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

TL;DR

Thermal annealing that removes oxygen from single-walled carbon nanotubes significantly enhances their ESR signal by increasing spin susceptibility, while spin transport remains unaffected, indicating oxygen's role as an acceptor compensating intrinsic defects.

Contribution

This study demonstrates that oxygen desorption can reversibly increase ESR signal in SWCNTs and provides insights into the role of molecular oxygen as an acceptor affecting spin susceptibility.

Findings

ESR signal increases fourfold after oxygen removal

Spin susceptibility follows Curie law from 4 K to 300 K

Oxygen desorption does not affect ESR linewidth or spin hopping frequency

Abstract

We have observed a nearly fourfold increase in the electron spin resonance (ESR) 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 ESR in SWCNTs can be reversibly altered via the molecular oxygen content in the samples. Independent of the presence of adsorbed oxygen, a Curie-law (spin susceptibility $\propto 1/T$) is seen from $\sim$4 K to 300 K, indicating that the probed spins are finite-level species. For both the pre-annealed and post-annealed sample conditions, the ESR linewidth decreased as the temperature was increased, a phenomenon we identify as motional narrowing. From the temperature dependence of the linewidth, we extracted an estimate of the intertube hopping frequency; for both sample conditions, we found this hopping frequency to be $\sim$100 GHz. Since the spin hopping frequency changes only slightly when oxygen is desorbed, we conclude that only the spin susceptibility, not spin transport, is affected by the presence of physisorbed molecular oxygen in SWCNT ensembles. Surprisingly, no linewidth change is observed when the amount of oxygen in the SWCNT sample is altered, contrary to other carbonaceous systems and certain 1D conducting polymers. We hypothesize that physisorbed molecular oxygen acts as an acceptor ($p$-type), compensating the donor-like ($n$-type) defects that are responsible for the ESR signal in bulk SWCNTs.