Enhanced thermal stability and spin-lattice relaxation rate of N@${\text{C}}_{60}$ inside carbon nanotubes

We studied the temperature stability of the endohedral fullerene molecule N@${\text{C}}_{60}$ inside single-wall carbon nanotubes using electron-spin-resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time $T_1$ of the encapsulated molecule is significantly shorter than that of the crystalline material, which is explained by the interaction of the nitrogen spin with the host nanotubes.

Figure 1

ESR spectra of the crystalline (a) and peapod (b) N@${\text{C}}_{60}:{\text{C}}_{60}$ multiplied by the temperature. The data are shown on different scales for the two kinds of materials. Note the magnified vertical scale for the high-temperature data. Solid curves are fit to the data. Asterisk indicates an impurity signal from the quartz tube.

Figure 2

Temperature dependence of the number of N@${\text{C}}_{60}$ spins, $N\left(T\right)$ normalized to the 400 K values, $○$: crystalline, $◼$: peapod. The longer averaged data for the peapod material at 580, 600, and 620 K are shown with asterisks. Dashed curve shows similar measurements on the crystalline material from Ref. 16. Note the logarithmic vertical scale.

Figure 3

Saturation curves of N@${\text{C}}_{60}$ and peapod N@${\text{C}}_{60}$ at 300 K. The ESR intensities are divided by the microwave magnetic field $B_1$ and are normalized to the values at the lowest power. Solid curves are calculated saturation curves with Eq. (1) and the parameters given in Table . Note that the solid curve for the crystalline material is calculated with relaxation times determined in spin-echo measurements in Ref. 12 with no further adjustable parameters.

Figure 4

Temperature dependence of the spin-lattice relaxation rate $1/T_1$ for peapod. We show the corresponding data for the crystalline material from Ref. 15 magnified by 10 with a dashed curve. Solid line is a guide to the eye.