STM Contrast, Electron-Transfer Chemistry, and Conduction in Molecules

Gaining insight into molecular tunnel junctions with a pocket calculator without I-V data fitting. Five-thirds protocol

The protocol put forward in the present paper is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs via off-resonant tunneling. The recipe is simple. The measured current-voltage curve I = I(V) should be recast as a curve of V5/3/I versus V. This curve exhibits two maxima: one at positive bias (V = Vp+), another at negative bias (V = Vp-). The values Vp+ > 0 and Vp- < 0 at the two peaks of the curve for V5/3/I at positive and negative bias and the corresponding values Ip+ = I(Vp+) > 0 and Ip- = I(Vp-) < 0 of the current is all information needed as input. The arithmetic average of Vp+ and |Vp-| in volt provides the value in electronvolt of the MO energy offset ε0 = EMO - EF relative to the electrode Fermi level (|ε0| = e(Vp+ + |Vp-|)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as γ = (4/5)(Vp+ - |Vp-|)/(Vp+ + |Vp-|) sign (ε0). Even the low-bias conductance estimate, G = (3/8)(Ip+/Vp+ + Ip-/Vp-), can be a preferable alternative to that deduced from fitting the I-V slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this "five-thirds" protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen)

We present a strong correlation of the Ni(salophen) structure and properties measured in single-molecule vs bulk quantities and in ultra high vacuum vs solution phase. Under a scanning tunneling microscope (STM), Ni(salophen) forms a self-assembled monolayer (SAM) on Au(111) at 23 °C with molecular structure identical to that of the X-ray crystallographic measurement. The HOMO and LUMO levels are determined using elastic tunneling spectroscopy at the single-molecule level with confirmation by monolayer-quantity ultraviolet photoelectron spectroscopy (UPS) and by cyclic voltammetry (CV) measurements. The STM-determined HOMO-LUMO gap of 3.28 eV and (HOMO-1)-HOMO gap of 0.36 eV form a new foundation for the selection of hybrid functionals with a simple basis set to be effective in accurately calculating single-molecule Ni(salophen) frontier MO levels. Our results suggest that microscopy-based experiments on a surface, along with free-molecule gas-phase calculations, can provide useful insights into the physical properties of metal(salen) complexes, especially when such direct measurements are not available in solution.

Nanoparticle Linker-Controlled Molecular Wire Devices Based on Double Molecular Monolayers

Highly conductive molecular wires are an important component for realizing molecular electronic devices and have to be explored in terms of interactions between molecules and electrodes in their molecular junctions. Here, new molecular wire junctions are reported to enhance charge transport through gold nanoparticle (AuNP)-linked double self-assembled monolayers (SAMs) of cobalt (II) bis-terpyridine molecules (e.g., Co(II)(tpyphS)₂ ). Electrical characteristics of the double-SAM devices are explored in terms of the existence of AuNP. The AuNP linker in the Co(II)(tpyphS)₂ -AuNP-Co(II)(tpyphS)₂ junction acts as an electronic contact that is transparent to electrons. The weak temperature dependency of the AuNP-linked molecular junctions strongly indicates sequential tunneling conduction through the highest occupied molecular orbitals (HOMOs) of Co(II)(tpyphS)₂ molecules. The electrochemical characteristics of the AuNP-Co(II)(tpyphS)₂ SAMs reveal fast electron transfer through molecules linked by AuNP. Density functional theory calculations reveal that the molecular HOMO levels are dominantly affected by the formation of junctions. The intermolecular charge transport, controlled by the AuNP linker, can provide a rational design for molecular connection that achieves a reliable electrical connectivity of molecular electronic components for construction of molecular electronic circuits.

