Electrochemical Potential Control Research Articles

Characteristics of the Raman spectra of single-walled carbon nanotube bundles under electrochemical potential control

Resonant Raman scattering spectra of single-walled carbon nanotube-sodium dodecyl sulfate (SWNT-SDS) bundles adsorbed on Au electrodes have been investigated in aqueous electrolytes. Raman intensities of the radial breathing mode (RBM) with 785-nm laser excitation were monitored at different electrode potentials between -0.5 and +0.8 V relative to the SCE. Six resolved RBM peaks assignable to different diameter tubes all decreased in intensity when the electrode was positively biased, because of depletion of valence-band electrons associated with resonant excitation. The attenuation occurred at more positive potentials for narrower-diameter tubes with higher RBM frequencies consistent with their larger bandgaps. The results suggest the Fermi level is equilibrated in bundled SWNTs in contrast with the large Fermi-level shifts reported for isolated SWNTs.

Voltammetry and In Situ Scanning Tunneling Microscopy of Cytochrome c Nitrite Reductase on Au(111) Electrodes

Escherichia coli cytochrome c nitrite reductase (NrfA) catalyzes the six-electron reduction of nitrite to perform an important role in the biogeochemical cycling of nitrogen. Here we describe NrfA adsorption on single-crystal Au(111) electrodes as an electrocatalytically active film in which the enzyme undergoes direct electron exchange with the electrode. The adsorbed NrfA has been imaged to molecular resolution by in situ scanning tunneling microscopy (in situ STM) under full electrochemical potential control and under conditions where the enzyme is electrocatalytically active. Details of the density and orientational distribution of NrfA molecules are disclosed. The submonolayer coverage resolved by in situ STM is readily reconciled with the failure to detect nonturnover signals in cyclic voltammetry of the NrfA films. The molecular structures show a range of lateral dimensions. These are suggestive of a distribution of orientations that could account for the otherwise anomalously low turnover number calculated for the total population of adsorbed NrfA molecules when compared with that determined for solutions of NrfA. Thus, comparison of the voltammetric signals and in situ STM images offers a direct approach to correlate electrocatalytic and molecular properties of the protein layer, a long-standing issue in protein film voltammetry.

Formation of stable nanowires from ferromagnetic metals using 2-butyne-1,4-diol

We mechanically fabricated Fe, Co, and Ni nanowires showing conductance quantization under electrochemical potential control. The effects of 2-butyne-1,4-diol and saccharin on the stability of the nanowires were investigated by measuring the conductance and the stretched length of the nanowires. Nanowires of these ferromagnetic metals were stabilized by adding 2-butyne-1,4-diol. In Co, a monoatomic contact was stretched 0.4 nm in length, indicating the formation of a highly stable monoatomic contact of ferromagnetic metals. The improved stability of the nanowires could originate from specific adsorption of organic molecules on the contacts. Mechanical properties of these metals were considered to explain the origin of the metal-dependent stability of the nanowires.

Coronene-Iodine Coadsorbed Adlayers on Au(111) Surfaces Promoted by Electrochemical Potential Control

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Fabrication of stable Pd nanowire assisted by hydrogen in solution

We have mechanically fabricated a Pd nanowire in solution under electrochemical potential control. A clear feature appeared in the conductance histogram when the electrochemical potential of the Pd wire was kept at the hydrogen evolution potential. Conductance traces showed the Pd wire was stretched 0.4nm in length just before breaking, suggesting that at least two Pd atoms might contribute to the formation of the Pd wire. The results indicate that a certain atomic configuration of the Pd nanowire is stabilized by hydrogen. We discuss the stabilization mechanism due to changes in bond strengths caused by hydrogen adsorption or incorporation.

Oxygen-free in situ scanning tunnelling microscopy

Scanning tunneling microscopy under full electrochemical potential control (in situ STM) has been used extensively as an efficient method to characterize microstructures at solid/liquid interfaces at the atomic and molecular levels. However, under ambient conditions oxygen may interfere in open in situ STM systems by diffusion through the solutions. Such interference can be serious for oxygen sensitive systems both for the target molecules themselves and for chemical linker molecules bound to the electrode surface for example, via Au–S bonding. This is strikingly illustrated in the present report. We present here oxygen-free in situ STM, in which the samples are contained in an argon atmosphere which removes oxygen in the system. Interference from oxygen is profoundly reduced in this setup and demonstrated by three examples. Two of these are in situ STM of the intermediate-size sulfur containing organic molecule cysteamine and the biomolecule homocysteine. The third example is the iron–sulfur protein ferredoxin (Pyrococcus furiosus). Monolayers of these molecules on Au(1 1 1) are imaged to molecular resolution and the images compared in the presence and absence of oxygen. In all the cases, the STM image quality of the monolayer structures has improved significantly in the absence of oxygen. Reasons for the drastic effects of dioxygen based on both chemical surface reactivity and electron scattering effects are discussed. These observations are important in general for STM of oxygen sensitive chemical and biological samples.

