Uniformization of silica particles by theory directed rate-zonal centrifugation to build high quality photonic crystals

• As-synthesized silica particles were uniformed by density gradient centrifugation.
• A theoretical equation was derived to control the centrifugal condition.
• Silica particles from 80 nm to 5 μm were successfully uniformed within 1 min.
• The polydispersity index was improved from 0.084 to 0.007 for 260 nm silica particles.
• High quality photonic crystals were assembled with uniformed silica particles.

Non-uniform size distribution can be observed in the synthesis of nanoparticles. There is thus a compelling need for post-synthesis separation of nanoparticles. Herein, a theory directed rate-zonal centrifugation method was proposed to unify the as-synthesized silica spheres. A formula that quantitatively links the centrifugal time with centrifugal conditions was derived. Under the guidance of the formula, various silica particles with size from 80 nm up to 5 μm were successfully uniformed and/or separated in ca. 1 min by centrifugation. The polydispersity index improved largely, e.g., from 0.116, 0.084, 0.071 and 0.102 to 0.013, 0.007, 0.006 and 0.009 for 80 nm, 260 nm, 740 nm and 5 μm silica particles, respectively. The separation time and efficiency were easily adjusted by regulating the centrifugal speed, gradient formulation and fractionation size. With the uniformed particles, better photonic crystals (PCs) were assembled, with stopband width largely sharpened, e.g., from 66 nm to 16 nm for a 260-nm-silica-assembled PC, and stopband reflectivity extensively enhanced from 75% up to even 99%. This method can also be extended to unify other spherical particles.

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