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Abstract:
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.

Nanoscience: Weak force. Strong effect.

Austria | Posted on November 19th, 2007

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.

Due to a server upgrade image and text will not be available online until
Tuesday, 20th November 2007, 09.00 a.m. CET onwards:
http://www.fwf.ac.at/en/public_relations/press/pv200711-en.html
Image available on request.

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).

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Contacts:
Scientific Contact:
Prof. Helmut Sitter
Institut für Halbleiter- und Festkörperphysik
Johannes Kepler Universität Linz
T +43 / 732 / 2468 - 9623
E

Austrian Science Fund FWF:
Mag. Stefan Bernhardt
Haus der Forschung
Sensengasse 1
1090 Wien
T +43 / 1 / 505 67 40 - 8111
E

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Vienna, 19th November 2007

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