Symposium NM2 : 2D Layers and Heterostructures beyond Graphene—Theory, Preparation, Properties and Devices

In the last 5 years there has been exciting discoveries in the area of 2D materials that unveiled new physical phenomena and created new technological opportunities. However, material quality remains a limiting factor in making 2D materials into new markets. In this seminar I will cover several aspects of this growing research area.

Atomically-thin two dimensional (2D) materials and their van der Waals hetero-structures (vdWHs) have been studied intensively because of their enormous potential for future optoelectric devices [1-5]. In vdWHs building from transition metal dichalcogenides (TMDs), efficient charge separation [6] and formation of long-lived interlayer excitons [7] can be observed due to the type II band alignment. In addition, usage of interlayer excitons is thought to be the key to realize the exciton valleytronics [8]. In this study, we report the modulation of intralayer and interlayer exciton PL properties of vdWHs composed from 1L-MoS₂ and 1L-MoSe₂ by means of the temperature control and field effect gating.
1L-MoS₂/1L-MoSe₂ vdWH was fabricated using transfer method by poly dimethylsiloxane (PDMS) film [9]. This hetero-structure becomes n-type because of initially doped electrons. We measured PL spectra of 1L-MoS₂/1L-MoSe₂ vdWH at different temperature from 5K to 300K. Intralayer trion emission peaks from 1L-MoS₂ (Peak X: ~1.83 eV) and 1L-MoSe₂ (Peak Y: ~1.64 eV) are observed in the whole temperature region. On the other hand, interlayer PL peak (Peak I: ~1.45 eV) appears remarkably at low temperature below 120 K. Thermal dissociation of interlayer excitons due to the small exciton binding energy (~ 80 meV) is the possible mechanism to understand this thermal crossover behavior.
We also measured PL spectra of 1L-MoS₂/1L-MoSe₂ hetero-structure with changing applied gate voltage at 50 K. Interlayer exciton PL intensity (Peak I: ~1.45 eV) is increased gradually with increasing the negative voltage despite the small change of intralayer trion PL intensity in 1L-MoSe₂ (Peak Y: ~1.64 eV). In the large negative voltage region, electron density of each layer becomes small due to the compensation of initially doped electrons in by field effect doping. This could enhance the charge separation between layers and formation of interlayer excitons.

Reference
[1] B. Radisavljevic et al., Nat. Nanotechnol. 6, 147 (2011). [2] A. K. Geim et al., Nature 499, 419 (2013).[3] H. Fang et.al., PNAS 111, 6198 (2013). [4] S. Mouri et al., Nano Lett. , 13, 59442013.
[5] S. Mouri et al., Phys. Rev. B. 90, 155449(1) (2014). [6] X. Hong et.al., Nat. Nanotehnol. 9, 682 (2014). [7] P. Rivera et.al., Nat. Commun. 6 6242 (2015). [8] P. Rivera et.al., Science 351,688 (2016). [9] F. Cebellos et al., ACS Nano, 8, 12717, (2014).

We present experimental electronic and thermoelectric transport properties of NaSn₂As₂. NaSn₂As₂ is new 2-D system, with Na atom embedded between the Sn-As layers. The electrical resistivity and magnetoresistance are measured from 2K to 300K in magnetic fields from – 7 to + 7 Tesla. The samples show typical metallic behavior with its resistivity increasing with temperature. The observed magnetoresistance and Hall resistivity curves versus magnetic field are evidence for a 2-carrier system: the Hall curve has different slopes at low and high field. The high-field slope gives an overall carrier concentration that is consistent with about 1 hole per formula unit. The thermal and thermoelectric transport properties are measured in both a Quantum-Design PPMS system and a self-built system from 2 to 420 K. The Seebeck coefficient is negative (about -10μV/K ) at room temperature. Since the Hall coefficient is mostly positive, this is also a strong indication of two-carrier conduction. The Nernst coefficient is quite small at room temperature. These results are consistent with recent band structure calculations. A quantitative fit between band structure and data will be presented.

Supported by NSF EFRI grant 1433467

Properties of low-dimensional materials are highly dependent on their atomic structures. Correlation between the atomic structure and physical properties of 2D materials is of general interest for the fundamental researches and technological applications. Here I present some examples for atomic-scale imaging and spectroscopy of various low-dimensional materials with interrupted periodicities and attempt to correlate with their optical and transport properties. Nitrogen defects and their chemical dynamics of graphene are corroborated at individual atom basis [1]. Defects and phase transitions of single-layered dichalcogenides (MX₂) are corroborated in situ [2]. In plane anisotropy of transport properties in single-layered group VII dichalcogenides (ReS₂ and ReSe₂) is recently reported [3]. Optical properties near the hetero-interface of the other transition metal dichalcogenides are proved to be accesible by high energy resolution EELS [4, 5]. Eventually single atom magnet at grapheme atomic defects will be proposed [6].

[1] Y.-C. Lin et al., Nano Letters, 15 (2015) pp.7408-7413
[2] Y.-C. Lin et al., Nature Communications, 6:6736 (2015)
[3] Y.-C. Lin,et al., ACS Nano (2015) DOI: 10.1021/acsnano.5b04851
[4] L. Tizei et al., Phys. Rev. Lett., 114 (2015) 107601
[5] L. Tizei et al., Appl. Phys. Lett., 108 (2015) 163107
[6] Y.-C. Lin et al., Phys. Rev. Lett., 115 (2015) 206803
[7] This work is partially supported by a JST research acceleration programme.

Monolayer transition metal dichalcogenides (TMDs) such as MoS₂ or WS₂ are semiconductors with degenerate, yet inequivalent k-points labeled K and K’ that define the direct bandgap. The valence band maximum in each valley has only one spin state in which the spins are opposite for K and K’. Consequently, one can selectively populate each valley independently with circularly polarized light and determine the valley populations via the polarization of emitted light. Monitoring changes in emitted polarization, therefore provide insights into the fundamental processes of intervalley scattering. Here we probe the degree of circular polarization of the emitted photoluminescence as a function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. Monolayer flakes of MoS₂ and MoSe₂ show a strong depolarization as the excitation energy is increased. However, WS₂ maintains significant polarization for high excitation energies, even at room temperature when properly prepared. We discuss the behavior of the polarization in terms of various phonon assisted intervalley scattering processes, and the role electron capture of the trion has on suppressing the intervalley scattering process. This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141.

The potential of MoS₂ mono- and few-layers in electronics, optoelectronics, and flexible devices requires the fundamental understanding of their properties as a function of strain. While previous reports have studied mechanically exfoliated flakes, tensile strain experiments on chemical vapor deposition (CVD)-grown few-layered MoS₂ has not been examined hitherto, although CVD is a state of the art synthesis technique with clear potential for scale-up processes. In this report, we used CVD-grown terrace MoS₂ layers to study how the number and size of the layers affected the physical properties under uniaxial and biaxial tensile strain. Interestingly, we observed significant shifts in both the Raman in-plane mode (as high as -5.2 cm^-1) and photoluminescence (PL) energy (as high as -88 meV) for the few-layered MoS₂ under ~1.5% applied uniaxial tensile strain. The observed results were compared to monolayers and few-layers of MoS₂ previously reported. We also observed slippage between the layers which resulted in a hysteresis of the Raman and PL spectra during further applications of strain. Through DFT calculations, we contended that this random layer slippage was due to defects present in CVD-grown materials. This work demonstrates that the properties of CVD-grown few-layered MoS₂ studied here can be tuned under strain as well as, if not better than, it’s exfoliated monolayered counterpart.

ReS₂ has recently emerged as a new member in the rapidly expanding family of two-dimensional materials. Unlike MoS₂ or WSe₂, the optical and electrical properties of ReS₂ are not isotropic due to the reduced symmetry of the crystal. This additional degree of freedom could be harnessed in future optoelectronic devices. Here, we shed light on the anisotropic optical behavior of ReS₂ using Raman and photoluminescence (PL) spectroscopy.

Raman measurements in the ultralow frequency range (< 50cm^-1) allow us to access the layer breathing modes (LBM) and shear modes (LSM) of the material which stem from rigid-layer oscillations. The layer dependence of their peak positions enables an easy determination of the number of layers in a crystal and can be readily reproduced by means of a monoatomic chain model. By varying the angle between the linearly polarized laser and the in-plane crystal axis, we are able to reveal an energetic shift of the LSM which is directly linked to the in-plane anisotropy of the shear modulus in this material.
Furthermore, we perform layer-dependent PL measurements on ReS₂ down to 4K. Thereby, we reveal three different peak features, which also possess anisotropic behavior. Their energetic position distinctively depends on the number of layers, which emphasizes the electronic coupling between individual layers.
Finally, we elaborate on the possibility of using monolayer ReS₂ as an atomically thin and anisotropic substrate in van der Waals heterostructures.

Single layers of transition metal dichalcogenides (TMDCs) like MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂ and WTe₂ can be produced by simple mechanical exfoliation. Offering a direct bandgap at the K-points in the Brillouin zone, they represent a promising semiconductor material for flexible and transparent optoelectronic applications.
Due to inversion symmetry breaking together with strong spin-orbit-interaction, the valley and spin degrees of freedom are coupled in monolayer TMDCs. Via circularly polarized optical excitation, an efficient polarization of the K⁺ or the K^- valley can be generated. Here, we investigate the dynamics of these coupled spin-valley polarizations in mechanically exfoliated monolayers of different TMDCs by means of photoluminescence spectroscopy and time-resolved Kerr rotation (TRKR). By varying the excitation energy, we observe a zoo of different excitonic features directly visible in the TRKR data, including an energetic splitting of the singlet and triplet state of the negatively charged exciton resonance in single-layer WS₂. The sign of the conduction band spin splitting, which is opposite for Mo and W compounds, has significant influence on the valley dynamics and on the generation of long-lived dark excitons, whose radiative recombination is forbidden by the spin-dependent selection rules. The results indicate a maximum achievable valley-lifetime in these materials exceeding one nanosecond at low temperatures.

Transition metal dichalcogenides (TMDs) van der Waals (vdWs) heterostructures present fascinating optical and electronic phenomena, and bear tremendous significance for electronic and optoelectronic applications. The behavior of excitons in vdWs heterostructures depends on both the electron-electron interactions and charge transfer at the hetero-interface. However, to what extent the carrier densities of counterparts and band alignment determine the photoluminescence properties of vdWs heterostructures remains unrevealed. Here, we tune the photoluminescence properties of monolayer MoS₂/graphene and WSe₂/graphene heterostructures by modulating the carrier densities of counterparts and contact barrier at the interface via electrochemical gating. It is manifested that PL intensities of excitons in MoS₂/graphene can be tuned by more than two-order of magnitude, and the blueshift of exciton peaks up to 40 meV is observed. By extracting the carrier density of MoS₂ by electric potential distribution model, and the Schottky barrier by first-principle calculations, we find that the controllable carrier density in MoS₂ plays a dominant role on PL tuning at negative gate bias, while interlayer relaxation of excitons induced by the Schottky barrier has a major contribution at positive gate bias. This is further verified by controlling the tunneling barrier and screening field across MoS₂ by inserting self-assembled monolayers (SAMs) at the interface. In contrast, upon tuning the photoluminescence property of WSe₂/graphene and WSe₂/MoS₂/graphene heterostructures by virtue of electric field, it is demonstrated that the interlayer relaxation of excitons at the hetero-interface in WSe₂/graphene, which plays a dominant role in PL tuning, is even stronger than that in MoS₂/graphene and WSe₂/MoS₂ heterostructures, and In addition, it is discovered that the interlayer coupling between monolayer WSe₂ and graphene is enhanced under high electric field, which breaks the momentum conservation of first order Raman-allowed phonons in grapheme, yielding the enhanced Raman scattering of defects in graphene. Our results suggest that the interplay between electric field and vdWs heterostructures may provide versatile approaches to tune the intrinsic electronic and optical properties of the heterostructures. Moreover, these findings will benefit to understand the effects of many-body interactions and hetero-interface on optical properties of vdWs heterostructures.

Two-dimensional (2D) layered materials with diverse properties have attracted significant interest in the past decade. The layered materials discovered so far have covered a wide, yet discontinuous electromagnetic spectral range from semimetallic graphene, insulating boron nitride, to semiconductors with bandgaps from middle infrared to visible light. Here, we introduce new layered semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into long-wavelength infrared (LWIR) regime and cannot be readily reached by other layered materials. Moreover, polarization-resolved infrared absorption and Raman studies reveal in-plane anisotropic properties of b-AsP. In the last part, we will discuss detection of long-wavelength infrared lights using b-AsP transistors.

References.
[1] Liu, B. et al., Advanced Materials, 2015, 27, 4423.

Atomically thin black phosphorus, or phosphorene, has emerged as a promising two-dimensional material because it exhibits higher charge carrier mobility than transition metal dichalcogenides, and its finite band gap allows for higher on-off current ratios than in graphene-based transistors. We recently demonstrated that passivated phosphorene, that has not been affected by degradation due to air exposure and oxidation, exhibits nearly symmetric electron and hole charge transport. This makes phosphorene particularly promising for applications that require p-type and n-type conduction regions in the same parent semiconductor, such as diodes and logical gates. I am going to present recent experimental work on the controlled doping of few-layer black phosphorus using metal adatoms. The developed method allows us to demonstrate a scalable CMOS-like logical inverter, which uses a single phosphorene crystal for both the p-type and n-type regions, does not require local electrostatic gating or different contact metals, and operates at matched input-output voltages with excellent inverter gain.

The fabrication of two-dimensional topological insulators (TIs) has applications in the development of spintronics and the realization of dissipation-free conduction channels. Unlike most of the current selection of known TIs, stanene, a hexagonal monolayer of Sn similar to graphene, has been predicted to be an ideal candidate that is two-dimensional and possesses topological edge states with a large enough bandgap (>0.1 eV) to allow operation of devices at room temperature. Here we report the growth of high-quality stanene on Bi₂Te₃ (111) using molecular beam epitaxy (MBE). For films grown in an MBE system at the synchrotron, we observe wavelength dependent scattering along Bi₂Te₃ crystal truncation rods with a strong modulation at the Sn K-edge, indicating a crystalline Sn monolayer coherently strained to the Bi₂Te₃ substrate. These measurements reveal further structural details of the stanene films and can differentiate between the various polymorphs of monolayer stanene.

Graphene sheets are thermally and structurally stable, and can be isolated by mechanical cleavage of graphite samples [1]. Recently, Shunhong Zhang et al [2] proposed a new class of carbon-based nanostructures, named ‘penta-graphene’. Starting from penta-graphene, we propose another structure with same topology of penta-graphene formed only by silicon atoms, which we call penta-silicene. In this work, we report thermodynamical, structural stability and vibrational properties of penta-silicene nanoflakes. To obtain the atomistic trajectories we have used General Utility Lattice Program [GULP][3] with reactive force field ReaxFF[4]. Preliminary results show that the penta-silicene monolayer does not maintain layer structure’s under temperature effects (there is a trend to occur a self-scroll of penta-silicene sheet), but maintain structural stability until 1000K. Thermodynamical results from DFT(Density Functional Theory) calculations using Dmol3[5,6] although indicate that penta-silicene is more energetically favorable to synthesize than penta-graphene present itself in a metastable state, and free energy curves show that around 400K the penta-silicene formation reaction should occur spontaneously. Further investigations will show potential applications of penta-silicene nanoflakes as nano-mechanical devices.

[1] Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451.
[2] Zhang, Shunhong; Zhou, Jian; Wang, Qian; et al., PNAS, 112, 2372 (2015).
[3] J.D. Gale, JCS Faraday Trans., 93, 629 (1997).
[4] A.C.T. van Duin, S. Dasgupta, F. Lorant and W.A. Goddard III, J. Phys. Chem. A, 105, 9396-9409 (2001).
[5] Delley, B. J. Chem. Phys. 1990, 92, 508.
[6] Delley, B. J. Chem. Phys. 2000, 113, 7756.

According to a recent Thomson-Reuters study of the 10 hottest research fronts in Physics, “Silicene growth and properties” is ranked fourth, but first in condensed matter physics. This reflects the huge interest on the emerging synthetic group IV 2D materials, namely silicene, germanene and stanene, which are artificially created, since they have no parent crystal in nature, at variance with graphene, which descents from graphite. In this talk, I will discuss recent advances in the synthesis, properties and applications of these novel 2D materials, from monolayers to multilayers including their one-dimensional nanoribbon counterpart.

Despite unique and promising properties including high carrier mobility and conspicuous in-plane anisotropy, black phosphorus (BP) and phosphorene stability is hampered by apparent, severe crystal deterioration upon exposure to oxygen and water. Herein, we demonstrate that field-effect transistors (FETs) based on MoS2-passivated black phosphorus (BP) or bilayer (2L) phosphorene van der Waals (vdW) heterostructures may provide an effective solution to addressing this challenge. The trilayer MoS2 passivation is shown to enhance 75% of the photoresponse of a 2L-phosphorene optoelectronic device when no gate voltage is applied, and retain the conductivity of the BP channel without degradation. In this work, we explore four thicknesses for MoS2-passivated BP FETs at 1.5 (assigned to bilayer), 5, 13, and 20 nm, to explore the effect of the MoS2 passivation layer on the device stability and electrical characteristics both in the dark and under illumination (λ = 600 nm). When in contact with a trilayer MoS2, the photoluminescence intensity of the 2L BP crystal decreases by 29%, suggesting that a built-in electric field forms at the BP-MoS2 p-n interface that could help dissociate photo-generated electron-hole pairs, thereby reducing the probability of the recombination events. The effectiveness of a few-layer MoS2 as a vdW protection layer is tested by exposing BP-MoS2 vdW vertical heterostructures to the air for up to 3 weeks as well as by annealing at a high temperature (350°C) in an inert argon environment. Then, the heterostructure FETs are fabricated so that direct comparisons between exposed and MoS2-passivated BP regions on the same flake can be made. We find that MoS2 passivation layer gives rise to only an insignificant degradation in the transport characteristics in the dark, with the trend where its impact decreases as the thickness of the underlying BP crystal increases. Upon illumination, the MoS2-passivated regions of the 2L BP device show enhanced photoresponse compared with the exposed regions by 75 % with no applied gate voltage. Thus, 2D MoS2 thin films may aid in the realization of the future transparent, flexible BP electronic and optoelectronic devices by acting not only as an atomically thin passivation layer but as a photoresponse enhancer.

Synapses are functional links between neurons through which“information” flows in the entire neural network. Developing solid state devices with novel functionalities that can mimic the operation of biological synapse is critical for building artificial synaptic network, which has many applications in image recognition, sensing and hardware implementation of machine learning algorithms. In this work, we demonstrate two novel synaptic device concepts based on the unique properties of black phosphorus (BP) for applications in neuromorphic computing. In the first device, the anisotropic transport property of BP is utilized to realize connection heterogeneity in artificial synaptic networks. Here, we demonstrated the first BP based transistor-type synaptic device, which offers intrinsic heterogeneity in its synaptic characteristics directly resulting from its low crystalline symmetry. The device can mimic complex synapse connectivity in biological neural network. Synaptic connection weight change in the Y-direction device is 1.75 times higher than that of the X-direction device. This heterogeneity in synaptic device characteristics originates from the intrinsic difference in the transport properties of BP along different in-plane directions of its crystal. Moreover, the native PO_x of BP is carefully analyzed using high-resolution cross-sectional scanning tunneling electron microscopy (STEM) together with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). The native oxide PO_x is utilized as the charge trapping layer, giving rise to the synaptic behavior in the device in response to positive and negative pulses applied at the gate. Furthermore, temperature dependent measurements were carried out to characterize the time constant for the post-synaptic current decay. In the second device, we demonstrate a reconfigurable synaptic device based on a highly tunable BP-semiconductor heterojunction. Key features of biological synapses such as long-term plasticity with heterogeneity, including long-term potentiation (LTP)/depression and spike-timing-dependent plasticity, are mimicked. BP synaptic device shows LTP with the time constant on the order of hundreds of seconds, which is ideal for mimicking long-term memory or learning functions in biological systems. Furthermore, the BP-semiconductor heterojunction synaptic device can be dynamically re-configured between excitatory and inhibitory responses as a result of the highly tunable electrical characteristics of the junction. We further demonstrated a simple compact heterogeneous axon-multi-synapses network using BP synapse. With the additional tuning of synaptic device behavior enabled by the anisotropy of the BP channel and the re-configurability resulting from the moderate bandgap of BP, BP synaptic devices are promising for a new generation of neuromorphic electronics to emulate the complex heterogeneity in biological neural network.

Due to its direct and tunable band gap and high carrier mobility, two-dimensional (2D) black phosphorus (BP) is promising for applications in thermal-imaging, thermos-electrics, fiber optics communication, photovoltaics and so on. Current few-layer BP fabrication methods include mechanical exfoliation, plasma thinning, liquid phase exfoliation and ion bombardment-free plasma etching. However, none of these methods have a good control on the BP thickness. It is crucial to find a scalable and controllable method to fabricate few-layer BP for industry applications.

In this paper, we report a study of a thermal thinning method. Layer-by-layer uniform sublimation of BP flakes (about 90 nm thick originally) was observed in the temperature range from 500 K to 600 K in nitrogen ambient, and the sublimation rate was temperature dependent. We monitored thinning process according to the changes of peak intensity ratio of , by performing in-situ Raman spectroscopy measurements with 442 nm excitation wavelength. Via thermal sublimation thinning, BP flakes were successfully reduced to 7 nm on polyimide substrates and to 12 nm on Si/SiO₂ substrates. Raman spectroscopy, atomic force microscope (AFM) measurements of these samples showed good crystallinity, uniformity and integrity.

Furthermore, we use angle-resolved polarized Raman spectroscopy (ARPRS) with 442 nm excitation wavelength to measure the out-of-plane Raman mode and in-plane mode before and after the thinning process. The maximum Raman intensity ratio of peak over peak was obtained when the incident polarization was aligning to the zigzag crystallographic direction. Experimentally, this ratio is better measured using 442 nm excitation compared to longer excitation wavelength reported previously. This is because that the secondary Raman peaks in the zigzag direction with 488 nm or longer excitation wavelengths make the determination of the crystal direction and the peak less accurate. Raman tensor elements a and c were measured for BP on polyimide and on SiO₂/Si substrates. Our results support that the phase difference between a and c elements is zero for 10 to 200 nm BP samples studied.

In summary, we report that the sublimation thinning method to be a promising method in the fabrication of high quality few-layer BP in large scale. We also report that Raman signal intensity ratio of peak over peak measured with a 442 nm excitation is effective and convenient in determining BP orientation.

Two-dimensional black phosphorus (i.e., phosphorene) has attracted significant attention due to its interesting optical and electronic properties. However, production methods for phosphorene have been limited due to its high chemical reactivity that leads to rapid oxidization in ambient conditions¹. To minimize this degradation pathway, solution-based exfoliation techniques have been developed using anhydrous organic solvents². While this approach avoids exposure to oxidizing species, it has relatively poor exfoliation yield and results in relatively thick black phosphorus flakes. To overcome these limitations, we have exfoliated black phosphorus using deoxygenated aqueous surfactant solutions, which results in phosphorene down to the monolayer limit as confirmed by transmission electron microscopy, electron diffraction, Raman spectroscopy, and photoluminescence spectroscopy³. Furthermore, X-ray photoelectron spectroscopy confirms that phosphorene is not oxidized or otherwise chemically degraded following processing in deoxygenated water. The resulting phosphorene dispersions are shown to be compatible with centrifugal separation methods (e.g., density gradient ultracentrifugation (DGU)), which allows for enrichment of the thinnest and largest lateral area phosphorene flakes. DGU-processed phosphorene flakes have been incorporated into field-effect transistors and show device metrics that meet or exceed the performance of micromechanically exfoliated black phosphorus.

This work was supported by National Science Foundation (NSF) Grant DMR-1505849.

1. Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K. S.; Cho, E.; Sangwan, V. K.; Liu, X. L.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Nano Lett. 2014, 14, 6964-6970.
2. Kang, J.; Wood, J. D.; Wells, S. A.; Lee, J. H.; Liu, X. L.; Chen, K. S.; Hersam, M. C. ACS Nano 2015, 9, 3596-3604.
3. Kang, J.; Wells, S. A.; Wood, J. D.; Lee, J. H.; Liu, X.; Ryder, C. R.; Zhu, J.; Guest, J. R.; Husko, C. A.; Hersam, M. C. Proc. Natl. Acad. Sci. U. S. A. 2016, 10.1073/pnas.1602215113.

Ultra-low dimensional MoX₂ (X = S, Se) has been the subject of considerable research efforts owing to its novel optoelectronic properties that emerge as a result of quantum confinement—namely, MoX₂ materials transition from an indirect to direct band gap at the atomic limit. Yet, while properties are highly dependent on the number of monolayers assembled into a multi-layer structure, synthetic techniques that can reliably produce uniform large area samples of a precise thickness are still in their nascence. This makes experimental exploration of the parameter space a formidable challenge. Our lab has developed a modified physical vapor deposition technique whereby atomically precise layers of selenide-based two-dimensional material precursors are deposited and then gently heated to self-assemble into metastable large area nanolaminates. We have shown that we can reliably prepare films of a desired thickness between 2 and 24 layers. Additionally, we have demonstrated that selenium vapor annealing can be used to decrease defect concentration in fully formed films. Constituent layers are partially decoupled from one another due to rotational disorder within the stack, which we consider to be an important degree of freedom in the evolution of transition metal dichalcogenide heterostructures. This serves to preserve photoluminescence and other low dimensional optical signatures even in films up to 24 layers thick. This synthetic platform enables us to create and study homologous series of two-dimensional materials heterostructures with rational chemical and structural changes.

MoS₂ has attracted much interest because of its two-dimensional structure as well as tunable optical, electrical, and mechanical properties for next generation electronic and electro-optical devices. We have achieved facile fabrication of MoS₂ layers on CdS nanowires by cation exchange in solution at room temperature, atomic images of the MoS₂/CdS interface and also observed their extraordinary magnetic properties. We established the atomic structure of the MoS₂/CdS heterostructure by performing first-principles density functional geometry optimizations and also STEM-ADF image simulations. Spin-polarized electronic structure calculations for the MoS₂/CdS heterostructure within the density functional theory with the generalized gradient approximation reveal that the magnetism in the MoS₂/CdS heterostructure stems from the ferromagnetic MoS₂ monolayer next to the MoS₂/CdS interface. The ferromagnetism is attributed to the partial occupation of the Mo d_x²-_y²/d_xy conduction band in the interfacial MoS₂ monolayer caused by charge transfer from the outer MoS₂ monolayers due to the mixed covalent-ionic bonding among the MoS₂ and CdS monolayers near the MoS₂/CdS interface. The present findings of the ferromagnetic MoS₂ monolayer with large spin polarization at the MoS₂-semiconductor interface suggest a new route for fabrication of the transition metal dichalcogenide-based magnetic semiconductor multilayers for applications in spintronic devices.

Vapour deposition techniques have gained a lot of interest for growth of two dimensional (2D) materials[1-4]. In the recent past there has been a surge in the number of researchers studying atomic planes of other Van der Waals solids and heterostructures created by stacking layers with complementary characteristics to achieve novel functionality [5]. For successful scaling up of prototypical applications demonstrated to date, technologies and processes for large area deposition of these materials need to be developed. Here we present the technology employed and study of the impact of process parameters on a chemical vapour deposition (CVD) process for the production of single-layer MoS₂ using a gas-phase S precursor (H₂S) and solid Mo precursor (MoCl₅). Strategies for optimising crystalline quality via direct control of deposition variables and the impact of process parameters on defect density is analysed qualitatively using Raman spectroscopy [6]. We also present the characteristics of CVD grown MoS₂ on different substrates and investigate the use of graphene as a substrate for MoS₂ growth which opens an avenue for growth of 2D heterostructures.

References

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The energy bandgap is a property that is central to the functionality of a semiconductor. Finding new ways to obtain precise control on the bandgap energy, particularly in emerging electronic materials, is a continuing pursuit with significant scientific and technological implications. We demonstrate that in a layered semiconductor a distinct path for band engineering is accessible through its surfaces.¹ In gallium telluride (GaTe), a layered monochalcogenide with a direct gap of 1.65 eV, exposure to air induces a transformation of the bandstructure akin to that achieved by alloying in conventional semiconductors. Here, we report on such transformation as observed through several surface and bulk characterization techniques. After prolonged exposure of GaTe to air, the rise in sub-bandgap absorption leads to the emergence of a new indirect bandgap below 0.8 eV. Density Functional Theory (DFT) calculations modeled the new indirect bandgap to the intercalation and chemisorption of oxygen molecules to the tellurium-terminated surfaces of each layer. X-ray diffraction (XRD) near the surface of GaTe shows a small increase in the inter-layer spacing over time, supporting the model of an intercalated species. Interestingly, XRD also helps to illustrate that the chemisorption process results in a crystal structure that is at once layered and disordered. Thermal annealing of transformed GaTe leads to the formation of gas bubbles between the layers, further confirming the intercalation of gas molecules like oxygen. Remarkably, localized partial recovery of the original direct gap is achieved by further thermal annealing. These results demonstrate that reversible band engineering is accessible in a layered semiconductor through its surfaces.
This work was principally supported by the National Science Foundation Graduate Research Fellowships Program (Grant No. DGE-1106400). Experiments were supported by the Electronic Materials Program (EMAT). EMAT and XPS experiments at the Molecular Foundry were funded by the DOE’s Office of Basic Energy Sciences under Contract No. DE-AC02-05CH11231. Computational resources were partly provided by TUBITAK ULAKBIM, High Performance and Grid Computing Center (TR-Grid e-Infrastructure). Part of this work was performed at the Stanford Nano Shared Facilities (SNSF).

[1] J.J. Fonseca, S. Tongay, M. Topsakal, A.R. Chew, A.J. Lin, C. Ko, A.V. Luce, A. Salleo, J. Wu, O.D. Dubon, Adv. Mater. 2016, DOI: 10.1002/adma.201601151

Transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), have attracted attention due to striking mechanical and electronic properties in their two dimensional (2D) structures. The potential of these 2D materials in various electronics, optoelectronics, and flexible devices requires a fundamental understanding of the effects of strain on the electronic, magnetic and optical properties. Particularly important is the response of chemical vapor deposition (CVD)-grown structures that comprise of a few layers. Recent experiments on strain effects of CVD-grown few-layered MoS₂ structures suggest that the electronic properties of these structures can be tuned under strain. The strain response of such few-layered structures, however, is determined by the number of layers as well as the presence of intrinsic defects such as vacancies and interstitials. In this study, the effect of the presence of various defects on the modifications in the local electronic structure and barriers for the strain relaxation (sliding) of the bilayer MoS₂ structures are studied at the atomic scales using density functional theory (DFT). DFT calculations suggest that the presence of a local defect lowers the energy barrier required to slip, thus increases the possibility of slippage locally in regions with defects. The effects of strain on the energy barrier as well as the electronic properties of different types of defect structures (vacancies and interstitials) will be presented. The understanding of the links between strains and defect structures, and electronic properties in bilayered MoS₂ structures will allow for the possibility of unprecedented performance improvements for the miniaturized electronic devices.

As a new class of hybrid systems, graphene non-covalently functionalized with smart polymers offer improved solubility and mechanical properties. The adsorption, interactions and smart behaviors, however, are not yet well understood at the atomic level. In this study, interaction of a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), with graphene in aqueous solution has been studied using molecular dynamics (MD) simulations. In order to examine the effect of graphene composition, bare graphene (G) as well as oxygenated graphene (graphene oxide GO) were investigated. The dynamic process of physical adsorption of PNIPAM onto G and GO was studied, followed by examining the temperature-responsive behavior of the hybrid systems. PNIPAM could spontaneously anchor to the surfaces of both G and GO at low temperature based on various available interactions. Configuration of PNIPAM on G, however, proved to be different from that on GO which resulted in distinct responsive behaviors upon temperature increase as discussed based on the ruling interactions and the solvation behaviors. The results obtained are practical in bottom-up design and manipulation of multi-functional hybrid stimuli-responsive systems with optimized properties.

The ability to control the self-organization of nanoscale domains at solid surfaces is vital to miniaturization of functional devices and further development of modern technology. Although it has been suggested that two-dimensional materials could give rise to a new disruptive technology, experimental evidence of bottom-up self-organization of nanostructures in two-dimensional materials is still very rare. Particularly intriguing is the evolution and arrangement of nanoscale domains within a two-dimensional layer made of boron, carbon and nitrogen (BCN). Previous work performed on ruthenium reported the evolution of graphene submonolayers subjected to progressive exposure to borazine vapors [1].
Here, we report the first observation of spontaneous formation and self-assembly of graphene quantum dot (QD) arrays embedded in a two-dimensional BCN alloy grown in ultra-high-vacuum conditions. The growth method consists of simultaneously dosing C and BN precursor molecules on a clean iridium surface. The formed QDs exhibit a highly regular size of approximately 2.3nm, while the periodicity of the array can be tuned as a function of carbon fraction. We propose that the segregation and self-assembly are consistent with minimization of the energy of the two-dimensional BCN system.
A full set of complementary in-situ microscopy and spectroscopy techniques (including scanning tunnelling microscopy (STM), X-ray photoemission spectroscopy and low-energy electron microscopy [2] and diffraction) is used to fully characterize the nanoscale domains present within the BCN monolayer. In particular, close STM investigations reveal that the graphene QD arrays behave like a 2D superlattice, showing topological defects, such as vacancies and dislocation-like defects, similar to the case of 3D lattices.
The presence of periodic arrays and spatial confinement of the domains observed here in the BCN monolayer paves the way to an extreme form of materials design. Such structures may exhibit peculiar and potentially useful electron transport phenomena, and the optoelectronic properties of such layers could be used in a large number of applications in several fields, with the additional perspective of tuning the superstructures through control of the growth conditions.
Ultimately, as self-assembly is a general phenomenon (i.e., not specific to graphene or hexagonal boron nitride) and very common in nature, it is likely that similar evolutions are also found for other 2D alloys or heterostructures.

References and notes
[1] Lu, J. et al., Order–disorder transition in a two-dimensional boron–carbon–nitride alloy. Nature communications 2013, 4.
[2] This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. “DE-SC0012704”

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently attracted a lot of attention as a new class of semiconductors widely believed to be promising candidates for various optoelectronic applications. Owing to the 2D confinement and reduced dielectric screening, these systems support tightly bound electron-hole pairs, excitons, even at the room temperature. Commonly, excitons are created through the direct photon absorption or the injection of electrons and holes, which respectively lead to the phenomena of photo (PL) and electroluminescence (EL). A very different way to produce excitons in TMDCs is through near-field coupling that enables non-radiative energy transfer (NRET) from the proximal quantum emitters such as nanocrystal quantum dots (NQDs).
In this work, we prepare and examine hybrid nanostructures comprised of the monolayer MoS2 flakes and dense nanocrystal films. Time-resolved photoluminescence (PL) and femtosecond transient absorption (TA) spectroscopies are used to directly observe, for the first time, the dynamic evolution of excitonic signatures in monolayer MoS2 as it is accepting energy from the NQDs. The experimental data derived on hybrids exhibit nanosecond-scale kinetics absent in pristine MoS2 which is the time scale consistent with measured and evaluated NRET rates. Importantly, the data unequivocally demonstrate highly effective excitonic sensitization of monolayer MoS2 in the hybrid structures as manifested by nearly 10-fold emission enhancement of the MoS2 PL intensity in hybrid structures vs. PL emission resulting from the direct photon absorption in the bare MoS2 reference monolayers. We register this enhancement in both PL and TA data. Monolayer TMDCs are known to possess very attractive properties such as the direct gap, however the number of excitons produced in them by the given-power direct photoexcitation is restricted due to the limited amount of light absorption, fast recombination losses and associated low quantum yield. Our findings indicate that excitonic sensitization of TMDCs in ET-coupled hybrids can be used to overcome such limitations while preserving the monolayer attributes. This should extend the range of optoelectronic application opportunities for monolayer dichalcogenide systems, further enhanced by the possibility of electric manipulation of the strength of near-field coupling.

We investigate, through first-principles calculations, the electronic band structure - including the spin-orbit coupling - of single-layer M₃(Org)₂ metal-organic polymers, where the metal atoms (M= Ni, Pt, Cu and Au) are at kagome lattice sites, and Org is a triangular organic linker (either THT - 1,2,5,6,9,10-triphenylenehexathiol, or bis-dithiolene). Several materials within this family have already been synthesized [1-3]. This family of materials contains, in its electronic structure, spin-orbit gaps that could allow their use in quantum spin Hall effect devices [4,5]. We find that the partial inclusion of exact exchange in the calculations (beyond a semilocal exchange-correlation level) is essential for a quantitative, and even qualitative, description of the electronic structure of the materials with M=Ni and Pt: upon inclusion of exact exchange, the predicted fundamental band gap of these semiconductor materials increases to more than twice, and the predicted spin-orbit gaps change by as much as 44%. Even the qualitative description of the valence bands of these materials changes upon inclusion of exact exchange. We also find that the magnitudes of the spin orbit gaps are not monotonic with the increase of the atomic number of the metal atom, from Ni to Au, due to the changes in the localization of the gapped states at the metal site.

[1] R. Dong et al., Angewandte Chemie, 2015, 54, 12058.
[2] J. Cui and Z. Xu, Chem. Commun., 2014, 50, 3986.
[3] T. Kambe et al., J. Am. Chem. Soc., 2013, 135, 2462.
[4] X. Zhang, Z. Wang, M. Zhao and F. Liu, Phys. Rev. B, 2016, 93, 165401.
[5] Z. F. Wang, N. Su, F. Liu, Nano Lett., 2013, 13, 2842.

Organizing gold nanoparticles into highly-ordered architectures is crucial to fabricate functional materials with tunable properties. Salts-induced spontaneous formation of 2D Gibbs monolayers of thiolated single-stranded DNA-capped gold nanoparticles (ssDNA-AuNPs) at the vapor–liquid interface is characterized by in situ surface sensitive X-ray scattering and spectroscopy techniques. Synchrotron-based grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) demonstrate that non-base-pairing ssDNA-AuNPs in aqueous solution spontaneously accumulate at the vapor–liquid interface and form a single layer in the presence of divalent salts (i.e., MgCl₂ or CaCl₂). The monoparticle layer crystallizes into a long-range ordered hexagonal superlattice above a threshold concentration. At similar levels of ionic strength to the threshold concentration, we find that monovalent (NaCl) or trivalent (LaCl₃) salts are less effective than MgCl₂ or CaCl₂ in inducing interfacial self-assembly and crystallization. X-ray near-total-reflection fluorescence measurements of the same samples provide direct evidence of surface accumulation of AuNPs, and more importantly the surface enrichment of the divalent cations, namely, Ca²⁺. Quantitative analysis suggests that the divalent cations screen the charge of DNA and at a threshold salt concentration, the hydrophobic effect mainly arising from the hexyl-thiol (commonly used to modify ssDNA for capping AuNPs) overcomes the DNA’s affinity to water. Our study points to a new direction, where hydrophilic/hydrophobic effects can be used to design and construct organic and inorganic structures at interfaces. Exploiting this concept, poly(acrylic acid) (PAA) instead of DNA grafted to gold nanoparticles via alkane thiol are used to assemble nanoparticles at interfaces in the presence of MgCl₂ .

