Reconstruction of Cu(111) Induced by a Hyperthermal Oxygen

May 14, 2008 - Kousuke Moritani , Michio Okada , Yuden Teraoka , Akitaka Yoshigoe and Toshio Kasai. The Journal of Physical Chemistry A 2009 113 (52),...
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8662

J. Phys. Chem. C 2008, 112, 8662–8667

Reconstruction of Cu(111) Induced by a Hyperthermal Oxygen Molecular Beam Kousuke Moritani,†,* Michio Okada,‡,§ Yuden Teraoka,† Akitaka Yoshigoe,† and Toshio Kasai‡ Synchrotron Radiation Research Center, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan, Department of Chemistry, Graduate School of Science, Osaka UniVersity, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: March 14, 2008

This paper reports a study on the surface reconstruction on Cu(111) induced by a hyperthermal oxygen molecular beam (HOMB) at room temperature (RT). HOMB incidence at translational energies (Ei) g 0.5 3 2 eV induced surface reconstruction to the |-1 2| structure for an O coverage (Θ) g 0.27 monolayer (ML). On the other hand, long-range-ordered structures were not formed even at Θ ≈ 0.4 ML for the backfilling of thermal O2 at RT. No surface reconstruction was induced at Ei e 0.23 eV where the attainable Θ value was 0.27 ML for the O2 exposures below 1018 molecules/cm2. Translational energy above 0.5 eV is required for the dissociative adsorption of O2 and the resulting surface reconstruction at Θ g 0.27 ML. The O-1s XPS peak for HOMB incidence at RT was resolved into two components, 529.4 and 528.9 eV, at Θ g 0.27 ML, which can be assigned to the O atoms occupying the three-fold hollow sites on unreconstructed Cu(111) and 3 2 3 2 four-coordinated sites on the |-1 2| structure, respectively. Annealing the |-1 2| reconstructed surface at 620 K decreased Θ to ∼0.27 ML and induced (√13R46.1° × 7R21.8°), the so-called “29” superstructure. 1. Introduction The physical and chemical properties of a material surface depend on the surface structures. Therefore, adsorbate-induced reconstructions of transition metals have been widely investigated.1 Cu oxides play important roles in a wide range of applications, including solar cells,2 microelectronics,3 and catalysis for oxidation.4 These applications have motivated us to study O-induced reconstructions on the low-index faces of Cu. The oxidation of Cu(111) is not well understood, although numerous experimental5–25 and theoretical26–29 studies have been devoted to this topic. On Cu(111), an O2 molecule adsorbs dissociatively at room temperature (RT).6–8 Because the high coordination of the closely packed Cu(111) face makes it stable, it is anticipated that the reactivity of gaseous O2 is lower on (111) than on open (001) and (110).9 The dissociative adsorption of O2 on Cu(111) is accompanied by complex reconstructions, and thus, the O-induced surface structures and the surface oxide formation processes on Cu(111) remain controversial. Some groups have reported that long-range-ordered structures are not formed at RT in O2 backfilling.10–14 The saturation coverage of O has been estimated as ∼0.29 monolayer (ML) using nuclear reaction analysis (NRA).11 Moreover, it has been suggested that chemisorbed O occupies a three-fold hollow site12,13 and causes the lateral expansion of the copper lattice.13–15 To form ordered structures in O2 backfilling, a high temperature is necessary. Several O-induced structures on Cu(111) have been reported at elevated temperatures.16–20 Furthermore, low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) have revealed the (√13R46.1° × 7R21.8°) and * Corresponding author. Present address: Department of Mechanical and System Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan. E-mail: [email protected]. † Japan Atomic Energy Agency. ‡ Osaka University. § PRESTO.

