Influence of shear strain on the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdW heterostructure

CONTEXT

This study investigates the dynamic stability of monolayers MoS2, WS2, and MoS2/WS2 van der Waals heterostructures (vdWHs) and the influence of shear strain on their electronic properties. The computational results of the binding energy and phonon dispersion demonstrate the excellent dynamic stability of MoS2/WS2 vdWHs. The MoS2/WS2 vdWH, with a type-II band alignment and an indirect bandgap, reduces electron-hole recombination, enhancing the efficiency and performance of optoelectronic devices. Under shear strain, the bandgap size and type of monolayers MoS2, WS2, and MoS2/WS2 vdWHs were effectively modulated, along with the interlayer charge redistribution in the MoS2/WS2 vdWHs. This work reveals the tunability of the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdWHs under shear strain, offering new possibilities and solutions for developing optoelectronic devices, sensors, and related fields.

METHODS

This work employed the CASTEP module within the Materials Studio software package for first-principles calculations. Ultrasoft pseudopotentials were employed during geometry optimizations to account for ion-electron interactions using the GGA-PBE functional for exchange-correlation potentials. The electronic configurations of the S, Mo, and W atoms were chosen as their typical arrangements: (3s2p4), (4s2p6d55s1), and (5s2p6d46s2), respectively. A vacuum layer of 20 Å was added to avoid interactions between the atomic layers. A cutoff energy of 500 eV was set for structural optimization and self-consistent calculations, with k-point grids of 6 × 6 × 1 and 9 × 9 × 1. During the structural optimization process, the energy convergence criterion was set to 1 × 10-5 eV, and the thresholds for interatomic forces and stresses were set to 0.01 eV/Å and 0.01 GPa, respectively. Grimmer's DFT-D2 correction accounted for the interlayer vdW interactions in the MoS2/WS2 vdWH, while the phonon dispersion was calculated using the linear response method.

From Two-, One-, to Zero-Dimensional Vacancies: A Densification Pattern for a Typcial Transition-Metal Dichalcogenide of TiSe2

In the areas of condensed matter physics, geoscience, material science, and inorganic chemistry, how the crystal structures evolve under an external field such as high-pressure is a fundamental question. By taking TiSe₂ as the case, we investigate the phase transformations of the layered transition-metal dichalcogenides (TMDs) under high-pressure. The ambient 6-fold P-3m1 TiSe₂ undergoes a transformation into the monoclinic 8-fold coordinated C2/m phase at 15 GPa and then into the hexagonal 9-fold Fe₂P-type structure at 34 GPa. The above phase transitions can be unitedly described as the evolution of the vacancies: from a layered structure with two-dimensional (2D) vacancies to the structure with one-dimensional (1D) and zero-dimensional (0D) vacancies. The proposed densification model of TiSe₂ reveals the processes how the symmetry breaking phase of spatial chemical bonding restores the symmetry under the isotropic external pressure.

2D Materials and Heterostructures at Extreme Pressure

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Co-segregation behavior of Sc and Zr solutes and their effect on the Al Σ5 (210) [110] symmetrical tilt grain boundary: a first-principles study

The research into the co-segregation behavior of Sc and Zr solutes and their effect on the mechanical properties of the Al Σ5 (210) [110] grain boundary was carried out by first principles calculations. It is concluded that Sc and Zr both have a powerful driving force to segregate to the grain boundary, and based on the most negative segregation energy, first one Sc atom segregates to the grain boundary, then one Zr atom and finally another Sc atom. The grain boundary energy, strengthening/embrittling energy, fracture energy and theoretical tensile peak stress all demonstrate that Sc and Zr solutes have a strengthening effect on the grain boundary, which can be attributed to the combination of the "chemical effect" (charge density variation) and the "structural effect" (atomic arrangement change), especially the migration of the first Sc-segregated site toward the grain boundary, which not only brings stronger Sc/Zr-Al bonds instead of weak Al-Al bonds but also leads to the shrinkage and charge accumulation of the great vacuum area between two grains. This work sheds light on the underlying mechanism of the better mechanical performance due to the co-addition of Sc and Zr solutes at the atomic and electronic levels.

Bilayer MSe2 (M = Zr, Hf) as promising two-dimensional thermoelectric materials: a first-principles study

Motivated by experimental synthesis of two-dimensional MSe₂ (M = Zr, Hf) thin films, we investigate the thermoelectric transport properties of MSe₂ (M = Zr, Hf) bilayers by using first-principles calculations and Boltzmann transport theory.

