Multiple Coordination of Chromium Ion Luminescence: A Strategy for Designing Ultra-broadband NIR Long Persistent Luminescent Materials

Abstract

Near-infrared (NIR) long persistent luminescent (LPL) materials have attracted the interest of many researchers as they have potential applications in many aspects. However, majority of studies on Cr3+ ion-doped LPL materials have focused on Cr3+ in an octahedral site, and the luminescence is limited to the short-wavelength NIR-I region (700–900 nm), which is detrimental to fully explore Cr3+ ion-doped LPL materials with potential applications. In this work, a novel ultra-broadband NIR LPL material, Na2CaGe6O14 (NCGO):x%Cr3+, was successfully designed and synthesized, covering the luminescence range of 600–1200 nm and having the best afterglow duration of more than 10 h. Combining the luminescence lifetime with the low-temperature spectrum, it was concluded that the luminescence of NCGO:Cr3+ consists of the co-emission of Cr3+ in octahedra and tetrahedra. And it was confirmed by electron paramagnetic resonance (EPR) spectrum and X-ray absorption near-edge spectrum (XANES). The application prospects of NCGO:x%Cr3+ in many aspects were investigated in detail. This work could not only give a reference for researchers to study Cr3+ luminescence in multiple coordination but also provide a new strategy for obtaining new ultra-broadband NIR LPL materials.

Simultaneous Luminescence Color Tuning and Spectral Broadening of Cr3+ Near-Infrared Emission for Night Vision Application

“…It can be seen that the energy of 2 E g → 4 A 2g transition of the strong field is marginally affected by variations in the crystal field. , However, in a weak crystal field, the 4 T 2g → 4 A 2g transition occurs as a broad emission (peak1) and is more easily influenced by the surrounding crystal field environment, which causes the emission adjustment. As shown in previous reports, the emission of Cr 3+ in a tetrahedral crystal field also behaves as a wideband emission (peak2) in the NIR region. , The crystal field strength D q and Racah parameter B can be obtained via the following equations: where E ( 4 A 2g → 4 T 2g ) is the energy of 4 A 2g → 4 T 2g transition, and E ( 4 A 2g → 4 T 1g ) is the energy gap between 4 A 2g and 4 T 1g . Based on the PLE and PL spectrum of the AGO:0.7% Cr 3+ phosphor, the values of D q and B are calculated to 1636.7 and 818.4 cm –1 , respectively.…”

“…As shown in previous reports, the emission of Cr 3+ in a tetrahedral crystal field also behaves as a wideband emission (peak2) in the NIR region. 10,37 The crystal field strength D q and Racah parameter B can be obtained via the following equations: (4)…”

Cr3+-Doped NIR Phosphor with Well-Defined Stokes/Anti-Stokes Phonon Sidebands for a Ratiometric Thermometer and Thermally Stable NIR LEDs

“…There are plenty of NIR luminescent materials, including organic dyes, quantum dots, and ion-activated phosphors. − Among them, the last one offers an unparalleled advantage in long-term stability. Cr 3+ has emerged as a star activator for its efficient and adjustable NIR emission and its excitation band matching blue LED chips. , With a d 3 electronic configuration, Cr 3+ needs to fit in octahedral crystal fields to achieve NIR emission from the 2 E g / 4 T 2g → 4 A 2g transition. , Therefore, those hosts with substitutable octahedral sites can construct NIR phosphors through doping Cr 3+ , such as LiIn 2 SbO 6 :Cr 3+ , MgSi 2 O 6 :Cr 3+ , and LiGa 5 O 8 :Cr 3+ . − Although many Cr 3+ -doped phosphors have been realized, the luminescent properties of Cr 3+ are not entirely satisfying. In particular, thermal stability, which characterizes the emission at elevated temperature, is crucial for the applications in NIR LEDs and optical thermometry, because it affects not only the emission profile but also the signal intensity. − Mn 4+ -doped phosphors with the same electronic configuration as Cr 3+ often have an anti-Stokes phonon sideband to improve their thermal stability .…”

Excitation Strategy of Infrared Persistent Phosphors via Upconversion Charging and Persistent Energy Transfer

“…Infrared persistent luminescence phenomenon has become especially attractive recently owing to its self-sustained emission at ambient temperature as well as its infrared emission wavelength. − Among the persistent phosphor systems reported, Cr 3+ -activated infrared phosphors have aroused considerable interest from their broad prospects for biomedical imaging, in vivo cell tracking, information storage, sensors, and anticounterfeiting. − Also, development of infrared persistent phosphors activated by Pr 3+ , Nd 3+ , Yb 3+ , or Ni 2+ has become a growing concern. − In most reports, attention is paid to optimize the emission performance of these infrared persistent phosphors. In contrast, research on excitation for the charging infrared persistent phosphors is relatively lacking.…”