Stepwise Molecular Confinement Enabling Persistent Room-Temperature Phosphorescence in Blue Graphene Quantum Dots with Quantum Yield Exceeding 60%

Graphene quantum dots (GQDs) face major challenges in solid-state luminescent applications due to aggregation-induced quenching, surface defects, and dominant nonradiative decay pathways. In this study, a stepwise molecular hybridization strategy was developed to simultaneously enhance solution fluorescence and solid-state room-temperature phosphorescence (RTP) of GQDs. GQDA (graphene quantum dot–APTES) introduces siloxane-related hybridization through Si–O–Si and possible Si–O–C interactions, while UGQD (urea–GQD) provides nitrogen-related surface passivation and hydrogen-bond-assisted confinement. The fully hybridized UGQDA system combines both structural rigidification and molecular passivation within a cooperative hybrid framework. As a result, UGQDA exhibits bright fluorescence in solution together with persistent solid-state RTP, addressing the common limitation in carbon nanomaterials where fluorescence and phosphorescence are typically optimized separately. The optimized UGQDA hybrid achieves a photoluminescence quantum yield (PLQY) of approximately 60% at 2.0 wt % GQD loading, compared with 3.2% for pristine GQDs, together with enhanced photoluminescence intensity in both solution and solid states. The hybrid system additionally exhibits prolonged room-temperature afterglow lasting up to 10 s and phosphorescence lifetimes approaching 0.9 s under ambient conditions. The enhanced optical behavior is consistent with reduced nonradiative relaxation, increased structural confinement, and progressive reduction of the singlet–triplet energy gap (ΔE_ST) from 0.23 eV in pristine GQDs to 0.15 eV in UGQDA. These findings demonstrate that cooperative molecular hybridization provides an effective strategy for developing heavy-metal-free phosphorescent GQD materials for optical security, anticounterfeiting, and advanced luminescent applications.