Thermoelectric power of a network of 6-nm Bi nanowires in a porous Vycor glass matrix

Semiconductor quantum wires are a promising thermoelectric material because of the potential enhancement of the figure of merit in low dimensional materials. Recently, Heremans reported semiconducting behavior of the resistance and very large enhancements of the thermoelectric power of composites containing Bi nanowires with diameters of 9 nm and 15 nm embedded in silica and alumina matrices. The results are interpreted in terms of quantum confinement. We studied the magnetic field dependent resistance and Seebeck coefficient of a high density network of 6 nm diameter wires of Bi and Bi doped with 0.14 at.% of Te embedded in a well characterized monolithic porous silica (porous Vycor glass from Corning) in order to observe the expected properties. R increases weakly for decreasing temperature for these composites. The observed behaviour corresponds closely to that found by Heremans. However, in contrast to Heremans' results, we find that the composites's thermoelectric power is metallic (d|S|/dT> 0) and of the order of magnitude of the thermoelectric power of bulk Bi. Our results are interpreted in terms of a model of surface charges that spoil the semimetal-to-semiconductor transition of quantum wires.