Ⅱ-Ⅵ族半導體奈米晶粒(量子點)之合成與光學性質分析

Abstract

本論文以研究Ⅱ-Ⅵ族半導體奈米晶粒(量子點)的合成、光學性質和結構為主，並探討形成核殼結構時對於光學性質的影響，從紫外光-可見光吸收光譜圖和光激發光光譜圖、X-ray繞射圖譜、穿透式電子顯微鏡照片來了解Ⅱ-Ⅵ族奈米晶粒的光學性質及其結構。 以化學膠體合成法來製備CdSe與CdSe/ZnSe核殼結構之量子點，在固定的Cd/Se莫耳數比、相同的反應溫度下，動力學控制利用反應時間的長短來合成不同粒徑的量子點，並且經由同一時間的成核和成長機制來達到粒徑分佈均勻。 在光學性質方面，CdSe與CdSe/ZnSe核殼結構之量子點均呈現量子限量化效應，即表示隨著粒子尺寸的增加，能隙(energy gap)變小，在光吸收和光激發圖譜中有紅位移(red shift)現象，此外，藉由紫外光-可見光吸收光譜圖，利用公式估算粒徑大小。在發光量子效率(quantum yield)的測量上，以coumarin 6為標準品，是一種有機染料，從光吸收和光激發圖譜得到的測量值代入公式，計算得到Q.Y.值，經由Q.Y.值的增加，間接證明CdSe/ZnSe部分核殼結構形成。 在結構性質方面，CdSe量子點為wurtzite結構，屬於六方最密堆積，從XRD的圖形來看，的確符合理論值，但在短時間的反應下，CdO的peak出現，表示CdO反應不完全。CdSe/ZnSe核殼結構的XRD圖形，peak沒有位移的現象但有變寬的趨勢，只能證明ZnSe部分成長在CdSe的表面，未達到完整的核殼結構。在TEM方面，只有針對反應時間為20分鐘的CdSe，依照圖中比例，約略估計粒徑大小在5nm左右，與吸收圖譜計算的結果，大約符合，至於CdSe/ZnSe核殼結構量子點，需要HR-TEM才能看到晶格影像，直接證明核殼結構的形成。

The synthesis, optical and structure properties of Ⅱ-Ⅵ group semiconductor nanocrystal (quantum dots) are investigated and the core-shell structures of the quantum dots exhibit interesting effects on optical properties. To comprehend optical and structure properties through UV-Vis absorbance spectra, photoluminescence spectra, X-ray diffraction spectra, TEM pictures. To prepare quantum dots of CdSe and CdSe/ZnSe core-shell structure in chemical colloidal synthesis. At fixed Cd/Se molar ratio, the same reaction temperature, Kinetic control reaction time to produce different size quantum dots. The narrow size distribution were reached due to the mechanism of nucleation and growth. In optical properties, CdSe and CdSe/ZnSe core-shell structure have quantum confinement effects. It means the band gap of nanocrystals decreases as their size increases. There is red shift phenomenon in absorbance and PL spectra. In addition, we can use a formular to compute particle size. Photoluminescence quantum yield values were measured relative coumarine 6. Coumarin 6 is an organic dye. To obtain Q.Y. values, we need the data from absorbance and PL spectra. It could indirectly prove the formation of CdSe/ZnSe core-shell structure via the raise of Q.Y. In structure properties, CdSe is wurtzite structure, hexagonal closet packing. It conforms with theoretical values in XRD pattern. In XRD pattern, there are CdO’s peaks. It means that CdO reacts incompletely. The peaks are not shift but broading in CdSe/ZnSe core-shell structure’s XRD pattern. We figure CdSe overcoated with a potion ZnSe. The whole core-shell structure may not form yet. There are only TEM pictures of reaction time equal to twenty minutes. From these pictures, we estimate the particle sizes equal to 5nm. The result is to fit in with the data from absorbance spectra. As for CdSe/ZnSe core-shell structure, we need HR-TEM to see lattice images to prove directly the formation of core-shell structure.