Switchable Bragg Reflections via Controllable Inner Particle Motion in Yolk-Shell Colloidal Crystals

Abstract

Yolk-shell particles consist of a hollow shell enclosing a mobile inner particle. Colloidal crystals made from yolk-shell particles are a unique structure, in which the disorder of highly scattering inner particles can be controlled, which allows for optical switching. In this work, yolk-shell particles were synthesized and assembled into an ordered structure. External alternating current (AC) electric fields were used to control the inner particle motion, as observed by confocal microscopy and optical reflection measurements. The colloidal crystal of yolk-shell particles showed long-range order due to the assembled shells but decreased short-range order due to the Brownian motion of inner particles. Using an AC electric field (25 V/mm), all inner particles moved electrophoretically, resulting in ordered inner-particle arrangements. This enabled the fast, reversible switching of the Bragg reflection intensities. Next, we investigated how a decreased short-range order when the field is off influenced the switchability. The largest optical intensity change was achieved with a high ionic strength (10 mM) and a small core-to-shell size ratio (∼0.3). Our proof-of-concept results show promise that with further optimization, even more strongly switchable photonic crystals can be achieved in this way.