Atomic structure of surfaces

LEED can be used to determine the atomic structure of surfaces, surface structural disorder, and to some extent, surfiice morphology, as well as changes in structure with time, temperature, and externally controlled conditions like deposition or chemical reaction. Some examples are briefly discussed here ... 

High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfacest and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. 

The actual achievement of STM greatly exceeds this expectation. Details of surface electronic structures with a spatial resolution of 2 A are now routinely observed. Based on the obtained electronic structure, the atomic structures of surfaces and adsorbates of a large number of systems are revealed. Furthermore, the active role of the STM tip through the tip-sample interactions enables real-space manipulation and control of individual atoms. An era of experimenting and working on an atomic scale arises. 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

Atomic structures of surfaces of non-metallic materials (a) Silicon surfaces... 

II. The Atomic Structure of Surfaces. Structures of Low and High Miller Index Crystal Surfaces. 5... 

Scanning tunnelling microscopy (STM) [33, 34] is widely used for investigations of the local atomic structure of surfaces. A probe "tip" is scanned across the surface revealing the positions of individual atoms. With its ability to achieve atomic resolution and, in most cases, distinguish between chemical species, the STM has provided key insights into the nature of alloy formation on surfaces. Both the static and dynamic properties of surface alloys can be probed with the STM. For the system of Pb on Cu, STM measurements were first to show the existence of surface alloy phases unambiguously and identify many of their stmctural properties [20-22, 35]. 

Physical adsorption on the (001) face of MgO also attracted considerable attention in recent years (see short review and references in Ref. [26]). It provides another opportunity to test methods of adsorption potential calculation which can be used later to simulate adsorption on adsorbents with less reliable atomic structure of surfaces like amorphous oxide. There is a large and rapidly changing electric field near the surface of MgO which should be much stronger than in silicalite due to small cations of Mg " " and larger ionicity of MgO in comparison to Si02. Thus calculations with polar and quadrupole molecules which were carried out on that surface (see Ref. [26] and references therein) necessarily employ methods which may useful for computer simulations on amorphous oxides. 

EIM Eield Ion Microscopy Surface metals, alloys very sharp tip (He gas above sample) Ele ions + high electric field produce image -0.1 nm 0.1-2 nm Atomic structure of surface 34,42... 

Analysis of the Local Atomic Structure of Surface Layers from SEFS Experimental Data... 

But in spite of all the advantages of the SEFS method as compared to both diffraction and spectroscopic methods of structure analysis, this technique has not yet been applied to analyze the local atomic structure of surfaces and thin films. This is explained by the diversity and complexity of the processes forming the secondary electron spectrum and the corresponding fine structures, and the resulting difficulties in their theoretical description and in the mathematical formalization of the problem of determining local atomic structure parameters from the experimental data. 

Extended X-ray absorption fine EXAFS Interference effects during X-ray Atomic structure of surfaces and... 

John M. Vohs is the Carl V.S. Patterson Professor and chair of the Department of Chemical and Biomolecular Engineering at the University of Pennsylvania. He joined the faculty there after receiving a B.S. degree from the University of Illinois and a Ph.D. from the University of Delaware. Dr. Vohs research interest is in the field of surface and interfacial science, particularly the relationships between the local atomic structure of surfaces and their chemical reactivity. His work on structure-activity relationships for metal-oxide catalysts, especially those used for selective oxidation reactions and automotive emissions control systems, is widely known. In recent years, he has collaborated in the development of solid-oxide fuel cells that run on readily available hydrocarbon fuels, such as natural gas and diesel. Dr. Vohs has received numerous honors, including an NSF Presidential Young Investigator Award and two Union Carbide Research Innovation Awards, vohs seas.upenn.edu)... 

---