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Advanced in EELS Instrumentation and Analysis: High-Speed Spectroscopy, with Extended Energy and Dynamic Range
Pēdējās izmaiņas veiktas:
08.05.2017
Figures 1 a): EELS colorized chemical maps of S L2,3 at 165 eV when present in the sulfate chemical phase in blue and in sulfide in red. Both chemical phases of S are distributed across the analyzed area in the TEM specimen; b) S L2,3-edges spectrum when the S is in the sulfate chemical phase; c) S L2,3-edges spectrum when the S is in the sulfide chemical phase.
Figures 2: a) EELS colorized eleme ntal maps of Zr L2,3-edges at 2222eV in red and the Ce M4,5-edges at 883 eV in green and blue for the Ce 4+ and Ce 3+ respectively. The interface YSZ / Ce2O3 look fairly nice and flat unlike the interface Ce2O3 / CeO2 that looks quite rough. The different chemistry of the CeOx is spatially resolved at atomic level; b) background removed Ce M4,5-edges spectra extracted from the areas in the sample where Ce is in the 3+ and 4+ oxidation state respectively. The shape is different showing the different chemistry. Particular thanks go to Prof Peter A. Van Aken at Max Plank Institute in Stuttgart Germany for providing access to the microscope installation and the TEM specimen used for the experiment.

Paolo Longo (Gatan, Inc.)

The acquisition of high-quality EELS data in the transmission electron microscope (TEM) presents many challenges not experienced by most TEM acquisition modes. The central challenges are dose efficiency and dynamic range. For EELS, the range of intensities of interest in a single spectrum can often span 6 to 7 orders of magnitude making recording problematic. Since the spectrum is recorded in parallel, EELS acquisition can be very dose efficient but only if the acquisition device can be read out quickly and efficiently.

To address these issues, we have developed a next generation post-column energy filter, the GIF Quantum, which excels at energy filtered imaging but also incorporates several new features that allow the optimal collection of energy-loss spectra generated by the high-brightness electron sources currently available. Key features of the GIF Quantum include a new CCD camera design that achieves high spectra readout rates (1kHz) with very little overhead, and a system of electrostatic deflectors that allows the nearly simultaneous (<10μs delay) recording of dual energy-loss ranges with microsecond exposure control. These deflectors enable the optimized acquisition of both high-energy core-loss electrons together with the zero-loss and low-loss electron signal.

In this talk, I will present details and advantages afforded by these new developments and show application data collected under optimized conditions from different materials. As example in this abstract, Figure 1 shows the chemical distribution maps of the S L2,3-edges at 165 eV in both the sulphate and sulphide chemical phases respectively in blue and red. As shown in Figures 1 b,c the shape of the S L2,3 is quite different when the S is in the sulphate or sulphide chemical phase. There are distinct regions in the analysed area where the S presents different chemistry. Such chemical maps were acquired in just about 4 minutes using an exposure time per spectrum of 5ms and this proves the high sensitivity and speed of the camera fitted to the GIF Quantum.

Figure 2a shows the elemental distribution map of Ce and Zr. These maps in addition to only compositional information they deliver chemical information. The different oxidation state of Ce is chemically and atomically resolved. The substrate seems to contain only CeO2 with Ce in the oxidation state 4+ whereas the interface with the Yttria-Stabilized Zirconia (YSZ) seems to contain Ce2O3 with the Ce in the oxidation state of 3+. The Ce chemical maps were obtained by means of multiple linear least square (MLLS) using the Ce M4,5-edges at 883 eV reference spectra with the Ce in the oxidation state 3+ and 4+ respectively. Differences in the shape of the Ce M spectrum can be easily observed easily resolved even with an energy resolution of 2 – 3 eV. This is shown in Figure 2b that reports the background removed Ce M4,5-edges spectra extracted from the region across the interface and in the bulk respectively. The chemical maps shown in Figure 2a were acquired in less than 2 minutes. Again, this proves the sensitivity and speed of the GIF Quantum camera and also the power of EELS capable to acquire compositional information as well as chemical.