Transport properties of few-layer NbSe2: From electronic structure to thermoelectric properties

Abstract

4-layer NbSe2 is grown on SiO2 by molecular beam epitaxy. The in-situ X-ray photoelectron spectroscopy measurements suggest an Nb-rich stoichiometry (Nb1+xSe2) likely due to the intercalation of Nb atoms in between the NbSe2 layers. The metallic nature of the samples is confirmed using scanning tunneling microscopy and local density of state measurements as well as band structure calculations. This metallic nature is consistent with the small measured Seebeck coefficient and large electrical conductivity values. A change of sign in the Seebeck coefficient is observed in the bulk single crystal sample at 50 K, and in the polycrystalline few-layer sample at 120 K. Since the samples are metallic, this change of sign is the result of a change in the density of state slope at the Fermi level. The temperature dependence of the measured Seebeck coefficient matches with theoretical calculations for 4-layer NbSe2. The room temperature Seebeck coefficient is negative, but when oxidized, that of the few-layer sample changed to positive. The in-plane thermal conductivity of the few-layer samples is measured using the heat diffusion imaging method at low temperatures and is (32 ± 10) W/m∙K at 200 K.