Thermal Stability Improvement of Cr³⁺‐Activated Broadband Near‐Infrared Phosphors via State Population Optimization

Abstract

Technological progress has accelerated the researches into broadband near-infrared (NIR) luminescent materials as next-generation intelligent NIR light sources; however, poor thermal stability restricts the applications of NIR phosphors. Herein, new insights into Cr 3+ -activated NIR phosphors with improved thermal stability are provided. The photoluminescence intensity originating from the 4T2→4A2 broadband emission of CaLu 2 Al 4 SiO 12 :Cr 3+ (CLAS:Cr) with optimal electron occupation ( 4 T 2 / 2 E) increases up to 118% at 475 K compared with that at room temperature, which is the best for Cr 3+ -activated broadband NIR phosphor. Accordingly, new ideas for the design of thermally stable phosphors are proposed by optimizing the thermal population between 4 T 2 and 2 E states and electron migration from 2 E to 4 T 2 state. Finally, multifunctional applications of CLAS:Cr in NIR fluorescence intensity ratio thermometry, X-ray detection, NIR phosphor-converted light-emitting diodes, and bio-imaging are demonstrated.

The broadband emission of Cr3+-doped CaY2Mg2Ge3O12 and its applications for NIR detectors

“…According to the equations, the Dq / B of Cr1 is 2.11 and that of Cr2 is 2.04, demonstrating the weak crystal fields around both Cr 3+ sites ( Dq / B < 2.3), and then producing broadband emission from the 4 T 2g → 4 A 2g transition. 35,36 Generally speaking, the crystal field strength is tetrahedron > octahedron > dodecahedron. 37 Therefore, the crystal field of the Cr1 site stronger than that of Cr2 means that Cr1 comes from the octahedral site [MgO 6 ] and Cr2 locates at the dodecahedral site [Ca/YO 8 ] (Tanabe–Sugano diagram in Fig.…”

Highly Quantum Efficient and Thermally Stable Near‐Infrared‐Emitting K‐β‐Al2O3:Cr3+ Phosphor

“…[37,38] With the doping of Ga 3+ , the crystal field strength decrease and the energy difference between 2 E and 4 T 2 becomes larger, which weakens process 1, therefore, process 3 with a shorter lifetime gradually becomes dominant, and the average lifetime decreased. [39][40][41] The non-radiative transition may also contribute to the lifetime decreases decrease, as shown in Figure 4, with the doping of Ga 3+ , the thermal stability decreased, which is closely related to the increased non-radiative transition. [16] The thermal stability variation will be discussed next part.…”

Broadband Near-Infrared Luminescence from Mo4+ in Zero-Dimensional Perovskite Cs2Zr(Cl,Br)6 with an Exceptionally High Quantum Efficiency and Thermal Stability

“…The widely adopted strategy to produce broadband NIR luminescence is to select NIR emission centers with large Stokes shifts, which are intrinsically related to the different crystal-field (CF) interactions with the crystalline environment of the excited and ground states. For example, the NIR emission of the octahedrally coordinated Cr 3+ (3d 3 ) ion is due to the 4 T 2 → 4 A 2 transition. − The 4 T 2 and 4 A 2 states come from the t 2 2 e 1 and t 2 3 CF configurations, respectively, in which the interactions of the 3d electrons with the crystalline environment are quite different, resulting in distinct equilibrium geometries and hence strongly Stokes-shifted broadband emissions. − Likewise, the broadband NIR emission of Sb 3+ /Bi 3+ -containing materials originates from the 3 P 1 → 1 S 0 transition of Sb 3+ /Bi 3+ ; the two states derive from s 1 p 1 and s 2 electron configurations, respectively, interacting differently with the CF environment. , Although the successful realization of the Stokes-shifted broadband NIR luminescence has been reported in numerous materials, there are still limitations to their practical applications regarding their low quantum efficiency and poor thermal stability. − This is because the large equilibrium geometry difference between the two states facilitates nonradiative relaxation through a thermally activated crossover from the excited to the ground state. On the other hand, if the excited and ground states derive from the same electron or CF configuration having similar interactions with the crystalline environment, nonradiative relaxation through thermal crossover would be restricted, which would improve their luminescence quantum efficiency and suppress thermal quenching, but simultaneously the emission band would be narrow.…”