Janus structures of the C 2h polymorph of gallium monochalcogenides: first-principles examination of Ga2XY (X/Y = S, Se, Te) monolayers.

Group III monochalcogenide compounds can exist in different polymorphs, including the conventional D 3h and C 2h phases. Since the bulk form of the C 2h-group III monochalcogenides has been successfully synthesized [Phys. Rev. B: Condens. Matter Mater. Phys. 73 (2006) 235202], prospects for research on their corresponding monolayers have also been opened. In this study, we design and systematically consider a series of Janus structures formed from the two-dimensional C 2h phase of gallium monochalcogenide Ga2XY (X/Y = S, Se, Te) using first-principles simulations. It is demonstrated that the Janus Ga2XY monolayers are structurally stable and energetically favorable. Ga2XY monolayers exhibit high anisotropic mechanical features due to their anisotropic lattice structure. All Janus Ga2XY are indirect semiconductors with energy gap values in the range from 1.93 to 2.67 eV. Due to the asymmetrical structure, we can observe distinct vacuum level differences between the two surfaces of the examined Janus structures. Ga2XY monolayers have high electron mobility and their carrier mobilities are also highly directionally anisotropic. It is worth noting that the Ga2SSe monolayer possesses superior electron mobility, up to 3.22 × 103 cm2 V-1 s-1, making it an excellent candidate for potential applications in nanoelectronics and nanooptoelectronics.

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 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.