Quenching of quantum dot luminescence under light irradiation
Size-tunability of the absorption and fluorescence properties of colloidal quantum dots (QDs) makes them an attractive material for modern science and technology. Size-controllable optical properties of QDs allow their applications in multiplexed biolabeling, solar cells, light emitting diodes, catalysis, etc. QDs may gradually replace organic dyes from these fields because of their higher linear absorption cross-sections, photoluminescence (PL) quantum yields (QY), and photostabilities. However, in contrast to organic dyes, QDs are known to be susceptible to environmental factors such as oxygen; in addition, intense laser irradiation is known to alter their PL QY. Therefore, understanding of the mechanisms and factors causing instability of QD optical properties is one of the most important issues for the development of new generations of QDs and QD-based devices.
In this study, we have investigated the influence of long-term visible-light irradiation on the PL QY of QDs in order to model their storage and/or operation under common laboratory or industrial conditions. Two types of light sources with the same light power output (15 mW/cm2) were used for QD irradiation: a 40 W incandescent lamp and an array of light-emitting diodes with a radiation wavelength of 405 nm. Three types of core/shell QDs with different shell compositions and thicknesses, CdSe/ZnS(3ML), CdSe/ZnS(1ML)/CdS(1ML)/ZnS(1ML), and CdSe/CdS(7ML)/ZnS(1ML), all with initial PL QYs exceeding 70%, were used to estimate the effects of the shell thickness and confinement potential on the stability of their PL. QD solutions in chloroform with an optical density at the first excitonic transition maximum of 0.1 were placed into 10-mm light path quartz cuvettes and irradiated under continuous stirring in the ambient atmosphere. Along with the irradiated samples, reference samples of the same solutions were stirred in the dark for the same time intervals.
Irradiations of QDs with an incandescent lamp and a LED array have revealed two parallel counteracting processes: an increase in and quenching of the PL. The kinetic curves of these processes are shown in Figure 1.
In the case of CdSe/ZnS(3ML) QDs, irradiation with an incandescent lamp caused a slow PL decline to 48% of the initial PL QY value, while irradiation with LEDs caused a rapid PL quenching to 52% followed by slow restoration of luminescence to 48% of the initial level.
Multishell CdSe/ZnS/CdS/ZnS QDs exhibited a similar kinetic of PL QY decrease, but the degree of PL quenching was much lower. Thus, irradiation with an incandescent lamp caused a 10% drop of PL, while irradiation with a LED array resulted in a 18% loss of PL QY followed by restoration to 90% of the initial PL QY during the remaining period of irradiation.
Thick-shell CdSe/CdS(7ML)/ZnS QDs exhibited a totally different response to irradiation with both light sources, without considerable PL quenching. These QDs were found to be almost completely stable under long-term irradiation; under irradiation from either source, a slight general growth of PL QY to 104% of the initial value was observed. The reference samples representing the same solutions of QDs incubated in the dark did not exhibit detectable variation of PL QY, thus proving that the observed effects originated from light irradiation rather than from other possible environmental factors.
Figure 1. Quantum dot luminescence quenching under light irradiation.
Transfer of the excited charge carriers from the core of QDs to the surface ligands or the surrounding molecules is known to be an efficient pathway of QD PL quenching . We suppose that the difference in charge transfer rates is the reason of the observed differences in the PL quenching kinetics of the QDs studied. This difference can be related to the variation in (i) the shell thickness or (ii) the confinement potentials determined by the structure and composition of the shells. Specifically, a 3ML-thick ZnS shell ensures a high potential barrier of moderate length for electrons and holes to tunnel through and to escape the QD core. The mutishell structure provides a slightly higher potential barrier than a 3ML-thick ZnS shell does, due to the strong quantum confinement effect in monolayer-thick shell layers , while having the same barrier length. Finally, thick-shell CdSe/CdS(7ML)/ZnS QDs have the lowest energy of potential barrier created by CdS, but the barrier length is 2.5 times larger than that in other QDs. From the obtained results, one can conclude that the barrier length has a much stronger effect on the stability of QD colloidal solutions to irradiation, and a sufficiently thick shell could completely protect QDs from photoinduced degradation.
The difference in PL quenching kinetics under irradiation from different light sources can be attributed to different spectral overlaps of the light source emissions and QD absorption spectra. The entire incident photon flux from the blue LED array falls within the absorption range of all types of QDs, while a major part of the incandescent lamp irradiation cannot be absorbed by QDs. However, we can conclude that, even in the latter case, significant quenching of QD PL can occur under long-term irradiation.
It is noteworthy that the rate of restoration of QD PL seems to be independent of irradiation in all the three cases (Figure 1) and is likely to be caused by a different factor. Identification of this factor will be the subject of further research.
The results obtained in this work could be important for application-driven design of novel QDs. The high stability of PL QY is an advantage in engineering of photostable fluorescent biolabels or QD-LEDs; on the other hand, QDs that are prone to fast photoinduced PL quenching could be efficient tags in high-resolution stimulated emission depletion (STED) optical microscopy.
This study was supported by the Russian Science Foundation, grant no. 14-13-01160. We thank Vladimir Ushakov for the help in preparation of the manuscript.
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