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Valentina DE RENZI

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

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2022 - Disclosing the Microscopic Picture: An Interdisciplinary Investigation of Friction and Wetting Using a Gecko-Inspired Tape [Articolo su rivista]
Scorzoni, C.; Goldoni, G.; Rota, A.; De Renzi, V.

We propose an innovative approach to teaching friction. Our approach aims to educate students on its microscopic nature by highlighting its origin in intermolecular interactions. We have designed a teaching sequence (TS) based on a set of experimental investigations of the properties of a gecko-inspired tape at different length scales. The TS has been conceived to unravel the peculiar behavior of this man-made, commercially available biomimetic material and to train students to identify the connection between the micrometer-scale patterning and the peculiar tribological properties. Specifically, our approach compares friction (and adhesion) to wetting, occurring at solid/solid and solid/liquid interfaces, respectively. The aim is to scaffold a correct mental model of real interfaces and disclose the common origin of both phenomena in intermolecular interactions. The TS has been devised according to the design-based research scheme and it was inspired by the Investigative Science Learning Environment (ISLE) and the 5E paradigms. It has been tested and tuned with students at level 3 in the International Standard Classification of Education (ISCED), during several on-campus stages. We report here the details and results of pre- and post-tests, which demonstrate the effectiveness of our method. Specifically, we measure success in terms of the students' comprehension of the link between contact area and friction and of the role of intermolecular forces. We are confident that the learning experience with our TS will lead students to recognize the enormous potential impact of surface patterning in technological applications, in a curiosity-driven manner that will likely result in students' interest in quantitative studies of science and technology.

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

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

2020 - Vibrational signature of the graphene nanoribbon edge structure from high-resolution electron energy-loss spectroscopy [Articolo su rivista]
Cavani, N.; De Corato, M.; Ruini, A.; Prezzi, D.; Molinari, E.; Lodi Rizzini, A.; Rosi, A.; Biagi, R.; Corradini, V.; Wang, X. -Y.; Feng, X.; Narita, A.; Mullen, K.; De Renzi, V.

Bottom-up approaches exploiting on-surface synthesis reactions allow atomic-scale precision in the fabrication of graphene nanoribbons (GNRs); this is essential for their technological applications since their unique electronic and optical properties are largely controlled by the specific edge structure. By means of a combined experimental-theoretical investigation of some prototype GNRs, we show here that high-resolution electron energy-loss spectroscopy (HREELS) can be successfully employed to fingerprint the details of the GNR edge structure. In particular, we demonstrate how the features of HREEL vibrational spectra-mainly dictated by edge CH out-of-plane modes-are unambiguously related to the GNR edge structure. Moreover, we single out those modes which are localized at the GNR termini and show how their relative intensity can be related to the average GNR length.

2019 - CoTPP molecules deposited on graphene/Ni (111): Quenching of the antiferromagnetic interaction induced by gold intercalation [Articolo su rivista]
Corradini, V.; Candini, A.; Klar, D.; Biagi, R.; De Renzi, V.; Lodi Rizzini, A.; Cavani, N.; Del Pennino, U.; Wende, H.; Otero, E.; Affronte, M.

In this work, we investigated the effect of Au-intercalation on the magnetic coupling between a sub-monolayer of Co-Tetraphenylporphyrin molecules and a graphene-covered Ni(111) single crystal. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and its field dependence were probed. Cobalt strongly couples antiferromagnetically to the nickel substrate, also through the graphene layer. The intercalation of graphene with gold leads to a complete removal of this coupling. Published under license by AIP Publishing.

2018 - Probing magnetic coupling between LnPc2 (Ln=Tb, Er) molecules and graphene / Ni(111) substrate with and without Au-intercalation: role of the dipolar field [Articolo su rivista]
Corradini, Valdis; Candini, Andrea; Klar, David; Biagi, Roberto; de Renzi, Valentina; Lodi Rizzini, Alberto; Cavani, Nicola; del Pennino, Umberto; Klyatskaya, Svetlana; Ruben, Mario; Velez-Fort, Emilio; Kummer, Kurt; Brookes, Nicholas B.; Gargiani, Pierluigi; Wende, Heiko; Affronte, Marco

Lanthanides (Ln) bis-phthalocyanine (Pc), the so called LnPc2 double decker, are promising class of molecules, with well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on the graphene/Ni(111) substrate, and how they modify when an Au layer is intercalated between the Ni and graphene. X-ray absorption spectroscopy (XAS), linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold intercalated or not. Two types of LnPc2 molecules (Ln=Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easyplane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). On the contrary, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggests the possible importance of the magnetic dipolar field contribution in determining the magnetic behaviour.

2017 - Lateral Fusion of Chemical Vapor Deposited N = 5 Armchair Graphene Nanoribbons [Articolo su rivista]
Chen, Zongping; Wang, Hai I.; Bilbao, Nerea; Teyssandier, Joan; Prechtl, Thorsten; Cavani, Nicola; Tries, Alexander; Biagi, Roberto; DE RENZI, Valentina; Feng, Xinliang; Klã¤ui, Mathias; De Feyter, Steven; Bonn, Mischa; Narita, Akimitsu; Mullen, Klaus

Bottom-up synthesis of low-bandgap graphene nanoribbons with various widths is of great importance for their applications in electronic and optoelectronic devices. Here we demonstrate a synthesis of N = 5 armchair graphene nanoribbons (5-AGNRs) and their lateral fusion into wider AGNRs, by a chemical vapor deposition method. The efficient formation of 10- and 15-AGNRs is revealed by a combination of different spectroscopic methods, including Raman and UV-vis-near-infrared spectroscopy as well as by scanning tunneling microscopy. The degree of fusion and thus the optical and electronic properties of the resulting GNRs can be controlled by the annealing temperature, providing GNR films with optical absorptions up to 2250 nm.

2017 - Probing optical excitations in chevron-like armchair graphene nanoribbons [Articolo su rivista]
Denk, Richard; Lodi-Rizzini, Alberto; Wang, Shudong; Hohage, Michael; Zeppenfeld, Peter; Cai, Jinming; Fasel, Roman; Ruffieux, Pascal; Berger, Reinhard Franz Josef; Chen, Zongping; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus; Biagi, Roberto; De Renzi, Valentina; Prezzi, Deborah; Ruini, Alice; Ferretti, Andrea; Prezzi, Deborah

The bottom-up fabrication of graphene nanoribbons (GNRs) has opened new opportunities to specifically tune their electronic and optical properties by precisely controlling their atomic structure. Here, we address excitation in GNRs with periodic structural wiggles, the so-called chevron GNRs. Based on reflectance difference and high-resolution electron energy loss spectroscopies together with ab initio simulations, we demonstrate that their excited-state properties are of excitonic nature. The spectral fingerprints corresponding to different reaction stages in their bottom-up fabrication are also unequivocally identified, allowing us to follow the exciton build-up from the starting monomer precursor to the final GNR structure

2016 - Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates [Articolo su rivista]
Marocchi, Simone; Candini, Andrea; Klar, David; Van Den Heuvel, Willem; Huang, Haibei; Troiani, Filippo; Corradini, Valdis; Biagi, Roberto; DE RENZI, Valentina; Klyatskaya, Svetlana; Kummer, Kurt; Brookes, Nicholas B.; Ruben, Mario; Wende, Heiko; DEL PENNINO, Umberto; Soncini, Alessandro; Affronte, Marco; Bellini, Valerio

We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices.

2016 - Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate [Articolo su rivista]
Candini, A; Klar, D.; Marocchi, S.; Corradini, V.; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; Troiani, F.; Bellini, V.; Klyatskaya, S.; Ruben, M.; Kummer, K.; Brookes, N. B.; Huang, H.; Soncini, A.; Wende, H.; Affronte, Marco

Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc 2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.

2016 - Synthesis of Graphene Nanoribbons by Ambient-Pressure Chemical Vapor Deposition and Device Integration [Articolo su rivista]
Chen, Zongping; Zhang, Wen; Palma, Carlos Andres; LODI RIZZINI, Alberto; Liu, Bilu; Abbas, Ahmad; Richter, Nils; Martini, Leonardo; Wang, Xiao Ye; Cavani, Nicola; Lu, Hao; Mishra, Neeraj; Coletti, Camilla; Berger, Reinhard; Klappenberger, Florian; Kläui, Mathias; Candini, Andrea; Affronte, Marco; Zhou, Chongwu; DE RENZI, Valentina; DEL PENNINO, Umberto; Barth, Johannes V.; Räder, Hans Joachim; Narita, Akimitsu; Feng, Xinliang; Müllen, Klaus

Graphene nanoribbons (GNRs), quasi-one-dimensional graphene strips, have shown great potential for nanoscale electronics, optoelectronics, and photonics. Atomically precise 47 8 GNRs can be "bottom-up" synthesized by surface-assisted assembly of molecular building blocks under ultra-high-vacuum conditions. However, large-scale and efficient synthesis of such GNRs at low cost remains a significant challenge. Here we report an efficient "bottom-up" chemical vapor deposition (CVD) process for inexpensive and high-throughput growth of structurally defined GNRs with varying structures under ambient-pressure conditions. The high quality of our CVD-grown GNRs is validated by a combination of different spectroscopic and microscopic characterizations. Facile, large-area transfer of GNRs onto insulating substrates and subsequent device fabrication demonstrate their promising potential as semiconducting materials, exhibiting high current on/off ratios up to 6000 in field-effect transistor devices. This value is 3 orders of magnitude higher than values reported so far for other thin-film transistors of structurally defined GNRs. Notably, on-surface mass spectrometry analyses of polymer precursors provide unprecedented evidence for the chemical structures of the resulting GNRs, especially the heteroatom doping and heterojunctions. These results pave the way toward the scalable and controllable growth of GNRs for future applications.

