Clouds of electrons cause neighborhood conflict among molecules

New results of great importance for organic electronic components

Phthalocyanine molecules on the silver surface from the bird's eye view (lower part) and in the side view (upper part). The red and green colored areas indicate the charge distribution of the electrons. The "electron clouds" in silver mentioned in the text are shown in yellow in the upper part of the picture. © Christian Kumpf
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The contact between metals and organic molecules plays a major role in the electronics of the future. Würzburg physicists have now discovered an effect that could decisively improve such contacts. They report on their findings in the current issue of the journal Nature Physics.

Electronic components based on organic materials are increasingly being used in modern technology. In the form of organic light-emitting diodes, so-called OLEDs, they are already used in mobile phone displays and in television sets. OFETs - organic field-effect transistors - are of interest for applications that require low storage densities or short-term use, such as electronic watermarks, barcodes or disposable sensors, for example in medicine.

Less power and lower costs

These components are not yet capable of such high performance as their comparable competitors, which are made of silicon. But they have the unbeatable advantage that they are much cheaper. How their performance can be improved is researched by scientists all over the world. Würzburg physicists have now possibly taken a big step forward in this way - by a coincidence.

"That was an unexpected effect. We did not expect that, "says Christian Kumpf, formerly of the University of Würzburg, and now at Forschungszentrum Jülich.

A silver crystal is vaporized

For their investigations, the physicists around Kumpf have prepared an approximately one centimeter silver crystal with an extremely clean surface. They then applied to this crystal a thin layer of organic molecules by evaporating the material in vacuo in a small crucible and depositing on the crystal. Metal phthalocyanines, a class of compounds characterized by a nitrogen-carbon ring structure and a central metal atom, were used. display

The surprise came in the following study of the geometric structure of this molecular layer on the silver surface. Normally, such organic molecules arrange themselves on metallic surfaces in such a way that they seek the greatest possible contact with the metal and always in close proximity to each other want to stay with the other molecules. The molecules 'linger, they show an attractive interaction with their neighbors, ' says Kumpf.

The metal thus forms islands of organic matter that grow continuously and eventually collide. There are usually very many, and thus especially small islands whose boundaries are very irregular. These limits, so-called grain boundaries, limit the performance of the components, for example their electrical conductivity, if they occur very frequently.

"Electronic Clouds" ensure repulsion

The phthalocyanines behaved quite differently: The molecules do not work, but they also align themselves flatly on the silver, but at the greatest possible distance from their neighbors says Kumpf. The physicists first had to search for the reason for this unexpected behavior. They got busy with the electrons.

"We were able to prove that the organic molecules and the silver exchange electrical charges, ie electrons, " says Kumpf. As a result, "electron clouds" are formed in the silver crystal below the phthalocyanine molecules, which repel each other. These forces are greater than the forces of attraction otherwise known as "van der Waals forces".

This discovery may not sound spectacular at first glance. However, in addition to its fundamental importance - such a behavior has never been observed for such molecules - it may also have major consequences for organic electronic components, "says Kumpf.

Because the molecules are distributed absolutely uniformly, they form a perfectly ordered, homogeneous layer of unprecedented size on the silver surface. The negative effect of the many grain boundaries is significantly reduced. "This is possibly a very big step towards improved performance, " says Kumpf.

Promising results

However, further steps are necessary until applicability: "You need a thicker crystal of organic molecules so that the components can work effectively, " says Kumpf.

He is already working with the physicists from Würzburg - with promising results. Meanwhile, the working group is able to apply even thicker layers of phthalocyanine molecules of the required quality on the silver crystal.

(idw - University W rzburg, 13.01.2009 - DLO)