Novel fabrication of NIR-vis upconversion NaYF₄:Ln (Ln = Yb, Er, Tm) crystal layers by a flux coating method

High quality upconverting NaYF₄:Ln (Ln = Yb, Er, Tm) crystal layers were successfully fabricated directly onto glass substrates at a relatively low temperature of 350 °C by a novel two-step flux coating method. First, mixed pastes of solute (NaF and YF₃) and dopants (LnF₃) were coated on glass substrates by simple bar-coating, and then flux pastes (NaNO₃ or NaNO₃-NaF) were coated onto the dopant-containing solute layer. After the two-step coating, raw material-coated substrates were heated at 350 or 400 °C in an electric furnace. To remove the remaining flux, the crystal layers were washed with warm water. Finally, high quality, nanotextured NaYF₄:Ln crystal layers with good adhesion were directly grown onto the glass substrates. These crystal layers consisted of densely packed idiomorphic NaYF₄:Ln crystals. In particular, individual NaYF₄:Ln crystals were typically shaped as hexagonal prisms. The NaYF₄:Ln crystal phases depended strongly on the heating conditions, i.e., temperature and time. The NaNO₃ and NaNO ₃-NaF fluxes were effective for the growth of high-quality idiomorphic NaYF₄:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF₄:Ln crystal layers were controlled by changing dopant types and ratios. The NaYF₄:10%Yb,1%Er, NaYF ₄:50%Yb,1%Er, NaYF₄:20%Yb,1%Tm and NaYbF₄:1%Er crystal layers emitted green, orange, blue and red fluorescences, respectively, under 980 nm laser irradiation.