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Materials for new technologies

Ab-initio characterization of hydrogen-functionalized graphene derivatives on copper and quartz substrate
Graphene The study of the interface between hydrogen-functionalized graphene and catalyst metal surfaces can be pivotal to assess the feasibility of direct CVD growth methods for this material. We investigated the adhesion of graphane, a double-side-hydrogenated graphene derivative, to a Cu(111) surface by using ab initio calculations. The magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. The effects the adhesion to either Cu(111) or alpha-quartz (0001) surface on the magnetic properties of graphone have been investigated using an exchange-correlation functional corrected for long range interactions. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature.
  • Francesco Buonocore, ENEA
Ab-initio characterization of graphene on amorphous copper
Graphene Graphene production process can be optiized by accurate ab-initio characterization of the interface betwen graphene and the substrate. Amorphous copper is experimentally indicated as one of the most interesting substrate. Formation energies, charge transfer, electronic band structure and induced strains are computed and interpreted.
  • Francesco Buonocore, ENEA
Shear-band formation in Cu64Zr36 metallic glass under uniaxial compression
CuZr Cu64Zr36 metallic glasses are characterized by extensive Molecular Dynamics simulations. An amorphous sample is produced by quenching from the melt. The topological short-range order is analyzed by the Voronoi tessellation method. The most prominent atomic arrangements in the glass are Cu-centered icosahedra. Subsequently, the sample was uniaxially compressed to investigate the mechanical deformation in metallic glasses. A stress-strain curve was measured, which shows stress drops in the plastic regime that might stem from shear-transformation zones. Then a larger Cu64Zr36 metallic glass was simulated to study shear-band formation in metallic glasses. An annealing step at sufficiently high temperature is needed to get rid of any periodicity introduced by the replicas. Shear bands are formed under uniaxial compression and visualized using the atomic local shear strain. In the early stages of yielding several shear bands are observed, while one shear band dominates at larger plastic deformation.
  • Bernd Schonfeld, Jerome Zemp, Univ. of Zurich
Ab-initio calculations of Nb3Sn superconducting properties under applied mechanical strain
Nb3Sn Using calculations from first principles based on the density functional theory, we have studied the strain sensitivity of superconducting Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multi-filamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain both on the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electronphonon coupling, the superconducting characteristic critical temperature of the material Tc, has been calculated by means of the Allen-Dynes modification of the McMillan formula. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed Tc curve, and compared to experimental data from multi-filamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions of the bulk, perfectly binary and stoichiometric Nb3Sn. Understanding the origin of this difference might open potential pathways towards the improvement of the strain tolerance in such systems.
  • G. De Marzi and L. Morici, ENEA Frascati
Glass polymorphism in amorphous germanium probed by first-principles computer simulations
The low-density (LDA) to high-density (HDA) transformation in amorphous Ge at high pressure is studied by first-principles molecular dynamics simulations in the framework of density functional theory. Previous experiments are accurately reproduced, including the presence of a well-defined LDA-HDA transition above 8 GPa. The LDA-HDA density increase is found to be about 14%. Pair and bond-angle distributions are obtained in the 0-16 GPa pressure range and allowed us a detailed analysis of the transition. The local fourfold coordination is transformed in an average HDA sixfold coordination associated with different local geometries as confirmed by coordination number analysis and shape of the bond-angle distributions.
  • G. Mancini, F. Iesari, A. Di Cicco, Univ. of Camerino
  • M. Celino, ENEA
First principles calculations of the electronic properties of bismuth-based nanostructures
Interface Ab initio calculation of the electronic properties of bismuth-based surfaces and nanostructures are introduced. Special focus is given to applications (e.g. in thermoelectric devices) and to quantum confinement effects. Due to the complex crystal structure and semi-metallic band structure of bulk bismuth, the occurrence of quantum confinement as dimension are reduced down to few nm is quite debated in the literature, with particular reference on a semiconductor-to-semimetal transition upon increasing the material dimensions.
  • G. Cantele, D. Ninno, Univ. of Naples
Electronic properties at complex interfaces
Interface The study of electronic processes and barrier heights at semiconductor-semiconductor and semiconductor-metal interfaces, of interest for applications in (nano)electronics, energy conversion and optoelectronic devices, is presented. Two different systems are chosen, namely, the SrTiO3-TiO2 interface and graphene nanoribbons covalently attached to a metallic substrate. Fundamental properties, following the formation of the interface, are: charge transfer and interface dipole, band offset, role of defects. It is shown that the local chemistry and the interface stoichiometry play a fundamental role in determining the interface electronic properties. As such, a deep understanding at microscopic level is needed, as provided by accurate first principle approaches.
  • G. Cantele, D. Ninno, Univ. of Naples
A Hybrid Particle-Field Coarse-Grained Molecular Model for Pluronics Water Mixtures
The triblock-copolymers of poly(ethylene oxide)m-poly(propylene oxide)n-poly(ethylene oxide)m (PEOm-PPOn-PEOm), are an important family of amphiphilic polymers. The hydrophilic-lipophilic character of these block-copolymers can be tuned varying the blocks length and the molecular weight of both, PEO and PPO blocks. Such adaptability allowed to employ such copolymers in many fields, like foaming, detergency, dispersion stabilization, emulsification, lubrication, cosmetic formulation, modification of surface for biocompatibility for medical applications. To study the phase behavior of such systems we need to reach time and length scales on the order of magnitude of microsec and nm. We present a computational study of the phase behavior of binary Pluronic-water mixtures (L62 and L64), based on the hybrid particle-field molecular dynamics method.
  • Antonio De Nicola, Giuseppe Milano, Univ Salerno
Ab-initio characterization of Zr43O86 clusters:
ZrO Ab-initio molecular dynamics simulations are used to optimize the atomic configuration of zirconium oxide clusters. Surface properties and reactivity are computed and compared with recent experimental results. The good agreement with experiments allows to extend this approach to greater and different clusters.
  • Roberto Grena, ENEA

