In situ scanning tunneling microscopy: New insight for electrochemical electrode-surface investigations

The successful expansion which the scanning tunneling microscopy (STM) has had is dependent on its ability to examine surfaces on a sub-nanometric scale and on providing in situ (i.e. in the presence of bulk electrolyte) sample examination. In addition to the ability to study metals and semiconductors in vacuo, the application of the technique to surfaces in contact with an electrolytic solution has prompted increased interest amongst electrochemists. We discuss herein the technique, with particular reference to advances in electrochemical applications. A new scanning tunneling microscope for operation in electrolytic environments is described. Atomic force microscopy, scanning electrochemical microscopy and scanning ion-conducting microscopy are compared with the STM.     

Isolated supramolecules on surfaces studied with scanning tunneling microscopy

To date, supramolecular chemistry is an ever growing research field owing to its crucial role in molecular catalysis, recognition, medicine, data storage and processing as well as artificial photosynthetic devices.Different isolated supramolecules were prepared by molecular self-assembly on surfaces. This review mainly focuses on supramolecular aggregations on noble metal surfaces studied by scanning tunneling microscopy, including dimers, trimers, tetramers, pentamers, wire-like assemblies and Sierpin′ ski triangular fractals. The variety of self-assembled structures reflects the subtle balance between intermolecular and molecule–substrate interactions, which to some extent may be controlled by molecules, substrates and the molecular coverage. The comparative study of different architectures helps identifying the operative mechanisms that lead to the structural motifs. The application of these mechanisms may lead to novel assemblies with tailored physicochemical properties.     

Electric-field-induced changes in scanning tunneling microscopy images of metal surfaces

Electric-field-dependent changes of scanning tunneling microscopy (STM) images of metal surfaces, metallic surface alloys, and ultra-thin magnetic films are predicted. We present a two-state model that demonstrates the general conditions leading to an inversion of the corrugation pattern as a function of field strength, and relate the effects to field-induced changes of the potential barrier. This image reversal for a pure metal surface corresponds to a change from a normal to an anticorrugated STM image. For ordered surface alloys, a switch of the imaged chemical species is possible and may even cause a change from a p(1×1) to a c(2×2) image at different fields. We explicitly demonstrate the effects by first-principles calculations of the Fe(001) surface.     

Identification of single-strand DNA by in situ scanning tunneling microscopy

We report here in situ scanning tunneling microscopy imaging of hexaammineruthenium (II)/(III) (RuHex) binding to single-strand DNA oligonucleotide with 13 bases immobilized in a mixed monolayer on a single-crystal Au(111) surface. RuHex does not bind to the other component in the monolayer, mercaptohexanol. Images before and after addition of RuHex show a strong contrast increase, suggesting that single-stranded DNA is, indeed, conducting when a redox group is bound to the strand. When compared with previous results, the data also suggest that the mechanism of domain formation of the immobilized DNA is independent of the sequence.     

Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.     

Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.     

Atomic-scale scanning tunneling microscopy study of plasma-oxidized ultrahigh-modulus carbon fiber surfaces

In the present work, scanning tunneling microscopy (STM) was employed to study the surface modification of ultrahigh modulus carbon fibers at the atomic level by oxygen plasma. As detected by STM, the distinctive feature of the fresh, untreated surface was the general presence of atomic-scale arrangements in different degrees of order (from atomic-sized spots without a clearly ordered disposition to triangular patterns identical to those typical of perfect graphite). Following fiber exposure to the plasma, the STM images showed evidence of the abstraction of carbon atoms from random locations on the fiber surface, giving rise to the development of defects (i.e., structural disorder), which in turn were the places where oxygen could be introduced during and after the plasma etching. It was observed that the most effective treatments in terms of extent of surface structural modification (disordering) and uniform introduction of oxygen were those carried out for just a few ( approximately 3) minutes. Considerably shorter exposures failed to provide a homogeneous modification and many locations on the fiber surface remained unaltered, retaining their original atomic-scale order, whereas longer treatments did not bring about further structural changes to the surface and only led to fiber consumption. These results are consistent with previous X-ray photoelectron spectroscopy measurements on these fibers and provide an atomic-level understanding of the saturation effect observed in the surface oxygen concentration of this and other types of carbon fibers with plasma oxidation. Such understanding may also prove helpful for the accurate control and optimization of fiber-matrix interaction in composite materials.     

