One-dimensional band-like electronic states on Ni(755) surfaces studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation

J O U R N A L OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 707-710

One-dimensional band-like electronic states on Ni(755) surfaces studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation H. Namba a, K. Yamamoto b, T. Ohta b, H.

Kuroda c

aDepartment of Physics, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-77, Japan bGraduate School of Science, University of Tokyo, Hongo, Bunkyoku, Tokyo 113, Japan CResearch Institutefor Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan

Abstract New electronic states localized at step edges on Ni(755) surfaces are found by angle-resolved ultraviolet photoelectron spectroscopy. Characteristics of those states are examined and compared with the previous results of Ni(7911) surfaces. Onedimensional band states are found at the step edges on the Ni(755) surface. © 1998 Elsevier Science B.V. Keywords: One-dimensional electronic states; Step; Photoelectron spectroscopy; Nanostructure

1. Introduction Structures in nanometer scale (nanostructures) appeared or manufactured on surfaces are interesting targets in surface and materials science. It is well known that one-dimensional arrays of steps and/or kinks are naturally formed on high Miller index surfaces or so-called stepped surfaces [1]. Step edges on those surfaces are composed from a few atomic rows and separated by v e r y narrow atomic planes. Those can be regarded as a limiting case of nanostructures on surfaces. Thus, examining electronic states related with surface steps is very important. In our previous studies by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation (SR), new electronic states appearing on the stepped surface of Ni(7911) ( = 5(111) x ( - 101)) were found. Based on the experimental results of the dispersion relations of the step electronic states perpendicular and parallel

to the step rows, we concluded that the step electronic states are localized at the step edges and thus have one-dimensional characteristics along steps [2,3]. In this paper, we report the SR-ARUPS results on Ni(755) ( = 6(111) x (100)) surfaces. The shape of the step edges on (755) is smooth as compared with the zigzag shape of those on (7911). The terrace planes of both stepped surfaces have a similar atomic arrangement to a (111) plane.

2. Experimental and results Experiments were carried out with an ARUPS apparatus of a beamline ADES-400 (VG Scientific) installed at BL7-B in Photon Factory, National Laboratory for High Energy Physics. The photon energy range of this beamline is from 5 to 50 eV [4]. Stepped surfaces of Ni(755) were obtained by spark cutting a single crystal rod (Johnson Matthey,

0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00203-X

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H. Namba et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 707-710

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Fig. 1. Geometry of the present ARUPS measurements.

5N purity) and polished in air. Sample surfaces were cleaned by repeated cycles of 500 eV-Ar + sputtering and annealing at 800°C under ultra high vacuum. The step periodicity on the clean surface was confirmed by means of the incident energy dependence of the low energy electron diffraction pattern. Hydrogen gas (6N purity) was dosed on the sample surface for detecting the surface sensitive electronic states in ARUPS. The geometry of the ARUPS measurements is shown in Fig. 1. The electric vector of the incident light and the detection of photoelectrons are in a plane perpendicular (case I) or parallel (case II) to

the steps. The incident angle of SR was fixed to 10° measured from the surface normal in the present measurements. Fig. 2 shows the typical photoelectron spectra measured on the clean and the hydrogen adsorbed (0.1 Langmuir of H 2 ) surfaces in case I, and also the result of the difference of those spectra. Two prominent peaks in the difference spectrum are found at the binding energy of 0.10 and 0.26 eV measured from the Fermi level, respectively. The latter peak is in good agreement with the surface electronic states on the planer surface of Ni(111), as discussed in the case of the Ni(7911) surface [2,5]. So it is unambiguous that the electronic states at 0.26 eV are the surface electronic states of the terrace planes on the Ni(755) surface. Another surface sensitive peak at 0.16 eV is due to the new electronic states appearing at the step on the surface. We denoted the step and the terrace electronic states as Ss and St, respectively. The step electronic states are overlapping with the bulk electronic states denoted as B2, as analyzed in the previous reports. On the Ni(7911) surface, the quenching rate of St by hydrogen adsorption as well as alkali metal adsorption is faster than that of St. This suggests that the activation of adsorption at step sites is due to a faster formation of the chemisorption bond of the adsorbate

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Binding E n e r g y Fig. 2. Photoelectron spectra measured on the clean (crosses) and the hydrogen adsorbed (circles) surfaces at + 10° from the terrace normal upward to the steps on the Ni(755) surfaces. Difference curve of [adsorbed] - [clean] is also shown (dots). The photon energy was 10 eV.

