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

Md Sherajul Islam; Ashraful Hossain Howlader; Rongkun Zheng; Catherine Stampfl; Jeongwon Park; Akihiro Hashimoto

doi:10.1021/acsomega.2c02792

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

ACS Omega. 2022 Jul 18;7(30):26591-26600. doi: 10.1021/acsomega.2c02792. eCollection 2022 Aug 2.

Authors

Md Sherajul Islam^{1

2}, Ashraful Hossain Howlader³, Rongkun Zheng³, Catherine Stampfl³, Jeongwon Park^{4

2}, Akihiro Hashimoto⁵

Affiliations

¹ Department of Electrical and Electronic Engineering, Khulna University of Engineering and Technology, Khulna 9203, Bangladesh.
² Department of Electrical and Biomedical Engineering, University of Nevada, Reno, Nevada 89557, United States.
³ School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia.
⁴ School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa ON K1N 6N5, Canada.
⁵ Graduate School of Engineering, University of Fukui, Fukui 910-8507, Japan.

Abstract

We explored the mixing effect of ¹⁰B 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 ¹⁰B isotopes and atomic vacancy densities ranging from 0 to 30%. With increasing isotope densities, we noticed a blue shift of the Raman-active A₁ 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 A₁ peak in the phonon density of states. High-energy optical phonons were localized as a result of both ¹⁰B isotopes and the presence of mixing defects. We found an asymmetrical nature of the localization length with increasing ¹⁰B 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 (¹⁰B 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.