Using Host Composition And Energy Transfer To Control The Luminescence Properties Of Zn₂GeO₄ And Zn₃Al₂Ge₂O₁₀

In this paper,serials of rare earth ions RE(RE = Ce³⁺,Eu³⁺,Tb³⁺,Dy³⁺)and transition metal ion Cr³⁺ doped Zn₂GeO₄ and Zn₃Al₂Ge₂O₁₀ are synthesized using a high-temperature solid-state method.Luminescent properties can be tuned by different substitutions,energy transfer and controlling the ratio of cations in host,and the reason of tunable luminescent properties are analysed,as follows.（1）Rare earth ions RE(RE = Ce³⁺,Eu³⁺,Tb³⁺,Dy³⁺)doped Zn₂GeO₄ by substituting Zn²⁺ or Ge⁴⁺ are synthesized,and the luminescent properties are investigated.Herein,Zn2-x Ge O4:x Ce³⁺ and Zn2Ge1-x O4:x Ce³⁺ can show two different emission bands due to the 5d-4f transitions of Ce³⁺ ions.The CIE chromaticity coordinates and the luminescence photographs of Zn2-x Ge O4:x Ce³⁺ and Zn2Ge1-x O4:x Ce³⁺ exhibit different characteristics.The emission bands appear blueshift and redshift due to the combination of crystal field splitting and nephelauxetic effect.However,for Zn2-a Ge O4:a R and Zn2Ge1-a O4:a R(R = Eu³⁺,Tb³⁺,Dy³⁺),the 4f-4f characteristic transitions of Tb³⁺,Eu³⁺ and Dy³⁺ are observed,and the emission and excitation peaks are not influenced by the crystal field.（2）Transition metal ion Cr³⁺ doped Zn₂GeO₄ by substituting Zn²⁺ or Ge⁴⁺ are synthesized,and a new strategy has been proposed to suppress the native emission of Zn Ge2O4 by controlling the defect types.There are two kinds of defects,i.e.positive and negative defects,which are formed when Cr³⁺ substituting for Zn²⁺ and Ge⁴⁺ ions in Zn Ge2O4,respectively.And the results present that the host emission can be more effectively suppressed by increasing the number of negative defects.In addition,it is surprising that a rare green light can be obtained from Cr³⁺ doped Zn₂GeO₄ by adjusting crystal field.（3）Zn₂GeO₄:Tb³⁺,Eu³⁺ are synthesized,and the luminescent properties are investigated.Under the ultraviolet（UV）radiation,Zn₂GeO₄ host presents a broad emission band from 350 to 600 nm,and the emission band at 430 nm is originated from the V’o,Zn i˙,VGe and V”Zn native defects.Eu³⁺ doped Zn₂GeO₄ has low absorption in the near-UV and blue region,and there is no ET from host to Eu³⁺ ions.However,the ET from Zn₂GeO₄ to Eu³⁺ may be realized by utilizing Tb³⁺ as the ET bridge to connect Eu³⁺ and Zn₂GeO₄ host.Zn₂GeO₄:Tb³⁺,Eu³⁺ exhibits a broad absorption band in the range of 250-450 nm,which could be excited by near-UV or blue LED chip.Moreover,the transformation process of crystal structure and defects were investigated by the thermoluminescence and photoluminescence spectra of Zn₂GeO₄:Tb³⁺,Eu³⁺.（4）Zn₃Al₂Ge₂O₁₀:Cr³⁺ are synthesized,and the luminescent properties are investigated.Under excitation of 400 nm,Zn₃Al₂Ge₂O₁₀:Cr³⁺ can emit a red emission band at 694 nm in the range of 650-750 nm,which is caused by 2E → 4A2 transition of Cr³⁺.Under 262 nm excitation,Zn₃Al₂Ge₂O₁₀:Cr³⁺ can produce a weakly broad emission band in the range of 350-600 nm,and the emission intensity of Zn₃Al₂Ge₂O₁₀:Cr³⁺ can be enhanced by tuning the ratio of Ca²⁺ to Zn²⁺,especially,Zn₃Al₂Ge₂O₁₀:Cr³⁺ can present a bluish white emission and long afterglow properties by introducing Ca²⁺.The bluish white luminescence is caused by increasing the intrinsic defects（V’’Zn,V’’Ca and V’’o）.And Ca vacancies can assist to stabilize the O vacancies;therefore,Zn₃Al₂Ge₂O₁₀:Cr³⁺ show the long afterglow characteristics.It enriched the luminescent properties of Zn₃Al₂Ge₂O₁₀:Cr³⁺ by introducing Ca²⁺,since Zn₃Al₂Ge₂O₁₀:Cr³⁺ shows different characteristics under different radiation excitation.