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Luca PASQUALI

Professore Associato presso: Dipartimento di Ingegneria "Enzo Ferrari"


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Pubblicazioni

2021 - Analysis of External and Internal Disorder to Understand Band‐Like Transport in n‐Type Organic Semiconductors [Articolo su rivista]
Stoeckel, Marc‐Antoine; Olivier, Yoann; Gobbi, Marco; Dudenko, Dmytro; Lemaur, Vincent; Zbiri, Mohamed; Guilbert, Anne A. Y.; D'Avino, Gabriele; Liscio, Fabiola; Migliori, Andrea; Ortolani, Luca; Demitri, Nicola; Jin, Xin; Jeong, Young‐Gyun; Liscio, Andrea; Nardi, Marco‐Vittorio; Pasquali, Luca; Razzari, Luca; Beljonne, David; Samorì, Paolo; Orgiu, Emanuele
abstract

Charge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e., related to interactions with the dielectric layer), especially for n-type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low-frequency (


2021 - Analysis of Resonant Soft X-ray Reflectivity of Anisotropic Layered Materials [Articolo su rivista]
Pasquali, Luca; Mahne, Nicola; Giglia, Angelo; Verna, Adriano; Sponza, Lorenzo; Capelli, Raffaella; Bonfatti, Matteo; Mezzadri, Francesco; Galligani, Emanuele; Nannarone, Stefano
abstract

We present here a method for the quantitative prediction of the spectroscopic specular reflectivity line-shape in anisotropic layered media. The method is based on a 4 × 4 matrix formalism and on the simulation from the first principles (through density functional theory—DFT) of the anisotropic absorption cross-section. The approach was used to simulate the reflectivity at the oxygen K-edge of a 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) thin film on Au(111). The effect of film thickness, orientation of the molecules, and grazing incidence angle were considered. The simulation results were compared to the experiment, permitting us to derive information on the film geometry, thickness, and morphology, as well as the electronic structure.


2021 - Enhancement of X-ray-Excited Red Luminescence of Chromium-Doped Zinc Gallate via Ultrasmall Silicon Carbide Nanocrystals [Articolo su rivista]
Beke, D.; Nardi, M. V.; Bortel, G.; Timpel, M.; Czigany, Z.; Pasquali, L.; Chiappini, A.; Bais, G.; Rudolf, M.; Zalka, D.; Bigi, F.; Rossi, F.; Bencs, L.; Pekker, A.; Markus, B. G.; Salviati, G.; Saddow, S. E.; Kamaras, K.; Simon, F.; Gali, A.
abstract

X-ray-activated near-infrared luminescent nanoparticles are considered as new alternative optical probes due to being free of autofluorescence, while both their excitation and emission possess a high penetration efficacy in vivo. Herein, we report silicon carbide quantum dot sensitization of trivalent chromium-doped zinc gallate nanoparticles with enhanced near-infrared emission upon X-ray and UV-vis light excitation. We have found that a ZnGa2O4 shell is formed around the SiC nanoparticles during seeded hydrothermal growth, and SiC increases the emission efficiency up to 1 order of magnitude due to band alignment that channels the excited electrons to the chromium ion.


2021 - Production Strategies of TiNx Coatings via Reactive High Power Impulse Magnetron Sputtering for Selective H2 Separation [Articolo su rivista]
Mortalò, Cecilia; Deambrosis, Silvia Maria; Montagner, Francesco; Zin, Valentina; Fabrizio, Monica; Pasquali, Luca; Capelli, Raffaella; Montecchi, Monica; Miorin, Enrico
abstract

This scientific work aims to optimize the preparation of titanium nitride coatings for selective H-2 separation using the Reactive High Power Impulse Magnetron Sputtering technology (RHiPIMS). Currently, nitride-based thin films are considered promising membranes for hydrogen. The first series of TiNx/Si test samples were developed while changing the reactive gas percentage (N-2%) during the process. Obtained coatings were extensively characterized in terms of morphology, composition, and microstructure. A 500 nm thick, dense TiNx coating was then deposited on a porous alumina substrate and widely investigated. Moreover, the as-prepared TiNx films were heat-treated in an atmosphere containing hydrogen in order to prove their chemical and structural stability; which revealed to be promising. This study highlighted how the RHiPIMS method permits fine control of the grown layer's stoichiometry and microstructure. Moreover, it pointed out the need for a protective layer to prevent surface oxidation of the nitride membrane by air and the necessity to deepen the study of TiNx/alumina interface in order to improve film/substrate adhesion.


2021 - Spin control using chiral templated nickel [Articolo su rivista]
Mishra, Suryakant; Pasquali, Luca; Fontanesi, Claudio
abstract

This Letter reports an original spin valve device that is based on a chiral templated nickel material. Chirality in Ni is induced by exploiting co-electrodeposition of an organic chiral template. In this specific case, the chiral templating is enantiopure tartaric acid (TA). Facile electrodeposition (co-deposition) in ambient conditions produces a nickel chiral-templated material. Z-shaped magnetoresistance curves, switching sign as a function of TA handedness, prove the peculiar ferromagnetic character induced by the presence of a chiral compound. Synchrotron measurements using circular polarized light, x-ray natural circular dichroism, confirm the chirality of the Ni in the TA/Ni composite. Density functional theory calculation proves the existence of a strong electronic delocalization involving the tartaric acid and Ni. The significant finding of this Letter is that chiral templated Ni paves the way for future spin valve, which will be able to control the spin without an external magnetic field (as indeed foreseen within the chiral induced spin selectivity-effect framework).


2021 - Wavy graphene sheets from electrochemical sewing of corannulene [Articolo su rivista]
Bruno, Carlo; Ussano, Eleonora; Barucca, Gianni; Vanossi, Davide; Valenti, Giovanni; Jackson, Edward A.; Goldoni, Andrea; Litti, Lucio; Fermani, Simona; Pasquali, Luca; Meneghetti, Moreno; Fontanesi, Claudio; Scott, Lawrence T.; Paolucci, Francesco; Marcaccio, Massimo
abstract

The presence of non-hexagonal rings in the honeycomb carbon arrangement of graphene produces rippled graphene layers with valuable chemical and physical properties. In principle, a bottom-up approach to introducing distortion from planarity of a graphene sheet can be achieved by careful insertion of curved polyaromatic hydrocarbons during the growth of the lattice. Corannulene, the archetype of such non-planar polyaromatic hydrocarbons, can act as an ideal wrinkling motif in 2D carbon nanostructures. Herein we report an electrochemical bottom-up method to obtain egg-box shaped nanographene structures through a polycondensation of corannulene that produces a new conducting layered material. Characterization of this new polymeric material by electrochemistry, spectroscopy, electron microscopy (SEM and TEM), scanning probe microscopy, and laser desorption-ionization time of flight mass spectrometry provides strong evidence that the anodic polymerization of corannulene, combined with electrochemically induced oxidative cyclodehydrogenations (Scholl reactions), leads to polycorannulene with a wavy graphene-like structure.


2020 - Correlation between crystal purity and the charge density wave in 1T−VSe2 [Articolo su rivista]
Sayers, C. J.; Farrar, L. S.; Bending, S. J.; Cattelan, M.; Jones, A. J. H.; Fox, N. A.; Kociok-Köhn, G.; Koshmak, K.; Laverock, J.; Pasquali, L.; Da Como, E.
abstract

We examine the charge density wave (CDW) properties of 1T-VSe2 crystals grown by chemical vapor transport (CVT) under varying conditions. Specifically, we find that upon lowering the growth temperature (Tg < 630◦C), there is a significant increase in both the CDW transition temperature and the residual resistance ratio (RRR) obtained from electrical transport measurements. Using x-ray photoelectron spectroscopy, we correlate the observed CDW properties with stoichiometry and the nature of defects. In addition, we have optimized a method to grow ultrahigh-purity 1T-VSe2 crystals with a CDW transition temperature TCDW = (112.7 ± 0.8) K and maximum residual resistance ratio RRR ≈ 49, which is the highest reported thus far. This work highlights the sensitivity of the CDW in 1T-VSe2 to defects and overall stoichiometry and the importance of controlling the crystal growth conditions of strongly correlated transition metal dichalcogenides.


2020 - Exchange Interactions Drive Supramolecular Chiral Induction in Polyaniline [Articolo su rivista]
Mishra, S.; Kumar, A.; Venkatesan, M.; Pigani, L.; Pasquali, L.; Fontanesi, C.
abstract

The focus of this paper is on the intermolecular interaction active between polyaniline (PANI) and 10-camphorsulfonic acid (10CSA). Enantiopure 10CSA, present in the electropolymerization solution, promotes chiral induction in the supramolecular polyaniline polymer (cPANI). Tight integration of experimental data (circular dichroism, CD, near edge X-ray absorption spectra, NEXAFS, conductive probe atomic force microscopy, CP-AFM) and theoretical [density functional theory, (DFT)] results allows to unfold the nature of the electronic interaction between PANI and 10CSA and to shed light on the physical interactions inducing the chiral character to bulk pristine non-chiral PANI: eventually yielding cPANI. The electropolymerization follows a “wet chemistry” method: electrochemical polymerization of aniline in the co-presence in bulk solution of enantiopure 10-camphorsulfonic acid (10CSA). The latter is exploited as chirality inductor. The method of integration between experimental results with ab-initio theoretical calculations, strongly suggests that the chiral induction exerted by the CSA stems from exchange interaction between CSA and PANI.


2020 - Magnetic depth profiling of the Co/C60 interface through soft X-ray resonant magnetic reflectivity [Articolo su rivista]
Verna, Adriano; Bergenti, Ilaria; Pasquali, Luca; Giglia, Angelo; Albonetti, Cristiano; Dediu, Valentin; Borgatti, Francesco
abstract

We have probed the structural and magnetic properties of a ferromagnetic/organic interface constituted by a polycrystalline Co layer deposited on a fullerene thin film through resonant soft X-ray reflectivity measurements. The fitting analysis of the reflectivity indicates the formation of a sharp interface with limited intermixing and a null remanent magnetization in a ∼1 nm thick region of the Co film at the interface with C60. This information contributes to elucidate the role of organic–inorganic interfaces in the charge and spin transport inside organic spintronic devices.


2020 - Oligothiophene‐Based Phosphonates for Surface Modification of Ultraflat Transparent Conductive Oxides [Articolo su rivista]
Timpel, Melanie; Nardi, Marco V.; Wegner, Berthold; Ligorio, Giovanni; Pasquali, Luca; Hildebrandt, Jana; Pätzel, Michael; Hecht, Stefan; Ohta, Hiromichi; Koch, Norbert
abstract

The self-assembly of electroactive organic molecules on transparent conductive oxides is a versatile strategy to engineer the interfacial energy- level alignment and to enhance charge carrier injection in optoelectronic devices. Via chemical grafting of an aromatic oligothiophene molecule by changing the position of the phosphonic acid anchoring group with respect to the organic moiety (terminal and internal), the direction of the main molecular dipole is changed, i.e., from parallel to perpendicular to the substrate, to study the molecular arrangement and electronic properties at the organic–inorganic interface. It is found that the observed work function increase cannot solely be predicted based on the calculated molecular dipole moment of the oligothiophene-based phosphonates. In addition, charge transfer from the substrate to the molecule has to be taken into account. Molecular assembly and induced electronic changes are analogous for both indium-tin oxide (ITO) and zinc oxide (ZnO), demonstrating the generality of the approach and highlighting the direct correlation between molecular coverage and electronic effects.


2020 - Spin dependent electrochemistry: Focus on chiral vs achiral charge transmission through 2D SAMs adsorbed on gold [Articolo su rivista]
Innocenti, Massimo; Passaponti, Maurizio; Giurlani, Walter; Giacomino, Agnese; Pasquali, Luca; Giovanardi, Roberto; Fontanesi, Claudio
abstract

The efficiency of charge transmission in chiral compared to achiral molecular systems, of structurally related compounds, is probed via cyclic voltammetry measurements. For such a purpose five different thiols have been selected two achiral (i.e. 3-mercaptopropionic acid (MPA) and 8-mercapto-1-octanol (M8O)) and three chiral (i.e. L- and D-cysteine (Lcys, Dcys) and D-penicillamine (Dpen)). These compounds are used to form 2D self-assembled monolayers (SAM) on gold. Cyclic voltammetry (CV) measurements, by using the potassium hexacyanoferrate(III)/potassium hexacyanoferrate(II) redox couple (Fe3Fe2), are exploited to probe the charge transfer ability of the electrode|SAM|solution interface. In particular, MPA, Lcys and Dpen compounds are se- lected due to their quite similar structural and geometrical characteristics (virtually the same molecular length and terminal groups). M8O is a longer SAM forming thiol, which was selected as a reference blocking electrode compound. The comparison of the cyclic voltammetry data shows that a better ability in the charge transmission is obtained when chiral SAM are used; namely in the case of L-cysteine and D-penicillamine. This result can be related to the spin filtering ability of chiral compounds: chiral induced spin selectivity (CISS) effect. As rational- ized on the basis of theoretical values of optical rotation (OR), calculated at the CAMB3LYP/6-31G(d,p) level of the theory.


2019 - Characterization and assessment of the potential toxicity/pathogenicity of fibrous glaucophane [Articolo su rivista]
Di Giuseppe, D.; Harper, M.; Bailey, M.; Erskine, B.; Della Ventura, G.; Ardit, M.; Pasquali, L.; Tomaino, G.; Ray, R.; Mason, H.; Dyar, M. D.; Hanuskova, M.; Giacobbe, C.; Zoboli, A.; Gualtieri, A.
abstract

In California, the metamorphic blueschist occurrences within the Franciscan Complex are commonly composed of glaucophane, which can be found with a fibrous habit. Fibrous glaucophane's potential toxicity/pathogenicity has never been determined and it has not been considered by the International Agency for Research on Cancer (IARC) as a potential carcinogen to date. Notwithstanding, outcrops hosting fibrous glaucophane are being excavated today in California for building/construction purposes (see for example the Calaveras Dam Replacement Project - CDRP). Dust generated by these excavation activities may expose workforces and the general population to this potential natural hazard. In this work, the potential toxicity/pathogenicity of fibrous glaucophane has been determined using the fibre potential toxicity index (FPTI). This model has been applied to a representative glaucophane-rich sample collected at San Anselmo, Marin County (CA, USA), characterized using a suite of experimental techniques to determine morphometric, crystal-chemical parameters, surface reactivity, biodurability and related parameters. With respect to the asbestos minerals, the FPTI of fibrous glaucophane is remarkably higher than that of chrysotile, and comparable to that of tremolite, thus supporting the application of the precautionary approach when excavating fibrous glaucophane-rich blueschist rocks. Because fibrous glaucophane can be considered a potential health hazard, just like amphibole asbestos, it should be taken into consideration in the standard procedures for the identification and assessment of minerals fibres in soil and air samples.


2019 - Interpretation of linear dichroism at S L2,3 x-ray absorption edges of small organic molecules at surfaces [Articolo su rivista]
Capelli, Raffaella; Pasquali, Luca
abstract

Resonant soft x-ray absorption was applied at the S L2,3 (2p) edges of thin films of 1,4-benzenedimethanethiol (BDMT) on gold and of sexithiophene (6 T) on flat and patterned CaF2 surfaces. Linear dichroism effects were clearly observed depending on the orientation of the electric field vector of the incoming radiation. They were related to the organization of the molecules. For the interpretation of the angular dependencies of the spectral features, density functional theory (DFT) simulations of the absorption cross sections were calculated. It was possible to confirm the generally accepted chemisorption geometry of BDMT on Au(111), composed of molecules nearly vertical at the surface with one of the S atoms participating in the bonding with Au and the other left pending at the outermost surface and contributing mostly to the measured x-ray absorption. For 6 T on CaF2 a new chemisorption phase was identified on ridge-patterned CaF2(110), coexisting with a previously known phase given by 6 T crystallites highly oriented along the ridges. The new phase being composed of molecules oriented perpendicular to the ridges and lying on their edges.


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 - Quantitative resonant soft x-ray reflectivity from an organic semiconductor single crystal [Articolo su rivista]
Capelli, R.; Da Como, E.; Kociok-Köhn, G.; Fontanesi, C.; Verna, A.; Pasquali, L.
abstract

Resonant soft X-ray reflectivity at the carbon K-edge was applied to a trigonal tetracene single crystal. The angular resolved reflectivity was quantitatively simulated describing the tetracene crystal in terms of its dielectric tensor, which was derived from the anisotropic absorption cross section of the single molecule, as calculated by density functional theory. A good agreement was found between the experimental and theoretically predicted reflectivity. This allows us to assess the anisotropic optical constants of the organic material, probed at the carbon K-edge, in relation to the bulk/surface structural and electronic properties of the crystal, through empty energy levels.


2019 - Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics [Articolo su rivista]
Fontanesi, C.; Como, E. D.; Vanossi, D.; Montecchi, M.; Cannio, M.; Mondal, P. C.; Giurlani, W.; Innocenti, M.; Pasquali, L.
abstract

Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped n-type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon, via two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s−1 range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant (KET). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.


2019 - Structure Model and Toxicity of the Product of Biodissolution of Chrysotile Asbestos in the Lungs [Articolo su rivista]
Gualtieri, A. F.; Lusvardi, G.; Pedone, A.; Di Giuseppe, D.; Zoboli, A.; Mucci, A.; Zambon, A.; Filaferro, M.; Vitale, G.; Benassi, M.; Avallone, R.; Pasquali, L.; Lassinantti Gualtieri, M.
abstract

Asbestos is a commercial term indicating six natural silicates with asbestiform crystal habit. Of these, five are double-chain silicates (amphibole) and one is a layer silicate (serpentine asbestos or chrysotile). Although all species are classified as human carcinogens, their degree of toxicity is still a matter of debate. Amphibole asbestos species are biopersistent in the human lungs and exert their chronic toxic action for decades, whereas chrysotile is not biopersistent and transforms into an amorphous silica structure prone to chemical/physical clearance when exposed to the acidic environment created by the alveolar macrophages. There is evidence in the literature of the toxicity of chrysotile, but its limited biopersistence is thought to explain the difference in toxicity with respect to amphibole asbestos. To date, no comprehensive model describing the toxic action of chrysotile in the lungs is available, as the structure and toxic action of the product formed by the biodissolution of chrysotile are unknown. This work is aimed at fulfilling this gap and explaining the toxic action in terms of structural, chemical, and physical properties. We show that chrysotile's fibrous structure induces cellular damage, mainly through physical interactions. Based on our previous work and novel findings, we propose the following toxicity model: inhaled chrysotile fibers exert their toxicity in the alveolar space by physical and biochemical action. The fibers are soon leached by the intracellular acid environment into a product with residual toxicity, and the dissolution process liberates toxic metals in the intracellular and extracellular environment.


