Maurizio Casarin

Role and effective treatment of dispersive forces in materials: Polyethylene and graphite crystals as test cases.

A semiempirical addition of dispersive forces to conventional density functionals (DFT‐D) has been implemented into a pseudopotential plane‐wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT‐D/plane‐wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.

Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-p-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C-C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, p-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly(p-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling.

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction.

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.

Electronic spectroscopy of trivalent lanthanide ions in lead zinc borate glasses

Abstract New zinc borate glasses of composition 4PbO·2ZnO·5B 2 O 3 · and 2PbO·4ZnO·5B 2 O 3 doped with Pr 3+ , Nd 3+ , Eu 3+ , Dy 3+ , Ho 3+ and Er 3+ were prepared. Absorption and luminescence spectra were measured and the Judd–Ofelt parameters were calculated. The spectroscopic behaviour appears to be strongly influenced by the presence of the highly polarizable Pb 2+ ion. The values of stimulated emission cross sections for selected laser transition appear to be relatively high, suggesting that these materials can be considered as interesting candidates for optical applications.

Fullerene/Porphyrin Multicomponent Nanostructures on Ag(110): From Supramolecular Self-Assembly to Extended Copolymers

A novel two-step bottom-up approach to construct a 2D long-range ordered, covalently bonded fullerene/porphyrin binary nanostructure is presented: in the first place, reversible supramolecular interactions between C60 and 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin are exploited to obtain large domains of an ordered binary network, subsequently a reaction between fullerene molecules and the amino-groups residing on porphyrin units, triggered by thermal treatment, is used to freeze the supramolecular nanostructure with covalent bonds. The resulting nanostructure resists high temperature treatments as expected for an extended covalent network, whereas very similar fullerene/porphyrin nanostructures held together only by weak interactions are disrupted upon annealing at the same or at lower temperatures.

Absorption and luminescence spectroscopy of Nd3+ and Er3+ in a zinc borate glass

Abstract 4ZnO · 3B 2 O 3 glasses doped with Nd 3+ and Er 3+ were investigated by optical spectroscopy. Emission transition probabilities, radiative life-times and fluorescence branching ratios for several excited states of the Nd 3+ and Er 3+ ions were estimated from the room temperature absorption spectra. The stimulated emission cross sections σ p for the near infrared laser transitions of Nd 3+ and Er 3+ were obtained from laser-excited luminescence spectra. The values of σ p are comparable with those shown by glasses used in solid state laser applications.

Magnetic Properties and Vapochromic Reversible Guest-Induced Transformation in a Bispyrazolato Copper(II) Polymer: an Experimental and Dispersion-Corrected Density Functional Theory Study

Dispersion-corrected density functional theory (DFT-D) calculations, Electron Spin Resonance spectroscopy (EPR), and variable temperature magnetic moment measurements were used to investigate the structure and the electronic/magnetic properties of bispyrazolato-copper(II) coordination polymer and of its hydration product. The Cu(II) ions are antiferromagnetically coupled through the sigma system of the pyrazolate rings in both compounds. Theoretical electron density maps reveal that water molecules interact simultaneously and to a comparable extent with two Cu(II) centers (through the electronegative O end) and two pyrazolate rings (through the partly positively charged H atoms), which is compatible with the observed internuclear distances. DFT-D calculations indicate that low kinetic barriers are involved in the rearrangement of the host structure.

A theoretical study of the H2O and H2S chemisorption on Cu2O(111)

Abstract Density functional theory coupled to the molecular cluster approach has been used to study the bonding of two Bronsted acids (H 2 X, X=O and S) to the Cu 2 O(111) non-polar surface. Both molecular and dissociative chemisorption have been considered. The interaction between surface Cu(I) Lewis acid sites and the nucleophilic X end of the undissociated H 2 X has been investigated for different molecular orientations, i.e., with the molecular plane either perpendicular (atop ⊥ ) or parallel (atop ‖ ) to the surface. As far as the dissociative chemisorption is concerned, both partial and total deprotonation of H 2 X have been considered. For both acids, the atop ‖ chemisorption corresponds to the absolute minimum, even if the partial deprotonation of H 2 S is found isoenergetic to H 2 S atop ‖ .

An angle-scanned photoelectron diffraction study on the surface relaxation of ZnO (0001)

Abstract An angle-scanned X-ray photoelectron diffraction (XPD) study of the polar Zn-terminated ZnO(0001) surface has been carried out in order to look for a hypothetical inward relaxation of the outermost Zn layer. From the comparison of the O 1s polar scan with single-scattering-cluster (SSC) simulations it is clearly evident that the surface is bulk terminated and any relaxation is ruled out. Moreover, the capability of the XPD technique in determining the surface polarity (atomic termination) by inspection of θ/φ 2D plots without the aid of theoretical simulations has been demonstrated.

An LCAO-LDF study of the chemisorption of H2O and H2S on ZnO(0001) and ZnO(101̄0)

Local density functional theory coupled to the molecular-cluster approach has been used to study the molecular and dissociative chemisorption of H2O and H2S on the relaxed ZnO(0001) and ZnO(1010) surfaces. The geometrical parameters and vibrational frequencies of the adsorbate have been computed. The agreement with experiment is good. Theoretical outcomes indicate that the basicity of the Lewis-base site available on the (0001) surface (the oxygen ions of the second layer partly exposed at the surface) is negligible, and that both H2O and H2S exist even on relaxed ZnO(0001) only as undissociated molecules. At variance to this, the dissociative chemisorption of H 2 S on relaxed ZnO(1010) is highly favoured, while as far H2O is concerned, dissociative and molecular chemisorption energies are very close to each other.