Carbon Nanotubes Self-Assemble Into Tiny Transistors

Carbon nanotubes can be used to make very small electronic devices, but they are difficult to handle. University of Groningen (Netherlands) scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes. The results look deceptively simple: a self-assembled transistor with nearly 100 percent purity and very high electron mobility. But it took ten years to get there. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor.

polymer wrapped nanotube

In our previous work, we learned a lot about how polymers attach to specific carbon nanotubes, Loi explains. These nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon. ‘Depending on the way the sheets are rolled up, they have properties ranging from semiconductor to semi-metallic to metallic.’ Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

We had the idea of using polymers with thiol side chains some time ago‘, says Loi. The idea was that as sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. While Loi was working on the problem, IBM even patented the concept. ‘But there was a big problem in the IBM work: the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them.’

Loi’s solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. ‘What we have now shown is that this concept of bottom-up assembly works: by using polymers with a low concentration of thiols, we can selectively bring semiconducting nanotubes from a solution onto a circuit.’ The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.

Over the last years, we have created a library of polymers that select semiconducting nanotubes and developed a better understanding of how the structure and composition of the polymers influences which carbon nanotubes they select’, says Loi. The result is a cheap and scalable production method for nanotube electronics. So what is the future for this technology? Loi: ‘It is difficult to predict whether the industry will develop this idea, but we are working on improvements, and this will eventually bring the idea closer to the market.’

The results were published in the journal Advanced Materials on 5 April.

How To Produce Graphene Massively

With properties that promise faster computers, better sensors and much more, graphene has been dubbed the ‘miracle material’. But progress in producing it on an industrial scale without compromising its properties has proved elusive. University of Groningen (Netherlands) scientists may now have made a breakthrough.

Graphene is a special material with crystals that are just one atom thick. Electrons pass through it with hardly any resistance at all, and despite being very flexible, it is stronger than any metal. The discoverers of graphene, Andre Geim and Konstantin Novoselov, famously made it by peeling graphite with Scotch tape until they managed to isolate a single atomic layer: graphene. It won them the 2010 Nobel Prize in Physics.

The challenge is to find a substrate that not only preserves the properties of graphene, but also enables scalable production’, says Stefano Gottardi, PhD student at the University of Groningen Zernike Institute for Advanced Materials. A good candidate is chemical vapour deposition. Here heat is used to vaporize a carbon precursor like methane, which then reacts with a catalytically active substrate to form graphene on its surface. A transition metal is normally used as the substrate. However, not only does the transition metal act as a support, but it also tends to interact with the graphene and modify – or even deteriorate – its outstanding properties. To restore these properties after growth on the metal, the graphene has to be transferred to a non-interacting substrate. Gottardi and his colleagues have managed to successfully grow graphene on copper oxide. This achievement together with an in-depth characterization of graphene’s properties will be published in Nano Letters.


Drink Coffee To Fight Cancer

A team of researchers from the University of Groningen – Netherlands – and the Université de Bourgogne – France – stated that combining a caffeine-based compound with a small amount of gold could be used as an anticancer agent.
Angela Casini, Ewen Bodio, Michel Picquet and colleagues note that caffeine and certain caffeine-based compounds have recently been in the spotlight as possible anticancer treatments. But drinking gallons of coffee, sodas and energy drinks isn’t the solution. And the regular caffeine in these drinks would start to have negative effects on healthy cells, too, at the levels necessary to kill cancerous ones. Gold also can wipe out cancer cells, but, like caffeine, it can harm healthy cells. So, the research team put the two together into certain configurations to see whether the new caffeine-based gold compounds could selectively stop cancer cells from growing without hurting other cells. They made a series of seven new compounds, called caffeine-based gold (I) N-heterocyclic carbenes, in the laboratory and studied them. The scientists found that, at certain concentrations, one of the compounds of the series selectively killed human ovarian cancer cells without harming healthy cells. In addition, the compound targeted a type of DNA architecture, called “G-quadruplex,” that is associated with cancer.
What we need is to design precisely a compound which will present a maximum of efficiency to destroy cancerous cells without harming healthy tissues“, says Ewen Bodio, from the Université de Bourgogne, one of the co-authors of the study. “We did test in laboratory on tissues and cancerous cells. The next step will be the administration of the compound to mice. If the results are positive, then perhaps after 5 years we will try human tests“.

The findings are published in the ACS journal Inorganic Chemistry .