2019 - Understanding adhesion of gold conductive films on sodium-alginate by photoelectron spectroscopy [Articolo su rivista]
Capelli, Raffaella; Maccagnani, Piera; Dinelli, Franco; Murgia, Mauro; Bertoldo, Monica; Montecchi, Monica; Doyle, Bryan P.; Carleschi, Emanuela; Pasquali, Luca
abstract

Ultrathin layers of gold, from 2 to 25 nm of nominal coverage, have been deposited on sodium-alginate biopolymer foils applying two alternative approaches: low power sputtering and thermal evaporation. The morphology of the deposited layers was obtained by means of atomic force microscopy. In the early stages of growth, thermal evaporation gives rise to a top surface resembling the underlying substrate, whereas low power sputtering produces a topography characterized by smoother areas. This indicates that the film growth occurs in different ways. X-ray photoelectron spectroscopy with two photon energies, corresponding to Al Kα and Ag Lα photons, was used to get information on the chemistry at the interface and on the degree of intermixing between Au and sodium-alginate. While no chemical modifications with respect to the bare materials could be detected, the evolution of the intensities of the relevant core levels of Au and sodium alginate (Au 4f and Na 1s in particular) indicated a strong intermixing in the case of films deposited by low power sputtering. This is further supported by optical measurements. The observed behaviour can be correlated with the enhanced adhesion of sputtered films compared to thermally evaporated ones.


2018 - A novel combined experimental and multiscale theoretical approach to unravel the structure of SiC/SiO: Xcore/shell nanowires for their optimal design [Articolo su rivista]
Morresi, Tommaso; Timpel, Melanie; Pedrielli, Andrea; Garberoglio, Giovanni; Tatti, Roberta; Verucchi, Roberto; Pasquali, Luca; Pugno, Nicola Maria; Nardi, Marco Vittorio; Taioli, Simone
abstract

In this work we propose a realistic model of nanometer-thick SiC/SiOxcore/shell nanowires (NWs) using a combined first-principles and experimental approach. SiC/SiOxcore/shell NWs were first synthesised by a low-cost carbothermal method and their chemical-physical experimental analysis was accomplished by recording X-ray absorption near-edge spectra. In particular, the K-edge absorption lineshapes of C, O, and Si are used to validate our computational model of the SiC/SiOxcore/shell NW architectures, obtained by a multiscale approach, including molecular dynamics, tight-binding and density functional simulations. Moreover, we present ab initio calculations of the electronic structure of hydrogenated SiC and SiC/SiOxcore/shell NWs, studying the modification induced by several different substitutional defects and impurities into both the surface and the interfacial region between the SiC core and the SiOxshell. We find that on the one hand the electron quantum confinement results in a broadening of the band gap, while hydroxyl surface terminations decrease it. This computational investigation shows that our model of SiC/SiOxcore/shell NWs is capable to deliver an accurate interpretation of the recorded X-ray absorption near-edge spectra and proves to be a valuable tool towards the optimal design and application of these nanosystems in actual devices.


2018 - Influence of domestic and environmental weathering in the self-cleaning performance and durability of TiO2 photocatalytic coatings [Articolo su rivista]
Erika Iveth Cedillo-Gonzáleza, ; Barbieri, Virginia; Paolo, Falcaro; Torres-Martínez, Leticia M.; Isaías, Juárez-Ramírez; Laura, Villanova; Montecchi, Monica; Pasquali, Luca; Siligardi, Cristina
abstract

Weathering of photocatalytic TiO2 coatings represents an important issue for the successful application of TiO2- based self-cleaning materials. Photocatalytic efficiency of the as-prepared materials is crucial for commercialization; however, changes in the coating performance due to weathering become a critical factor for practical applications. Moreover, chemical durability should be considered as weathering can promote the release of photocatalyst nanoparticles, which can pollute the environment and be hazardous for human health. In this study, two photocatalytic TiO2 coatings with different microstructures (namely compact and mesoporous) were exposed to chemical treatments to simulate domestic and environmental weathering. Results show that dense TiO2 coatings with a slow photocatalytic activity are suitable for domestic applications as minimum leaching of photoactive material was observed. Conversely, once exposed to chemical solutions commonly present in domestic environments, the initially highly active mesoporous TiO2 coatings showed a dramatic drop of the selfcleaning performance and a significant release of nanoparticles in the surrounding environment. It is expected that the results reported here will be of particular relevance for the construction sector, as the manuscript discloses important knowledge for the development of TiO2-based self-cleaning materials once exposed to indoor or outdoor environments.


2018 - Is fibrous ferrierite a potential health hazard? Characterization and comparison with fibrous erionite [Articolo su rivista]
Gualtieri, Alessandro F.; Gandolfi, Nicola Bursi; Passaglia, Elio; Pollastri, Simone; Mattioli, Michele; Giordani, Matteo; Ottaviani, Maria Francesca; Cangiotti, Michela; Bloise, Andrea; Barca, Donatella; Vigliaturo, Ruggero; Viani, Alberto; Pasquali, Luca; Gualtieri, Magdalena Lassinantti
abstract

Fibrous erionite is classified by the International Agency for Research on Cancer (IARC) as carcinogenic substance to humans (Group 1). In the areas where it is present in the bedrock, it may cause environmental exposure, and both professional and environmental exposures are possible when the bedrock is used for industrial applications (e.g., building materials). For health and environment protection, prevention is a priority action. In this framework, the recent guidelines of the Consensus Report of the Weinman International Conference on Mesothelioma suggest identifying locations where potentially hazardous mineral fibers (like erionite) are found in the environment, to prevent environmental exposure. The present study will show that one such potentially hazardous mineral fiber might be fibrous ferrierite. Here, the mineralogy, chemical-physical properties, and surface activity of a hydrothermal fibrous ferrierite from Monte Lake British Columbia (Canada) and a diagenetic fibrous ferrierite from Lovelock, Nevada (U.S.A.), were investigated using a combination of "state of the art" experimental methods including optical microscopy, electron microscopy and microprobe analysis, laser ablation-inductively coupled plasma-mass spectrometry (for the trace elements), vibrational spectroscopy, electron paramagnetic resonance, and synchrotron powder diffraction. The chemical-physical properties of these fibrous ferrierites (morphometric parameters, specific surface area, chemical composition with special attention to metals, mainly iron) that prompted adverse effects in vivo were compared to those of the positive carcinogenic standard fibrous erionite-Na from Jersey, Nevada (U.S.A.). The results of our study have demonstrated that, although there are differences in the crystal chemistry and genetic environment, ferrierite samples exhibit outstanding similarities with fibrous erionite samples: both fibrous erionite and fibrous ferrierite may occur in large amounts as microcrystalline fibrous-asbestiform phases in diagenetic rocks with fibers of breathable sizes. For both zeolites, iron is not structural but is associated with impurities lying at the surface of the fibers. Moreover, data useful to understand the surface activity of these fibrous ferrierites were collected. As far as hydrothermal sample is concerned, the EPR data indicate the presence of hydrophilic (SiO-, AlO-, SiOH) and hydrophobic (Si-O-Si) interacting surface groups able to bind the charged CAT1 probes at close sites and attract the probes in the water pools formed into the fiber aggregates. A high percentage of CAT1 probes weakly interacting with the surface due to competition with metal ions were observed for surface of the diagenetic sample. CAT8 probes were less adsorbed by its surface if compared to the diagenetic sample but the more charged surface provided a stronger binding strength for the diagenetic sample compared to the hydrothermal one. In summary, the results of this study indicate that fibrous ferrierite may represent a potential health hazard and, applying the precautionary principle, it should undergo a procedure of toxicity testing.


2018 - Oxygen Impurities Link Bistability and Magnetoresistance in Organic Spin Valves [Articolo su rivista]
Bergenti, Ilaria; Borgatti, Francesco; Calbucci, Marco; Riminucci, Alberto; Cecchini, Raimondo; Graziosi, Patrizio; MacLaren, Donald A.; Giglia, Angelo; Rueff, Jean Pascal; Céolin, Denis; Pasquali, Luca; Dediu, Valentin
abstract

Vertical crossbar devices based on manganite and cobalt injecting electrodes and a metal-quinoline molecular transport layer are known to manifest both magnetoresistance (MR) and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the MR and magnetic modulation of bistability. To explain the device functionality, we identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with the chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrate under voltage application, resulting in a modification of the electronic properties of the organic material and of the oxidation state of the interfacial layer with the ferromagnetic contacts. Variable oxygen doping of the organic molecules represents the key element for correlating bistability and MR, and our measurements provide the first experimental evidence in favor of the impurity-driven model describing the spin transport in organic semiconductors in similar devices.


2018 - Role of gallium diffusion in the formation of a magnetically dead layer at the Y3Fe5O12/Gd3Ga5O12 epitaxial interface [Articolo su rivista]
Suturin, S. M.; Korovin, A. M.; Bursian, V. E.; Lutsev, L. V.; Bourobina, V.; Yakovlev, N. L.; Montecchi, M.; Pasquali, L.; Ukleev, V.; Vorobiev, A.; Devishvili, A.; Sokolov, N. S.
abstract

We have clarified the origin of a magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700 degrees C onto a gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG/GGG interface is demonstrated by means of composition depth profiling performed by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and x-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of a Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and x-ray reflectometry studies of the YIG/GGG epitaxial system.


2018 - Spin-dependent electrochemistry: Enantio-selectivity driven by chiral-induced spin selectivity effect [Articolo su rivista]
Gazzotti, Mirko; Arnaboldi, Serena; Grecchi, Sara; Giovanardi, Roberto; Cannio, Maria; Pasquali, Luca; Giacomino, Agnese; Abollino, Ornella; Fontanesi, Claudio
abstract

Spin-Dependent Electrochemistry (SDE) is a new paradigm in electrochemistry where the electrochemical response of a chiral electrode|solution interface is studied as a function of spin-polarized current. In this work, the SDE concept is further developed exploring the use of the “chiral imprinting” concept, which is implemented in two different, complementary, ways i) a chiral compound in bulk solution to obtain chiral-induced spin selectivity effect at the ferromagnetic (FM) electrode surface ii) conversely, a chiral-ferromagnetic (CFM) hybrid working electrode is produced: nickel is electrochemically co-deposited with a chiral compound, L-ta or D-(−)-tartaric acid, which is added to the electrodeposition bath; this allows to obtain a chiral co-deposited nickel-tartaric acid (Ni-LTA or Ni-DTA) working electrode. As a further innovation, the ferromagnetic working electrode is prepared by direct Ni electrodeposition on the north, or south, pole of a permanent magnet. The electrochemical response of these two chiral imprinted systems is studied by comparing cyclic voltammetry (CV) curves. The latter are recorded in the potential range relevant to the Ni(III)/Ni(II) electrochemical equilibrium, and also in the presence of glucose in bulk solution. An impressive variation in peak potentials is found when comparing CVs recorded on the north, versus south, pole of the magnet (in particular, when the co-deposited CFM working electrode is used). These results are properly rationalized within the Chiral-Induced Spin Selectivity (CISS) effect.


2018 - Synthesis, spectroscopic and electrochemical characterization of Co(II)-terpyridine based metallopolymer [Articolo su rivista]
Zanardi, C.; Zanfrognini, B.; Morandi, S.; Terzi, F.; Pigani, L.; Pasquali, L.; Seeber, R.
abstract

A new oligothienyl-based metallopolymer including Co(terpyridine)2complex units is synthesised by electrochemical oxidation of the relevant monomer. The chemical structure of the polymer chains and the electronic interaction between the organic portions and the metal centres are investigated by spectroscopic (X-rays photoelectron spectroscopy included), electrochemical, and spectroelectrochemical techniques. Thanks to the low potential of the Co(III)/Co(II) redox couple in the Co(terpyridine)2complex and to the presence of oligothienyl spacer, strong electron interaction between the organic backbone and the metal centres have been demonstrated.


2018 - Versatile and Scalable Strategy to Grow Sol-Gel Derived 2H-MoS2 Thin Films with Superior Electronic Properties: A Memristive Case [Articolo su rivista]
Nardi, Marco V.; Timpel, Melanie; Ligorio, Giovanni; Zorn Morales, Nicolas; Chiappini, Andrea; Toccoli, Tullio; Verucchi, Roberto; Ceccato, Riccardo; Pasquali, Luca; List-Kratochvil, Emil J. W.; Quaranta, Alberto; Dirè, Sandra
abstract

Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), show peculiar chemical/physical properties that enable their use in applications ranging from micro- and nano-optoelectronics to surface catalysis, gas and light detection, and energy harvesting/production. One main limitation to fully harness the potential of MoS2 is given by the lack of scalable and low environmental impact synthesis of MoS2 films with high uniformity, hence setting a significant challenge for industrial applications. In this work, we develop a versatile and scalable sol-gel-derived MoS2 film fabrication by spin coating deposition of an aqueous sol on different technologically relevant, flexible substrates with annealing at low temperatures (300 °C) and without the need of sulfurization and/or supply of hydrogen as compared to cutting-edge techniques. The electronic and physical properties of the MoS2 thin films were extensively investigated by means of surface spectroscopy and structural characterization techniques. Spatially homogenous nanocrystalline 2H-MoS2 thin films were obtained exhibiting high chemical purity and excellent electronic properties such as an energy band gap of 1.35 eV in agreement with the 2H phase of the MoS2, and a density of states that corresponds to the n-type character expected for high-quality 2H-MoS2. The potential use of sol-gel-grown MoS2 as the candidate material for electronic applications was tested via electrical characterization and demonstrated via the reversible switching in resistivity typical for memristors with a measured ON-OFF ratio ≥102. The obtained results highlight that the novel low-cost fabrication method has a great potential to promote the use of high-quality MoS2 in technological and industrial-relevant scalable applications.


2018 - ZnO Functionalization: Metal–Dithiol Superstructures on ZnO(0001) by Self-Assembly [Articolo su rivista]
Tong, Yongfeng; Jiang, Tingming; Qiu, Shunli; Koshmak, Konstantin; Giglia, Angelo; Kubsky, Stefan; Bendounan, Azzedine; Chen, Lin; Pasquali, Luca; Esaulov Vladimir, A.; Hamoudi, Hicham
abstract

We present a study of the functionalization of a monocrystalline ZnO surface using aromatic dithiols. The aim is to obtain a sulfur-terminated self-assembled monolayer (SAM), which is then used for creation of further molecular superstructures with intercalated metal atoms. These metal-molecule self-assembled structures are characterized by high-resolution X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure (NEXAFS) measurements. Formation of a 5,5- bis(mercaptomethyl)-2,20- bipyridine (BPD) SAM on ZnO(0001) is demonstrated using a protocol developed by us earlier for dithiol assembly on gold, allowing production of standing-up SAMs with free SH groups. Thereafter the formation of a metal intercalated dithiol super lattice is achieved by first grafting metal atoms (Ag or Ni) and then attaching a second BPD molecular layer. Metal atoms bind to both sulfur and the pyridine nitrogens within the SAM. Clear changes in the valence band region near the Fermi level are observed, and the highest occupied system orbital positions are determined along with work function evolution.


2018 - 3D reconstruction of pentacene structural organization in top-contact OTFTs via resonant soft X-ray reflectivity [Articolo su rivista]
Capelli, Raffaella; Nardi, Marco Vittorio; Toccoli, Tullio; Verucchi, Roberto; Dinelli, Franco; Gelsomini, Carolina; Koshmak, Konstantin; Giglia, Angelo; Nannarone, Stefano; Pasquali, Luca
abstract

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.


2017 - Adsorption and Thermal Stability of 1,4 Benzenedimethanethiol on InP(110) [Articolo su rivista]
Alarcón, Leonardo Salazar; Cristina, Lucila J.; Jia, Juanjuan; Chen, Lin; Giglia, Angelo; Pasquali, Luca; Sánchez, Esteban A.; Esaulov, Vladimir A.; Grizzi, Oscar
abstract

Self-assembly of dithiol molecules is of interest because these can be used as linkers between metallic or semiconductor entities and thus employed in molecular electronics and plasmonic applications, or for building complex heterostructures. Here we focus on dithiol self-assembly by evaporation in vacuum, a method that could circumvent the dithiol oxidation that can occur in solution. We present a high resolution X-ray photoelectron spectroscopy (XPS) and an ion scattering study of adsorption and desorption of 1,4-benzenedimethanethiol on InP(110) as a function of exposure and sample temperature. Results for InP are compared to those on Au(111) and found to differ due to formation of a thick BDMT layer at room temperature, resulting from extra molecules sticking on top of the self-assembled monolayer. This may play an adverse effect in some afore-mentioned applications as in molecular electronics. We furthermore study the evolution of the dithiol film with sample temperature and the elements remaining at the surface after annealing and delineate initial coverage dependent effects.


2017 - Buried Interfaces Effects in Ionic Conductive LaF3–SrF2 Multilayers [Articolo su rivista]
Koshmak, Konstantin; Banshchikov, Alexander; Ciancio, Regina; Orgiani, Pasquale; Borgatti, Francesco; Panaccione, Giancarlo; Giglia, Angelo; Céolin, Denis; Rueff, Jean Pascal; Sokolov, Nikolai S.; Pasquali, Luca
abstract

In multiphase/multilayer solid electrolytes, the composition, reactivity, and structure of interfaces between materials and phases play a fundamental role for fast ion-conduction. Here, the properties of buried interfaces in prototypical fast ion-conducting LaF3/SrF2 epitaxial multilayers are investigated. Photoelectron spectroscopy—both with soft-X and high-energy photons—is applied to separate composition and reactivity of buried interfaces with respect to the outermost surface. X-ray reflectivity, high-energy electron diffraction, X-ray diffraction, atomic force and transmission electron microscopies are used to study morphology, layer crystallinity, epitaxy relations, and buried interface structure. It is found that while the alternated layers present good crystallinity and high lattice matching, with formation of almost ideal sharp interfaces, buried interfaces show a sizeable reduction of the energy barrier for F vacancy formation with respect to bare materials. A density higher by a factor of six of fluorine vacancies is observed at buried interfaces in multilayers with respect to the bare materials. This is correlated to the formation of space charge regions, favoring ion conduction. The formation of F depleted La fluoride regions at interfaces is also promoted by annealing. This is associated to the increase of ion conductivity in annealed heterostructures reported in literature.


