Photothermoelectric effects at and near individual grain boundaries in gold

Thermoelectricity is best known for thermocouples, where a voltage is generated when heating the interface of two materials with different Seebeck coefficients. In metals, the electronic Seebeck coefficient depends on the electrical conductivity which can be manipulated at the nanoscale to create single metal thermocouples. We will present scanning photothermoelectric measurements of the simplest single metal thermocouple: a single grain boundary between two single-crystal gold nanowires. Unlike a traditional thermocouple, where heating the grain boundary results in the largest voltage, the photovoltage as a function of laser position changes polarity at the grain boundary, varying on length scales much larger than the laser size. Modeling suggests that these results are consistent with long-scale Seebeck coefficient gradients within the crystals. Electron back-scatter diffraction relates the voltages to the relative crystallographic orientation across the boundary and x-ray probes provide insight of strain within the device. We propose how thermovoltages can probe areas of impurities, strain, and other intrinsic irregularities that may not otherwise be detected using traditional electronic transport measurements.