Visualization of Compositional Fluctuations in Complex Oxides Using

Energy-filtered images of the elements of interest were acquired to obtain the two-dimensional elemental distributions within polycrystalline samples ...
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Chem. Mater. 2002, 14, 135-143

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Visualization of Compositional Fluctuations in Complex Oxides Using Energy-Filtering Transmission Electron Microscopy I. Rom,† F. Hofer,*,† E. Bucher,‡ W. Sitte,‡ K. Gatterer,§ H. P. Fritzer,§ and A. Popitsch| Research Institute for Electron Microscopy, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria, Institute of Physical Chemistry, University of Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria, Institute of Physical and Theoretical Chemistry, Graz University of Technology, Rechbauerstrasse 12, A-8010 Graz, Austria, and Institute of Chemistry, Karl-Franzens-University of Graz, Schubertstrasse 1, A-8010 Graz, Austria Received April 12, 2001. Revised Manuscript Received August 7, 2001

Energy-filtering transmission electron microscopy (EFTEM) was used to evaluate the purity and phase composition of perovskite- and spinel-type oxides, using La1-xSrxCoO3-δ (x ) 0.5, 0.6) and CdCr2-xInxO4 (x ) 0.05, 0.5) prepared via a mixed oxide route, the glycine nitrate process and a wet chemical method, respectively. Energy-filtered images of the elements of interest were acquired to obtain the two-dimensional elemental distributions within polycrystalline samples at high lateral resolution. The presence of other crystalline phases in addition to the expected perovskite and spinel phases is easily recognized in the elemental distribution images. Additionally, those secondary phases were identified using spectroscopical techniques available in the transmission electron microscope, namely, energydispersive X-ray spectrometry (EDXS) and electron energy loss spectroscopy (EELS). The present investigation demonstrates that energy-filtering transmission electron microscopy is a powerful technique for assessing homogeneity of materials with nanometer resolution and high-detection sensitivity.

1. Introduction Perovskite compounds have been extensively investigated their interesting physical and technological properties, such as ferroelectricity, piezoelectricity, hightemperature superconductivity, magnetic behavior, and catalytic activity.1-3 Several preparation techniques have been proposed for synthesizing these complex oxides, including the mixed oxides route, spray pyrolysis, sol-gel, solvent combustion methods (like the glycine nitrate process). The family of chromium chalcogenide spinels has also aroused considerable technological interest over the last 20 years.4-7 This is mainly due to their semiconducting and magnetic properties that may be tuned gradually by cation substitution. * To whom correspondence should be addressed. E-mail: [email protected]. † Research Institute for Electron Microscopy, Graz University of Technology. ‡ University of Leoben. § Institute of Physical and Theoretical Chemistry, Graz University of Technology. | Karl-Franzens-University of Graz. (1) Petrov, A. N.; Kononchuk, O. F.; Andreev, A. V.; Cherepanov, V. A.; Kofstad, P. Solid State Ionics 1995, 80, 189. (2) Mizusaki, J. Solid State Ionics 1992, 52, 79. (3) Sitte, W.; Bucher, E.; Benisek, A.; Preis, W. Spectrochim. Acta A 2001, 57, 2071. (4) Villain, J. Z. Phys. 1979, B33, 31. (5) Alba, M,; Hammann, J.; Nogues, M. Physica 1981, 107B, 627. (6) Shimizu, Y.; Kusano, S.; Kuwayama, H.; Tanaka, K.; Egashiva, M. J. Am. Ceram. Soc. 1990, 73, 818. (7) Honeybourne, C. L.; Rasheed, R. K. J. Mater. Chem. 1996, 6, 277.

