Heavy Halide Nanomaterials as Next-Generation Hosts for Lanthanide Upconversion

Activating inorganic hosts with lanthanide ions enables nonlinear phenomena, such as photon upconversion, photon avalanche, and quantum cutting, thereby expanding the potential of nanomaterial-based technologies. To maximize the optical performance of lanthanide dopants, it is essential to develop optimal host materials, especially those with low cutoff phonon energies. Heavy halide compositions, with their low phonon energies, have tantalized researchers with the potential to boost lanthanide brightness but are hampered by inherent stability issues. However, thanks to advancements in colloid and surface chemistry, these issues can now be addressed. In this Perspective, we outline research on heavy halide hosts favorable for lanthanide doping and photon upconversion, highlight the outstanding challenges when introducing lanthanides into these hosts, and identify as-yet unexplored directions that may lead to significant performance gains in brightness, nonlinear process efficiency, and color-tuning. Inspired by early research on lanthanide upconversion in bulk heavy halides and ongoing studies on lead halide perovskites, we provide insight into the development of lanthanide-activated heavy halides - from nanocrystal synthesis and surface and composition engineering to leveraging emerging approaches in tuning optical properties, including automated and computational materials discovery. Ongoing research on doped heavy halides will expand the variety of lanthanide-based nonlinear materials, unlock unique optical properties, and deliver greater value to applications in color and energy conversion, imaging, security, and photonics.