Self-assembled gold nanoparticles on functionalized gold (111) studied by scanning tunneling microscopy

Nanogold colloidal solutions are prepared by the reduction of HAuClO4 with sodium citrate and sodium borohydride.4-Aminothiophenol (ATP) self-assembled monolayers (SAMs) are formed on gold(111) surface,on which gold nanopartides are immobilized and a sub-monolayer of the particles appears.This sub-monolayer of gold nanopartides is characterized with scanning tunneling microscopy (STM),and a dual energy barrier tunneling model is proposed to explain the imageability of the gold nanopartides by STM.This model can also be used to construct multiple energy barrier structure on solid/ liquid interface and to evaluate the electron transport ability of some organic monolayers with the aid of electrochemical method.     

In situ scanning tunneling microscopy of oxide-supported metal clusters: nucleation,growth, and thermal evolution of individual particles

An experimental approach was developed for imaging the nucleation and growth of individual oxide-supported nanoparticles and their subsequent in situ chemical and thermal treatments by scanning tunneling microscopy (STM). The potential of the method is demonstrated for Au nanoparticles supported on a reduced TiO(2) substrate where a cluster-by-cluster comparison is made of the morphological evolution and stability of nanoparticles during their nucleation and thermal annealing. Using this methodology the details of the nucleation and growth kinetics can be directly observed.     

Single carbon dioxide molecules on surfaces studied by low-temperature scanning tunneling microscopy

The chemical conversion of carbon dioxide (\(\hbox {CO}_2\)) has been intensively studied because the molecule is responsible for global warming. Rational design of catalysts plays an important role in converting \(\hbox {CO}_2\) into value-added compounds. Understanding the interaction between \(\hbox {CO}_2\) and surfaces of catalysts is a prerequisite to preparing high-performance catalysts. This review focuses on the investigations of \(\hbox {CO}_2\) molecules on single crystalline surfaces studied by low-temperature scanning tunneling microscopy. Molecular adsorption, diffusion, and conversion on metal surfaces, metal oxide surfaces, and surfaces decorated by metal-organic frameworks are summarized.     

Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM) of electrodispersed gold electrodes

Images of large active surface (electrodispersed) gold electrodes obtained by electroreduction of an oxide layer grown by applying a fast periodic square wave potential to polycrystalline specimens, have been obtained by STM and SEM. For the first time a correlation of imaging data of both microscopes can be established which allows one to find different structural details of these electrodes. The roughening increase produced by the electrochemical activation is explained tentatively through a generalized structural model consisting of an overlayer of sticking spheres of about 10 nm average diameter leaving inner interconnected channels of nearly the same average diameter and penetrating in the overlayer structure. The size of the spherical units is comparable to that of metal clusters involving the optimal ratio of surface to bulk atoms associated with the greatest catalytic activity.     

Single-electron charging features of larger, dodecanethiol-protected gold nanoclusters: electrochemical and scanning tunneling microscopy studies

