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Suche nach „[Stirner] [Thomas]“ hat 139 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces

    Computational Materials Science, vol. 43, no. 4, pp. 769-801

    2008

    Abstract anzeigen

    A theoretical study of the non-equilibrium behaviour of colloidal particle monolayers under the influence of the displacement of a single, optically trapped colloidal particle is presented. Two different types of monolayer (i.e., one relatively rigid and the other relatively soft) confined to a liquid–liquid interface are investigated using the Stokesian dynamics simulation technique. An effective dipole–dipole interaction is assumed to act between the colloidal particles and viscous drag forces acting on the particles are taken into account. The calculations reveal periodic oscillations of the net force on the trapped particle for the soft monolayer and highly non-linear, non-monotonic variations for the rigid monolayer. The highly non-linear variations of the net force for the rigid monolayer are concomitant with rapid, cooperative particle rearrangements and large oscillations in the global orientational order parameter of the monolayer. These findings, combined with the results of optical-tweezer experiments, should be useful for the rheology of liquid interfaces and the investigation of Pickering emulsions.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Molecular dynamics simulation of the (0001) α-Al2O3 and α-Cr2O3 surfaces

    Surface Science, vol. 601, no. 5, pp. 1358-1364

    2007

    Abstract anzeigen

    A simple, rigid pair-potential model is applied to investigate the dynamics of the (0 0 0 1) α-Al2O3 and α-Cr2O3 surfaces using the molecular dynamics technique. The simulations employ a two-stage equilibration process: in the first stage the simulation-cell size is determined via the constant-stress ensemble, and in the second stage the equilibration of the size-corrected simulation cell is continued in the canonical ensemble. The thermal expansion coefficients of bulk alumina and chromia are evaluated as a function of temperature. Furthermore, the surface relaxation and mean-square displacement of the atoms versus depth into the slab are calculated, and their behaviour in the surface region analysed in detail. The calculations show that even moderate temperatures (∼400 °C) give rise to displacements of the atoms at the surface which are similar to the lattice mismatch between α-alumina and chromia. This will help in the initial nucleation stage during thin film growth, and thus facilitate the deposition of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates.

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    Zeitschriftenartikel

    C. Ellis, M.-O. M. Piepenbrock, D. Dong, R. Kelly, Thomas Stirner, M. Carrasco-Or, M. O’Neill

    Superlattices of organic/inorganic semiconductor nanostructures from liquid-crystal templates

    Physical Review B - condensed matter and materials physics, vol. 75

    2007

    DOI: 10.1103/PhysRevB.75.035207

    Abstract anzeigen

    A lyotropic liquid-crystal-templated superlattice of inorganic CdSe semiconductor nanostructures within an organic semiconductor polymer-network film was fabricated by a self-assembly process. The film is characterized with small-angle x-ray diffraction experiments and ultraviolet-visible absorption measurements. Empirical pseudopotential calculations of the effective CdSe nanocrystal band gap show that the experimental observations are consistent with the formation of CdSe nanostructures inside lyotropic liquid-crystal nanotubes. A potential application of these nanocomposite supramolecular arrays is demonstrated via a photoconductive device.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    M.-O. M. Piepenbrock, S. Kelly, Thomas Stirner, M. O’Neill

    Growth dynamics of CdTe nanoparticles in liquid and crystalline phases

    Journal of the American Chemical Society (JACS), vol. 129, no. 24, pp. 7674-7679

    2007

    Abstract anzeigen

    Normally the size dependence of the chemical potential is used to explain the growth dynamics of semiconductor nanoparticles. Instead we show that very small CdTe nanoparticles continue to grow at high dilution, the growth rate is virtually independent of monomer concentration, nucleation continues after the growth of larger particles has saturated, and the growth rate has a much greater nonlinear dependence on particle size than predicted by theory. We suggest that nanoparticle growth is fast in the liquid phase and then saturates as the particles change phase from liquid to crystal at a threshold size which depends on the growth temperature and not the monomer concentration. The photoluminescence quantum efficiency becomes high when tellurium is depleted in the reaction solution giving a cadmium enriched surface.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0001) α-Cr2O3 templates

