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We report on the self-organized patterning of polycrystalline metal films supported on dielectric substrates using defocused Ar{sup +} ion-beam sputtering at glancing incidence. The data demonstrate that a nonflat initial surface morphology does plays a critical role in triggering self-organization during the early stages, where the distribution of polycrystalline grains initially imposes a nonstochastic spatial modulation of the ion impact sites. A crossover to the conventional erosive regime observed for single crystals sets in at longer sputtering times. Surprisingly, the vertical and lateral modulation of the pristine surface profile of the nonflat films results in a substantial reduction in the ion dose required for the formation of the nanoscale patterns, in comparison to the case of an initially flat film.

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.

We have experimentally determined the magnitude of the in-plane magnetocrystalline anisotropy of monatomic Fe steps on the surface of nanopatterned $\mathrm{Fe}∕\mathrm{Ag}(001)$ films. The films have been nanopatterned by the ion-sculpting technique, in which grazing-incidence ion sputtering is exploited to induce the formation of nanometer-scale surface ripples oriented along the ion-beam direction. We quantitatively assessed the ripple morphology in order to determine the density of Fe monatomic steps parallel to the ripple ridges. By correlating the morphology with the measured uniaxial magnetic anisotropy, we obtained the strength of the magnetocrystalline anisotropy of Fe monatomic steps ${\ensuremath{\kappa}}_{u}^{\mathit{mc}}=(6.5\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕\mathrm{cm}$.

Using a quartz crystal microbalance technique, we have measured the interfacial viscosity of nitrogen monolayers deposited on very homogeneous Pb(111) surfaces. At temperatures below 15?K, no dissipation is detected, suggesting that the N2 films are rigidly coupled to the oscillating electrode. By raising the temperature close to 20?K, we find sliding of the nitrogen film for coverages above about 0.5?nominal layers. The observed temperature dependence is in good qualitative agreement with recent calculations on the static friction of a nitrogen slab in contact with a crystalline Pb substrate.

We have investigated the surface kink contribution to the in-plane magnetic anisotropy of nanostructured ultrathin Co/Cu(001) and Fe/Ag(001) films. We have exploited the ion sculpting technique to induce the formation of nanosized surface ripples misaligned with respect to the high-symmetry crystallographic directions, thus obtaining a controlled distribution of surface steps and kinks departing from the fourfold-symmetric equilibrium one. The imbalance in the densities of the differently oriented kinks thus generated allows to underscore the magnetic anisotropy of atoms at kink sites. We find that atoms at kink sites at the surface of Fe/Ag(001) films give an overall uniaxial contribution to the in-plane magnetic anisotropy with easy axis oriented along the directions, while no sizable kink contribution has been observed in the case of Co/Cu(001) films.

M. Goryl,1 F. Buatier de Mongeot,2 F. Krok,1 A. Vevecka-Priftaj,3 and M. Szymonski1,* 1Center for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy, and Applied Computer Science, Jagiellonian University, ulica Reymonta 4, 30-059 Krakow, Poland 2Dipartimento di Fisica, Universita di Genova, Via Dodecaneso 33, 16146 Genova, Italy 3Department of Physics, Polytechnic University of Tirana, Sheshi “Nene Tereza,” No. 4, Tirana, Albania Received 8 March 2007; revised manuscript received 21 May 2007; published 17 August 2007

The possibility of modifying the magnetic properties of ultrathin films by means of ion sculpting has been verified for Co/Cu(0 0 1) and Fe/Ag(0 0 1). For both these systems, ion sputtering at grazing incidence performed in a proper, material-dependent, temperature range induces the formation of ripples at the surface of the films. Following ripple formation, the magnetic anisotropy in the plane of the films undergoes dramatic changes brought about by the out-of-equilibrium distribution of defect sites induced by the ripple structure itself.

