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Roberto BIAGI

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

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Pubblicazioni

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

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

2023 - Tuning the Electronic Response of Metallic Graphene by Potassium Doping [Articolo su rivista]
Marchiani, D.; Tonelli, A.; Mariani, C.; Frisenda, R.; Avila, J.; Dudin, P.; Jeong, S.; Ito, Y.; Magnani, F. S.; Biagi, R.; De Renzi, V.; Betti, M. G.
abstract

Electron doping of graphene has been extensively studied on graphene-supported surfaces, where the metallicity is influenced by the substrate. Herewith we propose potassium adsorption on free-standing nanoporous graphene, thus eluding any effect due to the substrate. We monitor the electron migration in the x* downward-shifted conduction band. In this rigid band shift, we correlate the spectral density of the x* state in the upper Dirac cone with the associated plasmon, blue-shifted with increasing K dose, as deduced by electron energy loss spectroscopy. These results are confirmed by the Dirac plasmon activated by the C 1s emitted electrons, thanks to spatially resolved photoemission. This crosscheck constitutes a reference on the correlation between the electronic x* states in the conduction band and the Dirac plasmon evolution upon in situ electron doping of fully free-standing graphene.

2022 - Graphite/epoxy composite for building Bipolar Plates [Articolo su rivista]
Spinelli, Luca; Roncaglia, Fabrizio; Biagi, Roberto; di Bona, Alessandro; Romagnoli, Marcello; Mucci, Adele
abstract

2021 - Development of Graphite-Epoxy Composites for Bipolar Plates in PEM Fuel Cells [Abstract in Atti di Convegno]
Roncaglia, Fabrizio; Spinelli, Luca; Imperato, Manuel; Biagi, Roberto; Romagnoli, Marcello; di Bona, Alessandro; Mucci, Adele
abstract

Graphite-Epoxy composites can be a good alternative to metals and metal alloys to build Bipolar Plates (BPs),1 that are important components of Proton Exchange Membrane Fuel Cells (PEMFC), mainly used in hydrogen-powered electric vehicles. We are currently working2 on the preparation of graphite-epoxy composites, suitable for manufacturing BPs meeting the technical targets for 2025.3 Among the overall properties expected for BPs, we are mainly focussing on conductivity, flexural strength and permeability and we are tuning the preparation steps, i.e. composite formulation, mixing and molding, trying to optimize these properties. We compared different resin to filler ratios, dry and wet mixing, mechanical and magnetic stirring and different temperature and pressure ranges. A two-level full factorial Design Of Experiment (DOE) approach was performed to analyze the molding parameters. We observed substantial changes in the properties of the composites, depending on the type of graphite, the mixing method, the epoxy resin to filler ratio and the molding pressure, temperature and time. The results of these studies will be presented.

2021 - Graphite-epoxy composites for fuel-cell bipolar plates: Wet vs dry mixing and role of the design of experiment in the optimization of molding parameters [Articolo su rivista]
Roncaglia, Fabrizio; Romagnoli, Marcello; Incudini, Simone; Santini, Elena; Imperato, Manuel; Spinelli, Luca; di Bona, Alessandro; Biagi, Roberto; Mucci, Adele
abstract

Bipolar plates (BPs) are key components of Proton Exchange Membrane Fuel Cells mainly employed in hydrogen-powered electric vehicles. Here, a reliable and detailed experimental method to prepare graphite-epoxy composites suitable for manufacturing BPs is reported. Dry and wet mixing procedures were compared and a simple composition was optimized, with regard to electrical conductivity. The adoption of wet mixing of the components and the choice of the conductive filler were the main factors that contributed to the achievement of good electrical and mechanical properties. The addition of a small percentage of carbon black as a secondary filler was also advantageous. The effects of molding parameters (pressure, temperature, and time) on a graphite-epoxy composite of fixed-composition were modeled using a Design Of Experiments approach, which provided valuable information for future improvements. Conductivity values well above the US DOE requirements were obtained

2021 - Graphite/epoxy composite for building Bipolar Plates [Abstract in Atti di Convegno]
Spinelli, L.; Roncaglia, F.; Biagi, R.; di Bona, A.; Romagnoli, M.; Mucci, A.
abstract

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.
abstract

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.
abstract

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.

