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Carbon film electrodes can often be used without pretreatment, and their fabrication allows for flexibility in size and shape and for mass production. In this work, we are exploring layered structures comprised of thin films of carbon on gold (eC/Au) prepared by electron-beam evaporation. These extremely flat films are not pyrolyzed and are comprised of mainly amorphous carbon but still exhibit reasonable conductivity due to the underlying gold layer. eC/Au electrodes, without any pretreatment, yield similar heterogeneous electron-transfer rates for benchmark redox systems and significantly lower background current in comparison with polished glassy carbon. Interestingly, they show insignificant adsorption of quinones, which is uncommon for most carbon electrode materials. However, eC/Au is still prone to adsorption of airborne hydrocarbons when exposed to ambient air like most graphitic materials. With reproducibly fast electron transfer kinetics, low background current, negligible adsorption, and ultraflat surface, eC/Au films are a promising candidate for electrochemical and sensor applications. [ABSTRACT FROM AUTHOR]

Presents a study that determined the effects of fabrication variables on the surface of silver particles. Characterization of the silver microparticles; Impact of fabrication conditions on the morphology of the microparticles; Analysis of the Raman spectrum of the silver microparticles.

Ted Kuwana was not only a scientific leader in electrochemistry and related fields, but he was also an excellent citizen and unselfish contributor to the careers of many other scientists, myself included. [ABSTRACT FROM AUTHOR]

Two layers of molecular oligomers were deposited on flat carbon electrode surfaces by electrochemical reduction of diazonium reagents, then a top contact applied to complete a solid-state molecular junction containing a molecular bilayer. The structures and energy levels of the molecular layers included donor molecules with relatively high energy occupied orbitals and acceptors with low energy unoccupied orbitals. When the energy levels of the two molecular layers were similar, the device had electronic characteristics similar to a thick layer of a single molecule, but if the energy levels differed, the current voltage behavior exhibited pronounced rectification. Higher current was observed when the acceptor molecule was biased negatively in eight different bilayer combinations, and the direction of rectification was reversed if the molecular layers were also reversed. Rectification persisted at very low temperature (7 K), and was activationless between 7 and 100 K. The results are a clear example of a "molecular signature" in which electronic behavior is directly affected by molecular structure and orbital energies. The rectification mechanism is discussed, and may provide a basis for rational design of electronic properties by variation of molecular structure. [ABSTRACT FROM AUTHOR]

Molecular junctions consisting of a Ru(bpy)3 oligomer between conducting carbon contacts exhibit an exponential dependence of junction current on molecular layer thickness (d) similar to that observed for other aromatic devices when d < 4 nm. However, when d > 4 nm, a change in transport mechanism occurs which coincides with light emission in the range of 600-900 nm. Unlike light emission from electrochemical cells or solid-state films containing Ru(bpy)3, emission is bipolar, occurs in vacuum, has rapid rise time (<5 ms), and persists for >10 h. Light emission directly indicates simultaneous hole and electron injection and transport, possibly resonant due to the high electric field present (>3 MV/cm). Transport differs fundamentally from previous tunneling and hopping mechanisms and is a clear "molecular signature" relating molecular structure to electronic behavior. [ABSTRACT FROM AUTHOR]

Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction. [ABSTRACT FROM AUTHOR]

The article discusses heterogeneous electron transfer (ET) rates of ordered graphite electrodes. Several factors contributing to electrochemical kinetics of carbon electrodes are noted, including redox state, surface density, and electrode edge plane sites. Other topics include the preparation of defect-free graphite surfaces, the electronic kinetic variations between various types of carbon electrodes, and electrocatalytic redox reactions such as oxidation, deposition, and dioxygen reduction.

