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The preparation of ultra-thin chromium–vanadium oxides on Cu(100) studied by XPS and LEED
Surface Science 445 (2000) 80–88 www.elsevier.nl/locate/susc
The preparation of ultra-thin chromium–vanadium oxides on Cu(100) studied by XPS and LEED A. Maetaki, M. Yamamoto, H. Matsumoto, K. Kishi * Department of Chemistry, School of Science, Kwansei-Gakuin University, Nishinomiya, Hyogo 662-8501, Japan Received 29 June 1998; accepted for publication 4 October 1999
Abstract Ultra-thin chromium and chromium–vanadium oxides have been grown on Cu(100) and characterized by XPS, X-AES and LEED. The chromium oxide was prepared by the oxidation of deposited chromium atoms on Cu(100) at 310 K, followed by annealing at 673 K in vacuum. At an oxide coverage of one layer, a weak LEED pattern ascribable to CrO(111) surface was observed. The chromium oxide containing Cr3+ was formed as well. At a coverage of more than two layers, a Cr O (111) surface was formed. When the Cu(100) substrate covered with the chromium 2 3 oxide was heated in oxygen (~1.3×10−5 Pa) at >473 K, a Cu+-oxide was formed on top of the chromium oxide. The ultra-thin chromium oxide was stabilized as a Cr2+-oxide between the copper substrate and the Cu+-oxide. The chromium–vanadium oxide assumed a surface structure like VO(111) at the total (chromium and vanadium) coverage of one layer and a surface like V O (111), similar to Fe O (111) structure at the larger coverage. The addition of the 3 4 3 4 vanadium oxide to the chromium oxide increased the order of the surface structure of the oxide and prevented the copper substrate from the oxidation by the heating in oxygen (~1.3×10−5 Pa) at 723 K. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chromium oxide; Copper oxide; Low-energy electron diffraction (LEED); Surface structure; Vanadium oxide; X-ray photoelectron spectroscopy ( XPS )
1. Introduction Ordered ultra-thin films of metal oxides on another metal substrate have been investigated in connection with the surface science studies of the structure and chemical states of surfaces of bulk metal oxides [1–14]. A new structure and chemical state are also expected for the ultra-thin metal oxide films, which are stable only when contacting with metallic substrates. * Corresponding author. Tel.: +81-798-54-6384; fax: +81-798-51-0914. E-mail address: [email protected] ( K. Kishi)
The surface structures of the chromium oxide films grown on Pt(111)[10,11] and on Cr(110)[15– 20] have been investigated. The chromium oxide on Pt(111) gives a surface structure such as Cr O (111) or Cr O (111), depending on the 3 4 2 3 thickness from a submonolayer to more than eight monolayers. The chromium oxide prepared on Cu(110) assumes a structure of CrO(111) at monolayer coverage and Cr O (111) at a coverage 2 3 of more than two layers [21]. The surface structures, CrO(111) and Cr O (111), are incommensu2 3 rate with the Cu(110) substrate with a corrugated surface structure. In the present study, the ultrathin chromium and chromium–vanadium oxides
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grown on Cu(100) were investigated by XPS, X-AES and LEED, and the changes in the structures and chemical states of the chromium oxide by the addition of the vanadium oxide were discussed.
2. Experimental The preparation of ultra-thin chromium, vanadium and chromium–vanadium oxides were performed in a chamber (base pressure: 8×10−8 Pa) equipped with LEED optics ( ULVAC ) and an XPS spectrometer (Shimadzu). The surface of the Cu(100) (from Metal Crystals and Oxides Ltd., Cambridge, UK ) was cleaned by Ar+ sputtering and annealing up to 800 K and exposed to O 2 (1×10−4 Pa, 5 min) at 310 K before deposition of metals. Chromium atoms were evaporated onto the substrate at 320 K from a small grain of chromium (>99.9% purity) clipped by tantalum sheet heated by a tungsten filament, and vanadium atoms were from a tungsten filament wound with a thin vanadium wire (>99.7%). The chromium oxide was formed by exposing the deposited chromium atoms to O (1×10−5 Pa for 50 s on 2 Cu(100) and 2×10−5 Pa for 200 s on vanadium oxide) at 310 K and subsequent annealing at 673 and 723 K, respectively, for 15 min in vacuum. The ordered vanadium oxide was prepared by heating the deposited vanadium atoms for 15 min in O of 5×10−5 Pa at 623 K on Cu(100) or in 2 O of 2×10−5 Pa at 723 K on chromium oxide. 2 O gas (99.99%) from Takachiho Kagaku Kogyo 2 was introduced to the chamber through a needle tube in close proximity to the sample surface. The binding energies of core electrons were calibrated with respect to the Cu 2p peak 3/2 (932.7 eV ) of the metallic Cu. The photoelectrons by AlKa source were collected at an angle of 60° with respect to the surface normal. The Cr 2p 3/2 peaks were obtained by subtracting the background due to Auger peaks of Cu or Cu+ from the recorded spectrum. The background feature from Cu+ was obtained from Cu O formed on 2 Cu(100) after exposure to air.
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3. Results 3.1. Growth of ultra-thin chromium oxide on Cu(100) XP spectra were taken for the chromium oxide prepared in successive increments on the Cu(100) surface to monitor the deposition and growth of the oxide. Fig. 1 shows the plots of the Cr 2p , 3/2 O 1s and Cu 2p peak intensities as a function of 3/2 the total deposition time of the chromium. The linear lines changing in gradient at ~13 min suggest the growth of the chromium oxide mostly two-dimensionally up to the inflection point. The coverage of the chromium oxide at ~13 min is designated as 1.0h hereafter. The Cu 2p peak CrO 3/2 and the CuL M M Auger peaks (designated 3 4,5 4,5 as CuLMM hereafter) showed no spectral change except for attenuation of the intensity. The Cr 2p peaks at 576.0 eV, predominant up to 3/2 0.7h (Fig. 2a and c) and at 577.0 eV at CrO ~1.6h (g) can be assigned to Cr2+ and to CrO Cr3+, respectively, because the Cr 2p binding 3/2 energies are 574.2 eV for Cr metal, 576.0 eV for Cr2+, and 577.0 eV for Cr3+ in Cr O (111)/Cr(110) [15] and 577.2 eV for Cr3+ 2 3 in Cr O /NiAl O [22]. The LEED pattern (A) of 2 3 2 4
Fig. 1. Plots of the Cr 2p , O 1s and Cu 2p intensities of the 3/2 3/2 chromium oxide on Cu(100) as a function of the deposition time (total ) of the chromium atoms. The intensities at 0 min are for the oxygen preadsorbed Cu(100).
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Fig. 2. Variation of the Cr 2p and O 1s spectra for the chro3/2 mium oxide on Cu(100) as a function of the deposition time of the chromium atoms.
