Manipulation of Luminescence via Surface Site Occupation in Ln³⁺-Doped Nanocrystals

Ln³⁺-doped (Ln = lanthanide) nanocrystals are garnering strong interest for their potential as optical materials in various applications. For that reason, a thorough understanding of photophysical processes and ways to tune them in these materials is of great importance. This study, using Eu³⁺-doped Sr₂YF₇ as a well-suited model system, underscores the (not unexpected) significance of surface site occupation of Ln³⁺ and also challenges the prevailing views about their contribution to the luminescence of the system. High-temperature cation exchange and epitaxial shell growth allow nanocrystals to exclusively feature Eu³⁺ residing at the surface or in the interior, thereby separating their spectral responses. Meticulous experiments reveal that nanocrystals with high doping concentrations exhibit luminescence primarily from surface Eu³⁺, in contrast to the popular belief that luminescence from surface Ln³⁺ is largely negligible. The present study shows, on the one hand, the necessity to revise common ideas and also reveals the potential for manipulating the luminescence of such materials through an, until now, unperceived way of surface engineering.