Browsing by Author "Mulder, FM"

Now showing 1 - 9 of 9
  • No Thumbnail Available
    Item
    Density functional calculations of potential energy surface and charge transfer integrals in molecular triphenylene derivative HAT(6)
    (Springer, 2010-03) Zbiri, M; Johnson, MR; Kearley, GJ; Mulder, FM
    We investigate the effect of structural fluctuations on charge transfer integrals, overlap integrals, and site energies in a system of two stacked molecular 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT(6)), which is a model system for conducting devices in organic photocell applications. A density functional based computational study is reported. Accurate potential energy surface calculations are carried out using an improved meta-hybrid density functional to determine the most stable configuration of the two weakly bound HAT(6) molecules. The equilibrium parameters in terms of the twist angle and co-facial separation are calculated. Adopting the fragment approach within the Kohn-Sham density functional framework, these parameters are combined to a lateral slide, to mimic structural/conformational fluctuations and variations in the columnar phase. The charge transfer and spatial overlap integrals, and site energies, which form the matrix element of the Kohn-Sham Hamiltonian are derived. It is found that these quantities are strongly affected by the conformational variations. The spatial overlap between stacked molecules is found to be of considerable importance since charge transfer integrals obtained using the fragment approach differ significantly from those using the dimer approach. © 2010, Springer.
  • No Thumbnail Available
    Item
    Direct view on nanoionic proton mobility
    (Wiley-Blackwell, 2011-04-22) Chan, WK; Haverkate, LA; Borghols, WJH; Wagemaker, M; Picken, SJ; van Eck, ERH; Kentgens, APM; Johnson, MR; Kearley, GJ; Mulder, FM
    The field of nanoionics is of great importance for the development of superior materials for devices that rely on the transport of charged ions, like fuel cells, batteries, and sensors. Often nanostructuring leads to enhanced ionic mobilities due to the induced space-charge effects. Here these large space-charge effects occurring in composites of the proton-donating solid acid CsHSO4 and the proton-accepting TiO2 or SiO2 are studied. CsHSO4 is chosen for this study because it can operate effectively as a fuel-cell electrolyte at elevated temperature while its low-temperature conductivity is increased upon nanostructuring. The composites have a negative enthalpy of formation for defects involving the transfer of protons from the acid to the acceptor. Very high defect densities of up to 10% of the available sites are observed by neutron diffraction. The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders-of-magnitude enhanced mobility in the nanostructured composites of TiO2 or SiO2, both in crystalline CsHSO4 and an amorphous fraction.© 2011, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.com
  • No Thumbnail Available
    Item
    Dispersive kinetics in discotic liquid crystals
    (American Physical Society, 2010-11-22) Kruglova, O; Mulder, FM; Kearley, GJ; Picken, SJ; Stride, JA; Paraschiv, I; Zuilhof, H
    The dynamics of the discotic liquid-crystalline system, hexakis (n-hexyloxy) triphenylene (HAT6), is considered in the frame of the phenomenological model for rate processes proposed by Berlin. It describes the evolution of the system in the presence of the long-time scale correlations in the system, and we compare this with experimental quasielastic neutron scattering of the molecular assembly of HAT6 in the columnar phase. We interpret the parameters of this model in terms of nonextensive thermodynamics in which rare events in the local fast dynamics of some parts of the system control the slower dynamics of the larger molecular entity and lead to a fractional diffusion equation. The importance of these rare local events to the overall dynamics of the system is linked to the entropic index, this being obtained from the data within the model approach. Analysis of the waiting-time dependence from momentum transfer reveals a Lévy distribution of jump lengths, which allows us to construct the van Hove correlation function for discotic liquid-crystalline system. © 2010, American Physical Society
  • No Thumbnail Available
