Structure Of Transition Metal Dichalcogenides Research Articles

Molecular Intercalation‐Induced Two‐Phase Evolution Engineering of 1T and 2H‐MS2 (M = Mo, V, W) for Interface‐Polarization‐Enhanced Electromagnetic Absorbers

AbstractPolarization at interfaces is an important loss mechanism for electromagnetic wave (EMW) attenuation, though the motion behavior of carriers in interfaces composed of different types of conductors has yet to be investigated. Tuning the phase structure of transition metal dichalcogenides (TMDs) MS2 (M = Mo, V, W) by organics small molecule intercalation to achieve the modulation of interfacial types is an effective strategy, where 1T‐MS2 exhibits metallic properties and 2H‐MS2 has semiconducting properties. To exclude the contribution of the intrinsic properties of TMDs materials, three TMDs (MoS2, VS2, and WS2), which also possess phase transitions, are investigated. Among them, the 1T‐MS2 composite exhibits excellent EMW absorption performance under the synergistic effect of interfacial polarization and conduction loss. 1T‐MoS2/MOF‐A exhibits the best EMW absorption performance with an RLmin of −61.07 dB at a thickness of 3.0 mm and an EAB of 7.2 GHz at 2.3 mm. The effectiveness of the modulation of the interfacial polarization using 1T‐phase and 2H‐phase MS2 is demonstrated, which is important for the analysis of the carrier motion behavior during the interfacial loss.

Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Engineering Single-Layer Hollow Structure of Transition Metal Dichalcogenides with High 1T-Phase Purity for Hydrogen Evolution Reaction.

Rational design and controllable synthesis of hollow structures based on transition metal dichalcogenides (TMDs) have gained tremendous attention in the field of clean energy. However, the general synthetic strategies to fabricate single-layer hollow structures of TMDs, especially with unconventional phases (e.g., 1T or 1T'), still pose significant challenges. Herein, a scalable method is reported for the synthesis of single-layer hollow spheres (SLHS) of TMDs with high 1T-phase purity by etching bismuth (Bi) cores from pre-synthesized Bi@TMDs core-shell heterostructures including SLHS-1T-MoS2 , SLHS-1T-MoSe2 , SLHS-1T-WS2 , and SLHS-1T-WSe2 . Additionally, the etched Bi ions can be adsorbed on the single-layer TMDs shells in the form of single atoms (SAs) via the Bi─S bond. Due to the benefits of the single-layer hollow structure, high conductivity of 1T phase, and synergistic effect of Bi SAs and TMDs supports, the fabricated SLHS-1T-MoS2 exhibits superior electrocatalytic performance for hydrogen production. This work provides a way to manufacture advanced functional materials based on the single-layer hollow structures of 1T-TMDs and to expand their applications.

Electronic Janus lattice and kagome-like bands in coloring-triangular MoTe2 monolayers

Polymorphic structures of transition metal dichalcogenides (TMDs) host exotic electronic states, like charge density wave and superconductivity. However, the number of these structures is limited by crystal symmetries, which poses a challenge to achieving tailored lattices and properties both theoretically and experimentally. Here, we report a coloring-triangle (CT) latticed MoTe2 monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and ordered mirror-twin-boundaries into a pristine monolayer via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with theoretical calculations reveal that the monolayer has an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a Te pseudo-sublattice, and shares the identical geometry with the Mo5Te8 layer. Dirac-like and flat electronic bands inherently existing in the CT lattice are identified by two broad and two prominent peaks in STS spectra, respectively, and verified with density-functional-theory calculations. Two types of intrinsic domain boundaries were observed, one of which maintains the electronic-Janus-lattice feature, implying potential applications as an energy-tunable electron-tunneling barrier in future functional devices.

