of thin (10-60 nm) flakes of MoO3 exfoliated to different metallic surfaces (Au, Cr, Al). Despite the fact that normal Raman spectra of this material feature at least 12 well distinguishable peaks within 100-1100 cm-1 range, only one peak at 995 cm-1 corresponding to out-of-plane Mo-O vibration was strongly enhanced in TERS spectra of MoO3 crystals. It should be expected and actually has been observed in TERS spectra of 2D semiconductors that out-of-plane modes got preferential enhancement, but the degree to which it happens in MoO3 is outstanding. Such mode selectivity in TERS spectra of 2D materials may be useful for mode assignment in case there are multiple possibilities for a specific band.
Another unexpected effect was observed in the course of TERS characterization of WSe₂ and MoSe₂ exfoliated to gold and chromium. It is well known that TERS produces the strongest enhancement in so-called gap mode, when a thin sample is sandwiched between a plasmonic tip and a plasmonic substrate, usually silver or gold. To our surprise, TERS spectra of both WSe₂ and MoSe₂ crystals exfoliated to chromium showed very similar intensities of characteristic Raman bands compared to samples exfoliated to gold, although the background spectra obtained from the bare metal areas were much weaker for chromium compared to gold. Calculations of the optical field intensity of a TERS probe over gold and chromium surface confirmed that we indeed observed a gap mode TERS response on non-plasmonic chromium substrate. This important observation expands the choice of substrates suitable for high quality TERS characterization of 2D materials.

Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions resulting in dense IX ensembles thermalized to the lattice temperature. Besides IX ensemble in III-V heterostructures [1,2], transition metal dichalcogenides (TMD) exhibit superior potential for studying interacting IX ensembles due to a strong light matter interaction together with a large exciton binding energy in these 2D materials [3]. Hetero-bilayers from these materials have a type II band alignment driving an efficient charge transfer between the two layers that results in the formation of spatially separated electron-hole pairs.
We report the observation of a doublet structure in the low-temperature photoluminescence of IX in heterostructures consisting of monolayer MoSe₂ and WSe₂. Both peaks exhibit long photoluminescence lifetimes of several tens of nanoseconds up to 100 ns verifying the interlayer nature of the excitons [4]. The energy and line width of both peaks show unusual temperature and power dependences. While the low-energy peak dominates the spectra at low power and low temperatures, the high-energy peak dominates for high power and temperature. We explain the findings by two kinds of interlayer excitons being either indirect or quasi-direct in reciprocal space [4]. Encapsulation in hexagonal boron nitride results in very narrow emission lines for the direct excitons[5], as well as the multiplet IX emission lines found in TMD hetero-bilayers [6]. Stark effect devices allows for the manipulation of the excitons by external electric fields [6]. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.


We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) via excellence cluster “Nanosystems Initiative Munich” as well as DFG projects WU 637/4-1 and HO3324/9-1.

References
[1] S. Dietl, et al., Phys. Rev. B. 95, 085312 (2017).
[2] S. Dietl, et al., Superlattices and Microstructures 108, 42-50 (2017).
[3] U. Wurstbauer, et al., J. Phys. D: Appl. Phys. 50, 173001 (2017).
[4] B. Miller, et al., Nano Lett. 17 (9), 5229–5237 (2017).
[5] J. Wierzbowski et al. Scientific Reports 7, 12383 (2017).
[6]. F. Sigger et al. (2018).

Two dimensional (2D) post-transition metal chalcogenides is an emerging group of promising materials for high-performance electronic and optoelectronic devices [1,2]. However, the synthesis of this group of 2D materials with a lateral dimension of > 50 µm has been a challenge [3]. In this work we present a facile way to synthesis 2D indium sulfide (In₂S₃) from sulfurization of the surface oxide layer of a melted indium metal. 2D In₂S₃ is determined to feature p-type semiconducting behavior with a direct bandgap of ∼2.9 eV, potentially offering the broad detection range from UV to visible blue light region. The 2D In₂S₃ based photodetector exhibits a very high photoresponsivity of 8364.4 AW^-1 with an excellent external quantum efficiency of 3.706710⁴% and a detectivity of 4.4205 × 10¹⁰ Jones. The synthesis technique is facile, scalable and holds promise for creating atomically thin semiconductors at wafer scale. Furthermore, the impressive optoelectronic properties of 2D In₂S₃ represent it a suitable candidate for future generation optical and electronic devices.
References
[1] Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesman, A. S. R.; Russo, S. P.; Lau, D. W. M.; Xu, Z.-Q.; Bao, Q.; Kavehei, O.; Gibson, B. C.; Dickey, M. D.; Kaner, R. B.; Daeneke, T.; Kalantar-Zadeh, K. Wafer-Scale Two-Dimensional Semiconductors from Printed Oxide Skin of Liquid Metals. Nat. Commun. 2017, 8, 14482.
[2] Xu, M., Liang, T., Shi, M. & Chen, H. Graphene-like two-dimensional materials. Chem. Rev. 113, 3766–3798 (2013).
[3] Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 7, 699–712 (2012).

Limeng Ni, Uyen Huynh, Alexandre Cheminal, Tudor H. Thomas, Ravichandran Shivanna, Aditya Sadhanala, Ture Hinrichsen, Akshay Rao
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Email: [email protected]

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons remains an open question. Here, we investigate two widely used materials, namely butylammonium lead iodide (CH₃(CH₂)₃NH₃)₂PbI₄ and hexylammonium lead iodide (CH₃(CH₂)₅NH₃)₂PbI₄, both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm^-1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 cm^-1 and 137 cm^-1. Using the determined optical phonon energies, we analyzed PL broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence lineshape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials. Strong exciton-phonon coupling in 2D perovskites can be a benefit, for instance, it can accelerate polariton relaxation and realize polariton lasing with very low threshold. It could also provide interesting opportunities for the development of broadband, short-pulsed lasers.
I will also show our recent results on exciton-photon coupling and polaritons in layered perovskite crystals in high-Q microcavities. We found out that the coupling strength and the Rabi splitting energy are affected by quantum well thickness.

Transition metal dichalcogenides (TMDs) are promising candidates for emerging applications such as transparent and flexible optoelectronics and electronics. Understanding the impact of defects on material properties, such as luminescence efficiency and charge carrier mobility, is essential for the advancement of these materials. However, it is challenging to observe discrete defects in monolayer TMDs at the nanometer-scale and to directly correlate defects with material properties. This is due to a lack of techniques capable of facile observation of defects in monolayers. Here, we demonstrate the ability to observe electronically active defects in monolayer TMDs using conductive atomic force microscopy (AFM) in ambient conditions, and we correlate defect density with local optoelectronic and electronic properties. We briefly discuss our AFM-based sample preparation technique which creates clean, homogeneous samples that enable us to investigate the intrinsic optical and electronic properties. We find that CVD-grown WS₂ samples have up to an order of magnitude variation in defect density within a single triangular grain. We also find that the photoluminescence (PL) intensity is inversely proportional to defect density. We develop a model assuming non-radiative recombination occurs when excitons collide with defects, and we find good agreement between the model and experiments. To investigate electronic properties, we use kelvin probe force microscopy to obtain spatial maps of electrostatic potential in operating TMD transistors. The local potential gradient is proportional to the local resistivity, which allows us to observe variations in resistivity across the channel. We find that regions with low PL intensities exhibit large potential gradients. This suggests that the defects responsible for decreased PL intensity are also responsible for decreased electrical conductivity. In summary, we use a combination of conductive AFM, PL measurements, and kelvin probe force microscopy to understand the impact of defects on optoelectronic and electronic properties in monolayer TMDs. Our work demonstrates a simple and general methodology for correlating defects with various material properties in two-dimensional materials, which is essential for optimizing material and device performance.

Two dimensional layered materials, such as graphene, and transition metal dichalcogenides (TMD) have great potential for gas sensing applications due to its high surface to volume ratio. Ultrasensitive chemical detection using single-layer graphene and single- and few-layer MoS₂have been reported. Unfortunately, graphene based sensors without proper surface modification suffer from lack of selectivity; TMD sensors show good selectivity but not long term stability. Recently, hexagonal boron nitride (h-BN) nanomaterials, eg. nanosheets and nanotubes, have been predicted to be an effective material for reversible gas adsorption with high selectivity once charged. By modifying the charge state of the BN nanomaterials, adsorption and desorption of gas on BN can be controlled. This makes the whole process controllable and suitable for gas sensing. However, despite the encouraging theoretical discovery, sensing with h-BN is difficult to realize experimentally due to its electric insulating nature with large band gap (around 5.8 eV).
In this research, we report on the controlled synthesis of high surface area h-BN coated graphene aerogel and its application for selective gas sensing. Hybrid BN/graphene aerogel is synthesized using graphene aerogel as a template and boric acid and ammonia as boron and nitrogen source, respectively. A thin layer of BN can be uniformly coated on graphene aerogel while maintaining the macro- and nanoscale morphologies and mechanical properties of the aerogel. With thin BN and graphene combination, charge transfer between BN and graphene can be greatly improved compared to bare h-BN alone. Sub ppm level detection limit for NH₃ is shown, while negligible cross sensitivity to tens ppm of NO₂, 2000 ppm CO₂ and 50000 ppm hydrogen. The low detection limit, high selectivity and high stability makes the sensor suitable for practical sensing application.

Since its isolation in 2014, few-layer black phosphorus (BP) has been under intensive research efforts, due to its many interesting material properties such as the combination of high mobility and sizable band gap, in-plane anisotropy and band structure modulation by electric field or strain. Many of the predicted properties can only be revealed in high mobility samples, in which the intrinsic phenomena are not obscured by impurities, and which are essential for further applications in any quantum device structures. Here I will present our recent works on ultra-high quality few-layer BP transistors, including the creation of double quantum wells, Landau level gaps in samples with demonstrated strong in-plane anisotropy, high quality integer quantum Hall effect and the first observation of fractional quantum Hall effect in BP. These results pave the way to the study of even more fragile fractional quantum Hall states or more complicated quantum structures in BP.

Transition-metal dichalcogenide (TMDC) has become an invaluable material platform for condensed matter research owing to its intriguing properties and functionalities emerging at monolayer limit. There, conventional mechanical exfoliation plays an central role for sample fabrication, while bottom-up synthesis by molecular-beam epitaxy (MBE) has expanded a lineup of materials under investigation even to hardly cleavable compounds, providing a promising route to further exploration of novel quantum phenomena emerging at monolayer limit including heterostructures. However, those MBE-based approaches have been mostly focused on spectroscopic studies using conducting graphene substrates, whereas transport studied have been less performed despite its essential importance due to difficulties in growing high quality TMDC thin films on insulating substrates.

