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Charge dynamics and electronic structure of pi-conjugated systems
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
2002 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The research presented in this theses deals with the electronic properties of 1rconjugated molecular systems. Charge dynamics has been studied in order to get a better understanding of the physical processes behind charge transport in these materials, intended to be used in organic molecular electronic devices. The systems were investigated theoretically employing quantum mechanical methods.

Carbon based materials have been shown to exist in several different forms in addition to the well known phases of graphite and diamond. The relatively rare combination of hard and elastic properties has been found in carbon nitride materials. These materials have the specific structure of crosslinked, curved graphite layers incorporated with nitrogen atoms. To understand and improve such properties, it is necessary to investigate the chemical environment of nitrogen within the material. In the work included in this thesis, the binding energy of the N(ls) level has been calculated for different model molecules. This was done using the .6.-SCF method. The results, identifying both sp2 and sp3 bonded nitrogen, were compared with experimental X-ray photoelectron spectroscopy data.

In recent years the interest in molecular electronics has grown tremendously. It is now possible to measure the electrical conductance through individual molecules. One of the key issues is to have a good contact between the molecule or molecular wire and the metal electrode. For this purpose a thiol group is used as an interconnecting bridge. The work presented as a part of this thesis investigates the interaction of a small sulphur bonded molecule with a gold substrate. Using ab initio Hartree-Fock theory, it was shown that the electronic system of the gold surface interacts with the conducting states of the molecular wire at certain energies. Thus such a gold-thiol bridge allows for efficient charge transfer.

The process of charge transport through the molecular wire itself - or more generally, any system of conjugated polymers - is a subject of huge interest. The discovery that these light weight, mechanically flexible materials can be doped to high electrical conductivity was the starting point of this research field about 25 years ago. The focus has now shifted towards the semiconducting properties. Conjugated polymers are currently being used as active materials in electronic devices such as light-emitting diodes and solar cells. The main part of the research described in this thesis is aimed at investigating the motion of polaronic excitations in model polymer systems. The well known Su-Schrieffer-Heeger model has been applied with the additional potential of an external electric field. It was observed that charge delocalize over several chains and the transport properties are strongly dependent on the interchain coupling and the field strength. For the application of conjugated polymers in solar cells, a mixture with C60 fullerenes have proven very successful. This molecule tends to separate a electron-hole pair on the polymer chain in such a way that the negative charge is moved over to the C60 and from there, further away in the system.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2002. , p. 87
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 783
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-143552ISBN: 917373456X (print)OAI: oai:DiVA.org:liu-143552DiVA, id: diva2:1164856
Public defence
2002-12-06, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2018-01-17Bibliographically approved

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CiteExportLink to record
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