| 1. |
- Mason, M.G., et al.
(författare)
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Interfacial chemistry of Alq3 and LiF with reactive metals
- 2001
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 89:5, s. 2756-2765
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure. © 2001 American Institute of Physics.
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| 2. |
- Crispin, Annica, et al.
(författare)
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Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:18, s. 8159-8167
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors was studied by means of photoelectron spectroscopy. With lithium atoms as dopants, two charges were stored on the same spiro branch in the form of bipolarons, for spiro-quarterphenyl and spiro-sexiphenyl. For doping with the sodium atoms, the size of the counter ions made it less energetically desirable to store two charges onto a single branch, and the charged species were polarons independent of the level of doping which was confirmed by optical absorption data.
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| 3. |
- Linares, Mathieu, et al.
(författare)
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On the interface dipole at the pentacene-fullerene heterojunction : A theoretical study
- 2010
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:7, s. 3215-3224
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C60 molecules (and of the resulting local electrical fields sensed by the molecular units).
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| 4. |
- Keil, M., et al.
(författare)
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High levels of alkali-metal storage in thin films of hexa-peri-hexabenzocoronene
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:24, s. 10854-10860
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Tidskriftsartikel (refereegranskat)abstract
- The results of the doping of molecular solid films of the medium-sized aromatic hydrocarbon hexa-peri-hexabenzocoronene (HBC), using lithium or sodium atoms were presented. The evolution of the valence band electronic structure was monitored by x-ray photoelectron spectroscopy and studied by ultraviolet photoelectron spectroscopy. Results indicated that n-doping depended on the type of alkali-metal atom employed and on the molecular order of the film. The smaller Li ion was found to have a stronger influence on the electronic structure than Na ions. These results revealed the potential use of the graphene materials in lithium-ion batteries with a high charge-storage capacity.
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| 5. |
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| 6. |
- Bröms, P., et al.
(författare)
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Optical absorption studies of sodium doped poly(cyanoterephthalylidene)
- 1994
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 67:1-3, s. 93-96
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Tidskriftsartikel (refereegranskat)abstract
- The effects of doping poly(cyanoterephthalylidene) with sodium in ultrahighvacuum been studied by optical absorption spectroscopy. Upon doping, new optical transitions are observed within the bandgap; the characteristics of these transitions are consistent with the formation of bipolarons. The optical absorption results are confirmed by direct measurements of the doping-induced gap states using ultraviolet photoelectron spectroscopy.
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| 7. |
- Crispin, Xavier, et al.
(författare)
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Characterization of the interface dipole at organic/metal interfaces
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:27, s. 8131-8141
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Tidskriftsartikel (refereegranskat)abstract
- In organics-based (opto)electronic devices, the interface dipoles formed at the organic/metal interfaces play a key role in determining the barrier for charge (hole or electron) injection between the metal electrodes and the active organic layers. The origin of this dipole is rationalized here from the results of a joint experimental and theoretical study based on the interaction between acrylonitrile, a p-conjugated molecule, and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces is investigated experimentally by photoelectron spectroscopies, while quantum mechanical methods based on density functional theory are used to study the systems theoretically. It appears that the interface dipole formed at an organic/metal interface can be divided into two contributions: (i) the first corresponds to the "chemical" dipole induced by a partial charge transfer between the organic layers and the metal upon chemisorption of the organic molecules on the metal surface, and (ii) the second relates to the change in metal surface dipole because of the modification of the metal electron density tail that is induced by the presence of the adsorbed organic molecules. Our analysis shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chemical potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the organic layer.
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| 8. |
- Crispin, Xavier, 1972-, et al.
(författare)
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Electronic delocalization in discotic liquid crystals : A joint experimental and theoretical study
- 2004
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:38, s. 11889-11899
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Tidskriftsartikel (refereegranskat)abstract
- Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2′,3′-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper π-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the π-electrons along the discotic stacks.
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| 9. |
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| 10. |
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| 11. |
- Greczynski, G., et al.
(författare)
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Electronic structure of pristine and sodium doped poly(p-pyridine)
- 2001
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 114:9, s. 4243-4252
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Tidskriftsartikel (refereegranskat)abstract
- Ultraviolet spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) were used to study the valence electronic structures of pristine and sodium doped poly (p-pyridine) (PPY). The UPS spectra were analyzed by studying the density-of-valence-states (DOVS) derived from quantum chemical calculations. The electronic band structure of the PPY chains was also theoretically investigated using the valence effective Hamiltonian (VEH) method. The theoretical approach was found to be more accurate in describing the electronic structure of PPY.
