Atomic Adsorption of Oxygen on Cu(l 11) and Cu(l 10)

Transcription

1 Appl. Phys. A 41, (1986) Applied.o.,.. Physics A "" Surfaces 9 Springer-Verlag 1986 Atomic Adsorption of Oxygen on Cu(l 11) and Cu(l 10) W. Jacob* Physikalisches nstitut der Universitfit, Am Hubland, D-8700 Wiirzburg, Fed. Rep. Germany V. Dose Max-Planck-nstitut fiir Plasmaphysik, D-8046 Garching/Mfinchen, Fed. Rep. Germany A. Goldmann Laboratorium fiir Festk6rperphysik, Universit/it-Gesamthochschule, D-4100 Duisburg, Fed. Rep. Germany Received 25 April 1986/Accepted 10 June 1986 Abstract. We have studied angle-resolved inverse photoemission (hco = 9.7 ev) after room temperature adsorption of oxygen on Cu(111) and CU(110). On Cu(111) exposure to 500 L induces a band (3.0 ev above Er at F) which shows clear dispersion (1.0eV) to higher energies for off normal incidence. Since no LEED superstructure is seen for that system, our results present strong evidence for the presence of short-range surface order. Two adsorbate bands are identified (2.8 ev and 6.3 ev at F) on Cu(110) p(2 x 1)-O. Our results are in good agreement with a long-bridge adsorption site. PACS: 73.20r, 79.20Kz The dissociative adsorption of oxygen on Cu(111) and Cu(ll0) at room temperature has attracted much attention in recent years and a number of experimental studies have yet been performed [1-12]. Despite these considerable efforts, the atomic chemisorption on the two well-understood and (if clean) unreconstructed surfaces is far from being explained. n fact, extremely controversial conclusions have been derived concerning the adsorption geometry, and not very much is known about the electronic states. Exposure of Cu(lll) to 02 at room temperature does not produce any ordered overlayer as judged by LEED [1-5]. Although Auger electron spectroscopy (AES) detects increasing oxygen signals until a saturation occurs at ~800L 02[3, 5] - which corresponds to about 0.5 monolayer coverage in terms of the number of substrate surface atoms [1, 5] - no change of the * New address: Max-Planck-nstitut gir Plasmaphysik, D-8046 Garching/Mtinchen, FR Germany work function (A~b< 15meV) could ever be observed [3, 5]. Proposed structural models based on various experimental methods, include a simple disordered overlayer on a smooth (111) - surface [1], a disordered overlayer on a reconstructed (roughened and disordered) substrate [5], disordered adsorption within the outermost ordered (111)-substrate layer [4, 5] and disordered subsurface oxygen in unspecified sites [3, 4 3. Adsorption of oxygen on Cu(110) leads to a large variety of ordered overlayers, depending = except of course from the oxygen dose - on parameters like substrate temperature during exposure and annealing times and temperatures after exposure [3, 12]. There is some agreement now that exposure of Cu(110) to 10 to 102L O2 at room temperature leads to a (2 x 1)-O overlayer on a reconstructed substrate [6-12]. The coverage corresponding to the saturated (2 x 1) structure is half a monolayer [9, 12] and a work function increase A~b of about 0.3 ev is reported [8]. Most

2 146 W. Jacob et al. experiments derive "long-bridge" adsorption sites in the "1" real space direction along [001], while in the "2" direction along [170] every second Cu-row is missing ("missing row model") [7, 11] or displaced perpendicular to the surface ("buckled row model") [9, 13]. The oxygen atoms in the long-bridge position are either located above the outermost Cu plane [10], or below it [6]. Thus a number of conflicting geometries are offered. n this situation we thought it interesting to apply the recently matured technique of angle-resolved inverse photoemission [14-17] to the Cu(lll)/O and Cu(110)p(2 x 1)-O systems. Our experimental method yields information about the empty oxygen-derived electronic states. Although, of course, structural details can be inferred only indirectly from the electronic energy bands, we hoped to be able to rule out some of the proposed structural models. n the following we present our results and proposed interpretation. t turns out that our data contribute an additional piece to a complicated puzzle. 