In this work we report on modification of electrical and optical properties of extended thin films of single-wall carbon nanotubes induced by iodination from gaseous phase. High resolution ...transmission electron microscopy and Raman data have revealed formation of different types of polyiodide species or one-dimensional iodine crystals (depending on pristine nanotube geometry) inside nanotubes. UV–vis–NIR optical absorption spectra of iodinated nanotubes demonstrated a clear suppression of the optical absorption band corresponding to the first electron transition for semiconducting nanotubes (with the factor determined by the nanotube geometry). It was interpreted as a result of charge transfer from nanotubes to polyiodide species formed inside them. Because of this effect the Fermi level shifted down into the valence band, and the nanotube conductivity type was changed. For filled nanotube films the metallic type of temperature-dependent electrical resistance behavior was observed at elevated temperatures. A reduction of the electrical resistance of pristine films by an order of magnitude (from 1550Ohm/sq to 270Ohm/sq; from 865Ohm/sq to 150Ohm/sq; from 700Ohm/sq to 70Ohm/sq at 300K) has been observed. The obtained sheet resistances are comparable with those for the most popular today material for transparent conductive electrodes – indium tin oxide.
Electrical transport mechanisms of 2D carbon nanotube networks are presently under intensive studies. The related experimental data are ambiguous and controversial. We report on terahertz-infrared ...spectra of optical conductivity and dielectric permittivity of thin transparent films composed of pristine and CuCl- or iodine-doped single-walled carbon nanotubes (SWCNTs) measured in the frequency range from 7 to 25 000 cm−1 and at temperatures from 5 to 300 K. Controversially to the existing results, we have not observed a clear signature of the so-called terahertz conductivity peak. Instead, a typical metallic-like frequency- and temperature-dependent behavior of the conductivity and permittivity has been discovered. It was attributed to the high quality interconnected SWCNT network providing the almost free pathways for charge carriers. Applying Drude conductivity model, we have determined the temperature and doping dependences of effective parameters of the carriers in the films: plasma frequency, scattering rate, mobility, mean-free path. The obtained results demonstrate a great potential of the material in the field of electromagnetic applications at frequencies up to few terahertz.
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The thermal transport properties of random network, single-walled carbon nanotube (SWNT) films were assessed using Raman spectroscopy. Two types of SWNT films were investigated: single-layer and ...stacked. The single-layer films were fabricated by aerosol chemical vapour deposition and subsequent direct dry deposition, while the stacked films were prepared by placing the single-layer films on top of one another. The anisotropy of the network structures of each of these films was evaluated based on the angular dependence of the optical absorbance spectra. The results show that the anisotropy of the films decreases with increasing film thickness in the case of the single-layer films, and that the film anisotropy is preserved during the stacking process. The sheet thermal conductance is proportional to the SWNT area density in the case of stacked films, but is reduced with increasing thickness in the case of single-layer films. This effect is explained by a change in the network morphology from a two-dimensional anisotropic structure to the more isotropic structure. This work demonstrates the fabrication of low-density films with high sheet thermal conductance through the stacking of thin SWNT films.
In this paper we show the advantages of transparent high conductive films based on filled single-wall carbon nanotubes. The nanotubes with internal channels filled with acceptor molecules (copper ...chloride or iodine) form networks demonstrating significantly improved characteristics. Due to the charge transfer between the nanotubes and filler, the doped-nanotube films exhibit a drop in electrical sheet resistance of an order of magnitude together with a noticeable increase of film transparency in the visible and near-infrared spectral range. The thermoelectric power measurements show a significant improvement of air-stability of the nanotube network in the course of the filling procedure. For the nanotube films with an initial transparency of 87% at 514 nm and electrical sheet resistance of 862 Ohm sq-1 we observed an improvement of transparency up to 91% and a decrease of sheet resistance down to 98 Ohm sq-1. The combination of the nanotube synthesis technique and molecules for encapsulation has been optimized for applications in optoelectronics.
The environmental impact of the heavy metals contained in the combustion product ash depends on the speciation of the heavy metals and the size distributions of the heavy metals in the ash. ...Therefore, the behavior of cadmium, lead, copper, and zinc was studied experimentally during circulating fluidized bed combustion (CFBC) of Swedish forest residue. The size distributions and concentrations of the heavy metals in the fly ash particles and in the gas phase were determined by low-pressure impactors and filters upstream of the convective back pass at 830 °C. Downstream of the convective back pass at T = 150 °C, the size distributions were determined. The fly ash from CFBC was found to contain two separate particle classes. Fine particles (D p < 0.5 μm) consisted mainly of KCl, and coarse particles (D p > 0.5 μm) contained as major elements Ca and Si. Major fraction of all the studied heavy metals were found in the coarse fly ash particles at location 1 at 830 °C; 7−26% of Pb, 24−27% of Cu, 1−8% of Cd, and less than 1% of Zn were found in the gas phase. The gas-to-particle conversion route for Cd, Pb, and Cu was found by chemical surface reaction, probably with silicates. None of the studied heavy metals were enriched in the fine particles at the inlet of the electrostatic precipitator.
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