A novel surface-confined C-C coupling reaction involving two carbene molecules and a water molecule was studied by scanning tunneling microscopy in real space. Carbene fluorenylidene was generated from diazofluorene in the presence of water on a silver surface. While in the absence of water, fluorenylidene covalently binds to the surface to form a surface metal carbene, and water can effectively compete with the silver surface in reacting with the carbene. Water molecules in direct contact with fluorenylidene protonate the carbene to form the fluorenyl cation before the carbene can bind to the surface. In contrast, the surface metal carbene does not react with water. The fluorenyl cation is highly electrophilic and draws electrons from the metal surface to generate the fluorenyl radical which is mobile on the surface at cryogenic temperatures. The final step in this reaction sequence is the reaction of the radical with a remaining fluorenylidene molecule or with diazofluorene to produce the C-C coupling product. Both a water molecule and the metal surface are essential for the consecutive proton and electron transfer followed by C-C coupling. This C-C coupling reaction is unprecedented in solution chemistry. © 2023 American Chemical Society. All rights reserved.

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification. © 2023, The Author(s).

Solvents are increasingly known to influence chemical reactivity. However, the microscopic origin of solvent effects is scarcely understood, particularly at the individual molecule level. To shed light on this, we explored a well-defined model system of water (D2O) and carbon monoxide on a single-crystal copper surface with time-lapsed low-temperature scanning tunneling microscopy (STM) and ab initio calculations. Through detailed measurements on a time scale of minutes to hours at the limit of single-molecule solvation, we find that at cryogenic temperatures CO-D2O complexes are more mobile than individual CO or water molecules. We also obtain detailed mechanistic insights into the motion of the complex. In diffusion-limited surface reactions, such a solvent-triggered increase in mobility would substantially increase the reaction yield. © 2023 American Chemical Society.

Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study the tautomerization reaction of an H2Pc molecule adsorbed on a Ag(100) surface. The presence of two hydrogen atoms in the cavity of the H2Pc molecule enforces the existence of two molecular tautomers. It causes a reduction from 4- to 2-fold symmetry in STM images that can be recorded as two current states over the H2Pc molecule with a high-to-low current state ratio of ∼1.2. These findings are confirmed by the spatial distributions of the all-atom electron charge density calculated by using DFT and transmission maps together with tunneling current ratios (∼1.2) determined from the nonequilibrium Green's function transport calculations. Therefore, we demonstrate that an H2Pc molecule adsorbed on a Ag(100) surface is a good candidate for a bistable molecular conductance switch since neither the presence of the Ag(100) surface nor that of the STM tip alters the tautomerization. © 2022 American Chemical Society.

The adsorption orientation of molecules on surfaces influences their reactivity, but it is still challenging to tailor the interactions that govern their orientation. Here, we investigate how the substituent and the surface structure alter the adsorption orientation of halogenated benzene molecules from parallel to tilted relative to the surface plane. The deviation of the parallel orientation of bromo-, chloro-, and fluorobenzene molecules adsorbed on Cu(111) and Cu(110) surfaces is determined, utilising the surface selection rule in reflection-absorption infrared spectroscopy. On Cu(111), all three halogenated molecules are adsorbed with their molecular plane almost parallel to the surface at low coverages. However, they are tilted at higher coverages; yet, the threshold coverages differ. On Cu(110), merely bromo- and chlorobenzene follow this trend, albeit with a lower threshold for both. In contrast, fluorobenzene molecules are tilted already at low coverages. The substantial influence of the halogen atom and the surface structure on the adsorption orientation, resulting from an interplay of molecule-molecule and molecule-surface interactions, is highly relevant for reactivity confined to two dimensions. © 2022 the Owner Societies.

Although the formation of insulating ionic layers on metal surfaces has been well studied, their growth mechanisms are still controversial. Here, we report several innovative approaches to trigger in situ growth to understand this growth mechanism. The modification of the layer and its in situ growth is followed by time-lapsed scanning tunneling microscopy at room temperature with atomic resolution. The NaBr molecules form bilayer and trilayer islands when deposited at room temperature. These stable layers begin to disintegrate when the voltage exceeds the threshold voltage during scanning. The molecules released from the modified layer subsequently attach to the preexisting layer in a predefined scan region. Scanning of two neighboring trilayer islands traps the mobile molecule between them, leading to their coalescence. Time-lapsed measurements offer a step-by-step realization of the in situ controlled growth of an ionic layer at the atomic scale. © 2022 American Chemical Society.

The distribution of water on metal supported oxides is an important step in understanding heterogeneous catalysis such as in the water gas shift reaction. Here, we study water structures on Ag(111)-supported graphitic zinc oxide islands by variable temperature scanning tunneling microscopy around 150 K and ab initio calculations. Water clusters, accumulating on the ZnO islands, are confined to the hcp regions of the ZnO moiré pattern. A significantly higher cluster density at the island border is related to the dimensions of its capture zone. This suggests an upward mass transport of the water from the supporting metal to the ultrathin oxide film, increasing the water density at the active metal-oxide border. © 2022 American Chemical Society. All rights reserved.

Halogen bonding has recently been recognized as an interaction whose relevance is on par with hydrogen bonding. While observed frequently in solution chemistry, the significance of halogen bonds in forming extended supramolecular structures on surfaces is less explored. Herein, we report on the self-assembly of chlorobenzene molecules adsorbed on the Cu(111) surface into nanosized clusters at submonolayer coverages, where the molecular planes are close to parallel to the surface. A comprehensive study of the role of intermolecular interactions through both halogen and weak hydrogen bonds on nanocluster formation is presented, gained by combining the results of temperature-programmed desorption, reflection-absorption infrared spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Based on an unprecedented precise determination of the molecules’ orientation within the clusters, the binding motifs that lead to the formation and stability of nanoclusters with magic sizes are identified and explained. A complex and delicate interplay of halogen bonds with weak hydrogen bonds, van-der-Waals forces, and surface–adsorbate interactions leads to a preference for hexamers and tetramers with an observable propensity for halogen bonding over weak hydrogen bonding when adsorbed to the Cu(111) surface. © 2021 American Chemical Society

Water nucleation on alkali precovered metal-supported oxide surfaces is an important step in understanding water as one of the reactants in alkali-assisted heterogeneous catalysis. For instance, alkali metals as catalyst dopants enhance the water-gas shift reaction that catalyzes on ZnO-metal nanostructures. Here, we investigate the hydration of cesium on a Ag(111)-supported graphitic zinc oxide ultrathin film using scanning tunneling microscopy at (160 ± 30) K. Upon hydrating the pristine graphitic ZnO film, the water forms well-separated clusters on the hcp regions of the ZnO moiré pattern at water coverages below 85% ML. In the presence of cesium on the fcc regions of the ZnO moiré pattern, the water clusters coalesce across hcp regions at water coverages above ∼32% ML, forming unique one-dimensional water-Cs chains along the high-symmetry directions of the ZnO moiré pattern. Our study demonstrates that the alkali doping of an oxide surface alters the dimensionality of water structures redirecting it partially to other adsorption regions, possibly influencing its reactivity. © 2022 American Chemical Society.