H. Namba et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 707-710

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Binding Energy(eV) Fig. 3. The angular distribution of photoelectron spectra measured on the clean surface in perpendicular to the steps (case I). + and denote the direction upward and downward to the steps, respectively. The direction of [111] (normal to the terrace planes) is fixed as 0. The photon energy was 10 eV.

with the step electronic states than that at the terrace planes. In the present case on the Ni(755) surface, the quenching rates of Ss and St by hydrogen molecule adsorption are comparable. This result suggests that enhancement of the adsorption activity due to the step electronic states is very small. Fig. 3 shows the angular dependence of the photoelectron spectra measured in case I at the photon energy of 10 eV. The binding energy of St is shifted as a function of the emission angles of photoelectrons. The binding energy of St is 0.26 eV at +10 ° and 0.70 eV at - 1 0 ° from the direction of [111]. On the other hand, the peak position of Ss is almost constant. The accurate binding energy of Ss was estimated by calculating each difference spectrum as shown in Fig. 2, because, in the raw data, the peak Ss

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Binding Energy (eV) Fig. 4. The angular distribution of photoelectron spectra measured on the clean surface in parallel to the steps (case I). The direction of [755] (normal to the sample surface) is fixed as 0. The photon energy was 10 eV.

overlaps with that of B2. This constant binding energy of Ss means the flat dispersion of the step electronic states in the direction perpendicular to the steps. This implies that the step electronic states are localized at the step edges, which shows onedimensional characteristics of the step electronic states. Fig. 4 shows the angular dependence of the photoelectron spectra measured in case II at the same photon energy of Fig. 3. Although the binding energy shift of Ss is not so clearly seen in the raw data shown in the figure, the binding energy of Ss estimated from the difference spectra (not shown) is certainly shifted from 0.15 at 0 ° to 0.34 eV at +35 ° measured from the direction of [755]. This slight shift shows the dispersion of Ss along the steps. The peak St shows the binding energy shift from 0.6 at 0 to 0.9 eV at +35 from the [755] direction.

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H. Namba et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 707-710

3. Discussion On the Ni(755) surface, new electronic states localized at the step edges are found in the present experiments of SR-ARUPS. The step electronic states are dispersive along the steps. The activity of hydrogen adsorption related with the step electronic states is not so different from that of the terrace planes. The first finding is easily understood from the results of the Ni(7911) surfaces. On the Ni(7911) surface, the step and the terrace electronic states are found at the binding energy of 0.05 and 0.35 eV, respectively. This agrees with the present results of 0.1 and 0.26eV. The small shifts of the binding energy found between two kinds of the stepped surfaces may be due to the different atomic arrangements on two surfaces. The second result suggests one very interesting characteristic of the step electronic states. On the Ni(7911) surfaces, the dispersion of the step electronic states along the steps is very small [3]. So we tentatively concluded that the step electronic states are atomic-like rather than band-like. On the Ni(755) surface, the step electronic states are certainly dispersed along the steps, which suggests the formation of a free electron-like band. Only fourfold sites with a similar atomic arrangement of the (111) plane are periodically arranged at the step. The step on the Ni(7911) surface is composed from fourfold and threefold sites which are alternately arranged along the step. The step on the Ni(755) surface is much smoother than that of the (7911) surface. In this case the wave function of the step electronic states can

easily extend along the step, and can then form onedimensional band states on the (755) surface. On the (7911) surface, overlapping of the wave function must be disturbed by the zigzag shape of the step, which results in the atomic-like dispersion. It is experimentally shown that the shape of the step edges is a very important factor in obtaining onedimensional band states on the surface. It is very important to notice that the last finding is related to a variation of catalytic activity due to the different shapes of the steps. Catalytic activity is a very complex term which is composed of many factors, such as an atomic shape of reaction sites, atomic distance between neighboring sites, and electronic states. So it is difficult to tell whether the variety of the hydrogen adsorption found on two stepped surfaces is only due to the different nature of the step electronic states. However, the electronic factor in the hydrogen adsorption must play a very important role.

References [1] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994, p. 47. [2] H. Namba, N. Nakanishi, T. Yamaguchiand H. Kuroda, Phys. Rev. Lett. 71 (1993) 4027. [3] H. Namba, N. Nakanishi, T. Yamaguchi, T. Ohta and H. Kuroda,, Surf. Sci. 357/358 (1996) 238. [4] H. Namba, M. Masuda, H. Kuroda, T. Ohta and H. Noda, Rev. Sci. Instrum. 60 (1989) 1917. [5] F.J. Himpsel, J.A. Knapp and D.E. Eastman, Phys. Rev. B19 (1979) 2919.