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Studies of iron and iron oxide layers by electron spectroscopes
Applied Surface Science 252 (2005) 330–338 www.elsevier.com/locate/apsusc
Studies of iron and iron oxide layers by electron spectroscopes B. Lesiak a,*, A. Jablonski a, J. Zemek b, P. Jiricek b, M. Cˇernˇansky´ b a
b
Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 16253 Prague 6, Czech Republic Received 20 August 2004; accepted 24 December 2004 Available online 16 March 2005
Abstract Thin iron oxide layers prepared ‘‘in situ’’ in the ultra high vacuum on polycrystalline iron substrate were investigated by electron spectroscopy methods—X-ray photoelectron spectroscopy (XPS) and elastic peak electron spectroscopy (EPES), using spectrometer ADES-400. The texture and the average grain size of the iron substrate foil have been examined by glancing angle X-ray diffraction (XRD). Qualitative and quantitative estimation of investigated oxide layers was made using (i) the relative sensitivity factor XPS method, (ii) comparison of binding energy shifts of Fe 2p photoelectron line and (iii) non-linear fitting procedure of Fe 2p photoelectron lines. Both, sputter-clean polycrystalline iron substrate and finally grown Fe2.2O3 layer, were investigated by the EPES method to measure the electron transport parameters used for quantitative electron spectroscopy, such as the electron inelastic mean free path (IMFP) values. The IMFPs were measured in the electron kinetic energy range 200–1000 eV with the Cu standard. The surface excitation parameters using Chen and Werner et al. approaches were evaluated and applied for correcting these IMFPs. The discrepancies between the evaluated parameters obtained using the above quantitative and qualitative approaches for characterising the iron oxide layers were discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Iron; Iron oxide layers; Fe2.2O3; Fe2O3; Layer thickness; X-ray photoelectron spectroscopy (XPS); Elastic peak electron spectroscopy (EPES); Cu standard; Inelastic mean free path (IMFP); Surface excitation
1. Introduction Metal-oxides constitute an important class of materials that are involved in environmental science, electrochemistry, biology, chemical sensors, magnetism, heterogeneous catalysis, where metal-oxides are * Corresponding author. Tel.: +48 22 343 3432; fax: +48 226325276. E-mail address: [email protected] (B. Lesiak).
catalysts used for synthesis of many organic compounds via selective oxidation, dehydrogenation, isomerization and other chemical processes [1–8]. The basic understanding of the relationship between the catalytic function of oxide materials, their crystallographic structure and chemical composition in the nearest surface region is necessary for developing the catalysts. The main goal consists in distinguishing qualitatively and quantitatively the two possible oxidation states Fe2+ and Fe3+. The oxide layers
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.055
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investigated in the present work has been recently studied using the XPS qualitative and quantitative analysis for determining the chemical oxidation state, Tougaard QUASES and Werner et al. SESSA approaches for investigating the oxides layer thickness and depth distribution function measurements [9]. For the purpose of quantitative electron spectroscopy, like XPS, Auger electron spectroscopy (AES), reflection energy electron loss spectroscopy (REELS), the basic electron parameter such as the electron inelastic mean free path (IMFP) is required. Values of the IMFPs for Fe, derived from the transmission optical data and a theoretical model, valid for the bulk of the investigated material, can be evaluated from Tanuma et al. optical data [10] and TPP-2M [11] predictive formula, which is based on the fits of Tanuma et al. IMFPs [10]. With the use of TPP-2M [11] formula the IMFPs for any complex inorganic and organic compound can be evaluated. The G1 of Gries [12] predictive formulae based on calculated IMFPs of Tanuma et al. [10] and on atomistic model can be also used for evaluating the IMFPs in elemental solids and complex samples. Another theoretical model applied to optical data developed by Kwei et al. [13] allows evaluating the IMFPs for Fe. Experimental determination of the IMFPs valid for the surface, differing from those for the bulk due to inelastic scattering and the relaxation processes, as well as density, can be determined using the electron elastic peak spectroscopy (EPES) described in details elsewhere [14]. Several authors evaluated the IMFPs for Fe using the EPES method based on the measurements of electron backscattering probability [15–19], however these values have not been evaluated for Fe oxides. Lesiak et al. [15] evaluated the IMFPs in Fe from the experimental intensities and Monte Carlo model accounting for electron multiple elastic scattering. Werner et al. [17,18] IMFPs in Fe and other 23 elemental solids were derived from the experimental intensities on the basis of a simple physical model of elastic backscattering that accounts for bulk elastic and inelastic scattering in the electron energy range 50–3400 eV. For energies above 200 eV the IMFPs were well described by Bethe equation for inelastic scattering, as found earlier by analysis of optical scattering data with the aid of linear theory. The inelastic scattering of electrons was also considered when evaluating the IMFPs in Fe by Fujita et al. [19].
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In the present work, the iron foil and the Fe oxide layers of various thicknesses are characterised using the XRD, XPS and EPES methods. The XPS is applied for estimating the composition of the investigated surfaces. The photoelectron Fe 2p lines allow analysing qualitatively Fe oxidation state from the binding energy shifts. Determination quantitatively and qualitatively of metallic and oxidised Fe in oxide layers of various thickness proceeds using the nonlinear fitting procedure of the Fe 2p line from metallic and finally deposited thick Fe2.2O3 iron oxide [9]. Besides, the IMFPs in Fe and Fe2.2O3 iron oxide, in the electron kinetic energy range 200–1000 eV are determined using the Cu standard by the EPES method [14]. The measured EPES IMFPs in Fe and iron oxide are corrected for surface excitation using Chen [20] and Werner et al. [21] corrections, and compared to Tanuma et al. [10] and G1 of Gries predictive formula IMFPs [12].
