| 1. |
- Bantikassegn, W., et al.
(författare)
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Absence of Schottky barrier formation in junctions of Al and polypyrrole-polyelectrolyte polymer complexes
- 1993
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 224:2, s. 232-236
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of conducting polypyrrole doped with large polymeric anions of polystyrene-sulphonate are electrochemically prepared to study the metal/polymer junctions. Aluminium and gold contacts are vacuum deposited to form metal/polymer/gold sandwich structures for current-voltage characterization. Photoelectron spectroscopy, using UV and X-ray photons, is carried out to investigate the possible causes of current limitation in the Al/PPy(PSS) junction.
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| 2. |
- Bantikassegn, W., et al.
(författare)
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Proceedings of the International Conference on Science and Technology of Synthetic Metals Electronic properties of polypyrrole (polystyrene-sulphonate)/metal junctions
- 1993
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Ingår i: Synthetic metals. - 0379-6779 .- 1879-3290. ; 55:1, s. 36-42
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Tidskriftsartikel (refereegranskat)abstract
- The nature of polymer/metal interfaces is decisive for the operation of polymer based electronic devices. At such interfaces charge transport may be affected by barrier formation, or by formation of insulating interfaces of various types. We have prepared thin films of conducting polypyrrole doped with large polymeric anions of polystyrenesulphonate for studies in metal/polymer junctions. Aluminium and gold contacts are vacuum deposited to form metal/polymer/gold sandwich structures. The current-voltage characteristics show that the interface between polypyrrole and gold is ohmic with no current limitation. However, the aluminium/polypyrrole interface forms highly resistive and nonohmic contacts. Photoelectron spectroscopy using UV and X-ray photons reveals a decrease of the work function upon Al deposition, reactions between Al and the sulphonate anions, and immediate oxidation of the aluminium upon exposure to oxygen. These observations corroborate the interpretation that the current limitation found at Al/polypyrrole junctions is due to formation of insulating aluminium oxide, not excluding reactions between the metal and dopant. It is also pointed out that interfaces between reactive metals and polymers are prone to such oxide interface formation, considering the high diffusivity of oxygen in many polymers.
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| 3. |
- Barta, P., et al.
(författare)
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Temperature evolution of the electronic band structure of the undoped and doped regioregular analog of poly(3â€alkylthiophenes) : A spectroscopic and theoretical study
- 1994
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 100:2, s. 1731-1741
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Tidskriftsartikel (refereegranskat)abstract
- The undoped and ferric chloride p‐doped ‘‘head‐to‐head’’ ‘‘tail‐to‐tail’’ analog of poly(3‐decylthiophene) obtained from substituted bithiophenes, i.e., poly(4,4’didecyl‐2,2’bithiophene)‐PDDBT has been investigated. The samples were studied by means of ultraviolet photoelectron spectroscopy,optical absorption measurements, and polymerbands structure calculations. Experiments were carried out at different temperatures between 100 °C and −180 °C. The results indicate that PDDBT is nonplanar in the undoped state. No temperature effects have been observed in the π‐electron properties. In the doped state, however, lowering of the temperature results in an increase of the system planarity that modifies the electronic band structure. This effect has been shown to be fully reversible with temperature.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Dannetun, Per, et al.
(författare)
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New Results on Metal-Polymer Interfaces
- 1993
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Ingår i: Molecular Crystals and Liquid Crystals. - : Taylor & Francis. - 1542-1406 .- 1563-5287. ; 228:1, s. 43-48
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Tidskriftsartikel (refereegranskat)abstract
- New results on studies of the early stages of formation of the aluminum-poly(p-phenylenevinylene) interface are presented.
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| 8. |
- 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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| 9. |
- 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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| 10. |
- 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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