Anomalous electron spin resonance behavior of single-walled carbon nanotubes

We have studied the electron spin resonance (ESR) of single-walled carbon nanotubes (SWNTs) both in their pristine state and after irradiation with fast electrons in order to detect the signal of conduction electrons. No metallic Pauli contribution was observed in the global spin susceptibility, the ESR signal of the conduction electrons is undetectable because it is broadened by magnetic impurities. We measured a paramagnetic contribution from localized states, with an effective Curie constant decreasing when the temperature increases, following a deactivation law of the type $A-B\exp (-E_a∕k_{B}T)$. This behavior is supposed to be characteristic of semiconducting SWNTs interacting with metallic impurities from the catalyst.

Figure 1

$X$-band ESR spectrum at $T=10\mathrm{K}$ of $3\mathrm{C}∕{\mathrm{cm}}^2$ electron-irradiated SWNTs, fitted by two Lorentzian lines. The use of two Lorentzian facilitates the analysis but is not expected to reflect the presence of two distinct paramagnetic centers. We believe in fact that the line has a hyper-Lorentzian shape reflecting a distribution of relaxation times in a weakly coupled spin system. The inset shows a large scan $(0–6000\mathrm{G})$. The irradiation-induced signal near $3300\mathrm{G}$ $(g=2.0028)$ is superimposed on a large ferromagnetic background coming from magnetic catalytic nanoparticles.

Figure 2

Temperature dependence of the total integrated intensity $I$ of the fitted Lorentzian lines for the pristine SWNT sample (+), a sample irradiated at $2\mathrm{C}∕{\mathrm{cm}}^2$ (엯), and another one irradiated at $3\mathrm{C}∕{\mathrm{cm}}^2$ (•). Each sample shows specific temperature dependence of intensity, which is similar for the two fitted lines in each sample. The full line shows the expected curve for localized spins described by the Curie law.

Figure 3

Temperature dependence of the effective Curie constant $C_{\mathrm{eff}}=TI$, for the three samples: (+) pristine, (엯) $2\mathrm{C}∕{\mathrm{cm}}^2$, (•) $3\mathrm{C}∕{\mathrm{cm}}^2$. $C_{\mathrm{eff}}$ has been divided by its value at $200\mathrm{K}$ for a better comparison between the samples. Full lines represent a fit by $A-B\exp (-E_a∕k_{B}T)$ to simulate a deactivation process. The values of $A∕B$ and $E_a$ are reported in Table . $C_{\mathrm{eff}}$ in graphitic materials usually shows a straight line of positive slope (the Pauli term) intercepting the $y$ axis at the Curie constant.

Figure 4

$C_{\mathrm{eff}}$, normalized by the sample mass, is reported in arbitrary units as a function of the irradiation dose, at $10\mathrm{K}$ (full circles) and $200\mathrm{K}$ (open circles).

Figure 5

Width of the two fitted Lorentzian lines for the three samples: (+) pristine, (엯) $2\mathrm{C}∕{\mathrm{cm}}^2$, (•) $3\mathrm{C}∕{\mathrm{cm}}^2$