  Graphene, due to its superior stretchability, exhibits rich structural
deformation behaviors and its strain-engineering has proven useful in modifying
its electronic and magnetic properties. Despite the strain-sensitivity of the
Raman G and 2D modes, the optical characterization of the native strain in
graphene on silica substrates has been hampered by excess charges interfering
with both modes. Here we show that the effects of strain and charges can be
optically separated from each other by correlation analysis of the two modes,
enabling simple quantification of both. Graphene with in-plane strain randomly
occurring between -0.2% and 0.4% undergoes modest compression (-0.3%) and
significant hole doping upon thermal treatments. This study suggests that
substrate-mediated mechanical strain is a ubiquitous phenomenon in
two-dimensional materials. The proposed analysis will be of great use in
characterizing graphene-based materials and devices.