(√73R5.8° × √21R - 10.9°) structures.16–19 These superstructures are denoted by “29” and “44” structure, respectively, because they have 29 and 44 times larger unit cells than the (1 × 1) unit cell, respectively. For the 29 structure, multipledomain19 and single-domain20 structures have been reported. STM measurements have also revealed that the two-dimensional (2D) oxide of the 29 structure is initiated at the step edge and grows in the lateral direction.19,20 The saturation coverage of O on the 29 structure has been elucidated to 0.52 ML with NRA.17 These studies have proposed a one layer Cu2O(111) structure, which consists of three O and two Cu layers. A normal incidence X-ray standing wavefield absorption (NIXSW) study has suggested that the Cu2O-like local geometry exists in the disordered chemisorption phase of O/Cu(111) and that this geometry should be a precursor to Cu2O formation on Cu(111).21 On the other hand, Judd et al. have reported a significantly 3 2 different |-1 2| LEED pattern even at RT for high O2 exposures 5 (>10 L; 1 L ) 1.33 × 10-4 Pa · s) and have proposed that the structure involves a pseudo-Cu(001) structure where O atoms occupy four-fold hollow sites.22 This ordered LEED pattern has also been observed in our previous studies of oxygen adsorption using hyperthermal oxygen molecular beam (HOMB) at RT.23,24 Toomes et al. have reported that O with a four-fold Cu coordinated local geometry exists on O/Cu(111) using a photoelectron diffraction (PhD) investigation of the disordered 3 2 phase.25 However, the exact structure of the |-1 2| structure and what induces this structure remain unclear. Recently, HOMB-induced oxidation of low-index Cu surfaces at RT has been reported.23,24,30–33 The energy range of 0.1-2 eV for HOMB is comparable to the activation barrier of surface processes, for example, the dissociation of a gaseous molecule and diffusion of adatom on transition metals. However, the HOMB energy range is much lower than the required energy for sputtering.34 Therefore, it is speculated that HOMB can control surface process without damaging the substrate. To date, we have demonstrated that HOMB can induce collision-induced

10.1021/jp800713u CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008

Reconstruction of Cu(111) Induced by Molecular Beam

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absorption on Cu surfaces30 and the effective dissociative chemisorption of O2.31–33 Thus, highly energetic molecules can efficiently control the surface processes and furthermore, induce surface reconstruction.23,24 However, the role of energetic O2 in the HOMB-induced reconstruction has not been clearly understood. In this paper, we demonstrate that HOMB incidence at translational energies (Ei) g 0.5 eV at an O coverage (Θ) of 0.4 ML induces the surface reconstruction to triple-domain |3-1 22| structures, but that O2 at Ei e 0.23 eV does not induce surface reconstruction. This translational energy of 0.5 eV is higher than the theoretically calculated activation barrier of 0.2 eV26 for the dissociation. On the other hand, the energetic Ar beam induce no reconstructions of O-preadsorbed Cu(111). This result suggests that the chemical processes contribute to the surface reconstruction in addition to the mechanical energy transfer into the substrate during the collision. Enough translational energy is required for O2 to overcome the activation barrier of dissociation, and the resulting energy dissipation in an exothermic reaction induces the reconstruction. After annealing at 620 3 2 K, the |-1 2| structure changed to the 29 structure. These experimental results reveal that HOMB can induce the metastable surface structure which cannot be produced in the thermal equilibrium process. 2. Experimental Section All experiments were performed with a surface reaction analysis apparatus (SUREAC2000) constructed in BL23SU at SPring-8.35 The surface reaction analysis chamber was equipped with an electron energy analyzer (OMICRON EA125) and a Mg/Al KR twin-anode X-ray source (OMICRON DAR400). A supersonic-molecular-beam apparatus, which had a differentially pumped chamber, was conjunct with the soft-X-ray synchrotron beamline. A differentially pumped quadrupole mass spectrometer, which was used to analyze the molecular species in the HOMB, was located on the line of the molecular beam axis. The base pressure of the surface reaction chamber was about 2 × 10-8 Pa. The details of the experimental apparatus are described elsewhere.36 The 15 × 15 mm2 Cu(111) sample was cleaned by repetition of 1.5 keV Ar+ sputtering and annealing at 870 K until impurities were not detected by XPS with synchrotron radiation and the low-energy electron diffraction (LEED) showed a sharp (1 × 1) pattern with a low background. The thermal energy (25 meV) O2 molecules were dosed on Cu(111) by backfilling an O2 atmosphere at 6.7 × 10-6 to 1.3 × 10-3 Pa. The hyperthermal energy (g0.1 eV) O2 molecules were dosed by HOMB incidence along the surface normal. The details of HOMB are described elsewhere.23,33 Briefly, the energies of O2 molecules are varied by seeding in He and/or Ar and by changing the nozzle temperature. The nozzle temperature was 300 K except for the 1.0 eV HOMB, which was generated at a nozzle temperature of 1400 K. The beam diameter at the sample position was about 9 mm. The flux density of O2 at the sample position was experimentally estimated for each experimental condition using an estimation method described elsewhere.37 Under our operating conditions, the typical flux density of O2 molecules at the sample surface was 7 × 1013, 2 × 1014, 1 × 1015, and 3 × 1014 molecules · cm-2 · s-1 at Ei ) 0.1, 0.23, 0.5, and 1.0 eV, respectively. The background pressure and parasite O2 pressure during the HOMB incidence were ∼5 × 10-6 and ∼5 × 10-8 Pa, respectively. After dosing with the proper amount of O2, the high-resolution XPS spectra were measured using synchrotron radiation with a photon energy of 890 eV

