Effect of size variation on the cathodoluminescence characteristics of graphene quantum dots

Cathodoluminescence (CL) has been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The size dependence of CL peak wavelength is very analogous to that of photoluminescence (PL) peak wavelength unusually showing non-monotonic behaviors having a maximum at d = ∼17 nm. The CL behaviors can therefore be attributed to the novel feature of GQDs, i.e., the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at d = ∼17 nm as d increases. However, the peak wavelengths of CL are especially much smaller than those of PL at both ends in the size range of GQDs, possibly resulting from the recombination of the electron-beam-excited e-h pairs at higher energy states before thermalization due to fast carrier-carrier scattering dominating over electron-phonon scattering in graphene.