Electrochemical scanning tunneling microscopy

The electrochemical scanning tunneling microscope was the first tool for the investigation of solid-liquid interfaces that allowed in situ real space imaging of electrode surfaces at the atomic level. Therefore it quickly became an important addition to the repertoire of methods for the determination of the local surface structure as well as the dynamics of reactions and processes taking place at surfaces in an electrolytic environment. In this short overview we present several examples to illustrate the powerful capabilities of the EC-STM, including the observation of clean metal surfaces as well as the adsorption of thin metal layers, specifically adsorbed anions and non-specifically adsorbed organic cations. In several cases the electrode potential has a significant influence on structure and reactivity of the surface that can be explained by the observations made with the EC-STM.     

Direct observations of changes in surface structures by scanning tunneling microscopy

Recently, we have studied the interaction of reactive adsorbates H, C, O, and S with Ni and Cu surfaces by scanning tunneling microscopy (STM). In this paper, we briefly illustrate and discuss how such studies provide significant insight into the understanding of dynamic surface processes such as adsorbate-induced restructuring, surface reactions, and adsorbate-adsorbate interactions. The STM results demonstrate that there is a strong coupling between the chemisorption/reaction process and the distortion of the metal surface.     

An examination of surface waving on mercury film electrodes using scanning tunneling microscopy

Mercury film electrodes (mfe) have been prepared on several substrates and examined using scanning tunneling microscopy (STM). Surface waving was found to be at least one order of magnitude smaller in amplitude than that observed on bulk mercury drops. A solid/liquid/gaseous interface was observed, as was the amplitude of surface waves under various imaging conditions. No resonance condition was observed in these systems. A simple system for exciting surface waves was developed, and used to investigate surface waving of mercury during exposure to various conditions. The source of these waves remains unknown.     

Spin-polarized scanning tunneling microscopy: breakthroughs and highlights

The principle of scanning tunneling microscopy, an imaging method with atomic resolution capability invented by Binnig and Rohrer in 1982, can be adapted for surface magnetism studies by using magnetic probe tips. The contrast mechanism of this so-called spin-polarized scanning tunneling microscopy, or SP-STM, relies on the tunneling magneto-resistance effect, i.e. the tip-sample distance as well as the differential conductance depend on the relative magnetic orientation of tip and sample. To illustrate the working principle and the unique capabilities of SP-STM, this compilation presents some key experiments which have been performed on various magnetic surfaces, such as the topological antiferromagnet Cr(001), a double-layer of Fe which exhibits a stripe- domain pattern with about 50 nm periodicity, and the Mn monolayer on W(110), where the combination of experiment and theory reveal an antiferromagnetic spin cycloid. Recent experimental results also demonstrate the suitability of SP-STM for studies of dynamic properties, such as the spin relaxation time of single magnetic nanostructures.     

Characterization by scanning tunneling microscopy of silver oxydehydrogenation catalysts

A silicon carbide-supported silver catalyst used in the oxydehydrogenation of ethylene glycol to glyoxal has been studied by scanning tunneling microscopy. The surface morphology depends upon reaction conditions. Silver particles normally sinter into large plates covering the support. However, in the presence of diethylphosphite there is a chemical erosion which results in a tortuous and fractal-like surface.     

Characterization of surfaces of carbon fibres by scanning tunneling microscopy

Summary The experimental results obtained by scanning tunneling microscopy (STM) studies of different carbon fibres are presented and discussed. The comparative analysis of the STM images at scales from hundreds of nanometers down to atomic scale reveals the differences of surface features for carbon fibres processed from different precursors, polyacrylonitrile fibres and pitch. The high temperature treatment of carbon fibres — the so-called graphitization process — as used to improve the stress modulus induces drastically increased ordering phenomena at the atomic level. Structural information obtained by STM on the surface of the fibres as well as in their cross sectional areas is discussed in comparison with known results of diffraction studies. STM appears to be the new powerfull technique for the detailed structural studies of surfaces of carbon fibres. The perspectives of these studies are under discussion.     

Vibration-assisted upconversion of molecular luminescence induced by scanning tunneling microscopy

We investigate the effects of coupling between a molecular exciton, which consists of an electron and a hole in a molecule, and a surface plasmon (exciton-plasmon coupling) on the electron transitions of the molecule using nonequilibrium Green’s function method. Due to the exciton-plasmon coupling, excitation channels of the molecule arise in the energy range lower than the electronic excitation energy of the molecule. It is found that the electron transitions via these excitation channels give rise to the molecular luminescence and the vibrational excitations at the bias voltage lower than the electronic excitation energy of the molecule. Our results also indicate that the vibrational excitations assist the emission of photons, whose energy exceeds the product of the elementary charge and the bias voltage, (upconverted luminescence).     

