Luminescence of ZnS: Dy,Cu, ZnS: Pr,Cu and ZnS: Ho,Cu phosphors depending on the activator and coactivator concentration

Abstract

The study of rare-earth activated ZnS phosphors is of considerable interest in view of the theoretical interpretation of lanthanide (Ln) ion spectra in crystals as well as in connection with the possible application of these luminescent materials in practice, especially in electroluminescent devices. In many papers phosphors are studied where only Ln ions are incorporated as activators. In some of the papers (e.g. [1-6]) Li + , Cu + or F ions are also introduced as charge-compensating coactivators. However, sufficiently complete information, particularly about the influence of the activator and coactivator concentrations on the luminescent properties, under the same conditions of phosphor preparation, has not yet been obtained. This letter deals with the dependence of the photoand electroluminescence intensity and spectrum of ZnS doped with dysprosium, praseodymium or holmium and coactivated with copper on the activator and coactivator concentrations. Results for ZnS phosphors activated with other rare earth ions will be reported elsewhere. All samples studied were obtained by firing polycrystalline ZnS with Ln(NO3)3 and CuSO4 added at 1100 ° C for 1 h in an H2S flow. The luminescent-grade ZnS used was precipitated with HzS from a purified ZnSO4 solution and deoxidated by heating for 1 h at 500°C in HzS before the introduction of the activator and coactivator. In the absence of copper the changes in the emission intensity and spectrum of photoexcited ZnS : Dy with increasing activator concentration are given in Table I and Fig. l a. Spectral measurements at higher resolution show that a broad band appears at about 515 nm in addition to the lines due to f-f transitions (Fig. lb). A band with similar spectral position has been ascribed to oxygen-containing dysprosium centres [7], whereas in [1] broadband emission at 440 to 445nm was observed and radiative electron transitions in DyznVzn associates were assumed. When the amount of dysprosium was raised to 8 x 10 2mo1% the 515 nm band became strongly predominant: the 486 and 573 nm line groups were superimposed on them with low intensity and the lines around 668 and 753 nm vanished (Fig. la, curve 2). Introduction of 2 x 10 2mo1% copper as coactivator led to an enhancement of the photoluminescence efficiency by more than an order of magnitude (Table I); this is probably due to copper -* dysprosium energy transfer [3] as well as to a decrease in the Vzn concentration as a result of charge compensation through incorporated cuprous ions. Supposing first that the quantum yield for f f emission in Dyzn-V(,'. centres is low compared with Dyz. and Dyz,-Cuzn, and secondly that the 515 nm band emission is created by donor-acceptor transitions in the Dyzn-Vzn associates, one may explain the increase of the line emission as well as the established strong decrease of the relative intensity of the 515 nm band with the rise of the copper concentration, Ccu. The diminution of the luminescence intensity at Ccu > 2 x 10 .-2 to 4 x 10-2mol % (Table I) could be caused by quenching interaction between the copper ions. The strongest photoluminescence is registered in the presence of 2 x 10 2mo1% copper a n d 2 x 10 2 t o 4 x 10-2mo1% dysprosium. The considerable effect of incorporated copper on the photoluminescence spectrum is seen in Fig. la (curves 3 and 4); the spectra in Fig. lb clearly show the appearance of a new line at 482 nm and well-expressed changes in the relative intensities of the dysprosium lines: a long-wave shift of the line group at 573 nm occurs as a result of intensity redistribution and the line groups at 668 and 753 nm do not disappear even at high CDy when copper is present.