  We studied the photoluminescence (PL) properties of graphene quantum dots
(GQDs) separated by size-exclusion high performance liquid chromatography. The
size separation of GQDs was confirmed by transmission electron microcopy
images. PL excitation maps of chromatographically-separated GQDs show four
distinct emission peaks at 330, 440, 520, and 600 nm, respectively. The
dominant luminescence features of the separated GQDs show discrete change
depending on the overall size of GQDs, indicating that PL variation occurs
because of differences in density, shape, and size of sp2 fragments available
in the GQDs. On the basis of the experimental results of PL peak wavelength and
pH dependence, the emission is attributed to quasi-molecular PL from the
fragments composed of a few aromatic rings with oxygen containing functional
groups.