In this report, we demonstrate the single-electron charging features of larger-sized (ca. 3.72 nm) Au nanoclusters protected with dodecanethiol [approximate composition, Au1415(RS)328] using combined electrochemical and scanning tunneling microscopic (STM) studies. In particular, these nanoclusters show a highly populated single-electron charging peak in voltammetric experiments, where the calculated capacitance is in good agreement with the experimentally obtained value of 1.6 aF. In comparison to the voltammetric studies, STM measurements over a single Au particle on the highly oriented pyrolytic graphite surface reveal nonlinear current-voltage (I-V) characteristics with a large central gap, signifying single-electron-transfer features. The I-V results demonstrate a clear Coulomb blockade effect with a central gap of around 0.2 eV, which is in good agreement with the orthodox theory for the double barrier tunnel junction system. The standard heterogeneous electron-transfer rate constant estimated from impedance measurements is found to be of 7.97 x 10(-6) cm.s(-1), suggesting that the process is very sluggish. Furthermore, diffusion coefficient (Dc) values calculated from chronoamperometry and impedance measurements are in good agreement with theoretically calculated values using the modified Stokes-Einstein equation. The electron-transfer rate constant estimated from cyclic voltammograms of adsorbed monolayer protected Au nanoclusters is found to be about 2 s(-1), which is slower than that reported for its smaller analogues.     

Cadmium underpotential deposition on Cu(111) in situ scanning tunneling microscopy

Atomically resolved in situ STM images are presented for an underpotentially deposited (upd) cadmium layer on a Cu(111) electrode from a 10(-4) M CdCl2/10(-2) M HCl solution. The observed moiré-like structure seen in the images is analysed by means of an algebraic model for this long-range superstructure. A structure model for the upd layer is developed which reflects all features of the observed moiré pattern. Furthermore the height modulation was simulated by a hard-sphere model for the Cd overlayer and shows remarkable agreement with the detailed tunneling current density distribution of the measured STM images. The existence of translational and rotational domains is demonstrated. The results are also compared and shown to be fully consistent with previous (ex situ) low-energy electron diffraction (LEED) observations of this system. The mechanism of Cd upd involves a dynamic site exchange between preadsorbed Cl- anions and adsorbing Cd2+ cations as previously concluded from ex situ X-ray photoelectron (XPS) and low-energy ion scattering (LEIS) measurements.     

Efficient transport of gold atoms with a scanning tunneling microscopy tip and a linker molecule

A thiophene-containing molecule attached to a scanning tunneling microscopy (STM) tip is used to transport gold atoms on a Au(111) surface. The molecule contains eight thiophene rings and therefore has sulfur atoms that are known to bind to gold atoms. Using a gold-coated tip, the molecules previously deposited on the surface bind to the lower-coordination gold atoms of the tip. When that tip is used to scan the surface, the still free thiophene rings (not all of the sulfur atoms bind to the tip) can attach to gold atoms from the surface and drag them along the scanning direction, depositing them either at the position where the tip changes its scanning direction or where the tip encounters an "up step", whichever event occurs first.     

Adsorption and in situ scanning tunneling microscopy of cysteine on Au(111): Structure, energy, and tunneling contrasts

The amino acid L-cysteine (Cys) adsorbs in highly ordered (3 square root of 3 x 6) R30 degrees lattices on Au(111) electrodes from 50 mM ammonium acetate, pH 4.6. We provide new high-resolution in situ scanning tunneling microscopy (STM) data for the L-Cys adlayer. The data substantiate previous data with higher resolution, now at the submolecular level, where each L-Cys molecule shows a bilobed feature. The high image resolution has warranted a quantum chemical computational effort. The present work offers a density functional study of the geometry optimized adsorption of four L-Cys forms-the molecule, the anion, the neutral radical, and its zwitterion adsorbed a-top-at the bridge and at the threefold hollow site of a planar Au(111) Au12 cluster. This model is crude but enables the inclusion of other effects, particularly the tungsten tip represented as a single or small cluster of W-atoms, and the solvation of the L-Cys surface cluster. The computational data are recast as constant current-height profiles as the most common in situ STM mode. The computations show that the approximately neutral radical, with the carboxyl group pointing toward and the amino group pointing away from the surface, gives the most stable adsorption, with little difference between the a-top and threefold sites. Attractive dipolar interactions screened by a dielectric medium stabilize around a cluster size of six L-Cys entities, as observed experimentally. The computed STM images are different for the different L-Cys forms. Both lateral and vertical dimensions of the radical accord with the observed dimensions, while those of the molecule and anion are significantly more extended. A-top L-Cys radical adsorption further gives a bilobed height profile resembling the observed images, with comparable contributions from sulfur and the amino group. L-Cys radical a-top adsorption therefore emerges as the best representation of L-Cys adsorption on Au(111).     

