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Molecular Electronics: A Theoretical Study of Electronic Structure of Bulk and Interfaces
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics . Linköping University, The Institute of Technology.
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 [en]
molecular electronics, electron transport, disorder, localization, long-range correlation, organometallic magnets, interface dipole
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-6938ISBN: 91-85457-82-5 (print)OAI: oai:DiVA.org:liu-6938DiVA: diva2:22058
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
List of papers
1. Anderson localization in two-dimensional disordered systems
Open this publication in new window or tab >>Anderson localization in two-dimensional disordered systems
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.

Keyword
Transport, Disorder, Organic metals
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13868 (URN)10.1016/S0379-6779(03)00125-5 (DOI)
Available from: 2006-06-28 Created: 2006-06-28
2. The effect of lont-range correlation on the metal-insulator transition in disordered pentacene
Open this publication in new window or tab >>The effect of lont-range correlation on the metal-insulator transition in disordered pentacene
Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-13869 (URN)
Available from: 2006-06-28 Created: 2006-06-28 Last updated: 2010-01-13
3. Electron Localization in DNA
Open this publication in new window or tab >>Electron Localization in DNA
2003 (English)In: Nano Letters, ISSN 1530-6984, Vol. 3, no 10, 1417-1420 p.Article in journal (Refereed) Published
Abstract [en]

Electron localization lengths in different DNA sequences have been calculated numerically using the transfer matrix method. It is shown that electronic states associated with guanine can reach fairly long localization lengths in disordered guanine-cytosine double strands if both intra- and interstrand π−π interactions are considered. For DNA sequences containing all four bases, the electronic states become localized to very few base pairs. In particular, the human chromosome 22 and λ-DNA, both with approximately equal concentrations of the four bases, show insulating behavior.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13870 (URN)10.1021/nl034631d (DOI)
Available from: 2006-06-28 Created: 2006-06-28
4. Coulomb interactions in rubidium-doped tetracyanoethylene: a model system for organometallic magnets
Open this publication in new window or tab >>Coulomb interactions in rubidium-doped tetracyanoethylene: a model system for organometallic magnets
Show others...
2004 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 16, 165208- p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure of tetracyanoethylene (TCNE) has been studied both in its pristine state and upon stepwise rubidium intercalation, by UV and x-ray photoelectron spectroscopy as well as with theoretical calculations. The intercalated system may serve as a model for TCNE-based organometallic magnets, of which the electronic structure remains largely unknown. Rubidium is found to n-dope the TCNE molecules forming Rb+TCNE- with almost complete charge transfer. Calculations show a spin splitting of the former highest occupied molecular orbital level upon Rb doping. We see no evidence for the formation of doubly charged TCNE molecules. A gap opens up at the Fermi energy for Rb+TCNE- due to on-site Coulomb interactions. We estimate the on-site Coulomb interaction of amorphous TCNE doped with Rb to be ∼2 eV.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13871 (URN)10.1103/PhysRevB.69.165208 (DOI)
Available from: 2006-06-28 Created: 2006-06-28 Last updated: 2017-12-13
5. TDAE chemisorbed on gold
Open this publication in new window or tab >>TDAE chemisorbed on gold
2008 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 31Article in journal (Refereed) Published
Abstract [en]

DFT calculations on tetrakis(dimethylamino)ethylene (TDAE) interacting with a gold surface have been performed. A monolayer of TDAE deposited on a gold surface creates an interface dipole, which decreases the barrier for electron injection from a gold contact into an (organic) electroactive material. This was studied by simulating the complex in two different ways, using a slab model and using a gold cluster surface. These approaches are shown to be complementary: the cluster results apply to the situation of isolated molecules and the slab results describe the case of interacting TDAE molecules on the gold surface. We found that there is a transfer of around one electronic charge per TDAE to the gold in the limit of non-interacting TDAE molecules. This charge transfer results in the formation of an interface dipole and a corresponding lowering of the work function of the surface. The lowering of the work function increases with increasing coverage and is as large as 2.4 eV for a monolayer of TDAE on gold. Due to depolarization effects, the charge transfer in this state is reduced to 0.56 times the electronic charge.

Place, publisher, year, edition, pages
Bristol, United Kingdom: Institute of Physics Publishing (IOPP), 2008
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13872 (URN)10.1088/0953-8984/20/31/315008 (DOI)000257759600025 ()
Available from: 2006-06-28 Created: 2006-06-28 Last updated: 2017-12-13Bibliographically approved

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