Symposium NM07—Beyond Graphene 2D Materials—Synthesis, Properties and Device Applications

Valley excitons has proven to play a significant role in the photoconductivity and light emission processes of two-dimensional (2D) Molybdenum Disulfide (MoS₂). Exploring ways to manipulate energy levels of excitons opens the possibility to tune quantum degrees of freedom such as electron spin, layer pseudospin, and valley pseudospin. In this contribution, we first introduce a new method to produce single-layer sheets of MoS₂ using miscible organic solvents to minimize the exfoliation energy in the fabrication process. The atomically thin membranes were suspended on four different epitaxial substrates and analyzed using Raman spectroscopy, Photoluminescence (PL), Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). We report on the strain induced from the lattice mismatch between the substrate and the material, correlate the effect this has on the binding energy of MoS₂, and lastly explore the impact substrates have on the optical properties of MoS₂ by measuring the exciton binding energy, spin-orbit coupling, and optical and electronic band gap. This work serves as a foundational piece to assist researchers with choosing a formidable substrate to build field effect transistors.

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS₂ and WSe₂, results in two energetically equivalent domain alignments, often referred to as 0^o and 60^o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to play a key role in domain alignment by altering the energy landscape for nucleation and adatom diffusion.

Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe₂ and WS₂ monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.3^o toward <110>, consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850^oC to 1000^oC, was used to achieve epitaxial films using W(CO)₆, H₂Se and H₂S as precursors in a H₂ carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

Two-dimensional (2D) heterostructures combining several individual 2D materials provide unique platforms to create unprecedented physical properties, thereby exploring new applications. In particular, heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications. Although several methods have been developed to produce in-plane heterostructures of graphene and h-BN through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we demonstrated a new chemical route in which the Pt substrate plays a catalytic role. We also proposed that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enabled the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate [1]. It means that we could fabricate spatially controlled in-plane heterostructures of h-BN and graphene.
We expanded the spatially controlled conversion of h-BN to graphene on an array of Pt nanoparticles (NPs) to realize an array of uniform GQDs embedded in an h-BN sheet. A uniform Pt NP array was formed on a SiO₂/Si substrate with the aid of self-patterning diblock copolymer micelles, and the h-BN sheet was transferred on the Pt NPs array, followed by the conversion of h-BN on Pt to GQDs. The size of the obtained GQDs corresponded with the sizes of the Pt NPs, because of the selective conversion of h-BN on top of Pt NPs. Uniform and precisely controlled size of the GQDs ranging from 7 to 13 nm was achieved. Finally, we demonstrated electron transport by the size-controlled GQDs isolated by insulating h-BN like a Coulomb blockade, indicating that the splitting energy of the GQD is 70–140 meV, compatible with its dimension [2].
In addition, a new photoluminescence peak in the GQD/h-BN heterostructures was observed at 410 nm. This blue-emitting photoluminescence occurs at 1D heterojunctions of h-BN and graphene, which is originated from the localized energy states at the disordered boundaries of h-BN and graphene [3].
[1] G. Kim, et al., Nano Letters 15, 4769 (2015)
[2] G. Kim, et al., Nature Communications 10, 230 (2019)
[3] G. Kim, et al., Nature Communications 11, 5359 (2020)

Transition Metal Dichalcogenide (TMDC) materials such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) exhibit unique optoelectronic properties. This makes TMDCs particularly promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, the vertically-oriented configuration of TMDCs could be advantageous as compared to the conventional horizontal configuration, because the inherent broken symmetry of the vertically orientated MoS2 and WS2 would favor an enhanced nonlinear response. Further, these attractive optoelectronic properties of vertically-oriented MoS2 and WS2 can be augmented by fabricating heterostructures based on their combination. Up to now, most of these heterostructures have been fabricated by utilizing transfer techniques or by performing separate chemical vapor deposition (CVD) steps for each material.

As a starting point, we show that the direct sulfurization of predeposited Mo metal results in vertically orientated MoS2 (v-MoS2) nanosheets. A systematic study on these v-MoS2 nanosheets shows that the sulfurization rate is strongly dependent on the reaction temperature and reaction time. By systematically varying the reaction time and temperature we were able to determine both the activation energy and the diffusion constant for the sulfurization process. We verify an enhanced nonlinear response in the resulting v-MoS2 nanostructures as compared to their horizontal counterparts.

Building upon these results, we demonstrate the feasibility of creating vertically-oriented TMDC heterostructures via a one-step CVD process without relying on transfer techniques, utilizing the direct sulfurization of molybdenum (Mo) and tungsten (W) heterostructures and multilayers. By depositing different configurations of Mo and W metal and carefully controlling the sulfurization depth we can design different TMDC heterostructures and multilayers. Utilizing FIB tomography and 3D reconstructions, we determine that the grain boundaries at the interface between the Mo and W metals are continuous and that, after sulfurization, these continuous metal grains facilitate the synthesis of in-plane vertical heterostructures. HR-TEM analysis confirms that these in-plane heterostructures are continuous and uniformly distributed along the interface.

This versatile method of fabricating in-plane and vertically aligned MoS2 and WS2 heterostructures can be further extended to different arrangements of TMDC heterostructures and multilayers, thus represents a steppingstone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.

h-BN is an eminent member of the 2D materials family with exceptional and peculiar properties such as atomically smooth surface, ultra-high flexibility, and transparency. h-BN is an attractive candidate for application in flexible electronics, optoelectronics, and electro-mechanics devices. Moreover, h-BN encapsulated 2D materials ^[1] demonstrate elevated throughput and enhanced physical properties.

h-BN is a binary element material produced in various shapes via the CVD process ^[2]. The coalescence of the different shape and orientation domains generate a range of grain boundaries (GBs) that can be composed of N-N, B-B, and N-B (5, 7) defects ^[3]. Indeed, defective GBs lead to high leakage current and low mechanical strength, making it unsuitable for electro/mechanical devices and other applications. Therefore, the synthesis of large-area and defect-free h-BN is pivotal. Attempts have been made to grow large-area h-BN relying on pre-treatment of the substrate ^[4], the alloyed substrate ^[5], water vapors ^[2], and monocrystalline substrate ^[6]. These processes have shown promising results, but the impact of oxygen to reduce the nucleation density has not been widely scrutinized in the literature.

In this work, we investigated the impact of oxygen on the growth of large-area h-BN. CVD growth of h-BN was carried out on polycrystalline copper (Cu) foils using ammonia borane as a precursor. Our investigation suggests that pre-oxidation of Cu foils by a few hundred ppm of oxygen leads to an increase of the h-BN domains size by at least a factor of 5 when compared to the h-BN domains synthesized by sole hydrogen annealing of Cu foils ^{[4, 6]}. Typical samples contain the various shapes of domains with a maximum lateral size of hundreds of microns (> 100 μm), which have not been reported so far.

The quality of h-BN is confirmed by Raman, XPS, Tof-SIMS, and UV-Vis spectroscopies. An intense peak is observed in the frequency range of 1360-1370 cm^-1 that is the typical E_2g mode of h-BN reported in the literature. The binding energy of boron and nitrogen is about 190 eV and 398 eV, respectively. We also confirm the stoichiometry of h-BN, i.e., the N: B ratio is close to 1 in good agreement with high-quality h-BN growth. UV-Vis spectroscopy shows strong absorption around 202 nm that corresponds to the 6.13 eV optical bandgap. The combined results of Raman spectra with UV-Vis spectroscopy confirm the formation of h-BN and reject the possibilities of other crystallographic BN structures such as r-BN, c-BN, and w-BN. The tunneling current was measured for hundreds of metal/h-BN/metal structures with an overlap area of 25 μm². For a typical device, the thickness of h-BN is 2.6 nm, as confirmed by AFM. The normalized current density was found to be 1.79 nA/μm² at a DC bias of 1 V, proving the good insulator potential of the synthesized h-BN.

The achieved large areas of grown h-BN are attributed to an optimum concentration of intrinsic carbon and organic species present in the Cu foil. These species presumably play a critical role in determining the size of the h-BN domains. An excessive degree of oxidation deteriorates surface quality during the process, leading to voids on the substrate and gives rise to defective growth, whilst an inadequate degree of oxidation results in smaller domains (< 5-10 μm). There is a tradeoff between the residual impurities and the h-BN grain size that can be tuned by the degree of oxidation of Cu foils, as demonstrated in this work.


References

1- J. Holler et al., 2D Mater. 7, 015012, (2019).
2- L. Wang et al., Mater. Chem. Front. 1, 1836-1840, (2017).
3- J. Strand et al., J. Phys.: Condens Matt., 32, 055706, (2019).
4- R. Y. Tay et al., Nano Lett., 14, 839-846, (2014).
5- G. Lu et al., Nat. Comm., 6, 6160, (2015).
6- L. Wang et al., Nat., 570, 91-95, (2019).

The monolayered 2D nanosheets of layered inorganic solids (layered metal oxides, layered double hydroxides, layered metal chalcogenides, and graphene) attract intense research interest because of their versatile roles in multifunctional nanohybrids applicable for energy and environmental technologies. The monolayered 2D nanosheets of inorganic solids can be synthesized by soft-chemical exfoliation reaction of the pristine layered materials. A great diversity in the chemical compositions and crystal structures of inorganic nanosheets provides this class of materials with a wide spectrum of physical properties and functionalities. The inorganic nanosheets can be used as powerful building blocks for exploring high performance hybrid photocatalysts and electrocatalysts. These materials can play a role as catalytically active components as well as conductive additives for improving the catalyst performance of hybridized species. In this talk, several practical examples of 2D monolayered nanosheet-based photocatalysts and electrocatalysts active for solar fuel production will be presented together with the discussion about the relationship between catalyst performance and chemical bonding nature.

Euclidean geometry is the fundamental mathematical framework of classical crystallography. Traditionally, layered materials are grown on flat substrates, but growing Euclidean crystals on non-Euclidean surfaces has rarely been studied. Here, we present a general model describing the growth of layered materials with screw-dislocation spirals on non-Euclidean surfaces and show that it leads to continuously twisted multilayer superstructures. This model is experimentally demonstrated by growing supertwisted spirals of tungsten disulfide (WS₂) and tungsten diselenide (WSe₂) draped over nanoparticles near the centers of spirals. Microscopic structural analysis shows that the crystal lattice twist is consistent with the geometric twist of the layers, leading to moiré superlattices between the atomic layers, which further influence the electronic structures of the material. The preliminary studies on the physical properties of such multilayer twisting 2D materials will be discussed. Our results introduce new concepts in the growth of exotic nanomaterials and a rational strategy for potentially controlling the twist angle in layered materials from direct synthesis, which opens up new opportunities for the study of twisted moiré superlattices and chirality-related properties.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) monolayers are considered promising for future extreme device downscaling. For wafer scale manufacturing of advanced logic and memory devices, dense arrays of single-crystal, globally aligned TMD monolayer nanoribbons that can be controllably grown, are highly desired. Demonstrated top-down approaches to form such nanoribbons require large-area, single-crystal TMDs and damage-free etching processes, which are not available currently. Bottom-up growth methodologies for TMD nanoribbons have been reported to individually achieve control of layer number, single-crystallinity, local alignment, and dimensionalities. However, controlled nanoribbon growth with all aforementioned properties synergistically remains a major challenge. In this talk, we will demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and predominantly single-crystalline MoS₂ nanoribbons on β-gallium (III) oxide (β-Ga₂O₃) (100) substrates. Experimental observation and density function theory (DFT) simulation indicate that the presence of intrinsic ledges on β-Ga₂O₃ (100) leads to the reduction in binding energy. The two previously indistinguishable and nearly degenerate configurations are now remarkably uncoupled from each other by ~2 eV, overcoming the atomic registry—the van der Waals (vdW) epitaxy constraint and thus ensuring the mono-orientated nucleation. Meanwhile, potential energy surface (PES) mapping sheds light on a surface diffusion limited pathway along the ledge, hence driving the energetically preferred and directionally modulated growth of aligned MoS₂ domains into single-crystalline nanoribbons. The stitching of unidirectional seeds into continuous, single-crystal and -orientated MoS₂ nanoribbons was confirmed by second harmonic generation (SHG) and dark field-scanning transmission electron microscopy (DF-STEM). The MoS₂ nanoribbons can be readily transferred to arbitrary substrates while the underlying β-Ga₂O₃ can be re-used after mechanical exfoliation. Prototype MoS₂ nanoribbon-based field-effect transistors exhibit high on/off ratios of 10⁸ and an averaged room temperature electron mobility of 65 cm²V^-1s^-1, both on par with values reported for mechanically exfoliated MoS₂ devices_. We further demonstrate, for the first time, the LDE growth as a general strategy for single-crystal p-type WSe₂ nanoribbons and lateral n-p-n heterostructures made of p-WSe₂ and n-MoS₂ nanoribbons. Our findings pave the way to direct the growth of single-crystalline TMDs and their heterostructures for futuristic electronics applications.

Tailoring the specific stacking sequences (polytypes) of layered materials represents a powerful strategy to identify and design novel physical properties. While nanostructures built upon transition-metal dichalcogenides (TMDs) with either the 2H or 3R crystalline phases have been routinely studied, our knowledge of those based on mixed 2H/3R polytypes is far more limited. Here we report on the characterization of mixed 2H/3R free-standing WS₂ nanostructures displaying a flower-like configuration by means of advanced transmission electron microscopy. We correlate their rich variety of shape-morphology combinations with relevant local electronic properties such as their edge, surface, and bulk plasmons. Electron energy-loss spectroscopy combined with machine learning reveals that the 2H/3R polytype displays an indirect band gap with E_BG = 1.6 eV. Further, we identify the presence of energy-gain peaks in the EEL spectra characterized by a gain-to-loss ratio I_G / I_L > 1. Such property could be exploited to develop novel cooling strategies for atomically thin TMD nanostructures and devices built upon them. Our results represent a stepping stone towards an improved understanding of TMDs nanostructures based on mixed crystalline phases.

Two-dimensional transition metal dichalcogenides (TMDCs) have attracted widespread attention due to their prospective for the next-generation non-silicon electronics and optoelectronics. Among these two-dimensional TMDCs research, the most highlight has been given to single-layer TMDCs because of its intriguing electrical and optical properties. However, the fabrication demands and the physics of TMDCs suggest that few-layer TMDCs may be more attractive than single layer TMDCs for the industrial manufacturers because few-layer TMDCs equit more density of states for application in electronics, better light absorption and wider spectral response in optical application. To realize full potential of few-layer TMDCs, there is still much effort in the scalable and layer-controllable growth of highly crystalline TMDCs need to be achieved. In this work, WSe₂ is treated as a representative of common TMDCs growth, and we report an experimental observation that strain presenting in the first WSe₂ layers plays a critical role in the determination of growth manners for the following layers. Frank–van der Merwe Growth mode can be realized through inducing built-in tensile strain in first WSe₂ layers during the CVD process. Moreover, we demonstrate the stacking angle between the successive layers and the first layers by second harmonic generation (SHG) measurement. The foundational understanding of growth mechanism is assessed by a comprehensive density functional theory (DFT) simulation. This work provides a crucial hint to realize uniform layer control over the large scale, which is an important step for TMDCs towards real applications.

One of the primary objectives of nanoscience is the precise control over the processing-structure-property relationship intrinsic to traditional materials science. Recently, the concept of dimensionality, and the engineering of materials’ structure and properties via changes in dimensionality, has emerged as an additional degree of freedom within this paradigm. To this end, the isolation of single atomic-planes (e.g. graphene and the transition metal dichalcogenides) and chains (e.g. transition metal trichalcogenides, TMTs) from quasi-low-dimensional materials has been extremely fruitful. The archetypal quasi-1D materials family is the TMTs, commonly referred to as MX₃ compounds, with M a transition metal and X a chalcogen. Typically for these materials, 1D MX₃ chains are weakly coupled by interchain vdW interactions to form coherent, but highly anisotropic, three-dimensional crystals. The bulk synthesis and properties of the TMTs has been well explored and they are canonical examples of superconductors and charge density wave materials. This structural motif is common across a range of M and X atoms (e.g. NbSe₃, HfTe_3,TaS₃), but not all M and X combinations are stable. Here, we demonstrate the successful synthesis of three previously unreported MX₃ TMT compounds: NbTe₃, VTe₃, and TiTe₃. This is accomplished through nano-confined growth within the cavity of multi-walled carbon nanotubes (MWCNTs). Depending upon the inner diameter of the encapsulating MWCNTs, specimens ranging from many chains, to few chains (2-3), and even single chain, can be isolated and studied. The MWCNT sheath stabilizes the chainlike morphology, enabling synthesis and characterization with transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy (STEM). It is found that few-chain specimens of the new TMTs can exhibit a coordinated interchain spiraling, while the single-chain limit exhibits a trigonal anti-prismatic (TAP) rocking distortion, which we experimentally resolve for the first time here. First principles calculations give insight into the integral role that the encapsulating CNT plays in stabilization and provide information regarding the electronic structure of the new materials.

Ruddlesden-Popper (RP) phase lead halide perovskites have emerged as a new class of 2D semiconductors that exhibit tunable electronic and optical properties, potentially offering unlimited heterostructure configurations for exploration. However, the promise of such heterostructures has not been fulfilled, because halide perovskites’ highly mobile and fragile crystal lattices make controllable direct synthesis or van der Waals integration extremely difficult. Here, we report the direct growth of large-area free-standing nanosheets of diverse phase-pure RP perovskites with thickness down to a monolayer at the solution-air interface and a gentle and reliable approach for transferring and stacking these nanosheets. These advances enabled the deterministic fabrication of arbitrary vertical heterostructures and multi-heterostructures of different RP perovskites with unprecedented structural degrees of freedom that define the electronic structures of the heterojunctions. Such rationally designed heterostructures exhibit interesting interlayer properties such as interlayer carrier transfer and reduction of photoluminescence linewidth, and would enable the exploration of exciton physics and optoelectronic applications.

Accumulation of highly toxic oxyanions such as chromate and arsenate in ground and surface waters from the geological processes and/or man-made pollution has led to a wide range of health and environmental problems. Thus, the development of innovative materials that can selectively remove target toxic oxyanions has attracted enormous research interest in academia and industry. Ionic 2D covalent organic frameworks (iCOFs) offer a unique means to effectively incorporate well-defined anion binding motifs into chemically robust, highly periodic, low-density polymeric materials. Structural modifications can be easily implemented to fine-tune the local environments of anion binding sites inside iCOFs providing insights into the selectivity and efficiency of the ion exchange process. A series of guanidinium-based 2D iCOFs have been developed to systematically examine the impacts of steric, electrostatic, and H-bond groups on the anion exchange process of chromate and arsenate anions. Interesting conclusions include: 1) well-position H-bonds seem to play a crucial role in determining the weak binding of arsenate anions, 2) electrostatic tuning significantly impacts the nature and incorporation of guanidinium sites in iCOFs, 3) moderate steric alteration imposes little influences on chromate and arsenate selectivity. Results and knowledge from this study are expected to aid the development of new guanidinium-based polymeric and composite anion exchange materials for toxic oxyanion remediation and other extraction applications.

As a result of their unique electronic, optical, and physical properties, two-dimensional (2D) materials are actively being explored for applications in optoelectronics [1], neuromorphic computing [2], quantum information science [3], and energy technologies [4]. With exceptionally high surface-to-volume ratios, 2D materials are highly sensitive to their environment, resulting in a strong dependence of their properties on substrate effects, extrinsic adsorbates, and interfacial defects. Furthermore, the integration of 2D materials into heterostructure devices introduces further demands for controlling interfaces with atomic precision. With this motivation, this talk will explore emerging efforts to understand and utilize interfacial chemical functionalization to influence the properties of 2D heterostructures. For example, organic adlayers can tailor chemical reactivity to enable conformal atomic layer deposition of pinhole-free encapsulation layers that mitigate the deleterious effects of ambient exposure, particularly for ambient-unstable 2D materials such as black phosphorus and monochalcogenides [5]. The integration of organic self-assembled monolayers with 2D semiconductors also allows for tailoring of electronic and optical properties such as photoinduced charge separation in fullerene/InSe heterojunctions [6] and mixed-dimensional excitonic states in phthalocyanine/MoS₂ heterojunctions [7]. By exploiting spatially inhomogeneous surface chemistry, seamless lateral 2D heterointerfaces can also be realized including perylene/borophene [8], graphene/borophene [9], and concentric borophene superlattices [10], each of which show atomically sharp electronic interfaces as confirmed by ultrahigh vacuum scanning tunneling microscopy and spectroscopy. Overall, by providing precise tailoring of interfaces, chemical functionalization presents opportunities for improved functionality in 2D heterostructure devices.

[1] M. E. Beck and M. C. Hersam, ACS Nano, 14, 6498 (2020).
[2] V. K. Sangwan and M. C. Hersam, Nature Nanotechnology, 15, 517 (2020).
[3] X. Liu and M. C. Hersam, Nature Reviews Materials, 4, 669 (2019).
[4] S. Padgaonkar, et al., Accounts of Chemical Research, 53, 763 (2020).
[5] S. A. Wells, et al., Nano Letters, 18, 7876 (2018).
[6] S. Li, et al., ACS Nano, 14, 3509 (2020).
[7] S. H. Amsterdam, et al., ACS Nano, 13, 4183 (2019).
[8] X. Liu, et al., Science Advances, 3, e1602356 (2017).
[9] X. Liu, et al., Science Advances, 5, eaax6444 (2019).
[10] L. Liu, et al., Nano Letters, 20, 1315 (2020).

Two-dimensional (2D) transition metal dichalcogenides (TMDs) monolayers have been considered promising for future device scaling. For wafer-scale manufacturing, dense arrays of single-crystal, globally aligned TMD monolayer nanoribbons are desired for advanced logic and memory devices. The top-down approach to form such nanoribbons requires large-area, single-crystal TMD, and deliberate lithography/etching processes, which are not available currently. The bottom-up growth approaches toward TMD nanoribbons have been reported to individually achieve the control of layer number, single-crystallinity, local alignment, and dimensionalities. However, the nanoribbon growth with all the above-mentioned properties synergistically remains a major challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of centimeter-long, self-aligned, monolayer, and predominantly single-crystalline MoS₂ nanoribbons on β-gallium(III) oxide (β-Ga₂O₃) (100) substrates. Experimental observations and density functional theory (DFT) simulation suggest that nucleation and growth of the nanoribbons follow the β-Ga₂O₃ ledges. The stitching of unidirectional seeds into continuous, single-crystal, and -orientation MoS₂ nanoribbons was confirmed by second harmonic generation (SHG) and dark field-scanning transmission electron microscopy (DF-STEM). The MoS₂ nanoribbons can be readily transferred to arbitrary substrates while the underlying β-Ga₂O₃ can be re-used after mechanical exfoliation. Our prototype MoS₂ nanoribbon-based field-effect transistor exhibits an on-off ratio of 10⁸ and room temperature carrier mobility of 65 cm²V^-1s^-1 comparable to those mechanically exfoliated benchmarks. We further demonstrate the LDE growth of p-type WSe₂ nanoribbons and lateral heterostructures made of p-n WSe₂-MoS₂ nanoribbons. Our findings pave the way to miniaturization of single-crystalline TMDs and their heterostructures with potential applications not available by other means.

