Synthesis of nanoamorphous germanium and its transformation to nanocrystalline germanium
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A simple reaction between a mild reducing agent such as a trialkoxysilane and GeIV species such as germanium tetraalkoxides in a room-temperature water/ alcohol solution produces silica-coated ultrasmall (2–3 nm) amorphous germanium nanoparticles (na-Ge/SiO2). The initial reaction involves the straightforward hydrolysis and condensation of the precursors, Ge(OCH2CH3)4 and (CH3CH2O)3SiH, where the reaction rate depends on the water concentration in the reaction medium. These processes can be further accelerated by adding acid to the reaction medium or carrying out the reaction at higher temperatures. At low water contents (up to 50% water/ethanol) and low acid concentrations, the reaction proceeds as a clear solution, and no precipitation is observed. The initially colorless clear solution progressively changes to pale yellow, yellow, orange, red, and finally dark red as the na-Ge particles grow. Evaporation of the solvent yields a reddish-brown powder/monolith consisting of na-Ge, embedded in an encapsulating amorphous silica matrix, na-Ge/SiO2. The formation of na-Ge proceeds extremely slowly and follows a first-order dependence on both water concentration and diameter of the na-Ge particles under the reaction conditions used. Annealing of the na-Ge/SiO2 powder under an inert atmosphere at 600 °C produces ultrasmall germanium nanocrystals (nc-Ge) embedded in amorphous silica (nc-Ge/SiO2). Freestanding, colloidally stable nc-Ge is obtained by chemical etching of the encapsulating silica matrix.