Modulation of Optical Behavior in MoS2/WS2 Heterobilayers Using Hydrostatic Pressure

The two-dimensional van der Waals (vdW) heterostructure composed of transition metal dichalcogenides (TMDs) has great photoelectric properties because of different energy band arrangements and interactions between layers. The lattice structure and photophysical properties of TMDs can be regulated through pressure, thus altering the properties of interlayer excitons. This study’s chemical vapor deposition method grew MoS2/WS2 vertical heterostructure. Its vibration mode signal and interactions between layers under pressure were investigated by pressure-variable Raman and photoluminescence (PL) spectra. We found that an inflection point appears at 1.22 GPa-1.89 GPa, and the PL peak changed with the variation trend of pressure. It is believed that the energy band transformation of MoS2/WS2 vertical heterostructures is induced by pressure. Our findings provide valuable information which is conducive to the development of new functional optoelectronic devices.

Adsorption of NO2 and NH3 on single-atom (Co, Pd, Pt)-decorated 2H-MoS2 monolayer: A DFT study

The single-atom decoration has attracted significant research interest by virtue of its critical role in modulating the electronic structure of transition metal dichalcogenides, and has been widely used in the fields of single-atom catalysis. Herein, we investigated the structures, electronic properties, and gas adsorption performance of single-atom (Co, Pd, Pt)-decorated 2H-MoS2 monolayer for adsorption of typical toxic gases, NO2 and NH3, using spin-polarized density functional theory calculations. The results indicate that the decorated metal atoms substantially improved the activity and electronic properties of the MoS2 monolayer, and further enhanced the adsorption and sensing performance for NO2 and NH3. The Pd-MoS2 monolayer is found to be a promising highly efficient gas sensing material for both gases in the temperature range of 400–500 K. On the other hand, the Co-MoS2 and Pt-MoS2 monolayers are not suitable for gas sensing applications, however, they can be considered as gas capture materials. These findings privide valuable references for the design and development of single-atom decorated MoS2 monolayer as high-performing gas sensing or scavenging materials for environmental monitoring and other related applications.

Effect of Large-Area Exfoliation and Etching on the Electrical Behavior of Transition-Metal Dichalcogenide Field-Effect Transistors

Large-area layer transfer was used to fabricate multilayer transition-metal dichalcogenide (TMDC) thin-film transistor (TFT) arrays with areas of (40 × 40 μm2) from single-crystal flakes. The effects of plasma etching of the TFT backchannel surface and bulk defects in the layers on the electrical performance and stability of the n-channel depletion-mode TFTs were investigated. Few-layer structures (∼3 monolayers) were fabricated using a dry etching process to thin multilayer (∼60–90 nm thick) TMDC structures. The etching improved the threshold voltage of the TFTs, resulting in a positive threshold voltage shift of +40 V after etching the backchannel, correlating to a bulk trap density of approximately 1 × 1016 cm–3 eV–1 per monolayer. Etching the MoS2 surface resulted in a threshold voltage shift of 0.2 V per nanometer of MoS2 removed (for MoS2 thicknesses >15 nm). For MoS2 layers etched to <15 nm, the threshold voltage changed to ∼1.8 V per nanometer. An observed degradation of the carrier transport and electrical stability of these samples were found to be due to the proximity of the etched surface approaching the active channel region of the device. The results reveal the performance trade-offs of fabricating large-area arrays of few-layer TMDC TFTs using a mechanical exfoliation and dry etching approach.

Chirality dependent electromechanical properties of single-layer MoS2 under out-of-plane deformation: a DFT study.

2D transition metal dichalcogenides (TMDs) demonstrate significant promise in logic circuits and optoelectronic devices because of their unique structures and excellent semiconductor properties. However, they inevitably undergo out-of-plane deformation during practical applications due to their ultra-thin structures. Recent experiments have shown that out-of-plane deformation significantly affects the electronic structures of 2D TMDs. However, the underlying physical mechanism is largely unknown. Therefore, it is critical to have a deeper understanding of out-of-plane deformation in 2D TMDs to optimize their applications in different fields. Currently, one of the most pressing matters that requires clarification is the chirality dependence of out-of-plane deformation in tuning the electromechanical properties of 2D TMDs. In this work, using single-layer MoS2 as a probe, we systematically investigate the effects of out-of-plane deformation along different chirality directions on the bond length, bending stiffness, electric polarization, and band structure of 2D TMDs by employing first-principles calculations based on density functional theory. Our results indicate that the bond length, bending energy, polarization strength, and band gap size of single-layer MoS2 are isotropic under out-of-plane deformation, while the band gap type is closely related to the direction of deformation. Our study will provide an essential theoretical basis for further revealing the structure-performance relationship of 2D TMDs.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications.

