Lanthanide Doped Metal Halide Materials Based on 4 f -4 f Electronic Transitions: From Luminescence Regulation to Versatile Applications

Conspectus Lanthanide doped metal halide luminescent materials (MHLMs) have recently emerged as a research focus, combining the rich energy level transitions of lanthanide ions with the versatile structural tunability of metal halides. The materials enable the development of highly tunable luminescent systems with diverse emission wavelengths and modes. By contrast with conventional lead-based luminescent materials, many MHLM systems provide more environmentally friendly alternatives. Consequently, lanthanide activated MHLMs have demonstrated significant potential in a wide range of applications, including anticounterfeiting technologies, light-emitting diodes (LEDs), as well as near-infrared (NIR) and X-ray detection. Compared with organic dye-based fluorescent labels, these materials exhibit superior photostability and color performance, characterized by higher color purity and spatial resolution. Among the 17 rare-earth elements, luminescence primarily arises from either 4f-5d or 4f-4f transitions. Different from the 4f-5d transitions, lanthanide ions exhibiting 4f-4f transitions are relatively insensitive to the external environment. Therefore, theys usually display characteristic narrow emission bands with fixed peak positions and superior spectral stability. In addition, the 4f-4f transitions are parity-forbidden, resulting in intrinsically small absorption cross sections and relatively low luminescence efficiencies. With the rapid development of MHLMs, substantial efforts have recently been devoted to developing strategies such as compositional engineering, codoping approaches, low-dimensional structural design, and defect engineering, leading to significant advances in enhancing luminescence properties. However, there are few systematic reports on the understanding and modulation of lanthanide luminescence based on 4f-4f transitions in MHLMs, which not only limits the predictability of lanthanide activating strategies but also increases the experimental cost associated with empirical optimization of luminescent performance. In this review, we comprehensively summarize the recent research progress of our group on lanthanide doped MHLMs based on 4f-4f electronic transitions. We begin by introducing the design principles underlying the enhancement of luminescent properties in lanthanide doped MHLMs, including energy transfer (ET), self-trapped exciton emission (STE), and ion emission, thereby providing a theoretical framework for optimization and prediction of optical characteristics. Additionally, based on these design principles, we propose specific strategies for luminescence modulation, such as compositional substitution, dimensional tailoring, core–shell structure and environmental factors, which enable the modification of emission wavelengths and improvement of material stability. Finally, considering the unique luminescent features of different material system, we comprehensively discussed the potential applications of our group’s and other groups’ studies in various optoelectronic fields. This review aims to provide perspectives based on our group’s research on regulating the optical properties of lanthanide ions with 4f-4f transitions and to improve the predictability of their luminescence behavior.