For a detailed characterization of these oxide materials, for example, with regard to their thermodynamic and kinetic properties as well as for their potential technological application, it is essential to obtain homogeneous single phases (monophase products) or at least have accurate information on their exact chemical composition. Therefore, it is necessary to evaluate phase purity and composition of the materials prior to further investigations or usage. The primary characterization technique routinely used to evaluate the purity and phase composition of crystalline solid state materials is powder X-ray diffraction (XRD). A “clean” diffraction pattern with peaks from only a single phase is generally considered indicative of a phase-pure (and consequently homogeneous) material. However, comparatively small amounts of contaminating phases may not be observed by XRD because of overlapping peaks or small crystallite size or amorphous phases. Additionally, XRD cannot detect elemental concentration variations within a given crystalline phase (e.g., growth pattern or zoning) or features such as crystal morphology. Another well-known technique for evaluating the homogeneity of materials is energy-dispersive X-ray (EDX) elemental distribution mapping using a scanning electron microscope (SEM). Its information is complementary to that from XRD, specifically with respect to elemental homogeneity, phase purity, and phase formation. SEM/EDX makes use of characteristic X-rays

10.1021/cm011108w CCC: $22.00 © 2002 American Chemical Society Published on Web 12/01/2001

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emitted when a sample is exposed to an electron beam to map the elemental distribution of the flat surface of a solid material. This technique has been routinely used to explore a variety of materials in chemistry, geology, and biology. In materials science, for example, X-ray elemental mapping has been used to study local element distribution, diffusion processes, and chemical reactions at the micrometer scale.8 Unfortunately, this method has its limitations in quantitative microanalysis, as it lacks the resolution needed for the fine-scale microstructure of many materials. Because the samples investigated in the SEM are usually thick, electron beam broadening in the samples limits the analytical resolution to the micrometer range. Another disadvantage lies in the long data acquisition times (up to an hour) necessary for obtaining high-quality maps and hence problems with radiation damage and specimen drift. In this work we will show that energy-filtered transmission electron microscopy (EFTEM) can be used to evaluate the purity, stoichiometry, and phase composition of technical materials. This method allows sample characterization on the nanometer scale if a sufficiently thin sample can be prepared from the material.9 Sample thickness for EFTEM typically ranges from 10 to 200 nm, but nevertheless such a sample contains many unit cells, in fact, many more than are probed by surfacesensitive techniques. EFTEM is an element-specific method, which can be applied not only to crystalline specimens but also to amorphous specimens. Energy filtering of the transmitted electrons allows measurement of elemental distributions of thin samples at high spatial resolution (nanometer range) and within short acquisition times (between seconds and minutes). Another important factor is the possibility of combining EFTEM with advanced analytical techniques, such as electron energy-loss spectrometry (EELS) and also energy-dispersive X-ray spectrometry (EDXS), which allows sample characterization in great detail and at a spatial resolution similar to that of EFTEM. When compared with EDXS, EELS offers improved spatial resolution and better sensitivity, especially for light elements. Because of the higher energy resolution, EELS peaks seldom overlap, but the detection limit for heavy elements by EELS is inferior to EDXS. So both methods contribute valuable information, complementing each other well. In materials science EFTEM elemental mapping (in combination with analytical techniques) has successfully been used, for example, to detect secondary phases in steel specimens,10,11 to investigate grain boundaries in ceramics12 as well as phase distributions in composites.13 The method has also been applied to semiconductor device problems.14 However, this technique has not (8) Houk, C. S.; Page, C. J. Adv. Mater. 1996, 8, 173. (9) Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope; Plenum Press: New York, 1996. (10) Warbichler, P.; Hofer, F.; Hofer, P.; Letofsky, E. Micron 1998, 29, 63. (11) Hofer, F.; Warbichler, P.; Buchmayr, B.; Kleber, S. J. Microsc. 1996, 184, 163. (12) Grogger, W.; Hofer, F.; Warbichler, P.; Feltz, A.; Ottlinger, M. Phys. Status Solidi 1998, 166, 315. (13) Brydson, R.; Hofer, F.; Upadhyaya, D.; Kothleitner, G.; WardClose, M.; Tsakiropoulos, P.; Froes, S. Micron 1996, 27, 107. (14) Hofer, F.; Warbichler, P.; Grogger, W.; Leitner, O. Microsc. Semicond. Mater., Inst. Phys. Conf. Ser. 1999, 164, 35.