1. Experimental All results presented below are "isochromat" spectra: Electrons with kinetic energy E~i, impinge on the sample at an angle 0 with respect to the surface normal, in a mirror plane of the substrate lattice. An isochromat spectrum represents the photon yield at the fixed photon detection energy hr~=9.7ev during a scan of Eki n. The experimental details have been described in detail elsewhere [18, 19]. Clean samples of Cu(ll 1) and Cu(110) were prepared by the usual repeated cycles of argon ion bombardment and thermal treatments, for details see [20]. The samples were then characterized by AES and LEED. An excellent low background LEED pattern was observed and no contaminants could be identified using retarding-field AES. The experimental setup is incorporated in a stainless steel UHV chamber pumped by an ion pump and a turbomolecular pump. Exposures to oxygen ( %) were made, with sample and gas at or slightly above room temperature, by filling the vacuum chamber to pressures between and 5 * 10.5 Pa. For exposures >1 L the ion pump was switched off and only the turbomolecular pump was used to maintain a steady oxygen flow at constant oxygen partial pressure. A quadrupole mass spectrometer served to measure the oxygen pressure and the adsorbate purity during the experiments. No LEED superstructure could be observed for exposures < 103L on Cu(111), in agreement with all other previous investigations. The Cu(110) sample was exposed to oxygen typically between 10 and 20L. Subsequent heating to about 370K for 5-10min produced a very sharp p(2 x 1)-O LEED pattern, in agreement with reports from different other studies. 2. Results and Discussion 2.1. Oxygen on Cu(111) sochromat spectra showing bulk and surface derived features of clean Cu(lll) have been presented and discussed in detail elsewhere E20]. Coveragedependent spectra taken at 0 = 4 ~ are reproduced in Fig. 1, upper half, together with corresponding difference spectra in the lower half. Upon oxygen adsorption the prominent peak So, which is due to a Cu(111) surface state [-20, 23] is strongly attenuated. Simultaneously an adsorbate-induced peak labeled A is appearing at about 3.0eV above E v. Whether the shoulder $1, which is due to radiative transitions into a series of image potential states on clean Cu(lll) [20,23] is quenched or if only the minimum around 3 ev is getting filled upon oxygen adsorption, cannot be distinguished unambiguously. Also a new peak appears at about 0.8 ev, which is either correlated with the oxygen surface, or was hidden before below S o. Figure 2 reproduces some angle-dependent results for clean Cu(lll), dashed lines, and after an oxygen exposure of 500 L. This exposure yields saturation of peak A and higher doses (< 104L) do not bring out J i 1 i i i r, i ~, Cu(111) =4" i i i i i i i i E - E F lev) Fig. 1. nverse photoemission spectra from a clean and oxygen covered Cu(111) surface. Oxygen exposures range from 50 to 100 L. The lower continuous curves are the difference spectra oxygen covered minus clean. The angle of electron incidence is 0=4 ~

3 _ Atomic Adsorption of Oxygen on Cu(111) and Cu(110) 147 Cu (111) / LJ 2 - iii 6 ~ ) ~.z.z ~ \. ~ ' " Cu1111)/02 - ' ~ sy./~ E.--Z.m /1 r- e" 0 2 Z, 6 8 E - EF(eV) Fig. 2. Dispersion of emission features from clean and oxygen covered Cu(111) samples. The arrows mark the oxygen induced features. Spectra from the oxygen covered sample are amplified by roughly a factor of two with respect to clean surface spectra (dashed curves). The wave-vector component k,, is varied along /~/~ and FS~' new features. The results of Fig. 2 clearly exhibit a dispersion of peak A, from 3.0 ev at 0 =0 ~ to about 4 ev at 0= 23 ~ From Fig. 2 and other spectra not shown here we can directly obtain the energy dispersion E(k,), where the component of the electron momentum parallel to the surface is defined by k, = sin 0 * [(2m/h 2) (E - E e + he) - ~b)] t/2. Here ~ is the work function and m the rest mass of the electron. Figure 3 summarizes our E(k,,) results. Band A is located within a projected bulk band gap of the clean (and unreconstructed) Cu(l 11) surface. The most interesting aspect is the dispersion of about 1.0eV when going from/~ to k, ~ 0.6 ~-1. Such a dispersion clearly contradicts the assumption of disordered oxygen adsorption. t is therefore necessary to check the consistency of our results with other information. Angle-resolved photoemission studies of the Cu(111)/O system do not exist to our knowledge. Angle-integrated results report occupied bonding orbitals of oxygen - observed as rather broad features of low intensity - at about 5.7 ev [8] or 6.2eV [21], respectively, below E E. Electron O.S ~' 4-- kill.