Zero-Disturbance Sensitization Strategy for Ultra-High- Efficiency Cr3+ Near-Infrared Phosphors

Technical Summary: Zero-Disturbance Sensitization Strategy for Ultra-High-Efficiency Cr³⁺ Near-Infrared Phosphors

Problem Statement
Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are critical for applications in intelligent sensing, biological imaging, and night vision. While Cr³⁺-activated garnet-type materials are promising candidates due to their tunable emission and compatibility with blue LEDs, they face significant bottlenecks. These include insufficient excitation energy utilization, severe non-radiative relaxation at high temperatures, and relatively low internal quantum efficiency (IQE). Furthermore, traditional strategies to enhance performance, such as rare-earth ion co-doping, often introduce negative side effects. These include emission peak shifts, spectral band changes, fluorescence lifetime attenuation, and lattice distortion, which compromise the intrinsic luminescence characteristics and thermal stability of the Cr³⁺ emitters. The challenge lies in developing a sensitization strategy that significantly boosts efficiency without disturbing the original luminescence properties.

Methodology
The authors synthesized a series of Cr³⁺ single-doped and Cr³⁺/Tb³⁺ co-doped Ca₃Gd₂Ga₂Ge₂O₁₂ (CGGGO) phosphors using a high-temperature solid-state reaction method. The study employed a "zero-disturbance sensitization" strategy, introducing Tb³⁺ ions as an efficient energy transfer mediator within the rigid garnet host lattice.

Key experimental and analytical steps included:

• Structural Characterization: X-ray diffraction (XRD) with Rietveld refinement was used to verify phase purity and crystal structure. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analyzed morphology and elemental distribution. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) confirmed oxidation states and local coordination environments.
• Optical Characterization: Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured under 260 nm (UV) and 450 nm (blue) excitation. Fluorescence decay curves were analyzed to determine lifetimes. Internal quantum efficiency (IQE) was measured using an integrating sphere.
• Thermal and Theoretical Analysis: Temperature-dependent luminescence (80–500 K) assessed thermal stability. Crystal field parameters were calculated to understand the local environment of Cr³⁺. Density functional theory (DFT) calculations provided electronic band structure insights.
• Device Fabrication: A NIR pc-LED was fabricated by coating the optimal phosphor onto a commercial 450 nm InGaN LED chip to evaluate electroluminescence (EL), output power, and night-vision imaging capabilities.

Key Contributions and Results

1. Zero-Disturbance Sensitization: The introduction of Tb³⁺ into CGGGO:Cr³⁺ resulted in a significant enhancement of luminescence intensity without altering the emission peak position (~795 nm), spectral shape, or fluorescence lifetime. Specifically, emission intensity increased by 28% under 260 nm excitation and 14% under 450 nm excitation.
2. Ultra-High Efficiency: The co-doped sample achieved an internal quantum efficiency (IQE) of 97%, a substantial improvement over the 86.5% observed in the single-doped sample. The single-doped sample itself exhibited a high IQE of 86.5%.
3. Dual-Pathway Energy Transfer Mechanism: The study elucidated a mechanism where Tb³⁺ acts as an energy capture and transfer medium. Under 260 nm excitation, electrons are pumped to Tb³⁺ ground sublevels, thermally activated to intermediate levels, and subsequently transferred to Cr³⁺ via the conduction band. This dual-pathway mechanism enhances energy utilization efficiency without creating new quenching centers.
4. Structural Integrity: Rietveld refinement and XRD confirmed that Tb³⁺ incorporation did not induce lattice distortion or phase impurities. The ionic radius tolerance calculations indicated that Cr³⁺ preferentially occupies octahedral [GaO₆] and tetrahedral [Ga/GeO₄] sites, while Tb³⁺ co-doping maintains this structural stability.
5. Thermal Stability and Device Performance: The co-doped phosphor retained 72.6% of its room-temperature emission intensity at 400 K. The fabricated NIR pc-LED demonstrated stable electroluminescence, a maximum output power of 26 mW at 320 mA, and successful night-vision imaging performance, capturing concealed objects in dark environments.

Significance
The paper claims that this work provides a universal and reliable "zero-disturbance sensitization strategy" for designing efficient luminescent materials. By demonstrating that Tb³⁺ can significantly boost the performance of Cr³⁺-based NIR phosphors without degrading their intrinsic optical properties or thermal stability, the study offers a new approach to overcoming the limitations of traditional co-doping methods. The authors assert that this strategy not only yields a high-performance NIR phosphor suitable for advanced optoelectronic applications but also provides theoretical guidance for the design of future high-efficiency luminescent materials. The successful fabrication of a stable, high-output NIR pc-LED device validates the practical potential of this approach for real-world applications in night vision and security systems.