2. Elastic peak electron spectroscopy (EPES) method The EPES method [14] combines the experimental and theoretical procedures to evaluate the IMFPs in the surface region of solids by comparing the measured and calculated electron elastic backscattering probability. The theoretical model of elastic backscattering is typically implemented by a Monte Carlo (MC) scheme. We assume that the solid has a uniform composition and atomic density within the surface region submitted to analysis. The electron trajectory can be considered as a ‘‘random walk’’ of an electron in the volume of the solid, where the electron direction is changed after each elastic collision. Both, largeangle and small-angle scattering events are taken into account. The direction of an electron leaving the solid is calculated after considering all collisions, described by the polar and azimuthal angles, along the trajectory in the solid. The distribution of polar scattering angles after a single scattering event is described by the elastic scattering cross-section. The azimuthal scattering angles are assumed to follow the uniform distribution in the range from 08 to 1808. The linear step lengths between elastic electron collisions follow the exponential distribution. The electron trajectory is followed from its point of entry into the solid until it
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emerges, or until its length becomes so large that the probability of passing it without energy loss is negligibly small. The MC algorithm is considered to be the most accurate. However, due to its slow convergence to obtain a reasonable accuracy for a particular experiment, a considerable computational effort is necessary. Usually, the electron trajectories to be generated should be 107. Selection of electron elastic scattering crosssections as a function of electron energies is important [22]. Version 3.0 [23] contains the differential elastic scattering cross-sections derived from the wave function obtained from the Dirac–Hartree–Fock potential. In the present calculations the differential elastic scattering cross-sections from database NIST 3.0 were used [24]. The MC simulations require the following input parameters: the electron kinetic energy, the primary electron incidence angle, the backscattered electron emission angle and the solid angle of the analyser. For each electron energy, the number of electron elastic backscattering intensities at the investigated sample is simulated assuming the set of input values of IMFPs ˚ , called a calibration curve. ranging from 5 to 300 A Experiment provides the ratios of backscattered intensities. Then, the ratio of electron elastic backscattering intensities at the sample and the standard is simulated. For standard, the recommended IMFPs is applied [14]. Comparison of simulated calibration curves to the measured electron backscattering probabilities (area under the elastic peak) or their ratios with respect to the selected standard, allows estimating the required IMFP for investigated samples.
3. Experimental 3.1. Sample preparation The iron foil 25 mm 25 mm, 0.001 mm thickness of purity >99.85% (Goodfellow, UK) was used. Repeated sputter cleaning and annealing of foil surface was made ‘‘in situ’’ by Ar+ beam (5 keV, 1 105 A/cm2, 608 with respect to the surface normal) at sample temperature from room temperature to 350 8C. Iron oxide films of nanometric thickness were grown ‘‘in situ’’ at successive molecular oxygen exposition at pressure from 1 104 to 1 102 Pa,
Table 1 Comparison of quantitative XPS results for FeOx oxide layers of various thicknesses: (a) stoichiometry obtained from quantification of O 1s and Fe 2p XPS line areas; (b) thickness obtained from O 1s and Fe 2p peak areas using QUASES and SESSA software [9]; (c) metallic iron and iron oxide contribution obtained from non-linear fitting procedure applied to the XPS lineshape of Fe 2p transition FeOx spectrum (a) Stoichiometry
(1) (2) (3) (4) (5) (6) (7) (8)
Fe2.1O3 Fe2.5O3 Fe2.1O3 Fe2.0O3 Fe2.0O3 Fe2.2O3 Fe2.0O3 Fe1.8O3
(b) Overlayer thickness [9]
(c) Results of non-linear fitting (contribution)
QUASES
No back ground
Linear background
Fe
Fe2.2O3
Fe
Fe2.2O3
56 47 36 34 29 31 24 9
44 53 64 66 71 69 76 91
56 49 39 35 34 32 24 13
44 51 61 65 66 68 76 87
0.19 0.34 1.09 1.22 1.49 1.56 1.67 1.71
SESSA
0.15 0.26 0.68 0.85 1.27 1.41 1.55 1.75
at a sample temperature from room temperature to 240 8C, and at different times of exposition. The oxide layers thickness evaluated by XPS, Tougaard—QUASES [25], Werner et al.—SESSA [26] packages (summarized in Table 1) and ‘‘ex situ’’ by electron probe microanalysis (JOEL JXA-733) have been evaluated and described in details elsewhere [9]. 3.2. X-ray diffraction (XRD) texture and grain size evaluation The iron polycrystalline foil annealed up to 350 8C and submitted to Ar+ ion surface cleaning was investigated by recording the diffraction patterns using Cu Ka radiation with the flat-cassette camera in the back- and forward-reflection arrangement. The grain size has been evaluated using the glancing angle XRD method within the information depth of 10 mm [27], i.e. within the whole Fe foil. 3.3. X-ray photoelectron spectroscopy (XPS) Experiments were carried out using ADES-400 photoelectron spectrometer (V.G. Scientific, UK). The photoelectrons were excited using Al Ka radiation (hn = 1486.6 eV). The half-cone acceptance angle of
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the hemispherical analyser was 4.18. Spectra were recorded in constant energy mode at pass energy 100 or 20 eV. The XPS measurements were carried out at the X-ray incidence angle 708 and photoelectron emission angle of 08 with respect to the surface normal. For quantitative and qualitative analysis the set of spectra in the energy region Fe 2p, O 1s, C 1s, Ar 2p were recorded for a sputter cleaned Fe surface. After each oxidation step, similar spectra were recorded to estimate quantitatively and qualitatively the surface iron oxides. 3.4. Elastic peak electron spectroscopy (EPES) Experiments were carried in ADES-400 spectrometer (V.G. Scientific, UK) at the surface of sputtercleaned Fe and oxidised Fe samples and sputter-cleaned Cu standard. The electron source beam current was 0.1–1.0 mA, and beam spot diameter 4 mm. The electron backscattering intensities were recorded at the primary electron kinetic energies 200, 250, 300, 350, 400, 500, 750 and 1000 eV with the electron source impact normal to the sample surface, the electron emission angle 508 with respect to the surface normal and the half-cone angle 4.18. The typical full width at half-maximum (FWHM) of elastic peak in the whole energy range was 0.5 eV. The area under the elastic peak was calculated after subtracting the Shirley background.
