New Sb2Te3-xSex monolayers with high electron mobilities and wide absorption range

As a traditional thermoelectric material with high thermoelectric performance at room temperature, antimony telluride (Sb$_2$Te$_3$) has been widely used in energy applications like power generation and refrigeration. By employing the "atomic transmutation" method, three new kinds of Sb$_2$Te$_3$-based monolayers of $\alpha-$Sb$_2$Te$_2$Se, $\alpha-$Sb$_2$TeSe$_2$ and $\beta-$Sb$_2$TeSe$_2$ are designed, which are expected to possess high thermoelectric performance due to the quantum-confinement effects. In this work, by using the \textit{ab-initio} calculations, we systematically study electronic structures, vibration modes, optical, transport and thermoelectric properties for the three kinds of monolayers, and find that they are indirect-bandgap semiconductors materials with chemical and thermodynamic stability up to 700 K ($\alpha-$Sb$_2$TeSe$_2$) or 900 K ($\alpha-$Sb$_2$Te$_2$Se and $\beta-$Sb$_2$TeSe$_2$). The bandgaps are around 1.0 eV with five nearly denegerate peaks in valence bands. The three S...