Abstract
Large-area single-crystal monolayers of two-dimensional (2D) materials such
as graphene and hexagonal boron nitride (h-BN) can be grown by chemical vapour
deposition (CVD). However, the high temperatures and fast timescales at which
the conversion from a gas-phase precursor to the 2D material appear, make it
extremely challenging to simultaneously follow the atomic arrangements. We
utilise helium atom scattering to discover and control the growth of novel 2D
h-BN nanoporous phases during the CVD process. We find that prior to the
formation of h-BN from the gas-phase precursor, a metastable $(3\times3)$
structure is formed, and that excess deposition on the resulting 2D h-BN leads
to the emergence of a $(3\times4)$ structure. We illustrate that these
nanoporous structures are produced by partial dehydrogenation and
polymerisation of the borazine precursor upon adsorption. These steps are
largely unexplored during the synthesis of 2D materials and we unveil the rich
phases during CVD growth. Our results provide significant foundations for 2D
materials engineering in CVD, by adjusting or carefully controlling the growth
conditions and thus exploiting these intermediate structures for the synthesis
of covalent self-assembled 2D networks.