Surface sensitization

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

The experimental study of detergency ranges from the very practical to the molecular surface-sensitive approaches discussed in this book. In more practical studies, model... 

Surfaces are investigated with surface-sensitive teclmiques in order to elucidate fiindamental infonnation. The approach most often used is to employ a variety of techniques to investigate a particular materials system. As each teclmique provides only a limited amount of infonnation, results from many teclmiques must be correlated in order to obtain a comprehensive understanding of surface properties. In section A 1.7.5. methods for the experimental analysis of surfaces in vacuum are outlined. Note that the interactions of various kinds of particles with surfaces are a critical component of these teclmiques. In addition, one of the more mteresting aspects of surface science is to use the tools available, such as electron, ion or laser beams, or even the tip of a scaiming probe instrument, to modify a surface at the atomic scale. The physics of the interactions of particles with surfaces and the kinds of modifications that can be made to surfaces are an integral part of this section. 

Electrons are extremely usefiil as surface probes because the distances that they travel within a solid before scattering are rather short. This implies that any electrons that are created deep within a sample do not escape into vacuum. Any technique that relies on measurements of low-energy electrons emitted from a solid therefore provides infonuation from just the outenuost few atomic layers. Because of this inlierent surface sensitivity, the various electron spectroscopies are probably the most usefid and popular teclmiques in surface science. 

Powell C J, Jablonski A, Tilinin I S, Tanuma S and Penn D R 1999 Surface sensitivity of Auger-electron spectroscopy and x-ray photoelectron spectroscopy J. Eiectron Spec. Reiat. Phenom. 98-9 1... 

Experimental investigations of the model system of dye molecules adsorbed onto surfaces of polystyrene spheres have finuly established the sensitivity and surface specificity of the SHG method even for particles of micrometre size [117]. The surface sensitivity of die SHG process has been exploited for probing molecular transport across the bilayer in liposomes [118], for measurement of electrostatic potentials at the surface of small particles [119] and for imaging... 

There are several approaches to gain the required surface sensitivity with diffraction methods. We review several of these here, emphasizing the case of solid/vacuum interfaces some of these also apply to other interfaces. 

One fiirther method for obtaining surface sensitivity in diffraction relies on the presence of two-dimensional superlattices on the surface. As we shall see fiirtlrer below, these correspond to periodicities that are different from those present in the bulk material. As a result, additional diffracted beams occur (often called fractional-order beams), which are uniquely created by and therefore sensitive to this kind of surface structure. XRD, in particular, makes frequent use of this property [4]. Transmission electron diffraction (TED) also has used this property, in conjunction with ultrathin samples to minimize bulk contributions [9]. 

As we have seen, the electron is the easiest probe to make surface sensitive. For that reason, a number of hybrid teclmiques have been designed that combine the virtues of electrons and of other probes. In particular, electrons and photons (x-rays) have been used together in teclmiques like PD [10] and SEXAFS (or EXAFS, which is the high-energy limit of XAES) [2, Hj. Both of these rely on diffraction by electrons, which have been excited by photons. In the case of PD, the electrons themselves are detected after emission out of the surface, limiting the depth of sampling to that given by the electron mean free path. 

There are a few other surface-sensitive characterization techniques that also rely on the use of lasers. For instance surface-plasmon resonance (SPR) measurements have been used to follow changes in surface optical properties as a fiinction of time as the sample is modified by, for instance, adsorption processes [ ]. SPR has proven usefiil to image adsorption patterns on surfaces as well [59]. 

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Because a set of binding energies is characteristic for an element, XPS can analyse chemical composition. Almost all photoelectrons used in laboratory XPS have kinetic energies in the range of 0.2 to 1.5 keV, and probe the outer layers of tire sample. The mean free path of electrons in elemental solids depends on the kinetic energy. Optimum surface sensitivity is achieved with electrons at kinetic energies of 50-250 eV, where about 50% of the electrons come from the outennost layer. 

Owing to the limited escape depth of photoelectrons, the surface sensitivity of XPS can be enlianced by placing the analyser at an angle to the surface nonnal (the so-called take-off angle of the photoelectrons). This can be used to detemiine the thickness of homogeneous overlayers on a substrate. 

Figure Bl.25.5. (a) XPS spectra at take-off angles of 0° and 60° as measured from the surface nonnal from a silicon crystal with a thin layer of Si02 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity of XPS increases, (b) Plot of...

The strong point of AES is that it provides a quick measurement of elements in the surface region of conducting samples. For elements having Auger electrons with energies hr the range of 100-300 eV where the mean free path of the electrons is close to its minimum, AES is considerably more surface sensitive than XPS. 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

Depth sensitivity is an equally important consideration in the analysis of surfaces. Techniques based on the detection of electrons or ions derive their surface sensitivity from the fact that these species cannot travel long distances in soflds without undergoing interactions which cause energy loss. If electrons are used as the basis of an analysis, the depth resolution will be relatively shallow and depend on both the energy of the incident and detected electrons and on characteristics of the material. In contrast, techniques based on high energy photons such as x-rays will sample a much greater depth due... 

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

Figure 10 shows a comparison of Scofield s calculated values with experimentally measured values (24) which, in addition to (, are dependent on spectrometer transmission function. The overall agreement between the calculated and experimental values is quite good. The far rightjy-axis in Figure 10 indicates the experimentally accessible surface sensitivities in monolayers (ML) as a function of atomic number. For most elements, sensitivities on the order of 1% of a ML are achievable. 

Xps is a surface sensitive technique as opposed to a bulk technique because electrons caimot travel very far in soHds without undergoing energy loss. Thus, even though the incident x-rays penetrate the sample up to relatively large depths, the depth from which the electron information is obtained is limited by the "escape depth" of the photoemitted electrons. This surface sensitivity of xps is quantitatively defined by the inelastic mean free path parameter which is given the symbol X. This parameter is defined to be the distance an electron travels before engaging in an interaction in which it experiences an energy loss. 

The latest portable Brinell testers are spring-loaded, hand-held, and digitized to read direcdy in Brinell hardness units. Their accuracy is questionable because of extreme surface sensitivity and they are not in fact Brinell testers but RockweU indenters caUbrated to read in Brinell numbers. 

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