Amorphous semiconducting film containing nanometer particles of CuTCNQ: preparation, characterization and electrical switching property

An amorphous functional organometallic semiconducting thin film containing nanometer particles of a CuTCNQ charge-transfer complex was prepared and characterized by UV-VIS-NIR spectral analysis, Fourier transform infrared (FTIR) spectral analysis, Fourier transform Raman spectral analysis and I–V characterization. All of the spectral analyses are identical to those of both chemically synthesized pure CuTCNQ powder produced by the reaction of CuI and TCNQ, and the solution-grown polycrystalline films. I–V characteristics showed a memory switching property in the amorphous film material sandwiched between two metal electrodes of Al and Cu at a electric field strength of about 7 000 V cm⁻¹.     

Local feedback mode of scanning electrochemical microscopy for electrochemical characterization of one-dimensional nanostructure: theory and experiment with nanoband electrode as model substrate

Local feedback mode is introduced as a novel operation mode of scanning electrochemical microscopy (SECM) for electrochemical characterization of a single one-dimensional (1D) nanostructure, for example, a wire, rod, band, and tube with 1-100-nm width and micrometer to centimeter length. To demonstrate the principle, SECM feedback effects under diffusion limitation were studied theoretically and experimentally with a disk probe brought near a semi-infinitely long band electrode as a geometrical model for a conductive 1D nanostructure. As the band becomes narrower than the disk diameter, the feedback mechanism for tip current enhancement is predicted to change from standard positive feedback mode, to positive local feedback mode, and then to negative local feedback mode. The negative local feedback effect is the only feedback effect that allows observation of a 1D nanostructure without serious limitations due to small lateral dimension, available tip size, or finite electron-transfer rate. In line-scan and approach-curve experiments, an unbiased Pt band electrode with 100-nm width and 2.6-cm length was detectable in negative local feedback mode, even using a 25-microm-diameter disk Pt electrode. Using a 2-microm-diameter probe, both well-defined and defected sites were observed in SECM imaging on the basis of local electrochemical activity of the nanoband electrode. Noncontact and spatially resolved measurement is an advantage of this novel SECM approach over standard electrochemical approaches using electrodes based on 1D nanostructure.     

Scanning electrochemical microscopy. 47. Imaging electrocatalytic activity for oxygen reduction in an acidic medium by the tip generation-substrate collection mode

The oxygen reduction reaction (ORR) in acidic medium was studied on different electrode materials by scanning electrochemical microscopy (SECM) operating in a new variation of the tip generation-substrate collection mode. An ultramicroelectrode tip placed close to the substrate electrode oxidizes water to oxygen at a constant current. The substrate is held at a potential where the tip-generated oxygen is reduced and the resulting substrate current is measured. By changing the substrate potential, it is possible to obtain a polarization (current-potential) curve, which depends on the electrocatalytic activity of the substrate material. The main difference between this mode and the classical feedback SECM mode of operation is that the feedback diffusion process is not required for the measurement, allowing its application for studying the ORR in acidic solutions. Activity-sensitive images of heterogeneous surfaces, e.g., with Pt and Au electrodes, were obtained from the substrate current when the x-y plane was scanned with the tip. The usefulness of this technique for imaging electrocatalytic activity of smooth metallic electrodes and of highly dispersed fuel cell-type electrocatalysts was demonstrated. The application of this method to the combinatorial chemical analysis of electrode materials and electrocatalysts is discussed.     

Mass-production of single-crystalline device arrays of an organic charge-transfer complex for its memory nature

Controllable synthesis of single-crystalline lamellae of copper tetracyano-p- quinodimethane (CuTCNQ, phase II) is achieved, and mass-produced devices or device arrays based on symmetrical Cr/Au gap electrodes are fabricated in situ. The devices exhibit semiconductor properties important for the understanding of CuTCNQ.     

Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.     

Assessment of the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only approximately 1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.     