Water diffusion across the surfaces of materials is of importance to disparate processes such as water purification, ice formation, and more. Despite reports of rapid water diffusion on surfaces the molecular level, details of such processes remain unclear. Here, with scanning tunneling microscopy, we observe structural rearrangements and diffusion of water trimers at unexpectedly low temperatures (<10 K) on a copper surface, temperatures at which water monomers or other clusters do not diffuse. Density functional theory calculations reveal a facile trimer diffusion process involving transformations between elongated and almost cyclic conformers in an inchworm-like manner. These subtle intermolecular reorientations maintain an optimal balance of hydrogen-bonding and water–surface interactions throughout the process. This work shows that the diffusion of hydrogen-bonded clusters can occur at exceedingly low temperatures without the need for hydrogen bond breakage or exchange; findings that will influence Ostwald ripening of ice nanoclusters and hydrogen bonded clusters in general. © 2021 American Chemical Society

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.

Microscopic insight into interactions is a key for understanding the properties of heterogenous interfaces. We analyze local attraction in noncovalently bonded Xe-Cs+ aggregates and monolayers on Cu(111) as well as repulsion upon electron transfer. Using two-photon photoemission spectroscopy, scanning tunneling microscopy, and coupled cluster calculations combined with an image-charge model, we explain the intricate impact Xe has on Cs+/Cu(111). We find that attraction between Cs+ and Xe counterbalances the screened Coulomb repulsion between Cs+ ions on Cu(111). Furthermore, we observe that the Cs 6s electron is repelled from Cu(111) due to xenon's electron density. Together, this yields a dual, i.e., attractive or repulsive, response of Xe depending on the positive or negative charge of the respective counterparticle, which emphasizes the importance of the Coulomb interaction in these systems. © 2021 American Physical Society.

The restriction imposed by the lattice structure of different surfaces is used to investigate the influence of the distance between two monomers on their ability to bind to each other. We compare the interaction of ammonia monomers at two distinct distances imposed by the surface structure of a Cu(511) high-index surface to that of a Cu(110) low-index surface using low-temperature scanning tunneling microscopy, inelastic tunneling spectroscopy, and density functional theory. Frustrated translational and rotational modes, the Mulliken and Bader charge analyses, and electrostatic potential mapping indicate chemisorption of ammonia monomers on both surfaces, with their dipoles oriented perpendicular to the surface plane. At a larger intermolecular distance of around 0.51 nm on step edges of Cu(511), the monomers slightly repel each other due to electrostatic repulsion. At a shorter distance of around 0.36 nm perpendicular to the close-packed rows on Cu(110), a noticeable charge transfer between adjacent monomers indicates binding, that is, dimer formation in parallel orientation. This binding energy of the molecules compensates for the electrostatic repulsion. Our results outline how the choice of the surface structure may be utilized to alter the intermolecular interaction of solvent molecules and to enforce or suppress dimer formation. ©

We investigate the polymorphism of complexes formed by the hydration of a functionalized azobenzene molecule by low-temperature scanning tunneling microscopy. Under conditions at which the water-less azobenzene molecules remain as monomers on Au(111), co-adsorption of water leads to water-azobenzene complexes. These complexes prefer to adopt linear arrangements of the azobenzene mediated by its functionalized end groups. Such structures may serve as model systems for investigating the influence of a solvent on a surface reaction. © 2021 Author(s).

Though largely influencing the efficiency of a reaction, the molecular-scale details of the local environment of the reactants are experimentally inaccessible hindering an in-depth understanding of a catalyst's reactivity, a prerequisite to maximizing its efficiency. We introduce a method to follow individual molecules and their largely changing environment during a photochemical reaction. The method is illustrated for a rate-limiting step in a photolytic reaction, the dissociation of CO2 on two catalytically relevant surfaces, Ag(100) and Cu(111). We reveal with a single-molecule resolution how the reactant's surroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to a volcano-like reactivity. Our unprecedented local view during a photoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH

We investigate superstructures formed by CO2 on Ag(100) and Cu(111) from small clusters forming at 21 K up to multilayers grown at 43 K by low temperature scanning tunneling microscopy. On both surfaces, CO2 nucleates only at defects, here at co-adsorbed CO. At the lower adsorption temperature, superstructures of different symmetry coexist on both surfaces at submonolayer coverage, while the superstructures formed at the higher adsorption temperature differ largely for the two surfaces. On Ag(100), the CO2 monolayer exhibits a long-range order interrupted by antiphase domain boundaries. On Cu(111), a random distribution of domain structures of different symmetry leads to a monolayer without long-range order. Surprisingly, the degree of ordering is inverted for the 2nd layer of CO2. On Ag(100), the coexistence of different superstructures in the 2nd layer leads to reduced long-range order. On Cu(111), a hexagonal 2nd layer exhibits long-range order. A layer of a similar superstructure, hexagonal with long-range order, exists as the 3rd layer of Ag(100). Despite the different substrates, a multitude of common structural features of CO2 exist. Hexagonal layers grow with a long-range order on less ordered layers on both surfaces. Our results suggest that the preferred structure of a CO2 layer is hexagonal. © 2020 the Owner Societies.

