Building vertical gold nanorod arrays to enhance upconversion luminescence of <i>β</i>-NaYF<sub>4</sub>: Yb<sup>3+</sup>/Er<sup>3+</sup> nanocrystals
Authors: Gao Wei , Wang Bo-Yang , Han Qing-Yan , Han Shan-Shan , Cheng Xiao-Tong , Zhang Chen-Xue , Sun Ze-Yu , Liu Lin , Yan Xue-Wen , Wang Yong-Kai , Dong Jun ,   School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an 710121, China
Published: December 2020

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@SiO<sub>2</sub>@NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> composite structures with different thickness values of SiO<sub>2</sub> isolation layer is successfully built from self-assembled gold nanorods, steamed SiO<sub>2</sub>, 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 NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> 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@SiO<sub>2</sub>@NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> composite structure are studied by using a confocal microscope spectroscopic test system, and regulated by changing the thickness of SiO<sub>2</sub> isolation layer. The results indicate that the UC emission intensity of NaYF<sub>4</sub>:20%Yb<sup>3+</sup>/2%Er<sup>3+</sup> 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@SiO<sub>2</sub>@NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+ </sup>composite structure with red UC emission is constructed in the same way. It can be found that the UC emission intensity of NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+</sup> 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<sup>3+</sup> ion dynamic process indicates that the Er<sup>3+</sup> ion transition rate is accelerated due to the coupling from UC emission peaks and gold nanorod absorption peaks in GVA@SiO<sub>2</sub>@NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+ </sup>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.