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  • Journal article
    Parkes MA, Tompsett DA, d'Avezac M, Offer GJ, Brandon NP, Harrison NMet al., 2016,

    The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study.

    , Physical Chemistry Chemical Physics, Vol: 18, Pages: 31277-31285, ISSN: 1463-9084

    Yttria stabilized zirconia (YSZ) is an important oxide ion conductor used in solid oxide fuel cells, oxygen sensing devices, and for oxygen separation. Doping pure zirconia (ZrO2) with yttria (Y2O3) stabilizes the cubic structure against phonon induced distortions and this facilitates high oxide ion conductivity. The local atomic structure of the dopant is, however, not fully understood. X-ray and neutron diffraction experiments have established that, for dopant concentrations below 40 mol% Y2O3, no long range order is established. A variety of local structures have been suggested on the basis of theoretical and computational models of dopant energetics. These studies have been restricted by the difficulty of establishing force field models with predictive accuracy or exploring the large space of dopant configurations with first principles theory. In the current study a comprehensive search for all symmetry independent configurations (2857 candidates) is performed for 6.7 mol% YSZ modelled in a 2 × 2 × 2 periodic supercell using gradient corrected density functional theory. The lowest energy dopant structures are found to have oxygen vacancy pairs preferentially aligned along the ⟨210⟩ crystallographic direction in contrast to previous results which have suggested that orientation along the ⟨111⟩ orientation is favourable. Analysis of the defect structures suggests that the Y(3+)-Ovac interatomic separation is an important parameter for determining the relative configurational energies. Current force field models are found to be poor predictors of the lowest energy structures. It is suggested that the energies from a simple point charge model evaluated at unrelaxed geometries is actually a better descriptor of the energy ordering of dopant structures. Using these observations a pragmatic procedure for identifying low energy structures in more complicated material models is suggested. Calculation of the oxygen vacancy migration activat

  • Journal article
    Petrov PK, Zou B, Walker C, Wang K, Tileli V, Shaforost O, Harrison N, Klein N, Alford Net al., 2016,

    Growth of Epitaxial Oxide Thin Films on Graphene

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    The transfer process of graphene onto the surface of oxide substrates is well known.However, for many devices, we require high quality oxide thin films on the surface ofgraphene. This step is not understood. It is not clear why the oxide should adopt theepitaxy of the underlying oxide layer when it is deposited on graphene where there isno lattice match. To date there has been no explanation or suggestion of mechanismswhich clarify this step. Here we show a mechanism, supported by first principlessimulation and structural characterisation results, for the growth of oxide thin films ongraphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a grapheneand show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillarspots that enable the epitaxial growth of STO thin films on the surface of thegraphene layer. This study, and in particular the suggestion of a mechanism forepitaxial growth of oxides on graphene, offers new directions to exploit thedevelopment of oxide/graphene multilayer structures and devices.

  • Journal article
    Carta D, Salaoru I, Khiat A, Regoutz A, Mitterbauer C, Harrison NM, Prodromakis Tet al., 2016,

    Investigation of the Switching Mechanism in TiO2-Based RRAM: A Two-Dimensional EDX Approach

    , ACS Applied Materials & Interfaces, Vol: 8, Pages: 19605-19611, ISSN: 1944-8244

    The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).

  • Journal article
    Korotana RK, Mallia G, Fortunato NM, Amaral JS, Gercsi Z, Harrison NMet al., 2016,

    A combined thermodynamics and first principles study of the electronic, lattice and magnetic contributions to the magnetocaloric effect in La0.75Ca0.25MnO3

    , Journal of Physics D: Applied Physics, Vol: 49, ISSN: 0022-3727

    Manganites with the formula La1−x Ca x MnO3 for 0.2  <  x  <  0.5 undergo a magnetic field driven transition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. An isotropic expansion of the La0.75Ca0.25MnO3 cell at the phase transition has been observed experimentally. It is expected that there will be a large entropy change at the transition due to its first order nature. Doped lanthanum manganite (LMO) is therefore of interest as the active component in a magnetocaloric cooling device. However, the maximum obtained value for the entropy change in Ca-doped manganites merely reaches a moderate value in the field of a permanent magnet. The present theoretical work aims to shed light on this discrepancy. A combination of finite temperature statistical mechanics and first principles theory is applied to determine individual contributions to the total entropy change of the system by treating the electronic, lattice and magnetic components independently. Hybrid-exchange density functional (B3LYP) calculations and Monte Carlo simulations are performed for La0.75Ca0.25MnO3. Through the analysis of individual entropy contributions, it is found that the electronic and lattice entropy changes oppose the magnetic entropy change. The results highlighted in the present work demonstrate how the electronic and vibrational entropy contributions can have a deleterious effect on the total entropy change and thus the potential cooling power of doped LMO in a magnetocaloric device.