Figure 1. O-coverage dependence of O-1s XPS spectra for 0.5 eV HOMB incidence along the surface normal at RT on a Cu(111) surface. Peak components of A and B are reproduced by a single Voigt component.

and the LEED patterns were observed at RT. All measurements were performed at RT. 3. Results 3.1. Oxidation by HOMB Incidence at RT. Figure 1 shows the typical evolution of the O-1s XPS spectra on Cu(111) during 0.5 eV HOMB incidence along the surface normal. The Θ value was determined by integrating the O-1s XPS peak, and Θ was calibrated using the O-1s XPS spectrum of Cu(100)-(22 × 2)R45°-O (0.5 ML) and by considering the face-dependent atomic density. For Θ < 0.27 ML, an O-1s peak, which fitted well with a single component of the Voigt function, appeared at a binding energy of 529.4 eV (peak A). The intensity of peak A increased as Θ increased up to ∼0.27 ML, which is the tentative saturation coverage in backfilling of thermal O2 at RT.23,24 Moreover, a new O-1s component appeared at 528.9 eV (peak B) for Θ g 0.27 ML. Peak B increased, while peak A decreased with increasing Θ, and finally the surface became saturated at Θ ) ∼0.4 ML. Parts a and b of Figure 2 show the observed LEED pattern at Θ ) 0.42 ML for the 0.5 eV HOMB incidence and a 3 2 schematic representation of the diffraction spots for the |-1 2| pattern, respectively. The dotted lines represent unit cells for a single domain. The triple-domain |3-1 22| LEED pattern was clearly observed at Θ ) 0.42 ML. It should be noted that the saturation coverage by the HOMB at Ei e 0.23 eV was 0.27 ML, and only peak A appeared in XPS and a (1 × 1) LEED pattern was observed. These results suggest that energetic O2 induces the 3 2 reconstruction to the |-1 2| superstructure on Cu(111) at Θ g 0.27 ML.

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Figure 2. (a) LEED pattern (Ep ) 86.5 eV) of the Cu(111) surface at Θ ) 0.42 ML prepared by 0.5 eV HOMB incidence at RT. (b) Schematic representation of the reciprocal lattice of the structure. Dotted lines represent unit cells for a single domain, which were intensified after annealing. (See text.)

Figure 3. O-uptake curves for 1.0, 0.5, 0.23, and 0.1 eV HOMB incidence along the surface normal and for the thermal O2 exposure on Cu(111) at RT. Uptakes are determined by integrating the O-1s XPS spectra. The ML unit represents the number of atoms per surface Cu atom.

Figure 3 shows the translational energy dependence of the O-uptake curves during the HOMB incidence and thermal O2 exposure. Each uptake curve was produced from the integration of the O-1s XPS. The slope of the uptake curve represents the rate of oxygen adsorption. The uptake curves for HOMB showed the characteristic tendency with the translational energy of O2. The rate of oxygen adsorption for HOMB was smaller at Ei ) 0.1 eV, but greater at Ei g 0.23 eV than that for thermal O2 exposure. A more important point is that the saturation coverage was only 0.27 ML for HOMB at Ei e 0.23 eV, while the Θ value increased up to 0.4 ML for HOMB at Ei g 0.5 eV. For the backfilling of thermal O2, the surface was tentatively saturated at Θ ) ∼0.27 ML, and a very large exposure of ∼2 × 1019 molecules · cm-2 was required to attain a coverage of Θ ) 0.4 ML. It should be noted that an ordered structure was not observed in the LEED pattern during thermal O2 exposure even at Θ ) 0.4 ML.38 These results suggest that a sufficient translational energy O2 is required in order for oxygen to induce reconstruction on a Cu(111) surface at Θ g 0.27 ML. 3.2. Surface Structure at Elevated Temperature. Figure 4 show the O-1s XPS peaks and the corresponding LEED

