Nanostructured metals subject to local optical interrogation can generate open-circuit photovoltages potentially useful for energy conversion and photodetection. We report a study of the photovoltage as a function of illumination position in single metal Au nanowires and nanowires with nanogaps formed by electromigration. We use a laser scanning microscope to locally heat the metal nanostructures via excitation of a local plasmon resonance and direct absorption. In nanowires without nanogaps, where charge transport is diffusive, we observe voltage distributions consistent with thermoelectricity, with the local Seebeck coefficient depending on the width of the nanowire. In the nanowires with nanogaps, where charge transport is by tunneling, we observe large photovoltages up to tens of mV, with magnitude, polarization dependence, and spatial localization that follow the plasmon resonance in the nanogap. This is consistent with a model of photocurrent across the nanogap carried by the nonequilibrium, "hot" carriers generated upon the plasmon excitation. Table of Contents Figure:In the Seebeck effect, a diffusive conductor subject to a temperature gradient develops an open-circuit potential difference along its length, to counter the thermal diffusion of carriers. 1 When the temperature gradient is driven by optical excitation, this thermoelectric effect may be leveraged for photodetection. 2,3 Advances in the development of nanostructured materials have created new opportunities to improve thermoelectric response. 4,5 In thin metal films and nanowires, the proximity of the surfaces is detrimental to thermoelectricity, as surface scattering reduces electron and phonon mean free path, which lowers both electronic and "phonon drag" contributions to the Seebeck coefficient. [6][7][8][9] This sensitivity to boundary scattering makes it possible to create single-material thermocouples by controlling its geometry on the length scale of a few times the electronic mean free path. 10,11 The thermoelectric power of metals can also be modified by the creation of the atomic-scale junctions for which the Seebeck coefficient is determined by the energy dependence of the transmission properties of the junction. 12,13 Singlemolecule junctions have been intensely studied recently following proposals for enhanced thermoelectric properties due to quantum interference effects, [14][15][16][17][18] though simple tunneling junctions are not expected to show particularly large Seebeck response. 19,20 The conventional treatment of thermoelectric effects assumes that electron and lattice temperatures are the same; however this may not be the case in optically driven systems where light can generate short-lived, high energy carriers. [21][22][23][24][25] Hot electrons generated in plasmonic nanostructures were demonstrated to create additional photocurrent in molecular and tunnel junctions. [26][27][28] The effects of nonthermal electronic distributions on thermoelectric properties in plasmonic metal nanostructures have been ...

Nanoscale structuring holds promise to improve thermoelectric properties of materials for energy conversion and photodetection. We report a study of the spatial distribution of the photothermoelectric voltage in thin-film nanowire devices fabricated from single metal. A focused laser beam is used to locally heat the metal nanostructure via a combination of direct absorption and excitation of a plasmon resonance in Au devices. As seen previously, in nanowires shorter than the spot size of the laser, we observe a thermoelectric voltage distribution that is consistent with the local Seebeck coefficient being spatially dependent on the width of the nanostructure. In longer structures, we observe extreme variability of the net thermoelectric voltage as the laser spot is scanned along the length of the nanowire.The sign and magnitude of the thermoelectric voltage is sensitive to the structural defects, metal grain structure, and surface passivation of the nanowire. This finding opens the possibility of improved local control of the thermoelectric properties at the nanoscale.

We report a method to electrically detect heating from excitation of propagating surface plasmon polaritons (SPP). The coupling between SPP and a continuous wave laser beam is realized through lithographically defined gratings in the electrodes of thin film gold "bow tie" nanodevices. The propagating SPPs allow remote coupling of optical energy into a nanowire constriction. Heating of the constriction is detectable through changes in the device conductance and contains contributions from both thermal diffusion of heat generated at the grating and heat generated locally at the constriction by plasmon dissipation. We quantify these contributions through computational modeling and demonstrate that the propagation of SPPs provides the dominant contribution. Coupling optical energy into the constriction via propagating SPPs in this geometry produces an inferred temperature rise of the constriction a factor of 60 smaller than would take place if optical energy were introduced via directly illuminating the constriction. The grating approach provides a path for remote excitation of nanoconstrictions using SPPs for measurements that usually require direct laser illumination, such as surface-enhanced Raman spectroscopy.

Efficient hot electron generation in plasmonic nanostructures is of particular interest. Distinguishing between hot carrier generation and other competing effects due to direct absorption and heating can be challenging. Here, we report a study of the open-circuit photovoltage in thin-film gold nanostructures as a function of illumination position. Comparison is made between direct illumination of a nanowire constriction or electromigrated tunneling junction and remote excitation via the use gratings in the surrounding electrode to excite propagating surface plasmon polaritons (SPPs). Photovoltage response in continuous nanowire constrictions is dominated by photo-thermoelectric effects, with grating illumination demonstrating that it is possible to achieve a nontrivial temperature distribution at the constriction due to SPP excitation. Direct illumination of tunneling gaps had previously demonstrated enhanced photovoltages due to hot electron tunneling. We reconfirm this and find that the photovoltages generated by illuminating the gratings after electromigration are enhanced up to a factor of 100 compared to their premigration values. The polarization dependence and polarity of the signal with illumination at the grating provide evidence that the SPPs couple with local plasmon modes at the gap, producing hot electron current via plasmon decay, and a corresponding open-circuit voltage develops to enforce the open-circuit condition of no net current. Variations in such measurements show the sensitivity of SPP-local mode coupling to the structural details of the junctions.

The electronic Seebeck response in a conductor involves the energy-dependent mean free path of the charge carriers and is affected by crystal structure, scattering from boundaries and defects, and strain. Previous photothermoelectric (PTE) studies have suggested that the thermoelectric properties of polycrystalline metal nanowires are related to grain structure, although direct evidence linking crystal microstructure to the PTE response is difficult to elucidate. Here, we show that room temperature scanning PTE measurements are sensitive probes that can detect subtle changes in the local Seebeck coefficient of gold tied to the underlying defects and strain that mediate crystal deformation. This connection is revealed through a combination of scanning PTE and electron microscopy measurements of single-crystal and bicrystal gold microscale devices. Unexpectedly, the photovoltage maps strongly correlate with gradually varying crystallographic misorientations detected by electron backscatter diffraction. The effects of individual grain boundaries and differing grain orientations on the PTE signal are minimal. This scanning PTE technique shows promise for identifying minor structural distortions in nanoscale materials and devices.