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Atomic geometry determination of FeO(0 0 1) grown on Ag(0 0 1) by low energy electron diffraction
Surface Science 601 (2007) 1239–1245 www.elsevier.com/locate/susc
Atomic geometry determination of FeO(0 0 1) grown on Ag(0 0 1) by low energy electron diffraction E.L. Lopes, G.J.P. Abreu, R. Paniago, E.A. Soares, V.E. de Carvalho *, H.-D. Pfannes Departamento de Fı´sica, ICEx, UFMG, CP702, Belo Horizonte, Minas Gerais, Brazil Received 12 September 2006; accepted for publication 14 December 2006 Available online 19 December 2006
Abstract A low energy electron diffraction (LEED) investigation of the structure of the surface of an FeO(0 0 1) thin film grown on Ag(0 0 1) is presented. The results show that this surface has an almost bulk termination structure with a very small rumple on the first layer, which agrees with the structure found in other studies carried out on the (0 0 1) surface of oxides that have rock-salt structure. Evidences that may support a linear behaviour of the topmost layer rumple with the oxide lattice constant are also discussed. 2006 Elsevier B.V. All rights reserved. Keywords: Surfaces; Structural determination; LEED; Metal oxides
1. Introduction In the last years, there has been an increasing interest in the study of metal oxides surfaces due to their technological applications [1,2]. Oxides are important in catalytic reactions where they play the role of inert support for catalytic metal clusters or even as catalytic material [3,4]. Also, oxides are the most important material to construct gas sensor devices [5]. It is believed that in all these applications the main processes are governed by the surface properties. However, in contrast to the extensive research done on the properties of metal and semiconductor surfaces, investigations of oxide surfaces are lacking. Under the several reasons for this lack are the difficulties of preparing high quality surfaces and, as the majority of the techniques in surface analysis uses electrons or ions as a probe, the insulating nature of many oxide materials. The development of the technique of growing crystalline films on metallic substrates under ultra-high-vacuum conditions has opened new possibilities for studying oxide surfaces. Oxide films grown on metallic substrates and *
Corresponding author. E-mail address: vagner@fisica.ufmg.br (V.E. de Carvalho).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.031
showing good surface quality have been produced at several thicknesses [6]. So far, metal surfaces that are relatively inert with respect to oxidation like Pt(1 1 1), Ag(1 1 1), Ag(0 0 1) and Au(1 1 1) have been the best choices for the substrate. Iron oxides are important in many technological applications and processes, e.g., they are useful as catalysts in a number of industrial processes and widely used in magnetic storage industry and represent most of the corrosion products of iron and steel. The rock salt phase (wu¨stite) FeO, an anti-ferromagnetic insulator, which is not stable in the bulk at temperatures below 580 C is of special interest. Several studies on FeO films have been reported, however, the majority is dedicated to the investigation of the magnetic properties and very few works treat the problem of the surface structure of the films. As far as we know, only the FeO(1 1 1) surface has been object of structural investigation. The production of ordered iron oxide grown on a substrate other than Fe bulk crystals, may allow us to study the influence of the substrate symmetry on the growth orientation of the iron oxide, preferential nucleation site, growth morphology and adhesion properties between oxide and substrate. Heteroepitaxial FeO films were first produced by Vurens et al. [7]. They observed that
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FeO(1 1 1) films grew layer-by-layer on Pt(1 1 1) and Pt(1 0 0). After that several studies have been performed where the influence of the thickness of the film on the resulting phase has been investigated. Weiss and Somorjai [8] reported a qualitative LEED study on FeO(1 1 1) film grown on Pt(1 1 1) by first depositing iron and oxidizing it afterward. At 1 monolayer (ML) coverage a stable hexagonal (1 · 1) structure, with a threefold symmetry, with a unit cell 15% larger than that of Pt(1 1 1) and the presence of an (8 · 8) superstructure has been observed. At thicker films (8 ML) a (2 · 2) reconstructed surface was observed, exhibiting a sixfold symmetry indicating the existence of two domains rotated by 60 with respect to each other. In a similar work, Ritter et al. [9] used LEED and STM techniques to study iron oxide films on Pt(1 1 1). They found that up to 2 ML the oxide grows as a monolayer of FeO(1 1 1) and at higher coverage the film changes to Fe3O4(1 1 1). The FeO(1 1 1) layers consist of hexagonal close-packed iron–oxygen bilayers that are laterally expanded with respect to the bulk FeO structure and slightly rotated against the platinum substrate and are oxygen terminated, resulting in a Moire´ pattern that is clearly seen in the STM image. A similar superstructure formation is observed for FeO(1 1 1) films grown on Ru(0 0 0 1). Up to 4 ML of FeO on Ru(0 0 0 1) a very well ordered and highly oriented (8 · 8) superstructure is observed, corresponding to the coincidence of 7 FeO units with 8 Ru atoms [10]. On Pt(1 0 0) substrate Ritter et al. [11] observed for a monolayer FeO film a superstructure c(2 · 10) with respect to the substrate (1 · 1) unit cell. With increasing film thickness (up to 8 ML) the LEED showed an hexagonal pattern typical of the Fe3O4(1 1 1) surface. They carried out a quantitative LEED analysis of the FeO(1 1 1) phase and concluded that the oxide consists of a bi-layer terminated in oxygen atoms over an iron layer but either the iron and ˚ , respecthe oxygen layers are buckled of 0.5 and 0.3 A tively. Moreover, in a recent work Waddill and Ozturk [12] reported a study of an epitaxial iron oxide film grown on Ag(1 1 1). They first grew Fe films up to 10 monolayers and then proceeded with oxidation what resulted in poorly ordered FeO(1 1 1) films. In a second method, they sequentially produced sub-monolayer Fe films followed by oxidation. By this procedure they obtained a much better ˚ thickness. Thicker crystallographic order up to about 10 A oxide films presented a coexistence of Fe3O4(1 1 1) domains. In the present work, we present the results of a low energy electron diffraction (LEED) atomic geometry investigation of the FeO(0 0 1) surface of an insulating epitaxial film grown on Ag(0 0 1) under ultra-high-vacuum conditions. The results show that this surface has a nearly bulk terminated structure with a very small rumple on the first layer, what is in agreement with the very small relaxations found in other studies carried out on oxides of the same rock-salt structure. We also present evidences that may support a linear behaviour of the topmost layer rumple with the oxide lattice constant.
