Thermoelectric Properties of Bismuth and Silicon Nanowires

Abstract

Thermoelectric materials convert temperature differences into electricity and vice versa. Such materials utilize the Seebeck effect for power generation and the Peltier effect for refrigeration. In the Seebeck effect, a temperature gradient across a material causes the diffusion of charged carriers across that gradient, thus creating a voltage difference between the hot and cold ends of the material. Conversely, the Peltier effect explains the fact that when current flows through a material a temperature gradient arises because the charged carriers exchange thermal energy at the contacts. Thermoelectrics perform these functions without moving parts and they do not pollute. This makes them highly reliable and more importantly attractive as renewable energy sources, especially at a time when global warming is a growing concern. However, thermoelectrics find only limited use because of their poor efficiency. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit,ZT = S²σT/κ , where S is the thermoelectric power, defined as the thermoelectric voltage, V, produced per degree temperature difference ΔT , σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature. To maximize ZT, S must be large so that a small temperature difference can create a large voltage, σ must be large in order to minimize joule heating losses, and κ must be small to reduce heat leakage and maintain a temperature difference. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another. The best commercially available thermoelectric devices are alloys of Bi2Te3 and have a ZT of 1 which corresponds to a carnot efficiency of ~10%. My research has focused on achieving efficient thermoelectric performance from the single component systems of bismuth and silicon nanowires. Bismuth nanowires are predicted to undergo a semi-metal to semiconductor transition below a size of 50 nm which should increase the thermopower and thus ZT. Limited experimental evidence by other groups has been acquired to support this claim. Through electric field gating measurements and by tuning the nanowire size, we have shown that no such transition occurs. Instead, surface states dominate the electric transport at a size smaller than 50 nm and bismuth remains a semimetal. Bulk silicon is a poor thermoelectric due to its large thermal conductivity. However, silicon nanowires may have a dramatically reduced thermal conductivity. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk silicon are achieved over a broad temperature range, including a ZT ~ 1 at 200K. Independent measurements of S, σ, and κ, combined with theory, indicate that the improved efficiency originates from phonon effects. The thermal conductivity is reduced and the thermopower is enhanced. These results are expected to apply to other classes of semiconductor nanomaterials.