Colloids of superparamagnetic shell: synthesis and self-assembly into 3D colloidal crystals with anomalous optical properties

Abstract

Silica microspheres were coated with Fe₃O₄ and carbon by the decomposition of ferrocene in an acetone system containing hydrogen peroxide. The system was heated at the temperature of 100 °C for the first 24 h and then 180 °C for the next 24 h. The as-prepared superparamagnetic colloids of SiO₂@Fe₃O₄@C were dispersed in acetone and efficiently self-assembled into centimetre-sized 3D magnetic colloidal crystals under the induction of a 0.2 T magnetic field. Both optical microscope image and reflection spectrum show a blue color of the colloidal crystals, demonstrating photonic crystal features. Because of the thinness of the shell and the large difference in reflective index between the core and shell, a new pseudogap whose reflection cannot be predicted by Bragg's law was generated.