Name: fabricating nanogaps by nanoskiving
Uploaded: 2013-05-13T13:00:00
Description: Watch this Scientific Journal Video about Fabricating Nanogaps by Nanoskiving at JoVE.com

This work is part of the Joint Solar Programme (JSP) of Hyet Solar and the Stichting voor Fundamenteel Onderzoek der Materie FOM, which is part of the Netherlands Organization for Scientific Research (NWO).

1. Preparation of a Block for Sectioning
<ol>
 <li>Treat a technical-grade 3" silicon wafer in an air plasma cleaner for 30 sec and then expose it to (tridecafluoro-1,1,2,2,-tetrahydrooctyl)trichlorosilane vapor for one hour. Note: This step is necessary prior to step 1.4 to prevent the epoxy from adhering to the silicon wafer. </li>
 <li>Deposit a layer of gold (usually 100 nm-thick, which defines the width of the wires) through a Teflon master (that defines the length of the resulting wires; 0...

<ol>
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In this paper we demonstrated the fabrication of nanogap structures using nanoskiving. This experimentally simple method enables the production of nanostructures at the rate of about one per second, with control over all three dimensions. The gap-size is defined by incorporating either sacrificial layers of aluminum and silver or self-assembled monolayers of dithiols (which affords a resolution as small as &Aring;). The nanostructures can be positioned by hand on any arbitrary substrate and they are directly electrical...

This paper describes the fabrication of electrically addressable, high-aspect-ratio nanowires of gold separated by gaps of single nanometers using vacuum-deposited aluminum and silver as a sacrificial spacer layers for gaps > 5 nm and self-assembled monolayers (SAMs) of alkanedithiols for gaps as small as 1.7 nm. We fabricated these nanostructures without a clean room or any photolithographic processes by sectioning sandwich structures of gold separated by a sacrificial spacer using an ultramicrotome, a form of edge lithography known as nanoskiving.1-3 This method is a combination of the deposition of thin metal films and sectioning using an ultramicrotome. The main step in nanoskiving is slicing thin sections with an ultramicrotome equipped with diamond knife which is attached to a boat full of water to produce slabs that are as thin as ~ 30 nm. Ultramicrotomes are used extensively for the preparation of thin samples for imaging with optical or electron microscopy and many of the most experience practitioners of ultramicrotomy come from a biological or medical background. There are several methods of fabricating nanogaps including mechanical break junctions,4 electron-beam lithography5, electrochemical plating,6, 7 electromigration,8 focused ion beam lithography,9 shadow evaporation,10 scanning probe and atomic force microscopy,11 on-wire lithography,12 and molecular rulers.13 All of these methods have their own characteristics and applications but producing and addressing nanogaps both in useful numbers and with precise control over the dimensions of the gap remains a challenge. In addition these methods have high operating costs, they are limited to the class of materials that can survive the etching processes, and are limited in resolution. Nanoskiving enables the rapid fabrication of electrically-addressable nanowires with spacings of single nanometers on the bench-top. We are interested in the rapid prototyping of nanostructures for Molecular Electronics, for which the nano-fabricated electrodes do not require specialized or time-consuming techniques;14 once a block is made, it can produce hundreds of thousands of nanostructures, (serially) on demand. However, the technique is not limited to SAMs or Molecular Electronics and is a general method for preparing a gap between two nanostructures. In this paper we use silver, aluminum, and SAMs as sacrificial layers to produce gaps of various sizes between gold nanowires, but the technique is not limited to these materials (or to metallic nanowires). The wires are pick-and-place and are compatible with magnetic alignment, thus they can be placed on arbitrary substrates.15 Another strength of nanoskiving is that it affords control over all three dimensions. The dimensions of the samples are determined by the topography of the substrate (X), the thickness of the deposited film (Y) and the thickness of the slab produced by the ultramicrotome (Z). Figure 1 summarizes the procedure used to produce the nanowires with the defined spacing. Gold features (1-2 mm in length) are deposited by evaporation through a Teflon mask onto a silicon substrate. Epofix (Electron Microscopy Sciences) epoxy pre-polymer is poured over the entire wafer, covering the gold features, when the epoxy is cured, the epoxy is separated from the wafer (i.e. via template stripping); the gold features remain adhered to the epoxy. For metallic sacrificial layers, aluminum or silver is evaporated with the desired thickness through the Teflon mask with an offset of 200 - 500 &mu;m over the gold features. To produce sub-5 nm gaps, a SAM is formed by submerging the gold features in a 1 mM ethanolic solution of the appropriate dithiol overnight. A second set of gold (or another metal) is deposited by placing the Teflon shadow mask over the first layer of gold features (covered in silver, aluminum or a SAM) with an offset of 200 - 500 &mu;m with respect to the first evaporation. This offset will eventually define the longest dimension of the gap, and it can be accurately measured using a micro-ruler before embedding the entire structure in epoxy for sectioning. Then the whole structure is embedded in a block of epoxy which then could be ready for sectioning with the ultramicrotome. The sample arm holds the prepared block as the diamond knife advances towards it in controlled steps that will define the thickness of the slabs. The resulting section floats on the water in the boat.

