| 1. |
- Brede, Jens, et al.
(författare)
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Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates
- 2009
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Ingår i: NANOTECHNOLOGY. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 20:27, s. 275602-
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Tidskriftsartikel (refereegranskat)abstract
- A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.
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| 2. |
- Broitman, E, et al.
(författare)
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Water adsorption on fullerene-like carbon nitride overcoats
- 2008
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 517:3, s. 1106-1110
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Tidskriftsartikel (refereegranskat)abstract
- Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous carbon nitride (a-CNx) films, widely used as protective overcoats in computer disk drive systems, with fullerene-like carbon nitride (FL-CNx) and amorphous carbon (a-C) films. Films with thickness in the range 10-300 run were deposited on quartz crystal substrates by reactive DC magnetron sputtering. A quartz crystal microbalance placed in a vacuum chamber was used to measure the water adsorption. Electron paramagnetic resonance (EPR) has been used to correlate water adsorption with film microstructure and surface defects (dangling bonds). Measurements indicate that the amount of adsorbed water is highest for the pure a-C films and that the FL-CNx films adsorbed less than a-CNx. EPR data correlate the lower water adsorption on FL-CNx films with a possible lack of dangling bonds on the film surface. To provide additional insight into the atomic structure of defects in the FL-CNx, a-CNx and a-C compounds, we performed first-principles calculations within the framework of Density Functional Theory. Emphasis was put on the energy cost for formation of vacancy defects and dangling bonds in relaxed systems. Cohesive energy comparison reveals that the energy cost formation for dangling bonds in different configurations is considerably higher in FL-CNx than for the amorphous films. These simulations thus confirm the experimental results showing that dangling bonds are much less likely in FL-CNx than in a-CNx and a-C films.
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| 3. |
- Broitman, E., et al.
(författare)
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Water adsorption on phosphorous-carbide thin films
- 2009
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 204:6-7, s. 1035-1039
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Tidskriftsartikel (refereegranskat)abstract
- Amorphous phosphorous-carbide films have been considered as a new tribological coating material with unique electrical properties. However, such CPx films have not found practical use until now because they tend to oxidize/hydrolyze rapidly when in contact with air. Recently, we demonstrated that CPx thin films with a fullerene-like structure can be deposited by magnetron sputtering, whereby the structural incorporation of P atoms induces the formation of strongly bent and inter-linked graphene planes. Here, we compare the uptake of water in fullerene-like phosphorous-carbide (FL-CPx) thin films with that in amorphous phosphorous-carbide (a-CPx), and amorphous carbon (a-C) thin films. Films of each material were deposited on quartz crystal substrates by reactive DC magnetron sputtering to a thickness in the range 100-300 nm. The film microstructure was characterized by X-ray photoelectron spectroscopy, and high resolution transmission electron microscopy. A quartz crystal microbalance placed in a vacuum chamber was used to measure their water adsorption. Measurements indicate that FL-CPx films adsorbed less water than the a-CPx and a-C ones. To provide additional insight into the atomic structure of defects in the FL-CPx and a-CPx compounds, we performed first-principles calculations within the framework of density functional theory. Cohesive energy comparison reveals that the energy cost formation for dangling bonds in different configurations is considerably higher in FL-CPx than for the amorphous films. Thus, the modeling confirms the experimental results that dangling bonds are less likely in FL-CPx than in a-CPx and a-C films.
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| 4. |
- 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
-
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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| 5. |
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| 6. |
- Furlan, Andrej, et al.
(författare)
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Fullerene–like CPx : A first–principles study of the relative stability of precursors and defect energetics during synthetic growth
- 2006
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 515:3, s. 1028-1032
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Tidskriftsartikel (refereegranskat)abstract
- Inherently nanostructured CPx compounds were studied by first-principles calculations. Geometry optimizations and cohesive energy comparisons show stability for C3P, C2P, C3P2, CP, and P4 (P2) species in isolated form as well as incorporated in graphene layers. The energy cost for structural defects, arising from the substitution of C for P and intercalation of P atoms in graphene, was also evaluated. We find a larger curvature of the graphene sheets and a higher density of cross-linkage sites in comparison to fullerene-like (FL) CNx, which is explained by differences in the bonding between P and N. Thus, the computational results extend the scope of fullerene-like thin film materials with FL-CPx and provide insights for its structural properties.
