July 14, 2008 Confocal microscope image of a self-assembled monolayer of a polychlorotriphenyl methyl radical patterned on a quartz surface. This multifunctional molecule behaves as an electroactive switch with optical and magnetic response.
Tiny electronically active chemicals can be made to form ordered layers on a surface. These nanostructured layers may one day be used to build the components of electronics devices, such as transistors and switches, for a future generation of powerful computers based on molecules rather than silicon chips.
Speaking at the European Materials Research Society (EMRS) meeting in Strasbourg, SONS II scientist Marta Mas-Torrent explained the potential of nanotechnology: "Currently, there is a great interest in employing functional molecules as building blocks for preparing devices since this will facilitate the move towards device miniaturization."
On this scale, manipulating nanoscopic components requires skill and determination but by exploiting molecular self assembly, the researchers hope to build ordered layers just a single molecule thick using microcontact printing techniques borrowed from the electronics industry.
They are now creating different arrangements of monolayers on gold, silica, and other materials.
Mas-Torrent works with Nuria Crivillers and Concepcio Rovira in Jaume Veciana's group at CSIC, in Barcelona, Spain, and is a member of the Fun-SMARTs project of ESF's SONS initiative. In her talk, which won the symposium's most original research work, sponsored by Advanced Materials, she explained the importance of multifunctional organic radicals, molecules with a spare electron, such as polychorotriphenylmethyl (PTM) radicals, which can undergo self-assembly into these organised layers.
Organic free radicals are usually highly reactive because of their spare electron. The moment they come into contact with another molecule the electron triggers an often-explosive chemical reaction. PTM radicals are different because their spare electron is shrouded by bulky chlorine atoms that hinder any explosive behaviour.
PTM radicals are often highly coloured and exhibit fluorescence in the red region of the visible spectrum, colour and fluorescence always have the potential to be exploited in optical electronics devices. Just as importantly, PTM radicals are also electroactive. This means they can be easily and reversibly reduced (or oxidized) to their positively or negatively charged (cationic or anionic) species. The different oxidised and reduced forms of PTM are different colours but neither oxidized nor reduced form is magnetic or fluorescent.
Mas-Torrent explained the relevance of this clutch of changeable properties for her self-assembled monolayers (SAMs). "The preparation of SAMs functionalised with PTM radicals on substrates results in multifunctional surfaces which are electrochemically, optically and magnetically active," she said, "We have demonstrated that these SAMs can be used as chemical and electrochemical redox switches with optical and magnetic responses."
Mas-Torrent and her colleagues did not stop with standalone SAMs. They have now added long hydrocarbon side-chains to their PTMs and found that these can also self-assemble on a graphite surface. They then studied behaviour at the interface between the graphite surface and a liquid and found that the self-assembly process is hierarchical and can give rise to complex three-dimensional ordered nanostructures that form double rows composed by a magnetic core of radicals surrounded by the side-chains.
By modifying a surface with molecules that can switch between two states - bistable compounds - the team hopes to open up the possibility of using these systems in memory devices. Surfaces functionalised with PTM radicals will allow them to fabricate multifunctional surfaces which can be interconverted between two states that exhibit different optical and magnetic properties that can be used as read-out mechanisms.
"The ultimate goal is to employ these radical building blocks to construct nanometre-scale devices addressed to specific applications," explains Mas-Torrent. By immobilizing them on specially prepared surfaces they could control and observe electrical and magnetic behaviour and in the future perhaps hook them up to input and output devices.
Key to the team's success is the collaborative possibilities opened up by the program. "Veciana's group started working on the functionalisation of surfaces after the collaboration initiated with the group of Reinhoudt from the MESA+ Research Institute in Twente within the SONS Programme," explains Mas-Torrent. "The combination of the expertise of surface functionalisation from Twente with the expertise of functional molecules of Barcelona emerged in our recent results focused on the functionalisation of different surfaces with multifunctional molecules (paramagnetic, electroactive and fluorescent) which can act as molecular switches," Mas-Torrent adds.
Mas-Torrent concedes that "much more fundamental research works need to be carried out" before applications become available. "We hope that in the future, molecular devices will play an important technological role in our society," she adds.
Research support was provided by the European Science Foundation (ESF) through the EUROCORES programne SONS 2 (Self-Organised NanoStructures).
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