Surface etching by ion sputtering can be used to pattern surfaces. Recent studies using the high-spatial-resolution capability of the scanning tunnelling microscope revealed in fact that ion bombardment produces repetitive structures at nanometre scale, creating peculiar surface morphologies ranging from selfaffine patterns to 'fingerprint'-like and even regular structures, for instance waves (ripples), chequerboards or pyramids. The phenomenon is related to the interplay between ion erosion and diffusion of adatoms (vacancies), which induces surface re-organization. The paper reviews the use of sputter etching to modify 'in situ' surfaces and thin films, producing substrates with well defined vertical roughness, lateral periodicity and controlled step size and orientation.

We show that dissociative oxygen adsorption on Ag͑001͒ induces below room temperature a missing row 2ͱ2ϫͱ2 reconstruction of the substrate. As demonstrated by the analysis of the photoelectron diffraction patterns, the oxygen atoms sit thereby in a c(2ϫ2) arrangement in the previous fourfold hollow sites nearly coplanar with the Ag atoms, while rows of substrate atoms are removed along the ͓100͔ directions. Annealing the crystal above 350 K restores the p(1ϫ1) symmetry and the oxygen moves to 0.6 Å above the fourfold hollow site. It becomes then more oxidic in nature, as demonstrated by the shift of the O 1s level from 530.3 eV to 528.3 eV. The phase transition affects also the O 2s and O 2p levels as well as the surface component of Ag 3d 5/2 . The vibrational frequency of the oxygen adatoms against the surface decreases at the phase transition, in accord with the larger adsorption distance. The higher temperature phase is active towards CO and C 2 H 4 oxidation, while the low-temperature phase is not. When cooling the sample below room temperature the reconstructed phase is restored. The time constant of this process as well as the chemical reactivity of the high-temperature phase are weakly reproducible since they depend on the previous history, i.e., presumably on the subsurface oxygen content of the sample.

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