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Molecular electronics : a theoretical study of electronic structure in molecular crystals and surfaces
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2007 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

  

This thesis deals with theoretical studies of the electronic structures of molecules in organic crystal and molecules adsorbed on metal surfaces. Both studies with first principle calculations and classical mechanics were performed. The materials include pentacene and bis(9-cyclohexylimino-l-oxyphenalenyl)boron and the

Tetrakis( dimethylamino )ethylene (TDAE) molecule chemisorbed on a gold surface. Molecular crystals of pentacene are reported to have the highest field-effect mobility values for organic thin film field effect transistors, and are therefore suitable to be the active material in such devices. The increase in conductivity obtained experimentally by doping with alkali metals showed great promise, and we therefore performed calculations on pentacene crystals doped with potassium. Phase transitions occured upon doping, and the herringbone arrangement of the pristine pentacene molecules relaxes towards a 1r stacked structure. Charge transfer between the crystal and the potassium occurs, which shifts the Fermi level into the lowest unoccupied molecular orbitals bands and turns the crystal metallic. The study of bis(9-cyclohexylimino-l-oxyphenalenyl)boron, a biphenalenyl based neutral radical, was performed in order to investigate the electronic properties since experimental studies had shown simultaneous electrical, optical and magnetical bistability. The system is quasi one-dimensional, as shown from band structure calculations.. The injection of charge carriers from metals to organic semiconductors is one of the most central processes when it comes to the performance of organic devices. The interface between the metal and the semiconductor is one of the device parameters that will significantly influence the device performance. The addition of a molecular layer on the interface can, by the introduction of a dipole, reduce the energy barrier for charge injection. ·we found that a molecular layer of TDAE deposited on a gold surface results in a charge transfer from the molecule to the surface which reduced the charge injection barrier by as much as 2.4 eV.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2007. , 34 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1315
Series
LIU-TEK-LIC, 24
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-38440Local ID: 44368ISBN: 978-91-85831-76-0 (print)OAI: oai:DiVA.org:liu-38440DiVA: diva2:259289
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2013-11-14
List of papers
1. Structural and electronic transitions in potassium doped pentacene
Open this publication in new window or tab >>Structural and electronic transitions in potassium doped pentacene
2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 18Article in journal (Refereed) Published
Abstract [en]

We calculate the ground state geometrical structure of potassium-intercalated pentacene lattices using molecular mechanics and the density-functional theory. Both methods result in a structural phase transition in going from the pristine form to the intercalated state with one potassium ion per pentacene molecule. The phase transition is characterized by a sliding of adjacent pentacene molecules relative to each other. The electronic properties of this phase is studied with the density-functional theory. As a result of the geometrical changes, the - overlap in the direction perpendicular to the molecular planes of the layered pristine pentacene structure increases substantially and many of the electronic bands show strong dispersion in this direction. The Fermi energy of the doped phase appears in the middle of the conduction band where the density of states is maximum. The bandwidth of the conduction band is 0.7  eV.

Keyword
potassium, organic semiconductors, density functional theory, solid-state phase transformations, conduction bands, Fermi level, electronic density of states, ground states
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14063 (URN)10.1103/PhysRevB.73.184114 (DOI)
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2017-12-13
2. Electronic structure calculations of the phenalenyl-based neutral radical conductor bis(9-cyclohexylimino-1-phenalenyl) boron
Open this publication in new window or tab >>Electronic structure calculations of the phenalenyl-based neutral radical conductor bis(9-cyclohexylimino-1-phenalenyl) boron
2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 15Article in journal (Refereed) Published
Abstract [en]

The solid state of free radicals of bis(9-cyclohexylimino-1-oxyphenalenyl) boron is one of few neutral molecular systems which exhibit high conductivity at room temperature. The system is crystalline but highly anistotropic. We have performed band structure calculations based on the density functional theory on this system. It is evident from these studies that the system is quasi-one-dimensional with strong intermolecular interactions along one crystal direction. The bandwidth is 0.424 eV along this direction as compared to 0.055 eV perpendicular to it. Despite the quasi-one-dimensional character of the system, no signature of a Peierls distortion towards a dimerized state is observed. We argue that this is due to the fact that the Fermi energy lies slightly below the middle of the conduction band. In this case the electronic driving force for the distortion to occur is weakened and overcome by the restoring force of the lattice. Also the optical properties of the system have been investigated and the origin of the low lying excitations in the system has been clarified.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-50104 (URN)10.1103/PhysRevB.74.155111 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12
3. 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
In thesis
1. Molecular electronics : a theoretical study of electronic structure in molecular crystals and surfaces
Open this publication in new window or tab >>Molecular electronics : a theoretical study of electronic structure in molecular crystals and surfaces
2007 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

  

This thesis deals with theoretical studies of the electronic structures of molecules in organic crystal and molecules adsorbed on metal surfaces. Both studies with first principle calculations and classical mechanics were performed. The materials include pentacene and bis(9-cyclohexylimino-l-oxyphenalenyl)boron and the

Tetrakis( dimethylamino )ethylene (TDAE) molecule chemisorbed on a gold surface. Molecular crystals of pentacene are reported to have the highest field-effect mobility values for organic thin film field effect transistors, and are therefore suitable to be the active material in such devices. The increase in conductivity obtained experimentally by doping with alkali metals showed great promise, and we therefore performed calculations on pentacene crystals doped with potassium. Phase transitions occured upon doping, and the herringbone arrangement of the pristine pentacene molecules relaxes towards a 1r stacked structure. Charge transfer between the crystal and the potassium occurs, which shifts the Fermi level into the lowest unoccupied molecular orbitals bands and turns the crystal metallic. The study of bis(9-cyclohexylimino-l-oxyphenalenyl)boron, a biphenalenyl based neutral radical, was performed in order to investigate the electronic properties since experimental studies had shown simultaneous electrical, optical and magnetical bistability. The system is quasi one-dimensional, as shown from band structure calculations.. The injection of charge carriers from metals to organic semiconductors is one of the most central processes when it comes to the performance of organic devices. The interface between the metal and the semiconductor is one of the device parameters that will significantly influence the device performance. The addition of a molecular layer on the interface can, by the introduction of a dipole, reduce the energy barrier for charge injection. ·we found that a molecular layer of TDAE deposited on a gold surface results in a charge transfer from the molecule to the surface which reduced the charge injection barrier by as much as 2.4 eV.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2007. 34 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1315
Series
LIU-TEK-LIC, 24
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-38440 (URN)44368 (Local ID)978-91-85831-76-0 (ISBN)44368 (Archive number)44368 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2013-11-14

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Citation style
  • apa
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  • modern-language-association-8th-edition
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  • Other style
More styles
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  • de-DE
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  • nn-NB
  • sv-SE
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More languages
Output format
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