White-light emission of blue-luminescent graphene quantum dots by europium (III) complex incorporation

Traditionally, graphene quantum dots are prepared by fragmentation of graphene sheets into the nanoscale particles of controlled sizes followed by band gap adjustment by doping with electron-donating elements. Our novel synthetic aqueous arc discharge process has been developed to produce the blue-luminescent graphene quantum dots (bGQDs). The resulting bGQDs are ∼15 nm in diameter and the amount of oxygen-including functional groups can be controlled to 27.4% and 30.8% at 1 and 4 A of a current level, respectively. The presence of a band gap is confirmed by using scanning tunneling microscopy/spectroscopy (STM/STS). Additionally, we investigated the effect of oxygen doping levels on the band gap by photoluminescence (PL) behaviors and a density functional theory (DFT). The PL emission is red-shifted from 397 to 425 nm corresponding to the amount of oxygen-including functional groups in bGQDs and the DFT calculation confirms the decrease in a band gap from ∼2.0 to ∼1.7 eV due to electron donation from oxygen. In addition, our quantum dots have promising applications for practical use in optoelectronics devices. For example, tris-dibenzoylmethane mono-1,10-phenanthroline-europium (III) (Eu^IIIDP) is incorporated with bGQDs for white-light emission and is shown to be successfully fabricated into light-emitting polymer films.