We investigate the adsorption geometry of a three-dimensional organic molecule, anilino-nitro azobenzene, within hydrogen-bonded supramolecular structures on Au(111) by low temperature scanning tunneling microscopy. Therein, three conformational isomers exist, completely planar trans and cis-isomers and a non-planar, but surface adapted cis-isomer. The anilino-end of the molecule is planar for all isomers. In contrast, the nitro-end of the cis-isomer is only planar, if the nitro-end of the molecule forms hydrogen bonds. Our study pinpoints the subtle balance between molecule-substrate and molecule-molecule interaction in adsorption-induced bond-angle distortion that drive partial or full planarization of the molecule. © 2019 Elsevier B.V.

Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-Temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation. Copyright © 2020 American Chemical Society.

We investigate the shape of monatomic high Cu islands on a Cu(111) surface by variable-temperature scanning tunneling microscopy between 110 K and 240 K. Low temperature dendrites evolve towards more compact shapes at increasing temperature; finally reaching the equilibrium shape of a hexagon with rounded corners. Time-lapsed imaging at increasing temperature reveals the onset of shape change to be at ≈170 K, corresponding to the onset of edge and corner diffusion of atoms along the island's borders. Despite a substantial variation for individual islands at each temperature, the mean fractal dimension increases monotonously between 170 K up to 240 K, from the smallest to the largest values feasible for islands grown on surfaces. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

We follow the decay of two-dimensional Ag nanoclusters, called islands, on Cu-Ag core-shell supports by variable low temperature scanning tunneling microscopy in the temperature range between 160 and 260 K. We reveal two qualitatively different types of decay mechanisms, either linear in time, indicative of an interface-limited decay, or non-linear in time, indicative of diffusion-limited decay. In contrast to conventional decay on monometallic supports, the decay exponent of the diffusion-limited decay depends on temperature; it varies by one order of magnitude. Moreover, the decay rate decreases with increasing temperature. This unusual behaviour is traced back to the temperature-dependent shell of the core-shell support. © 2019 The Royal Society of Chemistry.

A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.

We investigate the induced growth of a Ag layer on a Cu(111) surface by variable low-temperature scanning tunneling microscopy between 100 and 140 K at submonolayer coverage. Without any interference by the scanning process, the Ag atoms form a two-dimensional gas on the Cu(111) surface. Imaging the surface at elevated voltage induces nucleation and growth of one-dimensional Ag stripes of monolayer height, eventually filling the surface of the imaged area completely. The stripes consist of rods of atoms with a preferential length of (1.88 ± 0.10) nm, corresponding to approx. seven or eight Ag atoms on eight to nine Cu hollow sites. At a ratio of approximately 1:3, rods of double length are the second most observed species. The rods stack in the 112 directions at the √3 distance of Cu(111). Although all equivalent three surface directions are observed, their abundance is not equally distributed, such that the rod direction aligned with the fast scanning direction predominates. At slow growth rates, it is possible to create a striped pattern with one surface direction only. Copyright © 2019 American Chemical Society.

We investigate the decay of Ag islands on Cu(111) by variable low temperature scanning tunneling microscopy between 195 and 250 K. Such islands exhibit a misfit dislocation pattern forming (8×8) to (10×10) superstructures because of a major lattice mismatch between silver and copper. The decay of islands smaller than 200nm2 alternates between a slower and a faster decay. It is slower for specific island sizes, in particular those with magic numbers of superstructure unit cells. We relate these changes to the complexity of the heteroepitaxial decay, involving a deconstruction of the misfit dislocation pattern and a simultaneous diffusion of several adspecies during decay. © 2019 American Physical Society.

We report the influence of lithium ions on binding and structure of water nanoislands on Au(111) by temperature-programmed desorption and variable-temperature scanning tunneling microscopy. Water coverages between a fraction and full bilayer and two lithium coverages (<0.15% ML) are explored. Lithium enhances selectively the binding of some of the water molecules on precovered Au(111) as compared to water on pristine Au(111), which is revealed by an increase of the water desorption temperature by approx. 10 K. Surprisingly, the effect of lithium on the structure of water is much more extended than expected from these desorption experiments. A small amount of lithium changes the structure of water nanoislands drastically compared to those on pristine Au(111). On pristine Au(111), water ice grows in the form of crystalline islands that are two or three bilayers high. On Li precovered Au(111), the islands are more corrugated, at a 5 times broader apparent height distribution and much smaller, at a 4 times smaller area distribution. These changes reflect the influence of lithium as a structure maker, or kosmotrope, on water. Our study provides unprecedented real-space information of the influence of a kosmotrope on the water structure at the nanoscale. We utilize its kosmotropic behavior to provide real-space images of desorption. © Copyright © 2019 American Chemical Society.

If a material grows on another material with a largely different lattice constant, which of the two adapts for an energetically favorable growth? To tackle this question, we investigate the growth of Ag on Cu(111) by variable temperature scanning tunneling microscopy. The structures grown between 120 and 170 K are remarkably different from those grown between 200 and 340 K. The low-temperature structure is rectangular-like and consists of stacked rods, 7 to 8 Ag atoms long, which form a superstructure without long-range order. This structure covers the whole surface prior to nucleation of further layers. The high-temperature structure is hexagonal and consists of misfit dislocations forming 8 × 8 to 10 × 10 superstructures. For this structure, second layer nucleation sets in far before the closure of the first monolayer. While both structures are driven by the large lattice misfit between the two materials, the growing Ag layer adapts to the Cu surface at low temperature, while the Cu surface adapts to the growing Ag layer at higher temperature. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

Low-temperature scanning tunneling microscopy was used to follow the formation of a solvation shell around an adsorbed functionalized azo dye from the attachment of the first water molecule to a fully solvated molecule. Specific functional groups bind initially one water molecule each, which act as anchor points for additional water molecules. Further water attachment occurs in areas close to these functional groups even when the functional groups themselves are already saturated. In contrast, water molecules surround the hydrophobic parts of the molecule only when the two-dimensional solvation shell closes around them. This study thus traces hydrophilic and hydrophobic properties of an organic molecule down to a sub-molecular length scale. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have observed the inversion of the solvation environment of a one-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9-diazofluorene on Ag(111) yields highly oriented supramolecular chains, which are then exposed to water molecules. The annealing of dry and water-decorated chains results in diametrically opposed outcomes. While the former simply leads to an increase in chain length and number, the latter results in a complete loss of order and produces water clusters decorated with the organic molecule. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

We follow the diffusion of CO molecules on Cu(111) by time-lapsed low-temperature scanning tunneling microscopy. The diffusivity of individual CO molecules oscillates with the distance to its nearest neighbor due to the long-range interaction mediated by the surface state electrons. The markedly different wavelengths of the oscillation at a coverage of 0.6% ML as compared to the one at 6% ML coverage correspond to two different wavelengths of the surface state electrons, consistent with a shift of the surface state by 340 meV. This surprisingly large shift as compared to results of averaging methods suggests a local modification of the surface state properties. © 2018 American Physical Society.