3 2 patterns after annealing the |-1 2| surface (Θ ) 0.36 ML) prepared with the 0.5 eV HOMB at RT. After annealing at 520 K, peaks A and B coexisted in the O 1s XPS peak as shown in Figure 4a, but the triple-domain LEED pattern changed to a sharp single-domain pattern as shown in Figure 4c. It should 3 2 be noted that the single-domain |-1 2| structure and the multiple domains of a new long-range structure coexist in the LEED pattern of Figure 4c. The highlighted LEED spots in parts c and d of Figure 4 correspond to the same spots. However, the LEED pattern of the long-range structure in Figure 4d could not be assigned in this work because the spots were very weak. After annealing at 620 K, peak B disappeared almost completely, while peak A increased, as shown in Figure 4b. Θ decreased to 0.28 ML, which corresponds to the saturation coverage for the 3 2 HOMB incidence at Ei e 0.23 eV. On this surface, the |-1 2| structure disappeared, but the single-domain 29 structure newly appeared, as shown in Figure 4e.39 Figure 4f shows a schematic representation of the reciprocal lattice. The O-1s XPS peak and its corresponding LEED pattern at the elevated temperatures suggest that peak B corresponds to the emission from the O 3 2 atoms on the |-1 2| structure, while peak A corresponds to that on the other structures and includes the three-fold hollow site and the 29 structure, which have the hexagonal symmetry of the Cu atoms.17–21

4. Discussion 4.1. Oxidation Processes by HOMB. We measured the O-1s XPS peaks on Cu(111) and produced uptake curves by the evolution of the O-1s peaks during HOMB incidence and thermal O2 exposure. From the uptake curves, the evolution of Θ on the Cu(111) surface can be separated into three stages: Θ < 0.27 ML, Θ ) 0.27-0.4 ML, and Θ > 0.4 ML. In the initial stage at Θ < 0.27 ML, the O-1s peak appeared at the binding energy of 529.4 eV as Θ increased, and an ordered structure was not observed in the LEED pattern. In this region, O2 adsorbs dissociatively on a three-fold hollow site without surface reconstruction. Theoretical26,29 and spectroscopic studies12,13 support O adsorption on the three-fold hollow site at low Θ. Thus, peak A can reasonably be assigned to O atoms at the three-fold hollow site. In addition, a theoretical study has indicated that the binding energy of O atoms on Cu(111) decreases sharply as Θ increases above 0.25 ML due to the repulsive interaction between the adsorbates on the (111) terrace.29 The saturation coverage of 0.27 ML at Ei e 0.23 eV

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Figure 4. O-1s XPS spectra for annealing the O/Cu(111) sample (Θ ) 0.36 ML) at (a) 520 and (b) 620 K. The O/Cu(111) sample was prepared 3 2 by 0.5 eV HOMB incident along the surface normal at RT. Similar O-1s spectra at Θ ) 0.42 ML as shown in Figure 1 and the |-1 2| LEED pattern, which had triple unique domains as shown in Figure 2, were observed before annealing. (c) LEED pattern at the primary energy of 86.5 eV and (d) primary energy of 48.5 eV taken after annealing at 520 K. Circled areas indicate the same spots. (e) LEED pattern at a primary energy of 86.5 eV was taken after annealing at 620 K. (f) Reciprocal lattice.