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| 12. |
- Karazazi, Y., et al.
(författare)
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Influence of contact geometry and molecular derivatization on the interfacial interactions between gold and conjugated wires
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 387:4-6, s. 502-508
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Tidskriftsartikel (refereegranskat)abstract
- Self-assembled monolayers made of thiolated conjugated wires attached on gold surfaces currently attract a considerable interest in the field of nanoelectronics. The interactions taking place at the metal/molecule interface govern the electronic structure of the complex, and hence the barriers for charge injection from the electrodes to the molecules. Considering benzenethiol as a prototype molecule, we investigate here the way the electronic structure is affected by the nature of the anchoring site of the sulfur atom on the gold surface and by the relative orientation of the molecule with respect to the surface. We also assess whether the changes in the molecular electronic properties upon substitution are similar for the isolated molecule and for the molecule attached on the gold surface. Our results provide strong evidences that, in order to introduce functionalities and/or improve charge injection in molecular devices, the electronic properties of conjugated molecular wires can be tailored by derivatization independently of the metal electrodes. copy,
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| 13. |
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| 14. |
- Bureau, C., et al.
(författare)
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DPT modeling of Stark-Tuning effect : CO on polarized Pd(100) as a probe for double-layer electrostatic effects in electrochemistry
- 2000
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Ingår i: QUANTUM SYSTEMS IN CHEMISTRY AND PHYSICS, VOL 2. - : Springer. - 0792359704 ; , s. 169-192
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Konferensbidrag (refereegranskat)abstract
- The lifetime of chemisorbed radical anions produced in the electroreduction of vinylic molecules is thought to play a decisive part in the mechanism accounting for the production of grafted films in electropolymerization reactions. With the ultimate purpose of evaluating these lifetimes, we propose a one-dimensional model taking into account the interface bond, the anion/metallic surface image charge potential, and the anion/polarized-surface electrostatic repulsion. Orders of magnitude are known for the parameters entering in these terms, except for the latter. In the present work, this term is described using the Gouy-Chapmann model for the electrochemical double layer. Comparing our theoretical DFT predictions on Stark-Tuning effect of CO on Pd(100) with experiment, we can discuss on the legitimacy of a phenomenological linear relationship between the (microscopic) surface electric field and the (macroscopic) electrode potential. The slope of this relationship, termed the electric field rate, in (V.cm(-1)).V-1, turns out to be numerically equivalent to the characteristic length of the double layer, whatever the underlying model. Our calculated rates, carried out within the Gouy-Chapmann approximation, are in acceptable agreement with previous experimental estimates. First insights into our electropolymerization reactions suggest that the presumed intermediate chemisorbed radical-anions may have a borderline stability, i.e. a largely non negligible lifetime on the surface.
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| 15. |
- Crispin, Xavier, et al.
(författare)
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Chemisorption of acrylonitrile on the Cu(100) surface: A local density functional study
- 1999
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:7, s. 3237-3251
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of chemically grafting polyacrylonitrile onto transition metal electrodes via electropolymerization leads to promising applications in the fields of corrosion protection or metal surface functionalization. The initial step of the electrografting mechanism is the adsorption of the acrylonitrile monomer on the metal surface from solution. Here, we investigate theoretically this adsorption process on the copper (100) surface; Density Functional Theory is used in the Local Spin Density approximation to describe the electronic and structural properties of acrylonitrile adsorbed on copper clusters. The chemisorption of acrylonitrile on the copper surface is confirmed experimentally via X-Ray Photoelectron Spectroscopy. The thermodynamic characteristics of the adsorption process are also studied via statistical mechanics. Finally, determining the influence of the copper cluster size on the adsorption of acrylonitrile allows to extrapolate the properties of the acrylonitrile/Cu(100) surface from those of acrylonitrile/copper clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)70231-X].
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| 16. |
- Crispin, Xavier, et al.