Although the wetting of hydrophilic surfaces on the macroscale is a well-known phenomenon, we here develop a microscopic understanding of the more recently discovered complete covering of hydrophobic surfaces by a uniform water layer. For this aim, we deposit D2O on Ag(111) at two different temperatures, 20 and 96 K, and investigate the geometry of the layers at coverages up to four bilayers (BL) by low-temperature scanning tunneling microscopy. In the coverage range up to 0.5 BL, the ice grows in the form of islands that differ largely in size, shape, and density, but surprisingly not in their height. Moreover, the water fills the layer with islands of the same thickness of three to four bilayers at both temperatures. The different island shapes and densities in the coverage range before coalescence are attributed to details in the interaction between water nanoclusters and activated cluster diffusion at the higher growth temperature of 96 K, as visualized in time-lapsed series of scanning tunneling microscopy images. © Copyright 2018 American Chemical Society.

We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the π∗ orbitals of the halobenzenes from where they are transferred to σ∗ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules. © 2018 American Physical Society.

We observe the transformation of fractal ice islands grown at 96 K to compact ones annealed at 118 K and compare those to compact islands grown directly at 118 K. The low-temperature grown islands form a four bilayer high wetting layer. The annealing causes a crystallization and reshaping of the islands and a substantial increase in height and roughness in particular at higher coverage. Moreover, it leads to a dewetting of the ice film. The islands grown at the higher temperature show qualitative similarities to the annealed ones at smaller nucleation density. However, their orientation with respect to the surface differs by 30° as compared to the annealed islands. © 2017 American Chemical Society.

We investigate the influence of the annealing temperature on the evolution of the ice nanoclusters’ geometry by means of low-temperature scanning tunneling microscopy. The clusters, grown at 110 K on Ag(100), gradually increase in height and their shape becomes more compact during annealing at 120 K, 125 K, and 130 K. The increase in height indicates an upward mass transport and reflects a stronger water-water than water-surface bonding. The change in shape, quantitatively expressed as an increase in fractal dimension, is driven by a reduction of the total energy of the step edge. The significant changes in geometry induced by a relatively mild temperature increase underline the importance of temperature for the shape and all properties influenced by this shape of hydrogen-bonded clusters of water ice. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2) meV, considerably smaller than the one for monomer diffusion, at (72±1) meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3) meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity. © 2018 American Physical Society.

Scanning tunneling microscopy is sensitive to surface adsorbates to a much lower impurity level than most other surface science techniques. Even under the best vacuum and preparation conditions, a very low concentration of depressions of unknown origin is often observed in STM images of the coin metal surfaces. We outline a procedure to identify impurities by apparent height spectroscopy; a technique that can be easily performed by standard scanning tunneling microscopes. Apparent height spectroscopy, performed with a low-temperature scanning tunneling microscope, records the apparent height of an adsorbate with respect to the surface level over an extended voltage range at distinct voltage intervals. The spectra show characteristic features that can be used to identify adsorbates. We exemplify our method for two common impurities on Cu(111), oxygen atoms and carbon monoxide molecules. We reveal three characteristic differences in the apparent height spectroscopy of the two adsorbates: the dark region, the voltage of contrast reversal, and the onset of the lowest unoccupied molecular orbital. Each of these features is characteristic for the specific adsorbate/substrate system; giving three possibilities to identify the two species. The procedure can easily be extended to other impurities. © 2018 Author(s).

Different conformations of organic molecules are often almost isoenergetic, giving rise to their coexistence at finite temperature. In the gas phase, pressure was used to shift the equilibrium toward desired conformers. Here we use lateral pressure, enforced by increased coverage, to change the relative abundance of conformers of astraphloxin, an industrial dye, on Ag(100). We show in a variable temperature scanning tunnelling microscopy study that higher coverages enforce the predominance of one conformer only, leading to a homoconformational superstructure. The necessary conformational changes are possible at temperatures as low as 120 K. © 2017 American Chemical Society.

Scanning tunneling microscopy offers the opportunity to image water ice formation on surfaces on the molecular scale, from single water molecules up to closed water layers. The variation of the growth conditions leads to a large variety of ice structure, both amorphous and crystalline ones. Our studies open up the possibility to investigate solvation of surfaces as well as of single adsorbed molecules and the influence of this solvation on photo reactions on the molecular scale. The results will contribute to understanding the influence of solvents onto reactions within the hot topic of solvation science. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Scanning tunneling microscopy offers the opportunity to image water ice formation on surfaces on the molecular scale, from single water molecules up to closed water layers. The variation of the growth conditions leads to a large variety of ice structure, both amorphous and crystalline ones. Our studies open up the possibility to investigate solvation of surfaces as well as of single adsorbed molecules and the influence of this solvation on photo reactions on the molecular scale. The results will contribute to understanding the influence of solvents onto reactions within the hot topic of solvation science. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase. © 2017, The Author(s).

Mobile molecules crossing freely underneath the scanning tip of a scanning tunneling microscope create a uniform diffusive noise, making the identification of single molecules on the surface a challenge. We demonstrate the possibility of detecting mobile molecules on a surface by scanning tunneling microscopy and reveal how the diffusive noise is created. Additionally, we show that a molecule caught in the tip sample junction allows us to explore the potential energy surface of the system. Finally, voltage pulses disturb the mobile molecules, causing the loss of that ability. They also allow the creation of islands on the surface. Most of the investigations were done for Co- and Cu-phthalocyanine (Pc) on Ag(100). However, the concept is limited to neither Pc molecules nor Ag(100), as shown for a different organic molecule, astraphloxin, on Cu(111).

