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SERGIO VALERI

SENIOR PROFESSOR presso: Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica


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Pubblicazioni

2020 - Optical and electronic properties of silver nanoparticles embedded in cerium oxide [Articolo su rivista]
Pelli Cresi, J. S.; Silvagni, E.; Bertoni, G.; Spadaro, M. C.; Benedetti, S.; Valeri, S.; D'Addato, S.; Luches, P.
abstract

Wide bandgap oxides can be sensitized to visible light by coupling them with plasmonic nanoparticles (NPs). We investigate the optical and electronic properties of composite materials made of Ag NPs embedded within cerium oxide layers of different thickness. The electronic properties of the materials are investigated by X-ray and ultraviolet photoemission spectroscopy, which demonstrates the occurrence of static charge transfers between the metal and the oxide and its dependence on the NP size. Ultraviolet-visible spectrophotometry measurements show that the materials have a strong absorption in the visible range induced by the excitation of localized surface plasmon resonances. The plasmonic absorption band can be modified in shape and intensity by changing the NP aspect ratio and density and the thickness of the cerium oxide film.


2020 - Polar Step-Driven Metal Nucleation and Growth: The Ag/ZnO(10(1)over-bar0) Case [Articolo su rivista]
Benedetti, S; Valenti, I; Valeri, S; Castilla, S; Touze, E; Bronstein, Y; Toumar, A; Finocchi, F; Lazzari, R
abstract

The morphology, epitaxy, band alignment, and optical response of silver nanoparticles on ZnO(10 (1) over bar0) have been analyzed by microscopy, electron diffraction, photoemission and differential reflectivity. At 700 K, clusters tend to nucleate and grow exclusively along [010]-oriented polar steps and corners and not on flat terraces or nonpolar [001] steps. This preference leads to a specific epitaxy Ag[1 (1) over bar0](111) II ZnO[010] (10 (1) over bar0) with a large lattice mismatch along the dense row of the metal. The experiments have been rationalized through ab initio simulations. A much lower adsorption energy is obtained for the O-terminated steps with a charge transfer from silver to oxygen that corroborates the variation of ionization energy observed in photoemission and the presence of cationic silver. This anisotropy of growth reverberates in the plasmonic response of the metallic particles. The growth and epitaxy of metals on polar (0001) and nonpolar (10 (1) over bar0) orientations of ZnO are discussed in terms of surface/step polarity compensation in the light of the present findings.


2020 - Tribological response of laser-textured steel pins with low-dimensional micrometric patterns [Articolo su rivista]
Fiaschi, G.; Di Lauro, M.; Ballestrazzi, A.; Rota, A.; Biscarini, F.; Valeri, S.
abstract

Our purpose was to study the tribological eects of surface texturing at the mesoscale, generally disregarded. We realized hemispherical dimples on 100Cr6 steel pins using a ns-pulsed Nd:YAG laser. Both radius and depth of the patterns fall in the very-low part of the micrometric scale, ranging respectively from 10 to 13 m and from 4 to 6.5 m. To map Stribeck curves, we used a pin-on-disk apparatus and a commercial motor oil as lubricant. The results conrm that the post-texturing lapping process is crucial to achieve a reduction in the coecient of friction. These patterns show a tribological eect mainly in mixed lubrication regime, and partially in hydrodynamic regime. Even very small dierences in radius and depth may induce very dierent response, evidencing the need of very ne tuning of the geometrical characteristics of the patterns. The lower friction was recorded for larger and deeper dimples. Friction reduction for increasing dimples density has been found as well. Endurance tests in boundary lubrication regime show a more stable coecient of friction and a larger mean lifetime of patterned surfaces with respect to polished one. Furthermore, water-contact-angle tests show a weak hydrophobic eect for high aspect ratio patterns. However, no correlation was observed between wettability and tribological response.


2019 - A chemical, mechanical, and tribological analysis of DLC coatings deposited by magnetron sputtering [Articolo su rivista]
Fiaschi, G.; Rota, A.; Ballestrazzi, A.; Marchetto, D.; Vezzalini, E.; Valeri, S.
abstract

Diamond-like carbon is one of the most studied and used solid lubricants on the market. Despite this large use and its outstanding mechanical and tribological properties, there are still some unclear aspects related to its self-lubricant properties, and some drawbacks in the deposition methods. We deposited soft DLC films on Si(100), iron, and stainless steel substrates by PVD magnetron sputtering technique with a Cr/CrN adhesive interlayer. The DLC films were characterized from a chemical, mechanical, and tribological point of view. Our aim was to connect the coating chemical and mechanical characteristics to the different conditions used for the deposition, such as discharge power and substrate-target distance. We found a stronger sp(3) dependence on the discharge power for DLC deposited closer to the target. The tribological results did not depend on the chosen substrate-target distance, but rather on the hardness of the substrate. This could be ascribed to the better mechanical coupling of soft DLC films on harder substrates.


2019 - Core-Shell Charge Transfer in Plasmonic Fe@Ag Nanoparticles on MgO Film [Articolo su rivista]
Benedetti, S.; Valenti, I.; Valeri, S.
abstract

In this work we report the interfacial charge transfer between the Fe core and Ag shell in self-organized nanoparticles on MgO films on Mo(001). Predeposited Fe nanoparticles organize in a square network with long-range order on the oxide surface guided by the MgO coincidence lattice. When Ag is added, it covers the Fe nanoparticles, forming a shell. By means of XPS and UPS we show that a charge transfer occurs between the Fe core and the Ag shell, determining the oxidation of part of the Fe atoms and a negative charging of the Ag shell. This is confirmed by band bending and core level shifts. As a consequence of the Fe@Ag morphology and composition the plasmonic response of the nanoparticles is modified with respect to pure Ag nanoparticles.


2019 - Dynamics of the interaction between ceria and platinum during redox processes [Articolo su rivista]
Luches, P.; Gasperi, G.; Sauerbrey, M.; Valeri, S.; Falta, J.; Flege, J. I.
abstract

The work is focused on understanding the dynamics of the processes which occur at the interface between ceria and platinum during redox processes, by investigating an inverse catalytic model system made of ceria epitaxial islands and ultrathin films supported on Pt(111). The evolution of the morphology, structure and electronic properties is analyzed in real-time during reduction and oxidation, using low-energy electron microscopy and spatially resolved low-energy electron diffraction. The reduction is induced using different methods, namely thermal treatments in ultra-high vacuum and in H2 as well as deposition of Ce on the oxide surface, while re-oxidation is obtained by exposure to oxygen at elevated temperature. The use of two different epitaxial systems, continuous films and nanostructures, allows determining the influence of platinum proximity on the stabilization of the specific phases observed. The factors that limit the reversibility of the observed modifications with the different oxidation treatments are also discussed. The obtained results highlight important aspects of the cerium oxide/Pt interaction that are relevant for a complete understanding of the behavior of Pt/CeO2 catalysts.


2019 - Highly efficient plasmon-mediated electron injection into cerium oxide from embedded silver nanoparticles [Articolo su rivista]
Pelli Cresi, J. S.; Spadaro, M. C.; D'Addato, S.; Valeri, S.; Benedetti, S.; Di Bona, A.; Catone, D.; Di Mario, L.; O'Keeffe, P.; Paladini, A.; Bertoni, G.; Luches, P.
abstract

The coupling with plasmonic metal nanoparticles (NPs) represents a promising opportunity to sensitize wide band gap oxides to visible light. The processes which come into play after the excitation of localized surface plasmon resonances (LSPRs) in the NPs largely determine the efficiency of the charge/ energy transfer from the metal NP to the oxide. We report a study of plasmon-mediated energy transfer from mass-selected silver NPs into the cerium oxide matrix in which they are embedded. Femtosecond transient absorption spectroscopy is used to probe the dynamics of charge carrier relaxation after the excitation of the LSPR of the silver nanoparticles and to evaluate the plasmon-mediated electron transfer efficiency from the silver nanoparticles to the cerium oxide. High injection efficiencies in the 6-16% range have been identified for excitation between 400 and 600 nm. These high values have been explained in terms of plasmon-mediated direct electron injection as well as indirect hot electron injection from the NPs to the oxide. The information obtained provides an important contribution towards a knowledge- driven design of efficient cerium oxide based nanostructured materials for solar to chemical energy conversion.


2019 - Mesoporous bioactive glasses doped with cerium: Investigation over enzymatic-like mimetic activities and bioactivity [Articolo su rivista]
Nicolini, V.; Malavasi, G.; Lusvardi, G.; Zambon, A.; Benedetti, F.; Cerrato, G.; Valeri, S.; Luches, P.
abstract

In this work, we investigate the ability of mesoporous bioactive glasses (MBGs) modified with cerium ions (Ce3+/Ce4+) to act as catalase and superoxide dismutase (SOD) mimetic materials. We have previously reported that the catalytic properties of bioactive Ce-containing glasses based on 45S5 Bioglass® and synthesized via melting are influenced by: i) composition (presence/absence of P2O5); ii) Ce3+/Ce4+ molar ratio. The introduction of cerium species drastically decreased the bioactivity in terms of Hydroxyapatite formation during bioactivity tests in vitro. We thus decided to add cerium to MBGs, a class of glasses with improved bioactivity with respect to classical molten glasses. MBGs exhibit a high surface area and their reactivity is increased with respect to the molten glasses; they are able to induce the formation of Hydroxyapatite over the surface within shorter times with respect to the 45S5. The catalase and SOD mimetic activity tests revealed that the Ce-MGBs are able to act as mimetic materials for the two enzymes. Both Infrared Spectroscopy and X-ray diffraction analysis have confirmed the presence of Hydroxyapatite over both 80SiO2–15CaO–5P2O5, and 80SiO2–20CaO MBGs samples modified by 5.3% mol of CeO2; simultaneously the glasses maintain a good catalase activity. Moreover, the 80SiO2–15CaO–5P2O5 potential bioactive glasses showed SOD mimetic activity. These results highlight that it is possible to obtain a glass with both antioxidant and bioactivity properties.


2019 - Nanoscale frictional properties of ordered and disordered MoS2 [Articolo su rivista]
Serpini, E.; Rota, A.; Valeri, S.; Ukraintsev, E.; Rezek, B.; Polcar, T.; Nicolini, Patrizia
abstract

The present work aims to understand the sliding of ordered/disordered molybdenum disulfide against itself by combination of nanoscale sliding experiments and atomistic simulations. Tribological experiments were performed using lateral force microscopy with tips covered by a thin sputtered MoS2 film. Nanoscale contact area between the MoS2-coated tips and MoS2 samples opened up the possibility for close comparison with classical molecular dynamics simulations. Our simulations replicated well the coefficient of friction obtained by experiments for various contact conditions and shed light on nanoscale sliding of both crystalline and amorphous MoS2. Experimental sliding at humid environment demonstrated detrimental effect of water molecules on friction. However, such effect was much less pronounced when compared to that observed in macroscopic sliding experiments.


2019 - Physical Synthesis and Study of Ag@CaF 2 Core@Shell Nanoparticles: Morphology and Tuning of Optical Properties [Articolo su rivista]
D'Addato, Sergio; Vikatakavi, Avinash; Spadaro, Maria Chiara; Valeri, Sergio; Pasquali, Luca
abstract

Pre-formed Ag nanoparticles (NPs) and Ag@CaF 2 core–shell NPs are physically synthesized using DC magnetron-based NP source and deposited on Si-SiO x wafers. The samples are prepared by co-depositing Ag nanoparticles and CaF 2 produced by an evaporation source, or by sequential deposition method, i.e., by depositing in a sequence a CaF 2 buffer layer, the Ag NPs generated by the NP source and a capping CaF 2 layer. The supported films are characterized by Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Surface Differential Reflectivity (SDR). SEM shows that Ag NPs deposited directly on Si-SiO x tend to diffuse and to agglomerate, affecting the size distribution of the nanostructures. The presence of a CaF 2 buffer layer between Ag and Si-SiO x limits this effect, while XPS reveals electrical charging, caused by the insulating nature of the CaF 2 continuous film. The surface plasmon resonance behavior for different samples is analyzed using SDR with p-polarized light. There is a clear evidence of a blue shift in the plasmon excitation due to the presence of CaF 2 on Si, which can represent a potential advantage for the technological applications in photovoltaics and optoelectronics.


2019 - Reducibility of Ag- and Cu-Modified Ultrathin Epitaxial Cerium Oxide Films [Articolo su rivista]
Gasperi, G.; Brugnoli, L.; Pedone, A.; Menziani, M. C.; Valeri, S.; Luches, P.
abstract

The functionality of cerium oxide, and in particular its reactivity, can be significantly altered by the addition of diluted cationic species with different electronic properties as compared to cerium. We investigate the modifications induced by Ag and Cu as modifier cations in cerium oxide ultrathin epitaxial films. The reducibility is assessed by following the modifications of the oxidation state of surface Ce ions by X-ray photoemission spectroscopy, during thermal treatments in ultrahigh vacuum and oxygen partial pressure. A significantly higher reducibility is observed in Ag- and Cu-modified films as compared to pure CeO2 films of the same thickness. The thermal stability of the cation modifier concentration and the changes of the surface structure with the reducing treatments are also discussed. The modifications induced in the material are explained by comparison with density functional theory calculations, which indicate that the oxygen vacancy formation energy is significantly modified by the addition of Ag or Cu in the cerium oxide matrix. The obtained results are of help in view of a rational design of catalysts with optimized performance.


2019 - Tribological Properties of High-Speed Uniform Femtosecond Laser Patterning on Stainless Steel [Articolo su rivista]
Gnilitskyi, Iaroslav; Rota, Alberto; Gualtieri, Enrico; Valeri, Sergio; Orazi, Leonardo
abstract

In this work, an analysis of the tribological performance of laser-induced periodic surface structures (LIPSS) treated X5CrNi1810 stainless steel was conducted. The approach followed by authors was to generate LIPSS-patterned circular tracks, composed of radial straight grooves with uniform angular periodicity. This permitted to measure the tribological properties in a pin-on-flat configuration, keeping fixed the orientation between the grooves and the sliding direction. A Stribeck curve was measured, as well as the consequent wear. A deep analysis of the sub-surface conditions after LIPSS generation was moreover performed using Focused Ion Beam (FIB) cross-section.


2018 - Ageing effects on electrical resistivity of Nb-doped TiO 2 thin films deposited at a high rate by reactive DC magnetron sputtering [Articolo su rivista]
Casotti, D.; Orsini, V.; di Bona, A.; Gardonio, S.; Fanetti, M.; Valant, M.; Valeri, S.
abstract

We report on the long-term stability of electrical resistivity in Nb-doped TiO 2 thin films grown at a high rate by a reactive DC magnetron sputtering from metallic targets. The high deposition rate is obtained by an active control of the oxygen flow during the growth process. Film microstructure and preferential orientation of the crystallites are controlled by the total working pressure in the film growth process. After a heat treatment in vacuum, the film resistivity is in a 10 −3 Ω cm range and the optical transmission higher than 80% in the visible region. While the film is stable when kept under dry nitrogen, significant ageing has been observed when the material is exposed to air. In this case, the DC resistivity steadily increases and fractures form throughout the film. The ageing process is discussed in terms of the evolution of the film microstructure and/or the oxygen exchange through on the film surface. Oxygen uptake from ambient air is confined to a shallow surface region. It is possible that this mechanism triggers the formation/propagation of the fractures that predominantly contribute to the increase in film resistivity.


2018 - Friction and Adhesion of Different Structural Defects of Graphene [Articolo su rivista]
Tripathi, M.; Awaja, F.; Bizao, R. A.; Signetti, S.; Iacob, E.; Paolicelli, G.; Valeri, S.; Dalton, A.; Pugno, N. M.
abstract

Graphene structural defects, namely edges, step-edges, and wrinkles, are susceptible to severe mechanical deformation and stresses under tribo-mechanical operations. Applied forces may cause deformation by folding, buckling, bending, and tearing of these defective sites of graphene, which lead to a remarkable decline in normal and friction load bearing capacity. In this work, we experimentally quantified the maximum sustainable normal and friction forces, corresponding to the damage thresholds of the different investigated defects as well as their pull-out (adhesion) forces. Horizontal wrinkles (with respect to the basal plane, i.e., folded) sustained the highest normal load, up to 317 nN, during sliding, whereas for vertical (i.e., standing) wrinkles, step-edges, and edges, the load bearing capacities are up to 113, 74, and 63 nN, respectively. The related deformation mechanisms were also experimentally investigated by varying the normal load up to the initiation of the damage from the defects and extended with the numerical results from molecular dynamics and finite element method simulations.


2018 - Role of cerium oxide in bioactive glasses during catalytic dissociation of hydrogen peroxide [Articolo su rivista]
Benedetti, Francesco; Amidani, Lucia; Pelli Cresi, Jacopo Stefano; Boscherini, Federico; Valeri, Sergio; D'Addato, Sergio; Nicolini, Valentina; Malavasi, Gianluca; Luches, Paola
abstract

The addition of cerium oxide to bioactive glasses, important materials for bone tissue regeneration, has been shown to induce multifunctionality, combining a significant bioactivity with antioxidant properties. We provide a real time investigation of the evolution of the electronic properties of highly diluted cerium ions in a liquid environment containing hydrogen peroxide - the most abundant reactive oxygen species in living cells. This challenging task is undertaken by means of high-energy resolution fluorescence detected by X-ray absorption near-edge spectroscopy at the Ce L-3 edge. We investigate samples with variable compositions and different morphologies. We relate the observed spectroscopic changes not only to variations in the concentration of the two Ce oxidation states in the samples, but also to changes in the local atomic environment of Ce ions, providing a clear picture of the role of cerium ions in the dissociation of hydrogen peroxide. The obtained results contribute to the understanding of the mechanisms that come into play in the process and provide a basis for the optimization of the functionalities of this class of materials.


2017 - Cerium Oxide Epitaxial Nanostructures on Pt(111): Growth, Morphology and Structure [Articolo su rivista]
Sauerbrey, M.; Gasperi, G.; Luches, P.; Falta, J.; Valeri, S.; Flege, J. I.
abstract

Cerium oxide in combination with metals is a very important material for catalysis. We present an in-depth characterization of the growth and morphology of cerium oxide in the form of an inverse model catalyst on Pt(111). Using state of the art low energy electron microscopy (LEEM) and microprobe low energy electron diffraction we provide a detailed in-situ insight into the nucleation and growth dynamics of this model system in different experimental conditions. By probing the unoccupied band structure of the sample during the growth process using intensity-voltage LEEM we also obtain information on the growth dynamics of the deposited ceria overlayer. The results represent an important basis in view of reactivity studies on well controlled ceria epitaxial systems.


2017 - Cerium-doped bioactive 45S5 glasses: spectroscopic, redox, bioactivity and biocatalytic properties [Articolo su rivista]
Nicolini, Valentina; Malavasi, Gianluca; Menabue, Ledi; Lusvardi, Gigliola; Benedetti, Francesco; Valeri, Sergio; Luches, Paola
abstract

The ability of Ce-containing bioactive glasses, based on 45S5 Bioglass®, to inhibit oxidative stress in terms of reduction in hydrogen peroxide and superoxide (O2−), by mimicking the catalase and superoxide dismutase activity is reported in this work. The characterization is performed on the powders of pristine glasses and after the soaking in H2O2solutions and simulated body fluid. The glass samples are analysed by XPS, XRD, UV–Vis and FT-IR. The best catalyst activities are obtained for the glass with the highest content of cerium (H_5.3 = 5.3 mol% of CeO2in the nominal glass composition), and the best Ce3+/Ce4+ratio in terms of catalase mimetic activity is found to be a function of H2O2concentration. Moreover, the detailed study of the surface during the mimic enzymatic activity tests shows the formation of a Ca-P-rich layer on the glass surface, where the presence of Ce ions favours the formation of CePO4. The phosphate in turn inhibits the formation of hydroxyapatite, decreasing the bioactivity of the glass with the highest of CeO2in the glass composition. This work shows the effect of Ce3+/Ce4+ratio towards the catalase mimetic activity and for the first time the superoxide dismutase mimetic activity of Ce-containing 45S5-derived glasses.


2017 - Contraction, cation oxidation state and size effects in cerium oxide nanoparticles [Articolo su rivista]
Cresi, Jacopo Stefano Pelli; Chiara Spadaro, Maria; D'Addato, Sergio; Valeri, Sergio; Amidani, Lucia; Boscherini, Federico; Bertoni, Giovanni; Deiana, Davide; Luches, Paola
abstract

An accurate description of the structural and chemical modifications of cerium oxide nanoparticles (NPs) is mandatory for understanding their functionality in applications. In this work we investigate the relation between local atomic structure, oxidation state, defectivity and size in cerium oxide NPs with variable diameter below 10 nm, using x-ray absorption fine structure analysis in the near and extended energy range. The NPs are prepared by physical methods under controlled conditions and analyzed in morphology and crystalline quality by high resolution transmission electron microscopy. We resolve here an important question on the local structure of cerium oxide NPs: we demonstrate a progressive contraction in the Ce-O interatomic distance with decreasing NP diameter and we relate the observed effect to the reduced dimensionality. The contraction is not significantly modified by inducing a 4%-6% higher Ce3+ concentration through thermal annealing in high vacuum. The consequences of the observed average cation-anion distance contraction on the properties of the NPs are discussed.


2017 - Investigation of Ni@CoO core-shell nanoparticle films synthesized by sequential layer deposition [Articolo su rivista]
Spadaro, Maria Chiara; Luches, P.; Benedetti, Francesco; Valeri, Sergio; Turchini, S.; Bertoni, G.; Ferretti, A. M.; Capetti, E.; Ponti, A.; D'Addato, Sergio
abstract

Films of Ni@CoO core-shell nanoparticles (NP Ni core size d ≈ 11 nm) have been grown on Si/SiOx and lacey carbon supports, by a sequential layer deposition method: a first layer of CoO was evaporated on the substrate, followed by the deposition of a layer of pre-formed, mass-selected Ni NPs, and finally an overlayer of CoO was added. The Ni NPs were formed by a magnetron gas aggregation source, and mass selected with a quadrupole mass filter. The morphology of the films was investigated with Scanning Electron Microscopy and Scanning Transmission Electron Microscopy. The Ni NP cores have a shape compatible with McKay icosahedron, caused by multitwinning occurring during their growth in the source, and the Ni NP layer shows the typical random paving growth mode. After the deposition of the CoO overlayer, CoO islands are observed, gradually extending and tending to merge with each other, with the formation of shells that enclose the Ni NP cores. In situ X-ray Photoelectron Spectroscopy showed that a few Ni atomic layers localized at the core-shell interface are oxidized, hinting at the possibility of creating an intermediate NiO shell between Ni and CoO, depending on the deposition conditions. Finally, X-ray Magnetic Circular Dichroism at the Ni L2,3 absorption edge showed the presence of magnetization at room temperature even at remanence, revealing the possibility of magnetic stabilization of the NP film.


2017 - Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO [Articolo su rivista]
Benedetti, S.; Valenti, I.; di Bona, A.; Vinai, G.; Castan-Guerrero, C.; Valeri, S.; Catellani, A.; Ruini, A.; Torelli, P.; Calzolari, A.
abstract

The conduction and optoelectronic properties of transparent conductive oxides can be largely modified by intentional inclusion of dopants over a very large range of concentrations. However, the simultaneous presence of structural defects results in an unpredictable complexity that prevents a clear identification of chemical and structural properties of the final samples. By exploiting the unique chemical sensitivity of Hard X-ray Photoelectron Spectra and Near Edge X-ray Absorption Fine Structure in combination with Density Functional Theory, we determine the contribution to the spectroscopic response of defects in Al-doped ZnO films. Satellite peaks in O1s and modifications at the O K-edge allow the determination of the presence of H embedded in ZnO and the very low concentration of Zn vacancies and O interstitials in undoped ZnO. Contributions coming from substitutional and (above the solubility limit) interstitial Al atoms have been clearly identified and have been related to changes in the oxide stoichiometry and increased oxygen coordination, together with small lattice distortions. In this way defects and doping in oxide films can be controlled, in order to tune their properties and improve their performances.


2017 - Steering the magnetic properties of Ni/NiO/CoO core-shell nanoparticle films: The role of core-shell interface versus interparticle interactions [Articolo su rivista]
Ponti, Alessandro; Ferretti, Anna M.; Capetti, Elena; Spadaro, Maria Chiara; Bertoni, Giovanni; Grillo, Vincenzo; Luches, Paola; Valeri, Sergio; D’Addato, Sergio
abstract

Supported core-shell Ni/NiO/CoO nanoparticle (NP) films were obtained by deposition of preformed and mass-selected Ni NPs on a buffer layer of CoO, followed by a top CoO layer. The resulting NPs have core/shell morphology, with a McKay icosahedral Ni core and a partially crystalline CoO shell. X-ray photoelectron spectroscopy evidenced the presence of a thin NiO layer, which was shown to be between the Ni core and the CoO shell by elemental TEM mapping. CoO and NiO shells with different thickness values were obtained, allowing us to investigate the evolution of the magnetic properties of the NP assemblies as a function of the oxide shell thickness. Both exchange-coupling and magnetostatic interactions significantly contribute to the magnetic behavior of Ni/NiO/CoO NP films. After the Ni/NiO/CoO NPs are cooled in a weak magnetic field, they have blocking temperature higher than room temperature because of strong magnetostatic interactions, which support the formation of a spin-glass-like state below similar to 250 K. Exchange coupling dominates the magnetic behavior after the NPs are cooled in a strong magnetic field. The exchange bias (EB) is in the 0.17-2.35 kOe range and strongly depends on the CoO thickness (0.4-2.7 nm), showing the onset of the EB at the few-nanometer scale. The switching field distribution showed that the EB opposes the magnetization reversal from the direction along the cooling field but it does not significantly ease the opposite process. The EB depends on t(CoO) only for t(NiO) <= 0.5 nm, but when NiO is 0.7 nm thick it strongly interacts with CoO and a large increase of the EB and coercivity is observed.


