On the upconversion fluorescence in carbon nanodots and graphene quantum dots.

Upconversion fluorescence has been frequently cited as an important feature in carbon nanodots (CNDs) and graphene quantum dots (GQDs); and some mechanisms and potential applications have been proposed. Contrary to such a general belief, we demonstrate in this report no observable upconversion fluorescence based on five different synthesized CNDs and GQDs. We confirm that the so-called upconversion fluorescence actually originates from the normal fluorescence excited by the leaking component from the second diffraction in the monochromator of the fluorescence spectrophotometer. Upconversion fluorescence can be identified by measuring the excitation intensity dependence of the fluorescence.