Characteristics of sulfur atoms adsorbed on Ag(100), Ag(110), and Ag(111) as probed with scanning tunneling microscopy: experiment and theory

In this paper, we report that S atoms on Ag(100) and Ag(110) exhibit a distinctive range of appearances in scanning tunneling microscopy (STM) images, depending on the sample bias voltage, V_S. Progressing from negative to positive V_S, the atomic shape can be described as a round protrusion surrounded by a dark halo (sombrero) in which the central protrusion shrinks, leaving only a round depression. This progression resembles that reported previously for S atoms on Cu(100). We test whether DFT can reproduce these shapes and the transition between them, using a modified version of the Lang-Tersoff-Hamann method to simulate STM images. The sombrero shape is easily reproduced, but the sombrero-depression transition appears only for relatively low tunneling current and correspondingly realistic tip-sample separation, d_T, of 0.5-0.8 nm. Achieving these conditions in the calculations requires sufficiently large separation (vacuum) between slabs, together with high energy cutoff, to ensure appropriate exponential decay of electron density into vacuum. From DFT, we also predict that an analogous transition is not expected for S atoms on Ag(111) surfaces. The results are explained in terms of the through-surface conductance, which defines the background level in STM, and through-adsorbate conductance, which defines the apparent height at the point directly above the adsorbate. With increasing V_S, for Ag(100) and Ag(110), we show that through-surface conductance increases much more rapidly than through-adsorbate conductance, so the apparent adsorbate height drops below background. In contrast, for Ag(111) the two contributions increase at more comparable rates, so the adsorbate level always remains above background and no transition is seen.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Floppy molecules as candidates for achieving optoelectronic molecular devices without skeletal rearrangement or bond breaking

Molecular species investigated as possible candidates for molecular photoswitches often toggle between two (low and high conductance) conformations implying skeletal rearrangement, bond breaking, and substantial changes of molecular length. All these represent shortcomings that impede the switching speed and straightforward incorporation in nanodevices. In the present paper we propose a mechanism wherein the photoinduced switching is from a nonplanar conformation to a planar conformation, and involves neither skeletal rearrangement nor bond breaking or significant molecular length changes. Specifically, by choosing typical floppy molecules consisting of two benzene or benzene-like rings that can easily rotate relative to each other, we present results of both ab initio and DFT quantum chemical calculations demonstrating that the lowest electronic excitation corresponds to a planar molecular conformation (φ = 0), in contrast to the nonplanar ground state characterized by φ ≠ 0. Because the low bias conductance scales as G ∝ cos² φ, the planar conformation has a higher conductance than the non-planar conformation, acting therefore as ON and OFF states of the molecular switch, respectively. We analyze recent experimental data on illuminated single-molecule junctions (E.-D. Fung et al., Nano Lett., 2017, 17, 1255) and show that the measured photoinduced conductance enhancement is consistent with the presently proposed mechanism. Furthermore, based on recent results demonstrating the substantial impact of the SAM coverage on the twisting angle (I. Bâldea, Faraday Discuss., 2017, 204, 35) we show that a photoinduced conductance enhancement can be much stronger than the rather modest enhancement obtained in the aforementioned experiment.

Stable negative differential resistance in porphyrin based σ–π–σ monolayers grafted on silicon

Two Si–porphyrin hybrid monolayers showed room temperature negative differential resistance (NDR) property. The monolayer with a fluorophenyl porphyrin moiety showed a better peak-to-valley ratio due to compact packing.

Single-molecule electronics: from chemical design to functional devices

The use of single molecules in electronics represents the next limit of miniaturisation of electronic devices, which would enable to continue the trend of aggressive downscaling of silicon-based electronic devices.

Adsorption and assembly of ions and organic molecules at electrochemical interfaces: nanoscale aspects

We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.