    Item
    Dynamic solubility limits in nanosized olivine LiFePO(4)
    (American Chemical Society, 2011-07-06) Wagemaker, M; Singh, DP; Borghols, WJH; Lafont, U; Haverkate, LA; Peterson, VK; Mulder, FM
    Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties. © 2011, American Chemical Society
  • No Thumbnail Available
    Item
    Electronic and vibronic properties of a discotic liquid-crystal and its charge transfer complex
    (AIP Publishing, 2014-01-06) Haverkate, LA; Zbiri, M; Johnson, MR; Carter, EA; Kotlewski, A; Picken, SJ; Mulder, FM; Kearley, GJ
    Discotic liquid crystalline (DLC) charge transfer (CT) complexes combine visible light absorption and rapid charge transfer characteristics, being favorable properties for photovoltaic (PV) applications. We present a detailed study of the electronic and vibrational properties of the prototypic 1:1 mixture of discotic 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) and 2,4,7-trinitro-9-fluorenone (TNF). It is shown that intermolecular charge transfer occurs in the ground state of the complex: a charge delocalization of about 10−2 electron from the HAT6 core to TNF is deduced from both Raman and our previous NMR measurements [L. A. Haverkate, M. Zbiri, M. R. Johnson, B. Deme, H. J. M. de Groot, F. Lefeber, A. Kotlewski, S. J. Picken, F. M. Mulder, and G. J. Kearley, J. Phys. Chem. B116, 13098 (2012)], implying the presence of permanent dipoles at the donor-acceptor interface. A combined analysis of density functional theory calculations, resonant Raman and UV-VIS absorption measurements indicate that fast relaxation occurs in the UV region due to intramolecular vibronic coupling of HAT6 quinoidal modes with lower lying electronic states. Relatively slower relaxation in the visible region the excited CT-band of the complex is also indicated, which likely involves motions of the TNF nitro groups. The fast quinoidal relaxation process in the hot UV band of HAT6 relates to pseudo-Jahn-Teller interactions in a single benzene unit, suggesting that the underlying vibronic coupling mechanism can be generic for polyaromatic hydrocarbons. Both the presence of ground state CT dipoles and relatively slow relaxation processes in the excited CT band can be relevant concerning the design of DLC based organic PV systems. © 2014 AIP Publishing LLC.
  • No Thumbnail Available
    Item
    Hydrogen adsorption strength and sites in the metal organic framework MOF5: comparing experiment and model calculations
    (Elsevier, 2008-07-03) Mulder, FM; Dingemans, TJ; Schimmel, HG; Ramirez-Cuesta, AJ; Kearley, GJ
    Hydrogen adsorption in porous, high surface area, and stable metal organic frameworks (MCF's) appears a novel route towards hydrogen storage materials [N.L. Rosij Eckert, M. Eddaoudi, D.T. Voclakj Kim M., O'Keeffe, O.M. Yaghi, Science 300 (2003) 1127: J.L.C. Rowsell, A.R. Millward, K. Sung Park O.M. Yaghi J,,. Am. Chem. Soc. 126 (2004) 5666; G. Ferey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-Guegan, Chem. Commun. (2003) 2976; T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry T., Bataille, G. Ferey, Chem. Eur. J. 10 (2004) 1373]. A prerequisite for such materials is sufficient adsorption interaction strength for hydrogen adsorbed on the adsorption sites of the material because this facilitates successful operation under moderate temperature and pressure conditions. Here we report detailed information on the geometry of the hydrogen adsorption sites, based on the analysis of inelastic neutron spectroscopy (INS). The adsorption energies for the metal organic framework MOF5 equal about 800 K for part of the different sites, which is significantly higher than for narroporous carbon materials (-550 K) [H.G. Schimmel, GJ. Kearley, M.G. Nijkamp, C.T. Visser, K.P. de Jong, F.M. Mulder, Chem. Eur. J. 9 (2003) 4764], and is in agreement with what is found in first principles calculations [T. Sagara, J. Klassen, E. Ganz, J. Chem. Phys. 121 (2004) 12543; F.M. Mulder, T.J. Dingernans, M. Wagemaker, G.J. Kearley, Chem. Phys. 317 (2005) 113]. Assignments orthe INS spectra is realized using comparison with independently published model calculations [F.M. Mulder, T.J. Dingemans, M. Wagemaker, G.J. Kearley, Chem. Phys. 317 (2005) 113] and structural data IT. Yildirim, M.R. Hartman, Phys. Rev. Lett. 95 (2005) 215504]. © 2008, Elsevier Ltd.
