SPM Aarhus 150

Extremely Stable and Highly Versatile SPM Aarhus 150 SPECS for Ultimate Scanning Probe Microscopy Applications

The SPM Aarhus 150 is an outstandingly stable and time saving instrument. A specially designed variable temperature scanner platform of 3 kg mass with integrated low noise liquid nitrogen (LN2) cooling device guarantees the uncompromised superior SPM performance. Special care was taken to decouple the flow cooler from the sample stage and yet ensure permanent cooling connection between them. For this dedicated flexible copper braids are used to couple the extra heavy scanner platform to the flow cryostat without affecting excellent stability of the SPM Aarhus. A tight mechanical and thermal contact between sample holder and SPM stage allows for the extremely accurate sample temperature control and stability. A typical cool down times of less than 60 min to the temperatures below 130K are achieved. A typical time span of 20 min from insertion of a sample at room temperature to “ready for SPM” at below 130 K and has been shown. For temperature ramps counter heating of the sample is possible even to elevated temperatures up to 400 K. A comparably low LN2 consumption can be realized in operation as well as during the fast cool down of the cryostat from room temperature. About 20 l of LN2 is consumed during initial cool down while the typical LN2 consumption during operation at 130 K is about 10 litres per hour. Through the unification of the SPM mechanics into one STM/AFM unit, the SPM Aarhus 150 can easily be upgraded to AFM by using the KolibriSensor™. The SPM Aarhus 150 sets a new standard by showing the highest thermal stability at variable temperatures between 90 and 400K without compromising its original mechanical stability. A direct in-situ optical access allows for the sample illumination and investigation of light induced processes. Additionally an evaporation port permits in-situ deposition on the sample surface and investigation of the growth processes during scanning.

KEY FEATURES

  • Extreme stability
  • Highest productivity
  • In-situ tip preparation
  • Capable of video speed scanning
  • KolibriSensor for combined nc-AFM/STM
  • SPC 260 or Nanonis™ control system
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MADE FOR THESE METHODS

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SPECIFICATIONS

SPM Aarhus 150
Mounting
Mounting Flange

150CF

Control Electronics

  • Nanonis
  • SPC 260

Performance
Temperature Stability

Better than ±2 K (150 K...400 K)

Sensitivity z Range

±175 nm

Drift Rate

< 0.05 nm/min (vert), < 0.15 nm/min (lat)

Stability

< 10 pm

Operation
In-Situ Access

Specular and evaporation

Scan Range

1.500 nm x 1.500 nm

Temerature Control

2 controlled subsystems for sample & scanner

Operating Modes

  • STM
  • AFM

Sensors

  • STM tip
  • KolibriSensor

Optional Accessories

  • STM tips
  • Kolibri sensors
  • Sample holders

Required Accessories

  • STM tips
  • Kolibri sensors
  • Sample holders

Working Pressure

10-11 to 10-7 mbar

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RELATED PRODUCTS

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APPLICATION NOTES

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PUBLICATIONS

  1. (2013) Imaging Reactions of Acetone with Oxygen Adatoms on Partially Oxidized TiO2(110)

    Understanding the interaction of O2 with ketones on metal oxide surfaces is important for the photo-oxidation of toxic organic molecules. The consecutive reaction steps of acetone molecules with oxygen adatoms (Oa's) on partially oxidized TiO2(110) surfaces have been studied using high-resolution scanning tunneling microscopy (STM) at 300 K. The sequential isothermal STM images reveal two types of acetone–Oa species as a result of reactions of acetone with an oxygen adatom and a bridging bound oxygen vacancy (VO). One such species is the Ti5c-bound acetone–Oa diolate formed from Ti5c-bound acetone reacting with Oa. The diolate is mobile at 300 K and can assist the diffusion of surface Oa by exchanging the acetone oxygen with the Oa. The second acetone–Oa species is the VO-bound acetone–Oa complex formed from a VO-bound acetone reacting with an Oa located on the neighboring Ti row. The VO-bound complex is stationary at 300 K. This species has not been reported previously.



    Y. Xia, J. Ye, P. Murray, A. Ali, Q. Geb, Z. Zhang
    Phys. Chem. Chem. Phys. 15, 13897-13901
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  2. (2012) The role of surface defects in large organic molecule adsorption: substrate configuration effects

    The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4′-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4′-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed.



    T. Waldmann, C. Nenon, K. Tonigold, H. E. Hoster, A. Groß, R. J. Behm
    Physical Chemistry Chemical Physics 14 (30), pp. 10726-10731
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  3. (2012) Oxidation of an organic adlayer: a bird’s eye view

    The reaction of O2 with an adlayer of the oligopyridine 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)-pyridine-2-yl)pyrimidine (2,4′-BTP), adsorbed on the (111) surfaces of silver and gold and on HOPG – which can be considered as a model system for inorganic|organic contacts – was investigated by fast scanning tunneling microscopy (video STM) and dispersion corrected density functional theory (DFT-D) calculations. Only on Ag(111), oxidation of the 2,4′-BTP adlayer was observed, which is related to the fact that under the experimental conditions O2 adsorbs dissociatively on this surface leading to reactive O adatoms, but not on Au(111) or HOPG . There is a distinct regiospecifity of the oxidation reaction caused by intermolecular interactions. In addition, the oxidation leads to a chiral ordering. The relevance of these findings for reactions involving organic monolayers is discussed.



