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Alberto ROTA

Ricercatore t.d. art. 24 c. 3 lett. B presso: Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica


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Pubblicazioni

2021 - Experimental investigation on a novel approach for laser surface hardening modelling [Articolo su rivista]
Orazi, L.; Rota, A.; Reggiani, B.
abstract

Laser surface hardening is rapidly growing in industrial applications due to its high flexibility, accuracy, cleanness and energy efficiency. However, the experimental process optimization can be a tricky task due to the number of involved parameters, thus suggesting for alternative approaches such as reliable numerical simulations. Conventional laser hardening models compute the achieved hardness on the basis of microstructure predictions due to carbon diffusion during the process heat thermal cycle. Nevertheless, this approach is very time consuming and not allows to simulate real complex products during laser treatments. To overcome this limitation, a novel simplified approach for laser surface hardening modelling is presented and discussed. The basic assumption consists in neglecting the austenite homogenization due to the short time and the insufficient carbon diffusion during the heating phase of the process. In the present work, this assumption is experimentally verified through nano-hardness measurements on C45 carbon steel samples both laser and oven treated by means of atomic force microscopy (AFM) technique.


2020 - Mechanical Properties and Metallurgical Features of New Green NiTi Reciprocating Instruments [Articolo su rivista]
Generali, Luigi; Malovo, Anastasiya; Bolelli, Giovanni; Borghi, Alessia; La Rosa, Giusy Rita Maria; Puddu, Pietro; Lusvarghi, Luca; Rota, Alberto; Consolo, Ugo; Pedullà, Eugenio
abstract

To evaluate the properties of two nickel-titanium (NiTi) reciprocating endodontic instruments (commercially known as Procodile and Reziflow), a total of 40 size 25 and 0.06 taper new Procodile and Reziflow instruments (n = 20) were subjected to cyclic fatigue tests (60° angle of curvature, 5-mm radius) at 20 °C and 37 °C and a torsional test based on ISO 3630-1. The fracture surface of each fragment was examined. The morphological, mechanical, chemical, thermal, and phase composition characteristics of the files were investigated by field-emission gun scanning electron microscopy (FEG-SEM) equipped with an energy-dispersive X-ray (EDX) detector, focused ion beam analysis (FIB), micro-Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Auger electron spectroscopy (AES). Reziflow showed higher cyclic fatigue resistance than Procodile at 37 °C (p < 0.05). The maximum torsional strength of Procodile was lower than that of Reziflow (p < 0.05). No difference was found between their angular rotations to fracture (p > 0.05). SEM, FIB, Micro-Raman, and AES analyses revealed the presence of an Nb/Nb2O5 coating on the Procodile surface. DSC and XRD analysis confirmed that both files consist of an almost austenitic phase structure at 37 °C. The cyclic fatigue resistance of Procodile and Reziflow significantly decreases upon exposure to body temperature.


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

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


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

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


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

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


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

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


2018 - Magnetic characterization of cobalt nanowires and square nanorings fabricated by focused electron beam induced deposition [Articolo su rivista]
Venturi, Federico; Gazzadi, Gian Carlo; Tavabi, Amir H; Rota, Alberto; Dunin-Borkowski, Rafal E; Frabboni, Stefano
abstract

The magnetic properties of nanowires (NWs) and square nanorings, which were deposited by focused electron beam induced deposition (FEBID) of a Co carbonyl precursor, are studied using off-axis electron holography (EH), Lorentz transmission electron microscopy (L-TEM) and magnetic force microscopy (MFM). EH shows that NWs deposited using beam energies of 5 and 15 keV have the characteristics of magnetic dipoles, with larger magnetic moments observed for NWs deposited at lower energy. L-TEM is used to image magnetic domain walls in NWs and nanorings and their motion as a function of applied magnetic field. The NWs are found to have almost square hysteresis loops, with coercivities of ca. 10 mT. The nanorings show two different magnetization states: for low values of the applied in-plane field (0.02 T) a horseshoe state is observed using L-TEM, while for higher values of the applied in-plane field (0.3 T) an onion state is observed at remanence using L-TEM and MFM. Our results confirm the suitability of FEBID for nanofabrication of magnetic structures and demonstrate the versatility of TEM techniques for the study and manipulation of magnetic domain walls in nanostructures.


