Violet upconversion emission of sol–gel neodymium-doped GeO2–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.