Doped semiconductor nanoparticles for possible daytime radiative cooling applications

Daytime radiative cooling has become a promising passive cooling technique in recent years, showing great potential in building-related applications to reduce the power consumption of air conditioning. Currently used mid-infrared emitters mainly rely on phonons in crystals, molecular vibrations in polymers (which have limited tunability) and other sophisticated photonic and plasmonic structures (which rely on expensive techniques). In this work, we propose the use of doped semiconductor nanoparticles as selective emitters, whose infrared optical properties can be readily tuned using the doping concentrations. The theoretical simulation shows that wide bandgap semiconductors with a bandgap greater than 3.5 eV can suppress solar absorption effectively. The plasma frequencyf(p)should be approximately 2400 cm(-1)to have enhanced emissivity in the atmospheric window for nanoparticles embedded in a medium with electric permittivity of 2.3. The complex refractive index of the metamaterial is calculated with the effective medium theory and the spectral and angular emissivity is estimated with the help of optical theories. A metamaterial containing 5% (volume fraction) doped ZnO nanoparticles is predicted to exhibit a cooling power greater than 100 W m(-2)and a temperature drop of more than 10 degrees C under a wide range of ambient temperatures.