2017 - Structure of active cerium sites within bioactive glasses [Articolo su rivista]
Benedetti, Francesco; Luches, Paola; D'Addato, Sergio; Valeri, Sergio; Nicolini, Valentina; Pedone, Alfonso; Menziani, Maria Cristina; Malavasi, Gianluca
abstract

The inclusion of small amounts of cerium within the matrix of bioactive glasses, a class of materials used for bone tissue reparation and regeneration, adds a very important antioxidant property. In this work we investigate the local structure around cerium ions in cerium oxide doped bioactive phosphosilicate and silicate glasses, by X-ray absorption fine structure at the Ce K-edge, combined with classical molecular dynamics (MD) simulations. The local structure of the active Ce sites results significantly different from the one in bulk cerium oxide phases. A contracted Ce–O first shell distance with respect to bulk oxides is detected, in full agreement with the results of MD simulations. The cerium environment in the glass matrix at different stages of the reaction with H2O2 does not show significant modifications, in spite of the very high catalytic activity. The accurate description of the active Ce sites within bioactive glasses reported in this work is an important step toward an atomic scale understanding of the material functionalities.


2017 - Surface passivation by graphene in the lubrication of iron: A comparison with bronze [Articolo su rivista]
Marchetto, Diego; Restuccia, Paolo; Ballestrazzi, Antonio; Righi, Maria Clelia; Rota, Alberto; Valeri, Sergio
abstract

It has been recently reported that graphene is able to significantly reduce the friction coefficient of steel-on-steel sliding contacts. The microscopic origin of this behavior has been attributed to the mechanical action of load carrying capacity. However, a recent work highlighted the importance of the chemical action of graphene. According to this work graphene reduces the adhesion of iron interfaces by reducing the surface energy thanks to a passivation effect. The aim of the present work is to clarify the still debated lubricating behavior of graphene flakes. We perform pin-on-disc experiments using liquid dispersed graphene solution as a lubricant. Two different materials, pure iron and bronze are tested against 100Cr6 steel. Raman spectroscopy is used to analyze the surfaces after the friction tests. The results of these tests prove that graphene flakes have a beneficial effect on the friction coefficient. At the same time they show a tendency of graphene to passivate the native iron surfaces that are exposed during sliding as a consequence of wear.


2017 - The role of humidity and oxygen on MoS2 thin films deposited by RF PVD magnetron sputtering [Articolo su rivista]
Serpini, Elisabetta; Rota, Alberto; Ballestrazzi, Antonio; Marchetto, Diego; Gualtieri, Enrico; Valeri, Sergio
abstract

MoS2 is usually applied as thick films (1 μm and above) on sliding counterparts to decrease friction and wear. Thick films of MoS2 generally grow columnar-like and the bending or fracture of these columns during sliding are supposed to be the origin of the good tribological performances. In the present work, we studied the tribological behavior of 200 nm MoS2 films prepared by RF magnetron sputtering, with emphasis on the friction mechanisms. We performed ball-on-disc tests at different values of residual humidity, in pure oxygen atmosphere and at different temperatures, in order to disentangle the role of water and molecular oxygen during sliding. We found that, despite the fact that the inner structure of these thin films is not lamellar, the tribological behavior is similar to thicker ones. The friction in absence of humidity is well below that in standard conditions and the lifetime of the film is strongly enhanced. We observed similar performances in humid air while keeping the sample at 75 °C. Our results clearly demonstrated that absorbed water causes the deterioration of the lubricating properties of the films, while film oxidation plays only a marginal role.


2016 - AFM-based tribological study of nanopatterned surfaces: The influence of contact area instabilities [Articolo su rivista]
Rota, A.; Serpini, Elisabetta; Gazzadi, G. C.; Valeri, Sergio
abstract

Although the importance of morphology on the tribological properties of surfaces has long been proved, an exhaustive understanding of nanopatterning effects is still lacking due to the difficulty in both fabricating 'really nano-' structures and detecting their tribological properties. In the present work we show how the probe-surface contact area can be a critical parameter due to its remarkable local variability, making a correct interpretation of the data very difficult in the case of extremely small nanofeatures. Regular arrays of parallel 1D straight nanoprotrusions were fabricated by means of a low-dose focused ion beam, taking advantage of the amorphization-related swelling effect. The tribological properties of the patterns were detected in the presence of air and in vacuum (dry ambient) by atomic force microscopy. We have introduced a novel procedure and data analysis to reduce the uncertainties related to contact instabilities. The real time estimation of the radius of curvature of the contacting asperity enables us to study the dependence of the tribological properties of the patterns from their geometrical characteristics. The effect of the patterns on both adhesion and the coefficient of friction strongly depends on the contact area, which is linked to the local radius of curvature of the probe. However, a detectable hydrophobic character induced on the hydrophilic native SiO2 has been observed as well. The results suggest a scenario for capillary formation on the patterns.


2016 - CATALYTIC BIOACTIVE GLASSES: CATALASE MIMETIC ACTIVITY, AN EXAMPLE [Articolo su rivista]
Malavasi, Gianluca; Nicolini, Valentina; Gambuzzi, Elisa; Menabue, Ledi; Lusvardi, Gigliola; Pedone, Alfonso; Benedetti, Francesco; Luches, Paola; D'Addato, Sergio; Valeri, Sergio
abstract

The ability of a Ce-containing bioactive glasses to inhibit oxidative stress in terms of reduction of H2O2, by mimicking the catalase enzyme activity is demonstrated for the first time. The antioxidant properties of bioactive glasses containing CeO2 have been evaluated by following the degradation of hydrogen peroxide with time after immersion in H2O2 aqueous solutions with different concentration. XPS, UV-Vis and magnetic measurements allowed us to determine the Ce3+/Ce4+ ratio (bulk and surface) and to correlate it with the ability of the samples to show catalase mimetic activity. We have found that the bioactive glass (23.2Na2O-25.7CaO-43.4SiO2-2.4P2O5-5.3CeO2) immersed in 0.1M H2O2 aqueous solution is able to degrade 90% of it in one week.


2016 - Dopant-Induced Diffusion Processes at Metal-Oxide Interfaces Studied for Iron- and Chromium-Doped MgO/Mo(001) Model Systems [Articolo su rivista]
Benedetti, Stefania; Nilius, Niklas; Valeri, Sergio; Tosoni, Sergio; Albanese, Elisa; Pacchioni, Gianfranco
abstract

X-ray photoelectron spectroscopy reveals spontaneous atom diffusion from a Mo(001) support into a MgO thin film doped with transition-metal ions. The amount of interfacial mixing depends on the nature of the dopants and is considerably larger for Fe than for Cr impurities. DFT calculations find the reason for Mo diffusion in the ability of the dopants to change oxidation state. Cr exclusively occurs as 3+ ion in the rocksalt lattice, whereby the charge mismatch to native Mg2+ ions is compensated for by Mg vacancies. Iron, on the other hand, switches its thermodynamically preferred configuration from 3+ to 2+ with increasing temperature. As a result, Mo atoms from the support move into the Mg vacancies upon sample annealing and become oxidized via charge transfer into the Fe3+ species. Our study unravels a new charge-compensation scheme in doped oxides that proceeds via chemical intermixing at a metal-oxide interface. The mechanism may rationalize the often observed inactivity of doped oxides in charge-transfer reactions.


2016 - Electronic properties of epitaxial cerium oxide films during controlled reduction and oxidation studied by resonant inelastic X-ray scattering [Articolo su rivista]
Gasperi, Gabriele; Amidani, Lucia; Benedetti, Francesco; Boscherini, Federico; Glatzel, Pieter; Valeri, Sergio; Luches, Paola
abstract

We investigated the evolution of the electronic structure of cerium oxide ultrathin epitaxial films during reduction and oxidation processes using resonant inelastic X-ray scattering at the Ce L3 absorption edge, a technique sensitive to the electronic configurations at the 4f levels and in the 5d band thanks to its high energy resolution. We used thermal treatments in high vacuum and in oxygen partial pressure to induce a controlled and reversible degree of reduction in cerium oxide ultrathin epitaxial films of different thicknesses. Two dominant spectral components contribute to the measured spectra at the different degrees of oxidation/reduction. In ultrathin films a modification of the electronic properties associated with platinum substrate proximity and with dimensionality is identified. The different electronic properties induce a higher reducibility in ultrathin films, ascribed to a decrease of the surface oxygen vacancy formation energy.


2016 - Friction of rough surfaces on ice: Experiments and modeling [Articolo su rivista]
Spagni, Alberto; Alice, Berardo; Marchetto, Diego; Gualtieri, Enrico; Nicola, M. Pugno; Valeri, Sergio
abstract

Over a century of scientific research on the sliding friction of ice has not been enough to develop an exhaustive explanation for the tribological behavior of frozen water. It has been recognized that ice shows different friction regimes, but a detailed description of all the different phenomena and processes occurring at the interface, including the effect of surface roughness of both the ice and the antagonist material is still missing. In this work the effect of surface morphology on the friction of steel/ice interfaces is studied. Different degrees of random roughness on steel surfaces are introduced and the friction coefficient is measured over a wide range of temperature and sliding velocity. Correlation between the surface roughness and the lubrication regime and friction coefficient is discussed. A theoretical model is developed in order to explain this correlation, and to control the tribological behavior of the system by a proper selection of surface roughness parameters.


2016 - Influence of defect distribution on the reducibility of CeO2-x nanoparticles [Articolo su rivista]
Spadaro, Maria Chiara; Luches, Paola; Bertoni, Giovanni; Grillo, Vincenzo; Turner, Stuart; Van Tendeloo, Gustaaf; Valeri, Sergio; D'Addato, Sergio
abstract

Ceria nanoparticles (NPs) are fundamental in heterogeneous catalysis because of their ability to store or release oxygen depending on the ambient conditions. Their oxygen storage capacity is strictly related to the exposed planes, crystallinity, density and distribution of defects. In this work a study of ceria NPs produced with a ligand-free, physical synthesis method is presented. The NP films were grown by a magnetron sputtering based gas aggregation source and studied by high resolution- and scanning-transmission electron microscopy and x-ray photoelectron spectroscopy. In particular, the influence of the oxidation procedure on the NP reducibility has been investigated. The different reducibility has been correlated to the exposed planes, crystallinity and density and distribution of structural defects. The results obtained in this work represent a basis to obtain cerium oxide NP with desired oxygen transport properties.


2016 - Tribological characteristics of few-layer graphene over Ni grain and interface boundaries [Articolo su rivista]
Tripathi, Manoj; Awaja, Firas; Paolicelli, Guido; Bartali, Ruben; Iacob, Erica; VALERI, Sergio; Ryu, Seunghwa; Signetti, Stefano; Speranza, Giorgio; Pugno, Nicola Maria
abstract

The tribological properties of metal supported few layered graphene (FLG) depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the wetting and frictional characteristics of the FLG system. We individually measured friction characteristics of FLG covering grains and grain boundaries of polycrystalline Ni metal substrate via an Atomic Force Microscopy (AFM) probe. The friction coefficient of FLG covered at grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer/s. Finite element simulations show good agreement with experiments and led to rationalize the observations. Thus, with grain boundaries the FLG tribology can be effectively tuned.


2016 - Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation [Articolo su rivista]
Tacchi, S.; Pini, M. G.; Rettori, A.; Varvaro, G.; Di Bona, A.; Valeri, Sergio; Albertini, F.; Lupo, P.; Casoli, F.
abstract

The effect of graded anisotropy on static and dynamic magnetic properties of Ar+-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by Ar+ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing α, a decreasing thickness of the hard L10 phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (ν0≈6 GHz), which could be reproduced assuming the presence of a nonzero "rotatable" anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.


2015 - Atomic Scale Structure and Reduction of Cerium Oxide at the Interface with Platinum [Articolo su rivista]
Luches, P.; Giordano, L.; Grillo, V.; Gazzadi, G. C.; Prada, S.; Campanini, M.; Bertoni, G.; Magen, C.; Pagliuca, Federico; Pacchioni, G.; Valeri, Sergio
abstract

A detailed atomic scale description of the interfaces between cerium oxide and metals is necessary for a clear understanding of interfacial interactions, and it may open the way to the optimization of the properties of the combined material in view of its applications. In this study the interface between cerium oxide epitaxial fi lms and the (111) surface of platinum is studied by aberration-corrected scanning transmission electron microscopy and by ab initio density functional theory calculations. Local modifi cations of the registry between the cerium oxide and platinum lattices and the occurrence of nanometric platinum islands at the interface are detected by high-angle annular dark fi eld imaging. Moreover, the preferential adsorption sites of Ce and O atoms at the interface are determined by a comparison of the images acquired in annular bright-fi eld mode and the results of density functional theory calculations. The presence of a non-negligible concentration of Ce 3+ ions at the interface is detected by electron energy loss spectroscopy and it is rationalized in terms of interfacial charge transfer as evidenced by the calculations. This work provides an insight into the atomic and electronic structure of the technologically relevant CeO 2 /Pt interface and it helps to clarify the properties of the combined material.


2015 - Chromium-Doped MgO Thin Films: Morphology, Electronic Structure, and Segregation Effects [Articolo su rivista]
Benedetti, Stefania; Nilius, Niklas; Valeri, Sergio
abstract

Incorporation of Cr into a crystalline MgO(001) thin film has been investigated by means of scanning tunnelingmicroscopy, X-ray photoelectron spectroscopy, and diffraction. For this purpose, samples of different Cr content (0−33 at %) and postannealing treatments (300−1050 K) have been analyzed. The Cr impurities mainly adopt a 3+ oxidation state, which renders the formation of compensating Mg vacancies necessary to maintain charge neutrality. Only at 33 at % doping amount are Cr6+ species detected in the as-grown films, indicating the development of a metastable CrO3 phase. At low doping level, the Cr ions fully dissolve in the MgO lattice, while segregation toward the surface is observed for Cr-rich films. In the latter case, a new surface oxide develops that is characterized by a lower binding energy of the Mg 1s and Cr 2p core levels and a (2 × 1) superstructure with respect to weakly doped MgO. In combination with a distinct 4-fold symmetric XPD pattern, the new surface phase resembles Mg chromite (MgCr2O4) growing on top of the heavily doped MgO film. Our study provides insight into the technologically relevant transition region between doped binary and ternary oxides.


2015 - Electrical, optical, and electronic properties of Al:ZnO films in a wide doping range [Articolo su rivista]
Valenti, Ilaria; Benedetti, Stefania; Di Bona, Alessandro; Lollobrigida, Valerio; Perucchi, Andrea; Di Pietro, Paola; Lupi, Stefano; Valeri, Sergio; Torelli, Piero
abstract

The combination of photoemission spectroscopies, infrared and UV-VIS absorption, and electric measurements has allowed to clarify the mechanisms governing the conductivity and the electronic properties of Al-doped ZnO (AZO) films in a wide doping range. The contribution of defect-related in-gap states to conduction has been excluded in optimally doped films (around 4 at. %). The appearance of gap states at high doping, the disappearance of occupied DOS at Fermi level, and the bands evolution complete the picture of electronic structure in AZO when doped above 4 at. %. In this situation, compensating defects deplete the conduction band and increase the electronic bandgap of the material. Electrical measurements and figure of merit determination confirm the high quality of the films obtained by magnetron sputtering, and thus allow to extend their properties to AZO films in general.


2015 - Electrospun Fibers Containing Bio-Based Ricinoleic Acid: Effect of Amount and Distribution of Ricinoleic Acid Unit on Antibacterial Properties [Articolo su rivista]
Totaro, G.; Paltrinieri, L.; Mazzola, G.; Vannini, M.; Sisti, L.; Gualandi, C.; Ballestrazzi, A.; Valeri, S.; Pollicino, A.; Celli, A.; Di Gioia, D.; Focarete, M. L.
abstract

Novel bio-based and biodegradable electrospun membranes, containing bactericidal ricinoleic acid (RA) units either by copolymerization with butylene succinate (BS) units or by blending PRA and PBS homopolymers, are characterized in their chemical, thermal, mechanical and surface properties. In the blends E. coli and S. aureus mortality rate increases with PRA amount, while the copolymer shows lower bactericidal activity compared to the blend with similar composition. XPS demonstrates that in the blends an enrichment of RA units at the fiber surface occurs during electrospinning due to phase separation, while in the copolymer RA units are homogeneously dispersed along fiber cross-section.


2015 - Evidence of catalase mimetic activity in ce(3+)/ce(4+) doped bioactive glasses [Articolo su rivista]
Nicolini, Valentina; Gambuzzi, Elisa; Malavasi, Gianluca; Menabue, Ledi; Menziani, Maria Cristina; Lusvardi, Gigliola; Pedone, Alfonso; Benedetti, Francesco; Luches, Paola; D'Addato, Sergio; Valeri, Sergio
abstract

The ability of Ce-containing bioactive glasses to inhibit oxidative stress in terms of reduction of hydrogen peroxide, by mimicking the catalase enzyme activity is demonstrated here for the first time. The antioxidant properties of three bioactive glasses containing an increasing amount of CeO2 have been evaluated by following the degradation of hydrogen peroxide with time after immersion in H2O2 aqueous solutions with different concentration. XPS and UV-vis measurements allowed us to determine the Ce(3+)/Ce(4+) ratio in the bulk and on the glass surface, and to correlate it with the ability of the samples to show catalase mimetic activity. Interestingly, we have found that the bioactive glass with composition 23.2Na2O-25.7CaO-43.4SiO2-2.4P2O5-5.3CeO2 immersed in 0.1 M H2O2 aqueous solution is able to degrade 90% of it in 1 week. The reduction in bioactivity of the glasses with increasing CeO2 content is here rationalized in terms of a lower amount of phosphate groups available for the hydroxyapatite layer formation, after binding with cerium ions. In fact, classical molecular dynamics simulations revealed that the addition of CeO2 leads to the formation of cerium phosphate rich regions. The formation of an insoluble CePO4 crystalline phase is also observed by XRD analysis after thermal treatment of the glass samples.


2015 - Frictional Transition from Superlubric Islands to Pinned Monolayers [Articolo su rivista]
Pierno, M.; Bruschi, L.; Mistura, G.; Paolicelli, G.; Di Bona, Alessandro; Valeri, Sergio; Guerra, R.; Vanossi, Andrea; Tosatti, E.
abstract

Friction characteristics of graphene deposited on different substrates have been studied by atomic force microscopy (AFM). In particular, we compared mechanically exfoliated graphene transferred over Si/SiO2 with respect to monolayer (ML) graphene grown in our laboratory by low temperature chemical vapor deposition on Ni(111) single crystal. Friction force measurements by AFM have been carried out as function of load under different environment conditions, namely vacuum (10−5 Torr), nitrogen and air. The typical decrease of friction force with increasing number of layers has been observed on graphene over Si/SiO2 in all environment including vacuum. Continuum mechanical approximation has been used to analyze the friction versus load curves of ML graphene on Ni(111). Analysis shows that Derjaguin–Mueller–Toporov model is in good agreement with our experimental data indicating that overall behavior of the interface graphene–Ni(111) is relatively rigid respect to out of plane deformations. This result is consistent with the structural characteristics of the interface since graphene grows in registry with Ni(111) surface with covalent bonding character. Finally, the shear strength and the work of adhesion of the two systems with respect to AFM tip in vacuum have been compared. The result of this procedure indicates that shear strength and work of adhesion measured on graphene–Si/SiO2 interface are always greater than those on graphene–Ni(111) interface.


2015 - Influence of size, shape and core–shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiOx [Articolo su rivista]
D'Addato, Sergio; Pinotti, Daniele; Spadaro, MARIA CHIARA; Paolicelli, Guido; Grillo, Vincenzo; Valeri, Sergio; Pasquali, Luca; Bergamini, Luca; Corni, Stefano
abstract

Ag and Ag@MgO core-shell nanoparticles (NPs) with a diameter of d = 3-10 nm were obtained by physical synthesis methods and deposited on Si with its native ultrathin oxide layer SiOx (Si/SiOx). Scanning electron microscopy and transmission electron microscopy (TEM) images of bare Ag NPs revealed the presence of small NP aggregates caused by diffusion on the surface and agglomeration. Atomic resolution TEM gave evidence of the presence of crystalline multidomains in the NPs, which were due to aggregation and multitwinning occurring during NP growth in the nanocluster source. Co-deposition of Ag NPs and Mg atoms in an oxygen atmosphere gave rise to formation of a MgO shell matrix surrounding the Ag NPs. The behaviour of the surface plasmon resonance (SPR) excitation in surface differential reflectivity (SDR) spectra with p-polarised light was investigated for bare Ag and Ag@MgO NPs. It was shown that the presence of MgO around the Ag NPs caused a red shift of the plasmon excitation, and served preserve its existence after prolonged (five months) exposure to air, realizing the possibility of technological applications in plasmonic devices. The Ag NP and Ag@MgO NP film features in the SDR spectra could be reproduced by classical electrodynamics simulations by treating the NP-containing layer as an effective Maxwell Garnett medium. The simulations gave results in agreement with the experiments when accounting for the experimentally observed aggregation.


2015 - Morphology, structural properties and reducibility of size-selected CeO2- x nanoparticle films [Articolo su rivista]
Spadaro, MARIA CHIARA; D'Addato, Sergio; Gasperi, Gabriele; Benedetti, Francesco; Luches, Paola; Grillo, Vincenzo; Bertoni, Giovanni; Valeri, Sergio
abstract

Non-stoichiometric ceria nanoparticles (NPs) were obtained by a gas aggregation source with a magnetron and were mass-selected with a quadrupole mass filter. By varying magnetron power, Ar gas flow, and the length of the aggregation tube, NPs with an average diameter of 6, 9, and 14 nm were synthesized and deposited onto a substrate, thus obtaining NP films. The morphology of the films was studied with scanning electron microscopy, while high resolution transmission electron microscopy was used to gain a deeper insight into the atomic structure of individual NPs. By using X-ray photoelectron spectroscopy we analyzed the degree of reduction of the NPs of different diameters, before and after thermal treatments in vacuum (reduction cycle) and in O2 atmosphere (oxidation cycle) at different temperatures. From this analysis we inferred that the size is an important parameter only at intermediate temperatures. As a comparison, we evaluated the reducibility of an ultra-thin ceria film with the same surface to volume ratio as the 9 nm diameter NPs film, observing that NPs are more reducible than the ceria film.


2015 - Nanoscale frictional behavior of graphene on SiO2and Ni(111) substrates [Articolo su rivista]
Paolicelli, G.; Tripathi, M.; Corradini, V.; Candini, A.; Valeri, S.
abstract

Friction characteristics of graphene deposited on different substrates have been studied by atomic force microscopy (AFM). In particular, we compared mechanically exfoliated graphene transferred over Si/SiO2 with respect to monolayer (ML) graphene grown in our laboratory by low temperature chemical vapor deposition on Ni(111) single crystal. Friction force measurements by AFM have been carried out as function of load under different environment conditions, namely vacuum (10-5 Torr), nitrogen and air. The typical decrease of friction force with increasing number of layers has been observed on graphene over Si/SiO2 in all environment including vacuum. Continuum mechanical approximation has been used to analyze the friction versus load curves of ML graphene on Ni(111). Analysis shows that Derjaguin-Mueller-Toporov model is in good agreement with our experimental data indicating that overall behavior of the interface graphene-Ni(111) is relatively rigid respect to out of plane deformations. This result is consistent with the structural characteristics of the interface since graphene grows in registry with Ni(111) surface with covalent bonding character. Finally, the shear strength and the work of adhesion of the two systems with respect to AFM tip in vacuum have been compared. The result of this procedure indicates that shear strength and work of adhesion measured on graphene-Si/ SiO2 interface are always greater than those on graphene-Ni(111) interface.


2015 - Structure and Morphology of Silver Nanoparticles on the (111) Surface of Cerium Oxide [Articolo su rivista]
Benedetti, Francesco; Luches, Paola; Spadaro, Maria Chiara; Gasperi, Gabriele; D'Addato, Sergio; Valeri, Sergio; Boscherini, Federico
abstract

The structure of Ag nanoparticles of different size, supported on the cerium oxide (111) surface, was investigated by X-ray absorption fine structure at the Ag K-edge. The results of the data analysis in the near and extended energy range are interpreted with the help of the results obtained by X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements and allow to obtain a detailed atomic scale description of the model system investigated. The Ag nanoparticles have an average size of a few tens of angstroms, which increases with increasing deposited Ag amount. The nanoparticles show a slight tendency to nucleate at the step edges between different cerium oxide layers and they have a face centered cubic structure with an Ag-Ag interatomic distance contracted by 3-4% with respect to the bulk value. The interatomic distance contraction is mainly ascribed to dimensionality induced effects, while epitaxial effects have a minor role. The presence of Ag-O bonds at the interface between the nanoparticles and the supporting oxide is also detected. The Ag-O interatomic distance decreases with decreasing nanoparticle size.


2015 - Structure, morphology and reducibility of epitaxial cerium oxide ultrathin films and nanostructures [Articolo su rivista]
Luches, Paola; Valeri, Sergio
abstract

Cerium oxide is a very interesting material that finds applications in many different fields, such as catalysis, energy conversion, and biomedicine. An interesting approach to unravel the complexity of real systems and obtain an improved understanding of cerium oxide-based materials is represented by the study of model systems in the form of epitaxial ultrathin films or nanostructures supported on single crystalline substrates. These materials often show interesting novel properties, induced by spatial confinement and by the interaction with the supporting substrate, and their understanding requires the use of advanced experimental techniques combined with computational modeling. Recent experimental and theoretical studies performed within this field are examined and discussed here, with emphasis on the new perspectives introduced in view of the optimization of cerium oxide-based materials for application in different fields.


