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Journal articleChang V, Camino B, Noakes TCQ, et al., 2020,
Theoretical study of the influence of hydrides on the performance of Mg and Y photocathodes
, Journal of Applied Physics, Vol: 127, Pages: 1-9, ISSN: 0021-8979Our understanding of material properties in the broadest sense is based on our ability to observe and disentangle underlying mechanisms. This has been aided enormously by the discovery and exploitation of synchrotron radiation. The next generation of light sources will be based on free electron lasers with potentially much greater light intensity and time resolution. This requires the development of new photocathode materials with high quantum efficiency (QE) and low emittance that are chemically and mechanically robust. One prospect is the use of yttrium (Y) and/or magnesium (Mg) thin films, but here, a fundamental understanding of the photoemission process from realistic materials is lacking. Observations of photoemissive performance would appear to contradict simple models. It is well known that materials with a lower work function are expected to facilitate photoemission, but the measured QE of Mg is higher than that of Y despite its nominal work function (3.7 eV) being significantly higher than that of Y (3.1 eV). In this work, these apparently anomalous observations are explained and rationalized by combining a simple three-step model of photoemission with large scale density functional theory calculations to predict the QE for realistic models of both materials in a special chemical environment. This approach allows us to identify the material parameters that govern the efficiency of the photoemission process. A detailed comparison with the experimental data suggests that, in this case, hydride formation on the Y surface, invisible to most experimental probes, nevertheless has a surprisingly large influence and reduces the photoemission significantly.
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Book chapterTseng HH, Serri M, Harrison N, et al., 2020,
Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation
, Porphyrin Science by Women in 3 Volumes, Pages: 924-931Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn-Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) ~1.9 × 10<sup>-17</sup> cm<sup>2</sup>/s is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.
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Journal articleTseng H-H, Serri M, Harrison N, et al., 2019,
Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation
, Journal of Porphyrins and Phthalocyanines, Vol: 23, Pages: 1515-1522, ISSN: 1088-4246Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) ∼1.9×10−17cm2/s is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.
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Journal articleAhmad EA, Chang H-Y, Al-Kindi M, et al., 2019,
Corrosion protection through naturally occurring films: new insights from iron carbonate
, ACS Applied Materials and Interfaces, Vol: 11, Pages: 33435-33441, ISSN: 1944-8244Despite intensive study over many years, the chemistry and physics of the atomic level mechanisms that govern corrosion are not fully understood. In particular, the occurrence and severity of highly localized metal degradation cannot currently be predicted and often cannot be rationalized in failure analysis. We report a first-principles model of the nature of protective iron carbonate films coupled with a detailed chemical and physical characterization of such a film in a carefully controlled environment. The fundamental building blocks of the protective film, siderite (FeCO3) crystallites, are found to be very sensitive to the growth environment. In iron-rich conditions, cylindrical crystallites form that are highly likely to be more susceptible to chemical attack and dissolution than the rhombohedral crystallites formed in iron-poor conditions. This suggests that local degradation of metal surfaces is influenced by structures that form during early growth and provides new avenues for the prevention, detection, and mitigation of carbon steel corrosion.
