Sergio D'ADDATO - personale UniMoRe

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Sergio D'ADDATO

Professore Associato
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

Pubblicazioni

2024 - Initial stages of growth and electronic properties of epitaxial SrF2 thin films on Ag(1 1 1) [Articolo su rivista]
Borghi, M.; Mescola, A.; Paolicelli, G.; Montecchi, M.; D'Addato, S.; Vacondio, S.; Bursi, L.; Ruini, A.; Doyle, B. P.; Grasser, T.; Pasquali, L.
abstract

2023 - Injecting Electrons into CeO2 via Photoexcitation of Embedded Au Nanoparticles [Articolo su rivista]
Spurio, E.; Pelli Cresi, J. S.; Ammirati, G.; Pelatti, S.; Paladini, A.; D'Addato, S.; Turchini, S.; O'Keeffe, P.; Catone, D.; Luches, P.
abstract

The electron injection efficiency and the steady state absorptance at different photon energies for a composite system made of Au NPs embedded in a cerium oxide matrix are reported. Cerium oxide can be coupled with plasmonic nanoparticles (NPs) to improve its catalytic properties by visible-light absorption. The present work is a study of the ultrafast dynamics of excited states induced by ultraviolet and visible-light excitation in Au NPs combined with cerium oxide, aimed at understanding the excitation pathways. The data, obtained by femtosecond transient absorption spectroscopy, show that the excitation of localized surface plasmon resonances (LSPRs) in the Au NPs leads to an ultrafast injection of electrons into the empty 4f states of the surrounding cerium oxide. Within the first few picoseconds, the injected electrons couple with the lattice distortion forming a polaronic excited state, with similar properties to that formed after direct band gap excitation of the oxide. At sub-picosecond delay times, we observed relevant differences in the energetics and the time dynamics as compared to the case of band gap excitation of the oxide. Using different pump energies across the LSPR-related absorption band, the efficiency of the electron injection from the NPs into the oxide was found to be rather high, with a maximum above 30%. The injection efficiency has a different trend in energy as compared to the LSPR-related static optical absorptance, showing a significant decrease in low energies. This behavior is explained considering different deexcitation pathways with variable weight across the LSPR band. The results are important for the design of materials with high overall solar catalytic efficiency.

2023 - Suppression of grain boundary contributions on carrier mobility in thin Al-doped ZnO epitaxial films [Articolo su rivista]
Magrin Maffei, Riccardo; di Bona, Alessandro; Sygletou, Maria; Bisio, Francesco; D'Addato, Sergio; Benedetti, Stefania
abstract

2022 - Adhesion, mobility and aggregation of nanoclusters at surfaces: Ni and Ag on Si, HOPG and graphene [Articolo su rivista]
D'Addato, S.; Perricone, F.; Paolicelli, G.
abstract

Abstract: An experimental investigation of Ag and Ni nanoparticles (NPs) deposited on Silicon with its native oxide, on highly oriented pyrolytic graphite and on graphene flakes is reported. The NPs were physically synthesized with a magnetron based gas aggregation source and the produced beam was mass-filtered and deposited in vacuum on the substrates. The study was concentrated on the morphology for the different cases, shedding some light on the interaction of pre-formed NPs with surfaces, a crucial aspect both of technological and scientific relevance. The nature of adhesion can be strongly influenced by the intrinsic properties of the surface (like for instance the energetics of interaction between the NP surface atoms and the first layers of the substrate) and/or the extrinsic properties, like the presence of defects, step edges, impurities and other irregularities. After adhesion, the NPs mobility and their mutual interaction are very relevant. In this work, the study was concentrated on NP/surface morphology, by using atomic force microscopy, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Article highlights: Morphology of physically synthesized metal Nano-Particles (NPs) on Si, HOPG and Graphene was investigated. The NPs were pure Ag and Ni. Coalescence, diffusion and self-aggregation and preferential adhesion were observed, with possible applications in sensor technology.Possible explanations are: NP softness, NP/surface bonding interaction and presence of contaminant species molecules between NP.

2022 - Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces [Articolo su rivista]
Vikatakavi, Avinash; Benedetti, Stefania; Righi, Giulia; Magri, Rita; D'Addato, Sergio; Luches, Paola; Selloni, Annabella
abstract

2022 - Loading with Biomolecules Modulates the Antioxidant Activity of Cerium-Doped Bioactive Glasses [Articolo su rivista]
Lusvardi, Gigliola; Fraulini, Francesca; D'Addato, Sergio; Zambon, Alfonso
abstract

: In order to identify new bioactive glasses (BGs) with optimal antioxidant properties, we carried out an evaluation of a series of cerium-doped BGs [Ce-BGs─H, K, and mesoporous bioactive glasses (MBGs)] loaded with different biomolecules, namely, gallic acid, polyphenols (POLY), and anthocyanins. Quantification of loading at variable times highlighted POLY on MBGs as the system with the highest loading. The ability to dismutate hydrogen peroxide (catalase-like activity) of the BGs evaluated is strongly correlated with cerium doping, while it is marginally decreased compared to the parent BG upon loading with biomolecules. Conversely, unloaded Ce-BGs show only a marginal ability to dismutate the superoxide anion (SOD)-like activity, while upon loading with biomolecules, POLY in particular, the SOD-like activity is greatly enhanced for these materials. Doping with cerium and loading with biomolecules give complementary antioxidant properties to the BGs investigated; combined with the persistent bioactivity, this makes these materials prime candidates for upcoming studies on biological systems.

2022 - Morphology and Optical Properties of Gas-Phase-Synthesized Plasmonic Nanoparticles: Cu and Cu/MgO [Articolo su rivista]
D’Addato, Sergio; Lanza, Matteo; Boiani, Anthea; Spurio, Eleonora; Pelatti, Samuele; Paolicelli, Guido; Luches, Paola
abstract

In this paper, an investigation of the properties of Cu and Cu/MgO nanoparticles (NPs) is presented. The NPs were obtained with gas-phase synthesis, and the MgO shells or matrices were formed via the co-deposition method on inert substrates. SEM and AFM were used to investigate the NP morphology on Si/SiOx, quartz, and HOPG. The Cu NPs revealed flattening of their shape, and when they were deposited on HOPG, diffusion and formation of small chains were observed. The embedding of Cu NPs in MgO was confirmed by TEM and EDX maps. XPS showed that Cu was in its metallic state, regardless of the presence of the surrounding MgO. UV–Vis revealed the presence of an intense localized surface plasmon resonance (LSPR) for Cu/MgO and for “bare” NPs. These results confirmed the role of MgO as a protective transparent medium for Cu, and the wavelength position of the LSPR in the Cu/MgO system was consistent with calculations. The wavelength position of the LSPR observed for “bare” and post-oxidized Cu NPs was probably affected by the formation of copper oxide shells after exposure to air. This study paves the way for the use of Cu/MgO NPs as plasmonic nanomaterials in applications such as photovoltaics and sensor technology.

2021 - Ag/mgo nanoparticles via gas aggregation nanocluster source for perovskite solar cell engineering [Articolo su rivista]
Caleffi, M.; Mariani, P.; Bertoni, G.; Paolicelli, G.; Pasquali, L.; Agresti, A.; Pescetelli, S.; Carlo, A. D.; De Renzi, V.; D'Addato, S.
abstract

Nanocluster aggregation sources based on magnetron-sputtering represent precise and versatile means to deposit a controlled quantity of metal nanoparticles at selected interfaces. In this work, we exploit this methodology to produce Ag/MgO nanoparticles (NPs) and deposit them on a glass/FTO/TiO2 substrate, which constitutes the mesoscopic front electrode of a monolithic perovskite-based solar cell (PSC). Herein, the Ag NP growth through magnetron sputtering and gas aggregation, subsequently covered with MgO ultrathin layers, is fully characterized in terms of structural and morphological properties while thermal stability and endurance against air-induced oxidation are demonstrated in accordance with PSC manufacturing processes. Finally, once the NP coverage is optimized, the Ag/MgO engineered PSCs demonstrate an overall increase of 5% in terms of device power conversion efficiencies (up to 17.8%).

2021 - Ultrafast Dynamics of Plasmon-Mediated Charge Transfer in Ag@CeO2 Studied by Free Electron Laser Time-Resolved X-ray Absorption Spectroscopy [Articolo su rivista]
Pelli Cresi, Jacopo Stefano; Principi, Emiliano; Spurio, Eleonora; Catone, Daniele; O'Keeffe, Patrick; Turchini, Stefano; Benedetti, Stefania; Vikatakavi, Avinash; D'Addato, Sergio; Mincigrucci, Riccardo; Foglia, Laura; Kurdi, Gabor; Nikolov, Ivaylo P; De Ninno, Giovanni; Masciovecchio, Claudio; Nannarone, Stefano; Kopula Kesavan, Jagadesh; Boscherini, Federico; Luches, Paola
abstract

Expanding the activity of wide bandgap semiconductors from the UV into the visible range has become a central goal for their application in green solar photocatalysis. The hybrid plasmonic/semiconductor system, based on silver nanoparticles (Ag NPs) embedded in a film of CeO2, is an example of a functional material developed with this aim. In this work, we take advantage of the chemical sensitivity of free electron laser (FEL) time-resolved soft X-ray absorption spectroscopy (TRXAS) to investigate the electron transfer process from the Ag NPs to the CeO2 film generated by the NPs plasmonic resonance photoexcitation. Ultrafast changes (<200 fs) of the Ce N4,5 absorption edge allowed us to conclude that the excited Ag NPs transfer electrons to the Ce atoms of the CeO2 film through a highly efficient electron-based mechanism. These results demonstrate the potential of FEL-based TRXAS measurements for the characterization of energy transfer in novel hybrid plasmonic/semiconductor materials.

