Assembly, conductivity, and chemical reactivity of sub-monolayer gold nanoparticle junction arrays

Assemblies of gold nanoparticles (nominal 20 nm in diameter) and poly-(diallyldimethylammonium chloride) (PDDAC) are formed on tin-doped indium oxide (ITO) and glass substrates in a layer-by-layer deposition process. Electron microscopy imaging suggests clustering and sub-monolayer formation even after multiple deposition cycles. Voltammetric characterisation of the gold-PDDAC assemblies demonstrates at low coverage a facile electron transport perpendicular to the film but essentially insulating characteristics laterally across an inter-electrode gap of 40 μm. However, gentle removal of the organic assembly components (PDDAC) in a room temperature UV-ozonolysis process allows the array of “clean” gold–gold junctions to become electrically conducting due to (i) random multiple tunnel junction pathways and (ii) ionic conductivity through a thin water layer. In this room temperature ozone-cleaned state, the gold assembly is considerably more electrically conducting when compared to thermally cleaned films. The crucial effect of humidity on the resistivity and capacitive currents for gold nanoparticle junction arrays is demonstrated. The gold nanoparticle films readily react with thiols and dithiols from the gas phase which results in a dramatic increase in resistivity. The process is fully reversible and the sensor re-usable after UV-ozonolysis cleaning. Measurements are reported for a range of dithiols with different carbon chain lengths demonstrating that tunnel junction effects are likely to be responsible for the electrical conductivity.