Two-dimensional single-layered covalent organic framework (2D sCOF) is a typical class of graphene-like monolayer. Scientists are dedicated to the synthesis of 2D sCOF to prepare two graphene-like materials with novel electronics, sensing, catalysis and so on. We have explored the preparation of 2D sCOF based on Schiff base reaction. 2D sCOF containing porphyrin active groups have been constructed, and found that the preferential adsorption is one key factor to get high-quality 2D sCOF.

Two-dimensional (2D) materials, such as graphene and atomically thin transition metal dichalcogenide (TMDC) semiconductors, are promising materials for nanophotonics and optoelectronics. In particular, direct band-gap for a monolayer, near-unity photoluminescence, high electron mobility make these materials attractive candidates for ultrathin and lightweight photovoltaics. Here, we explore theoretically and experimentally light trapping within such 2D materials and their heterostructures. We show that near-unity light absorption across the visible spectrum above the semiconductor direct gap band edge can be achieved in a few unit cells of a heterostructure superlattice of monolayer TMDC alternated with hexagonal boron nitride (hBN).
Despite strong light interaction with TMDCs, a free-standing monolayer absorbs only 5-15% of the visible light. At the same time, an efficient solar cell requires near-perfect light harvesting. Therefore broadband, angle-insensitive light-trapping designs are needed to realize highly efficiencient photovoltaic cells in two-dimensional TMDC materials.
In this work we study periodic heterostructures made of monolayer TMDCs (MoS2, WSe2 and WS2) paired with few layers hBN on top of a silver backreflector. We show both theoretically and experimentally that light absorption in such heterostructures may be dramatically enhanced. In particular we demonstrate that in just three unit cells of monolayer MoS2 – 10 nm hBN over 70 % of light absorption is attained from 400nm to 500nm. For a stack comprised of 10 unit cells we achieve near unity absorption over a broader spectral range. We further show that the spectral response may be tuned and controlled by tailoring the thickness of hBN layers. We underline key mechanisms responsible for such a dramatic enhancement of light absorption and demonstrate that the heterostructure may successfully be described by an effective medium theory.
Finally, we discuss potential strategies for efficient light absorption in just one TMDC layer.

Atomically thin semiconductors such as Graphene, boron nitride, MoS₂ and MoSe₂ are showing exotic physical properties including high electron mobility, valley polarization, photoluminescence and mechanical strength. Known as transition metal dichalcogenides (TMD), MoS₂ and MoSe₂ are considered as the possible alternative for the graphene and extensively attract huge research interest. Compared to zero band gap for the graphene, TMDs show a finite and thickness dependent band gap and preserve common properties with graphene which make them more suitable for different electronic and devices applications. To really understand their potential, their electrical, thermal, optical and mechanical properties need to be investigated on monolayer scale. Here we used a suspended microdevice to measure the basal thermal conductivity of suspended monolayer MoSe₂. The measurements are performed on intrinsic, as synthesized MoSe₂ and substitutionally doped tungsten doped MoSe₂ samples. The results reveals nanostructure behavior for the thermal conductivity and they are in agreement with the reported values from first principle calculations. Also the results show more than 50% decrease in the thermal conductivity due to the phonon scattering by tungsten atoms. To better understand the transport mechanism, a theoretical modeling based on Boltzmann Transport Equation and Molecular Dynamics is performed to evaluate the different phonon scattering and their contribution in the lattice thermal conductivity.

Monolayer MoS₂ has been shown to be an interesting material for optoelectronics with interesting spin-valley physics; however, the thermoelectric properties of MoS₂ have not been studied as comprehensively. In this work, we show that 2D MoS₂ possesses a favorable bandstructure to serve as a backbone for thermoelectrics with opportunities to tune it further. In particular, the thermoelectric powerfactor defined as S²σ in bilayer MoS₂ is seen to be as high as 8.5 mW/m.K² (comparable to benchmark thermoelectric materials) and we attribute this to a large density-of-states effective mass. We also perform careful measurements of temperature dependent four-probe conductivity and seebeck coefficient and comment on the Metal-Insulator Transition, as well as measure the transport effective mass. We also show that the density-of-states effective mass can be tuned by controlling the number of layers of 2D MoS₂.

Abstract
Transition metal dichalcogenides (TMDs) are one of recent research themes because of high structural stability and easy growth method¹. However, some reports claimed that the grain boundaries of TMDs can be easily degraded by oxygen in water^2-3, UV^4-5, ozone⁶, and heating in ambient condition⁶. Moreover, monolayers of TMDs undergo dramatic aging effects including severe cracking and quenching of photoluminescence (PL) in ambient condition for several months⁷ even though it is not reacted immediately like black phosphorus. In this presentation, we demonstrate the photo-degradation of WSe₂ by laser (532 nm) exposure. The photo-degradation was monitored by changes of Raman and PL intensities. The photo-degradation of WSe₂ in air occurred at laser power more than 0.5 mW and did not occur at as low as 0.1 mW. However, the photo-degradation of WSe₂ with a water droplet was accelerated and happened even at laser power of 0.1 mW. An interesting thing is that the encapsulation of WSe₂ with h-BN layer (h-BN-covered WSe₂) prevented the photo-degradation of WSe₂. In our experiments, single-layer h-BN was not enough to perfectly prevent the photo-degradation of WSe₂, but 3 layers of h-BN showed perfect prevention of the photo-degradation. We also showed that photo-degradation of MoSe₂ was prevented by encapsulation with h-BN layers. This work can be used as a general strategy to improve the stability of 2D materials for real applications.


Reference
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Recently, two dimensional (2D) transition metal dichalcogenides (TMDC) have drawn significant attention due to their unique optical, electrical and mechanical properties that are different from bulk. However, desired progress in device fabrication has been limited because of the challenge in large scale production of these materials. Here, we present a scalable and facile method of growing WS₂ and MoS₂ in the form of customized chemical vapor deposition (CVD) at atmospheric pressure. Differently with the reported methods, a sputtered metallic W or Mo film was used to deposit a thin base layer on a sapphire substrate. The thin layer was sulfurized under a N₂ + H₂S flow at an elevated temperature in the CVD chamber. Thickness of the metal layer and sulfurization time were found to be critical factors in determining the area and quality of the resultant 2D layers. For example, monolayer MoS₂ was obtained by sulfurization of ~10 nm thickness of Mo at 750 ^oC for 10 min. The growth of uniform and large-area (~10x10 mm²) 2D semiconductor may cover more versatile applicability in electronic and optoelectronic devices.

Binary quasi two-dimensional (2D) crystals i.e. van-der Waal heterostructures comprised of Transition metal dichalcogenide semiconductor and graphene monolayers are emerging as novel class of materials relevant for energy efficient memory and optoelectronic device applications. However, the growth of these quasi 2D crystals over large area is still in rudimentary stages. Here we report the direct growth of uniform monolayer WS₂ films over a large area (~50 mm²) upon bilayer graphene via chemical vapor deposition method. The uniform deposition was confirmed by Raman mapping and SEM spectroscopy. The Photoluminescence (PL) spectrum of as grown graphene/WS₂ films exhibited significant quenching in PL intensity, however, the PL band position corresponding to “A” exciton remained unchanged. The room temperature Raman spectrum revealed a red shift of ~4 cm^-1 in E_2g frequency indicating the presence of lattice stress in graphene/WS₂ heterostructure, which could alter the optical band gap nature of 1L-WS₂ from direct to indirect. Nevertheless, the strong optical absorption of 1L-WS₂ facilitates the quantum efficiency of graphene/WS2 heterostructures. Therefore, we studied the room temperature photodetector characteristics of the graphene/WS₂ hybrids field effect devices (FET) in back gate configuration. The photocurrent (I_ph) showed a sharp decrease in the presence of light (λ =514.5 nm) indicating p-type nature of graphene. The saturation magnitude of I_ph was gate bias dependent. The measured gain and responsivity of the phototransistor was found to be ~10⁶ and ~10⁴ A/W, respectively at room temperature. Thereof the direct synthesis of the large area semiconducting TMDCs monolayer upon graphene paves the way to scalable fabrication of optically active binary hybrids.

The emergence of two-dimensional (2D) materials has led to tremendous interest in the study of graphene (Gr) and transition metal dichalcogenides (TMDCs). Among these TMDCs, the study of gallium selenide (GaSe) and molybdenum disulfide (MoS₂) have gained increasing attention due to its promising optical, electronic, and optoelectronic properties [1]. Furthermore, the formation of these TMDCs on graphene for 2D-heterojunctions has become the focus of 2D materials research for advanced electronic, and photonic devices. While it is convenient to create such 2D heterojunctions by the top-down exfoliation and transfer approaches, these 2D heterojunctions are random in orientation and contain impurities. On the other hand, chemical vapor deposition (CVD) has shown great promises to form high-quality 2D heterojunctions.

Here we describe selective growth of GaSe and MoS₂ on single crystalline, monolayered Gr domains. Despite of the large lattice mismatch between GaSe and graphene, we found that GaSe crystals with triangular shapes and stoichiometry composition (Ga:Se ~ 1:1) are predominantly grown on monolayer Gr. Electrical properties of the Gr/GaSe heterostructures were also studied using Kelvin probe force microscopy. The energy diagram constructed based on the experimental data confirmed the Schottky barrier junction of the GaSe/Gr domain. On the other hand, we also observed preferential growth of 2D MoS₂ crystals on Gr. The origin of preferential growth of GaSe and MoS₂ on graphene will be discussed based on the results of high-resolution scanning transmission electron microscopy (HR-STEM), sticking coefficient, and surface energy analysis. Apparently the growth mechanism of these simple heterostructures is interesting and very important towards more sophisticated controlled synthesis of complex and functional 2D heterostructures.

Y.K.Y. acknowledges the support from the National Science Foundation, Division of Materials Research (Award No. 1261910).

[1] M. Ye, D. Winslow, D. Zhang, R. Pandey and Y. K. Yap, "Recent Advancement on the Optical Properties of Two-Dimensional Molybdenum Disulfide (MoS2) Thin Films," Photonics 2015, 2(1), 288-307.

Transition Metal Dichalcogenides (TMDs) have attracted considerable attention for use in flexible electronics, coatings, nanocomposites, photonics, charge transportation layers, and catalysis applications due to their complimentary properties relative to other low dimensional nanomaterials (e.g. graphene, BN, aluminosilicates, phosphenes, etc.). While CVD can afford large monolayer TMDs for heterogeneous integration into microelectronics, other technologies require solution processible materials such as those produced via liquid phase and chemical exfoliation techniques. However, these methods, which have been extensively developed for MoS₂ and WS₂, are energy intensive (i.e. requiring harsh chemicals and solvents, failure prone equipment (sonicator), and long processing times (>48 hr per sample) with low total yields of single-to-few layer product (1-5%). As an alternative, we report a bulk exfoliation method that is effective for Group V, VI, VII, and VIII TMD materials. We demonstrate successful exfoliation of MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, NbSe₂, and ReS₂ from their corresponding bulk powders with yields approaching 80% of suspended flakes, and >10% mono-to-few layer product. Single-crystallinity is preserved (high resolution TEM), and thus two-dimensionally confined optical properties, including photoluminescence, (e.g. ReS₂, WSe₂) are observed. The exfoliation mechanism relates to the reactivity of TMD edge sites and the in-situ formation of anionic polyoxometalates (POMs). Adsorption of POMs to the TMD surface leads to columbic repulsion between layers and their separation. The exfoliated dispersions are stable in a variety of previously inaccessible solvents (acetonitrile, acetone, ethanol), allowing for facile formulation and ink development. Exploiting edge-site reactivity of TMDs is a new methodology for the preparation of high quality, exfoliated TMDs in high yields, and will enable exploration of surface functionalization for sensing and catalysis as well as nano-heterostructure production of TMDs beyond MoS₂.

Van der Waals layered structures, notably the transitional metal dichalcogenides (TMDs) and TMD-based heterostructures, have recently attracted immense interest due to their unique physical properties and potential applications in electronics, optoelectronics, and energy harvesting. Despite the recent progress, it is still a challenge to perform comprehensive characterizations of critical properties of these layered structures, including crystal structures, chemical dynamics, and interlayer coupling, using a single characterization platform. In this study, we successfully developed a multimodal nonlinear optical imaging method to characterize these critical properties of molybdenum disulfide (MoS₂) and MoS₂-based heterostructures. Our results demonstrate that MoS₂ layers exhibit strong four-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic generation (SHG) nonlinear optical characteristics. We believe this is the first observation of FWM and SFG from TMD layers. All three kinds of optical nonlinearities are sensitive to layer numbers, crystal orientation, and interlayer coupling. The combined and simultaneous SHG/SFG-FWM imaging is not only capable of rapid evaluation of crystal quality and precise determination of odd-even layers but also provides in situ monitoring of the chemical dynamics of thermal oxidation in MoS₂ and interlayer coupling in MoS₂-graphene heterostructures. This method has the advantages of versatility, high fidelity, easy operation, and fast imaging, paving the way for comprehensive characterization of van der Waals layered structures for fundamental research and practical applications.

Two-dimensional semiconductors have a great potential for applications in field-effect transistors, lithium ion batteries, or photovoltaic devices. Especially the IV-VI semiconducting layered structure tin(II)sulfide (SnS) is very attractive due to its natural abundance and low toxicity. In combination with promising electrical properties like the narrow band gap of 1.3 eV, excellent hole mobility, and high absorption coefficient, this two-dimensional material is highly suitable for thin-film photovoltaic devices.

Here we show how two-dimensional SnS nanosheets with different morphologies can be synthesized. It is possible to modify the shape of nanosheets by using 1-octadecene as ligand which causes the development of rectangularly formed two-dimensional nanosheets instead of hexagonally formed SnS. Via a simple wet-chemical approach, including oleylamine and oleic acid in the presents of hexamethyldisilazane (HMDS), it is possible to get hexagonally shaped two-dimensional SnS nanosheets. The injection of 1-octadecene led to the formation of rectangularly shaped SnS nanosheets. X-ray diffraction measurements show that both, the hexagonally shaped nanosheets and the rectangularly shaped nanosheets, possess the same crystal structure. HRTEM and electron-diffraction analysis approve the determination of the growth directions of rectangularly and hexagonally formed nanosheets, respectively. Hence, we get the information in which directions the sheets are growing and the role of 1-octadecene as ligand. Furthermore, aliquots were taken during the reaction which were then analyzed to get an understanding of the growth mechanism of hexagonally shaped as well as rectangularly shaped SnS nanosheet.

Since HMDS is essential for the formation of two-dimensional SnS nanosheets, we propose the mechanistic function of HMDS for the first time.

Two-dimensional (2D) layered materials have been widely explored recently due to the advantageous electronic, optical, and mechanical properties which are revealed when they are exfoliated from their bulk structures. One family of 2D materials, transition metal carbides, nitrides and carbonitrides termed “MXenes”, are synthesized by selective extraction of ‘A’ elements, such as aluminum, from their MAX phase precursors. [1, 2] MXenes exhibit many valuable properties, such as high mechanical strength, hydrophilic nanosheet surfaces, and high electronic conductivities. Moreover, MXenes can be delaminated into nanometer-thick sheets, enabling further solution-processing to fabricate films, electrodes, inks, and more. [3] Titanium carbide MXene (Ti₃C₂T_x), one of the most studied MXenes, has been primarily dispersed in aqueous media, which despite abundant benefits, limits the material from expanding into many functional applications such as mixing with other nanomaterials or polymers to form composites, formulation of inks for additive manufacturing, and fabrication of micro-electronic devices, where organic solvents may be essential.

We report on the first study of the dispersion of Ti₃C₂T_x nanosheets in organic solvents. Stable MXene dispersions were achieved with polar organic solvents such as N, N dimethylformamide, N-methyl-2-pyrrolidone, propylene carbonate, and ethanol. The stability of the nanosheets dispersions was monitored over time by digital photographs, concentration, and particle size measurements. Additionally, solvent parameters, such as surface tension, boiling point, viscosity, as well as Hansen and Hildebrand solubility parameters were correlated with Ti₃C₂T_x dispersion stability, providing information that could be useful for formulating solutions of this and other MXenes in solvents or solvent mixtures. This new understanding of how Ti₃C₂T_x behaves in organic solvents provides critical information necessary to facilitate the production and use of Ti₃C₂T_x in ways not explored previously, opening many avenues for future investigation.


References:

1. Naguib, M., et al. Advanced Materials 2011, 23, (37), 4248-53.
2. Naguib, M., et al. Advanced Materials 2014, 26, (7), 992-1005.
3. Mashtalir, O., et al. Nature Communications 2013, 4, 1716.

In recent years, layered transition metal dichalcogenides (TMDs) have attracted a great deal of interest due do their unique properties such as ease of energy band engineering by creating new Van der Waals heterostructures. Such exciting properties of TMDs make them promising for a wide range of electronic and optoelectronic applications. However, the transformation of basic science studies into viable device technologies necessitates the large-area synthesis of device-quality TMDs. Therefore, there has been an enormous research activity to enable large-area synthesis of various TMDs, including molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), etc. Despite the large body of work on synthesis of MoS₂, little has been done to study the large-area growth of WS₂ and its electronic properties.
Here, we report reproducible synthesis of large monolayer WS₂ flakes in excess of 200μm using chemical vapor deposition (CVD). The CVD growth of WS₂ was performed using solid precursors (i.e. WO₃ and sulfur powder) in nitrogen ambient. In addition to the monolayer WS₂, we demonstrate the synthesis of multi-layer WS₂ flakes, achievable mostly by controlling the growth time. We have thoroughly examined the structural properties of the WS₂ flakes using photoluminescence (PL), Raman spectroscopy, and atomic force microscopy (AFM). Interestingly, our PL and Raman mapping results reveal the homogeneity of the monolayer WS₂ flakes, which is conceivably attributed to the small mismatch in the coefficient of thermal expansions (CTE) of WS₂ and Si. Additionally, we have fabricated a large number of back-gated and top-gated WS₂ field-effect transistors (FETs) to gain insight into the key electronic properties of the CVD WS₂ films. For top-gated devices, we developed an atomic layer deposited HfO₂ film as the gate dielectric.
Temperature dependent studies were carried out on WS₂ FETs to extract the field-effect mobility (μ_FE) and the Schottky barrier height at the metal/semiconductor interface. To eliminate the effect of large parasitic series resistance at the contacts on the mobility calculations, the intrinsic channel conductance was measured using four-point gated devices. The μ_FE of WS₂ was deduced to be ~48cm²/V.s at room temperature, which gradually increased to ~300cm²/V.s at 50°K. In particular, μ_FE was found to follow a power-law temperature dependence (µT^-γ) with γ ~1.5, suggesting the dominance of phonon scattering in these devices. Capacitance-voltage (C-V) measurements were also performed to probe the HfO₂/WS₂ interface properties. The C-V measurements reveal a high density of interface traps, which consequently results in the significant charge trapping in the channel and the underestimation of the charge mobility.
In summary, we have successfully demonstrated the CVD growth of high-quality single and multi-layer WS₂ flakes on SiO₂/Si substrates. To our knowledge, our monolayer CVD WS₂ has the highest mobility reported to-date in the literature.

Transition metal dichalcogenide (TMD) monolayers are atomically thin and chemically stable films that present optical and electrical properties different from their bulk counterpart. There is an increasing interest in these materials [particularly MX₂ (where M = Mo, W and X = S, Se)] due to their potential optoelectronic applications. Much advances have been done in understanding the physical properties of these 2D materials. However, the production of high quality large area TMD films in a reliable and controlled way is still a technological challenge. Very few groups have reported high-quality monolayer TMD films deposited over large areas. The most common and simple approach uses a chemical vapor deposition (CVD) reactor were the precursors are evaporated from solid sources and deposited on a substrate at high temperature, this method does not allow a high degree of flexibility to tune the growth parameters. In this work, we propose a modified CVD process were the solid precursors are placed in separated inner tubes avoiding the source cross-contamination and depletion. The setup allows independent control of the temperature of each precursor and the substrate, as well as the type of gas carrier, flow and pressure. We studied the influence of the different growth parameters on the thickness and crystal quality of the films. The samples were characterized using a combination of SEM, TEM, Raman spectroscopy, photoluminescence, and atomic force microscopy in order to determine the number of layers; and the homogeneity and crystalline quality of the films. By tuning the growth parameters different kinds of structures were observed, including the formation of suspended networks of nano-filaments interconnected by few-layer sheets of TMDs, different island morphologies, as well as continuous large-area TMD films. The proposed method also allows the combination of different metallic sources, in order to synthesize 2D alloys and doping (simultaneous evaporation) or heterostructures (sequential evaporation).

Modulations of electrons, phonons, and spins render symmetry breaking in strongly correlated materials. From a delicate competing between the degrees of freedom arise unconventional macroscopic phenomena such as high-T_C superconductivity and colossal magnetoresistance. We choose charge density wave (CDW) to be the model system and study structural symmetry breaking induced by electron modulation, as a step toward better understanding of the intricate quantum phenomena. We take full advantage of state-of-the-art analytical TEM techniques that allow in-depth study of local structural asymmetry and thickness measurement, to probe CDW in few-layer 1T-TiSe₂.

Analytical TEM has been used to perform full-scale characterization of few-layer 1T-TiSe₂, a quasi-two-dimensional material that belongs to the strongly correlated system. At cryogenic temperature, we demonstrate the commensurate 2×2×2 superlattice using electron diffraction to confirm the CDW phase transition, and study the periodic lattice distortion (PLD) unit-cell-by-unit-cell. The distribution of structural modulation can be traced in the real space by raster scanning the convergent electron beam, which shows domain patterns, suggesting phase separation. Electron energy-loss spectroscopy performed at cryogenic temperature will be discussed to understand how electronic structures are affected by structural modulations.

Two-dimensional (2D) heterostructure devices are excellent candidates for flexible and transparent next-generation photodetectors. An ideal device structure enables both a fast photoresponse time and high quantum efficiency. The geometrical factors and materials properties that dictate the time-dependent photoresponse of vertical graphene/carbon nanotube (CNT) film/multilayer MoS₂/Ti-Au heterojunction photovoltaic devices are identified by spatially mapping the spectral and temporal dependencies of the photocurrent. Devices consisted of an ultrathin semiconducting single walled CNT thin-film that forms a vertical p-n junction with MoS₂ exhibiting external quantum efficiencies (EQE) of up to 2% at zero bias. The origin of the gate-dependent photoresponse is explained by considering the graphene/CNT junction as metal semiconductor junction with a tunable Schottky barrier. The photocurrent was measured with a nanosecond time resolution by exciting devices with a pulsed laser diode and recording the amplified photocurrent as a function of time with a digital sampling oscilloscope. The photoresponse time of 6 ns, which is one of the fastest reported in 2D photodetectors, is attributed to the short recombination lifetime of MoS₂ and the fast transport times. The recombination lifetime (4 ns) is slower than the electron transit sweep time (<0.9 ns) and therefore limits the photoresponse time. Intrinsic and extrinsic factors that determine the gain and photoresponse time, including the geometry and applied electric fields, were explored. The fall time decreases under an applied positive drain bias, indicating that photocarriers escape the active region more quickly as the electric field across the MoS₂ layer increases. This understanding provides the principles necessary to improve the EQE while maintaining the fast photoresponse time, which could provide performance advantages relative to existing device technologies.

Monolayer molybdenum disulfide (MoS₂) has emerged as an excellent 2D model system because of its two inequivalent, direct-gap valleys that lead to exotic bound and excited states. Here we focus on one such bound state, the negatively charged trion. Unlike excitons, trions can radiatively decay with non-zero momentum by kicking out an electron, resulting in an asymmetric trion photoluminescence (PL) peak with a long low-energy tail. As a consequence, the peak position does not correspond to the zero momentum trion energy. By including the trion's long tail in our analysis we are able to accurately separate the exciton from the trion contributions to the PL spectra. According to theory, the asymmetric energy tail has both a size-dependent and a temperature-dependent contribution. Analysis of the temperature-dependent data reveals the effective trion size, consistent with literature, and the temperature dependence of the band gap and spin-orbit splitting of the valence band. Finally, we observe signatures of Pauli-blocking of the trion decay.

The valley degree of freedom offers another way to tune carrier transport within 2D transition metal dichalcogenides (TMDCs) such as MX₂ (where for e.g. M = Mo, W, X = S, Se) apart from charge and spin degrees of freedom. In particular, the breaking of spatial inversion symmetry in such non-centrosymmetric 2D crystals whilst maintaining time-reversal symmetry offers a ready test-bed for exploring the physics of spin-valley coupling and new device concepts in valleytronics [1]. Further, in bilayer systems the additional coupling to layer polarization offers yet another dimension of control for novel device concepts [1]. The opportunity to exploit such valley or layer pseudospin has therefore triggered a flurry of recent research. Here we will share recent developments in our group toward the detection and tuning of valley polarization in 2D TMDCs, including band structure studies using a newly installed angle-resolved photoemission spectroscopy system and customized micro-photoluminescence system (with circular dichroism capabilities). The correlation between the experimental results and the theoretical modelling by ab initio band structure calculations will also be discussed.
[1] X. Xu, W. Yao, D. Xiao, and T. F. Heinz, Nature Physics 10, 343 (2014).

The bandgap of two-dimensional MoS2 can be modulated by changing layer numbers or introducing strain. As a result, MoS2 with different layers show different optical and electrical properties, which makes it a promising material in the field of optoelectronic devices, logical devices, detectors, and etc. However, the further study and application of MoS2 with different layers have been blocked by the slow progress in the synthesis side, especially for the layer-controlled growth through the widely-used CVD method. Here, we propose a one-step method to grow large area layer-controlled MoS2 film. We develop a new strategy to form a homogeneous layer of precursor for MoS2, which is key to the high quality CVD synthesis. The large area uniform MoS2 film has been obtained in controlled layer number without residues. Optical microscope images, Raman mapping and photoluminescence mapping results show that monolayer, bilayer and a few layer large area uniform MoS2 film can be synthesized by using our method. Our method may pave a way for the large-area growth of other 2D materials or for in situ form of van der Waals heterostructures in a better controlled way.

Recently, the transition metal dichalcogenides (TMDCs) have caught interest as two dimensional (2D) layered materials like graphene. In general, TMDCs are the layered materials with strong in-plane bonding and weak out-of-plane interactions and have a stoichiometric MX₂ bond which M is a transition metal element and X is a chalcogen atom. These materials exhibits a layered structure, X-M-X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms. Among various TMDCs materials, tungsten disulfide(WS₂) is one of the 2D materials that can compete with other TMDCs because of its interesting optical and electronic properties. WS₂ is a semiconductor with direct band gap in the visible spectrum (1.4 eV for monolayer), and it shows a strong spin-orbit coupling and high carrier mobility. These characteristics of WS₂ can be considered as a good candidate for next-generation nano-electronic devices.
In this study, we investigated the synthesis of WS₂ thin film by H₂S annealing with WNC thin film. WNC thin film was deposited on SiO₂ substrates by remote plasma atomic layer deposition (RPALD) using (MeCp)W(CO)₂(NO) as a tungsten precursor and N₂ plasmas as a reactant. We carried out sulfur annealing process at various conditions to synthesize WS₂ thin film. In order to analyze the characteristics of WS₂ thin films, X-ray diffraction (XRD), Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS), and Transmission Electron Microscope (TEM) were performed. When we conducted the H₂S annealing depending on temperatures and times, the XRD and Raman spectra were examined to observed the phase formation of WS₂. And we examined the crystallinity of WS₂ as the annealing temperature increased. XPS analysis was performed to examine the chemical binding of thin films and the changes of tungsten chemical bonding states. TEM images were also taken to explore the layered structure of WS₂ film.

2-dimensional transition metal dichalcogenide (TMDC) materials such as MoS₂ have attracted much research interest for future electronics and optoelectronics due to their unique properties. In this presentation, we report, for monolayer MoS₂, one can selectively deform the part of monolayer by laser irradiation. We argue the difference in the thermal expansion between the substrate and MoS₂ can result in the mechanical deformation. In addition, we bring attention to the periodic modulation of the structure generated in the monolayer by the laser irradiation. The strain engineering might provide a useful tool with enhancing the performance of electronic and optoelectronic devices.

Among 2-dimensional (2D) materials beyond graphene, monolayer (1L) transition metal dichalcogenides (TMDs) have 1-2 eV direct band gaps, with applications in nanoelectronics and optoelectronics. While MoS₂ is the most studied and easiest TMD to synthesize,¹ recent results suggest that 1L diselenides (MoSe₂ and WSe₂), have higher mobility^2,3 and lower band gap, being potentially more desirable in practical applications. However, all reported TMD selenides grown by chemical vapor deposition (CVD) on SiO₂ to date either produce large but sparse crystals or continuous films with small nanocrystalline grains.

In this work, we use a seeding layer to synthesize 1L high-density diselenide crystals up to 100 μm in size for the first time. We demonstrate growth directly on SiO₂ as well as other dielectrics such as hafnia and alumina. Such direct, uniform growth on the most important transistor gate dielectrics suggests that these films can be made continuous with large grains and immediately made into electronic devices without the need for layer transfers.

We present a spectroscopic study of our MoSe₂ and WSe₂ 1L crystals grown on SiO₂ by CVD. The TMDs are grown at 760 Torr in an Ar atmosphere, using solid Se and MoO₃ or WO₃ as precursors, H₂ gas as a reducing agent, and perylene-3,4,9,10 tetracarboxylic acid tetrapotassium salt (PTAS) as the seeding molecule. After synthesis, Raman and photoluminescence (PL) spectra are taken for several crystals to observe variation in phonon energy and optical band gap energy, both as a function of crystal size and distance from the solid precursor to confirm that the optical gap and phonons have energies in accordance with those expected for these materials.^4,5

On average, we observe MoSe₂ to photoluminesce at 1.52 eV and to exhibit E'', A₁', E', and A'' modes near 171, 240, 285, and 356 cm^-1 respectively. WSe₂, by contrast, photoluminesces at 1.61 eV and has nearly degenerate A₁' and E' modes at 249 cm^-1. It is likely that the PL peaks of our as-grown 1L TMDs are redshifted compared to exfoliated samples due to the tensile strain in the film after cooling to room temperature, post-synthesis. In addition to the first-order Raman peaks, we also observe bands of higher-order Raman modes in MoSe₂, including the LA(M) mode near 144 cm^-1, and many of its overtones and multi-phonon peaks in bands from 258 to 302 and 427 to 449 cm^-1. This work represents the first step in synthesizing and characterizing large-grain 2D diselenides for large-area electronic and optical applications.

1 Y.-H. Lee, et al, Adv. Mater. 24, 2320 (2012).
2 X. Wang, et al., ACS Nano 8, 5125 (2014).
3 J.-K. Huang, et al., ACS Nano 8, 923 (2014).
4 P. Soubelet, et al., Phys. Rev. B 93, 155407 (2016).
5 P. Tonndorf, et al., Opt. Express 21, 4908 (2013).

Recently, various monolayer transition metal dichalcogenides (TMDs) have been gaining popularity for their diverse properties. At the same time, in literature conflicting measurements are reported for the step height of various 2D materials [1]. The step height is the height from the substrate to first layer of the TMD and is not necessarily equal to the interlayer spacing of the bulk material. Atomic force microscopy (AFM) is ubiquitously used to perform step height measurements in TMD with a range of results.
There are two physical variables with tapping mode AFM that can be adjusted to scan a sample: the oscillation amplitude and the force. The amplitude determines the distance the tip travels from the highest to lowest point and the force is related to the distance between the tip and the surface, which can also be expressed as a percentage of the oscillation amplitude. Through appropriate adjustment of the driving voltage and the set point (which controls the force), we have demonstrated that the AFM can be properly tuned to provide accurate step height measurements. By adjusting both parameters, it has been seen that the step height is refined to a condensed range more precise than previous literature ranges.
In this experiment, the Nanonics Imaging model MV-2000 atomic force microscope was used to scan the step heights, while the MV-1000 was used for force measurements to convert the set point voltage to a force. The MV-2000 used a glass fiber tip, while the MV-1000 used a silicon tip. We tested chemical vapor deposition (CVD)-grown MoS₂, WS₂, and WSe₂, as well as exfoliated MoS₂ and WS₂.
While keeping one parameter constant, the TMDs were scanned with a selective range of set points and oscillation amplitudes. TMD step height measurements were performed from substrate to material and from material to the same material. Step heights were determined using the WSxM software by the histogram method. Low oscillation amplitude while maintaining force resulted in large step height measurements above 10 nanometers which decreased approximately logarithmically as the amplitude increased. The step height to which this graph approached was determined to be the valid step height for a monolayer. Several repetitions produced errors bars on this value smaller than the range reported across a wide literature survey.
Our tuning of the two physical parameters of oscillation amplitude and force for tapping mode AFM has clarified the uncertainty of the step height measurements of TMDs and provides guidance for researchers to obtain valid step measurements for 2D materials.

[1] Shearer, C. J., Slattery, A. D., Stapleton, A. J., Shapter, J. G. & Gibson, C. T. Accurate thickness measurement of graphene. Nanotechnology 27, 125704 (2016).

Fabrication of two-dimensional (2D) heterostructure of transition metal dichalcogenides (TMDs) promises a range of opportunities to broaden the physical properties of 2D semicondcutor materials. Especially, the lateral heterostructures where two different atomic monolayers are connected in-plane could exhibit tunable optoelectronic properties. The formation of TMDs lateral heterojunction has been demonstated recently via chemical vapor deposition (CVD) method. Nevertheless, solution-based synthesis of such 2D heterostructures has remained challenging. Here we report the colloidal synthesis of stable heterostructure of tungsten/molybdenum diselenide (WSe₂/MoSe₂) by controlling the delivery of precursors during the crystal growth. 2D heterostructures are grown via a two-step process: (1) we introduce W precursor (W(CO)₆, 0.1 mmol) into 5 mL of oleic acid and heat to 350 ^oC under Ar condition. Then, Se precursor (Ph₂Se₂, 0.2 mmol) dissolved in 2 mL of oleic acid is slowly injected and heated for 4 hr to continue the growth of WSe₂ nanosheets, then naturally cooled down to room temperature. (2) Mo precursor (Mo(CO)₆, 0.1 mmol) dissolved in oleic acid is sebsequently injected into the solution and heated to 370 ^oC, followed by the injection of 0.2 mmol of Se precursor in oleic acid, and MoSe₂ growth is continued for 2 hrs. We confirm the growth morphology of the heterostructure, which confirmed by using SEM, TEM, EDX, and Raman spectroscopy, is strongly dependent on the temperature ramping rate and the Se precursor injection. The effect of the reaction medium other than oleic acid on the heterostructure morphology (i.e., vertical vs lateral) will be discussed. We also propose a simple mechanism showing the favorable nucleation sites for the heterostructure formation via interaction between precursors and oleic acid. Our results suggest new methods to create TMDs heterostructures and highlight the importance of ligand chemistry governing the colloidal synthesis of semiconducting TMDs structures.

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied as a counterpart to graphene, as semiconducting 2D materials with direct bandgaps. However, its reliable fabrication via chemical vapor deposition (CVD) for practical applications has proven challenging. There have been extensive efforts towards increasing the monolayer domain size via control of growth parameters such as flow rates, growth pressures, growth temperatures, and ramp rates. [1, 2] Although effects of pre-treatment and plasma enhancement have been extensively studied on the catalyst/substrate used in fabricating nanomaterials via CVD such as carbon nanotubes [3] and graphene [4], pre-treatments for CVD-grown TMDs and their resulting effects remain largely unexplored.
In this work, we investigated the effect of various plasma (e.g., oxygen, argon, CF₃, and SF₆), wet etching (e.g., KOH), and organic solvent (e.g., IPA, acetone) pretreatments on either the growth substrate or the source substrate. MoO₃ or WO₃ was deposited onto SiO₂ substrates via physical vapor deposition (PVD) as the Mo or W sources, respectively; pretreatment of this “source substrate” was performed with organic solvents by varying the solvent concentration, soaking duration, and drying method before initiating growth. Following its pretreatment, the source substrate was placed in direct contact (face-to-face) with a growth substrate that had likewise undergone pretreatment, but with plasma or wet etching. Located upstream was a crucible containing S (or Se) powder that sublimates into gaseous form as the tube is heated, supplying the S or Se that combines with the Mo or W from the source substrate, and thus MoS₂, MoSe₂, WS₂ or WSe₂ is deposited atop the growth substrate. Both the etching and solvent soaking pretreatment parameters were optimized by analyzing the growth results for monolayer domain size, surface coverage, and to reduce the amount of bilayer and multilayer defects.
The pretreated growth substrates were characterized using atomic force microscope (AFM), contact angle based surface energy measurements, and x-ray photoelectron spectroscopy (XPS) to determine the changes in surface energy and the corresponding contributions associated with silanol group density and surface roughness. Similarly, the solvent treated source substrates were also characterized to determine the presence of compound residues post-evaporation, and thus, elucidating the influences of substrate pretreatments on growth of TMD monolayers.
[1] Cong, C. et al. Adv. Opt. Mater. 2, 131–136 (2014).
[2] Zhang, Y. et al. ACS Nano 7, 8963–8971 (2013).
[3] Cantoro, M. et al. Diamond & Related Materials 15, 1029-1035 (2006).
[4] Jia, C. et al. Scientific Reports 2, 707 (2012).

Heterostructures made by two-dimensional (2D) materials can have distinct optical and electrical properties for advanced electronics and optoelectronics [1]. Transition metal dichalcogenides (TMDs) are semiconductors with direct band gaps, when they are made into atomically thin monolayers. Chemical vapor deposition (CVD) growth may enable large-scale production of these heterostructures in the future [2, 3].
In this work, we demonstrated the epitaxial growth of disparate vertical heterostructures of TMDs by direct growth of one TMD monolayer on top of a previously grown TMD via CVD. We have achieved six different combinations of bilayer vertical heterostructures composed of either WS2, WSe2, or MoS2. The growth of the first layer of TMD was carried out at 800 to 850 C , while the second layer of TMD was grown at a lower temperature around 650 to 750 C. The higher temperature is to achieve a larger coverage for first layer TMDs, and the lower temperature is to protect the first grown layer during the growth of the second layer. The two important points in CVD growth of TMDs are the maximum temperature and time to supply sulfur or selenium vapor. The maximum temperature can be controlled by the programmable CVD, while the time to supply vapor can be controlled by the distance of the crucible from the heat source inside the CVD vacuum tube. Furthermore, we established a function of sulfur powder melting time related to the environment temperature and air humidity to predict a more accurate distance according to the environmental conditions.
Although the bilayer TMD heterostructures can also be made by stacking different 2D materials through layer transfer (polymer stamping), the epitaxial growth of heterostructures uniquely enables in-phase growth (i.e., aligned crystal orientation) and a cleaner interface between layers. With cleaner and repeatable results, the CVD growth of heterostructures has the potential for large-scale production for future industrial applications.

1. Ge, M., Mei, G., Xiaoyan, G. & Junfeng, Z. Transport properties of in-plane MoS2 heterostructures from lateral and vertical directions. Journal of Atomic and Molecular Sciences 7, 33–41 (2016).
2. Gong, Y. et al. Vertical and in-plane heterostructures from WS2/MoS2 monolayers. Nat.Mater. 13, 1135–1142 (2014).
3. Gong, Y. et al. Two-Step Growth of Two-Dimensional WSe2/MoSe2 Heterostructures.Nano Lett. 15, 6135–6141 (2015).

