Selenide Chains and Rings

Elemental semiconductor clusters encaged in zeolites provide a valuable opportunity for gaining a fundamental understanding of semiconductor clusters because stoichiometry is not a concern in the synthesis. Selenium is of interest because it has an intermediate electrical conductivity and a negative coefficient of resistivity in the dark hence it is markedly photoconductive. It has uses in, for example, photoelectric devices and xerography. When Se is sorbed into a molecular sieve, it gives markedly different optical absorption spectra from those of the bulk material. 

Since the discovery of its stable soccerball structure [240] and of the arc-discharge preparation method for synthesizing large quantities of Cjo (buckminsterfull-erene, Fig. 4-16) and other fullerenes, [241] research on these materials has been progressing at a furious pace. The chemistry of the fullerenes has been recently summarized, [242, 243] and new reviews are rapidly making even rather recent ones out of date. 

Of great interest was the discovery of hi temperature superconductor properties in Cgo doped with combinations of alkali and other metals. [244, 245] These can be viewed as insertion compounds with the metal center occupying octahedral and tetrahedral interstitial ates in the fee lattice of Qo- Charge transfer occurs from the metal center to a vacant conduction band formed from interaction between the Cw and metal valence orbitals in the dose packed lattice. Since the fullerenes have stable hdlow cage structures, there is a possibility that dusters might be synthesized and stabilized within them. These hypothetical materials might be expected to have novel properties, induding superconductivity. 

The molecular scale cages of crystalline aluminosilicate zeolites (molecular sieves) are uniform, microscopic, and solvent-like media which can be used for synthesis of new materials, often called nanodusters. Ousters nthesized in the cages indude metal carbonyl, metal, and metal oxides. The best characterized of these are those dusters that are also well known in molecular chemistry, that is, such 

The presence of such clusters in zeolites has been well documented by a variety of spectroscopic methods, but there is still no evidence that clusters have been made in pure form in any zeolite. The highest reported yields are roughly 80%. Most of characterizations of the species in the cages were not quantitative. 

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