Ge nanoparticles growth in Ge-doped sol-gel silica by e-beam exposure

Silica xerogels doped with Ge(IV), substituting for Si(IV) in the oxide network, are prepared from tetraethylorthosilicate and germanium-tetraethoxide. The sintering process is carried out in reducing atmosphere at 700 - 900°C by reaction with H2. Raman spectroscopy and high resolution transmission electron microscopy (TEM) show that reactions with H2 give rise, in the porous silica network, to uncontrolled islands of crystallites of elemental cubic germanium with average size of 50 nm. Sintering process in reducing H2 atmosphere at temperatures just below the phase separation, at about 610°C, gives materials where Ge atoms are dispersed in the matrix in conditions of incipient clustering. Evidences of segregation of germanium nanocrystals are observed with electron irradiation during TEM analysis. Furthermore, the electron beam induced precipitation leads to the formation of isolated quantum dots-like nanocrystals (5-6 nm in diameter) and with narrower size dispersion. The ranges of suitable temperature and germanium concentration are analysed, as well as the size dispersion of the resulting Ge nanophases.