Sócrates O. Dantas

Molecular-dynamics simulations of carbon nanotubes as gigahertz oscillators.

Recently, Zheng and Jiang [Phys. Rev. Lett. 88, 045503 (2002)]] have proposed that multiwalled carbon nanotubes could be the basis for a new generation of nano-oscillators in the several gigahertz range. In this Letter, we present the first molecular dynamics simulation for these systems. Different nanotube types were considered in order to verify the reliability of such devices as gigahertz oscillators. Our results show that these nano-oscillators are dynamically stable when the radii difference values between inner and outer tubes are of approximately 3.4 A. Frequencies as large as 38 GHz were observed, and the calculated force values are in good agreement with recent experimental investigations.

Sign Change of Poisson's Ratio for Carbon Nanotube Sheets

Most materials shrink laterally like a rubber band when stretched, so their Poissons ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poissons ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poissons ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poissons ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poissons ratio is associated with a large positive Poissons ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poissons ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

Gigahertz nanomechanical oscillators based on carbon nanotubes

We report molecular dynamics studies of carbon nanotubes as mechanical gigahertz oscillators. Our results show that different oscillatory regimes exist but that sustained oscillations are possible only when the radii difference values of the inner and outer tubes are . Frequencies as large as 87?GHz were obtained. Calculated force and frequency values are in good agreement with estimated data from recent experimental investigations.

Experimental realization of suspended atomic chains composed of different atomic species

Research into nanostructured materials frequently relates to pure substances. This contrasts with industrial applications, where chemical doping or alloying is often used to enhance the electrical or mechanical properties of materials1. However, the controlled preparation of doped nanomaterials has been much more difficult than expected because the increased surface-area-to-volume ratio can, for instance, lead to the expulsion of impurities (self-purification)2. For nanostructured alloys, the influence of growth methods and the atomic structure on self-purification is still open to investigation2,3. Here, we explore, experimentally and with molecular dynamics simulations, to what extent alloying persists in the limit that a binary metal is mechanically stretched to a linear chain of atoms. Our results reveal a gradual evolution of the arrangement of the different atomic elements in the narrowest region of the chain, where impurities may be expelled to the surface or enclosed during elongation.

HMDSO plasma polymerization and thin film optical properties

Departamento de Fisica e Quimica Faculdade de Engenharia UNESP, 12500-000 Guaratingueta, SP

Modeling the auxetic transition for carbon nanotube sheets

A simple model is developed to predict the complex mechanical properties of carbon nanotube sheets (buckypaper) [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 modication.

Resonance Raman investigation and semi-empirical calculation of the natural carotenoid bixin

Abstract A detailed resonance Raman investigation of the natural carotenoid bixin (6,6′-diapo-ψ-ψ′-carotenedioic acid monomethyl ester) was undertaken in chloroform solution. The excitation profiles of four fundamentals, one overtone and one combination band were obtained and calculated by the transform method within the standard assumptions. A simple model of displaced harmonic oscillators reproduced the profiles satisfactorily, in contrast to the more elaborate models previously used in the case of 1,3,5-hexatriene. In addition, the time-dependent formalism was used to reproduce the optical absorption spectrum of bixin, and together with the transform method, to calculate the displacement parameters. Use was made of semi-empirical calculations via MOPAC6 and ZINDO to gain further insight into the bond length variations in the excited electronic state.

Quantum mechanical investigation of the tautomerism in the azo dye Sudan III

The tautomerism in Sudan III bis-azo dye has been analyzed using ab initio Hartee-Fock (HF) and second-order Moller-Plesset perturbatim theory (MP2) and density functional (B3LYP) methods. Gas-phase and solution calculations were performed to investigate the solvent effect on the azo (OH)! hydrazone (NH) tautomeric equilibrium. The azo (OH) tautomer was found to be preferred in gas phase at the HF level of theory. The inclusion of the electronic correlation (MP2) shifted the equilibrium toward the hydrazone (NH) form. The NH isomer was also found to be more favorable in the gas phase according to the B3LYP results. In solution the equilibrium is shifted toward the NH tautomer as the dielectric constant of the medium increases. The energy barrier for the intramolecular proton transference was calculated and the value found suggested a strong hydrogen bond. The B3LYP and MP2 activation Gibbs free energies were very close and much lower than the HF value. The ultraviolet/visible electronic spectra for the minima and transition state (TS) structures were calculated and compared with the experimental data. The theoretical band positions obtained considering the TS geometry were found to best agree with the experimental data. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 1076-1086, 2000

Transmission electron microscopy and molecular dynamics study of the formation of suspended copper linear atomic chains

We report high resolution transmission electron microscopy and classical molecular dynamics simulation results of mechanically stretching copper nanowires conducting to linear atomic suspended chains (LACs) formation. In contrast with some previous experimental and theoretical work in literature that stated that the formation of LACs for copper should not exist our results showed the existence of LAC for the [111], [110], and [100] crystallographic directions, being thus the sequence of most probable occurence.

A semi–empirical study on the electronic structure of ellipticines

Abstract Ellipticine and some of its derivatives are organic molecules with intense antitumor activity. In this work we report a semi–empirical study on the electronic structure of ellipticine and some derivatives. Geometrical and spectroscopic aspects were investigated through the use of the well–known PM3 (Parametric Method 3) and ZINDO (Zerner’s Intermediate Neglect of Differential Overlap) methods. Comparing PM3 and previously reported AM1 (Austin Method One) results, we found out that overall PM3 results are better. The ZINDO simulated absorption spectra compare well to the available experimental data. We have observed a very simple correlation between the dipole moment value (DM) and the antitumor activity. These results suggest a way to improve and design new antitumor ellipticine derivatives, as well as, to select promising molecules from untested groups.