Thermal and electrical conductivity of size-tuned bismuth telluride nanoparticles.

Quantum-confined semiconductors composed of heavy elements hold great promise as thermoelectric materials. An increase in the density of states near the Fermi level due to quantum confinement effects and an increased scattering of boundary phonons due to nanostructuring can lead to an increase in the dimensionless figure of merit, ZT, which is defined as s 2 STk � 1 . [1,2] Here, s is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and k is the thermal conductivity. To realize the highest ZTquantum-confined material possible from a conventional thermoelectric semiconductor material such as bismuth telluride, meeting several criteria are important. First, it is reasonable to expect that uniform, optimal size quantum domains will lead to the highest ZT at a given doping level. The electronic structure of the semiconductor, which determines the thermoelectric power, depends on the degree of quantum confinement and thus the domain size. Second, the interfacial electrical transport must be optimized. The presence of impurities between the domains/particles typically leads to barriers to transport that reduce the electrical conductivity. Third, while optimizing electrical transport between domains, it is important that the quantum architecture not be destroyed, or the phonon scattering will decrease and the thermal conductivity will increase. Fourth, it is well knownthatproperdopingiscriticaltotheperformanceofbulk