Contact mode atomic force microscopy (AFM) was used to intentionally scratch a monolayer 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. PPF surfaces modified by reduction of diazonium ions of stilbene, biphenyl, nitrobiphenyl, terphenyl, and nitroazobenzene (NAB) were scratched and their modification layer thicknesses determined. For single-scan derivatizations of 1 mM diazonium ions to -0.6 V versus Ag+/Ag, the biphenyl and stilbene monolayers exhibited thicknesses close to those expected for true monolayers. However, more extensive derivatization resulted in multilayers up to 6.3 nm thick for the case of NAB. Such multilayers imply that electrons are transmitted through the growing film during diazonium reduction, despite the fact that electron tunneling would not be expected to be operative over such long distances. The results are consistent with a conductance increase in the growing film, which yields a partially conductive layer that can support further diazonium ion reduction and additional layer growth.

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.

We report herein the fabrication of a molecular junction in which a thin (8−15 Å) layer of oriented organic molecules is positioned between two electronic conductors. The molecular layer becomes a component in an electronic circuit and exhibits properties that depend strongly on molecular structure. Bonding between the carbon substrate and the molecular layer is covalent and conjugated, and thus differs fundamentally from that of the widely studied self-assembled monolayers of alkane thiols on metal surfaces. The chemisorbed molecular layer is densely packed and stable and does not contain the tunneling barrier imposed by a sulfur atom. The current/voltage behavior of methyl-phenyl, n-butyl phenyl, tert-butyl phenyl, and stilbene monolayers between pyrolytic carbon and mercury indicates a negligible pinhole density and shows weak dependence on temperature. The action of a nitroazobenzene molecular junction as a bistable switch is demonstrated, and switching behavior persisted for many on−off cycles and over a period of at least 14 h. Carbon-based molecular junctions represent a new paradigm for molecular electronics, which shows promising electronic behavior and is amenable to low cost, benchtop processing.

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 Å and 47 Å thickness, and junctions were examined over the temperature range of +80 °C to −50 °C. Junction resistances were a strong function of molecular length and structure, with mean resistances for 0.78 mm2 junctions of 34.4 Ω, 13.8 KΩ, and 41.0 KΩ 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 °C but thermally activated above 10 °C. 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 °C, 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.

Mono- and multilayers of nitroazobenzene (NAB), azobenzene (AB), nitrobiphenyl (NBP), biphenyl (BP), and fluorene (FL) were covalently bonded to flat pyrolyzed photoresist films (PPF) by electrochemical reduction of their diazonium derivatives. The structure and orientation of the molecular layers were probed with ATR-FT-IR and Raman spectroscopy. A hemispherical germanium ATR element used with p-polarized light at 65 degrees incidence angle yielded high signal/noise IR spectra for monolayer coverage of molecules on PPF. The IR spectra are dominated by in-plane vibrational modes in the 1000-2000-cm(-1) spectral range but also exhibit weaker out-of-plane deformations in the 650-1000-cm(-1) region. The average tilt angle with respect to the surface normal for the various molecules varied from 31.0 +/- 4.5 degrees for NAB to 44.2 +/- 5.4 degrees for FL with AB, NBP, and BP exhibiting intermediate adsorption geometries. Raman intensity ratios of NAB and AB for p- and s-polarized incident light support the conclusion that the chemisorbed molecules are in a predominantly upright orientation. The results unequivocally indicate that molecules electroreduced from their diazonium precursors are not chemisorbed flat on the PPF surface, but rather at an angle, similar to the behavior of Au/thiol self-assembled monolayers, Langmuir-Blodgett films, and porphyrin molecules chemisorbed thermally on silicon and PPF from alkyne and alkene precursors.