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Journal articleNapier IA, Chang V, Noakes TCQ, et al., 2019,
From electronic structure to design principles for photocathodes: Cu-Ba alloys
, Physical Review Applied, Vol: 11, Pages: 064061-1-064061-13, ISSN: 2331-7019Producing a metal photocathode with a low work function (WF), low emissivity, and high quantum efficiency is a matter of importance in the design of the next generation of free-electron laser facilities. General rules for the design of appropriate materials are currently unclear and difficult to elucidate from observations of structure-composition relationships of known photocathodes. In this work, high-quality density-functional-theory electronic structure calculations and a simple physical model are employed to develop design rules for photocathodes based on metallic alloys. A theoretical study of metal alloys for photocathode applications is presented, in which high WF, stable copper is paired with low WF, unstable barium in two alloys, Cu13Ba and CuBa. Surfaces terminating in a plane of Ba atoms have a lower computed surface energy than those terminating in Cu atoms due to surface segregation of the larger Ba atoms. This results in a significant surface dipole due to the interatomic charge transfer from the differences in electronegativity of the species. The details of the surface structure determine the direction of the dipole and thus have a strong influence on the computed WF. The computed WF of the Cu13Ba Ba-terminated (100) surface is even lower than that of pure Ba, at 1.95 eV. The computed quantum efficiency (QE) of the best-performing pure Cu surface is 5.86×10−6, whereas the best-performing Cu13Ba surface terminates in a plane of Ba atoms and has a significantly increased QE of 5.09×10−3. A surface terminating in two planes of Ba atoms, the (001) surface of CuBa, has an even higher computed QE of 1.38×10−2.
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Journal articleIgnatans 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: 1932-7447Perovskites 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.
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Journal articleMartinez-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: 2296-2646Anatase 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).
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Journal articleHilderbrand M, Abualnaja F, Makwana Z, et al., 2019,
Strain engineering of adsorbate self-assembly on graphene for band gap tuning
, Journal of Physical Chemistry C, Vol: 123, Pages: 4475-4482, ISSN: 1932-7447Recent interest in functionalized graphene has been motivated by the prospect of creating a two-dimensional semiconductor with a tunable band gap. Various approaches to band gap engineering have been made over the last decade, one of which is chemical functionalization. In this work, a predictive physical model of the self-assembly of halogenated carbene layers on graphene is suggested. Self-assembly of the adsorbed layer is found to be governed by a combination of the curvature of the graphene sheet, local distortions, as introduced by molecular adsorption, and short-range intermolecular repulsion. The thermodynamics of bidental covalent molecular adsorption and the resultant electronic structure are computed using density functional theory. It is predicted that a direct band gap is opened that is tunable by varying coverages and is dependent on the ripple amplitude. This provides a mechanism for the controlled engineering of graphene’s electronic structure and thus its use in semiconductor technologies.
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Journal articleRafols i Belles C, Selim S, Harrison NM, et al., 2019,
Beyond band bending in the WO3/BiVO4 heterojunction: insight from DFT and experiment
, Sustainable Energy and Fuels, Vol: 3, Pages: 264-271, ISSN: 2398-4902Heterojunction photocatalysts can significantly enhance the efficiency of photocatalytic water splitting. It is well known that the key to such improvements lies at the interfacial region where charge separation occurs. Understanding the origins of this interfacial enhancement can enable the design of better performing water splitting devices. Therefore, in this work, a novel theoretical–experimental approach is developed for the study of photocatalytic heterojunctions using the model system – WO3/BiVO4, where it has been shown that the quantum efficiency of water splitting can approach unity at certain wavelengths. Our photoelectrochemical measurements of this heterojunction show a significantly enhanced performance over its separate components when illuminated through the BiVO4 side but not the WO3 side. This is indicative of more efficient electron transfer (i.e. from BiVO4 to WO3) than hole transfer (i.e. from WO3 to BiVO4) across the junction. Our classical band bending model of this junction predicts noticeable interfacial barriers, but could not explain the reduced performance under back illumination. Our atomistic model was used to investigate the effect of interfacial reconstructions and chemical interactions on the electronic structure of the system. The model reveals a non-staggered valence band, in contrast to the staggered conduction band, due to strong hybridization of valence band orbitals in both materials across the interface. This non-staggered valence band does not provide an energetic driving force for charge separation for hole transfer (i.e. from WO3 to BiVO4 under back illumination). Hence, a significant improvement in performance is only observed under front illumination. This combined approach, using both experiment and theory, results in a more complete understanding of a heterojunction photocatalyst system and provides unique insight into the interfacial effects that arise when two semiconductor materials are brought together
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Journal articleMartinez-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: 1932-7447An 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.