2021 - Zno thin films growth optimization for piezoelectric application [Articolo su rivista]
Polewczyk, V.; Magrin Maffei, R.; Vinai, G.; Lo Cicero, M.; Prato, S.; Capaldo, P.; Dal Zilio, S.; Di Bona, A.; Paolicelli, G.; Mescola, A.; D'addato, S.; Torelli, P.; Benedetti, S.
abstract

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.

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 - Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation [Articolo su rivista]
Benedetti, S.; Righi, G.; Luches, P.; D'Addato, S.; Magri, R.; Selloni, A.
abstract

We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce3+ ions, O vacancies, and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria toward H2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electrochemical devices.

2020 - Ultrafast Formation of Small Polarons and the Optical Gap in CeO2 [Articolo su rivista]
Pelli Cresi, J. S.; Di Mario, L.; Catone, D.; Martelli, F.; Paladini, A.; Turchini, S.; D'Addato, S.; Luches, P.; O'Keeffe, P.
abstract

The ultrafast dynamics of excited states in cerium oxide are investigated to access the early moments of polaron formation, which can influence the photocatalytic functionality of the material. UV transient absorbance spectra of photoexcited CeO2 exhibit a bleaching of the band edge absorbance induced by the pump and a photoinduced absorbance feature assigned to Ce 4f → Ce 5d transitions. A blue shift of the spectral response of the photoinduced absorbance signal in the first picosecond after the pump excitation is attributed to the dynamical formation of small polarons with a characteristic time of 330 fs. A further important result of our work is that the combined use of steady-state and ultrafast transient absorption allows us to propose a revised value for the optical gap for ceria (Eog = 4 eV), significantly larger than usually reported.

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 - 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 - Ultrafast electron-lattice thermalization in copper and other noble metal nanoparticles [Articolo su rivista]
Mongin, Denis; Maioli, Paolo; Burgin, Julien; Langot, Pierre; Cottancin, Emmanuel; D'Addato, Sergio; Canut, Bruno; Treguer, Mona; Crut, Aurélien; Vallée, Fabrice; Del Fatti, Natalia
abstract

Using time-resolved ultrafast pump-probe spectroscopy we investigated the electron-lattice energy transfer in small copper nanospheres with diameters ranging from 3.2 to 23 nm, either embedded in a glass or dispersed in a solvent. Electron-lattice scattering rate is shown to increase with size reduction, in agreement with our previous results obtained on gold and silver nanoparticles in the low excitation regime. We attribute this effect to the reduction of the screening efficiency of electron-phonon interactions close to the nanoparticle surface. To understand the discrepancy between the results on the electron-lattice scattering in different metals reported in the literature (reduction, no dependence or increase with nanoparticle size), we discuss the experimental conditions required for the accurate determination of electron-lattice energy transfer time from time-resolved investigations in the weak and strong excitation regimes and present power-dependent experiments on gold nanospheres in solution. Our findings are derived from a theoretical analysis based on the two-temperature model predictions and on a complete modeling of the nanoparticle transient extinction cross-section through the resolution of Boltzmann equation in the presence of hot electrons.

2018 - Low pressure bottom-up synthesis of metal@oxide and oxide nanoparticles: control of structure and functional properties [Articolo su rivista]
D’Addato, Sergio; Spadaro, Maria Chiara
abstract

Experimental activity on core@shell, metal@oxide, and oxide nanoparticles (NPs) grown with physical synthesis, and more specifically by low pressure gas aggregation sources (LPGAS) is reviewed, through a selection of examples encompassing some potential applications in nanotechnology. After an introduction to the applications of NPs, a brief description of the main characteristics of the growth process of clusters and NPs in LPGAS is given. Thereafter, some relevant case studies are reported: Formation of native oxide shells around the metal cores in core@shell NPs. Experimental efforts to obtain magnetic stabilization in magnetic core@shell NPs by controlling their structure and morphology. Recent advancements in NP source design and new techniques of co-deposition, with relevant results in the realization of NPs with a greater variety of functionalities. Recent results on reducible oxide NPs, with potentialities in nanocatalysis, energy storage, and other applications. Although this list is far from being exhaustive, the aim of the authors is to provide the reader a descriptive glimpse into the physics behind the growth and studies of low pressure gas-phase synthesized NPs, with their ever-growing potentialities for the rational design of new functional materials.

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.

2018 - Scanning tunneling microscopy and photoemission studies of self-organised Ag nanostructures on the N-modified Cu(001) surface [Articolo su rivista]
Finetti, Paola; Ferrari, Luisa; D'Addato, Sergio
abstract

There has been a strong interest in methods of creating nanometer scale structures and in particular forming one and two-dimensional electron confinement structures. Self -organisation has been recognised as a promising way for growing large nanostructure domains with sufficiently regular size and spacing as required for the observation of quantum well states. We investigated the electronic properties and the morphology of Ag nano structures on c(2 x 2)-N/Cu(001) surface. This system is an example of epitaxial growth confined on nanoscale regions due to the occurrence of an adsorbate induced reconstruction. Using a combination of Scanning Tunneling Microscopy and Angle Resolved Photoemission Spectroscopy techniques we were able to determine the morphology and the growth mode of Ag on N -modified Cu(001) surface and the occurrence of quantum size effects in the electron properties of Ag nanostripes and nanoislands, evidenced in the observation of quantum well states.

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 - Editorial [Articolo su rivista]
Dinescu, M.; D'Addato, S.; Giapintzakis, I. J.; Wojcik, M.
abstract

2017 - In-Flight Analysis [Capitolo/Saggio]
D'Addato, Sergio
abstract

Some important experimental techniques used to characterize “in flight” nanoparticles (NPs) generated by gas physical synthesis are reviewed. This kind of studies presents some advantages with respect to the ones on supported NPs, as the NPs are free from interactions with the substrate, which can modify their original properties. The chapter is focused on the studies of structure, electronic and magnetic properties following a historical approach, from the first experiments performed in the 1970s and 1980s to very recent works with innovative methods and instruments, like third generation synchrotron radiation sources and free electron lasers. This activity opens new opportunities for fundamental studies and applications of these building blocks of advanced materials.

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 - 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 CeâO 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.

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 - 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 - Reducible oxides as ultrathin epitaxial films [Capitolo/Saggio]
Luches, Paola; D'Addato, Sergio
abstract

This chapter reviews and discusses recent work on two-dimensional films of reducible oxides supported on metal substrates. In general, peculiar chemical and structural phases, different from the bulk ones, can be stabilized depending on the oxygen chemical potential, on kinetic processes and on the specific substrate used. A peculiarity of reducible oxides is that the observed phases can often be reversibly transformed one into the other by applying reducing and oxidizing treatments.

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 - 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 - 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 - 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.

2014 - A brief tutorial for the STEM-CELL software [Articolo su rivista]
Grillo, Vicnenzo; Rotunno, Enzo; Campanini, Marco; Chiara Spadaro, Maria; D'Addato, Sergio
abstract

TEM and STEM analytical studies are going in toward a stronger involvement of computing for the image interpretation. Geometric Phase Analysis [1], for example, has proved to be a useful tool to evaluate the strain in different structures. TEM Image simulations are another very important tool to be compared with complicated structures to obtain quantitative interpretation of the contrast.

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.

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.

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 - Surface X-ray diffraction analysis of Fe nanostructured films grown on c(2×2)-N/Cu(100) [Articolo su rivista]
D'Addato, Sergio; F., Borgatti; R., Felici; P., Finetti
abstract

We report the results of a Surface X-Ray Diffraction (SXRD) study of Fe nanostructured films depositedon c(2×2)-N/Cu(100) at room temperature (RT), with Fe coverage ΘFe=0.5ML and ΘFe=1 ML. Thec(2×2)-N/Cu(100) surface is an example of self-organised system, that can be used for growth of arraysof metal nano-islands and organic molecules assemblies. We chose two different values of N coverage,ΘN=0.3 ML and ΘN=0.5 ML, the second value corresponding to N saturation. We monitored the presenceof surface diffraction peaks in hk scans and we performed Crystal Truncation Rods (CTR) analysiswith ROD fitting programme. In the case of ΘN=0.5 ML, i.e. at saturation coverage, the CTR could be fittedwith one surface domain with p4gm(2×2) symmetry. In the surface cell adopted, N atoms occupy fourfoldhollow sites, with Fe (intermixed with Cu) giving rise to a “clock” reconstruction previously observedon iron nitride films obtained by co-deposition and annealing. This result is an indirect confirmation of Nsurface segregation on top of the Fe films, occurring during the growth at RT. When subsaturation N coverage(ΘN=0.3 ML) is used as a substrate for Fe deposition, the best results could be obtained with amodel where two surface domains are present: the first one corresponds to a surface cell with Fe sittingin four-fold hollow sites on bare Cu areas, with possible interdiffusion in the second lattice. The seconddomain is assigned to growth of Fe on the N-covered square islands occurring once the bare Cu areasare fully covered. The SXRD analysis on N-covered surface domains shows that the mechanism of reconstructionand of N segregation on top layer is already active at RT for all N-coverage values.

