Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties

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• 作者：Fan Zhang ; Renchao Che ; Xiaomin Li ; Chi Yao ; Jianping Yang ; Dengke Shen ; Pan Hu ; Wei Li ; Dongyuan Zhao
• 刊名：Nano Letters
• 出版年：2012
• 出版时间：June 13, 2012
• 年：2012
• 卷：12
• 期：6
• 页码：2852-2858
• 全文大小：476K
• 年卷期：v.12,no.6(June 13, 2012)
• ISSN：1530-6992

文摘

Lanthanide-doped upconversion nanoparticles have shown considerable promise in solid-state lasers, three-dimensional flat-panel displays, and solar cells and especially biological labeling and imaging. It has been demonstrated extensively that the epitaxial coating of upconversion (UC) core crystals with a lattice-matched shell can passivate the core and enhance the overall upconversion emission intensity of the materials. However, there are few papers that report a precise link between the shell thickness of core/shell nanoparticles and their optical properties. This is mainly because rare earth fluoride upconversion core/shell structures have only been inferred from indirect measurements to date. Herein, a reproducible method to grow a hexagonal NaGdF₄ shell on NaYF₄:Yb,Er nanocrystals with monolayer control thickness is demonstrated for the first time. On the basis of the cryo-transmission electron microscopy, rigorous electron energy loss spectroscopy, and high-angle annular dark-field investigations on the core/shell structure under a low operation temperature (96 K), direct imaging the NaYF₄:Yb,Er@NaGdF₄ nanocrystal core/shell structure at the subnanometer level was realized for the first time. Furthermore, a strong linear link between the NaGdF₄ shell thickness and the optical response of the hexagonal NaYF₄:Yb,Er@NaGdF₄ core/shell nanocrystals has been established. During the epitaxial growth of the NaGdF₄ shell layer by layer, surface defects of the nanocrystals can be gradually passivated by the homogeneous shell deposition process, which results in the obvious enhancement in overall UC emission intensity and lifetime and is more resistant to quenching by water molecules.

Keywords:

Nanocrystal; core/shell; upconversion; luminescence