Voltammetry and in situ scanning tunnelling spectroscopy of osmium, iron, and ruthenium complexes of 2,2':6',2'-terpyridine covalently linked to Au(111)-electrodes

We have studied self-assembled molecular monolayers (SAMs) of complexes between Os(II)/(III), Fe(II)/(III), and Ru(II)/(III) and a 2,2',6',2'-terpyridine (terpy) derivative linked to Au(111)-electrode surfaces via a 6-acetylthiohexyloxy substituent at the 4'-position of terpy. The complexes were prepared in situ by first linking the terpy ligand to the surface via the S-atom, followed by addition of suitable metal compounds. The metal-terpy SAMs were studied by cyclic voltammetry (CV), and in situ scanning tunnelling microscopy with full electrochemical potential control of substrate and tip (in situ STM). Sharp CV peaks were observed for the Os- and Fe complexes, with interfacial electrochemical electron transfer rate constants of 6-50 s(-1). Well-defined but significantly broader peaks (up to 300 mV) were observed for the Ru-complex. Addition of 2,2'-bipyridine (bipy) towards completion of the metal coordination spheres induced voltammetric sharpening. In situ STM images of single molecular scale strong structural features were observed for the osmium and iron complexes. As expected from the voltammetric patterns, the surface coverage was by far the highest for the Ru-complex which was therefore selected for scanning tunnelling spectroscopy. These correlations displayed a strong peak around the equilibrium potential with systematic shifts with increasing bias voltage, as expected for a sequential two-step in situ ET mechanism.     

Electrochemical and spectroscopic investigations of immobilized de novo designed heme proteins on metal electrodes.

On the basis of rational design principles, template-assisted four-helix-bundle proteins that include two histidines for coordinative binding of a heme were synthesized. Spectroscopic and thermodynamic characterization of the proteins in solution reveals the expected bis-histidine coordinated heme configuration. The proteins possess different binding domains on the top surfaces of the bundles to allow for electrostatic, covalent, and hydrophobic binding to metal electrodes. Electrostatic immobilization was achieved for proteins with lysine-rich binding domains (MOP-P) that adsorb to electrodes covered by self-assembled monolayers of mercaptopropionic acid, whereas cysteamine-based monolayers were employed for covalent attachment of proteins with cysteine residues in the binding domain (MOP-C). Immobilized proteins were studied by surface-enhanced resonance Raman (SERR) spectroscopy and electrochemical methods. For all proteins, immobilization causes a decrease in protein stability and a loosening of the helix packing, as reflected by a partial dissociation of a histidine ligand in the ferrous state and very low redox potentials. For the covalently attached MOP-C, the overall interfacial redox process involves the coupling of electron transfer and heme ligand dissociation, which was analyzed by time-resolved SERR spectroscopy. Electron transfer was found to be significantly slower for the mono-histidine-coordinated than for the bis-histidine-coordinated heme. For the latter, the formal heterogeneous electron-transfer rate constant of 13 s(-1) is similar to those reported for natural heme proteins with comparable electron-transfer distances, which indicates that covalently bound synthetic heme proteins provide efficient electronic communication with a metal electrode as a prerequisite for potential biotechnological applications.     

Electrochemistry of self-assembled monolayers of the blue copper protein Pseudomonas aeruginosa azurin on Au(111)

We report the self-assembly and electrochemical behaviour of the blue copper protein Pseudomonas aeruginosa azurin on Au(111) electrodes in aqueous acetate buffer (pH=4.6). The formation of monolayers of this protein is substantiated by electrochemical measurements. Capacitance results indicate qualitatively that the protein is strongly adsorbed at sub-μM concentrations in a broad potential range (about 700 mV). This is further supported by the attenuation of a characteristic cyclic voltammetric peak of Au(111) in acetate solution with increasing azurin concentration. Reductive desorption is clearly disclosed in NaOH solution (pH=13), strongly suggesting that azurin is adsorbed via its disulphide group. An anodic peak and a cathodic peak associated with the copper centre of azurin are finally observed in the differential pulse voltammograms. These peaks are, interestingly, indicative of long-range electrochemical electron transfer such as paralleled by intramolecular electron transfer between the disulphide anion radical and the copper atom in homogeneous solution, and anticipated by theoretical frames. Together with reported in situ scanning tunnelling microscopy (STM) results they constitute the first case for electrochemistry of self-assembled monolayers of azurin, even redox proteins. This integrated investigation provides a new approach to both structure and function of adsorbed redox metalloproteins at the molecular level.     

