Localization of the Optical Phonon Modes in Boron Nitride Nanotubes: Mixing Effect of 10 B Isotopes and Vacancies.

We explored the mixing effect of 10 B isotopes and boron (B) or nitrogen (N) vacancies on the atomic vibrational properties of (10,0) single-wall boron nitride nanotubes (BNNTs). The forced oscillation technique was employed to evaluate the phonon modes for the entire range (0-100%) of 10 B isotopes and atomic vacancy densities ranging from 0 to 30%. With increasing isotope densities, we noticed a blue shift of the Raman-active A 1 phonon peak, whereas an increased density of mixed or independent B and N vacancies resulted in the emergence of a new low-frequency peak and the annihilation of the A 1 peak in the phonon density of states. High-energy optical phonons were localized as a result of both 10 B isotopes and the presence of mixing defects. We found an asymmetrical nature of the localization length with increasing 10 B isotope content, which corresponds well to the isotope-inherited localization length of carbon nanotubes and monolayer graphene. The localization length falls abruptly with the increase in concentration of both atomic vacancies (B or N) and mixing defects ( 10 B isotope and vacancies). These findings are critical for understanding heat conduction and nanoscopic vibrational investigations such as tip-enhanced Raman spectra in BNNTs, which can map local phonon energies.