Synthesis of meso-nitrophenylporphyrins covalently linked to a polyphenylene chain bearing methoxy groups

A convenient procedure for the synthesis of polyphenylene porphyrin dyads is described. The dyads consist of a meso-nitrophenylporphyrin covalently linked to a polyphenylene chain by an amide bond. The final phenyl group in the chain bears electron donor methoxy groups. The π-conjugated chain was obtained in excellent yield via a Wittig–Horner reaction, which produces a new double bond (E isomer) incorporating either — COOCH 3 or E isomer) incorporating either — CON ( OCH 3)( CH 3) functional groups. Expansion of the conjugated chain involves the reduction of these groups to aldehydes followed by a second Wittig–Horner reaction. 5-(4-Aminophenyl)-10,15,20-tris(4-nitrophenyl)porphyrin was synthesized from meso-(4-nitrophenyl)dipyrromethane. The E isomer) incorporating either — NO 2 groups in para positions of the peripheral phenyl rings give extra electron-withdrawing character to the porphyrin macrocycles. Coupling of the polyphenylene acids with aminoporphyrin results in the desired dyads. These dyads have moieties with different electron donor–acceptor properties. This present strategy may be easily used for preparation of other similar dyad derivatives. These compounds can be suitable for photodynamic therapy.

Tunneling spectroscopy of organic monolayers and single molecules

Basic concepts in tunneling spectroscopy applied to molecular systems are presented. Junctions of the form M-A-M, M-I-A-M, and M-I-A-I'-M, where A is an active molecular layer, are considered. Inelastic electron tunneling spectroscopy (IETS) is found to be readily applied to all the above device types. It can provide both vibrational and electron spectroscopic data about the molecules comprising the A layer. In IETS there are no strong selection rules (although there are preferences) so that transitions that are normally IR, Raman, or even photon-forbidden can be observed. In the electronic transition domain, spin and Laporte forbidden transitions may be observed. Both vibrational and electronic IETS can be acquired from single molecules. The negative aspect of this seemingly ideal spectroscopic method is the thermal line width of about 5 k(B)T. This limits the useful measurement of vibrational IETS to temperatures below about 10 K. In the case of most electronic transitions where the intrinsic linewidth is much broader, useful experiments above 100 K are possible. One further limitation of electronic IETS is that it is generally limited to transitions with energy less than about 20,000 cm(-1). IETS can be identified by peaks in d(2) I/dV (2) vs bias voltage plots that occur at the same position (but not necessarily same intensity) in either bias polarity.Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I'-M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dI/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic width of the ionization and affinity transitions, OMTS can be applied at temperatures above 500 K.This is not a comprehensive review of more than 20 years of research and there are many excellent papers that are not cited here. An absence of a citation is not a reflection on the quality of the work.

Electronic communication in dinuclear C(4)-bridged tungsten complexes

The dinuclear tungsten carbyne [X(CO)(2)(dppe)WC(4)W(dppe)(CO)(2)X] (dppe = 1,2-bis(diphenylphosphino)ethane; X = I (3), Cl (7)) complexes were prepared from the bisacetylide precursor Li(2)[(CO)(3)(dppe)WC(4)W(CO)(3)(dppe)] (2) via oxidative replacement of one CO group at each tungsten center with a halide substituent. The iodide ligand in 3 could be substituted with isothiocyanate or triflate resulting in [X(CO)(2)(dppe)WC(4)W(dppe)(CO)(2)X] complexes (X = NCS (8), OTf (9)). Substitution of two and all four CO ligands in 3 was achieved via subsequent photolytic or thermal activation with dppe. The "half-substituted" complex [I(CO)(2)(dppe)WC(4)W(dppe)(2)I] (11) allows reversible one-electron oxidation which results in the monocationic species [I(CO)(2)(dppe)WC(4)W(dppe)(2)I][PF(6)] (11[PF(6)]). The "all-dppe substituted" complex [I(dppe)(2)WC(4)W(dppe)(2)I] (10) possesses two reversible redox states leading to the stable monocationic [I(dppe)(2)WC(4)W(dppe)(2)I][PF(6)] (10[PF(6)]) and the dicationic [I(dppe)(2)WC(4)W(dppe)(2)I][PF(6)](2) (10[PF(6)](2)) compounds. The complexes 2, 3, [W(CO)(3)(dppe)(C[triple bond]CPh)(I)] (4), [X(CO)(2)(dppe)W[triple bond]C-C(Me)=C(Me)-C[triple bond]W(dppe)(CO)(2)X] (X = I (5), Cl (6)), 7, 8, 10, 11 and 11[PF(6)] were characterized by single crystal X-ray diffraction. The electronic properties of complexes 10, 10[PF(6)], 10[PF(6)](2), as well as of compounds 11 and 11[PF(6)], were investigated using cyclic voltammetry (CV), EPR, IR, near-IR spectroscopy, and magnetization measurements. These studies showed that the [W][triple bond]C-C[triple bond]C-C[triple bond][W] canonical form of the bridged system with strong tungsten-carbon interaction contributes significantly to the electronic coupling in the mixed-valent species 10[PF(6)] (comproportionation constant K(c) = 7.5 x 10(4)) and to the strong antiferromagnetic coupling in the dicationic complex 10[PF(6)](2) (exchange integral J = -167 cm(-1)). In addition, the rate for electron transfer between the tungsten centers in 10[PF(6)] was evaluated by near-IR and IR studies.