We have recently established a versatile route to layer-by-layer epitaxial growth of millimeter-scale single-crystalline TMDC thin films on insulating sapphire substrates by MBE [1], opening the door for exploration of transport properties of various TMDCs and their heterostructures at nanometer scale. In this presentation, we will introduce our growth recipe in detail, together with an interesting aspect of van der Waals epitaxy that is missing in conventional epitaxy with strong substrate-film interaction. We will also present our recent achievements in transport studies including realization of spin-momentum-locked ‘Ising superconductivity’ in millimeter-scale ultrathin films based on group-V TMDCs that are suitable for further investigation of 2D superconductivity, observation of emergent ferromagnetism with anomalous Hall effect in strongly-correlated TMDCs that are missing in their bulk counterparts, as well as development of van der Waals heterostructures exhibiting intriguing interface transport phenomena in combination with a variety of TMDC thin films.

[1] M. Nakano et al., Nano Lett. 17, 5595 (2017).

Layered topological insulators (TIs), such as Bi₂Se₃ and Bi₂Te₃, host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, which can be utilized for nano-electronic, spintronic, and quantum computing applications. Here we present a strategy to fabricate TI-magnetic insulator (MI) heterostructures with high-quality interfaces, and experimentally measure the magnetoresistance (MR) at various temperatures. We observe rich hysteretic MR features including sharp MR switching and large, broad MR bumps (a few percent in magnitude), which we attribute to domain wall switching and out-of-plane anisotropic MR in the surface state, respectively. The out-of-plane magnetic anisotropy leads to band gap opening in the TI surface state, confirmed by an analytical diffusive transport model. These results provide both fundamental insights into the mechanism of magnetic proximity and spin exchange interactions in layered heterostructures, and a route towards energy efficient spintronic device applications.

We demonstrate direct writing of self-ali 18 gned electrical circuitry with a scanning light probe on a two-dimensional (2D) van der Waals (vdW) semiconductor at the time scale of a minute. Light illumination over Au-electrode patterns on n-type 2H-MoTe₂ 2D channels locally convert them into p-type ones by creating adatom-vacancy clusters as electron acceptors in the host lattice. We provide direct evidence of such a microscopic doping mechanism by atomic scale imaging and spectroscopy. This real-time writing process is precisely controllable within a minute, in that diffusive doping profiles can be controlled at the sub-micrometer scale, and doping concentrations are tunable to vary the channel sheet resistance over four orders of magnitudes. As such, we assembled both n- and p-doped channels within the same atomic planes to fabricate 2D device arrays of n-p-n (p-n-p) bipolar junction transistor (BJT) amplifiers and radial p-n photovoltaic cells in high performances. This doping method can be potentially used to fabricate designer 2D circuits based on atomically thin vdW semiconductors in arbitrary shapes.

Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. So far semiconducting 2D materials attracted most interests; correlated 2D materials, on the other hand, received little attention. Vast opportunities exists in correlated 2D materials: the reduced dimensionality may lead to novel properties that are vastly different from that in the bulk, and the exposed surface makes these materials highly tunable. In this talk I will discuss the emerging opportunities in correlated 2D materials. In particular, I will talk about three correlated 2D materials that we found particularly interesting – 1T-TaS₂ with tunable charge density wave phases, and 2D ferromagnet Fe₃GeTe₂. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.

The remarkably strong and environmentally sensitive Coulomb interaction at the atomically thin limit allows for modification of the quasiparticle bandgap and exciton binding energy of two-dimensional (2D) materials through modification of the dielectric environment [1]. The change in the non-local or distance-dependent dielectric screening strongly modulates the free-carrier bandgap energy and excited exciton states compared to the ground state exciton transition. By probing the inhomogeneous linewidth broadening of the excited exciton transition using optical spectroscopy, we have identified dielectric disorder as a significant source of electronic disorder at the 2D limit.

We infer that within the diffraction-limited, sub-micron area of study there exists a spatially disordered dielectric environment. This in turn leads to spatially inhomogeneous quasiparticle bandgaps on the sub-micron level. Our results imply that the variation in the bandgap induced by dielectric disorder can be over 100 meV in monolayer WS₂ and WSe₂ samples that have not been encapsulated in hexagonal boron nitride (h-BN), but are supported on typical substrates like SiO₂. This variation decreases to the order of 10 meV upon h-BN encapsulation. While there is widespread interest to improve the optical and electronic properties of atomically thin materials by substrate surface passivation or encapsulation in a wide bandgap insulator like h-BN, the underlying microscopic picture has not been fully explored. From our study, we suggest dielectric disorder as a mechanism likely to play a significant role.

[1] A. Raja et al. Nature communications, 8 (2017), 15251

The crystal orientation of an exfoliated black phosphorous flake is determined by purely electrical means. A sequence of three resistance measurements on an arbitrarily shaped flake with five contacts determines the three independent components of the anisotropic in-plane resistivity tensor, thereby revealing the crystal axes, with the help of simple geometric transformations including conformal mapping. The crystallographic orientation deduced from this all-electrical conformal five-contact method is confirmed with polarized Raman spectroscopy. The resistivity anisotropy ratio is observed to decrease linearly with increasing temperature T and carrier density reaching a maximum ratio of 3.0 at low temperatures and densities, while mobility indicates impurity scattering at low T and acoustic phonon scattering at high T. In addition, we examine the disorder-related transient conductivity, and observe that the commonly observed hysteresis in both electrical and photoluminescence studies of 2D materials can be characterized as a heavy-tail transient response to a step-function excitation. Dispersive diffusion equations successfully fit the transients in both the pristine and highly disordered sample limits, and a microscopic model for the response is provided, based on the continuous-time random walk model. Finally, we observe for the first time a generalized scaling behavior for the gated conductivity of 2D materials with disorder strength.
*PHYS. REV. LETT. 120, 086801 (2018).

Two dimensional (2D) materials belong to a large family of anisotropic compounds which have strong, covalent bonds within a layer while between layers there are only weak van der Waals interactions that can be overcome, obtaining molecularly thin sheets. Such a dimensionality reduction has a profound impact on properties, that vary strongly with the number of atomic layers. From the moment of graphite exfoliation into an atomically thin monolayer, known today as graphene [1], two dimensional materials have been of main interest for a variety of electronic applications.

Whereas graphene applications in electronics has thus far been hindered by its non-existent band gap, layered semiconductors are studied as potential candidates for future devices. Many Transition Metal Dichalcogenides (TMDs) including MoS₂, MoSe₂, WS₂, WSe₂ have been thoroughly investigated, however in order to meet rising demands new families of 2D semiconductors are studied. One family of such is the transition metal thiophosphates, denoted MPS_x, for x=3 or 4; for example bulk crystals of CrPS₄ - chromium thiophosphate – which has been examined in the past for applications in lithium batteries. Nowadays, this compound has once again gained scientific interest due to its optical anisotropic properties and the possibility to obtain and study its few- and monolayer systems [2].

In this research, bulk crystals of CrPS₄ were obtained by vapor transport synthesis (furnace method), followed by structure and composition confirmation via different techniques, for example Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), Powder X-Ray Diffraction (PXRD) and Raman spectroscopy. Optical properties, such as band gap and optical transitions were investigated by Solid State UV-VIS Spectroscopy, PhotoAcoustic Spectroscopy (PAS) [3] and Modulated Spectroscopy (MS) [3]. Then, bulk crystals of chromium thiophosphate (CrPS₄) were exfoliated in liquid by ultrasounds to obtain few layers systems and photoconductivity measurements were used to ascertain photoactive properties, both of re-stacked films and bulk crystals.


Acknowledgments:
This work was supported by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN-642656)
This work was performed within the grant of the National Science Centre Poland (OPUS 11 no. 2016/21/B/ST3/00482).
S.J.Z. also acknowledges the support within the ETIUDA 5 grant from National Science Center Poland (no. 2017/24/T/ST3/00257).

References:
[1] Novoselov et.al., Science, vol 306, no. 5696 (2004)
[2] Lee et.al., ACS Nano, vol. 11, no. 11 (2017)
[3] Zelewski & Kudrawiec, Scientific Reports, vol. 7 no. 15365 (2017)

Two dimensional (2D) transitional metal dichalcogenides (TMD) such as molybdenum disulfide (MoS₂) have been demonstrated to be excellent semi-conductors for ultra-thin FETs. However, the main challenging for improving the performance is the contact resistance for all 2D materials. The direct deposition of metals can introduce substantial damage to ultra-thin TMD materials, which leads to high contact resistance. Studies have shown that creation of van der Waals contacts by dry transfer (graphene¹, hBN², metal³) method reduces damage to the TMDs. Here, we report the realization of ultra-clean van der Waals contact between Indium and 2D materials. From scanning transmission electron microscopy images, we can see the interface is atomically sharp with no detectable chemical interaction. This ultra-clean van der Waals contact can also be translated to good device performance for 2D materials such as MoS₂, WS₂, WSe₂, and NbS₂. Because Indium is a soft metal, we demonstrate that alloys of Pd, Pt, and other metals can be formed to vary the work function so that both hole and electron injection can be facilitated. We have measured the work function of the alloys using Kelvin force microscopy and correlated them with band offsets in WSe₂ devices.


Reference:
1. Cui, X. et al. Multi-terminal transport measurements of MoS₂ using a van der Waals heterostructure device platform. Nat. Nano. 10, 534–540 (2015).
2. Cui, X. et al. Low-Temperature Ohmic Contact to Monolayer MoS2 by van der Waals Bonded Co/h-BN Electrodes. Nano Lett. 17, 4781–4786 (2017).
3. Liu, Y. et al. Approaching the Schottky–Mott limit in van der Waals metal-semiconductor junctions. Nature 1 (2018). doi:10.1038/s41586-018-0129-8

Reducing friction and wear is essential in industrial sliding applications. Layered materials like graphite and boron nitride are extremely effective as solid lubricants and have been extensively applied¹. More recently, exfoliating these layered materials yields 2D materials that have surfaces with very low coefficients of friction². These materials show promise for reducing friction in nano- and micro-electromechanical systems. However, they could also benefit larger scale applications, and exploiting the thinness and low friction properties of 2D materials requires further development³.
Here we study the deposition of hexagonal boron nitride (hBN) onto steel using chemical vapour deposition from the solid precursor ammonia borane. We find that hBN forms nanostructured, multilayer films when using higher precursor temperatures. These films yield a lower coefficient of friction on the surface under lubrication than those with no coating. We also investigate the effect of pre-plating the surface with copper and nickel, as these are more well-defined hBN growth surfaces.
We also study composites made from the electrodeposition of hBN with nickel. These composites are examined using atomic force microscopy to understand how the surface wears under repeated sliding. The hBN incorporates into the nickel film, again reducing the surface coefficient of friction. These techniques could help give industrially scalable coating techniques to help reduce friction in sliding applications.
1. Scharf, T. W. & Prasad, S. V. Solid lubricants: A review. J. Mater. Sci. 48, 511–531 (2013).
2. Lee, C. et al. Frictional Characteristics of Atomically Thin Sheets. Science (80-. ). 328, 76–80 (2010).
3. Qi, Y., Liu, J., Zhang, J., Dong, Y. & Li, Q. Wear Resistance Limited by Step Edge Failure: The Rise and Fall of Graphene as an Atomically Thin Lubricating Material. ACS Appl. Mater. Interfaces 9, 1099–1106 (2017).

