Ligand-Mediated Photobrightening and Photodarkening of CdSe/ZnS Quantum Dot Ensembles

Abstract

Variation of the steady-state optical properties of quantum dots (QDs) caused by photoinduced charge transfer is a widely known phenomenon which hinders QD applications requiring long-term stability of the optical signal, for example, bioimaging, singlephoton sources, molecular tracking, and lasing. Intense light irradiation may provoke QD photodarkening, when their photoluminescence (PL) quantum yield (QY) is reduced, or photobrightening, when the QY increases and QD PL becomes brighter. In order to understand the nature of these processes and to determine the optimal operating conditions for QDs with different core sizes and shell thicknesses, we have systematically studied the photodarkening and photobrightening phenomena for different wavelengths and intensities of irradiation. The results have shown that changes in the core size only slightly alter the effect of light irradiation on the QD optical properties, whereas the irradiation wavelength and shell thickness are the major factors that determine the variation of the QD PL QY. We have found that the observed photoinduced processes are independent of the irradiation intensity, which proves the single-photon origin of these processes. We have also used comparative data on irradiation of QD solution and QDs embedded in polymer matrices to develop a general model describing the mechanism of the QD sensitivity to intense irradiation and elucidating the role of surface ligands in this mechanism. Our findings determine the strategy for increasing QD optical stability and optimization of operational conditions for QD-based devices.

Hybrid fluorescent liquid crystalline composites: directed assembly of quantum dots in liquid crystalline block copolymer matrices

“…The CdSe/ZnS QDs that were used in our study were synthesized according to the previously reported procedure. 43 The QDs were approximately 5 nm in diameter and contained oleylamine (OLA) surfactant on their surface as a ligand.…”

“…CdSe/ZnS quantum dots (QDs) of approximately 5 nm in diameter covered with oleylamine and surrounded by TOPO matrix were synthesized according to the previous report. 43 The LC di and triblock copolymers as well as the random copolymers were synthesized by reversible addition-fragmentation transfer (RAFT) polymerization as described previously (Scheme S1 †). 39 The controlled synthesis of copolymers was performed using three different RAFT agents, namely S,S 0 -bis(methyl-2-isobutyrate)trithiocarbonate 51 for the ABA block copolymers, commercially available CPDDTC for the diblock copolymers and random copolymers, and a specially designed RAFT agent 40 (Fig.…”

Tempo-spectral multiplexing in flow cytometry with lifetime detection using QD-encoded polymer beads

“…The radiative lifetime of such II/VI QDs is about 20-40 ns [52][53][54] , and the longest components in the luminescence decay kinetics of the QDs used here are within this time range. This suggests, that despite some QDs having a high photoluminescence quantum yield, the average lifetimes of both QD ensembles are, however, shortened due to a large proportion of QDs with a lower photoluminescence quantum yield [55][56][57] . As previously mentioned, the more pronounced emission wavelength dependence of the decay kinetics of QDG shown in Fig.…”

Enhanced spontaneous emission from two-photon-pumped quantum dots in a porous silicon microcavity

“…Moreover, the use of semiconductor quantum dots (QDs) with uniquely high two-photon absorption cross-sections compared to conventional dyes [14,15] allows one to reach the unprecedentedly high values of fluorescence contrast [16]. In addition, QDs are the excellent probes for sensing due to their wide one-and two-photon absorption [17,18] and narrow PL spectra, high quantum yield [19], and excellent photostability [20,21].…”