Enhancement of the Electron Spin Resonance of Single-Walled Carbon Nanotubes by Oxygen Removal

We have observed a nearly 4-fold increase in the electron spin resonance (BR) signal from an ensemble of single-walled carbon nanotubes (SWCNTs) due to oxygen desorption. By performing temperature-dependent ESR spectroscopy both before and after thermal annealing, we found that the BR in SWIM can be reversibly altered via the molecular oxygen content in the samples. Independent of the presence of adsorbed oxygen, a Curie law (spin susceptibility proportional to 1/T) is seen from similar to 4 to 300 K, indicating that the probed spins are finite-level species. For both the pre-annealed and post-annealed sample conditions, the ESR line width decreased as the temperature was increased, a phenomenon we Identify as motional narrowing. From the temperature dependence of the line width, we extracted an estimate of the intertube hopping energy; for both sample conditions, we found this hopping energy to be similar to 1.2 meV. Since the spin hopping energy changes only slightly when oxygen is desorbed, we conclude that only the spin susceptibility, not spin transport, is affected by the presence of physisorbed molecular oxygen in SWCNT ensembles. Surprisingly, no line width change is observed when the amount of oxygen in the SWCNT sample is altered, contrary to other carbonaceous systems and certain 1D conducting polymers. We hypothesize that physisorbed molecular oxygen acts as an acceptor (p-type), compensating the donor-like (n-type) defects that are responsible for the BR signal in bulk SWCNTs.