2017 - Controlling In-Plane Isotropic and Anisotropic Orientation of Organic Semiconductor Molecules on Ionic Fluoride Dielectrics [Articolo su rivista]
Jiang, Tingming; Koshmak, Konstantin; Giglia, Angelo; Banshchikov, Alexander; Sokolov, Nikolai S.; Dinelli, Franco; Capelli, Raffaella; Pasquali, Luca
abstract

alpha-Sexithiophene (6T) ultrathin films have been grown on CaF2(111)/Si(111) planar surfaces and on CaF2(110)/Si(001) ridged surfaces by molecular beam epitaxy. The growth mode has been studied by means of atomic force microscopy (AFM), photoemission, and near edge X-ray absorption fine structure (NEXAFS). AFM reveals a substantial difference in the film morphology on the two substrates: on CaF2(111) large islands with flat terraces form with no in-plane preferential growth direction; on CaF2(110) narrow and elongated islands develop following the substrate corrugation. Photoemission and X-ray absorption at Ca L-2,L-3 and F K edges indicate that the interaction with the substrate is negligible. Near-edge X-ray absorption (NEXAFS) flanked by DFT calculations of the angular-resolved absorption cross section of 6T at the carbon K-edge reveal that the molecules on both substrates have their long axis vertically oriented with respect to the substrate plane. In addition, in-plane anisotropy of the molecular orientation has been observed on CaF2(110), and it has been interpreted in terms of well aligned molecules in the elongated islands.


2017 - Correction: Functionalization of SiC/SiOX nanowires with a porphyrin derivative: A hybrid nanosystem for X-ray induced singlet oxygen generation (Molecular Systems Design and Engineering (2017) DOI: 10.1039/c7me00005g) [Articolo su rivista]
Tatti, R.; Timpel, M.; Nardi, M. V.; Fabbri, F.; Rossi, F.; Pasquardini, L.; Chiasera, A.; Aversa, L.; Koshmak, K.; Giglia, A.; Pasquali, L.; Rimoldi, T.; Cristofolini, L.; Attolini, G.; Varas, S.; Iannotta, S.; Verucchi, R.; Salviati, G.
abstract

The authors regret an error in the name of the 5th author which was incorrectly shown as R. Rossi instead of F. Rossi. The corrected list of authors and affiliations for this paper is shown here. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.


2017 - Functionalization of SiC/SiOx nanowires with a porphyrin derivative: a hybrid nanosystem for X-ray induced singlet oxygen generation [Articolo su rivista]
Tatti, R.; Timpel, M.; Nardi, M. V.; Fabbri, F.; Rossi, R.; Pasquardini, L.; Chiasera, A.; Aversa, L.; Koshmak, K.; Giglia, A.; Pasquali, Luca; Rimoldi, T.; Cristofolini, L.; Attolini, G.; Varas, S.; Iannotta, S.; Verucchi, R.; Salviati, G.
abstract

Singlet oxygen has attracted great attention in physical, chemical, as well as biological studies, mainly due to its high reactivity and strong oxidising properties. In this context, hybrid nanosystems comprised of (inor- ganic) X-ray absorbing nanostructures and (organic) light-sensitive material (photosensitizers) can poten- tially overcome the limitations of visible light penetration in matter. A deep investigation of the interface of such hybrid nanosystems for X-ray induced generation of singlet oxygen is key to better understand the processes at the hybrid interface, and to control the energy transfer from inorganic to organic counter- parts, which ultimately leads to enhanced singlet oxygen generation. Here, we demonstrate that SiC/SiOx core/shell nanowires functionalized with the tetrakisIJpentafluorophenyl)porphyrin can act as a highly prom- ising and viable strategy to generate singlet oxygen, making this novel hybrid nanosystem attractive for ap- plications in photocatalysis and nanomedical applications. Using different excitation sources (i.e., electrons, visible light, and X-rays) our findings prove that SiC/SiOx core/shell nanowires show X-ray excited optical luminescence, and that optical emission of the photosensitizer is largely enhanced by the nanowires, yield- ing an efficient energy transfer. A consequent singlet oxygen production of the functionalized nanowires is demonstrated after X-ray excitation in a clinical linear accelerator. These findings will provide an insight in developing an effective route to the molecular functionalization of SiC/SiOx core/shell nanowires and their potential use as singlet oxygen generators.


2017 - Long-lived nonthermal electron distribution in aluminum excited by femtosecond extreme ultraviolet radiation [Articolo su rivista]
Bisio, Francesco; Principi, Emiliano; Magnozzi, Michele; Simoncig, Alberto; Giangrisostomi, Erika; Mincigrucci, Riccardo; Pasquali, Luca; Masciovecchio, Claudio; Boscherini, Federico; Canepa, Maurizio
abstract

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev, Phys. Rev. Lett. 107, 165003 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.165003], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior.


2016 - Case studies on the formation of chalcogenide self-assembled monolayers on surfaces and dissociative processes [Articolo su rivista]
Tong, Yongfeng; Jiang, Tingming; Bendounan, Azzedine; Harish, Makri Nimbegondi Kotresh; Giglia, Angelo; Kubsky, Stefan; Sirotti, Fausto; Pasquali, Luca; Sampath, Srinivasan; Esaulov, Vladimir A.
abstract

This report examines the assembly of chalcogenide organic molecules on various surfaces, focusing on cases when chemisorption is accompanied by carbon–chalcogen atom-bond scission. In the case of alkane and benzyl chalcogenides, this induces formation of a chalcogenized interface layer. This process can occur during the initial stages of adsorption and then, after passivation of the sur- face, molecular adsorption can proceed. The characteristics of the chalcogenized interface layer can be significantly different from the metal layer and can affect various properties such as electron conduction. For chalcogenophenes, the carbon–chalcogen atom- bond breaking can lead to opening of the ring and adsorption of an alkene chalcogenide. Such a disruption of the π-electron system affects charge transport along the chains. Awareness about these effects is of importance from the point of view of molecular elec- tronics. We discuss some recent studies based on X-ray photoelectron spectroscopy that shed light on these aspects for a series of such organic molecules.


2016 - Electronic structure of CuTPP and CuTPP(F) complexes: a combined experimental and theoretical study II [Articolo su rivista]
Mangione, Giulia; Sambi, Mauro; Carlotto, Silvia; Vittadini, Andrea; Ligorio, Giovanni; Timpel, Melanie; Pasquali, Luca; Giglia, Angelo; Nardi, Marco Vittorio; Casarin, Maurizio
abstract

The unoccupied electronic structure of thick films of tetraphenylporphyrin and tetrakis(pentafluorophenyl)- porphyrin Cu(II) complexes (hereafter, CuTPP and CuTPP(F)) deposited on Au(111) has been studied by combining the outcomes of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with those of spin-unrestricted time-dependent density functional (TD-DFT) calculations carried out either within the scalar relativistic zeroth order regular approximation (ZORA) framework (C, N and F K-edges) or by using the Tamm– Dancoff approximation coupled to ZORA and including spin–orbit effects (Cu L2,3-edges). Similarly to the model- ling of NEXAFS outcomes pertaining to other Cu(II) complexes, the agreement between theory and experiment is more than satisfactory, thus confirming the open-shell TD-DFT to be a useful tool to look into NEXAFS results pertinent to Cu(II) compounds. The combined effect of metalation and phenyl (Ph) fluorine decoration is found to favour an extensive mixing between Phs* and pristine porphyrin macrocyle (pmc) pmcp* virtual levels. The lowest lying excitation in the C and N K-edge spectra of both CuTPP and CuTPP(F) is associated with a ligand- to-metal-charge-transfer transition, unambiguously revealed in the CuTPPN K-edge spectral pattern. Moreover, the comparison with literature data pertaining to the modelling of the Cu(II)L2,3 features in the phthalocyanine– Cu(II) (CuPc) complex provided further insights into how metal-to-ligand-charge-transfer transitions associated with excitations from 2pCu(II) AOs to low-lying, ligand-based p* MOs may contribute to the Cu(II) L2,3-edge inten- sity and thus weaken its believed relationship with the Cu(II)–ligand symmetry-restricted covalency. Despite the coordinative pocket of CuTPP/CuTPP(F) mirroring CuPc, the ligand-field strength exerted by the phthalocyanine ligand on the Cu(II) centre is experimentally found and theoretically confirmed to be slightly stronger than that experienced by Cu in CuTPP and CuTPP(F). On the whole, the obtained results complement those published in the near past by the same group on the occupied and empty states of the H2TPP and H2TPP(F) free ligands as well as on the occupied states of both CuTPP and CuTPP(F), thus providing the final piece to get a thorough description of electronic perturbations associated with the metalation and the Ph halogen decoration of H2TPP.


2016 - Quantitative resonant soft x-ray reflectivity of ultrathin anisotropic organic layers: Simulation and experiment of PTCDA on Au [Articolo su rivista]
Capelli, R.; Mahne, N.; Koshmak, K.; Giglia, A.; Doyle, B. P.; Mukherjee, S.; Nannarone, S.; PASQUALI, Luca
abstract

Resonant soft X-ray reflectivity at the carbon K edge, with linearly polarized light, was used to derive quantitative information of film morphology, molecular arrangement, and electronic orbital anisotropies of an ultrathin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) film on Au(111). The experimental spectra were simulated by computing the propagation of the electromagnetic field in a trilayer system (vacuum/PTCDA/Au), where the organic film was treated as an anisotropic medium. Optical constants were derived from the calculated (through density functional theory) absorption cross sections of the single molecule along the three principal molecular axes. These were used to construct the dielectric tensor of the film, assuming the molecules to be lying flat with respect to the substrate and with a herringbone arrangement parallel to the substrate plane. Resonant soft X-ray reflectivity proved to be extremely sensitive to film thickness, down to the single molecular layer. The best agreement between simulation and experiment was found for a film of 1.6 nm, with flat laying configuration of the molecules. The high sensitivity to experimental geometries in terms of beam incidence and light polarization was also clarified through simulations. The optical anisotropies of the organic film were experimentally determined and through the comparison with calculations, it was possible to relate them to the orbital symmetry of the empty electronic states.


2015 - Electrowetting of nitro-functionalized oligoarylene thiols self-assembled on polycrystalline gold [Articolo su rivista]
Casalini, Stefano; Berto, Marcello; Bortolotti, Carlo Augusto; Foschi, Giulia; Operamolla, Alessandra; Di Lauro, Michele; Omar, Omar Hassan; Liscio, Andrea; Pasquali, Luca; Montecchi, Monica; Farinola, Gianluca M; Borsari, Marco
abstract

Four linear terarylene molecules (i) 4-nitro-terphenyl-4″-methanethiol (NTM), (ii) 4-nitro-terphenyl-3″,5″-dimethanethiol (NTD), (iii) ([1,1';4',1″] terphenyl-3,5-diyl)methanethiol (TM), and (iv) ([1,1';4',1″] terphenyl-3,5-diyl)dimethanethiol (TD) have been synthesized and their self-assembled monolayers (SAMs) have been obtained on polycrystalline gold. NTM and NTD SAMs have been characterized by X-ray photoelectron spectroscopy, Kelvin probe measurements, electrochemistry, and contact angle measurements. The terminal nitro group (-NO2) is irreversibly reduced to hydroxylamine (-NHOH), which can be reversibly turned into nitroso group (-NO). The direct comparison between NTM/NTD and TM/TD SAMs unambiguously shows the crucial influence of the nitro group on electrowetting properties of polycrystalline Au. The higher grade of surface tension related to NHOH has been successfully exploited for basic operations of digital μ-fluidics, such as droplets motion and merging.


2015 - Erratum: 1,4-Benzenedimethanethiol Interaction with Au(110), Ag(111), Cu(100), and Cu(111) Surfaces: Self-Assembly and Dissociation Processes (Journal of Physical Chemistry C (2014) 118:46 (26866-26876) DOI: 10.1021/jp509184t) [Articolo su rivista]
Jia, J.; Giglia, A.; Flores, M.; Grizzi, O.; Pasquali, L.; Esaulov, V. A.
abstract


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 - New one-step thiol functionalization procedure for Ni by self-assembled monolayers [Articolo su rivista]
Fontanesi, Claudio; Tassinari, Francesco; Parenti, Francesca; Cohen, Hagai; Mondal, Prakash Chandra; Kiran, Vankayala; Giglia, Angelo; Pasquali, Luca; Naaman, Ron
abstract

This article reports on a facile and fast strategy for the self-assembled monolayer (SAM) functionalization of nickel surfaces, employing cyclic voltammetry (CV) cycling of a suitable tailored solution containing the species to be adsorbed. Results are presented for ultrathin films formed on Ni by 1-hexadecanethiol (C16), L-cysteine (L-cys), and the polymethyl (2R)-3-(2,2'-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate (PCT-L) thiophene-based chiral polymer. The effective formation of high-quality ultrathin organic films on the nickel was verified both electrochemically and by exploiting typical surface characterization techniques such as contact angle, ellipsometry, atomic force microscopy (AFM), polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS).


2015 - On sulfur core level binding energies in thiol self-assembly and alternative adsorption sites: An experimental and theoretical study [Articolo su rivista]
Jia, Juanjuan; Kara, Abdelkader; Pasquali, Luca; Bendounan, Azzedine; Sirotti, Fausto; Esaulov, Vladimir A.
abstract

Characteristic core level binding energies (CLBEs) are regularly used to infer the modes of molecular adsorption: orientation, organization, and dissociation processes. Here, we focus on a largely debated situation regarding CLBEs in the case of chalcogen atom bearing molecules. For a thiol, this concerns the case when the CLBE of a thiolate sulfur at an adsorption site can be interpreted alternatively as due to atomic adsorption of a S atom, resulting from dissociation. Results of an investigation of the characteristics of thiol self-assembled monolayers (SAMs) obtained by vacuum evaporative adsorption are presented along with core level binding energy calculations. Thiol ended SAMs of 1,4-benzenedimethanethiol (BDMT) obtained by evaporation on Au display an unconventional CLBE structure at about 161.25 eV, which is close to a known CLBE of a S atom on Au. Adsorption and CLBE calculations for sulfur atoms and BDMT molecules are reported and allow delineating trends as a function of chemisorption on hollow, bridge, and atop sites and including the presence of adatoms. These calculations suggest that the 161.25 eV peak is due to an alternative adsorption site, which could be associated to an atop configuration. Therefore, this may be an alternative interpretation, different from the one involving the adsorption of atomic sulfur resulting from the dissociation process of the S-C bond. Calculated differences in S(2p) CLBEs for free BDMT molecules, SH group sulfur on top of the SAM, and disulfide are also reported to clarify possible errors in assignments.


2015 - Tuning the Electronic Structure of Graphene by Molecular Dopants: Impact of the Substrate [Articolo su rivista]
Christodoulou, Christos; Giannakopoulos, Angelos; Ligorio, Giovanni; Oehzelt, Martin; Timpel, Melanie; Niederhausen, Jens; Pasquali, Luca; Giglia, Angelo; Parvez, Khaled; Müllen, Klaus; Beljonne, David; Koch, Norbert; Nardi, Marco V.
abstract

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced (∼1.3 eV) on Cu compared to quartz (∼1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F6TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene.


2015 - XAS of tetrakis(phenyl)- and tetrakis(pentafluorophenyl)-porphyrin: an experimental and theoretical study [Articolo su rivista]
Nardi, Marco Vittorio; Verucchi, Roberto; Pasquali, Luca; Giglia, Angelo; Fronzoni, Giovanna; Sambi, Mauro; Mangione, Giulia; Casarin, Maurizio
abstract

The unoccupied electronic structure of tetrakis(phenyl)-and tetrakis(pentafluorophenyl)-porphyrin thick films deposited on SiO2/Si(100) native oxide surfaces has been thoroughly studied by combining the outcomes of near-edge X-ray absorption fine structure spectroscopy at the C, N, and F K-edges with those of scalar relativistic zeroth order regular approximation time-dependent density functional theory calculations carried out on isolated molecules. Both experimental and theoretical results concur to stress the electronic inertness of pristine porphyrin macrocycle based 1s(C) -> pi* and 1s(N) -> pi* transitions whose excitation energies are substantially unaffected upon fluorination. The obtained results complement those published by the same group about the occupied states of both molecules, thus providing the missing tile to get a thorough description of the halide decoration effects on the electronic structure of the tetrakis(phenyl)-porphyrin.


2014 - Chalcogen Atom Interaction with Palladium and the Complex Molecule–Metal Interface in Thiol Self Assembly [Articolo su rivista]
Jia, Juanjuan; Bendounan, Azzedine; Chaouchi, Karine; Kubsky, Stefan; Sirotti, Fausto; Pasquali, Luca; Esaulov, Vladimir A.
abstract

In the case of reactive metals, on adsorption of organic chalcogenide molecules like thiols, chalcogenide-C bond scission can occur. Thus, the high reactivity of Pd leads to initial thiol dissociation and formation of a complex PdS interface layer on which thereafter thiol self-assembled monolayers (SAM) can form. In this context we investigate in detail the adsorption of S, Se, alkanethiols, and aromatic dithiols on Pd by photoemission with synchrotron radiation. The nature of the PdS and PdSe layers formed is studied, and thiol adsorption on Pd(111), PdS, and PdSe surfaces is investigated, along with interface characteristics. After initial strong sulfidation (selenization) in Na2S(Se) solutions, a well-ordered surface PdS (PdSe) layer can be obtained by annealing. For S, annealing leads to formation of a PdS (root 7 x root 7)R19.1 degrees layer, whereas for Se, large domains of this structure are formed. Experiments suggest that in thiol adsorption the Pdsulfide interface is not simply similar to the (root 7 x root 7)R19.1 degrees PdS layer but that modifications in this surface sulfide layer are induced. A similar effect is observed on the selenide interface layer. In addition, 1,4-benzenedimethanethiol adsorption on Pd is investigated with the aim of creation of thiol-terminated dithiol molecular layers. Unlike the case of surfaces like Au, no clear indication of a standing-up, thiol-terminated SAM was found. X-ray radiation damage effects are reported.


