Study On The Preparation And Optimized Luminescence Characteristics Of LED Red/Blue Phosphors For Plant Growth

With the development of modern agricultural technology,in order to meet the needs of plants for illumination under non natural conditions such as greenhouse,artificial light source is usually needed as supplementary light source.At present,the commonly used incandescent lamp,sodium lamp,high pressure mercury lamp and other light sources have low light supplement efficiency and large heat dissipation,and their emission spectrum covers 400-700 nm in the visible region.The growth of plants needs blue light of about 450 nm and red light of about 600-750 nm.If the ordinary incandescent lamp and other artificial light sources are used to illuminate the plants,it is bound to cause a waste of energy.Light emitting diode（LED）has the advantages of small size,long lifetime,small heat dissipation,low cost and easy adjustment of spectrum,which has been widely studied in recent years.Therefore,the development of LED phosphors for plant growth is necessary.In this paper,red and blue phosphors for plant growth are prepared by using Cr³⁺and Ce³⁺as activating ions.Luminescence characteristics can be controlled by introducing the luminescence center and the cation substitution of host.Luminescence mechanism of the activated ions is analyzed in detail.It provides a new idea for the design of phosphors for plant growth,main works are as follows:（1）Cr³⁺ions doped Zn₃Al₂Ge₂O₁₀ red-emitting phosphors for plant growth are prepared by a high-temperature solid-state method.Under the excitation of 470 nm,phosphor shows two emission bands:the narrow emission band at 705 nm is caused by the spin-forbidden ²E→⁴A₂ of Cr³⁺ions;another 721 nm broad emission band is attributed to the spin-forbidden ⁴T₂（⁴F）→⁴A₂ transition.In order to enhance the emission intensity of Cr³⁺ions,make it more compatible with the red light required for plant growth.Luminescent properties of Zn₃Al₂Ge₂O₁₀:Cr³⁺are improved by introducing a new luminescence center(Ce³⁺,Dy³⁺,Tb³⁺,Eu³⁺,Mn²⁺)into the material.By comparing the co-doping of rare earth ions and transition metal ions,luminescence intensity of Cr³⁺ions is enhanced in varying degrees when different kinds of rare earth ions are introduced as new luminescence centers.Among them,when Tb³⁺or Eu³⁺ions are co-doped with Cr³⁺ions,the luminescence intensity of Cr³⁺is twice of the initial intensity.（2）Aiming at the problem of poor matching between the emission spectrum of Zn₃Al₂Ge₂O₁₀:Cr³⁺and the absorption wavelength of the phytochrome P_FR in plants,using Ga³⁺and Al³⁺with different radii instead of Al³⁺to change the crystal field environment of Cr³⁺and control the luminescence.We design and synthesize Zn₃（Al Ga）₂Ge₂O₁₀:Cr³⁺solid solution.The solid solution is illustrated by the density functional theory calculation,X-ray powder diffraction（XRD）and High resolution transmission electron microscopy（HR-TEM）.The changes in the band gap width of Zn₃（Al Ga）₂Ge₂O₁₀ are compared,and it is proved that cation substitution of host does not affect the electronic transition direction of Cr³⁺ions.By adjusting the Cr³⁺doping concentration in Zn₃（Al Ga）₂Ge₂O₁₀ solid solution,the luminescence properties of material are improved,so that the emission spectrum is more matched with the absorption spectrum of the phytochrome P_FR.The effects of Ga³⁺or In³⁺substitution for Al³⁺on the luminescence characteristics of Zn₃Al₂Ge₂O₁₀:Cr³⁺are analyzed by emission spectra,excitation spectra and decay curves.（3）Ba₂B₂O₅:Ce³⁺blue phosphors for plant growth are synthesized by a high-temperature solid-phase method.Under the excitation of 360 nm ultraviolet light,the sample emitted blue light at 443 nm.Aiming at the problem of the mismatch between excitation spectrum of Ba₂B₂O₅:Ce³⁺and the near-ultraviolet chip,and the inconsistency between the emission spectrum and the wavelength required for photosynthesis,the emission spectrum and excitation spectrum of material show red-shift or blue-shift using Sr²⁺,Ca²⁺,Zn²⁺and Mg²⁺instead of Ba²⁺,respectivily.The excitation band of Ba_xCa_zB₂O₅:Ce³⁺shifts to 400 nm by introducing Ca²⁺ions,so that the synthesized phosphor can be better matched with the n-UV chip.Therefore,Ba_xCa_zB₂O₅:Ce³⁺can be expected to be applied to near-ultraviolet-based blue LEDs for plant growth.Variations of the luminescence characteristics for Ba_xSr_yB₂O₅:Ce³⁺（BSBO）,Ba_xCa_zB₂O₅:Ce³⁺（BCBO）,Ba_xZn_uB₂O₅:Ce³⁺（BZBO）and Ba_xMg_vB₂O₅:Ce³⁺（BMBO）phosphors are analyzed by crystal field splitting and nephelauxetic effect.In addition,the temperature dependence test is used to analyze the thermal stability of the material.This paper provides a new idea for improving the luminescent characteristics of phosphors,and has guiding significance for the development of LED for plant growth lighting.