扫描电子显微镜与扫描隧道显微镜联用装置

在ＫＹＫＹ－１０００Ｂ型扫描电子显微镜上所开发的与其联用的袖珍型扫描隧道显微镜主要有四个部分：（１）减震阻尼装置,（２）隧道探针,（３）探针扫描与逼近装置,（４）电子控制与图象采集系统。它的分辨率约为１ｎｍ,并用它观察了半导体光栅与硅上金膜的细微结构。     

Rastertunnelmikroskopie an chiralen Molekülen: Nanochemie

The imaging and manipulation capabilities of the scanning tunnelling microscope (STM) render possible a novel nanoscale chemistry based on experiments with single molecules. Herein, we address several aspects of a nanoscale stereochemistry using the STM. As an example, we investigate 1‐nitronaphthalene on Au(111). 1‐Nitronaphthalene becomes chiral upon planar adsorption on the metal surface. High‐resolution STM images reflect the asymmetric electronic structure of the molecules and allow for the determination of the absolute configuration of any individual molecule within complex molecular structures. At medium coverage, spontaneous breaking of the chiral symmetry results in the formation of homochiral conglomerates, while at high coverage racemic structures prevail. Finally, the tip of the STM is used to separate “supramolecule‐by‐supramolecule” a racemic mixture of chiral 1‐nitronaphthalene aggregates into the enantiopure compounds.     

Direct observation of the gating step in protein electron transfer: electric-field-controlled protein dynamics

Heterogeneous electron transfer of proteins at biomimetic interfaces is characterized by unusual distance dependences of the electron-transfer rates, whose origin has been elusive and controversial. Using a two-color, time-resolved, surface-enhanced resonance Raman spectroelectrochemical approach, we have been able to monitor simultaneously and in real time the structure, electron-transfer kinetics, and configurational fluctuations of cytochrome c electrostatically adsorbed to electrodes coated with self-assembled monolayers. Our results show that the overall electron-transfer kinetics is determined by protein dynamics rather than by tunnelling probabilities and that the protein dynamics in turn is controlled by the interfacial electric field. Implications for interprotein electron transfer at biological membranes are discussed.     

Inelastic interactions of tunnel electrons with surfaces

Inelastic interactions of electrons emitted from the tip of a scanning tunnelling microscope (STM) are used to desorb individual hydrogen atoms from a Ge(111) surface. It is observed that the inelastic interactions depend not only on the electron energy and the current intensity but also on the electron emission regime of the STM tip. Quite surprisingly, it is found that tunnel electrons interact inelastically much less efficiently than field emitted electrons even though the electrons are in resonance with the Ge-H unoccuppied orbital.     

Metal-molecule-metal junctions in Langmuir-Blodgett films using a new linker: trimethylsilane

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.     

Potential barriers for electron tunnelling in low-temperature aqueous glasses (comparison of the computer simulation model with experiments)

The experimental kinetic data on the trapped electron decay in 6 M NaOH aqueous glass doped with electron scavengers were analysed. The electron decay curves obtained by the computer simulation under assumption of the simple tunnelling mechanism of the electron transfer were fitted to the experimental decays. It was found that for a group of scavengers the optimization procedure works well and gives the average barrier height for electron tunnelling between 1.26 and 1.42 eV. There is however a numerous group of scavengers for which the simple tunnelling mechanism does not provide adequate simulated kinetics of the trapped electron decay.     

Hierarchical self-assembly of designed 2 x 2-alpha-helix bundle proteins on Au(111) surfaces

