Abstract: The possible use of nanopores for single DNA molecules biosensing has been demonstrated, but much remains to do in order to develop advanced engineered devices with enhanced stability, and controlled geometry and surface properties. Here we present morphological and electrical characterization of solid state silicon nitride nanopores fabricated by focused ion beam direct milling and chemically functionalized by probe oligonucleotides, with the final aim of developing a versatile tool for biosensing and gene expression profiling.
Abstract: Nuclear factor-kappaB (NF-kappaB) is a ubiquitous transcription factor involved in the pro-inflammatory response to several factor, and in auto-inflammatory diseases. The usual methods for detection of NF-kappaB DNA binding activity are the electrophoretic mobility shift assay (EMSA), and enzyme-linked immunosorbent assay (ELISA). Here we report a development of a quantitative atomic force microscopy (AFM) based technique, for the analysis of NF-kappaB DNA binding activity. NF-kappaB target sequence DNA has been employed to mica functionalization in order to set up a surface able to capture transcriptionally active NF-kappaB protein complexes from cell lysates, with the aim to detect DNA binding capacity of NF-kappaB from low amount of biological samples such as biopsy. We were able to obtain images of the captured complex on the surface and furthermore we carried out an AFM images quantification. We were able to quantify relative and absolute quantities of NF-kappaB at pico-Molar proteins concentration range from cultured cell samples and from biological fluid cells permitting us to estimate NF-kappaB binding activity. The results obtained by AFM imaging have been compared and validated with EMSA. The present work represents the first quantification approach by AFM analysis. The results and the method may be used toward development of NF-kappaB based bio-diagnostic nano-device. Copyright (c) 2010 Elsevier B.V. All rights reserved.
Abstract: Single-wall carbon nanotubes (SWCNTs) could be promising delivery vehicles for cancer therapy. These carriers are generally introduced intravenously, however, little is known of their interactions with endothelial cells, the cells lining vessels and mediating clearance of nanoparticles. Here we show that SWCNTs of 1 to 5 mu m in length, both "pristine" and functionalized by oxidation, had limited toxicity for endothelial cells in vitro as determined by growth, migration morphogenesis, and survival assays. Endothelial cells transiently took up SWCNTs, and several lines of data indicated that they were associated with an enhanced acidic vesicle compartment within the endothelial cells. Our findings of SWCNT interactions with endothelial cells suggest these may be optimal vehicles for targeting the vasculature and potential carriers of anti-angiogenic drugs. The implications on their biological activity must be taken into account when considering the use of these nanoparticles for therapeutic delivery of drugs. From the Clinical Editor: Interactions of single walled carbon nanotubes (SWCNTs) with endothelial cells following IV administration remains unclear. Functionalized and naive SWCNTs of 1-5 mm in length had limited toxicity to endothelial cells in vitro. Endothelial cells transiently took up SWCNTs and were associated with an enhanced acidic vesicle compartment within the cells. These findings suggest that SWCNTs may be promising vehicles for targeting the vasculature and potential carriers of anti-angiogenic drugs. (C) 2010 Elsevier Inc. All rights reserved.
Abstract: With a quartz-crystal microbalance technique we have studied the nanofriction of Ne monolayers at temperatures below 6.5 K and in ultrahigh-vacuum conditions deposited on metallic surfaces plated with heavy rare-gases multilayers. Covering the electrode with one layer of Kr or Xe increases the slippage of a Ne monolayer by a factor close to 3. Such a behavior has been observed with smooth lead electrodes as well as with rougher gold ones. The lubrication effect does not improve with thicker overlayers.
Abstract: Recently, nanopore technology has been introduced for genome analysis. Here we show results related to the possibility of preparing "engineered solid state nanopores". The nanopores were fabricated on a suspended Si3N4 membrane by Focused Ion Beam (FIB) drilling and chemically functionalized in order to covalently bind oligonucleoticles (probes) on their surface. Our data show the stable effect of DNA attachment on the ionic current measured through the nanopore, making it possible to conceive and develop a selective biosensor for gene expression profiling. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: In the present work a macroporous brushite bone cement for use either as an injected or mouldable paste, or in the shape of preformed grafts, has been investigated. Macropores have been introduced by adding to the powder single crystals of mannitol which worked as a porogen. The size of the crystals was in the range of 250-500 mu m in diameter, suitable for cell infiltration, with a shape ratio between 3 and 6. From compression tests on cylindrical samples an elastic modulus in the range 2.5-4.2 GPa and a compressive strength in the range 17.5-32.6 MPa were obtained for a volume fraction of macropores varying between 15 and 0%. Thus the compressive strength exceeded in all tests the maximum value currently attributed to cancellous bone. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: 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.
Abstract: A quantitative surface reconstruction technique has been developed for the geometric characterization of three-dimensional structures by using a combined focused ion beam-scanning electron microscopy (FIB-SEM) instrument. A regular pattern of lines is milled at normal incidence on the sample to be characterized and an image is acquired at a large tilt angle. By analyzing the pattern under the tilted view, a quantitative estimation of surface heights is obtained. The technique has been applied to a test sample and nanoscale resolution has been achieved. The reported results are validated by a comparison with atomic force microscopy measurements.
Abstract: The leak devices most frequently used to calibrate leak-detection instruments are permeation leaks. These devices are very sensitive to temperature and can only be used with helium. The physical-leak types that could overcome this limits are prone to clog and their minimum size (about 1 mu m) limits their applicability in the lowest flow range. Here, the authors propose a fabrication technique by means of focused-ion beam with which, in suitable materials, they are able to produce nanometer orifices. These devices [Universitagrave degli Studi di Genova, Italian Patent No., TO2008A000683 (18 September 2008)] work in the molecular-flow regime up to atmospheric pressure and do not clog. Other advantageous characteristics are the possibility of obtaining leak rates in the range equal to those of the permeation type and the linear dependence of the throughput on the inlet pressure.
Abstract: In recent years there has been a rapid increase in nanotechnology applications to medicine in order to prevent and treat diseases in the human body. The established and future applications have the potential to dramatically change medical science. The present paper will give a few examples that could transform common medical procedures.
Abstract: Films of the molecular nanomagnet Mn12ac with thicknesses spanning the nm to mm range have been deposited using a customised vacuum spray deposition technique. The films have been characterized by AFM, XPS, magnetic and magneto-optical measurements, indicating that the deposition procedure does not damage the Mn12ac molecules. As a result, the films show magnetic properties similar to those of the parent molecular material down to the nm thickness range, as revealed by magneto-optical methods. Vacuum spray deposition therefore represents a promising approach for the preparation under controlled vacuum conditions of ultrathin films of molecular nanomagnets supported on transparent dielectric substrates, making possible the magneto-optical readout of the magnetisation state of the Mn12ac film.
Abstract: We report on contact mechanics experiments addressing the role of surface roughness on interfacial stiffness and adhesion. Colloidal atomic force microscopy probes, based on poly( dimethylsiloxane) microparticles, are pressed against ceramic substrates with different roughnesses; the applied load, deformation rate and dwell time being separately controlled. We observe a clear dependence of load - deformation curves and pull- off forces on roughness values, likely arising from morphological modulation of the contact area; remarkably this affects the contact stiffness, which is found to decrease for rougher junctions. The emergence of purely geometrical effects for poly( dimethylsiloxane) rough contacts extends previous findings on plastically deformed self- affine surfaces and demonstrates the efficient tuning of contact response through a proper design of surface morphology.