2015 - Quasiparticle spectrum and plasmonic excitations in the topological insulator Sb2Te3 [Articolo su rivista]
Nechaev, I. A.; Aguilera, I.; DE RENZI, Valentina; Di Bona, A.; LODI RIZZINI, Alberto; Mio, A. M.; Nicotra, G.; Politano, A.; Scalese, S.; Aliev, Z. S.; Babanly, M. B.; Friedrich, C.; Blügel, S.; Chulkov, E. V.

We report first-principles GW results on the dispersion of the bulk band-gap edges in the three-dimensional topological insulator Sb2Te3. We find that, independently of the reference density-functional-theory band structure and the crystal-lattice parameters used, the one-shot GW corrections enlarge the fundamental band gap, bringing its value in close agreement with experiment. We conclude that the GW corrections cause the displacement of the valence-band maximum (VBM) to the Γ point, ensuring that the surface-state Dirac point lies above the VBM. We extend our study to the analysis of the electron-energy-loss spectrum (EELS) of bulk Sb2Te3. In particular, we perform energy-filtered transmission electron microscopy and reflection EELS measurements. We show that the random-phase approximation with the GW quasiparticle energies and taking into account virtual excitations from the semicore states leads to good agreement with our experimental data.

2015 - Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition [Articolo su rivista]
Gondoni, P.; Mazzolini, P.; Russo, V.; Diani, M.; Amati, M.; Gregoratti, L.; DE RENZI, Valentina; Gazzadi, G. C.; Martí Rujas, J.; Li Bassi, A.; Casari, C. S.

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows tuning electrical resistivity over a very wide range (10- 1 - 106 Ω cm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests.

2014 - Alla scoperta delle nanotecnologie [Monografia/Trattato scientifico]
DE RENZI, Valentina; Goldoni, Guido; Annamaria, Lisotti

La fisica alla nanoscala

2014 - Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface Mediated by the Extended States of Graphene [Articolo su rivista]
Candini, Andrea; Bellini, Valerio; Klar, David; Corradini, Valdis; Biagi, Roberto; DE RENZI, Valentina; Kummer, Kurt; Brookes, Nicholas B.; DEL PENNINO, Umberto; Wende, Heiko; Affronte, Marco

The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular dichroism, the field-dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8 K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots.

2014 - Nitrocatechol/ZnO Interface: The Role of Dipole in a Dye/Metal-Oxide Model System [Articolo su rivista]
Arnaud, Gaelle Francoise; DE RENZI, Valentina; DEL PENNINO, Umberto; Biagi, Roberto; Corradini, Valdis; Calzolari, Arrigo; Ruini, Alice; A., Catellani

The electronic properties of a prototype system suitable for dye-sensitized solar cell applications are investigated both experimentally and theoretically by means of electron spectroscopies (high-resolution electron energy loss spectroscopy, HREELS, and ultraviolet and X-ray photoemission spectroscopies, UPS and XPS) and first-principles density functional theory (DFT)-based calculations. The comparison of HREELS and UPS data with the DFT results allows the microscopic description of electronic structure modifications upon interface formation, and provides a quantitative evaluation of the ionization energy and electron affinity changes induced by functionalization: these variations can be associated to the electric dipole of the functional species and, thus, to the formation of an interface dipole layer.

2014 - Surface Investigation on Gd4M8(M = Zn, Ni) Single Molecule Coolers [Articolo su rivista]
Corradini, Valdis; Ghirri, Alberto; Candini, Andrea; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Dotti, Gianluca; Otero, Edwige; Hooper, Thomas N.; Inglis, Ross; Brechin, Euan K.; Affronte, Marco

Proving the preservation of functionality at the molecular scale is still a major challenge, both for the choice of suitable derivatives and protocols and for the employed experimental tools and methods. By using a combined scanning probe (AFM and STM) and spectroscopic methodology (XPS, XAS and XMCD) we show that Gd4M8 derivatives (with M=Zn2+, Ni2+) are robust molecular units which preserve their electronic, magnetic and thermodynamic properties when deposited on a metallic surface. Namely we measured entropy variation ΔS=[S(6T)-S(0T)] exceeding 8R (20 J Kg-1 K-1) at 4K in isolated Gd4Ni8 molecules dispersed on Au(111) surface and show a viable route to exploit large magnetocaloric effect at single molecule level.

2013 - Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films [Articolo su rivista]
David, Klar; Svetlana, Klyatskaya; Andrea, Candini; Bernhard, Krumme; Kurt, Kummer; Philippe, Ohresser; Valdis, Corradini; DE RENZI, Valentina; Biagi, Roberto; Loic, Joly; Jean Paul, Kappler; DEL PENNINO, Umberto; Affronte, Marco; Heiko, Wende; Mario, Ruben

The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 moleculescouple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface.

2013 - Educational pathways through nanoscience: nitinol as a paradigmatic smart material [Articolo su rivista]
Lisotti, Annamaria; DE RENZI, Valentina; Carlo Andrea, Rozzi; Elena, Villa; Franca, Albertini; Goldoni, Guido

We developed an educational path based on nitinol, a shape memory alloy which conveniently exemplifies the smart material concept, i.e., a material that performs a predetermined, reversible action in response to a change in the environment. Nitinol recovers a given shape, changes its resistivity drastically and modifies its elastic properties if subjected to a temperature change in a convenient range. Here, the properties are verified with laboratory protocols appropriate to a high-school environment. Use of mobile electronic devices is also suggested. The collected electrical and mechanical properties are analysed within a didactic path which emphasizes their common physical origin, i.e., the martensitic transition. Moreover, the peculiarities of this solid-to-solid transformation are put in correspondence with the apparently unrelated but more familiar liquid–vapour transition. The relationship with possible applications is emphasized by measuring the efficiency of using a nitinol spring as an actuator.

2013 - Electrochemically assisted grafting of asymmetric alkynyl(aryl)iodonium salts on glassy carbon with focus on the alkynyl/aryl grafting ratio [Articolo su rivista]
Florini, Nicola; M., Michelazzi; Parenti, Francesca; Mucci, Adele; Sola, Marco; C., Baratti; De Renzi, Valentina; K., Daasbjerg; S. U., Pedersen; Fontanesi, Claudio

Glassy carbon (GC) surfaces are functionalized exploiting the electrochemical reduction of asymmetric iodonium salts of the general formula [4-Zsingle bondC6H4single bondI+single bondCtriple bond; length of mdashC(CH2)4Cl, Z = NO2, Br, F, H, or CH3]. The range of Z groups was selected aiming to examine the Carylsingle bondI bond energy as a function of the “electron withdrawing/electron donating” ability of the Z-group, with the ultimate purpose of controlling the alkynyl/aryl grafting ratio. The electroreduction mechanism and the characteristics of the grafted surface are studied by means of cyclic voltammetry and X-ray photoelectron spectroscopy. It is shown that the alkynyl/aryl grafting ratio decreases with the increasing electron withdrawing nature of Z, which induces a weakening of the Carylsingle bondI bond. Ab initio DFT vertical electron affinities (EAv) allowed to rationalize the electroreduction potentials of both the bulk and surface functionalised redox couples. Moreover, the alkynyl/aryl grafting ratio is assessed, at a molecular level, on the basis of ab initio DFT potential energy surfaces.

2012 - Competing Pathways in N-Allylurea Adsorption on Si(111)-(7 × 7) [Articolo su rivista]
DE RENZI, Valentina; Arnaud, Gaelle Francoise; DEL PENNINO, Umberto

Functionalization of silicon surfaces with Nallylurea(CH2CH−CNH−CO−NH2) represents a valuable strategy to obtain covalently bonded Si−C interfaces with amino and/or carbonyl termination. In this work, we studied N-allylurea adsorption on the Si(111)-(7 × 7) surface by combining X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) with high resolution energy loss spectroscopy (HREELS) measurements. XPS core level analysis provides information on the molecular attachment process. Si−C covalent bonding is evidenced by the presence of a C 1s component at 284.8 eV, while interaction through N−Si bonding is proved by the presence of a N 1s component at 397.8 eV. Three different adsorption mechanisms are envisaged: (I) [2 + 2]-like cycloaddition occurring at the rest atom−adatom dimer through cleavage of the vinyl group, (II) Si−N bonding at adatom sites upon cleavage of NH2 and rearrangement of the ureic group to form an imidol species (−NC−OH), with release of a H atom, and (III) hydrosilylation at adatom sites, through cleavage of the vinyl group and involvement of H atoms provided by reaction II.

2012 - Electronic and Magnetic Properties of Mn12 Molecular Magnets on Sulfonate and Carboxylic Acid Prefunctionalized Gold Surfaces [Articolo su rivista]
Moro, Fabrizio; Biagi, Roberto; Valdis, Corradini; Marco, Evangelisti; Gambardella, Alessandro; DE RENZI, Valentina; DEL PENNINO, Umberto; Eugenio, Coronado; Alicia Forment, Aliaga; Francisco M., Romero

Structural, electronic, and magnetic properties of [Mn12O12(bet)16(EtOH)4](PF6)14·4CH3CN·H2O (in short Mn12bet, bet = betaine = +N(CH3)3-CH2−COO−) singlemolecule magnets (SMMs) deposited on previously functionalized gold surfaces have been investigated. Self-assembled monolayers (SAMs) either of sodium mercaptoethanesulfonate (MES) or mercaptopropionic acid (MPA) are used as functionalization to avoid the direct interaction between the Mn12bet molecules and the Au surface with the aim of preserving the main functional properties of the molecules. Scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) analysis show deposited Mn12bet SMMs well-isolated from each other and uniformly distributed on both MES and MPA SAMs. X-ray absorption spectroscopy (XAS) studies show that the oxidation state of the mixed-valence Mn12bet core is largely reduced to Mn2+ when molecules are deposited on MES-SAM, whereas in the case of MPA-SAM the relative weights of Mn2+, Mn3+, and Mn4+ in the Mn12bet core are preserved. Despite the substantial retaining of their electronic properties, the magnetization of Mn12bet molecules deposited on MPA-SAM measured by X-ray magnetic circular dichroism (XMCD) is perturbed with respect to the pristine molecules.