Self-assembled monolayers of biomolecules on atomically planar surfaces offer the prospect of complex combinations of controlled properties, e.g., for bioelectronics. We have prepared a novel hemi-4-alpha-helix bundle protein by attaching two alpha-helical peptides to a cyclo-dithiothreitol (cyclo-DTT) template. The protein was de novo designed to self-assemble in solution to form a 4-alpha-helix bundle, whereas the disulfide moiety enables the formation of a self-assembled monolayer on a Au(111) surface by opening of the disulfide, thus giving rise to a two-step self-assembly process. The 2 x 2-alpha-helix bundle protein and its template were studied by X-ray photo electron spectroscopy (XPS), electrochemical methods, and electrochemical in situ scanning tunneling microscopy (in situ STM). XPS showed that the cyclo-DTT opens on adsorption to a gold surface with the integrity of the 2 x 2-alpha-helix bundle proteins retained. The surface properties of the DTT and 2 x 2-alpha-helix bundle protein adlayer were characterized by interfacial capacitance and impedance techniques. Reductive desorption was used to determine the coverage of the adlayers, giving values of 65 and 16 muC cm(-2) for DTT and 2 x 2-helix, respectively. The 2 x 2-alpha-helix bundle protein adlayers were imaged by in situ STM. The images indicated a dense monolayer according with the voltammetric data. No long-range order could be detected, but two clearly distinct STM contrasts were assigned to 2 x 2-alpha-helix bundle protein molecules oriented in parallel and antiparallel conformations. The template molecule DTT alone forms highly ordered 30-40 nm domains, giving an adlayer density which agreed well with the coverage determined by voltammetry. This could be exploited in STM imaging of mixed DTT/2 x 2-alpha-helix bundle protein monolayers, with clearly distinct STM patterns of the two components.     

Dynamics of vacancy defects in 2-D crystalline monolayers under chiral smectic thin films studied by scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) can be used to image adsorbed organic molecules in real space and real time. The technique seems especially well suited for imaging 2-D crystalline monolayers formed under liquid crystal films. In addition to observing perfect 2-D crystals, STM provides the ability to observe crystal defects, and to observe how these defects evolve over time. In this study two different vacancy defects in 2-D lamellar monolayers of chiral liquid crystal molecules under bulk smectic films were observed in situ. Both vacancies showed dynamic behaviour and an unexpected transport anisotropy.     

Probing organic layers on the TiO2(110) surface

In this work, we use first principles simulations to provide features of the dynamic scanning force microscopy imaging of adsorbed organic layers on insulating surfaces. We consider monolayers of formic (HCOOH) and acetic (CH(3)COOH) acid and a mixed layer of acetic and trifluoroacetic acids (CF(3)COOH) on the TiO(2)(110) surface and study their interaction with a silicon dangling bond tip. The results demonstrate that the silicon tip interacts more strongly with the substrate and the COO(-) group than the adsorbed acid headgroups, and, therefore, molecules would appear dark in images. The pattern of contrast and apparent height of molecules is determined by the repulsion between the tip and the molecular headgroups and by significant deformation of the monolayer and individual molecules. The height of the molecule on the surface and the size of the headgroup play a large role in determining access of the tip to the substrate and, hence, the contrast in images. Direct imaging of the molecules themselves could be obtained by providing a functionalized tip with attraction to the molecular headgroups, for example, a positive potential tip.     

In situ electrochemical scanning tunnelling microscopy investigation of structure for horseradish peroxidase and its electricatalytic property

Immobilization of protein molecules is a fundamental problem for scanning tunnelling microscopy (STM) measurements with high resolution. In this paper, an electrochemical method has been proved to be an effective way to fix native horseradish peroxidase (HRP) as well as inactivated HRP from electrolyte onto a highly oriented pyrolytic graphite (HOPG) surface. This preparation is suitable for both ex situ and in situ electrochemical STM (ECSTM) measurements. In situ STM has been successfully employed to observe totally different structures of HRP in three typical cases: (1) in situ ECSTM reveals an oval-shaped pattern for a single molecule in neutral buffer solution, which is in good agreement with the dimension determined as 6.2×4.3×1.2. nm³ by ex situ STM for native HRP; (2) in situ ECSTM shows that the adsorbed HRP molecules on HOPG in a denatured environment exhibit swelling globes at the beginning and then change into a V-shaped pattern after 30 min; (3) in situ ECSTM reveals a black hole in every ellipsoidal sphere for inactivated HRP in strong alkali solution. The cyclic voltammetry results indicate that the absorbed native HRP can directly catalyse the reduction of hydrogen peroxide, demonstrating that a direct electron transfer reduction occurred between the enzyme and HOPG electrode, whereas the corresponding cyclic voltammograms for denatured HRP and inactivated HRP adsorbed on HOPG electrodes indicate a lack of ability to catalyse H₂O₂ reduction, which confirms that the HRP molecules lost their biological activity. Obviously, electrochemical results powerfully support in situ STM observations.     

