Magneto - Magnetic Bifunctional Core - Shell Nanocomposites Based On Dual Mode Excitation

In this paper, NaYF₄:Yb³⁺,Er³⁺ as a core and NaGdF₄:Eu³⁺ as a shell are chosen to investigate their upconversion and downconversion luminescence properties.Subsequently, these surfaces of NaYF₄:Yb³⁺,Er³⁺ particles are coated by Na_4-3xGd_xF₄:Eu_0.02³⁺（x= 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6） shell materials to form core-shell type nano-composite materials. Finally, the effects of Gd³⁺ contents inside their shell materials on down-/up-conversion luminescent properties and magnetic properties in nano-composite materials are carefully studied.Firstly, Yb³⁺ and Er³⁺ co-doped Na YF4 nanoparticles were synthesized using thermal decomposition reaction. So did NaGdF₄:Eu³⁺ particles. These nanoparticles were measured by TEM, XRD, and fluorescence spectrophotometry to characterize their morphology, crystal structure and upconversion luminescence. The results indicate that upconversion luminescence appear at 550 and 660 nm in NaYF₄:Yb³⁺,Er³⁺ nanoparticles under a 980 nm laser excitation; Characteristic downconversion luminescence of NaGdF₄:Eu³⁺ particles are obtained under the direct（394 nm） and host（274 nm）excitations.Secondly, NaYF₄:Yb³⁺,Er³⁺@Na_4-3xGd_xF₄:Eu_0.02³⁺（x= 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6）nano-composite materials were also synthesized using thermal decomposition reaction.Their morphology, crystal structure and upconversion luminescence were characterized.The results show those as follows. The down-conversion luminescent intensities of these nano-composite materials were depended on the increase of Gd³⁺ contents inside their shell materials. Their upconversion luminescent intensities were also improved with the increase of Gd³⁺ contents. It’s worth noting that the strongest down-/up-conversion luminescent intensities were obtained when Gd³⁺ contents reached up to50 mol%. This can be attributed to the contributions of micro-crystal fields inside the core materials and the cooperative ligand fields between the core materials and shell interfaces. For the former, the unique micro-crystal fields can contribute to the down-conversion luminescent improvement of the doped Eu³⁺ in shell structures; for the latter, the cooperative ligand fields can activate the ‘dormant’ Yb³⁺ and Er³⁺ ions near or on the surfaces of the nanoparticles.Thirdly, magnetic properties of these nano-composite materials were affected by the Gd³⁺ contents. Their paramagnetic were also improved with the increase of Gd³⁺contents.