    T. Waldmann, D. Künzel, H.E . Hoster, A. Groß, R. J. Behm
    Journal of the American Chemical Society 134 (21), pp. 8817-8822
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  4. (2012) Stabilization of Large Adsorbates by Rotational Entropy: A Time‐Resolved Variable‐Temperature STM Study

    Investigating the dynamics in an adlayer of the oligopyridine derivative 2‐phenyl‐4,6‐bis(6‐(pyridine‐2‐yl)‐4‐(pyridine‐4‐yl)pyridine‐2‐yl)pyrimidine (2,4′‐BTP) on Ag(111) by fast scanning tunneling microscopy (video‐STM), we found that rotating 2,4′‐BTP adsorbates coexist in a two‐dimensional (2D) liquid phase (β‐phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α‐phase) and β‐phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature‐dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid‐size and large adsorbates.



    T. Waldmann, J. Klein, H. E. Hoster, R. J. Behm
    ChemPhysChem 14 (1), pp. 162-169
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  5. (2012) Quantum Confinement Drives Macroscopic Stress Oscillations at the Initial Stage of Thin Film Growth

    Functionalization of thin-film heterostructures on the basis of their electrical, optical and magnetic properties, requires precise control of the film stresses that develop during the growth process. By using real-time in situ stress measurements, the present study reveals strikingly that the in-plane film stress oscillates with increasing film thickness at the initial stage of epitaxial Al(111) film growth on a Si(111)−√3×√3−Al surface, with a periodicity of 2 times the Fermi wavelength of bulk Al and a stress variation from maximum to minimum as large as 100 MPa. Such macroscopic stress oscillations are shown to be caused by quantum confinement of the free electrons in the ultrathin epitaxial metal film. The amplitude, period, and phase of the observed stress oscillations are consistent with predictions based on the free electron model and continuum elasticity.



    D. Flötotto, Z. Wang, L. P. H. Jeurgens, E. J. Mittemeijer
    Phys. Rev. Lett. 109, 045501
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  6. (2012) An STM study of the initial oxidation of single-crystalline zirconium surfaces

    The microstructural development of thin (thickness < 10 nm) oxide layers grown on Zr surfaces by thermal oxidation was investigated by in-vacuo STM and XPS. To this end, single-crystalline Zr(0001) and Zr(100) surfaces were prepared under UHV conditions by a cyclic treatment of ion-sputtering and in-vacuo annealing steps and then exposed to dry O2(g) in the temperature range of 300–450 K (at pO2 = 1×10− 4 Pa). Oxidation proceeds by the fast formation of a dense arrangement of tiny oxide nuclei, which cover the entire Zr surface. The initial oxide cluster size is about 1.2 ± 0.1 nm. The transport processes on the oxidizing surface become promoted with increasing temperature and thereby the oxide clusters rearrange into bigger agglomerates with increasing oxidation time. At the same time, a long-range atomic order develops in the oxide overgrowths, as evidenced from the emergence of a bonding/non-bonding fine structure in the resolved oxide-film upper valence band, as measured in-situ by XPS.



    G. Bakradze, L .P. H. Jeurgens, E. J. Mittemeijer
    Surface Science 606 (9-10), pp. 846-851
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  7. (2011) Surface preparation of Si(1 0 0) by thermal oxide removal in a chemical vapor environment

    The preparation of Si(1 0 0) surfaces in chemical vapor environments suitable for subsequent epitaxial III–V integration by chemical vapor deposition (CVD) involving metal-organic precursors was investigated by surface sensitive instruments accessible through a dedicated sample transfer to ultra high vacuum (UHV). Using X-ray photoelectron spectroscopy for inspection of the chemical surface composition, we verified the ability to obtain clean Si(1 0 0) free of oxygen or other contaminants. Annealing for 30 min in a pure hydrogen atmosphere of 950 mbar pressure was found to be sufficient if the surface temperature reached at least 950 °C. We characterized the crucial annealing step comprehensively regarding reliability, dependency on essential process parameters (such as annealing time and surface temperature). Our results verified significant differences to established Si(1 0 0) UHV preparation routines and therefore indicated a major influence of the process gas in the SiO2 removal process, hence we also considered a chemically active role of the hydrogen ambient in the deoxidation reaction. A complementary assessment of the general structure and the atomic configuration of our CVD-prepared Si(1 0 0) surfaces included low energy electron diffraction and scanning tunnelling microscopy and confirmed atomically flat surfaces with two-domain (2×1)/(1×2) reconstruction typical for completely deoxidized Si(1 0 0) as well as for a monohydride termination.



