The ordered domain patterns that form spontaneously in a wide variety of chemical and physical systems as a result of competing interatomic interactions can be used as templates for fabricating nanostructures. Here we describe a new self-assembling domain pattern on a solid surface that involves two surface structures of lead on copper. The evolution of the system agrees with theoretical predictions, enabling us to probe the interatomic force parameters that are crucial to the process.

We report the first experimental evidence of the cyclic form of ozone, found in three air stable surface reconstructions of MgO (111) annealed above 1450 ± C. The MgO ͑111͒-͑ p 3 3p 3 ͒R30 ± surface consists of equilateral oxygen trimers while the MgO ͑111͒-͑2 3 2͒ and MgO ͑111͒-͑2 p 3 3 2 p 3 ͒R30 ± surfaces are periodic arrangements of trimers and single oxygen atoms. The oxygen trimers appear to be centered over underlying Mg atoms. The structures fit transmission electron diffraction data better than neutral plane faceting based models proposed for the polar MgO (111) surface. [S0031-9007(98)07807-7] PACS numbers: 61.14.Rq, 61.16.Bg, 68.35.BsThe stability of polar oxide surfaces has long been a problematic question in surface science. A bulk terminated polar surface has an infinite surface energy because alternating layers of oppositely charged ions produce a large dipole moment perpendicular to the surface [1]. For the model MgO (111) polar oxide surface theoretical results have pointed to two similar solutions for this problem: microscopic faceting into neutral ͕100͖ planes upon annealing [2-5] and surface reconstructions, which are essentially faceting to neutral planes but on an atomic scale [5][6][7].The microscopic faceting model has long been supported by low energy electron diffraction (LEED) and scanning electron microscopy (SEM) of 1200 ± C annealed MgO (111) surfaces [8,9] which revealed micron-sized triangular facets. These facets were interpreted to be neutral ͕100͖ planes. It has been recently shown, however, that these facets are much shallower vicinal ͕111͖ planes introduced by acid etching in sample preparation [10]

The surface structure of MgO͑111͒-͑1 ϫ 1͒ bulk and thinned single crystals have been investigated by transmission and reflection high-energy electron diffraction, low-energy electron diffraction ͑LEED͒, and x-ray photoelectron and Auger electron diffraction. The ͑1 ϫ 1͒ polar surface periodicity is observed both after 800°C annealing in air and also after oxygen plasma cleaning and annealing in ultrahigh vacuum. The x-ray photoelectron spectroscopy and diffraction results were analyzed by simulations based on path-reversed LEED theory and by first-principles calculations to help distinguish between different mechanisms for the stabilization of this extremely polar oxide surface: ͑1͒ stabilization by adsorption of a hydrogen monolayer; maintaining the insulating nature of the surface and ͑2͒ stabilization of the clean O or Mg terminated 1 ϫ 1 surface by interlayer relaxations and two-dimensional surface metallization. The analysis favors stabilization by a single OH layer, where hydrogen sits on top of the O ions with O-H bond distance of 0.98Å. The in-plane O and Mg positions fit regular rocksalt sites, the distance between the topmost O and Mg plane is 1.04 Å, contracted by ϳ14% with respect to bulk MgO distance of 1.21 Å, while the interlayer separation of the deeper layers is close to that of bulk, contracted by less than 1%. The presence of a monolayer of H associated with the terminal layer of oxygen reduces significantly the surface dipole and stabilizes the surface.

Using low-energy electron microscopy, we find that Cu and Pb, arranged in single atomic layers on the Cu(111) surface, self-assemble into ordered, nanoscale domain patterns. The pattern type, feature size, and degree of long-range order vary controllably with surface composition and temperature. The continuous evolution of the domain structures from circular islands to stripes to `inverted' islands with increasing Pb coverage agrees with theoretical predictions and simulations based on the existence of competing long- and short-range interactions. The details of the self-assembly process depend on a number of factors including temperature, surface morphology, and the presence of small amounts of sulfur.

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