Developing highly sensitive solid-state chemical sensors for detecting biological, environmental and chemical agents is a very important issue. To obtain very high sensitivity, two factors are simultaneously required: (i) low electrical noise, which can be derived from high conductivity, and (ii) strong signal, originating from abundant analyte adsorption sites. However, as the introduction of reactive sites on the surface significantly degrades the electrical conductivity, these two factors are always in a trade-off relation. Thus, none of the previous sensor materials were able to simultaneously satisfy the two characteristics.
Here, we demonstrate state-of-the-art metallic Ti₃C₂T_x MXene gas sensors by employing a vacuum filtration method. The abundant surface functionalities and high conductivity of Ti₃C₂T_x enable the Ti₃C₂T_x sensor to overcome previous trade-off limitations, substantially outperforming typical two-dimensional (2D) material gas sensors in two important aspects. First, it has a very high sensitivity and shows a limit of detection (LOD) of 50~100 parts per billion (ppb) at room temperature, which is one of the lowest LOD ever reported. Second, the extremely low noise of metallic Ti₃C₂T_x yields a signal-to-noise ratio (SNR) that is two orders of magnitude higher than that of other 2D materials. In addition, we have observed that Ti₃C₂T_x sensors have higher selectivity toward VOC gases over acidic gases, which is a rare property. In-situ X-ray diffraction (XRD) measurements were used to verify that the intercalation of gas molecules into Ti₃C₂T_x layers plays a fundamental role in detecting gas molecules. The interlayer swelling induced by the intercalation of gas molecules can explain not only Ti₃C₂T_x sensors’ selectivity, but also their universal positive response for various gas molecules. It is noteworthy that more than 30 MXenes are now accessible, and we expect that this study will open the door for a large family of MXene sensors.

Layered 2D materials have garnered huge interest due to their inherent chemical, optical and electronic properties. In addition to the individual material property the weak inter-layer bonding, characteristic of 2D materials, allows for variable stacking of a combination of these materials; forming heterostructures. These heterostructures can provide pathways to develop all 2D electronics or enhancing the novel properties of the resultant stack. Current research using transferred exfoliated flakes has demonstrated the rich landscape of possible properties and applications of these heterostructures. However, to study these heterostructures without the hindrance of transfer process contaminants, bottom-up growth techniques which can provide pristine interfaces are needed. To understand and control the bottom-up assembly process, it is imperative to investigate the impact of defects in a layered substrate on the growth of other 2D materials. The defects can act as nucleation sites, which in turn can be used to control the density and the domain size of the growing 2D films.

In this work we investigate the effect of defects in epitaxial graphene on the growth of transition metal dichalcogenides like WS₂. Epitaxial graphene was grown on on-axis 6H (0001) SiC substrate. The defect density of epitaxial graphene was controlled by exposing it to a helium plasma for different durations. The pristine and plasma treated epitaxial graphene was exposed to a H₂S environment and subsequently to a WS₂ growth environment. The H₂S treatment and WS₂ growth was carried out at 50 Torr at temperatures ranging between 700-1000°C. The H₂S and W(CO)₆ precursor flow rate was maintained at 400 sccm and 5.7×10^-4 sccm, respectively. The results show that the defect generation, and subsequent sulfur incorporation was higher when the buffer layer was present between the SiC substrate the graphene layers. The higher defect generation is attributed to the distortion in the bonds of the top graphene layer when the interfacial layer is partially bonded to SiC. In addition, the duration of plasma treatment directly controls the nucleation density of WS₂ domains, which is a crucial factor for controlling the domain size. Another key observation was that the plasma treatment modified the WS₂ nucleation sites from step edges to terraces. This impact on the nucleation site was, however, temperature dependent. Additional details about the role of the buffer layer, the interaction of the defects with the precursors and the impact on the nucleation site and density will also be presented.

Valleytronics provide an additional degree of freedom in optoelectronic devices by having multiple non-equivalent “valleys” in the reciprocal(k)-space bandstructure of a valleytronic material. Such valleys obey certain selection rules and can be selectively polarized by optical means. Until recently, research in valleytronics has mostly been conducted using two-dimensional transition metal dichalcogenides. However, such systems can only be probed at low dimensions and/ or with the application of certain experimental conditions, such as cryogenic temperatures and/or strong electric or magnetic field. This inevitably pose a plethora of practical challenges that create a high barrier in advancing the technology towards practical applications.
In our recent work, we show experimental evidences of the valley effect in a bulk, ambient, and bias-free model system of Tin(II) sulphide (SnS). We elucidate the direct access and identification of different sets of valleys, based primarily on the selectivity in absorption and emission of linearly polarized light, and demonstrate strong optical dichroic anisotropy of up to 600% and nominal polarization degrees of up to 96% for the two valleys with band-gap values 1.28 and 1.48eV, respectively; the ease of valley selection further manifested in their non-degenerate nature. Such discovery enables a new platform for better access and control of valley polarization.
There are two non-degenerate local band gaps along two high symmetry axes, ΓY and ΓX, in the orthogonal crystal structure of SnS (and most IV-VI monochalcogenides). By looking at the compositions of the conduction and valence bands, we obtained the point groups that describe the k points along ΓY and ΓX, and the IRs of the respective bands and show, via the electric dipole approximation, that the transitions at ΓY and ΓX will only occur with excitation using y and x-polarized light respectively. This forms the basis for valley selectivity.
Experimentally, we demonstrate that Tauc plots of absorption measurement results give band-gap values of 1.48 and 1.28eV, clearly distinguishing the two sets of valleys. We also see that SnS is able to maintain a α_zigzag/α_armchair more than 200% for a range of more than 0.2eV and also possesses a maximum α_zigzag/α_armchair more than 600%.
The polarization dependence of two PL peaks at 817 and 995nm, respectively, is clearly observed; under parallel polarization, the 817nm peak maximizes at the polarization that minimizes the 995nm peak, and vice versa, demonstrating a 90° phase shift between the peaks and hence the orthogonality in polarization selectivity of the two sets of valleys responsible for such PL emissions.

Emerging graphene quantum dots (GQDs) have attracted great attention due to their excellent resistance to photobleaching and tunable band gap, making them promising candidates for future light-emitting devices. However, in the solid-state, π-interaction-induced aggregation-caused quenching (ACQ) of photoluminescence (PL) in GQDs makes realization of high performance device challenging.
Here we propose application of GQD incorporated boron oxynitride (GQD@BNO), fabricated by a simple microwave-assisted method, on alternating-current powder electroluminescent device (ACPEL) as an active material. The BNO, comprising an interfacial, zone-joining boric oxide and turbostratic boron nitride, is an ideal matrix material for incorporating GQDs for three reasons: i) the presence of abundant surface oxygen functional groups facilitates intermolecular interaction with GQDs, ii) the relatively high refractive index (1.6-1.8) can enhance PL quantum yields (PL-QYs), iii) the creation of a localized dielectric environment is advantageous for high-field induced electrical excitation of embedded GQDs.
With these advantages, blue-luminescent GQDs were successfully incorporated into a BNO matrix via intermolecular hydrogen bonding to form GQD@BNO, which showed 8-fold enhanced PL-QY compared to that of the same amount of GQDs in water. The PL-QY enhancement results from the increase in the spontaneous emission rate of GQDs due to the surrounding BNO matrix, which provides a high refractive index environment and fluorescence energy transfer from the larger gap BNO donor to the smaller gap GQD acceptor. In addition, the size effect of GQD@BNO and the GQD loading ratio in BNO on the photo-physical properties shed light on the in-depth carrier dynamics of GQD in a BNO matrix. The robust solid-state PL properties made the GQD@BNO an ideal active material for ACPEL devices, whose luminance exceeded 283 cd m^-2 for the first time. This successful demonstration shows great potential of GQDs in the field of low cost, eco-friendly EL devices.

The applied uniaxial stress can break the original symmetry of a material, providing an experimentally feasible way to alter material properties. Here, we explore the effects of uniaxial stress along an arbitrary direction on mechanical and electronic properties of phosphorene, showing the enhancement of inherent anisotropy. Basic physical quantities including Young’s modulus, Poisson’s ratio, band gap, and effective carrier masses under external stress are all computed from first principles using density functional theory, while the final results are presented in compact analytical forms.

Manipulation of thermal transport is in increasing demand as heat transfer plays a critical role in a wide range of practical applications, such as efficient heat dissipation in nanoelectronics and heat conduction hindering in solid-state thermoelectrics. It is well established that the thermal transport in semiconductors and insulators (phonons) can be effectively modulated by structure engineering such as external mechanical strain or materials processing. While three-dimensional bulk solid materials usually exhibit decreased lattice thermal conductivity upon mechanical stretching and enhanced thermal transport by compression, the thermal response of two-dimensional (2D) materials to mechanical strain is not that simple. Generally speaking, perfectly planar atomically-thin materials with graphene as representative have reduced thermal transport ability when tensile strain is applied. In contrast, some 2D materials with intrinsic buckled structure will possess enhanced thermal conductivity when being stretched. However, many exceptions exist in other 2D materials. For example, by performing comparative study of thermal transport in two-dimensional group III-nitrides (h-BN, h-AlN, h-GaN) and graphene, we found that the thermal conductivity of all three monolayer group III-nitrides is tremendously enhanced, especially for h-AlN and h-GaN (up to one order of magnitude). By deeply analyzing the orbital projected electronic structure, we establish a microscopic picture of the lone-pair electrons driving strong phonon anharmonicity in group III-nitrides. We also found that the lone-pair electrons do not necessarily lead to enhanced thermal conductivity in strained penta-like 2D materials, which suggests the complexity of thermal response of 2D materials to mechanical strain. Our findings offer perspectives of modulating thermal transport properties of broad 2D materials for applications such as thermoelectrics, thermal circuits, and nanoelectronics.

Besides of graphene, two-dimensional (2D) van der Waals (vdW) layered materials have been intensively studied over the last eight years especially in the field of electronic and optoelectronic devices. Even though many interesting next-generation device physics like spin transport, charge density wave, and topological insulator have been revealed in various 2D vdW materials, conventional device configurations like field-effect transistor, Schottky, and p-n diode implemented with the 2D vdW materials have also drawn researchers’ attention due to its similarity to the modern electronic and optoelectronic applications. Particularly, the Schottky and p-n junction based devices are essential elements of energy harvesting applications such as photovoltaic solar cell. In this sense, various surface doping methods have been used to tune the electronic and optoelectronic properties of the WSe₂ sheets, but combination effects associated with different surface dopants and its detailed studies are still lacking.
Here, we have demonstrated a Schottky junction-based photovoltaic few-layer WSe₂ device with an Al₂O₃ dielectric deposition near source electrode and a molecular doping (NDP-9) near drain electrode selectively. Scanning photocurrent microscopy (SPCM) with a 514-nm-wavelength laser confirmed that a highly asymmetric photocurrent generation was occurred in the device, locally only at the WSe₂/source electrode contact. Based on the SPCM and photoresponse results with various bias conditions from the devices, it was revealed that this asymmetric photocurrent was strong and its generation mechanism was originated from the enhanced Schottky junction at the WSe₂/source electrode by the proper Al₂O₃ dielectric deposition and chemical doping. A peak photo-responsivity of ~20 mA W^-1 as a photodetector and a peak external quantum efficiency of ~0.2 % as a photovoltaic power generator have been extracted. It is believed that our devices can pave the way for developing a van der Waals two-dimensional material-based Schottky junction solar cell.