& Fig. 3. Dispersion of the oxygen induced emission features marked by the arrow in Fig. 2 as a function ofk,. For definition of the surface Brillouin-zone points 2~ and 2~' see e.g. [28] energy loss spectra (ELS) show one weak loss near 9.6eV [8] or 9.3eV [4], which is characteristic of chemisorbed oxygen on Cu(111) [22]. The observed loss indicates transitions into an empty oxygenderived band at eV above Er, in excellent agreement with our result. The Cu(111)/O system was recently also studied by high-resolution electron energy loss spectroscopy (HRELS) [4]. The HRELS evidence suggests that the oxygen atoms are adsorbed at three-fold hollow sites of the (111) surface. The observed dispersion of peak A (Fig. 3) can only be interpreted by an oxygen-oxygen interaction within an ordered overlayer. Since no LEED superstructure was ever observed, the dispersion of the oxygen induced empty electronic state must be attributed to short-range order of the overlayer. This can easily be reconciled with the LEED experience, since the dispersion of electronic bands observed in photoemission and inverse photoemission is sensitive only to the next few neighbours, while LEED patterns register almost perfect order on distances of the experimental coherence length, about 102 A for our equipment. For a further discussion of the short-range order observed here we remember that a p(2 x 1)-O overlayer is observed on Ni(111) [24], leading to empty oxygen states between 1.1 ev at F and about 2.8 ev at k, = 0.7 A- 1 [25]. Since the lattice constants of the two fcc-metals agree within 2.5%, we may assume similar adsorption geometries. n fact, the dispersion of A in Fig. 3 is fully compatible with assumed short-range (2x 2)-0 order: in that case E(k,) should be symmetrical about the zone boundaries of the (2 x 2) surface Brillouin zone, which are indicated in Fig. 3 as a -! )~A nd M a, respectively. Note, however, that our data are not accurate enough to rule out a (1/~ x ]//3) R30 ~ short-range arrangement. n the latter case the boundaries would be located at k, 0.82 A-1.

4 148 W. Jacob et al. Figure 3 suggests an alternative interpretation of band A. ts dispersion E(k,,) would be compatible within the error bars with band Sz after a rigid downshift of about 1.3 ev. However, S~ is an image potential state pinned in energy about 0.6 ev below the vacuum level Ev [20]. Several other experiments failed [3, 5] to observe a work function change of Cu(lll) upon oxygen adsorption. Therefore such an interpretation can be ruled out. Similar arguments defeat an identification of A as being a shifted So band: this band occurs below Ee at ft. Angle-resolved photoemission studies clearly demonstrate [26] that this band does not shift in energy after oxygen exposures comparable to the present work. We summarize that A can be assigned to an empty oxygen derived band. ts dispersion clearly demonstrates short-range order of atomic oxygen on Cu 0 11) Oxygen on Cu(11 O) Spectra from clean Cu(110) were reported earlier in [20-. We therefore concentrate in the following on the oxygen induced changes in spectra taken for electron incidence with k,, along the [170] and [-001] bulk directions, i.e. the F-3~ and fly directions of the Brillouin zone of the Clean Cu(110) surface. For a definition of the surface Brillouin zone, see [Ref. 27, Fig. 3]. Figure 4 reproduces coverage-dependent spectra taken at 0 = 35 ~ along fly. The peaks labeled B 1 and S t are of bulk and surface origin [20], respectively, and the rapid quenching of $1 as compared to B1 is clearly evident. With increasing oxygen dose the adsorbatederived feature A develops. Between exposures of 10 and 20 L of 02, a well-defined (2 x 1) superstructure is O = 35 ~ [ 2L 6' 'i' ';'' ENERGY E-E F {ev) Fig. 4. nverse photoemission spectra from clean and oxygen covered Cu(ll0) surfaces. The angle of eiectron incidence is 0=35 ~ (k,, along FY~) E-E F (ev) Fig. 5. nverse photoemission spectra from a Cu(110) surface with an ordered p(2 x 1) oxygen overlayer. The wave-vector component k,, is varied along the F'7 direction observed by LEED after the substrate was kept at 370K for about 