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155 results on '"Gaspari, Roberto"'

1. Thermal Stability and Anisotropic Sublimation of Two-Dimensional Colloidal Bi2Te3 and Bi2Se3 Nanocrystals

Author

Buha, Joka, Gaspari, Roberto, Castillo, Antonio Esau Del Rio, Bonaccorso, Francesco, and Manna, Liberato

Subjects
Physics - Chemical Physics
Abstract

The structural and compositional stabilities of two dimensional 2D Bi2Te3 and Bi2Se3 nanocrystals, produced by both colloidal synthesis and by liquid phase exfoliation, were studied by in situ transmission electron microscopy TEM during annealing at temperatures between 350 and 500 C. The sublimation process induced by annealing is structurally and chemically anisotropic and takes place through the preferential dismantling of the prismatic 011-0 type planes, and through the preferential sublimation of Te or Se., Comment: 23 pages, 5 figures, May 27, 2016 Letter pubs.acs.org/NanoLett Nano Letters, May 27, 2016

Published
2016
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2. Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals

Author

Tu, Renyong, Xie, Yi, Bertoni, Giovanni, Lak, Aidin, Gaspari, Roberto, Rapallo, Arnaldo, Cavalli, Andrea, De Trizio, Luca, and Manna, Liberato

Subjects
Physics - Chemical Physics
Abstract

Cu2-xTe nanocubes were used as starting seeds to access metal telluride nanocrystals by cation exchanges at room temperature. The coordination number of the entering cations was found to play an important role in dictating the reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e. with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe or HgTe nanocrystals with Cu2-xTe/CdTe or Cu2-xTe/HgTe Janus-like heterostructures as intermediates. The formation of Janus-like architectures was attributed to the high diffusion rate of the relatively small tetrahedrally coordinated cations, which could rapidly diffuse in the Cu2-xTe NCs and nucleate the CdTe (or HgTe) phase in a preferred region of the host structure. Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe phases rather than the more stable zinc-blende ones, indicating that the anion framework of the starting Cu2- xTe particles could be more easily deformed to match the anion framework of the metastable wurtzite structures. As hexagonal HgTe had never been reported to date, this represents another case of metastable new phases that can only be accessed by cation exchange. On the other hand, the exchanges involving octahedrally coordinated ions (i.e. with coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline PbTe or SnTe nanocrystals with Cu2-xTe@PbTe or Cu2-xTe@SnTe core@shell architectures at the early stages of the exchange process. In this case, the octahedrally coordinated ions are probably too large to diffuse easily through the Cu2-xTe structure: their limited diffusion rate restricts their initial reaction to the surface of the nanocrystals, where cation exchange is initiated unselectively, leading to core@shell architectures., Comment: 11 pages, 7 figures in J. Am. Chem. Soc, 13 May 2016

Published
2016
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11. A theoretical investigation of the (0001) covellite surfaces.

Author

Gaspari, Roberto, Manna, Liberato, and Cavalli, Andrea

Subjects
*SURFACES (Technology), *COVELLITE, *DENSITY functional theory, *SURFACE energy, *COPPER sulfide, *ELECTRON work function
Abstract

We report on the properties of the (0001) covellites surfaces, which we investigate by periodic slab density functional theory calculations. The absolute surface energies have been computed for all bulk terminations, showing that surfaces terminated by the flat CuS layer are associated with the lowest surface energy. Cleavage is predicted to occur across the [0001] interlayer Cu-S bond. The surfaces obtained by lowest energy cleavage are analyzed in terms of the atomic vertical relaxation, workfunction, and surface band structure. Our study predicts the presence of a shallow pz-derived surface state located 0.26 eV below the Fermi level, which is set to play an important role in the surface reactivity of covellite. [ABSTRACT FROM AUTHOR]

Published
2014
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12. Extended diffusion theory: Recovering dynamics from biased/accelerated molecular simulations.

Author

Rapallo, Arnaldo, Gaspari, Roberto, Grasso, Gianvito, and Danani, Andrea

Subjects
*MOLECULAR dynamics, *SPACE exploration, *ARTIFICIAL satellite attitude control systems, *SIMULATION methods & models, *SYSTEM dynamics
Abstract

