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Surface Science 295 (1993) 427-432 North-Holland

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The unoccupied electronic structure of Na/Cu( 110) D. Tang, C. Su and D. Heskett Department

of Physics,

University of Rhode Islmd, Kingston, Ri 02881, USA

Received 3 March 1993; accepted for publication 11 June 1993

Using the technique of inverse photoemission spectroscopy (IPES), we have measured the unoccupied electronic states of sodium on Cu(ll0) as a function of Na dose on the Cu(ll0) surface at room temperature. An Na-induced state appears for Na coverages above 0.08 ML for normal incidence, which we assign as the Na unoccupied 3p level. A second peak appears for coverages greater than 1 ML near the 7 point. The adsorption of Na also causes shifts and attenuation of Cu(ll0) surface states. We compare our results with studies of related systems.

1. Introduction The adsorption of alkali-metal atoms on metal surfaces has been of a great deal of interest to experimentalists and theorists for many years [l81. These systems are simple chemiso~tion systems which can serve as the first step towards investigations of the general properties of metal adsorption on surfaces. In experimental studies of these systems, inverse photoemission spectroscopy (IPES) has proven to be a valuable technique. The systems Na/Cu(lll) [9], Na/Al(lll) [lO,ll], K/Ag(llO) [El, Na/Ni(llO) [131, Li/Be(~l) [14-161, and Na/Ni(lll) [171 among others have all been investigated using IPES. In this paper we use this technique to investigate Na/Cu(llO) at room temperature. We have recently performed photoemission measurements of the Na core levels for the systems Na/Cu(lll) [18], Na/Ni(lll) 1181, and Na/Cu(llO) 1191, and inverse photoemission measurements of Na/Ni(lll) 1171. The binding energy shifts of the NaZp core level versus Na coverage on Ni(l11) and Cu(ll1) and of the unoccupied Na3p levels are quite similar 19,171. In contrast, the binding energy of the Na2p core level has a qualitatively and quantitatively different behavior for Na/Cu(llO), which we have related to the Na-induced missing row recon-

struction of this surface. It is particularly interesting, therefore, to determine what happens to the unoccupied states. We can also compare our results of Na/Cu(llOI with a previous study by Memmel et al. 1131of the system of Na/Ni(llO), as well as to other alkali/metal systems.

2. Experimental The measurements reported here were performed in an ultra high vacuum system equipped with an angle-resolved inverse photoemission spectrometer (IPESI, Auger electron spectrometer (AES), and low energy electron diffraction (LEEDS apparatus. The base pressure in the chamber was approximately 1 X 10-i’ Torr. The IPES measurements were performed in the isochromat mode. The electron gun used in the experiment was modeled after the design of Erdman and Zipf [201, with a sample current of N 1.2 PA. The emitted photons were detected with an I,-He filled Geiger-Muller tube detector equipped with an SrF, entrance window. The overall energy resolution was u 0.4 eV. The detector was positioned at 45” with respect to the normal of the sample surface. The incident angle 0 was changed by rotating the electron gun. The Cu(ll0) single-crystal surface was cleaned

0039”6028/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

428

D. Tang et al. / The unoccupied electronic structure of Na / Cu(ll0)

by argon ion sputtering and annealing to 700 K for 5 min. Its cleanliness was checked by IPES and AES. Sodium was evaporated onto the sample at room temperature from a commercial SAES getter source equipped with a shutter and collimation. The pressure during evaporation stayed in the low lo-*’ Torr range. We have determined work function change and LEED patterns here and in two separate investigations of this system [19]. Na relative coverages were determined by correlating the work function change to LEED patterns and to Na2p core level intensities [193.

