The European Commission has reported on the MONA-LISA project into the properties of organic semiconductors as dimensions are reduced using unconventional fabrication techniques. The findings of the investigation could lead to novel nanoelectronic applications.
When the microprocessor revolution was in its infancy in the late 1970s, it was dubbed ‘a solution looking for a problem’. The same cannot be said of organic semiconductors. Application engineers have already thought up novel ideas, such as e-tags that would replace barcodes and which would hold much more data and be capable of two-way data transfer.
Organic semiconductors can be employed in flexible devices that continue to operate even if they are deformed. They could be applied to curved surfaces, such as the side of a can of food or a bottle, to tag the product’s history and to monitor the food inside and display the level of freshness. Smart textiles incorporating these devices could warn the wearer of life-threatening situations. But before these ideas become viable, however, a lot of basic research needs to be carried out, and the MONA-LISA project is making a significant contribution.
This Commission-funded project is studying the transport of energy and charge in organic semiconductors as the size of the layer is made smaller and smaller, both laterally and vertically. The problem takes the investigation beyond the transport properties of thin films to the transport across single molecular domains.
“The project started with the use of a prototype molecule for organic semiconductors called sexythiophene, which is a well-known organic semiconductor,” explains project co-ordinator Dr Fabio Biscarini of the Italian National Research Council (CNR) Institute for Nanostructured Materials Studies.
“The first field effect transistor to incorporate an organic molecule in place of silicon was developed more than ten years ago. We choose this molecule because it’s one of the best in terms of mobility and we know how to control its organisation – the way the molecules are ordered – in a device. We have the know-how on the growth of thin films using this material, and we decided to use it as a kind of benchmark material for seeing what changes in the size and ordering would make to the transistor’s performance.
“Later in the project, we used different types of material such as pentatcene, which has exhibited the best performance, and quatrothiophene, which has four units instead of six, as well as soluble homologues of sexy- and quarterthiophene. We explored these materials by growing ultra thin films with a technique known as organic molecular beam deposition – a highly controlled sublimation in an ultra-high vacuum – and compared them with other materials either sublimed or deposited by spin casting.”
The MONA-LISA project also addressed the problem of the control of the material’s properties through varying its size. Here the most challenging task was to make a device from the material, make it smaller and smaller, and then connect it to the outside world.
“A very important activity in MONA-LISA concerned the fabrication of nanostructures, patterns and devices, such as field effect transistors, on the nanometre scale,” says Biscarini. “We were not just moving from, say, 1 micron to 10 nm length scales. We needed to explore the whole range of dimensions in between to see if there is some critical length that makes significant changes or improvements in the transport properties. As we move from the micron to nanometre scale, we have to move the technological platform away from photolithography; we have been exploring various techniques for making this transition.
“A whole range of fabrication techniques have been used in MONA LISA: serial (electron beam and scanning probe lithography) and parallel (micro-contact printing and nano imprinting), the latter being suitable for scaling up towards mass production of nanostructures and devices. This has also meant integrating different know how, from organic electronics to nanofabrication, that eventually has led to a wide cross-fertilisation in the research interests of the groups.”
Bringing together key players“This is a quite complex project that brings together key players both from the European Union and the Newly Associated States, and in many ways it’s not unlike an FP6 Integrated Project,” observes Biscarini.“It’s hard to imagine how a project as large as this could be supported by a single institution. The four academic groups working on the project are: University of Wuerzburg (DE), University of Wuppertal (DE), University of Algarve (PT) and Nova Gorica Polytechnic (SL). There are three national research institutes: the CNR, the Spanish Consejo Superior de Investigaciones Científicas (CSIC) and the Polish Academy of Science. Finally, we have a major industrial partner, Philips Research Laboratories, which is working on polymer electronics. Philips, in particular, will be looking towards the application of this technology for large area fabrication.A patent application has already been made on behalf of CNR and CSIC, and a second is in preparation. “We have demonstrated the ability to manufacture high-performance transistors using ultra thin films of only one or two monolayers thick. These films are very stable and their cycled operation is highly reproducible,” claims Biscarini.