Solvent effects on the magnetic-field-dependent reaction yields of photogenerated radical ion pairs.

A pronounced solvent viscosity and polarity dependence of the magnetic field effect was found for polymethylene-linked radical ion pairs generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH2)n-N,N-dimethylaniline, with n = 7–16. This is interpreted in terms of the general radical pair mechanism, i.e. the nuclear hyperfine interaction mechanism including a spin-exchange interaction, modulated by the stochastic folding motion of the linking CH2 chain which leads to a “motional deformation” of the reaction yield spectra

Increasing the templating effect on a bulk insulator surface: from a kinetically trapped to a thermodynamically more stable structure

Molecular self-assembly, governed by the subtle balance between intermolecular and molecule-surface interactions, is generally associated with the thermodynamic ground state, while the competition between kinetics and thermodynamics during its formation is often neglected. Here, we present a simple model system of a benzoic acid derivative on a bulk insulator surface. Combining high-resolution noncontact atomic force microscopy experiments and density functional theory, we characterize the structure and the thermodynamic stability of a set of temperature-dependent molecular phases formed by 2,5-dihydroxybenzoic acid molecules, self-assembled on the insulating calcite (10.4) surface. We demonstrate that a striped phase forms before the thermodynamically favored dense phase, indicating a kinetically trapped state. Our theoretical analysis elucidates that this striped-to-dense phase transition is associated with a distinct change in the chemical interactions involved in the two phases. The striped phase is characterized by a balance between molecule-molecule and molecule-substrate interactions, reminiscent of the molecular bulk. In contrast, the dense phase is formed by upright standing molecules that strongly anchor to the surface with a comparatively little influence of the intermolecular interactions, i.e., in the latter case the substrate acts as a template for the molecular structure. The kinetic trapping stems from a relatively strong intermolecular interaction between molecules in the striped phase that need to be broken before the substrate-templated dense phase can be formed. Thus, our results provide molecular level insights into two qualitatively different bonding motifs of a simple organic molecule on a bulk insulator surface. This understanding is mandatory for obtaining predictive power in the rational design of molecular structures on insulating surfaces. © 2016 American Chemical Society

Understanding atom movement during lateral manipulation with the STM tip using a simple simulation method

Kühnle A, Meyer G, Hla SW, Rieder K-H. Understanding atom movement during lateral manipulation with the STM tip using a simple simulation method. Surface Science. 2002;499(1):15-23.We report on a fast simulation method to investigate the movement of an atom induced by the tip during lateral manipulation with a scanning tunneling microscope. The simulation is based on a model assuming the atom moving in the combined potential of tip and surface. The pathway of the tip is subdivided in small steps, and the atomic position for each step is calculated by an iterative algorithm searching for the closest energetic minimum. The method is demonstrated for manipulation on the (111) surface of an fcc metal. Our model calculations predict which energetic minima of the surface are attained by the atom during manipulation. The details of the modelled manipulation curves allow a precise description of the atomic pathway in dependence on manipulation direction and positioning of the tip relative to the atom. Furthermore, the simulation predicts a transition from the so-called pulling to sliding manipulation mode upon reducing tip-surface distance, well in agreement with general experimental observations. To test our algorithm we present experimental results for the manipulation of iodine on Cu(I 11) along the [211] direction and compare them to simulated manipulation curves. The comparison allows for a complete understanding of all details in atomic movements during manipulation along a complicated path. (C) 2001 Published by Elsevier Science B.V

Controlled lateral manipulation of single diiodobenzene molecules on the Cu(111) surface with the tip of a scanning tunnelling microscope

Hla SW, Kühnle A, Bartels L, Meyer G, Rieder KH. Controlled lateral manipulation of single diiodobenzene molecules on the Cu(111) surface with the tip of a scanning tunnelling microscope. Surface Science. 2000;454:1079-1084.We report on the controlled lateral manipulations of adsorbed single diiodobenzene molecules on the Cu(111) surface with a scanning tunnelling microscope (STM) tip at 20 K. The molecular motions in this experiment are mainly induced by the attractive interaction between the tip and the molecule. Even though the leading manipulation mode is 'pulling', a continuous 'sliding' mode can also be induced if we use higher tip-molecule interaction forces. During the manipulation process, the molecules can follow the tip with hops of single or double copper-atomic-site distances and in some cases 'hop-scotch' type movements can also be observed. (C) 2000 Elsevier Science B.V. All rights reserved

