Tunable colloidal photonic crystals

TL;DR

This study reveals that aqueous suspensions of charge-stabilized, fluorinated colloids can produce tunable, angle-independent structural colors across the visible spectrum, despite their low refractive index contrast, by adjusting concentration.

Contribution

It demonstrates the unexpected photonic properties of low-refractive-index colloids and shows how to tune structural colors continuously through concentration changes.

Findings

Suspensions become partially transparent with strong, angle-independent color.

Structural colors can be tuned across the entire visible range.

Concentrated suspensions exhibit Bragg peaks and glassy phases.

Abstract

Spherical colloids arranged in a crystalline order are known to produce structural colors. The intensity and brilliance of such photonic crystals require high size-monodispersity of the colloids, a low number of lattice defects and disorder, as well as a relatively large refractive index contrast between the scattering colloids and the continuous background. Here we present the unexpected photonic properties of aqueous suspensions of charge-stabilized, 186 nm large, fluorinated colloids with a refractive index of 1.37. Employing reflectivity, optical observation, small angle x-ray scattering measurements and reflectivity modeling, we demonstrate that these suspensions become partially transparent while showing strong, almost angle-independent color in reflection despite the very small refractive index difference. Under certain conditions, additional sharp Bragg reflections are observed. We were able to tune the observed structural colors continuously across the entire visible range by simply changing the volume fraction of these colloidal suspensions, which show a white appearance when dilute, structural color and Bragg peaks when concentrated enough to form Wiegner crystal, and angle-independent color when very concentrated and in a glassy phase.