Search

The coadsorption of porphyrin molecules (TMPyP: tetra(N-methyl-4-pyridyl)-porphyrin), sulfate anions and copper on a Au(111) electrode was investigated by the use of cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy. With decreasing electrode potential the following sequence of surface phases was found: (I) an ordered $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ structure on the unreconstructed Au(111)-(1 × 1) surface; (II) a disordered SO42−-layer on the still unreconstructed Au(111)-(1 × 1); (III) a $$\left( {\sqrt 3 \times \sqrt 3 } \right)R30^\circ$$ coadsorption structure of 2/3 ML Cu and 1/3 ML SO42−; (IV) a completed 1 ML Cu covered by a layer of mobile, i.e. not imaged, SO42− anions, moreover, a coadsorption layer of disordered porphyrin molecules and still mobile SO42− anions; (V) overpotentially deposited Cu-multilayers terminated by the well known Moire-type modulated $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ structure (similar to bulk Cu(111)) and covered by a dense layer of flat lying TMPyP molecules showing a growing square as well as hexagonally ordered arrangement, and at even more negative potential values and low Cu concentrations in the solution (VI) a pseudomorphic underpotentially deposited Cu-monolayer covered by a $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ layer and a dense, ordered porphyrin layer ontop. The formation of the various phases is driven by the potential dependent surface charge density and the resultant electrostatic interaction with the respective ions. A severe imbalance between the copper deposition and desorption current in the CV spectra suggests also the formation of CuTMPyP-metalloporphyrin on the surface which diffuses into the bulk solution.

The early stages of electrodeposition of polypyrrole (PPy) on an Au(111) surface are studied using in-situ scanning tunneling microscopy under the electrochemical control of the process. Shape, size, nucleation density and dimensionality of the early growth of the polymer phase was of interest. Electropolymerization took place in aqueous electrolytes containing multicharged anions. The diameter of the smallest detected surface particulates was of the order of a few A in Na3AlF6 solution and a few tens of A in pyrrole - H2SO4 solution. In H2SO4 solution, the very early stages of the electropolymerization lead to flat islands with a height of about 2.0 A and a diameter of several tens of Angstrom on Au(111) terraces, which indicates the dominance of two-dimensional polypyrrole structures. The surface population of these islands observed in sulphuric acid increases with the number, range, and decreasing potential scan rate of voltage cycles. At later stages and higher coverages the STM images show a further growth of overlapping PPy globules, which change from two- to three-dimensional structures. In aqueous Na3AlF6 solution, the STM images showed lifting of the Au(111) surface reconstruction, which may be due to the chemisorption of OH−. The early polypyrrole deposition in aqueous sodium hexafluoroaluminate, visible with nano-scale horizontal resolution, results predominantly in a 2D growth of the polymer globules having about 2.0 A in height on the onset of the polymer deposition.

The adsorption of sulfate anions ( S O 4 2 − ) and copper cations on Au(1 1 1) from CuSO4 containing H2SO4 solution was investigated by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). The underpotential deposition from solution results first in the formation of 2/3 monolayer of copper with the well known ( 3 × 3 ) structure of sulfate followed by the completion of the first copper monolayer at lower potentials. Just after formation of 1ML of Cu STM reveals a disordered surface. At more negative potential values, already in the overpotential deposition region, the STM images reflect a ( 3 × 7 ) sulfate structure on terraces of the first pseudomorphic Cu layer, a ( 3 × 3 ) -like structure on terraces of the second uncompleted Cu layer with sulfate anions residing in atomic vacancies, and the known Moire structure on terraces of the multi-layer Cu deposit. The formation of the ( 3 × 7 ) sulfate structure on pseudomorphic monolayer Cu terraces on Au(1 1 1) is discussed in the context of results obtained for sulfate on bare Au(1 1 1) and Cu(1 1 1) electrodes in H2SO4 solution.