Co growth on Cu(111) was investigated at several temperatures between 120 K and 300 K by variable-temperature fast-scanning tunneling microscopy at submonolayer coverage. Islands nucleate heterogeneously at step edges and homogeneously on terraces. The height and area distribution difference between these two types of differently nucleated islands is attributed to a step edge alloy. Furthermore, the transformation from one-monolayer high islands to two-monolayer high islands is followed in time-lapsed sequences between 145 and 165 K. A surprising low-energy barrier for upward mass transport of Eupward≈(0.15±0.04) eV is determined for islands on terraces. At 120 and 150 K, the terrace islands are pure Cu; in contrast, at room temperature, terrace islands larger than ≈120 nm2 alloy at their border. © 2017 American Physical Society.

The diffusion of carbon monoxide molecules on Cu(111) is investigated in time-lapsed scanning tunneling microscopy in a temperature range from 30 to 38 K. An asymptotic theory of adsorbate diffusion predicted a trio interaction that changes the diffusion barrier of three particles diffusing in close proximity beyond the change induced by the long-range interaction between three pairs of molecules. Distance-dependent variations in the diffusion energy confirm this theoretical prediction. In future, the theory can better assist experiments for a broader exploration, not only for diffusion, but also for nucleation and reaction. © 2016 American Chemical Society.

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co-adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta- and the para-isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta- isomer differently from its gas-phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The complete bilayer is commonly considered as the termination of the (0001) surface of hexagonal ice. Experiments on thin crystalline ice structures grown on Cu(111) demonstrated a termination by admolecule structures on top of the bilayer. Modeling of complex admolecule terminations including admolecule clusters and decorated hexagon adrows within density functional theory and high-resolution STM imaging are combined for the structural analysis and to reveal possible causes for the apparent distinction. A dominant admolecule structure that appears during a short anneal at 130 K is identified as an arrangement of water dimer and trimers. By the combined approach, detailed models for decorated hexagon adrows are derived. Such structures possess low energy; however, the proton-ordered bilayer is more favorable at a small margin. Yet, energetically unfavorable bonding of water, for example, in thin ice films may drive the formation of admolecule terminations, for which kinetic effects still are an important factor. The results also shine light on the edge termination of bilayer islands. © 2015 American Chemical Society.

We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption. © 2016 AIP Publishing LLC.

Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively. © 2016 Author(s).

Studies of supported water clusters provide a means for understanding the initial stages of heterogeneous ice nucleation in diverse areas as atmospheric chemistry and astrophysics. Despite the importance of non-perfect ice structures in these fields, research focused on crystalline ice structures. Here, we report real-space observations of fractal ice islands grown between 89 K and 119 K. The island shape changes linearly from the most fractal dimension of 5/3 to the fractal dimension close to the one of an equal-sided hexagon. The mere linear increase is assigned to a shape dependent sticking coefficient of mobile ice clusters to the fractal islands. Our study reveals the complexity involved in formation of fractal ice structures. © 2016 Elsevier B.V.

Meta-dichlorobenzene is adsorbed on Ge(001) and investigated by low temperature scanning tunneling microscopy. The molecule is altered between two adsorption sites by inelastic electron tunneling manipulation. These adsorption sites differ largely in conductivity. The necessary energy for switching the molecule between the sites and its polarity dependence indicate that the manipulation is initiated by the electronic excitation of the molecule. © 2015 the Owner Societies.

We investigate astraphloxine, an industrial dye, on two metal surfaces, Au(111) and Ag(111). Low-temperature scanning tunneling microscopy with submolecular resolution in comparison to semiempirical calculations reveal that only two of the nine possible conformers of this molecule are adsorbed. The two conformers adsorb via one of their indol groups, which serves as a platform that decouples the rest of the molecule from the surfaces. A change from one to the other conformer is demonstrated by injecting inelastic electrons from the tunneling tip selectively into individual molecules. © 2015 American Chemical Society.

We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment. © 2015 American Chemical Society.

The motion of D<inf>2</inf>O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy. The diffusion distance histogram reveals a squared diffusion lattice that agrees with the primitive unit cell of the (100) surface. From the Arrhenius dependence, we derive the diffusion energy, the pre-exponential factor, and the attempt frequency. The mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion. © 2015 American Chemical Society.

The growth of Cu on Ag(100) is investigated by low-temperature scanning tunneling microscopy. Exchange diffusion of Cu deposited onto Ag(100) leads to small pure Cu islands and larger islands consisting of a CuAg alloy in room temperature growth. The ratio of the different types of islands depends on terrace widths up to 100 nm. This surprisingly long-range dependence is correlated to the density of the surface alloy. We thus reveal that the exchange diffusion barrier is influenced by terrace widths far beyond quantum size confinement. © 2015 American Physical Society.

A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations. © 2015 American Physical Society.

Sodium chloride is adsorbed at low temperature on Cu(111) and investigated by scanning tunneling microscopy. The ramified shape of the lowerature grown islands and early second layer nucleation suggest diffusion-limited aggregation of crystallites as corroborated by atomically resolved images of the highly disordered salt clusters. A comparison to single crystalline islands grown at room temperature on Ag(111) reveals differences in bond length and angle and thus crystal order for the lowerature phase. © 2015 American Chemical Society.

A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.

We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces. © 2015 American Chemical Society.

We explore co-deposition of water and 4, 4′-dihydroxy azobenzene on Ag(111) by low-temperature scanning tunneling microscopy at different water-to-azobenzene ratios. At all ratios, the water interacts with the hydroxyl end groups of the molecule replacing the direct hydrogen bonding. The change in bonding reduces the azobenzene density as compared to the one in the closed-packed waterless azobenzene structure. At intermediate water-to-azobenzene ratios, pores are formed in the azobenzene layer at nanometer distance from the water. At high water-to-azobenzene ratios, a water superstructure with a 1.4 nm × 1.4 nm unit cell develops. Our results point to a method to vary the density of an organic layer by tuning the amount of an inorganic additive. © 2015 AIP Publishing LLC.

We present a very compact molecular photoswitch on the technologically important Si(100) surface. Its adsorption configuration is determined by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study. The mechanisms of the isomerization reactions are discussed in view of DFT calculations and proven by in situ light irradiation.

Collective vibrational modes of crystal lattices, called phonons, determine fundamental material properties, such as their thermal and electrical conductivities. Bulk phonon spectra are influenced by point defects. More recently, the importance of phonons on nanostructures has come into the focus of attention. Here we show a spatially resolved phonon spectra of point defects that reveal distinctly different signatures for a cavity alone and an impurity atom fully integrated into the surface as opposed to one placed into a cavity. The spectra are indicative for delocalized phonons and localized vibrations, respectively, as confirmed by theory. © 2014 Macmillan Publishers Limited. All rights reserved.

