Low-temperature scanning tunneling microscopy and high-resolution photoemission

Responsible: Heinz Hövel (CV: {pdf}),
Coworkers: Stefanie Duffe (Postdoc)
Niklas Grönhagen, Sabrina Hennes, Lukas Patryarcha (PhD students),
Karl Bauer, Kolja Mende, Natalie Miroslawski (Diploma students)

email: <last name> @ physik.uni-dortmund.de

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Contents

Projects

Mass selected clusters on surfaces

Metalclusters in nanopits

Production of nanopits using focused ion beams

Rare gas adsorbate layers

Other projects

Experimental method and setup

Publications

PhD and Diploma theses

Lecture notes

Public outreach etc.

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Projects

Mass selected clusters on surfaces
(in cooperation with Bernd von Issendorff, Univ. Freiburg)

Clusters are particles between atom and bulk, with particle sizes between 1 nm and 100 nm, this corresponds to from about 10 up to 10 billion atoms per cluster. Their investigation is the basis for the rapidly developing nanotechnology as a continuation of the technically relevant miniaturization. The production of clusters itself is already a complex process.

In this project (supported by the DFG, SPP 1153) the combined results of tunneling spectroscopy and of photoelectron spectroscopy performed on size selected large clusters in tunneling contact with a surface as well as of photoelectron spectroscopy on the same clusters in the gas phase will be used to clarify the electronic structure of the cluster/surface system and the nature of charge transfer processes between the cluster and the surface. Furthermore this direct comparison of the three different techniques is expected to improve the understanding of the results of tunneling spectroscopy on metal particles.

Figure 1 shows the experimental setup, in which the low-temperature surface science facility with scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS) is combined with a cluster machine consisting of a magnetron sputter gas aggregation source, a differential pumping stage with a cryo pump and a high transmission infinite range mass selector. For these samples low-temperature STM and UPS will be compared with photoelectron spectroscopy for the same clusters in a free cluster beam.

Figure 1

As a new and promising substrate system for the softlanding of mass selected clusters we investigated a monolayer of C₆₀ molecules on an Au(111) surface. Figure 2 shows on the left side mass selected silver clusters with 309±3 atoms deposited on this substrate (image size: 65 × 65 nm2, height scale: 2.75 nm). All the clusters have the same height, which gets obvious in comparison to the right side of Figure 2, which shows islands of similar size grown by the deposition of Ag atoms (image size: 50 × 50 nm2, height scale: 2.0 nm). More details can be found in publication no.35 and in a recent talk given on the Symposium on Size Selected Clusters 2007.

Figure 2

 

Metalclusters in nanopits

Since several years we use the method of controlled growth of silver and gold clusters in nanometer sized pits, which were prepared in advance on a graphite surface. With a controlled defect production and a following oxidation process we produced 'nanopits', with a depth of one atomic layer and a width of some few nanometers on a graphite surface. The edges of these nanopits can be used as nucleation centers for a well-defined cluster growth. The cluster-size is controlled by the amount of material, which is deposited by physical vapor deposition on the surface. Figure 3 displays two examples. The image on the left side shows small gold clusters, with an average height of 2.1 nm. On the right side more gold was deposited (effective coverage 0.920 monolayers) which led only to a small increase in cluster height and a more lateral cluster growth with hexagonal (111) facets on top of the clusters. The images were measured with low-temperature ultrahigh vacuum scanning tunneling microscopy at a temperature of T=5 Kelvin.

Figure 3

 

Production of ordered nanometer sized pits with Focused Ion Beams (FIB)
(in cooperation with Raith GmbH, Dortmund)

In previous experiments with metal clusters we used nanopits distributed at random locations on a graphite surface. In this project a focused beam of gallium ions is used to produce the defects at localized positions on the substrate. The “ionLiNE” Focused Ion Beam (FIB) facility can achieve a resolution better than 10 nm. The oxidation of a sample structured with FIB allows to measure areas structured with very small ion intensity or the penetration depth of the incident ions.

Figure 4 shows an image, measured with STM, of nanopits produced by the oxidation of defects placed with FIB in a square pattern with 300 nm pitch. In addition an oxidized grain boundary is visible as a black line crossing in vertical direction together with subsurface defects as white features.

