Tuning the Upconversion Luminescence via Excitation-Power Control in Single β-NaYF4:Yb3+/Ho3+ Microcrystals

Upconversion nanomaterials have been extensively applied in many fields owing to their unique optical and electronic properties. In this work, β-NaYF₄:Yb³⁺/Ho³⁺ microcrystals were synthesized by an improved hydrothermal method. The morphology and the composition of the prepared microcrystals were experimentally investigated in detail by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Under the excitation of a 980 nm continuous-wave (CW) laser, the green (541 nm) and red (644 nm) upconversion luminescence (UCL) of Ho³⁺ through the energy transfer upconversion processes assisted by the Yb³⁺ ions were experimentally studied at the single-particle level. With the gradual increase of excitation intensity in the highly doped Yb³⁺ microcrystals, the red UCL was enhanced significantly more than the green UCL and was dominant, resulting in the UCL color finally tuning to pure red. Comparatively, for the lightly doped Yb³⁺ microcrystals, the green UCL exhibited a relatively similar increase to the red UCL. The populations of the Yb³⁺ and Ho³⁺ ions and transitions of the UCL were systematically demonstrated through calculation of the energy levels. Notably, a new UCL at 580 nm was observed, which was ascribed to the transition of ⁵F₁,⁵G₆ → ⁵I₇.