  The catalyst-free synthesis of graphene on dielectrics prevents the damage
induced by the transfer process. Although challenging, to master this synthesis
would boost the integration of graphene on consumer electronics since defects
hinder its optoelectronic properties. In this work, the influence of the
different surface terminations of c-plane sapphire substrates on the synthesis
of graphene via plasma-enhanced chemical vapour deposition (PECVD) is studied.
The different terminations of the sapphire surface are controlled by a plasma
etching process. A design of experiments (DoE) procedure was carried out to
evaluate the major effects governing the etching process of four different
parameters: i.e. discharge power, time, pressure and gas employed. In the
characterization of the substrate, two sapphire surface terminations were
identified and characterized by means of contact angle measurements, being a
hydrophilic (hydrophobic) surface the fingerprint of an Al- (OH-) terminated
surface, respectively. The defects within the synthesized graphene were
analysed by Raman spectroscopy. Notably, we found that the ID/IG ratio
decreases for graphene grown on OH-terminated surfaces. Furthermore, two
different regimes related to the nature of graphene defects were identified and
depending on the sapphire terminated surface are bound either to vacancy or
boundary like defects. Finally, studying the density of defects and the
crystallite area, as well as their relationship with the sapphire surface
termination paves the way for increasing the crystallinity of the synthesized
graphene.
