Atomic-scale surface science phenomena studied by scanning tunneling microscopy

Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO₂, the nature of the catalytically active edge sites on MoS₂ nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.     

Atomic-scale spin-polarized scanning tunneling microscopy applied to Mn3N2(010)

Atomic-scale spin-polarized scanning tunneling microscopy is demonstrated in the case of the unique surface spin structure of Mn3N2(010) at 300 K. We find that the surface spin structure is manifested as a modulation of the normal atomic row height profile. The atomic-scale spin-polarized image is thus shown to contain two components, one the normal, nonpolarized part, and the other the magnetic, spin-polarized part. A method is presented for separating these two spatially correlated components, and the results are compared with simulations based on integrated local spin density of states calculated from first principles.     

Selective analysis of molecular states by functionalized scanning tunneling microscopy tips

Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the pi states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.     

Defects in graphite may be magnetic and magnetostrictive as revealed by scanning tunneling microscopy

We use scanning tunneling microscopy to measure magnetic field induced strains in highly oriented pyrolytic graphite. This is done by using a scanning tunneling microscope with some magnetic components, which however do not produce an observable response within our resolution in the case of pure (99.999%) paramagnetic or diamagnetic metals (at the low field strengths applied). We study also ferromagnetic metals with this method for comparison. We find a relatively large (similar to that of permalloy) magnetostrictive response of graphite for the low applied field. The data shows saturation of the strain and also that the strain observed is localized and is not the cumulative strain from the mounted edge of the sample to the position of measurement, implying that volume is not conserved with the strains. We believe that the observed strains correspond to a signal of a ferromagnetic material and in this case may be due to the defects observed on the graphite planes.Received: 25 May 2004, Published online: 24 September 2004PACS: 68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM) - 75.80. + q Magnetomechanical and magnetoelectric effects, magnetostriction - 81.05.Uw Carbon, diamond, graphite     

Bulk electronic structure of metals resolved with scanning tunneling microscopy

We demonstrate that bulk band structure can have a strong influence in scanning tunneling microscopy measurements by resolving electronic interference patterns associated with scattering phenomena of bulk states at a metal surface and reconstructing the bulk band topology. Our data reveal that bulk information can be detected because states at the edge of the surface-projected bulk band have a predominant role on the scattering patterns. With the aid of density functional calculations, we associate this effect with an intrinsic increase in the projected density of states of edge states. This enhancement is characteristic of the three-dimensional bulk band curvature, a phenomenon analog to a van Hove singularity.     

Resolving noncollinear magnetism by spin-polarized scanning tunneling microscopy

We present a density functional theory (DFT) investigation of magnetically frustrated Mn monolayers deposited on the triangular lattice of the Cu(1 1 1) surface. Noncollinear magnetic structures are treated on the basis of the vector spin-density formulation of the DFT. The spin-polarized scanning tunneling microscope operated in the constant-current mode is proposed as a powerful tool to investigate these complex magnetic structures.     

Atomic-scale studies of impurities in superconductors with a scanning tunneling microscope

Imaging and spectroscopy with a cryogenic scanning tunneling microscope (STM) have been used to study the local variation of electronic states on the atomic scale in both low-T_c and high-T_c superconductors. These experiments provide an atomic-scale perspective of impurity scattering by directly probing the localized excitations that are induced by individual impurities.     

First-principles theory of scanning tunneling microscopy

Scanning tunneling microscopy/spectroscopy (STM/STS), which has been so epoch-making in surface science experiments introduced many challenging problems also to the theory of condensed matter physics. Recent progress in theories of STM/STS contributed to revealing the relation between the atomic structure of the tip and the STM/STS data, and to clarify various strange phenomena observed. The present article reviews various important issues of the fundamentals of STM/STS from theoretical view points.

After surveying the so far presented theoretical approaches, the first-principles simulation method based on the microscopic electronic state of both the sample surface and the tip is introduced. Several examples of the simulation such as graphite and Si surfaces, are described. Some novel phenomena of the microscopic tunnel system of STM such as the negative differential resistance in STS and single electron tunneling through fine supported particles are also discussed, as well as the many-body effect or electron-phonon coupling effect on STM/STS.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

The image potential in scanning transmission electron microscopy and scanning tunneling microscopy

Abstract

We sketch developments in the theory of the self-energy of charged particles moving near condensed matter surfaces. Some applications to experimental results from spectroscopy with electrons localized in microprobe beams and to electrons tunneling across a gap between two metals are considered.     

Electronic structure of MgO-supported Au clusters: quantum dots probed by scanning tunneling microscopy

We investigate via density functional theory (DFT) the appearance of small MgO-supported gold clusters with 8 to 20 atoms in a scanning tunneling microscope (STM) experiment. Comparison of simulations of ultrathin films on a metal support with a bulk MgO leads to similar results for the cluster properties relevant for STM. Simulated STM pictures show the delocalized states of the cluster rather than the atomic structure. This finding is due to the presence of s- derived delocalized states of the cluster near the Fermi energy. The properties of theses states can be understood from a jellium model for monovalent gold.     

Measurement of gold surface self-diffusion by scanning tunneling microscopy

Measurement of gold surface self-diffusion by the method of surface profile decay, using a scanning tunneling microscope (STM) have been done on a polycrystalline gold film deposited on a glass substrate. The peak-to-peak surface roughness was measured as a function of annealing time after annealing at 170 °C with a special pan-cake furnace in the STM. The gold surface diffusion coefficient at 170 °C can then be extracted from these measurements.     

Detection of surface acoustic waves by scanning tunneling microscopy

A new method for the investigation of ultrasonic waves on surfaces of solids based on scanning tunneling microscopy is presented. A sinusoidal high frequency signal is added to the tip voltage. Hence the tunneling current contains a component whose frequency is the difference of the frequencies of the acoustic wave field and the ac tip voltage. Amplitude and phase of this component carry the full information about the wave field.     

Optical pump-probe scanning tunneling microscopy for probing ultrafast dynamics on the nanoscale

The development of a method for exploring the ultrafast transient dynamics in small organized structures with high spatial resolution is expected to be a basis for further advances in current science and technology. Recently, we have developed a new microscopy technique by combining scanning tunneling microscopy (STM) with ultrashort-pulse laser technology, which enables the visualization of ultrafast carrier dynamics even on the single-atomic level. A nonequilibrium carrier distribution is generated using ultrashort laser pulses and its relaxation processes are probed by STM using the optical pump-probe method realized in STM by the pulse-picking technique. In this paper, the fundamentals of the new microscopy technique are overviewed.