Silicon- and germanium-based sheet polymers and zintl phases

We have summarized the present knowledge about the layered Zintl phases of Ca with Si and Ge and about the sheet polymers obtained from them. In particular, we have been able to demonstrate in recent years the high quality epitaxial growth of CaSi2, CaGe2, and Ca(Si1-xGex)2 alloys on crystalline Si and Ge substrates, for the first time synthesizing the alloy Zintl phases. By topotactic transformation of these Zintl phases, a variety of formerly unknown sheet polymer compounds was obtained, most notably polygermyne and the alloy polygermanosilynes. With the alloy sheet polymers, we now have Si- and Ge-based materials available which show a strong visible photoluminescence tunable from 2.4 to 1.3 eV. While the variation of the luminescence through ligand substitution in sheet polymers has been known for a long time, we have shown that substitution of backbone atoms can also be used to tune the optical properties of these substances. Nevertheless, the issues raised in the last section ensure that sheet polymers will be an interesting and rewarding research field for years to come.