Study of electron transfer across the liquid/ice-like matrix interface by scanning electrochemical microscopy

In this work, we report the findings of a study on scanning electrochemical microscopy (SECM) to investigate the interfacial electron-transfer (ET) reaction between the 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ*-) in 1,2-dichloroethane and ferricyanide in an ice-like matrix (a mixture of insulting ice and conductive liquid) under low temperatures. Experimental results indicate that the formed liquid/ice-like matrix interface is superficially similar in electrochemical characteristics to a liquid/liquid interface at temperatures above -20 degrees C. Furthermore, imaging data show that the surface of the ice-like matrix is microscopically flat and physically stable and can be applied as either a conductive or an insulting substrate for SECM studies. Perchlorate ion was selected as the common ion in both phases, the concentrations of which controlled the interfacial potential difference. The effect of perchlorate concentration in the DCE phase on interfacial reactions has been studied in detail. The apparent heterogeneous rate constants for TCNQ*- oxidation by Fe(CN)6(3-) in another phase under different temperatures have been calculated by a best-fit analysis, where the experimental approach curves are compared with the theoretically derived relationships. Reaction rate data obey Butler-Volmer formulation before and after the freezing point, which is similar to most other known cases of ET reactions at liquid/liquid interfaces. However, there is a sharp change observed for heterogeneous rate constants around the freezing point of the aqueous phase, which reflects the phase transition. At temperatures below -20 degrees C, surface-confined voltammograms for the reduction of ferricyanide were obtained, and the ice-like matrix became an insulating one, which indicates that the aqueous phase is really a frozen phase.     

Scanning electrochemical microscopy of quinoprotein glucose dehydrogenase

The activity of immobilized glucose dehydrogenase (GDH), a typical PQQ-dependent quinoprotein, was studied qualitatively and quantitatively by scanning electrochemical microscopy (SECM). PQQ-dependent GDH is of interest because of its high activity and independence of dissolved oxygen in catalyzing the transfer of electrons from glucose to an electron mediator. Biotinylated glucose dehydrogenase was bound to streptavidin-coated paramagnetic beads (surface concentration > or = 1.8 x 10(-11) mol cm(-2)) which were deposited as microscopic microspots on a hydrophobic surface. The catalytic activity of immobilized GDH was mapped in SECM feedback mode and generation-collection mode using ferrocenemethanol, ferrocenecarboxylic acid, p-aminophenol, and ferricyanide as electron mediators, respectively. The apparent steady-state kinetics of catalysis were measured under conditions of high d-glucose concentration using the theory developed for the SECM feedback and generation collection (GC) modes. In feedback mode, curves of the kinetically controlled substrate current against normalized distance were plotted, and it was found that GDH catalysis follows pseudo-first-order kinetics. In GC mode detection, the catalysis follows zero-order kinetics in the presence of high concentration of both substrates for GDH. The turnover rate obtained for immobilized GDH is lower than that of native GDH but much higher than that generally observed for glucose oxidase.     

Tuning reaction processes for the synthesis of micron and nanometer sized, single crystalline lamellae of copper 7,7,8,8-tetracyano-p-quinodimethane (Phase II) with large area

Two simple methods have been demonstrated to obtain large area, single crystalline lamellae of copper-7,7,8,8-tetracyanoquinodimethane (CuTCNQ). The formation of the lamellae was a result of fine tuning of the processes during the synthesis processes of CuTCNQ phase II. This facile synthesis of large area single crystalline lamellae suggests bright prospects for the study and understanding of the electrical switching of CuTCNQ by using single crystals of its phase II, and future applications of the material in memory and switching devices. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale.     

Scanning electrochemical microscopy of model neurons: constant distance imaging

Undifferentiated and differentiated PC12 cells were imaged with the constant-distance mode of scanning electrochemical microscopy (SECM) using carbon ring and carbon fiber tips. Two types of feedback signals were used for distance control: the electrolysis current of a mediator (constant-current mode) and the impedance measured by the SECM tip (constant-impedance mode). The highest resolution was achieved using carbon ring electrodes with the constant-current mode. However, the constant-impedance mode has the important advantages that topography and faradaic current can be measured simultaneously, and because no mediator is required, the imaging can take place directly in the cell growth media. It was found that vesicular release events do not measurably alter the impedance, but the depolarizing solution, 105 mM K+, produces a dramatic impedance change such that constant-distance imaging cannot be performed during application of the stimulus. However, by operating the tip in the constant-height mode, cell morphology (via a change in impedance) and vesicular release could be detected simultaneously while moving the tip across the cell. This work represents a significant improvement over previous SECM imaging of model neurons, and it demonstrates that the combination of amperometry and constant-impedance SECM has the potential to be a powerful tool for investigating the spatial distribution of neurotransmitter release in vitro.     

