Spontaneous self-assembly of conductive molecular linkages between gold nanoelectrodes from aryl diisocyanides

The adsorption of 4,4'-biphenyldiisocyanide (BPDI) and 4,4 ''-terphenyldiisocyanide (TPDI) is studied on gold films from solution as a function of coverage using attenuated total internal reflection infrared (ATR-IR) spectroscopy to identify the nature and determine the orientation of the adsorbed species. At low coverages, both BPDI and TPDI adsorb with their molecular planes parallel to the surface and exhibit a single isocyanide stretching frequency consistent with the formation of gold-linked BPDI and TPDI oligomer chains, analogous to similar oligomers found when using 1,4-phenylene diisocyanobenzene (1,4-PDI) on gold. A vibrational mode due to a free isocyanide appears at higher BPDI and TPDI exposures, indicating that adsorbed BPDI and TPDI have reoriented to form eta(1)-adsorbed species, in a similar behavior to that found previously for 1,4-PDI on gold. It has been demonstrated that gold-PDI oligomer chains provide conductive bridges between gold nanoparticles on mica, in which the conductivity initially increases with 1,4-PDI dose as the number of oligomer linkages increases, but then decreases once again as the 1,4-PDI adopts a perpendicular orientation to disrupt the conductive chains. Similar behavior is found for BPDI and TPDI, where the dose at which a conductivity decrease is noted corresponds to the appearance of the free isocyanide stretching mode in the ATR-IR spectra. The sheet resistance R of the conductive films was measured as a function of sample temperature where it was found to vary linearly as ln(R) versus 1/root T. The slopes of such plots for gold nanoparticle films with different initial conductivities are used to estimate the tunneling barrier for gold nanoparticles linked by BPDI- and TPDI-derived oligomers. The results suggest that the barrier increases in the order TPDI > BPDI > 1,4-PDI. These results indicate that the ability to link between gold nanoparticles on surfaces by forming gold-containing oligomer chains with phenyl-linked diisocyanide is a quite general phenomenon.