Browsing by Author "Pakarinen, O. H."

Now showing 1 - 7 of 7
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    First principles study of adsorption, diffusion and charge stability of metal adatoms on alkali halide surfaces
    (2008) Hakala, M. H.; Pakarinen, O. H.; Foster, A. S.; Department of Applied Physics
    In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.
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    First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces
    (American Physical Society (APS), 2008) Hakala, M. H.; Pakarinen, O. H.; Foster, Adam S.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.
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    High-resolution scanning force microscopy of gold nanoclusters on the KBr (001) surface
    (American Physical Society (APS), 2006) Pakarinen, O. H.; Barth, C.; Foster, Adam S.; Nieminen, Risto M.; Henry, C. R.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    In this study we use a combination of dynamic scanning force microscopy experiments and first-principles simulations to study the imaging process of gold nanoclusters adsorbed on the (001) surface of KBr. In previous experiments atomic resolution was readily obtained on the KBr substrate. However, it was not possible to resolve atoms within the clusters themselves. This correlates with imaging simulations we present here using several different probable tip models: measurable contrast was readily achieved on the KBr surface and on the gold (001) surface, but simulations on the clusters demonstrated poor contrast for all tips. We further consider the role of cluster charging in the tip-surface interaction, and the role that surface defects play in the properties of adsorbed clusters.
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    Role of van der Waals forces in the adsorption and diffusion of organic molecules on an insulating surface
    (American Physical Society (APS), 2009) Pakarinen, O. H.; Mativetsky, J. M.; Gulans, A.; Puska, Martti J.; Foster, Adam S.; Grutter, P.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science
    The adsorption and diffusion of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on a nanostructured KBr (001) surface were investigated by combining noncontact atomic force microscopy (NC-AFM) and first-principles calculations. Atomically resolved measurements demonstrate trapping of PTCDA molecules in intentionally created rectangular monolayer-deep substrate pits and a preferential adsorption at kink sites. In order to understand the experimental results, we found that it was essential to include a first-principles treatment of the van der Waals interactions. We show that at some sites on the surface, 85% of the molecular binding is provided by van der Waals interactions, and in general it is always the dominant contribution to the adsorption energy. It also qualitatively changes molecular diffusion on the surface. Based on the specificity of the molecular interaction at kink sites, the species of the imaged ionic sublattice in the NC-AFM measurements could be identified.
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    Simulating atomic force microscopy imaging of the ideal and defected TiO2 (110) surface
    (American Physical Society (APS), 2003) Foster, Adam S.; Pakarinen, O. H.; Airaksinen, J. M.; Gale, J. D.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    In this study we simulate noncontact atomic force microscopy imaging of the TiO2 (110) surface using first-principles and atomistic methods. We use three different tip models to investigate the tip-surface interaction on the ideal surface, and find that agreement with experiment is found for either a silicon tip or a tip with a net positive electrostatic potential from the apex. Both predict bright contrast over the bridging oxygen rows. We then study the interaction of this tip with a bridging oxygen vacancy on the surface, and find that the much weaker interaction observed would result in vacancies appearing as dark contrast along the bright rows in images.
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    The so-called dry laser cleaning governed by humidity at the nanometer scale
    (AIP Publishing, 2008) Grojo, D.; Delaporte, Ph.; Sentis, M.; Pakarinen, O. H.; Foster, Adam S.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    Illumination with single nanosecond pulses leads to the detachment of silica particles with 250nm radii from siliconsurfaces. We identify two laser-energy dependent cleaning regimes by time-of-flight particle-scattering diagnostics. For the higher energies, the ejection of particles is produced by nanoscale ablation due to the laser field enhancement at the particle-surface interface. The damage-free regime at lower energy is shown to be governed by the residual water molecules, which are inevitably trapped on the materials. We discuss the great importance that the humidity plays on the cleaning force and on the adhesion in the experiments.
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    Towards chemical identification in atomic-resolution noncontact AFM imaging with silicon tips
    (American Physical Society (APS), 2003) Foster, Adam S.; Gal, A. Y.; Airaksinen, J. M.; Pakarinen, O. H.; Lee, Y. J.; Gale, J. D.; Shluger, A. L.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    In this study we use ab initio calculations and a pure silicon tip to study the tip-surface interaction with four characteristic insulating surfaces: (i) the narrow gap TiO2 (110) surface, (ii) the classic oxide MgO (001) surface, (iii) the ionic solid CaCO3 (101¯4) surface with molecular anion, and (iv) the wide gap CaF2 (111) surface. Generally we find that the tip-surface interaction strongly depends on the surface electronic structure due to the dominance of covalent bond formation with the silicon tip. However, we also find that in every case the strongest interaction is with the highest anion of the surface. This result suggests that, if the original silicon tip can be carefully controlled, it should be possible to immediately identify the species seen as bright in images of insulating surfaces. In order to provide a more complete picture we also compare these results to those for contaminated tips and suggest how applied voltage could also be used to probe chemical identity.