Molecular precursor method, synthesis

Gci c j,Si tSnj, layers grown over time directly on Si wafers. We also describe synthesis of films and nanometer-scale islands of Sii (Ge ( grown on Si(l 00) substrates via a unique single-source molecular precursor method. This new approach allows precise control of concentration and structure at the atomic level and it is particularly useful for development of composi-tionally homogeneous and uniform assemblies of nanoscale structures. 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

Nanocrystals are zero-dimensional particles and can be prepared by several chemical methods, typical of them being reduction of salts, solvothermal synthesis and the decomposition of molecular precursors, of which the first is the most common method used... 

Decomposition of single molecular precursors provides convenient and effective routes for the synthesis of metal chalcogenide nanocrystais. In this method, a molecular complex consisting of both the metal and the chalcogen is thermally decomposed in a coordinating solvent. For example dithiocarbamates and diseleno-carbamates have been found to be good air stable precursors for sulfides and selenides of Cd, Zn and Pb. Nanocrystais of Cd, Hg, Mn, Pb, Cu, and Zn sulfides have been obtained by thermal decomposition of metal hexadecylxanthates in hexadecylamine... 

A very useful method leading to highly sterically hindered /3-diketones with R1 = CRR(OMe), employed for the synthesis of alkaline-earth metal-containing molecular precursors for CVD, is based on the condensation of ketones with alkynes, followed by the reaction of the product with acyl chlorides to give a-acetylinic ketones. Finally the a-acetylinic ketones react with amines yielding aminovinylketones and, after hydrolysis, the resulting /3-diketones (Equation (5)) 50... 

A large number of binary ceramics such as carbides, nitrides, and oxides have been obtained by the decomposition of metal-organic precursors. The following text presents some selected examples to demonstrate the promises and limitations of molecular precursor approach. A complementary account of the synthesis of metal oxide nanoparticles from solution-based methods is given in Chapter 12.03. 

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