Opal-Hematite and Opal-Magnetite Films: Lateral Infiltration, Thermodynamically Driven Synthesis, Photonic Crystal Properties

Abstract

Hematite (α-Fe 2 O 3 ) was infiltrated into opal film pores without depositing the oxide onto the outer film surface by the developed method of lateral infiltration of an aqueous solution of iron nitrate under capillary forces. The thermodynamically driven syntheses of either Fe 3 O 4 (magnetite) or α-Fe were performed in the opal pores using α-Fe 2 O 3 as a precursor. The opal-α-Fe 2 O 3 and opal-Fe 3 O 4 films were characterized by scanning electron microscopy, X-ray diffraction, reflectance, and transmittance measurements. The photonic band gap red-shifted gradually as the number of lateral infiltration runs increased. The maximum filling degrees both of opal-α-Fe 2 O 3 and of opal-Fe 3 O 4 films, calculated from the reflection spectra, appeared to be similar and equal to ∼55% of the pore volume because of very close values of molar volumes of the oxides. It was found that the reversible chemical transformation of fillers in opal pores α-Fe 2 O 3 ↔ Fe 3 O 4 ↔ α-Fe 2 O 3 changes only the filler dielectric constant but does not practically produce structural defects that could affect the photonic crystal properties of the composite. A combination of lateral infiltration technique and thermodynamically driven synthesis is a cost-effective approach to produce three-dimensional photonic crystals based on opal films filled with A 3 B 5, A 2 B 6, and A 4 B 6 semiconductors, oxides, and metals.