group-VIB Transition Metal Dichalcogenides Research Articles

High‐Pressure‐Induced Lattice and Electronic Structural Transformations in Multilayer ZrS2

Compared with Group VIB transition metal dichalcogenides (TMDs), Group IVB TMDs, specifically ZrX2, have exhibited superior carrier mobility, rendering them highly promising for developing innovative electron devices. The application of high‐pressure has been identified as an effective approach for altering the physical properties of TMDs by manipulating their lattice and electronic structures. This subject has attracted considerable attention within the scientific community. In this investigation, in situ high‐pressure Raman scattering and UV‐visible absorption spectra analysis are conducted to examine the lattice and electronic structural changes in multilayer ZrS2. Raman spectroscopy analysis reveals two phase transitions occurring at 3.65 and 10.95 GPa. Concurrently, the optical absorption findings reveal a distinct transformation in both the optical energy gap and Urbach energy at 3.34 GPa, highlighting the accompanying lattice and electronic structural changes in ZrS2 under high‐pressure. Moreover, these lattice and electronic structural changes are found to be irreversible, indicating the potential utility of compressed ZrS2 in optoelectronic devices.

Moiré Dirac fermions in transition metal dichalcogenides heterobilayers

Abstract Monolayer group-VIB transition metal dichalcogenides (TMDs) feature low-energy massive Dirac fermions, which have valley contrasting Berry curvature. This nontrivial local band topology gives rise to valley Hall transport and optical selection rules for interband transitions that open up new possibilities for valleytronics. However, the large bandgap in TMDs results in relatively small Berry curvature, leading to weak valley contrasting physics in practical experiments. Here, we show that Dirac fermions with tunable large Berry curvature can be engineered in moiré superlattice of TMD heterobilayers. These moiré Dirac fermions are created in a magnified honeycomb lattice with its sublattice degree of freedom formed by two local moiré potential minima. We show that applying an on-site potential can tune the moiré flat bands into helical ones. In short-period moiré superlattice, we find that the two moiré valleys become asymmetric, which results in a net spin Hall current. More interestingly, a circularly polarized light drives these moiré Dirac fermions into quantum anomalous Hall phase with chiral edge states. Our results open a new possibility to design the moiré-scale spin and valley physics using TMD moiré structures.

The growth mechanism and intriguing optical and electronic properties of few-layered HfS2.

Due to electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB (Hf and Zr) TMDs have become intriguing materials in next-generation nanoelectronics. Therefore, the growth of few-layered hafnium disulfide (HfS2) on c-plane sapphire as well as on a SiO2/Si substrate has been demonstrated using chemical vapour deposition (CVD). The structural properties of HfS2 were investigated by recording X-ray diffraction patterns and Raman spectra. The XRD results reveal that the layers are well oriented along the (0001) direction and exhibit the high crystalline quality of HfS2. The Raman spectra confirm the in-plane and out-plane vibration of Hf and S atoms. Moreover, the HfS2 layers exhibit strong absorption in the UV to visible region. The HfS2 layer-based photodetector shows a photoresponsivity of ∼1.6, ∼0.38, and ∼0.21 μA W-1 corresponding to 9, 38, and 68 mW cm-2, respectively under green light illumination and is attributed to the generation of a large number of electron-hole pairs in the active region of the device. Besides, it also exhibits the highly crystalline structure of HfS2 at high deposition temperature. The PL spectrum shows a single peak at ∼1.8 eV and is consistent with the pristine indirect bandgap of HfS2 (∼2 eV). Furthermore, a few layered HfS2 back gate field-effect transistor (FET) is fabricated based on directly grown HfS2 on SiO2/Si, and the device exhibits p-type behaviour. Thus, the controllable and easy growth method opens the latest pathway to synthesize few layered HfS2 on different substrates for various electronic and optoelectronic devices.