To understand the mechanism of bias-induced resistance switching observed in polypyrrole (PPy) based solid state junctions, in situ UV-vis absorption spectroscopy was employed to monitor oxidation states within PPy layers in solution and in PPy/metal oxide junctions. For PPy layers in acetonitrile, oxidation led primarily to cationic polaron formation, while oxidation in 0.1 M NaOH in H2O resulted in imine formation, caused by deprotonation of polarons. On the basis of these results in solution, spectroelectrochemistry was used to monitor bias-induced formation of polarons and imines in PPy layers incorporated into solid state carbon/PPy/Al2O3/Pt junctions. A positive bias on the carbon electrode caused PPy oxidation, with the formation of polaron and imine species strongly dependent on the surrounding environment. The spectral changes associated with polarons or imines were stable for at least several hours after the applied bias, while a negative bias reversed the absorbance changes back to the initial PPy spectrum. These results indicate that PPy can be oxidized in nominally solid state devices, and the formation of stable polarons is dependent on the tendency for deprotonation of the polaron to the imine. Since PPy conductivity depends strongly on the polaron concentration, monitoring its concentration is critical to determining resistance switching mechanisms. Furthermore, the importance of ion mobility and OH- generation through H2O reduction at the Pt contact are discussed. [ABSTRACT FROM AUTHOR]

Abstract: We introduce a simple thermal oxidation technique for decreasing feature sizes of nanoimprint lithography (NIL) masters. During oxidation, the dimensions of negative features are reduced (e.g., gaps become narrower), and the dimensions of positive features increase (e.g., lines become wider). We demonstrate that positive feature sizes can also be reduced after oxidation by selective etching of the oxide. We show that 74nm gaps can be reduced to 10nm and 226nm lines can be narrowed to 55nm. The reduction in feature size achieved in both positive and negative structures directly translates into increased imprint resolution, and we demonstrate improved resolution in a complete NIL pattern transfer using thermally oxidized NIL masters. [Copyright &y& Elsevier]

Virtually all types of molecular electronic devices depend on electronically addressing a molecule or molecular layer through the formation of a metallic contact. The introduction of molecular devices into integrated circuits will probably depend on the formation of contacts using a vapour deposition technique, but this approach frequently results in the metal atoms penetrating or damaging the molecular layer. Here, we report a method of forming ‘soft’ metallic contacts on molecular layers through surface-diffusion-mediated deposition, in which the metal atoms are deposited remotely and then diffuse onto the molecular layer, thus eliminating the problems of penetration and damage. Molecular junctions fabricated by this method exhibit excellent yield (typically >90%) and reproducibility, and allow examination of the effects of molecular-layer structure, thickness and contact work function. [ABSTRACT FROM AUTHOR]

The article offers information on advanced carbon electrode materials for molecular electrochemistry. It mentions that since the application of Sir Humphrey Davy of graphite electrodes for the electrochemical production of alkali metals, carbon materials have been widely employed in both industrial and analytical electrochemistry. It is also posed that carbon electrodes are also used in metal production and energy storage in batteries and supercapacitors.

In situ Raman spectra of chemisorbed azobenzene (AB) monolayers on glassy carbon (GC) electrodes were observed under potentiostatic conditions in acetonitrile (ACN) with tetrabutyl-ammonium tetrafluoroborate (TBA-BF4). The Raman intensities of these spectra were high below −1000 mV, and this is attributed to the change in absorbance of AB on GC. In this paper, we describe chemisorbed AB molecules on GC electrode surfaces under potentiostatic conditions. [ABSTRACT FROM AUTHOR]

The article reports on the growth of molecular electronic devices and some of the problems relating to the characterization of electronic components with molecular dimensions. Pursuing these devices is motivated by the extension of Moore's law. It discusses the important advantages of incorporating molecules into electronic circuits, its functions and the techniques used. It presents an overview of the importance of photolithography in the practical realization of any electronic device and the cost benefits of molecular electronics compared to silicon devices.

