Since the discovery of carbon nanotubes a lot of experimental and theoretical work has appeared investigating and analyzing their structural and electronic properties [1-3]. Especially single walled carbon nanotubes (SWNTs) were proposed both as microcavities for storage [4-9] and as ideal molecular wires for microelectronic applications [10]. Furthermore the functionalization of the SWNT surface was found to affect both their storage capacity [9] and their electronic properties [10-12].

Hydrogen has been recognized as an ideal energy carrier but has not been used yet to a large extent. One of the major problems is the difficulty of efficient storage. In the beginning metal alloys were tested for storage tanks but even though they have sufficient storage capacity, they are expensive and heavy for commercial production focused on mobile applications. In the recent years, carbon based materials attracted attention due to the discovery of novel carbon nanomaterials like fullerenes, nanofibers, and nano-tubes [1-3]. Especially SWNTs, which have diameters of typically a few nanometers, have been suggested as suitable materials for gas storage [4]. Since pores of molecular dimensions can adsorb large quantities of gases, hydrogen can condense to high density inside narrow SWNTs even at room temperature [5]. The high hydrogen uptake of these materials suggests that they can be used as hydrogen-storage materials for fuel-cell electric vehicles [6-8].

Recently it was experimentally proved that the electronic and transport properties of SWNTs are extremely sensitive to their exposure to gas molecules [10-12]. Especially oxygen attracted a lot of interest due to the importance of its interaction with SWNTs both in their synthesis by purification and their functionalization for controlling their electronic properties. Considering that all previous experimental studies of SWNTs have used samples exposed to air, the results of these measurements must be carefully reevaluated.

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