4. Results 4.1. XRD and XPS analyses The value of average grain size in iron foil annealed up to 350 8C estimated by XRD was 100 nm, whereas the exhibited texture of the iron foil was found very weak. Atomic concentrations of iron and/or oxygen in sputter cleaned iron and the iron oxide layer were determined from Fe 2p and O 1s peak areas after inelastic background subtraction, assuming a simple model of a semi-infinite solid of homogeneous composition, using the relative sensitivity factor approach [28], the peak areas corrections for photoelectric cross-sections and asymmetry [29], the total mean free paths of electrons in the transport approximation, and the measured transmission function of the spectrometer [30]. The resulting
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atomic composition for sputter clean Fe was: Fe— 97 at.%, C—2 at.%, Ar—1 at.%, and for the final Fe oxide: Fe—42.9 at.%, O—57.1 at.%, indicating average stoichiometry Fe2.2O3. In addition, for thinner oxide films, the finite film thickness was accounted for in quantitative analysis (see Table 1). The qualitative analysis on the basis of binding energies for Fe 2p3/2 electrons indicated metallic Fe8 state (binding energy 706.9 eV) and Fe3+ oxide state (binding energy 710.9 eV) [9,28]. 4.2. Separation of oxide and substrate contribution from the XPS Fe 2p lines Exemplary Fe 2p XPS spectra representing two chemical Fe states—metallic Fe8 and finally deposited thick Fe oxide layer of stoichiometry Fe2.2O3, with thickness exceeding the Fe 2p photoelectron information depth (ID), are shown in Fig. 1(a). The respective spectra representing thin Fe oxide layers deposited on Fe foil, having thickness smaller than Fe 2p photoelectron ID, are shown in Fig. 1(b). The non-linear fitting procedure was applied to fit the Fe 2p photoelectron spectra from thin FeOx oxide layers, to estimate quantitatively the contribution from metallic iron and iron oxide. In the above fitting procedure the Fe 2p photoelectron spectra measured at sputter cleaned metallic Fe8 foil and finally deposited thick Fe oxide layer of stoichiometry Fe2.2O3 without and after linear background subtraction were applied. The results of fitting procedure representing the percentage contribution from measured spectra, are compared in Table 1. Results of analysis for evaluating the stoichiometry of FeOx iron oxide layers of various thickness (Fig. 1(b)) were based on quantification of O 1s and Fe 2p photoelectron lines peak areas recorded at thin iron oxide layers, accounting for percentage contribution of Fe 2p signal calculated from the non-linear fitting procedure described above. These values of estimated stoichiometry applying the procedure described above for every thin iron oxide layer are indicated in Table 1. 4.3. The IMFPs using the EPES method The model of MC simulating the electron transport in a solid, does not account for electron inelastic
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Fig. 1. Photoelectron Fe 2p spectra. (a) Sputter-cleaned Fe substrate and thick Fe2.2O3 oxide layer. (b) Iron oxide FeOx layers of various thicknesses.
surface excitation experienced by an electron with energy E incoming and leaving the surface at an incidence angle, uin, and emission angle, uout, measured with respect to the surface normal. The influence of the surface excitations on the calculated electron elastically backscattered intensity can be corrected by the surface excitation parameter. The surface excitation parameter, SEP, denoted by PS(E), represents the average number of surface excitations of electron in a single surface crossing. Since in EPES analysis the electron crosses the surface twice, total surface excitation parameter for incoming, PS(E, uin), and outcoming, PS(E, uout), electrons defined as surface excitation parameter, total SEP or PS(E), is the following [31]: PS ðEÞ ¼ PS ðE; uin Þ þ PS ðE; uout Þ The SEPs can be evaluated using the model proposed by Chen [20] valid for rectilinear surface crossing or a semi-empirical approach of Werner et al. [21] accounting for deflections during surface crossing.
Using a model dielectric function, the SEP have been calculated for several elements and semiconductors by Chen [20] according to equation: PS ðE; uÞ ¼ ach E1=2 cos1 u where ach is the material dependent fitting parameter. Using extensive measurements of reflection energy loss spectra (REELS) for selected elements and semiconductors, Werner et al. [21] extracted the SEP values from the ratio of the number of electrons that induced a surface excitation to those elastically reflected. The SEPs can be evaluated from the following: PS ðE; uÞ ¼ 1=ð0:173aH E1=2 cos u þ 1Þ where aH is the material dependent fitting parameter. Corrections for surface excitation should be applied to the measured or MC calculated intensity ratios. The probability that an electron incoming and leaving the solid does not produce any surface excitation in a backscattering experiment is expressed by exp[PS(E)].
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Fig. 2. Ratios of electron elastic backscattering intensities measured at investigated samples with respect to Cu standard (uncorrected, Chen [20] and Werner et al. [21] corrected): (a) metallic iron; (b) thick Fe2.2O3 oxide layer.
Then, the MC simulated electron elastic backscattering probabilities should be multiplied by exp[PS(E)] to be compared to the measured elastic peak areas, or the measured peak areas should be divided by exp[PS(E)]. The measured ratios of electron backscattered intensity from Fe versus the Cu standard and Fe2.2O3 versus the Cu standard are compared with those corrected using Chen [20] and Werner et al. [21] approaches in Fig. 2(a) and (b), respectively. The material parameters for Fe and Cu are taken from Refs. [20,21]. For oxides, however, they are not available in the literature. These parameters are expected to be smaller for oxides in comparison to elemental solids [32]. To receive the material parameter for iron oxide we used a semi-empirical formula derived by Werner et al. [21] providing a rough estimate of the parameter for any material. Calculation was carried out for Fe2O3 with a mean number of valence electrons 6.8 per atom and atomic density of 9.862 1022 cm3. The atomic density was calculated from the volume density of Fe2O3 using the SESSA software [26]. The value of material parameter for Fe2O3 was aH/aNFE = 1.5851. The energy dependence of the IMFPs, uncorrected and corrected using Chen [20] and Werner et al. [21] SEPs in Fe is shown in Fig. 3(a), whereas the energy dependence of the IMFPs in Fe oxide is shown in Fig. 3(b). The measured EPES IMFPs in Fe in the electron energy range 200–1000 eV are compared to Tanuma et al. [10], G1 of Gries [12] predictive
formulae IMFPs and Werner et al. [17,18] IMFPs (Fig. 3(a)). The measured EPES IMFPs in iron oxide (uncorrected, partially corrected for the Cu SEP, and fully corrected for the Cu and Fe2O3 SEPs) are compared to the G1 of Gries [12] predictive formula IMFPs (Fig. 3(b)). The scatter between the EPES measured and calculated IMFPs is evaluated from estimating the root-mean-square deviation (RMS) and percentage deviation (R) using equations listed below [14]: " #1=2 r 1 X 2 RMS ¼ ðlmeas lcalc Þ r j¼1 X 1 r lmeas lcalc R ¼ 100 r j¼1 lmeas where lmeas and lcalc are the EPES measured and calculated IMFPs, and r the number of the IMFPs accounted for in the procedure. This scatter is listed in Table 2.