Carbon/nitroazobenzene (NAB)/titanium/gold molecular electronic junctions with active thicknesses of 7-8 nm were constructed having partially transparent Ti/Au top contacts, which permitted in situ monitoring of molecular structure with Raman spectroscopy for applied biases between +3 and -3 V. Deposition of the Ti/Au top contacts resulted in spectral changes similar to those accompanying NAB reduction in a conventional spectroelectrochemical experiment. Upon application of +3 V (carbon relative to Ti), the spectrum indicated reoxidation of the NAB reduction product, and this redox cycle could be repeated at least three times. When a voltage excursion to -2 or -3 V occurred, the spectra indicated irreversible loss of the nitro group, and a dramatic but reversible decrease in Raman intensity over the entire shift range examined. Negative applied voltage causes formation of reduced NAB and a high oxidation state of titanium, while positive voltage forms oxidized NAB and injects electrons into the titanium oxide layer. The spectral changes were correlated with current/voltage curves in order to probe the mechanism of rectification and conductance switching reported previously. Overall, the combination of spectroscopic and voltammetric results implies a conduction mechanism involving both NAB and titanium oxide, possibly mediated by the injection of carriers into the semiconducting titanium oxide, or by reduction of an insulating titanium oxide to a more conductive form. [ABSTRACT FROM AUTHOR]

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K2Cr2O7. Raman spectra acquired in and near corrosion pits allowed identification of two products of CrVI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an AlIII–CrVI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al3+ and CrO42-. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a CrIII–CrVI mixed oxide, formed by partial reduction of CrVI to CrIII by exposed Al metal or by H2. At the pitting potential, a mixture of AlIII and CrIII mixed oxides was observed, with the AlIII species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by CrVI are considered. In particular, the concentration of CrVI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion. [Copyright &y& Elsevier]

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 (1-3 nm) 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-1 region, and the 1000- 1700-cm-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 4.5 nm), the intensities of the NO2 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 tenninal NO2 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. [ABSTRACT FROM AUTHOR]

In situ Raman spectroscopy was used to monitor 4-nitroazobenzene (NAB) in an electrochemical cell, both as a free molecule and as a chemisorbed monolayer on a glassy carbon (GC) electrode surface. Reduction of free NAB exhibited two well-defined voltammetric couples in acetonitrile, and the accompanying spectral changes supported a mechanism involving two successive 1-e transfers. Raman spectra of NAB chemisorbed to GC via diazonium ion reduction were obtained in acetonitrile with a high-sensitivity, linefocused CCD spectrometer. The chemisorbed NAB spectra were quite different from the free NAB spectra, and were sufficiently strong to monitor as a function of applied potential. In the potential range of +400 to -800 mV vs Ag/Ag[sup +], the intensity of the Raman bands associated with the phenyI-NO[sub 2] moiety varied, implying an electronic interaction between the π system of the graphitic substrate and the chemisorbed NAB molecules. Negative of -800 mV, a 1-e[sup -] voltammetric reduction peak was observed, which was reversible on the positive voltage scan. This peak was accompanied by significant spectral changes, particularly the loss of the N=N and NO[sub 2] stretches. The spectra are consistent with formation of a quinoid structure containing a C=C double bond between the NAB and the graphitic surface. The electron transfer and spectral changes occurred over a wider potential range than expected for a conventional Nernstian equilibrium, but did not appear to be broadened by slow electron-transfer kinetics. The results imply a significant perturbation of electron transfer between the GC and the monolayer, caused by strong electronic coupling between the graphitic π system and the NAB orbitals. Rather than a discrete electron transfer to a free molecule, the electron transfer to chemisorbed NAB is more gradual, and is presumably driven by the electric field at the electrode/solution interface. [ABSTRACT FROM AUTHOR]

Insights into the role of chromate on aluminum alloys corrosion. Anodic and cathodic inhibition by chromate; Characteristics of chromate.

Examines the electroanalytical performance of carbon films with near-atomic flatness. Use of pyrolized photoresist to characterize the carbon films; Surface properties of the pyrolized photoresist films; Comparison of the pyrolyzed films with glass carbon.

Examines the heterogeneous electron transfer kinetics for several catechols. Role of a layer catechol as an electrocatalyst for solution-phase redox component; Effect of electrostatic attraction of repulsion between the surface and redox system on the magnitude of inhibition; Observance of slow electron transfer in an inert film.

