Doped amorphous solids

Finally it is appropriate to mention the Scher-Montroll treatment (66) of anomalous charge transport in doped amorphous solids and polymers as an example of stochastic non-Markoffian rate expressions which result in the empirical Cole-Cole circular arc function. The memory function used is not contrary to some opinions purely empirical as Scher and Lax (67) have derived a similar function by consideration of transitions between randomly distributed trapping sites and evaluation of overlap integrals for the transition probability. (Some further discussion of this approach and... 

The two extremes of ordering in solids are perfect crystals with complete regularity and amorphous solids that have little symmetry. Most solid materials are crystalline but contain defects. Crystalline defects can profoundly alter the properties of a solid material, often in ways that have usefial applications. Doped semiconductors, described in Section 10-, are solids into which impurity defects are introduced deliberately in order to modify electrical conductivity. Gemstones are crystals containing impurities that give them their color. Sapphires and rubies are imperfect crystals of colorless AI2 O3, red. 

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

ILECTRONIC TRANSPORT IN AMORPHOUS SOLIDS and in random organic media in particular has been the subject of vigorous scientific activity for more than 2 decades. The approach taken in this chapter is to integrate recent studies of electronic transport in polysilanes with the extensive earlier work on poly(N-vinylcarbazole) (PVK) and molecularly doped polymers (MDPs) (i,e., systems in which transport-active molecular species are dispersed in an inert binder) and thereby allow the distinctive features of transport in the Si-based systems to emerge. 

Figure 5 Heat capacity of doped (solid circles) and pure (open circles) amorphous solid water...

So far experiments with H2Pc-doped pure rare gas matrices have been reported. As discussed above, the transition from a polycrystalline to an amorphous solid can be studied in a single system using condensed rare gas mixtures, on which we will focus now. 

Allen S.D., Garmire E., Bass M., Packer B. Calorimetric measurement ofLiNbOs waveguide absorption losses. Appl. Phys. Lett. 1979 34 435 37 Almeida R.M., Vasconcelos H.C., Gongalves M.C., Santos L.F. XPS and NEXAFS studies of rare-earth doped amorphous sol-gel films. J. Non-Cryst. Solids 1998 232-234 65-71 Almeida R.M. Sol-gel planar waveguides for integrated optics. J. Non-Cryst. Solids 1999 259 176-181... 

The absorption and emission spectra of crystalline or amorphous solids can be varied within wide spectral ranges by doping them with atomic or molecular ions [5.121]. The strong interaction of these ions with the host lattice causes broadenings and shifts of the ionic energy levels. The absorption spectrum shown in Fig.5.72b for the example of alexandrit, depends on the polarization direction of the pump light. Optical pumping of excited states... 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

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