Abstract: The focused ion beam (FIB) microscope is a tool that has a Widespread use in the field of material science because it is able to micromachining with high resolution imaging thus therefore enhancing it broad range of both fundamental and technological applications in material science. The FIB is based oil a beam of Ga ions Which sputter the sample enabling precise machining at the nanometer/micrometer scale. The FIB instruments received particular attentions in the 1980s when the semiconductor industry used it as offline equipment for mask or circuit repair, but only in the 1990s the FIB was used in research laboratory. Nowadays there are commercial instruments (Dual Beam FIB / SEM) that integrate the precision cross section power of a FIB with the high resolution imaging of an SI-M creating a powerful cross section and imaging tool. The combined SEM capability allows for real time monitoring of the FIB cuts with a higher resolution.
Abstract: 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.
Abstract: 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 Nimetal clusters. For this purpose uniform Ni metal films, of different thicknesses, were deposited onto an oxidized Si(00 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. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: We present experimental results on simultaneous surface nanostructuring and optical activation of lithium fluoride crystals by 800 eV off-normal Ar+ sputtering. Our data demonstrate that the formation of periodic nanostructures is accompanied by the efficient production of stable electronic defects, optically active in the green and red parts of the visible spectra, thus providing the possibility to conceive and fabricate advanced insulating substrates. (C) 2006 Elsevier B.V. All rights reserved.
Abstract: The onset of in-plane uniaxial magnetic anisotropy in ion-sculpted ultrathin Fe/Ag(001) 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. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The possibility of modifying the magnetic properties of ultrathin films by means of ion sculpting has been verified for Co/Cu(001) and Fe/Ag(001). 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. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: We report on the synthesis and characterization of elastomer colloidal AFM probes. Poly(dimethylsiloxane) microparticles, obtained by water emulsification and cross-linking of viscous prepolymers, are glued to AFM cantilevers and used for contact mechanics investigations on smooth substrates: in detail cyclic loading-unloading experiments are carried on ion-sputtered mica, the deformation rate and dwell time being separately controlled. We analyze load-penetration curves and pull-off forces with models due respectively to Zener; Maugis and Barquins; and Greenwood and Johnson and account for bulk creep, interfacial viscoelasticity, and structural rearrangements at the polymer-substrate interface. A good agreement is found between experiments and theory, with a straightforward estimation of colloidal probes' material parameters. We suggest the use of such probes for novel contact mechanics experiments involving fully reversible deformations at the submicrometer scale.
Abstract: We report for the first time on muscovite mica surfaces nanostructured by a low-energy defocused Ar ion beam: ripple structures self-organize on macroscopic areas, with wavelength and roughness in the range 40-140 nm and 0.5-15 nm respectively, according to ions dose. In detail we address structural and chemical variations of the surface layer induced by sputtering. X-ray Photoelectron Spectroscopy (XPS) survey spectra reveal selective sputtering and Al surface enrichment whereas Atomic Force Microscopy (AFM) force-spectroscopy experiments indicate reduced charging of irradiated specimens under aqueous electrolyte solutions. Such experimental evidences contribute to clarify the chemical and physical properties of nanostructured mica samples, in view of their potential use as templates for aligned deposition of organic molecules and investigations on nanolubrication phenomena. (C) 2006 Elsevier B.V. All rights reserved.
Abstract: 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 N-2 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.
Abstract: 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.
Abstract: Production, characterization and control of alignment degree of molecules are of importance for investigating in detail the stereodynamics of elementary processes involving elastic, inelastic and reactive events and also to prepare gas-phase species for selective surface scattering investigations. The focus here is on collisional alignment in supersonic seeded molecular beams, a technique which shows perspectives on the applications, offering appealing features for 'duty cycle' and intensity characteristics. Attention will be addressed to recent stereodynamical studies carried out on hydrocarbon molecules in the gas phase and on applications of such aligned beams to surface scattering studies.
Abstract: We report experimental results on the morphological characterization of WC coatings deposited by High-Velocity-Oxygen-Fuel (HVOF) thermal spraying technique. Optical microscopy has been used to characterize film microstructure from the millimeter down to the micrometer scale; surface roughness and correlation length have been estimated as a function of film thickness. We have observed that analysed specimens display a self-affine morphology with Hurst exponent H=(0.33 +/- 0.05), in agreement with the scaling behaviour associated to the Kardar-Parisi-Zhang stochastic growth equation and to discrete ballistic deposition models. Our findings suggest that the introduction of scaling arguments within refined theoretical frameworks, realistically treating local splats interactions, could allow to directly predict the morphological evolution of coatings, partially overcoming the intrinsic complexity of the HVOF thermal spray process. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers. (c) 2006 American Institute of Physics.
Abstract: We present results on simultaneous nanostructuring and optical activation of lithium fluoride crystals by 800 eV 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 30 nm and a mean height of 3 nm. 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.
Abstract: Consecutive transitions between regular and periodic nanostructures induced by ion bombardment on the Cu(110) surface have been studied as a function of the sputtering parameters (primary ion energy, substrate temperature, and ion flux). The morphologies can vary from the well-known ripple patterns oriented along the two main symmetry directions of the fcc(110) substrate, to mounded structures, and include the far from equilibrium rhomboidal pyramid motif recently observed on the Rh(110) surface. The dependence of the nanostructure facet slope and lateral separation from ion energy allows us to identify a morphological regime accessed for low ion energy, below 500 eV, corresponding to the formation of rhomboidal pyramids. The dependence from the ion flux and substrate temperature bears strong similarities with a growth experiment, and is determined by the relaxation of isolated adatom and/or vacancy clusters created in the topmost surface layer by an ion impact. The selection of preferential step orientation and slopes follows from a delicate balance between the diffusion currents along the two main diffusion channels, the <$(1) over bar $ > and the < 001 >.
Abstract: 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.
Abstract: In this work we present our results concerning the formation of self-organized nanoscale structures during the bombardment with a low-energy defocused Ar ion beam. We studied glass surfaces because of their physical properties, technological interest and cheapness. The evolution of sample surface was studied ex situ by atomic force microscopy. We found, in agreement with Bradley and Harper, a morphology characterized by a regular ripple structure with the wave vector perpendicular or parallel to the ion beam direction. This structure periodicity was found to vary in the range 90-350 nm with a linear time evolution. In order to gain further information about the sputtering process and for comparison with the existing continuum theories of surface erosion, we studied the scaling behaviour of surface roughness. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: We study the morphological evolution of surfaces during ion sputtering and we compare their dynamical corrugation with aeolian ripple formation in sandy deserts. We show that, although the two phenomena are physically different, they must obey similar geometrical constraints and therefore they can be described within the same theoretical framework. The present theory distinguishes between atoms that stay bounded in the bulk and others that are mobile on the surface. We describe the excavation mechanisms, the adsorption and the surface mobility by means of a continuous equation derived from the study of dune formation on sand. We explore the spontaneous development of ordered nanostructures and explain the different dynamical behaviours experimentally observed in metals or in semiconductors or in amorphous systems. We also show that this novel approach can describe the occurrence of rotation in the ripple direction and the formation of other kinds of self-organized patterns induced by changes in the sputtering incidence angle.
