An artist's impression of a self-assembled layer of functionalized buckyballs. The fullerenes attach to the metal surface, while the glycol-ether tails induce self-assembly of a bilayer. Image: Xinkai Qiu, Stratingh Institute for Chemistry, University of Groningen.
An artist's impression of a self-assembled layer of functionalized buckyballs. The fullerenes attach to the metal surface, while the glycol-ether tails induce self-assembly of a bilayer. Image: Xinkai Qiu, Stratingh Institute for Chemistry, University of Groningen.

Organic self-assembled monolayers (SAMs) have been around for over 40 years, and the most widely used variety is based on thiols (sulfur-containing groups) bound to a metal surface. But although these thiol SAMs are very versatile, they are also chemically unstable. When exposed to air, they become oxidized and break down within a single day.

Scientists at the University of Groningen in the Netherlands have now created SAMs using the spherical carbon nanomaterials called buckyballs, also known as fullerenes, functionalized with 'tails' of ethylene glycol. These molecules produce self-assembled monolayers that have all the properties of thiol SAMs but remain chemically unchanged for several weeks when exposed to air, making them much easier to use in research and in devices. The scientists report their work in a paper in Nature Materials.

Self-assembled monolayers are dynamic structures, explains Ryan Chiechi, associate professor of organic-materials chemistry and devices at the University of Groningen. “These monolayers self-repair and the molecules will continually find the most efficient packing,” he says. “Furthermore, all processes are reversible, and it is possible to change their composition.” This distinguishes SAMs from other monolayers that are used to functionalize surfaces: “These are often very stable, but they don't self-assemble and lack the dynamics of SAMs.”

SAMs based on the binding of thiols to metal are widely studied and used. Applications of SAMs range from the control of wetting and adhesion on surfaces, to creating chemical resistance in lithography, to sensor production or nanofabrication. They can also be used to produce molecular electronics.

“Electric current will pass through such a monolayer by quantum tunneling,” says Chiechi. “And small modifications to the molecular layer can alter the tunneling properties. Through such chemical tailoring, it is possible to create new types of electronics.”

The problem with thiol-based SAMs, however, is that they are sensitive to oxidization when exposed to air. Without protection, they will not last a single day. “This means that you need all kinds of equipment to keep the air out when working with these SAMs for molecular electronics,” explains Chiechi. “It also makes it difficult to use them in a biological context.”

This is where the new buckyball-based SAMs come in. In a joint effort, scientists from the Stratingh Institute for Chemistry and the Zernike Institute for Advanced Materials at the University of Groningen have discovered and characterized the properties of glycol-ether functionalized fullerenes. The buckyballs adhere more strongly to metal surfaces than thiols, and because the glycol-ether tails are polar, they naturally form a bilayer when immersed in an organic solvent.

“You simply put the metal in a solution of these functionalized buckyballs and the bilayer will form through self-assembly,” says Chiechi. Furthermore, SAMs prepared in this way are very resistant to oxidization: when left exposed to air, they will remain intact for at least 30 days.

“Our results strongly suggest that the tails of the molecules are intertwined. This results in a stable and very dynamic structure where molecules are free to move, which is typical for a SAM,” says Chiechi.

The outer layer of these SAMs can be replaced by adding other functionalized groups. Chiechi and his colleagues connected spiropyrans (molecules that change shape when exposed to UV light) to the glycol-ether tail. By placing an electrode on the outer layer, they could measure quantum tunneling through the SAM. The scientists then showed that changing the shape of the spiropyran moiety with light also changed the conductance of the SAM by several orders of magnitude.

There are other alternatives for thiol-based SAMs but they all have limitations. “We believe that our SAMs have all the properties of thiol-based SAMs, with resistance to degradation by air as a large bonus,” concludes Chiechi. “Furthermore, we have shown that our system can be used to create molecular electronics.”

The system also appears to be a very useful platform for studying the behavior of SAMs. “You can do this on your lab bench without any need for protection,” says Chiechi. He thinks these SAMs might be useful for studying the behavior of bilayers in general, including the lipid bilayers that form cell membranes.

The ability to change the composition of the SAMs opens up interesting applications in molecular electronics. “This might be used to create a topological computer architecture, for neuromorphic computing,” suggests Chiechi.

Changes in the composition of the SAM could produce a memristor and possibly a system for stochastic computing, which uses the probabilities of 1s and 0s to represent numbers in a bitstream. “This could be represented by the fraction of one type of molecule in the SAM,” Chiechi says. Before this can become a reality, however, more work will have to be done to understand, for example, why the glycol-ether phase is such an efficient tunneling medium.

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.