Van der Waals force stops molecules

New findings on interactions at the interface

Thin layers of organic molecules can be bound to metallic support by weak attractive forces alone. © Claudia Ambrosch-Draxl
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The binding of certain organic molecules to metallic surfaces is solely due to the Van der Waals force, a weak force of attraction. In an organic device model, this force alone makes it possible to bind an organic film to a metallic surface. Now published in the journal Physical Review Letters, these new results necessitate the revision of numerous models of the physical interactions of thin films with their support material.

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Despite the fulfillment of complex functions, for example as computer chips, inorganic semiconductors are simply constructed, which places narrow limits on their application possibilities. Another is for semiconductors that use organic materials. The high flexibility of the organic molecules allows completely new applications. However, this advantage can only be used if its complexity, which is significantly higher than that of inorganic materials, is better understood.

Over and over

For the production of organic semiconductors, thin films of an electrically conductive organic material are applied to a carrier surface. It is important to understand the interactions at the interfaces between support material and organic material. This is exactly what an Austrian team from the National Research Network (NFN) "Interface controlled and functionalized organic thin films" at the Montanuniversität Leoben has made an important contribution to. Complex calculations have shown that a thin film of organic thiophene is held on a copper surface solely by Van der Waals forces. The adsorption energy of the team could be calculated with -0.50 eV.

The speaker of the NFN, Professor Helmut Sitter from the Institute of Semiconductor and Solid State Physics of the Johannes Kepler University (JKU) Linz, explains: "The Van der Waals force is a weakly interacting force between atoms that results from asymmetric charge distribution in the atoms. As we now know, their impact on extremely thin films of material used in the production of organic semiconductors is very significant and may be sufficient solely for bonding between materials. Because of its weakness, however, it has not yet been taken into account, or has only been considered inferior, in numerous methods used to calculate the interaction of different materials. "Display

Bonding behavior in thin layers explained

This seems to explain why the commonly used gradient approximation (GGA), often used for these purposes, could not satisfactorily explain the bonding behavior in thin films. In fact, these recently published results may explain long-known discrepancies between different experimental data and computational models for the interaction of thin layers.

The new data extends the basic understanding of interfacial interactions. At the same time, the influence of the Van der Waals force shows that in the calculated system no charges are transferred between the atoms of the organic and the carrier material. This is of crucial importance for the production and function of organic semiconductors.

(Science Fund FWF / University of Linz, 21.11.2007 - DLO)