In metal nanostructures under illumination, multiple different processes can drive current flow, and in an opencircuit configuration some of these processes lead to the production of open-circuit photovoltages. Structures that have plasmonic resonances at the illumination wavelength can have enhanced photovoltage response, due to both increased interactions with the incident radiation field, and processes made possible through the dynamics of the plasmon excitations themselves. Here we review photovoltage response driven by thermoelectric effects in continuous metal nanowires and photovoltage response driven by hot electron production and tunneling. We discuss the prospects for enhancing and quantifying hot electron generation and response via the combination of local plasmonic resonances and propagating surface plasmon polaritons.
Plasmonic structures can be used to enhance electromagnetic radiation, and nanoscale (<5 nm) gaps can increase this
enhancement even further. Fabrication of these desired structures involves using a relatively new, previously developed
self-aligned process to overcome typical electron beam lithography resolution limits. The resulting nanogap structures
have been shown to exhibit enhanced optical emission. This technique enables the fabrication of a large-area two-dimensional
matrix of such nanostructures which could prove useful for photovoltaics, plasmonically enhanced Raman
spectroscopy, biosensing, and other optoelectronic applications. Computational electromagnetic simulations of the
structures will prove useful for predicting behavior upon interaction with light and for experimental comparison.
We use simultaneous electronic transport and optical characterization measurements to reveal new information
about electronic and optical processes in nanoscale junctions fabricated by electromigration. Comparing electronic
tunneling and photocurrents allows us to infer the optical frequency potential difference produced by the
plasmon response of the junction. Together with the measured tunneling conductance, we can then determine
the locally enhanced electric field within the junction. In similar structures containing molecules, anti-Stokes and
Stokes Raman emission allow us to infer the effective local vibrational and electronic temperatures as a function
of DC current, examining heating and dissipation on the nanometer scale.
Contact resistances often contribute significantly to the overall device resistance in organic field-effect transistors (OFETs). Understanding charge injection at the metal-organic interface is critical to optimizing OFET device performance. We have performed a series of experiments using bottom-contact poly(3-hexylthiophene) (P3HT) OFETs in the shallow channel limit to examine the injection process. When contacts are ohmic we find that contact resistivity is inversely proportional to carrier mobility, consistent with diffusion-limited injection. However, data from devices with other electrode materials indicate that this simple picture is inadequate to describe contacts with significant barriers. A generalized transmission line method allows the analysis of nonohmic contacts, and we find reasonable agreement with a model for injection that accounts for the hopping nature of conduction in the polymer. Variation of the (unintentional) dopant concentration in the P3HT can significantly alter the injection process via changes in metal-organic band alignment. At very low doping levels, transport suggests the formation of a barrier at the Au/P3HT interface, while Pt/P3HT contacts remain ohmic with comparatively low resistance. We recently observed that self-assembled monolayers on the metal source/drain electrodes can significantly decrease contact resistance and maintain ohmic conduction under conditions that would result in nonohmic, high resistance contacts to untreated electrodes. Finally, we discuss measurements on extremely short channel devices, in the initial steps toward examining transport through individual polymer chains.