Electronic and Thermoelectric Properties of Transition-Metal Dichalcogenides

Using first-principles electronic structure calculations performed within the B1-WC hybrid functional, we study the thickness and strain dependency of electronic and thermoelectric (TE) properties of transition-metal dichalcogenides (TMDs). We consider both 2H (MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2) and 1T (SnS2, SnSe2, HfS2, HfSe2, HfTe2, ZrS2, ZrSe2) structures and identify those TMDs with a high TE potential (WSe2, MoTe2, and SnSe2). The thickness and strain significantly change the electronic properties near the forbidden band gaps. We rationalize at an atomic level these changes in terms of the interplay between in-plane bonding/antibonding X-X(M-M) interactions through sp(2) hybridization and the stronger antibonding/nonbonding M-X interactions due to sp(3)d and sp(3)d(2) hybridizations inside TMD layers (X, chalcogen; M, transition metal, Sn). Thickness and in-plane strain appear as effective ways to tune electronic band structures, increase the degeneracy of carrier pockets, and optimize the TE properties of TMDs. We estimate the anisopropy of carrier pockets and introduce the effective mass quality factor B-m for the maximization of TE performance at a given carrier density and temperature. High-potential TMDs have B-m and power factors comparable to PbTe and Bi2Te3.