Effect of surface treatment on photoluminescence of silicon carbide nanotubes

The inherent properties of silicon carbide as a wide band-gap semiconductor has enabled its use in most power devices for high frequency, high power, and high temperature applications. However, its application in optical devices has been hampered since it is an indirect-band-gap semiconductor which shows rather weak luminescence. Research advances in the field of nanotechnology have however shown that silicon carbide nanostructured materials exhibit different photoluminescence spectra as a result of the different structures of these materials. In spite of these advances, little has been reported on the photoluminescence properties of silicon carbide nanotubes.

Researchers at National Institutes for Quantum and Radiological Science and Technology and Tokyo University of Science investigated the impact that surface treatment has on the photoluminescence properties of silicon carbide nanotubes. Their research is now published in Applied Surface Science.

The authors formed the carbon-silicon carbide coaxial nanotubes by heating silicon powder with multi-walled carbon nanotubes. Next, the nanotubes were subjected to heat treatment in air whereby samples 1 and 4 were subjected to 700^°C for 2 hours, samples 2 and 5 to 800^°C for 4 hours, and samples 3 and 6 to 900^°C for 4 hours. Samples 4 to 6 were then treated in sodium hydroxide solution in order to modify the surface structure, after which they were treated in hydrochloric acid to completely remove the sodium ions from the nanotube surface. The as-formed nanotubes were then scattered in toluene and subjected to ultrasonic treatment.

The authors observed that all samples exhibited a broad x-ray diffraction peak which was indexed to multi-walled carbon nanotubes. Further, the intensities of this peaks were tiny as compared with those of silicon carbide, and they decreased as the temperature of heat treatment in air increased. Spectroscopy of silicon carbide nanotubes conducted before and after the heat treatment at 700^°C in air, showed that the nanotubes had 2 peaks which corresponded to carbon-silicon and carbon-carbon bonds which had binding energies of 282.6eV and 284.6eV respectively. The tests show that almost all the unreacted multi-walled carbon nanotubes were eliminated by heat treatment in air and that the remaining amount of elemental carbon decreased as the heat treatment temperature increased.

X-ray photoelectron spectroscopy of silicon carbide nanotubes, conducted with and without sodium hydroxide-hydrochloric acid treatment, showed that samples 1 to 3 had 2 peaks which corresponded to silicon-carbon and silicon-oxygen bonds which had binding energies of 100.6eV and 103.4eV respectively. Samples 4 to 6 however did not exhibit a peak that corresponds to silicon-oxygen bonds. From this the authors noted that silicon oxide layers on silicon carbide nanotubes were formed during heat treatment in air and they were eliminated through the subsequent sodium hydroxide-hydrochloric acid treatment.

The authors also observed that among the silicon carbide nanotubes, sample 1 exhibited a broad photoluminescence band, but which was small as compared with samples 4 to 6, while samples 2 and 3 exhibited no definite peaks. The stronger photoluminescence bands however emerged again in samples 4 to 6 which was after the removal of the silicon oxide layer through sodium hydroxide-hydrochloric acid treatment. The results show that the bands disappear as a result of the formation of silicon oxide layers on the silicon carbide nanotube surfaces.

The research team noted that the origin of photoluminescence was not related to a hydroxyl group adsorbed on defects. Additionally, the quantum confinement effect was observed to not be the sole reason for the observed blue-shifted photoluminescence peak energies.

Tomitsugu Taguchi and colleagues concluded in their study that the surface defects and surface state of silicon carbide nanotubes is closely related to their photoluminescence band. They suggest that the features of the photoluminescence bands can be controlled by varying their surface states which offers guidance for the formation of silicon carbide nanotube-based optical devices.