For the frequently observed edge enhancement and spatial nonuniformities of photoluminescence (PL) of WS₂ monolayer, even though tremendous effort has been put into, the fundamental physics behind remain elusive. Herein, we report that due to the chemisorption of oxygen atoms at the WS₂ edges, and the much higher electronegativity of oxygen compared to that of sulfur and tungsten atoms, electrons accumulate at the edges, leads to the n-doping behavior and higher electron concentrations near the edges than that at the interior region, giving rise to the strong trion fluorescence. Using the first principle calculations, it is proved that the edge exhibits n-doping semiconducting behavior instead of metallic behavior after oxygen adsorption at the edge.

Indium Selenide (InSe) has attracted significant attention due to large tunability in the band gap, high carrier mobility and unique anisotropic optical properties compared to other two- dimensional (2D), graphene, based systems for chemical and gas sensing applications in the hostile environments. The expanded research requires integrating the chemical and gas sensing capability with optical sensing function. Therefore, in this work, we report the integration and characterization of this material as field-effect transistor (FET) device and as an optical sensor. High-quality InSe thin flakes were prepared using the scotch-tape-based mechanical exfoliation method and then transferred to a cleaned 300 nm thermally grown SiO₂ wafer. The InSe thin flakes were identified via color contrast by optical microscopy and Atomic Force Microscope (AFM). A direct laser writer was used for pattering while the deposition of electrodes (5 nm Cr /50 nm Au) was done by thermal evaporation. Results indicate that the measured output characteristics (I_sd vs V_sd) of the device exhibits ohmic behavior. Different current amplitudes have been observed with different flake thickness, which indicates the thickness modulated conduction path. Also within a fixed thickness, the channel current modulation can go up to more than 2% with increasing the gate voltage by 0.5 V (by accessing the substrate as a gate). In additional, the optical spectrometer studies shows that few layer of InSe flakes can absorb the light throughout the visible range. Conductivity assessment of InSe flakes and more gate modulation analysis will be reported in the conference.

Two-dimensional semiconductor graphene-like materials are currently extremely developing materials. This is primarily due to their unique properties, which make it possible to create on their basis a large number of effective devices of nano- and optoelectronics. A separate promising niche for the use of these materials is the creation of valleytronics devices on their basis.
These materials are ideal candidates for studying the mechanisms of valleytronics and methods of its application. The charge carriers in these materials can be localized in two symmetric band valleys. However, optical absorption, carrier mobility, effective mass and other parameters of these materials directly depend on selected valley. Thus, valley selection may be accomplished by changing the parameters of the optical radiation excitation. This feature of these materials was the subject of presented work.
In this work we study two-dimensional flakes of transition-metal dichalcogenides - WSe₂. Flakes were obtained using a standard method of mechanical exfoliation on a Si/SiO₂ substrate, which ensured their uniformity and defectiveness. The substrate thickness was chosen on the basis of optimal conditions for the generation and detection of nonlinear optical processes in monolayer WSe₂ flakes.
To study the optical properties, a pump-probe technique combined with optical spectroscopy was used. Chosen technique allowed a complex analysis of the nanomaterials to be studied with high spatial resolution (400 nm) and at low temperature (4K). Thus, characteristic relaxation times of these structures were established. An analysis of the valley lifetimes direct measurement in single-WSe₂ layer using circularly polarized light was made.
The second optical harmonic generation dependence on the excited radiation energy and its power and the temperature of the WSe₂ flakes are identified experimentally.
The work was supported by Ministry of Education and Science of Russian Federation (State task no. 3.7335.2017/9.10 and grant No. 14.Z50.31.0034).

Atomically thin 2D crystals are promising channel materials for extremely scaled field-effect transistors (FETs). However, the challenge of contacting 2D materials, especially at the scaled contact lengths (L_c < 30 nm) required for future technologies, constitutes a major roadblock for realizing their full potential. Two mysteries have emerged from studying contact length scaling behavior in 2D FETs: the impact of 2D material thickness and contact gating. It is unclear how the thickness of 2D materials impacts the transfer length (length over which the majority of carriers are injected at the metal-semiconductor contact). There are many incongruent claims around this mystery, from both experimental and theoretical studies. For example, some theoretical studies claim the transfer length is ~1 nm for monolayer MoS₂ FETs, whereas some experimental studies demonstrate a transfer length ranging from 30 nm to 100 nm. The second mystery is the influence of contact gating on contact scaling. Most 2D FETs demonstrated thus far use a back-gate configuration, which allows for electrostatic modulation of the metal-2D as the channel is gated, creating a contact-gating effect. Contact gating could induce stronger carrier injection, but conclusive evidence for its actual impact on the transfer length remains unknown. Unraveling these two mysteries is pivotal in order to understand how carriers are transported in scaled metal-2D contacts.

In order to investigate these mysteries, we fabricated 2D devices having identical channel length, but with different contact lengths (from 15 nm to 100 nm). The channel material is CVD-grown MoS₂ with the thickness ranging from 1 to 4 layers, allowing us to study the effects of 2D crystal thickness based on monolayer increments. For this range of MoS2 thicknesses, we compared the devices in top and bottom gate configurations, to understand the impacts of contact gating on the transfer length and the contact scaling behavior. Furthermore, we benchmarked the contact scaling behavior of these various 2D FET configurations against the theoretical and experimental observations in the literature, providing a holistic picture of carrier transport at these metal-2D interfaces.

In two-dimensional van der Waals heterostructures, interactions between atomic layers dramatically change the vibrational properties of the hybrid system and demonstrate several interesting phenomena that are absent in individual materials. In this work, we have investigated the vibrational properties of the heterostructure between transition metal dichalcogenide (TMD) and hexagonal boron nitride (hBN) on gold film at low and high frequency ranges by Raman spectroscopy. Nineteen Raman modes have been observed from the sample, including a new interlayer coupling mode at 28.8 cm^-1. Compared to reported experimental results of WS₂ on Si/SiO₂ substrates, the Raman spectrum for WS₂ on hBN/Au emerges a blue shift of about 8 cm^-1. Furthermore, a remarkable enhancement of Raman intensity can be obtained when tuning hBN thickness in the heterostructure. Through systematic first-principles calculations, numerical simulations and analytical calculations, we find that the 28.8 cm^-1 mode originates from the shearing motion between monolayer TMD and hBN layers. In addition, the gold substrate and hBN layers form an optical cavity and the cavity interference effect enhances the obtained Raman intensity. Our research demonstrates novel vibrational modes of two-dimensional van der Waals heterostructure as an effective tool to characterize a variety of such van der Waals heterostructure and reveals a new method to enhance the Raman response of two dimensional materials.

The realization of quasicrystals has attracted a considerable attention due to their unusual structures and properties. The concept of quasicrystals in the atomically thin materials is even more appealing due to the in-plane covalent bonds and weak interlayer interactions. Here, it is demonstrated that 2D quasicrystals can be created/isolated from bulk phases because of long-range interlayer ordered aperiodic arrangements [1]. An ultrasonication-assisted exfoliation of poly-grained icosahedral Al–Pd–Mn quasicrystals at room temperature shows the formation of a large area of mono- and few layers in threefold quasi-crystalline plane. The formation of these layers from random grain orientation consistently indicates that the threefold plane is most stable in comparison to the twofold and fivefold planes in icosahedral clusters. The above experimental observations are further supported by theoretical simulations. The mono- and few-layered aperiodic planes render plentiful active sites for the catalysis of hydrogen evolution reaction. In order to understand the origin of structural stability of 3D metallic Al, Pd, and Mn atomic arrangement into 2D ones we carried out ab initio (DFT level, full structural relaxations) calculations. The structural models consisted of 2D layers initially in a threefold crystallographically icosahedral symmetry. These structures constitute the first demonstration of quasi-crystalline monolayer ordering in a free-standing thin layer without requiring the support of periodic or aperiodic substrate.

[1] T. P. Yadav et al. Adv. Funct. Mater., 1801181 (2018)

MXenes is an emerging class of two-dimensional transition metal carbides, nitrides and carbonitrides which exhibit large conductivity, ultrahigh volumetric capacitance, high threshold for light-induced damage and nonlinear optical transmittance, which have been suggested as candidates for energy storage, optical modulation and THz detection, among other applications [1]. Successful implementation of photonic and optoelectronic devises based on MXenes such as Ti₃C₂T_x requires knowledge of intrinsic carrier mobility in these materials as well as of the effects of photoexcitation on conductivity and ultrafast nonequilibrium dynamics of charge carriers. We report on equilibrium and non-equilibrium free carrier dynamics of Ti₃C₂T_x investigated by THz spectroscopic studies [2]. We have studied intrinsic THz conductivity as well as photoinduced changes in conductivity of a 16 nm-thick film of Ti₃C₂T_x nanoplates. We find that a high, ~ 2x10²¹ cm^-3 intrinsic charge carrier density and relatively high (~34 cm²/Vs) mobility of carriers within individual nanoplates results in a exceptionally large, ~ 46 000 cm^-1 absorption in the THz range, in agreement with prior theoretical predictions [3]. We also demonstrate that Ti₃C₂T_x conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared, 800 nm pulses suppresses the conductivity for hundreds of picoseconds. Experimental observation of the control over THz transmission and conductivity by photoexcitation suggests the promise for application of 2D Mxenes in THz modulation devices and variable electromagnetic shielding.


[1] B. Anasori, M.R. Lukatskaya, Y. Gogotsi, Nature Reviews Materials, 2 (2017) 16098.
[2] K.K. G. Li, Y. Dong, S. Chertopalov, A.M. Rao, V. Mochalyn, R. Podila, and L.V. Titova, 2D Materials, 5 (2018), https://doi.org/10.1088/2053-1583/aacb9e.
[3] Y.I. Jhon, M. Seo, Y.M. Jhon, Nanoscale, 10 (2018) 69-75.

A two-dimensional (2D) material placed on an atomically flat substrate can lead to the formation of surface nanobubbles trapping different types of substances. In this paper graphene nanobubbles of the radius of 7-34nm with argon atoms inside are studied using molecular dynamics (MD). All modeled graphene nanobubbles except for the smallest ones exhibit an universal shape, i.e., a constant ratio of a bubble height to its footprint radius, which is in an agreement with experimental studies and their interpretation using the elastic theory of membranes. MD simulations reveal that argon does exist in a solid close-packed phase, although the internal pressure in the nanobubble is not sufficiently high for the ordinary crystallization that would occur in a bulk system. The smallest graphene bubbles with a radius of 7nm exhibit an unusual “pancake” shape. Previously, nanobubbles with a similar pancake shape were experimentally observed in completely different systems at the interface between water and a hydrophobic surface.

