According to Empa, organic semiconductors are promising candidates as starting materials for the manufacture of cheap, large area and flexible electronic components such as transistors, diodes and sensors on a scale ranging from micro to nano. The challenge for the researchers was how to join components together with electrically conducting links and create an electronic circuit.
The research originated from an EU project called PHODYE instigated by Spanish physicist Angel Barranco and Empa colleagues. The aim of the project is to develop highly sensitive gas sensors based on fluorescing thin films which change colour and fluoresce on contact with certain gas molecules.
Barranco and Empa physicist Pierangelo Groening developed a plasma-deposition process in order to store fluorescing dye molecules such as metallo-proyphins, perylenes and phthalocyanines unmodified and at high concentrations in SiO2 or TiO2 layers. They discovered that if certain gas molecules were deposited on dye particles in the thin films, then these fluoresced at different wavelengths and the thin film changed colour as a result. If different dyes are used then the gases which are toxic to humans can be detected at very low concentrations.
Empa researchers then learnt how to manufacture nanowires with widely varying characteristics by appropriately selecting the starting molecule and the experimental conditions. Nanowires of metallo-phthalocyanine molecules have diameters of a mere 10 to 50nm and a length of up to 100microns. The new method revealed that by exactly controlling the substrate temperature, molecule flow and substrate treatment, the organic nanowires develop a previously unattained, perfectly monocrystalline structure.
Groening believes that the new process could not only provide nanowires for the gas sensors but also make it possible to create complex 'nanowire electric circuits' for electronic and optoelectronic applications such as solar cells, transistors and diodes. According to Groening, this is because the different types of nanowires can be combined as required to form networks with widely varying properties.
To achieve this lies a second step is required, in which the nanowires growing on the surface are 'decorated' with silver nanoparticles by a sputter-coating process. A target is bombarded with energetic ions, knocking off silver atoms which enter the gas phase and are deposited onto the nanowires. In a final step, the Empa team now grow more nanowires which, thanks to the silver particles, are in electrical contact with the original wires – the basis of an electrical circuit on the nanometer scale.
The first electrical conductivity measurements, made with the help of a four-tip scanning tunnel microscope in ultra high vacuum, exceeded the researchers' expectations, as the material is of an unusually high quality.
"This opens up the possibility of soon being able to manufacture organic semiconductor materials," Groening said. "And that, too, using a simple and economic process." The researchers have successfully synthesised increasingly more complex structures of nanowires, and managed to link these together.
Groening noted: "Take, for example, nanowires consisting of sections made with different starting molecules. If these molecules can transport either only positive or only negative charges, then a diode is created which allows current to flow in one direction alone."
Groening speculates that it is quite possible that one day components for nanolectronics and nanophotonics will be made using this technique.
