Electronic conductance behavior of carbon-based molecular junctions with conjugated structures

A novel molecular junction based on a monolayer between carbon and mercury "contacts" was investigated by examining current/voltage behavior as a function of temperature and monolayer thickness. Monolayers of phenyl, biphenyl, and terphenyl were covalently bonded to flat, graphitic carbon, then a top contact was formed with a suspended mercury drop. Similar molecular junctions were formed from multilayer nitroazobenzene (NAB) films of 30 A and 47 A thickness, and junctions were examined over the temperature range of +80 degreesC to -50 degreesC. Junction resistances were a strong function of molecular length and structure, with mean resistances for 0.78 mm(2) junctions of 34.4 Omega, 13.8 KOmega, and 41.0 KOmega for phenyl, biphenyl, and terphenyl junctions. The i/V characteristics of biphenyl and phenyl junctions were nearly independent of temperature, while those of terphenyl and NAB junctions were temperature independent below 0 degreesC but thermally activated above 10 degreesC. The results are consistent with a tunneling process at low temperature, where the molecular conformations are apparently fixed: For the thicker terphenyl and NAB junctions, the tunneling rate is sufficiently slow to observe a thermally activated conduction process at higher temperatures. The observed activation barriers of 0.3 to 0.8 eV are in the range expected for phenyl ring rotation, implying that the coplanar conformer of terphenyl has a significantly higher conductivity. Below 0 degreesC, the junction is presumably "frozen", with only a small fraction of terphenyl, molecules in the conductive conformation. Calculated HOMO-LUMO gaps for the planar and twisted conformations of terphenyl predict that the planar geometry is five times more conductive than the twisted conformation. In addition to presenting a new type of molecular electronic junction, the results bear on the widespread topic of electronic conductivity of organic molecules.