Rise and Decay of\nPhotoluminescence in Upconverting\nLanthanide-Doped Nanocrystals

Nanocrystals (NCs) doped with lanthanides are capable\nof efficient\nphoton upconversion, i.e., absorbing long-wavelength light and emitting\nshorter-wavelength light. The internal processes that enable upconversion\nare a complex network of electronic transitions within and energy\ntransfer between dopant centers. In this work, we study the rise and\ndecay dynamics of upconversion emission from β-NaYF<sub>4</sub> NCs codoped with Er<sup>3+</sup> and Yb<sup>3+</sup>. The rise dynamics\nof the red and green upconverted emissions are nonlinear, reflecting\nthe nonlinear nature of upconversion and revealing the mechanisms\nthat populate the emitting states. The excited-state decay dynamics\nare nonexponential. We unravel the underlying decay pathways using\nphotonic experiments. These reveal the contributions of different\nupconversion pathways visually, as each pathway exhibits a distinct\nresponse to systematic variation of the local density of optical states.\nMoreover, the effect of the local density of optical states on core-only\nNCs is qualitatively different from core–shell NCs. This is\ndue to the different balance between feeding and decay of the electronic\nlevels that produce upconverted emission. The understanding of the\nupconversion dynamics provided here could lead to better imaging and\nsensing methods relying on upconversion lifetimes or guide the rational\noptimization of the dopant concentrations for brighter upconversion.