2. Experimental and theoretical details The experiments were performed on a VG Escalab system with a base pressure of 1 · 1010 mbar and equipped with the standard facilities for sample preparation (cleaning, heating and deposition) and sample characterization (Auger, XPS and UPS) as well as with a computer controlled 4 grid LEED and a RHEED optics. The Ag(0 0 1) single crystal (from Monocrystal Company, with 99.995% purity, presenting a mirror like (0 0 1) surface oriented within ±0.5) was cleaned with several cycles of sputtering (Ar+ ions at 1.0 keV for 30 min) and annealing (up to 450 C for 5 min) until no trace of carbon, oxygen and sulphur contaminations could be detected by XPS and a sharp (1 · 1) LEED pattern could be observed. The FeO(0 0 1) films were grown on Ag(0 0 1) by evaporating high purity 57Fe from an OMICRON e-beam source while dosing a low pressure oxygen atmosphere. Since such procedure usually produces a mixture of FeO and Fe3O4 phases, various combinations of evaporation rate and oxygen pressure were carefully examined and as the best condition to obtain up to 90% of FeO, pO2 ¼ 1 107 mbar during 57Fe-evaporation and 6.8 · 1012 Fe – atoms/cm2 s impinging on the Ag(0 0 1) surface was determined. The film, consisting of 22 ML of FeO, was afterward annealed at 600 C during 10 min, which ensured good surface crystallinity. A complete study on the preparation of FeO and Fe3O4 films on Ag(0 0 1) will be published elsewhere [13]. The stoichiometry (Fe/O = 0.95 ± 0.05) and the presence of well resolved Fe2+ satellites of the Fe–2p1, 2p3 lines, at 730.3 eV and 716.2 eV, respectively, were checked by photoelectron spectroscopy, as shown in Fig. 1a and b. Another check of the FeO phase was done by in situ Conversion Electron Mo¨ssbauer Spectroscopy (CEMS) [14]. The CEMS spectrum shown in Fig. 1c was measured in situ with a 25 mCi – 57Co:Rh source in constant acceleration mode and accumulated during 12 h. The films were grown with 57 Fe in order to obtain a good statistics after this measuring time. The measured isomer shift (0.97 mm/s relative to a-Fe) and quadrupole splitting (0.52 mm/s) are in line with known values for Fe2+ in bulk (paramagnetic) FeO at room temperature. A spectrum taken at 100 K (not shown here) has evidenced the antiferromagnetic character of the film at low temperature (TNeel = 198 K for FeO-bulk). It is worth to point out that the photoelectron spectrum (Fig. 1b) has not shown Fe–2p1, 2p3 satellites typical of Fe3+ (at 733.3 eV and 719.2 eV). This indicates that if Fe3O4 coexists with FeO in the surface, it must be under the detectable limit of the technique, or Fe3O4 is buried under the FeO film and should not affect our LEED-analysis. In fact, the Mo¨ssbauer spectrum at room temperature has shown a small amount (approximately 15%) of Fe3O4, but Mo¨ssbauer spectroscopy is not as surface sensitive as photoelectron spectroscopy but has a probing depth of approximately 80 nm. The LEED patterns of the film were collected and recorded at room temperature and at normal incidence using
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Fig. 1. Photoelectron spectra (a and b) and Mo¨ssbauer spectra (c) of the 22 ML FeO(0 0 1) film. The thin lines correspond to a standard Mo¨ssbauer fitting. The central dublet corresponds to FeO, whereas the two sextets to Fe3O4.
