Symposium NT4 : Emerging Non-Graphene 2D Materials

Electrons in two-dimensional crystals with a honeycomb lattice structure possess a valley degree of freedom (DOF) in addition to charge and spin. These systems are predicted to exhibit an anomalous Hall effect whose sign depends on the valley index. Here, we report the observation of this so-called valley Hall effect (VHE). Monolayer MoS₂ transistors are illuminated with circularly polarized light, which preferentially excites electrons into a specific valley, causing a finite anomalous Hall voltage whose sign is controlled by the helicity of the light. No anomalous Hall effect is observed in bilayer devices, which have crystal inversion symmetry. Our observation of the VHE opens up new possibilities for using the valley DOF as an information carrier in next-generation electronics and optoelectronics. Time permitting, I will also discuss the properties of other novel 2D materials under study in our laboratory.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as monolayer MoS₂ and WS₂ are promising for developing truly atomic-scale light emission devices. Despite perfect surface passivation and strong exciton binding energy, the luminescence efficiencies of substrate-supported monolayers, however, are grossly poor compared to their suspended counterparts. Here we demonstrate that the luminescence efficiency of these monolayers is strongly dependent on the type of substrates and can be improved by orders of magnitude through substrate engineering. The origins for the substrate effects mainly lie in doping the monolayers as well as affecting their exciton dynamics, including facilitating the non-radiative recombination and slowing down the radiative decay. Using proper substrates like mica and Teflon can minimize the adverse effect of doping, and their luminescence efficiency can be largely tuned at room temperatures. These results provide useful guidance for the rational design of high-performance 2D TMDC light emission devices.

Energy transfer between excitonic systems separated by a small barrier layer plays an important role in determining the exciton decay dynamics in such systems. The ability to manage the direction of energy transfer in an intelligently engineered structure is of fundamental interest in semiconductor optoelectrnic devices. A vertical stack of semiconducting 2D crystals is effectively a coupled quantum well system that should allow exciton energy transfer across a sub-nanometer van der Waals gap via near field interaction. Previous studies on hetero-bilayers of group 6 transition metal dichalcogenides (TMDs) suggest that the interlayer charge transfer processes dominate exciton decay dynamics over energy transfer. In this talk, I will discuss our experimental observation of fast interlayer energy transfer in MoSe₂/WS₂ hetero-bilayer using photoluminescence excitation (PLE) spectroscopy. We find that the energy transfer is Förster-type involving excitons in the WS₂ layer resonantly exciting free electron-hole pairs in the MoSe₂ layer. Our results indicate that energy transfer competes with interlayer charge transfer with efficiencies exceeding 80% despite the type-II band alignment.

Atomically-thin transition metal dichalcogenide (TMD) semiconductors have generated great interest recently due to their remarkable physical properties. In particular, reduced screening in 2D has been predicted to result in dramatically enhanced Coulomb interactions that should cause giant bandgap renormalization and excitonic effects in single-layer TMD semiconductors. Here we present direct experimental observation of extraordinarily high exciton binding energy and band structure renormalization in a single-layer of semiconducting TMD [1]. We have determined the binding energy of correlated electron-hole excitations in monolayer MoSe₂ grown via molecular beam epitaxy [2] on bilayer graphene (BLG) by using a combination of high-resolution scanning tunneling spectroscopy and photoluminescence spectroscopy. We have measured both the quasiparticle electronic bandgap and the optical transition energy of monolayer MoSe₂/BLG, thus enabling us to obtain an exciton binding energy of 0.55 eV for this system, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. In a more highly screened environment (on top of graphite), we find that single layers of MoSe₂ show a strong 51% reduction in the exciton binding energy and an 11% reduction in the quasiparticle electronic gap, without significantly changing the optical gap. We have corroborated these experimental findings through ab-initio GW and Bethe-Salpeter equation calculations, which show that the large exciton binding energy arises from enhanced Coulomb interactions that lead to blue-shifting of the quasiparticle bandgap. We have also studied the role of interlayer coupling and layer-dependent carrier screening on the electronic structure [3] of few layer MoSe₂. We find that the electronic quasiparticle bandgap decreases by nearly 1 eV when going from one layer to three. Our findings paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. These results are of fundamental importance for the design and evaluation of room-temperature electronic and optoelectronic nanodevices involving single layers of semiconducting TMDs as well as more complex layered heterostructures.

References:

[1] M. M. Ugeda, A. J. Bradley, et al., Nature Materials 13, 1091 (2014).
[2] Y. Zhang, M. M. Ugeda, et al., Submitted (2015).
[3] A. J. Bradley, M. M. Ugeda, et al., Nano Letters 15, 2594 (2015).

Two-dimensional (2D) semiconductors, such as transitional-metal-dichalcogenide (TMD) monolayers, have aroused great attention because of the underlying quasiparticle physics and the promising optoelectronic applications such as atomically-thin light-emitting diodes and photodetectors. Even though considerable progress has been achieved recently, intrinsic and extrinsic excitonic states of TMD monolayers are still under discussion and require direct experimental exploration. Here, we report observations of unconventional excitonic emission states of monolayer WS₂ and MoS₂ under electrical doping and photoexcitation at low temperature. Through electrical and optical injection of charge carriers, tunable excitonic emission has been realized due to interplay of various excitonic states, and the crucial binding energies of trions, biexcitons and bound excitons have been obtained. Particularly, the superlinear emission and the unusual linear polarization behaviour of its photon energy opposite to that of exciton emission have been clearly observed, which is assigned to the presence of biexciton states, evidencing the strong Coulomb interactions in such 2D semiconductors. Extrinsic emission bands in monolayer WS₂ and MoS₂ have also been revealed and attributed to specific localized states resulting from impurities and structural defects, where the exact origin of such states has been discussed according to our experimental observations and theoretical calculations. In view of applications, we have developed a highly practical strategy to electrically manipulate the diverse excitonic emission in atomically thin semiconductors, which opens up many possibilities for design and improvement of 2D TMD-based optoelectronic devices.

Energy transfer is a well-known phenomenon commonly observed in molecular complexes, biological systems, and semiconductor heterostructures. In van der Waals heterostructures of 2D semiconductors, Förster-type energy transfer (ET) is expected to be efficient due to the parallel orientation and sub-nanometer separation of excitons in the neighboring layers. Here, we report an experimental observation of fast interlayer ET in MoSe₂/WS₂ heterostructures using photoluminescence excitation spectroscopy. We find that the ET is Förster-type involving excitons in the WS₂ layer resonantly exciting higher-order excitons in the MoSe₂ layer. Our results indicate that ET competes with interlayer charge transfer with efficiency exceeding 60% despite the type-II band alignment. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering. We anticipate that the ability to manage energy via ET at the sub-nanometer length scales in carefully designed heterostructures will allow realization of novel optical and optoelectronic device concepts.

Atomically thin transition metal dichalcogenides (TMDCs) have attracted great interests as a new class of two-dimensional (2D) direct bandgap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we investigate the modulation of optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules, nickel Pc (NiPc) and magnesium Pc (MgPc). Through energetically favorable electron transfer from the photo-excited TMDCs to the MPc, the photoluminescence (PL) emission characteristics can be selectively and reversibly engineered. NiPc molecules, whose reduction potential is positioned below the conduction band minimum (CBM) of MoSe2 and WSe2 monolayers, but higher than that of MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2 as its reduction potential is situated below the CBM of WSe2, but above that of MoS2 and MoSe2. The quenched PL emission can be fully recovered when MPc molecules are removed from the TMDC surfaces. We also find that photocurrents from TMDCs, probed by photoconductive atomic force microscopy (PC-AFM), are promoted when MPcs quench the PL, further supporting the photoinduced charge transfer mechanism. Our results will benefit design strategies for 2D inorganic-organic optoelectronic devices and systems with tunable properties and improved performances.

Two dimensional (2D) monolayer transition metal dichalcogenide (ML-TMDC) semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although the tightly bound exciton states lie at the heart of the optoelectronic properties of ML-TMDCs, surprising voids in the understanding of the manifold of these states persists. Here, we move beyond ground state excitonics, unifying somewhat contradictory observations by combining PLE spectroscopy, hyperspectral imaging, and DFT calculations of ML-MoS2, revealing anomalous optoelectronic behavior. Signatures of the 1S and 2S states of the A and B excitons are resolved and a reduction in PL quantum yield with increasing excitation energy is observed, confirming previous PLE studies. In contrast to the previous works however, we also observe subtle features in the PLE spectrum that are assigned to the quasiparticle bandgap and resulting onset of interband transitions, which is supported by complimentary density functional theory (DFT) calculations. Additionally, an “anomalous” increase in the experimental PLE spectrum at energies >2.6 eV diverges from previous results and expectations for a 2D quantum well with substantial many-body effects. This feature corresponds nicely to the step-like feature observed in the previous PCE measurements and likely arises from efficient coupling of excitations created in the band-nesting region of the Brillouin zone to the photoemissive direct gap excitons at the K-point. We find that these high-energy excitations can enhance emission from lower-energy states, such as trions or biexcitons. From hyperspectral PLE imaging of the single flakes of ML-MoS2, we find that the Stokes shift between excitation and emission resonances, a common metric for disorder in conventional 2D quantum wells, is significantly more uniform than the exciton energies, implying internal relaxation processes remain substantially more ordered than the exciton energy. Finally, the spatially resolved PLE spectroscopy reveals that spatial dispersion of the anomalous excitation resonances is correlated to the A and B exciton states, suggesting that these high-energy states are similarly excitonic in nature.

Black phosphorous (BP) is known as a most stable allotrope of phosphorous with its unique orthorhombic structure. Recent studies have shown that BP is an elemental layered material with strong in-plane bonds and a weak van der Waals interlayer interaction that make it possible to obtain layered structures through mechanical cleavage. The energy band gap of BP is reported to be 0.33 eV in the bulk and around 1.8 eV for a monolayer, and unlike transition metal dichalcogenides (TMD) such as MoS₂ and WSe₂, the energy band gap of BP retains its direct band gap character when the layer thickness varies. Moreover, the carrier mobility of BP can theoretically be as high as 20000 cm²/Vs. These fascinating properties mean that layer-structured BP is a promising semiconducting channel material for future nanoscale electronic devices. However, there are several technological obstacles that must be overcome to enable the actual implementation and integration of BP devices. The mechanical cleavage with tape of BP from the bulk is not as easily realized as for graphene and TMD materials making it difficult to control the layer thickness of the exfoliated film. Furthermore BP exfoliated from the bulk is not environmentally stable because it reacts with oxygen and water molecules and forms a rough surface containing impurities which act as charge trapping centers and carrier mobility scattering centers. As a result, considerable degradation of transistor performances, including decreases in mobility and threshold voltage shifts have been observed.
In an attempt to resolve these two issues, we tested in this study a plasma etching process in which the thickness of the layered BP film can be controlled by modulating the plasma etching time at the thinning rate of 2 layers /min. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface and that the remaining few-layer BP films exhibit excellent morphology and crystalline structure, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with polymethyl methacrylate (PMMA) immediately after the plasma etching process to protect the prepared BP films from water and oxygen molecules in air. With these techniques, a high field-effect mobility was achieved, 1150 cm²/Vs, with an I_on/I_off ratio of ~10⁵ at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its I-V characteristics and thus to exhibit excellent environmental stability over several weeks.

