Photobrightening of CdSe/ZnS/TOPO nanocrystals

We have investigated photobrightening effects of photoluminescence (PL) from CdSe/ZnS/TOPO nanocrystals (NCs) in ambient nitrogen gas saturated with water molecules (H2O/N2). The PL intensities from NC ensemble samples increase more than twice after 1200-s irradiation of cw light. These increases mostly recover to their previous values under continuous light irradiation when we evacuate the H2O/N2 atmosphere. These results indicate that the adsorption of H2O molecules onto NC surfaces provides some functions as to restore almost reversibly the PL-quenching defects or trap sites on or near the NC surfaces. To elucidate the mechanism of this restoring function microscopically, we have also measured time evolution of PL intensity profiles from single NCs both in H2O/N2 and in vacuum. While blinking behaviors of the PL intensity being observed in both environments, we have found significant increase of PL on-times in case of single NC observations in the H2O/N2 compared with those in vacuum. As for the blinking off-times, it is for the moment associated with the occasional events of carrier captures at the trap sites on or near the NC surfaces. Thus, we propose the photo-adsorption of H2O molecules inactivates these trap sites and decreases the occurrence of the capturing events, which in turn increase PL on-times and eventually provide the photobrightenings.