Abstract: Starting from their discovery in 1991, carbon nanotubes have attracted a great attention, thanks to their peculiar mechanical, electrical and elastic properties that could be used to realize new devices in many different fields. For nanotechnology applications it is very important to be able to control not only shape and position but also alignment and orientation of carbon nanotubes, both during the growth and after it. Here we present preliminary results obtained by depositing carbon nanotubes (CNT) solutions on ion sputtered quartz substrates. Atomic force microscopy (AFM) images allow to study both CNTs positioning on the "ripples" generated by Ar+ sputtering on the SiO2 surface and their radial deformation induced by the "rough" surface. Work is now in progress to optimize the sputtering parameters and solution treatment (purification and functionalization) in order to get single CNTs regularly arranged on a patterned surface. (c) 2004 Elsevier B.V. All rights reserved.
Abstract: We demonstrate that a combination of ion sputtering and soft lithography is an alternative and effective way of nanostructuring soft matter. We create self-organized nanoscale structures on a glass template by irradiating the surface with a defocused, low energy Ar ion beam. Capillary force lithography is then used to transfer the pattern, exploiting the glass transition of polymeric layers. In particular, we demonstrate the pattern transfer of a periodic 150 nm fipple structure onto an organic compound, This new, unconventional combination is then a low-cost strategy that opens the way to a variety of applications in the field of organic-based devices.
Abstract: 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(110) single crystal. Like the other (110) metal surfaces, previously investigated, also Rh(110) 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. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The control of spatial orientation of molecules has a great influence on the stereodynamics of elementary processes occurring both in homogeneous and heterogeneous phases. Nonpolar molecules have so far escaped direct experimental investigations because of their poor sensitivity to several external constraints. Recently, it has been shown that the collisional alignment produced in supersonic expansions coupled with molecular-beam velocity selection can help solve such problems. Here we show that the sticking probability of ethylene, a nonpolar molecule prototypical of unsaturated hydrocarbons, on an O-2-precovered Ag(001) surface is larger for molecules approaching in a helicopterlike motion than for those cartwheeling. A mechanism involving a weakly bound precursor state is suggested, with helicopter molecules having a lower chance of being scattered back into the gas phase than cartwheels when colliding with preadsorbed ethylene.
Abstract: We report on the experimental evidence of the role of rotational alignment of the gas-phase molecules in the interaction of propylene with Ag(001). Molecular alignment has been controlled by a velocity selection of the impinging molecules, flying in a supersonic seeded molecular beam. The experimental findings indicate that at low surface coverage the sticking probability is independent of molecular alignment, while when coverage exceeds few percent of a monolayer, molecules impinging rotating parallel to the surface (helicopter-like configuration) achieve a higher chance to be trapped than those which impinge rotating perpendicularly (cartwheels). The sudden appearance of a large stereodynamic effect suggests that the adsorption proceeds via a mobile precursor state and is tentatively correlated with a change in the configuration of the added propylene molecules, which adsorb tilted rather than flat at the surface.
Abstract: The changes in the surface composition of a Pt50Rh50(100) alloy due to an ongoing 2H(2) + O-2-2H(2)O chemical reaction have been studied in situ. Exploiting the high-energy resolution and surface sensitivity of synchrotron radiation core-level spectroscopy it was possible to monitor the population of the two transitions metals atoms at the gas-surface interface. Sequences of fast high-resolution core-level spectra of the Rh3d(5/2), Pt4f(7/2), and O1s core levels showed a continuous exchange of atoms between the first and subsurface layers. An unexpected Pt surface enrichement was found under slightly oxidizing conditions, opposite to what found in a highly oxidizing atmosphere.
Abstract: 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 3x10(-11) mbar, 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 1 h) and under UHV conditions, we have also developed a turbo pumped buffer chamber equipped with a cold trap. (C) 2005 American Institute of Physics.
Abstract: 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. (C) 2005 American Institute of Physics.
Abstract: We apply a theoretical approach, originally introduced to describe aeolian ripples formation in sandy deserts, to the study of surface instability in ion-sputtered surfaces. The two phenomena are distinct by several orders of magnitudes and by several physical mechanisms, but they obey similar geometrical constraints and therefore they can be described by means of the same approach. This opens a novel conceptual framework for the study of the dynamical surface roughening and ripple formation on crystal and amorphous surfaces during ion sputtering. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: 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.
Abstract: The causes of foxing, a rust-red spotting of engravings, books and archive documents, are not yet completely understood, but they are usually ascribed to mould growth and/or heavy-metal-induced degradation of cellulose and sizing materials. In the present work we report the use of attenuated total reflection Fourier transform infrared spectrometry, image analysis and atomic force microscopy as non-destructive tools for the surface analysis of foxing stains in respect of their chemical and physical characteristics.
Abstract: The nonstationary character of roughness is a widely recognized property of surface morphology and suggests modeling several solid surfaces by fractal geometry. In the field of contact mechanics, this demands novel investigations attempting to clarify the role of multiscale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of fractal surfaces by depth-sensing indentation. One class of experiments was conducted on organic thin films, load-displacement curves being acquired by atomic force microscopy using custom-designed tips. Another class of experiments focused on well-defined crystalline and mechanically polished ceramic substrates probed by a traditional nanoindenter. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the penetration depth is kept below the interface width.
Abstract: Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.
Abstract: The smoothening process of nanometer-scale ripples grown on the (110) surface of silver is investigated using thermal energy He atom scattering. Morphological equilibration the corrugated surface is followed in real time in the temperature range between 205 and 230 K. The mean ripple wavelength, Lambda, is observed to increase during surface recovery. To take this effect into account the decay time is assumed to scale as Lambda(3). Within this approximation the ripple amplitude is observed to decay linearly with time. The activation energy of the mechanism driving surface relaxation is estimated as (0.46 +/- 0.02) eV. The underlying rate limiting process, i.e. adatom detachment from the open <001> step edges is evidenced. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: 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. (C) 2004 American Institute of Physics.
Abstract: A light and compact mechanical velocity selector, of novel design, for applications in supersonic molecular-beam studies has been developed. It represents a simplified version of the traditional, 50 year old, slotted disks velocity selector. Taking advantage of new materials and improved machining techniques, the new version has been realized with only two rotating slotted disks, driven by an electrical motor with adjustable frequency of rotation, and thus has a much smaller weight and size with respect to the original design, which may allow easier implementation in most of the available molecular-beam apparatuses. This new type of selector, which maintains a sufficiently high velocity resolution, has been developed for sampling molecules with different degrees of rotational alignment, like those emerging from a seeded supersonic expansion. This sampling is the crucial step to realize new molecular-beam experiments to study the effect of molecular alignment in collisional processes. (C) 2004 American Institute of Physics.
Abstract: In metals, the surface curvature dependence of the sputtering yield and the presence of an extra energy barrier whenever diffusing adatoms try to descend step edges, produce a similar surface instability, which builds up regular patterns. By tuning the competition between these two mechanisms, it is possible to create self-organized structures of the size of few nanometers. Height, lateral distance and order of the structures change with the deposition parameters like ion energy, dose, incident angle and substrate temperature. The paper offers an overview of the experiments carried out and foresees possible applications of these results in the area of material science. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: 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.
