Fabrication of nanogaps and investigation of molecular junctions by electrochemical methods

In this work, an electrochemical method of fabricating and modifying nanogap electrodes on a silicon chip was developed. Electrochemical deposition was used to narrow a nanogap template and the electrodeposition process was optimized that the electrodeposited nanogap electrodes have needle-like surface morphology, which makes them suitable for assembling single Metal/Molecule/Metal molecular junctions. In addition, a setup combining mechanical controlled break junction (MCBJ) and electrochemical deposition was introduced, which combined the advantages of both techniques. By this setup, atomic scale junctions not only can be formed and broken repeatedly as those in MCBJ, but also can be regenerated easily by electrochemical deposition. A new type of molecular junctions based on electrostatic interactions was assembled. This type of molecular junctions can be used to investigate the charge transport properties across intermolecular electrostatic interfaces. The nanogap electrode arrays comprising twenty-nine pairs of facing nanoelectrodes were fabricated on a silicon chip by electron beam lithography. The nanogap electrodes had a width of 100 nm and the size of the nanogap formed by the facing nanogap electrodes was defined as 150 nm. These nanogap arrays cannot directly be used for assembling molecular junctions, but can be used as templates for following electrochemical modification. The electrochemical modification of the nanogap electrodes were performed in chloroauric acid solution and the morphology of the deposited gold clusters was investigated under various voltages. It was found that at low over-potentials (0.1 V), the deposited electrodes were observed to have needle-like surfaces and the radius of curvature of the nano-needle was as small as 2 nm [1]. The nanogaps formed by such nano-needles can reduce the possibility of integrating multiple molecules when assembling molecular junctions, and thus they are suitable to study transport properties through single molecules. The electrodeposited nanogaps have fixed gap sizes, that is not convenient for adjusting the nanogap size in situ. For this reason, the MCBJ techniques were introduced and combined with the electrochemical modification techniques. This setup features with low electrochemical leakage current(as low as 20 pA) at the electrolyte/substrate interfaces and makes it suitable to measure molecules of low conductivity in solutions. A new type of molecular junctions based on electrostatic interaction was assembled in solution on such electrochemical break junction set-up. The electrostatic interfaces play important roles in the biological macromolecule interactions and in the bio-sensor applications. The conductance through single molecular electrostatic interfaces is 3.4 x 10−5 G0, which is one order of magnitude smaller than the corresponding single covalent interface (3 x 10−4 G0), indicating that the electrostatic coupling is strong enough to form an electron transport pathway, but the electron transfer rate across the electrostatic interface is lower than the electron transfer rate across the covalent bond [2].