Figure 4

 

Rare gas adsorbate layers

Rage gases have a long tradition in serving as model systems in solid state physics due to their spherical symmetry and simple electronic structure. In surface science they are used as a model system for general principles of the geometric and electronic structure of two-dimensional adsorbates. The system 'xenon on graphite' has been extensively studied by various experimental techniques, mostly based on diffraction methods. With Low temperature STM we were able to perform a real-space study with atomic resolution (Figure 5, 16 nm x 16 nm, T = 5 K). With this we revealed new details for the structure and dynamics of the xenon monolayer.

Figure 5

The adsorption of a xenon monolayer on the silver (111) surface results in a modification of the surface state, a two dimensional electron state localized at the surface. Due to a shift of about 120 meV to higher energy it is no longer occupied with electrons. Therefore its signal in the photoelectron spectrum disappears. However, using scanning tunneling spectroscopy, we were able to investigate this modifaction in detail, because this method also gives information on unoccupied electron states. With the measurement of standing electron wave patterns, formed at defects and crystal step edges, we were able to compare the energy-dependent wavelength of the surface states on the clean and the xenon covered surface. A film sequence presenting these results can be found here.

 

Other projects

Investigation of molecular clusters (e.g. Fullerenes) on surfaces; Production and investigation of one-dimensional nanostructures (e.g. carbon-nanotubes); manipulation of nanostructures with the scanning tunneling microscope.

 

Experimental method and setup

The method of scanning tunneling microscopy (STM) was developed by G. Binnig and H. Rohrer at the IBM Zurich Research Laboratory (Nobel prize in physics, 1986). An atomically sharp tip is scanned over a sample surface by measuring the tunneling current, which varies strongly with the distance to the sample surface. In this way an image of the sample surface is produced which shows surface structures on an atomic scale. Measurements at room temperature and even under atmospheric conditions are possible. But for experiments in surface science it is absolutely necessary to work in ultra-high-vacuum (UHV) at a pressure below 10 exp -10 mbar, to keep the sample surface free from contamination by residual gas for a considerable measurement time. Novel phenomena become available to experiment if the measurements are performed at low temperatures near absolute zero. Then random thermal motion is stopped, which among others stabilizes the sample and the tip significantly. The additional technical problems, which arise at low temperatures, like vibration isolation or the choice of materials, were solved in a long-term collaboration by the IBM Research Laboratory in Zurich and OMICRON Nanotechnology GmbH. The combination of a low-temperature scanning tunneling microscope with additional methods for surface analysis (e.g. photoelectron spectroscopy with high energy resolution) in one UHV-apparatus allows investigating the various aspects of the geometric and electronic sample properties on one and the same sample.

Our two-chamber UHV-surface science facility contains a low-temperature scanning tunneling microscope (Omicron NanoTechnology GmbH) for temperatures below 5 Kelvin, an analysis-/ preparation-chamber with a manipulator which enables sample cooling and heating, sputtering, UHV-evaporators, photoelectron spectroscopy with high energy-resolution, Auger electron spectroscopy, electron diffraction (LEED).

Examples for photoelectron spectroscopy with high energy-resolution on a silver sample and atomic resolution at 4.9 Kelvin on a Au(111) surface, can be found here. A film sequence with low-temperature scanning tunneling spectroscopy maps which illustrates the standing wave pattern of surface state electrons on Ag(111) is shown here.

 