  • Journal article
    Martínez-Casado R, Chen VH, Mallia G, Harrison NMet al., 2016,

    A hybrid-exchange density functional study of the bonding and electronic structure in bulk CuFeS2

    , Journal of Chemical Physics, Vol: 144, ISSN: 1089-7690

    The geometric, electronic, and magnetic properties of bulk chalcopyrite CuFeS2 have been investigated using hybrid-exchange density functional theory calculations. The results are compared with available theoretical and experimental data. The theoretical description of the bonding and electronic structure in CuFeS2 is analyzed in detail and compared to those computed for chalcocite (CuS2) and greigite (Fe3S4). In particular, the behavior of the 3d electrons of Fe3+ is discussed in terms of the Hubbard-Anderson model in the strongly correlated regime and found to be similarly described in both materials by an on-site Coulomb repulsion (U) of ∼8.9 eV and a transfer integral (t) of ∼0.3 eV.

  • Journal article
    Inkpen MS, Du S, Hildebrand M, White AJP, Harrison NM, Albrecht T, Long NJet al., 2015,

    The unusual redox properties of fluoroferrocenes revealed through a comprehensive study of the haloferrocenes

    , Organometallics, Vol: 34, Pages: 5461-5469, ISSN: 1520-6041

    We report the synthesis and full characterization of the entire haloferrocene (FcX) and 1,1′-dihaloferrocene (fcX2) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1′-diyl). Finalization of this simple, yet intriguing set of compounds has been delayed by synthetic challenges associated with the incorporation of fluorine substituents. Successful preparation of fluoroferrocene (FcF) and 1,1′-difluoroferrocene (fcF2) were ultimately achieved using reactions between the appropriate lithiated ferrocene species and N-fluorobenzenesulfonimide (NFSI). The crude reaction products, in addition to those resulting from analogous preparations of chloroferrocene (FcCl) and 1,1′-dichloroferrocene (fcCl2), were utilized as model systems to probe the limits of a previously reported “oxidative purification” methodology. From this investigation and careful solution voltammetry studies, we find that the fluorinated derivatives exhibit the lowest redox potentials of each of the FcX and fcX2 series. This counterintuitive result is discussed with reference to the spectroscopic, structural, and first-principles calculations of these and related materials.

  • Journal article
    Chen VH-Y, Mallia G, Martinez-Casado R, Harrison NMet al., 2015,

    Surface morphology of CuFeS2: The stability of the polar (112)/((112)over-bar) surface pair

    , Physical Review B, Vol: 92, ISSN: 1550-235X

    The reconstruction and energetics for a range of chalcopyrite (CuFeS2) surfaces have been investigated using hybrid-exchange density functional theory. The stable nonpolar surfaces in increasing order of surface energy are (110), (102), and (114). In addition, the polar (112)/(112¯¯¯¯¯) surface pair was found to be remarkably stable with a surface formation energy that is only slightly higher than that of the (110) surface. The stability of (112)/(112¯¯¯¯¯) can be attributed to a combination of geometric and electronic mechanisms that result in the suppression of the electrostatic dipole perpendicular to the surface. Defect formation is a third mechanism that can further stabilize the (112)/(112¯¯¯¯¯) surface pair to an extent that it is thermodynamically preferred over the (110) surface. The stability of (112)/(112¯¯¯¯¯) means that regardless of the growth conditions, (112) and (112¯¯¯¯¯) facets are expected to have a significant presence in the surface morphology of CuFeS2.

