Electrospray‐Assisted Fabrication of Uniform Photonic Balls

Abstract

tion. A drop of Nylon 6,6±isopropanol suspension was deposited on a carbon-coated TEM grid. The grid was shadowed with a thin layer of platinum. A JEOL (1200 EX II) TEM with an accelerating voltage of 120 kV was used and calibration of the SAED spacing was done using TlCl standards. A Digital Instrument Nanoscope IIIA was used for AFM experiments. A drop of Nylon 6,6 crystal suspension was deposited on a carbon-coated glass slide. The force used by the cantilever was light enough to limit damage to the sample, yet obtain accurate surface features. The scanning rate was 1±3 Hz for the low magnification images (512 512 pixels per image). Arrays of close-packed, equal-sized microspheres have been used as templates for the micro-and nano-fabrication of optical materials. Colloidal suspensions of such monodisperse spheres can be especially useful in making optical materials because they can self-organize into ordered, hexagonally packed crystalline lattices in two or three dimensions. Received[1] These opaline lattices can exhibit optical insulating behavior (i.e., they can have a photonic bandgap (PBG)), similar to semiconductors in electronic devices, and are called photonic crystals. The optical insulating behavior of photonic crystals arises from the cooperative scattering of light from the ordered array of particles.[2] However, these opaline structures do not have a full PBG but only a few narrow stop bands. It is known that inverse opaline structures exhibit wider stop bandgaps, and can possess a full PBG when the refractive index contrast exceeds about 2.8. The inverse opaline structures are formed by infiltrating dielectric material into the interstices between the colloidal spheres and then dissolving the spheres away. [3] This produces an ordered foam of spherical cavities. The stop bands of opals or inverse opals with a face-centered cubic arrangement can be exploited to modify the spontaneous emission of organic dyes and semiconductor nanocrystals embedded in the voids of the colloidal crystals.[4]One of the important issues in the design of photonic crystals is the control of their uniformity in size and shape. This is required for the development of photonic devices, including waveguides and optical components, such as microlenses and beam splitters.[5±7] For other applications, photonic crystals in the form of colloidal clusters could be especially valuable, for example, as light scatterers, light diffusers, and pigments for electronic paper and electronic displays.[8±10] The practical use of these materials, however, requires the production of large quantities of monodisperse colloidal clusters. Here, we report a simple, single-step synthetic route for the generation of uniform colloidal aggregates and their inverse structures by electrospraying an aqueous colloidal suspension. Hereafter, both our opaline balls and their inverse structures COMMUNICATIONS

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Monodisperse Liquid-filled Biodegradable Microcapsules

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Colloidal Crystal Beads as Supports for Biomolecular Screening

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