Modulation of electronic properties from stacking orders and spin-orbit coupling for 3R-type MoS2

Rhombohedrally stacked layered transition metal dichalcogenides and their electrocatalytic applications

Layered transition metal dichalcogenides (TMDCs) are extensively investigated as catalyst materials for a wide range of electrochemical applications due to their high surface area and versatile electronic and chemical properties. Bulk TMDCs are van der Waals solids that possess strong in-plane bonding and weak inter-layer interactions. In the few-layer 2D TMDCs, several polymorphic structures have been reported as each individual layer can either retain octahedral or trigonal prismatic coordination. Among them, 1T (tetragonal), 2H (hexagonal) and 3R (rhombohedral) phases are very common. These polymorphs can display discrepancies in their catalytic activity as their electronic structure diverges due to different d orbital filling states. The broken inversion symmetry and large exposed edge sites of some of the 3R-phase TMDCS such as MoS2, NbS2 and TaS2 appear to be advantageous for electrocatalytic water reduction and battery applications. We describe recent studies in phase engineering of 2D TMDCs, particularly aiming at the 3R polytype and their electrocatalytic properties. Redox ability primarily depends on a distinct polymorphic phase in which TMDCs are isolated, and hence, with rich polymorphic structures being reported, numerous new catalytic applications can be realized. Phase conversion from 2H to 3R phase in some TMDCs enhances structural integrity and establishes robustness under harsh chemical conditions.

Shear Strain-Induced Two-Dimensional Slip Avalanches in Rhombohedral MoS2

Slip avalanches are ubiquitous phenomena occurring in three-dimensional materials under shear strain, and their study contributes immensely to our understanding of plastic deformation, fragmentation, and earthquakes. So far, little is known about the role of shear strain in two-dimensional (2D) materials. Here we show some evidence of 2D slip avalanches in exfoliated rhombohedral MoS2, triggered by shear strain near the threshold level. Utilizing interfacial polarization in 3R-MoS2, we directly probe the stacking order in multilayer flakes and discover a wide variety of polarization domains with sizes following a power-law distribution. These findings suggest that slip avalanches can occur during the exfoliation of 2D materials, and the stacking orders can be changed via shear strain. Our observation has far-reaching implications for the development of new materials and technologies, where precise control over the atomic structure of these materials is essential for optimizing their properties as well as for our understanding of fundamental physical phenomena.

Experimental Realization and Computational Investigations of B2S2 as a New 2D Material with Potential Applications

A new two-dimensional material B₂S₂ has been successfully synthesized for the first time and validated using first-principles calculations, with fundamental properties analyzed in detail. B₂S₂ has a similar structure as transition-metal dichalcogenides (TMDs) such as MoS₂, and the experimentally prepared free-standing B₂S₂ nanosheets show a uniform height profile lower than 1 nm. A thickness-modulated and unique oxidation-level dependent band gap of B₂S₂ is revealed by theoretical calculations, and vibration signatures are determined to offer a practical scheme for the characterization of B₂S₂. It is shown that the functionalized B₂S₂ is able to provide favorable sites for lithium adsorption with low diffusion barriers, and the prepared B₂S₂ shows a wide band photoluminescence response. These findings offer a feasible new and lighter member for the TMD-like 2D material family with potential for various aspects of applications, such as an anode material for Li-ion batteries and electronic and optoelectronic devices.

Modulating the stacking modes of nanosized metal-organic frameworks by morphology engineering for isomer separation

Modulating different stacking modes of nanoscale metal-organic frameworks (MOFs) introduces different properties and functionalities but remains a great challenge. Here, we describe a morphology engineering method to modulate the stacking modes of nanoscale NU-901. The nanoscale NU-901 is stacked through solvent removal after one-pot solvothermal synthesis, in which different morphologies from nanosheets (NS) to interpenetrated nanosheets (I-NS) and nanoparticles (NP) were obtained successfully. The stacked NU-901-NS, NU-901-I-NS, and NU-901-NP exhibited relatively aligned stacking, random stacking, and close packing, respectively. The three stacked nanoscale NU-901 exhibited different separation abilities and all showed better performance than bulk phase NU-901. Our work provides a new morphology engineering route for the modulation of the stacking modes of nano-sized MOFs and improves the separation abilities of MOFs.

Electrically Tunable Valley Dynamics in Twisted WSe_{2}/WSe_{2} Bilayers

The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSe_{2}/WSe_{2} bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40  ns) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from ∼80% in the n-doped regime to <5% in the p-doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

Bandgap engineering of few-layered MoS2 with low concentrations of S vacancies

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications. In this work, systematic density functional theory (DFT) calculations were carried out for few-layered 3R-MoS₂ with different concentrations of S vacancies. All results revealed that the defect energy levels introduced on both sides of the Fermi level formed an intermediate band in the band gap. Both the edges of the intrinsic and intermediate bands of the structures with the same type of vacancies were generally closer to the Fermi level, and the gaps decreased as the number of layers increased from 2 to 4. The preferentially formed S vacancies at the top layer and the transition of defect types from point to line led to similar indirect band gaps for 2- and 4-layered 3R-MoS₂ with a low bulk concentration (around 5%) of S vacancies. This is different from most reported results about transition metal dichalcogenide (TMD) materials that the indirect band gap decreases as the number of layers increases and the low concentrations of vacancies show negligible influence on the band gap value.

