Publications
45 results found
Zhou H, Mallia G, Harrison NM, 2022, Strain-Tuneable Magnetism and Spintronics of Distorted Monovacancies in Graphene, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 126, Pages: 19435-19445, ISSN: 1932-7447
Alaithan ZA, Mallia G, Harrison NM, 2022, Monomolecular Cracking of Propane: Effect of Zeolite Confinement and Acidity, ACS OMEGA, Vol: 7, Pages: 7531-7540, ISSN: 2470-1343
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- Citations: 5
Bower R, Wells MP, Johnson F, et al., 2021, Tunable double epsilon-near-zero behavior in niobium oxynitride thin films, Applied Surface Science, Vol: 569, Pages: 150912-150912, ISSN: 0169-4332
Ignatans R, Mallia G, Ahmad EA, et al., 2019, The effect of surface reconstruction on the oxygen reduction reaction properties of LaMnO3, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 123, Pages: 11621-11627, ISSN: 1932-7447
Perovskites have been widely studied for electrocatalysis due to the exceptional activity they exhibit for surface-mediated redox reactions. To date, descriptors based on density functional theory calculations or experimental measurements have assumed a bulk-like configuration for the surfaces of these oxides. Herein, we probed an initial exposed surface and the screened subsurface of LaMnO3 particles, demonstrating that their augmented activity toward the oxygen reduction reaction (ORR) can be related to a spontaneous surface reconstruction. Our approach involves high energy resolution electron energy loss spectroscopy for the fine structure probing of oxygen and manganese ionization edges under electron beam conditions that leave the structure unaffected. Atomic multiplet and density functional theory calculations were used to compute the theoretical energy loss spectra for comparison to the experimental data, allowing to quantitatively demonstrate that the particle surface layers are La-deficient. This deficiency is linked to equivalent tetrahedral Mn2+ sites at the reconstructed surface, leading to the coexistence of +3 and +2 oxidation states of Mn at the surface layers. This electronic and structural configuration of the as-synthesized particles is indirectly linked to strong adsorption pathways that promote the ORR on LaMnO3, and thus, it could prove to be a valuable design feature in the engineering of catalytic surfaces.
Martinez-Casado R, Todorovi M, Mallia G, et al., 2019, First principles calculations on the stoichiometric and defective (101) anatase surface and upon hydrogen and H2Pc adsorption: The Influence of electronic exchange and correlation and of basis set approximations, Frontiers in Chemistry, Vol: 7, ISSN: 2296-2646
Anatase TiO2 provides photoactivity with high chemical stability at a reasonable cost. Different methods have been used to enhance its photocatalytic activity by creating band gap states through the introduction of oxygen vacancies, hydrogen impurities, or the adorption of phthalocyanines, which are usually employed as organic dyes in dye-sensitized solar cells. Predicting how these interactions affect the electronic structure of anatase requires an efficient and robust theory. In order to document the efficiency and accuracy of commonly used approaches we have considered two widely used implementations of density functional theory (DFT), namely the all-electron linear combination of atomic orbitals (AE–LCAO) and the pseudo-potential plane waves (PP–PW) approaches, to calculate the properties of the stoichiometric and defective anatase TiO2 (101) surface. Hybrid functionals, and in particular HSE, lead to a computed band gap in agreement with that measured by using UV adsorption spectroscopy. When using PBE+U, the gap is underestimated by 20 % but the computed position of defect induced gap states relative to the conduction band minimum (CBM) are found to be in good agreement with those calculated using hybrid functionals. These results allow us to conclude that hybrid functionals based on the use of AE–LCAO provide an efficient and robust approach for predicting trends in the band gap and the position of gap states in large model systems. We extend this analysis to surface adsorption and use the AE–LCAO approach with the hybrid functional HSED3 to study the adsorption of the phthalocyanine H2Pc on anatase (101). Our results suggest that H2Pc prefers to be adsorbed on the surface Ti5c rows of anatase (101), in agreement with that seen in recent STM experiments on rutile (110).
