CASi-CVD · Stable Unsaturated Silicon Clusters as Nucleation Sites in Solution and the Gas Phase

Stable Unsaturated Silicon Clusters as Nucleation Sites in Solution and the Gas Phase

Executive Summary: Amorphous silicon (a-Si) is an important industrial material. It is used widely in electronic devices such as thin-film transistors (TFTs). Solar energy generation also relies heavily on a-Si for mass produced photovoltaic cells. Amorphous silicon is prepared via chemical vapour deposition (CVD) from silanes (e.g. SiH4), in which small, unsaturated silicon clusters are short-lived gas-phase intermediates. Significantly, such clusters are incorporated into the bulk a-Si and are critical in determining fundamental properties of the material, such as optical and electronic band-gaps. Crucial details of structure and bonding in these clusters are elusive. We sought to prepare stable derivatives of these unsaturated silicon clusters, explore their chemistry, and to demonstrate their use as CVD precursors in the deposition of a-Si. Our work built on the emerging chemistry of isolable, stable unsaturated silicon clusters. Using novel systematic synthetic approaches, we prepared stable unsaturated silicon cluster compounds as models for the unsaturated clusters present in CVD processes. We have been able to exploit the reactivity of these compounds to use them as solution-phase nucleation sites for cluster expansion, generating novel precursors for larger unsaturated cluster compounds. We successfully demonstrated proof of principle for the use of unsaturated silicon clusters in CVD processes. Furthermore, a remarkable solution-phase model for the growth of silicon clusters through unsaturated intermediates was discovered, and studied in depth. Significant additional results concerning the reaction chemistry of the smallest unsaturated silicon clusters, cyclotrisilenes, were also uncovered. One of the first examples of CO-activation by main-group species was explored. The project was highly successful, achieving a majority of the planned objectives as well as providing exciting results in unexpected (though related) areas. To date, five high-profile publications have been generated (see list). The fellow received strong training and gained skills in low-valent main-group chemistry, CVD processes, project/student supervision and DFT calculations. The project was terminated early due to the fellow obtaining an independent research position (a major aim of the IEF scheme). List of publications: i) Cowley, M. J., Abersfelder, K., White, A. J. P., Majumdar, M. & Scheschkewitz, D. Transmetallation reactions of a lithium disilenide. Chem Commun. 48, 6595–7 (2012). ii) Leszczyńska, K., Abersfelder, K., Mix, A., Neumann, B., Stammler, H., Cowley, M. J., Jutzi, P., & Scheschkewitz, D. Reversible base coordination to a disilene. Angew. Chem. Int. Ed. 51, 6785–8 (2012). iii) Ohmori, Y., Ichinohe, M., Sekiguchi, A., Cowley, M. J., Huch, V., Scheschkewitz, D. Functionalized Cyclic Disilenes via Ring Expansion of Cyclotrisilenes with Isocyanides. Organometallics 32, 1591–4 (2013). iv) Cowley, M. J., Huch, V., Rzepa, H. S., Scheschkewitz, D., Equilibrium between a cyclotrisilene and an isolable base adduct of a disilenyl silylene. Nature Chem. 5, 876-9 (2013) v) M. J. Cowley, Y. Ohmori, V. Huch, M. Ichinohe, A. Sekiguchi and D. Schechkewitz. Carbonylation of cyclotrisilenes. Angew. Chem. Int. Ed. DOI: 10.1002/anie201307450 (2013).

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