Abstract: The morphological equilibration of the rippled (I 10) 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. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: 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.
Abstract: The frictional properties of cluster assembled carbon films have been investigated on nanometric scale by friction force microscopy. The experiment was performed at low loads to avoid plastic deformation and wear of materials. We found that load-dependent measurements acquired on samples with different composition present excellent agreement with the Hertzian-plus-offset model. A quantitative comparison among these films and atom-assembled carbon compounds is finally presented. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We present a novel experimental technique devoted to the investigation of contact mechanics on mesoscopic scale. It consists of an atomic force microscope (AFM) equipped with custom-designed probes with integrated flat micrometric tips. Samples are normally compressed by the flat tips and load-displacement curves are acquired. The latter allow to investigate the mechanical response under a multi-asperity regime not accessible by conventional AFM. Preliminary results are reported for the contact mechanics of nanostructured carbon-based films having a self-affine fractal morphology. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The time evolution of the morphology of the Cu(110) surface during ion sputtering has been studied in situ and in real time by x-ray-based techniques. The surface was bombarded with Ar+ ions at an energy of 1 keV in the temperature range 100-320 K; the ion incidence angle was set out of the x-ray scattering plane in order to induce asymmetric nanostructures. The results show that the ripples or mounds formed by the ion sputtering evolve in time, changing the spatial periodicity and the local slope of the facets. The latter is strongly dependent on temperature; moreover the facets along the <1-10> direction evolve in a different way from those along <001>.
Abstract: We investigated the contact mechanics and friction forces between atomic force microscope (AFM) probes and self-affine fractal carbon films. We studied single-asperity contacts by means of conventional nanometric conical tips whilst custom-designed micrometric flat tips were adopted to form multiple junctions between the probe and the sample. By varying the externally applied load we found that the average frictional force follows a power-law behavior in the single-asperity regime and a linear behavior in the multi-asperity regime. The friction coefficient was the same for carbon specimens having different fractality. We also acquired quasi-static load-displacement curves on micrometric scale, revealing a strong dependence of the average indentation depth on the values of fractal parameters. A comparison of experimental data with contact theories for randomly rough surfaces is provided. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: 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 <1 (1) over bar0> 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 <1 (1) over bar0>-oriented ripples is measured to be (0.52+/-0.09) and (0.43+/-0.05) eV, respectively. The same underlying rate limiting process-i.e., adatom detachment from the open <001> step edge-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.
Abstract: Frictional properties of nanostructured carbon-based films, obtained by deposition of supersonic carbon clusters, have been investigated by friction force microscopy under ambient conditions. The experiment was performed at low loads to avoid plastic deformation and wear of materials. By analysing the load-dependent measurements acquired on samples with different composition, we deduced that the Hertzian-plus-offset model can take into account the frictional behaviour of these materials. A strong dependence of adhesive forces on the specific surface location was observed. A quantitative comparison among these films and atom-assembled carbon compounds is finally presented. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: 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 Ar+ ions at an energy of 1 keV in the temperature range 100-320 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 12degrees if measured along the [1 -1 0] direction and between 8 and 10degrees along [0 0 1].
Abstract: The evolution of the morphology of ion sputtered Ag(110) 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+. 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 90degrees by changing temperature. Also the slope of the mounds/ripples changes with temperature, ranging between 6degrees and 12degrees if measured along <1-10> and between 8degrees and 10degrees along <001>. (C) 2002 published by Elsevier Science B.V.
Abstract: The design and first tests of a new supersonic molecular beam are presented. The instrument is dedicated to study in a controlled way the interaction of atoms and molecules with solid surfaces in combination with high-resolution core level photoemission at the SuperESCA beamline of ELETTRA, the third-generation synchrotron radiation source in Trieste, Italy. The design of the instrument together with the characterization and optimization of the molecular flux and energy resolution are discussed. As a first example of application, we present the results of a time-resolved photoemission study (O-2+H2-->H2O reaction) on a PtRh single-crystal alloy. Time resolution of 100 ms/spectrum has been achieved by using a double-pass electron energy analyzer equipped with a novel 96 channel detector. (C) 2002 American Vacuum Society.
Abstract: 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 <1 (1) over bar0> 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 pro le, 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 <1 (1) over bar0> steps.
Abstract: The first stages of aggregation of low energy deposited carbon clusters on Ag(I 10) surface have been observed in situ by scanning tunnelling microscopy (STM). Sub-monolayer coverages of the substrate at room temperature have been obtained by using a supersonic neutral cluster beam with a mass distribution centered at an average of 900 atoms/cluster. We observe that clusters are able to diffuse on the surface and along steps, and organise themselves in compact islands principally located at step edges. Size of supported islands ranges from a few nm up to 40 nm. A statistical analysis based on several STM images shows that the island size follows a lognormal distribution law. Possible mechanisms involved in the diffusion and coalescence of carbon clusters on metallic surfaces are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Nanopatterns on metal surfaces can be easily created by ion sputtering. However, due to the fast diffusion processes characterizing these materials, the nanostructures are often unstable at room temperature and above. This effect prevents the use of such patterned substrates in nanotechnology applications. In this letter, we present a simple oxidation process able to stabilize these features durably. The method has been tested on Cu, but its generality suggests that it can be applied to many other metals. (C) 2002 American Institute of Physics.
Abstract: The surface morphology of silicon single crystal as a function of oxidation time has been investigated by Atomic Force Microscopy. The surface roughness corresponding to different stages of oxidation has been evaluated by the use of topographical images. It is observed that the root-mean square roughness increases with oxidation time. The distribution histogram of the surface structural elements is presented. To characterize the surface microstructure an attempt to use the fractal theory is made.
Abstract: 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.
Abstract: The morphology generated on Ag(110) and Ag(001) by 1 keV Ar+ sputtering at normal incidence has been studied by scanning tunnelling microscopy as a function of the substrate temperature T-S and the ion flux Phi. Since ion sputtering is a non-equilibrium process in which erosion competes with diffusion in determining the surface evolution, these macroscopic parameters can be used to tune the final surface morphology. Flat or rough surfaces as well as periodic structures have been observed on both substrates. On Ag(110), ion sputtering at 230 K and 320 K produces two well defined ripple patterns whose wave vectors are parallel to <1 (1) over bar0 > and < 001 > respectively, while on Ag(001) a periodic pattern of square islands has been observed over a wide range of substrate temperatures (240 K less than or equal to T-S < 440 K), Similar to the results reported in growth experiments, a flux increase produces a surface evolution qualitatively comparable with that obtained by lowering the temperature. The results are discussed in terms of a continuum model for the ion sputtering process.