Internal Architecture and Adsorption Sites of Violet Lander Molecules Assembled on Native and KBr‐Passivated InSb(001) Surfaces

The adsorption of individual Violet Lander molecules self‐assembled on the c(8×2) reconstructed InSb(001) surface in its native form and on the surface passivated with one to three monolayers of KBr is investigated by means of low‐temperature scanning tunneling microscopy (STM). Preferred adsorption sites of the molecules are found on flat terraces as well as at atomic step edges. For molecules immobilized on flat terraces, several different conformations are identified from STM images acquired with submolecular resolution and are explained by the rotation of the 3,5‐di‐tert‐butylphenyl groups around σ bonds, which allows adjustment of the molecular geometry to the anisotropic substrate structure. Formation of ordered molecular chains is found at steps running along substrate reconstruction rows, whereas at the steps oriented perpendicularly no intermolecular ordering is recorded. It is also shown that the molecules deposited at two or more monolayers of the epitaxial KBr spacer do not have any stable adsorption sites recorded with STM. Prospects for the manipulation of single molecules by using the STM tip on highly anisotropic substrates are also explored, and demonstrate the feasibility of controlled lateral displacement in all directions.     

Pressure-dependent Ni-O phase transitions and Ni oxide formation on Pt(111): an in situ STM study at elevated temperatures

Growth, atomic structure and O2 partial pressure dependent phase transitions of Ni-O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400-550 K) and variable O2 pressure. Depending on the substrate temperature, one-dimensional network-like Ni-O structures and islands with (7x1) and (4x2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2x2) reconstruction in a narrow O2 pressure range of 1.5-2x10(-6) mbar and can be monitored by in situ STM. Upon reduction of the O2 pressure to <10(-10) mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O2 pressure-dependent Ni-O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.     

Relative conductances of alkaneselenolate and alkanethiolate monolayers on Au{111}

The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.     

Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy

Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt₂₅Ni₇₅(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt₁₀Ni₉₀(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.     

Chemical analysis of PtxNi1?x alloy single crystal surfaces by scanning tunnelling microscopy

Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt₂₅Ni₇₅(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt₁₀Ni₉₀(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.     

A theoretical rationalization of a total inelastic electron tunneling spectrum: the comparative cases of formate and benzoate on Cu(111)

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals at the atomic scale. However, direct IETS-based chemical analysis remains difficult due to the selection rules that await a definite understanding. We present IETS simulations of single formate and benzoate species adsorbed in the same upright bridge geometry on a (111)-cleaved Cu surface. In agreement with measurements on a related substrate, the simulated IET-spectra of formate/Cu(111) clearly resolve one intense C-H stretching mode whatever the tip position in the vicinity of the molecular fragment. At variance, benzoate/Cu(111) has no detectable IET signal. The dissimilar IETS responses of chemically related molecules--formate and benzoate adsorbates--permit us to unveil another factor that complements the selection rules, namely the degree of the vacuum extension of the tunneling active states perturbed by the vibrations. As a consequence, the lack of a topmost dangling bond orbital is entirely detrimental for STM-based inelastic spectroscopy but not for STM elastic imaging.     

Coexistence of multiple conformations in cysteamine monolayers on Au(111)

The structural organization, catalytic function, and electronic properties of cysteamine monolayers on Au(111) have been addressed comprehensively by voltammetry, in situ scanning tunneling microscopy (STM) in anaerobic environment, and a priori molecular dynamics (MD) simulation and STM image simulation. Two sets of voltammetric signals are observed. One peak at -(0.65-0.70) V (SCE) is caused by reductive desorption of cysteamine. The other signal, at -(0.25-0.40) V consists of a peak doublet. The pH dependence of the latter suggests that the origin is catalytic dihydrogen evolution. The doublet feature is indicative of two distinct cysteamine configurations. Cysteamine monolayer formation from initial nucleation to a highly ordered phase has been successfully observed in real time using oxygen-free in situ STM. Random cellular patterns, disordered adlayer formation accompanied by high step edge mobility, and ultimately a highly ordered (square root 3 x 4) R30 degrees lattice are observed sequentially. Pits are formed due to enclosure of the mobile edges during the adsorption process. In the highly ordered cysteamine layer, each unit has two spots with apparent 0.6 A height difference in STM images. The coverage 5.7 +/- 0.1 x 10(-10) mol cm(-2) determined by voltammetry supports that the spots represent two individual cysteamine molecules. A priori MD and density functional simulations hold other clues to the image interpretation and indicate that the NH(3)(+) groups dominate the tunneling contrast. A wide range of interface structures, showing variations in the sulfur binding site and orientation, gauche and trans conformers, and especially hydrogen-bonding interactions, are examined, from which it is concluded that the adsorbate structure is controlled by interactions with the solvent rather than with the substrate.