    Surface Science, vol. 601, no. 21, pp. 5050-5056

    2007

    Abstract anzeigen

    Ab initio total energy Hartree–Fock calculations of ultrathin films of α-Al2O3 on (0 0 0 1) α-Cr2O3 templates are presented. The surface relaxation, the in-plane reconstruction and the surface and strain energies of the slabs are studied as a function of alumina film thickness. The surface Al layer is found to relax inwards considerably, with the magnitude of the inwards relaxation depending on the thickness of the ultrathin alumina film in a non-linear manner. The calculations also reveal that ultrathin films of alumina lower the surface energy of (0 0 0 1) α-chromia substrates. This indicates that the (0 0 0 1) α-chromia surface provides favourable conditions for the templated growth of α-alumina. However, increasing the alumina film thickness is found to give rise to a significant increase in strain energy. Finally, the electronic properties at the surface of the (0 0 0 1) α-Al2O3/α-Cr2O3 slabs are investigated. Here it is found that the alumina coating gives rise to an increase in the covalency of the bonds at the surface of the slabs. In contrast, the influence of an alumina layer on the electrostatic potential at the surface of the chromia slab is relatively minor, which should also be beneficial for the templated growth of α-alumina on (0 0 0 1) α-chromia substrates.

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    Zeitschriftenartikel

    S. Kitney, A. Löbbert, S. Kelly, Thomas Stirner, M. O’Neill

    Terminal end-group efficiency for the nematic phase using model bicyclo[2.2.2]octanes

    Liquid Crystals, vol. 34, no. 12, pp. 1357-1367

    2007

    Abstract anzeigen

    The synthesis is reported of new liquid crystals incorporating the 1,4-disubstituted bicyclo[2.2.2] octane ring and a series of substituents in a terminal position on the molecular core. The nature of the terminal substituent is varied from apolar with a small dipole moment to polar with a strong dipole moment. The angle of the dipole moment with respect to the molecular axis is also varied. An updated order of terminal group efficiency for substituents in a terminal position for the nematic phase is provided. The bicyclo[2.2.2] octane ring shields halogen substituents in a lateral position on phenyl rings attached to the bicyclooctane ring to a small degree and reduces the steric efects of these substituents, giving rise to high relative nematic - isotropic transition temperatures.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential

    Surface & Coatings Technology, vol. 201, no. 7, pp. 4201-4204

    2006

    Abstract anzeigen

    Alumina and chromia are very important materials in the surface coatings industry, e.g. for corrosion protection and as catalyst supports. The type of defects and the associated formation energy in these materials are of direct relevance to the surface stability and reaction kinetics. In the present work, a modified Matsui potential is applied to calculate the native point defect energies in α-Al2O3 and α-Cr2O3 based on the Mott–Littleton theory. Particular attention is paid to the convergence of the defect energies with the number of atoms surrounding the defect. The results show that the relative values of the defect formation energies are such that the Schottky energy is smaller than either of the Frenkel energies, which is in agreement with experimental data and recent results of first-principles calculations. The implications of these findings for diffusion mechanisms and the associated reaction kinetics are discussed briefly.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, Thomas Stirner

    Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3

    Surface & Coatings Technology, vol. 201, no. 7, pp. 4205-4208

    2006

    Abstract anzeigen

    There is considerable interest in the low-temperature growth of the stable α-phase of alumina, α-Al2O3, due to its superior chemical and mechanical properties compared to the other forms of alumina. Conventional methods for achieving this phase (such as chemical vapour deposition) utilise high temperatures which limits the range of substrate materials that can be beneficially coated. Recently, it has been reported that α-Al2O3 can be grown on α-Cr2O3 templates at a much lower substrate temperature (around 400 °C) by the RF magnetron sputtering deposition technique. Previous studies showed that both α-Al2O3 and α-Cr2O3 (0001) free surfaces relax considerably in comparison with the corresponding bulk structures. From the experimental point of view, a knowledge of the surface structure and stability of both materials will be of great help in determining the facet of α-Cr2O3 which best facilitates the growth of α-Al2O3. Here we present the results of first-principles Hartree–Fock calculations on the surface energy of the low-index planes of both α-alumina and α-chromia. The suitability of possible α-Cr2O3 facets as templates for the growth of α-Al2O3 is discusssed.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    A. Matthews, J. Sun, Thomas Stirner

    Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3

    Posterpräsentation

    International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006), San Diego, CA, USA

    2006

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    A. Matthews, J. Sun, Thomas Stirner

    Calculation of native defect energies in α-Al2O3 and α-Cr2O3 using a modified Matsui potential

    Posterpräsentation

    International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2006), San Diego, CA, USA

    2006

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    A. Matthews, J. Sun, W. Hagston, A. Leyland, Thomas Stirner

