Quantitative Mapping of Concentrations and Bonding States by Energy Filtering TEM
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ABSTRACT We have developed new methods to quantify the data acquired by electron spectroscopic imaging (ESI) in an energy filtering TEM. The analysis is based on recording series of energy filtered images across inner-shell loss edges or in the low-loss region. From the series of ESI images, electron energy loss (EEL) spectra can be extracted and subsequently analysed using standard EELS quantification techniques. From an ESI series one can measure the absolute amount (area density) of an element in the given sample area or the concentration ratios of one element with respect to other elements. Spectrum line-profiling has been shown to be an efficient way to acquire and present the information on the chemistry of an interface. The results obtained for different metallisation layer systems show that segregation in the monolayer range can still be analysed with high spatial resolution. For the study of the energy-loss near-edge structure (ELNES) a higher energy resolution is required. ESI series with narrow energy window width can be used to distinguish between different bonding states of a given element and is demonstrated for thin films of diamond and amorphous carbon. INTRODUCTION The increasing number of energy filtering transmission electron microscopes (EFTEMs) has given many microscopists the ability to apply the fast and very efficient tool of electron spectroscopic imaging (ESI) for analytical characterisation, rather than to record EELS spectra. Most commonly, ESI analysis is based on identifying the presence of a characteristic inner-shell loss edge of the element under investigation (for an overview see Reimer [1]). This can be accomplished by recording two or three ESI images at energy losses in the background region before the edge and the signal region just above the edge. The two methods mainly used are the three-window technique, which was first proposed by Jeanguillaume et al. [2], and the ratio map technique which was introduced by Krivanek et al. [3]. In this paper we will present results obtained with a new method for quantitative analysis which is based on a multi-window approach. Rather than acquiring only two or three ESI images, we acquire a series of ESI images around an inner-shell loss edge and, if required for quantification, in the low loss region [4]. The advantages of this method are: 1) information on the energy loss spectrum is obtained for each pixel in the image and the standard quantification methods developed for EEL spectra can be used for the analysis, 2) the background extrapolation and the signal integration regions can be extended over a large energy-loss range, and 3) the standard EELS methods for single scattering deconvolution and least squares fitting for overlapping edges can be used. From an ESI series, spectrum line profiles can be extracted and can be quantified e.g. in terms of the absolute thickness of thin segregation layers at interfaces. Furthermore, we show how the bonding information present in the near-edge fine structure can also be extracted from a series of
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