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Tuning the Layered Thickness of MoTe2 Thin Film for Dye‐Sensitized Solar Cells, UV and Visible Spectrum Photodetectors, and Hydrogen Evolution Reaction

Atomically thick van der Waals (VdW)‐bonded layered transition metal dichalcogenide (TMD) structures have influenced the investigation of novel phenomena for modern electronics and energy devices immensely. Herein, the fabrication of atomically thick tunable mollybednum ditelluride thin films on substrates through a chemical bath methodology is shown for the first time. The proposed methodology can be used to fine tune the atomic layer thickness of MoTe2 with highly modified surface characteristics, which is a greatly interesting feature for multifunctional nanodevice fabrication. Microscopic images confirm the atomic layer thickness‐tuned MoTe2. Furthermore, a photodetector assembled using few‐layer‐thick (S4) MoTe2 shows a high photoresponsivity of 3.21 A W−1 under ultraviolet light and an excellent detectivity of 5.4 × 1011 Jones, and a dye‐sensitized solar cell constructed using a photocathode made from S4 MoTe2 atomic layers shows a high power conversion efficiency of 8.44%, which is much greater than that of many other layered TMD structures. In addition, synthesized S4 MoTe2 atomic layers exhibit excellent hydrogen evolution characteristics in alkaline and acidic media. The resulting outcomes obviously establish the advantages of developed layered thick MoTe2 to produce the outstanding interface characteristics between the vdW‐bonded atomic layers to achieve outstanding energy and semiconductor devices.

Competing instabilities of the extended Hubbard model on the triangular lattice: Truncated-unity functional renormalization group and application to moiré materials

A simple yet paradigmatic model for the interplay of strong electronic correlations and geometric frustration is the triangular lattice Hubbard model. Recently it was proposed that moir\'e structures of transition metal dichalcogenides can be used to simulate extended versions that include non-local density-density interactions. We study competing instabilities of interacting electrons in such an extended Hubbard model on the triangular lattice near a filling where the density of states has a Van Hove singularity. We employ a truncated-unity functional renormalization group approach to investigate two cases: a paradigmatic minimally extended Hubbard model and a specific model with parameters that are applicable to hetero-bilayers of transition metal dichalcogenides. We unravel rich phase diagrams, including tendencies to spin-density-wave order and unconventional pairing, which can give rise to topological superconductivity. We classify the symmetry of the superconducting instabilities according to their irreducible representations and show that higher lattice harmonics are dominant when the nearest-neighbor interaction is sizable indicating pair formation between second-nearest neighbors. The phenomenological consequences can be enhanced spin and thermal quantum Hall responses in a topological superconductor.

WSe2-VSe2 Alloyed Nanosheets to Enhance the Catalytic Performance of Hydrogen Evolution Reaction.

Tuning the electronic structures of transition metal dichalcogenides (TMD) is essential for their implementation in next-generation energy technologies. In this study, we synthesized composition-tuned WSe2-VSe2 (W1-xVxSe2, x = 0-1) alloyed nanosheets using a colloidal reaction. Alloying the semiconducting WSe2 with VSe2 converts the material into a metallic one, followed by a 2H-to-1T phase transition at x = 0.7. Over a wide composition range, WSe2 and VSe2 are atomically immiscible and form separate ordered domains. The miscible alloy at x = 0.1 displayed enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. This trend was correlated with the d-band center via a volcano-type relationship. Spin-polarized density functional theory calculations consistently predicted the atomic immiscibility, which became more significant at the 2H-1T phase transition composition. The Gibbs free energy of H adsorption on the basal planes (Se or hole sites) and the activation barriers along the Volmer-Heyrovsky reaction pathway supported the enhanced HER performance of the alloy phase, suggesting that the dispersed V-doped structures were responsible for the best HER catalytic activity. Our study demonstrates how the atomic structure of TMD alloy nanosheets plays a crucial role in enhancing catalytic activity.

