Regioselective Growth of Colloidal Crystals Induced by Depletion Attraction.

Colloidal crystals display photonic stopbands that generate reflective structural colors. While micropatterning offers significant value for various applications, the resolution is somewhat limited for conventional top-down approaches. In this work, we introduce a simple, single-step bottom-up approach to produce photonic micropatterns through depletion-mediated regioselective growth of colloidal crystals. We employ lithographically-featured micropatterns with planar surfaces and nano-needle arrays as substrates. Heterogeneous nucleation is drastically suppressed on nano-needle arrays due to minimal particle-to-needles overlap of excluded volumes, while it is promoted on planar surfaces with large particle-to-plane volume overlap, enabling regioselective growth of colloidal crystals. This strategy allows high-resolution micropatterning of colloidal photonic crystals, with a minimum feature size as small as 10 μm. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, we can create secondary colors through structural color mixing by simultaneously crystallizing two different particle sizes into their own crystal grains, resulting in two distinct reflectance peaks at controlled wavelengths. Our simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications. This article is protected by copyright. All rights reserved.