主體效應對(Zn1-xCdx)(Ga1-yAly)2O4:R ( R = Mn, Cr, Tb, Eu )螢光體發光特性之研究

Abstract

本研究之構想係以Cd2+或Al3+離子，系統化地分別取代ZnGa2O4主體晶格中的Zn2+或Ga3+離子，以深入瞭解了解主體組成與晶體結構的變化，對具有尖晶石結構的ZnGa2O4:R (R = Mn2+, Cr3+, Tb3+, Eu3+)系列螢光體中發光特性的效應。 本研究發現：Cd2+或Al3+離子對主體晶格中的Zn2+或Ga3+離子的取代，在0≦x≦1或0≦y≦1之範圍，均能形成(Zn1-xCdx)Ga2O4或Zn(Ga1-yAly)2O4連續性固溶液。但Tb3+與Eu3+等離子之摻雜，因為受離子半徑不相匹配之限制，故兩離子之取代上限遠小於1 atom%，而且過量的摻雜通常會出現R3Ga5O12 (R = Tb, Eu)等雜相。 在光致發光光譜中，(Zn1-xCdx)Ga2O4自身活化的放射峰隨Cd2+離子取代量遞增其波長由438 nm偏移至538.4 nm，並而呈現紅位移之現象。而在摻雜過渡金屬離子系列樣品中，Zn(Ga1-yAly)2O4:Mn2+系列由於結晶場隨Al3+離子取代增加而變大，其應對於4T1→6A1之躍遷放射峰，因結晶場中能階之變化，其波長由503 nm偏移至513.4 nm。在(Zn1-xCdx)(Ga1-yAly)2O4:Cr3+系列樣品中，對應於波長為689 nm之2E→4A2躍遷，幾乎不受結晶場強度變化而偏移，所不同的是其振動模式細微結構有所差異。反之，在激發光譜中，本研究明顯觀察到結晶場強度對Cr3+離子之4A2→4T1(F)躍遷與A2→4T２躍遷能量的影響。在結晶場最強的ZnAl2O4:Cr3+與結晶場最弱之CdGa2O4:Cr3+兩螢光體之中，其對應於4A2→4T２躍遷的能量相差約1,902 cm-1。 此外，在摻雜稀土離子樣品中，其發光特性不易受結晶場影響，在(Zn1-xCdx)Ga2O4:Tb3+或Eu3+系列中，Tb3+或Eu3+離子不易進入晶格內，所以ZnGa2O4主體無法將所吸收的能量有效地轉移至Tb3+或Eu3+離子之上，導致(Zn1-xCdx)Ga2O4主體晶格之放射支配放射光譜。在Zn(Ga1-yAly)2O4:Eu3+系列中，當Al3+離子取代量增加時，對應於波長約為615 nm之5D0→7F2躍遷並無太大變化，此結果顯示Eu3+離子應處於一不具中心對稱之格位。在Zn(Ga1-yAly)2O4:Tb3+系列樣品中，當Al3+離子取代量增加時，位於綠光範圍之5D4→7FJ躍遷所對應之放射峰強度逐漸減弱，位於藍光範圍之5D3→7FJ躍遷所對應放射峰強度測漸漸增強。 另一方面，本研究由陰極射線發光光譜得知 (Zn1-xCdx)Ga2O4與Zn(Ga1-yAly)2O4兩系列中，主體自身活化之放射峰波長分別隨Cd2+或Al3+離子之取代而呈紅位移與藍位移，此可能是因不同主體結構因上述陽離子之摻雜造成能隙的變化所致。在掃瞄式電子顯微鏡影像微結構的分析顯示：(Zn1-xCdx)Ga2O4與Zn(Ga1-yAly)2O4兩系列主體螢光體晶粒形貌呈不規則的分佈，其平均粒徑分別約在0.1到0.5微米之間與0.5到1微米之間。

This research is attempted to investigate the effect of host compositions on the luminescent properties two series of spinel-type ZnGa2O4 by systematic substitution of Cd2+ and Al3+ for Zn2+ and Ga3+ in the host, respectively. Phases with nominal compositions of (Zn1-xCdx)Ga2O4:R (ZCGO:R) and Zn(Ga1-yAly)2O4:R (ZGAO:R) (R = Cr, Mn, Eu, Tb; 0 □ x or y □ 1.0) were synthesized as phosphors materials at 950 and 1300 ℃, respectively, to form continuous solid solutions. However, the substitution limit of Eu and Tb activators was estimated to be much lower than 1 atom% which was attributed to the size mismatch between Eu3+ or Tb3+ and host cations. With increasing substitution level of Cd2+ the emission wavelength (□em) of self-activated (Zn1-xCdx)Ga2O4 was observed to shift from 438 nm to 538.4 nm, as indicated by photoluminescence (PL) spectra data. Furthermore, with increasing Al3+ dopant content the □em corresponding to 4T1 □ 6A1 transition in Zn(Ga1-yAly)2O4:Mn2+ was found to shift from 503 nm to 514 nm due to stronger crystal field. On the other hand, the □em corresponding to 2E --> 4A2 transition in (Zn1-xCdx)(Ga1-yAly)2O4:Cr3+ was found to be independent of variation of crystal field, yet emission band with fine vibrational structure was observed in the ambient temperature PL emission spectra of Cr3+-doped phases. On the contrary, we have observed the strong dependence of the energy involved in the transitions of Cr3+ 4A2 □ 4T1(F) and 4A2 □ 4T2 on the strength of crystal field. The difference in energy of 4A2 □ 4T2 transition in the phases with the strongest crystal field (ZnAl2O4:Cr3+) and the weakest (CdGa2O4:Cr3+) was determined to be 1,902 cm-1. On the other hand, the luminescent properties of (Zn1-xCdx)Ga2O4:Tb3+ or Eu3+ were found to be independent of host composition variation due to Cd2+ doping. The emission due to self-activation in (Zn1-xCdx)Ga2O4 or Zn(Ga1-yAly)2O4 host lattice dominated in the PL emission spectra of rare-earth doped phases has been attributed to the fact of inefficient energy transfer from the host of ZnGa2O4 to activators such as Tb3+ or Eu3+. The □em of 615 nm corresponding to 5D0 □ 7F2 transition in the Zn(Ga1-yAly)2O4:Eu3+ series was found to be independent of Al3+ dopant content which indicates that Eu3+ occupied in a lattice site without inversion symmetry. The emission peak intensity of 5D4-->7FJ transition and that of 5D3-->7FJ transition in the Zn(Ga1-yAly)2O4:Tb3+ series was observed to decrease and enhance, respectively, as Al3+ content increases. The □em attributed to self-activation in (Zn1-xCdx)Ga2O4 and Zn(Ga1-yAly)2O4 was found to exhibit red and blue shift, respectively, with increasing content of Cd2+ or Al3+, as indicated by cathodoluminescence data. This observation could probably be attributed to the systematic change of energy gap of the host due to cation substitution. The grain morphology of both series of title phosphors was determined to be irregular and the average grain size was found to be submicron with diameter between 0.5 to 1 um, as indicated by SEM imaging analysis.