Supraballs Composed of Mie‐Resonant Silicon Nanospheres

Abstract In this work, a supraball composed of high‐refractive index ( n ∼4) silicon nanoparticles (Si NSs) with diameters ranging from 100 to 200 nm having Mie‐resonance in the visible range is fabricated via a solution‐based self‐assembly process. Structural characterization reveals the formation of highly spherical supraballs with diameters of 2 to 20 µm in which Si NSs are densely packed. Optical properties of individual supraballs made of different size Si NSs are systematically investigated. The main spectral feature of a densely packed supraball is well explained by a nanosphere‐layer model. The effect of Si NS density, i.e., inter‐NS distance, in a supraball on the optical response is studied by controlling the interparticle distance via two methods. Tuning the spacing between the NSs by oxidation reveals that the distance increase enhances the reflectance due to the appearance of the Mie resonance of individual Si NSs. The distance control by the formation of a thermoresponsive polymer shell on individual Si NSs in a supraball allows us to dynamically tune the interparticle distance through optically induced heating, and thus to dynamically control the reflectance spectrum.