Upconversion luminescence of β-NaYF4: Yb3+, Er3+@β-NaYF4 core/shell nanoparticles: Excitation power density and surface dependence

Coating a homogeneous layer outside the core nanoparticles has become a common method to improve the optical properties of nanoparticles. For rare earth ion-doped nanoparticles, although the homogeneous coating is found to enhance the upconversion luminescence, a large deviation in the reported enhancement amplitude, however, demonstrates the lack of a complete picture of the enhancement mechanism. In this work, we have performed steady-state and time-resolved spectroscopic studies on one of the most efficient upconversion nanosystems − β-NaYF4:Yb3+,Er3+ and β-NaYF4:Yb3+,Er3+@β-NaYF4 core/shell nanoparticles. Roles of the surface quenching centers, typically the high-frequency vibrational modes provided by the organic surfactants in the upconversion luminescence, are studied in detail. Our results show that excitation power density, once over a threshold, say similar to 150 W/cm(2) in this case, does have a non-negligible annealing effect, which may even lead to high luminescence upconversion intensity of the core nanoparticles compared to the shell-coated nanoparticles. The surface-related high-frequency vibrational modes play an important role in the upconversion process and in the laser annealing process, and the latter manifests itself in the difference of the laser annealing effect between the core and core/shell nanoparticles. From the upconversion luminescence kinetics analysis, it turns out that the luminescent centers of the core nanoparticle are severely quenched but homogeneous coating can effectively reduce the quenching, enhancing the upconversion luminescence. It is concluded that the upconversion emission spectrum, or more specifically the ratio between the red and green emissions, can be greatly altered by excitation power density for core nanoparticles but not for core/shell nanoparticles.