Reversible and non-reversible photo-enhanced luminescence in CdSe/ZnS quantum dots

We report on optical enhancement of the 655 nm photoluminescence (PL) band intensity in the core-shell CdSe/ZnS nano-particles (quantum dots) under a laser illumination. Kinetic curves and 80 K-to-room temperature dependences of the PL intensity reveal two parallel processes: a reversible enhancement when the PL intensity is recovered after the laser illumination is turned off, and a non-reversible permanent increase of the PL output. Experimental data evidence that the PL enhancement is attributed to the light-activated increase of the energy barrier for photo-generated carriers to escape quantum dot levels. A trap recharging and/or photo-chemical bond restructure play a possible role in the luminescence photo-enhancement.