MXenes are a family of two-dimensional (2D) transitional metal carbides and nitrides, obtained by selective etching of the A element from the parent MAX phases, where M is an early transition metal, A is an element of the A group, mostly group 13 or 14 of the periodic table (in particular, Al, Si, and Ga), and X stands for carbon and/or nitrogen. Their atomic-scale nature, scalable processing, and attractive electrical properties (up to 20,000 S/cm conductivity) make them strong candidates for a diverse range of applications such next generation energy storage systems, electromagnetic interference (EMI) shielding, antibacterial agents, and water purification. However, MXenes are solution-based 2D materials with abundant surface functional groups and intra-flake separations, which decrease the overall electrical conductivity in film form. This renders film annealing a critical post processing step.

While annealing of MXene films is typically performed in the 700-1500 °C range, we present a novel, athermal process to achieve similar results. We explore annealing with the electron wind force (EWF) at near-room temperature conditions in contrast to traditional heating methods. EWF is an atomic scale mechanical force that acts only in the defective regions, which is proposed to provide very high defect mobility. The process is demonstrated on nominally 5 microns thick, freestanding Ti₃C₂T_x films. We report 30-50% decrease in resistance after applying only 20 amp/mm² current density. The specimen temperature remains below 100 °C during the process. The resistance change was permanent and did not increase back when the current was removed. We acquired Raman spectroscopy on as-received and EWF-annealed specimens to observe remarkable decrease of intensity in the T_x, M-T_x and the Carbon regions. In the T_x (230−470 cm⁻¹) largest intensity drop occurred at wave shift of 286 cm^-1. This range represents the in-plane vibrations of surface groups attached to titanium atoms. Therefore, our remarkable intensity drop suggests elimination of the surface terminated bonds, which are helpful for the electrical conductivity. Further, the C region (580 – 730 cm⁻¹) represents carbon vibrations, where we observed remarkable intensity drops in the 600 and 720 cm^-1 peaks. We also observed remarkable decrease in the 123 cm^-1 peak, which represents the surface plasmon resonance. The most remarkable feature of this annealing process is that no increase in disordered carbon peaks (typically seen in high temperature annealing) were observed. Rather, the decreased peak intensity in the C region indicates loss of carbon atoms. The Raman results were verified by Energy-dispersive Spectroscopy performed in a SEM. We also prepared cross-sectional specimens to highlight the decrease of the intra-layer defects (mostly micron scale voids).

To summarize, we have demonstrated a low temperature (<100 °C) process to anneal MXene thick films to decrease their resistivity up to 50%. Raman and EDS analyses suggest that such improvement in electrical conductivity comes from the removal of the surface terminated bonds. The advantages of this new process are (a) low temperature will allow temperature sensitive substrates, such as polymers and (b) absence of high temperature induced formation of TiO₂ or disordered carbon.

Atomically thin two-dimensional (2D) crystals are promising channel materials for extremely scaled field-effect transistors (FETs) for the 2030 era [1]. In the quest of ultra-scaled transistors, both channel length (distance from source to drain contacts) and contact length (distance that the contacts overlap the 2D channel) must be scaled. However, contacting 2D materials at scaled contact lengths (L_c < 30 nm) has rarely been pursued or studied in-depth. In this work, we experimentally scaled contact length for Ni-contacted MoS₂ FETs and use asymmetrical contact measurements (ACM) as a new approach for characterizing the devices. We found that, contrary to most previous reports, top contacts can be scaled down to ~30 nm without noticeable degradation in contact resistance. Surprisingly, we also observed significant self-heating in scaled contacts in the saturation regime. While the first observation is promising for extremely scaled FET technologies, the second illustrates that current crowding in metal-2D contacts is a challenge toward the development for future scaled devices.
[1] IEEE International Roadmap for Devices and Systems. https://irds.ieee.org/ (2020).

Molybdenum disulfide (MoS₂) and MoS₂-based nanowire arrays have garnered significant attention recently due to several potential applications, including being used as interconnects for nanoelectronic devices, for efficient electron emission, their structural stability, and in connection promoting hydrogen evolution reaction. In particular, MoS₂ nanowires display decreasing resistivity with decreasing nanowire diameter. For this reason, relatively large electric currents can be sustained for nanowires with diameters in the tens of nanometer range, even though the resistivity of such nanosized MoS₂ nanowires increases to values larger than bulk MoS₂. This property makes MoS₂ nanowires promising candidates to be deployed as interconnects for nanodevices.

Furthermore, arranging MoS₂ nanowires into bundles whose axis is perpendicular to the substrate surface is a promising strategy for creating efficient electron emitters, where a lower voltage is required for electron extraction when decreasing the diameter of these nanowires. In order to create an ideal electron emitter device, the density of MoS₂ nanowires has to be such that the field enhancement effect of single nanowires is preserved while having a large density of MoS₂ nanowires to achieve a high electron current density.

Previous work in the fabrication of MoS₂ nanowires mostly focuses on bottom-up approaches based on hydrothermal synthesis, self-assembly, and chemical vapour deposition. However, these bottom-up methods lack precise control of the nanowire shape, nanowire-to-nanowire distance, and nanowire orientation. We propose a top-down approach for the fabrication of MoS₂ nanostructures which has not been widely explored. Here demonstrate the feasibility of a top-down approach to fabricate ordered arrays of MoS₂ nanowires where the shape, diameter, pitch, and length of these nanowires can be controlled in detail by means of the combination of sputter-deposition, electron beam lithography, and reactive ion etching.

Two-dimensional metal carbides and nitrides, known as MXenes, are the largest and yet quickly expanding family of 2D materials. Their diverse electronic, optical, and electrochemical properties have already distinguished them from other 2D materials. MXenes are produced by selective etching processes in which A layer atoms (i.e., Al, Si, Ga, etc.) are chemically removed from layered ceramics known as MAX phases or related layered carbides. MXenes have a general formula of M_n+1X_nT_x where M is a transition metal (i.e., Ti, V, Nb, Mo, etc.), X is carbon and nitrogen, n can be from 1 to 4, and T_x indicates to presence of mixed surface terminations (O, OH, Cl, F, etc.) on the surface of outer transition metal layers of MXenes. MXenes can also contain two or more transition metals in the M sites in a random solid solution (such as (Ti,V)₂CT_x) or ordered (in-plane or out-of-plane, such as Mo₂TiC₂T_x) structure, which further expands their range of compositions and therefore, properties. The surface functional groups of MXenes provide them with chemical stability and control their electronic properties. Titanium carbide MXene, Ti₃C₂T_x, has metallic conductivity reaching over 15,000 S cm^-1, while some other MXenes showed 2-3 orders of magnitude lower values. The tunable electronic properties of MXenes along with their high mechanical stiffness (Young’s modulus reaching ~0.4 TPa for Nb₄C₃T_x MXene) and high bending rigidity as well as rapid ion-transport properties, redox-active surfaces, tunable electrochemical and optical properties have rendered these materials as promising candidates for various high-tech applications such as transparent conductive electrodes (TCE) for organic light-emitting diodes, smart and conductive textiles for sensing and energy-storing wearable devices, thin-film transistors, biomedical and biosensing, photovoltaics, and high energy and power density supercapacitors. Herein, we aim to discuss synthesis of MXenes, as well as effect of synthesis process parameters on surface chemistry and properties. Scalability of MXene synthesis and its processing from aqueous solutions by a variety of techniques will be addressed in detail.

The development of direct-write nanopatterning approaches enabling the accurate and reliable production of nanoscale architecture is critical for exploiting the unique functionalities of materials at reduced length scales. In the last few decades, several atomic force microscopy (AFM) lithography techniques, which use a sharp probe to write on a solid surface, have been developed as cost-effective methods for patterning with nanoscale resolution. Common AFM lithography techniques usually exploit the meniscus formed around the tip-substrate contact to dissolve and transfer the reactants for surface patterning. This mechanism can be problematic in large scale production due to the restricted ink capacity, as well as the inconsistency in the transfer process by varying experimental and environmental factors. Here we highlight a new AFM patterning method with which an adsorbed ionic liquid (IL) monolayer on the substrate will be used as the ink material. The unique interfacial properties of ILs combined with electrical and mechanical stimuli can realize nanoscale patterning without the reliance of the meniscus for ink transfer. The chemical nature and physical properties of the deposited material will be evaluated.

Reducing the dimensionality of semiconducting transition metal dichalcogenides (TMDs) substantially alters their physical properties. Two-dimensional (2D) TMD crystals also exhibit electronic, magnetic, and optical properties which are sensitive to any manipulation of edge and strain states. We have introduced a rational chemical synthesis strategy that drastically improves control over the dimensions, morphology, and crystalline edges of 2D TMDs without the need for lithography and etching. TMD nanoribbons prepared through gas-phase growth on our designer surfaces exhibit atomically sharp edges and have enabled the exploration of anomalous exciton emission features. Moreover, we show that judicious assembly of mixed-dimensional architectures containing 2D TMD crystals and Si nanowires enables the introduction of nanoscale strain fields. Detailed nanoscale structural and optical characterization of these systems reveals unique edge and strain states that manifest localized photoluminescence with anomalous shifts in energy. Our results open a path toward the rational design of inorganic low-dimensional architectures with clear relevance for enabling future photonic and optoelectronic device studies.

2D crystals and superlattices possess extremely anisotropic structures originating from different in-plane intralayer and out-of-plane interlayer bondings. This extreme structural anisotropy leads to anisotropic transport of electrons, phonons and photons. Indeed, 2D materials have recently shown several exciting 2D electronic transport phenomena. However, its impacts on the transport of heat and light in different directions have not been fully explored. In this talk, I will discuss two recent results related to this topic. First, we will discuss extremely anisotropic thermal conduction in stacked multilayers of transition metal dichalcogenides, where interlayer rotations lead to air-like thermal insulation along the through-plane direction. This leads to a giant thermal conductivity anisotropy ratio ~ 900, which is much larger than that of graphite. Second, we will discuss the long-range (~cm) guiding and non-linear switching of 2D photonic waves using a "delta-waveguide" built based on wafer-scale MoS2. We show that this becomes an optical analog of quantum mechanical delta potential trap with a single trapped wavefunction. If time allows, we will also discuss how to realize these novel anisotropic properties by synthesizing and integrating other, molecule-based 2D materials.

The in-plane piezoelectricity or ferroelectricity of two-dimensional (2D) materials can vanish due to the appearance of inversion symmetry with increasing flake thickness, which drastically limits the development of their energy-harvesting application. On the other hand, although screw dislocation has long been regarded as an undesired line defect in three dimensional materials, screw dislocation driven growth in 2D materials could result in non-centrosymmetric structure even in bulk materials. The non-centrosymmetric structure in 2D materials can bring about plenty of intriguing properties such as vertical conductivity through the screw dislocation core and nonlinear optical generation, piezoelectricity and ferroelectricity and these properties might be further used in various applications like memory devices or piezoelectric generators. [1-3] However, despite the fact that the inversion symmetry breaking characteristic in spiral structure may the solve above-mentioned problem in 2D materials, the control of spiral growth remains immature owing to random occurrence of screw dislocation which limits the development of applications based on spiral 2D materials by poor nucleation site control and low percentage of spiral flakes.

In this research, a novel mechanism to achieve high percentage of spiral SnS flakes with superior control of nucleation position is demonstrated. By introducing atomic steps on substrates, the screw dislocation can be easily formed when SnS partially grows across these steps and leads to over 90% of spiral SnS flakes grown by physical vapor deposition (PVD). Furthermore, the preference for SnS to nucleate at steps can introduce remarkable nucleation site control of spiral growth even on substrates with artificially transferred graphene atomic steps. Through second harmonic generation (SHG) spectroscopy and cross sectional STEM analysis, it turns out that the spiral SnS structure exhibits inversion symmetry characteristic. Contrary to common understanding that spiral 2D material with single screw dislocation and Burgers vector equaling single layer thickness would show non-centrosymmetric structure, single-spiral SnS flakes with Burgers vector equaling the thickness of single layer SnS exhibits centrosymmetric structure. One possible reason that lead to centrosymmetric structure in spiral SnS could be ascribed to its ferroelectricity, [4] indicating that the orientation switching of SnS layer and the formation of AB stacking structure could be induced by strain.

This is the first work to control the formation position of spiral 2D materials and point out 2D materials with single spiral morphology and Burgers vector equaling single layer thickness do not guarantee non-centrosymmetric structure. High spiral flake percentage and precise control of nucleation sites in this study will facilitate future development of spiral 2D materials.

Reference:
[1] T. H. Ly, et al., Advanced Materials 2016, 28, 7723.
[2] L. M. Zhang, et al., Nano Letters 2014, 14, 6418.
[3] W. Z. Wu, et al., Nature 2014, 514, 470.
[4] M. H. Wu, et al., Nano Letters 2016, 16, 3236.

Transition metal dichalcogenides (TMDs) with lamellar structures similar to that of graphite have received significant attention because some of them are semiconductors with sizable bandgaps and are naturally abundant. This offers opportunities for fundamental and technological research in a variety of fields including catalysis, energy storage, sensing, and electronic devices. Among them, bulk WS₂ exhibits an indirect band gap of 1.2 eV, while single-layer WS₂ exhibits a direct band gap of 1.8 eV. In order to fully exploit this versatility, obtaining layers of this material over large areas with the desired properties (in particular the number of monolayers composing the stack) is mandatory. Despite recent advances in the growth techniques of such a material, further investigation is needed especially in the basic mechanisms involved in the growth of WS₂ layers aiming at expanding their applicability. In view of this scenario, we sulfurized sputtered WO₃ films on SiO₂/Si substrates, varying the processing parameters. The objective was i) to identify the role of hydrogen added to the carrier gas in the sulfurization process and ii) to determine sulfurization parameters that result in fully sulfurized WO₃ films. For that, WO₃/SiO₂/Si stacks were introduced in a quartz tube where they were heated for variable times and temperatures. Pure Ar and a Ar:H₂ gas mixture were used as carrier gases. Sulfur powder was independently heated. X-ray Photoemission Spectroscopy (XPS), Rutherford Backscattering Spectrometry (RBS), and Raman Spectroscopy were used to characterize the products of the sulfurization process and also to infer about W loss from the sample during sulfurization. Results evidence that H₂ promotes an efficient sulfurization of the WO₃ film. Using pure Ar as the carrier gas, higher temperatures must be employed aiming at a complete sulfurization. Moreover, W loss takes place during sulfurization, if S is not supplied in a specific moment of the sample’s heating ramp. This latter effect can be strongly suppressed by the use of H₂, which also significantly lowers the temperature needed for sulfurization. These results evidence a synthesis of WS₂ layers with a low thermal budget as well as details of the sulfurization process of WO₃ films. Such knowledge is fundamental for further improvements of WS₂ films’ properties and to new applications of this material.

Boron doped graphene and related materials, especially crystalline BC₃, have long been sought as promising materials for applications such as hydrogen and lithium-ion storage. Among the materials presented by experimentalists so far, boron presents as a notoriously difficult structural/chemical environment to unambiguously probe in the presence of carbon, frustrating the report of a successful synthesis of single-layer BC₃. Herein we report that the hexagonally symmetric C₆B₆ flower-like units that make up the structure of BC₃ exhibit a characteristic “breathing mode” detectable by Raman spectroscopy in the place of the usual D peak associated with graphene. This mode has a distinctly different dispersion relation, which is easily measured using a typical benchtop Raman spectrometer. Boron-substituted graphitic materials were synthesized by the direct route in the temperature range of 750 °C to 1100 °C across a range of compositions and levels of structural order, as characterized by a complement of techniques: Raman spectroscopy, X-ray diffraction, X-ray absorption spectroscopy, energy dispersive X-ray spectroscopy, and Auger electron spectroscopy. The dispersion relation was shown to be directly correlated with the structure and composition of the resulting materials, culminating in the detection of extended BC₃ units in samples of composition ~BC₅ prepared at 800 °C.

van der Waals (vdW) engineering of magnetism is a topic of increasing research interest in the community at present. In the first part of my talk, I will present and discuss our recent efforts¹ in manipulating the magnetic properties of quasi-two-dimensional layered vdW Mn₃Si₂Te₆ (MST) crystals upon proton irradiation as a function of fluence 1×10¹⁵, 5×10¹⁵, 1×10¹⁶, and 1×10¹⁸ H⁺/cm². We find that the magnetization is significantly enhanced by 53% and 37% in the ferrimagnetic phase (at 50 K) when the MST was irradiated with the proton fluence of 5×10¹⁵, both in ab and c plane, respectively. The ferrimagnetic ordering temperature and magnetic anisotropy retained even after proton irradiation.
In the second part of my talk, I will discuss on the electron spin resonance (ESR) properties² of CrCl₃ and CrI₃ single crystals upon photo-excitation in the visible range. We noticed remarkable changes in the ESR spectra upon illumination. In the case of CrCl₃, at 10 K, the ESR signal is shifted from g = 1.492 (dark) to 1.661 (light), line width increased from 376 to 506 Oe, and the signal intensity is reduced by 1.5 times. Most interestingly, the observed change in the signal intensity is reversible when the light is cycled on/off. We observed almost no change in the ESR spectral parameters in the paramagnetic phase (>20 K) upon illumination. Upon photo-excitation of CrI₃, the ESR signal intensity is reduced by 1.9 times; the g-value increased from 1.956 to 1.990; the linewidth increased from 1170 to 1260 Oe at 60 K. These findings are discussed by taking into account the skin depth, the slow relaxation mechanism and the appearance of low-symmetry fields at the photo-generated Cr²⁺ Jahn-Teller centers. Such an increase in the g-value as a result of photo-generated Cr²⁺ ions is further supported by our many-body wavefunction calculations. This work has the potential to extend to monolayer vdWs magnets by combining ESR spectroscopy with optical excitation and detection. Our work shows that it is possible to employ proton and photon excitation in tuning the magnetic properties of vdW crystals, and provide many opportunities to design desired magnetic phases.

¹L. M. Martinez, H. Iturriaga, R. Olmos, L. Shao, Y. Liu, Thuc T. Mai, C. Petrovic, Angela R. Hight Walker, and S. R. Singamaneni, Enhanced magnetization in proton irradiated, Mn3Si2Te6 van der Waals crystals, Appl. Phys. Lett. 116, 172404 (2020); doi: 10.1063/5.0002168
²S. R. Singamaneni, L. M. Martinez, J. Niklas, O. G. Poluektov, R. Yadav, M. Pizzochero, O. V. Yazyev, and M. A. McGuire, Light Induced Electron Spin Resonance Properties of van der Waals CrX₃ (X = Cl, I) Crystals, Applied Physics Letters 117, 082406 (2020); doi: 10.1063/5.0010888.

Confinement heteroepitaxy (CHet) is a fabrication technique for 2D heterostructure in which atoms are intercalated between defective epitaxial graphene and substrates of silicon carbide in a CVD process typically at 800 °C. The confined layers can be designed with the merit of intercalation in several combinations, including metals, refractory, nitrides, and oxides. With all of these combinations, the CHet layers attract interest for many next-generation 2D devices in the field of photonics, plasmonic, hot-electron transistors, and optical polarization substrates.

Using a double-corrected aberration electron microscope and many FIB cross-section samples, we noted that the CHet layers’ structure is not a superficial metallic layer on a perfect 6H-SiC stacking. We found that the SiC terminations vary from terrace-to-terrace, including a significant 3C-SiC termination on the topmost layer. Accordingly, this termination variation is found to affect the Chet layers structure and physical properties. For example, the 2D metals – called half-Van der Waals metals –exhibit a non-centrosymmetric structure with a lateral direction variance of the staking order, structure, and the number of layers, which adds to the vertical direction bonding variance; from covalent (SiC) to metallic (Ga or In) to Van der Waal (Graphene). This makes the 2D CHet surfaces a candidate for generating non-linear optical response and implemented as Surface-enhanced Raman Scattering substrates.

Besides, we observed a picometer-to-angstrom lattice spacing variation at the Si-metal interface. We also investigated epitaxial graphene samples, just before the intercalation process, to understand the Si-graphene and Si-carbon buffer layer interface. Here, we could provide the first direct microscopic evidence that the SiC termination induces Si sublimation from the topmost layer through controlled electron beam damage experiments. Our TEM observations were also consentient with many previous theoretical expectations that explained epitaxial graphene growth due to a pico-scale corrugation in the Si-face atoms due to a (√3 × √3)R30° reconstruction on hexagonal (0001) 6H-SiC.

With these observations, we could explain anomalous but repeatable STEM-HAADF images of 2D Gallium and indium interfaces. These images show a significant EELS signal of Ga and In co-located with the topmost SiC layer in addition to Si and C signal within the 2D Ga and In layer. This interface mixing is observed to be limited to the topmost SiC layer but could be propagated into few layers in severe cases. The Si-metal interface instabilities are essential to be fully understood to optimize the growth conditions to avoid them. However, the presence of few doped Si and C atoms within 2D Ga and In could also be exciting if it induces extra orbital hybridization that may generate local and confined magnetism from the metallic layer, which we are currently investigating.

Layered, two-dimensional (2D) semiconducting materials are promising candidates for thermoelectrics [1,2]. Among them, WSe₂ has one of the lowest thermal conductivities [3], in part due to its heavier atoms, which is an important consideration. In addition, WSe₂ is one of the few 2D materials that has been demonstrated as both n- and p-type (i.e. ambipolar) [4], thus satisfying the need of a thermoelectric generator with one material. Despite these promising material characteristics, only a few experimental studies on the thermoelectric properties of 2D WSe₂ have been performed [5,6], and their dependence on thickness and temperature is still unknown to date.

