Time-Resolved Photoluminescence Study of Colloidal CdSe/ZnS Quantum Dots
Date Issued
2012
Date
2012
Author(s)
Wang, Yu-Ju
Abstract
In this thesis, we use time-resolved photoluminescence data measured with up-conversion technique and time-correlated single photon counting system to study the changes of photoluminescence intensity and ultrafast carrier dynamics over time in CdSe/ZnS colloidal quantum dots and SiO2/CdSe/SiO2 sandwich structure.
Because the dangling bonds on the CdSe surface can be passivated by ZnS shell, there is no picosecond decay in the curve measured with up-conversion technique. Besides, the photoluminescence rise time in CdSe quantum dots is 537 fs.
CdSe quantum dots can be photooxidized in air. Oxidation-generated species can capture excited carriers. This effect and nonradiative recombination result in photoluminescence decay. In the SiO2/CdSe/SiO2 sandwich structure, oxygen is blocked by the outer SiO2 layer, thus photooxidation does not occur. The photoluminescence of the SiO2/CdSe/SiO2 sandwich structure is weaker. The CdSe surface was transformed and defects were generated during high temperature SiO2 growth process. This effect causes nonradiative recombination and weaker intensity. But we observed photo-induced photoluminescence enhancement in SiO2/CdSe/SiO2 sandwich structure. This photoluminescence enhancement is attributed to restructured surface and reduced defects.
Furthermore, we use μ-PL measurement system. In other words, CdSe quantum dots are irradiated with high power density. During high power density irradiation, carrier trapping causes nonradiative recombination and photodarkening. After darkening, photoluminescence enhancement is observed with low power density irradiation. This enhancement is also attributed to restructured surface.
Subjects
CdSe
quantum dot
colloidal quantum dot
time-resolved photoluminescence
Type
thesis
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