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Suche nach „[M.] [O’Neill]“ hat 27 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

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

    Growth dynamics of CdTe quantum dot ensembles in liquid and crystalline phases

    SPIE Optics and Photonics /Nanoscience and Engineering, San Diego, CA, USA

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    Vortrag

    W. Hagston, Thomas Stirner, S. Weston, M. O’Neill, J.H.C. Hogg, B. Lunn, D. Ashenford, P. Harrison

    Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures

    13th North American Conference on Molecular Beam Epitaxy, Stanford, CA, USA

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner, M. O’Neill, P. Harrison, W. Hagston

    Exciton dynamics in multiquantum well CdTe-Cd1-xMnxTe systems

    13th North American Conference on Molecular Beam Epitaxy, Stanford, CA, USA

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    S. Weston, M. O’Neill, J. Nicholls, J.H.C. Hogg, B. Lunn, D. Ashenford, K. Hieke, P. Harrison, W. Hagston, Thomas Stirner

    Comparative studies of excitons in magnetic asymmetric double quantum well structures

    Posterpräsentation

    3rd International Conference on Optics of Excitons in Confined Systems, Montpellier, Frankreich

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    Vortrag

    N. Cain, M. O’Neill, J. Nicholls, W. Hagston, D. Ashenford, Thomas Stirner

    Photoluminescence of highly excited CdTe/CdMnTe quantum wells

    European Quantum Electronics Conference (EQEC), Hamburg

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

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

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

    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.

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    Zeitschriftenartikel

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

    Dynamics of the size distribution of CdTe quantum dot ensembles during growth in liquid and crystalline phases

    Physical Chemistry Chemical Physics (PCCP), vol. 9, no. 7, pp. 1057-1061

    Abstract anzeigen

    We recently reported that the growth rate of colloidal CdTe nanoparticles decreases by orders of magnitude when the particles undergo a phase transition from liquid to crystal. The dynamics of nanoparticle growth are dominated by this factor rather than the size dependence of the chemical potential. Herein we discuss how the phase transition affects the size distribution and photoluminescence quantum efficiency of the nanoparticles. We suggest that the absorption linewidth is a better monitor of size distribution than the photoluminescence linewidth because the photoluminescence quantum efficiency, which affects the latter via energy transfer, varies substantially with reaction time. We find that the size distribution broadens in the early stages of growth possibly because of inhomogeneities in the phase transition radius or because particles nucleated at later times coalesce with nanocrystals. The quantum efficiency is enhanced when tellurium is depleted in the reaction solution, giving a cadmium-enriched surface. Batches with high initial tellurium and cadmium concentrations show a substantial amount of delayed nucleation, lower quantum efficiency and some anisotropic growth.

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    Zeitschriftenartikel

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

    Growth dynamics of CdTe nanoparticles in liquid and crystalline phases

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

    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.

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    Zeitschriftenartikel

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

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

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

    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.

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    Vortrag

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

    In-situ study of the ultraviolet irradiation of side-chain coumarin polymers using specular x-ray reflectivity

    Synchrotron Radiation in Polymer Science II Conference, Sheffield, Großbritannien

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    Zeitschriftenartikel

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

    Synthesis and investigation of nematic liquid crystals with flexoelectric properties

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

    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.

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    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

    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

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

    Electronic charge transport in extended nematic liquid crystals

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

    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.

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    Zeitschriftenartikel

    W. Hagston, S. Weston, M. O’Neill, J.H.C. Hogg, D. Ashenford, B. Lunn, Thomas Stirner, P. Harrison

    Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures

    Journal of Vacuum Science & Technology B, vol. 12, pp. 1146-1149

    DOI: 10.1116/1.587069

    Abstract anzeigen

    Novel methods of growing graded gap structures are described. In order to provide a check that the structures are indeed graded in the correct manner, magnetic ions in the structure have been employed. The good agreement between the predicted and observed Zeeman splitting of the corresponding light‐ and heavy‐hole exciton transitions, together with the magnetic field dependence of the Zeeman splittings and intensity of a forbidden heavy‐hole exciton transition, is taken as direct evidence that graded gap structures having the correct composition have indeed been fabricated.