Strain-engineering in two-dimensional transition metal dichalcogenide alloys

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive semiconductors for use in electronic, optoelectronic, and spintronic devices. This study examines how the electronic properties of 2D TMDs can be tuned for specific applications through a combination of alloying and applying strain. Group VIB TMDs (MoS2, MoSe2, WS2, and WSe2) are alloyed by mixing in the metal or chalcogen sublattices. Density functional theory is used to model the structures of the alloys at varying compositions and examine the electronic structure of the alloys under biaxial tensile and compressive strain. Alloying results in the continuous monotonic tuning of the direct bandgap between the limits of the pure components, with low bowing coefficients for all alloys. Applying strain results in a transition of the bandgap from direct to indirect at low values of tensile strain and higher values of compressive strain. Strain can also be used to increase or decrease the bandgap with low compressive strain or tensile strain, respectively. The shift rate, or the rate at which the bandgap changes with applied strain, changes monotonically with alloy composition. MoS2 is identified as the 2D TMD with the highest shift rate.

Phase Engineering of Metastable Transition Metal Dichalcogenides via Ionic Liquid Assisted Synthesis.

Metallic group VIB transition metal dichalcogenides (1T-TMDs) have attracted great interest because of their outstanding performance in electrocatalysis, supercapacitors, batteries, and so on, whereas the strict fabrication conditions and thermodynamical metastability of 1T-TMDs greatly restrict their extensive applications. Therefore, it is significant to obtain stable and high-concentration 1T-TMDs in a simple and large-scale strategy. Herein, we report a facile and large-scale synthesis of high-concentration 1T-TMDs via an ionic liquid (IL) assisted hydrothermal strategy, including 1T-MoS2 (the obtained MoS2 sample was denoted as MoS2-IL), 1T-WS2, 1T-MoSe2, and 1T-WSe2. More importantly, we found that IL can adsorb on the surface of 1T-MoS2, where the steric hindrance, π-π stacking, and hydrogen bonds of ionic liquid collectively induce the formation of the 1T-MoS2. In addition, DFT calculation reveals that electrons are transferred from [BMIM]SCN (1-butyl-3-methylimidazolium thiocyanate) to 1T-MoS2 layers by hydrogen bonds, which enhances the stability of 1T-MoS2, so the MoS2-IL performs with high stability for 180 days at room temperature without obvious change. Furthermore, the MoS2-IL exhibits excellent HER performance with an overpotential of 196 mV at 10 mA cm-2 in acid conditions.

Hydrothermal synthesis and controlled growth of group-VIB W metal compound nanostructures from tungsten oxide to tungsten disulphide.

Two-dimensional lateral group-VIB transition metal dichalcogenides (TMDs) have attracted much attention in the fast evolving field of advanced photoelectric functional materials, but their controllable fabrication is challenging. Herein, an emerging synthetic route for sulfurization of tungsten oxide was developed. During the hydrothermal reaction, the optimization of the precursor selection and synthesis parameters led to the tunable properties of WO3-WSxOy-WS2 nanostructures. The vulcanization was thermodynamically favorably at low temperatures and in an environment with a sufficient S source, wherein WO3 was reduced by H atoms to WO3-x, and S atoms were preferentially adsorbed on O vacancies. The WSxOy nanostructures have a narrow band-gap attributed to the effect of S on the valence band top and electronic density of states by density functional theory. The photocurrent response and charge transfer properties of WSxOy were improved due to the charge transport between WS2 and WO3. Understanding the formation and transformation of WS2 nanostructures in solution contributes to the discovery of the important structure-efficiency relationship, which may be extended to other TMDs systems. Hence, extensive research efforts are still needed to develop safer and more efficient synthesis and modification methods to fully utilize the distinctive advantageous properties of TMDs in the photoelectric field.

Self-Powered 2D Material-Based pH Sensor and Photodetector Driven by Monolayer MoSe2 Piezoelectric Nanogenerator.

