The quest of a reliable method for fabricating ordered atomic-scale structures is a prequisite for future atomic-scale technology. The interest in such nanostructured materials, consisting of building blocks of a small number of atoms, arises from their promising new optic, catalytic, magnetic and electronic poperties, which are fundamentally different from their macroscopic bulk counterparts: small is different. We succeeded in fabricating a sparse, self-assembled two-dimensional array of individual Ce adatoms (the "ultimate" building block) on a metal surface [1] by exploiting long-range interactions between adatoms mediated by surface state electrons. This has been achieved by evaporating in ultrahigh vacuum Ce atoms on a Ag(111) surface at a temperature below 10 K. Scanning Tunneling Microscopy (STM) images acquired at 3.9 K display a hexagonal superlattice, where the nearest-neighbor distance of 3.2 nm is near half the Fermi wavelength of the Ag(111) surface state electrons and corresponds roughly to the distance spanned by 11 Ag atoms in the substrate. The local density of states (LDOS) of this two-dimensional superlattice has been determined by Scanning Tunneling Spectroscopy (STS) and by tight-binding (TB) theoretical modeling. The scattering of Ag(111) surface state electrons by the Ce adatom superlattice leads to gap openings and to LDOS singularities at high-symmetry points of the surface mini-Brillouin zone which agree quantitatively with the characteristic features observed in differential conductance (dI/dV) spectra. The energy gain associated with the gap formation is maximal for superlattices with adatom distances in the range 2.3 - 3.5 nm, in excellent agreement with experiments. Finally, Ce is a magnetic atom, and such a superlattice of magnetic adatoms might be useful for the development of future atomic-scale magnetic devices.
[1]F. Silly, M. Pivetta, M. Ternes, F. Patthey, J.P. Pelz, and W.-D. Schneider, Phys.Rev.Lett. 92,016101(2004).

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1. Fabrication of ordered atomic-scale structures - a step towards future atomic-scale technology