Energy has been one of the most challenging issues during the past decades. The necessity of research in overcoming current energy challenges is crucial because of various reasons such as: limited sources of fossil fuels and carbon emission which leads to global warming and pollution. Electrochemistry is one of the most promising technologies towards environment-friendly and sustainable energy systems. However, it has advanced much more slowly over the last two decades than many companion fields. Fundamental physical parameters such as catalysts’ activity and electronic properties are now hindering major developments in electrochemistry and electrocatalysis. Therefore, new generation of materials are necessary for future applications. In this study, we report on synthesis and characterization of 2D transition metal dichalcogenide (TMDC) materials based on group V and VI of transition metals and their sulfides, selenides and tellurides.
These materials demonstrate excellent catalytic performance and much higher activity in oxygen reduction and evolution reactions in an aprotic medium with Li salts compared to the reported catalysts. We believe these materials can be utilized as new generation of electrocatalysts for the future developments in catalysis and energy storage systems.

In the last few decades, the advances and breakthroughs in graphene and related two-dimensional (2D) materials have been witnessed both scientific fundamentals and potential applications. Graphene’s unique electronic and optical properties have made it an attractive material for developing optoelectronic devices, such as solar cells and photodetectors.[1]

In this talk, I will introduce the graphene-semiconductor heterojunction and its photoelectric conversion from visible light to short-wave infrared (SWIR) light. The graphene can form a Schottky junction with Si, wherein the graphene can function as the transparent electrode and carrier transport layer. In this Schottky junction, Si can absorb the sunlight and contribute photo-generated carriers, so that this heterojunction can be used as a solar cell and a photodiode. [2-3] This solar cell is considered to be a promising demonstration and the power conversion efficiency is raised to 15.6% with electrical and optical improvement.[4-5] Furthermore, this graphene-based heterojunction can be utilized for the photodetectors from visible to SWIR light through structural design.[3,6] After employing the vertical built-in field used for trapping electrons and AuNPs-induced plasmonic effects for light absorption enhancement, graphene-based SWIR photodetectors exhibits the responsivity of 83 A/W with a fast rising time of 600 ns at the wavelength of 1.55 μm, which can overcome the low optical absorption of graphene, as well as the ultrashort lifetime of photoinduced carriers.[6] Finally, I will introduce some recent advances in graphene-2D heterojunctions photodetectors via interface engineering.[7]
(Thanks to the support of the JSPS KAKENHI Grant Number JP 17F17337. )
References
[1] X.M. Li* et al. Applied Physics Reviews 4, 021306 (2017).
[2] X.M. Li et al. Adv. Mater. 22, 2743-2748 (2010).
[3] X.M. Li et al. Small 12, 595-601 (2016).
[4] X.M. Li et al. Adv. Mater. 27, 6549–6574 (2015).
[5] Y. Song, X.M. Li et.al. Nano Lett. 15, 2104-2110 (2015).
[6] X.M. Li* et al. ACS Nano 11, 430-437 (2017).
[7] X.M. Li* et al. npj 2D Materials and Applications 1, 19 (2017).

Recent progress reveals the significance of metal dopants in transition-metal dichalcogenides (TMD) on reductive catalysis such as hydrogen evolution reaction. As the concentration of dopant increases, separately-distributed metal dopants tend to form energetically favored clustered structure. Higher concentration of dopant leads to phase separation and structural instability, demolishing the positive doping effect. As a result, the ideal performance of metal-doped TMD catalysts requires certain optimized doping concentration. In this work, we first realize controllable doping of various types of TMDs with desired concentration via a facile sulfurization process on homogenously mixed chloride precursors. The atomic structure of a series of doped TMDs was then unraveled by Raman and TEM techniques. Their catalytic performance was evaluated and the correspondence between atomic configuration, structural stability and catalytic activity was built to elucidate how doping booms the catalysis using TMDs to the greatest extent.

The incorporation of carbon nanostructures, such as graphene and carbon nanotubes, in the lattice of metals is desirable to take advantage of the superior mechanical and electrical properties of these graphitic nanostructures and the high density of electrons in metals. There have been numerous attempts to create composites of metals, such as copper or aluminum, with carbon nanostructures. These methods frequently require several steps and in aluminum produce the undesirable compound Al₄C₃. We use electrocharging assisted process which consist of the application of a high DC current to a mixture of liquid metal and carbon particles to incorporate carbon in the metal in a stable form. This method creates graphene nanoribbons and nanosheets in the metal lattice which form epitaxial nanostructures and gives rise to a 13 % increase in the electrical, and thermal conductivities and ~30% increase in mechanical properties of the composite compared to the pure metal or metal alloy. The graphitic structures bond with atoms in the metal making the composite very stable. We present a detailed investigation of the process and the effect on the properties of the composites. Raman spectra and mapping indicate that the carbon in the composite has sp2 character. Also, TEM imaging shows that the carbon forms graphene nanoribbons that extend along crystalline directions of the metal lattice. The improvement in electrical and thermal properties as well as the increase in tensile strength are due to the formation of the graphene nanoribbons.

This work is based upon work supported by the U. S. Department of Energy under Award No. DE-EE0008313. Dr. David Forrest is the Project Technology Manager and Debbie Schultheis is the Project Officer/Manager.

In the present decade, graphene, which is one of the large number of possible two-dimensional (2D) materials, is considered as a remarkable material for photonics and optoelectrics due to their specific properties such as gapless band structure which enables to modulate optical properties, ultra-fast carrier mobility which makes a fast modulation, a wide absorption ranges from ultra-violet (UV) to far-infrared (FIR), transparency and flexibility. However, a weak absorption properties and small built-in potential in a monolayer of carbon atoms have limited the properties such as an external quantum efficiency (EQE) range of ~ 0.1 - 1 % and a responsivity of a few of mAW^-1. Also, existing graphene-based photodetector with the lateral structure which has a photo response near graphene-metal junction is not an ideal for the harvesting efficient photons. To increase optoelectric properties in graphene, large number of devices integrating with other 2D transition metal dichalcognides (TMDCs) materials and Quantum dots (QDs) materials which have high absorption properties were developed recently. However, integration with these materials limits the absorption range due to their own bandgap even graphene has advantages in large absorption range. Here, we developed large absorption range to infra-red (IR) range graphene photodetector with metal/insulator/graphene heterostructure by controlling the Schottky barrier. The absorption range can be controlled by utilize with different materials by controlling Schottky barrier. In visible range, we used Au/h-BN/graphene heterostructure which the existence of the h-BN tunneling layer makes extremely low dark current ~10^-13 A. As the result, our photodetector has the high I_ph/I_dark over >20 contrast to reported lateral graphene photodetector which has under 5. Finally, we also show the possibility of the IR photodetector from Ni/NiO/graphene heterostructure

Black-phosphorus (BP) has emerged as a material of interest owing to its high carrier mobility and the presence of an intrinsic direct bandgap. Its thickness-dependent energy gap and highly anisotropic properties make it an important material to investigate amongst the family of two-dimensional (2D) materials. Few-layer BP has been a focus of several studies and is promising for applications in electronics, optoelectronics, energy storage, gas sensing, catalysis and chemical/biosensing. However, the ambient instability of BP remains the biggest hurdle in its progress. The fact that the material has to be stored and handled in an inert environment renders it to be unfavourable for practical implementation. To date, the solution to avoid degradation has been capping BP to minimize its interaction with the ambient environment. Here, we present a systematic investigation of the origins of oxidative degradation in few-layer black phosphorus (BP). Subsequently, we also propose an ionic liquid based approach to prevent ambient degradation of BP.
First, we conducted an in-depth investigation into the origins of degradation revealing that oxidation due to light causes degradation whereas humidity on its own does not cause any material and acts merely as a facilitator of photo-oxidation [1].
Subsequently, we determine the influence of discrete wavelengths ranging from UV to infrared on the degradation of BP. It is shown that the UV component of the spectrum is primarily responsible for the deterioration of BP in ambient conditions [2]. Based on these results, new insights into the degradation mechanism have been generated which will enable the handling and operating of BP in standard laboratory environments.
Finally, we designed an approach that allows this sensitive material to remain stable without requiring its isolation from the ambient environment [3]. We employ imidazolium-based ionic liquids (ILs) as quenchers of damaging oxidative species on the BP surface. This chemical sequestration strategy allows BP to remain stable for over thirteen weeks, while retaining its key electronic characteristics.
Besides, fundamental studies on the degradation of BP, we have also explored plasma thinning and defect engineering of BP layers.
References:
[1] S. Walia, et al,” 2D Materials, vol. 4, 2016, Article No. 015025.
[2] T. Ahmed, et al,” npj: 2D Materials and Applications, vol. 1, 2017, pp. 18.
[3] S. Walia, et al,” Advanced Materials, vol. 29, 2017, 1700152.

Graphene quantum dots (GQDs) have received tremendous attention because their band gap can be controlled. Although many approaches have been developed to fabricate GQDs, they are time-consuming and costly, and furthermore, precise control over the morphology and size distribution of GQDs remains challenging. Here, we demonstrate spatially controlled conversion of hexagonal boron nitride (h-BN) to graphene on an array of Pt nanoparticles (NPs) to realize an array of uniform GQDs embedded in an h-BN sheet. A uniform Pt NP array was formed on a SiO₂/Si substrate with the aid of self-patterning diblock copolymer micelles, and the h-BN sheet was transferred on the Pt NPs array, followed by the conversion of h-BN on Pt to GQDs. The size of the obtained GQDs corresponded with the sizes of the Pt NPs due to the selective conversion of h-BN on top of Pt NPs. Uniform and precisely controlled size of the GQDs ranging from 7 to 13 nm was achieved. Finally, we demonstrate electron transport by the size-controlled GQDs isolated by insulating h-BN like a Coulomb blockade, indicating that the splitting energy of the GQD is 80–160 meV, compatible with its dimension.

Many of two-dimensional materials show high electrical conductivity similar to Graphene. Especially, some of transition metal ditellurides has electrical conductivity as high as pure metals. Also, they can be transparent once they are in the form of a thin layer. Fortunately, they can be easily fabricated in the form of thin layer that consists of a few numbers of atomic layers. We attempt to fabricate thin films of nickel ditelluride in two ways (Intermetallic Target, Co-sputtering). Both of fabricated thin film crystal structure is Ni₁Te_2, Melonite, 98-004-3293. Our results prove that nickel ditelluride are one of promising candidates for transparent electrodes (existing electrode-200 μΩ×cm, 80%). First, 2 inch-compound targets were prepared by fabricating the intermetallic followed by SPS. Thin film (thickness 9nm) was deposited by Radio frequency sputtering (PVD) using the Intermetallic Target. Second, thin film (11nm) was deposited by co-sputtering a Ni and a Te target at Ni:Te=DC:RF. For two methods, we controlled sputtering power and deposition time and substrate heating conditions to maximize their electrical conductivity. Through TEM analysis proved that more grains with c-axis. Therefore, by after annealing and chemical exfoliation, Nickel ditellurides thin film grain size can be growth coarse and make more thin layer.