Metal nano clusters are of great importance in physical and chemical science. One simple method for obtaining metal clusters consists in the deposition of a metal film on an inert substrate followed by annealing at high temperature. After this procedure, the metal film reduces in small clusters, whose morphology depends on the annealing temperature and annealing time. In this work we present a grazing incidence small angle X-ray scattering (GISAXS) study carried out in situ to understand the nucleation and formation of Ni metal clusters. For this purpose uniform Ni metal films, of different thicknesses, were deposited onto an oxidized Si(0 0 1) substrate, and annealed at different temperatures in the range 400–800 K. Before annealing the samples were characterized by X-ray reflectivity measurements to exactly determine the values of the thickness and of the starting roughness. The GISAXS patterns show a surface roughness increase starting at about 600 K. By increasing the temperature the diffused intensity breaks in two lines around the reflectivity plane, indication of a characteristic length correlation of the roughness. This correlation length is maintained during the metal clusters formation.

The onset of in-plane uniaxial magnetic anisotropy in ion-sculpted ultrathin Fe/Ag(0 0 1) films has been investigated. Uniaxial anisotropy already develops at ion fluences where the self-organisation of the ripple structure has not yet started. The evolution of the strength of the uniaxial anisotropy as a function of ion fluence allows to identify the step unbalance at the ion-sculpted surface as the origin of the magnetic anisotropy.

Dynamic force microscopy (DFM), Kelvin probe force microscopy (KPFM) and scanning tunnelling microscopy/spectroscopy (STM/STS) have been used to study epitaxial nanostructures formed as a result of submonolayer deposition of gold on reconstructed InSb, InAs, and GaAs(001) surfaces. It has been found that both topography and composition of the structures are strongly dependent on the substrate temperature during evaporation. In the particular case of Au deposited on a c(8 × 2) InSb(001) surface at 300 K, the nanostructures have predominantly the form of rectangular islands with edges oriented along [110] and [] directions. Increasing the substrate temperature up to 600 K during deposition results in formation of narrow, long structures (nanowires) with an average length of 800 nm. The nanowires are created along the [110] direction of the reconstructed InSb(001) surface. High resolution KPFM and STS measurements indicate that nanowires of different heights may have different chemical composition and electronic structure.

We have experimentally determined the magnitude of the in-plane magnetocrystalline anisotropy of monatomic Fe steps on the surface of nanopatterned Fe/Ag(001) films. The films have been nanopatterned by the ion-sculpting technique, in which grazing-incidence ion sputtering is exploited to induce the formation of nanometer-scale surface ripples oriented along the ion-beam direction. We quantitatively assessed the ripple morphology in order to determine the density of Fe monatomic steps parallel to the ripple ridges. By correlating the morphology with the measured uniaxial magnetic anisotropy, we obtained the strength of the magnetocrystalline anisotropy of Fe monatomic steps kappa(mc)(u)=(6.5 +/- 1.5)x10(-10) erg/cm.

We have studied the nanofriction of Ne monolayers with a quartz-crystal microbalance technique at temperatures below 6.5 K and in ultrahigh-vacuum conditions. Very homogeneous and smooth lead electrodes have been physically deposited on a quartz blank at 150 K and then annealed at room temperatures. With such a Pb-plated quartz-crystal microbalance, we have observed a pronounced depinning transition separating a low-coverage region, where the film is nearly locked to the oscillating electrode, from a high-coverage region characterized by slippage at the solid-fluid boundary. Such a behavior has been found to be very reproducible. These data are suggestive of a structural depinning of the solid Ne film when it becomes incommensurate with the lead substrate, in agreement with the results of an extensive molecular-dynamics study.

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.

We have studied the nanofriction of Ne monolayers with a quartz-crystal microbalance technique at temperatures below 6.5 K and in ultrahigh-vacuum conditions. Very homogeneous and smooth lead electrodes have been physically deposited on a quartz blank at 150 K and then annealed at room temperatures. With such a Pb-plated quartz-crystal microbalance, we have observed a pronounced depinning transition separating a low-coverage region, where the film is nearly locked to the oscillating electrode, from a high-coverage region characterized by slippage at the solid-fluid boundary. Such a behavior has been found to be very reproducible. These data are suggestive of a structural depinning of the solid Ne film when it becomes incommensurate with the lead substrate, in agreement with the results of an extensive molecular-dynamics study.