In situ scanning probe microscopy and new perspectives in analytical chemistry

The resolution of scanning tunnelling microscopy (STM) and other scanning probe microscopies is unprecedented but the techniques are fraught with limitations as analytical tools. These limitations and their relationship to the physical mechanisms of image contrast are first discussed. Some new options based on in situ STM, which hold prospects for molecular- and mesoscopic-scale analytical chemistry, are then reviewed. They are illustrated by metallic electro-crystallisation and -dissolution, and in situ STM spectroscopy of large redox molecules. The biophysically oriented analytical options of in situ atomic force microscopy, and analytical chemical perspectives for the new microcantilever sensor techniques are also discussed.     

In situ scanning tunneling microscopy investigation of sulfur oxidative underpotential deposition on Ag(100) and Ag(110)

Underpotential (UPD) deposition of sulfur from Na(2)S solution in 0.1 M NaOH was studied on Ag(100) and Ag(110) using in situ scanning tunneling microscopy (STM). The cyclic voltammogram on Ag(100) presents two broad peaks, whereas three partial overlapping peaks and a sharper one are observed on Ag(110). STM measurements carried out during the whole UPD process show that progressively more compact structures are formed as the applied potential is scanned toward more positive potentials. More precisely, p(2×2), c(2×6), and c(2×2) were found on Ag(100) at E = -1.25, -1.0, and -0.9 V, respectively. Less definite conclusions can be drawn for the structures of S overlayers on Ag(110). However, the experimental findings are consistent with an incomplete p(2×1) at potentials preceding the sharp peak, and with a c(2×2) structure at E = -0.9 V vs Ag/AgCl, KCl(sat). The coverage values calculated on the basis of the hypothesized structures have been compared with the values obtained from chronocoulometric measurements at the most positive potentials investigated. Thus, the experimental coverage θ = 0.5 coincides with the coverage calculated for the c(2×2) structure found on Ag(110) at E = -0.9 V by STM, whereas the experimental coverage θ = 0.42 suggests that a mixture of structures c(2×6) and c(2×2) is formed on Ag(100).     

Optically transparent Au{111} substrates: flat gold nanoparticle platforms for high-resolution scanning tunneling microscopy

We demonstrate a new type of Au{111} substrate that is both atomically flat and optically transparent, which consists of solution-grown flat gold nanoparticles (FGNPs) deposited on indium tin oxide (ITO)-coated glass. We show that FGNPs are atomically flat single-crystal plates with large {111} faces that expose only 2-4 atomic layers. These FGNPs are excellent platforms for alkanethiol self-assembled monolayers (SAMs) and for high-resolution scanning tunneling microscopy (STM). Our supported FGNPs are also low-cost Au{111} substrates, employing only basic wet chemical techniques in preparation. This approach should be broadly applicable to other types of substrates for scanning probe microscopies.     

In situ scanning tunneling microscopy imaging of mercury film electrodeposited on Ir(1 1 1)

In situ scanning tunneling microscopy (STM) was used to examine multilayer Hg film electrodeposited on a well-ordered Ir(1 1 1) single crystal electrode in 0.1 M HClO₄ + 1 mM Hg(ClO₄)₂. Topography STM scans showed that the Ir(1 1 1) – supported Hg film electrode contained well-defined terraces separated by monatomic steps (Δz = 2.3 Å). The STM’s tip could be used to induce local dissolution of the Hg deposit under proper operating conditions and the depth of the etched pit informed directly the thickness of Hg deposit. Although in situ STM imaging with a tungsten tip could not result in atomic structure of bare Hg film in 0.1 M HClO₄, it discerned highly ordered iodine adlayers, represented by a (2 × 8√3)rect – I structure, on the Hg film in solution containing potassium iodide. These STM results suggested that the Hg substrate could have an ordered atomic structure.