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Figure 1. Electron energy-loss spectrum of La0.4Sr0.6CoO3-δ, showing the oxygen, cobalt, and lanthanum ionization edges.

Figure 2. Background subtraction for EFTEM elemental mapping with the positions of the energy windows needed for the calculation of the elemental maps.

previously been used either for comparison of products of different preparation techniques or for the routine evaluation of phase purity and homogeneity of technical materials. We will demonstrate the capabilities of the EFTEM method in analyzing La1-xSrxCoO3-δ and CdCr2-xInxO4 samples. 2. EFTEM Elemental Distribution Mapping 2.1. Principles of the Method. The basis of energy filtering is provided by electron energy loss spectroscopy (EELS),9 where incident fast electrons cause ionization of atoms through excitation of core electrons, resulting in characteristic ionization edges in the EELS spectrum (similar to X-ray absorption). The elements can then be identified by the energies of the ionization edges. Figure 1 shows an EELS spectrum of a La1-xSrxCoO3-δ sample with the specific ionization edges of cobalt, lanthanum, and oxygen. To image the distribution of an element, energy-filtered images using such characteristic edges in the EELS spectrum are recorded. However, for elemental mapping it is necessary to remove the background contribution to the image intensity because the ionization edges are superimposed on a strong background. To obtain elemental maps, we used the three window technique:15 Two energy-filtered background images in front of the edge and one energyfiltered image at the ionization edge of the element of interest have been recorded (Figure 2). Then, an extrapolated background image below the edge is calculated by fitting the pre-edge images with a power-law (15) Hofer, F.; Warbichler, P.; Grogger, W. Ultramicroscopy 1995, 59, 15.

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model. The extrapolated background is subtracted from the ionization edge image, thus giving the net elemental map.15 To increase the signal-to-noise ratio of the elemental maps, we have additionally calculated jump ratio images by dividing the post-edge image by a preedge image.15 In the present work we show jump ratio images that give in the case of the investigated samples the same information content as the elemental maps (calculated with the three-window technique). In the case of overlapping edges only the jump ratio method can be applied. However, it should be noted that the jump ratio images cannot be used for quantification and sometimes they are susceptible to background slope changes due to specimen thickness variations. Therefore, it is necessary that in every case elemental maps are acquired using the three-window method first, whereas the jump ratio images are then to be calculated afterward. Further image processing of the elemental maps is often required because, in addition to elemental contrast, contrast can also arise from mass thickness and diffraction effects. Especially the diffraction effects occurring in crystalline materials cause problems for reliable elemental mapping. These diffraction effects can be completely eliminated if the energy-filtered images are acquired under rocking beam illumination.16 If in a sample several elements are present, it is very important to reveal the spatial relationship between them. Two or three elemental images can be simply combined to form a color image by assigning each elemental image a red, green, or blue (RGB) color (see Figure 9). These RGB images give rapid information on phase distribution where the localization of the elements with respect to each other is also clearly evident. In this work this simple technique has been mainly used to show the spatial correlation of elements. Alternatively, correlation analysis can be applied using scatter diagrams that can be combined with other statistical methods such as principal component analysis and classification procedures.17,18 In principle, the number of elements used for the computation of scatter diagrams is not limited, but their straightforward visualization is possible only in two or three dimensions, and in practical work we restrict the scatter diagram analysis to two, or in certain cases, to three dimensions.19 2.2. Experimental. The TEM investigations were performed on a 200-kV transmission electron microscope (Philips CM20/STEM), operated with a LaB6 cathode. The microscope is equipped with an energy-dispersive X-ray spectrometer (HPGe-detector, Noran) and an electron energy filter (Gatan Imaging Filter, GIF),20 which can be used to record energy-filtered images and electron energy-loss spectra with the integrated slowscan CCD camera (acquisition times were typically on the order of 3-60 s/image). For elemental mapping the signal-to-noise-ratio (SNR) had to be optimized according to previously published recommendations.21,22 Images and spectra were processed with Gatan’s Digital (16) Hofer, F.; Warbichler, P. Ultramicroscopy 1996, 63, 21. (17) Bonnet, N. J. Microsc. 1998, 190, 2. (18) Hofer, F.; Grogger, W.; Kothleitner, G.; Warbichler, P. Ultramicroscopy 1997, 67, 83. (19) Grogger, W.; Hofer, F.; Kothleitner, G. Micron 1998, 29, 43. (20) Krivanek, O. L.; Gubbens, A. J.; Dellby, N.; Meyer, C. E. Microsc. Microanal. Microstruct. 1992, 3, 187. (21) Berger, A.; Kohl, H. Optik 1993, 92, 175. (22) Kothleitner, G.; Hofer, F. Micron 1998, 29, 349.