Gallium chalcogenides are new class of 2D materials with the chemical formula GaX where X are chalcogen atoms. In the bulk form, GaSe and GaTe are direct bandgap semiconductors displaying strong photoluminescence (PL) but become indirect gap semiconductors in the monolayer form. The direct to indirect transition happens because the valence band renormalizes itself to a “W” shape around the Γ point so that the valence band maximum (VBM) shifts from the Γ point. However, our recent experiments show that for few-layered GaX, the PL comes back at low temperatures and increases by many orders of magnitude as down to 4 K. This finding indicates the 2D GaX can still behave like a direct bandgap semiconductor at low temperatures due to dramatically weakened exciton-phonon interactions. To further study the exciton complexes in these materials, power dependent PL is measured at 5 K. The result shows that PL intensity is well fitted according to I=AP^α, where α varies in terms of different excitonic processes. Interestingly, in 2D GaX α is found to be different from its bulk form as well as 2D MoS₂, indicating a novel exciton recombination process. Based on the abnormal thermal quenching and power dependence of the PL, a three particle recombination process involving one electron and two holes is proposed for 2D GaX.

By combining electrical measurements, scanning Kelvin probe microscopy, and numerical electrical simulations, we find significant current crowding effect on two dimensional black phosphorus field-effect transistors. This current crowding can lead to localized Joule heating close to the metal contacts, and it is consistent with the features of the device failure observed in this study. Importantly, we find that the commonly used transmission-line model, in general, significantly underestimates the extent of the current crowding by 1 – 2 orders of magnitude, compared to our results. In addition, our approach allows us to determine the black phosphorus layer resistance and the specific contact resistance, an important parameter for evaluating electrical contacts, whereas the simple relation for calculating the specific contact resistance is invalid due to the current crowding. We find that the layer resistance increases significantly when the layer thickness decreases towards about 5 – 6 nm and remains relatively constant for thicker layers. It turns out that the specific contact resistance of Titanium contacts on black phosphorus field-effect transistors is about 0.3 Ωcm² in average when the layer thickness is larger than 7 nm, and significantly increases when the layer thickness is below 7 nm. The specific contact resistance also increases when the back gate voltage sweeps from -9 V to 9 V. These findings, which are likely to be relevant in other 2D materials, suggest the need to take into account the current crowding effect in designing novel 2D devices.

Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS₂ using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods. The pump-probe techniques described here also allow for studies of small, thermally-induced structural changes under quasi-equilibrium conditions, difficult to resolve with static heating approaches, including direct measurements of thermal boundary resistances across buried interfaces.

Atom-thick 2D materials such as Graphene and monolayer Molybdenum Disulphide (MoS₂) make very sensitive biosensors and detectors of gaseous organic analytes. Every atom on the surface is exposed, which maximizes the material’s sensitivity to molecules in the external environment. Through an efficient reproducible growth process that yields a high density of monolayer single crystal MoS₂ flakes, we are able to manufacture thousands of FET devices with a yield of 95% and average mobility of 36cm²V^-1s^-1 at room temperature in vacuum. With this method we were able to successfully perform bio and vapor sensing on our samples.
For biosensing, atomic length nickel-mediated linker chemistry is used to enable target binding events close to the MoS₂ surface to maximize sensitivity. A computationally redesigned μ-opioid receptor (MOR) is linked to the surface of the MoS₂, from which we were able to read out the concentrations of a synthetic opioid peptide with high MOR specificity. The data indicates binding affinities that match the values determined using traditional techniques.
For vapor sensing, we exposed an array of MoS₂ FETs to various analytes. Huge responses, up to 94% increase in the electrical conductivity, were observed upon exposure to 33vol% saturated dimethyl methylphosphonate (DMMP) vapor. These large responses were caused by the analyte molecules interacting with either the MoS₂ flake or the contacts between gold electrodes and the MoS₂ flake. Using Transmission Line Measurements (TLM), we studied the resistance of the MoS₂ flake and the resistance of the gold-MoS₂ contact as we exposed the sample to different vapor concentrations. This study strives to determine whether the responses to the analytes are driven by the MoS₂ flake or gold-MoS₂ contacts.
The combination of scalable array fabrication and rapid, precise readout enabled by the MoS₂ transistor offers the prospect of a solid-state drug testing platform for rapid readout of the interactions between novel drugs and their intended protein targets. Additionally, the large responses exhibited by MoS₂ devices upon exposure to various vapor analytes indicate a highly sensitive surface, which could potentially enable detection of small traces of dangerous compounds in the environment.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have generated significant research excitement due to their unique thickness-dependent electronic and optical properties, particularly their semiconducting direct bandgap that leads to photoluminescence, making them attractive for applications in next-generation electronics, optics, and chemical and biological sensing. Chemical functionalization is crucial for altering and tuning the properties of nanostructured materials such as graphene and carbon nanotubes in these types of applications, but is in its relative infancy for the TMDCs. In this work, we demonstrate robust biofunctionalization of pristine MoS₂ monolayers and bilayers with the fluorescent proteins GFP and mCherry. The fluorescent properties of both proteins are maintained and are simultaneously localized to the MoS₂ surface as shown by confocal fluorescence microscopy. Atomic force microscopy shows a uniform coverage of protein molecules, and Raman and PL mapping show that MoS₂ also retains its optical properties. The biofunctionalization process here relies on Ni-NTA tethering of polyhistidine-tagged proteins, as well as direct covalent functionalization of the pristine MoS₂ basal plane via aryl diazonium salts, which we also show for the first time. Previous methods of covalent modification of MoS₂ require harsh treatments such as lithium intercalation, defect engineering, or phase engineering, which disrupt many of the useful semiconducting and photoluminescent properties of the TMDCs. Instead, we demonstrate that diazonium functionalization of the pristine MoS₂ surface maintains and even enhances the MoS₂ photoluminescence (PL) two-fold. We confirm C–S bond formation by x-ray photoelectron spectroscopy (XPS) and we conduct detailed spatial mapping of the optical emission and surface coverage of reacted sites of the MoS₂ at time points ranging from 2 s to 6 h to derive a kinetic model of the covalent functionalization, which we find can be modeled by surface-limited second order reaction kinetics.

Atomically thin transition metal dichalcogenides (TMDCs) are two-dimension (2D) materials. Their unique electrical and optical properties make them an ideal platform for both fundamental studies as well as practical applications. In many cases, the ability to manipulate the optical properties of TMDCs via external modulation is highly desirable.
We have studied the photoluminescence (PL) response of both monolayer and bilayer tungsten disulfide (WS₂) under lateral electric field.¹ We demonstrate that monolayer and bilayer WS₂ responded oppositely towards the lateral electric field. PL of monolayer WS₂ can be substantially quenched under lateral electric fields while the emission of bilayer WS₂ was enhanced. In both cases, the spectral shifts were negligible. Temperature dependent measurements showed distinctly different behavior from that of the electric field, indicating that joule heating was not the dominant mechanism. Direct ionization and impact ionization were ruled out by comparing the theoretical threshold electric field strength with the experimental value. Considering the multi-valley band structure of tungsten disulfide, we attribute the field induced PL variations to photo-excited electron transfer from one conduction band extremum to another, resulting in the modification of recombination pathways. Such a physical process can only be observed in 2D TMDCs due to their exceptionally large exciton binding energy and the small energy difference between two conduction band extrema.
Laser treatment can also tune the emission spectrum of monolayer tungsten disulfide. We show that strong laser irradiation (>30 kW/cm²) can introduce additional luminescent features at 77 K. Before laser irradiation, the PL spectrum of monolayer WS₂ under low excitation power (~3 W/cm²)was dominated by exciton, trion and localized states (LS). However, after laser treatment, the emission intensity of LS was quenched substantially while a new PL peak with emission energy between LS and trion emerged under low excitation power (~3 W/cm²). However, this behavior can only be observed in monolayer WS₂ aged in ambient conditions, and thus is associated with defects. We found that the resultant PL shape evolved with laser irradiation time. It was a reversible process where PL spectrum can reverse back to original shape if thermal cycle was applied.
[1] Z. He, Y. Sheng, Y. Rong, G. Lee, J. Li, J. Warner. ACS Nano 2015, 9, 2740-2748.

Monolayer WS₂ is a direct band gap semiconductor in the visible spectral region and as part of the monolayer transition metal dichalcogenides (TMDs) family it is opening up new avenues for electronic and optoelectronic devices. Creating alternating layers of two-dimensional (2D) materials forms vertical heterostructures with diverse electronic and optoelectronic properties that could revolutionize future ultrathin devices. These 2D heterostructures can be realized by either transfer or direct growth.

One important aspect for atomically thin TMDs is its flexibility, which is better suited for strain engineering compared with other bulk semiconductors. Monolayer WS₂ grown by chemical vapor deposition (CVD) can have inbuilt strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS₂ and can be exploited in a wide range of applications. It has been predicted that tunable photonic devices and solar cells with better energy band matching can be realized by building multilayer 2D heterostructures based on stained TMDs. However, building layer-by-layer vertical heterostructures containing a strained TMD layer has been challenging due to the standard aqueous based transfer processes washing off the TMDs from the growth substrate. It is important to develop a non-aqueous transfer approach that allows other 2D crystals to be assembled on top of the strained TMDs to build multi-layered vertical heterostructures.

We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS₂, with Boron Nitride and Graphene. The 2D materials are all grown by CVD enabling large area vertical heterostructures to be formed. WS₂ monolayers grown by CVD directly on Si substrates with SiO₂ surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS₂. We show that stronger interactions between graphene and monolayer WS₂ occur when using non-aqueous based transfer, associated with sharper atomic interfaces from less contamination. This transfer method is suitable for layer-by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.

Hexagonal boron nitride (hBN) is a dielectric insulator with a two-dimensional layered structure, having a wide band gap energy of 5.6–6.0 eV. Due to its novel properties, which include a high mechanical strength, electric resistivity, thermal conductivity, and chemical inertness, as well as flexibility and transparency, hBN can provide many advantages to next-generation electronics and optoelectronics such as flexible and transparent transistors or transferrable light-emitting devices. While hBN layers can be prepared by mechanical cleavage methods from bulk crystals, their limited sizes and random thicknesses and orientations may not be suitable for many device applications. On the other hand, large-area hBN layers can be prepared by chemical vapor deposition (CVD); however, they usually exhibit polycrystalline structures with a typical average grain size of several microns. Grain boundaries or dislocations in hBN can affect its electronic or mechanical properties; several theoretical calculations have discussed the lower band gap energy and poorer mechanical strength of polycrystalline hBN. Accordingly, large-area single-crystal hBN layers are required to fully realize the potential merits of hBN. Here, we report the synthesis of single-crystal hBN (SC-hBN) layers by CVD.
The SC-hBN was grown on a 20 × 20 mm² Ni (111) substrate from a single ammonia-borane precursor by atmospheric pressure chemical vapor deposition (APCVD). A grown layer was transferred to arbitrary substrates by an electrochemical delamination technique, and the remaining Ni (111) substrate was repeatedly re-used. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV-Vis-IR spectroscopy confirmed that the grown films exhibited the characteristics of hexagonal boron nitride layers. Microstructural analysis by transmission electron microscopy (TEM) revealed that the grown layers were single-crystalline over their entire areas, presumably due to heteroepitaxial growth on the Ni (111) substrate. Atomic force microscopy (AFM) and electron backscatter diffraction (EBSD) studies on the Ni (111) substrate proved that there was no degradation after repeated growth and transfer cycles. The effect of growth parameters on the crystal structure of the grown hBN layers were investigated using reflection high-energy electron diffraction (RHEED).

