Room temperature photobrightening and low temperature photodarkening in two-dimensional CdSe nanoplatelets

Colloidal nanoplatelets are atomically flat quasi-two-dimensional colloidal particles of semiconductors which have been a topic of intense study for their optical properties and potential applications. Like for the previous generation quantum dots, the photostability of nanoplatelets remains a topic of concern that must be understood to reliably use them in devices. We have found that the photoluminescence intensity from CdSe nanoplatelets on continuous laser excitation displays a significant rise (photobrightening) over time at temperatures above 215 K. Conversely, a darkening effect is found below 215 K and the extent of photodarkening is systematically enhanced with the increase in the excitation power density. The darkening processes can be systematically reversed through sub-room temperature thermal annealing, suggesting an activated recovery process with a gap of about 55 meV. The photobrightening and photodarkening are inferred to be occurring on account of the photodamage and thermally assisted healing of oleic acid, the capping ligand commonly used for passivation and functionality. A significant enhancement of photobrightening is further observed around 250 K, in the vicinity of the phase transition of oleic acid. This work thus highlights the fundamental role of the ligand-surface interaction in controlling the photoluminescence efficiency of nanoplatelets.