Quenching-Resistant Solid-State Photoluminescence of Graphene Quantum Dots: Reduction of π-π Stacking by Surface Functionalization with POSS, PEG, and HDA

Graphene quantum dots (GQDs) have attracted great attention as next-generation luminescent nanomaterials due to the advantages of a low-cost process, low toxicity, and unique photoluminescence (PL). However, in the solid-state, the strong pi-pi stacking interactions between the basal planes of GQDs lead to aggregation-caused PL quenching (ACQ), which impedes practical application to light-emitting devices. Here, surface functionalized GQDs (F-GQDs) by polyhedral oligomeric silsesquioxane (POSS), poly(ethylene glycol) (PEG), and hexadecylamine (HDA) to reduce pi-pi stacking-induced ACQ is presented. The POSS-, PEG-, and HDA-functionalized GQDs show a significant enhancement in PL intensity compared to bare GQDs by 9.5-, 9.0-, and 5.6-fold in spin-coated film form and by 8.3-, 7.2-, and 3.4-fold in drop-casted film form, respectively. Experimental results and molecular dynamics simulations indicate that steric hindrance of the functionalization agent contributes to reducing the pi-pi stacking between adjacent GQDs and thereby enabling quenching-resistant PL in the solid-state. Moreover, the GQD-based white light-emitting diodes fabricated by mounting HDA-GQDs on a UV-LED chip exhibits efficient down-conversion for white light emission with a high color rendering index of 86.2 and a correlated-color temperature of 5612 K at Commission Internationale de l'eclairage coordinates of (0.333, 0.359).