Assembly-Induced Photon Confinement and Recirculation in Upconversion Superparticles

The controlled self-assembly of nanoparticles into microscale architectures with defined structures and emergent functionalities remains a critical challenge in nanoscience. Here, we develop an emulsion-mediated strategy to assemble lanthanide sodium fluoride upconversion nanoparticles (UCNPs) into smooth spherical superparticles. During self-assembly, the diffusion of solvent from UCNP-containing cyclohexane droplets into an isopropanol/H2O medium drives the progressive condensation of UCNPs into superparticles. The polarity of the extracting solvent serves as a key factor governing the self-assembly process. Solvents with permittivity comparable to that of isopropanol preserve spherical confinement, yielding uniform spherical superparticles, whereas solvents of lower or higher polarity disrupt droplet stability or suppress cyclohexane diffusion, leading to disordered or sheet-like assemblies, respectively. Furthermore, we reveal that the shape of UCNPs critically determines the surface roughness of the superparticles. Quasi-spherical UCNPs assemble into superparticles with smooth surface, whereas cubic and hexagonal platelet UCNPs generate rougher surface architectures. The resulting spherical superparticles confine upconversion emissions with guided and sustained propagation, enabling stimulated emissions with thresholds lower than 4 W/cm2 and quality factors exceeding 104. Moreover, hybrid superparticles composed of various lanthanide-doped UCNPs sustain stimulated emissions from visible to near-infrared. These findings promote pathways from nanoparticle building blocks through microscale self-assembly to stimulated emissions, and platforms for integrated photonic devices.