Provides insights into quinone electron-transfer mechanisms by examining several catechols and hydroquinone. Use of native and modified glassy carbon (GC) surfaces in the examination; Observation on carbon electrode behavior; Inhibition of electron transfer from catechol by phenyl groups on GC surface; Role of adsorption in electron-transfer process.

Compares hydrogen-modified glassy carbon electrodes prepared by hot filament method with polished glassy carbon in terms of surface structure, stability, and electrode kinetics. Lack of significant difference in voltammetry for dopamine and ascorbic acid; Decrease in electron-transfer rate for hydrogen modified carbon due to the absence of carbonyl sites.

Reports on the use of Raman spectroscopy to observe chemisorbed nitrophenyl on glassy carbon and highly ordered pyrolytic graphite and stereoscopic observation of the electrochemical reduction of surface nitrophenyl groups. Surface derivatization; Surface spectra; Surface coverage.

Examines several types of carbon electrode surfaces using Raman spectroscopy and Raman imaging. What variations in the intensity ratio of the D and E2g Raman bands across the carbon surface indicated; Application of Raman microspectroscopy to highly ordered pyrolytic graphite and glassy carbon surfaces; Reference to the inherent heterogeneity of the carbon surface.

Studies the control of electron transfer kinetics at glassy carbon (GC) electrodes by specific surface modification. GC surface preparation; Polished GC; Monolayer-coated GC; Low-oxide GC; Specific chemisorption to functional groups; Systems which are relatively insensitive to surface chemistry; Systems dependent on surface oxides.

Discusses Raman spectroscopy of normal and diseased human breast tissues. Medical diagnostic techniques useful for the screening of breast cancer; Requirement that variations in spectra from normal biopsy specimens be smaller than those occurring between normal and abnormal tissue; Peak frequencies for Raman features.

Investigates the Raman spectra of normally fluorescent materials obtained by quenching the fluorescence through adsorption on carbon surfaces. Effect of energy transfer from the excited state to the carbon surface on fluorescence; Observation of strong resonance enhancement of Raman scattering; Advantages of the porous graphite substrate.

The electronic properties of molecular junctions of the general type carbon/molecule/TiO2/Au were examined as examples of “molecular heterojunctions” consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing Al2O3 instead of fluorene, and those with only the TiO2 layer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiO2 layer, and occurred in all three junction types studied. Conduction in PPF/TiO2/Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This “memory” effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to “dynamic doping” of the TiO2 layer by the imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration. [ABSTRACT FROM AUTHOR]

The internal potential profile and electric field are major factors controlling the electronic behavior of molecular electronic junctions consisting of ∼1–10 nm thick layers of molecules oriented in parallel between conducting contacts. The potential profile is assumed linear in the simplest cases, but can be affected by internal dipoles, charge polarization, and electronic coupling between the contacts and the molecular layer. Electrochemical processes in solutions or the solid state are entirely dependent on modification of the electric field by electrolyte ions, which screen the electrodes and form the ionic double layers that are fundamental to electrode kinetics and widespread applications. The current report investigates the effects of mobile ions on nominally solid-state molecular junctions containing aromatic molecules covalently bonded between flat, conducting carbon surfaces, focusing on changes in device conductance when ions are introduced into an otherwise conventional junction design. Small changes in conductance were observed when a polar molecule, acetonitrile, was present in the junction, and a large decrease of conductance was observed when both acetonitrile (ACN) and lithium ions (Li+) were present. Transient experiments revealed that conductance changes occur on a microsecond–millisecond time scale, and are accompanied by significant alteration of device impedance and temperature dependence. A single molecular junction containing lithium benzoate could be reversibly transformed from symmetric current–voltage behavior to a rectifier by repetitive bias scans. The results are consistent with field-induced reorientation of acetonitrile molecules and Li+ ion motion, which screen the electrodes and modify the internal potential profile and provide a potentially useful means to dynamically alter junction electronic behavior. [ABSTRACT FROM AUTHOR]

