Browsing by Author "Astala, R."

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    Nanoindentation of silicon surfaces: Molecular-dynamics simulations of atomic force microscopy
    (American Physical Society (APS), 2000) Astala, R.; Kaukonen, M.; Nieminen, Risto M.; Heine, T.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    We investigate the atomic-scale details of atomic force microscopy through a quasistatic molecular dynamics simulation together with a density-functional-based tight-binding method. The changes in the AFM tip shape, the size of the tip-sample contact area, as well as the microscopic hardness and Young’s moduli of silicon {111},{110},{100} surfaces are studied. Furthermore, the effects of hydrogen termination of the surface and of subsurface vacancies on hardness and Young’s modulus are discussed.
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    Properties of small carbon clusters inside the C60 fullerene
    (2002-06-15) Astala, R.; Kaukonen, M.; Nieminen, Risto; Jungnickel, G.; Frauenheim, T.; Aalto University; Department of Applied Physics; Paderborn University
    We present the results of an atomic-scale simulation of the confinement of small carbon clusters inside icosahedral C60 fullerene. We carefully investigate the incorporation of various clusters into C60 including chains, rings, and double ring configurations, and have analyzed both the energetics and the resulting geometries. The calculations have been performed employing the density-functional-based tight-binding methodology within the self-consistent charge representation. We find that certain carbon cluster configurations that are unstable as free molecules become stabilized inside C60. By adding single atoms into random positions inside the fullerene shell we establish an upper limit for the filling of C60 with carbon. When the number of atoms inside the fullerene exceeds ten we observe bonding to the surrounding shell and, hence, a gradual transition of the fullerene towards an sp3 rich but locally disordered carbon system.
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    Properties of small carbon clusters inside the C60 fullerene
    (American Physical Society (APS), 2002) Astala, R.; Kaukonen, M.; Nieminen, Risto M.; Jungnickel, G.; Frauenheim, T.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    We present the results of an atomic-scale simulation of the confinement of small carbon clusters inside icosahedral C60 fullerene. We carefully investigate the incorporation of various clusters into C60 including chains, rings, and double ring configurations, and have analyzed both the energetics and the resulting geometries. The calculations have been performed employing the density-functional-based tight-binding methodology within the self-consistent charge representation. We find that certain carbon cluster configurations that are unstable as free molecules become stabilized inside C60. By adding single atoms into random positions inside the fullerene shell we establish an upper limit for the filling of C60 with carbon. When the number of atoms inside the fullerene exceeds ten we observe bonding to the surrounding shell and, hence, a gradual transition of the fullerene towards an sp3 rich but locally disordered carbon system.
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    Simulations of diamond nucleation in carbon fullerene cores
    (American Physical Society (APS), 2001) Astala, R.; Kaukonen, M.; Nieminen, Risto M.; Jungnickel, G.; Frauenheim, Th.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    Recent experiments have shown that heavy ion or electron irradiation induces the nucleation of diamond crystallites inside concentric nested carbon fullerenes, i.e., bucky onions. This suggests that the fullerene acts as a nanoscopic pressure shell. In this paper we study the formation of tetrahedrally bonded carbon inside a prototype icosahedral two-shell fullerene by means of atomic-scale computer simulations. After the simulated irradiation, we can identify regions in which almost all carbon atoms become sp3 bonded. Additionally, we observe a counteracting tendency for the carbon atoms to form shell-like substructures. To shift the balance between these two processes towards diamond nucleation strongly nonequilibrium conditions are required.