  The integration of two-dimensional (2D) materials with functional non-2D
materials such as metal oxides is of key importance for many applications, but
underlying mechanisms for such non-2D/2D interfacing remain largely elusive at
the atomic scale. To address this, we here investigate the nucleation stage in
atomic layer deposition (ALD) of the important metal oxide HfO2 on chemical
vapor deposited graphene using atomically resolved and element specific
scanning transmission electron microscopy (STEM). To avoid any deleterious
influence of polymer residues from pre-ALD graphene transfers we employ a
substrate-assisted ALD process directly on the as grown graphene still
remaining on its Cu growth catalyst support. Thereby we resolve at the atomic
scale key factors governing the integration of non-2D metal oxides with 2D
materials by ALD: Particular to our substrate-assisted ALD process we find a
graphene-layer-dependent catalytic participation of the supporting Cu catalyst
in the ALD process. We further confirm at high resolution the role of surface
irregularities such as steps between graphene layers on oxide nucleation.
Employing the energy transfer from the scanning electron beam to in situ
crystallize the initially amorphous ALD HfO2 on graphene, we observe HfO2
crystallization to non-equilibrium HfO2 polymorphs (cubic/tetragonal). Finally
our data indicates a critical role of the graphene's atmospheric adventitious
carbon contamination on the ALD process whereby this contamination acts as an
unintentional seeding layer for metal oxide ALD nucleation on graphene under
our conditions. As atmospheric adventitious carbon contamination is hard to
avoid in any scalable 2D materials processing, this is a critical factor in ALD
recipe development for 2D materials coating. Combined our work highlights
several key mechanisms underlying scalable ALD oxide growth on 2D materials.
