Synthesis And Properties Of Rare Earth Fluorides Micro-Nanocrystals

Lanthanide-doped upconversion materials have generated a tremendous amount of interest.In constrast to organic fluorophores and semiconductor nanocrystals,they exhibit superior chemical and optical characteristics,such as low toxicity,large Stokes shift,high quantum yields,long lifetime,weak background luminescence,and good resistance to photo-bleaching.In this dissertation,low temperature molten salt method were applied to fabricate rare earth fluoride upconversion materials and upconversion luminescence properties were also studied.The main research content and results are as follows.1.Low-temperature molten-salt method and the NH₄NO₃ flux were devleoped to synthesize quasispherical cubic BaGdF₅:Er³⁺/Yb³⁺nanocrystals with the average diameter of 30 nm.The influences of the reaction time,reaction temperature and doped concentrations of Er³⁺/Yb³⁺on the crystal phase structure,morphology and size of the resultant samples were systematically investigated.The optimum doping concentrations of Er³⁺and Yb³⁺ion are 2mol%and 20mol%,respectively.The introduce of Ce³⁺in BaGdF₅:Er³⁺/Yb³⁺nanocrystals deduced a huge variation of the upconversion emission.The strong upconversion emission are observed for the first time in the ultraviolet region of 330³80nm due to the overlap of⁴G_7/2/²K_15/2/⁴G_9/2?⁴I_15/2 transitions of Er³⁺and blue one locating at 440⁴80 nm attributing to ⁴F_3/2/⁴F_5/2?⁴I_15/2 and ⁴G_7/2/²K_15/2/⁴G_9/2?⁴I_13/23/2 transitions of Er³⁺,respectively.This work not only demonstrates a rational approach to tailor upconversion emission?red,blue and ultraviolet?by tuning doping ions and concentration,but also for better understanding of their essential upconversion luminescence properties.2.NaBiF₄:Er³⁺/Yb³⁺nanocrystals were synthesized via the low-temperature molten-salt method in NH₄NO₃ flux.The influences of the usage amount of NaF and NH₄NO₃ flux,doped concentrations of Er³⁺and Yb³⁺,the reaction time and reaction temperature on the crystal phase structure,morphology and size of the resultant samples were systematically investigated.The morphology,size,doped concentrations of Er³⁺and Yb³⁺,pumping power,and temperature dependences of upconversion emissions were studied.The optimum doping concentrations of Er³⁺and Yb³⁺ion are3 mol%and 20mol%,respectively.The NaBiF₄:3mol%Er³⁺/20mol%Yb³⁺crystal exhibits the strongest green upconversion emission,which was observed under several mW 980 nm excitation at room temperature.The energy transfer between Bi³⁺and Er³⁺during upconversion emission process was also discussed at length.This work is significant not only to provide one novel synthesis method of Bi-based fluorideupconversion materials,but also for the better understanding of the energy transfer from Bi³⁺to Er³⁺.