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Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.

The diffusion coefficient (D(O(2))), solubility (C(O(2))), and electrochemical behavior of oxygen reduction were investigated by cyclic voltammetry and transient amperometry on a Pt ultramicroelectrode in aqueous solutions containing various concentrations of NaOH (1-12 M). The results show that both D(O(2)) and C(O(2)) decrease as the solution viscosity (eta) increases significantly with increasing concentration of NaOH. The Stokes-Einstein relationship (D(O(2)) vs 1/eta) is followed, yielding a radius for the O(2) molecule, 2.8 A, that does not change over the concentration range of NaOH studied. From results reported previously for C(O(2)) in more dilute NaOH solutions and the new results here, the number of electrons, n, involved in the first step of the oxygen reduction reaction (ORR) was found to change with the concentration of NaOH, from n = 2 at low NaOH concentrations (1-2 M) to n = 1 at high concentrations (6 M), in line with the changing water activity. Scanning electrochemical microscopy (SECM) was employed as a sensitive tool to investigate the electrochemical behavior of the product of the ORR in 10 M NaOH solution, O(2)(*-), as a function of potential. The SECM approach curves depend on the substrate bias and state of the Pt substrate surface, and the apparent rate constant for the redox couple of O(2)/O(2)(*-) is determined to be about 2.6 x 10(-4) cm/s in this solution.

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

We demonstrate the feasibility of semiconductor deposition via the electrochemical reduction of gaseous precursors by the use of an anhydrous proton-conducting membrane, the solid acid CsHSO4, at 165 °C. This membrane electrode assembly was operated within the oxidation of hydrogen on a porous Pt anode and the deposition of Si or Ge under bias at the cathode from chloride-based gaseous precursors; SiCl4 and GeCl4 in an Ar flow with a reduction potential over -1.0 V (vs RHE).

We report excellent correlations between the first negative threshold potentials (V(TH)s) for electric conduction, electrochemical potentials, and computed lowest unoccupied molecular orbital energies in a series of phenylene-ethynylene oligomers bearing a sulfur-based anchoring unit and different electroactive substituents on the central benzene ring. The theoretical and electrochemical results strongly suggest that the peaks observed in the i-V curves have a true molecular origin and are associated with distinct unoccupied molecular levels of the compounds that are strongly localized on the central ring (except for compound I). This localization might account for the existence of a long-lived radical-anion state that permits lateral electron hopping and leads to charge trapping and storage.

The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. The i-V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i-V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V(TH), for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor beta of hexadecanethiol was high, ranging from 1.3 to 1.4 A(-1), and was nearly independent of the tip bias. The beta-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 A(-1) for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 A(-1) for dinitro oligo(phenylene) thiol, VI, at a -3.0 V tip bias. Both the V(TH) and beta values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, approximately 25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.

By using specially constructed nanometer tips of sharpened Pt-Ir wire in a wax sheath, small numbers of molecules (1−10) can be trapped between the tip and a substrate. Repeated electron transfers of an electroactive molecule as it shuttles by diffusion between tip and substrate produce a current (∼0.6 pA/molecule) that can be used to detect the trapped molecules. The tip electrode size and shape can be found from the electrode approach curves (current vs tip-to-substrate distance) based on approximate equations and digital simulations. Analysis of the observed fluctuating currents by autocorrelation, spectral density, and probability density functions is also described.

A p-type boron arsenide photoelectrode was prepared from a material consisting of a thin layer of boron arsenide on a boron substrate. The structure of the material was identified using X-ray diffraction and scanning electron microscopy, and the surface composition was determined by means of X-ray photoelectron spectroscopy. The electrode was found to be photoactive under both visible light and UV-vis irradiation and displayed a photocurrent of ~0.1 mA/cm(2) under UV-vis irradiation at an applied potential of -0.25 V vs Ag/AgCl. Mott-Schottky plots for this boron arsenide electrode displayed an estimated flat-band potential near the onset photopotential. The estimated indirect band gap, as determined from incident photon-to-electron conversion efficiency plots, is 1.46 ± 0.02 eV.

