Researchers have long sought to understand and mimic the biological systems in nature that contain multi-scale hierarchical nanostructures. One of the major obstacles in reproducing such biological systems is that there is no straightforward way to fabricate ultra-long, tiny, and free-standing nanowire structures over a relatively large surface area at a high rate and a low cost.
Published on the cover of Nanotechnology (Issue 16, Volume 25, 165301), researchers Ke Du, Ishan Wathuthanthri, Yuyang Liu, and Professor Chang-Hwan Choi at Stevens Institute of Technology teamed up with Professor Yong Tae Kang at Korea University have successfully created such artificial hierarchical nanostructures by utilizing a simple plasma etching technique for the first time.
“Our research is very exciting and for graduate students to see their work published on the cover of Nanotechnology is very important,” says Du. “If you publish in a journal such as this, your peers see you as a leader in the field.”
Dr. Chang-Hwan Choi explains that low-cost and simple, large-area (wafer-scale) nanopatterning techniques are essential to make the meaningful and broader impact of nanostructures in many science and engineering applications. The maskless oxygen plasma etching technique reported in the paper, exploiting the self-masking effects associated in the etching processes, will allow the fabrication of polymer nanostructures over a large area to be low-cost and high-rate.
“In this research, we have identified the key mechanisms associated with the self-masking effects,” says Choi. “Such advanced understanding will make the simple nanopatterning techniques more useful and controllable, opening up new application possibilities using polymer nanostructures such as superhydrophobic surfaces, biomaterials, energy, photonics/optics, and sensors."
For several years, scientists have argued the formation mechanism for high-aspect-ratio random nanostructures in plasma etching. One hypothesis is that large surface roughness is the main reason for the formation of polymer nanowires. However, K. Du et al. used atomic force microscope (AFM) to prove that the polymer surface has a surface roughness of less than 1 nm, which might not be enough to contribute to the formation of polymer nanowires. Instead, x-ray photoelectron spectroscopy (XPS) was used and showed that impurities including antimony, fluorine, aluminum, silicon, and their compound materials exist on the polymer surface after etching. Since all those impurities cannot be etched by oxygen plasma, they may serve as effective etching masks during the oxygen plasma etching and induce the formation of polymer nanowires.
The technique introduced by K. Du et al. provides exciting advancement in nanofabrication. In order to achieve sub 50 nm nanostructures, it normally requires low throughput and expensive facilities including electron beam lithography (EBL) and focused ion beam (FIB). However, the fabrication of sub 50 nm nanowires by using oxygen plasma etching is a fast and relatively inexpensive technique that could significantly reduce the costs for the fabrication of MEMS devices, optical devices, and microfluidic devices.
Non-wetting superhydrophobic surfaces are one of the immediate applications for the fabrication technique. Due to the lower solid fraction at the interface, more air can be trapped in the micro and nanostructures. This could reduce the surface energy at the interface and prevent liquid wetting.