(författare)
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Understanding the initial stages of polymer grafting on metals : A photoelectron spectroscopy study of acrylonitrile adsorption on transition metal surfaces
- 2001
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - 0368-2048 .- 1873-2526. ; 121:1-3, s. 57-74
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Tidskriftsartikel (refereegranskat)abstract
- X-ray and UV photoelectron spectroscopies show that acrylonitrile is chemisorbed on iron, nickel and copper polycrystalline surfaces via the carbon and nitrogen atoms. Depending on the conditions used, different adsorption geometries are found. The molecules can either be adsorbed flat on the surface and chemically bound by a (2pp)-(3d/4s) overlap via both the C=C double bond and the C=N nitrile group or they can be adsorbed perpendicular to the surface via a covalent interaction between the nitrogen lone pair and the 3d-4s levels of the metals. Analysis of the XPS data obtained on molecular mono-layers chemisorbed on metal surfaces emphasizes the importance of initial-state effects (charge transfer upon chemisorption, contribution of the metal surface dipole) and final-state effects (metal screening and polarization effect within the mono-layer). The correlation between the XPS and UPS data illustrates the importance of the metal surface dipole in understanding the workfunction changes upon molecular adsorption on metal surfaces. © 2001 Elsevier Science B.V. All rights reserved.
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| 17. |
- Dannetun, Per, et al.
(författare)
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Evolution of Charge-Induced Gap States in Short Diphenylpolyenes as Studied by Photoelectron Spectroscopy
- 1994
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455 .- 0022-3654 .- 1541-5740. ; 98:11, s. 2853-2858
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of doping-induced electronic states within the otherwise forbidden energy gap has been studied as a function of the polyene length in a series of diphenylpolyenes. The chemical and electronic structures ha ve been studied using both X-ray and ultraviolet photoelectron spectroscopies. The results are interpreted with the help of quantum chemical calculations, performed using the semiempirical Austin Model 1 and valence effective Hamiltonian methods. The molecules studied area series of diphenylpolyenes, DPx, with x = 1-7 C=C double bonds in the pol yene part of the molecule. Since the frontier or bitals of the diphenylpolyenes are localized on the polyene chain portion of the molecule, there is a high degree of separation of the phenyl and polyene parts of the 11"-systems. Hence, many chemical and electronic properties of diphenylpolyenes are expected to be similar to those of short-chain trans-polyacetylene. For the longer molecules, n = 6 or 7, the present results indicate the presence of doubly charged, interacting soliton-antisoliton pairs, which appear as two new energy levels in the otherwise forbidden energy gap. In diphenyldecaheptaene to stilbene, i.e. 1 ≤ x ≤ 5, however, a singly charged state is formed at intermediate doping levels, after which the soliton-antisoliton pairs appear for the fully doubly charged systems. These results show that, remarkably, even for very short polyene segments, charges transferred are stored in the form of ( confined) solitons.
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| 18. |
- Dannetun, Per, et al.
(författare)
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Interface formation between poly(2,5-diheptyl-p-phenylenevinylene) and calcium : implications for light-emitting diodes
- 1994
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 67:1-3, s. 133-136
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Tidskriftsartikel (refereegranskat)abstract
- The early stages of metal/polymer interface formation between calcium and poly(2,5-diheptyl-p-phenylenevinylene) (PDHPV) have been studied using both X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Charge transfer is observed from the metal atoms to the polymer; as a result the calcium atoms at the interface are ionic, and negative bipolarons appear as the charge-carrying species on the polymer chains. This n-type doping of PDHPV by calcium leads to the appearance of new electronic states in the polymer bandgap. The calcium atoms appear to diffuse into the near surface region of the polymer, rather than forming a well-defined overlayer on the organic films.
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| 19. |
- Dannetun, Per, et al.
(författare)
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Proceedings of the International Conference on Science and Technology of Synthetic Metals The chemical and electronic structure of the interface between aluminum and conjugated polymers or molecules
- 1993
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 55:1, s. 212-217
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between aluminum and α-ω-diphenyltetradecaheptaenee (DP7), α-sexithienyl (6T), and poly(p-phenylenevinylene) (PPV), respectively have been studied using both X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS). The UPS valence band spectra, are interpreted with the help of quantum chemical calculations based upon Modified Neglect of Diatomic Overlap (MNDO), Valence Effective Hamitonian (VEH) and ab initio Hartree-Fock methods. DP7 is a model molecule for polyacetylene, while 6T is a model molecule (an oligomer) of polythiophene. The results indicate that aluminum reacts strongly with the surfaces of all of the materials studied. The π-electronic structure of each material was strongly modified. Furthermore, aluminum reacts preferentially with the polyene partof DP7, with the vinylene part of PPV, and with the α-carbons of the thiophene nits of 6T.
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| 20. |
- Dannetun, Per, et al.