Abstract: The electronic and atomic structures of the interfaces formed upon deposition of Gd and Dy onto HOPG(0001) at substrate temperatures of 20 and 300 K have been studied by AES, UPS, XPS, EELS, and LEED. At both temperatures studied, Gd/HOPG interface reproduces atomic structure of HOPG(0001) up to 20 Angstrom thicknesses. The analysis performed by AES favors a quasi layer-by-layer growth and diffusion of Gd and Dy into interplane space of graphite even at 20 and 300 K. The structure of Dy/HOPG interface is different from that of the Gd/HOPG system: at thick coverages (45 Angstrom) when only Dy features an observed by electron spectroscopy, the LEED pattern typical of clean HOPG(0001) coexists with superstructural rings. Annealing of the interfaces at temperatures up to 1300 K induces, most probably, formation of the ordered carbide-like systems with geometrical structure analogous to that of pristine graphite for Gd-C and superstructure of root3 x root3 type for Dy-C systems. (C) 2001 Elsevier Science BN. All rights reserved.
Abstract: 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 me V. 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.
Abstract: 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.
Abstract: The submonolayer homoepitaxial growth on Ag(1 1 0) is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The evolution of island density is studied as a function of temperature at given flux (approximate to0.0025 ML s(-1)) and coverage (0.16 ML) to evaluate the energy barriers and the binding energies for the in-channel and cross-channel diffusion. The experimental results are compared with kinetic Monte Carlo simulation. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We show that in presence of subsurface oxygen, CO oxidation of oxygen precovered Ag(001) modifies the surface in such a way that CO adsorption is stabilized up to crystal temperatures of 160 K. When diffusion is activated new surface oxide phases form, characterized by oxygen both in surface and in subsurface sites and by a high density of slates just below the Fermi level. Subsurface oxygen accumulation might therefore be relevant for the understanding of the origin of the pressure gap observed for ethylene epoxidation and other important catalytic reactions.
Abstract: We investigated O-2 adsorption on Ag(001) in the presence of defects induced by Ne+ sputtering at different crystal temperatures, corresponding to different surface morphologies recently identified by scanning tunneling microscopy. The gas-phase molecules were dosed with a supersonic molecular beam. The total sticking coefficient and the total uptake were measured with the retarded reflector method, while the adsorption products were characterized by high resolution electron energy loss spectroscopy. We find that, for the sputtered surfaces, both sticking probability and total O-2 uptake decrease. Molecular adsorption takes place also for heavily damaged surfaces but, contrary to the flat surface case, dissociation occurs already at a crystal temperature, T, of 105 K. The internal vibrational frequency of the O-2 admolecules indicates that two out of the three O-2(-) moieties present on the flat Ag(001) surface are destabilized by the presence of defects. The dissociation probability depends on surface morphology and drops for sputtering temperatures larger than 350 K, i.e., when surface mobility prevails healing the defects. The latter, previously identified with kink sites, are saturated at large O-2 doses. The vibrational frequency of the oxygen adatoms, produced by low temperature dissociation, indicates the formation of at least two different adatom moieties, which we tentatively assign to oxygen atoms at kinks and vacancies. (C) 2001 American Institute of Physics.
Abstract: 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 O-2 On Ag(110) and on Ag(111). The mechanisms of Inelastic electron scattering by adsorbed O-2 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.
Abstract: We have investigated the homoepitaxial growth of Ag(110) in the multilayer regime. After deposition of 30 monolayers of Ag at a temperature of 210 K a ripplelike surface instability is produced and the ridges of the ripples, as well as the majority steps, are found to be parallel to (1 (1) over bar 0) which is the thermodynamically favored orientation. As the deposition temperature is decreased to 130 K, an unexpected 90 degrees switch of the ripple orientation is observed. The ridges of the ripples and the steps are in this case parallel to (100). In the intermediate temperature range a checkerboard of rectangular mounds results. We interpret our results in terms of the peculiar hierarchy of interlayer and intralayer diffusion barriers present on the anisotropic Ag(110) surface.
Abstract: A tribological investigation of nanostructured carbon films performed with atomic force microscope is presented. Surface morphology is self-affine, with different correlation lengths corresponding to an inhomogeneous distribution of deposited carbon clusters. After reducing topographical effects, the friction coefficient mu turns out to be similar to 0.15 (in air), with a slight dependence on the cluster distribution.
Abstract: 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 < 150 K, it saturates at a value corresponding to an average terrace width of less than two atomic spacings. The experimental measurements are compared with recent kinetic Monte Carlo simulation results on the same system and a good agreement is found. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: 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 O-2 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. (C) 2000 American Institute of Physics. [S0021-9606(00)71415-2].
Abstract: Surface morphologies of nanostructured carbon films, produced by supersonic cluster beam deposition, have been investigated by atomic force microscopy. From topographical images, we evaluated the surface roughness corresponding to different scan lengths and thicknesses, and deduced that the images examined are self-affine. Experimental scaling exponents and the hypothesis of non-destructive primeval cluster aggregation suggest that quenched noise effects may be important in the process of growth of the nanostructured surfaces. (C) 2000 Published by Elsevier Science B.V. All rights reserved.
Abstract: The electronic structure and growth mode of C-60 molecules deposited on Ag(001) have been studied as a function of deposition parameters and annealing temperature. The measurements show that C-60 molecules are chemically bound to the Ag(001) surface even when deposited at 150 K, and the bond properties do not change significantly after annealing up to 670 K. In the resulting ordered C-60 overlayer, a mixed contrast of the buckyballs, as seen by scanning tunneling microscopy, is discussed in terms of non-equivalent orientations of the adsorbed molecules. (C) 2000 Published by Elsevier Science B.V. All rights reserved.
Abstract: We show that dissociative oxygen adsorption on Ag(001) induces below room temperature a missing row 2 root 2x root 2 reconstruction of the substrate. As demonstrated by the analysis of the photoelectron diffraction patterns, the oxygen atoms sit thereby in a c(2x2) arrangement in the previous fourfold hollow sites nearly coplanar with the Ag atoms, while rows of substrate atoms are removed along the [100] directions. Annealing the crystal above 350 K restores the p(1X1) symmetry and the oxygen moves to 0.6 Angstrom above the fourfold hollow site. It becomes then more oxidic in nature, as demonstrated by the shift of the O1s level from 530.3 eV to 528.3 eV. The phase transition affects also the O2s and O2p 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 highs temperature phase is active towards CO and C2H4 oxidation, while the low-temperature phase is nor. 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.
Abstract: We show that vibrational energy can be used to tune the chemisorption state of an adsorbed molecule. When dosing C2H4 with a supersonic molecular beam on Ag(0 0 1) we demonstrate by vibrational spectroscopy that, with increasing nozzle temperature, TN, firstly no stable adsorption occurs, then a pi -bonded configuration is populated, switching finally to a more strongly, undissociated, di-sigma -bonded state. Direct dissociation is observed at still higher T-N with the formation of dehydrogenated radicals. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: 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. (C) 1999 American Institute of Physics. [S0003-6951(99)03747-X].
Abstract: The time evolution of adatom and vacancy islands on an Ag(110) surface is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The islands are created by Ar+ ion sputtering at low temperatures and at very low fluxes (similar to 0.004 ML s(-1)). The area of both kinds of island decreases linearly as a function of time, at least for small islands. The experimental results are compared with kinetic Monte Carlo simulations. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Abstract: We show that the population of the rotational degrees of freedom of gas molecules affects their adsorption probability also for a physisorbed state. The effect, expected to be very general, is demonstrated for the case of ethylene interaction with Ag(001), when the physisorption probability is suppressed for molecules produced with a hot nozzle, characterized by a substantially larger average rotational quantum number. [S0031-9007(99)09333-3].
