A team from the University of Alberta has developed a new way of creating molecular junctions, which are ultra-thin layers of organic molecules sandwiched between two carbon electrodes. The discovery could lead to novel electrochemical sensors or other molecular electronic devices.

While silicon is currently the basis for all computer chips, many organic molecules have unique electronic properties that silicon lacks. “Molecules can do things like chemical sensing and interaction with light or the local atmosphere,” says Richard McCreery, a chemistry professor and senior research officer at the National Institute for Nanotechnology at U of A. “And if you make the layers as thin as a single molecule, the interactions are governed by quantum mechanics, which leads to more new properties,” McCreery says. 

Molecules with a diazonium group on one end and an alkyne group on another can serve as a ‘primer’ to build up ultra-thin layers of electronically active molecules. In this example ferrocene is added to a layer of ethynylbenzene diazonium salt. The method could allow for new types of chemical sensors like ‘electronic noses.’  Photo credit: Richard McCreery

McCreery’s group creates single-molecule layers on conductive carbon surfaces by reacting them with organic compounds that contain diazonium (N2+) groups. Electrons conducted through the carbon reduce the diazonium to inert nitrogen gas, while the rest of the molecule forms a stable carbon-carbon bond. “That’s fine, except diazonium reduction is limited to a certain class of aromatic molecules,” says McCreery. His group’s latest innovation, published in the Journal of the American Chemical Society, makes use of molecules with a diazonium group on one end, and alkyne (C ≡ CH) groups on the other. Alkynes are known to participate in so-called ‘click chemistry’ reactions making it simple and easy to add other molecular layers above the first. “You can think of it as a primer, just like in paint, except the primer layer is only one or two nanometres thick,” says McCreery. The primer layer could be topped with a wide range of molecular structures before adding the top contact, which is also made of carbon.
The ability to build up the junction layer by layer could allow for novel electronic devices. For example, ‘electronic noses’ could change their conductivity by reacting with toxins or other biomolecules. “It’s like a box of parts, each of which gives you unusual electronic behaviours,” he says. “When you combine them, you can do things that you could never do with silicon.”