Investigation of gold nanoparticles immobilized on the surface of pyrite by scanning probe microscopy,scanning tunneling spectroscopy,and X-ray photoelectron spectroscopy

The characteristics of gold spontaneously deposited on the surface of pyrite from an HAuCl₄ solution at room temperature are investigated by scanning probe microscopy, scanning tunneling spectroscopy, atomic-force microscopy, and X-ray photoelectron spectroscopy with synchrotron excitation. Within minutes after the onset of the deposition, gold is deposited in the form of metallic particles with a diameter ranging from 8 to 15 nm, which, in turn, subsequently form agglomerates with sizes up to several hundred nanometers. It is revealed that the Au 4f _7/2 lines in the X-ray photoelectron spectra of the gold samples are shifted as compared to those for bulk gold and that tunneling in scanning tunneling spectra is suppressed. The effects caused, apparently, by the Coulomb blockage are unusually pronounced for such relatively large particles and decrease rather slowly upon the aggregation of particles.     

Periodic, pearl chain-like nanostructure observed by scanning tunneling microscopy

A periodic, pearl chain-like nanostructure is observed in ultrahigh vacuum scanning tunneling microscopy (UHV-STM) of chromium filled carbon nanotubes. The structure displays one dimensional periodic morphology, with periodicity of ∼3.3 nm. Atomically resolved STM image of the pearl chain structure shows hexagonal periodicity. The origin of the pearl chain structure is discussed.     

Imaging the structure and porosity of active carbons by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed for a comparative structural characterization, down to the atomic scale, of a representative set of porous carbons with different adsorption characteristics and prepared either by activation (physical or chemical) or templating techniques. The studied materials included a chemically activated, supermicroporous high surface area carbon, two activated carbons containing both micro- and mesopores synthesized by physical activation, an ultramicroporous carbon molecular sieve, and an ordered microporous carbon templated in the nanochannels of zeolite Y. In general, good agreement was found between the porous structures as imaged by STM and the porous texture derived from gas adsorption data for all the carbons investigated. The activated carbon samples were dominated by networks of slit type micropores and, in some cases, by mesopores of varied morphologies. By contrast, the zeolite-templated carbon exhibited rounded micropore morphology, and the carbon molecular sieve displayed a rather featureless conformation dominated by voids only below 1 nm wide. The structural differences observed by STM were interpreted in terms of the different preparation procedures of the studied carbons. In particular, the templated carbon consisted of minute clusters about 1 nm in diameter that were interpreted to be formed within the extremely confined, microporous spaces of the zeolite Y template.     

Interfacial self-assembly of amino acids and peptides: scanning tunneling microscopy investigation

Proteins play important roles in human daily life. To take advantage of the lessons learned from nature, it is essential to investigate the self-assembly of subunits of proteins, i.e., amino acids and polypeptides. Due to its high resolution and versatility of working environment, scanning tunneling microscopy (STM) has become a powerful tool for studying interfacial molecular assembly structures. This review is intended to reflect the progress in studying interfacial self-assembly of amino acids and peptides by STM. In particular, we focus on environment-induced polymorphism, chiral recognition, and coadsorption behavior with molecular templates. These studies would be highly beneficial to research endeavors exploring the mechanism and nanoscale-controlling molecular assemblies of amino acids and polypeptides on surfaces, understanding the origin of life, unravelling the essence of disease at the molecular level and deeming what is necessary for the "bottom-up" nanofabrication of molecular devices and biosensors being constructed with useful properties and desired performance.     

Easily made and handled carbon nanocones for scanning tunneling microscopy and electroanalysis

Well-formed carbon nanocones at the ends of micrometer-diameter carbon fibers (CFs) were fashioned into functional tips for scanning tunneling microscopy (STM) and miniaturized voltammetric sensors. Sharpening of single graphite filaments was achieved by simple DC electrochemical etching in 0.1 N NaOH. Operated as STM tips, pointed CFs resolved in air the contour and surface morphology of a nanoscopic Au line pattern and imaged in vacuum a Si (1 1 1) surface with clear atomic resolution. Subjecting already etched CFs to tip-sparing insulation with electrodeposited paint produced conical carbon ultramicroelectrodes (UMEs) with effective radii down to about 900 nm. Comparative cyclic voltammetry trials in alkaline, neutral and acidic solutions showed that the conical carbon UME’s had a wider practical potential window for electroanalytic applications than, for instance, Pt disk UMEs. The CF-based conical sensors described here are exceptionally easy to make with simple laboratory equipment and perform well in STM topography imaging and voltammetry. The inherent simplicity of sensor production widens the field of potential users, and offers clear advantages over existing types of UMEs, in particular those based on carbon nanotubes, which are especially hard to handle in an optical microscope setting.     