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    Beitrag (Sammelband oder Tagungsband)

    N. Cain, M. O’Neill, J. Nicholls, W. Hagston, D. Ashenford, Thomas Stirner

    Photoluminescence of highly excited CdTe/CdMnTe quantum wells

    Proceedings of the 1996 European Quantum Electronics Conference, EQEC'96, Hamburg, 08.-13.09.1996

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    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

    ISBN: 978-0750309233

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    Zeitschriftenartikel

    S. Weston, J. Nicholls, M. O’Neill, P. Harrison, W. Hagston, J.H.C. Hogg, B. Lunn, D. Ashenford, Thomas Stirner

    Comparative studies of excitons in magnetic asymmetric double quantum well structures

    Journal de Physique IV, vol. 3, no. C5, pp. 401-404

    DOI: 10.1051/jp4:1993585

    Abstract anzeigen

    Four different asymmetric double quantum well structures in the dilute magnetic semiconductor system Cd1-xMnx Te have been investigated. Theoretical calculations are presented which show that the observed photoluminescence (PL) and photoluminescence excitation (PLE) energies are consistent with an anticrossing of the lowest two hole states in one of the double quantum well structures when subjected to an external magnetic field.

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    Zeitschriftenartikel

    Thomas Stirner, M. O’Neill, P. Harrison, W. Hagston

    Exciton dynamics in multiquantum well CdTe-Cd1-xMnxTe systems

    Journal of Vacuum Science & Technology B, vol. 12, no. 2, pp. 1150-1152

    DOI: 10.1116/1.587070

    Abstract anzeigen

    The exciton dynamics associated with the transfer of energy from the light‐hole state to both the heavy‐hole state and to a donor bound exciton (D0X) are investigated utilizing time‐resolved spectroscopy, in a multiquantum well sample of CdTe–Cd1-xMnxTe containing 75 Å wells and 150 Å barriers. The results show that the relaxation time of the light‐hole to the heavy‐hole state occurs, at low temperature, in a time ≤40 ps whereas the combined drift/diffusion of the excitons in the plane of the quantum well and their subsequent trapping at donor states occurs in a time ∼80 ps. Theoretical calculations show that the light‐hole–heavy‐hole carrier relaxation time is consistent with confined (acoustic) phonon relaxation and that the equivalent in‐plane mobility is consistent with interface roughness scattering.

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    Zeitschriftenartikel

    Thomas Stirner, N. Kirkman, L. May, C. Ellis, J. Nicholls, S. Kelly, M. O’Neill, J.H.C. Hogg

    CdTe nanocrystals: synthesis, optical characterisation and pseudopotential calculation of the bandgap

    Journal of Nanoscience and Nanotechnology, vol. 1, no. 4, pp. 451-455

    DOI: 10.1166/jnn.2001.053

    Abstract anzeigen

    CdTe nanocrystals were synthesized in aqueous solution using 1-thioglycerol and 2-mercaptoethanol as surface stabilizers. The nanocrystals were characterized by means of X-ray powder diffraction and UV-vis absorption measurements. The UV-vis absorption spectra exhibit two distinct transition lines. Comparison of the experimental measurements with the results of the empirical pseudopotential calculations of the CdTe nanocrystals showed that the lower energy absorption line can be assigned to the heavy-hole exciton transition, whereas the higher energy absorption line can be attributed to the light-hole exciton transition.

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    Zeitschriftenartikel

    P. Chen, J. Nicholls, M. O’Neill, Thomas Stirner, J.H.C. Hogg, B. Lunn, D. Ashenford

    Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures

    Journal of Applied Physics, vol. 84, pp. 5621-5625

    DOI: 10.1063/1.368609

    Abstract anzeigen

    The role of biexcitons has been studied in two optically pumped quantum well(QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18%QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3%QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.