Abstract: We report evidence for the occurrence of sub-surface oxygen incorporation during the dissociation process of admolecules. The process is observed for O-2 chemisorbed on Ag(001) at a surface temperature of 100 K, where thermally activated sub-surface diffusion is inactive. The phenomenon occurs both when dissociation is induced thermally or by electron bombardment. EELS peaks corresponding to the vibration of adatoms and of sub-surface oxygen are observed. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: The electronic and atomic structures of the interface formed upon deposition of Gd onto (0001) surface of highly oriented pyrolytic graphite (HOPG) have been studied by, electron spectroscopy. It was found thai; during deposition at 20 and 300 K, Gd/BOPG interface reproduces atomic structure of HOPG (0001) surface up to 5-7 ML thicknesses. The analysis of the interface by Auger electron spectroscopy favors a quasi layer-by-layer Gd growth onto HOPG. Annealing of the interface at temperatures up to 1200-1300 K leads to the formation of a stable Gd-C carbide-like system.
Abstract: The structure of submonolayer C-60 films deposited on Ag(001) is studied as a function of the substrate temperature in the range 100-700 K. Morphological aspects such as island shape and preferential nucleation sites are in agreement with the theory of submonolayer deposition. A peculiar irreversible transition which leads to the onset of brightness differences between molecules of the same film is observed at around 300 K. This effect is ascribed to electronic differences due to non-equivalent orientation of the C-60 molecules on the surface rather than to murphological differences induced by a surface reconstruction. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: The surface morphology generated by ion sputtering on a Cu(110) crystal has been investigated by Scanning Tunneling Microscopy (STM). A ripple structure is observed for all the considered values of the incident ion beam angle theta (0 degrees < theta < 70 degrees). In particular, normal:sputtering produces a well defined ripple structure whose wave vector rotates from (001) to (1 (1) over bar 0) by increasing the substrate temperature. Moreover, for theta = 45 degrees the ripple wavelength lambda increases in time following a scaling law lambda proportional to t(z), with z = 0.26 +/- 0.02. These results are described by a continuum equation which includes, in addition to the surface curvature dependent erosion terms, a diffusion term that takes into account both the surface anisotropy and the effect of an Ehrlich-Schwoebel barrier.
Abstract: We reinvestigated non-dissociative chemisorption of O-2 on Ag(001) by the King and Wells reflectivity method and high-resolution electron energy loss spectroscopy over a wide range of impact energies of O-2. In analogy to O-2/Ag(110), we find that the sticking probability has a maximum at a total energy of 0.7 eV corresponding to the onset of direct inelastic scattering from the repulsive part of the chemisorption potential. Below it, normal energy scaling is approximately obeyed, whereas above it, a breakdown of such behaviour is observed. The decrease in sticking probability at high impact energy is more pronounced for O-2/Ag(001) than for O-2/Ag(110). (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: The morphology of C60 films deposited on a Ag(001) single crystal has been studied using an UHV-STM. The C60 molecules are arranged in a quasi-hexagonal c(6 x 4) structure, having the long axis along the (110) direction of the Ag surface. Due to the existence of two equivalent directions, the C60 film presents different domains, separated by dislocation lines. In each domain, peculiar structures of bright C60 molecules have been resolved. These structures are due not to geometric effects, but to nonhomogeneous charge distributions and different chemical bonding with the Ag substrate. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: A review of ripple formation on solid surfaces induced by ion sputtering is presented. We review in detail very recent development in this field showing that the ripple morphology, generated by ion bombardment at normal incidence on (110) metal surfaces, is a consequence of the effect both of a Schwoebel barrier on inter-layer diffusion processes and of the anisotropic intra-layer mass transport which generates a ripple instability which can overcome the one due to ion erosion.
Abstract: An STM study of the morphologies generated by 1 keV Ar+ sputtering in the 180-500 K temperature range is presented. It is shown that the sputtering temperature, T-s, can be used to lune the final surface structure. At low temperatures, the surface is characterized by a random distribution of vacancy islands. When T-s is increased, square-shaped vacancy islands with straight edges oriented along the principal crystallographic directions are observed. Depth, periodicity and dimension grow as a function of the substrate temperature until, for T-s greater than or equal to 440 K, an almost Aat surface, resulting from a layer-by-layer removal, appears. A simple model, based on the microscopic surface diffusion processes, is also presented in which the importance of vacancy mobility is strongly stressed. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
Abstract: A brief review of our measurements of the sticking probability of O-2 on Ag single crystal surfaces is presented. On Ag(110) and Ag(001) the non dissociative chemisorption probability behaves similarly increasing by three orders of magnitude as the oxygen translational energy is increased from 90 to 800 meV. The temperature dependence of the dissociative sticking probability allows to distinguish two different pathways to dissociation : on Ag(110) molecules can dissociate at terrace sites while on Ag(001) only special sites, tentatively identified with kinks, are effective. On Ag(111) the sticking probability is below the experimental sensivity. We finally report on the more recent results about the possibility to desorb or dissociate chemisorbed O-2 even at low temperature by collision with hyperthermal Xe atoms. (C) 1998 Elsevier Science Ltd. All rights reserved.
Abstract: We have investigated desorption and dissociation of O-2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms by high resolution electron energy loss spectroscopy and supersonic molecular beam technique. The cross section for both processes increases rapidly both as a function of angle of incidence and of total impact energy of the inert gas atom. While the increase with energy is expected, the increase with the angle is somewhat surprising arid is sensibly larger than observed for previously investigated systems. The cross section for desorption decreases moreover with coverage. In the limit of high impact energy and high coverage its value is always larger than the one for dissociation. The branching ratio between the two processes depends thereby on energy and angle of incidence of the inert gas atom. Atomic oxygen is not removed under any impact condition, because of its larger binding energy. In order to explain the experimental results, molecular dynamics simulations have been performed using a simple model including multiple scattering, We find that the angular dependence of the cross section is determined by surface corrugation and by multiple scattering which suppresses desorption at normal incidence while the energetic threshold is determined by energy loss to the substrate. (C) 1998 American Institute of Physics.
Abstract: The evolution of the Cu(110) surface morphology during low temperature (180 K) ion sputtering was studied as a function of the incident ion beam angle theta by means of scanning tunneling microscopy. The morphology was dominated by a ripple structure with the wave vector parallel or perpendicular to the direction of the incident beam. The time evolution of the interface shows that the ripple wavelength increases in time following a scaling law lambda proportional to t(z), with z = 0.26 +/- 0.02. These results are ascribed to the effect of a Schwoebel barrier on the interlayer diffusion of the recoiling atoms produced during ion sputtering.
Abstract: Surface morphology of a Cu(110) crystal, generated by ion sputtering, has been investigated by scanning tunneling microscopy. Different from recent theoretical predictions and experimental results, normal sputtering produces a well defined ripple structure whose wave vector rotates from [001] to [1 (1) over bar 0] increasing the substrate temperature. Off-normal sputtering at low temperature (180 K) generates ripples whose orientation depends on both ion direction and surface azimuthal orientation. These results are described by a continuum equation which includes both surface curvature dependent erosion terms and diffusion terms accounting for surface anisotropy and Ehrlich-Schwoebel barriers. [S0031-9007(98)07159-2].