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Journal articleJoshi GR, Cooper K, Zhong X, et al., 2018,
Temporal evolution of sweet oilfield corrosion scale: Phases, morphologies, habits, and protection
, Corrosion Science, Vol: 142, Pages: 110-118, ISSN: 0010-938XElectrochemical measurements and substrate analysis have been employed to study the corrosion of iron in sweet solution (pH = 6.8, T = 80 °C) over a period of 288 h. Correlated with decreasing corrosion rate, diffraction, microscopy, and spectroscopy data reveal the evolution of adhered sweet corrosion scale. Initially, it is comprised of two phases, siderite and chukanovite, with the latter affording little substrate protection. Subsequently, as the scale becomes highly protective, siderite is the sole component. Notably, siderite crystals are concluded to display a somewhat unexpected habit, which may be a trigger for local breakdown of protective sweet scales.
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Journal articleChang V, Noakes TCQ, Harrison N, 2018,
Work function and quantum efficiency study of metal oxide thin films on Ag(100)
, Physical Review B, Vol: 97, ISSN: 2469-9950Increasing the quantum efficiency (QE) of metal photocathodes is in the design and development of photocathodes for free-electron laser applications. The growth of metal oxide thin films on certain metal surfaces has previously been shown to reduce the work function (WF). Using a photoemission model B. Camino [Comput. Mater. Sci. 122, 331 (2016)CMMSEM0927-025610.1016/j.commatsci.2016.05.025] based on the three-step model combined with density functional theory calculations we predict that the growth of a finite number of MgO(100) or BaO(100) layers on the Ag(100) surface increases significantly the QE compared with the clean Ag(100) surface for a photon energy of 4.7 eV. Different mechanisms for affecting the QE are identified for the different metal oxide thin films. The addition of MgO(100) increases the QE due to the reduction of the WF and the direct excitation of electrons from the Ag surface to the MgO conduction band. For BaO(100) thin films, an additional mechanism is in operation as the oxide film also photoemits at this energy. We also note that a significant increase in the QE for photons with an energy of a few eV above the WF is achieved due to an increase in the inelastic mean-free path of the electrons.
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Journal articleSavazzi 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: 1948-7185Graphene 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.
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Conference paperHarrison N, 2018,
Computational characterisation of catalysts in reactive environments: Phase stability, surface compostion, structure and reaction sites
, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727 -
Journal articleChandavar S, Goetz JT, Hicks K, et al., 2018,
Double K<i><sub>S</sub></i><SUP>0</SUP> photoproduction off the proton at CLAS
, PHYSICAL REVIEW C, Vol: 97, ISSN: 2469-9985- Cite
- Citations: 7
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Journal articleAdhikari KP, Deur A, El Fassi L, et al., 2018,
Measurement of the <i>Q</i><SUP>2</SUP> Dependence of the Deuteron Spin Structure Function <i>g</i><SUP>1</SUP> and its Moments at Low <i>Q</i><SUP>2</SUP> with CLAS
, PHYSICAL REVIEW LETTERS, Vol: 120, ISSN: 0031-9007- Cite
- Citations: 18
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Working paperJong U-G, Yu C-J, Ri G-C, et al., 2018,
Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites
, Publisher: ROYAL SOC CHEMISTRY- Cite
- Citations: 120
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Journal articleAkbar Z, Roy P, Park S, et al., 2017,
Measurement of the helicity asymmetry <i>E</i> in ω→ π<SUP>+</SUP>π<SUP>-</SUP>π<SUP>0</SUP> photoproduction
, PHYSICAL REVIEW C, Vol: 96, ISSN: 2469-9985- Cite
- Citations: 22
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Journal articleFersch RG, Guler N, Bosted P, et al., 2017,