an Omicron LEEDStar video system in the range of 40– 500 eV. The intensity versus energy curves (LEED-I(V) curves) were collected for 23 diffracted beams. After averaging the symmetrically equivalent beams, the data were reduced to 5 inequivalent beams covering a total energy range of 1192 eV. The curves were then normalized with respect to the current of the incident beam and smoothed using a 5-points least-squares cubic polynomial algorithm. The LEED quantitative theoretical analysis was carried out using the method of symmetrized automated tensor LEED [15] with the programs associated to calculate the scattering phase shifts using the approximation of the muffin-tin potential. A self-consistent Dirac–Fock approach was used in order to compute the atomic orbitals for each element. In addition, 10 relativistic phase shifts (lmax = 9)
were evaluated by numerical integration of the Dirac equation, using the Mattheiss’ prescription to compute the muffin-tin potential. Although FeO is an ionic compound, we used neutral phase shifts. It is well known that the structural fit does not depend too much on non-structural parameters, provided their values are reasonable, as was carefully verified by Barbieri et al. [16]. To simulate the difference in atomic radii, different muffin-tin radii for both elements were tested, with the following restriction to avoid Fe O overlap of the spheres: rO muf þ r muf ¼ d nn , where r muf is the Fe oxygen and rmuf the iron muffin-tin radius, respectively, and dnn the nearest–neighbour distance. The Debye temperature for bulk was fixed at a value of 450 K [20] and the real part of the inner potential was simultaneously optimised with the structural parameters while the complex
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part was fixed at 9.0 eV. The simulated LEED-I(V) curves were compared with the experimental data using the Pendry reliability R-Factor [17] and all the calculations were performed on an AMD Athlon (1.6 GHz) computer running Scientific Linux [18]. The Ag(0 0 1) substrate has not been considered in the calculations because the FeO(0 0 1) film used in this analysis was thick enough to completely cover the substrate. The structural parameters optimized were the vertical positions of the anion and cation of the first four layers. This allowed us to investigate the rumple and distance beC tween layers. The rumple is defined as (di ¼ DzA i Dzi ) A C where Dzi and Dzi are, respectively, the anion and the cation vertical displacements from the ideal bulk positions of the ith-layer. The distance between layers is defined as Ddij = (Di Dj), where Di and Dj are the layers relaxations, C defined as Di ¼ 12 ðDzA i þ Dzi Þ. 3. Results and discussion The LEED pattern for the clean Ag(0 0 1) and for the FeO(0 0 1) film after annealing are presented in Fig. 2. As
it is shown in this figure a well defined LEED pattern was obtained for the FeO(0 0 1) indicating a good crystallinity of the film. The initial stage of the LEED analysis involved the investigation of the surface lattice constant of the film (aFeO 0 ). The value reported in the literature is approximately ˚ [12,9] for a single crystal and therefore an investiga3.04 A tion of the film lattice constant was performed, in order to see whether the substrate lattice has any influence, as observed in ultra-thin films [9,19]. For this analysis, a series of calculations were carried out assuming different values ˚ (3.3% compression) to 3.21 A ˚ (5.5% of aFeO from 2.94 A 0 ˚ expansion) in steps of 0.03 A (1%). For each value of aFeO 0 , the neutral phase shifts are calculated by adjusting the muffin-tin radius of the elements as described in Section 2. For each value of the film lattice constant a full structural optimization was performed starting form the bulk positions and assuming the Debye temperature of the surface equal to that of the bulk. In this optimization, 8 structural parameters were varied (what is equivalent to relax 4 layers). The Rp factor as a function of the lattice constant is shown in Fig. 3a. The final structures for all tested lattice
Fig. 2. LEED pattern of the clean Ag(0 0 1) (left) and FeO(0 0 1) film (right) at 127 eV.
O FeO ˚ (b). Fig. 3. Behaviour of the Rp factor as a function of the lattice constant (a), and as a function of the ratio rFe ¼ 3:09 A muf =r muf for a0
E.L. Lopes et al. / Surface Science 601 (2007) 1239–1245
constants were bulk terminated, considering the error bars of the parameters calculated using the variance of the Rp factor. As can be seen in Fig. 3a, a well defined minimum ˚ (1.6% expanwas obtained for aFeO ¼ ð3:09 0:09Þ A 0 sion), which agrees with the values found in the literature. We believe that at this thickness the film is completely relaxed but as the LEED technique probes only the first layers of the sample we do not have information about the behaviour of the lattice parameter in deeper layers. Therefore, based on these results nothing can be said about the presence or not of residual strain in the film. This information could be obtained by performing a RHEED or X-ray diffraction experiments at different stages of the film growth. Using the lattice constant obtained in this first step, a O grid search was performed over the ratio rFe muf =r muf from Fe 1/3 (which is equivalent to rmuf ¼ 0:25d nn and rO muf ¼ 0:75d nn ) up to 3 (which is equivalent to rFe muf ¼ 0:75d nn O Fe and rO muf ¼ 0:25d nn ) in steps of 0.05dnn for r muf and rmuf . At this stage, the same structural (8 vertical positions) and non-structural (real part of inner potential) parameters fitted previously were optimized, with the Debye temperature of the surface equal to that of the bulk. The behaviour of the Rp factor is shown in Fig. 3b. As can be seen from this figure, the Rp exhibits a shallow minimum at rFe muf = rO corresponding to rO and muf ¼ 0:81, muf ¼ 0:55d nn Fe rFe muf ¼ 0:45d nn , but it is nearly constant for r muf = rO muf > 0:81. With the in-plane lattice constant and the muffin-tin radii determined, the Debye temperature of the two elements Fe of the first layer (HO 1 and H1 ) was varied independently, from 200 K up to 900 K in steps of 25 K. Again, for each O value of HFe 1 and H1 , a full optimization of the parameters was performed using the best value of the lattice constant and muffin-tin radii found previously. The results of this optimization can be seen in Fig. 4. The Rp factor shows a
Fig. 4. Rp factor as a function of the first layer Debye temperatures (HO 1 and HFe 1 ). The greyscale corresponds to changes in the Rp factor from 0.23 (black) up to 0.37 (light grey).