2014 - CRYSTAL CHEMISTRY AND SURFACE CONFIGURATIONS OF TWO IRON-BEARING TRIOCTAHEDRAL MICA-1M POLYTYPES [Articolo su rivista]
Elmi, Chiara; Brigatti, Maria Franca; Guggenheim, S.; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano
abstract

The aim of this paper is to explore the crystal chemical features of both the bulk and the uppermost surface layers of two trioctahedral Fe-rich mica-1M (space group C2/m) polytypes: a tetra-ferriphlogopite from an alkali-carbonatic complex near Tapira (Belo Horizonte, Minas Gerals, Brazil) and a Fe2+-bearing phlogopite also containing Fe3+ in tetrahedral sites from the Kovdor carbonatite-bearing alkaline-ultrabasic complex (Kola Peninsula, Russia). The chemical formulae are [XII](K0.99) [VI](Fe2+0.08 Fe3+0.15 Mg2.76 Ti0.01) [IV](Fe3+0.82 Si3.18) O10.37 (F0.24 OH1.39) and [XII](K0.94 Na0.06) [VI](Fe2+0.17 Fe3+0.05 Mg2.75 Mn0.01 Ti0.05) [IV](Fe3+0.16 Al0.84 Si3.00) O10.21 (F0.35 OH1.44) for tetra-ferriphlogopite and Fe-bearing phlogopite, respectively. The two minerals show a pronounced difference in tetrahedral composition, where Fe for Si substitution is at 20.5% in tetra-ferriphlogopite and at 4% in Fe-bearing phlogopite. Fe3+ substitution produces an increase of the tetrahedral sheet thickness and of mean tetrahedral edge lengths in tetra-ferriphlogopite with respect to Fe-bearing phlogopite. The tetrahedral rotation angle (α) changes remarkably from tetra-ferriphlogopite (α = 10.5°) to the Fe-bearing phlogopite (α = 8.5°), thus indicating a significantly greater tetrahedral ring distortion in the tetra-ferriphlogopite with respect to the Fe-bearing phlogopite. The interconnection between α and tetrahedral flattening angle () may indirectly suggest the variation of  due to Fe for Si tetrahedral substitution. Crystal chemical modifications at the mineral surface were investigated via X-ray Photoelectron Spectroscopy by comparing data related to the surface to data originating from the bulk. The local population at the anionic position can be obtained by considering that the binding energy of photoelectron peaks is dependent on the chemical state of atoms and their local environment. If oxygen is substituted by fluorine, the binding energy of Mg was observed to shift to higher values. By considering the Mg1s binding energies of tetra-ferriphlogopite and Fe-bearing phlogopite, Mg is preferentially coordinated to F and not to OH groups. This is consistent with higher BE values observed for Fe-rich phlogopite characterized by higher F content with respect to tetra-ferriphlogopite.


2014 - Crystal chemistry and surface configurations of two polylithionite-1M crystals [Articolo su rivista]
Elmi, Chiara; Brigatti, Maria Franca; Guggenheim, S.; Pasquali, Luca; Montecchi, Monica; Nannarone, S.
abstract

This paper explores the crystal chemical features of the bulk and the outermost (001) surface layers of two trioctahedral Li-rich mica-1M (space group C2) polytypes, i.e. a polylithionite (MLG-114) from Li-mica granitic pegmatite at St. Austell (SW England) and a Fe2+-rich polylithionite (Ch-140) from a rhyolite at Profitis Ilias, Chios Island, Greece. Structural formulae are [xii](K0.952Na0.019Rb0.019) [vi](Al1.034Li1.459Fe2+0.389Fe3+0.046Mn0.038Mg0.002Zn0.002Ti0.001) [iv](Al3+0.477 Si3.523)O10.081(F1.735OH0.184) and [xii](K0.992Na0.014) [vi](Al0.980Li1.028Fe2+0.787Fe3+0.022 Mn0.059Mg0.052Zn0.010Ti0.024) [iv](Al3+0.857Si3.143) O10.095 (F1.617 OH0.288) for MLG-114 and Ch-140, respectively. Each mineral is characterized by a high F content in the anion site and has tetrahedral and octahedral compositions related to the exchange vector [vi]Li-1[iv]Si-1 [vi]Fe2+ [iv]Al. Unit cell dimensions are a = 5.251(1), b = 9.066(2), c = 10.087(2) Å; β = 100.694(5)° for polylithionite MLG-114 and a = 5.282(1), b = 9.121(3), c = 10.080(3) Å; β = 100.764(5)° for Ch-140. Crystal structure refinements (agreement factors are R = 3.58% and R = 3.75% for MLG-114 and Ch-140, respectively) demonstrate that the [vi]Li-1[vi]Fe2+ [iv]Si-1[iv]Al exchange vector produces a decrease in the lateral dimensions of the tetrahedral and octahedral sheets. The decrease in basal oxygen distances results from the effect of the strain caused by the orientation of opposing tetrahedral sheets within a 2:1 layer. The decrease reduces the strain so that the basal oxygen plane can remain nearly planar. Changes in these dimensions via distortions of the tetrahedral basal-oxygen ring (α = 3.3° and α = 4.1° for MLG-114 and Ch-140, respectively) are limited. Octahedral M1 and M3 sites are similar in size and much larger than M2 and the mean electron count is M3 < M1 < M2 in MLG-114 and M1  M2 < M3 in Ch-140. Al preferentially occupies the M2 site, whereas Fe and Li are nearly disordered between M1 and M3 sites with a slight preference of Fe for the M1 site in MLG-114 and for M3 site in Ch-140. Element concentrations on the (001) surface, obtained through X-ray photoelectron spectroscopy (XPS) high-resolution spectra for Si2p, Al2p, Fe2p, K2p, Li1s, and F1s core levels, indicates that a greater amount of lithium and a smaller amount of potassium characterize the surface with respect to the bulk. The decrease in K content, commonly observed in micas, is related to its location on the cleavage surface, because the cation must be distributed equally between the two (001) surfaces generated upon cleavage. The increase in Li content on or near the (001) cleavage surface suggests a preference for cleavage near lithium-enriched regions. The surface structure of the polylithionite crystals suggests that Al, Li, and Fe cations maintain coordination features at the surface similar to the bulk. Silicon, however, which is generally in four-fold coordination, shows also a small number of [1]-fold coordinated components at a binding energy of 99.85 eV.


2014 - Electrochemical fabrication of surface chemical gradients in thiol self-assembled monolayers with tailored work-functions [Articolo su rivista]
Fioravanti, Giulia; Lugli, Francesca; Gentili, Denis; Mucciante, Vittoria; Leonardi, Francesca; Pasquali, Luca; Liscio, Andrea; Murgia, Mauro; Zerbetto, Francesco; Cavallini, Massimiliano
abstract

The studies on surface chemical gradients are constantly gaining interest both for fundamental studies and for technological implications in materials science, nanofluidics, dewetting, and biological systems. Here we report on a new approach that is very simple and very efficient, to fabricate surface chemical gradients of alkanethiols, which combines electrochemical desorption/partial readsorption, with the withdrawal of the surface from the solution. The gradient is then stabilized by adding a complementary thiol terminated with a hydroxyl group with a chain length comparable to desorbed thiols. This procedure allows us to fabricate a chemical gradient of the wetting properties and the substrate work-function along a few centimeters with a gradient slope higher than 5°/cm. Samples were characterized by cyclic voltammetry during desorption, static contact angle, XPS analysis, and Kelvin probe. Computer simulations based on the Dissipative Particle Dynamics methods were carried out considering a water droplet on a mixed SAM surface. The results help to rationalize the composition of the chemical gradient at different position on the Au surface.


2014 - High-Energy X-ray Photoemission and Structural Study of Ultrapure LaF3Superionic Conductor Thin Films on Si [Articolo su rivista]
Koshmak, Konstantin; A., Banshchikov; T., Vergentev; Montecchi, Monica; D., Céolin; J. P., Rueff; N. S., Sokolov; Pasquali, Luca
abstract

LaF3 films in the 5−40 nm thickness range were grown on Si(111) by molecular beam epitaxy. The substrates were kept at 450 °C during deposition. The films were investigated by high- energy X-ray photoemission flanked by conventional X-ray photoemission, reflection high-energy electron diffraction, and atomic force microscopy. The film growth was layer-by-layer. The surface of the films presented flat terraces, ∼100 nm wide, separated by monatomic steps, reproducing the morphology of the substrate. La 3d, F 1s, O 1s, and Si 2p core levels and valence band were measured by high-energy photoemission to investigate the reactivity of the system and the surface and bulk composition of the films, following varying sample treatments (X-ray irradiation, sputtering, heating). The fresh prepared films resulted of high purity, with no traces of reaction or intermixing at the buried interface between the substrate and the trifluoride. The X-ray beam was seen to induce F depletion at the surface and promote oxide formation. F depletion enhancement was obtained through Ar ion sputtering. An irreversible variation of the film composition was finally observed for samples heated above 300 °C, with the development of La oxides and oxofluorides. These effects were related to the high mobility of F ions in the LaF3 lattice and to the high tendency of defects formation involving F sites.


2014 - Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors (OLETs) [Articolo su rivista]
Capelli, R.; Dinelli, F.; Gazzano, M.; D’Alpaos, R.; Stefani, A.; Generali, G.; Riva, M.; Montecchi, Monica; Giglia, A.; Pasquali, Luca
abstract

Herein is described a multidisciplinary approach to understand the per- formance limitations of small molecule organic light emitting transistors (OLETs) based on a layered architecture, an innovative architecture poten- tially competitive with the state of the art and more flexible for spectral emis- sion control. The processes of charge injection and field-effect transport at metal/organic and organic/organic interfaces are analysed using microscopic and spectroscopic techniques in coordination. Atomic force microscopy and ultrasonic force microscopy are employed to characterize the interface mor- phology and the initial growth stages of organic films where charge transport actually occurs. X-ray diffraction and near edge X-ray dichroic absorption with linearly polarised light allow to determine the unit cell packing and the molecular orientation at the active organic interfaces, as well as the amount of non-ordered domains. Moreover, chemical reactivity at the interfaces are measured by X-ray photoelectron spectroscopy. It is found that a strong reac- tion occurs at the metal-organic interfaces, with molecular fragmentation. Additionally, the transport properties strongly depend on the nature of the materials forming the organic stack. Specifically, amorphous conjugated films as bottom layers can promote an increased molecular disorder in the upper active layer, with a concomitant deterioration of the conduction properties.


2014 - Structural and electronic properties of anisotropic ultrathin organic films from dichroic resonant soft x-ray reflectivity [Articolo su rivista]
Pasquali, Luca; Subhrangsu, Mukherjee; Terzi, Fabio; Angelo, Giglia; Nicola, Mahne; Koshmak, Konstantin; Vladimir, Esaulov; Chiara, Toccafondi; Maurizio, Canepa; Stefano, Nannarone
abstract

We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28° relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived.


2014 - Surface Modification of ZnO(0001)–Zn with Phosphonate-Based Self-Assembled Monolayers: Binding Modes, Orientation, and Work Function [Articolo su rivista]
Melanie, Timpel; Marco V., Nardi; Stefan, Krause; Giovanni, Ligorio; Christos, Christodoulou; Pasquali, Luca; Angelo, Giglia; Johannes, Frisch; Berthold, Wegner; Paolo, Moras; Norbert, Koch
abstract

We used partially fluorinated alkyl and aromatic phosphonates as model systems with similar molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface. The introduced surface dipole moment allows tailoring the ZnO work function to tune the energy levels at the inorganic−organic interface to organic semiconductors, which should improve the efficiency of charge injection/extraction or exciton dissociation in hybrid electronic devices. By employing a wide range of surface characterization techniques supported by theoretical calculations, we present a detailed picture of the phosphonates’ binding to ZnO, the molecular orientation in the SAM, their packing density, as well as the concomitant work function changes. We show that for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding.


2014 - Tuning the Work Function of Graphene-on-Quartz with a High Weight Molecular Acceptor [Articolo su rivista]
C., Christodoulou; A., Giannakopoulos; M. V., Nardi; G., Ligorio; M., Oehzelt; L., Chen; Pasquali, Luca; M., Timpel; A., Giglia; S., Nannarone; P., Norman; M., Linares; K., Parvez; K., Müllen; D., Beljonne; N., Koch
abstract

Ultraviolet and X-ray photoelectron spectroscopies in combi- nation with density functional theory (DFT) calculations were used to study the change in the work function (Φ) of graphene, supported by quartz, as induced by adsorption of hexaazatriphenylene−hexacarbonitrile (HATCN). Near edge X-ray absorption fine structure spectroscopy (NEXAFS) and DFT modeling show that a molecular-density-dependent reorientation of HATCN from a planar to a vertically inclined adsorption geometry occurs upon increasing surface coverage. This, in conjunction with the orientation- dependent magnitude of the interface dipole, allows one to explain the evolution of graphene Φ from 4.5 eV up to 5.7 eV, rendering the molecularly modified graphene-on-quartz a highly suitable hole injection electrode.


2014 - Workfunction tuning of graphene with organic molecules [Abstract in Atti di Convegno]
Christodoulou, Christos; Giannakopoulos, Angelos; Nardi, Marco Vittorio; Ligorio, Giovanni; Oehzelt, Martin; Chen, Liping; Heimel, Georg; Ovsyannikov, Ruslan; Pasquali, Luca; Timpel, Melanie; Monti, Oliver; Giglia, Angelo; Nannorone, Stefano; Parvez, Khaled; Linares, Mathieu; Norman, Patrick; Muellen, Klaus; Beljonne, David; Koch, Norbert
abstract

Graphene has conquered the field of Cutting Edge Technology as the ultimate next generation material. However, in order to be used widely in applications, one should be able to tune its electronic properties (i.e. work function). This may be achieved by deposition of electron acceptor or donor molecules on the surface of Graphene. In this work, we investigate the interaction between Graphene and an organic molecule, hexaazatriphenyl hexacarbonitrile (HATCN). HATCN is a strongly electron deficient molecule widely used in organic LEDs for hole injection [1,2]. By means of first principle computational techniques, we study the evolution in the work function of Graphene due to the adsorption of HATCN, as a function of the relative orientation and density of the doping molecules. Our modeling work points to a change from a lying-down to a standing-up configuration as the coverage increases, which is also observed in HATCN layer growth experiment over gold (111) and silver(111) surfaces [3,4]. The preferential standing-up configuration is confirmed by simulated Near Edge X-ray Absorption Fine Structure Spectra (NEXAFS) in excellent agreement with experimental data.


2014 - 1,4-Benzenedimethanethiol Interaction with Au(110), Ag(111), Cu(100), and Cu(111) Surfaces: Self-Assembly and Dissociation Processes [Articolo su rivista]
Jia, Juanjuan; Giglia, Angelo; Flores, Marcos; Grizzi, Oscar; Pasquali, Luca; Esaulov, Vladimir A.
abstract

In this work, we study systematically the evaporative adsorption under high vacuum conditions of 1,4-benzenedimethanethiol (BDMT) onto different metal surfaces: Ag(111), Au(110), Cu(100), and Cu(111). The study is carried out by photoemission using synchrotron radiation. In the case of Ag(111) and Au(110), at low exposures, a lying down BDMT phase is formed, with both S atoms attached to Ag and Au. A standing up phase is attained after a large exposure, above several hundred thousand Langmuir. However, also a mixed complex over layer appears to be formed, attributable to molecules sticking on top of the SAM. In the case of Au(110), heating leads to BDMT desorption with some degree of SC bond scission, and some S atoms are left in different adsorption sites with mainly two different core level binding energies. On Ag(111), after heating the sample, BDMT desorbs and also sulfidation of Ag occurs. In the case of Cu(100) and Cu(111), SC bond cleavage occurs already after initial adsorption. Lost S from BDMT molecules is adsorbed on Cu. Surface passivation occurs and only then BDMT adsorption takes place, with an interface with Cu enriched with sulfur.


2013 - And Yet it Moves! Microfluidics Without Channels and Troughs [Articolo su rivista]
Francesca, Lugli; Giulia, Fioravanti; Denise, Pattini; Pasquali, Luca; Montecchi, Monica; Denis, Gentili; Mauro, Murgia; Zahra, Hemmatian; Massimiliano, Cavallini; Francesco, Zerbetto
abstract

A simple, versatile, rapid, and inexpensive procedure based on the immersion method is developed to fabricate chemical gradients on chemically activated Si/SiO2 surfaces by a trichloro (1H,1H,2H,2H-perfluorooctyl) silane self-assembly monolayer (SAM). Contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy data based on the intensity of the signals of C1s and F1s, which progressively increase, indicate that the surface is characterized by the presence of increasing amounts of the SAM along the gradient direction. Experimental conditions are optimized by maximizing the variation of the contact angle of water drops at the starting and the ending points of the gradient. The application of the chemical gradient to droplet motion is demonstrated. The results are rationalized by dissipative particle dynamics simulations that well match the observed contact angles and the velocities of the drops. The simulations also show that the intrinsic nature of the gradient affects the velocity of the motion.


2013 - Highly luminescent yttria–silica core–shell nanoparticles by the sacrificial polymer shell method [Articolo su rivista]
M., Ghahari; P., Fabbri; F., Pilati; Pasquali, Luca; Montecchi, Monica; R., Aghababazadeh
abstract

This paper describes the correlation existing between the photoluminescence of core–shell SiO2@Y2O3:Eu3+ nanoparticles synthesised by a method based on a sacrificial polymer-shell intermediate, and the thickness of that polymer shell. The method is based on the synthesis of a covalently bonded sacrificial polymer shell by polymerization of methacryl oxyethyl isocyanate (MOI) onto silica cores; the polymer shell was demonstrated to act as a promoter and stabilizer of the adsorption of yttrium and europium ions on the surface of silica, and its thickness plays an important role on the final luminescent intensity of the hybrid particles, which is related to the adhesion and continuity of the lanthanide shell of SiO2@Y2O3:Eu3+ nanoparticles. Yttria shells were obtained with thickness varying in the range 3–20 nm by tuning the polymer shell thickness in the range 0–50 nm. Results demonstrate that photoluminescence of SiO2@Y2O3:Eu3+ nanoparticles prepared via the sacrificial polymer shell method is much higher with respect to nanoparticles obtained by a traditional procedure.


2013 - Lying-Down to Standing-Up Transitions in Self Assembly of Butanedithiol Monolayers on Gold and Substitutional Assembly by Octanethiols [Articolo su rivista]
Juanjuan, Jia; Subhrangsu, Mukherjee; Hicham, Hamoudi; Nannarone, Stefano; Pasquali, Luca; Vladimir A., Esaulov
abstract

Self-assembly of butanedithiol (C4DT) mono- layers (SAMs) on gold and substitutional assembly by octanethiols from a lying-down doubly tethered phase of butane dithiol, were investigated in a vacuum evaporation experiment by high-resolution photoemission. The intent was to address the question of transitions from a lying-down to a standing-up phase in dithiol self-assembly and whether a standing-up phase of short chain dithiol can be formed. The lying-down doubly tethered dithiol phase, with both sulfur atoms attached to gold, was formed by evaporation and then exposed to octanethiol molecules. A mixed SAM composed of standing-up octanethiol and dithiol molecules is found to be formed. The degree of dithiol replacement, as evidenced by the increasing thickness of the SAM, is found to augment slowly with increasing exposure to octanethiol. A standing-up C4DT phase was found to be formed upon rapid very large exposure to C4DT vapors. Differences in valence band photoemission between the standing-up and lying-down phases were noted and accounted for theoretically. In all cases, measurements indicate more than one possible adsorption configuration as evidenced by different sulfur binding energies.