    H. Döscher, S. Brückner, A. Dobrich, C. Höhn, P. Kleinschmidt, T. Hannappel
    Journal of Crystal Growth 315 (1),pp. 10-15
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  8. (2011) Imaging an ionic liquid adlayer by scanning tunneling microscopy at the solid| vacuum interface

    Structure formation in an ionic liquid adlayer: First molecularly resolved scanning tunneling microscopy images of an ionic liquid adlayer ([Py1,4]+ [FAP]− (see image)), evaporated on a Au(111) surface, resolve a molecular pattern at 210 K with a distinct short range order, indicating a 2D solid, while at room temperature, the mobility of the adlayer is too high to resolve molecular features, as expected for a 2D liquid.



    T. Waldmann, H. H. Huang, H. E. Hoster, O. Höfft, F. Endres, R. J. Behm
    ChemPhysChem 12 (14), pp. 2565-2567
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  9. (2011) Growth of an oligopyridine adlayer on Ag (100)–A scanning tunnelling microscopy study

    The growth behaviour of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4′-BTP) on Ag(100) in the sub-monolayer regime was investigated by variable temperature scanning tunneling microscopy under ultra-high vacuum conditions. Over the entire coverage range, the molecules are adsorbed in a flat lying configuration, with preferential orientations with respect to the 〈110〉 direction of the surface. The azimuth angles are derived using a previously introduced algorithm that fits the positions of the intramolecular N atoms geometrically to the underlying surface lattice (“points-to-lattice fit”) [H.E. Hoster et al., Langmuir 2007, 23, 11570], indicating that the orientation of the admolecules and thus of the adllayer structure with respect to the Ag(100) surface lattice is determined by the 2,4′-BTP−Ag(100) interaction, while intermolecular interactions are decisive for the structure of the adlayer. The results will be compared to other adsorption systems.



    T. Waldmann, C. Nenon, H. E. Hoster, R. J. Behm
    Physical Chemistry Chemical Physics 13 (46), pp. 20724-20728
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  10. (2011) Highly Ordered Assembly of Single-Domain Dichloropentacene over Large Areas on Vicinal Gold Surfaces

    Defining pathways to assemble long-range-ordered 2D nanostructures of specifically designed organic molecules is required in order to optimize the performance of organic thin-film electronic devices. We report on the rapid fabrication of a nearly perfect self-assembled monolayer (SAM) composed of a single-domain 6,13-dichloropentacene (DCP) brick-wall pattern on Au(788). Scanning tunneling microscopy (STM) results show the well-ordered DCP SAM extends over hundreds of nanometers. Combining STM results with insights from density functional theory, we propose that a combination of unique intermolecular and molecule-step interactions drives the DCP SAM formation.



    J. Wang, I. Kaur, B. Diaconescu, J.-M. Tang, G.P . Miller, K. Pohl
    ACS Nano 5 (3), pp. 1792–1797
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  11. (2010) Large area chiral supramolecular self-assembly of rubrene has been achieved on Au(111) surface at room temperature

    Large area chiral supramolecular self-assembly of rubrene has been achieved on Au(111) surface at room temperature. The basic building block of such self-assembled layers consists of two double-layered Y-shape supramolecular structures containing eight twisted rubrene molecules. Chirality is maintained and transferred from the bottom layer to the top layer in the successive molecular layers for up to five layers. Such chiral multilayers can be potential candidates in enantioselective catalysis and chiral separations.



    L. Wang, H. Kong, X. Song, X. Liu, H. Wang
    Phys. Chem. Chem. Phys., 12, 14682-14685
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  12. (2010) Structure and oscillatory multilayer relaxation of the bismuth (100) surface

    We present a combined experimental and theoretical study of the surface structure of single crystal Bi(100) via scanning tunneling microscopy (STM), low-energy electron diffraction intensity versus energy (LEED-IV) analysis and density functional theory (DFT). We find that the surface is unreconstructed and shows an unusually large oscillatory multilayer relaxation down to the sixth layer. This unexpected behavior will be explained by a novel mechanism related to the deeply penetrating electronic surface states. STM reveals wide (100) terraces, which are separated by two-layer high steps in which the shorter of the two interlayer spacings is terminating this surface, consistent with the LEED structural analysis and DFT.



    J. Sun, J. Wang, J. Wells, Y. M. Koroteev, G. Bihlmayer, E.V. Chulkov, Ph. Hofmann, K. Pohl
    New Journal of Physics 12, 063016
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  13. (2010) Substrate registry in disordered layers of large molecules

    We present a statistic evaluation of the azimuth orientations of flat‐adsorbed oligopyridine molecules in disordered adlayers on Au(111) and (111) oriented Ag‐adlayers on Ru(0001). On both surfaces, we find a strong preference for a set of twelve angles, which belong to one specific, unsymmetrical alignment and its symmetry equivalents. These angles are also those that exclusively occur in more densely packed, ordered structures on the same surfaces. We describe a geometric fitting algorithm, which correctly predicts these angles, and which only requires the substrate lattice and the positions of the nitrogen atoms within the flat‐adsorbed molecule as input parameters. Such predictions are particularly valuable to reduce the parameter space in structure simulations [C. Rohr, M. Balbás Gambra, K. Gruber, E. C. Constable, E. Frey, T. Franosch, B. A. Hermann, Nano Lett. 2009, 10, 833].