The emerging two-dimensional (2D) layered materials are promising candidates for high-performance and energy efficient, flexible transparent electronics. Graphene is widely studied for transistors, sensors, and optoelectronic applications but is limited because of its lack of band gap. Beyond graphene, transition metal dichalcogenides (TMDCs) such as MoS₂ and WSe₂ are attractive because alloying, thickness, or strain engineering can be used to tune their band gap. Here, piezoresistive strain sensing with transparent and flexible atomically thin molybdenum disulfide (MoS₂) field effect transistors (FETs) made from highly uniform large-area synthesized films is demonstrated. The change in the MoS₂ current-voltage characteristics with mechanical strain is observed to be consistent with a change in band gap as measured using optical reflection spectroscopy. The magnitude of the band gap change agrees with previous theoretical and spectroscopic studies. In addition, the gauge factor or the sensitivity of the MoS₂ strain sensor is found to be adjustable via gate biasing, which changes the Fermi level of the MoS₂. The gate-tunable piezoresistivity is useful to adjust the sensitivity in practical sensing applications. The results show that MoS₂ can lead to a new generation of transparent and flexible strain and force sensors.

Transition metal dichalcogenides such as MoSe₂ have attracted great attention due to their desirable properties for electronic and optoelectronic applications. MoSe₂ field-effect transistors (FETs) showed promising device characteristics such as high on/off ratio (10⁶) and field-effect mobility (50-200 cm²/Vs). However, strategies to optimize device performance have not been established yet. In this presentation, we explore the possibility of improving MoSe₂ FET performance using atomic-layer-deposited Al₂O₃ passivation layers on MoSe₂ channels. By comparing the effect of Al₂O₃ layers on the device characteristics of bottom-gate multilayer MoSe₂ FETs, we observe the consistent enhancement of field-effect mobility and current-voltage hysteresis. These results suggest that the deposition of Al₂O₃ passivation layers on top of MoSe₂ channel can be an efficient method of optimizing the device performance of MoSe₂ FETs.

The fabrication of a top-gated transition metal dichalcogenide (TMD) FET requires a uniform and pinhole free gate dielectric. However the direct deposition of gate dielectrics on TMD materials is challenging due to the absence of surface dangling bonds. To deposit dielectrics with low leakage and low equivalent oxide thickness (EOT) on WSe₂, a flat-lying titanyl phthalocyanine (TiOPc) monolayer, deposited via molecular beam epitaxy (MBE), was used as a seed layer for atomic layer deposition (ALD) on the WSe₂ surface. ALD pulses of trimethyl aluminum (TMA) and H₂O resulted in the uniform deposition of Al₂O₃ on the TiOPc/WSe₂, confirmed by AFM and cross-sectional TEM. After depositing 50 cycles of Al₂O₃ onto the TiOPc/multilayer WSe₂, a top-gated FET was fabricated. The top gate leakage current was measured as 0.046 pA/µm2 at 1 V gate bias with 2.2 nm EOT, which is lower leakage current than all previous results on TMD with similar EOT. A subthreshold swing of 80 mV/dec was measured. While the TiOPc/WSe₂ was deposited by MBE, other phthalocyanines can be readily deposited from solution, making the functionalization technique more accessible. For example, to make a Cobalt Crown ether Phthalocyanine (CoCrPc) monolayer on WSe₂, 3 uM of CoCrPc/benzene-ethanol (9:1 v/v) solution was prepared. Two droplets of the solution were deposited onto a freshly cleaved, bulk WSe₂ substrate. After the solvent was completely evaporated, the sample was annealed at 200 C for 30 min in a N₂ filled glove box. An Al₂O₃ layer was deposited by 50 cycles of ALD using TMA and water at 120 C. The surface of the Al₂O₃ film was analyzed with ambient Atomic Force Microscopy (AFM), showing that the film is dense and uniform and has no pinholes detectable by AFM.

Two-dimensional (2D) materials have attracted a great attention since the rise of graphene. Recently, black phosphorus (BP), its monolayer known as phosphorene, has been highlighted as a promising 2D material that has properties comparable to that of graphene and other 2D materials. Beside its excellent electronic properties, BP has an advantage over graphene in the presence of direct bandgap. As each layer of phosphorene is held to each other by van der Waals interaction, BP flakes of high crystallinity can be easily transferred using mechanical exfoliation method. However, preparing BP film with desired thickness by mechanical exfoliation is still an issue. Fine control of BP thickness is essential for the fabrication of BP-based electronic devices because the characteristics of BP, such as bandgap, is highly sensitive to its thickness. In our experiments, we employed UV-ozone treatment on exfoliated BP flake to control its thickness by etching each BP layer. BP flakes were prepared on SiO₂/Si substrates using mechanical exfoliation method. The in situ optical characteristics of BP flakes were observed with Raman spectroscopy as UV-illumination time increased. Likewise, the thickness-dependent electrical properties of BP-based field-effect transistors were studied in situ. We investigated the changes in various electrical properties, including mobility and on/off ratio, as the BP thickness varied. The obtained results would be reproducible and controllable using our suggested method. Further results and discussion will be presented in detail.

Composites offer a facile means to tailor the properties of hybrid, dissimilar material systems for applications ranging from optoelectronics to strain sensors. Two-dimensional (2D) layered materials have attracted significant interest in recent years with the isolation of graphene through the mechanical exfoliation of graphite more than a decade ago. The 2D materials family is also rich with diverse compositions that offer semiconducting characteristics. In this work, we have explored the prospects of MoS₂ and WS₂, both of which are semiconducting 2D materials, for potential composite applications. In order to form 2D materials composites we have to first disperse them chemically in solution. MoS₂ and WS₂ powders were oversaturated in N-Methyl-2-pyrrolidone (NMP) solution at 37.5 mg/mL and sonicated at room temperature (RT) for sonication times ranging from 30 minutes to close to 24 hours. After solution processing, the samples with the 2D flakes were transferred to an Isopropyl Alcohol (IPA) bath for particle size distribution analysis.
We have observed significant changes in particle size distribution spanning two orders of magnitude as a function of the sonication conditions. Specifically, the observed changes in particle size distribution for MoS₂ and WS₂ powders ranged from 44 microns down to 0.409 microns, and 148 microns down to 0.409, respectively, as compared to the untreated materials. Structural analysis was conducted using the SEM and X-Ray diffraction. The structural analysis using the SEM revealed morphological signatures between the two materials, where the MoS₂ flakes had a randomly oriented distribution with occasional triangular flakes. In the case of the WS_2, regardless of the sonication conditions, the flakes seemed to have a characteristic 120° angular distribution at the vertices, representing a rhombus with concave edges. The XRD analysis showed a minute shift in the characteristic peaks that maybe due to strain-induced effects as a result of the solution processing. Optical characterization of the materials was also conducted using Raman Spectroscopy to validate the average layer number resulting from the solution dispersions and the spatial and compositional uniformity of the two material samples. After the material characterization studies on the parent MoS₂ and WS₂ were conducted, we integrated these materials with a polymer matrix to form a hybrid composite system. The optical, mechanical and electrical properties of the 2D-polymer composites were investigated for their potential utility in optoelectronic and strain sensing devices.

Surface migration of foreign atoms is expected to be promoted for 2D materials, which are virtually all-surface materials. An accompanying issue is chemical reactivity of these migrating atoms, which can dramatically alter the atomic structure and properties of 2D materials. Unfortunately, very little is known about these phenomena and the role of temperature and current density. In this study, we examine freestanding WSe₂-graphene heterostructure as well as multi-layer graphene for the effect of heat and charge flow by performing insitu experiments inside the transmission electron microscope. We observe selenium decompose and then silicon atoms migrate from the silicon micromachined test structure to the freestanding specimen. At high temperatures, this initiate rapid degradation through the formation of tungsten disilicide and silicon carbide formation. The role of current flow is to enhance the migration of silicon from the sample holder and to knock-out the selenium atoms. Experimental results show that purely thermal loading requires about 150 C higher temperature than that under combined electrical and thermal loading. We also present evidence of moderate current density, when accompanied with high temperature, promoting migration of foreign atoms on the surface of multi-layer graphene. Our in situ transmission electron microscope experiments show migration of silicon atoms at temperatures above 800 C and current density around 4.2x10⁷ A/cm². The silicon atoms are observed to react with the carbon atoms in the multi-layer graphene to produce silicon carbide at 900-1000 C temperature. In absence of electrical current, there is no migration of silicon and only pyrolysis

In order to realize the potential applications of layered TMDs, TMD materials have to be exposed to ambient air for device fabrication and characterization. Therefore, it is critical to understand the effect of air exposure on layered TMD materials. The effect of air exposure on MBE 2H-WSe₂/HOPG was elucidated on the atomic scale via scanning tunneling microscopy and spectroscopy. The 2H-WSe₂ was grown by molecular beam epitaxy (MBE) on HOPG. The WSe₂ nucleation is initiated at both the step edges and the terraces of HOPG, consistent with mixing of step flow growth mode and Volmer-Weber growth mode. High resolution STM images revealed nearly zero vacancy defects or dislocations in WSe₂ and the band gap of a WSe₂ monolayer was determined to be 2.18 ± 0.03 eV by STS. After exposure of air to WSe₂/HOPG, only the edges of the WSe₂ layer were oxidized, while the terraces of the WSe₂ layer were nearly intact. ; Iin the air exposed WSe₂ terraces, zero air induced defects were observed and the band gap was nearly identical to that of as-decapped WSe₂ ML (2.07 ± 0.05 eV by STS). Conversely, exposure of air to WSe₂/HOPG induced topological transitions from smooth edges of WSe₂ to bright protrusions along the edges of WSe₂ and a significant decrease in tunneling current at the edges of WSe₂ ML compared to the edge of as-decapped WSe₂ ML. It is hypothesized that the oxidation of the WSe₂ edges originates from the dangling bonds on the atoms at the WSe₂ edges. ; Iin high resolution STM images of as-decapped WSe₂, a bright brim was observed along the WSe₂ edge, originating from excess charge states of dangling bonds at the WSe₂ edge. In the STS of the as-decapped WSe₂ edge, a very narrow band gap was observed, as well as a shifted Fermi level towards the valence bands. However, this electronic edge state is passivated by the oxidation of edges in ambient air. The data is consistent with oxidation passivating defect states at the edges of WSe₂ which will be beneficial for device fabricating.Consequently, the present study can serve as a milestone to develop processes for device fabrication based on TMD materials, and elucidate the effects of the presence of electronic edges states on device performance.

