Interaction between edge-localized spins and molecular oxygen in multishell nanographites derived from nanodiamonds

Adsorbed molecular oxygen significantly affects magnetic properties of nanographites. The present work reports on EPR study of multishell nanographite samples obtained from nanodiamonds by prolonged heat treatment. Remarkable differences in EPR spectra and temperature dependences of spectral parameters (integral intensity and linewidths) are observed for both well evacuated/annealed and air saturated samples. At room temperature (RT) air free samples show Lorentzian-like singlet lines narrowing on temperature decrease. RT spectra of oxygen rich samples demonstrate much broader asymmetric signals that, at lower temperatures, reveal superposition of broad (> 90%) and narrow (< 10%) lines. On cooling the broad component starts fast broadening whereas the narrow one remains of the same linewidth. Air free samples demonstrate saturation trend of peak intensities (Ipp) on microwave power (P) increase, whereas air rich samples reveal significant upward deviations from the non-saturating linear behavior in Ipp(P) plots — so-called “anti-saturation”. This deviation occurs due to the narrowing of EPR lines on power rise. The effect may be interpreted in terms of selective and reversible removal of the main broadening agent – oxygen – from the location of uncoupled electron spins.