Metal chalcogenides (MCs) with two-dimensional (2D) atomic layers have evolved as premier materials for functional devices, including WS₂-based field-effect transistors, Bi₂Te₃-based topological insulators, and Bi₂Se₃-based thermoelectric converters. Despite being highly studied, MCs continue to be predominantly synthesized via solid-state reactions at high temperature (>1000 °C) or liquid-phase reactions, which afford microcrystal powders detrimental to low-cost processing of devices over large areas.
Bismuth sulfide (Bi₂S₃) is an attractive 2D-layered, visible light-absorbing semiconductor composed of non-toxic, abundant elements. However, improving the quality of a Bi₂S₃ thin film for device applications while maintaining its intrinsic electronic properties is a challenge, as conventional film fabrication processes require a trade-off due to the uncontrolled nucleation and growth steps. Herein, we report a novel procedure for Bi₂S₃ film formation involving spin-coating of a precursor solution of bismuth salt and sulfur-source organic molecule followed by crystallization under diluted H₂S gas. This two-step process produced a large-grained (< 400 nm), smooth (surface roughness = 1.7 nm), and highly pure Bi₂S₃ film with a layer-stacked structure on a substrate. Most importantly, the thin film exhibited excellent photo-sensing performance as a photoresistor with improved photoconductance and on-off ratio compared to those prepared by conventional methods. The obtained high on-off ratio is a result of high photoconductance and low dark-current, supported by high crystallinity, morphology and purity. Our approach provides a versatile route for the development of metal sulfide semiconductors for optoelectronic devices.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit unusually superior and tunable electrical, optical, and chemical properties unattainable in conventional inorganic thin films. The intrinsically layered van der Waals (vdW) crystallinity of 2D TMDs coupled with their extremely small thickness suggest that such property advantages can be further promoted by integrating them onto the substrates of novel functionalities and unconventional geometries. A prerequisite for the full exploration of such unprecedented opportunities is to precisely transfer large wafer-scale 2D TMDs onto targeted functional substrates without compromising their intrinsic material quality of as-grown states. Herein, we present a water-assisted “green” approach to efficiently separate, transfer, and integrate 2D TMD layers onto virtually arbitrary substrates without using additional chemicals. We demonstrate the successful integration of 2D TMDs onto a variety of unconventional substrates including paper, wood, and plastics as well as their layer-by-layer integration with controlled morphology. The success of this facile 2D layer integration is attributed to the surface energy imbalance in between as-grown 2D TMDs and their growth substrates (i.e., silicon dioxide (SiO₂), which efficiently facilitates the penetration of water underneath the grown 2D layers. We further extend this novel 2D layer integration approach to develop large-scale 2D/2D heterostructures as well as 2D flexible photodetectors, which confirms its versatility for a wide range of device applications.

The intrinsically anisotropic layered structure of two-dimensional (2D) transition metal dichalcogenides (2D TMDs) enables a variety of intriguing material properties which strongly depend on the physical orientation of constituent 2D layers. For instance, 2D TMDs with vertically-aligned 2D layers exhibit numerous dangling bonds on their 2D layer edge sites, offering tremendous opportunities for a wide range of applications which demand high physical/chemical surface reactivity. Moreover, such property advantages can be further promoted as far as 2D TMDs with controlled layer orientation can be integrated onto the unconventional substrates of tailored geometry and functionality while their structural integrity is well retained. Herein, we report large-area (>2 cm²) three-dimensionally (3D) structured 2D MoS₂ layers with vertically-aligned layers integrated on mechanically flexible elastomeric substrates. We synthesized 2D MoS₂ with vertically-aligned layers on silicon dioxide/silicon (SiO₂/Si) 3D pillars-patterned substrates. The 3D-ordered vertical 2D MoS₂ layers were subsequently transferred and integrated onto flexible polydimethylsiloxane (PDMS) substrates, enabled by the water-assisted layer transfer method developed in our group. The structural integrity of the vertical 2D MoS₂ layers was characterized by extensive spectroscopy and microscopy before and after the transfer and was confirmed to well retain. We demonstrated mechanically flexible humidity sensors based on the 3D-ordered 2D vertical MoS₂ layers and identified their superior sensitivity, which is attributed to the combined effect of increased surface area and enhanced 2D edge density.

The realization of wafer-scale transition metal dichalcogenide monolayer films^[1] and the successful exploration of 2D TMDs library^[2] are two major milestones in the recent efforts on the synthesis of 2D materials. Interestingly, NaCl is used in both cases to reduce the nucleation density and melting points of oxides. However, the presence of alkali metals is detrimental in conventional silicon-based semiconductors due to high rates of ion diffusion through the bulk, leading to degradation in device performance and reliability. In 2D materials, like traditional dopants such as rhenium and niobium, alkali metals, such as potassium K⁺, results in degenerate doping of MoS₂. Therefore, it is critical to comprehensively evaluate the impacts of NaCl on the properties of synthetic 2D materials. In this work, we elucidate NaCl-induced heterogeneities in synthesis, photonic response and electrical performance of MoS₂ films, and demonstrate that salt-assisted synthesis of epitaxial MoS₂ monolayers degrades electrical device performance. First, we develop the NaCl free large area (2×2 cm²) epitaxy of MoS₂, evident from in-plane X-ray diffraction (XRD) and utilize it to compare to the growth with NaCl. Atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman, and photoluminescence (PL) indicate that growth of monolayer films without NaCl are crystalline and uniform across sapphire substrates without island growth, while two distinct growth modes (island and layer-by-layer) and rates are identified when NaCl is utilized. The heterogeneous growth kinetics due to NaCl leads to photonic heterogeneities related to non-uniform monolayer strain. Moreover, utilizing NaCl introduces a 1.5x decrease in mobility, 2x increase in subthreshold slope, and 100x reduction in on/off ratio due to electronic trap states. Finally, comparison of transport properties between devices fabricated using as-grown and transferred films reveals that interface coupling (charge transfer, trap states) can dominate the device performance, indicating the importance of substrate engineering to realize the high performance, synthetic 2D based devices.
Reference
[1] K. Kang, S. Xie, L. Huang, Y. Han, P. Y. Huang, K. F. Mak, C.-J. Kim, D. Muller, J. Park, Nature 2015, 520, 656.
[2] J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.-H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B. I. Yakobson, Q. Liu, K. Suenaga, G. Liu, Z. Liu, Nature 2018, 556, 355.

Thermoelectricity, based on the Seebeck effect, is an alternative for power generation and refrigeration due to its ability to directly convert thermal into electrical energy. Here, we study thermoelectric properties of exfoliated Li_xMoS₂ grown using Chemical Vapour Transport (CVT), where a mixture of 1T' and 2H phases coexist in a single sample due to the 2H to 1T' phase transition induced by Lithium (Li) intercalation. We show that as-fabricated Li_xMoS₂ devices are p-type in nature with high electrical conductivity due to its metallic 1T' phase, and that in-situ thermal annealing of Li_xMoS₂ in vacuum induces a p-type to n-type transition as a result of delithiation. A peak power factor of 18 uW/mK² is observed in an effective medium of Li_xMoS₂ where reasonable electrical conductivity is maintained by the metallic 1T’ phase and the Seebeck coefficient is controlled by the semiconducting 2H phase. By refining the thermal annealing temperature and annealing time that controls de-lithiation of Li_xMoS₂, we hypothesize that a higher thermoelectric powerfactor can be achieved, showing an additional knob to tune energy dependent scattering in these novel materials.

Monolayer (ML) transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe₂) are particularly interesting in the field of nanoscale optoelectronic devices because of its direct band gap, high carrier mobility and high ON/OFF ratio. However, atomically thin material layers have limited ability to absorb and emit radiation because of their low cross-sectional area.
To enhance the photoresponsivity of ML WSe₂, a strong light absorbing materials such as PbS quantum dots (QDs) or organolead halide perovskite were introduced. However, these materials are toxic and unstable under ambient condition because of the presence of Pb compounds or organic materials. Therefore, it is a huge challenge for finding a low cost, non-toxic and stable material to create a hybrid structure with ML WSe₂ to improve its optoelectronic performance.
In this work, we report a facile method for fabricating a hybrid structure that consists of ML WSe₂ covered with Nitrogen-doped graphene quantum dots (ML WSe₂/N-GQDs). The PL intensity of ML WSe₂ is enhanced drastically because of the reduction of the positive trion and enhancement of the neutral exciton formations through electron transfer from N-GQDs. The improved photoresponsivity in ML WSe₂/N-GQDs photodetector is attributed to strong light absorption and charge transfer from N-GQDs to ML WSe₂. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. Notably, this hybrid photodetector exhibits good stability under ambient condition.

2-dimensional (2D) material field effect transistors (FETs) are promising devices to overcome the problems of conventional devices with their new attractive electrical properties. Since 2D material FETs have very similar structures to Junctionless transistors (JLTs) with no junctions requiring heavily-doped silicon as a channel, the analysis of the conduction mechanism of 2D material FETs based on the conduction mechanism of JLTs is considered as an effective method. JLTs are a special type of FETs that operate in accumulation mode using bulk conduction mechanism instead of conventional inversion mode.

Prior studies show that, in a 2D material FETs with a multi-layered channel, the conduction path is formed on the surface of the channel material if the electrodes are fabricated on the surface of the channel. In this study, multi-layered 2D material FETs with edge-contacted channel using molybdenum disulfide (MoS₂) were fabricated to investigate the effect of bulk conduction. The changes of transfer characteristics with the electrodes structure of FETs were observed, which can be explained by carriers directly injected at the interface between electrodes and edges of the channel. For the further analysis, mobility degradation factors using transconductance (g_m) and Low Frequency Noise (LFN) of FETs were evaluated. FETs with electrodes on the edge of the channel show less decrease of g_m curve than the ones with electrodes on the surface of the channel. The change of the current fluctuation was observed in case of edge contacted FETs, attributed to the contribution of bulk conduction.

These results indicate the reduction of surface roughness scattering from the bulk conduction mechanism in multi-layered structures. An enhancement of bulk conduction was also observed in FETs with electrodes on the edge of the channel in LFN Characteristics. Our study suggests an effective method to analyse the mobility degradation factors depending on conduction path in multi-layered channel regime.

