【作者】 郑金桔；

【导师】 赵家龙； 郑著宏；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 凝聚态物理， 2010， 博士

【摘要】 量子点由于其发光效率高、稳定性好及发射光波长易于调控等特点在生物荧光标记、生物芯片、光电子器件、太阳能电池等应用上发挥出越来越大的潜能。过渡金属掺杂的ZnS/ZnSe量子点在基质选择上迎合了绿色环保的要求,同时相对于非掺杂量子点它具有零自吸收,更好的光化学稳定性,更宽的发射光谱范围等特点而得到广泛的关注。本论文利用高温分解法和飞秒激光消融法制备了高效的稳定的过渡金属(Mn、Cu)掺杂的半导体纳米晶(ZnSe:Cu/Mn、ZnS:Cu/Mn),对纳米晶的生长动力学和发光机理进行了深入的研究。在量子点的制备、光电子性质研究中获得如下创新性的研究结果:1.用高温分解法中的成核掺杂方式可重复地合成发光效率高达40-50%的单分散的MnS/ZnS核壳(或者称为ZnS:Mn)量子点。通过分析MnS晶核的大小以及ZnS壳层厚度对量子点发光效率的影响,得到比较小MnS晶核和充分扩散的扩散层以及足够厚的ZnS壳层是制备高发光效率的ZnS:Mn纳米晶的必要条件。另一方面,我们通过生长掺杂方式制备了发光效率达10%的Cu掺杂的ZnSe量子点。对合成过程中的关键因素进行了初步的讨论。2.用成核掺杂方式可控的制备出不同壳层厚度的MnS/ZnS量子点,利用光致发光光谱和荧光衰减光谱研究了量子点的壳层依赖的发光机制。得到发光效率随壳层厚度的增加而明显增加的主要原因是量子点中的ZnS基质到Mn离子的能量传递效率提高和处于激发态的Mn离子的非辐射复合率降低这两方面因素引起。实验结果表明,通过控制量子点的壳层厚度可以实现量子点质量优化。3.通过对不同壳层厚度的量子点在不同温度下进行热处理,分析影响Mn离子扩散的因素,进而推导在纳米粒子生长过程中Mn离子的扩散机制。实验结果表明,当热处理温度高于220℃时,从荧光淬灭和发光寿命变短证明了Mn离子在热处理过程中由纳米晶的内部扩散到表面。这种扩散在量子点壳层比较薄的时候容易进行,并随热处理温度升高而加快。进而我们推测在纳米晶的生长过程中,Mn离子的扩散主要发生在最初包覆比较薄的ZnS壳层的过程中。基于上面关于扩散过程的推测优化实验路线,制备出高效的量子点,证明了对扩散过程分析的正确性。4.对所制备的量子点通过表面配体交换实现由油相到水相的转换,得到发光效率达到30%的水溶性MnS/ZnS量子点。分析了配体交换过程对不同壳层厚度的量子点的发光效率的影响并对其做出合理的解释。把其应用于生物荧光标记,实验证明水溶性量子点表面具有可与生物分子偶联的功能化基团,并适合用于生物学检测。5.用飞秒激光消融法制备了分散性好的水溶的ZnS:Cu量子点。据我们文献调研所知,这是首次利用激光直接消融掺杂的体相靶材料来制备掺杂量子点。利用X-Ray衍射谱(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)、紫外可见吸收光谱(Uv-Vis)、荧光光谱(PL)、荧光衰减光谱等表征方法证明,制备的ZnS:Cu纳米晶具有跟靶材料一样的闪锌矿结构并观测到来自Cu杂质能级的发光,而且所制备的量子点尺寸随着激光功率密度的增加而减少。更多还原

【Abstract】 Semiconductor nanocrystals (NCs) have been widely studied for their fundamental properties and applications, mostly as tunable emitters for biomedical labeling, light emitting diodes, lasers, and sensors. Despite their apparent advantages versus organic dyes, the intrinsic toxicity of cadmium has casted a doubtful future for this promising field. Zinc chalcogenide doped with transition metal ions may generate a cadmium-free nanocrystal emitter and the large ensemble Stokes shift can avoid the reabsorption or the self-absorption process and make the system ideal for different optical applications. We synthesized stable, small, efficient transition metal ions doped Zinc chalcogenide quantum dots by femtosecond laser ablation as well as nucleation-doping strategy. We study on their growth kinetics and luminescent property. The original works are organized as follows:1. We synthesized MnS/ZnS core/shell quantum dots (QDs) or called ZnS:Mn QDs via hot solution phase chemistry using nucleation-doping strategy. Efficient PL of Mn ions in the QDs with quantum yield (QY) of about 40-50% is demonstrated in the resulting QDs. The experimental results indicated that highly efficient luminescent ZnS:Mn NCs could be obtained by controlling the diffusion of Mn ions into the ZnS shell via annealing the core/shell NCs and using small sized MnS cores In addition, we also prepared ZnSe:Cu QDs with the PL QY=10% using growth-doping strategy.2. We studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell QDs synthesized using nucleation-doping strategy. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs with thick ZnS shell can be understood in terms of significantly enhanced energy transfer from the ZnS shell to Mn ions and slightly decreased nonradiative relaxation rate from Mn ions to surface states/traps of the ZnS shell by the surface passivation of the QDs with a thick ZnS shell.3. We studied the diffusion of Mn ions in MnS/ZnS nanocrystals by steady-state and time-resolved photoluminescence spectroscopy. Based on the evolution of the Mn dopant photoluminescence (PL) and its lifetime, we confirmed that the Mn ions can diffuse to the surface of the ZnS shell from the MnS core under long annealing at the temperature above 220°C, resulting in a decrease in PL QY and lifetime. It is found that the diffusion process at the initial growth of the ZnS shell is a key factor for the interface layer between the MnS core and ZnS shell, thus determine the quality of the obtained nanocrystals. We thus modified the synthesis procedure as follows to obtain highly efficient luminescent MnS/ZnS QDs: growing a thin ZnS shell on a small sized MnS core at low temperature, annealing the resulting NCs for effectively diffusing Mn ions into the ZnS shell at high temperature to form a ZnS:Mn diffusion layer, and overcoating a thicker ZnS shell as a passivating layer.4. The ligand exchange of the MnS/ZnS QDs was carried out to transfer the QDs into a water solution by using mercaptopropionic acid for biomedical applications. Recognition of a biotin pattern by QDs conjugated with avidine was carried out to illustrate the suitability of these efficient, stable, small, and water soluble QDs as biomedical labeling reagents.5. We prepared Cu-doped ZnS (ZnS:Cu) QDs in de-ionized water by femtosecond laser ablation of a bulk ZnS:Cu target. The obtained QDs exhibit good colloidal stability and water solubility, showing narrow and symmetric Cu-related emission. The mean size of the quantum dots varies from 2.1 to 4.0 nm by changing the laser fluence, resulting in a redshift of the emission peak from 375 to 400 nm. The experimental results demonstrate that femtosecond laser ablation is an effective method for preparing doped QDs.更多还原