Upconversion emission and its color modulation of the ZnS: Ho³⁺, Yb³⁺, Mn²⁺ nanomaterials

The ZnS: Ho³⁺, Yb³⁺, Mn²⁺ nanomaterials fabricated by the co-precipitation method have the zinc-blende structure with the crystallite size about 90 ∼ 100 nm. The X-ray diffraction (XRD), the scanning electron microscope (SEM) and the transmission electron microscope (TEM) characterizations confirm that the doping of the Mn²⁺ has less effect on the structure of the ZnS: Ho³⁺, Yb³⁺, Mn²⁺ nanomaterials. With the excitation of a 978 nm laser, the upconversion (UC) emissions of the ZnS: Ho³⁺, Yb³⁺, Mn²⁺ nanomaterials include two emission bands located at 551 and 664 nm, which correspond to the transitions ⁵F₄ (⁵S₂)→⁵I₈ and ⁵F₅→⁵I₈ of the Ho³⁺ ions. The UC emission characterization confirms that the UC emission of the ZnS: Ho³⁺, Yb³⁺ nanomaterials can be modulated by the doping of the Mn²⁺ ions, the increase of the incident laser power and the change of the laser pulse-width. The red emission of the ZnS: Ho³⁺, Yb³⁺ nanomaterials particle will increase significantly with the incorporation of the Mn²⁺ ions. The incident laser power and the laser pulse-width also can modulate the intensity ratio G/R of the green to the red. With the increase of the incident laser power, the emission intensity ratio G/R will also increase. With the increase of laser pulse-width from 0.2 to 0.8 ms, the above intensity ratio G/R will decrease significantly. The modulation between the green and red emission is due to the energy transfer from the ⁵F₄ (⁵S₂) state to the ⁵F₅ state of the Ho³⁺ ions through the state ⁴T₁ of the Mn²⁺ ions.