    A simple transferable interatomic potential model for binary oxides applied to bulk α-Al2O3 and the (0001) α-Al2O3 surface

    Journal of Crystal Growth, vol. 290, no. 1, pp. 235-240

    2006

    Abstract anzeigen

    A simple transferable potential for binary, highly ionic oxides is derived from a previous work. In this new potential the van der Waals terms involving cations and the cation–cation short-range repulsive interactions do not appear explicitly. The potential parameters of this new model are optimised for α-Al2O3α-Al2O3, MgO and CaO. Good agreement between theory and experiment is obtained for the structural parameters and lattice energies. The potential is then employed to study the relaxation of the Al-terminated (0 0 0 1) α-Al2O3α-Al2O3 surface as well as the dynamics of this surface at non-zero temperatures using the molecular dynamics method. Here it is shown that the new potential predicts a relaxation of the (0 0 0 1) α-Al2O3α-Al2O3 surface which is consistent with experiment and shell model calculations. Finally, the calculated dynamical features of the (0 0 0 1) α-Al2O3α-Al2O3 surface are explained in terms of a combination of the loss of translational symmetry and the resulting relaxation of the surface atoms.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    M.-O. M. Piepenbrock, Thomas Stirner, M. O’Neill

    A low-temperature synthesis for organically soluble HgTe nanocrystals exhibiting near-infrared photoluminescence and quantum confinement

    Journal of the American Chemical Society (JACS), vol. 128, no. 21, pp. 7087-7090

    2006

    Abstract anzeigen

    A new low-temperature, one-pot method is introduced for the preparation of organically passivated HgTe nanocrystals, without the use of highly toxic precursors. The nanocrystals show bright photoluminescence in the infrared telecommunication windows about 1300 and 1550 nm with quantum efficiencies between 55 and 60%. They have a zinc blende structure with a mean particle diameter of 3.4 nm, thus exhibiting quantum confinement effects. Particle growth is self-limited by temperature quenching, so a narrow size distribution is obtained. The measured size of the particles agrees with calculations using the pseudopotential method.

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    Zeitschriftenartikel

    M. Aldred, G. Mehl, P. Vlachos, K. Woon, Thomas Stirner, M. O’Neill

    Electronic charge transport in extended nematic liquid crystals

    Chemistry of Materials, vol. 18, no. 9, pp. 2311-2317

    2006

    Abstract anzeigen

    We report a systematic study of charge transport in a range of low-molar-mass and extended (having at least six aromatic rings) nematic liquid crystals, some of which are reactive mesogens, with a high degree of shape anisotropy, i.e., the length-to-width (aspect) ratio is exceptionally high. We demonstrate that the hole mobility is independent of the macroscopic, but not microscopic, ordering of the nematic and isotropic phases of these nematic liquid crystals with a long, rigid, and extended aromatic molecular core, because no discontinuity is observed at the transition between these phases. A room-temperature mobility of up to 1.0 × 10-3 cm2 V-1 s-1 is obtained in the nematic phase, which is attributed to the short intermolecular distances between the highly polarizable but rigid long aromatic cores. We show that the intermolecular separation can be easily fine-tuned by changing the lateral and terminal aliphatic groups of these nematic liquid crystals. Hence, the charge mobility can be varied by up to 2 orders of magnitude without altering the core structure of the molecules, and this chemical fine control could be used to limit hole transport and so provide better charge balance in organic light-emitting diodes. X-ray diffraction is used to obtain the intermolecular separation and shows local lamellar order in the nematic phase.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    N. Kirkman, J. Wild, K. Bartle, P. Tuffin, Thomas Stirner, M. O’Neill

    Synthesis and investigation of nematic liquid crystals with flexoelectric properties

    Chemistry of Materials, vol. 17, no. 25, pp. 6354-6360

    2005

    DOI: 10.1021/cm051682y

    Abstract anzeigen

    We describe the synthesis and characterization of a series of model nematic liquid crystals with transverse dipole moments used to study the flexoelectric effect in guest−host mixtures with commercial liquid crystal host. The flexoelectric coefficicent of the mixtures, containing only 10% by weight of the dopant, are up to 6 times higher than those of the pure hosts. The length, bend angle, and dipole moments of the molecules are systematically varied to investigate any correlations with the flexoelectric effect. We find that the flexoelectric coefficients increase with molecular length, are inversely correlated with the bend angle, and are independent of the dipole moment of the dopant. Although these findings seem to contradict predictions from dipolar flexoelectric theories, they can be reconciled by considering the properties of both the guest and host in the mixture. Thiophenes and dimesogens show particularly large flexoelectric effects. This work should inform the molecular design of new materials with enhanced flexoelectric properties.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Sun, Thomas Stirner

