“We were looking for a way to build a simpler molecular rectifier than the one suggested by Aviram and Ratner in 1974, where they used a more complicated molecule with a donor and an acceptor moiety linked by a short bridge,” Dominique Vuillaume told nanotechweb.org. “More generally, our group is exploring chemical self-assembly to fabricate nanoscale and molecular devices since this is a simple and versatile approach at a low cost.”
Vuillaume and colleagues made the devices by allowing vinyl-terminated alkyl chains to self-assemble into monolayers on a silicon substrate. They used chains of three different lengths - heptadec-16-en-1-trichlorosilane, tetradec-en-1-trichlorosilane, and oct-7-en-1-trichlorosilane.
The team oxidized the vinyl end-groups to give -COOH-terminated monolayers and then reacted the -COOH end-groups with one of two alcohols (phenylmethanol or thien-3-ylmethanol). This resulted in σ-π self-assembled monolayers (SAMs). To make the second electrode, the scientists evaporated a 10 nm-thick layer of aluminium on top of the molecules.
“This demonstrates a new way of making molecular rectifying diodes by using a geometrical asymmetry of the position of the π-electron group between the two electrodes of the electrode/molecule/electrode junction,” explained Vuillaume.
Testing the current-voltage characteristics of the various Si/σ-π SAM/Al junctions revealed that the rectification ratio (ratio of current density at -1 V to current density at 1 V) was between 2 and 37. The value did not appear to depend on the chemical nature of the two end-groups or the alkyl spacer length. The threshold voltage for rectification, meanwhile, was in the range -0.3 to -0.9 V.
The team says that its σ-π molecular diodes compete very well with the more sophisticated molecular diodes reported previously.
“Possible applications are in low-cost nanoscale electronics and in testing the feasibility of molecular circuits,” said Vuillaume. “Simple logic circuits based on diodes and resistors could be fabricated by combining chemical self-assembly and nanoscale patterning techniques. The sequential self-assembly we used may also be repeated to build other monolayers on top of the first one, leading to a possible 3D circuit architecture. Finally, the fact that these molecular diodes are made on a silicon substrate may lead to the implementation of molecular devices in hybrid circuits.”
One of the next steps for the research team is to reduce the experimental dispersion of the devices’ properties and to improve the fabrication yield. They are particularly keen to increase the typical rectification ratio to 30-40. “As usual in such organic monolayer devices, taking more care in making the top metal electrode will probably solve a part of this aim,” added Vuillaume. “Also, we are exploring chemical approaches to improving the structural organization of the pi-electron groups in the upper part of the device.”
The scientists, who reported their work in Nano Letters, are also investigating using other pi-electron groups, with different electronic structures, to control diode properties.