The large piezoelectricity of monolayer MoSe2, which is predicted to be stronger than that of all of the other group VIB transition-metal dichalcogenides (including MoS2), has only been theoretically investigated. Here, we report experimental evidence of in-plane piezoelectricity in MoSe2. Monolayer single-crystalline MoSe2 flake derived from chemical vapor deposition demonstrates a peak output voltage of 60 mV at 0.6% strain, which is ∼50% larger than that of MoS2. Piezoelectric signal along the armchair orientation of MoSe2 is ∼6 times larger than that along the zigzag orientation, indicative of strong anisotropic piezoelectricity. Piezoelectric nanogenerator based on a single MoSe2 flake illustrates remarkable electromechanical conversion ability, and thus is able to noninvasively monitor vital health signs, such as respiratory rate and heart rate. Despite the extremely small size, MoSe2 nanogenerator is able to drive pH sensor based on MoS2 and photodetector based on MoS2/WSe2 heterojunction due to the outstanding piezoelectricity of MoSe2 and the ultralow power consumption of two-dimensional (2D) material sensors. The self-powered, solely 2D-material-based sensor units demonstrate superb sensing performance. Therefore, the discovery of piezoelectricity in monolayer MoSe2 provides a route for achieving self-powered atomic-scale electromechanical systems that could stimulate further fundamental research and potential applications.

Origin of robust out-of-plane ferroelectricity in d1T-MoS2 monolayer

The d1T-MoS2, distorted-1T group-VIB transition metal dichalcogenides monolayer, is considered as promising atomically thin out-of-plane ferroelectric materials. We study the origin of the ferroelectricity in d1T-MoS2 monolayer using first-principles calculations and the Landau theory of phase transition. In contrast to conventional improper ferroelectrics, we find that the polarization has dependence on both primary and secondary modes. It turns out that the charge imbalance between chalcogen atoms at different symmetry sites is the source for the out-of-plane polarization without any out-of-plane displacement in the primary mode. The secondary mode following the primary mode cancels partially the polarization and secures it against the depolarization field. We show that the polarization of d1T-MoS2 is robust to external electric fields and can be manipulated by the uniaxial strain.

Optical Nonlinearity of ZrS₂ and Applications in Fiber Laser.

Group VIB transition metal dichalcogenides (TMDs) have been successfully demonstrated as saturable absorbers (SAs) for pulsed fiber lasers. For the group comprising IVB TMDs, applications in this field remain unexplored. In this work, ZrS2-based SA is prepared by depositing a ZrS2 nanostructured film onto the side surface of a D-shaped fiber. The nonlinear optical properties of the prepared SA are investigated, which had a modulation depth of 3.3% and a saturable intensity of 13.26 MW/cm2. In a pump power range of 144–479 mW, the Er-doped fiber (EDF) laser with ZrS2 can operate in the dual-wavelength Q-switching state. The pulse duration declined from 10.0 μs down to 2.3 μs. The single pulse energy reached 53.0 nJ. The usage of ZrS2 as a SA for pulse generation in fiber lasers is presented for the first time. Compared to the experimental results of dual-wavelength Q-switched fiber lasers with two-dimensional (2D) materials, our laser performance was better. Our work indicates that the group comprising IVB TMD ZrS2 has bright prospects for nonlinear optical applications.

First-principle high-throughput calculations of carrier effective masses of two-dimensional transition metal dichalcogenides * * Project supported by the National Natural Science Foundation of China (Nos. 11404131, 11674121), the Program for JLU Science and Technology Innovative Research Team, and the Special Fund for Talent Exploitation in Jilin Province of China.

Two-dimensional group-VIB transition metal dichalcogenides (with the formula of MX2) emerge as a family of intensely investigated semiconductors that are promising for both electronic (because of their reasonable carrier mobility) and optoelectronic (because of their direct band gap at monolayer thickness) applications. Effective mass is a crucial physical quantity determining carriers transport, and thus the performance of these applications. Here we present based on first-principles high-throughput calculations a computational study of carrier effective masses of the two-dimensional MX2 materials. Both electron and hole effective masses of different MX2 (M = Mo, W and X = S, Se, Te), including in-layer/out-of-layer components, thickness dependence, and magnitude variation in heterostructures, are systemically calculated. The numerical results, chemical trends, and the insights gained provide useful guidance for understanding the key factors controlling carrier effective masses in the MX2 system and further engineering the mass values to improve device performance.