2012 - Functionalization of glassy carbon surface by means of aliphatic and aromatic amino acids. An experimental and theoretical integrated approach [Articolo su rivista]
Vanossi, Davide; Benassi, Rois; Parenti, Francesca; Tassinari, Francesco; Giovanardi, Roberto; Florini, Nicola; DE RENZI, Valentina; Arnaud, Gaelle Francoise; Fontanesi, Claudio

Glassy Carbon (GC) electrode surfaces are functionalized through electrochemical assisted grafting, in oxidation regime, of six amino acids (AA): -Alanine (-Ala), L-Aspartic acid (Asp), 11-aminoundecanoic acid (UA), 4-Aminobenzoic acid (PABA), 4-(4-Amino-phenyl)-butyric acid (PFB), 3-(4-Amino-phenyl)-propionic acid (PFP). Thus, a GC/AA interface is produced featuring carboxylic groups facing the solution. Electrochemical (cyclic voltammetry and electrochemical impedance spectroscopy) and XPS techniques are used to experimentally characterize the grafting process and the surface state. The theoretical results are compared with the experimental evidence to determine, at a molecular level, the overall grafting mechanism. Ionization Potentials, Standard Oxidation Potentials, HOMO and electron spin distributions are calculated at the CCD/6-31G* level of the theory. The comparison of experimental and theoretical data suggests that the main electroactive species is the “zwitterionic” form for the three aliphatic amino acids, while the amino acids featuring the amino group bound to the phenyl aromatic moiety show a different behaviour. The comparison between experimental and theoretical results suggests that both the neutral and zwitterionic forms are present in the acetonitrile solution in the case of 4-(4-Amino-phenyl)-butyric acid (PFB) and 3-(4-Amino-phenyl)-propionic acid.

2012 - Magnetic Anisotropy of Cr7Ni Spin Clusters on Surfaces [Articolo su rivista]
Valdis, Corradini; Alberto, Ghirri; Elena, Garlatti; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; Valerio, Bellini; Stefano, Carretta; Paolo, Santini; Grigore, Timco; Richard E. P., Winpenny; Affronte, Marco

The problem of the experimental and theoretical determination of magnetic anisotropy in isolated molecular spin clusters is addressed here. To this end, the case of molecular Cr 7 Ni rings sublimated in ultrahigh vacuum conditions and assembled in an ordered fashion on Au(111) surface is addressed and investigated using X-ray magnetic dichroism (XMCD) and theoretical calculations. Fixing the experimental conditions at a temperature T = 8 K and a magnetic fi eld of 5 T, the angular-dependence of the dichroic signal reveals an easy-axis anisotropy for the Ni magnetization along the direction perpendicular to the ring while the magnetization of the whole Cr 7 Ni molecule is preferentially aligned within the ring plane.

2011 - Self-assembled monolayer of Cr7Ni molecular nanomagnets by sublimation. [Articolo su rivista]
Ghirri, Alberto; Corradini, Valdis; Bellini, Valerio; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Julio C., Cezar; Christopher A., Muryn; Grigore A., Timco; Richard E. P., Winpenny; Affronte, Marco

We show, by complementary spectroscopic and STM analysis, that Cr7Ni derivativesare suitable to be sublimed in UHV conditions. Cr7Ni-bu weakly bonds to gold surface and can diffuserelatively freely on it, forming monolayers with hexagonal 2D packing. Conversely, by adding afunctional thiol group to the central dibutylamine, a covalent bond between the molecule andsurface gold adatoms is promoted, leading to a strong molecular grafting and the formation of adisordered monolayer. These two examples demonstrate the possibility to control the assembly of alarge molecular complex, as rationalized by DFT calculations that establish different energy scales inthe deposition processes. Moreover, low-temperature XMCD sprectra show that the magneticfeatures of Cr7Ni rings deposited in UHV on gold remain unchanged with respect to those of thecorresponding bulk sample.

2011 - Surface supramolecular organization of a terbium (III) double-decker complex on graphite and its single molecule magnet behavior [Articolo su rivista]
Gonidec, Mathieu; Biagi, Roberto; Corradini, Valdis; Moro, Fabrizio; DE RENZI, Valentina; DEL PENNINO, Umberto; Summa, Domenico; Muccioli, Luca; Zannoni, Claudio; Amabilino, David; Veciana, Jaume

The two dimensional self-assembly of a terbium (III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) from solution (by casting) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular 2-D nanocrystals on the surface, that are aligned with the graphite symmetry axes. Molecular Dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experimental results, showing that − after diffusion on the graphite surface − the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism (XMCD), confirming that the compounds maintain their properties as single molecule magnets when they are in close interaction with the graphite surface.

2010 - Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism [Articolo su rivista]
Moro, Fabrizio; Valdis, Corradini; Marco, Evangelisti; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; Julio C., Cezar; Ross, Inglis; Constantinos J., Milios; Euan K., Brechin

We report on a comparative study of electronic and magnetic properties of Mn6 single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S ¼ 4 andhave been functionalized with 3-tp-CO2 (3-thiophene carboxylate, tpc) ligands and characterized thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3 edges shows the modification of the spectral lineshape in the sMLs respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules deposited on gold surface. In spite of this, the expected MnIII oxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only spin part because of the quenched orbital moment. Moreover, variable temperature and variable XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives.

2010 - On the electroreduction mechanism of iodonium salts on glassy carbon electrodes [Abstract in Atti di Convegno]
K., Daasbjerg; S. U., Pedersen; DE RENZI, Valentina; Florini, Nicola; Parenti, Francesca; Bortolotti, Carlo Augusto; Vanossi, Davide; Fontanesi, Claudio

Glassy carbon (GC) surfaces can be functionalized exploiting the electrochemical reduction of iodonium salts of general formula [RIR’]+ [1]. The overall mechanism could be roughly sketched as: IR’ bond cleavage: [RIR’]+ + e + GC  R’GC + IR Route (1) IR bond cleavage: [RIR’]+ + e + GC  RGC + IR’ Route (2) Upon electroreduction the I-R or the I-R’ bond dissociate, leading to a neutral closed shell organic iodide and an open shell radical, the latter reacts with the GC electrode (grafting). Several factors can influence the R/R’ ratio grafted on the GC surface. In fact, different amounts of the R and R’ radicals can be formed depending on the electronic structure of the neutral open shell [R-I-R’] • radical. Moreover, the different radicals can exhibit different reactivity toward the carbon surface, as well as different electrochemical stabilities (the radical itself could be reduced to a negative closed shell form). To clarify the interplay of the various factors affecting the final surface functionalization, a number of iodoniums has been considered and experimental evidences (electrochemical and XPS) are compared with theoretical results calculated at the DFT level of the theory (electron affinities, potential energy surfaces of competitive reaction pathways).

2010 - X-ray absorption and magnetic circular dichroism investigation of Bis(phthalocyaninato)terbium single-molecule magnets deposited on graphite [Articolo su rivista]
Biagi, Roberto; J., Fernandez Rodriguez; M., Gonidec; A., Mirone; V., Corradini; Moro, Fabrizio; DE RENZI, Valentina; DEL PENNINO, Umberto; J. C., Cezar; D. B., Amabilino; J., Veciana

Bis-phthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures -- which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets (SMMs) on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), performed at low temperature and high magnetic field at the M4,5 edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (sub-monolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behaviour can be related to the hindrance of the counter-ion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperature suggests that the magnetic properties of molecules are preserved when adsorbed onto the graphite surface.

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

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 - Successful grafting of isolated molecular Cr7Ni rings on Au(111) surface [Articolo su rivista]
V., Corradini; Moro, Fabrizio; Biagi, Roberto; DE RENZI, Valentina; DEL PENNINO, Umberto; V., Bellini; S., Carretta; P., Santini; V. A., Milway; G., Timco; R. E. P., Winpenny; Affronte, Marco

We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7Ni ringsdeposited on Au111 by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show thatthe grafting of the Cr7Ni rings onto the gold surface does not affect the oxidation state and the local symmetryof the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is dueto a reduction in the exchange coupling constants that, however, preserves the structure of the low-energylevels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measure-ments on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slightring distortion suggesting possible explanation for the observed magnetic behavior. These results show thatcomplex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must beexamined in individual cases.

2009 - Understanding the electronic properties of molecule/metal junctions: the case study of thiols on gold [Articolo su rivista]
DE RENZI, Valentina

In this work, I review our recent results on the electronic properties of thiolate/metal interface. Work function change, molecular level alignment and interfacial state formation of the methylthiolate/ Au(1 1 1) and dimethyl-disulfide/Au(1 1 1) interfaces are investigated by means of UPS and XPS. A complete picture of the electronic properties of both the dimethyl-disulfide (DMDS) weakly-adsorbed and the methylthiolate (MT) chemisorbed phases is obtained by direct quantitative comparison between the experiment and theory. The modifications of the electronic properties of the DMDS thin film, induced by a MT buffer layer are also discussed. Moreover, the cysteine/Au(1 1 1) system is investigated as an example of interface formed by molecules with large intrinsic dipole moment. Finally, surface inhomogeneity and its influence on the interface electronic properties is briefly discussed, in light of the important concept of local work function.