    T. Waldmann, R. Reichert, H. E. Hoster
    ChemPhysChem 11, 1513
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  14. (2010) Hydrogen Bonding Controls the Dynamics of Catechol Adsorbed on a TiO2(110) Surface

    Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly.



    S.-C. Li, L.-N. Chu, X.-Q. Gong, U. Diebold
    Surface Science 328, 882
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  15. (2010) 2D Random Organization of Racemic Amino Acid Monolayers Driven by Nanoscale Adsorption Footprints: Proline on Cu(110)

    Amino acids find their feet: Scanning tunneling microscopy of racemic (R,S)‐proline on Cu(110) reveals rows of random chiral amino acid sequences, showing that organization is not governed by molecular chirality. Instead, the system is dictated by a strict heterochiral adsorption‐footprint template, in which each adsorption position can be occupied by either enantiomer (see picture), resulting in a random solid solution in 2D.



    M. Forster, M. S. Dyer, M. Persson, R. Raval
    Angew. Chem. Int. Ed. 49 (13), pp. 2344-2348
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  16. (2009) Local ordering and electronic signatures of submonolayer water on anatase TiO2(101)

    The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO2 photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO2 over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO2, rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO2 polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule–anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2×2) superstructure of molecular water.



    Y. He, A. Tilocca, O. Dulub, A. Selloni, U. Diebold
    Nature Materials 8, pp. 585–589
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  17. (2009) Nucleation and growth of 1D water clusters on rutile TiO2(011)-2x1

    We present a combined experimental and theoretical study of the adsorption of water on the rutile TiO2(011)-2×1 surface, whose “brookite (001)-like” reconstruction has been recently elucidated. By using scanning tunneling microscopy and density functional theory calculations, we provide evidence that water adsorbs weakly on the stoichiometric surface, while hydroxyls resulting from water dissociation at surface O vacancies act as nucleation centers for the growth of H-bonded water clusters that are confined in one dimension.



    Y. He, X. Gong, O. Dulub, A. Selloni, U. Diebold
    J. Phys. Chem. C 113 (24), pp. 10329–10332
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  18. (2009) Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2 (110)

    This letter reports (i) the enhanced thermal stability (up to 1060 °C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced.



    A. Naitabdi, F. Behafarid, B. Roldan Cuenya
    Applied Physics Letters 94, 083102
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  19. (2008) Templated growth of an ordered array of organic bidimensional mesopores

    We report on a method to fabricate a porous two dimensional (2D) array of porphyrins on c(2×2)N∕Cu(001) with pore sizes larger than 5nm, larger than the reported sizes for hydrogen-bonded or coordination porous organic networks. When deposited on the square nanopattern created by partial nitridation of the Cu(001) surface, the porphyrin molecules prefer to adsorb on clean copper instead of adsorbing on the CuN islands, forming a porous 2D array. This nanopatterning technique can be straightforwardly extended to other molecular species to form the pore walls since its working principle only depends marginally on the nature of the intermolecular interactions.



    D. Écija, M. Trelka, C. Urban, P. de Mendoza, A. Echavarren, R. Otero, J. M. Gallego, R. Miranda
    Appl. Phys. Lett. 92, 223117
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  20. (2008) Surface Structure of Sn-doped In2O3(111) thin films by STM

    High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ~1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.



    E. H. Morales, Y. He, M. Vinnichenko, B. Delley, U. Diebold
    New J. Phys. 10 125030
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  21. (2008) Molecular Conformation , Organizational Chirality, and Iron Metalation of meso-Tetramesitylporphyrins on Copper(100)

    We report on the conformation and self-assembly properties of meso-tetramesitylporphyrin on Cu(100). The results show that the presence of the mesityl groups limits the interaction between the porphyrin ring and the copper surface, contributing to the high porphyrin mobility at room temperature. At low temperatures it is the substrate which determines the molecule orientation. The intermolecular interaction is also very weak, and only for high coverages do the porphyrins self-assemble to form large islands with two different mirror symmetric unit cells. The porphyrins can be Fe metalated by sublimation of Fe at room temperature on a porphyrin overlayer deposited on the copper surface.



    D. Ecija, M. Trelka, C. Urban, P. de Mendoza, E. Mateo-Martı, C. Rogero, J. A. Martın-Gago, A. M. Echavarren, R. Otero, J. M. Gallego, Rodolfo Miranda,
    J. Phys. Chem. C 112 (24), pp. 8988–8994
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  22. (2008) Evidence for the Predominance of Subsurface Defects on TiO2 Anatase(101)

    Scanning tunneling microscopy (STM) images taken on a freshly cleaved anatase TiO2(101) sample show an almost perfect surface with very few subsurface impurities and adsorbates. Surface oxygen vacancies are not typically present but can be induced by electron bombardment. In contrast, a reduced anatase (101) crystal shows isolated as well as ordered intrinsic subsurface defects in STM, consistent with density functional theory (DFT) calculations which predict that O vacancies (VO’s) at subsurface and bulk sites are significantly more stable than on the surface.