Martinez-Casado R, Mallia G, Harrison NM, et al., 2018, First-principles study of the water adsorption on Anatase(101) as a function of the coverage, Journal of Physical Chemistry C, Vol: 122, Pages: 20736-20744, ISSN: 1932-7447
An understanding of the interaction of water with the anatase(101) surface is crucial for developing strategies to improve the efficiency of the photocatalytic reaction involved in solar water splitting. Despite a number of previous investigations, it is still not clear if water preferentially adsorbs in its molecular or dissociated form on anatase(101). With the aim of shedding some light on this controversial issue, we report the results of periodic screened-exchange density functional theory calculations of the dissociative, molecular, and mixed adsorption modes on the anatase(101) surface at various coverages. Our calculations support the suggestion that surface-adsorbed OH groups are present, which has been made on the basis of recently measured X-ray photoelectron spectroscopy, temperature-programmed desorption, and scanning tunneling microscopy data. It is also shown that the relative stability of water adsorption on anatase(101), at different configurations, can be understood in terms of a simple model based on the number and nature of the hydrogen bonds formed as well as the adsorbate-induced atomic displacements in the surface layers. These general conclusions are found to be insensitive to the specific choice of approximation for electronic exchange and correlation within the density functional theory. The simple model of water–anatase interactions presented here may be of wider validity in determining the geometry of water–oxide interfaces.
Savazzi F, Risplendi F, Mallia G, et al., 2018, Unravelling some of the structure-property relationships in graphene oxide at low degree of oxidation, Journal of Physical Chemistry Letters, Vol: 9, Pages: 1746-1749, ISSN: 1948-7185
Graphene oxide (GO) is a versatile 2D material whose properties can be tuned by changing the type and concentration of oxygen-containing functional groups attached to its surface. However, a detailed knowledge of the dependence of the chemo/physical features of this material on its chemical composition is largely unknown. We combine classical molecular dynamics and density functional theory simulations to predict the structural and electronic properties of GO at low degree of oxidation and suggest a revision of the Lerf–Klinowski model. We find that layer deformation is larger for samples containing high concentrations of epoxy groups and that correspondingly the band gap increases. Targeted chemical modification of the GO surface appears to be an effective route to tailor the electronic properties of the monolayer for given applications. Our simulations also show that the chemical shift of the C-1s XPS peak allows one to unambiguously characterize GO composition, resolving the peak attribution uncertainty often encountered in experiments.
Wells MP, Zou B, Doiron BG, et al., 2017, Tunable, Low Optical Loss Strontium Molybdate Thin Films for Plasmonic Applications, Advanced Optical Materials, Vol: 5, ISSN: 2195-1071
Strontium molybdate (SrMoO3) thin films are grown epitaxially on strontium titanate (SrTiO3), magnesium oxide (MgO), and lanthanum aluminate (LaAlO3) substrates by pulsed laser deposition and possess electrical resistivity as low as 100 µΩ cm at room temperature. SrMoO3 is shown to have optical losses, characterized by the product of the Drude broadening, ΓD, and the square of the plasma frequency, ωpu2, significantly lower than TiN, though generally higher than Au. Also, it is demonstrated that there is a zero-crossover wavelength of the real part of the dielectric permittivity, which is between 600 and 950 nm (2.05 and 1.31 eV), as measured by spectroscopic ellipsometry. Moreover, the epsilon near zero (ENZ) wavelength can be controlled by engineering the residual strain in the films, which arises from a strain dependence of the charge carrier concentration, as confirmed by density of states calculations. The relatively broad tunability of ENZ behavior observed in SrMoO3 demonstrates its potential suitability for transformation optics along with plasmonic applications in the visible to near infrared spectral range.
Korotana RK, Mallia G, Fortunato NM, et 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.
Martínez-Casado R, Chen VH, Mallia G, et 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.
Chen VH-Y, Mallia G, Martinez-Casado R, et 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.
Ahmad EA, Tileli V, Kramer D, et 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.
