A Fluorescence Correlation Spectroscopy, Steady-State, and Time-Resolved Fluorescence Study of the Modulation of Photophysical Properties of Mercaptopropionic Acid Capped CdTe Quantum Dots upon Exposure to Light

Abstract

Light-induced modulation of the fluorescence behavior of mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) in aqueous solution is studied by a combination of fluorescence correlation spectroscopy (FCS) and steady-state and time-resolved fluorescence techniques. These investigations reveal a dramatic variation in the fluorescence properties of the QDs under exposure to light. In the FCS measurement, a large decrease in amplitude and change in shape of the correlation curves are observed with increasing excitation power. The change in the shape of the correlation curves, particularly at short lag time, e.g., a faster relaxation at high excitation power, is attributed to the increasing contribution of the off state of the QDs. Interestingly, despite this increasing contribution of the off state, which reduces the effective number of emitters in the observation volume and hence should increase the amplitude of the correlation curve, the latter actually decreases at high excitation power. This apparent contradiction is resolved by considering light-induced transformation of the dark QDs to bright QDs due to surface passivation of the QDs with increasing excitation power. Enhancement of the steady-state fluorescence intensity under light irradiation, both in aerated and deaerated environments, supports the mechanism of passivation of the surface trap states by photoadsorption of water molecules. Fluorescence lifetime data is also shown to be consistent with this light-induced surface passivation mechanism.