Abstract: The growth of Ag-O rows on a Ag(110) surface has been studied at room temperature by using a UHV-STM. The evolution of the typical (n x 1) (n = 8,7,...,2) structures has been analyzed as a function of O-2 exposure. At the beginning, the (Ag-O) rows are randomly distributed on the surface. Increasing the exposure, many (n x 1) phases coexist. The evolution of the system is due to a global rearrangement of the existing rows, which can move along the [1 (1) over bar 0] direction. A model for this mechanism is proposed.
Abstract: The dynamics of the dissociative adsorption of O-2 on Ag(001) were investigated with a supersonic molecular beam source and electron energy loss spectroscopy versus surface temperature. Contrary to the case of Ag(110) where dissociation occurs at the atomic terraces and has a high probability, for Ag(001) we find that only 0.44% of the adsorbed molecules dissociate at room temperature. An Arrhenius analysis indicates that the process is thermally activated and the activation energy coincides with the energy for generating kinks, which are thus identified as the active sites. The interplay between a local geometry similar to a (110) site and enhanced charge transfer to the antibonding molecular orbitals is responsible for the pronounced reactivity of such sites. Molecules adsorbed at (001) terrace sites instead desorb with a high probability. (C) 1997 Published by Elsevier Science B.V.
Abstract: A new UHV variable temperature STM has been constructed. A "beetle type" STM is coupled with a Liquid Helium cryostat, allowing us to vary the sample temperature in the range 60-500 K. The base pressure in the UHV chamber is 10(-10) mbar. The STM is able to obtain atomic resolution on metal surfaces such as Ag(110). The Z-piezo parameters to correct the intrinsic distortion have been calculated. This apparatus is used to study gas adsorption on metal surfaces. As an example, we present recent results on the O-2-Ag(110) system. At room temperature, we observe the well known (n x 1) structures, with the Ag-O rows growing along the <001> direction. At low temperatures, the O-2 molecules are first randomly adsorbed on the surface, but at increasing coverage they are organised in short rows (approximate to 50 Angstrom) along the <1(1) over bar0$> direction. (C) 1997 Elsevier Science Ltd.
Abstract: The coverage dependence of the vibrational modes of O-2 chemisorbed on Ag(001) was studied by electron energy loss spectroscopy. We observe the dipole active modes associated to the intramolecular stretch and to the molecule-surface vibrations. Moreover two further features are present in the EEL spectra, due to the anharmonicity of the molecular potential: the combination-band of the stretching and of the molecule-surface vibration, and the first overtone of the O-O stretching mode. From the EELS intensity of the O-O stretching mode versus coverage the dynamical charge transfer e* = 0.61 +/- 0.02 e(-) is determined which corresponds to a dynamical dipole moment mu=0.16+/-0.01 D. The O-2-Ag vibrational frequency is found to decrease with coverage, suggesting that a weakening of the molecule-surface bond takes place. This observation is confirmed by thermal desorption spectroscopy. The anharmonicity parameter x(e) of the O-O potential increases significantly upon chemisorption, in analogy to O-2-Ag(110) and O-2-Pt(111). The intensity of the overtone peak is analyzed within the Morse approximation, allowing us to discriminate the contributions arising from the mechanical anharmonicity, and from the nonlinearity of the dynamical charge flow. A correlation is observed between the magnitude of e* and the contribution of the latter: term to the overtone intensity. (C) 1997 American Institute of Physics.
Abstract: 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-Bag 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)1013.3+/-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).
Abstract: The morphology of the Ag(110) surface after Ar+ ion sputtering has been studied by a variable temperature scanning tunneling microscope. For ion energies greater than 800 eV and in the temperature range 270 < T-s < 320 K (where T-S is the sputtering temperature), a peculiar ripple structure well aligned along the [1(1) over bar0$] direction and independent of the ion beam angle has been observed. A simple model of this effect, based on the anisotropy and on the temperature hierarchy of the Ag adatom and vacancy surface diffusion, explains the observed features.
Abstract: 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. (C) 1997 American Institute of Physics.
Abstract: We have investigated desorption and dissociation of O-2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms provided by a supersonic molecular beam. We find that the cross section for both processes increases rapidly with impact energy and with angle of incidence of the inert gas atom. A model is presented which accounts for the observed energy threshold and for the trend in the cross section. (C) 1997 Elsevier Science B.V.
Abstract: O-2 adsorption on Ag(001) at 100 K has been investigated by HREELS. Contrary to previous reports we could resolve 4 oxygen related peaks, 3 of which are due to the internal stretching vibration as confirmed by isotope labelling. The lowest frequency mode at 63 meV is assigned to molecular oxygen chemisorbed at defect sites. The further two features are very close to each other in frequency, at 79 and 84 meV. We assign them to molecules chemisorbed in a peroxidic state in two different adsorption sites. Initially both sites are filled simultaneously while at higher coverage adsorption in the higher frequency site is favoured.
Abstract: We investigated the dynamics of the adsorption of O-2 on Ag(001) with the supersonic molecular beam technique combined with EEL spectroscopy, TDS and the reflection detector technique. The initial sticking coefficient S-0 was measured as a function of the impact energy and of the angle of incidence of the molecules at crystal temperatures of 100 and 300 K, corresponding to molecular and dissociative adsorption, respectively. Both regimes are characterized by the same distribution of energy-barriers. This indicates that, in analogy to the case of our previous investigation of O-2-Ag(110), the main reaction path involves activated adsorption into the molecular well, followed eventually by thermally induced dissociation for T-s>150 K. Again in analogy to O-2-Ag(110), the physisorbed state plays no role in the chemisorption process.
Abstract: The coverage dependence of electron-energy loss intensity (EELS) and of the frequency of the O-Ag(110) stretching vibration are re-examined and compared with other EELS and infrared reflection absorption spectroscopy (IRAS) data. The analysis of the EELS intensity in our experiment yields a charge transfer q=0.51+/-0.03 e(-), which compares well with independent EELS estimates but is at variance with the much smaller IRAS value reported by Peng and Reutt-Robey [Surf. Sci. 336 (1995) L755]. Our value can account for most of the observed frequency shift with coverage, indicating that no chemical shift occurs; in contrast with the recent theory of Schimizu and Tsukada [Surf. Sci. 295 (1993) L1017], the opposite conclusion would be supported by IRAS. The (surprising) inconsistency of EELS in dipole scattering and IRAS could be indicative that the physical basis of the two spectroscopies is not unique, as assumed so far.
Abstract: We have investigated the interaction of oxygen with Ag(111) by using a supersonic molecular beam in the impact energy range 93-800 meV. At 105 K, contrary to the results of Carley et al. [Surf. Sci. 238 (1990) L467], we find no evidence for O-2 adsorption even after very high O-2 exposures (similar to 25000 L) indicating that for a clean surface the sticking probability S is lower than 6 x 10(-7) far the whole impact energy range. At room temperature dissociative oxygen adsorption occurs at E(i) = 0.80 eV, with S approximate to 9 x 10(-7). The data show however evidence that the adsorption process is mediated also in this case by adsorbed OH so that S is even smaller for the clean surface.