Photocurrents generated by illumination of carbon-based molecular junctions were investigated as diagnostics of how molecular structure and orbital energies control electronic behavior. Oligomers of eight aromatic molecules covalently bonded to an electron-beam deposited carbon surface were formed by electrochemical reduction of diazonium reagents, with layer thicknesses in the range of 5-12 nm. Illumination through either the top or bottom partially transparent electrodes produced both an open circuit potential (OCP) and a photocurrent (PC), and the polarity and spectrum of the photocurrent depended directly on the relative positions of the frontier orbitals and the electrode Fermi level (EF). Electron donors with relatively high HOMO energies yielded positive OCP and PC, and electron acceptors with LUMO energies closer to EF than the HOMO energy produced negative OCP and PC. In all cases, the PC spectrum and the absorption spectrum of the oligomer in the molecular junction had very similar shapes and wavelength maxima. Asymmetry of electronic coupling at the top and bottom electrodes due to differences in bonding and contact area cause an internal potential gradient which controls PC and OCP polarities. The results provide a direct indication of which orbital energies are closest to EF and also indicate that transport in molecular junctions thicker than 5 nm is controlled by the difference in energy of the HOMO and LUMO orbitals. [ABSTRACT FROM AUTHOR]

Electrochemical deposition of aromatic organic molecules by reduction of diazonium reagents enables formation of molecular layers with sufficient integrity for use in molecular electronic junctions of interest to microelectronics. Characterization of organic films with thicknesses in the 1-10 nm range is difficult with Raman spectroscopy, since most molecular structures of electronic interest have Raman cross sections which are too small to observe as either thin films on solid electrodes or within intact molecular junctions. Layer formation on a 10 nm thick Ag island film on a flat carbon surface (eC/Ag) permitted acquisition of structural information using surface enhanced Raman spectroscopy (SERS), in many cases for molecules with weak Raman scattering. Raman spectra obtained on eC/Ag surfaces were indistinguishable from those on carbon without Ag present, and the spectra of oligomeric molecular layers were completely consistent with those of the monomers. Layer growth was predominantly linear for cases where such growth was sterically allowed, and linear growth correlated strongly with the line width and splitting of the C?C phenyl ring stretches. Molecular bilayers made by successive reduction of different diazonium reagents were also observable and will be valuable for applications of 1-20 nm organic films in molecular electronics. [ABSTRACT FROM AUTHOR]

The article reports that Richard L. McCreery has been selected as an associate editor of the journal "Analytical Chemistry." McCreery is currently Professor of Chemistry at Ohio State University. McCreery research involves spectroscopic probes of electrochemical processes and molecular electronics, with the goal of relating surface structure to chemical and electronic behavior.

Two layers of molecular oligomers were deposited on flat carbon electrode surfaces by electrochemical reduction of diazonium reagents, then a top contact applied to complete a solid-state molecular junction containing a molecular bilayer. The structures and energy levels of the molecular layers included donor molecules with relatively high energy occupied orbitals and acceptors with low energy unoccupied orbitals. When the energy levels of the two molecular layers were similar, the device had electronic characteristics similar to a thick layer of a single molecule, but if the energy levels differed, the current voltage behavior exhibited pronounced rectification. Higher current was observed when the acceptor molecule was biased negatively in eight different bilayer combinations, and the direction of rectification was reversed if the molecular layers were also reversed. Rectification persisted at very low temperature (7 K), and was activationless between 7 and 100 K. The results are a clear example of a "molecular signature" in which electronic behavior is directly affected by molecular structure and orbital energies. The rectification mechanism is discussed, and may provide a basis for rational design of electronic properties by variation of molecular structure. [ABSTRACT FROM AUTHOR]