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

2019 - INVESTIGATION OF MOLDING PARAMETERS ON GRAPHITE/EPOXY COMPOSITE-BASED BIPOLAR PLATES [Relazione in Atti di Convegno]
Roncaglia, F.; Di Bona, A.; Biagi, R.; Romagnoli, M.; Degrandis, D.; Mucci, A.
abstract

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

2018 - 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
abstract

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
abstract

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
abstract

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
abstract

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
abstract

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.

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
abstract

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
abstract

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
abstract

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
abstract

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 - Magnetic Cooling at a Single Molecule Level: a Spectroscopic Investigation of Isolated Molecules on a Surface [Articolo su rivista]
Valdis, Corradini; Alberto, Ghirri; Andrea, Candini; Biagi, Roberto; DEL PENNINO, Umberto; Gianluca, Dotti; Edwige, Otero; Fadi, Choueikani; Robin J., Blagg; Eric J. L., Mcinnes; Affronte, Marco
abstract

A sub-monolayer distribution of isolated molecular Fe-14(bta)(6) nanomagnets is deposited intact on a Au(111) surface and investigated by X-ray magnetic circular dichroism spectroscopy. The entropy variation with respect to the applied magnetic field is extracted from the magnetization curves and evidences high magnetocaloric values at the single molecule level.

2013 - Spin and orbital configuration of metal phthalocyanine chains assembled on the Au(110) surface [Articolo su rivista]
Pierluigi, Gargiani; Giorgio, Rossi; Biagi, Roberto; Valdis, Corradini; Maddalena, Pedio; Sara, Fortuna; Arrigo, Calzolari; Stefano, Fabris; Julio Criginski, Cezar; N. B., Brookes; Maria Grazia, Betti
abstract

The spin and orbital configuration of magnetic metal phthalocyanines (MPcs) deposited on metallic substrates are strongly influenced by the rehybridization of the molecular states with the underlying metal. FePc, CoPc, and CuPc isolated molecules are archetypal systems to investigate the interrelationship between magnetic moments and orbital symmetry after deposition on a metallic substrate. MPcs form long-range ordered chains self-assembled along the reconstructed channels of the Au(110) surface. X-ray magnetic circular dichroism from the L2,3 absorption edges of Fe, Co, and Cu shows that the orbital and spin configuration are strongly modified upon adsorption on the Au(110) surface if the orbitals responsible of the magnetic moment are involved in the interaction process. The magnetic moment for a single layer of molecular chains is completely quenched for the CoPc molecules, fully preserved for the CuPc and reduced for the FePc ones. The modified magnetic configuration is confined to the very interface layer, i.e., to the MPc molecules bound to the metal substrate up to the compact packing of the single layer. The different response can be rationalized in terms of the symmetry/orientation of the metal-ion d states interacting with the substrate states, as indicated by density functional theory calculations in agreement with experimental findings.

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
abstract

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 - 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
abstract

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.

2012 - Magnetic and entanglement properties of molecular Cr2nCu2 heterometallic spin rings [Articolo su rivista]
Lorusso, Giulia; V., Corradini; A., Ghirri; Biagi, Roberto; DEL PENNINO, Umberto; Siloi, Ilaria; F., Troiani; G., Timco; R. E. P., Winpenny; Affronte, Marco
abstract

We investigate the low-temperature magnetic and entanglement properties of a series of molecular Cr2nCu2 heterometallic spin rings (withn=4,5,6). These are cyclic spin systems, consisting of two Cu2+(s=1/2) ions, coupled by two antiferromagnetic segments of nCr3+(s=3/2) ions. Thermodynamic measurements (magnetization, susceptibility, and specific heat) allow us to determine the total spin of the ground state and to estimate the spin-Hamiltonian parameters related to magnetic anisotropy. X-ray spectroscopies (XAS and XMCD) are used to probe the local magnetization of the Cr and Cu ions, and provide results that are consistent with the bulk magnetization data. We finally investigate the relation between heterometallicity and entanglement in these prototypical spin systems. In particular, we focus on the spatial modulation of entanglement induced by the Cu defect spins and on the long-distance entanglement between them induced by the two Cr chains.