Publications

1. H. Hövel, P. Grosse, W. Theiss
   „Analysis of photoacoustic IR spectra of aerogel and silica powder“
   Journal of Non-Crystalline Solids 145, 159 (1992)
2. H. Hövel, S. Fritz, A. Hilger, U. Kreibig, M. Vollmer
   „Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping“
   Physical Review B 48, 18178 (1993) {pdf}
3. U. Kreibig, A. Hilger, H. Hövel, M. Quinten
   „Optical Properties of free and embedded metal clusters: Recent results“
   in: „Large Clusters of Atoms and Molecules“, ed. T.P. Martin,
   (Kluwer, 1996), 475
4. U. Kreibig, M. Gartz, A. Hilger, H. Hövel
   „Mie-plasmon spectroscopy: A tool of surface science“
   in: „Fine Particles Science and Technology“, ed. E. Pelizzetti,
   (Kluwer, 1996), 499
5. A. Relitzki, A. Hilger, H. Hövel, U. Kreibig, D. Schumacher, H. Winkes
   „Deposition of silver clusters on silver surfaces: Influences on the electrical resistance“
   in: „Science and Technology of Atomically Engineered Materials“, eds. P. Jena, S.N. Khanna, B.K. Rao
   (World Scientific, Singapore, 1996), 453
6. U. Kreibig, M. Gartz, A. Hilger, H. Hövel
   „Surface analysis by cluster-plasmon spectroscopy“
   in: „Science and Technology of Atomically Engineered Materials“, eds. P. Jena, S.N. Khanna, B.K. Rao
   (World Scientific, Singapore, 1996), 403
7. H. Hövel, Th. Becker, A. Bettac, B. Reihl, M. Tschudy, E.J. Williams
   „Controlled cluster condensation into preformed nanometer-sized pits“
   J. Appl. Phys. 81, 154 (1997) {pdf}
8. H. Hövel, Th. Becker, A. Bettac, B. Reihl, M. Tschudy, E.J. Williams
   „Crystalline structure and orientation of gold clusters grown in preformed nanometer-sized pits“
   Appl. Surf. Sci. 115, 124 (1997) {pdf}
9. H. Hövel, A. Hilger, I. Nusch, U. Kreibig
   „Experimental determination of deposition induced cluster deformation“
   Z. Phys. D 42, 203 (1997) {pdf}
10. T. Becker, H. Hövel, M. Tschudy, B. Reihl
    „Applications with a new Low-Temperature UHV STM at 5K“
    Appl. Phys. A 66, S27 (1998) {pdf}
11. H. Hövel, T. Becker, D. Funnemann, B. Grimm, C. Quitmann, B. Reihl
    „High-Resolution Photoemission Combined with Low-Temperature STM“
    J. Electron Spectros. Rel. Phenom. 88-91, 1015 (1998) {pdf}
12. U. Kreibig, M. Gartz, A. Hilger, H. Hövel
    „Optical Inverstigations of Surfaces and Interfaces of Metal Clusters“
    in: „Advances in Metal and Semiconductor Clusters, Vol. 4, Cluster Materials“, ed. M. A. Duncan,
    (JAI Press, 1998), 345
13. H. Hövel, B. Grimm, M. Pollmann, B. Reihl
    „Cluster-Substrate Interaction on a Femtosecond Timescale Revealed by a High-Resolution Photoemission Study of the Fermi-Level Onset“
    Phys. Rev. Lett. 81, 4608 (1998) {pdf}
14. H. Hövel, B. Grimm, M. Pollmann, B. Reihl
    „Femtosecond dynamics of final-state effects in the valence band photoemission of silver clusters“
    The European Physical Journal D9, 595 (1999) {pdf}
15. B. Grimm, H. Hövel, M. Pollmann, B. Reihl
    „Physisorbed Rare-Gas Monolayers: Evidence for Domain-Wall Tilting“
    Phys. Rev. Lett. 83, 991 (1999) {pdf}
16. H. Hövel, L.S.O. Johansson, B. Reihl
    „Fundamentals of Adsorbate-Surface Interactions“
    in: „Metal Clusters at Surfaces“, ed. K.H. Meiwes-Broer, Springer Series in Cluster Physics (2000), 37
17. B. Grimm, H. Hövel, M. Bödecker, K. Fieger, B. Reihl
    „Observation of Domain-Wall Dynamics in Rare-Gas Monolayers at T = 5 K“
    Surf. Sci. 454-456, 618 (2000) {pdf}
18. H. Hövel, B. Grimm, M. Bödecker, K. Fieger, B. Reihl
    „Tunneling spectroscopy on silver clusters at T = 5 K: size dependence and spatial energy shifts“
    Surf. Sci. 463, L603 (2000) {pdf}
19. H. Hövel
    „Clusters on surfaces: High resolution spectroscopy at low temperatures“
    Appl. Phys. A 72, 295 (2001) {pdf}
20. H. Hövel, B. Grimm, B. Reihl
    „Modification of the Shockley-Type Surface State on Ag(111) by an Adsorbed Xenon Layer“
    Surf. Sci. 477, 43 (2001) {pdf}
21. C. Kennerknecht, H. Hövel, M. Merschdorf, S. Voll, W. Pfeiffer
