Search Results (124)

Hydrogen from water splitting is seen as a promising future energy source. Pt electrochemical catalysts with an ideal hydrogen evolution reaction (HER) performance face problems relating to their cost and scarcity. Research into transition metal dichalcogenides (TMDs) as alternative catalysts is in demand. In our work, H adsorption on monolayer MoTe₂ is investigated at different sites and rates. Through structure and charge distribution analysis, it is found that uniform charge distribution facilitates H adsorption. In addition, the enhanced electronic density of states and reduced band gap calculated by the electronic energy band structure are advantageous for H adsorption. And the Mo edge of MoTe₂ is sensitive to the H adsorption rate. Finally, the H adsorbed on the sites is stable at 600 K, as shown in molecular dynamics (MD) calculations. Our work provides a further mechanism for H adsorption on MoTe₂. Full article

SnSe₂, as a prominent member of the post-transition metal dichalcogenides, exhibits many intriguing physical phenomena and excellent thermoelectric properties, calling for both fundamental study and potential application in two-dimensional (2D) devices. In this article, we realized the molecular beam epitaxial growth of SnSe₂ films on a $\sqrt{3}\times \sqrt{3}$-Sn reconstructed Si(111) surface. The analysis of reflection high-energy electron diffraction reveals the in-plane lattice orientation as SnSe₂[110]//$\sqrt{3}$-Sn [112]//Si [110]. In addition, the flat morphology of SnSe₂ film was identified by scanning tunneling microscopy (STM), implying the relatively strong adsorption effect of $\sqrt{3}$-Sn/Si(111) substrate to the SnSe₂ adsorbates. Subsequently, the interfacial charge transfer was observed by X-ray photoemission spectroscopy. Afterwards, the direct characterization of electronic structures was obtained via angle-resolved photoemission spectroscopy. In addition to proving the presence of interfacial charge transfer again, a new relatively flat in-gap band was found in monolayer and few-layer SnSe₂, which disappeared in multi-layer SnSe₂. The interface strain-induced partial structural phase transition of thin SnSe₂ films is presumed to be the reason. Our results provide important information on the characterization and effective modulation of electronic structures of SnSe₂ grown on $\sqrt{3}$-Sn/Si(111), paving the way for the further study and application of SnSe₂ in 2D electronic devices. Full article

Two-dimensional transition metal dichalcogenides have attracted considerable attention in electrocatalytic hydrogen evolution due to their unique layered structures and tunable electronic properties. However, prior research has predominantly focused on the intrinsic catalytic activity of planar few-layer structures, which offer limited exposure of edge-active sites due to their restricted two-dimensional geometry. Moreover, van der Waals interactions between layers impose substantial barriers to electron transport, significantly hindering charge transfer efficiency. To overcome these limitations, this study presents the innovative synthesis of high-quality single-screw WS₂ with a 5° dislocation angle via physical vapor deposition. Second harmonic generation measurements revealed a pronounced asymmetric polarization response, while the selected area electron diffractionand atomic force microscopy elucidated the material’s distinctive screwed dislocation configuration. In contrast to planar monolayer WS₂, the conical/screw-structured WS₂—formed through screw-dislocation-mediated growth—exhibits a higher density of exposed edge-active catalytic sites and enhanced electron transport capabilities. Electrochemical performance tests revealed that in an alkaline medium, the screwed WS₂ nanosheets exhibited an overpotential of 310 mV at a current density of −10 mA/cm², with a Tafel slope of 204 mV/dec. Additionally, under a current density of 18 mA/cm², the screwed WS₂ can sustain this current density for at least 30 h. These findings offer valuable insights into the design of low-cost, high-efficiency, non-precious metal catalysts for hydrogen evolution reactions. Full article

Janus-structured transition metal dichalcogenides (TMDs) demonstrate remarkable electronic, optical, and catalytic characteristics owing to their distinctive asymmetric configurations. In this study, we comprehensively analyze the stability of Janus SWSe containing common vacancy defects through first-principles calculations. The findings indicate that the Gibbs free energy for the hydrogen evolution reaction (HER) is notably decreased to around 0.5 eV, which is lower compared with both pristine SWSe and traditional MoS₂ monolayers. Importantly, the introduction of external strain further improves the HER efficiency of defect-modified Janus SWSe. This enhancement is linked to the adaptive relaxation of localized strain by unsaturated bonds in the defect area, leading to unique adjustable patterns. Our results clarify the fundamental mechanism driving the improved HER performance of SWSe via strain modulation, offering theoretical insights for designing effective HER catalysts using defective Janus TMDs. Full article

