Weak force – strong effect

Austrian researchers warn that models of the interaction between the organic films and metallic substrates used in plastic electronics will have to be refined in the light of new data on intermolecular forces and their role in bonding thin layers of materials.

Adsorption of a thiopene organic molecule onto a copper surface via a van der Waals interaction alone

Organic semiconductors are manufactured by attaching thin films of an electrically conductive organic material to a metallic carrier surface. Organic electronics are highly flexible, but they are also more complex than their inorganic counterparts. A detailed understanding of the materials’ physical and chemical characteristics is required before they can be exploited to the full.

Using density functional theory (DFT), researchers at the University of Leoben have found that a thin film of organic thiophene is bonded to a copper surface by the van der Waals force alone, and they have calculated the adsorption energy involved.

DFT is widely used in materials science, but it is a challenge to incorporate van der Waals forces into the theoretical framework. Study leader Priya Sony and co-workers have tackled the problem by employing a recent addition the DFT toolbox which allows a proper treatment of van der Waals interactions. With this approach it is now possible to predict the correct binding energies, and grasp the underlying physical processes.

“This opens the possibility to reliably predict the interface of such weakly interacting systems,” says study co-author Peter Puschnig. “This is important for understanding thin film growth mechanisms, as well as electronic charge transport through the interfaces of organic optoelectronic devices.”

“Understanding and designing interface properties, and the wetting of organic molecules on metals, is a central issue for the optimisation of optoelectronic devices such as LEDs and solar cells, which are based on plastic materials,” comments Graz University physicist Ulrich Hohenester. “The prospect that such optimisation can now be reliably performed by computer simulations will certainly boost research activities in this field.”

Further reading: “Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype”, Sony et al., Phys. Rev. Lett. 99, 176401 (2007).

Figure: Adsorption of a thiopene organic molecule onto a copper surface via a van der Waals interaction alone (source: Peter Puschnig/Montanuniversität Leoben).

Article first published in Nanomaterials News.