    „Surface plasmon assisted photoemission from Au nanoparticles on Graphite“
    Appl.Phys.B 73, 425 (2001)
22. H. Hövel, M. Bödecker, B. Grimm, C. Rettig
    „Growth mechanisms of carbon nanotubes using controlled production in ultrahigh vacuum“
    J. Appl. Phys. 92, 771 (2002)
    (selected for the July 8, 2002 issue of the Virtual Journal of Nanoscale Science & Technology) {pdf}
23. C. Rettig, M. Bödecker, H. Hövel
    „Carbon-nanotubes on graphite: alignment of lattice structure“
    J. Phys. D: Appl. Phys. 36, 818 (2003) {pdf}
24. I. Barke, H. Hövel
    „Confined Shockley surface states on the (111) facets of gold clusters“
    Phys. Rev. Lett. 90, 166801 (2003)
    (selected for the May 5, 2003 issue of the Virtual Journal of Nanoscale Science & Technology) {pdf}
25. H. Hövel, I. Barke
    „Large noble metal clusters: electron confinement and band structure effects“
    New J. Phys. 5, 31 (2003) {pdf}
26. H. Hövel, I. Barke, H.-G. Boyen, P. Ziemann, M.G. Garnier, P. Oelhafen
    „Photon energy dependence of the dynamic final-state effect for metal clusters at surfaces“
    Phys. Rev. B 70, 045424 (2004) {pdf}
27. Thomas Andreev, Ingo Barke, Heinz Hövel
    „Adsorbed rare-gas layers on Au(111): Shift of the Shockley surface state studied with ultraviolet photoelectron spectroscopy and scanning tunneling spectroscopy“
    Phys. Rev. B 70, 205426 (2004) {pdf}
28. M. Paulus, R. Fendt, C. Sternemann, C. Gutt, H. Hövel, M. Volmer, M. Tolan, K. Wille
    „An internet-based synchrotron experiment for students measuring the X-ray magnetic circular dichroism of a PtFe alloy“
    Journal of Synchrotron Radiation, 12, 246 (2005)
29. T. Irawan, I. Barke, H. Hövel
    „Size dependent morphology of gold clusters grown on nanostructured graphite“
    Appl. Phys. A 80, 929 (2005) {pdf}
30. Marina Pivetta, Francois Patthey, Ingo Barke, Heinz Hövel, Bernard Delley, Wolf-Dieter Schneider
    „Gap opening in the surface electronic structure of graphite induced by adsorption of alkali atoms“
    Phys. Rev. B 71, 165430 (2005)
31. T. Irawan, D. Boecker, F. Ghaleh, C. Yin, B. v.Issendorff, H. Hövel
    „Metal clusters on rare gas layers - growth and spectroscopy“
    Appl. Phys. A 82, 81 (2006) {pdf}
32. Heinz Hövel, Ingo Barke
    „Morphology and electronic structure of gold clusters on graphite: scanning-tunneling techniques and photoemission “
    Progress in Surface Science 81, 53 (2006) {pdf}
33. F. Ghaleh, R. Köster, H. Hövel, L. Bruchhaus, S. Bauerdick, J. Thiel, R. Jede
    „Controlled fabrication of nanopit-patterns on a graphite surface using focused ion beams and oxidation“
    J. Appl. Phys. 101, 044301 (2007) {pdf}
34. Heinz Hövel, Mario DeMenech, Mirko Bödecker, Christian Rettig, Ulf Saalmann, Martin E. Garcia
    „Tip-induced distortions in STM imaging of carbon nanotubes“
    The European Physical Journal D 45, 459 (2007)
35. Stefanie Duffe, Thomas Irawan, Markus Bieletzki, Torsten Richter, Benedikt Sieben,
    Chunrong Yin, Bernd von Issendorff, Michael Moseler, Heinz Hövel
    „Softlanding and STM imaging of Ag561 clusters on a C60 monolayer“
    The European Physical Journal D 45, 401 (2007) {pdf}
36. M. Rohmer, F. Ghaleh, M. Aeschlimann, M. Bauer, H. Hövel
    „Mapping the femtosecond dynamics of supported clusters with nanometer resolution“
    The European Physical Journal D 45, 491 (2007)
37. Stefanie Duffe, Niklas Grönhagen, Lukas Patryarcha, Ben Sieben, Chunrong Yin, Bernd von Issendorff,
    Michael Moseler, Heinz Hövel
    „Penetration of thin C60 films by metal nanoparticles“
    Nature Nanotechnology, published online April 2010, DOI: 10.1038/NNANO.2010.45
38. Ben Wortmann, Kolja Mende, Stefanie Duffe, Niklas Grönhagen, Bernd von Issendorff,
    Heinz Hövel
    „Ultraviolet photoelectron spectroscopy of supported mass selected silver clusters“
    Phys. Status Solidi B, published online March 2010, DOI: 10.1002/pssb.200945586