Two-dimensional (2D) organic-inorganic hybrid perovskite is a re-emerging material with strongly excitonic absorption and luminescence properties that are attractive for optoelectronics. Similar to other excitonic 2D semiconductors, the excitons in 2D perovskite nanosheets are expected to exhibit near-field coupling with nearby excitonic systems. Here we report on an experimental observation of excitonic energy transfer in van der Waals heterostructures consisting of 2D hybrid perovskite ([C6H5C2H4NH3] 2 PbI4) and monolayer WS2. We prepared the heterostructure using the dry-transfer technique with the WS2 layer encapsulating the perovskite sheet to protect it from degradation. Photoluminescence excitation spectroscopy with WS2 exciton emission in the detection channel reveals a distinct ground exciton absorption feature of perovskite, evidencing energy transfer from perovskite to WS2. At the resonance peak of perovskite, the emission from WS2 was enhanced by a factor of 2~5. This observation suggests that the energy transfer occurs not only from the top-most layer but also from the lower layers of perovskite. We further discuss the emergence of a peculiar sub-gap low energy emission peak which may be attributed to the interfacial hybridization between WS2 and the organic moiety of perovskite.

When the silicon metal-oxide-semiconductor field effect transistor (MOSFET) was first introduced in integrated circuits, it shows strong scalability and the complexity of circuits increases exponentially in the past several decades according to the Moore’s law. However, as the characteristic dimension of MOSFET reaches the thermal and quantum limit, scaling down the device from lateral and vertical directions becomes increasingly difficult and costly. Driven by the urgent demand for continuously shrinking the size of transistors, great efforts have been made on exploring low dimensional electronic materials, such as nanowires, carbon nanotubes (CNTs) and two-dimensional (2D) materials as building blocks for constructing smaller devices during the past two decades. Although electronic devices based on individual nanomaterials have exhibited high performance, there is still a huge gap towards integrating these single devices into practical circuits that can compete with state-of-the-art silicon chips. Here we present an alternative device manufacturing strategy in which the ultrathin electronic components in the active layer are chemically synthesized and integrated simultaneously in a single step and each component is connected via covalent bonds instead of physical interfaces. As a proof of the concept, we synthesized arrays of logic devices and radio frequency devices through the phase-patterned growth of ultrathin atomic layers. The obtained circuits composed of semiconducting phase as channels and metallic phase as contacts and interconnects were ready to work after the synthesis process and exhibited high performance. Moreover, we successfully constructed multilayered devices with vertical interconnections to show the potential of our strategy on fabricating three-dimensional (3D) integrated circuits (IC) with ultimate circuit densities in vertical space. This approach makes the design and optimization of electronic devices much more flexible and preserves the intrinsic performance of the materials to the largest extent and therefore, provides a new technology for manufacturing high performance integrated circuits beyond silicon.

Probing the microstructure of transition metal dichalcogenide (TMDC) monolayers is of paramount importance to understand the optical and electrical properties, and to facilitate a controllable application in optoelectronic devices [1]. Industry-scale fabrication on TMDC monolayers seems within reach due to the rapid development of chemical vapor deposition (CVD) [2], yet visualization of spatial structure variations in nondestructive and high-throughput manner remains challenging [3,4].

Here we report how grain boundaries, strain fields, and grain orientation can be identified unambiguously with combined lateral force microscopy (LFM) and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS₂) monolayers. In LFM images, sharp line-shape contrasts appear and persist after sample transfer, and their locations coincide with regions having photoluminescence (PL) intensity fluctuation, corresponding to inherent structural defects, i.e. grain boundaries. Differently, diffuse-shape contrasts disappearing after sample transfer solely arise from changes of the friction coefficient under strain. TSM is demonstrated to be superior in identifying grain orientation since the shear stress signal varies with respect to the scan vector due to the elastic anisotropy of the crystalline sample. Further, angle-dependent TSM measurements enable us to acquire the fourth-order elastic constants of monolayer WS₂ experimentally.

Our study exemplifies a rapid and non-destructive approach to visualize the microstructure of TMDC monolayers, insights into their elastic properties, thus providing an accessible tool to support the development of advanced two-dimensional optoelectronic devices.

References
［1］A. M. van der Zande, P. Y. Huang, D. A. Chenet, T. C. Berkelbach, Y. You, G.-H. Lee, T. F. Heinz, D. R. Reichman, D. A. Muller, J. C. Hone, Nature Materials 2013, 12, 554.
［2］Z. Cai, B. Liu, X. Zou, H.-M. Cheng, Chemical Reviews 2018, 10.1021/acs.chemrev.7b00536.
［3］X. Yin, Z. Ye, D. A. Chenet, Y. Ye, K. O’Brien, J. C. Hone, X. Zhang, Science 2014, 344, 488.
［4］J. Wang, H. Yu, X. Zhou, X. Liu, R. Zhang, Z. Lu, J. Zheng, L. Gu, K. Liu, D. Wang, L. Jiao, Nature Communications 2017, 8, 377.

Monolayered molybdenum disulfide (MoS₂) is one of the most promising materials for next-generation electronic/optoelectronic devices because of its prominent piezoelectric property that can modulate Schottky barrier height and control transport behaviors without applying any external gate bias. Although temperature dependency of current transport in monolayered MoS₂ has been predicted in a theoretical study based on 2D thermionic equation, experimental result about temperature dependencies of piezoelectric/piezophotoelectric effect in monolayered MoS₂ has not been reported yet. Several researchers have studied piezoelectric properties of monolayered MoS₂ only at room temperature. Here, we focused on temperature dependence of the piezoelectric effect in CVD grown monolayered MoS₂. In this study, transport behaviors of monolayered MoS₂ on polyethylene terephthalate (PET) as flexible substrate under strain from 0 to 0.3 % in generally accepted operating temperature region from 273 K to 333 K were examined. Thermocouple was used to directly measure the temperature of such MoS₂. We observed that strain-generated piezoelectric charges in our MoS₂ affected the transport behavior by modulating SBH at the interface between electrodes and MoS₂. In addition, we found that the current and SBH had different tendencies depending on sample temperature. A dominant effect of the current behavior with the strain used in our monolayered MoS₂ depending on sample temperature was also determined by measuring photocurrent under the strain. The piezoelectric effect was found to be dramatically increased when sample temperature was decreased from 320 K to 270 K.

Abstract:
Transitional metal dichalcogenides (TMDC) such as WS₂ has drawn immense attention recently owing to their thickness depended band gap energy which is missing among other two-dimensional materials like graphene or carbon nitride. However, widespread application of semiconducting WS₂ is hampered by their low yield production methods. Among the two-dimensional (2D) TMDC, WS₂ is very reluctant to undergo liquid exfoliation than other. So, it is a new challenge to synthesize mono or few layer WS₂ flakes or nanocrystals of controlled size in large scale and simple way. WS₂ shows similar kind of band gap modulation like MoS₂ with the layer number attractive for opto-electronic device application. Here, we have synthesized bi-to-few layers WS₂ in solution from bulk WS₂ by Li-ion intercalation technique using lithium halide. In brief, WS₂ bulk powders and LiX (anhydrous) were mixed in a hexane solution. The mixture was sonicated for 30 minutes in an ultrasonic bath sonicator. The black mixture was washed several times with N, N-Di methylformamide (DMF) to remove unreacted lithium halides and further sonicated for 15 minutes to obtain yellowish green solution. In addition, WS₂ nanocrystals of different sizes have been synthesized by sonication induced fragmentation method. The structural and morphological properties of the as synthesized WS₂ nanocrystals were investigated by different spectroscopic and microscopic techniques such as TEM, AFM, RAMAN, XRD, XPS. We have observed that the photoluminescent property of the nanocrystals changes dramatically compared to the 2D flakes of WS₂. We have employed ZnO as n-type and WS₂ nanocrystals of 2-3 nm as p-type to fabricate heterojunction light emitting diode. The fabricated device shows a broad band emission from 400 to 700 nm at 0.9 mA. In addition, charge storage ability of WS₂ nanocrystals is investigated by integrating flexible solid state symmetric supercapacitor using acetylene black as a conducting filler and PVA-H₃PO₄ mixture as gel electrolyte. The fabricated supercapacitor shows 28 mF/cm² areal capacitance at a current density of 0.1 mA/cm². This result paves the way to fabricate future optoelectronic devices.


Ref:
A. Ghorai, A. Midya, R. Maiti, S. K Ray Exfoliation of WS₂ in the semiconducting phase using a group of lithium halides: a new method of Li intercalation Dalton Transaction, 2016, 45, 14979-14987
A. Ghorai, S. Bayan, N Gogourla, A. Midya, S K Ray Highly Luminescent WS₂ Quantum Dots/ZnOHeterojunctions for Light Emitting Devices ACS Applied Materials & Interfaces 2017, 9,558–565.
A.Ghorai A. Midya, S. K Ray Superior charge storage performance of WS ₂ quantum dots in a flexible solid state supercapacitor, New Journal of Chemistry 2018, 42 (5), 3609-3613

T-phase tungsten disulfide (T-WS₂) is firstly suggested as a potential candidate for carbon-free cathode of Li-air batteries (LABs) to replace conventional carbon-based materials. Using density functional theory calculations, it was revealed that the electron transfer caused by the adsorption of oxygen reduction reaction (ORR) intermediates (i.e. LiO₂ and Li₂O₂) enhances the W – W clustering due to the electronic instability induced by additional charge, and it causes a phase transition to the new phases of T-WS₂ (named as DT-WS₂ and ZT-WS₂). Moreover, in terms of electrochemical performances, DT-WS₂ and ZT-WS₂ have a small discharge overpotential of 0.84 V and 0.91 V, respectively. For this reason, DT-WS₂ and ZT-WS₂ are predicted to have comparable electrochemical performance with well-known carbon-based cathodes. In addition, it is found that the adsorptions of intermediates induce the semiconductor to metal transition in DT-WS₂ and the increase of carrier concentration in ZT-WS₂, resulting in high electrochemical performance. It is therefore confirmed that DT-WS₂ and ZT-WS₂ are strong candidates as a carbon-free cathode based on two-dimensional materials for LABs.