One of the key challenges in the controlled chemical vapor deposition (CVD) of two-dimensional (2D) tungsten dichalcogenide crystals is achieving a steady flow of tungsten source in the vapor phase. Due to the high sublimation point of tungsten oxide precursors, CVD of tungsten dichalcogenides can be achieved only at high temperatures (>1000 ^oC) or at low pressures. We demonstrate atmospheric pressure CVD of WSe₂ and WS₂ monolayers at moderate temperatures (700 ~ 850 ^oC) using alkali metal halides (MX where M= Na or K and X=Cl, Br or I) as the growth promoters. Chemical analysis of the precursor after a growth run reveals that alkali metal halides react with tungsten oxide at elevated temperatures to form volatile tungsten oxyhalide species during growth, which leads to efficient delivery of the precursor to the growth substrates. The monolayer crystals obtained in this method were found to be highly symmetric, large in size (up to 200 μm), and free of unintentional doping due to alkali metal and halogen atoms. The monolayer crystals also exhibited excellent field-effect transistor (FET) performances with tunable polarity, large current on/off ratios (~10⁷), and high field-effect mobilities (electron and hole mobilities of 102 and 26 cm² V^-1 s^-1 for WSe₂ and electron mobility of ~14 cm² V^-1 s^-1 for WS₂ devices), further indicating their high quality.

Reference：Li, S. et al., Applied Materials Today 1 (2015) 60–66

Two-dimensional transition metal dichalcogenides (TMDC) have recently been under intense investigation due to their direct bandgaps and potential uses in high speed electronic devices. Despite being the most thoroughly studied TMDC, the mechanisms dominating the carrier dynamics in monolayer MoS₂ remain poorly understood. Much work to date has been performed on multilayer films, which have an indirect bandgap and reduced Coulombic interactions as compared to monolayer MoS₂. Other studies have focused on monolayer exfoliated flakes, which are not scalable for industrial applications.

In this work, we present transient optical absorption and time-resolved terahertz spectroscopy (TRTS) measurements of large area (~ 1 cm²) monolayer films of MoS₂ deposited on dielectric substrates via chemical vapor deposition. We report excited state dynamics that are surprisingly similar to those previously reported for multilayer MoS₂ in that they can be described by two decay components: one operative on sub-picosecond times scales and the other in 10's of picoseconds. Combining these two techniques allow for the contributions to the two-component excited state dynamics to be identified.

Terahertz conductivity measurements reveal that charge carriers, and not excitons, are responsible for the sub-picosecond dynamics. The frequency-dependent dielectric function is characteristic of localized charges in a polycrystalline material with sub-micron grains. Sub-picosecond lifetime photo-induced conductivity suggests that charges are rapidly trapped. This process complicates the excited state spectrum, which shows changes in line-width and spectral shifts associated with bandgap renormalization in addition to simple state-filling effects. We also report conductivity originating from sub-optical gap excitation, which reveals the existence of mid-gap trap states that may originate from surface defects or grain boundaries. These dynamics are largely insensitive to excitation density, wavelength, temperature and choice of substrate. We assign the slow excited state dynamics to the interband exciton lifetime, which lengthens from 50ps in monolayer films to 150ps in multilayer films due to the transition from direct to indirect bandgap. We see no signature from trions, suggesting that surface adsorbates neutralize the intrinsic n-type behavior.

Our results help add clarity to several conflicting reports of the optoelectronic properties of MoS₂, providing explanations for the sub-picosecond excited state dynamics as well as the absence of trions. Our findings suggest that trapping dominates the transient optical properties of monolayer MoS₂. They also suggest that CVD grown films hold potential for high-speed optoelectronic applications.

Black phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus with several unique properties that has recently been exfoliated to atomically thin flakes. On the downside, BP atomic layers are observed to irreversibly degrade into phosphorus oxoacids upon exposure to humid air. However, in more robust configurations such as films, composites, and embedded structures, BP can potentially be utilized in many applications. In this study, we fabricated films of stacked black phosphorus nanoflakes made by liquid exfoliation and explored their chemical sensing characteristics upon exposure to different families of chemicals such as ketons, benzenes, alcohols, and water vapor. Interestingly, an ultra-sensitive and selective response was observed toward water vapor which provided for a trace-level moisture detection capability. The drain current of the sensors is observed to modulate by nearly 4 orders of magnitude when the humidity level changes from 10% to 85%. Evidenced by our spectroscopic and control experiments, we propose that the operation principle of the BP film sensors is based on a change in the leakage ionic current caused by auto-ionization of water molecules and ionic solvation of the phosphorus oxoacids produced on moist BP surfaces. Concurrently, our stability tests reveal that the response of the BP film sensors remain quite stable after prolonged exposures to ambient conditions (tested up to 3 months). This study opens up the route for utilizing films of BP stacked flakes in many potential sensing applications as well as energy conversion devices.

After discovery of graphene, there have been increasing interest in other two-dimensional (2D) materials that possess unique electrical and optical properties. The characteristics of 2D materials include both layered structures and heterostructures of transition metal dichalcogenides, silicone, germanene, and black phosphorous (BP). Among them BP is considered to be an interesting material due to its intrinsic band gap nature, strong in-plane anisotropy, and high electron mobility. These properties make BP as a promising material for applications in electronics, optoelectronics and clean energy. However, there are two main issue with BP, i.e., method of synthesis and degradation under moisture. In the present study, we report the facile synthesis method of few layer BP flakes by treating red phosphorous and further utilized BP flakes to make hetero-structure on single layer graphene. The structural and chemical properties of BP-graphene hetero-structure were evaluated using Raman spectroscopy. The electrical performance of heterojunctions will be discussed.

The structural, electronic and magnetic properties of MoS₂ single layer doped with 4f rare earth metal (RE) atoms Sm, Eu, Gd, Tb and Dy in the form of Mo₁₁S₂₄RE supercells were investigated using Density Functional Theory implemented within ADF-BAND. Our results with Pedrew Burke Ernzerhof (PBE) exchange correlational functional used within generalized gradient approximation (GGA) showed that variation of bond length is maximum around impurity atom in 2D Eu:MoS₂ and they are minimum near Sm impurity. The results indicated the appearance of impurity levels in the band gap of MoS₂ that lead to band gap narrowing. Analyzing the difference of spin charge densities, the magnetic moment of 3.3 µ_B, 8.1 µ_B, 8.5 µ_B, 6.8 µ_B and 6.4 µ_B was seen for Sm-, Eu-, Gd-, Tb- and Dy-doped structures, respectively. The findings of the work point out that the band gap and magnetic properties of 2D MoS₂ structures can be tuned by RE 4f doping to make it p-type degenerate semiconductor for device grade applications.

Graphene being a zero band-gap semimetal [1] intrinsically possesses very high mobility [1,2] but with very low on/off ratio [2], whereas two-dimensional MoS₂ being a direct band-gap semiconductor of around 1.9 eV [3] has low mobility [4,5], but can reach much higher on/off ratio [5]. Therefore, they are expected to complement each other in graphene/MoS₂ heterostructures with high enough mobility and reasonable on/off ratio [6]. Here we present a simple device model based on the Dirac equation for carriers in graphene and effective-mass model for carriers in MoS₂ to explain the experimental results for IV curves of graphene/MoS₂ heterostructures theoretically. The carrier transfer from graphene to MoS₂ is described by the relaxation process involving electron-phonon scattering. The IV curves are simulated according to the transport equations in semiconductors with the use of quasi-equilibrium distribution of carriers in leads. The dependence of transport characteristics on temperature, bias, and gate-voltage with and without optical pumping is simulated. The theoretical results are compared with available experimental data and good agreement is obtained with the use of a few physical parameters, which all fall into reasonable range.
[1] M. I. Katsnelson and K. S. Novoselov, Graphene: new bridge between condensed matter physics and quantum electrodynamics, Solid State Commun. 143, 3 (2007).
[2] J.-H. Chen et al., Intrinsic and extrinsic performance limits of graphene devices on SiO₂, Nat. Nanotechnol. 3, 206 (2008); F. Chen et al., Dielectric Screening Enhanced Performance in Graphene FET, Nano Lett. 9, 2571 (2009).
[3] K. F. Mak et al., Atomically thin MoS₂: a new direct-gap semiconductor, Phys. Rev. Lett. 105, 136805 (2010); A. Splendiani et al., Emerging photoluminescence in monolayer MoS2, Nano Lett. 10, 1271 (2010).
[4] B. Radisavljevic et al., Single-layer MoS₂ transistors, Nat. Nanotechnol, 6, 147 (2011); Y. Zhang et al., Ambipolar MoS₂ Thin Flake Transistors, Nano Lett. 12 (3), 1136 (2012).
[5] B. Radisavljevic and A. Kis, Mobility engineering and a metal-insulator transition in monolayer MoS₂, Nat. Mater. 12, 815 (2013); K.-K. Liu et al., Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates, Nano Lett. 12, 1538 (2012); C. R. Wu et al., Multilayer MoS₂ Prepared by One-time and Repeated Chemical Vapor Depositions: Anomalous Raman Shifts and Transistors with High ON/OFF Ratio, J. of Phys. D: Appl. Phy. 48 (43) 435101 (2015).
[6] H. Tian et al., Novel field-effect Schottky barrier transistors based on graphene-MoS₂ heterojunctions, Sci. Rep., 4, 5951 (2014); M. Y. Lin et al., Toward Epitaxially Grown Two-Dimensional Crystal Hetero-Structures: Single and Double MoS₂/Graphene Hetero-Structures by Chemical Vapor Depositions, Appl. Phys. Lett. 105 (29), 073501 (2014).

Piezoelectric properties of transition metal dichalcogenide monolayers are important properties for the application of this kind of 2D material in various fields and nonlinear piezoelectric coefficients are vital since large strains are often observed for 2D materials. In this works, first principle’s calculation in the frameworks of density functional theory were performed to investigate the piezoelectric properties of TMX₂ (TM=Mo,W, X=S,Se,Te) monolayers. Polarization of the 1H phases of TMX₂ were obtained for a series of strains from -5% to 5% and a nonlinear relationship was found. Piezoelectric coefficients were obtained by fitting the simulated data and nonlinear terms of piezoelectric coefficients must be included to fit the simulated polarization results. The piezoelectric coefficients e₁₁ are 370.5, 410.8 and 537.0 pC/m for MoS₂, MoSe₂ and MoTe₂, and 267.7, 309.7 and 407.9 pC/m for WS₂, WSe₂ and WTe₂, respectively. The nonlinear terms e₁₁₁ and e₁₂₂ are -817 and -509 pC/m for MoS₂, -813 and -367 pC/m for MoSe₂, -1206 and -540 pC/m for MoTe₂, respectively. The coefficient e₁₁ and e₁₂ have almost the same value but opposite sign, but e₁₁₁ and e₁₂₂ are independent.

Fabrication of electrical and opto-electronic devices with vertically stacked transition metal dichalcogenides (TMDCs), leads to interfaces that are misoriented. Prior experimental and theoretical studies of misorientation in graphene bilayers demonstrated that a few degrees of misorientation is sufficient to decouple the low energy states of the individual layers. Experimental and ab-initio calculations have shown the bandgap of misoriented bilayer MoS₂ remains indirect. Some specific trends of the interlayer distance and band gaps have also been researched. However, the transport properties across the misoriented interface of the bilayer TMDCs are currently unknown. The coherent interlayer transmission across two stacks of MoS₂ are calculated for unrotated and rotated MoS₂ bilayers using ab-initio calculations and non-equilibrium Greens functions. The energy dependence of the interlayer transmission is analyzed. The effect of rotation angle on the electron and hole interlayer resistance is determined.