In this work, we conduct thickness- and temperature- dependent in-plane thermoelectric measurements of WSe₂ films. In order to tune the Fermi level and modulate the carrier density in the channel, we use an ionic-liquid, 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), for electrolyte gating. WSe₂ flakes are exfoliated onto a glass substrate and contacted from the top with 50 nm of evaporated Pd. By using ionic-liquid gating, we are able to sweep across a large range of carrier densities with relatively low applied gate bias. Furthermore, the low thermal conductivity of both the ionic-liquid and substrate contribute to a larger lateral temperature gradient along the WSe₂, which is important for an accurate measurement of the Seebeck voltage. The thermoelectric measurements are conducted in vacuum, with on-chip Pd metal lines serving as resistive heaters and thermometers.

We sweep the gate voltage and measure the in-plane Seebeck coefficient (S_n,p) and electrical conductance of WSe₂ flakes with thicknesses from ~10 to 90 nm between 296 to 400 K. At room temperature, we measure S_p up to 700 μV/K for holes and S_n down to -400 µV/K for electrons in the same device, which are some of the largest reported for sub-100 nm WSe₂ [5,6]. In addition, we measure power factor values up to 50 µW/K²/cm, which are comparable to bulk Bi₂Te₃, a common commercially used thermoelectric material, at room temperature [7]. We examine the trends of thermoelectric performance vs. electron and hole density as well as temperature and observe an increasing power factor with decreasing flake thickness. Overall, our results demonstrate the potential for WSe₂ to eventually compete with bulk materials for use in thermoelectric energy harvesters and we contribute to further understanding the fundamental properties of WSe₂.

[1] M. Dresselhaus et al., Adv. Mat., 19, 1043 (2007)
[2] J. Wu et al., Adv. Elec. Mat., 4, 1800248 (2018)
[3] A. Mavrokefalos et al., Appl. Phys. Lett., 91, 171912 (2007)
[4] S. Das et al., Appl. Phys. Lett., 103, 103501 (2013)
[5] J. Pu et al., Phys. Rev. B, 94, 014312 (2016)
[6] M. Yoshida et al., Nano Lett., 16, 2061 (2016)
[7] I Witting et al., Adv. Elec. Mat., 5, 1800904 (2019)

Owing to its favorable solution processability, the development of a stable dispersion of two-dimensional (2D) boron nitride (BN) has received significant attention for cutting-edge optic/electronic applications. Herein, we report an efficient method to disperse BN nanosheets (BNNSs) in polar solvents via dual noncovalent interactions using pyrene-tethered poly(4-vinylpyridine) (P4VP-Py). As a dispersion agent, P4VP-Py enables dual-functionalization with BNNS through π−π and Lewis acid−base interactions arising from the pyrene and pyridine moiety, respectively, resulting in highly stable BNNS dispersions in different solvents. The blend of P4VP-Py-functionalized BNNS with the pristine P4VP matrix resulted in increased thermal conductivity and dielectric constant combined with superior thermal stability by forming a compatible interface between the P4VP-Py matrix and the BNNS adjacent to the P4VP-Py. We demonstrated that dual noncovalent functionalization of BNNSs based on molecular design presents a strategy to achieve high dispersion of 2D materials into various media, ranging from polar solvents to solid matrices, for the expansion of advanced optic/electronic applications using BNNSs.

Tungsten disulfide (WS₂), as a member of two-dimensional transition metal dichalcogenides family, has attracted great attention due to its tunable band structure, excellent electrical transportation, unique optical properties and good air-stability. Despite the fact that WS₂ is a potential candidate for next-generation channel materials, preparing wafer-scale WS₂ film (8 inch or larger) with good uniformity and film quality remains challenging. Moreover, the bandgap of WS₂ with different layer numbers was tunable, which implied the necessity to precisely control the thickness. Chemical vapor deposition (CVD) is a common method to achieve high quality WS₂ films [1].However, it is difficult to synthesize 2-inch or larger size WS₂ films with precisely controllable thickness. Atomic layer deposition (ALD) is an effective method to achieve wafer-scale WS₂ film. The self-limiting growth mode enabled the thickness of WS₂ film to be accurately controlled through changing the cycle numbers. The S sources of most ALD WS₂ research works were H₂S, which was highly toxic and dangerous. In addition, F-based precursor were used in many studies[2-5], resulting in an accelerated degradation of deposition system due to the highly corrosive nature of F-based precursors. In this work, low-toxic and less-corrosive tungsten hexachloride and hexamethyldisilathiane ((CH₃)₃SiSSi(CH₃)₃, HMDST) were used as W and S precursors, respectively, to achieve wafer-scale and thickness-controllable WS₂ films. The annealing process was carried out to improve the crystallinity of WS₂ film at 800^oC for 2h. The XPS spectra, Raman spectra and plane-view and cross-sectional TEM indicated the good quality and controllable thickness of WS₂ film. Moreover, arrays of field-effect transistors based on WS₂ channel were further fabricated showing homogeneous electrical properties. The on/off ratio of WS₂ FET was nearly 10⁵ and the average electron mobility of WS₂ FET was 3.5 cm²V^-1s^-1. Our results paved a new path to synthesize wafer-scale and thickness-controllable WS₂ film in use of low-toxic and less-harmful precursors through ALD process, while the electrical properties demonstrated the attractive and promising potentials for WS₂ films in future nano-electronic device integrations and applications.
References
[1] C.Y. Lan, Z.Y. Zhou, Z.F. Zhou, et al. "Wafer-scale synthesis of monolayer WS₂ for high-performance flexible photodetectors by enhanced chemical vapor deposition", Nano Res., 2018, 11, (6), pp. 3371-3384.
[2] A. Delabie, M. Caymax, B. Groven, et al. "Low temperature deposition of 2D WS2 layers from WF₆ and H₂S precursors: impact of reducing agents", Chem. Commun., 2015, 51, (86), pp. 15692-15695.
[3] B. Groven, M. Heyne, A. Nalin Mehta, et al. "Plasma-Enhanced Atomic Layer Deposition of Two-Dimensional WS₂ from WF₆, H₂ Plasma, and H₂S", Chem. Mat., 2017, 29, (7), pp. 2927-2938.
[4] B. Groven, A.N. Mehta, H. Bender, et al. "Two-Dimensional Crystal Grain Size Tuning in WS₂ Atomic Layer Deposition: An Insight in the Nucleation Mechanism", Chem. Mat., 2018, 30, (21), pp. 7648-7663.
[5] B. Groven, A.N. Mehta, H. Bender, et al. "Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates", J. Vac. Sci. Technol. A, 2018, 36, (1), pp. 11.

An effective chemical way to optimize the electrochemical functionalities of layered inorganic solids is developed by controlled restacking of exfoliated 2D nanosheet (NS). The fine-control of the stacking number and oxygen defect of restacked inorganic NSs can be achieved by employing diverse intercalants having various ionic sizes and charge densities. In contrast to conventional expectation, the supercapacitor electrode and electrocatalyst performances of inorganic NSs are well-correlated with their stacking numbers and oxygen defects, rather than with their basal spacings. The application of appropriate intercalant ion is quite effective in enhancing the electrode/electrocatalyst performances of restacked NSs via the creation of oxygen defect and the decrease of stacking number, resulting in improvement of catalysis kinetics and charge transfer property, and electrochemical surface area (ECSA). The present study highlights that the controlled restacking of exfoliated inorganic NSs can provide an effective way to optimize its electrochemical functionalities.

The interstratified superlattices of graphitic carbon nitride (g-C₃N₄)-molybdenum disulfide (MoS₂) monolayers with improved bifunctional catalytic performance are synthesized by an electrostatically-driven self-assembly between oppositely-charged exfoliated nanosheets (NSs). In contrast to other exfoliated inorganic NSs, the cationic form of g-C₃N₄ NS can be obtained via the tuning of suspension pH, which is applicable as a new type of cationic building block for superlattice nanohybrids. The resulting superlattice nanohybrids display strong interfacial interaction between restacked g-C₃N₄ and MoS₂ NSs, leading to creation of nitrogen vacancy, stabilization of 1T'-MoS₂ phase, and efficient interfacial electronic transition between the interstratified NSs. The superlattice g-C₃N₄-MoS₂ nanohybrids display remarkably enhanced bifunctionality as electrocatalysts for hydrogen evolution reaction and photocatalysts for visible light-induced N₂ fixation with high selectivity. The beneficial effect of hybridization on catalyst performances is attributable to the promoted adsorption of H⁺/N₂, the provision of many active sites, and the enhancement of charge transfer kinetics, the charge separation, and the visible light absorptivity. The present study highlights that the application of g-C₃N₄ NS as a cationic building block provides valuable opportunity to widen the library of multifunctional NS-based superlattice nanohybrids.

Graphene and Boron nitride are 2D materials that has been widely investigated due to their excellent features and potential applications in optoelectronic devices because of their high absorbance and transmittance properties. Graphene has been grown by several methods such as chemical vapor deposition (CVD), chemical or plasma exfoliation from natural graphite, mechanical cleavage (exfoliation) from natural graphite, microwave synthesis, etc.
CVD has demonstrated to be the most remarkable process for large-scale graphene fabrication. When the thermal CVD process is carried out in a resistive heating furnace, it is known as thermal CVD, and when the process consists of plasma assisted growth, it is called plasma enhanced CVD or PECVD. In this work it is presented graphene growth by hot filament CVD. Here methane gas it is used as a carbon source which is decomposed with a filament at high temperature. In this technique it is possible to obtain a single to few layers graphene by adjusting the growth parameters. To characterize graphene one of the main techniques used is Raman spectroscopy. A nondestructive tool for investigating atomic vibrational properties. Using the important features such as the D, G and 2D band of graphene, can be distinguish the number of layers, the presence of defects, edge orientation and to monitor the temperature. In this work will be presented how the Raman spectra change depending on the substrate where graphene has been grown or transferred, as well as the dependence on the temperature of the G band in bilayer graphene. Also, hexagonal-boron nitride (h-BN) has been known as the best substrate dielectric for studying 2D physics of graphene and for the high-performance of graphene electronics, due to its atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. H-BN have been used to encapsulate graphene to avoid the impurities and as well to tailor the electronic properties of graphene. In this work, it will be presented the synthesis of h-BN and its structural characterization.

Although the doping of graphene grown by chemical vapor deposition is crucial in graphene-based electronics, non-invasive methods of n-type doping have not been widely investigated in comparison with p-type doping methods. We developed a convenient and robust method for the non-invasive n-type doping of graphene, wherein electrons are directly injected from sodium anions into the graphene. This method involves immersing the graphene in solutions of [K(15-crown-5)₂]Na prepared by dissolving NaK alloy in 15-crown-5 solution. The n-type doping of the graphene was confirmed by down-shifted G and 2D bands in Raman spectra and by the Dirac point shifting to a negative voltage. The electron-injected graphene showed no sign of structural damage, exhibited higher carrier mobilities than that of pristine graphene, and remained n-doped for over a month of storage in air. In addition, we demonstrated that electron injection enhances noncovalent interactions between graphene and metallomacrocycle molecules without requiring a linker, as used in previous studies, suggesting several potential applications of the method in modifying graphene with various functionalities.

Abstracts
Chemical vapor deposition (CVD) of 2D transition metal dichalcogenides (TMDCs) always involves the conversion of vapor precursors to solid products in a vapor-solid-solid (VSS) growth mode (e.g., WO₃ + S → WS₂ + SO₂). This often requires very high temperatures to sublimate metal oxide precursors (e.g., WO₃).
Our pioneering work on salt-assisted CVD (Salt 1.0 technique) enables the growth of 2D WS₂ and WSe₂ monolayers in a mild condition (lower temperature and atmospheric pressure) [1,2]. In the last five years, the use of alkali (alkaline earth) metal halides (AH, A = Li, Na, K, Ba, Ca; H = F, Cl, Br, I) in CVD has demonstrated great success in growing tens of atomically thin metal chalcogenides, graphene & h-BN monolayers [3,4]. This is due to the formation of volatile MO_uCl_v and non-volatile NaM_yO_z when alkali (alkaline earth) metal halides react with metal oxides. They are highly efficient precursors for growing 2D TMDC monolayers.
The recent discovery and use of non-volatile molten salts in CVD (Salt 2.0 technique) trigger the vapor-liquid-solid (VLS) growth of 1D/2D TMDC monolayers [5]. The Salt 2.0 technique shows great improvements in the high-efficient and reproducible growth of large-area, uniform, and high-quality 2D TMDC monolayers. The Salt 2.0 technique also demonstrates great potentials in growing 2D TMDC materials in the following aspects: wafer-scale single crystals, patterns, heterostructure, and alloys [6]. It represents a new trend in the CVD growth of 2D TMDC materials.

References
[1] S. Li, S. Wang, D.-M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg and G. Eda, Appl. Mater. Today 1 60-66 (2015).
[2] D. Bradley, Mater. Today 18 533 (2015).
[3] J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.-H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B. I. Yakobson, Q. Liu, K. Suenaga, G. Liu and Z. Liu, Nature 556 355 (2018).
[4] C. Liu, X. Xu, L. Qiu, M. Wu, R. Qiao, L. Wang, J. Wang, J. Niu, J. Liang, X. Zhou, Z. Zhang, M. Peng, P. Gao, W. Wang, X. Bai, D. Ma, Y. Jiang, X. Wu, D. Yu, E. Wang, J. Xiong, F. Ding and K. Liu Nat. Chem. 11, 730 (2019).
[5] S. Li, Y.-C. Lin, W. Zhao, J. Wu, Z. Wang, Z. Hu, Y. Shen, D.-M. Tang, J. Wang, Q. Zhang, H. Zhu, L. Chu, W. Zhao, C. Liu, Z. Sun, T. Taniguchi, M. Osada, W. Chen, Q. Xu, A. T. S. Wee, K. Suenaga, F. Ding and G. Eda, Nat. Mater. 17 535 (2018).
[6] S. Li, Y.-C. Lin, X.-Y. Liu, Z. Hu, J. Wu, H. Nakajima, S. Liu, T. Okazaki, W. Chen, T. Minari, Y. Sakuma, K. Tsukagoshi, K. Suenaga, T. Taniguchi and M. Osada, Nanoscale 11 16122 (2019).

Recently, due to the physical limitations of silicon-based transistors, studies using 2-dimensional (2D) materials as next-generation transistors have become more active. Graphene, widely known as a representative next-generation 2D material, has attracted a great deal of attention due to its high electrical conductivity, thermal conductivity and stiffness coefficient. However, since it does have zero-bandgap, it is limited to use as a semiconductor. Molybdenum disulfide (MoS₂), one of the TMD materials consisting of chalcogenide elements on both sides of the transition metal, has been reported to have n-type property, a direct band gap of 1.9 eV in a single layer and high absorptivity. Synthesis of MoS₂ films with a different carrier type is a very desirable technology because it can overcome the scaling issues in the further CMOS technology. However, most doping techniques of MoS₂ are difficult to satisfy well-controlled the number of layers, and large-scaled uniformity at the same time. Herein, we propose that an outstanding method with two steps enables us to obtain 2-inch wafer sized MoS₂ thin film with controlled the number of layers and even metal-doped MoS₂ with different carrier type in significantly short time. Various experiments have verified the quality of grown MoS₂ including thickness-dependent electrical behavior, and especially an innovative TEM analysis has confirmed an existence of dopants regardless of an atomic size.

Solution synthesis may prove to be one of the most scalable synthesis methods of 2D materials providing that the thermodynamic and kinetic drivers that determine the phase, nucleation, and growth kinetics, and the degree of perfection, can be understood and controlled. A combination of in situ characterization and simulations of crystallization pathways demonstrated the dependance of specific pathways on local chemical environments. Using well-defined model systems these studies identified the factors for particle morphology control during homogeneous nucleation and growth and highlighted the importance of interfacial precursor kinetics and surface stress in inducing asymmetric growth. In particular, fast ion deposition kinetics at twin planes was shown to promote the growth of nanoplates in pure precursor solution. In ligand assisted homogeneous and heterogeneous growth ion- and ligand-induced interfacial strain relief was found to control the growth of asymmetric nanoparticles and single-layer metal organic framework films, respectively. The detailed analysis of the dependence of crystallization pathways on synthesis parameters (pH, precursor and ligand concentrations, temperature) and the systematic approaches to control synthesis of 2D materials using homogeneous and heterogeneous solution-based synthesis will be discussed.

Strain plays a key role in defining both the electronic and chemical behavior or materials. The properties of two-dimensional (2D) materials are particularly susceptible to strain and the controllable creation and analysis of strain in such systems is critically important to understand their behavior. In recent years, moiré superlattices that are formed by a small lattice mismatch or the azimuthal misorientation (interlayer twist) of two 2D layers have emerged as some of the most fascinating platforms to probe a host of emergent phenomena. Twisted bilayer graphene, with one sheet twisted relative to another, results in a moiré superlattice with a wavelength that is inversely proportional to the twist angle. Because of the variation in stacking of AA vs AB/BA sites in the two graphene layers, the individual sheets strain from the graphene hexagonal lattice in order to minimize free energy. This talk will discuss our work to precisely map this atomic reconstruction and measure the resultant strain in twisted bilayer graphene using a new methodology called Bragg interferometry based on 4-dimensional scanning transmission electron microscopy (4D-STEM). Intralayer strain and reconstrcution mechanics are determined from mapping the displacement vectors between the graphene lattices by acquiring diffraction patterns at each position of the scanning electron probe. By quantitatively mapping strain tensor fields we uncover two distinct regimes of structural relaxation—in contrast to previous models depicting a single continuous process—and we disentangle the electronic contributions of the rotation modes that comprise this relaxation. Further, we find that applied heterostrain accumulates anisotropically in saddle point regions to generate distinctive striped shear strain phases. Our results thus establish the reconstruction mechanics underpinning the twist angle dependent electronic behaviour of twisted bilayer graphene, and provide a new framework for directly visualizing structural relaxation, disorder, and strain in any moiré material.

It is known that the electronic quality of monolayer transition metal dichalcogenides (TMDs) is strongly compromised by the presence of chalcogen vancancy defects. These defects are commonly present in densities of > 10¹³ cm^-2 and result in unintentional n-type doping and sub-gap optical emission features. In this study, we report suppression of sulfur vacancy formation during chemical vapor deposition of monolayer WS₂ by means of intentional in-situ Re-doping. Using aberration-corrected scanning transmission electron microscopy (STEM), we reveal that the sulfur vacancy density consistently decreases by up to 52% with increasing Re content. Interestingly, despite the possible electron-doping behavior of substitutional Re, we find the Re-doped WS₂ to be less n-type, indicating the suppresion of unintentional doping effects of sulfur vacancies and high activation energy of Re impurities. We further show that substrate-induced exciton line broadening and sub-gap emission features are strongly suppressed in Re-doped samples, demonstrating their high electronic quality.

Chemical and biological sensing using 2-D nanomaterials has been an area of intense investigation including inorganic materials that exhibit high sensitivity, however it has been limited as a field due to the lack of selectivity among analytes. While 2D inorganic materials can readily decipher between donor/acceptor groups, strategies to incorporate selectivity in these devices is crucial. Organic 2D materials known as covalent organic frameworks are large porous and repeated organic frameworks which have been shown to act as selectivity agents due to functionalization and porosity. This research focuses on the association of COFs with 2-D nanomaterials using top-down microwave-assisted activation of TMDs and deposition of COF particles. Several known COF species have been examined in order to understand the fundamental surface interactions between 2-D nanomaterials and COFs. Kinetics and mechanistic studies were used to determine the manner of association of the inorganic and organic substrates.

Electronic properties of 2-dimensional (2D) van der Waals materials can be modified by adsorbing and intercalating metal atoms. Controlled aggregation of isolated atoms into clusters and other low-dimensional features can be used to generate new functional properties with potential applications in electronic devices and energy storage and conversion. Our ability to utilize these possibilities are predicated on the development of new synthesis strategies that, in turn, require detailed understanding of diffusion and aggregation pathways of adsorbed species on surfaces of these materials.
Here we report on binding, diffusion, and aggregation of transition metal atoms (Pd, Cu, Fe) adsorbed on a prototype 2D transition metal dichalcogenide (TMD) WTe₂ investigated using ab initio (density functional theory) simulations. While we find four distinct surface binding sites, the diffusion activation barriers between them differ dramatically, creating an asymmetry in surface migration. Metal dimers are only marginally more stable (~0.1 eV) than isolated adsorbed atoms; however, clusters of three atoms are stabilized by as much as ~0.4 eV. Further increase of the cluster size up to 11 atoms is not significantly favored on the non-defective surface, which suggests that such clusters can decompose under mild thermal treatment. In contrast, adsorption of metal atoms at the Te vacancy sites is strongly favored. This is particularly evident in the case of Pd, which is slightly more electronegative than Te. As the size of the vacancy bound Pd cluster increases, the amount of the electron charge transferred to the cluster from the surface Te increases as well. This charge redistribution is accompanied by Pd atoms displacing Te atoms from their lattice sites leading to significant lattice damage. Finally, we investigated pathways for Te substitution with the adsorbed metal species on pristine surfaces. Our results suggest that formation of transient Pd dimers and trimers can trigger Te-to-Pd charge transfer sufficient to destabilize surface Te atoms and induce Pd for Te substitution. These results are consistent with the formation of two types of small Pd clusters on WTe₂ observed using scanning tunneling microscopy and their responses to thermal treatment. We discuss the similarities and the differences in behavior of the adsorbed metal species and their effect on the electronic properties of WTe₂ as well as implications for functionalization of other TMDs.