Conductance bistability of gold nanowires at room temperature

Quantized conductance behavior of gold nanowires was studied under electrochemical potential control. We fabricated $1\text{\ensuremath{-}}\mathrm{nm}$-long monoatomic wires in solution at room temperature. Electrochemical potential significantly affected the stability of the monoatomic wire and fractional conductance peak occurrence in the conductance histogram. We revealed that the hydrogen adsorption on gold monoatomic wires was a decisive factor of the fractional peak, which was originated from the dynamic structural transition between two bistable states of the monoatomic wire showing the unit and the fractional values of the conductance. We could tune the stability of these bistable states to make the fractional conductance state preferable.

In situscanning tunnelling spectroscopy of inorganic transition metal complexes

Redox molecules with equilibrium potentials suitable for electrochemical control offer perspectives in nanoscale and single-molecule electronics. This applies to molecular but also towards higher sophistication such as transistor or diode function. Most recent nanoscale or single-molecule functional systems are, however, fraught with operational limitations such as cryogenic temperatures and ultra-high vacuum, or lack of electrochemical potential control. We report here cyclic voltammetry (CV) using single-crystal Au(111)- and Pt(111)-electrodes and electrochemical in situ scanning tunnelling microscopy (STM) of a class of Os(II)/(III)- and Co(II)/(III)-complexes, the former novel molecular electronics. The complexes are robust, with ligand groups suitable for linking the complexes to the Au(111)- and Pt(111)-surfaces via N- and S-donor atoms. The data reflect monolayer behaviour. Interfacial ET of the Os-complexes is fast, kET(0) > or = 10(6) s(-1), while the Co-complex reacts much more slowly, kET(0) approximately (1-3) x 10(3) s(-1). In STM of the Os-complexes shows a maximum in the tunnelling current/overpotential relation at constant bias voltage with up to 50-fold current rise. The peak position systematically the bias voltage and equilibrium potential, in keeping with theoretical frames for two-step electron transfer (ET) of in situ STM of redox molecules. The molecular conductivity behaves broadly similarly. The Co-complex also shows a tunnelling spectroscopic feature but much weaker than the Os-complexes. This can be ascribed much smaller interfacial ET rate constant, again caused by large intramolecular nuclear reorganization and weak electronic coupling to the substrate electrode. Overall the has mapped the properties of target molecules needed for stable electronic switching, possible importance in molecular electronics towards the single-molecule level, in room temperature condensed matter environment.

Electrochemical Potential Control of Stretched Length of Au Nanowire in Solution

Abstract Gold nanowire was mechanically fabricated in 0.1 M Na2SO4 solution under electrochemical potential control. Stretched length of the gold nanowire was evaluated from the length of the last conductance plateau showing a unit conductance quantum, 1G0 = 2e2/h. Estimated wire length changed reversibly with the electrochemical potential. We have shown that the stability of the gold atomic wire could be controlled by the electrochemical potential.

Mechanical fabrication of metal nano-contacts showing conductance quantization under electrochemical potential control

We have mechanically fabricated Ni and Cu nano-constrictions in solution to study their quantized conductance behavior under electrochemical potential control. Conductance quantization was observed at both metals in solution at room temperature for the first time. The conductance of Cu nano-constriction was quantized in units of G 0 ( = 2 e 2 / h ) . A sharp 1 G 0 peak was observed in the conductance histogram. For Ni, a rather broad peak at 1–1.5 G 0 was observed in the histogram. The conductance quantization behavior was discussed by comparing previously documented results of nano-constrictions fabricated in air or ultra-high vacuum conditions, with those fabricated in solution.

Electrochemical potential control of isolated single-walled carbon nanotubes on gold electrode

Spectral features of Raman scattering of isolated single-walled carbon nanotubes (SWNT) on gold surface in aqueous solution was investigated under electrochemical potential control. Spectrum of the radial breathing mode of SWNT was dependent upon the electrode potential. Change in the intensity of the spectrum was reversible. Distinct difference in the potential dependence was observed at SWNT with different diameter. Absolute potential of the Fermi level of individual tubes was estimated from the potential dependence. Observations suggest that the work function of the tube becomes larger in a manner inversely proportional to the diameter of SWNT. Linewidth of the spectrum also showed potential dependence at the metallic tubes. Characteristics of the spectral changes induced by electrochemical doping were correlated to the difference in electronic density of individual SWNT with distinct diameter and chirality.

