Molecular electronics promises a new generation of ultralow-energy information technologies, based around functional molecular junctions where two electrodes are bridged by just a few molecules. How molecules exactly behave in a real junction however is still not well understood, since interactions with the electrode materials and neighbouring molecules at the nanoscale is difficult to model and probe experimentally. Many studies so far have characterised in detail the electrical response of molecular junctions by statistical analysis of many repeated measurements, but without monitoring in real time the behaviour of individual devices. The main difficulty of in situ measurements is the absence of characterisation tools that can provide direct access to the dynamics of nm-sized junctions during device operation. This thesis explores two novel approaches used to create optically accessible nanoscale junctions. One method is based on graphene electrodes, used to fabricate extended junctions with a metal oxide spacer. These graphene junctions are found to behave as memristive devices, where a solid state redox reaction releases gas from the oxide spacer that remains trapped under the graphene, resulting in an actuating mechanism. Displacement of the surface layers can be optically tracked in real time using metallic nanoparticles deposited on the sample, whose plasmonic coupling with the bottom electrode is modulated by the actuation mechanism. The second method used to construct nanoscale junctions is more appropriate for molecular junctions, and is based on electrical contacting of individual metallic nanoparticles with a conductive transparent cantilever. Nanoparticles are deposited on a molecular monolayer and represent one electrode of a molecular junction, and at the same time allow to optically probe the junction itself by enabling plasmonic confinement of light to volumes <100nm$^3$. Darkfield and Raman spectroscopy are performed on single nanoparticle junctions in real time while voltage is applied, and a modulation of the optical response with voltage is observed, revealing that molecules undergo conformational changes during device operation.
plasmonics, nanoparticles, molecular junctions, molecular electronics, electrical contacting, nanotechnology
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