Identifying efficient and physically meaningful descriptors is crucial for the rational design of hydrogen evolution reaction (HER) catalysts. In this study, we systematically investigate the HER activity of transition metal dichalcogenide (TMD) monolayers by combining density functional theory (DFT) calculations and machine learning techniques. By exploring the relationship between key electronic properties, including the conduction band minimum (CBM), p_z band center, and hydrogen adsorption free energy (ΔG_*H), we establish a strong linear correlation between the CBM and ΔG_*H, identifying the CBM as a reliable and physically meaningful descriptor for HER activity. Furthermore, this correlation is validated in vacancy-defected TMD systems, demonstrating that the CBM remains an effective descriptor even in the presence of structural defects. To enable the rapid and accurate prediction of the CBM, we develop an interpretable three-dimensional model using the Sure Independence Screening and Sparsifying Operator (SISSO) algorithm. The SISSO model achieves a high predictive accuracy, with correlation coefficients (r) and coefficients of determination (R²) reaching 0.98 and 0.97 in the training and 0.99 and 0.99 in the validation tests, respectively. This study provides an efficient computational framework that combines first-principles calculations and machine learning to accelerate the screening and design of high-performance TMD-based HER catalysts. Full article

In two-dimensional (2D) materials research, exfoliating 2D transition metal dichalcogenides (TMDs) from their growth substrates for device fabrication remains a significant challenge. Current methods, such as those involving polymers, metals, or chemical etchants, suffer from limitations like contamination, defect introduction, and a lack of scalability. Here, we demonstrate a selenium capping-based exfoliation technique. Its advantage lies in its ability to enable the clean, contamination-free exfoliation and transfer of TMD films. We successfully exfoliated and transferred monolayer and multilayer TMD films, including WSe₂ and MoSe₂. The selenium capping layer not only enables seamless exfoliation but also protects the film from oxidation, as confirmed by X-ray photoelectron spectroscopy and Raman spectroscopy. This approach is versatile and applicable to a range of TMDs and thicknesses, paving the way for the high-quality, scalable integration of 2D materials into nanoelectronic devices. Full article

We perform a theoretical investigation of the electron–surface optical phonon (SOP) interaction in Van der Waals heterostructures (vdWHs) formed by monolayer graphene (1LG) and transition metal dichalcogenides (TMDCs), using eigenenergies obtained from the tight-binding Hamiltonian for electrons. Our analysis reveals that the SOP interaction strength strongly depends on the specific TMDC material. TMDC layers generate localized SOP modes near the 1LG/TMDC interface, serving as effective scattering centers for graphene carriers through long-range Fröhlich coupling. This interaction leads to resonant coupling of electronic sub-levels with SOP, resulting in Rabi splitting of the electronon energy levels. We further explore the influence of different TMDCs, such as WS₂, WSe₂, MoS₂, and MoSe₂, on transport properties such as SOP-limited mobility, resistivity, conductivity, and scattering rates across various temperatures and charge carrier densities. Our analysis confirms that at elevated temperatures and low carrier densities, surface optical phonon scattering becomes a dominant factor in determining resistivity. Additionally, we investigate the Auger recombination process at the 1LG/TMDC interface, showing that both Auger and SOP scattering rates increase significantly at room temperature and higher, ultimately converging to constant values as the temperature rises. In contrast, their impact is minimal at lower temperatures. These results highlight the potential of 1LG/TMDC-based vdWHs for controlling key processes, such as SOP interactions and Auger recombination, paving the way for high-performance nanoelectronic and optoelectronic devices. Full article