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 - 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 - 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 - 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.

2008 - Growth and study of Ni nanoparticles films deposited on inert subtrates [Articolo su rivista]
D'Addato, S.; Gragnaniello, L.; Di Bona, A.; Rota, A.; Vavassori, P.; Valeri, S.
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.

2007 - Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis [Articolo su rivista]
D'Addato, Sergio; Gunnella, R; Borgatti, F; Felici, R; Finetti, P.
abstract

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage ΘFe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At ΘN < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe–Fe (or Fe–Cu) bond length values varying between R1 = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R1 = 2.58 ± 0.01 Å at ΘFe = 0.5 ML, ΘN = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe (R = 2.48 Å), and compatible with fcc Fe (R1 = 2.52 Å) and fcc Cu (R1 = 2.55 Å). At the Fe lowest coverage (ΘFe = 0.5 ML) the dependence of R1 on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (ΘN = 0.5 ML) the XAFS analysis has to keep into account the Fe–N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.

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.

2005 - Molecular orientation of 2-mercaptobenzoxazole adsorbed on Cu(1 0 0) surface [Articolo su rivista]
Bussolotti, F.; D'Addato, Sergio; Allegretti, F.; Dhanak, V.; Mariani, C.
abstract

The long range ordered p(2 × 2) self-assembled monolayer formed by dosing 2-mercaptobenzoxazole (MBO, C7H5NOS) on the Cu(1 0 0) surface at room temperature has been investigated by means of near edge X-ray adsorption fine structure (NEXAFS) technique at the sulphur K-edge. NEXAFS S–K-edge spectra have been recorded at different incident photon angles with respect to the Cu(1 0 0) surface, using linearly polarised synchrotron radiation. A clear spectral feature has been observed close to the absorption edge, with higher intensity at grazing incidence, assigned to the σ*(S–C) resonance, corresponding to the transition from the S 1s core level to the σ*(S–C) antibonding molecular state. The angular dependence of the σ*(S–C) resonance intensity clearly indicates that the S–C axis is nearly perpendicular to the surface (13° ± 13°), confirming the major role of S in bonding to the surface, and the influence of the π–π interaction between the molecules in driving the almost upright position at saturation coverage.

2005 - Self-assembly of an aromatic thiolate on Cu(1 0 0): The local adsorption site [Articolo su rivista]
Allegretti, F.; Woodruff, D. P.; Dhanak, V. R.; Mariani, C.; Bussolotti, F.; D'Addato, Sergio
abstract

The long-range ordered p(2 × 2) self-assembled monolayer formed by dosing 2-mercaptobenzoxazole (MBO, C7H5NOS) on the Cu(1 0 0) surface at room temperature has been structurally characterised by means of normal incidence X-ray standing wavefield absorption (NIXSW) at both (2 0 0) and (1 1 1) reflections. The results show that the S atom, through which the adsorbed molecular thiolate species bonds to the surface, has an interlayer spacing of 1.33 ± 0.05 Å relative to the extended bulk scatterer planes (2 0 0) parallel to the surface and occupies the four-fold coordinated hollow site, with no evidence of major reconstruction. These results are entirely consistent with previous findings for a number of thiolates formed on (1 0 0) noble metal surfaces. The role of the head-group to surface potential corrugation and intermolecular interactions in determining the adsorption site and ordering of this aromatic molecular adsorbate is discussed.

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 - 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 - 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.

2003 - Structure properties of nanostructured Fe films grown on c(2x2)N/Cu(100) self organised surface [Articolo su rivista]
Finetti, P.; D'Addato, Sergio; Felici, R.; Gunnella, R.; D'Addato, S.
abstract

The growth of Fe on the c(2x2)N/Cu(100) surface was studied with XAFS assisted by LEED. Previous STM work had shown that atomic N on Cu(100) forms periodic arrays of c(2x2) N islands with a size of about 5x5 nm channelling the growth of Fe onto the clean Cu areas. From XAFS multishell analysis we describe the growth in terms of fcc phase with strong lattice distortion, deviation from the layer by layer growth, and presence of surface complex phases, some with different geometry. The differences between the Fe growth on the clean and N saturated Cu(100) surface are explained in terms of Fe-N atoms interaction, and we could obtain the adsorption geometry of Fe on N-saturated surface.

2002 - Centenarians in good health conditions [Articolo su rivista]
Motta, M.; Maugeri, D.; Malaguarnera, M.; Capurso, A.; Colacicco, A. M.; Solfrizzi, V.; Bonafe', M.; Barbi, C.; Gaddi, A.; D'Addato, S.; Sangiorgi, Z.; Trabucchi, M.; Boffelli, S.; Rozzini, R.; Rapisarda, R.; Tomasello, F. B.; Bennati, E.; Ferito, L.; Frantone, A.; Zoccolo, A.; Perticone, F.; Nardi, L.; Berardelli, M.; De Benedictis, G.; Falcone, E.; De Luca, M.; Casotti, G.; Monti, D.; Petruzzi, E.; Sorbi, S.; Grassi, E.; Latorraca, S.; Bertolini, S.; Agretti, M.; Costelli, P.; Nicita Mauro, V.; Basile, G.; Mari, D.; Duca, F.; Terrazzi, P.; Bosi, E.; Manzoni, M.; Salvioli, G.; Baldeli, M. V.; Neri, M.; Cossarizza, A.; Troiano, L.; Pini, G.; Varricchio, M.; Gambardella, A.; Prolisso, G.; Frada', G.; Barbagallo, M.; Pollina, R.; Passeri, M.; Sansoni, P.; Lavagetto, G.; Ferrari, E.; Battegazzore, C.; Molla, G.; Senin, U.; Cherubini, A.; Polidori, M. C.; Marigliano, V.; Bauco, C.; Borriello, C.; Deiana, L.; Carru, C.; Pes, G. M.; Baggio, G.; Forconi, S.; Boschi, S.; Guerrini, M.; Fabris, F.; Cappa, G.; Ferrario, E.
abstract

The increased survival of the population represents the most relevant demographic phenomenon during the end of the 20th century. Aging of the population is correlated with two factors: (i) reduction of the birth rate, and (ii) decrease of mortality rate. The progressive increase of the advanced age suggested the necessity of analyzing absolutely new aspects and facts, namely the properties of centenarians. The true centenarian subjects are in good clinical conditions, therefore, we can call them healthy centenarians. This is the reason why we can classify the centenarians in 3 groups on the basis of the criteria evaluating the clinical and physical conditions of them (groups A, B, C, being in good, moderate or poor clinical conditions, respectively). The healthy centenarian subject had overcome the negative environmental factors, and therefore, his/her survival is accompanied by a slow and gradual loss of the reserves of functional capacities, i.e., the exhaustion of the factors being the true determinants of the longevity.

2002 - Electronic properties of CaF2 nanodimensional islands on Si(001): An MDS and UPS study [Articolo su rivista]
Pasquali, Luca; S., Suturin; N., Sokolov; G., Selvaggi; D'Addato, Sergio; Nannarone, Stefano
abstract

The joint use of metastable deexcitation spectroscopy and angle resolved ultraviolet photoemission gives a valuable contribution to the comprehension of the physical-chemical reactions occurring during the formation of an interface between different materials. This approach has been applied to study the evolution of the surface valence band of CaF, deposited on Si(001) at different temperatures. Under suitable growth conditions, CaF2 nanostructures of different shape and size can be obtained. Information regarding the chemical reactions taking place within the first deposited layer and the dependence of the electronic properties on film thickness was derived. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Growth of epitaxial Yb silicide on Si(100) studied by metastable atom deexcitation spectroscopy and photemission [Articolo su rivista]
Pasquali, L.; D'Addato, S.; Nannarone, S.
abstract

Films of different Yb thickness are deposited on Si(100) 2x1 and are annealed at increasing high temperature. The evolution of the valence-band electronic properties is followed with metastable deexcitation spectroscopy and photoemission. Different surface-structure phases are observed at progressively high annealing temperature. A 3x1 ordered phase is observed by low-energy electron diffraction (LEED) after annealing of the films at about 640 degreesC and it is associated to the formation of a stable silicide. Correspondingly, electronic states assigned to the Yb 6s-5d hybridized band and to Si 3s and 3p bands in the compound are observed. A further increase of the annealing temperature causes the system to evolve rapidly. At T>650 degreesC the LEED pattern shows a 3x2 periodicity. Above 700 degreesC, the films present a character similar to that of the Si substrate.

