构建垂直金纳米棒阵列增强NaYF4:Yb^3+/Er^3+纳米晶体的上转换发光

Building vertical gold nanorod arrays to enhance upconversion luminescence of β-NaYF4:Yb^3+/Er^3+ nanocrystals

以金纳米棒垂直阵列(gold-nanorods vertical array,GVA)为衬底,SiO2为隔离层,构建GVA@SiO2@NaYF4:Yb^3+/Er^3+纳米复合结构.在近红外980 nm激发下,通过改变中间隔离层SiO2的厚度,研究GVA对NaYF4:Yb^3+/Er^3+纳米晶体上转换发光的调控规律.实验结果表明,当SiO2层的厚度增大至8 nm时,Er^3+离子整体的上转换发射强度增大近8.8倍,且红光强度增强尤为明显,约为16.2倍.为了进一步证实GVA对Er^3+离子红光发射的增强效果,以红光发射为主的NaYF4:40%Yb^3+/2%Er^3+纳米晶体为对象展开研究,发现Er^3+离子红绿比由1.84增加到2.08,证实该复合结构更有利于提高红光的发射强度.通过对其光谱特性、发光动力学过程的研究并结合其理论模拟,证实了上转换发光的增强是由激发与发射增强共同作用,而激发增强占据主导地位.采用该套复合体结构实现上转换荧光发射的增强,不仅有效地利用了贵金属的等离激元共振特性,而且对深入理解等离激元增强上转换发光的物理机理提供理论依据.

The plasmon resonance effect is one of the effective ways to enhance the upconversion(UC)luminescence,which is realized by enhancing the electromagnetic field from incident light interacting with free electrons of AuNRs surface.In this work,a series of GVA@SiO2@NaYF4:Yb3+/Er3+composite structures with different thickness values of SiO2 isolation layer is successfully built from self-assembled gold nanorods,steamed SiO2,and spin-coating rare-earth nanocrystals.The results of the SEM indicate that the size of gold-nanorods is approximately 22 nm in diameter and 65 nm in length.The X-ray diffraction and transmission electron microscope results demonstrate that the NaYF4:Yb^3+/Er^3+ nanocrystals possess hexagonal-phase structure with a size of about 20 nm.Under 980 nm near-infrared(NIR)excitation,the UC emission characteristics of GVA@SiO2@NaYF4:Yb^3+/Er^3+ composite structure are studied by using a confocal microscope spectroscopic test system,and regulated by changing the thickness of SiO2 isolation layer.The results indicate that the UC emission intensity of NaYF4:20%Yb^3+/2%Er^3+ nanocrystals is enhanced by about 8.8 times,and the enhancement factor of red UC emission intensity is about 16.2.In order to further prove the enhancement effect of the red UC emission,the GVA@SiO2@NaYF4:40%Yb^3+/20%Er^3+ composite structure with red UC emission is constructed in the same way.It can be found that the UC emission intensity of NaYF4:40%Yb^3+/20%Er^3+ ^nanocrystals is enhanced by 8.7 times and the red UC emission intensity is raised by about 9.7 times under the 980 nm NIR excitation.The corresponding excitation enhancement mechanism is simulated according to the power excitation dependence.And it is found that the rate of UC emission decreases and the R/G ratio also decreases with the excitation pump power increasing.The analysis of the above results shows that the excitation enhancement plays a leading role and is accompanied by emission enhancement.Meanwhile,the study of Er^3+ ion dynamic process indicates that the Er^3+ ion transition rate is accelerated due to the coupling from UC emission peaks and gold nanorod absorption peaks in GVA@SiO2@NaYF4:40%Yb^3+/20%Er^3+ composite structure.The enhancement mechanism of UC emission is also simulated,which further proves that the excitation enhancement is dominant.This kind of composite structure can not only help us to further understand the physics mechanism of the plasmon-enhanced UC luminescence but also promote the applications of rare-earth materials in medical imaging and fingerprint recognition.