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| 7. |
- Furlan, Andrej, 1974-, et al.
(författare)
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Synthesis of phosphorus-carbide thin films by magnetron sputtering
- 2008
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Ingår i: physica status solidi (RRL) - Rapid Research Letters. - : Wiley InterScience. - 1862-6254. ; 2:4, s. 191-193
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Tidskriftsartikel (refereegranskat)abstract
- Phosphorus-carbide, CPx (0.025≤x≤0.1) thin films have beensynthesized by magnetron sputtering from pressed graphite-phosphorustargets. The films were characterized by X-ray photoelectron spectroscopy,transmission electron microscopy and diffraction, andnanoindentation. CP0.02 exhibits C-P bonding in an amorphous structure with elements of curved grapheneplanes, yielding a material with unique short range order. These features are consistent with what has been predicted by our results of theoreticallymodeled synthetic growth of CPx. The films are mechanicallyresilient with hardness up to 24 GPa and elastic recovery upto 72%.
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| 8. |
- Gueorguiev, Gueorgui Kostov, 1968-, et al.
(författare)
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First-principles calculations on the curvature evolution and cross-linkage in carbon nitride
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 410:4-6, s. 228-234
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations were utilized to study the formation mechanisms and structural features of fullerene-like carbon nitride (FL CN x). Cohesive energy comparisons reveal the energy cost for different defects arising from substitution of C for N as a function of the nitrogen concentration. In FL CNx, combinations of pentagons and heptagons compete in causing graphene sheet curvature during the addition of CN-precursors and single species. Also, cross-linkage between graphene layers in FL CNx can be explained by the bond rotation due to incorporated N atoms. The computational results agree with recent experimental observations from the growth of FL CNx thin films. © 2005 Elsevier B.V. All rights reserved.
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| 9. |
- Gueorguiev, G.K., et al.
(författare)
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First-principles calculations on the role of CN precursors for the formation of fullerene-like carbon nitride
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 401:1-3, s. 288-295
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations were utilized to study the formation mechanisms of the fullerene-like structure in carbon nitride compounds in the context of aligned incorporation of CN precursors along edges of evolving sheets during vapor phase synthesis. The incorporation of CN dimers and cyanogen molecules promotes an initial stabilization of pentagon defects, necessary for the observed bending of graphene multilayers in these compounds. The theoretical results corroborate recent experimental findings suggesting that preformed CxNy species, besides C and N atoms and ions, play an important role during the growth of FL CNx thin films by reactive magnetron sputtering. © 2004 Elsevier B.V. All rights reserved.
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| 10. |
- Gueorguiev, Gueorgui Kostov, et al.
(författare)
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First–principles calculations on the structural evolution of solid fullerene–like CPx
- 2006
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 426:4-6, s. 374-379
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Tidskriftsartikel (refereegranskat)abstract
- The formation and structural evolution of fullerene-like (FL) carbon phosphide (CPx) during synthetic growth were studied by first-principles calculations. Geometry optimizations and comparison between the cohesive energies suggest stability for solid FL-CPx compounds. In comparison with fullerene-like carbon nitride, higher curvature of the graphene sheets and higher density of cross-linkages between them is predicted and explained by the different electronic properties of P and N. Cage-like and onion-like structures, both containing tetragons, are found to be typical for fullerene-like CPx. Segregation of P is predicted at fractions exceeding ~20 at.%.
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| 11. |
- Gueorguiev, Gueorgui Kostov, 1968-, et al.
