Angle-independent colours from spray coated quasi-amorphous arrays of nanoparticles: combination of constructive interference and Rayleigh scattering

Angle-independent colours have enormous potential in buildings, displays and sensors. Here, we have demonstrated angle-independent full-colour films by spray-coating monodispersed silica nanoparticles (NPs) of different sizes (100-340 nm in diameter). Ethanol and isopropanol with low surface tension and medium volatility were selected as solvents to spray-coat nanoparticles, forming quasi-amorphous arrays on various substrates. The angle-resolved reflection and scattering measurements showed that the real sample colour matched the colour from interference of reflected light at the specular angle but did not match the scattering peak position. Further study and colour fitting suggests that the observed colour of the spray-coated nanoparticle assembly arose from a combination of constructive interference and Rayleigh scattering. Moreover, we attempted to enhance the colour saturation by replacing air with water between the black background and the coating, in order to suppress reflection from the background.