Giant bulk photovoltaic effect driven by the wall-to-wall charge shift in WS2 nanotubes

The intrinsic light–matter characteristics of transition-metal dichalcogenides have not only been of great scientific interest but have also provided novel opportunities for the development of advanced optoelectronic devices. Among the family of transition-metal dichalcogenide structures, the one-dimensional nanotube is particularly attractive because it produces a spontaneous photocurrent that is prohibited in its higher-dimensional counterparts. Here, we show that WS2 nanotubes exhibit a giant shift current near the infrared region, amounting to four times the previously reported values in the higher frequency range. The wall-to-wall charge shift constitutes a key advantage of the one-dimensional nanotube geometry, and we consider a Janus-type heteroatomic configuration that can maximize this interwall effect. To assess the nonlinear effect of a strong field and the nonadiabatic effect of atomic motion, we carried out direct real-time integration of the photoinduced current using time-dependent density functional theory. Our findings provide a solid basis for a complete quantum mechanical understanding of the unique light–matter interaction hidden in the geometric characteristics of the reduced dimension.

Electronic Band Tuning and Multivalley Raman Scattering in Monolayer Transition Metal Dichalcogenides at High Pressures.

Transition metal dichalcogenides (TMDs) possess spin-valley locking and spin-split K/K' valleys, which have led to many fascinating physical phenomena. However, the electronic structure of TMDs also exhibits other conduction band minima with similar properties, the Q/Q' valleys. The intervalley K-Q scattering enables interesting physical phenomena, including multivalley superconductivity, but those effects are typically hindered in monolayer TMDs due to the large K-Q energy difference (ΔEKQ). To unlock elusive multivalley phenomena in monolayer TMDs, it is desirable to reduce ΔEKQ, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we use high pressure to tune the electronic properties of monolayer MoS2 and WSe2 and probe K-Q crossing and multivalley scattering via double-resonance Raman (DRR) scattering. In both systems, we observed a pressure-induced enhancement of the double-resonance LA and 2LA Raman bands, which can be attributed to a band gap opening and ΔEKQ decrease. First-principles calculations and photoluminescence measurements corroborate this scenario. In our analysis, we also addressed the multivalley nature of the DRR bands for WSe2. Our work establishes the DRR 2LA and LA bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. Conversely, their intensity could potentially be used to monitor the presence of compressive or tensile strain in TMDs. Furthermore, the ability to probe K-K' and K-Q scattering as a function of strain shall advance our understanding of different multivalley phenomena in TMDs such as superconductivity, valley coherence, and valley transport.

Pressure-induced structural transition, metallization, and topological superconductivity in PdSSe

Pressure not only provides a powerful way to tune the crystal structure of transition metal dichalcogenides (TMDCs) but also promotes the discovery of exotic electronic states and intriguing phenomena. Structural transitions from the quasi-two-dimensional layered orthorhombic phase to three-dimensional cubic pyrite phase, metallization, and superconductivity under high pressure have been observed experimentally in TMDCs materials PdS2 and PdSe2. Here, we report a theoretical prediction of the pressure-induced evolutions of crystal structure and electronic structure of PdSSe, an isomorphous intermediate material of the orthorhombic PdS2 and PdSe2. A series of pressure-induced structural phase transitions from the layered orthorhombic structure into an intermediate phase, then to a cubic phase are revealed. The intermediate phase features the same structure symmetry as the ambient orthorhombic phase, except for drastic collapsed interlayer distances and striking changes of the coordination polyhedron. Furthermore, the structural phase transitions are accompanied by electronic structure variations from semiconductor to semimetal, which are attributed to bandwidth broaden and orbital-selective mechanisms. Especially, the cubic phase PdSSe is distinct from the cubic PdS2 and PdSe2 materials by breaking inversion and mirror-plane symmetries, but showing similar superconductivity under high pressure, which is originated from strong electron-phonon coupling interactions concomitant with topologically nontrivial Weyl and high-fold Fermions. The intricate interplay between lattice, charge, and orbital degrees of freedom as well as the topologically nontrivial states in these compounds will further stimulate wide interest to explore the exotic physics of the TMDCs materials.

