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Electronic Structure and Transport Properties of Carbon Based Materials
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]

In the past decade the interest in molecular electronic devices has escalated. The synthesis of molecular crystals has improved, providing single crystals or thin films with mobility comparable with or even higher than amorphous silicon. Their mechanical flexibility admits new types of applications and usage of electronic devices. Some of these organic crystals also display magnetic effects. Furthermore, the fullerene and carbon nanotube allotropes of carbon are prominent candidates for various types of applications. The carbon nanotubes, in particular, are suitable for molecular wire applications with their robust, hollow and almost one-dimensional structure and diverse band structure. In this thesis, we have theoretically investigated carbon based materials, such as carbon nanotubes, pentacene and spiro-biphenalenyl neutral radical molecular crystals. The work mainly deals with the electron structure and the transport properties thereof. The first studies concerns effects and defects in devices of finite carbon nanotubes. The transport properties, that is, conductance, are calculated with the Landauer approach. The device setup contains two metallic leads attached to the carbon nanotubes. Structural defects as vacancies and bending are considered for single-walled carbon nanotubes. For the multi-walled carbon nanotubes the focus is on inter-shell interaction and telescopic junctions. The current voltage characteristics of these systems show clear marks of quantum dot behaviour. The influence of defects as vacancies and geometrical deformations are significant for infinite systems, but in these devices they play a minor role. The rest of the studies concern molecular crystals, treated with density-functional theory (DFT). Inspired by the enhance of the electrical conductivity obtained experimentally by doping similar materials with alkali metals, calculations were performed on bundles of single-walled carbon nanotubes and pentacene crystals doped with potassium. The most prominent effect of the potassium intercalation is the shift of Fermi level in the nanotube bands. A sign of charge transfer of the valence electrons of the potassium atoms. Semi-conducting bundles become metallic and metallic bundles gain density of states at the Fermi level. In the semi-conducting pristine pentacene crystals structural transitions occur upon doping. The herringbone arrangement of the pristine pentacene molecules relaxes to a more π-stacked structure causing more dispersive bands. The charge transfer shifts the Fermi level into the lowest unoccupied molecular orbital band and turns the crystal metallic. Finally, we have studied molecular crystals of spiro-biphenalenyl neutral radicals. According to experimental studies, some of these materials show simultaneous electrical, optical and magnetical bistability. The electronic properties of these crystals are investigated by means of DFT with a focus on the possible intermolecular interactions of radical spins.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2006.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1001
Keyword [en]
Molecular crystals, Organic crystals, Nano technique, Density-Functional Theory (DFT), Fermi
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-7544ISBN: 9185497118 (print)OAI: oai:DiVA.org:liu-7544DiVA: diva2:22557
Public defence
2006-01-27, Schr¨odinger, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2017-12-12
List of papers
1. Effect of bending and vacancies on the conductance of carbon nanotubes
Open this publication in new window or tab >>Effect of bending and vacancies on the conductance of carbon nanotubes
2000 (English)In: Physical Review B, ISSN 1050-2947, Vol. 62, no 11, 7639-7644 p.Article in journal (Refereed) Published
Abstract [en]

Electron transport through nanotubes is studied theoretically using the Landauer formalism. The studies are carried out for finite metallic nanotubes that bridge two contacts pads. The current is observed to increase stepwise with the applied voltage. Each step corresponds to resonance tunneling including one single-particle eigenstate of the nanotube. Moderate bending of the nanotube results in a shift of the single-particle levels but the overall current remains essentially unaffected. For large bending, however, the π electron system becomes more disturbed, which introduces backscattering and a marked decrease in the conductivity along the tube. A single carbon vacancy in the nanotube is shown to have very small effect on the conductivity in the center of the metallic band whereas, by increasing the defect concentration the conductivity decreases in the same way as for the strongly bent tubes.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14060 (URN)10.1103/PhysRevB.62.7639 (DOI)
Available from: 2006-10-05 Created: 2006-10-05
2. Intershell conductance in multiwall carbon nanotubes
Open this publication in new window or tab >>Intershell conductance in multiwall carbon nanotubes
2003 (English)In: Physical Review B, ISSN 1050-2947, Vol. 67, no 7, 075406- p.Article in journal (Refereed) Published
Abstract [en]

A computational study of the electron transport properties of multiwall carbon nanotubes (MWNT’s) is presented. The study is focused on the role of intershell interactions and the resistance associated with transport between neighboring shells. The tight-binding approximation is used to describe finite double-wall nanotubes [(10,10)@(5,5)] connected to two semi-infinite metallic leads. The current-voltage characteristics are calculated from the multichannel Landauer formula. When contact is made to the outer shell the calculations show that the current is distributed over both coaxial NT’s but the overall resistance is independent if the inner tube is present or not. A device in which the leads are contacted to different coaxial shells of the MWNT (telescopic junction) is also investigated. The resistance of this system depends crucially on the extent of delocalization of the wave functions over several NT’s.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14061 (URN)10.1103/PhysRevB.67.075406 (DOI)
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2009-05-15
3. Electronic structure calculations of potassiumintercalated single-walled carbon nanotubes
Open this publication in new window or tab >>Electronic structure calculations of potassiumintercalated single-walled carbon nanotubes
2005 (English)In: Physical Review B, ISSN 1050-2947, Vol. 72, no 12, 125420-125428 p.Article in journal (Refereed) Published
Abstract [en]

We present results from density-functional theory calculations on the geometrical and electronic structure of potassium-intercalated (4,4) armchair and (7,0) zigzag single-walled carbon nanotubes. Intercalation of potassium results in notable changes in the geometrical structure, in particular in the zigzag system in which the carbon–carbon bond lengths in the unit cell vary between 1.40 Å and 1.45 Å. The most prominent effect of K intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from potassium to the carbon nanotube. In the case of the potassium-intercalated (7,0) nanotube the band structure and the position of the Fermi energy indicate a very good metallic conductor. The (4,4) nanotube has the potential to become superconducting due to the very high density of states at the Fermi energy which is obtained at high intercalation densities.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14062 (URN)10.1103/PhysRevB.72.125420 (DOI)
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2009-05-15
4. 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
5. Electronic structure calculations of a phenalenyl-based neutral radical conductor
Open this publication in new window or tab >>Electronic structure calculations of a phenalenyl-based neutral radical conductor
Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-14064 (URN)
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2010-01-13

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