2008 - Electronic Structure of a Mn6 Single Molecule Magnet (S=4) grafted on Au(111) [Articolo su rivista]
DEL PENNINO, Umberto; V., Corradini; Biagi, Roberto; DE RENZI, Valentina; Moro, Fabrizio; D. W., Boukhvalov; G., Panaccione; M., Hochstrasser; C., Carbone; C. J., Milios; E. K., Brechin

Single molecule magnets (SMMs) form a new class of magnetic materials consisting of identical nanoscaleparticles that can show magnetization in the absence of a magnetic field. We have experimentally and theoreticallyinvestigated the low-spin (S=4) member of the Mn6 SMM family, properly functionalized with two3-thiophenecarboxylate (3tpc) ligands in order to graft it on to a Au(111)surface. We report the theoreticaldensity of states calculated within the local density approximation (LDA) scheme accounting for the on-siteCoulomb repulsion (LDA+U) for U values ranging from 0 to 8 eV. On the experimental side, by exploitingresonant photoemission at the Mn 2p edge, we were able to single out the Mn 3d derived states in the valenceband energy region for a submonolayer distribution of Mn6-3tpc deposited on Au(111). From the comparisonbetween the experimentally derived 3d density of states and the theoretical one, we found that the bestagreement occurs for a U value of 4 eV. From the binding energy of Mn 2p3/2 core line, measured in situ, wealso derived a value for the 2p-3d correlation energy of about 5 eV—in agreement with previousdetermination.

2008 - Grafting derivatives of Mn6 single-molecule magnets with high anisotropy energy barrier on Au(111) surface [Articolo su rivista]
Moro, Fabrizio; V., Corradini; M., Evangelisti; DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto; C. J., Milios; L. F., Jones; E. K., Brechin

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn6family (general formula [MnIII6O2(R-sao)6(O2C-th)2L4-6], where R ) H (1) or Et (2), HO2C-th ) 3-thiophenecarboxylic acid, L ) EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface.Complex 1 exhibits spin ground-state S ) 4, as the result of ferromagnetic coupling between the twoantiferromagnetic MnIII3 triangles, while slight structural changes in complex 2, switch the dominant magneticexchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S ) 12 and, consequently,the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magneticsusceptibility measurements show that the functionalized complexes preserve the main magnetic propertiesof the corresponding not-functionalized Mn6 clusters (i.e., total spin value and magnetic behavior as a functionof temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes,the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-rayphotoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved aftergrafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters witha slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positionsdemonstrate, for both derivatives, the strength of the grafting with the gold surface.

2008 - Structural study of CH3S self-assembled monolayers on Au(111) [Articolo su rivista]
Bracco, G; Cavanna, D; Thea, S.; DE RENZI, Valentina

We present preliminary results of an experimental investigation of ordered phases of CH3S chemisorbed on Au(111). The self-assembled monolayer has been grown by dosing dimethyl disulfide in ultrahigh vacuum at different substrate temperatures between 200 K and 320 K and following different protocols. The monolayers have been characterized by means of low-energy He atom scattering with time-of-flight detection in a temperature range between 150 K and 300 K. The observed diffraction patterns show that the main periodicity is well-described by the (3 x 4) overlayer of the Au(111) lattice, coexisting with the (root 3 x root 3) periodicity, in agreement with previous results obtained by means of low-energy electron diffraction.

2008 - Very-low energy vibrational modes as a fingerprint of H-bond network formation: L-Cysteine on Au(111) [Articolo su rivista]
DE RENZI, Valentina; L., Lavagnino; V., Corradini; Biagi, Roberto; DEL PENNINO, Umberto; M., Canepa

The ultrahigh vacuum adsorption of cysteine layers on the Au(111) surface has been studied by means ofX-ray photoelectron (XPS) and high-resolution energy loss spectroscopies (HREELS). Room-temperaturedeposition determined the formation of a quite heterogeneous first layer, where both weakly and stronglybound molecules coexist. Deposition at a slightly higher temperature (330 K) led instead to the formation ofa homogeneous, self-assembled monolayer made of molecules chemisorbed through a thiolate bond. In thelatter case, HREELS measurements have been interpreted in terms of a well-organized H-bond network madeof zwitterionic molecules. Two vibrational modes, denoted as N and H modes, respectively, have been identifiedas distinguishing features of the homogeneous monolayer obtained at 330 K. The N mode lies at 3350 cm-1and is attributed to a stretching vibration of the N-H· · ·O bond. The H mode, observed at 74 cm-1 for fullmonolayer coverage, is assigned to a collective vibration of the two-dimensional H-bond network. At halfmonolayercoverage, the H mode has been observed at 55 cm-1. This red-shift indicates a coverage dependenceof the H-mode frequency, which clearly supports its intermolecular origin. This finding is a nice example ofthe extreme sensitivity of low-frequency vibrational modes to the details of molecule-molecule interactions.

2008 - X-ray magnetic circular dichroism investigation of spin and orbital moments in Cr8 and Cr7Ni antiferromagnetic rings [Articolo su rivista]
V., Corradini; Moro, Fabrizio; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; S., Carretta; P., Santini; Affronte, Marco; J. C., Cezar; G. TIMCO AND R. E. P., Winpenny

We investigated the electronic and magnetic properties of thick films of molecular Cr8 and Cr7Ni antiferromagneticrings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroismXMCD. We determined the local symmetries, the electronic configuration, and the values of orbital and spinmoments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation betweenthe Cr and Ni spins in the Cr7Ni molecular ring switches from antiferromagnetic to ferromagnetic withincreasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separateevaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function oftemperature and magnetic field. The magnetic behaviors experimentally observed are compared with the resultsof spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering andlow-temperature specific-heat measurements. The very good agreement between experimental data and calculationsis a clear indication of the integrity of molecules. The temperature dependence of the ion magneticmoments results from the interplay between Zeeman and isotropic-exchange contributions, and is well capturedby the theoretical model.

2007 - An Ab-initio theoretical study of the electrochemical grafting process of alkynil(aryl)iodonium salts on glassy carbon surfaces [Relazione in Atti di Convegno]
K., Daasbjerg; DEL PENNINO, Umberto; DE RENZI, Valentina; Fontanesi, Claudio; Parenti, Francesca; Rastelli, Augusto

The behaviour of alkynil(aryl)iodonium salts upon electrochemical reduction, on glassy carbon electrodes, is here studied by comparing both theoretical and experimental results. In particular, experimental results are obtained by means of cyclic voltammetry and chronocoulometric measurements as well as by collecting XPS spectra, while the standard electrode potential and different dissociation paths (I-C, arylic, compared to the I-C, alkynil, bond cleavage) have been characterized theoretically at the B3LYP/3-21g** level of the theory (the solvation free Gibbs energy of the cation and of the neutral radical species have been calculated by using the CPCM method).

2007 - Isolated heterometallic Cr7Ni rings grafted on Au(111) surface [Articolo su rivista]
V., Corradini; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; Gambardella, Alessandro; Affronte, Marco; Ca, Muryn; Ga, Timco; Rep, Winpenny

A study of the deposition of heterometallic antiferromagnetically coupled rings onto gold surfaces is reported. Twonew {Cr7Ni} rings, [NH2nPr2][Cr7NiF8(3-tpc)16] (1) (where 3-tpc ) 3-thiophenecarboxylate) and [nBuNH2CH2CH2SH][Cr7NiF8(O2CtBu)16] (2) have been made and structurally characterized. They have been deposited from the liquidphase on Au(111) and the adsorbed molecules compared by means of scanning tunneling microscopy (STM) andX-ray photoemission spectroscopy (XPS). In both cases a two-dimensional distribution of individually accessible{Cr7Ni} heterometallic rings on the gold surface has been obtained, exploiting the direct grafting of sulfur-functionalizedclusters. There is a competition between the chemisorption of the {Cr7Ni} clusters and a thiolic self-assembledmonolayer (SAM) formed by free ligands. In 2, the presence of a single sulfur ligand should force the molecule tograft with the ring axis normal to the surface. The cluster stability in the STM images and the S-2p energy positionsdemonstrate, for both functionalizations, the strength of the grafting with the gold surface.

2007 - Molecular level alignment and work function change of dimethyl-disulfide physisorbed thin films on Au(111) [Articolo su rivista]
DE RENZI, Valentina

The formation and the electronic properties of a dimethyl–disulfide (DMDS) thin film adsorbed at low temperature on the Au(111) surface has been studied by means of Ultraviolet Photoemission Spectroscopy. The dependence of the binding energy of the molecular states as a function of coverage has been analyzed and compared to the corresponding work function change. A deviation from the expected rule for the molecular level alignment is observed and attributed to the differences between local and average work function.The effect of a buffer thiolated monolayer, obtained by dosing DMDS at room temperature, on the evolution of the interface electronic properties is also analyzed and discussed.