Ion sculpting of ultrathin Fe/Ag(001) films induces the self-assembled formation of nanometric surface ripples oriented along the ion beam direction and the corresponding onset of an in-plane uniaxial contribution to magnetic anisotropy. The authors show that fine tuning of the ion dose impinging on the film allows to tailor the in-plane magnetic anisotropy of such films from purely biaxial in the as-grown state to purely uniaxial. The magnitude of magnetic anisotropy in the pure uniaxial state can be tailored by varying the Fe film thickness prior to irradiation. (c) 2006 American Institute of Physics.

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.

We have developed and tested a vacuum suitcase which allows to transport samples under ultrahigh vacuum (UHV) conditions. The suitcase is pumped by a new performant pumping system based on a getter pump, and it reaches an ultimate pressure lower than 3×10−11mbar, which is 2 orders of magnitude better than in the existing projects. Furthermore it has no need for a continuos power supply, no electric or magnetic field, low weight, low cost, and compactness. In order to transfer the sample from the suitcase to the main experimental chamber, in a short time (about 1h) and under UHV conditions, we have also developed a turbo pumped buffer chamber equipped with a cold trap.

We report on the growth of {111}-faceted Co nanocrystals endowed with fcc structure during multilayer epitaxial deposition on a flat Cu(110) substrate. Lateral ordering of the Co nanocrystals into a periodic array is achieved if the Co deposition is performed on a Cu(110) substrate, prepatterned by ion sculpting. While for low coverages the Co film growth is conformal, reproducing the underlying arrangement of the Cu substrate pattern, for film thicknesses above 16 monolayers, size-selected {111}-faceted Co nanocrystals nucleate in registry with the underlying pattern periodicity.

Metal surfaces can be easily nanopatterned via ion sputtering: mounds or ripples can be created depending on the surface symmetry and temperature. However, in many cases these structures are unstable at room temperature and above, due to the adatom fast diffusion. This fact prevents the use of such systems as substrate or nanostamps for a technological implementation. In this paper we present a spot profile analysis low energy electron diffraction (SPA-LEED) study on the nanopatterning of a Rh(1 1 0) single crystal. Like the other (1 1 0) metal surfaces, previously investigated, also Rh(1 1 0) shows for increasing temperatures a transition between different rippled morphologies. The main advantage of this system is its stability at room temperature. From SPA-LEED data we can measure the structural features (average periodicity and local faceting) of the observed rippled structures.

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.

We have investigated the growth of surface nanostructures on a Co/Cu(001) film and the growth of Co films on a nanostructured Cu(001) substrate as well as the effect of nanoscale pattern formation on the film magnetic properties. Here we demonstrate by scanning tunneling microscopy measurements and magneto-optic Kerr effect hysteresis curves that low-temperature grazing-incidence ion sputtering can be used to induce the formation of nanoscale ripples which reduce the four-fold symmetry of the Co film to two-fold, thus generating a strong in-plane uniaxial magnetic anisotropy. The nanostructures and the associated uniaxial magnetic anisotropy were found to be stable up to room temperature.

We have investigated the correlation between morphology and magnetic anisotropy in nanostructured Co films on Cu(001). The formation of nanoscale ripples by ion erosion is found to deeply affect the magnetic properties of the Co film. A surface-type uniaxial magnetic anisotropy with easy axis parallel to the ripples is observed. The origin of the magnetic anisotropy has been identified with the modification of thermodynamic-step distribution induced by ripple formation. At higher ion doses, when Co ripples detach and crystalline nanowires form, a strong enhancement of the magnetic anisotropy due to magnetostatic contributions is observed.

