Laser-based diagnostics applied to the study of BN nanotubes synthesis

The boron nitride nanotubes (BNNTs) synthesis, using CO2-laser vaporization of a BN target under nitrogen gas, is investigated by UV-laser induced fluorescence (LIF) of the vapor phase and UV-Rayleigh scattering (RS) of the gas-suspended nanoparticles. The LIF signal from B atoms is mainly detected in the 1.5 mm-thick region above the BN target. It originates from a boron-rich vapor region confined near the hot boron droplet formed at the target surface. Then, recombination between hot boron and N2 gas occurs through a fast condensation process as revealed by both the depletion of B atoms from the vapor phase and the RS signal arising from the grown BN nanoparticles. Fluorescence spectra exhibit a strong peak at 250 nm due to boron fluorescence and mainly to nanoparticles Rayleigh scattering. A narrow peak is observed at 210 nm and a broader peak at 189 nm. These bands are tentatively assigned to fluorescence or photoluminescence (PL) from gaseous or solid BN species respectively since both gas and solid phases coexist in the plume due to the rapid cooling process. Two very weak bands occur at 308 nm and 350 nm. They are related to PL of defects bands from BN nanostructures on the basis of ex situ PL spectra of h-BN crystallites and multi-wall BNNTs. Detection of oxygen impurities is shown feasible through LIF from BO radical which is detected just above the BN target evaporated under vacuum pressure (approximately 1 mbar). An optical diagnostic strategy is demonstrated from these first in situ observations during BNNTs synthesis.