Two-dimensional molybdenum disulfide (MoS₂) has attracted great attention owing to its superior device performances. Various fabrication techniques of MoS₂ thin film are previously introduced, and especially chemical vapor deposition (CVD) can produce high-quality thin film. CVD is mainly classified into two types. The first is regarded as a two-step CVD, in which metal Mo precursor is initially deposited by e-beam evaporation and then sulfurized into MoS₂. The second is regarded as a one-step CVD, wherein gaseous Mo and S precursors are simultaneously introduced and react to form MoS₂. Among them, two-step CVD is suitable for fabrication of large area uniform thin film. In general, elemental sulfur powder is mainly used for sulfurization; however, its low vapor pressure at a low temperature region largely limit the reaction rate. Though hydrogen sulfide is also widely used for two-step CVD, its high toxicity becomes an obstacle. Contrastingly, di-tertiary-butyl disulfide [(t-C₄H₉)₂S₂], which is a much safer organic precursor without toxicity and combustibility, has a sufficiently high vapor pressure at low temperature (2.7 kPa at 84°C). In this study, we fabricated MoS₂ thin films by two-step CVD using (t-C₄H₉)₂S₂ and investigated the effects of temperature and gas flow rate on the formation. Moreover, the decomposition reaction of (t-C₄H₉)₂S₂ was examined by density functional theory (DFT).
Metal Mo thin films were deposited on SiO₂/Si substrates by e-beam evaporation and sulfurized using (t-C₄H₉)₂S₂ at 200-440°C to form MoS₂. The flow rate of gas phase (t-C₄H₉)₂S₂ was controlled by bubbling temperature and set to 7.05E-6 and 1.41E-5 mol/min. The MoS₂ reaction rate was investigated by S/Mo ratio, which was calculated by Mo 3d and S 2p XPS peak area ratio. DFT simulation with B3LYP/3-21G was performed using PC GAMESS/Firefly [1, 2] in order to calculate the enthalpy change for a decomposition. As a result, the decomposition temperature of (t-C₄H₉)₂S₂ is estimated approximately 300°C and S/Mo ratio approached to ideal stoichiometric composition (S/Mo=2.0) with increase in the sulfurization temperature and the gas flow rate, indicating that high-speed film formation can be achieved by optimization of the parameters. From the DFT calculation, the optimum decomposition reaction of (t-C₄H₉)₂S₂ in H₂ atmosphere was determined as (t-C₄H₉)₂S₂+H₂→2H₂S+2C₄H₈, suggesting suppression of carbon contamination and unintentional elemental sulfur deposition.
[1] A. A. Granovsky, Firefly version 8, www http://classic.chem.msu.su/gran/firefly/index.html
[2] M. W. Schmidt, et al., J. Comput. Chem. 14, 1347-1363 (1993).

Sustainable generation and storage of energy is arguably the biggest challenge facing society. Investment into energy research is considerable (e.g. ~€2.5billion in EU FP7). One possible solution is hydrogen, due to its high energy density compared to other means of energy storage such as batteries. However, there are many problems associated with hydrogen, both in production and storage, which need to be overcome before it can be widely used. Much of this research is into the catalysts and storage materials that play an important part. Metal carbides and nitrides which, traditionally, are desirable for their mechanical properties are now being investigated for these materials.

Catalysis composition and structure are usually one of the main problems with heterogeneous catalysts frequently requiring two or more components. In catalysts based on metals such as Au or Pt, this involves straightforward chemistry: there are a plethora of methods for depositing metal nanoparticles on supports. For more advanced materials, such as combinations of two different ceramics, the controlled synthesis of nanocomposites (e.g. oxide+carbide) is more challenging. Historically, the synthesis of ceramic composites is achieved simply by mechanical mixing limiting surface area.

Recently, our group has developed a series of one-pot routes to ceramic nanocomposites. These methods use biopolymers to preorganize metal cations into strong aqueous gels. On heating, the polymer network controls the nucleation and growth of nanoparticles. Simultaneous formation of two phases (e.g MgO/Fe₃C) can be achieved, through selective carbothermal reduction. These materials and their derivatives have shown high activity in important catalytic processes such as the oxygen reduction reaction and methanol steam reforming. Based on these promising results our ambitious goal is a general tunable route to create nanocomposites of any two components, including metal oxides, carbides, nitrides and metals. In order to achieve this, we are studying all aspects of the biopolymer nanocomposite synthesis, such as the mechanism of biopolymer control of nanoparticle growth using SANS and the simultaneous formation of multiple crystalline phases using in situ synchrotron XRD. Furthermore, we have selected several target systems to study in more detail, specifically to test whether our optimized synthesis methods could make a significant impact on catalytic activity.

Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) have been actively explored in recent years on account of the wide range of potential applications by virtue of various active sites such as edge sites, sulfur defects, and vacancies. More recently, it has been reported that vertically aligned MoS₂ with edge-rich structure enables highly enhanced gas adsorption properties toward NO₂, suggesting that edge sites of TMD materials might lead to considerable improvement in gas sensing performance. In addition, carbon nanofibers are known to be able to detect NO₂ gas molecules working at room temperature. In this regard, it could be an efficient strategy to combine carbon nanofibers and active sites of mono-layered TMDs for highly enhanced gas sensors to sense NO₂ particularly at room temperature. Herein, we propose multitubular carbon nanofibers functionalized with edge sites of mono-layered WS₂ nanoflakes concentrated on the surface (WS₂@MTCNFs) via copolymer-electrospinning.

For the copolymer-electrospinning, polyacrylonitrile (PAN) and (NH₄)₂WS₄ were blended together before styrene-acrylonitrile (SAN) was added into the solution, which leads to microstructure splitting inside the solution into two regions of PAN and SAN. The two-step heat treatment was carried out to complete the crystallization of WS₂ nanoflakes and the carbonization of the PAN while the SAN burns out, forming multiple tubular pores inside of carbon nanofibers. Mono-layered nanoflakes in carbon nanofibers can be obtained by c-axis confined growth of WS₂ in the carbon matrix, resulting in no stacking of WS₂ single layers. For comparison, carbon nanofiber and WS₂ embedded carbon nanofibers were synthesized for sensing test toward NO₂ gas.

This newly proposed design of the WS₂@MTCNFs exhibits improved response (15% at 1 ppm of NO₂) as compared to that of pure CNFs (2% at 1 ppm of NO₂) at room temperature. Moreover, selectivity of WS₂@MTCNFs toward NO₂ was highly enhanced by adding edge sites of mono-layered WS₂ in the surface. With respect to recovery characteristic, WS₂@MTCNFs show much enhanced recovery properties, particularly in the low concentration range (200 ppb–1 ppm). Limit of detection (LoD) was exponentially plotted and obtained as 0.29% at 10 ppb. The superior sensing property can be ascribed to edge sites of mono-layered WS₂ on the surface and high surface area by nano-structuring of carbon nanofibers via copolymer-electrospinning.

Photodetectors based on two-dimensional materials, which provide scalable active interfaces, high performance in photoresponse, high flexibility, etc., have aroused huge attention. Particularly, graphene hybrid phototransistors, which are composed of a graphene sheet as carrier transport channel and an efficient light absorption material as photo-active layer, highly likely give rise to their giant photoconductive gains. However, their response speeds are dramatically compromised at the expense due to the large number of trap states at the interfaces.
Herein, by intercalating a large-area atomically thin MoS₂ film into a graphene/silicon vertical junction, we have developed a prototype tunneling phototransistor, which exhibits a record-fast response (within 30 ns) and a high responsivity (~3×10⁴ A/W at 635 nm illumination) across visible to the near infrared spectral range (400 nm-1100 nm). The bulk silicon serves as an optical active layer and the MoS₂ film acts as a passivation layer for reducing surface states and offering an ultra-thin layer for the carrier tunneling. Photo-excited carrier transfer process driven by the ultra-fast quantum tunneling effect rather than by the carrier drift in the depletion region enables a superior response speed, while the responsivity retains high. The carrier tunneling process is systematically investigated in terms of its dependences on the built-in field, the tunneling layer thickness and temperature. This intriguing tunneling phototransistor with both high speed and high responsivity, as well as with large photo active area, offers significant potential in practical applications among existing integrated optoelectronic devices.

The growth of single layer tin (stanene) on a Bi₂Te₃ substrate has engendered a great deal of interest, in part due to stanene’s predicted exotic properties. In particular, stanene has attracted lot of attention owing to its tremendous promise in topological insulation, large-gap 2D quantum spin hall states, lossless electrical conduction, enhanced thermoelectricity, and topological superconductivity. Most of the previous work on stanene has focused on its electronic properties. Atomistic investigations of growth mechanisms (needed to guide synthesis), phonon transport (crucial for designing thermoelectrics), and thermo-mechanical behavior of stanene are scarce. This paucity is primarily due to the lack of inter-atomic potentials that can accurately capture atomic interactions in stanene. To address this, we have developed a bond-order potential (BOP) based on Tersoff’s formalism that can accurately capture bond breaking/formation events, structure, energetics, thermodynamics, phonon frequencies, thermal conductivity, and mechanical properties of single layer tin. We determine the BOP parameters by fitting to a training dataset containing (a) structure, (b) equation of state (energy vs area), (c) elastic constants, and (d) phonon dispersion of stanene obtained from our density functional theory calculations. To optimize this potential, we employed a hybrid global optimization scheme based on genetic algorithms and Nelder-Mead simplex. Finally, we employed our newly developed BOP to study anisotropy in thermal conductivity of stanene sheets, temperature induced rippling, as well as dependence of anharmonicity and thermal conductivity on temperature.

We have studied the Raman spectra of few-layer MoTe₂ in both ultralow-frequency and high-frequency regions. In the ultralow-frequency region, the frequencies of C and LB modes agree well with the prediction based on the LCM in which only nearest interlayer coupling is considered. The intensity of the lowest-frequency LB mode is much stronger than that of the C mode. This phenomenon is opposite to the reported results for few-layer MoS₂ and WSe₂. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe₂. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe₂ (NL MoTe₂; N is an integer). Under the resonant excitation condition, we observe N–dependent Davydov components in ML MoTe₂, originating from the Raman-active A^′₁ (A²_1g) modes at ～172 cm⁻¹. More than two Davydov components are observed in NL MoTe₂ for N >4 by Raman spectroscopy. The N-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency A^′₁ (A²_1g) modes. The different resonant profiles for the two Davydov components in 3L MoTe₂ indicate that proper excitation energy of ～1.8 − 2.2 eV must be chosen to observe the Davydov splitting in ML MoTe₂. Our work presents a simple way to identify layer number of ultrathin MoTe₂ flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe₂ and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

Graphene, an atomically thick layer of carbon atoms arranged in a hexagonal lattice, has attracted a lot of interest in basic and in applied physics. With the advent of h-BN crystals, it was possible to improve graphene device quality and uncover many interesting quantum effects of Dirac fermions. On the other hand, heterostructures prepared using other 2D materials do not lead to considerable improvements of graphene devices quality, but they allow the development of other non linear electronics elements. For instance, a silicon graphene heterojunction shows photovoltaic properties, but it is a challenging to improve its efficiency since it requires a control of the doping of the graphene. An solution for this issue is to use chemically modified graphene, but instability limits its application. Here we show that graphene devices prepared on top of Talc (a flat and clean 2D substrate) have a systematic, stable high p-type doping, reaching charge density larger than n = 1x10^-13 cm^-2_, and requiring no energy consumption. We present a combination of electrical measurements, Raman spectroscopy and DFT calculations to elucidate the origin of the doping induced in graphene. Even though our devices have atomically flat and clean surfaces, our data show universal conductance fluctuations at low temperatures and field effect mobility comparable to graphene/SiO₂ devices. Finally, we expect that the spontaneous and stable p-doped induced in graphene/Talc heterostrutures will be useful for improving optoelectronic devices, like photovoltaic solar cells and pn-junctions.

Acknowledgements: FAPEMIG, CAPES, CNPQ, INCT/Nanocarbono, Rede de Nano-Instrumentação and Pós-graduação em Física da UFMG.

In this presentation, the electrical and magnetic transport properties of magnetic tunneling junctions (MTJs) based on two-dimensional (2D) transition-metal dichalcogenide MX₂ (M=Mo, W; X=S, Se) monolayers are reported. In the MTJ devices, the MX₂ serves as the tunneling layer. The 2D MX₂ were grown using chemical vapor deposition. As-grown 2D MX₂ monolayers were transferred and fabricated into MTJ devices using lithography process. Ferromagnetic metals such as Co and permalloy were employed as top and bottom layers in the MTJ devices. Spin valve effect was clearly observed at low- and room-temperatures. Annealing effect on the MTJ device was studied. Effect of an antiferromagnetic layer such as IrMn coupling with one of the ferromagnetic layers in the MTJ devices was studied. This 2D MTJ study is likely to pave the way to further increase the density of non-volatile memories based on MTJs and spin-transfer torque devices.

The remarkable optical and electronic properties of WS₂ and MoS₂, two members of the layered transition metal dichalcogenide materials, continue to be the focus of a great deal of work in the 2D materials community. However, the speed of progress in this field is hampered by challenges in materials synthesis. In this talk, we will describe an improved chemical vapor transport route to growth of WS₂ and MoS₂ monolayers by using water vapor as a transport agent for WO₃ and MoO₃. The gas phase water concentration has a direct correlation to the resulting morphology, structure and optical properties and greatly improves the uniformity of grown materials.

To date, chemical vapor transport is the predominant synthetic method for the direct growth of monolayer WS₂ and MoS₂. Typical experiments consist of elemental sulfur and metal oxide powders loaded in a furnace along with a growth substrate, relying on the vapor pressure of each precursor species at elevated temperature to achieve growth. Reports of growth in the literature demonstrate varying results with similar conditions. Some of this is inherent to the simplicity of the growth technique, which belies the complexity in controlling the many factors which impact growth uniformity, quality, and reproducibility. However, as was reported by Miller and Neugebauer in 1949 [1], the vapor pressures of WO₃ and MoO₃ are very sensitive to the presence of water vapor – water reacts with these and many other metal oxides to form volatile metal hydroxide species [2]. As a result, the water vapor content and, therefore, volatility of transition metal oxide precursor plays an important and, as of yet, unexplored role.

To reduce variability in the transition metal oxide source, we used atomic layer deposition to define precise amounts of WO₃ and MoO₃ on thermal oxide coated silicon wafers. As expected, the volatility of these transition metal oxides can be effectively controlled by varying the water vapor concentration on the ppm scale. At low water vapor content (~ 0 ppm) metal oxide films are chalcogenized in place, showing little to no evidence of vapor transport, while higher water concentrations (> 200 ppm) result in minimal residual metal disulfide after growth. Under optimized conditions, highly luminescent, triangular monolayer WS₂ and MoS₂ islands with good island-to-island uniformity can be grown directly on a metal oxide coated substrate or onto a bare substrate placed downstream of a source. The use of controlled amounts of water vapor is a new knob by which to tune growth of these materials, and these results demonstrate a possible route to improved material quality and reproducibility of chemical vapor transport of W and Mo containing transition metal dichalcogenides.

[1] T. Millner and J. Neugebauer, Nature 163, 601 (1949).
[2] P.J. Meschter, E.J. Opila, and N.S. Jacobson, Annu. Rev. Mater. Res. 43, 559 (2013).

New materials and processes¹ are needed to improve the performance of existing devices or enable new ones,^1-6 which are also environmentally benign. In this context, graphene and other 2d crystals, thanks to their excellent and complementary properties, are emerging as promising materials.^1-6 In particular, the assembly of such 2D crystals (heterostructures) will provide a rich toolset for the creation of new, customised materials.^1,2 However, a key requirement for applications such as flexible (opto)electronics and energy storage and conversion is the development of industrial-scale, reliable, inexpensive production processes,² while providing a balance between ease of fabrication and final material quality with on-demand properties.
Liquid-phase exfoliation^2,4 is offering a simple and cost-effective pathway to fabricate various 2D crystal-based (opto)electronic and energy devices, presenting huge integration flexibility compared to conventional methods. Here, I will show our scaling up approach for the solution processing of 2D crystal based on wet-jet milling of layered materials. Moreover, I will present an overview of 2D crystals for flexible and printed (opto)electronic and energy applications, from the fabrication of large area electrodes³ to devices integration.^6-12
References
<>1.F. Bonaccorso, et al., Mater. Today 15, 564 (2012).
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A. L. Palma, et al., Nano Energy 22, 349 (2016).

Molybdenum disulphide (MoS₂) is emerging as one of the most promising 2D materials amongst the class of transition metal dichalcogenides. Its ultrathin layered structure and a direct band gap of 1.9 eV in the monolayer regime makes MoS₂ a very suitable candidate for a wide array of future (opto-) electronic applications. One of the major current challenges is to synthesize high quality MoS₂ with accurate thickness control over a large area. Atomic layer deposition (ALD) offers precise thickness control at the atomic level with excellent wafer scale uniformity. ALD therefore could be instrumental in realizing large area 2D MoS₂ with monolayer growth control and could therefore be a superior alternative to the most widely used synthesis techniques like exfoliation and chemical vapor deposition.
In this contribution, we use a combination of ALD and thermal sulphurization to synthesize 2D-MoS₂. In the first step, ultrathin films of MoO₃ were deposited by PE-ALD using halide-free chemistry on thermal SiO₂ substrates at 200 °C with a growth rate of 0.8 Å/cycle [1]. The TEM analysis revealed that the ALD grown MoO₃ films as thin as down to ~2 nm were closed and uniform in nature. The second step involved atmospheric pressure thermal sulphurization at 600°C - 700°C which transformed the ALD grown MoO₃ ultrathin films into mono- or few-layered MoS₂ films. There was a clear correlation between thickness of these resulting final MoS₂ thin films and initial thickness of ALD grown MoO₃ films which could be precisely controlled just by tuning the number of ALD cycles.
Raman spectroscopy, as performed on various thicknesses of MoS₂ films showed a clear variation in the frequency difference value (Δk) between the two vibrational modes observable at 408 cm^-1 and 382 cm^-1 for MoS₂ in bulk regime. The ‘Δk’ monotonically decreased down to 20.64 cm^-1 with decreasing layer thickness which corresponds to a monolayer. The photoluminescence spectroscopy results were in line with these results, showing a strong peak at ~1.9 eV corresponding to the direct band gap transition for the thinnest (few- to mono-layer) samples. Furthermore, XPS analysis showed the core level binding energy values for Mo3d and S2p identical to the atomically thin MoS₂ films reported in the literature. These results demonstrate the viability of our two-step approach based on thermal sulphurization of ALD grown metal oxide for the controllable, large area synthesis of 2D MoS₂ thin films.

References
1 Vos et al., J. Vac Sci Tech. A, 2016, 34, (1), pp. 01A103, DOI: 10.1116/1.4930161

Two-dimensional (2D) gallium telluride (GaTe) has attracted much attention recently, due to its extremely high responsivity, fast response time, and promising thermoelectric performance. Different from most commonly studied 2D materials, GaTe has in-plane anisotropy and a unique low symmetry C_2h³. Investigating the in-plane optical anisotropy, including electron-photon and electron-phonon interactions of GaTe is essential in realizing its application in optoelectronics and thermoelectrics. In this work, the anisotropic light-matter interaction in the low-symmetry material GaTe is studied using anisotropic optical absorption and Raman spectroscopies as probes. Our polarized optical absorption spectroscopy reveals the weak anisotropy for optical absorption of visible light in multilayer GaTe. Polarized Raman spectroscopy proves to be effective in probing the crystalline orientation of GaTe, uncovers the existence of anisotropy in the electron-phonon interaction for certain phonon modes, and shows the non-trivial dependence of Raman anisotropy on flake thickness, photon and phonon energies. Such non-trivial dependence of Raman anisotropy can be explained by our theoretical analyses employing first-principles calculations and group theory. These studies are a crucial step towards the applications of GaTe especially in optoelectronics and thermoelectrics, and provide a general methodology for the study of light-matter interaction anisotropy in GaTe and other layered materials with in-plane anisotropy.

Transition metal dichalcogenide (TMD) heterostructures based on the van der Waals stacking of dissimilar 2D TMD layers represent a new class of 2D materials which are envisioned to exhibit extraordinary materials properties unattainable from their individual counterparts. Efforts to grow the TMD van der Waals heterostructures have been presently limited by uncontrolled size, distribution, location, and number of grown 2D layers. In this talk, we present a wafer-scale (> few cm²) growth of TMD van der Waals heterostructures composed of vertically-stacked few-layer only 2D MoS_2/WS₂ by a modified chemical vapor deposition (CVD) sulfurization of metal seed layers ¹. The structural integrity of the heterostructures was studied by transmission electron microscopy (TEM) which revealed the atomically abrupt and seamless heterointerfaces of 2D MoS_2/WS₂ spanning over the entire growth substrate. The electrical quality of the 2D MoS_2/WS₂ heterojunctions was assessed by the vertical transport measurement across the individual 2D layers transferred onto a flexible insulating substrate. An excellent diode-like behavior was manifested by a high current rectification ratio of ~10³, further confirming the high quality of the heterostructures. We also demonstrate a wafer-scale transfer of 2D MoS₂/WS₂ heterostructures by directly growing them on transferable substrates with metal sacrificial layers which are easily separated in water. Our new growth approach is believed to enable novel 2D electronic/optoelectronic devices whose integration is compatible with existing wafer-scale manufacturing technologies.
¹N. Choudhary, Y. Jung et al., Sci. Rep. 6, 25456 (2016)

Photodetection is of paramount importance to extensive realms spanning imaging, sensing and communication. However, traditional materials confront severe challenges, notably low optical absorption, omnipresent surface dangling bonds and high cost. 2D van der Waals materials (vdWMs) represent lamellar structures featuring vdW interaction among molecular layers, inside which atoms are covalently bonded. Thus, they manifest dangling-bone-free surface and excellent mobility. In addition, vdWMs contain a variety of materials with abundant attributes spanning semimetals, semiconductors and insulators. Consequently, they serve as great platform for fundamental research and multifunctional photoelectrical applications. Moreover, novel physics such as indirect-direct bandgap transition and valley splitting, emerge when their thickness is downsized to atomic scale. Currently, there are mainly two approaches for their production, top-down exfoliation and bottom-up chemical vapor deposition. However, the former suffers inconstant size and shape, and the latter suffer high temperature and low efficiency.
To address these issues, we exploited pulsed laser deposition (PLD) to grow vdWMs, which exhibits unique superiority beyond above approaches. First, it is a precursor-free physical vapor deposition approach with little contamination. Additionally, thickness of products can be controlled by adjusting the pulse number. Furthermore, the pulsed laser can ionize the majority of materials, which restrains asynchronous evaporation, rendering perfect reproduction of target components onto substrate. In our works, various centimeter-scale layered vdWMs have been produced. The resulting WS₂ photoresistor demonstrates stable photoswitching and responsivity of 0.5 A/W.¹ Then, strategies were developed to further improve the performance. By leverage on the suppression of deep-level defect states via alloy engineering, the Mo_0.5W_0.5S₂ photodetector achieves a superior responsivity of 5.8 A/W.² In addition, taking advantage of the passivation effect of Bi₂Te₃, an excellent responsivity of 30.7 A/W for a WS₂/Bi₂Te₃ photodetector is demonstrated.³ On the other hand, benefit from the gapless surface state of Bi₂Te₃, the Bi₂Te₃/Si heterojunction responses to Terahertz illumination.⁴ By engineering the heterointerface with a selective blocking WS₂ layer, the figures-of-merit are substantially improved.⁵ In summary, these findings stress that the PLD-grown 2D vdWMs constitute a new paradigm for next-generation photodetection.

1. J. D. Yao, Z. Q. Zheng, J. M. Shao, G. W. Yang, Nanoscale, 2015, 7, 14974-14981.
2. J. Yao, Z. Zheng, G. W. Yang, ACS Appl. Mater. Interfaces, 2016, 8, 12915-12924.
3. J. Yao, Z. Zheng, G. W. Yang, J. Mater. Chem. C, 2016, under consideration.
4. J. Yao, J. Shao, Y. Wang, Z. Zhao, G. Yang, Nanoscale, 2015, 7, 12535-12541.
5. J. Yao, Z. Zheng, J. Shao, G. Yang, ACS Appl. Mater. Interfaces, 2015, 7, 26701-26708.

It has become possible in recent years to fabricate and manipulate two-dimensional nanomaterials in the laboratory that are as thin as one to few atomic layers. The reduced dimensionality gives rise to unique physical and chemical properties that differ from those of traditional bulk materials, and intriguing physical properties have been found for these few-layer systems. When one constructs the van der Waals heterosturcturs by stacking identical or dissimilar layers together, Moiré patterns appear due to the presence of lattice mismatch or a finite relative rotation between the layers. These Moiré patterns will modify the electronic and structural parameters as a function of position. In this talk, I will focus on a few representative systems, including twisted bilayer graphene and heterostructures of transition metal dichalcogenides that exhibit properties ranging from coupled Dirac particles to normal semiconductors. I will discuss our recent theoretical and computational studies to explore the connections between the local structural and electronic properties.

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their ultra-thin structures, similar to graphene, but with unique semiconducting properties. However, CVD growth of these materials typically result in forming non-uniform polycrystalline monolayers or isolated single crystalline monolayers at random locations on substrates.
In this work we demonstrated a localized growth and chip-scale transfer of single crystalline monolayers of 4 different TMDs such as MoS₂, MoSe₂, WS₂ and WSe₂. To grow TMDs only on predefined locations, we created MoO₃ or WO₃ patterns via photolithography and subsequent lift-off followed by physical vapor deposition (PVD) of 5nm of MoO₃ or WO₃ on a silicon dioxide substrate (growth chip) prior to growth . The bare silicon dioxide substrate (bottom chip) was etched using KOH to increase the surface energy. The MoO₃/WO₃ substrate was then placed atop the KOH-etched substrate face to face. Argon (60 sccm) and hydrogen (40 sccm) gases were flown through the growth tube with sulfur powder placed upstream in the furnace [1]. Finally, the TMD crystal growth was carried out at 850 Celsius. The growth of TMDs with surface treatment on the bottom chip enhanced the monolayer growth on the source chip. Higher surface energy on the bottom chip attracts excess MoO₃/WO₃ gaseous molecules from the growth chip, enhancing the monolayer growth.
Furthermore, we demonstrated a low-residue target transfer of grown TMDs using PMMA and thermal tape. After dropping PMMA solution on the growth chip, the chip was left in an ambient condition to dry. The PMMA-coated source chip floated in 30% KOH solution facing up for 20 min. The KOH etched the SiO₂, and the Si substrate sunk, leaving the PMMA/TMD layers floating on the KOH solution. The PMMA/TMD layers were then bonded to thermal tape with a 5 mm×5 mm square window; the window allowed the process of alignment as well as a full exposure of PMMA to acetone during the polymer removal process, thereby significantly reducing residue after transfer.
Our localized technique allows predefined growth, largely eliminating post lithography involving polymer residue on the layer interface. In combination with a reported metal based transfer method [2], one can realize polymer-free target transfer.

[1] K. K. and E. H. Yang, “The Contact-Growth Method – Direct and Pre-patterned Synthesis of 2 Dimensional Heterostructures,” Provisional Patent, US 62/183, June 23, 2015, (2015).
[2] Z. Lin, Y. Zhao, C. Zhou, R. Zhong, X. Wang, Y. H. Tsang, and Y. Chai, “Controllable Growth of Large-Size Crystalline MoS2 and Resist-Free Transfer Assisted with a Cu Thin Film.,” Scientific reports, 5 (October), 18596, (2015).

Ultrasonic-assisted liquid phase exfoliation allows scalable production of 2-dimensional materials, such as transition metal dichalcogenides (TMDs). Though their dispersions can be inkjet printed to develop proof-of-concept devices, their sub-optimal fluidic properties present challenges in reliable and uniform material deposition. Here, we present a universal formulation of functional TMD inks that allows for low temperature curing and reliable, uniform printing on a range of substrates without pre- or post print treatments.

The TMDs (MoS₂, WS₂ and MoSe₂) are exfoliated in water, followed by an exchange into a binary solvent ink carrier for formulation. The carrier and material composition is tuned to achieve optimal fluidic properties for inkjet printing, with γ∼28 mNm^-1, η∼2.4 mPa.s and ρ∼0.8 gcm^-3. This ensures stable jetting of droplets without any satellite droplet formation, observable from the printer stroboscopic camera. The low γ of the ink ensures good wetting of untreated Si/SiO₂, glass and PET. Immediately after printing, the carrier solvents at the contact edge of the droplet evaporate faster than the rest of the droplet. This induces an outward flow of the solvents, which carries the dispersed materials to the droplet edge. However, the difference in evaporation rate of the two-solvent components results in a second flow of solvents in the opposite direction to rebalance the overall ink composition across the droplet. This results in a highly uniform deposition of TMDs across the printed patterns. We demonstrate this by time-dependent measurement of steadily declining contact angles, showing a suppression of 'pinning' of droplet edges and non-uniform deposition. In addition to the fluidic properties, we also determine the optimal print conditions (drop spacing, curing temperature, and printed line edge roughness) to achieve highly uniform and repeatable print patterns at < 60 ^oC. Through linear and non-linear optical raster scanning, we demonstrate high (> 95%) spatial uniformity of the printed patterns. The optical absorbance and thickness of the printed MoS₂, WS₂ and MoSe₂ samples also vary linearly with print repetitions, with < 2.5% and < 2% standard deviation, respectively. Our TMD inks are free from stabilizers and binders, avoiding use of treatments such as high temperature annealing (e.g. 400 ^oC) after printing to remove impurities or stabilizers, offering exciting opportunities for scalable manufacture of their optoelectronic and photonic devices.

1T′ MoTe₂ has recently sparked great interest due to its novel properties and promising electronic applications. The scalable production of high-quality, large-area two-dimensional 1T′ MoTe₂ is challenging but crucial both for fundamental research and applications. Here we synthesize high-quality large-area few-layer 1T′ MoTe₂ films with high homogeneity by the controlled tellurization of a MoO₃ film. The resulting 1T′ MoTe₂ film has resistivity value even lower than that reported for bulk 1T′ MoTe₂. We find that the Mo precursor plays a key role in determining the quality and morphology of the as-grown 1T′ MoTe₂. Furthermore, the amount of Te strongly influences the phase of the MoTe₂ grown from MoO₃. The investigation of the role of the Mo precursor and the amount of Te for the growth of MoTe₂ provides insights into the controllable synthesis and phase engineering of MoTe₂. Our growth method paves the way towards studies of the exotic properties of 1T′ MoTe₂ and the scalable production of high-quality 1T′ MoTe₂-based applications.

Energy Barriers and Contact Resistances in 2D Materials:
First Principles Calculations and Experiment
Su Ying Quek<sup>1

1</sup>Department of Physics and Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore


In order for 2D materials to be useful in electronic and optoelectronic devices, it is critical to know their contact resistance when interfaced with metallic contacts, as well as how the energy levels in different components of the device line up with one another. This energy level alignment (ELA) determines the energy barrier experienced by the electron or hole carriers as they move from one part of the device to another. In the first part of this work, we show, using two examples, that the presence of different types of 2D materials can influence in a non-trivial way the ELA. First, we show that by inserting graphene at the interface between MoS₂ and nickel, the Schottky barrier for electron injection from nickel electrodes into MoS₂ can be reduced significantly. This is primarily due to a large reduction in the work function of the electrode as a result of charge transfer. A contact resistance of 200 Ohm-microns is achieved experimentally – a 20-fold reduction from nickel-MoS₂ interfaces.[1] Second, we ask if a semiconducting 2D interlayer can effectively screen an organic-metal interface. We consider the prototypical molecule, PTCDA, which self-assembles as a 2D monolayer on single-layer WSe₂ placed on a graphite substrate. We find that the WSe₂ layer can substantially, but not completely, screen the organic-metal interface, resulting in a sizeable reduction in the molecular electronic gap. Our predicted ELAs are in good agreement with experiment.[2] In the second part of this work, we add a different spin to our results by considering the effect of electron spin on the contact resistance at the graphene-metal interface. Taken together, our work provides fundamental insight into the influence of charge transfer, electron spin and electronic screening on the ELA and contact resistance involving 2D materials.

[1] ACS Nano 9, 869 (2015)
[2] ACS Nano 10, 2476 (2016)

This work was done in collaboration with the experimental groups of John Thong and Andrew Wee in NUS. We gratefully acknowledge funding from the Singapore National Research Foundation, Prime Minister’s Office, under the NRF Research Fellowship (NRF-NRFF2013-07) and under its medium-sized centre program.

Graphene, and other emerging two-dimensional materials, offer a unique platform for nano-mechanics due to their very low mass density, high elastic strength, exceptionally stiff in-plane Young’s modulus, strong adhesion and flexibility. These properties have motivated the adoption of graphene in the next generation of nano-mechanical devices. In this contribution, we theoretically demonstrate strong mechanical response and deformation of graphene sheets via optical forces. We study single layer graphene and a two-layer graphene stack (with large separation) and show that tunable attractive and repulsive forces can be generated optically using near-field excitations. We identify that the evanescently coupled guided resonances strongly enhance the optical forces, which can lead to the formation of localized blisters in the graphene sheets. This can be done by coupling the sheets to the external radiation of a dipole source or a guided mode injected parallel to them. We report mechanical deformations of several nanometers per microwatts of optical power. Our study points towards new routes for mechanical actuation of graphene and for a novel class of planar light force devices providing a flexible optoelectronics platform. Guided modes in graphene structures enable a rich set of phenomena and can add new dimensions to the concept of “straintronics” due to the presence of both attractive and repulsive resonances. There are several degrees of freedom and possibilities to explore. The ability to manipulate guided resonances offers exciting opportunities for tailoring complex force patterns that could be used for applications such as compensation for Casimir forces to avoid stiction in nanoelectromechanical devices, flattening wrinkles in the deposition process of graphene, and generation of deformation patterns in graphene.

Compared with ferromagnetism, ferroelectricity or ferroelasticity may survive at a relatively higher temperature in low dimensions as long as the crystal structure can be maintained. However, there are very few study on two-dimensional (2D) ferroelectric [1] or ferroelastic [2] materials at present, let alone multiferroic materials that exhibit two ferroic order parameters simultaneously. However, compared with ferromagnetism, ferroelectricity or ferroelasticity may survive at a relatively higher temperature in low dimensions as long as the crystal structure can be maintained.Phosphorene and phosphorene analogues such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calculation evidence that these monolayers are potentially the long-sought 2D materials that can combine electronic transistor characteristic with non-volatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultra-high reversible strain, while SnS, SnSe, GeS, GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their notable structural anisotropy enables ferroelastic or ferroelectric switching readily readable via electrical, thermal, optical, mechanical or even spintronic detection upon the swapping of the zigzag and armchair direction. Additionally, it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temperature, suggesting high potential for practical device application [3].


[1] Wu, M.; Burton, J.; Tsymbal, E. Y.; Zeng, X. C.; Jena, P. J. Am. Chem. Soc. 2012, 134: 14423; Phys. Rev. B 2013, 87, 081406(R).
[2] Wu, M.;Fu, H.; Zhou, L.; Yao, K. L.; Zeng, X. C. Nano Lett. 2015, 15: 3557.
[3] Wu, M.; Zeng, X. C. Nano Lett. 2016, 16: 3236.

Phosphorene, the 2D phosporus-based analogue of graphene, has
attracted recently attracted intense attention due its outstanding
properties. However, synthesis of its 3D parent compound -- black phosphorus -- has
remained challenging, hampering further research and development of
phosphorene-based applications. In this work, we used
first-principles calculations to investigate the fundamentals of the
synthesis of black phosporus from the more common white and red
phosphorus forms. We find that the kinetics of the red-to-black and
white-to-black phosphorus transformations are closely connect to the
thermodynamic energy differences between these phosphorus
allotropes. Based on the obtained understanding, we suggest new
procedures for rapid synthesis of black phosphorus and discuss how
these first-principles based predictions compare to the experimental
results.

Transition metal phosphates (TMPs) are a class of quasi-2D materials consisting of group 4, 5, 12, 13, and 14 metal cations linked through -RPO₃ groups, where R is a surface functional group. Studied in the past for their unique materials properties that include ease of surface functionalization, solution-based exfoliation, and their ability to readily form layered materials with organic compounds, electronic and thermal properties have not been studies in depth. Utilizing first principles density functional theory calculations and machine learning methods we report on a range of key material properties including electronic and thermal properties not present in literature. Our results indicate that these materials, owing to their range of stoichiometries, have excellent electronic and thermal properties. Further, through surface functionalization and doping of the material we present ways in which these properties can be finely tuned; our results indicate promising properties for solar, thermal electric, and catalytic applications.

2D materials have received great interest for electronics and energy applications, yet reaching their full potential is still limited by a number of challenges. Here I will show our recent progress on using theory and first-principles methods to help tackle these challenges, including passivating/engineering^1-3 defects, reducing the contact resistance⁴, improving the performance of 2D-materials based Li-ion batteries^5,6, Na-ion batteries⁷, and designing highly-active and cost-effective 2D catalysts for hydrogen fuel production⁸, as well as discovering new functional material⁹. I will also discuss their connections with experiments.

1. Liu, Y.; Xu, F.; Zhang, Z.; Penev, E. S., et al. Nano Lett. 2014.
2. Liu, Y.; Stradins, P.; Wei, S.-H. Angew. Chem. Int. Ed. 2016, 55, 965-968.
3. Liu, Y.; Xiao, H.; Goddard, W. A. Nano Lett. 2016, 16, 3335–3340.
4. Liu, Y.; Stradins, P.; Wei, S.-H. Science Advances 2016, 2.
5. Liu, Y.; Wang, Y. M.; Yakobson, B. I.; Wood, B. C. Phys. Rev. Lett. 2014, 113, 028304.
6. Liu, Y.; Artyukhov, V. I.; Liu, M.; Harutyunyan, A. R., et al. J. Phys. Chem. Lett. 2013, 4, 1737-1742.
7. Liu, Y.; Merinov, B. V.; Goddard, W. A. Proc. Natl. Acad. Sci. USA 2016, 113, 3735-3739.
8. Liu, Y.; Wu, J.; Hackenberg, K. P.; Zhang, J., et al. arXiv:1608.05755 2016.
9. Liu, Y.; Penev, E. S.; Yakobson, B. I. Angew. Chem. Int. Ed. 2013, 52, 3156-3159.

The structural, electronic, and diffusive properties of copper and silver intercalated molybdenum disulfide are investigated from first-principles density functional theory calculations. Metal atoms are inserted in the van der Waals gap of 2H-MoS₂ at tetrahedral and octahedral sites producing structure of the form M_xMoS₂ (M=Cu and Ag), where the concentration (x) of metal atoms is varied between 1.3% and 100%. We find that tetrahedral sites are energetically favorable for copper intercalates, while octahedral sites favor the intercalation of silver atoms. Analysis of the electronic structure shows an increase of states at the Fermi level, resulting in a semiconductor-to-metal transition with increasing metal intercalation. Transition state theory calculations show relatively low activation energies for the diffusion of copper and silver in MoS₂ with values of 0.3eV and 0.4eV, respectively. Additionally, the energetics, electronic and magnetic properties of doped transition metal dichalcogenide (TMDC) monolayers with group IIIA-VIIA species are systematically investigated. We find that some of these species can be used to dope the 2D semiconductor p- or n-type; moreover, most of the studied dopants induce a ferromagnetic ordering to the TMDC.

Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) which possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS₂. Various DFT energies are predicted from the layer-dependent MoS₂, 2D flake-size related mono- and bilayer MoS₂, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS₂ flake in a small 2D size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS₂ flake becomes stable with increasing 2D size. The critical 2D flake-size of phase stability between mono- and bilayer MoS₂ is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temperature-composition (P-T-x) growth windows are predicted for MoS₂, indicating that the formation of MoS₂ flake with reduced size appears in the middle but close to the lower T and higher P “Gas + MoS₂” phase region. It further suggests that Mo diffusion is a controlling factor for MoS₂ growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS₂ energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available experiments, and the present data provide quantitative insight into the controlled growth of 2D layered MoS₂.

Transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are layered materials capable of growth to one monolayer thickness via chemical vapor deposition (CVD). Such CVD methods, while powerful, are notoriously difficult to extend across different reactor types and conditions, with subtle variations often confounding reproducibility, particularly for 2D TMD growth. In this work, we formulate the first generalized TMD synthetic theory by constructing a thermodynamic and kinetic growth mechanism linked to CVD reactor parameters that is predictive of specific geometric shape, size, and aspect ratio from triangular to hexagonal growth, depending on specific CVD reactor conditions. We validate our model using experimental data from Wang et al. (Chem. Mater., 2014, 26 (22)) that demonstrate the systemic evolution of MoS₂ morphology down the length of a flow CVD reactor where variations in gas phase concentrations can be accurately estimated using a transport model (C_Sulfur = 9–965 μmol/m³; C_MoO3 = 15–16 mmol/m³) under otherwise isothermal conditions (700 ^oC). A stochastic model which utilizes a site-dependent activation energy barrier based on the intrinsic TMD bond energies and a series of Evans-Polanyi relations leads to remarkable, quantitative agreement with both shape and size evolution along the reactor. The model is shown to extend to the growth of WS₂ at 800 ^oC and MoS₂ under varied process conditions. Finally, a simplified theory is developed to translate the model into a “kinetic phase diagram” of the growth process. The predictive capability of this model and its extension to other TMD systems promise to significantly increase the controlled synthesis of such materials.

Graphene and the two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in such semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally different than the corresponding bulk materials due to the fact that surface and bulk coincide when going from 3D down to 2D. Interestingly, one can take advantage of the latter property by using ordered supramolecular layers in order to tune charge transport and optical properties of such 2D materials. Hence, the electrical properties of graphene can be strongly influenced by the presence of physisorbed molecules, which induce charge transfer and doping[1]. In this context, supramolecular chemistry makes it possible to precisely tune the doping effect via the formation of ordered self-assembled monolayers (SAMs) of molecules embedding different functional groups[2].
In my presentation, I will give an example of a more general physical scenario: the charge transport and the doping on graphene can be tuned through ad-hoc engineering of the supramolecular assemblies physisorbed on its surface. A very fine photoreactivity process allows to achieve molecules featuring the same assembly motif on the surface but different terminal groups which contribute to doping the graphene in a novel way.
This approach will give an easy example of all the potential hidden in van der Waals heterostructures composed of supramolecular lattices physisorbed onto graphene.


[1] Li et al. Nanoscale 5, 9640(2013); S.-L. Li, K. Tsukagoshi, E. Orgiu, P. Samori Chem. Soc. Rev. 45, 118 (2016).
[2] G. Preston, et al. Chem Soc Rev. 42, 3(2012)

Straining few-layers MoS2 and MoO3 films to both high magnitudes and with a spatially varying design is currently eagerly sought, as it enables precise manipulation of the band structure and electrical transport within these materials. Optoelectronic, electronic and nanoelectromechanical systems applications would benefit from having ultra-thin materials with tunable electronic properties over a large area, and supported by an inexpensive substrate. We demonstrate a facile thermal annealing process to locally strain-engineer few-layers Mo-compounds over areas as large as 50x50 m2. In our approach, spatially controllable strain is achieved, during thermal annealing, by in situ delamination and conformal contact with a textured Si substrate. We take advantage of well-established techniques to process large-area Si substrates in ordered arrays of ultra-sharp tips. In addition, we harness the extreme flexibility of few-layers MoS2, and MoO3 which can sustain very high elastic strain, without undergoing any plastic deformation. Specifically, we perform a scaffold-assisted transfer of mechanically exfoliated MoS2 to bulk Si patterned into 2D arrays of ultra-sharp tips. We obtain textured Si over 1x1 cm2 areas via a multi-step approach beginning with interference lithography, metal evaporation, and lift-off. These processes yield a 2D array of circular Cr pads. Next, we perform reactive ion-etching of bulk Si. Cr serves as hard mask for Si during a dry etching process yielding Si nano-cones. Removal of the Cr mask followed by thermal oxidation results in lateral thinning of the nano-cones, and formation of ultra-sharp tips with height ranging from 350 to 500 nm, and base of ~350 nm, as measured by scanning electron microscopy (SEM). Mechanically exfoliated MoS2 flakes are adhered to textured Si via a scaffold-assisted transfer. MoS2 thickness, as estimated by off-axis SEM is ~100-1000 nm. Next we perform thermal annealing in a quartz tube furnace in Ar/air mixed atmosphere, at 250°C. The pressure in the tube is set to ~1.5 mTorr. During annealing, few-layers Mo-compounds at the interface with the textured Si delaminate in situ and conform to the substrate topography. The remaining portion of the MoS2 flake stays intact, and it is removed by mechanical exfoliation. Delamination and guided self-assembly of few layers Mo-compounds yield a spatially varying strain field within a single material. We demonstrate that different annealing conditions result in guided self-assembly of a variety of Mo-compounds. The geometry, structural quality and spatially varying strain are characterized by optical contrast, SEM, raman and photoluminescence spectroscopy.

he optical transitions of monolayer graphene were substantially tuned by electrical gating and thus, various graphene-based photonic devices such as high-speed waveguide-integrated electroabsorption modulator were demonstrated. Although efficient controls of desired optical properties in graphene-based passive devices have been successfully achieved, further studies are still needed in active photonic devices. In particular, it remains a challenge to achieve the efficient loss control of graphene-based nanocavities and the switching of the resultant lasing operation. In this work, we demonstrated on/off switching of graphene-integrated single- and double-cavity photonic crystal lasers by electrical gating of the monolayer graphene sheet. The optical loss of graphene on top of photonic crystal cavities was efficiently controlled with varying gate voltage V_g, which was confirmed in the transmittance measurement of graphene and the numerical simulation of optical loss of a cavity mode. Optically pumped lasing action was successfully demonstrated from a graphene-integrated single photonic crystal cavity at V_g smaller than -0.6 V, exhibiting a low lasing threshold of ~480 mW, whereas lasing was not observed at V_g larger than -0.6 V due to the intrinsic optical loss of graphene. In addition, in the double-cavity photonic crystal lasers with graphene, individual switching of each cavity with a separate graphene sheet was achieved and the two lasing actions were controlled independently, despite the close distance of ~2.2 mm between the cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices would pave the way to demonstrate high-contrast and ultra-compact photonic integrated circuits.

The Raman scattering spectroscopy is a technique of choice to study lattice dynamics in semiconductor nanostructures. Additionally the resonant excitation of the Raman scattering results in rich spectra, which reflect the coupling of phonon modes to electronic states excited resonantly in a crystal. The Raman scattering in semiconductor transition metal dichalcogenides (TMDs) resonant with the A and B excitons related to the fundamental bandgap has been thoroughly studied.
Much less is known on the effect of excitation deep within the bands, in resonance with higher-energy minima of the TMDs bandstructure. The Raman scattering excited under such conditions in thin MoTe2 layers results in a complicated pattern of the spectra due to out-of-plane (A1g/A1’) vibrations. Davydov-split modes of the vibrations can be observed. Their number and the energy splitting reflect van der Waals interactions between monolayers of MoTe2.

We report on the effect of temperature (5K to 300K) on the Raman scattering due to A1g/A1’ modes associated with the out-of-plane modes in 1L, 2L, 3L, and 4L MoTe2. The temperature-evolution of the modes critically depends on the flake thickness. Most striking is the evolution of the A1g mode intensity observed in 2L MoTe2. The intensity decreases with decreasing temperature down to 200K. The A1g mode vanishes from the Stokes scattering spectrum in the temperature range between 150K and 200K. The peak recovers at lower temperatures and at T=5K it becomes three times more intense that at room temperature.
Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe2 in which tellurium atoms in all three layers vibrate in-phase. On the contrary, the intensity of the other out-of-plane Raman-active mode in which vibrations of tellurium atoms in the central layer of 3L MoTe2 are shifted by 180° with respect to the vibrations in outer layers, only weakly depends on temperature. Similar although weaker effect can be observed in 4L MoTe2.

We relate the observed non-monotonic temperature evolution of the Raman scattering spectrum to the resonance of the excitation light with an electronic excitation in MoTe2.
The observed quenching of the Raman scattering is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed “antiresonance” is related to the electronic excitations at the M point of the Brillouin zone in few-layer MoTe2.

Tremendous research interest has been dedicated to the rechargeable lithiumion batteries (LIBs) for the upcoming era of portable electronics and electric vehicles (EVs). Most commercial LIBs utilize graphite as anode material due to its low cost, flat average potential and long cycle life. However, its specific capacity of 372 mA h g^-1 results in a low device energy density far below EVs requirements. So far, various novel materials have been extensively studies for LIBs anodes, including alloys and transition metal oxides. Although most of them possess significantly larger capacity, they suffer from either poor cycling due to volume change or sluggish kinetics stemmed from slow ion diffusivity or intrinsic poor electron conductivity.
Some transition metal sulfides have also been considered highly promising for high-performance anode materials. Among various candidates, molybdenum disulfide (MoS₂) possesses a similar layered structure to graphite but a much larger interlayer spacing of 6.15 Å by stacking together through van der Waals interactions, facilitating Li⁺ intercalation without significant volume expansion. However, MoS₂ still suffers from fast structural deterioration and poor electrical/ionic conductivity, resulting in unsatisfactory cycling and rate capability in LIBs application. Therefore, the development of novel highly stable MoS₂-based materials with fast kinetics remains challenging, owing to the lack of a ration design from molecular level. Moreover, it is also critical to correlate the performance with materials structure, and to understand the chemistry behind before its future practical applications.
Herein, we demonstrate a facile solvothermal synthesis of nanocomposites consisting few-layered MoS₂ and covalently sulfur-doped graphene (MoS₂/SG) with excellent electrochemical performance. We focus on not only the development of MoS₂-based electrode materials but also the materials design based on both structure and chemistry considerations. The sulfur atoms covalently bonded to graphene sheets and effectively bridging 2D few-layered MoS₂ and graphene enable high robustness of the composite materials. Moreover, the intimate contact of MoS₂ and highly conductive graphene provides efficient electron transfer pathways, while the high surface of assembled 2D materials allows fast access to active materials. The unique composite architecture derived from the “bridging effect” ensures the electrode with an exceptional cycling stability and rate capability, which is also interpreted by the density functional theory calculations. A capacity retention of 92.3% can be achieved after 2000 cycles at a current density of 10 A g^-1; even at a high current density of 20 A g^-1, the electrode still possesses a specific capacity of 766 mA h g^-1. This composite material with excellent electrochemical properties synthesized via a facile solvo-thermal approach holds great promise for high-performance LIBs.

Transition metal phosphorus trisulfides (MPS₃) are a new class of layered materials. Some of the bulk MPS₃ materials (FePS₃, MnPS₃ and NiPS₃) have been studied as magnetic materials with antiferromagnetic phase transitions. Recently, there are some reports that atomically thin MPS₃ samples can be prepared by mechanical exfoliation from bulk crystals. [1,2] However, there has not yet been a report for magnetic ordering in atomically thin samples. We synthesized bulk FePS₃ crystals by vapor transport method and prepared atomically thin samples on SiO₂/Si substrates by mechanical exfoliation. Dramatic changes in Raman spectra are observed at the critical temperature due to magnetic ordering effects. By performing temperature dependent polarized Raman spectroscopy, we investigated antiferromagnetic phase transitions in FePS₃. The thickness dependence of the transition temperature up to monolayer samples are studied in detail.
References
[1] C. Kuo et al., Sci. Rep. 6, 20904 (2016).
[2] K.-z Du et al., ACS Nano 10, 1738 (2016).

Transition metal dichalcogenides have emerged as a promising candidate for nanoelectronic devices to overcome limitations of Si-based devices. However, most of the researches have been focusing on MoX₂ and WX₂ (X = S, Se, and Te), which are made using rare earth elements. On the other hand, field-effect transistors using SnS₂ could be fabricated with a lower budget because of its lower synthesis temperature and abundance of the element Sn. Therefore, in this research, we use SnS₂ as the channel material to demonstrate a high-performance field-effect transistor with graphene electrodes encapsulated between hBN. All 2D materials used in this research were mechanically exfoliated from as-received bulk materials and dry-transferred to reduce impurities as low as possible. Our result showed that a high on/off ratio of more than 5X10⁸, a mobility of ~65 cm²/Vs and a subthreshold swing of ~100 mV/dec was observed with the hBN gate dielectric and the graphene gate electrode. We also compared various contact structures using various combinations of metal (Ni/Au) and bottom/top graphene source/drain electrodes to analyze the difference of lateral/vertical charge conduction.

Channel doping issue is one of the hottest challenges in the research of 2D transition metal dichalcogenides (TMDs). In this study, we did doping Co precursor with Tungsten Disulfide(WS2) channel which was get by mechanical exfoliation on SiO2/Si substrate. WS2 was dipped into Co precursor and heated under vacuum condition. Conventional chemical and electrical measurements were progressed by various local probe investigations like Raman spectroscopy, X-ray Photoelectron Spectroscopy(XPS), and Semiconductor Characterization system(SCS). Electrical changes with mobility, on/off ratio, shift in the threshold voltage were investigated. Atomic Force Microscopy (AFM) was also employed for finding reason of changes. Components of doped WS2 was confirmed by XPS and Raman. Also 1/f noise which is modeled with CNF-CMF(Carrier number fluctuation - Carrier mobility fluctuation) and hooge mobility fluctuation was analyzed. Both model is well fitted with 1/f noise data of WS2 FET.

A facile method was developed for the doping of graphene nanosheets
grown via chemical vapor deposition (CVD) for all graphenebased
organic hybrid devices. Graphene oxide (GO) functionalized
with organic ligands was employed as a dopant for CVD graphene. The
doping effects from functionalized-GO with various work function
(WF) values were verified by ultraviolet photoelectron spectroscopy
and Raman spectroscopy analysis. In addition, the study also
demonstrated easily controlled photodetectors based on WF-tuned
graphene and photosensitive organic thin layers. A hybrid deposition
system that combines thermal evaporation and vapor-phase metalation
was utilized for the preparation of tunable photosensitive
organic layers such as metalloporphyrin. This work represents a major
breakthrough in the manufacturing of advanced all graphene-based
hybrid devices such as flexible devices, organic transistors, and
chemical sensors.

Atomically sharp WSe₂ and MoS₂ lateral heterojunctions have been synthesized using a two-step epitaxial growth method ^[1]. The 4.5% lattice mismatch between WSe₂ and MoS₂ makes strain and dislocations along those junctions unavoidable. By using atomic resolution high angle annular dark-field scanning transmission electron microscopy, followed by geometric phase analysis, we observed periodic misfit dislocations (5|7 pairs) along the junctions. More interestingly, those dislocations are able to migrate from the junction into the first grown material, WSe₂, and grow a several unit-cell-wide trail in the form of MoS₂ nanowire behind the dislocation core. This observation of dislocation abnormalities allows us to engineer dislocations to embed one dimensional quantum wells with coherent strain into two dimensional materials.

[1] Li, M.-Y., et al. (2015) Science, 349(6247), 524–528.

2D heterojunctions enabled by combining 2D TMDs of dissimilar band structures form a basis for a variety of 2D electronic devices that function at near atomic length scales. Conventional approaches to create the heterojunctions based on the manual assembly of individually isolated 2D layers have suffered from uncontrolled junction geometries with limited yield and spatial inhomogeneity. In this work, I will present new ways to fabricate 2D TMD heterojunctions with precisely-defined junction geometries by using chemical and physical means. In the first part of the presentation, I will introduce a new chemical synthesis method to grow large-scale 2D heterostructures composed of few layer-only, vertically-stacked 2D MoS₂ and WS₂. These new 2D MoS₂/WS₂ heterostructures were produced over a few cm² maintaining their atomically sharp 2D heterointerfaces, confirmed by transmission electron microscopy characterizations. Electrical transport measurements revealed a diode-like behavior with clear current rectification, further confirming the formation of high-quality 2D heterojunctions. In the next part of the presentation, I will introduce the fabrication of lateral 2D heterojunctions on few layer 2D MoS₂ flakes by using an oxygen plasma treatment. A specific exposure of oxygen plasma converts pristine MoS₂ to oxygen-rich MoO_x-MoS₂ compound, which leads to the formation of heterojunctions at the interface of MoS₂/MoO_x-MoS₂. A clear current rectification is observed across the heterojunction due to the band offset between the pristine and the oxygen-exposed MoS₂. The intrinsic controllability of the presented methods to realize heterojunctions in 2D semiconductors would open new avenues in designing unconventional 2D electronic devices with multifunctionalities.
¹N. Choudhary, Y. Jung et al., Sci. Rep. 6, 25456 (2016)
²N. Choudhary, Y. Jung, et al., J Phys.: Condensed Matter, In Press (2016)

We demonstrate that interfacial alloying can be achieved between semiconducting WSe₂ transition metal dichalcogenide (TMD) channels and metallic NbSe₂ TMD contact layers, forming metallic Nb_xW_1-xSe₂ inter-facial layers that aid in forming excellent electrical con-tact between the semiconducting and metallic TMD re-gions. This interfacial transition Nb_xW_1-xSe₂ structure considerably lowers the potential barrier height of the junction between 2D semiconductor-metal regions leading to significantly improved performance of the WSe₂ based transistor device. The creation of such composi-tion engineered transition regions across 2D junctions between dissimilar TMD domains could be most im-portant in the design and fabrication of 2D atomic layer devices.

Transition metal dichalcogenides (TMDs) two-dimensional layered materials (2D) with a superior electrical, optical, and mechanical properties are considered as promising next generation 2D materials in electronic and optoelectronic applications. Tungsten diselenide (WSe₂) one of those TMDs materials with p-type dominant semiconductor characteristics exhibit a unique indirect to direct band gap transition as decreasing from bulk to monolayer. For the further applications of those TMDs materials, doping is required to control their intrinsic properties like carrier type and range of doping.

In this study, we demonstrate in situ doping of monolayer tungsten diselenide (WSe₂) with phosphorus compounds via vapor phase deposition techniques. We use the one inch and one zone furnace with promoter to synthesis and the shape of flake becomes star shape to hexagonal shape at same growth condition. The proposed doping method is investigated in detail through X-ray photoelectron spectroscopy (XPS), electrical measurements (I_ds-I_gs), and Raman spectroscopy. XPS characterization of flakes with of without growth using phosphorus compounds also show chemical shift to lower BEs. Then, monolayer transistor fabricated using photolithography method show the enhancement of p-type conductivity and can be applied at transparent flexible device.

Dirac materials hold great promise for future high frequency device integration. This stems from the high mobility of Dirac electrons and their robustness against disorder. In particular, topological insulators such as Bi₂Se₃ have recently attracted interest due to their tunable Dirac nature. In such materials, a Dirac surface state disperses in the band gap of semiconducting bulk states. A gate voltage can then be used to tune the chemical potential into and out of the Dirac state. This makes topological materials attractive materials for transistor integration. In this study, growth of few-quintuple layer Bi₂Se₃ nanostructures on exfoliated hexagonal boron nitride (h-BN) is performed via physical vapor transport (PVT) in a tube furnace. Microflakes with thickness varying between 2 nm and 100 nm are obtained. The material is characterized using X-ray diffraction, micro-Raman spectroscopy and scanning electron and atomic force microscopy. X-ray diffraction indicates heteroepitaxial growth with the exclusive appearance of (0 0 3n) Bi₂Se₃ peaks in the spectra. Micro-Raman spectroscopy yields Raman active peaks characteristic of the R-3m structure of Bi₂Se₃. The relative Raman peak intensity suggests a suppression of in-plane lattice vibrations for thinner flakes.

The material is then partially encapsulated with a top BN layer, contacted, and integrated into a waveguide suitable for radiofrequency transport measurements up to 40GHz. The top BN is used as a dielectric gate, and the admittance versus frequency spectrum is measured for different gate voltages. This allows us to extract variations in the device capacitance and the channel resistance of Bi2e3 as a function of the gate voltage through a single measurement, and map out the gate voltage dependence of electron transport in Bi₂Se₃. Our realization paves the way for future high frequency transistor devices based on topological materials.


Work funded by ANR LabEx grant ENS-ICFP (ANR-10-LABX-0010/ANR-10-IDEX-0001-02 PSL). JD also partially funded by the Northeastern University Presidential Global Scholarship.

Field effect transistors with graphene channels (GFET) were studied. The graphene channels were deposited over well separated semiconductor quantum dots (QDs), which were imbedded in a periodic hole structure. The photo-electronic and electrical properties of this structure were studied. The fluorescence of the dots was affected by both the drain-source and gate bias while at plasmonic resonance conditions. Raman-scattering of the graphene under bias was also assessed.

Graphene, a single layer of crystalline graphite, exhibits high conductivity, chemical inertness, mechanical robustness and unusual dispersion relations. Free-standing, single layer, or bi-layer graphene have been studied when deposited over anodized aluminum oxide (AAO) substrates. The periodic array of nano-porous AAO could support surface plasmon/polariton (SPP) mode, which affected the photoluminescence of the QDs imbedded in them.

Our structures were assembled by anodizing aluminum on SiO₂/Si substrates. The QDs were imbedded in the hole pattern in the anodized film. Then, we transferred a single layer of graphene on top of the QDs imbedded AAO. As a result of a two-step anodization, 50-100 nm thick perforated AAO layer was formed on the SiO₂/Si substrate. The diameter of the holes was 25-30 nm with a pitch of ca 100 nm. The Si served as the back-gate electrode.

The fluorescence of the QDs imbedded AAO films was assessed with a 35 mW Ar ion laser at 488 nm in a confocal arrangement. The GFET was placed on a rotatable platform to allow a study of the fluorescence as a function of incident angle. The peak fluorescence of the QDs varied as a function of gate bias V_gs as well as the drain-source bias V_ds. When the incident, or emission wave-vector were at resonance with the perforated hole pattern, the fluorescence maximized due to an efficient coupling between the plasmonic mode and the QDs.

A typical Raman spectra of graphene has two major bands: the G band, which is the result of a Stokes scattering with one emitted phonon and the 2D band, which is the result of a double resonant scattering with two emitted phonons. We studied the Raman shift of our GFETs as a function of the bias V_g and V_ds at resonance plasmonic conditions.

In summary, the florescence of QDs interfaced with graphene on the AAO substrates were studied by varying the bias voltage and the tilt angle. Raman scattering spectra of the GFETS were assessed as a function of the bias conditions, as well.

Since the discovery of graphene, a number of 2D nanomaterials, such as transition metal dichalcogenides, hexagonal boron nitride, and metal oxides, have been found and attracted great interest due to their extraordinary electrical, optical and mechanical properties. In order to utilize those 2D nanomaterials in nanodevices, they need to be transferred from an original substrate to a target substrate in a clean and efficient way, which is one of the most challenging technical issues.
In the present work, we report selective pick-up and transfer (SPT) method using polydimethylsiloxane (PDMS) and demonstrate 2D electronic devices using the SPT method. This SPT method allows PDMS to pick up only a single 2D nanomaterial selectively from the original substrate, then, to release the material on to the specific area in the target substrate without touching other materials around. Both picking-up and transferring processes could be achieved at the same PDMS by modulating the adhesion strength between PDMS and 2D nanomaterials. As a result, this method does not require any sacrificial layers or chemical etching processes that can leave residues on the devices. Furthermore, our SPT method is CMOS-compatible and applicable to arbitrary substrates and materials.

Low-density nanostructured foams from different materials have been synthesized in recent years. However, most of these nanofoams present low mechanical and thermal stabilities, thus often limiting most of their engineering applications. Also, their large-scale fabrication has been proven to be a very difficult technical challenge. Recently [1], it was reported ultra light foams consisting of graphene oxide membranes decorated with boron nitride platelets. This new material exhibit enhanced mechanical and thermal properties, thus simultaneously solving two of the major problems of most carbon-base nanofoams. However, there are still many open questions regarding the precise origin of this enhanced behavior. In this work we investigated the thermal conductivity (TC) of 3D model foams composed of stacked graphene oxide (GO) membranes decorated with hexagonal boron nitride (h-BN) platelets. In order to carry out this study we used fully atomistic reactive molecular (MD) dynamics simulations with the well-known ReaxFF force field, and the MD simulations were performed using the LAMMPS (Large–scale Atomic/Molecular Massively Parallel Simulator) software [2]. In order to investigate the thermal stability of the foam we calculated the TC in a temperature range of 200-1000K. Our results show that the presence of the h-BN flakes increases the foam thermal conductivity in comparison to the pristine graphene oxide structure, as well as, produce increase structural stability. We attribute these enhancements to the strong interfacial interaction between GO and h-BN, which results in globally (more planar) stiffer structure, which increases the phonon propagation along a larger area. Also, these ‘stiffer’ motifs produce more organized 3D layered structures, which also contributes to higher thermal conductivity.
Acknowledgements: FAPESP 16/02086-8
[1] S. Vinod et al., Nature Commun. v5, 4541 (2014).
[2] S. Plimpton, J. Comput. Phys. v117, 1 (1995).

By considering driven diffusive dynamics of exciton polaritons (a quantum superposition of cavity photons and quasi-two-dimensional excitons) in an optical microcavity with an embedded MoS₂ monolayer, we determine an experimentally relevant range of parameters, at which room-temperature superfluidity of a polariton gas can be observed. It is shown that the superfluid transition occurs in a trapped polariton gas at a laser pumping power of P > 600 mW and a trapping potential strength of k > 50 eV/cm². Then, we propose a simple analytic model that provides a useful estimate, which determines the conditions for the observation of room-temperature polariton superfluidity in a cavity with an embedded MoS₂ monolayer. We also study the utility of a patterned microcavity for controllable polariton guiding that enables to transmit optical signals in polaritonic circuits. By numerically solving the Boltzmann equation for a gas of interacting polaritons in a normal state at room temperatures and the Gross-Pitaevskii equation for a dipolariton Bose-Einstein condensate in superfluid state at low temperatures, we pinpoint the range of parameters, at which polaritons can be driven in a controllable way in polaritonic channels formed in a patterned microcavity. Finally, we propose a few design guidelines for optical transistors and switches based on Y- and Ψ-shaped channels in a cavity with an embedded MoS₂ monolayer.

Hexagonal boron nitride (h-BN) has shown incredible potential as a 2D dielectric, an atomically abrupt tunnel barrier, a substrate for graphene growth, and a deep-UV emitter. Currently the lack of single-crystal wafer-scale h-BN films is a major bottleneck for producing h-BN devices. To this date, h-BN thin film growth methods have focused on catalytically self-limiting growth on transition metal substrates. Unfortunately, these metals are not available in single crystals of suitable size and quantity. A solution would be to explore possible h-BN epitaxy on more conventional, non-metallic substrates. By growing using molecular beam epitaxy (MBE) or chemical beam epitaxy (CBE) on these common substrates, in situ characterization of nucleation and growth can provide feedback for optimizing growth conditions and exploring the growth process.

In this talk, progress of growing h-BN by MBE and CBE methods on non-transition metal substrates will be presented. In particular, 6H-SiC(0001) was chosen as a candidate substrate due to its crystalline quality, compatibility with current fabrication processes, and potential a coincident lattice match. For MBE growths, boron was deposited by electron-beam evaporation and a RF nitrogen source provided active nitrogen species. CBE depositions were achieved through thermal decomposition of borazine at high temperatures, without the use of a catalytic surface. Film stoichiometry and surface chemistry was measured by in situ x-ray photoemission spectroscopy (XPS). Near stoichiometric depositions have been achieved by both MBE and CBE, as determined by B 1s:N 1s peak area ratios. Nuclei and growth surfaces were investigated by scanning probe microscopy and scanning electron microscopy. Triangular nuclei on SiC surfaces show promising signs of epitaxial growth. Different substrate terminations and growth parameters were compared to determine their effect on h-BN quality. In particular, epitaxial graphene formed through in situ evaporation of Si from the SiC surface was explored as a growth surface. Finally, other substrates such as c-plane sapphire were also investigated as potential epitaxial substrates for these high-temperature UHV growths.

We report on the preparation of high quality wafer-scale p-type WS₂ thin films with a carrier mobility as high as 63 cm²/Vs. The WS₂ films were grown by chemical vapor deposition (CVD) method which involves sulfurization of oxide thin films deposited on sapphire substrate. The existence of a mixture of randomly oriented WS₂ crystallites, with the c axes oriented parallel to (type I) and perpendicular to (type II) the substrate, greatly limits the large-scale transfer of WS₂ thin films. This issue is addressed using a 5 nm Ni texture promoter which significantly changed the microstructure of the WS₂ thin film from a mixture of type I and type II crystallites to large type II crystals. The transfer of wafer-scale WS₂ thin films onto different substrates was achieved by an etching-free technique that took advantage of the hydrophobic property of WS₂ and hydrophilic property of sapphire. This induces water molecules to penetrate into the interface leading to the separation of the WS₂ thin film from the sapphire substrate. The transferred films were free of wrinkles, cracks, or polymer residues. WS₂ films grown without the Ni promoter can only be exfoliated using corrosive chemical etching solution, such as HF or KOH resulting in significant damages to the material. The advantage of our wafer-scale WS₂ thin film transfer for heterojunction fabrication was demonstrated by the high quality p-n junctions fabricated by transfer of the p-type WS₂ onto an n-type GaN with superior performances compared to the directly-grown WS₂/GaN heterojunctions.

Two-dimensional titanium carbides, Ti₃C₂, named MXenes, have been synthesized from MAX phase (Ti₃AlC₂) through selective etching of the A element by a treatment of hydrofluoric acid (HF). However, until now, under the aqueous environment with presence of both HF and H₂O molecules, the exposed Ti surfaces of the isolated Ti₃C₂ layers are initially terminated with F or/and OH groups (Ti₃C₂T_x), which can be influenced on their intrinsic physical property. To prepare “metal (M)”-terminated surface, the chemically induced reduction method of MXene in solution and at room temperature has been sought for a long time. Here, we report a novel reducing agent system (alkali metal-amine system, lithium ethylenediamine) that allows for an efficient, one-pot reduction of Ti₃C₂T_x into Ti₃C₂ under solution-phase at room temperature. The reducing agent system provided de-laminated MXenes by intercalation chemistry, simultaneously, resulting in highly conducting MXene paper.

Single layer TMDCs are optically interesting due to their coinciding band edges at the K point in contrast to their indirect gap multilayer counterparts. However, being atomically thin limits the extent of their absorption as well as emission. Consequently, though a number of postulates do exist to tune the quasi particles (viz. exciton or trion), realization of effective ways to control the intensity of photoluminescence for practical purposes still remains challenging. There has been a number of ongoing attempts to tune PL of MX2 (M = Mo, W and X = S, Se) with plasmonic structures with high electric field confinement. These methods require extremely specialized fabrication techniques and the pinning down the underlying mechanism is also quite challenging. As an alternative and effective approach, we use thin (~10-30 nm), amorphous oxides films (by pulsed laser deposition (PLD)) as a capping layer to obtain screening or charge doping across the interface. The advantage with this approach is the huge range of material availability with wide variety of electronic configuration and energy-level (band) distribution, all of which can be deposited by PLD technique. This enables the realization of various extents of charge screenings and (or) doping. Added to that, this technique allows the use of variable sizes of shadow mask to control the area of deposition which enables the opportunity to pattern the 2D layer. We benefit from the fact that MoS₂ grown in our CVD method has PL intensity an order of magnitude higher than exfoliated samples. This enables us to realize different extents of quenching mechanisms which can be effectively distinguished. The quality of the MoS₂ layer is probed after oxide deposition by Raman spectroscopy which shows that there is no material degradation. The extent of effective charge doping on MoS₂ is estimated from carrier density measurement and the ellipsometry measurement gives the nature of charge injection and screening mechanism. We demonstrate a patterned MoS₂ surface with variable PL by varying the local capping layers. We believe, this is an easy and affordable way to control light matter interaction in TMDCs.

Two-dimensional materials, such as graphene and transition metal dichalcogenides, are a promising class of nanomaterials for next generation electronics, photovoltaics, electrocatalysts, sensors, and optoelectronic devices. Molybdenum disulfide (MoS₂) is of particular interest due to its direct bandgap in the visible spectrum, high electron mobility, and chemical stability. In this work, we demonstrate that alterations in the density of surface hydroxyl groups on silicon dioxide substrates can control nucleation and growth in molybdenum disulfide thin films produced by atmospheric-pressure chemical vapor deposition. The extent of MoS₂ nucleation is linearly correlated to the density of surface hydroxyl groups. Controlling the density of surface hydroxyl groups on the initial substrate provides a method of growing patterned molybdenum disulfide. Furthermore, we establish that the surface density of hydroxyl groups on silicon dioxide (SiO₂) is altered using conventional gallium focused ion beam (FIB) patterning. Upon gallium-ion beam exposure, the number of hydroxyl groups generated on the surface is directly proportional to the ion dosage. In addition, we show a two-step growth method, where molybdenum trioxide is first vaporized and nucleated on the SiO₂ substrate, including preferentially in the FIB-hydroxylated regions. A second sulfurization step results in large-area monolayer MoS₂, as confirmed by Raman spectroscopy and photoluminescence. Our two-step approach to MoS₂ synthesis provides robust control over the parameters in powder vaporization, which greatly increases reliability and reproducibility of this widely used method for monolayer synthesis. This work establishes a means of patterning large-area monolayer MoS₂ on silicon dioxide substrates, which is a critical step for realizing applications in imaging, catalysis, biosensing, chemical detection, electronics and optoelectronics.

Ultrathin two-dimensional nanosheets raise a rapidly increasing interest due to their unique dimensionality-dependent properties. Most of the two-dimensional materials are obtained by exfoliation of layered bulk materials or are grown on substrates by vapor deposition methods. To produce free-standing NSs with defined phase and shape, solution-based colloidal methods are emerging as promising routes. In this work, we demonstrate free-standing, ultrathin CdSe nanosheets with controllable shape and phase. The key of our approach is the use of 1-bromoheptane as co-ligand in hot-injection synthesis. Increasing concentrations of 1-bromoheptane can tune the shape from sexangular to quadrangular to triangular and the phase from zinc blende to wurtzite. The structural and optical properties of the resulting CdSe NSs are characterized by transmission electron microscopy, UV-vis absorption, fluorescence and X-ray diffraction.

BN nanosheets (BNNSs), graphene-like 2D nanomaterials of hexagonal boron nitride (h-BN), are promising as fillers in advanced polymeric composites such as a gas barrier film and heat dissipating sheet because of its gas impermeable properties and high thermal conductance. Toward their application, large lateral size is preferable. Besides, mass production method is also an important issue. However, sonication-assisted liquid exfoliation takes long processing time of 24-96 hours, and obtained BNNSs was relatively small size of several hundreds nanometers.
In this study, we developed novel production method for BNNSs with a large lateral size by using supercritical fluid (SCF). In the previous work, we have conducted graphene exfoliation by using SCF and indicated that SCF method can yield graphene with high monolayer content [1,2]. By applying these knowledges, we conducted BNNSs production from h-BN particles. The aqueous dispersion of h-BN particles was injected into the heated reactor by a high pressure pump and treated in the supercritical fluid condition. Then, the dispersion flows to a cooling region, where it is quenched room temperature. We repeated the series of heat treatment to enhance exfoliation efficiency.
As a result, we succeeded in peeling off h-BN. By using transmission electron microscopy, we confirmed that the lateral size of many samples was approximately 4-6 μm and thickness of these was approximately 5-20 nm. BNNSs produced by this method have a quite large area and a high aspect ratio, which cannot be obtained by the conventional method. The productivity was reached to 200 mg/h by using the flow-type reactor. This method will contribute to the mass production of BNNSs for its useful applications.
[1]D. Rangappa et al. , Chem. Eur. J. 16, 6488-6494 (2010). [2]T. Tomai et al. , Appl. Phys. Lett. 100, 233110 (2012).

Single photon emissions have been found in atomic thin semiconductors such as WSe₂ flakes and monolayer hexagonal boron nitride. The observed sharp peaks were attributed to the exciton emissions localized at impurity states or edge states of the flakes. Here, we report on single photon emissions in monolayer WSe₂ by tuning the hydrostatic pressure at a temperature of 8 K. It is noted that the intensity of the broad emission peaks at zero pressure gradually decrease and disappear with increasing pressure. Instead, several narrow emission lines occur. The second-order correlation function measurements reveal the single photon characteristics of the narrow emission lines. The linewidth and lifetime of single photon emissions are approximately 1 meV and 28 ns, respectively.

The rapid growth in two dimensional (2D) materials research is due to their ability to demonstrate exceptional (and controllable) 2D electrical and optical properties with associated applications in FETs and memory devices, optical and chemical sensors, and energy harvesting. Key to the successful development and application of these 2D materials is the in-depth characterization of their fundamental structural, electronic and optical properties. In this paper we discuss the advanced characterization of a family of 2D materials important for device applications, including strained ReS₂, SnSe device structures and black phosphorous thin films. Both 2D material and 2D device structures will be characterized via such methods as micro UPS, nano LEEM and micro-diffraction, as well as HRTEM/STEM (aberration corrected and EELS) and SAXS/GISAXS structural characterization and EDX elemental mapping. The relevance for each characterization method relative to the materials and device systems, and the roadmap for advanced characterization tool development for 2D materials will be discussed.

Layered WTe₂ has recently exhibited giant magnetoresistance¹, and has been predicted to be a Weyl semimetal² and to exhibit a quantum spin Hall state in the monolayer limit³. Thus, large-scale direct growth of WTe₂ nanostrctures or thin films is highly desired, but has yet to be demonstrated due to its low bonding energy, compared with other transition metal dichalcogenides. Here, two-step process atmospheric pressure chemical vapor deposition was developed to achieve direct synthesis of large-scale WTe₂ thin films. Its semi-metallic and polycrystalline nature has been studied by transport measurements. Owing to the stable T_d structure, active basal plane and Te edge sites, and high conductivity, our thin films exhibit very low Tafel slope of~ 40 mV/dec in hydrogen evolution reaction (HER), which is comparable with that of the best WS₂ HER catalysts. Our study suggests WTe₂ is a promising candidate as a high performance and low-cost HER electrocatalyst.
1. Ali, M. N.; Xiong, J.; Flynn, S.; Tao, J.; Gibson, Q. D.; Schoop, L. M.; Liang, T.; Haldolaarachchige, N.; Hirschberger, M.; Ong, N. P.; Cava, R. J., Large non-saturating magnetoresistance in WTe₂. Nature 2014, 514 (7521), 205.
2. Soluyanov, A. A.; Gresch, D.; Wang, Z. J.; Wu, Q. S.; Troyer, M.; Dai, X.; Bernevig, B. A., Type-II Weyl semimetals. Nature 2015, 527 (7579), 495-498.
3. Qian, X. F.; Liu, J. W.; Fu, L.; Li, J., Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 2014, 346 (6215), 1344-1347.

