| 摘要: | 半導體量子點材料因具有廣泛的發光波長與高量子效率，其製備與改質的研究已引起廣泛討論。與傳統的螢光染料相較，量子點有集中的光譜、連續的吸收光譜以及高的量子效率，使它們更適合做為螢光材料。其中II-VI族的合金量子點可由組成比例控制其波長，並具有較其他量子點更強的晶格結構而受到重視。近年的研究多致力於發展其應用，卻少有研究嘗試以提高結晶度、改變配位體比例來進一步提升量子點的光學性質。 在本研究中，高品質的ZnCdSe合金量子點經由高溫有機金屬製程來合成。量子點在成核、成長反應後快速冷卻至較低的溫度(100-250 oC)中退火，並觀察其光學性質的變化狀況。製程中以被廣泛利用的十六胺(HDA)與三正辛基氧磷(TOPO)為配位體，探討兩者比例對於量子點光學性質的影響。所製備ZnCdSe量子點的尺寸大小、結構、組成比例、表面化學鍵結、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)、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)、紫外光可見光吸收光譜儀(UV-vis)與螢光光譜儀(FL)作系統性的分析。 實驗中發現，配位體的比例和溫度皆會對波長與量子效率造成極大影響。在320 oC下，隨著HDA比例由0增加到100 %，放射波長由590 藍移至520 nm；而在270 oC下，同樣的條件卻使波長由504 紅移至543 nm。另外當HDA含量由0增加到75 %時，ZnCdSe的量子效率可由<1變化到80 % 之多。此現象源自TOPO與HDA不同的空間障礙。在原子組成方面，當未使用HDA於製程中時，量子點中的鋅原子比例顯著地由35-56 % 下降至15-19 %。 當ZnCdSe量子點在成核、成長之後，並降到100-250 oC進行熱處理一段時間則會使量子效率有明顯的提升。在不同的熱處理溫度比較中，150 oC熱處理可最大效率地提升發光效率，其量子效率由80提升至95 %並保持固定的放射波長。同一樣品在52天的儲存過程中保有的發光效率也大於未經熱處理ZnCdSe量子點。此外，FTIR的分析發現量子點表面配位體的比例與製程中所加入的配位體比例一致，在儲存過程中表面的配位體比例也幾乎不隨時間改變。 The preparation and modification of semiconductor quantum dots (QDs) are very attractive research topics due to their wide range of emission wavelength and high quantum yield (QY). Compared to the traditional fluorescent dyes, QDs have narrow emission peak, continuous absorption spectrum, and high QY, which make QDs as appropriate fluorescence materials. II-VI group alloyed QDs with tunable emission wavelength which can be changed by controlling their compositions, and stronger lattice structure than other QDs, are studied recently. In these years, studies have been focused on their applications. Few attentions have been paid on the optimization through increasing crystallinity or changing the ligands ratio during the preparation of QDs. In the study, high quality ZnCdSe QDs are synthesized by the high temperature organometallic procedure. QDs are soon cooled down to lower temperature (100-250 oC) and annealed after nucleation and growth. The variations on optical properties are observed in the process. The widely used Hexadecylamine (HDA) and trioctylphosphine oxide (TOPO) are chosen for the ligands in the procedure, and the effect of ligands ratio is also investigated in the study. Their particle sizes, structures, compositions, surface chemical states, QYs and optical properties are systemically analyzed by high resolution spectrometer (HRTEM), X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Fourier transform infrared spectroscopy (FTIR), UV-visible absorption spectroscopy (UV-vis), and fluorescence spectroscopy (FL), respectively. It is found that the ligands ratios and temperatures both significantly influence the emission wavelength and QY of the QDs. While HDA ratio raises from 0 to 100 %, emission wavelength blue-shifts from 590 to 520 nm in 320 oC but red-shifts from 504 to 543 nm in 270 oC. Additionally, when HDA content increases from 0 to 75 %, the variation on QY of ZnCdSe QDs changes from <1 to 80 % due to the different steric hindrance of TOPO and HDA. In terms of atomic compositions, the ratio of zinc atoms obviously decreases from 35-56 mol% to 15-19 mol% when HDA is absence in the procedure. The QY has obvious enhancement if QDs are annealed in 100-250 oC for a period of time after the nucleation and growth. For the samples annealed at various temperatures, annealing process in 150 oC can most effectively enhance the emission efficiency with an increase of QY from 80 to 95 % without changes of emission wavelength. The sample also maintains a better QY performance during the 52 day storage. Besides, it is found that the ratio of surface ligands is proportional to the ligands ratio initially added in the procedure, and is almost unchanged with storage time. |