Structural Control of Gold Atomic Wires by Changing Electrochemical Potential

Gold nano wires were mechanically fabricated in solution to study their quantized conductance behavior under the electrochemical potential control. A nano wire 1 nm in length showing a unit value of the quantized conductance (G0 = 2e2/h) was fabricated in solution at room temperature by choosing appropriate electrochemical potential and electrolyte. As well as a unit value, several fractional values also appeared at the quantization. Intensity of the fractional conductance peak at 0.6 G0 in the histogram increased as the electrochemical potential of the wire became negative. Conductance trace proves that the fractional peak in the histogram is due to reversible conductance fluctuation between a unit (1 G0) and the fractional value (0.6 G0). These experimental results strongly suggest the formation of a gold mono-atomic wire showing reversible phase transition between a dimerized state and an equal spacing state. Possibility regarding the mono-atomic wire as a predecessor for Pierls transition in 1 D metal system was discussed.

Characteristics of Raman features of isolated single-walled carbon nanotubes under electrochemical potential control

Highly dispersed isolated single-walled carbon nanotubes (SWNT) on gold electrode were prepared to study the effect of electrochemical doping on their Raman features. Spectrum of the radial breathing mode of SWNT was dependent upon the electrode potential. Change in the intensity of the spectrum was reversible. Distinct difference in the potential dependence was observed at SWNT with different diameter. Linewidth of the spectrum also showed potential dependence at the metallic tubes. Characteristics of the spectral changes induced by electrochemical doping were correlated to the difference in electronic density of individual SWNT.

Electrochemical control of CO/NO ligand exchange in a triruthenium cluster monolayer assembled on a gold electrode surface.

A highly selective ligand exchange reaction is realized in the self-assembled monolayer (SAM) of a triruthenium cluster on a gold electrode surface under precise electrochemical potential control. CO as well as NO molecules, which are known to play key roles in many chemical, biological, and environmental systems, can be efficiently introduced into the SAM by electrochemically tuning the electronic state of the Ru site. These unique surface reactions are more convenient and efficient than conventional ligand exchange reactions in solution and could be used for the elucidation of the electron-transfer mechanism in a biological system as well as in the development of molecular sensors and devices.

Long-Range Order of Organized Oligonucleotide Monolayers on Au(111) Electrodes

Oligonucleotides modified by a hexamethylene linker group adsorb on gold electrodes via Au-S bond formation. We have obtained novel data for adsorption of thiol-modified (HS) single-strand HS-10A and double-stranded HS-10AT oligonucleotides and for analogous thiol-free 10A (A = adenine) and 10T (T = thymine) nonspecifically adsorbed as reference molecules. Mercaptohexanol has served as a second reference molecule. The data are based on cyclic and differential pulse voltammetry, interfacial capacitance data, and in situ scanning tunneling microscopy (STM) directly in an aqueous buffer solution, with electrochemical potential control of both the sample electrode and the tip. All the data are based on single-crystal, atomically planar Au(111)-electrode surfaces. The high sensitivity of such surfaces provides accurate HS-10A and HS-10AT electrode coverages on the basis of the reductive desorption of the Au-S bond. The coverage is high and in keeping with dense monolayers of adsorbed HS-10A and HS-10AT in an upright or tilted orientation, with the oligonucleotide backbone repelled from the strongly negatively charged electrode surface. Adsorbed thiol-free 10A only gives a Au(111)-reconstruction peak, while 10T shows a subtle pattern involving pronounced voltammetric adsorption peaks indicative of both nonspecific adsorption via single thymine units and potential-dependent structural reorganization in the surface layer. In situ STM supports these findings at the molecular level. In situ STM of HS-10A discloses large, highly ordered domains at strongly negative sample potentials. Reversible domain formation and disordering could, moreover, be controlled by an electrochemical potential variation in the negative and positive directions, respectively. 10A and 10T did not form ordered adsorbate domains, substantiating that domain formation rests on adsorption of thiol-modified oligonucleotide adsorption in an upright or tilted orientation. The comprehensive, high-resolution information reported may hold prospects for single-molecule electronic conduction and molecular-scale mapping of oligonucleotide hybridization.