2012 - Structural Phases of Ordered FePc-Nanochains Self-Assembled on Au(110) [Articolo su rivista]
Maria Grazia, Betti; Pierluigi, Gargiani; Carlo, Mariani; Biagi, Roberto; Jun, Fujii; Giorgio, Rossi; Andrea, Resta; Stefano, Fabris; Sara, Fortuna; Xavier, Torrelles; Manvendra, Kumar; Maddalena, Pedio
abstract

Iron-phthalocyanine molecules deposited on the Au(110) reconstructed channels assemble into one-dimensional molecular chains, whose spatial distribution evolves into different structural phases at increasing molecular density. The plasticity of the Au channelsfirst induces an ordered phase with a 5×5 symmetry, followed by a second long-range ordered structure composed by denser chains with a 5×7 periodicity with respect to the bare Au surface, as observed in the low-energy electron-diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD) patterns. The geometry of the FePc molecular assemblies in the Au nanorails is determined by scanning tunneling microscopy (STM). For the 5×7 phases, the GIXRD analysis identifies a“4-3”rows profile along the [001] direction in the Au surface and an on-top FePc adsorption site, further confirmed by density functional theory (DFT) calculations. The latter also reveals the electronic mixing of the interface states. The chain assembly is driven by the molecule−molecule interaction and the chains interact with the Au nanorails via the central metal atom, while the chain−chain distance in the different structural phases is primarily driven by the plasticity of the Au surface.

2011 - Oxo-centered carboxylate-bridged trinuclear complexes deposited on Au(111) by a mass-selective electrospray. [Articolo su rivista]
Corradini, Valdis; C., Cervetti; Ghirri, Alberto; Biagi, Roberto; DEL PENNINO, Umberto; G. A., Timco; R. E. P., Winpenny; Affronte, Marco
abstract

We developed an apparatus for nondestructive in vacuum deposition of mass-selected fragile Cr based metal trinuclear complexes, by modifying a commercial Mass Spectrometer containing an electrospray ionization source. Starting from a solution, this system creates a beam of ionized molecules which is then transferred into an evacuated region where the molecules can be mass selected before deposition. To verify the system efficiency, we deposited sub monolayers of oxo-centered carboxylate-bridged trinuclear complexes (Cr3 and Cr2Ni) on Au(111) surface. By XPS and STM we determined the deposited molecule stoichiometry and the surface coverage. The results show that this apparatus is works well for the in vacuum deposition of molecular nanomagnets and, thanks to its reduced dimensions, it is portable.

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
abstract

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
abstract

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
abstract

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 - Electronic and Magnetic Study of Polycationic Mn12 Single-Molecule Magnets with a Ground Spin State S =11 [Articolo su rivista]
Juan M., Clemente Juan; Eugenio, Coronado; Alicia Forment, Aliaga; Alejandro Gaita, Arino; Carlos Gim enez, Saiz; Francisco M., Romero; W., Wernsdorfer; Biagi, Roberto; V., Corradini
abstract

The preparation, magnetic characterization, and X-ray structures of two polycationic Mn12 single-molecule magnets[Mn12O12(bet)16(EtOH)4](PF6)14 3 4CH3CN3 H2O (1) and [Mn12O12(bet)16(EtOH)3(H2O)](PF6)13(OH) 3 6CH3CN3 EtOH3H2O (2)(bet = betaine = (CH3)3Nþ-CH2-CO2 ) are reported. Interestingly, bond valence sum calculations extracted from X-ray diffraction data indicate the presence of two Mn 2+ ions in the Mn12 core for both 1 and 2.This finding is confirmed by X-ray absorption spectroscopy (XAS) measurements. A complete magnetic characterization,including subkelvin micro-SQUID magnetometry and inelastic neutron scattering (INS) measurements, permits to extract theparameters of the giant spin Hamiltonian of these polycations. Compared with the archetypal Mn12 acetate, an increase in thevalue of the ground spin state from S=10 to S=11 together with a decrease in the effective energy barrier, is observed for 1 and 2. Such a result is consistent with the reduction of two Mn 3+ to the less anisotropic Mn 2+ ion in the structures.