Thermal energy He atom scattering is used to investigate the recovery of a nanometer-scale corrugation on the (110) surface of silver. Periodic and remarkably well-ordered rippled structures with ridges oriented along the $〈001〉$ and $〈11\ifmmode\bar\else\textasciimacron\fi{}0〉$ azimuthal directions are grown by ion sputtering at grazing incidence and at a crystal temperature of 210 K. Hence, morphological equilibration of the corrugated surface is investigated in real time in the temperature range between 200 and 230 K. The activation energy for the mechanism driving surface relaxation of $〈001〉,$ and $〈11\ifmmode\bar\else\textasciimacron\fi{}0〉\ensuremath{-}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{d}$ ripples is measured to be $(0.52\ifmmode\pm\else\textpm\fi{}0.09)$ and $(0.43\ifmmode\pm\else\textpm\fi{}0.05)\mathrm{eV},$ respectively. The same underlying rate limiting process\char22{}i.e., adatom detachment from the open $〈001〉$ step edge\char22{}is suggested. Finally, the ripple amplitude is observed to decay with time following the inverse linear behavior, independently of the ripple orientation and of the substrate temperature. This result disagrees with the predictions of the one-dimensional model by Israeli and Kandel where an exponential decay law is obtained for the case of attachment-detachment limited kinetics.

We have investigated the correlation between morphology and magnetic anisotropy in nanostructured Co films on Cu(001). The formation of nanoscale ripples by ion erosion is found to deeply affect the magnetic properties of the Co film. A surface-type uniaxial magnetic anisotropy with easy axis parallel to the ripples is observed. The origin of the magnetic anisotropy has been identified with the modification of thermodynamic-step distribution induced by ripple formation. At higher ion doses, when Co ripples detach and crystalline nanowires form, a strong enhancement of the magnetic anisotropy due to magnetostatic contributions is observed.

The morphological equilibration of the rippled (1 1 0) surface of silver is investigated using thermal energy He atom scattering. After ion sputtering at grazing incidence and at a crystal temperature around 210 K a well defined ripple structure with ridges oriented along the incoming ion beam direction is observed. The evolution with time of this structure is followed at different surface temperatures in the range between 170 and 220 K to gain information on the ripple stability and on the diffusion processes contributing to ripple decay.

The time evolution of the morphology of the Ag(110) surface during ion sputtering has been studied in situ and in real time by x-ray based techniques. The surface was bombarded with ${\mathrm{Ar}}^{+}$ ions at an energy of 1 keV in the temperature range 100\char21{}320 K. Grazing-incidence x-ray scattering measurements have been carried out in order to characterize the shape and the time evolution of the regular structures (mounds or ripples) created on the surface. The results show that the periodicity (i.e., the average separation between features) increases as function of the sputtering time, following a power-law behavior. Moreover, the slope of the mounds/ripples depends on temperature, ranging between 6 and 12\ifmmode^\circ\else\textdegree\fi{} if measured along the 〈1 -1 0〉 direction and between 8 and 10\ifmmode^\circ\else\textdegree\fi{} along 〈0 0 1〉.

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 self-affine 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 have investigated the growth of Ag(001) in the multilayer regime by high-resolution electron diffraction (SPA-LEED). Ag deposition at temperatures above 180 K results in the growth of approximately square mounds with a predominant 10 step termination which is thermodynamically favored. Surprisingly, when the deposition temperature is reduced below 130 K, instead of observing a random roughening of the step profile, as expected in a self-affine growth regime, the proliferation of 001 oriented facets is observed. We attribute our observation to the de-activation of corner crossing, as well as to the presence of easy pathways for interlayer diffusion down the {111} facets which are formed at 10 steps.

