Red afterglow and luminescence arising from defects in CaS:Eu²⁺, Tm³⁺

Abstract

CaS:Eu2+, Tm3+ is a phosphor known to emit a long afterglow of red emission (650 nm) when excited by blue light (450 nm). It shows a long afterglow time of 700 s for Eu = 0.05% and Tm = 2%. The mechanism of this afterglow is investigated using time-resolved fluorescence (TR-F) spectroscopy from the nanosecond to millisecond region. At room temperature, it is not possible to investigate shallow levels because of the effects of thermal vibrations. The mechanism of the emission characteristics at room temperature would be affected by these levels that can be observed only at low temperatures. Therefore, the samples are cooled to 15 K for the TR-F measurements. The host material CaS emits blue light (420 nm) arising from sulfur defects, and the typical decay time is measured to be 6 ms. This blue emission becomes stronger when Tm3+ is doped. Furthermore, the doped Eu ions emit a broad red spectrum at 650 nm originating from the Eu2+-specific 4f65d1–4f7 transition. When the excitation is ceased, the red emission decays with a fast time constant of 0.6 μs. This value is a typical decay time for Eu2+. This red emission has multiple decay time constants, and a component with a decay time of 6 ms appears. This 6 ms decay time is the same as that of the blue emission from the sulfur defects, which have an important role on the red afterglow.

Photostimulated luminescence excited by infrared LEDs in CaS:Eu2+ red afterglow phosphors

“…Charge defects which were decreased by Na doping were suggested to serve as major trapping centers and to be responsible for the afterglow. In this study, small concentration of Eu0.1% is used, because PL efficiency and afterglow for Eu0.1% were found to be better than for Eu1% [16,32]. phosphors.…”

“…Timeresolved fluorescence spectroscopy and EPR study suggested that intrinsic S vacancies V S in the CaS host matrix are the probable reasons for the afterglow [32]. The trap levels located at 0.3 eV below the bottom of the conduction band of CaS cause the afterglow, and these trap levels are considered to be associated with V S [16,32]. The EPR signal of V S appears near 351 mT [32][33][34], and the data are plotted in figure 6.…”

“…225)) was obtained. In some cases, diffraction patterns of small traces of oxides appear, but no correlation between the afterglow properties and the signals from oxides was found [16,32]. Thus the peaks of Ca, S and Cl are considered in figure 5.…”

“…Figure 6 shows EPR spectra in CaS and CaS:Cl 12%, in order to investigate defect states in CaS phosphors. Timeresolved fluorescence spectroscopy and EPR study suggested that intrinsic S vacancies V S in the CaS host matrix are the probable reasons for the afterglow [32]. The trap levels located at 0.3 eV below the bottom of the conduction band of CaS cause the afterglow, and these trap levels are considered to be associated with V S [16,32].…”

Alkaline-Earth Chalcogenide Nanocrystals: Solution-Phase Synthesis, Surface Chemistry, and Stability

“…Alkaline-earth chalcogenides are indirect, wide band gap semiconductors of growing interest for optical applications . Doping them with rare earth ions results in luminescent phosphors suitable for bioimaging, , light-emitting diodes (LEDs), , thermal sensors, and photovoltaics . Among their different binary phases, alkaline-earth monochalcogenides, collectively referred to as AeCh or IIa-VIa semiconductors, are predicted to exhibit favorable thermoelectric properties in both their rock salt and hexagonal monolayer forms. , While these monochalcogenide phases traditionally adopt the rock salt structure, related polychalcogenide phases such as BaSe 2 and BaSe 3 adopt lower symmetry structures with distinct electronic properties (Figure ).…”