Upconversion emission enhancement of Gd3+ ions induced by surface plasmon field in Au@NaYF4 nanostructures codoped with Gd(3+)-Yb(3+)-Tm(3+) ions.

Au nanoparticles (NPs) attached β-NaYF(4) nanocrystals codoped with Gd(3+)-Yb(3+)-Tm(3+) were synthesized by a facial solution method. The UV-vis-near-infrared absorption spectrum shows typical surface plasmon resonance band of Au NPs in addition to the characteristic absorption peaks of Yb(3+) ion. X-ray diffraction and selected area electron diffraction results indicate the existence of Au NPs. The transmission electron microscopic image reveals the formation of Au@NaYF(4) nanostructures. Enhanced ultraviolet (UV) upconversion luminescence (UCL) was observed in the nanostructures under the excitation of 980-nm infrared laser. The largest enhancement factor was obtained as 76 for the (6)I(J)→(8)S(7/2) emission of Gd(3+) ions, which was much larger than those emission enhancement factors of Tm(3+). It is for the first time to our knowledge that the emission enhancement of Gd(3+) ions was obtained. Local field enhancement induced by Au NPs was found to be responsible for the UCL enhancement, which is the further experimental evidence of local field enhancement theory. Magnetic measurements of the Au@NaYF(4) nanostructure indicated it would have potential application in magnetic resonance imaging.