2002 - Surface electronic states of Yb silicide ultrathin films studied with He metastable deexcitation spectroscopy [Articolo su rivista]
D'Addato, Sergio; Pasquali, Luca; Nannarone, Stefano
abstract

We studied the growth of Yb silicide films on Si(100) with Metastable Deexcitation Spectroscopy (MDS) assisted by Low Energy Electron Diffraction (LEED). Different surface structure phases are observed at progressively high annealing temperature. Annealing at T<550 degreesC produces Yb-rich disordered compounds, with metallic character shown on MDS. A 3 X 1 ordered phase is observed by low energy electron diffraction (LEED) after annealing of the films at about 640 T and it is associated with the formation of a stable silicide. Correspondingly, electronic states assigned to the Yb 6s-5d hybridised band and to Si 3s and 3p bands in the compound are observed in MDS. A further increase in the annealing temperature causes the system to evolve rapidly. Above 700 degreesC, the films present a character similar to that of the Si substrate. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Variations in the lifetime of 3d hole states in ultrathin Fe films grown on Cu(100) deduced from the LMM Auger spectra of Fe [Articolo su rivista]
D'Addato, Sergio; Luches, P.; Gotter, R.; Floreano, L.; Cvetko, D.; Morgante, A.; Newton, A.; Martin, D.; Unsworth, P.; Weightman, P.
abstract

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe lms grown on Cu(100)have been measured using photon excitation energies above and below the L2 ionisation edge, and incoincidence with the Fe 2p3=2 and Fe 2p1=2 photoelectrons. A comparison of the relative intensities ofthe satellites and main Auger transitions indicates that the M4;5 holes of L3M4;5 double hole states arelocalised for longer than the L3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparableto that of L3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by theresults of the coincidence experiments. The reasons for the variation in the lifetime of M4;5 holes arediscussed in terms of changes in the number of Fe{Fe neighbours and the density of states at the Fermienergy in these systems.

2001 - An extended X-Ray Absorption Fine Structure study of Mn ultrathin films grown on Cu(100) [Articolo su rivista]
D'Addato, Sergio; Finetti, P.
abstract

Ultrathin Mn ®lms grown at room temperature on Cu(1 0 0) have been studied with extended X-ray absorption ®nestructure (EXAFS) assisted by low energy electron diraction (LEED). At a ®lm coverage H 0:5 monolayers (ML),corresponding to the presence of a c2 2 superstructure in the LEED pattern, we obtained values of the bond lengthand of the eective coordination number which are consistent with the formation of a surface MnCu alloy, in agreementwith previously published results. At increasing coverage the degradation of the LEED pattern with the disappearenceof the spots at H 6 ML and the absence of a clear multishell signal in the EXAFS results point towards a progressivedecrease of long-range crystal order, probably caused by the strain of Mn ®lms growing in a strongly distorted lattice.

2001 - EXAFS analysis of ultrathin Fe films grown on Ni(100) [Articolo su rivista]
D'Addato, Sergio; Marassi, L; Luches, P; Gazzadi, Gc; Pasquali, Luca; Verucchi, R; Finetti, P; Nannarone, Stefano
abstract

We performed an extended X-ray absorption fine structure (EXAFS) investigation of ultrathin Fe films grown in situ. on Ni(1 0 0) surface. From the first shell analysis of the EXAFS oscillations we obtained values of the bond length, of the effective coordination number and of the Debye-Waller factor that are compatible with a model where there is coexistence of face centred cubic phase (with tetragonal distortion) and of body centred cubic (bcc) phase in the films. We also performed theoretical simulations of the EXAFS data including signals from higher order shells with multiple scattering effects. From these simulations we obtained that the experimental data can be well reproduced with a linear combination of spectra, one reproducing the bulk bcc Fe structure and one obtained by a face centred tetragonal (fct) lattice structure with an interlayer distance d = 1.9 A. In the model the relative weights of the two spectra change with an increasing value for the weight corresponding to the bcc phase. At 20 ML, the weight for the fct phase (30-40%) is still considerable. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Formation of CaF2 nanostructures on Si(001) [Articolo su rivista]
Pasquali, Luca; D'Addato, Sergio; Selvaggi, G; Nannarone, Stefano; Sokolov, Ns; Suturin, Sm; Zogg, H.
abstract

Initial stages of epitaxial growth and formation of CaF2 nanostructures on Si(001) were studied. A variety of nanostructures were grown including ultrathin two-dimensional layers at 750 degreesC, quasi-one-dimensional stripes at 650 degreesC and well-ordered dots at lower growth temperatures. Atomic force microscopy and reflection high-energy electron diffraction were used to measure the nanostructure shape and lattice orientation. The evolution of the surface electronic structure under different growth conditions was studied by ultraviolet photoelectron spectroscopy and metastable de-excitation spectroscopy. The leading role of the wetting layer in high-temperature formation of the fluorite-silicon interface was established.

2001 - Growth of Yb silicide on Si(100): structure and electronic properties as a function of annealing temperature [Articolo su rivista]
Pasquali, Luca; D'Addato, Sergio; Turdi, D; Nannarone, Stefano
abstract

The growth of Yb on Si(100) from 1 to 12 ML as a function of the annealing temperature has been monitored with low energy electron diffraction (LEED) and the electronic properties were studied by He I photoemission and He metastable deexcitation spectroscopy (MDS). The annealing temperature (ranging from 590 degreesC to 640 degreesC) induces a 3 x 1 double domain LEED pattern. Correspondingly, a change in the electronic properties is observed, with the appearance of well defined s-p type features in MDS and UPS. This stage corresponds to the formation of a well ordered silicide film. At increasing temperature Yb is progressively diluted in a Si rich surface. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Interface magnetometry in a (Fe-6A/Ni-24A)(10) multilayer [Articolo su rivista]
Capelli, R; D'Addato, Sergio; Gazzadi, Gc; Pasquali, Luca; Verucchi, R; Nannarone, Stefano; Dudzik, E; Mirone, A; Sacchi, M.
abstract

The interplay between atomic geometry and magnetic properties of Fe atoms at the Fe-Ni(1 1 1) interface was studied. To this end dichroic optical absorption and reflectivity (both in energy and theta -2 theta angular scan mode) were carried out in the L-2,L-3 edge region of Fe in (Fe-6 (Angstrom)/Ni-24 (Angstrom)(1 1 1))(10) multilayer while the atomic geometry of the Fe site at the interface was studied for 1 atomic layer of Fe on Ni(1 1 1) by photo-electron diffraction tin angular scan mode and back-scattering regime). The Fe atom was found to be chemisorbed in an fee hollow site and to stack pseudomorphically to Ni(1 1 1). Its magnetic moment was found to be in the range 2.9 +/- 0.9-1.93 +/- 0.6 mu (B). Possibility of surface anisotropy is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Structural and magnetic properties of self-assembled nanoscale Fe islands on Cu(100) [Articolo su rivista]
P., Finetti; V. R., Dhanak; C., Binns; K. W., Edmonds; S. H., Baker; D'Addato, Sergio
abstract

We have grown nanoscale Fe islands on the c(2x2)N/Cu(100) surface at room temperature. Previous scanning tunneling microscopy studies have shown that adsorption of atomic N at subsaturation coverage forms periodic arrays of c(2x2) N islands each with a size of about 5x5 nm. At room temperature these islands can channel the growth of other transition metals such as Fe onto the remaining clean Cu areas. The result is a regular array of Fe islands separated by N-covered islands. The size and shape of the Fe islands depends both on Fe and N coverage. We have studied the structural properties of the Fe islands by means of low energy electron diffraction and Fe K-edge surface extended X-ray absorption fine structure. From these data we estimate that we are able to produce Fe column-like islands thicker than 10 atomic layers. The Fe atoms are arranged in an fee lattice with tetragonal distortion as observed in Fe/Cu(100) films. The magnetic properties of the Fe islands have been probed by means of magnetic linear dichroism in the angular distribution and magnetic circular dichroism of photoelectrons from the Fe 3p core levels. Our data from Fe islands shows that they can be remanently magnetised parallel or perpendicular to the surface after applying a field of B=0.3 T, indicating cubic anisotropy.

2000 - Growth of Fe ultrathin films on Ni(111): structure and electronic properties [Articolo su rivista]
D'Addato, Sergio; Pasquali, Luca; Gazzadi, Gc; Verucchi, R; Capelli, R; Nannarone, Stefano
abstract

We investigated the evolution during film growth of the electronic properties of Fe/Ni(111) ultrathin films with electron energy loss spectroscopy from the valence band and from the Fe L-2,L-3 ionization edges, relating them with structural modifications monitored with low energy electron diffraction. At ca. 4 monolayers of Fe a fee to bce phase transition was observed with the major changes in the electronic structure occurring in the 3d band region. (C) 2000 Elsevier Science B.V. All rights reserved.

2000 - Magnetic profile of Ni/Fe/Ni trilayers [Articolo su rivista]
G. B., Fratucello; A. M., Prandini; Luches, Paola; D'Addato, Sergio; A. J., Patchett; Nannarone, Stefano
abstract

The magnetic structure of Ni(1 1 1)/Fe/Ni trilayer was studied by Mossbauer spectroscopy. Two kinds of samples were studied differing each other in the substrate preparation, in the evaporation method of Fe-57 and in vacuum condition. Fabrication by thermal evaporation in HV on a Ni buffer layer grown on mica and by electron beam bombardment in UHV on cleaned Ni(1 1 1) single crystal was used. The Ni substrate morphology was characterised by STM. Similar magnetic structure was found for the two systems, consisting of two well-defined features of the hyperfine field occurring at high value (similar to 28 T) and low one (<8 T). The high-field component was assigned to Fe atoms close to Fe-Ni interface with high-spin ferromagnetic state, and the low-field component to Fe atoms inside the Fe layer with low-spin antiferromagnetic state.