Na

Coverag

1.5 ML

1 ML 0.75 ML 0.5 ML

0.25 ML

3. Results and discussion In fig. 1, we present inverse photoemission spectra of Na/Cu(llO) for normal incidence of the electron beam as a function of Na coverage. Coverage determination was based on work function measurements and LEED pattern changes, which are presented in more detail elsewhere [19]. The lowermost curve shows the spectrum of clean Cu(ll0). The weak peak labeled S, at 4 eV above the Fermi level is a clean Cu(ll0) surface state originally reported by Jacob et al. [211. This state should correctly be called an image surface “resonance”, since it falls within the projected bulk bands of the surface Brillouin zone (SBZ) for k,, = 0. Probably for this reason, it is rather weak as compared with the image states of Cu(ll1) [9], Ni(lll) [17,22,23], and other surfaces. As Na is dosed onto the surface of the Cu(ll0) crystal, an Na-induced peak appears at a coverage of 0.08 ML (labeled 3~). This peak grows gradually in intensity and shifts towards the Fermi level with increasing Na coverage. The peak gradually decreases in intensity after the Na coverage has increased over 0.5 ML. At 0.75 ML, we can only detect a small peak 1 eV above the Fermi level. We are not sure if this feature should be assigned to the 3p peak or not. In fig. 2, we plot the energy of this Na-induced unoccupied state as a function of Na coverage. This peak shifts by 1 eV towards the Fermi level before it disappears. For comparison, we plot the energy of a similar Na-induced peak for the sys-

Na/Cu( 110) K//=0 hv=9..5eV

/ I

I

I

I

EF= 0

2

4

6

Energy Above Fermi Level (eV)

Fig. 1. Inverse photoemission spectra of Na/Cu(llO) at hv = 9.5 eV as a function of Na coverage. Spectra were taken at normal incidence of the electron beam. Note that 1 ML corresponds to saturation coverage of the first layer. The tick marks indicate the state labeled S, and the Na3p state.

tern of Na/Ni(lll). For both surfaces, we assign this peak as an unoccupied Na3p state, though we cannot rule out hybridization with the Na3s level, as suggested by the calculations of Ishida for Na/jellium [8]. The main difference between these systems is that the energy shift is much smaller for Na/Cu(llO) (- 1 eV) than for Na/Ni(lll) ( - 3 eV) (both shown in fig. 2) or for Na/Cu(lll) ( _ 2.4 eV) from a previous study by Dudde et al. [9]. We believe that this is related to the missing row reconstruction of the Na/Cu(llO) surface.

D. Tang et al. / The unoccupied electronic structure of Na / Cu(ll0)

0.2

0.6

0.4

Na Coverage

I

429

Cu(ll0) (labeled S,) is attenuated with increasing Na coverage, but with no shift in the energy of the peak. We assign the peak labeled B, as a bulk peak of the Cu(ll0) substrate, in agreement with previous assignments [21,25]. Its energy shift as a function of Na dose is due to the fact that the peak is a rather highly dispersive peak for the clean surface 1211. With increasing Na coverage, the work function changes, causing the k,, probed to shift for a fixed electron gun incident angle, as was used for the data plotted in fig. 3. In fig. 4, we present spectra of Cu(ll0) as a function of Na coverage at the y point in the

0.8

(ML)

Fig. 2. Plot of the energies of the unoccupied Na-induced peaks as a function of Na coverage for the systems Na/Cu(llO) and Na/Ni(lll) for normal incidence of the electron beam.

In an STM study of K/Cu(llO), Schuster et al. [24] found that the alkali atoms substitute for copper atoms and sit in copper positions even at low K coverage. In these adsorption sites, the individual atoms should be somewhat shielded from each other by the surrounding Cu surface atoms. Therefore, for the case of Na/Cu(llO) at low Na coverage, we expect that the Na-Na interactions will be smaller than in the case of Na atoms on the top of the surface, as for Na/Cu(lll) or Na/Ni(lll), which could cause a reduction in the magnitude of any energetic shifts which involve Na-Na interactions. Specifically, this could account for the smaller total shift in energy of the unoccupied Na 3p level for Na/Cu(llO) versus Na/Cu(lll) and Na/Ni(lll) as noted above, as well as a smaller coverage-dependent shift in the binding energy of the Na2p core level and in the workfunction change for Na/Cu(llO) versus Na/Cu(lll), as we have reported previously [19]. In fig. 3, we present spectra of the Cu(ll0) surface as a function of Na coverage near the x point. The lowest curve is the clean Cu(ll0) spectrum, and is in good agreement with previous investigations [21,25]. From this set of spectra, we can see that the main effect at this point in the SBZ of adding Na is that the surface state of

0.66UL 0.7UL

Na/Cu( 110) 9= 50’ along rX t-w-9.5eV

w=

I

I

I

I

I

I

0

2

4

6

0

10

Energy Above Fermi Level (eV)

Fig. 3. Inverse photoemission spectra of Na/Cu(llO) at hv = 9.5 eV as a function of Na coverage. Spectra -- were taken near the x point at 50” off normal along the P-X direction. The clean surface state is labeled S,.