Motivated by some incomprehensible aspects of the electro-deposition of copper on a Au(111) surface from copper sulfate containing sulfuric acid solution published in the literature, we have revisited this system using cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Specifically, we have examined the different stages of Cu deposition associated with 2/3 ML, 1 ML, supposedly 5/3 ML and multilayer coverages, and present a consistent structure model for the coadsorbed sulfate overlayers on the copper deposits of varying thickness. At a Cu coverage of 2/3 ML the coadsorbed sulfate anions exhibit the well known (3×3)R30° structure. In the regime where the full pseudomorphic Cu monolayer is supposed to exist first the formation of a disordered sulfate layer is found, which only at more negative potentials transforms into a (3×7)R19.1° sulfate structure (on the first completed Cu monolayer). In view of the (3×7)R19.1° structures found on both the Au(111) and the bulk Cu(111) surface a (3×7)R19.1° structure on a pseudomorphic Cu monolayer on Au(111) appears much more likely than a (3×3)R30° structure as claimed in the literature. Moreover, this (3×7)R19.1° structure on 1ML Cu/Au(111) could only be found at rather negative potentials and, in particular, low Cu concentrations in the solution. Further deposition of Cu then leads again to the formation of a (3×3)R30° structure on a 2/3 filled and slightly contracted second Cu layer, i.e. at a local copper coverage of 5/3 ML. Consistent with this contraction the STM images indicate a weak Moire superstructure of the second layer. The formation of this (3×3)R30° structure is again rationalized by the adsorption of sulfate anions in atomic vacancies within the second Cu layer, similar to what was previously established for a copper coverage of 2/3 ML on Au(111). Cu islands, which exhibit multilayer thickness show the characteristic sulfate induced Moire structure similar, but not identical, to that known from bulk Cu(111).

A controlled electron transfer can be employed as a tool to drive the crafting of long-range self-assembled nanostructures of electro-active species on surfaces. In this work, we show this approach with the formation of an ordered bi-layer of tetra(N-methyl-4-pyridyl)-porphyrin molecules (TMPyP) on an iodine-modified Au(100) electrode studied by means of Cyclic Voltammetry (CV) and in situ Electrochemical Scanning Tunneling Microscopy (EC-STM). This bi-layer exists only towards negative electrode potentials between the first reduction potential of the adsorbed molecules and the desorption potential of the iodine. Starting a cathodic potential sweep at positive potentials, where the surface is covered with an adsorbed monolayer of TMPyP molecules which did not undergo any previous electrochemical reaction yet (Ref. Surfaces 2018, 1, 3 - 17), the formation of the ordered bi-layer coincides with the first reduction step of the adsorbed molecules; and it disappears again in a joint desorption process with the iodine. Reversal of the potential sweep leads to the opposite observations. Depending on the actual potential value as well as the scan direction up to five different competing ordered bi-layer phases can be identified. Synergistic effects between the modified substrate and the adsorbate are determined to be responsible of the corresponding structures, and at the same time surface-mediated catalytic effects shifting the first reduction step of the adsorbed molecules to a more positive potential in comparison to the reduction potential of the species from the bulk solution are observed. Moreover, based on detailed potentiodynamic STM measurements, a mechanism is also proposed to explain these observations.

In this contribution, we provide a view on the role of halide interlayers in the self-assembly of layers of organic molecules at metal-electrolyte interfaces with molecular resolution. In particular, the ordering of tetra(4-trimethylammoniophenyl) porphyrin molecules in cationic form, abbreviated as [H2TTMAPP]4+, has been studied on a Cu(111) electrode surface in aqueous solution containing chloride-, bromide- or iodide-anions using a combination of cyclic voltammetry (CV) and in-situ electrochemical scanning tunneling microscopy (EC-STM). On the one hand the obtained results unveil that the degree of ordering of the self-assembled layer of [H2TTMAPP]4+ cations on the Cu(111) surface changes significantly when changing the halide anion in the working electrolyte, and, thereby, the buffer layer between the Cu(111) surface and the adsorbed [H2TTMAPP]4+ molecular cations. Under the same experimental conditions a fully ordered layer is observed on the chloride precovered Cu(111) surface, but less ordering and even incomplete monolayer formation are found on the bromide and iodide terminated Cu(111) surface, respectively; the degree of order decreases with decreasing electronegativity of the underlying halide. On the other hand potential driven reduction of the organic species lowers the charge density of the organic cations themselves causing a desorption of the reduced molecules at even lower potentials from any of the three differently halide modified electrode surfaces. Both observations underline the importance of electrostatic interactions in the adsorption and structure formation of the porphyrin molecules on the halide modified surfaces.

