  In this work, we perform a follow-up and comprehensive study on the
structural details and formation mechanism of chemical vapor deposition (CVD)
synthesized one-dimensional (1D) van der Waals heterostructures. Edge
structures, nucleation sites, crystal epitaxial relationships are clearly
revealed using transmission electron microscopy (TEM). This is achieved by the
direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which
enabled a transfer-free and non-destructive access to many intrinsic structural
details. In particular, we have distinguished different shaped boron nitride
nanotube (BNNT) edges, which are confirmed, by electron diffraction at the same
location, to be strictly associated with its own chiral angle and polarity. We
also demonstrate the importance of surface cleanness and isolation for the
formation of perfect 1D heterostructures. Furthermore, we elucidate the
handedness correlation between SWCNT template and BNNT crystals. This work not
only provides an in-depth understanding of this new 1D heterostructure material
group, but also, in a more general perspective, serves as an interesting
investigation on crystal growth on highly curved (radius of a couple of nm)
atomic substrates.