Probing heterogeneous electron transfer at an unbiased conductor by scanning electrochemical microscopy in the feedback mode

The theory of the feedback mode of scanning electrochemical microscopy is extended for probing heterogeneous electron transfer at an unbiased conductor. A steady-state SECM diffusion problem with a pair of disk ultramicroelectrodes as a tip and a substrate is solved numerically. The potential of the unbiased substrate is such that the net current flow across the substrate/solution interface is zero. For a reversible substrate reaction, the potential and the corresponding tip current depend on SECM geometries with respective to the tip radius including not only the tip-substrate distance and the substrate radius but also the thickness of the insulating sheath surrounding the tip. A larger feedback current is obtained using a probe with a thinner insulating sheath, enabling identification of a smaller unbiased substrate with a radius that is approximately as small as the tip radius. An intrinsically slow reaction at an unbiased substrate as driven by a SECM probe can be quasi-reversible. The standard rate constant of the substrate reaction can be determined from the feedback tip current when the SECM geometries are known. The numerical simulations are extended to an SECM line scan above an unbiased substrate to demonstrate a "dip" in the steady-state tip current above the substrate center. The theoretical predictions are confirmed experimentally for reversible and quasi-reversible reactions at an unbiased disk substrate using disk probes with different tip radii and outer radii.     

Scanning electrochemical microscopy. 46. Shielding effects on reversible and quasireversible reactions

An approximate theory for the feedback mode of the scanning electrochemical microscope (SECM) is developed to interpret the effects of substrate shielding on an ultramicroelectrode tip during a recording of iT versus d curves (approach curves) for reversible and quasireversible kinetics at a substrate surface. The resulting expressions for the tip current, iT, show a good fit to more accurate SECM simulations as well as to the experimental response of a reversible and quasireversible reaction. SECM shielding experiments thus give an interesting new insight into SECM approach curves over electrodes at different potentials, which suggest possible applications to measuring heterogeneous kinetics for fast reactions and diffusion coefficient determination.     

Scanning electrochemical microscopy. 55. Fabrication and characterization of micropipet probes

The fabrication and characterization of novel micropipet probes for use in scanning electrochemical microscopy (SECM) are described. These can be used to dispense small (pL) amounts of a solution while monitoring the electrochemical response at a substrate and at a ring electrode tip on the micropipet probe. The probes were constructed by insulating gold-coated borosilicate micropipets with electrophoretic paint and exposing a ring electrode at the tip by heat treatment. Characterization of the probes was performed using scanning electron microscopy, cyclic voltammetry, and SECM approach curve experiments. Routine construction of tips with diameters of the order of 3 microm was possible using this technique. The probes exhibited stable steady-state currents and positive and negative feedback approach curves that agreed with those predicted by theory. Demonstrative SECM imaging experiments were performed using a picodispenser to continuously dispense an electroactive solution (ferrocenemethanol) to the SECM cell while the probe was located within a few micrometers of a Pt substrate surface. Oxidation of the dispensed electroactive solution was performed at the substrate, and feedback currents were measured at the probe tip by holding the gold ring at a reducing potential. This mode of tip-dispensing SECM was used to obtain images of a platinum substrate electrode while monitoring both the substrate current and the feedback current at the probe.     

Use of catechol as selective redox mediator in scanning electrochemical microscopy investigations

The use of catechols, and more specifically of dopamine, as a specific redox mediator for scanning electrochemical microscopy (SECM) investigations was evaluated in the challenging situation of an ultrathin layer deposited on a conductive substrate (carbon materials). Experiments show that dopamine is a well-adapted redox system for SECM in feedback mode and in unbiased conditions. Used as a redox mediator, catechol permits the investigations of modified surfaces without an electrical connection of the sample thanks to fast charge transfer kinetics but with a surface selectivity that does not exist in classical outer-sphere redox mediators. The interest of catechol in SECM as a sensitive redox mediator is exemplified by monitoring several modification steps of an ultrathin (<1 nm) hierarchically porous organic monolayer deposited on carbon substrates. For quantitative analysis, the SECM approach curves using dopamine could simply be characterized with an irreversible electron transfer kinetics model in a large range of pH.     