2013 - Proximity effects and exchange bias in Co/MnF2(111) heterostructures studied by x-ray magnetic circular dichroism [Articolo su rivista]
S. M., Suturin; V. V., Fedorov; A. G., Banshchikov; D. A., Baranov; K. V., Koshmak; P., Torelli; J., Fujii; G., Panaccione; K., Amemiya; M., Sakamaki; T., Nakamura; M., Tabuchi; PASQUALI, Luca; N. S., Sokolov
abstract

Cobalt nano-structured ultrathin films were grown on orthorhombic MnF2 by molecular beam epitaxy on CaF2 epitaxial layers deposited on Si(111) substrates. The Co film was grown at room temperature. It was found to be polycrystalline, forming nano-islands with height ≈ diameter ≤ 10 nm. X-ray absorption evidences the chemical stability of the Co/MnF2 interface. Remarkably, x-ray magnetic circular dichroism (XMCD) demonstrates that the Co induces a net magnetization on the Mn ions close to the interface. The magnetic moments of these Mn ions couple antiparallel to the Co and rotate upon field reversal following the magnetization of the Co both below and high above the Ne ́el temperature of MnF2 (TN = 67 K). The density of coupled Mn moments is found to be temperature dependent, with an equivalent thickness of ∼1.5 MnF2 monolayers at 20 K, decreasing to about ∼0.5 ML as the temperature is raised to 300 K. Interestingly, the intensity of the Mn XMCD signal appears to be related to the coercivity of the Co layer. This behavior is interpreted in terms of the competition between thermal fluctuations, exchange coupling between Co and Mn at the interface and, at low temperature, the antiferromagnetic order in MnF2.


2013 - Sodian muscovite-2M1: crystal chemistry and surface features [Articolo su rivista]
Elmi, Chiara; Brigatti, Maria Franca; Guggenheim, S.; Pasquali, Luca; Montecchi, Monica; Malferrari, Daniele; Nannarone, Stefano
abstract

his study explores crystal chemical features of a sodian muscovite from Antarctica, both in the bulk of the mineral and at the uppermost surface layers. These features may support a deeper comprehension of processes affecting mineral surface and mineral physical properties. The muscovite under analysis is characterized by limited celadonite and paragonite substitutions and shows the following chemical formula: [XII](K1.73 Na0.27) [VI](Fe2+ 0.14 Al3+ 3.77 Mg 0.07 Ti0.01) [IV](Al1.91 Si6.09) O20 (F0.11 OH3.89). The sample is monoclinic, 2M1 polytype, with symmetry C2/c and unit cell parameters a = 5.1969(1), b = 9.0138(3), c = 20.0835(7) A, and β = 95.763(2)°. The two tetrahedral sites, T1 and T2, present similar distances (>T1.O< = 1.643A and >T2.O< = 1.647 A) and distortion parameter values, evidencing a disordered distribution of Si and Al at tetrahedral sites. The tetrahedral ring is significantly distorted (α = 11.2°) and the tetrahedral basal oxygen atom plane is remarkably wavy (δz = 0.224 A).Crystal chemical modifications at mineral surface were investigated by X-ray photoelectron spectroscopy. The interlayer cation K is eight-fold coordinated both in the bulk and at the surface, and its amount decreases at the surface. Sodium substitutes K and its coordination at the mineral surface is six. This evidence, together with a significant Na increase at the surface, is consistent with Na clustering close to the cleavage plane.


2013 - Studies of the interface of conducting polymers with inorganic surfaces [Articolo su rivista]
TERZI, Fabio; PASQUALI, Luca; SEEBER, Renato
abstract

Many of the properties of multi-material systems and relevant devices depend on the interfaces between the different components. This review focuses on characteriza- tion of the interfaces between intrinsically conducting poly- mers and inorganic materials consisting of metals and metal oxides. These materials are chosen because of their impor- tance in several analytical applications. Although use of conducting polymers and metals or metal oxides in analyt- ical systems, specifically in sensing, is well established, the number of novel materials used for analytical purposes is continuously increasing. This further increases the possible number of effective combinations of different materials within multicomponent systems. As a consequence, innova- tive characterization techniques have become as important as more conventional techniques. On the other hand, sophis- ticated characterisation techniques are increasingly wide- spread and, consequently, also readily accessible. This critical review is not an exhaustive discussion of all possible analytical techniques suitable for characterization of interfaces. It is, instead, limited to an overview of the most effective, relatively widespread techniques, emphasising their most significant recent advances. Critical analysis of the individual techniques is complemented by a few selected examples.


2012 - Electrochemical method for pickling and passivation of austenitic steel welds [Articolo su rivista]
Giovanardi, Roberto; Poli, Giorgio; Pasquali, Luca; Montecchi, Monica; L., Filippetti; G., Orlando
abstract

The aim of this work is to develop an innovative process for pickling and passivating tungsten inert gas welds on austenitic stainless steel in order to obtain complete removal of non-protective oxides and to passivate the metal surface. In particular, the treatment intends to increase both corrosion resistance and surface appearance of the steel without making use of traditional pickling solutions (which usually contain high concentrations of strong acids). The treatment, based on an electrochemical approach, uses specific electrolytes, and to evaluate their effect in terms of corrosion resistance and surface finish, different solutions were prepared and tested using low concentrations of weak organic acids, e.g. citric, oxalic, sulphamic, gluconic and lactic acids. This aspect is very important to reduce the hazardousness of the solution compared to the traditional pickling acids.


2012 - In-situ graphene oxide reduction during UV-photopolymerization of graphene oxide/acrylic resins mixtures [Articolo su rivista]
P., Fabbri; L., Valentini; S., Bittolo Bon; D., Foix; Pasquali, Luca; M., Montecchi; M., Sangermano
abstract

The preparation of electrically conductive acrylic resins containing reduced graphene oxide (rGO) by photopolymerization is presented. The synthesis consists of a single-step procedure starting from a homogeneous water dispersion of GO, which undergoes reduction induced by the UV radiation during the photopolymerization of an acrylic resin. The role played by the amount of radical photoinitiator added to the resin has been evaluated in relation to the in-situ reduction of GO, that was monitored by X- ray photoelectron spectroscopy. Results show that the UV-induced photopolymerization of acrylic resins with added GO gives rise to conductive acrylic composites thanks to the simultaneous reduction of GO to rGO and crosslinking of the resin. On this basis UV-induced photopolymerization is proposed as a sustainable strategy for the production of conductive graphene/polymer composites.


2012 - Photoemission and X-Ray Absorption Study of the Interface between 3,4-Ethylenedioxythiophene Related Derivatives and Gold [Articolo su rivista]
Pasquali, Luca; Terzi, Fabio; B., Doyle; Seeber, Renato
abstract

The chemisorption of 3,4-ethylenedioxythiophene (EDOT) derivatives on Au(111) single crystal and polycrystalline surfacesis studied with valence band photoemission and near edge x-ray absorption (NEXAFS). Different EDOT derived molecules are considered, namely EDOT, its dimer (bi-EDOT), 3’,4’-ethylenedioxy-2,2’:5’,2’’-terthiophene (TET) and the relevant electro-generated polymer, PEDOT, at different stages of substrate coverage. The interpretation of the spectra is carried out comparing with the DFT calculations of the densities of states of different possible reaction products that can be formed at the interface. Simulation of the NEXAFS spectrum of EDOT and comparison with literature work on thiophene and ologothiophene films are used for the interpretation of the X-ray absorption spectra of bi-EDOT and PEDOT as a function of the electro-polymerization time. Valence band features that are associated to EDOT products are identified together with new structures that are ascribed to reactive dissociation products at the interface. In particular the presence of thiophene/oligothiophene molecules is observed, together with possible alkyl chains. NEXAFS indicates that the thiophene rings tend to adopt a planar configuration at the interface.


2011 - Atomic and electronic structure of ultrathin fluoride barrier layers at the oxide/Si interface [Articolo su rivista]
PASQUALI, Luca; MONTECCHI, Monica; NANNARONE, Stefano; F., Boscherini
abstract

A SrF2 ultrathin barrier layer on Si(001) is used to form a sharp interface and block reactivity and intermixing between the semiconductor and a Yb2O3 overlayer. Yb2O3/Si(001) and Yb2O3/SrF2/Si(001) interfaces grown in ultra high vacuum by molecular beam epitaxy are studied by photoemission and x-ray absorption fine structure. Without the fluoride interlayer, Yb2O3/Si(001) presents an interface reacted region formed by SiOx and/or silicate compounds, which is about 9 A ̊ thick and increases up to 14–15 A ̊ after annealing at 500–700 ◦C. A uniform single layer of SrF2 molecules blocks intermixing and reduces the oxidized Si region to 2.4 A ̊ after deposition and to 3.5 A ̊ after annealing at 500 ◦C. In both cases we estimate a conduction band offset and a valence band offset of ∼1.7 eV and 2.4 eV between the oxide and Si, respectively. X-ray absorption fine structure measurements at the Yb LIII edge suggest that the Yb oxide films exhibit a significant degree of static disorder with and without the fluoride barrier. Sr K edge measurements indicate that the ultrathin fluoride films are reacted, with the formation of bonds between Si and Sr; the Sr–Sr and Sr–F interatomic distances in the ultrathin fluoride barrier film are relaxed to the bulk value.


2011 - Crystal chemistry and surface features of a 2M1 paragonitic muscovite [Abstract in Rivista]
Elmi, Chiara; Brigatti, Maria Franca; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; Malferrari, Daniele
abstract

The paragonitic muscovite considered [XII](K1.722 Na0.251 Ba0.010) [VI](Fe2+0.140 Al3+3.769 Cr3+0.003 Mg0.074 Ti0.012 Mn0.002) [IV](Al1.897 Si6.103) O20 (F0.109 OH3.891) came from Antarctica, it is monoclinic, 2M1 polytype, with symmetry C2/c and unit cell parameters a = 5.1969(1), b = 9.0138(3), c = 20.0835(7) (Å) and β = 95.763(2) (°).By using a multi-analytical approach this study describes the crystal chemical features of muscovite determined both in the bulk and on the uppermost surface layers. The bulk structure was refined by single crystal X-ray diffraction and the final refinement yielded the following agreement factor R = 0.0299. The two tetrahedral sites, T1 and T2, present similar distances (<T1-O> = 1.643 Å and <T2-O> = 1.647 Å) and distortion parameter values, thus evidencing a disordered distribution of [IV]Si and [IV]Al in tetrahedral sites. Tetrahedral ring is significantly distorted (α = 11.2°) and the tetrahedral basal oxygen atoms plane is remarkably wavy (Δz = 0.224Å). tetrahedral ring distortion affects not only the topology of the cleavage plane, but also the coordination of the interlayer cation, which reduces from twelve to eight. Crystal chemical modifications at mineral surface were investigated via X-ray Photoelectron Spectroscopy (XPS), by comparing data related to surface to the ones characterizing the bulk. A peculiarity of muscovite from Antarctica is a limited Na for K interlayer substitution, which was found in the bulk as well as on the mineral surface. It is well known that in micas the coordination of the interlayer cation can vary from twelve to six, depending on the distortion of the hexagonal tetrahedral ring, as measured by  angle, as observed and predicted by Weiss et al. [1].The interlayer cation K (binding energy = 293.36 eV) is confirmed to be eight-fold coordinated [2]. Na (binding energy = 1071.02 eV) substitutes K both in the bulk and at the surface. Na coordination, at mineral surface, reduces from eight to six [3]. The six-fold coordination presented by Na is consistent with  ≈ 16°, which is typical for paragonite, whose interlayer is completely occupied by Na. Chemical composition at the surface was compared to bulk chemical composition obtained by electron microprobe analysis. This comparison highlights a decrease in K content, which can be explained by its location on the cleavage surface, since the cation is expected to be distributed equally along the two surfaces generated after cleavage, and an increase in Na which can be related to Na domains close to the cleavage surface.These aspects may relate muscovite cleavage processes to the presence of “defects”, or better in-homogeneities in layer crystal chemistry. This fact may thus suggest that Na content in paragonitic muscovite could not be only attributed to a solid solution mechanism, where this cation substitutes for K, but rather to the presence of Na-dominant clusters.


2011 - High-temperature vesuvianite: crystal chemistry and surfaceconsiderations [Articolo su rivista]
Elmi, Chiara; Brigatti, Maria Franca; Pasquali, Luca; Montecchi, Monica; Laurora, Angela; Malferrari, Daniele; Nannarone, Stefano
abstract

A multi-methodical approach has been appliedfor characterizing the bulk and surface crystal chemical featuresof a high-temperature vesuvianite crystal from skarns ofMount Somma-Vesuvius Volcano (Naples, Italy). Vesuvianitebelongs to the space group P4/nnc with unit cellparameters a = 15.633(1)A ° , c = 11.834(1)A ° and chemicalformula (Ca18.858 Na0.028 Ba0.004 K0.006 Sr0.005 h0.098)19.000(Al8.813 Ti0.037 Mg2.954 Mn0.008 Fe0.1142? Fe1.3753? Cr0.008B0.202)13.511 Si18.000(O0.261 F0.940 OH7.799)9.000. Structurerefinement, which converges at R = 0.0328, demonstrates astrong positional disorder down the fourfold axes, indicatingthat the Y1 site is split into two positions (Y1A and Y1B)alternatively occupied. However, because of X4 proximity toY1B and Y1A, X4 cannot be occupied if Y1B or Y1A are.OverallY1 occupancy (Y1A ? Y1B) reaches approximately0.5, as common in vesuvianite and occupancy of Y1B site isextremely limited. Moreover, T1 position, limitedly occupied,accommodates the excess of cations generally related toY position. A small quantity (0.202 apfu) of boron is sited atthe T2 site that, like T1, is poorly occupied. The determination of the amount of each element on the (100)vesuvianite surface, obtained through X-ray photoelectronspectroscopy high-resolution spectra in the region of the Si2p,Al2p, Mg1s, and Ca2p core levels, evidences that a greateramount of aluminum and a smaller amount of calciumcharacterize the surface with respect to the bulk. Althoughboth of these features require further investigation, we mayconsider the Al increase can be related to preferential orientationof Al-rich sites on the (100) plane. Furthermore, thesurface structure of vesuvianite suggests that Al, Ca, and Mgcations maintain coordination features at the surface similarto the bulk. Silica, however, while presenting fourfold coordination,shows also a [1]-fold small coordinated componentat binding energy 99.85 eV, due to broken Si–O bonds at thesurface. The presence of eight- and nine-coordinated Cacations is suggested by a large XPS feature resulting from themerging of Ca2p3/2 and Ca2p1/2 peaks at 348.45 and352.05 eV, respectively.


2011 - Local structure at interfaces between hydride-forming metals: A case study of Mg-Pd nanoparticles by x-ray spectroscopy [Articolo su rivista]
L., Pasquini; F., Boscherini; E., Callini; C., Maurizio; Pasquali, Luca; Montecchi, Monica; E., Bonetti
abstract

The structure at the interface between elements or phases that exhibit different hydrogen (H) binding energies exerts a profound influence on the thermodynamics of H in nanophase materials. In this paper, we study the local structure at the Mg/Pd interface in Mg nanoparticles with partial Pd coating, and we map its evolution in response to annealing and H sorption. This task is accomplished by x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, also including in situ experiments, with the support of crystallographic information from x-ray diffraction. It is shown that the initial Pd surface layer reacts with Mg at relatively low temperatures, leading to irreversible formation of a Mg-rich intermetallic phase Mg6Pd. Due to the high Mg-H binding energy, this phase reversibly transforms, upon H absorption, into a nanophase mixture of magnesium hydride and a Pd-rich intermetallic with H in solid solution, MgPdHδ . These reversible structural changes are discussed with reference to recent calculations that highlight their relevance to the thermodynamics of the metal-hydride transition. The picture drawn here might be relevant to other multiphase materials presently investigated in the field of hydrogen-related science and technology.


2011 - New Insights on the Interaction between Thiophene Derivatives and Au Surfaces. The Case of 3,4-Ethylenedioxythiophene and the Relevant Polymer [Articolo su rivista]
Terzi, Fabio; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; A., Viinikanoja; T., Aaritalo; M., Salomaki; J., Lukkari; B. P., Doyle; Seeber, Renato
abstract

The nature of the interface between electrogen-erated poly(3,4-ethylenedioxythiophene) and the Au substrateis studied in detail. In particular, the adsorption of the relevantmonomer, namely, 3,4-ethylenedioxythiophene, is investigatedand compared with that of other thiophene derivatives. Di!er-ent deposition procedures have been adopted: very thin "lms ofthe thiophene derivatives have been obtained through chemi-sorption processes from vapor and liquid phases, on Au poly-crystalline substrates, Au nanoparticles possessing di!erent size,and a Au(111) single crystal. Di!erent techniques, operatingboth in situ and ex situ, have been employed for the characterization of these deposits, that is, X-ray photoemission and surface- enhanced Raman spectroscopy. The results show that the poly(3,4-ethylenedioxythiophene)/metal interface is far from being simply constituted by unreacted molecules in contact with the substrate; rather, the formation of oligothiophene species and sulfur atoms at the interface has been ascertained.


2011 - UPS, XPS, and NEXAFS Study of Self-Assembly of Standing 1,4 Benzenedimethanethiol SAMs on Gold [Articolo su rivista]
PASQUALI, Luca; TERZI, Fabio; SEEBER, Renato; NANNARONE, Stefano; D., Datta; C., Dablemont; H., Hamoudi; M., Canepa; V. A., Esaulov
abstract

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.


2010 - Additive nanoscale embedding of functional nanoparticles on silicon surface [Articolo su rivista]
M., Cavallini; F. C., Simeone; F., Borgatti; C., Albonetti; V., Morandi; C., Sangregorio; C., Innocenti; F., Pineider; E., Annese; G., Panaccione; Pasquali, Luca
abstract

We present a novel additive process, which allows the spatially controlled integration of nanoparticles (NPs) inside silicon surfaces. The NPs are placed between a conductive stamp and a silicon surface; by applying a bias voltage a SiO2 layer grows underneath the stamp protrusions, thus embedding the particles. We report the successful nanoembedding of CoFe2O4 nanoparticles patterned in lines, grids and logic structures.


