Anomalous Seebeck Coefficient in Sulfur-Substituted Bismuth Telluride

Abstract

Seebeck coefficient is determined by the asymmetry of density of states across Fermi level and their occupancy. These parameters are, in turn, essentially governed by the band structure and carrier concentration of the materials. In the present study, sulfur was substituted in versatile bismuth telluride thermoelectric material in subatomic ratio. The successful substitution in the resulting compounds, viz., Bi 2 Te 2.85 S 0.15 and Bi 2 Te 2.7 S 0.3 , was confirmed using Xray diffraction with Rietveld refinement and X-ray photoelectron spectroscopy. High resolution-transmission electron microscopy showed an ultrathin platy morphology in nanopowder and a randomly oriented high grain density at nanometer scale in the consolidated pellet. The sulfur substitution resulted in polarity reversal and enhanced value of the Seebeck coefficient at subzero temperatures in the bismuth telluride matrix. This anomaly in the sign and value of the Seebeck coefficient was analyzed using temperature-dependent Hall measurement, scanning tunneling spectroscopy, and first-principle computation using density functional theory coupled with Boltzmann transport theory. The analysis showed that the band-structure variations due to the localized polarity induced by the higherelectronegativity sulfur atom at Te I sites and suppression of antisite defects may be the prominent reasons for causing this anomaly. This study may also pave the way for the design of thermoelectric materials for enhanced performance in subzero temperature range for solid-state refrigeration application.