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications.

Hydrogenated-Graphene-Encapsulated Graphene: A Versatile Material for Device Applications

Graphene and its heterostructures exhibit interesting electronic properties and are explored for quantum spin Hall effect (QSHE) and magnetism-based device applications. In present work, we propose a heterostructure of graphene encapsulated by hydrogenated-graphene, which could be a promising candidate for a variety of device applications. We have carried out DFT calculations on this system to check its feasibility to be a versatile material. We found that electronic states of multilayer pristine graphene, especially the Dirac cone, an important feature to host QSHE, can be preserved by sandwiching it by fully hydrogenated graphene. The interference of electronic states of hydrogenated graphene was insignificant with those of graphene. States of graphene were also found to be stable upon application of an electric field up to ±2.5 V/nm. For device applications, multilayer graphene or its heterostructures are required to be deposited on a substrate, which interacts with the system opening up a gap at the Dirac cone making it less suitable for QSHE applications, and hydrogenated graphene can prevent it. Magnetization in these hydrogenated-graphene-sandwiched graphene systems may be induced by creating vacancies or distortions in hydrogenated graphene, which was found to have a minimal effect on graphene's electronic states, thus providing an additional degree of manipulation. We also performed a set of calculations to explore its applicability for detecting some molecules. Our results on trilayer graphene encapsulated by hydrogenated graphene indicate that all these observations can be generalized for systems with a larger number of graphene layers, indicating that multilayer graphene sandwiched between two hydrogenated graphene is a versatile material that can be used in QSHE and sensor devices.

Optical control of the layer degree of freedom through Wannier-Stark states in polar 3R MoS2

Electrons in two-dimensional layered crystals gain a discrete positional degree of freedom over layers. We propose the two-dimensional transition metal dichalcogenide homostructure with polar symmetry as a prototypical platform where the degrees of freedom for the layers and valleys can be independently controlled through an optical method. In 3R MoS₂, a model system, the presence of the spontaneous polarization and built-in electric field along the stacking axis is theoretically proven by the density functional theory. The K valley states under the electric field exhibit Wannier-Stark type localization with atomic-scale confinement driven by double group symmetry. The simple interlayer-dynamics-selection rule of the valley carriers in 3R homostructure enables a binary operation, upward or downward motion, using visible and infrared light sources. Together with the valley-index, a 2 [Formula: see text] 2 states/cell device using a dual-frequency polarized light source is suggested.

Thermally driven homonuclear-stacking phase of MoS2 through desulfurization

Engineering phase transitions or finding new polymorphs offers tremendous opportunities for developing functional materials. We reveal that the thermally driven desulfurization of single-crystalline MoS2 samples improves transport properties by reducing the band gap and further induces metallization. Semi-desulfurization, i.e., removal of the topmost S layer, results in the placement of the exposed Mo layers directly on top of the following sub-layers, together with the bottom S layer of the top layer. This homonuclear (AA) stacking derived from the AA' stacking of the hexagonal (2H) phase is retained even after further desulfurization of the remaining bottom S layer, i.e., full desulfurization of the top layer. Our findings fundamentally explain why the 2H phase of TMDs is characterized by AA' stacking.

Modulation of Hydrogen Evolution Catalytic Activity of Basal Plane in Monolayer Platinum and Palladium Dichalcogenides

With an appropriate catalyst, hydrogen evolution reaction (HER) by water splitting can be used to produce hydrogen gas. Recently, layered transition-metal dichalcogenides have been proposed as alternative HER catalysts. However, a significant challenge is how to obtain the high-density active sites. With first-principle calculations, we explore the possibility of defect engineering to trigger the HER catalytic activity of basal plane by analyzing monolayer PdSe₂, PtSe₂, PdTe₂, and PtTe₂. It is found that the double-vacancy DV_Se (DV_Te) and B-doping can modulate appropriately the interaction between H and basal plane and improve the HER activity of these transition-metal dichalcogenides. Especially, the B-doping with high concentration can increase enormously the density of active sites on basal plane.

Stacking change in MoS2 bilayers induced by interstitial Mo impurities

We use a theoretical approach to reveal the electronic and structural properties of molybdenum impurities between MoS₂ bilayers. We find that interstitial Mo impurities are able to reverse the well-known stability order of the pristine bilayer, because the most stable form of stacking changes from AA’ (undoped) into AB’ (doped). The occurrence of Mo impurities in different positions shows their split electronic levels in the energy gap, following octahedral and tetrahedral crystal fields. The energy stability is related to the accommodation of Mo impurities compacted in hollow sites between layers. Other less stable configurations for Mo dopants have larger interlayer distances and band gaps than those for the most stable stacking. Our findings suggest possible applications such as exciton trapping in layers around impurities, and the control of bilayer stacking by Mo impurities in the growth process.