Photonic and Quantum Interactions of Atomic-Scale Junctions

The interaction of light with nanoscale matter can produce highly useful benefits, such as color-changing or adaptive materials, ultra-sensitive chemical measurements, and increased understanding of chemical behavior, due to quantum effects observed at small length scales. Experimental realization of such nanostructures is extremely difficult due to the limitations of current fabrication techniques. This research employs nanoscale electrochemistry as a fabrication, sensing, and tuning method to probe the photonic/quantum interactions of bimetallic atomic-scale junctions (ASJs). Significant results demonstrate the capability of nanoelectrochemistry as a nanofabrication technique capable of creating previously-inaccessible nanostructures, such as a negative-index optical metamaterial based on a nanoparticle array with ε < 0 for a range of wavelengths in the visible spectrum. With suitable application of voltage across an electrolyte containing Ag+ ions, a nanoscale conductive filament may be formed through electrodeposition. These filaments may be formed and dissolved at specified spatial coordinates in an ion-conducting solid polymer electrolyte using a conductive atomic-force microscope (C-AFM). This C-AFM method was employed to gather statistics of filament formation and dissolution times, granting insight into the kinetics of ion transport within the electrolyte. Experimentation and simulation showed that nanoparticles, otherwise electrically inaccessible within the electrolyte, act as bipolar electrodes and influence filament behavior within a large area of the electrolyte. Fabrication of self-assembled nanoparticle stacks was demonstrated by a novel nanopore templating method and used with an auto-aligned C-AFM program to form filaments through stacks of nanoparticles Widely-spaced nano-sized holes in metal films, or nanoapertures, block transmission of propagating light, acting as zero-mode waveguides (ZMWs). Closely-spaced nanoapertures behave not as ZMWs but acquire a different, aggregate response, acting as a plasmonic metamaterial which may selectively transmit light as a function of pore spacing and geometry. Electrodeposition of a dissimilar metal onto the conductive nanohole film, acting an electrode, creates a metamaterial with a tunable optical response proportional to charge transfer by decreasing hole size. Used as a reporter in a closed-bipolar electrode configuration, detection of analyte in a separate electrochemically-coupled cell was demonstrated with the metamaterial acting as an optical readout of charge transfer, a capability which may enhance future point-of-care medical diagnostics. Filaments formed in tightly confined conditions, such as inside nanopores of diameter < 100 nm, show unexpected behavior in formation/dissolution time as well as filament morphology, which suggests exceptionally altered ion transport behavior in confined electrolytes. This technique provides valuable examination of ionic whole-system behavior, indicating avenues for future research with similar systems such as batteries, resistive memory, and ionic membranes. Nanoelectrochemistry thus provides a valuable glimpse into the underlying principles and kinetics that govern electrochemical systems, as well as promising capabilities in the nanofabrication of optical metamaterials, optoelectronics, and other advanced devices, which is highly significant to both NASA and the community at large.