suggests that O2 cannot overcome the activation barrier, which is enhanced by the repulsive interaction. In this region, the kinetic energy of O2 is one of the important parameters for the rate of oxygen adsorption. The theoretical study26 proposed the reaction path of the O2 dissociation via a molecularly chemisorbed precursor on Cu(111). The activation barrier for the dissociation of O2 was estimated as 0.20 eV/O2 at the most favorable configuration of O2 and surface sites.26 In this activated adsorption system, thermal O2 with a kinetic energy of 25 meV cannot directly overcome this activation barrier. The reaction model via the precursor states of O2 on Cu(111) has been also suggested by an experimental study.8 For the 0.1 eV HOMB incidence, the rate of oxygen adsorption decreased compared with that for thermal O2 exposure. Impinging O2 with sufficient kinetic energy will be scattered from the surface before the impinging O2 is trapped on the surface. On the other hand, the rates of oxygen adsorption for the 0.23, 0.5, and 1.0 eV HOMB were larger than that for thermal O2 exposure. Because the translational energy above 0.23 eV is sufficient to overcome the activation barrier of 0.20 eV, O2 can dissociate in a direct process, which enhances the rate of oxygen adsorption. For the 1.0 eV HOMB generated by the nozzle at 1400 K, the effect of vibrationally and rotationally excited states in the incident beam cannot be neglected in the dissociative adsorption. These excited internal states, in addition to the high translational energy, may assist the dissociation via a direct adsorption process.33 In the second stage at Θ ) 0.27-0.4 ML, HOMB irradiation at Ei g 0.5 eV was accompanied with the surface reconstruction 3 2 3 2 to the |-1 2| structure. For the |-1 2| LEED pattern, a pseudoCu(001)-O model where O atoms are placed in four-fold hollow sites on a Cu(001)-like structure formed on the Cu(111) surface has been proposed.22 Although a pseudo-(001) reconstruction on the (111) surface has been observed for C, N, and S adsorption on Cu and Ni,1 such reconstruction is not favorable

for O/Cu(111) because the stability on the surface and in the bulk for the O/Cu system is dominated by -Cu-O-Cu- chains along the [100] direction, which are unavailable on the Cu(111) surface. However, a structure with a four-coordinated O atom 3 2 has been suggested for the |-1 2| structure because a binding energy of 528.9 eV for peak B agrees very well with that for O atoms adsorbed at the four-fold hollow site on a unreconstructed Cu(001) surface at low coverage.30 Such a Cu(001)-like reconstruction has also been suggested in the photoelectron diffraction (PhD) study of the disordered chemisorption phase on O/Cu(111) by Toomes et al.25 In their structure model, O atoms were placed in four-fold coordinated hollow sites on the (001)-type Cu overlayer above a unreconstructed (111) surface. Figure 5 schematically shows the tentative structural model for the reconstructed O/Cu(111) surface induced by HOMB at RT. In this stage, oxidation by the thermal O2 exposure proceeded rather gradually. In our experiments, ordered structures were not observed even at 0.4 ML for the thermal O2 exposure, suggesting that HOMB induces surface reconstruction differently from the thermal O2 process. Details of the HOMB-induced process are discussed in Section 4.2. In the final stage at Θ > 0.4 ML, oxidation required a larger O2 translational energy. The attainable saturation coverage for the 0.5 eV HOMB was about 0.4 ML. In previous studies, we have demonstrated that higher energetic O2, which is supplied by 2.3 eV HOMB, increases the Θ value even above 0.4 ML.23,24 In the region above 0.4 ML, the Cu2O component begins to grow by a collision-induced absorption process.30 4.2. Effect of Translational Energy of O2 in the HOMBInduced Reconstruction. We found that HOMB at Ei g 0.5 3 2 eV could induce the reconstruction to the |-1 2| structure at Θ g 0.27 ML, but it was hard to produce reconstruction by thermal O2 exposure. Although Judd et al. have found that a huge thermal O2 dose of 106 L (∼1021 molecules · cm2) is necessary at RT to induce |3-1 22| reconstruction,22 our experimental condi-

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3 2 Figure 5. Structural models for the |-1 2| oxide phase on the Cu(111) surface. Large gray circles and small black circles represent the Cu and O atoms, respectively. The intensity of shading in the copper atoms on the top layer indicates the relative heights of these atoms. The dotted 3 2 lines represent a |-1 2| unit cell.