  • No Thumbnail Available
    Item
    Hydrogen in porous tetrahydrofuran clathrate hydrate
    (Wiley-VCH Verlag Berlin, 2008-06-23) Mulder, FM; Wagemaker, M; van Eijck, L; Kearley, GJ
    The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier.([1]) Storage within solid-state clathrate hydrates,([2-4]) and in the clathrote hydrate of tetrohydrofuran (THF), has been recently reported.([5,6]) In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that-in contrast to previous reports([5,6]) - hydrogen (deuterium) occupies the small cages of the clathrote hydrate only to 30% (at 274 K and 90.5 bar). Such a D-2 load is equivalent to 0.27 wt. % of stored H-2. In addition, we show that a surplus of D2O results in the formation of additional D2O ice Ih instead of in the production of sub-stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. [6]). Structure-refinement studies show that [D-8]THF is dynamically disordered, while it fills each of the large cages of [D-8]THF center dot 17D(2)O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above approximate to 220 K, the H-storage characteristics of the clathrate hydrate have similarities with those of surface-adsorption materials, such as nanoporous zeolites and metal-organic frameworks,([7,8]) but at lower temperatures, the adsorption rates slow down because of reduced D-2 diffusion between the small cages. © 2008, Wiley-VCH Verlag Berlin
  • No Thumbnail Available
    Item
    Molecular modelling of ground- and excited-states vibrations in organic conducting devices: hexakis(n-hexyloxy)triphenylene (HAT(6)) as case study
    (CSIRO Publishing, 2010-03-26) Zbiri, M; Johnson, MR; Haverkate, LA; Mulder, FM; Kearley, GJ
    In order to gain insight into fundamental aspects of organic photocell materials, we have calculated ground and excited electronic-state structures and molecular vibrations for an isolated HAT6 molecule (hexakis(n-hexyloxy)triphenylene). Excited-state calculations are carried out using time-dependent density functional theory and frequencies are evaluated analytically using coupled perturbed Kohn–Sham equations. These model calculations have been validated against new infrared and ultraviolet data on HAT6 in solution. The main allowed valence excitation, having the largest oscillator strength, is chosen for the structural and vibrational investigations. Comparison with the ground-state vibrational dynamics reveals surprisingly large spectral differences. In addition, the alkoxy tails, which are usually considered to play only a structural role, are clearly involved in the molecular vibrations and the structural distortion of the excited electronic state compared with the ground state. The tails may play a more important role in charge separation, transport and excited-state relaxation than was previously thought. In this case, chemical modification of the tails would allow vibrational and related properties of organic photocell materials to be tailored. © 2010, CSIRO Publishing
  • No Thumbnail Available
    Item
    On the morphology of a discotic liquid crystalline charge transfer complex
    (American Chemical Society, 2012-10-01) Haverkate, LA; Zbiri, M; Johnson, MR; Deme, B; de Groot, HJM; Lefeber, F; Kotlewski, A; Picken, SJ; Mulder, FM; Kearley, GJ
    Discotic liquid crystalline (DLC) charge transfer (CT) complexes, which combine visible light absorption with rapid charge transfer characteristics within the CT complex, can have a great potential for photovoltaic applications when they can be made to self-assemble in a bulk heterojunction arrangement with separate channels for electron and hole conduction. However, the morphology of some liquid crystalline CT complexes has been under debate for many years. In particular, the liquid crystalline CT complex built from the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) and discotic molecules has been reported to have the TNF "sandwiched" either between the discotic molecules within the same column or between the columns within the aliphatic tails of the discotic molecules. We present a detailed structural study of the prototypic 1:1 mixture of the discotic 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6) and TNF. Nuclear magnetic resonance (NMR) line widths and cross-polarization rates are consistent with the picosecond time scale anisotropic thermal motions of the HAT6 and TNF molecules previously observed. By computational integration of Rietveld refinement analyses of neutron diffraction patterns with density experiments and short-range structural constraints from heteronuclear 2D NMR, we determine that the TNF molecules are vertically oriented between HAT6 columns. The data provide the insight that a morphology of separate hole conducting channels of HAT6 molecules can be realized in the liquid crystalline CT complex. © 2012, American Chemical Society.