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Figure 3. XRD patterns of La0.5Sr0.5CoO3-δ prepared via the mixed oxide route (a) and glycine nitrate process (b).

Micrograph software package. All images and spectra were corrected for dark current and gain variation. However, they were not corrected for the blurring caused by the point spread function of the CCD detector. The drift between successive images was corrected using a cross-correlation algorithm. The X-ray spectra have been quantified with the CliffLorimer method23 using sensitivity factors which have been measured by means of thin film standards. The specimens for the TEM investigations were embedded in epoxy resin and subsequently prepared to electron transparency following the standard TEM powder preparation techniques with final low angle ion milling. With this procedure large and thin flat specimens with an average thickness of about 20 to 80 nm were obtained. 3. Mapping of La1-xSrxCoO3-δ Specimens The perovskite-type oxide La1-xSrxCoO3-δ has potential application, for example, as electrode material for high-temperature fuel cells, as a catalyst, or as a gas sensor.24 For the investigation of the transport and electrocatalytic properties of this material, it is essential to synthesize a homogeneous strontium-rich lanthanum cobaltite. A number of preparation routes are available for these complex oxides, for example, the mixed oxides route by using La2O3, SrCO3, and Co3O4 as starting materials,25 which are sintered in air, or the glycine nitrate process,26 where glycine is added to aqueous solutions of the appropriate nitrates, followed by a heat treatment until the onset of combustion and a calcination and sintering process. In this work, products of conventional (mixed oxides) and glycine nitrate methods for the production of La1-xSrxCoO3-δ are compared. (23) Cliff, G.; Lorimer, G. W. J. Microsc. 1975, 103, 203. (24) Petrov, A. N.; Cherepanov, V. A.; Kononchuk, O. F.; Gavrilova, L. Y. J. Solid State Chem. 1990, 87, 69. (25) Chick, L. A.; Pederson, L. R.; Maupin, G. D.; Bates, J. L.; Thomas, L. E.; Exarhos, G. J. Mater. Lett. 1990, 10, 6. (26) Bucher, E.; Jantscher, W.; Benisek, A.; Sitte, W.; Preis, W.; Rom, I.; Hofer, F. Solid State Ionics 2001, in press.

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Figure 4. La0.5Sr0.5CoO3-δ prepared via the mixed oxide route; (a) TEM bright field image and elemental distribution images of (b) cobalt (Co-L2,3 edge) and (c) lanthanum (La-M4,5 edge), exhibiting the occurrence of a cobalt-rich phase (particle A), the nominal La0.5Sr0.5CoO3-δ phase (particle B) and a lanthanum-rich phase (particle C); (d) EDX spectra from La0.5Sr0.5CoO3-δ sample regions A, B, and C.

