Tracing the origin of near-infrared emissions emanating from manganese (II)

This research entailed the co-doping of 2% Ce³⁺ and 20% Mn²⁺ into the classic garnet structure Y₃Al₅O₁₂ (YAG). Figure 1 illustrates that Ce³⁺ and Mn²⁺ are seamlessly integrated into the YAG crystal structure without causing any structural deformation. To determine the valence state of Mn, the team utilized analytical methods including X-ray Absorption Near Edge Structure (XANES), X-ray Photoelectron Spectroscopy (XPS), and low-temperature (77 K) Electron Paramagnetic Resonance (EPR). These assessments affirmed that Mn within the YAG structure exists in its divalent (+2) state. Notably, in the excitation spectrum of YAG:20% Mn²⁺, as depicted in Figure 1b, monitoring the NIR emission at approximately 750 nm uncovers a distinctive narrowband absorption peak at roughly 400 nm. This peak originates from the transition of Mn²⁺ from the ⁶A₁⁶(S) to the [⁴A₁(⁴G), ⁴E(⁴G)] states. The fluorescence lifetime of the red emission at 600 nm in YAG:2%Ce³⁺,20%Mn²⁺ stands at merely 0.69 ms, whereas the NIR emission, approximately at 750 nm, boasts a fluorescence lifetime of up to 13.4 ms. This suggests that the NIR emission of Mn²⁺ is probably attributed to its occupation of high-symmetry sites within the YAG lattice, whereas the red emission stems from Mn²⁺ occupying distorted lattice sites.

 

Guided by the above assumption, as shown in Figure 2, the team utilized Density Functional Theory (DFT) to predict the energy level positions of both the ground and excited states of Mn²⁺ ions occupying distorted dodecahedral and octahedral sites. Meanwhile, based on Extended X-ray Absorption Fine Structure (EXAFS) fitting performed on YAG:2%Ce³⁺,20%Mn²⁺, it was determined that the NIR emission of YAG:2%Ce³⁺,20%Mn²⁺ stems from Mn²⁺ occupation of octahedral sites, whereas the red emission arises from its occupation of dodecahedral sites.

 

This study fabricated a phosphor-converted LED (pc-LED) by combining a blue LED chip with the NIR-emitting phosphor YAG:2%Ce³⁺,20%Mn²⁺, which converts blue light into NIR emission. As depicted in Figure 3, the emission intensity of the pc-LED device exhibits a gradual increase with the augmentation of the drive current. Additionally, the NIR output power and photoelectric efficiency of the pc-LED device were measured across various current levels.

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