2007 - Ordered phases and temperature behavior of CH3S self-assembled monolayers on Au(111) [Articolo su rivista]
D., Cavanna; G., Bracco; DE RENZI, Valentina; V., Corradini; Biagi, Roberto; DEL PENNINO, Umberto

Abstract. We present the results of an experimental characterization of the ordered phases of CH3S chemisorbed on Au(111) and of their temperature dependence. The CH3S self-assembled monolayer has been grown by dosing dimethyl disulfide (DMDS) in ultrahigh vacuum at different substrate temperatures between 200 and 320 K and has been characterized by means of low energy He atom scattering (HAS) with time of flight detection and low energy electron diffraction (LEED) in a temperature range between 150 and 320 K. Upon following an appropriate dosing–annealing procedure, two ordered coexisting phases, i.e. a (3 × 4) and a periodicity, have been observed using HAS, in agreement with previous findings. Moreover, a new protocol consisting of a single-step dosing of DMDS at higher pressures (10-5 mbar range) has been found to produce both coexisting phases without any need of the annealing step. The temperature behaviour of the ordered phases has been studied using LEED, showing that a first-order phase transition occurs at ~320 K.

2007 - Self-assembling of Mn12 molecular nanomagnets on FIB-patterned Au dot matrix [Articolo su rivista]
V., Corradini; DEL PENNINO, Umberto; Biagi, Roberto; DE RENZI, Valentina; Gambardella, Alessandro; G. C., Gazzadi; Candini, Andrea; Zobbi, Laura; Cornia, Andrea

AbstractWe have developed a novel strategy to build arrays of magnetic nanodots on the 100 nm scale, which exploits the potentialities of both bottom-up and top-down approaches, by self-assembling sulfur-functionalized Mn12 single molecule magnets (SMMs) on patterned Au dot matrices nanofabricated by FIB (focus ion beam). In this way, we demonstrate the capability to assemble SMMs in ordered arrays, where the magnetic information can be easily addressed, being the single bit represented by a 2D distribution of few hundred Mn12 clusters, grafted on top of each 100 × 100 nm2 Au dot. Moreover, the chosen Mn12 functionalization is expected to favour a preferential orientation of the grafted molecule with the easy magnetization axis normal to the surface.

2006 - Interfacial electrostatics of self-assembled monolayers of alkane thiolates on Au(111): Work function modification and molecular level alignments [Articolo su rivista]
R., Rousseau; DE RENZI, Valentina; R., Mazzarello; Marchetto, Diego; Biagi, Roberto; S., Scandolo; DEL PENNINO, Umberto

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.

2006 - Valence band resonant photoemission of Mn-12 single molecules grafted on Au(111) surface [Articolo su rivista]
DEL PENNINO, Umberto; DE RENZI, Valentina; Biagi, Roberto; V., Corradini; Zobbi, Laura; Cornia, Andrea; D., Gatteschi; F., Bondino; E., Magnano; M., Zangrando; M., Zacchigna; A., Lichtenstein; D. W., Boukhvalov

In the present work, we address the determination of the electronic structure of a monolayer of Mn-12 clusters, grafted on Au(1 1 1) substrate by a suitable functionalization, by means of X-ray absorption and resonant valence band photoemission (RESPES) spectroscopies taken across the Mn 2p-3d absorption edge. The absorption edge of the Mn-12 monolayer is compared with that - reported in literature - of bulk Mn-12, showing that the deposition procedure does not significantly affect the intimate nature of the Mn-12 cluster. Quantitative comparison between RESPES spectra data allow us to extract the Mn 3d density of states from the Mn-12-monolayer valence band spectrum, which is dominated by the Au 5d states. The shape of the experimentally obtained valence band DOS is in good agreement with the theoretical DOS, obtained by first-principle LDA + U calculations, confirming the importance of correlation effects in the determination of the electronic properties of the Mn-12 cluster. (c) 2006 Elsevier B.V. All rights reserved.

2005 - Chemically homogeneous, silylated surface for effective DNA binding and hybridization [Articolo su rivista]
Alessandrini, Andrea; DE RENZI, Valentina; L., Berti; I., Barak; P., Facci

We report on a method for covalent immobilization of 5 ´-thiol-modified single strand DNA probes, onto oxygen exposing surfaces by exploiting surface derivatization by 3-mercaptopropyltrimethoxysilane and subsequent intermolecular disulfide bond formation. The various steps in the formation of the molecular edifices have been characterized by X-ray photoelectron spectroscopy, quartz crystal microbalance and atomic force microscopy under liquid. Surface reaction kinetics of thiol-modified DNA probes with thiol-bearing silanes turned out to be a second-order one, possibly due to the presence of both free thiol and S-S dimers in solution. The ability of immobilized single strand DNA to bind the complementary strand has been tested and confirmed by quartz crystal microbalance measurements. The presented DNA immobilization method appears to be applicable to any surface bearing exposed hydroxyl moieties.

2005 - Isolated Mn-12 single-molecule magnets grafted on gold surfaces via electrostatic interactions [Articolo su rivista]
E., Coronado; A., Forment Aliaga; F. M., Romero; V., Corradini; BIAGI, Roberto; DE RENZI, Valentina; GAMBARDELLA, Alessandro; DEL PENNINO, Umberto

Electrostatic interactions drive the adsorption of polycationic single-molecule magnets onto anionic monolayers self-assembled on gold surfaces. Well-isolated magnetic clusters have been deposited and characterized using scanning tunneling microscopy and X-ray photoemission spectroscopy.

2005 - Metal work function changes induced by organic adsorbates: A combined experimental and theoretical study [Articolo su rivista]
DE RENZI, Valentina; R., Rousseau; Marchetto, Diego; Biagi, Roberto; DEL PENNINO, Umberto; S., Scandolo

The role of molecular dipole moment, charge transfer, and Pauli repulsion in determining the workfunctionchange (DeltaPhi) at organic-metal interfaces has been elucidated by a combined experimental andtheoretical study of (CH3S)2/Au(111) and CH3S/Au(111). Comparison between experiment and theoryallows us to determine the origin of the interface dipole layer for both phases. For CH3S/Au(111), Delta Phican be ascribed almost entirely to the dipole moment of the CH3S layer. For (CH3S)2/Au(111), a Paulirepulsion mechanism occurs. The implications of these results on the interpretation of Delta Phi in the presenceof strongly and weakly adsorbed molecules is discussed.

2005 - On the electroreduction of Cr(VI) aqueous solutions on iron and copper cathodes [Articolo su rivista]
Giovanardi, Roberto; E., Soragni; Fontanesi, Claudio; DE RENZI, Valentina; DEL PENNINO, Umberto; M. L., Foresti

The electroreductive behaviour of CrO3 +H2SO4 aqueous solution is studied on poly-crystalline iron and copper cathodes. Theelectrochemical results have been also compared with XPS, AFM and SEM findings collected ‘‘ex situ’’ on emersed electrodes andan electrochemical mechanism featuring adsorbed Cr(IV) and Cr(III) species is proposed. In the case of iron a main role is played bythe passive state of the electrodic surface caused by the presence of a protective mixed iron/chromium oxide layer and by its electrochemicaldissolution. This passive to active transition of the electrodic surface and the occurrence of an irreversible reductivecurrent peak, in a potential range where the deposition of metallic chromium does not yet occur, suggests that the electrodic surfaceplays an active role in the electrochemical process. A major difference is found between Fe and Cu cathodes, concerning the kineticmechanism of formation of adsorbed species.

2004 - Ordered (3 x 4) high-density phase of methylthiolate on Au(111) [Articolo su rivista]
DE RENZI, Valentina; R., Di Felice; Marchetto, Diego; Biagi, Roberto; DEL PENNINO, Umberto; A., Selloni

The formation of ordered phases of dimethyl-disulfide on the Au(111) surface has been investigated by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and state-of-the-art density-functional theory (DFT) periodic supercell calculations. The LEED diffraction pattern, obtained after a production method that includes two-step dosing and prolonged postdeposition annealing, unambiguously corresponds to a novel phase that consists of (3 x 4) domains coexisting with the as-deposited (root3 x root3)R30degrees structure. XPS measurements indicate that the coverage of the new (3 x 4) superstructure is the same as that of the (root3 x root3)R30degrees phase. In both phases, the binding energy of the S 2p(3/2) core-level peak is found to be 162.2 eV, corresponding to the formation of a thiolate layer. The DFT calculations allow us to identify a viable metastable (3 x 4) structure where the S headgroups of the CH3S radicals select distinct adsorption sites; three quarters of them adsorb at bridge sites and one quarter at top sites. The relative energetics of the (3 x 4) and (root3 x root3)R30degrees configurations suggest that the two structures may coexist on the surface, in agreement with experimental data.

2004 - Organized single-molecule magnets: direct observation of new Mn12 derivatives on gold [Articolo su rivista]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; Pacchioni, Mirko; Zobbi, Laura; D., Bonacchi; A., Caneschi; D., Gatteschi; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; L., Gurevich; AND H. S. J., VAN DER ZANT

Gold adsorbates of the dodecamanganese(III,IV) single-molecule magnet (SMM) [Mn12O12(L)16(H2O)4] whereL=16-(acetylthio)hexadecanoate have been prepared and investigated by X-ray photoelectron spectroscopy andscanning tunneling microscopy (STM). The successful imaging of Mn12 molecules by STM represents a first step towardthe magnetic addressing of individual SMMs and the development of molecule-based devices for magnetic informationstorage.