Fig. 3 was observed at ~1.0h , which was overCrO lapped with the substrate pattern at the lower coverage. The change in chemical state of the chromium oxide (~1.0h ) by the annealing temperature in CrO vacuum is shown in Fig. 4. The Cr 2p peak at 3/2 576.0 eV for Cr2+ increased with increase in the temperature up to 673 K, associated with a strengthening of the LEED spots. The annealing at 723 K resulted in a weakening of the LEED spots and an increase in the peak at 577.0 eV for Cr3+ ( Fig. 4g). The annealing at 773 K in vacuum was, however, required at 2.5h to gain an CrO ordered chromium oxide giving pattern (B) in Fig. 3, (앀3×앀3)R30° structure of pattern (A), and the Cr 2p peak at 577.0 eV for Cr3+ much 3/2 stronger in intensity than the peak at 576.0 eV for Cr2+. The annealing of the chromium oxide/Cu(100) in oxygen at 723 K produced a different surface oxide. The heating of the 1.5h chromium oxide CrO on Cu(100) in O of 1×10−5 Pa at 723 K for 2 15 min increased the intensity of the CuLMM Auger peak at 916.6 eV (kinetic energy), being characteristic of Cu O [23], at the expense of the 2 peak at 918.6 eV for Cu0 [(b)–(c) in Fig. 5], revealing the formation of Cu+-oxide at the surface. The Cr 2p peak at 576.0 eV for Cr2+ was 3/2 observed as a main peak. The total Cr 2p peak 3/2 intensity decreased by ~55% while the Cu 2p 3/2
Fig. 3. LEED patterns of ultra-thin oxides on Cu(100). (A) ~1.0h chromium oxide. Two hexagonal domains are scheCrO matically presented by spots of filled and open circles. (B) >2.5h chromium oxide. (C ) 0.9h vanadium oxide. (D) CrO VO 1.8h vanadium oxide. ( E ) 0.9h chromium oxide on VO CrO 0.9h vanadium oxide. VO
peak intensity at 932.7 eV increased from 24% of the clean Cu(100) to 65%. The LEED pattern was similar to pattern (A) but with stronger and sharper spots. 3.2. Interaction between ultra-thin oxides of chromium and vanadium 3.2.1. Preparation of chromium oxide on vanadium oxide and vanadium oxide on chromium oxide The vanadium oxide on Cu(100) gives the VO(111) structure at !1.0h and the VO V O (111) structure at 1.0h , giving the LEED 3 4 VO
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Fig. 4. Variation of the Cr 2p and O 1s spectra of ~1.0h 3/2 CrO chromium oxide on Cu(100) as a function of the heating temperature in vacuum.
patterns (C ) and (D), respectively, in Fig. 3 [13]. The 1.0h represents a VO(111) overlayer, conVO sisting of V2+–O2− bilayer completely covering the Cu(100) surface [13]. The growth of the chromium oxide on the well-ordered VO(111)/Cu(100) was carried out to investigate the interaction between the oxides. The preparation of 0.3h chromium oxide on the 0.9h CrO VO VO(111)/Cu(100) changed the V 2p peak at 3/2 515.1 eV (assigned to V2+ [13]) into the one at
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516.0 eV (assigned to V3+[13]) with a shoulder around 514.8 eV ( Fig. 6a and b). The Cr 2p 3/2 peak at 575.8 eV for Cr2+ was stronger than the peak at 577.2 eV for Cr3+. The sharp LEED pattern (C ) of Fig. 3 changed into pattern ( E ), corresponding to a 2×2 structure of the VO(111). The subsequent increment (total 0.6h ) of the CrO chromium oxide resulted in the sharpening of the LEED pattern between 80 and 150 eV, but the third growth of the chromium oxide (total 0.9h ) made the pattern diffuse slightly, together CrO with an increase in the Cr 2p peak at 577.0 eV 3/2 for Cr3+ as shown in Fig. 6d. The decrease in V 2p peak intensity was very small compared to 3/2 the attenuation of the Cu 2p peak by 46% after 3/2 the formation of the chromium oxide, indicating a mixing of the chromium oxide into the vanadium oxide to some extent. The heating of the chromium oxide (0.9h ) on the VO(111) CrO (0.9h ) at 723 K did not change the CuLMM VO Auger spectra and the LEED pattern even in O 2 of 5×10−5 Pa. Next, the vanadium oxide was prepared on ~1.0h chromium oxide/Cu(100). The LEED CrO pattern (A) gradually varied into the pattern like
Fig. 5. CuL M M Auger, Cr 2p and O 1s spectra for (a) clean Cu(100), (b) 1.5h chromium oxide, and (c) after subsequent 3 4,5 4,5 3/2 CrO heating at 723 K in 1×10−5 Pa O for 15 min. The Cr 2p spectra (b)∞ and (c)∞ were obtained by subtraction of the background 2 3/2 due to Cu and Cu+ from (b) and (c), respectively.
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Fig. 6. V 2p , Cr 2p and O 1s spectra for (a) 0.9h vanadium oxide and after subsequent growth of (b) 0.3 (c) 0.6 and (d) 3/2 3/2 VO 0.9h chromium oxide. CrO
( E) after deposition of the vanadium oxide from 0.2h to 0.6h . The V 2p peak for V3+ was VO VO 3/2 observed at 516.2 eV, and the Cr 2p peaks for 3/2 Cr2+ and Cr3+ were at 575.8 and 577.0 eV, respectively, as shown in Fig. 7. The peaks around 520 eV
Fig. 7. V 2p and Cr 2p spectra for (a) 1.0h chromium 3/2 3/2 CrO oxide and after subsequent growth of (b) 0.2 (c) 0.4 and (d) 0.6h vanadium oxide. VO
in the V 2p spectra were due to the O 1s peak 3/2 by AlKa radiation. Further growth of the vana3,4 dium oxide made the LEED pattern diffuse. No change was observed with the CuLMM Auger spectra during the heating in oxygen. 3.2.2. Oxidation of mixture of vanadium and chromium atoms The chromium–vanadium oxide was prepared in a different way. Vanadium atoms (equivalent to 0.5h ) deposited on the oxygen chemisorbed VO Cu(100) surface were oxidized by exposure to O 2 (1×10−5 Pa) at 310 K for 200 s. Chromium atoms (equivalent to 0.5h ) were subsequently deposited CrO on the surface and oxidized by exposure to O 2 (1×10−5 Pa) for 200 s. Annealing of the oxide at 723 K for 15 min in vacuum gave a strong LEED pattern like the VO(111). The Cr 2p peak at 3/2 575.8 eV for Cr2+ and the V 2p peak at 515.2 eV 3/2 for V2+ were predominant. The subsequent growth of the same amount of chromium–vanadium oxide under the same conditions changed the LEED pattern into the 2×2 pattern. The Cr3+ was comparable in the amount with the Cr2+. The V 2p peak at 516.2 eV for V3+ was observed 3/2 with a weak shoulder at the lower binding energy side.
