Effect of Electrochemical Treatment on Electrical Conductivity of Conical Carbon Nanotubes

Interaction of conical carbon nanotubes (CNTs) with hydrogen during electrochemical treatment and its effect on their electronic properties was studied. The temperature dependencies of electroconductivity of initial and electrochemically hydrogenated conical CNTs were investigated by using four-probe van der Pauw method. The studies revealed that the electrochemical hydrogen absorption leaded to a significant reduction in the electroconductivity of conical carbon nanotubes. We assume that these changes can be associated with a decrease in the concentration of charge carriers as a result of hydrogen localization on the carbon π-orbitals, the transition from sp2 to sp3 hybridization of conical CNTs band structure, and, therefore, a metal-semiconductor-insulator transition

Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study

In crystals of La2CuO4.02 oxygen diffusion can be limited to such small length scales, that the resulting phase separation is invisible for neutrons. Decomposition of the 139La NQR spectra shows the existence of three different regions, of which one orders antiferromagnetically below 17K concomitantly with the onset of a weak superconductivity in the crystal. These regions are compared to the macroscopic phases seen previously in the title compound and the cluster-glass and striped phases reported for the underdoped Sr-doped cuprates.Comment: 4 pages, RevTeX, 5 figures, to be published in PR

Ni-substituted sites and the effect on Cu electron spin dynamics of YBa2Cu{3-x}NixO{7-\delta}

We report Cu nuclear quadrupole resonance experiment on magnetic impurity Ni-substituted YBa$_2$Cu$_{3-x}$Ni$_x$O$_{7-\delta}$. The distribution of Ni-substituted sites and its effect on the Cu electron spin dynamics are investigated. Two samples with the same Ni concentration $x$=0.10 and nearly the same oxygen content but different $T_c$'s were prepared: One is an as-synthesized sample (7-$\delta$=6.93) in air ($T_c$$\approx 80 K$), and the other is a quenched one (7-$\delta$=6.92) in a reduced oxygen atmosphere ($T_c$$\approx 70 K$). The plane-site $^{63}$Cu(2) nuclear spin-lattice relaxation for the quenched sample was faster than that for the as-synthesized sample, in contrast to the $^{63}$Cu(1) relaxation that was faster for the as-synthesized sample. This indicates that the density of plane-site Ni(2) is higher in the quenched sample, contrary to the chain-site Ni(1) density which is lower in the quenched sample. From the analysis in terms of the Ni-induced nuclear spin-lattice relaxation, we suggest that the primary origin of suppression of $T_c$ is associated with nonmagnetic depairing effect of the plane-site Ni(2).Comment: 7 pages, 5 figure

CVD growth of carbon nanostructures from zirconia: mechanisms and a method for enhancing yield.

By excluding metals from synthesis, growth of carbon nanostructures via unreduced oxide nanoparticle catalysts offers wide technological potential. We report new observations of the mechanisms underlying chemical vapor deposition (CVD) growth of fibrous carbon nanostructures from zirconia nanoparticles. Transmission electron microscope (TEM) observation reveals distinct differences in morphological features of carbon nanotubes and nanofibers (CNTs and CNFs) grown from zirconia nanoparticle catalysts versus typical oxide-supported metal nanoparticle catalysts. Nanofibers borne from zirconia lack an observable graphitic cage consistently found with nanotube-bearing metal nanoparticle catalysts. We observe two distinct growth modalities for zirconia: (1) turbostratic CNTs 2-3 times smaller in diameter than the nanoparticle localized at a nanoparticle corner, and (2) nonhollow CNFs with approximately the same diameter as the nanoparticle. Unlike metal nanoparticle catalysts, zirconia-based growth should proceed via surface-bound kinetics, and we propose a growth model where initiation occurs at nanoparticle corners. Utilizing these mechanistic insights, we further demonstrate that preannealing of zirconia nanoparticles with a solid-state amorphous carbon substrate enhances growth yield.This material is based upon work supported by the National Science Foundation under Grant No. 1007793 and was also supported by Airbus group, Boeing, Embraer, Lockheed Martin, Saab AB, Hexcel, and TohoTenax through MIT’s Nano- Engineered Composite aerospace STructures (NECST) Consortium. This research was supported (in part) by the U.S. Army Research Office under Contract W911NF-13-D-0001. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF Award No. ECS-0335765. CNS is part of Harvard University. This work was carried out in part through the use of MIT Microsystems Technology Laboratories. Stephan Hofmann acknowledges funding from EPSRC under grant EP/H047565/1. Piran Kidambi acknowledges the Lindemann Trust Fellowship.This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/ja509872y

To the Intrinsic Magnetism of the Bi<inf>108</inf>Sn<inf>0.02</inf>Sb<inf>0.9</inf>Te<inf>2</inf>S Topological Insulator

© 2019, Pleiades Publishing, Inc. Using Electron Spin Resonance spectroscopy together with the Superconducting Quantum Interference Device magnetometry, we found that the intrinsic magnetic moments, originating from the nonmagnetic structural defects of Bi1.08Sn0.02Sb0.9Te2S topological insulator form the superparamagnetic state. It represents an array of nanoscale single domain ferromagnets randomly distributed in the nonmagnetic media. Their net magnetic polarization in the absence of external magnetic field is completely averaged out. Single domain ferromagnetic particles at elevated temperatures behave magnetically in a manner analogous to the Langevin paramagnetism of moment bearing atoms. The main distinction is that the moment of the particle may be 102—103 times the atomic moment

To the Intrinsic Magnetism of the Bi<inf>108</inf>Sn<inf>0.02</inf>Sb<inf>0.9</inf>Te<inf>2</inf>S Topological Insulator

© 2019, Pleiades Publishing, Inc. Using Electron Spin Resonance spectroscopy together with the Superconducting Quantum Interference Device magnetometry, we found that the intrinsic magnetic moments, originating from the nonmagnetic structural defects of Bi1.08Sn0.02Sb0.9Te2S topological insulator form the superparamagnetic state. It represents an array of nanoscale single domain ferromagnets randomly distributed in the nonmagnetic media. Their net magnetic polarization in the absence of external magnetic field is completely averaged out. Single domain ferromagnetic particles at elevated temperatures behave magnetically in a manner analogous to the Langevin paramagnetism of moment bearing atoms. The main distinction is that the moment of the particle may be 102—103 times the atomic moment

Fuel Cell Electrodes Based on Carbon Nanotube/Metallic Nanoparticles Hybrids Formed on Porous Stainless Steel Pellets

The preparation of carbon nanotube/metallic particle hybrids using pressed porous stainless steel pellets as a substrate is described. The catalytic growth of carbon nanotubes was carried out by CVD on a nickel catalyst obtained by impregnation of pellets with a highly dispersive colloidal solution of nickel acetate tetrahydrate in ethanol. Granular polyethylene was used as the carbon source. Metallic particles were deposited by thermal evaporation of Pt and Ag using pellets with grown carbon nanotubes as a base. The use of such composites as fuel cell electrodes is discussed