Odd–even effect and bandgap modulation by C-H doping in armchair nanoribbons of monolayer WS[formula omitted

Breaking the symmetry in out-of-plane direction in monolayer structures to trigger peculiar electronic features has long been predicted. Inspired by the recent experimental progress on the doping of carbon-based units in monolayer transition-metal dichalcogenide (TMD), we investigate the electronic structure and transport of armchair WS2 nanoribbons doping with C-H units in one face, using first-principle calculations. An odd–even effect is found, where the ribbons doped with even number of C-H units exhibit semiconducting behavior, and that with even number are metallic. With the increase of ribbon width, the band gap is found to decrease smoothly and finally trigger a semiconducting-metallic transition. Moreover, a periodically oscillatory variation of band gap is observed. Analysis shows it is resulted by the asymmetric edge effect induced by Janus-face doping. Our findings provide another way to modulate the electronic structure of TMDs, and can be extended to other TMD structures.

Comprehensive Study of Lithium Adsorption and Diffusion on Janus Mo/WXY (X, Y = S, Se, Te) Using First-Principles and Machine Learning Approaches.

The structural asymmetry of two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs) produces internal dipole moments that result in interesting electronic properties. These properties differ from the regular (symmetric) TMD structures that the Janus structures are derived from. In this study, we, first, examine adsorption and diffusion of a single Li atom on regular MX2 and Janus MXY (M = Mo, W; XY = S, Se, Te) TMD structures at various concentrations using first-principles calculations within density functional theory. Lithium adsorption energy and mobility differ on the top and bottom sides of each Janus material. The correlation between Li adsorption energy, charge transfer, and bond lengths at different coverage densities is carefully examined. To gain more physical insight and prepare for future investigations into regular TMD and Janus materials, we applied a supervised machine learning (ML) model that uses clusterwise linear regression to predict the adsorption energies of Li on top of 2D TMDs. We developed a universal representation with a few descriptors that take into account the intrinsic dipole moment and the electronic structure of regular and Janus 2D layers, the side where the adsorption takes place, and the concentration dependence of adatom doping. This representation can easily be generalized to be used for other impurities and 2D layer combinations, including alloys as well. At last, we focus on analyzing these structures as possible anodes in battery applications. We conducted Li diffusion, open-circuit voltage, and storage capacity simulations. We report that lithium atoms are found to easily migrate between transition-metal (Mo, W) top sites for each considered case, and in these respects, many of the examined Janus materials are comparable or superior to graphene and regular TMDs. In addition, we report that the side with higher electronegative chalcogen atoms is suitable for Li adsorption and only MoSSe and MoSeTe can be suitable for full coverage of Li atoms on the surface. Bilayer Li adsorption was hindered due to negative open-circuit voltage. Bilayer Janus structures are better suited for battery applications due to less volumetric expansion/contraction during the discharge/charge process and having higher storage capacity. Janus monolayers undergo a transition from semiconducting to metallic upon adsorption of a single Li ion, which would improve anode conductivity. The results imply that the examined Janus structures should perform well as electrodes in Li-ion batteries.

Engineering MoTe2 and Janus SeMoTe nanosheet structures: First-principles roadmap and practical uses in hydrogen evolution reactions and symmetric supercapacitors

