Gold Surface Plasma Element Enhances The Upconversion Luminescence Properties Of Rare Earth Ion Doped NaYF 4 Nanoparticles

Rare earth doped upconversion nanocrystals can convert two or more photons with lower energy into one photon with higher energy,which have unique potential application in biological assays and medical imaging due to their sharp emission bandwidths,large anti-Stokes shifts,low auto-fluorescence background and high penetration depth.However,upconversion emissions of rare earth doped nanocrystals mainly are attributed to the parity-forbidden 4f-f electron transitions,resulting in their low UC emission efficiencies.Surface plasmon resonance around the metal particles can generate significantly and highly localized electric fields,resulting in the luminescence enhancement of UC nanocrystals in the vicinity of metal particles.However,the surface plasmon resonance bands of metal structures are located in the visible region.This severely limits its UC emission enhancement factor because the surface plasmon of metal nanoparticles cannot couple with near infrared excitation light(980 nm).Consequently,it is important to tune plasmon resonance absorption of metal particles to near infrared region.Rare earth ions doped luminescent nanoparticles have attracted much attention from the biologists due to their potential applications as a biomarker.Among numerous rare earth ions doped UCL nanoparticles,the visible UCL nanoparticles excited at the 980 nm were extensively and mainly reported.However,the 980 nm excitation is overlapped with the absorption of water molecules in biological issues,which can result in the overheating of biological issues.Thus the significant cell death and tissue damage occurred under the excitation of 980 nm.In contrast to the UCL nanoparticles upon the 980 nm excitation,the Nd3+ sensitized NaYF4:Yb3+,Er3+nanoparticles can generate the UCL under the excitation of 808 nm.Previous investigations demonstrated that the UCL of Nd3+ sensitized NaYF4 nanoparticles excited at the 808 nm is more suitable for the biological applications,due to avoiding the overheating of biological issues caused by 980 nm excitation light.However,one of the major challenges limiting the applications of the UCL of rare earth ions doped nanoparticles excited at 808 nm is their low UCL efficiency.In this work,the tunable plasmonic Au films were used to improve the UCL of Nd3+ sensitized NaYF4:Yb3+,Er3+nanoparticles.In this work,the Au films with papilla Au nanoparticles were prepared by using polystyrene microsphere arrays as template,which exhibited ultra-broad plasmon resonance absorption.The influence of Au films with ultra-broad plasmon resonance absorption on the UC emission of NaYF4:Yb3+,Er3+ nanoparticles was investigated.More than 90-fold upconversion emission enhancement of NaYF4:Yb3+,Er3+ nanoparticles was obtained on the Au films with papilla Au nanoparticles,This new Au films may be used to other phosphors,which may have potentialapplications in photonic devices such as lasers and solid state imaging devices.Furthermore,the results showed that the enhancement factors and mechanism of the UCL of Nd3+ sensitized nanoparticles are associated with the tunable plasmonic properties of Au films.The maximum enhancement factors of green,red UCL of NaYF4:Nd3+,Yb3+,Er3+ nanoparticles excited at the 808 nm are 6 and 5.8-factors,respectively,on the ultra-broad plasmonic Au film.The differentiation of UCL enhanced mechanism of NaYF4:Nd3+,Yb3+,Er3+ nanoparticles on the tunable plasmonic Au films was observed.The enhanced UCL of NaYF4:Nd3+,Yb3+,Er3+nanoparticles on the narrow plasmonic Au film was from the enhanced excitation field and increasing of radiative decays rate,and the energy transfer enhancement was also obtained on ultra-broad plasmonic Au films besides the enhanced excitation field and the increasing of radiative decays rate.