Tailoring Upconversion Emissions through Core-Shell and Local Structure Engineering

ConspectusFeaturing ladder-like arranged 4f energy levels, lanthanide-doped upconversion nanoparticles (UCNPs) are capable of generating ultraviolet, visible, and near-infrared anti-Stokes emissions via sequential multiphoton absorption under low-power near-infrared excitation. However, these intrinsic emissions often fall short of meeting the diverse demands of applications, such as high emission efficiency, multicolor output, tunable lifetime, and highly nonlinear response. In recent years, core-shell architecture and local structure engineering have emerged as effective strategies for modulating energy transfer pathways and transition behaviors of lanthanide ions, enabling precise and multidimensional tailoring of the emission characteristics of UCNPs.On the one hand, constructing a core-shell architecture that spatially separates distinct lanthanide species enables reconfiguration of the interionic energy transfer pathways, thereby allowing fine control over upconversion emissions. Typically, the shell, composed of optically inert ions, can act as a protective layer that shields the luminescent centers in the core from surface quenchers, thereby minimizing energy dissipation and substantially enhancing upconversion emissions. Moreover, when multiple sensitizers and activators are compartmentalized into separate shells, the resulting network of lanthanide ions facilitates long-range energy migration. This leads to diversified emission profiles, along with flexible tunability in the emission color and lifetime. Furthermore, by introducing an inert interlayer to suppress interlayer energy crosstalk, non-interfering luminescent regions can be integrated into a single nanoparticle, leading to excitation orthogonalized upconversion emissions. On the other hand, as the 4f-4f transitions of lanthanide ions are sensitive to the site symmetry, precise control over the local structure offers an effective route for tailoring upconversion emissions. Local structure encompasses parameters such as local symmetry, interionic distances, interstitial ions, vacancies, etc., which can be engineered through adjusting the composition, exerting external fields, and modulating interfacial strain. Notably, upconversion luminescence arises from a cascade of absorption, energy transfer, and emission processes, all of which are governed by the local structural environment. Consequently, strategic engineering of the core-shell and local structure allows efficient modulation of upconversion emissions, including their emission intensity, selectivity, lifetime, and nonlinear behavior.In this Account, we summarize the recent advances in tailoring upconversion emissions through these two distinct yet complementary strategies: core-shell architecture and local structure engineering. We begin by elucidating how the rational design of core-shell architectures allows precise control over energy transfer pathways, leading to tunable emission profiles, enhanced emission intensity, and excitation orthogonalization. We then discuss how modulation of the local coordination environment and perturbation of the crystal field influence the 4f-4f transitions and emission behaviors. Key progress in synthetic methodologies, energy transfer mechanisms, and multidimensional emission modulations is highlighted. Finally, emerging opportunities in core-shell structure fabrication and local structure engineering are presented, offering perspectives on the design of efficient upconversion systems. This Account is expected to provide the upconversion community with a clearer blueprint toward precise luminescence control for diverse optical and multidisciplinary applications.