EELFS SPECTROSCOPY APPLIED TO MATERIALS SCIENCE

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1 EELFS SPECTROSCOPY APPLIED TO MATERIALS SCIENCE A.N. Maratkanova, Yu.V. Ruts, D.V. Surnin, D.E. Guy Physical-Technical Institute, Ural Branch of RAS 132 Kirov St., Izhevsk Abstract In this paper we report the history of the electron Extended Energy Loss Fine Structure (EELFS) spectroscopy in brief, quite a novel technique for studying local atomic structure of materials. Fine structures above ionization edges studied by EELFS technique are similar to those measured in X-ray absorption spectra and have an extension of several hundreds ev above a threshold and a period of about tens ev. The analysis of these extended structures gives local structural information (partial interatomic distances, coordination numbers, backscattering amplitudes, phase shifts, etc.). EELFS technique has been proved to be a powerful tool for local structure investigations of clean surfaces and chemisorbed species. Numerous papers have been published to demonstrate the applicability of EELFS technique in the determination of the structure of different compounds and different metals deposited on clean surfaces. Application of EELFS technique provides great progress to materials science in regard to the atomic structure study. In this paper we give some examples of studying different materials by EELFS both in the transmission and reflection mode using the results obtained by different authors including ours. 1. INTRODUCTION In the last few years the papers related to studying atomic structure of materials have been of great interest for specialists working in the field of solid state physics, solid state chemistry and materials science. The increased interest is due to the fact that the atomic structure of any material determines directly its physical, mechanical and technological properties. Therefore, thorough study of materials atomic structure allows further optimization of their properties, such as strength, plasticity, corrosion resistance and etc. as well as their prediction. To determine an atomic arrangement in bulk crystalline samples single-crystal diffraction techniques are routinely used. As a result, one may obtain some information about the symmetry of an unit cell and its parameters. Obtaining such kind of information one is faced with a problem of the samples without a long-range order where diffraction techniques are unsuccessful. Another problem is to study the atomic structure of individual blocks, domains, grain boundaries, intermetallic compounds and other inhomogeneous inclusions contained in the bulk material composition. Studying the atomic structure of material surface is also of great importance, since the surface structure has an essential effect on the properties of the whole material, such as physical, chemical, mechanical and all others. When studying the structure of metals and their behavior during different kinds of effects (thermal, magnetic and electric fields, mechanical effort, radiation exposure) it is extremely important to obtain some information about structure and properties of just first few atomic surface layers. It is natural that such kind of information can be obtained using high resolution and sensitive techniques. Extended Energy Loss Fine Structure (EELFS) is one of them and allows direct study of the local atomic structure. EELFS technique [1, 2] detects and analyses oscillations of electron emission intensity extended for several hundreds of ev above core-edge thresholds. There is a close analogy of EELFS technique with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy based on the same interference processes which occur above a core level ionization edge in the X-ray absorption coefficient [3]. The difference is - 1 -

2 the use of electrons as a valid alternative to X-rays to ionize core electrons. The utilization of electrons for structural EXAFS-like spectroscopy has been demonstrated for the first time by Ritsko et al. [4] and later by Kincaid et al. [5] for the K-edge of graphite and by Leapman et al. [6] for heavier elements like chromium. Fig. 1. Schematic picture of the various inelastic excitation processes induced in a solid surface. 