High-temperature molecular beam epitaxy of hexagonal boron nitride monolayers on carbon nanotubes

We report the growth of hexagonal boron nitride (hBN) layers on multiwalled carbon nanotubes (MWCNTs) by high-temperature molecular beam epitaxy (HT-MBE). Using a variety of characterisation techniques, including transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy (AC-STEM-EELS), we demonstrated the epitaxy of boron nitride on the external surfaces of MWCNTs. This process formed coaxial heterostructures consisting of an inner multiwalled carbon nanotube coated with a few monolayers of hBN. The high structural quality of the carbon surface of the MWCNTs enabled smooth, uniform deposition of several monolayers of hBN. We proposed nucleation at defects on the MWCNT surface, followed by a step-flow-like "wrapping" mechanism for the hBN epitaxy around the carbon nanotubes. Additionally, boron nitride nanotubes grew from the open step edges at the tips of the multiwalled carbon nanotubes, facilitating the formation of circular carbon nanotube - boron nitride nanotube (BNNT) heterojunctions. Spatially-resolved photoluminescence (PL) spectroscopy in the UV-C spectral range confirmed the excellent structural quality of the hBN monolayers on the CNTs. The PL spectrum of BNNTs exhibited a line around 5.8 eV, which corresponds to the phonon replica in bulk hBN associated with phonon-assisted recombination involving optical phonons. This PL spectrum provides the first evidence of intrinsic radiative exciton recombination in quasi-one-dimensional hBN, with an estimated indirect bandgap of approximately 5.95 eV.