构建核壳结构增强Ho3+离子在镥基纳米晶中的红光上转换发射

Enhanced red upconversion fluorescence emission of Ho3+ ions in NaLuF4 nanocrystals through building core-shell structure

本文主要以具有六方相结构的NaLuF4:Yb3+/Ho3+/Ce3+纳米晶体为核,采用外延生长法构建具有同质结构的NaLuF4:Yb3+/Ho3+/Ce3+@NaLuF4:Yb3+核壳纳米晶体.借助X-射线衍射仪及透射电子显微镜对样品的晶体结构、形貌及尺寸进行表征.在近红外光980nm激光激发下,通过构建核壳结构及有效调控外壳中敏化离子Yb3+离子的掺杂浓度,实现Ho3+离子在NaLuF4纳米晶体中的红光发射增强.实验结果表明:在相同的激发条件下,具有核壳结构的NaLuF4:Yb3+/Ho3+/Ce3+@NaLuF4:Yb3+纳米晶体的红光发射均得到了增强,同时,当外壳中Yb3+离子的掺杂浓度为10.0%时,其上转换红光发射强度最强,为NaLuF4:Yb3+/Ho3+/Ce3+晶体核红光发射强度的5.8倍.根据其光谱特性及发光动力学过程,讨论了同质壳及壳中敏化离子掺杂浓度变化对其发光特性的影响规律.这种具有较强红光发射的核壳结构纳米晶体在生物医学、防伪编码、多色显示等领域具有较大的应用前景.

A series of the hexagonal-phase NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0% Ce3+@NaLuF4:x%Yb3+ core-shell(CS)nanocrystals with codoping different Yb3+ ions in the shell is successfully built by a sequential growth process.The crystal structures and morphologies of samples are characterized by X-ray diffractometer and transmission electron microscope.With the Yb3+ ion concentration increasing from 0% to 15% in NaLuF4 shell,none of the crystal structures,sizes,and morphologies of the samples changes obviously because of the similarity in ionic radius between Yb3+ and the ions in shell and the low doping concentration.Under 980 nm near-infrared(NIR)excitation,the NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ core nanocrystal produce green and red UC emission.And the red UC emission intensity is higher than green emission intensity.This is because two effective crossrelaxation processes happen between Ho3+ and Ce3+ ions,which results in the enhancement of the red emission.However,the overall emission intensity of NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ nanocrystal decrease compared with that of the NaLuF4:20.0%Yb3+/2.0%Ho3+ nanocrystal.Thus,to further enhance the red UC emission intensity in NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ nanocrystal,the NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+@NaLuF4:x%Yb3+ CS nanocrystal are prepared for blocking the excitation and emission energy,transmitting surface quenching center and getting more excitation energy through doping Yb3+ ions in NaLuF4 shell.It can be clearly seen that the red UC emission intensity of CS nanocrystal first increases and then decreases with Yb3+ ion concentration increasing.Meanwhile,the corresponding red-to-green ratio increases from 4.9 to 5.6.The highest red UC emission intensity is observed in each of the NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+@NaLuF4:10%Yb3+ CS nanocrystal because the Ho3+ ions get more energy through the following three ways:1)Yb3+(core)-Ho3+(core);2)Yb33+(shell)-Ho3+(core);3)Yb3+(shell)-Yb3+(core)-Ho3+(core).Thus,building CS nanocrystals is one of the most effective approaches in order to improve the UC efficiency by suppressing the non-radiative decay of activators in the core and getting more excitation energy through different energy transfer ways.These NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+@NaLuF4:Yb3+ CS nanocrystals with red UC emission have great potential applications in biological field and multi-primary color.