2. PECULIARITIES OF EELFS Compared to EXAFS spectroscopy EELFS technique has both advantages and disadvantages. One of the most important advantages is its high site-selectivity due to using electrons to ionize a core level that allows the local atomic structure study of individual constituent parts of material studied, namely grains, intermetallic inclusions, domains and etc. EELFS technique offers some unique advantages with respect to the diffraction techniques such as low energy electron diffraction (LEED) [7] because structural information can be obtained about the systems which exhibit only a short-range order as well as those with a long-range periodicity. Moreover, light elements sensitivity gives an opportunity to study bulk and surface oxides, carbides and others, processes of their formation including gas adsorption. One of the most attractive aspects of EELFS technique is the possibility to use the experimental equipment available in the most of materials science or surface science laboratories (standard Auger electron microprobe for surface EELFS in reflection mode or a standard transmission electron microscope equipped with an attachment for electron energy loss spectra). Besides, close analogy with EXAFS allows using the same mathematical treatment for EELFS spectra to extract structural information, namely Fourier transformation and regularization methods [8-11]. Among the main drawbacks of EELFS technique there is certainly the low contrast of the spectra, since EELFS features are generally 10 20% of the corresponding coreedge thresholds intensities as shown in the schematic picture of various inelastic excitation processes induced in the solid surface upon interaction with a primary electron beam and measured in the back-scattered electron yield (Fig. 1). One more disadvantage is the superposition of EELFS structures with other electronic processes. 3. EELFS MECHANISM The phenomenology of EELFS process is similar to that of the photoelectron which generates EXAFS features. To clarify the point, we will show this process compared with EXAFS one (Fig. 2). In the case of EXAFS the atom excited by the X-ray photon emits a photoelectron. This photoelectron is scattered coherently by the nearest- neighbor atoms. This process gives rise to oscillatory fine structures observed in the energy (!ϖ ) range above the core level ionization edge (E α ). EXAFS contains information about the local atomic structure. The oscillatory behavior of the absorption coefficient within single-scattering formalism is given in the case of a K-edge by the following equation [8, 9]: l N j rj χ( k) = ( 1) Fj( k, π)sin ( 2krj + ϕ j( k) + 2δ l ) exp 2 σ jk +, 2 j krj λ( k) (1) - 2 -

3 where k is the wave vector of the photoelectron, l is the final angular momentum, r j is the distance between the excited central atom and its surrounding neighbors, N j is the number of the atoms of type j around the central atom at the distance r j. F(k,π) is the amplitude of the backscattering of the photoelectron by an j- neighbor atom. The sine term includes the neighbor atom phase shift, ϕ j ( k), and central atom phase shift, δ 0 l. The exponential term takes into account the thermal vibration of the atoms in the solid (Debye-Waller term) and the damping due to inelastic losses of the photoelectron during 2 its passing, where σ j is the mean-square displacement and λ(k) is the electron mean free path. In the case of EELFS an atom is ionized by a beam of monoenergetic primary electrons, which loses some energy at least equal to that necessary to ionize a core electron in the medium. The energy distribution of these inelastic electrons shows the same features as the X-ray absorption coefficient distribution in EXAFS as presented schematically in Fig.2. Except for the difference in the ionization mechanism EELFS process is the same as EXAFS one and it may also be described by Eq.(1). 4. BRIEF HISTORY OF EELFS Since the time of the first experimental paper devoted to EELFS study of the local atomic structure of graphite in the transmission mode [5], where the possibility of using electrons for structural EXAFS-like spectroscopy was demonstrated, the technique development has made great progress. It has been done in three main directions. First of all, there are a lot of experimental works concerning the local atomic structure study of different materials. The main focus is the accumulation of rich experience and comparison of EELFS results with those obtained by EXAFS. Beside the noticeable number of experimental measurements, EELFS technique has also attracted attention from the theoretician s point of view. Much effort has been devoted particularly to understanding the fundamental mechanisms which give rise to the analogy and difference between EXAFS and EELFS [12-14] spectroscopies. One of the main theoretical problems is the applicability of the dipole selection rule. The analysis of this problem has been carried out by several authors on different systems. On the basis of the theoretical calculations [15-20] and experimental EELFS results [21, 22] it has been shown that the dipole selection rule can be used within at least first approximation. Finally, different techniques of the analysis of EELFS spectra to extract structural information, namely, the structural lattice parameters, the backscattering amplitudes and the phase shifts, have been developed. The most widely used ones are the Fourier transformation and the Tikhonov regularization method [8-10, 11]. 5. ANALYSIS OF EELFS DATA Fig.2. Energy diagram of the EELFS and EXAFS processes

