Lattice thermal conductivity of topological insulator Bi2Se3 nanocrystals: comparison from theoretical and experimental

Abstract

Lattice thermal conductivity (κ_L) calculations using the Wiedemann-Franz law involve electrical conductivity, which introduces an error in the actual value of κ_L. We have adopted a non-contact measurement technique and calculated the κ_L from the temperature and power-dependent Raman spectra of the Bi₂Se₃ nanocrystals with truncated hexagon plate morphology stabilized in a hexagonal crystal structure. The hexagon plates of Bi₂Se₃ are 37 to 55 nm thick with lateral dimensions around 550 nm. These Bi₂Se₃ nanocrystals show three Raman lines, which agree with the theoretical prediction of A11g, E2g and A21g modes. Although the first-order thermal coefficient (-0.016) of Bi₂Se₃ nanocrystals is quite low, the room temperature κ_L ∼1.72 W m^-1 K^-1 is close to the value obtained from the simulation adopting a three-phonon process. The phonon lifetime of Bi₂Se₃ nanocrystals observed between ∼0.2 ps and 2 ps confirmed carrier-carrier thermalization with a small contribution from electron-electron and intraband electron-longitudinal-optical-phonon relaxation. The variations of phonon lifetime, Gruneisen parameter and κ_L of the mode frequencies outline the crucial role of the anharmonicity and acoustic-optical phonon scattering in reducing the κ_L of Bi₂Se₃. The non-contact measurements and relevant thermal property parameters open up exciting opportunities to address the anharmonic effects in other thermoelectric materials for obtaining a high figure of merit.