The Rare-earth Ions Doped Fluoride Nanocrystals And Investigation Of Their Mid-infrared Fluorescence

Mid-infrared?2⁵ ?m?fiber laser locates in the transparent atmospheric transmission window,covers many kinds of important molecular characteristic spectral lines,and has strong H2 O absorption band,as a result,it has found wide application in national defense,medical therapy,industry and communication,etc.So far,the research of mid-infrared fluorescence and laser materials center on the rate-earth ions doped glass materials,glass ceramic and singlecrystal materials.However,the self-defects of these conventional materials,such as weak crystal field,limited host materials,difficult to draw fiber,has made mid-infrared fiber laser materials have reached a bottleneck state.Thus,it is extremely urgent to search a new solid state host for mid-infrared fluorescence and laser.Compared with conventional materials,nanocrystal-glass composite materials could break through the bondage of conventional materials,and provide a new research direction for mid-infrared fiber laser materials.Because of much attention on phonon energy and hydroxyl content for mid-infrared fluorescence and laser host materials,the key of forming nanocrystal-glass composite materials is selecting suitable mid-infrared nanocrystals.As one of high efficiency host materials,NaYF₄ basically satisfy the requirement of mid-infrared fluorescence and laser on host materials,therefore,NaYF₄ is also a kind of ideal mid-infrared host materials.Thus,the aim of this dissertation is to prepare Er³⁺-doped NaYF₄ nanocrystals with controllable crystalline phase and morphology under high temperature,which is hoped can be used to form nanocrystal-glass composite materials.Due to owning lower phonon energy,?-NaYF₄ is researched first in this dissertation.Monodispersed Er³⁺-doped ?-NaYF₄ nanorods are prepared by utilizing hydrothermal method.Pumped by 980 nm laser diode,Er³⁺: ².7 ?m mid-infrared fluorescence is obtained in ?-NaYF₄ nanocrystals.After sequent calcined at high temperature,organic groups are removed thoroughly,and enhanced Er³⁺: ².7 ?m mid-infrared fluorescence is realized.During the calcination process,the phase transformation of ?-NaYF₄:Er³⁺ nanocrystals destroys the morphology of nanocrystals,and influence the property of nanocrystals combining with glass.And the phase transformation process and mechanism is discussed deeply via XRD and TGDSC analysis.Er³⁺-doped ?-NaYF₄ nanocrystals are prepared via co-precipitation method in this dissertation.The nanocrystals are quasi-spherical with the particle size of ¹00 nm.Er³⁺: ².7 ?m mid-infrared fluorescence has an obvious enhancement after high temperature calcination,but the XRD and TG-DSC results demonstrate,during the increasing temperature process,?-NaYF₄:Er³⁺ nanocrystals would undergo twice phase transformations,resulting in bigger crystal size and agglomerated morphology of phosphor materials,and making it difficult to comelt with glass matrix.On this basis,we have investigated the effects of thermal treatment on crystalline phase,morphology and fluorescence properties of ?-NaYF₄:Er³⁺ nanocrystals.Moreover,according to XRD and fluorescence spectra results,the thermal stability between ?-NaYF₄ and ?-NaYF₄,and the mid-infrared fluorescence property of Er³⁺ in two kinds of crystal structures of NaYF₄ are discussed deeply.Utilizing the hydrolysis of tetraethoxysilane,?-NaYF₄:Er³⁺ nanocrystals are coated by amorphous SiO2.The optimal coating thickness confirmed by changing the addition of TEOS is ⁵0 nm.XRD and SEM results indicate that controllable crystalline phase and morphology can be realized by SiO2 coating under the condition of ensuring Er³⁺: ².7 ?m fluorescence output,and the controllable temperature can reach 700 oC.After etching SiO2 shells by hydrofluoric acid?HF?,further enhanced Er³⁺: ².7 ?m fluorescence is realized.Finally,?-NaYF₄:Er³⁺ nanospheres with high Er³⁺: ².7 ?m fluorescence efficiency are obtained in this dissertation.