The effects of electron and hole injection on the photoluminescence of CdSe/CdS/ZnS nanocrystal monolayers

The photoluminescence (PL) of 2D monolayers of CdSe/CdS/ZnS semiconductor nanocrystals (NCs) deposited on gold substrates and incorporated into electrochemical cells has been studied. By combining simultaneous cyclic voltammetry and confocal microscopy it is demonstrated that when a positive potential is applied to the film in an acetonitrile electrolyte, the PL is irreversibly quenched. This is irrespective of whether the samples are under an inert atmosphere or exposed to air or water vapor. When a negative potential is applied under nitrogen, quenching is also observed; however, it is reversible. Conversely when a negative potential is applied to the NC films in aerated acetonitrile, the PL intensity increases. The enhancement of the PL is stable for at least 180 s while the potential is held at - 1.0 V (vs Ag quasi-reference electrode). When the potential is removed the PL intensity returns to the starting value. These results clearly indicate that photobrightening and charge carrier injection are coupled processes. On the basis of these data, we propose a simple kinetic model that explains the origins of photobrightening.