Two-dimensional (2D) heterostructures hold the promise for future atomically-thin electronics and optoelectronics due to their diverse functionalities. While heterostructures consisting of different transition metal dichalcogenide monolayers with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) or edge epitaxy, constructing heterostructures from monolayers of layered semiconductors with large lattice misfits still remains challenging. Here we report a two-step chemical vapor deposition (CVD) synthesis to grow vertically stacked GaSe/MoSe₂ misfit bilayer heterostructures with well-aligned lattice orientation in an incommensurate superlattice exhibiting Moiré patterns in atomically-resolved scanning transmission electron microscopy images. In addition, we show that the growth of lateral heterostructures results in interesting laterally-striped, vertically-stacked bilayer heterojunctions where GaSe and MoSe₂ domains meet. We report that these misfit GaSe/MoSe₂ heterobilayers form p−n junctions which exhibit effective transport and separation of photo-generated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These new 2D misfit layered heterostructures, characterized by their Moiré superlattices, open the door to new two-dimensional “building blocks” for not only optoelectronic applications, but a wide range of other physical phenomena ranging from interfacial magnetism, superconductivity, and Hofstadter’s butterfly effect.

Synthesis science sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Materials characterization conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. X.L. and M.L. acknowledge support from ORNL Laboratory Directed Research and Development. J.L. and S.T.P. were supported by Department of Energy grant DE-FG02-09ER46554. W.Z. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.

The M₂AX phases, a family of layered ceramic compounds, have recently been chemically exfoliated to form two-dimensional transition metal carbides, known simply as M₂Xenes. In this work, 10,530 bulk compounds belonging to this large class of layered ceramics are screened for their thermodynamic stability using a high throughput first-principles scheme. The objective is to determine which of these bulk compounds are theoretically available as precursors for the formation of new M₂Xenes. For the 48 experimentally synthesized M₂AX phases we predict energetic stability or small positive (< 30 meV/atom) energetic metastability against energy competing phases. In addition to benchmarking the bulk formation energy of the 48 existing M₂AX phases, we predict 267 new bulk compounds with exothermic stability in excess of 100 meV/atom with respect to their most stable competing phases. These stable M₂AX precursors can then be chemically exfoliated by aqueous HF to form two-dimensional M₂XO₂ (M₂Xene) nanosheets. We generate and overlay Pourbaix diagrams for the bulk MAX phase precursors and the 2D M₂Xenes to determine overlapping solution conditions which will simultaneously result in etching of the M₂AX phase and passivation of the M₂Xene. These Pourbaix diagrams affirm the solution conditions already used to produce all 5 existing M₂Xene nanosheets, and predict that only Al-based M₂AX phases can be etched with HF to form M₂Xenes. Our predictions offer insight into the experimental conditions that are most likely to result in successful synthesis of both new and existing compounds; for example, the application of a suitable negative electrochemical potential during synthesis appears to be one way to stabilize most M₂Xenes against dissolution. These results are expected to serve as a map for experimentalists aiming to design new MXene compositions from scratch.

We report the observation of the local band structure of topological surface states in Bi_1.5Sb_0.5Te_1.7Se_1.3 using scanning tunneling microscopy/spectroscopy (STM/STS). The energy-momentum dispersion relation is locally deduced by extracting the Landau level (LL) energies, which are formed in a high magnetic field, from the STS data. Spatial variation of LLs revealed a shift of the Dirac point energy and a change in Fermi velocity at the nanometer scale. The structure of the potential fluctuation was not correlated with the topography, which indicated the Te/Se substitution did not induce the potential shift because of their same valence. The results show that composition control in Bi_2-xSb_xTe_3-ySe_y is an efficient method for doping topological surface states without the creation of additional charge states from the substitution.

Black phosphorus (BP) and the analagous isoelectronic IV-VI semiconductors that form the layered herzenbergite crystal structure (e.g. SnS, SnSe, GeS, GeSe) are a relatively unexplored family of 2D materials for device fabrication. These layered materials have been found to display band gaps that are sensitive to the number of atom layers (e.g. 0.3 - 1.0 eV for black phosphorus and 1.1 - 2.0 eV for SnS). They have been identified as candidate materials for a range of applications in electronic devices, sensors and photovoltaics. Both black phosphorus and SnS can be readily exfoliated in NMP and related solvents to form nanosheets with 2 - 5 atomic layers and flake diameters in the range 100 - 1000 nm. If these nanosheets remain stable in dispersion they are potential 2D inks for device fabrication by inkjet printing.

Here we present two studies exploring the use of inkjet printing in fabricating all printed devices from 2D BP (phosphorene) and 2D SnS nanosheets. In order to facilitate drying after printing without the need for heat treatment, we have formulated inks using acetonitrile (CNCH3), which has a boiling point of 82 °C, allowing facile solvent removal after printing at low temperatures. NMP, by comparison, has a boiling point of 202 °C, which requires a high drying temperature (>150 °C) or high vacuum to successfully remove the solvent completely. Inks, which are stable against sedimentation for several days, have been prepared with solid loadings around 1 mgml-1 and with appropriate rheology and surface tension to be deposited by inkjet printing.

All devices were fabricated using an in-house designed and built laboratory scale inkjet printing platform (MPP 1000) equipped with a piezoelectric actuated inkjet print head of internal diameter 60 μm (MJ-ATP-01-60-8MX, Microfab Technologies Inc., Plano, TX, USA). All samples were printed on a silver interdigitated electrode pattern also previously deposited by inkjet printing. After printing the devices are dried in dry N2 at 80 °C. Device performance was characterised electrically using a probe station and its chemical composition and structure studied using Raman spectroscopy and FTIR to determine the integrity of the nanosheets after deposition. The BP device showed extreme sensitivity to moisture and can be used as a highly sensitive humidity detector. Under high humidity conditions the BP shows permanent change in its properties but this can be recovered by a subsequent heat treatment in dry nitrogen. The SnS device shows promising results as a photodetector. These experiments demonstrate the utility of inkjet printing as a technique for the fabrication of electronic devices from a range of 2D materials.

Recent progress in the synthesis and characterization of novel 2D materials beyond graphene also known as van der Walls solids has been exceptional. The applicability of these materials for future technologies appear remarkably promising. In addition to single layers, heterostructures of dissimilar 2D materials (stacked layers) also offer unique opportunities. We will discuss current state of the art in the 2D-stacked layers (homogeneous, heterogeneous) and hybrid structures. Synthesis, properties , and possible devices that can be made with these materials with multifunctionality will be emphasized. Basic research issues such as strain engineering in 2D materials and imaging will be discussed in addition to some specific examples of near term applications such as synthetic electronic textiles, power-energy etc.

Among the 2D transition metal dichalcogenides (TMDs), MoS2 has attracted the most attention due to its structure-dependent electronic properties and potential for a wide range of applications. Yet, in order for 2D MoS2 to be technologically relevant, improved understanding of its properties and phase stability is needed, as well as efficient means to control them. Further, many applications will require high quality large-area films. In this talk, I will discuss our recent computational and experimental work towards understanding and controlling MoS2 phases, through both substitutional doping and chemical functionalization, and tuning junctions between 2 dimensional materials through structural and electronic modulation. In addition, I will show how the use of sulfurization of Atomic Layer Deposited (ALD) oxides could be instrumental to the production of clean, layer controlled, large area two-dimensional materials through control of the ALD nucleation regime.

Silicon dominates semiconductor technology not simply because of its properties as a semiconductor, which are actually quite ordinary, but because of its compounds. Silicon processing technology is as much about the mutual processing compatibility of silicon oxide, silicides, and silicon nitride as it is about silicon itself. Thus, silicon telluride seems an obvious choice for bringing a two-dimensional (2D) silicon chalcogenide into the realm of applied electronic materials.

We report the synthesis of high-quality single-crystal layered silicon telluride, Si2Te3, in multiple morphologies that can be exfoliated to form single 2D monolayers. Silicon telluride is a p-type semiconductor with a phololuminescence peak at 641nm. It enjoys chemical and processing compatibility with other silicon-based material including amorphous SiO2 but is chemically sensitive to its environment. We also show the potential for the chemical tunability of this material from a semiconductor to a semi-metal through intercalation of zero-valent metals such as copper.

Transition metal dichalcogenides (TMDs) have generated significant interest for numerous applications including sensors, flexible electronics, heterostructures and optoelectronics due to their interesting, thickness-dependent properties. Despite recent progress, the synthesis of high-quality and highly uniform TMDs on a large scale is still a challenge. In this talk, synthesis routes for WSe₂ and MoS₂ that achieve monolayer thickness uniformity across large area substrates with electrical properties equivalent to geological crystals will be described. Controlled doping of 2D semiconductors is also critically required. However, methods established for conventional semiconductors, such as ion implantation, are not easily applicable to 2D materials because of their atomically thin structure. Redox-active molecular dopants will be demonstrated which provide large changes in carrier density and workfunction through the choice of dopant, treatment time, and the solution concentration. Finally, several applications of these large-area, uniform 2D materials will be described including heterostructures, biosensors and strain sensors.

Two-dimensional (2D) crystals such as transition metal dichalcogenides (TMDs) along with other families of layered materials including graphene, SnSe₂, GaSe, BN etc, has attracted intense attention from the scientific community. One monolayer of such materials represent the thinnest “quantum wells”. Numerous novel material properties and device concepts have been discovered, proposed and demonstrated lately. However, the low internal photoluminescence efficiency (IPE, In this talk, I will share our progress and the challenges we face in terms of preparing, characterizing these 2D crystals and their heterostructures using both mechanical exfoliation as well as molecular beam epitaxy.

Inspired by the success of graphene, alternative non-graphene 2-dimensional (D) materials have become the focus of intense research due to their unique physical and chemical properties. This presentation highlights our recent progress on development of a new stress driven synthesis of crystalline nanowires and nanosheets from spherical nanoparticles. Starting with ordered spherical nanoparticle arrays, we compress the nanoparticle arrays through controlled pressure fields. In situ high-pressure small-angle synchrotron X-ray scattering studies show that mechanical compression induces oriented ordering of the nanoparticle arrays. When the nanoparticles approach each other along the compression axis, enhanced deviatoric stress drives oriented consolidation of the nanoparticles into crystalline nanorods, nanowires, and nanosheets. Depending on structural dimension of the starting nanoparticle arrays, dimensions such as the diameter of the nanowires and thickness of the nanosheets can be tuned. Applications in materials from metals (Au, Ag, etc) to semiconductors (CdSe, PbS, etc) have been demonstrated. In situ TEM and optical characterizations correlate structural and optical properties of the resulted 1-2D nanostructured materials. Unlike prior methods in materials synthesis processes, such as Ostwald ripening that relies on nucleation and growth, stress driven synthesis involves direct consolidation of nanoparticles into 1-2D materials under mechanical compression offering a facile alternative tool for design and fabrication of advanced functional materials with controlled nanostructures and compositions.

