Fabrication and optical properties of 3D composite photonic crystals of core–shell structures
Introduction
Three-dimensional (3D) photonic crystals have attracted an increasing interest in the past few years, because of their various potential applications in photonics and electronics [1]. A promising approach to fabricate 3D photonic crystals with photonic band gap is self-assembling growth of spherical colloidal particles [2], [3]. However, the fabrication of high-quality 3D colloidal crystals is still a technical challenge, because a large amount of intrinsic defects could be formed during the self-assembling growth process. To fabricate high-quality colloidal crystals films, many methods have been developed, including vertical deposition method [4], convective self-assembly deposition [5], electrophoretic deposition [6], etc. Among these, the vertical deposition method provides a relatively simple approach to fabricate high-quality colloidal crystals films. Many efforts have been made using this method, however, some conventional photonic materials, such as SiO2 and polymers, do not have a full photonic band gap in visible range because of their relatively low refractive indexes (1.4–1.6). A great deal of novel work of fabricating self-assembly 3D photonic crystals have been performed by choosing different materials, such as Fe2O3 [7], ZnO [8], ZnS/SiO2 [9], [10], and CdTe/Latex composite structure [11]. Among those materials, ZnO is a promising candidate for fabricating 3D composite photonic crystals in visible region because of its high refractive index (2.1–2.2) and wide applications in optical and electric devices [12], [13].
In this paper, 3D composite photonic crystals SiO2/ZnO of core–shell structures were fabricated by a two-stage deposition method. The synthetic process of SiO2/ZnO core–shell structures was reported in detail. The periodic arrays of both the pre-deposited SiO2 and SiO2/ZnO core–shell structures were characterized by SEM. The optical properties of the core–shell structures were also studied and a photonic band gap was found in the (1 1 1) direction.
Section snippets
Experimental
3D photonic crystals SiO2/ZnO of core–shell structures were fabricated on borosilicate glass (BSG) substrate by a two-stage deposition method. Prior to the deposition, all the substrates were preprocessed by first being soaked in 0.1 mol/L chromate acid cleaning solution for 12 h, then rinsed with ethanol and dried in air. The SiO2 colloidal crystals were synthesized on BSG substrate following the Stober–Fink–Bohn method [14]. In a typical reaction, silica sol was obtained at 35 °C with constant
Results and discussion
Fig. 1 indicates the transmission electron microscopy (TEM) image of SiO2 colloidal spheres dispersed in acetone solvent. It can be clearly seen that these SiO2 colloidal spheres are in the homogeneous particle sizes with a diameter of 250 nm. The SiO2 colloidal spheres, self-assembled into crystalline colloidal arrays in acetone, are monodisperse due to electrostatics repulsive interactions between the individual spherical particles. Agglomerated particles are not observed. After annealing, the
Conclusions
In this work, 3D composite photonic crystals SiO2/ZnO of core–shell structures were fabricated onto borosilicate glass substrate by a two-stage deposition method. For the first time, acetone was used as dispersant to reduce the cycle of deposition. SEM measurements show that both the pre-deposited SiO2 and SiO2/ZnO core–shell periodic arrays are in the close-packed arrangement, oriented with their (1 1 1) axis parallel to the substrate. Optical measurements reveal that optical properties depend
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