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We present a combined experimental and theoretical study of photoinduced current in molecular junctions consisting of monolayers of nitroazobenzene oligomers chemisorbed on carbon surfaces and illuminated by ultraviolet-visible light through a transparent electrode. Experimentally observed dependence of the photocurrent on light frequency, temperature, and monolayer thickness is analyzed within first-principles simulations employing the Hubbard nonequilibrium Green's function diagrammatic technique. We reproduce qualitatively correct behavior and discuss mechanisms leading to the characteristic behavior of dark and photoinduced currents in response to changes in bias, frequency of radiation, temperature, and thickness of molecular layer.

This paper presents a method to derivatize a wide variety of substrate materials that are frequently used in spectroscopic characterizations with molecular layers through the reduction of aromatic diazonium reagents. The method relies on an ultrathin (5 nm) layer of a reactive metal (e.g., Ti or Al) deposited as a primer that subsequently mediates the reduction of aromatic diazonium reagents from acetonitrile solution. Following surface modification, the Ti can be oxidized to provide a passivated support surface. Raman, Infrared, UV-vis and X-ray photoelectron spectyoscopic techniques are used to characterize the molecular layers on the metal primer surface. When a Ti primer layer is derivatized via diazonium reduction, the molecule is shown to be present on the ultrathin Ti layer on Au, Al, quartz, Si/Si[O.sub.x], glass, and polyethylene surfaces. For molecules bound to a Ti primer, the molecular layer was found to be stable to sonication in acetone or acetonitrile, a 1 h exposure to boiling water, and a 30 rain exposure to 0.1 M acid or base. The approach also permits spectroscopic characterization of buried thinfilm molecular layers on optically transparent substrates after deposition of thick top metal contacts.

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-FLIR 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.sup.-1] spectral range but also exhibit weaker out-of-plane deformations in the 650-1000-[cm.sup.-1] region. The average tilt angle with respect to the surface normal for the various molecules varied from 31.0 [+ or -] 4.5[degrees] for NAB to 44.2 [+ or -] 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.

A facile method for the preparation of thin-film carbon electrodes by electron beam evaporation onto highly doped silicon is presented. The physical and electrochemical properties of these films both before and after postdeposition pyrolysis are investigated. Raman spectroscopy establishes the amorphous structure of the nonpyrolyzed carbon films and confirms the formation of graphitic carbon after pyrolysis at 1000[degrees]C. Scanning force microscopy reveals the root-mean-square roughness of nonpyrolyzed films to be ~1 [Angstrom], while pyrolyzed films exhibit an increased roughness of ~4 [Angstrom]. The electrochemical behavior of the electrodes resembles glassy carbon, with measured heterogeneous electron-transfer rate constants among the highest measured for thin carbon films. These carbon film electrodes will potentially find applications in such fields as molecular electronics and scanning probe microscopy of adsorbed species.

Molecular junctions consisting of nitroazobenzene (NAB) chemisorbed to a substrate of pyrolyzed photoresist film (PPF) and a top contact of vapor-deposited titanium were examined with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of the NAB layer varied from submonolayer to 4.5 nm, and a thin (1-3nm) overlayer of Ti was deposited by electron beam deposition. Without Ti, the NAB surface Raman spectrum was sufficiently strong to observe previously unreported modes in the [500-1000-cm.sup.-1] region, and the [1000-1700-cm.sup.-1] region was sufficiently strong to observe the effects of metal deposition. Upon Ti deposition, the intensities of NAB modes associated with the nitro group decreased significantly, and the XPS indicated formation of a Ti--N bond. For the thicker NAB layers (1.9 and 4.5nm), the intensities of the N[O.sub.2] Raman modes partially recovered over a several-day period, but they remain depressed or absent in the submonolayer sample. The results indicate a reaction between condensing Ti atoms and the terminal N[O.sub.2] group, probably to form a Ti--nitroso linkage between NAB and Ti. The result is a molecular junction with covalent bonding at both ends in the form of a C--C bond between PPF and NAB and a Ti--N bond to the top contact. The structural implications of the current results are interpreted in the context of recently reported functioning PPF/NAB/Ti molecular electronic junctions. In particular, the reaction between Ti and the nitro group appears to prevent short circuits resulting from incursion of Ti into the NAB layer.

Pyrolyzed photoresist films (PPF) are introduced as planar carbon electrodes in a PDMS-quartz hybrid microchip device. The utility of PPF in electroanalytical applications is demonstrated by the separation and detection of various neurotransmitters. PPF is found to form a stable, low-capacitance, durable layer on quartz, which can then be used in conjunction with a microchip capillary electrophoretic device. Sinusoidal voltammetric detection at PPF electrodes is shown to be very sensitive, with a detection limit (S/N = 3) of 100 nM for dopamine, corresponding to a mass detection limit (S/N = 3) of 2 amol. The selectivity of analysis in the frequency domain is demonstrated by isolating each individual signal in a pair of analytes that are chromatographically unresolved. Effectively decoupling the electrophoresis and electrochemical systems allows the electrodes to be placed just inside the separation channel, which results in efficient separations (80 000-100 000 plates/m).