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4. Discussion 4.1. Structure and chemical state of chromium oxide on Cu(100) The LEED pattern (A) of Fig. 3 for the 1.0h chromium oxide, which grew two-dimenCrO sionally, can be resolved into two hexagonal meshes, as shown in Fig. 3, and assigned to the chromium oxide of a hexagonal structure with two domains at 90° aligned along the [0,1,1] and [0,−1,1] directions of the Cu(100) surface, respectively. In contrast, only one domain was formed for the chromium oxide on Cu(110) [21]. The difference reveals an influence exerted on the angular orientation of the overlayer by the structure of the substrate surface. Pattern (A) is similar to that for the 1.0h VO(111) on Cu(100), pattern (C ) VO in Fig. 3, although the spots for the chromium oxide were not sharp and strong. The clearest spots of the pattern (A) were obtained when the Cr2+/Cr3+ ratio was largest after the heating at 623–673 K in vacuum ( Fig. 4), indicating a correlation between the pattern (A) and Cr2+. The chromium oxide showing the pattern (A) was then assigned to a CrO(111) with the same structure as the VO(111), as illustrated by Fig. 8a. The lattice constant for the CrO(111) was calculated as 0.30 nm. In the case of bulk ionic oxides such as CrO and VO, the polar surfaces of CrO(111) and VO(111) are inherently unstable due to the divergence of the electrostatic surface potential [24–
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26 ]. However, the CrO(111) at 1.0h and CrO VO(111) at 1.0h on the Cu(100) is considered VO to be stabilized by a compensation of its electrostatic dipole by an image dipole in the copper substrate, as has been discussed for a similar hexagonal close-packed FeO overlayer with oxygen on top (slightly distorted) on Pt(100)-hex-R0.7° [5]. The CrO(111) surface may be partly stabilized by the adsorption of OH−, as in the case for NiO [27], as suggested by the tailing of the O 1s peak around 531.5 eV. The weaker spots of pattern (A) than those for the VO(111) and the presence of the Cr3+ suggests that patches of islands of Cr O - and/or Cr O -like oxides coexisted with 3 4 2 3 the CrO(111) because the untypical Cr2+-oxide is rather unstable. The larger attenuation of the Cu 2p peak intensity at 1.0h , 52% in Fig. 1, 3/2 CrO than 43% for the VO(111) [13] is probably due to the coexistence of the oxides at a higher oxidation state. The formation of the (앀3×앀3)R30° structure of the CrO(111) in Fig. 3B, at the coverage of 2.5h , implies a commencement of the formation CrO of Cr O (111) surface, as shown by Fig. 8b. This 2 3 was in accordance with the predominant Cr 2p 3/2 peak at 577.0 eV for Cr3+. The presence of the weak peak at 576.0 eV for Cr2+ means the coexistence of the chromium oxide containing Cr2+. The lattice constant obtained from pattern (C ), 0.52 nm, is slightly larger than the 0.50 nm for the Cr O (111) of the bulk oxide. The much more 2 3
Fig. 8. Schematic representation of oxide structures. (a) VO(111) or Cr(111), (b) Cr O (111) and (c) one termination of the 2 3 Fe O (111) surface [36 ]. 3 4
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ordered Cr O (111) was formed on Cu(110) at a 2 3 coverage greater than 2h under the same prepaCrO ration conditions [21].
bulk Cu O is usually unstable at a temperature 2 such as 723 K in vacuum[35]. 4.3. Formation of chromium–vanadium oxide
4.2. Segregation of copper oxide Heating of the 1.5h chromium oxide on CrO Cu(100) in O (1×10−5 Pa) at 723 K for 15 min 2 resulted in the oxidation of the copper surface to Cu+ ( Fig. 5). When the Cu(100) surface with no chromium oxide overlayer was treated with oxygen under the same conditions, a reconstructed (앀2×앀2)R45° structure induced by chemisorbed oxygen [28–31] was obtained. The Cu LMM Auger peak showed no formation of Cu+. The ultra-thin chromium oxide facilitated the oxidation of the Cu(100) surface in oxygen at a low pressure such as 1×10−5 Pa, whereas the oxidation of the copper substrate was not observed for the thin oxide overlayers of vanadium [13], titanium [32] and iron [33] on the Cu(100) after heating in oxygen under the same conditions. The large increase in Cu 2p intensity at 932.7 eV can be 3/2 interpreted by an accumulation of Cu+-oxide at the topmost layer on the chromium oxide because the Cu 2p peak for metallic copper mostly over3/2 laps with that for the Cu+-oxide; the chemical shift of the Cu 2p peak for Cu O is 0.1 eV [34] 3/2 2 or 0.3 eV [23]. If the Cu+ ions locate under the chromium oxide, the intensity of the Cu 2p peak 3/2 at 932.7 eV will not increase. The decrease in the Cr 2p peak also supports the presence of the 3/2 Cu+ on the chromium oxide. The main Cr 2p peak at 576.0 eV (Fig. 5) 3/2 indicates that the chromium oxide of Cr2+ was stabilized between the copper substrate and the Cu+ oxide overlayer. The sharp and intense LEED pattern like (A) after the segregation of the Cu+ indicates the formation of well-ordered Cu+-oxide grown probably epitaxially on the CrO(111). The stabilization of the surface structure was not due to the adsorption of OH− since the O 1s peak changed into a single peak at 530 eV due to O2−. The Cu+-oxide layer was stable on the chromium oxide during heating at 723 K for 15 min in vacuum, although the surface of the
The formation of the 0.6h chromium oxide CrO on the 0.9h vanadium oxide changed the VO VO(111) structure into the 2×2 structure, as shown by the change in the LEED pattern, from (C ) to ( E ) in Fig. 3, as observed during the increase in the vanadium oxide coverage on Cu(100) [13]. A similar change in the structure with the oxide coverage was observed for iron oxide on Pt(100) and (111), from FeO(111) to Fe O (111) [2]. The 3 4 present 2×2 surface structure suggests a formation of the mixed vanadium–chromium oxide with a surface structure like V O (111), Cr O (111) and 3 4 3 4 Fe O (111). One of the candidates for the surface 3 4 structure of the vanadium–chromium oxide is one such as that in Fig. 8c, which, it has been concluded, is one of the two terminations for the (111) surface of magnetite ( Fe O ) [36 ]. In the 3 4 chromium–vanadium oxide at 0.3h , the V3+ CrO was larger than V2+ ( Fig. 6b) in number of ions, while the Cr2+ was larger than Cr3+. The increase in the ratio of the Cr3+ with the increase in the amount of the chromium oxide (c and d) is correlated with the diffusion of the LEED pattern at 0.9h (total ) since the V O and Cr O have a CrO 2 3 2 3 different structure and lattice constant to those of the V O and Cr O . 3 4 3 4 The surface structure resembling Fe O (111) 3 4 was also formed for the 0.6h vanadium oxide VO on the ~1.0h chromium oxide, as shown by the CrO sharp and strong LEED pattern ( E), although the ordered Cr O (111) structure could not be 3 4 obtained on Cu(100) for the pure chromium oxide. Most of the vanadium ions were in the 3+ state, but nearly half of the chromium ions were in 2+ ( Fig. 7). Annealing (in a vacuum) of the chromium (0.5h ) and the vanadium (0.5h ) oxides that CrO VO had been oxidized beforehand at 310 K gave a well-ordered structure like VO(111)/Cu(100). Most of the chromium and vanadium ions were in a divalent state, as shown by the Cr 2p peak 3/2 at 575.8 eV and the V 2p peak at 515.2 eV, 3/2 indicating that the mixing of the vanadium ions
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stabilized the Cr2+ state and then the structure like VO(111), even at such a low total coverage. The subsequent growth of the same amount of chromium–vanadium mixed oxide gave the stable 2×2 structure of the VO(111). Most of the vanadium ions were in the V3+ state ( V 2p peak at 3/2 516.2 eV ), and the Cr3+ (Cr 2p peak at 3/2 576.8 eV ) was comparable in the amount with the Cr2+(at 576.0 eV ). The oxidation of the copper substrate was not found during heating in O when the chromium 2 oxide on Cu(100) was mixed with the vanadium oxide, indicating that the addition of vanadium oxide to chromium oxide can prevent oxidation of the Cu substrate.