5. Discussion and conclusions The thickness of thin Fe oxide layers estimated previously using QUASES and SESSA software were between 0.19 and 1.71 nm and 0.15 and 1.75 nm, respectively [9]. Since the mean escape depth (MED) of the O 1s photoelectrons travelling in iron oxide is
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Fig. 3. Comparison of IMFPs measured using Cu standard: (a) Fe; (b) thick Fe2.2O3 oxide layer. Closed symbols: uncorrected EPES IMFPs. Open symbols: Werner et al. [21] corrected EPES IMFPs. Small open symbols: Chen [20] corrected IMFPs. Solid line: Tanuma et al. [10]. Dotted line: Gries [12]. Dashed line: Werner et al. [17,18].
typically 0.7 and 0.8 nm [9], the Fe 2p spectra recorded at every layer should consist of contribution from metallic Fe and Fe oxide. The final thickness of the Fe2.2O3 oxide layer estimated by electron microprobe analysis (EPMA) was 6.3 nm [9]. No contribution from metallic Fe substrate was detected in Fe 2p photoelectron spectrum recorded at Fe2.2O3 layer.
The results of quantification of thin iron oxide layers indicate their stoichiometry ranging from Fe1.8O3 to Fe2.5O3, with the average value of stoichiometry Fe2.07O3 (Table 1). Then, for every thickness of iron oxide layer the stoichiometry approaches the stoichiometry for thick iron layer. The results of non-linear fitting of Fe 2p photoelectron lines from metallic Fe and thick Fe2.2O3 oxide
Table 2 Comparison of deviations between the EPES determined IMFPs and the reference IMFPs ˚) Sample IMFPs RMS (A R (%)
Reference IMFPs
Fe Fe
Uncorrected Uncorrected
0.74 0.75
6.37 8.59
SEP (Chen) [20] SEP (Werner) [21]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.09 1.06 1.00
9.32 9.20 9.77
Tanuma et al. [10]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.16 1.14 1.06
10.13 9.97 10.12
Gries [12]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.28 1.33 1.89
14.73 15.21 20.26
Werner et al. [17,18]
Fe2.2O3
Uncorrected
0.20 3.06 3.06
1.77 26.49 26.90
SEP (Werner) [21] Cu SEP (Werner) [21] Cu SEP (Chen) [20]
Fe2.2O3
Uncorrected SEP (Werner) Cu SEP (Werner) Cu SEP (Chen)
1.73 1.54 1.45 1.47
16.26 14.24 17.46 18.28
Gries [12]
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layers to the Fe 2p photoelectron lines recorded at thin iron oxides indicate increasing content of iron oxide from 44 to 91 at.%, with the content of metallic Fe decreasing from 56 to 9 at.% (Table 1). These quantitative results remain in a reasonable agreement for the spectra proceeded without background subtraction and after linear background subtraction (Table 1). The dependence of IMFPs in metallic Fe and Fe2.2O3 layer on energy shown in Fig. 3(a) and (b), respectively, indicates good accuracy of measured and Tanuma et al. [10], predictive formula G1 of Gries [12] IMFPs and Werner et al. [17,18] IMFPs. The statistical error, including background subtraction, has been evaluated previously as not exceeding 2% [33,34], whereas the statistical error in Monte Carlo procedure approaches 1% [14]. Therefore, the source of systematic error is related with the model of a solid, accuracy of electron elastic scattering cross-sections and accuracy of input parameters into the Monte Carlo algorithm, i.e. density and geometry of analysis. The EPES measured IMFPs corrected for surface excitation using Chen [20] and Werner et al. [21] approach do not differ substantially from the uncorrected values (Fig. 3, Table 2). These corrections result in percentage deviation between 6.37% and 8.59% for Chen [20] and Werner et al. [21] approach, respectively (Table 2). Percentage deviations between EPES measured and Tanuma et al. [10] IMFPs in metallic Fe range from 9.20% to 9.77%, whereas between the measured and Gries [12] IMFPs—from 9.97% to 10.13% (Table 2). For thick Fe2.2O3 oxide layer the difference between EPES measured and G1 of Gries [12] IMFPs is larger (Table 2). Surface excitation corrections made for Fe2.2O3 iron oxide using Werner et al. [21] approach result in decreasing the IMFPs with respect to these values by the G1 of Gries [12] predictive formula (Fig. 3(b)). Deviations between the EPES IMFPs in metallic iron due to surface excitation corrections (6.37%) exceed these deviations due to corrections for Fe2.2O3 oxide layer (1.77%) (Table 2). Surface excitation corrections in Fe2.2O3 iron oxide change deviations between the EPES IMFPs in Fe2.2O3 and the G1 of Gries [12] IMFPs in the range 14.24–16.26% (Table 2). Due to similarity of material parameters for Fe and Fe oxide resulting from the Werner et al. [21] approach, the percentage deviation between the uncorrected and
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corrected IMFPs is rather low, i.e. 1.77% (Table 2). Application of SEP corrections only to the Cu standard results in percentage deviations between the uncorrected IMFPs for Fe2.2O3 from 26.49%, for Werner et al. [21] approach, to 26.9%, for Chen [20] approach (Table 2). These deviations decrease when introducing SEP corrections for Fe2.2O3. Deviation between the SEP corrected IMFPs by Werner et al. [21] in Fe2.2O3 and Cu, received using the EPES method and IMFPs by predictive formula G1 of Gries [12] decrease to 14.24%. Deviation between the EPES IMFPs in Fe2.2O3, when applying uncorrected IMFPs for iron oxide and corrected IMFPs for Cu standard using Werner et al. approach [21] and IMFPs by Gries [12], decreases to 17.46%. From this comparison we can conclude that SEP corrections for iron oxides cannot be neglected. Results in the present work indicate possibility of quantitative characterization of complex thin layer on substrate systems. The non-linear fitting method was found to be a reasonable tool. The IMFPs evaluated using the EPES method remain in agreement with the values from Tanuma et al. [10] and predictive formula G1 of Gries [12]. Surface excitation corrections resulting from Chen [20] and Werner et al. [21] approach do not affect the measured EPES IMFPs in Fe and Fe2.2O3, profoundly. This results from similar values of surface excitation parameters for Fe, Fe2.2O3 and Cu [21]. Better accuracy for IMFPs in Fe2.2O3 iron oxide may be obtained accounting for accuracy of surface density value which may be different from the bulk density value.
Acknowledgements The authors (JZ and PJ) acknowledge the support of the project GACR 202/02/237 and Institutional Research Plan no. Avøz 10100521. The author (BL) acknowledges the support of the project Surphare G5MA-CT-2002-04034-WP6.
