High-aspect-ratio (HAR) structures have become highly desirable motifs for scientists and engineers. Many examples of HAR materials have been found in nature, such as the hair on geckos’ feet, and there are many potential applications for materials based on these structures. Arrays of HAR nanoneedles with diameters of close to 20 nm have previously been produced by nanoimprint lithography using nanopore molds. Chien-Chong Hong and colleagues at the National Tsing Hua University and National Nano Device Laboratories in Taiwan have now used specially developed silicon ‘nanograss’ molds to cast arrays of polymer nanoneedles smaller than 20 nm in diameter.1

A nanoneedle coating of cyclic olefin copolymer prepared using a silicon nanograss mold is highly transparent and superhydrophic, causing droplets of water to stand almost spherical on its surface.

Hong and his co-workers have been working on creating a disposable microfluidic biochip. Such applications demand coatings with high hydrophobicity and transparency, and the research team had been investigating a cheap and simple fabrication method that could be used to prepare transparent and hydrophobic surfaces with tunable surface properties. HAR nanostructures such as nanopillars provide high hydrophobicity, but existing methods for their synthesis reduce the transparency of the biocompatible organic materials used for such coatings. “We realized that by scaling down the microstructure of cyclic olefin copolymer, we could achieve superhydrophobicity without reducing transparency,” explains Hong.

Cyclic olefin copolymer has excellent biochemical and optical properties, but fabricating superhydrophic surfaces using it has proved challenging. “Silicon nanopore arrays can be used as a mold to create nanopillars, but it is difficult to get the plastic to flow into the pores,” says Hong. Taking an alternative approach, the team instead prepared silicon wafers of nanoneedles by hydrogen plasma etching followed by surface treatment to facilitate demolding. The resulting nanograss mold could be used to hot-press a complementary plastic surface. The plastic chosen, cyclic olefin copolymer, is fluid enough to diffuse among the blades of nanograss.

Nanoneedle arrays of varying aspect ratios could be made by controlling the temperature of the mold during hot-pressing. With the mold temperature set at 60 °C, above the glass transition temperature of the plastic, the researchers created arrays of HAR nanoneedles, which could be separated from the silicon mold by etching away the silicon with tetramethylammonium hydroxide.