Showing papers by "Sven Stafström published in 1999"
TL;DR: The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq3) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium as discussed by the authors.
Abstract: The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq3) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq3 molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq3 molecule.
133 citations
TL;DR: In this article, the decay of photoexcitations in polythiophene chains has been studied in solid solutions of the polymer from room temperature to 4 K. A strong blue shift of the emission spectrum is observed in th...
45 citations
TL;DR: In this paper, the binding energies of the N 1s level in hard and elastic CNx films are investigated by means of theoretical studies of model molecules, where the core electron binding energies are determined using the delta self-consistent field method.
Abstract: Binding energies of the N 1s level in hard and elastic CNx films are investigated by means of theoretical studies of model molecules. Results for the model systems are obtained from ab initio Hartree–Fock calculations, where the core electron binding energies are determined using the delta self-consistent-field method. The theoretical results are compared with experimental x-ray photoelectron spectroscopy data in order to understand the microstructure of the CNx films. Both sp2 and sp3 bonded nitrogen have been identified. The presence of nitrogen bonded to graphite edges is also likely.
38 citations
TL;DR: In this paper, a C60 molecule sandwiched between a metal surface and a metal tip was simulated with an external pressure, and it was found that the observed increase in conductance cannot be explained in terms of the changes in the electronic structure of the C60 molecules alone.
Abstract: Using a tight-binding model we have studied the electronic transmission through a C60 molecule sandwiched between a metal surface and a metal (scanning tunnelling microscope) tip. By simulating compression of C60 we have interpreted an experimental study of the variation of the conductance through a C60 molecule with an applied external pressure. We found that the observed increase in conductance cannot be explained in terms of the changes in the electronic structure of the C60 molecule alone. Effects related to the metal/molecule contact, i.e. the strength of the metal/C60 interaction and the shape of the molecular orbitals in the tip, are in fact more important for the conductance. In view of this we discuss the importance of interference effects in the tip/molecule coupling.
27 citations
TL;DR: The complete electronic and vibrational structure of thin films of bithiophene in undoped form and after alkali doping have been investigated experimentally and theoretically as discussed by the authors, and the formation of a quinoidic structure on Cs doping is well reflected in high-resolution electron energy loss spectra (HREELS), which is due to the stiffened C-C interring bond as also predicted by theory.
Abstract: The complete electronic (occupied and unoccupied states) and vibrational structure of thin films of bithiophene in undoped form and after alkali doping have been investigated experimentally and theoretically. The ultraviolet (UV) photoemission and inverse photoemission spectra demonstrate clearly the filling of the LUMO of bithiophene in the doping process due to charge transfer from the alkali atoms, and density functional theory (DFT) calculations confirm the formation of dianion (bipolaronlike) states in the HOMO–LUMO band gap at high doping levels, respectively. High-resolution electron energy loss spectra (HREELS) have been used to probe the vibrational structure of the bithiophene films, and the formation of a quinoidic structure on Cs doping is well reflected in HREELS by the advent of an intense vibrational structure at 206 meV, which is due to the stiffened C–C interring bond as also predicted by theory.
12 citations
TL;DR: In this article, the metal-insulator phase diagram for trans-polyacetylene was calculated as a function of doping concentration and interchain interaction strength, and the effect of finite chain lengths was studied.
Abstract: Using a tight-binding Hamiltonian the metal-insulator phase diagram for trans-polyacetylene was calculated as a function of doping concentration and interchain interaction strength. The phase boundary for the periodic system coincides with the gap closing, which occurs for certain combinations of critical values for the doping concentration and the interchain interaction strength. The values found are in good agreement with the experimentally observed increase in the Pauli susceptibility. To simulate disorder in the polymer, the effect of finite chain lengths was studied. This type of disorder pushes the metal/insulator phase boundary towards the metallic side of the phase diagram. An increase in the doping concentration and/or interchain interaction is shown to reduce the localizing effects of disorder effectively. For realistic values of the interchain interaction strength the number of chain breaks needed to localize the states at the Fermi energy is quite small, of the order of a few percent. The localization length is found to be substantially longer than the conjugation length of the polymer.
10 citations
TL;DR: In this paper, a phenyl-sulphur molecule is attached to gold surfaces and can be used as interconnects of molecular wires in nano electronics in order to understand electronic transport through the sulphur-gold bridge.
4 citations
TL;DR: In this article, the problem of two polyene segments of variable lengths to which we have added two electrons was studied and the energy difference as a function of chain lengths for the two cases was calculated and a crossover from polarons to intra chain soliton -antisolitons was found.
2 citations
TL;DR: In this article, the energy spectrum and bond orders of a single-bonded C60 dimer were found to be correlated with the results of Valence Effective Hamiltonian calculations, leading to the general conclusion that each electron added to C60 stimulates the creation of one single bond to a neighbouring C60 anion.
Abstract: Single-bonded fulleride polymers of one-, two- and three-dimensional phases have been investigated theoretically using the Huckel model. Parameters for the sp3 hybridized carbon atoms forming the bonds have been found by fitting the energy spectrum and bond orders of the bonding atoms of a single-bonded C60 dimer to the results of Valence Effective Hamiltonian calculations. Our calculations strongly support previous results and lead to the general conclusion that each electron added to C60 stimulates the creation of one single bond to a neighbouring C60 anion. Furthermore, despite the fact that the intermolecular bonds are formed between two sp3 hybridized carbon atoms, all the polymer phases exhibit extended states.
2 citations
TL;DR: In this article, the parameters for the sp3 hybridized carbon atoms forming the bonds have been found by fitting the energy spectrum and bond orders of the bonding atoms of a single bonded C60 dimer to the results of VEH calculations on model systems of polymers.
1 citations
TL;DR: In this paper, the electronic transmission through a C 60 molecule with emphasis on the contact geometry, contact strength and the geometry of the molecule was studied, and interference effects related to the metal/molecule contact as well as the contact strength are in fact very important for the conductance.