Thermal emission of colloid-based plasmonic structures for radiative cooling
Radiative cooling is an approach to cool down the temperature of objects, such as buildings, by selective thermal radiation. The cooling efficiency is maximized when the system emits thermal radiation only in the wavelength range from 8-13 µm, where the atmosphere is transparent. Photonics and plasmonics have been utilized to engineer the emissivity of materials for radiative cooling and a lot of nano-micro structures have been proposed. However, multiple-times sputtering or electron beam lithography are required to fabricate such nanostructured systems. Consequently, it is difficult to produce large-area device efficiently. Hybrid materials comprising polymer and silica nanoparticles have also been reported to overcome those fabrication difficulties, but it is difficult to control the emissivity.
To realize a high performance scalable radiative cooler, we designed a plasmonic structure, which can be easily fabricated by colloidal lithography. We investigate this structural motif by finite element method simulations using COMSOL Multiphysics and transfer these results into experimental prototypes. Our proposed structure is composed of an Au reflector, micron-sized SiO2 spheres, and a thin layer of Indium Tin Oxide (ITO). Our results show that by using the right particle size and thickness of ITO, it is possible to specifically tune the emission properties in the desired wavelength range. We provide structural guidelines for selective and strong emissivity in the wavelength range from 8 to 13 µm. This broadband selective emissivity is attributed to Fano resonances induced by the interaction between the plasmon resonance of the ITO semishell and the phonon resonance of the SiO2 sphere.