Microscopically inhomogeneous electronic and material properties arising during thermal and plasma CVD of graphene
Graphene layers were prepared on copper substrates by thermal chemical vapor deposition (CVD) and microwave (MW) plasma CVD processes. Atomic force microscopy in topography, phase imaging, and conductivity detection (C-AFM) as well as Raman micro-spectroscopy are employed to analyze the graphene layers on a microscopic scale in an as-deposited state on copper. By correlating these data we identify microscopic inhomogeneities (on a micrometer and nanometer scale) in material and electrical properties of both types of graphene. The inhomogeneities are attributed to (i) large but defects including flakes of plasma CVD graphene and (ii) high quality but small flakes of thermal CVD graphene. Moreover, the plasma CVD graphene contains large density of non-conductive openings (0.1–1 μm in size) where no graphene is detected. The obtained data explain two orders of magnitude lower electrical conductivity of the plasma CVD graphene measured macroscopically after substrate transfer. Implications for optimizing the graphene growth processes are discussed.