Thermal annealing activates amplified photoluminescence of germanium metabolically doped in diatom biosilica

There is significant interest in the fabrication of germanium (Ge) doped silica for optoelectronic device applications. In this study, highly photoluminescent Ge centers, metabolically doped into diatom biosilica, are activated by thermal annealing in air. Diatoms are single celled photosynthetic algae that make silica shells called frustules. These frustules possess intricate features and patterns on the nano- and micro-scale. A two-stage photobioreactor cultivation strategy is used to biologically fabricate diatom biosilica doped with Ge, ranging from 0.24 to 0.96 weight percent Ge. X-Ray photoelectron spectroscopy (XPS) and electron diffraction show that a mixture of amorphous germanium dioxide (GeO2) and germanium oxide (GeO) is doped into the frustule structure. Annealing in air thermally converts the amorphous GeO2 to GeO, commensurate with an enhancement in the photoluminescence. Thermal gravimetric analysis (TGA) and photoluminescence of annealed biosilica with and without Ge confirm that the photoluminescence originates from GeO photoluminescent centers, and not from the inherent photoluminescence of the biosilica. This is the first study to thermally activate and characterize highly photoluminescent Ge centers metabolically doped into diatom biosilica.