Ferromagnetism in magnesium chloride monolayer with an unusually large spin-up gap

The primary research target of the rapidly evolving spintronic industry is to design highly efficient novel materials that consume very low power and operate with high speed. Main group based ferromagnetic half-metallic materials are very promising due to their long spin-relaxation time. In recent years, the discovery of superconducting state with high critical temperature in a magnesium based system (MgB₂) invigorated researchers due to its simple crystal structure and intriguing results, leading to its use as a good material for large scale application in electronic devices. Here, we report ferromagnetism and strong half-metallicity in another Mg-based system, which can be a promising material for spintronics based devices rather than for electronic devices (such as MgB₂). Based on the first principle calculations, we report here a series of magnetic half-metallic magnesium chloride based monolayers [Mg_0.89δ_0.11Cl₂, Mg_0.78δ_0.22Cl₂, and Mg_0.67δ_0.33Cl₂ (MgCl₃)]. This MgCl₃ phase has a similar pattern as that in CrI₃, which has drawn remarkable attention worldwide as the first intrinsic 2D magnet. These magnesium chloride monolayer based systems are 100% spin-polarized, and promising for scattering-less transport due to strong half-metallicity and large spin-up gap (∼6.135-6.431 eV). The unusually large spin-up gap in our proposed system may shield spin current leakage even in nanoscale device. Further investigation explores a ferromagnetic ordering in Mg_0.89δ_0.11Cl₂ with a Curie temperature of 250 K, which makes the system viable for operation at temperatures slightly lower than the room temperature. High magnetic anisotropy energy (MAE) in Mg_0.89δ_0.11Cl₂ (452.84 μeV) indicates that the energy required to flip the spin is high, and therefore inhibits spin fluctuation. These results suggest a promising way to discover MgCl₂-based 2D spin valves, GMR, TMR and other spintronics devices.

Pressure-induced phase transition, metallization and superconductivity in ZrS2

Zirconium disulfide (ZrS2) is an exemplary case among layered materials that exhibit unusual electronic and vibrational properties, with applications in potential photovoltaic and single-layer transistor materials. Here, we examine the effect of pressure on the structural stability, phonon dispersion, electronic properties and electron-phonon coupling of ZrS2 using first-principles calculations. Our results unravel that ZrS2 undergoes several pressure-induced phase transformations from the ambient-pressure P3[combining macron]m1 structure to a monoclinic P21/m structure at 2.0 GPa, to an orthorhombic Immm structure at 5.6 GPa, and to a tetragonal I4/mmm structure at 25.0 GPa. The electronic band calculations indicate that the layered P3[combining macron]m1 and P21/m structures are narrow-gap semiconductors. The gaps of the above two phases, which are normal semiconductors, decrease with pressure. Our results show that ZrS2 reaches the metallic state by a P21/m → Immm phase transition and keeps its metallic state in the I4/mmm phase. A pressure-driven evolution of the topological Fermi surface has been uncovered. The electron-phonon coupling results identify superconducting states in both metallic Immm and I4/mmm structures. Our research shows that pressure is efficient in the modulation of the bonding states, crystal structures and electronic properties of ZrS2, which will stimulate further high-pressure structural and conductive measurements.

Electronic structure of ZrX2 (X = Se, Te)

The electronic structure of the ZrX₂ (X = Se, Te) compounds has been studied using photoelectron, resonant photoelectron and X-ray absorption spectroscopy, theoretical calculations of the X-ray absorption spectra, and density of electronic states. It was found that the absorption spectra and valence band spectra are influenced by the chalcogen type. The results of the multiplet calculation of the Zr⁴⁺ atom show that the change in the splitting in the crystal field, which is described by the 10Dq parameter, is due to the change in the ratio of covalent and ionic contributions to the chemical bond. The resonance band near the Fermi level in the valence band spectra is observed for ZrTe₂ in the Zr 3p-4d resonant excitation mode. The extent of photon energy indicates the charge localization on the Zr atom. Similar resonance band for ZrSe₂ is absent; it indicates the presence of a gap at the Fermi level.

Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX₂; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin-orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7-3.1 and 1.9-3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M⁴⁺ and X^2-) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo⁴⁺ < W⁴⁺) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S^2- < Se^2-). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS₂, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression.

Ultra-narrow blue phosphorene nanoribbons for tunable optoelectronics

We report optoelectronic properties of ultra-narrow blue phosphorene nanoribbons (BPNRs) within the state-of-the-art density functional theory framework.

Electronic properties of layered phosphorus heterostructures

By using ab initio approaches, the electronic properties of vertical heterostructured compounds of different structural phases of layered phosphorus have been studied.

Electric field induced insulator to metal transition in a buckled GaAs monolayer

We investigate the electronic properties of two-dimensional buckled honeycomb GaAs in the presence of an external electric field using first principles calculations.

Tuning Dirac points by strain in MoX2nanoribbons (X = S, Se, Te) with a 1T′ structure

We report the tunability of the topological edge state by strain engineering in quasi-one-dimensional nanoribbons of 1T′-MoX₂with X = (S, Se, Te) through first-principles calculations.

Electronic structure engineering of various structural phases of phosphorene

We report the tailoring of the electronic structures of various structural phases of phosphorene (α-P, β-P, γ-P and δ-P) based homo- and hetero-bilayers through in-plane mechanical strains, vertical pressure and transverse electric field by employing density functional theory.