(författare)
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Reactions of low work function metals Na, Al, and Ca on α,ω-€diphenyltetradecaheptaene. Implications for metal/polymer interfaces
- 1994
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 100:9, s. 6765-6771
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Tidskriftsartikel (refereegranskat)abstract
- The interactions between different low work function metals aluminium,calcium and sodium, and α,ω‐diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π‐electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic‐like defect for Ca/DP7.
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| 21. |
- Dannetun, Per, et al.
(författare)
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The chemical and electronic structure of the interface between aluminum and polythiophene semiconductors
- 1993
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 99:1, s. 664-672
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the chemical nature and the electronic structure of the interface between a low work function metal,aluminum, and a conjugated polymersemiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core‐level x‐ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymerelectronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl‐substituted poly‐3‐octylthiophene and (ii) the α‐sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals
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| 22. |
- Dannetun, Per, et al.
(författare)
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The evolution of charge-induced gap states in degenerate and non-degenerate conjugated molecules and polymers as studied by photoelectron spectroscopy
- 1994
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 67:1, s. 81-86
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of ultraviolet photoelectron spectroscopy (UPS) studies of the interaction between sodium and conjugated systems for a series of diphenylpolyees and diffrent oligomers of poly(p-phenylenevinylene) (PPV). The diphenylpolyenes include molecules containing two (i.e., stilbene) to 14 carbon atoms in the polyene part; stilbene itself can also be considered as a phenyl-capped monomer of PPV. Furthermore, a PPV oligomer with three phenylene units, as well as PPV itself, has been studied. The experimental results are interpreted with the help of quantum-chemical calculations using the Hartree-Fock semi-empirical Austin Model 1 (AM1) and valence-effective Hamiltonian (VEH) methods. An important result is that all the systems react strongly with sodium; at high doping levels two new doping-induced states are detected above the valence band edge of the pristine material. In the case of saturation-doped diphenylpolyenes (i.e., two sodiums per molecule), the new states can be discussed in terms of soliton-antisoliton pairs confined within the polyene part of the molecules; in contrast, the self-localized states induced in PPV and its oligomers have to be referred to as bipolarons.
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| 23. |
- Fredriksson, C., et al.
(författare)
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Theoretical studies of the aluminum/poly(p-phenylene vinylene) interface
- 1993
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 57:2, s. 4632-4637
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Tidskriftsartikel (refereegranskat)abstract
- A molecular quantum chemical approach is used to study the aluminum on PPV (Poly(p-Phenylene Vinylene)) interface. We focus on modifications to the chemical and electronic structure of the polymer upon interaction with a submonolayer of aluminum. A model system, trans-stilbene, is taken to investigate the nature of the Al-PPV bonding. Energetically favorable conformations are then used as prototypes to study the evolution of the electronic structure as modified by the reaction with aluminum. Results at the ab initio Hartree-Fock level indicates that Al atoms react with the vinylene linkage rather than the phenyl groups at early stages of interface formation.
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| 24. |
- Fredriksson, C., et al.
(författare)
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Theoretical studies of the aluminum/trans-polyacetylene interface : Calculations of model systems and comparison to experiment
- 1993
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Ingår i: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 57:2-3, s. 4590-4595
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between Al atoms and trans-polyacetylene has been studied quantum chemically at the ab initio Hartree-Fock level using oligomeric model systems. Investigations of the Al-polyacetylene bonding and modifications to the chemical and electronic structure of model systems for polyacetylene upon interaction with Al atoms are reported. The density-of-states is calculated for a polyene chain interacting with a pair of Al atoms. The results are discussed in relation to photoelectron spectra taken during Al deposition on an oligomeric model for polyacetylene (a diphenylpolyene with 7 C=C bonds in the polyene segment).
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| 25. |
- Greczynski, G., et al.
(författare)
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Electronic structure of poly(9,9-dioctylfluorene) in the pristine and reduced state
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:4, s. 1700-1706
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of a polymer of interest in polymer-based light emitting devices, poly(9,9-dioctyl-fluorene), was studied using a combined experimental-theoretical approach. Results were compared with those of equivalent studies of the electronic structure of two related conjugated polymers: poly(p-phenylene) and a latter-type poly (p-phenylene) (LPPP). Finally, it was shown that electrons added to the polymer system lead to the formation of polarons at low doping levels, and bipolarons at high doping levels. The energies of the polaron and bipolaron states were measured directly, and agree with the theoretical modeling of the added electrons.
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