Browsing by Author "Johnston, Karen"

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    Adsorption structures of phenol on the Si (001)-(2 × 1) surface calculated using density functional theory
    (American Physical Society (APS), 2010) Johnston, Karen; Gulans, Andris; Verho, Tuukka; Puska, Martti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    Several dissociated and two nondissociated adsorption structures of the phenol molecule on the Si(001)-(2×1) surface are studied using density functional theory with various exchange and correlation functionals. The relaxed structures and adsorption energies are obtained and it is found that the dissociated structures are energetically more favorable than the nondissociated structures. However, the ground state energies alone do not determine which structure is obtained experimentally. To elucidate the situation core level shift spectra for Si 2p and C 1s states are simulated and compared with experimentally measured spectra. Several transition barriers were calculated in order to determine, which adsorption structures are kinetically accessible. Based on these results we conclude that the molecule undergoes the dissociation of two hydrogen atoms on adsorption.
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    Adsorption structures of phenol on the Si(001)-(2×1) surface calculated using density functional theory
    (2010-06-21) Johnston, Karen; Gulans, Andris; Verho, Tuukka; Puska, Martti J.; Department of Applied Physics
    Several dissociated and two nondissociated adsorption structures of the phenol molecule on the Si(001)−(2×1) surface are studied using density functional theory with various exchange and correlation functionals. The relaxed structures and adsorption energies are obtained and it is found that the dissociated structures are energetically more favorable than the nondissociated structures. However, the ground state energies alone do not determine which structure is obtained experimentally. To elucidate the situation core level shift spectra for Si 2p and C 1s states are simulated and compared with experimentally measured spectra. Several transition barriers were calculated in order to determine, which adsorption structures are kinetically accessible. Based on these results we conclude that the molecule undergoes the dissociation of two hydrogen atoms on adsorption.
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    Computational study of (111) epitaxially strained ferroelectric perovskites BaTiO3 and PbTiO3
    (American Physical Society (APS), 2008) Oja, Riku; Johnston, Karen; Frantti, Johannes; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    The phase transition behavior of PbTiO3 and BaTiO3 under (111) epitaxial strain is investigated using density-functional theory calculations. From tensile strains of +0.015 to compressive strains of −0.015, PbTiO3 undergoes phase transitions from C2 through two Cm phases and then to R3m. The total polarization is found to be almost independent of strain. For the same range of strains BaTiO3 undergoes phase transitions from a single Cm phase, through R3m and then to R3¯m. In this case the application of compressive strain inhibits and then completely suppresses the polarization on transition to the nonpolar R3¯m phase.
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    Influence of van der Waals forces on the adsorption structure of benzene on silicon studied using density functional theory
    (American Physical Society (APS), 2008) Johnston, Karen; Kleis, Jesper; Lundqvist, Bengt I.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    Two different adsorption configurations of benzene on the Si(001)−(2×1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure.
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    Polymer adhesion
    (2007-08-02) Johnston, Karen; Nieminen, Risto M.; Department of Applied Physics
    The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si (001) - (2×1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid, H2 C O3, and propane, C3 H8, dissociate on contact with the surface. Passivation of the Si surface with H atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.
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    Polymer adhesion: First-principles calculations of the adsorption of organic molecules onto Si surfaces
    (American Physical Society (APS), 2007) Johnston, Karen; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si(001)−(2×1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid, H2CO3, and propane, C3H8, dissociate on contact with the surface. Passivation of the Si surface with H atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.