 

PhD and Diploma theses

1. Marco Schaffhöfer: „Untersuchungen an nanostrukturierten Oberflächen mittels Rastertunnelmikroskopie“, Diplomarbeit, Universität Dortmund, Mai 1997 {pdf}
2. Michael Pollmann: „Geometrische und elektronische Struktur von adsorbierten Fullerenschichten“, Diplomarbeit, Universität Dortmund, September 1998 {pdf}
3. Kristina Fieger: „Wachstum und Morphologie von Goldclustern auf Oberflächen untersucht mit STM und UPS“, Diplomarbeit, Universität Dortmund, August 1999 {pdf}
4. Mirko Bödecker: „Neues Verfahren zur Herstellung von Kohlenstoff-Nanoröhren und ihre Untersuchung mit dem STM“, Diplomarbeit, Universität Dortmund, August 1999 {pdf}
5. Burkhard Grimm: „Tunnelspektroskopie und Photoemission bei tiefen Temperaturen an Edelgas-Modellsystemen und Nanostrukturen“, Dissertation, Universität Dortmund, Februar 2000 {pdf}
6. Ingo Barke: „Edelgasschichten auf der Au(111)-Oberfläche: Präparation und lokale Tunnelspektroskopie“, Diplomarbeit, Universität Dortmund, September 2001 {pdf}
7. Christian Rettig: „Wechselwirkung von Clustern und Nanoröhren mit Oberflächen: Messung der geometrischen und elektronischen Struktur“, Diplomarbeit, Universität Dortmund, Oktober 2001 {pdf}
8. Thomas Andreev: „Herstellung und Untersuchung von Edelgas-Adsorbatschichten: Einfluss auf die geometrische und elektronische Struktur der Oberfläche“, Diplomarbeit, Universität Dortmund, Januar 2003 {pdf}
9. Thomas Irawan: „Morphologie und elektronische Struktur von Edelmetallclustern hergestellt durch gesteuertes Wachstum auf HOPG“, Diplomarbeit, Universität Dortmund, September 2003 {pdf}
10. Daniel Boecker: „Cluster auf Oberflächen: Herstellung neuer Substratsysteme und Charakterisierung der Cluster-Deposition“, Diplomarbeit, Universität Dortmund, Oktober 2004 {pdf}
11. Ingo Barke: „Morphology and Electronic Structure of Gold Clusters on Graphite“, Dissertation, Universität Dortmund, Dezember 2004 {pdf}
12. Christian Rettig: „Bleinanostrukturen auf mit Wasserstoff passiviertem Silizium (111): Elektrochemische Abscheidung und Vakuumaufdampfverfahren“, Dissertation, Universität Dortmund, Juni 2005 {pdf}
13. Farhad Ghaleh: „Messungen mit Photoelektronen Spektroskopie an Metallclustern auf Edelgasschichen“, Diplomarbeit, Universität Dortmund, Juni 2005 {pdf}
14. Stefanie Krause: „Massenselektierte Cluster deponiert auf Oberflächen“, Diplomarbeit, Universität Dortmund, März 2006 {pdf}
15. Markus Bieletzki: „Rastertunnelmikroskopie an Clustern auf Edelgasfilmen“, Diplomarbeit, Universität Dortmund, Juni 2006 {pdf}
16. Thomas Irawan: „ Geometric and Electronic Properties of Size-Selected Metal Clusters on Surfaces “, Dissertation, Universität Dortmund, September 2006 {pdf}
17. Robert Köster: „Defekterzeugung mit fokussierten Ionenstrahlen zur Herstellung von Nanostrukturen“, Diplomarbeit, Universität Dortmund, September 2006 {pdf}