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

Layer-by-layer (LBL) growth allows for novel architectures of thin films such as the ultra-fine tuning of thickness, hetero-stacking of multiple materials, and building of quantum well structures. In order to achieve LBL growth, molecular beam epitaxy and atomic layer deposition have been popularly used under Frank-van der Merwe mode. However, these processes can only be applied to specific materials with limited surface energy and under restricted kinetic conditions. Here, we introduce a novel method for wafer scale artificial LBL stacking with monolayer thick semiconducting films as basis materials. Specifically, our process includes i) wafer scale growth of monolayer MoS₂, WS₂, MoSe₂ and WSe₂ using MOCVD, ii) separation of films from growth substrate using dry peel-off process, iii) stacking of multiple layers using van der Waals forces. Low interaction between the MOCVD grown films and substrates allows us to achieve high delamination yield (>99.5%) of monolayer films from substrates. Our LBL stacking process was performed under vacuum without chemical etching or polymer supporter. Therefore, it guarantees an ultra-clean interface without air bubble or chemical contamination. We demonstrated the layer number (N) control by multiple stacking from N=1 to N=20, and vertical heterostructures of MoS₂, WS₂, MoSe₂ and WSe₂ monolayer films. We confirm the ultra-clean interface of each layer by photoluminescence and cross sectional scanning transmission electron microscopy.

Experimental and theoretical studies of atomically thin quasi two-dimensional (2D) materials and their nanostructures have revealed that these systems can exhibit highly unusual behaviors. Owing to their reduced dimensionality, quasi-2D materials present opportunities for manifestation of concepts/phenomena that may not be so prominent or have not been seen in bulk materials. Symmetry, many-body interaction, and substrate screening effects often play a critical role in shaping qualitatively and quantitatively their electronic, transport and optical properties, and thus their potential for applications. In this talk, we present theoretical studies on quasi-2D systems such as monolayer and few-layer transition metal dichalcogenides and metal monochalcogenides, as well as other 2D crystals going beyond graphene. Several phenomena are discussed, including novel exciton behaviors, tunable electrical transport and magnetic properties, and the important influence of substrate screening. We investigate their physical origins and compare theoretical predictions with experimental data.

This work was supported in part by the National Science Foundation and the U.S. Department of Energy.

The chemical vapor deposition (CVD) growth of atomic layer transition metal dichalcogenides (TMDs) is a collection of recipes. However, no comprehensive understanding of growth mechanism has been reached due to the challenges from the complexity of reactants and reactions. In current study, we focused on the growth of MoSe₂ with MoO₃ and Se as the precursors and compared the influence of cooling condition on the growth process. We found the existence of both MoSe₂ and nanoparticles on the growth substrate and we were able to build a multi-step growth model of MoSe₂ by analyzing the content and distribution of nanoparticles. Our mechanism was unambiguously supported by ab initio molecular dynamics simulations, annealing experiment and feedstock modeling. Our study provides the first comprehensive understanding of CVD process of atomic layer TMDs with a unique growth model fundamentally different from the growth of graphane, nanoparticle or nanowire system.

Transition metal dichalcogenides have been studied for their interesting properties at the monolayer limit. In particular, monolayers of MoS₂, MoSe₂, WS₂, and WSe₂ are direct bandgap semiconductors with strong light-matter interactions via tightly-bound excitons. In addition, their hexagonal crystal structure and lack of inversion symmetry leads to degenerate valleys at opposite corners of the reduced Brillouin zone. Combined with large spin-orbit coupling (>150meV), the valley degree of freedom is coupled to carrier spin, creating potential applications in novel optoelectronics, next-generation logic, and quantum information. In most cases, monolayers of transition metal dichalcogenides are created via mechanical exfoliation. With small flake size and randomized deposition, a method of growth is needed to make mass production and commercialization possible. While large-scale growth of TMD monolayers typically makes use of chemical vapor deposition, we show that high quality tungsten disulfide can be grown with a much simpler process that eliminates the need for precise stoichiometry and growth conditions. Using physical vapor deposition, a low-cost tube furnace was used to create monolayers of tungsten disulfide of high quality. The quality of these monolayers is confirmed by optical and electrical measurements, with some device applications discussed.

It is of great interest and importance for materials design to uncover, through computational and theoretical modeling, the following relationships: {basic atomic interactions à structure/morphology à functionality (including electronic)}. We will discuss recent examples from low-dimensional materials, where we seem to achieve satisfactory degree of understanding, mostly focusing on nucleation and islands shapes [1], grain boundaries and dislocations [2], heterojunctions [3], catalysis [4], and even predictive 2D boron synthesis [5].
[1] V. Artyukhov et al. Phys. Rev. Lett. 114, 115502 (2015); V. Artyukhov, Z. Hu et al. unpublished.
[2] X. Zou et al. Nano Lett. 15, 3495 (2015); A. Aziz et al. Nature Comm. 5, 4867 (2014); Y. Liu et al. Nano Lett. 14, 6782 (2014).
[3] Y. Gong et al. Nature Mater. 13, 1135 (2014); A. Kutana, H. Yu et al. unpublished.
[4] Y. Liu et al. Phys. Rev. Lett. 113, 028304 (2014); X. Zou et al. Acc. Chem. Res. 48, 73 (2015).
[5] Y. Liu et al. Angew. Chemie, 52, 3156 (2013); Z. Zhang et al., Angew. Chemie, 127, 13214, DOI: 10.1002/anie.201505425 (2015).

Two dimensional (2D) nanomaterials such as graphene and transition-metal dichalcogenides (TMDCs) have attracted tremendous attention over recent years due to their unique properties and potential for numerous applications. In this talk, I will provide an overview of our research efforts in the solution-based exfoliation of 2D layered materials, such as graphene, MoS2 and WS2 where we have chemically exfoliated 2D layered materials for their utility in ink-jet printing. In ink-jet printing, it is necessary to have fluids which possess the right combination of viscosity and surface energy, and a range of organic solvents were explored where the viscosity was engineered through chemical means. The 2D MoS2 and graphite solutions were then patterned onto rigid and flexible substrates and the electrical transport and optical properties of these printed structures were then characterized. The structural characteristics of the solution-dispersed 2D flakes were also analyzed by considering the role of sonication on the particle size distribution which revealed morphological signatures between MoS2 and WSe2 flakes. The 2D solution dispersions were then integrated with organic materials such as polymers for the realization of composite structures for potential sensing applications. In addition, we will also comment on excitonic behavior that we have noted in solution-dispersed 2D materials where measurements on the samples were conducted using a spectrophotometer spanning the ultra violet to near-infra red. Other materials that we have examined include the synthesis of graphene, MoS2 and WSe2 using (CVD), in addition to exploring the intriguing optoelectronic properties of black phosphorus where measurements were made over a range of cryogenic temperatures spanning 350 K to 6 K.

A novel bottom-up method to create heterostructures of WS₂ and WSe₂ has been developed. The synthetic method involves the precursors Tungsten Hexacarbonyl (W (CO)₆), Hydrogen Disulfide (H₂S) and Hydrogen Diselenide (H₂Se). Transition metal precursors were pre-deposited lithographically at controlled locations. Selective growth of WS₂, WSe₂ and their heterostructures was achieved on SiO₂, Sapphire and h-BN. Extensive characterization by Raman spectroscopy, atomic force microscopy, scanning and transmission electron microscopies reveled polycrystalline nature of these films which can be grown as thin as mono layer. This method enabled the fabrication of field effect transistors and photo-detectors with performances comparable to those observed on CVD grown and exfoliated samples

Although the one-atom-think crystal like graphene and h-BN have fantastic properties and attracted tremendous interests in these years, there are a lot of 2D materials beyond graphene to be explored. Using chemical solid reaction and chemical vapour deposition methods, we have successfully synthesized a wide spectrum of 2D materials (both single crystals and few layers), including

1. Binary 2D materials: Borides (h-BN, WB), TMDs (MoS₂, WSe₂, MoSe₂, WSe₂, MoTe₂, WSe₂, ReS₂, ReSe₂, PtS₂, PtSe₂, PdS₂, PdSe₂, NbSe₂, SnS₂, SnSe, SnSe₂, TiS₃, HfSe₃, HfTe₃, TiSe₂, TaTe₂, TaSe₂), and others (InSe, In₂Se₃, GaSe, SrSi₂, Ta3S₂, BiI₃, PbI₂), etc.
2. Ternary and multi-component 2D materials: B_xC_yN_z, Mo_xW_1-xS₂, MoWTe₄, MoS_2xSe_2(1-x), WSe_2xTe_2(1-x), ReS_2xSe_2(1-x), Ta₂NiS₅, Ta₂NiSe₅, Ta₂ISe₈, Ti_xTa_1-xS₂, Ti_xNb_1-xS₂, Ta₃Pd₃Te₁₄, NiPS₃, FePS₃, ZnIn₂S₄, Ta₂SeI, V₂AlC, W₂AlC, CuIP₂S₄, Tl₂Mn₂O₇
3. Heterostructured 2D materials: Graphene/h-BN, MoS₂/WS₂, WSe₂/MoSe₂
4. Organ/Inorganic heterostructures: MoS2/Rubrene, Organic Perovskite/2D

Potential applications of 2D materials have been developed, such as 2D anisotropic electronics (FETs, resonators and photodetectors), energy harvester, lithium ion battery, high performance catalyst and wearable devices etc. These applications pave a promising way to the large scale applications of 2D materials.

References:
1. Lin Niu, Xinfeng Liu, Chunxiao Cong etc. Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-matter Interactions, Advanced Materials, 2015
2. Jiadong Zhou, Qingsehng Zeng, Danhui lv etc. Controlled Synthesis of High-quality Monolayered a-In2Se3 via Physical Vapor Deposition, Nano Lett, 2015, 15, 6400
3. Chaoliang Tan, Peng Yu, Yanling Hu etc. High-Yield Exfoliation of Ultrathin Two-Dimensional Ternary Chalcogenide Nanosheets for Highly Sensitive and Selective Fluorescence DNA Sensors, JACS, 2015, 137, 10430
4. Fucai Liu, Wai Leong Chow, Xuexia He, etc. Van der Waals p-n Junction Based on Organic-Inorganic Heterostructure , Advanced Functional Materials, 2015, 25, 5865
5. Zheng Liu, Matin Amani, Sina Najmaei, PM Ajaya, Jun Lou etc. Strain and Structure Heterogeneity in MoS2 Atomic Layers Grown by Chemical Vapor Deposition. Nature Communications 2014, 5, 5246
6. Z. Liu, Y.J. Gong et al., Ultrathin high-temperature oxidation-resistant coatings of hexagonal boron nitride, Nature Communications, 2013, 4, 2541.
7. S. Najmaei,* Z. Liu,* W. Zhou etc. Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nature Materials 2013, 12, 754-759.
8. Z. Liu, L. Ma, G. Shi, W. Zhou, etc. In-plane heterostructures of graphene and hexagonal boron nitride with controlled domain sizes. Nature Nanotechnology 2013, 8, 119-24.

