📄 chemistry

Color tunability and temperature sensing in transparent Dy3+/Eu3+ co-doped KGd2F7 oxyfluoride glass ceramics

Transparent Dy³⁺/Eu³⁺ co-doped KGd₂F₇ oxyfluoride glass-ceramics were successfully developed to achieve tunable yellow-to-orange-red emission and high-precision optical thermometry through efficient quadrupole–quadrupole energy transfer, making them promising candidates for multicolor displays and temperature sensing applications.

Original authors: Zhushen Jiang, Huimin Ding, Youjun Cai, Kaikai Ren, Xin Wang, Yuepin Zhang

Published 2026-08-21

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CC BY 4.0

Original authors: Zhushen Jiang, Huimin Ding, Youjun Cai, Kaikai Ren, Xin Wang, Yuepin Zhang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Lighting and displays have long relied on materials that glow when struck by energy, but finding a single substance that can change its color on demand while also acting as a precise thermometer has been a significant challenge for scientists. The key to this challenge lies in rare-earth elements, a group of metals known for their ability to emit very specific colors of light when excited. When these elements are trapped inside a solid material, they can transfer energy between one another, allowing researchers to mix colors much like a painter blends pigments. However, the environment surrounding these atoms is crucial; if the material is too rigid or disordered, the light is lost as heat. To solve this, scientists often turn to glass-ceramics, a hybrid material that combines the transparency of glass with the ordered atomic structure of crystals. By carefully designing these materials, researchers can create a host that protects the glowing atoms, allowing them to shine brightly and interact efficiently, opening the door to devices that can do more than just illuminate a room.

In a recent study, a team of researchers from Ningbo University in China created a new type of transparent glass-ceramic designed to do exactly this. They started with a mixture of common glass ingredients, including silica and boron, and added specific amounts of dysprosium and europium, two rare-earth elements. This mixture was melted at a very high temperature and then cooled rapidly to form a clear glass. To transform this glass into a glass-ceramic, the team heated it again at a lower temperature for several hours. This second heating step caused tiny crystals to form inside the glass without making it cloudy. Using powerful microscopes, the researchers confirmed that these crystals, made of a compound called KGd2F7, were perfectly round and incredibly small, measuring about 15.7 nanometers across. Because they are so much smaller than the wavelength of visible light, they do not scatter the light, leaving the final material as clear as a windowpane.

The true magic of this material lies in how the two rare-earth elements work together. When the researchers shined a near-ultraviolet light onto the sample, the dysprosium ions absorbed the energy and began to glow, primarily emitting yellow and blue light. However, the team observed that the dysprosium did not keep all that energy for itself. Instead, it passed a significant portion of it to the nearby europium ions. This transfer of energy caused the europium to glow intensely in a deep orange-red color. By simply changing the amount of europium added to the mix, the researchers could smoothly shift the color of the light emitted by the material. With very little europium, the glass glowed a warm yellow. As they increased the amount of europium, the color shifted through orange and settled into a rich, pure red. This ability to tune the color continuously makes the material a strong candidate for creating custom lighting or high-quality displays where specific colors are needed.

Beyond just changing colors, the researchers discovered that this material could also measure temperature with remarkable precision. They found that the two colors emitted by the material—yellow from the dysprosium and red from the europium—reacted differently to heat. As the temperature rose, the yellow light faded much faster than the red light. By comparing the brightness of these two colors, the material could act as a built-in thermometer. The team tested this property across a wide range of temperatures, from 25°C up to 275°C. They found that the material was highly sensitive, capable of detecting tiny changes in heat. In fact, at room temperature, the system could distinguish temperature differences as small as 0.021 degrees Celsius. This level of precision suggests that the material could be used in situations where accurate, non-contact temperature readings are vital, such as in biological monitoring or environmental sensing.

The study also looked closely at how the energy moved from one atom to another to ensure the process was efficient. By measuring how long the light lasted after the initial energy pulse, the team calculated that the energy transfer between the two elements was highly effective, reaching an efficiency of over 55 percent. They determined that this transfer happened through a specific type of interaction between the electrical fields of the atoms, rather than through simple collisions or other mechanisms. This finding confirms that the crystal structure they created provides an ideal environment for these rare-earth ions to communicate. The results indicate that this transparent glass-ceramic is not just a laboratory curiosity but a practical material that combines the ability to produce tunable, high-quality light with the function of a high-precision sensor, offering a versatile solution for future optical technologies.

