Electric Field and Strain Tuning of 2D Semiconductor van der Waals Heterostructures for Tunnel Field-Effect Transistors

Enhancing magnetic coupling in MN4-graphene via strain engineering

MN4-embedded graphene (MN4-G) layers, with transition metal elements M, are experimentally accessible two-dimensional (2D) materials and show great potential for stable nanoscale magnetization. In these materials, the exchange couplings between magnetic atoms are predominantly governed by Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, exhibiting an unusual prolonged decay of r-n, where r is the M-M separation distance, and 0.5 ≤ n ≤ 2. In this paper, we explore the effects of induced strain on the electronic and magnetic properties of MN4-G layers through ab initio density functional theory. We employ a specific method to apply strain by positioning atoms from one layer within the equilibrium structure of another layer, thereby inducing strain in the form of either tension or compression. The induced strain results in an approximate ±0.4% variation in the unit-cell area of the MN4-G lattice. Our findings reveal that while the exchange coupling mechanism remains unaffected, the strength, amplitude, and decay rate of the RKKY coupling are significantly influenced by the induced strain. Notably, the CoN4-G layer exhibits a remarkable increase in the strength and oscillation amplitude of the RKKY coupling, along with a reduced decay rate. Additionally, the electronic and magnetic properties of the CuN4-G layers remain unchanged under induced strain.

Exploring the versatility of MoSe2/WS2 heterostructures

Two-dimensional materials and their combined heterostructures have paved the way for numerous next-generation electronic and optoelectronic applications. Herein, we performed first principles calculations to computationally design the MoSe2/WS2 heterostructure and consider its geometric structure, electronic properties and contact behavior, as well as the effects of the electric fields and strain. Our results show that the MoSe2/WS2 heterostructure is energetically, thermodynamically and mechanically stable. Depending on the stacking configurations, the MoSe2/WS2 heterostructure could form type-I or type-II band alignment. The versatility in contact behavior makes the MoSe2/WS2 heterostructure attractive in electronics and optoelectronics. The combination of the MoSe2/WS2 heterostructure also leads to an enhancement in the adsorption efficiency and the carrier mobility compared with the constituent components. More interestingly, our findings demonstrate that the electric field can induce the transition between type-I and type-II band alignments, as evident by the experimental measurement [J. Kistner-Morris, A. Shi, E. Liu, T. Arp, F. Farahmand, T. Taniguchi, K. Watanabe, V. Aji, C. H. Lui and N. Gabor, Nat. Commun., 2024, 15, 4075]. Additionally, we also find that strain can also induce the transition between type-I and type-II band alignments and lead to the transition from semiconductor to metal in the MoSe2/WS2 heterostructure. Our findings prove that the MoSe2/WS2 heterostructure holds significant potential for developing next-generation electronics.

Strain and Electric Field Engineering of G-ZnO/SnXY (X, Y = S, Se) S-Scheme Heterostructures for Photocatalyst and Electronic Device Applications: A Hybrid DFT Calculation

Using hybrid density functional theory calculations, we systematically study the biaxial strain and electric field modulated electronic properties of g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe S-scheme van der Waals heterostructures (vdWHs). g-ZnO/SnS2 and g-ZnO/SnSSe are found to be promising photocatalysts for water splitting with high solar-to-hydrogen efficiencies, even under acidic, alkaline, and high-stress conditions. The strain effect on the bandgaps of g-ZnO/SnXY is explained in detail according to the correlation between geometry structure and orbital hybridization of SnXY, which could help understand the strain-induced band structure evolutions in other SnXY (X, Y = S, Se)-based vdWHs. It is surprising that under an external electric field, g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe can offer the occupied nearly free-electron (NFE) states. In many materials, NFE states are usually unoccupied and is not conducive to the charge transport. The NFE state in g-ZnO/SnSe2 is the most sensitive to the electric field and might be promising electron transport channel in nanoelectronic devices. g-ZnO/SnSe2 might also have application potential in gas sensors and high-temperature superconductors.

Systematic DFT Modeling van der Waals Heterostructures from a Complete Configurational Basis Applied to γ-PC/WS2

Periodic boundary conditions in density functional theory (DFT)-based modeling of bilayer van der Waals heterostructures introduce an artificial lock to a metastable configuration. Depending on the initial supercell, geometric optimization may reach local energy minima at a fixed twist-angle in a restricted strain-space. In this work, an algorithm was introduced for generating a complete scope of ways to combine two monolayer unit cells into a common supercell. In its application to γ-PC/WS2, 18,123 bilayer supercells were derived, for which the constituting monolayers possessed isotropic strains, anisotropic strains, or intralayer shear strains. Based on analysis, 45 isotropically strained configurations were carefully chosen for optimization by DFT. Geometric and energetic features and band structures were revealed and compared, following the variations at different strains and twist-angles. As such, this case study brought to resolution the impacts of supercell construction on DFT's outcomes and the merits of in-depth screening of the different options. Repetitions and extensions to the demonstrated approach may be applied to characterize van der Waals heterostructures and derivatives in the future.

Properties of Layered TMDC Superlattices for Electrodes in Li-Ion and Mg-Ion Batteries

In this work, we present a first-principles investigation of the properties of superlattices made from transition metal dichalcogenides for use as electrodes in lithium-ion and magnesium-ion batteries. From a study of 50 pairings, we show that, in general, the volumetric expansion, intercalation voltages, and thermodynamic stability of vdW superlattice structures can be well approximated with the average value of the equivalent property for the component layers. We also found that the band gap can be reduced, improving the conductivity. Thus, we conclude that superlattice construction can be used to improve material properties through the tuning of intercalation voltages toward specific values and by increasing the stability of conversion-susceptible materials. For example, we demonstrate how pairing SnS2 with systems such as MoS2 can change it from a conversion to an intercalation material, thus opening it up for use in intercalation electrodes.