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very well defined minimum at HO 1 ¼ ð450 300Þ K (bulk value) and HFe ¼ ð350 300Þ K (22% lower than that of 1 the bulk). Fe Then, using these best values for HO 1 and H1 , a new optimization procedure was taken and the final structure shows a small rumple in the topmost layer (d1) of (3.9 ± 3.2)% with the oxygen ions moving towards the vacuum side, and all the other structural parameters optimized are essentially the same as the bulk values. Specifically, the change from the bulk values are: Dd12 = (2.4 ± 4.6)%, Dd23 = (3.2 ± 4.8)% and Dd34 = (0.7 ± 5.7)%, with a Rp factor equal to (0.23 ± 0.06). In Fig. 5 a side view of the surface structure of the FeO(0 0 1) film (left) and the good agreement between theoretical and experimental LEED-I(V) curves (right) are shown. As can be seen from Table 1 and considering the uncertainty of our results, the topmost layer rumple of FeO from our study is in agreement with that of most of other structural studies carried out on the (0 0 1) surface of other oxides with rock-salt structure. All studies indicate rumples on the topmost layer. The surface rumples as a function of the bulk lattice constant is shown in Fig. 6. In this plot all the results presented in Table 1 except the MEIS results for NiO [23] and MnO [26] single crystals have been used. This plot suggests that a linear dependence of the surface rumple with the bulk lattice constant may exist. A similar behaviour is observed for the (1 1 0) surface of III–V semiconductors [32]. A positive rumple – corresponding to oxygen atoms lying outwards – for all the rock-salt oxides surfaces is expected based on the model proposed by Goniakowski and Noguera [30,31]. According to this model, the rumple is the result of the dissymmetry in the anion and the cation displacements due to the fact that the second-neighbour electron hopping process can be different on the anion and the cation sublattices. Depending on the atomic orbitals symmetry in each crystalline structure, effective attractive or repulsive forces will be acting on surface anions and cations giving rise to a positive or a negative rumpling. In the case of the rock-salt oxides, the anion–anion electron delocalization processes increase the total energy and, at the surface, the bond length can expand in order to decrease the energy. On the other hand, the cation–cation contribution is negative and consequently the energy can be lowered by contraction of the bond length. Then, an expansion of the O–O bond length and a contraction of the cation–cation bond length produce a positive rumple. Also, when the oxygen atom is larger than the metal atom, the cation–cation contribution to the energy is much smaller compared to that of oxygen–oxygen processes. Therefore, the rumple strength must vary with the effective metal ion size and, as a consequence, a crystal lattice parameter dependence may be expected. So, according to this model, the FeO(0 0 1) surface should present a positive rumple with the strength defined by the iron atom radius. These predictions seem to be confirmed by considering the results presented in the Fig. 6.
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<100>
A C 1 Δ i = 2 ( Δz i + Δz i ) C A δi = Δz i – Δz i
<110>
O–2 Fe+2 δ1
Fig. 5. (Left) Side view of the FeO(0 0 1) surface structure. The oxygen and iron ions are represented by white and black spheres, respectively. (Right) LEED-I(V ) curves for the FeO(0 0 1) film on Ag(0 0 1). Thin solid lines: theoretical curves for the best model obtained. Thick solid lines: experimental curves.
Table 1 The topmost layer rumple determined for (0 0 1) surface of oxides with rock-salt structure obtained by several experimental and theoretical techniques reported on literature ˚) Oxide d1 (A Technique/method FeO CoO NiO NiO NiO MnO MnO MnO MgO MgO MgO CaO
0.09 ± 0.07 0.06 ± 0.04 0.014 0.05 ± 0.05 0.10 ± 0.01 0.01 0.10 ± 0.04 0.08 ± 0.02 0.024 0.02 ± 0.01 0.04 ± 0.04 Less than 0.04
LEED (this work) LEED [21] Hartree–Fock [22] LEED [19] MEIS [23] Ab initio [24] LEED [25] MEIS [26] Hartree–Fock [22] GIXS [27] LEED [28] LEED [29]
4. Conclusions In this work low-energy electron diffraction has been used to determine the surface structure of FeO(0 0 1) thin films grown on Ag(0 0 1). A good agreement between experiment and theory was obtained as can be seen from the obtained value of the Rp factor. The results show that this surface has a small rumple of (3.9 ± 3.2)% with the oxygen ions moving towards the vacuum and a Debye temperature of the two elements of the first layer equal to (450 ± 300) K for oxygen and (350 ± 300) K (22% lower than bulk) for iron atoms. These results are consistent with the small relaxations and rumples observed for the (0 0 1) surface of other rock-salt oxides reported in the literature, and the rumple strength seems to vary linearly with the lattice constant. Acknowledgements We thank CNPq and FAPEMIG, Brazilian research agencies, for financial support. References
Fig. 6. Rumple of different rock-salt metal oxides as a function of their lattice constant.
[1] V. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, 1994. [2] J. Wollschla¨ger, D. Erdo¨s, H. Goldbach, R. Ho¨pken, K.M. Schro¨der, Thin Solid Films 400 (2001) 1. [3] H.J. Freund, H. Kuhlenbeck, V. Staemmler, Prog. Surf. Sci. 59 (1996) 283. [4] M. Ba¨mer, H.J. Freund, Prog. Surf. Sci. 61 (1999) 127. [5] S.A. Hoover, The Nanoparticles 2002, Conference Proceedings. [6] S.A. Chambers, Surf. Sci. 39 (2000) 105. [7] G.H. Vurens, V. Maurice, M. Salmeron, G.A. Somorjai, Surf. Sci. 268 (1992) 170. [8] W. Weiss, G.A. Somorjai, J. Vac. Sci. Technol. A 11 (4) (1993) 2138. [9] M. Ritter, W. Ranke, W. Weiss, Phys. Rev. B 57 (1998) 7240.
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G. Ketteler, W. Ranke, PB 66, (2002) 033405. M. Ritter, H. Over, W. Weiss, Surf. Sci. 371 (1997) 245. G.D. Waddill, O. Ozturk, Surf. Sci. 575 (2005) 35. G.J.P.Abreu, H.-D.Pfannes, R. Paniago, to be published. J.R. Gancedo, J.Z. Da´valos, M. Gracia, J.F. Marco, Hyperfine Interact. 110 (1997) 41. Barbieri/Van Hove SATLEED package.. A. Barbieri, W. Weiss, M.A. Van Hove, G. Somorjai, Surf. Sci. 302 (1994) 259. J.B. Pendry, J. Phys. C 13 (1980) 937. . M. Caffio, B. Cortigiani, G. Rovida, A. Atrei, C. Giovanardi, A. di Bona, S. Valeri, Surf. Sci. 531 (2003) 368. S. Stolen, R. Glo¨ckner, F. Gronvold, T. Atake, S. Izumizawa, Am. Mineral. 81 (1996) 973. J. Wang, K.M. Schindler, W. Widdra, H. Neddermeyer, ICSOS8 Abstract Book 1 (2005) 141.