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 - A Surface Extended X-ray Absorption Fine Structure investigation of Fe islands grown on c(2x2) N/Cu(100) surface [Articolo su rivista]
D'Addato, Sergio; Binns, C.; Finetti, P.
abstract

We present the results of a surface extended X-ray absorption fine structure (SEXAFS) investigation of the growthof Fe islands on the c(2×2) N/Cu(100) surface for different N doses. It is known from scanning tunnelling microscopy(STM) experiments that at subsaturation coverage adsorbed N atoms on Cu(100) form square islands of c(2×2)periodicity and about 5 nm×5 nm in size. These islands can self-organize in regular patterns. From the persistence ofthe c(2×2) low energy electron diffraction patterns obtained for any Fe and N coverage we deduce (in agreementwith previous STM results) that Fe islands grow on surface areas which are not passivated by N atoms. In this studywe estimate that we were able to produce Fe islands thicker than 10 atomic layers, a value higher than that observedusing STM. The first shell analysis of SEXAFS data shows that Fe atoms are arranged in an fcc lattice with tetragonaldistortion, as observed in Fe/Cu(100) films, with the formation of bilayers and/or coalescence in small islands at thelowest coverage studied (one monolayer).

1999 - The Structure of ?3x?3 R30° Iodine on Pd(111) Surface Studied by Normal Incidence X-Ray Standing Wavefield Absorption [Articolo su rivista]
V. R., Dhanak; A. G., Shard; D'Addato, Sergio; AND A., Santoni
abstract

The surface structure of 63=63.R308 I-Pd 111.has been investigated by normal incidence X-ray standing waveabsorption. The measurements show that the site of adsorption is at the three-fold fcc hollow, with a layer spacing of 2.16 A°above the surface. The location of the I atoms at the fcc hollow site is consistent with halogen adatom adsorption on theneighbouring fcc 111.transition metal surfaces.

1998 - Assessment of sense of taste in Italian centenarians [Abstract in Rivista]
Baggio, G.; Dalla Vestra, M.; Donazzan, S.; Barbagallo Sangiorgi, G.; Barbagallo, M.; Frada, G.; Bertolini, S.; Agretti, M.; Costelli, P.; Bosi, E.; Manzoni, M.; Tomasello, F. B.; Capurso, A.; Colacicco, A. M.; Solfrizzi, V.; Fabris, F.; Cappa, G.; Ferrario, E.; Forconi, S.; Guerrini, M.; Boschi, S.; Franceschi, C.; Cossarizza, A.; Monti, D.; Gaddi, A.; D'Addato, S.; Galletti, C.; Giarelli, L.; Cavalieri, F.; Stanta, G.; Mari, D.; Duca, F.; Ferrazzi, P.; Marigliano, V.; Bauco, C.; Cacciafesta, M.; Masotti, G.; Marchionni, N.; Petruzzi, E.; Mattace, R.; Motta, M.; Pansini, L.; Motta, L.; Rapisarda, R.; Receputo, G.; Passeri, M.; Fagnoni, F.; Sansoni, P.; Salvioli, G.; Baldelli, M. V.; Neri, M.; Senin, U.; Cherubini, A.; Polidori, M. C.; Trabucchi, M.; Boffelli, S.; Rozzini, R.; Varricchio, M.; Gambardella, A.; Paolisso, G.
abstract

The Italian multicentric study on centenarians (IMSC) was aimed at assessing the level of preservation of the sense of taste, and at estimating to what extent the recognition of various gustative stimuli can give satisfaction and information regarding the surrounding environment for the centenarians. Taste sensitivity has been qualitatively established in a group of 126 Italian centenarians (mean age 101.9 ± 1.4 years) and compared to that of a group of 100 elderly subjects (mean age 70.5 ± 5.0 years). All the individuals included in the study groups had a mini mental state examination (MMSE) score larger than 19. The results revealed that taste sensitivity is significantly reduced in the centenarians; as a matter of fact, the elderly control subjects are able to perceive taste stimuli in 84.25 ± 3.40% of the testing, while the centenarians were successful only in 74.23 ± 6.19% of the experiments (P < 0.001). Furthermore, elderly subjects can correctly recognize taste stimuli in 70% of the testings, while correct recognition amounted only to 49.25% in cases of centenarians assessed. In spite of these differences between the elderly and centenarians, the latter are still able to perceive and recognize taste stimuli adequately, and as a consequence, are able to obtain information on tastes and receive a sufficient sensory stimulation through the tasting pathway.

1998 - Charge transfer and redistribution in the formation of the K/GaP(110) interface: a photoelectron spectroscopy study [Articolo su rivista]
D'Addato, Sergio; P., Bailey; J. M. C., Thornton; D. A., Evans
abstract

We present the results of a photoelectron spectroscopy study of the K/GaP(110) interface grown at T = 120 K. We have investigated the system looking at the Ga 3d and P 2p core level spectra by analysing their lineshape. This analysis shows that the transferred charge in the adatom-substrate bonding formation affects the electronic and chemical environment of both surface Ga and P atoms. The results suggest relevant differences with the intuitive picture of strong ionic localized bonding between the alkali atom and surface Ga. They are instead more consistent with a delocalized redistributed valence charge between the adsorbate and the substrate. We also conclude that the system at Theta = 1 ML is metallic, with an increasing disorder and inhomogeneity at the highest coverage studied. The similarities and differences with the Na/GaP(110) and the other alkali metal/GaAs(110) systems are discussed.

1997 - Core level analysis of the K/GaP(110) interface [Articolo su rivista]
D'Addato, Sergio; P., Bailey; J. M. C., Thornton; D. A., Evans
abstract

The results of a photoelectron spectroscopy study of the K/GaP(110) interface grown at T=120 K are presented for the first time. The analysis shows a big core level shift at lower binding energies (Delta E=1.6 eV at a coverage theta=0.1 ML) for the P 2p core level spectra. We interpret this result in terms of a charge redistribution affecting both types of atoms of the first substrate layer, suggesting some differences from the intuitive picture of strong ionic localised bonding between the alkali and the surface Ga atom.

1997 - Removal of the clock reconstruction of Ni(100)-(2x2) p4g-N by coadsorption of K: A spot-profile-analysis low-energy-electron-diffraction and angle-resolved ultraviolet-photoemission-spectroscopy study [Articolo su rivista]
P., Finetti; M. J., Scantlebury; J. R., Mercer; D'Addato, Sergio; R., Mcgrath
abstract

Previous work has revealed that the (2x2)p4gstructure formed by adsorption of N on Ni(100) is a clock reconstruction. A coverage of 0.5 ML of N atoms located in alternate fourfold hollow sites on the surface results in an expansion of each hollow by means of rotation of the Ni atoms in alternate clockwise and counterclockwise directions. Here we report on the removal of this substrate reconstruction by coadsorption of K. The structural changes induced by the alkali metal on the N precovered Ni(100) surface have been monitored by means of spot-profile-analysis low-energy-electron diffraction. The chemisorption bonding of N on Ni(100) and K on Ni(100) has been investigated by means of angle-resolved ultraviolet-photoemission spectroscopy. The band structure of the adsorbate-induced states is presented. The changes in the dispersive behavior of the N-derived bands upon coadsorption of K have been related to the structural changes. Our data suggest a mechanism for the removal of the reconstruction based on the K-induced restoration of the screening of the repulsive forces between the topmost Ni atoms.

1997 - Structural and magnetic properties of Ni/gamma-Fe/Ni multilayers [Articolo su rivista]
A. M., Prandini; G. B., Fratucello; D'Addato, Sergio; Nannarone, Stefano
abstract

The crucial point in the characterization of magnetic interfaces is the knowledge of the morphology and the structure of the films. Ni/gamma-Fe/Ni multilayers were grown on three different buffer layers. Structure and morphology were investigated by X-ray diffraction and scanning tunneling microscope. Mossbauer spectroscopy was used to provide local information on both structure and magnetism of the buried Fe layers.

1997 - Surface phase transitions of Ge(111)c(2x8) studied by electron energy loss spectroscopy [Articolo su rivista]
Pasquali, Luca; D'Addato, Sergio; Tagliavini, L; Prandini, Am; Nannarone, Stefano
abstract

We report the results of an electron energy loss spectroscopy (EELS) investigation of the Ge(111)c(2 x 8) surface at different temperatures, covering the three different phases previously observed for this system (c(2 x 8)[-](1 x 1)[-] surface premelting). In order to discriminate between bulk and surface losses, we used different primary beam energies (50-500 eV). Four loss features are revealed to be affected by the temperature. Results are discussed on the basis of a comparison with previous optical spectroscopy data and of the dielectric theory of EELS applied to a slab geometry.

