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 $7\mathrm{K}$) 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 $25\left(2\right){\mathrm{cm}}^{-1}$ 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.

Figure 1

cw-ESR and ED-ESR spectra at low temperatures: (a) pristine ACF. The ED-ESR signal is recorded in the form of absorption curve, whereas cw-ESR signal is recorded as the first derivative of the absorption. (b) In the cw-ESR spectrum of $\mathrm{ACF}+\mathrm{NB}$ the three components can be distinguished (Refs. 23, 35), whereas in ESE experiments the echo signal can be induced with spins in narrowest line only. The ED-ESR line is differentiated for better comparison with cw-ESR spectrum.

Figure 2

Temperature dependence of the spin-lattice relaxation rate $1∕T_1$ for ACF and $\mathrm{ACF}+\mathrm{NB}$ samples. The solid lines are the best fits to Eq. (1) with thermally activated process having activation energy $24{\mathrm{cm}}^{-1}$.

Figure 3

Electron spin echo amplitude decay in ACF and $\mathrm{ACF}+\mathrm{NB}$ at $22\mathrm{K}$. The recovery is exponential and shown by the solid lines resulting from Eq. (2).

Figure 4

Temperature dependence of the ESE dephasing rate $1∕T_M$ ($T_M$ is the phase memory time). The exponential increase in $1∕T_M$ is due to thermally activated motion with activation energy $27{\mathrm{cm}}^{-1}$ for ACF and $44{\mathrm{cm}}^{-1}$ for $\mathrm{ACF}+\mathrm{NB}$.