tions only required an O2 dose of ∼1 × 1017 molecules · cm2. On the other hand, it has also been reported that thermal O2 exposure at RT produces the disordered phase and that the 29 structure is produced after subsequent annealing above ∼600 3 2 K, while reconstruction to the |-1 2| structure cannot be 16,17,19,20 reproduced. The dissociative adsorption occurs in the normal-energy scaling for various diatomic molecules.40 On the other hand, the total Ei of O2 contributes to the observed HOMB-induced reconstruction, because the 0.5 eV HOMB in the oblique angle incident at 40° from the surface normal, where the normal energy is only 0.2 eV,41 induced the reconstruction of Cu(111). The total energy scaling can be explained by the large surface corrugation of interaction potential42 during the island formation (vide infra). In order to clarify the possible mechanism of HOMB-induced reconstruction, we measured the O-1s spectra and the LEED pattern after the 0.5 eV Ar beam incidence along the surface normal on Cu(111) of Θ ) 0.27 ML, which was prepared by thermal O2 exposure at RT. In this case, reconstruc3 2 tion to the |-1 2| structure was not induced on the surface, suggesting that HOMB-induced reconstruction cannot be explained by a process where the energetic O2 dissipates its translational energy only mechanically to the disordered phase on the terrace at Θ ) 0.27 ML and the resulting local heating 3 2 induces the reconstruction to the |-1 2| structure. One possible reaction model for the HOMB-induced reconstruction process is as follows. The growth of the |3-1 22| structure is initiated from the oxygen adsorbed at step edges because peak B does not increase before the tentative saturation coverage of 0.27 ML. On the Cu(111) surface, STM studies have indicated that the step edges play a crucial role for oxide growth19,20 as these studies have revealed that the step edge along the [11j0] direction initiates the nucleation of 2D oxide. Moreover, the role of the open step for Cu2O formation has been revealed on a Cu(410) surface.43 The simultaneous increase in peak B and

Moritani et al. decrease in peak A in Figure 1 suggests that HOMB promotes 3 2 the growth of the |-1 2| structure from the step edges onto the (111) terrace. The Ei dependence of the O-uptake curves suggests that there is a threshold of energy for HOMB-induced reconstruction between 0.23 and 0.5 eV. However, the translational energy of 0.5 eV is probably not high enough to induce the reconstruction via mechanical energy transfer, because an energetic Ar beam incidence induced no reconstructions to the 3 2 |-1 2| structure. The translational energy probably assists O2 to overcome the activation barrier of dissociation at the step edge and consequently combines with the low coordinated Cu atom. The translational energy of 0.5 eV is enough to overcome the barrier, even though the O-O repulsion will increase the dissociation barrier from the theoretical 0.2 eV26 at a high Θ value. The chemical energy released in the chemisorption of O atom, which achieves several electron volts,26 is locally dissipated at the low-coordinated Cu atoms to possibly break Cu-Cu bonds and produce Cu adatom and Cu-O components. A similar step formation process induced by adsorption has also been reported on CO/Pt(110) where a CO molecule, which is attracted to a step atom by an energy comparable to the step formation energy, induces disordering.44 Many experimental43,45–47 and theoretical27,48 studies have indicated that O atom prefers to adsorb at the four-fold hollow site in the {001} faceted step edge where Cu-Cu spacing exists along [100], which is the most favorable direction for -Cu-O-Cu- row formation. Cu adatoms and Cu-O components, which are mobile even at RT,49 stack at this step edge to form suitable Cu-Cu spacing for copper oxide, although some lattice mismatches to the Cu(111) face remain. 4.3. Surface Reconstruction at the Elevated Temperature. In the HOMB-induced process at RT, the |3-1 22| structure, which has triple possible domains, grew on the surface. The structure including triple domains became streaky upon annealing and changed to the single-domain phase at 470 K, which implies that Cu and O atoms of the oxide may be mobile at higher temperature. Mobile species in the oxide formation process have also been suggested by STM studies.19,20 The mobile species migrate on the terrace and aggregate with the oxide island to produce a 2D oxide. The single-domain oxide structure may grow on a slightly disoriented surface, which has the high density of step edges in a specific direction. On such a slightly tilted surface, the migration length necessary to encounter the edge of an oxide island is relatively short in the tilted direction compared with that of the other directions. Therefore, a specific domain of the oxide grows preferentially. This assumptive reaction model can explain the oxide formation process where a single-domain oxide forms at elevated temperature, such as that observed in our study. 3 2 After annealing at 620 K, the |-1 2| structure changed to the 29 structure. The 29 structure has a structure similar to that of bulk Cu2O(111),17,18 and thus, the 29 structure is a more stable 3 2 oxide structure than the |-1 2| structure, which has a lattice mismatch between the Cu(001)-like overlayer and the Cu(111) substrate. Moreover, it is plausible that thermal expansion of the (111) lattice at high temperature may cause reconstruction to the 29 structure. These results suggest that HOMB can induce the metastable surface structure which cannot be produced in the thermal equilibrium process. 5. Conclusion We measured the high-resolution O-1s XPS spectra on a Cu(111) surface and observed the corresponding LEED pattern during HOMB incidence and thermal O2 exposure at RT. Our

