Intervalence charge transfer of Cr³⁺

Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

image: a, PL spectra of LaMgGa₁₁-xO₁₉:xCr³⁺ (x=0-2) under 440 nm excitation. b, Luminescence intensities of NIR-Ⅰ and NIR-Ⅱ versus Cr³⁺ concentration; c, PLE spectra of LaMgGa₁₁-xO₁₉:0.7Cr³⁺ monitoring at 890 and 1200 nm. The excitation signals result from the transitions of isolated Cr³⁺ ion. d, Cryogenic (80 K) UV-Vis-NIR diffuse reflectance curve validating that no absorption of Cr⁴⁺ ion can be traced. e, XPS curves of Cr₂O₃, LaMgGa₁₁-xO₁₉:0.2Cr³⁺, and LaMgGa₁₁-xO₁₉:0.7Cr³⁺ samples validating no chemical shift. f, EPR curves of LaMgGa₁₁-xO₁₉:0.2Cr³⁺ and LaMgGa₁₁-xO₁₉:0.7Cr³⁺ samples. In (f), the broad resonance signal with g of 1.96 is attributed to the Cr³⁺-Cr³⁺ pair, indicating strong interaction between Cr³⁺ ions. view more

Credit: by Shengqiang Liu, Jingxuan Du, Zhen Song, Chonggeng Ma, Quanlin Liu

The near-infrared (NIR) spectrum contains characteristic vibrational absorption bands of numerous organic functional groups. NIR phosphor-converted light-emitting diodes (pc-LEDs) have gathered increasing interests in fields including non-destructive testing and night vision. In 2016, Osram reported the first NIR pc-LED, SFH4735, while with low output power (16 mW @ 350 mA) and limited wavelengths. Furthermore, luminescent contrast agents operating within the second biological imaging window (1000-1800 nm) exhibit lower tissue absorption and scattering coefficients in contrast to the traditional first window (750-950 nm), thereby enabling enhanced detection depth and improved imaging signal-to-noise ratio. Significantly, the luminescence of Cr3+ via engineering the crystal field environment is located in the NIR-Ⅰ region, as illustrated by the Tanabe-Sugano diagram. The presence of Cr4+ ([Ar]3d2) is capable of extending the emission to the NIR-Ⅱ region, but the efficiency is subpar due to poor luminescence thermal quenching at room temperature. In contrast, phosphors doped with lanthanide ions typically exhibit narrow-band multiplets emission, making spectral tuning a challenging task. Hence, it becomes crucial to investigate methods for achieving broadband NIR-Ⅱ luminescence through ion doping and structural composition.

In a new paper published in Light Science & Application, a team of researchers, led by Professor Quanlin Liu from School of Materials Sciences and Engineering, University of Science and Technology Beijing, China, and co-workers have developed the first-ever NIR-Ⅱ broadband luminescence based on intervalence charge transfer (IVCT) of Cr3+-Cr3+ → Cr2+,Cr4+ in magentoplumbite-type LaMgGa11O19. Based on heavily incorporation of Cr3+ ion, LaMgGa11O19 exhibits dual-emission (NIR-Ⅰ, 890 nm and NIR-Ⅱ, 1200 nm) with a full width at half maximum (FWHM) of 626 nm and luminescence external efficiency of 18.9%. They further observed the luminescence anti-thermal quenching behavior (432% @ 290 K vs @80 K) of target NIR-Ⅱ luminscence.

They observed the NIR-Ⅰ luminescence at low concentration of Cr3+ ions, whereas the NIR-Ⅱ luminescence appears as the concentration of Cr3+ ions increases to 0.5. With a high doping concentration of Cr3+ ions, the excitation and absorption signals of Cr4+ ions cannot be traced. Additionally, in contrast to the Cr4+ ions, they discovered significantly longer luminescence decay lifetime (2.3 ms) associated with this anomalous NIR-Ⅱ luminescence. The potential application of LaMgGa11O19:Cr3+ phosphor as a light-emitting converter in non-destructive analysis, tissue penetration, and long-distance night vision is demonstrated via fabricating a NIR pc-LED.

Light Science & Applications

10.1038/s41377-023-01219-x

25-Jul-2023

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