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.
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 behavior 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.
Original
article
“Reconstruction
of Molecular Orbital Densities from Photoemission Data"