Dynamical properties are of great importance in determining the behavior of synthetic and natural molecules, but capturing them by computational methods is a nontrivial task. Very often the time scales of the relevant phenomena are far beyond the typical time windows accessible by classical Molecular Dynamics (MD) simulations, currently limited to the order of microseconds on standard laboratory workstations. On the other hand, biased and accelerated simulations allow for fast and thorough exploration of the molecular conformational space, but they lose the dynamic information. The problem of recovering dynamics from biased/accelerated simulations is a very active field of research, but no totally robust/reliable solutions have been given yet. In this paper it is shown how the Smoluchowski equation, in the framework of Diffusion Theory (DT), can be used to bridge this gap, and dynamical properties, in the form of time correlation functions (TCFs), can be extracted also from such kind of simulations. DT is first extended (EDT) to express the mobility tensors entering the Smoluchowski operator in terms of a recently introduced unified and regularized Rotne–Prager–Yamakawa approximation, [P. J. Zuk, E. Wajnryb, K. A. Mizerski, P. Szymczak, J. Fluid. Mech. 2014, 741, R5, 1–13] also involving mixed rotation‐translation contributions, and rotation‐rotation terms beside the classical translation‐translation ones, so far used in DT. Then, the method is applied to recover the dynamics of a nontrivial example of a peptide in explicit water from the first 200 ns of a Replica Exchange Molecular Dynamics simulation, which is a popular computational method that destroys the long time dynamics. EDT dynamics were found to favorably compare against those coming from a standard MD simulation of the same system, requiring a time window of 30 μs to converge. This result shows that EDT is a tool of practical value to recover the long time dynamics of systems in diffusive regimes from biased/accelerated simulations, to be exploited in those cases when direct evaluation by standard MD is unfeasible. [ABSTRACT FROM AUTHOR]

Published
2021
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16. Formulation of improved basis sets for the study of polymer dynamics through diffusion theory methods.

Author

Gaspari, Roberto and Rapallo, Arnaldo

Subjects
*DIFFUSION, *MOLECULAR dynamics, *CONDUCTION electrons, *ELECTRON emission, *QUANTUM theory
Abstract

In this work a new method is proposed for the choice of basis functions in diffusion theory (DT) calculations. This method, named hybrid basis approach (HBA), combines the two previously adopted long time sorting procedure (LTSP) and maximum correlation approximation (MCA) techniques; the first emphasizing contributions from the long time dynamics, the latter being based on the local correlations along the chain. In order to fulfill this task, the HBA procedure employs a first order basis set corresponding to a high order MCA one and generates upper order approximations according to LTSP. A test of the method is made first on a melt of cis-1,4-polyisoprene decamers where HBA and LTSP are compared in terms of efficiency. Both convergence properties and numerical stability are improved by the use of the HBA basis set whose performance is evaluated on local dynamics, by computing the correlation times of selected bond vectors along the chain, and on global ones, through the eigenvalues of the diffusion operator L. Further use of the DT with a HBA basis set has been made on a 71-mer of syndiotactic trans-1,2-polypentadiene in toluene solution, whose dynamical properties have been computed with a high order calculation and compared to the “numerical experiment” provided by the molecular dynamics (MD) simulation in explicit solvent. The necessary equilibrium averages have been obtained by a vacuum trajectory of the chain where solvent effects on conformational properties have been reproduced with a proper screening of the nonbonded interactions, corresponding to a definite value of the mean radius of gyration of the polymer in vacuum. Results show a very good agreement between DT calculations and the MD numerical experiment. This suggests a further use of DT methods with the necessary input quantities obtained by the only knowledge of some experimental values, i.e., the mean radius of gyration of the chain and the viscosity of the solution, and by a suitable vacuum trajectory, with great savings in computational time required. This offers a theoretical bridge between the experimental static and dynamical properties of polymers. [ABSTRACT FROM AUTHOR]

Published
2008
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17. Ab initio structure determination of Cu2- xTe plasmonic nanocrystals by precession-assisted electron diffraction tomography and HAADF-STEM imaging

Author

European Research Council, European Commission, Mugnaioli, Enrico, Gemmi, Mauro, Tu, Renyong, David, Jeremy, Bertoni, Giovanni, Gaspari, Roberto, Trizio, Luca de, Manna, Liberato, European Research Council, European Commission, Mugnaioli, Enrico, Gemmi, Mauro, Tu, Renyong, David, Jeremy, Bertoni, Giovanni, Gaspari, Roberto, Trizio, Luca de, and Manna, Liberato

Abstract

We investigated pseudo-cubic Cu2–xTe nanosheets using electron diffraction tomography and high-resolution HAADF-STEM imaging. The structure of this metastable nanomaterial, which has a strong localized surface plasmon resonance in the near-infrared region, was determined ab initio by 3D electron diffraction data recorded in low-dose nanobeam precession mode, using a new generation background-free single-electron detector. The presence of two different, crystallographically defined modulations creates a 3D connected vacancy channel system, which may account for the strong plasmonic response of this material. Moreover, a pervasive rotational twinning is observed for nanosheets as thin as 40 nm, resulting in a tetragonal pseudo-symmetry.

