Georg Hermann Simon

Measuring the Charge State of Point Defects on MgO/Ag(001)

A detailed understanding of surface defects is highly desirable, e.g. to clarify their role as active sites in catalysis. Here localized defects on the surface of MgO films deposited on Ag(001) are investigated. Since the electronic structure of color centers depends on their local position, spectroscopic signals are highly convoluted and often difficult to disentangle. In this study we aimed to obtain morphological and spectroscopic information on single color centers at a microscopic level with frequency modulated dynamic force microscopy (FM-DFM) and scanning tunneling microscopy (STM) in an ultrahigh vacuum and at low temperature. Four of the major and in literature mostly discussed defect types on MgO have been characterized by their charge state and finally identified by the complementary application of FM-DFM and STM in combination with density functional theory results.

Probing adsorption sites on thin oxide films by dynamic force microscopy

Site specific atomically resolved dynamic force spectroscopy measurements were performed on a thin MgO film grown on Ag(001). The microscope is operated in ultrahigh vacuum at low temperature to ensure defined imaging condition, high stability, and drift reduction. Atomically resolved dynamic force microscopy images have been combined with site specific frequency shift versus distance measurements. The frequency shift is measured as a function of z and the lateral displacement. With these measurement characteristics the authors are currently probing inequivalent surface sites on thin MgO films to extract atomic-scale information on surface chemical reactivity and possible adsorption sites for metal atoms and small clusters.

Direct measurement of the attractive interaction forces on F0 color centers on MgO(001) by dynamic force microscopy.

Defect sites on oxide surfaces play a dominant role in surface chemistry. The direct atomistic study of these sites is important but very difficult. We have mimicked the adsorbate-defect interaction by a dynamic force microscope tip measuring the interaction with a color center (F(0)) on the MgO(001) surface. The experimental findings, complemented by density functional theory calculations, show a highly attractive adsorbate-defect interaction and a charge transfer at a critical distance.

Atomic structure of the ultrathin alumina on NiAl(110) and its antiphase domain boundaries as seen by frequency modulation dynamic force microscopy

Atomically resolved frequency modulation dynamic force microscopy (FM-DFM) images of the ultrathin alumina film on NiAl(110) are presented. Images show in detail the surface unit cell, both types of antiphase domain boundaries (translation-related domain boundaries) and lateral displacements within these types of boundaries. Due to the loss of translational symmetry at the boundary, structures of even increased complexity are revealed. Lateral models for these local arrangements have been created on the basis of adjusted unit cell structures. FM-DFM produces on this surface a contrast of extraordinarily high surface sensitivity. It matches the topmost oxygen layer even with respect to topographic height, which adds the third dimension to the analysis. With this the antiphase domain boundaries are shown to be shallow depressions. Furthermore, new symmetry aspects have been found in the topography of these boundaries. The local structure of the film surface shows evidence of substrate influence in its topography and the domain boundary network shows indications that its growth behaviour is affected by this interaction in its very details beyond sheer appearance. Presented results can be linked to the relation between growth and structure of an emerging class of structurally related ultrathin alumina films.

Defects in oxide surfaces studied by atomic force and scanning tunneling microscopy

Summary Surfaces of thin oxide films were investigated by means of a dual mode NC-AFM/STM. Apart from imaging the surface termination by NC-AFM with atomic resolution, point defects in magnesium oxide on Ag(001) and line defects in aluminum oxide on NiAl(110), respectively, were thoroughly studied. The contact potential was determined by Kelvin probe force microscopy (KPFM) and the electronic structure by scanning tunneling spectroscopy (STS). On magnesium oxide, different color centers, i.e., F0, F+, F2+ and divacancies, have different effects on the contact potential. These differences enabled classification and unambiguous differentiation by KPFM. True atomic resolution shows the topography at line defects in aluminum oxide. At these domain boundaries, STS and KPFM verify F2+-like centers, which have been predicted by density functional theory calculations. Thus, by determining the contact potential and the electronic structure with a spatial resolution in the nanometer range, NC-AFM and STM can be successfully applied on thin oxide films beyond imaging the topography of the surface atoms.

