Effect of vacancies on thermopower of molybdenum disulfide monolayers

Abstract

A detailed theoretical investigation of the effect of scattering of electrons and phonons by lattice vacancies in molybdenum disulfide (MoS2) monolayers (MLs) on diffusion, Sd, and phonon-drag, Sg, components of thermoelectric power (TEP), S, is presented over a wide-temperature range (1 < T < 300 K) using the Boltzmann transport formalism. The diffusion component is assumed to be influenced, not only by vacancies via short-range and Coulomb disorder scattering, but also by charged impurities (CIs) and acoustic and optical phonons. In the case of Sg, the phonons are considered to be scattered, besides the vacancies, by sample boundaries, substitutional isotopic impurities, as well as other phonons via both N- and U-processes. Numerical calculations of Sd and Sg, as functions of temperature and vacancy defect density are presented for MoS2 MLs with ns = 1017 m−2 supported on SiO2/Si substrates. The role of carrier scatterings by mono-sulfur and mono-molybdenum vacancies in influencing the overall electron and phonon relaxation rates and in determining Sd and Sg are investigated. The behavior of Sd and Sg is found to be noticeably influenced by vacancy scattering. The influence on Sd is seen to be more for mono-sulfur vacancies for densities lesser than 1%. The influence, is to enhance Sd slightly for MLs with realizable CI concentrations. On the other hand, Sg is found to depend sensitively on the vacancy disorder for T < 50 K; a S-vacancy density of 0.1% is found to suppress the characteristic peak of Sg by almost 60%. The extent of reduction in the characteristic peak of Sg, observable in low temperature measurements of S, can provide information about defect density. The calculations demonstrate that defect engineering of MoS2 ML systems can be used to tune their thermoelectric performance. A need for detailed experimental studies is suggested.