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[22] M.D. Towler, N.M. Harrison, McCarthy, Phys. Rev. B 52 (1995) 5375. [23] T. Okazawa, Y. Yagi, Y. Kido, Phys. Rev. B 67 (2003) 195406. [24] H. Momida, T. Oguchi, J. Phys. Soc. Jpn. 72 (2003) 588. [25] E.A. Soares, R. Paniago, V.E. de Carvalho, E.L. Lopes, G.J.P. Abreu, H.-D. Pfannes, Phys. Rev. B 73 (2006) 35419. [26] T. Okazawa, Y. Kido, Surf. Sci. 556 (2004) 101. [27] O. Robach, G. Renaud, A. Barbier, Surf. Sci. 401 (1998) 227. [28] M.R. Welton-Cook, W. Berndt, J. Phys. C: Solid State Phys. 15 (1982) 5691. [29] M. Prutton, J.A. Ramsey, J.A. Walker, M.R. Welton-Cook, J. Phys. C: Solid State Phys. 12 (1979) 5271. [30] C. Noguera, Physics and Chemistry at Oxide Surfaces, Cambridge University Press, 1995. [31] J. Goniakowski, C. Noguera, Surf. Sci. 323 (1995) 129. [32] W. Mo¨nch, Semiconductor Surfaces and Interfaces, Springer, 1995.
Atomic geometry determination of FeO(0 0 1) grown on Ag(0 0 1) by low energy electron diffraction E.L. Lopes, G.J.P. Abreu, R. Paniago, E.A. Soares, V.E. de Carvalho *, H.-D. Pfannes Departamento de Fı´sica, ICEx, UFMG, CP702, Belo Horizonte, Minas Gerais, Brazil Received 12 September 2006; accepted for publication 14 December 2006 Available online 19 December 2006
Abstract A low energy electron diffraction (LEED) investigation of the structure of the surface of an FeO(0 0 1) thin film grown on Ag(0 0 1) is presented. The results show that this surface has an almost bulk termination structure with a very small rumple on the first layer, which agrees with the structure found in other studies carried out on the (0 0 1) surface of oxides that have rock-salt structure. Evidences that may support a linear behaviour of the topmost layer rumple with the oxide lattice constant are also discussed. 2006 Elsevier B.V. All rights reserved. Keywords: Surfaces; Structural determination; LEED; Metal oxides
1. Introduction In the last years, there has been an increasing interest in the study of metal oxides surfaces due to their technological applications [1,2]. Oxides are important in catalytic reactions where they play the role of inert support for catalytic metal clusters or even as catalytic material [3,4]. Also, oxides are the most important material to construct gas sensor devices [5]. It is believed that in all these applications the main processes are governed by the surface properties. However, in contrast to the extensive research done on the properties of metal and semiconductor surfaces, investigations of oxide surfaces are lacking. Under the several reasons for this lack are the difficulties of preparing high quality surfaces and, as the majority of the techniques in surface analysis uses electrons or ions as a probe, the insulating nature of many oxide materials. The development of the technique of growing crystalline films on metallic substrates under ultra-high-vacuum conditions has opened new possibilities for studying oxide surfaces. Oxide films grown on metallic substrates and *
Corresponding author. E-mail address: vagner@fisica.ufmg.br (V.E. de Carvalho).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.031
showing good surface quality have been produced at several thicknesses [6]. So far, metal surfaces that are relatively inert with respect to oxidation like Pt(1 1 1), Ag(1 1 1), Ag(0 0 1) and Au(1 1 1) have been the best choices for the substrate. Iron oxides are important in many technological applications and processes, e.g., they are useful as catalysts in a number of industrial processes and widely used in magnetic storage industry and represent most of the corrosion products of iron and steel. The rock salt phase (wu¨stite) FeO, an anti-ferromagnetic insulator, which is not stable in the bulk at temperatures below 580 C is of special interest. Several studies on FeO films have been reported, however, the majority is dedicated to the investigation of the magnetic properties and very few works treat the problem of the surface structure of the films. As far as we know, only the FeO(1 1 1) surface has been object of structural investigation. The production of ordered iron oxide grown on a substrate other than Fe bulk crystals, may allow us to study the influence of the substrate symmetry on the growth orientation of the iron oxide, preferential nucleation site, growth morphology and adhesion properties between oxide and substrate. Heteroepitaxial FeO films were first produced by Vurens et al. [7]. They observed that
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FeO(1 1 1) films grew layer-by-layer on Pt(1 1 1) and Pt(1 0 0). After that several studies have been performed where the influence of the thickness of the film on the resulting phase has been investigated. Weiss and Somorjai [8] reported a qualitative LEED study on FeO(1 1 1) film grown on Pt(1 1 1) by first depositing iron and oxidizing it afterward. At 1 monolayer (ML) coverage a stable hexagonal (1 · 1) structure, with a threefold symmetry, with a unit cell 15% larger than that of Pt(1 1 1) and the presence of an (8 · 8) superstructure has been observed. At thicker films (8 ML) a (2 · 2) reconstructed surface was observed, exhibiting a sixfold symmetry indicating the existence of two domains rotated by 60 with respect to each other. In a similar work, Ritter et al. [9] used LEED and STM techniques to study iron oxide films on Pt(1 1 1). They found that up to 2 ML the oxide grows as a monolayer of FeO(1 1 1) and at higher coverage the film changes to Fe3O4(1 1 1). The FeO(1 1 