2015 - Tunability of exchange bias in Ni@NiO core-shell nanoparticles obtained by sequential layer deposition [Articolo su rivista]
Spadaro, Maria Chiara; D'Addato, Sergio; Luches, Paola; Valeri, Sergio; Grillo, Vincenzo; Rotunno, Enzo; Roldan, Manuel A; Pennycook, Stephen J; Ferretti, Anna Maria; Capetti, Elena; Ponti, Alessandro
abstract

Films of magnetic Ni@NiO core-shell nanoparticles (NPs, core diameter d ≅ 12 nm, nominal shell thickness variable between 0 and 6.5 nm) obtained with sequential layer deposition were investigated, to gain insight into the relationships between shell thickness/morphology, core-shell interface, and magnetic properties. Different values of NiO shell thickness ts could be obtained while keeping the Ni core size fixed, at variance with conventional oxidation procedures where the oxide shell is grown at the expense of the core. Chemical composition, morphology of the as-produced samples and structural features of the Ni/NiO interface were investigated with x-ray photoelectron spectroscopy and microscopy (scanning electron microscopy, transmission electron microscopy) techniques, and related with results from magnetic measurements obtained with a superconducting quantum interference device. The effect of the shell thickness on the magnetic properties could be studied. The exchange bias (EB) field Hbias is small and almost constant for ts up to 1.6 nm; then it rapidly grows, with no sign of saturation. This behavior is clearly related to the morphology of the top NiO layer, and is mostly due to the thickness dependence of the NiO anisotropy constant. The ability to tune the EB effect by varying the thickness of the last NiO layer represents a step towards the rational design and synthesis of core-shell NPs with desired magnetic properties.


2015 - ZnO nanostructure formation on the Mo(001) surface [Articolo su rivista]
Valenti, Ilaria; Benedetti, Stefania; Di Bona, Alessandro; Valeri, Sergio
abstract

In this paper we have determined the role of oxidizing pressure and annealing temperature in the formation of ZnO nanostructures on the Mo surface. ZnO has been grown on Mo(001) by means of physical vapor deposition of Zn in molecular oxygen pressure. Combining STM, XPS, and XRD we have studied the relation between morphology, composition, and structure. ZnO grows as a film with (101¯1) preferential orientation up to a thickness of 10 Å, catalyzed by the Mo surface that provides oxygen available for oxide formation and limited by the residual polarity of the surface. Increasing the deposited amount, the role of the Mo interface becomes less effective, and part of the Zn remains metallic, forming clusters on the surface that catalyze the further growth of ZnO in the form of polycrystalline nanostructures. Surfactant Zn is removed by annealing, and peculiar oxide nanostructures are left on the surface. Increasing O pressure or reducing Zn flux allows control of nanostructure density and shape, while the continuous film underneath does not modify.


2014 - Biovetri come potenziali drug delivery systems ed impianti con proprietà antiossidanti [Capitolo/Saggio]
Nicolini, Valentina; Malavasi, Gianluca; Ferrari, Erika; Benedetti, Francesco; Luches, Paola; Lusvardi, Gigliola; Castagnetti, Mattia; Valeri, Sergio; Saladini, Monica; Menabue, Ledi
abstract

Fra i biomateriali (“materiali progettati per essere usati a contatto con tessuti viventi, organismi o microorganismi”, definizione IUPAC), i biovetri ricevono un interesse sempre maggiore. Tali materiali trovano infatti largo impiego in chirurgia odontoiatrica ed ortopedica, nella produzione di impianti dentali, protesi o riempitivi ossei. Attualmente l’interesse è rivolto soprattutto ai biovetri di seconda e terza generazione: biovetri che oltre a formare un legame chimico coi tessuti che li circondano, sono capaci di indurre una specifica risposta a livello molecolare nei tessuti stessi, come l’osteoinduzione. I biovetri possono essere prodotti con processi ad alte temperature (metodo per fusione) o a basse temperature con i metodi Sol-Gel, ottenendo in questo caso biovetri porosi o con il metodo EISA ottenendo biovetri mesoporosi. Attualmente presso il nostro laboratorio due sono le linee di ricerca sui biomateriali: i) sintesi di biovetri Sol-Gel (BGSG) ed EISA (MBGSG) e verifica della possibilità di impiegarli come drug delivery systems (DDS) di molecole con proprietà antitumorali, quali i curcuminoidi; ii) sintesi per fusione di biovetri a partire dal vetro 45S5, drogati con CeO2 al fine di verificare la Catalase e SOD mimic-like activity di tali sistemi. Sui vetri porosi e mesoporosi, caricati con curcumina ed un suo derivato (K2T21) sono stati effettuati test di rilascio statico e dinamico in SBF. I rilasci sono stati quantificati ed in entrambi i casi le concentrazioni raggiunte dai farmaci nell’organismo sono nell’ordine del micromolare, ovvero sufficienti per manifestare l’attività antitumorale. Per ciò che concerne la valutazione della Catalase mimic-like activity, sono stati condotti test con H2O2 ed è stato verificato che i biovetri ottenuti per fusione manifestano questa proprietà. Sono inoltre in atto misure di rilascio di cerio in acqua e soluzione di SBF.


2014 - Comparing the deposition mechanisms in suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) deposition of yttria-stabilised zirconia (YSZ) [Articolo su rivista]
Joulia, Aurélien; Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; Valeri, Sergio; Vardelle, Michel; Rossignol, Sylvie; Vardelle, Armelle
abstract

Thermal spraying using liquid feedstock has emerged as a promising technology for the deposition of finely structured ceramic coatings. In order to provide a comparative assessment of the deposition mechanisms occurring when spraying suspension or solution feedstock, suspensions of 300 nm-sized ZrO2–4.5 mol.% Y2O3 particles dispersed in water and in ethanol and solutions of zirconium and yttrium salts, corresponding to ZrO2–4.5 mol.% Y2O3 and ZrO2–8 mol.% Y2O3 stoichiometries, were processed by plasma spraying using different parameter settings. In-flight diagnostics of sprayed droplets, together with the morphological, microstructural and phase analysis of individual lamellae collected onto polished substrates, performed by SEM, FIB, AFM and micro-Raman spectroscopy, led to the identification of deposition mechanisms, which were subsequently verified through the characterisation of complete coating layers.


2014 - Controlled growth of Ni/NiO core–shell nanoparticles: Structure, morphology and tuning of magnetic properties [Articolo su rivista]
D'Addato, Sergio; Spadaro, MARIA CHIARA; Luches, Paola; Grillo, V.; Frabboni, Stefano; Valeri, Sergio; Ferretti, A. M.; Capetti, E.; Ponti, A.
abstract

We performed a detailed study of Ni/NiO core–shell nanoparticles (NP) obtained with a gas aggregation source. The NP oxide shells were produced by oxidizing the NP with different procedures: deposition in oxygen atmosphere, post-annealing in air, sequential deposition of (a) first NiO layer, (b) Ni NP and (c) third NiO Layer. X-ray photoelectron spectroscopy from Ni 2p core-level gave information about the chemical state of Ni in the core and in the oxide shell, while scanning electron microscopy was used for investigation of the NP morphology. High quality scanning transmission electron microscopy in high angle annular dark field mode data demonstrated core–shell structure also for NiO/Ni NP/NiO samples. Fieldcooled/ zero-field-cooled magnetization curves and field-cooled isothermal hysteresis cycles at T = 5 K were recorded by a SQUID magnetometer. In this way, the relation between magnetic properties and oxide shell structure was assessed, showing the role played by the control of the formation of oxide on the exchange bias and interparticle magnetic interaction.


2014 - Structural and morphological modifications of thermally reduced cerium oxide ultrathin epitaxial films on Pt(111) [Articolo su rivista]
Luches, P.; Pagliuca, F.; VALERI, Sergio
abstract

The modifications of the stoichiometry, morphology and surface structure of cerium oxide ultrathin films induced by thermal treatments under vacuum and oxygen partial pressure were studied using in situ X-ray photoemission spectroscopy, scanning tunnelling microscopy and low energy electron diffraction. The effect of the film nominal thickness, heating temperature and heating time on the degree of reduction of the film was investigated. The reduction is more relevant on the film surface, where different ordered surface structures were observed at different degrees of reduction for very thin films. The obtained results are discussed taking into account the dimensionality of the oxide and the effects of the proximity of the Pt substrate. After reduction it was always possible to re-oxidize the films back to their original oxidation state by thermal treatment under oxygen-rich conditions.


2013 - Anisotropy-graded magnetic media obtained by ion irradiation of L10 FePt [Articolo su rivista]
A., di Bona; P., Luches; F., Albertini; F., Casoli; P., Lupo; L., Nasi; D'Addato, Sergio; G. C., Gazzadi; Valeri, Sergio
abstract

We show that Ar+ irradiation can be used effectively to transform a chemically ordered FePt L10 homogeneous thin film into an anisotropy-graded composite media with tunable magnetic response. This can be exploited to produce magnetic media with high thermal stability and moderate coercivity with potential in high-density magnetic recording applications. The depth distribution of the chemical order parameter, which controls the magnetic switching mechanism of the system, has been determined by high-resolution transmission electron microscopy. The irradiation-induced modifications of the material have been modeled using Monte Carlo simulations for ion transport in solids. The magnetic properties and coupling regimes of the resulting exchange-coupled systems are discussed.


2013 - Controlled co-deposition of FePt nanoparticles embedded in MgO: a detailed investigation of structure and electronic and magnetic properties [Articolo su rivista]
D'Addato, Sergio; V., Grillo; A., di Bona; P., Luches; Frabboni, Stefano; Valeri, Sergio; P., Lupo; F., Casoli; F., Albertini
abstract

Films of FePt nanoparticles (NP) embedded in MgO were obtained by controlled co-deposition of FePt NP pre-formed by a gas aggregation source and of Mg evaporated in Oxygen atmosphere. Control over the deposition rate permitted to pass from FePt@MgO core-shell morphology to a film of MgO embedding FePt NP. Transmission Electron Microscopy (TEM) and High Resolution-TEM revealed the core-shell structure of the NP, with FePt core (of average diameter <d>= 4.75 nm) presenting a multi-twinned icosahedral structure, and MgO partially in crystalline form. The functional effect of MgO shell in shielding the FePt core from external oxidation was shown with XPS. Upon controlled annealing transition from A1 to L10 ordering could be obtained, with structural and morphological re-arrangement. The magnetic hysteresis loops obtained from Alternating Gradient Field Magnetometry at room temperatures show a “wasp-waist” shape, with small values of coercive field (Hc=300-1400 Oe), decreasing at increasing amount of co-deposited MgO.


2013 - Interfacial interaction between cerium oxide and silicon surfaces [Articolo su rivista]
Pagliuca, Federico; P., Luches; Valeri, Sergio
abstract

Abstract The interaction of cerium oxide films with Si substrates is investigated by means of X-ray photoelectron spectroscopy. Cerium oxide films of different thickness have been grown at room temperature by reactive deposition on different Si surfaces, namely Si(111), Si(100) and thermally oxidized Si(100). We show that cerium oxide and silicon form a silicate phase of subnanometric thickness and that the interfacial phase composition and thickness is similar on the (111) and (100) Si surfaces. The silicate phase formed at the interface contains cerium in the 3+ oxidation state, while silicon presents different oxidation states up to 4+. With a thermal annealing in O2 at 1040K the interface reaction proceeds and the silicate phase evolves in stoichiometry. We demonstrate the stability of the silicate phase towards oxidation after exposure to atomic or molecular oxygen or air. The presence of a thick thermal oxide layer on the Si surface partially limits the extent of the reaction. Highlights ► A subnanometric silicate phase forms at the interface between cerium oxide and Si. ► The silicate phase evolves in thickness and composition after thermal treatments. ► The interfacial reaction has the same extent on the (111) and (100) Si surfaces. ► The interfacial reaction is partially limited by the presence of a silicon oxide. ► The silicate phase is stable towards oxidizing treatments.


2013 - NiO/Fe(001): Magnetic anisotropy, exchange bias, and interface structure [Articolo su rivista]
E., Młyńczak; P., Luches; Valeri, Sergio; J., Korecki
abstract

The magnetic and structural properties of NiO/Fe epitaxial bilayers grown on MgO(001) were studied using magnetooptic Kerr effect (MOKE) and conversion electron Mössbauer spectroscopy (CEMS). The bilayers were prepared under ultra high vacuum conditions using molecular beam epitaxy with oblique deposition. Two systems were compared: one showing the exchange bias (100ML-NiO/24ML-Fe), ML stands for a monolayer, and another where the exchange bias was not observed (50ML-NiO/50ML-Fe). For both, the magnetic anisotropy was found to be complex, yet dominated by the growth-induced uniaxial anisotropy. The training effect was observed for the 100ML-NiO/24ML-Fe system and quantitatively described using the spin glass model. The composition and magnetic state of the interfacial Fe layers were studied using 57Fe-CEMS. An iron oxide phase (Fe3+ 4Fe2+ 1O7), as thick as 31 Å, was identified at the NiO/Fe interface in the as-deposited samples. The ferrimagnetic nature of the interfacial iron oxide film explains the complex magnetic anisotropy observed in the samples.


2013 - Orbital anisotropy in paramagnetic manganese oxide nanostripes [Articolo su rivista]
S., Altieri; F., Allegretti; W., Steurer; S., Surnev; M., Finazzi; V., Sessi; N. B., Brookes; Valeri, Sergio; F. P., Netzer
abstract

We have detected a clear magnetic anisotropy in the temperature and field dependent Mn L2,3 magnetic circular dichroism (MCD) spectra of 28 A wide paramagnetic manganese oxide nanostripes, which we relate to ˚ the presence of a largely unquenched orbital moment revealed by MCD sum rule analysis. Polarization, angle, and temperature dependent linear dichroism spectra indicate that the observed anisotropy is related to a depletion of 3d states with ml = ±2 orbital quantum number in the ground state of the paramagnetic system. Strongly anisotropic electrostatic coupling provides an internal source of spatial symmetry breaking which might generally yield paramagnetic anisotropy phenomena in 3d oxide nanostructures.


2013 - Origin of Hydrophobicity in FIB-Nanostructured Si Surfaces [Articolo su rivista]
ROTA, Alberto; TRIPATHI, MANOJ; GAZZADI, gian carlo; VALERI, Sergio
abstract

Surface morphology has been demonstrated to influence the tribological properties at different scales, but the phenomena which occur at the nano-scale have not been completely understood. The present study reports on the effect of Focused Ion Beam nano-patterning on coefficient of friction (CoF) and adhesion of Si(001) surface covered by native oxide. Regular arrays of nano-grooves reduce both CoF and adhesion, related to a hydrophobic character of the patterned surface, but this effect disappears as soon as the separation among the nano-structures approaches the microscopic scale. The dependence of this hydrophobic effect on the pitch is not linked to the corresponding contact area. It has been found that each nano-structure is surrounded by a low-friction region which extends some hundreds nm from it. For pitch of 125 and 250nm these low-friction regions completely overlap, generating a consistent decrease in CoF and adhesion, while for pitch of 500 and 1000nm its effect is negligible. The low-friction regions were not observed in humidity free ambient, indicating that they are the origin of patterns hydrophobicity.


2013 - Steering the Growth of Metal Ad-particles via Interface Interactions Between a MgO Thin Film and a Mo Support [Articolo su rivista]
Stefania, Benedetti; Fernando, Stavale; Valeri, Sergio; Claudine, Noguera; Hans Joachim, Freund; Jacek, Goniakowski; Niklas, Nilius
abstract

The coincidence lattice formed between a crystalline MgO film and a Mo(001) support is found to be an ideal template to produce long-range ordered ensembles of Fe and Cr particles for electronic, magnetic and chemical applications. The structural and electronic properties of this super-lattice are analyzed by means of scanning tunnelling microscopy and density functional theory. The different registers of atoms at the interface induce periodic changes in the MgO lattice parameter, in the metal-oxide binding length, and the workfunction. These variations modulate the adsorption landscape for Cr and Fe atoms and give rise to the observed ordering phenomena. Preferred nucleation and growth is revealed in those regions of the coincidence lattice that have a small lattice mismatch with the ad-particles and enable electron-transfer processes with the support.


2013 - Structure of Ultrathin CeO2Films on Pt(111) by Polarization-Dependent X-ray Absorption Fine Structure [Articolo su rivista]
P., Luches; Pagliuca, Federico; Valeri, Sergio; F., Boscherini
abstract

We present a study of the structure of cerium oxide ultrathin films supported on Pt(111), focused on the evolution of the epitaxial strain in films of different thickness. The stoichiometry and oxidation state of the films are determined by X-ray photoemission spectroscopy, and the surface structure, measured by low-energy electron diffraction, is compared with the results obtained by the analysis of X-ray absorption fine structure measurements at the Ce L3 edge, exploiting the polarization dependence of the cross section to probe the in-plane and the out-of-plane atomic correlations. The obtained results allow to establish the epitaxial relation between the cerium oxide film and the Pt substrate and give an accurate evaluation of the cerium oxide layer structure. The 2 ML films have a fluorite structure which is compressed in the (111) plane. The measured compression is compatible with the assumption of a coincidence lattice between overlayer and substrate, in which three CeO2 surface unit cells match four Pt unit cells. The films’ three-dimensional structure is compared with the one expected assuming the bulk-phase elastic constants. The strain is released when the film thickness is increased to 10 ML, and the lattice parameters assume the bulk values.


2013 - Tribology of patterned Si surface from the nano-to the micro-scale [Articolo su rivista]
Rota, A.; Tripathi, M.; Gazzadi, G. C.; Valeri, S.
abstract


2012 - Assembly and structure of Ni/NiO core–shell nanoparticles [Articolo su rivista]
D'Addato, Sergio; Vincenzo, Grillo; Altieri, Salvatore; Frabboni, Stefano; Valeri, Sergio
abstract

In this work it is reported a detailed investigation of the structure in Ni/NiO core–shell nanoparticles (NP). An experimental set-up was realized for the preparation and the study of pre-formed NP films. NiO shell was obtained with controlled dosing of O2 gas in the experimental system. A comparison of HR-TEM experimental images with theoretical simulations shows that the Ni NP core has a regular multitwinned icosahedral structure, composed with single crystal tetrahedra with (1 1 1) faces. NiO phase is clearly observed forming islands on the NP surface. In order to better investigate the oxide shell, the exit wave reconstruction method was applied to the images. It was found evidence of oxide island formation with direct or opposite (twinned) stacking on Ni (1 1 1) surfaces.


2012 - Controlled AFM detachments and movement of nanoparticles: gold clusters on HOPG at different temperatures [Articolo su rivista]
TRIPATHI, MANOJ; Guido, Paolicelli; D'ADDATO, Sergio; VALERI, Sergio
abstract

The effect of temperature on the onset of movement of gold nanoclusters (diameter 27 nm)deposited on highly oriented pyrolytic graphite (HOPG) has been studied by atomic forcemicroscopy (AFM) techniques. Using the AFM with amplitude modulation (tapping modeAFM) we have stimulated and controlled the movement of individual clusters. We show how,at room temperature, controlled detachments and smooth movements can be obtained forclusters having dimensions comparable to or smaller than the tip radius. Displacement ispractically visible in real time and it can be started and stopped easily by adjusting only oneparameter, the tip amplitude oscillation. Analysing the energy dissipation signal at the onset ofnanocluster sliding we evaluated a detachment threshold energy as a function of temperaturein the range 300–413 K. We also analysed single cluster thermal induced displacement andcombining this delicate procedure with AFM forced movement behaviour we conclude thatdetachment threshold energy is directly related to the activation energy of nanoclusterdiffusion and it scales linearly with temperature as expected for a single-particle thermallyactivated process.


2012 - Depth-dependent magnetization reversal and spin structure of Fe/NiO exchange-coupled epitaxial bilayers [Articolo su rivista]
P., Luches; L., Pasquini; S., Benedetti; Valeri, Sergio; R., Rüffer; F., Boscherini
abstract

We investigate the depth-dependent magnetization reversal and spin structure of Fe/NiO epitaxial bilayers before and after field cooling. Using nuclear resonant scattering of synchrotron radiation we selectively probe the spin structure of the interfacial layers in contact with NiO. We provide a direct evidence that magnetization reversal involves a rotation in the film plane perpendicular to the applied field, except for the interfacial Fe layers. Moreover, we show that the spin structure of the ferromagnetic layer is not modified by field cooling and that the exchange bias in the Fe layer is independent of the distance from the antiferromagnetic phase.


2012 - Nature of Ag Islands and Nanoparticles on the CeO2(111) Surface [Articolo su rivista]
Paola, Luches; Federico, Pagliuca; Valeri, Sergio; Francesc, Illas; Gloria, Preda; Gianfranco, Pacchioni
abstract

Ag nanoparticles have been deposited on stoichiometric and or reduced thin CeO2 films grown on Pt(111). The nucleation and growth of the Ag nanoparticles has been characterized by STM and XPS (X-ray photoemission spectroscopy) measurements complemented with DFT calculations on Ag atoms, clusters, and extended layers deposited on slab models of the CeO2(111) surface. The XPS spectra clearly show a reduction of the ceria support by Ag deposition (formation of Ce3+ ions). This is accompanied by a positive shift of the Ag 3d core levels, in which final state effects related to the finite size of the Ag deposits come into play. The DFT calculations support the view of a direct electron transfer from the Ag clusters and nanoparticles to the ceria support. Other possible origins of the reduction of the ceria substrate, like the occurrence of oxygen reverse spillover on the Ag nanoparticles, are ruled out based on energy considerations.


2011 - Ag Surface Diffusion and Out-of-Bulk Segregation in CrN-Ag Nano-Composite Coatings [Articolo su rivista]
INCERTI, Luca; ROTA, Alberto; BALLESTRAZZI, Antonio; GUALTIERI, Enrico; VALERI, Sergio
abstract

CrN-Ag nanocomposite coatings are deposited on Si(100) wafers and 20MnCr5 steel disks in amixed Ar+N2 atmosphere by reactive magnetron sputtering. Structure, composition and morphologywere investigated by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES),X-ray Photoemission Spectroscopy (XPS), X-ray Diffraction (XRD) and Focused Ion Beam (FIB)cross sectional analysis. The as deposited film matrix is mainly composed by CrN phase (78%), buta relevant part (28%) is composed by Cr2N. Ag agglomerates in the CrN matrix forming elongatedgrains 200–400 nm wide and 50–100 nm high, which extends on the top of CrN columns. At thesurface Ag aggregates into two different structures: large tetrahedral crystalline clusters, with typicaldimension ranging from 200 to 500 nm, and smaller Ag nanoparticles with diameter of 15–25 nm.The annealing in N2 atmosphere up to 500 C does not affect size and distribution of the Aggrains in the sub-surface region, while it induces a size increase of the bigger Ag clusters onthe surface, mainly related to Ag surface diffusion and clusters coalescence. Annealing at highertemperature leads to an evident Ag out-of-bulk segregation, generating Ag depleted voids in thenear-surface region, and further increasing of the Ag clusters size at the surface. Tribological testson as deposited CrN-Ag film reveal a coefficient of friction against a steel ball reduced with respectto CrN film, probably related to the presence of Ag which acts as solid lubricant, but the coatingis removed after a very short sliding distance. The poor mechanical properties of the realized Agbasedcoatings are confirmed by lower hardness and Young modulus values with respect to pureCrN.


2011 - Assembly and Fine Analysis of Ni/MgO Core/Shell Nanoparticles [Articolo su rivista]
D'Addato, Sergio; Vincenzo, Grillo; Altieri, Salvatore; Frabboni, Stefano; Francesca, Rossi; Valeri, Sergio
abstract

We obtained assemblies of Ni/MgO core/shell nanoparticles (NPs) by using a special experimental setup, in which a beam of preformed Ni nanoclusters, obtained by sputtering and aggregation, is co-deposited with evaporated Mg on a substrate, in a controlled O(2) atmosphere. In situ X-ray photoelectron spectroscopy (XPS) and ex situ energy-filtered transmission electron microscopy (EF-TEM) show that the Ni core remains metallic, whereas Mg, located in the shell, is completely oxidized. Detailed high-resolution transmission electron microscopy and geometric phase analyses reveal the multitwinned icosahedral structure of the Ni core and details the arrangement of the MgO shell. This combined effort of controlled assembly of NPs and fine analysis offers intriguing possibilities in the design of nanoscale materials.


2011 - Competition between Polar and Nonpolar Growth of MgO Thin Films on Au(111) [Articolo su rivista]
Benedetti, S.; Nilius, N.; Torelli, P.; Renaud, G.; Freund, Hj; Valeri, Sergio
abstract

We report a growth study of MgO thin films on an Au(111) support, performed with scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy-electron and X-ray-diffraction techniques. Depending on the deposition temperature, the O-2 partial pressure, and the availability of water during oxide formation, two growth regimes can be distinguished. At high oxygen pressure, the MgO mainly adopts a square-lattice configuration and exposes the nonpolar (001) surface, whereas at low O-2 pressure a hexagonal lattice develops that resembles the (111) surface of rocksalt MgO. For films beyond the monolayer limit, the emerging electrostatic dipole along the MgO(111) direction becomes important for the film morphology. Depending on the preparation conditions, the system takes either structural or adsorption-mediated routes to remove the polarity. Whereas surface roughening is identified as main polarity-compensation mechanism at perfect vacuum conditions, hydroxylation becomes important if water is present during oxide growth.