A theory of redox-mediated electron tunneling through an electrochemical two-center contact

The dependences of the tunnel current on the overpotential and bias voltage for a symmetric electrochemical contact involving two redox groups are calculated. The main physical situations involving various combinations of the strengths of the electronic coupling of the redox groups with each other and with the electrodes are considered in detail. The treatment is more rigorous and complete as compared with previous work. In particular, totally adiabatic transitions are discussed and the potential distribution in the tunnel gap is taken into account. It is shown that the system under consideration manifests negative differential resistance and rectification. A new effect is predicted in the current/overpotential dependence, namely the appearance of two maxima. The experimental data of (Tran et al 2006 Faraday Discuss. 131 197) are addressed. It is concluded that they are compatible with the assumption on a strong screening of the electric potential within the tunnel gap.

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.

SERS and tunneling spectroscopy investigation of iron-protoporphyrin IX adsorbed on a silver tip

Iron-protoporphyrin IX adsorbed on a scanning tunneling microscopy silver tip is investigated by combining surface enhancement Raman (SERS) and tunneling spectroscopies down to single molecule regime. Both the Raman signals and the tunneling current intensity reveal fast switching between the iron oxidation states and present analogous, significant fluctuations in time. The results point out the occurrence of a strong electronic coupling between the molecule and the metal, and may contribute to elucidate the dynamical phenomena at the molecule-metal interface, deserving some nanotechnological interest.

The study of charge transport through organic thin films: mechanism, tools and applications

In this paper, we discuss the current state of organic and molecular-scale electronics, some experimental methods used to characterize charge transport through molecular junctions and some theoretical models (superexchange and barrier tunnelling models) used to explain experimental results. Junctions incorporating self-assembled monolayers of organic molecules - and, in particular, junctions with mercury-drop electrodes - are described in detail, as are the issues of irreproducibility associated with such junctions (due, in part, to defects at the metal-molecule interface).

A scanning tunneling microscopy study of self-assembled nickel(II) octaethylporphyrin deposited from solutions on HOPG

The adsorption of nickel(II) octaethylporphyrin (NiOEP) from benzene and chloroform solutions on highly ordered pyrolytic graphite (HOPG) was investigated with a scanning tunneling microscope (STM) operated in ambient conditions. STM images show that NiOEP self-assembles on the graphite surface and that the molecules lie flat and form 2D lattices with spacings of 1.58 +/- 0.03 nm by 1.46 +/- 0.06 nm with a lattice angle of 69 degrees +/- 4 degrees averaged over both solvents. We were unable to eliminate the possibility that one unit cell distance is twice the above-reported distance. The corresponding molecular packing density, 4.5 +/- 0.3 x 10(13) molecules/cm(2), was essentially the same for benzene and chloroform solution deposition. These results differ somewhat from the structure revealed by high-resolution STM images of NiOEP on Au (111). The lack of apparent height (image intensity) in the constant current STM image of the alkane region of alkane-substituted metal porphyrins is attributed to a combination of changes in alkane configuration relative to the ring and associated changes in electronic coupling with HOMO and LUMO.

