Importance of oxides in carbon/molecule/metal molecular junctions with titanium and copper top contacts

Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper on monolayers of nitroazobenzene (NAB), biphenyl, and nitrobiphenyl (NBP), and multilayers of NAB and NBP covalently bonded to an sp(2) carbon substrate. The electronic behavior of Ti junctions was extremely dependent on residual gas pressure during E-beam deposition, due to the formation of a disordered Ti oxyhydroxide deposit. The junction resistance decreased with decreasing residual gas pressure, and the hysteresis and rectification observed previously for relatively high deposition pressure was absent for pressures below 5 x 10(-7) Torr. Deletion of the molecular layer resulted in low-resistance junctions for both high and low deposition pressures. Replacement of the Ti with Al with otherwise identical deposition conditions resulted in insulating junctions with much higher resistance and no rectification. Ti junctions made at low residual gas pressure had resistances and current/voltage characteristics similar to those of junctions with Cu top contacts, with the latter exhibiting high yield and good reproducibility. The current/voltage characteristics of both the Ti and Cu junctions fabricated with low residual gas pressure were nonlinear and showed a strong dependence on the molecular layer thickness. The hysteresis and rectification previously observed for junctions fabricated at relatively high residual gas pressure depend on the combination of the NAB layer and the semiconducting TiOx film, with the TiOx layer conductivity depending strongly on formation conditions. Rectification and hysteresis in NAB/TiOx junctions may result from either redox reactions of the NAB and TiOx layers, or from electron injection into the conduction band of Ti oxide. (c) 2005 The Electrochemical Society.