We describe the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions on a NaBH(4)-treated Pt ultramicroelectrode (UME). We observe single NP events through the enhanced current by electrocatalytic water oxidation, when IrO(x) contacts the electrode and transiently sticks to it. The overall current transient consists of repeated current spikes that return to the background level, superimposed on a current decay, rather than the staircase response seen where an NP sticks on the UME. Here each event produces a unique current spike (or "blip"). The frequency of the spikes was directly proportional to the particle concentration, and the peak current increased with the applied potential. The observed current is very sensitive to the material and surface state of the measuring electrode; a NaBH(4)-treated Pt UME was important in obtaining reproducible results.

Electrogenerated chemiluminescence (ECL) can be obtained from an electrochemically oxidized highly oriented pyrolytic graphite electrode or a graphene oxide suspension in an aqueous solution containing 0.1 M NaClO(4), phosphate buffer (PBS) (pH = 7.0) and 13 mM tri-n-propylamine. Single ECL events from individual graphene oxide nanosheets can be observed. The observation of ECL in such systems may lead to the development of a new class of carbon-based nanomaterials for potentially highly sensitive analytical applications.

We introduce single-molecule spectroelectrochemistry (SMS-EC), a powerful new technique for studying electrochemical kinetics in highly heterogeneous systems. This technique uses fluorescence single-molecule spectroscopy to indirectly measure electrochemical kinetics one molecule at a time, offering for the first time the distribution of key electrochemical variables, such as the half-wave potential, E1/2, not just the ensemble averages. In SMS-EC, the potential of the working electrode of an electrochemical cell is linearly scanned while simultaneously measuring the florescence intensity, Ifl(t), of individual single molecules as a function of time in a wide-field microscope. SMS-EC is used herein to study the oxidation at an indium tin oxide (ITO) electrode of single molecules of the organic conjugated polymer F8BT. The results reveal both excited singlet state and ground state oxidation of F8BT. The latter process occurs over a narrow distribution of single-molecule half-wave potential values, indicating a relatively uniform electrochemical potential at the electrode.

The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. Peak-shaped i-V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i-V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, V(TH), for electric conduction. For a SAM of 2',5'-dinitro-4,4'-bis(phenylethynyl)-1-benzenethiolate, VII, V(TH) was nearly independent of the tip material [Ir, Pt, Ir-Pt (20-80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (beta values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both V(TH) and beta of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the "memory" effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.

We demonstrate a novel and powerful method to study electrogenerated chemiluminescence (ECL) of single nanoparticles (NPs) (r = 25 +/- 15 nm) of a conjugated polymer, F8BT, on an ITO electrode in the presence of a co-reactant, such as tri-n-propylamine (TPrAH) in acetonitrile solution. The results reveal that the maximum formation rate of ECL of individual NPs is achieved after a long "build-up" time (10-40 s after pulse application). The high number of detected ECL photons from individual NPs (1500 photons during 100 s) highlights the potential of this technique as a very sensitive analytical method. Additionally, TPrAH acts as a very efficient protecting agent against irreversible electrochemical processes occurring in F8BT, as found in photoluminescence studies. This protection mechanism probably involves the neutralization of holes at the particle surface via electron transfer by both TPrAH and TPrA radical (TPrA*).

The single crystalline nanobelts were successfully fabricated with an ionic compound by a simple reprecipitation method. The compound used is the water-insoluble derivative of tris(bipyridine) Ru(II), [Ru(bpy)2(4,4'-(CH3(CH2)14COO)2-bpy)](ClO4)2. The prepared nanobelts show an enhanced fluorescence emission and relatively strong electrogenerated chemiluminescence (ECL), that have potential analytical applications. More interesting, ECL of a single nanobelt deposited on an ultramicroelectrode was observed. The observation of ECL in such nanostructures leads to the development of a new class of ECL systems that may prove useful for a variety of purposes.

A simple approach to prepare and characterize biomaterial-based electrocatalysts for oxygen reduction was carried out. Poly-l-histidine was used as a matrix and ligand to complex Cu2+ to mimic the active sites of laccases. A modified glassy carbon (GC) electrode with Cu2+-poly-l-histidine complex decreases the oxygen reduction overpotential as compared with the bare GC electrode. An array of Cu2+-poly-l-histidine spots with different compositions was deposited on a GC substrate, and their catalytic activity for oxygen reduction was evaluated by a scanning electrochemical microscopy-based screening technique. The electrocatalytic activities of complexes for oxygen reduction strongly depended on the mole ratio of Cu2+ to poly-l-histidine and the applied potential of the substrate.