Molybdenum disulphide (MoS₂) is a transition metal dichalcogenide (TMD) semiconductor that has unique optical and electronic properties making it attractive for future electronic and photonic devices [1]. Due to the quantum confinement effect in 2D MoS₂, it has a tuneable band gap which increases as the size decreases. Bulk MoS₂ has an indirect band gap of ~1.2eV, while 2D MoS₂ has a direct band gap of ~1.8eV. In this work, we present MoS₂ nanoparticles (NPs) synthesized using a chemical exfoliation method and investigate the optical properties . First, we disperse 0.5 grams of MoS₂ powder in 50 mL of N-Methyl-2-pyrrolidone (NMP). Next, sonication using a probe set at 50% amplitude, and 10/2 seconds duty cycle for 6 hours in an ice bath maintained at 0 °C. The dispersion is then centrifuged at 1500 rpm for 60 min to remove unexfoliated particles, then at higher speed (7500 rpm) for 30 min to remove soluble impurities from the dispersion. The NMP is then removed, and the MoS₂ nanoparticles are filtered and re-dispersed in 25 mL of IPA. In order to first characterize the chemically exfoliated MoS₂ (ce-MoS₂) nanoparticles, we first deposit them on 3 cm x 3 cm pieces of fused silica by spin coating. Three coats of 100 µL were deposited with a total of 300 µL. The samples were left to dry for 1 hour between each coat at room temperature. The spin coating was conducted at 150 rpm for 40s. Optical microscopy of the samples under ultra-violet (UV) illumination showed a homogenous distribution of the MoS₂ NPs, exhibiting a red photoluminescence (PL) which is correlated to the quantum-confinement effect in MoS₂ at the nanoscale where direct band gap transitions can occur with a stable tunable PL depending on the NPs size. The films showed a smooth layer at low particle densities with cluster sizes below the resolution of the microscope (< 300 nm). At higher particle densities, larger clusters begin to form, with an approximate size of 1 μm and above, with a majority of clusters in the range of 2-5 μm in diameter.
Both reflection and transmission spectra are measured after each coat using a UV–VIS spectrophotometer (Lambda 850, PerkinElmer) to investigate the behaviour of MoS₂ NPs, and the effect of the number of coats applied on the reflectance of the substrate. The corresponding reflection spectra were captured as a function of the incident wavelength, ranging from 250 nm to 1500 nm. Coated samples show a severe reduction in transmission to almost ~0% at UV-Vis (250 to 850 nm) and ~ 10% at NIR (850 to 1500 nm) after three coats. However, reflectance was around ~20% for UV-Vis and increased to ~50% at NIR. Which got absorption to increase from ~0% to ~80% at UV-Vis and ~40% at NIR after the third coat. We also deposited the MoS₂ on low temperature PECVD Ge films grown on (100) P doped (n-type) Si described in detail in our previous work [2]. A large drop in reflectance below 800 nm is seen which can be attributed to absorption in the MoS₂ nanoparticles. Above 800 nm, reflectance increases. This asymmetrical behaviour, which will be investigated further, can be explained by increased scattering in the non-absorption region. To summarize, in this work, we investigate the optical properties of chemically exfoliated MoS₂ NPs on fused silica and PECVD Ge films. The results show a large reduction in reflectance due to absorption in the MoS₂ in the UV-Vis region. Moreover, these results show that MoS₂ can have potential applications in photonics and PV-based devices.

[1] Singh, E., Singh, P., Kim, K. S., Yeom, G. Y., & Nalwa, H. S. (2019). Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics. ACS Applied Materials & Interfaces, 11(12), 11061-11105.
[2] Ghada H. Dushaq, Mahmoud S. Rasras, Ammar M. Nayfeh, "Low temperature deposition of germanium on silicon using Radio Frequency Plasma Enhanced Chemical Vapor Deposition," Thin Solid Films, page 585-592, 2017

Recent discoveries of gate-tunable magnetism in ferromagnetic two-dimensional (2D) materials such as CrI₃and Fe₃GeTe₂ highlight the unique potential of this class of materials for novel spintronic devices. Understanding the interplay between magnetic order, free electron density, and electric field in the 2D limit is essential for uncovering the full potential of these materials. In this talk, we will first discuss electrical control of magnetism in Cr₂Ge₂Te₆ (CGT), a van der Waals ferromagnetic semiconductor, in an electric double-layer transistor (EDLT) geometry. We show that degenerately electron-doped CGT exhibits enhanced Curie temperature of up to 200 K, and rotation of its easy axis from out-of-plane to in-plane orientation. We demonstrate that similar changes in Curie temperature and easy axis can be induced by Na intercalation, which is accompanied by heavy electron doping of the host. We will further discuss our exploration of magnetism in other van der Waals materials such as NbFeTe₂ and magnetically doped transition metal dichalcogenides.

The emergent magnetic two-dimensional (2D) materials provide ideal solid-state platforms for a broad range of applications including miniaturized spintronics and magnetoelectric sensors. Owing to the general environmental sensitivity of 2D magnets, the understanding of ambient effects on 2D magnetism is critical. Apparently, the nature of itinerant ferromagnetism potentially makes metallic 2D magnets insensitive to environmental disturbance. Nevertheless, our systematic study showed that the Curie temperature of metallic 2D Fe₃GeTe₂ decreases dramatically in the air but thick Fe₃GeTe₂ exhibits self-protection. Remarkably, we found the air exposure effectively promotes the formation of multiple magnetic domains in 2D Fe₃GeTe₂, but not in bulk Fe₃GeTe₂. Our first-principles calculations support the scenario that substrate-induced roughness and tellurium vacancies boost the interaction of 2D Fe₃GeTe₂ with the air. Our elucidation of the thickness-dependent air-catalyzed evolution of Curie temperatures and magnetic domains in 2D magnets provides critical insights for chemically decorating and manipulating 2D magnets.

The unique opto-electronic properties of two-dimensional monolayer of phosphorus (namely phosphorene) makes them lucrative for a variety of applications in nano-scale electronic devices, including field-effect transistors, photovoltaic junctions and thin-film solar cells. In particular, the lure of phosphorene arises from its semiconducting nature, thickness-dependent electronic band gap (1.5 eV in monolayer to 0.3 eV in bulk), high hole mobility (~10⁵ cm²/V/s), and large drain current modulation. The properties of phosphorene can be further engineered by precise introduction of defects via ion bombardment or electron-irradiation. For instance, isolated mono-vacancies can induce hole doping as well as local magnetic moments; which can be leveraged to achieve specific functionality in opto-electronic devices. Despite this promise, precise defect engineering of phosphorene via ion/electron beams has remained challenging due to lack of fundamental understanding of the atomic-scale mechanisms underlying production, accumulation, and subsequent annealing of defects during irradiation. Indeed, previous first-principles modeling and electron microscopy studies provide insights into the atomic structure of defects, their thermodynamic stability, and threshold ion energy required to produce them. However, the effect of dose on defect accumulation, atomistic details of temporal evolution of defects, and defect dynamics under irradiation as well as subsequent annealing are largely unclear. Here, we employed reactive molecular dynamics (MD) simulations to unravel the dynamical atomic-scale processes underlying formation, accumulation, and re-organization/annealing of defects during Ar-ion bombardment of phosphorene. We found that radiation dosage strongly influences the type of defects that form, as well as their subsequent annealing. At low doses (10¹³ Ar⁺ions/cm² at 25 keV), the predominant defects that form are isolated mono-vacancies or Stone-Wales type defects that largely retain the planarity of the sheet. Interestingly, even small voids (1-2 nm) that form at these low doses can be healed by subsequent annealing; such healing proceeds via local re-arrangement of rings. Beyond a critical dosage (10¹⁴ Ar⁺ ions/cm² at 25 keV), large nanopores form whose edges are stabilized by formation of 3D structures consisting of P₄ tetrahedra. During subsequent annealing, these 3D structures facilitate the coalescence of nanopores, which further degrades the structure. These results will be discussed in the context of designing novel routes to precisely tune opto-electronic properties of phosphorene-based devices using ion beams.

Two-dimensional transition metal dichalcogenides show great potential as a new class of atomically thin semiconductors for electronics and optoelectronics. Understanding the atomistic origin of defects in these materials and their impact on the electronic properties, as well as finding viable ways to dope them is matter of intense scientific and technological interest. In particular, controlling defects could be envisioned as a strategy for the design of ad-hoc electronic and optoelectronic properties. Here, we demonstrate a new integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve a nanoscale control of the local thermal activation of defects in monolayer MoS₂. The tc-SPL activated nanopatterns can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-mm spatial control and a rectification ratio over 10⁴. Doping and defects formation mechanisms are elucidated at the molecular level by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. The p-type doping of locally heated MoS₂ in HCl/H₂O atmosphere is found to be related to the rearrangement of sulfur atoms and the formation of new protruding covalent S-S bonds on the surface, which produce a band structure with p-character. Alternatively, local heating MoS₂ in N₂ produces n-character.

Pending

Recently, two-dimensional covalent-organic-frameworks (COFs), an structural analogue of graphene, have attracted great interests because of their tailorable structures lacking in other 2D materials. The topological design principles of monomers enable researchers to control the pore size and the unit cell geometry of 2D COFs allowing for highly tunable functionalities and properties. The unique structural features make 2D COFs possess exciting multifunctional properties with broad applications in liquid/gas separation, water filtration, energy storage/conversion, catalysis and ionic conduction, etc. However, the understanding of its mechanical properties and fracture mechanisms remains elusive. Here we report a quantitative in-situ tensile study of ultrathin COFs films. The fracture strength was measured to be 0.75±0.34 GPa, and the tensile modulus was measured to be 10.38±3.42 GPa, with a nominal density of 0.393 g/cc, thus having specific strength equivalent to Kevlar(2 GPa cc/g), and specific modulus comparable to titanium alloys (23 GPa cc/g). Additionally, the fracture toughness was measured to be 0.55±0.09 MPa√m, and it was found that the crack propagation could be insensitive to the pre-crack when the size of pre-crack is below a critical value, leading to intriguing flaw insensitivity in such ultrathin nanomaterials. This work provides in-depth insights into the fracture properties of 2D COF films and lays a foundation for their future applications.

The absence of surface dangling bonds on two-dimensional (2D) materials mitigates materials compatibility issues of heterostructuring. Conventional materials synthesis on 2D materials have been prepared by van der Waals (vdW) and remote epitaxy techniques. In principle conventional or other 2D materials epitaxially grown on a 2D materials can be feasibly delaminated from the 2D layer as a substrate via applying mechanical force. Previous reports have shown that semiconductor nanomaterials and thin films for device applications can be prepared on 2D materials, especially graphene. Then the grown materials were detached from the host substrate, and applied for flexible devices. However the 2D layer in between the grown materials and the host substrate has not been thoroughly investigated. The 2D layer as a substrate or interlayer for vdW and remote epitaxy techniques has been considered as damaged. Thus there was no systematic characterizations of 2D layer after the epitaxy procedures and no attempt to recycling the 2D layer multiple times.
In the presentation recycling graphene and transition metal dichalcogenides layers for vdW and remote epitaxy will be demonstrated and discussed. Surface morphology and optical characteristics of the recycled 2D layers along repetition of the vdW and remote epitaxy were studied. Moreover the electrical/optical characterizations of semiconductors/2D heterostructures were performed to obtain insights of 2D/conventional materials interfacial properties.

Monolayer molybdenum disulfide (MoS₂) is an attractive semiconductor for nano-optoelectronic devices due to its direct bandgap, open-air stability, and potential for large-scale synthesis. To optimize the electrical performance of these devices for applications such as photovoltaics, the relationship between the semiconductor and metal contacts must be fully understood and optimized.

The Schottky barrier height, denoted as Φ_B, is a potential energy barrier that exists at the junction of a metal and a semiconductor due to the difference in work functions. This directly impacts device performance, as a higher barrier decreases minority carrier transport since electrons cannot easily flow back into the semiconductor. We report here the experimental measurements of Φ_B between monolayer MoS₂ and a number of metals with varying work functions and process conditions. Large-area monolayer MoS₂ is grown using chemical vapor deposition (CVD) and metal contacts are deposited using electron beam evaporation on patterns written by electron beam lithography.

Different metals have different electrical performances and Schottky barriers resulting from their interaction with a semiconductor. The metals used for contacts are Ti and Sc (low work functions) and Pt (high work function). Here, the barrier height is extracted using the thermionic emission model, which implies that the Schottky barrier is temperature dependent. With an applied source-drain bias of 1 V, a source-drain sweep is taken for the nanoscale transistors over the gate voltage range -30 V to 30 V. These sweeps are repeated at 25 K temperature increments over 100-400 K. This data is plotted on an Arrhenius plot, with logarithmic gate voltages on the y-axis and the inverse of temperature on the x-axis. The initial linear slopes are related to the barrier height using the thermionic emission model, which results in an approximate barrier vs gate voltage plot from which the actual Schottky barrier height is extracted.^1,2 So far, we have experimentally determined the Schottky barrier height of titanium, Φ_Ti = 0.21. We will be repeating this process with the other metal contacts listed above to fully document how our CVD grown 2D MoS₂ interacts with these metals.

Thermal treatments have been used for decades to improve electrical performance in traditional III-V solar cells and semiconductor devices. Building on this, we posit that there are optimal annealing parameters that can improve the desired transport properties in electron and hole-selective Schottky barriers between 2D materials and metals. This annealing is done using a Rapid Thermal Annealer (RTA) to expose the device to high temperatures for short periods of time. Since RTA is scalable, it directly translates to manufacturing for large area 2D PV. By comparing the Schottky barrier heights without annealing and after various annealing conditions, we will establish a better understanding of how this process impacts the metal-semiconductor junction. The ability to tune the barrier height allows devices to be further customized with improved performance.

Understanding the barrier of the MoS₂-metal junction will allow for more complex devices to be designed and fabricated, including but not limited to photovoltaics, photo-emitters, photodetectors, etc. Future work entails optimizing device preparation and treatment to result in Schottky barriers for different 2D electronic applications. These measured barrier heights are also being included in a computational device model to study various MoS₂-based optoelectronic devices.


¹ Das, S., Chen, H., Penumatcha, A. V., & Appenzeller, J. (2012). High Performance Multilayer MoS₂ Transistors with Scandium Contacts. Nano Letters, 13(13), 100-105
² Wang, W., Liu, Y., Tang, L., Jin, Y., Zhao, T., & Xiu, F. (2014). Controllable Schottky Barriers between MoS₂ and Permalloy. Scientific Reports, Sci Rep 4, 6928 (2014)

Metallic transition-metal dichalcogenides (TMD) are rich material systems in which the interplay between strong electron-electron and electron-phonon interactions results in a variety of instabilities such as charge density waves (CDWs). While most metallic TMDs exhibit coexistence of superconductivity and CDWs, 2H-NbS₂, with a superconducting transition temperature (T_C) of ~6K, shows no CDW order. Most recently, the existence of an incommensurate CDW pinned by atomic impurities (such as vacancies) has been reported in 2H-NbS₂. Substitutional dopants, depending on their atomic number and electronic state, can considerably affect electronic behavior in these systems.
Here, we substitutionally dope 2H-NbS₂ with heavy atoms via chemical vapor transport. Using aberration-corrected scanning transmission electron microscopy (STEM) imaging, we confirm the 2H phase of NbS₂ and determine the distribution and concentration of dopant atoms. Using a combination of low temperature transport measurements and density functional theory calculations, we demonstrate how heavy dopant atoms can modify the collective electronic states (such as superconductivity and CDW) in 2H-NbS₂.

The near-infrared (NIR) region of the electromagnetic spectrum presents optimal characteristics for imaging of complex (biological) samples due to reduced light scattering, absorption, phototoxicity and autofluorescence. However, despite their clear potential over commonly employed visible fluorophores, only few NIR fluorescent materials are known so far, and even less are suitable for biomedical applications. For this reason, it is of great interest to identify novel NIR fluorophores and use them for biomedical applications.
Here, we exfoliated a class of layered silicates: Egyptian Blue (CaCuSi₄O₁₀, EB), Han Blue (BaCuSi₄O₁₀, HB) and Han Purple (BaCuSi₂O₆, HP). These pigments fluoresce in the NIR (λ_emi ≈ 920-950 nm) with a long excited state lifetime (τ ≈ 10-100 µs). By milling, tip sonication and several centrifugation steps, small and monodisperse 2D nanosheets (NS) could be exfoliated. The morphology of such NS was then fully characterized by atomic force (AFM) and scanning electron microscopy (SEM). Most interestingly, the intense NIR fluorescence emission of the bulk counterparts is retained in these nanostructures. In comparison to state-of-the-art fluorophores, EB-NS show no bleaching while displaying outstanding fluorescence intensity. These qualities of EB-NS enabled us to inject them into systems of biological relevance such as developing Drosophila embryos, and perform in vivo single-particle tracking and microrheology measurements. Furthermore, because of their high biocompatibility, it is possible to use them for NIR imaging in plants. [1] The above mentioned three silicate NS are very bright and can be imaged through several cm of tissue phantoms, which demonstrates the potential for (bio)photonics. Finally, we used the microsecond fluorescence lifetimes of EB-NS, HB-NS and HP-NS for micro- and macroscopic fluorescence lifetime imaging (FLIM). The results show that lifetime engineering of these silicate nanostructures is possible and can be used for lifetime-encoded imaging. [2]
In summary, we present a new exfoliation route that yields NIR fluorescent nanosheets with high potential for bioimaging and photonics.

Literature:
[1] G. Selvaggio et al., Nat. Commun. 11, 1495 (2020)
[2] G. Selvaggio et al., https://doi.org/10.26434/chemrxiv.13350728.v1 (2021)

The variegated family of two-dimensional materials comprises crystals that have attracted great interest for diverse characteristics and peculiarities. Among them, semiconducting transition-metal dichalcogenides (TMDs) possess alluring optoelectronic and spin properties when reduced to the single layer. In particular, TMD monolayers are characterised by a direct bandgap, resulting in an efficient light emission in the visible/infrared range, which renders them appealing for optoelectronic devices. Furthermore, a strong spin-orbit coupling makes them interesting candidates for valley- and spin-tronics. Indeed, the inherent plane-confined nature of these materials -coupled to their exceptional mechanical flexibility and robustness- makes them highly sensitive to external stimuli. Methods to tailor their unique properties on demand have been thus sought after, and protocols based on controllable external perturbations such as mechanical deformations have shown promise in this respect.

Here, we present a novel technique to induce controllable strain-fields in TMD monolayers and study their effect on the optical and spin properties. Localised strains are created via low-energy hydrogen-ion irradiation of bulk TMDs, leading to the production and accumulation of molecular hydrogen in the first interlayer region. The trapped gas coalesces, leading to a local blistering of the material, and thus to the formation of one-monolayer-thick micro/nano-bubbles, which stud the crystal surface and locally turn the dark bulk material into an efficient light emitter. [1] Electron-beam-lithography-based approaches allowed us to achieve control over the formation process of the bubbles, and to create them in ordered arrays and with the desired dimensions (from few tens of nm to few microns). [1-2] These bubbles are durable and incredibly robust [3] and host complex strain fields, evaluated numerically, that cause dramatic changes in the TMD optoelectronic properties. Photoluminescence steady-state and time-resolved studies enabled the characterisation of the strain-induced band-structure modifications and revealed intriguing phenomena, such as bandgap crossovers enabling the creation of exciton states with long lifetimes. [4] Magneto-optical experiments allowed us to achieve unprecedented information on the spin and valley properties of strained TMDs, and led to the observation of hybridisation mechanisms between different band states.

Recently, the interest for TMD monolayers has surged due to the possibility to stack them via weak van der Waals forces to create heterostructures. TMDs have also been coupled with other semiconducting two-dimensional materials, such as post-transition-metal chalcogenides. Novel excitonic states -referred to as interlayer excitons- have been observed in TMD-based heterostructures, in which the two oppositely charged carriers forming the exciton are localised in different monolayers. Here, we exploit the same approach used for the formation of single monolayer bubbles to strain-tune the electronic properties of novel van der Waals heterostructures, and show how strain can play a crucial role by changing the relative band alignment of the stacked materials.

Our results unveil unprecedented information on the strain effects on TMDs and their heterostructures, the understanding of which represents an essential step towards their integration into flexible electronic devices.

[1] D. Tedeschi, E. Blundo, et al., Adv. Mater. 31, 1903795 (2019).
[2] E. Blundo et al., Adv. Mater. Interfaces 7, 2000621 (2020).
[3] C. Di Giorgio et al., Adv. Mater. Interfaces 7, 2001024 (2020).
[4] E. Blundo et al., Phys. Rev. Res. 2, 012024 (2020).

Nickel ferrite nanoparticles were subjected to mechanochemical activation for ball milling times ranging from 0 to 12 hours. The milling was performed with and without the addition of equimolar concentrations of graphene nanoparticles. Characterization of resulting nano-powders was undertaken by Mossbauer spectroscopy and magnetic measurements. The hyperfine magnetic field was studied as function of milling time for octahedral and tetrahedral sites. An additional quadrupole split doublet represented the occurrence of superparamagnetic particles in the as-obtained and milled specimens. A new phase was obtained in the graphene-milled set of samples, which could be assigned to carbon-rich particles. The degree of inversion and canting angle were derived from the Mossbauer measurements and studied as function of ball milling time. The degree of inversion was found to decrease with milling time, especially for the set without graphene and evidenced a transition from inverse to normal spinels. The canting angle decreased with time for the graphene milled nanoparticles. The recoilless fraction was determined as function of milling time and was consistent with the observation – for the first time in literature – of a distribution of recoilless fractions in the studied specimens. The saturation magnetization, remanence magnetization and coercive field were derived from the hysteresis loops, recorded at 5K and 5T. The zero-field-cooling-field-cooling measurements were obtained in a magnetic field of 200 Oe and the blocking temperature was determined. Our results show new features of the behavior of nickel ferrite nanoparticles under mechanochemical activation with and without graphene.
Cobalt ferrite nanoparticles were exposed to mechanochemical activation, with and without equimolar amounts of graphene nanoparticles, for time periods ranging from 0 to 12 hours. Their structural and magnetic properties were detailed from Mossbauer spectroscopy and magnetic measurements. The Mossbauer spectrum corresponding to the unmilled cobalt ferrite powder was analyzed using 2 sextets, corresponding to the tetrahedral and octahedral sites of ferrites. The rest of the spectra was deconvoluted using an additional quadrupole-split doublet, with an abundance close to 30% and was assigned to superparamagnetic particles. Moreover, the spectra corresponding to milling with graphene at the longest times needed a third sextet, which could be assigned to iron carbide. The degree of inversion was determined from the Mossbauer spectra and found to decrease with milling time, both for the set with and that without graphene. The canting angle was derived and studied as function of the ball milling time for both sets of samples. Hysteresis loops were recorded at 5 K in an applied magnetic field of 5 T and was found to exhibit a constricted shape. Magnetization was plotted as function of temperature in the range 5-300 K with an applied magnetic field of 200 Oe using zero-field-cooling-field-cooling (ZFC-FC) measurements. These made it possible to determine the blocking temperature of the samples. Our data exhibit new characteristics of the cobalt ferrite nanopowders milled with and without graphene nanoparticles.