5. Conclusions Ultra-thin chromium oxide and chromium– vanadium mixed oxide films grown on Cu(100) have been characterized by means of XPS, X-AES and LEED. The chromium oxide prepared by exposing the deposited chromium atoms to O 2 (~20 L) and subsequent annealing at 673 K in vacuum shows two-dimensional growth of the oxide. Up to ~1.0h , the chromium oxide is CrO formed mainly as CrO(111), showing the diffuse hexagonal LEED pattern. The ratio of the coexisting Cr3+-oxide increases with increasing coverage. The Cr O (111) is formed at more than 2.5h to 2 3 CrO give the (앀3×앀3)R30° structure of the CrO(111). The copper substrate is not oxidized at any of the steps. The chromium oxide/Cu(100) surface is not stable in oxygen at a high temperature such as 673 K. The Cu+–oxide segregates onto the CrO(111), although the oxidation of the copper is not found for the ultra-thin Ti, V and Fe oxide films on Cu(100) under the same conditions. The Cu+-oxide overlayer is stable in UHV at a high temperature such as 673 K. The mixing of the vanadium ions into the chromium oxide leads to the formation of wellordered and stable chromium–vanadium oxides, whose structures are similar to the VO(111) or VO(111)-(2×2), depending on the amounts of the
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oxide, and prevents oxidation of the copper substrate.
References [1] A. Barbieri, W. Weiss, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 302 (1994) 259. [2] G.H. Vurens, V. Maurice, M. Salmeron, G.A. Somorjai, Surf. Sci. 268 (1992) 170. [3] W. Weiss, A. Barbieri, M.A. Van Hove, G.A. Somorjai, Phys. Rev. Lett. 71 (1993) 1848. [4] W. Weiss, G.A. Sormorjai, J. Vac. Sci. Technol. A 11 (1993) 2138. [5] M. Ritter, H. Over, W. Weiss, Surf. Sci. 371 (1997) 245. [6 ] H. den Daas, M. Passacantando, L. Lozzi, S. Santucci, P. Picozzi, Surf. Sci. 317 (1994) 295. [7] H. den Daas, O.L.J. Gijzeman, J.W. Geus, Surf. Sci. 285 (1993) 15–26. [8] A.B. Boffa, H.C. Galloway, P.W. Jacobs, J.J. Benı´tez, J.D. Batteas, M. Salmeron, A.T. Bell, G.A. Sormojai, Surf. Sci. 326 (1995) 80. [9] W.S. Oh, C. Xu, D.Y. Kim, D.W. Goodman, J. Vac. Sci. Technol. A 15 (1997) 1710. [10] L. Zhang, M. Kuhn, U. Diebold, Surf. Sci. 375 (1997) 1. [11] L. Zhang, M. Kuhn, U. Diebold, J. Vac. Sci. Technol. A 15 (1997) 1576. [12] K.B. Lewis, S.T. Oyama, G.A. Somorjai, Surf. Sci. 233 (1990) 75. [13] K. Kishi, Y. Hayakawa, K. Fujiwara, Surf. Sci. 356 (1996) 171. [14] K. Kishi, K. Fujiwara, J. Electron Spectrosc. Relat. Phenom. 85 (1997) 123. [15] C. Xu, M. Hassel, H. Kuhlenbeck, H.-J. Freund, Surf. Sci. 258 (1991) 23. [16 ] C.A. Ventrice Jr., D. Ehrlich, E.L. Garfunkel, B. Dillmann, D. Heskett, H.-J. Freund, Phys. Rev. B 46 (1992) 12892. [17] H. Kuhlenbeck, C. Xu, B. Dillmann, M. Hassel, B. Adam, D. Ehrlich, S. Wohlrab, H.-J. Freund, U.A. Ditzinger, H. Neddermeyer, M. Neuber, M. Neumann, Ber. Bunsenges Phys. Chem. 96 (1992) 15. [18] M. Bender, I.N. Yakovin, H.-J. Freund, Surf. Sci. 365 (1996) 394. [19] P. Michel, C. Jardin, Surf. Sci. 36 (1973) 478. [20] S. Ekelund, C. Leygraf, Surf. Sci. 40 (1973) 179. [21] A. Maetaki, K. Kishi, Surf. Sci. 411 (1998) 35. [22] A.M. Venezia, C.M. Loxton, J.A. Horton, Surf. Sci. 225 (1990) 195. [23] D. Briggs, M.P. Seah ( Eds.), Practical Surface Analysis, second ed., Vol. 1, Wiley, New York, 1990. [24] P.W. Tasker, J. Phys. C 12 (1979) 4977. [25] D. Wolf, Phys. Rev. Lett. 68 (1992) 3315. [26 ] C.A. Ventrice Jr., Th. Bertrams, H. Hannemann, A. Brodde, H. Neddermeyer, Phys. Rev. B 49 (1994) 5773. [27] D. Cappus, M. Hasßel, E. Neuhaus, M. Heber, F. Rohr, H.-J. Freund, Surf. Sci. 337 (1995) 268. [28] M. Wuttig, R. Franchy, H. Ibach, Surf. Sci. 213 (1989) 103.
88
A. Maetaki et al. / Surface Science 445 (2000) 80–88
[29] H.C. Zeng, R.A. McFarlane, K.A.R. Mitchell, Surf. Sci. 208 (1989) L7. [30] M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, T. Lindner, J.C. Somers, D.E. Ricken, A.M. Bradshaw, Surf. Sci. 236 (1990) 1. [31] F.M. Leibsle, Surf. Sci. 337 (1995) 51. [32] T. Maeda, Y. Kobayashi, K. Kishi, Surf. Sci. 436 (1999) 249.
[33] A. Maetaki, S. Kadowaki, T. Tawa, K. Kishi, submitted for publication. [34] P.E. Larson, J. Electron Spectrosc. Relat. Phenom. 4 (1974) 213. [35] D.L. Cocke, G.K. Chuah, N. Kruse, J.H. Block, Appl. Surf. Sci. 84 (1995) 153. [36 ] A.R. Lennie, N.G. Condon, F.M. Leibsle, P.W. Murray, G. Thornton, D.J. Vaughan, Phys. Rev. B 53 (1996) 10244.
