A Study on the Novel Synthesis and Luminescent Properties of Alkaline-Earth Sulfide Phosphors
Dr. Teng-Ming Chen
|Abstract:||以CaS與SrS為主體的螢光材料常應用在陰極射線管及電激（EL）發光顯示器中。但由於硫離子在水中易形成鹼土金屬硫酸鹽及亞硫酸鹽，難以形成CaS與SrS，故文獻中常以固態法合成，而少有以濕化學製程合成的報導。本研究以化學溶膠法及微乳液法等兩種濕化學製程成功地合成此二系列螢光體，並以固態法所合成的樣品做對照，其中以微乳液法所合成的樣品粒徑最小，可達300 ~ 500 nm。
此外，本論文在CaS與SrS中分別摻雜1 mol %的Ce3+、Eu3+、Sm3+三種稀土離子作為活化劑，以觀察並比較其發光特性。本研究發現SrS：Ce3+之發射峰波長相對於CaS：Ce3+呈現藍位移現象，且在（Ca1-xSrx）S：Ce3+（x = 0 ~ 1）固溶液中，當Sr2+取代量越多時，藍位移現象越明顯，此乃因Ce3+離子的發光是由5d → 4f電子遷移所造成，其發光特性受主體晶場強度變化影響大。此外， CaS：Eu的放射則呈現紅光的帶狀光譜，本研究推測此乃由Eu3+的特性紅光線狀光譜與Eu2+的藍綠光的帶狀光譜所形成，故Eu2+與Eu3+可能共存於此螢光體中。發橘紅光的Sm3+無論在CaS或SrS中，其發射光譜相近，此乃因Sm3+離子的發光是由4f → 4f電子遷移所造成，由於4f電子易受外層5d、6s電子屏蔽，故Sm3+之發光受主體影響小。
Phosphors with MS (M = Ca, Sr) as host lattice have been commonly utilized in the cathode ray tube and electroluminescence displays. However, reports regarding wet-chemical synthesis of MS phosphors have been rarely reported as compared to that from solid-state routes and the rationalization can presumably be attributed to the formation of stable metal sulfates and sulfites in aqueous solutions. We have synthesized the phosphors of MS phases by chemical colloid and microemulsion methods, respectively, and investigated the dependence of synthetic methodology on microstructure and luminescent properties of both pristine and rare-earth-activated MS phases. In addition, to investigate the luminescence of rare earth-activated alkaline sulfide phosphors we have prepared three series of MS:1%A (M = Ca, Sr; A = Ce3+, Eu3+, Sm3+) phosphors. As indicated by photoluminescence (PL) spectra, the emission wavelength (lem) attributed to the 5d → 4f transition of Ce3+ was found to exhibit an apparent blue shift when x increases from 0 to 1.0 for the series of (Ca1-xSrx)S:Ce3+ (0 < x < 1) phases. This observation can presumably be rationalized by the weakening of crystal field that was experienced by Ce3+ luminescence center when Sr2+ content increases. The emission of CaS:Eu was found to exhibit a red emitting band that consists of line emissions attributed to Eu3+ in the red region and a band emission attributed to Eu2+ in the blue-green spectral region. Furthermore, the both CaS:Sm3+ and SrS:Sm3+ were found to be orange-emitting due to 4f → 4f transition regardless of the type of host lattice, which was attributed by the shielding effect of 4f electrons by outer 5d and 6s shells. To stabilize metal sulfide particles, we have attempted to carry out the passivation of CaS and SrS by coating the phosphor surface with poly(methylmethacrylic acid) (PMMA) and, however, the crystallinity and PL intensity were found to deteriorate for passivated MS and MS:A phosphors.
|Appears in Collections:||Thesis|