Molecular electronics provides a route to extend silicon-based technology and,
as shown here, self-assembly offers a simple, scalable, geometrically-independent method of creating molecular layers on gold and silicon substrates. Moreover, reaction of chemical components at the surface of the film provides a convenjent method of building complex electroactive molecules and overcomes the solubility problem with long molecular wires. Using an in-situ stepwise synthetic approach, in this case by coupling components via imine groups, a monolayer can be extended to form complex structures. With careful selection of components, functionality can be introduced and electrical properties tuned.
Quartz crystal microbalance and X-ray photoelectron spectroscopy were used
to verify the assembly of molecules onto gold-coated substrates and the subsequent presence of imino groups. The electrical characteristics of various wires were investigated, using scanning tunnelling spectroscopy alongside an additional method that has been developed, using an electromagnetic cantilever. A two-component two-part wire, containing electron-accepting and electron-donating moieties, demonstrated weak rectifying characteristics following the Aviram-Ratner scheme with a current ratio of 4 at ± 1 V. Other systems investigated, for example, by assembling up to seven component units, produce symmetrically conducting molecular wires. Current magnitudes through these wires are ca. 1 nA at ±1 V using a scanning probe.
Significant to molecular electronics, this stepwise imino-coupling technique has
been used to bridge electrode gaps. For the first time, using silicon structures,
molecules have been covalently bonded to the top and bottom electrodes and then linked to form a molecular bridge. Empty devices, and those with electrodes coated using the aldehyde terminated 4-ethynlybenzaldehyde anchoring molecule exhibit leakage currents in the sub-picoampere range. Bridging occurred after the .insertion of different amino-terminated molecules, creating wires that were either ca. 3 nm or 6 nm in length. Surface roughness was relied on to produce gaps to match the molecules. The devices exhibit symmetrical I-V curves with lower limiting currents of 12 nA and 10 nA respectively at ± 1 V, which probably correspond to a single bridge or a few connecting molecules. Gold nanogaps have been bridged previously, but this is the first time that a silicon nanogap bas been bridged and the molecule covalently bonded to each silicon electrode.