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Studies of iron and iron oxide layers by electron spectroscopes B. Lesiak a,*, A. Jablonski a, J. Zemek b, P. Jiricek b, M. Cˇernˇansky´ b a
b
Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 16253 Prague 6, Czech Republic Received 20 August 2004; accepted 24 December 2004 Available online 16 March 2005
Abstract Thin iron oxide layers prepared ‘‘in situ’’ in the ultra high vacuum on polycrystalline iron substrate were investigated by electron spectroscopy methods—X-ray photoelectron spectroscopy (XPS) and elastic peak electron spectroscopy (EPES), using spectrometer ADES-400. The texture and the average grain size of the iron substrate foil have been examined by glancing angle X-ray diffraction (XRD). Qualitative and quantitative estimation of investigated oxide layers was made using (i) the relative sensitivity factor XPS method, (ii) comparison of binding energy shifts of Fe 2p photoelectron line and (iii) non-linear fitting procedure of Fe 2p photoelectron lines. Both, sputter-clean polycrystalline iron substrate and finally grown Fe2.2O3 layer, were investigated by the EPES method to measure the electron transport parameters used for quantitative electron spectroscopy, such as the electron inelastic mean free path (IMFP) values. The IMFPs were measured in the electron kinetic energy range 200–1000 eV with the Cu standard. The surface excitation parameters using Chen and Werner et al. approaches were evaluated and applied for correcting these IMFPs. The discrepancies between the evaluated parameters obtained using the above quantitative and qualitative approaches for characterising the iron oxide layers were discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Iron; Iron oxide layers; Fe2.2O3; Fe2O3; Layer thickness; X-ray photoelectron spectroscopy (XPS); Elastic peak electron spectroscopy (EPES); Cu standard; Inelastic mean free path (IMFP); Surface excitation
1. Introduction Metal-oxides constitute an important class of materials that are involved in environmental science, electrochemistry, biology, chemical sensors, magnetism, heterogeneous catalysis, where metal-oxides are * Corresponding author. Tel.: +48 22 343 3432; fax: +48 226325276. E-mail address: [email protected] (B. Lesiak).
catalysts used for synthesis of many organic compounds via selective oxidation, dehydrogenation, isomerization and other chemical processes [1–8]. The basic understanding of the relationship between the catalytic function of oxide materials, their crystallographic structure and chemical composition in the nearest surface region is necessary for developing the catalysts. The main goal consists in distinguishing qualitatively and quantitatively the two possible oxidation states Fe2+ and Fe3+. The oxide layers
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.055
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investigated in the present work has been recently studied using the XPS qualitative and quantitative analysis for determining the chemical oxidation state, Tougaard QUASES and Werner et al. SESSA approaches for investigating the oxides layer thickness and depth distribution function measurements [9]. For the purpose of quantitative electron spectroscopy, like XPS, Auger electron spectroscopy (AES), reflection energy electron loss spectroscopy (REELS), the basic electron parameter such as the electron inelastic mean free path (IMFP) is required. Values of the IMFPs for Fe, derived from the transmission optical data and a theoretical model, valid for the bulk of the investigated material, can be evaluated from Tanuma et al. optical data [10] and TPP-2M [11] predictive formula, which is based on the fits of Tanuma et al. IMFPs [10]. With the use of TPP-2M [11] formula the IMFPs for any complex inorganic and organic compound can be evaluated. The G1 of Gries [12] predictive formulae based on calculated IMFPs of Tanuma et al. [10] and on atomistic model can be also used for evaluating the IMFPs in elemental solids and complex samples. Another theoretical model applied to optical data developed by Kwei et al. [13] allows evaluating the IMFPs for Fe. Experimental determination of the IMFPs valid for the surface, differing from those for the bulk due to inelastic scattering and the relaxation processes, as well as density, can be determined using the electron elastic peak spectroscopy (EPES) described in details elsewhere [14]. Several authors evaluated the IMFPs for Fe using the EPES method based on the measurements of electron backscattering probability [15–19], however these values have not been evaluated for Fe oxides. Lesiak et al. [15] evaluated the IMFPs in Fe from the experimental intensities and Monte Carlo model accounting for electron multiple elastic scattering. Werner et al. [17,18] IMFPs in Fe and other 23 elemental solids were derived from the experimental intensities on the basis of a simple physical model of elastic backscattering that accounts for bulk elastic and inelastic scattering in the electron energy range 50–3400 eV. For energies above 200 eV the IMFPs were well described by Bethe equation for inelastic scattering, as found earlier by analysis of optical scattering data with the aid of linear theory. The inelastic scattering of electrons was also considered when evaluating the IMFPs in Fe by Fujita et al. [19].
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In the present work, the iron foil and the Fe oxide layers of various thicknesses are characterised using the XRD, XPS and EPES methods. The XPS is applied for estimating the composition of the investigated surfaces. The photoelectron Fe 2p lines allow analysing qualitatively Fe oxidation state from the binding energy shifts. Determination quantitatively and qualitatively of metallic and oxidised Fe in oxide layers of various thickness proceeds using the nonlinear fitting procedure of the Fe 2p line from metallic and finally deposited thick Fe2.2O3 iron oxide [9]. Besides, the IMFPs in Fe and Fe2.2O3 iron oxide, in the electron kinetic energy range 200–1000 eV are determined using the Cu standard by the EPES method [14]. The measured EPES IMFPs in Fe and iron oxide are corrected for surface excitation using Chen [20] and Werner et al. [21] corrections, and compared to Tanuma et al. [10] and G1 of Gries predictive formula IMFPs [12].
