Unleashing the Impact of Topological Surface States on the Thermoelectric Properties of Granular Sb2Te3 Thin Films Deposited on Flexible Substrates

Abstract

Between thermoelectric materials, topological insulators (TIs) such as Sb₂Te₃ can effectively decouple phonon and electronic transport. Recent works mostly focused on TI composites or superlattices, where the contribution of the topological surface states (TSS) to the thermoelectric properties is overshadowed by other mechanisms such as energy filtering or electronic band reorganization. Here, we investigate efficient thermoelectric Sb₂Te₃ polycrystalline thin films deposited on plastic foil. Magneto-transport studies show that the presence of TSS in more granular films is responsible for the 2-orders of magnitude higher electronic conductivity compared to thick films owing to larger crystalline domains (> 100 nm). The prevalence of the bulk states in thick films reduces both their thermal and electronic conductivity; however, they are responsible for an increase in the Seebeck coefficient. Overall, we show that to achieve higher thermoelectric performance of single-component TI films, it is necessary to tune the relative contribution of topological and bulk states. This will potentially allow for the development of cost-effective thermoelectric generators, reducing the complexity of competitive systems based on multicomponent heterostructures.

Keywords: MOCVD; Sb2Te3; Topological insulators; flexible thermoelectrics; thin film.

1

Chemical and morphological analysis of the as grown and annealed Sb₂Te₃ film with a thickness of 130 nm. a) XRD pattern. b) Raman spectra. c) and d) XPS spectra of the as grown and annealed film, respectively.

2

AFM images of the non-annealed films with a thickness of 40 nm (a) and 130 nm (b) and of the annealed films with a thickness of 40 nm (c and e) and 130 nm (d and f). g) Grains diameter distribution measured per each investigated film thickness before and after annealing at 300 °C. h) Line profile showing the higher granularity of the 40 nm thick films compared to the thicker 130 nm film.

3

a) and b) Electrical resistivity measured as a function of temperature for the as grown and annealed films with different thicknesses (a) 40 and (b) 130 nm. Temperature dependence of the electrical conductivity (c), Seebeck coefficient (d) and P.F. (e).

4

MC measurements. a) Energy normalized MC acquired on the annealed films (B oriented at 45° with respect to the sample plane; thickness 40 and 130 nm; 6–7 K) (Lines connecting data points indicate the data trend). b) HLN-fit of the magnetoconductance data acquired from the 40 nm annealed film.