Tensile properties of graphene nanotube hybrid structures : a molecular dynamics study

Graphene has been reported with record-breaking properties which have opened up huge potential applications. A considerable research has been devoted to manipulate or modify the properties of graphene to target a more smart nanoscale device. Graphene and carbon nanotube hybrid structure (GNHS) is one of the promising graphene derivates, while their mechanical properties have been rarely discussed in literature. Therefore, such a studied is conducted in this paper basing on the large-scale molecular dynamics simulation. The target GNHS is constructed by considering two separate graphene layers that being connected by single-wall carbon nanotubes (SWCNTs) according to the experimental observations. It is found that the GNHSs exhibit a much lower yield strength, Young’s modulus, and earlier yielding comparing with a bilayer graphene sheet. Fracture of studied GNHSs is found to fracture located at the connecting region between carbon nanotubes (CNTs) and graphene. After failure, monatomic chains are normally observed at the front of the failure region, and the two graphene layers at the failure region without connecting CNTs will adhere to each other, generating a bilayer graphene sheet scheme (with a layer distance about 3.4 Å). This study will enrich the current understanding of the mechanical performance of GNHS, which will guide the design of GNHS and shed lights on its various applications.

Prediction of thermal expansion properties of carbon nanotubes using molecular dynamics simulations

The axial coefficients of thermal expansion (CTE) of various carbon nanotubes (CNTs), i.e., single-wall carbon nanotubes (SWCNTs), and some multi-wall carbon nanotubes (MWCNTs), were predicted using molecular dynamics (MDs) simulations. The effects of two parameters, i.e., temperature and the CNT diameter, on CTE were investigated extensively. For all SWCNTs and MWCNTs, the obtained results clearly revealed that within a wide low temperature range, their axial CTEs are negative. As the diameter of CNTs decreases, this temperature range for negative axial CTEs becomes narrow, and positive axial CTEs appear in high temperature range. It was found that the axial CTEs vary nonlinearly with the temperature, however, they decrease linearly as the CNT diameter increases. Moreover, within a wide temperature range, a set of empirical formulations was proposed for evaluating the axial CTEs of armchair and zigzag SWCNTs using the above two parameters. Finally, it was found that the absolute value of the negative axial CTE of any MWCNT is much smaller than those of its constituent SWCNTs, and the average value of the CTEs of its constituent SWCNTs. The present fundamental study is very important for understanding the thermal behaviors of CNTs in such as nanocomposite temperature sensors, or nanoelectronics devices using CNTs.

Mathematical models on the impact of noise and dyadic molecular structures on the properties of a cardiac myocyte

In cardiac myocytes (heart muscle cells), coupling of electric signal known as the action potential to contraction of the heart depends crucially on calcium-induced calcium release (CICR) in a microdomain known as the dyad. During CICR, the peak number of free calcium ions (Ca) present in the dyad is small, typically estimated to be within range 1-100. Since the free Ca ions mediate CICR, noise in Ca signaling due to the small number of free calcium ions influences Excitation-Contraction (EC) coupling gain. Noise in Ca signaling is only one noise type influencing cardiac myocytes, e.g., ion channels playing a central role in action potential propagation are stochastic machines, each of which gates more or less randomly, which produces gating noise present in membrane currents. How various noise sources influence macroscopic properties of a myocyte, how noise is attenuated and taken advantage of are largely open questions. In this thesis, the impact of noise on CICR, EC coupling and, more generally, macroscopic properties of a cardiac myocyte is investigated at multiple levels of detail using mathematical models. Complementarily to the investigation of the impact of noise on CICR, computationally-efficient yet spatially-detailed models of CICR are developed. The results of this thesis show that (1) gating noise due to the high-activity mode of L-type calcium channels playing a major role in CICR may induce early after-depolarizations associated with polymorphic tachycardia, which is a frequent precursor to sudden cardiac death in heart failure patients; (2) an increased level of voltage noise typically increases action potential duration and it skews distribution of action potential durations toward long durations in cardiac myocytes; and that (3) while a small number of Ca ions mediate CICR, Excitation-Contraction coupling is robust against this noise source, partly due to the shape of ryanodine receptor protein structures present in the cardiac dyad.

Coarse-Grained Molecular Dynamics Simulation of the Aggregation Properties of Multiheaded Cationic Surfactants in Water

The aggregation property of multiheaded surfactants has been investigated by constant pressure molecular dynamics (MD) simulation in aqueous medium. The model multiheaded surfactants contain more than one headgroup (x = 2, 3, and 4) for a single tail group. This increases the hydrophilic charge progressively over the hydrophobic tail which has dramatic consequences in the aggregation behavior. In particular, we have looked at the change in the aggregation property such as critical micellar concentration (cmc), aggregation number, and size of the micelles for the multiheaded surfactants in water. We find with increasing number of headgroups of the Multiheaded surfactants that the cmc values increase and the aggregation numbers as well as the size of the micelles decrease. These trends are in agreement with the experimental findings as reported earlier with x = 1, 2, and 3. We also predict the aggregation properties of multiheaded surfactant With four headgroups (x = 4) for which no experimental studies exist yet.

