Siloxene structure

The stability and possible routes for the synthesis of these hypothetical tubular siloxene structures are strongly supported by the calculated strain energy as a function of tube diameter. The energy difference between a tube and the corresponding layer structure converges in proportion to 1/D towards the value of the reference structure (strain energy zero) as the diameter D increases see Figure 17.7. 

Our own data obtained from energy dispersive X-ray (EDX) analysis on epitaxial siloxene films confirm this general result. Figure 1 shows the typical EDX spectrum of a 1 pm-thick siloxene film on Si(l 11), obtained by exposure to concentrated HCl at 0 °C for one hour and washed in acetone. Whereas Ca is obviously completely removed from the sample, the film still contains some Cl not entirely washed out. From the spectrum, the Si 0 ratio is determined as 2.1 1. Thus, the formation of (Si2HOH)n is confirmed to be favored compared to that of related compounds with a lower or higher oxygen content. Indeed, siloxene is relatively stable in ambient atmosphere, in contrast to polysilyne (SiH) , which is reported to combust spontaneously in air [10]. Since in the ideal siloxene structure the single Si layers are exclusively terminated by H on one side and by OH on the... 

It should be noted that other structures, such as siloxene [De6] or polysilane [Ta4], that have been proposed to play a role in the luminescence mechanism of PS have also been studied on a theoretical level. 

Partially functionalized cyclopolysilanes recently attracted attention as model substances for siloxene and luminescent silicon. The yellow luminescent silicon is formed by the anodic oxidation of elemental silicon in HF-containing solutions and may be used for the development of silicon-based materials for light-emitting structures which could be integrated into optoelectronic devices77. Because the visible photoluminescence of... 

Since only polymers obtained from the cyclic starting materials 5, 6, 7, 10, 11 and 12, which are likely to have siloxene-like structures, exhibit color and fluorescence, the polysilane ring seems to be essential for the exceptional optical properties of siloxene. This is in agreement with the original idea of Kautsky, who assumed the cycle to be the chromophore. 

Kautsky342) called it siloxene and found the structure to be a polymeric layered compound with six membered silicon-rings linked by oxygen. The... 

Let us return now to the Wohler compound. What light does the chemistry of siloxene shed on the structure of this compound The Wohler compound is formed from CaSi2 with HC1 in high concentration in light and air. Under these conditions, siloxene forms halogen derivatives which react with water to hydroxysiloxenes. 

Crystalline silicon is the most widely used semiconductor material today, with a maiket share of above 90%. Because of its indirect electronic band structure, however, the material is not able to emit light effectively and therefore carmot be used for key applications like light-emitting diodes or lasers. Selected one- or two-dimensional silicon compounds like linear or branched polysilylenes [1] or layered structures like siloxene [2], however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid-state polymer with a sheet-like layered structure and an empirical formula Si H (OH) , in particular, is considered as an alternative material for Si-based liuninescent devices. Detailed studies of stmctural and photophysical properties of the material, however, are strraigly impeded by its insolubility in organic solvents. 

Summary Several linear and cyclic silanes (four-, five- and six-membered rings) with silicon-halogen or silicon-triflate functions were prepared and hydrolyzed to polymeric structures similar to Wohler siloxene and Kautsky siloxene. Optical investigations on the fluorescence of these polymers were carried out. The color and the fluorescence of the polymers are influenced by the ring size and the kind of substituents. Depending on the starting material the fluorescence maxima range from 400 to 550 nm. 

To elucidate the question whether or not cyclosilane structures are essential for the color and fluorescence of the polymers, linear and cyclic silanes with silicon halogen or -triflate functions were prepared and hydrolyzed to siloxene-like structures. 

Figure 15.1. Structures of the trigonal-rhombohedral tr6 modifications of the Zintl phases CaSi2 and CaGe2, of the sheet polymers polygermyne (GeH) and siloxene (Si2HOH) and of the polygermyne calcium dihydroxide intercalation compound (Ca(OH)2(GeH)2) . In each case, a full unit cell in the c direction is shown.

A prerequisite for an understanding of the luminescence properties of siloxene is to understand its solid-state structure. In the following section, the present knowledge about the structure of siloxene is briefly reviewed, and this is followed by a short summary of current studies aimed at rebuilding siloxene-like structures from well-defined molecular precursors to obtain structurally well defined sub-elements of the siloxene lattice. Particular attention will be devoted to the question as to whether the fluorescence properties of siloxene can be matched by selectively prepared siloxene sub-units or substituted cyclopolysilanes. 

---