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The electronic properties of single-walled carbon nanotubes are shown here to be extremely sensitive to the chemical environment. Exposure to air or oxygen dramatically influences the nanotubes' electrical resistance, thermoelectric power, and local density of states, as determined by transport measurements and scanning tunneling spectroscopy. These electronic parameters can be reversibly "tuned" by surprisingly small concentrations of adsorbed gases, and an apparently semiconducting nanotube can be converted into an apparent metal through such exposure. These results, although demonstrating that nanotubes could find use as sensitive chemical gas sensors, likewise indicate that many supposedly intrinsic properties measured on as-prepared nanotubes may be severely compromised by extrinsic air exposure effects.
Many carbon materials have excellent molecular adsorption and sieving properties (1). Carbon nanotubes in particular, because of their size, large surface area, and hollow geometry, are being considered as prime materials for gas adsorption (2-4), Li storage (5, 6), and selective molecular filtering (7, 8). Independently, the results of numerous theoretical and experimental studies suggest that single-walled carbon nanotubes (SWNTs) behave as nearly ideal one-dimensional quantum wires (9-12). However, virtually no attention has been given to the possible interdependence of gas adsorption and electrical quantum conductance in nanotubes, despite the large surface area of these materials.
We show here that the measured electronic properties of nanotubes [electrical resistance R, thermoelectric power S, and local density of states N(E)] are in fact exceedingly sensitive to environmental conditions, namely gas exposure (13), and can be reversibly "tuned" simply by exposure to air or oxygen. Isolated, apparently semiconducting nanotubes can be converted into apparent metals through room-temperature exposure to oxygen. Hence the electronic properties of a given nanotube are not specified only by the diameter and chirality of the nanotube but depend critically on gas exposure history. Because virtually all previous experimental studies of SWNTs have used samples exposed to air (and perhaps to other contaminants as well from aqueous solution purification or cutting processes), the results of those measurements must be carefully reevaluated before firm conclusions are drawn, especially with respect to the theoretically predicted behavior of idealized pure nanotubes.
The SWNTs used in our experiments were grown by the conventional laser ablation method (14). Material from different synthesis runs yielded similar results. In general, SWNT-rich material taken...