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  4. Synthesis and Applications of Semiconductor Nanocrystallites
 
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Synthesis and Applications of Semiconductor Nanocrystallites

Date Issued
2005
Date
2005
Author(s)
Cheng, Chiu-Ting
DOI
en-US
URI
http://ntur.lib.ntu.edu.tw//handle/246246/51896
Abstract
In this thesis, we focus on the syntheses and characterization of various II-VI compound semiconductor nanocrystallites, and type-II quantum dots (QDs) in chief. Our long-term goal is to exploit QDs with chemical modifications in fundamental approaches such as relaxation dynamics and energy/charge transfer processes, or practical applications such as bio labeling. Prior to my work at present, a general review in terms of lasting progress, basis and application of nanoscience is elaborated in Chapter 1. In Chapter 2, syntheses of CdTe/CdSe type-II quantum dots (QDs) using CdO and CdCl2 as precursors for core and shell, respectively, are reported. Characterization was made via near-IR interband emission, TEM, EDX, and XRD. Femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core (in CdTe/CdSe) emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 1.96, 1.44 to 1.07 ×1012 s-1. The finite rates of the initial charge separation are tentatively rationalized by the low electron-phonon coupling, causing small coupling between the initial and charge-separated states. In Chapter 3, syntheses of CdSe/ZnTe/ZnS (core/shell/shell) type-II quantum dots (QDs) are reported. Structural characterization was made via TEM, EDX, XPS and XRD. Photophysical properties were investigated via the interband emission (CdSe  ZnTe) and its associated quantum efficiency as well as relaxation dynamics. In comparison to the weak emission (f ~ 4 x 10-3 in toluene) of CdSe/ZnTe (3.9/0.5 nm), capping ZnS (0.4 nm in thickness) enhances the CdSe  ZnTe interband emission by ~30 folds (f ~ 0.12), whereas the peak wavelength shifts only slightly to the red. By encapsulating dihydrolipoic acid (DHLA), water soluble CdSe/ZnTe/ZnS was also prepared, and it exhibited an interband emission at ~930 nm with an emission yield of ~0.1. Femtosecond pulse excitation (ex ~ 1200 nm) measurement estimated a two-photon absorption cross section, , of ~70  1050 cm4 s photon1 for DHLA capped CdSe/ZnTe/ZnS in water, supporting its suitability for the use as near infrared (NIR) dyes in biomedical imaging.
Subjects
半導體
奈米粒子
量子點
semiconductors
nano paeticles
quantum dots
Type
thesis
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