In recent years, monolayer transition metal dichalcogenides (TMD) have garnered widespread interest from the research community across the world due to their exciting optical properties and strong light-matter interaction. As the optical properties of these materials are extremely sensitive to external mechanical stimuli, controlled static strain has been successfully applied to tune the light-matter interaction in these materials [1]. However, the interaction of excitons with the travelling strain and piezoelectric field generated by surface acoustic wave remains widely unexplored. Here, we experimentally investigate exciton transport in monolayer WSe₂ under surface acoustic wave.
We generate high-frequency Rayleigh type surface acoustic wave (SAW) by patterning interdigitated electrodes (IDTs) on 128⁰ Y-cut piezoelectric lithium niobate (LiNbO₃) substrate using standard photolithography methods. The SAW resonators are designed for a resonance frequency of 600 MHz (SAW wavelength ~ 6 μm). We characterize the transport of excitons in hexagonal boron nitride (hBN) encapsulated WSe₂ monolayer using phase locked time correlated single photon counting (TCSPC). Upon RF excitation the travelling hydrostatic strain wave of the SAW modulates the conduction and valance band of the monolayer WSe₂ in opposite phase resulting in localized potential pockets, commonly referred as type – II bandgap modulation [2]. The resulting potential pockets can capture and result in long-range transport of excitonic species.
We observed strong coupling of monolayer exciton species to the travelling strain field generated by the surface acoustic wave that resulted in remote recombination of the transported excitons. Using measured exciton lifetime of 1.27 ns, we estimate the exciton velocity under acoustic modulation to be 0.78x10³ ms^-1. Using experimentally reported value of strain mobility in monolayer WSe₂ [3], we estimate the hydrostatic strain on the monolayer to be 0.4%. Our observations show that the transport distance is mostly limited by the intrinsic exciton mobility and lifetime in monolayer WSe₂.
References:
[1] C. Martella, C. Mennucci, A. Lamperti, E. Cappelluti, F. B. de Mongeot, and A. Molle, Adv. Mater. (2018).
[2] J. Rudolph, R. Hey, and P. V. Santos, Phys. Rev. Lett. 99, 1 (2007).
[3] D. F. Cordovilla Leon, Z. Li, S. W. Jang, C. H. Cheng, and P. B. Deotare, Appl. Phys. Lett. (2018).

Binary layered semiconductors, especially transition metal dichalcogenides (TMDs), are among the most studied van der Waals (vdW) semiconductors. Recently ternary layered materials are drawing more attention due to their attractive properties.[1]

Amid ternary vdW semiconductors, iron phosphorus trisulfide - FePS₃ - is an interesting material. It belongs to a family of transition metal phosphorus trisulfides (TMTs) with general formula MPS₃. FePS₃ is magnetic semiconductor, absorbing in near IR, and it is also relatively stable in exfoliated form. Large FePS₃ crystals (almost centimeter size) have been obtained in big amounts (more than 70% chemical yield) via chemical vapor transport (CVT). Bulk material is investigated by X-Ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Optical properties are carefully studied with temperature change. The results are further corroborated with DFT calculations. FePS₃ is both mechanically and liquid exfoliated and obtained products are compared by X-Ray diffraction (XRD) and then by conventional transmission electron microscopy (cTEM), carefully proceeded for chosen crystallographic directions with high-resolution analysis for both kinds of samples.[2] Here, a simple, one-step protocol for mechanical exfoliation directly onto transmission electron microscope grid is used as an efficient sample preparation method.[1,2,3] Finally, atomic resolution elemental maps are registered with high-resolution scanning transmission electron microscope (HR-STEM) and reveal directly the atom columns arrangement.[2]

At last, the large-scale project will be briefly described, where vdW materials are used for the first time for tunable X-Ray emission. The X-Ray radiation is produced from two combined mechanisms [called parametric coherent bremsstrahlung (PCB) collectively]: parametric X-ray radiation (PXR) and coherent bremsstrahlung (CBS). FePS₃ with other ternary vdW materials CrPS₄, MnPS₃, CoPS₃, NiPS₃, and also binary WSe₂, are mechanically exfoliated and transferred to electron microscopy grids. Next, they are irradiated by an electron beam inside a transmission electron microscope. The tunability of energy of emitted X-Ray is achieved both by adjusting electrons acceleration voltage or using different material as the emitter.[3]

References:
[1] A.K. Budniak, N.A. Killilea, S.J. Zelewski, M. Sytnyk, Y. Kauffmann, Y. Amouyal, R. Kudrawiec, W. Heiss, E. Lifshitz, “Exfoliated CrPS₄ with promising photoconductivity”, Small, 2020, 16 (1), 1905924
[2] A.K. Budniak, S.J. Zelewski, M. Birowska, T. Wozniak, T. Bendikov, Y. Kauffmann, Y. Amouyal, R. Kudrawiec, E. Lifshitz, “Spectroscopic and structural investigation of bulk and exfoliated iron phosphorus trisulfide - FePS₃”, manuscript in preparation
[3] M. Shentcis, A.K. Budniak, X. Shi, R. Dahan, Y. Kurman, M. Kalina, H. Herzig Sheinfux, M. Blei, M. Kamper Svendsen, Y. Amouyal, S. Tongay, K. Sommer Thygesen, F.H.L. Koppens, E. Lifshitz, F. J. García de Abajo, L.J. Wong, I. Kaminer, “Tunable free-electron X-ray radiation from van der Waals materials”, Nature Photonics, 2020, 14 (11), 686-692

Heterostructures built from atomically thin 2D crystals, and bound by the van der Waals force, are well known to exhibit attractive optoelectronic properties and exotic physics. For instance, the strong Coulomb interactions can create bind charges across an interface, creating interlayer excitons with longer lifetimes. However the quantum confinement of electrons in quasi-1D structures creates distinctly different possibilities than available in 2D, as can be seen by contrasting graphene (a semi-metal in 2D with no excitonic absorption) and carbon nanotubes (semiconducting for certain chiralities, and featuring strong excitonic absorption).

Here we report the structure, composition and optoelectronic properties of a 1D van der Waals heterostructure consisting of nano-scale coaxial cables [1,2]. Carbon nanotubes (CNTs) formed the “core” of the cable, and were wrapped by atomically-thin nanotubes of the insulator boron nitride, and then by molybdenum disulfide (MoS₂). The high quality of the composite was directly evident on the atomic scale from high-resolution transmission electron microscopy, and on the macroscopic scale by a study of its equilibrium and ultrafast optoelectronics [2].

We used THz spectroscopy to establish the good conductance of the CNT cores within the coaxial nano-cable, which can be therefore be regarded as a radial metal/insulator/semiconductor heterojunction. Via optical pump, THz probe spectroscopy we investigated the electron mobility of the MoS₂ nanotubes, finding that it was comparable to that of high-quality atomically-thin 2D crystals of MoS₂. The high mobility of the MoS₂ nanotubes highlights the huge potential of the coaxial “hetero-nanotube” growth platform for nanoscale optoelectronic devices [3].

Finally, we addressed the intriguing question: can the optical properties of such coaxial nano-cables be described by the average properties of each material in isolation, or do interactions between the materials lead to unique new properties? To investigate, we used multi-colour infrared pump, visible probe spectroscopy to selectively create excitons in the carbon nanotubes, while probing the response of the A and B excitons in the MoS₂ nanotube sheath. We observed a rapid and strong excitonic response of the MoS₂ to the presence of excitons in the carbon nanotube core. This observation is surprising as in the single-photon, single particle picture a photon from the infrared pump pulse (0.6eV) does not give an electron sufficient energy to cross the MoS₂ excitonic gap (1.9eV). Further, charge transfer processes are prohibited by the intermediate BN insulating layer. However, our experimental findings can be understood by considering the strong Coulomb correlations between the carbon nanotube core and the MoS₂ sheath, which created the intertube excitonic and biexcitonic absorption features observed in the transient spectra. This first observation of intertube excitons, analogous to the interlayer excitons of 2D heterostructures, further underscores the potential of 1D van der Waals materials for basic science and future applications.


[1] Xiang et al., Science (2020) 367, 537-542, DOI: 10.1126/science.aaz2570
[2] Burdanova et al., Nano Letters (2020) 20, 3560-3567, DOI: 10.1021/acs.nanolett.0c00504
[3] Xiang and Maruyama (2021), Heteronanotubes: Challenges and Opportunities. Small Sci. 2000039, DOI: 10.1002/smsc.202000039

When two superconductors are connected through a ferromagnet, spin configuration of the transferred Cooper pairs can be modulated due to the exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction, depending on the thickness of the ferromagnetic weak link. In this talk, we present Josephson phase engineering in van der Waals heterostructures of atomically thin magnetic insulator Cr2Ge2Te6 sandwiched between NbSe2 van der Waals superconductors. Employing a superconducting quantum interference device based on our magnetic insulator Josephson Junctions, we reveal a doubly degenerate nontrivial Josephson phase originating from the magnetic barrier. We find that these unusual magnetic Josephson junctions are formed by momentum conserving tunneling of Ising Cooper pairs of NbSe2 across the magnetic domains of atomically thin Cr2Ge2Te6. The doubly degenerate ground states in magnetic insulator Josephson junction provide a quantum two-level system which can be utilized as a new component for superconducting quantum devices

Ferromagnetism arises from the interplay of the Coulomb repulsion between electrons and their fermionic statistics. The vast majority of magnets consist of ordered arrangements of the electron spins stabilized by the spin orbit interaction. In my talk, I will describe a new class of magnets based on the spontaneous alignment of electron orbitals. Such orbital ferromagnetism may be a generic phenomena, but has, to date, found its fullest expression in graphene heterostructures in which the two dimensional orbits of electrons in distinct momentum space valleys provide the underlying degree of freedom for magnetic order. These magnetic degrees of freedom arise directly from the band wavefunctions, making orbital magnets exquisitely sensitive to both the design of the electronic wavefunctions as well as in situ control parameters. For example, in systems in which interlayer lattice mismatches lead to a moire superlattice potential, the resulting superlattice band structure may feature nontrivial topology, which in conjunction with orbital magnetism can give rise to precise quantization off the Hall effect at zero magnet field. Remarkably, the conduction electrons themselves can also directly influence the magnetic order, leading to field-effect switchable magnetic moments. Finally, I will conclude with our progress towards using orbital magnetism to engineer "topologically ordered" states at zero magnetic field, in which the electron splits into fractionally charged anyons.

In the family of two-dimensional (2D) transition metal dichalcogenide (TMD) materials, molybdenum disulfide (MoS₂) has received immerse attention owing to its desirable electrical, chemical, and mechanical properties. Similar to single-crystalline MoS₂ nanostructures, polycrystalline MoS₂ structures also possess many desirable properties for applications, such as outstanding catalytic properties. Recently we reported an innovative and scalable approach to synthesize MoS₂ nanoribbons with tunable dimensions and excellent dispersibility in suspension [1]. The obtained polycrystalline MoS₂ nanoribbons exhibit high chemical purity which endows them with much-enhanced surface reactivity compared to those of their single-crystal counterparts. They can be readily grafted with UV-triggered click-chemistry ; they effectively react and remove mercury contaminates from water. They also possess an ultrafast optoelectronic response from 450 nm to 750 nm that is the same as the single-crystal forms. In this presentation, we highlight these properties and describe in detail our newly obtained unpublished result on the electronic properties of the MoS₂ nanostructures and how these determine their manipulation and electro-rotation behavior. We show that the nanoribbons can be manipulated efficiently in a high-frequency electric field. They propel along arbitrary paths on a 2D surface, assemble rapidly on prepatterned microelectrodes, and rotate clockwise and counter-clockwise. Their mechanical behaviors reflect their semiconductor electronic type, distinct from those of metals or insulators, which provides additional support to their optoelectronic characterizations. The fabrication, manipulation, assembly, and unique properties of the obtained MoS₂ is useful for device fabrication, and applications of the TMD family.

[1] Huang, Y., Yu, K., Li, H., Liang, Z., Walker, D., Ferreira, P., Fischer, P., Fan, D., "Scalable Fabrication of Molybdenum Disulfide Nanostructures and their Assembly", Adv. Mater., 2003439, 2020.

Monolayer transition metal dichalcogenides (TMDs) are two-dimensional materials with exceptional optical properties such as high oscillator strength, valley related excitonic physics, efficient photoluminescence, and several narrow excitonic resonances. However, above effects have been so far explored only for structures produced by techniques involving mechanical exfoliation and for the best results, encapsulation in hBN, both proceedures inevitably inducing considerable large-scale inhomogeneity. On the other hand, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of interesting optical effects.

We report for the first time on the MBE-grown TMD exhibiting narrow and fully resolved spectral lines of neutral and charged exciton. Moreover, our MBE-grown TMD exhibits unprecedented high spatial homogeneity of optical properties, with variation of the exciton energy as small as 0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth [1,2] is presented for MoSe2 and includes extremely slow growth rate, the use of atomically flat hexagonal boron nitride (hBN) substrate and the annealing at very high temperature. Importantly, good optical properties are achieved for as-grown sample, without any post growth exfoliation and encapsulation in hBN. This novel recipe opens a possibility of MBE growth of TMD and their heterostructures with optical quality, dimensions and homogeneity required for optoelectronic applications.

[1] W. Pacuski, M. Grzeszczyk, K. Nogajewski, A. Bogucki, K. Oreszczuk, J. Kucharek, K.E. Polczynska, B. Seredynski, A. Rodek, R. Bozek, T. Taniguchi, K. Watanabe, S. Kret, J. Sadowski, T. Kazimierczuk, M. Potemski, P. Kossacki; Nano Letters 20, (2020) pp. 3058-3066.
[2] Z. Ogorzalek, B. Seredynski, S. Kret, A. Kwiatkowski, K. P. Korona, M. Grzeszczyk, J. Mierzejewski, D. Wasik, W. Pacuski, J. Sadowski and M. Borysiewicz; Nanoscale 12 (2020) pp. 16535-16542.

Light matter interaction between excitonic resonances in atomically thin TMDCs, which are highly susceptible to electrostatic stimulus, and cavity polaritonic modes can enable novel electro-optic functionalities. In this work, we investigated electrical control of strong light-matter interactions in molybdenum ditelluride via opto-electronic modulation of an h-BN encapsulated monolayer MoTe₂ gated heterostructure designed also as a ‘Salisbury-screen’ photonic resonant absorber.

Using low temperature reflection contrast and photoluminescence measurements at different Fermi-level values, we observed strong modulation of the A 1s, 2s and B 1s exciton and trion peaks. Our results can be primarily understood from increased electronic doping which gives rise to enhanced elastic Coulomb scattering and screening – resulting in oscillator strength shifts from excitons to trions and also spectral broadening of the individual resonances. Moreover, many body effects leading to band-structure renormalization were seen from the changes in the binding energy of the different excitonic species with increased charge density. We performed transfer matrix calculations to obtain the complex dielectric function of MoTe₂ at each Fermi level position. Notably, we observed unity-order refractive index modulation at the A1s exciton. Additionally, we also studied the influence of optical pumping by performing laser fluence dependent emission measurements. Spectral broadening of the excitonic features was observed, indicating exciton-exciton interaction at higher pump powers. Our results shed light on the strongly correlated many-body interactions in MoTe₂ and suggest paths for integration into active devices such as near infrared electro-optical modulators.

Graphene has attracted the attention of the scientific community for the last two decades, thanks to an astounding set of properties such as high thermal conductivity (5000 W/mK), superior electron mobility (250,000 cm²/V-s), high modulus of elasticity (~1 T Pa), and ultra-high flexibility. Graphene’s C-C bonds are sp² hybridized providing superior mechanical stability in the basal plane directions. Graphene appears as the perfect material constituent for a wide range of application fields, like in flexible electronics, in electromechanical devices, and in reinforced lightweight composites that rely on both electrical and mechanical performances. It remains crucial to determine the latter properties under realistic and well-controlled conditions to guide the fail-safe design of graphene-based applications.

While the mechanical behavior of graphene has been very much investigated numerically for both perfect and defected lattice structures [1,2], most experimental works remain partly inconclusive regarding the fracture resistance. Nanoindentation of a suspended single layer (SLG) and of multilayer graphene (MLG) on top of holes [3,4], and uniaxial tensile test using the so-called push-to-pull device, are mostly used to deform graphene [8,9]. Testing graphene using a valid fracture mechanics approach is very complex due to the nature of the fabrication, the difficulty of pre-cracking, and the delicate data analysis. Two reports on the fracture toughness of SLG have been published [5,6], while other data are for MLG [7, 8]. The aforementioned works have provided insight into the fracture mechanisms of graphene, using a notch as a starter crack. This can potentially lead to an overestimation of the fracture toughness with a dependence on the notch root radius, which is difficult to deconvolute. Furthermore, the notch is produced by a focus ion beam, which may introduce damage. These tensile and fracture tests relied on external equipment either for loading or sensing, leading to cumbersome manipulation and allowing only a few specimens to be tested, preventing statistical analysis.

In the present work, we report two novel extensions of methods developed in our laboratory to study the mechanical and fracture properties of freestanding SLG. The method relies on the use of residual tensile-stress involved in a beam layer called the actuator beam. This beam is attached to the graphene specimen. Removing the sacrificial layer induces the release of internal stress in the actuator, pulling then on the graphene specimen. Hence, the use of external (electro-)mechanical actuation is avoided. In a first configuration, the actuator layer is pulling on a dog-bone shape or rectangular graphene specimen, from which the uniaxial stress-strain curve is extracted up to fracture [10]. In a second configuration, two actuator beams pull on a notched specimen but using a crack arrest method to determine the fracture toughness suppressing the problem of notch blunting effect at crack initiation [11]. Benefiting from the lithography process, numerous devices have been fabricated hence providing statistically representative data. Our investigation shows fracture strains larger than 10% and tensile strength larger than 100 GPa for specimens with an area larger than 160 mm². Brittle fracture behavior is observed with fracture toughness value equal to about 4 MPa m^0.5 in agreement with literature.

References
1. F. Liu et al., Phys. Rev. B 76, 2007
2. A. Shekhawat et al., Nat. Commun. 7, 10546, 2016.
3. C. Lee et al., Science 321, 2008.
4. H.I. Rasool et al., Nature Commun. 4, 2013.
5. Y. Hwangbo et al., Sci. Rep. 4, 2014.
6. P. Zhang et al., Nature Commun. 5, 2014.
7. X. Wei et al., Nano Lett. 15, 2015.
8. B. Jang et al., Extrem. Mech. Lett. 14, 2017.
9. K. Cao et al., Nat. Commun, 11, 92020.
10. M. Coulombier et al., thin films. Scr. Mater. 62, 2010.
11. S. Jaddi et al., JMPS, 123, 2019.

The large anisotropy of the bonding states in layered transition metal dichalcogenides (TMDs) provides an additional dimension to modulate materials properties [1,2]. For example, relatively weak interlayer van der Waals forces lead to distinctly different electronic, mechanical, vibrational, and electrochemical properties along the c-axis of MoS₂ compared to the basal plane. Efforts have been focused on probing photoinduced structural dynamics in the basal plane, where strong covalent bonding dominates the response of the material [3,4]. Comparatively few ultrafast studies have shown the different nature of photoexcited structural dynamics along the c-axis as compared to the in-plane behaviors [5,6]. In addition, defects play an important role in the nucleation of coherent strain waves studied with the bright-field imaging modality of ultrafast electron microscopy (UEM) [7-9].

Here, we report the combined study of direct imaging of photoinduced localized c-axis acoustic-phonon dynamics of a free-standing, multilayer MoS₂ flake with UEM and finite element transient deformation analysis with COMSOL. With nanometer-picosecond spatiotemporal resolutions in bright-field imaging, we observe and quantify the emergence and oscillation of two spatially localized interlayer breathing modes separated by a 10-nm thick step edge, with the difference in frequency due to the different thicknesses in the two respective regions. The onset of coherent contrast dynamics in the thicker region of the terrace is delayed by 2 to 3 ps, indicating a more rapid structural response to photoexcitation in the thin region of the terrace. Time-dependent finite element analysis on a terrace of the same dimensions as the specimen shows excellent agreement with experimental results. Photoexcitation was modeled by applying a spatially varying strain profile along the c-axis calculated based on the absorption of multilayer MoS₂ and the thickness of the terrace, leading to a discontinuity of the c-axis strain profile at the step edge. Both experiment and simulation captured a gradual decrease of 1 to 2 GHz of the oscillation frequencies starting 50 to 100 nm away from the step edge on both sides of the terrace, likely due to the additional force component along the c-axis opposed to wave propagation because of the distortion of the basal-plane covalent bonds. These results provide insights into the local, nanoscale nature of transient lattice deformations and the associated impacts on phonon dynamics in layered materials [10].

[1] A. P. Nayak, et al., Nature 5, (2014) 3731.
[2] S. Tongay, et al., Nano Lett. 13, (2013) 2831.
[3] E. M. Mannebach, et al., Nano Lett. 15, (2015) 6889.
[4] D. R. Cremons, et al., Nat. Commun. 7, (2016) 11230.
[5] A. J. McKenna, et al., Nano Lett. 17, (2017) 3952.
[6] E. M. Mannebach, et al., Nano Lett. 17, (2017) 7761.
[7] D. R. Cremons, et al., Struct. Dyn. 4, (2017) 044019.
[8] S. A. Reisbick, et al., J. Phys. Chem. A 124, (2019) 1877.
[9] Y. Zhang, et al., Nano Lett. 19, (2019) 8216.
[10] This material is based upon work supported by the National Science Foundation under Grant No. DMR-1654318. This work was partially supported by the National Science Foundation through the University of Minnesota MERSEC under Award Number DMR-2011401.

Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCS-Eliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the single-layer limit, where electron-electron interactions are dramatically enhanced. Here we report experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in single-layer NbSe₂, a model strongly correlated 2D material. Our high-resolution spectroscopic measurements reveal a characteristic spin resonance excitation in the density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance, observed along with higher harmonics, gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (T_C and H_C2), which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation between the energy of the spin resonance and its harmonics (Ω_n) and the local superconducting gap (Δ), which invokes a pairing of electronic origin associated with spin fluctuations. Our findings demonstrate the fundamental role that electronic correlations play in the development of superconductivity in 2D transition metal dichalcogenides, and open the tantalizing possibility to explore unconventional superconductivity in simple, scalable and transferable 2D superconductors.

