Characterization of the Dynamics of Photoluminescence Degradation in Aqueous CdTe/CdS Core-Shell Quantum Dots

Abstract

We investigate the effects of the excitation power on the photoluminescence spectra of aqueous CdTe/CdS core-shell quantum dots. We have focused our efforts on nanoparticles that are drop-cast on a silicon nitride substrate and dried out. Under such conditions, the emission intensity of these nanocrystals decreases exponentially and the emission center wavelength shifts with the time under laser excitation, displaying a behavior that depends on the excitation power. In the low-power regime a blueshift occurs, which we attribute to photo-oxidation of the quantum dot core. The blueshift can be suppressed by performing the measurements in a nitrogen atmosphere. Under high-power excitation the nanoparticles thermally expand and aggregate, and a transition to a redshift regime is then observed in the photoluminescence spectra. No spectral changes are observed for nanocrystals dispersed in the solvent. Our results show a procedure that can be used to determine the optimal conditions for the use of a given set of colloidal quantum dots as light emitters for photonic crystal optical cavities.

Size-Dependent Photobleaching Mechanism and Kinetics Induced by Nanosecond Laser Pulses in Colloidal Semiconductor Quantum Dots

“…As can be seen in Figure , the magnitude of the wavelength shift is directly proportional to the laser power for all samples, and we did not notice any transition from an emission blue-shift regime (at low-power excitation) to an emission red-shift regime (at high-power excitation), as reported in ref for CdTe/CdS QDs drop-cast on a silicon nitride substrate. Actually, the cited work was performed in a solid-state environment (dried droplets deposited on a solid substrate), while our results were conducted in solution.…”

“…Otherwise, photobleaching is the opposite of the photoenhancement phenomenon, i.e., the PL intensity decreases over the excitation time at the threshold power. Results reported in the literature show that the PL photobleaching in water-soluble QDs may be correlated with the change in pH, temperature, and O 2 concentration. − Although such references have investigated PL photobleaching in QDs, they have focused on cw-laser or broadband light sources. To the best of our knowledge, the occurrence of photobleaching in nanosized colloidal particles subjected to pulsed laser excitation has not been fully explored yet.…”

High-Q Microcavity Enhanced Optical Properties of CuInS2/ZnS Colloidal Quantum Dots toward Non-Photodegradation

“…Because of the power induced broadening, the linewidth of PL spectra increases generally with increasing excitation power without considering the time evolutions, as shown in Figure 8(a). In order to see the clear change of the PL spectra, we define ∆λ and ∆FWHM as the difference between the first spectrum and the rest spectra of the central emission peak [51]. The ∆λ and ∆FWHM as a function of excitation time with different pumping powers are shown in Figures 7(b)-7(f) and Figures 8(b)-8(f) respectively.…”

“…In order to see the clear change of the PL spectra, we define Δλ and Δfwhm as the difference between the first spectrum and the rest of the spectra of the central emission peak. 51 The Δλ and Δfwhm as a function of excitation time with different pumping powers are shown in Figure 7b−f and Figure 8b−f, respectively. The PL blue shifts at a low excitation power exposed in air can be explained by photooxidation.…”