Upconversion And Downconversion Luminescent Properties Of Rare-earth-doped NaYF₄ By Hydrothermal Synthesis

Rare-earth (RE³⁺)-doped NaYF4 have regarded as promising materials for their potential applications in lighting and display, infrared inspection and biological labeling, etc., due to its low phonon energy, high refractive index, optical transparency and excellent luminescence properties. Herein, this Dissertation will present a facile hydrothermal approach to synthesis the RE³⁺-codoped NaYF4 phosphors. The main objective of this work is to investigate the structure and luminescence properties of NaYF4 phosphors, to elucidate its mechanisms responsible for the upconversion (UC) and downconversion (DC) process, and to examine its suitability as potential UC and DC phosphors.This Dissertation will start with Charpter 1 that presents a brief overview on the developments of RE³⁺-doped phosphors, UC and DC phenomena in phosphors are highlighted. Charpter 2 will describe the theoretical basis and experimental techniques.Charpter 3 will focus on the preparation and properties of Er³⁺/Yb³⁺ codoped LiYF4,NaYF4 and BaYF5 by hydrothermal route. The products are characterized by X-ray diffractometer, scanning electron microscope, and photoluminescence spectroscopy. With the excitation of 980 nm laser diode (LD), all samples present the intense red/green upconversion emissions by fixed the Er³⁺ concentration at 2 mol.%, the optimal Yb³⁺ doping concentrations of these three host lattices are found to be 10, 20, and 20 mol.%, respectively. Compared with the same testing condition, alternative spectral splitting modes are obtained. Moreover, NaYF4:Er³⁺,Yb³⁺ exhibits the strongest intensity both in green and red UC emissions while LiYF4:Er³⁺,Yb³⁺ has advantage in the green UC emission, and BaYF5:Er³⁺,Yb³⁺ shows a weaker UC performance than two former. Upon excitation at 448 nm, the near infrared (NIR) luminescence is realized by Er³⁺: 4F7/2 + Yb³⁺: 2F7/2→Er³⁺: 4I11/2 + Yb³⁺: 2F5/2; Er³⁺: 4I11/2 + Yb³⁺: 2F7/2→Er³⁺: 4I15/2 + Yb³⁺: 2F5/2 or Er³⁺: 4I11/2→Er³⁺: 4I15/2. The downconversion process of NaYF4:Er³⁺,Yb³⁺ from 4F7/2 level has a very low efficiency due to fast multiphonon relaxation from 4F7/2 to 4S3/2 via the intermediate 2H11/2 level. While upon excitation at 377 nm, the red light in 653 nm enhances by the efficient cross relaxation process Er³⁺: 4G11/2 + Yb³⁺: 2F7/2→Er³⁺: 4F9/2 + Yb³⁺: 2F5/2, which results in NIR quamtum cutting (QC) process: absorpting a 377 nm photon to emit a red 653 nm photon and an infrared 1000 nm photon.In charpter 4, UC and DC performance of NaYF4:Ho³⁺,Yb³⁺ will be investigated. Upon excitation with 980 nm LD, intense blue, green and red emissions at 483 nm, 540 nm, 644 nm and 749 nm can be clearly observed, which assign to the transitions of 5F3→5I8,5S2→5I8,5F5→5I8 and 5S2→5I7, respectively. The luminescence intensity increase rapidly and have no quenching before the doping level arrives at 30 mol.%. Upon excitation at 448 nm, for NaY0.99F4: Ho0.01, we can observe the NIR emissions at 966 nm and 1012 nm corresponding to the 5F5→5I7 and 5S2→5I6 transitions besides the visible emissions. The visible emissions decrease when introduce the Yb³⁺ ions, while the NIR emissions enhance for the cross relaxation processes of Ho³⁺: 5S2 + Yb³⁺: 2F7/2→Ho³⁺: 5I6 + Yb³⁺: 2F5/2 and Ho³⁺: 5F5 + Yb³⁺: 2F7/2→Ho³⁺: 5I7 + Yb³⁺: 2F5/2, then generate the transition Yb³⁺:2F5/2→2F7/2 at 978 nm.Charpter 5 will report an efficient cooperative downconversion in Pr³⁺/Yb³⁺ codoped NaYF4 phosphors. Upon excitation with 443 nm, the intense emission band at 482 nm, 522 nm, 606 nm, 640 nm, 688 nm and 721 nm in visible region assign to the electronic transitions of 3P0→3H4, 3P0→3Hi (i= 5, 6), 3Fj (j= 2, 3, 4), respectively. Furthermore, Yb³⁺ ions emit two NIR photons at 977 nm through cooperative energy transfer from Pr³⁺ to Yb³⁺. By changing Yb³⁺ doping concentration, the highest QE has been found to be 133.6% before reaching concentration quenching threshold.