| 1. |
- Carlsson, Tobias
(författare)
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Brownian Dynamics Simulations of Macromolecules : Algorithm Development and Polymers under Confinement
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I have used computer simulations to study the structure and dynamics of grafted polymers during confinement. These systems are of importance for understanding e.g. colloidal stability and surface coatings. We have used Brownian dynamics simulations with the polymers modeled as discrete wormlike chains allowing for a variable persistence length as well as different non-bonded interactions. The size and shape of the chains are characterized by the radius of gyration and the degree of oblateness/prolateness, and the entanglement is followed by calculating the mean overcrossing number. Starting in the dilute regime with a single polymer mushroom we have investigated how the rate of compression and solvent quality effects the behaviour of a compressed chain. In the brush regime, we investigated how the surface coverage effects the behaviour during compression. For low coverages the chains have the possibilty to increase their lateral extension during confinement but in general, the chains have a low inter-entanglement, as they strive to keep their integrity during the confinement process.To go from a polymer brush to the construction of a connected network, we have developed a method to construct a closed network without using periodic boundary conditions by building the network on a sphere in R4. In this way we avoid the restrictions of periodicity at the cell boundaries. We finally also show how to develop the idea of using spherical boundary conditions, by presenting a novel algorithm for simulating diffusion on a spherical surface. The method is more stable and allows for larger time steps, compared to commonly used methods in computer simulations.
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| 2. |
- Kebede, Getachew, et al.
(författare)
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Red-shifting and blue-shifting OH groups on metal oxide surfaces : towards a unified picture
- 2018
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 20:18, s. 12678-12687
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Tidskriftsartikel (refereegranskat)abstract
- We analyse the OH vibrational signatures of water molecules and hydroxide ions on thin water films on MgO(001) and CaO(001), using DFT-generated anharmonic potential energy surfaces. We find that the OH stretching frequencies of intact water molecules on the surface are always downshifted with respect to the gas-phase species while the OH– groups are either upshifted or downshifted. Despite these differences, the main characteristics of the frequency shifts for all three types of surface OH groups (OHw, OsH and OHf) can be accounted for by one unified expression involving the in situ electric field from the surrounding environment, and the molecular properties of the vibrating species (H2O or OH–). The origin behind the different red- and blueshift behaviour can be traced back to the fact that the molecular dipole moment of a gas-phase water molecule increases when an OH bond is stretched, but the opposite is true for the hydroxide ion. We propose that familiarity with the relations presented here will help surface scientists in the interpretation of vibrational OH spectra for thin water films on ionic crystal surfaces.
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| 3. |
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| 4. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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DFT-based Monte Carlo Simulations of Impurity Clustering at CeO2(111)
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 121:28, s. 15127-15134
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Tidskriftsartikel (refereegranskat)abstract
- The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F-)(n)(Ce3+)(n) nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups are noticeably different.
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| 5. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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Oxygen Vacancy Clustering at the Ceria(111) surface
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Oxygen vacancy clustering at the ceria(111) surface have been studied with both force-field and density function theory methods. Two of the methods predict that stable clusters of surface oxygen vacancies should form on these surface, as seen in numerous experimental studies. We propose that vacancy clustering of both surface and sub-surface vacancies follow the simple principle of sharing their Ce(III) neighbors. For surface oxygen vacancies this leads to compact clusters separated by one surface lattice constant. On the other hand for sub-surface this leads to sparse clusters separated by two surface lattice constants.
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| 6. |
- Mitev, Pavlin, et al.
(författare)
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Calculation of anharmonic OH phonon dispersion curves for the Mg(OH)(2) crystal
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:3, s. 034120-
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Tidskriftsartikel (refereegranskat)abstract
- Anharmonic OH phonon dispersion curves have been calculated for the Mg(OH)(2) crystal. A crystal Hamiltonian was set up for the vibrational problem, where the coordinates consists of the bond lengths of two hydroxide ions in the central unit cell. Its two-dimensional potential energy surface was constructed from first principle calculations within the density functional theory approximation. Dispersion curves were calculated by diagonalizing the Hamiltonian in a basis of singly excited crystal functions. The single particle functions used to construct the crystal states were taken from a Morse oscillator basis set. These well chosen functions made it possible to restrict calculations to include only very few functions, which greatly contributed to a transparent presentation of the underlying theory. All calculations could be done analytically except for the calculation of a few integrals. We have compared our results with those of a series of harmonic lattice dynamics calculations and have found that the anharmonicity shifts the IR and Raman dispersion curves downward appreciably and slightly changes the energy differences between both curves. From an analysis of the harmonic results we conclude that incorporating the coupling between OH stretching motion and the motion of their centers of mass will appreciably change the overall features of the dispersion curves. Extension of the anharmonic model along these lines will cause no problem to the theoretical approach presented in this paper.
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| 7. |
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| 8. |
- Wolf, Matthew J., et al.
(författare)
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Fluorine clusters at CeO2(111) - A DFT+U and Monte Carlostudy
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- STM experiments on CeO2(111) reveal depressions in the surface oxygen sub-lattice which are observed to form clusters of various shapes and sizes [1].While these depressions were assumed to be oxygen vacancies, subsequent DFTcalculations have indicated that clusters of oxygen vacancies are energeticallyunstable [2-4]. Recently, we showed theoretically that fluorine impurities shouldappear almost identical to oxygen vacancies in STM experiments, but that theirproperties are more in line with those of the defects observed in experiments [5].Here, I will present the results of a further investigation into the distribution ofF impurity clusters at CeO2(111), using a combination of DFT+U calculations,and Monte Carlo sampling based on a simple but accurate pair potential whichwas fitted to the DFT results. The distribution is characterised in terms of thenumber of clusters of a certain size, and also on their topology, i.e. whetherthey are compact or open/linea r. Our results compare favourably with theexperiments, and also exhibit some interesting physics in their own right.[1] F. Esch et al., Science 309, 752 (2005).[2] J. Conesa, Cat. Today 143, 315 (2009).[3] C. Zhang et al., Phys. Rev. B 79, 075433 (2009).[4] X.-P. Wu & X.-Q. Gong, Phys. Rev. Lett. 116, 086102 (2016).[5] J. Kullgren, M. J. Wolf et al., Phys. Rev. Lett. 112, 156102 (2014).
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