Activities of vacancy defects in carton nanotubes were directly monitored by in situ high-resolution transmission electron microscopy at elevated temperatures. Adatom-vacancy pair defects were first prolific due to the knock-on damage, and then the induced vacancies indeed grew up to 1 to 2nm in the size by the following Joule heating. Surprisingly, these large vacancies, or "holes", tend to migrate and coalesce with each other to form even larger ones. It suggested that the activation barrier was substantially lowered due to the contributions of an electromigration and/or irradiation effect.

Vacancy Migrations in Carbon Nanotubes. Jin, C., Suenaga, K., Iijima, S.: Nano Letters, 2008, 8[4], 1127-30