Theoretical Investigation Of Model Polymers For Eumelanins. Ii. Isolated Defects

We investigate here defects in initially ordered polymers of 5,6-indolequinone, in one or other redox form. The defects studied include aggregation of the carboxyl radical into one skeleton monomer, the aggregation of a host monomer in a lateral misplaced position, and faults in the polymerization sequencing. Our study is carried out, as in the first paper I, through Hückel π-electron theory, and results are compared to the perfect structures in I. Our results indicate that the end-type defect suggested as an electron capture center in I is not deactivated by these other structural defects, and that new capture centers might be introduced that could also be responsible for the acceptor behavior of melanins. © 1990 American Institute of Physics.9342848285

Theoretical Investigation Of Model Polymers For Eumelanins. I. Finite And Infinite Polymers

We investigate the electronic structure of ideal ordered polymers of 5,6-indolequinone, in one or other redox form. These molecules are the most abundant constituents of eumelanin, the pigment appearing in human skin. Our study is carried out through Hückel π-electron theory, which allows us to follow the trends in electronic structure from a single monomer - the isolated molecules - to finite polymers of up to 10 units, and to infinite polymers. We have chosen different polymerization directions which produce semiconducting chains. The comparison between finite and infinite polymers is very useful and leads us to propose a model that accounts for some of the known properties of eumelanins. © 1990 American Institute of Physics.9242630263

Polymerization Of 5,6-indolequinone: A View Into The Band Structure Of Melanins

We present for the first time a study of finite and infinite polymers of 5,6-indolequinone. We show that the band structure of the infinite polymer presents semiconductor characteristics, and that the intrinsic paramagnetism of eumelanin can be modeled through electron trapping at deep "end- effect" defects. © 1988 American Institute of Physics.8864088409

Spatially Variable Reaction In The Formation Of Anodically Grown Porous Silicon Structures

In porous silicon formations there is an increase of dissolution rate at the fluorine-covered sites of the silicon surface due to the presence of excess electrons coming from oxidation of molecular hydrogen at the passivated (hydrogen-covered) sites. The dissolution rate increase in the presence of excess charge at the fluorine-covered sites is experimentally measured and a theoretical investigation is carried out by a semiempirical Hartree-Fock calculation. This spatially variable dissolution generates the porous silicon surface. © 1995 American Institute of Physics.78159059

A Parametric Method 3 (pm3) Study Of Trans-stilbene

We report a comparative modified neglect of diatomic overlap (MNDO), Austin method one (AM1), and parametric method 3 (PM3) study of trans-stilbene (tS) in its ground, excited (singlet and triplet), and ionic (positive and negative polarons and bipolarons) states. We have also calculated the barrier for ring rotation about the backbone single bond. Our results show that PM3 geometries are superior to MNDO and AM1, at least for tS. PM3 predicts, in contrast with MNDO, AM1 and even ab initio 3-21G, a coplanar structure for tS, in accordance with recent experimental data. Singlet and triplet energies obtained from heats of formation are in surprisingly good agreement with experimental data. © 1993 American Institute of Physics.9843016302

Rotational Dynamics And Polymerization Of C60 In C60 -cubane Crystals: A Molecular Dynamics Study

We report classical and tight-binding molecular dynamics simulations of the C60 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 C60 fullerenes show rotational motions. Fullerenes perform "free" rotational motions at short times (1 ps), small amplitude hindered rotational motions (librations) at intermediate times, and rotational diffusive dynamics at long times (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. 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Indication Of Unusual Pentagonal Structures In Atomic-size Cu Namwires

