Violet upconversion emission of sol-gel neodymium-doped GeO ₂-SiO₂ thin films via organically modified silane precursors

Nd³⁺-doped GeO₂-SiO₂ thin films are prepared by a sol-gel spin-coating process for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with gemanium isopropoxide are used as matrix precursors. Thermal gravimetric analysis, UV-visible spectroscopy, and Fourier transform infrared spectroscopy are used to study the structural and optical properties of the thin films. The results indicate that crack-free and high transparency in the visible and near infrared range thin films with a thickness of about 0.7 μm can be obtained by a single spin-coating process after a heat treatment at 500 °C. A strong UV absorption region at short wavelength ∼200 nm, accompanied with a shoulder peak at ∼240 nm due to the neutral oxygen monovacancies defects, is also identified. The effect of Nd³⁺ doping concentration on up-conversion emission of the thin films is studied. An intense room-temperature violet up-conversion emission at 397 nm is observed from the thin film with an optimum Nd³⁺ concentration of 0.5 mol% upon excitation with a xenon lamp at the wavelength of 586 nm. In addition to this intense violet emission, a relative weak ultraviolet emission at 372 nm and a blue emission at 469 nm are also observed.