Transition metal dichalcogenides (TMDs), such as tungsten diselenide (WSe₂), are expected to be used in next-generation optoelectronic devices due to their unique properties. In this study, we developed a simple method of using ethanol to scroll monolayer WSe₂ nanosheets into nanoscrolls. These nanoscrolls have a quasi-one-dimensional structure, which enhances their electronic and optical properties. The characterization confirmed their unique structure, and the photodetectors made of these nanoscrolls have high sensitivity to polarized light, with anisotropy ratios of 1.3 and 1.7 at wavelengths of 638 nm and 808 nm. The enhanced light response is attributed to the large surface area and quantum wire-like behavior of the nanoscrolls, making them suitable for advanced polarization-sensitive devices. Full article

We theoretically investigated the electron–surface optical phonon interaction across the long-range Fröhlich coupling in monolayer transition metal dichalcogenides, such as WS₂, WSe₂, MoS₂, and MoSe₂ monolayers, on $\mathrm{S}\mathrm{i}\mathrm{C}$ and hexagonal BN dielectric substrates. We employed the effective Hamiltonian in the ${K_{+}(K}_{-})$ valley of the hexagonal Brillouin zone to assess the electronic energy shifts induced by the interaction between electronic states and surface polar optical phonons. Our results indicate that the interaction between electrons and surface optical phonons depends upon the polar nature of the substrate. We have also calculated the polaronic oscillator strength, as well as the polaronic scattering rate of the lower polaron state in monolayer WS₂, WSe₂, MoS₂, and MoSe₂ on $\mathrm{S}\mathrm{i}\mathrm{C}$ and hexagonal BN dielectric substrates. As a result, we have theoretically proved the following: firstly, the enhancement of the polaronic scattering rate with temperature, and secondly, the notable influence of the careful selection of surrounding dielectrics on both the polaronic oscillator strength and the polaronic scattering rate. Thus, optimal dielectrics would be those exhibiting both elevated optical phonon energy and a high static dielectric constant. Full article

Materials exhibiting charge density waves are attracting increasing attention owing to their complex physics and potential for applications. In this paper, we present a computational, first principles-based study of the Janus monolayer of 1T-TaSSe transition metal dichalcogenide. We extensively compare the results with those obtained for parent compounds, TaS₂ and TaSe₂ monolayers, with confirmed presence of $\sqrt{13}\times \sqrt{13}$ charge density waves. The structural and electronic properties of the normal (undistorted) phase and distorted phase with $\sqrt{13}\times \sqrt{13}$ periodic lattice distortion are discussed. In particular, for a normal phase, the emergence of dipolar moment due to symmetry breaking is demonstrated, and its sensitivity to an external electric field perpendicular to the monolayer is investigated. Moreover, the appearance of imaginary energy phonon modes suggesting structural instability is shown. For the distorted phase, we predict the presence of a flat, weakly dispersive band related to the appearance of charge density waves, similar to the one observed in parent compounds. The results suggest a novel platform for studying charge density waves in two-dimensional transition metal dichalcogenides. Full article

The surface symmetry of the substrate plays an important role in the epitaxial high-quality growth of 2D materials; however, in-depth and in situ studies on these materials during growth are still limited due to the lack of effective in situ monitoring approaches. In this work, taking the growth of MoSe₂ as an example, the distinct growth processes on Al₂O₃ ($11\overline{2}0$) and Al₂O₃ (0001) are revealed by parallel monitoring using in situ reflectance anisotropy spectroscopy (RAS) and differential reflectance spectroscopy (DRS), respectively, highlighting the dominant role of the surface symmetry. In our previous study, we found that the RAS signal of MoSe₂ grown on Al₂O₃ ($11\overline{2}0$) initially increased and decreased ultimately to the magnitude of bare Al₂O₃ ($11\overline{2}0$) when the first layer of MoSe₂ was fully merged, which is herein verified by the complementary DRS measurement that is directly related to the film coverage. Consequently, the changing rate of reflectance anisotropy (RA) intensity at 2.5 eV is well matched with the dynamic changes in differential reflectance (DR) intensity. Moreover, the surface-dominated uniform orientation of MoSe₂ islands at various stages determined by RAS was further investigated by low-energy electron diffraction (LEED) and atomic force microscopy (AFM). By contrast, the RAS signal of MoSe₂ grown on Al₂O₃ (0001) remains at zero during the whole growth, implying that the discontinuous MoSe₂ islands have no preferential orientations. This work demonstrates that the combination of in situ RAS and DRS can provide valuable insights into the growth of unidirectional aligned islands and help optimize the fabrication process for single-crystal transition metal dichalcogenide (TMDC) monolayers. Full article