4 EELFS spectra are generally measured as a function of the first and second derivative of the electron yield N(E) to increase their contrast range. The structural analysis of EELFS spectra can be divided into the following different steps, first three of them being the same for different treatment techniques: 1) preliminary background subtraction to extract oscillatory parts, 2) integration of the whole EELFS spectrum to obtain the N(E), 3) conversion of the energy data into a wave-number form, 4) Fourier transformation or regularization to obtain the atomic pair correlation function (PCF), 5) inverse Fourier transformation to filter EELFS oscillation associated with the given coordination shell to obtain the envelope function and phase shift. A typical EELFS spectrum in the reflection Intensity, arb. units Energy Loss, ev Fig.3. a) Typical EELFS spectrum above the Fe M 23-edge in the reflection mode. The dashed line is the background. b) The extracted fine structure above the Fe M -edge. mode with the extracted oscillatory part is represented in Fig.3. There is a distinct background superimposed on the energy loss structures. After removing this background the data should be numerically integrated and the extended fine structure should be isolated from all other contributions not related to the atomic structure of the material studied, especially the core loss feature and the near-edge structure. The extension of the data energy range depends on the element and the excited edge under study. For example, the K-edges of light elements (C, O, N) extend for no more than ev because of a monotone decrease of the backscattering amplitudes of these elements as a function of wave vector [23]. The next step for EELFS analysis is the conversion of the energy data into a wave number k(å -1 ) grid according to the relation: 1 1 m E( ev) k( Å ) = 2 2 2! (2) where E = E E 0 ; E is the measured energy loss of the scattered electron and E 0 is the binding energy of the core electron. Further mathematical treatment is aimed at obtaining the atomic PCF using the Fourier transformation according to: kmax 1 2ikr FR ( ) = χ() ke dk, 2π k min 23 (3) where k min and k max are the lower and upper limits of the data range. The regular algorithms can also be used. Various peaks in the obtained atomic PCF are related to different neighbors in the nearest surrounding of the atom ionized. The area of different peaks correlates with the coordination numbers, and their half-width at half-height correlates with the mean-square displacement. 6. SOME APPLICATIONS OF EELFS IN MATERIALS SCIENCE b a, - 4 -

5 We review here the experimental results obtained by EELFS spectroscopy. The most attractive prospects of using EELFS technique are connected with the possibility to study the processes on the solid surface. Most works made by EELFS technique relate to studying the local atomic structure of adsorbates and chemisorbed species on metals and semiconductors surface. In particular, the first papers on EELFS in the reflection mode were devoted to the study of chemisorption processes of oxygen on Ni(100) [25] and carbon on Ni(111) [26, 27] surfaces to obtain interatomic distances between the chemisorbed atoms and substrate as well as studying bond lengths evolution in oxidation, carbidisation. For example, the structural parameters of the graphite carbon overlayer on a Ni(111) substrate has been determined by using EELFS in the reflection mode [28]. It has been found that graphite carbon is very similar to a graphite single-crystal plane, but slightly expanded (~2%), with C-C 1 and C-C 2 distances of 1.45 ± R (Å) beam energy and 2.50 ± 0.03Å, respectively. In Fig. 4 C overlayer 1 st Ni layer the experimental spectrum χ(k) extracted above the graphite carbon K-edge (a) and the Fourier transform of this spectrum (b) are presented. The inset shows the Fourier transform of graphite single crystal obtained with the same experimental conditions. The (b) graphite overlayer floats at 2.80 ± 0.08Å above the Ni(111) face that is shown in Fig.5. It has been found that the Ni-Ni distance remains identical (Fig.6), showing that the substrate is not affected by the growth of the graphite overlayer. The processes of R (Å) oxidation, carbidisation and nitridization of different materials surfaces were studied by many authors [28, 29] using EELFS. EELFS Fig.5. (a) Fourier transform of an EXAFS model technique in the reflection mode has turned calculation for the system C /Ni(111) around graph out