The orientation of water molecules on water bilayers is investigated on Cu(111) by a combination of scanning tunneling microscopy and density functional theory. Theory predicts that the application of a field reorients the adsorbed water molecules at a distance of close to a nanometer from the surface. Experimental evidence is presented for this prediction. Furthermore, the process differs strongly between adsorption on two and on three ordered layers. We propose that these results give insight into the behavior of the diffusive layer close to electrodes. So simple? Since the basic idea of ultrahigh-vacuum (UHV) electrochemical modeling emerged, it has been claimed that UHV model experiments are too simple because they do not include the electrode potential. This combined scanning tunneling microscopy and density functional theory study gives insight into the influence of the electric field on single molecules in the diffusive layer. A field reorients adsorbed water molecules on water bilayers on Cu(111) at a distance of about 1nm from the surface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface. © 2014 American Physical Society.

We outline a possibility to determine the adatom density of individual nano-objects via measurement of their electronic structure. For this aim, the nonlinear shift of image potential states measured on individual Cu nanoclusters on Ag(100) by low-temperature scanning tunneling spectroscopy is carefully analyzed. The quantitative analysis is confirmed by density-functional theory calculations. A peak width analysis furthermore reveals whether the clusters are purely metallic or alloyed. © 2014 American Physical Society.

This study investigates the interface state electron dispersion relation between NaCl(100) islands and Ag(111) dependent upon NaCl island size. Both onset energy and effective mass are size dependent. However, these dependencies are relevant at different island sizes. We trace back this effective mass dependency to a misfit-induced strain based on atomically resolved images. Our results open up new avenues for the development of nanodevices by tuning the effective electron mass via strain of the insulating component. © 2013 American Chemical Society.

We demonstrate that Ag adatoms are capable of stabilizing negatively charged copper-phthalocyanine (CuPc) molecules in a Ag-CuPc network at room temperature. For this aim, the structure of the Ag-CuPc coordination network at different molecule-adatom densities is investigated experimentally by scanning tunneling microscopy and theoretically by firstprinciples calculations. The islands formed at saturation adatom density, close to the source of adatoms, consist of a closed-packed layer without voids. The islands formed at lower adatom density consist of an irregular arrangement of larger entities, named subunits, mainly (CuPc)4Ag and (CuPc)6Ag2, which are interconnected in the same fashion as the CuPc molecules in the closed-packed layer. Silver adatoms in the subunits and between them differ by the number of molecules they link. The Ag-CuPc networks are stabilized, because the adsorption energy of CuPc molecules increases due to the presence of adatoms. © 2014 American Chemical Society.

Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4?-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule. © 2014 American Chemical Society.

This article reviews manipulation of single molecules by scanning tunnelling microscopes, in particular vertical manipulation, lateral manipulation, and inelastic electron tunnelling (IET) manipulation. For a better understanding of these processes, we shortly review imaging by scanning tunnelling microscopy - as a prerequisite to detect the manipulated species and to verify the result of the manipulation - as well as scanning tunnelling spectroscopy and IET spectroscopy which are used to chemically identify the molecules before and after the manipulation that employs the tunnelling current. This article reviews manipulation of single molecules by scanning tunnelling microscopes, in particular vertical manipulation, lateral manipulation, and inelastic electron tunnelling manipulation. For a better understanding of these processes, the authors shortly review imaging by scanning tunnelling microscopy - as a prerequisite to detect the manipulated species and to verify the result of the manipulation - as well as scanning tunnelling spectroscopy and inelastic electron tunnelling spectroscopy which are used to chemically identify the molecules before and after the manipulation that employs the tunnelling current. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

NaCl islands on Ag(111) are investigated by low temperature scanning tunneling microscopy and spectroscopy. The thermodynamically stable growth mode consists of bilayer-high rectangular-shaped islands that are (100) terminated with a large band gap. Deviations from this bulk-like (100) growth are induced by surface defects as intrinsic step edges and point defects in the supporting Ag(111) surface. The interface between NaCl(100) and Ag(111) induces an interface state that is completely depopulated with its onset at (92 ± 4) meV. The influence of the Ag surface-induced defects on the interface state is discussed. © 2013 American Chemical Society.

Scanning tunneling microscopy offers the exciting possibility to manipulate individual molecules by vibrational excitation via inelastically tunneling electrons. The electrons transfer energy into molecular vibrational modes, leading to breakage or formation of individual bonds. It is challenging to precisely control intramolecular changes by this process. We demonstrate that for 4,4′-dihydroxyazobenzene adsorbed on Au(111) or Ag(111), the manipulation facilitates rotation of the OH end groups around the C-O bond between metastable states; this corresponds to a reorientation of the hydrogen, the ultimate limit of a conformational change within a molecule. © 2013 American Chemical Society.

A beetle type stage and a flexure scanning stage are combined to form a two stages scanning tunneling microscope (STM). It operates at room temperature in ultrahigh vacuum and is capable of scanning areas up to 300 μm × 450 μm down to resolution on the nanometer scale. This multi-scale STM has been designed and constructed in order to investigate prestructured metallic or semiconducting micro- and nano-structures in real space from atomic-sized structures up to the large-scale environment. The principle of the instrument is demonstrated on two different systems. Gallium nitride based micropillars demonstrate scan areas up to hundreds of micrometers; a Au(111) surface demonstrates nanometer resolution. © 2012 American Institute of Physics.

The DNA base thymine is deposited at 100 K on Cu(111) and investigated and manipulated by low-temperature scanning tunneling microscopy at 5 K. At submonolayer coverage paired rows are observed. At monolayer coverage a hexagonal commensurate self-assembled layer with the methyl group pointing away from the surface forms. A reversible local manipulation of molecules within the self-assembled layer is demonstrated. This manipulation is interpreted as an out-of-plane relaxation of molecules within the layer induced by the change of the adsorption geometry of individual molecules between two meta-stable orientations. A positive field of 2-4 V leads to this local change in the molecular arrangement, while a field larger than 4 V restores the original geometry.

