Regioselective Growth of Colloidal Crystals Induced by Depletion Attraction

Abstract

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, a simple, single-step bottom-up approach is introduced to produce photonic micropatterns through depletion-mediated regioselective growth of colloidal crystals. Lithographically-featured micropatterns with planar surfaces and nano-needle arrays as substrates are employed. 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 mu m. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, secondary colors can be created 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. The simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications. Photonic micropatterns are created through single-step growth of colloidal crystals on lithographically prepatterned substrates with planar surfaces and nano-needle arrays. When the colloidal assembly is mediated by depletion attraction, the planar surfaces effectively cause heterogeneous nucleation and crystal growth, whereas the nano-needle arrays inhibit it. This results in regioselective crystallization on the planar areas, yielding high-resolution structural-color graphics.image