2018 - Superhydrophilic properties driven by highly-regular laser-induced periodic structures on Si surface [Relazione in Atti di Convegno]
Gnilitskyi, Iaroslav; Rota, Alberto; Orazi, Leonardo
abstract

In this work an investigation about the effects of high-speed, highly-regular laser induced periodic surface structures (HR-LIPSS) on wetting properties of silicon surface are presented and discussed. In order to investigate the influence of chemistry on wetting properties of textured surfaces, treatments were performed both under air and N2 shielding gas. The morphology was investigated by Scanning Electron Microscope (SEM), the chemistry by energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy and the wetting properties by camera recording. The results demonstrate substantial effects of HR-LIPSS on wetting properties of silicon surface. The hydrophilic Si surface becomes superhydrophilic after femtosecond nanopatterning and the addition of N2 in laser treated zone essentially influences the chemistry of the surface, this permits to control the water flowing and makes HR-LIPSS more flexible, bringing up to a new level of control of wettability properties for several applications.


2017 - Multifunctional properties of high-speed highly uniform femtosecond laser patterning on stainless steel [Relazione in Atti di Convegno]
Gnilitskyi, Iaroslav; Rota, Alberto; Ctvrtlik, Radim; Serro, Ana Paula; Gualtieri, Enrico; Orazi, Leonardo
abstract

Highly-regular laser-induced periodic surface structures essentially change surface properties of treated surfaces. Here, effect of HR-LIPSS on optical, tribological, mechanical and wetting properties of stainless steel were systematically investigated.


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

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


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

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


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

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


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

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


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


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

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


2011 - Characterization of a new cobalt precursor for focused beam deposition of magnetic nanostructures [Articolo su rivista]
Gazzadi, gian carlo; J. J. L., Mulders; P., Trompenaars; Ghirri, Alberto; Rota, Alberto; Affronte, Marco; Frabboni, Stefano
abstract

The electrical and magnetic properties of nanowires deposited from cobalt tricarbonyl nitrosyl (Co(CO)(3)NO) precursor by focused electron beam- and focused ion beam-induced deposition (FEBID and FIBID) have been investigated. As-deposited nanowires have similar Co content, around 50-55 at.%, but different electrical behaviour: FEBID nanowire is highly resistive (6.3 m Omega cm at RT) and non-metallic at low T, while the FIBID one has much lower resistivity (189 mu Omega cm at RT) and it is metallic. The magnetic properties, tested with magnetoresistance measurements, reveal a non-magnetic behaviour for both nanowires. After 400 degrees C annealing in vacuum FEBID wire is much less resistive (62 mu Omega cm at RT) and recovers the metallic behaviour at low T, and both FEBID and FIBID wires display ferromagnetic behaviour. Structural analysis by low energy-scanning transmission electron microscopy (LE-STEM) suggests that coarsening and interconnection of the Co nanograins are responsible for the improvement in electrical and magnetic properties.


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


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

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


2006 - Au island growth on a Si(111) vicinal surface [Articolo su rivista]
Rota, Alberto; A., Martinez Gil; G., Agnus; E., Moyen; T., Maroutian; B., Bartenlian; R., Mégy; M., Hanbücken; P., Beauvillain
abstract

Au island nucleation and growth on a Si(111) 7 · 7 vicinal surface was studied by means of scanning tunneling microscopy. The surfacewas prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255–430 C substratetemperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au–Si reconstructedislands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density,size and position on substrate temperature and on Au coverage is investigated. At 340 C and above, hemispherical 3D islandsnucleate systematically on the step edges.


