Jan 29, 2014
Electrons in chains
Semiconducting π-conjugated polymers are promising materials for applications such as light-emitting diodes, field-effect transistors, photovoltaics and optoelectronics devices thanks to their easily tuneable electrical, optical and magnetic properties. The organic materials are also easy to fabricate and are mechanically strong. Now, a team of researchers at the University of California at Berkeley and the Lawrence Berkeley National Laboratory has come up with a new way to make these polyacetylene-derived polymers and has also analysed individual chains of oligo-(E)-1,1′-bi(indenylidene) using advanced scanning probe microscopy techniques. The study has revealed how oligomer chains actually form and provides new insights into the relationship between the chemical structure and the electronic properties of the chain molecules.
“Our work will help in the design of devices based on conducting polymers with tailored electronic properties,” team member Alexander Riss told nanotechweb.org, “and our new approach to make these polymers may even provide a route to making new graphene-based nanostructures on insulating substrates.”
The team, led by Michael Crommie and Felix Fischer, produced individual chains of oligo-(E)-1,1′-bi(indenylidene) by heating enediyne precursors followed by radical step-growth polymerization. This is the first time that a surface-supported polyacetylene derivative has been synthesized in such a chemical reaction. The researchers then used scanning tunnelling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to characterize the semiconducting polymer. These techniques rely on a very sharp probe, or metal tip, that is brought extremely close to the sample (less than a few atomic diameters away).
Extended electronic state
In STM, measuring the quantum-mechanical tunnelling current between the tip and sample provides information on the electronic states of the polymer. nc-AFM, for its part, can be used to directly image the chemical structure of molecules at the surface of the sample. For these techniques to work properly, specially modified tips are needed and various measurement parameters such as the tip-sample distance must be controlled, explains Riss. “Our experiments reveal a detailed picture of how the polymer chains form,” he says. “We have observed an extended electronic state emerging in the oligomer chains. The energy of this state correlates with the length of the chains.”
These low-energy electronic states develop thanks to an efficient π-orbital overlap between monomer building blocks, he adds, and increased spatial delocalization is intimately associated with a decrease in oligomer electronic energy, as confirmed by our theoretical simulations.
The California team says that it will now be looking at synthesizing graphene nanoribbons on insulating surfaces using its new chemical approach. “Graphene nanoribbons (unique materials that go from being semimetal to semiconducting as their width decreases) should show a variety of interesting and technologically useful properties,” said Riss. The materials could be used in high-performance nanoelectronics devices, such as high-frequency transistors and sensors, and could also be ideal as interconnects in nanoelectronics circuits.
The present work is detailed in Nano Lett. DOI: 10.1021/nl403791q.
Unzipped graphene reveals its secrets (May 2011)
'Atomic collapse' seen in graphene (Mar 2013)
Patterning and alignment of π-conjugated polymer (May 2010)
Conjugated polymers make strong candidates for future spintronic applications (Nov 2011)
About the author
Belle Dumé is contributing editor at nanotechweb.org