Reconstruction of Cu(111) Induced by Molecular Beam results suggest the dominant contribution of Ei of O2 to the oxidation processes. HOMB at Ei g 0.23 eV enhances the dissociative adsorption of O2. For HOMB at Ei e 0.23 eV, the attainable saturation coverage was 0.27 ML and no surface reconstructions were induced. The translational energy Ei g 0.5 eV induced further 3 2 oxidation, accompanied with the reconstruction to the |-1 2| structure at Θ g 0.27 ML. This translational energy of 0.5 eV is higher than the theoretically calculated activation barrier of 0.2 eV26 for the dissociation. In this process, the translational energy assists the dissociation of O2, and as a result, the released chemical energy in the chemisorption of O atom is dissipated locally and induces the reconstruction. A binding energy of 528.9 eV for peak B agrees well with O at the four-fold hollow site on Cu(001), which suggests a pseudo-Cu(001)-O structure 3 2 is formed on the Cu(111) surface. The |-1 2| structure changes to the 29 structure, which is the most thermally stable phase on O/Cu(111). Hence, this study successfully demonstrates that the pseudoCu(001) structure formation is selectively induced by HOMB at Ei g 0.5 eV on a Cu(111) surface. Moreover, we suggest that a hyperthermal molecular beam-induced reconstruction is a general phenomenon, which induces a surface process and consequently controls the surface phase by a thermal nonequilibrium process. Acknowledgment. All the experiments were performed using SUREAC2000 in BL23SU at SPring-8. The authors are thankful to Drs. Y. Saitoh and S. Fujimori for their help with the operation of the monochromatic system at the beamline. K.M. and M.O. gratefully acknowledge the Hyogo Science and Technology Association. M.O. is also supported by PRESTO of the Japan Science and Technology Agency. References and Notes (1) Woodruff, D. P. J. Phys.: Condens. Matter 1994, 6, 6067, and references therein. (2) Musa, A. O.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305. (3) Ohba, T. Appl. Surf. Sci. 1995, 91, 1. (4) Sadana, A.; Katzer, J. R. Ind. Eng. Chem. Fundam. 1974, 13, 127. (5) Simmons, G. W.; Mitchell, D. F.; Lawless, K. R. Surf. Sci. 1967, 8, 130. (6) Spitzer, A.; Lu¨th, H. Surf. Sci. 1982, 118, 136. (7) Rajumon, M. K.; Prabhakaran, K.; Rao, C. N. R. Surf. Sci. 1990, 233, L237. (8) Sueyoshi, T.; Sasaki, T.; Iwasawa, Y. Surf. Sci. 1996, 365, 310. (9) Oustry, A.; Lafourcade, L.; Escaut, A. Surf. Sci. 1973, 40, 545. (10) Habraken, F. H. P. M.; Kieffer, E. P.; Bootsma, G. A. Surf. Sci. 1979, 83, 45. (11) Wiegel, M.; Balkenende, A. R.; Gijzeman, O. L. J.; Leibbrandat, G. W. R.; Habraken, F. H. P. M. Surf. Sci. Lett. 1991, 254, L428. (12) Dubois, L. H. Surf. Sci. 1982, 119, 399. (13) Haase, J.; Kuhr, H. J. Surf. Sci. 1988, 203, L695. (14) Niehus, H. Surf. Sci. 1983, 130, 41. (15) Luo, B.; Urban, J. J. Phys.: Condens. Matter 1991, 3, 2873. (16) Jensen, F.; Besenbacher, F.; Lægsgaad, E.; Stensgaard, I. Surf. Sci. 1991, 259, L774.

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