Selective Direct Growth of Atomic Layered HfS2 on Hexagonal Boron Nitride for High Performance Photodetectors

Hafnium disulfide (HfS2) has attracted significant interest because of the predicted excellent electronic properties superior to group VIB transition metal dichalcogenides. On the other hand, atomically thin hexagonal boron nitride (h-BN) is an ideal dielectric substrate for optoelectronic applications of other 2D materials. Here, for the first time, we report the direct growth of high-quality atomic layered HfS2 on few-layer h-BN transferred on SiO2/Si substrates by chemical vapor deposition. It was found that the HfS2 layers are selectively grown on h-BN rather than on SiO2/Si. Density functional theory calculations are performed to help understand the mechanism of selective growth of HfS2. Furthermore, the photodetectors based on the HfS2/h-BN heterostructures exhibit excellent visible-light sensing performance, such as a high on/off ratio of more than 105, an ultrafast response rate of about 200 μs, a high responsivity of 26.5 mA W–1, and a competitive detectivity exceeding 3 × 1011 Jones, superior to...

Space‐Confined Chemical Vapor Deposition Synthesis of Ultrathin HfS2 Flakes for Optoelectronic Application

AbstractDue to the predicted excellent electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB TMDs have enormous potential in nanoelectronics. Here, the synthesis of ultrathin HfS2 flakes via space‐confined chemical vapor deposition, realized by an inner quartz tube, is demonstrated. Moreover, the effect of key growth parameters including the dimensions of confined space and deposition temperature on the growth behavior of products is systematically studied. Typical as‐synthesized HfS2 is a hexagonal‐like flake with a smallest thickness of ≈1.2 nm (bilayer) and an edge size of ≈5 µm. The photodetector based on as‐synthesized HfS2 flakes demonstrates excellent optoelectronic performance with a fast photoresponse time (55 ms), which is attributed to the high‐quality crystal structure obtained at a high deposition temperature and the ultraclean interface between HfS2 and the mica substrate. With such properties HfS2 holds great potential for optoelectronics applications.

Stabilization of 1T′ phase WTe2 by scalar relativistic effect

Among semiconducting hexagonal (1H), metallic octahedral (1T), and semimetallic distorted octahedral (1T′) phases that group-VIB transition metal dichalcogenides (TMDCs) may have, the thermodynamically stable structure is known to be the 1H phase. The exception is WTe2, for which it is 1T′ phase. However, the origin of the stable 1T′ phase WTe2 is unknown. Here, we study the electronic property of group-VIB TMDCs using first-principles calculations to find the origin of the 1T′ phase of WTe2. We find that the scalar relativistic effect in W stabilizes the 1T′ phase WTe2 against 1H-WTe2. The Fermi surface of the electron pocket of WTe2 is sensitively changed as the relativistic effect is turned on and off in W pseudopotentials. Calculated phonon dispersion exhibits drastic softening in the phonon mode at the M point, which corresponds to the nesting vector qM on the Fermi surface. We show that difference in occupation of s- and d-orbitals is a key parameter that determines the shape and size of the electron pocket, and thus the stable phase of group-VIB TMDCs between 1H or 1T′ phases.

Atomic and Electronic Structures of WTe2 Probed by High Resolution Electron Microscopy and ab Initio Calculations

Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials that have attracted growing interest because of their unique electronic and optical properties. Under ambient conditions, most TMDs generally exhibit 2H or 1T structures. Unlike other group VIb TMDs, bulk crystals and powders of WTe2 exist in a distorted 1T structure (Td) at room temperature and have semimetallic properties. There is so far a lack of direct atom-by-atom visualization, limiting our understanding of this distorted 2D layered material system. We present herein atomic resolution images of Td structured WTe2. The Td structure can be distinguished in the three major orientations along the [100], [010], and [001] zone axes. Subtle structural distortions are detected by atomic resolution imaging, which match well with the optimized structure relaxed by ab initio calculations. The calculations also showed that both crystal field splitting and charge density wave (CDW) interactions contribute to the stabilization ...

ChemInform Abstract: Electronic Structures and Theoretical Modelling of Two‐Dimensional Group‐VIB Transition Metal Dichalcogenides

AbstractReview: 215 refs.

Valley excitons in two-dimensional semiconductors

Abstract Monolayer group-VIB transition-metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibit remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges and the valley-dependent optical selection rules for interband transitions. Here, we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition-metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.

Evolution of electronic structure as a function of layer thickness in group-VIB transition metal dichalcogenides: emergence of localization prototypes.