Redox Controlled Partition and Spatial Distribution of Solvent and Salt in Electroactive Polyvinylferrocene Films

In situ neutron reflectivity measurements have been made on spin cast polyvinylferrocene (PVF) films maintained under electrochemical potential control. The data show that the interiors of both oxidized and reduced films are close to homogeneous. The results show that neutron reflectivity measurements, including isotopic substitution, can provide novel insights into film composition and structure at the electrode/polymer/solution interface. Isotopic substitution of the solvent has allowed the contributions of the solvent and the solute to the total film composition to be separated. Responses were determined to variations in the imposed potential (a constraint at the electrode/polymer interface) and the electrolyte concentration (a constraint at the polymer/solution interface). Solvent entry occurs upon oxidation, resulting is a more diffuse polymer/solution interface. Only at very high concentrations is salt permeation into PVF films sufficiently high to be considered as a possible source of co-ions for r...

Electrochemical microstructuring of ultra-thin passive films on aluminium

Special optical sensors require thin film devices with a layer structure of the type glass/Al/Al 2O 3 with thicknesses of oxide and metal in the range of some nm. Structures comparable with a printed circuit board were written by electrochemical potential control with a scanning droplet device. Dots, lines, or areas of different thickness ratios Al/Al 2O 3 or even with gradients can be written. Insulating lines become possible by complete oxidation of the Al metal.

Electrochemical Potential Dependence of an Ultrafast Nonequilibrium Electron Dynamics at Au(111) Electrode/Aqueous Solution Interfaces Modified with Alkanethiols

Femtosecond transient reflectivity measurements were performed for Au( 111) electrode/aqueous solution interfaces under electrochemical potential control. Electrode interfaces modified with and without a self-assembled monolayer of alkanethiols were studied in aqueous solutions of HClO 4 and H 2 SO 4 , Dependence on electrochemical potential, interface modification, and kind of solute anions are observed concerning the transient reflectivity response in 3ps. For Au electrodes in an aqueous HClO 4 solution, the rate of the initial relaxation at an unmodified interface, 6-10 ps - 1 , decreased with the electrochemical potential at the potential range where a water dipole rearranged. The rate for alkanethiol-modified interfaces showed a tendency depending on the hydrocarbon chain length. A relative deposited energy from absorbed photon energy was estimated, indicating that interface modification with alkanethiols restricted energy dissipation from the interface. Our trial could contribute not only to develop a study of interaction between a substrate and adsorbed molecules in an ultrafast time scale but also to elucidate elementally processes of electrochemical reactions such as oxidation/reduction reactions at an electrode interface.

Mechanisms of molecular electronic rectification through electronic levels with strong vibrational coupling

We present a new view and an analytical formalism of electron flow through a donor–acceptor molecule inserted between a pair of metal electrodes. The donor and acceptor levels are strongly coupled to an environmental nuclear continuum. The formalism applies to molecular donor–acceptor systems both in vacuum or air, and in aqueous solution under electrochemical potential control. Multifarious patterns of rectified electron flow from the negatively to the positively biased electrode arise. The electronic interaction between the donor and acceptor fragments, mutually and with the electrodes, can be weak, corresponding to the fully diabatic limit. The rectification process then reduces to a sequence of vibrationally relaxed single-electron transfer steps. In the limits where the interactions are strong, denoted as the partially and fully adiabatic limits, the character of the rectification process is different, and electron flow proceeds coherently, without vibrational relaxation. In still another class of mechanisms the electronic level broadening of either donor or acceptor from the adjacent electrode is so strong that it is comparable to the vibrational broadening. The process then reduces to a three-level transition similar to STM of large redox molecules. Recent data for rectification in hexadecyl-quinolinium tricyanodimethanide monolayers by Metzger and co-workers [J. Am. Chem. Soc. 119, 10455 (1997); Acc. Chem. Res. 32, 950 (1999)], are discussed in terms of the reported views and formalism.

Fractional conductance quantization in metallic nanoconstrictions under electrochemical potential control

We study the electrical conductance of gold nanoconstrictions by controlling the electrochemical potential. At positive potentials, the conductance is quantized near integer multiples of G0(2e(2)/h) as shown by well-defined peaks in the conductance histogram. Below a certain potential, however, additional peaks near 0.5G(0) and 1. 5G(0) appear in the histogram. The fractional conductance steps are as stable and well defined as the integer steps. The experimental data are discussed in terms of electrochemical-potential-induced defect scattering and Fermi energy shift, but a complete theory of the phenomenon is yet to be developed.