Preparation And Properties Of Rare Earth Ion Doped Fluoride Upconversion Luminescencc Materials

The up-conversion luminescent material is an important rare earth luminescentmateiral. Up-conversion （UC） phosphors have been investigated extensively becauseof their potential application in various fields，such as medical，technology ofanti-counterfeiting, three-dimensional display, communication, detection and so on.Yb³⁺ and Er³⁺ co-doped NaYF4with hexagonal phase is considered as one of the mostefficient host lattices for performing infrared-to-visible photon conversion. Due totheir potential applications as sensitive bio-probes, research on NaYF4:Yb³⁺, Er³⁺,especially preparation and up-conversion fluorescence of their nanocrystals hasbecome active recently. Although certain progress has been made in this area, thetheoretical system on upconversion luminescent material is not perfect. More studiesare needed. For example, the mechanism for ofrming nanoparticles and research onsurface modification need to be investigated in detail.In this project, Yb³⁺ and Er³⁺ co-doped MYF4（M二Na, Li, K） with different size and+structures were prepared. After that, effect of different Yb3concentration, differentEr³⁺ concentration, different calcination temperature and time, different structures anddifferent host lattices on the up-conversion luminescence of MYF4:Yb³⁺, Er³⁺ wereinvestigated in detail respectively and the optimal conditions for preparingup-conversion luminescence material with better pefrormance was explored. Thelight-emitting mechanism of the up-conversion luminescence was further discussed.The effect of Yb³⁺ concentration on the up-conversion luminescence of NaYF4：Yb³⁺, Er³⁺ was investigated. Results showed that Yb³⁺ increased the luminous intensityof the green light when its amount was not too high. When the concentration of Yb³⁺increases too much, it led to a decrease in intensity of green light, accompanying byan increase in intensity of red light. In detail, the function of Yb³⁺ is very small when the concentration of Yb³⁺ is very low, because theexcited-state absorption process occurs under very small Yb³⁺ concentration, thenthe energy transfer process becomes weak. Therefore, even Yb³⁺ can absorbphotons of980nm under this situation, it can’t transfer the energy to Er³⁺. Yb³⁺could only works when its concentration reaches a certain degree. The effect of Er³⁺concentration on the up-conversion luminescence of NaYF4:Yb³⁺, Er³⁺ wasinvestigated. With the increase of Er³⁺ concentration, the green emission intensityincrease first, then decreases remarkably. The intensity of the green emission showsthat2mol%Er³⁺ doping gives the strongest luminescence. When the concentrationof Er³⁺ is too high, green light quenched obviously. The effect of different crystalphase on color emission was also explored. Results showed that the hexagonal phaseemits strong and pure green light. The presence of the impurity phase declined theintensity of the green light. Existence of the cubic phase NaYF4generated a stronginteraction to emit strong red. Different crystal phase causes different splitting of thecrystal field, which affects the energy transfer between Yb³⁺ and Er³⁺. In otherwords, different crystal phase will affect the efficiency of up-conversion luminescent.