Mie-Resonant Rutile TiO₂ Nanospheres for Unclonable Structural Colors

Monodisperse carbon-embedded rutile TiO2 nanospheres were synthesized through the sol–gel reaction of titanium alkoxide in concert with dodecylamine and the subsequent thermal annealing process in inert gas. Since dodecylamine was uniformly carbonized in nanospheres, the growth of rutile crystals was limited to ∼50 nm in the thermal annealing process, which enabled us to keep the spherical shape of nanospheres. Due to the size and shape uniformity, colloidal films of these rutile TiO2 nanospheres showed unique Mie-resonant structural colors, in which multiple scattering was suppressed due to carbon embedded in nanospheres. Due to the high refractive index of rutile TiO2, there were multiple backscattering peaks in the visible region, which make it extremely difficult to replicate the colorimetric information when they were utilized as encryption inks. Finally, we demonstrated that IR-reflective carbon-embedded rutile TiO2 nanospheres can be used as automotive paints, which can be distinguishable at night for light detection and ranging (LiDAR) systems.