Crystal symmetry breaking in few quintuple Bi2Te3 nanosheets: applications in nanometrology of topological insulators and low-temperature thermoelectrics.

Bismuth telluride (Bi2Te3) and its associated compounds are the best bulk thermoelectric (TE) materials known in present day. In addition, stacked two-dimensional (2D) layers of Bi2Te3 have attracted brawny interest due to topologically protected surface state property. The authors herein report results of micro-Raman spectroscopy study of the "graphene-like" crystalline Bi2Te3 nanosheets with a thickness of a few atoms (few-quintuples) synthesized by convenient solvothermal route which is chiefly attractive from physics point of view. It is investigated that the optical phonon mode A1u, which is not-Raman active in bulk Bi2Te3 crystals, appears in atomically-thin nanosheets due to crystal-symmetry breaking in few quintuples layers (FQLs) and can be used in nanometrology of topological insulators (TIs). It is also suggested that sheets thinning to FQLs and tuning of Fermi level can help in achieving TI surface transport regime with enhance thermoelectric power. From seebeck measurements, Bi2Te3 sample exhibit p-type conduction having higher TE power at low temperature (40 K). Thus, Bi2Te3 nanosheets with strong spatial confinement of charge carriers are beneficial for TE devices. The developed technology for producing 2D layers of -Te(1)-Bi-Te(2)-Bi-Te(1)- creates an thrust for exploration of TIs and their possibility in practical applications.