Photoluminescence from amino functionalized graphene quantum dots prepared by electrochemical exfoliation method in the presence of ammonium ions

This study elucidates how the nitrogen functionalities affect the UV-visible absorption and photoluminescence (PL) emission of N-modified graphene quantum dots (NGQDs). An electrochemical exfoliation technique incorporated with an in situ amino-functionalization is adopted to synthesize N-modified graphene nanosheets (GNs) with different amidation levels. The applied voltage plays a crucial role in tuning the N/C atomic ratio on the GN product. The maximal N/C atomic ratio reaches 8.4% when exfoliating the GNs at 5 V. The nitrogen functionalities on the GNs include (i) amide/amino functional groups, decorated to the edge sites of GNs, and (ii) pyridinic/pyrrolic/quaternary N, planar to the GNs. The tunable PL emission is strongly affected by the n-pi* transitions, forming an interstate transition. The N chemical doping in C=C sp(2) domains and amino edge functional groups strongly influences the bonding and anti-bonding of molecular orbits, leading to different electronic transition n-states. Thus, the N dopant and edge N functionalities serve as new radiative sites to tune the PL emission. The surface coverage of sp(2) carbon bonding decreased with an increase in the number of N functional groups, inducing a red shift. Therefore, the color-tunable emission from blue to greenish color and the increase of absorption in the visible region can be done by tuning appropriate amidation level on the NGQD specimens.