    Molecular dynamics simulation of the structural configuration of binary colloidal monolayers

    Langmuir, vol. 21, no. 14, pp. 6636-6641

    2005

    DOI: 10.1021/la050402q

    Abstract anzeigen

    Molecular dynamics simulations of binary colloidal monolayers, i.e., monolayers consisting of mixtures of two different particle sizes, are presented. In the simulations, the colloid particles are located at an oil-water interface and interact via an effective dipole-dipole potential. In particular, the influence of the particle ratio on the configurations of the binary monolayers is investigated for two different relative interaction strengths between the particles, and the pair correlation functions corresponding to the binary monolayers are calculated. The simulations show that the binary monolayers can only form two-dimensional crystals for certain particle ratios, for example, 2:1, 6:1, etc., while, for example, for a particle ratio of 7:1 the monolayers are found to be in a disordered, glassy state. The calculations also reveal that in analogy to the Wigner lattice the configurations are very sensitive to the relative interaction strength between the particles but not to the absolute magnitude of the interaction strength, even when particle size effects are taken into account. Consequently, it is argued that a comparison between the calculated configurations and actual binary particle monolayer systems could provide useful information on the relative interaction strength between large and small particles. Possible mechanisms giving rise to disparities in the interaction strength between large and small particles are described briefly.

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    Zeitschriftenartikel

    P. Jackson, W. Duffy, S. Kelly, G. Bergmann, Thomas Stirner, M. O’Neill, J.H.C. Hogg, G. Clark

    Photoinduced changes of surface order in coumarin side-chain polymer films used for liquid crystal photoalignment

    Applied Physics Letters, vol. 87

    2005

    DOI: 10.1063/1.2009070

    Abstract anzeigen

    Specular x-rayreflectivity probes morphological changes in a crosslinkable coumarin photoalignment polymer film resulting from ultraviolet irradiation. An ordered surface layer with density oscillations compatible with planar side-chain alignment is obtained before irradiation. The ordering is enhanced in the early stages of crosslinking. This is attributed to the photoinduced increase of mobility of the side-chains resulting from the creation of free volume by the crosslinking process. The expansion of the thin film confirms that free volume is created. The surface ordering decreases with prolonged ultraviolet irradiation because of increased material viscosity resulting from a high crosslinked density. The implications of surface ordering on liquid crystal photoalignment are discussed.

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    Zeitschriftenartikel

    N. Kirkman, W. Hagston, Thomas Stirner

    A new method for investigating the surface tension from molecular dynamics simulations applied to liquid droplets

    Computational Materials Science, vol. 30, no. 5, pp. 126-130

    2004

    DOI: 10.1016/j.commatsci.2004.01.020

    Abstract anzeigen

    A new method for investigating surface tension, using molecular dynamics simulations, is proposed. The method uses the inter-particle forces to characterise the surface region. The average particle energy in this region is then employed to evaluate the surface tension. The main advantage of the proposed method is its applicability to any surface geometry and multi-component systems. For the purpose of illustration, and in view of its practical importance, the model is then applied to spherical droplets. The droplets contain 500, 1000, 2000 and 4000 particles respectively, acting under the influence of a Lennard-Jones pair potential. The effects of temperature and droplet size on the surface tension are investigated. Finally, the calculations are compared with experimental and theoretical results in the literature

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    Vortrag

    J. Sun, Thomas Stirner

    Computer simulation of binary oxides

    Invited Lecture

    International Workshop on Hard Metallic Phases, Linköping, Schweden

    2004

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Computer simulation of colloidal particle monolayers at liquid-liquid interfaces

    Simulation and Modelling of Colloidal Systems (Meeting of the RSC Colloids and Interface Science Group, the RSC Statistical Mechanics and Thermodynamics Group and the IoP Complex Fluids Group), Sheffield, Großbritannien

    2004

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Beitrag (Sammelband oder Tagungsband)

    S. Kelly, Thomas Stirner, M. O’Neill

    Cadmium chalcogenide nanocrystals

    Chapter 4

    The handbook of electroluminescent materials, Bristol, UK; Philadelphia, PA

    2004

    ISBN: 978-0750309233