Copper-based heterogeneous catalysts are used in several of the chemical processes related to the production and consumption of hydrogen, including methanol synthesis, the water gas shift reaction and preferential oxidation, where they demonstrate good yields and much lower costs than comparable noble-metal catalysts. There is consequently much interest in the fundamental understanding of the interaction between hydrogen and the surfaces of the copper-based catalysts.
In terms of fundamental studies, Cu single crystal based model catalysts are used for understanding reactions at the atomic level. Cu(111), for instance, can be oxidized to form Cu₂O/Cu(111) one layer 2D thin films under ultra-high vacuum (UHV) conditions. This presentation probes the hydrogenation process of a model Cu₂O/Cu(111) catalyst system combining traditional UHV studies with novel high pressure studies using complementary ex situ and in situ techniques, including scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS). At high temperatures and low H₂ pressures, the reduction is fast; whereas at room temperature and ambient H₂ pressures, the intermediate phases co-exist on the surface after a long induction period before the surface is fully reduced to Cu(111). An autocatalytic feature is clearly demonstrated by the ambient pressure STM measurements. We propose the importance of under-coordinated copper atoms for the observed reactivity.
We have extended this in situ hydrogenation studies to Cu₂O/Cu(111) supported 2D ceria nano-islands model catalysts by ambient pressure STM. This allows us to directly identify the topographical changes during the hydrogenation process. The observed ceria-copper interface expansion promotes the understanding of the high activity of ceria-copper catalysts for methanol synthesis from CO₂.

Electronic materials with nontrivial band topology hold great promise for realizing novel devices with low power consumption and heat dissipation. Here using first-principles approach, we go beyond binary tranition metal dichalcogenides, and predict that a class of layered ternary transition metal chalcogenides (TTMC) MM'Te₄ exhibits dual topological characteristics: quantum spin Hall insulators in their 2D monolayers and topological Weyl semimetals in their 3D noncentrosymmetric crystals upon van der Waals (vdW) stacking. Remarkably, by tuning the vdW spacing, one can create and annihilate Weyl fermions, and realize the transition between Type-I and Type-II Weyl fermions. In addition, the ternary nature of TTMCs offers greater tunability of electronic structure by controlling different stoichiometry and valence charges. TTMCs, therefore, could be an ideal materials system for exploring quantum spin Hall effect and topological phase transition, and may open up new avenues for both two-dimensional and topological materials research. (Reference: http://arxiv.org/abs/1605.03903)

We carried out Raman measurements on few-layer MoSe₂ up to 8 layers by using 8 different excitation energies from 1.58 to 3.81 eV. An additional peak is observed near the A_1g peak (~ 240 cm⁻¹) in bulk MoSe₂ ^[1]. In the case of few-layer MoSe₂, we observe that the A_1g mode peak splits into multiple peaks, which are dependent on the thickness of the sample, due to so-called ‘Davydov splitting’. The weak van der Waals interaction between equivalent layers results in the splitting of the intra-layer vibration modes. In addition, the intensities of the split peaks are dependent on the excitation energies due to the exciton-phonon coupling. When the excitation energy is closed to the C exciton energy (2.71–2.81 eV), the intensities of these peaks are enhanced. We also compare the excitation energy dependent intensities of the other vibration modes, the E_1g, E_2g¹, A_2u, 2LA, shear and breathing modes, to study the exciton-phonon coupling. The intensity of each mode shows a different resonant behavior due to excitonic resonance effects.

References
[1] Dahyun Nam et al., Sci. Rep. 5, 17113 (2015).

Reversible polymorphism of monolayer transition-metal dichalcogenides has currently attracted much attention from both academic and applied perspectives, e.g. for the formation of intimate contacts and memory applications. Of special interest is MoTe₂, where the stable semiconducting 2H and metastable (semi)metallic 1T′ phases have a rather small energy difference implying the low-energy cost of such a transition. In this work, using first-principles calculations, we demonstrate that there exists a previously unknown phase of MoTe₂, namely a distorted trigonal prismatic phase with alternating shorter and longer bonds and bond angles, that is formed in the electronically excited state due to population inversion. This phase, which is unstable and decays to the ground 2H state after cessation of the excitation, is metallic and can act to lower the energy barrier on the way to the metastable 1T ′ phase. In addition to providing a deeper understanding into the physics of the 2H-1T′ phase transition, the presence of this phase in MoTe₂ can be used for fabrication of ultra-fast optical modulators and related devices.

There is a great thirst for replacing indium tin oxide (ITO) due to indium’s cost and diffusivity into active layers plus its low throughput. Other than that, ITO is brittle and cannot be used in flexible electronics. Many research groups around the globe are working on synthesis of new composite materials to replace ITO. Therefore, our research group has developed a new 2D materials based composite which can be a suitable candidate for replacing ITO. Herein, we report on the elaboration and transportation properties of a sandwich like 2-dimensional Ti₃C₂T_x (MXene)/Graphene composite through alternating electrospray of MXene and Graphene materials. All deposition parameters including flow rate, stand-off distance, applied potential, nozzle to and fro movement and stable Taylor cone-jet covering area have optimized using nozzle 8 for electrohydrodynamic atomization of the dispersions namely Ti₃C₂T_x (MXene) and graphene. The structural and electrical properties were systematically investigated with respect to the graphene content. The surface roughness of the samples has shown to decrease considerably after graphene integration. Electrical measurements show an increase in electrical conductance and Hall mobility with respect to the graphene content. Electrical conductance has increased from 2967 S/cm to 9.5x10⁴ S/cm and Hall mobility has increased from 9.70 to 54.58 cm²/Vs for only 2.5 wt. % of graphene, rendering Ti₃C₂T_x. This MXene based composite material is one of the most electrically conductive to date. This conductive composite can be applied in thin film electronic devices for achieving better efficiencies.

Layered two-dimensional materials have obtained interest because of to their unique properties since the isolation of the graphene in 2004[1]. The metal-dichalcogenides materials also have attracted interest due to their potential use for components of next-generation electric devices[2]. There are two different phases in metal-dichalcogenides materials such as 2H, which is semiconducting material and stable at room temperature, and 1T', which is semimetallic material. The molybdenum ditelluride (MoTe₂) have been reported their electrical properties with different phases[3]. In this study, the in-plane thermal conductivity of 2H-MoTe₂ with 8.8 nm thickness had been measured by the suspended heater method[4]. The phonon dispersion relation of bulk and monolayer MoTe₂ had been calculated based on density functional theory. The increased anharmonicity with low-lying optical branches in bulk MoTe₂ causes the thermal conductivity reduction. This result may help to understand phonon transport in layered two-dimension materials.

Reference:
1. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.
2. Chhowalla, M., et al., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nature Chemistry, 2013. 5(4): p. 263-275.
3. Keum, D.H., et al., Bandgap opening in few-layered monoclinic MoTe2. Nature Physics, 2015. 11(6): p. 482-U144.
4. Kim, J., Effect of spatial confinement on thermal transport in nanomaterials, in Department of Mechanical Engineering. 2016, Yonsei University.

Surface condition of the substrate plays a crucial role in crystal growth. In layered system, it has become more significant because modification of the surface can enhance the interaction between substrate and precursor. Transition metal dichalcogenides (TMDs) have been studied about the surface condition treated by promoter which enables its monolayer growth. In fact, the origin for interaction enhancement is still ambiguous although a number of growth reports has been carried out by introducing promoter. Here, we report that the role of promoter of TMDs, especially MoS₂, and what kind of materials can be act as promoter in our system. We found out that transition metal is bound with promoter and SiO₂ to form mixed phase, which is proved by set of X-ray photoelectron spectroscopy (XPS) experiments. Also, this growth mechanism is thermodynamically expected by the Gibbs free energy changes. We applied the platform to demonstrate the growth of as-known materials and their heterostructures to prove the compatibility with previous reports. Our study therefore not only provides the insight for investigating growth mechanism of TMDs, but also warns decomposition of the insulating (SiO₂) surface for electronics.

Transition metal dichalcogenides (TMDs) belong to the family of van der Waals bonded 2D materials, which have attracted tremendous interest from scientists around the world since the first isolation of graphene on SiO₂/Si substrate in year 2005¹. Among all the TMDs, molybdenum disulfide (MoS₂) is particularly noteworthy due to its excellent performance as a field effect transistor², promising applications in photonics, optoelectronics, piezoelectric devices³, and the control of valley polarization and coherence in single- and multi-layer MoS₂⁴. As a promising candidate for the next-generation electronics, large-scale single- and few-layer MoS₂ grown by chemical vapor deposition (CVD) method^5,6 is an important advancement towards the technological implementation of this material. However, compared to exfoliated MoS₂, CVD grown atomically thin MoS₂ often contains structural defects such as point defects, grain boundaries⁷, mixed stacking sequences⁸ and dislocations. These local structural defects can significantly impact the macroscopic properties of MoS₂, and are presumably caused by the high-temperature growth condition of CVD method. To establish the process-structure correlation, in-situ monitoring the growth process of MoS₂ at high temperature is the key. Here, we demonstrate a novel way to investigate this growth process in-situ by atomic-resolution scanning transmission electron microscopy (STEM). In this setup, high-energy electron beam serves as a cutting tool and constantly supplies Mo and S atoms from the neighboring material, while new MoS₂ layers are growing from the supplied atoms at high temperatures. During this process, atomic-resolution STEM is used to capture the structural evolution of the newly grown MoS₂. Grain boundary migration, stacking sequence selection and collective defects formation are observed, and detailed mechanisms for these structural evolution processes will be discussed in combination with ab initio molecular dynamics simulations.
¹ K.S. Novoselov, A.K. Geim et al., Science 80, 306, 666 (2004).
² B. Radisavljevic et al., Nat. Nanotechnol. 6, 147 (2011).
³ H. Zhu et al., Nat Nano 10, 151 (2015).
⁴ R. Suzuki et al., Nat. Nanotechnol. 9, 611 (2014).
⁵ A.M. van der Zande et al., Nat. Mater. 12, 554 (2013).
⁶ S. Najmaei et al., Nat. Mater. 12, 754 (2013).
⁷ W. Zhou et al., Nano Lett. 13, 2615 (2013).
⁸ A. Yan et al., Physical Review B 93, 4 (2016).

Two-dimensional (2D) energy materials exhibit distinctly different characteristics from their bulk counterparts, providing a number of exciting opportunities for fundamental studies and technological applications. Ultrathin VOPO₄ nanosheets could be a promising new 2D graphene-like material with greatly enhanced electrochemical properties due to the synergic advantages of high redox potential and the 2D layered structure. Here we will present our recent studies of intercalation chemistry in ultrathin VOPO₄ nanosheets for advanced energy storage devices including flexible pseudocapacitors, Li-ion and Na-ion batteries. The ultrathin VOPO₄ nanosheets exhibit high capacities, high rate capabilities and good cyclabilities for alkali ions storage due to the large contribution from intercalation pseudocapacitance. The demonstrated alkali-ion intercalation pseudocapacitance represents a promising direction for developing battery materials with promising high rate capability.

Two dimensional (2D) transition metal (M) carbides and nitrides (MXenes) are expanding rapidly with more than 20 materials already synthesized in that system and structures and properties of more than 30 other MXenes theoretically predicted. MXenes’ versatile chemistry renders their properties tunable for a variety of applications, such as energy storage devices, reinforcement for composites, water purification, catalysts in chemical industry, bio- and gas-sensors, lubricants, photocatalysts, electromagnetic shielding, etc.
A 2D flake of MXene consists of two, three or four layers of an early transition metal (M: Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo) that are interleaved with carbon and/or nitrogen (X) layers (M₂X, M₃X₂ and M₄X₃). Due to their synthesis from aqueous environment, they have surface termination of OH, O and F. It has been shown recently that two different transition metals can occupy different M layers, giving an additional way to control MXenes’ electronic, magnetic, optical, and electrochemical properties. For example, replacing the two outer Ti layers in a metallic Ti₃C₂ MXene with Mo turns it into a semiconductor Mo₂TiC₂. Here, a theoretical and experimental overview of how changing the ordering of two different transition metals can change MXenes from metals to semiconductors, and change their properties from nonmagnetic to magnetic and their electrochemical behavior from capacitive to a battery-like will be discussed.

Energy band alignment of atomically thin transition metal dichalcogenides (TMD) are investigated via photoemission spectroscopic analysis, suggesting versatile work function for monolayer TMD samples that is considerably substrate-dependent. The influence of underlying substrate on the carrier density of atomically thin 2D crystal verifies its fickle Fermi energy to allow aligned vacuum level under electrical contact, upholding the prerequisite of constructing band alignment by Anderson’s rule. As a result, quantitative agreement of valence band offset in various van der Waals heterostructures is manifested between experimental measurements and theoretical prediction deduced from Anderson’s rule, justifying the extraordinary interlayer coupling developed between atomically abrupt interfaces such as MoS₂/WS_2, MoS₂/WSe₂, and MoSe₂/WSe₂. This behavior typifies the unique property of 2D crystals to enable a new route for designing heterojunction-based devices; likewise it postulates the possibility of tuning electronic property of 2D crystal by supporting substrates, providing a facile strategy to diversify heterojunction-based electronic/optoelectronic devices.

Lateral, or in-plane, 2D material heterostructures such as h-BN/graphene are subject to the formation of interface misfit dislocations as a result of the lattice mismatch between the two materials. As in 3D materials, the formation of strain-relieving dislocations in 2D heterostructures is energetically favorable only when the dimensions of the in-plane "film" and "substrate" are large enough that the strain relief justifies the energetic cost of the defect. Here we consider several possible misfit dislocation core reconstructions and compute critical thicknesses for 2D material lateral heterostructures as a function of the thickness ratio of the "film" (e.g. h-BN) and the "substrate" (e.g. graphene). We show how the climb motion of a misfit dislocation relates to the relief of mismatch strain, and to the dislocation core structure, namely whether the core reconstructs in the 8-6 or 5-7 geometry, or whether it dissociates into partial dislocations, as seen in some recent experimental work. Our analysis of dislocation structure is shown to be consistent with moire patterns observed experimentally. We quantify the energetics of the various dislocation structures, and present an analysis that leads to identification of both thermodynamically and kinetically limited critical thickness definitions. The effect of temperature on the relationships between the two definitions is also analyzed. Our analysis makes it possible to identify structures and growth regimes in which it is possible to achieve defect-free heterostructure design over significantly larger length scales.

Atomically smooth van der Waals materials are structurally stable in monolayer and few layers but some are susceptible to the oxygen-rich environment. In particular, recently emerging materials such as black phosphorus and perovskite have revealed stronger environmental sensitivity than other two-dimensional layered materials, often obscuring the interesting intrinsic electronic and optical properties. Unleashing the true potential of these materials requires the oxidation-free sample preparation that protects thin flakes from air exposure. Here, we fabricated the few-layer hafnium disulfide (HfS₂) field effect transistors (FETs) using an integrated vacuum cluster system and study their electronic properties and stability under ambient conditions. By performing all the device fabrication and characterization procedure under oxygen- and moisture-free environment, we found that AA-stacked few-layer HfS₂-FETs display excellent on/off ratio (I_on/I_off ~ 10⁷) with reduced hysteresis compared to the FETs prepared under ambient conditions. Oxidation of HfS₂ occurs uniformly over the entire area, increasing film thickness by 250% at a prolonged oxidation time of > 120 hours, while defects on the surface are the preferential initial oxidation sites. We further demonstrated that the stability of the device in the air is significantly improved by passivation FETs with BN in a vacuum cluster.

Our elastic model of ErAs disordered GaAs/AlAs superlattices exhibits a local thermal conductivity maximum as a function of length due to exponentially suppressed Anderson-localized phonons. By analyzing the sample-to-sample fluctuations in the dimensionless conductance, g, the transition from diffusive to localized transport is identified as the crossover from the multi-channel to single-channel transport regime g=1. Single parameter scaling is shown to hold in this crossover regime through the universality of the probability distribution of g that is independent of system size and disorder strength.

Among 2-dimensional (2D) van der Waals materials, black phosphorus (BP) has recently received great attentions due to its outstanding properties, such as a tunable band gap and high hole mobility.¹ While BP has the fascinating physical properties, it has a disadvantage. BP is easily oxidized and it hinders researchers from observing the pristine properties of BP. Thus, the experimental observations on electrical properties of pristine BP is lacking. The electrostatic force microscopy (EFM) is a useful technique to figure out the electrostatic properties in a nanoscale, such as surface potential, local charge distribution and local dielectric constant due to its high spatial resolution. Moreover, since the oxidation process of BP could be continuously observed in the topology image of EFM, pristine and oxidized properties of BP can be separated.
In this study, we performed the EFM measurements on the BP flake with various thickness to investigate the surface potential, localized charge distribution and dielectric constant depending on the oxidation. As the oxidation progresses, we successfully observed that the surface potential and the dielectric constant of BP were significantly decreased. In contrast to the band gap of BP, however, the obvious dependence of the surface potential on the thickness has not been revealed in the thickness range over 10 nm.

Reference
(1) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9 (5), 372–377.

Three-dimensional (3D) graphene network reinforced Cu composite was synthesized using various size of Cu particles with chemical vapor deposition (CVD). Firstly, the shape of Cu disk was prepared by the axial compaction of Cu particles in a mould with a double action oil hydraulic press, and CVD was conducted on the Cu disk at appropriate temperature, finally the 3D graphene reinforced Cu composite was obtained. In order to show the 3D graphene network, Cu metal within the Cu composite was removed with etching solution. In the room-temperature electrical test for the Cu composite, it is confirmed that its conductivity is comparable to that of OFHC. Therefore, if the average size and shape of 3D graphene network are controllable, then this 3D graphene network reinforced Cu composite is thought to have various applications in the field of structural materials which require good electrical and heat conductivities, good attrition resistance, as well as good corrosion resistance.

Functionalized graphene is promising for many chemical and biological sensor applications, such as electrochemical detection of glucose and solution pH sensors. The chemical behavior of bilayer graphene is relatively unknown compared to monolayer graphene, which has been far more widely studied. Bilayer graphene is less chemically reactive than monolayer graphene, however, bilayer graphene is more attractive for electronic applications rather than monolayer graphene because of its tunable band gap. In this research, bilayer graphene was synthesized by hot-filament vapor deposition (HFCVD) uniformly on copper foils, the high quality of the bilayer graphene was confirmed by Raman spectroscopy mapping. For the functionalization of large area bilayer graphene, manganese chloride molecules were used as precursor and a xenon lamp as the light source of the photochlorination process. The samples obtained were analyzed using TEM, FTIR, Raman spectroscopy, and sheet resistance to study the resulting changes in the physical and electronic structures.

We develop nanoscale graphene wigglers (or serpentine ribbons) on Ge substrates to investigate a novel approach for the excitation of terahertz (THz) radiation. As a result of a DC voltage applied across the serpentine conductor, carriers traveling along the sinusoidal path are expected to periodically accelerate/decelerate and correspondingly radiate, according to a cyclotron-like mechanism. The record large mobilities recently reported for graphene on Ge provide a sufficiently long mean free path to enable efficient radiation based on the proposed mechanism. In our work, we use a simple time-of-flight and interference argument to estimate the radiation frequency produced by a graphene wiggler on a Ge substrate. In the simple case of a sinusoidal trajectory of period L and amplitude A, an emission frequency in the THz range requires wigglers with L<100 nm, and A of the order of ~50 nm. In addition, we estimate that the wiggler period, L, needs to be reduced to ~20 nm. In addition, high A/L ratios are desired to obtain significant levels of radiated power. Based on our theoretical analysis, we fabricate graphene wigglers by transfer to Ge gratings with 45-75 nm half-pitch, and with amplitudes of 50-100 nm, respectively. We achieve nanoscale gratings on Ge using a combination of electron beam (e-beam) lithography, hard masking, and dry etching. E-beam lithography allows us to carefully control the pitch of the Ge grating. Hard-masking and dry etching enable transfer to bulk Ge of patterns with high aspect ratio between amplitude and period. We perform scaffold-assisted transfer of single-layer graphene grown via chemical vapor deposition (CVD) on Cu. Next, we remove the scaffold and treat the graphene/Ge combination in ethylene glycol to facilitate graphene conformation to the nanoscale pattern. The graphene sheet is ultimately shaped into an array of nanoribbons via top-down processing. Scanning electron microscopy and atomic force microscopy confirm the conformal contact between graphene and Ge gratings on a 500x500 μm² scale. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show no significant effect of the transfer process on the quality and structural integrity of the graphene. Finally, we use van der Pauw and Hall measurements to estimate carrier concentration and carrier mobility (and mean free path) in the wiggled graphene film.

Monolayer Transition Metal Dichalcogenides (TMDs), such as MoS2 and WS2, are direct band gap semiconductors that emit light in the visible spectrum and expand the possible applications of 2D materials beyond what graphene alone can achieve. The properties of these TMDs depends heavily on the presence of defects and dopants. I will present recent results on the atomic resolution analysis of MoS2 and WS2 using aberration corrected transmission electron microscopy. The reconstructions around sulfur vacancies will be presented, showing subtle differences in the bonding and compression strain from the atomic loss of sulfur atoms. The behaviour of sulfur line vacancies at high temperature (800oC) has been examined using an in-situ heating holder, showing drastically different dynamics and structure compared to the room temperature behaviour. I will present a detailed examination of the bonding structure of the atomically sharp zigzag edges of MoS2, as well as edges terminated with MoS nanowires. The presence of dopant atoms in MoS2 and WS2 will be discussed, in the form of subsitutions in the lattice replacing Mo in MoS2, as well as surface bound adatom dopants. These results help the understanding of the exact structural form of defects in TMDs, their evolution and transformation, and subsequent stability, which has important consquences for interpreting defect bound exciton emission in photoluminescence and charger carrier scattering in electronic transport measurements.

2D materials represented by graphene, h-BN, and transition metal dichalcogenides (TMD) are of great interest among the researchers all over the world for its intriguing physical properties and its application toward various devices. When exploited in device materials such as a channel material in transistors, the band gap has a great impact on its performance. TMD, unlike graphene, exhibits band gap which prevent current leakage suppressing excessive power consumption. As well as its favorable properties in terms of use in device, its physical properties have attracted much of attention such as the shift from direct to indirect band gap between single-layer and multi-layer, the possibilities to tune the band gap, and so on. Synthesizing TMD alloys enables the chance to tune the band gap. There have been reports on alloying both the metal (Mo_1-xW_xS₂) [1] and the chalcogen (MoS_2(1-x)Se_2x ) [2] parts of the material synthesized by chemical vapor deposition. In this study, we report the synthesis of MoS_2(1-x)Te_2x by co-sputtering deposition and effect of mixture on its band gap. The band gap is reported to vary, under simulation, roughly between 1.0 eV to 1.8 eV for single layer not taking into account of so-called bowing effect [3].
MoS_2(1-x)Te_2x was synthesized by sputter deposition, co-sputtering MoS₂ and MoTe₂ target at the same time. The deposition was carried out at room temperature, enabling the deposition of MoS_2(1-x)Te_2x mixture alloy. The film thickness was controlled as 6.5 nm (10L). Sputtering conditions were as follows: RF power = 100 W (for both targets), Ar pressure = 0.3 Pa, sample holder rotation speed = 20 rpm. Following the deposition the sample was subjected to vacuum annealing with the temperature up to 600 °C for crystallization.
The composition ratio was investigated with X-ray photoelectron spectroscopy and determined to be Mo:S:Te = 1:1.2:0.3. Band gap was estimated from Tauc plot using its absorption coefficient obtained by analysis with spectroscopic ellipsometry and the sample showed band gap of 1.11 eV. The resulting band gap was lower than that of bulk MoS₂ and higher than that of bulk MoTe₂ suggesting mixture of both
materials was achieved to form MoS_1.2Te_0.3 by co-sputtering.

Reference:
[1] H. Liu, et al., Nanoscale 6, 624 (2014)
[2] J. Mann, et al., Adv. Mater. 26, 1399 (2014)
[3] H. P. Komsa and A. V. Krasheninnikov, J. Phys. Chem. Lett.. 3, 3652 (2012)

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have emerged as a promising material system for next-generation of optoelectronic devices due to the myriad of new physics observed at the monolayer limit as well as their ability to form lattice-match free heterostructures and high tunability by electric field and strain. However, the primary figure-of-merit for optoelectronic devices, the room-temperature photoluminescence quantum yield (QY), is extremely poor for these new materials. Here we will present an air-stable solution-based chemical treatment by an organic superacid which uniformly enhances the photoluminescence and minority carrier lifetime of MoS₂ monolayers by over two orders of magnitude resulting in a final QY of over 95% with a longest observed lifetime of 10.8±0.6 nanoseconds [1]. Through studying the luminescence QY over a large pump dynamic range in the chemically treated samples, biexcitonic recombination is identified as the primary intrinsic non-radiative pathway at high exciton concentrations. We will also present on the applicability of this treatment method on other TMDC systems [2] as well as MoS₂ prepared by chemical vapor deposition. Obtaining perfect optoelectronic monolayers opens the door for highly efficient light emitting diodes, lasers, and solar cells based on 2D materials.
[1] M. Amani, D.-H. Lien, D. Kiriya, J. Xiao, A. Azcatl, J. Noh, S. R. Madhvapathy, R. Addou, S. K. C., M. Dubey, K. Cho, R. M. Wallace, S.-C. Lee, J.-H. He, J. W. Ager III, X. Zhang, E. Yablonovitch, A. Javey, "Near-Unity Photoluminescence Quantum Yield in MoS2", Science, 350, 1065-1068, 2015.
[2] M. Amani, P. Taheri, R. Addou, G. H. Ahn, D. Kiriya, D.-H. Lien, J. W. Ager III, R. M. Wallace, A. Javey. "Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides", Nano Letters, 16 (4), 2786–2791, 2016.

To synthesize two dimensional (2D) transition metal dichalcogenides (TMDs), the substrate is necessary and the substrate properties have direct impact on the structural,^1–3 electrical,^4,5 and optoelectronic properties of TMDs.^6,7 Therefore, understanding the interactions between the MoS₂ films and the substrate is critical. Here, we report the first successful doping of Mn into monolayer MoS₂ and discuss the substrate impact on the efficacy of doping. Mn is not able to be doped into monolayer MoS₂ on sapphire and SiO₂ by in-situ powder vaporization, but it is able to be doped when the substrate is inert (i.e. graphene). The dangling bonds on sapphire and SiO₂ is believed to be responsible for the preclusion of doping. Beyond the doping, engineering the sapphire substrate properties can be an effective route to achieve epitaxial growth of MoS₂. Here we use surface characterization methods (contact angle test, AFM and XPS) to correlate the sapphire surface properties and MoS₂ properties. It turns out that higher surface energy is critical for the epitaxial MoS₂ growth on sapphire. Considering the energy balance of MoS₂ orientation on a substrate,¹ higher substrate surface energy enables MoS₂ to overcome the energy barrier between the symmetry-equivalent and non-equivalent orientation. Interestingly, strong charge transfer (coupling) between MoS₂ and sapphire is also observed in the epitaxial MoS₂. Ultra-fast decay time is identified by time-resolved photoluminescence measurement, indicating the insufficient radiative recombination. Decoupling techniques used to reduce the substrate-film interactions will be described, including the impact of the decoupling on device performance, which demonstrates a >100x increase in mobility after decoupling.
(1) Ji, Q.; Kan, M.; Zhang, Y.; Guo, Y.; Ma, D.; Shi, J.; Sun, Q.; Chen, Q.; Zhang, Y.; Liu, Z. Nano Lett. 2015, 15 (1), 198-205.
(2) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.-C.; Ajayan, P. M.; Lou, J. Nat. Mater. 2013, 12 (8), 754–759.
(3) Van der Zande, A. M.; Huang, P. Y.; Chenet, D. a; Berkelbach, T. C.; You, Y.; Lee, G.-H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A; Hone, J. C. Nat. Mater. 2013, 12 (June), 554–561.
(4) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat. Nanotechnol. 2011, 6 (March), 147–150.
(5) Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lopez-Sanchez, O.; Krasnozhon, D.; Chen, M.-W.; Gillet, P.; Morral, A. F. i; Radenovic, A.; Kis, A. ACS Nano 2015, 9 (4), 4611-4620.
(6) Li, Y.; Qi, Z.; Liu, M.; Wang, Y.; Cheng, X.; Zhang, G.; Sheng, L. Nanoscale 2014, 6 (24), 15248–15254.
(7) Buscema, M.; Steele, G. a.; van der Zant, H. S. J.; Castellanos-Gomez, A. Nano Res. 2015, 7 (4), 561–571.

In order to improve material properties, nanostructuring, alloying, doping or creating a material surface / interface are the common methods. Studies of those process influences on 2D nanomaterials and their properties are important, because they offer an opportunity to explore new structure-property relationships in the 2D nanomaterials and generate the next generation of nanomaterials to meet materials performance requirements. Two-dimensional transition metal dichalcogenides (2D-TMDs) are of broadening research interest due to their novel physical, electrical, and thermoelectric properties. Having the chemical formula MX₂, where M is a transition metal and X is a chalcogen, there are many possible combinations to consider for materials-by-design exploration. However, to develop these materials into wide-scale use, it is crucial to comprehensively understand the compositional affects. We applied various criteria, such as supply risk and restriction, environmental implication, structural stability, and processing, to narrow down the candidate list from 62 to 18 potential TMD compounds. We then implemented a combination of first-principles approach and Boltzmann transport theory, coupled with experimental measurements, to quickly screen electrical and thermal properties for 18 TMD compounds. Our results identified key factors to optimize MX₂ compositions for desired performance. We paid particular attention to substitutional doping and heterostructure effects on electrical and thermal properties of transition metal dichalcogenide TMD materials. We found that atomic weight, radius, oxidation state and interfacial lattice mismatching play key roles to optimize the properties. In the presentation, we will discuss their effects and identify the structure-property relationships for materials development with desired properties and performance.

We have investigated methanol absorption on MoS₂(0001) by using photoemission and inverse photoemission spectroscopy. The absorption of methanol on MoS₂ results in the formation of methoxy [1], evident from the change of the valence and conduction band as well as the core level shift of the carbon and oxygen. Both the binding energies of oxygen and carbon are shifted by 0.7 eV to lower binding energy. While adsorption and conversion to methoxy is exothermic at a MoS₂ defect [1,2], an activation barrier is experimentally evident in methanol adsorption on MoS₂. The DFT calculations demonstrates that the reaction products generated in the process of the methoxy formation could create more defects and more defects could reduce the activation energy to adsorption at the higher methoxy coverages on MoS₂, as is observed. To better understand the surface electronic structure, the symmetries of the occupied of MoS₂(0001) bands have been identified by using symmetry selection rules applied to angle-resolved photoemission, which suggests bonding is dominated by S p_z and Mo d_z2 weighted orbitals frontier orbitals.
This work was supported by the U.S. DOE through grant DE-FG02-07ER15842
[1] Y.-Y. Chen, M. Donga, Z. Qina, X.-D. Wen, W. Fan, J. Wang, J. Molecular Catalysis A: Chemical 338, 44–50 (2011)
[2] D. Le, T.B. Rawal, and T.S. Rahman, J. Physical Chemistry C 118, 5346-5351 (2014)

Alloy engineering has the potential to increase the flexibility and versatility of transition metal dichalcogenides for two-dimensional heterostructure device applications. This process generates composition-dependent material properties that differ from those of pure samples, and thus extends their functional capabilities. Mo_1-xW_xTe₂ alloys are particularly interesting because they may exhibit a potentially tunable structural transition that is useful for applications such as phase change memory. In addition, they are predicted to host nontrivial electronic states as a type-II Weyl semimetal. Previous studies have made efforts to understand the band structure of Mo_1-xW_xTe₂ alloys, but a detailed and systematic characterization of electron-phonon and phonon-phonon interactions is lacking.
Here, we explore the evolution of the vibrational properties of Mo_1-xW_xTe₂ (0 ≤ x ≤ 1) with alloy composition. Co-polarized and cross-polarized Raman measurements at room temperature are compared to definitively assign the symmetry of all modes. This analysis allows us to extract the relative Raman tensor components for each mode, and we find it necessary to allow for the presence of a complex interaction term in the Raman tensors. We also document the change in frequencies and linewidths for all modes with alloy composition, which correspond to the structural phase changes in the alloys on the micron length scale. Polarized Raman measurements also allow us to observe both the presence of two hybrid modes as well as the development of an asymmetrical line broadening of a W associated peak in only the alloyed samples. Interestingly, the hybrid modes agree with infrared active modes calculated for MoTe₂ and WTe₂ in other studies, but may also be related to disorder of the lattice. On the other hand, the asymmetrical line broadening of the W associated peak increases as the percentage of W is decreased, and thus may indicate disorder in the crystal lattice on the sub-micron length scale. The results of this study provide an understanding of the composition dependence of the vibrational properties and crystal structure of Mo_1-xW_xTe₂ alloys.

MXene materials, i.e., transition metal carbides or nitrides, have recently gained interest as a developing class of 2D materials with applications geared towards energy storage, catalysis, and electronic devices. To better understand the physiochemical and electronic properties, detailed atomic resolution structural analysis of monolayer MXene was investigated using a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density functional theory (DFT). Large area Ti₃C₂T_x MXene flakes, were synthesized and the type and concentration of atomic scaled defects were analyzed. Ti vacancies and Ti vacancy clusters were found to be the most prevalent defects. Using DFT we calculate the energy required to create these defects and confirmed the defects are mainly on the surface layers from HF etching. DFT also suggests that the surface defects alter the termination groups and C atoms in close proximity. The surface defects, however, do not change the metallic behavior according to the density of state. To measure conductivity field effect devices were fabricated from single and multi-layer Ti₃C₂T_x flakes. Our devices confirmed that single layer and multi-layer Ti₃C₂T_x are metallic conductors with conductivity greater than 1T MoS₂ and other reported metallic 2D materials, and comparable to graphene. Our results thus shed light on the future nano-electronic application using 2D metallic MXene single layers.


Research supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Aberration-corrected STEM imaging was conducted as part of a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy Office of Science User Facility.

Two-dimensional (2D) crystals such as graphene and transition metal dichalcogenides exhibit unique optoelectronic properties that are absent in their bulk counterparts. Interestingly, these properties can be tuned via defect-engineering, doping, and surface functionalization.(1) Although creating defects or impurity doping in 2D crystals could negatively impact their optoelectronic properties, they can also lead to unexpected enhancements or even lead to new optoelectronic phenomenon. One such example are defect states in monolayer (ML) WSe₂ acting as single-photon emitters with very narrow emission peak width (~meV) in comparison with their intrinsic emission peaks (~meV).(2) While there have been a number of works on defect-mediated optoelectronic phenomenon in ML crystals, defect engineering in van der Waals (vdW) heterostructures is still in their infancy. In this presentation, we demonstrate that introducing oxide defects in a WSe₂/EG vdW heterostructure leads to new tunneling phenomena that could potentially impact a variety of novel devices such as resonant tunneling diodes or memristors. Pristine WSe₂ atomic layers were grown on epitaxial graphene (EG) via a metallic-organic chemical vapor deposition process to form a synthetic vertical WSe₂/EG heterostructures. Oxygen defects were subsequently introduced into the WSe₂/EG via UV-Ozone treatment at room temperature. We find that the WSe₂ terrace is more robust against oxidization compared to WSe₂ edges, and many-layer WSe₂ is more resistant to oxidization than monolayers. Furthermore, scanning tunneling spectroscopy (STS) reveals their band structures are significantly different compared to their pristine counterpart. This band structure modification via oxide-defect formation also significantly impacts the vertical electrical transport of the WSe₂/EG heterostructure. The electrical measurements indicate an unexpected, defect-mediated, non-linear current-voltage (I-V) relationship, while the pristine ones exhibit more traditional I-V properties. The nonlinear curves exhibit record negative differential resistance (NDR) values, with significantly narrower tunneling peak width (<30 mV) and some of the highest reported peak-to-valley ratios (P/V~20). Furthermore, large hysteresis in the I-V curves in oxidized-WSe₂/EG layers are very similar to metal-oxide-metal-based and grain-boundary-induced memristors.(3,4) This study shows how defects in vdW heterostructures can dominate their electronic properties and also demonstrates that defect-engineering in vdW heterostructures may be an important route forward for developing new optoelectronic phenomenon.
(1) Z. Lin et al., 2D Mater. 3, 022002 (2016)
(2) C. Chakraborty et al., Nat. Nanotechnol. 10, 507–511 (2015)
(3) D. B. Strukov et al., Nature 453, 80-83 (2008)
(4) V. K. Sangwan et al., Nat. Nanotechnol. 10, 403–406 (2015)

This talk will first provide an overview of different defects in 2-Dimensional materials including graphene and Chalcogenides. In particular, we will focus on: 1) defining the dimensionalities and atomic structures of defects; 2) pathways to generating structural defects during and after synthesis and, 3) the effects of having defects on the physico-chemical properties and applications. We will then discuss the synthesis of large-area, high-quality monolayers of nitrogen-, boron- and silicon-doped graphene sheets on Cu foils using ambient-pressure chemical vapor deposition (AP-CVD). Scanning tunneling microscopy (STM) and spectroscopy (STS) demonstrate that defects in doped graphenes arrange in different geometrical configurations exhibiting different electronic and chemical properties. Interestingly, these doped layers can be used as efficient molecular sensors and electronic devices. In this context, Graphene enhanced Raman spectroscopy will be introduced and it will explained that by doping graphene with Nitrogen, the Fermi level (E_F) of graphene shifts, and if this shift aligns with the lower unoccupied molecular orbital (LUMO) of a molecule, charge transfer would be enhanced, thus significantly amplifying the molecule's vibrational Raman modes. Concentrations as low as 10^-11 mol/L of different dye molecules can be detected using GERS. Finally, we will discuss the controlled synthesis and assembly of chalcogenide monolayers on different substrates. The electronic performance of monolayers of MoS2, WS2 and hetero-systems operating under flexural strain will also be presented. Our findings demonstrates that it is now possible to use chalcogenide layers for the fabrication of flexible electronic devices, however, defect control is required to tailor their performance.

In recent years tremendous efforts have been devoted to the research on two dimensional materials. Among them transition metal dichalcogenides (TMDs) have attracted significant attention owing to their unique structures, remarkable properties, and great potential for a wide range of applications in electronics, optoeletronics, valleytronics, catalysis, etc. The synthesis of high quality large area mono- and few-layer TMD materials is highly desirable for their applications. In this talk I will present the chemical vapour deposition (CVD) approach we have developed to synthesis these TMD materials, two examples, MoS2 and MoTe2 will be discussed.

