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ALESSANDRO DI BONA

Docente a contratto
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2023 - Graphite-epoxy composites for bipolar plates: The gas permeability issue [Articolo su rivista]
Roncaglia, Fabrizio; Mucci, Adele; Romagnoli, Marcello; Spinelli, Luca; DI BONA, Alessandro; Biagi, Roberto
abstract

To sustain Hydrogen Economy, proton exchange membrane (PEM) devices are expected to play a central role in both the generation of clean hydrogen and its efficient employment. Bipolar plates are an essential part of PEM devices, therefore investigations directed to technical improvement and cost reduction of these components deserve considerable attention. This work represents an extension of our previous study on graphite-epoxy composites suitable for manufacturing bipolar plates and based on the use of cheap and commercially available materials. Through appropriate changes in the formulation of the composites and in the processing conditions, we succeeded in obtaining materials with full technical compliance, in terms of electrical conductivity and gas permeability. The processability and productivity of the method were improved as well, through the reduction of molding times. The variation of the properties of the samples with the molding pressure was analyzed.

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

2023 - Thermoelectric and Structural Properties of Sputtered AZO Thin Films with Varying Al Doping Ratios [Articolo su rivista]
Isram, Muhammad; MAGRIN MAFFEI, Riccardo; Demontis, Valeria; Martini, Leonardo; Forti, Stiven; Coletti, Camilla; Bellani, Vittorio; Mescola, Andrea; Paolicelli, Guido; Rota, Alberto; Benedetti, Stefania; DI BONA, Alessandro; Ribeiro, Joana M.; Tavares, C. J.; Rossella, Francesco
abstract

2023 - Unveiling the Thermoelectric Performances of Zn1−xFexSe Nanoparticles Prepared by the Hydrothermal Method [Articolo su rivista]
Isram, Muhammad; Demontis, Valeria; Magrin Maffei, Riccardo; Abbas Khan, Najaf; di Bona, Alessandro; Benedetti, Stefania; Amin, Nasir; Mahmood, Khalid; Rossella, Francesco
abstract

Fe2+-doped ZnSe nanoparticles, with varying concentrations of Fe2+ dopants, were prepared by the hydrothermal method and investigated using a multi-technique approach exploiting scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy, as well as measurement of the electrical transport properties and Seebeck coefficient (S). The doped nanoparticles appeared as variable-sized agglomerates on nanocrystallites upon SEM investigation for any doping level. Combined XRD and Raman analyses revealed the occurrence of a cubic structure in the investigated samples. Electric and thermoelectric (TE) transport investigations showed an increase in TE performance with an increase in Fe atom concentrations, which resulted in an enhancement of the power factors from 13 µWm−1K−2 to 120 µWm−1K−2 at room temperature. The results were also dependent on the operating temperature. The maximum power factor of 9 × 10−3 Wm−1K−2 was achieved at 150 °C for the highest explored doping value. The possible applications of these findings were discussed.

2022 - Doping-Dependent Optical Response of a Hybrid Transparent Conductive Oxide/Plasmonic Medium [Articolo su rivista]
Sygletou, Maria; Benedetti, Stefania; DI BONA, Alessandro; Canepa, Maurizio; Bisio, Francesco
abstract

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate.

2021 - Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles [Articolo su rivista]
Sygletou, Maria; Benedetti, Stefania; Ferrera, Marzia; Marco Pierantozzi, Gian; Cucini, Riccardo; Della Valle, Giuseppe; Carrara, Pietro; De Vita, Alessandro; DI BONA, Alessandro; Torelli, Piero; Catone, Daniele; Panaccione, Giancarlo; Canepa, Maurizio; Bisio, Francesco
abstract

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

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.

2019 - EFFECT OF COMPRESSION MOLDING PARAMETERS ON GRAPHITE/EPOXY COMPOSITE BIPOLAR PLATES [Poster]
Roncaglia, Fabrizio; DI BONA, Alessandro; Imperato, Manuel; Biagi, Roberto; Romagnoli, Marcello; Mucci, Adele
abstract

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 - INVESTIGATION OF MOLDING PARAMETERS ON GRAPHITE/EPOXY COMPOSITE-BASED BIPOLAR PLATES [Relazione in Atti di Convegno]
Roncaglia, F.; Di Bona, A.; Biagi, R.; Romagnoli, M.; Degrandis, D.; Mucci, A.
abstract

In order to obtain a material suitable for manufacturing bipolar plates for PEM Fuel Cells the effect of molding parameters (pressure, temperature and time), relative to a graphite/epoxy composite of fixed composition, were studied and modeled by means of a two-level full factorial Design Of Experiment approach. In-plane conductivity and mechanical strength were the dependent variables.

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

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

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

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

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

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

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

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

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

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

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

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

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 - Focused ion beam patterned Hall nano-sensors [Articolo su rivista]
Candini, A.; Gazzadi, G. C.; di Bona, A.; Affronte, M.; Ercolani, D.; Biasiol, G.; Sorba, L.
abstract

By means of focused ion beam milling, we fabricate Hall magnetometers with active areas as small as 100 × 100 nm2. The constituent material can either be metallic (Au), semimetallic (Bi) or doped bulk semiconducting (Si doped GaAs). We experimentally show that Au nano-probes can work from room temperature down to liquid helium with magnetic flux sensitivity &lt; 10- 1 Φ0. © 2006 Elsevier B.V. All rights reserved.

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

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

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

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

1999 - Epitaxial growth of ultrathin Fe films on Ni(001): A structural study [Articolo su rivista]
Luches, P.; Gazzadi, G. C.; Di Bona, A.; Marassi, L.; Pasquali, L.; Valeri, S.; Nannarone, S.
abstract

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

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

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