1996 - A Cooper minimum photoemission study of the Au95Pd5 alloy [Articolo su rivista]
D'Addato, Sergio; N. J., Brooks; J. M. C., Thornton; P., Unsworth; P., Weightman; L., Duo; M., Sancrotti
abstract

We present the results of a valence band (VB) photoemission study of a AU(95)Pd(5) alloy in a photon energy range covering the Cooper minima of the Pd 4d and Au 5d photoionization cross sections, (hv = 90-280 eV). The spectra show the presence of the Pd 4d-derived virtual bound state (VBS) which we find to lie between the Au 5d band and the Fermi level with a binding energy of 1.45 eV. The intensity of this peak goes through a minimum at hv = 140 eV. The results are discussed in terms of the spatial distortion of the Pd 4d wavefunction in the alloy. Subtractions between the alloy and metallic Au spectra at different photon energies reveal marked differences in the Au 5d part of the valence band. These differences are ascribed to modifications in the density of states caused by the presence of the impurity and to the variation in the photon energy dependence of the cross section of different parts of the VB spectrum.

1996 - Coverage-dependent azimuthal alignment of SO2 on Ag(110) [Articolo su rivista]
A., Gutierrezsosa; J. F., Walsh; C. A., Muryn; P., Finetti; G., Thornton; A. W., Robinson; D'Addato, Sergio; S. P., Frigo
abstract

The polarisation dependence of S R-edge NEXAFS has been used to study the adsorption geometry of SO, on Ag(110). The molecular plane is azimuthally aligned along the [001] azimuth at 1/3 monolayer coverage, but rotated from [001] by about 55 degrees at 1/2 monolayer, with the molecule upright at both coverages.

1995 - AN INVESTIGATION OF THE UNOCCUPIED P-SYMMETRY STATES IN AG-CONTAINING AND PD-CONTAINING SYSTEMS VIA L-](L-1) HIGH-RESOLUTION NEAR-EDGE X-RAY-ABSORPTION SPECTROSCOPY [Articolo su rivista]
L., Duo; R., Cosso; G., Curro; D'Addato, Sergio; D. A., Gregory; P., Unsworth; P., Weightman; M., Sancrotti
abstract

In this work, the first systematic study of the low-lying, unoccupied p-derived states in Pd- and Ag- containing systems is reported. Ag and Pd M(4,5) near-edge X-ray absorption spectroscopy measurements from bulk Ag and Pd binary alloys, namely Cu50Pd50, Cu80Pd20, Ag50Pd50 and Ag80Pd20, and of the Ag/Si(111)2 x 1 interface were performed. Recent studies have indicated that this kind of spectroscopy can provide valuable information on the unoccupied p-projected density of states, via a 3d --> 5p dipole allowed transition.

1995 - AN X-RAY-ABSORPTION SPECTROSCOPY STUDY OF THE K/GAAS(110) INTERFACE [Articolo su rivista]
D'Addato, Sergio; D. A. C., Gregory; A. W., Robinson; A., Santaniello; C., Hellwig; C., Jung; D. A., Evans
abstract

We report the results of an X-ray Absorption Spectroscopy (XAS) investigation on the K L2,3 edges of the K/GaAs(110) interface at different coverages. The behaviour of the structure corresponding to the K 2p-->4s transition suggests a dispersed bandlike nature of the 4s-derived states from lower coverage. The two main features of the spectra, corresponding to the transition from the spin-orbit split K 2p core levels to the empty d-projected states, vary significantly with coverage. A least square fitting analysis of the XAS was possible, revealing the presence of two components that change their relative intensity and linewidth at increasing coverage. We discuss these features in terms of a final state effect in the photoabsorption process.

1995 - COPPER L3-M4,5M4,5 AUGER AND AUGER SATELLITE STRUCTURES IN POLYCRYSTALLINE CU50PD50 ALLOY [Articolo su rivista]
P., Unsworth; N., Brooks; J. M. C., Thornton; M., Sancrotti; D'Addato, Sergio; L., Duo; P. T., Andrews; P., Weightman
abstract

The Cu L3M4,5M4,5 and Cu L8M4,5 - M4,5M4,5M4,5 Auger vacancy satellite spectra of polycrystalline Cu50Pd50 have been measured using tunable monochromatic x-rays from a synchrotron. The photon energy dependence of the Cu Auger satellite lineshape has been recorded for increasing photon energies. The Auger and Auger vacancy satellite spectra of Cu in Cu50Pd50 are found to be identical to those of pure Cu. This result is discussed in terms of the electronic structure of CuPd alloys.

1995 - PERFORMANCE OF THE SOFT-X-RAY DOUBLE-CRYSTAL MONOCHROMATOR ON BEAMLINE-4.2 AT THE SRS, DARESBURY LABORATORY [Articolo su rivista]
A. W., Robinson; D'Addato, Sergio; V. R., Dhanak; P., Finetti; G., Thornton
abstract

The performance of a new beamline, designated BL 4.2 at the SRS, Daresbury Laboratory, is described. The beamline covers the energy range 640<hn< 10 keV, using a double crystal monochromator.

1995 - THE LOCALIZATION OF 3D HOLE STATES IN FE AND FEAL STUDIED BY AUGER VACANCY SATELLITE SPECTROSCOPY [Articolo su rivista]
P., Unsworth; N., Brooks; J. M. C., Thornton; M., Sancrotti; D'Addato, Sergio; L., Duo; P. T., Andrews; P., Weightman
abstract

For Ni, Cu and Zn the occurrence of the L2L3M4,5 Coster-Kronig transitions give rise to L3M4,5-M4,5M4,5 Auger vacancy satellite transitions to low kinetic energy of the main L3M4,5M4,5 transitions. In pure Fe the intensity of the Auger vacancy satellites are negligibly small even though L2L3M4,5 Coster-Kronig occur, a fact which has been attributed to the delocalisation of the M4,5 (3d) hole state within the lifetime of the L3 hole. In this work we present Auger spectra for Fe and FeAl excited using radiation obtained on station 5U.1 of the Daresbury synchrotron. Our spectra support earlier work regarding the relative lifetime of the L3 and M4,5 hole state in Fe though our spectra are different in detail. We also show that the Fe 3d hole is significantly more localised in FeAl than in Fe.

1994 - Charge-Transfer satellites in K L 23 XAS Data for K/Si(111)-(2 × 1): Evidence for strong ionic bonds [Articolo su rivista]
D'Addato, S.; Ramaker, D. E.; Cosso, R.; Gregory, D.; Morrison, T. P.; Unsworth, P.; Duo, L.; Panaccione, G.; Nannarone, S.; Weightman, P.
abstract

We report K L 23 edge X-ray absorption spectra (XAS, both normal and grazing incidence) and photoelectron spectra (XPS) for K/Si(111)-(2 × 1) at three different coverages (0.1, 0.3 and 0.5 ML). The data show features which exhibit a very strong variation in both the energy, intensity, and line shape with incidence angle and coverage. Some of these features correspond to charge-transfer satellites and give a measure of the hybridization of the ground state. They indicate very ionic bonding. © 1994 IOP Publishing Ltd.

1994 - Feasibility of experiments with circularly polarised light on beamline 4.1 at the SRS [Articolo su rivista]
P., Finetti; A. W., Robinson; G., Thornton; D'Addato, Sergio
abstract

We present the results of a feasibility study for upgrading of beamline 4.1 at the SRS to a configuration that allows experiments with circularly polarised light. The upgrading project is based on the well-known of bending magnets that light emitted out of the storage ring plane is elliptically polarised. The photon flux at the sample and the degree of circular polarisation have been calculated, as a function of the photon energy, for different values of the out-of-plane solid angle from which the light emitted by an ideal bending magnet at the SRS could be collected by beamline 4.1.

1994 - Temperature-dependent growth of the Na/GaP(110) interface [Articolo su rivista]
D. A., Evans; P., Bailey; J. M. C., Thornton; D'Addato, Sergio
abstract

The adsorption of Na onto GaP(110) is markedly different when the substrate is held below 120 K and above 140 K. At 120 K, the development of core and valence level photoemission spectra is consistent with charge transfer between the alkali metal and the surface atoms of a chemically undisrupted substrate. However, at 140 K, similar measurements show very different spectral changes, resembling more closely a reactive interface, involving interaction between the Na and bulk GaP atoms

1993 - AN X-RAY-ABSORPTION FINE-STRUCTURE STUDY OF GE(001)(2X1)-S [Articolo su rivista]
K., Newstead; A. W., Robinson; D'Addato, Sergio; A., Patchett; N. P., Prince; R., Mcgrath; R., Whittle; E., Dudzik; I. T., Mcgovern
abstract

The Ge(001)(2 x 1)-S overlayer formed by dosing clean Ge(001) with H2S at room temperature and annealing to remove H has been studied using surface extended X-ray absorption fine structure (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS). The results indicate that the S atoms sit in bridge sites on the surface with a bond length of 2.36 +/- 0.05 angstrom. This is consistent with the breaking of the Ge-Ge surface dimers, and is thus an example of the phenomenon of adsorbate-induced dereconstruction. Comparison is made with a previous study of this system using angle-resolved photoemission fine structure (ARPEFS) and with experimental and theoretical studies of the related Ge(001)(1 x 1)-S overlayer formed by dosing with S2.

