  Using a scanning electron microscope, we observed a reproducible, discrete
distribution of secondary electron intensity stemming from an atomically thick
graphene film on a thick insulating substrate. The discrete distribution made
it possible to uniquely relate the secondary electron intensity to the number
of graphene layers. Furthermore, we found a distinct linear relationship
between the relative secondary electron intensity from graphene and the number
of layers, provided a low primary electron acceleration voltage was used. Based
on these observations, we propose a practical method to determine the number of
graphene layers in a sample. This method is superior to the conventional
optical method in its capability to characterize graphene samples with
sub-micrometer squares in area on various insulating substrates.