Indium Selenide (InSe) belongs to the III-VI group of van der Waals material, which are characterized by strong in-plane bonds and weak van der Waals coupling between the layers. Each layer consists of four sub-layers in the form of Se-In-In-Se. The weak coupling allows for mechanical exfoliation of quasi-2D films and investigation of their properties. We fabricated two types of few-layer InSe field-effect transistors (FETs): with protective Boron Nitride (BN) capping and without it. The capped and uncapped devices demonstrated a reproducible hysteresis effect, which allowed for changing the transport type from p- to n- and vice versa depending on the direction of the gate voltage (V_G) sweep. The drain current (I_D) was dependent on the sweeping speed of the applied gate voltage (V_G). The low temperature measurements indicated that the observed effects can be due to deep level traps that capture and emit electrons at different rates depending on temperature. Obtained results can be useful for proposed applications of III-VI materials in electronics and photo-detectors.
The work at UC Riverside was supported, in part, by the Emerging Frontiers of Research Initiative (EFRI) 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications (NSF 005400) and by the Semiconductor Research Corporation (SRC) and Defense Advanced Research Project Agency (DARPA) through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME).

Several two-dimensional (2D) layered transition metal dichalcogenides (TMD) exhibit interesting charge density wave (CDW) effects at relatively high temperature. Thinning down these materials to their atomic limit opens opportunity to explore physical phenomena that are not present in bulk crystals. It has been demonstrated that the phase transition temperature between different CDW phases can be dramatically changed as the thickness reduces from bulk to atomically thin [1-2]. In this work we study CDW formation in the thin films of 1T-TaS₂ with the thickness of 6-9 nm using Raman spectroscopy. To avoid oxidation and degradation of TaS₂ films we protect them by capping with a layer of h-BN. The thin films show clear CDW signature peaks at the non-commensurate (NC) – incommensurate (IC) transition temperature of 350 K. We used in-situ Raman spectroscopy to demonstrate that the CDW phase transitions can be triggered electrically, exhibiting a threshold switching phenomenon. The obtained results are important for understanding the electrically- induced phase transition in this CDW system. The threshold switching in 1T-TaS₂ is a potentially useful function in various electronic applications, e.g. dynamic random access memory.

This work was supported in part by NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications (NSF 005400).

[1] P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, Nano Letters, 12, 5941 (2012).
[2] R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nano Letters, 15, 2965 (2015).

Transition metal dichalcogenides (TMDC’s), in their 2-D forms, have been shown to possess incredible electrical and optical properties. In particular, monolayer TMDC’s possess direct bandgaps in the visible spectrum, making them candidates for light harvesting and emitting, as well as hydrogen catalysis. It is predicted that MoS₂ solar cells could have a power density 3 orders of magnitude higher than existing solar cells [1]. The promise of large-area heterostructure devices has motivated a global research effort to produce wafer-scale and high quality monolayer films with highly controlled synthesis. To achieve this growth, several variations of Chemical Vapor Deposition (CVD) have emerged, which offer scalable production of monolayer MoS₂, although at a generally lower crystal quality than exfoliated samples [2]. A wafer-scale CVD method that can reliably produce TMDC crystals with high-quality photoluminescence (PL) and transport properties will allow for fabrication, and eventually commercial-scale production, of TMDC light collectors and emitters. We now present our results on a CVD format that modifies the approach of Wu et al [3] and others and demonstrates precise layer control, allowing for a systematic study of the layer-by-layer transformation of MoS₂ using Raman, luminescence, and electronic measurements. Our growth uses MoS₂ powder as the sulfur source in a CVD growth chamber under the influence of Ar carrier gas, enabling control over vapor transport. MoS₂ growth substrates were pre-deposited with Molybdenum films via electron beam evaporation in one-angstrom increments ranging from 0.1nm to 1.1nm. We have shown that approximately 5Å of starting Mo yields a monolayer of MoS₂, 10Å Mo yields a bilayer, and the intermediate thicknesses show the transition phases from monolayer to bilayer. The growths show a PL peak at 677nm, which is characteristic of monolayer MoS₂, that is strongest for the 5 angstrom Mo precursor sample. The PL magnitude attenuates as the Mo thickness increases past 0.5nm, showing a transition from direct to indirect bandgap as a bilayer expands over the monolayer. Raman spectroscopy also indicates monolayer MoS₂ on the 0.5nm sample, with characteristic Raman peaks of E_2g at 382cm^-1 and A_1g at 404cm^-1. The bilayer sample shows Raman peaks at 383cm^-1 and 406cm^-1, as expected. Electronic characterization was done by fabricating bottom-gated transistors; first measurements of the carrier mobility of the monolayer sample show semiconducting behavior, as desired, with mobility increasing by an order of magnitude from monolayer to bilayer samples. The grown MoS₂ using this CVD technique shows potential for highly controlled wafer-scale growth that can be optimized for use in heterostructures as light-emitters and solar cells.

[1] Grossman et Al, Nano Lett 13, 3664-3670 (2013)
[2] Choi et al, Appl. Phys. Lett. 012104 (2015)
[3] Wu et al, Appl. Phys. Lett. 105, 072105 (2014)

Since the isolation of graphene, a monolayer of sp2-bonded carbon atoms arranged in a hexagonal lattice, two-dimensional (2D) layered materials have attracted a great deal of attention due to their outstanding mechanical, optical and electronic properties. The research areas of interest for these new materials include exploring their novel properties, developing scalable approaches to synthesize these materials, and integrating them into a new generation of nanodevices. The utilization of 2D materials in devices has many advantages, which includes scaled materials to the limit of atomic-scale membranes, and the potential to form device structures on flexible and transparent substrates, among others. Transition metal dichalcogenides (TMDs) monolayers in particular have received increasing attention in recent years, especially molybdenum disulfide (MoS₂), which is one of the most well explored compound in these family of two dimensional materials. Both of these materials are intrinsic semiconductors with direct band gap characteristics, making them attractive for optoelectronic devices. In this work we present the synthesis of MoS₂ using chemical vapor deposition, where we have varied the synthesis parameters by looking at the role of the transport carrier gas, growth temperature and pressure; we have compared the structure and quality of the CVD synthesized MoS₂ and compared the characteristics with those obtained for mechanically exfoliated flakes from the bulk crystal. The MoS₂ quality has been analyzed using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. In addition, the electronic characterization was done at cryogenic temperatures using a state-of-the-art vacuum probe stage, and current-voltage characteristics were captured using an ultra low-noise semiconductor parameter analyzer (Keysight B1500A). In addition, in our work we have also conducted stability analysis of MoS₂ by heating the samples in air in an oxidizing environment, and measured the structural characteristics as well as the photoelectric response of the aged materials. Finally, we will comment and compare our work with previous results reported in the literature.

Here, we report high-performance black phosphorus (BP) field-effect transistors with channel lengths down to 20 nm fabricated using a facile angle evaporation process. By controlling the evaporation angle, the channel length of the transistors can be reproducibly controlled to be anywhere between 20 to 70 nm. The as-fabricated 20 nm top-gated BP transistors exhibit respectable on-state current (174 µA/µm) and transconductance (70 µS/µm) at a V_DS of 0.1 V. Owing to the use of two-dimensional BP as the channel material, the transistors exhibit relatively small short channel effects, preserving a decent on-off current ratio of 10² even at an extremely small channel length of 20 nm. Additionally, BP Schottky diodes with asymmetric metal contacts have been demonstrated using gold and aluminium electrodes. The device exhibits rectifying characteristics with current rectification ratio up to 1.5×10³. The effect of channel length on the electrical characteristics of BP Schottky diodes has also been studied and the results reveal that the device loses its rectifying behavior (rectification ratio = 1.37) at ultrashort channel length of around 30 nm. The transition from rectifying to nonrectifying characteristics at extremely small channel length is attributed to the electric-field-induced barrier thinning, which results in significantly increased tunneling current under reverse bias. Using the BP Schottky diodes with relatively long channel length (~1 µm), photodetectors with fast response time of less than 2 ms have been demonstrated.

Surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials, with their high spatial confinement, can open up new opportunities for enhanced light-matter interaction, super lenses, subwavelength metamaterial, and novel photonic devices. In situ characterization of these polaritonic excitations in different applications requires a versatile optical imaging and spectroscopy tool with nanometer spatial resolution.
Through a non-invasive near-field light-matter interaction, scattering-type scanning near-field optical microscopy (sSNOM) provides a unique way to selectively excite and locally detect electronic and vibrational resonances in real space. By collecting scattered light from the area underneath the atomic force microscope (AFM) tip, local optical properties of the sample can be measured with spatial resolution limited by the tip radius of ~20nm. The full near-field optical response of the plaritonic resonances can be measured with phase resolved detection, providing both amplitude and phase information.

Here we demonstrate the technique by imaging both SPhPs on hexagonal boron nitride (hBN) and SPPs on graphene with tunable QCL and CO₂ laser sources. Amplitude and phase near-field optical images provide complementary information for characterizing the complete polaritonic resonances. >90 deg phase shift of SPhPs are observed on hBN, indicating strong light-matter coupling.
By integrating with broadband and ultrafast light sources, sSNOM can be further used to study nonlinear properties associated with these polaritonic resonances.

The heteroepitaxial growth of Au on MoS₂, a layered van der Waals bonded dichalcongenide, is analyzed. It is argued that the weak coupling between the layers in the dichalcogenides enables the first substrate layer to deform elastically almost independently from the substrate layers below, and hence enables epitaxial growth for a larger mismatch than might otherwise be expected. Linear, continuum elasticity theory and density functional theory are used to show that a {111} oriented Au film is the preferred over an {001} and a second {111} Au film that is rotated relative to the MoS₂ (rotated {111}), despite the fact that the {111} orientation leads to a much higher elastic strain. During the initial stages of growth, the rotated {111} orientation is favored over the {111} and {001} orientation. As the Au film grows thicker (above 10 layer of Au grown), the elastic relaxation of the first layer of the substrate leads to a reduction in the elastic energy of the growing film. This reduces the elastic energy difference between these three orientations enabling the {111} orientation to remain the most stable one for all film thicknesses above 10 layer of Au. This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

The realization of 2D layer based, next generation electronic applications essentially depends on a reproducible, large-scale production of 2D layers via chemical vapor deposition (CVD). To fabricate electronic devices, it is necessary to transfer 2D layers onto different substrates with high efficiency, large area and high quality. Currently, the most commonly used transfer methods rely on polymethyl-methacrylate (PMMA) to support the 2D films and to prevent folding while the growth substrate is etched.¹ However, PMMA is not suitable for the transfer of high quality 2D films, because it is easy to introduce contamination due to residues of PMMA thus resulting in degradation of the intrinsic properties and the reliability of devices based on 2D materials as well as some difficulties in constructing multiple layer heterostructure of 2D materials. Thus, further study of heterostructure of 2D materials using layer-by-layer transferring methods has been greatly hampered. Other transferring methods which do not involve PMMA have been also demonstrated.^2,3 However, so far they are still limited on the size, uniformity, degree of freedom and quality of transferred 2D materials. In this work, we report a development of a facile transfer technique for 2D materials by adding a water-soluble polyvinyl alcohol (PVA) layer in-between PMMA and 2D materials grown on the rigid substrate. This technique allows not only effective transfer to a target substrate with a high degree of freedom but also etching-free PMMA-assisted transfer while minimizing the effects of contamination on the overall device performance. We applied our universal technique to transfer different 2D materials grown on various rigid substrates, i.e. graphene on copper foil, h-BN on platinum and MoS₂ on SiO₂ substrates. We then investigated the quality of 2D materials transferred by different transfer techniques such as substrate etching, bubbling transfer and oxide layer etching as reported elsewhere in the past. FET made using graphene transferred by our transfer technique exhibited a negative shift of charge neutrality point close to zero. Furthermore, both graphene and MoS₂ FETs showed higher mobility and current modulation than those FETs prepared by conventional transfer technique, mainly due to the elimination of PMMA contaminants. Our results demonstrated the transferred 2D materials are of high quality, and that the developed transfer method is so versatile that even multilayer stacking of heterostructure of 2D materials can be reliably performed over a wafer-scale.