  • Journal article
    Ahmad EA, Tileli V, Kramer D, Mallia G, Stoerzinger KA, Shao-Horn Y, Kucernak AR, Harrison NMet al., 2015,

    Optimizing Oxygen Reduction Catalyst Morphologies from First Principles

    , Journal of Physical Chemistry C, Vol: 119, Pages: 16804-16810, ISSN: 1932-7455

    Catalytic activity of perovskites for oxygen reduction (ORR) wasrecently correlated with bulk d-electron occupancy of the transition metal. Weexpand on the resultant model, which successfully reproduces the high activity ofLaMnO3 relative to other perovskites, by addressing catalyst surface morphology asan important aspect of the optimal ORR catalyst. The nature of reaction sites onlow index surfaces of orthorhombic (Pnma) LaMnO3 is established from FirstPrinciples. The adsorption of O2 is markedly influenced by local geometry andstrong electron correlation. Only one of the six reactions sites that result from experimentally confirmed symmetry-breakingJahn−Teller distortions is found to bind O2 with an intermediate binding energy while facilitating the formation of superoxide, animportant ORR intermediate in alkaline media. As demonstrated here for LaMnO3, rational design of the catalyst morphology topromote specific active sites is a highly effective optimization strategy for advanced functional ORR catalysts.

  • Journal article
    Parkes MA, Refson K, d'Avezac M, Offer GJ, Brandon NP, Harrison NMet al., 2015,

    Chemical descriptors of yttria-stabilized zirconia at low defect concentration: an ab initio study.

    , Journal of Physical Chemistry A, Vol: 119, Pages: 6412-6420, ISSN: 1520-5215

    Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energ

  • Journal article
    Kucernak ARJ, 2015,

    Electrochemical Characterization and Quantified Surface Termination Obtained by LEIS and XPS of Orthorhombic and Rhombohedral LaMnO<sub>3</sub> Powders

    , Journal of Physical Chemistry C, Vol: 119, Pages: 12209-12217, ISSN: 1932-7455

    LaMnO3 powder synthesized by glycine combustion synthesis with the rhombohedral and orthorhombic structures has been characterized by the combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), while the electrocatalytic activity for the oxygen reduction reaction is measured with the rotating disk electrode (RDE) method. Quantification of the surface terminations obtained by LEIS suggests that the orthorhombic LaMnO3 crystallites are near thermodynamic equilibrium as surface atomic ratios compare well with those of equilibrium morphologies computed by a Wulff construction based on computed surface energies. Both rhombohedral and orthorhombic structures present the same La/Mn atomic ratio on the surface. Electrochemical activity of the two structures is found to be the same within the error bar of our measurements. This result is in disagreement with results previously reported on the activity of the two structures obtained by the coprecipitation method [Suntivich et al. Nat. Chem. 2011, 3 (7), 546], and it indicates that the preparation method and the resulting surface termination might play a crucial role for the activity of perovskite catalysts.

  • Journal article
    Tseng H-H, Serri M, Harrison NM, Heutz Set al., 2015,

    Thin film properties of tetracyanoquinodimethane (TCNQ) with novel templating effects

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 8694-8699, ISSN: 2050-7526
  • Journal article
    Martinez-Casado R, Usvyat D, Mallia G, Maschio L, Casassa S, Ellis J, Schuetz M, Harrison NMet al., 2014,

    Diffraction of helium on MgO(100) surface calculated from first-principles

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 21106-21113, ISSN: 1463-9076
  • Journal article
    Patel M, Sanches FF, Mallia G, Harrison NMet al., 2014,

    A quantum mechanical study of water adsorption on the (110) surfaces of rutile SnO2 and TiO2: investigating the effects of intermolecular interactions using hybrid-exchange density functional theory

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 21002-21015, ISSN: 1463-9076
  • Journal article
    Sanches FF, Mallia G, Liborio L, Diebold U, Harrison NMet al., 2014,

    Hybrid exchange density functional study of vicinal anatase TiO2 surfaces

    , PHYSICAL REVIEW B, Vol: 89, ISSN: 1098-0121
  • Journal article
    Martinez-Casado R, Usvyat D, Maschio L, Mallia G, Casassa S, Ellis J, Schuetz M, Harrison NMet al., 2014,

    Approaching an exact treatment of electronic correlations at solid surfaces: The binding energy of the lowest bound state of helium adsorbed on MgO(100)

    , PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950
  • Journal article
    Korotana R, Mallia G, Gercsi Z, Liborio L, Harrison NMet al., 2014,

    Hybrid density functional study of structural, bonding, and electronic properties of the manganite series La1-xCaxMnO3 (x=0, 1/4, 1)

    , PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950
  • Journal article
    Serri M, Wu W, Fleet LR, Harrison NM, Hirjibehedin CF, Kay CWM, Fisher AJ, Aeppli G, Heutz Set al., 2014,