The preparation of ultra-thin chromium–vanadium oxides on Cu(100) studied by XPS and LEED A. Maetaki, M. Yamamoto, H. Matsumoto, K. Kishi * Department of Chemistry, School of Science, Kwansei-Gakuin University, Nishinomiya, Hyogo 662-8501, Japan Received 29 June 1998; accepted for publication 4 October 1999
Abstract Ultra-thin chromium and chromium–vanadium oxides have been grown on Cu(100) and characterized by XPS, X-AES and LEED. The chromium oxide was prepared by the oxidation of deposited chromium atoms on Cu(100) at 310 K, followed by annealing at 673 K in vacuum. At an oxide coverage of one layer, a weak LEED pattern ascribable to CrO(111) surface was observed. The chromium oxide containing Cr3+ was formed as well. At a coverage of more than two layers, a Cr O (111) surface was formed. When the Cu(100) substrate covered with the chromium 2 3 oxide was heated in oxygen (~1.3×10−5 Pa) at >473 K, a Cu+-oxide was formed on top of the chromium oxide. The ultra-thin chromium oxide was stabilized as a Cr2+-oxide between the copper substrate and the Cu+-oxide. The chromium–vanadium oxide assumed a surface structure like VO(111) at the total (chromium and vanadium) coverage of one layer and a surface like V O (111), similar to Fe O (111) structure at the larger coverage. The addition of the 3 4 3 4 vanadium oxide to the chromium oxide increased the order of the surface structure of the oxide and prevented the copper substrate from the oxidation by the heating in oxygen (~1.3×10−5 Pa) at 723 K. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chromium oxide; Copper oxide; Low-energy electron diffraction (LEED); Surface structure; Vanadium oxide; X-ray photoelectron spectroscopy ( XPS )
1. Introduction Ordered ultra-thin films of metal oxides on another metal substrate have been investigated in connection with the surface science studies of the structure and chemical states of surfaces of bulk metal oxides [1–14]. A new structure and chemical state are also expected for the ultra-thin metal oxide films, which are stable only when contacting with metallic substrates. * Corresponding author. Tel.: +81-798-54-6384; fax: +81-798-51-0914. E-mail address: [email protected] ( K. Kishi)
The surface structures of the chromium oxide films grown on Pt(111)[10,11] and on Cr(110)[15– 20] have been investigated. The chromium oxide on Pt(111) gives a surface structure such as Cr O (111) or Cr O (111), depending on the 3 4 2 3 thickness from a submonolayer to more than eight monolayers. The chromium oxide prepared on Cu(110) assumes a structure of CrO(111) at monolayer coverage and Cr O (111) at a coverage 2 3 of more than two layers [21]. The surface structures, CrO(111) and Cr O (111), are incommensu2 3 rate with the Cu(110) substrate with a corrugated surface structure. In the present study, the ultrathin chromium and chromium–vanadium oxides
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grown on Cu(100) were investigated by XPS, X-AES and LEED, and the changes in the structures and chemical states of the chromium oxide by the addition of the vanadium oxide were discussed.
2. Experimental The preparation of ultra-thin chromium, vanadium and chromium–vanadium oxides were performed in a chamber (base pressure: 8×10−8 Pa) equipped with LEED optics ( ULVAC ) and an XPS spectrometer (Shimadzu). The surface of the Cu(100) (from Metal Crystals and Oxides Ltd., Cambridge, UK ) was cleaned by Ar+ sputtering and annealing up to 800 K and exposed to O 2 (1×10−4 Pa, 5 min) at 310 K before deposition of metals. Chromium atoms were evaporated onto the substrate at 320 K from a small grain of chromium (>99.9% purity) clipped by tantalum sheet heated by a tungsten filament, and vanadium atoms were from a tungsten filament wound with a thin vanadium wire (>99.7%). The chromium oxide was formed by exposing the deposited chromium atoms to O (1×10−5 Pa for 50 s on 2 Cu(100) and 2×10−5 Pa for 200 s on vanadium oxide) at 310 K and subsequent annealing at 673 and 723 K, respectively, for 15 min in vacuum. The ordered vanadium oxide was prepared by heating the deposited vanadium atoms for 15 min in O of 5×10−5 Pa at 623 K on Cu(100) or in 2 O of 2×10−5 Pa at 723 K on chromium oxide. 2 O gas (99.99%) from Takachiho Kagaku Kogyo 2 was introduced to the chamber through a needle tube in close proximity to the sample surface. The binding energies of core electrons were calibrated with respect to the Cu 2p peak 3/2 (932.7 eV ) of the metallic Cu. The photoelectrons by AlKa source were collected at an angle of 60° with respect to the surface normal. The Cr 2p 3/2 peaks were obtained by subtracting the background due to Auger peaks of Cu or Cu+ from the recorded spectrum. The background feature from Cu+ was obtained from Cu O formed on 2 Cu(100) after exposure to air.
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3. Results 3.1. Growth of ultra-thin chromium oxide on Cu(100) XP spectra were taken for the chromium oxide prepared in successive increments on the Cu(100) surface to monitor the deposition and growth of the oxide. Fig. 1 shows the plots of the Cr 2p , 3/2 O 1s and Cu 2p peak intensities as a function of 3/2 the total deposition time of the chromium. The linear lines changing in gradient at ~13 min suggest the growth of the chromium oxide mostly two-dimensionally up to the inflection point. The coverage of the chromium oxide at ~13 min is designated as 1.0h hereafter. The Cu 2p peak CrO 3/2 and the CuL M M Auger peaks (designated 3 4,5 4,5 as CuLMM hereafter) showed no spectral change except for attenuation of the intensity. The Cr 2p peaks at 576.0 eV, predominant up to 3/2 0.7h (Fig. 2a and c) and at 577.0 eV at CrO ~1.6h (g) can be assigned to Cr2+ and to CrO Cr3+, respectively, because the Cr 2p binding 3/2 energies are 574.2 eV for Cr metal, 576.0 eV for Cr2+, and 577.0 eV for Cr3+ in Cr O (111)/Cr(110) [15] and 577.2 eV for Cr3+ 2 3 in Cr O /NiAl O [22]. The LEED pattern (A) of 2 3 2 4
Fig. 1. Plots of the Cr 2p , O 1s and Cu 2p intensities of the 3/2 3/2 chromium oxide on Cu(100) as a function of the deposition time (total ) of the chromium atoms. The intensities at 0 min are for the oxygen preadsorbed Cu(100).
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Fig. 2. Variation of the Cr 2p and O 1s spectra for the chro3/2 mium oxide on Cu(100) as a function of the deposition time of the chromium atoms.
