Identification and Quantification of Charge Transfer in CaAl₂O₄:Eu²⁺,Nd³⁺ Persistent Phosphor

Abstract

Reversible charge transfer between lanthanide ions is identified as storage mechanism for the current workhorse persistent phosphors. Present evidence relies on sophisticated X‐ray absorption spectroscopy to detect the reversible charge transfer. Here, simple optical spectroscopy is used to study the charge transfer in one of the benchmark persistent phosphors CaAl2O4:Eu2+,Nd3+. Based on the observation that both the trapping and de‐trapping processes in CaAl2O4:Eu2+,Nd3+ are thermally activated, forward charge transfer from Eu2+ to Nd3+ and backward charge transfer from Nd2+ to Eu3+ are identified. The percentages of Eu2+ and Nd3+ involved in the charge transfer are >10%, which exceeds the previous estimates in other persistent phosphors. Furthermore, this strategy offers additional advantage of site selectivity, which enables the identification of the distinct contributions of different Nd3+ sites to the charge transfer. These findings underline the significance of reversible charge transfer in persistent phosphors and move towards a complete understanding of persistent luminescence mechanism.

Broadband sensitized near‐infrared emission in Eu2+‐Nd3+ co‐doped BaAl2O4 as a potential spectral modulator for silicon solar cells

“…Although Ce 3+ has the characteristics of broadband sensitization, there is a problem of low photon utilization between the two absorption peaks [12,14]. Nd 3+ possesses abundant energy levels, which facilitates the acquisition of more energy from the sensitizer due to the resonance energy transfer effect [15,16].…”

Design, Synthesis, and Characterization of a Novel Blue-Green Long Afterglow BaYAl3O7:Eu2+, Nd3+ Phosphor and Its Anti-Counterfeiting Application

“…Since 1996, when the first report appeared about the strong long afterglow luminescent materials SrAl 2 O 4 :Eu 2+ , Dy 3+ , long afterglow rare-earth luminescent materials have been widely investigated. In SrAl 2 O 4 :Eu 2+ , Dy 3+ phosphor, Eu 2+ serve as the luminescence center, while Dy 3+ ions are thought to strongly enhance the sustained luminescence intensity [9,10].…”

Strontium Aluminate Persistent Luminescent Single Crystals: Linear Scaling of Emission Intensity with Size Is Affected by Reabsorption

“…Persistent phosphors are luminescent materials that can emit light long after the excitation has ceased. , This technologically interesting feature follows from a photoinduced charge transfer between the activator dopant (e.g., Eu 2+ ) and a defect, often the trivalent lanthanide co-dopant, like Dy 3+ or Nd 3+ , , that acts as a metastable charge trapping center. Extensive research on these materials started after the development of green-emitting SrAl 2 O 4 :Eu,Dy a few decades ago .…”