|
Nanoscience: Weak Force. Strong Effect Download The van der Waals force, a weak attractive force, is solely responsible for binding certain organic molecules to metallic surfaces. In a model for organic devices, it is this force alone that binds an organic film to a metallic substrate. This data, recently published in Physical Review Letters, represents the latest findings from a National Research Network (NRN) supported by the Austrian Science Fund FWF. These findings mean that numerous calculation models for the physical interactions between thin films and their carrier materials will need to be revised. Although they fulfil complex functions when used, for example, as computer chips, inorganic semiconductors have a simple construction that greatly limits their application. The same does not apply to semiconductors made of organic materials. Because organic molecules are extremely flexible, they can be used in a whole new range of applications. However, before this advantage can be exploited to the full, scientists need to have a better understanding of the far greater complexity of these materials over their inorganic counterparts. Up & Down Organic semiconductors are manufactured by applying thin films of an electrically conductive organic material to a carrier surface. When carrying out this process, it is important to understand the interactions that occur at the interfaces between the carrier material and the organic material. A team from the "Interface controlled and functionalised organic thin films" National Research Network (NRN) at the University of Leoben has made an important contribution to scientific understanding in precisely this field. Using complex calculations, the team has been able to show that a thin film of organic thiophene is held on to a copper surface solely by the van der Waals force. The team calculated that the adsorption energy involved is -0.50 eV. The spokesperson for the NRN, Prof. Helmut Sitter from the Institute of Semiconductor and Solid State Physics at Johannes Kepler University (JKU) in Linz, explains: "The van der Waals force is a weakly interacting force between atoms that occurs as a result of asymmetric charge distribution in atoms. We now know that this exerts a highly significant influence on the kinds of extremely thin material films used to manufacture organic semiconductors. Indeed, this force can successfully bind the materials entirely on its own. However, due to its weakness, several previous methods used to calculate the interactions between different materials have attached only minor importance to this force, or have ignored it altogether." This would also seem to provide some explanation for why the generalized gradient approximation (GGA) often used in such instances has been unable to satisfactorily explain the bonding behaviour in thin layers. In fact, these newly published results could explain the discrepancies that have long been found between various experimental data and models for calculating the interaction between thin layers. Publications, Prizes, Products The new data adds to our fundamental understanding of the interactions that take place at interfaces. The influence of the van der Waals force also indicates that no charge is transferred between the atoms of the organic materials and their substrates in the calculated system. This finding is of key significance to the production and functionality of organic semiconductors. Several articles in the Advanced Materials journal this year demonstrate how research carried out by members of the NRN maintains a steady focus on practical applications. As a result of one such article, the Institute of Experimental Physics at JKU won the official Innovation Prize of the Province of Upper Austria. It is no surprise that three spin-off companies – run almost exclusively by graduates from the Institutes involved in the NRN – have already been established as a direct result of the findings. One of these companies, Nanoident, was declared "Entrepreneur of the Year 2007" by Ernst & Young Austria. Prof. Sitter believes that all of these achievements, together with an article by the NRN published in SCIENCE in the summer of this year, prove how this National Research Network has successfully combined fundamental research, applied research and technology transfer – with the support of the FWF. Original publication: Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype, P. Sony, P. Puschnig, D. nabok & C. Ambrosch-Draxl. Phys. Rev. Lett. 99, 176401 (2007). Scientific contact: Austrian Science Fund FWF: Copy Editing and Distribution: Unusual Nanoscale Mound Formation Download Terrace-like elevations of just a few nanometres can form during production of organic thin films made from electrically conductive material. This phenomenon was previously only known from inorganic materials and is crucially important for future production of a new generation of semi-conductor components based on organic thin films. The data now published in the first July edition of SCIENCE was collated as part of a national research network funded by the Austrian Science Fund FWF. Inorganic semi-conductors have a simple construction and have made high-performance computers possible. In contrast, organic semi-conductors are complex but enable production of innovative electronic circuits, as vividly demonstrated by the first prototypes for roll-up screens. Yet these benefits of organic semi-conductors can only be fully harnessed when the response of their organic molecular layer – whose thinness is crucial in functional terms – is better understood. The national research network (NRN) “Interface controlled and functionalised organic thin films” of the Austrian Science Fund FWF is contributing to precisely this understanding. Microscopic Height Measurement In the latest issue of SCIENCE, a team from the NRN has now been able to show that organic molecules spread out on a carrier material in a previously unknown form to create thin electrically conductive films. As Prof. Christian Teichert from the Institute of Physics at the University of Leoben explains: "Totally surprising diffusion behaviour at step edges formed during film growth was observed on the films of the organic substance parahexaphenyl produced by solid state physicists from Graz University of Technology. The molecules here come into contact with a diffusion barrier, which leads to the other molecules piling up. Although a diffusion barrier of this nature is well known in inorganic, atomically structured films – it is called the Ehrlich-Schwoebel barrier in honour of its inventors – it had not previously been observed for organic materials." The team in Leoben used scanning force microscopy to better understand this hitherto unknown behaviour of the organic molecules. This enabled precise measurement of the nano mounds at the step edges. Evaluation of the data thus obtained led to a further surprise. The shape of the nano elevations is strongly reminiscent of the terraced mounds encountered in mining. The team was struck by the fact that the terrace height of 2.6 nm almost exactly matches the length of a molecule of parahexaphenyl. The conclusion from this is that the molecules align themselves upright within a terrace. However, it was also shown that the lower terraces are somewhat lower in height than those above. Project team member Dr. Gregor Hlawacek explains this phenomenon: "The data from the measurement allowed us to calculate the Ehrlich-Schwoebel barrier for this case. It also transpired that the molecules of the lower terraces are deposited at an angle. As a result, the terrace height here diminishes relative to the angle of inclination." Energy-saving Measure at Nano Level The measured values were used to perform computer simulations in the Chair of Atomistic Modelling and Design of Materials. These were not only able to confirm the experimental values for the diffusion barriers but also revealed that the parahexaphenyl molecules are bent in diffusion. This was surprising as bending requires expansion of the bonds in the molecule, which is in fact avoided owing to the energy required. However, in this way, the diffusing molecule can maintain bonds to neighbouring molecules more effectively than a rigid molecule, so that bending is overall the more energy-saving mechanism. For the team from Leoben and Graz, these findings are extremely exciting as producing organic thin-film transistors requires closed films of such upright molecules. Improved understanding of the fundamental forces that bring this about will enable them to be manipulated and thus used in a controlled way. This NRN is therefore making a direct contribution to the future production of a new generation of semi-conductor components. Text and image material available from 08:00 GMT on Wednesday 09.07.08 at: englisch http://www.fwf.ac.at/en/public_relations/press/pv200807-en.html Original publication: Characterization of Step-Edge Barriers in Organic Thin-Film Growth, G. Hlawacek, P. Puschnig, A. Winkler, C. Ambrosch-Draxl & C. Teichert. Science (2008), 108-111. Scientific contact: FWF Austrian Science Fund: Editor/publisher: Vienna, 08 July 2008 |
Page last modified on August 26, 2008, at 11:11 PM