Structure of clean surfaces

Over the past several years the surface structures of several clean monatomic solid surfaces and a variety of adsorbed atoms on solid surfaces have been determined by LEED (7). This field of study is now called surface crystallography and is one of the most rapidly growing fields of surface science. By studying the atomic surface structure of clean surfaces and adsorbed molecules, the nature of the surface chemical bond can be explored in a systematic manner. 

The structure of clean surfaces of polyatomic solids (alloys, halides, oxides, sulfides, etc.) should be explored along with molecular solid surfaces (organic systems). More open, rough surfaces with high Miller indices should be investigated, including the structure of atoms and molecules bonded to steps and kinks on surfaces. Such sites are known to be key for some important surface chemical processes, but little is known of their structure. 

The technique of Low Energy Electron Diffraction (leed) is well suited to the observation of surface monolayers and the structure of clean surfaces. Farnsworth et 31,60.69,70 gre3.t deal of work in this field, obtaining measure-... 

The most widely used technique to get information on the electronic structure of clean surfaces, nanostructures on surfaces, or even molecules adsorbed on surfaces is ultraviolet photoelectron spectroscopy (UPS). The difficulty of this method, when applying it to clusters on surfaces, is to obtain sufficient spectral contrast between the low number of adsorbed clusters and the substrate [45]. Thus, electron energy loss spectroscopy (EELS) is more successfully used as a tool for the investigation of the electronic structure of supported clusters. An interesting test case for its suitability is the characterization of supported monomers, i.e., single Cu atoms on an MgO support material [200], as this system has been studied in detail before with various surface science techniques [201-204]. The adsorption site of Cu on MgO(lOO) is predicted... 

The Dynamic Lattice Model of Surfaces. The Experimental Facts. Let us now review what is known about the structure of clean surfaces without adsorbates. There are three phenomena that have been identified relaxation, reconstruction and the presence of steps and kinks. 

Clean, Low-Indexed Surfaces. The structures of clean surfaces are of fundamental interest because they are the basis for more complicated systems offering practical applications. On their own, clean surfaces have importance as quasi-2D systems, which can show special effects like relaxations, reconstructions, phase transitions, surface-specific defects, local mass fluctuations, and roughening transitions. In the following we concentrate on face-centered cubic (fee) metals. The geometry of the three low-indexed fee surfaces is shown in Figure 44. 

Using these techniques, the atomic surface structures of clean surfaces and adsorbate monolayers have been determined. Surface composition can be verified to better than 1% of a monolayer (10 atoms/cm or less). The oxidation states of surface atoms can also now be verified. 

Fig. XVI-8. (a) The quasi-hexagonal surface structure of clean Pt(lOO) surface, (b) Adsorption of CO lifts this reconstruction to give the structure corresponding to the termination of (100) planes (from LEED studies). [Reprinted with permission from G. Ertl, Langmuir, 3, 4 (1987) (Ref. 56). Copyright 1987, American Chemical Society.]...

Surfaces are found to exliibit properties that are different from those of the bulk material. In the bulk, each atom is bonded to other atoms m all tliree dimensions. In fact, it is this infinite periodicity in tliree dimensions that gives rise to the power of condensed matter physics. At a surface, however, the tliree-dimensional periodicity is broken. This causes the surface atoms to respond to this change in their local enviromnent by adjusting tiieir geometric and electronic structures. The physics and chemistry of clean surfaces is discussed in section Al.7.2. 

The study of clean surfaces encompassed a lot of interest in the early days of surface science. From this, we now have a reasonable idea of the geometric and electronic structure of many clean surfaces, and the tools are readily available for obtaining this infonnation from other systems, as needed. 

Because LEED theory was initially developed for close packed clean metal surfaces, these are the most reliably determined surface structures, often leading to 7 p factors below 0.1, which is of the order of the agreement between two experimental sets of 7-V curves. In these circumstances the error bars for the atomic coordinates are as small as 0.01 A, when the total energy range of 7-V curves is large enough (>1500 eV). A good overview of state-of-the-art LEED determinations of the structures of clean metal surfaces, and further references, can be found in two recent articles by Heinz et al. [2.272, 2.273]. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

Schlier, R. E. and Farnsworth, H. E. Structure and adsorption characteristics of clean surfaces of germanium and silicon. Journal of Chemical Physics 30, 917 (1959). 

LEED studies of clean surfaces have revealed that most of these surfaces, if prepared under proper conditions, are ordered on an atomic scale and exhibit sharp diffraction beams and high diffraction beam intensities. Metal, semiconductor, alkali halide, inert gas, and organic crystal surfaces have been studied this way, and all exhibit ordered surface structures. 

As a consequence of the identical crystallographic structure and the very similar ionic radii, MgO and NiO or CoO form uniform solid solutions Mg(i X)MxO (0 < x < 1), where M is Ni or Co (25). Real crystals are often covered by strongly adsorbed water and carbon dioxide, and thermal treatments in vacuo are needed to clean the surfaces and create the surface coordinative unsaturation mentioned previously (in particular the fourfold and threefold coordinated sites are cleaned only at very high temperatures). In this connection, it is evident that the surface chemistry of clean surfaces is primarily determined by the presence of these... 

A BRIEF REVIEW of the research in semiconductor surface physics is presented. Emphasis is placed on die limits of present theory and the importance of knowing the composition and structure of the surface of interest. The feasibility of new experimental approaches to the study of surfaces such as nuclear magnetic resonance and quadrupole res -onance is discussed. A review of recent developments in an understanding of the energy level diagram of the cleaned germanium surface is reviewed. 

A variety of model catalysts have been employed we start with the simplest. Single-crystal surfaces of noble metals (platinum, rhodium, palladium, etc.) or oxides are structurally the best defined and the most homogeneous substrates, and the structural definition is beneficial both to experimentalists and theorists. Low-energy electron diffraction (LEED) facilitated the discovery of the relaxation and reconstruction of clean surfaces and the formation of ordered overlayers of adsorbed molecules (3,28-32). The combined application of LEED, Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), field emission microscopy (FEM), X-ray and UV-photoelectron spectroscopy (XPS, UPS), IR reflection... 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

---