2010 - Grafting molecular Cr7Ni rings on a gold surface [Articolo su rivista]
Corradini, Valdis; Ghirri, Alberto; Del Pennino, Umberto; Biagi, Roberto; Victoria A., Milway; Grigore, Timco; Floriana, Tuna; Richard E. P., Winpenny; Affronte, Marco
abstract

Molecular {Cr7Ni} rings have shown several ideal features for the observation of quantum phenomena and they appear suitable candidates for qubits encoding at low temperatures. We have exploited different functionalization pathways to graft molecular {Cr7Ni} rings onto a Au(111) surface from the liquid phase and we report a comparative analysis of the results obtained by STM, XPS, XAS and XMCD experimental techniques.

2010 - Localized and Dispersive Electronic States at Ordered FePc and CoPc Chains on Au(110) [Articolo su rivista]
Betti, Maria Grazia; Gargiani, Pierluigi; Frisenda, Riccardo; Biagi, Roberto; Cossaro, Albano; Verdini, Alberto; Floreano, Luca; Mariani, Carlo
abstract

Iron and cobalt phthalocyanines assemble on the Au(110) surface lying parallel to the surface, as deduced bynear-edge X-ray absorption fine structure (NEXAFS) taken with linearly polarized radiation at the C and NK edges. The molecular chains, firmly anchored to the underlying metal surface, arrange in long-range orderedrows with a (5 × 3) symmetry along the [001] azimuthal direction at completion of the first single layer. Theinteraction process is mainly determined by the d orbitals associated to the central Fe and Co atoms, asobserved by valence band photoemission and NEXAFS at the Fe and Co L2,3 edges. The spin and orbitalconfiguration of the FePc and CoPc molecules is strongly influenced by the interface with a charge transferfrom the underlying metal to the out-of-plane empty states located at the Fe and Co centers of the molecules.This interaction process induces electronic states located at the interface, localized on the central metal atomsand close to the Fermi level (0.2 eV binding energy for FePc and 0.7 eV for CoPc) without energy dispersion,as deduced by angular-resolved photoemission. On the contrary, a delocalized state has been observed withdispersion along the molecular chains, mainly due to the overlapping of the π charge of the macrocyclesligands mediated by the Au substrate.

2010 - Probing local magnetization in molecular heterometallic Cr2Cu trimer [Articolo su rivista]
Lorusso, Giulia; V., Corradini; A., Candini; A., Ghirri; Biagi, Roberto; Del Pennino, Umberto; S., Carretta; E., Garlatti; P., Santini; G., Amoretti; G., Timco; R. G., Pritchard; R. E. P., Winpenny; Affronte, Marco
abstract

We have extensively studied the magnetic features of a heterometallic molecular trimer, Cr2Cu. We havecharacterized molecular Cr2Cu by low-temperature ac susceptibility and specific-heat measurements and thisallowed us to determine the microscopic parameters of the spin Hamiltonian. Then, we used x-ray magneticcircular dichroism to selectively probe local symmetries, electronic configuration, orbital, and spin magneticmoments of the magnetic ions. We found that at low temperatures only the ground state S=3/2 is occupied andthe spin of Cu is found opposite to that of Cr in agreement with spin-Hamiltonian calculations.

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
abstract

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 - 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
abstract

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.

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
abstract

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
abstract

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 - 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
abstract

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
abstract

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 - 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
abstract

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 - 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

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
abstract

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
abstract

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
abstract

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 - 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
abstract

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 - Isolated single-molecule magnets on native gold [Articolo su rivista]
L., Zobbi; M., Mannini; M., Pacchioni; G., Chastanet; D., Bonacchi; Zanardi, Chiara; Biagi, Roberto; DEL PENNINO, Umberto; D., Gatteschi; Cornia, Andrea; R., Sessoli
abstract

The incorporation of thioether groups in the structure of a Mn-12 single-molecule magnet, [Mn12O12(L)(16)(H2O)(4)] with L = 4-(methylthio) benzoate, is a successful route to the deposition of well-separated clusters on native gold surfaces and to the addressing of individual molecules by scanning tunnelling microscopy.

