Research

The stabilization of highly reactive compounds and the synthesis of technologically important materials (such as semiconductors) from molecular precursors are the two main areas of our research. We are particularly interested in finding low temperature routes for the deposition of thin films. Current work concentrates on thin films of copper, silver, gold and semiconductors (mostly silicon and germanium).

These films are an integral part of all microelectronic circuits and have a large potential for photo voltaic application ("solar cells"). Research in our group can be grouped into five largely interdependent fields:

1. Carbenes and their "heavy" relatives (R2M: with M = C, Si, P+, Ge, Sn and transition metals).
2. Stabilization of highly reactive species through bulky amido substituents.
3. Synthesis of novel volatile compounds as CVD precursors for microelectronic materials
4. Generation of Ge-Si and Ge-C thin films for semiconductor and solar cell technology, synthesis of new precursors for electronic grade thin silicon films.
5. Generation of superhard, oxidation resistant films based on boron and titanium.

 


1. Carbenes

We studied the stability of Cyclic diaminocarbenes 1 for a number of increasingly bulky alkyl substituents R

The stability of the carbenes is critically dependent on the steric bulk of the substituents and this distinguishes carbenes 1 from the aromatically stabilized Arduengo carbenes. For R = tBu, the carbene 1 can be distilled without decomposition. For smaller substituents R, dimerization is observed.

 

Alternative methods of synthesis, details of the carbene dimerization as well as reactions of the carbenes and olefins are currently investigated by Shilpi Gupta, Jose Rodezno, and Sebastien Fournier-Bidoz.


2. Silylenes

Like other reactive intermediates, Silylenes R2Si: are typically generated and studied in situ. The silylenes H2Si: and Cl2Si: play a key role in the growth of semiconductor silicon films.

 

We obtained the first stable dicoordinate silicon compound, the cyclic amide 1 in 1994 and have added other related compounds to the list (2 - 4).

Stable silylenes of type 1 intrigue us by a rare combination of very high thermostability (up to 500 oC) and high reactivity. The silylene 1 is stabilized by aromaticity and are currently exploring the extent of aromatic stabilization in isoelectronic compounds of phosphorus, boron and transition elements.

 

Transition metal silylene complexes R2Si=M, sila-imines R2Si=N and many other exotic silicon compounds that were so far accessible only with great difficulty can now easily be obtained from 1.

 Stable diamino-silylenes 1 can be obtained by reduction [M. K. Denk et al., J. Am. Chem. Soc. 1994, 116, 2691]. The reaction conditions as well as the substituents R can influence the yield and the outcome of the reduction reaction. Insight into the mechanism of the dihalosilane reduction was gained from an analysis of the product spectrum and from the reverse reaction, the oxidation of stable silylenes 1 by halogen donors. Alternative methods of synthesis for 1, details of the reduction and oxidation of stable silylenes and the reaction of 1 with metal halides will be reported. Silylenes are dicoordinate silicon compounds R2Si: Like other reactive intermediates such as Carbenes, Radicals etc. they have to be generated and studied in situ. The silylenes H2Si: and Cl2Si: play a key role in the growth of semiconductor silicon films.

We obtained the first stable dicoordinate silicon compound, the cyclic amide 1 in 1994 and have added other related compounds to the list (2 - 4).

Stable silylenes of type 1 intrigue us by a rare combination of very high thermostability (up to 500 oC) and high reactivity. We were able to show that 1 is stabilized by aromaticity and are currently exploring the extent of aromatic stabilization in isoelectronic compounds of phosphorus, boron and transition elements.

Transition metal silylene complexes R2Si=M, sila-imines R2Si=N and many other exotic silicon compounds that were so far accessible only with great difficulty can now easily be obtained from 1.

The stabilization of silylenes 1 has steric as well as electronic contributions. In aresent study we investigated the role of steric stabilization for diamino-carbenes 3. These carbenes can easily be obtained from the corresponding thioureas by reduction.

For R = Me, Et and iso-Pr, the carbenes slowly dimerize to the olefins but for R = For R = tert-Bu, the carbene is stable indefinitely [17].


Phosphenium Cations

Phosphenium cations are isoelectronic to Silylenes. We were interested if aromaticity would be beneficial in the stabilization of phosphenium cations.