A study of the structural and quantum conductance properties of atomic-size copper nanowires generated by mechanical stretching was presented. The time-resolved electron microscopy observations and molecular dynamics simulations were used to derive the atomistic evolution. The quantum transport behavior was analyzed by means of conductance measurements and theoretical calculations. The formation of an unusual and highly pentagonal Cu nanowires with a diameter of ∼0.45 nm and ∼4.5 conductance quanta was also shown.93121261031-126103-4Agraït, N., Yeyati, A.L., Van Ruitenbeek, J.M., (2003) Phys. Rep., 377, p. 81Gulseren, O., Ercolessi, F., Tosatti, E., (1998) Phys. Rev. Lett., 80, p. 3775Kondo, Y., Takayanagi, K., (2000) Science, 289, p. 606Oshima, Y., Onga, A., Takayanagi, K., (2003) Phys. Rev. Lett., 91, p. 205503Ohnishi, H., Kondo, Y., Takayanagi, K., (1998) Nature (London), 395, p. 780Yanson, A.I., (1998) Nature (London), 395, p. 783Rodrigues, V., Fuhrer, T., Ugarte, D., (2000) Phys. Rev. Lett., 85, p. 4124Rego, L.G.C., Rocha, A.R., Rodrigues, V., Ugarte, D., (2003) Phys. Rev. B, 67, p. 045412Rodrigues, V., (2002) Phys. Rev. B, 65, p. 153402Krans, J.M., (1995) Nature (London), 375, p. 767Kondo, Y., Takayanagi, K., (1997) Phys. Rev. Lett., 79, p. 3455Rodrigues, V., Ugarte, D., (2003) Nanowires and Nanobelts, 1, p. 177. , edited by Z. L. Wang Kluwer, DordrechtRodrigues, V., Bettini, J., Silva, P.C., Ugarte, D., (2003) Phys. Rev. Lett., 91, p. 096801Marks, L.D., (1994) Rep. Prog. Phys., 57, p. 603Urban, J., (1998) Cryst. Res. Technol., 33, p. 1009Lisiecki, I., (2000) Phys. Rev. B, 61, p. 4968Emberly, E.G., Kirczenow, G., (1998) Phys. Rev. B, 58, p. 10911(1999) Phys. Rev. B, 60, p. 6028Reinhard, D., (1997) Phys. Rev. Lett., 79, p. 1459Sen, P., (2002) Phys. Rev. B, 65, p. 235433Cleri, F., Rosato, V., (1993) Phys. Rev. B, 48, p. 22Tomànek, D., Aligia, A.A., Balseiro, C.A., (1985) Phys. Rev. B, 32, p. 5051Coura, P.Z., (2004) Nano Lett., 4, p. 1187EPAPS Document No. E-PRLTAO-93-010436, E-PRLTAO-93-010436. , http://www.aip.org/pubservs/epaps.html)orfromftp.aip.orginthedirectory/ epaps, Figure 4 snapshot of a Cu nanowire being elongated along the [110] axis. A direct link to this document may be found in the online article's HTML reference section. The document may also be reached via the EPAPS homepage See the EPAPS homepage for more informatio

Low-density Three-dimensional Foam Using Self-reinforced Hybrid Two-dimensional Atomic Layers

Low-density nanostructured foams are often limited in applications due to their low mechanical and thermal stabilities. Here we report an approach of building the structural units of three-dimensional (3D) foams using hybrid two-dimensional (2D) atomic layers made of stacked graphene oxide layers reinforced with conformal hexagonal boron nitride (h-BN) platelets. The ultra-low density (1/400 times density of graphite) 3D porous structures are scalably synthesized using solution processing method. A layered 3D foam structure forms due to presence of h-BN and significant improvements in the mechanical properties are observed for the hybrid foam structures, over a range of temperatures, compared with pristine graphene oxide or reduced graphene oxide foams. It is found that domains of h-BN layers on the graphene oxide framework help to reinforce the 2D structural units, providing the observed improvement in mechanical integrity of the 3D foam structure. © 2014 Macmillan Publishers Limited.5Reinfried, M., Hybrid foams-A new approach for multifunctional applications (2011) Adv. Eng. Mater., 13, pp. 1031-1036Studart, A.R., Gonzenbach, U.T., Tervoort, E., Gauckler, L.J., Processing routes to macroporous ceramics: A review (2006) Journal of the American Ceramic Society, 89 (6), pp. 1771-1789. , DOI 10.1111/j.1551-2916.2006.01044.xReddy, E.S., Schmitz, G.J., Superconducting foams (2002) Supercond. Sci. 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Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles

publisher: Elsevier articletitle: Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles journaltitle: Carbon articlelink: http://dx.doi.org/10.1016/j.carbon.2017.04.018 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.publisher: Elsevier articletitle: Mechanical properties and fracture patterns of graphene (graphitic) nanowiggles journaltitle: Carbon articlelink: http://dx.doi.org/10.1016/j.carbon.2017.04.018 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.This work was supported in part by the Brazilian Agencies CNPq, CAPES and FAPESP. The authors would like to thank the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID Grant 2013/08293-7. N.M.P. is supported by the European Research Council PoC 2015 “Silkene” No. 693670, by the European Commission H2020 under the Graphene Flagship Core 1 No. 696656 (WP14 “Polymer Nanocomposites”) and under the Fet Proactive “Neurofibres” No. 732344

Crystalline Networks With Unusual Predicted Mechanical And Thermal Properties

MOST materials shrink laterally and become less dense when stretched. Materials that both expand laterally (that is, have negative Poisson's ratio) and densify when stretched are of interest both from the fundamental and the practical points of view1-5. A few monocrystalline phases with negative Poisson's ratio are known3,4, but these do not densify when stretched. Here we present molecular-mechanics calculations for some hypothetical phases of carbon which exhibit both kinds of behaviour. The properties derive from the presence of bonds that act as hinges in extended helical chains. Other unusual properties of these phases include negative thermal expansion, dopant-controlled porosity and low-temperature polymorphism. Such structures can be envisaged for polyacetylene, polydiacetylene, polyphenylene and (BN)x phases, as well as for variants of some known, structurally related inorganic phases.365644873473