Surface electronic structure, techniques

Potential energy surface for a chemical reaction can be obtained using electronic structure techniques or by solving Schrodinger equation within Born-Oppenheimer approximation. For each geometry, there is a PE value of the system. 

Preliminary models of the surface topography, for example, can be determined by atomic-probe methods, ion-scattering, electron diffraction, or Auger spectroscopy. The chemical bonds of adsorbates can be estimated from infrared spectroscopy. The surface electronic structure is accessible by photoelectron emission techniques. In case the surface structure is known, its electronic structure has to be computed with sophisticated methods, where existing codes more and more rely on first principles density functional theory (DFT) [16-18], or, in case of tight-binding models [19], they obtain their parameters from a fit to DFT data [20]. The fit is not without ambiguities, since it is unknown whether the density of states used for the fit is really unique. 

The most widely used technique for finding surface electronic structure is, in fact, to treat a thin slab (typically 5 or so atomic layers thick) rather... 

The various forms of photoelectron spectroscopy presently available permit a straightforward determination of occupied and unoccupied surface states. The most comprehensive and authoritative collection of reviews is in the book edited by Feuerbacher et al. [44], while Ertl and Kiippers [15] also provide useful information. Here, we will only attempt to summarize how the principal versions of the technique can be used in the determination of surface electronic structure. In this context the crucial factor is that photoemission spectra represent a direct manifestation of the initial and final density of states of the emitting system. Because selection rules (matrix element effects) can be involved in the transition, the state densities may not always correspond to those derived from the band structure, but in practice there is frequently a rather close correspondence. 

In this section we discuss briefly a few important experimental techniques used to obtain information about surface electronic structure. The four techniques that we describe have been selected because of their use to interpret features of surface chemical bonds. In the following section, we show that some of the commonly accepted interpretations are oversimplified and give misleading information. More importantly, we also present correct interpretations based on the analysis of the results of cluster calculations. In the present section, we will describe the physical properties that the techniques that we discuss are used to measure for a detailed discussion of the measurement techniques themselves, see, for example. Woodruff and Delchar. ... 

The chapter shall begin with brief discussions of a few basic concepts of crystalline soHds, such as an introduction to the common crystal types, and the cohesive properties of solids. Following this, the most widely used electronic structure technique for interrogating the properties of solids and their surfaces, namely density functional theory (DFT) is introduced. We then discuss cohesion in bulk metals and semiconductors in more depth before reaching the main body of the chapter, which involves a discussion of the atomic structures of crystalline... 

The interpretation of RAS spectra from single-crystal surfaces is not as straightforward as it is for, for example, XPS. This is because the response of the surface depends on the complex dielectric function (a quantity that is difficult to calculate from first principles even for well-characterized materials) for both the bulk of the sample and the surface region. Also, in common with other techniques that are sensitive to surface electronic structure, the existence of intrinsic surface states and surface-modified bulk states compHcates matters. However, absence of a firm theoretical framework for predicting RAS spectra has not necessarily impeded the application of RAS in various fields. An empirical approach, often supported by other techniques that provide information on the electronic transitions responsible for RAS spectral features, can allow surface changes to be studied even without a complete understanding of the RAS spectra. 

STM found one of its earliest applications as a tool for probing the atomic-level structure of semiconductors. In 1983, the 7x7 reconstructed surface of Si(l 11) was observed for the first time [17] in real space all previous observations had been carried out using diffraction methods, the 7x7 structure having, in fact, only been hypothesized. By capitalizing on the spectroscopic capabilities of the technique it was also proven [18] that STM could be used to probe the electronic structure of this surface (figure B1.19.3). 

Other artifacts that have been mentioned arise from the sensitivity of STM to local electronic structure, and the sensitivity of SFM to the rigidity of the sample s surface. Regions of variable conductivity will be convolved with topographic features in STM, and soft surfaces can deform under the pressure of the SFM tip. The latter can be addressed by operating SFM in the attractive mode, at some sacrifice in the lateral resolution. A limitation of both techniques is their inability to distinguish among atomic species, except in a limited number of circumstances with STM microscopy. 

Although physical studies of the electronic structure of surfaces have to be performed under UHV conditions to guarantee clean uncontaminated samples, the technique does not require vacuum for its operation. Thus, in-situ observation of processes at solid-gas and solid-liquid interfaces is possible as well. This has been utilized, for instance, to directly observe corrosion and electrode processes with atomic resolution [5.2, 5.37]. 

The availability of high-intensity, tunable X-rays produced by synchrotron radiation has resulted in the development of new techniques to study both bulk and surface materials properties. XAS methods have been applied both in situ and ex situ to determine electronic and structural characteristics of electrodes and electrode materials [58, 59], XAS combined with electron-yield techniques can be used to distinguish between surface and bulk properties, In the latter procedure X-rays are used to produce high energy Auger electrons [60] which, because of their limited escape depth ( 150-200 A), can provide information regarding near surface composition. 

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