2. Elastic peak electron spectroscopy (EPES) method The EPES method [14] combines the experimental and theoretical procedures to evaluate the IMFPs in the surface region of solids by comparing the measured and calculated electron elastic backscattering probability. The theoretical model of elastic backscattering is typically implemented by a Monte Carlo (MC) scheme. We assume that the solid has a uniform composition and atomic density within the surface region submitted to analysis. The electron trajectory can be considered as a ‘‘random walk’’ of an electron in the volume of the solid, where the electron direction is changed after each elastic collision. Both, largeangle and small-angle scattering events are taken into account. The direction of an electron leaving the solid is calculated after considering all collisions, described by the polar and azimuthal angles, along the trajectory in the solid. The distribution of polar scattering angles after a single scattering event is described by the elastic scattering cross-section. The azimuthal scattering angles are assumed to follow the uniform distribution in the range from 08 to 1808. The linear step lengths between elastic electron collisions follow the exponential distribution. The electron trajectory is followed from its point of entry into the solid until it
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emerges, or until its length becomes so large that the probability of passing it without energy loss is negligibly small. The MC algorithm is considered to be the most accurate. However, due to its slow convergence to obtain a reasonable accuracy for a particular experiment, a considerable computational effort is necessary. Usually, the electron trajectories to be generated should be 107. Selection of electron elastic scattering crosssections as a function of electron energies is important [22]. Version 3.0 [23] contains the differential elastic scattering cross-sections derived from the wave function obtained from the Dirac–Hartree–Fock potential. In the present calculations the differential elastic scattering cross-sections from database NIST 3.0 were used [24]. The MC simulations require the following input parameters: the electron kinetic energy, the primary electron incidence angle, the backscattered electron emission angle and the solid angle of the analyser. For each electron energy, the number of electron elastic backscattering intensities at the investigated sample is simulated assuming the set of input values of IMFPs ˚ , called a calibration curve. ranging from 5 to 300 A Experiment provides the ratios of backscattered intensities. Then, the ratio of electron elastic backscattering intensities at the sample and the standard is simulated. For standard, the recommended IMFPs is applied [14]. Comparison of simulated calibration curves to the measured electron backscattering probabilities (area under the elastic peak) or their ratios with respect to the selected standard, allows estimating the required IMFP for investigated samples.
3. Experimental 3.1. Sample preparation The iron foil 25 mm 25 mm, 0.001 mm thickness of purity >99.85% (Goodfellow, UK) was used. Repeated sputter cleaning and annealing of foil surface was made ‘‘in situ’’ by Ar+ beam (5 keV, 1 105 A/cm2, 608 with respect to the surface normal) at sample temperature from room temperature to 350 8C. Iron oxide films of nanometric thickness were grown ‘‘in situ’’ at successive molecular oxygen exposition at pressure from 1 104 to 1 102 Pa,
Table 1 Comparison of quantitative XPS results for FeOx oxide layers of various thicknesses: (a) stoichiometry obtained from quantification of O 1s and Fe 2p XPS line areas; (b) thickness obtained from O 1s and Fe 2p peak areas using QUASES and SESSA software [9]; (c) metallic iron and iron oxide contribution obtained from non-linear fitting procedure applied to the XPS lineshape of Fe 2p transition FeOx spectrum (a) Stoichiometry
(1) (2) (3) (4) (5) (6) (7) (8)
Fe2.1O3 Fe2.5O3 Fe2.1O3 Fe2.0O3 Fe2.0O3 Fe2.2O3 Fe2.0O3 Fe1.8O3
(b) Overlayer thickness [9]
(c) Results of non-linear fitting (contribution)
QUASES
No back ground
Linear background
Fe
Fe2.2O3
Fe
Fe2.2O3
56 47 36 34 29 31 24 9
44 53 64 66 71 69 76 91
56 49 39 35 34 32 24 13
44 51 61 65 66 68 76 87
0.19 0.34 1.09 1.22 1.49 1.56 1.67 1.71
SESSA
0.15 0.26 0.68 0.85 1.27 1.41 1.55 1.75
at a sample temperature from room temperature to 240 8C, and at different times of exposition. The oxide layers thickness evaluated by XPS, Tougaard—QUASES [25], Werner et al.—SESSA [26] packages (summarized in Table 1) and ‘‘ex situ’’ by electron probe microanalysis (JOEL JXA-733) have been evaluated and described in details elsewhere [9]. 3.2. X-ray diffraction (XRD) texture and grain size evaluation The iron polycrystalline foil annealed up to 350 8C and submitted to Ar+ ion surface cleaning was investigated by recording the diffraction patterns using Cu Ka radiation with the flat-cassette camera in the back- and forward-reflection arrangement. The grain size has been evaluated using the glancing angle XRD method within the information depth of 10 mm [27], i.e. within the whole Fe foil. 3.3. X-ray photoelectron spectroscopy (XPS) Experiments were carried out using ADES-400 photoelectron spectrometer (V.G. Scientific, UK). The photoelectrons were excited using Al Ka radiation (hn = 1486.6 eV). The half-cone acceptance angle of
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the hemispherical analyser was 4.18. Spectra were recorded in constant energy mode at pass energy 100 or 20 eV. The XPS measurements were carried out at the X-ray incidence angle 708 and photoelectron emission angle of 08 with respect to the surface normal. For quantitative and qualitative analysis the set of spectra in the energy region Fe 2p, O 1s, C 1s, Ar 2p were recorded for a sputter cleaned Fe surface. After each oxidation step, similar spectra were recorded to estimate quantitatively and qualitatively the surface iron oxides. 3.4. Elastic peak electron spectroscopy (EPES) Experiments were carried in ADES-400 spectrometer (V.G. Scientific, UK) at the surface of sputtercleaned Fe and oxidised Fe samples and sputter-cleaned Cu standard. The electron source beam current was 0.1–1.0 mA, and beam spot diameter 4 mm. The electron backscattering intensities were recorded at the primary electron kinetic energies 200, 250, 300, 350, 400, 500, 750 and 1000 eV with the electron source impact normal to the sample surface, the electron emission angle 508 with respect to the surface normal and the half-cone angle 4.18. The typical full width at half-maximum (FWHM) of elastic peak in the whole energy range was 0.5 eV. The area under the elastic peak was calculated after subtracting the Shirley background.