430

D. Tang et al. / The unoccupied electronic structure of Na /Cu(llO)

surface Brillouin zone of the clean surface. The lowest curve is the clean Cu(llO) spectrum and agrees with previous investigations [21,25] as well as a calculation by Redinger et al. [26]. If we track the surface peak labeled S, versus Na coverage, this peak shifts away from the Fermi level to higher energy for low Na coverages. For higher Na coverages above 0.25 ML, this peak turns around and shifts towards the Fermi level, disappearing at an Na coverage of - 0.75 ML. This general behavior is very similar to that of Na/Ni(llO) and Na/Ag(llO) [13,271 for the room temperature results. We would like to point out that, as discussed pre~ously with regards to peak B, in fig. 3, the effect of the changing work function due to Na dosing also causes a change in k,, for a fixed incident angle. Therefore, we also recorded a set of IPES spectra at the y point as a function of Na in which we adjusted the electron gun direction to track k . The results in this case were quite similar to fig”. 4 . In fig. 4, we can also see a weak peak (labeled 1,) at 3.5-4.5 eV above the Fermi level in the Na coverage range of 0.25 to 1 ML. A similar somewhat more intense peak was detected for Na/Ni(llO) at room temperature by Memmel et al. [13,27] and was assigned as an image state of the Na/Ni(llO) system. In agreement with this study, we similarly assign peak I, to an image state of Cu(llO), which we were, however, unable to detect at low Na coverage and on the clean Cu(ll0) surface. At a coverage above 1 ML, we also observed a new peak at 1.3 eV above the Fermi level (labeled I,). We tentatively assign this as an image state of the Na overlayer. We do not know the detailed structure of the Na in the second layer (only a weak (1 x 1) LEED pattern was observed), but island growth in the second layer could account for the fact that two image states (one from the monolayer and one from the second layer) could be detected at 13= 1.28 ML. In fig. 5b, we plot the energies of the Na-induced peaks I, and I,, and the Cu(ll0) surface state S, versus Na coverage. For comparison, we also plot the energy of the Ni(ll0) surface state versus Na coverage [131 in the same figure and the workfunction change of Na/Cu(llO) in fig. Sa. The LEED pattern changes we observed for

Na/Cu(llO) are indicated in these figures. Note, in particular, the very similar behavior of peak S, for Na/Ni~llO) 1131 and the present results of Na/Cu(llO). As described above, according to the STM study of K/Cu(llO) by Schuster et al. 1241, the alkali atoms substitute for copper atoms and sit in copper positions even at low K coverage. As the coverage increases, the LEED pattern changes to a (1 X 2) pattern, and a missing row reconstruction is observed. As the alkali coverage increases further, the number of top layer Cu

. . ,^

Na

NB/CU( 110)

Covera<

35’ along rY hv=9.5 eV B=

1.5Ml

1.28

MI 1 MI

0.72

MI

0.48

MI

0.40

ML

0.32

ML

0.25ML 0.16

ML

O.lZML

I

I

I

I

EF= 0

2

4

6

1

8

Energy Above Fermi Level (eV) Fig. 4. Inverse photoemission spectra of Na/Cu(llO) at hv = 9.5 eV as a function of Na coverage. Spectra were taken near the ij- point at 35” off normal along the f-9 direction. The Cu(llO) surface state is labeled S,. I, and I, are Na-induced features.

D. Tang et al. / The unoccupied electronic structure of Na /Cu(llO)

the shift in the unoccupied Ni(ll0) peaks at y at room temperature. The similar results we have observed for Na/Cu(llO) at y suggest that the same analysis should apply to this system as well. While this model can account for peak shifts near the q point, it would probably fail to account for our results at 52. In this case we observed only an attenuation of the copper surface state with no shift in the peak position versus sodium coverage (fig. 3). We suggest that the difference in behavior between the y and x

atoms will continue to decrease as they are replaced by Na (in our case). With this picture in mind, Memmel et al. [271 have presented a simple model calculation of the Na-induced changes they observed in their IPES spectra of Na/Ni(llO) and Na/Ag(llO) at room temperature. In their model, the effect of the addition of the Na adlayer, which causes the surface states to drop in energy, is opposed by the removal of Ni atoms, which has the opposite effect. They were qualitatively and semi-quantitatively able to account for o.o*

