Effect of Gas Adsorption on the Electrical Properties of Single-Walled Carbon Nanotubes Mats
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INTRODUCTION Since their discovery in 1991 [1], carbon nanotubes have been the subject of intensive research, motivated by the intrinsic richness and diverse applications potential. Their conductive properties depend drastically on both the diameter and chirality of the hexagonal carbon lattice along tubes. In fact, a slight change in the winding of hexagons along the tube can transform the electronic properties of the tube from metallic to a large gap semiconductor
[2]. Several groups have reported electrical resistivity measurement results for mats of singlewall carbon nanotube (SWNT). Around the ambient temperature, this conductivity exhibits metallic (i.e., positive dR/dT) or non-metallic (i.e., negative dR/dT) behavior depending on the experimental preparation (arc discharge or laser ablation) [3-5] and measurements conditions. Due to their high porosity, these materials are extremely sensitive to doping and/or gas adsorption. In this paper, we present results on the influence of the surrounding gas nature on the electrical resistivity of carefully outgassed mats of such samples. EXPERIMENTAL SET-UP Four-probe electrical resistance (E.R.) measurements were performed in a high vacuum chamber on SWNT mats. These samples were produced by the electric arc method [6]. A graphite rod containing powder of catalyst particles (Ni:Y:C) was vaporized under a He atmosphere at 660 mbar. Collecting the collaret part, we performed purification by successively using the following processes: acid treatment, cross flow filtration and annealing under nitrogen atmosphere up to 1200'C during 6 hours [7]. The final product is a compacted mat which contains a multitude of cleaned and compacted SWNT bundles with only a small density of metallic carbides or oxides. The 0.2x2x25mm 3 samples were cut and placed in the E.R. measurement chamber which was then carefully outgassed (base pressure < 3.10-5 mbar). This chamber was divided in two compartments connected to the pumping system: the first one contains the sample holder and its heater; the second one is isolated from the other parts of the chamber allowing the 173 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
preparation and thermalization of the high purity (99.995%) gases before introduction -at a final pressure of -1,000mbar- in the measurement chamber. E.R. measurements were done at 25°C by using a I0mA DC current. Parasite thermoelectric effects were compensated. The sample temperature was measured by a platinum resistor placed very near from the carbon nanotube (CNT) mat. Introduction or pumping of gases were done rapidly : the pressure in the reactor was varying between the working pressure (P) of -1,000mbar and 1mbar in less than one second. We also studied the influence of minute quantities (in ppmV) of H2 0 vapor by injecting few microgrammes of water of ultra-high purity in the intermediate chamber. Nitrogen was then injected in the reactor until the total and final pressure reached 1,000mbar. Before each refilling of the reactor with a new gas
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