(författare)
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Nano-wire formation by self-assembly of silicon-metal cage-like molecules
- 2008
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 458:1-3, s. 170-174
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Tidskriftsartikel (refereegranskat)abstract
- By using DFT calculations we show that nano-wires consisting of endohedral MSi12 cage-like molecules are stable especially for light transition metal elements (M = Fe, Ni, Co, Ti, V, and Cu). The nano-wire assemblies are stabilized by the metal atoms located along their principal axes and can be seen as close Si-based analogues of C nanotubes, but with hexagonal cross-section due to the D6h-symmetry of their MSi12 building blocks. Independently on M, with the increase in the length of a (MSi12)m nano-wire, its HOMO-LUMO gap decreases gradually. The metallic behavior of (MSi12)m defines them as possible conductive components for self-assembled nano-devices. © 2008 Elsevier B.V. All rights reserved.
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| 12. |
- Gueorguiev, Gueorgui Kostov, et al.
(författare)
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Silicon-metal clusters : Nano-templates for cluster assembled materials
- 2006
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 515:3, s. 1192-1196
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Tidskriftsartikel (refereegranskat)abstract
- The structure, cohesive energy and electronic properties of MSin clusters were studied by first-principles calculations as a function of size (n) and species (M). We investigated 168 different clusters, containing from 1 to 14 Si atoms together with one transition metal atom among 12 different elements: Ti, Zr, Hf, V, Nb, Ta, Ni, Pd, Pt, Cu, Ag, Au. Clusters with n = 7, 10, 12 appear as local maxima in cohesive energy, independently of the metal involved. This, together with our previous findings for MSin (containing 12 other transition metal elements), establishes a systematic behavior. For most metals, MSi12 and MSi10 (the smallest endohedral species) are highly symmetric and exhibit local (with respect to their neighbors in size) stability. Thus, besides practically all MSi12 clusters (exceptions being HfSi12, ZrSi12), also some MSi10 such as VSi10, NiSi10, PdSi10, NbSi10 and AgSi10, are promising candidates as building blocks for cluster assembled materials. Electronic properties of structurally equivalent clusters depend markedly on the transition metal involved, providing the means to tailor pre-defined properties when designing extended phases. © 2006 Elsevier B.V. All rights reserved.
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| 13. |
- Hansson, Anders, et al.
(författare)
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Electronic structure calculations of potassiumintercalated single-walled carbon nanotubes
- 2005
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Ingår i: Physical Review B. - 1050-2947 .- 1094-1622. ; 72:12, s. 125420-125428
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
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| 15. |
- Hansson, Anders, et al.
(författare)
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Structural and electronic transitions in potassium doped pentacene
- 2006
-
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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| 16. |
- Hultell (Andersson), Magnus, 1978-
(författare)
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Electron-Lattice Dynamics in pi-Conjugated Systems
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we explore in particular the dynamics of a special type of quasi-particle in pi-conjugated materials termed polaron, the origin of which is intimately related to the strong interactions between the electronic and the vibrational degrees of freedom within these systems. In order to conduct such studies with the particular focus of each appended paper, we simultaneously solve the time-dependent Schrödinger equation and the lattice equation of motion with a three-dimensional extension of the famous Su-Schrieffer-Heeger (SSH) model Hamiltonian. In particular, we demonstrate in Paper I the applicability of the method to model transport dynamics in molecular crystals in a region were neither band theory nor perturbative treatments such as the Holstein model and extended Marcus theory apply. In Paper II we expand the model Hamiltonian to treat the revolution of phenylene rings around the sigma-bonds and demonstrate the great impact of stochastic ring torsion on the intra-chain mobility in conjugated polymers using poly[phenylene vinylene] (PPV) as a model system. Finally, in Paper III we go beyond the original purpose of the methodology and utilize its great flexibility to study radiationless relaxations of hot excitons.