Reactivity Of Pph3 Toward Ruiiruiiicl(O2car)4 - Syntheses, Molecular-Structures, And Spectroscopic And Electrochemical Properties Of Ruiiruiii(Oh2)Cl(Mecn)(O2car)4(Pph3)2 And Ruii2(Oh2)(Mecn)2(O2car)4(Pph3)2

By the reaction of Ru2Cl(O2CAr)4 (1) and PPh3 in MeCN-H2O the diruthenium(II,III) and diruthenium(II) compounds of the type Ru2(OH2)Cl(MeCN)(O2CAr)4(PPh3)2 (2) and Ru2(OH2)(MeCN)2(O2CAr)4(PPh3)2 (3) were prepared and characterized by analytical, spectral, and electrochemical data (Ar is an aryl group, C6H4-p-X; X = H, OMe, Me, Cl, NO2). The molecular structure of Ru2(OH2)Cl(MeCN)(O2CC6H4-p-OMe)4(PPh3)2 was determined by X-ray crystallography. Crystal data are as follows: triclinic, P1BAR, a = 13.538 (5) angstrom, b = 15.650 (4) angstrom, c = 18.287 (7) angstrom, alpha = 101.39 (3)-degrees, beta = 105.99 (4)-degrees, gamma = 97.94 (3)-degrees, V = 3574 angstrom 3, Z = 2. The molecule is asymmetric, and the two ruthenium centers are clearly distinguishable. The Ru(III)-Ru(II), Ru(III)-(mu-OH2), and Ru(II)-(mu-OH2) distances and the Ru-(mu-OH2)-Ru angle in [{Ru(III)Cl(eta-1-O2CC6H4-p-OMe)(PPh3)}(mu-OH2)(mu-O2CC6H4-p-OMe)2{Ru(II)(MeCN)(eta-1-O2CC6H4-p-OMe)(PPh3)}] are 3.604 (1), 2.127 (8), and 2.141 (10) angstrom and 115.2 (5)-degrees, respectively. The compounds are paramagnetic and exhibit axial EPR spectra in the polycrystalline form. An intervalence transfer (IT) transition is observed in the range 900-960 nm in chloroform in these class II type trapped mixed-valence species 2. Compound 2 displays metal-centered one-electron reduction and oxidation processes near -0.4 and +0.6 V (vs SCE), respectively in CH2Cl2-TBAP. Compound 2 is unstable in solution phase and disproportionates to (mu-aquo)diruthenium(II) and (mu-oxo)diruthenium(III) complexes. The mechanistic aspects of the core conversion are discussed. The molecular structure of a diruthenium(II) compound, Ru2(OH2)(MeCN)2(O2CC6H4-p-NO2)4(PPh3)2.1.5CH2Cl2, was obtained by X-ray crystallography. The compound crystallizes in the space group P2(1)/c with a = 23.472 (6) angstrom, b = 14.303 (3) angstrom, c = 23.256 (7) angstrom, beta = 101.69 (2)-degrees, V = 7645 angstrom 3, and Z = 4. The Ru(II)-Ru(II) and two Ru(II)-(mu-OH2) distances and the Ru(II)-(mu-OH2)-Ru(II) angle in [{(PPh3)-(MeCN)(eta-1-O2CC6H4-p-NO2)Ru}2(mu-OH2)(mu-O2CC6H4-p-NO2)2] are 3.712 (1), 2.173 (9), and 2.162 (9) angstrom and 117.8 (4)-degrees, respectively. In both diruthenium(II,III) and diruthenium(II) compounds, each metal center has three facial ligands of varying pi-acidity and the aquo bridges are strongly hydrogen bonded with the eta-1-carboxylato facial ligands. The diruthenium(II) compounds are diamagnetic and exhibit characteristic H-1 NMR spectra in CDCl3. These compounds display two metal-centered one-electron oxidations near +0.3 and +1.0 V (vs SCE) in CH2Cl2-TBAP. The overall reaction between 1 and PPh3 in MeCN-H2O through the intermediacy of 2 is of the disproportionation type. The significant role of facial as well as bridging ligands in stabilizing the core structures is observed from electrochemical studies.

