Hexagonal β-Na(Y,Yb)F4 based core/shell nanorods: epitaxial growth, enhanced and tailored up-conversion emission

Abstract

To meet the increasing requirement, much effort has been devoted to enhance the emission intensity and tailor the emission color of rare earth phosphors. However, limited contributions have been made to the up-conversion (UC) of nanorods by complete epitaxial growth on each facet to achieve this requirement. In this study, we propose a facile epitaxial growth route to grow anisotropic hexagonal β-NaYF₄:Yb³⁺/Ho³⁺@β-NaYbF₄:Er³⁺, β-NaYF₄:Yb³⁺/Ho³⁺@β-NaYF₄, and β-NaYbF₄:Er³⁺@β-NaYF₄ core/shell nanorods, which were realized by adding hexagonal β-NaYF₄:Yb³⁺/Ho³⁺ or β-NaYbF₄:Er³⁺ nanorods as a core-nanostructure into a solution containing cubic α-NaYbF₄:Er³⁺ or α-NaYF₄ nanoparticles as the shell-precursor. During epitaxial growth-induced phase transformation, the precursor nanoparticles disappeared gradually in the solution and consequently corresponding β-phased shell yielded on each outer facet of each β-phased nanorod core. Eventually, the nanorod core was covered completely with a uniformly grown β-NaYbF₄:Er³⁺ or β-NaYF₄ shell. The UC emission of either β-NaYF₄:Yb³⁺/Ho³⁺ or β-NaYbF₄:Er³⁺ core can be enhanced by the outer shell due to the decrease in the number of surface defects. In addition, tailored UC emissions could be obtained by controlling the shell components and thickness, typically in the core/shell nanorods of β-NaYF₄:Yb³⁺/Ho³⁺@β-NaYbF₄:Er³⁺. The tunable colors with improved emission in these core/shell nanorods may find wider applications in multicolor labeling and anti-counterfeiting.