Very recently, a new type of two-dimensional layered material, ${\mathrm{MoSi}}_2{\mathrm{N}}_4$, was fabricated that is semiconducting with weak interlayer interaction, high strength, and excellent stability. We systematically investigate the effect of vertical strain on the electronic structure of ${MA}_2{Z}_4$ ($M=\text{Ti/Cr/Mo}, A=\mathrm{Si}, Z=\text{N/P}$) bilayers. Taking bilayer ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ as an example, our first-principles calculations show that its indirect band gap decreases monotonically as the vertical compressive strain increases. Under a critical strain around 22%, it undergoes a transition from semiconductor to metal. We attribute this to the opposite energy shift of states in different layers, which originates from the built-in electric field induced by the asymmetric charge transfer between two inner sublayers near the interface. Similar semiconductor to metal transitions are observed in other strained ${MA}_2{Z}_4$ bilayers, and the estimated critical pressures to realize such transitions are within the same order as semiconducting transition metal dichalcogenides. The semiconductor to metal transitions in the family of ${MA}_2{Z}_4$ bilayers present interesting possibilities for strain-induced engineering of their electronic properties.

The recently synthesized ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayer [ Science 369, 670 (2020)] exhibit outstanding environmental stability, moderate band gap, and excellent mechanical properties, which opens up a new avenue for the explorations of two-dimensional (2D) ${\mathrm{MA}}_2{\mathrm{Z}}_4$ materials. Inspired by this finding, we perform comprehensive structural predictions of ${\mathrm{MSi}}_2{\mathrm{N}}_4$ monolayers with lanthanide and actinide metals at the M site. Using the CALYPSO structural search method and first-principles calculations, we identify seven ${\mathrm{MSi}}_2{\mathrm{N}}_4$ monolayers with robust ambient stabilities, four of which are metals (M = Tm, Lu, Pa, Np) and three are semiconductors (M = Ce, Th, U). Of particular interest is the ${\mathrm{ThSi}}_2{\mathrm{N}}_4$ monolayer, which is an indirect-gap semiconductor with ultra-high electron mobility of 14384 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Our results enrich the 2D ${\mathrm{MA}}_2{\mathrm{Z}}_4$ family and offer insights into the design and synthesis of novel multifunctional materials.

The alloy strategy through the A- or X-site is a common method for experimental preparation of high-performance and stable lead-based perovskite solar cells. As one of the important candidates for lead-free and stable photovoltaic absorbers, the inorganic antiperovskite family has recently been reported to exhibit excellent optoelectronic properties. However, the current reports on the design of antiperovskite alloys are rare. In this work, we investigated the previously overlooked electronic property (e.g., conduction band convergence), static dielectric constant, and exciton binding energy in inorganic antiperovskite nitrides by first-principles calculations. Then, we revealed a linear relationship between the tolerance factor and various physical quantities. Guided by the established structure–composition–property relationship in six antiperovskite nitrides X₃NA (X²⁺ = Mg²⁺, Ca²⁺, Sr²⁺; A^3– = P^3–, As^3–, Sb^3–, Bi^3–), for the first time, we designed a promising antiperovskite alloy Mg₃NAs_0.5Bi_0.5 with a quasi-direct band gap of 1.402 eV. Finally, we made a comprehensive comparison between antiperovskite nitrides and conventional halide perovskites for pointing out the future direction for device applications.

In this paper, we theoretically investigated the electronic properties of monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ by combining first-principles calculations and symmetry analyses. Spin-orbital coupling resulted in band splitting, whereas a horizontal mirror symmetry constrained the spin polarization to be along the $z$ direction. In addition, a three-band tight-binding model was constructed to describe the low-energy quasiparticle states of monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$, which can be generalized to strained $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ and its derivatives. The calculations using the tight-binding model showed an undamped $\sqrt{q}$-dependent plasmon mode, consistent with the results of first-principles calculations. The developed model is suitable for future theoretical and numerical investigations of low-energy properties in $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ family materials. Furthermore, the study of the electronic properties of monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ paves a way for its applications in spintronics and plasmonics.

Inorganic-organic hybrid lead halide perovskites have shown great prospects in solar cells in recent years, and their power conversion efficiency (PCE) has reached up to $25.5\%$. However, Pb toxicity and poor ambient stability have remained two unsolved urgent problems. To address these problems, more and more researchers have begun to focus on the application of inorganic lead-free perovskites in the photovoltaic field. Herein, several cerium-based lead-free inorganic chalcogenide perovskites $A{\mathrm{CeTe}}_3$ $(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$ are proposed as suitable optical absorbers for thin-film solar cells based on first-principles calculations. The distorted-phase $A{\mathrm{CeTe}}_3$ perovskites are predicted as direct-band-gap semiconductors with a gap value of 1.02–1.13 eV. Moreover, the distorted $A{\mathrm{CeTe}}_3$ not only possess a strong optical absorption coefficient $({10}^5–{10}^6{\mathrm{cm}}^{-1})$, superior to ${\mathrm{MAPbI}}_3 ({\mathrm{MA}}^{+}={\mathrm{CH}}_3{\mathrm{NH}}_3^{+})$, but also have excellent PCE, especially for ${\mathrm{BaCeTe}}_3$, whose PCE can reach $\sim 29\%$ even in a thin film $\left(1\mu \mathrm{m}\right)$. All these characteristics render distorted $A{\mathrm{CeTe}}_3$ perovskites promising photovoltaic materials.