Nanostructuring Rh(110) Surfaces by Ion Etching
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0960-N01-02
Nanostructuring Rh(110) Surfaces by Ion Etching Alessandro Molle, Andrea Toma, Corrado Boragno, Ugo Valbusa, and Francesco Buatier de Mongeot Dipartimento di Fisica, Università di Genova, Via Dodecaneso, 33, Genova, 16146, Italy
ABSTRACT The ion irradiation of the Rh(110) surface results in the self-organised formation of various nano-structured morphologies like ripples, mounds, pyramids which have been thoroughly studied as a function of the incidence angle and of the impact energy of the impinging ions. A study of the evolution of the surface ripples at various impact energies above the hot-spot threshold, has been rationalized in terms of a contribution due to an ion-induced surface diffusion mechanism. In the very low ion incidence regime, where the formation of hot spots following ion impact is inhibited, the formation of a rhomboidal pyramid pattern is singled out and attributed to the predominant reorganization of surface adatom and vacancies produced in the topmost surface layers. The metastable rhomboidal pyramid pattern, was recently proven to have extraordinary chemical reactivity since it is endowed with a very high density of undercoordinated step sites runnin along the very open azimuthal direction.
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INTRODUCTION
Nanostructuring transition metals via ion irradiation is a well-known result descending from the competition between the diffusive relaxation of the collision-induced surface defects (adatoms and vacancies) and the curvature dependent erosion rate [1]. The two terms can be distinctly discriminated by acting on the incidence angle of the ion beam. At grazing angles the etching removal due to the ion impacts prevails therein forcing the surface destabilization towards rippled patterns with ridges aligned along the projection of the beam in agreement with the linear theory due to Bradley and Harper (BH) [2]. Tuning the ripple orientation by means of the ion beam directionality leads to a non-equilibrium distribution of majority steps parallel to the ripple ridges which, for instance, can be used as a template for a subsequent deposition of a hetroepitaxial overlayer in order to artificially tailor the film morphology with a prescribed lateral ordering in the nanometer scale. As an example, this configuration has been recently adopted in the case of a ferromagnetic Co film supported on a Cu(001), patterned until disconnected Co nanowires were formed parallel to the ion beam projection in correspondence of which the easy axis of the magnetic anisotropy switched from biaxial to uniaxial along the controlled direction of the ripple ridges [3]. Furthermore, a similar procedure has been pursued in the case of the Co growth on a pre-patterned Cu(110) substrate in order to achieve a position controlled nucleation of Co nanocrystals [4]. Below a critical angle θ* (roughly 60° from the normal to the initially flat surface plane) the diffusive regime sets in provided thermally activated diffusion is sufficiently efficient and consequently the rippled configurations reflect the symmetry o
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