Feb 18, 2009
Building cavities in microspheres and nanospheres
Microfeatured and nanofeatured structures have attracted great attention due to their size-dependent physical effects and potential applications. In particular, many novel and functional structures with cavities, such as nanobowls and nanorings, exhibit particular optical properties, magnetic properties, plasmon resonance, negative refractive index, etc., not only due to the scale and material features, but also due to the interior cavity and the feature size of the cavities.
Various advanced lithographic technologies and nonlithographic methods have been applied to create microscale and nanoscale structures, and it is noteworthy that many of these preparation methods use self-assembled colloidal crystals as masks or templates to fabricate multifarious patterns and functional materials, due to the ordered arrays of interstices and cost-effectiveness, which is well developed process known as colloidal lithography (CL). Combining CL with versatile methods, nanobowls and nanorings made of a large variety of materials, including polymers, metals and semiconductors have typically been obtained. For example, rings or crescent moons of metal have been prepared by metal deposition, and subsequent removal of colloidal spheres. Polymer or semiconductors rings have been fabricated via combining capillary forces, dewetting, or edge spreading lithography.
In addition, reactive ion etching (RIE) also provides a versatile tool for the nanomachining of colloidal crystals. Anisotropic RIE has been employed not only to change the size of colloidal spheres, but also to etch colloidal crystals in an anisotropic fashion. Nanoholes could also be built in colloidal spheres, which can be applied to the fabrication of functional composite particles.
However, it is noteworthy that it is difficult to control the topography of the cavity, which determines the properties and applications. In their paper, published in Nanotechnology, Bai Yang and co-corkers present a simple method of fabricating silica bowls and silica rings based on soft-lithography and anisotropic RIE using polymer as mask, and investigated the changing process in detail. The morphology of the cavity in the silica structure undergoes transformation in three steps: (1) increase of the diameters of the opening of the cavities; (2) increase of the depth of the cavities; and (3) change in geometry of the resulting structure. As a result, silica bowls or silica rings could be obtained by controlling the etching stage, and this approach can be extended to a wide range, from a micrometer to a nanometer, based on the silica spheres. Moreover, these structures afford opportunities and space for fabricating various functional materials and applications.
About the author
Professor Bai Yang is director of the State Key Lab of Supramolecular Structure and Materials , College of Chemistry, Jilin University, Changchun, China