Fig. 3 was observed at ~1.0h , which was overCrO lapped with the substrate pattern at the lower coverage. The change in chemical state of the chromium oxide (~1.0h ) by the annealing temperature in CrO vacuum is shown in Fig. 4. The Cr 2p peak at 3/2 576.0 eV for Cr2+ increased with increase in the temperature up to 673 K, associated with a strengthening of the LEED spots. The annealing at 723 K resulted in a weakening of the LEED spots and an increase in the peak at 577.0 eV for Cr3+ ( Fig. 4g). The annealing at 773 K in vacuum was, however, required at 2.5h to gain an CrO ordered chromium oxide giving pattern (B) in Fig. 3, (앀3×앀3)R30° structure of pattern (A), and the Cr 2p peak at 577.0 eV for Cr3+ much 3/2 stronger in intensity than the peak at 576.0 eV for Cr2+. The annealing of the chromium oxide/Cu(100) in oxygen at 723 K produced a different surface oxide. The heating of the 1.5h chromium oxide CrO on Cu(100) in O of 1×10−5 Pa at 723 K for 2 15 min increased the intensity of the CuLMM Auger peak at 916.6 eV (kinetic energy), being characteristic of Cu O [23], at the expense of the 2 peak at 918.6 eV for Cu0 [(b)–(c) in Fig. 5], revealing the formation of Cu+-oxide at the surface. The Cr 2p peak at 576.0 eV for Cr2+ was 3/2 observed as a main peak. The total Cr 2p peak 3/2 intensity decreased by ~55% while the Cu 2p 3/2
Fig. 3. LEED patterns of ultra-thin oxides on Cu(100). (A) ~1.0h chromium oxide. Two hexagonal domains are scheCrO matically presented by spots of filled and open circles. (B) >2.5h chromium oxide. (C ) 0.9h vanadium oxide. (D) CrO VO 1.8h vanadium oxide. ( E ) 0.9h chromium oxide on VO CrO 0.9h vanadium oxide. VO
peak intensity at 932.7 eV increased from 24% of the clean Cu(100) to 65%. The LEED pattern was similar to pattern (A) but with stronger and sharper spots. 3.2. Interaction between ultra-thin oxides of chromium and vanadium 3.2.1. Preparation of chromium oxide on vanadium oxide and vanadium oxide on chromium oxide The vanadium oxide on Cu(100) gives the VO(111) structure at !1.0h and the VO V O (111) structure at 1.0h , giving the LEED 3 4 VO
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Fig. 4. Variation of the Cr 2p and O 1s spectra of ~1.0h 3/2 CrO chromium oxide on Cu(100) as a function of the heating temperature in vacuum.
patterns (C ) and (D), respectively, in Fig. 3 [13]. The 1.0h represents a VO(111) overlayer, conVO sisting of V2+–O2− bilayer completely covering the Cu(100) surface [13]. The growth of the chromium oxide on the well-ordered VO(111)/Cu(100) was carried out to investigate the interaction between the oxides. The preparation of 0.3h chromium oxide on the 0.9h CrO VO VO(111)/Cu(100) changed the V 2p peak at 3/2 515.1 eV (assigned to V2+ [13]) into the one at
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516.0 eV (assigned to V3+[13]) with a shoulder around 514.8 eV ( Fig. 6a and b). The Cr 2p 3/2 peak at 575.8 eV for Cr2+ was stronger than the peak at 577.2 eV for Cr3+. The sharp LEED pattern (C ) of Fig. 3 changed into pattern ( E ), corresponding to a 2×2 structure of the VO(111). The subsequent increment (total 0.6h ) of the CrO chromium oxide resulted in the sharpening of the LEED pattern between 80 and 150 eV, but the third growth of the chromium oxide (total 0.9h ) made the pattern diffuse slightly, together CrO with an increase in the Cr 2p peak at 577.0 eV 3/2 for Cr3+ as shown in Fig. 6d. The decrease in V 2p peak intensity was very small compared to 3/2 the attenuation of the Cu 2p peak by 46% after 3/2 the formation of the chromium oxide, indicating a mixing of the chromium oxide into the vanadium oxide to some extent. The heating of the chromium oxide (0.9h ) on the VO(111) CrO (0.9h ) at 723 K did not change the CuLMM VO Auger spectra and the LEED pattern even in O 2 of 5×10−5 Pa. Next, the vanadium oxide was prepared on ~1.0h chromium oxide/Cu(100). The LEED CrO pattern (A) gradually varied into the pattern like
Fig. 5. CuL M M Auger, Cr 2p and O 1s spectra for (a) clean Cu(100), (b) 1.5h chromium oxide, and (c) after subsequent 3 4,5 4,5 3/2 CrO heating at 723 K in 1×10−5 Pa O for 15 min. The Cr 2p spectra (b)∞ and (c)∞ were obtained by subtraction of the background 2 3/2 due to Cu and Cu+ from (b) and (c), respectively.
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Fig. 6. V 2p , Cr 2p and O 1s spectra for (a) 0.9h vanadium oxide and after subsequent growth of (b) 0.3 (c) 0.6 and (d) 3/2 3/2 VO 0.9h chromium oxide. CrO
( E) after deposition of the vanadium oxide from 0.2h to 0.6h . The V 2p peak for V3+ was VO VO 3/2 observed at 516.2 eV, and the Cr 2p peaks for 3/2 Cr2+ and Cr3+ were at 575.8 and 577.0 eV, respectively, as shown in Fig. 7. The peaks around 520 eV
Fig. 7. V 2p and Cr 2p spectra for (a) 1.0h chromium 3/2 3/2 CrO oxide and after subsequent growth of (b) 0.2 (c) 0.4 and (d) 0.6h vanadium oxide. VO
in the V 2p spectra were due to the O 1s peak 3/2 by AlKa radiation. Further growth of the vana3,4 dium oxide made the LEED pattern diffuse. No change was observed with the CuLMM Auger spectra during the heating in oxygen. 3.2.2. Oxidation of mixture of vanadium and chromium atoms The chromium–vanadium oxide was prepared in a different way. Vanadium atoms (equivalent to 0.5h ) deposited on the oxygen chemisorbed VO Cu(100) surface were oxidized by exposure to O 2 (1×10−5 Pa) at 310 K for 200 s. Chromium atoms (equivalent to 0.5h ) were subsequently deposited CrO on the surface and oxidized by exposure to O 2 (1×10−5 Pa) for 200 s. Annealing of the oxide at 723 K for 15 min in vacuum gave a strong LEED pattern like the VO(111). The Cr 2p peak at 3/2 575.8 eV for Cr2+ and the V 2p peak at 515.2 eV 3/2 for V2+ were predominant. The subsequent growth of the same amount of chromium–vanadium oxide under the same conditions changed the LEED pattern into the 2×2 pattern. The Cr3+ was comparable in the amount with the Cr2+. The V 2p peak at 516.2 eV for V3+ was observed 3/2 with a weak shoulder at the lower binding energy side.