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.sup.+]/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.

Glassy carbon (GC) surfaces modified with monolayers of biphenyl and nitrobiphenyl molecules were examined as voltammetric electrodes for ferrocene, benzoquinone, and tetracyanoquinodimethane electrochemistry in acetonitrile. The modified electrodes exhibited slower electron transfer than unmodified GC, by factors that varied with the monolayer and redox system. However, after a negative potential excursion to ~-2.0 V versus [Ag.sup.+]/Ag, the modified electrodes exhibited much faster electron-transfer kinetics, approaching those observed on unmodified GC. The effect is attributed to an apparently irreversible structural change in the biphenyl or nitrobiphenyl monolayer, which increases the rate of electron tunneling. The transition to the 'ON' state is associated with electron injection into the monolayer similar to that observed in previous spectroscopic investigations and causes a significant decrease in the calculated HOMO-LUMO gap for the monolayer molecule. Once the monolayer is switched ON, it supports rapid electron exchange with outer-sphere redox systems, but not with dopamine, which requires adsorption to the GC surface. The increase in electron-transfer rate with electron injection is consistent with an increase in electron tunneling rate through the monolayer, caused by a significant decrease in tunneling barrier height. The ON electrode can reduce biphenyl-or nitrobiphenyldiazonium reagent in solution to permit formation of a second modification layer of biphenyl or nitrobiphenyl molecules. This 'double derivatization' procedure was used to prepare tetraphenyl- and nitrotetraphenyl-modified electrodes, which exhibit significantly slower electron transfer than their biphenyl and nitrobiphenyl counterparts. A 'switching' electrode may have useful properties for electroanalytical applications and possibly in electro-catalysis. In addition, the ON state represents an 'organic electrode' in which electron transfer occurs at an interface between an organic conductor and a solution rather than an interface between a solution and a metal or carbon electrode.

Plasmonic hot carriers with a nonthermal distribution of kinetic energies have opened up new avenues in photovoltaics, photodetection and photocatalysis. While several articles have reported ultrafast hot electron injection from coinage metals into n-type semiconductors across Schottky barriers and efficient subsequent utilization of injected hot electrons, reports of hot hole harvesting are comparatively rare due to the difficulty in forming Schottky junctions between p-type semiconductors and high work function metals. In this work, we report the fabrication, characterization and theoretical calculations of a novel integrated multiferroic-plasmonic system comprising YMn2O5 nanowires decorated on their surface with Ag nanoparticles (NPs). A Schottky barrier for holes exists at the YMn2O5–Ag hetero-interface and hot holes are injected from Ag across this barrier. The synthesized hybrid along with bare Ag NPs were tested for Raman surface photocatalytic reduction of 4-NBT (4-nitrobenzenethiol) to DMAB (p,p′-dimercaptoazobenzene) where the composite demonstrated superior activity compared to the bare metal. Ultraviolet photoelectron spectroscopy (UPS) revealed a significantly reduced work function of the composite compared to the pristine Ag, indicative of more energetic hot electrons on the surface of the composite required for efficient photoreduction. Density functional theory (DFT)-based calculations revealed localization of molecular orbitals supportive of a possible hole transfer from YMn2O5 to Ag and a reorganization of electronic states, which promotes increased phonon-mediated sp-intraband damping of the plasmon. DFT results also indicated a purely electronic contribution to the ferroelectric polarization of YMn2O5 over and above the ionic contribution, which originated from the magnetic polarization of O2p states. [ABSTRACT FROM AUTHOR]

A molecular junction based on a monolayer between carbon and mercury contacts is investigated by examining current/voltage behavior as a function of temperature and monolayer thickness. The results are consistent with a tunneling process at low temperature, where the molecular conformations are apparently fixed.

The study of heterogeneous electron transfer at carbon electrodes is described with reference to its self-catalysis by catechols and quinones. The results offer explanations for the catechol behavior.

The electrochemical oxidation of dopamine, 4-methylcatechol, dihydroxyphenylacetic acid, dihydroxyphenyl ethylene glycol, and hydroquinone was examined on several native and modified glassy carbon (GC) surfaces. Treatment of polished GC with pyridine yielded small (delta)E(sub p) values for cyclic voltammetry of all systems studied, implying fast electron-transfer kinetics. Changes in surface oxide coverage had little effect on kinetics, nor did the charge of the catechol species or the solution pH. Small (delta)E(sub p) values correlated with catechol adsorption, and surface pretreatments that decreased adsorption also increased (delta)E(sub p). Electron transfer from catechols was profoundly inhibited by a monolayer of nitrophenyl or (trifluoromethyl)phenyl (TFMP) groups on the GC surface, so that voltammetric waves were not observed. The (delta)E(sub p) increased monotonically with surface coverage of TFMP groups. The results indicate that catechol adsorption to GC is required for fast electron transfer for the redox systems studied. Unlike Ru(NH(sub 3))(sub 6)(super 3+/2+), chlorpromazine, methyl viologen, and several others, electron tunneling through monolayer films was not observed for the catechols. The results are not consistent with an electrontransfer mechanism involving proton transfer or electrostatic interactions between the catechols and surface sites on the GC surface. The vital role of adsorption in the electron-transfer process is currently under study but appears to involve changes in the inner-sphere reorganization energy.