The modelling of long-range ordered nanostructures is still a major issue for the scientific community. In this work, the self-assembly of redox-active tetra(N-methyl-4-pyridyl)-porphyrin cations (H2TMPyP) on an iodine-modified Au(100) electrode surface has been studied by means of Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) with submolecular resolution. While the CV measurements enable conclusions about the charge state of the organic species, in particular, the potentio-dynamic in situ STM results provide new insights into the self-assembly phenomena at the solid-liquid interface. In this work, we concentrate on the regime of positive electrode potentials in which the adsorbed molecules are not reduced yet. In this potential regime, the spontaneous adsorption of the H2TMPyP molecules on the anion precovered surface yields the formation of up to five different potential-dependent long-range ordered porphyrin phases. Potentio-dynamic STM measurements, as a function of the applied electrode potential, show that the existing ordered phases are the result of a combination of van der Waals and electrostatic interactions.

In this article, we focus on demonstrating the utility of scanning probe methods in the imaging of chemical reactions. We first highlight the utility of different imaging methods and highlight their advantages and drawbacks. Subsequently, we select a number of examples to illustrate different surface processes including adsorption, dissociation, diffusion and rotation of adsorbed molecules, formation of reaction intermediates, and conclude with complex reactions. In these examples, we mainly focus on the STM, which is most extensively employed as a method of choice. To limit the complexity of the article we have selected only a few systems for the discussion. In particular, elemental steps in the reactions of water, alcohols, and diols on TiO 2 (110) surface are utilized to illustrate the power of imaging techniques in our understanding of surface chemistry. We also provide a brief outlook on both current and future challenges in this exciting area of research.

The adsorption and electrochemical reaction of chloride anions at copper surfaces in hydrochloric acid solution is reviewed. Based on results from cyclovoltammetry, in situ scanning tunneling microscopy, X-ray diffraction, and reflectance anisotropy spectroscopy, as well as ex situ photoelectron spectroscopy and ion scattering spectroscopy measurements a unified description is given for the relative reactivity of the three low-index copper surfaces Cu(111), Cu(100), and Cu(110). Their reactivity toward chloride anions increases—not unexpectedly—in the order Cu(111)

Functional organic layers on solid surfaces play an important role in modern material science. Their deposition from solution is the method of choice for all those molecules which are not intact volatile. Electrochemical deposition using the respective molecular ions adds the electric potential of the substrate electrode as an extra parameter—besides concentrations, temperature, etc.—in order to control the growth and structure of the layers. Ordered layers stand out due to their particularly homogeneous properties.

Scanning tunnelling microscopy (STM) was used to investigate the nucleation and growth of palladium clusters on two different, ultrathin, epitaxial, titania films grown on a Pt3Ti(111) surface. The first oxide phase, z'-TiOx, is anisotropic and consists of parallel stripes separated by trenches. Defects (i.e., oxygen vacancies) in this structure are confined to these trenches and act as nucleation sites. Therefore, the Pd clusters are mostly arranged in unidirectional rows along the trenches, creating a template effect. The second phase, w'-TiOx, exhibits a hexagonal, long range, (7 × 7)R21.8°, Moiré-type superstructure with fewer and shallower defects, making the template effect less discernible.

On-surface polymerization realized at the solid-liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of ?-conjugated polymer formation at the interface between an iodine-modified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and ?-conjugated final product. The I/Au(111) substrate enhances the formation of ?-conjugated polymers, as established comparing their production on the surface and in the bulk solution.

