  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 wepredict a new category of 2D monolayers named tellurene,
composed of the metalloid element Te, with stable 1T-MoS2-like ( {\alpha}-Te),
and metastable tetragonal (\b{eta}-Te) and 2H-MoS2-like ({\gamma}-Te)
structures. The underlying formation mechanism of such tri-layer arrangements
is uniquely 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
semiconductor-like (e.g.,S). In particular, the {\alpha}-Te phase can be
spontaneously obtained from the magic thicknesses truncated along the [001]
direction of the trigonal structure of bulk Te. Furthermore, both the {\alpha}-
and \b{eta}-Te phases possess electron and hole mobilities much higher than
MoS2, as well as salient optical absorption properties. These findings
effectively extend the realm of 2D materials to group-VI monolayers, and
provide a new and generic formation mechanism for designing 2D materials.
