“For molecular electronics to become viable, a simple, reliable, parallel method of fabricating molecular junctions at the wafer scale is necessary,” James Kushmerick of the Naval Research Laboratory told nanotechweb.org. “Magnetic directed-assembly is very promising from this perspective, since thousands of junctions can be formed simultaneously.”
To carry out their technique, Kushmerick and colleagues used 1.5 µm-diameter silica microspheres coated on one side with a 50 nm layer of nickel and 10 nm layer of gold. The nickel makes the spheres magnetic while the gold coating provides an oxide-free surface for connecting to organic molecules.
The researchers aligned the spheres on arrays of magnetic probe tips separated by a 1 µm gap. They also applied an external magnetic field to get the best results. The locally intense magnetic field in the gap between the tips attracted the metallized microspheres. Around 60% of the gaps became filled by a single bead while 20-30% gained two or three beads and 10-20% remained empty.
To create molecular junctions, the scientists coated the gold surface of the magnetic probe tips with self-assembled monolayers of undecanethiol (C11), oligo(phenylene ethynylene), or oligo(phenylene vinylene). These materials are potential candidates for molecular wires.
Adding a microsphere created two molecular junctions in series. Electrical measurements enabled the scientists to estimate that around 300 molecules were contacted at each sphere-electrode contact. If two microspheres bridged an electrode gap, parallel molecular junctions resulted.
“This work demonstrates how relatively simple techniques of optical lithography, vapour metallization, and solution-based self-assembly processes can be used to create molecular junctions a few nanometres in thickness quickly, accurately and in high yield,” said David Long, formerly at the Naval Research Laboratory and now at Geo-Centers. “In addition, use of magnetic entrapment can be considered a ‘soft’ self-assembly technique which will not chemically damage or alter the active molecules trapped in fabricated devices, thus giving greater confidence to a researcher as to exactly what is being measured.”
The team says that the technique also offers potential for making hybrid complementary metal-oxide semiconductor/molecular electronic devices.
“We intend to use magnetic directed-assembly to screen potential molecular electronic components,” said Kushmerick. “To date, a small number of molecular structures have been investigated for molecular electronic applications. The ability to rapidly create thousands of junctions will enable us to screen larger libraries of molecules.”
The researchers reported their work in Applied Physics Letters.