title: Shielding Upconversion by Surface Coating: A Study of the Emission Enhancement Factor.
abstract: Surface coating is a commonly used strategy to enhance upconversion emissions by shielding the luminescent core from surface quenching. In this work, we provide insights into the effect of surface coating on upconversion by investigating NaYF4 :Yb/Er nanoparticles and the corresponding NaYF4 :Yb/Er@NaYF4 core-shell nanoparticles, as a function of dopant concentration of Yb(3+) and excitation power. We observe declining emission enhancement factors with decreasing Yb(3+) concentration and increasing excitation power. Our mechanistic investigations suggest that the phenomenon originates from stepwise excitation in the upconversion process, as well as energy hopping among the Yb(3+) dopants. This increased understanding of the effect of surface coating on upconversion should be important towards the rational design of lanthanide-doped core-shell nanoparticles for various applications.
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, P. R. China.
authors.forename: Tianying
authors.initials: T
authors.lastname: Sun
authors.affiliation: State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China.
authors.forename: Ronghua
authors.initials: R
authors.lastname: Ma
authors.affiliation: State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China. qiaoxus@zju.edu.cn.
authors.forename: Xvsheng
authors.initials: X
authors.lastname: Qiao
authors.affiliation: State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China.
authors.forename: Xianping
authors.initials: X
authors.lastname: Fan
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, P. R. China. fwang24@cityu.edu.hk.|City University of Hong Kong, Shenzhen Research Institute, Shenzhen, 518057, P. R. China. fwang24@cityu.edu.hk.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
keywords: core-shell structures; energy transfer; luminescence; nanoparticles; upconversion