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
abstract

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.

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
abstract

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
abstract

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]
Cornia, Andrea; FABRETTI COSTANTINO, Antonio; M., Pacchioni; Zobbi, Laura; D., Bonacchi; A., Caneschi; D., Gatteschi; Biagi, Roberto; DEL PENNINO, Umberto; DE RENZI, Valentina; L., Gurevich; H. S. J., VAN DER ZANT
abstract

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
abstract

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
abstract

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.

2003 - Thin film SiC epitaxy on Si(111) from acetylene precursor [Relazione in Atti di Convegno]
DE RENZI, Valentina; Biagi, Roberto; DEL PENNINO, Umberto
abstract

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.

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
abstract

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.

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
abstract

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 - Single particle and collective excitations of a two-dimensional electron gas at the Cs/InAs(110) Surface [Articolo su rivista]
Biagi, Roberto; Corradini, Valdis; Mariani, Carlo; DEL PENNINO, Umberto; Bertoni, G.; Betti, Maria Grazia
abstract

The accumulation space-charge region at a semiconductor surface has been studied by a joint investigationof the plasmon excitation and the spectral density of the quasi-two-dimensional electron gas ~Q2DEG!. Theanalysis has been performed by means of high-resolution electron-energy-loss spectroscopy and highresolutionultraviolet photoemission, respectively. The accumulation layer was produced by depositing tinyamounts of Cs on the InAs~110! surface. By using a semiclassical dielectric model, the formation of theQ2DEG in the subsurface region was unambiguously proved by a satisfactory description of the coverage andprimary energy dependence of the collective excitations. The characteristic parameters of the Q2DEG, i.e.,charge density and width of the space-charge region, are determined. These results are in very good agreementwith the values deduced by self-consistently solving the Poisson and Schro¨dinger coupled equations, whichalso give the eigenvalue spectrum and spectral density as measured by photoemission.

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
abstract

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.
abstract

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 - High-frequency phonon modes on stepped and kinked Cu surfaces: Experiments and theory [Articolo su rivista]
Abdelkader, Kara; Pavlin, Staikov; TALAT S., Rahman; Jrg, Radnik; Biagi, Roberto; HANS JOACHIM, Ernst
abstract

Using electron energy loss spectroscopy, we have found step-localized vibrational modes with frequencieshigher than those of the bulk phonons on Cu(532) and Cu(211). From structural and lattice dynamical calcu-lations based on many-body interaction potentials, we trace the origin of these modes to local effects involvingsurface relaxations and resulting stiffening of force constants between the step (or kink-site) atom and its‘‘bulk’’ nearest neighbor. Such modes are not observed on Cu(511) and Cu(17,1,1), although they are pre-dicted in the theoretical calculations. We discuss the impact of these results on systems with bond-length–bond-order correlation.

1998 - Charge accumulation layer induced by potassium adsorption on hydrogen terminated Si(111)(1x1) [Articolo su rivista]
Biagi, Roberto; Rettighieri, L; DEL PENNINO, Umberto; Panella, V; Dumas, P.
abstract

By ultraviolet photoemission spectroscopy and high-resolution electron energy loss spectroscopy we have detected the formation of a charge accumulation layer at the surface of a hydrogenated Si(111)(1 x 1) single crystal at very low potassium coverages. By UPS we have followed the evolution of the band edges relative to the Fermi level, leading to a downwards band-bending (BB) around 340 meV, at about 0.05 of the K saturation coverage. This means that the conduction band bottom is below the Fermi level, producing the accumulation of charge carriers just below the surface. At increasing coverages the BB retains its downward trend, reducing its value to 100 meV at K saturation. The presence of an accumulation layer, consisting of a quasi two-dimensional electron gas, is unambiguously confirmed by the anomalous behaviour of the quasi-elastic peak in the HREEL spectra. Its width strongly increases at the lowest coverage and remains quite large up to the coverage at which UPS shows that the Fermi level leaves the CB minimum. (C) 1998 Elsevier Science B.V. All rights reserved.