10rain. Samples of angle-dependent spectra are displayed in Figs. 5 and 6 to visualize the prominence (or weakness, respectively) of the spectral features observed on Cu(l10)p(2 x 1)-O. We summarize our results in the form of an E(k,,)-plot in Fig. 7. Features labeled B1 and B 3 in Fig. 7 are of bulk origin [20] and need not be further discussed here. On clean Cu(110) we had observed two surface bands located in the gap of the projected bulk bands around the ~-point. These bands are indicated in Fig. 7 by the dashed lines labeled "1" and "3". nspection of Fig. 7 suggests that the surface bands labeled A3 and A4 are the former surface bands now shifted by 0.5 ev (A4) and 0.7 ev (A3) due to the presence of the oxygen overlayer and the oxygen-induced reconstruction of the substrate. We recall that along fly the periodicity of the surface mesh remains unmodified. There is thus no reason that the surface states supported by the projected bulk band gap around Y should disappear completely. Consequently, emission branches A 3 and A4 are unlikely to arise from oxygen derived empty bands. An analogous surface band reported 1-20] in the gap around the J< point is indicated by the dashed line "2" in Fig. 7. However, the (2 1)-reconstruction changes the periodicity along FX(F'X'F" of the 2 x 1 zone) and probably also removes the unoccupied surface state at J~. Therefore we do not expect to

5 Atomic Adsorption of Oxygen on Cu(111) and Cu(110) 149 to z Ll E-EF(eV) Fig. 6. nverse photoemission spectra from a Cu(111) surface with an ordered p(2 x 1) oxygen overlayer. The wave-vector component k,, is varied along the FJ~ direction observe a shifted band "2". nstead, a new band labeled A 6 appears. t exhibits a definitely different dispersion around )( and this observation further rules out a rigid shift of band "2". We interpret A 6 as one branch of an oxygen-induced band labeled A2 near s ts periodicity, as indicated by the solid line through the data points, is clearly consistent with the overlayer geometry. The bands labeled A1 are also assigned to transitions into oxygen derived empty states. Band A1 along UY' shows a large dispersion of about 2.4eV. Along F'X ~' the data points are not sufficient to determine a dispersion. However, the results are consistent with a much smaller dispersion (of < 0.5 ev as indicated by the solid fine) than that observed along UY'. This should be expected because the oxygenoxygen real-space distance is larger by a factor of V~ along that direction. No unique interpretation is proposed for band As. On clean Cu(ll0) a band was observed there which is displayed as the dashed line labeled "4" in Fig. 7. t had been explained [20] by transitions into a bulk band that converts into a splitoff surface state on approaching the projected gap. The A5 data points are consistent with the assumption that this band 5 is shifted to slightly higher energies by the reconstruction and the presence of the (2 x 1)-overlayer. However, the A5 data can be equally well explained by a superposition of the former bulk band transition with transitions into oxygen-derived orbitals belonging to the A1 band. This obvious ambiguity can only be resolved in future experiments using tunable photon energy, to discriminate bulk against surface transitions. Next we compare our results to earlier work. The data of Fig. 4 show that the energetic position of A1 is independent of the oxygen coverage. Figure 7 demonstrates a large dispersion of A1 with k,,, indicative of strong oxygen-oxygen interaction. Both observations can be explained consistently if the adsorption proceeds by the growth of islands having the (2 x 1) structure. Such a growth mode had already been concluded from helium beam scattering studies [7]. Angle-integrated photoemission [8] observes an occupied bonding orbital of oxygen near 6 ev below Er. Results from ELS reveal [8] that the appearance of this oxygen peak is accompanied by one broad characteristic loss between 9.3 and about 10eV. This ELS peak is ascribed [8] to an electronic transition of oxygen in its atomic form as concluded from gas phase data. Clearly this ELS result is fully consistent also with the interpretation of the A1 band, compare Fig. 7. A recent angle-resolved photoemission study [13] observed dispersion of those occupied oxygen orbitals -Cu(110) p (2 xl)- 0 A2 -- ~ ~ s ~ ~E v A3 ~ ~ [001] ~ k,t (,&-l) r4~b [11" Fig. 7. Summary of dispersions of oxygen induced emission features on Cu(110)

