Structural study of the reconstructed Ir(110)-(1 x 2) surface by low-energy electron diffraction

Solid State Communications, Vol. 30, pp. 47—49. Pergamon Press Ltd. 1979. Printed in Great Britain. STRUCTURAL STUDY OF THE RECONSTRUCTED Ir(l lO)-(l x 2) SURFACE BY LOW-ENERGY ELECTRON DIFFRACTION C.-M. Chant MA. Van Hove, W.H. Weinberg~and ED. Williams1 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA. (Received 27 November 1978 by G. Burns) The structure of the reconstructed Ir(1 10)—(l x 2) surface has been analyzed by low-energy electron diffraction. Three models proposed for the reconstructed Ir(1 10)—(1 x 2) surface the missing row model, the paired row model and the buckled surface model were tested. Based on a comparison between experimental data consisting of intensity-voltage profiles for ten half-order beams and eight integral-order beams and the calculated curves, the missing row model with a topmost interlayer spacing of 1.22 ±0.07 Ais the preferred structure. —

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THIS COMMUNICATION reports the first structural analysis, by low-energy electron diffraction (LEED), of an irreversibly reconstructed clean metal surface. Very recently, a preliminary analysis of the low temperature (below room temperature) c(2 x 2) structure of the W(100) surface has been carried out [1]. However, the limited number of beams examined (five) and the

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equivocal agreement between theory and experiment did not permit a definitive “structural determination” of this reconstructed surface. It has been known for some time from LEED studies that a clean Ir(1 10) surface __________________________________________

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________________________________________ Fig. 2. Top views of a hard-spheres representation of (a) the paired row model; (b) the buckled surface model; and (c) the missing row model. The x- andy-directions are in the plane of the crystal surface. The corresponding side views are shown in (d)—(f), respectively. The zdirection is perpendicular to the crystal surface.

§ Camille and Henry Dreyfus Foundation TeacherScholar, and Alfred P. Sloan Foundation Fellow. National Science Foundation Predoctoral Fellow, 47

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displays a (1 x 2) structure, illustrated schematically in Fig. 1, indicating a reconstructed surface superlattice with ay-dimension twice that of the bulk lattice [2]. Different models for this reconstructed surface have been proposed. The three most common and simplest ones are the missing row model, the paired row model and the budded surface model, In the missing row model, alternate rows of atoms are absent on the surface. The paired row model suggests that every two adjacent rows of first layer atoms are paired to form one row. In the buckled surface model, adjacent rows of the first layer are relaxed in opposite directions perpendicular to the crystal surface. Figure 2 shows schematic hard-spheres drawings of these three models proposed for the reconstructed Ir(1 10)—(1 x 2) surface. Previous investigations of the unreconstructed

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E. eV Fig. 3. Comparison between the theoretical I—V spectra for the missing row model (with a modified inner potential of 10 eV and a topmost interlayer spacing of 1.22 A) and the experimental I—V spectra of integral-order beams from the Ir(1 10)—(1 x 2) surface. 0 = 0 corresponds to normal incidence.

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E,eV Fig. 4. As in Fig. 3, except for half-order beams. Ir(1 10) surface [31suggest that the transformation of the impurity-stabilized (1 x 1) structure to the (1 x 2) structure involves an extensive atomic rearrangement of the surface. A priori, this evidence makes the missing row model a highly possible candidate for the reconstructured surface since its formation would involve movement of entire rows of atoms. The Ir sample, a randomly oriented single crystal rod, was aligned within 40 of the (110) orIentation by the back-reflection Laue X-ray method and was cut and polished mechanically using standard technIques [3]. Mter Ar” bombardment and a series of treatments in 5 x 1O”~tort oxygen at 800K, followed by brief annealing in vacuum at 1600K, the surface was shown to

