Ultrathin Dielectric Nanocoatings Enable Multifunctional High-Emissivity Silicon Surfaces

Controlling surface emissivity is crucial for thermal-photonic technologies requiring outdoor stability, yet combining high emissivity with environmental durability remains challenging. Here, we systematically investigate ultrathin dielectric nanocoatings, titanium dioxide, Teflon, diamond, boron nitride, and alumina, deposited on flat and microstructured doped silicon, chosen as examples of highly emissive surfaces with different spectral and angular profiles. Using experimental measurements and theoretical modeling, we demonstrate that these nanocoatings not only preserve but can enhance broadband emissivity across 250 nm to 20 µm, while providing broad control over wettability. Diamond-coated surfaces exhibit the highest broadband emissivity and angular isotropy in the mid-infrared, whereas TiO₂ introduces controlled spectral features due to phonon-polariton resonances. Our findings show that ultrathin nanocoatings provide a versatile route to tailor surface functionality while maintaining high radiative performance, positioning nanocoated silicon as a scalable, multifunctional platform for radiative cooling, solar energy harvesting, and infrared emission control under harsh outdoor conditions.