| 1. |
- Böhlin, Johan, et al.
(författare)
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Effect of dynamic disorder on charge transport along a pentacene chain
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:8, s. 085209-
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Tidskriftsartikel (refereegranskat)abstract
- The lattice equation of motion and a numerical solution of the time-dependent Schrodinger equation provide us with amicroscopic picture of charge transport in highly ordered molecular crystals. We have chosen the pentacene single crystal as a model system, and we study charge transport as a function of phonon-mode time-dependent fluctuations in the intermolecular electron transfer integral. For comparison, we include similar fluctuations also in the intramolecular potentials. The variance in these energy quantities is closely related to the temperature of the system. The pentacene system is shown to be very sensitive to fluctuation in the intermolecular transfer integral, revealing a transition from adiabatic to nonadiabatic polaron transport for increasing temperatures. The extension of the polaron at temperatures above 200 K is limited by the electron localization length rather than the interplay between the electron transfer integral and the electron-phonon coupling strength.
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| 2. |
- Böhlin, Johan, et al.
(författare)
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Electronic structure calculations of the phenalenyl-based neutral radical conductor bis(9-cyclohexylimino-1-phenalenyl) boron
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:15
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Hansson, Anders, et al.
(författare)
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Structural and electronic transitions in potassium doped pentacene
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:18
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Hjort, M, et al.
(författare)
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Disorder-induced electron localization in metallic carbon nanotubes
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 63:11
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Tidskriftsartikel (refereegranskat)abstract
- Anderson localization in carbon nanotubes has been studied using the transfer matrix method. A new method was developed for which transfer matrices could be obtained for both chiral and achiral tubes. The localization length scales Linearly with the tube diameter but shows no dependence on tube chirality. The conductance in the nanotubes can be significantly altered by functionalization, an effect that could make them applicable as sensors. DOI: 10.1103/PhysRevB.63.113406.
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| 5. |
- Hjort, Mattias, et al.
(författare)
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Localization in quasi-one-dimensional systems
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:8, s. 5245-5250
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Tidskriftsartikel (refereegranskat)abstract
- A general method for obtaining transfer matrices in quasi-one-dimensional systems is presented. We used the method to study Anderson localization in trans-polyacetylene, polyparaphenylene, polyparaphenylene vinylene, and polythiophene. The electron localization length for the polymers as a function of disorder was calculated from the Lyapunov exponents. The polymers are shown to exhibit different sensitivity to disorder, which could be explained in terms of different dimensionality of the polymeric systems. We also give an explanation to the recently observed differences between electron and hole intrachain mobilities in alkoxy derivatives of polyparaphenylene-vinylene, as being a result of the electron-donating nature of the alkoxy substituents.
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| 6. |
- Hjort, Mattias, et al.
(författare)
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Modeling vacancies in graphite via the Huckel method
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:20, s. 14089-14094
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Tidskriftsartikel (refereegranskat)abstract
- It is known that when irradiating a graphite surface with ions the predominant source of defects are vacancies, Vacancies are also formed in a growing graphite sheer and cannot be filled. The theoretical work that has been carried out so far on the properties of these defects has only involved quite small model systems. Calculations on graphite, or closely related carbon phases like nanotubes, with randomly distributed vacancies require very large systems allowing only the simplest model to be used. We have used the Huckel method to model vacancies, both ordinary and hydrogenated, by fitting the Huckel energy levels to those resulting from a valence effective Namiltonian calculation. The three atoms neighboring a vacancy are treated as pseudo-pi orbitals and our calculations suggest that they together contribute with one extra electron to the system. Using this model optical and electronic properties of a graphite sheet as a function of vacancy density have been calculated.
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| 7. |
- Hultell (Andersson), Magnus, et al.
(författare)
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Impact of ring torsion dynamics on intrachain charge transport in conjugated polymers
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:1, s. 014302-
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Tidskriftsartikel (refereegranskat)abstract
- Based on an approach including both the time-dependent Schrodinger equation and an effective Newtons equation for the ionic motion, we study the impact of ring torsion dynamics on the intrachain charge transport process in conjugated polymers. As model systems we have used single chains of poly(para-phenylene-vinylene). Without any external electric field, the dynamics of the phenyl ring torsion is the dominant property controlling intrachain charge propagation. The charge is coupled to both ring torsions and bond lengths distortions, which results in a significantly more localized polaron state than in a planar chain. In the presence of an electric field, the charge can breach the barriers caused by ring torsions, a process that involves nonadiabatic effects and a temporary delocalization of the polaron state.
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| 8. |
- Hultell (Andersson), Magnus, et al.
(författare)
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Impact of ring torsion on the intrachain mobility in conjugated polymers
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:10, s. 104304-
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Tidskriftsartikel (refereegranskat)abstract
- Wehave developed a fully three-dimensional model based on the solutionof the time-dependent Schrödinger equation for studies of polaron mobilityin twisted polymer chains. Variations in ring torsion angles alonga conjugated polymer chain are shown to have a strongeffect on the intrachain charge carrier mobility. An increase inring torsion between two neighboring monomers can cause electron localizationand then result in a transition of the type oftransport from adiabatic polaron drift to nonadiabatic polaron hopping. Inparticular, we show the sensitivity for such a transition inthe case of random variations in the ring torsion anglesalong a poly(phenylene vinylene) chain. The effective energy barrier associatedwith the change in torsion angle also depends on theapplied electric-field strength, and by increasing the field strength atransition back to adiabatic transport can be obtained.
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| 9. |
- Johansson, Anders A., et al.
(författare)
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Nonadiabatic simulations of polaron dynamics
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:23
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Tidskriftsartikel (refereegranskat)abstract
- Polaron drift in an external electric field and the dynamics following insertion of an electron have been simulated in a one-dimensional conjugated polymer chain. The simulations were performed by solving the time-dependent Schrödinger equation and the lattice equation of motion simultaneously and nonadiabatically. We observed a step-like transition from sonic to supersonic polaron velocities around a field strength of 0.14 mV/Å. This behavior is observed to be directly related to decoupling of the acoustic and optical characters of the lattice vibrations associated with the polaron. Thermalization of hot (excited) electrons was shown to occur at a subpicosecond time scale. This process is highly nonadiabatic and strongly coupled to the lattice vibrations.
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| 10. |
- Johansson, A, et al.
(författare)
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Interchain charge transport in disordered pi-conjugated chain systems
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:8
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Tidskriftsartikel (refereegranskat)abstract
- Intermolecular charge transport under the influence of an external electric field is simulated in systems of conjugated polymer chains. Disorder is included in our model by varying the interchain coupling. Without the external field present the charge is extended over several chains in regions of strong interchain coupling, i.e., diffusion across regions of low interchain coupling is not possible. In the presence of a sufficiently strong external electric field, the electrons can drift (tunnel) through such regions. The average time the charge spends in the most disordered regions is therefore negligible compared to the residing time of the electron in the more ordered regions.
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