一、二六族化合物半導體奈米粒子其粒徑與雙光子吸收截面積探討 二、二六族化合物半導體奈米材料於聲學聲子性質研究

Abstract

由於量子限量化效應的影響，奈米材料表現出與塊材不同的特性，諸如材料 的光電磁等物理特性，甚至因為在奈米材料其表面積對體積的比值很大，進而影 響其化學性質。本篇論文主要分成兩部分，第一部分探討奈米材料的雙光子躍遷 的特性。第二部分則探討當空間被侷限的時候。聲學聲子呈現所謂的量子化的效 應。 論文中的第一章主要是回顧了早期其他的研究群對奈米材料的基礎特性進行 探討，因為奈米材料其尺度介在巨觀塊材與分子之間，在探討其能帶結構時需考 慮限量化效應的有想，早期理論上探討引入了粒子在一維盒中的模型、有效粒子 質量等來解釋奈米材料的電子結構。另外，並舉出奈米材料在光電元件上應用的 前瞻性。第二章則是回顧文獻中合成奈米材料的方法，利用膠體化學的方式合成 了硒化鎘與硒化碲來應用在第三章雙光子吸收截面積與粒徑關係探討，接著利用 膠體化學的方式來合成硒化鎘─硒化碲核─殼奈米材料與硒化鎘，於第四章中進 行限量化的聲學聲子探討。另外嘗試用高溫裂解醋酸鋅合成粒徑單一性的氧化鋅 奈米顆粒，期望未來能探討兆赫波吸收特性。並於第二章中討論調控不同的界面 活性劑比例來進行奈米材料的形狀控制，因為特殊形狀的奈米結構在不同的光電 元件有不同的應用性與前景。

Part 1: The size-dependent two photon absorption (TPA) cross section of II-VI semiconductor quantum dots (QDs) has been investigated. As the size of the QDs increased, the TPA cross section was found to be empirically related via a power-law proportionality of 3.5 and 5.6 to the diameters of CdSe and CdTe QDs. These results can be rationalized by a theoretical model of two-photon excitation in a system incorporating excitons and defects. Part 2: With spatial confinement, low dimensional systems such as QDs and nanowires (NWs) exhibit not only electronic but also acoustic energy quantization. In 1882, Lamb had already studied the acoustic normal modes confined in a homogeneous free sphere. Two types of modes, torsional and spheroidal (SPH), were derived from the stress-free boundary condition on the spherical surface. Taking advantage of the charge separation structure in the CdSe/CdTe core-shell type-II QDs, we experimentally observed and verified the existence the resonance-enhanced dipolar interaction between terahertz (THz) photons and their corresponding eigenmode (l=1). We also observed that the piezoelectricity of CdSe QDs can induced THz photon absorption related to inactive breathing mode (l=0). From our THz absorption spectrum, we found that the corresponding absorption cross section of CdSe/CdTe QDs shows a D4 (D: diameter) dependence, and the frequency of absorbed photons is inversely proportional to D of the QDs and agrees with that of dipolar active mode. The absorption peak of CdSe QDs which related to breathing mode also shows that the frequency of peak is inversely to the D of the CdSe QDs. We can covert the THz photons into a phonon of the same frequency through charge separation of CdSe/CdTe and the piezoelectricity of CdSe.