Visible light emission was observed for molecular junctions containing 5-19 nm thick layers of aromatic molecules between carbon contacts and correlated with their current--voltage behaviors. Their emission was compared to that from Al/AlOx/Au tunnel junctions, which has been previously attributed to transport of carriers across the AlOx layer to yield "hot carriers" which emit light as they relax within the Au contact. The maximum emitted photon energy is equal to the applied bias for the case of coherent tunneling, and such behavior was observed for light emission from AlOx and thin (<5 nm) molecular junctions. For thicker films, the highest energy observed for emitted photons is less than eVapp and exhibits an energy loss that is strongly dependent on molecular layer structure and thickness. For the case of nitroazobenzene junctions, the energy loss is linear with the molecular layer thickness, with a slope of 0.31 eV/nm. Energy loss rules out coherent tunneling as a transport mechanism in the thicker films and provides a direct measure of the electron energy after it traverses the molecular layer. The transition from elastic transport in thin films to "lossy" transport in thick films confirms that electron hopping is involved in transport and may provide a means to distinguish between various hopping mechanisms, such as activated electron transport, variable range hopping, and Poole Frankel transport. [ABSTRACT FROM AUTHOR]

Abstract: A large number of nonvolatile memory devices have been reported with both inorganic and organic components, and many of these involve changes in device resistance between a high conductivity “ON” state and a low conductivity “OFF” state. The mechanism of memory action in many of these devices is uncertain, and may be based on many phenomena, including redox reactions, metal filament formation, charge storage in “floating gates”, and redistribution of oxide vacancies. We report here a Raman spectroscopic probe of organic polymer memory devices which permits direct monitoring of the doping state and conductivity of polythiophene in a 3-terminal device. The polymer conductance is controlled by voltage pulses between the source and gate electrodes in FET geometry, while the conductance state is read out by a separate circuit between source and drain. The conductance was directly correlated with the Raman determination of the density of polarons in the polymer film, which was shown to control both the “electroforming” process and the conductance switching in working memory devices. The polymer conductance change requires a redox counter-reaction at the gate electrode, and atmospheric effects on performance indicate that water and oxygen reduction are involved. The observations are consistent with a redox process between the gate and source electrodes which modulates the polaron concentration and source–drain conductivity. This mechanism provides a framework for optimization of the device by changing its composition and geometry, particularly the identity of the redox counter-reaction and control of ion mobility. [Copyright &y& Elsevier]

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density—voltage (J—V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 109 cycles in air at 100 °C without a significant change in J—V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics. [ABSTRACT FROM AUTHOR]

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 ran) 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 spectroscopic techniques are used to characterize the molecular layers on the metal primer surface. When a Ti primer layer is derivalized via diazonium reduction, the molecule is shown to be present on the ultrathin Ti layer on Au, Al, quartz, Si/SiOx, 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 min exposure to 0.1 M acid or base. The approach also permits spectroscopic characterization of buried thin- film molecular layers on optically transparent substrates after deposition of thick top metal contacts. [ABSTRACT FROM AUTHOR]

Molecular electronic junctions consisting of a 20 nm thick layer of polypyrrole (PPy) and 10 nm of TiO2 between conducting layers of carbon and gold were investigated as potential nonvolatile memory devices. By making the polymer layer much thinner than conventional polymer electronic devices, it is possible to dynamically oxidize and reduce the polypyrrole layer by an applied bias. When the electrode in contact with the PPy is biased positive, oxidation of the PPy occurs to yield a conducting polaron state. The junctions exhibit a large increase in conductance in response to the positive bias, which is reversed by a subsequent negatively biased pulse. Switching between the conducting and nonconducting state can occur for pulses at least as short as 10 µs, and the conducting state persists after a positive bias pulse for at least 1 week. The read/write/read/erase cycle may be repeated for at least 1700 cycles, although with an error rate of ~3% due mainly to an incomplete "erase" step. The speed and retention of the PPy/TiO2 junctions are far superior to those of the analogous fluorene/TiO2 devices lacking the polymer, and the conductance changes are absent if SiO2 is substituted for TiO2. The observations are consistent with "dynamic doping" of the solid-state polymer layer, with the possible involvement of adventitious mobile ions. Although the speed of the current polymer/TiO2 junctions is slower than commercial dynamic random access memory, their retention is ~5 orders of magnitude longer. [ABSTRACT FROM AUTHOR]

Presents information on mono- and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy for intentionally scratching a monolayer deposited on a pyrolyzed photoresist film. Statistical process devised to avoid user bias in determining the monolayer thickness; Implication of multilayers on electrons transmission through the growing film during diazonium reduction; Consistency of the results 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.

