A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes

The nucleation of carbon nanotubes on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. This technique closely follows the conditions of the synthesis of carbon nanotubes by chemical vapor deposition. The possible formation of a carbon cap on the catalyst particle is studied as a function of the carbon chemical potential, for particles of different size, either crystalline or disordered. We show that these parameters strongly influence the structure of the cap/particle interface which in turn will have a strong effect on the control of the structure of the nanotube. In particular, we discuss the presence of carbon on surface or in subsurface layers

Quantum Simulations of One-Dimensional Nanostructures under Arbitrary Deformations

A powerful technique is introduced for simulating mechanical and electromechanical properties of one-dimensional nanostructures under arbitrary combinations of bending, twisting, and stretching. The technique is based on a novel control of periodic symmetry, which eliminates artifacts due to deformation constraints and quantum finite-size effects, and allows transparent electronic structure analysis. Via density-functional tight-binding implementation, the technique demonstrates its utility by predicting novel electromechanical properties in carbon nanotubes and abrupt behavior in the structural yielding of Au7 and MoS nanowires. The technique drives simulations markedly closer to the realistic modeling of these slender nanostructures under experimental conditions.Comment: 9 pages, 8 figures, 1 vide

Structure, stability and stress properties of amorphous and nanostructured carbon films

Structural and mechanical properties of amorphous and nanocomposite carbon are investigated using tight-binding molecular dynamics and Monte Carlo simulations. In the case of amorphous carbon, we show that the variation of sp^3 fraction as a function of density is linear over the whole range of possible densities, and that the bulk moduli follow closely the power-law variation suggested by Thorpe. We also review earlier work pertained to the intrinsic stress state of tetrahedral amorphous carbon. In the case of nanocomposites, we show that the diamond inclusions are stable only in dense amorphous tetrahedral matrices. Their hardness is considerably higher than that of pure amorphous carbon films. Fully relaxed diamond nanocomposites possess zero average intrinsic stress.Comment: 10 pages, 6 figure

Triggering one dimensional phase transition with defects at the graphene zigzag edge

One well-known argument about one dimensional(1D) system is that 1D phase transition at finite temperature cannot exist, despite this concept depends on conditions such as range of interaction, external fields and periodicity. Therefore 1D systems usually have random fluctuations with intrinsic domain walls arising which naturally bring disorder during transition. Herein we introduce a real 1D system in which artificially created defects can induce a well-defined 1D phase transition. The dynamics of structural reconstructions at graphene zigzag edges are examined by in situ aberration corrected transmission electron microscopy (ACTEM). Combined with an in-depth analysis by ab-initio simulations and quantum chemical molecular dynamics (QM/MD), the complete defect induced 1D phase transition dynamics at graphene zigzag edge is clearly demonstrated and understood on the atomic scale. Further, following this phase transition scheme, graphene nanoribbons (GNR) with different edge symmetries can be fabricated, and according to our electronic structure and quantum transport calculations, a metal-insulator-semiconductor transition for ultrathin GNRs is proposed.Comment: 6 pages, 4 figure

A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes [Working paper]

The nucleation of carbon nanotubes on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. This technique closely follows the conditions of the synthesis of carbon nanotubes by chemical vapor deposition. The possible formation of a carbon cap on the catalyst particle is studied as a function of the carbon chemical potential, for particles of different size, either crystalline or disordered. We show that these parameters strongly influence the structure of the cap/particle interface which in turn will have a strong effect on the control of the structure of the nanotube. In particular, we discuss the presence of carbon on surface or in subsurface layers

Molecular Dynamics Simulation of Chemical Vapor Deposition of Amorphous Carbon: Dependence on H/C Ratio of Source Gas

By molecular dynamics simulation, the chemical vapor deposition of amorphous carbon onto graphite and diamond surfaces was studied. In particular, we investigated the effect of source H/C ratio, which is the ratio of the number of hydrogen atoms to the number of carbon atoms in a source gas, on the deposition process. In the present simulation, the following two source gas conditions were tested: one was that the source gas was injected as isolated carbon and hydrogen atoms, and the other was that the source gas was injected as hydrocarbon molecules. Under the former condition, we found that as the source H/C ratio increases, the deposition rate of carbon atoms decreases exponentially. This exponential decrease in the deposition rate with increasing source H/C ratio agrees with experimental data. However, under the latter molecular source condition, the deposition rate did not decrease exponentially because of a chemical reaction peculiar to the type of hydrocarbon in the source gas.Comment: accepted by Jpn. J. Appl. Phys. (2008

