Nanoskiving: A New Method to Produce Arrays of Nanostructures

Entry by Pichet Adstamongkonkul, AP 225, Fall 2011

Reference:

Title: Nanoskiving: A New Method to Produce Arrays of Nanostructures

Authors: Q. Xu, R. M. Rioux, M. D. Dickey, G. M. Whitesides

Journal: Accounts of chemical research, December 2008, Vol. 41, No. 12

Keywords: nanoskiving, microtome, top-down approach

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Summary

Nanoskiving is a novel fabrication technique which combines the deposition of thin films on a master substrate, with sectioning using ultramicrotome. By this technique, one can create a pre-defined, freestanding nanostructures on nonplanar surfaces, which is challenging for the conventional top-down techniques, such as photolithography and e-beam lithography, although there are limitations of cost, availability of facilities and material. The master stamp, however, can be created from those conventional procedures.

Methodology

First, an epoxy solution is deposited onto the poly(dimethylsiloxane) (PDMS) stamp, which can be either flat or topographically patterned, and allowed to cure, forming a layer of epoxy. A nanometer-thick film of metal is then deposited on the epoxy layer by any deposition technique, and another layer of epoxy is deposited on top, resulting in an epoxy block with an embedded thin film. The block is then cut using an ultramicrotome, usually made of glass or diamond knife, producing sections with the thickness of less than 100 nm. Eventually, the sections are transferred to another solid substrate and the epoxy is removed by using oxygen plasma, leaving behind the nanostructure on the surface.

Considerations

Embedded Materials Selection

The embedding materials need to have appropriate mechanical properties for sectioning process to be performed in the room temperature. Some desirable properties:

• Have large Young's modulus in order to withstand force from ultramicrotome (i.e. "brittle" materials like poly(methylmethacrylate) (PMMA), polystyrene, epoxy-based resins)
• Have flexibility to allow section bending at an angle <math>\sim 90^o</math> during sectioning.
• Must provide support to the embedded material
• Must be removed easily and quickly by etching
• Must adhere to the embedded material sufficiently to prevent delamination during sectioning

Embedded Material Choices

The mechanical properties of the embedded materials are also important. If the material is malleable, there won't be much damage inflicted on the material from sectioning. However, the more-brittle material would crack or even break, the problems which could be alleviated by using proper selection of knife and embedding material.

Several techniques used to deposited the material depend on the type of material to be deposited;

• Metal
  • Physical vapor deposition
  • Chemical vapor deposition
  • Atomic layer deposition

• Organic materials - i.e. conductive and electroactive polymers
  • (best method) Spin-coating

Knife can be made from a pool of materials.

• Commonly made from glass or high-quality, natural diamond (more expensive)
  • Glass knives are typically for one-time use, as they dull quickly
  • High quality sections would require diamond knife with the radius of curvature of the sharp edge of 3-6 nm
  • Diamond knives have longer durability.
• Knives can be dulled fast, if used on materials they are not suited for.

Ultramicrotome sectioning

There are two postulated mechanisms possibly involve in generating nanometer-thick slaps.

• Direct-sectioning ("true-sectioning"): the edge of the knife remains in contact with the epoxy block. This is believed to be the primary mechanism for soft, plastic materials
• Cleavage-fracture: the edge of the knife initiates a crack that propagates in the sample block. This is thought to be responsible for sectioning hard, brittle materials

For the collection of the sections, the so-called wet sectioning technique is the most common, as the sections float on the water surface, which reduces damage by reducing the friction between the back edge of the knife and the epoxy section. The transfer of the sections to solid substrates can be done by either using the collection loop to form water meniscus to support the sections, or submerging the substrate underneath the sections and pulling the substrate toward the sections.

Applications

The fabrication of more complex nanostructures can be tailored using the nanoskiving technique by

• Choose the topography of the substrate
• Rationally select the orientation

This thus makes the nanoskiving technique, in principle, a nanomanufacturing process.

As photolithography defines one dimension, the dimension can be varied and controlled with the limits of the patterning techniques. In perpendicular sectioning, one advantage is that a single block can be sectioned to form multiple samples. In contrast, the parallel sectioning generates large arrays of nanostructures in a single section. However, only a limited number of individual sections can be generated from a single sample because of the constraint on the aspect ratio of the features fabricated by photolithography. The limitation can be overcome by using other fabrication techniques.

Nanoskiving can be used to fabricate:

• Frequency-selective surface-plasmonic nanowires
• Array of nanoslits
• Addressable nanoelectrode embedded in a polymer matrix
• Stack of multilayer structures - functional quasi-3D materials