Fig. 1. (a) SEM image with insert showing XRD pattern of as-grown C₇₀ nanotubes and (b) Raman spectrum with 830-nm laser used as excitation line of as-grown C₇₀ nanotubes.

Fig. 2. PL spectrum of as-grown C₇₀ nanotubes irradiated by 514.5-nm laser with power 0.2 mW, with insert showing UV-Vis absorption spectrum of as-grown C₇₀ nanotubes, and red and blue dashed lines denoting two fitted peaks of the PL band of C₇₀ nanotubes.

Fig. 3. PL spectra of pristine C₇₀ nanotubes irradiated by 514.5-nm laser with 0.2 mW, 1 mW, 2 mW, 5 mW, and 10 mW, respectively.

Fig. 4. (a) Laser power-dependent center positions of PL peak and (b) laser-power-dependent intensity ratio of peak B to peak A of C₇₀ nanocrystals.

Fig. 5. Raman spectra of C₇₀ nanotubes irradiated by (a) 514.5-nm laser and (b) 830-nm laser, respectively. Lasers with various powers are employed for comparison, and characterized E2′ Raman peak for samples irradiated with different values of laser power are shown in inserts.

Fig. 6. (a) PL spectra of C₇₀ nanotubes irradiated by 514.5-nm laser with power of 10 mW (black) and C₇₀ nanotubes treated under 2.0 GPa, 700 K (red). PL spectrum of the laser irradiated sample is fitted to two peaks: Peak A (red dashed curve) and peak B (green dashed curve). (b) Raman spectra of C₇₀ nanotubes irradiated by laser with 10-mW power and HPHT treated under 2.0 GPa, 700 K.