Vol. 30, No. 1

Ir(l l0)—(1 x 2) SURFACE BY LOW-ENERGY ELECTRON DIFFRACTION

contain less than 2 at.% carbon which was shown not to influence the surface structure [the intensity—voltage (I—V) beam profiles were unchanged when occasionally more and/or less carbon was present on the surface] and no other impurities detectable by Auger electron spectroscopy. The clean (110) surface, after further annealing at 1600K in vacuum, exhibited a (1 x 2) superstructure. Eighteen LEED I—V spectra consisting of ten halforder beams and eight integral-order beams were collected with a rotatable Faraday cup at approximately 2 eV intervals. In all cases, the incident electron beam was normal to the surface. The achievement of normal incidence was verified by the satisfactory agreement between equivalent non-specular beams. To confirm that the data are reproducible, ten spectra were retaken after repolishing the Ir crystal. The agreement between these two independent sets of data is excellent, A convergent perturbative scheme known as the layer-doubling method [4]was used for the theoretical calculations. The atomic potential used for Ir is a band structure potential [5] and includes full Slater exchange. Symmetry properties of the beams at normal incidence were exploited in the calculations. Eight phase shifts and an equivalent of a maximum of 170 beams were used. The real part of the inner potential (the muffin-tin-zero) was assumed to be 15 eV, and this quantity was allowed to vary by a rigid shift of the energy scale in the comparison between theoretical and experimental I—V spectra. Aconstant inelastic damping of 5 eV was used. Comparison between the experimental data and the results of the calculations using the missing row model, the paired row model and the buckled surface model show that the missing row model is the best one considered. In the calculations of the LEED I—V spectra for the missing row model, the topmost layer spacing of the Ir(l 10) surface was allowed to relax from 15% (percentage contraction of the bulk interlayer spacing of 1385 A) to + 5% (expansion) in steps of 5%. The comparisons between the experimental I—V spectra and the theoretical I—V spectra obtained from the missing row model calculations with a topmost interlayer spacing of 1.22 A 10%), modified with an inner potential of 10 eV are displayed in Figs. 3 and 4 for the integralorder beams and the half-order beams, respectively. —

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Ofthe eight integral-order beams, six show very good agreement between theory and experiment; whereas two, the (11) and the (21) beams, agree less well from 70 to 100 eV and from 180 to 210 eV, respectively. Among the ten half-order beams, eight exhibit very good correspondence with experiment, but the (0 3/2) beam exhibits only mediocre agreement, while the calculated (1 1/2) beam shows some minor disagreement with the experimental data. The minor disagreement between theory and experiment for these beams may be due to roughness of the (110) surface [6, 7]. However, the missing row model, with a topmost interlayer spacing of 1.22 ± 0.07 A, is certainly the most probable structure for the (1 x 2) reconstructed surface of those tested, based on the agreement between experiment and theory for the majority of the beams. A detailed R-factor analysis, developed by Zanazzi and Jona [8], will be used to determine quantitatively the level of agreement between theory and experiment for the different models considered [9]. Work is in progress also to include a slight movement (row pairing) of the second layer within the framework of the missing row model [9]. Acknowledgements This work was supported by the Army Research Office (Durham) under Grant No. DAHCO4-750l70, and by the Donors of the Petroleum Research Fund administered by the American Chemical Society (Grant No. 9309-AC5, 7). —

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

R.A. Barker, PJ. Estrup, F. Jona & P.M. Marcus, 5~~t1 State Commun. 25, 375 (1978). K. Christmann & G. Ertl, Z Naturf 28a, 1144 (1973). C.-M. Chan, S.L. Cunningham, K.L. Luke, W.H.. Weinberg & S.P. Withrow, Surf Sd. 78, 15 (1978). J.B. Pendry, Low-Ene,~Electron Diffraction, Academic, London (1974). G.O. Arbman & S. Hornfelt,J. Phys. F2, 1033 (1972). E. Zanazzi, F. Jona, D.W. Jepsen & P.M. Marcus, I. Phy& ClO, 375 (1977). M. Maglietta, E. Zanazzi, F. Jona, D.W. Jepsen & P.M. Marcus, (1977). E. Zanazzi & J. F. Phys. Jona, ClO, Surf 3287 Scm. 62,61(1977). C.-M. Chan, MA. Van Hove, W.H. Weinberg & ED. Williams (in preparation).