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| 17. |
- Hultell (Andersson), Magnus, 1978-
(författare)
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Electron-lattice dynamics in π-conjugated systems
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis concerns the dynamics in π-conjugated hydrocarbon systems. Due to the molecular bonding structure of these systems there exists a coupling between the electronic system and the phonons of the lattice. If this interaction, which is referred to as the electron-phonon (e-ph) coupling, is sufficiently strong it may cause externally introduced charge carriers to self-localize in a polarization cloud of lattice distortions. These quasi-particles are, if singly charged, termed polarons, the localization length of which, aside from the e-ph coupling strength, also depend upon the structural and energetic disorder of the system. In disordered systems localization is strong and transport is facilitated by nonadiabatic hopping of charge carriers from one localized state to the next, whereas in well-ordered systems, where extended states are formed, adiabatic transport models apply.Despite great academic efforts a unified model for charge transport in π-conjugated systems is still lacking and further investigations are necessary to uncover the basic physics at hand in these systems. The call for such efforts has been the main guidelines for the work presented in this thesis and are related to the topics of papers I-IV. In order to capture the coupled electron-lattice dynamics, we use a methodological approach where we obtain the time-dependence of the electronic degrees of freedom from the solutions to the time-dependent Schrödinger equation and determine the ionic motion in the evolving charge density distribution by simultaneously solving the lattice equation of motion within the potential field of the ions. The Hamiltonian used to describe the system is derived from an extension of the famous Su-Schrieffer-Heeger (SSH) model extended to three-dimensional systems.In papers I-III we explore the impact of phenylene ring torsion on delocalization and transport properties in poly(para-phenylene vinylene) (PPV). The physics that we are particularly interested in relates to the reduced electron transfer integral strength across the interconnection between the phenylene rings and the vinylene groups upon ring torsion. Keeping this in mind, we demonstrate in paper I the impact of static ring torsion on intrachain mobility and provide a detailed analysis of the influence of the potential barriers (due to consecutive ring torsion) on the nature of charge carrier propagation. In paper II we extend our initial approach to include also the dynamics of ring torsion. We show that without any externally applied electric field, this type of dynamics is the dominant property controlling intrachain propagation, but that when an external electric field is applied, charge carriers may traverse the potential barriers through a process that involves nonadiabatic effects and a temporary delocalization of the polaron state. Finally, in paper III we study the impact of the lattice dynamics on the electron localization properties in PPV and show that the phenylene ring torsion modes couples strongly to the electronic wave function which gives rise to electron localization at room temperature.In papers IV and V we focus on the dynamics of molecular crystals using a stack of pentacene molecules in the single crystal configuration as a model system, but study, in paper IV, the transport as a function of the intermolecular interaction strength, J. We observe a smooth transition from a nonadiabatic to an adiabatic polaron drift process over the regime 20<J<120 meV. For intermolecular interaction strengths above J≈120 meV the polaron is no longer stable and transport becomes band-like. In paper V, finally, we study the internal conversion processes in these systems, which is the dominant relaxation channel from higher lying states. This process involves the transfer of energy from the electronic system to the lattice. Our results show that this process is strongly nonadiabatic and that the relaxation time associated with large energy excitations is limited by transitions made between states of different bands.
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| 18. |
- 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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| 19. |
- 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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| 20. |
- Hultell (Andersson), Magnus, 1978-, et al.
(författare)
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Nonradiative relaxation processes in molecular crystals
- 2008
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Ingår i: Journal of Luminescence. - : Elsevier BV. - 0022-2313 .- 1872-7883. ; 128:12, s. 2019-2026
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Tidskriftsartikel (refereegranskat)abstract
- Internal conversion is the dominant relaxation channel from higher lying excited states in molecular crystals and involves the transfer of energy from the electronic system to the lattice. In this work, we present results from simulations of the nonradiative relaxation process with an emphasis on both intra- and interband transitions. We find the internal conversion process to be strongly nonadiabatic and the associated relaxation time in the case of large energy excitations to be limited by the transitions made between states of different bands.
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| 21. |
- Hultell (Andersson), Magnus, et al.
