Design of Sn-doped cadmium chalcogenide based monolayers for valleytronics properties

Abstract

Valleytronics, that uses the valley index or valley pseudospin to encode information, has emerged as an interesting field of research in two-dimensional (2D) systems with promising device applications. Spin-orbit coupling (SOC) and inversion symmetry breaking leads to spin-splitting of bands near the energy extrema (valleys). In order to find a new 2D material useful for valleytronics, we have designed hexagonal planar monolayers of cadmium chalcogenides (CdX, X = S, Se, Te) from the (111) surface of bulk CdX zinc blende structure. Band structure study reveals valence band local maxima at symmetry point K and its time reversal conjugate point K$\textquotesingle$. Application of SOC initiates spin-splitting in the valleys that lifts the energy degeneracy and shows strong valley-spin coupling character. We have substituted two Cd atoms in the planar monolayers by Sn atoms which increases the spin-splitting significantly. The structural, dynamic, mechanical and thermal stability of all the monolayers has been confirmed. Values of formation energies indicate that it may be feasible to synthesize the Sn-doped CdSe and CdTe monolayers using bottom-up approach. Zeeman-type spin-splitting is observed in the valley region and Rashba spin-splitting is observed at the $\Gamma$ point for Sn-doped CdSe and CdTe monolayers. Berry curvature values are more in all the Sn-doped monolayers than the pristine monolayers. These newly designed monolayers are thus found to be suitable for valleytronics applications. Sn-doped monolayers show band inversion deep in the valence and conduction bands between Sn~$s$ and $p$ and X~$p$ states but lack topological properties.