Brillouin light scattering from carbon nanotube arrays

Abstract

Brillouin light scattering spectroscopy was used to investigate the vibrations in two carbon nanotube arrays (one with 200 nm - 300 nm of the nanotubes protruding above the alumina substrate surface and one with the top of nanotubes flush with the alumina surface) and two alumina templates with holes (non-annealed and annealed). The carbon nanotube array with nanotubes protruding above the surface was exposed to vacuum and various gases (H ₂ , CH₄, Ar, Kr, CF₄, SF₆, C₂F₆ and air). For this sample at least three Brillouin peaks were observed for free spectral range up to 50 GH z. The Brillouin modes were at the frequency shifts of ~1.3 GHz, ~ 5 GHz and ~ 7 GHz. None of the modes exhibited surface character. One additional peak of Gaussian profile and zero frequency shift, originating from the gas surrounding the sample was also noticed. The line shapes and intensities of the peaks depended on the gaseous environment to which the sample was exposed. The most significant changes in intensity and frequency shifts were observed when H₂ was used as an environment. Brillouin scattering experiments performed on the sample of carbon nanotube array with zero exposed length and on non-annealed and annealed alumina templates that represent intermediate steps in the fabrication of carbon nanotube arrays aided in the determination of the character and origin of the peaks observed in the carbon nanotube array spectra. The values of phonon velocities were determined using a coarse approximation. It is possible that these peaks originate due to the bulk modes propagating in the array. In this case the phonon velocities were estimated to be a few hundreds meters per second for the peak at ~1.3 GHz and between 1000-2000 m/s for peaks at 5 and 7 GHz, depending on the gas surrounding the sample. It is also conceivable that the peaks are transverse, longitudinal and/or twist (torsional) modes that propagate in individual tubes.