VOLUME 66, NUMBER 13 PHYSICAL REVIEW LETTERS 1 APRIL 1991 Role of Regular Steps on the Formation of Missing-Row Reconstructions: Oxygen Chemisorption on Ni(771) O. Haase, R. Koch, M. Borbonus, and K. H. Rieder Fachbereich Physik, Freie Universitat Berlin, Arnimallee 14, D-1000 Berlin 33, Germany (Received 9 August 1990) Scanning tunneling microscopy and LEED reveal that on regularly stepped Ni(771) the oxygen-driven missing-row reconstruction of the (110) terraces proceeds in a very transparent manner: Substrate atoms can move to adjacent terraces without crossing steps, where they line up with remaining atom rows to form linear 0-Ni chains. On the resulting double-width terraces perfect missing-row structures all over the surface are formed in contrast to liat Ni(110); to surmount the double-step barrier, elevated temperatures (~ 250'C) a"e required. PACS numbers: 68. 35.8s, 61.16.Di, 82. 65.My The (1 && 2) reconstructions of the (110) surfaces of Au and Pt have received considerable interest in the last de- cade. Prior to the establishment of the missing-row structure' with every second close-packed metal row along [110] missing, alternate models like the buckled- row and sawtooth structures have been considered; these models circumvent the problem of explaining the large mass transport involved in building up (and lifting) the missing-row arrangement. The (I x 2) reconstruc- tions can be induced on the clean unreconstructed (110) surfaces of Ag, Cu, Ni, and Pd by alkali-metal adsorp- tion. It is by now also well established that oxygen chemisorption on Cu(110) and Ni(110) induces a missing-row-type (2 x 1) reconstruction with every second metal row perpendicular to the close-packed rows, i.e. , along [001], removed; oxygen locations in or near the long bridges were established by several methods. ' With the scanning tunneling microscope, investigations on the local atomic rearrangements upon lifting the (1 X2) missing-row reconstruction on Pt(110) by CO adsorption and upon formation of the oxygen- induced (2X I) on Cu(110) (Ref. 10) were reported. In the latter case it was pointed out that random steps play an important role: Steps deliver diffusing atoms to ter- races where they are trapped by oxygen atoms and forced to form long Cu-0 chains; in this way the previ- ously proposed missing-row-added-row model for the oxygen-induced formation of the (2X 1) structure could be visualized. We are presently pursuing a program employing main- ly scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to investigate the role of steps in the buildup of missing-row structures in a sys- tematic manner by varying step orientations and dis- tances. On Au(430) we previously observed" that the steps perpendicular to the close-packed rows cause a re- markable surface mobility along [110]: After surface preparation by sputtering and annealing it takes — 20 h until at room temperature a stable state is reached, characterized by hillocks of — 2 nm height terminated 0.25 : AS::: :;4K ' 0 [110] (a) (b) (c) FIG. 1. Sphere models of (a) side and top views of ideal Ni(771) and (b), (c) top views of possible (2X 1) reconstruc- tions of the missing-row type [dashed circles indicate positions from which Ni atoms have moved; the oxygen atoms which drive the reconstruction occupy the long bridge sites, as indi- cated in (c) by solid circlesl. Units are in nm. The step-down direction, from the top to the bottom, is maintained in the oth- er figures. by (111) and (100) facets and the miscut of 8. 1 against the (110) plane taken up in a rather irregular manner. In this Letter we present results on a stepped surface on which we could influence the missing-row formation in a defined way, namely, by varying the coverage of a chemisorption species able to drive the reconstruction. We investigated the adsorption of oxygen on Ni(771), a stepped (110) surface which up to 500'C we observed to exist in the bulk truncated form: (110) terraces of length 3.5a (a =0.352 nm being the lattice constant of Ni) are separated by (111) steps of monatomic height as shown in Fig. 1(a); the angle between the macroscopic (771) surface and the (110) terraces is 5. 8'. By choos- ing Ni(771) we were — in analogy to the case of Au(430) — guided by the expectation that the missing- row reconstruction forms in the following simple way: Metal atoms or metal-oxygen chains on a given terrace can move along [001] as indicated in Fig. 1(b) without having to surmount a step. A particularly stable situa- tion could thus be reached at oxygen coverages near 1991 The American Physical Society 1725