Phase engineering enhanced upconversion luminescence in ultrasmall Li+-doped Cs2ZrF6:Yb/Er nanocrystals for ratiometric optical thermometry

Lanthanide-doped ultrasmall upconversion nanocrystals exhibit unique optical properties that differ significantly from bulk materials, making them highly promising for diverse applications. However, their widespread applications have long been impeded by extremely weak photoluminescence due to severe surface quenching effect. Herein, we present a Li+-doping-induced phase engineering strategy to achieve significantly enhanced upconversion luminescence in ultrasmall Cs2ZrF6:Yb/Er nanocrystals (NCs). While initial Li+ doping improves crystallinity, yielding up to a 63-fold emission increase, further doping triggers a phase transition to form heterophase nanocrystals consisting of trigonal Cs2ZrF6 and tetragonal LiYbF4. The synergistic effect of improved crystallinity and reduced local symmetry around lanthanide ions results in a 302-fold boost in emission intensity, even as the particle size decreases to ~6.1 nm. Moreover, these ultrasmall nanocrystals display anomalous anti-thermal quenching behavior, with luminescence intensity increasing as temperature rises from 303 to 483 K, driven by defect-mediated energy repopulation. This work not only offers a robust approach for fabricating ultrasmall, high-brightness NCs but also establishes phase engineering as a pivotal mechanism for modulating local crystal field, paving the way for high-performance, thermally stable ratiometric nanothermometry.