Scanning electrochemical microscopy. 38. Application of SECM to the study of charge transfer through bilayer lipid membranes

The use of the scanning electrochemical microscope (SECM) to probe the kinetics of charge-transfer processes at bilayer lipid membranes (BLM) is presented. Analysis of the SECM tip response demonstrates that an unmodified BLM behaves as an insulator, whereas a BLM doped with iodine shows some positive feedback. The SECM technique thus allows one to probe processes at a BLM and determine the kinetics of the charge-transfer process. The SECM can also be used to determine the shape of the BLM.     

Batch fabrication of atomic force microscopy probes with recessed integrated ring microelectrodes at a wafer level

A batch fabrication process at the wafer-level integrating ring microelectrodes into atomic force microscopy (AFM) tips is presented. The fabrication process results in bifunctional scanning probes combining atomic force microscopy with scanning electrochemical microscopy (AFM-SECM) with a ring microelectrode integrated at a defined distance above the apex of the AFM tip. Silicon carbide is used as AFM tip material, resulting in reduced mechanical tip wear for extended usage. The presented approach for the probe fabrication is based on batch processing using standard microfabrication techniques, which provides bifunctional scanning probes at a wafer scale and at low cost. Additional benefits of batch fabrication include the high processing reproducibility, uniformity, and tuning of the physical properties of the cantilever for optimized AFM dynamic mode operation. The performance of batch-fabricated bifunctional probes was demonstrated by simultaneous imaging micropatterned platinum structures at a silicon dioxide substrate in intermittent (dynamic) and contact mode, respectively, and feedback mode SECM. In both, intermittent and contact mode, the bifunctional probes provided reliable correlated electrochemical and topographical data. In addition, simulations of the diffusion-limited steady-state currents at the integrated electrode using finite element methods were performed for characterizing the developed probes.     

Scanning electrochemical microscopy. 57. SECM tip voltammetry at different substrate potentials under quasi-steady-state and steady-state conditions

We discuss SECM tip voltammetry, where a UME tip is held above a conductive substrate within about a tip radius and a tip voltammogram is recorded as its potential is slowly scanned while the substrate is held at a fixed potential. When the potential of the substrate is changed, the series of steady-state tip voltammograms provide information about the reactants and products. When the potential of the substrate, ES, is set so that the reaction at the substrate is opposite to that at the tip (the usual SECM conditions), a total positive feedback (tpf) tip voltammogram is recorded. When the substrate potential is set to values where the reaction at the substrate is the same as that occurring on the tip, the tip is shielded from the species in the bulk solution. Depending upon the substrate potential, this can cause total shielding (ts) or a voltammogram that is the result of partial feedback/partial shielding (pf-ps). The result is a series of tip voltammograms that are characterized by tpf, pf-ps, or ts, depending upon ES. Experimental tip voltammograms resulting from the reversible reduction of TCNQ and oxidation of ferrocene in MeCN are reported. These are compared with those from simulations and approximate equations developed to describe the features of the tip voltammograms generated under tpf, ts, or pf-ps conditions. The effect of the diffusion coefficient ratio on the ability of the UME tip to reach a true steady state is also addressed and possible applications, e.g., obtaining information about the reversibility of an electrochemical reaction, the product of an electrochemical reaction, the stability of that product, or the diffusion coefficients of the electroactive species, are discussed.     

Scanning electrochemical microscopy with slightly recessed nanotips

Slightly recessed nanoelectrodes were prepared by controlled etching of nanometer-sized, flat Pt electrodes. By using high-frequency (e.g., 2 MHz) ac voltage, the layer of Pt as thin as greater, approximately >3 nm was removed to produce a cylindrical cavity inside the insulating glass sheath. The etched electrodes were characterized by combination of voltammetry and scanning electrochemical microscopy (SECM) to determine the radius and the effective depth of the recess. The theory was developed for current versus distance curves obtained with a recessed tip approaching either a conductive or an insulating substrate. Good agreement between the theoretical and experimental approach curves indicated that recessed nanotips are suitable for quantitative feedback mode SECM experiments.