### Browsing by Author "Gulans, Andris"

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Item 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 ScienceSeveral 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.Item 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 PhysicsSeveral 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.Item Bound and free self-interstitial defects in graphite and bilayer graphene: A computational study(American Physical Society (APS), 2011) Gulans, Andris; Krasheninnikov, Arkady V.; Puska, Martti J.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe role of self-interstitials in the response of layered carbon materials such as graphite, bilayer graphene and multiwalled carbon nanotubes to irradiation has long remained a puzzle. Using density-functional-theory methods with an exchange and correlation functional which takes into account the interlayer van der Waals interaction in these systems without any material-specific empirical parameters, we study the energetics and migration of single- and di-interstitials in graphite and bilayer graphene. We show that two classes of interstitials, “bound” and “free,” can coexist. The latter are mobile at room and lower temperatures, which explains the experimental data and reconciles them with the results of atomistic simulations. Our results shed light on the behavior of graphite and carbon nanotubes under irradiation and have implications for irradiation-mediated processing of bilayer graphene.Item Electronic structure of boron nitride sheets doped with carbon from first-principles calculations(American Physical Society (APS), 2013) Berseneva, Natalia; Gulans, Andris; Krasheninnikov, Arkady V.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceUsing density functional theory with local and non-local exchange and correlation (XC) functionals, as well as the Green's function quasiparticle (GW) approach, we study the electronic structure of hexagonal boron nitride (h-BN) sheets, both pristine and doped with carbon. We show that the fundamental band gap in two-dimensional h-BN is different from the gap in the bulk material, and that in the GW calculations the gap depends on the interlayer distance (separation between the images of the BN layers within the periodic supercell approach) due to the non-local nature of the GW approximation, so that the results must be extrapolated to infinitely large separations between the images. We further demonstrate by the example of carbon substitutional impurities that the local and hybrid XC functionals give a qualitatively correct picture of the impurity states in the gap. Finally, we address the effects of many important parameters, such as the choice of chemical potential, and atom displacement cross sections for the substitutional process during electron-beam-mediated doping of h-BN sheets with carbon atoms. Our results shed light on the electronic structure of pristine and doped h-BN and should further help to optimize the postsynthesis doping of boron nitride nanostructures stimulated by electron irradiation.Item Linear-scaling self-consistent implementation of the van der Waals density functional(American Physical Society (APS), 2009) Gulans, Andris; Puska, Martti J.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceAn efficient linear-scaling approach to the van der Waals density functional in electronic-structure calculations is demonstrated. The nonlocal correlation potential needed in self-consistent calculations is derived in a practical form. This enables also an efficient determination of the Hellmann-Feynman forces on atoms. The numerical implementation employs adaptive quadrature grids in real space resulting in a fast and an accurate evaluation of the functional and the potential. The approach is incorporated in the atomic orbital code SIESTA. The application of the method to the S22 set of noncovalently bonded molecules and comparison with the quantum chemistry data reveal an overall agreement but show that different exchange functionals should be used for different types of bonds.Item Linear-scaling self-consistent implementation of the van der Waals density functional,(2009-05-13) Gulans, Andris; Puska, Martti; Nieminen, Risto M.; Department of Applied Physics; Electronic Properties of MaterialsAn efficient linear-scaling approach to the van der Waals density functional in electronic-structure calculations is demonstrated. The nonlocal correlation potential needed in self-consistent calculations is derived in a practical form. This enables also an efficient determination of the Hellmann-Feynman forces on atoms. The numerical implementation employs adaptive quadrature grids in real space resulting in a fast and an accurate evaluation of the functional and the potential. The approach is incorporated in the atomic orbital code SIESTA. The application of the method to the S22 set of noncovalently bonded molecules and comparison with the quantum chemistry data reveal an overall agreement but show that different exchange functionals should be used for different types of bonds.Item Time-frequency component of the GreenX library: minimax grids for efficient RPA and GW calculations(Open Journals, 2023-10-03) Azizi, Maryam; Wilhelm, Jan; Golze, Dorothea; Giantomassi, Matteo; Panadés-Barrueta, Ramón L.; Delesma, Francisco A.; Buccheri, Alexander; Gulans, Andris; Rinke, Patrick; Draxl, Claudia; Gonze, Xavier; Department of Applied Physics; Computational Electronic Structure Theory; Université Catholique de Louvain; University of Regensburg; TU Dresden; Humboldt University of Berlin; University of LatviaItem van der Waals Bonding in Layered Compounds from Advanced Density-Functional First-Principles Calculations(American Physical Society (APS), 2012) Björkman, Torbjörn; Gulans, Andris; Krasheninnikov, Arkady V.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; COMP Centre of Excellence; COMP Centre of Excellence; Perustieteiden korkeakoulu; School of ScienceAlthough the precise microscopic knowledge of van der Waals interactions is crucial for understanding bonding in weakly bonded layered compounds, very little quantitative information on the strength of interlayer interaction in these materials is available, either from experiments or simulations. Here, using many-body perturbation and advanced density-functional theory techniques, we calculate the interlayer binding and exfoliation energies for a large number of layered compounds and show that, independent of the electronic structure of the material, the energies for most systems are around 20 meV/Å2. This universality explains the successful exfoliation of a wide class of layered materials to produce two-dimensional systems, and furthers our understanding the properties of layered compounds in general.