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Anderson localization in two-dimensional disordered systems
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics . Linköping University, The Institute of Technology.
2003 (English)In: Synthetic Metals, ISSN 0379-6779, Vol. 139, no 2, 239-244 p.Article in journal (Refereed) Published
Abstract [en]

Using the transfer matrix method we have calculated the localization length in a 2D (two dimension) rectangular lattice with both on-site and off-diagonal disorder. Using finite size scaling we show that systems with off-diagonal disorder are much more sensitive to disorder than the system with on-site disorder, e.g. at the same amount of disorder (W=w=2) the localization length for on-site disorder is 104 times longer than for off-diagonal disorder. We consider both isotropic and anisotropic systems, where the latter can be considered as a model for an organic crystal. In the anisotropic case the maximum localization length is at the band center, the isotropic system has a maximum away from the band center.

Place, publisher, year, edition, pages
2003. Vol. 139, no 2, 239-244 p.
Keyword [en]
Transport, Disorder, Organic metals
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13868DOI: 10.1016/S0379-6779(03)00125-5OAI: oai:DiVA.org:liu-13868DiVA: diva2:22053
Available from: 2006-06-28 Created: 2006-06-28
In thesis
1. Molecular Electronics: A Theoretical Study of Electronic Structure of Bulk and Interfaces
Open this publication in new window or tab >>Molecular Electronics: A Theoretical Study of Electronic Structure of Bulk and Interfaces
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with theoretical studies of the electronic structure of molecules used in the context of molecular electronics. Both studies with model Hamiltonians and first principle calculations have been performed. The materials studied include molecular crystals of pentacene and DNA, which are used as active material in field-effect transistors and as tentative molecular wires, respectively. The molecular magnet compound TCNE and surface modification by means of chemisorption of TDAE on gold are also studied.

Molecular crystals of pentacene are reported to have the highest field-effect mobility values for organic thin film field-effect transistors. The conduction process in field-effect transistors applications occurs in a single layer of the molecular crystal. Hence, in studies of transport properties molecular crystals of pentacene can be considered as a two dimensional system. An open question of these system is if the charge transport is bandlike or if as a result of disorder is a hopping process. We address this question in two of the included papers, paper I and paper II.

The conducting properties of DNA are of interest for a broad scientific community. Biologist for understanding of oxidatively damaged DNA and physicist and the electronics community for use as a molecular wire. Some reports on the subject classifies DNA as a conductor while other report insulating behavior. The outcome of the investigations are heavily dependent on the type of DNA being studied, clearly there is a big difference between the natural and more or less random sequence in, e.g., λ-DNA and the highly ordered syntethic poly(G)-poly(C) DNA. It has been suggested that long-range correlation would yield delocalized states, i.e., bandlike transport, in natural DNA, especially in the human chromosome 22. In paper III we show that this is not the case. In general our results show that DNA containing an approximately equal amount of the four basis is an insulator in a static picture.

An emerging research field is spintronics. In spintronic devices the spin of the charge carrier is as important as the charge. One can envision a device where spin alone is the carrier of information. In realizing spintronic devices, materials that are both magnetic and semiconducting are needed. Systems that exhibit both these properties are organic-based magnets. In paper IV the electronic structure of the molecular magnet compound TCNE is studied, both experimentally and theoretically.

The injection of carriers from metal contacts to organic semiconductors is central to the performance of organic based devices. The interface between the metal contact and the organic material has been pointed out to be one of the device parameters that most significantly influences the device performance. This relates to the process of injection of charge carriers in to the organic material. In some contact and organic material combinations the energy barrier for charge injection can be very high. The barrier can be reduced by modify the interface dipole, this is achieved by a monolayer of adsorbed molecules at the interface. The molecule TDAE chemisorbed on gold is studied in paper V.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2006
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 991
Series
Keyword
molecular electronics, electron transport, disorder, localization, long-range correlation, organometallic magnets, interface dipole
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-6938 (URN)91-85457-82-5 (ISBN)
Public defence
2006-01-13, Planck, Fysikhuset, IFM, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2006-06-28 Created: 2006-06-28 Last updated: 2009-06-08

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Unge, MikaelStafström, Sven

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