Electron-beam (e-beam) deposition of carbon on a gold substrate yields a very flat (0.43 nm of root-mean-square roughness), amorphous carbon film consisting of a mixture of sp[sup 2]- and sp[sup 3]-hybridized carbon with sufficient conductivity to avoid ohmic potential error. E-beam carbon (eC) has attractive properties for conventional electrochemistry, including low background current and sufficient transparency for optical spectroscopy. A layer of KCl deposited by e-beam to the eC surface without breaking vacuum protects the surface from the environment after fabrication until dissolved by an ultrapure electrolyte solution. Nanogap voltammetry using scanning electrochemical microscopy (SECM) permits measurement of heterogeneous standard electron-transfer rate constants (k°) in a clean environment without exposure of the electrode surface to ambient air. The ultraflat eC surface permitted nanogap voltammetry with very thin electrode-to-substrate gaps, thus increasing the diffusion limit for k° measurement to >14 cm/s for a gap of 44 nm. Ferrocene trimethylammonium as the redox mediator exhibited a diffusion-limited k° for the previously KCl-protected eC surface, while k° was 1.45 cm/s for unprotected eC. The k° for Ru(NH3)6 [sup 3+/2+] increased from 1.7 cm/s for unprotected eC to above the measurable limit of 6.9 cm/s for a KCl-protected eC electrode. [ABSTRACT FROM AUTHOR]

Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO- LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts. [ABSTRACT FROM AUTHOR]

The photocurrent spectra for large-area molecular junctions are reported, where partially transparent copper top contacts permit illumination by UV--vis light. The effect of variation of the molecular structure and thickness are discussed. Internal photoemission (IPE), a process involving optical excitation of hot carriers in the contacts followed by transport across internal system barriers, is dominant when the molecular component does not absorb light. The IPE spectrum contains information regarding energy level alignment within a complete, working molecular junction, with the photocurrent sign indicating transport through either the occupied or unoccupied molecular orbitals. At photon energies where the molecular layer absorbs, a secondary phenomenon is operative in addition to IPE. In order to distinguish IPE from this secondary mechanism, we show the effect of the source intensity as well as the thickness of the molecular layer on the observed photocurrent. Our results clearly show that the IPE mechanism can be differentiated from the secondary mechanism by the effects of variation of experimental parameters. We conclude that IPE can provide valuable information regarding interfacial energetics in intact, working molecular junctions, including clear discrimination of charge transport mediated by electrons through unoccupied system orbitals from that mediated by hole transport through occupied system orbitals. [ABSTRACT FROM AUTHOR]

Bilayer molecular junctions were fabricated by using the alkyne/azide "click" reaction on a carbon substrate, followed by deposition of a carbon top contact in a crossbar configuration. The click reaction on an alkyne layer formed by diazonium reduction permitted incorporation of a range of molecules into the resulting bilayer, including alkane, aromatic, and redox-active molecules, with high yield (>90%) and good reproducibility. Detailed characterization of the current-voltage behavior of bilayer molecular junctions indicated that charge transport is consistent with tunneling, but that the effective barrier does not strongly vary with molecular structure for the series of molecules studied. [ABSTRACT FROM AUTHOR]

Quantum interference in cross-conjugated molecules embedded in solid-state devices was investigated by direct current–voltage and differential conductance transport measurements of anthraquinone (AQ)-based large area planar junctions. A thin film of AQ was grafted covalently on the junction base electrode by diazonium electroreduction, while the counter electrode was directly evaporated on top of the molecular layer. Our technique provides direct evidence of a large quantum interference effect in multiple CMOS compatible planar junctions. The quantum interference is manifested by a pronounced dip in the differential conductance close to zero voltage bias. The experimental signature is well developed at low temperature (4 K), showing a large amplitude dip with a minimum >2 orders of magnitude lower than the conductance at higher bias and is still clearly evident at room temperature. A temperature analysis of the conductance curves revealed that electron–phonon coupling is the principal decoherence mechanism causing large conductance oscillations at low temperature. [ABSTRACT FROM AUTHOR]

