Ripple rotation in multilayer homoepitaxy

Applications of metal surfaces nanostructured by ion beam sputtering

We review results relative to the formation of regular nanoscale patterns on metal substrates exposed to defocused ion beam irradiation. Particular emphasis is placed on work which demonstrates the possibility of controllably modifying chemico-physical properties of the material by tailoring the nanoscale morphology during IBS patterning. Starting from the well-established results found on single-crystal model systems, we show how the controlled modification of the atomic step termination can deeply affect chemical reactivity or magnetic anisotropy. We then look in greater detail at the more recent attempts focused on the extension of IBS patterning on supported polycrystalline metal films, a promising class of systems in view of potential applications. A modification of the functional properties of metal films can also be obtained by forcing a shape anisotropy of the nanostructures. The modification of the optical response of polycrystalline metal nanowires supported on anisotropic templates produced by IBS provides a clear example of this.

Synchrotron x-ray scattering from metal surfaces nanostructured by IBS

Ion beam sputtering (IBS) can induce the formation of ordered nanostructures, whose properties depend on ion flux, sputtering angle, sample temperature, sample structure, surface symmetry, etc. For the comprehension of the time evolution of the formed nanostructure morphology it is necessary to perform in situ real time studies. In this review we shall describe results obtained using x-ray based techniques at synchrotron facilities to study in situ the time and temperature evolution of metal surfaces nanopatterned by ion sputtering. Different techniques, such as x-ray reflectivity, grazing incidence small angle x-ray scattering and x-ray surface diffraction have been used, each of them providing complementary information for the determination of the surface structure and morphology. In this review, we present some experiments done in recent years to show how these methods contributed to our understanding of the IBS process on metal surfaces.

Dislocation dynamics and surface coarsening of rippled states in the epitaxial growth and erosion on (110) crystal surfaces

Rippled one-dimensionally periodic structures are commonly seen in the experimental studies of the epitaxial growth and erosion on low symmetry rectangular (110) crystal surfaces. Rippled states period (wavelength) and amplitude grow via a coarsening process that involves motion and annihilations of the dislocations disordering perfect periodicity of these structures. Unlike the ordinary dislocations in equilibrium systems, the dislocations of the growing rippled states are genuinely traveling objects, never at rest. Here, we theoretically elucidate the structure and dynamics of these far-from-equilibrium topological defects. We derive fundamental dislocation dynamics laws that relate the dislocation velocity to the rippled state period. Next, we use our dislocations velocity laws to derive the coarsening laws for the temporal evolution of the rippled state period lambda and the ripple amplitude w (surface roughness). For the simple rippled states on (110) surfaces, we obtain the coarsening law lambda approximately w approximately t{2/7} . Under some circumstances however, we find that these states may exhibit a faster coarsening with lambda approximately w approximately t{1/3} . We also discuss the dislocations in the rectangular rippled surface states for which we derive the coarsening law with lambda approximately w approximately t{1/4} . The coarsening laws that occur at the transition from the rippled to the rhomboidal pyramid state are also discussed, as well as the crossover effects that occur in rippled states in the proximity of this transition on (110) crystal surfaces.

Vertical asymmetry and the ripple-rotation transition in epitaxial growth and erosion on (110) crystal surfaces

Vertical (up-down) asymmetry is ubiquitous feature of the nonequilibrium statistical mechanics of realistic growing interfaces. Yet, the actual role of vertical asymmetry (VA) in epitaxial growth on crystal surfaces is still elusive. Is vertical asymmetry a primary or secondary factor in epitaxial growth and erosion? Can vertical asymmetry alone produce major qualitative effects on long length scale interface morphologies? To address these questions in depth, we theoretically discuss the effects of vertical growth asymmetry on far-from-equilibrium interfacial morphologies occurring in the epitaxial growth and erosion of (110) crystal surfaces. We theoretically elucidate the so-called ripple rotation transition on the Ag(110) crystal surface [F. B. de Mongeot; Phys. Rev. Lett. 84, 2445 (2000), G. Constantini, J. Phys.: Condens. Matter 13, 5875 (2001)], as the transition between the rectangular rippled states (checker-board structures of alternating rectangular pyramids and pits). We show that the experimental surface diffraction data seen in this transition can be understood only by invoking vertical growth asymmetry. In the proximity of the transition point, we find that vertical asymmetry itself produces an interface morphology yielding a four-lobe near in-phase diffraction pattern having four peaks along the principal axes of the (110) surface, in accord with the experiments on Ag(110). Moreover, on the two sides of the ripple rotation transition, we find two exotic interface states induced by vertical asymmetry, which correspond well /to the interface morphologies seen on Ag(110). We document our results by numerical simulations and by analytic arguments. Our theoretical findings, in combination with experiments, provide the first rigorous evidence that VA plays a significant role in epitaxial growth and erosion on crystal surfaces.