The co-adsorption of submono-, mono- and multi-layers of Cu with sulfate anions on a Au(111) electrode surface was investigated in electrochemical environment (0.1 mM CuSO 4 + 0.1 M H 2 SO 4 ) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). Correlated with the STM investigations the CV measurements indicate co-adsorption/-desorption processes of Cu of submono-, mono- and multi-layer coverages with sulfuric acid species on Au(111). The formation of a quasi-hexagonal Moire superstructure on terraces of copper multilayers was observed in situ by STM. In detail the observed Moire-structure is similar but not identical to the one observed on the (111) surface of bulk copper. High resolution STM images show the formation of a 3 × 7 -like sulfate structure on all multilayer copper terraces.

Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium.

The adsorption of redox-active 5,10,15,20-Tetrakis(4-trimethylammoniophenyl) porphyrin tetra(p-toluenesulfonate) molecules, abbreviated as [H2TTMAPP]4+, on a chloride pre-covered Cu(111) electrode surface was studied in an electrochemical environment by means of combined cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). The [H2TTMAPP]4+ molecules adsorb spontaneously on the c p × 3 Cl/Cu(111) surface and form a highly ordered 2D monolayer once the supporting electrolyte (10 mM HCl) is replaced by the molecule containing electrolyte (10 mM HCl + 0.1 mM H2TTMAPP) at a potential of E = + 20 mV vs. RHE (reversible hydrogen electrode). This ordered phase undergoes a phase transition into a disordered one once the [H2TTMAPP]4+ molecules pass through the first reduction step at negative potentials. In contrast, at positive potentials, the copper corrosion process, taking place at step-edges with predominant 2 ¯ 11 directions, is not prevented by the presence of the [H2TTMAPP]4+ adlayer.

The adsorption of sulfide on a Au(100) electrode from a 0.01 M NaOH + 0.5 mM Na2S electrolyte was studied by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). Starting with a sulfur free electrode surface at low potentials the subsequent formation of a p(2 × 2)-S, a c(2 × 6)-S and a c(2 × 2)-S phase was observed during potential increase. Only the p(2 × 2)-S (ΘS = 0.25 ML) phase appears as a single species phase; the c(2 × 6)-S (ΘS = 0.33 ML) and the c(2 × 2)-S (ΘS = 0. 5 ML) phase are accompanied by the simultaneous appearance of additional “dimer” particles. Concomitant changes of the step morphology and the results of the electrochemical characterization strongly suggest that these additional particles contain both, sulfur and gold atoms. Upon further potential increase the formation of a phase consisting of ring-like units was found, which is well known for Au(111) surfaces in the presence of sulfur. This phase is, by analogy, interpreted as a gold sulfide compound formed under electrochemical conditions.

The time dependence of the formation of lotus wax tubules after recrystallization from various chloroform-based solutions on an HOPG surface at room temperature was studied by atomic force microscopy (magnetic AC mode) taking series of consecutive images of the formation process. The growth of the tubules oriented in an upright fashion follows a sequential rodlet→ring→tubule behavior. The influence of a number of factors, e.g., different wax concentration in chloroform, the additional presence of water, or salts [(NH4)2SO4, NH4NO3] or a mixture of salt/water in the solution on the growth rate and orientation of the tubules is also investigated. Different wax concentrations were found to have no effect on the growth rate or the orientation of tubules in none of the solutions. The presence of water, however, considerably increased the growth rate of tubule formation, while the presence of salt was again found to have no effect on growth rate or orientation of tubules.

Morphological changes of a bare Cu(110) substrate in 10 mM HCl aqueous solution have been studied using cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (EC-STM), and reflectance anisotropy spectroscopy (RAS). At cathodic potentials more positive than the hydrogen evolution reaction, a bare copper surface (1 × 1) structure is found by EC-STM. At anodic potentials more negative than the copper(II) dissolution reaction, a furrowed structure is found. The governing factor that rules Cu(110)-Cl interface processes is discussed as an interplay among Cl(-) adsorption/desorption, the dynamic rearrangement of the surface atoms on the substrate, and strain in order to reduce the surface energy. The information provided by EC-STM and RAS complements that of CV, supplies detailed information on the surface morphology, and correlates peaking Faraday currents to structural modifications. Furthermore, RAS and EC-STM show changes in the surface appearance in a potential range where no specific charge transfer is observed. CV indicates that the Cu(110) surface chemistry compares much better to that of amorphous Cu than to that of the more stable (100) and (111) surfaces, respectively.