1) layers consist of hexagonal close-packed iron–oxygen bilayers that are laterally expanded with respect to the bulk FeO structure and slightly rotated against the platinum substrate and are oxygen terminated, resulting in a Moire´ pattern that is clearly seen in the STM image. A similar superstructure formation is observed for FeO(1 1 1) films grown on Ru(0 0 0 1). Up to 4 ML of FeO on Ru(0 0 0 1) a very well ordered and highly oriented (8 · 8) superstructure is observed, corresponding to the coincidence of 7 FeO units with 8 Ru atoms [10]. On Pt(1 0 0) substrate Ritter et al. [11] observed for a monolayer FeO film a superstructure c(2 · 10) with respect to the substrate (1 · 1) unit cell. With increasing film thickness (up to 8 ML) the LEED showed an hexagonal pattern typical of the Fe3O4(1 1 1) surface. They carried out a quantitative LEED analysis of the FeO(1 1 1) phase and concluded that the oxide consists of a bi-layer terminated in oxygen atoms over an iron layer but either the iron and ˚ , respecthe oxygen layers are buckled of 0.5 and 0.3 A tively. Moreover, in a recent work Waddill and Ozturk [12] reported a study of an epitaxial iron oxide film grown on Ag(1 1 1). They first grew Fe films up to 10 monolayers and then proceeded with oxidation what resulted in poorly ordered FeO(1 1 1) films. In a second method, they sequentially produced sub-monolayer Fe films followed by oxidation. By this procedure they obtained a much better ˚ thickness. Thicker crystallographic order up to about 10 A oxide films presented a coexistence of Fe3O4(1 1 1) domains. In the present work, we present the results of a low energy electron diffraction (LEED) atomic geometry investigation of the FeO(0 0 1) surface of an insulating epitaxial film grown on Ag(0 0 1) under ultra-high-vacuum conditions. The results show that this surface has a nearly bulk terminated structure with a very small rumple on the first layer, what is in agreement with the very small relaxations found in other studies carried out on oxides of the same rock-salt structure. We also present evidences that may support a linear behaviour of the topmost layer rumple with the oxide lattice constant.
2. Experimental and theoretical details The experiments were performed on a VG Escalab system with a base pressure of 1 · 1010 mbar and equipped with the standard facilities for sample preparation (cleaning, heating and deposition) and sample characterization (Auger, XPS and UPS) as well as with a computer controlled 4 grid LEED and a RHEED optics. The Ag(0 0 1) single crystal (from Monocrystal Company, with 99.995% purity, presenting a mirror like (0 0 1) surface oriented within ±0.5) was cleaned with several cycles of sputtering (Ar+ ions at 1.0 keV for 30 min) and annealing (up to 450 C for 5 min) until no trace of carbon, oxygen and sulphur contaminations could be detected by XPS and a sharp (1 · 1) LEED pattern could be observed. The FeO(0 0 1) films were grown on Ag(0 0 1) by evaporating high purity 57Fe from an OMICRON e-beam source while dosing a low pressure oxygen atmosphere. Since such procedure usually produces a mixture of FeO and Fe3O4 phases, various combinations of evaporation rate and oxygen pressure were carefully examined and as the best condition to obtain up to 90% of FeO, pO2 ¼ 1 107 mbar during 57Fe-evaporation and 6.8 · 1012 Fe – atoms/cm2 s impinging on the Ag(0 0 1) surface was determined. The film, consisting of 22 ML of FeO, was afterward annealed at 600 C during 10 min, which ensured good surface crystallinity. A complete study on the preparation of FeO and Fe3O4 films on Ag(0 0 1) will be published elsewhere [13]. The stoichiometry (Fe/O = 0.95 ± 0.05) and the presence of well resolved Fe2+ satellites of the Fe–2p1, 2p3 lines, at 730.3 eV and 716.2 eV, respectively, were checked by photoelectron spectroscopy, as shown in Fig. 1a and b. Another check of the FeO phase was done by in situ Conversion Electron Mo¨ssbauer Spectroscopy (CEMS) [14]. The CEMS spectrum shown in Fig. 1c was measured in situ with a 25 mCi – 57Co:Rh source in constant acceleration mode and accumulated during 12 h. The films were grown with 57 Fe in order to obtain a good statistics after this measuring time. The measured isomer shift (0.97 mm/s relative to a-Fe) and quadrupole splitting (0.52 mm/s) are in line with known values for Fe2+ in bulk (paramagnetic) FeO at room temperature. A spectrum taken at 100 K (not shown here) has evidenced the antiferromagnetic character of the film at low temperature (TNeel = 198 K for FeO-bulk). It is worth to point out that the photoelectron spectrum (Fig. 1b) has not shown Fe–2p1, 2p3 satellites typical of Fe3+ (at 733.3 eV and 719.2 eV). This indicates that if Fe3O4 coexists with FeO in the surface, it must be under the detectable limit of the technique, or Fe3O4 is buried under the FeO film and should not affect our LEED-analysis. In fact, the Mo¨ssbauer spectrum at room temperature has shown a small amount (approximately 15%) of Fe3O4, but Mo¨ssbauer spectroscopy is not as surface sensitive as photoelectron spectroscopy but has a probing depth of approximately 80 nm. The LEED patterns of the film were collected and recorded at room temperature and at normal incidence using
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Fig. 1. Photoelectron spectra (a and b) and Mo¨ssbauer spectra (c) of the 22 ML FeO(0 0 1) film. The thin lines correspond to a standard Mo¨ssbauer fitting. The central dublet corresponds to FeO, whereas the two sextets to Fe3O4.
