Molecular substructures of the silicon crystal lattice

The development of microelectronics toward higher integration density is to encounter some fundamental physical limits in the near future. As a consequence research efforts have been made to develop molecular electronics (in contrast to nowadays solid state electronics). One aspect of these studies deals with the investigation of small sub- structures of the silicon lattice, as for instance the generation of silicon nano-crystals. While such materials are mainly synthesized employing physical techniques (for example vapor deposition, epitaxial growth and others) the research outlined in the current proposal is pursuing an approach to molecular precursor materials which are subjected to rearrangement conditions. The advantages of this entry are its high selectivity, simple characterization of the materials and especially the flexibility. In some preliminary work to the project it has been shown that the first example of an all sila-adamantane, which is the smallest substructure of the silicon lattice (and actually also the smallest possible silicon nano crystal) could be synthesized. The obtained molecule is a tricyclic, unstrained compound which can be subjected to further derivatisation reactions. Taking this as a starting point, the chemistry and physical properties of the sila-adamantanes will be studied. These compounds can be considered to be the missing link between bulk silicon and the class of polymeric organosilicon compounds (polysilanes). This state demands for a thorough examination of physical (spectroscopic) and chemical properties and comparison with solid state bulk silicon. Also the functionalization of the sila-adamantanes, as well as the synthesis of higher substructures of the silicon lattice will be investigated. By the introduction of atoms others than silicon (especially phosphorus and boron) into the scaffold of the adamanatanes the physical process of doping shall be studied at a molecular level.

Polymers have played an important role in our lives over the last 100 years. As synthetic materials they are ubiquitously present. While the typical conception about polymers is that they consist of chains of carbon, there are also polymers known, which consist of other elements such as silicon or germanium. Silicon itself is also omnipresent nowadays as it serves as the basic material for contemporary electronics. In the course of the rapid miniaturization of electronic devices, polymeric chains of silicon with organic residues, so called polysilanes are an interesting research area, for the development of molecule-based electronics. The arrangement of atoms in crystalline silicon (as it is used for electronic purposes) follows a diamond lattice. As the spatial orientation in the solid state is important for the conduction of electrons, it also needs to be considered in an attempt to substitute solid state silicon with polymeric chains of silicon atoms. Accordingly, synthetic efforts to prepare polysilanes need to take this into consideration. The project Molecular Substructures of the Silicon Crystal Lattice was on one hand concerned with methods for the synthesis of selected sections of solid state silicon and on the other hand with the study of the properties of these molecules. The synthetic focus was put on a catalytic reaction which allows polysilanes to rearrange the position of the silicon atoms within the molecule to obtain a more stable structure. This increased stability often correlates with an orientation which is similar to that in the silicon crystal lattice. A class of compounds where the scaffold atom arrangement resembles the diamond lattice is that of the adamantanes. The catalytic mentioned reaction now could convert a suitable precursor to the first known example of a polysilane-adamantane molecule. Another distinct feature of the studied rearrangement reaction, which was not know so far, is its ability to promote cyclization of the molecule with concurrent elimination of a molecule fragment. As cyclic molecules are more restricted in their intramolecular motion, the spatial immobilization of the atoms is stronger than in chain-type molecules. As germanium is another important semiconductor element attempts were made to introduce germanium atoms into polysilanes and the rearrangement behaviour of the obtained molecules was studied. It was shown that germanium atoms move into positions, where they are mainly surrounded by silicon or other germanium atoms and avoid bonds to carbon. This reaction behaviour allows the directed synthesis of chains consisting of germanium atoms with a silicon surface. In cooperation with an Irish research group the possibilities of obtaining nano-wires of a silicon-germanium alloy from our germylated polysilanes were studied. Quite unexpected nano-wires with a crystalline germanium core- and an amorphous silicon-oxide shell were formed. Such hetero core-shell structures are of high interest for the development of novel electronic circuitry.