Enriching libraries of high-aspect-ratio micro- or nanostructures by rapid, low-cost, benchtop nanofabrication

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Wiki Entry by Daniel Rubin, AP225, 11/27/2012

General Information

Authors: Philseok Kim, Wilmer E. Adorno-Martinez, Mughees Khan, Joanna Aizenberg

Title: Enriching Libraries of High-Aspect-Ratio Micro- or Nanostructures by Rapid, Low-Cost, Benchtop Nanofabrication

Publication: P. Kim, et al. Enriching Libraries of High-Aspect-Ratio Micro- or Nanostructures by Rapid, Low-Cost, Benchtop Nanofabrication. Nature Protocols, 7, 2, 311

Key Words: high-aspect-ratio, low cost, nanofabrication

Summary

The Aizenberg Lab has become well known in the field of soft-lithography, specifically with respect to the fabrication of high-aspect-ratio (HAR) nanostructures. Traditional synthesis of HAR nanostructures relies on the use of top down lithographic methods. While these methods work and provide a lot of functionality, they are slow and expensive. Often, the property that one seeks (super-hydrophobicity, specific cell response, optical activity), etc relies on a very specific pattern with specifically sized-features. To create the complete combinatorial set of feature shape and size by traditional lithographic methods is a daunting task.

Figure: 1 - Overview and comparison of different STEPS methods.

In a recent study, the Aizenberg lab demonstrated that soft-lithography methods can be used to replicate the traditional silicon HAR substrates. By first providing a positive, and then a negative mold, polymer based copies of the original can be made quickly and inexpensively. They can begin to augment the patterns by stretching or shearing the polymer-based mold from which they cast the replicate structures. However, there is always a need for more specific control of design. That work is presented in this Nature Protocols... protocol.

Utilizing a method called STEPS (structural transformation by electrodeposition on patterned substrates), Kim et all are able to transform a single HAR substrate into a variety of different substrates. Figure 1 depicts the process by which different substrates are created. In each case, a metallic layer is laid on the polymer substrate and is used to electrodeposit conductive polymer. This polymer 'grafting' to the surface allows for the HARs to be expanded in a number of different ways. In STEPS I and STEPS IV, the metallic layer is sputter coated from above and evenly coats the structures. The variation in the eventual structures is due to variation in the electrodeposition conditions. In Steps II and III, the metallic layer is placed on the surface by evaporative methods that allow it to collect specifically on the lower portion of the structures. It can be symmetrically associated with the structure (leads to a post-to-cone transition STEP II), or associated with only one side (leads to bent structures - STEP III).

Figure: 2 - Example SEM images of STEPS modified structures.

All of these variations can be seen in Figure 2 by scanning electron microscopy. The applications of these structures, as described herein, are to control wetting, ice formation, mechanical properties, optical properties, and biological properties. Effectively, everything that you would investigate by creating substrates of different types by top-down methods.

Discussion

In this report, Kim et al. present an interesting new twist on HAR nanostructure fabrication. By grafting polymers to the surface of the structures and polymerizing them in different conditions, they are able to quickly augment their surface geometry in a number of different ways. These modifications have real impacts on the optical, mechanical, wetting behavior.

Reference

P. Kim, et al. Enriching Libraries of High-Aspect-Ratio Micro- or Nanostructures by Rapid, Low-Cost, Benchtop Nanofabrication. Nature Protocols, 7, 2, 311