Dip-coated silicon nanospheres give curved objects a glossy colour that ultraviolet light does not fade
Materials scientists have built a dip-coating route that lays a single, even layer of sub-micron silicon spheres over complex three-dimensional objects. The colour comes from Mie resonances inside each particle rather than from a pigment or a periodic photonic crystal, so the hue stays saturated from every viewing angle and holds up under intense ultraviolet light. It is a way of colouring a shaped object whose brightness does not rest on a dye that can bleach.
Science··Evening
A single layer of spheres laid down by dipping
Structural colour is a hue that comes from the way a surface scatters light, with no dye involved. Materials scientists have now built a dip-coating route that leaves one even layer of silicon spheres, each smaller than a micrometre, across complex three-dimensional objects. Because the layer is laid down by dipping rather than patterned onto a flat substrate, it follows a curved or irregular shape, and it stays a single particle thick over the whole piece.[1]
Each particle carries the colour on its own
The colour originates in Mie resonance, named for the scattering theory that describes it: incoming light drives electric and magnetic multipole oscillations inside each individual particle, which silicon supports because its refractive index is high. That origin differs from the periodic photonic crystals used in much structural-colour work, where the repeating arrangement of the structure sets the hue. Here the single sphere is the optical element, so the coating does not depend on an ordered lattice of neighbours.[1]
The hue holds its angle and its brightness
On the finished surface that mechanism removes iridescence, the colour shift a viewer sees when the angle changes, so the hue reads the same from any direction and stays saturated instead of washing out. The coating also keeps its colour under intense ultraviolet light, the exposure that bleaches pigment, because a Mie resonance rests on the size, shape and refractive index of the particle. The result is a glossy, angle-independent finish on an arbitrary curved surface.[1]
