To Luminesce or to Change Valence? Insight into the Wavelength Dependency of the Reversible Valence Switching of Europium in Sr₃SiO₅

Abstract

A good control over the valence state of dopants in luminescent materials or phosphors is important for the development of highly efficient phosphors for white light-emitting diodes (LEDs). Detailed spectroscopic studies allow us to reveal optically induced charge transfer processes and elucidate the underlying mechanisms in phosphors with additional functionalities such as photochromism or persistent luminescence. However, the spectroscopic study of the valence switching of europium has scarcely been reported. Here, we report on the Sr3SiO5:Eu phosphor, in which photo-reduction (Eu3+ → Eu2+) and photo- or thermal-oxidation (Eu2+ → Eu3+) reactions are demonstrated. The variation of the illumination wavelength influences the efficiency of both the photo-reduction/oxidation and the accompanying dynamic process, especially when the two opposite reactions occur simultaneously. Temperature-dependent annealing indicates a large trap depth for the electron trapped by Eu3+. The good stability of Eu2+ obtained by photo-reduction and the repeatability of the Eu2+/Eu3+ valence switching are confirmed as well. Furthermore, the application of optical information storage is demonstrated based on this phosphor. The results of this work may not only improve the understanding of Eu2+/Eu3+ valence change during illumination but also allow the development of new functional luminescent materials.

Unlocking The Inherent Luminescence of Eu2+/Eu3+ by Light‐Induced Oxidation for Invisible Optical Storage

“…Therefore, we conclude that the oxidation of Eu 2+ is the primary factor contributing to the degradation of luminescence under UV irradiation. [ 7–9 ]…”

“…Therefore, we conclude that the oxidation of Eu 2+ is the primary factor contributing to the degradation of luminescence under UV irradiation. [7][8][9] In addition, the luminescence quenching of Eu 2+ induced by UV irradiation was partially restored through thermal treatment in air (Figure 4f; Figure S12, Supporting Information). This observed recovery during treatment in air aligns with the processes of carrier filling and release mediated by defects rather than the Eu 3+ →Eu 2+ transitions, indicating the possibility of an additional mechanism contributing to luminescence quenching.…”

“…[ 6 ] Recently, it was discovered that the inherent luminescence of Eu 2+ /Eu 3+ offers the unique ability to finely adjust PL properties (intensity and color) via a pre‐irradiation process under a fixed wavelength at different time. For example, in a series of Eu 3+ doped luminescent materials [(Ba,Sr) 2 SiO 4 :Eu 3+,[ 7 ] Ba 3 MgSi 2 O 8 :Eu 3+ , [ 8 ] and Sr 3 SiO 5 :Eu 3+ , [ 9 ] etc], their PL emission intensities of the Eu 3+ and Eu 2+ ions can be dynamically tuned by the UV or X‐ray‐induced redox reactions of Eu 3+ →Eu 2+ , accompanied by the changes in the color of the emitted light from initial red to blue. This inherent tunability of Eu 3+ /Eu 2+ enables the dynamic modulation of multicolor displays, showing potential in optical information storage compared with static color changes.…”

Insight into the electron transfer and anti-thermal quenching of europium doped Li4SrCa(SiO4)2

“…[2][3][4] In certain cases, illumination of the phosphor can also result in optically-induced oxidation or reduction of Eu 2+ and Eu 3+ , respectively, due to an electron transfer between the activator ion and another dopant ion or intrinsic defect such as a vacancy. [4][5][6][7] If this electron transfer occurs between two metal ions differing only in oxidation state, the process is called intervalence charge transfer (IVCT). 8 However, such IVCT between Eu 2+ and Eu 3+ usually manifests itself as a quenching effect when the two Eu ions are incorporated on the same crystallographic site, or as so-called anomalous emission in a few exceptions, making it difficult to be identified.…”