    Y. He, O. Dulub, H. Cheng, A. Selloni, U. Diebold
    Phys. Rev. Lett. 102, 106105
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  23. (2007) Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles

    The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300 °C. A subsequent thermal treatment at 700 °C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs’ surface. Annealing at 900 °C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900 °C. At 1000 °C, Au desorption was observed.



    A. Naitabdi, B. Roldan Cuenya
    Appl. Phys. Lett. 91, 113110
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  24. (2007) Epitaxial growth, magnetic properties and lattice dynamics of Fe nanoclusters on GaAs(001)

    Epitaxial bcc-Fe(001) ultrathin films have been grown at ∼50 °C on reconstructed GaAs(001)−(4×6) surfaces and investigated in situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and 57Fe conversion electron Mössbauer spectroscopy (CEMS). For tFe = 1 ML (monolayer) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With increasing tFe the Fe clusters first connect along the [−110], but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at tFe = 4 ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures TB of 62 ± 5, 80 ± 10, and 165 ± 5 K for tFe = 1.9, 2.2, and 2.5 ML, respectively. At T



    B. Roldan Cuenya, A. Naitabdi, E. Schuster, R. Peters, M. Doi, W. Keune
    Phys. Rev. B 76, 094403
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  25. (2007) Direct Visualization of Enantiospecific Substitution of Chiral Guest Molecules into Heterochiral Molecules Assemblies at Surfaces

    Site‐specific chiral recognition in a two‐dimensional heterochiral structure leading to highly enantiospecific substitution by a chiral guest was observed by scanning tunneling microscopy (STM). Thus, in the case of succinic acid (SU) on Cu(110), which forms two chiral motifs denoted D‐ and L‐SU, (R,R)‐tartaric acid ((R,R)‐TA=R) only substitutes L sites (see picture; in the STM image R appears as “slots” aligned −20° to the [001] direction).



    N. Liu, S. Haq, G. R. Darling, R. Raval
    Angew. Chem. Int. Ed. 46 (40), pp. 7613-7616
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  26. (2006) Enantiospecific Adsorption of Cysteine at Chiral Kink Sites on Au(110)-(1x2)

    Enantiospecific adsorption of cysteine molecules onto chiral kink sites on the Au(110)-(1×2) surface was observed by scanning tunneling microscopy. l- and d-cysteine dimers were found to adopt distinctly different adsorption geometries at S kinks, which can be understood from the need to reach specific, optimum molecule−substrate interaction points. Extended, homochiral domains of l/d-cysteine were furthermore observed to grow preferentially from R/S kinks. The results constitute the first direct, microscopic observation of enantiospecific molecular interaction with chiral sites on a metal single-crystal surface.



    A. Kühnle, T. R. Linderoth, F. Besenbacher
    J. Am. Chem. Soc. 128 (4), pp. 1076–1077
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  27. (2006) Chiral switching by spontaneous conformational change in adsorbed organicmolecules

    Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified8 and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.



    S. Weigelt, C. Busse, L. Petersen, E. Rauls, B. Hammer, K. v. Gothelf, F. Besenbacher, T. R. Linderoth
    Nature materials 5, pp. 112-117
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  28. (2006) Through the looking glass

    A mechanism for switching the chirality of molecules adsorbed on gold surfaces offers a thermally activated approach to growing two-dimensional crystals of a single enantiomer.



    N. Richardson
    Nature materials 5, pp. 91-92
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  29. (2006) Azobenzene on Cu(110): Adsorption Site-Dependent Diffusion

    Azobenzene and its derivatives can undergo reversible trans−cis isomerizations when irradiated with light, making them potential candidates for optically sensitive materials and devices. The adsorption and diffusion of azobenzene on the Cu(110) surface was investigated with a variable-temperature scanning tunneling microscope. The trans-isomer was observed and found to occupy two adsorption geometriesan energetically stable and a metastable state. Diffusion occurred along the closed-packed [1 −1 0] direction of the surface, and the diffusivity for the two adsorption states was found to differ by approximately 1 order of magnitude.



    J. A. Miwa, S. Weigelt, H. Gersen, F. Besenbacher, F. Rosei, T. R. Linderoth
    J. Am. Chem. Soc. 128 (10), pp. 3164–3165
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  30. (2006) Scanning tunneling microscopy manipulation of complex organic molecules on solid surfaces

    Organic molecules adsorbed on solid surfaces display a fascinating variety of new physical and chemical phenomena ranging from self-assembly and molecular recognition to nonlinear optical properties and current rectification. Both the fundamental interest in these systems and the promise of technological applications have motivated a strong research effort in understanding and controlling these properties. Scanning tunneling microscopy (STM) and, in particular, its ability to manipulate individual adsorbed molecules, has become a powerful tool for studying the adsorption geometry and the conformation and dynamics of single molecules and molecular aggregates. Here we review selected case studies demonstrating the enormous capabilities of STM manipulations to explore basic physiochemical properties of adsorbed molecules. In particular, we emphasize the role of STM manipulations in studying the coupling between the multiple degrees of freedom of adsorbed molecules, the phenomenon of molecular molding, and the possibility of creating and breaking individual chemical bonds in a controlled manner, i.e., the concept of single-molecule chemistry.