6 150 W. Jacob et al. that are directed along /~Y by about 1.8eV. By contrast, only a small dispersion of <0.2eV was resolved along FX. This strong directional anisotropy closely parallels our results for A1 of about 2.4 ev and <0.5 ev, respectively. Our observations are thus in full agreement with the proposed long-bridge sites for the oxygen atoms. n particular, they strongly support the analysis given in [13], which concludes that the O-Cu bond is highly directional along/~y. Our observation of bulk transitions characteristic of unreconstructed Cu(110), and the experimental evidence that the ~" gap is essentially retained after formation of the oxygen overlayer, point against a drastic rearrangement of the Cu surface geometry. Finally we compare our results with those obtained from Ni(110)p(2 x 1)-O. The corresponding empty adsorbate band disperses from 3.3 ev at F by 1.2eV to k,,-~0.6a -1 along/~y,, while no dispersion could be resolved along FJ((k,,<0.5 A-1). We point out the obvious analogy with our A~ band, which shows dispersion of about 1.6 ev and < 0.5 ev, respectively, for the corresponding k,, intervals. Therefore, we propose to assign emission feature Aa to an "antibonding" oxygen derived orbital, as expected from simple surface-molecule pictures. No interpretation within this framework, however, is available yet for the adsorbate-induced band A Summary and Conclusion We have studied angle-resolved inverse photoemission spectra (hco=p.7ev) from surfaces of Cu(lll) and Cu(ll0) covered with oxygen at room temperature Exposure of Cu(111) to 500 L of O2 induces an empty band, which shows clearly resolved dispersion from 3.0eV above EF at k,,=0 to about 4.0eV at k,,=0.6a -1 along F~r'. Since no LEED pattern is observed for that system, our results present strong evidence for the existence of short-range surface order. Exposure of Cu(110) to typically 15L of Oz and subsequent annealing at 370K produces the well known (2 x 1)-O LEED pattern. For this system two empty adsorbate bands are identified (2.8eV and 6.3 ev at ~ which show different amounts of disper- sion along different directions on the surface. Although the existence of two empty bands cannot be explained at present, we show that our results are fully consistent with a long-bridge adsorption site of the oxygen atom. Acknowledgement. This work has been financially supported by the,,deutsche Forschungsgemeinschaft". References 1. F.H.P.M. Habraken, E.Ph. Kiefer, G.A. Bootsma: Surf. Sci. 83, 45 (1979) 2. F.H.P.M. Habraken, C.M.A.M. Mesters, G.A. Bootsma: Surf. Sci. 97, 264 (1980) 3. A. Spitzer, H. Lfith: Surf. Sci. 118, 136 (1982) 4. L.H. Dubois: Surf. Sci. 119, 399 (1982) 5. H. Niehus: Surf. Sci. 130, 41 (1983) 6. R.P.N. Bronckers, A.G.J. de Wit: Surf. Sci. 112, 133 (1981) 7. J. Lapujoulade, Y. Le Cruer, M. Lefort, Y. Lejay, E. Maurel: Sure Sci. 118, 103 (1982) 8. A. Spitzer, H. Lfith: Surf. Sci. 118, 121 (1982) 9. R. Feidenhansl,. Stensgaard: Sure Sci. 133, 453 (1983) 10. U. D6bler, K. Baberschke, J. Haase, A. Puschmann: Phys. Rev. Lett. 52, 1437 (1984) 11. H. Niehus, G. Comsa: Surf. Sci. 140, 18 (1984) 12. G.R. Gruzalski, D.M. Zehner, J.F. Wendelken, R.S. Hathcock: Surf. Sci. 151, 430 (1985) 13. R.A. Didio, D.M. Zehner, E.W. Plummer: J. Vac. Sci. Technol. A2, 852 (1984) 14. N.V. Smith: Vacuum 33, 803 (1983) 15. V. Dose: Progr. Surf. Sci. 13, 225 (1983) 16. F.J. Himpsel, Th. Fauster: J. Vac. Sci. Technol. A 2, 815 (1984) 17. V. Dose: J. Phys. Chem. 88, 1681 (1984) 18. V. Dose: Appl. Surf. Sci. 22/23, 338 (1985) 19. K. Desinger, V. Dose, M. G16bl, H. Scheidt: Solid State Commun. 49, 479 (1984) 20. W. Jacob, V. Dose, U. Kolac, Th. Fauster, A. Goldmann: Z. Physik B (1986) (in press) 21. R.C. Baetzold: Surf. Sci. 95, 286 (1980) 22. F. Solymosi, J. Kiss: Surf. Sci. 104, 181 (1981) 23. N.V. Smith: Appl. Surf. Sci. 22/23, 349 (1985). 24. F.J. Himpsel, Th. Fauster: Phys. Rev. Lett. 49, 1583 (1982) 25. W. Altmann, K. Desinger, M. Donath, V. Dose, A. Goldmann, H. Scheidt: Surf. Sci. 151, L 185 (1985) 26. A. Goldmann: (Unpublished results) 27. K. Desinger, V. Dose, A. Goldmann, W. Jacob, H. Scheidt: Surf. Sci. 154, 695 (1985) 28. E.W. Plummer, W. Eberhardt: Adv. Chem. Phys. 46, 594 (1982)