Pseudo-Superlattices of Bi 2 Te 3 Topological Insulator Films with Enhanced Thermoelectric Performance
- PDF / 260,110 Bytes
- 5 Pages / 432 x 648 pts Page_size
- 68 Downloads / 240 Views
Pseudo-Superlattices of Bi2Te3 Topological Insulator Films with Enhanced Thermoelectric Performance V. Goyal, D Teweldebrhan and A.A. Balandin Nano-Device Laboratory, Department of Electrical Engineering and Materials Science and Engineering Program, Bourns College of Engineering, University of California, Riverside, California 92521 USA. ABSTRACT It was recently suggested theoretically that atomically thin films of Bi2Te3 topological insulators have strongly enhanced thermoelectric figure of merit. We used the “graphene-like” exfoliation process to obtain Bi2Te3 thin films. The films were stacked and subjected to thermal treatment to fabricate pseudo-superlattices of single crystal Bi2Te3 films. Thermal conductivity of these structures was measured by the “hot disk” and “laser flash” techniques. The room temperature in-plane and cross-plane thermal conductivity of the stacks decreased by a factor of ~2.4 and 3.5 respectively as compared to that of bulk. The strong decrease of thermal conductivity with preserved electrical properties translates to ~140-250% increase in the thermoelectric figure if merit. It is expected that the film thinning to few-quintuples, and tuning of the Fermi level can lead to the topological insulator surface transport regime with the theoretically predicted extraordinary thermoelectric efficiency. INTRODUCTION Bismuth Telluride (Bi2Te3) and its alloys are known as the best thermoelectric (TE) materials [1] with room temperature thermoelectric figures of merit ZT=S2σT/(Ke+Kl) ~ 1, where S is the Seebeck coefficient, σ is electrical conductivity, T is the absolute temperature, Ke and Kl are the electron and phonon (lattice) contributions to the thermal conductivity. It has been suggested by theoretical predictions that a drastic enhancement in ZT can be achieved in low-dimensional structures either from confinement-induced increased electron density of states near the Fermi level EF [2-3] or reduction of Kl due to the phonon – boundary scattering or modification of the phonon spectrum [4]. Most recently, it was shown that the Bi2Te3 related materials such as Bi2Se3 and Sb2Te3 are topological insulators (TIs). TIs are materials with a bulk insulating gap and conducting surface states that are topologically protected against scattering by the time-reversal symmetry [5-6]. It was shown theoretically that ZT can be strongly enhanced in Bi2Te3 thin-film TIs provided that the Fermi level is tuned to ensure the surface transport regime and the films are thin enough to open a gap in the “Dirac cone” dispersion on the surface [7]. At the same time, thermoelectric applications require a sufficient quantity of material, i.e. “bulk”, i.e. the single quintuples would hardly be practical. For this reason, we studied the stacks of the exfoliated films, which were put on top of each other and subjected to thermal treatment.
127
EXPERIMENTAL DETAILS The “pseudo-superlattices” were fabricated by the stacking of individual single-crystal Bi2Te3 films exfoliated using the “graphene-like” mechanical exfoliati
Data Loading...
