Luminescence of (Ca, La)S:Dy

Photoluminescence of CaS: Dy and Ca_1-xLa_xS: Dy prepared in Na₂CO₃-S and K₂CO₃-S flux was investigated. The excitation spectrum of CaS: Dy consists of two dominant bands at 260 and 280-300 nm. Excitation band at 260 nm is assigned to the band gap energy of CaS, and the 280-300 nm band to the excitation from the acceptor level originated from V_Ca(2+). By analyzing the influence of x in the spectra of Ca_1-xLa_xS: Dy, the peaks from f-f transition of Dy and bands from the host matrix could be resolved and assigned. Emission spectrum of CaS: Dy prepared in Na₂CO₃-S flux has sharp peaks from f-f transition, in addition to two broad bands at 525 and 580 nm. The broad band spectrum is associated with defects and vacancies of host material, that is, the peak at 525 nm is from the defect pairs of M_Ca(2+)³⁺-V_Ca(2+) and V_S(2)-V_Ca(2+) and the peak at 580 nm is from the pairs of M_Ca(2+)³⁺-Na_Ca(2+)⁺ and V_S(2-)-Na_Ca(2+)⁺. The sharp emission peaks at 486, 573, and 665 nm could be assigned to the transition of ⁴F_9/2→⁶H_15/2,⁴F_9/2→⁶H_13/2, and ⁴F_9/2→⁶H_11/2 of Dy³⁺, respectively. However, emission spectrum of CaS: Dy prepared in K₂CO₃-S flux, shows only the f-f transition of Dy³⁺. The dependence of the emission intensity ratio, I_yellow/I_blue, of CaS: Dy on Dy³⁺ concentration and flux was explained.