Ballistic thermoelectric properties of monolayer semiconducting transition metal dichalcogenides and oxides

Abstract

Combining first-principles calculations with Landauer-B\"uttiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely ${\mathrm{MX}}_{2}$ with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature $p$-type thermoelectric figure of merit ($ZT$) of 1.57 is obtained for $2\mathrm{H}\ensuremath{-}{\mathrm{HfSe}}_{2}$, which can be as high as 3.30 at $T=800$ K. Additionally, $2\mathrm{H}\ensuremath{-}{\mathrm{ZrSe}}_{2}, 2\mathrm{H}\ensuremath{-}{\mathrm{ZrTe}}_{2}$, and $2\mathrm{H}\ensuremath{-}{\mathrm{HfS}}_{2}$ have considerable $ZT$ values (both $n$- and $p$-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their $ZT$ values to below 1 at room temperature.