Electron spin relaxation and quantum localization in carbon nanoparticle: Electron spin echo studies

We present pulsed electron spin resonance measurements of activated carbon fibers (ACFs) for pristine samples and after adsorption of nitrobenzene in the fibers' porous structure. Two-level tunneling state (TLS) model is discussed in order to define the nature of paramagnetic centers and to explain the electron spin-lattice relaxation mechanism in the studied system. Electron spin echo decay is dominated by spin diffusion at low temperatures (below 7 K) and for higher temperatures the echo dephasing rate is governed by thermal excitations of the TLS model. The model enables us to treat the nanographitic units being the basic structural units of ACF, as quantum wells (dots) separated by potential barriers of the order of 25(2) cm(-1) determined from temperature dependence of the spin-lattice relaxation rate. Stimulation of these barriers leads to spin localization within nanographitic units. A possible control of the potential barriers is the crucial point for the problem of nanographitic unit system treated as a quantum dot matrix.