Ordered ultrathin titanium oxide films have been produced under ultrahigh vacuum (UHV) conditions by oxidation of a Pt3Ti(111) single crystal at elevated temperatures. Depending on substrate temperature, oxygen dosage, and partial pressure, four different titania phases have been observed. All phases have been investigated by low- energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Two commensurate phases with a rect-(6 × 3√3) and a hex-(7 × 7)R21.8° superstructure, respectively, are obtained at low oxygen pressures. Both structures form homogeneous films, which wet the complete surface and are stable against further annealing. At high oxygen partial pressures two incommensurate structures can be prepared. The first one contains several holes to release stress arising from the lattice mismatch between the oxide film and substrate. The second one shows a very rough surface morphology and is, like the other incommensurate phase, unstable against thermal treatment. The structures of the different phases are very similar to the structures found for the systems TiOx/Pt(111), VOx/ Rh(111), and VOx/Pd(111), which have been described extensively in the literature.

Scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES) have been used to investigate the growth morphology of ultra-thin Pb films on the Ni3Al(111) face at room temperature. A previous study [K. Miśków and A. Krupski Appl Surf Sci 273, 2013, 554] using low-energy electron diffraction (LEED) and real time Auger intensity recording has demonstrated that an initial two-dimensional growth of the first Pb monolayer thick ‘wetting layer’ takes place. With further deposition and for T = 300 K, flat three atomic-layer-high islands are grown. Above 350 K, the Stranski–Krastanov growth mode was observed. In the current study, the analysis of STM measurements indicate and confirm that for coverage θ = 1.0 ML two-dimensional growth of the first Pb monolayer took place. Above θ > 1.0 ML, a three-dimensional growth of the Pb islands was observed with a strongly preferred atomic-scale ‘magic height (N),’ hexagonal shape and flat-tops. At coverage θ = 3.5 ML, only islands containing N = 3, 5, 7 and 11 atomic layers of Pb are observed. At the higher coverage θ = 5.5 ML, three types of regular hexagonal islands with side lengths of 25, 30 and 45 nm are observed. Furthermore, three different island adsorption configurations rotated by 10° ± 1° and 30° ± 6° with respect to each other were observed. After an annealing at T = 400 K of 5.5 ML of lead deposited at RT on the Ni3Al(111) the morphology of the surface changes. Post-anneal, islands of Pb are observed above the ‘wetting layer’ with an estimated average size and diameter of 768 nm2 ± 291 nm2 and 38.17 ± 6.56 nm and constant uniform height of two atomic layers (N = 2).

In the present work the effect of the intramolecular steric flexibility on the structural self-assembly of organic cations and their redox activity at a chloride precovered Cu(100) electrode is investigated. In particular the adsorption of 1,1′-dibenzyl-4,4′-(propane-1,3-diyl)dipyridinium (C3-DBDP) is studied by means of cyclic voltametry (CV), in situ scanning tunneling microscopy (EC-STM) and ex situ X-ray photoelectron spectroscopy (XPS) and the experimental results are compared to previously published findings on related bipyridinium (“viologen”) molecules. The CV measurements reveal a loss of the redox activity of the more flexible C3-DBDP2 + compared to dibenzylviologen (DBV2 +), as the first electron reduction step of C3-DBDP2 + does not appear within the potential window of the Cu(100), but is shifted below the hydrogen evolution regime. This reduced redox activity is the result of the lifting of the extended π-system of the bipyridinium core by introducing the propyl chain between the two pyridinium rings. In agreement with this result, XP spectra prove that the C3-DBDP2 + cations retain their initial dicationic charge within the entire potential window in solution but also upon adsorption on the Cl-c(2x2)/Cu(100) substrate, where they are found to form an inter-linked stripe phase. The building blocks of each stripe are attributed to one pyridinium-benzyl moiety, which represents half of one C3-DBDP2 + molecule. The resulting consecutive arrangement of half C3-DBDP2 + molecules along one stripe is stabilized by electrostatic attraction between the positively charged pyridinium rings and the negatively charged π-system of the benzyl rings.

