2017年10期电子期刊

为了增强β-NaGdF₄：Yb³⁺，Tm³⁺纳米晶的上转换发光，克服外延增长钝化壳增大尺寸的不足，利用阳离子交换法制备核壳纳米结构，研究了样品在980 nm激发下的上转换发光性质。首先，利用高温热分解法制备了直径为10 nm的β-NaGdF₄：Yb³⁺，Tm³⁺纳米晶；然后，将制备的纳米晶与Gd³⁺在油酸-十八烯混合溶液中在300℃进行交换反应。实验结果表明，随着表面Yb³⁺和Tm³⁺被Gd³⁺取代，钝化壳的形成抑制了内部Yb³⁺的表面去激发过程，增强了内部Yb³⁺（²F₅）→ Tm³⁺（³H₅，³F_2，3）的能量传递，上转换发光逐渐增强。交换30 min后，Tm³⁺的³H₄ →³H₆近红外发光增强达到最大，为对照样品的6.5倍，而尺寸基本保持不变。在生物成像方面，上转换纳米晶的尺寸必须与生物分子相匹配，同时发光强度要高，阳离子交换法既能增强近红外发光，又能保持原来小的尺寸，在生物成像领域具有很好的应用前景。

NIR Enhancement of β-NaGdF4:Yb3+,Tm3+ Nanocrystals via Cation Exchange Reaction

In order to enhance upconversion luminescence of β-NaGdF4:Yb3+, Tm3+ nanocrystals and meanwhile overcome the particle size enlargement caused by epitaxial growth technique, NaGdF4:Yb3+,Tm3+@NaGdF4 core-shell nanostructure was prepared using cation exchange strate-gy. The upconversion luminescence of the core-shell nanocrystals was investigated under 980 nm la-ser excitation. The oleate-capped β-NaGdF4:Yb3+,Tm3+ nanocrystals with diameter about 10 nm were firstly prepared by thermal decomposition procedure. Then, the cation exchange reaction of the nanocrystals with Gd3+ was performed in l-octadecene and oleic acid mixture solution at 300℃. Ex-perimental results show that the cation exchange strategy has significantly enhanced upconversion lu-minescence brightness of the nanocrystals, which attribute to the suppression of inner Yb3+ de-exci-tation by the cation exchange shell and the enhancement of energy transfer from Yb3+( 2 F5 ) to Tm3+( 3 H5 ,3 F2,3 ) inside of the nanocrystals. The maximum improvement of NIR emission for Tm3+3 H4→3 H6 transition is achieved with 6. 5 times than that of the contrast sample after 30 min of ex-change reaction. It demonstrates that the cation exchange strategy can not only enhance NIR lumi-nescence of the nanocrystals, but also well retain small particle size. It provides a simple and con-venient way to development of high brightness upconversion nanocrystas with comparable in size to biomolecules, which has enormous applications in biomedical imaging fields.