By first-principles calculations, we find that valleytronic and piezoelectric properties coexist in Janus MoAZ₃H ML materials, providing wide applications in the fields of valleytronics and energy conversion for MoAZ₃H MLs.

On the basis of density functional theory (DFT) calculations, we propose a stable two-dimensional (2D) monolayer phosphorus carbide (PC) with a GaSe-like structure, which has intriguing electronic and optical properties. Our calculated results show that this 2D monolayer structure is more stable than the other allotropes predicted by Tománek et al. [Nano Lett., 2016, 16, 3247–3252]. More importantly, this structure exhibits superb optical absorption, which can be mainly attributed to its direct band gap of 2.65 eV. The band edge alignments indicate that the 2D PC monolayer structure can be a promising candidate for photocatalytic water splitting. Furthermore, we found that strain is an effective method used to tune the electronic structures varying from direct to indirect band-gap semiconductor or even to metal. In addition, the introduction of one carbon vacancy in such a 2D PC structure can induce a magnetic moment of 1.22 µ_B. Our findings add a new member to the 2D material family and provide a promising candidate for optoelectronic devices in the future.

In this paper we investigate the electronic and optical dielectric properties of lateral and vertical heterostructures composed of two-dimensional (2D) $\mathrm{Zr}{\mathrm{S}}_2$ and $\mathrm{Hf}{\mathrm{S}}_2$ monolayers based on density-functional theory. First, we show that the bulk and monolayer $\mathrm{Zr}{\mathrm{S}}_2$ and $\mathrm{Hf}{\mathrm{S}}_2$ as well as the vertical ${\left(\mathrm{Zr}{\mathrm{S}}_2\right)}_m/{\left(\mathrm{Hf}{\mathrm{S}}_2\right)}_n$ heterostructures are indirect band-gap semiconductors, while the lateral heterostructures exhibit an indirect to direct band-gap transition. Then we demonstrate that the optical properties of the bulk and monolayer $\mathrm{Hf}{\mathrm{S}}_2$ and $\mathrm{Zr}{\mathrm{S}}_2$ are strongly anisotropic; for the bulk $\mathrm{Hf}{\mathrm{S}}_2$ and $\mathrm{Zr}{\mathrm{S}}_2$, the in-plane components of the dielectric function are negative in a certain frequency band, where they can work as naturally hyperbolic metamaterials. Interestingly, the vertical heterostructures also possess a hyperbolic region, whose position and width can be tunable with the thickness ratio of constituents. It is also found that the $\left(\mathrm{Zr}{\mathrm{S}}_2\right)/\left(\mathrm{Hf}{\mathrm{S}}_2\right)$ vertical heterostructures can enhance spontaneous emission and about 100-fold improvement of the Purcell factor is obtained. These results prove the feasibility of 2D material heterostructures to realize tunable hyperbolic metamaterials; the heterostructures present a promising opportunity for the practical applications in light-generation technologies.