Published
2018

24. Real space crystallography of a complex metallic alloy: high-angle annular dark-field scanning transmission electron microscopy of o-Al4(Cr,Fe)

Author

Gaspari, Roberto, Erni, Rolf, Arroyo, Yadira, Parlinska-Wojtan, Magdalena, Dshemuchadse, Julia, Pignedoli, Carlo A., Passerone, Daniele, Schmutz, Patrick, and Beni, Alexandra

Subjects
Complex metallic alloys, Al alloys, Real-space crystallography, Quasicrystal approximants, High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), Engineering & allied operations, ddc:620
Abstract

Journal of Applied Crystallography, 47 (3), ISSN:0021-8898, ISSN:1600-5767

Published
2014

27. Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance

Author

Urso, Carmine, primary, Barawi, Mariam, additional, Gaspari, Roberto, additional, Sirigu, Gianluca, additional, Kriegel, Ilka, additional, Zavelani-Rossi, Margherita, additional, Scotognella, Francesco, additional, Manca, Michele, additional, Prato, Mirko, additional, De Trizio, Luca, additional, and Manna, Liberato, additional

Published
2016
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36. Linking structure and function at the surface of selected metal-based solids through atomistic simulations

Author

Gaspari, Roberto, Parrinello, Michele, and Spaldin, Nicola

Subjects
METALLE UND LEGIERUNGEN (METALLURGIE), Chemistry, METALS AND ALLOYS (METALLURGY), ADSORPTION + ABSORPTION + DESORPTION, DICHTEFUNKTIONALE (QUANTENCHEMIE U. QUANTENMECHANIK), RASTERTUNNELMIKROSKOPE, RTM + RASTERTUNNELMIKROSKOPIE, TOPOCHEMIE, DENSITY FUNCTIONALS (QUANTUM CHEMISTRY AND QUANTUM MECHANICS), SCANNING TUNNELING MICROSCOPES, STM + SCANNING TUNNELING MICROSCOPY, TOPOCHEMISTRY, ddc:540
Published
2012

37. Hollow and Porous Nickel Cobalt Perselenide Nanostructured Microparticles for Enhanced Electrocatalytic Oxygen Evolution

Author

Shinde, Dipak V., Trizio, Luca De, Dang, Zhiya, Prato, Mirko, Gaspari, Roberto, and Manna, Liberato

Abstract

Nickel and cobalt-based chalcogenides are an interesting class of electrochemically active compounds. Herein we report a two-step synthesis of uniform hollow nickel cobalt perselenide nanostructured microparticles with a corrugated surface, randomly spaced pores, and a tunable composition. In the first step, we synthesized Ni–Co acetate hydroxide prism-shaped microparticles, which were subsequently used as templates for the selenization process at temperatures as low as 80 °C. The substitution of acetate and hydroxide anions with selenide anions at a low temperature eventually produced partially amorphous hollow microparticles due to the nanoscale Kirkendall effect. Thin films of the as-synthesized samples behaved as highly active catalysts for the electrochemical oxygen evolution reaction (OER). Among those, Ni0.88Co1.22Se4hollow microparticles showed excellent performances, exhibiting a 10 mA/cm2current density at a modest overpotential of 320 mV, a high turnover frequency of 0.146 S1–, and a long-term operational stability (over 100 h of continuous operation).

Published
2024
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43. Cu3-xP Nanocrystals as a Material Platform for Near-Infrared Plasmonics and Cation Exchange Reactions

Author

De Trizio, Luca, primary, Gaspari, Roberto, additional, Bertoni, Giovanni, additional, Kriegel, Ilka, additional, Moretti, Luca, additional, Scotognella, Francesco, additional, Maserati, Lorenzo, additional, Zhang, Yang, additional, Messina, Gabriele C., additional, Prato, Mirko, additional, Marras, Sergio, additional, Cavalli, Andrea, additional, and Manna, Liberato, additional

Published
2015
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45. Ab InitioStructure Determination of Cu2–xTe Plasmonic Nanocrystals by Precession-Assisted Electron Diffraction Tomography and HAADF-STEM Imaging

Author

Mugnaioli, Enrico, Gemmi, Mauro, Tu, Renyong, David, Jeremy, Bertoni, Giovanni, Gaspari, Roberto, De Trizio, Luca, and Manna, Liberato

Abstract

We investigated pseudo-cubic Cu2–xTe nanosheets using electron diffraction tomography and high-resolution HAADF-STEM imaging. The structure of this metastable nanomaterial, which has a strong localized surface plasmon resonance in the near-infrared region, was determined ab initioby 3D electron diffraction data recorded in low-dose nanobeam precession mode, using a new generation background-free single-electron detector. The presence of two different, crystallographically defined modulations creates a 3D connected vacancy channel system, which may account for the strong plasmonic response of this material. Moreover, a pervasive rotational twinning is observed for nanosheets as thin as 40 nm, resulting in a tetragonal pseudo-symmetry.

Published
2018
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