This communication reports the findings of the investigation of the electrochemical (EC) oxidation of the important bimolecular guanosine (Gs) by scanning electrochemical microscopy (SECM) using carbon fiber ultramicroelectrodes (CF-UMEs) as the probe and substrate. The first attempt is to try to gain a steady-state voltammogram for EC oxidation of Gs at the CF-UME probe in aqueous buffer solutions with three different pH values. Experimental results indicate that due to serious adsorption of Gs on the CF-UME surface, an "S-shaped" steady-state voltammetric curve, which is required for SECM studies, cannot be obtained in aqueous solutions. To solve this adsorption problem, a series of experiments for studying the EC behavior of Gs in DMF are carried out. A well-defined "S-shaped" steady-state cyclic voltammogram (CV) could be achieved at the CF-UME in DMF containing 0.1M TBAPF6 as the supporting electrolyte. By combining several EC techniques, including cyclic voltammetry at glassy carbon (GC) macroelectrode and CF-UMEs, and chronoamperometry, the general chemical characteristics and EC behavior of Gs in DMF solution are studied. Furthermore, SECM detection of Gs*+, the radical cation of Gs electrogenerated in its first oxidation, is carried out by using feedback and tip generation/substrate collection modes in a nanogap configuration. Gs*+ has been electrochemically detected for the first time, with an estimated lifetime of

