Prediction of giant electroactuation for papyruslike carbon nanoscroll structures: First-principles calculations

We study by first-principles calculations the electromechanical response of carbon nanoscroll structures. We show that although they present a very similar behavior to carbon nanotubes in their axial deformation sensitivity, they exhibit a radial response upon charge injection which is up to one order of magnitude larger. In association with their high stability, this behavior makes them a natural choice for a new class of very efficient nanoactuators.74

Rotational dynamics and polymerization of C(60) in C(60)-cubane crystals: A molecular dynamics study

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)We report classical and tight-binding molecular dynamics simulations of the C(60) fullerene and cubane molecular crystal in order to investigate the intermolecular dynamics and polymerization processes. Our results show that, for 200 and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations, while C(60) fullerenes show rotational motions. Fullerenes perform "free" rotational motions at short times (less than or similar to 1 ps), small amplitude hindered rotational motions (librations) at intermediate times, and rotational diffusive dynamics at long times (greater than or similar to 10 ps). The mechanisms underlying these dynamics are presented. Random copolymerizations among cubanes and fullerenes were observed when temperature is increased, leading to the formation of a disordered structure. Changes in the radial distribution function and electronic density of states indicate the coexistence of amorphous and crystalline phases. The different conformational phases that cubanes and fullerenes undergo during the copolymerization process are discussed.1296IMMP/MCTIN/MCTTHEO-NANOConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Surface morphologies in GaAs homoepitaxy: Mound formation and evolution

Atomic force microscopy has been used to observe surface morphologies during growth of GaAs films on GaAs(001) by chemical beam epitaxy. Mound formation is observed at the beginning of GaAs growth as a function of the surface prior to deposition. GaAs substrates exhibit a large density of pits and cracks after usual thermal treatment employed for oxide desorption. On this kind of surface mounds form and coalesce as film thickness increases; surface planarization is eventually achieved-at this point, morphologies are typically those expected from two-dimensional growth. In this sense we observe that monolayer island size distribution is determined by the kinetic conditions used for the growth; nucleation sites and island spatial distribution, however, are strongly influenced by the topography of the initial surface where the film is deposited even for films thousands of monolayers thick. The final morphologies present wide terraces and few monolayer islands on top of them independent of growth conditions. This picture agrees with theoretical results where negligible step edge barriers are considered.5841947195

Elastic properties of graphyne-based nanotubes

Graphyne nanotubes (GNTs) are nanostructures obtained from rolled up graphyne sheets, in the same way carbon nanotubes (CNTs) are obtained from graphene ones. Graphynes are 2D carbon-allotropes composed of atoms in sp and sp2 hybridized states. Similarly to conventional CNTs, GNTs can present different chiralities and electronic properties. Because of the acetylenic groups (triple bonds), GNTs exhibit large sidewall pores that influence their mechanical properties. In this work, we studied the mechanical response of GNTs under tensile stress using fully atomistic molecular dynamics simulations and density functional theory (DFT) calculations. Our results show that GNTs mechanical failure (fracture) occurs at larger strain values in comparison to corresponding CNTs, but paradoxically with smaller ultimate strength and Young's modulus values. This is a consequence of the combined effects of the existence of triple bonds and increased porosity/flexibility due to the presence of acetylenic groups

Modeling the auxetic transition for carbon nanotube sheets

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)A simple model is developed to predict the complex mechanical properties of carbon nanotube sheets (buckypaper) [L. J. Hall et al., Science 320, 504 (2008)]. Fabricated using a similar method to that deployed for making writing paper, these buckypapers can have in-plane Poisson's ratios changed from positive to negative, becoming auxetic, as multiwalled carbon nanotubes are increasingly mixed with single-walled carbon nanotubes. Essential structural features of the buckypapers are incorporated into the model: isotropic in-plane mechanical properties, nanotubes preferentially oriented in the sheet plane, and freedom to undergo stress-induced elongation by both angle and length changes. The expressions derived for the Poisson's ratios enabled quantitative prediction of both observed properties and remarkable new properties obtainable by structural modification.7811National Science Foundation [DMI-0609115]Air Force Office of Scientific Research [FA9550-05-C-0088]Lintec CorporationFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)National Science Foundation [DMI-0609115]Air Force Office of Scientific Research [FA9550-05-C-0088

Geometric and electronic structure of carbon nanotube networks: 'super'-carbon nanotubes

Structures of the so-called super-carbon nanotubes are proposed. These structures are built from single walled carbon nanotubes connected by Y-like junctions forming a 'super'-sheet that is then rolled into a seamless cylinder. Such a procedure can be repeated several times, generating a fractal structure. This procedure is not limited to carbon nanotubes, and can be easily modified for application to other systems. Tight binding total energy and density of states calculations showed that the 'super'-sheets and tubes are stable and predicted to present metallic and semiconducting behaviour.17361762

Topologically Closed Macromolecules Made of Single Walled Carbon Nanotubes-'Super'-Fullerenes

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We propose and theoretically investigated a new class of topologically closed macromolecules built using single walled carbon nanotubes. These macromolecules are based on the fullerene architecture. Classical molecular dynamics simulations were used to predict their stability, thermal, vibrational, and mechanical properties. These macromolecules, named 'super'-fullerenes, present high porosity, low density (similar to 1 g/cm(3)), and high surface area (congruent to 2500 m(2)/g). Our results predict gas phase specific heat of about 0.4 Jg(-1)K(-1) at room temperature and high flexibility under compressive strains. These properties make these hypothetical macromolecules good candidates for gas storage material and biomolecular sieves.10743784383Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2007/03923-1