2010 - Ceria-containing frit for luster in modern ceramic glaze [Articolo su rivista]
Siligardi, Cristina; Montecchi, Monica; Montorsi, Monia; Pasquali, Luca
abstract

The main goal of this work was to study the formation of theluster effect in a glaze manufactured from a ceria-containingfrit. The glazes obtained under industrial firing were studied usingX-ray photoelectron spectroscopy, X-ray diffraction at roomtemperature and high temperature, and scanning electron microscopywith energy-dispersion spectroscopy. The mechanismof the luster effect on the surface was explained. The resultsindicated that the glaze presents iridescent colors on the surface,mainly associated with the formation of a very thin layer ofCeO2 crystals.Moreover, the CeO2 crystals show a preferentialorientation and different dimensions. The CeO2 crystallizationon the glaze was also studied using a hot-stage X-ray technique.


2010 - Crystal chemistry, surface morphology and X-ray photoelectron spectroscopy of Fe-rich osumilite from Mt. Arci, Sardinia (Italy) [Articolo su rivista]
Elmi, Chiara; Brigatti, Maria Franca; Pasquali, Luca; Montecchi, Monica; Laurora, Angela; Malferrari, Daniele; Nannarone, Stefano
abstract

Microprobe anal., single crystal X-ray diffraction, XPS, at. force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) Å and chem. formula (K0.729)C (Na0.029)B (Si10.498 Al1.502)T1 (Al2.706 Fe0.2942+)T2 (Mg0.735 Mn0.091 Fe1.1842+)AO30. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe2+ content, unlike the c parameter which does not seem to be affected by chem. compn. The detn. of the amt. of each element on the mineral surface, obtained through XPS high-resoln. spectra in the region of the Si2p, Al2p, Mg1s and Fe2p core levels, suggests that Fe presents Fe2+ oxidn. state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si4+ and Si1+ components, resp., both in tetrahedral coordination. The binding energy of Al2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site.


2010 - The relationships of bulk structure, surface structure, chemistry, and physical properties of mineral phases with six- and eight-membered silicate rings [Abstract in Atti di Convegno]
Elmi, Chiara; Brigatti, Maria Franca; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; Laurora, Angela; Malferrari, Daniele
abstract

The goal of this research is to better understand the relationship between chemical composition and atomic structure (both bulk and surface structure) of minerals having six-membered silicate rings, such as osumilite and eight-membered silicate rings, such as vesuvianite. Osumilite under investigation occurs in thin fissures within a rhyolite from the volcanic massif of Mt. Arci, Sardinia (Italy). Vesuvianite sample is from skarns of Somma-Vesuvius volcano (Italy) instead.The osumilite and vesuvianite bulk structures were refined via single crystal X-ray diffraction. Osumilite sample is hexagonal, with symmetry P6/mcc and unit cell parameters a = 10.1550(6) c = 14.306(1) (Å). The structural formula is (K0.729)C (Na0.029)B’ (Si10.498 Al1.502)T1 (Al2.706 Fe2+0.219 Mg0.075)T2 (Mg0.660 Mn0.091 Fe2+1.258)AO30. When compared to literature data [1], osumilite sample from Mt. Arci is characterized by a significantly high Fe2+ content (1.478 apfu).X-ray photoelectron spectroscopy (XPS) enabled the crystal chemical characterization of osumilite surface, where iron was still observed in its divalent state, thus confirming a similarity in osumilite surface and bulk chemical composition. A further evidence of Fe oxidation state at osumilite surface is provided from X-ray absorption spectroscopy (XAS) at Fe-L2,3 edge.Vesuvianite is a complex tetragonal silicate. Vesuvianite ideal formula is Ca19(Al,Mg)13(B, Si)18O68(OH, O, F)10 and crystallizes, generally, in the space group P4/nnc. Vesuvianite presents in nature two different structural type: a high temperature type (at about 1000 °C) and a low temperature type (at about 900 °C). To recognize which type of vesuvianite, our sample belongs to, we studied its thermal decomposition via X-ray powder diffraction method by using a Philips X’Pert PRO diffractometer equipped with X’Celerator area detector. Finally we could assure that our sample is a high temperature vesuvianite with high symmetry P4/nnc.Unlike osumilite, vesuvianite surface is very complex to study because of its roughness on given faces of the crystal. However we could obtain preliminary information on the chemical composition of the surface via XPS and subsequently compare these data to bulk chemical composition. Moreover, unlike osumilite sample, vesuvianite presents Fe in octahedral site both in divalent and trivalent state, as confirmed by XAS experiments performed at the L2,3 edge of iron.


2009 - Adsorption of 3,4-ethylenedioxythiophene (EDOT) on noble metal surfaces: a photoemission and x-ray absorption study [Articolo su rivista]
Pasquali, Luca; Terzi, Fabio; Montecchi, Monica; B., Doyle; J., Lukkari; Zanfrognini, Barbara; Seeber, Renato; Nannarone, Stefano
abstract

The adsorption of 3,4-ethylenedioxythiophene (EDOT) on Au and Pt surfaces is studied by core level and valence band photoemission using synchrotron radiation and by near edgeX-ray absorption spectroscopy.To closer simulate real applications, the films are grown from aqueous solution at room temperature and are compared to an ‘ideal’ film prepared by dosing the molecules from the vapour phase on a clean Au(1 1 1) single crystal. The S 2p, C 1s and O 1s levels show multiple components which are associatedto molecular fragmentation. NEXAFS confirms that fragmentation takes place at the surface. Thiophene species as well as alkyl chains and Sn species are identified as the most probable fragmentation products.


2009 - Formation and distribution of compounds at the Ru - Si(001) ultrathin film interface [Articolo su rivista]
Pasquali, Luca; N., Mahne; Montecchi, Monica; V., Mattarello; Nannarone, Stefano
abstract

Interface formation between Ru and Si(001) has been studied by x-ray and ultraviolet photoemissions. The film properties were investigated, for metal deposition at room temperature, as a function of the Ru film thickness and as a function of the annealing temperature of a thick grown film. From the evolution of the Ru and Si core levels, we find that alloying takes place at the interface during growth and estimate the thickness of the intermixed region to be of the order of 15–20 ML. Annealing at increasingly high temperatures causes the formation of different silicide phases, which are discussed in relation to theoretical and experimental data on related transition metal silicides.


2009 - Growth dynamics of L-cysteine SAMs on single-crystal gold surfaces: a metastable deexcitation spectroscopy study [Articolo su rivista]
M., Canepa; L., Lavagnino; PASQUALI, Luca; R., Moroni; F., Bisio; DE RENZI, Valentina; S., Terreni; L., Mattera
abstract

We report on a metastable deexcitation spectroscopy investigation of the growth of L-cysteine layers deposited under UHV conditions on well-defined Au(110)-(1 × 2) and Au(111) surfaces. The interaction of He∗ with molecular orbitals gave rise to well-defined UPS-like Penning spectra which provided information on the SAM assembly dynamics and adsorption configurations. Penning spectra have been interpreted through comparison with molecular orbital DFT calculations of the free molecule and have been compared with XPS results of previous works. Regarding adsorption of first-layer molecules at room temperature (RT), two different growth regimes were observed. On Au(110), the absence of spectral features related to orbitals associated with SH groups indicated the formation of a compact SAM of thiolate molecules. On Au(111), the data demonstrated the simultaneous presence, since the early stages of growth, of strongly and weakly bound molecules, the latter showing intact SH groups. The different growth mode was tentatively assigned to the added rows of the reconstructed Au(110) surface which behave as extended defects effectively promoting the formation of the S–Au bond. The growth of the second molecular layer was instead observed to proceed similarly for both substrates. Second-layer molecules preferably adopt an adsorption configuration in which the SH group protrudes into the vacuum side.


2009 - Growth of N,N′-Bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxdiimide Films on Ag (111) [Articolo su rivista]
Laura N., Serkovic Loli; Hicham, Hamoudi; J., Esteban Gayone; M., Luz Martiarena; Esteban A., Sánchez; Oscar, Grizzi; Pasquali, Luca; Nannarone, Stefano; Bryan P., Doyle; Céline, Dablemont; Vladimir A., Esaulov
abstract

We have studied the self-assembly characteristics, the electronic structure, and the thermal stability of thin N,N′-bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) films grown on Ag (111) by a broad set of surface science analysis techniques. The deposition of single molecules to the formation of very thin films performed at room temperature under ultrahigh-vacuum (UHV) conditions was followed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS), and was modeled by density functional theory (DFT) calculations. STM pictures show that substrate step edges are the starting point for molecule adsorption, which is followed by formation of islands and eventually a monolayer. The monolayer is composed of large domains with ordered molecules with their perylene-3,4,9,10- tetracarboxdiimide (PTCDI) core lying almost parallel to the surface. For further exposure, multilayer stacked domains are formed. For thicker films molecular order could not be established. Upon heating it is found that the multilayer is stable up to 150 °C where a rapid desorption takes place, followed by the dissociation of the molecules, leaving an ordered monolayer of presumably perylene core type molecules.


2009 - Inertial sensor surface properties for LISA Pathfinder and their effect on test mass discharging [Articolo su rivista]
M. O., Schulte; D. N. A., Shaul; D., Hollington; S., Waschke; T. J., Sumner; P. J., Wass; Pasquali, Luca; Nannarone, Stefano
abstract

Maintaining the low acceleration noise target for LISA Pathfinder requires the removal of electrostatic charge from the test masses. The charge management device (CMD) has been designed to remove charge either continuously with a low noise impact, or intermittently at high discharge rates. Recent measurements and simulations have highlighted the sensitivity of the discharge performance to the inertial sensor surface properties. In response to this Imperial College London (ICL) and the University of Trento (UTN) have initiated a laboratory program to characterize the properties of representative surfaces in detail. The aim of this program is to ensure that the behaviour of the surfaces used in flight is well understood and that the discharge simulations and pre-flight measurements are representative of the in-flight performance. The discharge process has been simulated, taking into account surface properties and we use simulation results to understand the experimental results from the test mass discharge experiments performed using the Trento torsion pendulum. Finally, we describe a new concept to implement redundancy and ruggedness in a UV-LED based design for LISA, incorporating recent advances made for LISA Pathfinder charge management.


2009 - Lead free Cu-Containing Frit for Modern Metallic Glaze [Articolo su rivista]
Siligardi, Cristina; Montecchi, Monica; Montorsi, Monia; Pasquali, Luca
abstract

A lead-free frit containing high amount of copper and its glazeconceived for fine porcelain stoneware tiles were investigated.The glazes obtained under industrial firing were studied using Xrayphotoelectron spectroscopy, X-ray diffraction, optical andscanning electron microscopy with energy dispersion spectroscopy.The mechanism of the red color formation on the surfacewas explained. The results indicated that the glaze presents ametallic gloss and iridescent colors on the surface, mainly associatedto the formation of CuO crystallites and Cu metallicparticles. Moreover, the Cu glaze showed a good chemical durability;this is particularly interesting because commercial Cufrits usually present poor chemical properties.


2009 - Lustre Ce-glazes: scientific aspects of materials having high aesthetic properties [Relazione in Atti di Convegno]
Siligardi, Cristina; Montecchi, Monica; Pasquali, Luca; Claudio, Ansaloni
abstract

Three frits conceived for high porosity single firing tiles containing different amounts of cerium oxide and their glazes were investigated. The glazes obtained under industrial firing were studied using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersion spectroscopy (EDS). The mechanism of the lustre effect on the surface was explained. The results indicate that the glaze presents iridescent colours on the surface, mainly associated to the formation of a very thin layer of CeO2 crystals. Moreover, the CeO2 crystals present a preferential orientation and different dimensions.


2009 - Osumilite from Mount Arci (Sardinia, Italy): bulk and surface crystal chemistry [Abstract in Rivista]
Elmi, Chiara; Brigatti, Maria Franca; Pasquali, Luca; Montecchi, Monica; Laurora, Angela; Malferrari, Daniele; Nannarone, Stefano
abstract

This study is focused on osumilite crystal chemistry and on its surface chemical characteristics and properties. Osumilite belongs to the milarite mineral group. This mineral, with the ideal formula XIICIXB’2VIA2(T2)3IV(T1)12O30 crystallizes in the space group P6/mcc. T1 is mainly occupied by Si and to a smaller amount by Al, T2 is usually occupied by Al with Fe and little Mg, whereas A octahedron mostly contains Mg and Fe. The ring arrangement defines structural channels occupied by the twelve-coordinated C site which is characterized by the presence of alkaline cations (Na, K).The osumilite from Mt Arci (Sardinia) occurs in thin fissures and small cavities within a rhyolite from the volcanic massif of Mt. Arci, Sardinia. The mineral forms regular dark blue hexagonal prismatic-tabular crystals associated to trydimite and sporadic phlogopite crystals. The osumilite structure was refined by single crystal X-ray diffraction and the final refinement yielded the following agreement factors R = 0.0199. The sample studied is hexagonal, with symmetry P6/mcc and unit cell parameters a = 10.1550(6) c = 14.306(1) (Å). The structural formula is (K0.729)C (Na0.029)B’ (Si10.498 Al1.502)T1 (Al2.706 Fe2+0.219 Mg0.075)T2 (Mg0.660 Mn0.091 Fe2+1.258)AO30. In respect of data from literature [2], osumilite sample from Mt. Arci is characterized by a significantly high Fe content (1.478 apfu).In order to study the morphology of osumilite from Mt. Arci, an atomic force microscopy (AFM) filtered images of osumilite was performed. The (001) surface roughness of osumilite is about 0.40 nm, thus it confirms the presence of the channels defined by the double rings of tetrahedra. The distance between two dark areas, defining channel position, is 1 nm, which well agree with the unit cell parameter a value.X-ray photoelectron spectroscopy (XPS) analysis was carried out to measure chemical composition and quantity of each element on the osumilite surface. Higher resolution spectra in the region of Si2p, Al2p, Mg1s and Fe2p core levels were measured. From data collected, we could say that the osumilite surface is chemically well characterized and the elemental composition of surface totally corresponds to the bulk chemical composition. Moreover, comparing our results to data from literature we obtained also crystal chemical information on Fe, Mg, Si and Al high resolution spectra, compared to the crystal chemical characteristics of the bulk.In order to confirm the valence of Fe and the position of this element in the A site, X-ray absorption spectroscopy (XAS) was also made in the L2,3 edge of iron comparing data from literature.


2008 - Adsorption geometry variation of 1,4-benzenedimethanethiol self-assembled monolayers on Au(111) grown from the vapor phase [Articolo su rivista]
Pasquali, Luca; Terzi, Fabio; Seeber, Renato; B. P., Doyle; Nannarone, Stefano
abstract

1,4-benzenedimethanethiol was chemisorbed from the vapor phase onto Au(111). The chemisorption geometry, molecular orientation, and bonding properties were studied at different degrees of surface coverage by photoelectron spectroscopy, metastable deexcitation spectroscopy, and near-edge x-ray absorption fine structure spectroscopy at the carbon K edge. Two main chemisorption regimes were identified: at low coverage the molecules adopt a flat configuration, then, as the molecular density of the first layer increases, the reduction of the available chemisorption sites induces the newly bonded molecules to assume a vertical alignment, with only one of the sulphur head groups interacting with the substrate. Experimental results were interpreted on the basis of theoretical calculations that we performed on the free molecule concerning the molecular orbitals' density of states and simulated x-ray absorption. (c) American Institute of Physics.


2008 - Molecular states of polyacenes grown on noble metal surfaces [Relazione in Atti di Convegno]
M., Pedio; B., Doyle; N., Mahne; A., Giglia; Nannarone, Stefano; Montecchi, Monica; Pasquali, Luca
abstract

Here we present a combined photoemission (UPS), metastable deexcitation (MDS) and optical absorption (NEXAFS) at C K-edge study of molecular states of polyacenes grown on Ag(111) and Au(111), from submonolayer to multilayer thicknesses. We focus on the evolution of the HOMO and LUMO molecular states induced by the adsorption from submonolayer to monolayer thickness and we find a different redistribution of these states in the various systems formed at RT: while a strong redistribution of the molecular states takes place in Pn/Ag(111) and Tc/Ag(111) interface, a weaker interaction is indicated for Tc/Au(111).


2008 - Parallel-local anodic oxidation of silicon surfaces by soft stamps [Articolo su rivista]
Albonetti, C; Martinez, J; Losilla, NS; Greco, P; Cavallini, M; Borgatti, F; MONTECCHI, Monica; PASQUALI, Luca; Garcia, R; BISCARINI, FABIO
abstract

We investigate the fabrication of nanometric patterns on silicon surfaces by using the parallel-local anodic oxidation technique with soft stamps. This method yields silicon oxide nanostructures 15 nm high, namely at least five times higher than the nanostructures made with local anodic oxidation using atomic force microscopy, and thanks to the size of the stamp enables one to pattern the surface across a centimetre length scale. To implement this technique, we built a machine to bring the metallized polydimethylsiloxane stamp in contact with the silicon surface, subsequently inserted in a sealed chamber with controlled relative humidity. The oxide nanostructures are fabricated when a bias voltage of 36 V is applied between the stamp and the silicon for 2 min, with a relative humidity of 90%. The flexibility of the stamp enables a homogeneous conformal contact with the silicon surface, resulting in an excellent reproducibility of the process. Moreover, by means of two subsequent oxidations with the same stamp and just rotating the sample, we are able to fabricate complex nanostructures. Finally, a detailed study of the oxidation mechanism, also using a finite element analysis, has been performed to understand the underlying mechanism.


2007 - BEAR: a Bending Magnet for Emission Absorption and Reflectivity [Articolo su rivista]
Nannarone, Stefano; A., Giglia; N., Mahne; A., De Luisa; B., Doyle; F., Borgatti; M., Pedio; Pasquali, Luca; G., Naletto; M. G., Pelizzo; G., Tondello
abstract

The BEAR (Bending Magnet for Absorption Emissionand Reflectivity) apparatus is presented. The main partsof the apparatus including the transport optics and theexperimental end stations are essentially described. Anumber of scientific results are presented dealing withon going activity at BEAR. They include optical propertiesof materials, studies of buried interfaces, diffuse interfacescattering of light and the determination of electronicstructure and local geometry of a chemisorbedmolecule on a metal surface.