    R. Otero, F. Rosei, F. Besenbacher
    Annu. Rev. Phys. Chem. 57, pp. 497-525
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  31. (2006) Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy

    The relationship between the structural/morphological and electronic properties of size-selected gold nanoparticles was investigated using scanning tunneling microscopy and spectroscopy. The nanoparticles were synthesized by inverse micelle encapsulation and were dip-coated on TiO2∕Ti(15nm)∕Si(111). Annealing in vacuum to 500 °C resulted in the removal of the polymer and the formation of an ultrathin TiC support. Significant changes in the electronic local density of states (LDOS) of the nanoparticles, in particular, the onset of nonmetallic behavior, were observed with decreasing particle size. The nanoparticle-support interactions were studied and evidence for substrate-induced modifications in the LDOS of interfacial gold atoms is found.



    A. Naitabdi, L. K. Ono, B. Roldan Cuenya
    Applied Physics Letters 89, 043101
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  32. (2006) Revisiting the Structure of the p(4x4) Surface Oxide on Ag(111)

    Scanning tunneling microscopy (STM) and density-functional theory are used to reexamine the structure of the renowned p(4×4)−O/Ag(111) surface oxide. The accepted structural model [C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000)] is incompatible with the enhanced resolution of the current STM measurements. An “Ag6 model” is proposed that is more stable than its predecessor and accounts for the coexistence of the p(4×4) and a novel c(3×5√3)rect phase. This coexistence is an indication of the dynamic complexity of the system that until now has not been appreciated.



    J. Schnadt, A. Michaelides, J. Knudsen, R. T. Vang, K. Reuter, E. Lægsgaard, M. Scheffler, F. Besenbacher
    Phys. Rev. Lett. 96, 146101
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  33. (2006) L-Cysteine Adsorption Structures on Au(111) Investigated by Scanning Tunneling Microscopy under Ultrahigh Vacuum Conditions

    Adsorption structures formed upon vapor deposition of the natural amino acid l-cysteine onto the (111) surface of gold have been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. Following deposition at room temperature and at cysteine coverages well below saturation of the first monolayer, we found coexistence of unordered molecular islands and extended domains of a highly ordered molecular overlayer of quadratic symmetry. As the coverage was increased, a number of other structures with local hexagonal order emerged and became dominant. Neither of the room temperature, as-deposited, ordered structures showed any fixed rotational relationship to the underlying gold substrate, suggesting a comparatively weak and nonspecific molecule−substrate interaction. Annealing of the cysteine-covered substrate to 380 K lead to marked changes in the observed adsorption structures. At low coverages, the unordered islands developed internal order and their presence started to perturb the appearance of the surrounding Au(111) herringbone reconstruction. At coverages beyond saturation of the first monolayer, annealing led to development of a (√3 × √3)R30° superstructure accompanied by the formation of characteristic monatomically deep etch pits, i.e., the behavior typically observed for alkanethiol self-assembled monolayers on Au(111). The data thus show that as-deposited and thermally annealed cysteine adsorption structures are quite different and suggest that thermal activation is required before vacuum deposited cysteine becomes covalently bound to single crystalline Au(111).



    A. Kühnle, T. R. Linderoth, M. Schunack, F. Besenbacher
    Langmuir, 22 (5), pp. 2156–2160
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  34. (2003) Oxygen-Mediated Diffusion of Oxygen Vacancies on the TiO2(110) Surface

    Defects such as oxygen vacancies play a crucial role in the surface properties of transition metal oxides. By means of time-resolved, high-resolution scanning tunneling microscopy, we unraveled an adsorbate-mediated diffusion mechanism of oxygen vacancies on rutile TiO2(110). Adsorbed oxygen molecules mediate vacancy diffusion through the loss of an oxygen atom to a vacancy and the sequential capture of an oxygen atom from a neighboring bridging oxygen row, leading to an anisotropic oxygen vacancy diffusion pathway perpendicular to the bridging oxygen rows.



    R. Schaub, E. Wahlström, A. Rønnau, E. Lægsgaard, I. Stensgaard, F. Besenbacher
    Science 299, pp. 377-379
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  35. (2003) Chemistry of one-dimensional metallic edge states in MoS2 nanoclusters

    Nanostructures often have unusual properties that are linked to their small size. We report here on extraordinary chemical properties associated with the edges of two-dimensional MoS2 nanoclusters, which we show to be able to hydrogenate and break up thiophene (C4H4S) molecules. By combining atomically resolved scanning tunnelling microscopy images of single-layer MoS2 nanoclusters and density functional theory calculations of the reaction energetics, we show that the chemistry of the MoS2 nanoclusters can be associated with one-dimensional metallic states located at the perimeter of the otherwise insulating nanoclusters. The new chemistry identified in this work has significant implications for an important catalytic reaction, since MoS2 nanoclusters constitute the basis of hydrotreating catalysts used to clean up sulfur-containing molecules from oil products in the hydrodesulfurization process.