The submonolayer growth of NaCl bilayer high-rectangular shaped islands on Ag(111) is investigated at around room temperature by using low temperature scanning tunneling microscopy. The growth at the step edges is preferred. Two kinds of islands are observed. They either grow with their non-polar edge at the step edge of Ag(111) or the islands overgrow in a carpet-like mode with the polar direction parallel to the edge. In the latter case, the Ag step is rearranged and considerable, while the NaCl layer is bent. This study clarifies the nature of the interaction of an alkali halide nanostructure with a metal step edge. © 2012 IOP Publishing Ltd.

We investigate diffusion and decay of adatom and vacancy islands on Ag(100) between ≤10 and ≤1000 nm2 in size at room temperature by fast scanning tunneling microscopy. Adatom and vacancy islands decay in the diffusion and in the attachment limit, respectively. This adatom-driven kinetics confirms the existence of an Ehrlich-Schwoebel barrier on Ag(100). The dependence of the diffusivity of vacancy islands on island size is consistent with a kink-dominated periphery diffusion mechanism. Quantitative differences to previously published work are discussed. © 2012 American Physical Society.

The photoisomerization properties of an azobenzene derivative on a thin insulating NaCl layer on Ag(111) are investigated with a low-temperature scanning tunneling microscope and density functional calculations. Illumination with UV light at 365 nm induces the reversible direct isomerization of the adsorbed species, while visible light does not lead to any changes. This unexpected behavior cannot be explained by the change of the electronic structure upon adsorption on the inert surface. It is rationalized in terms of electrostatic interactions caused by the atomistic details of the surface. © 2012 American Physical Society.

Diffusion and decay of alloyed Cu/Ag islands are investigated in the size range from 1 to 40nm2 on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above 7nm2 show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than 4nm2 diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the Cu/Ag islands. © 2011 American Physical Society.

We present a successful attempt of decoupling a dye molecule from a metallic surface via physisorption for enabling direct photoisomerization. Effective switching between the isomers is possible by exposure to UV light via the rotation pathway. © 2011 The Royal Society of Chemistry.

We have investigated vibrational spectra of nitrobenzene molecules adsorbed on Cu(111) by low temperature inelastic electron tunneling spectroscopy. This molecule, which should support 39 internal modes, only gives rise to seven peaks in the spectra. We outline a comparison with ensemble IR data and interpret the small number of vibrational peaks by the superposition of a multitude of almost isoenergetic vibrational modes. The non-detectability of further modes cannot be understood in terms of symmetry considerations. Additional modes in the spectra are attributed to external molecularmetal vibrations. © 2011 IOP Publishing Ltd.

Single anilino-nitro-azobenzene molecules are investigated on the reconstructed Au(111) surface by low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and inelastic electron tunneling manipulation. The molecules attach with their anilino end to the point dislocation of the reconstructed surface and with their nitro end to a gold adatom. These complexes are induced to rotation, diffusion, and isomerization by injecting electrons into the lowest unoccupied molecular orbitals or by injecting holes into the highest occupied molecular orbitals. The reaction yield thereby reflects the spatial distribution of these orbitals. This suggests that frontier orbitals and their spatial extent can be mapped via molecular reactions induced by transient population of these orbitals by tunneling electrons. © 2011 American Chemical Society.

We report real space imaging measurements of inelastic Friedel oscillations. The inelastic electron tunneling spectroscopy, using scanning tunneling microscopy, around dimers of dichlorobenze adsorbates on Au(111) surface display clear spatial modulations that we attribute to inelastic scattering at the molecular sites caused by molecular vibrations. Due to local interactions between the adsorbate and the surface states, the molecular vibrations generate a redistribution of the charge density at energies in a narrow range around the inelastic mode. Our experimental findings are supported by theoretical arguments. © 2011 American Chemical Society.

A regular step, a dislocation slip step and a step formed by the emergence of a split edge dislocation (SED) to the surface influence the local density of states close to the onset of the surface state as investigated by scanning tunnelling spectroscopy at low temperature.The onset of the surface state shifts close to the regular step and the dislocation slip step by approximately 15 meV towards the Fermi energy.Additional maxima above the onset are only observed if a second step leads to confinement.In both cases, the conductivity decreases close to the step.However, an increase in conductance above the surface state onset is observed close to the SED step.Furthermore, a variety of additional states are discernable.Thus, different types of steps lead to markedly different changes in the local electronic structure on surfaces.© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Molecular electronics offers a promising way for constructing nano-electronic devices in future with faster performance and smaller dimensions. For this aim, electronic switches are essential as basic components for storage and logical operations. The main requirements for a molecular switch are reversibility and bistability. This necessitates the existence of at least two different thermally stable forms of a molecule that may be changed repeatedly from one state to the other one through an external stimulus. The transition should then be connected to a measurable change in molecular properties. The development of such molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nanoelectronic circuits. Since isomers may differ significantly in physical and chemical properties, isomerisation opens a way for a molecular switch. In this article, an overview is provided over those isomerisation reactions of single molecules adsorbed on surfaces that are investigated with a scanning tunnelling microscope and that have a potential as a molecular switch in future molecular electronics. These are mainly, but not exclusively, constitutional, configurational, and geometric isomerisation reactions. The external stimulus is either light or the possible interaction with the tip of a scanning tunnelling microscope, i.e. electrons, electric field, or mechanical force. Some reactions are similar to those observed for the molecule in the liquid phase, but some are observed or even possible only on a surface. The detailed investigation of the isomerisation yield dependence on several parameters gives insight into the underlying processes of the reaction. © 2011 Elsevier Ltd. All rights reserved.

The electronic structure of 4-anilino-4′-nitroazobenzene superstructures formed on Au(111) at 250 K is investigated by low temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and dI/dV mapping at 5 K. Changes in the dI/dV maps of this push-pull molecule reflect the spatial distribution of the frontier orbitals on the molecular scale. Spectra of the trans- and the cis-isomer differ between themselves and in different parts of supramolecular assemblies. The relative importance of these differences is discussed. © 2011 American Institute of Physics.

