Fabrication and characterisation of DNA-templated copper nanowires

Abstract

This thesis describes four approaches developed towards the fabrication of conductive 1-dimensional copper nanostructures, or nanowires, using a DNA-templating strategy. Cu-DNA nanowires are of interest for miniaturised interconnects in microprocessors. The chemical identity of the nanowires was characterised using X-ray Photoelectron Spectroscopy (XPS) and powder X-ray Diffraction (XRD) methods. Structural investigations were performed using Fourier-Transform Infra-Red (FTIR) Spectroscopy n+ and Atomic Force Microscopy (AFM) to study the Cu -DNA interaction mode(s) and nanowire size/morphologies, respectively. The electrical properties of nanowires were elucidated using Electrostatic Force Microscopy (EFM), Conductive-AFM (C-AFM) and a two-probe semiconductor device analyser for recording current-voltage (i-V) relationships. One method describes a non-aqueous route to the formation of 1-D Cu nanostructures. + This was achieved by doping of surface-immobilised DNA (the template) with Cu ions from Cu(CHCN).PF followed by chemical reduction to the zero-valent metal using 346 phenylsilane and organic solvent, under inert conditions. Metallic copper was confirmed to have formed on DNA and copper hydroxide (Cu(OH)) was also identified as a 2 surface overlayer on this material. The final structures were ~6 nm in height and show complete coverage of the template. The material was polycrystalline due to observations of a densely packed series of Cu nanoparticles along the template axis. However, electrical studies indicated these structures to be highly resistive. Thermal annealing of the templated material resulted in a considerable structural transformation, from tightly packed particles to the formation of a sporadic array of larger clusters well separated along the structure length. 2+ Another approach involved an aqueous solution-based synthesis, whereby Cu from Cu(NO) and ascorbic acid were added to a solution of DNA. This resulted in 32 nanowires, ~7 nm in height, which were mostly continuous in morphology and 2+ noticeably absent of inter-particle boundaries. The Cu :DNA(phosphate) ratio (n) was found to be critical for the formation of smooth nanowires (n= 0.05) as opposed to aggregated assemblies of material (n> 0.1). Chemical characterisation of the reaction product confirmed the presence of metallic copper as well as a surface layer of Cu(OH) Nanowires were confirmed to be conductive and i-V measurements gave a2. conductivity value of 0.94 Scm ; the first recorded conductance for copper templated on DNA, over micron length scales. Attempts were made to passivate the nanowires formed in solution by attachment of thiol molecules (p-mercaptobenzoic acid) onto the copper surface. This modification 0 resulted in the formation of a Cu(I)-thiolate layer on Cu and significantly protected the nanowires from oxidation, resulting in the formation of almost pure metallic Cu -1 nanowires. The electrical conductivity (1.01 Scm ) was similar to that obtained for the -1 unprotected wires (0.94 Scm ). This suggested that, although the degree of oxidation was minimised significantly, other factors must be more accountable for causing resistance in these wires such as surface and/or grain boundary scattering. Finally, a physical-based approach towards metallisation of DNA was performed using a vacuum deposition method. This was achieved by suspending single molecules of DNA between two electrodes, across a trench etched into the substrate. This was followed by metallisation of the sample by evaporation of copper in a sealed vacuum chamber. Electron Microscopy data showed that nanowires lay taut across the trench and were highly continuous in coverage. This process resulted in a working two- terminal nanowire device which was conductive due to nanowire(s) bridging between insulating gaps.