to be useful to study the growth of thin the carbon K-edge. (b) Fourier transform of the epitaxial films and metal/silicon interface experimental EELFS spectrum above the C K-edge. formation where metals were cobalt, titanium, chromium and iron [30-34]. Unlike other diffraction techniques, such as LEED and Fourier Transform (arb. units) (a) Carbon K-edge 2.8 Å Carbon Nickel arb. units Fourier Transform k (Å -1 ) (a) (b) C graph /Ni(111) Carbon K-edge Graphite C K-edge R (Å) Fig.4. (a) EELFS signal extracted above the K-edge of the graphitic carbon formed on Ni(111). (b) Fourier transform of the above spectrum. The inset shows the Fourier transform of the graphite single crystal obtained with the same primary - 5 -

6 photoelectron diffraction, EELFS allows one to study not only the early stages of the interface formation when long-range order holds, but also any other compound formation obtained as a result of interaction and interdiffusion with the substrate. In these works it was shown that EELFS is extremely sensitive to any change in the local atomic structure on passing from the initial silicide to the formation of a pure film by increasing the amount of the evaporated metal. Since the time of the first experimental EELFS work, reported by M. Kincaid et al., on graphite and amorphous carbon [5] this material is still one of the most intensively studied one. Many authors [35-39] have studied different forms of carbon by EELFS technique to probe the structure parameters. Recently EELFS technique has been used to evaluate the Fourier Transform (arb. units) (a) (b) Ni(111) M -edge C /Ni(111) graph Ni M -edge R (Å) 23 quality of thin polycrystalline diamond films grown on smooth silicon substrates using the hot filament chemical vapor deposition (HF-CVD) technique as compared with the natural diamond [39]. In Fig.7 the extended fine structure above the carbon K-edge for (a) polycrystalline diamond thin film is shown in comparison with other modifications of carbon. Fig.9 shows the radial distribution functions obtained by applying the Fourier transform to the extended energy loss spectra of the diamond particle and diamond film presented in Fig.8, (a) and (b), respectively The values obtained for the nearest neighbors in the two diamond samples agree well with the known Fig.6. Fourier transform of the EELFS spectrum crystallographic ones of 1.54Å for the first above the Ni(111) M nearest neighbors and 2.51Å for the second 23-edge for (a) clean surface and (b) surface covered with graphitic carbon. nearest neighbors of cubic diamond. Fei et al. [40] discussed EELFS results obtained on metallic glasses containing boron, Fe 80 B 20 and Fe 78 Si 5 B 17 comparing the structural results with pure boron. Lee, Tong and Montano [41] used EELFS technique in combination with UPS and EELS spectroscopies to study the deposition of iron on MgO(100) as well as to identify the surface compounds formed after reaction of CO/H 2 (1:1). EELFS analysis of the oxygen K-edge of the Mg(100) surface with two monolayers of iron suggests that the iron atoms bond preferentially with the oxygen at the surface. With increasing Fe coverages (four monolayers) EELFS spectrum shows that the deposited iron is oxidized after the reaction of CO/H 2. Higher iron coverages result in carbidization of the surface. Carbon deposition was at the same time monitored above the carbon K-edge and different carbide forms were found depending on iron coverages. Changes of the materials local atomic structure due to different heat treatments were studied using EELFS technique. For example, EELFS spectra above Al K- and Ag M 4,5 -edges for the Al-20%Ag alloy after high temperature aging and for pure components of the alloy were Fig.7. Extended fine structure above the C K-edge for (a) polycrystalline diamond thin film, (b) natural diamond, (c) highly oriented pyrolitic graphite, (d) amorphous carbon. Intensity, arb. units (a) Energy Loss, ev R, Å (b) Fig.8. Radial distribution functions F(R) as obtained from the direct Fourier transform of the extended fine structures shown in Fig.7. measured [42]. The comparison between the experimental atomic PCFs and model ones showed the decomposition state of the solid solution. The binary alloy after high temperature aging becomes the aluminum matrix with the zones enriched with silver atoms. The Al-Al o Fig.9. (a) Extended fine structure above the C K-edge - 6 in - cementite after heating up to 1050 C followed by o isothermal tempering at 500 C for 1 min (dashed line); the same heat treatment plus an additional o annealing at 700 C for 20 hours (dashed-dotted); the model calculation (solid). (b) Radial distribution function around C atoms as obtained from Fourier transform of the extended fine structures shown in Fig.8.

