The Hidden Effects of Particle Shape and Criteria for Evaluating the Upconversion Luminescence of the Lanthanide Doped Nanophosphors

The dependences of upconversion luminescence (UCL) on the particle size, shape, and inorganic−ligand interface of the hexagonal (β)-phase NaYF4:Yb,Er upconversion nanophosphors (β-UCNPs) were studied. The photophysical data, e.g., relative UCL intensity, power-dependent UCL, green to red emission intensity ratio (f g/r), and dynamic luminescence lifetimes of the prism-, plate-, and rod-shaped β-UCNPs as a function of surface to volume (SA/Vol) ratio were investigated. It was found that the UC properties can be attributed to not only the surface effects by comparing the SA/Vol ratios but also the particle shapes. At the comparable SA/Vol and ion (Yb/Er) doping ratios (20%/2%), the prism-shaped UCNPs showed much larger UCL intensity and smaller saturation power than those of the rod-shaped UCNPs. The shape effects on the UCL were also confirmed from the f g/r and photodynamic results on the fluorescence decay time. We attribute the shape effects to the different fractional lattice components in an anisotropic orientational β-nanocrystal in which the a-axis and c-axis have the crystal planes of ⟨101̅0⟩ and ⟨0001⟩ families, respectively, with different d-spacing distances. Therefore, the differently shaped nanocrystals with identical SA/Vol ratios could have different lattice energy and multiphonon relaxation processes. Aiming at clarifying the current contradictory observations on the UCL, three general criteria, namely, (i) a particle size window with a consistent shape, (ii) systematic investigation of f g/r as a function of the excitation power, and (iii) comparable UC hosts and surface properties, for assessing the UCL of the UCNPs are proposed. The observations reported here add an alternative point of view of understanding the energy transfer in the rare earth doped NPs, which should be useful for testing theories of optical confinement of the lanthanide-doped NPs and provide new strategies for both fundamental and technological interest like those well-known colloidal semiconductor quantum dots and rods.