    High-temperature antiferromagnetism in molecular semiconductor thin films and nanostructures

    , NATURE COMMUNICATIONS, Vol: 5, ISSN: 2041-1723
  • Journal article
    Zuehlsdorff TJ, Hine NDM, Spencer JS, Harrison NM, Riley DJ, Haynes PDet al., 2013,

    Linear-scaling time-dependent density-functional theory in the linear response formalism

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 139, ISSN: 0021-9606
  • Conference paper
    Patel M, Mallia G, Harrison NM, 2013,

    The structure of water on rutile TiO2(110) for applications in solar hydrogen production: towards a predictive model using hybrid-exchange density functional theory

    , MRS Spring Meeting 2013
  • Conference paper
    Korotana R, Mallia G, Gercsi Z, Harrison NMet al., 2013,

    A hybrid-exchange density functional study of Ca-doped LaMnO3

    , 12th Joint MMM-Intermag Conference, Publisher: AMER INST PHYSICS, ISSN: 0021-8979
  • Journal article
    Mukhopadhyay S, Finnis MW, Harrison NM, 2013,

    Electronic structures and phonon free energies of LaCoO3 using hybrid-exchange density functional theory

    , PHYSICAL REVIEW B, Vol: 87, ISSN: 1098-0121
  • Journal article
    Wu W, Rochford LA, Felton S, Wu Z, Yang JL, Heutz S, Aeppli G, Jones TS, Harrison NM, Fisher AJet al., 2013,

    Magnetic properties of copper hexadecaphthalocyanine (F16CuPc) thin films and powders

    , JOURNAL OF APPLIED PHYSICS, Vol: 113, ISSN: 0021-8979
  • Journal article
    Ahmad EA, Mallia G, Kramer D, Kucernak AR, Harrison NMet al., 2013,

    The stability of LaMnO3 surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst

    , JOURNAL OF MATERIALS CHEMISTRY A, Vol: 1, Pages: 11152-11162, ISSN: 2050-7488
  • Journal article
    Parkes M, Refson K, d'Avezac M, Offer G, Brandon N, Harrison Net al., 2013,

    Determining Surface Chemistry and Vibrational Properties of SOFC Anode Materials Through ab initio Calculations

    , SOLID OXIDE FUEL CELLS 13 (SOFC-XIII), Vol: 57, Pages: 2419-2427, ISSN: 1938-5862
  • Journal article
    Risplendi F, Cicero G, Mallia G, Harrison NMet al., 2013,

    A quantum-mechanical study of the adsorption of prototype dye molecules on rutile-TiO2(110): a comparison between catechol and isonicotinic acid

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 15, Pages: 235-243, ISSN: 1463-9076
  • Journal article
    Patel M, Mallia G, Liborio L, Harrison NMet al., 2012,

    Water adsorption on rutile TiO2(110) for applications in solar hydrogen production: A systematic hybrid-exchange density functional study

    , PHYSICAL REVIEW B, Vol: 86, ISSN: 2469-9950
  • Journal article
    Ahmad EA, Mallia G, Kramer D, Tileli V, Kucernak AR, Harrison NMet al., 2012,

    Comment on "2D Atomic Mapping of Oxidation States in Transition Metal Oxides by Scanning Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy"

    , PHYSICAL REVIEW LETTERS, Vol: 108, ISSN: 0031-9007
  • Journal article
    Ferretti A, Mallia G, Martin-Samos L, Bussi G, Ruini A, Montanari B, Harrison NMet al., 2012,

    Ab initio complex band structure of conjugated polymers: Effects of hydrid density functional theory and GW schemes

    , PHYSICAL REVIEW B, Vol: 85, ISSN: 1098-0121
  • Conference paper
    Wu W, Fisher AJ, Harrison NM, Wang H, Wu Z, Gardener J, Heutz S, Jones T, Aeppli Get al., 2012,

    Theoretical modeling of the electronic structure and exchange interactions in Cu(II)Pc

    , International Conference on Strongly Correlated Electron Systems (SCES), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
  • Journal article
    Ahmad EA, Liborio L, Kramer D, Mallia G, Kucernak AR, Harrison NMet al., 2011,

    Thermodynamic stability of LaMnO3 and its competing oxides: A hybrid density functional study of an alkaline fuel cell catalyst

    , PHYSICAL REVIEW B, Vol: 84, ISSN: 2469-9950

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