New C 2h phase of group III monochalcogenide monolayers AlX (X = S, Se, and Te) with anisotropic crystal structure: first-principles study

In this paper, we introduce a new phase of two-dimensional aluminum monochalcogenide, namely -AlX (X = S, Se, and Te). With the space group, -AlX possesses a large unit cell containing 8 atoms. The 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 -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 -AlX are found to be direct band gap semiconductors, which are compared with the indirect band gap semiconductors of available -AlX. Particularly, the transition from direct to indirect band gap is observed in -AlX when a compressive biaxial strain is applied. Our calculated results indicate that -AlX exhibits anisotropic optical characteristics and its absorption coefficient is high. Our findings suggest that -AlX monolayers are suitable for applications in next-generation electro-mechanical and anisotropic opto-electronic nanodevices.