Raman spectroscopic studies of graphene have attracted much interest. The
G-band Raman intensity of a single layer graphene on Si substrate with 300 nm
SiO2 capping layer is surprisingly strong and is comparable to that of bulk
graphite. To explain this Raman intensity anomaly, we show that in addition to
the interference due to multiple reflection of the incident laser, the multiple
reflection of the Raman signal inside the graphene layer must be also accounted
for. Further studies of the role of SiO2 layer in the enhancement Raman signal
of graphene are carried out and an enhancement factor of ~30 is achievable,
which is very significant for the Raman studies. Finally, we discuss the
potential application of this enhancement effect on other ultra-thin films and
nanoflakes and a general selection criterion of capping layer and substrate is
given.Comment: 13 pages, 3 figures to be published in Applied Physics Letter
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