Dielectrophoretic Assembly of Switchable Two-Dimensional Photonic Crystals with Specific Orientation
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Dielectrophoretic Assembly of Switchable Two-Dimensional Photonic Crystals with Specific Orientation Simon O. Lumsdon1, Eric W. Kaler2 and Orlin D. Velev3 1. DuPont Central Research & Development, Experimental Station, Wilmington, DE 19880 2. Department of Chemical Engineering, University of Delaware, Newark, DE 19716 3. Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695
ABSTRACT
One and two-dimensional colloidal crystals are assembled from aqueous suspensions of latex and silica microspheres in an alternating electric field. These crystals of size up to 25 mm2 are formed in the gap between two planar gold electrodes. They have specific axis orientation parallel to the direction of the applied field without the need for expensive micropatterned templates. The field gradient causes the particles to accumulate on the surface between the two electrodes, align into rows along the field direction, and then crystallize into hexagonal arrays. The lattice spacing can be controlled via electrostatic repulsion. The system can find application in switchable photonic devices.
INTRODUCTION
Three-dimensional (3D) and two-dimensional (2D) photonic crystals, with periodicity comparable to the wavelength of light, are able to manipulate the flow of light by forming bandgaps that prevent propagation at certain frequencies.1 Crystals have been fabricated by the inexpensive assembly of colloidal microspheres followed by replication of the structure into continuous matrices.2-4 Practical application of these self-assembled “opals” has been limited, however, because the assembly process is slow and the resulting samples are often conglomerates of many crystalline domains. Large crystals have been formed using micropatterned templates5-8 but this relies on sedimentation and diffusion, polymer addition and capillary forces to arrange the particles. It would be desirable to compliment templated assembly with simpler and more rapid processes. Crystal assembly has been accelerated by applying
Downloaded from https://www.cambridge.org/core. La Trobe University, on 26 Oct 2019 at 12:36:46, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-776-Q6.8
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electric fields to particles in a thin cell. A constant field causes the particles to be attracted towards the oppositely charged electrode9-11 and an alternating field brings the particles together.12,13 Crystals assembled in this manner however, have random orientation and polycrystalline domains. In this paper, the crystal is assembled between two parallel electrodes deposited on a glass surface in the same horizontal plane. This geometry allows rapid electrically driven assembly with long-range ordering and specific orientation.
EXPERIMENTAL DETAILS
Distilled water was passed through a Millipore Milli-Q Plus water purification system, a resistivity of 18.2 MΩ.cm was obtained. NaCl (99+%) and glycerol (99+%) were purchased from Fisher Scientific. The polystyrene lat
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