  We report catalyst-free direct synthesis of vertical graphene nanosheets
(VGNs) on SiO2/Si and quartz substrates using microwave electron cyclotron
resonance - plasma enhanced chemical vapor deposition. The evolution of VGNs is
studied systematically at different growth stages. Raman analysis as a function
of growth time reveals that two different disorder-induced competing mechanisms
contributing to the defect band intensity. The VGNs grown on SiO2/Si substrates
predominantly consists of both vacancy-like and hopping defects. On the other
hand, the VGNs grown on quartz substrates contain mainly boundary-like defects.
XPS studies also corroborate Raman analysis in terms of defect density and
vacancy-like defects for the VGNs grown on SiO2/Si substrates. Moreover, the
grown VGNs exhibit a high optical transmittance from 95 to 78 % at 550 nm and
the sheet resistance varies from 30 to 2.17 kohms/square depending on growth
time.