In metal surfaces a build-up of a regular pattern during ion sputtering is produced by two different mechanisms that produce a similar surface instability: the surface curvature dependence of the sputtering yield and the presence of an extra energy barrier whenever diffusing adatoms try to descend step edges. By tuning the competition between erosion- and diffusion-induced surface re-organization, it is possible to investigate new phenomena like the rotation of ripple orientation on an anisotropic fcc(110) substrate and the pattering of a fcc(001) substrate from a mound-like to a ripple structure. Similar phenomena occur in the case of homoepitaxial growth. In the multilayer regime [Ag on Ag(110)] the rotation of a ripple-like pattern was observed with changing surface temperature. The phenomenon is related to the peculiar hierarchy of inter- and intra-layer diffusion barriers present on the anisotropic Ag(110) substrate. Homoepitaxial growth on Ag(001) in the multilayer regime is studied with STM. The height, lateral distance, and order of the mound-like structures change with the deposition temperature. The surface morphologies obtained after sputtering and homoepitaxial deposition of Ag(001) have strong similarities. The general approach considers sputtering as the negative of homoepitaxial deposition, i.e. as a deposition of vacancies which can eventually have an asymmetry in their diffusivities. This paper shows that this picture is oversimplified because the dominant effect of ion sputtering is the formation of clusters of adatoms and vacancies.

The evolution of the morphology of ion sputtered Ag(1 1 0) has been studied by X-ray technique at the European Synchrotron Radiation in Grenoble and by scanning tunneling microscope. The surface was bombarded with Ar þ ions at an energy of 1 keV, changing the temperature in the range 100–320 K. The erosion is never layer by layer in the explored temperature range. On the contrary, regular structures (ripples) can be created on the surface, and their orientation rotates by 90 by changing temperature. Also the slope of the mounds/ripples changes with temperature, ranging between 6 and 12 if measured along h1 � 10 i and between 8 and 10 along h 00 1i. 2002 Published by Elsevier Science B.V.

We report on the formation of high aspect ratio anisotropic nanopatterns on crystalline GaAs (100) and Si (100) substrates exploiting defocused Ion Beam Sputtering assisted by a sacrificial self-organised Au stencil mask. The tailored optical properties of the substrates are characterised in terms of total reflectivity and haze by means of integrating sphere measurements as a function of the morphological modification at increasing ion fluence. Refractive index grading from sub-wavelength surface features induces polarisation dependent anti-reflection behaviour in the visible-near infrared (VIS-NIR) range, while light scattering at off-specular angles from larger structures leads to very high values of the haze functions in reflection. The results, obtained for an important class of technologically relevant materials, are appealing in view of photovoltaic and photonic applications aiming at photon harvesting in ultrathin crystalline solar cells.

We present a scanning tunneling microscopy study of the direct comparison between homoepitaxial deposition and surface ion sputtering on the Ag(001) system. At a temperature of 200 K, sputtering results in mound formation similar to the epitaxy case, while at higher temperatures an erosive regime sets in with the appearance of regular square pits. Contrary to the conventional wisdom, which considers ion sputtering as a deposition of vacancies, the analysis of single ion impact events reveals that the process produces both adatom and vacancy clusters. The key parameter determining the temperature dependence of surface morphology turns out to be the mobility of the adatom clusters which exceeds that of vacancy clusters.

We demonstrate that double ionisation of the Ag atoms, leading to the formation of d-holes, takes place in the initial oxidation stage of Ag(001). The complex formed by the Ag2+ ion, two oxygen adatoms and one oxygen atom in the octahedral interstitial gives rise to a dipole active electron energy loss at 130 meV. Signature of the electronic origin of this loss is the inverse isotope shift and the peculiar temperature dependence of the energy loss, which can be reproduced by a theoretical model.

This article gathers together a collection of recent experimental studies of the adsorption of oxygen on (001), (110) and (111) crystal surfaces of silver with special emphasis on the negative ion states of this model system for oxygen adsorption. These investigations were performed in a network entitled `Negative ion resonances of adsorbed molecules' supported financially by the European Union within the `Human capital and mobility programme'. The kinetics and thermodynamics of adsorption are investigated by measuring the sticking coefficient and by thermal desorption spectroscopy (TDS). The vibrational spectra provided by high-resolution electron energy loss spectroscopy (HREELS) are used to analyse the adsorbed species (physisorbed and chemisorbed) in the case of O2 on Ag(110) and on Ag(111). The mechanisms of inelastic electron scattering by adsorbed O2 are further investigated with special reference to the negative ion resonances (NIRs), formed by electron capture, which are involved in the electron-molecule collision process.

