Fabricating Nanogaps by Nanoskiving

Summary

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.

Abstract

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.

Introduction

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.....

Protocol

1. Preparation of a Block for Sectioning

1. 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.
2. 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; 0.5 mm, 1 mm, or 1.5 mm) onto the pre-treated silicon wafer.
3. Cover the entire wafer with ~ 8.5 ml of Epofix epoxy pre-polymer and cure for t....

Results

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 .......

Discussion

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 Å). The nanostructures can be positioned by hand on any arbitrary substrate and they are directly electrical.......

Materials

List of materials used in this article

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