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4. Discussion 4.1. Structure and chemical state of chromium oxide on Cu(100) The LEED pattern (A) of Fig. 3 for the 1.0h chromium oxide, which grew two-dimenCrO sionally, can be resolved into two hexagonal meshes, as shown in Fig. 3, and assigned to the chromium oxide of a hexagonal structure with two domains at 90° aligned along the [0,1,1] and [0,−1,1] directions of the Cu(100) surface, respectively. In contrast, only one domain was formed for the chromium oxide on Cu(110) [21]. The difference reveals an influence exerted on the angular orientation of the overlayer by the structure of the substrate surface. Pattern (A) is similar to that for the 1.0h VO(111) on Cu(100), pattern (C ) VO in Fig. 3, although the spots for the chromium oxide were not sharp and strong. The clearest spots of the pattern (A) were obtained when the Cr2+/Cr3+ ratio was largest after the heating at 623–673 K in vacuum ( Fig. 4), indicating a correlation between the pattern (A) and Cr2+. The chromium oxide showing the pattern (A) was then assigned to a CrO(111) with the same structure as the VO(111), as illustrated by Fig. 8a. The lattice constant for the CrO(111) was calculated as 0.30 nm. In the case of bulk ionic oxides such as CrO and VO, the polar surfaces of CrO(111) and VO(111) are inherently unstable due to the divergence of the electrostatic surface potential [24–
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26 ]. However, the CrO(111) at 1.0h and CrO VO(111) at 1.0h on the Cu(100) is considered VO to be stabilized by a compensation of its electrostatic dipole by an image dipole in the copper substrate, as has been discussed for a similar hexagonal close-packed FeO overlayer with oxygen on top (slightly distorted) on Pt(100)-hex-R0.7° [5]. The CrO(111) surface may be partly stabilized by the adsorption of OH−, as in the case for NiO [27], as suggested by the tailing of the O 1s peak around 531.5 eV. The weaker spots of pattern (A) than those for the VO(111) and the presence of the Cr3+ suggests that patches of islands of Cr O - and/or Cr O -like oxides coexisted with 3 4 2 3 the CrO(111) because the untypical Cr2+-oxide is rather unstable. The larger attenuation of the Cu 2p peak intensity at 1.0h , 52% in Fig. 1, 3/2 CrO than 43% for the VO(111) [13] is probably due to the coexistence of the oxides at a higher oxidation state. The formation of the (앀3×앀3)R30° structure of the CrO(111) in Fig. 3B, at the coverage of 2.5h , implies a commencement of the formation CrO of Cr O (111) surface, as shown by Fig. 8b. This 2 3 was in accordance with the predominant Cr 2p 3/2 peak at 577.0 eV for Cr3+. The presence of the weak peak at 576.0 eV for Cr2+ means the coexistence of the chromium oxide containing Cr2+. The lattice constant obtained from pattern (C ), 0.52 nm, is slightly larger than the 0.50 nm for the Cr O (111) of the bulk oxide. The much more 2 3
Fig. 8. Schematic representation of oxide structures. (a) VO(111) or Cr(111), (b) Cr O (111) and (c) one termination of the 2 3 Fe O (111) surface [36 ]. 3 4
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ordered Cr O (111) was formed on Cu(110) at a 2 3 coverage greater than 2h under the same prepaCrO ration conditions [21].
bulk Cu O is usually unstable at a temperature 2 such as 723 K in vacuum[35]. 4.3. Formation of chromium–vanadium oxide
4.2. Segregation of copper oxide Heating of the 1.5h chromium oxide on CrO Cu(100) in O (1×10−5 Pa) at 723 K for 15 min 2 resulted in the oxidation of the copper surface to Cu+ ( Fig. 5). When the Cu(100) surface with no chromium oxide overlayer was treated with oxygen under the same conditions, a reconstructed (앀2×앀2)R45° structure induced by chemisorbed oxygen [28–31] was obtained. The Cu LMM Auger peak showed no formation of Cu+. The ultra-thin chromium oxide facilitated the oxidation of the Cu(100) surface in oxygen at a low pressure such as 1×10−5 Pa, whereas the oxidation of the copper substrate was not observed for the thin oxide overlayers of vanadium [13], titanium [32] and iron [33] on the Cu(100) after heating in oxygen under the same conditions. The large increase in Cu 2p intensity at 932.7 eV can be 3/2 interpreted by an accumulation of Cu+-oxide at the topmost layer on the chromium oxide because the Cu 2p peak for metallic copper mostly over3/2 laps with that for the Cu+-oxide; the chemical shift of the Cu 2p peak for Cu O is 0.1 eV [34] 3/2 2 or 0.3 eV [23]. If the Cu+ ions locate under the chromium oxide, the intensity of the Cu 2p peak 3/2 at 932.7 eV will not increase. The decrease in the Cr 2p peak also supports the presence of the 3/2 Cu+ on the chromium oxide. The main Cr 2p peak at 576.0 eV (Fig. 5) 3/2 indicates that the chromium oxide of Cr2+ was stabilized between the copper substrate and the Cu+ oxide overlayer. The sharp and intense LEED pattern like (A) after the segregation of the Cu+ indicates the formation of well-ordered Cu+-oxide grown probably epitaxially on the CrO(111). The stabilization of the surface structure was not due to the adsorption of OH− since the O 1s peak changed into a single peak at 530 eV due to O2−. The Cu+-oxide layer was stable on the chromium oxide during heating at 723 K for 15 min in vacuum, although the surface of the
The formation of the 0.6h chromium oxide CrO on the 0.9h vanadium oxide changed the VO VO(111) structure into the 2×2 structure, as shown by the change in the LEED pattern, from (C ) to ( E ) in Fig. 3, as observed during the increase in the vanadium oxide coverage on Cu(100) [13]. A similar change in the structure with the oxide coverage was observed for iron oxide on Pt(100) and (111), from FeO(111) to Fe O (111) [2]. The 3 4 present 2×2 surface structure suggests a formation of the mixed vanadium–chromium oxide with a surface structure like V O (111), Cr O (111) and 3 4 3 4 Fe O (111). One of the candidates for the surface 3 4 structure of the vanadium–chromium oxide is one such as that in Fig. 8c, which, it has been concluded, is one of the two terminations for the (111) surface of magnetite ( Fe O ) [36 ]. In the 3 4 chromium–vanadium oxide at 0.3h , the V3+ CrO was larger than V2+ ( Fig. 6b) in number of ions, while the Cr2+ was larger than Cr3+. The increase in the ratio of the Cr3+ with the increase in the amount of the chromium oxide (c and d) is correlated with the diffusion of the LEED pattern at 0.9h (total ) since the V O and Cr O have a CrO 2 3 2 3 different structure and lattice constant to those of the V O and Cr O . 3 4 3 4 The surface structure resembling Fe O (111) 3 4 was also formed for the 0.6h vanadium oxide VO on the ~1.0h chromium oxide, as shown by the CrO sharp and strong LEED pattern ( E), although the ordered Cr O (111) structure could not be 3 4 obtained on Cu(100) for the pure chromium oxide. Most of the vanadium ions were in the 3+ state, but nearly half of the chromium ions were in 2+ ( Fig. 7). Annealing (in a vacuum) of the chromium (0.5h ) and the vanadium (0.5h ) oxides that CrO VO had been oxidized beforehand at 310 K gave a well-ordered structure like VO(111)/Cu(100). Most of the chromium and vanadium ions were in a divalent state, as shown by the Cr 2p peak 3/2 at 575.8 eV and the V 2p peak at 515.2 eV, 3/2 indicating that the mixing of the vanadium ions
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stabilized the Cr2+ state and then the structure like VO(111), even at such a low total coverage. The subsequent growth of the same amount of chromium–vanadium mixed oxide gave the stable 2×2 structure of the VO(111). Most of the vanadium ions were in the V3+ state ( V 2p peak at 3/2 516.2 eV ), and the Cr3+ (Cr 2p peak at 3/2 576.8 eV ) was comparable in the amount with the Cr2+(at 576.0 eV ). The oxidation of the copper substrate was not found during heating in O when the chromium 2 oxide on Cu(100) was mixed with the vanadium oxide, indicating that the addition of vanadium oxide to chromium oxide can prevent oxidation of the Cu substrate.