1993 - L -> (L-1) AGM4,5 XAS INVESTIGATION AT THE AG/SI(111)2 X-1 INTERFACE [Articolo su rivista]
M., Sancrotti; L., Duo; R., Cosso; D'Addato, Sergio; D. A., Gregory; P., Unsworth; P., Weightman
abstract

We have studied the evolution of the low-lying unoccupied Ag5p-derived electron states at the Ag/Si(111)2 x 1 interface as a function of the Ag coverage via (l --> (l - 1)) Ag M4,5 X-ray absorption spectroscopy. The measured line-shapes are discussed in terms of a transition of the overgrown adlayer from two- to three-dimensional character.

1993 - STRONG EVOLUTION OF THE P-PROJECTED EMPTY DENSITY OF STATES IN PD-AL ALLOYS - AN M4,5 X-RAY-ABSORPTION-SPECTROSCOPY INVESTIGATION [Articolo su rivista]
L., Duo; M., Sancrotti; G., Curro; A., Ruocco; D'Addato, Sergio; R., Cosso; P., Unsworth; P., Weightman
abstract

We present a near-edge x-ray-absorption investigation on the Pd M4,5 edge of Pd, Pd0.85Al0.15, PdAl, and PdAl3, and on the Ag M4,5 edge of Ag. A comparison with theoretical results for the pure elements Pd and Ag indicates this kind of spectroscopy to be related to the p-projected density of states above the Fermi level via a --> 3d --> 5p dipole allowed excitation. A direct estimation of the Ag M4,5 core-hole life-time width value (0.4 +/- 0.05 eV) is furthermore accomplished. The dramatic changes of the investigated line shapes on going from pure Pd to an increasing dilution of Pd atoms into an AI matrix host suggest a strong participation of the p states to the formation of the chemical bonding which has been undervalued up to now.

1992 - ADSORBATE-INDUCED DE-RECONSTRUCTION IN THE INTERACTION OF H2S WITH GE(001) 2X1 [Articolo su rivista]
K., Newstead; A. W., Robinson; D'Addato, Sergio; A., Patchett; N. P., Prince; R., Mcgrath; R., Whittle; E., Dudzik; I. T., Mcgovern
abstract

The room temperature adsorption of H2S on Ge(001) 2 x 1 has been studied using surface-extended x-ray-absorption fine structure, low-energy electron diffraction and Auger electron spectroscopy. The Ge(001) 2 x 1-S system formed by a saturation dose followed by an anneal to remove H has a S-Ge bond length of 2.36 +/- 0.05 angstrom, with S sitting in twofold bridge sites, consistent with breaking of the Ge-Ge dimer reconstruction. Further adsorption of H2S at low or elevated temperatures is kinetically unfavourable, and it is proposed that this is consistent with the surface Ge atoms sitting in non-ideal rather than bulk-terminated positions.

1992 - ELECTRON-ENERGY LOSS AND AUGER-SPECTROSCOPY OF THE YB-GAP(110) INTERFACE [Articolo su rivista]
A., Ruocco; S., Iacobucci; N., Gambacorti; D'Addato, Sergio; Nannarone, Stefano; L., Duo; M., Sancrotti; R., Cosso; P., Weightman
abstract

The Yb-GaP(110) interface formation was studied by EELS (electron energy loss spectroscopy) and AES (Auger electron spectroscopy). The EELS was performed in the 2-27 eV loss range at different coverages ranging from 0.5 to 20 ML (monolayer) with a primary beam energy of 100 eV. Losses typical of the interface were found at approximately 3.5 and 6 eV. The EEL data are characterized by the persistence of the 20 eV loss associated with the Ga 3d level. The AES P L2,3VV data were taken at 1 and 2 ML as a function of the take-off angle. Lineshape changes were observed both versus coverage and versus angle, allowing insight into the bonding at the P site versus coverage and depth. A model of the morphology of the interface during the growth process is presented characterized by Yb-P compounds formation and Ga segregation at the surface.

1992 - P L2, 3 VV AUGER-SPECTROSCOPY AT THE GAP(110)/YB INTERFACE - ANALYSIS OF THE CHEMICAL-BOND [Articolo su rivista]
M., Sancrotti; L., Duo; R., Cosso; D'Addato, Sergio; A., Ruocco; Nannarone, Stefano; P., Weightman
abstract

We report a P L2,3VV Auger line-shape study of the early stages of the GaP(110)/Yb interface formation. Strong similarities with the evolution displayed by the Si L2,3VV Auger profiles measured from the Si(111)/Yb interface and nearly-d0 metal silicides (Ca-silicides, Gd-silicides) is pointed out and exploited to determine the character of the P 3p-derived valence states in the Yb phosphide-like reaction products. A comparison, at different Yb coverages, with the self-folded partial density of states calculated for stoichiometric YbP is moreover performed. On this basis, the chemical bond is mainly described in terms of the formation of P 3p-Yb 5d hybrids.

1992 - P-DERIVED VALENCE STATES AT THE REACTIVE GAP(110)/YB INTERFACE VIA P L2,3VV AUGER LINE-SHAPE SPECTROSCOPY [Articolo su rivista]
M., SANCROTTI; L., DUO; R., COSSO; D'ADDATO, Sergio; A., RUOCCO; NANNARONE, Stefano; P., WEIGHTMAN
abstract

We report on the evolution of the P L2,3 VV Auger line shape with increasing Yb deposition onto the GaP(110) surface. Strong affinity with Si L2,3 VV profiles measured from the Si(111)/Yb interface and nearly-d0 metal silicides (Ca silicides, Gd silicides) is pointed out and fruitfully exploited to model the character of the P 3p-derived valence states in the Yb phosphidelike reaction products. The chemical bond is thus ascribed mainly to the formation of P 3p - Yb 5d hybrids.

1992 - SATELLITE STRUCTURES OF THE NICKEL L3 VV AUGER SPECTRUM IN NICKEL SILICIDES - A SYNCHROTRON RADIATION INVESTIGATION [Articolo su rivista]
R., Cosso; M., Sancrotti; L., Duo; D'Addato, Sergio; S., Iacobucci; G., Panaccione; Nannarone, Stefano; G., Curro; P., Unsworth; P., Weightman
abstract

We report the first synchrotron radiation investigation of the satellite structures of the Ni L3M4,5M45, Auger spectrum in the Ni compounds Ni2Si and NiSi2. The line shape of the satellites is extracted for both compounds for a set of increasing ionizing energies; the procedure used to generate the difference spectra is described. A comparison of the two sets of satellites with each other and with results reported in the literature for the case of pure Ni allows us to identify the changes in the line shape due to the different chemical environment. Possible mechanisms leading to the formation of these structures are discussed.

1992 - SYNCHROTRON-RADIATION INVESTIGATION OF THE CHEMICAL-DEPENDENCE OF THE VACANCY-SATELLITE STRUCTURE OF THE NIL3VV SPECTRA IN NI SILICIDES [Articolo su rivista]
G., CURRO; R., COSSO; M., SANCROTTI; L., DUO; D'ADDATO, Sergio; NANNARONE, Stefano; S., IACOBUCCI; G., PANACCIONE; P., WEIGHTMAN
abstract

We report a synchrotron-radiation investigation of the satellite structures of the Ni L3M4,5M4,5 Auger spectrum in the Ni compounds Ni2Si and NiSi2. The line shape of the satellites is extracted and analyzed for both compounds and compared to the corresponding structures in Ni metal as reported in the literature for a set of increasing photon energies; the procedure used to analyze the Auger data is described. The spectra analysis allows us to study the evolution in the line shape and in the energy position of the satellite structures in connection with the different chemical environment. The physical origin of these structures is discussed.

1991 - EELS CROSS-SECTION CALCULATIONS ON SI(111)2X1 [Articolo su rivista]
D'Addato, Sergio; Nannarone, Stefano; R., Delsole
abstract

A quantitative calculation of the EEL spectra of Si(111)2 x 1 has been performed. We reproduced the aximuthal dependence, the characteristic behaviour at low primary energies and the effect of the analyzer angular acceptance observed in existing experimental results.

1991 - EMPTY STATES OF THE NI/SI(111)7X7 INTERFACE [Articolo su rivista]
J. Y., Veuillen; T. T., Nguyen; D'Addato, Sergio; Nannarone, Stefano
abstract

Up to now, the evolution of the empty states of the metal/silicon interface has received only little attention. It is however necessary to investigate both occupied and unoccupied states in order to get a complete picture of the interface electronic structure. We present a combined X-ray photoemission (XPS)/bremstrahlung isochromat spectroscopy (BIS) study of the Ni/Si(111)7 x 7 interface formation at room temperature. Valence and conduction band densities of states, as well as metal and Si core levels, were recorded for coverages (theta) ranging from 1 to 100 ML. Above 1 ML, the data reveal an intermixed phase. Its composition is close to Ni2Si at low coverage (theta = 3-6 ML), as testified by both XPS and BIS spectra. (For higher coverages, we found evidence for composition changes induced by the e-beam.) In the 1 ML range the situation seems to be more complex, with possible specific empty interface states.

