Super-resolution fluorescence quenching microscopy of graphene.

Lately, fluorescence quenching microscopy (FQM) has been introduced as a new tool to visualize graphene-based sheets. Even though quenching of the emission from a dye molecule by fluorescence resonance energy transfer (FRET) to graphene happens on the nanometer scale, the resolution of FQM so far is still limited to several hundreds of nanometers due to the Abbe limit restricting the resolution of conventional light microscopy. In this work, we demonstrate an advancement of FQM by using a super-resolution imaging technique for detecting fluorescence of color centers used in FQM. The technique is similar to stimulated emission depletion microscopy (STED). The combined "FRET+STED" technique introduced here for the first time represents a substantial improvement to FQM since it exhibits in principle unlimited resolution while still using light in the visible spectral range. In the present case we demonstrate all-optical imaging of graphene with resolution below 30 nm. The performance of the technique in terms of imaging resolution and contrast is well described by a theoretical model taking into account the general distance dependence of the FRET process and the distance distribution of donor centers with respect to the flake. In addition, the change in lifetime for partially quenched emitters allows extracting the quenching distance from experimental data for the first time.