Abstract: We studied the dissociation of molecular oxygen adsorbed on Ag(110) by electron-energy-loss spectroscopy. We observe that the dissociation of the adsorbed molecules at 150 K is connected to increased surface disorder. The elastic reflectivity is recovered at 200 K independently of coverage when the added row reconstruction of the substrate sets in.
Abstract: The chemisorption dynaMiCS Of O2 and Ag(110) was studied by molecular beams and high resolution electron energy loss spectroscopy by measuring the initial sticking probability S0 as a function of the molecular translational energy E(i) and angle of incidence theta(i) along [001] and [110BAR]. S0 scales as E(i)cos(n)theta(i) with n = 1.1 along (001) and n = 1.6 along [110BAR]. This indicates that parallel momentum has a role in promoting adsorption. The effect is connected to the amplitude of the corrugation seen by the O2 molecule.
Abstract: Phonon transmission through superconducting In, Sn, and Pb films (with thicknesses of 4 and 8 mu m) is studied with a heat-pulse technique using a semiconductor Si:P detachable bolometer as the phonon detector. Below the critical temperature of the films, ballistic-phonon propagation is observed. From the dependence of the phonon transmission on the incident power and film thickness, we calculate the mean free path of phonons at the gap frequency. The phonon transmission amount changes when a magnetic field is applied and is also different if the field is parallel or perpendicular to the film. By a simple model based on the Landau laminar model for intermediate states, the critical field for the films can be calculated. Good agreement (for In and Sn) with the theoretical values is obtained.
Abstract: We investigated the dynamics of the adsorption of O-2 on Ag(110) with the molecular beam technique combined with EEL spectroscopy and with the method of King and Wells. The initial sticking coefficient S-0 is reported for molecules impinging along both high symmetry azimuthal directions ([001]) and [1(1) over bar0$] as a function of total energy and angle of incidence of the molecules and crystal temperature. The initial sticking coefficient is anisotropic for low as well as for room temperature adsorption. The dependence of the sticking coefficient on the temperature of the sample, T-s, indicates that adsorption takes place directly into the molecular well. Dissociation is eventually induced thermally for T-s>150 K. The physisorbed state plays no role in the chemisorption process in the investigated impact energy range.
Abstract: We report on the dependence of the sticking coefficient S of O-2 on Ag(110) on oxygen coverage, for molecular and dissociative adsorption. S is found to diminish strongly with increasing coverage. The initial dependence is linear and for dissociative adsorption the slope depends on crystal temperature. The data are indicative of a coverage dependence of the adsorption barrier height, which is caused by the change of the surface work function with coverage. The analysis of the temperature variation of the dependence of S on coverage allows to extract the variation of the ratio of the pre-exponential factors and of the height of the barriers for desorption and dissociation from the molecularly chemisorbed state.
Abstract: The frequency shift in the stretching mode of peroxo-like O2 on Ag(110) is studied by EELS. The anharmonicity parameter and an estimate of the dissociation energy are obtained from the frequency of the 0 --> 2 vibrational transition. A comparison with already published data for free and physisorbed oxygen is given. We find that chemisorption strongly reduces the dissociation energy and that the anharmonicity is larger than in the case of physisorption.
Abstract: Oxygen adsorption on Ag(110) is investigated as a function of coverage by high resolution electron energy loss spectroscopy. We find for the O-Ag(110) stretching frequency a value of 39.1 meV from the limit of low coverage up to the (4 x 1) structure. The frequency is then gradually shifted upwards, whereby an increase of 0.9 meV is observed when the reconstruction changes from the (3 x 1) to the (2 x 1) phase. We show that the shift is explained by a dipolar theory. The dependence of the frequency on coverage is discussed with reference to the model recently proposed by Schimizu and Tsukada [Surf. Sci. Lett. 295 (1993) L1017] who attributed the shift to fine changes in the atomic positions.
Abstract: 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.
Abstract: We report on high-resolution electron-energy-loss measurements of surface-plasmon losses on Ag(001) performed for crystal temperatures T in the range 100-400 K. Both the surface-plasmon frequency and linewidth are found to depend on T. The surface-plasmon frequency decreases with increasing T, nearly independently of wave vector; the linewidth increases by 7 x 10(-5) eV/K. The shift in energy is explained by the thermal expansion of the crystal, while the linewidth is influenced by the surface-plasmon-phonon interaction.
Abstract: The dependence of surface-plasmon damping versus plasmon momentum and energy has been measured for the Ag(011) surface with angle-resolved electron energy loss spectroscopy. The data are compared with previous results obtained for Ag(001). At small transferred momenta, q parallel-to, we observe a nearly linear dependence of the energy loss width with q parallel-to due to the growing phase space available for e-h pair creation. However the very efficient damping mechanism observed for q parallel-to beyond 0.10 angstrom-1 is not present for Ag(011). We demonstrate that it is due the onset of interband transitions between filled and empty surface states. Two such states are indeed present near the XBAR point of Ag(001) as demonstrated by inverse photoemission experiments and electronic band structure calculations. The energy gap between the two bands can thus be determined by EELS with a greater precision than by photoemission.
Abstract: Measurements are reported of surface plasmon energy and dispersion carried out by electron energy loss spectroscopy on Ag (110) along both high-symmetry crystallographic directions. For small q(parallel-to) the data show a linear and positive dispersion with a strong azimuthal dependence that is twice as large along the [001] direction as along [110BAR].
Abstract: We present analysis of electron-energy loss spectra of Ag(001) using first-principles surface phonon and multiple scattering inelastic cross-section calculations. Excellent agreement with experiment is obtained. The study reveals a new longitudinal surface phonon at MBAR which causes the measured loss peak to shift as the incident energy is varied. These results demonstrate the importance of using accurate theory for proper interpretation of EELS spectra and indicate that force-constant models with adjustable parameters may be misleading.
Abstract: In this paper a UHV apparatus for studying adsorption of gases on surfaces will be described. The system is specifically designed for measuring the sticking coefficient of a molecule on the surface and its dependence on energy and angle of incidence. The method is based on the use of an electron energy loss spectrometer which detects the amount of the adsorbed species as a function of the exposure of the surface to a supersonic nozzle beam which deposits the molecular species at different energies and angles of incidence. Measurements of the sticking probability of O2 on Ag(001) are presented showing the capability of the method.
Abstract: A new amplifier, designed to operate with fast semiconducting bolometers, is presented. Based on current detection of the bolometer signal, it allows one to obtain good sensitivity with response times of about 10 ns without using cryogenic electrical components. The amplifier was used to detect heat pulses propagating in a Si crystal.
Abstract: The paper describes a procedure for measuring the initial sticking coefficient of gases on clean surfaces. The method is based on the determination of the rate of increase of coverage with total exposure to impinging particles using a combination of an Electron Energy Loss Spectrometer with a nozzle beam source. The adsorption of O2 on Ag (001) has been studied. For a crystal temperature of 300 K and for an O2 beam impinging on the crystal at normal incidence and at an energy of 88 meV, the adsorption is dissociative and we find an initial sticking coefficient of (7.9 +/- 2.8) 10(-4).
