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In this work, we applied multiscale modeling to investigate electrical conductivity of carbon nanotube (CNT) enhanced polymer composites. The multiscale approach is based on a combination of first-principles calculations of contact resistance between CNTs using Green’s functions approach and statistical calculation of CNTs ensemble conductivity using the Monte Carlo percolation model. The results of first-principles calculations show strong dependence of contact resistance between CNTs on the angle φ between nanotubes axes: for (5,5) CNTs contact resistance increases by two orders of magnitude as φ changes from φ = 0 to φ = π/4. This angular dependence of contact resistance has strong influence on conductivity of CNTs ensemble, decreasing composite conductivity by about an order of magnitude. We stress that obtained conductivity is the upper theoretical limit for an ideal CNT composite, and experimental data with larger values of conductivity should be attributed to positive influence of some non-idealities in composite. [ABSTRACT FROM AUTHOR]

We report the photoinduced conductivity changes measured on a system composed of single-wall carbon nanotubes (SWNTs) modified by a semiconducting poly(amidoamine) dendrimer (PAMAMC) characterized by highly aromatic end groups. Under illumination hole injection into SWNTs prevails over the photoinjection of electrons from PAMAMC to SWNT holes, and film photoconductivity is observed. This system was incorporated as an electroactive component within a conducting polymer [poly(3-octylthiophene)], providing improvements in the photoelectrical properties of the composite. Such supramolecular structures consisting of dendrimer-functionalized carbon nanotubes provide the means for an approach toward the preparation of photoactive materials of high current interest. [ABSTRACT FROM AUTHOR]

The adsorption of NO2 molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO2 exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO2 molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT+NO2 system, considering semiconducting nanotubes with different diameters and introducing a Stone–Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT+NO2 system, while close to the adsorption site new states appear very close to the Fermi level. [ABSTRACT FROM AUTHOR]

In this work we have developed a method to covalently attach butylamine molecules to the sidewall of single-walled carbon nanotubes (SWNTs) starting from fluorinated SWNTs (F-SWNTs) obtained by CF4 plasma treatment at room temperature. Scanning tunneling microscopy images demonstrate that after the sidewall fluorination of SWNTs some bundles with a lower number of single tubes are present. The subsequent amine functionalization of the tubes has been demonstrated by infrared spectroscopy and thermogravimetric measurements. The chemical adsorption of butylamine molecules on F-SWNTs has been also investigated by electrical transport measurements in thin-film transistor geometry. The electrical conductance of SWNTs decreases after adsorption of butylamine. The amine groups in the molecule are electron donating and the observed photoconductivity of amino-functionalized F-SWNTs is presumably due to the injection of electrons from amine groups to SWNTs. [ABSTRACT FROM AUTHOR]

In this work a combined experimental and theoretical study on carbon nanotube (CNT) based system for gas sensing applications is reported. Carbon nanotubes thin films have been deposited by plasma-enhanced chemical vapor deposition on Si[sub 3]N[sub 4]/Si substrates provided with Pt electrodes. Microstructural features as determined by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy highlight the growth of defective tubular carbon structures. The electrical resistivity of the CNT film shows a semiconductinglike temperature dependence and a p-type response with decreasing electrical resistance upon exposure to NO[sub 2] gas (100 ppb). No response has been found by exposing the film to CO gas in the temperature range between 25 and 250 °C. In order to obtain a theoretical validation of the experimental results, the equilibrium position, charge transfer, and density of states are calculated from first principles for the CNT+CO and CNT+NO[sub 2] systems. Our spin-unrestricted density functional calculations show that NO[sub 2] retains its spin-polarized state upon adsorption. Both CO and NO[sub 2] molecules adsorb weakly on the tube wall, with essentially no charge transfer between the tube and molecules. The electronic properties of CNTs are sensitive to the adsorption of NO[sub 2], due to an acceptorlike peak close to the tube valence-band maximum, while they are insensitive to the CO adsorption. According to the experimental findings, our theoretical results suggest that gas-induced modification of the density of states close to the Fermi level might significantly affect the transport properties of nanotubes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The growth behavior of carbon nanotubes (CNTs), produced by radio frequency plasma enhanced chemical vapor deposition, is studied here as a function of the CH[SUB4]/N[SUB2] ratio and Ni catalyst layer thickness. Scanning electron microscopy shows that by employing a suitable deposition (methane) to etching (nitrogen) gas fatio, it is possible to obtain the growth of nanotubes with a limited presence of amorphous carbon on the substrate surface. In particular a progressive transition from random to aligned CNTs is observed when nitrogen is added to the plasma atmosphere. The electronic structure was then investigated by C Is and valence band photoemission spectroscopy. The results show a shift of the overall spectral to a higher-binding-energy side and a larger density of the states at the Fermi level indicating the formation of metallic aligned tubes with increasing nitrogen fraction in the plasma atomsphere. The electronic structure indicates the insertion of odd-member rings in the graphene network leading to an improvement ofthe mechanical properties. The thickness of the Ni layer has a strong influence on the CNTs growth showing maximum value for nanotube formation at 20 nm. [ABSTRACT FROM AUTHOR]

Hydrogenated amorphous carbon (a-C:H) films were deposited, at room temperature, from a CH[sub 4]/Ar plasma produced by a radio frequency (rf) glow discharge system at 13.56 MHz, and different power values. X-ray reflectivity and Brillouin light scattering have been used to investigate the relationship between deposition conditions and elastic properties of hydrogenated amorphous carbon films. The composition of the initial gaseous mixture and the rf power input are shown to affect significantly both the mechanical and the microstructural properties of deposited films. As the fraction of methane in the feed gas is increased, both the film density and the phase velocity of the surface Rayleigh wave exhibit an enhancement, thus indicating the production of a harder material. On the other hand, x-ray reflectivity measurements suggest the occurrence of a higher degree of structural order in the sp[sup 2] lattice. Experimental findings are discussed in terms of the different chemical composition of the plasma. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

With the continuous advance of modern electronics, the demand for nonvolatile memory cells rapidly grows. In order to develop post-silicon electronic devices, it is necessary to find innovative solutions to the eco-sustainability problem of materials for nonvolatile memory cells. In this work, we realized a resistive memory device based on graphene oxide (GO) and GO/cellulose nanocrystals (CNC) thin films. Aqueous solutions of GO and GO with CNC have been prepared and drop cast between two metal electrodes. Such thin-film based devices showed a transition between low and high conductivity states upon the forward and backward sweeping of an external electric field. This reversible current density transition behavior demonstrates a typical memory characteristic. The obtained results open an easy route for electronic information storage based on the integration of nanocrystalline cellulose onto graphene based devices. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) deposited by plasma-enhanced chemical vapor deposition on Si[sub 3]N[sub 4]/Si substrates have been investigated as resistive gas sensors for NO[sub 2]. Upon exposure to NO[sub 2], the electrical resistance of the CNTs was found to decrease. The maximum variation of resistance to NO[sub 2] was found at an operating temperature of around 165°C. The sensor exhibited high sensitivity to NO[sub 2] gas at concentrations as low as 10 ppb, fast response time, and good selectivity. A thermal treatment method, based on repeated heating and cooling of the films, adjusted the resistance of the sensor film and optimized the sensor response to NO[sub 2]. [ABSTRACT FROM AUTHOR]

The adsorption of NO(2) molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO(2) exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO(2) molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT + NO(2) system, considering semiconducting nanotubes with different diameters and introducing a Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT + NO(2) system, while close to the adsorption site new states appear very close to the Fermi level.