Novel technique opens path to low-cost 3D nanomanufacturing

The silicon nanomolds were fabricated at the Birck Nanotechnology Centre in Purdue's Discovery Park by a research group led by Minghao Qi, an associate professor of electrical and computer engineering.

"It is counter-intuitive to use silicon for moulds because it is a pretty brittle material compared to metals," Qi said. "However, after we deposit an ultrathin layer of aluminum oxide on the nanomolds, it performs extremely well for this purpose. The nanomolds could be reused many times without obvious damage. Part of the reason is that although the strain rate is very high, the shock pressure applied is only about 1-2 gigapascals."

The shapes were shown to have an "aspect ratio" as high as five, meaning the height is five times greater than the width, an important feature for the performance of plasmonic metamaterials.

"It is a very challenging task from a fabrication point of view to create ultra-smooth, high-fidelity nanostructures," Qi noted. "Normally when metals recrystallise they form grains and that makes them more or less rough. Previous trials to form metal nanostructures have had to resort to very high pressure imprinting of crystalline metals or imprinting amorphous metal, which either yields high roughness in crystalline metals or smooth surfaces in amorphous metals but very high electrical resistance. For potential applications in nanoelectronics, optoelectronics and plasmonics you want properties such as high precision, low electromagnetic loss, high electrical and thermal conductivity. You also want it to be very high fidelity in terms of the pattern, sharp corners, vertical sidewalls, and those are very difficult to obtain. Before Gary's breakthrough, I thought it unlikely to achieve all of the good qualities together."

The paper was authored by Purdue doctoral students Huang Gao, Yaowu Hu, Ji Li, and Yingling Yang; researcher Ramses V. Martinez from Harvard and Madrid Institute for Advanced Studies; Purdue research assistant professor Yi Xuan, Purdue research associate Chunyu Li; Jian Luo, a professor at the University of California, San Diego; Qi and Cheng.

Future research may focus on using the technique to create a roll-to-roll manufacturing system, which is used in many industries including paper and sheet-metal production and may be important for new applications such as flexible electronics and solar cells.

The work was supported by the National Science Foundation, National Institutes of Health, Defence Threat Reduction Agency, Office of Naval Research and the National Research Council.

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