Bicrystals and Bowties: Photothermoelectric and plasmonic effects of gold nanoscale
Evans, Charlotte Irene
Doctor of Philosophy
The interaction of light with gold dramatically changes when the size of the gold devices is reduced to the nanoscale. The free electron gas in the metal can collectively oscillate with the electric field of the incident laser in an excitation called plasmons which can result in dramatic enhancements of the local electric field. Electronic transport and open circuit voltage (photothermoelectric) measurements can be powerful tools to characterize a gold nanoscale device heated by an incident laser. This laser can be raster-scanned to probe the response of the device as a function of laser position. The thermoelectric effect has been well-studied since its discovery by Seebeck in the 1820s. A resurgence of thermoelectric studies have been published recently manipulating the thermoelectric properties of thin metal film devices via nanostructuring. Substantial variation of the local Seebeck coefficient has recently been observed in polycrystalline nanowires of uniform thickness and width, a regime where the Seebeck coefficient is generally regarded as a constant value. A study of the photothermoelectric voltage signal of single crystalline wires and bicrystal devices, two single crystals in contact at an individual grain boundary, provides evidence that grain structure does not substantially contribute to the local Seebeck coefficient. Instead, the photovoltages of these single and bicrystal devices are well-correlated with local variations in strain as detected by electron back-scatter diffraction measurements and with variations in platinum impurity concentration. Therefore, the photovoltage measurements are demonstrated to have sensitivity to intrinsic variations in nanoscale devices that are otherwise difficult to detect using traditional electronic transport and imaging techniques. In nanoscale molecular junctions, the large enhancements of the local electric field due to local surface plasmon resonances allows for single-molecule Raman measurements, which probe the inelastic energy exchanges between incident photons and the vibrational modes of the molecule. These optical measurements can be combined with electronic transport measurements to detect the energy exchanges between the vibrational and electronic energy states in the molecule. However, the plasmonic resonances that allow for the single-molecule sensitivity also result in high local heating making these energy exchanges difficult to detect. We will discuss one potential way to mitigate the local heating: remote excitation of the nanojunction using propagating surface plasmon polaritons. Electronic transport measurements estimate the local temperature rise of the device due to direct and remote excitation. Large enhancements of the open circuit photovoltages in nanogaps due to hot electron photocurrents from non-radiative plasmon decay are observed. The open circuit photovoltages are then used to probe the interactions of the propagating surface plasmon polaritons with the local modes in the nanogap, demonstrating that the open circuit photovoltages and electronic transport methods are powerful tools to not only characterize nanoscale devices but also to probe the plasmonic properties of the devices.