Hofer, F.

Kothleitner, G.

Schlögl, R.

Su, D.

MPG.PuRe

EELS Measurements on Selected Transition Metal Oxides with a New HighEnergy-Resolution SpectrometerG. Kothleitner*, F. Hofer*, D.S. Su** and R. Schlögl*** Research Institute f. Electron Microscopy (FELMI), Graz University of TechnologySteyrergasse 17, A-8010 Graz, Austria**Department of Inorganic Chemistry, Fritz-Haber-Institute, Max-Planck SocietyFaradayweg 4-6, D-14195 Berlin, GermanyAlthough it is well known that EELS near edge fine structures are a powerful tool for studying chemicalbonding at the nanometer level, important applications were prohibited due to the limitation of the initialenergy width of the source which is typically 0.6 eV for Schottky emitters. If the energy resolution can beimproved towards the 0.1 eV regime, near edge fine structure details can be revealed not previouslyobservable by EELS in common instruments.To address this, FEI / Gatan has constructed a monochromized 200 kV (S)TEM which aims at 0.1 eV energyresolution in EELS. The improved energy resolution is due to several factors: (a) The energy spread of aSchottky field emitter (typically 0.6 eV) is reduced by a Wien filter monochromator positioned directly afterthe field emission gun. (b) The 200 kV high tension tank was improved by adding mechanical as well aselectrical damping elements [1], (c) and a high resolution energy filter delivers more stable electronics as wellas improved electron optics [2]. The electron optics of the spectrometer as installed at the FELMI nowcomprises additional multipol lenses in front of the magnetic prism to take out 3rd and some of the 4th orderspectral aberrations. The electron optical performance of the spectrometer can be represented by itsisochromatic surface as displayed in fig. 1. It shows the relationship between the spectrometer collectionangle (implicitly expressed by the size of the indicated apertures) and the focussing error of the spectrometer.The filter can now accept maximum scattering angles, which are about 70% larger compared to conventionalpost-column filters without compromising energy resolution. Larger spectrometer entrance apertures incombination with further reduced camera lengths result in a system with considerably improved collectionefficiency for EELS measurements, giving spectra of higher quality especially on dose sensitive samples.The main objective of this study is to check the performance of the new spectrometer on a monochromized /unmonochromized 200kV FEG (S)TEM with a well characterized sample and to compare these results withprevious experiments. For this purpose, vanadium oxide (V2O5) was chosen for its challenging fine-structureat the V 2p(3/2) and V 2p(1/2) peaks and for its known sensitivity on reduction under the electron beam [3,4].V2O5 has an orthorhombic structure, with Vanadium being surrounded by six oxygen atoms, forming adistorted octahedron. The L3 and L2 peaks are further split into doublet features as a consequence of anoctahedral ligand-field splitting, which divide the d-state into t2g and eg orbitals. Although the L3 and L2-edgeof TiO2 (the neighbouring d0 oxide) show well-resolved t2g-eg features, the energy separation for V2O5 doesnot have the same value and in fact is only in the order of the energy resolution of a typical FEG TEM ataround 0.8 eV [5].Experimental spectra of V2O5 taken on different are displayed in fig. 2. The main spectral features of the VL2,3 edge can be revealed even with a LaB6 TEM. However, most fine structure details are only observablewith field-emission instruments, where the monochromized TEM exhibits a significantly better separation ofthe spectral features. The paper intends to demonstrate the usefulness of the newspectrometer/monochromators in its application to technologically relevant transition metal oxides.References:1. Tiemeijer, P.C., van Lin, J.H.A., and de Jong, A.F.,Microsc. Microanal. 7 (2), 1130-1131.2. Brink, H.A. et al. (2001), Microsc. Microanal. 7 (2), 908-909.3. Hebert, C. et al. (2001), Abstractsbook Dreländertagung f. Elektronenmikroskopie, 34.4. Su, D. S. et al.(2001), Microsc. Microanal. 7 (2), 440-441.5. Chen, J. G. (1997), Surface Science Reports 30, 1-152.Fig. 1: Extrapolated experimental isochromatic surface of the high resolution EELS spectrometer at 200 kV.Apertures are indicated by circles. Measurements indicate that for a practical total energy resolution of 0.3 eVon a monochromated microscope, a 3mm aperture can still be used.Fig. 2: Experimental EELS spectra from V2O5 recorded on a FEI CM20/ LaB6 / GIF200 (top), a T20F FEGwith a high resolution spectrometer (middle) and a T20F / monochromator (bottom). The t2g-eg ligand fieldsplitting of 0.8 eV comes out clearly on the monochromated microscope (sample courtesy of D.S. Su).Acknowledgement:B. Freitag and P. Tiemeijer from FEI Eindhoven are gratefully acknowledged for giving access to themonochromated microscope.1eV0.6eV0.25eV

EELS measurements on selected transition metal oxides with a new high energy-resolution spectrometer

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