Tailoring Plasmon-Enhanced Light-Matter Interaction

Plasmons are the collective oscillation of free electrons in materials. They concentrate light into nanoscale volumes and trigger optical processes in nearby materials. My thesis is devoted to the understanding of optical processes that are mediated by localized surface plasmons. The fundamental excitation of plasmonic modes and the enhancement of optical absorption and Raman scattering in nanoscale materials are studied using experimental and theoretical approaches. I introduce a novel type of plasmonic excitation in layered films of metallic nanoparticles. Because of field retardation, incident light induces antiparallel dipoles in adjacent layers of metallic nanoparticles exciting a dark interlayer plasmon. It benefits from reduced radiative damping and efficient light absorption as I demonstrate with simulations and experiments. The self-assembled nanoparticle films pave the way for large-area coatings with tunable plasmon resonances. An application is the decay of plasmons into hot charge carriers that trigger photocatalytic reactions in molecules. I propose dark interlayer plasmons as ideal excitation channels for hot electrons because of their small radiative damping. Using plasmonic nanostructures for photodetection and sensing requires an understanding of the interaction with adjacent materials. I introduce microscopic theories for the enhancement of optical absorption and Raman scattering by localized surface plasmons. The plasmonic near field of nanoparticle arrays induced non-vertical optical transitions in graphene in dependence of the periodicity of the plasmonic lattice. For plasmon-enhanced Raman scattering I developed a general theoretical framework using perturbation theory. It provides analytic expressions for the enhanced Raman cross section. In a molecular dipole coupled to a plasmonic nanoparticle the enhancement is strongly affected by interference between different scattering channels. Plasmon-enhanced Raman scattering is an ideal tool to study the properties of materials interfaced with plasmonic nanostructures. I analyzed nanoscale strain and doping in graphene on top of a gold nanostructure. I developed a method for separating the contributions from strain and doping in the Raman spectrum of graphene, which is applicable to graphene on arbitrary substrates and in arbitrary strain configurations.Ziel dieser Arbeit ist es, ein besseres Verständnis von optischen Prozessen zu erlangen, die durch lokalisierte Oberflächenplasmonen gesteuert werden. Dafür habe ich grundlegende Anregungsmechanismen von Plasmonmoden, sowie die Verstärkung von optischer Absorption und Ramanstreuung, mit experimentellen und theoretischen Methoden untersucht. Ich stelle eine neuartige plasmonische Anregung in geschichteten Filmen von metallischen Nanopartikeln vor. Dieses dunkle Plasmon besteht aus antiparallelen plasmonischen Dipolen in den Nanopartikeln benachbarter Lagen und kann aufgrund von Feldretardierung direkt mit Licht angeregt werden. Ich zeige mit Experimenten und Simulationen, dass diese Anregung eine reduzierte Strahlungsdämpfung aufweist und zu einer ausgeprägten, durchstimmbaren Lichtabsorption im nahinfraroten Spektralbereich führt. Da die Nanopartikelfilme mittels Selbstorganisation von Nanopartikeln hergestellt werden können, eignen sie sich für die großflächige Beschichtung von Oberflächen. Aufgrund der unterdrückten radiativen Dämpfung sind dunkle Plasmonen in Nanopartikelfilmen ein idealer Anregungskanal für heiße Elektronen, mit Anwendungen in der Fotokatalyse. Mit mikroskopischen Theorien habe ich die Interaktion von plasmonischen Nanostrukturen mit angrenzenden Nanomaterialien untersucht. Ich zeige, dass das plasmonische Nahfeld eines Gitters von Goldnanopartikeln nicht-vertikale optische Übergänge in Graphen anregt. Die Auswahlregeln für diese Übergänge hängen von der Periodizität der plasmonischen Nanostruktur ab. Die mikroskopische Theorie führt zu einem besseren Verständnis der Photostromentstehung in Graphen-basierten optoelektronischen Detektoren. Als Zweites stelle ich ein allgemeines Konzept zur Beschreibung von plasmon-verstärkter Ramanstreuung mit Störungstheorie vor. Die analytischen Ausdrücke aus dieser Theorie eignen sich, um die Abhängigkeit der plasmonischen Verstärkung von der Anregungsenergie zu untersuchen. Mittels einer Implementierung für ein Molekül nahe eines plasmonischen Nanopartikels zeige ich, dass die Verstärkung stark von der Interferenz verschiedener Streuprozesse beeinflusst wird. Plasmon-verstärkte Ramanstreuung ist ideal, um zu untersuchen, wie Materialeigenschaften von einer angrenzenden plasmonischen Nanostruktur beeinflusst werden. Das zeige ich für Materialverspannungen und Dotierung in Graphen durch eine Gold-Nanostruktur. Ich habe dafür eine allgemeine Methodik entwickelt, mit der die Beiträge von Verspannung und Dotierung zum Ramanspektrum von Graphen voneinander getrennt und quantifiziert werden können. Diese eignet sich zur Auswertung von unbekannten Verspannungs-Konfigurationen in Graphen auf verschiedensten Substraten