Browsing by Author "Kaukonen, M."
Now showing 1 - 12 of 12
Results Per Page
Sort Options
Item Atomic-scale modeling of the ion-beam-induced growth of amorphous carbon(American Physical Society (APS), 2000) Kaukonen, M.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe results of a detailed molecular-dynamics study of the growth of amorphous carbon (a−C) are reported. Carbon atoms with kinetic energies between 10 and 150 eV are deposited on a−C surface originating from bulk a−C. Earlier simulation results of an optimal energy window at 40–70 eV are confirmed. Additionally, it is found that the growth rate is at maximum at around 40 eV. At low implantation energies (Ebeam≈10 eV), the growth of amorphous carbon takes place on the surface. At higher energies, the growth proceeds increasingly in the subsurface region by global film expansion and single atom diffusion towards the surface. Scattering events (e.g., the deposited atom does not adsorb to the surface) at intermediate energies Ebeam≈100 eV result in a densification of the growing film. Moreover, at Ebeam≈150 eV, nonpermanent diamond formation is observed.Item Dynamic relaxation of the elastic properties of hard carbon films(AIP Publishing, 1997) Hirvonen, J.-P.; Koskinen, J.; Kaukonen, M.; Nieminen, Risto M.; Scheibe, H.-J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe effect of enhanced atomic mobility on the growth of hard carbon films was examined. Tetrahedrally bonded amorphous carbon films were deposited by condensing energetic carbon ions using an arc-discharge deposition method. The deposition temperature varied between 50 and 400 °C. The dependence of elastic properties on deposition temperature was examined by determining the frequency-dependent propagation velocity of ultrasonic surface acoustic waves induced by a laser. A remarkable decrease in elastic coefficient was revealed above the deposition temperature of 300 °C and complete relaxation was obtained at 400 °C. This observation was analyzed by using a simple model which was in turn supported by molecular dynamics simulations. The relaxation turns out to be a thermally activated, dynamic process with an activation energy of 0.57 eV. Possible relaxation mechanisms associated with the migration of atoms or defects on a growing surface are discussed.Item Effect of N and B doping on the growth of CVD diamond (100):H(2×1) surfaces(1998) Kaukonen, M.; Sitch, P.K.; Jungnickel, G.; Nieminen, R.M.; Pöykkö, S.; Porezag, D.; Franheim, Th.; Department of Applied PhysicsThe doping of the chemical vapor deposition (CVD)-diamond (100):H(2×1) surface with B and N has been studied using the density-functional tight-binding method. In agreement with recent experimental results, B doping is found to lower the abstraction energies and remove diffusion barriers along the diamond growth pathway proposed by Harris and Goodwin [J. Phys. Chem. 97, 23 (1993)]. In contrast, the Harris-Goodwin mechanism is less favorable with N doping, casting doubt on its validity in this case. We therefore propose a growth pathway on N-doped CVD diamond (100):H(2×1) surfaces. This involves a dimer opening reaction and requires less H abstraction reactions compared to the Harris-Goodwin mechanism.Item Growth of (110) diamond using pure dicarbon(American Physical Society (APS), 2001) Sternberg, M.; Kaukonen, M.; Nieminen, Risto M.; Frauenheim, Th.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe use a density-functional-based tight-binding method to study diamond growth steps by depositing dicarbon species onto a hydrogen-free diamond (110) surface. Subsequent C2 molecules are deposited on an initially clean surface, in the vicinity of a growing adsorbate cluster, and finally near vacancies just before completion of a full new monolayer. The preferred growth stages arise from C2n clusters in near ideal lattice positions forming zigzag chains running along the [1¯10] direction parallel to the surface. The adsorption energies are consistently exothermic by 8–10 eV per C2, depending on the size of the cluster. The deposition barriers for these processes are in the range of 0.0–0.6 eV. For deposition sites above C2n clusters the adsorption energies are smaller by 3 eV, but diffusion to more stable positions is feasible. We also perform simulations of the diffusion of C2 molecules on the surface in the vicinity of existing adsorbate clusters using a constrained conjugate gradient method. We find migration barriers in excess of 3 eV on the clean surface, and 0.6–1.0 eV on top of graphenelike adsorbates. The barrier heights and pathways indicate that the growth from gaseous dicarbon proceeds either by direct adsorption onto clean sites or after migration on top of the existing C2n chains.Item Locally activated Monte Carlo method for long-time-scale simulations(American Physical Society (APS), 2000) Kaukonen, M.; Peräjoki, J.; Nieminen, Risto M.; Jungnickel, G.; Frauenheim, Th.