Controlling the electronic properties of single-wall carbon nanotubes by chemical doping
Introduction
High-yield synthesis of carbon single-wall nanotubes (SWNTs) 1, 2 has made it possible to investigate the electronic properties of macroscopic amounts of material 3, 4 and to show the possibility of decreasing its resistivity by more than one order of magnitude when it is exposed to potassium or bromine vapours [5].
In a previous Letter, we have shown that it is possible to tune the Fermi level of SWNTs, and thus to modify their electronic structure by changing their density of states [6]. This result was obtained by performing redox reactions between SWNTs thin films and solutions of organic radical-anions and monitoring the change in the electronic structure of individual tubes by optical absorption experiments. In the present Letter, we report on the modifications of the electronic properties of the bulk material undergone by the same redox reactions. The use of macroscopic amounts of SWNTs allows the determination the number of electrons transferred onto the sample, leading to the relationship between both conductivity and charge carrier concentration and the Fermi level of SWNTs. All the reported experiments and their interpretations depend on the fact that the redox potential and the Fermi level are identical, except for the reference level [7].
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Experiment
SWNTs were synthesized by laser vaporization and purified by successive acidic treatments, cross flow filtrations and vacuum annealing, leading to negligible amounts of carbonaceous particles and metal catalysts, with respect to SWNTs [8]. The samples used in the present study, typically of the order of 1–2 mg, were cut in the same bucky paper of ∼15 mg, resulting from the purification process.
All the experiments were performed in a glass apparatus sealed under high vacuum. The sample was fixed
Results and discussion
The effect of doping SWNTs with the radical-anion of naphthalene on the resistance of a sample is shown in Fig. 1. As soon as the doping solution is brought into contact with the sample, its resistance drops, then slightly decreases, and reaches a plateau when total charge transfer is achieved. The resistance of the doped sample is 1/15 of that of the pristine sample and its chemical composition, determined from the change in the optical absorption spectrum of the doping solution before and
Conclusions
Single-wall carbon nanotubes can be reversibly doped by redox reactions with no stages of intercalation. We established and quantified the one to one map between both Fermi level, charge carrier concentration and electrical conductivity. The van Hove singularities characteristic of a one-dimensional system, seem to disappear in intercalated SWNTs, probably because of the loss of long-range ordering.
Acknowledgements
We are grateful to A.G. Rinzler and R.E. Smalley for supplying the SWNT material.
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