Size-Dependent Surface Evolution Controls Luminescence in Ln3+-Doped Nanocrystals

Lanthanide-doped luminescent nanocrystals (NCs) hold significant promise for bioimaging, theranostics, catalysis, sensing, lighting, and displays, motivating an in-depth understanding of their optical properties. This work reveals the profound, yet widely disregarded, impact of surface structure changes on the luminescence of growing NCs through a model study of Eu3+ surface-exchanged Sr2YF7 NCs. Contrary to expectation, a stark drop in luminescence intensity exceeding several orders of magnitude was observed as the particle size increased. Atomic-resolution electron microscopy imaging shows that the NCs evolve from spheroidal to cuboidal morphologies during growth, with a shift in preferentially exposed crystal facets from {111} to {200} that renders lattice distortion less favorable. High-resolution optical spectroscopy probing surface Eu3+, together with energy-dispersive X-ray spectroscopy and reaction kinetics study, reveals a high extent of Eu3+ exchange in the small spheroidal NCs with lattice distortion extending far from the surface into the interior. This starkly contrasts the situation in the large cuboidal NCs, where the extent of Eu3+ residing in the surface region is severely limited, thereby causing the dramatic drop in luminescence. These findings demonstrate surface structure control as a powerful strategy for tailoring the luminescence properties of Ln3+-doped NCs and other nanomaterials for advanced applications.