Since the advent of graphene, many new two-dimensional (2D) materials have been either fabricated or theoretically proposed. These new materials, the so called 2D materials beyond graphene, represent a revolutionary new development in the science and technology of materials. Silicene, a 2D allotrope of silicon, is one of these materials. Compared to the normal 3D solids, the electronic as well as phononic characteristics of these materials are quite unusual. It has a stronger spin-orbit coupling, which gives rise to quantum spin Hall effect. Further, it has an electrically tunable band gap, a property useful in the design of field effect transistors at room temperatures. Hence, there is a strong interest in silicene for solid state devices for diverse applications such as thermal management, thermoelectric energy conversion, quantum computing, and spintronic devices.
A reliable and robust mathematical model is necessary for industrial development of new materials. It is needed for development of their characterization and testing systems and for virtual experimentation for providing answers to “what if” type questions. The conventional modelling techniques based upon the continuum approximation of solids are not applicable to 2d materials. One obvious reason is that the solid has almost 0 thickness. This introduces a discontinuity in the derivatives of field quantities, in the direction of the thickness. Hence all components of the standard continuum parameters, like stress and strain tensors, are not rigorously defined for 2d materials. Modeling 2d materials is, therefore, a multiscale many body problem, which must be valid at the atomistic scales.
Further, it has been recognized that the performance and stability of quantum materials are sensitive to the presence of lattice defects. Lattice defects induce displacement field in the solid, which strongly affects thermal, optical and electronic parameters of the solid. The elastic displacement field due to a point defect in the plane of the solid has a logarithmic behavior, which introduces a size effect in the material parameters, which must be accounted for in modeling and interpretation of measurements. Hence a discrete atomistic scale model needs to be large in order to avoid spurious size effects.
Finally, the model should be able to simulate low frequency phonons and their interaction with the electrons, which make a significant contribution to the thermal and possibly even quantum response of the solid. All these requirements make modeling of silicene a highly challenging task for which special techniques are needed. A conventional molecular dynamics based model is computationally inefficient for modeling low frequency phonons in large crystallites because of rather poor mathematical convergence.
At NIST, we have developed a multiscale phonon Green’s function method, which is very efficient for modeling the phononic response of 2d quantum materials. Our method can simulate a million atoms over an extended time range on an ordinary desktop computer. The power of the Green’s function method arises from the fact that we retain up to second order terms in the expansion of the lattice Hamiltonian in terms of the atomic displacements. In contrast, only the first order terms are retained in standard molecular dynamics. The second order term is crucial for phonon applications because the phonon energies are determined by the second order term.
In this talk I will briefly describe our Green’s function method and its application to calculation of phonon related properties of 2d quantum materials with and without lattice defects, such as vacancy, lithium, phosphorous, and boron. Numerical results will be presented for silicene.

Electron emission devices with ultrahigh emission current density and ultrafast response are instrumental to applications including but not limited to electron microscopy, high power microwave generation, and free electron lasers. A key limitation to achieving the ultrahigh emission current density and ultrafast response lies in the 3D nature of electron emitters. In a 3D photoemitter, the photons are absorbed at a finite depth and the excited electrons need to transit to the emitting surface before they can be extracted. During this transit process, these high energy electrons lose their excess energy due to electron-phonon and electron-electron scattering. In a 2D material like graphene, the excited electrons are always at the emitting surface and therefore the transit length is effectively eliminated. In a recent experimental study, it has been demonstrated experimentally that a waveguide integrated graphene photoemitter can absorb nearly 100% of the photons traveling through the waveguide and the excited electrons can be extracted by applying a dc electric field with a quantum efficiency that exceeds 5 orders of magnitude compared to that of multiphoton and strong field photoemitters. In this work, we have performed a theoretical investigation of the performance limits of such a photoemitter by solving for the Monte Carlo Boltzmann Transport Equation (MCBTE) accounting for the scattering processes in graphene as well as the tunneling rates into the vacuum as a function of absorbed optical power density, photon energy and applied dc electric field. The rates of different scattering and tunneling processes were calculated by quantum mechanical theory using Fermi’s golden rule. We have shown that the theoretical model can reproduce the experimental results with good quantitative agreement. Our results predict two regimes of operation for graphene hot electron emitters: (a) single hot electron emission at lower power densities (< 10⁹ W/m²) and (2) ensemble hot electron emission at higher power densities (> 10⁹ W/m²) where the electronic temperature of graphene increases beyond the lattice temperature. In the ensemble hot electron emission regime, it is possible to achieve a quantum efficiency that exceeds 100% with a response time below a picosecond. This theoretical study demonstrates an experimental roadmap towards realizing high efficiency electron emission with large emission current densities and ultrafast response times.

Monolayer MoTe2, one of the 2D transition metal dichalcogenide (TMD) materials, exhibits two stable structural phases: semiconducting 2H phase and metallic 1T′ phase. The dynamic control of the transition between these two phases on a single atomically thin sheet holds promise for a variety of revolutionary device applications. Particularity, stress could be utilized to dynamically modulate such phase transition. To date, the atomistic and kinetic mechanism of the phase transition under stress is not clear. In this study, the finite deformation nudged elastic band method and density functional theory are applied to determine the phase transition barriers and pathways of monolayer MoTe2 as a function of applied stress. It is found that the stress can greatly influence the thermodynamics and kinetics of the phase nucleation and propagation. The results shed light on the phase engineering of 2D TMD materials with stress at the atomic leve

Low-dimensional systems such as graphene and planar h-BN sheets exhibit anomalous size-dependent thermal transport. Non-equilibrium atomistic simulations using a constant heat current or a temperature gradient are ideal for analyzing this size dependency. Most analyses to date have employed the Fourier representation with complex normal mode coordinates to describe the phonon modes; while this representation is sufficient for computing the phonon dispersion relationships or spectral energy density, it lacks a precise capability to express the modal energy, and more importantly, a real modal heat current along a specified wave-vector. In this work, we adopt our recent non-equilibrium molecular dynamics approach [J. Chem. Phys., 151, 104110 (2019)], which employs a set of real asymmetric normal mode amplitudes that can unambiguously define the exact modal heat currents in a given direction, to extract the size-dependent modal thermal conductivity in planar SiC and h-BN sheets under a temperature gradient. We capture both the left-going and right-going modal heat current and phonon population independently, which is theoretically impossible with the traditional complex normal mode coordinates, and contrast their size-dependent behavior in planar SiC and h-BN sheets. While a modified Tersoff potential is used to describe the interatomic interactions in the planar SiC, the recently developed EXTEP potential is employed for h-BN. We anticipate that our approach would particularly be useful to characterize thermal transport in low-dimensional systems and molecular junctions where bulk or effective thermal properties are ill-defined or difficult to access.

The doping of two-dimensional (2D) semiconductors has been intensively studied toward modulating their electrical, optical, and magnetic properties. While ferromagnetic 2D semiconductors hold promise for future spintronics and valleytronics, the origin of ferromagnetism in 2D materials remains unclear. Here, we present that substitutional Fe-doping into MoS₂ and WS₂ monolayers to induce magnetic properties. The Fe-doped monolayers were directly synthesized via chemical vapor deposition. A Fe₃O₄ powder was sandwiched between a bare SiO2/Si substrate and the transition metal oxide-deposited substrate for the growth. After loading the substrates and precursors, Ar gas was supplied, and the furnace temperature was increased up to 900 °C. Fe substitutional doping was successfully achieved, as confirmed using scanning transmission electron microscopy. While both Fe:MoS₂ and Fe:WS₂ showed PL quenching and n-type doping, Fe dopants in WS₂ monolayers were found to assume deep-level trap states, in contrast to the case of Fe:MoS₂, where the states were found to be shallow. Unlike MoS₂ monolayers showing ferromagnetic behavior, WS₂ monolayers showed paramagnetic behavior upon Fe-doping. These contrasting magnetic behaviors caused by paired Fe atoms can be attributed to paired Fe atoms generating more accumulated electric charge in Fe:WS2 by pulling the Fe atoms apart. These findings allow us to control the material’s magnetic properties layer-by-layer and can be applied for spintronics.

Quasi-Fermi level splitting (Δµ) of any material under illumination symbolizes the maximum possible open-circuit voltage (Voc) that a photovoltaic device can reach if made out of that material in ideal conditions. We have experimentally quantified the Δµ (Voc) values of various monolayer transitional metal dichalcogenides (TMDs). Briefly, we employed optical non-destructive techniques of micro-photoluminescence and micro-absorption spectroscopy with reference to the generalized Planck law of emission. We report that the photovoltaic devices made of monolayer WS₂, MoS₂, WSe₂, and MoSe₂ can possibly yield Voc values of up to ~1.4, 1.12, 1.06, and 0.93 V at 1 Sun illumination, respectively. We further show some of the different possible ways to manipulate the reported Voc performance using external voltage and chemical molecules of aggregation-induced emission in Type-I heterostructures.

Nanocarbon materials have been widely used for nanoelectronics and other energy-related applications. In the kinetic relationship, the phonon (lattice) thermal conductivity is given as k_L~C_pv_gΛ, with C_p as the phonon specific heat, v_g as phonon group velocity, Λ as phonon mean free paths (MFPs). Within general composite films made of reduced graphene oxides (rGO) and carbon nanotubes (CNTs), the existence of many nanocontacts results in a k reduction but the loss in k can be overshadowed by the significant advantages of using bulk composites for large-scale applications. Detailed thermal analysis of composite films made of rGO and CNTs is still lacking, particularly for the individual in-plane k_‖ contribution from each constituent material within the complicated 3D nanocarbons. In this work, composite films consisting of rGO nanosheets and varied weight percentage of single-wall CNTs (SWCNTs) are synthesized and studied for their in-plane thermal conductivities, in which increased SWCNTs percentage leads to a reduced k_‖. Different from pristine graphene and other composite nanocarbon films with decreased thermal conductivities above 300 K, the in-plane thermal conductivities of these composite films are found to follow the trend of the specific heat of graphene from 100 K to 400 K, i.e., monotonously increasing at elevated temperatures. For investigated samples, the extracted k_‖ can be matched by a scaled curve for the product C_pv_g summed up for all phonon modes that are computed for pristine graphene. This indicates that the rGO nanosheets contribute significantly to the k_‖ but their MFPs are largely restricted by the dense SWCNT-graphene junctions and additional graphene-graphene nanocontacts. Such a trend has seldom been observed for nanocarbon. This unique temperature dependence of thermal conductivities is attributed to the so-called small-nanostructure-size (SNS) limit, at which the phonon MFPs are simply restricted as the structure size. This SNS limit is often found at cryogenic temperatures where the majority bulk phonon MFPs are much longer than the sample size. But in this study, the intrinsically long phonon MFPs within the initial graphene sheets and the dense SWCNT-graphene/graphene-graphene nanocontacts result in k≈ k_L following the SNS limit even up to 400 K. The highest in-plane thermal conductivity among samples with different synthesis conditions is 62.8 W/mK at 300 K which is significantly higher than amorphous materials with a similar temperature dependence of thermal conductivities. Such a high thermal conductivity, combined with its unique temperature dependency, can be ideal for applications such as flexible film-like thermal diodes based on the junction between two materials with a large contrast for their temperature dependence of the thermal conductivity.

Electric double-layer capacitors (EDLCs) are one of the most promising candidates for environmentally friendly and sustainable energy storage systems due to their balanced energy-power density compared to conventional batteries and capacitors. As key materials for EDLCs, group-14 elemental two-dimensional materials (ETDMs) possess extremely large surface area for doping and ion adsorption, which is advantageous for maximizing the charge storage. However, one of the bottlenecks that limits the performance as EDLCs is the ETDMs’ lack of density of states (DOS) near the Fermi level (E_F) that can store the charge transferred from the ions. In this study, to provide additional DOS near the E_F, Ti-doping was introduced to the ETDMs. The density functional theory (DFT) calculation revealed that additional DOS was provided to ETDMs by the d band of doped Ti atom. Since the surface charge densities increased due to the newly formed DOS near the E_F, the quantum capacitances were also enhanced. In addition, among the Ti-doped ETDMs, Ti-doped silicene is found to have superior characteristics at the low voltage range. These findings may provide practical guidelines for improving the performance of EDLCs.

The effect of Oxygen (O) doping in Zigzag Graphene Nano ribbons (GNR) has been investigated in terms of their stability and electronic properties, using density-functional theory-based ab-initio approach. The Zigzag GNR has been subjected to mono vacancy and decorated with three possible sites of oxygen impurities around the MonoVacancy (MV). The stability of the systems has been analysed in terms of cohesive energy, and the electronic properties in terms of band structure, and density of states profiles. The cohesive energy calculations suggest that formation of the pyridine-type defects with oxygen doping, have nearly the same stability in the case of monovacancy introduced system. The electronic bandstructure and density of state profile observed that the metallicity of the ZGNR enhances with O doping, validated through increase in density of states at the fermi level. This enhanced metallicity in ZGNR, defends its application as metallic electrodes as well as interconnects in electronic industry.

For more than 70 years, Graphite Intercalated Compounds (GICs) have been extensively studied, wherein LiC6 was discovered as a lead material for high-capacity electrical energy storage. With the recent surge of compact electronic devices in commercial production, we present a novel fabrication technique for ultra-thin LiC6 devices. Here we present a new transfer method, termed “bifacial float transfer”, which enables large-scale multilayer graphene transfer from both sides of a nickel growth catalyst [1]. Bifacial float transfer has the potential to reduce graphene production costs by transferring chemically vapor deposited (CVD) graphene films from both sides of their native nickel substrate, with one graphene film transferred to a polymer and the other graphene film transferred to another target substrate. In traditional transfer methods, the graphene on the “non-preferred” side, that is, the bottom of the substrate, is removed with oxygen plasma before removal of the metal catalyst in etchant solution. Although this treatment prevents undesired aggregation of the graphene films, it fails to utilize both sides of CVD-grown graphene. In this talk, we compare the quality of graphene transferred from both sides onto target glass and polymer substrates. The results of optical microscopy, confocal Raman spectroscopy, atomic force microscopy, and electronic transport measurements suggest that the quality of the multilayer graphene on the “non-preferred” side does not differ significantly from that of the “preferred” side. This method will allow more efficient and cost-effective use of graphene by doubling the usable graphene per area of growth substrate, and by eliminating the need for intermediate sacrificial transfer substrates such as poly(methyl methacrylate). By modifying the air-liquid interface, we also attempt similar polymer-free transfer of progressively thinner multilayer graphene films. The fabricated samples are then lithiated using the in-situ short-circuit direct contact method [2]. The ultra-thin LiC6 exhibits metallic behaviour at room temperature with low sheet resistance and improved optical transparency in the visible spectrum [3]. We present preliminary results on the fabrication and characterization of these large-scale, stable LiC6 films.
[1] Andrade J. A. et al., Adv. Funct. Mater., 2005103 (2020)
[2] Shellikeri A. et al., J. of Elec. Soc., 164, 14 (2017)
[3] Bao W. et al., Nat. Communication, 5 , 4224 (2014)

Electron emission devices are important for applications in electron microscopy, free electron lasers, radio frequency (RF) amplifiers, and accelerators. Photoemission plays an important role in realizing high efficiency electron emission by exciting lower energy electrons above the workfunction barrier. There are three fundamental limitations to improving the efficiency of photoemission from cathodes: (1) low optical absorption (2) large absorption depth leading to a large transit length to the emitting surface for the excited electrons and (3) high workfunction barrier to extracting the electrons. As a result, most of the works on photoemitters have focused ultraviolter photons with energy that exceeds the workfunction barrier (> ~5 eV), multiphoton absorption, and strong field photoemission processes to realize electron emission. In this work, we have addressed these limitations by using a waveguide integrated graphene electron emitter excited by subworkfunction photons (3.06 eV). While monolayer graphene absorbs only 2.303% of the free space incident light, a waveguide integrated graphene can absorb nearly 100% of the light traveling through the waveguide. In addition, the monolayer thickness of graphene ensures that the excited electrons are always at the emitting surface which eliminates the transit length and reduces the energy loss in electrons due to electron-phonon scattering and electron-electron scattering. This waveguide integrated graphene electron emitter absorbs photons at 3.06 eV which is significantly smaller than the workfunction barrier in graphene (~4.5 eV) and a dc electric field then assists these electrons to tunnel out of graphene before they lose energy. This device experimentally demonstrates an efficiency exceeding the efficiency of multiphoton and strong field photoemitters available in literature by 5 orders of magnitude. Finally, we have modified the graphene surface by introducing a coating of LaB₆ nanoparticles which have a low workfunction (~2.7 eV). This coating of LaB₆ nanoparticles further reduces the workfunction of graphene to 3.5 eV resulting in a tenfold increase in the hot electron emission efficiency. Therefore, we have addressed all three fundamental limitations of a photoemitter by (a) using the waveguide integrated structure that allows ~100% optical absorption (b) using monolayer graphene that eliminates transit length for excited electrons, and (3) reducing the workfunction barrier with a LaB₆ nanoparticle coating. The strategy to this work can be extended to optical resonators which allows higher optical power densities and other 2D materials including transition metal dichalcogenides which may allow a further increase the efficiency of hot electron emission.

Topological order is intimately connected to the presence of translational symmetries - these result in symmetry-protected phases in quantum materials and the resulting classification of topological materials. However, recent experiments have hinted at the presence of topologically protected states even in the absence of translational symmetry. In this talk, I will discuss how theoretical calculations have uncovered topological phases in materials without translational symmetry -- namely in a quasicrystal material and in an amorphous material. I will first describe how a two-dimensional Ta-Te chalcogenide quasicrystal and approximant hosts a layer-tunable topologically nontrivial electronic bandstructure. Next, I will show how introducing disorder into the BiTeI layered family of Rashba materials can induce a trivial-to-nontrivial topological phase transition. Finally, I will discuss the amorphous limit of this case where theory and ARPES measurements suggest topologically protected surface states in amorphous Bi2Se3.

Identifying intrinsic low-dimensional ferromagnets with high transition temperature and electrically tunable magnetism is crucial for the development of miniaturized spintronics and magnetoelectrics. Recently, monolayer CrI₃ and Cr₂Ge₂Te₆ semiconductors were found to exhibit long-range two-dimensional ferromagnetism with relatively low Curie transition temperature. Here we present our theoretical study of van der Waals layered magnetic semiconductors CrSBr and CrSeBr. Using first-principles electronic structure theory and renormalized spin-wave theory, we predict that monolayer CrSBr and CrSeBr exhibit highly anisotropic electronic structure with bandgap of 1.66 eV and 0.78 eV. More importantly, monolayer CrSBr and CrSeBr possess high Curie temperature of ~150K which was recently verified by experiments, well beyond that of CrI₃ and Cr₂Ge₂Te₆. The origin of the high Curie temperature is attributed to strong anion-mediated superexchange interaction and sizable spin-wave excitation gap due to large exchange and single-ion anisotropy. Our results also show that it may be possible to switch magnetization easy axis via electrostatic doping can, suggesting the possibility of realizing spin field effect transistor controlled by electrostatic doping. Finally, we will also discuss linear and nonlinear optical properties of van der Waals layered CrSBr and CrSeBr. Monolayer CrSBr and CrSeBr semiconducting ferromagnets offer long-desired alternatives to dilute magnetic semiconductors and provide unprecedented opportunities for 2D spintronics such as spin valves and spin FETs.

Atomically thin (two-dimensional, 2D) semiconductors have shown great potential as the fundamental building blocks for next-generation electronics. However, all the 2D semiconductors that have been experimentally made so far have room-temperature electron mobility lower than that of bulk silicon, which is not understood. Here, by using first-principles calculations and reformulating the transport equations to isolate and quantify contributions of different mobility-determining factors, we show that the universally low mobility of 2D semiconductors originates from the high “density of scatterings,” which is intrinsic to the 2D material with a parabolic electron band. The density of scatterings characterizes the density of phonons that can interact with the electrons and can be fully determined from the electron and phonon band structures without knowledge of electron-phonon coupling strength. Our work reveals the underlying physics limiting the electron mobility of 2D semiconductors and offers a descriptor to quickly assess the mobility. [1,2,3]
[1] L. Cheng, C. Zhang, Y. Liu, PRL 2020, DOI: 10.1103/PhysRevLett.125.177701 (Editor’s suggestion)
[2] L. Cheng, C. Zhang, Y. Liu, JACS, 2019, DOI: 10.1021/jacs.9b05923
[3] L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871

First-principles theoretical analysis of the catalytic activity of van der Waals hetero-structures of 1H-MoS₂ and graphene substituted with three chemical types of nitrogen species (i) Graphitic (G), (ii) Pyridinic (Pn), and (iii) Pyrrolic (Pr), for application in catalysis of hydrogen evolution reaction (HER) has been presented. Graphitic and pyrrolic N substituents result in n-type electronic structure, whereas substitution of pyridinic N imparts p-type electronic character to the hetero-structure. Work functions () of the hetero-structures suggest that graphitic N-graphene:MoS₂ hetero-structure () is expected to be effective in catalysing the reduction of H⁺ to evolve H₂. 1H-MoS₂ monolayer in the hetero-structure contributes by enabling increased H₂O adsorption and offsetting the band edge energies optimal for the catalytic activity. Near optimum Gibbs free energy of H-adsorption () were obtained for graphitic ( ~ 0.29 eV) and pyrrolic ( ~ −0.2 eV) N-graphene:MoS₂ hetero-structures. Our work showcases how catalytic and electronic properties of the N-doped graphene:MoS₂ hetero-structure depends on the chemical identity of N-sites and uncovers a route to 2D hetero-structures with high catalytic activity.

We have utilized data mining approaches to elucidate over 1000 2D materials and several hundred 3D materials consisting of van der Waals bonded 1D subcomponents, or molecular wires. We find that hundreds of these 2D materials have the potential to exhibit observable piezoelectric effects, representing a new class of piezoelectrics. A further class of layered materials consists of naturally occurring vertical hetero structures, i.e. . bulk crystals that consist of stacks of chemically dissimilar van der Waals bonded layers like a 2-D super lattice. We further combine this data set with physics-based machine learning to discover the chemical composition of an additional 1000 materials that are likely to exhibit layered and two-dimensional phases but have yet to be synthesized. This includes two materials our calculations indicate can exist in distinct structures with different band gaps, expanding the short list of two-dimensional phase change materials. We find our model performs five times better than practitioners in the field at identifying layered materials and is comparable or better than professional solid-state chemists. Finally, we find that semi-supervised learning can offer benefits for materials design where labels for some of the materials are unknown.