While new species and properties of two-dimensional (2D) materials are being reported with extraordinary regularity, a significant bottleneck in the field is the ability to controllably produce and process material. To date, proof-of-principle 2D material based device fabrication has involved the isolation of monolayers followed by making metal contact with the film, commonly using e-beam lithography. Regardless of the technique used to produce monolayer, devices are then typically fabricated using lithography and metal deposition, which may require further processing in order to obtain good metal-semiconductor contacts. Here, we envision and present a chemical vapor deposition process whereby semiconductor monolayer is selectively grown only in deterministically patterned regions leaving the metallic patterns conductive after the growth process. By using different combinations of metallic pattern and oxide based precursor, heterostructured MoS2/WS2 growth has been observed as well. Monolayer films, with lateral extent of up to hundreds of microns are controllably grown on and around patterned regions of bulk transition metals. The materials display strong luminescence, monolayer Raman signatures, and relatively large crystal domains. In addition to producing high optical quality monolayer material deterministically and selectively over large regions, the metallic patterns remain conductive and therefore have the advantage of providing as-grown metallic contacts to the material, offering a path for device fabrication and large scale production.

Transition metal dichalcogenides (TMDs) provide a family of materials with a variety of physical, chemical and electrical properties that are especially suited for novel applications in the fields of electronics, optics and photovoltaics. However, similar to the initial work done on graphene, much of the work has focused on mechanically exfoliated samples. In this talk we provide a basic model of the growth of TMDs using vapor transport. We use Raman and Photoluminescence spectroscopy as tools to understand the defects in vapor transport grown materials and possible routes to improve material quality. These techniques are also used to understand the effect of strain on TMDs. Novel techniques such as Microwave Impedance Microscopy (MIM) are used to show how elemental deficiencies in TMDs can lead to changes in local electrical properties. Using vapor transport grown material we show that high room temperature mobilities (70 cm²/Vs), current densities ( > 200 μA/μm) and high on/ off ratios (> 10⁷) can be attained. We also demonstrate that vapor transport grown material can be useful for high frequency applications ( fT = 6.7 GHz, fmax = 5.3 GHz). Finally we show that MIM maps obtained in the on vs off states of TMD devices clearly show localized insulator to metal transition.

Two-dimensional (2D) materials, such as Graphene, hBN and MoS₂, are promising candidates in a number of advanced functional and structural applications, owing to their exceptional electrical, thermal and mechanical properties. Understanding mechanical properties of 2D materials is critically important for their reliable integration into future electronic, composite and energy storage applications. However, it has been a significant challenge to quantitatively measure mechanical responses of 2D materials, due to technical difficulties in the nanomechanical testing of atomically thin membranes. Additionally, actively tuning functional properties of 2D materials by stress/strain is a very attractive approach for engineering these atomically thin components in practical applications. In this talk, we will discuss our recent effort to determine the engineering relevant fracture toughness of graphene with pre-existing defects, rather than the intrinsic strength that governs the uniform breaking of atomic bonds in perfect graphene. Our combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Strategies on how to improve the fracture resistance in graphene will be discussed. Also in this talk, we report the atomic structure and morphology of the grains and their boundaries in the CVD grown polycrystalline molybdenum disulfide atomic layers. The implications of the effects of defects such as grain boundaries and interfaces on functional properties of 2D atomic layers will be discussed. Finally, the concept of strain engineering in 2D materials will also be discussed and demonstrated.

Transition metal dichalcogenides (TMDs) have been gathering much attention as a next-generational material instead of graphene. In particular, molybdenum dichalcogenide (MoX₂) (X:S,Se) was expected as an energy device material for secondary batteries and hydrogen evolution. As an alternative of a cathode material in secondary batteries, MoX₂ nanosheets with a large specific surface area are preferable for a cation adsorption. In addition, it is supposed that an interlayer spacing is controlled by changing a ratio of elemental sulfur and selenium and will affect an ability of cation storage between layers. Lately, several liquid and solvothermal processes are proposed for the fabrication of MoX₂ nanosheets. However, MoX₂ fabricated by means of those processes do not have the large specific surface area and its solid solution, Mo (S_xSe_1-x) ₂, has not been fabricated.
In this study, we succeeded in the fabrication of flower-like MoX₂ nanosheet with large specific surface area by using SCF chalcogenization process for metal-oxide precursor^1,2). Nano-sized MoX₂ fabrication is based on a chemical reaction of MoO₃, elemental sulfur (S₈), selenium (Se), and NaBH₄ under a supercritical ethanol atmosphere in a hastelloy reactor at 400°C for 2h. NaBH₄, S₈ and Se are dissolved into supercritical ethanol, and deoxydation and chalcogenization reactions of MoO₃ simultaneously proceed. After the reaction, the reactor was rapidly cooled down in water at room temperature.
The SEM and TEM analyses confirmed that a morphology of the obtained MoX₂ was a flower-like sphere with the diameter of ~100 nm. These spheres were consisted of the MoX₂ nanosheets with ~10 layered structure. Results of Raman spectrum analyses support the existence of multilayer MoX₂ nanosheets. We suppose that the decrease in the flower size was caused by an increasing affinity of MoX₂ to Ethanol in the different pressure conditions. MoX₂ flower size decreased as ethanol density increased. Moreover, we can synthesize a solid solution of Mo (S_xSe_1-x)₂ by changing the ratio of sulfur and selenium source. The WDX analysis showed the composition of sulfur and selenium in MoX₂ was controllable.
Performances of MoX₂ as a cathode material of Li-ion battery were measured in discharge and charge tests between cutoff voltages of 0.5 and 3.0 V at a current density of 100 mAg^-1 by using CR2032-type coin cells: working electrode, MoX₂ (80 wt%), conductivity agent (10 wt%, acetylene black), and binder (10 wt%, PVDF); counter and reference electrodes, Li metal; electrolyte, a solution of 1M LiPF₆ in EC/DEC. It was confirmed that MoX₂ was working well during charge-discharge cycles and its capacity showed 65% of theoretical capacity at 5^th cycle.

Refenrences
Y. Nakayasu, T. Tomai, N. Oka, I. Honma, Appl. Phys. Express, 8, 021201 (2015).
T. Tomai, Y. Nakayasu, M. Yanaka, I. Honma, J. Supercrit. Fluids, 101, 48 (2015).

Copper sulfide, which has been extensively researched, is considered a p-type semiconductor with a 1.5 eV bandgap, exhibits promising thermo-electric properties, and has been mostly used for harvesting light in photovoltaic devices. Using a simple chemical vapor deposition (CVD) process, we synthesized large samples of layered-crystalline copper sulfide. By exposing copper foils or copper wires to an atmosphere containing sulfur to temperatures ranging from 250°C to 850°C, we transformed the copper material into copper sulfide in a one-step anneal under inert gas flow. Using scanning electron microscopy (SEM), we observed crystallinity with diverse original grain morphologies with specific symmetry and orientations such as triangles, pyramids, etc. Each morphology correlated to specific process parameters. One of the copper sulfide samples showed a dominant phase of Cu₉S₅, as shown by extensive high-resolution electron transmission microscopy (HRTEM) and XRD characterizations. HRTEM and atomic force measurements (AFM) showed that the synthesized copper sulfide is a layered material and that we were able to isolate single layers. We are currently exploring the specific electrical properties of this layered compound.

Developing new 2D materials beyond graphene offers the expansion of the properties of 2D systems and enhances their utility in electronics and opto-electronic applications. Monolayer transition metal dichalcogenides (TMDs), MoS₂ and WS₂, are direct band gap semiconductors. Interfacing these TMDs with other 2D materials can lead to heterostructures with ideal excitonic properties for opto-electronics. I will discuss recent work on the formation of ionic metal chloride (CuCl) 2D crystals epitaxially templated on the surface of monolayer MoS₂. This new 2D crystal has no bulk layered form and exists purely through strong interactions with the uniform periodic surface of the MoS₂ lattice. The template, monolayer MoS₂, was grown by chemical vapour deposition (CVD) using previously reported methods^1,2 and the detailed investigations on its structural defects have been published³. This novel heterostructured 2D system is studied at the atomic level using aberration corrected transmission electron microscopy at the low accelerating voltage of 80kV. Dynamics of discrete rotations of the CuCl crystal are observed, maintaining epitaxial correlations to the MoS₂ lattice directions. Strain maps are produced from displacement maps and used to track real time variations of local atomic bonding and defect production. New properties of the heterostructure system will be discussed and their potential impact on opto-electronic applications.
References
1. Wang, S. et al. Shape Evolution of Monolayer MoS₂ Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 26, 6371–6379 (2014).
2. Wang, S., Pacios, M., Bhaskaran, H. & Warner, J. H. Substrate Control for Large Area Continuous Films of Monolayer MoS₂ by Atmospheric Pressure Chemical Vapor Deposition. Nanotechnology 27, 085604 (2016).
3. Wang, S., Lee, G.-D., Lee, S., Yoon, E. & Warner, J. H. Detailed Atomic Reconstruction of Extended Line Defects in Monolayer MoS₂. ACS Nano 10, 5419–5430 (2016).

Ultrathin van der Waals materials such as transition metal dichalcogenide (TMD) have attracted tremendous interest until recently owing to their direct band gaps, rich excitonic dynamics, and valley polarization characteristics. In addition, the lack of dangling bonds on the surface of these materials promotes the formation of chemically stable and atomically sharp heterointerfaces. In-plane or vertically stacked heterostructure based on TMDs are emerging as an alternative to silicon-based optoelectronics. However, large-scale fabrication of these layered heterostructures are still technically challenging, even though several proof-of-principle based report exist. Especially, controlling the formation of atomically thin in-plane, periodic superlattice, and site-specific doping without the use of sophisticated nanofabrication tools can revolutionize the fabrication of next generation optoelectronic nanodevice. In this study, we developed a simple yet generic approach for laser induced chemical modification of TMD materials. In this post-growth approach, thin-layered material of WSe₂ has been locally converted to WS₂ in a homemade sealed mini-chamber, which allows laser irradiation of the sample in a controlled sulfur-rich atmosphere and simultaneous Raman and PL characterization. The site-specific conversion from WSe₂ to WS₂ was carried out using a focus laser beam (532 nm) that induce local heating followed by chemical exchange with the reactive sulfur atmosphere. The whole process was monitored by mapping the temporal evolution of specific Raman active modes during the in-situ conversion steps. The process can be tuned thereby choosing appropriate laser power, exposure time, and reactive gas environment. The complete conversion of WSe₂ to WS₂ was further confirmed by collecting the photoluminescence spectra at the desired exposed site. The chemical exchange is not fully understood but it is expected that the laser serve two major functions, that is, creation of Se vacancies in the TMD materials, and cracking of sulfur containing molecules from the gas environment. The released S molecules are then incorporated into the Se vacancy sites. The optimization of the above mentioned parameters will allow to develop techniques based on photo-induced chemical reactions for local doping, alloying and the fabrication of in-plane TMD heterostructure.

We demonstrate that electrodes assembled from chemically exfoliated nanosheets of metallic 1T phase MoS₂ can be used for electrochemical actuation. Previously, we reported that 1T phase MoS₂ is attractive for electrodes in high energy and high power density supercapacitors [1]. High charge storage performance was attributed to high electrical conductivity, surface charges and dynamic expansion of layers via cation intercalation. Such expansion behavior can also be utilized to transform directly to mechanical energy. Accordingly, we have fabricated bimorph actuator with MoS₂ and analyzed the electromechanical behavior in various electrolytes. Our findings show that charge storage induces a reversible in-plane electrode expansion up to 0.8 %, generating enough mechanical force to bend bimorph actuator and lift masses 100 times heavier than its own weight for a few millimetres. Such high actuation performance even at high frequencies can be attributed to high conductivity , fast proton insertion and comparably higher elastic modulus of MoS₂ layers.

References:
[1] Acerce M., Voiry D., Chhowalla M., Metallic 1T phase MoS₂ nanosheets as supercapacitor electrode materials,Nature Nanotechnology, 2015,10, 313–318.

Black phosphorus (BP) has recently joined the two-dimensional material family as a promising candidate for photonic applications. Compared with gapless graphene, the moderate bandgap (~0.3 eV) of BP thin films allows for suppressed off-state current and low noise photodetection. BP thin films also offer much stronger photon absorption than monolayer 2D materials such as graphene and transition metal dichalcogenides. However, BP’s unique potential compared with other layered materials in the mid-infrared wavelength range has not been revealed. In this work, we demonstrate BP thin film photodetectors operating in a broad wavelength range, spanning from 532 nm to 3.39 µm. It is experimentally clarified that dominant photocurrent generation mechanism of BP thin film under low incident light power is the photo-gating effect, which results in large photoconductive gain above 10⁴ at 532 nm. At 3.39 µm, the BP thin film photodetector exhibits an external responsivity of up to 82 A/W. Noise measurements further show that such BP thin film photodetectors are capable of detecting mid-infrared light in the pico-watt range at room temperature. The impressive performances, including high photoconductive gain, relatively high speed in kilo-hertz range and at the same time low detection limit, can be coalesced to realize chip scale mid-IR sensors and imagers operating at low light levels. In addition, we show that BP’s low-crystalline symmetry leads to a photoresponse related to both the incident light polarization and the photocurrent collection direction, which may be utilized for applications that require polarization sensitive detection schemes.

ACKNOWLEDGEMENTS
We thank the financial support from the Air Force Office of Scientific Research (FA9550-14-1-0277) and the National Science Foundation (EFMA-1542815).

The size reduction and economics of integrated circuits, captured since the 1960’s in the form of Moore’s Law, is under serious challenge. Current industry roadmaps reveal that physical limitations include reaching aspects associated with truly atomic dimensions, and the cost of manufacturing is so significant that only 2 or 3 companies can afford to develop leading edge capabilities. To address the physical limitations, 2D materials such as graphene, phosphorene, h-BN, and transition metal dichalcogenides have captured the imagination of the electronics community for advanced applications in nanoelectronics and optoelectronics. The ideal materials properties have much appeal, but the reality of defects, impurities, and materials integration constraints will surely compromise the intrinsic performance of such device technologies. This talk will present a sample of our recent work examining defects, impurities, passivation, as well as the process integration challenges and potential solutions.
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA, the SWAN Center, a SRC center sponsored by the Nanoelectronics Research Initiative and NIST, and the US/Ireland R&D Partnership (UNITE) under the NSF award ECCS-1407765.

Next-generation electronics calls for new materials beyond silicon, aiming at increased functionality, performance, and scaling in integrated circuits. In this respect, two-dimensional gapless graphene and semiconducting transition metal dichalcogenides have emerged as promising candidates due to their atomic thickness and chemical stability. However, difficulties in the precise spatial control during their assembly currently impede actual integration into devices. Here, we report on the large scale, spatially-controlled synthesis of heterostructures made of single-layer semiconducting MoS₂ contacting conductive graphene. Transmission electron microscope studies reveal that the single-layer molybdenum disulphide nucleates at the graphene edges. Critically, this geometry allows graphene to serve as a metal contact and inject current into the MoS₂. These chemically-assembled atomic transistors exhibit high transconductance (10 µS), on-off ratio (~10⁶), and mobility (~17 cm² V⁻¹ s⁻¹), while the graphene also lowers the contact resistance of MoS₂ compared to traditional metals by an order of magnitude. The precise site selectivity from atomically-thin conducting and semiconducting crystals enables us to exploit these heterostructures to assemble two-dimensional logic circuits, such as a NMOS inverter with a high voltage gain (up to 70).

Due to their extraordinary optoelectronic properties, 2-D materials have been identified as promising materials for integrated photonics. However, most 2-D material-integrated photonic devices demonstrated to date are fabricated by transferring a layer of 2-D material on top of already fabricated photonic structures, which limits full utilization of their capability. Here we introduce a new photonic integration approach via direct deposition and fabrication of chalcogenide glass photonic devices on 2-D materials. We have applied the new process to fabricate high-performance, broadband on-chip graphene-based optical polarizers with a high contrast ratio of 740 dB/cm leveraging the remarkable optical anisotropy of graphene, and thermo-optic switches with a record heating efficiency of 10 nm/mW using in-waveguide low-loss (20 dB/cm) graphene transparent electrodes. The low processing temperatures of chalcogenide glasses further enables monolithic integration on plastics and the first waveguide-integrated graphene photodetector on flexible substrates. Last but not least, we have also demonstrated monolithic integration of chalcogenide photonic components on several other 2-D materials including WSe₂, WS₂, and MoTe₂. The glass-on-2-D-material approach therefore provides a facile universal route for photonic integration based on 2-D materials.

In this talk, I will discuss our recent work in developing novel electronic and optoelectronic devices based on the anisotropic properties of low-symmetry two-dimensional (2D) materials, including black phosphorus (BP) and rhenium diselenide (ReSe₂). High mobility, narrow gap BP thin film (0.3 eV in bulk) fill the energy space between zero-gap graphene and large-gap TMDCs, making it a promising material for ultra-low power digital logic, as well as near and mid-infrared optoelectronics. Most importantly, its anisotropic nature within the plane of the layers allow for the realization of conceptually new electronic and photonic devices. Here, I will first present our work in understanding the fundamental electronic and optical properties of low-symmetry 2D materials such as BP and ReSe₂. Our recent demonstration of a vertical BP transistor for ultralow power digital logic will then be discussed. Taking advantage of the unique properties of BP to realize the switching mechanism, the device can operate at a supply voltage of only 0.1 V with a steep sub-threshold slope below 45 mV/dec, breaking the 60 mV/dec fundamental limit defined by the energy dependence of electron density. Furthermore, I will also discuss our work on a novel application of anisotropic BP device for emulating hetero-synaptic behavior for neuromorphic computing. I will conclude with remarks on promising future research directions of low-symmetry electronics based on anisotropic 2D materials and how their novel properties is expected to benefit the next-generation electronics and photonics technologies.

Transition metal dichalcogenides (TMDs) have unique physical and electronic properties that make them attractive candidates for various electronic applications. However, the integration of TMDs in devices require low resistance contacts to the material. One promising candidate is graphene, demonstrated to produce low-barrier contacts to molybdenum disulfide (MoS₂) as well as allow the fabrication of “all-2D” electronics. Previously, graphene and other 2D materials have been overlaid onto various TMDs to produce vertical 2D heterostructures. However, this requires transfer techniques that are not scalable for fabrication of large arrays of devices. Recent developments in direct growth of heterostructures to overcome this challenge show that lateral heterojunctions between various TMDs or graphene-hexagonal boron nitride is possible, but very little is known regarding growth of graphene-TMD lateral heterojunctions. Here, we demonstrate that directly grown graphene-MoS₂ heterojunctions can exhibit unique transport properties based on the graphene carrier type.

Epitaxial graphene (EG) grown on silicon carbide (SiC) is patterned using conventional lithography techniques. MoS₂ is then grown directly on the SiC substrate using powder vaporization. The MoS₂ exhibits preferential growth outwards from the edge of the EG patterns and on the SiC substrate, producing lateral heterostructures without requiring transfer methods. Furthermore, the ability of EG to be modulated from n-type to p-type through hydrogenation gives this method the flexibility of producing p-n junctions or low-barrier contacts to both p- and n- type TMDs [Y.C. Lin et al, Nanoscale 8, 8947]. We demonstrate that the interface exhibits no direct bonding between the MoS₂ and EG, but is rather a localized vertical stack of pristine MoS₂ over the EG edge. The clean interface due to the direct growth process leads to a low-resistance contact between the EG and MoS₂. In addition, the contact between hydrogenated p-type EG and MoS₂ is revealed to have a significantly higher barrier to transport compared to the contact between as-grown EG and MoS₂. The electronic transport of the lateral graphene contact to MoS₂ is characterized for both doping polarities of graphene and benchmarked to vertical EG-to-MoS₂ and metal-to-MoS₂ contacts. Arrays of n-type metal oxide field effect transistors (MOSFETS) fabricated using this method is demonstrated. Furthermore, the morphology of the MoS₂ film, dependant on the effects of the lithographic process on the SiC substrate, is revealed to affect the transport properties through the device. It is envisaged that the direct-growth process can be extended to TMDs beyond MoS_2, providing a scalable, clean process for integration of various TMDs with low-barrier contacts for device applications.

Broad spectral range optical detection is of high interest for technological applications such as imaging, sensing, communication and spectroscopy. Two-dimensional (2D) materials are very promising for such applications due to their high optical absorption, wide detection range and material flexibility.
In this talk, graphene / silicon Schottky diodes made of chemical vapor deposited (CVD) graphene on n-type Si substrates will be discussed. The effects of incident light intensity and wavelength are investigated. The diodes exhibit good rectification and high sensitivity to light intensity. A broad spectral response of 60 - 407 mA/W is measured from ultraviolet to near infrared light [1]. In contrast to graphene, molybdenum disulfide (MoS₂) is an n-type semiconducting 2D material. Monolayer MoS₂ has a direct band gap of ~1.8 eV, whereas bulk MoS₂ has an additional indirect band gap of ~1.3 eV. MoS₂/Si hybrid diodes made with multilayer, CVD grown MoS₂ yield a maximum spectral response of 8.6 mA/W [2].
Furthermore, the hybrid integration of large area CVD single- and bilayer graphene as transparent conductive electrodes with amorphous silicon (a-Si) will be discussed for applications as multispectral photodetectors (MS-PDs). A 300 % enhancement of the detectors’ spectral response is observed in the ultraviolet region compared to reference devices with conventional aluminum doped zinc oxide electrodes. The maximum responsivity of these multispectral PDs can be tuned in their wavelength from 320 nm to 510 nm by external biasing, which allows single pixel detection of UV to visible light. The material combination of graphene and a-Si enables flexible diodes on polyimide substrates. Going from single to bilayer graphene boosts the maximum photoresponsivity of these flexible diodes from 134 mA/W to 239 mA/W [2].

Monoclinic (1T') MoTe₂ exhibits a structural phase transition to an orthorhombic phase (T_d) at approximately 250 K, breaking inversion symmetry. Recent studies have demonstrated that T_d-MoTe₂ phase exhibits nontrivial electronic properties such as pressure-induced superconductivity and a Type-II Weyl semimetallic phase. However, thus far there have been no studies exploring the vibrational properties of T_d-MoTe₂ or how they evolve across the T_d→1T' structural phase transition.

In this work, we use temperature-dependent Raman spectroscopy to examine the anharmonic behaviors of four prominent optical phonon modes in T_d-MoTe₂. The frequency of all modes exhibits a redshift that is linear with temperature from 100 K to 200 K, in agreement with the Grüneisen model. However, we also observe non-linearity in some modes below 100 K, which we find is consistent with anharmonic contributions to the phonon self-energy arising from the decay of an optical phonon into multiple acoustic phonons. This results suggest that phonon-phonon interactions can dominate anharmonic effects in the T_d phase, especially at lower temperatures. Furthermore, there is a significant change in the intensity and linewidths of high frequency Raman modes at around 250 K, which correlates well with the T_d→1T' structural phase transition. These results provide important information regarding the role of phonon-phonon interactions in T_d-MoTe₂ and pave the way for future studies exploring the effects of layer-number, strain, and superconductivity on the phonon anharmonicity.

Though metal oxides are widely used in the fabrication of many advanced devices, few reports have discussed their properties in 2D limit. Here we are reporting the discovery of a new 2D materials system, 2D tin monoxide (SnO). Due to the dipole-dipole interaction of lone pair originating from Sn 5s electrons, a Van der Waals gap of 2.52 Angstrom forms along [001] crystallographic direction of the layers which results in the 2D nature of SnO. High resolution transmission electron microscopy (HR-TEM) demonstrates the layer structure of 2D SnO films grown using a pulsed laser deposition method. Raman spectroscopic and X-ray photoelectron spectroscopic (XPS) analysis confirmed the formation of phase pure SnO. Optical transmittance shows the band-gap variation of SnO films from bulk to 2D layered films. Back gated field effect transistors (FET) using few layer SnO channels grown on SiO₂ substrates were successfully fabricated. These FETs show typical p-channel conduction with field effect mobility ranging from 0.05-1.9 cm²/Vs. Field effect mobility varies with the number of SnO layers and decreases on either sides of the optimum layer numbers (12), which is explained based on charge screening and interlayer coupling in layered materials.

We present the synthesis of the first two-dimensional (2D) transition metal nitride (Ti₄N₃), a new member of the family of 2D transition metal carbides and nitrides, known as MXenes. Nitride MXenes have been predicted to exist and have several potential advantages over their carbide counterparts, but their synthesis had previously not been reported. Transition metal nitrides are known to have higher electronic conductivities than carbides and can potentially offer better performance for transformation optics and metamaterial devices. Previously reported MXene synthesis methods involve the selective etching of the interlayer “A” element from the MAX phases in aqueous acidic solutions. We present a molten fluoride salt etching procedure that removes Al from a Ti₄AlN₃ powder precursor at 550 °C under an argon atmosphere. The multilayered two-dimensional nitride MXene was also delaminated to produce few-layered nanosheets and monolayers of Ti₄N₃T_x, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti₄N₃ and terminated Ti₄N₃T_x were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti₄N₃ are predicted to be metallic. Moreover, bare Ti₄N₃ is expected to exhibit magnetic properties, which is significantly reduced when functional groups are present.

Boron nitride (BN) is unknown in nature and only became available commercially in the latter half of the 20^th century. It is analogous to carbon in having both a cubic (diamond-like) phase (cBN) and a hexagonal (graphite-like) phase (hBN) [1]. cBN is second only to diamond in hardness, has a wide band-gap, good thermal conductivity and unlike diamond, can be doped both p- and n-type, making it suitable for high-power electronic applications. The high thermal and chemical stability and wide band-gap of BN have been exploited in the fabrication of deep-ultraviolet light emitting devices.

The electronic and optical properties of BN are heavily influenced by defects but due to the complex microstructure in BN, it is a challenge to reliably identify these defects. X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) are complimentary methods which can reveal the local bonding environments in materials. The near edge structure (NES), which occurs on the characteristic absorption edges is particularly sensitive to defects but its interpretation is not always straight forward. In this work, we employ ab initio theoretical modeling using CASTEP [2] to calculate the electronic structure and NES of important defects in BN. The theoretical results are compared with XAS and EELS results from crystalline, turbostratic, annealed and irradiated BN. Our work demonstrates that the microstructure and bonding in complex materials such as BN can be characterized using their NES.

[1] “Synthesis and Properties of Boron Nitride” (Mat. Sci. For., 54-55), eds J.J.Pouch & S.A.Alterovitz, trans tech publication (1991).
[2] S.J. Clark, M.D. Segall, C.J. Pickard, P.J. Hasnip, M.I.J. Probert, K. Refson and M.C. Payne, Zeitschrift für Kristallographie, 220, 567-570 (2005).

Hexagonal boron nitride (h-BN) is the two-dimensional, insulating allotrope of boron nitride with a planar structure analogous to graphene. Films of h-BN have been used as insulating spacers [1], encapsulating layers [2], substrates for electronic devices [3,4], oxidation and corrosion-resistant coatings [5,6], surfaces for growth of other 2D nanomaterials, such as graphene [7] and WS₂ [8], and recently as a single photon source [9]. The need for large-area, uniform h-BN has led to a multitude of studies synthesizing h-BN by chemical vapor deposition (CVD) [10-13]. While these studies have examined the low pressure and atmospheric pressure regimes and found conditions in both regimes that leads to thin h-BN films, little mechanistic information is available about the CVD growth of h-BN from ammonia−borane, especially at different growth pressures.

This study analyzes the optical, chemical, and electrical properties of CVD grown hexagonal boron nitride (h-BN) using the precursor ammonia-borane (H₃N–BH₃) as a function of Ar/H₂ background pressure (P_TOT) [14]. Films grown at P_TOT ≤ 2.0 Torr are uniform in thickness, highly crystalline, and consist solely of h-BN. At larger P_TOT, with constant precursor flow, the growth rate increases, but the resulting h-BN is more amorphous, disordered, and sp³-bonded. We attribute these changes in h-BN grown at high pressure to incomplete thermolysis of the H₃N–BH₃ precursor from a passivated Cu catalyst. A similar increase in h-BN growth rate and amorphization is observed even at low P_TOT if the H₃N–BH₃ partial pressure is initially greater than the background pressure P_TOT at the beginning of growth. h-BN growth using the H₃N–BH₃ precursor reproducibly can give large-area, crystalline h-BN thin films, provided that the total pressure is under 2.0 Torr and the precursor flux is well-controlled. Growth protocols for well-controlled h-BN synthesis require a careful balance between precursor flux, growth rate, and the catalytic properties of the growth surface in order to avoid incorporation of undesirable precursor byproducts in the grown h-BN film.

[1] L. Britnell, et al., Nano Lett. 12, 1707 (2012).
[2] A. Mayorov, et al., Nano Lett. 11, 2396 (2011).
[3] C.R. Dean, et al., Nat. Nano. 5, 722 (2010).
[4] P.J. Zomer, et al. Appl. Phys. Lett. 99, 232104 (2011).
[5] L. Li, et al., Adv. Mater. Interfaces 1, 1300132 (2014).
[6] Z. Liu, et al., Nat. Commun. 4, 2541 (2013).
[7] W. Yang, et al., Nat. Mater. 12, 792 (2013).
[8] M. Okada, et al., ACS Nano 8, 8273 (2014).
[9] T. Trong, et al. Nat. Nano. 11, 37 (2016).
[10] Y. Shi, et al. Nano Lett. 10, 4134 (2010).
[11] L. Song, et al., Nano Lett. 10, 3209 (2010).
[12] K. Kim, et al., Nano Lett. 12, 161 (2012).
[13] A. Ismach, et al., ACS Nano 6, 6378 (2012).
[14] J. Koepke, J. Wood, et al., Chem. Mater. Article ASAP (2016).

The spectrum of 2D and layered materials “beyond graphene” has been continually expanding. The realization of wide bandgap (E_g) 2D materials “beyond hexagonal boron nitride (hBN)”, however, has been limited. Along similar lines to initial theoretical discovery and subsequent experimental synthesis of “beyond graphene” 2D materials (i.e. silicene and borophene), theoretical studies have suggested that indium nitride (InN), gallium nitride (GaN), and aluminum nitride (AlN) take on a 2D graphitic structure with a thickness tunable energy E_g (~0.7-7.0 eV) due to quantum confinement. Despite the extensive computational discovery of 2D materials, the experimental synthesis of wide E_g 2D nitrides “beyond hBN” on technologically relevant substrates still remains elusive. We have developed a novel growth scheme, known as Migration Enhanced Encapsulated Growth (MEEG)¹, which utilizes the mechanism of intercalation via defects in graphene to stabilize wide E_g 2D materials that are not layered in bulk crystals. We demonstrate for the first time that 2D GaN not only can be stabilized, but also exhibits unique structural, optical and electrical properties from that of bulk material.
Here we elucidate the mechanism of 2D nitride formation and discuss the ability of the interface of quasi-free standing epitaxial graphene (QFEG) in providing sufficient thermodynamic stabilization of the (direct E_g ~5 eV) 2D buckled structure of GaN (R3m space group symmetry). In the case of 2D GaN, a layer of gallium intercalates between the hydrogenated QFEG and the SiC substrate. The intercalated bilayer of gallium is converted to a quintuple monolayer of 2D GaN via nitrogen intercalation from decomposed NH₃. Our density functional theory (DFT) calculations suggest that the atomic structure in 2D nitrides considerably impacts the stability and bandstructure. We verify the atomic structure by directly resolving the nitrogen and gallium atomic columns in 2D GaN using aberration corrected scanning TEM (STEM) in annular bright field (ABF) mode with supported ABF-STEM simulations. Our DFT calculations predict an energy E_g for 2D GaN in the range of 4.79-4.89 eV which correlates well with experimental results from UV-visible reflectance, absorption coefficient and low loss EELS measurements. Vertical transport measurements suggest 2D GaN acts as a Schottky barrier between graphene and SiC. High resolution x-ray photoelectron spectroscopy demonstrates that 2D GaN is stable in air for at least 24 hours after removal of the graphene cap.
We expand our novel growth scheme to the formation of other 2D nitrides, such as 2D InN, broadening the range of accessible E_g energies of 2D materials well into the deep UV. Recognizing the impact of 2D nitrides, it can be expected that the addition of 2D GaN will enable new avenues for scientific exploration and electronic/optoelectronic device development.
¹Graphene stabilization of two-dimensional gallium nitride Al Balushi Z.Y. et al. arXiv:1511.01871

In search of new layered and two dimensional materials, here, we consider nitrogen-rich transition metal, TiN₂ as a candidate material isotypic to MoS₂. Owing to the difficulty of synthesizing binary nitrogen-rich transition metal compounds, we first studied the properties of ternary compounds of the form MTiN₂ where M is an early alkali metal (Li, Na, and K). We then focused on how the binary TiN₂ can be obtained by chemical or electrochemical deintercalation of M in MTiN₂. Due to previously reported difficulties with complete deintercaltion of Li from LiMoN₂, in addition to Li, we also considered Na and K. These larger alkali metal could affect the likelihood of anti-site defects formation as well as the diffusion rates at low metal concentrations. Additionally, we studied the properties of bulk, bilayer, and monolayer of binary TiN₂. We found that TiN₂ is a direct band gap semiconductor in both the monolayer and bulk limits with no significant influence of interlayer interaction on the electronic structure. Interlayer binding energy in TiN₂ is smaller than in the known materials MoS₂ and MoSe₂.

Gallium nitride (GaN) is a wide-bandgap semiconductor at the center of light-emitting diodes and high power electronics. Interestingly, thinning GaN to a single layer leads to an atomic reconstruction that results in the enhancement of its band gap to 5 eV (compared to 3.4 eV in bulk form)¹. The ability to achieve 2D nitrides could therefore open up a new realm in the “Beyond Graphene” arena by providing a route to wide bandgap and ultra-wide bandgap 2D materials. This possibility is now becoming a reality, with recent demonstrations of 2D-GaN via intercalation techniques¹ and ammonolysis of mechanically exfoliated gallium selenide (GaSe)². However, to fully realize 2D nitrides for future applications, controllable and reliable synthesis methods must be developed. Here, we demonstrate that it is possible to directly synthesize 2D GaN through ammonlysis of ultra-thin GaSe synthesized via vapor phase deposition.

Powder vaporization of GaSe was carried out at temperatures between 700°C and 850°C, resulting in monolayer GaSe grown on SiO₂ with domain sizes as large as 20 µm. Photoluminescence measurements indicate the bandgap of GaSe is 3 eV, verifying it is monolayer.^3,4 Additionally, x-ray photoelectron spectroscopy confirms that Ga and Se are present in a 1:1 ratio, and that bonding in the grown material corresponds to that of GaSe. To convert GaSe to GaN, GaSe was exposed to ammonia for 30 minutes at 600°C. X-ray photoelectron spectroscopy has confirmed an exchange between nitrogen and selenium, resulting in the formation of GaN. However, significant etching of the monolayer GaSe also occurs, leaving primarily multi-layer GaN behind. To eliminate GaSe monolayer etching, we employ a protective graphene capping layer prior to ammonolysis, which provides a means to directly realize monolayer GaN. We will discuss the role that nitrogen precursor, graphene capping, and processing conditions play in Se/N exchange and GaN realization. Finally, we will also discuss the impact processing has on the optoelectronic properties of this newly discovered material.

1. Balushi, Z. Y. Al et al. Graphene stabilization of two-dimensional gallium nitride. arXiv Prepr. arXiv1511.01871 (2015). at
2. Sreedhara, M. B., Vasu, K. & Rao, C. N. R. Synthesis and Characterization of Few-layer Nanosheets of GaN and Other Metal Nitrides. Zeitschrift für Anorg. und Allg. Chemie 640, 2737–2741 (2014).

Defect engineering in solid-state materials is a rapidly progressing field with applications for quantum information processing, nanophotonics, and nanoscale sensing. While the majority of efforts have focused on three-dimensional wide-bandgap semiconductors such as diamond and silicon carbide, low-dimensional materials hosting single spins and single-photon sources can offer a unique physical platform due to intrinsic spatial confinement and the ability to create multifunctional layered materials. Recently, single-photon emitters with visible photoluminescence (PL) have been reported in the wide-bandgap van der Waals-bound material hexagonal boron nitride (h-BN), though many questions about the chemical and electronic structure of the defects remain.

We report on the creation and characterization of single-photon emitters in free-standing single-crystal h-BN membranes, where substrate interaction effects are eliminated. Exfoliation of single-crystal h-BN onto patterned SiO₂/Si substrates is followed by oxygen plasma cleaning, electron beam irradiation, and annealing in an inert atmosphere in order to induce the creation of fluorescent defects. Confocal scanning fluorescence microscopy enables the identification of isolated, visible single-photon source which are robust and stable over timescales of several months in ambient conditions. Their optical properties vary widely, including their PL spectra, lifetimes, brightness, visibility, as well as the orientation of their absorptive and emissive dipoles. Defects uniformly exhibit strongly polarized excitation and emission along differing directions, and a statistical analysis of the polarization dependence indicates a tentative correlation between the optical dipole orientation of some defects and the crystallographic axes of the h-BN host. An analysis of defect spectra show similarities in vibronic coupling for emitters with widely-varying energies. Photon autocorrelation measurements on single emitters exhibit photon bunching at intermediate times, indicative of metastable states with different lifetimes. The distinct spectral shapes and photon emission statistics suggest that several different classes of emitters can exist simultaneously in free-standing h-BN, whether they be different defects, different charge states of the same defect, or the result of differing local environments.

This work is supported by the Army Research Office (W911NF-15-1-0589) and NSF MRSEC (DMR-1120901).

Single photon emitter (SPE) in a solid state system, such as nitrogen-vacancy centers in diamond, have been investigated due to its broad applications in quantum technologies [1]. Recently, bright and robust SPE in low dimensional hexagonal boron nitride (hBN) were been reported [2]. Low dimensional materials are advantageous for photonic-integrated systems since they can be easily integrated with a large variety of photonic structures. The SPE in hBN has a broad inhomogeneous broadening across ~200 nm, which has been associated with local strain variations [3]. Here we supply supporting evidence of the strain-based inhomogeneous distribution. We demonstrate a strong dependence of the emission energy of hBN SPEs on local strain, which we vary deterministically in experiments. The SPEs were created by focused He ion irradiation of hBN flakes transferred onto polycarbonate substrates. The ability to spectrally tune bright and photostable SPEs in hBN opens a new possibility for engineered atom-like emitters and their integration with photonic integrated circuits and photonic crystal cavities.

[1] M. W. Doherty et al, New Journal of Physics, 13 (2011)
[2] T. Tran et al, Nature Nanotech 11, 37 (2015)
[3] T. Tran et al, arXiv:1603.09608 (2016)

Layered hexagonal boron nitride (hBN) is a wide band gap semiconductor (> 6 eV), which attracts a growing interest for its strong UV photoluminescence properties [1,2]. In addition, it is used in the context of engineering of 2D heterostructures. In particular electron mobility of graphene is known to be preserved when graphene is supported by a h-BN film. The optical properties of bulk hBN as well as BN nanotubes are governed, in the energy range 5.2-6 eV, by strong excitonic effects. They have been studied recently, but the experiments are difficult because of the necessity to work in the far UV range [3].

