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Fig. 7.18 (a) Calculated emission wavelength (nm) using TDDFT method in vacuum as a function
of the diameter of GQDs. The solid line is the linear fitting of zigzag-edged GQDs (G1–G6).
The indicated diameter is the average of the horizontal and vertical dimensions. (b) Emission
wavelength of oxidized GQD (G4) as a function of the coverage of –OH and –COOH groups.
(Reproduced from Ref. [141] with permission of the Royal Society Chemistry)
indicating that the entire visible light emissions (400–770 nm) may be covered
by the diameter of GQD while varying from 0.89 to 1.80 nm. The red shift of
the emission wavelength, as the size increases, is due to the decrease in band gap
resulting from π-electron delocalization [141]. Moreover, such a band gap reduction
may also be caused by the fabrication of functional groups (Fig. 7.18b). Graphene
with either zigzag or armchair edges may influence the fluorescence of GQDs;
for instance, the 1.27 and 2.06 nm armchair-edged GQDs emit at blue-shifted
wavelength of ∼450.5 and 678.2 nm, respectively, whereas the zigzag-edged GQDs