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

Abstract

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 the 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) the phosphorene-like (Pmn2 1 ), which has already been synthesized, and (ii) the SiTe-type (P 3m1), that has been much less explored. Except for SnS, we find that the SiTe-type is the lowest energy structure and with 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, the MQs 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 Pmn2 1 phase, the MQs have wider band gaps and show 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/composition, making the refractive index a distinguishing parameter for identifying the different MQs. Finally, the 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.

Tuning Excitonic Properties of Monochalcogenides via Design of Janus Structures

“…First, we validate our calculations by comparing our equilibrium lattice parameters with available published data. 67 , 70 , 71 The lattice parameters for non-Janus structures shown in Table 1 are in good agreement with the reported values of 3.65 Å for SGeGeS, 3.81 Å for SeGeGeSe, 3.95 Å for SSnSnS, and 4.09 Å for SeSnSnSe obtained from DFT simulations at the same level as those adopted in our work.…”

“…Figure depicts ball-and-stick models for selected 2D monolayers, which have a hexagonal unit cell, as shown in Figure a. These structures originate from previously investigated centrosymmetric group-IV monochalcogenides with space group P 3̅ m 1. − However, the formation of Janus structures breaks the inversion-center symmetry (which is classified through the use of the Atomic Simulation Environment (ASE) tool) resulting in novel optoelectronic characteristics . Each material comprises four atomic layers, with two triangular layers of group-IV atoms sandwiched between triangular chalcogen lattices.…”