Temperature and concentration dependence of adsorption properties of methane in NaY: a molecular dynamics study

Molecular dynamics calculations on methane sorbed in NaY (Si/Al = 3.0) employing realistic methane-methane and methane-zeolite intermolecular potential functions at different temperatures (50, 150, 220, and 300 K) and concentrations (2, 4, 6, and 8 molecules/cage) are reported. The thermodynamic results are in agreement with the available experimental data. Guest-guest and guest-host radial distribution functions (rdfs), energy distribution functions, distribution of cage occupancy, center-of-cage-center-of-mass (coc-com) rdfs, velocity autocorrelation functions for com and angular motion and the Fourier transformed power spectra, and diffusion coefficients are presented as a function of temperature and concentration. At 50 K, methane is localized near the adsorption site. Site-site migration and essentially free rotational motion are observed at 150 K. Molecules preferentially occupy the region near the inner surface of the alpha-cage. The vibrational frequencies for the com of methane shift toward higher values with decreasing temperature and increasing adsorbate concentration. The observed frequencies for com motion are 36, 53, and 85 cm-1 and for rotational motion at 50 K, 95 and 150 cm-1 in agreement with neutron scattering data. The diffusion coefficients show a type I behavior as a function of loading in agreement with NMR measurements. Cage-to-cage diffusion is found to be always mediated by the surface.

Sorbate properties and cage-to-cage diffusion of argon in NaCaA: a molecular dynamics study

Detailed molecular dynamics simulations of argon in zeolite NaCaA are reported. Thermodynamic, structural, and dynamical properties of the sorbate as a function of temperature have been obtained. The properties calculated include various site-site radial distribution functions, different energy distribution functions, selfdiffusion coefficients, the power spectra, and properties relating to cage-to-cage diffusion. The results suggest that sorbate is delocalized above 300 K. Both modes of cage-to-cage diffusion-the surface-mediated and centralized diffusion-are associated with negative barrier heights. Surprisingly, rate of cage-to-cage diffusion is associated with negative and positive activation energies below and above 500 K. The observed differences in the behavior of the rate of cage-to-cage diffusion between Xe-NaY and Ar-NaCaA systems and the nature of the potential energy surface are discussed. Presence of sorbatezeolite interactions results in significant enhancement in the rate of cage-to-cage diffusion and rate of cage visits. It is shown that properties dependent on the long-time behavior such as the diffusion coefficient and the rate of cages visited exhibit the expected Arrhenius dependence on temperature.

Sorbate properties of xenon in cloverite: A molecular dynamics study

Molecular dynamics calculations on xenon adsorbed in the cubic cavity of cloverite, a gallophosphate, is presented. Guest-host radial distribution functions, guest-host energy distribution functions, power spectra, and diffusion coefficients for xenon are reported at 397, 494, and 716 K. Results suggest that xenon is highly mobile at 700 K. A shift in the peak position of the power spectra toward lower frequency is observed with increase in temperature. (C) 1994 Academic Press, Inc.

Synthesis and catalytic properties of cobalt- and molybdenum-containing mesoporous MCM-41 molecular sieves

Mesoporous MCM-41 type silicas containing molybdenum and cobalt have been prepared with pore sizes in the range 30-38 Angstrom and 54-59 Angstrom. Catalytic properties of these materials have been examined with respect to the oxidation of cyclooctene and aniline.

Structural and optical properties of nonpolar (11-20) a-plane GaN grown on (1-102) r-plane sapphire substrate by plasma-assisted molecular beam epitaxy

We report the structural and optical properties of a-plane GaN film grown on r-plane sapphire substrate by plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction was used to determine the out-of-plane and in-plane epitaxial relation of a-plane GaN to r-plane sapphire. Low-temperature photoluminescence emission was found to be dominated by basal stacking faults along with near-band emission. Raman spectroscopy shows that the a-GaN film is of reasonably good quality and compressively strained. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Molecular self-assembly of cadmium-triazolate complexes via hydrogen bonding: Synthesis, structures and photoluminescent properties

Two new cadmium coordination polymers namely Cd(HAmTrz-COO)(4)(NH4+)(2)] 1; and Cd(HAmTrz)(2)I-2](n) 2; (HAmTrz-COOH = 3-amino-1,2,4-triazole-5-carboxylic acid), have been prepared based on HAmTrz-COOH as ligand. The crystal structures of 1 and 2 have been determined by single-crystal X-ray diffraction technique. In coordination-complex 1 four triazole ligands coordinate via N1 nitrogen leading to a tetrahedral geometry around cadmium ion, while in 2 the ligand prefers to coordinate to the metal in a bidentate bridging mode. The structures of both the coordination polymers can be envisaged as 3D hydrogen bonded networks. Thermogravimetric analysis shows that 2 is more stable than 1 owing to different coordination numbers of cadmium atoms. Photoluminescence properties of both the compounds have been investigated in the solid state. (C) 2011 Elsevier B.V. All rights reserved.

Substrate nitridation induced modulations in transport properties of wurtzite GaN/p-Si (100) heterojunctions grown by molecular beam epitaxy

Phase pure wurtzite GaN films were grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) were found to be phase mixtured, containing both cubic (beta) and hexagonal (alpha) modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN films grown with silicon nitride buffer layer grown at 800 degrees C when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 degrees C. Core-level photoelectron spectroscopy of Si(x)N(y) layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism was observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibited superior rectifying nature with reduced trap concentrations. Lowest ideality factors (similar to 1.5) were observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3658867]