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) [4]. The excitons are mainly composed of electron-hole pair transitions around the high symmetry point K. The valence and conduction bands in this region can be well approximated by pure Nitrogen and Boron Bloch states respectively. As a result, the tight-binding model surprisingly accurate - with only a few adjustable parameters fitted to the ab initio energies - while remaining simple enough to offer physical insight.

We analyse the first five excitons of single layer hBN, corresponding to the first three bright peaks and two dark states. We perform a symmetry analysis of the full wavefunctions in direct and reciprocal space. Optical selection rules are derived which are in agreement with ab-initio results. 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. Furthermore, we analyse the Davydov splitting of the excitons into a sequence of bright and dark excitons as one goes from the single layer of hBN to few-layer systems and – ultimately – makes the transition to bulk hBN.
[1] P. Jaffrennou el al., Phys. Rev. B, 77 (2008) 235422.
[2] ] L. Wirtz et al., Phys. Rev. Lett. 100 (2008).
[3] L. Schué el al., Nanoscale, 8 (2016) 6986.
[4] T. Galvani et al., arXiv :1605.09581

The ability to convert photons of different wavelengths directly into mechanical motion is of significant interest in many energy conversion and reconfigurable technologies. Only few materials exist that can provide a direct chromatic mechanical response. Here we demonstrate a reversible and chromatic mechanical response in MoS2 nanocomposites between 405 nm to 808 nm with large stress release. The wavelength dependent actuation is as a result of the band structure of few layer MoS2 affecting optical absorption and subsequent mechanical response. Applying uniaxial tensile strains to the MoS2 crystals in the nanocomposite resulted in spatially varying energy levels inside the nanocomposite that enhanced the broadband optical absorption upto 2.3 eV and subsequent mechanical response. The unique photomechanical response in MoS2 based nanocomposites is a result of the rich d electron physics not available to sp bonded materials such as graphene and carbon nanotubes.

Monolayer Transition Metal Dichalcogenides (TMDs) provides a new two-dimensional (2D) platform for exploring the fundamental dynamics of excitons in low-dimensions. Even though monolayer TMDs have attracted attention due to their strong light-matter interactions and layer-dependent direct band gaps, monolayer TMDs heterostructures are still unknown in respect to quantum phenomena such as 2D confinement and tunneling effect. In order to understand quantum many body systems such as excitons, we investigated interlayer excitons generated by Coulomb force across TMDs Heterostructures using micro-photoluminescence spectroscopy (micro-PL), Photoluminescnece excitation spectroscopy (PLE) and Time-resolved Photoluminescence spectroscopy (TRPL).

As a result, we observe that interlayer exciton PL peak is blue-shifted and interlayer PL intensities become stronger as temperature decreases. These facts demonstrate that interlayer exciton dynamics are highly related to nonradiative recombination with thermal fluctuation. We further perform TRPL to investigate interlayer exciton lifetime. It shows that interlayer exciton lifetime is longer than intralayer exciton lifetime by three orders of magnitude, which is theoretically anticipated using density functional theory. In conclusion, we report interlayer exciton dynamics in monolayer TMDs heterostructures in terms of stacking order, the number of layers, temperature and photo-carrier injection. The tunable interlayer exciton gives us tremendous chances for photovoltaic application thanks to its spatially indirect band configuration and long lifetime.

Transition metal dichalcogenides (TMDs) have attracted significate attention as promising atomically thin semiconductors owing to their two-dimensional (2D) structures, finite bandgaps and reasonably high mobility. With the van der Waals interaction, TMDs can be flexibly integrated with different materials without the limitation of lattice matching, which opens up the possibility of next-generation functional electronic and optoelectronic devices, such as atomically-thin-body field-effect transistors (FETs), TMD/TMD or TMD/graphene heterostructures and devices. ¹ Compared with the widely studied MoS₂, WS₂ has similar properties but very limited research. However, the performance of WS₂ FETs is mainly restricted by the large contact resistance at the metal/WS₂ junction. Both density functional theory (DFT) calculations² and experimental work³ have demonstrated that a strong Fermi level pinning exists near the midgap of WS₂. Therefore, the commonly used method of using metals with a work-function close to the conduction band or valence band of semiconductors to reduce the Schottky barrier heights (SBHs) has become ineffective.

To solve this problem, we present a novel approach of reducing the SBHs based on the graphene/WS₂ heterostructures and the modulation of WS₂ bandgap. By increasing the number of layers, the WS₂ bandgap and the SBHs with Au/Cr can be reduced.⁴ At the same time, the Dirac point of graphene is aligned to the bottom of the conduction band of WS₂, and the barrier height between graphene and WS₂ can be easily tuned by graphene’s work function.^5,6 The SBH of graphene/WS₂ heterojunction has been studied using DFT supercell calculations to investigate the effect of graphene on tuning the SBH of metal/WS₂ contact. Experimentally, our WS₂ FETs were prepared using WS₂ flakes with different numbers of layers by micromechanical exfoliation. Monolayer graphene was synthesized by chemical vapor deposition (CVD)⁷ and then transferred onto WS₂ flakes to form graphene/WS₂ heterostructures. By modulating the number of WS₂ layers and inserting monolayer graphene between the metal and WS₂ interface, a very low contact resistance could be achieved, which would enhance the device performance.

1. Duan, X., Wang, C., Pan, A., Yu, R. & Duan, X. Chem. Soc. Rev. 44, 8859–8876 (2015).
2. Guo, Y., Liu, D. & Robertson, J. ACS Appl. Mater. Interfaces 7, 25709–25715 (2015).
3. Park, W. et al. Adv. Electron. Mater. 2, (2015).
4. Kim, H. H.-C. et al. ACS Nano 9, 6854–6860 (2015).
5. Georgiou, T. et al. Nat. Nanotechnol. 8, 100–3 (2013).
6. Yu, Y.-J. et al. Nano Lett. 9, 1–5 (2009).
7. Wu, X. et al. Sci. Rep. 6, (2016).

Electromagnetic interference (EMI) shielding have been attracted much attention for a wide range of applications in the modern high-power electronics, portable devices, and self-driving cars, as the highly integrated and high-speed wireless communication devices suffer from undesirable electromagnetic interference effect that not only deteriorates the performance of the devices but also brings serious concern on harmful health problem to human. For an EMI shielding material to be effective it should have high electrical conductivity. Until now, metal shrouds were the material of choice to combat EMI pollution. However, metal fillers add additional weight and with susceptibility to corrosion making them less desirable. As a result, lightweight, low-cost, high strength and easily fabricated shielding materials are desired. In this presentation, I would like to briefly demonstrate that sulfur-doped graphenes and transition metal carbides (MXenes) can be considered as the best candidates for EMI shielding materials. Sulfur-doping on graphene induces strong n-doping effect that gives rise to the strong improvement in electrical conductivity. MXenes are a family of two dimensional (2D) transition metal carbides and nitrides, with a formula of M_n+1X_nT_x, where M is an early transition metal (e.g. Ti and Mo), X is carbon and/or nitrogen, T is surface functional groups such as (–OH, =O and –F). This combination gives MXenes exceptional electrical conductivity. The large electrical conductivity of sulfur-doped graphenes and transition metal carbides (MXenes) is responsible for the exceptional EMI SE performance.

There is a large interest in establishing cheap, scalable processes for producing low dimensional semiconducting dichalcogenide films for electronic application. In this work, well exfoliated MoS₂ dispersions were prepared through two step liquid phase exfoliation process with N-methyl-pyrrolidone (NMP) and Isopropanol (IPA). The obtained exfoliated MoS₂ flakes were characterized by microscopy (TEM and SEM), Uv - Vis and Raman spectroscopy.
Bottom gate thin film transistors (TFTs) based on exfoliated MoS₂ film were fabricated by using spray coating techniques. The deposition process was optimized to get uniform and percolated MoS₂ film with different thicknesses. Directly after layer deposition, transistors show minor conductivity. However, depositing additional PMMA layer on top shows large improvement in electrical characteristics, i.e. proper switching behavior with changing gate voltage. Interpretation is that the PMMA layer improves the inter-flake contact and enables proper percolation. Further investigation on polymer electrolyte-gated MoS₂ transistors is carried out to improve contact resistance and mobility. This low-cost and scalable solution-based fabrication process will promote the application of dichalcogenides in future nanoelectronic devices.

MoS₂ has attracted considerable attention as an electronic material, due to its large bandgap (1.8 eV for monolayers) and two-dimensional atomic layer structure. While MoS₂ nanoribbons may eventually be used in applications for nanoelectronics and chemical sensing, only a few publications report fabrication of MoS₂ nanoribbon transistors. Here, we present the fabrication of single-layer MoS₂ nanoribbon transistors using tip-based nanofabrication. The fabrication starts with the transfer of chemical vapor deposition grown MoS₂ monolayers onto 270 nm thick SiO₂ on a substrate of highly p-doped Si. Then, we deposit 5 nm Ni/30 nm Au contact electrodes on top of MoS₂ monolayers. These electrodes form the source and drain of the MoS₂ device. Next, a heated atomic force microscope cantilever tip directly writes polystyrene (PS) nanoribbons on MoS₂ monolayers across electrodes. The PS nanoribbons serve as an etch mask and further oxygen plasma etching removes unmasked MoS₂. The width of the fabricated MoS₂ nanoribbons can be controlled over the range 30 nm to 600 nm, as measured using electron beam microscopy. We fabricated and tested several back-gated monolayer MoS₂ field effect transistors (FET). The results for a single 480 nm wide nanoribbon device are as follows. Before nanoribbon patterning, our monolayer MoS₂ FET had a current on/off ratio of 2x10⁴ and field-effect mobility of 3.45 cm²/Vs in ambient conditions. After nanoribbon patterning, the device has a high contact resistance, which we attribute to the constriction at the nanoribbon-electrode interface. The current on/off ratio was about 2x10², and the device field-effect mobility was 0.07 cm²/V. Subsequent deposition of 24 nm Al₂O₃ resulted in a current on/off ratio of 8x10⁴ and field-effect mobility of 9.65 cm²/Vs.

Van der Waals heterostructures based on two-dimensional (2D) materials have attracted considerable research interest in the past few years. In contrast to conventional heterostructures formed by covalent bonds that are generally associated with atomic/ionic inter-diffusion, the van der Waals heterostructure has an abrupt transition between the two materials and a sharp gradient of carrier concentration across the interface¹. Their unique structural and physical properties have enabled new possibilities for a large array of novel devices and applications ranging from vertical tunneling transistors², barristors³ to optoelectronic applications⁴. However, few have investigated their interaction with gaseous molecules and application as chemical or biological sensors.
In this work, we demonstrate a van der Waals heterostructure chemical sensor based on few-layered black phosphorus (BP) and molybdenum diselenide (MoSe₂) flakes. Due to the atomically thin nature of 2D materials, surface adsorption of gas molecules can effectively modulate the band alignment at the BP/MoSe₂ interface and, correspondingly, the electron transport characteristics of the device, making it a highly sensitive detector for chemical and physical adsorptions. Compared with sensors made of homogeneous nanomaterials on the same substrate, our device demonstrates a marked enhancement in detection limit and sensitivity by orders of magnitude for NO₂ detection. Kelvin probe force microscopy (KPFM) analysis confirms that the total built-in potential at the hetero-interface dramatically increases after exposure of NO₂ and provides direct evidence of the changes in band alignment due to NO₂ adsorption. Finite element model based on the quantitative KPFM results reveals that the modulation of barrier height in MoSe₂, which is induced by the modulation of both the total built-in potential and the ratio between majority carrier concentrations of both materials, is responsible for the enhanced sensitivity. Our work demonstrates the potential of van der Waals heterostructure as a fundamentally new platform for sensing applications and also provides insights into the interactions between gaseous molecules and 2D heterostructures.
References:
1. Fang, H. et al. Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides. Proc. Natl. Acad. Sci. U. S. A. 111, 6198–202 (2014).
2. Britnell, L. et al. Field-effect tunneling transistor based on vertical graphene heterostructures. Science 335, 947–950 (2012).
3. Yang, H. et al. Graphene barristor, a triode device with a gate-controlled Schottky barrier. Science 336, 1140–1143 (2012).
4. Xia, F., Wang, H., Xiao, D., Dubey, M. & Ramasubramaniam, A. Two-dimensional material nanophotonics. Nat. Photonics 8, 899–907 (2014).

1T'-Tungsten ditelluride (WTe₂), an important member of the transition metal dichalcognide family, has attracted much attention recently due to the observtion of the extraordinarily large and non-saturating magneto-resistance (XMR) of its bulk crystals at low temperatures and superconductivity under high pressure. Here, we have carried out magneto-transport experiment on ultra-thin WTe₂ field effect transistors that are far away from charge neutrality (in the heavily electron-doped regime). We found that the magnetoresistance (MR) of the samples is tunable by gate voltage, and the two-fluid model phenomenologically captured most of the physics in this regime. By tuning the 2D electron-hole imbalance from 8.2 × 10¹⁷m^-2 to 3.3 × 10¹⁷m^-2, we were able to change the MR of the devices by 900%. The change of MR could be as large as 2.2 × 10⁶ % if we could restore the mobility of device to that of its bulks crystals. Our result shows the potential of ultra-thin WTe₂ in the applications such as magnetic field sensors, read heads in high density hard disks, random access memories, and galvanic isolators.

With the rise of 2D materials in the past decade, there has been a multitude of new materials each exhibiting its own unique properties and characteristics. Many of these 2D materials complement each other in a way that enhances the performance of the films greatly, as in the example of hexagonal boron nitride enhancing the mobility of graphene films. More recently phosphorene has been shown to demonstrate some very desirable properties, including a direct band gap on the order or 1 eV and mobility values of up to 1000 cm²/Vs. Recent work has shown that a 2D p-n junction can be formed by stacking multilayer p-type black phosphorus and n type monolayer MoS₂, giving rise to a p-n diode. In this work we create and characterize p-n heterojunctions using monolayer to few layer p-type phosphorene and n-type monolayer MoS₂ films. Furthermore, we compare the use of both graphene and ITO as transparent electrodes for these devices. Using atomic force microscopy (AFM) and current mapping AFM (CAFM) we are able to accurately study the p-n junction characteristics under photoillumination with respect to phosphorene thickness and transparent electrode material. In addition, we create phosphorene-MoS2 heterojunction FET’s and electrically characterize them with respect to varying phosphorene thickness. The results demonstrate that by varying the bandgap of phosphorene with respect the number of layers, these p-n junctions have potential as broadband detectors.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000

Among two-dimensional materials, group IV elemental sheets such as graphene, silicene, germanene and stanene have been intensively investigated using first principles calculation^[1]. Recently, stanene has been proposed as topological insulator^[2], which has attracted great attention and motivated researchers to study a variety of properties of this material. Here, we perform electron-phonon coupling calculation of stanene combined with density functional theory and density functional perturbation theory with Wannier interpolation approach in order to obtain band structure, phonon dispersion, and most importantly, electron-phonon coupling matrix for all the phonon branches including out-of-plane acoustic (ZA), transverse acoustic (TA), longitudinal acoustic (LA), out-of-plane optical (ZO), transverse optical (TO) and longitudinal optical (LO) modes. We find that the electron and hole mobility are ~3650 cm²V^-1s^-1 and ~3800 cm²V^-1s^-1. The deformation potential constants for acoustic phonon mode and optical phonon mode are found to be one order of magnitude less than those of graphene. Although the smaller deformation potential constant in stanene than the value in graphene would indicate the higher mobility in the deformation potential approximation, instead the obtained mobility is about 100 times lower than that in graphene. This result can be described by the smaller slope of the band structure near the Fermi level, where more states in k space can couple with phonon modes, and by the phonons with smaller frequency, which causes larger contributions to scattering. We also find that the ZA phonon dominates the carrier scattering process. The origin of the strong coupling with the ZA mode is broken horizontal mirror symmetry due to the buckling of stanene.

[1] S. Balendhran, S. Walia, H. Nili, S. Sriram, and M. Bhaskaran, Small 11, 640 (2015).
[2] Y. Xu, B. Yan, H.-J. Zhang, J. Wang, G. Xu, P. Tang, W. Duan, and S.-C. Zhang, Phys. Rev. Lett. 111, 136804 (2013).

In the rapid scaling of Si Metal Oxide Semiconductor Field Effect Transistor (MOSFET), the channel resistance (R_ch) has been decreased continuously. On the other hand, the contact resistance (R_c), one of the main source of the external resistance (R_ext), has been kept relatively unchanged. As a result, R_c became one of the dominant factors limiting the performance of MOSFET.
One of the main cause of R_c is Schottky barrier between metal and Si in source and drain. Schottky barrier height (SBH) can be predicted by the Schottky-Mott rule from the difference between the metal work function and the conduction band edge of semiconductor. In principle, R_c can be reduced as lowering SBH with metal with proper work function. However, observed SBH’s in Si differ significantly from the predicted values, due to the pinning effect of Si.
The current approach to lower R_c is to change the carrier flow mechanism to tunneling emission. To increase the tunneling current, heavily doped Si was used to reduce the barrier width. However, this method is approaching its limitation because of the dopant solubility in Si, short channel effect, and dopant profile control. As an alternate option, metal-insulator-semiconductor (MIS) structure was presented. The interface insulator is working as a tunnel barrier and de-pinning material. However, this technique is lack of thermal stability and thickness controllability of insulator layer.
In this work, we propose a new method to reduce the SBH and R_c at the metal/Si interface with the insertion of semiconducting 2D materials, such as transition metal dichalcogenides. In this approach, an interface dipole formed at the metal-2D interface modulate the work function of metal to align with the conduction band edge of Si, leading to the reduction of SBH and the R_c. We obtain SBH of less than 0.2 eV with a proper choice of interlayer materials. We also simulate the Schottky barrier height of 2D inserted metal/Si interfaces and the results are in good agreements with experimental results.

Two-dimensional transition metal dichalcogenides, such as MoS₂ and MoSe₂, have potential applications in various electronic and optoelectronic devices, including field-effect transistors, photovoltaics and light-emitting diodes. Controllable synthesis of heterojunction between different transition metal dichalcogenides is a crucial step for the realization of these devices. Compared to vertical heterojunctions, lateral heterojunctions between transition metal dichalcogenides are less explored. Vapour transport growth with alternating precursor vapours is the most popular method to synthesize lateral heterojunctions, but the pattern of the fabricated heterojunction is limited. In this work, we propose an approach to fabricate monolayer MoS₂-MoSe₂ lateral heterojunction with lithographically defined patterns in a simple tube furnace system. Additionally, this approach is also compatible with traditional planar device fabrication. Monolayer MoSe₂ flakes are exfoliated and patterned with electron beam lithography. A patterned mask layer is subsequently formed by deposition of Al₂O₃. After annealing in sulfur vapor, MoSe₂ at the unmarked region is converted to MoS₂, forming the lateral heterojunction. After removal of Al₂O₃ mask by wet etching, back-gate field-effect transistors are fabricated on the MoS₂-MoSe₂ lateral heterojunction. The lateral heterojunction formation is studied with Raman and photoluminescence spectroscopy as well as device characterization.

As devices become ultra-thin, a new problem arises for their electrodes: there is a need for ultra-thin electrodes with good electrical and mechanical properties. As the thickness of metals become less than 20 nm, their quality can decrease e.g. the electrical resistance increases and their chemical stability deteriorates.
Graphene, by being an atom thick and having good electron mobility has the potential to be a good electrode. However, it is easily contaminated and could degrade due high temperature treatment and/or oxidation.
This work explore the possibility to develop ultra-thin electrodes by combining both graphene and a thin metal layer. The metal would provide resistance for the graphene against oxidation at temperatures, whilst the graphene and metal would support one another mechanically. The overall conductivity in theory should increase, as the structure’s electrical equivalent circuit could be considered as two resistors in connected in parallel, where the total resistance is lower than any of the individual resistances.
Bilayers were manufactured on 90 nm of silicon dioxide buffered silicon substrates. Single or double layer CVD made graphene was transferred onto the substrate using standard “wet” method. Five different metals were used to produce flat metal layers with thicknesses up to 6-10nm. E-beam evaporation and magnetron sputtering methods (carried out at room and elevated temperatures) were used for deposition.
The electrode structures’ quality was controlled by optical microscopy, AFM and X-ray diffraction; Raman spectroscopy was employed to monitor the graphene layer quality, while four-probe current-voltage and Hall effect measurements were used to analyse samples’ electrical properties.
It was found that the electrodes containing silver and those using titanium formed at room temperature had lover conductivity. The high temperature titanium samples exhibited an improved conductivity. It was also observed that the magnetron sputtering method damages the graphene layer, which results in high resistivity of the bilayer structure.
The applicability of these bilayer structures as electrodes in RF tuneable devices and next-generation memory elements will also be discussed.

Two-dimensional group IV layers beyond graphene, as silicene, germanene and the Sn-based stanene, have been recently synthesized by molecular beam epitaxy. Density Functional Theyory (DFT) calculations predict low-buckled structures for these 2D nanosheets, with a hexagonal honeycomb conformation, typical of the graphene-like surfaces. The buckling parameter d increases from Si to Sn-based layers, with a maximum predicted of 0.92 Å for stanene [1]. High-buckled structures for these materials resulted to be unstable. We have previously shown that for silicene and germanene, the origin of the buckled structure resides on the pseudo Jahn-Teller puckering distortion, resulting from non-adiabatic effects. It has been shown that hexagermabenzene, the single hexagonal unit of germanene, is subject to a strong vibronic coupling whose origin is the pseudo Jahn-Teller effect [2]. This coupling resulted to be around ten times larger than the one obtained for hexasilabenzene [3]. For stanene, an additional effect needs to be considered in order to understand the origin of buckling: the spin-orbit coupling (SOC). This SOC contributes to open an electronic band gap, enabling the use of these layers as nanoelectronic components. In this work we present an analysis based on DFT in the Zeroth-Order Regular Approximation (ZORA) for both scalar relativistic and spin-orbit versions that quantify the influence of the spin-orbit coupling in the puckering of Sn₆H₆. Also, under the linear vibronic coupling model between the ground and the lowest excited states, we present the pseudo Jahn-Teller contribution. The scalar ZORA approximation is used to perform time-dependent DFT calculations in order to incorporate the low-energy excitations contributions. Our model leads to the determination of the coupling constants and predicts simultaneously the Adiabatic Potential Energy Surface behavior for the ground and excited states around the maximum symmetry point. These values allow us to compare the Jahn-Teller relevance in buckling with the other group IV layers [2,3].

We are thankful with the computer facilities of the Dirección General de Cómputo y de Tecnologías de Información y Comunicación (DGTIC-UNAM) for providing supercomputing resources on “Miztli” through Projects SC16-1-IR-23 and SC16-1-IG-83.


[1] B. van den Broek et al, Two-dimensional hexagonal tin: ab initio geometry, stability, electronic structure and functionalization, 2D Mater. 1 (2014) 021004
[2] J. R. Soto, B. Molina and J.J. Castro, Strong Pseudo Jahn-Teller effect on the single hexagonal unit of germane. DOI: http://dx.doi.org/10.1557/adv.2016.14
[3] J.R. Soto, B. Molina, J.J. Castro, Reexamination of the origin of the pseudo Jahn-Teller puckering instability in silicene, Phys. Chem. Chem. Phys. 17, (2015), 7624

WS₂ quantum dots (QDs) were effectively prepared through the simple solvothermal treatment of WS₂ nanosheets under relatively mild conditions (100^oC, 1 h). As-prepared WS₂ QDs had 2.8 nm in average size with a lateral spacing of 0.27 nm. They exhibited strong absorption in visible range and a highly enhanced quantum yield (2.73%) for fluorescence emission with excitation wavelength dependency. The strong fluorescence of WS₂ QDs was then selectively quenched by dopamine while other neurotransmitters and interferences such as norepinephrine, epinephrine, ATP, ascorbic acid, and uric acid did not influence their fluorescence. The WS₂ QDs was able to detect dopamine at 20 nM in serum. The sensing mechanism responsible for the selective detection of dopamine was also demonstrated in this work.

Ultrathin two-dimensional semiconductors obtained from layered transition-metal dichalcogenides such as molybdenum disulfide (MoS₂) are promising for ultimately scaled transistors beyond Si. Although the shortening of the semiconductor channel is widely studied, the narrowing of the channel, which should also be important for scaling down the transistor, has been examined to a lesser degree thus far. In this study, the impact of narrowing on MoS₂ flakes was investigated according to the channel-width-dependent Schottky barrier heights at metal contacts. Narrower channels were experimentally found to possess a higher Schottky barrier height, which is ascribed to the edge-induced band bending in MoS₂. Theoretical analyses based on Poisson’s equation showed that the edge-induced effect can be alleviated by a high dopant impurity concentration, but this strategy should be limited to channel widths of roughly 0.7 μm because of the impurity-induced charge-carrier mobility degradation. Therefore, proper termination of the dangling bonds at the edges should be necessary for aggressive scaling with layered semiconductors.

Transition metal dichalcogenides (TMD) are a class of earth abundant and inexpensive materials which have emerged to be promising catalysts for the hydrogen evolution reaction (HER). Molybdenum disulfide (MoS₂) is a TMD that has been shown to be an excellent catalyst for the HER and a crucial step towards implantation of hydrogen energy technology. Starting from powder, MoS₂ can be exfoliated into nanosheet form or can be synthesized into nanostructure form. The resulting ink or paste is then tested in a typical three-electrode electrochemical systems. Many have demonstrated that enhanced performance for the HER is possible by increasing the amount of exposed edge active sites, while the basal plane remains relatively inactive. Alternatively, MoS₂ and similar materials can also be synthesized in single crystal form using Chemical Vapor Deposition (CVD) which offers higher quality MoS₂. The resulting CVD MoS₂ samples are triangular in form, single layer and are 10-20 µm in size. Some efforts have been made to measure the electrocatalytic activity of clusters of CVD MoS₂, however it is difficult to determine the behavior of a single triangle of MoS₂ due to its microscale size. Using electron beam lithography, we have fabricated microcells of a well-defined surface area. After initial deposition of metal contacts, the entire substrate is covered by electron beam resist such as poly(methylmethacrylate) (PMMA). By exposing specific areas to the electron beam, we can remove microscale areas of electron beam resist that will leave bare areas of the sample to the electrolyte. This opens a window in the electron beam resist that allows the electrolyte to contact the CVD MoS₂. In this way, measurements of individual CVD single layer samples of MoS₂ can be made. Electron beam lithography also provides us discrete control of what areas are exposed to the electrolyte. This allows us to specifically measure the activity of the edge of a CVD sample and compare directly to the basal plane. By measuring a single CVD MoS₂ triangle crystal, we are able to gain some insight into the intrinsic electrochemical behavior of MoS₂. This technique can be applied to selectively measure the electrocatalytic activity of other two dimensional materials and allow us to observe microscale electrochemical interactions.

Ultra-thin Black phosphorus(BP), phosphorene was introduced to the family of 2D semiconductor as a new potential material on recent study and gets deep interest. BP is consist of a single element (P) and it can be exfoliated from a bulk Van der Waals crystals structure by using the conventional scotch-tape method. The single-layer BP has a band gap of ~1.8eV, which decrease dramatically with adding the number of layers to about 0.3eV in the bulk. Unlike other semiconducting TMDs, it has direct band gap independent on all number of layers that makes it a promising material in optoelectronics application. Mechanically exfoliated BP has good electrical properties, showing high mobility 200 ~ 1000 cm²V^-1s^-1 at room temperature, 10²-10⁵ current on/off ratio at 10nm thickness of BP, broad band gap.
We investigate the degradation pattern of Black phosphorus(BP) field effect transistor(FETs) was investigated by using an mechanically exfoliated BP that react O₂ and water vapor in ambient condition, degradation. The BP FETs was electrically measured every 20 minutes(1cycle) in the air, the total cycle is 100. We show electrical changes with Mobility, On/off ratio, Current and a significant positive shift in the threshold voltage. We extracted the current level at V_gs-V_th = 0,-10,-20 and fitting with Swiss-cheese model. This model suggested that Swiss-cheese model is well fitted with degradation pattern of BP FETs. Also, we researched degradation pattern of 1/f noise in sus&non-suspended BP FET. It is modeled with CNF-CMF(Carrier number flcutuation-Carrier mobility fluctutation) and hooge mobility fluctuation. Both model is well fitted with 1/f noise data of deteriorated BP FET.

Optoelectronic sensors that can switch the photocurrent polarity by the wavelength of incident light are one of the important building blocks of novel optical logic gates, color sensors, and photocatalysts.[1] Observations of the wavelength-dependent switching of the photocurrent polarity were reported for various photoelectrochemical devices. These devices, we call wavelength-dependent bipolar photodetectors (WBPDs), consist of hetero nanostructures as optical sensitizers with liquid electrolytes that transports photoexcited carriers.[2] The behaviors of switchable photocurrent polarity in the devices are caused by the difference of optical properties of two materials used in the devices. In other words, transition wavelengths of the output polarity are determined by the optical properties of two types of materials in the devices. Moreover, the response time of current switching against the incident light in some devices mentioned above is typically as slow as sub-second because of low carrier mobility in liquid electrolytes. For future optoelectronic devices such as logic gate devices, extremely higher response time is required.
In this study, to resolve these issues, we proposed an all-solid-state WBPD without any electrolyte.[2] The proposed device is composed of single semiconductor with band bendings at both the front and rear surfaces, and have high-speed response. We have fabricated WS₂-based WBPDs. The device exhibited bipolar output currents that depend on the incident light wavelength. The photocurrent response times (i.e., the rise and fall times) were confirmed to be less than 30 and 270 μs, respectively. These values are significantly shorter than those of other WBPDs reported to date. From a device simulation, we verified the WBPD operation of various semiconductors with bifacial band bendings and confirmed that the threshold wavelength, at which the polarity changes, depends on the thicknesses of the semiconductors.
In the session, detail experimental and simulation results will be also shown.
[1] K. Szacilowski, Chem. Rev. 108 (2008) 77.
[2] T. Ikuno et al. Appl. Phys. Exp. 9 (2016) 062201.

Transition metal dichalcogenides (TMDs) are promising materials for low dimensional electrical, optical and flexible applications. Therefore, their properties and processing are widely examined.
Various methods to achieve one atomic layer of these TMDs have been used and among these, liquid phase exfoliation technique is one of the effective methods to produce large quantities of two dimensional materials. However, usability of this technique have been limited to making ink for confirming the size of a flake or inkjet printing. One of the issues using TMD is large area application with uniformity. TMD ink and spraying can be applied on large area with excellent uniformity and photolithography is a general semiconductor processing for manufacturing.
To the best of our knowledge, no one succeeded in fabricating top gate transistors using photolithography and only MoS2 ink without any other two dimensional materials including graphene.
It should be noticed that ionic current by residual solvent such as N-methyl-pyrrolidone (NMP) should be removed when using ink-based processing.
By spraying MoS₂ ink and thermal annealing, a large area film with improved and uniform conductivity but without residual solvent. Spraying was performed on 1.5x1.5 cm² SiO2/Si substrate placed on a hot plate at 250 °C to rapidly remove solvent. The maximum thickness of the generated film is less than 100 nm when 1 ml MoS2 ink is applied on the size of 1.5 x 1.5 cm². After fabrication process, transmission line measurment (TLM) was conducted to estimate sheet and contact resistance. Sheet resistance was dramatically improved after 700 °C thermal annealing under N₂ atmosphere (1.6E5 ohm/sq) compared to after 400 °C drying only (8.0E8 ohm/sq). Also the thickness and sheet resistance are very uniform in all locations. The maximum current level is approximately 400 μA at source-drain voltage of 10 V when channel length is 10 μm.
Although XPS results shows that MoO₃ is created after deposition, micro-Raman spectra does not show any Raman active modes of MoO₃ that means the amount of MoO₃ is very small and its electrical effect can be ignored. The estimated field-effect electron mobility is approximately 0.25 cm²/V.s.
Detailed results of XPS, Photoluminescence, Raman spectra and electrical properties of thin film transistors with top gate will be discussed.

Novel two-dimensional materials from the group of layered transition metal dichalcogenides (TMD) have recently attracted scientific interest for their unusual physical properties.
An interesting quality is their strong optical response that stems from the quantum confinement of electrons and holes in the quasi-2D geometry. The modified screening strongly enhances the binding of electron-hole pairs leading to binding energies of the lowest-lying excitonic transition of magnitude 0.2-1.0 eV in Mo and W based TMDs [1,2]. Despite this, a closer analysis of the nature of the excitonic transitions contributing to observed absorption peaks is still lacking so far.

We have thus performed ab initio calculations of the theoretical absorption spectra including electron-hole interaction for a range of mono-, bi- and trilayer transition metal dichalcogenides in trigonal-prismatic and octahedral phases and analyzed the k-space representation and spatial extend of the dominant excitons for excitation energies up to 3 eV. Our results suggest that changes of the electronic structure from variation of the chalcogen atoms and interlayer interactions lead to a noticeable qualitative evolution of the dominant transitions and can be explained by band nesting between valence and conduction bands [3].
[1] H. M. Hill et al., Nano Letters 15, 2992 (2015)
[2] A. Chernikov at al., Phys. Rev. Lett. 113, 076802 (2014)
[3] R. Gillen et al., accepted at IEEE J. Sel. Top. Quantum Electron., arXiv:1605.01972 (2016)

Atomically thin transition metal dichalcogenides (TMDs) are promising materials for applications in optoelectronics and catalysis. MoS2 nanostructures are rising candidates for the replacement of Pt catalysts in water splitting. Focusing on the hydrogen evolution reaction part of this process and on how hydrogen (H) interacts with MoS2 nanostructures, either free-standing or on a graphene substrate, we study H adsorption on such nanostructures with DFT calculations [1]. Results for the stability and for H binding on several configurations, from 2D infinite monolayers to quasi-1D MoS2 ribbons and quasi-0D MoS2 flakes, are presented. We calculate the adsorption energy of H atoms on various sites of the MoS2 nanostructures, notably at Mo and S active edges. Comparing free-standing and MoS2/graphene heterostructures we find that when the graphene substrate induces considerable strain on the nanostructures, H adsorption energies are significantly modified. The effect of strain, which is known to modify TMDs' electronic and dielectric properties [2], is analyzed. The changes in H binding are correlated with the strain-induced nanostructure modifications, showing that H adsorption can be tuned with strain.
[1] A.E. Maniadaki, G. Kopidakis, Phys. Status Solidi RRL 10, 453 (2016).
[2] A.E. Maniadaki, G. Kopidakis, I.N. Remediakis, Solid State Commun. 227, 33 (2016).

Van der Waals (vdw) heteroepitaxial stacking growth of dissimilar layered materials can form a new class of superlattices, and thus hold promises for new two-dimensional (2D) electronic and optical phenomena in solid state. Conceptually the 2D vdw heteroepitaxial growth does not strictly require the close lattice-match between the constituent 2D crystals, due to the weak vdw interlayer interactions. However, it is yet to be experimentally verified. Here we report the two different growth mechanisms of vdw heteroepitaxial Bi₂Te₃/Sb₂Te₃ few-layers stacking by choosing different substrates. We show that the growth mode becomes the layer-by-layer growth (Volmer-Weber growth) on h-BN substrates, and the 3D island growth (Frank-van der Merwe growth) on SiO₂/Si. We find that the compressive strain in the substrates imposed by the lattice-mismatch plays a crucial role to determine different growth modes in these 2D nucleation kinetics models. Our work suggests general implications for large-area 2D stacking growth of various layered materials

Zero-valent metal intercalation provides unique chemical control of a 2D layered material resulting in increased conductivity, enhanced catalytic activity, and chemically tunable transparency. While zero-valent intercalation is fairly new, deintercalation of these zero-valent metals has been difficult to achieve. Here, we demonstrate a solution-based facile approach to deintercalate zero-valent copper and tin from Bi₂Se₃, GeS, MoSe₂, NbSe₂ and lots of other 2D layered chalcogenides using a one-step comproportionation reaction. The transparency of a layered material can be reversibly controlled through this chemistry providing a general route to chemically tailor 2D chalcogenide layered materials.

Aerogels are ultra-low density, highly porous materials that are of great interest for a wide variety of applications including sensing, energy storage, and catalysis. Previously two-dimensional (2D) materials such as graphene and graphene oxide have been widely reported as being ideal for the formation of such aerogels. However, despite structural similarities the wide variety of other 2D materials such as transition metal dichalcogenides (TMD) and boron nitride (BN) there have been very few reports of their use in aerogel formation. Our group has recently demonstrated a method of producing aerogels from pristine graphene, and composites with other nanocarbons, through a process of room temperature freeze gelation (RTFG) [1]. The exfoliated material is first dispersed in a suitable organic solvent, such as phenol, which is capable of forming a stable dispersion while having a melting transition temperature slightly above room temperature and a high vapour pressure above the solid at room temperature to allow the solvent to sublime. This allows us to produce a material that is highly processable, able to be injection moulded as well as ink jet and 3D printed.

In this work we have produced ultra-low density, high surface area aerogel composites containing one or more TMDs, BN, as well as graphene, through a process of RTFG. Dispersions of each 2D material were first produced by ultrasonication in specially chosen solvents, before monolithic aerogels were produced using phenol as the solvent. The effect of polymeric binders to increase the mechanical strength and reduce fragility was also investigated. Aerogels were produced using each 2D material individually, or as composites containing multiple components to allow for a synergistic effect. The materials were first characterized by SEM as well as by Raman spectroscopy, XPS, XRD, and BET surface area measurements. These aerogel materials were then tested for applications including electrochemical energy storage (e.g. supercapacitors) and catalytic hydrogen evolution (HER). These inorganic aerogels were also tested for use in ink jet and 3D printing.

[1] Adv. Mater. 2016, In Press.

Recent research has shown that in addition to the composition and arrangement of atoms in materials, dimensionality plays a crucial role in determining their fundamental properties. This has been most strikingly highlighted over the past few years with two-dimensional (2D) graphene, which exhibited exotic condensed-matter phenomena that were absent in bulk. Compared with other known 2D materials such as transition metal oxides including titania- and perovskite-based oxides and boron nitride (BN), transition metal dichalcogenides (TMDCs) show a wide range of electronic, optical, mechanical, chemical and thermal properties that have been explored by researchers for decades. There is at present a resurgence of scientific and engineering interest in TMDCs in their atomically thin 2D forms because of recent advances in sample preparation, optical detection, transfer and manipulation of 2D materials, and physical understanding of 2D materials learned from graphene.
Herein, we introduce a facile route, chemical synthesis, to prepare photoluminescent MoSe₂ QDs using ammounium molybdate ((NH₄)₆Mo₇O₂₄) and selenium dioxide (SeO₂) as precursors. Raman scattering performances were proving their layer thickness, structure formation and their surface homogeneity. The atomic layer thickness was precisely controlled by adjusting deposition time, which was declared by AFM results. TEM analyses were also supporting our claim of layer thickness existence. Photoluminescence spectral results revealed that 1.4 nm layer thickness MoSe₂ QDs showing their better luminescent performance. SEM analyses confirmed their continuous and uniform surface nature of MoSe₂ QDs. Our novel and simple route paves the way for the controlled synthesis of high-quality TMDC materials, which is an attractive option for the applications in electronic and optoelectronic devices.