Two-dimensional materials having a van der Waals (vdW) gap have attracted considerable attention for future electronics and optoelectronics applications because of their ultimate thickness limit. One of the most intriguing properties of standard transition metal dichalcogenides (TMDs) is that they show a direct optical transition as a monolayer, while they possess an indirect band gap in the bulk counterpart. Even though they possess interesting properties, materials exploration of TMDs is limited to the composition of MX₂, where M is a transition metal and X represents a chalcogen element. In this study, we propose HfGeTe₄ as a new ternary transition metal chalcogenide [1].
It was reported that bulk HfGeTe₄ was a layered structure having a zigzag vdW gap, which is a striking difference from a standard flat vdW gap. Therefore, it is expected that such the zigzag shape allows a higher cohesive energy between layers. The total energies were calculated by a density functional theory (DFT) code CASTEP as a function of inter-layer distances, and it was found that HfGeTe₄ has the highest cohesive energy than other typical TMDs like MoS₂ or MoTe₂. This would enable the stronger adhesion with metal or insulator layers that has advantage for industry.
The band structures were calculated for bulk and monolayer HfGeTe₄ using a screened exchange (sX) hybrid functional. The bulk has a band gap of about 0.67 eV and a transition occurs from the Γ point in the valence band maximum (VBM) to the Y point in the conduction band minimum (CBM), indicating an indirect semiconductor. On the other hand, the direct transition is realized at the Γ point in ML with a band gap of 1.3 eV. Furthermore, it was also found that models more than 2ML show the indirect transition, demonstrating the same trend with TMDs. The band gap decreases with increasing number of layers toward the bulk limit. Based on the current study, the materials exploration of layered chalcogenides will dramatically extend from binary to ternary systems that may lead to the discovery of novel materials for future electronics applications.
[1] Y. Saito and J. Robertson. APL Mater. 6, 046104 (2018)

Recent nanofluidic experiments using single nanopores or nanotubes have shown considerable potential for harvesting osmotic power from the Gibbs free-energy of mixing fresh and salt water. In particular, experiments with single boron nitride nanotubes have reported large surface charge in aqueous solution, and osmotic power densities up to several kW/m² when extrapolated to macroscopic membranes having closely packed pores. However, no macroscopic BN-nanotube or nanopore membranes have been fabricated with such high power densities.
Here, we demonstrate the osmotic-power-generation performance of macroscopic-scale boron-nitride nanopore membranes (BN-AAO) fabricated from anodized alumina (AAO) templates. Thin h-BN layers are chemical-vapor deposited into the template pores without excessive hBN on the top surface, ensuring that most pores remained open. Scanning confocal Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) show the high quality of the hBN layers in the AAO pores. The membranes can exceed 100 cm² in size and have ~30 nm pores at a density of ~10⁸ pores/cm².We investigate ion transport and osmotic power generation of the macroscopic BN-AAO membranes at different molarity ratios and pH conditions. Despite the relatively large pore diameter of 30 nm, the BN-AAO membranes show highly selective transport, with electrophoretic transport rates differing by a factor of ~74.5 between positively and negatively charged fluorescent ions. The power generation per unit pore area increased as the salt concentration and pH increased. The maximum power density of 27.3 W/m² for pH=11 and a 1 M:1 mM KCl molarity difference is an order of magnitude higher than existing macroscopic ion-selective membranes. Notably, these BN-AAO membranes, with large yet highly selective pores, appear to have unique advantages in terms of the trade-off between membrane fouling performance versus selectivity and osmotic power generation. Such nanopore membranes may ultimately enable commercially viable reverse electrodialysis for efficient osmotic power generation.

Characterization of the thickness and continuity of wide band gap 2D materials with monolayer sensitivity over large areas has proven to be very challenging. A prime example is 2D hexagonal boron nitride (hBN). Unlike the case of metallic and semiconducting 2D materials (graphene, transition metal dichalcogenides etc.), optical contrast methods applied to hBN suffer from the lack of visible absorption in the material; Raman spectral signatures are weak and often not conclusive; and electrical measurements are not possible due to the high electrical resistivity. In this contribution, we will demonstrate an experimental method that enables thickness and continuity mapping of large-area hBN monolayers and bilayers transferred to Si/SiO₂ substrates (thickness range: 0-7 Å). The proposed method is based on spectroscopic ellipsometry measurements and subsequent fitting based on an appropriate optical model. The method has sub-monolayer thickness sensitivity, is relatively fast, non-destructive, and can be easily automated. The hBN thicknesses measured in this study have been confirmed by Raman spectroscopy, x-ray photoemission spectroscopy, and by a series of ellipsometry control experiments. The key step for obtaining reliable results is the selection of an appropriate optical model to fit the ellipsometry data. The optical model must include the commonly observed polymer residuals from the transfer process. In this contribution, we will also present a workflow of our experimental procedure, so that other researchers can extend this characterization method to other 2D materials and hopefully accelerate their development.

Rhenium disulfide (ReS₂) differs fundamentally from other group VI transition metal dichalcogenides (TMDs) due to its low structural symmetry, which results in its optical and electrical anisotropy. Although vertical growth is observed in some TMDs under special growth conditions, vertical growth in ReS₂ is very different in that it is highly spontaneous and substrate independent. In this study, the mechanism that underpins the thermodynamically favorable vertical growth mode of ReS₂ is uncovered. It is found that the governing mechanism for ReS₂ growth involves two distinct stages. In the first stage, ReS₂ grows parallel to the growth substrate, consistent with conventional TMD growth. However, subsequent vertical growth is nucleated at points on the lattice where Re atoms are “pinched” together. At such sites, an additional Re atom binds with the cluster of pinched Re atoms, leaving an undercoordinated S atom protruding out of the ReS₂ plane. This undercoordinated S is “reactive” and binds to free Re and S atoms, initiating growth in a direction perpendicular to the ReS₂ surface. The utility of such vertical ReS₂ arrays in applications where high surface to volume ratio and electric field enhancement are essential, such as surface enhanced Raman spectroscopy and solar based disinfection of bacteria is demonstrated.

[1] Gao et al. Nano Letters, 16, 3780 ( 2016).
[2] Q. Zhang et al. Adv. Mater., 28, 2616 (2016)

Current-transport characteristics rely mainly on the relative band alignment offset at the 2D heterojunction. Hence, devices based on heterojunctions offer diverse functionality upon stacking 2D materials with different electron affinities followed by different bandgaps. In this view, band-structure alignment can be classified into three types of heterojunctions: straddling-gap (type-I), staggered-gap (type-II) and broken-gap (type-III). In particular, broken-gap heterojunctions are interesting as there is no overlap between energy bands of two stacked materials, which can incur some exotic phenomena. Till now tunable current-transport across broken-gap heterojunction have been achieved only if combination of employed 2D materials is varied. In this view, it appears that diverse current-transport across broken-gap heterojunction is primarily a material dependent phenomenon. Here we report that BP-based broken-gap heterojunction could display tunable current-transport characteristics. To elucidate our findings, we have chosen BP/ReS₂ van der Waals heterostructures due to following reasons: (i) In addition to opposite polarity of majority charge carriers i.e. p-type BP and n-type ReS₂, BP/ReS₂ forms type-III broken-gap alignment at the heterojunction and (ii) BP flakes show substantial dependence of work function on their respective thickness. In the Phosphorene (BP)/Rhenium disulfide (ReS₂) heterojunction, we exploit BP work function tunability with varying flake thickness to demonstrate that BP-based broken-gap heterojunction could display diverse current-transport such as gate tunable rectifying p-n junction diodes (current rectification ~ 10⁶), Esaki diodes (peak to value current ratio ~ 4.4), backward-rectifying diodes and non-rectifying devices as a consequence of band-structure alignment at the heterojunction. Optoelectronic experiments on Esaki diode and p-n junction diode clearly reveal that diversity of current-transport derives from the work function modulation of BP. The p⁺-n-p junction made of thick-BP/ReS₂/thin-BP functions as multi-value inverters, and shows the behavior of bipolar junction transistor (BJT) with common-emitter current gain up to 50. Such tunability could be beneficial in the development of compact electronic circuits with devices of multifunctionality reducing the needs of multiple materials. Current-transport tunability due to Φ_BP modulation and p⁺-n-p BJT with high gain, which benefits from the high hole-mobility of BP, demonstrates the high potential for a new class of scalable and multifunctional devices.

Heterometallic structures containing metal 4+ ions are interesting molecules for the creation of (multi)functional 2D materials such as high quality graphene-like 2D MX₂ (M = transition metal, X= S, Se) structures, given their unique physical properties for optical, optoelectronic and electronic devices. However, traditional metal precursors like halides, oxides, and thiometallates implicate several disadvantages such as low solubility and volatility and the use of hazardous coreactants such as H₂S. Therefore, in this study, a novel class of [M^IV(SEtN(Me)EtS)₂] single-source precursors was designed for a variety of tetravalent transition metals including tungsten(IV), molybdenum(IV), titanium(IV) and vanadium(IV). These complexes were formed by simple synthetic protocols and characterized by a combination of spectroscopic techniques, single crystal x-ray analysis and TG-DTA experiments. For instance, the controlled decomposition of the well-characterized [W(SEtN(Me)EtS)₂] complex during chemical vapor deposition (CVD) experiments resulted in the formation of highly crystalline 2D layered WS₂ thin films without the necessity for further purification steps. As-prepared films were subjected to atomic scale imaging and x-ray analysis to confirm the synthesis of phase-pure MX₂ materials. Dianionic, tridentate chelating pincer type XNX-ligands (X=S, Se) have thus opened up a new field of precursor engineering and allow an efficient approach towards the synthesis of scalable van der Waals heterostructured materials such as 2D transition metal dichalcogenides that are expected to play an important role for future energy applications.

Non-noble metal catalysts such as transition metal dichalcogenides (TMDs) are being studied as alternatives to Pt for sustainable production of H₂ through the electrochemical hydrogen evolution reaction (HER). Layered TMDs like WS₂ are promising candidates for HER catalysts when engineered into nanostructures with exposed reactive edge sites. In this work, we demonstrate a novel and highly reproducible approach based on plasma-enhanced atomic layer deposition (PEALD) to maximize the density of such reactive edge sites.

PEALD is a cyclic deposition technique based on sequential use of self-limiting precursor and plasma-activated co-reactant steps. Controlling the various parameters during the plasma exposure step of PEALD allows tailoring of material properties (morphology, composition etc.) which can significantly influence the HER performance. Sub-nanometer thickness control and low processing temperatures are some of the other key merits of PEALD.

In our work, two types of nanostructured WS₂ with distinct material properties were synthesized at a low temperature of 300 °C by using two different plasmas as co-reactant during PEALD. Transmission electron microscopy (TEM) studies revealed that the application of a H₂S+Ar plasma resulted in the growth of WS₂ ‘nanosheets’ while the addition of H2 to the plasma i.e. H₂S+Ar+H₂ resulted in the growth of WS₂ ‘fins’, for the same tungsten precursor. The WS₂ nanosheets comprised of closely packed sheets with their edges predominantly terminating on the top surface, while the WS₂ fins comprised of high surface area open structures that tapered out on the top surface. Rutherford backscattering spectroscopy (RBS) studies indicated excess S content for the WS₂ nanosheets with a S:W ratio of 2.2, while a S deficiency was observed for the WS₂ fins with a S:W ratio of 1.9. Moreover, in line with TEM studies, X-ray diffraction (XRD) studies showed differences in preferential orientation of the crystals for WS₂ nanosheets and WS₂ fins.

The HER performance of WS₂ nanosheets was significantly better than of WS₂ fins when comparing the overpotential required to reach a current density of 10 mA/cm². A relatively lower overpotential of ~390 mV was sufficient for the nanosheets while a significantly higher overpotential of ~460 mV was required for the fins. In line with the HER results, copper underpotential depositions on the respective nanostructures revealed a higher number (3x) of reactive edge sites for the WS₂ nanosheets when compared with the WS₂ fins. To further enhance the HER performance, the catalytically superior WS₂ nanosheets were grown on top of the high surface-area WS₂ fins. This WS₂ layered stack (nanosheets on fins) showcased the best HER performance in our work (overpotential of ~365 mV). To summarize, we demonstrate how PEALD can be used as a new approach to nanoengineer and enhance the HER performance of WS₂ by maximizing the density of reactive edge sites at low processing temperature (300 °C).