Temporally dynamic photopolymerization of ${\text{C}}_{60}$ molecules encapsulated in single-walled carbon nanotubes

The photopolymerization of carbon-60 $\left({\text{C}}_{60}\right)$ molecules encapsulated inside single-walled carbon nanotube $\left(en{\text{-C}}_{60}\right)$ is investigated through far-field and near-field Raman spectroscopies. In general, a moderately intense laser irradiation invokes complete photopolymerization of nonencapsulated bulk ${\text{C}}_{60}$ molecules (bulk-${\text{C}}_{60}$). However, in contrast, we observed in our experiments that the $en{\text{-C}}_{60}$ molecules never get completely polymerized, even under long and strong irradiation. In both far-field and near-field Raman measurements, we observed evidences of simultaneous occurrence of polymerization and depolymerization of $en{\text{-C}}_{60}$ during the process of irradiation. The temporal fluctuation of Raman intensity associated with ${\text{C}}_{60}$-monomer peak confirmed that $en{\text{-C}}_{60}$ molecules jump back and forth between the polymerized and depolymerized states. The results have been discussed through the unique movement of ${\text{C}}_{60}$ molecules inside nanotube, and through the interaction between ${\text{C}}_{60}$ molecules with the inner wall of the nanotube in $en{\text{-C}}_{60}$ sample, which generates frictional heat, responsible for the scission of polymeric bonds between neighboring ${\text{C}}_{60}$ molecules. The frictional scission competes with the photopolymerization process, resulting in a temporally dynamic situation where the sample can never get completely polymerized even under strong irradiation.

Figure 1

TEM image of ${\text{C}}_{60}$ encapsulated in an isolated SWNT. The contrast in this image depicts the presence of closely packed ${\text{C}}_{60}$ molecules inside the tube, as illustrated by the dotted circles in a zoomed area in the inset. This is also evident that there are no ${\text{C}}_{60}$ molecules on the outer wall of the SWNT.

Figure 2

(Color online) (a) Raman spectra of bulk-${\text{C}}_{60}$ sample in the spectral range of $A_g\left(2\right)$ mode, before (lower curve) and after (upper curve) laser irradiation. The blue spectrum shows a monomer peak at $1467{\text{cm}}^{-1}$ while the red spectrum shows a polymer peak $1457{\text{cm}}^{-1}$. (b) Laser irradiation power dependence of the amount of polymerization, quantified by the intensity ratio $I_{1467}/I_{1457}$, where $I_{1467}$ and $I_{1457}$ represent Raman intensities of the modes at 1467 and $1457{\text{cm}}^{-1}$, respectively. The exposure time, the accumulation time and the observation laser power for Raman measurements were 1 min, 5 min, and $8\mu \text{W}$, respectively.

Figure 3

(Color online) (a) Raman spectra of $en{\text{-C}}_{60}$ sample in the spectral range of $A_g\left(2\right)$ mode, measured with two different observation laser powers of $8\mu \text{W}$ (blue curve) and $500\mu \text{W}$ (red curve). In order to focus our attention on the ${\text{C}}_{60}$ vibrational modes, the SWNT background signal was removed. The $A_g\left(2\right)$ mode in blue spectrum is observed at $1464{\text{cm}}^{-1}$, which is downshifted by $3{\text{cm}}^{-1}$ in comparison with bulk-${\text{C}}_{60}$ sample due to encapsulation. This peak is still observed in the red spectrum, which, in addition, shows a polymer peak at $1458{\text{cm}}^{-1}$, as illustrated by the dotted curves. (b) Irradiation power dependence of the monomer peak intensity. The exposure time, the accumulation time and the observation laser power were 1 min, 20 min, and $8\mu \text{W}$, respectively. (c) Time dependence of the intensity ratio $I_{1464}/I_{1458}$, where $I_{1464}$ and $I_{1458}$ represent Raman intensities of the modes at 1464 and $1458{\text{cm}}^{-1}$, respectively. All measurements were performed under $500\mu \text{W}$ irradiation with an accumulation time of 5 min from the same spot of the sample.

Figure 4

(Color online) (a) Near-field Raman spectra of $en{\text{-C}}_{60}$ sample in the spectral range of $A_g\left(2\right)$ mode. Blue curve shows normal Raman spectrum (far-field) while the red curve shows TERS spectrum (near-field). Due to poor sensitivity in these experiments, the $A_g\left(2\right)$ mode cannot be observed in the far-field, however, it appears clearly in the TERS spectrum due to the field enhancement. (b) Time dependence of TERS intensity of the monomer peak at $1464{\text{cm}}^{-1}$ obtained from a fixed sample position. The spectra were recorded under $400\mu \text{W}$ observation power with accumulation time of 2 s. The temporal fluctuation of this peak becomes stronger and frequent when measured from a small area $\left(\sim 30\text{nm}\right)$ of the sample in TERS experiments.