5. Conclusions Ultra-thin chromium oxide and chromium– vanadium mixed oxide films grown on Cu(100) have been characterized by means of XPS, X-AES and LEED. The chromium oxide prepared by exposing the deposited chromium atoms to O 2 (~20 L) and subsequent annealing at 673 K in vacuum shows two-dimensional growth of the oxide. Up to ~1.0h , the chromium oxide is CrO formed mainly as CrO(111), showing the diffuse hexagonal LEED pattern. The ratio of the coexisting Cr3+-oxide increases with increasing coverage. The Cr O (111) is formed at more than 2.5h to 2 3 CrO give the (앀3×앀3)R30° structure of the CrO(111). The copper substrate is not oxidized at any of the steps. The chromium oxide/Cu(100) surface is not stable in oxygen at a high temperature such as 673 K. The Cu+–oxide segregates onto the CrO(111), although the oxidation of the copper is not found for the ultra-thin Ti, V and Fe oxide films on Cu(100) under the same conditions. The Cu+-oxide overlayer is stable in UHV at a high temperature such as 673 K. The mixing of the vanadium ions into the chromium oxide leads to the formation of wellordered and stable chromium–vanadium oxides, whose structures are similar to the VO(111) or VO(111)-(2×2), depending on the amounts of the
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oxide, and prevents oxidation of the copper substrate.
References [1] A. Barbieri, W. Weiss, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 302 (1994) 259. [2] G.H. Vurens, V. Maurice, M. Salmeron, G.A. Somorjai, Surf. Sci. 268 (1992) 170. [3] W. Weiss, A. Barbieri, M.A. Van Hove, G.A. Somorjai, Phys. Rev. Lett. 71 (1993) 1848. [4] W. Weiss, G.A. Sormorjai, J. Vac. Sci. Technol. A 11 (1993) 2138. [5] M. Ritter, H. Over, W. Weiss, Surf. Sci. 371 (1997) 245. [6 ] H. den Daas, M. Passacantando, L. Lozzi, S. Santucci, P. Picozzi, Surf. Sci. 317 (1994) 295. [7] H. den Daas, O.L.J. Gijzeman, J.W. Geus, Surf. Sci. 285 (1993) 15–26. [8] A.B. Boffa, H.C. Galloway, P.W. Jacobs, J.J. Benı´tez, J.D. Batteas, M. Salmeron, A.T. Bell, G.A. Sormojai, Surf. Sci. 326 (1995) 80. [9] W.S. Oh, C. Xu, D.Y. Kim, D.W. Goodman, J. Vac. Sci. Technol. A 15 (1997) 1710. [10] L. Zhang, M. Kuhn, U. Diebold, Surf. Sci. 375 (1997) 1. [11] L. Zhang, M. Kuhn, U. Diebold, J. Vac. Sci. Technol. A 15 (1997) 1576. [12] K.B. Lewis, S.T. Oyama, G.A. Somorjai, Surf. Sci. 233 (1990) 75. [13] K. Kishi, Y. Hayakawa, K. Fujiwara, Surf. Sci. 356 (1996) 171. [14] K. Kishi, K. Fujiwara, J. Electron Spectrosc. Relat. Phenom. 85 (1997) 123. [15] C. Xu, M. Hassel, H. Kuhlenbeck, H.-J. Freund, Surf. Sci. 258 (1991) 23. [16 ] C.A. Ventrice Jr., D. Ehrlich, E.L. Garfunkel, B. Dillmann, D. Heskett, H.-J. Freund, Phys. Rev. B 46 (1992) 12892. [17] H. Kuhlenbeck, C. Xu, B. Dillmann, M. Hassel, B. Adam, D. Ehrlich, S. Wohlrab, H.-J. Freund, U.A. Ditzinger, H. Neddermeyer, M. Neuber, M. Neumann, Ber. Bunsenges Phys. Chem. 96 (1992) 15. [18] M. Bender, I.N. Yakovin, H.-J. Freund, Surf. Sci. 365 (1996) 394. [19] P. Michel, C. Jardin, Surf. Sci. 36 (1973) 478. [20] S. Ekelund, C. Leygraf, Surf. Sci. 40 (1973) 179. [21] A. Maetaki, K. Kishi, Surf. Sci. 411 (1998) 35. [22] A.M. Venezia, C.M. Loxton, J.A. Horton, Surf. Sci. 225 (1990) 195. [23] D. Briggs, M.P. Seah ( Eds.), Practical Surface Analysis, second ed., Vol. 1, Wiley, New York, 1990. [24] P.W. Tasker, J. Phys. C 12 (1979) 4977. [25] D. Wolf, Phys. Rev. Lett. 68 (1992) 3315. [26 ] C.A. Ventrice Jr., Th. Bertrams, H. Hannemann, A. Brodde, H. Neddermeyer, Phys. Rev. B 49 (1994) 5773. [27] D. Cappus, M. Hasßel, E. Neuhaus, M. Heber, F. Rohr, H.-J. Freund, Surf. Sci. 337 (1995) 268. [28] M. Wuttig, R. Franchy, H. Ibach, Surf. Sci. 213 (1989) 103.
88
A. Maetaki et al. / Surface Science 445 (2000) 80–88
[29] H.C. Zeng, R.A. McFarlane, K.A.R. Mitchell, Surf. Sci. 208 (1989) L7. [30] M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, T. Lindner, J.C. Somers, D.E. Ricken, A.M. Bradshaw, Surf. Sci. 236 (1990) 1. [31] F.M. Leibsle, Surf. Sci. 337 (1995) 51. [32] T. Maeda, Y. Kobayashi, K. Kishi, Surf. Sci. 436 (1999) 249.
[33] A. Maetaki, S. Kadowaki, T. Tawa, K. Kishi, submitted for publication. [34] P.E. Larson, J. Electron Spectrosc. Relat. Phenom. 4 (1974) 213. [35] D.L. Cocke, G.K. Chuah, N. Kruse, J.H. Block, Appl. Surf. Sci. 84 (1995) 153. [36 ] A.R. Lennie, N.G. Condon, F.M. Leibsle, P.W. Murray, G. Thornton, D.J. Vaughan, Phys. Rev. B 53 (1996) 10244.