Recent advancements in van der Waals-bonded atomically layered structures have tremendously shaped the possibilities of exploring novel materials and physical phenomena for practical modern applications. The astonishing physicochemical characteristics inspired by the Janus transition-metal dichalcogenides (TMDs) of SeMoTe owing to their asymmetric atomic arrangements were deliberately chosen for study by first-principles calculations with an elaborate comparison of their paternal MoSe 2 and MoTe 2 structures. The experimental synthesis was made by a cost-effective one-pot reaction methodology to engineer MoTe 2 and SeMoTe alloy nanosheet structures. For the water-splitting hydrogen evolution reaction, the SeMoTe alloy nanosheet structures evolved low overpotentials and Tafel slopes in alkaline and acidic media, which proved their persistent atomic arrangements regardless of the pH. To extend their uses, symmetric capacitors were made using MoSe 2 , MoTe 2 , and SeMoTe active electrodes, and found their high specific energy of 41.3 W h kg −1 at a specific power of 4.0 kW kg −1 with 367 F g −1 capacitance at a current density of 1.0 A g −1 for the Janus SeMoTe nanosheet structure. The observed results not only discover the incredible potential of Janus SeMoTe nanosheets for green energy production and symmetric storage devices but also facilitate novel phenomena to engineer asymmetrically stretched TMD structures for future smart applications. Asymmetrically atoms arranged SeMoTe layered structure synthesized by one-pot reaction. Symmetric capacitance of 367 F g −1 at 1 A g −1 with 41.3 W h kg −1 at 4.0 kW kg −1 explored for SeMoTe electrode. SeMoTe alloy evolved low overpotentials and Tafel slopes in base and acidic media for HER. First principles calculation derived the density of states, Gibbs energy and quantum capacitance. • Asymmetrically arranged SeMoTe layered structure derived by one-pot reaction. • Exclusive comparison of SeMoTe electrochemical characteristic with MoTe 2 and MoSe 2 . • First principles calculation proved the enhanced density of states and low Gibbs energy. • Symmetric capacitance of 460 F g −1 at 1 A g −1 with 40.8 W h kg −1 at 4.5 kW kg −1 . • SeMoTe alloy evolved low overpotentials and Tafel slopes in base and acidic media.

A Facile Liquid Phase Exfoliation of Tungsten Diselenide using Dimethyl Sulfoxide as Polar Aprotic Solvent to Produce High‐quality Nanosheets

AbstractTwo‐dimensional (2D) ultra‐thin nanosheets of semiconducting layered structure of transition metal dichalcogenides (TMDs) are in high demand for fundamental studies as well as potential applications in optoelectronic devices, advanced display devices and many more. The real challenge in this area is to produce large scale nanosheets with a single stable phase.Herein, we report a top‐down strategy for liquid‐phase exfoliation of bulk tungsten diselenide (WSe2) powder to produce hexagonal (2H) few layers (2–3 layers) of nanosheets by ultra‐sonication method using a highly‐efficient dimethyl sulfoxide (DMSO) as polar aprotic solvent. As produced nanosheets are further characterized by different spectroscopic techniques such as Raman and ultraviolet‐visible (UV‐Vis) spectroscopy to validate the quality of exfoliated WSe2 nanosheets and estimated the direct optical band gap after exfoliation of WSe2. Further, the Raman spectroscopy evidences the transition from bulk to few layers of WSe2 nanosheets. The TEM/HRTEM images and XPS results also confirm the good quality and efficient exfoliation of WSe2 nanosheets. Moreover, the estimated optical band gap from UV‐Vis and Tauc plot confirm the transition occurs from indirect band gap (∼1.27 eV) to direct optical band gap ∼1.55 eV after exfoliation of WSe2 nanosheets. Hence, the proposed method provides a simple, large scale and inexpensive exfoliation approach to produce high‐quality single phase 2H‐WSe2 nanosheets.

Probing valley population imbalance in transition metal dichalcogenides via temperature-dependent second harmonic generation imaging

Degenerate minima in momentum space—valleys—provide an additional degree of freedom that can be used for information transport and storage. Notably, such minima naturally exist in the band structure of transition metal dichalcogenides (TMDs). When these atomically thin crystals interact with intense laser light, the second harmonic generated (SHG) field inherits special characteristics that reflect not only the broken inversion symmetry in real space but also the valley anisotropy in reciprocal space. The latter is present whenever there exists a valley population imbalance (VPI) between the two valleys and affects the polarization state of the detected SHG. In this work, it is shown that the temperature-induced change of the SHG intensity dependence on the excitation field polarization is a fingerprint of VPI in TMDs. In particular, pixel-by-pixel VPI mapping based on polarization-resolved raster-scanning imaging microscopy was performed inside a cryostat to generate the SHG contrast in the presence of VPI from every point of a TMD flake. The generated contrast is marked by rotation of the SHG intensity polar diagrams at low temperatures and is attributed to the VPI-induced SHG.