Solution Chemical Routes to Two-Dimensional Bismuth Tellurides for Thermoelectric Applications

Abstract

Bismuth telluride and its alloys are currently the best thermoelectric materials known at room temperature and are therefore used for portable solid-state refrigeration. If the thermoelectric figure of merit ZT could be improved by a factor of 3 or more, quiet and rugged solid-state devices could eventually replace conventional compressor-based cooling systems. In order to test the theoretical prediction that low dimensional materials could enhance ZT due to reduced thermal conductivity,¹ we are developing solution processing methods to make two-dimensional materials. Bismuth telluride and its p-type and n-type alloys have layered structures consisting of 5 atom thick Te-Bi-Te-Bi-Te sheets. Lithium ions are intercalated into the layered materials using liquid ammonia. Lithium intercalated Bi₂Te₃ has a higher conductivity and lower Seebeck coefficient than pristine Bi₂Te₃ likely due to electron transfer from the lithium. The intercalated materials can be exfoliated in water to form colloidal suspensions with relatively narrow particle size distributions. The layers are then deposited onto substrates, which after annealing at low temperatures, form highly c-axis oriented thin films. The low dimensional materials are characterized with powder X-ray diffraction, scanning electron microscopy, inductively coupled plasma and dynamic light scattering.