Probing of thermal transport in 50 nm thick PbTe nanocrystal films by time-domain thermoreflectance

Abstract

Bottom-up fabrication of thermoelectric (TE) materials from colloidal nanocrystal (NC) building blocks can substantially increase their TE efficiency, for example, by reducing lattice thermal conductivity. In this work, 10 nm spherical phase pure oleate-capped PbTe NCs with narrow size distribution were synthesized and employed to fabricate 50 nm thick films on insulating SiO2/Si substrates. The spin-coating, with subsequent ligand exchange procedure, was applied to enhance coupling interactions between the NCs. Using dark conductivity measurements, we confirmed the semiconducting behavior and the Schottky-type electrical field-dependent conductivity mechanism in the resultant thin films. The thermal transport in the thin film was probed by means of a time-domain thermoreflectance method. For this purpose, we used a customized state-of-the-art system based on a picosecond thermoreflectance instrument, which enables area-selective analysis with spatial resolution down to 5 mu m. The results show that the as-fabricated PbTe NC films exhibit ultralow thermal conductivity of 0.9 W m(-1) K-1 at 300 K. The transport property findings suggest potential in the proposed quick and cost-effective spin-coating strategy for bottom-up fabrication of nanostructured TE films from high-quality colloidal NC building blocks.