Formation of germanium nanoparticles in silica glass studied by optical absorption and X-ray absorption fine structure analysis

We examined the relation between the 3.1 eV emission band and local structure for Ge+ implanted silica glass by means of photoluminescence, optical and X-ray absorption spectroscopies. In the 2 × 1015 cm−2 implanted sample, a new emission band around 2.7 eV was observed, the origin of which was assigned to the B2α oxygen deficient center and/or small Si clusters in silica. When the Ge+ fluence exceeded 2 × 1016 cm−2, a sharp and intense 3.1 eV emission band replaced the 2.7 eV band. We found that the intense 3.1 eV PL occurred by the prolonged X-ray irradiation onto the 2 × 1015 cm−2 implanted sample. UV–vis absorption and XAFS spectroscopies suggested that the aggregation of atomically dispersed tetravalent (Ge(IV)) atoms into Ge(0) clusters of ∼1 nm exhibit strongly correlation with the generation of the 3.1 eV PL. Such nano- and/or subnano-size Ge(0) clusters formed by the X-ray radiation were oxidized and decomposed again to the isolated Ge(IV) atoms, while those produced by the higher Ge+ fluence were stable against the exposure to air.