Effect of host matrix on Yb3+ concentration controlled red to green luminescence ratio
Authors: Zhang Xiang-Yu  , Wang Dan  , Shi Huan-Wen  , Wang Jin-Guo  , Hou Zhao-Yang  , Zhang Li-Dong  , Gao Dang-Li  ,   College of Science, Chang'an University, Xi'an 710064, China;,   College of Science, Xi'an University of Architecture and Technology, Xi'an 710055, China
Published: December 2020

Rare earth doped upconverting micro/nanoparticles with controlled size and structure,which are excited by near-infrared light and emit the visible light,possess many applications especially in the areas of biomedicine and photonics devices.There is no universally favored spectral profile in a variety of specific applications.We expect upconversion (UC) nanoparticles with the tunable spectral behavior to meet the demand for actual applications.Although the UC emission wavelengths are strictly limited by the electronic structure of the dopant,the spectral profile could be varied by many factors such as the structure,size,and crystallization. Varying matrix host is the most convenient approach to dynamically tuning UC that is essential for a variety of studies.However,this approach suffers a significant constraint due to insensitive response of most dopant luminescence centers to matrix host.In this paper,a facile EDTA-assisted hydrothermal approach is developed to the shape-selective synthesis of fluoride microcrystals including NaYF4 rods,LiYF4 octahedrons,and YF3 cuboid brick,by only tuning the pH of the mother liquid.The UC spectra of a series of Yb3+/Er3+-doped fluoride particles with the different shapes and phases are investigated in detail under a near-infrared co-focused laser excitation.The effects of matrix hosts on UC luminescence attributed to the 4f-4f transitions of the Er3+ ions in a single particle are amplified through elevating Yb3+ concentration.The associated tuning mechanisms are explored by using the power dependent UC luminescence and the temporal evolutions of up/down-conversion emission spectra. Mechanistic investigation reveals that the sensitive response of Er3+ UC emission to matrix host stems from maximal use of the various channels populated luminescence levels.It is well known that the population and depopulation of the luminescence levels strongly depend on the excitation power density,the energy level structure of electron,the ratio of the population ions between the two levels,maximum phonon energy and phonon density.The matrix plays the most important role in both the population and depopulation of the luminescence levels mediated by modifying the radiation relaxation probability and non-radiation relaxation probability via varying lattice symmetry and phonon energy.However,the fine modification of the matrix by doping is not always effective to luminescence tuning.In the current study,comparing with LiYF4 and YF3 matrixes,it is interestingly found that NaYF4 matrix can effectively tune the intensity ratio of red to green luminescence from 0.48 to 6.11 by varying Yb3+ concentration from 0 to 98% particle.The result indicates that the multiple aspects in the UC process could be influenced by Yb3+ doping NaYF4 matrix structure.We believe that Yb3+/Er3+ codoped NaYF4 matrixes with various Yb3+ concentrations will result in applications in displays,biological imaging,chemical sensing and anticounterfeiting.