Tuesday, November 17, 2009
For the first time, it has been possible to measure electron
density in individual molecular states using what is known as the
photoelectric effect. Now published in Science, this method
represents a key building block in the development of organic
semiconductor elements.
Supported by the Austrian Science Fund
FWF, the success of this project rested on the mathematical
transformation of the measured data. This made it possible to
interpret the distribution of the electrons and draw conclusions
about the potential properties of organic semiconductor
elements.
|
| A recent Science paper sheds a new light on
electron density in individual molecular states. |
Ultra-thin films made of organic molecules form the basis of
future semiconductor technologies. Because organic molecules are
extremely flexible, they can be used in a whole new range of
applications, making it equally possible to create pliable screens
and cost-effective solar cells. However, apart from these everyday
applications for organic semiconductors, the most important task is
to gain a better understanding of the interactions between organic
materials and inorganic carrier substances. A team from the
Universities of Graz and Leoben has now succeeded in developing a
means of doing just that.
TIGHTLY PACKED
"The properties of an organic molecule are defined to a large
extent by specific electron states", explains Dr. Peter Puschnig of
the Chair of Atomistic Modelling and Design of Materials at the
University of Leoben, who led the research. He adds: "If we can
determine their distribution within the molecule accurately, then
we will be able to better understand how organic semiconductor
components work and thus increase their efficiency." Until now,
there has been a lack of effective methods of measuring this
electron distribution. Dr. Puschnig and his team have therefore
succeeded in making significant progress.
The team's achievement is based on the use of the photoelectric
effect. This enables individual electrons to be "knocked out" of
organic molecules. As part of this project, an organic molecule was
exposed to ultraviolet light that emitted sufficient energy to
separate individual electrons from the molecules. The direction and
speed of the electrons thus released were then measured using
highly-sensitive detectors, generating the basic data required to
calculate the electron distribution within the molecule. As part of
this process, Prof. Michael Ramsay and his team from the University
of Graz used a hexaphenyl film just one molecule thick that had
been applied to a copper surface. The team from Graz carried out
the actual measurements at the Berliner Elektronen-Speicherring
Gesellschaft für Synchrotronstrahlung (BESSY, Berlin Electron
Storage Ring Society for Synchrotron Radiation).
A CALCULATED RESULT
Commenting on the evaluation of this data, Dr. Puschnig says:
"It revealed a quite characteristic distribution of the electrons
emitted. However, it initially proved difficult to interpret this
distribution and it seemed it would be impossible to link the
measured data to the original electron distribution in the
molecule." It was only by using special mathematical
transformations (Fourier Transformation) that the team was able to
establish that the measured electron distribution matched that of
the molecule. As the distribution was in this instance already
known from calculations carried out as part of the density
functional theory, it was possible to test and confirm the
viability of the new method.
This new method is particularly valuable as it means measuring
the behaviour of electrons at the interfaces between organic
semiconductors and metals is now relatively easy and highly
accurate. The study "Interface controlled and functionalised
organic thin films" supported by the FWF as part of the National
Research Network (NFN) is thus making a fundamental contribution to
future applications of organic semiconductors.
SOURCE