Probing the tunable optical properties of highly luminescent functionalized graphene quantum dots as downconverters for superior detection of ultraviolet radiation

Detection and utilization of ultraviolet (UV) radiation are of paramount importance as commonly used Si photodetectors and photovoltaic cells suffer from poor UV response. Herein, we demonstrate the applications of graphene quantum dots (GQDs) as downconverters to sensitize Si photodiodes for extended and enhanced UV detection down to 300 nm and facilitating distinct visible response to UV wavelength-specific excitation. Stable blue (B-) and orange-red (R-) emitting nitrogen functionalized GQDs, synthesized by cost-effective solvothermal method, exhibit high photoluminescence (PL) quantum yield (57-62%), strong UV absorption, and large Stokes shift (130-250 nm). A scalable threefold improvement in UV response (330 nm) with a relative enhancement factor of 200% is recorded from a Si photodetector externally sensitized with visible-blind B-GQDs. The engineered combination of B- and R-GQDs in a tandem fashion exhibits distinct visible emission and facilitates bareeye recognition of UV sub-bands. The present study provides a new pathway of UV detection with or without integrated devices by employing thin layers of solution-processed GQDs.