Optical cadmium compounds

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

Second order non-linear optical properties have been reported for a variety of TTF donor-acceptor compounds <02T7463> and the palladium complex 84 is a room-temperature semiconductor <02CL936>. Preparation of the zinc and cadmium compounds 85 has been reported <02CC1474> and aromatic fused TTFs such as 86 form thin films with useful electrical properties <02JAP265466>. A ferromagnetic interaction occurs in the salt of a TTF... 

More recently, the dangerous organometallic precursors (especially dimethylcad-mium) have been replaced by les s dangerous cadmium compounds, such as CdO [99] or Cd-acetate [260], both of which can be dissolved inTOPO or ODE in the presence of carboxylic or phosphonic acids. This makes the high-temperature synthesis of nanocrystals in organic solvents much easier (and therefore accessible), without losing any of the excellent optical properties of the particles. 

Quantum dots (QDs) are nanosized semiconductor particles of group II-VI or III-V main group elements with diameters of less than 10 nm [123]. These were first characterized in 1983 by Brus [124] as small semiconductor spheres in a colloidal suspension. QDs due to their nanoscale dimensions are subject to strong quantum confinement, which results in unique optical properties. Thus, in the last two decades, synthesis of QDs has attracted a lot of attention and generated a large number of publications [125]. A variety of methods have been proposed and implemented for synthesis of QDs. Of these methods, colloidal synthesis is the most accessible method for producing QDs suspended in solution. Colloidal synthesis of CdSe QDs is shown in Fig. 4. A cadmium compound is heated to 320°C... 

Table 4.1-157 Phonon frequencies/wavenumbers at symmetry points for cadmium compounds. Cadmium oxide (CdO), Fundamental optical-mode frequencies cadmium sulfide (CdS), 25 K cadmium selenide (CdSe), 300 K, from infrared and Raman spectroscopy cadmium tellmide (CdTe), 300 K, from inelastic neutron scattering...

The cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

The chalcogenides are binary compounds of a chalcogen (i.e., the elements of Group Ilb zinc, cadmium, mercury) with a less electropositive element, such as those of Group VIb (oxygen, sulfur, selenium, and tellurium). This section covers the sulfides, selenides, andtellurides. Oxides are reviewed above in Ch. 11. Most of the chalcogenides have useful optical characteristics and their applications are usually found in optics. 

The values of the bond ionicily can be used to predict the forbidden band width of zinc and cadmium diphosphides this width should be 1.5-2 eV. This prediction is supported by the recent [19, 20] measurements of the optical properties of these compounds which yield the optical width of the forbidden band AE = 2 eV for ZnP2 [19] and CdP2 [20]. 

Careful analysis of both powder X-ray dilfraction and EXAFS spectroscopic data located the cadmium sulfide as (CdS)4 cubes occupying the space within sodalite cages, with the Cd ions coordinated to framework oxygen atoms (Fig. 6.10). Furthermore, the clusters were observed to order between adjacent sodalite cages, to give superclusters or a superlattice structure. In subsequent work, a variety of compounds and elements have been prepared as well-defined clusters within zeolite frameworks, including metal oxides, selenides and phosphides, and these have been studied mainly with the view of determining the effects of cluster size on optical and electronic properties. 

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