4. Results 4.1. XRD and XPS analyses The value of average grain size in iron foil annealed up to 350 8C estimated by XRD was 100 nm, whereas the exhibited texture of the iron foil was found very weak. Atomic concentrations of iron and/or oxygen in sputter cleaned iron and the iron oxide layer were determined from Fe 2p and O 1s peak areas after inelastic background subtraction, assuming a simple model of a semi-infinite solid of homogeneous composition, using the relative sensitivity factor approach [28], the peak areas corrections for photoelectric cross-sections and asymmetry [29], the total mean free paths of electrons in the transport approximation, and the measured transmission function of the spectrometer [30]. The resulting
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atomic composition for sputter clean Fe was: Fe— 97 at.%, C—2 at.%, Ar—1 at.%, and for the final Fe oxide: Fe—42.9 at.%, O—57.1 at.%, indicating average stoichiometry Fe2.2O3. In addition, for thinner oxide films, the finite film thickness was accounted for in quantitative analysis (see Table 1). The qualitative analysis on the basis of binding energies for Fe 2p3/2 electrons indicated metallic Fe8 state (binding energy 706.9 eV) and Fe3+ oxide state (binding energy 710.9 eV) [9,28]. 4.2. Separation of oxide and substrate contribution from the XPS Fe 2p lines Exemplary Fe 2p XPS spectra representing two chemical Fe states—metallic Fe8 and finally deposited thick Fe oxide layer of stoichiometry Fe2.2O3, with thickness exceeding the Fe 2p photoelectron information depth (ID), are shown in Fig. 1(a). The respective spectra representing thin Fe oxide layers deposited on Fe foil, having thickness smaller than Fe 2p photoelectron ID, are shown in Fig. 1(b). The non-linear fitting procedure was applied to fit the Fe 2p photoelectron spectra from thin FeOx oxide layers, to estimate quantitatively the contribution from metallic iron and iron oxide. In the above fitting procedure the Fe 2p photoelectron spectra measured at sputter cleaned metallic Fe8 foil and finally deposited thick Fe oxide layer of stoichiometry Fe2.2O3 without and after linear background subtraction were applied. The results of fitting procedure representing the percentage contribution from measured spectra, are compared in Table 1. Results of analysis for evaluating the stoichiometry of FeOx iron oxide layers of various thickness (Fig. 1(b)) were based on quantification of O 1s and Fe 2p photoelectron lines peak areas recorded at thin iron oxide layers, accounting for percentage contribution of Fe 2p signal calculated from the non-linear fitting procedure described above. These values of estimated stoichiometry applying the procedure described above for every thin iron oxide layer are indicated in Table 1. 4.3. The IMFPs using the EPES method The model of MC simulating the electron transport in a solid, does not account for electron inelastic
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Fig. 1. Photoelectron Fe 2p spectra. (a) Sputter-cleaned Fe substrate and thick Fe2.2O3 oxide layer. (b) Iron oxide FeOx layers of various thicknesses.
surface excitation experienced by an electron with energy E incoming and leaving the surface at an incidence angle, uin, and emission angle, uout, measured with respect to the surface normal. The influence of the surface excitations on the calculated electron elastically backscattered intensity can be corrected by the surface excitation parameter. The surface excitation parameter, SEP, denoted by PS(E), represents the average number of surface excitations of electron in a single surface crossing. Since in EPES analysis the electron crosses the surface twice, total surface excitation parameter for incoming, PS(E, uin), and outcoming, PS(E, uout), electrons defined as surface excitation parameter, total SEP or PS(E), is the following [31]: PS ðEÞ ¼ PS ðE; uin Þ þ PS ðE; uout Þ The SEPs can be evaluated using the model proposed by Chen [20] valid for rectilinear surface crossing or a semi-empirical approach of Werner et al. [21] accounting for deflections during surface crossing.
Using a model dielectric function, the SEP have been calculated for several elements and semiconductors by Chen [20] according to equation: PS ðE; uÞ ¼ ach E1=2 cos1 u where ach is the material dependent fitting parameter. Using extensive measurements of reflection energy loss spectra (REELS) for selected elements and semiconductors, Werner et al. [21] extracted the SEP values from the ratio of the number of electrons that induced a surface excitation to those elastically reflected. The SEPs can be evaluated from the following: PS ðE; uÞ ¼ 1=ð0:173aH E1=2 cos u þ 1Þ where aH is the material dependent fitting parameter. Corrections for surface excitation should be applied to the measured or MC calculated intensity ratios. The probability that an electron incoming and leaving the solid does not produce any surface excitation in a backscattering experiment is expressed by exp[PS(E)].
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Fig. 2. Ratios of electron elastic backscattering intensities measured at investigated samples with respect to Cu standard (uncorrected, Chen [20] and Werner et al. [21] corrected): (a) metallic iron; (b) thick Fe2.2O3 oxide layer.
Then, the MC simulated electron elastic backscattering probabilities should be multiplied by exp[PS(E)] to be compared to the measured elastic peak areas, or the measured peak areas should be divided by exp[PS(E)]. The measured ratios of electron backscattered intensity from Fe versus the Cu standard and Fe2.2O3 versus the Cu standard are compared with those corrected using Chen [20] and Werner et al. [21] approaches in Fig. 2(a) and (b), respectively. The material parameters for Fe and Cu are taken from Refs. [20,21]. For oxides, however, they are not available in the literature. These parameters are expected to be smaller for oxides in comparison to elemental solids [32]. To receive the material parameter for iron oxide we used a semi-empirical formula derived by Werner et al. [21] providing a rough estimate of the parameter for any material. Calculation was carried out for Fe2O3 with a mean number of valence electrons 6.8 per atom and atomic density of 9.862 1022 cm3. The atomic density was calculated from the volume density of Fe2O3 using the SESSA software [26]. The value of material parameter for Fe2O3 was aH/aNFE = 1.5851. The energy dependence of the IMFPs, uncorrected and corrected using Chen [20] and Werner et al. [21] SEPs in Fe is shown in Fig. 3(a), whereas the energy dependence of the IMFPs in Fe oxide is shown in Fig. 3(b). The measured EPES IMFPs in Fe in the electron energy range 200–1000 eV are compared to Tanuma et al. [10], G1 of Gries [12] predictive
formulae IMFPs and Werner et al. [17,18] IMFPs (Fig. 3(a)). The measured EPES IMFPs in iron oxide (uncorrected, partially corrected for the Cu SEP, and fully corrected for the Cu and Fe2O3 SEPs) are compared to the G1 of Gries [12] predictive formula IMFPs (Fig. 3(b)). The scatter between the EPES measured and calculated IMFPs is evaluated from estimating the root-mean-square deviation (RMS) and percentage deviation (R) using equations listed below [14]: " #1=2 r 1 X 2 RMS ¼ ðlmeas lcalc Þ r j¼1 X 1 r lmeas lcalc R ¼ 100 r j¼1 lmeas where lmeas and lcalc are the EPES measured and calculated IMFPs, and r the number of the IMFPs accounted for in the procedure. This scatter is listed in Table 2.