For the mixed oxide route La2O3, SrCO3, and Co3O4 powder have been mixed for 15 min in a ball mill, pressed to a pellet, and fired for 15 h in air at 1000 °C. This treatment was repeated three times and the last firing was done at 1400 °C for 20 h. The XRD patterns in Figure 3 reveal minor impurities in the case of the material prepared by this route and a single homogeneous phase for the sample prepared via the glycine nitrate route. Figure 4 shows the bright field image of a La0.5Sr0.5CoO3-δ sample prepared via the mixed oxide route and the corresponding cobalt and lanthanum jump ratio images (using the Co-L2,3 and La-M4,5 ionization edges), respectively. From the elemental distribution maps it is evident that at least three different phases are present in the sample. The cobalt elemental map reveals a bright region (region A) indicating Co enrichment. The lanthanum map, on the other hand, shows no detectable lanthanum content for region A, but

indicates lanthanum enrichment in region C. The local chemical composition of the different phases (regions A, B, and C as indicated in Figure 4a) was measured using EDXS (Figure 4d). The spectra were analyzed quantitatively, showing that the composition of the main material (particles B) is very close to the (desired) nominal composition x ) 0.5 (La ) 22.7, Sr ) 24.6, Co ) 52.7 in at. % of the cations), whereas particle A is cobalt oxide containing La and Sr (La ) 2.1, Sr ) 4.0, Co ) 93.9 in at. % of the cations). The quantitative analysis of region C yields La ) 36.6, Sr ) 32.2, and Co ) 31.2 in at. % of the cations. Figure 4a shows a bright field image of La1-xSrxCoO3-δ with the nominal composition x ) 0.6 obtained via the glycine nitrate process. Electron energy-filtered images were recorded using the O-K, Co-M2,3, La-N4,5, and Sr-M4,5 core level ionization edges in the electron energy-loss spectrum (Figure 5b-d). From the elemental distribution images (the results for oxygen are not

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Figure 5. La0.4Sr0.6CoO3-δ prepared via the glycine nitrate process; (a) TEM bright field image and elemental distribution images of (b) cobalt (Co-M2,3 edge), (c) lanthanum (La-N4,5 edge), and (d) strontium (Sr-M4,5 edge), revealing the uniform composition of the sample.

included in Figure 5, but confirm the overall trend) it is clearly revealed that the sample is uniform in composition. For both materials, EFTEM distribution images have been recorded from different ion-milled specimens. Within one specimen several regions have been analyzed, confirming the results mentioned above. In agreement with the XRD results, the glycine nitrate process (compared to the conventional synthesis from the oxides) yields single-phase perovskite materials.20 This is an important aspect because small departures from the stoichiometric A/B ratio (A ) La, Sr; B ) Co) from unity can result in the formation of unwanted secondary phases and hence influence the materials’ properties significantly. 4. Mapping of CdCr2-xInxO4 Specimens CdCr2O4 is an antiferromagnetic material and has been shown to have excellent sensing properties for NO and NO2.27 When the paramagnetic Cr3+ centers are gradually replaced by diamagnetic In3+, the long-range order decays into short-range order and finally disappears completely at a critical In3+ content. To study the magnetic properties (e.g., magnetic percolation phenomena) of these spinels, it is essential to have homogeneous (27) Lu, G.; Miuva, N.; Yamazoe, N. J. Mater. Chem. 1997, 7, 1445.

Figure 6. XRD patterns of (a) CdCr1.5In0.5O4 sample revealing two spinel phases and (b) CdCr1.95In0.05O4 sample; both samples were synthesized from high-purity nitrates.

and well-defined phases. The spinels CdCr2-xInxO4 with x ) 0.5 and 0.05 were synthesized from high-purity nitrates. Aqueous solutions of the stoichiometric mixtures were evaporated to dryness and then heated in an alumina crucible at 773 K for 5 h to decompose the nitrates. The metal oxides were fired at 1000 K. The resulting fine powder was pressed into pellets at 5 MPa and sintered in an argon atmosphere at 1170 K for 40 h. The XRD pattern of the first sample (x ) 0.5) showed

