Effects of Microwave Absorption on Long and Short Single-Walled Carbon Nanotubes at 10-6 Torr

Carbon nanotubes have been observed to emit ultraviolet, visible and infrared radiation when exposed to microwaves. We have performed experiments in which both short (0.5 μm–2 μm) and long (5 μm–30 μm) single and double-walled carbon nanotubes were exposed to 2.46 GHz microwaves at a pressure of ~ 10-6 Torr. Structural modifications of the carbon nanotubes due to microwave absorption have been studied using the Raman spectroscopy G-band and D-band intensities, which suggest that microwave irradiation at relatively low pressure results in an increase in nanotube defects, especially in the case of the long nanotubes. Furthermore, a comparison of the spectra of the radiation emitted from the nanotubes suggests that the longer nanotubes emitted radiation of much greater intensity than the shorter nanotubes. Based on the results of the experiments and results described in previous reports, the ultraviolet, visible and infrared radiation emitted as the result of microwave absorption by carbon nanotubes seems to be primarily blackbody radiation emitted due to Joule heating. However, the presence of several broad photopeaks in the spectra of the emitted radiation (which do not seem to be related to gases absorbed by the nanotubes or the presence of catalyst particles) suggest that emissions are not the result of Joule heating alone.

Defect Photoluminescence of TiO2 Nanotubes

Photoluminescence (PL) spectroscopy of nanocrystalline TiO2 using ultraviolet light excitation reveals a range of intra-bandgap defect states which emit at visible wavelengths. In this study we use 350 nm excitation to probe the luminescent defect states of TiO2 nanotubes fabricated by anodization of titanium. The nanotubes show a broad visible luminescence from 450 to 700 nm with a peak at 520-550 nm or 2.4-2.3 eV. The intensity of nanotube PL is orders of magnitude lower than that of nanoparticulate anatase and P25 (mixed-phase anatase/rutile) films of comparable thickness. Similar to the PL of the nanoparticles, the nanotube PL is increased by vacuum annealing. The nature of the nanotube defects is investigated through shifts in the intensity and shape of the PL spectra in hole or electron scavenging environments. We find the PL intensity of the nanotubes to be less dependent on environment than that of conventional TiO2 nanoparticles. We conclude that there are two inter-related reasons for decreased intensity and decreased environmental dependence of PL from TiO2 nanotubes as compared to nanoparticles: decreased density of defect states and improved carrier transport.