Electron transport and redox reactions in carbon-based molecular electronic junctions.

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.

Carbon/molecule/metal molecular electronic junctions: the importance of "contacts".

Molecular electronic junctions fabricated by covalent bonding onto a graphitic carbon substrate were examined with Raman spectroscopy and characterized electronically. The molecular layer was a 4.5 nm thick multilayer of nitroazobenzene (NAB), and the top contact material was varied to investigate its effect on junction behavior. A 3.0 nm thick layer of copper, TiO2, or Al(III) oxide (AlO(x)) was deposited on top of the NAB layer, followed by a 7.0 nm thick layer of gold. Copper "contacts" yielded molecular junctions with low resistance and showed a strong dependence on molecular structure. Carbon/ NAB/AlO(x)/Au junctions exhibited high resistance, with current densities three orders of magnitude less than those for analogous Cu junctions. However, Raman spectroscopy revealed that the NAB layer was reduced when the carbon substrate was biased negative, to a product resembling that resulting from electrochemical reduction of NAB. Carbon/ NAB/TiO2/Au junctions showed rectifying J/V behavior, with high conductivity to electrons able to enter the TiO2 conduction band. Substitution of azobenzene for nitroazobenzene yielded junctions with similar spectroscopic and electronic behavior to NAB, indicating that the nitro group is not essential for rectification. The results are interpreted in terms of the energy levels of the molecule relative to those of TiO2. The combination of a covalently bonded molecular layer and a semiconducting oxide yields unusual electronic properties in a carbon/molecule/semiconductor/Au molecular junction.

Electrooxidation of ascorbic acid on a polyaniline-deposited nickel electrode: surface modification of a non-platinum metal for an electrooxidative analysis.

The electrooxidation of ascorbic acid (H2A), which does not occur on a bare Ni electrode, has been shown to take place on a polyaniline (PANI)-coated Ni electrode in aqueous electrolytes of a wide pH range. The characteristic voltammetric peak of PANI in 0.1 M H2SO4 at 0.2 V vs SCE corresponding to the transformation of leucoemeraldine to emeraldine gradually diminishes with an increase in concentration of H2A as a result of adsorption. This peak disappears before the appearance of another peak corresponding to the oxidation of H2A at a concentration of 1 mM. The irreversible oxidation current of H2A exhibits a linear dependence on the concentration. The effect of adsorption of H2A on PANI has been shown to increase the voltammetric peak current. A study on the variation of the PANI thickness and its influence on the voltammetric oxidation of H2A has led to an optimum thickness of 1.6 microm. The oxidation currents on the porous PANI/Ni electrode have been found to be several times higher at lower potentials in comparison with the data of a Pt electrode. The reaction has also been studied by ac impedance spectroscopy. In alkaline electrolytes, the Nyquist impedance plot is characterized by two semicircles instead of a single semicircle in acidic electrolytes. Thus, Ni, which is a non-platinum metal, has been found to be useful, by surface modification with PANI, for electrooxidation of H2A. The data are reproducible in the electrolytes of a wide pH range, thus suggesting a good stability, reusability and a long life for the PANI/Ni electrodes.