1997 - Modifications of space charge layer on InP(110) surface induced by atomic hydrogen at very low exposures [Articolo su rivista]
DEL PENNINO, Umberto; Biagi, Roberto; Mariani, Carlo; L., Siller; A., Amoddeo; F. PROIX AND C. A., Sebenne
abstract

The HREEL spectrum of InP(110) in the quasi-elastic peak region is strongly modified by the exposure to atomic hydrogen. From a detailed analysis of these modifications we have shown that (i) cleavage induced states in InP can have acceptor-like nature, as an “unavoidable” small upwards band bending was always present even on mirror-like cleaved surfaces; (ii) atomic hydrogen at very low exposures can neutralize these acceptor states, reducing to almost zero the pre-existing band bending; (iii) at high exposures, H induces new acceptor states inside the gap and, consequently, a new and larger band bending occurs at the surface, which reaches about 0.6 eV above 1000 L.

1996 - Cesium-induced electronic states and space-charge-layer formation in Cs/InSb(110) interface [Articolo su rivista]
BETTI, Maria Grazia; BIAGI, Roberto; DEL PENNINO, Umberto; MARIANI, Carlo; Pedio, M.
abstract

We report a study of the electronic properties of Cs overlayers on the narrow-band-gap III-V semiconductor InSb(110) as determined by means of photoemission and high-resolution electron-energy-loss spectroscopy. The electric and electronic properties of Cs-deposited on InSb(110) present two distinguishable phases, which can be related to the morphological transition observed in the scanning tunneling microscopy from one-dimensional chains to the two-dimensional (2D) Cs layer. The earlier stages of Cs adsorption induce an accumulation layer, while an additional Cs deposition results in a depletion of carriers in coincidence with the appearance of Cs-induced states in the semiconductor gap. When a 2D layer of cesium is formed on the InSb(110) surface, the interface is insulating with the surface band gap at 0.65 eV, larger than the underlying InSb bulk gap (0.175 eV at room temperature).

1995 - QUASI-2-DIMENSIONAL ELECTRON-GAS AT SUBMONOLAYER COVERAGES OF CS ON INSB(110) [Articolo su rivista]
Betti, Maria Grazia; Biagi, Roberto; DEL PENNINO, Umberto; Mariani, Carlo
abstract

We present a complementary study of the space charge layer formation and band bending determination of caesium adsorption on InSb(110), by means of photoemission and high-resolution electron energy loss spectroscopy. At extremely low Cs coverage (below 0.01 monolayer), the Fermi level gets pinned into the InSb conduction band and an accumulation layer is formed. The quasi-two-dimensional (2D) behaviour of the accumulated carriers is experimentally deduced from the dispersion of the free-carrier plasmon. Eventually, further Cs deposition causes a reversed Fermi level pinning, with the formation of a depletion layer at saturation coverage.

1995 - Space charge layer at metal/InSb(110) interfaces [Articolo su rivista]
Betti, Maria Grazia; Martinelli, V; Biagi, Roberto; DEL PENNINO, Umberto; Mariani, Carlo
abstract

Space charge layer formation and collective excitations of the Cs/InSb(110) and the Bi/InSb(110) interfaces have been studied by means of high resolution electron energy loss spectroscopy (HREELS). The influence of bismuth is effective in slightly enlarging the depletion layer thickness, while the alkali metal induces an accumulation layer up to the formation of one-dimensional (1D) chains. When a 2D Cs layer is formed the space charge layer is depleted of carriers. The influence of the atomic geometry on the Fermi level pinning is considered.

1994 - 2D vs. 3D behaviour of free-carriers gas in delta-doped p-type GaAs(100). [Articolo su rivista]
Biagi, Roberto; DEL PENNINO, Umberto
abstract

The free-carrier-induced plasma excitation in delta-doped p-type GaAs(001) has been studied by means of high-resolution electron-energy-loss spectroscopy (HREELS). Several samples, with different values of doping and depth of the dopant layer, have been investigated at various primary-beam energies. The HREEL spectra show a strong dependence on the doping level. We were able to reproduce satisfactorily all the measured spectra using a suitable dielectric model of a classically confined free-carrier gas, pointing out the two-dimensional character of the free-carrier gas in the samples having the two lowest dopings. On the contrary, a characteristic three-dimensional behavior of the plasma excitation is exhibited by the most doped sample.