References
Garcia et al. Nano Lett. 12, 4449-4454, 2012.
Yang et al. Small 11, 175-181, 2015.
Banszerus et al. Sci. Adv. 1-6, 2015.

Phosphorene (PE) - the newly discovered 2D derivative of Phosphorus - has an inherent band gap and a high current on/off ratio. Manipulating strain in PE films - strain engineering (SE) – will offer the opportunity to further tailor their electronic properties. In particular, their atomically thin nature should give these materials a small but nonzero bending modulus, allowing them to conform to nearly any substrates, and to form folds with no sharp edges. Using objective boundary conditions [1] (OBC) coupled [2] with self-consistent charge density functional tight binding (DFTB), we calculated the bending rigidity of PE and its 2D allotropes (β, γ and δ), by modeling bent PE as large diameter nanotubes (PNTs). OBCs not only allow for drastic reductions in the number of atoms in simulations but also enable simulations of chiral PNTs, which is impossible with periodic boundary conditions. At the same time, the method describes how bending influences the electronic structure. We establish a robust platform for achieving SE for anisotropic 2D films. From in-plane stretching we find that PE along with γ- and δ- PE allotropes are orthotropic in nature, whereas β-PE is isotropic. Using results from our calculations and orthotropic thin shell model we develop equivalent continuum structure (ECS) for PE and its allotropes upon bending, having a definite finite thickness and elastic moduli. As expected for orthotropic phases, the elastic stiffnesses and elastic moduli of ECS for PNTs/bent PE depend on the direction of bending but interestingly, the thickness does too. The developed ECS can be used for performing finite element simulations of PE films on substrates.

[1] Dumitrica and James, “Objective molecular dynamics”, J. Mech. and Phys. of Solids 55 (10), 2206-2236 (2007).
[2] Nikiforov et al., “Ewald summation on a helix: A route to self-consistent charge density-functional based tight-binding objective molecular dynamics”, J. Chem. Phys. 139 (9), 094110 (2013).

Two-dimensional transition metal dichalcogenides (TMDs) have recently come under intense investigation as building blocks for van der Waals heterostructure electronics. One of the most promising TMDs is MoS₂, which transitions from an indirect bandgap (1.3 eV) in its bulk state to a direct band gap (1.8 eV) in its single layer state making it suitable for optoelectronic and transistor applications. The synthesis of high quality single layer MoS₂ on large substrates, however, remains a challenge. Although capable of producing the highest quality material, mechanical exfoliation is is limited by small surface areas and is not scalable [1]. Chemical vapor deposition (CVD) is also widely utilized but exhibits a lack of thickness control, poor process stability, and requires high deposition temperatures (typically above 650 °C) [2]. Atomic layer deposition (ALD) is natural technique for the synthesis of 2D materials. ALD is a CVD technique in which reactants are introduced to the chamber sequentially rather than simultaneously. Sequential self-limiting surface reactions allow for precise thickness control, high conformality, and scalability to large surface areas. ALD of single layer MoS₂ has been reported recently. However, depositions were limited to approximately 2.5 cm x 2.5 cm area sapphire substrates and high temperature (800 °C) post deposition anneals were required to yield highly crystalline MoS₂ films [3].
Here we demonstrate low temperature ALD of monolayer to few layer MoS₂ using purge separated cycles of MoCl₅ and H₂S precursors at reactor temperatures of up to 475 °C. ALD is achieved uniformly across 150 mm diameter SiO₂/Si and quartz substrates, avoiding the need for expensive sapphire substrates. Raman scattering studies show clearly the in-plane (E¹_2g) and out-of-plane (A_1g) modes of MoS₂. The separation of the E¹_2g and A_1g peaks is shown to be a function of the number of ALD cycles, shifting closer together with fewer layers. Raman polarization indicates that the MoS₂ crystals are oriented parallel to the surface. X-ray photoelectron spectroscopy (XPS) shows that the stoichiometry is improved by post deposition annealing in a sulfur ambient and results in the sharpening of the E¹_2g and A_1g peaks. Sulfur annealing also results in the appearance of the band edge photoluminescence peak and the spin orbit splitting peaks, both indications of monolayer MoS₂. Finally, high resolution transmission microscopy (TEM) confirms the atomic spacing of monolayer MoS₂.
[1] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nature 7, 669-712 (2012).
[2] T. J. Larrabee, T. E. Mallouk and D. L. Allara, Rev. Sci. Instrum., 84, 014102 (2013).
[3] X. Huang, Z. Zeng, H. Zhang, Chem. Soc. Rev. 42,1934 (2014).

Monolayer molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) are graphene-like two-dimensional materials with remarkable physical and mechanical properties which have provided them with a wide range of potential applications in advanced nanodevices and nanomaterials. Understanding the phonon thermal transport properties of monolayer MoS2 and h-BN is of importance in their successful utilization in nanodevices and provides further insights regarding the thermal transport properties of graphene-like two-dimensional materials. Such understanding will also allow us to tailor the thermal conductivity of two-dimensional materials through phonon engineering. We Use reverse nonequilibrium molecular dynamics simulations (RNEMD) to study the thermal conductivity of monolayer hexagonal boron nitride nanoribbons and monolayer molybdenum disulfide as a function of length, width, strain and edge chirality.
Our results show that the thermal conductivity of both MoS2 and h-BN is considerably lower than the thermal conductivity of graphene. Similar to graphene, by increasing the length of the ribbons, their thermal conductivity increases. Thermal conductivity of ribbons is significantly affected by the edge chirality of the ribbons; zigzag ribbons have a higher thermal conductivity than armchair ribbons with similar length and width. The strain impact on the thermal properties of two-dimensional materials is not the same. Longitudinal tensile strain has negligible effects on the thermal conductivity of MoS2 ribbons. However, longitudinal strains have an anomalous impact on the thermal conductivity of h-BN; thermal conductivity of h-BN nanroibbons increases under the application of tensile strain. This is in direct contrast with what is observed in bulk materials and some other two-dimensional materials such as graphene as the application of tensile strain on such materials leads to a reduction of their thermal conductivity. Investigation of the phonon-dispersion curves of two-dimensional boron nitride at different strains indicates that under tensile strain out-of-plane phonon acoustic branch stiffens. The stiffening of the out-of-plane acoustic branch leads to an increase in the thermal conductivity.

Currently tremendous research focuses on different transistor designs for beyond-CMOS technology. Among the latest designs, the tunnel field-effect transistor (TFET) is considered a promising candidate since it offers a much improved on-off current ratio and low power consumption. The TFET operation is based on the band-to-band tunneling (BTBT), instead of thermionic emission of conventional CMOS operation in the sub-threshold regime, which can provide steeper subthreshold slope. However, only few TFETs have been demonstrated with subthermionic subthreshold slope and most of them have much lower ON-currents than the CMOS. As the discovery of two-dimensional (2-D) materials recently, 2-D semiconductors have been proposed and demonstrated as TFET channel materials with supreme electrostatics and much smaller tunneling width, which can improve the subthreshold slope and the on-state BTBT rate. At the same time, the band offsets of the 2-D materials also need to be considered and different heterojunctions can be engineered to reduce the tunneling barrier height to further increase the ON-current. Currently, the band offsets of most 2-D materials are based on theoretical predictions, and have not been experimentally measured yet.
Internal photoemission (IPE) is a robust and accurate technique to determine barrier height at a solid/solid interface, in particular as a most common application to semiconductor/insulator and metal/insulator interfaces. Conventional IPE on TFET structure has been recently proved possible where IPE process was specifically tailored to enhance sensitivity of electron photoemission from each semiconductor component of the heterojunction over a large band gap insulator. Consequently, the barrier height at the semiconductor heterojunction can be deduced.
This talk is intended to provide a basic introduction to IPE and describe the approach used in an IPE measurement to determine band offsets of various 2-D materials that have been proposed in TFET design, i.e. SnSe₂, WSe₂, MoS₂, etc. In particular, we will demonstrate the method of band offset determination applied to the Au-Al₂O₃-SnSe₂ test structure, where the 40 nm heavily-doped n-type SnSe₂ layer is grown on GaAs substrate by molecular beam epitaxy (MBE). The band offset from SnSe₂ conduction band minimum to Al₂O₃ conduction band minimum is determined by the threshold of the cube root of IPE yield and found to be 3.5 eV, which confirms that SnSe₂ has a larger electron affinity than most semiconductors and can be combined with other semiconductors to form near broken-gap heterojunctions with low barrier heights. To identify the source of photoemission, imaginary part of the pseudo-dielectric function of the 2-D material is also measured by spectroscopic ellipsometry (SE) and aligned with optical features observed in yield plot. We will also present the results from the measurements and data analysis from other 2-D materials being considered for TFET structure.

Ultra-thin two-dimensional (2D) semiconducting materials possess a combination of large, tunable electronic bandgaps, optical transparency, and mechanical flexibility, and will likely revolutionize electronic devices such as wearable sensors and flexible displays. A primary step in the development of such devices with integrated 2D materials is the development of scalable, transfer-free synthesis over large areas at low temperatures. Electrically insulating amorphous transition metal dichalcogenide (TMD) films can be deposited via physical vapor deposition on large area flexible substrates at room temperature, and crystallized with subsequent illumination with light. Focused laser light with a power density of ~1 kW cm² is suitable for writing micron scale features in semiconducting transition metal dichalcogenides on polymer substrates. Broad band illumination from a xenon lamp can also be used over the large substrate areas (> 100 cm²), or passed through a physical mask to print features only in desired locations. The semiconducting properties of 2D MoS₂ and WS₂ materials synthesized in this way have been characterized using conductive atomic force microscopy, and other techniques to observe the expected temperature dependence on electrical conductivity. Structure and composition of the materials can be controlled by altering the incident fluence as well as by controlling the ambient environment during illumination, as verified by Raman spectroscopy, X-ray photoelectron spectroscopy, cross-sectional and plan view transmission electron spectroscopy, and other techniques. Multiple layers of 2D materials can also be treated in this way. For example, both layers in a MoS₂/WS₂ heterostructure of 10 nm total thickness on a polymer (PDMS) substrate were crystallized upon laser illumination. Diverse 2D architectures and devices built from illumination-based crystallization techniques will be highlighted.

Attachment of biomolecules, such as peptides, to 2D materials is an attractive approach for enhancing sensitivity and selectivity in molecular sensing applications. The peptides provide receptor sites allowing detection of specific analytes, while the high surface-to-volume ratio associated with molecularly thin 2D materials increases the sensitivity to very small changes in surface potential associated with adsorption of individual molecules. The critical step toward novel 2D molecular sensors is identification and understanding of the structure and chemistry of peptide-transition metal dichalcogenide interfaces. In this study, phage display techniques were employed to identify peptides that selectively bind to 2D targets such as graphene and MoS₂ in the forms of micro scale fine powder, bulk crystals, and ultra-thin films (