Novel two-dimensional Janus ß-Ge2XY (X/Y = S, Se, Te) structures: first-principles examinations.

Two-dimensional (2D) structures can stably exist in different allotropes. In this manuscript, we propose a new series of Janus structures based on the ß-phase of germanium monochalcogenides, namely, ß-Ge2XY (X/Y = S, Se, and Te) monolayers. Our calculations indicate that Janus ß-Ge2XY monolayers have a stable crystal structure and possess anisotropic mechanical properties. At the ground state, ß-Ge2XY monolayers are semiconductors with a large bandgap and their electronic properties depend strongly on a biaxial strain. Strains not only change the bandgap but can also lead to a change in the bandgap characteristic, namely transitions from indirect to direct bandgap. Our findings not only introduce a new structure of germanium chalcogenide compounds but also show that they have superior physical properties suitable for applications in nanoelectronics.

Strain-induced phase transitions and high carrier mobility in two-dimensional Janus MGeSN2 (M = Ti, Zr, and Hf) structures: first-principles calculations.

In this study, we construct new 2D Janus MGeSN2 (M = Ti, Zr, and Hf) monolayers and systematically investigate their electronic band structures under applied biaxial strain. Their crystal lattice and electronic as well as transport properties are also examined based on the first-principles calculations and deformation potential theory. The results show that the MGeSN2 structures have good dynamical and thermal stability, and their elastic constants satisfy the criteria of Born-Huang also indicating the good mechanical stability of these materials for experimental synthesis. Our calculated results indicate that the TiGeSN2 monolayer exhibits indirect-bandgap semiconductor characteristics whereas the ZrGeSN2 and HfGeSN2 monolayers exhibit direct-bandgap semiconductor characteristics. Importantly, the biaxial strain shows significant influences on the electronic energy band structures of the monolayers in the presence of a phase transition from semiconductor to metal, which is an important feature of these materials for their application in electronic devices. All three structures exhibit anisotropic carrier mobility in both x and y transport directions, suggesting their great potential for application in electronic devices.