Multivalency-Driven Formation of Te-Based Monolayer Materials: A Combined First-Principles and Experimental study

Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable $1\mathrm{T}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$-like ($\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Te}$), and metastable tetragonal ($\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Te}$) and $2\mathrm{H}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$-like ($\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Te}$) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Te}$ phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the $\ensuremath{\alpha}$- and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Te}$ phases possess electron and hole mobilities much higher than ${\mathrm{MoS}}_{2}$. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.