2011 - Growth and morphology of metal particles on MgO/Mo(001): A comparative STM and diffraction study [Articolo su rivista]
Benedetti, S.; Myrach, P.; di Bona, A.; Valeri, Sergio; Nilius, N.; Freund, H. J.
abstract

Using scanning tunnelling microscopy and photoelectron diffraction, we have analyzed the nucleation and growth behavior of simple (Mg and Ca), transition (Ni and Fe), and noble metals (Ag and Au) on MgO films grown on Mo(001). With the exception of gold, the particles develop interface planes that are in registry with the surface-oxygen lattice of the MgO(001). To achieve this commensurability, the particles expose facets that differ from the low-energy surfaces of the respective metals and adopt crystallographic structures that are unknown in the bulk materials. This peculiar growth behavior demonstrates the importance of interfacial interactions between the metal deposits and the MgO film, despite the inert nature of the oxide support. The observed equilibrium shapes of the particles have been analyzed with a simple growth model that includes the interfacial interactions, the surface energies, and the elastic distortion of the particle lattice to reach commensurability with the MgO(001). Experimental particle densities have been explained with a scheme that considers formation of metal cations at certain MgO defects to be the initial nucleation step.


2011 - HRTEM and HAADF analysis of Ni Multi-Twinned Nanoparticles [Articolo su rivista]
V., Grillo; D'Addato, Sergio; Altieri, Salvatore; Frabboni, Stefano; Valeri, Sergio
abstract

In this work, we report our HR-TEM results on Ni and Ni/NiO core shell nanoparticles.


2011 - Interfacial magnetic structure in Fe/NiO(001) [Articolo su rivista]
P., Luches; L., Pasquini; S., Benedetti; V. Bellini, V.; Valeri, Sergio; Manghi, Franca; R., Ruffer; F., Boscherini
abstract

Using nuclear resonant scattering of synchrotron radiation and density functional theory calculations we haveresolved the magnetic properties of the different Fe phases present at the Fe/NiO(001) interface, an epitaxialferromagnetic/antiferromagnetic system. We have detected the presence of an interfacial antiferromagneticFeO-like phase with a significantly increased magnetic moment compared to the case of a sharp interface.Already a few atomic layers above the interface, the Fe atoms have a bulk-like metallic character and the reversalof their magnetization is strongly influenced by the antiferromagnetic layer.


2011 - Laterally confined magnetic nanometric structures [Capitolo/Saggio]
Valeri, Sergio; DI BONA, Alessandro; Gazzadi, gian carlo
abstract

Basic and applied aspects of magnetic nanostructures are presented and discussed, with emphasis on the state of the art fabrication methods and modeling.


2011 - Morphology, Stoichiometry, and Interface Structure of CeO2 Ultrathin Films on Pt(111) [Articolo su rivista]
Luches, P.; Pagliuca, F.; Valeri, Sergio
abstract

Studies of model systems based on cerium oxide are important to improve current understanding of the properties of ceria-based materials, which find wide application based on the ability of cerium oxide to store, release, and transport oxygen. We report a study of CeO2 ultrathin films grown on the Pt(111) surface by reactive deposition of Ce using molecular or atomic oxygen as the oxidizing gas. High-temperature treatments in O-2 allowed us to obtain epitaxial structures with a very good quality in terms of morphology, stoichiometry, and structure. The cerium oxide films have a very flat morphology with terraces several tens of nanometers wide. The stoichiometry of the films is mainly CeO2, and the concentration of Ce3+ ions in the film can be reversibly increased by temperature treatments. We propose that the Pt substrate oxidation has a determinant role for the epitaxial stabilization of ceria films.


2011 - Nanostructured self-lubricating CrN-Ag films deposited by PVD arc discharge and magnetron sputtering [Articolo su rivista]
Incerti, Luca; A., Rota; Valeri, Sergio; A., Miguel; J. A., García; R. J., Rodríguez; J., Osés
abstract

CrN-Ag nanostructured coatings are deposited onto low-alloy steel substrates by means of PhysicalVapour Deposition (PVD) reactive magnetron sputtering and PVD reactive arc discharge evaporation. Thetwo different kinds of film have been characterized morphologically, chemically and tribologically.Depending on the used technique, as-deposited film shows different morphology, but in both cases, Agnano-clusters are present on the surface. The annealing leads to coarsening of superficial Ag clusters andsegregation out of the CrN matrix, depending on the temperature. After 600 C annealing the surface iscovered by an almost continuous layer of Ag, no matter which deposition technique is used. Tribologicaltests at different temperatures show that the lowest coefficient of friction appears at 600 C for bothcoatings. The analysis of the wear tracks reveals that such a low friction is related to continuous Agsegregation out-of-CrN matrix, which enables self-lubrication.


2011 - Role of Roughness Parameters on the Tribology of Randomly Nano-Textured Silicon Surface [Articolo su rivista]
GUALTIERI, Enrico; N., Pugno; ROTA, Alberto; SPAGNI, Alberto; E., Lepore; VALERI, Sergio
abstract

This experimental work is oriented to give a contribution to the knowledge of the relationship among surface roughness parameters and tribological properties of lubricated surfaces; it is well known that these surface properties are strictly related, but a complete comprehension of such correlations is still far to be reached. For this purpose, a mechanical polishing procedure was optimized in order to induce different, but well controlled, morphologies on Si(100) surfaces. The use of different abrasive papers and slurries enabled the formation of a wide spectrum of topographical irregularities (from the submicro- to the nano-scale) and a broad range of surface profiles. An AFM-based morphological and topographical campaign was carried out to characterize each silicon rough surface through a set of parameters. Samples were subsequently water lubricated and tribologically characterized through ball-on-disk tribometer measurements. Indeed, the wettability of each surface was investigated by measuring the water droplet contact angle, that revealed a hydrophilic character for all the surfaces, even if no clear correlation with roughness emerged. Nevertheless, this observation brings input to the purpose, as it allows to exclude that the differences in surface profile affect lubrication. So it is possible to link the dynamic friction coefficient of rough Si samples exclusively to the opportune set of surface roughness parameters that can exhaustively describe both height amplitude variations (Ra, Rdq) and profile periodicity (Rsk, Rku, lc) that influence asperity-asperity interactions and hydrodynamic lift in different ways. For this main reason they cannot be treated separately, but with dependent approach through which it was possible to explain even counter intuitive results: the unexpected decreasing of friction coefficient with increasing Ra is justifiable by a more consistent increasing of kurtosis Rku.


2011 - Size-dependence transition from nano-friction to micro-friction [Poster]
Rota, Alberto; Gualtieri, Enrico; Tripathi, Manoj; Valeri, Sergio
abstract

The interpretation of tribological processes is presently very different for nano-scale and micro-scale phenomena. This is mainly related to the presence of adhesive forces, which are not negligible at the nano-scale and, in some cases, dominate the tribological behaviour of the system. The morphological characteristics of sliding surfaces have been demonstrated to strongly influence both the nano-scale and the micro-scale tribology [1-4]. The phenomena which rule these properties are different depending on the dimensions of surface micro/nano-structures, on the contact area and on the applied load. At the micro-scale the presence of a regular array of micro-cavities could lead to the reduction of friction and wear, due to the enhancement of the hydrodynamic lift-off effect and to the role played by the micro-cavities as lubricant reservoir and debris pocket [1, 2]. At the nano-scale parallel nano-grooves induce a hydrophobic character to the surface, resulting in a reduction of the coefficient of friction and adhesion [3, 4]. The aim of the present study is to determine the critical size at which the transition from nano- to micro-scale regimes occurs, investigating the effect of surface patterning on the tribological properties of Si(100). With this purpose the Si flat surface have been patterned by means of Focused Ion Beam with regular arrays of parallel grooves 50nm wide and 4nm deep, varying the pitch from 125nm to 1m. Adhesion and friction have been studied as a function of the pitch by means of AFM in air and in vacuum, using a dedicated flat tip sensitive to the integrated effect of the nano-structures on the tribology of the system. The hydrophobicity of 125nm pitch array, already described in ref. 4, decreases between 400 and 500nm pitch, resulting almost absent for larger values. This finding identifies 400-500nm pitch interval as the threshold between nano- and micro-scale regimes. This research has been supported by PRRIITT (Regione Emilia Romagna) and Centro Interdipartimentale per la Ricerca Applicata e i Servizi nella Meccanica Avanzata e nella Motoristica (Università di Modena e Reggio Emilia). [1] A. Kovalchenko et al., Tribol. Int. 38 (2005) 219. [2] A. Borghi et al., Wear 265 7-8 (2008) 1046. [3] Y. Ando, Tribo. Lett. 19 (2005) 29. [4] D. Marchetto et al., Wear 268 (2010) 488.


2011 - Spontaneous Oxidation of Mg Atoms at Defect Sites in an MgO Surface [Articolo su rivista]
Stefania, Benedetti; Niklas, Nilius; Philipp, Myrach; Valenti, Ilaria; Hans Joachim, Freund; Valeri, Sergio
abstract

Scanning tunnelling microscopy and X-ray photoelectron spectroscopy have been used to study the adsorption of Mg on 12 ML thick MgO films grown either on Ag(001) or Mo(001) supports. The initially arriving Mg atoms spontaneously oxidize on the MgO surface, whereas metallic aggregates only form at higher exposure. The total amount of cationic Mg is considerably larger on the Mo- compared to the Ag-supported oxide films. The charge transfer is suggested to be induced by Mg attachment to MgO line defects with high electron-trapping potential. The density of those defect lines and hence the oxidation power of the film is governed by the lattice mismatch with the underlying metal support.


2011 - Structure and stability of nickel/nickel oxide core-shell nanoparticles [Articolo su rivista]
D'Addato, Sergio; V., Grillo; Altieri, Salvatore; R., Tondi; Valeri, Sergio; Frabboni, Stefano
abstract

The results of a combined x-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HR-TEM) study of Ni nanoparticles (NP), before and after oxidation, are presented. An experimental set-up was realized for the preparation and study of pre-formed NP films, concentrating the attention on Ni NP in the diameter range between 4 and 8 nm. The XPS data were taken in situ from NPs after different stages of oxidation, including controlled dosing of O(2) gas in the experimental system and exposure to the atmosphere. The Ni 2p structure is a combination of spectra from metallic Ni in the NP core and from the oxide shell. The signal from the NP core was observed even for samples after exposure to air. From the comparison of HR-TEM experimental images with theoretical simulations, it was found that the Ni NP core has a regular multitwinned icosahedral structure, composed of single-crystal tetrahedra with (111) faces. The NiO phase is clearly observed forming islands on the NP surface.


2010 - Depth-resolved magnetic characterization of Fe films on NiO(001) [Articolo su rivista]
Luches, P.; Benedetti, S.; Pasquini, L.; Boscherini, F.; Zajac, M.; Korecki, J.; Rüffer, R.; Valeri, Sergio
abstract

We used nuclear hyperfine spectroscopies and a 57Fe probe layer approach to study the depth-dependentmagnetic properties of ultrathin Fe films on NiO(0 0 1), a system exhibiting exchange bias. Conversionelectron Msbauer spectroscopy and nuclear resonance scattering of synchrotron radiation wereemployed. The samples were two Fe films with a thickness (8–10 ML) slightly above the critical thicknessfor the onset of ferromagnetism at room temperature, in which a 2 ML-thick probe layer, enriched in the57Fe Msbauer isotope, was embedded at different depths from the Fe/NiO interface. Both techniquesindicate that inside the film Fe has a metallic character, while at the interface with NiO different Fephases are present. The main conclusion is that already a few monolayers from the interface with NiOthe magnetic properties of Fe are bulk-like.


2010 - FIB assisted study of plasma sprayed splat–substrate interfaces: NiAl–stainless steel and alumina–NiAl combinations [Articolo su rivista]
Bolelli, Giovanni; Kazi, Sabiruddin; Lusvarghi, Luca; E., Gualtieri; Valeri, Sergio; P. P., Bandyopadhyay
abstract

This paper deals with the splat–substrate and intersplat interfaces of splats produced from Ni–5 wt.% Al and alumina based ceramic powders. The cross section of the splat–substrate interface was studied using a focused ion beam (FIB) assisted Field Emission Scanning Electron Microscope (FE-SEM). The metallic splat showed good interfacial integrity apart from a few interfacial pores resulting from entrapped gases and edge curl resulting from quenching stresses. The ceramics showed generally well adherent interfaces for both single and multilayer splats. Good adherence of both alumina splats was attributed to the improved wettability provided by the bond coat. One kind of alumina splats, namely, those obtained from agglomerated nanostructured powder was found to be free from crack and edge curl. This was attributed to the presence of a glassy phase in this splat, which can absorb quenching stresses.


2010 - Growth of antiferromagnetic oxide thin films [Capitolo/Saggio]
Valeri, Sergio; Luches, Paola; Altieri, Salvatore
abstract

The most recent studies on NiO and CoO films are presented and discussed. Relatively less studied AF binary oxides, such as MnO, FeO andFe2O3 are also reviewed. Some special topics, such as the oxide-substrate interface and films exposing polar surfaces, are discussed. Finally, some trends for future studies in this field are presented


2010 - Hydrophobic effect of surface patterning on Si surface [Articolo su rivista]
Marchetto, Diego; Rota, Alberto; L., Calabri; Gazzadi, Gian Carlo; Menozzi, Claudia; Valeri, Sergio
abstract

Adhesion and friction force between flat Si tip and nano-patterned Si surface have been investigated byatomic force microscopy (AFM) in air and high vacuum conditions (10−5 Torr) at room temperature (RT) and at 180 ◦C. The pattern consists of parallel grooves realized on a Si(0 0 1) single crystal by focused ion beam (FIB) milling. The FIB technique was also used to modify a commercial AFM Si probe in order to obtain a flat tip, suitable for tribological characterization of patterned surfaces, since it presents an extended contact area. In high vacuum the coefficient of friction (CoF) on the pristine surface is about 64% lower than in ambient condition, related to the presence of a water meniscus. The situation is completely different on the patterned surface where CoF does not change appreciably in the different analyzed conditions. The adhesive force related to the presence of a water layer is lower on the patterned surface with respect to the pristine surface. These findings suggest that patterning induces a hydrophobic character to the Si surface. © 2009 Elsevier B.V. All rights reserved.


2010 - Magnetic couplings and exchange bias in Fe/NiO epitaxial layers [Articolo su rivista]
Luches, P.; Benedetti, S.; di Bona, A.; Valeri, Sergio
abstract

We report an investigation of the magnetic properties of Fe films epitaxially grown on NiO layers with _001_orientation. The results are interpreted with the help of a morphological characterization of the system. Theonset of ferromagnetic behavior for the Fe layer is observed at 8 ML and it is ascribed to the coalescence of theislands, formed by Fe at the first stages of the growth, into larger interconnected islands. At larger Fe layerthickness the magnetization reversal is explained by taking into account the interplay between a unidirectionalanisotropy, due to coupling with NiO, and the uniaxial growth anisotropy of Fe, which develops around 50 MLFe layer thickness. No significant chemical modification of the interface is observed for thermal treatments upto 540 K. Exchange bias can be induced only for NiO layer thickness above 25 ML and its value can be tunedby changing the ferromagnetic to antiferromagnetic layer thickness ratio, reaching 220 Oe. No training effectis observed for this system.


2010 - Morphology and Magnetic Properties of size-selected Ni nanoparticle films [Articolo su rivista]
D'Addato, Sergio; Gragnaniello, Luca; Valeri, Sergio; Rota, Alberto; DI BONA, Alessandro; F., Spizzo; T., Panozaqi; S. F., Schifano
abstract

We present the results of a study on the morphology and magnetic properties of size-selected Ni nanoparticles films grown on Si/SiOx substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The diameter d of the produced particles ranged between 3 and 10 nm. The morphology of the films, with average thickness t varying from t = 0.5 up to t = 7nm, was studied with Atomic Force Microscopy and Scanning Electron Microscopy, combining in this way information about height and lateral topography. We observed the presence of some small aggregates made of 2 o 3 particles at the early stage of film formation, probably due to some degree of cluster diffusion on the substrate, and particle average flattening. For increasing values of t, large agglomerates are formed in the films, resulting in a porous structure. Information about the magnetic properties was obtained with Field Cooled-Zero Field Cooled (FC/ZFC) magnetization curves. We observed a reversibility-irreversibility transition at temperatures 70 K < TI < 80 K, and a significant deviation from the superparamagnetic behavior at T>TI, even for the lowest coverage studied (t = 2 nm for ZFC/FC measurements, <d> = 5.5 nm). A comparison with Montecarlo simulations of the FC/ZFC curves reveals the concurrence between inter-particle exchange interaction and single particle random anisotropy.


2010 - Secondary electron yield enhancement by MgO capping layers [Articolo su rivista]
S., Altieri; M., Finazzi; H. H., Hsieh; H. J., Lin; C. T., Chen; Valeri, Sergio; L. H., Tjeng
abstract

The yield of secondary electrons emitted from an epitaxial three monolayer (3 ML) NiO(1 0 0)/Ag(1 0 0) film excited by soft X-ray linearly polarized synchrotron radiation at the Ni L2,3 absorption threshold has been measured for different values of the thickness of a MgO(1 0 0) capping layer. Compared with the as grown 3 ML NiO(1 0 0)/Ag(1 0 0) film, we observe a significant enhancement by about a factor 1.2 of the secondary electron emission for the capped 8 ML MgO(1 0 0)/3 ML NiO(1 0 0)/Ag(1 0 0) sample. A further substantial yield enhancement by a factor 1.6 with respect to the uncapped NiO sample is observed after deposition of an additional 8 ML MgO(1 0 0) film, for a total capping layer thickness of 16 ML. The observed secondary electron yield enhancement is discussed in terms of modified electronic structure, surface work function changes, and characteristic electron propagation lengths. 2009 Elsevier B.V. All rights reserved.


2010 - Tribology and wettability of nano-machined silicon rough surfaces [Poster]
Incerti, Luca; Ballestrazzi, Antonio; Rota, Alberto; Gualtieri, Enrico; Valeri, Sergio
abstract

1 – Introduction The more and more pressing demand for eliminating liquid lubricants in mechanical devices, due to their environmental costs, represents an important challenge for the tribological community. Low-friction coatings, like layered materials (graphite, Mo2S), soft metals (Ag, Au), non-layered metal oxides (MoO3, V2O5), can substitute liquid lubricants, but presently they are characterized by poor mechanical properties. On the other hand, hard ceramic coatings, like CrN, TiN, TiCN, TiAlN, have been studied for the last three decades for their high hardness, chemical inertness, high oxidation resistance and low wear rate. The aim of our study is to couple the properties of hard ceramics with those of soft metals low friction coatings. The nanostructuring of CrN-Ag ternary coating represents an appealing solution. 2 – Main Body of Text We studied the tribo-mechanical properties of CrN-Ag ternary coatings, where Ag clusters are dispersed in a CrN thick film. Some preliminary tests were also performed on a similar system, composed by Ag/CrN multilayers. CrN-Ag composite coatings were deposited on Si(100) and on a steel (20MnCr5) using PVD Magnetron Sputtering in a mixed N2+Ar atmosphere (P=5mtorr), with temperature ranging from RT to 500°. Structure, composition and morphology of both systems were characterized by Auger Electron Spectroscopy, X-rays diffraction and cross sectional analysis (depth-profiling AES, FIB cross section). The co-deposition of CrN and Ag in N2 atmosphere leads to the formation of a composite nanostructured coating, where Ag clusters behave as solid lubricant in the hard CrN bulk. Tribo-mechanical properties were measured with a ball-on-disc tribometer and a Berckovich nanoindenter. 3 – Conclusion By tuning the preparation conditions we were able to control the size, the distribution and the self organization of Ag nanoclusters in the CrN matrix, thus controlling frictional behavior.


2009 - Controlling cluster dynamics at the nano scale [Articolo su rivista]
G., Paolicelli; Rovatti, Massimo; A., Vanossi; Valeri, Sergio
abstract

Gold nanoclusters deposited on highly oriented pyrolitic graphite are selectively detached andmoved as a function of their size using the atomic force microscope. Control is obtained working inamplitude modulation and by tuning the interaction strength between oscillating tip and clusters.Weshow that fundamental controlling parameter is the energy dissipation that can be adjusted varyingthe tip amplitude oscillation and monitoring the phase shift signal. We characterize the energydetachment threshold of nanoclusters with sizes of 24 and 42 nm of diameter, then we preciselyinduce and control the movement of clusters with diameter below 40 nm within a mixed deposition. © 2009 American Institute of Physics.


2009 - EFFETTO DELLO SPESSORE SULLA RESISTENZA AD USURAE CORROSIONE DI RIVESTIMENTI WC-CoCr DEPOSITATI SU LEGA DI Al TRAMITE TERMOSPRUZZATURA HVOF [Articolo su rivista]
Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; Valeri, Sergio
abstract

Si sono esaminati gli effetti dello spessore e della presenza/assenza di un sottile bond coat in Ni sulla resistenzaad usura e a corrosione di rivestimenti in materiale cermet WC-CoCr, depositati tramite termospruzzaturaHVOF su piastre in lega di alluminio AA 6082T6. Riporti con quattro diversi spessori, compresi nell’intervallo50 μm – 150 μm, sono stati prodotti eseguendo, rispettivamente, un totale di 2, 3, 4 e 5 scansioni consecutivedella torcia davanti al substrato. All’aumentare del numero di scansioni, la porosità del rivestimento cermet(sia con, sia senza bond coat) diminuisce; il cambiamento più sensibile si verifica fra i riporti depositati con2 e 3 passaggi della torcia. Per spiegare questo fenomeno, si sono analizzati i meccanismi di deposizione disingole lamelle di WC-CoCr, esaminandole con tecnica Focused Ion Beam (FIB). All’aumento di densità delrivestimento corrispondono un incremento di microdurezza, un miglioramento della resistenza all’usura perstrisciamento (misurata mediante test ball-on-disk) ed una maggior protezione del substrato contro la corrosione(valutata mediante spettroscopia di impedenza elettrochimica e monitoraggio del potenziale di libera corrosione).Rispetto a strati anodizzati, prodotti sulle stesse piastre, tutti riporti WC-CoCr (indipendentemente dallospessore) sono molto più resistenti ad usura ma meno protettivi contro la corrosione del substrato.


2009 - Fe self-organization on stepped MgO surfaces [Articolo su rivista]
Benedetti, S; Gragnaniello, Luca; Franchini, Mirko; Torelli, P; Valeri, Sergio
abstract

In this work we present a study of Fe self-organization on thesurface of stepped MgO as a function of deposition temperatureand thickness. The Fe morphology results in being very muchinfluenced by the presence on the MgO terraces of a roughnessdue to exposure of the sample to the air. Because of the additionalsites introduced by the MgO roughness, at room temperature Fenucleates indistinctly on steps and terraces. Dramatic changes inthe Fe nucleation appear on increasing the deposition temperatureto 800 K. Here Fe diffuses on the MgO terraces and 40% of theclusters nucleate on step edges. This effect is more evident ondecreasing the amount of Fe deposit.' 2008 Elsevier Ltd. All rights reserved.


2009 - Fe/NiO(100) and Fe/MgO(100) interfaces studied by X-rayabsorption spectroscopy and non-linear Kerr effect [Articolo su rivista]
S., Colonna; A., Cricenti; P., Luches; Valeri, Sergio; F., Boscherini; J., Qi; Y., Xue; N., Tolk
abstract

A structural characterization of the Fe/NiO(100) and Fe/MgO(100)interfaces is reported. Polarization-dependent X-ray absorptionspectroscopy has been employed to explore the local atomicstructure of the interfaces. A strong difference between the twosystems is observed: the Fe/MgO(100) interface does not showa significant Fe oxidation or oxygen diffusion while a buckledFeO like layer is observed at Fe/NiO(001). Moreover, while on theMgO(001) substrate the tetragonal strain is released at 10 ML Fethickness, a residual epitaxial strain can be observed on a film ofthe same thickness on NiO(001). A magnetic characterization of theFe/NiO(001) system by linear and non-linear magneto-optical Kerreffect measurements is also reported and discussed.' 2009 Elsevier Ltd. All rights reserved.


2009 - Growth of oxide-metal interfaces by atomic oxygen: Monolayer of NiO(001) on Ag(001) [Articolo su rivista]
Rota, A.; Altieri, S.; Valeri, S.
abstract

We have tested the use of atomic oxygen to prepare 3d oxide-metal interfaces for which standard reactive deposition techniques based on molecular oxygen fail in providing well-defined chemical composition and controlled atomic structure and morphology. Using monolayer NiO(001) on Ag(001) as a model system, we find that NiO(001)/Ag(001) films grown by atomic oxygen have a two-dimensional highly stoichiometric (1×1) structure and a uniform monoatomic thickness, while those grown by conventional O2 have a nonstoichiometric (2×1) structure or a three-dimensional morphology. Atomic oxygen may provide a practical way to prepare 3d oxide-metal interfaces with highly controlled stoichiometry, structure, and morphology. © 2009 The American Physical Society.


2009 - Image charge screening: A new approach to enhance magnetic ordering temperatures inultrathin correlated oxide films [Articolo su rivista]
S., Altieri; M., Finazzi; H. H., Hsieh; M. W., Haverkort; H. J., Lin; C. T., Chen; Frabboni, Stefano; G. C., Gazzadi; Rota, Alberto; Valeri, Sergio; L. H., Tjeng
abstract

We have tested the concept of image charge screening as a new approach to restore magnetic orderingtemperatures and superexchange interactions in correlated oxide ultrathin films. Using a three-monolayerNiO100 film grown on Ag100 and an identically thin film on MgO100 as model systems, we observedthat the Néel temperature of the NiO film on the highly polarizable metal substrate is 390 K while that of thefilm on the poorly polarizable insulator substrate is below 40 K. This demonstrates that screening by highlypolarizable media may point to a practical way toward designing strongly correlated oxide nanostructures withgreatly improved magnetic properties.


