Distance-dependent electron hopping conductivity and nanoscale lithography of chemically-linked gold monolayer protected cluster films

Abstract

Films of monolayer protected Au clusters (MPCs) with mixed alkanethiolate and ω-carboxylate alkanethiolate monolayers, linked together by carboxylate–Cu 2+–carboxylate bridges, exhibit average edge-to-edge cluster spacings that vary with the numbers of methylene segments in the alkanethiolate ligand as determined by a combined atomic force microscopy (AFM)/UV-Vis spectroscopy method. The electronic conductivity ( σ EL) of dry films is exponentially dependent on the cluster spacing, consistent with electron tunneling through the alkanethiolate chains and non-bonded contacts between those chains on individual, adjacent MPCs. The calculated electronic coupling factor ( β) for tunneling between MPCs is 1.2 Å −1, which is similar to other values obtained for tunneling through hydrocarbon chains. Electron transfer rate constants measured on the films reflect the increased cluster–cluster tunneling distance with increasing chainlength. The MPC films are patterned by scanning the surface with an AFM or scanning tunneling microscopy (STM) tip under appropriate conditions. The patterning mechanism is physical in nature, where the tip scrapes away the film in the scanned region. Large forces are required to pattern films with AFM while normal imaging conditions are sufficient to produce patterns with STM. Patterns with dimensions as small as 100 nm are shown. Subsequent heating (300 °C) of the patterned surfaces leads to a metallic Au film that decreases in thickness and is smoother compared to the MPC film, but retains the initial shape and dimensions of the original pattern.