an Omicron LEEDStar video system in the range of 40– 500 eV. The intensity versus energy curves (LEED-I(V) curves) were collected for 23 diffracted beams. After averaging the symmetrically equivalent beams, the data were reduced to 5 inequivalent beams covering a total energy range of 1192 eV. The curves were then normalized with respect to the current of the incident beam and smoothed using a 5-points least-squares cubic polynomial algorithm. The LEED quantitative theoretical analysis was carried out using the method of symmetrized automated tensor LEED [15] with the programs associated to calculate the scattering phase shifts using the approximation of the muffin-tin potential. A self-consistent Dirac–Fock approach was used in order to compute the atomic orbitals for each element. In addition, 10 relativistic phase shifts (lmax = 9)
were evaluated by numerical integration of the Dirac equation, using the Mattheiss’ prescription to compute the muffin-tin potential. Although FeO is an ionic compound, we used neutral phase shifts. It is well known that the structural fit does not depend too much on non-structural parameters, provided their values are reasonable, as was carefully verified by Barbieri et al. [16]. To simulate the difference in atomic radii, different muffin-tin radii for both elements were tested, with the following restriction to avoid Fe O overlap of the spheres: rO muf þ r muf ¼ d nn , where r muf is the Fe oxygen and rmuf the iron muffin-tin radius, respectively, and dnn the nearest–neighbour distance. The Debye temperature for bulk was fixed at a value of 450 K [20] and the real part of the inner potential was simultaneously optimised with the structural parameters while the complex
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part was fixed at 9.0 eV. The simulated LEED-I(V) curves were compared with the experimental data using the Pendry reliability R-Factor [17] and all the calculations were performed on an AMD Athlon (1.6 GHz) computer running Scientific Linux [18]. The Ag(0 0 1) substrate has not been considered in the calculations because the FeO(0 0 1) film used in this analysis was thick enough to completely cover the substrate. The structural parameters optimized were the vertical positions of the anion and cation of the first four layers. This allowed us to investigate the rumple and distance beC tween layers. The rumple is defined as (di ¼ DzA i Dzi ) A C where Dzi and Dzi are, respectively, the anion and the cation vertical displacements from the ideal bulk positions of the ith-layer. The distance between layers is defined as Ddij = (Di Dj), where Di and Dj are the layers relaxations, C defined as Di ¼ 12 ðDzA i þ Dzi Þ. 3. Results and discussion The LEED pattern for the clean Ag(0 0 1) and for the FeO(0 0 1) film after annealing are presented in Fig. 2. As
it is shown in this figure a well defined LEED pattern was obtained for the FeO(0 0 1) indicating a good crystallinity of the film. The initial stage of the LEED analysis involved the investigation of the surface lattice constant of the film (aFeO 0 ). The value reported in the literature is approximately ˚ [12,9] for a single crystal and therefore an investiga3.04 A tion of the film lattice constant was performed, in order to see whether the substrate lattice has any influence, as observed in ultra-thin films [9,19]. For this analysis, a series of calculations were carried out assuming different values ˚ (3.3% compression) to 3.21 A ˚ (5.5% of aFeO from 2.94 A 0 ˚ expansion) in steps of 0.03 A (1%). For each value of aFeO 0 , the neutral phase shifts are calculated by adjusting the muffin-tin radius of the elements as described in Section 2. For each value of the film lattice constant a full structural optimization was performed starting form the bulk positions and assuming the Debye temperature of the surface equal to that of the bulk. In this optimization, 8 structural parameters were varied (what is equivalent to relax 4 layers). The Rp factor as a function of the lattice constant is shown in Fig. 3a. The final structures for all tested lattice
Fig. 2. LEED pattern of the clean Ag(0 0 1) (left) and FeO(0 0 1) film (right) at 127 eV.
O FeO ˚ (b). Fig. 3. Behaviour of the Rp factor as a function of the lattice constant (a), and as a function of the ratio rFe ¼ 3:09 A muf =r muf for a0
E.L. Lopes et al. / Surface Science 601 (2007) 1239–1245
constants were bulk terminated, considering the error bars of the parameters calculated using the variance of the Rp factor. As can be seen in Fig. 3a, a well defined minimum ˚ (1.6% expanwas obtained for aFeO ¼ ð3:09 0:09Þ A 0 sion), which agrees with the values found in the literature. We believe that at this thickness the film is completely relaxed but as the LEED technique probes only the first layers of the sample we do not have information about the behaviour of the lattice parameter in deeper layers. Therefore, based on these results nothing can be said about the presence or not of residual strain in the film. This information could be obtained by performing a RHEED or X-ray diffraction experiments at different stages of the film growth. Using the lattice constant obtained in this first step, a O grid search was performed over the ratio rFe muf =r muf from Fe 1/3 (which is equivalent to rmuf ¼ 0:25d nn and rO muf ¼ 0:75d nn ) up to 3 (which is equivalent to rFe muf ¼ 0:75d nn O Fe and rO muf ¼ 0:25d nn ) in steps of 0.05dnn for r muf and rmuf . At this stage, the same structural (8 vertical positions) and non-structural (real part of inner potential) parameters fitted previously were optimized, with the Debye temperature of the surface equal to that of the bulk. The behaviour of the Rp factor is shown in Fig. 3b. As can be seen from this figure, the Rp exhibits a shallow minimum at rFe muf = rO corresponding to rO and muf ¼ 0:81, muf ¼ 0:55d nn Fe rFe muf ¼ 0:45d nn , but it is nearly constant for r muf = rO muf > 0:81. With the in-plane lattice constant and the muffin-tin radii determined, the Debye temperature of the two elements Fe of the first layer (HO 1 and H1 ) was varied independently, from 200 K up to 900 K in steps of 25 K. Again, for each O value of HFe 1 and H1 , a full optimization of the parameters was performed using the best value of the lattice constant and muffin-tin radii found previously. The results of this optimization can be seen in Fig. 4. The Rp factor shows a
Fig. 4. Rp factor as a function of the first layer Debye temperatures (HO 1 and HFe 1 ). The greyscale corresponds to changes in the Rp factor from 0.23 (black) up to 0.37 (light grey).