2009 - Increasing nanohardness and reducing friction of nitride steel by surface laser texturing [Articolo su rivista]
Gualtieri, Enrico; Borghi, A; Calabri, L; Pugno, N; Valeri, Sergio
abstract

Laser surfacetexturingisoftenemployedtoimprovetribologicalperformancesofmatingsurfaces.Theprincipalaimofthepresentworkistostudylocalmodificationsinducedbylaserbeamduringthetexturingprocess(microdimpling),onthemechanicalpropertiesofa30NiCrMo12nitridesteel.A brieftribologicalcharacterizationwasinitiallymadeinordertoverifytheimprovementoftribologicalbehaviourensuredbymicrodimpling.Frictionmeasurementswerecarriedoutinlubricatedflat-on-flatandround-on-flatconfigurations.A sub-surfacecharacterizationwasperformedbyfocusedionbeamcross-sectionandimaging.Modifiedmateriallayerswerediscoveredincorrespondenceoflaser-affectedzonesarounddimples,where graindimensionswereobservedtobereduced.Toquantifythisphenomenon,anewtheoreticalenergetic modelwasdeveloped.Hardness measurementswereperformedbyatomicforcemicroscopenanoindentation.Thisapproach allowedtoobserveanhardeningeffectmovingfrombulkmaterialtodimpleedges.Therefore, thetheoreticalmodelwasintegratedwiththeHall–Petch’slawinordertoquantifytheobservedhardeningbehaviour.& 2008ElsevierLtd.Allrightsreserved.


2009 - Morphology-induced magnetic phase transitions in Fe deposits on MgO films investigated withXMCD and STM [Articolo su rivista]
P., Torelli; S., Benedetti; P., Luches; Gragnaniello, Luca; J., Fujii; Valeri, Sergio
abstract

The interplay between magnetic properties and morphology of thin iron deposits on MgO films grown onAg001 was investigated by means of x-ray magnetic circular dichroism and scanning tunneling microscopySTM measurements as a function of the Fe thickness and after postgrowth thermal treatments. The as-grownFe deposits display a sharp transition as a function of the Fe thickness, corresponding to the development offerromagnetism at around 4.5 ML. The ferromagnetic phase can be turned into a superparamagnetic phase bymeans of a thermal treatment. STM measurements allowed us to ascribe the onset of ferromagnetism to thetransition from a three-dimensional to a two-dimensional growth mode of the iron deposit and to explain thesuperparamagnetic behavior in the annealed film as due to the formation of a collection of well-separatedsquarediron particles. Moreover, using the particle shape and size distribution measured by STM, we calculateda value of the magnetic anisotropy of the Fe particles, which is 1 order of magnitude larger than the bulkiron one. This increase is mainly ascribed to the role of surface anisotropy in the Fe nanoparticles and also therole of the dipolar interactions between particles is discussed.


2009 - The analytical relations between particles and probe trajectories in atomic force microscope nanomanipulation [Articolo su rivista]
RAO, A; GNECCO, E; MARCHETTO, Diego; MOUGIN, K; SCHÖNENBERGER, M; VALERI, Sergio; MEYER, E.
abstract

A new approach to the problem of manipulation of nanoparticles on a surface is proposed. Exploitinganalogies with traditional scattering theory, we show how the geometry of the particles, andtheir interaction with the underlying surface, could be determined from measurable quantities likethe angle of deflection and the spacing between consecutive scan lines in atomic force microscopyexperiments. An analytical formula is derived and applied to spherical gold particles moved on flatand patterned silicon surfaces.


2009 - Thermally Sprayed Coatings as Interlayersfor DLC-Based Thin Films [Relazione in Atti di Convegno]
Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; F., Pighetti Mantini; F., Pitacco; Valeri, Sergio; H., Volz
abstract

This article examines the usefulness of a thick thermally sprayed interlayer (plasma-sprayed Ni-50%Cr,plasma-sprayed Al2O3-13%TiO2, or high-velocity oxygen-fuel-sprayed WC-17%Co) for enhancing thewear resistance and the corrosion protectiveness of a diamond-like carbon (DLC)-based thin filmdeposited onto a carbon steel substrate. Scratch tests indicate that the Al2O3-13%TiO2 and WC-17%Cointerlayers definitely increase the critical spallation load of the thin film, but the Al2O3-13%TiO2interlayer itself undergoes brittle fracture under high-contact loads. Accordingly, during ball-on-disktests at room temperature, no cracking and spallation occur in the DLC-based film deposited onto theWC-17%Co interlayer, whereas the one onto the Al2O3-13%TiO2 interlayer is rapidly removed becausethe interlayer itself is fractured. At 300°C, by contrast, the DLC-based film on the Al2O3-13%TiO2interlayer offers the best tribological performance, possibly thanks to the increased toughness of theceramic interlayer at this temperature. Electrochemical polarization tests indicate that the thin film/WC-Co systems possess the lowest corrosion current density.


2009 - X-ray photoemission study of the charge state of Au nanoparticles on thin MgO/Fe(001) films [Articolo su rivista]
P., Torelli; S., Benedetti; P., Luches; Valeri, Sergio; E., Annese; L., Giordano; G., Pacchioni
abstract

The charge state of Au nanoparticles deposited on MgO/Fe(001) ultrathin films has been investigated experimentally by means of X-ray Photoemission Spectroscopy and theoretically by performing Density Functional Theory (DFT) calculations. The binding energy of the Au 4f7/2 core level peak has been studied as a function of the Au coverage and of the MgO film thickness. The Au 4f7/2 peak presents a high binding energy value (about 600 meV higher than bulk Au) for the smallest Au nanoparticles deposited on thick MgO films. The binding energy shift is only 200 meV for the very thin MgO films, indicating a different electronic configuration for the Au particles inthis case. For larger Au coverages the binding energy of the Au 4f7/2 peak converges to the bulk value for both thick and thin MgO films. In view of the results from DFT calculations, we show that this behavior is consistent with the occurrence of a charge flow from the MgO/Fe(001) interface to the Au nanoparticles. We also show that on ultrathin MgO films, the Au 4f7/2 photoemission linewidth is much sharper than for thicker oxide films, a result which is rationalized based on the different growth mode of Au nanoclusters when a charge transfer process takes place.


2008 - AFM investigation of tribological properties of nano-patterned silicon surface [Articolo su rivista]
Marchetto, Diego; Rota, Alberto; L., Calabri; G. C., Gazzadi; C., Menozzi; Valeri, Sergio
abstract

The tribological properties of a nano-patterned Si surface have been investigated in ambient condition by atomic force microscopy (AFM). Thepattern, consisting of parallel grooves, was realized on a Si(0 0 1) single crystal via focused ion beam (FIB) milling. The same technique was usedto modify a commercial Si probe in order to obtain a flat tip, suitable for the tribological characterization of the patterned surface, since it presentsan extended contact area. The dependence of the lateral force on the normal load has been studied, varying the sliding speed and the fast scandirection with respect to the pattern orientation. Lateral force versus normal load curves show a linear trend except in the region closed to thepull-off limit. In this region adhesive effects related to ambient condition strongly affect the process. This behavior is more evident on the pristinesurface. Experimental results show that the friction coefficient is sensibly lower on the patterned area than on the unpatterned one. A non-negligiblewear of the patterns related to the sliding of the flat tip has been detected.© 2007 Elsevier B.V. All rights reserved.


2008 - AFM nanoindentation: tip shape and tip radius of curvature effect on the hardness measurement [Articolo su rivista]
L., Calabri; N., Pugno; C., Menozzi; Valeri, Sergio
abstract

AFM nanoindentation is nowadays not so widespread for the study of mechanical properties of materials at the nanoscale. 'Nanoindenter' machines are presently more accurate and more standardized. However, AFM could provide interesting features such as imaging the indentation impression right after the load application.In this work a new method for nanoindentation via AFM is proposed. The use of AFM allows hardness measurement with standard sharp AFM probes and a simultaneous high-resolution imaging (which is not achievable with standard indenters—cube corner and Berkovich). How the shape of the indenter and the tip radius of curvature affect the hardness measurement is here analysed with three different approaches: experiments, numerical simulations and theoretical models. In particular the effect of the tip radius of curvature, which is not negligible for the real indenters, has been considered both in the nature of the indentation process, than in the practice of imaging with AFM.A final theoretical model has been developed, that includes the effect of the tip radius of curvature as well as variable corner angle. Through this model we have been able to define a correction factor which permits us to evaluate the actual hardness of the material, once the radius of curvature of the tip is measured.


2008 - Adhesion detachment and movement of gold nanoclusters induced by dynamic atomic force microscopy [Articolo su rivista]
Paolicelli, G; Mougin, K; Vanossi, A; Valeri, Sergio
abstract

Via dissipation processes in dynamic atomic force microscopy, we have investigate,in controlled ambient conditions, adhesion and frictional properties of deposited nanoclusters.Specifically, we have considered gold nanoclusters with nominal diameter of 13 and 24 nmphysisorbed on silicon substrate. The manipulation experiment has shown unambiguously thatthe Amplitude Modulation-AFM method (AM-AFM) and the adopted calibration procedure letus to distinguish and measure the size dependence of the energy detachment threshold fordeposited objects down to the nanometer scale. Moreover, allowing to switch easily and withhigh repeatability between imaging and manipulation during the AFM scans, this operationalmethod proves to be a promising tool for inducing controlled spatial displacements at theselength scales.


2008 - Controlled manipulation of thiol-functionalised gold nanoparticles on Si(100) by Dynamic Force microscopy [Relazione in Atti di Convegno]
G., Paolicelli; K., Mougin; A., Vanossi; Valeri, Sergio
abstract

Mechanical control of nanometer size objects and the dynamic behaviour at thislength scale are subjects of growing interest. One promising approach to operate and performquantitative measurements in this regime is to use dissipation processes in atomic forcemicroscopy. We obtained a controlled manipulation of thiol-functionalised gold nanoparticleson silicon dioxide and a measurements of the energy depinning threshold as a function ofnanoparticles characteristics by using the AFM microscope in a particular dynamic regime.Detailed procedure and preliminary results will be described in this contribution.


2008 - Effect of the indentation depth on the evaluation of mechanical properties of thin film [Articolo su rivista]
L., Calabri; Mantovani, Sara; L., Rettighieri; Valeri, Sergio
abstract

Hardness measurement of thin films is a major topic. At the nanoscale the interplay between substrate and coating during indentation process is not well known. Atomic Force microscopy nanoindentation has been used to adress this problem. We measured the hardness of thin coatings deposited on a steel substrate, varhing the indentation depth and observing the effect of the substrate-coating interaction on the hardness evaluation. Focused ion beam cross-sections of the indented region and numerical simulations also show a relevant contribution of the substrate to the hardness evaluation.


2008 - Growth and study of Ni nanoparticles films deposited on inert substratets [Articolo su rivista]
D'Addato, Sergio; Gragnaniello, Luca; A., di Bona; Rota, Alberto; P., Vavassori; Valeri, Sergio
abstract

We report the results of a study on Ni-clusters assembled films deposited on Si(100) and MgO(100) surfaces. The films were produced by deposition of pre-formed Ni nanoparticles generated by a gas aggregation source. The samples topography and growth mode were investigated with SEM, AFM, STM and XPS. MOKE technique was used to determine magnetization curves at T=300 K, T=100 K and T=50K. The films show random paving growth mode, where clusters assemble in agglomerates. From MOKE data we observe a behaviour that can be ascribed to the occurrence of Super Correlated Spin Glass phase at T=300 K, and soft ferromagnetic hysteresis cycles at low temperature, possibly related with a different magnetic phase.


2008 - Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam: two-dimensional perpendicular patterns [Articolo su rivista]
F., Albertini; L., Nasi; F., Casoli; S., Fabbrici; P., Luches; G. C., Gazzadi; A., di Bona; P., Vavassori; Valeri, Sergio; Contri, Sara Federica
abstract

Focused ion beam was utilized to locally modify magnetism and structure of L10 FePt perpendicularthin films. As a first step, we have performed a magnetic, morphological, and structural study ofcompletely irradiated FePt films with different Ga+ doses 11013 –41016 ions/cm2 and ionbeam energy of 30 keV. For doses of 11014 ions/cm2 and above a complete transition from theordered L10 to the disordered A1 phase was found to occur, resulting in a drop of magneticanisotropy and in the consequent moment reorientation from out-of-plane to in-plane. The lowesteffective dose in disordering the structure 11014 ions/cm2 was found not to affect the filmmorphology. Taking advantage of these results, continuous two-dimensional 2D patterns ofperpendicular magnetic structures 250 nm dots, 1 m dots, 1 m-large stripes were produced byfocused ion beam without affecting the morphology. The 2D patterns were revealed by means ofmagnetic force microscopy, that evidenced peculiar domain structures in the case of 1 m dots.© 2008 American Institute of Physics.


2008 - Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth [Articolo su rivista]
F., Casoli; L., Nasi; F., Albertini; S., Fabbrici; C., Bocchi; F., Germini; P., Luches; Rota, Alberto; Valeri, Sergio
abstract

This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thinfilms epitaxially grown on MgO100 at intermediate temperatures 380 °C. It focuses on theeffects obtained by in situ annealing the FePt films after deposition. We have demonstrated that theannealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with largeperpendicular anisotropy up to 3107 erg/cm3 and high perpendicular squareness and remanenceratio both higher than 0.85. At the same time, we have found a peculiar morphology evolution byincreasing the annealing time: the interconnected mazelike grains evolve towards interconnecteddotlike grains of reduced size down to 10 nm. Due to the interconnection at the grains basis, theincrease in the film perpendicular coercivity with the annealing process is moderate up to 4.1 kOe,leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further steptowards the understanding of the properties evolution with annealing, we have investigated themagnetic domain pattern and analyzed the domain-period dependence on the anisotropy constant incomparison with the behavior expected by the Kooy–Enz theory C. Kooy and U. Enz, Philips Res.Rep. 15, 7 1960. The main magnetic properties and reduced grain size obtained after the in situannealing are promising to realize perpendicular recording media with reduced bit size, high thermalstability, and suitable switching field.


2008 - Self-organized growth of Ni clusters on a cobalt-oxide thin film induced by a buried misfit dislocation network [Articolo su rivista]
P., Torelli; E. A., Soares; G., Renaud; Gragnaniello, Luca; Valeri, Sergio; X., Guo; P., Luches
abstract

We report on a clear tendency toward organized growth, at room temperature, of Ni nanoislands depositedon a CoO(001) nanopatterned surface. The 9.2 nm square surface patterning consists in a periodic displacementfield induced at the surface by an underlying interfacial dislocations network that is buried at the interfacebetween the 5 nm thick CoO(001) thin film and the Ag(001) substrate. Grazing incidence small and wide anglex-ray scattering GISAXS and GIXD performed in situ, during growth, reveal that the nucleation of Niparticles is driven by the underlaying dislocation distribution. The tendency for organization is confirmed byscanning tunneling microscopy STM, which also reveals a quite narrow size distribution of the Ni nanoparticlesaround 5 nm width and 0.6 nm height.


2008 - Structure and morphology of thin MgO films on Mo(001) [Articolo su rivista]
S., Benedetti; P., Torelli; Valeri, Sergio; H. M., Benia; N., Nilius; G., Renaud
abstract

Using a combination of reciprocal and real-space techniques, the structural evolution and its effect on thesurface morphology is investigated for MgO films of 1–30 ML thickness epitaxially grown on Mo(001). Thestrain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formationof an ordered network of interfacial misfit dislocations aligned along the MgO (110) directions, particularlyevident after annealing the film at 1070 K. A dislocation periodicity of about 60 Å has been determined bymeans of grazing incidence x-ray diffraction. The dislocations induce a tilting of the surface that appears inelectron diffraction along the (100) MgO directions for thin films and changes to (110) directions when theoxide thickness increases. Scanning tunneling microscopy (STM) shows the presence of a regular pattern onthe surface below 7 ML thickness associated to the dislocation network. With increasing thickness, screwdislocations connected by nonpolar steps appear on the oxide surface. Thanks to the combination of differentdiffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgOfilms on Mo(001) can be drawn.


2008 - Tribological effects of surface texuring on nitride steel for automotive engine applications [Articolo su rivista]
A., Borghi; Gualtieri, Enrico; Marchetto, Diego; L., Moretti; Valeri, Sergio
abstract

This study reports on the effects of surface modification by laser texturing on tribological performances of nitride steel for automotive engine applications. In particular, a comparison of tribological properties on untextured and textured 30NiCrMo12 nitride steel was made among hydrodynamic, mixed and boundary lubrication regimes.Ordered arrays of circular dimples were created on nitride steel surfaces by Laser Surface Texturing. Friction experiments were performed using a pin-on-disc apparatus at sliding speeds ranging from 1 to 12 cm∙s-1 and normal applied loads ranging from 1 to 10 N. Static counterpart was a custom pin of 100Cr6 steel. Two different configurations were studied: “single drop” and “dry contact”. Long sliding condition tests and Stribeck curves were obtained. Finally, Secondary Electron Microscopy imaging, Energy Dispersive Spectroscopy and Auger Electron Spectroscopy investigations were carried out to study morphology and chemistry at surfaces, before and after tribological tests.Maximum effects of laser surface texturing on friction coefficient and wear have been observed in single drop condition at high normal applied load and for long sliding time.


2007 - Arrays of metal nanostructures produced by focussed ion beam [Articolo su rivista]
P., Luches; A., di Bona; Contri, Sara Federica; G. C., Gazzadi; P., Vavassori; F., Albertini; F., Casoli; L., Nasi; S., Fabbrici; Valeri, Sergio
abstract

We present a study of the magnetic properties of arrays of nanostructures produced in a focussed ion beam-scanning electron microscope dual beam system. The single magnetic units have been isolated either by direct removal of parts of the metallic film or by local modification of the film magnetic properties. The final quality of the shape and the residual damage strictly depend on beam parameters (spot size and pixel dwell time) and on the swelling properties of the patterned materials. On square Fe(001) elements with a well-defined intrinsic (magnetocristalline) and shape- and size-induced (shape plus configurational) anisotropy we show that the overall magnetic anisotropy is not a mere superposition of the individual contributions. We also demonstrate that with ion irradiation doses below the milling threshold L1(0) FePt films with perpendicular magnetic anisotropy undergo a transition from the magnetically hard L1(0) phase to the magnetically soft A1 phase leading to an out-of-plane to in-plane spin reorientation. The magnetic properties of the planar arrays obtained by local modification of the film are compared to arrays of sculpted structures of the same material.


2007 - Deformation and Adhesion of Elastomer Poly(dimethylsiloxane) Colloidal AFM Probes [Articolo su rivista]
R., Buzio; A., Bosca; S., Krol; Marchetto, Diego; Valeri, Sergio; U., Valbusa
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 loading-unloading cycles and adhesion forces are acquired on ion-sputtered mica, the deformation rate and dwell time being separately controlled. We discuss load-penetration curves and pull-off forces based on Zener and Maugis-Barquins models, accounting for adhesion hysteresis effects related to 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 elastic deformations at the sub-micrometer scale.


2007 - Effects of structural nonplanarity on the magnetoresistance of Permalloy circular rings [Articolo su rivista]
Vavassori, P; Busato, A; Chiapatti, A; DI BONA, A; Valeri, Sergio; Metlushko, V; AND ILIC, B.
abstract

We have measured magnetoresistance in single, 1 micron external diameter, Permalloy Ni80Fe20circular rings with inner diameters of 150, 300, and 600 nm and film thickness of 25 nm. The ringstructures were deposited on 25-nm-thick, 250-nm-wide Au nanocontacts previously fabricated ona SiO2 /Si substrate by e-beam lithography. Using a four contact geometry, we studied thedependence of the magnetoresistance on the direction of the applied field. The experimental datawere compared with simulations based on the conventional anisotropic magnetoresistance effect andon numerical simulations of the current distribution within the samples combined withmicromagnetic simulations of the field dependent magnetization profile. Two different approacheswere used for the simulations: one assumes flat magnetic structures with the Au contacts depositedon top of them; in the second approach the simulations were carried out using nonplanar magneticstructures deposited on the Au contacts. The magnetoresistance curves calculated using the secondapproach differ considerably from those obtained assuming flat magnetic structures and reproducethe experimental curves. The results presented demonstrate that the nonplanarity of the magneticstructures affects profoundly the magnetoresistive behavior of the ring structures. © 2007 AmericanInstitute of Physics.


2007 - Ferromagnetic-Antiferromagnetic Fe/NiO(100) interface studied by non-linear Kerr effect [Articolo su rivista]
S., Colonna; F. Ronci, A. Cricenti; P., Luches; Valeri, Sergio; J., Qi; Y., Xu; J. K., Miller; N., Tolk
abstract

In this work, the magnetic properties of the Fe/NiO (100) interface at room temperature is investigated using non-linear magnetoopticalKerr effect. Nickel oxide is a promising material for technological applications due to its Ne´el temperature (525 K) occurring wellabove the room temperature. Non-linear Kerr rotation measurements have demonstrated to be an effective way to investigate a ferromagnetic/antiferromagnetic interface.The measurements here reported demonstrate that the magnetic properties of the studied interface strongly depend on the Fe layerthickness. In fact, a clear hysteresis loop is observed only for a thickness higher than 5 ML, whereas for the 2 ML sample the hysteresisloop completely disappears. These results are in agreement with previous structural characterizations of the interface. 2007 Elsevier B.V. All rights reserved.


2007 - Grain size reduction and magnetic properties improvement by in-situ annealing of FePt epitaxial thin films [Articolo su rivista]
F., Albertini; L., Nasi; F., Casoli; S., Fabbrici; P., Luches; Rota, Alberto; Valeri, Sergio
abstract

FePt thin films were epitaxially grown by RF sputtering on MgO (1 0 0) substrates at 550 1C, followed by in situ annealing at the sametemperature. The in situ annealing was found to improve the magnetic characteristics: i.e., the increase in magnetic squareness up to 0.9and increase in the ratio between anisotropy and coercivity (e ¼ anisotropy field/coercive field ¼ 35.9 for 115 min annealing).Remarkably, a decrease in grain size was also found to occur by increasing the annealing time.r 2007 Elsevier B.V. All rights reserved.


2007 - Growth and study of 3d Ni nanoparticles films deposited on inert substrates [Poster]
D'Addato, Sergio; Gragnaniello, Luca; DI BONA, Alessandro; Rota, Alberto; Valeri, Sergio
abstract

The interest in metal nanostructured films has grown in the last years because of their fascinating physical properties and their potentiality in various applications, like magnetic recording industry and tribology. As an example, smaller particles are required in order to realize advanced magnetic memory units. This request constitutes an important challenge, because of the super-paramagnetic limit for the density of recorded bits, which makes conventional recording media with three-dimensional particles unstable [1]. We performed a study of 3d (Cu, Ni and Fe) metal nanoparticles films grown on oxidized Si and MgO inert substrates. The deposition has been obtained by making use of a recently developed experimental system, with a gas aggregation cluster source, a quadrupole mass filter and a deposition chamber [2,3]. The source can produce high flux of particles with variable size distribution in a range from 1 to 10 nm. We produced films of Cu, Ni and Fe particles of different size (from 3 to 5 nm average diameter) and thickness values, from 1 to 10 nm corresponding to regimes where the nanoparticles are sparse or they constitute a nanostructured film. The samples have been characterized ex-situ with scanning electron microscopy, atom force microscopy, scanning tunneling microscopy and electron spectroscopy techniques to check morphology and composition. Magneto Optical Kerr Effect (MOKE) experiments have been performed on Ni grown on MgO(100) to investigate the role played by the magnetic dipole interaction between the particles. Results have shown that samples are paramagnetic at Room Temperature. We also performed a low temperature MOKE experiment on a 7 nm thick film of Ni nanoparticles with 5 nm average diameter grown on MgO(100), and we obtained a ferromagnetic hysteresis at T=100 K, revealing magnetic blocking at this temperature. [1] see, for instance S. D. Bader, Rev. of Mod. Phys. 78 (2006) 1 and ref.s therein. [2] C. Binns, SUrf. Sci. Rep. 44 (2001) 1. [3] C. Binns et al., J. Phys. D: Appl Phys. 38 (2005) R357.


2007 - Hydrolysis at MgO(100)/Ag(100) oxide-metal interfaces studied by O 1s x-ray photoelectron and Mg KL{23}L{23} Auger spectroscopy [Articolo su rivista]
S., Altieri; Contri, Sara Federica; Valeri, Sergio
abstract

Using O 1s x-ray photoelectron and Mg KL23L23 Auger spectroscopy, we demonstrate that monolayerMgO100/Ag100 exposed at 300 K to H2O vapor pressure of 10−8 Torr is strongly hydrated up to 0.8 H2Oper MgO formula unit, half of the water molecules being dissociatively adsorbed at the fourfold coordinatedsurface oxide sites. These dramatic hydrolysis and molecular adsorption phenomena are strikingly differentfrom those observed at perfect bulklike MgO100 surfaces, and provide strong experimental support to theconcept of unprecedented interactions and properties at oxide-metal interfaces.


2007 - Magnetic anisotropy engineering in square magnetic elements [Articolo su rivista]
DI BONA, A; Contri, Sara Federica; GAZZADI G., C; Valeri, Sergio; Vavassori, P.
abstract

Square magnetic elements with side in the 100–500nm range have been fabricated using the focused ion beam (FIB) milling techniquefrom a 10 nm thick, single-crystal Fe film, epitaxially grown on MgO(0 0 1). Thanks to the good crystal quality of the film, magneticelements with well-defined magnetocrystalline anisotropy have been prepared, while the fine control of the size and shape of the magnetsallows for the effective engineering of the anisotropic behavior of the magnetostatic energy that determines the so-called configurationalanisotropy. Micromagnetic calculations and experiments show that the angular dependence of the transverse susceptibility has a strongdependence on the material parameters as well as on the static applied field. This allows the effective engineering of the total anisotropyof the magnets.r 2007 Elsevier B.V. All rights reserved.


