Pairs of Gold Electrodes with Nanometer Separation Performed over SiO$_2$ Substrates with a Molecular Adhesion Monolayer

TL;DR

This paper presents a room-temperature electromigration technique to create nanometer-scale gold electrode pairs on SiO2 substrates with a molecular adhesion monolayer, enabling the fabrication of single-molecule electronic devices.

Contribution

The study introduces a simple, room-temperature method for fabricating nanogaps using electromigration on gold wires with a molecular adhesion layer, avoiding low-temperature processes.

Findings

Achieved ~1 nm gaps between gold electrodes after electromigration.

Successfully fabricated and tested single-molecule junctions with 1,4-Benzenedimethanethiol.

Eliminated residual metal interlinks and simplified the fabrication process.

Abstract

Pairs of electrodes with nanometer separation (nano-gap) are achieved through an electromigration-induced break-junction (EIBJ) technique at room temperature. Lithographically defined gold (Au) wires are formed by e-beam evaporation over oxide coated silicon substrates silanized with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and then subjected to electromigration at room temperature to create a nanometer scale gap between the two newly formed Au electrodes. The Si-O-Si covalent bond at the SiO$_2$ surface and the Au-sulphur (Au-S) bond at the top evaporated Au side, makes MPTMS as an efficient adhesive monolayer between SiO$_2$ and Au. Although the Au wires are initially 2$\mu$m wide, gaps with length $\sim$1nm and width $\sim$5nm are observed after breaking and imaging through a field effect scanning electron microscope (FESEM). This technique eliminates the presence of any residual metal interlink in the adhesion layer (chromium or titanium for Au deposition over SiO$_2$) after breaking the gold wire and it is much easier to implement than the commonly used low temperature EIBJ technique which needs to be executed at 4.2 K. Metal-molecule-metal structures with symmetrical metal-molecule contacts at both ends of the molecule, are fabricated by forming a self-assembled monolayer of -dithiol molecules between the EIBJ created Au electrodes with nanometer separation. Electrical conduction through single molecules of 1,4-Benzenedimethanethiol (XYL) is tested using the Au/XYL/Au structure with chemisorbed gold-sulfur (Au-S) coupling at both contacts.