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very well defined minimum at HO 1 ¼ ð450 300Þ K (bulk value) and HFe ¼ ð350 300Þ K (22% lower than that of 1 the bulk). Fe Then, using these best values for HO 1 and H1 , a new optimization procedure was taken and the final structure shows a small rumple in the topmost layer (d1) of (3.9 ± 3.2)% with the oxygen ions moving towards the vacuum side, and all the other structural parameters optimized are essentially the same as the bulk values. Specifically, the change from the bulk values are: Dd12 = (2.4 ± 4.6)%, Dd23 = (3.2 ± 4.8)% and Dd34 = (0.7 ± 5.7)%, with a Rp factor equal to (0.23 ± 0.06). In Fig. 5 a side view of the surface structure of the FeO(0 0 1) film (left) and the good agreement between theoretical and experimental LEED-I(V) curves (right) are shown. As can be seen from Table 1 and considering the uncertainty of our results, the topmost layer rumple of FeO from our study is in agreement with that of most of other structural studies carried out on the (0 0 1) surface of other oxides with rock-salt structure. All studies indicate rumples on the topmost layer. The surface rumples as a function of the bulk lattice constant is shown in Fig. 6. In this plot all the results presented in Table 1 except the MEIS results for NiO [23] and MnO [26] single crystals have been used. This plot suggests that a linear dependence of the surface rumple with the bulk lattice constant may exist. A similar behaviour is observed for the (1 1 0) surface of III–V semiconductors [32]. A positive rumple – corresponding to oxygen atoms lying outwards – for all the rock-salt oxides surfaces is expected based on the model proposed by Goniakowski and Noguera [30,31]. According to this model, the rumple is the result of the dissymmetry in the anion and the cation displacements due to the fact that the second-neighbour electron hopping process can be different on the anion and the cation sublattices. Depending on the atomic orbitals symmetry in each crystalline structure, effective attractive or repulsive forces will be acting on surface anions and cations giving rise to a positive or a negative rumpling. In the case of the rock-salt oxides, the anion–anion electron delocalization processes increase the total energy and, at the surface, the bond length can expand in order to decrease the energy. On the other hand, the cation–cation contribution is negative and consequently the energy can be lowered by contraction of the bond length. Then, an expansion of the O–O bond length and a contraction of the cation–cation bond length produce a positive rumple. Also, when the oxygen atom is larger than the metal atom, the cation–cation contribution to the energy is much smaller compared to that of oxygen–oxygen processes. Therefore, the rumple strength must vary with the effective metal ion size and, as a consequence, a crystal lattice parameter dependence may be expected. So, according to this model, the FeO(0 0 1) surface should present a positive rumple with the strength defined by the iron atom radius. These predictions seem to be confirmed by considering the results presented in the Fig. 6.
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<100>
A C 1 Δ i = 2 ( Δz i + Δz i ) C A δi = Δz i – Δz i
<110>
O–2 Fe+2 δ1
Fig. 5. (Left) Side view of the FeO(0 0 1) surface structure. The oxygen and iron ions are represented by white and black spheres, respectively. (Right) LEED-I(V ) curves for the FeO(0 0 1) film on Ag(0 0 1). Thin solid lines: theoretical curves for the best model obtained. Thick solid lines: experimental curves.
Table 1 The topmost layer rumple determined for (0 0 1) surface of oxides with rock-salt structure obtained by several experimental and theoretical techniques reported on literature ˚) Oxide d1 (A Technique/method FeO CoO NiO NiO NiO MnO MnO MnO MgO MgO MgO CaO
0.09 ± 0.07 0.06 ± 0.04 0.014 0.05 ± 0.05 0.10 ± 0.01 0.01 0.10 ± 0.04 0.08 ± 0.02 0.024 0.02 ± 0.01 0.04 ± 0.04 Less than 0.04
LEED (this work) LEED [21] Hartree–Fock [22] LEED [19] MEIS [23] Ab initio [24] LEED [25] MEIS [26] Hartree–Fock [22] GIXS [27] LEED [28] LEED [29]
4. Conclusions In this work low-energy electron diffraction has been used to determine the surface structure of FeO(0 0 1) thin films grown on Ag(0 0 1). A good agreement between experiment and theory was obtained as can be seen from the obtained value of the Rp factor. The results show that this surface has a small rumple of (3.9 ± 3.2)% with the oxygen ions moving towards the vacuum and a Debye temperature of the two elements of the first layer equal to (450 ± 300) K for oxygen and (350 ± 300) K (22% lower than bulk) for iron atoms. These results are consistent with the small relaxations and rumples observed for the (0 0 1) surface of other rock-salt oxides reported in the literature, and the rumple strength seems to vary linearly with the lattice constant. Acknowledgements We thank CNPq and FAPEMIG, Brazilian research agencies, for financial support. References
Fig. 6. Rumple of different rock-salt metal oxides as a function of their lattice constant.
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