Available online 21 August 2012Keywords:Self-assemblyPorphyrinCyclic voltammogramScanning tunneling microscopy Combined Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM)studies were employed to compare the self-assembly of redox-active 5,10,15,20-Tetrakis (4-trimethyl am-monium phenyl) porphyrin tetra (p-toluenesulfonate), abbreviated as [H 2 TTMAPP] +4 on Cu(100) andCu(111) electrode surfaces in 10 mM HCl solution. Under these conditions, both surfaces are chloridepre-covered with a c(2×2) Cl- and cpffiffiffi3pCl-layer on Cu(100) and Cu(111), respectively. On both sur-faces highly ordered [H 2 TTMAPP] +4 layers are spontaneously formed. The short range molecular arrange-ment is quadratic in nature on both surfaces which can be described by affiffiffiffiffiffi58pffiffiffiffiffiffi58pR23° unit cell onthe c(2×2) Cl-Cu(100), and a (3×4) unit cell on the cpffiffiffi3pCl-Cu(111) substrate. Large scale domainsare formed on both surfaces the orientation of which reflects the symmetry of the substrate, namely onCu(100) [Cu(111)] equivalent domains are rotated by 90° [120°], respectively. Thus, the different symmetryof the respective substrate surface has an influence on the otherwise self-assembled organic molecules.© 2012 Elsevier B.V. All rights reserved.

The adsorption of metal-free phthalocyanine molecules on an anisotropic Au(1 1 0)(1 × 2) surface has been studied with ultraviolet (UV) photoemission, low-energy electron diffraction and low-temperature scanning tunneling microscopy. In all cases, the molecules form rows in the [1 [Formula: see text] 0] direction, i.e. along the troughs of the reconstructed substrates. However, depending on the exposure and adsorption temperature, the substrate maintains (1 × 2)- or transforms into a (1 × 3)-reconstruction, and the molecular separation along the rows shrink from six to five times the Au-Au interatomic distance. The results are in agreement with previous density functional theory (DFT) calculations.

Metal-free phthalocyanine (Pc) molecules adsorbed on the Au(110) surface have been studied both experimentally (STM, LEED) and with density functional calculations. A strong interaction between substrate and adsorbate is observed. On the one hand, a clear template effect of the anisotropic substrate is observed: already at low coverages, the Pc molecules adsorb in various typical row patterns. On the other hand, the molecular adsorption modifies the substrate: at coverages higher than 0.25 monolayers, the usual (1 × 2) reconstruction is converted to a (1 × 3) reconstruction. First principle DFT calculations yield adsorption geometries that agree with the measured STM images and adsorption energies in the range of 2–3 eV. The adsorption leads to covalent and van der Waals interactions between adsorbate and substrate and is accompanied by a considerable charge transfer.

The redox behavior and potential dependent adsorption structure of heptyl viologen (1,1'-Diheptyl-4,4'-bipyridinium dichloride, DHV(2+)) on a Cu(1 0 0) electrode have been investigated by Cyclic Voltammetry (CV), in situ Electrochemical Scanning Tunneling Microscopy (EC-STM) and Infrared Reflection Absorption Spectroscopy (IRRAS). Current peaks in CV measurements of the redox-active DHV molecule are ascribed to two one-electron transfer steps and phase transformation processes. A highly ordered 2D dot-array structure emerges on the c(2 x 2)-Cl modified Cu(1 0 0) electrode surface between +200 and -100 mV in STM images. One-electron reduction of the dications DHV(2+) around -150 mV causes a phase transition from a dot to a stripe pattern which has a bi-layer structure in the STM images. With a further decrease of the applied potential, the structure of the adlayer undergoes a further change from the bi-layer phase to a close-packed monolayer of stripes. An IRRAS analysis on the orientation of the bipyridine moieties reveals a slight tilt of the long molecular axis with respect to the surface in the dicationic phase, but a parallel orientation in the monocationic striped phases. Additionally a large fraction of the hydrocarbon chains are in gauche conformation with low order among them. Only in the bi-layer stripe phase evidence for a marginal order between the alkyl chains was found. Calculation of the average angle of the alkyl chains with respect to the surface normal reveals an orientation along the surface within the dot structure but an outward tilt within the closed packed monolayer due to space limitation between the stripes. (C) 2011 Elsevier B.V. All rights reserved.

