Quantum multilayer systems

It was pointed out earlier that photosensitivity may be realized not only by the inclusion of dyes in a polymer matrix, but also by means of multilayered system production. For TPA dispersed in a polycarbonate, sensitized by a thin layer of vacuum-deposited amorphous selenium, the quantum yield was equal to 0.7 at the electric field strength 6 x 10s V cm-1 [308]. The photoinjection efficiency of holes into polymer was equal to the efficiency in pure selenium. The spectra of the quantum efficiency is shown in Fig. 54. 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not... 

Addition of an inorganic sheet and a PVT-MV + layer results in efficient quenching of the porphyrin phosphorescence at 720 nm, due to electron transfer from excited porphyrin to MV +. Electron-transfer quantum yields were calculated as 0.58 and 0.76 for a-ZrP and HTiNbOs spacers, respectively. Figure 38 shows the steady-state emission spectrum of such a multilayer system. 

Lateral photoconductivity spectra and photofield electron emission of multilayer Ge/Si heterostructures with Ge quantum dots were studied at 77 and 300 K. Measurements of lateral photoconductivity spectra at 77 K showed the presence of localized states in the potential well. The photocurrent with limiting energies of 0.3 eV and 0.33 eV can be attributed to electron transitions from localized to delocalized states. A correlation between the regularities of the photoconductivity and photofield emission from such systems was revealed. 

In conclusion, photoelectric phenomena in multilayer quantum dot structures are determined by complicated multistage mechanism provided by the geometry of the system, properties of quantum dots and matrix as well as the interface between them and residual elastic strains in heterojunctions. 

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