CNT-Templated Microfabrication of Porous Silicon-Carbon Materials
Jun Song, David S. Jensen, David N. Hutchison, Brendan Turner, Taylor Wood, Andrew Dadson, Michael A. Vail, Matthew R. Linford, Richard R. Vanﬂeet and Robert C. Davis
Several functional properties of materials are impacted by the 3D shape of the material on both the micrometer scale (microscale) and the nanometer scale (nanoscale). The microscale patterning of a material affects several functional properties including ﬂuid ﬂow, ion and electron conductivity, the mechanical response, and interactions with electromagnetic ﬁelds and waves. Nanoscale structuring can dramatically change chemical-reaction rates and the surface-adsorption capacity, as the surface-to-volume ratio signiﬁcantly increases. Electronic material properties, including the band structure, recombination rates, and mobilities are strongly inﬂuenced by nanoscale structuring. Often, multiple physical properties are coupled and are jointly inﬂuenced by nanoscale structuring, as is the case for strained silicon: nanoscale strain control is used to produce higher mobilities than achievable in the bulk. Coupling between strain and electrochemical properties was observed in nanoscale silicon particles deposited on carbon nanotubes, which resulted in improved electrochemical cycling over bulk silicon anodes for lithium-ion batteries.
Since both micro- and nanoscale structuring inﬂuences functional material properties, the ability to control microscale shape in a variety of nanostructured materials will enable a wide range of applications. Precise 3D microscale structures have been widely fabricated by reactive ion etching (RIE), wherein bulk semiconductor, metal, and ceramic materials are micromachined into the desired shapes. Deep reactive ion etching has resulted in high-aspect-ratio structures in silicon. Vertically aligned carbon nanotubes grown from micropatterned catalyst layers result in high-aspect-ratio structures with vertical sidewalls. However, the as-grown nanotube density is low and the tubes are held to each other only by weak van der Waals forces, resulting in structures that are too weak to maintain their microscale shape when put in contact with ﬂuids. Recently, templated microfabrication of robust, high-aspect-ratio structures in silicon, silicon nitride, and carbon has been done using carbon-nanotube (CNT) frameworks. On the nanoscale, silicon materials including silicon nanowires (SiNWs), silicon nanotubes (SiNTs), and porous silicon have previously been fabricated by a variety of methods including chemical vapor deposition, template-assisted growth, solution-phase synthesis, and electrochemical etching. Vertically etched porous silicon has been used to produce microscale features with vertical nanopores by masking and plasma etching. Carbon nanotubes also have been used as a nanoscale template for various materials, including polymers, metals, metal alloys, and silicon. While these prior methods have been used to fabricate structures on the nano- and microscales, there has not been a general, ﬂexible method for 3D patterning that spans the entire range of sizes.
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