Excitonic effects in low dimensional systems
Graphene/Graphane In this report we briefly review the results that have been obtained in the last year also thanks to the use of the ENEA CRESCO facility. All the works concern the calculation of the electronic and optical properties of materials employing ab-initio theoretical methods based on Density Functional Theory (DFT), Time Dependent Density Functional Theory (TDDFT) and Many Body Perturbation Theory (MBPT). The report is organized as follows: an overview of the theoretical framework is given, together with a report of the important work of code parallelization, then results are reported. We present the results of two different studies concerning: i) the excited state properties of formamide, both isolated molecule and molecule in water solution; ii) the almost one-dimensional excitons in Si(111)2x1 surface. While the calculations running on the ENEA CRESCO facility have been mainly carried out by the authors of this report, the complete studies to which we will refer are due to the work of many collaborators, which appear as coauthors in the publications. Used codes: Quantum Espresso (www.quantum-espresso.org), EXC (http://etsf.polytechnique.fr/exc/), GW, DP (http://dp-code.org/).
  • Olivia Pulci, Univ. Tor Vergata, Roma
Crystalline and liquid Si3N4 characterization by first-principles molecular dynamics simulations:
Ni3Si4 Silicon nitride (Si3N4) has a wide range of engineering applications where its mechanical and electronic properties can be effectively exploited. In particular, in the microelectronics field, the amorphous silicon nitride films are used as charge storage layer in metal-alumina-nitride-oxide nonvolatile memory devices. Atomic structure of amorphous silicon nitride is characterized by high concentration of traps that control the electric behavior of the final device by the trapping-de-trapping mechanism of the electrical charge occurring in its traps. In order to have a deep understanding of the material properties and, in particular, the nature of the electrical active traps a detailed numerical characterization of the crystalline and liquid phases is mandatory. For these reasons first-principles molecular dynamics simulations are extensively employed to simulate the crystalline Si3N4 in its crystalline and liquid phases. Good agreement with experimental results is obtained in terms of density and formation enthalpy. Detailed characterization of crystalline Si3N4 electronic properties is performed in terms of band structure and band gap. Extensive first-principles molecular dynamics simulations are performed to obtain a reliable liquid sample. Detailed characterization of the atomic structure is achieved in terms of radial distribution functions and total structure factor.
  • Aurelio Mauri, MICRON
  • Massimo Celino, ENEA
Materials with high ionic conducibility:
Water Materials of the class AX2 (A=Si, Ge; X= O, Se, S) are studied in their liquid and amorphous phase in view of their application for sensors and batteries. Structural and electronic characterization are performed by using molecular dynamics code based on ab-initio scheme. In the case of SiSe2, the calculated structure factor is in very good agreement with experiments, as well as the number of corner- and edge-sharing tetrahedra. By focusing on the sequences of Si atoms linked via intra- and intertetrahedral bonds, we identify the predominant structural motifs. The sequences involving both corner- and edge-sharing connections are significantly more frequent than those formed exclusively by edge-shared Si atoms. Our results clarify a longstanding controversy on the structure of this prototypical disordered network-forming material.
  • Massimo Celino, ENEA
Undercooling liquid metals
We elucidate the role played by defective icosahedra on the stability of undercooled copper by using molecular-dynamics simulations. Our approach is substantiated by the level of agreement with experiments on a variety of structural properties. We show that not only perfect but also defective icosahedra, embedded in a disordered matrix, lower the local cohesive energy. This has the effect of stabilizing the liquid structure against crystallization. Our work rationalizes experimental findings by identifying the nature of those icosahedral subunits that contribute to the stability of the undercooled liquid.
  • Massimo Celino, ENEA
New elements are selected and studied for catalysis processes. Catalysis plays a fundamental role for the production of materials and molecules but it needs to be a safe and no polluntant process.
  • Luigi Cavallo, Univ. Salerno
Remote control of a Transmission Electron Microscopy (TEM)
A Transmission Electron Microscopy is installed in the ENEA Research Centre of Brindisi equipped with all the instrumentation for remote control and data acquisition. An original and customized software interface allows the microscope to be a resource of the ENEA Grid environment. In this way any ENEA Grid user, with a PC equipped with a simple interface, can use from everywhere the microscope.
  • Marco Vittori Antisari, ENEA
Structural materials:
A first-principles pseudopotential method is used to investigate the structural, elastic and phonon properties of ScAs and ScSb in their ambient B1(NaCl) and in high pressure B2 (CsCl) phases. The calculated lattice constants, static bulk modulus, first order pressure derivative of the bulk modulus and the elastic constants are reported in B1 and B2 structures and compared with available experimental and other theoretical results. The phonon properties of two compounds are compared among themselves which reveal that these compounds are predominantly metallic, due to degeneracy of optical frequencies at zone centre. At high pressure, near B1 to B2 transition, the LA mode at X-point softens leading to structural instability.
  • Bipul Rakshit, S.P. Sanyal, Univ. Barkatullah

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