Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)(2)](n)-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (beta similar to 0.19 nm(-1)), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)(2)](n)-Au and Au-[Ru(bpy)(3)](n)-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent beta-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.