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
abstract

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.

1993 - HREELS investigation of hydrogenated GaAs(110) surface [Articolo su rivista]
DEL PENNINO, Umberto; Mariani, Carlo; Biagi, Roberto; A., Amoddeo; F., Proix; C. A., Sebenne
abstract

The interaction of atomic hydrogen with the cleaved GaAs(110) surface has been investigated by high-resolution electron energy loss spectroscopy (HREELS), at primary energies of 5 and 15 eV, analysing the losses associated with three different mechanisms: (i) low-energy surface collective excitations (surface TO phonon and dopant-derived free-carrier plasmon); (ii) the stretching of the Ga-H and As-H bonds: and (iii) the electronic losses above the fundamental gap, involving both surface and bulk electronic states. The first kind of loss is very sensitive to H exposure and shows that hydrogen induces a band bending at the lowest exposures. The vibrational part of the spectrum indicates that the exposure of 104 L corresponds to a coverage of one monolayer and that H bonds to both Ga and As over the whole coverage range. The region of the electronic transitions indicates the disappearance of transitions of the clean surface and the appearance of new transitions characteristic of the H covered surface. At high exposures the growth of a very strong background is consistent with the presence of small metallic Ga clusters. This result is also consistent with the modifications, at the same exposures, of the low-energy region of the spectrum.

1992 - HOLE-PLASMON DAMPING ON HEAVILY DOPED P-TYPE GAAS(110) [Articolo su rivista]
Biagi, Roberto; Mariani, Carlo; DEL PENNINO, Umberto
abstract

The free-carrier-induced plasma excitation in heavily doped p-type GaAs(110) is studied by means of high-resolution electron-energy-loss spectroscopy. This surface hole-plasmon feature is very broad and approximately centered at 60 meV. The large broadening is due to the low hole mobility and its value depends on the energy of the incident-electron beam. Two causes can concur to determine this behavior: the momentum dependence of the Landau damping of the plasmon (due to the nonlocal nature of the free-carrier response) and the dependence of the hole mobility on the distance from the surface in the crystal. Measurements performed after increasing hydrogen exposures, which modify the subsurface charge region, allowed us to conclude that the momentum dependence of the Landau damping is the cause of the observed variation of the plasmon damping with the primary beam energy.

1992 - Investigation of the plasmon excitation on heavily doped p-type GaAs(110) surface. [Articolo su rivista]
DEL PENNINO, Umberto; Biagi, Roberto; Mariani, Carlo
abstract

We present high-resolution electron energy loss measurements on heavily doped p-type GaAs(110), exploiting a wide range of primary beam energies (E(p)). The spectra are remarkably different from those relative to the n-type samples, presenting very broad features and a large background up to several hundreds of meV. The use of different E(p) demonstrated the presence of an intrinsic dead layer at the clean semiconductor surface. We could fit the spectra by means of the three-layer model, obtaining a very good agreement. In particular, from the reproduced broad structure we deduced a large value for the plasmon damping, whose origin is discussed and related to the mobility.

1990 - Loss features in the HREEL spectra of the K/GaAs(110) system [Articolo su rivista]
DEL PENNINO, Umberto; Biagi, Roberto; Mariani, Carlo; Valeri, Sergio; Betti, Maria Grazia; J. A., Schaefer
abstract

We investigated the evolution of the space-charge layer at the GaAs(110) surface as a function of the potassium coverage by means of HREELS, looking at the surface coupled collective excitations (phonon-plasmon, i.e. plasmarons.

1990 - Vibrational and collective excitations on heavily doped p-type GaAs(110) surface [Articolo su rivista]
Biagi, Roberto; Mariani, Carlo; DEL PENNINO, Umberto
abstract

We investigated the space-charge layer at the surface of heavily doped GaAs(110) by means of HREELS, looking at the surface coupled collective excitations (phonon-plasmon, i.e. plasmarons). We could reproduce the general behaviour of the HREEL spectra based on a dielectric three-layer model, finding a strong value for the value of the plasmon damping.