1991 - EXPERIMENTAL AND THEORETICAL-STUDY OF THE PL2,3VV AUGER LINESHAPE OF GAP(110) [Articolo su rivista]
R., Cosso; L., Duo; M., Sancrotti; D'Addato, Sergio; A., Ruocco; Nannarone, Stefano; P., Weightman
abstract

A joint experimental and theoretical investigation of the PL2,3VV Auger transition on GaP(110) is presented. The theoretical data, calculated within a pure band-like picture, are compared to the experimental spectra. The angular momentum character of the five features constituting the lineshape is identified. Strong contributions to the experimental data from many-body effects may be ruled out due to the good overall agreement with the theoretical lineshape therefore supporting a single particle description for this Auger feature. We discuss effects such as Coulomb correlation, initial state core hole screening, self-energy corrections, that may possibly contribute to the relative energy shifts observed in one of the pp components of the compared spectra.

1991 - Inelastic-electron-scattering investigation of clean and hydrogen-exposed InP(110) surfaces [Articolo su rivista]
NANNARONE, Stefano; D'ADDATO, Sergio; BETTI, Maria Grazia; DEL PENNINO, Umberto; Chen, Yu; Samonto, P.; Lapeyre, G.
abstract

An electron-energy-loss spectroscopy (EELS) study on clean and hydrogenated InP(110) surfaces in the 1-6-eV loss-energy region is presented. We focused on the dependence of the spectra on the azimuthal angle between the plane of incidence and the [110 BAR] crystallographic direction-the direction of the zigzag atomic chains on the topmost semiconductor layer. An azimuthal dependence of the transition involving surface electronic states has been observed, as seen previously in other III-V semiconductor (110) surfaces. The results are compared with optical measurements, integrated EELS data, and surface band-structure calculations.

1991 - UNOCCUPIED 3D-DERIVED STATES IN NI SILICIDES VIA NI L2,3 X-RAY ABSORPTION-SPECTROSCOPY [Articolo su rivista]
M., Sancrotti; L., Duo; R., Cosso; D'Addato, Sergio; S., Iacobucci; G., Panaccione; A., Ruocco; Nannarone, Stefano; M., Surman; P., Unsworth; P., Weightman
abstract

We report Ni L2.3 X-ray absorption spectra as measured from bulk ingots of Ni2Si and NiSi2. A similar amount of unoccupied 3d character is found in the electronic structure of both silicides regardless of the differences occurring in the coordination local to the Ni sites. This is discussed in terms of the results of calculations of the partial density of states of Ni silicides and contrasted with recently reported ultraviolet inverse photoemission data which suggest complete depletion of the unoccupied Ni 3d-related states upon moving from Ni2Si to NiSi2.

1990 - BIS INVESTIGATION OF PD-SI(111)7X7 INTERFACE FORMATION [Articolo su rivista]
J. Y., Veuillen; T. T., Nguyen; R., Cinti; D'Addato, Sergio; S., Turchini; Nannarone, Stefano; A., Santaniello; G., Rossi
abstract

A combined BIS-XPS study of the Pd-Si(111)7x7 interface formation is presented. A Pd2Si-like distribution of partial density of empty states is found starting from the early stages of Si-Pd bond formation

1990 - HREELS INVESTIGATION OF CLEAN AND HYDROGEN-INP(110) SURFACES [Articolo su rivista]
Nannarone, Stefano; D'Addato, Sergio; Betti, Maria Grazia; DEL PENNINO, Umberto; Y., Chen; P., Samonto; G. J., Lapeyre
abstract

The results of a HREELS investigation of the electronic transitions in the 1-5 eV loss energy region on clean and hydrogen-exposed InP(110) are presented.

1990 - NEXAFS of the p(2 × 2) and (√3 × √3)R 30°-O-Ni(111) systems. A multiple scattering analysis [Articolo su rivista]
M., Pedio; L., Becker; B., Hillert; D'Addato, Sergio; J., Haase
abstract

The near-edge X-ray adsorption fine structure (NEFAXS) yields information on both the geometric and electronic structure. We performed NEXAFS studies on the O-Ni(111) system. Multiple scattering calculations for the NEXAFS have been performed using the ICXANES program which is based on a real space cluster method and includes full multiple scattering. The cluster is divided into concentric shells of scatterers around the absorbing atom. The scattering properties of each shell are described by phase shifts which are calculated from one-electron muffin-tin potentials. Best agreement between experiment and calculation was obtained for a three-fold coordinated fcc adsorption site on a Ni(111) surface.

1990 - OXYGEN ON NI(111) - A MULTIPLE-SCATTERING ANALYSIS OF THE NEAR-EDGE X-RAY-ABSORPTION FINE-STRUCTURE [Articolo su rivista]
M., Pedio; L., Becker; B., Hillert; D'Addato, Sergio; J., Haase
abstract

Oxygen chemisorption and oxide formation on a Ni(111) surface have been monitored by using the near-edge x-ray-absorption fine-structure technique. The adsorption site of oxygen in the p(2×2) and (√3 × √3 )R30° superstructures has been determined by a multiple-scattering analysis. In both structures the oxygen occupies a threefold-coordinated fcc site with a nearest-neighbor O-Ni bond length of 1.85±0.05 Å on a Ni(111) surface relaxed outwards by ∼0.15 Å.

1990 - Strong chemical reactivity at the early stages of Yb overgrowth on GaP(110): A synchrotron-radiation study [Articolo su rivista]
Lamberto, Duò; Massimo, Sancrotti; R., Cosso; D'ADDATO, Sergio; A., Ruocco; NANNARONE, Stefano; D., Norman; P., Weightman
abstract

The GaP(110)/Yb interface was studied with use of synchrotron radiation to excite the Ga 3d and the P 2p semiconductor core levels, the quasiatomic Yb 4f states together with the valence-band region, and the P L2,3VV Auger line shape in Yb coverages ranging from 0.2 to 5 monolayers. Strong reactivity was found at the interface region with semiconductor surface disruption and the formation of intermixed Yb-Ga-P products distributed on the substrate surface from the lowest coverage range investigated up to about 1 monolayer. Ga surface segregation with Yb-Ga alloying and Yb phosphide-like product trapping at the interface are dominant in the highest coverage range, resulting in a strong gradient concentration of the substrate chemical species, along a direction normal to the semiconductor surface. The reacted contributions in the Ga 3d, the P 2p, and the Yb 4f levels suggest a highly heterogeneous character of the growing interface with the presence of a multiplicity of atomic sites for each of the reacted atoms. Primarily divalent character of Yb atoms was found over all the coverage range investigated, as derived by Yb 4f spectroscopy. From the changes shown by the P L2,3VV line shape it was possible to model the evolution of the local density of states in Yb-P bonds in terms of Yb 5d- and P 3p-derived states mixing. The results are compared with the growth mechanisms of other III-V-semiconductorrare-earth-metal interfaces.

1990 - Yb interface growth on GaP(110): an electron spectroscopy investigation [Articolo su rivista]
L., Duó; M., Sancrotti; M., Newell; P., Weightman; D., Norman; R., Cosso; D'Addato, Sergio; A., Ruocco; Nannarone, Stefano
abstract

A photoemission study of Yb/GaP(110) interface at early stage of formation is presented, showing strong reactivity and interdiffusion.

1989 - ANISOTROPY OF THE ELECTRONIC-STRUCTURES OF THE GAP(110) SURFACE - A HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDY [Articolo su rivista]
Nannarone, Stefano; D'Addato, Sergio; J. A., Schaefer; C., Yu; G. J., Lapeyre
abstract

The anaisotropy of the electronic structures of clean GaP (110) surface has been studied y high-resolution electron-energy-loss spectroscopy (HREELS) for loss energies ranging from the bulk band gap up to 7 eV.

1989 - SURFACE ANISOTROPY OF III-V-COMPOUNDS [Articolo su rivista]
Nannarone, Stefano; D'Addato, Sergio; J. A., Schaefer; Y., Chen; J., Anderson; G. J., Lapeyre
abstract

The azimuthal dependence of the HREELS spectra of the (110) surfaces of GaAs and GaP was studied in the region of the electronic excitations, starting from the energy gap up to 7 eV.

1988 - CALCULATION OF SURFACE PHONON-DISPERSION ON NI(100) AND NI(100)+C(2X2) ALONG THE [010] DIRECTION BY MEANS OF THE MATCHING PROCEDURE .2. [Articolo su rivista]
J., Szeftel; A., Khater; F., Mila; D'Addato, Sergio; N., Auby
abstract

A detailed application of the matching procedure is presented in order to calculate the surface phonon dispersion on Ni(100) and Ni(100) + c(2x2)-O in the [010] direction.

1985 - DISPERSION EFFECTS AND ELECTRON-ENERGY LOSS OF SILICON SURFACE [Articolo su rivista]
D'Addato, Sergio; Nannarone, Stefano; M., Decrescenzi
abstract

A detailed calculation based on a phenomenological three-layer model of the electron energy loss spectrum of Si(111)2x1, in the region of dangling bond excitation, was performed.

Università degli studi di Modena e Reggio Emilia

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