Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi${}_2$N${}_4$ and WSi${}_2$N${}_4$ were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA${}_2$Z${}_4$, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi${}_2$P${}_4$ and MoSi${}_2$As${}_4$ by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi${}_2$Z${}_4$ (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi${}_2$Z${}_4$-based electronic and optoelectronic devices. We also build a monolayer MoSi${}_2$Z${}_4$-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi${}_2$Z${}_4$ and expands their potential application in nanoscale electronic and optoelectronic devices.

In this paper, we present a comprehensive study on the electronic and optoelectronic properties of indium monochalcogenide (InX with X = S, Se, Te) monolayers with and without strains. Our results show that InX monolayers are indirect semiconductors. Upon the application of strain, the band structures can be modulated and an indirect-to-direct bandgap transition is observed in an InSe monolayer. The electron mobility of up to 2.0 × 10³ cm² (V s)⁻¹ is quantitatively determined in the framework of deformation potential theory. Though the mobility of holes is relatively small, it can be greatly improved by introducing compressive strain, with a value up to 2.8 × 10³ cm² (V s)⁻¹. In addition, the performance of the photoresponse of InX monolayers is evaluated based on first-principles calculations. Under illumination, the InX based systems exhibit high photoresponsivity (R_ph = 0.18 A W⁻¹) and external quantum efficiency (EQE = 62.5%), which can be further enhanced via strain. Owing to such excellent electronic and optoelectronic merits, InX monolayers will become promising candidates for next-generation ultrathin and flexible electronic and optoelectronic devices.

As conventional Si-based devices approach their scaling limit, it is of great significance to find new materials for future electronic logic devices. The emerging two-dimensional (2D) materials with atomic thickness have attracted intense interests for their exotic properties. However, the presence of the Schottky barrier limits their applications, which is difficult to control over due to the Fermi level pinning effect. Therefore, searching for low resistance metal contact to 2D semiconductors becomes one of the most important topics. Here, we report that Ohmic contact can be realized in a monolayer InSe–Cu system. Based on the density functional theory combined with the nonequilibrium Green’s functions, the geometry, overlapping states, tunneling barrier, Schottky barrier, and band alignment at the interface of group-IB (Cu, Ag, and Au) with InSe monolayer are discussed in details. Our results reveal that Cu, the most common electrode used in the industry, shows great potential to form favorable contact with single layer InSe due to the strong interaction and high orbital overlapping. The calculated drain-source current versus bias voltage ( I − V ) curve exhibits linear behavior, indicating good Ohmic contact between the Cu electrodes and InSe channel. Our work may pave the way for design of next-generation ultrathin and flexible devices.

Lateral heterostructures have attracted a great deal of attention due to their advanced properties, which may open up unforeseen opportunities in materials science and device physics. Here, we demonstrate a novel type of lateral heterostructure within monolayer indium monochalcogenides. The thermal stability of the structure is obtained based on the ab initio molecular dynamics calculations. Our results reveal that the proposed lateral heterostructures have direct bandgaps, tunable electronic properties, and type-II band alignment. In addition, the predicted carrier mobilities exceed 10³ cm² (V s)⁻¹, which are 1–2 orders of magnitude higher compared to those of transition metal chalcogenide (TMD) materials. For the first time, the photoresponse and photovoltaic performance of such lateral heterostructures are evaluated based on the first-principles calculations. Upon illumination, the photoinduced current is generated throughout the heterojunction, with an external quantum efficiency up to 7.1%. These results make indium monochalcogenide lateral heterostructures promising candidates for next-generation of electronic and optoelectronic devices.

The intrinsic magnetism has long been pursued in two-dimensional (2D) materials down to one-atomic layer thickness. But only very recently, the intrinsic magnetism of monolayer CrI₃, Fe₃GeTe₂, FePS₃, VSe₂ and bilayer Cr₂Ge₂Te₆ are verified in experiment by optical measurement, Raman spectrum and conventional magnetism measurement. Among them, the intralayer exchange interaction of FePS₃ is antiferromagnetic while all the others are ferromagnetic. Most of the ferromagnetic orders in these materials are induce by super exchange interaction. Monolayer Fe₃GeTe₂ and VSe₂ exhibit metallic character while all the others are semiconductor or insulator. Stable spontaneous magnetization can exist in these monolayer 2D materials because of their strong anisotropy. The anisotropy is mostly from the strong spin–orbit coupling of heavy atoms (CrI₃, Cr₂Ge₂Te₆, Fe₃GeTe₂). Asymmetric lattice distortion (FePS₃) or the increased density of state near Fermi level (VSe₂) may also contribute to the anisotropy. The relationship between anisotropy and stable spontaneous magnetization are discussed based on spin wave theory and Mermin-Wagner theorem. About the application, spintronics may be the most direct benefitted field. Considering the relationship between conductance and magnetic structure, the applications related with the transport property are also widely investigated. Similarly, as the coupling between spin, phonon and photon are prominent in these magnetic 2D materials, the applications based on the magnetocaloric effect and magneto-optic effect are promising. And these magnetic 2D materials may be also applied as catalyst in water-splitting or electrode of supercapacitor.