Layered group-VIB transition metal dichalcogenides (with the formula of MX2) are known to show a transition from an indirect band gap in the thick n-monolayer stack (MX2)n to a direct band gap at the n = 1 monolayer limit, thus converting the system into an optically active material suitable for a variety of optoelectronic applications. The origin of this transition has been attributed predominantly to quantum confinement effect at reduced n. Our analysis of the evolution of band-edge energies and wave functions as a function of n using ab initio density functional calculations including the long-range dispersion interaction reveals (i) the indirect-to-direct band gap transformation is triggered not only by (kinetic-energy controlled) quantum confinement but also by (potential-energy controlled) band repulsion and localization. On its own, neither of the two effects can explain by itself the energy evolution of the band-edge states relevant to the transformation; (ii) when n decreased, there emerge distinct regimes with characteristic localization prototypes of band-edge states deciding the optical response of the system. They are distinguished by the real-space direct/indirect in combination with momentum-space direct/indirect nature of electron and hole states and give rise to distinct types of charge distribution of the photoexcited carriers that control excitonic behaviors; (iii) the various regimes associated with different localization prototypes are predicted to change with modification of cations and anions in the complete MX2 (M = Cr, Mo, W and X = S, Se, Te) series. These results offer new insight into understanding the excitonic properties (e.g., binding energy, lifetime etc.) of multiple layered MX2 and their heterostructures.

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides.

Atomically thin group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap in the visible frequency range, the pronounced spin-orbit coupling, the ultra-strong Coulomb interaction, and the rich physics associated with the valley degree of freedom. These 2D TMDs exhibit great potential for device applications and have attracted vast interest for the exploration of new physics. 2D TMDs have complex electronic structures which underlie their physical properties. Here we review the bulk electronic structures of these new 2D materials as well as the theoretical models developed at different levels, along which we sort out the understanding of the origins of a variety of properties observed or predicted.

Chemical vapour deposition of group-VIB metal dichalcogenide monolayers: engineered substrates from amorphous to single crystalline.

As structural analogues of graphene but with a sizeable band gap, monolayers of group-VIB transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se, Te, etc.) have emerged as the ideal two dimensional prototype for exploring fundamental issues in physics such as valley polarization, and for engineering a wide range of nanoelectronic, optoelectronic and photocatalytic applications. Recently, chemical vapour deposition (CVD) was introduced as a more efficient preparation method than traditional chemical or physical exfoliation options, and has allowed for the successful synthesis of large-area MX2 monolayers possessing a large domain size, high thickness uniformity and continuity, and satisfactory crystal quality. This tutorial review therefore focuses on introducing the more recent advances in the CVD growth of MX2 (MoS2, WS2, MoS2(1-x)Se2xetc.) monolayers via the sulphurisation/decomposition of pre-deposited metal-based precursors, or the one-step reaction and deposition of gaseous metal and chalcogen feedstocks. Differences in growth behaviour caused by commonly used amorphous SiO2/Si, and newly adopted insulating single crystal substrates such as sapphire, mica and SrTiO3, are also comparatively presented. Also discussed are the essential parameters that influence the growth of MX2, such as the temperature, the source-substrate distance and the composition of the carrier gas (Ar/H2). Finally, an assessment is provided for viable future pathways for fine-tuning of the domain size and orientation, thickness uniformity, and the bandgap of MX2 and its alloys.

Spin-resolved optical conductivity of two-dimensional group-VIB transition-metal dichalcogenides

We present an ab-initio study of the spin-resolved optical conductivity of two-dimensional (2D) group-VIB transition-metal dichalcogenides (TMDs). We carry out fully-relativistic density-functional-theory calculations combined with maximally localized Wannier functions to obtain band manifolds at extremely high resolutions and focus on the photo-response of 2D TMDs to circularly-polarized light in a wide frequency range. We present extensive numerical results for monolayer TMDs involving molybdenum and tungsten combined with sulphur and selenium. Our numerical approach allows us to locate with a high degree of accuracy the positions of the points in the Brillouin zone that are responsible for van Hove singularities in the optical response. Surprisingly, some of the saddle points do not occur exactly along high-symmetry directions in the Brillouin zone, although they happen to be in their close proximity.