New C2h phase of group III monochalcogenide monolayers AlX (X = S, Se, and Te) with anisotropic crystal structure: first-principles study

Abstract

In this paper, we introduce a new phase of two-dimensional aluminum monochalcogenide, namely C_2h-AlX (X = S, Se, and Te). With the C_2h space group, C_2h-AlX possesses a large unit cell containing 8 atoms. The C_2h phase of AlX monolayers is found to be dynamically and elastically stable based on the evaluation of its phonon dispersions and elastic constants. The anisotropic atomic structure of C_2h-AlX leads to a strong anisotropy in its mechanical properties with Young's modulus and Poisson's ratio strongly dependent on the directions examined in the two-dimensional plane. All three monolayers of C_2h-AlX are found to be direct band gap semiconductors, which are compared with the indirect band gap semiconductors of available D_3h-AlX. Particularly, the transition from direct to indirect band gap is observed in C_2h-AlX when a compressive biaxial strain is applied. Our calculated results indicate that C_2h-AlX exhibits anisotropic optical characteristics and its absorption coefficient is high. Our findings suggest that C_2h-AlX monolayers are suitable for applications in next-generation electro-mechanical and anisotropic opto-electronic nanodevices.