2007 - Bonding and orientation of 1,4-benzenedimethanethiol on Au(111) prepared from solution and from gas phase [Articolo su rivista]
Pasquali, Luca; Terzi, Fabio; Zanardi, Chiara; Seeber, Renato; N., Mahne; Nannarone, Stefano
abstract

The orientation and bonding of 1,4-benzenedimethanethiol molecules on Au(111) is studied by means of x-ray and ultraviolet (UV) photoemission, xray absorption and metastable deexcitation spectroscopy. The organic films are prepared both from solution and by exposing the clean substrate to the vapours of the substance in an evacuated environment. This leads to two different growth modes: when self-assembled monolayers (SAMs) are prepared from solution, the molecules tend to form a bilayer film with the molecules standing upright and with the molecular axis forming an angle of about 30 degrees with respect to the substrate normal; when growth is carried out from the gas phase, the molecules tend to assume at the earliest stages of exposure a flat-lying configuration, with both sulfur end-groups bonding to Au; at increasing exposure the surface coverage presents a saturation and the chemisorbed molecules tend to assume an upright arrangement.


2007 - Interface chemistry and epitaxial growth modes of SrF2 on Si(001) [Articolo su rivista]
Pasquali, Luca; Suturin, Sm; Kaveev, Ak; Ulin, Vp; Sokolov, Ns; Doyle, Bp; Nannarone, Stefano
abstract

Molecular beam epitaxy has been used to grow SrF2 thin films on Si(001). The growth modes have been investigated by atomic force microscopy, electron diffraction, and photoemission. Two principal growth regimes have been identified: (i) when deposition is carried out with the substrate held at a temperature of 700-750 degrees C, SrF2 molecules react with the substrate giving rise to a Sr-rich wetting layer on top of which three dimensional bulklike fluoride ridges develop; (ii) when deposition is carried out with the substrate held at 400 degrees C, a nanopatterned film forms with characteristic triangular islands. Results are compared to the growth mode of CaF2 on Si(001) under analogous deposition conditions. Morphological and structural differences between the two systems are associated with the larger lattice parameter of SrF2 with respect to CaF2, resulting in a larger mismatch with the Si substrate.


2007 - Structure and properties of 1,4-benzenedimethanethiol films grown from solution on Au(111): An XPS and NEXAFS study [Articolo su rivista]
Pasquali, Luca; Terzi, Fabio; Zanardi, Chiara; Pigani, Laura; Seeber, Renato; G., Paolicelli; Sm, Suturin; N., Mahne; Nannarone, Stefano
abstract

Core level photoemission and X-ray absorption at the carbon C 1s edge are applied to the study of chemisorption of 1,4-benzenedimethanethiol molecules oil Au(111) and to investigate the effect of the solvent used to prepare the organic films. 1,4-Benzeiledimethanethiol films were prepared in polar and non-polar solvents, i.e. ethanol, methanol and n-hexane. 1,4-Benzenedimethanethiol molecules are anchored to the substrate through a single S-Au bond; the molecules in the films tend to assume an upright orientation, the aromatic ring resulting tilted towards the direction perpendicular to the substrate plane. The films present an effective thickness corresponding to more than a single layer. Only small differences are observed among behaviours in different solvents. It is concluded that the filial properties and conformation of the 1,4-belizenedimethanethiol films are largely determined by the reciprocal interactions between the molecules and with the substrate. (c) 2007 Elsevier B.V. All rights reserved.


2006 - Cobalt on calcium fluoride: Initial stages of growth and magnetic properties [Articolo su rivista]
Pasquali, Luca; B. P., Doyle; F., Borgatti; A., Giglia; N., Mahne; M., Pedio; Nannarone, Stefano; A. K., Kaveev; A. S., Balanev; B. B., Krichevtsov; S. M., Suturin; N. S., Sokolov
abstract

Epitaxial cobalt films on CaF2(1 1 0) buffer layers on Si(0 0 1) were grown by molecular beam epitaxy. It is found that Co grows in a face-centered cubic lattice forming nanodimensional islands. The islands tend to align along the parallel grooves which characterize the self-patterned CaF2(1 1 0) surface grown on Si(0 0 1). Photoemission was used to probe the uniformity of the film and the occurrence of chemical reactions with the substrate. X-ray magnetic circular dichroism at the Co L2,3 edges was used to access the magnetic properties of the films. The thick Co film presents an in-plane easy magnetization axis along the directions of the grooves of the CaF2(1 1 0) surface.


2006 - FISICA A [Monografia/Trattato scientifico]
Nannarone, Stefano; Pasquali, Luca
abstract

Il testo rappresenta un manuale di introduzione alla Fisica classica per studenti di corsi di laurea scientifici di primo livello, ingegneria in particolare. Esso presenta una quadro essenziale ma completo, che partendo dai fondamenti, copre le aree della meccanica e dell'elettromagnetismo classici. Partendo dalla scrittura dell’equazione di Newton, introducendo l’origine delle forze ed a seguire fornendo il concetto di campo vengono man mano composti gli strumenti concettuali e formali con cui si imposta e si risolve il problema del moto sotto l’azione di forze. Lungo tale costruzione, risalto viene dato alle proprietà dei campi in quanto tali, giungendo alla scrittura delle equazioni di Maxwell in forma locale.


2006 - Perfluoropolyether-based organic-inorganic hybrid coatings [Articolo su rivista]
Fabbri, Paola; Messori, Massimo; Montecchi, Monica; Nannarone, Stefano; Pasquali, Luca; Pilati, Francesco; C., Tonelli; M., Toselli
abstract

Perfluoropolyether-based organic-inorganic hybrids were prepared by sol-gel process and applied as functional coatings onto glass substrates. Contact angle analysis and X-ray photoemission spectroscopy were carried out in order to characterize the surface of the prepared coatings. A strong hydrophobic and lipophobic character was observed for all tested compositions. The wettability behaviour was found to be dependent on both the coating technique and the reaction time of the sol-gel reaction. A strong surface segregation of perfluoropolyether segments was noted for all the prepared samples as evidenced by XPS analysis. On the basis of the obtained results, these materials could be used as functional coatings to prepare water and oil repellent glasses or other substrates.


2006 - Structural and photoemission studies of SrF2 adsorption on Si(001) [Articolo su rivista]
Pasquali, Luca; Suturin, Sm; Balanev, A; Kaveev, Ak; Sokolov, Ns; Doyle, Bp; Borgatti, F; Giglia, A; Mahne, N; Pedio, M; Nannarone, Stefano
abstract

The growth modes of SrF2 on Si(001) are investigated by AFM and ultraviolet photoemission. Two growth regimes are identified according to the substrate temperature during deposition, resulting in flat and ordered fluoride layers or in nano-patterned films with characteristic triangular islands. The flat layer growth obtained at high temperature is accompanied by molecular dissociation at the interface.


2005 - Calcium fluoride on Si(001): adsorption mechanisms and epitaxial growth modes [Articolo su rivista]
Pasquali, Luca; S. M., Suturin; V. P., Ulin; N. S., Sokolov; G., Selvaggi; A., Giglia; N., Mahne; M., Pedio; Nannarone, Stefano
abstract

Growth of CaF2 on Si(001) is studied as a function of the substrate temperature during deposition for coverages from fraction of a monolayer (ML) up to several monolayers. Structural and morphological studies using atomic force microscopy, low-energy electron diffraction, and reflection high-energy electron diffraction are combined with measurements of core-level photoemission and x-ray absorption. Bonding between CaF2 molecules and Si(001) substrates is followed by monitoring core-level shifts and x-ray absorption line shape. It is found that a dissociative reaction occurs at high deposition temperatures (similar to 750 degrees C), giving rise to a 1-ML-thick uniform wetting layer, which is bonded with the substrate through Ca atoms. This wetting layer changes the surface periodicity from double domain 2x1+1x2 to single domain 3x1. Three-dimensional CaF2 elongated islands develop on top of the wetting layer, with their (110) planes parallel to the Si surface plane. At temperatures below 600 degrees C no dissociative reaction takes place for CaF2; nanodimensional islands develop in the form of rectangular-based huts. The crystallographic orientation of these islands is parallel to that of the Si(001) substrate. The data are compared to results obtained on CaF2 deposited on Si(111).


2005 - Metastable He deexcitation at semiconductor interfaces [Articolo su rivista]
Pasquali, Luca; Nannarone, Stefano
abstract

A review is given of the application of metastable deexcitation spectroscopy (MDS) to the study of the interface formation between semiconductors and different materials. In particular we present an overview of the results obtained on nanostructured interfaces, where strain and reaction between the substrate and the overlayer atoms drive the growth mode and the morphology of the system. As prototypical examples we discuss the growth of CaF, on silicon and rare earths (Yb, Er) on silicon and gallium arsenide. The mechanisms and chemical reactions which bring to interface formation are examined on the basis of MDS results and their comparison with photoemission.


2005 - Perfluoropolyether-based organic-inorganic hybrid coatings: Preparation and surface characterisation [Articolo su rivista]
Messori, Massimo; M., Toselli; Pilati, Francesco; Fabbri, Paola; Pasquali, Luca; M., Montecchi; Nannarone, Stefano; C., Tonelli
abstract

Hydrophobic-oleophobic coatings were prepared by using the sol-gel process with organic-inorganic hybrids based on perfluoropolyether-polycaprolactone (PFPE-PCL) copolymers. Hybrid coatings were prepared by changing the organic-inorganic ratio, the PFPE/PCL ratio in the copolymer, and the reaction conditions. A complete surface characterisation was carried out by contact angle analysis and x-ray photoemission spectroscopy (XPS). In all cases the wettability of the coatings was found to be very low. Surface composition analysis showed that the surface segregation of the fluorinated PFPE segments was always high enough to give a very high hydrophobic character to the coating surface.


2005 - Perfluoropolyether-based Organic-Inorganic Hybrid Coatings: Preparation and Surface Characterization [Articolo su rivista]
Messori, Massimo; Toselli, M.; Pilati, Francesco; Fabbri, Paola; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; Tonelli, C.
abstract

not available


2005 - Perfluoropolyether-silica hybrids: preparation and surface characterization [Articolo su rivista]
Messori, Massimo; M., Toselli; Pilati, Francesco; Fabbri, Elena; Fabbri, Paola; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; C., Tonelli
abstract

α,ω-Triethoxysilane terminated perfluoropolyether-polycaprolactone block copolymers, TXCL(x)Si, were synthesised, characterised and subsequently reacted with tetraethoxysilane to prepare transparent organicinorganic hybrid materials by using the sol–gel process. Surface characterization was carried out through contact angle measurements. All the materials investigated (independently on both the reaction time of the sol–gel reaction before coating application and the bulk organic-inorganic ratio) showed a strong hydrophobic character when compared with non-fluorinated and fully inorganic control materials. The highest water contact angles were obtained when TXCL(x)Si oligomer used for the preparation of the hybrid contained short hydrogenated polycaprolactone segments. In all cases, the surface tension values indicated that the very surface at the air-coating interface showed almost completely perfluoropolyether segments.


2005 - Rivestimenti ibridi organici-inorganici a base di perfluoropolieteri: preparazione e caratterizzazione superficiale [Articolo su rivista]
M., Messori; M., Toselli; Pilati, Francesco; Fabbri, Paola; Pasquali, Luca; Montecchi, Monica; Nannarone, Stefano; C., Tonelli
abstract

not available


2005 - Synthesis and characterization of praseodymium-doped ceria powders by a microwave-assisted hydrothermal (MH) route [Articolo su rivista]
Bondioli, Federica; Ferrari, Anna Maria; Lusvarghi, Luca; Manfredini, Tiziano; Nannarone, Stefano; Pasquali, Luca; Selvaggi,
abstract

Nanocrystalline Pr-doped ceria powders were prepared for the first time by a microwave-assisted hydrothermal route. The effect of the microwave treatment in relation to the conventional hydrothermal technique was evaluated. The samples prepared were characterized in terms of composition, crystalline structure, particle shape and size distribution by X-ray diffraction, transmission electron microscopy (TEM) and specific surface area analysis (BET). The color properties of these solid solutions were also evaluated as a function of synthesis conditions and composition. Finally the oxidation state of the Pr cations and the relative quantities of Pr(IV) and Pr( III) were investigated by X-ray absorption at the M-4,M-5 Pr absorption edge.


2005 - 3-methylthiophene Self Assembled Monolayers on planar and nanoparticle Au surfaces [Articolo su rivista]
TERZI, Fabio; SEEBER, Renato; PIGANI, Laura; ZANARDI, Chiara; PASQUALI, Luca; NANNARONE, Stefano; M., FABRIZIO; S., DAOLIO
abstract

In this article the adsorption of 3-methylthiophene on planar and nanoparticle Au surfaces is investigated. The resulting systems are compared with a benchmark system based on 1-decanethiol. The characterization data collected evidence the formation of a packed 3-methylthiophene SAM on the planar surface. In particular, spectroscopic investigations suggest that 3-methylthiophene aromatic system is not adsorbed on the surface through the pi-electron system but rather through the S atom alone. On the other hand, the behavior of 3-methylthiophene on nanoparticle surfaces is notably different from that of the alkanethiol. Only a limited fraction of the surface of Au nanoparticles results to be actually coated after purification; this notwithstanding, the nanoparticle growth seems to be strongly influenced by the presence of such a labile encapsulating agent.


2004 - Initial stages of MBE growth and formation of CaF2/Si(001) high-temperature interface [Articolo su rivista]
Sokolov, Ns; Suturin, Sm; Ulin, Vp; Pasquali, Luca; Selvaggi, G; Nannarone, Stefano
abstract

It has been recently shown [Appl. Surf. Sci. 175-176 (2001) 619] that growth modes and epitaxial relations of CaF2 on Si(0 0 1) drastically depend on the growth temperature: below 500 degreesC 3D nucleation and CaF2(0 0 1)/Si(0 0 1) growth were observed; above 650 degreesC a wetting layer followed by the formation of CaF2 stripes and CaF2(1 1 0)/Si(0 0 1) growth have been found. In this work, electron diffraction, atomic force microscopy (AFM) and photoemission spectroscopy with use of synchrotron radiation were applied to study initial stages of growth and formation of the interface during CaF2 deposition on Si(0 0 1) surface at high temperature. AFM showed that initial (2 x 1 + 1 x 2) Si(0 0 1) surface transforms into anisotropic wetting layer serving as template for subsequent CaF2(1 1 0) growth. Photoelectron spectroscopy showed reduction of Ca2+ and electron transfer from the reduced calcium to the interface Si atoms with formation of negatively charged interface Si layer.


2004 - Photon reflectivity distributions from the LHC beam screen and their implications on the arc beam vacuum system [Articolo su rivista]
Mahne, N; Baglin, V; Collins, Ir; Giglia, A; Pasquali, Luca; Pedio, M; Nannarone, Stefano; Cimino, R.
abstract

In particle accelerators with intense positively charged bunched beams, an electron cloud may induce beam instabilities and the related beam induced electron multipacting (BIEM) can result in an undesired pressure rise. In a cryogenic machine such as the large hadron collider (LHC), the BIEM will introduce additional heat load. When present, synchrotron radiation (SR) may generate a significant number of photoelectrons, that may play a role in determining the onset and the detailed properties of the electron cloud related instability. Since electrons are constrained to move along field lines, those created on the accelerator equator in a strong vertical (dipole) field cannot participate in the e-cloud build-up. Therefore, for the LHC there has been a continuous effort to find solutions to absorb the photons on the equator. The solution adopted for the LHC dipole beam screens is a saw-tooth structure on the illuminated equator. SR from a bending magnet beamline at ELETTRA, Italy (BEAR) has been used to measure the reflectivities (forward, back-scattered and diffuse), for a flat and a saw-tooth structured Cu co-laminated surface using both white light SR, similar to the one emitted by LHC, and monochromatic light. Our data show that the saw-tooth structure does reduce the total reflectivity and modifies the photon energy distribution of the reflected photons. The implications of these results on the LHC arc vacuum system are discussed.


2004 - Prevention of plasticizer leaching from PVC medical devices by using organic-inorganic hybrid coatings [Articolo su rivista]
Messori, Massimo; A., Toselli; F., Pilati; Fabbri, Elena; P., Fabbri; Pasquali, Luca; S., Nannarone
abstract

α,ω-triethoxysilane terminated poly(ethylene oxide) (PEO-Si) was prepared and used to produce organic/inorganic hybrids by the sol–gel approach. These hybrids were used as coatings for flexible PVC tubes in order to reduce the plasticizer leaching from PVC medical devices. Extraction tests carried out with hexane indicated that all coating composition investigated are able to strongly reduce (about one order of magnitude) the leaching of di-ethylhexyl phthalate in hexane. The best results were obtained by an accurate balance of organic and inorganic phases content. XPS analysis showed a preferential segregation of silica onto the outer surface suggesting that a high inorganic content at the coating–extraction medium interface was present.


2004 - The BEAR Beamline at Elettra [Relazione in Atti di Convegno]
Nannarone, Stefano; F., Borgatti; A., Deluisa; B. P., Doyle; G. C., Gazzadi; A., Giglia; P., Finetti; N., Mahne; Pasquali, Luca; M., Pedio; G., Selvaggi; G., Naletto; M. G., Pelizzo; G., Tondello
abstract

The BEAR (Bending Magnet for Emission Absorption and Reflectivity) beamline is installed at the right exit of the 8.1 bending magnet at ELETTRA. The beamline - in operation since January 2003 - delivers linear and circularly polarized radiation in the 5 - 1600 eV energy range. The experimental station is composed of a UHV chamber for reflectivity, absorption, fluorescence and angle resolved photoemission measurements and a UHV chamber for in-situ sample preparation.


2004 - The UHV Experimental Chamber For Optical Measurements (Reflectivity and Absorption) and Angle Resolved Photoemission of the BEAR Beamline at ELETTRA [Relazione in Atti di Convegno]
Pasquali, Luca; A., De Luisa; Nannarone, Stefano
abstract

The experimental station of the BEAR (Bending magnet for Emission, Absorption and Reflectivity) beamline at ELETTRA (Trieste, Italy) is an UHV chamber conceived to fully exploit the spectroscopic possibilities offered by the light spot produced by the beamline. Spectroscopies include reflectivity (θ-2θ and diffuse), optical absorption, fluorescence and angle resolved photoemission. The chamber can be rotated around the beam axis to select the s (TE) or p (TM) incidence conditions and/or the position of the ellipse of polarization with respect to the sample. Photon detectors (e.g. photodiodes) and electron detector (hemispherical analyzer - 1° angular resolution, 20 meV energy resolution) cover about completely the full 2π solid angle above the sample surface in any light incidence condition.


