Up-conversion in rare earth-doped silica hollow spheres

In the present work, Er/Yb co-doped silica hollow spheres are prepared in a two-step process. In a first step, polystyrene-core is silica coated in situ by a modified Stöber sol–gel method and in the second one, the sacrificial polystyrene core is thermally removed. The core–shell and the hollow spheres are characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy (PL). PL measurements show up-conversion phenomena upon excitation at 975 nm, through the emission of blue (∼490 nm), green (∼523 nm and ∼536 nm) and red (∼655 nm) light. The up-conversion phenomena are discussed and modelled. The developed model explains the up-conversion phenomena of Er/Yb co-doped silica hollow spheres, with special agreement for high Yb/Er ratio.

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► UpC emission was detected at blue, green and red wavelengths, upon NIR excitation.
► Simulation explain UpC of Er:Yb materials, especially those with high Yb/Er ratio.
► Co-doped hollow spheres have potential for bio-imaging diagnostics.