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0

Na/Ni(llO)

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0.5

Third Layer

1.0

1.5

2.0

10)

2.5

Na Coverage (ML) Fig. 5. (a) The workfunction change of Na/Cu(llO) vs. Na coverage, taken from ref. [19]. Different symbols represent different experimentalruns. (b) The energy of Na-induced features I, and I,, a surface state S, of Na/Cu(llO) (solid symbols) from fig. 4, and the NKllO) surface state of Na/Ni(llO) (open circles) from the data of ref. [13] vs. Na coverage near the y point. LEED patterns for Na/Cu(llO) are indicated. The (1 X 1) pattern became weak for Na coverages greater than 1 ML.

432

D. Tang et al. / The unoccupied electronic structure of Na / Cu(llO)

surface states may be due to a difference in their orbital character. It would be interesting to apply the same analysis of Memmel et al. [27] to this state as a further test of their barrier-type model.

4. Summary

We have measured the unoccupied electronic states of Na/Cu(llO). At normal incidence of the electron beam we observed a new peak which we assign to the Na 3p level. This peak shifts towards the Fermi level with increasing Na coverage. Near the y point in the SBZ, we observed a Cu surface state which shifted with Na coverage in a manner very similar to the analogous state in a previous study of Na/Ni(llO). On the other hand, we observed no shift versus Na coverage in the energy of a second surface state at the )7 point. Many of our results can be accounted for only if the Na-induced missing row reconstruction of the surface is taken into account.

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[7] H. lshida and M. Tsukada, Surf. Sci. 169 (1986) 225. [S] H. Ishida, Phys. Rev. B 42 (1990) 10899. [9] R. Dudde, L.S.O. Johansson and B. Reihl, Phys. Rev. B 44 (1991) 1198. [lo] D. Heskett, K.H. Frank, E.E. Koch and H.J. Freund, Phys. Rev. B 36 (1987) 1276. [ll] K.H. Frank, H.J. Sagner and D. Heskett, Phys. Rev. B 40 (1985) 2767. [12] W. Jacob, E. Bertel and V. Dose, Phys. Rev. B 35 (1987) 5910. [13] N. Memmel, G. Rangelov, E. Bertel and V. Dose, Phys. Rev. B 43 (1991) 6938. [14] P.A. Bruhwiler, G.M. Watson, E.W. Plummer, H.-J. Sagner and K.-H. Frank, Europhys. Lett. 11 (1990) 573. [15] P.A. Bruhwiler, G.M. Watson, E.W. Plummer, H.-J. Sagner and K.-H. Frank, unpublished. [16] G.M. Watson, P.A. Bruhwiler, E.W. Plummer, H.-J. Sagner and K.-H. Frank, Phys. Rev. Lett. 65 (1990) 468. [17] D. Tang and D. Heskett, Phys. Rev. B 47 (1993) 10695. [18] X. Shi, D. Tang, D. Heskett, K.-D. Tsuei, H. Ishida, Y. Morikawa and K. Terakura, Phys. Rev. B 47 (1993) 4014; X. Shi, D. Tang, D. Heskett, K.-D. Tsuei, H. Ishida and Y. Morikawa, Surf. Sci. 290 (1993) 69. [19] C. Su, X. Shi, D. Tang, D. Heskett and K.-D. Tsuei, Phys. Rev. B., in press. [20] P.W. Erdman and E.C. Zipf, Rev. Sci. Instrum. 53 (1982) 225. [21] W. Jacob, V. Dose, U. Kolac and Th. Fauster, Z. Phys. B 63 (1986) 459. [22] S. Yang, K. Garrison and R.. Bartynski, Phys. Rev. B 43 (1991) 2025. [23] A. Goldmann, M. Donath, W. Altmann and V. Dose, Phys. Rev. B 32 (1985) 837. [24] R. Schuster, J.V. Barth, G. Ertl and R.J. Behm, Surf. Sci. 247 (1991) L229. [25] R. Bartynski, Ph.D. Thesis, University of Pennsylvania (1986). [26] J. Redinger, P. Weinberger, H. Erschbaumer, R. Podloucky, C.L. Fu and A.J. Freeman, Phys. Rev. B 44 (1991) 8288. [27] N. Memmel, G. Rangelov, E. Bertel and V. Dose, Surf. Sci. 251/252 (1991) 503.