Papers by Keyword: Polaron

Abstract: The energy levels of polaron in a wurtzite InxGa1-xN/GaN parabolic quantum well are investigated by adopting a modified Lee-Low-Pines variational method. The ground state energy, the transition energy and the contributions of different branches of optical phonon modes to the ground state energy as functions of the well width are given. The effects of the anisotropy of optical phonon modes and the spatial dependence effective mass, dielectric constant, phonon frequency on energy levels are considered in calculation. In order to compare, the corresponding results in zinc-blende parabolic quantum well are given. The results indicate that the contributions of the electron-optical phonon interaction to ground state energy of polaron in InxGa1-xN/GaN is very large, and make the energy of polaron reduces. For a narrower quantum well，the contributions of half-space optical phonon modes is large , while for a wider one, the contributions of the confined optical phonon modes are larger. The ground state energy and the transition energy of polaron in wurtzite InxGa1-xN/GaN are smaller than that of zinc-blende InxGa1-xN/GaN, and the contributions of the electron-optical phonon interaction to ground state energy of polaron in wurtzite InxGa1-xN/GaN are greater than that of zinc-blende InxGa1-xN/GaN. The contributions of the electron-optical phonon interaction to ground state energy of polaron in wurtzite InxGa1-xN/GaN (about from 22 to 32 meV) are greater than that of GaAs/AlxGa1-xAs parabolic quantum well (about from 1.8 to 3.2 meV). Therefore, the electron-optical phonon interaction should be considered for studying electron state in InxGa1-xN/GaN parabolic quantum well.

Abstract: Based on a tight binding model, we investigate impurity effect upon the stability of neutral and negatively or positively charged 1D conjugated polymer chains. Impurities are introduced by an attractive or a repulsive potential located at the lattice sites. The offsets of system energy due to light doping are calculated within adiabatic approximation. We show that doping position has significant impact upon system stability. A general picture of impurity distribution along the stretch direction of the polymer chain is obtained for both attractive and repulsive impurity potentials in neutral as well as in charged conjugated polymers. A polymer chain can generally be divided into edge, center and transition regions in terms of impurity distribution. It is found the static impurity distribution within a polymer is dominated by the strength and the sign of the impurity potential as well as whether the polymer chain is neutral or charged. Impurity distribution within the edge and the transition region is characterized by cluster and that within the center region by separation.

Abstract: A possible mechanism of polaron to bipolaron transition is investigated based on the well known SSH model. We have studied the dynamical process of collision between two positively charged polarons with same spin in the presence of external electric fields. It is found that when the relative velocity of collision is not very large, the collision leads to a direct transition of two polarons to a bipolaron.

Abstract: A soluble polypyrrole without any substituent groups was obtained by controlling the oxidation level of polypyrrole in the presence of oxygen. Upon the exposure to the 325 nm ultraviolet excitation from a helium-cadmium laser, thin films of the non-substituted polypyrrole could give off strong photoluminescence with its peak at about 530 nm. To elucidate the origin of the recorded strong photoluminescence, we measured the absorption spectra of the polypyrrole thin films with ultraviolet-visible absorption spectroscopy and our data indicated the appropriately oxidized polypyrrole are free of polarons. With the help of the electronic structures calculated with Hückel tight-binding method, the intense photoluminescence is interpreted in terms of the direct electronic transitions from the conduction band to valence band of the conjugated polymer.

Abstract: We investigate the dynamics of charged states (polarons) in bulk heterostructures of polythiophene and fullerene derivatives by means of photoinduced absorption spectroscopy, and is compared them with those in pristine polythiophene thin films. Polaron lifetimes in the bulk heterostructure and in polythiophene thin films at 6 K exceed 140 and 17 μs, respectively. The difference in lifetime indicates that charged states are better stabilized by the addition of the fullerene derivative. We also find that the dynamics of intrachain singlet excitons, which are not observed in the bulk heterostructure, are different from that of polarons in polythiophene thin films.