2007 - Magnetic linear dichroism studies of in situ grown NiO thin films [Articolo su rivista]
Krishnakumar, Sr; Liberati, Marco; Grazioli, C; Veronese, M; Turchini, S; Luches, P; Valeri, Sergio; Carbone, C.
abstract

We report thickness dependence of magnetic linear dichroism (MLD) of in situ grown NiO(001) films on Ag(001) substrate at the Ni L-2 absorption edge. Antiferromagnetic domains at the surface of NiO(001) films are found to be preferentially aligned in-plane. For films thinner than a critical thickness t(c) (20-40ML), we observe a softening of the in-plane magnetic domain alignments with increasing film thickness, arising from the strain-relaxation effects. Films thicker than t(c) exhibits a residual in-plane anisotropy, possibly related to the finite-thickness effects. (c) 2006 Elsevier B.V. All rights reserved.


2007 - Magnetoresistance of single Permalloy circular rings [Articolo su rivista]
P., Vavassori; A., Busato; A., Chiapatti; A., di Bona; Valeri, Sergio; V. Metlushko, B. Ilic
abstract

We have measured magnetoresistance in single, 1 mm external diameter, Permalloy (Ni80Fe20) circular rings with varied inner holediameter of 150, 300, and 600nm and film thickness of 25 nm. The Permalloy ring structures and the 10-nm-thick, 250-nm-wide Aunanocontacts were fabricated on a SiO2/Si substrate using e-beam lithography. Using a four contact geometry we studied the dependenceof the magnetoresistance on the direction of the applied field. The experimental data are explained by considering only the conventionalanisotropic magnetoresistance effect. Numerical simulations of the current distribution within the samples combined with micromagneticsimulations of the field dependent magnetization profile, yield good agreement with the experimental data. Upon increasing the innerhole diameter (viz. decreasing the ring width) the magnetoresistance measurements show a transition of the reversal process from the‘‘vortex nucleation–displacement–annihilation’’ sequence to the ‘‘onion state–reversed onion state’’ sequence, typical of narrownanorings.r 2007 Elsevier B.V. All rights reserved.


2007 - Metals on oxides: structure, morphology and interface chemistry [Articolo su rivista]
Valeri, Sergio; Benedetti, Stefania; P., Paola
abstract

In this paper we review our recent studies on the structure, morphology andinterface chemistry of different metal systems on simple rock-salt oxides,providing a basis for the understanding of their properties.Fe on both MgO and NiO grows in the cubic structure with the(001)Fe _ (001)oxide and [110]Fe _ [100]oxide orientation.The Fe/MgO interface is sharp at the atomic level up to 400 ◦C, thusshowing that MgO can allow the growth of ‘free standing’ Fe films and lowdimensionalstructures. Submicrometric Fe dots on MgO have been preparedby FIB milling and characterized by the magneto-optical Kerr effect. Atthe Fe/NiO(001) interface an Fe–Ni body centred tetragonal alloyed phase isformed on top of a two-dimensional FeO layer situated at the interface. TheFeO layer presents an expansion of the interplanar distance at the interfaceand a buckling in the Fe and O atomic positions. The presence of thestructurally distorted FeO phase is predicted by ab initio density functionaltheory calculations to increase the spin magnetic moment of Fe atoms by 0.6 μBcompared to the ideally abrupt interface. A reduction of NiO is also induced bythe deposition of (supposedly) inert Au films, which incorporate the metallicNi atoms. The reduction takes place at the low-coordination sites at the surfaceof the NiO film. The Co/MgO(001) system has also been studied. In all theinvestigated cases, given the lower surface free energy of the oxide surfacescompared to the metal ones, the deposited metals tend to form islands, whichcoalesce above a few monolayers.


2007 - Nano-structuration of CoO film by misfit dislocations [Articolo su rivista]
P., Torelli; E. A., Soares; G., Renaud; Valeri, Sergio; X. Guo, P. Luches
abstract

We have studied the nano-patterning of CoO film induced by misfit dislocation network at the interface with the Ag(001) substrate.Grazing incidence diffraction (GIXD), X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED) have beenused to characterize chemistry and structure of the CoO layers. The XPS spectrum of the Co 2p core level permitted to establish thestoichiometric growth of CoO. The structure of the CoO film together with the absorption sites of cobalt and oxygen atoms was determined,thanks to GIXD measurements. Moreover we have followed the evolution of the in-plane lattice constant of the CoO as a functionof the film thickness. It turns out that the CoO film growth starts with the same in-plane lattice constant of the Ag(001) substrate upto 3–4 ML; afterwards the in-plane parameter of CoO steadily increases before reaching a stable value of 2.98 A ˚ at 23 ML. During therelaxation process, at about 8 ML of film thickness, we observe the formation of a buried misfit dislocation network. These dislocations,that have a period of 9.2 nm for a film thickness of 23 ML, induce mosaicity in the CoO film which then appears as a regular distributionof tilted domains._ 2006 Elsevier B.V. All rights reserved.


2007 - Nanoindentation shape effect: experiments, simulations and modelling [Articolo su rivista]
L., Calabri; N., Pugno; Rota, Alberto; Marchetto, Diego; Valeri, Sergio
abstract

AFM nanoindentation is nowadays commonly used for the study of mechanicalproperties of materials at the nanoscale. The investigation of surface hardness ofa material using AFM means that the probe has to be able to indent the surface,but also to image it. Usually standard indenters are not sharp enough to obtainhigh-resolution images, but on the other hand measuring the hardness behaviourof a material with a non-standard sharp indenter gives only comparative resultsaffected by a significant deviation from the commonly used hardness scales.In this paper we try to understand how the shape of the indenter affectsthe hardness measurement, in order to find a relationship between the measuredhardness of a material and the corner angle of a pyramidal indenter. To achievethis we performed a full experimental campaign, indenting the same materialwith three focused ion beam (FIB) nanofabricated probes with a highly alteredcorner angle. We then compared the results obtained experimentally with thoseobtained by numerical simulations, using the finite element method (FEM), andby theoretical models, using a general scaling law for nanoindentation availablefor indenters with a variable size and shape.The comparison between these three approaches (experimental, numericaland theoretical approaches) reveals a good agreement and allowed us to find atheoretical relationship which links the measured hardness value with the shapeof the indenter.The same theoretical approach has also been used to fit the hardnessexperimental results considering the indentation size effect. In this case wecompare the measured data, changing the applied load.


2007 - Preparation and characterization of MgO stepped surfaces” [Articolo su rivista]
Benedetti, Stefania; P., Torelli; P., Luches; Gualtieri, Enrico; Valeri, Sergio
abstract

We present a study of the preparation procedure for stepped MgO surfaces which can be used as templates for the deposition ofmetallic nanostructures. A cleaved sample of MgO(001) was mechanically polished to reach the desired miscut angle along the [110]direction. Then a thermal annealing was performed. The effect of an intermediate chemical etching has been also studied. The surfacewas analyzed by means of contact AFM in air, LEED and XPS in UHV. The role of the chemical etching and the dependence ofthe final morphology on the annealing time and temperature were investigated. The influence of the miscut angle on the final surfacetopography is also briefly discussed._ 2006 Elsevier B.V. All rights reserved.


2007 - Structure and electronic properties of Fe nanostructures on MgO(001) [Articolo su rivista]
P., Luches; P., Torelli; Benedetti, Stefania; Ferramola, Elena; R., Gotter; Valeri, Sergio
abstract

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembledon MgO(001). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L23 branching ratio and changes in thesplitting, widths and relative intensities of the different final states in the L3M23M23 resonant Auger peak. Scanning tunnelling microscopyindicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronicproperties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionalityeffects are significant._ 2007 Elsevier B.V. All rights reserved.


2007 - Swelling process induced by Focused Ion Beam on magnetic layers [Poster]
Rota, Alberto; S. F., Contri; G. C., Gazzadi; S., Cottafava; Valeri, Sergio
abstract

The ability to prepare ordered arrays of micro/nano sized magnetic elements offers the chance to investigate magnetic properties at length scales previously inaccessible. Bi-dimensional ordered array of magnetic dots can be obtained through a number of different techniques. Patterning of extended magnetic layers or multilayers by Focused Ion Beam (FIB) is one of the most powerful approaches for nano-structuration on a large variety of materials due to the high spatial resolution, good process control and elevated flexibility. This enables a very high accuracy to be achieved in the refinement (shape, sharpness, orientation and dimension) of each individual element and of the separation between the elements. On the other hand ion interactions with solids produce several damage effects. These effects can be very severe in the case of crystalline solids and of nano-sized elements, and can influence magnetic properties of the systems (magneto-crystalline and configurational anisotropy). We studied by AFM and Depth Profile Auger analysis the ions effect on a submicron scale dots array obtained on a 10nm Fe/10nm NiO/MgO(001) multilayer capped with 10nm of MgO. Ion-induced local damage leads to a significant swelling effect on the nano-structures [1]. Our measurements suggest that swelling observed on the multilayer originates from substrate ion irradiation. Transmission Electron Microscopy and diffraction technique measurements should give structural information and clarify the relations between ion induced damage and swelling effect. [1] G. C. Gazzadi, P. Luches, S. F. Contri, A. di Bona, S. Valeri, Nucl. Instr. Meth. B 230 (2005) 512.


2006 - Focused Ion Beam induced swelling in MgO(001) [Articolo su rivista]
Rota, Alberto; Contri, Sara Federica; G. C., Gazzadi; S., Cottafava; Gualtieri, Enrico; Valeri, Sergio
abstract

The interplay between swelling and milling phenomena in determining the morphology of Focused Ion Beam (FIB)-processed MgO(001) was investigated by atomic force microscopy. At the early stages of ion irradiation.. before millinig erosion-is observed, MgO shows a relevant swelling behaviour with protrusion of the bombarded areas up to 6 nm for a dose of 5 x 10(16) ions cm(-2). The effect is mainly ascribed to subsurface defect accumulation, while the low Ga ions concentration, as measured by in-depth Auger analysis, seems to exclude a contribution from ion implantation. In order to explain and control the morphology of Fe/NiO FIB patterned sub-micron structures on MgO substrates, we have also investigated FIB effects on Fe(001) and NiO(001) single crystals. Absent or negligible swelling has been observed on these materials. (c) 2006 Elsevier B.V. All rights reserved.


2006 - Iron oxidation, interfacial expansion, and buckling at the Fe/NiO(001) interface [Articolo su rivista]
Luches, P; Bellini, V; Colonna, S; Di Giustino, L; MANGHI, Franca; VALERI, Sergio; Boscherini, F.
abstract

In order to provide a structural basis for a physical understanding of exchange bias in metal/magnetic-oxide interfaces, we have determined the structure of the Fe/NiO(001) interface by means of x-ray absorption spectroscopy and ab initio density functional theory calculations. A Fe-Ni alloyed phase on top of an interfacial FeO planar layer is formed. The FeO layer exhibits a 7% expanded interlayer distance and a 0.3 angstrom buckling; its presence is predicted to increase the spin magnetic moment of the interface Fe atoms by 0.6 mu(B), compared to the ideally abrupt interface.


2006 - Morphology and chemical activity at the Au/NiO interface [Articolo su rivista]
Benedetti, Stefania; P., Torelli; P., Luches; Rota, Alberto; Valeri, Sergio
abstract

We have grown An films of different thicknesses, ranging from 1 to 30 MonoLayers Equivalent, on 10 MLE of NiO deposited on Ag(1 0 0) single crystal. XPS and STM measurements were performed in order to study the chemical activity at the interface and the morphology of the system. The An growth on the NiO film starts in a 2D mode and becomes a 3D island growth after the formation of an almost complete layer. After An deposition, the formation of about 0.3 MLE of metallic Ni (Ni-0) is observed. The absence of significant attenuation of the PE signal of Ni-0 after successive An depositions indicates that Ni does not remain confined to the interface between An and NiO. The density of defects at the surface of the NiO film is shown to be fundamental in determining the chemical activity at the Au/NiO interface. 2006 Elsevier B.V. All rights reserved.


2006 - Morphology and optical properties of MgO thin films on Mo(001) [Articolo su rivista]
Benedetti, Stefania; Hm, Benia; N., Nilius; Valeri, Sergio; Hj, Freund
abstract

Thin MgO films with a nominal thickness ranging between 1 and 60 ML have been grown on a Mo(001) surface. The film morphology was studied by LEED and STM, revealing the presence of a coincidence pattern with the Mo support in the low coverage regime, a dislocation network at medium thickness and a rather flat and defect-poor MgO surface for thicker layers. The MgO optical properties were investigated as a function of film thickness by analyzing electro-luminescence spectra obtained via electron injection from the STM tip into well-defined surface areas. The spectra are characterized by two distinct emission bands at 3.1 and 4.4 eV. Their origin is discussed in the light of earlier photo-luminescence measurements on MgO nanocubes and smokes. (c) 2006 Elsevier B.V. All rights reserved.


2006 - The Fe/NiO interface studied by polarization dependent X – ray absorption spectroscopy [Articolo su rivista]
S., Colonna; P., Luches; Valeri, Sergio; F., Boscherini
abstract

In order to provide a structural basis for the modelling of the electronic and magnetic properties of the Fe/NiO(001) interface we haveperformed polarization dependent Fe K-edge X-ray absorption measurements. A multi-shell fit of the data is presented and discussed.We find that a 2 ML Fe film exhibits a complete tetragonal distortion of the unit cell and demonstrate the formation of a planar FeOlayer with expanded Fe–O distances perpendicular to the growth plane. We discuss a model for the interface with the FeO layer at theoxide–metal interface. At 10 ML thickness the tetragonal strain of the Fe film is partially released._ 2005 Elsevier B.V. All rights reserved.


2005 - A novel cylindrical microwave plasma source [Abstract in Atti di Convegno]
Garuti, M.; Balocchi, P.; Botti, C.; Veronesi, P.; Leonelli, C.; Rettighieri, L.; Balestrazzi, A.; Valeri, S.
abstract

Microwave activated plasma shows advantages on obtaining high-density coatings with advanced mechanical properties, since these kinds of plasma allow to activate precursor molecules with high binding energy. We have developed a novel pulsed microwave plasma source, whose shape allows to obtain a large toroidal plasma emission zone, with a constant emission rate on the whole circumference of the emitting source. This design allows the deposition on relatively large and oddshaped samples. Plasma can be active on a large pressure range (10-2 /10 Torr), with a base pressure of 10-6 Torr to ensure a good purity for mechanical coatings. The microwave source was designed in order to fit on any kind of vacuum system equipped with a ISO standard 200 mm flange, regardless on the mounting direction. The pulsed microwave source uses a solid state power supply able to pulse the magnetron up to 8 kW peak, with a square waveform having excellent dynamic characteristic.


2005 - Absence of oxide formation at the Fe/MgO(001) interface [Articolo su rivista]
P., Luches; Benedetti, Stefania; Liberati, Marco; F., Boscherini; I. I., Pronin; Valeri, Sergio
abstract

We present a study focussed at the atomic level characterization of the Fe/MgO(001) interface by means of X-ray photoelectron spectroscopy, X-ray photoelectron diffraction and X-ray absorption spectroscopy. The data show a good crystalline quality of the Fe films grown on MgO(001) at room temperature. The films have a bee structure with the Fe(001)//MgO(001) and Fe[110]//MgO[100] orientation. The Fe growth in the 1-10 ML thickness range proceeds by the formation of islands covering a fraction of the MgO surface. The oxidation of Fe and the consequent reduction of MgO can be excluded, thus proving that the interface is locally abrupt, with a very weak interaction between the two sides. The interface is stable with temperature up to 670 K.


2005 - Focused ion beam induced swelling in MgO(001) [Poster]
Rota, Alberto; S. F., Contri; G. C., Gazzadi; S., Cottafava; Valeri, Sergio
abstract

The ability to prepare ordered arrays of micro/nano sized magnetic elements off ers the chance to investigate magnetic properties at length scales previously inaccessible. Patterning of extended magnetic layers or multilayers by Focused Ion Beam (FIB) is one of the most powerful approaches for nano-structuration on a large variety of materials, due to its high spatial resolution, good process control and elevated fl exibility. MgO(001) is as a good substrate for UHV epitaxial growth of multilayers of magnetic interest, such as Fe/NiO: it is magnetically inert, it has a small lattice mismatch with respect to NiO and it is cheap and easy to prepare. In the present work we analyze ion-induced swelling effects on MgO(001) single crystals as they could infl uence the structure, morphology and, consequently, the properties of magnetic arrays prepared by FIB patterning of magnetic layers on MgO crystalline substrates. We studied by AFM the ions eff ects on the morphology of one micron size square dots array obtained on a 5–10 nm MgO/10 nm Fe/MgO(001) multilayers. A Fe interlayer has been deposited to avoid electronic charging that restricts ion beam effi ciency. 30 keV, 5x1016 Ga+cm-2 bombardment leads to a signifi cant swelling eff ect (4–6 nm protrusion) on the patterned areas. Electron diff raction and Auger depth profi le measurements suggest that the observed swelling mainly originates from MgO structural damage.


2005 - Initial stages of cobalt film growth on MgO(001) surface [Articolo su rivista]
I., Pronin; Da, Valdaitsev; As, Voronchikhin; Mv, Gomoyunova; Sf, Contri; Benedetti, Stefania; P., Luches; Valeri, Sergio
abstract

The initial stages of cobalt film growth on a MgO(001) surface was studied by methods of sample surface structure imaging by reflected electrons, low-energy electron diffraction, and Auger electron spectroscopy. The measurements were performed at room temperature for cobalt layer thicknesses up to 40 angstrom. It is established that cobalt film growth proceeds according to the island mechanism. In the interval of cobalt film thicknesses below similar to 10 angstrom, the dominating surface phase has the form of cobalt islands with an fcc structure; at greater layer thicknesses, the surface film consists predominantly of cobalt grains with an hcp structure.


2005 - Interfacial reactivity and morphology at the Au/NiO(001) interface [Poster]
Benedetti, Stefania; Luches, Paola; Torelli, P.; Rota, Alberto; Valeri, Sergio
abstract

The interest on metal-oxide interfaces is stimulated stimulated by the applications in many fields, such as catalysis, magnetic ecording, hard coatings. In the first stages of growth, gold nanoparticles dispersed on different oxides exhibit unusual electronic properties, depending on cluster size and gold-substrate interaction [1,2]. In particular a partial charge transfer can occur, especially when the substrate is defective since gold tends to nucleate on oxygen vacancies. Thus the properties of goldoxide systems are critically dependent on interface reactions, in turn influenced by substrate preparation. A deeper understanding of the reactivity at the gold-oxide interface is essential to determine and control properties and behaviour of supported gold clusters. In this work we present a study of Au nucleation on top of 10 ML NiO film on Ag(001). By means of XPS, XPD and STM/AFM we studied electronic properties and morphology of the first stages of growth. It was previously reported that deposition of 15 A Au on NiO/Ag(001) causes oxide reduction [3]. We provide detailed and quantitative description of the chemical interactions at this interface. From these results we aim to determine a relation between NiO reduction and morphological modifications (cluster formation and step decoration) in order to understand the driving force of the reduction process. Similar metal-oxide interfaces have been also studied [4,5] to identify a general trend in nucleation and reaction processes. In particular Fe, Pt on NiO and Fe, Pt, Au on MgO have been comparatively investigated. [1] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hkkinen, R.N. Barnett, U. Landman, J. Phys. Chem. A 103(1999) 9573 [2] Z. Yang, R. Wu, D.W. Goodman, Phys. Rev. B 61 (2000) 14066 [3] R. de Masi, D. Reinicke, F. Mller, P. Steiner, S. Hfner, Surf. Sci. 515 (2002) 523 [4] S. Benedetti, P. Luches, M. Liberati, S. Valeri, Surf. Sci. 572 (2004) L348 [5] P. Luches, S. Benedetti, M. Liberati, F. Boscherini, I.I. Pronin, S. Valeri, Surf. Sci., in press


2005 - Interplay between magnetocrystalline and configurational anisotropies in Fe(001) square nanostructures [Articolo su rivista]
P., Vavassori; D., Bisero; F., Carace; A., di Bona; G. C., Gazzadi; M., Liberati; Valeri, Sergio
abstract

We fabricated a set of arrays of single-crystal Fe micron and submicron square elements on MgO using a focused ion beam apparatus. The squares have different size (1-mu m and 500-nm side) and orientation with respect to the crystalline axes. The three patterns were magnetically characterized by means of magneto-optical Kerr effect microscopy/magnetometry, and the symmetry and strength of their magnetic anisotropy was determined through transverse susceptibility measurements performed with the modulated field magneto-optical anisometry technique. We observed that the overall anisotropy of the systems is determined by the interplay between the intrinsic magnetocrystalline anisotropy of the material and the anisotropic energy of the magnetic configurations determined by the lateral confinement (the so called configurational anisotropy). Depending on the relative orientation of the patterns with respect to the intrinsic magnetocrystalline anisotropy axes, the interplay can give rise to enhancement or reduction of the configurational contributions compared to the case of identical patterns made of isotropic material. Our results demonstrate that the strength of the latter contribution is comparable to the first one and can be used to create nanoelements with peculiar anisotropy symmetries.


2005 - Magnetocrystalline and configurational anisotropies in Fe nanostructures [Articolo su rivista]
P., Vavassori; D., Bisero; F., Carace; Liberati, Marco; A., di Bona; Valeri, Sergio; G. C., Gazzadi
abstract

Arrays of single-crystal Fe micron and submicron squares and disks, have been fabricated using a focused ion beam apparatus from a film epitaxially grown on MgO. The hysteresis loops of the patterned areas differ from those of the continuous film as a consequence of the different reversal determined by the lateral confinement of the Fe film. By means of modulated field magneto-optical anisometry measurements we studied the symmetry and the strength of the overall anisotropy. For the smaller square elements we observed a higher-order term in the overall anisotropy with eightfold symmetry arising from the configurational contribution. 2004 Elsevier B.V. All rights reserved


2005 - Morphology and chemical activity at the Au/NiO interface [Poster]
Benedetti, Stefania; P., Torelli; Luches, Paola; Rota, Alberto; Valeri, Sergio
abstract

The study of gold/oxide interfaces has increased recently due to its new found catalytic properties. Those properties are strongly dependent on the structure and morphology of the overlayer and on its interaction with the substrate [1, 2]. The tendency of Au to nucleate at oxygen vacancies [3] drives the growth of Au and favors charge transfer between the overlayer and the substrate. The morphology of the substrate surface and the fi rst stage of the Au growth thus result fundamental in determining the properties of gold/oxide systems. We present here a detailed characterization of the Au/NiO interface from a structural and morphological point of view by means of XPD, LEED and STM/AFM. We also investigated the chemical reactions at interface by XPS. The evolution of the Au/NiO system was studied as a function of the Au deposition in the range between 0 and 30 ML, with emphasis on the low deposition (<3 ML). XPS measurements aimed to a quantifi cation of the previously reported NiO reduction [4] induced by Au deposition. STM/AFM measurements were performed in order to establish a relationship between the growth mode and morphology of the Au overlayer and the Ni reduction at interface. We have also carried out similar experiments on the Fe/NiO [5] and Pt/NiO in order to investigate the general features of the metal/NiO systems. [1] Z. Yang et al. Phys. Rev. B 61, 14066 (2000). [2] L. Giordano et al. Surf. Sci. 471 (2001) 21. [3] Y. Zongxian et al. Phys. Rev. B 65, 155407, (2002). [4] R. de Masi et al. Surf. Sci. 515 (2002) 523. [5] P. Luches et al. Surf. Sci. in press.


2005 - Polar and non-polar domain borders in MgO ultrathin films on Ag(001) [Articolo su rivista]
A., Ferrari; S., Casassa; C., Pisani; S., Altieri; Rota, Alberto; Valeri, Sergio
abstract

The morphology of ultrathin epitaxial MgO layers reactively grown at 470 K and at 1ML min_1 on Ag(001) wasinvestigated by scanning tunnelling microscopy (STM). At deposition of 1.3 and 2.6 ML, square MgO domains ofabout (10 • 10) nm2 form. On mono- and bi-layer domains, edges are mainly oriented along the h110i oxide direction,corresponding to polar (P) borders, while on multi-layer domains also non-polar (NP) borders appear, correspondingto the h100i orientation. By using periodic models and ab initio DFT calculations, the energetics of the two orientationswas studied. Interaction with the metal substrate was found to strongly reduce the instability of P borders, although notenough to make the two orientations equally stable. The results are discussed, also with reference to conflicting experimentalevidence from other laboratories._ 2005 Elsevier B.V. All rights reserved.


2005 - Submicron-scale patterns on ferromagnetic–antiferromagnetic Fe/NiO layers by focused ion beam (FIB) milling [Articolo su rivista]
G. C., Gazzadi; P., Luches; Contri, Sara Federica; A., di Bona; Valeri, Sergio
abstract

With the aim of studying the magnetic properties of reduced-dimensionality magnetic systems we have patterned 250 nm- and 500 nm-size square elements on Fe/NiO layers by 30 keV Ga+ focused ion beam (FIB) milling, varying beam current and pixel dwell time. By high resolution scanning electron microscope (SEM) imaging and atomic force microscopy (AFM) analysis we found that island size decreases from the nominal value by increasing the beam current and features sharpness improves on increasing the dwell time. The top surface of the isolated features has a pronounced edge bending which may be as high as 9 nm with respect to the flat inner area of the island and decreases as dwell time grows. By varying the ion fluence we found that such a shape is related to a surface swelling effect occurring at low ion fluence in the irradiated areas. The swelling-related damage at the edges is expected to influence the magnetic properties of the patterned features.


