Hydrothermal Synthesis and Upconversion Properties of About 19 nm Sc 2 O 3 : Er 3+ , Yb 3+ Nanoparticles with Detailed Investigation of the Energy Transfer Mechanism

The Sc 2 O 3 : Er 3+ , Yb 3+ nanoparticles (NPs) with the size of about 19 nm were synthesized by a simple oleic acid-mediated hydrothermal (HT) process. X-ray diffraction (XRD), transmission electron microscopy (TEM), upconversion luminescence (UCL) spectra, and decay curves were used to characterize the resulting samples. The Sc 2 O 3 : Er 3+ , Yb 3+ NPs made by HT method exhibit the stronger UCL, of which the red UCL are enhanced by a factor of 4, in comparison with those samples prepared by solvothermal (ST) method at the same optimized lanthanide ion concentrations. The UCL enhancement can be attributed to the reduced surface groups and longer lifetimes. Under 980 nm wavelength excitation, the decay curves of Er 3+ : ( 2 H 11/2 , 4 S 3/2 ) → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 emissions for Sc 2 O 3 : Er 3+ , Yb 3+ NPs samples are both close to each other, resulting from the cross relaxation energy transfer from Er 3+ to Yb 3+ , followed by an energy back transfer within the same Er 3+ -Yb 3+ pair. Also, under the relatively low-power density, the slopes of the linear plots of log( I ) vs. log( P ) for red and green emissions are 2.5 and 2.1, implying the existence of three-photon processes. Our results indicate that Sc 2 O 3 : Er 3+ , Yb 3+ NPs is an excellent material for achieving intense UCL with small size in the biological fields.