Structural color via layer-by-layer deposition: layered nanoparticle arrays with near-UV and visible reflectivity bands

The same near-ultraviolet (NUV) (300-400 nm) light that facilitates photo-degradation in polymers is also a key component in animal communication, particularly in insects and birds. We developed a nature-inspired 1D photonic crystal structure that has simultaneous narrow stop bands in the NUV and visible wavelength range, resulting in a NUV reflector with tunable structural color in the visible region of the spectrum. The samples were prepared by high precision deposition of alternating stacks of SiO2 and more densely packed TiO2 nanoparticles via aqueous layer-by-layer (LbL) deposition. Multilayer thin films with eleven stacks achieved up to 75% reflectance with a 50 nm bandwidth and 65% reflectance with a 100 nm bandwidth for structural colors centered at 500 nm and 690 nm, respectively. Despite UV absorbance of the TiO2 nanoparticles and the supporting glass substrate, all samples exhibited a narrow NUV stop-band centered in the 300-400 nm range with a reflectance over 65%. Upon introduction of a defect mode, we also demonstrated the creation of a narrow reflectance dip associated with a localized state in the photonic band gap. Multilayer-based structural colors with NUV reflectivity can be used as pigments that can promote prevention of photo-degradation in coatings, in addition to bird and insect repelling functionality.