Highly Efficient Multicolor Up-Conversion Emissions and Their Mechanisms of Monodisperse NaYF4:Yb,Er Core and Core/Shell-Structured Nanocrystals

This paper comprehensively presents the highly efficient multicolor up-conversion (UC) emissions and related mechanisms of monodisperse NaYF4:Yb,Er core (α- and β-NaYF4:Yb,Er) and core/shell (α-NaYF4:Yb,Er@α-NaYF4 and β-NaYF4:Yb,Er@α-NaYF4) nanocrystals with controlled size (5−14 nm for α-phase nanocrystals and 20−300 nm for β-phase nanocrystals), chemical composition, and surface state. These nanoparticles were synthesized via a unique delayed nucleation pathway using trifluoroacetates as precursors in hot solutions of oleic acid/oleylamine/1-octadecene. With the naked eye and natural light the intense multicolor UC emissions (red, yellow, or green, without filters) can be observed in cyclohexane dispersions of as-prepared nanocrystals (1 wt %) excited by a 980 nm laser source (power density 1.22 W cm-2). On the basis of compositional optimization for β-NaYF4:Yb,Er nanocrystals, the intensity ratio of green to red emission (f g/r) reaches ca. 30, the highest value to our knowledge. The highly efficient multicolor UC emissions were aroused from the controlled crystallite size, phase, and associated defect state. α-NaYF4:Yb,Er and large β-NaYF4:Yb,Er nanocrystals displayed a normal two-photon UC process excited by a 980 nm NIR laser, while small β-NaYF4:Yb,Er nanocrystals showed an unusual partially three-photon process. Shell formation could remarkably decrease the surface defects and surface ligands influence and thus decrease the associated nonradiative decays. As a result, the core/shell-structured α-NaYF4:Yb,Er@α-NaYF4 and β-NaYF4:Yb,Er@α-NaYF4 nanocrystals exhibited greatly enhanced green emission intensity, f g/r, and saturation power with respect to their counterparts. The remarkable UC properties of the as-synthesized NaYF4:Yb,Er core and core/shell nanocrystals demonstrate that they are promising UC nanophosphors of adequate theoretical and practical interest.