Valentina De Renzi

Surface Supramolecular Organization of a Terbium(III) Double-Decker Complex on Graphite and its Single Molecule Magnet Behavior

The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. 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 experiment, 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, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface.

Self-assembled monolayer of Cr7Ni molecular nanomagnets by sublimation.

We show, by complementary spectroscopic and STM analysis, that Cr(7)Ni derivatives are suitable to be sublimed in UHV conditions. Cr(7)Ni-bu weakly bonds to gold surface and can diffuse relatively freely on it, forming monolayers with hexagonal 2D packing. Conversely, by adding a functional thiol group to the central dibutylamine, a covalent bond between the molecule and surface gold adatoms is promoted, leading to a strong molecular grafting and the formation of a disordered monolayer. These two examples demonstrate the possibility to control the assembly of a large molecular complex, as rationalized by DFT calculations that establish different energy scales in the deposition processes. Moreover, low-temperature XMCD sprectra show that the magnetic features of Cr(7)Ni rings deposited in UHV on gold remain unchanged with respect to those of the corresponding bulk sample.

Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films

Summary 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 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface.

Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism

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=4 and S=12 have been functionalized with 3-tp-CO2- (3-thiophene carboxylate, tpc) ligands and characterized as 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 with respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules are 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 given by spin part because of the quenched orbital moment. Moreover, variable temperature and variable field XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives.

Ordered phases and temperature behaviour of CH3S self-assembled monolayers on Au(111)

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.

Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates

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.

Lateral Fusion of Chemical Vapor Deposited N = 5 Armchair Graphene Nanoribbons

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.

An Ab‐Initio Theoretical Study of the Electrochemical Grafting Process of Alkynil(aryl)iodonium Salts on Glassy Carbon Surfaces

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).

Educational pathways through nanoscience: nitinol as a paradigmatic smart material

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.

Electronic and dielectric properties of Bi grown on GaAs(110)

Abstract 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 ≈ 2.7 × 10 18 cm −3 ). The low-energy electronic properties of a 700 A thick Bi crystal grown on GaAs(110) are also studied, and electronic interband transitions at 47 and ∼ 200 meV are singled out in semimetallic bismuth.