Scanning probe technology in metalloprotein and biomolecular electronics

The interfacing of man-made electronic components with specifically-folded biomacromolecules lies central not only to the development of sensory interfaces and potential new molecular-scale devices, but also enables us to analyse processes of great biological importance in a refined and controllable manner. Recent advances in both available technology, most notably optical and scanning probes in nature, and our understanding of suitable methodologies, have led us to the point where the characteristics of single biological molecules can be interrogated with good levels of reproducibility. We review here the application of scanning probe microscopy to the analysis of and experimentation on biological redox systems. Within this paper the tunnel transport characteristics, as assayed by both scanning tunnelling microscopy (STM) and conducting probe atomic force microscopy (AFM), of single metalloproteins are discussed. In a specific case study the electron transfer characteristics of the blue copper metalloprotein, azurin, are reported. The modulation of these properties under the influence of calibratable compressional force has also been examined in some detail. Work such as this enables one to reproducibly establish the conductance, barrier height, environmental sensitivity and electromechanical properties of these molecules.

Gating current flowing through molecules in metal–molecules–metal junctions

We have assembled two junctions that incorporate redox sites between Hg electrodes by different interactions. In the first junction, Hg-SAM-R//R-SAM-Hg, the redox site (R) are covalently linked to each electrode in self assembled monolayers (SAM-R). In the second junction, Hg-SAM//R//SAM-Hg, the redox sites dissolved in solution are trapped by electrostatic interaction at the SAM formed at the electrodes. The current flowing through these junctions can be controlled by adjusting the potential applied at the electrodes with respect to the redox potential of the species by using an electrochemical system. The current flowing in these two junctions is mediated by the redox sites through different mechanisms. In particular, the current flowing through the Hg-SAM-R//R-SAM-Hg junction occurs through a self exchange mechanism between the redox sites organized at each electrode, while the current flowing through the Hg-SAM//R//SAM-Hg junction is dominated by a redox-cycling mechanism. The systems described here are easy to assemble, well-characterized, yield reproducible data and make it easy to modify the electrical properties of the junctions by changing the nature of the redox centres. For these characteristics they are well suited for collecting fundamental information relevant to the fabrication of molecular switches.

Metalloprotein tunnel junctions: compressional modulation of barrier height and transport mechanism

Though the incorporation of sensory or potentially-switchable biological entities into electronic devices brings with it a number of complicating issues associated with hydration, structural complexity/delicacy, and low conductance, the possibility of resolving properties of fundamental importance (such as the influence of protein fold on conductance) at a molecularly-resolved level, are exciting. Our ability to analyse charge transport through a biological macromolecule remains, though, a significant practical and theoretical challenge. Though much information can be gained by carrying out such examinations at a molecular level, there exist few methods where such controlled analyses are, in fact, feasible. Here we report on the electron transport characteristics of a blue copper metalloprotein as characterized by conductive-probe atomic force microscopy. At very low imposed force, contact resistance is high, electrical contact unstable, and the junction undergoes dielectric breakdown at 1.1-1.5 GV m(-1). At increased applied force, the current-voltage characteristics are entirely reproducible and well-described by a Simmons (non-resonant) tunnelling model. Though highly resistive, observations demonstrate the ability of the protein matrix to mediate appreciable tunnelling current. Non-resonant behaviour is consistent with observations of bias-independent tunnelling imaging. In fitting observed transport characteristics to this model, it is possible to deconvolute barrier height and length at specific experimental conditions and, specifically, to monitor the modulation of these parameters by imposed compressional force. At higher field spectroscopic features assignable to metal based density of states are reproducibly observed. These vanish in a force regime where the tunnel barrier to direct tip-sample communication decreases.