Finding inexpensive and active catalyst is essential to produce renewable energy, hydrogen, in a sustainable method. MXenes have been reported as a potential hydrogen evolution reaction (HER) catalysts with the advantages of their unique surface properties and excellent electrical conductivity. Recent studies on MXenes, especially titanium carbide, have showed that nitrogen-doping improves the catalytic performances in HER. However, various nitrogen-containing by-products are formed on the surface of MXene during the doping processing, and the origins of catalytic performance variations or the theoretical optimal doping concentration have not been fully studied. In this study, to find the optimum nitrogen doping concentration of Ti₂CO₂, the adsorption energies of hydrogen and the conductivity, were investigated using the density functional theory (DFT) calculations. From the calculations of the doping formation energy, C-site of Ti₂CO₂ was found to be the most favored nitrogen substitution. To determine the optimum nitrogen concentration, the Gibbs free energy of hydrogen adsorption and the conductivity were calculated for various N doping concentration. As the doping concentration increased, both the Gibbs free energy of hydrogen adsorption and the conductivity were enhanced and reached an apex point at the specific doping concentration. As the doping concentration further increased, deterioration of the catalytic performance was expected as the values of the two properties decreased. Through these results, it can be inferred that the doping concentration of N at C site influences the catalytic properties of N-doped Ti₂CO₂ and that optimum catalytic performance can be achieved at the specific doping concentration. This study suggests a frame of systematic and theoretical predictions to optimize doping processing of MXene-based HER catalysts.

Vertically stacked, layered van der Waals (vdW) heterostructures offer the possibility to design materials, within a range of chemistries and structures, to possess tailored properties. Inspired by the naturally occurring mineral merelaniite, we investigate a vdW heterostructure composed of MoS₂ monolayer and PbS bilayer, using density functional theory. The results find such a heterostructure to be stable and possesses p-type semiconducting characteristics. Due to the heterostructure’s weak interlayer bonding, its carrier mobility is essentially governed by the constituent layers; the hole mobility is governed by the PbS bilayer, whereas the electron mobility is governed by the MoS₂ monolayer. Furthermore, we estimate the hole mobility to be relatively high (~10⁶ cm²V⁻¹s⁻¹), which can be useful for ultra-fast devices at the nanoscale.

Since the discovery of flat bands at magic angle in twisted bilayer graphene, the formation of flat bands has been predicted for several other materials, especially twisted transition metal dichalcogenides (TMD). WSe₂ possesses strong spin-orbit interaction which makes it different from other TMDs. Using a multiscale approach, based on classical force-fields for structural relaxation and density functional theory calculations for electronic structure, we study the effect of spin-orbit coupling on the formation of flat bands in relaxed twisted WSe₂. The spin-orbit interaction affects the flat bands with the twist angle close to 0° differently than those near 60°. The flat bands at valence and conduction band edge arise from K and Q points of the unit cell Brillouin zone respectively. For twist angle near 60°, we find that the flattening of the bands arising from K point is a result of the atomic reconstruction in the individual layers and not due to interlayer coupling. On the other hand, for twist angle close to 0°, the interlayer interaction between the two layers becomes important. Additionally, we also find flat bands folded from the Γ point of the unit cell Brillouin zone. The wave function localization of the flat bands matches well with STM measurements from existing literature [1]. Furthermore, we find that the atomistic spin-orbit splitting in moiré remains unchanged from the monolayer. We extend our study to strain-induced moiré in homobilayer TMD. The mismatch in lattice constant gives rise to moiré pattern. Relaxation of the system distributes the strain unevenly in the moiré lattice with increased strain at the highest energy stacking. Several well separated flat bands emerge at both valence and conduction band edges and we observe a significant reduction in the band gap. Thus, both twist and strain engineered moiré generate pathway to manipulate holes and electrons in TMD.
Reference:
1. Z. Zhang et al. Nat. Phys. doi:10.1038/s41567-020-0958-x (2020).

Ultra-clean van der Waals interfaces can be achieved between soft indium metal and monolayer 2D transition metal dichalcogenide semiconductors. Such interfaces lead to low contact resistance and n-type field effect transistors with high mobilities – in excess of 100 cm²-V^-1-s^-1. It has been, however, challenging to make similarly clean interfaces between refractory metals with high work functions to achieve efficient hole injection. Here, I will present our efforts on realizing p-type contacts using high work function metals and their alloys. This method preserves the ultra-clean interface between the monolayer semiconductor and alloy while increasing the work function so that p-type devices can be realized. We also demonstrate clean interfaces using metals such as Pd and Pt via direct deposition. These interfaces reveal low contact resistance and also high performance p-type devices.

Emerging technologies like stretchable electronics and straintronics require materials that are highly deformable yet electronically active. With high breaking strain, high electronic mobilities and diverse properties, two dimensional (2D) materials are ideal for such applications. Moreover, due to their low bending stiffness, 2D materials are excellent candidates for engineering three-dimensional (3D) nanostructures to relieve strain or engineer inhomogeneous strain under compression. In crumpled systems, highly deformed surfaces of 2D materials lead to applications in wearable electronics and multifunctional surfaces. Engineering inhomogeneous strain gradients into device architectures enables access and control of strain-dependent quantum states like single quantum emitters, exciton diffusion and asymmetric valley states.

To fully realize the potential of 2D materials for deformable electronics or straintronics, it is necessary to tailor the transfer of strain into the materials and resulting morphologies. Here we demonstrate tuning of the interfacial interaction between 2D membrane and elastomer substrate resulting in either crumpling through buckle delamination or conformal wrinkling, corresponding with low and high transfer of strain into the 2D membrane respectively. We then explore systems leveraging these tuned interactions resulting in strain resilient electronics from crumpled 2D heterostructures, and inhomogeneous or mechanically reconfigurable strain engineering from wrinkled 2D materials.

First, we demonstrate phototransistors from crumpled 2D heterostructures formed from graphene contacts to a monolayer transition-metal dichalcogenide (MoS₂, WSe₂) channel and quantify the membrane morphology and optoelectronic performance.[1] Under uniaxial compression, the heterostructure and constituent monolayers relieve the stress by creating nearly periodic delaminated folds whose spacing depends on the membrane type. The matched mechanical stiffness of the constituting layers allows the heterostructure to maintain a conformal interface through large deformations. Photoluminescence (PL) spectroscopy shows that the optical band gap of WSe₂ shifts by less than 2 meV under 15% biaxial crumpling, corresponding to a change in strain of less than 0.05%. Photocurrent revealed that the crumpled heterostructure behave as a phototransistor where photoresponsivity scaled as P^–0.38, with behavior similar to flat devices. Photocurrent maps reveal biaxial crumpling by 15% marginally increases the photoresponsivity, by only 20%. PL and photoresponse confirm that crumpling and delamination prevent the buildup of compressive strain leading to highly deformed materials and devices with similar performance to their flat analogs.

While crumpling leads to strain resilient structures, increased substrate interaction actually results in wrinkling and increased strain transfer. We demonstrate a method to tune the strain in 2D materials by wrinkling monolayer WSe₂ attached to a 15 nm thick Al₂O₃ support layer and compressing the heterostructure on a soft substrate.[2] The Al₂O₃ film stiffens the 2D material, enabling optically resolvable micron scale wrinkling rather than nanometer scale crumpling. Using PL spectroscopy, we show that the wrinkling introduces periodic modulation of the bandgap by 47 meV, corresponding with strain modulation from +0.67% tensile strain at the wrinkle crest to -0.31% compressive strain at the trough. Moreover, we show that cycling the substrate strain mechanically reconfigures the magnitude and direction of wrinkling and resulting band tuning.

Taken together, these results set a foundation for deformable all-2D heterostructure devices and circuitry for stretchable electronic applications, and articulate strategies for maximizing or minimizing strain transfer.

References:
[1] Hossain, M. A., et al. ACS Appl. Mater. Interfaces 2020, 12 (43), 48910–48916.
[2] Hossain, M. A., et al. Proceedings of SPIE 2020, 1146404, 1−7.

Mental stress negatively impacts more than 77% population in our modern societies. Continuous and objective assessment of stress level enables proactive management for depression, evasion, and suicidal tendency. Electrodermal activity (EDA), a.k.a. galvanic skin response (GSR), is an emotion-induced skin conductance change due to sweat gland activity. EDA has been widely used as a quantitative index of mental stress levels for decades. Palm has the highest density of eccrine sweat glands which are filled up under psychological stimuli, such as mental stress, primarily. However, state-of-the-art EDA monitors suffer from limitations such as obstructiveness, undesirable location, short-term wearability, motion artifacts, etc. Herein, we introduce a long-term wearable (e.g. 24 hours), high-fidelity palm EDA sensor using dry, sub-micron-thick graphene e-tattoos (GET). The filamentary-serpentine-shaped GET is so thin (only 150-nm-thick) and so soft that it is imperceptible optically, mechanically, and also physiologically. The GET can fully conform to the finest texture of human skin, resulting in an interface impedance even lower than wet Ag/AgCl gel electrodes as well as reduced susceptibility to motion artifacts. GET is laminated on the palm and extended to the wrist to connect to a wristwatch which provides data acquisition, storage, and wireless transmission. The orders of magnitude mismatch of stiffness and thickness between the GET and the wristwatch poses an outstanding challenge for the connection in between. We propose a simple yet effective solution named “heterogenous serpentine ribbon (HSR).” Due to the ultimate conformability of GET, simply laminating serpentine GET ribbon on an ultrathin serpentine metal ribbon without any interfacial adhesive could yield a conductive and stretchable interface. Compared with contacts between straight GET and metal ribbons, HSR offers a ten-fold strain reduction. To achieve a mechanically robust and reversible electrical connection between the ultrathin metal ribbon and the wristwatch, a soft interlayer is added in between to serve as a mechanical insulator but electrical conductor. We have successfully applied such GET-watch-based wearable EDA sensor for long-term, continuous, and ambulatory EDA sensing during our daily activities as well as during sleep. We argue that our HSR plus soft interlayer structure is a generic solution to robustly connect any ultrathin and ultrasoft e-tattoos to smartwatches, which will significantly extend the sensing modalities of future smartwatches.

Research on beyond graphene two-dimensional (2D) nanomaterials is becoming critically important for wide-ranging technological applications. In parallel with the discovery of new candidate materials and exploration of their unique characteristics, there are significant interests to rationally control the properties of 2D nanomaterials to meet requirements for energy-related applications. Electrocatalysis powered by renewable energy is expected to play a pivotal role in achieving sustainable energy goals. Recent advances in structural engineering have demonstrated their ability to overcome intrinsic material limitations, showing great potential for promoting catalytic activity and stability to a new level. Here we will discuss several important strategies to provide insights into application-oriented tailoring of material properties, and focus on our recent progress in developing 2D nanomaterials with tunable pore sizes or multidimensional interconnection for catalyzing several key electrochemical reactions, such as oxygen evolution, hydrogen evolution, and nitrogen reduction. Constructing porous and/or hierarchical 2D nanosheets enables improved ionic/electronic transport to achieve high active surface area for reactant adsorption/desorption, and meanwhile maintains high structural stability. These works demonstrate valuable insights, regarding material selection, structural modification and electronic tuning, into developing earth-abundant 2D electrocatalysts as alternatives to noble metal catalysts for efficient and stable electrocatalysis.

Emerging novel technologies, such as flexible, rollable, and wearable electronics, need to meet their special requirements such as ease of controlling electronic properties, flexibility, and transparency. To date, metal and semiconducting materials have been used for conventional electronics, but the materials are brittle and nontransparent. In contrast, two-dimensional (2D) materials such as graphene and transition dichalcogenides have demonstrated great potential for flexible electronics and transparent electrodes due to their unique mechanical and electronic properties. In order to fully utilize 2D materials for the next-generation of flexible electronics, geometric micropatterns of 2D materials on flexible substrates such as polymer substrates are essential to fulfill technological purposes. However, due to harsh conditions such as cleaning, developing, and etching processes in the existing lithography procedure, defining elaborate micropatterns of 2D materials using the conventional microelectromechanical systems on polymer substrates is difficult to achieve.
Here, we demonstrate a novel strategy to obtain micropatterned graphene on flexible substrates, which is an alternative to the conventional wet transfer method. Graphene microchannels, which are prepared by the conventional lithography process are transferred by direct polymer curing transfer. With this method, entirely flexible, transparent, self-activated graphene sensor arrays are fabricated on 4-inch wafer-scale polymer substrates. A sensor array with four sensors using micropatterned graphene channels with noble metal decoration provides different sensing information and discriminates chemical species. Finite element method simulations were employed to investigate the potential of this patterning technique. The calculation results demonstrate that the performance of the sensors can be highly enhanced when the active channels are suspended and nanoscaled. This study provides higher reliability compared to the present state-of-the-art flexible electronic fabrication techniques and also demonstrates future-oriented flexible chemical sensor platforms.

The effective and accurate sensing of dopamine (DA) has important implications due to its essential roles in the neurobiological system. The release process of dopamine is closely linked with neurological diseases such as Parkinson's, drug addiction, and attention deficit hyperactivity disorder (ADHD). Here we report a novel electrochemical biosensor based on atomically-thin tellurene, a 2D p-type semiconductor, for selective and sensitive detection of dopamine with a sensitivity for 2.81 mA/mM*cm², which is more superior to glassy carbon electrodes. Our device exhibits good linearity of oxidation current at the concentration of DA over a range of 1-2.875 mM, which presents the largest range reported in the literature. Enabled by the specific binding between Te and nitrogen atoms on biomolecules (DA), tellurene based sensors show greater selectivity of DA in the presence of uric acid and ascorbic acid. In addition, well-dispersed Au nanoparticles (Au-NPs) on 2D tellurene can enable excellent catalytic activity and a hospitable environment for biomolecules. We further demonstrated the successful sensing operation of tellurene/Au-NP based hybrid biosensors for the quantification of dopamine concentrations in human blood samples with satisfactory recoveries.

The growing importance of applications based on machine learning is driving the need to develop dedicated, energy-efficient electronic hardware. Compared with von-Neumann architectures, brain-inspired in-memory computing uses the same basic device structure for logic operations and data storage, thus promising to reduce the energy cost of data-centric computing significantly. Two-dimensional materials such as semiconducting MoS₂ could stand out as a promising candidate to face this obstacle thanks to their exceptional electrical and mechanical properties. Here, we show that wafer-scale grown MoS₂ can be used as an active channel material for developing logic-in-memory devices and circuits based on floating-gate field-effect transistors (FGFET). The conductance of our FGFETs can be precisely and continuously tuned, allowing us to use them as building blocks for reconfigurable logic circuits where logic operations can be directly performed using the memory elements. We show that this design can be simply extended to implement more complex programmable logic and functionally complete sets of functions. Our findings highlight the potential of atomically thin semiconductors for the development of next-generation low-power electronics.

Graphene nanoribbons (GNRs) have attracted considerable interest due to their largely modifiable electronic properties, including width-dependent bandgaps for armchair GNRs and spin-polarized edge states for GNRs with zigzag edges.^1,2,3,4 Manifestation of these properties requires atomically precise GNRs, which can be achieved through a bottom-up synthesis approach under ultrahigh vacuum conditions. We show that 5-atom wide armchair GNRs as well as pyrene-GNRs can be processed under ambient conditions and incorporated as the active material in a field effect transistor.^5,6 At room temperature, a film like behavior is observed while at cryogenic temperatures coulomb blockade and single electron tunnelling can be seen. Our recent results may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.

¹ Cai, J. et al., Nature 466, 470–473, (2010)
² Chen, Y.-C et al., ACS Nano 7, 7, 6123–6128, (2013)
³ Ruffieux, P. et al., Nature 531, 489–492, (2016)
⁴ Gröning, O. et al., Nature 560, 209–213, (2018)
⁵ El Abbassi, M. et al., ACS Nano 14, 5, 5754–5762 (2020)
⁶ Sun, Q. et al., Advanced Materials 32, 1906054 (2020)

Since the excitement about graphene, a monolayer of graphite, with its 2010 Nobel Prize, there has been extensive research in the synthesis of other non-carbon few/mono-layers exhibiting a variety of bandgaps and semiconducting properties (e.g., n or p type). The main approaches to deposit few/monolayers on a substrate are: (a) bottom-up synthesis from precursors using chemical vapor deposition (CVD) or (b) top-down exfoliation (liquid or mechanical) of bulk layered material. Using a Lego approach of superposing monolayers, we can envisage the fabrication of heterojunctions with original electronic behavior.
Here we show a combined bottom-up and top-down approach where (a) we synthesize in one step high yields of bulk layered materials by annealing a metal in the presence of a gas precursor (sublimated selenium from selenium powder) using chemical vapor deposition (CVD) and (b) we exfoliate and deposited (dropcast or Langmuir Blodgett) few/mono-layers on a substrate from a sonicated mixture of our material in a specific solvent. It is interesting to note that, besides the structure being 2D layered, the properties of the nanomaterials synthesized slightly differ from the materials with the same stoichiometry synthesized using conventional chemical methods (e.g., solvothermal).
In this talk, we will discuss the chemical synthesis, the very extensive characterizations, and the lessons we learned in making multiple metal selenides (Ag-Se, Cu-Se, W-Se, Mo-Se). We will see how we integrated these new materials into sensors and into electrochemical devices (see selected published papers below).

R. Konar, S. Das, E. Teblum, A. Modak, I. Perelshtein, J.J. Richter, A. Schechter*, and G.D. Nessim*
Facile and Scalable Ambient Pressure Chemical Vapor Deposition-Assisted Synthesis of Layered Silver Selenide (β-Ag2Se) on Ag foil as an Oxygen Reduction Catalyst in Alkaline Medium
Electrochimica Acta 370 (2021) 137709

A. Moumen, R. Konar, D. Zappa, E.Teblum, I.Perelshtein, G.D. Nessim*, and E. Comini*
Exfoliated 2D few-layered tungsten di-selenide (2H-WSe2) nanosheets for exceptionally sensitive and selective nitrogen dioxide (NO2) detection at room temperature
ACS Applied Materials and Interfaces (in press)

R. Konar, Rosy, I. Perelshtein, E. Teblum, M. Telkhozhayeva, M. Tkachev, J.J. Richter, E. Cattaruzza, A. Pietropolli Charmet, P. Stoppa, M. Noked*, G.D. Nessim*
Scalable synthesis of few-layered 2D tungsten di-selenide (2H-WSe2) nanosheets directly grown on tungsten (W) foil using ambient pressure chemical vapor deposition for reversible Li-ion storage
ACS Omega (inside cover) 5 (31), 19409-19421, July 2020

The importance of hexagonal boron nitride (BN) in the world of two-dimensional crystals (2D) is constantly increasing. This is due to the fact that it combines the properties of 2D materials, like a layered structure and possibility of exfoliation with those of classic nitrides – a wide bandgap and high chemical and physical resistance [1]. All of these aspects make it a perfect candidate for example for deep UV light source and detector, insulating barriers in van der Waals heterostructures or protecting layers for other, more sensitive materials [2]. Metal Organic Vapor Phase Epitaxy (MOVPE) is a very promising technique to obtain high quality boron nitride on the wafer-scale, which could facilitate the implementation of the before mentioned applications

The MOVPE growth of boron nitride requires careful selection of growth conditions – as this is a van der Waals material, effective synthesis is usually connected with a bad surface morphology and poor optical properties. Epitaxial material, oriented parallel to the sapphire substrate could be obtained basically with two different growth modes. The first mode is the Continuous Flow Growth (CFG) regime when the boron (TEB) and nitrogen (ammonia) precursors enter the reactor simultaneously, conducted at extremely high temperature (about 1300 °C) and with a large NH₃/TEB flow ratio resulting in thin, smooth BN layers [3]. The optical properties and surface morphology of such layers are satisfactory. Unfortunately the longer the growth, the more three-dimensional precipitates appear on the surface, thus the maximum thickness of a good quality layer, possible to obtain by this method is about 5 nm. The second growth regime is Flow-rate Modulated Epitaxy (FME) for which ammonia and TEB are delivered to the reactor in alternating pulses. This growth mode allows to obtain tens of nanometer thick BN but the supersaturation of boron in TEB pulses support the formation of structural defects, responsible for mid-gap luminescence [3].

The method we propose combines these two growth regimes by using CFG BN as a buffer layer for homoepitaxial growth of a FME layer on the top of it. This approach allows to obtain FME BN layers with much better optical and structural quality reaching a thickness of several tens of nanometers [4]. Moreover, this semi-homoepitaxial boron nitride exhibits high homogeneity on the entire two-inch surface. Further optimization of this method allows us to obtain better quality samples. The relationship between the growth parameters and the crystallographic structure, concentration of point defects and surface morphology of BN grown by this Combined Growth Mode (CGM) will be presented. Our results bring boron nitride significantly closer to large-scale applications.