Martinez-Casado R, Usvyat D, Mallia G, et 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
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- Citations: 10
Patel M, Sanches FF, Mallia G, et al., 2014, A quantum mechanical study of water adsorption on the (110) surfaces of rutile SnO<sub>2</sub> and TiO<sub>2</sub>: investigating the effects of intermolecular interactions using hybrid-exchange density functional theory, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 21002-21015, ISSN: 1463-9076
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- Citations: 20
Sanches FF, Mallia G, Liborio L, et al., 2014, Hybrid exchange density functional study of vicinal anatase TiO<sub>2</sub> surfaces, PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950
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- Citations: 14
Martinez-Casado R, Usvyat D, Maschio L, et 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
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- Citations: 15
Korotana R, Mallia G, Gercsi Z, et al., 2014, Hybrid density functional study of structural, bonding, and electronic properties of the manganite series La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> (x=0, 1/4, 1), PHYSICAL REVIEW B, Vol: 89, ISSN: 2469-9950
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- Citations: 10
Ahmad EA, Mallia G, Kramer D, et al., 2013, Erratum: The stability of LaMnO<inf>3</inf> surfaces: A hybrid exchange density functional theory study of an alkaline fuel cell catalyst (Journal of Materials Chemistry A (2013) (DOI:10.1039/c3ta11382e)), Journal of Materials Chemistry A, Vol: 1, ISSN: 2050-7488
Sanches FF, Mallia G, Harrison NM, 2013, Simulating constant current STM images of the rutile TiO<inf>2</inf> (110) surface for applications in solar water splitting, Materials Research Society Symposium Proceedings, Vol: 1494, Pages: 339-344, ISSN: 0272-9172
Solar water splitting has shown promise as a source of environmentally friendly hydrogen fuel. Understanding the interactions between semiconductor surfaces and water is essential to improve conversion efficiencies of water splitting systems. TiO2 has been widely adopted as a reference material and rutile surfaces have been studied experimentally and theoretically. Scanning Tunneling Microscopy (STM) is commonly used to study surfaces, as it probes the atomic and electronic structure of the surface layer. A systematic and transferable method to simulate constant current STM images using local atomic basis set methods is reported. This consists of adding more diffuse p and d functions to the basis sets of surface O and Ti atoms, in order to describe the long range tails of the conduction and valence bands (and, thus, the vacuum above the surface). The rutile TiO2 (HO) surface is considered as a case study. © 2013 Materials Research Society.
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
Korotana R, Mallia G, Gercsi Z, et al., 2013, A hybrid-exchange density functional study of Ca-doped LaMnO<sub>3</sub>, 12th Joint MMM-Intermag Conference, Publisher: AMER INST PHYSICS, ISSN: 0021-8979
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- Citations: 7
Risplendi F, Cicero G, Mallia G, et al., 2013, A quantum-mechanical study of the adsorption of prototype dye molecules on rutile-TiO<sub>2</sub>(110): a comparison between catechol and isonicotinic acid, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 15, Pages: 235-243, ISSN: 1463-9076
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- Citations: 19
Ahmad EA, Mallia G, Kramer D, et al., 2013, The stability of LaMnO<sub>3</sub> 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
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- Citations: 25
Ahmad EA, Mallia G, Kramer D, et al., 2013, The stability of LaMnO<sub>3</sub> surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst (vol 1, pg 11152, 2013), JOURNAL OF MATERIALS CHEMISTRY A, Vol: 1, Pages: 15555-15555, ISSN: 2050-7488
Patel M, Mallia G, Liborio L, et al., 2012, Water adsorption on rutile TiO<sub>2</sub>(110) for applications in solar hydrogen production: A systematic hybrid-exchange density functional study, PHYSICAL REVIEW B, Vol: 86, ISSN: 2469-9950
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- Citations: 22
Ahmad EA, Mallia G, Kramer D, et 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
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- Citations: 7
Ferretti A, Mallia G, Martin-Samos L, et al., 2012, <i>Ab initio</i> complex band structure of conjugated polymers: Effects of hydrid density functional theory and <i>GW</i> schemes, PHYSICAL REVIEW B, Vol: 85, ISSN: 2469-9950
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- Citations: 35
Ahmad EA, Liborio L, Kramer D, et al., 2011, Thermodynamic stability of LaMnO<sub>3</sub> 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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- Citations: 37
Bush IJ, Tomic S, Searle BG, et al., 2011, Parallel implementation of the <i>ab initio</i> CRYSTAL program: electronic structure calculations for periodic systems, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 467, Pages: 2112-2126, ISSN: 1364-5021
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- Citations: 32
Scaranto J, Mallia G, Harrison NM, 2011, An efficient method for computing the binding energy of an adsorbed molecule within a periodic approach. The application to vinyl fluoride at rutile TiO<sub>2</sub>(110) surface, COMPUTATIONAL MATERIALS SCIENCE, Vol: 50, Pages: 2080-2086, ISSN: 0927-0256
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- Citations: 45
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