Structure and photoluminescence of Ge nanoparticles embedded in SiO2 gel glasses fabricated at different temperatures

Ge nanocrystals embedded in bulk SiO2 glasses were prepared by combining the sol-gel process and the heat treatment in a H2 gas atmosphere. The size of Ge nanoparticles decreases with a decrease in the heat treatment temperature in a H2 gas atmosphere, and thus the crystalline Ge-Ge phonon vibration mode shifts to lower frequencies. The SiO2 gel glasses doped with Ge nanoparticles prepared at 500 °C showed a strong room temperature photoluminescence (PL) with peaks at 572, 609, 673, and 730 nm under 514.5 nm Ar+ laser excitation. When the heating temperature is increased to 600-700 °C, the peak positions of the PL spectra hardly vary. However, the PL intensity decreases tempestuously. The PL arises from the ultra small Ge clusters instead of nanocrystalline Ge with the diamond structure.