[1] K.S. Novoselov, A. Mishchenko, A. Carvalho, A.H. Castro Neto “2D materials and van der Waals heterostructures”. Science 353.6298, aac9439 (2016)
[2] N. Izyumskaya, D.O. Demchenko, S. Das, Ü. Özgür, V. Avrutin, H. Morkoç. “Recent Development of Boron Nitride towards Electronic Applications”, Advanced Electronic Materials 3, 5:1600485 (2017)
[3] K. Pakula, A. Dabrowska, M. Tokarczyk, R. Bozek, J. Binder, G. Kowalski, A. Wysmolek, R. Stepniewski ”Fundamental mechanisms of hBN growth by MOVPE” arXiv: 1906.05319 (2019)
[4] A.K. Dabrowska, M. Tokarczyk, G. Kowalski, J. Binder, R. Bozek, J. Borysiuk, R. Stepniewski, A. Wysmolek, ”Two stage epitaxial growth of wafer-size multilayer h-BN by metal-organic vapor phase epitaxy – a homoepitaxial approach”, 2D Mater., 8, 015017 (2021)

Skin-mountable microelectronic devices (i.e., electronic skin, e-skin) are garnering substantial interest for various promising applications including human−machine interfaces, bio-integrated devices, and personalized medicines^1-3. However, it remains a critical challenge to develop reliable, durable, and biocompatible e-skins to mimic the human somatosensory system in a full working range⁴. Herein, we present a multifunctional e-skin system with a heterostructured configuration that couples a vinyl hybrid silica nanoparticles (VSNP) modified polyacrylamide hydrogel (VSNP-PAM) with two-dimensional (2D) MXene. The VSNP-PAM hydrogel is engineered to achieve both high toughness and low hysteresis, serving as an elastic substrate for the e-skin that overcomes the toughness-hysteresis correlation. Then, a nano-bridging layer of 1D polypyrrole (Ppy) nanowires is applied to dynamically link the interfaces between the hydrogel substrate and the sensing layer of 2D MXene. These material configurations endow the e-skin with tunable sensing mechanisms, exhibiting an extraordinary stretchable working range (0−2,800%), ultrafast responsiveness (~90 ms) and outstanding resilience (~240 ms), good linearity (0−800%) and excellent reproducibility (>5,000 cycles). In parallel, this e-skin is capable of detecting, quantifying and remotely monitoring of stretching motions in multi-dimensions, pressure exertions (40 Pa-30 kPa), tactile and proximity sensing (from a distance of ~20 cm), temperature, and light variations, providing a promising platform for next-generation smart flexible electronics with a ultrabroad working range.
References:
1. Wang, S. et al., Nature 555, 83 (2018)
2. Larson, C. et al., Science 351, 1071-1074 (2016)
3. Gao, W. et al., Nature 529, 509 (2016)
4. McEvoy, M. A. & Correll, N. et al., Science 347, 1261689 (2015)

We experimentally demonstrate near-unity absorption in monolayer WS₂ via a van der Waals heterostructure coupled to an optical cavity. To achieve this perfect absorption at the excitonic frequency, we match the WS₂ radiative and non-radiative excitonic decay rates, which impedance matches the exciton to free space. With the presence of an Ag back mirror, this impedance matching condition results in perfect absorption at the excitonic frequency. We show that this impedance matching condition can be readily achieved at cryogenic temperatures by lowering the non-radiative decay rate due to homogeneous broadening but can also be achieved at room temperature by increasing the radiative decay rate, via Purcell enhancement of the excitonic emitter. We further show theoretically that perfect absorption with multiple excitonic species (e.g. WS₂ and WSe₂) is possible, suggesting that solar cells comprised of multiple excitonic perfect absorbers can achieve detailed balance photovoltaic efficiencies surpassing the Shockley-Queisser Limit. We find that monolayer 2D perfect absorbers have potential for highly efficient van der Waals optoelectronic and photovoltaic devices.

A simple and facile method to produce a hybrid nano-assembly of 3-dimensionally (3D) structured and palladium nanoparticles (PdNPs) embedded graphitic carbo with high-performance polymers would be highly desirable. All existing approaches to synthesize hybrid materials comprising graphene and metal NPs involve complicated processes; carbon nanomaterials are mostly fabricated based on chemical vapor deposition or reduction of graphene oxides methods and PdNPs are further integrated on carbon nanomaterials through thermal annealing or chemical processes. Recently, a scalable and fast laser photothermal method to generate laser-induced graphene (LIG) with high electrical conductivity and large specific surface area from commercial polyimide films has been demonstrated and triggered a significant research development in various applications, including energy storage, harvesting, and detection. The method has been further utilized to fabricate LIG based hybrids including heteroatoms-doped LIG, LIG-metal-sulfides nanocomposite materials, as well as LIG-metal-oxides nanocomposite materials for high-performance energy storage, detection, and generation. However, the application of the laser photothermal method in the fabrication of porous graphene-PdNPs nanocomposite material has yet been demonstrated especially for the hydrogen leakage system hence achieving the hybrid materials remains a challenge due to the processing requirements of high thermal/chemical energy, and the lengthy period of time consumption. Here, we report a one-step, fast laser photothermal nano-assembly method to synchronously Pd-NPs and porous graphene from polymer films of mixed Pd and carbon precursors. We used the polymers of intrinsic microporosity (PIM-1) as a precursor to obtaining porous graphene, which has polybenzodioxane structure with cyano groups, high solubility in a volatile solvent, and miscibility with inorganic materials enabling to fabricate homogeneous polymer films with evenly and highly dispersed Pd-ligands. We demonstrated a scalable laser photothermal method to form the nano-assembly of 3D porous graphene and PdNPs from the polymer film for wireless and flexible hydrogen detection systems with the limit of detection (LOD) of 1 ppm hydrogen gas concentration. With the combined advantages of high sensitivity/selectivity/surface of 3D porous graphene and rapid response/recovery speed of PdNPs, we could obtain high sensitivity with the fast response time at low H₂ concentration ranges (1-50 ppm). Moreover, the hybrids exhibit outstanding flexibility and durability under bending and twisting and the nano-assembly is further integrated with a wireless communication system via BLE protocol to transfer the monitored signal of conductance to the change of hydrogen concentrations to a mobile device. The innovative sensing technology based on laser nanomanufacturing of the nano-assembly for highly sensitive, flexible, wireless sensing systems has future potential for broad applications in various industries and military fields.

Photodetectors capable of detecting light in a broad spectrum is central to diversified optoelectronic applications in spectroscopy, remote sensing, imaging, and optical communication. Two-dimensional (2D) materials, such as graphene, borophene, phosphorene, and transition metal dichalcogenides (TMDs), are good candidates due to their unique semiconductor and structural characteristics. Tellurene is an emerging 2D semiconductor boasting characteristics potentially relevant for photodetection.
However, the small bandgap of tellurene makes it more suitable to detect long-wavelength light. Here, we demonstrate a highly sensitive, fast-response photodetector based on tellurene-CdS quantum dots mixed-dimension heterostructures, which enables the efficient separation of the photogenerated electron-hole pairs and inhibits the rate of undesirable recombination. The heterostructured device exhibits a low dark current (0.4 nA) and good quantum efficiency. A systematic characterization of the device performance indicates that such mixed-dimensional heterostructured photodetector exhibits superior photodetection performance with a broadened spectrum response, improved response speed, sensitivity and responsivity.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have received increased attention in recent years for optoelectronics due to their strong light-matter interactions. This includes ultra-thin molybdenum disulfide (MoS₂), which is a suitable material for flexible, wearable, and high specific power photovoltaics (PV)¹. Obtaining high efficiency PV through doping of 2D materials has been a challenge; however, Schottky PV is an alternative route to overcoming this challenge. In this work, we present a Schottky-type photovoltaic device using 2D MoS₂ films and asymmetric contacts.
Chemical vapor deposition (CVD) is utilized to synthesize MoS₂ films on sapphire substrate, with MoO₃ and S as the precursors, carried by Ar. High-quality monolayer films with sparse bi-layer regions are obtained, and film uniformity, optical performance, and electronic quality are confirmed through transmittance and photoluminescence spatial mapping. Ti contact MoS₂ transistors and photodetectors were fabricated from which an external quantum efficiency (EQE) of 25% at 620 nm laser illumination and a carrier mobility of 3.3 cm²/V.s was obtained, indicating high quality CVD grown MoS₂.
For the Schottky PV device, Ti is selected as one of the asymmetric metal contacts due to its work-function of 4.33 eV, while Pt is selected as the second contact due to its relatively high work-function of 5.64 eV. These Schottky PV device contacts are fabricated on SiO₂/Si substrates as an asymmetric interlocking-finger type design onto which the MoS₂ films are transferred using a surface energy assisted transfer technique. The MoS₂/metal configuration maximizes the PV active area by eliminating contact shading losses and utilizes lateral transport to minimize electrical shunts between contacts. A band offset of -0.9 eV arises between the Fermi levels of Ti and MoS₂ at their interface while an offset of 0.41 eV arise at the Pt-MoS₂ interface. The resulting selectivity of Ti to electrons and of Pt to holes in the Ti-Pt asymmetric contact device creates the condition for photo-generated carrier separation.
The 300 µm X 400 µm active area Schottky PV shows a V_OC of 260 mV under 660 nm laser illumination with a 1.07 uA/cm² J_SC, while 125 mV V_OC was obtained under 1 sun AM1.5G illumination using a solar simulator with J_SC of 0.17 uA/cm²). Device modeling using COMSOL Multiphysics (Semiconductor Module) shows that a V_OC of 900 mV and 41 uA/cm² [EMD1] J_SC is attainable for a Pt-Ti asymmetric contact 2D PV device. The low photocurrent and poor J_SC obtained from the fabricated asymmetric contact device can partly be attributed to the small diffusion length of carriers in these CVD-grown films, especially for holes. Furthermore, relatively large sheet and contact resistance was identified in the device through transmission line measurement (TLM). However, the relatively high V_OC and absorption, the promising carrier mobility obtained from the symmetric contact devices, and the device modeling all indicate that efficient photovoltaic devices are attainable.
Current efforts towards improving PV device performance include contact engineering, film growth/thickness, and device architecture optimization guided through spatial mapping of device properties. Atomistic investigation of interfaces through density functional theory, PV device modeling, and contact selection supported through Schottky barrier height extraction and work function measurements will also support the realization of high efficiency 2D PV.

¹Bernardi, M., Palummo, M., & Grossman, J. C. (2013). Extraordinary sunlight absorption and one nanometer thick photovoltaics using two-dimensional monolayer materials. Nano letters, 13(8), 3664-3670.

The fundamental properties of two-dimensional (2D) materials, as well as the quality of 2D films and device interfaces, give rise to inherent limitations in their device performance. The ultrathin nature and large surface area of two-dimensional (2D) materials make them quite sensitive to their surrounding environments, which enables their optical and electrical properties to be tuned through surface modification. Developing processes to controllably dope transition-metal dichalcogenides (TMDs) and their electrical contact interfaces are critical to achieving commercial viability for optical and electrical applications. Previously, we successfully expanded the application of electron-transfer doping using molecular reductants and oxidants to various TMDs, including monolayer MoS₂, MoSe₂, WS₂, and WSe₂. The chemical doping is introduced onto the 2D material by exposing it to solutions of molecular reductant (n-dopant) or oxidant (p-dopant). Both n- and p-type transport properties were achieved in all four monolayer-TMDs, and both non-degenerate and degenerate regimes are accessible.¹ The degree of doping can be finely controlled by choice of dopants, treatment time, and the concentration of doping solution. Detailed physical characterization was conducted to study the doping effect and to understand the underlying mechanisms. MoS₂ is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of the monolayer (ML) MoS₂ devices. Our study showed that p-type MoS₂ can only be achieved by combining molecular doping with high-work function Pd contacts.² Molecular doped p-FETs show effective hole mobilities of 2.3 cm² V^-1 S^-1, an on/off ratio exceeding 10⁶, and improved contact resistance of ≈482 kΩ μm. Devices with low work function Ti metal contacts only showed ambipolar behavior. It was shown that the contact resistance and Schottky barrier heights can be effectively modulated through doping of the channel materials and by changing the contact metal. A relatively low hole injection barrier of ≈156 meV was obtained for p-doped MoS₂ with Pd contacts. A MoS₂ inverter based on pristine (n-type) and p-doped monolayer MoS₂ was fabricated, demonstrating the potential application of 2D-based complementary electronic devices. In summary, we have demonstrated a solution-based charge transfer doping on several 2D semiconductors. This technique provides a simple yet effective route to tailor the band structure of the 2D materials and control the resulting electrical and optical properties.
References
1. S. Zhang, H. M. Hill, K. Moudgil, C. A. Richter, A. R. Hight Walker, S. Barlow, S. R. Marder, C. A. Hacker and S. J. Pookpanratana, Adv. Mater., 2018, 30, 1802991.
2. S. Zhang, S. T. Le, C. A. Richter and C. A. Hacker, Appl. Phys. Lett., 2019, 115, 073106.

Platinum (Pt) is commonly used as a catalyst in fuel cell production. Platinum being an expensive metal, makes commercialization of fuel cells uneconomical. Many research groups are working on finding the alternative catalytic material. Graphene and carbon based nanomaterials have potential to replace Pt as catalyst in polymer electrolyte membrane fuel cell (PEMFC) due to their properties such as broad electrochemically active surface area, fast carrier mobility, high conductivity, and ability to stabilize and increase the durability of fuel cells. Proton conducting Polymer Electrolyte Membranes (PEMs) are used in PEMFCs to allow protons (H+) to pass through the electrodes. An effective PEM should be able to facilitate ion transfer in fuel cells. Nafion is one of the most widely used electrolytes in PEMs due to its high ionic conductivity and highly resistance to chemical reactions such as corrosion. However, water retention in Nafion membrane give rise to fouling effect, which reduces the ion transfer and performance of PEMFC. The amount of water retained also causes the polymer to swell, therefore, decreasing the efficiency of fuel cells. To overcome this drawback, hydrophobic Polycaprolactone (PCL) is being considered a potential electrolyte in this study. The objective of our research is to use Graphene Oxide as catalyst in electrodes of PEMFC and find alternative for Nafion membrane by using electrospun PCL nanofibers instead. The use of carbon based nanomaterials has delivered commendable performance making commercialization of fuel cell products possible in near future.

An efficient interfacial interaction between Prussian blue (PB) nanocrystals and exfoliated inorganic nanosheets (NSs) enables to synthesize strongly-coupled composite materials with remarkably improved electrode functionality for supercapacitor. The highly porous nanohybrids of PB-inorganic NS are synthesized by crystal growth of PB nanocrystals on the inorganic NSs. The coordinative bonding between cyanide ligand of PB and inorganic NS induces significant interfacial charge transfer and the increase of the metal oxidation state of PB. The obtained nanocomposites possess expanded surface area than does the pristine PB material, indicating the enhancement of porosity by the hybridization. Of prime importance is that the present nanocomposites show larger specific capacitances with better cyclability and better stability than does the MnO₂ free-PB material, underscoring the beneficial effect of incorporated inorganic NS on the electrode functionality of PB. The present study clearly demonstrates that the composite formation of PB nanocrystals with inorganic NSs can provide an effective way to explore high performance electrode for supercapacitor.

Since graphene, the growing family of 2-dimensional (2D) materials spanning transition-metal dichalcogenides (TMDs), metal-organic frameworks (MOFs), oxides, and organics, has led to the broader development of nanomaterials and creative use of nanotechnology. Often, it is this reduced dimensionality that enables enhanced structure-property relationships necessary for advancing the field. Wearable flexible electronics is one research area that has seen tremendous growth in recent decades and has benefited by advances in 2D nanomaterials. Considered a fundamental building block, transistors are ubiquitous in modern technology. We demonstrate a novel approach for utilizing the piezoelectric and semiconducting properties of 2D zinc-oxide (ZnO) nanosheets (NS), synthesized via Ionic Layer Epitaxy (ILE), as strain-gated flexible transistors. We examine the effects of strain and immobile piezoelectric surface charge on the semiconducting properties of ZnO-NSs. Given the polar structure of ZnO, we show that the accumulation of piezoelectric charge can change depending on the crystallographic orientation with the respect to the gated contacts of the flexible transistor. As a consequence, the Schottky barriers at the metal-semiconductor interface can be asymmetrically modulated through the application of strain.

Combinining lead-free high-k materials and two dimensional (2D) semiconductor has been attracted many attentions owing to need for environmental-friendly and low-voltage operating devices. Especially, low voltage switching ferroelectric memory devices are rare in report. Here, we have successfully fabricated low-voltage operating field effect transistor (FET) and ferroelectric switching memory transistor using 2D MoS₂ channel and high-k/ferroelectric dielectric Ba_xS_1-xrTiO₃ (BST). Ferroelectricity of BST can be controlled by changing composition ratio of Ba and Sr in the BST film. In this study, Ba_0.5Sr_0.5TiO₃ (50BST, x=0.5) and Ba_0.8Sr_0.2TiO₃ (80BST, x=0.8) have been used as dielectric materials for low-voltage operating FET and nonvolatile ferroelectric memory transistor, respectively. Mechanically exfoliated MoS₂ flakes were transferred on PMMA brush-treated 50BST and 80BST oxide. The 5 nm-thin PMMA brush was used for reduction of the surface roughness of BST. As a result, we have successfully demonstrated extremly low voltage operating MoS₂ FET, which uses only 0.5 V gate voltage for ON/OFF switching due to high-k 50BST. And using 80BST, MoS₂ based ferroelectric memory transistor has been well operated in a nonvolatile manner under ±3 V pulses. Since employed high-k dielectric and ferroelectric oxides are lead-free in particular, the approaches applying high-k BST gate oxide for 2D MoS₂ FET are not only novel but also practical towards future low voltage nanoelectronics and green technology.

Two-dimensional (2D) semiconductors have attracted tremendous interest as an atomically thin channel for the continued transistor scaling. However, despite many proof-of-concept demonstrations, the full potential of 2D transistors remains elusive. To this end, the fundamental merits and technological limits of 2D transistors need a critical assessment, reality check and objective projection. In this talk, I will briefly review the promises and the current status of 2D transistors, and highlight the widely used device parameters (e.g., carrier mobility, contact resistance) could be frequently misestimated or misinterpreted, and may not be the most reliable performance metrics for benchmarking 2D transistors. We suggest the saturation or on-state current density, especially in the short channel limit, could provide a more reliable measure for assessing the potential of diverse 2D semiconductors, and should be applied for cross-checking different studies, especially when milestone performance metrics are claimed. We next summarize the key technical challenges in optimizing the channel, contacts, dielectric and substrate interfaces and outline the potential pathways to push the limit of 2D transistors; and lastly conclude with a prospect on the critical technical targets, the key technological hurdles to enable the lab-to-fab transition, and the potential opportunities arising in these atomically thin semiconductors.

In this work, we present the unique ultrathin platinum-based two-dimensional dichalcogenide (Pt-TMDs) based electronic tattoos. These ultrathin electronic tattoos are fabricated utilizing large-scale CVD-grown platinum diselenide (PtSe₂) and platinum ditelluride (PtTe₂). The growth is performed at moderate temperatures, at about 400°C, allowing for direct coating onto temperature-sensitive polymers, such as Kapton. Confirming the quality of TMD materials, we show a thorough characterization of PtSe₂ and PtTe₂’s electrical and electrochemical properties along with advanced healthcare-related applications. First of all, both PtSe₂ and PtTe₂ tattoos were found to have lower sheet resistance and electrode-skin impedance compared to graphene-based tattoos. Among themselves, PtTe₂ tattoos were found to be the most attractive, with sheet resistance as low as 13 Ohm/sq and electrode-to-skin impedance averaging at 4.94±1.61 kΩ (@10 kHz), clearly outperforming the state-of-the-art gold tattoos and even medical-grade Ag/AgCl gel electrodes. We used these ultrathin tattoos for a plurality of human physiological vital signs monitoring, such as the heart and the brain's electrical activity, muscle contractions, eye movements, and temperature. Moreover, to showcase an advanced application of the Pt-TMDs for human-machine interfacing, we built a multi-tattoo system that can easily distinguish eye movement and identify sight direction.

Two-dimensional (2D) materials are considered as ground-breaking foundations for novel optoelectronic devices owing to their tunable optoelectronic features, which rely on band/strain/defect engineering. However, early studies have limitations of precise tuning of new chemical composition correlated with electronic and crystal structure, which provides the space for developing a new strategy. In this context, we fabricated the novel non-stoichiometric Bi_x-Bi_2-xS_3-y 2D-layered materials by simple e-beam lithography. Furthermore, an in-situ HRTEM study reveals that newly formed Bi metallic nanocrystals on the edge of Bi_2-xS_3-y 2D layer show structural dynamics behavior followed by surface and structural transformation. We observed the nucleation, coagulation and Ostwald ripening process in the metallic nanocrystals with respect to the irradiation time. This functional material shows unique electronic structures, which can be manipulated by the non-stoichiometric ratio (Fermi level changes: p-type → n-type → quasi metallic). X-ray photoelectron spectroscopy studies supported the results of in-situ TEM analysis. Also, complete optoelectronic characteristics of newly synthesized materials were studied. We propose the possible mechanism involving the Auger decay of sulfur and local bond-breaking phenomenon in Bi₂S₃ material by considering the specimen heating effect and radiolysis process. This in-situ study opens the new corridor to the materials researchers to design the novel 2D transition metal chalcogenides materials by the one-step route and tune their physicochemical properties precisely.

The development of anodes for lithium-ion batteries (LIBs) has garnered considerable interest. In particular, anodes based on liquid-phase-exfoliated two-dimensional transition metal dichalcogenide (TMD) nanosheets are studied extensively because their intrinsic capacity is higher than graphite. As most semiconducting TMDs possess low electrical conductivity and lithium-ion diffusivity, costly processes are needed—such as the addition of conductive fillers, chemically converted metallic phase transformation, and topological nano-fabrication—to decrease the ion diffusion length and increase the surface area. Here, we introduce a novel conductor-free TMD nanosheet anode with graft-polymer ionic channels that ensures high stability and rate capability of the LIB. The fluorinated polymer binder grafted with ionomers allows not only the efficient exfoliation of TMD nanosheets in the liquid phase to guarantee stable sheet-to-sheet separation but also provides self-assembled ionic channels through which lithium ions in the electrolyte readily arrive close to the surface of the nanosheets. Efficient electrochemical reduction of lithium ions occurs on the surface of our simple binary anode of MoS₂ nanosheets, self-assembled with graft binder ionic channels, resulting in a high-performance LIB with stability (90% retention rate after 1,000 cycles), rate capability (50% at 5 A/g), and high cell capacity (933.1 mAh/g at 0.1 A/g). Our TMD anodes that do not require additional processes for enhancing electric as well as ionic conductivity offer a novel strategy for developing high performance large-scale TMD-based LIBs.

The light-matter interaction in the 2D materials can greatly be enhanced by exploiting their flexural strength. These materials either be twisted^[1] or wrinkled^[2] in order to decrease the substrate interaction. This generates enhanced carrier mobility in the suspended material and strong light confinement due to multiple internal reflections. Spirally rolling of 2D material (eg. MoS₂, WS₂) strictly restricts the electronic motion in a uniaxial direction. This encapsulated structure after getting hybridized with high yield QDs form type-II band alignment at the heterostructure interface. The photo-generated excitons after getting dissociated at the QD/2D interfaces transferred to the conductive 2D channel. The subsequent presence of QDs as a strong photoabsorptive material and multiple reflections at the inner wall of the nanocavity increase the photoconductivity in the nanoscale regime. Nanoscroll (NS) stimulates coherent lasing action with an unprecedently low lasing threshold and enhances the photosensitivity almost 3000 fold^[1] compared to the nanosheet counterpart. This spirally rolled helical structure predominately generates polarized photosensitivity due to preferential excitons localization along the circumference of the scroll. Our approach of QD hybridized NS formation thus enables feasible ways to discover exotic physical phenomena and to develop novel high-performance flexible devices toward realistic applications.
References:
[1] R. Ghosh et al., Small, 2003944. DOI: 10.1002/smll.202003944.
[2] R. Ghosh et al., ACS Appl. Mater. Interfaces 2019, 11, 26518. DOI: 10.1021/acsami.9b08294.