Ferrocenylundecanethiol Self-Assembled Monolayer Charging Correlates with Negative Differential Resistance Measured by Conducting Probe Atomic Force Microscopy

Electrical and mechanical properties of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers (SAMs) of electroactive 11-ferrocenylundecanethiol (FcC(11)SH) and a Pt-coated atomic force microscope (AFM) tip have been measured using a conducting probe (CP) AFM in insulating alkane solution. Simultaneous and independent measurements of currents and bias-dependent adhesion forces under different applied tip biases between the conductive AFM probe and the FcC(11)SH SAMs revealed reversible peak-shaped current-voltage (I-V) characteristics and correlated maxima in the potential-dependent adhesion force. Trapped positive charges in the molecular junction correlate with high conduction in a feature showing negative differential resistance. Similar measurements on an electropassive 1-octanethiol SAM did not show any peaks in either adhesion force or I-V curves. A mechanism involving two-step resonant hole transfer through the occupied molecular orbitals (MOs) of ferrocene end groups via sequential oxidation and subsequent reduction, where a hole is trapped by the phonon relaxation, is proposed to explain the observed current-force correlation. These results suggest a new approach to probe charge-transfer involving electroactive groups on the nanoscale by measuring the adhesion forces as a function of applied bias in an electrolyte-free environment.

Tip to substrate distances in STM imaging of biomolecules

STM images of single biomolecules adsorbed on conductive substrates do not reproduce the expected physical height, which generally appears underestimated. This may cause the tip to interfere with the soft biological sample during the imaging scans. Therefore, a key requirement to avoid invasive STM imaging is the knowledge, and the control, of the initial tip to substrate distance. This is connected to the setting of the tunnelling current and applied voltage, which define a tunnelling resistance. The height of the STM tip was measured by calibrating the tunnelling resistance, as a function of its vertical displacement until establishing a mechanical contact. At a tunnelling resistance of 4 x 10(9)Omega, distances of about 3 and 6 nm are estimated when flat Au substrates are imaged in water and in air, respectively. On such a ground, the relevance of the starting tip-substrate distance in determining a non-invasive imaging has been investigated for a plastocyanin mutant chemisorbed on Au(111) electrodes. At tunnelling distances sufficient to overcome the physical height of the imaged biomolecules, their lateral dimensions are found to be consistent with the crystallography, whereas they are significantly broadened for smaller distances.

Scanning tunneling spectroscopy investigation of self-assembled plastocyanin mutants onto gold substrates under controlled environment

The study of the electronic conduction through plastocyanin (PC) mutants assembled on a gold surface has been addressed by scanning tunneling spectroscopy. The two mutants exploit a single thiol group (PCSH) or a disulfide bridge (PCSS) to covalently bind at gold surface. The I-V measurements were performed by positioning the STM tip on top of a single molecule and sweeping the bias potential between +/-1 V, under both ambient and controlled atmosphere. For PCSS, under ambient conditions, asymmetric I-V characteristics were obtained, which disappear under nitrogen atmosphere. PCSH, instead shows a symmetric I-V relation in air and under nitrogen environment. Here, as factors underlying this distinct electron conductive behaviour, a potential role for hydration water molecules and for copper redox levels are discussed.

Wiring nanoparticles with redox molecules

Gold nanoparticles were used to make electrical contact to redox-active organic molecules. Viologen based dithiols were self-assembled from solution on Au(111) for use as tethers to attach nanoparticles to a conducting substrate. The topography and electrical properties of the resulting films were investigated by STM and STS and the orientation of these linkers was investigated by FTIR. Surface coverage increased with increasing reaction time, resulting in a change of film orientation from a flat to a more upright standing conformation. Gold nanoparticles attached to these self-assembled films were characterised by STM. It was possible to isolate a single redox-active molecule in an alkanethiol matrix and by subsequent attachment of a single gold nanoparticle the electrical properties of single wired molecules could be investigated. This method allowed the measurement of the conductivity of single molecules connecting a nanoparticle to the substrate chemically, thus forming stable electrical contacts at both ends.