| Popis: |
Contact mode atomic force microscopy (AFM) was used to intentionally scratch a monolayer of stilbene, biphenyl, nitrobiphenyl, terphenyl, and nitroazobenzene deposited on a pyrolyzed photoresist film (PPF). The force was set to completely remove the monolayer but not to damage the underlying PPF surface. A line profile determined across the scratch with tapping mode AFM permitted determination of the monolayer thickness from the depth of the scratch. A statistical process was devised to avoid user bias in determining the monolayer thickness, and was used to determine the thickness as a function of derivatization parameters. Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper onto monolayers of biphenyl, fluorene, and nitrobiphenyl, and multilayers of NBP covalently bonded to sp2 carbon substrates. 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. 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. 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 thickness of the molecular layer. Carbon/molecule/Cu molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene, biphenyl and nitrobiphenyl covalently bonded to flat, graphitic carbon. A “crossed wire” junction configuration provided high device yield and good junction reproducibility. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles, and exhibited negligible hysteresis. For junctions made with BP, FL, and NBP monolayers, the conductance varied by a factor of 40 at low voltage. Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thicknesses increased from 1.6 to 4.5 nm |