Zirconium oxide supported on Pd(100): Characterization by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) were used to characterize the structure of a model metal-supported dispersed metal oxide, ZrO₂ on Pd(100). STM images illustrate changes in the surface morphology of the ZrO₂ resulting from various chemical treatments. When the sample was treated in O₂, the ZrO₂ appeared as a smooth, featureless overlayer of varying thickness wetting the Pd. After treatment in H₂, the ZrO₂ formed non-wetting particles on the Pd, with a sharp Pd-ZrO₂ interface. TS provided a fingerprint that verified the presence of a semiconducting overlayer on a metallic support. These results appear to be consistent with X-ray absorption spectra of ZrO₂ supported on Pd black, reported elsewhere.     

Reduction potentials of heteropolyacid catalysts probed by scanning tunneling microscopy and UV-visible spectroscopy

Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.     

Fabrication and characterization of single crystal semiconductive diamond tip for combined scanning tunneling microscopy

Boron-doped type IIb diamond single crystal synthesized by the temperature gradient method under high pressure–high temperature (HPHT) conditions is proposed as a material for the tip of scanning nanoindentation–scanning tunneling microscopy (SN-STM) technique. Sequence of the procedures covering growing crystals with predetermined physical properties, selection of the synthesized crystals with the desired habit and their precise shaping in the form of Berkovich pyramid have been developed. Results of experimental studies demonstrated fabricated tip's ability to conduct combined nanoscale characterization of surface using indentation and scanning with the same tip.     

Fabricating and controlling molecular self-organization at solid surfaces: studies by scanning tunneling microscopy

This account presents a summary of recent work describing the control and fabrication of self-organized molecular adlayers on solid substrates. These results demonstrate that molecules, under appropriate conditions, will self-organize into well-ordered monolayers on various solid surfaces. Using scanning tunneling microscopy (STM) to probe the structure of these molecular architectures, it is possible to determine the surface quality to single molecule resolution. The surface structures can be controlled by external stimuli such as electrode potential and UV-light. The ability to control how these adlayers form is important for constructing surface molecular architectures with useful properties.     

Investigation of redox potential and negative differential resistance behavior of heteropolyacids by scanning tunneling microscopy

Nanoscale images and tunneling spectra of cation-exchanged or polyatom-substituted heteropolyacids (HPAs) were probed by scanning tunneling microscopy (STM) at room temperature. All the HPAs formed two-dimensional ordered arrays on graphite surfaces, and their molecular dimensions were in good agreement with values determined by X-ray crystallography. It was observed that the negative differential resistance (NDR) peak voltage measured by STM was closely related to the reduction potential of the corresponding bulk HPA. The higher the reduction potential of the HPA, the lower the applied voltage at which NDR was observed.     

Using scanning tunneling microscopy to characterize adsorbates and reactive intermediates on transition metal oxide surfaces

Scanning tunneling microscopy (STM) is demonstrated to be a powerful tool to characterize adsorption and reaction on oxide surfaces by imaging molecular adsorbates and reactive intermediates. The molecules were used to probe surface structure and to study surface reactivity spatially at the atomic level. Results for three systems are presented: alcohol adsorption on WO₃(0 0 1), carboxylates on the anatase polymorph of TiO₂, and propene adsorption on a PdO monolayer on Pd(1 0 0). When the alcohols were exposed to the WO₃(0 0 1)-c(2×2) surface at room temperature the molecules could not be imaged. Heating the surface to temperatures above a water desorption peak associated with alcohol deprotonation, however, allowed 1-propoxide to be imaged. The images reveal that the alkoxide has no preference for defects, rather it binds to W⁶⁺ ions exposed on the fully oxidized c(2×2) surface. Temperature-programmed desorption revealed that alkoxides at these sites undergo only dehydration reactions. To probe the structure of the unusual (1×4) reconstruction on anatase (0 0 1), formic and acetic acid adsorption were used. Following dissociative adsorption, both formate and acetate adsorb solely centered atop the bright rows that define the surface reconstruction, and the molecules are always at least two lattice constants apart. This result may be attributed to carboxylates bridge-bonded to Ti atoms at the center of the bright rows. This finding eliminates several suggested models of the reconstruction and suggests that a recently proposed ad-molecule model is a good representation of the surface structure. Propene was observed to initially randomly adsorb on the PdO monolayer. At higher coverages, however, the adsorbates cluster, disrupting the surface structure and causing the adsorption rate adjacent to the clusters to increase. Temperature-programmed reaction revealed that once propene adsorbs, the oxide monolayer catalyzes its oxidation at lower temperatures than metallic Pd, but that the propene sticking coefficient on the ordered oxide layer is a factor of 5 lower.