In this communication we report preliminary results with a tuning fork-based scanning probe technique combined with current-voltage (i-V) measurements, for the rapid characterization and screening of monolayer films in an inert atmosphere. We studied self-assembled monolayers (SAMs) of a variety of molecules with different structures that are being considered for possible application in molecular electronic devices and devised a high throughput analysis method for their characterization. There is currently a high level of interest in the electrical properties of isolated molecules especially in the use of unconventional substances and single molecules or SAMs to construct electronic devices.1-3 Advances in synthetic supramolecular chemistry, coupled with recent developments in device fabrication techniques and scanning probe techniques,4 allow single molecules to be manipulated and investigated electronically. Synthetic chemistry is mature enough to offer a huge range of molecular structures with different properties. A challenge has been to develop reliable and fast screening methods to characterize electronic properties of molecules and to be able to correlate the electrical behavior of the molecules with their structure. A few initial efforts have used long molecular wires across lithographically patterned proximal gold-coated probes separated by approximately 10 nm, but these studies were unreliable and not suitable for molecules shorter than the array gap.1,3 The Langmuir-Blodgett technique was also used to prepare a single molecular layer which was sandwiched between Al and Ti/Al contacts to form a device.5 Other efforts employed a nanopore arrangement or mechanically controllable break junctions6 where electronic measurements were performed between adjustable proximal point contacts. Still others have been performed in nanopores on a structure7 that has a metal top contact formed by vacuum evaporation, an active SAM, and a metal bottom contact. These techniques have provided interesting results, but the preparation of such nanostructures is time-consuming and fabrication-intensive. An attractive approach is to utilize a conducting atomic force microscope (AFM) tip8 as one of the contacts to form a metal-molecule-metal junction. We show here that a tuning fork-based scanning probe microscope (SPM)9 for rapid probe positioning combined with i-V measurements can be used to characterize and screen a large variety of molecules with different electrical properties. In the work reported here the molecules of interest were assembled on a flat gold substrate and then studied by this technique in a controlled environment. The basic principle of the device is illustrated in Figure 1. The tip, sharpened by electrochemical etching, as used for STM tips, is attached to a small tuning fork. The tuning fork is excited by an attached piezoelectric element, generally oscillating in the region of 33 to 100 kHz, and is used for rapid approach of the tip to the SAM. When the tip just contacts the SAM surface, the amplitude and frequency of the oscillation decrease, and this can be used to sense the presence of a surface. This same technique is used with many near-field scanning optical microscopy (NSOM) instruments to maintain tip position. Thus, the tip can be moved to the substrate and positioned fairly rapidly. The tip is then retracted slightly (about 10 nm) and moved to a different location on the SAM. The potential of the tip is swept with respect to the substrate over the desired potential range, and the current is recorded, as the tip is again approached toward the SAM, this time in small steps (e.g., 2 A). Before the tip contacts the molecules in the SAM, essentially no current flows. Upon contact, when the potential across the SAM containing electron-donating or electronwithdrawing groups attains a characteristic bias voltage, a current flows through the film. The magnitude of the current that flows in the i-V curve is a function of the conductance of the molecules. The synthesis of the compounds reported here and the preparation and characterization of the SAM have been described elsewhere10,11 (see Supporting Information). The i-V measurements were made in an argon atmosphere on seven compounds (Table 1).12 Figure 2 shows typical i-V characteristics of compounds I, II, and IV when the tip first contacts the surface of a SAM. For the alkylthiol (I) only the expected tunneling behavior at biases beyond about (4.8 V is seen (Figure 2A). Figure 2B shows the i-V curve of a SAM of 2′-ethyl-4,4′bis(phenylethynyl)-1-benzenethiolate (II) on gold. When the negative scan reaches about 2.8 V, a peaked current response of a few pA is observed. This type of response has been observed previously in similar studies with nanopore junctions and has been † The University of Texas at Austin. ‡ Rice University. (1) Tour, J. M. Acc. Chem. Res. 2000, in press. (2) Jortner, J.; Ratner, M. Molecular Electronics; Blackwell: London, 1997. (3) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950. (4) See, e.g.: (a) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (b) Semaltianos, N. G. Chem. Phys. Lett. 2000, 329, 76. (5) (a) Collier, C. P.; Wong, E. W.; Beloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (b) Wong, E. W.; Collier, C. P.; Beloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831. (6) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (7) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (8) (a) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 2970. (b) Kelley, T. W.; Granstrom, E. L.; Frisbie, C. D. AdV. Mater. 1999, 11, 261. (c) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523. (d) Alpersion, B.; Cohen, S.; Rubinstein, I.; Hodes, G. Phys. ReV. B 1995, 52, R17017. (e) Klein, D.; McEuen, P. Appl. Phys. Lett. 1995, 66, 2478. (9) Karrai, K.; Grober, R. D. Appl. Phys. Lett. 1995, 66, 1842. (10) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224. (11) Kosynkin, D.; Tour, J. M. Org. Lett., submitted for publication. (12) The number of molecules contacting the tip estimated from the tip radius (∼5 nm), the van der Waals radius (∼ 0.35 nm) of the molecules for a deformation depth of 0.2 nm of the SAM was about 35. Figure 1. Schematic representation of the measurement and formation of the metal-molecule-metal junction with a tuning fork-based SPM tip contacting the SAM on Au (not to scale). Ar atmosphere. 2454 J. Am. Chem. Soc. 2001, 123, 2454-2455

Fermi level pinning refers to a situation where the band bending in a semiconductor contacting a metal is essentially independent of the metal even for large variation in the work function of the metal. It was found that a similar situation sometimes results for a semiconductor contacting liquid electrolyte solutions containing redox couples having very different electrochemical potentials. Recently, workers in the field of semiconductor photoelectrochemistry have emphasized a limiting case of the model of the semiconductor/liquid interface where the drop across the semiconductor depends on applied potential; at equilibrium with the solution, the band bending is generally regarded as varying with changes in the solution potential by virtue of changes in the redox couple or simply changing the ratio of oxidized and reduced material. Fermi level pinning results in semiconductor/liquid interfaces which can be viewed as analogous to a Schottky barrier photocell in series with an electrochemical cell in that the extent to which a given redox process can be driven uphill is independent of the potential of the redox couple.n-GaAs, p-GaAs, p-GaAs, and p-Si are semiconductors that exhibit Fermi level pinning in liquid electrolyte solutions (CH/sub 3/CN/(n-Bu/sub 4/N)ClO/sub 4/) of redox reagents. Fermi level pinning has the disadvantagemore » in practical terms of limiting photovoltage in optical energy conversion applications, but such a phenomenon allows the use of a very wide range of solution couples. Since Fermi level pinning results from surface states, changes in the surface brought about by deliberate surface chemistry may change the surface states and hence the photovoltage in solid-state and liquid-junction solar devices.« less