【Domestic Papers】Boosting near-infrared mechanoluminescence from activated gallate solids by Ga₂O₃ heterojunction engineering for intelligent sensing

      Researchers from the China Jiliang University and Beijing University of Posts and Telecommunications have published a paper titled "Boosting near-infrared mechanoluminescence from activated gallate solids by Ga₂O₃ heterojunction engineering for intelligent sensing" in Applied Physics Reviews.

Background

      Mechanoluminescent (ML) materials can directly convert mechanical stimuli such as stretching, compression and friction into optical signals without external excitation sources, showing great application value in wireless sensing, anti-counterfeiting, bioimaging, artificial skin and other fields. Most commercial ML materials are concentrated in the visible spectrum (400–650 nm). When used in biological scenarios, they easily overlap with tissue autofluorescence and suffer from severe photon scattering in biological media, making it difficult to achieve deep-tissue imaging and high-resolution sensing. The near-infrared (NIR, 700–1700 nm) band has superior penetration depth and imaging contrast due to weak photon scattering and low tissue autofluorescence, making it an ideal band for bioimaging. As a NIR luminescent center, Cr³⁺ can realize broadband tunable emission in the NIR-I biological window through 3d electronic transitions. However, limited by the weak crystal field environment, Cr³⁺ exhibits broad and inefficient emission with low luminescence intensity. Current NIR ML materials generally suffer from low emission intensity, poor environmental stability and insufficient spectral coverage, which restrict their biomedical applications. Therefore, improving the NIR ML performance of Cr³⁺-based materials through material structure design has become the core research direction in this field.

Abstract

      Long-wavelength mechanoluminescent (ML) materials that convert mechanical stimuli into light hold promise for biomedical imaging and wireless sensing but are often restricted by low brightness and shallow tissue penetration. Here, a heterojunction engineering strategy is employed to enhance near-infrared ML emission in LiGa₅O₈/Ga₂O₃:Cr³⁺ composites, achieving a 416% intensity increase compared with the single-phase LiGa₅O₈. Incorporation of Ga₂O₃ introduces deeper electron traps and strengthens the crystal field around Cr³⁺ centers, thereby boosting emission efficiency in the 700–750 nm range. Density functional theory calculations reveal that orbital hybridization at the interface lowers recombination barriers and promotes radiative transitions. The optimized composite shows excellent aqueous stability over 240 h, while the flexible films fabricated with polydimethylsiloxane enable effective ML imaging through biological tissue, as verified using chicken joints. These findings demonstrate a promising pathway toward high-performance near-infrared ML materials for in vivo monitoring, biomedical imaging, wireless sensing, and intelligent sensing applications.

Highlights

      Ga₂O₃ heterojunction engineering is firstly adopted to modify LiGa₅O₈:Cr³⁺, and the NIR ML intensity is increased by 416% compared with the single phase;

      The synergistic enhancement mechanism of heterojunction interface regulating trap depth distribution, strengthening Cr³⁺ crystal field and reducing carrier recombination barrier is revealed;

      SiO₂-assisted stress transfer system is constructed to further improve luminescence intensity, mechanical and chemical stability;

      The flexible film realizes multifunctional applications in biological tissue penetration imaging, information encryption, electronic signature and 3D stress sensing.

Conclusion

      In summary, this study presents an in-depth investigation of stress-induced ML in LiGa₅O₈/Ga₂O₃:Cr³⁺ heterojunctions, with a focus on their application in flexible and stretchable optomechanical devices. By compositing Ga₂O₃ with LiGa₅O₈ and incorporating SiO₂ as an intensity-enhancing agent, we achieved a remarkable 416% increase in the ML intensity of the LiGa₅O₈/0.8Ga₂O₃:Cr³⁺ composite. This significant enhancement is attributed to a synergistic effect: an increased trap density combined with a more uniform trap depth distribution, which collectively facilitates more efficient charge carrier release and radiative recombination. An optimal LiGa₅O₈ to Ga₂O₃ ratio of 5:4 was identified for maximizing ML intensity. Further increases in the Ga₂O₃ fraction led to diminished performance, resulting from reduced trap uniformity and a rise in non-radiative processes. This heterojunction engineering strategy was successfully validated and demonstrated distinct advantages for several potential applications, including penetration through biological tissues (e.g., chicken skin), information encryption, and electronic signatures. Our results underscore the strong potential of LiGa₅O₈/Ga₂O₃:Cr³⁺ composites for next-generation biomedical sensing and anti-counterfeiting technologies.

Project Support

      This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LR24F050002) and the National Natural Science Foundation of China (62175225), with partial funding from the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie Actions COFUND, Physics for Future (Grant No. 101081515).