7 interatomic distance in the binary system is equal to that in the pure aluminum. Ag-Ag and Ag-Al partial interatomic distances agree well with the distances in the γ - phase of Ag 2 Al. The analysis of our own results obtained on the Fe 3 C samples after different heat treatments using EELFS above the carbon K-edge and the changes observed in the Mössbauer spectra of the steels annealed by various regimes [43] showed the difference in the nearest - neighbor surroundings of the iron atoms in the cementite lattice; i.e. some evolution of the cementite structure which is likely to be related to the rearrangement of the carbon atoms. 7. CONCLUSIONS EELFS technique turns out to be a unique probe of materials, since knowledge of local atomic structure, i.e., the species of atoms present and their locations, is essential to make progress in many scientific fields, in particular, in metallurgy and materials science. However, extracting this information with precision in the complicated, aperiodic materials of importance in modern science and technology is not easy, even though the subtle and refined experimental techniques is available. Over the past two decades EELFS technique has made great progress toward the goal of providing such information. But much work is currently to be done to make the tool more useful in extracting structural information of materials. The authors thank Russian Foundation for Basic Research (grants a and ). 1. De Crescenzi, M., Papagno, L. et al. Solid State Commun., 1981, vol. 40, p De Crescenzi, M., Chiarello, G. J. Phys. C, 1985, vol. 18, p Ritsko, J.J., Gibbons, C. et al. S.E. Phys. Rev. Lett., 1974, vol. 32, p Rehr, J.J., Albers, R.C. Rev. Mod. Phys., 2000, vol. 72, p Kincaid, B.M., Meixmer and Platzman, P.M. Rev. Lett., 1978, vol. 40, p Leapman, R.D., Grunes, L.A. et al. J. Chem. Phys., 1980, vol. 72, p D.P. Woodruff, Surf. Sci., 1986, 166, p Lytle, F.W., Sayers, D.E., Stern, E.A. Phys. Rev. B., 1975, vol. 11, p Lee, P.A., Citrin, P.H. et al. Rev. Mod. Phys., 1981, vol. 53, p Teo, B., Joy, D. EXAFS spectroscopy and Related Techniques. Plenum, New York, Tikhonov, A.N., Arsenin, V.Ya., in: J. Filtz (Eds). Methods of solution of ill-posed problem, 1977, 287 pp. 12. De Crescenzi, M., Chiarello, G. et al. Phys. Rev. B, 1984, vol. 29, p Mila, F., Noguera, C. J. Phys. C, 20 (1987) D.K. Saldin, Phys. Rev. Lett., 1988, vol. 60, p Fujikawa, T. J. Phys. Soc. Jpn., 1988, vol. 57, p Fujikawa, T., Takatoh, S., Usami, S. Jap. J. of Appl. Phys., 1989, vol. 28, p Yikegaki, T., Yiwata, N. et al. Jap. J. Appl. Phys., 1990, vol. 29, p Fujikawa, T. Surf.Sci., 1992, vol. 269/270, p Saldin, D.K., Yao, J.M. Phys. Rev. B., 1990, vol. 41, p Saldin, D.K., Ueda, Y. Phys. Rev. B., 1992, vol. 46, p De Crescenzi, M., Lozzi, L. et al. Surf.Sci., 1989, vol. 211/212, p Guy, D.E., Surnin, D.V. et al. J. Electron Spectrosc. Relat. Phenom. (to be published). 23. Teo, B. and Lee, P.A. J. Am. Chem. Soc., vol. 101, p Surnin, D.V., Guy, D.E. et al. J. Phys. IV France, 1997, vol. 7, p. C De Crescenzi, M., Antonangeli, F. et al. Phys. Rev. Lett., 1983, vol. 50, p Papagno, L., Caputi, L.S. Phys. Rev. B., 1984, vol. 29, p Rosei, R., De Crescenzi, M. et al. Phys. Rev. B., 1983, vol. 28, p De Crescenzi, M., Chiarello, G. et al. Solid State Commun., 1982, vol. 44, p

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