Mechanoluminescence (ML) is a type of luminescence that occurs when a material emits light in response to an external mechanical stimulus, such as scratching, pressing, or stretching. Unlike electroluminescence (EL) or photoluminescence (PL), ML does not require an electric energy or light excitation; instead, it directly converts mechanical energy into optical emission. Then, devices fabricated from ML materials are inherently passive. This characteristic represents an intelligent and energy-efficient light-emission mode. However, most current ML research mainly focuses on the study of single-matrix materials, whose stress-induced luminescence often lacks consistent brightness, with many showing limited ML performance. Enhancing ML intensity remains a critical technical challenge for practical applications. Furthermore, compared to the mature fields of PL and EL, ML heterojunction studies lack systematic investigations into down-conversion mechanisms and have not yet been applied to ML material design and fabrication. Therefore, exploring down-conversion in ML research is deemed necessary.
The Solution: Peng et al. developed high-performance ML heterojunctions by fabricating CaF2/CaZnOS heterojunctions and validating their efficacy through Tb3+ doping, achieving ML intensities ~2-fold that of the CaZnOS:0.5% Tb3+ reference sample. The incorporation of CaF₂ not only stabilized the heterojunction structure but also improved the optical transparency of polydimethylsiloxane (PDMS) films made from the composite. Notably, while co-doping Tb3+-Yb3+ reduced visible emission intensity compared to singly doped Tb3+ samples, near-infrared (NIR) emission intensity exceeded that of singly doped Yb3+ samples. The luminescence lifetime decreased progressively with increasing Yb3+ concentration, consistent with cooperative energy transfer-mediated Tb3+ luminescence quenching during down-conversion. The optimal Tb3+-Yb3+ energy transfer efficiency ( ηET ) was quantified as 66.9%, corresponding to a quantum efficiency ( ηQE ) of 166.9%. Extending this approach to the Pr3+-Yb3+ ion pair yielded comparable down-conversion ML performance, with an of 85.1% and ηQE of 185.1%. The observed cooperative energy transfer mechanism in lanthanide-doped heterojunctions provides a versatile platform for advancing down-conversion ML research across diverse material systems.
The Future: Future research will integrate artificial intelligence in designing advanced smart ML structures, combining semiconductor physics, materials science, chemistry, and information technology. High-efficiency ML structures can be created through techniques such as atomic layer deposition, chemical vapor deposition, and soft chemical methods for synthesizing epitaxial single-crystal heterojunctions. Additionally, one-dimensional whisker/fiber heterojunctions can enable both ML and mechanical reinforcement functions. These innovations will result in ML materials with broader pressure ranges, enhanced sensitivity, and faster response times, as well as efficient light energy conversion through up-conversion and down-conversion processes. In the future, ML materials will play a crucial role in industries like smart manufacturing, structural health monitoring, and healthcare. Once standardized, ML technology will provide unique advantages across diverse fields, driving technological innovation and progress.
The Impact: The development of the CaF2/CaZnOS heterojunction system not only introduces a novel heterojunction ML material system for enhancing ML, but also presents a new avenue for future research on downconversion ML. This advancement holds significant promise for the exploration of additional downconversion-related applications in the passive form of ML.
The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.
Reference: Tianlong Liang, Yuantian Zheng, Qi'an Zhang, Ziyi Fang, Mingzhi Wu, Yang Liu, Qidong Ma, Jiazhen Zhou, Maryam Zulfiqar, Biyun Ren, Yanze Wang, Jingnan Zhang, Xiaoyu Weng, Dengfeng Peng. Downconversion mechanoluminescence from lanthanide codoped heterojunctions[J]. Materials Futures, 2025, 4(2): 025701. DOI: 10.1088/2752-5724/add7f3
