Detailed microscopic observations of the structure of bulk colloidal crystals grown by sedimentation

Abstract

Since the actual and microscopic structure of hard-sphere-type colloidal crystals has not been fully understood, we performed detailed microscopic structural characterization of the sedimentation-grown opal-type colloidal crystals by conventional optical microscopy, laser conforcal microscopy, and secondary electron microscopy. It is directly proved for the first time, to the best of our knowledge, that iridescent striations originate from the repetition of twin structures and stacking faults in crystal structures. We propose “subdomain” as an intrinsic constitutional unit of a monocolored iridescent “domain” and the subdomain boundary is defined as the envelope of the edges of stacking faults. Every domain forming the bulk crystal contains a high concentration of slanted stacking faults at various tilt angles inherent to the domain. These observations show that the bulk colloidal crystals can be described in terms of five stages of hierarchical structure units, bulk crystal, domains, sub-domains, pristine single-crystal lamellae, and silica particles.