For semiconducting two-dimensional transition metal dichalcogenides (TMDs), the carrier transport properties of the material are affected by strain engineering. In this study, we investigate the carrier mobility of monolayer hafnium disulphide (HfS₂) under different biaxial strains by first-principles calculations combined with the Kubo–Greenwood mobility approach and the compact band model. The decrease/increase in the effective mass of the conduction band (CB) of monolayer HfS₂ caused by biaxial tensile/compressive strain is the major reason for the enhancement/degradation of its electron mobility. The lower hole effective mass of the valence bands (VB) in monolayer HfS₂ under biaxial compressive strain improves its hole transport performance compared to that under biaxial tensile strain. In summary, biaxial compressive strain causes a decrease in both the effective mass and phonon scattering rate of monolayer HfS₂, resulting in an increase in its carrier mobility. Under the biaxial compressive strain reaches 4%, the electron mobility enhancement ratio of the CB of monolayer HfS₂ is ~90%. For the VB of monolayer HfS₂, the maximum hole mobility enhancement ratio appears to be ~13% at a biaxial compressive strain of 4%. Our results indicate that the carrier transport performance of monolayer HfS₂ can be greatly improved by strain engineering. Full article

A fully quantum, numerically accurate methodology is presented for the simulation of the exciton dynamics and time-resolved fluorescence of cavity-tuned two-dimensional (2D) materials at finite temperatures. This approach was specifically applied to a monolayer WSe${}_2$ system. Our methodology enabled us to identify the dynamical and spectroscopic signatures of polaronic and polaritonic effects and to elucidate their characteristic timescales across a range of exciton–cavity couplings. The approach employed can be extended to simulation of various cavity-tuned 2D materials, specifically for exploring finite temperature nonlinear spectroscopic signals. Full article

Transition metal dichalcogenides (TMDs) are drawing significant attention due to their intriguing photoelectric properties, and these interesting properties are closely related to the number of layers. Obtaining layer-controlled and high-quality TMD is still a challenge. In this context, we use the salt-assisted chemical vapor deposition to grow multilayered MoSe₂ flake and characterize it by Raman spectroscopy, second harmonic generation, and photon luminescence. Spectroscopic analysis is an effective way to characterize the stacking order and optoelectronic properties of two-dimensional materials. Notably, the corresponding mapping reflects the film quality and homogeneity. We found that the grown continuous monolayer, bilayer, and trilayer of MoSe₂ sheets with different stacking orders exhibit distinctive features. For bilayer MoSe₂, the most stable stacking configurations are the AA’ and AB order. And the uniformity of the spectroscopy maps demonstrates the high quality of the stacked MoSe₂ sheets. Full article

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers exhibit unique physical properties, such as self-terminating surfaces, a direct bandgap, and near-unity photoluminescence (PL) quantum yield (QY), which make them attractive for electronic and optoelectronic applications. Surface charge transfer has been widely used as a technique to control the concentration of free charge in 2D semiconductors, but its estimation and the impact on the optoelectronic properties of the material remain a challenge. In this work, we investigate the optical properties of a WS₂ monolayer under three different doping approaches: benzyl viologen (BV), potassium (K), and electrostatic doping. Owing to the excitonic nature of 2D TMDC monolayers, the PL of the doped WS₂ monolayer exhibits redshift and a decrease in intensity, which is evidenced by the increase in trion population. The electron concentrations of $3.79\times {10}^{13}{\mathrm{cm}}^{-2}$, $6.21\times {10}^{13}{\mathrm{cm}}^{-2}$, and $3.12\times {10}^{12}{\mathrm{cm}}^{-2}$ were measured for WS₂ monolayers doped with BV, K, and electrostatic doping, respectively. PL offers a direct and versatile approach to probe the doping effect, allowing for the measurement of carrier concentration in 2D monolayer semiconductors. Full article