When scientists make crystals convert invisible ultraviolet radiation into useful near-infrared light, it's not magic but the cutting edge of modern photonics.
Luminescence is the ability of a material to emit light after absorbing energy. In the case of rare earth ions such as europium (Eu³⁺) and ytterbium (Yb³⁺), this glow arises from transitions of electrons within their 4f orbitals. These transitions between energy levels create the very glow that scientists strive to control and enhance2 .
Yttrium fluoride (YF₃) serves as an ideal "host lattice" for rare earth ions. Its crystalline structure readily accepts europium and ytterbium ions, and low phonon energy (crystal lattice vibrations) minimizes energy loss as heat, allowing for high luminescence efficiency3 .
The key process that researchers study is down-conversion (conversion down), where one high-energy photon (e.g., ultraviolet) is converted into one or more low-energy photons (e.g., infrared). This mechanism is particularly important for improving the efficiency of silicon solar cells, which most efficiently absorb radiation in the near-infrared range4 .
Eu³⁺ and Yb³⁺ ions enable efficient energy transfer through their unique electronic configurations.
YF₃ provides a stable matrix with low phonon energy, minimizing non-radiative energy losses.
The central achievement in this field was an experiment in which scientists successfully synthesized single-phase solid solutions Y₁₋ₓ₋ᵧEuₓYbᵧF₃ and determined the optimal ratios of europium and ytterbium ions to achieve maximum quantum yield of down-conversion luminescence4 .
Successfully created homogeneous solid solutions with controlled composition.
Determined ideal Eu³⁺:Yb³⁺ ratios for maximum efficiency.
Achieved 2.2% quantum yield for down-conversion luminescence.
Researchers used the high-temperature fusion method to obtain single-phase samples. The crystals had orthorhombic symmetry and corresponded to the β-YF₃ structure. The chemical composition of the finished samples was confirmed by energy-dispersive analysis and matched the nominal composition4 .
To measure luminescence, excitation at two different wavelengths — 266 nm and 296 nm — was used. This allowed investigation of different energy transfer channels within the crystal and identification of luminescence from both Eu³⁺ and Yb³⁺ ions4 .
The experiment revealed several important patterns. When excited at a wavelength of 266 nm, researchers recorded not only the expected luminescence of Eu³⁺ and Yb³⁺ ions but also luminescence of Eu²⁺ ions, indicating complex energy transfer processes in the system4 .
The most significant discovery was the determination of optimal activator ion ratios. The maximum quantum yield of ytterbium down-conversion luminescence in the near-infrared range was 2.2%. This indicator was achieved for compositions with Eu³⁺:Yb³⁺ ratios of 0.1:10.0 and 0.05:5.004 .
| Eu³⁺:Yb³⁺ Ratio | Excitation Wavelength | Quantum Yield |
|---|---|---|
| 0.1:10.0 | 266 nm | 2.2% |
| 0.05:5.00 | 266 nm | 2.2% |
| Other ratios | 266 nm | <2.2% |
| Excitation Wavelength | Observed Luminescence |
|---|---|
| 266 nm | Eu³⁺, Yb³⁺, Eu²⁺ |
| 296 nm | Eu³⁺, Yb³⁺ |
| Matrix | Phonon Energy | Luminescence Efficiency | Stability |
|---|---|---|---|
| YF₃ | Low | High | High |
| Oxide systems | High | Medium | Very high |
| NaYF₄ | Low | Very high | Phase transitions3 |
The obtained results demonstrate that it is precisely the ratio of activator ions, not just their presence in the crystal lattice, that determines the efficiency of the down-conversion process. This discovery has fundamental significance for the development of highly efficient phosphors for photonic applications.
Research in the field of luminescence of solid solutions based on yttrium fluoride opens the way to creating new generations of optoelectronic devices. Improving the efficiency of solar cells is just one of many potential applications. These materials can be used in biomedical imaging, radiation sensors, lasers, and display devices2 3 .
Enhancing efficiency by converting UV to IR light that silicon cells can utilize more effectively.
Enabling new contrast agents and imaging techniques with controlled luminescence properties.
Creating compact luminescent sensors for electromagnetic radiation with high sensitivity.
The study of luminescence of solid solutions based on yttrium fluoride doped with ytterbium and europium is a perfect example of how fundamental research into physicochemical processes in solids leads to the development of materials with unique properties. Subtle control of energy transfer processes at the atomic level opens up opportunities for creating technologies that recently seemed like science fiction. Light transformation continues to inspire scientists to search for new solutions in photonics and optoelectronics.