Technical Summary: Color Tunability and Temperature Sensing in Transparent Dy³⁺/Eu³⁺ Co-doped KGd₂F₇ Oxyfluoride Glass Ceramics

Problem Statement
The advancement of optoelectronic technologies, including solid-state lighting, flat-panel displays, and optical thermometry, necessitates high-performance luminescent materials that offer high efficiency, precise color tunability, and reliable temperature sensitivity. While inorganic glasses and glass-ceramics (GCs) are promising hosts due to their stability and compositional flexibility, optimizing the microstructure and energy transfer mechanisms to achieve simultaneous multicolor emission and high-precision thermometry remains a challenge. Specifically, there is a need for host matrices that combine the low phonon energy of fluorides with the structural stability of oxides to suppress non-radiative relaxation while enabling efficient energy transfer between rare-earth ions for color tuning and ratiometric sensing.

Methodology
The study employed a conventional melt-quenching technique followed by heat treatment to synthesize transparent oxyfluoride glass-ceramics (GCs) containing KGd₂F₇ nanocrystals. The precursor glass composition was defined as 65SiO₂-12H₃BO₃-15K₂CO₃-(10-y)GdF₃-10KF-xDy₂O₃-yEuF₃.

Synthesis: Raw materials were melted at 1570 °C, quenched to form precursor glass (PG), and subsequently annealed at 450 °C for 4 hours to induce the precipitation of nanocrystals.
Structural Characterization: X-ray diffraction (XRD) and transmission electron microscopy (TEM) were utilized to confirm phase formation, crystal structure, and nanocrystal size distribution.
Optical Characterization: Steady-state photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded to analyze emission properties and energy transfer mechanisms. Fluorescence lifetime measurements were conducted to quantify energy transfer efficiency.
Thermometric Analysis: Temperature-dependent emission spectra were recorded over a range of 298–548 K. The fluorescence intensity ratio (FIR) between Dy³⁺ and Eu³⁺ emissions was analyzed to determine absolute sensitivity ( $S_a$ ), relative sensitivity ( $S_r$ ), and temperature resolution ( $\delta T$ ).

Key Contributions and Results

Structural Confirmation: XRD and TEM analyses confirmed the successful formation of well-crystallized monoclinic KGd₂F₇ nanocrystals (space group I2/m) uniformly dispersed within the amorphous glass matrix. The average crystallite size was determined to be approximately 15.7 nm, a dimension small enough to minimize light scattering and ensure high optical transparency. The substitution of Gd³⁺ by Dy³⁺ and Eu³⁺ ions was confirmed to cause minor local lattice distortions without disrupting the overall crystal framework.
Color Tunability: By varying the Eu³⁺ concentration (0 to 1.2 mol%) while keeping Dy³⁺ fixed at 0.4 mol%, the emission color was continuously tuned from yellow to orange-red. This tunability resulted from the efficient energy transfer (ET) from Dy³⁺ (sensitizer) to Eu³⁺ (activator). As Eu³⁺ concentration increased, the yellow emission of Dy³⁺ (580 nm) decreased, while the red emission of Eu³⁺ (616 nm) increased, reaching a maximum at 1.2 mol% without concentration quenching.
Energy Transfer Mechanism: The ET mechanism was identified as a quadrupole–quadrupole interaction, verified by Dexter's theory and linear fitting of lifetime data. The energy transfer efficiency ( $\eta_{ET}$ ) increased with Eu³⁺ concentration, reaching a maximum of 55.6% at 1.2 mol% Eu³⁺. The decrease in the average fluorescence lifetime of Dy³⁺ (from 320.57 ns to 104.51 ns) with increasing Eu³⁺ content provided quantitative evidence of this transfer.
Optical Thermometry Performance: The sample co-doped with 0.4 mol% Dy³⁺ and 1.2 mol% Eu³⁺ exhibited distinct thermal quenching behaviors for the two ions, enabling ratiometric temperature sensing.
- Sensitivity: The system demonstrated a maximum absolute sensitivity ( $S_a$ ) of 0.372 % K⁻¹ and a maximum relative sensitivity ( $S_r$ ) of 0.388 % K⁻¹ at 298 K.
- Resolution: The temperature resolution ( $\delta T$ ) was calculated to be as low as 0.021 K at room temperature, indicating high precision.
- Stability: The thermometric performance remained stable across the 298–548 K range, with no abrupt fluctuations in resolution.

Significance
The paper posits that the Dy³⁺/Eu³⁺ co-doped KGd₂F₇ oxyfluoride glass-ceramics represent a promising candidate for dual-function applications. The material's ability to achieve continuous color tuning from yellow to orange-red makes it suitable for multicolor displays and customized lighting. Furthermore, its high sensitivity and low temperature resolution suggest significant potential for high-precision optical thermometry in fields such as biological sensing and environmental monitoring. The study provides a valuable guide for designing self-crystallized oxyfluoride GCs that leverage efficient rare-earth ion energy transfer for both photonic and sensing applications.

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