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.