Thermopower enhancement in nanowires via junction effects.

We present thermopower measurements on free-standing, straight and "junctioned" gold nanowires using a micromachined thermoelectric workbench. Measurements on straight 70 nm diameter gold nanowires show a thermopower similar to that of bulk gold; however for "junctioned" gold nanowires we observed a hitherto unreported peak in the thermopower near room temperature. The observed enhancement can be explained by approximating the "junctioned" nanowires as tunnel junctions in combination with Coulombic effect of the electrons crossing the junction. The electron transfer across the barrier can be expected to be stochastic in nature. Under thermal equilibrium conditions and in the absence of temperature gradient across the tunnel junction, the time averaged random fluctuation of the electrons across the tunnel junction results in a net zero voltage. However, in the presence of a temperature gradient across the junction, the time averaged fluctuation of the electrons across the junction is now offset by the tunnel junction thermoelectric effect and is measured by the lock-in amplifier. A hundredfold enhancement in the ZT of "junctioned" nanowires has been observed for the gold nanowires measured over several samples.