    J. V. Lauritsen, M. Nyberg, R. T. Vang, M. V. Bollinger, B. S. Clausen, H. Topsøe, K. W. Jacobsen, E. Lægsgaard, J. K. Nørskov, F. Besenbacher
    Nanotechnology 14, pp. 1-5
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  36. (2002) Chiral recognition in dimerization of adsorbed cysteine observed by scanning tunnelling microscopy

    Stereochemistry plays a central role in controlling molecular recognition and interaction: the chemical and biological properties of molecules depend not only on the nature of their constituent atoms but also on how these atoms are positioned in space. Chiral specificity is consequently fundamental in chemical biology and pharmacology and has accordingly been widely studied. Advances in scanning probe microscopies now make it possible to probe chiral phenomena at surfaces at the molecular level. These methods have been used to determine the chirality of adsorbed molecules, and to provide direct evidence for chiral discrimination in molecular interactions and the spontaneous resolution of adsorbates into extended enantiomerically pure overlayers. Here we report scanning tunnelling microscopy studies of cysteine adsorbed to a (110) gold surface, which show that molecular pairs formed from a racemic mixture of this naturally occurring amino acid are exclusively homochiral, and that their binding to the gold surface is associated with local surface restructuring. Density-functional theory calculations indicate that the chiral specificity of the dimer formation process is driven by the optimization of three bonds on each cysteine molecule. These findings thus provide a clear molecular-level illustration of the well known three-point contact model for chiral recognition in a simple bimolecular system.



    A. Kühnle, T. R. Linderoth, B. Hammer, F. Besenbacher
    Nature 415, pp. 891-893
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  37. (2002) Organic Molecules Acting as Templates on Metal Surfaces

    The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.



    F. Rosei, M. Schunack, P. Jiang, A. Gourdon, E. Lægsgaard, I. Stensgaard, C. Joachim, F. Besenbacher
    Science 296, pp. 328-331
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  38. (2001) A high-pressure scanning tunneling microscope

    We present the design and performance of a high-pressure scanning tunneling microscope (HP–STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10−10–1000 mbar) on a routine basis. The HP–STM is integrated in a gold-plated high-pressure cell with a volume of only ∼0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gases is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.



    E. Lægsgaard, L. Österlund, P. Thostrup, P. B. Rasmussen, I. Stensgaard, F. Besenbacher,
    Rev. of Scientific Instruments 72 (9), pp. 3537-3542
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  39. (2001) Atomic-Scale Structure of Dislocations Revealed by Scanning Tunneling Microscopy and Molecular Dynamics

    The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width at the surface, in agreement with the STM observations. Implications for surface-induced cross slip are discussed.



    J. Christiansen, K. Morgenstern, J. Schiøtz, K. W. Jacobsen, K.-F. Braun, K.-H. Rieder, E. Lægsgaard, F. Besenbacher,
    Phys. Rev. Lett. 88 (20), 206106
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  40. (2001) Bridging the Pressure Gap in Surface Science at the Atomic Level: H/Cu (110)

    The structural response of the Cu(110) surface to H2 gas pressures ranging from 10−13 to 1 bar is studied using a novel high-pressure scanning tunneling microscope (HP-STM). We find that at H2 pressures larger than 2 mbar the Cu(110) surface reconstructs into the ( 1×2) “missing-row” structure. From a quantitative analysis of the pressure dependence of the surface reconstruction, we conclude that Cu(110) responds identically to hydrogen at ultrahigh vacuum conditions and at atmospheric pressures. From the HP-STM data, we extract refined values for the adsorption and desorption rate constants



    L. Österlund, P. B. Rasmussen, P. Thostrup, E. Lægsgaard, I. Stensgaard, F. Besenbacher
    Phys. Rev. Lett. 86 (3), pp. 460-463
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  41. (2001) Anchoring of Organic Molecules to a Metal Surface: HtBDC on Cu(110)

    The interaction of largish molecules with metal surfaces has been studied by combining the imaging and manipulation capabilities of the scanning tunneling microscope (STM). At the atomic scale, the STM results directly reveal that the adsorption of a largish organic molecule can induce a restructuring of a metal surface underneath. This restructuring anchors the molecules on the substrate and is the driving force for a self-assembly process of the molecules into characteristic molecular double rows.



    M. Schunack, L. Petersen, A. Kühnle, E. Lægsgaard, I. Stensgaard, I. Johannsen, F. Besenbacher
    Phys. Rev. Lett. 86, pp. 456-459
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  42. (2001) Atomic-Scale Structure of Co-Mo-S Nanoclusters in Hydrotreating Catalysts

    By means of scanning tunneling microscopy (STM), it has been possible to obtain the first atomic-scale images of the Co–Mo–S structure present in hydrodesulfurization (HDS) catalysts. Information on the catalytically important edge structures has been obtained by synthesizing single-layer Co–Mo–S nanoclusters using the Au(111) herringbone structure as a template. It is observed that the presence of the Co promoter atoms causes the shape of the MoS2 nanoclusters to change from triangular to hexagonally truncated. This change in morphology appears to be driven by a preference for Co to be located at the S-edge of MoS2. The results also directly show that the presence of the Co atoms perturbs the local electronic environment of neighboring S atoms and this provides further insight into the effect of the promoter atoms.



