Surface analytical technique requirements

Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to emission of one or more of a variety of secondary particles such as electrons, photons, positive and secondary ions, and neutrals. Because the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary-secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application, some because of the restricted or specialized nature of the information obtained and others because of difficult experimental requirements. In this publication, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are given of the many others the use of which is less common but which - in appropriate circumstances, particularly in basic research - can provide vital information. 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

All materials will, to some degree, be subject to corrosion and oxidation by their environment, and the critical early stages of attack can often be understood through the use of surface analytical techniques. A similar approach is required to gain an understanding of the fundamental and applied aspects of surface catalysis, which is of great importance in the petrochemical industry. The microelectronics industry has also contributed to the development of modern surface analytical techniques, where there is a necessity to analyse dopant concentration profiles while retaining lateral resolution on the device of better than one micron. 

In this chapter we introduce high resolution diffraction studies of materials, beginning from the response of a perfect crystal to a plane wave, namely the Bragg law and rocking curves. We compare X-rays with electrons and neutrons for materials characterisation, and we compare X-rays with other surface analytic techniques. We discuss the definition and purpose of high resolution X-ray diffraction and topographic methods. We also give the basic theory required for initial use of the techniques. 

IR spectroscopy, one of the few surface analytical techniques not requiring a vacuum, provides a large amount of molecular information. The absorption versus frequency characteristics are obtained when a beam of IR radiation is transmitted through a specimen. IR is absorbed when a dipole vibrates naturally at the same frequency as the absorber, and the pattern of vibration is unique for a given molecule. Therefore, the components or groups of atoms that are absorbed into the IR at specific frequencies can be determined, allowing identification of the molecular structure. 

In spite of the high cost of secondary ion mass spectrometers and the technical expertise required, this sensitive microlocal and surface analytical technique can be applied successfully in forensic studies, e.g., for measurement of trace elements and inks on counterfeit banknotes.18... 

The development of surface analytical techniques such as LA-ICP-MS, GDMS and SIMS focuses on improvements to sensitivity and detection limits in order to obtain precise and accurate analytical data. With respect to surface analytical investigations, an improvement of spatial and depth resolution is required, e.g., by the establishment of a near field effect or the apphcation of fs lasers in LA-ICP-MS. There is a need for the improvement of analytical techniques in the (j,m and nm range, in depth profiling analysis and especially in imaging mass spectrometry techniques to perform surface analyses faster and provide more accurate data on different materials to produce quantitative 3D elemental, isotopic and molecular distribution patterns of increased areas of interest with high spatial and depth resolution over an acceptable analysis time. 

The susceptibility of solid surfaces to contamination often results in a requirement for an ultrahigh vacuum (UHV) chamber for preparation and observation of particular samples. For many materials, including metals such as platinum and nickel, adsorption of hydrocarbons and chemisorption of oxygen are quite fast at atmospheric pressure, and the surface must be isolated in UHV to prevent rapid degradation. In addition, a sample in UHV may be subjected to surface analytical techniques such as X-ray photoelectron and Auger spectroscopy to verify or corroborate Raman results. As a result, much of the early and well-characterized surface Raman experiments were carried out in UHV chambers operating below 10 torr (12). 

These techniques fall into two categories those considered as routine (e.g. atomic absorption and emission spectroscopy, X-ray fluorescence) and a growing number of microanalytical surface techniques (e.g. laser microprobe mass analysis [LAMMA] and sensitive high-resolution ion microprobe [SHRIMP]). Each analytical technique requires specific sample preparation prior to analysis, as summarised in Table 13.1. 

Most analytical techniques require the state of chemical equilibrium. At equilibrium, the rate of a forward process or reaction and that of the reverse process are equal. The photo at left shows the beautiful natural formation called "Frozen Niagra in Mammoth Cave National Park in Kentucky. As water seeps over the limestone surface of the cave, calcium carbonate dissolves in the water according to the chemical equilibrium... 

Providing a comprehensive characterization of pharmaceutical dosage forms requires a multi-instrumental approach as no single surface analytical technique can ascertain both the chemical and morphological nature of the surfaces. To this end, SPM, XPS, and ToF-SIMS have been used as complementary techniques with which to characterize the surface nature of dosage forms (141,143-145). 

A rate constant k is assigned to each surface chemical reaction. This is a schematic representation of the mechanism based upon analogous reactions of metal ion complexes in solution (see Purcell and Kotz, 1977, p, 659-669). Experimental determination of dissolved reactant and product concentrations [ArOH(aq), Mn Caq), etc.] can provide indirect information about the surface reaction [discussed in Stone (1986), Stone and Morgan (1987), and Stone (1987)]. Additional detail concerning the stoichiometry and structure of surface species will require the use of spectroscopic or other surface-analytical techniques. 

The application of laser light scattering techniques to molecular characterization of dielectric films offers the ability to directly probe chemical bonding within the film and at the film-substrate interface. Real-time measurements can be carried out under ambient conditions or in hostile environments allowing transient film stability studies to be conducted. Such laser-based techniques require only an optically clear line of sight between sample and analyzer and offer several advantages over the high vacuum surface analytical techniques commonly applied to film characterization. These include nondestructive measurement capability, rapid data acquisition time, and ability to use the optical properties of the sample to enhance the sensitivity of the measurement. 

Further aspects of surface analysis are the accuracy of determining composition or structural parameters. For practical purposes the possibility of combining different tools is very important. As an example, the STM is difficult to combine in situ with other techniques. Most surface analytical tools require ultrahigh-vacuum (UHV) for their operation, hence solid-liquid interfaces are hardly accessible. Another practical aspect concerns the analysis of surface processes, for... 

These physical sample preparation steps may be followed by one or more stages of chemical preparation. Table 4 outlines some analytical techniques commonly employed by geochemists and the sample preparation techniques required. More details are given in technique-specific articles. While some techniques such as EPMA, X-ray fluorescence (XRF), ion microprobe and laser ablation mass spectrometry require little preparation other than presentation of a clean, relatively flat surface, most techniques require either complete or partial digestion. 

All surface analytical techniques (with the exception of Rutherford back-scattering) require the use of ultrahigh vacuum environment. The analysis conditions are therefore rather limited and do not correspond to those normally used in ceramic processing where the powder experiences fairly prolonged exposure to the atmosphere. Although the techniques described here derive their usefulness from being truly surface sensitive, they can also be used to provide information... 

This book is about Mossbauer spectroscopy. We might wonder how it can be applied to study surfece layers a few nanometers thick. A surface analytical technique needs to meet two important requirements (i) it should be sensitive, that is, it should be able to detect a small number of atoms (while a cubic centimeter of solid material contains roughly substitute 10 atoms, a square centimeter of the same material contains a much smaller number of atoms, approximately 10 atoms), and (ii) it should be specific, that is, it should be able to separate the contributions of the surface from those of the bulk. 

As with most surface analytical techniques, the ability to produce a vacuum played a pivotal role in defining the introduction timeline of SIMS. The importance of vacuum is realized as this allows the passage of charged particles over the distances needed (typical flight path is one to several meters) and provides for a contamination free surface (required in the analysis of monolayer composition... 

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