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe present a technique for the structural optimization of atom models to study long time relaxation processes involving different time scales. The method takes advantage of the benefits of both the kinetic Monte Carlo (KMC) and the technimolecular dynamics simulation techniques. In contrast to ordinary KMC, our method allows for an estimation of a true lower limit for the time scale of a relaxation process. The scheme is fairly general in that neither the typical pathways nor the typical metastable states need to be known prior to the simulation. It is independent of the lattice type and the potential which describes the atomic interactions. It is adopted to study systems with structural and/or chemical inhomogeneity which makes it particularly useful for studying growth and diffusion processes in a variety of physical systems, including crystalline bulk, amorphous systems, surfaces with adsorbates, fluids, and interfaces. As a simple illustration we apply the locally activated Monte Carlo to study hydrogen diffusion in diamond.Item 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 ScienceWe 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.Item Nitrogen Doping of Amorphous Carbon Surfaces(American Physical Society (APS), 1999) Kaukonen, M.; Nieminen, Risto M.; Pöykkö, S.; Seitsonen, Ari P.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe surface properties of amorphous carbon ( a−C) are studied using first-principles electronic structure methods. The effect of nitrogen doping near the surface and, in particular, the effect of nitrogen on the work function is studied by doing a series of nitrogen substitutions near the surface. It is found that the work function is reduced by nitrogen doping of the a−C surface at “on top of the surface” sp1 and sp2 sites. Nitrogen doping by low energy ion bombardment is suggested as a doping method to minimize work function of the a−C surfaces.Item Nitrogen-doped carbon nanotubes under electron irradiation simulated with a tight-binding model(American Physical Society (APS), 2006) Loponen, T.; Krasheninnikov, A. V.; Kaukonen, M.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceExperiments show that nitrogen-doped carbon nanotubes subjected to the electron beam in a transmission electron microscope can easily lose dopant atoms and that overall they are less stable under electron irradiation than the pristine tubes. To understand the lower stability of nitrogen-doped nanotubes we use a density-functional-theory-based tight-binding model and simulate impacts of energetic electrons onto the nanotubes. We show that the dopant atom displacement energy and thus the electron threshold energy is lower for nanotubes with smaller diameter and that, independent of the nanotube diameter, the dopant nitrogen atoms can be displaced more easily than the host carbon atoms. Our results set a limit on the threshold electron energy for damage production in N-doped tubes and indicate that spatially localized electron irradiation of doped nanotubes can be used for local atomic and band structure engineering.Item 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 UniversityWe 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.Item 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 ScienceWe 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.Item 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 ScienceRecent 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.Item Structures of Thermal Double Donors in Silicon(American Physical Society (APS), 2000) Pesola, M.; Joo Lee, Young; von Boehm, J.; Kaukonen, M.; Nieminen, Risto M.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceAccurate total-energy calculations are used to study the structures and formation energies of oxygen chains as models for thermal double donors (TDD's) in Si. We find that the first three TDD's (TDD0–TDD2) consist of one four-member ring, with one or two adjacent interstitial O atoms. These metastable TDD's form bistable negative-U systems with the corresponding stable, electrically inactive staggered structures. The TDD3-TDD7 structures are found to consist of four-member rings with adjacent interstitial O atoms at both ends. The TDD's with a central “di-Y-lid” core are found to become energetically competitive with the four-member ring TDD's only for clusters larger than ten O atoms.