Ultrathin cerium oxide films have been prepared by oxidizing Ce-Pt/Pt(111) surface alloys with 65 L O2 at 900 K. According to low electron energy diffraction data, phonon spectra recorded by high resolution electron energy loss spectroscopy and scanning tunneling microscopy measurements the films are of fluorite-type CeO2(111) structure. They grow three dimensionally as islands, which are flat and up to several hundred square nanometers large. The film thickness varies between one and five layers. The band gap of the oxide films has been probed by scanning tunneling spectroscopy and turned out to be significantly smaller than for bulk CeO2.

Silicon carbide alloys are widely used in high-tech applications due to their interesting combination of chemical, mechanical and electronic properties. Growing thin films of this material in a simple and controlled way is a hot topic in modern material's science. In particular, the possibility to tailor the film properties just by tuning the deposition temperature would be an important progress. In the present work amorphous silicon–carbon alloys thin films have been deposited by electron beam sublimation of a poly-crystalline silicon carbide target in vacuum environment. The deposition temperature was varied from Room Temperature to about 1300 K. The resulting films were analyzed by means of Ultra High Vacuum–Atomic Force Microscopy (UHV–AFM) down to even atomic resolution. The observed features agree with literature data, e.g. interatomic bond lengths, as achieved by others methods, and the structural arrangements of silicon and carbon atoms as concluded from IR and Raman spectroscopy measurements carried out on the same samples. The results not only allow a correlation between film properties and deposition temperature but also support the notion of the UHV – AFM images of the amorphous surfaces being atomically resolved.

In this paper we present, for the first time, the results from Atomic Force Microscopy (AFM) surface studies from freshly fractured Baltic Amber samples, carried out under ultrahigh vacuum (UHV) conditions from micrometer to nanometer resolution. The micrometric AFM images provide a structural clue to the birefringent behavior occasionally observed with amber samples. Two-dimensional pair-distance distributions of the nanometric AFM images prove the completely amorphous structure of the material. This, together with the detection of individual motifs such as aromatic rings, supports the notion of amber being an amorphous polymeric organic network, consistent with the accompanying X-Ray Photoelectron spectroscopy (XPS) data. No nanocrystalline inclusions could be found. The results also show that it is possible to obtain atomically resolved AFM images from amorphous dielectric surfaces.

CO adsorption on clean and oxidized Pt3Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt3Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt3Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt3Ti(111) surfaces ( 220 L O2 exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.

A thin Al2O3/Ni3Al(111) film was prepared under ultrahigh vacuum conditions by surface oxidation of a Ni3Al(111) single crystal. Using scanning tunneling microscopy, it was found that the film does not cover the substrate entirely, which allows two surfaces and their adsorption properties to be investigated in a single study. The sample was subsequently exposed to the vapor of an organometallic compound, Ru3(CO)9(μ-SnPh2)3. The interaction between the ligand sphere of the adsorbate and the relatively inert oxide film favors diffusion rather than static adsorption; however, some coverage is observed also on alumina. Upon heating, the images of the sample surface suggest that the bimetallic centers of the molecules lose their ligands and nucleate as particles on the surface. On the oxide film, the particles grow three-dimensionally, whereas they do not go beyond monolayer thickness on the unoxidized surface areas. Particles can be found on the oxide even after heating the sample as high as 925 K, despite a ...