Self-trapping barrier

Another interesting situation arises in the ST of excitons in which the electron and hole are spatially separated and are localized on different filaments (polymers or quantum wires) or different planes (or quantum wells). In such structures the electron-hole Coulomb interaction changes when these filaments or planes are deformed. As a result a strong exciton-phonon interaction may exist, even if the individual quasiparticles (electron and hole) have very small interaction with the phonons. The theory of ST of this type of excitations may be found in Ref. (44). 

---

It will be seen that a barrier exists resisting self-trapping this has been observed as a time delay by Laredo et al (1981,1983) for holes in AgCl, indicating a barrier height of 1.8 meV. 

Another mechanism that has been proposed is that the carriers move as small polarons20. A small polaron is a carrier that is self trapped in a well created by the lattice distortion. This lattice distortion is formed when a carrier stays sufficiently long in a position to polarize the medium around it. The applied field can lower the polaron barrier in a PF fashion and increase the mobility. The polaron transport model is attractive in that the mobilities in this mechanism are not critically dependent on the sample preparation. 

An example is pyrene, which is a crystal with a rather strong exciton-phonon interaction and a barrier height U 262 cm-1. It was demonstrated that upon photogeneration of excitons in this crystal, even at low temperature, the process of self-trapping not always requires relaxation to the bottom of the free-exciton band (with k = 0 black arrows in Fig. 3.3), but sometimes takes place directly... 

Fig. 3.3. Ground state potential and asymmetric double-well potential associated with the phenomenon of exciton self-trapping, as a function of the coordinate rj that undergoes a strong displacement upon self-trapping. F is the bottom of the free-exciton band, in which the lattice is not distorted (77 = 0), S denotes the lowest self-trapped exciton state, and U is the barrier height. The luminescence from the self-trapped state is red-shifted relative to the free-exciton luminescence. Upon photoexcitation of the system, two pathways towards the self-trapped state occur. The first possibility is that the created excitons first relax towards the bottom of the free-exciton well, after which they may further relax to the self-trapped state through tunneling or a thermoactivated process. This pathway is indicated by the filled arrows. The second possibility is that high-energy (hot) excitons relax directly to the self-trapped state, as indicated by the open arrow. Reprinted with permission from Knoester et al. (47). Copyright Elsevier (2003).

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

An internal irradiation process leads to the appearance of self-sustained currents observable external current corresponds to internal radioactivity, as far as a surface Coulomb barrier for secondary electrons exists. Such a barrier can be destroyed by heating due to electric conductivity enlarging, which results in the involvement of secondary electrons in electric transport. The thermal-stimulated current, connected with traps liberation can be also observed at a fast heating. Spontaneous electric fields, connected with internal currents, can also be observed. Their strength achieves 1 kV/cm and more at a room temperature. 

---