Mapping the Optical Properties of CdSe/CdS Heterostructure Nanocrystals: The Effects of Core Size and Shell Thickness
Mapping the Optical Properties of CdSe/CdS Heterostructure Nanocrystals: The Effects of Core Size and Shell Thickness
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DOI：
10.1021/ja9030209
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发表时间：
2009-10-14
影响因子：
15
通讯作者：
Mulvaney, Paul
中科院分区：
文献类型：
--
作者：
van Embden, Joel;Jasieniak, Jacek;Mulvaney, Paul
在这里，我们提出了第一份关于 CdSe/CdS 异质结构纳米晶体的综合报告。研究了核尺寸和壳厚度对CdSe/CdS异质结构纳米晶光学性能的影响。我们报告了一种可靠的合成方法，可以在 CdSe 核上生长厚的 CdS 壳，尺寸范围为 2.5-4.7 nm。我们提供校准曲线，可确定各种芯尺寸的 CdS 壳厚度 (+/- 0.1 nm)，从而避免耗时的 HRTEM 分析。通过 HRTEM、XRD 和 SAIED 验证了壳的外延生长。原位反应测量显示镉和硫的平均每个颗粒 (p) 沉积率为 k(Cd) = 5.38 x 10(-25) mol s(-1) p(-1) 和 k(s) = 4.83 x 10(-24) mol s(-1) p(-1)。更快的硫沉积速率归因于生长培养基中不存在强硫结合配体。通过严格使用高分辨率透射电子显微镜，建立了异质结构的尺寸与其带边跃迁能量、量子产率和激发态寿命之间的直接联系。实验表明，核心样品的带边跃迁能量最初跨度约为 431 meV，在生长 6 个单层厚的 CdS 壳后，跨度仅为 163 meV。此外，发现带边跃迁能量的变化对纤芯尺寸极其敏感。所制备的核/壳的 QY 范围为 25% 至 60%。发现核/壳的 QY 和带边寿命取决于吸附到颗粒表面的配体。这些数据证明，尽管存在 2.2 nm 厚的 CdS 壳，但一个或两个电荷载体仍然可以到达颗粒表面。
Here we present the first comprehensive report on CdSe/CdS heterostructure nanocrystals. The effects of core size and shell thickness on the optical properties of CdSe/CdS heterostructure nanocrystals are investigated. We report a reliable synthetic method to grow thick CdS shells on CdSe cores with sizes ranging from 2.5-4.7 nm. We provide a calibration curve, which enables determination of CdS shell thickness (+/- 0.1 nm) over a wide range of core sizes, circumventing the need for time-consuming HRTEM analyses. Epitaxial growth of the shells was verified by HRTEM, XRD, and SAIED. In-situ reaction measurements revealed the average per particle (p) deposition rates for cadmium and sulfur to be k(Cd) = 5.38 x 10(-25) mol s(-1) p(-1) and k(s) = 4.83 x 10(-24) mol s(-1) p(-1). Faster sulfur deposition rates are attributed to the absence of strong sulfur binding ligands in the growth medium. Through the rigorous use of high resolution transmission electron microscopy, a direct link between the dimensions of the heterostructures and their band-edge transition energies, quantum yields, and excited state lifetimes is established. The experiments show that the band-edge transition energies of the core samples, which initially span approximately 431 meV, condense to span only 163 meV after the growth of a 6 monolayer-thick CdS shell. Furthermore, shifts in the band-edge transition energies were found to be extremely sensitive to core size. The QY of the as-prepared core/shells ranged from 25 to 60%. The QYs and band-edge lifetimes of the core/shells were found to depend upon the ligands adsorbed to the particle surface. These data prove that one or both of the charge carriers still has access to the particle surface despite the presence of a 2.2 nm thick CdS shell.