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Figure 7. CdCr1.5In0.5O4 sample synthesized from high-purity nitrates; (a) TEM image and jump ratio images of (b) cadmium (Cd-M4,5), (c) indium (In-M4,5), and (d) chromium (Cr-L2,3). The jump ratio images clearly reveal the occurrence of a chromiumand a cadmium-rich phase (region 1) and a phase enriched in indium and poor in chromium (region 2).

broadened lines for the CdCr2O4 spinel and a second phase appears, too (Figure 6a). The calculated lattice parameters suggest In3+-substituted CdCr2O4 and Cr3+doped CdIn2O4 spinel structures, which was also confirmed by selected area electron diffraction data. The XRD investigation of the spinel with the lower In concentration (x ) 0.05) reveals a single phase with sharp X-ray lines (Figure 6b). To analyze the actual chemical composition of the nanosized powder samples, analytical transmission electron microscopy has been used. Figure 7 shows a bright field TEM image of the spinel sample with the nominal composition CdCr1.5In0.5O4 (a) and the corresponding elemental distribution images of Cd (b), In (c), and Cr (d) obtained from the Cd-M4,5, In-M4,5, and CrL2,3 edges in the EELS spectrum. Because all the interesting ionization edges overlap (see EELS spectra in Figure 8), EFTEM elemental mapping is extremely difficult to record and to interprete. Therefore, reliable information can only be obtained by using the jump ratio method. In accordance with the XRD results, the EFTEM investigation clearly reveals the occurrence of two different phases, namely, a Cr- and Cd-rich phase (indicated as region 1 in Figure 7a) and an In-rich phase (region 2). Once the phases have been visualized in the EFTEM elemental maps, they can easily be analyzed with EDXS and EELS (Figure 8). The quantification of the EDX spectra reveals the composition of phase 1 (Cr

) 60.4, Cd ) 30.7, In ) 8.9) and phase 2 (Cr ) 14.7, Cd ) 32.8, In ) 52.5) (in at. % of the cations). Because of the severe overlapping of the ionization edges, the EELS spectra could not be used to derive the elemental concentrations. To show the spatial correlation of the elemental maps, we employed a scatter diagram analysis. Figure 9a shows the combination of the Cr-L2,3 and the In-M4,5 jump ratio images exhibiting several clusters that correspond to the chemical phases: The embedding material gives rise to a bright cluster in the upper left corner; more interesting is the cluster at a higher chromium concentration belonging to the chromium-rich spinel and a rather widespread cluster in the lower half of the scatter diagram corresponding to the indium-rich spinel. The separation of the clusters allows the creation of a phase map where each color represents a chemical phase with certain composition values (Figure 9b). The chromium-rich spinel is colored in yellow and the indium-rich phase in red. If the information from this map is combined with a thickness map that can also be recorded by EFTEM, it is possible to derive the volume fractions of the two main phases, which are in rough agreement with the XRD results. The scatter diagram approach is also compared with the simple RGB technique: For the RGB map in Figure 9c (RGB ) red-green-blue) the three-jump ratio images (Figure7b-d), each one representing one basic color

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indium concentration than the small particles. The EDX spectra (Figure 10) were analyzed quantitatively, showing that the large crystal has the composition 66.3 at. % Cr, 33.2 at. % Cd, and 0.5 at. % In (only cations), and the smaller particles contain 67.5 at. % Cr, 31.0 at. % Cd, and 1.5 at. % In (only cations), which is in close agreement with the nominal composition of the spinel with xIn ) 0.05. Our current results show that it is not possible to synthesize a homogeneous spinel phase up to the desired composition of xIn ) 0.5. But even at the very low indium concentration (xIn ) 0.05) two phases with slightly different indium concentrations were present. Further investigations of this material (including modification of the preparation route as well as studies of various compositions) are under progress. 5. Discussion

Figure 8. EDX spectra of phases 1 and 2 of the CdCr1.5In0.5O4 sample (as indicated in Figure 7a).

