小粒径同质/异质壳层结构NaGdF_4:3%Nd~(3+)纳米颗粒的近红外发光特性

采用共沉淀法制备了粒径小于5 nm的六方相NaGdF_4:3%Nd~(3+)纳米颗粒.纳米颗粒表面缺陷会使发光中心产生严重的淬灭,对其表面包覆适当厚度的壳层可以有效地减少发光淬灭,提高发光性能.对NaGdF_4:3%Nd~(3+)核心纳米颗粒分别进行同质和异质包覆并且通过调节核壳比制备了不同壳层厚度的NaGdF_4:3%Nd~(3+)@NaGdF_4和NaGdF_4:3%Nd~(3+)@Na YF4纳米颗粒,研究了不同的壳层厚度对核心纳米颗粒发光的影响,并对两种不同核壳结构纳米颗粒的发光性能进行了对比.在808 nm近红外光激发下,NaGdF_4:3%Nd~(3+)纳米颗粒发射出位于约866,893,1060 nm的近红外发射.与核心纳米颗粒相比,核壳结构的纳米颗粒的荧光强度增强,荧光寿命增长,并且随着壳厚的增加,荧光强度出现先增强后减弱、荧光寿命逐步增长的趋势.与相同条件下同质包覆的NaGdF_4:3%Nd~(3+)@NaGdF_4纳米颗粒相比,异质包覆的NaGdF_4:3%Nd~(3+)@NaYF_4纳米颗粒光谱荧光强度增强,寿命增长.

Near-infrared luminescence properties of small-sized homogeneous/heterogeneous core/shell structured NaGdF4:Nd3+ nanoparticles

In recent years, considerable researches have focused on the upconversion phosphor nanoparticles in the application of biomedical imaging, which emit visible light. Nevertheless, these kinds of nanoparticles limit the light penetration depth and imaging quality. The Nd³⁺ doped nanoparticles excited and emitted in a spectral range of 700-1100~nm can overcome those shortcomings. Furthermore, considering the applications of rare earth nanoparticles in biomedical imaging, smaller particle size is needed. However, the luminescence efficiencies of nano-structured materials are lower due to the inherent drawback of high sensitivity of Nd³⁺ ions to the surface defects. So, it is of vital importance for introducing a shell with low phonon energy to be overgrown on the surface of nanoparticles. According to the ratio of core material to the shell, core/shell structured nanoparticles are separated into “homogeneous” and “homogeneous”nanoparticles. And the shell material may influence the luminescence performance. In few reports there have been made the comparisons of luminescence performance of Nd³⁺ between heterogeneous and homogeneous core/shell nanoparticles. In the present work, small-sized hexagonal NaGdF₄:3%Nd³⁺ nanoparticles with an average size of sub-5~nm are synthesized by a coprecipitation method. To overcome the nanosize-induced surface defects and improve the luminous performance, the NaGdF₄:3%Nd³⁺ nanoparticles are coated with homogeneous and heterogeneous shells, respectively. Core/shell structured nanoparticles with different values of shell thickness are synthesized by using the core/shell ratios of 1:2, 1:4 and 1:6. The luminescence properties of the prepared nanoparticles are characterized by photoluminescence spectra and fluorescence lifetimes. Under 808~nm excitation, the NaGdF₄:3%Nd³⁺ nanoparticles exhibit nearinfrared emissions with sharp bands at ~866 nm, ~893 nm, ~1060 nm, which can be assigned to the transitions of ⁴F_3/2 to ⁴I_9/2, ⁴F_2/3 to ⁴I_11/2, respectively. The locations of emission peaks of the core/shell nanoparticles are in accordance with the those of cores while the fluorescence intensity increases significantly. In addition, the average lifetimes of Nd³⁺ ions at 866 nm of core/shell nanoparticles are longer than those of the cores, which indicates that the undoped shell can minimize the occurrence of unwanted surfac-related deactivations. Notably, comparing with the homogeneous NaGdF₄:3%Nd³⁺@NaGdF₄ nanoparticles, the fluorescence intensity of heterogeneous NaGdF₄:3%Nd³⁺@NaYF₄ nanoparticles is enhanced and their lifetimes become longer. It is due to the low stability of hexagonal NaYF₄, which suppresses the nucleation of the shell precursor and makes the shell able to be fully coated on the core. The decrease of electron charge density on the surface of core/shell nanoparticles is also beneficial to shell growth and crystallization. The high crystallinity of heterogeneous core/shell structured nanoparticles can eliminate negative influence of surface effect more efficiently. In addition, the phonon energy of NaYF₄ is lower than that of NaGdF₄, which leads to low possibility of non-radiative cross-relaxation between Nd³⁺ ions, thereby improving the luminescence efficiency in the near in frared emission.