ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS

118 ESRF

X-RAYS REVEAL THE ENERGY STORAGE MECHANISM OF GLOW-IN-THE-DARK MATERIALS

The mysterious greenish light from glow-in-the-dark materials has fascinated many, including physicists and chemists puzzled by the working principle. A combination of optical and X-ray spectroscopy has now revealed that a reversible electron transfer between the Eu2+ and Dy3+ impurities underlies the energy storage.

Glow-in-the-dark materials have attracted a lot of interest, scientific and other, since the renaissance. Nowadays, the brightest of all, strontium aluminate doped with small amounts of europium and dysprosium (SrAl2O4:Eu,Dy), can emit light up to 10 hours after initial charging and is therefore used on a large scale in safety applications, watch dials, toys and even glowing cycling roads.

The energy storage mechanism that underlies the afterglow has led to much debate in the scientific community working on these compounds. First, the europium ion (Eu2+) absorbs blue or near- UV incident light, forming a short-lived excited state which, in ordinary luminescent materials, decays radiatively, emitting the distinctive green light. However, in a persistent phosphor (Figure 102), the excited electron is transferred to a crystallographic defect, or a trap, before this radiative decay can occur. It is empirically known that the number of these traps is boosted by adding an appropriate second lanthanide ion, in this case dysprosium (Dy3+), into the phosphor. Its exact role in the afterglow mechanism is, however, still a mystery: does it trap the electron, forming an exotic Dy2+ ion, or does its size mismatch induce the necessary defects upon incorporation?

X-ray spectroscopy techniques are ideal tools to probe the valence state of the lanthanide dopants. Unfortunately, persistent phosphors are also strongly perturbed by the X-ray beam. When the beam is switched on, a bright luminescence emerges from the sample. An optical fibre was added to the setup, capturing the radioluminescence, which is composed of emission bands originating from europium and dysprosium. When the X-ray beam is switched off, the irradiated spot shows a strong afterglow, so the X-rays also lead to energy storage. The problem is that the traps are filled very fast, in the millisecond range for a high-brilliance beamline. However, a simple approach can overcome this experimental constraint.

At beamline ID26, a phosphor disk was rapidly spun, with the X-ray beam positioned 5 mm off-axis, resulting in a strongly reduced average X-ray flux impinging on the sample, prolonging the time to fill the traps from a few milliseconds to 10 seconds. To follow the movement of the electron, high-energy-resolution fluorescence- detected X-ray absorption near-edge structure spectroscopy (HERFD-XANES) was performed at the Eu L3 edge, monitoring the maximum of the Eu2+ and Eu3+ white lines. A reduction of the Eu2+ signal was found during the charging, along with an equal increase of the amount of Eu3+. The X-ray and optical spectra showed the same time-dependence, confirming that the energy storage process indeed involves the oxidation of the europium dopant.

The same experiment was subsequently repeated at the Dy L3 edge. This revealed the opposite behaviour (i.e., a decreasing Dy3+ signal and a simultaneously emerging Dy2+ white line). This also occurred on the same timescale as the charging of the persistent luminescence (Figure 103), demonstrating that the dysprosium co-dopants indeed act as the trapping centre of the electron originating from the europium luminescent centre.

Fig. 102: Persistent phosphors are widely used, for example in emergency signs. Now, it is shown that their working relies on a reversible electron transfer between Eu2+ and Dy3+.