Near surface techniques

Another technique is the nuclear reaction analysis of Lanford et al. (1976), which was applied to NAMs by Rossman et al. (1988), Skogby et al. (1990), Maldener et al. (2001), and Bell et al. (2003). This technique has the advantage of yielding absolute hydrogen concentrations. It is a near-surface technique in which the hydrogen concentration is measured as a function of depth. The spatial resolution of the technique is at the millimeter level Bell et al. (2003) prepared polished surfaces 5x5 mm in area. Maldener et al. (2001) state that 1 mm diameter samples can be analyzed. This technique has been used to calibrate absorption coefficients for IR spectroscopy (e.g., Maldener et al., 2001 Bell et al., 2003). Bell et al. (2003) applied this technique to hydrogen in olivine, and found some of the previous estimates of hydrogen concentration in olivine need to be revised upward by factors between 2 and 4. Such a correction cannot be applied uniformly to all previous studies, because their new calibrations are specific to polarized spectra. 

In 1972, Ziegler, et al. (p first reported the development of a near-surface technique complimentary to those mentioned above. The technique uses neutron reactions to measure absolute concentration versus depth profiles of a number of the light elements. Neutron... 

Near surface techniques, in contrast to true surface techniques, do not require an ultrahigh vacuum. Secondary Electron Microscopy-Eneigy Dispersive X-ray Spectroscopy (SEM-EDS) is an example of a particularly useful near surface technique that will be discussed in a later section. Micro surface electrochemical techniques, such as the Scaiming Kelvin probe, overcome these limitations and do give detailed electrochemical (rather than elemental) information about the system in question. 

Analysis of Surface Elemental Composition. A very important class of surface analysis methods derives from the desire to understand what elements reside at the surface or in the near-surface region of a material. The most common techniques used for deterrnination of elemental composition are the electron spectroscopies in which electrons or x-rays are used to stimulate either electron or x-ray emission from the atoms in the surface (or near-surface region) of the sample. These electrons or x-rays are emitted with energies characteristic of the energy levels of the atoms from which they came, and therefore, contain elemental information about the surface. Only the most important electron spectroscopies will be discussed here, although an array of techniques based on either the excitation of surfaces with or the collection of electrons from the surface have been developed for the elucidation of specific information about surfaces and interfaces. 

Electron Microprobe A.na.Iysis, Electron microprobe analysis (ema) is a technique based on x-ray fluorescence from atoms in the near-surface region of a material stimulated by a focused beam of high energy electrons (7—9,30). Essentially, this method is based on electron-induced x-ray emission as opposed to x-ray-induced x-ray emission, which forms the basis of conventional x-ray fluorescence (xrf) spectroscopy (31). The microprobe form of this x-ray fluorescence spectroscopy was first developed by Castaing in 1951 (32), and today is a mature technique. Primary beam electrons with energies of 10—30 keV are used and sample the material to a depth on the order of 1 pm. X-rays from all elements with the exception of H, He, and Li can be detected. 

The MOKE technique has a broad range of applications from the analysis of ultrathin films (less than about 2 nm) to the analysis of the near-surface region of bulk ferromagnets ... 

MOKE measurements can be made using relatively simple and inexpensive apparatus, compared to most other surface magnedc probes and surface analytical techniques. MOKE is usefid for the mj netic characterizadon of films of one to several monolayers, thin films, or the near-surface regions of bulk materials. MOKE has... 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

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. 

Nevertheless, in applications relevant for electrocatalysis and reactions that occur at solid-liquid interfaces, it has been essential to develop a methodology that can provide detailed insight into the surface and near-surface stmcture during the course of reaction. For that purpose, the in sim SXS diffraction technique, depicted in... 

Stipp S.L., Hochella M.F.Jr., Parks G.A., Leckie J.O. Cd2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES). Geochim Cosmochim Acta 1992 56 1941-1954. 

This chapter is concerned with the application of liquid state methods to the behavior of polymers at surfaces. The focus is on computer simulation and liquid state theories for the structure of continuous-space or off-lattice models of polymers near surfaces. The first computer simulations of off-lattice models of polymers at surfaces appeared in the late 1980s, and the first theory was reported in 1991. Since then there have been many theoretical and simulation studies on a number of polymer models using a variety of techniques. This chapter does not address or discuss the considerable body of literature on the adsorption of a single chain to a surface, the scaling behavior of polymers confined to narrow spaces, or self-consistent field theories and simulations of lattice models of polymers. The interested reader is instead guided to review articles [9-11] and books [12-15] that cover these topics. 

Information about the surface and interface structures in hexadecylamine-capped CdSe NC of 2 nm size has been obtained by a variety of 1H, 13C, 113Cd, and 77Se NMR techniques [342]. The 77Se CP-MAS-NMR spectrum showed five partially resolved peaks from surface or near-surface Se environments. It was possible to obtain 2D heteronuclear correlation (HETCOR) spectra between 1H and the other three nuclei despite the inherent sensitivity limitations (the 77Se- 3I-I HETCOR experiment required 504 h ). The latter experiment indicated that the methylene protons of the hexadecylamine chain interact with the surface Se atoms via a tilt of the chain toward the surface. The surface Se atoms were not seen to interact with thiophenol present, and it was suggested that thiophenol binds to a selenium vacancy at the surface. 

SIMS is by far the most sensitive surface technique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. The principle of SIMS is conceptually simple A primary ion beam (Ar+, 0.5-5 keV) is used to sputter atoms, ions and molecular fragments from the surface which are consequently analyzed with a mass spectrometer. It is as if one scratches some material from the surface and puts it in a mass spectrometer to see what elements are present. However, the theory behind SIMS is far from simple. In particular the formation of ions upon sputtering in or near the surface is hardly understood. The interested reader will find a wealth of information on SIMS in the books by Benninghoven et al. [2J and Vickerman el al. [4], while many applications have been described by Briggs et al. [5]. 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Among the most often used stress measurement methods for crystalline films is determination of the change in interplanar spacing of the crystallites in the film by x-ray diffraction. X-ray determination of near-surface strain and stress in bulk materials has a long history, dating back to the very early period of x-ray powder diffraction measurements and is a well-established technique. 

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