半導體奈米粒子合成及其應用

Abstract

在這個研究工作中，我們主要致力於II-VI族半導體奈米粒子的合成及其鑑定，主要是針對type-II的奈米粒子來探討它的光譜動力學和應用性。我們希望將來可以把奈米粒子做表面修飾研究它的緩解動力學、能量/電荷轉移的機制或是應用在生物標定上，所以在第一章裡我們主要是描述奈米科學從過去到現在一連串的發展過程，它的合成走向、基本原理及其應用方面。 在第二章節我們利用氧化鎘和氯化鎘做為核與層的前驅物，合成出放光位置在近紅外光區CdTe/CdSe type-II奈米粒子，我們經由各式儀器的鑑定，例如：TEM、EDX、XRD、XPS來確認我們的化合物是否為核殼奈米粒子，並且從核CdTe和核層CdTe/CdSe的放光光譜，來探討他們的緩解動力學，發現隨著CdTe的半徑愈大光誘發電子分離的速率跟著下降。我們主要是想要由CdTe/CdSe的電洞和電子分別被限制在CdTe（核）的價電帶和CdSe（殼）的傳導帶上，因此電子可以轉移到TiO2上，藉此應用在太陽能電池方面，來取代染料dye的存在。 之前，我們已經發表二層奈米粒子CdSe/ZnTe的相關論文，並探討它的一些光物理性質，最後一個章節中，我們發展三層的奈米粒子CdSe/ZnTe/ZnS（核/層/殼），其主要目的是希望藉由第三層ZnS的存在，來提高我們奈米粒子的量子產率、降低表面缺陷的存在。由於它的放光位置是在近紅外光區，對皮膚有較好的穿透性，再加上它的光穩定性很好，因而將此應用在生化影像方面，在未來，我們希望可以和生物醫學相關的實驗室合作，讓奈米粒子可以確實應用於人體上，幫助人們在檢測病變上有良好的效果。

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.