2004 - Chemical reactions and interdiffusion at the Fe/NiO(001) interface [Articolo su rivista]
Benedetti, Stefania; P., Luches; Liberati, Marco; Valeri, Sergio
abstract

The interaction between Fe and NiO(001) surface for increasing metal deposition at RT was studied by means of Xrayphotoelectron spectroscopy and X-ray photoelectron diffraction. NiO reduces to an extent that depends on theamount of deposited Fe, up to a maximum of 1ML for Fe deposition of 5ML or larger. The overlayer initially growsin a 3D island mode, with partial intermixing between the deposited Fe atoms and the metallic Ni atoms resulting fromthe reduction process. The intermixed region extends up to about 3ML. For 5ML Fe deposition, the islands coalesceinto a continuous film. 2004 Elsevier B.V. All rights reserved.


2004 - Evidence for interdot coupling in an array of micrometric Fe dots [Articolo su rivista]
D'Addato, Sergio; P., Vavassori; D., Bisero; Liberati, Marco; F., Feri; E., DI FABRIZIO; Valeri, Sergio
abstract

The magnetization reversal of both the continuous and patterned Fe films, grown in ultra high vacuum byatomicallycontrolled evaporation through a mask, was studied using magneto-optic Kerr magnetometry. The hysteresis loopsrecorded from continuous Fe film as a function of the direction of the applied field, evidence a well-defined uniaxialanisotropy, induced during the film deposition. The patterned area shows a magnetic anisotropy completely differentfrom that of the continuous film and related to the lattice symmetry. Scanning electron microscopy images of thepatterned area allows us to exclude effects related to deviations from circularityof the shape of the single dots. The closerelationship between the anisotropydirections and the pattern lattice axes indicates that the new anisotropyoriginatesfrom the interdot dipolar coupling interaction.r 2003 Elsevier B.V. All rights reserved.


2004 - Magnetism and morphology of Ni/Cu(100) and Ni-Fe-Ni/Cu(100) ultrathin films [Articolo su rivista]
G., Fratucello; P., Vavassori; Luches, Paola; Liberati, Marco; Valeri, Sergio
abstract

Ni films and Ni–Fe–Ni trilayers on Cu(1 0 0) have been grown and characterized in situ in their structure andmorphology. The magnetic properties have been investigated ex situ using MOKE. It is found that the magnetization isout-of-plane for both systems and that the presence of the Fe interlayer favors the in-plane reorientation. The effect ofthe Fe layer is interpreted on the basis of the competition between perpendicular interface and dipolar anisotropies. Amethod based on the coercive field for the orientation of the magnetization is presented.r 2003 Elsevier B.V. All rights reserved.


2004 - MgO/Ag interface structure and STM images from first principles [Articolo su rivista]
N., Lopez; Valeri, Sergio
abstract

The interface of MgO/Ags001d has been studied with density functional theory applied to slabs. We havefound that regular MgO films show a small adhesion to the silver substrate, the binding can be increased inoff-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of Oatoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanningtunneling microscopy signatures of these films is also analyzed in some detail. For defect free depositscontaining 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at largepositive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxidethey introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of theimage, the extra O stored at the interface can also be detected for very thin films.


2004 - Morphology of H2O dosed monolayer MgO(001)/Ag(001) [Articolo su rivista]
S., Altieri; Contri, Sara Federica; S., Agnoli; Valeri, Sergio
abstract

Combining scanning tunneling microscopy (STM) and X-ray photoelectron (XPS) as well as Auger spectroscopy, we establish a direct relationship between the morphology and the excitation spectra of ultrathin MgO(001) films on Ag(001), and we show that monolayer MgO(001) films do not exhibit significant morphology changes upon H2O dosage, with the constituent islands remaining monoatomically flat up to exposures of tens of Langmuir.


2004 - NiO and MgO ultrathin films by polarization dependent XAS [Articolo su rivista]
P., Luches; E., Groppo; D'Addato, Sergio; C., Lamberti; C., Prestipino; Valeri, Sergio; F., Boscherini
abstract

We present a comparative study of MgO and NiO ultrathin films on Ag(001) by polarization dependent X-ray absorption spectroscopy on both the metal and oxygen K-edges. The data have been analyzed using ab initio multiple-scattering calculations (FEFF code). We show that the films have the rock-salt structure with a negligible atomic interdiffusion with the substrate in all cases. Both MgO and NiO films are tetragonally strained to match the Ag substrate at low thickness and gradually relax their structure to the bulk one. The local in-plane and out-of-plane interatomic distances in the oxides have been accurately determined as a function of thickness and the elastic constants of the films have been found to be compatible with the bulk values. The oxygen absorption site has been found to be on top of Ag atoms, while the metal occupies the interstitial site in both systems. A significant increase of the interplanar distance at the interface between both MgO and NiO and the Ag substrate has been measured.


2004 - OK-edge x-ray absorption study of ultrathin NiO epilayers deposited in situ on Ag(001) [Articolo su rivista]
E., Groppo; C., Prestipino; C., Lamberti; R., Carboni; F., Boscherini; Luches, Paola; Valeri, Sergio; D'Addato, Sergio
abstract

In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni K edge, x-ray absorption spectroscopy (XAS) to probe the structure of ultrathin NiO epilayers deposited on Ags001d.In that experiment samples were measured ex-situ and a 5 ML-thick MgO cap was used to avoid the hydroxylation of the NiO film. In the present paper we report complementary O K edge XAS data on the same system; NiO epilayers, in the 3–50 ML thickness range, were grown in situ in the end station of the ALOISA beamline of the ELETTRA facility. A quantitative analysis of the data in the extended energy range has been performedusing multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the filmis rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain as a function of the film thickness T. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film, being the in- and out-of-plane nearest neighbor distances equal to ri=2.048±0.016 Å and r'=2.116±0.018 Å. These values are in agreement with the expected behavior for atetragonal distortion of the unit cell. The growth-induced strain is gradually released with increasing T: it is stillappreciable for 10 ML but is completely relaxed at 50 ML. Any significant intermixing with the Ag substratehas been ruled out. Combining O and Ni K edge results we can conclude that NiO films grow on Ags001d inthe O-on-Ag configuration, with an interface distance d=2.28±0.08 Å. This expansion of the interplanardistance is in agreement with recent ab initio simulations. A comparison with the similar MgO/Ags001d systemis also performed.


2004 - Oxygen-dosage effect on the structure and composition of ultrathin NiO layers reactively grown on Ag(001) [Articolo su rivista]
C., Giovanardi; A., Di Bona; Valeri, Sergio
abstract

NiO ultrathin films have been prepared under UHV conditions on Ag(001) substrate by metal deposition inO2 atmosphere. The films were prepared by deposition of the amount of metal correspondent to 1 ML monolayerof NiO in the presence of different oxygen-to-metal flux ratios, to investigate the oxygen dosage effecton the structure and composition of the growing layer and on the stoichiometry of Ni oxide. Thicker films (upto 20 ML) were also prepared. The structure has been monitored both in reciprocal and direct space bylow-energy electron diffraction and primary-beam diffraction-modulated electron emission. Core-level x-rayphotoemission spectroscopy has been used to study film chemistry and composition. At low coverage (below2 ML) a dramatic dependence of structure and composition on the oxygen-to-nickel flux ratio has beenobserved. Low oxygen dosage induces a (231) reconstruction in the 1-ML films that evolves to a (131)phase as the dosage and/or the film thickness increases. In the layers prepared at low oxygen dosage a largefraction of metallic Ni coexists with the Ni oxide, but the oxidized fraction largely prevails for high oxygendosage. The oxygen dosage during the growth also affects thicker films. The mosaic formation, which has beenascribed to misfit strain relaxation, is related to a low oxygen-to-nickel flux ratio.


2004 - X-ray absorption study at the Mg and O K edges of ultrathin MgO epilayers on Ag(001) [Articolo su rivista]
Luches, Paola; D'Addato, Sergio; Valeri, Sergio; E., Groppo; C., Prestipino; C., Lamberto; F., Boscherini
abstract

We determined the local atomic structure of MgO epilayers on Ag(001) by means of polarization-dependentx-ray absorption spectroscopy measurements at the Mg and O K edges. A quantitative analysis of the data inthe extended energy range has been performed using multiple scattering simulations.We found that, even in theultrathin limit, the local structure of the films is rocksalt and we obtained a quantitative evaluation of theaverage in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressivestrain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-planelattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion ofthe unit cell. This growth-induced strain is gradually released with increasing thickness and it is almostcompletely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansionof the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be inagreement with recent ab initio simulations. This work provides previously unavailable input for modeling thephysical properties of the system and supports the hypothesis that the different electronic properties of MgOfilms on Ag(001) are not related to structural or compositional differences at the ultrathin limit.


2002 - Initial stages of cobalt disilicide formation on silicon single crystals [Articolo su rivista]
M. V., Gomoyunova; I. I., Pronin; D. A., Valdaitsev; N. S., Faradzhev; Luches, Paola; Rota, Alberto; Valeri, Sergio
abstract

The initial stages of cobalt disilicide formation on Si(111) and Si(100)surfaces are studied using backscattered electron imaging of the near-surface atomicstructure. Both the reactive deposition and the solid phase epitaxy are investigatedin the coverage range of 1-10 ML of Co. The evidence for CoSi2 island formation atthe earliest stages of the process is found. The epitaxial orientations of disilicidelayers grown on Si(111) and Si(100) surfaces are determined.


2002 - Scanning tunnelling microscopy of MgO ultrathin films on Ag(001) [Articolo su rivista]
Valeri, Sergio; Altieri, Salvatore; DEL PENNINO, Umberto; DI BONA, Alessandro; Luches, Paola; Rota, Alberto
abstract

The morphology of ultrathin epitaxial MgO layers reactively grown on Ag~001! was investigated by usingscanning tunnelling microscopy. In the initial deposition stage Ag atoms are partially removed from thesubstrate and form extended monoatomic islands leaving vacancy islands in the substrate itself. On individualsubstrate terraces Ag is thereafter found at three atomic levels. For submonolayer deposition MgO condensatesin form of small islands of monoatomic height, located on the original substrate, on the protruding Ag islandsand on the vacancy islands as well. The largest Ag~001! fractional coverage by monoatomic MgO islands is70%. A limited amount of MgO bilayers or trilayers has also been detected ~about 1% fractional coverage!. Atthe nominal deposition of 1 ML flat, squared and connected MgO domains of about 10 nm in width form, withedges along the @110# directions. The actual substrate fractional coverage is about 85% and the occurrence ofbilayers and multilayers becomes significant ~about 30 and 5% fractional coverage, respectively!, resulting inthe formation of three-dimensional pyramidal MgO islands.


2002 - The Co/Si(111) interface formation: a temperature dependent reaction [Articolo su rivista]
P., Luches; Rota, Alberto; Valeri, Sergio; I. I., Pronin; D. A., Valdaitsev; N. S., Faradzhev; M. V., Gomoyunova
abstract

We have investigated the reaction of Co with the Si(111) surface both at room temperature (RT) and at high temperature (500-650 degreesC). The temperature evolution of the RT deposited 10 ML film has also been studied. The films, prepared by the different methods, have been structurally characterized by means of primary-beam diffraction modulated electron emission. Auger electron spectroscopy has been used to follow their stoichiometric evolution. For RT deposition the films have been found to have a B-type (180degrees rotated with respect to the underlying Si(111) surface) cubic structure with a Co content and an interlayer spacing increasing with thickness. After 650 degreesC annealing, the films are completely reacted and have an unstrained B-type CoSi2 structure. High temperature (500 degreesC) deposition of Co leads to the formation of stoichiometric CoSi2 films. Both annealed and high temperature grown films are found to be Si terminated.


2001 - Pulsed laser ablation of glassy carbon targets for the coating of ion accelerator electrodes [Articolo su rivista]
Rossi, M; Gabrielli, G; Papa, T; Piccirillo, S; Sessa, V; TERRANOVA M., L; Valeri, Sergio; Materassi, M.
abstract

The synthesis of hard carbon films grown by laser ablation of glassy-carbon targets is reported. The carbon films were deposited under various conditions using XeCl pulsed laser deposition at 10(-1) torr on Mo substrates kept at temperatures ranging from 200 to 500 degreesC. The structural features of the deposits were investigated by means of reflection high energy electron diffraction (RHEED) and Raman spectroscopy. Ion sputtering experiments have been performed using 1-keV Ar+ beams in order to verify the suitability of such coatings to protect spatial components against erosion by accelerated ions.


2000 - Structural analysis of Fe/Ni(001) films by photoelectron diffraction [Articolo su rivista]
Gazzadi, Gc; Luches, P; di Bona, A; Marassi, L; Pasquali, Luca; Valeri, Sergio; Nannarone, Stefano
abstract

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0-14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fee to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the bcc[111]parallel to fcc[110] in-plane orientation. Interlayer spacings of 2.05 +/- 0.068 Angstrom, 2.01 +/- 0.03 Angstrom, and 1.85 +/- 0.03 Angstrom were found in the bcc region, between bcc and fee layers and in the fee region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was 2.49 +/- 0.02 Angstrom, in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of 2.04 +/- 0.04 Angstrom and in plane n-n distance of 2.47 +/- 0.01 Angstrom. At 14 ML the PD pattern collected over a 94 degrees azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the bcc[111]parallel to fcc[110] alignment.


2000 - Structural analysis of epitaxial Fe films on Ni(001) [Articolo su rivista]
Gazzadi, Gc; Luches, P; D'Addato, Sergio; Marassi, L; Capelli, R; Pasquali, Luca; Valeri, Sergio; Nannarone, Stefano
abstract

The atomic geometry and growth mechanism of Fe films (0.5-20 monolayers (ML)) epitaxially grown on Ni(001) have been investigated by a multitechnique approach with the twofold aim of assessing the reliability of quick structural techniques (primary-beam diffraction modulated electron emission (PDMEE), secondary electron imaging (SEI)), suitable for on-line monitoring of film growth, and studying the structural evolution of the Fe/Ni system. To this end, samples were analysed by PDMEE and SEI together with synchrotron radiation photoelectron diffraction (PD), and extended x-ray absorption fine structure (EXAFS). Results show the effectiveness of the approach by quick techniques. In the early stage of growth, Fe arranges in a strained fcc(001) structure. Transition to the bce phase occurs through nucleation of bcc(110) domains with the bce < 111 > //fcc < 110 > orientation. Quantitative analysis based on PD and EXAFS data is also presented. (C) 2000 Elsevier Science B.V. All rights reserved.


1999 - Epitaxial growth of ultrathin Fe films on Ni(001): a structural study [Articolo su rivista]
Luches, P; Gazzadi, Gc; di Bona, A; Marassi, L; Pasquali, Luca; Valeri, Sergio; Nannarone, Stefano
abstract

We have investigated the atomic geometry of Fe films on a Ni(001) crystal in the thickness range 0-25 ML with the aim of following the transition from the fee pseudomorphic structure to the equilibrium bcc phase. The structural techniques used are primary-beam diffraction modulated electron emission (PDMEE) and low-energy electron diffraction (LEED). For film thicknesses up to 5 ML the films are fee, strained in the direction of film growth. For higher Fe coverages, the films begin a transition to the bulk bcc structure which is complete after 14 ML of Fe coverage. We have obtained evidence for intermixing between Fe and Ni in the first 3 ML. The orientational relationship between the bcc phase and the underlying fee substrate has also been investigated, finding a bcc (110) phase with four possible domains, each with one of the [111] directions parallel to an fcc [110] direction. (C) 1999 Elsevier Science B.V. All rights reserved.


1997 - Development, operation and analysis of bialkali antimonide photocathodes for high-brightness photo-injectors [Articolo su rivista]
A., di Bona; F., Sabary; S., Joly; P., Michelato; D., Sertore; C., Pagani; Valeri, Sergio
abstract

Different methods for the preparation of K-Cs-Sb photocathodes, suitable for electron sources in RF guns, are described in detail. The resulting material has been characterized by Auger depth profiling technique, and by its spectral response in the 250-600 nm range, as well. The results of lifetime experiments under operational and ''laboratory'' conditions are presented. The role of CO2 in reducing the operational lifetime of the photocathodes and that of O-2 in improving the long-wavelength response are investigated.


1997 - Early stage in low-energy ion-induced damage on InP(110) surface [Articolo su rivista]
Valeri, Sergio; Gazzadi, gian carlo; Rota, Alberto; DI BONA, Alessandro
abstract

The change in the short-range order created by ion milling in the near surface region of InP single crystals wasinvestigated by primary beam diffraction modulated electron emission (PDMEE). The very early stage of the damagecreation by low energy (0.6-1 keV) Ar ions in normal and oblique incidence was studied. A simple model based on theweighted combination of perfectly crystalline and completely amorphous regions was used to model the experimental results.Evidence of a subsurface nucleation of the amorphization process was found. We also found that the total sputtering yield ismarkedly dependent on the ion dose, being on the undamaged surface much larger than its steady state value. Low energyelectron diffraction (LEED) measurements were also performed to correlate long-range and short-range order removal by ionbombardment. Finally, the ion damage on the GaAs and InP surfaces was comparatively discussed.


1997 - Focusing-defocusing of keV electrons along [001] and [101] Fe atomic chains [Articolo su rivista]
Valeri, Sergio; A., di Bona; F., Borgatti
abstract

We have investigated the focusing and defocusing process of a primary electron wave along Fe atomic chains. To this end, we measured the intensity of the LVV Auger signal, as a function of the incidence angle of the primary beam, from a Co him buried in a Fe matrix. We found that maximum focusing efficiency takes place for six scatterers along [001]Fe atomic chains, then defocusing starts to set in. However, a significant anisotropy is still detectable at eleven scatterers deep in the bulk. The anisotropy depth profile we measured has been used to evaluate the anisotropy in more complex situations, such as thin films and multilayers.


1997 - Modulated electron emission by scattering-interference of primary electrons [Articolo su rivista]
Valeri, Sergio; Dibona, A.
abstract

We review the effects of scattering-interference of the primary, exciting beam on the electron emission from ordered atomic arrays. The yield of elastically and inelastically backscattered electrons, Auger electrons and secondary electrons shows a marked dependence on the incidence angle of primary electrons. Both the similarity and the relative importance of processes experienced by incident and excident electrons are discussed. We also present recent studies of electron focusing and defocusing along atomic chains. The interplay between these two processes determines the in-depth profile of the primary electron intensity anisotropy. Finally, the potential for surface-structural studies and limits for quantitative analysis are discussed, in comparison with the Anger electron diffraction (AED) and photoelectron diffraction (PD) techniques.


1997 - Surface and near surface structure of Fe-Co layers by scattering-interference of primary electrons [Articolo su rivista]
G. C., Gazzadi; A., di Bona; F., Borgatti; Rota, Alberto; Valeri, Sergio
abstract

We studied the Co/Fe bi- and multilayer growth on Fe single crystal using the primary-beam diffraction modulated electron emission (PDMEE) technique. This approach enables a chemical selected structural characterization of both surface and buried layers and interfaces to be performed. Co growth on the Fe(001) surface was studied in the 3–70 ML coverage range. The transition from the initial epitaxial bcc phase of the Co film to the stable hcp phase was found to occur rather abruptly at 25–30 ML coverage, a value larger than previously reported in the literature. Subsequent Fe deposition on the Co hcp film recovers the bcc structure in the sandwiched Co layer. However, removal of the Fe epilayer by mild sputtering results again in an hcp Co film.


1996 - Auger and x‐ray photoemission spectroscopy study on Cs2Te photocathodes [Articolo su rivista]
A., di Bona; F., Sabary; Valeri, Sergio; P., Michelato; D., Sertore; G., Suberlucq
abstract

Thin films of Cs2Te have been produced and analyzed by Auger depth profiling and x-ray photoemission spectroscopy (XPS). The formation of the photoemissive material passes through different phases, each of them has been characterized by XPS and by its total yield in the spectral region 3.5-5 eV. Copper and molybdenum substrates have been considered. While Mo behaves to all practical purposes like an ideal support for Cs2Te, strong diffusion from the substrate material into the photoemissive film has been observed on photocathodes fabricated on Cu, The ruggedness of the photocathodes has been tested by exposing them to a few hundred Langmuirs of different gases, namely O-2, CO2, CO, N-2, and CH4. The last three have no effect on the photocathode lifetime, while a substantial reduction of the quantum efficiency has been observed after the exposure to oxygen. The main reason for this is the formation of a thick cesium oxide layer at the surface of the photocathode. However, the oxygen pollution can be partially recovered by the combined effect of heating the photocathode at 230 degrees C and illuminating the poisoned material with the 4.9 eV radiation. No rejuvenation has been observed under the effect of the temperature or the radiation alone.


1995 - Element-specific, surface and subsurface structural analysis by scattering-interference of primary electrons [Articolo su rivista]
Valeri, Sergio; Dibona, A; Gazzadi, Gc; Borgatti, F.
abstract

We investigated the effects of scattering-interference of primary electrons on the secondary electron emission intensity fron ordered surfaces and interfaces. Because of the focusing-defocusing of the primary wave along atomic chains, maxima in the electron yield occur when the exciting beam is aligned with low index axes. Therefore the electron intensity distributions as a function of the angle of incidence of the primary beam can be interpreted as projected images of real space, local atomic arrangement. The process can be modelled in a single scattering cluster (SSC) approximation. Potential of primary-beam diffraction modulated electron emission (PDMEE) technique for surface and subsurface structural characterization is shown, for Co epitaxy on Fe(001).


1995 - SURFACE-SHIFT LOW-ENERGY PHOTOELECTRON DIFFRACTION - CLEAN AND HYDROGENATED GAAS(110) SURFACE-STRUCTURE RELAXATION [Articolo su rivista]
RUOCCO, A; BIAGINI, M; DIBONA, A; GAMBACORTI, N; VALERI, Sergio; NANNARONE, Stefano; SANTONI, A; BONNET, J.
abstract

Surface-shift low-energy photoelectron diffraction has recently been revealed as a powerful tool for the determination of surface structures. We have applied this method to study clean and hydrogenated (1/4 of monolayer) GaAs(110) surfaces. A recently developed renormalized multiple-scattering theory [M. Biagini, Phys. Rev. B 48, 2974 (1993)] has been applied to the interpretation of As 3d polar spectra. Numerical computations have been carried out for several surface geometries, corresponding to different buckling angles and surface bond lengths. We have obtained a buckling angle =26°and 6°for clean and hydrogenated surfaces, respectively. As far as surface bond lengths are concerned, changes within +2% for the clean surface and ideal bond lengths for the hydrogenated surface were found.


1994 - Angular anisotropy of electron-excited secondary electron emission [Articolo su rivista]
Valeri, Sergio; A., di Bona; G. C., Gazzadi
abstract

We studied the influence of the crystalline structure of III-V compound semiconductors on the intensity of the energy distribution of secondary electrons. Modulation of the amplitude of the primary electron waves within the crystal by electron-atom scattering results in a dependence of secondary emission intensity on the incidence angle. We measured the intensity anisotropy of the electron energy distribution in a wide energy range on either GaAs(110) and InP(110) surfaces, for different values of the primary beam energy between 0.6 and 5 keV. We focused our attention on the elastic and inelastic Auger emission and on the background intensity. The Monte Carlo method was used to simulate the sequences of events experienced by the primary and secondary electrons in the material. Anisotropy was assumed to arise in electron elastic focusing on atomic sites. The calculated anisotropy dependence on the electron kinetic energy is consistent with experimental results. Differences in anisotropy of electrons with different probing depth is related to the different relative importance of surface and bulk contributions to the overall emission intensity. Defocusing along forward scattering directions does not occur over a depth of several tens of angstrom, indicating that the defocusing length is longer than had been usually assumed for close-packed directions.


1994 - Focusing and defocusing in electron scattering along atomic chains [Articolo su rivista]
VALERI, Sergio; A., di Bona; G. C., Gazzadi
abstract

We investigated the degree of forward focusing and of multiple-scattering-induced defocusing for electrons in the keV range along regular chains of different atomic density in GaAs and GaP single crystals. The intensity angular anisotropy of elastic and inelastic (losses) Auger electrons was measured in the primary-beam diffraction-modulated electron-emission approach. In variance to the photoelectron-diffraction (PD) case, the possible influence of the energy-loss mechanism on the observed anisotropy can be disregarded and the in-depth distribution of the scattered intensity is independently determined. Both the focusing and defocusing lengths were found to be significantly larger than in previously reported PD results, and strongly dependent on the interatomic spacing. A comparative study of defocusing along the Ga [110] chains in GaAs and GaP gives experimental evidence of the influence of nearest-neighbor atomic rows (As and P, respectively) on the scattering processes.


1994 - PRIMARY-BEAM DIFFRACTION MODULATED ELECTRON-EMISSION (PDMEE) FOR SURFACE STRUCTURAL-ANALYSIS [Articolo su rivista]
Valeri, Sergio; DI BONA, A; Gazzadi, G. C.
abstract

Scattering interference of primary exciting electrons leads to a dependence of the Auger emission intensity on the beam incidence angle. For beam energy in the kiloelectron-volt range, maxima in the Auger yield occur when the primary beam is aligned with low-index crystal axes, therefore the Auger angular intensity distribution directly reflects the symmetries and the degree of local order. We investigated the potential for this effect, primary-beam diffraction modulated electron emission (PDMEE) to be used for surface structural characterization, in comparison with the Auger electron/photoelectron diffraction (AED/PD) technique. The angular intensity distributions of both low and high kinetic energy Auger electrons were measured on both GaAs and InP (110) surfaces. We show, by using highly collimated energetic electron beams, that the angular resolution is significantly improved in PDMEE with respect to AED/PD. Unlike the AED/PD case, the angular intensity distribution is independent of the kinetic energy of the Auger electrons, therefore low-energy, surface-sensitive electrons also directly reflect crystal symmetries, owing also to the fact that the specific momentum character of the Auger electrons can be neglected. Anisotropy in the intensity angular distribution as large as 70% was measured, comparable to AED/PD anisotropy values. Both a retarding field (low-energy electron diffraction optics) and a dispersion field cylindrical mirror analyser were used for PDMEE analysis, and instrumental performances are discussed comparatively.


