Self-assembled 3D photonic crystals from ZnO colloidal spheres

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Abstract

We present a novel method for the controlled synthesis of monodisperse ZnO colloidal spheres. These spheres are self-assembled into fcc periodic arrays. Optical measurements, including reflection-mode optical microscopy and transmission and single-domain reflection spectroscopy, reveal that the periodic arrays exhibit a photonic band gap in the (1 1 1) direction of the fcc lattice, and calculations are presented to estimate the effective value of the refractive index of the colloidal spheres. Finally, photoluminescence (PL) measurements show that the ZnO lasing thresholds are lower in periodic structures than in random arrays of identical spheres.

Introduction

Photonic crystals show a great deal of promise for applications in numerous types of devices in 1, 2, and 3D structures. The simplest devices are 1D structures consisting of alternating layers of high- and low-index materials. By carefully selecting the thickness of the alternating layers and the refractive indices of the materials, the structure can be engineered to reflect a selected range of wavelengths. Structures of this type form the basis for numerous devices including dielectric mirrors and vertical-cavity surface-emitting lasers [1].

2D structures show promise for integration on silicon. One significant problem with the evolution of photonic integrated devices is the ability to produce waveguides that can efficiently move photons across the surface of a chip. Specifically, traditional waveguide designs cannot include sharp bends without significant signal loss. 2D waveguide structures consisting of periodic 2D arrays of columns of a dielectric material have been demonstrated in which light can be efficiently guided around a 90° bend [2]. In addition to passive devices such as waveguides, there is also significant interest in devices formed from 2D periodic structures in optically active materials. Some research has demonstrated that it is possible to form a 2D defect-mode photonic band gap laser in an InGaAs thin film system. A photonic crystal is formed in the active layer by etching a periodic array of holes in the film. A defect is intentionally introduced into the photonic crystal which acts as a laser cavity, and provides the opportunity for coherent feedback [3].

A great deal of work is also underway in the area of 3D photonic crystals. Numerous techniques have been devised in an effort to produce periodic arrays of dielectric materials that can exhibit a photonic stop band. Some synthetic techniques are quite elaborate including complex, multi-layer lithography [4], multi-beam holographic lithography [5], optical interference methods [6], and production of so-called inverted opal structures [7], [8]. One of the simplest techniques, however involves colloidal self-assembly [9], [10], [11]. Essentially, monodisperse colloidal spheres will spontaneously assemble into periodic arrays under certain circumstances. Self-assembly does have some limitations; for example, colloidal spheres typically arrange into a close-packed FCC array, while it has been calculated that a diamond lattice would be more likely to produce an omnidirectional photonic band gap [12]. Also, thus far, most of the work performed in the area of self-assembled 3D photonic crystals has involved a few materials which are readily available as monodisperse colloidal spheres in sizes appropriate for photonic crystals including SiO2 and polymers, such as polystyrene and PMMA. While these materials do prove easy to assemble into FCC periodic arrays [11], their refractive indices are relatively low. In addition, while some studies have been performed in which emissive materials are added to the photonic crystal matrix [8], [10], [13], no work has explored the properties of photonic crystals formed directly from optically active materials.

Clearly, there is a great deal of novel work that can be performed in the area of self-assembled 3D photonic crystals simply by choosing different material systems. Van Blaaderen et al. have produced a number of interesting emissive materials as monodisperse colloidal spheres including Er3+-doped SiO2 [14], dye-doped PMMA [15], and SiO2/ZnS core/shell structures [16]. ZnO is another promising candidate for optically-active self-assembled photonic crystals because of its interesting optical properties. First, ZnO has a higher refractive index (2.1–2.2 in the visible regime) than other materials (1.4–1.5 for SiO2 and most polymers). In addition, ZnO has been found to be an efficient emitter, exhibiting lasing behavior in the near UV (λ∼385 nm) [17].

In the current work, we describe 3D photonic crystals formed from ZnO colloidal spheres. We describe the synthetic process used to produce monodisperse ZnO colloidal spheres over a broad range of sizes, and the technique used to produce photonic crystals from these colloids. We also explore the optical properties of our photonic crystals.

Section snippets

ZnO colloidal sphere synthesis

The ZnO colloidal spheres used in this work were produced by a reaction similar to that described by Jezequel et al. [18]. ZnO was formed by hydrolysis of zinc acetate dihydrate (ZnAc). In a typical reaction, 0.03 mol ZnAc was added to 300 ml diethylene glycol (DEG). This reaction solution was heated under reflux to 160 °C. Shortly after reaching the working temperature, precipitation of ZnO occurred. Jezequel et al., reported that it was possible to produce monodisperse ZnO powders of various

Optical characterization

Several types of measurements were performed in order to characterize the photonic band gap structures in the periodic ZnO arrays. Reflection-mode optical microscopy was used to observe the general color of the reflected light, large area transmission measurements were performed to detect the presence of a photonic stop band, and spatially-resolved reflection spectroscopy was performed to evaluate the quality of individual domains. In addition, we have performed simulations to evaluate the

Conclusions

In summary, we have developed a technique to produce monodisperse ZnO colloidal spheres. Our technique employs a two-step reaction, and allows close and predictable control of the size of the spheres during the secondary reaction by varying the amount of primary reaction supernatant added. We have demonstrated the production of particles ranging in size from ∼100 to 600 nm, and believe that it should be possible to go beyond this range.

We have self-assembled periodic arrays of these colloidal

Acknowledgements

This work is supported by the National Science Foundation under grant number ECS-9877113 and by the Northwestern University Materials Research Science and Engineering Center.

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