Influence of silica surface coating on optical properties of Er3+-Yb3+:YMoO4 upconverting nanoparticles

Abstract

Er3+-Yb3+:YMoO4@SiO2 core@shell upconversion (UC) nanoparticles (NPs) with its particle size ∼35nm and shell thickness ∼3nm synthesized via Stöber method have explored its thermal stability, particle morphology and optical properties. The optical band gap energy clearly shows the effect of surface coating of silica layer across the surface of core@shell NPs due to increase in the crystallite size. Due to presence of silica layer across the core NPs, the UC emission intensity corresponding to the 2H11/2, 4S3/2→4I15/2 transition has been increased about ∼10 times in the core@shell NPs than the core NPs. Temperature dependent study using thermally coupled 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions in the biological temperature range (300–368K) has been performed. The maximum sensitivity ∼2.36%K−1 at 368K, which is ∼12 times greater than the core NPs of the core@shell UC NPs has been reported. Thermal heat gain by the core@shell NPs within 311–343K temperature range is suitable for hyperthermia based cancer treatment has been reported. The core@shell NPs show good dispersibility in different biological solvents such as water, methanol and DMSO. Moreover, the core@shell NPs dispersed with biological solvents show good luminescence stability without interactions with solvent molecules which could minimize the thermal effect caused by the 980nm diode laser excitation. As a consequence, Er3+-Yb3+:YMoO4@SiO2 core@shell NPs have suitable applications for making optical devices namely upconvertors, LEDs, optical bioprobes, etc.