5. Discussion and conclusions The thickness of thin Fe oxide layers estimated previously using QUASES and SESSA software were between 0.19 and 1.71 nm and 0.15 and 1.75 nm, respectively [9]. Since the mean escape depth (MED) of the O 1s photoelectrons travelling in iron oxide is
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Fig. 3. Comparison of IMFPs measured using Cu standard: (a) Fe; (b) thick Fe2.2O3 oxide layer. Closed symbols: uncorrected EPES IMFPs. Open symbols: Werner et al. [21] corrected EPES IMFPs. Small open symbols: Chen [20] corrected IMFPs. Solid line: Tanuma et al. [10]. Dotted line: Gries [12]. Dashed line: Werner et al. [17,18].
typically 0.7 and 0.8 nm [9], the Fe 2p spectra recorded at every layer should consist of contribution from metallic Fe and Fe oxide. The final thickness of the Fe2.2O3 oxide layer estimated by electron microprobe analysis (EPMA) was 6.3 nm [9]. No contribution from metallic Fe substrate was detected in Fe 2p photoelectron spectrum recorded at Fe2.2O3 layer.
The results of quantification of thin iron oxide layers indicate their stoichiometry ranging from Fe1.8O3 to Fe2.5O3, with the average value of stoichiometry Fe2.07O3 (Table 1). Then, for every thickness of iron oxide layer the stoichiometry approaches the stoichiometry for thick iron layer. The results of non-linear fitting of Fe 2p photoelectron lines from metallic Fe and thick Fe2.2O3 oxide
Table 2 Comparison of deviations between the EPES determined IMFPs and the reference IMFPs ˚) Sample IMFPs RMS (A R (%)
Reference IMFPs
Fe Fe
Uncorrected Uncorrected
0.74 0.75
6.37 8.59
SEP (Chen) [20] SEP (Werner) [21]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.09 1.06 1.00
9.32 9.20 9.77
Tanuma et al. [10]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.16 1.14 1.06
10.13 9.97 10.12
Gries [12]
Fe
Uncorrected SEP (Chen) SEP (Werner)
1.28 1.33 1.89
14.73 15.21 20.26
Werner et al. [17,18]
Fe2.2O3
Uncorrected
0.20 3.06 3.06
1.77 26.49 26.90
SEP (Werner) [21] Cu SEP (Werner) [21] Cu SEP (Chen) [20]
Fe2.2O3
Uncorrected SEP (Werner) Cu SEP (Werner) Cu SEP (Chen)
1.73 1.54 1.45 1.47
16.26 14.24 17.46 18.28
Gries [12]
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layers to the Fe 2p photoelectron lines recorded at thin iron oxides indicate increasing content of iron oxide from 44 to 91 at.%, with the content of metallic Fe decreasing from 56 to 9 at.% (Table 1). These quantitative results remain in a reasonable agreement for the spectra proceeded without background subtraction and after linear background subtraction (Table 1). The dependence of IMFPs in metallic Fe and Fe2.2O3 layer on energy shown in Fig. 3(a) and (b), respectively, indicates good accuracy of measured and Tanuma et al. [10], predictive formula G1 of Gries [12] IMFPs and Werner et al. [17,18] IMFPs. The statistical error, including background subtraction, has been evaluated previously as not exceeding 2% [33,34], whereas the statistical error in Monte Carlo procedure approaches 1% [14]. Therefore, the source of systematic error is related with the model of a solid, accuracy of electron elastic scattering cross-sections and accuracy of input parameters into the Monte Carlo algorithm, i.e. density and geometry of analysis. The EPES measured IMFPs corrected for surface excitation using Chen [20] and Werner et al. [21] approach do not differ substantially from the uncorrected values (Fig. 3, Table 2). These corrections result in percentage deviation between 6.37% and 8.59% for Chen [20] and Werner et al. [21] approach, respectively (Table 2). Percentage deviations between EPES measured and Tanuma et al. [10] IMFPs in metallic Fe range from 9.20% to 9.77%, whereas between the measured and Gries [12] IMFPs—from 9.97% to 10.13% (Table 2). For thick Fe2.2O3 oxide layer the difference between EPES measured and G1 of Gries [12] IMFPs is larger (Table 2). Surface excitation corrections made for Fe2.2O3 iron oxide using Werner et al. [21] approach result in decreasing the IMFPs with respect to these values by the G1 of Gries [12] predictive formula (Fig. 3(b)). Deviations between the EPES IMFPs in metallic iron due to surface excitation corrections (6.37%) exceed these deviations due to corrections for Fe2.2O3 oxide layer (1.77%) (Table 2). Surface excitation corrections in Fe2.2O3 iron oxide change deviations between the EPES IMFPs in Fe2.2O3 and the G1 of Gries [12] IMFPs in the range 14.24–16.26% (Table 2). Due to similarity of material parameters for Fe and Fe oxide resulting from the Werner et al. [21] approach, the percentage deviation between the uncorrected and
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corrected IMFPs is rather low, i.e. 1.77% (Table 2). Application of SEP corrections only to the Cu standard results in percentage deviations between the uncorrected IMFPs for Fe2.2O3 from 26.49%, for Werner et al. [21] approach, to 26.9%, for Chen [20] approach (Table 2). These deviations decrease when introducing SEP corrections for Fe2.2O3. Deviation between the SEP corrected IMFPs by Werner et al. [21] in Fe2.2O3 and Cu, received using the EPES method and IMFPs by predictive formula G1 of Gries [12] decrease to 14.24%. Deviation between the EPES IMFPs in Fe2.2O3, when applying uncorrected IMFPs for iron oxide and corrected IMFPs for Cu standard using Werner et al. approach [21] and IMFPs by Gries [12], decreases to 17.46%. From this comparison we can conclude that SEP corrections for iron oxides cannot be neglected. Results in the present work indicate possibility of quantitative characterization of complex thin layer on substrate systems. The non-linear fitting method was found to be a reasonable tool. The IMFPs evaluated using the EPES method remain in agreement with the values from Tanuma et al. [10] and predictive formula G1 of Gries [12]. Surface excitation corrections resulting from Chen [20] and Werner et al. [21] approach do not affect the measured EPES IMFPs in Fe and Fe2.2O3, profoundly. This results from similar values of surface excitation parameters for Fe, Fe2.2O3 and Cu [21]. Better accuracy for IMFPs in Fe2.2O3 iron oxide may be obtained accounting for accuracy of surface density value which may be different from the bulk density value.
Acknowledgements The authors (JZ and PJ) acknowledge the support of the project GACR 202/02/237 and Institutional Research Plan no. Avøz 10100521. The author (BL) acknowledges the support of the project Surphare G5MA-CT-2002-04034-WP6.
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