We present a scanning tunnelling microscopy investigation of homoepitaxial growth on Ag(001) in the multilayer regime. The deposition of 33 monolayers (ML) of silver at a flux of 0.075 ML s −1 , produces mound-like structures whose height, lateral distance and order change with the deposition temperature T dep . In particular the temperature dependence of the surface roughness is characterised by a bell shape with a maximum around 200 K. The slope of the mound sides is found to increase monotonically with decreasing temperature until, for T dep

As recently demonstrated, the morphology of a surface can be modified on the mesoscopic scale by ion sputtering. Here we show by microscopy and spectroscopy that the chemical properties of the surface are strongly affected by nanostructuring and that surface reactivity can be tuned by changing surface morphology. For the otherwise inert Ag(001) surface significant O2 dissociation takes place on the nanostructured surface, thus allowing us to control the relative coverage of admolecules and adatoms. The dissociation probability is determined by the experimentally tunable density of kinks.

Surface etching by ion sputtering, besides producing equilibrium-oriented patterns similar to those obtained by molecular beam epitaxy (MBE), can also be used to pattern the surface along nonequilibrium orientations, thus extending the possibilities of MBE. By tuning the competition between ion erosion at grazing angles and diffusion-induced surface reorganization, it is, for example, possible to pattern a substrate characterized by a square symmetry with a well-ordered ripple structure running along any desired direction.

The results of a comparative STM and TPD/TPR study on the adsorption and oxidation of CO on pure Ru(0001) and on structurally and compositionally well-defined, bimetallic Pt/Ru(0001) surfaces are presented. The defect structure and local surface composition of the bimetallic substrates, which are produced by epitaxial growth of monolayer Pt islands or by Pt deposition and subsequent surface alloying, are characterized by high-resolution STM images with chemical contrast. TPD and TPR experiments for CO adsorption/oxidation on these surfaces show a distinct lowering of the CO adsorption energy on the bimetallic surfaces with respect to the pure Ru(0001) and Pt(111) surfaces, with the onset of CO desorption already at 230 K. The reduction in adsorption energy is attributed to an electronic modification of the deposit metal due to interaction with the Ru(0001) substrate. The bimetallic surface alloy catalyzes CO oxidation under UHV conditions. The reduced temperature for CO 2 desorption as compared to Pt(111) indicates an even lower barrier than on the latter surface, whereas the pure Ru(0001) surface is inert under these conditions. The results are discussed with respect to the superior CO tolerance of bimetallic PtRu catalysts in low-temperature fuel cells as compared to monometallic Pt catalysts, leading to a mechanistic explanation of that phenomenon distinctly diVerent from previous ideas. © 1998 Elsevier Science B.V. All rights reserved.

The coverage dependence of the sticking coefficient and temperature programmed desorption kinetics for molecular adsorption of oxygen on Ag(001) are investigated both experimentally and theoretically. The nearly exponential decrease of the sticking coefficient as a function of coverage by about five orders of magnitude is explained within a kinetic lattice gas model to be the result of strong nearest- and next-nearest-neighbor repulsions. The latter also shape the desorption spectra which are calculated within the same model under the assumption that fast surface diffusion maintains the adsorbate in quasiequilibrium during desorption. Because the desorption rate is proportional to the sticking coefficient under these conditions, we find a massive decrease of the pre-exponential factor in the Arrhenius parametrization of the rate as a function of coverage which explains the peculiar shape of the desorption spectra.

The relevant parameters which characterise molecular desorption of O 2 -Ag(001) have been determined under isothermal conditions in the low coverage limit, using the beam-flag method. We find a desorption order n=1.07±0.10, a desorption energy E des =0.50±0.12 eV and a pre-exponential factor v des =10 13.3±3.5 s -1 . Such values compare well with the thermal desorption parameters present in literature for molecular oxygen adsorbed on Ag(110). The much lower dissociative adsorption probability of O 2 on Ag(001) is therefore due to the existence of a different dissociation mechanism and not to a difference in the desorption probability compared with Ag(110).

