Crystal structure and electrical and optical properties of two-dimensional group-IV monochalcogenides

Two-dimensional (2D) semiconductor materials offer a platform for unconventional applications such as valleytronics, flexible nanoelectronics, and hosts of quantum emitters. Many of these materials and their electronic properties remain to be explored. Using ab initio simulations based on the density functional theory, we investigate group-IV monochalcogenides MQ (M=Si, Ge,Sn and Q=S, Se, Te), an emerging class of 2D materials, with two competing crystal structures: (i) phosphorenelike (Pmn21), which has already been synthesized, and (ii) SiTe-type (P¯3m1), which has been much less explored. Except for Sn, we find that the SiTe type is the lowest-energy structure and has higher structural stability, motivating efforts to synthesize this less explored P¯3m1 phase. Regarding the optoelectronic properties of these two phases, in the P¯3m1 phase, MQ compounds have band gaps around the sunlight spectrum peak and show narrower variations in band gap with the composition and higher absorption coefficients for lighter chalcogens. In contrast, in the Pmn21 phase, MQ compounds have wider band gaps and show a band gap variation of up to 72% with composition, higher absorption coefficients with Te atoms, and potential for valleytronics. In particular, SiS shows interesting high optical anisotropy among all the investigated materials. Furthermore, the optical spectra present peaks that are particular to each phase or composition, making the refractive index a distinguishing parameter for identifying the different MQ compounds. Finally, a phase transition from monolayer to bulk due to an interaction between the layers is observed. Thus, the present results straighten out the role of the crystalline phase in the optoelectronic properties of these monochalcogenides.
This article was originally published in Physical Review B. The version of record is available at: ©2023 American Physical Society
Querne, Mateus B. P., Jean M. Bracht, Juarez L. F. Da Silva, Anderson Janotti, and Matheus P. Lima. “Crystal Structure and Electrical and Optical Properties of Two-Dimensional Group-IV Monochalcogenides.” Phys. Rev. B 108, no. 8 (August 2023): 085409.