Determination of the proton spin structure functions for 0.05 < <i>Q</i><SUP>2</SUP> < 5GeV<SUP>2</SUP> using CLAS
, PHYSICAL REVIEW C, Vol: 96, ISSN: 2469-9985- Cite
- Citations: 42
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Journal articleJong UG, Yu CJ, Ri GC, et al., 2017,
Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites
, Journal of Materials Chemistry A, Vol: 6, Pages: 1067-1074, ISSN: 2050-7496The application of methylammonium (MA) lead halide perovskites, CH3NH3PbX3 (X = I, Br, Cl), in perovskite solar cells has made great recent progress in performance efficiency during recent years. However, the rapid decomposition of these materials in humid environments hinders outdoor application, and thus, a comprehensive understanding of the degradation mechanism is required. We investigate the effect of water intercalation and hydration of the decomposition and ion migration of CH3NH3PbX3 using first-principles calculations. We find that water interacts with PbX6 and MA through hydrogen bonding, and the former interaction increases gradually, while the latter hardly changes when going from X = I to Br and to Cl. Thermodynamic calculations indicate that water exothermically intercalates into the perovskite, and suggest that the water intercalated and monohydrated compounds are stable with respect to decomposition. More importantly, the water intercalation reduces the activation energies for vacancy-mediated ion migration, which become higher going from X = I to Br and to Cl. Our work indicates that hydration of halide perovskites must be avoided to prevent the degradation of solar cells upon moisture exposure.
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Journal articleWells 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: 2195-1071Strontium 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.
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Journal articleTautschnig MP, Harrison NM, Finnis MW, 2017,
A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina
, Acta Materialia, Vol: 132, Pages: 503-516, ISSN: 1359-6454A model for ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure is introduced. The grain structure is idealized as a lattice of identical hexagonal cells – a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. In the resulting Poisson-Nernst-Planck system of equations, the electrostatic potential is obtained from the Poisson equation in its integral form by summation. The model is used to interpret alumina membrane oxygen permeation experiments, in which different oxygen gas pressures are applied at opposite membrane surfaces and the resulting flux of oxygen molecules through the membrane is measured. Simulation results involving four mobile species, charged aluminum and oxygen vacancies, electrons, and holes, provide a complete description of the measurements and insight into the microscopic processes underpinning the oxygen permeation of the membrane. Most notably, the hypothesized transition between p-type and n-type ionic conductivity of the alumina grain boundaries as a function of the applied oxygen gas pressure is observed in the simulations. The range of validity of a simple analytic model for the oxygen permeation rate, similar to the Wagner theory of metal oxidation, is quantified by comparison to the numeric simulations. The three-dimensional model we develop here is readily adaptable to problems such as transport in a solid state electrode, or corrosion scale growth.
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Journal articleRimal D, Adikaram D, Raue BA, et al., 2017,
Measurement of two-photon exchange effect by comparing elastic e<SUP>±</SUP>p cross sections
, PHYSICAL REVIEW C, Vol: 95, ISSN: 2469-9985- Author Web Link
- Cite
- Citations: 51
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Conference paperHarrison NM, Maxted J, Harrison NM, 2017,
Modelling pathways for the rapid transfer to, and adoption of, emergent renewable energy technologies in African cities.