(författare)
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Polaron dynamics in highly ordered molecular crystals
- 2006
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 428:4-6, s. 446-450
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Tidskriftsartikel (refereegranskat)abstract
- From a numerical solution of the time-dependent Schrödinger equation and the lattice equation of motion we obtain a microscopic picture of polaron transport in highly ordered molecular crystals in the presence of an external electric field. We have chosen the pentacene single crystal as a model system, but study the transport as a function of the intermolecular interaction strength, J. We observe a smooth transition from a nonadiabatic to an adiabatic polaronic drift process over the regime 20 < J < 120 meV. For intermolecular interaction strengths above 120 meV the polaron is no longer stable and the transport becomes band like.
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| 22. |
- Jakobsson, Mattias, et al.
(författare)
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A Monte Carlo study of charge transfer in DNA
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:12, s. 125102-
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Tidskriftsartikel (refereegranskat)abstract
- A model describing charge (hole) transport in DNA has been developed. The individual charge transfer steps in the transport process are described by Marcus theory modified to account for electron delocalization over adjacent identical nucleobases. Such a modification, as well as introducing a distance dependence in the reorganization energy, is necessary in order to reach an agreement with the observed transfer rates in well defined model systems to DNA. Using previously published results as a reference for the reorganization energy and with the delocalization described within the Hückel model we obtain an excellent agreement with experimental data.
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| 23. |
- Jakobsson, Mattias, et al.
(författare)
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Hole mobility and transport mechanisms in lambda-DNA
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:15, s. 155102-
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a study of charge transport in lambda-DNA using a recently developed model based on Marcus theory and dynamic Monte Carlo simulations. The model accounts for charge delocalization over multiple adjacent identical nucleobases. Such delocalized states are found to act as traps for charge transport and therefore have a negative impact on the charge carrier (hole) mobility. Both the electric field and temperature dependence of the mobility in lambda-DNA is reported in this paper. Furthermore, the detailed information produced by the simulation allow us to plot the progress of a hole propagating through the DNA sequence and this is used to identify the bottlenecks that limits the charge transport process.
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| 24. |
- Jiang, Jun
(författare)
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A generalized quantum chemical approach for nano- and bio-electronics
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A generalized quantum chemical approach for electron transport in molecular devices is developed. It allows to treat the devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. Effects of molecular length and hydrogen bonding on the current-voltage (I-V) characteristics of molecular devices are discussed. An extension to include the vibration motions of the molecule has been derived and implemented. It provides the inelastic electron tunneling spectroscopy (IETS) of molecular devices with unprecedented accuracy, and reveals important information about the molecular structures that are not accessible in the experiment. The IETS is shown to be a powerful characterization tool for molecular devices. An effective elongation method has been developed to study the electron transport in nanoand bio-electronic devices at hybrid density functional theory level. It enables to study electronic structures and transportation properties of a 40 nm long self-assembled conjugated polymer junction, a 21 nm long single-walled carbon nanotubes (SWCNT), and a 60 basepairs DNA molecule. It is the first time that systems consisting of more than 10,000 electrons have been described at such a sophisticated level. The calculations have shown that the electron transport in sub-22 nm long SWCNT and short DNA molecules is dominated by the coherent scattering through the delocalized unoccupied states. The derived length dependence of coherent electron transport in these nanostructured systems will be useful for the future experiments. Moreover, some unexpected behaviors of these devices have been discovered.
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| 25. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
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AlGaInN metal-organic-chemical-vapor-deposition gas-phase chemistry in hydrogen and nitrogen diluents : First-principles calculations
- 2006
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 431:4-6, s. 346-351
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Tidskriftsartikel (refereegranskat)abstract
- Direct impact of H2 and N2 diluents on the metal-organic-chemical-vapor-deposition gas-phase chemistry in M(CH3)3/NH3 (M = Al, Ga, In) systems is identified in the framework of Density Functional Theory in terms of cohesive energy differences. While both diluents destabilize model reaction species, i.e. adducts, transition states and chain complexes, the effect is particularly strong with respect to N2 in the Al(CH3)3/NH3 system, and can be a factor to restrain the expansion of chain complexes that deplete the gas-phase from precursors. Theoretical results are supported by experimental evidences of higher growth rate and superior optical properties of AlN grown in N2 vs. H2 diluent. © 2006 Elsevier B.V. All rights reserved.
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