18. Torsten Richter: „Untersuchung von deponierten und gewachsenen Silberclustern auf Fullerenschichten mittels Rastertunnelmikroskopie“, Diplomarbeit, Universität Dortmund, März 2007 {pdf}
19. Benedikt Sieben: „Rastertunnelsmikroskopie an Clustersystemen“, Diplomarbeit, Universität Dortmund, September 2007
20. Lukas Patryacha: „Massenselektierte Silbercluster und aufgedampftes Silber auf C60/HOPG und C60/Au(111)“, Diplomarbeit, Universität Dortmund, August 2008
21. Niklas Grönhagen: „Nanostrukturen auf mit FIB strukturiertem Graphit“, Diplomarbeit, Universität Dortmund, Oktober 2008
22. Farhad Ghaleh: „Charakterisierung von Oberflächendefekten hergestellt durch fokussierte Ionenstrahlen und Exploration möglicher Anwendungen zum kontrollierten Wachstum von Nanostrukturen“, Dissertation, Universität Dortmund, Januar 2009
23. Stefanie Duffe: „Size-Selected Ag Clusters: Soft-Landing, Stability and Spectroscopy”, Dissertation, Universität Dortmund, April 2009
24. Sabrina Hennes: „Herstellung und Charakterisierung von Clustern in einer Überschallexpansion“, Diplomarbeit, Universität Dortmund, September 2009
25. Ben Wortmann: „Spektroskopie von massenselektierten Clustern“, Diplomarbeit, Universität Dortmund, September 2009
26. Stefan Balk: „Nanostrukturen auf mit FIB strukturiertem Graphit“, Diplomarbeit, Universität Dortmund, Dezember 2009
27. Niklas Grönhagen: „Spectroscopy of large metal clusters: comparison between clusters at surfaces and free clusters”, Dissertation, Universität Dortmund, laufende Arbeit
28. Lukas Patryarcha: „Graphene Nanostructures produced with focussed Ion Beams and Oxidation”, Dissertation, Universität Dortmund, laufende Arbeit
29. Lars Bruchhaus: „Development of a nano patterning FIB instrument for research applications and rapid prototyping with sub 10nm patterning capabilities”, Dissertation, Universität Dortmund, laufende Arbeit
30. Sabrina Hennes: „Eigenschaften und Reaktivität von Clustern untersucht mit Synchrotronstrahlung“, Dissertation, Universität Dortmund, laufende Arbeit
31. Kolja Mende: „Soft Landing und Spektroskopie von massenselektierten Clustern“, Diplomarbeit, Universität Dortmund, laufende Arbeit
32. Natalie Miroslawski: „Geometrisch und elektronisch magische Cluster“, Diplomarbeit, Universität Dortmund, laufende Arbeit
33. Karl Bauer: „Leiterbahn-Strukturen durch FIB Strukturierung und Wachstum“, Diplomarbeit, Universität Dortmund, laufende Arbeit

 

Lecture notes

1. Festkörperphysik (Solid state physics), Univ. Siegen (2005) {pdf}
2. Oberflächenphysik (Surface Science), Univ. Dortmund (2002...2006) {pdf} (additional part of lecture on neutron and X-ray scattering given by M. Tolan)

 

Public outreach etc.

NRW Meeting Surface Physics and Chemistry 2006

Article in “mundo” (Magazine of the Univ. Dortmund) {pdf}