2003 - Ag(100) surface density of states probed by metastable deexcitation spectroscopy: A comparison between experiment and theory [Articolo su rivista]
Pasquali, Luca; Sapet, Mc; Staicu Casagrande, Em; Cortona, P; Esaulov, Va; Nannarone, Stefano; Canepa, M; Terreni, S; Mattera, L.
abstract

The metastable deexcitation spectrum of Ag(100) is measured and information on the effective density of states probed by the metastable atom is obtained by deconvolution of the experimental spectrum, under the hypothesis that it reflects the self-convolution of the surface density of states. Layer resolved semi-empirical LCAO calculations are compared to the deconvolution curve, showing poor agreement as far as the branching ratios of sp versus d contributions are concerned. The agreement between theory and experiment strongly improves if an energy dependent exponential decay factor is included in the first layer calculated density of states to account for the energy dependent increased overlap between electronic states of the solid and metastable atom outside the surface.


2003 - Chlorine-induced modifications in the electronic structure of Ag surfaces: a metastable deexcitation spectroscopy and photoemission comparative study [Articolo su rivista]
Pasquali, Luca; Nannarone, Stefano; Canepa, M; Staicu Casagrande, Em; Esaulov, Va
abstract

Surface-sensitive spectroscopic techniques, namely metastable deexcitation spectroscopy (MDS) and ultraviolet photoemission (UPS), have been applied to investigate the effects of chlorine chemisorption on the electronic properties (surface density of states and charge density) of Ag(100), Ag(l 10) and Ag(l 11) surfaces. Initial stages of chemisorption, up to the formation of a saturated Cl overlayer, have been examined. In particular, MDS permitted us to observe at low Cl gas exposure a progressive depletion of the Ag (5s) charge due to transfer and bonding with Cl atoms. From both MDS and UPS it was possible to observe the development of Cl (3p) bonding and anti-bonding states, the amount of their splitting increasing with coverage. Differences between chemisorption at the three surfaces have been noticed and they have been justified in terms of the different adatom packing and possible formation of small AgCl clusters (especially for the Ag(111) surface).


2003 - Flame Retarding Poly(Methyl Methacrylate) with Nanostructured Organic-Inorganic Hybrids Coatings [Articolo su rivista]
Messori, Massimo; M., Toselli; Pilati, Francesco; Fabbri, Elena; P., Fabbri; S., Busoli; Pasquali, Luca; S., Nannarone
abstract

Organic–inorganic hybrid materials were prepared starting from tetraethoxysilane and α- or α,ω-triethoxysilane terminated poly(ε-caprolactone) (PCL–Si) using the sol–gel process. In all cases the formation of nanocomposites with a high level of interpenetration between organic and inorganic phases was noted. Poly(methyl methacrylate) slabs were dip-coated with PCL–Si/silica hybrids and a very strong increase of the flame resistance (also after UV irradiation) was noted for all coating compositions without marked differences with respect to hybrid compositions. This behavior was attributed to a preferential segregation of silica onto the outer surface, as evidenced by XPS analysis.


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, Luca; D'Addato, Sergio; Nannarone, Stefano
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 - Initial stages of CaF2 growth and wetting layer formation on Si[001] [Relazione in Atti di Convegno]
Sokolov, N. S.; Suturin, S. M.; Ulin, V. P.; Pasquali, L.; Selvaggi, G.; Nannarone, S.
abstract

It has been recently found that different types of CaF2 nanostructures can be formed on Si[001] substrates depending on the growth conditions. Below 500°C, nearly square-shaped dots nucleate on bare Si[001] surface; above 650°C, at submonolayer coverage, a so-called "wetting layer" forms. At higher coverage, formation of CaF2 stripes of a few nanometers in height and width takes place on top of the wetting layer. Interestingly, CaF2 [001]∥Si[001] in the dots and CaF2 [110]∥Si[001] in the stripes. In this work, initial stages of CaF2 epitaxial growth on Si[001] were studied by atomic force microscopy and photoemission spectroscopy with emphasis on formation and chemical composition of the wetting layer.


2002 - Structural transition in Fe ultrathin epitaxial films grown on Ni(111) [Articolo su rivista]
Gazzadi, Gc; Bruno, F; Capelli, R; Pasquali, Luca; Nannarone, Stefano
abstract

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni(111), has been performed in the 1.5-18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc(110) phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann (NW) in-plane orientation of the bcc(110) cell (<001>(bcc)parallel to<110>(fcc)) is favored over the Kurdjumov-Sachs (<1 (1) over bar1>(bcc)parallel to<1 (1) over bar0>(fcc)) orientation. The best-fit vertical interlayer distance between bcc(110) planes is d(NW)=2.11 Angstrom (+3.9% expansion) at 6 ML and relaxes to d(NW)=2.05 Angstrom (+1.0%) at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc(110) surface basis vectors changes from 67.7degrees to 69.0degrees, corresponding to -1.7% and -1.0% contractions of the surface cell area, respectively.


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.


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 - Interaction of hydrogen and oxygen with a chlorine covered Ag(111) surface [Articolo su rivista]
Staicu Casagrande, Em; Lacombe, S; Guillemot, L; Esaulov, Va; Pasquali, Luca; Nannarone, Stefano; Canepa, M.
abstract

Results of a study of changes in electron capture and loss rates in the interaction of hydrogen and oxygen atoms and anions with chlorine covered Ag(111) surfaces are reported for a wide range of coverages. Strong oscillatory changes in anion formation rates are observed as a function of chlorine coverage. These: changes are assigned to changes in local electronic structure and modifications in positions and widths of the anion levels at different phases of chlorine adsorption and in particular to interaction of the projectile with the adsorbed Cl(3p) levels. At high coverages, a higher electron capture probability is found, corresponding to the appearance of chloride islands.


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 - Magnetism and interlayer coupling in fcc Fe/Co films [Articolo su rivista]
Dallmeyer, A; Maiti, K; Rader, O; Pasquali, Luca; Carbone, C; Eberhardt, W.
abstract

The magnetism of epitaxial fee Fe films deposited on Co(100) and sandwiched between two Co(100) films was investigated by x-ray magnetic circular dichroism. The dependence of the Fe magnetism on the film thickness is complex and qualitatively similar on Co(100) and in fee Co/Fe/Co(100) trilayers. The fee Fe film magnetization presents a pronounced oscillation, suggesting a partial antiferromagnetic ordering in the 5-10 monolayer thickness range. The fee Fe films mediate an oscillatory, indirect coupling in Co/Fe/Co(100) structures that alternates in correspondence with the changes of the Fe magnetization.


2001 - Oscillatory interlayer coupling mediated by fcc-Fe/Co(100) films [Articolo su rivista]
Maiti, K; Dallmeyer, A; Malagoli, Mc; Carbone, C; Eberhardt, W; Rader, O; Pasquali, Luca; Banerjee, A; Turchini, S; Zennaro, S; Zema, N.
abstract

The magnetism of epitaxial fcc-Fe films deposited on Co(1 0 0) and sandwiched between two Co(1 0 0) films was investigated by X-ray magnetic circular dichroism at various temperatures. The dependence of the Fe magnetism on the film thickness is complex and qualitatively similar on Co(1 0 0) and in fcc-Co/Fe/Co(1 0 0) trilayers. The fcc-Fe film magnetization presents a pronounced oscillation suggesting a partial antiferromagnetic ordering in the 5-10 monolayers (ML) thickness range. The decrease in temperature down to 100 K does not exhibit any change in the magnetic properties of the Fe-film. The fcc-Fe films mediate an oscillatory, indirect coupling in Co/Fe/Co(1 0 0) structures that alternates in correspondence with the changes of the Fe magnetization. (C) 2001 Elsevier Science B.V. All rights reserved.


2001 - Positive ion neutralisation on chlorine covered silver surfaces [Articolo su rivista]
Guillemot, L; Staicu Casagrande, Em; Lacombe, S; Esaulov, Va; Pasquali, Luca; Nannarone, Stefano; Canepa, M; Vergara, L; Grizzi, O.
abstract

We report a study of the effect of chlorine adsorption on the interaction of positive ions with the silver surfaces. Neutralisation rates of He+ were measured and an ion scattering spectroscopy (ISS) study of Cl covered Ag(111) was performed. The Cl ISS peak remains vanishingly small during the Cl chemisorption submonolayer phase and then rapidly increases above 1.5 L. The neutralisation rate of He+ is found to increase during the first phase and then decrease above 1.5 L, an effect that we attribute to A.-Cl island formation. This points out a chemical environment dependance of the Cl detection with ISS due to changes in particular quasi-resonant neutralisation rates involving Cl(3s) core levels. In the case of H+, Ne+ and Ar+ neutralisation on Ag(111) (110) and (100) a striking oscillation of the neutralisation rate is found for the three surfaces at low coverages. A first minimum is reached around 0.5 to 2/3 of the full coverage and a maximum is then attained at full coverage. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and metastable deexcitation spectroscopy (MDS) measurements allow one to describe these features in terms of the modifications in Auger neutralisation (AN) rates due to changes of the density of states as adsorption changes from smaller to higher packing of adsorbed Cl. Beyond the saturation limit the neutralisation rate is found to decrease again on Ag(111) but remains constant on Ag(110) and Ag(100). This is ascribed to the fact that, in the exposure range reported, AgCl formation occurs only on the (111) face. On the two other faces an ordered chemisorption phase remains stable until higher coverages.


2001 - Surface electronic properties by metastable deexcitation spectroscopy [Articolo su rivista]
Nannarone, Stefano; Pasquali, Luca
abstract

Metastable deexcitation spectroscopy (MDS) is used to probe the surface electronic properties of semiconductor surfaces and different interfaces with semiconductors. Results are compared to photoemission data and calculations. The following case studies are treated: clean GaAs(110), clean Ge(111)c(2 x 8), Sb/GaAs(110), H:GaAs(110), Yb/GaAs(110), Yb/Si(100) and Ge(111) surface incomplete melting. For each system, MDS spectral features are related to surface valence band (VB) states.


2000 - Adsorbate phase transformations and the coverage-dependent oscillation of electron transfer probabilities [Articolo su rivista]
EM Staicu, Casagrande; L., Guillemot; S., Lacombe; Va, Esaulov; M., Canepa; L., Mattera; Pasquali, Luca; Nannarone, Stefano
abstract

We report a study of the effects of chlorine adsorption on the interaction of positive ions (H+, Ne+, Ar+ with a Ag(111) surface from the submonolayer Cl chemisorption to initial stages of AgCl formation. Cl adsorption on Ag(111) proceeds through different phases and we observed that the neutralization probabilities oscillate in this range, attaining an intermediate minimum at about 2/3 coverage and reach a maximum at full coverage. The subsequent appearance of AgCl phase again leads to a reduction in neutralization. These results are described in terms of changes in Auger neutralization rates due to modifications in the adsorbate density of states. (C) 2000 American Institute of Physics. [S0021-9606(00)71330-4].


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 - 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.


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

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


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

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


1999 - Resonant scattering of polarized soft X-rays for the study of magnetic oxide layers [Articolo su rivista]
M., Sacchi; C. F., Hague; S., Gota; E., Guiot; M., GAUTIER SOYER; Pasquali, Luca; S., Mrowka; E. M., Gullikson; J. H., Underwood
abstract

We report preliminary results that illustrate the potential of resonant scattering of polarized soft X-rays applied to the studyof the magnetic properties of oxide layers. As a test case, we study iron oxides, a class of materials of high technologicalinterest, especially when prepared in the form of thin epitaxial layers.


1999 - Soft-x-ray resonant scattering from V/Fe (001) magnetic superlattices [Articolo su rivista]
Sacchi, M; Mirone, A; Hague, Cf; Mariot, Jm; Pasquali, Luca; Isberg, P; Gullikson, Em; Underwood, Jh
abstract

The magnetic properties of Fe and V in (n ML V / 5 ML Fe) (001) superlattices (n = 1, 2, and 5 ML) have been investigated by measuring the magnetization dependent scattering of elliptically polarized soft x rays at various scattering angles and for photon energies across the 2p x-ray-absorption edges of both Fe and V. A quantitative estimate of the magnetic moment induced on vanadium is obtained from the analysis of the resonant scattering data. [S0163-1829(99)50842-4].


1998 - Optical constants of ferromagnetic iron via 2p resonant magnetic scattering [Articolo su rivista]
Sacchi, M; Hague, Cf; Pasquali, Luca; Mirone, A; Mariot, Jm; Isberg, P; Gullikson, Em; Underwood, Jh
abstract

We have determined the optical constants at the 2p edges of iron by measuring the Bragg scattering from a Fe/V superlattice using elliptically polarized photons whose energies were tuned across the Fe 2p resonances (700-730 eV). The analysis of the Bragg peak displacement as a function of photon energy and sample magnetization, together with standard absorption measurements, allowed to determine the complete dielectric tensor for Fe, including its off-diagonal terms.


1998 - Resonant ionization plus Auger neutralization in the study of the electronic structure of clean semiconductor surfaces [Articolo su rivista]
Pasquali, Luca; Canepa, M; Mattera, L; Nannarone, Stefano
abstract

Metastable atom de-excitation spectroscopy results on the surfaces of clean semiconductors are shown. De-excitation occurs through resonant ionization followed by Auger neutralization. After a review of the results obtained on the clean surface of GaAs(110) at room temperature, the evolution of the Ge(111) effective surface density of states (SDOS) as a function of temperature is discussed on the basis of the temperature-induced surface structure transitions reported in literature by different authors. (C) 1998 Elsevier Science B.V. All rights reserved.


1998 - Surface electronic structure of Ge(111) from 300 to 1100 K by metastable deexcitation spectroscopy [Articolo su rivista]
Pasquali, Luca; Nannarone, Stefano; Canepa, M; Mattera, L.
abstract

Metastable atom deexcitation spectroscopy is applied to the study of the temperature dependence of the electronic structure of the Ge(111)c(2X8) surface. The present work is stimulated by the debate on temperature induced surface phase transitions. In this field the application of high surface sensitive atomic beam spectroscopy appears to be extremely promising. Metastable deexcitation spectra are taken in the 300-1100 K temperature interval, i.e., up to similar to 100 K below the bulk melting point. Spectra show a monotonic variation with temperature. Restatom and adatom contributions are identified and their evolution with temperature is followed. In particular, the persistence of the adatoms up to the highest investigated temperatures and the progressive metallization of the surface, already visible since 670 K, are observed. Data seem to indicate more agreement with surface models where order is preserved at high temperatures.


1998 - X-ray magneto-optics and surface science (X-MOSS) beamline at ELETTRA [Articolo su rivista]
Nannarone, Stefano; Gazzadi, Gc; Pasquali, Luca; Sacchi, M; Pelizzo, Mg; Tondello, G.
abstract

The elements of the design of the X-MOSS (X-ray magneto-optics and surface science) beamline, to be installed at a bending magnet of the ELETTRA synchrotron radiation facility, are presented. The beamline is intended for optical study (reflectivity and absorption) of low-dimensional systems, typically surfaces and interfaces, with emphasis on magnetic systems. The optical range will extend from visible up to similar to 1400 eV with full control of linear and circular polarisation.


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 - Reactive and unreactive interfaces studied by means of metastable deexcitation spectroscopy [Articolo su rivista]
Pasquali, Luca; Fantini, P; Nannarone, Stefano; Canepa, M; Mattera, L.
abstract

The present paper is devoted to the application of metastable deexcitation spectroscopy (MDS) for studying interface formation with reference to Yb/GaAs(110) and Sb/GaAs(110). These systems are prototypical examples of intermixed disordered and ordered interfaces. In the discussion, emphasis is given to the potentiality offered by MDS in interface formation studies. A comparison with photoemission is discussed.


1995 - A MDS study of the Yb/GaAs(110) interface [Articolo su rivista]
Pasquali, Luca; Fantini, P; Nannarone, Stefano; Canepa, M; Mattera, L.
abstract

Metastable deexcitation spectroscopy (MDS) was applied to the study of the growth of the Yb/GaAs(110) interface. MD spectra taken at different coverages, from 0.04 ML to 30 ML, revealed the progressive metallization of the overlayer. Absence of 4f emission allowed to observe, for the first time, VB features of the forming interface up to similar to 8 ML.


1995 - GaAs(110) surface electronic structure by metastable deexcitation spectroscopy [Articolo su rivista]
PASQUALI, Luca; NANNARONE, Stefano; Canepa, M; Mattera, L.
abstract

Metastable deexcitation spectroscopy was applied to study the surface valence electronic structure of clean cleaved GaAs(110). Metastable deexcitation spectroscopy was flanked by angle-resolved photoemission. An effective surface density of states was derived from the experimental spectrum through deconvolution. Two groups of states were observed in the 0-4 and 5-8 eV range of binding energy, respectively. These features were ascribed to emission from surface states. A plane-by-plane tight-binding density-of-states calculation was performed. More quantitative insights were obtained by comparing experimental and theoretical results. The most prominent feature of the first group of states of deconvolution was assigned to surface state A(5). Contributions from states A(4), A(3), A(1)', and A(2)' were also observed. The doublet of the second group of features was identified with C-2 and C-1. Relative amplitudes of effective surface density of states were related to surface charge density.


1995 - SURFACE-DENSITY OF STATES OF SB/GAAS(110) AND HGAAS(110) BY METASTABLE DEEXCITATION SPECTROSCOPY [Articolo su rivista]
Pasquali, Luca; Plesanovas, A; Ruocco, A; Tarabini, Ac; Nannarone, Stefano; Abbati, I; Canepa, M; Mattera, L; Terreni, S.
abstract

Metastable deexcitation spectroscopy (MDS) was applied to the study of the electronic structure of Sb/GaAs(110) and H:GaAs(110) systems. For both systems valuable insights into the surface electronic structure (effective surface density of states and surface charge density) were obtained. A surface to volume ratio extremely higher than photoemission was obtained for MDS. For both systems new features, never observed by more conventional surface spectroscopies, were detected.


1994 - VALENCE-BAND STATES OF H-GAAS(110) [Articolo su rivista]
Plesanovas, A; Tarabini, Ac; Abbati, I; Kaciulis, S; Paolicelli, G; Pasquali, Luca; Ruocco, A; Nannarone, Stefano
abstract

The two-dimensional surface electronic band structure of H:GaAs(110) is studied by angular resolved ultraviolet photoemission spectroscopy along the XBAR'-MBAR and XBAR-MBAR symmetry lines and in the high symmetry points of the surface Brillouin zone. Three surface state bands are resolved in the first 5 eV below the upper valence band edge. A comparison with theoretical band structure calculations yields a satisfactory agreement, giving evidence of hydrogen induced GaAs(110) surface derelaxation. The clean GaAs(110) surface electronic band structure is studied comparing results with available theoretical and experimental data.