An in situ e.s.r. study of the intercalation of HNO₃ into highly oriented pyrolytic graphite: diffusion, charge transfer and strain fields

We present in situ e.s.r. studies as a function of exposure time to HNO₃. Detailed measurements at the very beginning of the intercalation process indicate a dramatic variation of the A/B ratio and intensity of the e.s.r. signal associated with the bulk HOPG. This can be interpreted by the formation of a macroscopic 'intercalation' layer and the advance of the interface separating this layer and the non-intercalated bulk HOPG. Using the theory of Zevin for the e.s.r. of layer-substrate systems, the results yield the thickness of the 'intercalation' layer, d, as a function of the exposure time, t. We demonstrate the relation d =(2Dt) ^{1 2} where D is the diffusion constant of the intercalant species. E.s.r. measurements throughout the entire intercalation process reveal the formation of a new e.s.r. line and a continuous shift of this line as a function of exposure time. This continuous variation may be interpreted as being due to charge transfer according to McClure and Yafet and Elliot, as well as to the formation of strain fields. The data strongly suggest that at the beginning of the intercalation process, the HNO₃ species do not form large clusters and that the diffusion fronts are probably not sharp.