Tellurene: a New Family of Two Dimensional Group-VI Monolayer Driven by Multivalency

The exploration of two-dimensional (2D) layered materials is of fundamental and practical importance in contemporary condensed matter physics. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, we predict a new family of 2D monolayers composed of a group-VI element of Te. It is revealed that the multivalency character of Te plays a crucial role in forming monolayers with the 1T-MoS$_{\mathrm{2}}$-like (termed $\alpha $-Te), tetragonal ($\beta $-Te), and 2H-MoS$_{\mathrm{2}}$-like ($\gamma $-Te) structures. We find that $\alpha $- and $\beta $-Te are semiconductors, while $\gamma $-Te is metal. For $\alpha $- and $\beta $-Te, the spin-orbit coupling effects give a transformation from an indirect band gap into a nearly direct and a direct band gap, respectively, leading to an optical absorption enhancement. Moreover, the semiconducting Te monolayers exhibit high electron and hole mobilities ranging from hundreds to thousands of cm$^{\mathrm{2\thinspace }}$V$^{-}^{\mathrm{1\thinspace }}$s$^{-}^{\mathrm{1}}$, superior to MoS$_{\mathrm{2}}$ monolayer. Our findings further extend the realm of 2D materials to include group-VI monolayers whose electronic properties are promising for potential applications in optoelectronics and electronics.