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

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


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

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


2006 - Top-down and bottom-up approaches for novel magnetic and plasmonic nanostructures [Capitolo/Saggio]
G., Agnus; T., Arnal; B., Blein; A., Fleurence; F., Gaucher; A., Martinez Gil; Rota, Alberto; B., Bartenlian; P., Beauvillain; M., Bibes; P. Gogol A. M., Haghiri Gosnet; Lecoeur, P. h.; A., Lesuffleur; T. Maroutian B., Vilquin; R., Mégy
abstract

The increasing demand for miniaturization in microelectronics has led to a gigantic progress in developing high resolution patterning techniques. Today, the gate length of a transistor approaches 50 nanometers, that is 10 times smaller than 15 years ago. Such miniaturization is based on the enormous progress of both the writing lithography and etching methods for producing nanostructures down to 50 nm inside a thin film. This is the top-down route at the scale of the « nanoworld ». In a parallel approach, researchers know how to favor the growth of nanostructures on surfaces presenting some roughness like atomic step bunches. A perfect control of such growth allows the formation of nanostructures with the exact desired size of a few tens of nanometers. This second route is called the “bottom-up” approach. It will be shown that both approaches can be combined to organize small nanostructures (~10 nm) on surface, to confine them at the desired place and finally to connect them to the “microworld”. Two different applications for spintronics and plasmonics will be presented and discussed.


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

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


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

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


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

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


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

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


2004 - Nano-patterned silicon surfaces for the self-organanized growth of metallic nanostructures [Articolo su rivista]
Martinez-Gil, A.; Rota, A.; Maroutian, T.; Bartenlian, B.; Beauvillain, P.; Moyen, E.; Hanbücken, M.
abstract

The aim of this work is to obtain a very regular alignment of metallic and magnetic nanostructureswith high density and a narrow size distribution via a self-organised growth process. The methodconsists of combining microelectronic processes which will provide a periodic surface pattern, withself-assembled growth on the so-obtained surface. The structured surface serves as a template forthe controlled positioning of the nanostructures. Concerning the silicon template preparation, thesurface periodicity should be of the same order of magnitude as the atomic surface diffusion lengthused during subsequent growth. These surface patterns were obtained on vicinal silicon surfaces byoptimising electron-beam nanolithography and reactive ion etching, to create arrays of nano-holestypically 50 nm in diameter with a 50 nm spacing. A subsequent sample annealing under ultrahighvacuum conditions is studied in order to obtain a corrugated surface mimicking the lithographypattern. As a model system, the growth of Au on vicinal Si(111) surfaces has been chosen. Afterhigh temperature annealing, clean Si surfaces present arrays of straight step bunches each separatedby ∼50 nm. We discuss the formation of Au islands on these step bunches during growth and/orannealing at a temperature between 300 and 500 ◦C.


2004 - Nano-patterned silicon surfaces for the self-organised growth of metallic nanostructures [Poster]
A., Martinez Gil; Rota, Alberto; T., Maroutian; B., Bartenlian; P., Beauvillain; E., Moyen; M., Hanbücken
abstract

The objective of this work is to obtain self-organised growth of magnetic and metallic nanostructures on nano-patterned silicon surfaces. In order to grow well-defined nanostructures with a narrow size distribution, clean crystalline interfaces and a controlled periodic arrangement, a promising and original method is to couple microelectronic processes which will provide a periodic surface pattern, with self-assembled growth on such a surface. The structured surface serves thus as template to control the position of the nanostructures obtained by self-organisation processes. Regarding the silicon templates preparation, we want a periodicity of the same order of magnitude as the atomic diffusion length during subsequent growth. By optimising e-beam nanolithography and reactive ion etching, we elaborated nano-holes arrays of typically 50 nm diameter with a 50 nm spacing on vicinal silicon surfaces. An annealing by direct current flow under ultra-high vacuum is then performed in order to obtain a corrugated surface mimicking the lithography pattern. It also ensures the surface cleanliness after the technological steps. For the metal deposition, we choose as a model system the growth of Au on vicinal silicon surfaces presenting arrays of step bunches 10 to 50 nm apart. The Au-Si alloy is well-known and forms a liquid eutectic at Tm = 363°C. We will discuss the formation of Au-Si islands on step edges during growth under and above Tm.


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

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


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

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


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

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


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

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


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

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