Self-organised synthesis of Rh nanostructures with tunable chemical reactivity

Nonequilibrium periodic nanostructures such as nanoscale ripples, mounds and rhomboidal pyramids formed on Rh(110) are particularly interesting as candidate model systems with enhanced catalytic reactivity, since they are endowed with steep facets running along nonequilibrium low-symmetry directions, exposing a high density of undercoordinated atoms. In this review we report on the formation of these novel nanostructured surfaces, a kinetic process which can be controlled by changing parameters such as temperature, sputtering ion flux and energy. The role of surface morphology with respect to chemical reactivity is investigated by analysing the carbon monoxide dissociation probability on the different nanostructured surfaces.

Interfacial states and far-from-equilibrium transitions in the epitaxial growth and erosion on (110) crystal surfaces

We discuss the far-from-equilibrium interfacial phenomena occurring in the multilayer homoepitaxial growth and erosion on (110) crystal surfaces. Experimentally, these rectangular symmetry surfaces exhibit a multitude of interesting nonequilibrium interfacial structures, such as the rippled one-dimensional periodic states that are not present in the homoepitaxial growth and erosion on the high symmetry (100) and (111) crystal surfaces. Within a unified phenomenological model, we reveal and elucidate this multitude of states on (110) surfaces as well as the transitions between them. By analytic arguments and numerical simulations, we address experimentally observed transitions between two types of rippled states on (110) surfaces. We discuss several intermediary interface states intervening, via consecutive transitions, between the two rippled states. One of them is the rhomboidal pyramid state, theoretically predicted by Golubovic [Phys. Rev. Lett. 89, 266104 (2002)] and subsequently seen, by de Mongeot and co-workers, in the epitaxial erosion of Cu(110) and Rh(110) surfaces [A. Molle, Phys. Rev. Lett. 93, 256103 (2004), and A. Molle, Phys. Rev. B 73, 155418 (2006)]. In addition, we find a number of interesting intermediary states having structural properties somewhere between those of rippled and pyramidal states. Prominent among them are the rectangular rippled states of long rooflike objects (huts) recently seen on Ag(110) surface. We also predict the existence of a striking interfacial structure that carries nonzero, persistent surface currents. Periodic surface currents vortex lattice formed in this so-called buckled rippled interface state is a far-from-equilibrium relative of the self-organized convective flow patterns in hydrodynamic systems. We discuss the coarsening growth of the multitude of the interfacial states on (110) crystal surfaces.

Self-organized formation of rhomboidal nanopyramids on fcc(110) metal surfaces

We report on the far from equilibrium self-organized morphologies obtained after Xe ion irradiation of the Rh(110) and Cu(110) surfaces. Here we experimentally identify by means of high resolution LEED a novel interfacial state characterized by a rhomboidal pyramid islanding with majority steps oriented along nonequilibrium low-symmetry directions. The formation of the novel rhomboidal pyramid state and the transition to the well-known rippled phases results from a delicate interplay of kinetic processes which are controlled by acting on temperature, ion flux, and impact energy.

Peculiar effects of anisotropic diffusion on dynamics of vicinal surfaces

We report on peculiar behaviors due to anisotropic terrace diffusion on step meandering on a vicinal surface. We find that anisotropy triggers tilted ripples. In addition, if the fast diffusion direction is perpendicular to the steps, the instability is moderate and coarsening is absent, while in the opposite case the instability is promoted, and interrupted coarsening may be observed. Strong enough anisotropy restabilizes the step for almost all step orientations. These findings point to the nontrivial effect of anisotropy and open promising lines of inquiries in the design of surface architectures.

Nanocrystal formation and faceting instability in Al(110) homoepitaxy: true upward adatom diffusion at step edges and island corners

Using atomic force microscopy and spot-profile analyzing low energy electron diffraction, we have observed the existence of a striking faceting instability in Al(110) homoepitaxy, characterized by the formation of nanocrystals with well-defined facets. These hut-shaped nanocrystals are over tenfold higher than the total film coverage, and coexist in a bimodal growth mode with much shallower and more populous surface mounds. We further use density functional theory calculations to elucidate the microscopic origin of the faceting instability, induced by surprisingly low activation barriers for adatom ascent at step edges and island corners.