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.

Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper on monolayers of nitroazobenzene ͑NAB͒, biphenyl, and nitrobiphenyl ͑NBP͒, 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 ϫ 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 TiO x film, with the TiO x layer conductivity depending strongly on formation conditions. Rectification and hysteresis in NAB/TiO x junctions may result from either redox reactions of the NAB and TiO x layers, or from electron injection into the conduction band of Ti oxide.

A major challenge in molecular electronics and related fields entails the fabrication of elaborate molecular architectures on electroactive surfaces to yield hybrid molecular/semiconductor systems. A method has been developed for the stepwise synthesis of oligomers of porphyrins linked covalently via imide units. A triallyl-porphyrin bearing an amino group serves as the base unit on Si(100), and the alternating use of a dianhydride (3,3',4,4'-biphenyltetracarboxylic dianhydride) and a porphyrin-diamine for reaction enables the rapid and simple buildup of oligomers composed of 2-5 porphyrins. The properties of these porphyrin "multad" films on Si(100) were interrogated using a variety of techniques. The charge densities of the redox-active porphyrin oligomers were determined via electrochemical methods. The stepwise growth was evaluated in detail via Fourier transform infrared (FTIR) spectroscopy and by selected X-ray photoelectron spectroscopic (XPS) studies. The morphology was probed via AFM methods. Finally, the thickness was evaluated by using a combination of ellipsometry and AFM height profiling, accompanied by selected XPS studies. Collectively, these studies demonstrate that high charge density, ultrathin, multiporphyrin films of relatively well-controlled thickness can be grown in a stepwise fashion using the imide-forming reaction. The increased charge densities afforded by the porphyrin multads may prove important for the fabrication of molecular-based information-storage devices. This bottom-up process for construction of surface-tethered molecular architectures complements the top-down lithographic approach for construction of functional devices with nanoscale dimensions.

Various aromatic and aliphatic alkynes and one alkene were covalently bonded to sp(2)-hybridized carbon surfaces by heat treatment in an argon atmosphere. X-ray photoelectron spectroscopy, Raman, and FTIR spectra of the modified surfaces showed that the molecules were intact after the 400 degrees C heat treatment but that the alkyne group had reacted with the surface to form a covalent bond. Alkynes with ferrocene and porphyrin centers exhibited chemically reversible voltammetric waves that could be cycled many times. Atomic force microscopy of the modified surfaces indicated a thickness of the molecular layer consistent with monolayer coverage, and surface coverage determined by voltammetry was also in the monolayer range. Raman spectroscopy of the porphyrin monolayers formed from a porphyrin alkyne showed no evidence for dimer formation, although multilayer formation may occur at undetected levels. FTIR spectra of the porphyrin-modified carbon surfaces were well-defined, similar to the parent molecule, and indicative of an average tilt angle between the porphyrin plane and the surface normal of 37 degrees . The bond between the molecular monolayer and the carbon surface was quite stable, withstanding sonication in tetrahydrofuran, mild aqueous acid and base, and repeated voltammetric cycling in propylene carbonate electrolyte. Heat treatment of alkynes and alkenes appears to be a generally useful method for modifying carbon surfaces, which can be applied to both aromatic and aliphatic molecules.