component of the color map (red ) cadmium, green ) indium, blue ) chromium), are combined. Now all elements are present in the same image and their localization with respect to each other is more evident. The phase distribution visible in the RGB image reveals the same information as the scatter diagram method. However, for the second sample with the nominal composition CdCr1.95In0.05O4, the X-ray diffraction pattern shows the existence of one spinel phase only. In the TEM samples it is easily possible to distinguish two morphologically different crystals, that is, many small crystals with diameters less than 0.5 µm and a few large crystals with diameters more than 2 µm. It was not possible to detect any compositional fluctuations by EFTEM because here this method is at its limit because of the overlapping of the ionization edges. However, a careful analysis using EDX revealed two different phases (Figure 10). The large particles yield a lower

The aim of this work was to demonstrate how EFTEM can be applied to evaluate the homogeneity of certain complex oxides prepared via different routes. From previous work it is already well-known that EFTEM elemental mapping can reveal compositional information in thin samples with nanometer resolution. Almost all elements ranging from lithium to plutonium can be analyzed. The main advantage of EFTEM is that many pixels can be acquired in one exposure, an important factor if extended areas of the specimen need to be analyzed. Furthermore, the acquisition times for the energy-filtered images (which are the basis for the elemental maps) are very short (from a few seconds to about 1 min). In principle, scanning transmission electron microscopy (STEM) with a fine focused electron probe scanning over the specimen can also be used to acquire elemental maps at nanometer resolution, if the apparatus is equipped either with an EELS spectrometer or an energy-dispersive X-ray spectrometer. In both cases the acquisition times for the STEM elemental map can be rather long (up to an hour). For example, we may wish to obtain information on the scale of 5 nm from a specimen field of width of 5 µm; this would require a minimum of 106 pixels, which can be recorded with a 1024 × 1024 pixel CCD array. It is difficult to acquire such a large number of data points in the STEM for two reasons. First, because the minimum readout time to obtain one EELS spectrum is currently in the range of several milliseconds (for a commercial parallel EELS spectrometer), the acquisition time would be prohibitively long. STEM recording times on the order of 1 h may be necessary to achieve adequate statistics in a large image and this may be inconvenient or expensive in terms of instrumental time. Here, the usual solution (for the STEM) is to reduce the amount of information recorded by decreasing the number of pixels. Second, the available current in the STEM is limited to about 1-20 nA by the requirement to form a small probe, whereas in the EFTEM currents on the order of more than 1 µA are available in the much larger spot size that illuminates all image picture points simultaneously. The alternative possibility to record elemental maps in the STEM with nanometer resolution is to collect the characteristic X-rays in the EDX spectrometer. How-

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Figure 9. Phase images of the CdCr1.5In0.5O4 sample; (a) scatter diagram composed from the Cr and In jump ratio images (from Figure 7); (b) chemical phase map derived from the scatter diagram; (c) RGB image composed of the Cd-M4,5, In-M4,5, and Cr-L2,3 jump ratio images (as given in Figure 7) with red ) Cd, green ) In, and blue ) Cr. The distribution of the In-rich phase is clearly visualized in both cases.

ever, for the same incident-beam current, EELS and EFTEM yield a count rate of core-loss electrons, which exceeds that of characteristic X-rays for two reasons. First, for low atomic number Z elements the generation rate of low-energy X-rays is very small because of the low X-ray fluorescence yield, which falls below 2% for Z < 11. Second, transmitted electrons which are used for EELS and EFTEM, are concentrated into a small angular range, thus allowing collection of almost all ionization events, whereas characteristic X-rays are emitted isotropically and the fraction recorded by an EDX detector is about 1%. For some very heavy elements (Hg-U) the EELS ionization cross sections can be rather low and then EDX mapping can be advantageous.28 On the other hand, the high background in the EELS spectrum is a disadvantage for EELS and EFTEM, which leads to a decreasing signal-to-noise-ratio for increasing specimen thickness and the method can only be applied, therefore, to relatively thin specimen areas (