An energetic beam of Xe atoms causes the dissociation of adsorbed O 2 molecules on cold Ag(110) or Pt(111) surfaces. This is a parallel channel to the previously observed collision induced desorption. Using EELS, mass spectrometry and TDS, the energy dependence of the collision induced dissociation and desorption cross sections was measured over a range of Xe beam energies. On both surfaces these events seem to have a common threshold energy of about 0.9 eV for Ag(110) and 1.2 eV for Pt(111). The branching ratios between desorption and dissociation, which are about 3 and 3.3 for Ag(110) and Pt(111), respectively, exhibit no or weak energy dependence.

We report on nanoscale patterning of GaAs (100) semiconductor substrates employing an ion projection through a self-organized stencil mask. The nanostructured mask, formed by ion beam sputtering of a polycrystalline Au film, allows driving the GaAs morphology strongly out of equilibrium. In a second stage, after the stencil mask is removed, we quantitatively follow the dynamical evolution of the forced system toward equilibrium by analyzing the evolution of the power spectral density of the height profiles and their slope and curvature distribution. Strong differences are observed by comparing the relaxation dynamics of GaAs surfaces which tend to smoothen with that of glass, a material which instead tends to non-linearly amplify the pattern once driven out of equilibrium by the stencil mask.

Si:P bolometers doped close to the metal insulator transition are suitable for detecting heat pulses owing to their fast response time, high responsivity [1] and possibility to work in magnetic field [2]. However, up to now these bolometers were realized directly on the surface of the same Si crystal in which the ballistic phonons propagated. This fact limited their application. To solve this problem a detachable Si:P bolometer has been realized using a superconducting thin film as coupling medium [3].

We report on self-organized patterning of polycrystalline noble metal films, supported on dielectric substrates, by defocused Ar+ ion beam irradiation. The initial surface morphology affects the formation of nanostructures, forcing the growth of ripples with a lateral periodicity imposed by the pristine polycrystalline grain distribution. At the early stages, the self-organization process is dominated by the shadowing of taller grains, while a crossover to the conventional erosive regime, observed for single crystals, sets in at longer sputtering times. The grain boundaries, although providing an additional bias for diffusion of mobile defects, do not affect the propagation of nanoscale ripples across individual grains.

We present results on simultaneous nanostructuring and optical activation of lithium fluoride crystals by 800eV off-normal Ar+ sputtering at different ion doses. The samples were studied by atomic force microscopy and optical spectroscopy. After ion irradiation smoothening of the initial random roughness is achieved and well-defined self-organized ripple structures appear, having a mean periodicity of 30nm and a mean height of 3nm. The simultaneous optical activation of the irradiated samples is due to the stable formation of electronic defects with intense photoluminescence in the visible spectral range.

We describe an apparatus for friction experiments on molecularly thin films carried out with the quartz crystal microbalance technique (QCM) in ultrahigh vacuum (UHV) and at a temperature as low as 4 K. Through the implementation in the UHV chamber of standard cryogenic procedures, the temperature of the quartz can be controlled with a resolution better than 5 mK below 10 K. The main chamber is provided with a stainless steel jacket that allows a quick change of the various temperature inserts (cryocooler head, liquid nitrogen insert, Peltier-cell stage, etc.) that span the working range 4–400 K, without ever breaking the vacuum. The crystal mounting is compatible with UHV conditions and with the Omicron standard and guarantees a good thermal and electrical contact over the entire temperature range. Once inserted in the copper sample holder, the quartz frequency and amplitude stabilities are found to be as good as the best recorded values achieved with QCMs mounted in standard high-vacuum, liquid bath cryostats. This apparatus can also be used for general QCM investigations in surface science, like the determination of film growth in deposition and sputter processes, the stoichiometry of submonolayer oxide films, and the measurement of adsorbate sticking probability.

A semiconductor detachable bolometer using a superconducting thin film as coupling medium is described. The film acts as a low pass band filter for the phonons having a frequency below the superconductive gap. A comparison between different coupling media is reported.