Critical investigation of up-conversion and dual emission from nitrogen functionalized graphene quantum dots

Up-conversion photoluminescence (UCPL) and dual-band photoluminescence (DBPL) are two widely-reported, exotic properties of graphene quantum dots (GQDs). However, both these phenomena can be associated with measurement artefacts. In case of excitation with monochromatic radiation derived from a white light source, the second order of the excitation or the emission signal can cause misleading impressions of UCPL and DBPL. Laser excitations can also generate spurious UCPL and/or DBPL signals due to leaking radiations. Using a spatially separated femtosecond pulsed laser, we find that in nitrogen-functionalized GQDs there is no evidence for DBPL although a real UCPL is hidden behind spurious signals.