Date on Master's Thesis/Doctoral Dissertation

8-2011

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Zamborini, Francis Patrick

Author's Keywords

Electrochemistry; Molecular switches; Surface enhanced Raman; Resistive switches; Molecular electronics; Hydrogen sensors

Subject

Nanostructures; Nanotechnology; Molecular electronics

Abstract

The research in this dissertation describes a simple electrochemical approach for fabricating nanoscale (metal/metal) or molecular (metal/polymer or self assembled monolayer (SAM)/metal) junctions. The fabrication involves metal deposition on one set of electrodes (El), where the metal grows and becomes connected to a second set of electrodes (E2) of an Au interdigitated array of electrodes with a 5 11m separation. The fabrication of molecular junctions involves deposition of a thin polymer or self assembled monolayer film on one set of electrodes (E2) prior to metal deposition on the other set of electrodes (El). The method is simple, low cost, highly parallel, and applicable to a wide variety of molecules and thin films with interesting electronic properties. Different metal/metal and metal/molecule or polymer/metal junctions such as El/nano Pd/E2, El/nano Pd/polyphenol/E2, El/Ag nanowire(NW)/E2, El/AgNW/polyphenol/E2 and El/AgNW/SAM/E2 junctions were fabricated using this electrochemical approach. The H2 sensing properties of El/nano Pd/E2, El/nano Pd/polypheno1/E2, and El/AgNW/polyphenol/E2 functionalized with Pd nanoparticles were explored. El/nano Pd/polypheno1/E2 junctions behaved as H2 switches above 1 % H2 (107 on/off ratio) while El/nano Pd/E2 and EI/Ag NW/polypheno1/E2 functionalized with Pd nanoparticles behaved as H2 sensors and detected H2 gas rapidly and reversibly down to 0.11 %. The resistance of El/AgNW/polyphenol/E2 junctions was controlled by electrodepositing polyphenol of different densities within the junction. This procedure did not lead to precise control over the resistance, but the number of polyphenol electrochemical cycles used during the synthesis did offer some general control over the resistance over 9 orders of magnitude. For El/AgNW/SAM/E2 junctions, where the SAM is comprised of alkanethiols, the resistance generally depends on the length of the alkane chain within the junction from C4 to C18. While the resistance was still not precisely controlled, the % of short circuits between the Ag NW and the E2 electrode decreases as the chain length of the SAM increases. This approach is useful for device fabrication, but better reproducibility is required for fundamental molecular electronics studies. The electrochemically deposited Ag NW contacts significantly enhance the Raman scattering signals for aminothiophenol (ATP) molecules in E/1/AgNW/ATP/E2 junctions. The extent of surface enhanced Raman spectroscopy (SERS) enhancement at several junctions within a device depends on the contact between the AgNW and ATP SAM coated electrode. The SERS enhancement was estimated to be 106 - 108, which was large enough to detect submonolayer coverages of A TP molecules diluted by a factor of 1000 with hexanethiol molecules. E1/AgNW/ATP/E2 junctions are useful as a platform for performing simultaneous conductivity and SERS measurements to correlate the electronic properties of molecules within the junction to its structural properties. E1/AgNW/E2 junctions with the AgNW/E2 contact broken electrochemically behave as resistance switching devices. These Ag filament-based switching devices exhibited reversible switching with an on/off ratio over 104 and endurance of at least 100 cycles.

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