Improving luminescence efficiency and thermal stability of NIR-emitting phosphors

Near-infrared (NIR)-emitting phosphor-converted light-emitting diodes (pc-LEDs) have attracted attention in emerging technology fields such as night-vision and bio-imaging. Currently, the development of NIR-emitting pc-LEDs has encountered a bottleneck due to the lack of blue-light excitable high-performance NIR-emitting phosphor materials.

Although Cr³⁺-activated phosphors stand out among numerous NIR-emitting phosphors and has recently made progress in realizing tunable broadband emission, major issues still remain their unsatisfactory luminescence efficiency and poor thermal stability.

A₃B₂C₃O₁₂-typed garnets are considered as promising host materials that can address these issues, because their compact coordinated environment and tunable structures may provide diverse luminescence properties including the required ones.

Unfortunately, there is a trade-off between the emission wavelength and efficiency as well as thermal stability. That is, the highly efficient and thermally stable NIR luminescence generally accompanied with short emission wavelength (< 750 nm). Challenge still lies in improving luminescence efficiency and thermal stability while maintaining original longer wavelength for Cr³⁺-doped NIR-emitting garnet phosphors.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Jun Lin from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Professor Guogang Li from China University of Geosciences have reported a "kill two birds with one stone" strategy to simultaneously improve luminescence efficiency and thermal stability of the NIR-emitting Ca₃Y_2-2x(ZnZr)_xGe₃O₁₂:Cr garnet system by chemical unit co-substitution, accompanied with only slight emission shift.

The developed NIR-emitting phosphors show potential applications in information encryption, bio-tissue imaging, and night-vision. This work provides a new insight for developing high-performance NIR-emitting phosphor materials.

To achieve high luminescence efficiency and thermal stability for Cr³⁺-doped garnet phosphors, two dominant factors should be considered. One is luminescence "killer" Cr⁴⁺ that shows intensive absorption in NIR region. Another crucial factor is structural rigidity.

In this work, the authors chose Ca₃Y₂Ge₃O₁₂ with a typical garnet structure as an initial host for chromium doping. Through a cation co-substitution of [Zn²⁺–Zr⁴⁺] for [Y³⁺–Y³⁺], a series of Ca₃Y_2-2x(ZnZr)_xGe₃O₁₂:Cr NIR-emitting phosphors were synthesized using a traditional high-temperature solid-state method. The underlying luminescence optimization mechanism of this garnet system were investigated.

The structural analysis and density functional theory (DFT) calculations indicate that chromium ions are very likely to enter the Ge⁴⁺ sites of Ca₃Y₂Ge₃O₁₂ in the form of tetravalent Cr⁴⁺ ions. The coexistence of Cr³⁺ and Cr⁴⁺ is demonstrated to be responsible for the low quantum efficiency of Ca₃Y₂Ge₃O₁₂:Cr. The designed co-substitution of smaller [Zn²⁺–Zr⁴⁺] for [Y³⁺–Y³⁺] plays an expected role as reductant, which promotes the transformation from Cr⁴⁺ luminescence killers to beneficial Cr³⁺ emission centers.

This result is also demonstrated by the diffuse reflectance spectra and Cr K-edge X-ray absorption near-edge structure spectra. The valence reduction is related to the successful reconstruction of the octahedral sites for Cr³⁺ ions. Moreover, the introduction of [Zn²⁺–Zr⁴⁺] unit also contributes to a rigid crystal structure.

These two aspects together achieve the simultaneous high internal quantum efficiency of 96% and excellent thermal stability of 89% at 423 K, which almost surpasses all of the reported Cr³⁺-doped garnet phosphors within similar emission region (770–820 nm). This proves the feasibility of the designed co-substitution in optimizing luminescence properties for Cr³⁺-doped garnet phosphors.

Furthermore, benefiting from the reconstructed rigid covalent structure, the acid resistance of the phosphor is also greatly improved. Inspired by this, information encryption with "burning after reading" is achieved. Finally, the fabricated NIR-emitting pc-LED shows promising applications in bio-tissue imaging and night-vision.

This work provides a new perspective of luminescence optimization by chemical unit co-substitution, and the revealed universal mechanism could motivate further exploration of high-performance Cr³⁺-doped NIR-emitting phosphor materials.