2003 - Direct Observation of Single-Molecule Magnets Organized on Gold Surfaces [Articolo su rivista]

A dodecamanganese(III,IV) cluster, [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH, is considered to be the prototype of a class of materials referred to as single-mol. magnets (SMMs), whose magnetic behavior is strongly reminiscent of bulk magnets. A method to deposit SMMs on a gold film and to observe them directly using scanning tunneling microscopy (STM) was demonstrated. To obtain suitably derivatized Mn12-type clusters, 16-sulfanylhexadecanoate ligands (L') were introduced around the Mn12 core by exploiting a well-known ligand-exchange reaction. However, treatment of I·4H2O·2AcOH with HL' in toluene or CH2Cl2 afforded intractable solids. The use of the corresponding acetyl-protected acid HL yielded a fully-substituted deriv. [Mn12O12(L)16(H2O)4] (II) which is highly sol. in org. solvents. Deposition of the nanoclusters on a Au(111) surface was carried out by incubating gold substrates in dild. solns. of compd. II in THF together with aq. NH4OH. STM anal. showed complete coverage of the gold substrate by a disordered layer of round particles with an apparent lateral size of ∼5.8 nm, which agreed with the size of compd. II obtained from mol. modeling studies (5.0-5.6 nm).

2003 - HREELS study of the adsorption mechanism and orientational order of 2-mercaptobenzoxazole on Cu(100) [Articolo su rivista]
F., Allegretti; DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto; G., Contini; V., DI CASTRO; C., Mariani; M. G., Betti; Fontanesi, Claudio

Organic molecules with sulfur-containing headgroups interacting with noble metal surfaces represent a fruitful class of systems with potential relevance in new device technology. Within this context, the room temperature adsorption of 2-mercaptobenzoxazole (MBO) on the Cu(1 0 0) surface has been studied by high-resolution electron energy-loss spectroscopy (HREELS) as a function of molecular exposure. At low coverage, the HREEL spectra present only one mode at 93 meV (750 cm(-1)), which is attributed to the out-of-plane C-H bending gamma(CH). Its off-specular angular dependence indicates that the molecules lie almost flat on the surface. At higher coverage, the MBO layer orders in a p(2 x 2) superstructure, and energy loss peaks at higher energies appear. These features can be assigned to in-plane vibrations indicating that the molecular axis tilts toward a more vertical position, thus allowing a closer molecular packing. Assignment of the HREELS peaks is confirmed by estimation of the gas phase normal modes of the free molecule, evaluated by ab initio methods. The absence of the N-H stretching mode in the HREEL spectra indicates that the H atom bonded to nitrogen in the gas-phase molecule is lost upon adsorption, suggesting that MBO molecule adsorbs on the Cu surface as a thiolate.

2003 - Photoemission investigation of the alkali-metal-induced two-dimensional electron gas at the Si(111)(1 X 1):H surface [Articolo su rivista]
Biagi, Roberto; Fantini, Paolo; De Renzi, Valentina; Betti, Maria Grazia; Mariani, Carlo; Del Pennino, Umberto

We investigate the formation of a two-dimensional electron gas (2DEG) at the hydrogen-passivated silicon (111) surface exposed to tiny amounts of alkali metals (K and Cs) by means of high-resolution ultraviolet photoemission spectroscopy. The accumulation layer derives from the quantization and filling of bulk states located around the conduction-band minimum. Direct photoemission from these states results in a well-defined and structureless feature, which becomes huge when the alkali metal is deposited at low temperature (150 K), indicating a much higher charge transfer associated with the growth at low temperature. Its shape is characterized by a tail which is attributed to the broadening of the 2DEG density of states due to the random fields set up by the ionized alkali metal at the surface, providing direct experimental evidence of tailing of two-dimensional subbands at a free semiconductor surface.

2002 - Study of the transition from the ideal Si(111)-H(1 x 1) surface to the (7 x 7) reconstruction by HREELS, UPS and LEED [Articolo su rivista]
DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto

The temperature-induced evolution of the Si(1 1 1)-H(1 x 1) surface towards the (7 x 7)-reconstruction has been studied by means of UPS, HREELS and LEED techniques. We found that H atom desorption occurs at about 500 degreesC, and the full development of the (7 x 7)-phase, checked by both UPS and LEED, occurs around 700 degreesC. In the valence band spectra, the (7 x 7)-related restatom peak is present already for annealing temperature of 465 degreesC, while the adatom-related peak starts to appear only around 600 degreesC and fully develops at 700 degreesC. A band bending is detected and its behavior discussed in relation to the surface state evolution. In the first annealing steps, the desorption-induced defects cause the shift of the surface Fermi level and the consequent upwards band bending, which reaches the maximum value of 0.47 eV for annealing to 465 degreesC. The subsequent evolution of the dangling-bond states into the (7 x 7) surface states determines the band bending decrease till 0.1 eV. (C) 2001 Elsevier Science B.V. All rights reserved.

2002 - Thin film SiC epitaxy on Si(111) from acetylene precursor [Relazione in Atti di Convegno]
DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto

A promising route for the production of SiC on Si surfaces exploits the use of small pi-bonded hydrocarbons, like acetylene, as carbon source. This choice in fact, allows maintaining the growth temperatures well below 1000 C. Starting from the hydrogenated Si(111)(1 x 1) surface can be a way to keep the Si substrate always below 800degreesC. In this way, however, to obtain a SiC film composed of large crystallites is not straightforward. We report on the epitaxial growth of 3C-SiC thin films on the Si(111)-(7x7) surface, obtained by dosing acetylene at 750degreesC. The growth was been followed by means of LEED, X-ray photoemission and Auger spectroscopy, as well as by high-resolution electron-energy loss spectroscopy. HREEL spectra show very strong features related to the SiC Fuchs-Kliewer phonon (and its replicas), XPS peaks show losses associated to the SiC bulk plasmon, while the LEED pattern is that of the 3C-SiC, confirming the crystalline character of the epitaxial film. The mean film thickness, as measured by ex-situ TEM, is of few hundreds Angstrom.

2001 - Acetylene adsorption on the Si(111)-(7X7) surface: Ultraviolet photoemission and high-resolution electron-energy-loss spectroscopies [Articolo su rivista]
De Renzi, Valentina; Biagi, Roberto; Del Pennino, Umberto

The room temperature adsorption and the thermal reaction of acetylene on the Si(111)-(7 x 7) surface has been investigated by means of ultraviolet-photoemission spectroscopy and high-resolution electron-energy-loss spectroscopy (HREELS). In the first stage of the adsorption process, the evolution of the restatom- and adatom-related electronic states as a function of acetylene exposure is well described by a modified di-sigma -bonding model, in which the metallic character of the (7 x 7) surface plays a fundamental role. For higher exposure, all the restatom. dangling bonds are saturated and the adsorption process continues through saturation of the remaining adatoms. In this phase, a molecular tilting is observed, possibly accompanied by a surface atom rearrangement. Upon annealing to 600 degreesC, acetylene dehydrogenation and partial desorption occur, as demonstrated by the disappearance of the C-H modes in HREELS and the reappearance of the adatom. and restatom dangling-bond states in valence-band spectra. At 700 degreesC the formation of a Si-C compound is observed. Atomic-force-microscopy topographic images taken ex situ after annealing to 800 degreesC show that the Si-C interface is constituted by islands with lateral dimension of similar to 300 Angstrom, while the overall surface roughness is 10-20 Angstrom.

2001 - Substrate reconstruction and electronic surface states: Ag(001) [Articolo su rivista]
Savio, L; Vattuone, L; Rocca, M; DE RENZI, Valentina; Gardonio, S; Mariani, C; DEL PENNINO, Umberto; Cipriani, G; Dal Corso, A; Baroni, S.

We present the result of a combined ab initio theoretical study and ultraviolet photoemission spectroscopy experiment of the electronic surface states of the clean and of the oxygen covered, unreconstructed and missing-row reconstructed, Ag(001) surface. For the clean and for the unreconstructed oxygen covered surface, theory and experiment find an electronic surface state at about 0.4 eV below the Fermi energy (EF) at the B point of the surface Brillouin zone. At the same point, theory predicts the existence of an empty state at +3 eV. Upon reconstruction, the photoemission signal of the filled state is strongly reduced and, according to calculations, the state shifts in energy above EF The surface interband transition (SIT) between the two states exists thus only for the unreconstructed surfaces and nearly matches the surface plasmon energy. Our investigation confirms therefore the correlation between the presence of this SIT and the anomaly of the surface plasmon dispersion curve of unreconstructed Ag(001). (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Temperature dependence of acetylene adsorption and reaction on Si(111)-(7 x 7) [Articolo su rivista]
DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto

The adsorption and reaction of acetylene dosed on the Si(111)-(7 x 7) surface has been studied by means of X-ray photoemission, Auger, and HREELS spectroscopies for different substrate temperatures. For exposure at temperature of 600 degreesC, only an extremely thin reacted layer can be obtained, which hinders further molecular reaction at the surface, acting as a barrier layer. The increase in surface diffusion occurring between 600 and 750 degreesC overrides the barrier layer effect, allowing the growth of a thicker SiC. (C) 2001 Elsevier Science B.V. All rights reserved.

2000 - Evaluation of alkali-induced band-bending inhomogeneity and charge transfer trend from core-level analysis [Articolo su rivista]
DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto; Pedio, M; Goldoni, A; Larciprete, R.

We present a method for evaluating the spatial inhomogeneity of adsorbate-induced band bending on semiconductors from the substrate core-level lineshape. As a case study we consider the first stages of accumulation layer formation due to K adsorption on the H:Si(lll) surface at 300 K. The observed Si 2p core-level lineshape variation is accounted for considering each randomly distributed adatom as a source,of a screened Coulomb potential. We determine the coverage: dependence of the band-bending distribution, and the parameters characterizing the local K-induced potential.