Supramolecular assemblies of 4-anilino-4′-nitroazobenzene are investigated on the Au(111) surface by low temperature scanning tunneling microscopy and spectroscopy with submolecular resolution. Adsorption at 250 K leads to three different structures that are formed via hydrogen bonds: a star structure and two types of line structures: a meandering and a zigzag line. The formation of these supramolecular assemblies is affected by the available space on the fcc domains of the reconstructed Au(111) substrate as well as by the two-dimensional chirality of the molecules on the surface. The star structure is enantiomerically pure, while both types of lines consist of a racemic mixture. Bonding between homochiral pairs differ from the one between heterochiral pairs in the position of the hydrogen bonds. Inside these supramolecular assemblies two configurations of the molecules are identified: An almost straight trans-configuration and a slightly bent cis*-configuration. The trans-configuration largely reflects the structure of this isomer in gas phase, while the cis*-configuration is two-dimensional on the surface in contrast to the three-dimensional gas phase cis-configuration. The reversible trans-cis* isomerization is induced by electron tunneling through the LUMO+1 state of the molecule, which is located at +2.9 V. © the Owner Societies.

This study investigates the surface chemistry and the ordering characteristics of coadsorbed hydrogen and chlorine atoms, generated by the exposure of the Si(100) surface to gas-phase HCl molecules at various substrate temperatures, by scanning tunneling microscopy (STM), core-level photoemission spectroscopy, and Monte Carlo simulation. Experimental results show that saturation exposure to HCl causes all surface dangling bonds to be terminated by the two fragments H and Cl atoms and that the number of H-terminated sites exceeds that of Cl-terminated ones by more than 10%. This finding suggests that, in addition to the dominant dissociative chemisorption, atomically selective chemisorption or atom abstraction occurs. STM images reveal that some Cl-terminated sites form patches with a local 2×2 structure at 110 K and that the degree of ordering is reduced as the substrate temperature increases. Results of Monte Carlo simulations demonstrate the importance of including dissociative fragment-adsorbate interactions during the random adsorption of diatomic molecules. Comparing the correlations between Cl-terminated sites identified from STM images and those predicted by simulation reveals two effective interaction energies of 8.5±2.0 and 3.5±2.0 meV between a dissociative fragment Cl atom and a nearest neighboring Cl adsorbates in the same dimer row and in the adjacent row, respectively. © 2010 The American Physical Society.

We investigate two fundamental steps of a nonadiabatic surface process, the photo-induced movement and approach of CO molecules on the Cu(111) surface, at a hitherto unachieved single-molecule level through scanning tunneling microscope imaging. For the close approach of two CO molecules, we not only determine the nonadiabatic diffusion barrier (87 meV), but also discover a femto-second-laser-induced transient attraction (30 meV) of the usually repelling CO molecules. © 2010 The American Physical Society.

Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10-9 per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I -t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change. © 2010 IOP Publishing Ltd.

The understanding of reaction intermediates in heterogeneous catalysis has important implications for the design of novel catalysts. We investigate the adsorption of CO2 on oxygen precovered Ag(100) at low temperature (17 K) by scanning tunneling microscopy and inelastic electron tunneling manipulation at 5 K. On the terraces, the adsorption leads to O-Ag-CO 2-Ag-O compounds with reduced binding of the oxygen to the surface as compared to the separately adsorbed molecules. The compound can be either dissociated into a bistable O-Ag-CO2 compound at 1.6 V, dissociated into its constituents at 2.2 V, or reacted at 6.5 V into a species, which we tentatively attribute to CO3. The thus obtained carbon trioxide or carbonate is an intriguing reaction intermediate, because it is not stable in the gas phase. Our detailed study of coadsorbed species outlines a possibility to investigate precursors of reactions that involve the substrate atoms. © 2010 American Chemical Society.

Scanning tunneling microscopy is used to investigate isomerization of amino-nitro-azobenzene on a thin NaCl layer on Ag(111) by inelastically tunneling electrons. A reversible isomerization between a planar trans and a three-dimensional cis form with two different thresholds is demonstrated. The isomerization characteristics are rationalized in terms of binding of the multipolar molecule to the ionic layer. This study shows the feasibility of a bistable single molecule switch on an insulator. © 2010 The American Physical Society.

(Figure Presented) Cannonball run: Preparation of CO2 has been achieved with CO and O2 coadsorbed onto a Cu(III) surface by illumination with 40 fs pulses of laser light at 400 nm. The hot adatom mechanism that follows O2 dissociation leads to a can-nonball trajectory of the product molecules and thus escape of CO2 from the reactant site to the bare terrace (see figure). It was thus demonstrated that non-adiabatic heterogeneous catalysis may be followed in real space. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The development of molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nanoelectronic circuits. With a scanning tunneling microscope (STM) it is possible to induce chemical reactions on a single molecule basis by electrons tunneling inelastically from the STM tip into a molecule. The method is based on high-resolution imaging at low temperature (5 K) that allows us to identify different groups within the molecule. Chemical reactions are induced by injecting, selectively, electrons into specific parts of the molecule. The success of the manipulation is visualized in the recorded tunneling current during the manipulation and in STM images taken afterwards. We review, here, isomerization of individual molecules adsorbed on metal surfaces. For chlorobenzene and azobenzene derivatives, the effects of different substitutional groups and different substrates are explored. © 2010 John Wiley & Sons, Ltd.

We investigate the distance distribution of two anorganic molecules (CO, H2O), one organic radical (parabenzyne), and one strongly dipolar molecule (ortho-dinitrobenzene) on the (111) faces of copper and silver. Above the onset of diffusion, their distribution is influenced by the surface state and oscillates. While CO, H2O, and para-benzyne show the expected oscillation period of ≈λF / 2, ortho-dinitrobenzene oscillates with λF / 4. The position of the first maximum in these oscillations is consistent with a perfect scatterer for the anorganic molecules, but inconsistent for the radical and the dipolar molecule. This observation is utilized to explain the double periodicity observed in the distance distribution of ortho-dinitrobenzene. © 2010 Elsevier B.V. All rights reserved. All rights reserved.

Scanning tunneling microscopy and dI/dV mapping reveal three types of interference patterns on Ag(100) and Ag(111) after implantation of argon or neon. These patterns originate from the scattering of bulk electrons between subsurface impurities and the surface. Above nanocavities, the interference pattern develop a rich internal structure. Above point impurities, two types of simpler patterns develop that differ in corrugation, shape, and energy dependence. Low corrugated patterns show a strong dependence on energy, while more corrugated patterns hardly vary with energy. In addition, only the high corrugated patterns reflect the symmetry of the surfaces, while the low corrugated patterns are circular. © 2010 The American Physical Society.