1993 - Auger electron emission by ion impact on solid surfaces [Articolo su rivista]
Valeri, Sergio
abstract

In this review, we examine recent work on the emission of Auger electrons by energetic ion collisions on solid surfaces. We give a detailed presentation of the results concerning the interpretation of the Auger lineshape, the identification and separation of different spectral contributions, and the evaluation of the relative importance of symmetric and asymmetric collisions in ionization and sputter-ejection processes. We stress the relevance of the excitation and detection geometry, and discuss the angular and energetic dependence of the Auger lineshape in terms of the Doppler effect. Emission from multi-elemental targets and from singly or multiply charged projectiles is also reviewed. Finally, a brief survey of computational studies in this area is presented.


1993 - Diffraction effects in Auger quantitative analysis on III–V compounds [Articolo su rivista]
Valeri, Sergio; A., di Bona; E., Nava
abstract

The anisotropy in surface and subsurface ionization due to the primary electron diffraction has been conflictually reported to play a minor or major role in Auger spectroscopy, and the influence of instrumental and physical parameters has been discussed. We report on the effects of incoming beam diffraction on the quantitative analysis of cleaved, sputtered and Cs covered III-V compounds. We show that measurements must be taken into account for crystalline specimen orientation with respect to the incident beam direction.


1993 - PLVV Auger lineshape modulation by incident beam diffraction in InP [Articolo su rivista]
Valeri, Sergio; A., di Bona
abstract

The electron excited PL2,3VV Auger spectrum of InP has been investigated as a function of the incident beam direction. Phosphorus lineshape has been found to be modulated by the scattering-interference of primary electrons via changes in the relative intensity of surface- and bulk-specific spectral features.


1991 - ION-BEAM EFFECTS ON THE SURFACE AND NEAR-SURFACE COMPOSITION OF TASI2 [Articolo su rivista]
Valeri, Sergio; Dibona, A; Ottaviani, Giampiero; Procop, M.
abstract

Low-energy (0.7-4.5 keV) ion bombardment effects on polycrystalline TaSi2 at sputter steady state and in various intermediate steps have been investigated, in the temperature range up to 550-degrees-C, to determine the time and temperature dependence of the altered layer formation. This in turn enables a better knowledge of the synergistic effects of the processes mentioned above. At low temperatures (T less-than-or-equal-to 410-degrees-C) the surface is silicon depleted, and the depletion is even more severe in the subsurface region up to a depth of several tens of angstroms; silicon preferential sputtering and radiation-enhanced segregation assisted by the displacement mixing-induced motion of atoms are assumed to be responsible for this composition profile, while thermally activated diffusion processes become operative above 410-degrees-C, reducing progressively the concentration gradient between the surface and the subsurface zone. The composition at different depths has been determined from Auger peaks of different kinetic energies, by varying the take-off angle and finally by sputter profiling at low ion energy the high energy processed surfaces. Quantitative analysis has been performed by XPS and AES by using the elemental standard method. by XPS and AES by using the elemental standard method.


1990 - AES and EELS study of alkali-metal adsorption kinetics on either cleaved or sputtered GaAs and InP (110) surfaces [Articolo su rivista]
Valeri, Sergio; M., Lolli; P., Sberveglieri
abstract

A detailed study at room temperature of the interfaces between thin alkali films and cleaved or sputtered processed (110) surfaces of GaAs and InP has been performed using Auger and electron energy loss spectroscopy. Attention has been focused on the interface growth kinetics, and on subsurface intermixing and segregation processes in the presence of different metals (Cs, Na and K) and substrates. The most relevant features in the interface formation are very similar in all cases. Alkali adatoms partially diffuse in the semiconductor subsurface. For K and Na on GaAs, this intermixing also results in an As outdiffusion. On sputter-prepared substrates, the interface formation rate is lower, the As outdiffusion on the contrary is larger and occurs also in the Cs/GaAs case. A comparison with high and low electronegativity metals on III–V semiconductors is made and the results are discussed. As far as interfacial reaction and products are concerned, we found that cations and anions are predominantly involved in the chemical bonding with the adatoms in GaAs and in InP, respectively.


1990 - AES, EELS AND XPS STUDY OF ION-INDUCED GAAS AND INP(110) SURFACE AND SUBSURFACE MODIFICATIONS [Articolo su rivista]
Valeri, Sergio; Lolli, M.
abstract

Surfaces of GaAs and InP(110) single crystals have been modified by ion bombardment at different energies. The modofications induced in the surface and subsurface regions have been investigated by different electron spectroscopies.


1990 - Loss features in the HREEL spectra of the K/GaAs(110) system [Articolo su rivista]
DEL PENNINO, Umberto; Biagi, Roberto; Mariani, Carlo; Valeri, Sergio; Betti, Maria Grazia; J. A., Schaefer
abstract

We investigated the evolution of the space-charge layer at the GaAs(110) surface as a function of the potassium coverage by means of HREELS, looking at the surface coupled collective excitations (phonon-plasmon, i.e. plasmarons.


1988 - Auger-electron emission induced by Ar+ impact on silicides [Articolo su rivista]
VALERI, Sergio; R., Tonini; G., Ottaviani
abstract

Impact of ions of several kilo-electron-volts (keV) with solids produces inner-shell excitations. The consequent decay results in the emission of Auger electrons whose spectra differ considerably from electrons or x-ray-excited spectra. Si (L23-derived) Auger emission, induced by Ar+ bombardment in pure Si and silicides of different metals (Cr,Ni,Pt) and stoichiometry (from NiSi2 to Ni3Si), has been studied by varying both the energy of the primary ions (in the 1–5-keV range) and the takeoff angle (from grazing to normal geometry). We separated the background of inelastic scattered electrons from the Auger electrons, and in a further step the contributions to the spectra coming from deexcitation inside and outside the solid. The portion of the Auger emission which originates from sputtered atoms (‘‘atomic spectrum’’) was found to depend on the target composition and takeoff angle, being larger in metal-rich silicides and at grazing geometry. The parents of ‘‘atomic’’ Auger electrons were found to be mainly neutral and excited sputtered atoms in Si and Si-rich silicides, while in metal-rich silicides there is a dominant contribution from sputtered ions. These results have been discussed in terms of the inner-shell ionization mechanism and the in-depth distribution of exciting collisions. The ‘‘atomic’’ spectral shape dependencies on ion energy and takeoff angle have been interpreted in terms of the Doppler shift imparted to the Auger-electron velocity by an anisotropic jet of high-energy parent atoms.


1987 - ADSORPTION OF MONO-VALENT METALS ON THE GaAs(110) SURFACE [Articolo su rivista]
Bertoni, Carlo Maria; C., Calandra; C., Mariani; Valeri, Sergio
abstract

We present a synchrotron radiation investigation on the electronic properties of the GaAs(110) surface covered with alkali metals (Cs, K). As and Ga core level spectra for the clean surface are compared with those obtained after metal deposition. Satellites structures at about 4 and 6 eV from the main peak are found, which can be interpreted in terms of the excitation of overlayer plasma modes. We also study comparatively the effect of K and Cs on the oxidation of the GaAs surface.


1987 - Atomic Effects in the Auger and Absorption Spectra Involving the M2,3 Levels in Cr and Cr-Compounds [Articolo su rivista]
DEL PENNINO, Umberto; CALANDRA BUONAURA, Carlo; L., Grandi; C., Mariani; Valeri, Sergio
abstract

We determined that in the Cr-M2,3VV spectra autoionization increases with oxygen exposure. These effects are interpreted due to multiplet term splitting.


1987 - ION-BEAM EFFECTS ON SURFACE AND SUBSURFACE COMPOSITION OF NEAR-NOBLE SILICIDES [Articolo su rivista]
Valeri, Sergio; G., Ottaviani
abstract

Modifications of Cr–silicide (Cr3Si and CrSi2) and Ni–silicide (NiSi2) surface and subsurface regions induced by an Ar ion beam were investigated by angle integrated Auger electron spectroscopy. The dependence of the surface modification on the ion beam energy and density was investigated between −80 and 400 °C. At room temperature (RT) Cr–silicides are metal enriched in the surface and subsurface region. Ion bombardment of a CrSi2 surface at high temperature (HT) produces Si enrichment with respect to RT, resulting in the formation of a CrSi layer. The Si enrichment occurs at a temperature lower than the temperature characteristic of purely thermal segregation. On the contrary, the Cr3Si surface composition is independent of the temperature. The NiSi2 surface is stoichiometric at RT; ion bombardment at HT produces a Si surface segregation in two distinct steps. On the basis of a careful line‐shape analysis of the Si core–valence–valence Auger transitions, we attribute the first step to the formation of an ordered, ∼1.4 Å thick, Si overlayer, the second one to the growth of a thicker (∼3 Å) amorphous Si layer.


1986 - Correlation and autoionization in Silicide Auger spectra [Articolo su rivista]
Bisi, Olmes; DEL PENNINO, Umberto; L., Grandi; Valeri, Sergio; Xi, Yiming
abstract

We measured the Auger CVV lineshape in nickel and chromium silicides and computed the corresponding two-particle spectra. A reasonable agreement between the measured and calculated spectra is obtained if correlation effects are taken into account. The autoionization contribution to the CVV Auger lines in chromium and in chromium silicides was also determined by comparing X-ray-excited spectra with electron-excited spectra and by studying the losses in the absorption edge region.


1986 - Electrical and structural characterization of Nb - Si thin film alloys [Articolo su rivista]
Nava, Filippo; P., Psaras; H., Takai; K. N., Tu; Valeri, Sergio; Bisi, Olmes
abstract

The structural and electrical properties of a Nb-Si thin alloy film as a function of temperature have been studied by Auger electron spectrometry, Rutherford backscattering spectroscopy, transmission electron microscopies, and in situ electrical resistivity and Hall coefficient measurements. The NbSi2,8 films were deposited by double electron-gun coevaporation onto oxidized silicon. For electrical measurements samples of a van der Pauw pattern were made through metallic masks. In the as-deposited state the coevaporated alloy film was amorphous. Upon annealing a precipitous drop in resistivity near 270°C has been determined to be the amorphous to crystalline phase transformation. The kinetics of the transformation has been determined by isothermal heat treatment over the temperature range of 224°C to 252°C. An apparent activation energy of 1.90 eV has been measured. The nucleation and growth kinetics in the crystallization process show a change in the power of time dependence from 5.5 to 2.4. The microstructures of films at various states of annealing have been correlated to the resistivity change. The crystalline NbSi2 shows an anomalous metallic behavior. The resistivity (p) versus temperature curve has a large negative deviation from linearity (dfl) and it approaches a saturation value (psat) as temperature increases. The resistivity data are fitted by two empirical expressions put forth to explain the resistivity behavior in A15 superconductors at low and high temperatures. One is based on the idea that ideal resistivity must approach some limiting value in the regime where the mean free path becomes comparable to the interatomic spacing and the other is based on a selective electron-phonon assisted scattering. In spite of the wide temperature range of analysis, it is not possible to choose one of them due to the fact that the best fit in both cases is nearly the same. The Hall coefficient (RH) changes sign from negative above ~250°K to positive below ~ 250°K.


1986 - Importance of Coulomb correlation in Silicide spectra [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Bisi, Olmes; DEL PENNINO, Umberto; Valeri, Sergio; Xi, Yiming
abstract

We present an experimental and theoretical study of the valence band spectra of Ni and Cr silicides. Evidence is provided that the mare features of both photoemission and Auger lineshapes cannot be understood if correlation effects are not included into the theoretical treatment


1986 - L2,3 Absorption Edges in Ni2Si [Articolo su rivista]
DEL PENNINO, Umberto; C., Mariani; VALERI, Sergio; G., Ottaviani; M. G., Betti; NANNARONE, Stefano; M., De Crescenzi
abstract

From the modifications in the Ni L2,3 absorption edges in the pure metal and in Ni2Si, information on the electronic structure of the silicide is deduced.


1985 - AES Study of Room Temperature Oxygen Interaction with Near Noble Metal-Silicon Compounds Surface [Articolo su rivista]
Valeri, Sergio; DEL PENNINO, Umberto; P., Lomellini; Ottaviani, Giampiero
abstract

We have carried out a comparative study of room temperature (RT) oxidation of near noble metal silicides similar in stoichiometry (M2Si) and electronic structure. Core-valence-valence (CVV) Auger line measurements on Ni2Si, Pd2Si and Pt2Si surfaces before and after exposure to 104 L of oxygen were performed. We compare the results with those for the oxidation of pure Si. In general oxygen interacts with Si atoms only. In Ni2Si, however, features ascribed to the onset of oxidation of Ni atoms appear in the Ni(MVV) line. In the Ni2Si and Pd2Si case, the Si reaction rate is increased with respect to that of pure Si, the strongest oxidation enhancement being obtained in Ni2Si. However, Si cannot be oxidized to SiO2 in these conditions; a SiOx (x &lt; 2) phase is formed. The Pt2Si oxidation behaviour is close to that of pure Si. The different catalytic effect of Ni, Pd and Pt on Si oxidation has been discussed. We conclude that the main effect of metal is to by-pass the kinetic bottleneck of pure Si oxidation, i.e. the dissociation of the O2 molecule at the silicide/gas interface. The effect of different exposure procedures was also underlined.


1985 - Real-space determination of atomic structure and bond relaxation at the NiSi2-Si(111) interface [Articolo su rivista]
E. J., van Loenen; J. W. M., Frenken; J. F., van der Veen; Valeri, Sergio
abstract

The atomic structure at the interface of a 180° rotated single-crystal NiSi2 film on Si(111) has been determined by a new thin-film ion-channeling method, using ultrahigh depth resolution. The Ni atoms at the interface are found to be sevenfold coordinated. The bonds across the interface are slightly contracted.


1985 - X-ray absorption near-edge structure and extended x-ray absorption fine-structure investigation of Pd silicides [Articolo su rivista]
M., De Crescenzi; E., Colavita; DEL PENNINO, Umberto; P., Sassaroli; VALERI, Sergio; C., Rinaldi; L., Sorba; NANNARONE, Stefano
abstract

X-ray absorption spectroscopy is used to investigate the near-L-edge structures of Pd in pure Pd and bulk Pd2Si and PdSi silicides. Possible many-body effects are suggested to explain the apparent discrepancy between the occurrence of L2,3 white lines and the 4d hole filling in Pd silicides. The interpretations of the extended fine structures and of the near-edge features are correlated with each other in order to find common support for a dynamical relaxation model for the L2,3 deep core holes of Pd silicides.


1984 - Correlation effects in valence band spectra of Nickel Silicides [Articolo su rivista]
Bisi, Olmes; CALANDRA BUONAURA, Carlo; DEL PENNINO, Umberto; P., Sassaroli; Valeri, Sergio
abstract

We discuss the role of the electronic correlation in determining the line shapes of valence-band photoemission and Auger spectra of Ni silicides. Theoretical one-particle spectra, with correlation effects included by a perturbation approach to a degenerated Hubbard Hamiltonian, are compared with experimental energy-distribution curves. It is shown that for these compounds the electronic correlation is of crucial importance for describing valence-band photoemission. We also present the experimental line shapes of Ni L3VV transitions and compare them with theoretical two-hole spectral densities. It is concluded that a band-theoretical approach can account for some important features of the experimental Auger spectra, but multiplet effects should be included for a more detailed description.


1984 - Oxygen Chemisorption and Oxide Formation on Ni Silicides Surfaces at Room Temperature [Articolo su rivista]
Valeri, Sergio; DEL PENNINO, Umberto; P., Lomellini; P., Sassaroli
abstract

The first stages of oxygen interaction with Ni silides surface up tp complete oxide formation have been investigated by electron spectroscopy


1983 - Effects of chemical environment in the lineshape of Silicon LVV Auger spectra of Nickel Silicides [Articolo su rivista]
DEL PENNINO, Umberto; P., Sassaroli; Valeri, Sergio; Bertoni, Carlo Maria; Bisi, Olmes; CALANDRA BUONAURA, Carlo
abstract

The Si L2,3VV Auger lineshape in nickel silicides of different compositions has been studied both experimentally and theoretically with the purpose of understanding the behaviour of silicon states in compounds where the Si atom has different local chemical environments. The experimental spectra provide evidence of significant modifications in the lineshapes, which are associated with changes in the distribution of the valence electrons. These modifications can be explained by theoretical calculations based on a single-particle description of the bulk electronic structure. Some discrepancies between theory and experiments can be attributed to approximations in the treatment of the final state of the Auger electron and to the neglect of surface effects in the electronic structure calculation.


1983 - Ion-beam-induced modification of Ni silicides investigated by Auger-electron spectroscopy [Articolo su rivista]
Valeri, Sergio; DEL PENNINO, Umberto; P., Sassaroli; G., Ottaviani
abstract

Surface modifications induced by Ar bombardment on Ni silicides have been investigated with the use of Auger-electron spectroscopy. The compounds considered are all the six predicted in the Ni-Si equilibrium phase diagram. The relative Si to Ni concentration varies between the surface and at a depth of a few tens of angstroms; the surface region is richer in Si than the near-surface region. Changes in the ion-beam energy in the range 0.5-5 keV also significantly affect the compositional and electronic properties of the surface and near-surface regions. By changing the ion-beam energies, the silicide surface can be modified in a controlled and reversible way, leading in some cases to the formation of a surface layer with composition and chemical properties characteristic of higher-Si-content Si-rich silicides. Surface segregation seems the dominant mechanism in the observed Si enrichment, and in the final product the chemical bonding plays an important role.


1983 - Oxidation behaviour of nickel silicides investigated by AES and XPS [Articolo su rivista]
Valeri, Sergio; DEL PENNINO, Umberto; P., Sassaroli
abstract

The oxygen interaction with Ni silicide surfaces in a wide range of composition, from NiSi2 to Ni3Si, has been investigated at room temperature by AES and XPS techniques. It has been found that the Si oxidation is enhanced over that of pure Si in all the Ni-Si compounds, and that the strongest oxidation occurs in Ni richer silicides. In general, oxygen bonds with Si leaving the metal unoxidized, but in Ni3Si evidence for the occurrence of Ni oxidation has been found at high exposures.


1982 - ELS INVESTIGATION OF THE SI-PD INTERFACE [Articolo su rivista]
DEL PENNINO, Umberto; P., Sasaroli; Valeri, Sergio; Nannarone, Stefano
abstract

The formation of the Si-Pd interface has been monitored by means of Electron Energy Loss Spectroscopy.


1982 - Spin‐polarized photoelectron emission study of AlxGa1−xAs alloys grown by molecular beam epitaxy [Articolo su rivista]
F., Ciccacci; S. F., Alvarado; Valeri, Sergio
abstract

We report on spin‐polarized photoelectron emission and quantum yield measurements performed on AlxGa1−x As alloys (0.08⩽x⩽0.65) which were grown by molecular beam epitaxy and activated in situ to negative electron affinity by exposure to Cs and O2. We investigate the electronic structure of the alloys near the band‐gap region. These alloys can be used as high‐ efficiency spin‐polarized photoelectron sources at photon energies matching convenient light sources.


1982 - Valence photoemission study of temperature dependent reaction products in Ni-Si interfaces and thin films [Articolo su rivista]
I., Abbati; L., Braicovich; B., De Michelis; DEL PENNINO, Umberto; Valeri, Sergio
abstract

Si(lll)-Ni interfaces were prepared by deposition of Ni onto Si(lll) held at liquid nitrogen temperature (LNT) and successively heated. We give angle integrated photoemission results (hν=21.2 eV) at LNT and at room temperature as a function of coverage (up to 10 monolayers). We also give photoemission results for bulk Ni2Si and NiSi prepared “in situ” by interdiffusion. The results are discussed in terms of the properties of the products formed in the interface reacted region.


1981 - Interlayer water and swelling properties of monoionic montmorillonites [Relazione in Atti di Convegno]
Alietti, Andrea; Brigatti, Maria Franca; DEL PENNINO, Umberto; Mazzega, Ezio; Valeri, Sergio; Poppi, Luciano
abstract

nd


1981 - Interlayer water and swelling properties of monoionic montmorillonites [Articolo su rivista]
DEL PENNINO, Umberto; Mazzega, Ezio; Valeri, Sergio; Alietti, Andrea; Brigatti, Maria Franca; Poppi, Luciano
abstract

The dehydration of two montmorillonites saturated by 10 different cations is investigated by X-ray spectroscopy, in different conditions of temperature and environmental relative humidity. We consider, in particular, the cation “solvatation” water and propose a model, on the basis of which we show that, in the case of the 15-Å structure the stability of the H20-cation-silicate complex depends essentially on the electrostatic energy of the water molecule dipole in the cation field. In the 12-A case, on the other hand, sometimes the energy change associated with the redistribution of the electrical charges of the cation and of the silicate seems to prevail, depending on the interlayer spacing. We also show a correlation between cationic radius and closest interlayer spacing in the conditions of extreme dehydration.


1981 - Spin polarized photoemission from molecular beam epitaxy-grown Be doped GaAs [Articolo su rivista]
Alvarado, S. F.; Ciccacci, F.; Valeri, Sergio; Campagna, M.; Feder, R.; Pleyer, H.
abstract

We have made a comparative study of the performance of spin polarized photocathodes based on molecular beam epitaxy (MBE) grown GaAs with Be-doping and on liquid phase epitaxy (LPE) grown GaAs with Zn-doping. The experiments were performed on GaAs (100), (110), and (111) surfaces atT-300 K. The photoelectron spin polarization (P) of the MBE-photocathodes is face dependent reaching 49% for the (111) surface, a value close to P=50% predicted by theory. In contrast for the LPE-photocathodes P is significantly lower (19≲P≲36%). Possible causes for the higher polarization of the MBE-photocathodes are investigated. The influence of the Zn- and Be-dopant is elucidated by theoretical model calculations which shows that replacing the Zn-dopant by Be reduces significantly the depolarization at very low temperatures, but not at room temperature. It is therefore concluded that theinterface region between the GaAs substrate and the MBE-GaAs layer grown on top of it strongly influences the performance of the source.


1981 - The oxygen effect in the growth kinetics of platinum silicides [Articolo su rivista]
F., Nava; Valeri, Sergio; G., Majni; A., Cembali; G., Pignatel; G., Queirolo
abstract

Silicide formation is strongly affected by the presence of oxygen contained in the metal film. A study has been performed by heating at various times and temperatures samples constituted by a thin Pt film sputter deposited on a silicon substrate. Four different oxygen concentrations were implanted in the Pt film ranging from 1 to 0.1 at.%. The presence of oxygen slows down the growth rate of Pt2Si. At 315 °C in the sample with an oxygen concentration of 1 at.%, about 1000 Å of Pt2Si are formed after 20 min of annealing. During the silicide formation, the oxygen originally contained in the platinum films segregates at the Pt2Si/Pt interface. This accumulation acts as a barrier for the transport of Pt and a new phase, PtSi, can form at the Pt2Si/Si interface. The dose necessary to stop the Pt2Si reaction is about (2–4)×1015 at./cm2 at 315 °C.


1980 - Energy band associated with dangling bonds in silicon [Articolo su rivista]
S., Mantovani; DEL PENNINO, Umberto; Valeri, Sergio
abstract

High-frequency capacitance measurements as a function of applied bias have been performed between 77 K and 250 K on Schottky Pd2Si-Si diodes, obtained from previously deformed n-silicon slices. Having used low dislocation densities and high-resistivity silicon we were able to check, in the range of temperatures investigated, the validity of the half-filled-band model, which assumes the existence of dangling bonds associated with edge-type dislocation. Good aggrement between theory and experiments was found, confirming that, with the edge dislocation in silicon, it can be associated with a one-dimensional energy band, of width 0.24 eV and a neutral level E0 at 0.40 eV above the top of the valence band; it was also confirmed that the edge dislocations are surrounded by cylindrical charge clouds, whose effective radius is comparable with the Debye screening length.


1980 - The electronic properties of the Si (111) - transition metals interfaces [Articolo su rivista]
I., Abbati; L., Braicovich; B., DE MICHELIS; Bisi, Olmes; CALANDRA BUONAURA, Carlo; DEL PENNINO, Umberto; Valeri, Sergio
abstract

We present the results of a joint experimental and theoretical investigation of the electronic properties of Silicon – transition metal interfaces. We carried out ultraviolet photoemission experiments on Si-Pt, Si-Ni and Si-Pd interfaces at different coverages. Our results indicate that these interfaces are reactive. Comparison with semi-empirical LCAO calculations allows to understand the main features of the observed spectra.


1980 - Viscosity of the Ice Surface Layer [Articolo su rivista]
Mantovani, Silverio; Valeri, Sergio; Loria, Arturo; DEL PENNINO, Umberto
abstract

By means of strain-gauges the viscous behaviour of the quasi-liquid layer at the Ice Ih surface below 0 C has been measured.


1978 - LIQUID-LIKE LAYER AT ICE SURFACE - DIRECT EXPERIMENTAL-EVIDENCE [Articolo su rivista]
Valeri, Sergio; S., Mantovani
abstract

Direct experimental proof of the existence of a liquid-like layer on the surface of ice crystals were obtained from measurements of the coefficient of thermal expansion of ice, by eans of strain gauges frozen on the surface. The liquid-like layer was found to exist in an appreciable temperature range below melting point.