The Downconversion Luminescence Properties Of Bi³⁺,Bi³⁺/Sm³⁺ Doped Compounds

Rare earth luminescence materials are promising alternative candidates for solid state laser,display,light-emitting diode,anti-counterfeiting,biomedicine imaging,and so on.They have attracted considerable attention due to their unique optical properties,such as high chemical stability,good thermal stability and high luminescence efficiency.It is well known that trivalent lanthanide ions doped rare earth materials as activators are widely used in various phosphors due to the f-f or f-d transitions of lanthanide ions.Under the excitation of ultraviolet wavelength,they show high luminescence efficiency,high color purity and sharp absorption transitions.However,their absorption band is too narrow and weak,and only a small part of visible spectral range can be used in practical applications.In order to realize effective absorption and broaden excitation band,appropriate sensitizers or activators with large absorption cross sections are desired.The metal Bi³⁺ion,as a potential candidate,has drawn considerable attentions due to the relatively strong and broad absorption band from ultraviolet to visible regions.Bi³⁺ion,as an activator,can produce absorption from ultraviolet to visible light to infrared wavelengths.And the photoluminescence properties of Bi³⁺ion are strongly affected by the crystal field.Therefore,it is extremely important to study the photoluminescence properties of Bi³⁺ion in different crystal structures.In addition,Bi³⁺ion as a sensitizer can transfer the absorbed energy to the activated ion,which not only enhances its luminescence intensity,but also adjusts the emission of phosphor.In the paper,the downconversion luminescence properties of Bi³⁺ion doped in different rare earth oxides and the tunable emission of Bi³⁺/Sm³⁺ions doped rare earth composite oxides have been investigated.The main contents are as follows:A series of Bi³⁺ion doped Ln₂O₃（Ln=Lu,Gd,La）phosphors have been prepared through a simple urea-based precipitation method.The ¹S₀→³P₁ transition position shift of Bi³⁺ion has been investigated in Bi³⁺-doped Ln₂O₃ phosphors.This is caused by difference of covalency and coordination number.The ¹S₀→³P₁ transition of Bi³⁺ion in Gd₂O₃ crystal shifts to lower energy position than that in Lu₂O₃ crystal for the same site due to the larger radius of Gd³⁺ion compared to Lu³⁺ion.However,the ¹S₀→³P₁ transition of Bi³⁺ion shifts to higher energy position owing to the increase of coordination number from cubic phase Lu₂O₃ and Gd₂O₃ crystals to hexagonal phase La₂O₃ crystal.Our work provides a reference to achieve various photoluminesence performances of Bi³⁺ion doped oxides.A series of Y₃Al₅O₁₂:Bi³⁺/Sm³⁺phosphors were prepared by a high-temperature solid-phase method.The luminescence properties and energy transfer process of Y₃Al₅O₁₂:Bi³⁺/Sm³⁺phosphors were studied.X-ray diffraction analysis result shows that the prepared samples belong to pure cubic phase and Ia-3d（No.230）space group.Under the excitation of282 nm ultraviolet light,the photoluminescence emission spectrum shows that the Y₃Al₅O₁₂:Bi³⁺phosphors exhibit broad emission band between 350-650 nm peaking at 420 nm.This is attributed to the ³P₁→¹S₀ emission of Bi³⁺ion.The optimized Bi³⁺ion concentration is 7mol%.Under 406 nm excitation,Y₃Al₅O₁₂:Sm³⁺phosphors exhibit orange-red emission at550-680 nm,and the optimized Sm³⁺ion concentration is 1.5 mol%.In the Bi³⁺/Sm³⁺ions co-doped Y₃Al₅O₁₂ phosphor,Bi³⁺ion can transfer energy to Sm³⁺ion,which enhances the emission intensity of Sm³⁺ion.The tunable emissions of samples are achieved through adjusting the doping concentration of Bi³⁺/Sm³⁺ions.The results show that the single-component Y₃Al₅O₁₂:Bi³⁺/Sm³⁺phosphor has a good application prospect for ultraviolet-excited white-light emitting diodes（WLEDs）.