A three terminal molecular memory device was monitored with in situ Raman spectroscopy during bias-induced switching between two metastable states having different conductivity. The device structure is similar to that of a polythiophene field effect transistor, but ethylviologen perchlorate was added to provide a redox counter-reaction to accompany polythiophene redox reactions. The conductivity of the polythiophene layer was reversibly switched between high and low conductance states with a "write/erase" (W/E) bias, while a separate readout circuit monitored the polymer conductance. Raman spectroscopy revealed reversible polythiophene oxidation to its polaron form accompanied by a one-electron viologen reduction. "Write", "read", and "erase" operations were repeatable, with only minor degradation of response after 200 W/E cycles. The devices exhibited switching immediately after fabrication and did not require an "electroforming" step required in many types of memory devices. Spatially resolved Raman spectroscopy revealed polaron formation throughout the polymer layer, even away from the electrodes in the channel and drain regions, indicating that thiophene oxidation "propagates" by growth of the conducting polaron form away from the source electrode. The results definitively demonstrate concurrent redox reactions of both polythiophene and viologen in solid-state devices and correlate such reactions with device conductivity. The mechanism deduced from spectroscopic and electronic monitoring should guide significant improvements in memory performance. [ABSTRACT FROM AUTHOR]

Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier. [ABSTRACT FROM AUTHOR]

Mono- and multilayers of nitroazobenzene (NAB), azobenzene (AD), 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° 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° for NAB to 44.2 ± 54° for FL with AD, NBP, and BP exhibiting intermediate adsorption geometries. Raman intensity ratios of NAB and AD 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. [ABSTRACT FROM AUTHOR]

A facile method for the preparation of thin-film carbon electrodes by electron beam evaporation onto highly doped silicon is presented. The physical and electro-chemical 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 graphic carbon after pyrolysis at 1000 °C. Scanning force microscopy reveals the root-mean-square roughness of nonpyrolyzed films to be ∼1 Å, while pyrolyzed films exhibit an increased roughness of ∼4 Å. The electro-chemical 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. [ABSTRACT FROM AUTHOR]

Examines glassy carbon surfaces modified with monolayers of biphenyl and nitrobiphenyl molecules as voltammetric electrodes for ferrocene, benzoquinone and tetracyanoquinodimethane electrochemistry in acetonitrile. Electron transfer of modified electrodes; Calculated parameters for monolayer molecules.

Eleven bilayer molecular junctions (MJs) consisting of two different 5–7 nm thick molecular layers between conducting contacts are investigated to determine how orbital energies and optical absorbance spectra of the oligomers affect the photocurrent (PC) response, the direction of photoinduced charge transport, and maximum response wavelength. Photometric sensitivity of 2 mA W−1 and a detection limit of 11 pW are demonstrated for MJs, yielding an internal quantum efficiency of 0.14 electrons per absorbed photon. For unbiased MJs, the PC tracks the absorption spectrum of the molecular layer, and is stable for >5 h of illumination. The organic/organic (O/O) interface between the molecular layers within bilayer MJs is the primary determinant of PC polarity, and the bilayer MJ mechanism is conceptually similar to that of a single O/O heterojunction studied in bilayers of much greater thickness. The charge transport direction of the 11 MJs is completely consistent with hole‐dominated transport of photogenerated carriers. For MJs illuminated while an external bias is applied, the PC greatly exceeds the dark current by factors of 102 to 105, depending on bias, bilayer structure, and wavelength. The bilayer MJs are amenable to flexible substrates, and may have applications as sensitive, wavelength‐specific photodetectors. [ABSTRACT FROM AUTHOR]