Cytokines and growth factors cross-link heparan sulfate

The glycosaminoglycan heparan sulfate (HS), present at the surface of most cells and ubiquitous in extracellular matrix, binds many soluble extracellular signalling molecules such as chemokines and growth factors, and regulates their transport and effector functions. It is, however, unknown whether upon binding HS these proteins can affect the long-range structure of HS. To test this idea, we interrogated a supramolecular model system, in which HS chains grafted to streptavidin-functionalized oligoethylene glycol monolayers or supported lipid bilayers mimic the HS-rich pericellular or extracellular matrix, with the biophysical techniques quartz crystal microbalance (QCM-D) and fluorescence recovery after photobleaching (FRAP). We were able to control and characterize the supramolecular presentation of HS chains—their local density, orientation, conformation and lateral mobility—and their interaction with proteins. The chemokine CXCL12α (or SDF-1α) rigidified the HS film, and this effect was due to protein-mediated cross-linking of HS chains. Complementary measurements with CXCL12α mutants and the CXCL12γ isoform provided insight into the molecular mechanism underlying cross-linking. Fibroblast growth factor 2 (FGF-2), which has three HS binding sites, was also found to cross-link HS, but FGF-9, which has just one binding site, did not. Based on these data, we propose that the ability to cross-link HS is a generic feature of many cytokines and growth factors, which depends on the architecture of their HS binding sites. The ability to change matrix organization and physico-chemical properties (e.g. permeability and rigidification) implies that the functions of cytokines and growth factors may not simply be confined to the activation of cognate cellular receptors

STM observations of a one-dimensional electronic edge state at steps on Cu(111)

Bartels L, Hla SW, Kühnle A, Meyer G, Rieder K-H, Manson JR. STM observations of a one-dimensional electronic edge state at steps on Cu(111). Physical Review B. 2003;67(20):205416.Scanning tunneling microscopy measurements across isolated straight step edges on a Cu(111) surface were carried out for biases between 100 mV and 5 V. In addition to the well known surface state oscillations, and at lower sample bias than the onset of the two-dimensional surface image state, a sharply defined linear protrusion, was observed at the top of the step faces. This linear feature is interpreted as a one-dimensional image state at the step, with its energy modified by a dipolar potential whose appearance is attributed to Smoluchowski smoothing of the electron density at the step edge

Atomic-scale chemistry: Desorption of ammonia from Cu(111) induced by tunneling electrons

Bartels L, Wolf M, Klamroth T, et al. Atomic-scale chemistry: Desorption of ammonia from Cu(111) induced by tunneling electrons. Chemical Physics Letters. 1999;313(3-4):544-552.We report on excitation experiments on individual ammonia molecules adsorbed on Cu(lll) using a low-temperature scanning tunneling microscope. Multiple electronic excitation of the ammonia-substrate bond can lead to the desorption of molecules from the substrate and their transfer to the STM tip apex. The dependency of the desorption yield on the tunneling current at different biases shows that the order of the desorption process correlates directly with the minimum number of electrons necessary to overcome the binding energy. In contrast to previous experiments, excitation with either polarity, i.e., electron and hole attachment, can cause desorption. Hartree-Fock calculations allow us to deduce from spectroscopical data that the desorption process is mediated by an ammonia modified Cu4s state near the Fermi level. (C) 1999 Elsevier Science B.V. All rights reserved

Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO2(110) surface

Enevoldsen GH, Pinto HP, Foster AS, et al. Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO2(110) surface. Physical Review B. 2008;78(4):045416.The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by the geometry and exact atomic structure of the tip apex. However, the tip state is an experimentally unknown parameter, and the lack of insight into the tip apex often limits the possibilities of extracting precise quantitative and qualitative atomistic information on the surface under inspection. From an interplay between simultaneously recorded nc-AFM and scanning tunneling microscopy (STM) data, and atomistic STM simulations based on multiple scattering theory, we demonstrate how the state of the scanning probe microscopy (SPM) tip in the experiments may be determined. The analysis of a large number of experimental SPM images recorded with different tips reveals that no general correlation exists between the contrast observed in the nc-AFM and the tunneling current (I-t) images on TiO2(110) surface. The exact state of the SPM tip must, therefore, be determined for each specific case, which is normally a very difficult endeavor. However, our analysis of the AFM contrast on TiO2(110) surface allows us to considerably reduce the number of tips to be considered in a full simulation. By carefully evaluating the contrast of a handpicked library of SPM tips, we manage to determine a very accurate model of the SPM tip used in an experiment for the first time. It is envisioned that the approach presented here may eventually be used in future studies to screen for and select a SPM tip with a special functionalization prior to imaging an unknown sample, and in that way facilitate precise modeling and chemical identification of surface species

Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy

Lauritsen JV, Foster AS, Olesen GH, et al. Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy. Nanotechnology. 2006;17(14):3436-3441.Atomic force microscopy in the non-contact mode (nc-AFM) can provide atom-resolved images of the surface of, in principle, any material independent of its conductivity. Due to the complex mechanisms involved in the contrast formation in nc-AFM imaging, it is, however, far from trivial to identify individual surface atoms or adsorbates from AFM images. In this work, we successfully demonstrate how to extract detailed information about defects and the chemical identity of adsorbates on a metal oxide surface from nc-AFM images. We make use of the observation that the apex of the AFM tip can be altered to expose either a positive or negative tip termination. The complementary set of images recorded with the two tip terminations unambiguously define the ionic sub-lattices and reveal the exact positions of oxygen vacancies and hydroxyl (OH) defects on a TiO2 surface. Chemical specificity is extracted by comparing the characteristic contrast patterns of the defects with results from comprehensive AFM simulations. Our methodology of analysis is generally applicable and may be pivotal for uncovering surface defects and adsorbates on other transition metal oxides designed for heterogeneous catalysis, photo-electrolysis or biocompatibility