Preparation And Photoluminescence Properties Of Trivalent Bismuth Ions Doped Tungstate And Vanadate Phosphors

With the growing threat of global climate change,energy crisis,and environmental issues,it is particularly important to look for new renewable energy or develop new energy-saving technology.Recently,white light-emitting diodes（WLEDs）have attracted wide attention because of their superior performance,such as high efficiency,energy conservation,environmental friendliness,small size,long lifetime,and unbreakability,thus been hailed as the green lighting source of the 21st century.Their promotion and use can reduce significantly global power requirements and the use of fossil fuels,resulting in the fall of CO₂ emission.Phosphor-converted WLEDs,especially those based on a blue LED chip and yellow phosphor,have attracted much attention and predominated in the lighting market of WLEDs owing to their simple packaging process,mature packaging technique,and low manufacturing cost.However,the combination strategy of a UV LED chip and multiple phosphors is still needed in order to meet the requirements of high color rendering.As the emission peak of GaN-based semiconductor chip expands toward shorter wavelength,the development of novel luminescent materials that can be effectively excited by NUV LED chips has become one of the most important and urgent challenges at present.So far,the efforts on phosphors mainly focus on rare earth(Eu²⁺,Eu³⁺,and Ce³⁺)doped systems such as sulfides,silicates,aluminates,and nitrides/oxynitride which suffer from some unavoidable problems（such as chemical and thermal stability,weather resistance,reabsorption among phosphors）and fail to meet the requirements for WLED lighting application.In this paper,we chose Bi³⁺as the activator,and developed a series of Bi³⁺-doped tungstate and vanadate phosphors.The photoluminescence（PL）properties of these phosphors were investigated systematically by using diversified characterization technique and computational method,demonstrating the intrinsic relationship between the crystal structure of host compounds and the PL property of Bi³⁺ions.The detailed research content and results are as follows:（1）We prescreen the candidate compound Zn WO₄ with high covalency by the dielectric chemical bond theory calculation and in consequence screen all possible types of defects due to Bi substitutions inside ZnWO₄ in aspects of PDOSs,formation energy and electronic configurations.DFT calculation shows Bi doped ZnWO₄ can promisingly create broad range tunable emission from visible to near infrared.Consequent experiments confirm this and tunable emission were observed between 400 to 800 nm as we modulated the environment temperatures,excitation schemes and energy transfer processes for these defects.The red luminescence from Bi _Zn³⁺was observed as excited by NUV light within 380-420 nm.In all,we believe that the combination of dielectric chemical bond theory calculation and DFT calculation can guide us to discover effectively new phosphors in the future.（2）We reported a La₃BWO₉:Bi³⁺yellow phosphor that can be efficiently excited by NUV light.PL spectroscopy analysis indicates that there are two Bi³⁺luminescent centers in La₃BWO₉:Bi³⁺phosphor,which is inconsistent with the previously reported hexagonal structure of La₃BWO₉,because only one La site in the structure can accommodate Bi³⁺ions.Combining the luminescent properties of Bi³⁺with Rietveld refinement,La₃BWO₉ is redefined as a trigonal structure with P3 space group in which threre are two independent crystallographic La sites.In addition,according to the extinction rule,the finding of the（0001）reflection further confirms the rationalization of P3 space group.So,the PL behaviors of Bi³⁺can act as a complementary tool to determine the real crystal structure especially when it is difficult to distinguish by traditional X-ray diffraction techniques.（3）We reported a GdVO₄:Bi³⁺yellow phosphor with an emission peak at 555 nm and an absolute quantum efficiency of 90.4%.However,the absorption of this phosphor in NUV region is too weak to meet the requirements for WLED lighting application.The enhanced near-UV absorption of Bi³⁺was realized by forming（Gd,Lu,Sc,La）VO₄:Bi³⁺solid solution via isovalent substitution and controlling Lu³⁺/Gd³⁺or Sc³⁺/Gd³⁺ratios.Meanwhile,the emission peak redshifts from 555 to 650 nm,implying a wide range of order tuning of PL.We expect this work can provide some guidance for the design of novel phosphors in the future.