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Sunday, February 17, 2008  

Rheological and conducting properties of polyethylene/carbon nanotube composites

Multi-walled carbon nanotube / polyethylene (MWCNT/PE) composites with several concentrations (0.5, 1, 2.5, 5, 7 wt %) of CVD grown nanotubes have been prepared by melt mixing. Melt rheological investigations indicate that the percolation threshold in polyethylene/carbon nanotube composites is between 1 and 2.5 wt% of CNTs for high density polyethylene (HDPE) and low density polyethylene (LDPE) matrix. A better CNT dispersion was obtained in the HDPE matrix. For both HDPE/CNT and LDPE/CNT samples researchers found a similar percolation threshold also in conductance, with a six orders of magnitude increase in electrical conductivity at room temperature between 1 and 2.5 wt% CNTs. Electrical bistability and reproducible rectification behaviour has been found for a HDPE/CNT composite below percolation threshold. Raman line shifts confirm the strong compressive forces associated with PE chains on MWCNT.
Source: PHYSICA E (2008), article in press.

Friday, February 15, 2008  

Growth of aligned carbon nanotubes from Ni nanoparticles prepared by ion implantation

The formation of dense Ni nanoparticles of the order of 10^11–10^12 cm-2 was achieved by ion implantation using FIB. Vertically aligned CNTs were synthesized from particles annealed at 700°C although the highest particle density was obtained after post-implantation annealing at 460°C. Carbon nanotubes were grown preferentially from particles larger than 10 nm under these CVD conditions and in the Ni/SiO2 system. Growth of vertically aligned CNTs from catalyst particles prepared by ion implantation could be a very useful technique for microelectronics applications.
Source: Diamond and Related Materials (2008), article in press

 

NO2 gas sensors based on single-wall carbon nanotubes

Researchers successfully fabricated and tested molecular gas sensors made of suspended SWNTs. They showed for two different semiconducting SWNTs a transient chemiresistor like change in conductivity when they are exposed to NO2. The full gate characteristic (−20V:20 V) of the SWNTs is highly dominated by a large hysteresis and it shows three prominent changes in the characteristic after the exposure to NO2. Researchers found a broadened hysteresis, suppression of the n-type conductivity for positive Vg and an increase in the p-type conductivity for negative Vg. These changes are related to dynamically charged and discharged trap states of NO2 molecules on and around the CNTs and additional static effects correlated to NO2 doping of the CNT.

Measurements of the decay times of the charged trap states show very long time constants that are comparable to the constants found for H2O. By waiting a longer time (≈30 min) than the decay times of the trap states before starting to measure, researchers could acquire a gate characteristic close to the intrinsic one. This enabled a coarse separation between dynamic and static effects. The resulting intrinsic gate characteristic measurement shows a static shift of the gate threshold voltage and a modification of the SB transparency. The measurements enable for the first time a quantitative comparison between the gate threshold shift and the saturation current in the transient measurements which leads to the conclusion that the suspended SWNT gas sensors operate as ChemFET rather than chemiresistors. Considering the application the researchers conclude that individual suspended s-SWNTs are highly sensitive to NO2, which can be exploited in applications such as fire detection, with operation at room temperature and at few nanoWatts.
Source: Sensors and Actuators B (2008), article in press

Friday, February 01, 2008  

Conductive and transparent thin films based on SWCNTs

Researchers fabricated transparent conductive thin films via the chemical assembly of ca-SWNTs with linker molecules onto an aminosilane-modified glass plate. Imaging by SEM showed uniform morphology and high-density multilayer structure over large areas. UV–vis spectroscopy analysis showed excellent transmittance in visible light region and good electrical conductivity. The thermal treatment (250 ◦C) significantly improved the transmittance (to greater than 90%) due to the removal of adsorbed organic molecules, and enhanced electrical conductivity because electron tunneling or hoping between ca-SWNT bundles became more effective by the elimination of the trapped organic materials.
The work function (measured by photoelectron spectroscopy) of the SWNT thin film was measured to 5.22 eV similar to the value of chemically shortened SWNT, which was slightly decreased to 5.12 eV by the thermal treatment. The excellent transmittance, good film uniformity and electric properties of these films demonstrate that such SWNT thin film can find a wide range of applications in optoelectronic devices requiring transparent electrode.
Source: Synthetic Metals 157 (2007) 997–1003

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