, Association of American Geographers Annual Meeting -
ReportAdjiman CS, Harrison NM, Weider SZ, 2017,
Molecular science and engineering: a powerful transdisciplinary approach to solving grand challenges
, Briefing paper, 1The concept of molecular science and engineering – melding a deep understanding of molecular science with an engineering mind-set – is emerging as a powerful way to create novel, effective and sustainable solutions to global grand challenges, such as the growing threat of antimicrobial resistance. By blurring the boundaries between scientificand engineering disciplines, in this holistic approach, final function and end-use requirements become an integral part of the underlying scientific research. Commercially ready materials can thus become a reality in an accelerated, flexible and economic manner. In other words, molecular science and engineering can fundamentally alter the way molecules are identified and designed for real-world usage. It is not enough to simply make molecules; we must make molecules work for a complex world.The notion of bringing researchers, industry and government communities together to work on grand challenges has a long and illustrious history – think, for instance, of the Manhattan Project, the industrial scale-up of penicillin and the Moon landings. More recently, the idea of ‘convergence’ – tackling grand challenges with a multifaceted array of scientists, engineers, clinicians and beyond – has become more formally recognised as a valuable way to stimulate societally important and ground-breaking research. Molecular science and engineering is a specific, yet far-reaching, part of this convergence landscape.Within the growing worldwide molecular science and engineering community, the Institute for Molecular Science and Engineering (IMSE) was founded in 2015 as Imperial College London’s newest Global Institute. The Institute’s overarching aim is to bring the College’s engineers, scientists, medics and business researchers together with awide array of external stakeholders – and to remove the boundaries between these disciplines – to find innovative molecular-based scie
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Journal articleBosted PE, Kim A, Adhikari KP, et al., 2017,
Target and beam-target spin asymmetries in exclusive pion electroproduction for <i>Q</i><SUP>2</SUP> > 1 GeV<SUP>2</SUP>. II. <i>ep</i> → <i>e</i>π<SUP>0</SUP> <i>p</i>
, PHYSICAL REVIEW C, Vol: 95, ISSN: 2469-9985- Cite
- Citations: 11
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Journal articleBedlinskiy I, Kubarovsky V, Stoler P, et al., 2017,
Exclusive η electroproduction at <i>W</i> > 2 GeV with CLAS and transversity generalized parton distributions
, PHYSICAL REVIEW C, Vol: 95, ISSN: 2469-9985- Cite
- Citations: 21
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Journal articleTileli V, Ahmad E, Webster R, et al., 2016,
Decoupling of valence and coordination number contributions at perovskite surfaces
, Pages: 934-935<jats:p> Perovskite oxide nanostructures are on the forefront of technology due to the wide spectrum of possible applications pertinent to renewable energy sources, such as water‐splitting, solar cells, fuel cells, batteries, and catalysis. In particular, the exceptional properties for the oxygen reduction reaction in catalysis have been detailed recently in a volcano plot and the results reveal that orthorhombic, Jahn‐Teller distorted LaMnO <jats:sub>3</jats:sub> perovskite nanoparticles are the leading, non‐noble metal candidate for enhanced catalytic activity on the cathode electrode of fuel cells [1]. Since the functional properties of these nanoparticles lie on their active surfaces, our approach involves a detailed structural and chemical evaluation of the surfaces on the atomic scale to determine what/where the reaction centres are. Subsequently, the morphology of the particles can be optimised to maximise the number of these reaction centres, allowing us to attain the highest possible performance of perovskite catalysts. </jats:p> <jats:p> From structural transmission electron microscopy (TEM) data it was determined that polar facets exist on crystallites, which lead to assumptions on possible surface reconstruction/relaxation. However, high resolution TEM indicated that the atomic terminations of several surfaces remained defect‐free up to the very surface with no visible reconstruction taking place [2], as shown in Figure 1. Next, the surface and subsurface of the working perovskite catalyst was probed by high spatial and temporal resolution electron energy‐loss spectroscopy (EELS) in scanning TEM mode. The results revealed that the surface shows different character than the bulk. Tan <jats:italic>et al.</jats:italic> has previously shown that different oxidation states of Mn can be probed at neighbouring sites in the same compound
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Journal articleParkes MA, Tompsett DA, d'Avezac M, et 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: 1463-9084Yttria 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
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Journal articleBosted PE, Biselli AS, Careccia S, et al., 2016,
Target and beam-target spin asymmetries in exclusive π<SUP>+</SUP> and π<SUP>-</SUP> electroproduction with 1.6-to 5.7-GeV electrons
, PHYSICAL REVIEW C, Vol: 94, ISSN: 2469-9985- Cite
- Citations: 6
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Computational Materials Science
