Visible Emission from GeO2 Nanowires: Site-Specific Insights via X-ray Excited Optical Luminescence

Crystalline GeO 2 nanowires synthesized by a flow deposition technique on a silica substrate emit intense and distinct luminescence. X-ray Excited Optical Luminescence (XEOL) stimulated by a synchrotron light source in the soft X-ray energy regime was used to monitor the optical emission from germanium oxide nanowires upon X-ray absorption with the excitation energy tuned across O K- and Ge L 3,2-edges. XEOL spectra vary significantly at different edges. At the O K-edge, luminescence is characterized by green (~520 nm) and red (~780 nm) emission related to oxygen vacancies in the core of the nanowires and to structural defects on their surface. At the Ge L 3,2-edge, emission is dominated by an intense violet peak at ~400 nm mostly associated with defect centers at the interface between adjacent nanowires and with the silica substrate. The increasing attenuation lengths of soft X-rays tuned at excitation energies across the O K-edge (~300 nm) and the Ge L 3,2-edge (~1 ?m) enable the sampling of defects placed at different depths in the nanosized GeO 2 structures, down to the nanowire-SiO 2 interface. The use of tunable XEOL makes it possible to correlate excitation energy with the site-specific origin of luminescence and has provided direct evidence for the behavior of different structural defects in the visible emission from the GeO 2 nanowires. These results are of key interest to exploit the nanodimensional oxides in practical device applications.