2000 - Evolution of one-dimensional Cs chains on InAs(110) as determined by scanning-tunneling microscopy and core-level spectroscopy [Articolo su rivista]
Modesti, S; Falasca, A; Polentarutti, M; Betti, Mg; DE RENZI, Valentina; Mariani, C.

We present a combined scanning tunneling microscopy and ultraviolet photoelectron spectroscopy investigation on the formation and evolution of one-dimensional (1D) Cs chains on the InAs(110) surface. At 40% of saturation coverage, the linear phase evolves into a two-dimensional (2D) cluster layer. Each Cs nanowire, several hundreds of angstroms long, is aligned along the [1(1) over bar 0] direction. The Cs chain phase presents different adsorption sites, as deduced by the Cs 5p core-level lineshape and STM. The coverage dependence of the mean interchain distance shows that the Cs nanowires repel each other in the [001] direction, even when they are about 50 Angstrom apart. A repulsive long-range dipole-like interaction influences the mesoscopic properties of the chain phase. The binding energy shift of the Cs core levels can be quantitatively explained, taking into account the Cs depolarization as a function of the chain-chain distance. (C) 2000 Elsevier Science B.V. All rights reserved.

2000 - Metal-induced gap states at InAs(110) surface [Articolo su rivista]
Betti, Mg; Bertoni, G; Corradini, V; DE RENZI, Valentina; Mariani, C.

High-luminosity and high-energy-resolution photoemission spectroscopy call provide direct observation of the spectral density of metal-induced states throughout the whole band gap, even at extremely low metal deposition. We present a study of the density of states of a two-dimensional electron gas induced in the InAs(110) conduction band by deposition of caesium, antimony and silver adatoms. We follow the density of states redistribution between the two-dimensional electron-gas channel and the appearance of metal-induced gap states. (C) 2000 Elsevier Science B.V. All rights reserved.

2000 - Occupied surface-state bands of the (1 x 2) ordered phase of Bi/InAs(110) [Articolo su rivista]
DE RENZI, Valentina; Betti, Mg; Mariani, C; Almeida, J; Grioni, M.

The ordered (1 x 2) monolayer of Bi on the InAs(110) surface is studied by means of angle-resolved high-resolution ultra-violet photoemission at room temperature. Four bands of Bi;induced surface states are singled out and their dispersion is mapped along the high-symmetry directions of the surface Brillouin zone. The highest occupied state S' lies inside the InAs(110) energy gap at 0.2 eV binding energy and spreads across the Fermi level, determining the semimetallic character of the system. The second-highest occupied state Sn is located at 0.83 eV binding energy (at <(Gamma)over bar>), while states Sm and SN are located near the internal gap edge at 2.57 eV and 3.3 eV binding energy, and present band dispersions along <(Gamma)over bar>(X) over bar of similar to 340 meV and 300 meV, respectively. The Pi-induced bands of the (1 x 2) phase are shifted in energy relative to the corresponding bands of the (1 x 1)-Bi/InAs(110) phase and their width is reduced. These differences are discussed in the light of the geometric structure of the two phases.

1999 - A high-resolution spectroscopy study on bidimensional ordered structures: the (1x1) and(1x2) phases of Bi/InAs(110) [Articolo su rivista]
DE RENZI, Valentina; Betti, Mg; Corradini, V; Fantini, P; Martinelli, V; Mariani, C.

The room-temperature growth of bismuth on the InAs(110) surface and the Bi(1 x 1) and Bi-(1 x 2) ordered phases have been studied by means of high-resolution ultraviolet photoemission and high-resolution electron-energy-loss spectroscopy A modified Stransky-Krastanov growth mode at room temperature and the stability of the (1 x 2)-symmetry phase in the 480-580 K annealing temperature range are deduced by quantitative analysis of the core-level data and Auger spectra. A Bi 5d and In 4d core-level analysis is presented, and discussed according to a recently proposed model for the (1 x 2) overlayer reconstruction. The formation of Bi-derived electronic states has been followed during the growth of the (1 x 1)-symmetry phase, showing a semiconducting behaviour at the monolayer coverage, with a well-defined gap stare at 0.39 eV binding energy related to the Bi p-like dangling bonds. The (1 x 2)-Bi phase is metallic, as indicated by the well-defined Fermi edge and by the metallic-like quasi-elastic peak tail in the energy-loss spectra. The metallicity and the electronic structure of the (1 x 2)-Bi phase are discussed in relation to the available geometric structure.

1999 - Anisotropic ordered Planar Growth of alpha- Sexithienyl Thin Films [Articolo su rivista]
Stefano, Prato; Luca, Floreano; Dean, Cvetko; DE RENZI, Valentina; Alberto, Morgante; Silvio, Modesti; Biscarini, Fabio; Roberto, Zamboni; C., Taliani

We show how the anisotropy of the (1 × 2) reconstructed Au(110) surface can be effectively used to achieve spontaneous ordered planar growth of rodlike conjugated oligomers uniaxially oriented on the substrate. The growth of the first few monolayers of α-sexithienyl (T6) films on the (1 × 2) reconstructed Au(110) surface in ultrahigh vacuum has been investigated by He atom scattering (HAS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). T6 molecules lie flat on the substrate forming long chains along the [110] direction of the Au rows. The interchain spacing is coverage dependent and uniaxially commensurate with the substrate along [001]. Three ordered structures with 12, 16, and 20 A periods are formed during the deposition of the first two T6 monolayers. This T6 overlayer acts as a template for the growth of at least five monolayers which are still planar and orientationally ordered.

1998 - A two-step optimized measurement for the phase shift [Articolo su rivista]
Chizhov, Av; DE RENZI, Valentina; Paris, Mga

A two-step detection strategy is suggested for the precise measurement of the optical phase shift. In the first step an unsharp, but unbiased, joint measurement of the phase and photon number is performed by heterodyning the signal field. Information coming from this step is then used for appropriate squeezing of the probe made to obtain a sharp phase distribution. Application to squeezed states leads to a phase sensitivity scaling as Delta phi similar or equal to N-1 relative to the total number of photons impinged onto the apparatus. Numerical simulations of the whole detection strategy are also presented.

1998 - Gap-state formation in two-dimensional ordered Bi layers on InAs(110) [Articolo su rivista]
Betti, Mg; Corradini, Valdis; DEL PENNINO, Umberto; DE RENZI, Valentina; Fantini, Paolo; Mariani, Carlo

The spectral density at the Fermi energy and the formation of adlayer-induced electronic states is studied by means of high energy resolution ultraviolet photoemission spectroscopy. A highly ordered (1x1)-Bi monolayer deposited on InAs(110) induces a well-resolved, occupied electronic state in the InAs surface gap, attributed to p-like dangling bonds at the Bi atomic chains. Appropriate annealing of a Bi multilayer produces a (1x2)-symmetry stable phase. Evolution of the spectral density close to the Fermi edge brings to light the metallicity of the (1 x 2)-Bi layer induced by Bi-Bi bonding in the atomic chains.

1997 - Temperature behavior of the (sqrt3 x sqrt3)R30-1CO and the (2 x 2)-3CO overlayers on Rh(111): A combined HAS and LEED investigation [Articolo su rivista]
Over, H.; Schwegmann, S.; Ertl, G.; Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Gotter, R.; Morgante, A.; Peloi, M.; Tommasini, F.; Zennaro, S.

The order-disorder phase transition of the Rh(111)-(√3 x √3)R30°-CO phase was investigated by He scattering. The critical temperature turned out to be 330±5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (<70 K) and various CO partial pressures there is evidence of the formation of a disordered second CO layer in equilibrium with the gas phase. The isosteric heat of CO adsorption in the second layer is about 6 kJ mol-1

1997 - The atomic geometry of the (2x2) phases of O and CO+O on Rh(111) [Articolo su rivista]
S., Schwegmann; H., Over; DE RENZI, Valentina; AND G., Ertl

The local adsorption geometries of the (2 × 2)-1O, (2 × 2)-2O, (2 × 2)-(O + CO) and (2 × 2)-(O + 2CO) phases on the Rh(111) surface have been investigated by analysing low-energy electron diffraction (LEED) intensity data. In all cases, the oxygen atoms were found to occupy the threefold fee site and the topmost layer spacing d12 of Rh(111) was found to be expanded by about 3%. Additional experiments with disordered oxygen overlayers prepared at low adsorption temperatures show a monotonic increase of d12 with oxygen coverage. An O coverage of θO ≈ 0.1 already lifts the contraction of 0.02 Å of the clean surface. This finding is remarkable as for Ru(0001) a substantial expansion of the first layer spacing (≈ 3%) is observed only at very high O coverages, e.g. for the (1 × 1)-O structure. For the high-coverage Rh(111)-(2 × 2)-2O phase a honeycomb arrangement of the O atoms can clearly be ruled out. The (2 × 2)-2O has rather to be regarded as a (2 × 1)-1O phase. In the mixed (2 × 2)-(O + CO) overlayer the CO molecule occupies the on-top position which is also expected from a reduced back-donation of (substrate) electron charge density due to coadsorbed O atoms. The presence of CO molecules weakens the Rh-O bond. Altogether, the structural findings nicely reflect the competition of CO and O for electron charge density at the Rh(111) surface. In the (2 × 2)-(O + 2CO) phase 60% of available hcp sites and 100% of available on-top sites in the (2 × 2) unit cell are occupied.

1996 - Ordering of of a prototypical conjugated molecular system during monolayer growth on the (1x2)Au(110) surface [Articolo su rivista]
Buongiorno Nardelli, M.; Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Gotter, R; Morgante, A; Peloi, M; Tommasini, F.; Danieli, R.; Rossini, S.; Taliani, C.; Zamboni, R.

