| 摘要: | 膠體硒化鎘(CdSe)量子點因具有潛力應用在固態發光，如發光二極體，光伏電池，生醫成像與雷射等，而使其製備與改質引起廣泛的研究。高品質CdSe量子點的合成方法已被廣泛報導，並且這些方法幾乎都使用正三辛基氧化膦(TOPO)為界面活性劑，然而，此正三辛基氧化膦在整個合成過程當中所扮演的角色並未被清楚的定位與討論。在本研究中，高品質的CdSe量子點經由高溫(320 oC)熱解有機金屬前驅物製程來合成。製程中以被廣泛使用的TOPO與十八胺(ODA)為界面活性劑，探討兩者比例對於CdSe量子點光學性質的影響。當樣品沒有添加TOPO會導致在溫度超過300 oC時有沉澱物產生，本研究亦探討此析出現象以及Cd前驅物時間的發展對其發光性質的影響。最後，將所製備的CdSe量子點存放在大氣環境下做長時間穩定性的測試。所製備CdSe量子點的形貌、結構、組成比例、表面化性、量子效率與光學性質分別以高解析穿透式電子顯微鏡(HRTEM)、X光繞射分析儀(XRD)與核磁共振光譜儀(NMR)、感應耦合電漿原子發射光譜分析儀(ICP-AES)、傅立葉紅外線光譜儀(FTIR)與光電子能譜儀(XPS)、紫外光可見光吸收光譜儀(UV-vis)與螢光光譜儀(FL)做系統性的分析。在不同比例的界面活性劑下製備的CdSe量子點具有不同的晶體結構，此乃因ODA當作包覆劑穩定形成閃鋅礦結構，而TOPO則形成纖鋅礦結構。此外，界面活性劑的比例也會影響其原子組成。當添加TOPO從0 增加到75 wt %時，由於含膦基的配位體傾向與鎘鍵結導致鎘與硒的原子比例從1.15增加到8.12，當添加TOPO達到100 wt %時，量子點經過相轉換以及表面重組的過程從閃鋅礦演變為纖鋅礦結構導致原子比例減少到2.72。此外，鎘與硒原子比例大的量子點呈現較慢的衰退速度，乃歸因於散佈在粒子表面的鎘錯合物可以有效的保護硒而不被氧化。當製備過程中沒有添加TOPO且溫度超過300 oC，ODA與羧酸基反應生成醯胺且氫氧化鎘為副產物，氫氧化鎘經過再結晶生成氧化鎘與金屬鎘混和物。另一方面，反應所生成的醯胺提供較佳的電子鈍化給量子點的表面，因此可得到較高發光效率的量子點。在TOPO與ODA比例為2:2時可製備出在50天穩定度測試時具有最佳穩定性與發光效率的量子點，此現象源自量子點具有最高的鎘與硒原子比。Recently, the preparation and modification of colloidal CdSe quantum dots (QDs) have attracted a lot of attention due to their potential applications in the field of solid state lighting, such as light-emitting diodes (LEDs), photovoltaics, bioimaging and laser, etc. Many successful preparation methods have been reported for the synthesis of high quality CdSe QDs. Most of these methods involve the use of trioctylphosphine oxide (TOPO) as surfactants to prepare CdSe QDs. However, the exact rule of such indispensable TOPO during the synthesis is still unclear.In this study, high quality CdSe QDs have been successfully synthesized by the high temperature pyrolysis organometallic procedure (320 oC). TOPO and Octadecylamine (ODA) are chosen as the surfactants in the procedure, and the effects of surfactants ratios on the optical properties of CdSe QDs are investigated in the study. The sample without TOPO addition could form the precipitates above 300 oC, and the effects of precipitation and temporal evolution of Cd precursor on the optical properties of CdSe QDs are also examined. Finally, long-term stability of the FL is studied for the as-prepared CdSe QDs stored under ambient conditions. The morphologies, structures, compositions, surface chemistry, quantum yields (QYs) and optical properties of CdSe QDs are systemically analyzed by high resolution spectrometer (HRTEM), X-ray diffraction (XRD) and nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy (UV-vis), and Fluorescence spectroscopy (FL), respectively.The obtained CdSe QDs synthesized with different surfactant ratios have different structures, in which ODA as a capping agent stabilizes the zinc blende structure while TOPO stabilizes the wurtzite phase. Besides, the surfactant ratios affect the Cd/Se atomic compositions. By adding 0 to 75 wt % of TOPO, the Cd/Se ratio increases from 1.15 to 8.12 owing to the preferential bonding of phosphonate ligands on Cd and then decreases to 2.72 when TOPO content increases to 100 wt %, which is caused by the phase transition and surface reconstruction. Moreover, CdSe QDs with larger ratio of Cd/Se displays the slower decay rate during the storage, suggesting that Cd-complexes can effectively protect Se from oxidation.When prepared above 300 oC without TOPO addition, CdO and Cd mixtures from recrystallization of Cd(OH)2 are formed, which can be attributed to that ODA combines with carboxyl groups to form amides, and the Cd(OH)2 is the by-product. On the other hand, the formation of amides can provide a good electronic passivation for the surface states of QDs. As a result, QDs with high QYs are obtained.The TOPO/ODA mass ratio of 2:2 seems to be an optimal condition during the 50 days storage to produce CdSe QDs with high stability and QY value due to their largest ratio of Cd/Se. |
