Photoresponse of bowtie nanojunctions
Evans, Kenneth Mellinger
Doctor of Philosophy
Plasmon resonant nanostructures provide a platform for controlling light on subwavelength lengthscales. Integrating plasmonic materials into dielectric environments, as well as its compliment – addressing nanoscale photonic elements with plasmon active geometries – is a challenging aspect of current research in wide variety of scientific disciplines including microscopy, photovoltaics, photonics, and catalytic chemistry. This thesis covers two experiments with the goal of electrically and optically addressing nanoscale volumes of semiconducting material using Au nanojunctions with plasmon resonant electrodes. The first measurement aims to use the large field enhancements in bowtie nanojunctions to trap semiconducting nanocrystals from solution. Trapped nanocrystals could then potentially span the gap between the structure's two electrodes to serve as an active optical and electrical region for a number of desirable photoresponsive measurements in single to few nanocrystals systems. We establish a numerical model simulating the force applied on nanocrystals in and around the nanogap as result of the structure's plasmon modes. We also provide experimental data of trapping events in bowtie nanogaps and measurements of the photocurrent generated in the resultant Au-nanocrystal devices. The challenges of this project, mostly related to ligand and surface chemistry, are discussed in detail. In the second experiment, we demonstrate plasmon-enhanced photoconduction in Au bowtie nanojunctions containing nanogaps overlaid with an amorphous Ge film. The role of plasmons in the production of nanogap photocurrent is verified by studying the unusual polarization dependence of the photoresponse. With increasing Ge thickness, the nanogap polarization of the photoresponse rotates 90 degrees, indicating a change in the dominant relevant plasmon mode, from the resonant transverse plasmon at low thicknesses to the nonresonant “lightning rod” mode at higher thicknesses. To understand the plasmon response in the presence of the Ge overlayer and whether the Ge degrades the Au plasmonic properties, we investigate the photothermal response (from the temperature-dependent Au resistivity) in no-gap nanowire structures, as a function of Ge film thickness and nanowire geometry. The film thickness and geometry dependence are modeled using a cross-sectional, finite element simulation. The no-gap structures and the modeling confirm that the striking change in nanogap polarization response results from redshifting of the resonant transverse mode, rather than degradation in the Au/Ge properties. We note remaining challenges in determining the precise mechanism of photocurrent production in the nanogap structures.
plasmonics; nanogap; photodetector