<table border="1">
<tbody>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Reagent/Material</td>
 </tr>
 <tr>
 <td>Epofix epoxy resin </td>
 <td> Electron Microscopy </td>
 <td> 1232 </td>
 <td> </td>
 </tr>
 <tr>
 <td></td>
 <td> Sciences </td>
 <td> </td>
 <td> </td>
 </tr>
 <tr>
 <td>Gold </td>
 <td> Schone Edelmetaal B.V</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Aluminum </td>
 <td> Umicore Materials AG </td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Silver </td>
 <td> Umicore Materials AG </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td>(tridecafluoro-1,1,2,2, </td>
 <td> ABCR GmbH co.KG</td>
 <td> 78560-45-9 </td>
 <td></td>
 </tr>
 <tr>
 <td>-tetrahydrooctyl) </td>
 <td> </td>
 <td> </td>
 <td> </td>
 </tr>
 <tr>
 <td>trichlorosilane </td>
 <td></td>
 <td> </td>
 <td> </td>
 </tr>
 <tr>
 <td>,12-dodecanedithiol </td>
 <td> Home-synthesised </td>
 <td> </td>
 <td>According to: Akkerman et. al., Nature. 441, 69-72 (2006)</td>
 </tr>
 <tr>
 <td>,14-tetradecanedithiol </td>
 <td> synthesized in house </td>
 <td> </td>
 <td> According to: Akkerman et. al., Nature. 441, 69-72 (2006)</td>
 </tr>
 <tr>
 <td>,16-hexadecanedithiol </td>
 <td> synthesized in house </td>
 <td> </td>
 <td>According to: Akkerman et. al., Nature. 441, 69-72 (2006)</td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Equipment</td>
 </tr>
 <tr>
 <td>Thermal deposition system </td>
 <td> home-built </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td>Ultramicrotome </td>
 <td> Leica Microsystems </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td>Dimanod knife ultra 35 </td>
 <td> Diatome </td>
 <td> DU3540</td>
 <td></td>
 </tr>
 <tr>
 <td>Dimanod knife ultra 45 </td>
 <td> Scimed GMBH </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td>Scanning electron microscope </td>
 <td> JOEL </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td>Source meter </td>
 <td> Keithley </td>
 <td> </td>
 <td></td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Table 1. Tables of Specific Reagents and Equipment.</td>
 </tr>
 </tbody>
</table>

fabricating nanogaps by nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

We prepared nanogap structures by incorporating two metallic sacrificial layers as the spacer: aluminum and silver. We etched these layers to obtain gaps of the desired thicknesses. As described in the Protocol section, after sectioning we exposed the structures containing silver to oxygen plasma, and those containing aluminum to aqueous HCl. Figure 2 shows scanning electron micrographs (SEMs) of the resulting nanowires with nanometer-scale separation. In both cases gaps are clearly visible and directly ...

Watch this Scientific Journal Video about Fabricating Nanogaps by Nanoskiving at JoVE.com

Fabricating Nanogaps by Nanoskiving

The fabrication of electrically addressable, high-aspect-ratio (> 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

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