Chemical-state information

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

P. N. Ross Jr. and K. A. Gaugler. Surf. Sci. 122, L579, 1982. Excellent example of chemical state information in CEELS. 

In principle all the X-ray emission methods can give chemical state information from small shifts and line shape changes (cf, XPS and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are the instrumental resolution of commercial systems is not adequate and the emission lines routinely used for elemental analysis are often not those most useftil for chemical shift meas-ure-ments. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission. 

The use and limitations of SIMS ion intensity distributions to provide quantitative and chemical state information for adsorption and reaction of small molecules at metal surfaces is discussed. We concentrate on well-defined surfaces where there is sufficient information on the adsorption system from other surface sensitive techniques to test the information content of SIMS. 

Tabic 5.5 Carbon content and chemical state information (from XPS). 

The primary technique for the surface analysis of polymers (3-4), including biomaterials (5-6) over the last decade has been X-ray photoelectron spectroscopy (XPS or GSCA). The technique has been employed to study the interfacial orientation, contamination, modifications, eg plasma treatments (7) and protein deposition on biomedical polymers ( ). While XPS provides valuable multi-element (except hydrogen) and chemical state information, the limited range... 

As discussed above It Is possible to distinguish between elemental and oxidized SI. For many elements AES spectral data can provide chemical state Information. Two elements that are Important In microelectronics, SI and Al, have dramatically different spectral features that allow relatively easy Identification of elemental vs. oxidized or nitrided states. A spectrum displayed In the N(E) form and showing peaks for both elemental and oxidized SI Is displayed In Figure 22. 

Maty other elements also demonstrate changes In spectral features that can be useful to the analyst. Not all of the changes are as dramatic as those discussed for Al and SI so greater effort may be required on the part of the analyst to obtain and Interpret the Information. Reviews of chemical state Information are available In the literature ( 15, 48 ). 

Typical XPS spectra are shownoin Figure 5. This technique samples a greater depth (about 30 A) and is able to give some detailed chemical state information about the composition ( ). Survey scans of the polymer reveal trace amounts of silicon contaminant, but represent only about 2% of the total surface area and can be subtracted from the final spectra. Silicones are... 

Modern XPS systems have the ability to analyze areas with a diameter of < 10 pm. This enables a surface to be mapped at high resolution and a plot of element distribution and chemical state information to be made. 

X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) are well established methods for the chenodcal characterisation of surfaces. The techniques combine well, where the quantitative elemental and chemical state information provided by XPS compliments the chemical and molecular structure information provided by ToFSIMS. The techniques probe only from the outermost few nanometres, which define and control the surface properties of many materials. 

There is the potential for chemical-state information in Auger spectroscopy for example, the oxidation state of silicon at an Si-Si02 interface may be ascertained [46]. EDX lacks chemical-state information. 

Alterations in polyamic acid-polyimide surfaces produced by the use of different methods of curing (oven vs. hot plate), the application of a silane adhesion promotor (y-aminopropyltriethoxysilane), and the deposition of copper have been studied using ESCA. Our objective was to develop chemical state information (elemental composition, chemical bonding features) about the modifications occurring in the outermost few tens of angstroms of polyimide films. The results show different C Is spectral features for hot plate vs. oven cured films but identical stoichiometries. The silane treated amic acid surface... 

Such core level shifts are well documented in handbooks and databases and are widely used to get chemical state information. But in some cases, the change in potential is insignificant although the valence electrons are changed considerably due to... 

XPS is probably the most widely used of the popular surface analysis methods that are generally available (the others being AES and SIMS). This is due to its unique advantages - that it readily gives chemical state information, is easily quantifiable, and applicable to insulators. The principal disadvantages are that it has lower spatial resolution although this has improved tremendously over the past decade and that it has a detection limit of 0.1 at.%. This latter value is rather better than AES but is well removed from that of SIMS which can be in the ngg range in favorable cases. 

Castle, J.E., Salvi, A.M. (2001) Chemical state information from the near-peak region of the X-ray photoelectron spectrirm./. Electron Spectrosc. Relat Phenom., 114-116,1103-1113. 

Most importantly, with regard to polymier surface analysis, XPS also prorides chemical state information particularly in the C Is region. Fi re 4 shows a high resolution scan of the C Is spectrum of poly lactic acid (PLA), and each of the three separate chemical emironments within the polymer gives rise to a peak in the C Is envelope. The fig ure also demonstrates a deconvolution of the envelope using Gaussian peaks and it is found that the three peaks have identical areas, reflecting the concentrations of these environments in the poly mer. 

In addition to its ability to quantify both elemental and chemical state information, XPS can be used to obtain additional film features such as thickness, growth mechanisms, and interface properties with relative simplicity from a single analysis (Hartmann and Lamb, 1997). Assuming the overlayer thin film is homogeneous in nature, the thickness (f) of the film can be calculated from the peak intensity of a particular peak from the thin film (7y) and the substrate (/J ), usually from a high-resolution... 

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