    J. V. Lauritsen, S. Helveg, E. Lægsgaard, I. Stensgaard, B. S. Clausen, H. Topsøe, F. Besenbacher
    Journal of Catalysis 197, pp. 1-5
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  43. (2001) Oxygen Vacancies as Active Sites forWater Dissociation on Rutile TiO2 (110)

    Through an interplay between scanning tunneling microscopy experiments and density functional theory calculations, we determine unambiguously the active surface site responsible for the dissociation of water molecules adsorbed on rutile TiO2(110). Oxygen vacancies in the surface layer are shown to dissociate H2O through the transfer of one proton to a nearby oxygen atom, forming two hydroxyl groups for every vacancy. The amount of water dissociation is limited by the density of oxygen vacancies present on the clean surface exclusively. The dissociation process sets in as soon as molecular water is able to diffuse to the active site.



    R. Schaub, P. Thostrup, N. Lopez, E. Lægsgaard, I. Stensgaard, J. K. Nørskov, F. Besenbacher,
    Phys. Rev. Lett. 87 (26), 266104-1
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  44. (1999) Palladium Nanocrystals on Al2O3: Structure and Adhesion Energy

    We have investigated the structure and morphology of nanosized palladium clusters supported by a thin Al2O3 film on NiAl(110) using scanning tunneling microscopy. Well-ordered clusters with a diameter above ≈40Å were imaged with atomic resolution, allowing a crystallographic identification of the cluster facets. A new method to obtain quantitative information on the work of adhesion (adhesion energy) of metal clusters deposited on oxides is introduced. For Pd on Al2O3 we obtain a value of Wadh=2.8±0.2J/m2. This result is at variance with values recently derived on the basis of ab initio density-functional theory.



    K. Højrup Hansen, T. Worren, S. Stempel, E. Lægsgaard, M. Bäumer, H.-J. Freund, F. Besenbacher, I. Stensgaard
    Phys. Rev. Lett. 83, 4120
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  45. (1999) Enhancement of surface self-diffusion of platinum atoms by adsorbed hydrogen

    Surface diffusion of atoms is an important phenomenon in areas of materials processing such as thin-film growth and sintering. Self-diffusion (that is, diffusion of the atoms of which the surface is comprised) has been much studied on clean metal and semiconductor surfaces. But in most cases of practical interest the diffusion happens on surfaces partly covered by atoms and molecules adsorbed from the gas phase. Adsorbed hydrogen atoms are known to be capable of both promoting and inhibiting self-diffusion, offering the prospect of using adsorbed gases to control growth or sintering processes. Here we derive mechanistic insights into this effect from observations, using the scanning tunnelling microscope, of hydrogen-promoted self-diffusion of platinum on the Pt(110) surface. We see an activated Pt–H complex which has a diffusivity enhanced by a factor of 500 at room temperature, relative to the other Pt adatoms. Our density-functional calculations indicate that the Pt–H complex consists of a hydrogen atom trapped on top of a platinum atom, and that the bound hydrogen atom decreases the diffusion barrier.



    S. Horch, H. T. Lorensen, S. Helveg, E. Lægsgaard, I. Stensgaard, K. W. Jacobsen, J. K. Nørskov, F. Besenbacher
    Nature 398, 134
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  46. (1997) Surface Diffusion of Pt on Pt(110): Arrhenius Behavior of Long Jumps

    The one-dimensional diffusion of Pt adatoms in the missing row troughs of the (1×2) reconstructed Pt(110) surface is monitored directly from atomically resolved time-lapsed scanning tunneling microscopy images. For this self-diffusion system, it is surprisingly found that not only jumps between nearest neighbor sites but also long jumps, i.e., jumps between next nearest neighbor sites, participate. The hopping rate for these long jumps is found to follow an Arrhenius dependence with an activation barrier for diffusion (Ed2=0.89eV) slightly larger than that for single jumps (Ed1=0.81eV).



    T. R. Linderoth, S. Horch, E. Lægsgaard, I. Steensgaard, F. Besenbacher
    Phys.Rev.Lett. 78, 4978
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  47. (1996) Scanning tunnelling microscopy studies of metal surfaces

    Scanning tunnelling microscopy (STM) has proved to be a fascinating and powerful technique in the field of surface science. The fact that sets the STM apart from most other surface sensitive techniques is its ability to resolve the structure of surfaces on an atomic scale, that is atom-by-atom, and furthermore its ability to study the dynamics of surface processes. This article presents a survey of recent STM studies of well characterized single crystal metal surfaces under ultra-high vacuum conditions. It particularly addresses STM investigations of clean metal surfaces, adsorbates on metal surfaces, adsorbate-induced restructuring of metal surfaces, chemical reactions on metal surfaces, metal-on-metal growth and finally studies of electron confinement and quantum size effects on metal surfaces.



    F. Besenbacher
    Rep. Prog. Phys. 59, pp. 1737-1802
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