Scalable, transfer-free growth of hexagonal boron nitride (h-BN) – an ideal interfacing platform for 2D heterostructures – is a critical prerequisite for the eventual manufacturability of 2D electronics. While such heterostructures currently require exfoliation or chemical transfer of h-BN resulting in small areas or significant interfacial impurities, here we demonstrate the growth of large-area, uniform, ultrasmooth and ultrathin h-BN directly on Si₃N₄/Si substrates. The h-BN film produced acts as an ultrasmooth and polymer-free dielectric interface for graphene/h-BN heterostructures, thus eliminating surface roughness induced charged impurity scattering observed with Si-based substrates, leading to a 3.5-fold increase in charge carrier mobility in graphene. Atomic-scale molecular dynamics simulations investigated the interactive stability of (BN)_XH_y complexes interfaced on Si₃N₄/Si and also on SiO₂/Si, and identified borazine-like structures as stable nucleating clusters for h-BN growth on silicon-based substrates. This single-step growth mechanism of h-BN for graphene heterostructures establishes a pathway for the design of complex and integrated 2D-heterostructured circuitry.

Ultra-thin films of selenide compound epilayers were deposited epitaxially by molecular beam epitaxy (MBE) on AlN(0001) / Si(111) templates and were studied structurally using high resolution transmission electron microscopy (HRTEM), image simulations, and geometrical phase analysis (GPA). The films comprised Bi₂Se₃, MoSe₂, and HfSe₂ epilayers, as well as composite heterostructures of these materials. Wurtzite AlN is a wide band gap semiconductor that strongly favors an excellent interfacial quality with the selenides under the employed growth conditions contrary to direct deposition on silicon that leads to interfacial amorphization and extended defects in the film. The structural observations were combined with angle-resolved photoelectron spectroscopy measurements.
Bi₂Se₃ is a topological insulator (TI), and the deposited films exhibited a surface Dirac cone making them promising for novel spintronics and quantum computing applications. HRTEM, combined with GPA, showed an epitaxial well-ordered interface with (0001) AlN. The interfacial periodicity was manifested by a 3:4 plane matching. No interdiffusion or chemical reaction was observed at the interface. High quality and large scale 2D films with thicknesses of 3 and 5 quintuple layers (QLs) were deposited [1]. The films contained only vertical and in-plane 180^o rotational domain boundaries.
In addition, high quality films of a few monolayers (MLs) of MoSe₂ and HfSe₂ compound semiconductors were deposited in extended scale by MBE directly on AlN(0001), showing promise for nanoelectronic device applications mediated by the van der Waals bonding [2]. In an alternative approach, Bi₂Se₃ was employed as buffer layer in order to maintain low growth temperatures that favor large scale manufacture. Furthermore, various combinations of alternating selenide layers were achieved, signifying a versatility towards advanced nanodevice possibilities and prospects for combined 2D semiconductor/TI applications. Cross sectional HRTEM, in conjunction with image simulations elucidated the interfaces between dissimilar materials. Variations in lattice spacings were obtained by GPA.

[1] P. Tsipas, E. Xenogiannopoulou, S. Kassavetis, D. Tsoutsou, E. Golias, C. Bazioti, G. P. Dimitrakopulos, Ph. Komninou, H. Liang, M. Caymax, A. Dimoulas, ACS Nano, 8, 6614 (2014).
[2] E. Xenogiannopoulou, P. Tsipas, K. E. Aretouli, D. Tsoutsou, S. A. Giamini, C. Bazioti, G.P. Dimitrakopulos, Ph. Komninou, S. Brems, C. Hughebaert, I. P. Radu, A. Dimoulas, Nanoscale, 7, 7896 (2015).

Acknowledgement: Work supported by the ERC Advanced Grant SMARTGATE-291260- and the National program of excellence (ARISTEIA-745) through the project TOP-ELECTRONICS.

In the present study, tensile behavior and thermal transport properties of monolayer molybdenum disulfide (MoS₂) have been studied using molecular dynamics simulation. We have investigated the effect of temperatures on tensile strength, and tensile strain on thermal conductivity of monolayer MoS₂. In case of all simulations, the energy minimized structure of MoS₂ was equilibrated with canonical ensemble i.e NVT (constant number of atoms, volume and temperature) and isothermal-isobaric ensemble i.e NPT (constant number of atoms, pressure and temperature) respectively. In order to investigate the effect of temperature on tensile properties of MoS₂, we have applied strain on the 5.0×5.0 nm² MoS₂ sheet. We have calculated the tensile properties at 300, 500, 700 and 900K temperatures, respectively. We have noticed that increment of temperature significantly reduces the tensile strength. We have applied the tensile strain both in armchair and zigzag direction of MoS₂ sheet to perceive the distinction in their tensile strength. Strain dependent thermal transport properties of ribbon-like MoS₂ with a dimension of 15.0×3.0 nm² have been investigated by applying both uniaxial and biaxial strain on MoS₂. The non-equilibrium molecular dynamics (NEMD) simulations were performed to calculate the thermal conductivity of MoS₂ .Our present study reveals that the tensile strain reduces the thermal conductivity of MoS₂. All the simulations were performed using the LAMMPS package. At the same time, we have designed and fabricated micro-electro-mechanical-system (MEMS) device which can be used to characterize two dimensional materials thermally and mechanically inside in-situ transmission electron microscope (TEM). The experimental proof of the simulation results is under process.

1T-TaS₂ is a layered material that exhibits rich charge density wave (CDW) phases, each of which has its own structural and electrical properties. Above 545K, 1T-TaS₂ is in a metallic phase with an unfilled conduction band. On cooling, this material begins to go through a series of first-order phase transitions. Below 545K, it enters into the incommensurate CDW (ICDW) phase. With temperature decreased to below 348K, further structure change leads to the nearly-commensurate CDW (NCCDW) phase. Finally, at below about 180K, the lattice structure changes again, resulting in the formation of commensurate CDW (CCDW) phase and a slight band-gap (~0.2eV) through Mott transition¹. It has been shown that this metal-insulating phase transition can be successfully switched through electrical means, which demonstrates potentials of fabricating the next-generation fast-speed switches or being applied into nonvolatile memory devices². In order to better understand NCCDW-CCDW phase transition in 1T-TaS₂ and take full advantage of the associated abrupt resistivity change, we have conducted a thorough study of substrate impact on 1T-TaS₂ structural evolutions by Raman spectroscopy. By exfoliating similar thickness of 1T-TaS₂ onto different substrates (SiO₂, sapphire, Graphene, VO₂ and SrVO₃), we have observed different Raman spectra evolutions, which indicates that NCCDW-CCDW phase transition can be affected by substrate’s interaction. Comparisons of surface structures and structural mismatch between substrate and 1T-TaS₂ indicate that surface roughness or strain is the dominate factor in controlling 1T-TaS₂ NCCDW-CCDW phase transition. In order to verify this assumption, we have patterned SiO₂ substrates with different surface structures and have exfoliated similar thickness of 1T-TaS₂ flakes on these patterned SiO₂ substrates. Based on Raman measurements, we found that flakes show similar NCCDW-CCDW evolution behaviors when the substrates underneath have similar surface structures. Thus, by patterning SiO₂ in a further systematic way, we are able to engineer the NCCDW-CCDW phase transition in 1T-TaS₂. Electrical measurements will provide further proof of these substrate effects and will allow for the realization of tunable NCCDW-CCDW phase transition in this material.

Applications of 2D materials in MEMS/NEMS and energy harvesting are emerging as a consequence of electromechanical properties that manifest at or near the monolayer limit. Many 2D materials, including transition metal dichalcogenides (TMDs) and hexagonal boron nitride, are intrinsically piezoelectric for single or certain small numbers of layers due to a lack of centrosymmetry. Graphene on the other hand is centrosymmetric and is not naturally piezoelectric, but can be coaxed to be so by introducing defects or by forming chemical bonds to other atoms. Monolayer molybdenum disulfide (MoS₂), a TMD, has been experimentally confirmed to exhibit both direct and converse piezoelectricity within the plane of its atoms. However, out-of-plane piezoelectricity is theoretically zero in MoS₂ due to its symmetry. In this study, we show that a non-zero out-of-plane electromechanical response can be measured and discuss some possible sources of this behavior.
High-quality chemical vapor deposited (CVD) MoS₂ is transferred from its growth substrate to a silicon/silicon dioxide substrate coated with gold. The MoS₂ is then probed using piezoresponse force microscopy (PFM). In this process, an atomic force microscope (AFM) with a conductive tip is used to apply alternating sinusoidal voltage across the MoS₂. Using a lock-in amplifier, the vertical deflection of the MoS₂ can be detected with the AFM probe tip at the same time as the voltage is being applied. Careful attention is needed to differentiate any background signal that is present. Using the phenomenon of contact resonance in which the electromechanical response is enhanced for voltages applied at the clamped mechanical resonant frequency of the AFM cantilever, the response of MoS₂ can be shown to differ from that of non-piezoelectric materials. It is also possible to differentiate the number of layers present in MoS₂ based on variations observed in PFM imaging. By mapping the electromechanical response over the surface of the sample, differences between the background gold signal, monolayer MoS₂ and multilayer MoS₂ can be clearly observed.
Some possible explanations will be given as to why an out-of-plane electromechanical response is observed even though the out-of-plane piezoelectric coefficients of MoS₂ are zero. One possibility involves flexoelectricity, in which a polarization field develops in response to strain gradients, and its inverse, in which a mechanical deformation is induced by electric field gradients.

The emergence of 2D materials in electronic devices has placed an increased emphasis on understanding their reliability and stability under current flow. To address this we present a phase-field model for the nanoscale evolution of voids in 2D materials. The model is applied to capture non-linear growth as observed in recent experiments on TiSe₂. Our work is an extension of a previous model that describes bulk diffusion limited growth due to the presence of excess vacancies in a solid. This model captures non-trivial features in dynamically driven void evolution recently observed on the surface of TiSe₂ via scanning tunneling microscopy. We focus mainly on the dynamics and nanoscale evolution of the system in an effort to capture the long time scale evolution of 2D materials under voltage biasing.

2D layered materials (2DLMs) have been studied immensely for its catalytic properties because of the large available surface area. Of these 2DLMs, Transition metal chalcogenides (TMDs) such as MoS₂, WS₂ and WSe₂ have shown exceptional performance when used as catalysts for hydrogen evolution reactions (HERs). In order to enhance the performance of these materials even further, defect incorporation and strain has been studied. This could be achieved by making alloys of various TMDs or by creating defects using oxygen plasma treatment or annealing in air. In this talk, we discuss the synthesis and stabilization of vertical structures of MoS₂ and WSe₂ on graphite paper. These synthesized TMDs can be compared to the current state of the art TMD HERs performance and has a Tafel slope of ~185 mV/Dec for MoS₂ and 76 mV/Dec for WSe₂. Using simple UV-Ozone treatments to the MoS₂ surface, the Tafel slope for MoS₂ can be lowered from 185 to 54 mV/Dec. This decrease in the Tafel slope can be attributed to increasing in the exposed surface area for the vertical MoS₂ structures and creating surface sulfur-oxygen bonding, thereby reducing the water contact angle from 160^o to 91^o. Although, similar treatments on WSe₂ showed an opposite behavior as the Tafel slope increased to 220 mV/Dec. This contrasting behavior was attributed to the type of the chalcogen atom that is attached on the surface when forming defects. Naturally, WSe₂ tends to oxidize to form WO_3-x species. If an alloy was made with sulfur instead of oxygen, it was shown to improve the performance of the WSe₂. Further understanding the effect of alloying was also performed with transition metal and similar enhancement was observed.

While in recent years, there has been a concentrated effort in achieving uniform electronic-grade transition metal dichalcogenide (TMDs) films over large-areas, there has been little investigation into the selective and patterned synthesis of these materials on traditional substrates¹. Developing a means for true bottom-up selective-area growth of these materials only in active device channel regions on device-ready substrates eliminates the need for a destructive and non-scalable film transfer process as well as top-down etching post-synthesis that can lead to domain edge degradation in ultra-scaled channels. Here, we demonstrate a facile and robust method for the selective-area growth of various semiconducting TMDs, with no special pre-seeding, and show that ultra-thin polymeric or amorphous boron nitride (aBN) surface functionalizations can decrease the substrate surface energy enough to preclude TMD layer nucleation and growth in pre-determined regions of the substrate. Importantly, the technique developed here not only allows for preclusion of TMD nucleation in functionalized regions, it also halts the lateral growth of TMDs that do nucleate in pristine substrate regions, allowing for exact pattern transfer to the resulting 2D film. We show that this polymer functionalization layer is achieved using standard lithography techniques, and our selective-area growth technique is compatible with multiple TMDs and multiple growth techniques including both metal organic chemical vapor deposition (MOCVD) and powder vaporization (PV). In addition, we have investigated the electrical properties of top-gated field-effect transistors (FETs) built on selectively grown MoS₂ channels on sapphire substrates realized in a streamlined fabrication process. In doing so, we have discovered a significant substrate impact on the mobility of epitaxial MoS₂ films directly grown on sapphire substrates, and we demonstrate methods for electronically decoupling MoS₂ films from their growth substrates for improved transistor performance. Lastly, we compare the effects of ionic-liquid-based gating methods versus traditional metal/ALD-dielectric top-gate stacks on FET performance for as-grown, decoupled, and transferred (onto SiO2/Si) films. By combining selective-area growth techniques with controlled lattice orientations via an epitaxial growth mechanism on sapphire², there is an opportunity for realizing bottom-up synthesis of grain-boundary-free nanoribbons with controlled edge-orientation.

(1) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.-C.; Ajayan, P. M.; Lou, J. Nat. Mater. 2013, 12, 754–759.
(2) Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazić, P.; Gibertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y.-C.; Krasnozhon, D.; Chen, M.-W.; Bertolazzi, S.; Gillet, P.; Fontcuberta i Morral, A.; Radenovic, A.; Kis, A. ACS Nano 2015, 9, 4611–4620.

Several recent studies have shown band-gap tuning of MoS₂ with single-layer thickness, from 1.2 eV indirect band gap for bulk material to a direct gap semiconductor with a 1.9 eV band gap for a single-layer MoS₂. The emerging strong photoluminescence in ultrathin MoS₂ layers, while comparing with the absence of luminescence in the bulk, is an indication of the transition from indirect band gap to direct band gap. This unique property results from modified electronic properties. Additionally, the phononic properties are extremely modified due to quantum size effect. However, there are limited reports on the confined phonon states in these structures. Thus, we present systematic studies of the confine phonon states in MoS₂ quantum dots of diameter ranging form 0.5nm to 10nm. We elucidate on the evolution of the phonon line shape with quantum dot diameter using a phenomenological modeling with envelop function that truncate the phonon wave at the surface of the quantum dot and also taking into account the diameter distribution of the quantum dots.

We present a systematic investigation of the changes in the phonon line shape of the first order modes of single to few layers of Tungsten Disulfide with laser flux. We observed a complicated dependence of the phonon line shape on the nature of the substrate and the thermal anchoring of the Tungsten Disulfide layers to the substrate. With increasing power density of the laser in a 1 μm focal spot, we see a clear growth of an asymmetry in the phonon line shape similar to what is observed in confined phonon state in quantum dots. The effects we observe in Tungsten Disulfide should be common to all TMDs and underscores the importance of demonstrating a flux-independent line shape when studying pure phonon confinement effects by Raman scattering.

Due to the discovery of graphene, increasing attention has been carried out on the two-dimensional (2D) materials. Recently, the exfoliation approach has been used to successfully create a new member in two-dimensional materials, consisting of transition metal carbides, nitrides, and carbonitrides, called MXene. These novel materials are produced by etching the A layer of MAX phases. The so-called MAX phases are layered ternary compound, with a general formula of M_n+1AX_n (n=1, 2, 3), where M represents the early d-block transition metals, A is the main-group element (predominantly IIIA or IVA), and X is either or both C and N atoms.
To this day, more than 10 MXenes have been synthesized by exfoliating the MAX phases , but the outer surfaces of the exfoliated layers are always terminated with other groups, such as F, OH,=O. So, the MXene nanomaterial usually be referred to as M_n+1X_nT_x, where T is the surface groups (F, OH, and/or O) and x is the number of terminations. In order to write simply and clearly, the M_n+1X_nT_x usually is abbreviated as M_n+1X_n, such as Ti₃C₂, Ti₃CN. Due to graphene-like morphology, the MXene has shown the potential application in the fields of hydrogen storage, lead adsorption and catalyst, lithium-ion battery and supercapacitor. So far, the majority of MXenes were only successfully produced by exfoliating the MAX phases with high concentration hydrofluoric acid (HF). However, the high concentration HF solution is very dangerous and harmful to health, and it is easy to cause serious environmental pollution. Recently, Halim et al. attempted the use of ammonium bifluoride(NH₄HF₂), as an etchant of MAX phases to produce the MXene, and found that the NH₄HF₂ can in lieu of the HF to etch the Ti₃AlC₂ film_, which was prepared through the epitaxial growth method. Comparing with the pressureless sintering, the production rate of epitaxial growth method is more lower and the cost is more high. However, there isn’t any report about how to etch MAX powder that was synthesized by the pressureless sintering to produce the MXene, using bifluoride as etchant.
In this paper, for fully understanding MXene synthesis process by etching in bifluoride solution, the MXenes(Ti₃C₂) were made by exfoliating Ti₃AlC₂ with NaHF₂,KHF₂,NH₄HF₂ at different temperature for different time. The morphology, structure and element composition and of the Ti₃C₂ were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). Furthermore, the electrochemical performance of Ti₃C₂ was investigated by the cyclic voltammetry (CV), galvanostatic charge-discharge(GC) and electrochemical impedance spectroscopy(EIS). The results demonstrated that Ti₃C₂ electrode possesses distinguished electrochemical performance: high specific capacitance, excellent cycling stability. Overall, the present study indicated an effective method to prepare the 2D MXene materials.

Publications concerning two-dimensional (2D) and layered materials “beyond graphene” encompass over 100 publications a day.^[1] However, our understanding of the process-structure-property relationship for 2D materials remains severely underdeveloped. For the case of molybdenum disulfide (MoS₂), the morphology has been experimentally and theoretically shown to vary from rounded (molybdenum rich) domains to equilateral triangular (sulfur rich) domains.^[2-3] These different morphologies can result in drastically different properties, which can be exploited for applications in catalytic reactions, digital electronics, optoelectronics, and energy storage applications.^[1,4-7] Powder vaporization (PV) synthesis of MoS₂ can yield vertical MoS₂ structures, however, these structures are often ignored when the morphology evolution of MoS₂ is discussed,^[2-3] thereby completely omitting a major part of the impact of the Mo:S ratio to the growth mode of MoS₂ during PV. Here we introduce a vertical-to-horizontal growth mode transition for MoS₂ that occurs in the presence of a molybdenum oxide partial pressure gradient.

Combining experimental and numerical simulation methods, the growth mode of MoS₂ is elucidated. We reveal that the molybdenum oxide to sulfur partial pressure ratio and its distribution are the primary parameters controlling the morphology of MoS₂ between lateral domains and vertical “fins” when metal-oxide precursors are utilized. Furthermore, transmission electron microscopy reveals that the growth of MoS₂ fins results from initial seeding of single crystalline molybdenum dioxide, followed by sulfurization from the substrate upward to form vertically oriented MoS₂. By tuning the metal-oxide to chalcogenide precursor partial pressures we demonstrate that not only can the lateral morphology of 2D materials be controlled, but is key to realizing vertically oriented 2D structures.

[1] G.R. Bhimanapati, et al. "Recent Advances in Two-Dimensional Materials Beyond Graphene." ACS nano 9.12 (2015): 11509-11539.

[2] S. Wang, et al. "Shape evolution of monolayer MoS2 crystals grown by chemical vapor deposition." Chemistry of Materials 26.22 (2014): 6371-6379.

[3] D. Cao, et al. "Role of Chemical Potential in Flake Shape and Edge Properties of Monolayer MoS2." The Journal of Physical Chemistry C 119.8 (2015): 4294-4301.

[4] D. Jariwala, et al. "Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides." ACS nano 8.2 (2014): 1102-1120.

[5] Y. Zhao, et al. "Synthesis of MoS2 and MoO2 for their applications in H2 generation and lithium ion batteries: a review." Science and Technology of Advanced Materials (2016).

[6] O. Lopez-Sanchez, et al. "Ultrasensitive photodetectors based on monolayer MoS2." Nature nanotechnology 8.7 (2013): 497-501.

[7] Q.H Wang, et al. "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides." Nature nanotechnology 7.11 (2012): 699-712.

We have investigated the light emission characteristics of two dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanosheets and its heterojunction with one dimensional (1D) ZnO nanorods. g-C₃N₄ nanosheets have been developed using chemical exfoliation of bulk g-C₃N₄ powder synthesized via simple pyrolysis technique. The formation of graphitic phase of C₃N₄ has been understood from the spectroscopic characterizations, while the microscopic studies indicate that the g-C₃N₄ nanosheets are composed of a few layers only. Again, the 2D-1D hybrid based on nanosheets and nanorods, has been fabricated through an aqueous solution growth method. It has been found that the atomistic bonding at the interface between g-C₃N₄ and ZnO has led to the formation of an isotype heterojunction of type-II nature. During photoluminescence studies, the bare nanosheets are found to exhibit strong blue light emission, which has also been witnessed in electroluminescence studies. On the other hand, the signature of white light emission is noticed for the 2D-1D hybrid system. As understood from the photoluminescence analysis, the emission spectrum gets broadened due to the events of interfacial charge transfer (from g-C₃N₄ to ZnO), and an interfacial radiative recombination. It is proposed that the white light is originated from the intermixing of the characteristic luminescence of both g-C₃N₄ and ZnO, along with the interfacial radiative recombination. Tuning the various parameters of such 2D/1D heterojunction based device can lead to fabrication of futuristic light emitting devices.

Ultrathin black phosphorus (BP) is one of the promising two-dimensional (2D) materials for future optoelectronic devices. Its chemical instability in ambient conditions and lack of a bottom-up approach for its synthesis necessitate efficient etching methods that generate BP films of designed thickness with stable and high quality surfaces. Herein, reporting a novel photochemical etching method, we demonstrate a controlled layer-by-layer thinning of thick BP films down to a few layers or a single layer, and confirm their unique Raman and photoluminescence characteristics. Ozone molecules generated by O₂ photolysis oxidize BP forming P₂O₅-like oxides. When the resulting phosphorus oxides are removed by water, the surface of BP with preset thickness is highly flat and self-passivated by surface oxygen functional groups strongly resisting ambient oxidation. This method provides a new fabrication strategy of BP and possibly other 2D semiconductors with bandgaps tuned by their thickness.

Over the past several years, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great attention due to its unique electrical, optical, and chemical properties. Especially, molybdenum disulfide (MoS₂) is one of the most promising semiconducting TMDs for future nanoelectronics^[1]. To obtain atomically thin MoS₂, various efforts such as mechanical cleavage, liquid/chemical exfoliation, and chemical/physical vapor deposition have been studied. Because of the multiple advantages of mass producibility, simplicity, solution processability, and high yield, the chemical exfoliation via Li-intercalation has been widely adopted. However, its practical application has been limited by several issues such as loss of semiconducting properties caused by structural phase transition from 2H (semiconductor) to 1T (metal), the poor stability of resultant Li-intercalated 1T MoS₂ (Li-1T MoS₂) dispersion,^[2] and restacking of exfoliated MoS₂. Here, we demonstrate that the one-step fabrication of highly stable 2H MoS₂ dispersion via solvothermal treatment in polar solvents using Li-1T MoS₂. During solvothermal treatment under mild temperature (180°C) for 5 h, restoration of 2H phase is accomplished with about 80% conversion ratio in Li-intercalated MoS_2. Especially, when the solvothermal treatment was conducted in n-methyl-2-pyrrolidone (NMP), MoS₂ was functionalized simultaneously by oxidation product of NMP terminated with carbonyl group. While the solvothermal treatment was performed using other polar solvents such as ethylene glycol, formamide and propyelene carbonate, the flakes are seriously agglomerated and dispersion stability was poor, the functionalized 2H-MoS₂ (fct-2H MoS₂) obtained by NMP shows the markedly improved long-term dispersion stability in aqueous solvent over 2 months. Moreover, the fct-2H MoS₂ with surface carbonyl group presents high sensitivity to humidity. The response of fct-2H MoS₂ humidity sensor was 12 times higher compared to that of Li-1T MoS₂ humidity sensors at RH = 80 %.

Van der Waals (vdW) superstructures formed by the ordered restacking of two-dimensional nanosheet lattices can lead to unique physical and electronic properties that are not available in the parent nanosheets. While many spectroscopic techniques are available to identify the restacking, Moiré patterns formed by the crystalline mismatch between adjacent nanosheets are the most direct visual proof for the presence of vdW superstructures. The pattern characteristics can also be used to quantify the intersheet rotation angle, a key parameter of the vdW superstructures. In this presentation, transmission electron microscopy (TEM) observation of hexagonal Moiré patterns with unusually large micrometer-sized lateral areas (up to ~1 µm2) and periodicities (up to ~50 nm) from restacking of liquid exfoliated hexagonal boron nitride nanosheets (BNNSs) will be discussed. This observation was attributed to the long range crystallinity and the contaminant-free surfaces of these chemically inert nanosheets. Parallel-line-like Moiré fringes with similarly large periodicities were also observed. Detailed simulations and experiments, for the first time, unambiguously revealed that the hexagonal patterns and the parallel fringes originated from the same rotationally mismatched vdW stacking of BNNSs and can be inter-converted by simply tilting the TEM specimen following designated directions. This finding may pave the way for further structural decoding of other 2D vdW superstructure systems with more complex Moiré images.

Over the past decade two-dimensional materials have garnered increasing interest due to their exceptional properties with a wide range of applications in electronics, optics and optoelectronics. Recently, quantum light emission has been demonstrated in both transition metal dichalcogenide (TMD) semiconductors [1] and insulators, such as hexagonal boron nitride (hBN) [2]. In contrast to TMDs, where single photon sources are attributed to bound excitons to impurities, emitters in hBN are associated with point-like defects of the crystal lattice. Similar to nitrogen-vacancy color centers in diamond, the atomic-like defects in hBN confine electronic levels within the large band gap which result in stable and extremely robust emitters.
Moreover, low dimensional hexagonal boron nitride can be stacked in heterostructures and integrated with more complex photonic structures.

In this work we present a technique for the production of single photon emitters in exfoliated hBN by using a focused ion beam, which allows for the implantation of defects with sub micrometer resolution.
Optical spectroscopy on individual emitters indicates high photon emission rate exceeding 10⁷ counts/sec at saturation. Second order correlation measurements yield antibunching (g2(0) < 0.5 ) up to 5 x 10⁶ counts/sec. This emitter is therefore one of the brightest room-temperature single photon source reported so far. Time resolved measurements indicate an excited state lifetime of τ ∼ 2 ns.

We also demonstrate that the hBN quantum emitters produced by ion beam implantation are preserved after transfer onto different substrates, paving the way for integration with optoelectronic and nanophotonic circuits.



[1] Srivastava, A. et al. Nature Nanotech. 10, 491–496 (2015); He, Y.-M. et al. Nature Nanotech. 10, 497–502 (2015); Koperski, M. et al. Nature Nanotech. 10, 503–506 (2015); Chakraborty, et al. Nature Nanotech. 10, 507–511 (2015)
[2] T. Tran et al, Nature Nanotech 11, 37 (2015)

Recently, Zang et .al [1] proposed (from a theoretical point of view) the existence of a new two-dimensional nanostructure carbon allotrope similar to graphene [2]. This new nanostructure was named penta-graphene, because it is composed only of carbon pentagons and resembling the Cairo pentagonal tiling. In the work performed by Zhang and co-workers shown that, the penta-graphene has a great thermal and mechanical stability in comparison to graphene.
Starting from penta-graphene, we propose another structure with same topology of penta-graphene formed only by silicon atoms, which we call penta-silicene. Present results show that the penta-silicene is more stable electronically than penta-graphene.
In this work, we report a comparative study using ab initio density functional methods on the behavior of structural and electronic properties of penta-graphene and penta-silicene under external electric field perpendicular to each sheet. Our results indicate that penta-graphene presents a decreasing gap against the increasing of external electric field strength. In addition, we observed a transition from indirect to direct band gap when the electric field strength reaches 3.3 V/nm. On the other hand, for penta-silicene, there was an electronic semiconductor-conductor transition under external electric field around 1.3 V/nm (using the same criteria as applied to penta-graphene). These preliminary results could feed further investigations for potential applications in opto-electronic devices.

[1] S. Zhang et. al, PNAS, 112, 2372-2377 (2015).
[2] K. S. Novoselov et. al, Science, 306, 666-669 (2004).

Smaller and lighter electronics have led to a universal issue: high heat density on the device chips. Heat sinks, blowing fans and cooling fluids are presently used for heat management in high-performance electronic devices and electrical components. Unfortunately metallic heat sink and coolants could not be applied directly on these devices due to the risk of an electrical short circuit. Therefore, electrically insulating but highly heat conducting materials that can be applied directly on devices are important for next generation on-chip heat management. Here we discuss about a novel class of boron nitride nanosheets (BNNSs) that can promote heat dissipation up to 250%.
Boron nitride (BN) phases including boron nitride nanosheets (BNNSs) and boron nitride nanotubes (BNNTs) have generated interest in nanotechnology for being chemically inert, mechanically robust, electrically insulating and thermally conducting [1, 2]. In this work, a comparative study of heat dissipation properties of different BN nanomaterials grown on silicon substrates are carried out under ambient conditions. Vertically-aligned Boron Nitride Nanotubes (VA-BNNTs) and vertically aligned Boron Nitride Nanosheets (VA-BNNSs) are synthesized on silicon substrates using catalytic chemical vapor deposition (CCVD) by growth vapor trapping method [3]. All samples are heated up to 140 ^oC and allowed to cool to room temperature in air. Results indicate that Si chips with VA-BNNSs has a cooling rate ~250% higher than a bare Si chip. On the other hand, Si chips with VA-BNNTs also has ~75% cooling rate faster than that of a bare Si chip. This means, VA-BNNSs can outperform VA-BNNTs by more than ~330% for on-chip heat management. This is explained by the full surface contact area of BNNSs with the hot Si chip and the VA wavy edges that offer large contact area with the surrounding cool air. A series of experiments further indicate that performance of VA-BNNSs are higher when the VA feature sizes are increased. Transmission electron microscopy (TEM) suggests that these VA-BNNSs are unique in the edge structures, making them hydrophilic, noticeably different from what have been reported in literatures. Furthermore, these VA-BNNSs can also be peeled and transferred to arbitrary substrates for heat management applications. Details of all these results and the cooling mechanism will be discussed in the meeting.
Y.K.Y. acknowledges the support from the National Science Foundation, Division of Materials Research (Award No. 1261910).
References:
1. Lee, C.H., et al., Introduction to B–C–N Materials, in BCN nanotubes and related nanostructures. 2009, Springer. p. 1-22.
2. Wang, J.S., et al., Low temperature growth of boron nitride nanotubes on substrates. Nano Letters, 2005. 5(12): p. 2528-2532.
3. Lee, C.H., et al., Effective growth of boron nitride nanotubes by thermal chemical vapor deposition. Nanotechnology, 2008. 19(45): p. 455605.

Lead halide perovskites have been shown to have fantastic material properties, with demonstrations of high quality solar cells, LEDs and photodetectors in the last several years. Recently, fabrication of lead halide perovskite nanoplatelets, two dimensional sheets with integer number of perovskite layers in the third dimension, have been demonstrated. These nanoplatelets are straightforward to make and demonstrate strong tunability of their energetics due to quantum confinement of the excited state, allowing for emission across the visible spectrum. In this work, we utilize these nanoplatelets as the emissive layer of a light emitting diode to generate quantum confined electroluminescence. By tuning the thickness of the nanoplatelets, we achieve electroluminescence from 570 nm to 700 nm for the methyl ammonium lead iodide system and from 440 nm to 525 nm for the methyl ammonium lead bromide system. This work demonstrates the excellent flexibility of these two dimensional perovskites, allowing for true red, green, and blue perovskite emission and paving the way towards an all-perovskite white LED.

Hexagonal boron carbonitride (h-BCN) presents itself as an interesting material system and has potential in electronic and lighting applications. h-BCN is a semiconductor and has electrical properties situated between boron nitride and graphite, while the three materials have the same hexagonal planar structure. In addition, the electrical properties of h-BCN can be tuned by changing the composition of its constituent elements. Here, we present the synthesis of h-BCN nanosheets by arc discharge and detailed characterization of these nanosheets including lattice parameters, composition by EDS in TEM, and size distributions.

The controlled introduction of atomic vacancies into materials allows for the modification of its mechanical, chemical, and other physical properties; it also offers a route for nanoscale fabrication of structures. In this poster, we study the formation of atomic vacancies in hexagonal boron nitride under electron irradiation. We show that by varying temperature from 450 to 900^o C and studying the formation of defects in-situ using aberration corrected transmission electron microscopy, the shape, size, and edge termination of defects can be controlled. We further use these results to explore the growth dynamics of vacancies and their potential applications.

Black phosphorus (BP) as a new family member of two-dimensional (2D) layered materials has attracted significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties.^[1-4] However, producing air-stable BP sensing devices is challenging because BP atomic layers degrade rapidly in ambient conditions. In this study, we explored the humidity sensing behavior of fully encapsulated BP field-effect transistors (FETs) by utilizing 6 nm thick atomic layer deposited (ALD) Al₂O₃ as an encapsulation layer. The encapsulated BP sensors exhibited good environmental stability without any noticeable degradation after 1 week exposure to ambient conditions. Moreover, the encapsulated BP sensors with 6 nm thick Al₂O₃ layer showed little sensitivity deterioration compared with unencapsulated ones. The conductivity of BP sensors increased upon humidity exposure indicating that water molecules can induce positive charges on BP surfaces. The capacitance-voltage (C-V) measurements further demonstrated that the operation principle is based on the electrostatic interaction between water molecules and BP leading to effective doping and increased conductivity. This study demonstrates the potential of utilizing atomically thin BP for future electronic and sensing device applications.
1. Ahmad N. Abbas, et al. ACS Nano 2015, 9, 5618-5624
2. Shumao Cui, et al. Nat. Commun. 2015, 6, 8632
3. Poya Yasaei, et al. ACS Nano 2015, 9, 9898-9905
4. Carmen C. Mayorga-Martinez, et al. Angew. Chem. Int. Ed. 2015, 54, 14317-14320

Graphdyine is a carbon allotrope characterized by having diacetylene chains connecting the hexagons of a graphene-like structure. It emerges as a promising selective membrane for a variety of applications because it is one of the most stable non-natural carbon allotropes and it was already experimentally realized [1]. The feasibility of using graphdyine is due to its single layer structure and uniformly distributed nanopores [2]. To study the applicability of graphdyine as a selective membrane, it is important to known how water behaves near its structure on equilibrium conditions, as well as, on extreme conditions. The dynamics of splashing of droplets on surfaces is an important area in general science and industry [3], dealing with the possible behaviors that a liquid could have on impacting surfaces at different velocities, shapes and sizes. In this work we have used molecular dynamics simulations to determine how water nanodroplets behave on impact at a graphdyine membrane. Using the well-known reactive force-field (ReaxFF) [4] we simulated graphdiyne suspended membranes impacted by nanodroplets of different radius (from 20 up to 50 Å) and at different velocities (from 200 up to 1000 m/s). Preliminary results show that despite the porous structure, the droplets present a impact dynamics quite similar to the one observed for graphene [5]. Under impact the droplets spread over the surface with a maximum contact radius proportional to the impact velocity. Due to the increased energy of impact, a crescent number of liquid molecules are able to pass through the nanopore sheets. Nevertheless, the cohesion of the molecules maintains the liquid with a hemispherical shape, and the system overall can reach a well-behaved vapor/liquid equilibrium.

[1] G. X. Li, Y. L. Li, H. B. Liu, Y. B. Guo, Y. J. Li, and D. B. Zhu, Chemical Communications, 46, 3256 (2010).
[2] Y. Jiao, A. Du, M. Hankel, Z. Zhu, V. Rudolph, and S. C. Smith, Chemical Communications, 47, 11843 (2011).
[3] A. L. Yarin, Annual Review of Fluid Mechanics, 38, 159 (2006).
[4] K. Chenoweth, A. C. T. van Duin, and W. a Goddard, Journal of Physical Chemistry A, 112, 1040 (2008).
[5] Y. M. Jaques, G. Brunetto, and D. S. Galvão, MRS Advances, 1, 675 (2016).

Catalytic activation with metal-free systems is in high demand for industrial processes, in order to accelerate chemical transformation while preventing high cost and leaching that occur with traditional metal catalysts. To achieve recyclability, heterogeneous catalysis should be possible on these metal-free systems, which remains a challenge. Monolayer hexagonal boron nitride (h-BN) may offer these characteristics for catalyzing alcohol synthesis reactions: recently, defects such as vacancies and dopants have been shown to significantly modify the electronic and chemical properties of two-dimensional monolayers, including in h-BN, to offer catalytically active sites for the conversion of synthetic gases (H₂ and CO₂) into higher alcohols.
Here we explore the role defects play in the activation of covalent bonds via chemisorption of the metal-free catalyst boron nitride for hydrogenation. In particular, we study the changes in optical and structural properties engendered by the introduction of selected defects in boron nitride. We introduce a protocol based on force microscopy and nanoscale infrared spectroscopy to explore the local interactions between the molecules involved in the reactions and the defects. We also investigate the role of edges in the catalytic processes. We correlate our results with fundamental processes such as adsorption, dissociation and diffusion unveiled using ab-initio approach based on the density functional theory with non-local van der Waals correction. Finally, we combine nanoscale experimental results and ab-initio calculations to reactor-scale analyses of the reaction products to determine factors that control the reactivity of defect-laden h-BN.

Efficient heat removal through the interface of monolayer MoS₂ and metal substrates is important for high performance and reliability of MoS₂-based electronic devices. We have developed an atomistic framework based on the first-principle Density Functional Theory (DFT) and the Atomistic Green’s Function (AGF) to elucidate the inherent connection between electronic structure and phonon properties at the interface of single-layer MoS₂ and metal substrates (Au and Sc). Strong chemical coupling at the interface of MoS₂ and Sc substrate results in a thermal boundary conductance (TBC) that is 19 times higher than that of the interface of MoS₂/Au. The redistribution of phonon density of states (DOSs) at MoS₂/Sc interface, correlated with interfacial electronic structure change, enhances interfacial phonon-phonon coupling and increases phonon transmission. We find that the charge transfer caused by the introduction of a metal substrate can affect the strength of the Mo-S bond. Furthermore, the effect of interfacial lattice-stacking configurations of MoS₂/Sc leads to a significant re-distribution of phonon DOSs and transmission at the interface. Results show that the weakening of the Mo-S bond strength due to charge redistribution and the resultant decrease in the force constant between Mo and S atoms keeps more phonons locating in the low-frequency region which leads to a 60 % decrease in TBC. The findings in this study demonstrate the inherent relationship among the interfacial electronic structure, the phonon distribution, and TBC, which helps us understand the mechanism of phonon transport at the MoS₂/metal interfaces. The results provide insights for the future design of MoS₂-based electronics and a way of enhancing heat dissipation at the interfaces of MoS₂-based nano-electronic devices.