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

[Image: see text] 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.