Atomic structure of surface defects in alumina studied by dynamic force microscopy: strain-relief-, translation- and reflection-related boundaries, including their junctions

We present an extensive atomic resolution frequency modulation dynamic force microscopy study of ultrathin aluminium oxide on a single crystalline NiAl(110) surface. One-dimensional surface defects produced by domain boundaries have been resolved. Images are presented for reflection domain boundaries (RDBs), four different types of antiphase domain boundaries, a nucleation-related translation domain boundary and also domain boundary junctions. New structures and aspects of the boundaries and their network are revealed and merged into a comprehensive picture of the defect arrangements. The alumina film also covers the substrate completely at the boundaries and their junctions and follows the structural building principles found in its unit cell. This encompasses square and rectangular groups of surface oxygen sites. The observed structural elements can be related to the electronic signature of the boundaries and therefore to the electronic defects associated with the boundaries. A coincidence site lattice predicted for the RDBs is in good agreement with experimental data. With 6 = 19 it can be considered to be of low-sigma

Atomic resolution on a metal single crystal with dynamic force microscopy

Atomically resolved frequency modulation dynamic force microscope (FM-DFM) images taken by a tuning fork setup on an Ag(001) surface are presented. The necessity of well chosen tip surface interaction parameters is demonstrated making atomically resolved FM-DFM images on metal surfaces scarce. The capability of our setup to measure frequency shift and simultaneously tunneling current gives insight into different tip trajectories and possible contrast formation mechanisms in FM-DFM and scanning tunneling microscopy. One explanation is related to local variations in the decaying signals, which may originate from different density of states contributions to tip-sample force and tunneling current.

A scanning tunneling microscopy observation of (√3x√3)R30° reconstructed Ni2P(0001)

A Ni2P(0001) single crystal surface has been studied in the framework of model catalysis with a low temperature scanning tunneling microscope (STM) under ultrahigh vacuum (UHV). We observed a previously unreported (√3×√3) R30° reconstruction and successfully recorded its atomically-resolved STM images. Two types of atomic arrangements have been found for this (√3×√3) R30° structure depending on annealing conditions during preparation. One shows a filled and the other one an empty network of polygons. Upon annealing to 940 K, only the latter empty type of structure has been observed.

Local work function differences at line defects in aluminium oxide on NiAl(110).

In recent years, atomic force microscopy has developed into an important tool to study surfaces with atomic resolution and is therefore applied in many different fields of science, for instance to understand surface chemistry on oxide surfaces. Apart from the topography of the surface atoms, further details about the surface can be determined. In the Kelvin probe force microscopy mode, the contact potential difference, that is, the difference between the work functions of the sample and of the tip of the microscope, can be locally measured. The work function is the minimum energy needed to remove an electron from the surface to a point outside of the surface. This is especially relevant for reactions on surfaces with electron transfer, like redox reactions. Recently, the differences of the work functions have been determined with atomic resolution on differently charged adatoms and in electrically charged defects. The possible catalytic activity of these sites may be related to the shift of the work functions. However, it has to be noted that these irregularities have been electrically charged, and differences of the work functions are therefore not unexpected. In this communication we want to report on the application of Kelvin probe force microscopy to a thin film without artificially produced defects, here aluminium oxide on NiAl(110). The experimental setup is a dual-mode low temperature (5 K) frequency modulation atomic force microscope (FM–AFM) and scanning tunnelling microscope (STM). FM–AFM detects the tip–sample interaction as a shift of the resonance frequency of the oscillating tip. A detailed description of the experimental setup can be found in refs. [12, 13]. The used sensor allows for simultaneous STM and AFM data acquisition. The feedback can be switched instantly between the STM and AFM mode. This means AFM and STM pictures of exactly the same position on the surface can be recorded with the same microscopic tip configuration, which shows the great advantage of this dual-mode device. The low temperature (5 K) during the experiments results in a very small thermal drift. 14] The metal tip is composed of 90 % platinum and 10 % iridium. The studied system, thin film aluminium oxide on NiAl(110), is composed of two oxygen and aluminium layers. The preparation is explained in detail in ref. [16] . The film grows in two reflection domains, A and B, which are tilted by 248 with respect to NiAl[1 1 0]. The structural defects on the film are reflection domain boundaries (A–B) and antiphase domain boundaries (A–A or B–B, abbreviated APDB). While the reflection domain boundaries occur randomly and rarely, APDBs occur regularly every roughly 8–10 nm. The APDBs release stress in the perfect oxide film which accumulates due to a small lattice mismatch between the oxide film and the NiAl along the [1 1 0] direction. An AFM picture with atomic resolution of the 0.5 nm thin film is shown in Figure 1. The oxygen

A portable microevaporator for low temperature single atom studies by scanning tunneling and dynamic force microscopy

Here, we present a microevaporator setup for single adatom deposition at low temperature, which is a prerequisite for most single atom studies with scanning probe techniques. The construction of the microevaporator is based on the tungsten filament of a modified halogen lamp, covered with the required adsorbate. Very stable evaporation conditions were obtained, which were controlled by the filament current. The installation of this microevaporator on a manipulator enabled its transportation directly to the sample at the microscope kept at 5 K. In this way, the controlled deposition of Li onto Ag(100), Li, Pd, and Au onto MgO/Ag(001) as well as Au onto alumina/NiAl(110) at low temperature has been performed. The obtained images recorded after the deposition show the presence of single Li/Au atoms on the sample surfaces as a prove for successful dispersion of single atoms onto the sample surface using this technique.