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.

Effect of Controlled Deposition of ZnS Shell on the Photostability of CdTe Quantum Dots as Studied by Conventional Fluorescence and FCS Techniques

“…However, in the case of CdTe/ZnS(2 ML)/MPA QDs, the G (0) value decreased only by a factor of 1.6 [with actual value decreasing from 0.43 (±0.03) to 0.26 (±0.03); Figure ] and the N value increased only from 3.85 (±0.28) to 9.07 (±0.38) for a similar change of the excitation power. Table , which presents a summary of the FCS data and shows a large fraction of the dark QDs at low excitation power, is consistent with the recent literature . Effective surface passivation by the ZnS shell is clearly evident from the fact that the fraction of dark QDs (68 %) in CdTe/ZnS(2 ML)/MPA is much lower than that in CdTe/MPA QDs (95 %).…”

“…Effective surface passivation by the ZnS shell is clearly evident from the fact that the fraction of dark QDs (68 %) in CdTe/ZnS(2 ML)/MPA is much lower than that in CdTe/MPA QDs (95 %). These dark fractions of CdTe/ZnS(2 ML)/MPA, which arise due to interfacial trap states between the core and shell, do not easily convert into bright one as the ZnS shell protects these interfacial trap states from photoadsorption of H 2 O molecules . As a result a much smaller decrease of the G (0) value compared with that of CdTe/MPA QDs was observed with increasing excitation power.…”

“…FCS experiments were carried out by using a time‐resolved confocal fluorescence microscope (MicroTime 200, PicoQuant). Details of the instrumentation are reported elsewhere . Pulsed diode laser ( λ ex =485 nm, FWHM=144 ps) was used for excitation of the sample.…”

“…High photostability and fluorescence efficiency are essential in most of these applications . However, both increasing emission intensity, due to photoactivation, and decreasing emission intensity, due to photocorrosion, of the QDs are observed upon exposure to light . Owing to their small size, the photostability, and consequently, the fluorescence efficiency of the QDs is strongly dependent on the surface quality .…”

Effect of light on stability of thiol-capped CdSe/ZnS quantum dots in the presence of albumin

“…However, no general consensus on the reasons of the short-term enhancement of PL has been reached yet. Some suggested mechanisms include the photoneutralization of the local charged centres inside and outside the QDs [24], the elimination of the topological surface defects [25][26][27], the passivation of the surface trap states by photoadsorbed molecules [10,11,28], for example, water molecules [29,30] and the photoinduced rearrangement of surface stabilizing agents [2,[31][32][33]. The reason why the MPA-capped QDs possessed higher values of the PL QY and enhanced stability than the TGA-capped QDs might be an unequal effect that these thiol ligands had on PL self-quenching.…”