Pretreatment of glassy carbon (GC) electrodes with 2-propanol, acetonitrile, or cyclohexane had a significant effect on electrode kinetics, adsorption, and capacitance. Reagent grade solvents slowed electron transfer rates for dopamine, ascorbic acid, [Fe.sup.3+/2+], and [Fe[(CN).sub.6].sup.3-/4-] and decreased adsorption of anthraquinone-2,6-disulfonate (AQDS) and methylene blue (MB). However, if activated carbon (AC) was present in the solvent during pretreatment, the result was increased electron transfer rates and adsorption for several commonly studied redox systems. The large surface area of AC acts as a 'getter' for solvent impurities and for species desorbed from the GC surface, leading to a carbon electrode surface with higher capacitance, higher adsorption of AQDS and MB, and faster electron-transfer rates for [Fe[(CN).sub.6].sup.3-/4-], dopamine, and ascorbic acid. In addition, the treated surfaces were more reproducible, and aged electrodes were reactivated by AC in 2-propanol. The results imply that large, polar organic impurities are present on the polished GC surface which are removed by the combination of an organic solvent and activated carbon. These impurities contain oxygen detectable by XPS and appear to be weakly catalytic toward the [Fe.sup.3+/2+] redox system.

Gas-phase modification of glassy carbon (GC) was investigated in an attempt to make a C-H-terminated surface that is resistant to oxidation. By using a hot filament technique, hydrogen radicals were generated from a flow of hydrogen gas, and then the radicals attacked glassy carbon electrode surfaces. The modified glassy carbon surfaces were characterized first by X-ray photoelectron spectroscopy, where the shape of the carbon 1s band shows a distribution of carbon oxidation states different from a fresh polished surface. The oxygen-to-carbon atomic ratio is low (

Various well-established and novel surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups. In addition, monolayers of several different adsorbates were formed on GC surfaces before electrochemical evaluation. Both the non-specific monolayer adsorbates and reagents which chemisorb to specific functional groups were observed on the surface with Raman and photoelectron spectroscopy. The various GC surfaces were then evaluated for their electron transfer reactivity with nine redox systems in aqueous electrolyte, including Ru[[([NH.sub.3]).sub.6].sup.2-/3+], Fe[(C[N.sub.6]).sup.3-/4-], ascorbic acid, and [[Fe.sub.aq].sup.3-/2+]. The nine systems were categorized according to their kinetic sensitivity to surface modification. Several, including Ru[[([NH.sub.3]).sub.6].sup.2+/3+], are insensitive to surface modifications and are considered outer sphere. [[Fe.sub.aq].sup.3+/2-], [[V.sub.aq].sup.2+/3+], and [[Eu.sub.aq].sup.2+/3+] are catalyzed by surface carbonyl groups and are very sensitive to the removal of surface oxides or derivatization of C=O groups. Ascorbic acid and Fe[[(CN).sub.6].sup.3-/4-] constitute a third group which are not catalyzed by oxides but which do require a specific surface interaction. A procedure for classifying redox systems by their kinetics on modified carbon surfaces is proposed.

Monolayers of 4-nitrophenyl radical, covalently attached to ordered graphite and glassy carbon surfaces, are analyzed by high sensitivity Raman spectroscopy. 4-Nitrobiphenyl, which is selected as a model compound for the nitrophenyl radical, exhibits most of the bands observed for nitrophenyl, such as 854, 1104, 1160 and 1340 per centimeter. Raman spectroscopic data and voltametric measurement reveal that the compound has a surface coverage of 6.5 x 10 to the -10th power mol/square cm on glassy carbon and 1.6 x 10 to the -10th power mol/square cm on the basal plane of pyrolytic graphite.

Raman spectra of normally fluorescent materials were obtained by quenching the fluorescence through adsorption on carbon surfaces. Energy transfer from the excited state to the carbon surface greatly reduced the fluorescence of rhodamine 6G and the near-IR laser dye IR125, and Raman spectra with high S/N were obtained from the adsorbed molecules. Strong resonance enhancement of Raman scattering was observed, and monolayer films of RGG were observed. RGG exhibited Langmuirian adsorption behavior, with saturation coverage occuring at [approximately] 0.5 mM in methanol. The technique was also successfully applied to bis(methylstyryl)benzene (BMB) and fluoranthene, which are less fluorescent but also much weaker scatterers. Since the laser wavelengths yielding resonance enhancement are in the same range as those producing fluorescence, adsorption to carbon permits elimination of fluorescence without loss of resonance enhancement. The porous graphite substrate has the added advantage of high microscopic surface area, low elastic background, and propensity to adsorb a wide range of organic materials. With care, it was possible to observe Raman features from monolayers of materials having cross sections comparable to that of benzene, such as the 1205 [cm.sup.-1] peak of BMB.