Anomalous photoluminescence in $\mathrm{BaS}:\mathrm{Eu}$

The photoluminescence emission properties of $\mathrm{BaS}:\mathrm{Eu}$ powders have been studied. At room temperature, several weak emission bands related to impurities or intrinsic defects are observed in the visible part of the spectrum. The strongest emission band is situated mainly in the infrared with a peak emission wavelength of $878\mathrm{nm}$ and is related to ${\mathrm{Eu}}^{2+}$ centers. From the viewpoint of the large Stokes shift, the large emission bandwidth, and the temperature quenching profile, the ${\mathrm{Eu}}^{2+}$ emission in $\mathrm{BaS}:\mathrm{Eu}$ is totally different than what can be expected from the analogy with the similar materials $\mathrm{CaS}:\mathrm{Eu}$ and $\mathrm{SrS}:\mathrm{Eu}$. Hence the emission in $\mathrm{BaS}:\mathrm{Eu}$ is anomalous. Photoluminescence excitation and electron paramagnetic resonance spectra show that the incorporation of ${\mathrm{Eu}}^{2+}$ in $\mathrm{BaS}$ is not radically different from $\mathrm{SrS}:\mathrm{Eu}$ or $\mathrm{CaS}:\mathrm{Eu}$. The appearance of infrared emission is related to the position of the $5d$ excited level of ${\mathrm{Eu}}^{2+}$ relative to the conduction band of $\mathrm{BaS}$, which leads to autoionization of the ${\mathrm{Eu}}^{2+}$ centers upon $4f^7\text{−}4f^{6}5d$ excitation and the formation of impurity trapped excitons. The influence of trap levels was studied by thermoluminescence and a major trap with an activation energy of $0.51\mathrm{eV}$ was found. The thermal quenching behavior was evaluated as well.

Figure 1

Normalized emission and excitation spectra obtained at room temperature for (a) $\mathrm{BaS}:\mathrm{Eu}[0.1\%]$, (b) $\mathrm{SrS}:\mathrm{Eu}[0.1\%]$, and (c) $\mathrm{CaS}:\mathrm{Eu}[0.1\%]$ prepared by sintering. Excitation spectra were recorded at the peak emission wavelength. Emission spectra were obtained at 425, 445, and $465\mathrm{nm}$ for $\mathrm{BaS}:\mathrm{Eu}$, $\mathrm{SrS}:\mathrm{Eu}$, and $\mathrm{CaS}:\mathrm{Eu}$, respectively.

Figure 2

Emission spectra of $\mathrm{BaS}:\mathrm{Eu}[0.1\%]$ as a function of measurement temperature. Excitation wavelength was $420\mathrm{nm}$. The inset shows the Arrhenius plot of the total emission intensity, with the solid line a fit to the experimental data points, yielding activation energies of 14 and $190\mathrm{meV}$.

Figure 3

Plot of $1000∕T_m$ as a function of $\ln (T_m^2∕\beta )$ for the determination of the trap energy by Hoogenstraaten’s method, derived from TL measurements on $\mathrm{BaS}:\mathrm{Eu}$ powder with heating rates of 0.5, 1.0, 2.0, and $5.0\mathrm{K}∕\min$. The inset shows the TL intensity monitored at $775\mathrm{nm}$ as a function of temperature for $\mathrm{BaS}:\mathrm{Eu}[1.0\%]$. A heating rate of $2.0\mathrm{K}∕\min$ was used.

Figure 4

Evolution of the light output of $\mathrm{BaS}:\mathrm{Eu}[1\%]$ powder at room temperature as a function of the excitation intensity (emission intensity monitored at $775\mathrm{nm}$ upon excitation at $425\mathrm{nm}$). The evolution of the light output as a function of time was observed at 3%, 6%, 12%, 25%, 50%, and 100% of the highest excitation intensity $I_0$ $(2\mathrm{mW}∕{\mathrm{cm}}^2)$.

Figure 5

Normalized emission spectra of undoped $\mathrm{BaS}$ powder (prepared by wet chemical synthesis) at an excitation wavelength of (a) $350\mathrm{nm}$ (RT), (b) $332\mathrm{nm}$ $\left(70\mathrm{K}\right)$, and (c) $350\mathrm{nm}$ $\left(70\mathrm{K}\right)$.

Figure 6

Normalized excitation spectra at $70\mathrm{K}$ of (a) undoped $\mathrm{BaS}$ powder (prepared by wet chemical synthesis) at an emission wavelength of $540\mathrm{nm}$ and (b) of $\mathrm{BaS}:\mathrm{Eu}[0.1\%]$ at an emission wavelength of $930\mathrm{nm}$.

Figure 7

Q band EPR spectra for $\mathrm{CaS}:\mathrm{Eu}$, $\mathrm{SrS}:\mathrm{Eu}$, and $\mathrm{BaS}:\mathrm{Eu}$ at room temperature. Powders were prepared by sintering and dopant concentration was 0.1%. The dotted lines are a guide to the eye.

Figure 8

Energy level scheme for $\mathrm{BaS}:{\mathrm{Eu}}^{2+}$ and $\mathrm{SrS}:{\mathrm{Eu}}^{2+}$, with the ground and excited state for ${\mathrm{Eu}}^{2+}$, the conduction band (CB) and valence band (VB), the exciton level (Exc), and the trap levels. The transitions $a$, $b$, and $c$ are referred to in the text.