The UHV deposition of α-hexathiophene (T6) oligomers on the 1 × 2 reconstructed Au(110) surface has been studied in situ. The evolution of the surface morphology during the deposition is followed by the He atom scattering method. A proliferation of Au monoatomic steps is observed during deposition, indicating a strong interaction between Au substrate and T6 molecules. The build-up of a well-ordered single monolayer is obtained with a rectangular unit cell. Its dimensions indicate that T6 molecules lie on the surface, aligned along the rows of close-packed Au atoms.

1996 - Ultra High Vacuum Single Layer Formation of alpha-hexathienyl on the (1x2)Au(110) Surface [Articolo su rivista]
M., BUONGIORNO NARDELLI; D., Cvetko; DE RENZI, Valentina; L., Floreano; R., Gotter; A., Morgante; M., Peloi; AND F., Tommasini; R., Danieli; S., Rossini; R. ZAMBONI AND C., Taliani

The formation of an α-hexathienyl (T6) single layer on the (1 × 2) Au(110) surface has been investigated by means of thermal He atom scattering (THAS) in ultra-high-vacuum (UHV) conditions. The monolayer (ML) structure has a rectangular surface unit cell with the T6 molecules lying flat on the surface and aligned parallel to the Au rows. The periodicity perpendicular to the Au rows is doubled with respect to the bare Au surface, while along the rows it is 27.4 ± 0.5 Å, corresponding to the molecular length. The correlation length of the overlayer structure depends upon substrate temperature during deposition and increases when the system is heated up to 653 K. The surface structure is irreversibly lost upon heating to temperatures higher than 673 K, but T6 molecules do not completely desorb up to about 773 K.

1995 - Electron density of (1x2)Pt(110) from He reflectivity measurements [Articolo su rivista]
Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Lausi, A.; Morgante, A.; Peloi, M.; Tommasini, F.; Kirsten, E.; Rieder, K. H.

The electron density of the (1×2)Pt(110) surface is determined by He reflectivity measurements and compared to the electron densities previously reported for (1×2)Au(110) and (1×2)Rh(110). With respect to the superposition of free atomic electron densities, significant charge depletion and enrichment are observed, respectively, on top of the surface atoms and in the threefold sites of the (111) microfacets.

1995 - Inverse growth kinetics on InSb(110) [Articolo su rivista]
Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Morgante, A.; Peloi, M.; Tommasini, F.; Chab, V.; Prince, K. C.

Using He atom scattering, layer-by-layer erosion of InSb(110) by low energy ion bombardment has been observed to proceed via nucleation of vacancy islands, island growth and coalescence. The mechanism is in full agreement with theoretical models developed for crystal growth, with diffusing adatoms substituted by diffusing vacancies. In particular it has been observed that coalescence of the vacancy islands sets in after the removal of small fractions of a monolayer and that the average terrace width increases with increasing number of sputtered monolayers.

1995 - Low-energy vibrations at the InSb(110) surface [Articolo su rivista]
Buongiorno Nardelli, M.; Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Morgante, A.; Peloi, M.; Tommasini, F.

We present a study of low-energy vibrations at the InSb(110) surface along the Y direction. Surface phonons have been measured by He inelastic scattering and a theoretical analysis has been performed by means of first-principles calculations. We have been able to distinguish the surface acoustic mode above the Rayleigh wave and to identify a surface resonance and an optical surface mode at higher energy. Theoretical calculations are shown to be in agreement with the detected phonon dispersion curves except for the surface resonance which is found 1 meV higher than calculated.

1995 - Terrace distribution during sputtering and recovery of InSb(110) studied by He-atom scattering [Articolo su rivista]
Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Morgante, A.; Peloi, M.; Tommasini, F.; Chab, V.; Prince, K. C.

A helium-atom scattering experiment is reported on the evolution of vacancy islands produced by low-energy ion bombardment on InSb (110). It is seen that island growth occurs through dynamical scaling during the sputtering process, in agreement with the scaling hypothesis that is the basis of nucleation and growth models. After interruption of the sputtering, the spatial distribution of steps on the surface (strictly related to the island size and distance) gradually modifies during the self-organizing surface recovery. In this case the surface reduces its free energy by decreasing the step density; the corresponding increase of the terrace width occurs in the absence of free vacancy supersaturation and the process is diffusion limited. This change of the growth mechanism causes the breaking of dynamical scaling.

1995 - Vacancy Island Nucleation and Inverse Growth of InSb(110) [Articolo su rivista]

He beam scattering measurements of the erosion of the InSb(110) surface by low-energy Ar+ bombardment are reported. Layer-by-layer erosion is observed for surface temperatures above 510 K and is found to proceed by nucleation of vacancy islands, island growth, and coalescence. The average island distance is measured at different stages of the erosion process and it is found to evolve in good agreement with the rate equations derived from models of island growth by atomic deposition. A general increase of the average terrace width is also observed with increasing number of removed layers. The activation barrier for intralayer diffusion of InSb dimer vacancies is found to be Ed=1.14±0.06 eV

1994 - Deconstruction and roughening transitions on (1x2)Pt(110) [Articolo su rivista]
Cvetko, D.; DE RENZI, Valentina; Floreano, L.; Morgante, A.; Tommasini, F.

We have investigated the Pt(110) surface by He atom scattering. The (1 x 2) deconstruction transition at 1087 K is shown to fall in 2D Ising universality class with antiphase domain walls proliferating predominantly along the [001] direction. The surface roughening is observed at 1160 ± 10K through a power law broadening of the anti-phase diffraction peak along the [001] direction. The two distinct phase transition confine the temperature range in which the surface is disordered but still bound to a common height level.

1994 - Space charge layer, metallization and collective excitations of the Bi/GaAs interface. [Articolo su rivista]
DE RENZI, Valentina; Biagi, Roberto; Betti, Maria Grazia; Mariani, Carlo

The collective excitations and the metallization of the Bi/GaAs(I 10) interface grown at room temperature, up to the completion of a few tens of a monolayer, have been studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS). Through analysis of the HREELS data, also by means of an appropriate semiclassical dielectric model, the modifications experienced by the substrate-related loss structures (Fuchs-Kliewer phonon and dopant-induced free-carrier plasmon) and by the quasielastic peak are related to changes in the dielectric response of the overlayer and in the semiconductor space-charge region. The influence of bismuth is effective in enlarging the depletion layer thickness, leaving the interface semiconducting at the monolayer-coverage scale. A band bending value of 0.56 eV is obtained at the coverage of one monolayer on the highly n-type doped sample (n approximately 2.7 X 10(18) cm-3); a determination free from any possible surface photovoltaic effect. At coverages greater than two monolayers, which corresponds to a structural transition, the intermediate structural phase becomes metallic, thus marking a clear semiconductor-metal transition. This intermediate metallic stage further develops towards the formation of actual semimetallic crystalline bismuth layers oriented with the basal plane parallel to the substrate surface.

DE RENZI, Valentina; Betti, M. G.; Mariani, C.

A high resolution electron-energy-loss spectroscopy investigation of the Bi/[n-GaAs(110)] interface grown at room temperature is presented. Analysis of the Fuchs-Kliewer phonon and of the dopant-induced free carrier plasmon is performed through an appropriate fit to the data, by using a dielectric model calculation. A Fermi level pinning of 0.6 eV is determined at one monolayer coverage for this semiconducting interface, and at the present doping level (n-type, n almost-equal-to 2.7 x 10(18) cm-3). The low-energy electronic properties of a 700 angstrom thick Bi crystal grown on GaAs(110) are also studied, and electronic interband transitions at 47 and approximately 200 meV are singled out in semimetallic bismuth.

1993 - Quantum size effects and temperature dependence of low-energy electronic excitations in thin Bi crystals [Articolo su rivista]
DE RENZI, Valentina; Betti, M. G.; Mariani, C.

The quantum size effect (QSE) and temperature dependence of the low-energy electronic properties of thin bismuth crystals are studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS) technique. Electronic interband transitions, taking place at the points L and T of the bismuth Brillouin zone (BZ), are distinctly brought into evidence at 47 and ∼200 meV, and the corresponding complex dielectric function in this energy region is determined. This provides direct experimental determination of the electronic transition from the Fermi level (EF) to the point T6+. The quantum size effect in thin Bi crystals is studied following the evolution of these electronic excitations as a function of the crystal thickness (110-2500). A downshift (upshift) of the transition at the point (T) of the BZ is measured by HREELS, and shown to be related to the Fermi-level modification as a function of thickness. This experimental finding is consistent with the theoretical predictions for the QSE-induced Fermi-level shift. In addition, the same experiment is performed as a function of temperature for crystals 750 thick, for which QSE's are not expected, and for crystals 200 thick which exhibit QSE's. A redshift by ∼12 meV of the lower-lying electronic excitation is measured on decreasing the temperature from 298 to 155 K. The energy shift is mainly due to the temperature dependence of the gap energy (Eg) in L. However, the temperature evolution of the occupancy of the filled and empty levels involved in the transition (through the width of the Fermi-Dirac distribution function around EF) cannot be neglected in fully determining the observed temperature dependence. These two causes are still determinant when the crystal is in a QSE regime.

Mariani, C; DE RENZI, Valentina; Compano, R.

We present a high resolution electron energy loss spectroscopy investigation of the Bi/[n-type GaAs(110)] system. Analysis of the elastic peak intensity, of the substrate's Fuchs-Kliewer phonon and dopant-induced free carrier plasmon, shows the influence exerted on GaAs by the first semiconducting Bi monolayer and by the subsequent semimetallic clusters at higher coverage.