Thermally stable high-contrast iridescent structural colours from silica colloidal crystals doped with monodisperse spherical black carbon particles

Monodisperse black carbon nanoparticles (average diameters: 272 and 511 nm) are prepared by the carbonization of monodisperse spherical π-conjugated polymer particles. Meanwhile, silica nanoparticles of different sizes are prepared by a conventional sol–gel process on the basis of the Stober method. The cast films fabricated from aqueous dispersion of silica nanoparticles show size-dependent iridescent structural colours of blue (212 nm), green (240 nm), yellow (261 nm), and red (270 nm). Remarkably, these structural colours show enhanced saturation when black carbon nanoparticles are mixed with a low ratio, such as 1 wt%. The reflection UV-vis spectra indicate that adding carbon particles causes an extreme reduction of reflectance in the visible wavelength region, while the peaks of the structural colours become enhanced in height with almost no shift. Scanning electron microscopy observations suggest that the carbon nanoparticles are well dispersed in the colloidal crystal films of silica nanoparticles but do not disturb the array arrangement. Hence, the iridescence of structural colour is retained. The angle-dependent reflectance spectra of the carbon nanoparticle-doped colloidal crystals mostly agree with theoretical predictions. Since the maverick carbon nanoparticles are thermally stable, the highly saturated structural colours persist up to 350 °C.