Near infrared to visible upconversion photoluminescence from Er/Yb co-doped MoS₂ nanosheets with tunable thickness

Er/Yb co-doped MoS₂ nanosheets exhibit efficient upconversion photoluminescence emission in the visible wavelength region at 524 nm, 541 nm, 650 nm and downconversion emission at 1124 nm under the excitation of 980 nm laser. These upconversion and downconversion emissions are proposed to be due to the ²H_11/2 - ⁴I_15/2 (524 nm), ⁴S_3/2 - ⁴I_15/2 (541 nm), ⁴F_9/2 - ⁴I_15/2 (650 nm) transitions of Er³⁺ and ²F_5/2 → ²F_7/2 (1124 nm) transition of Yb³⁺ ions respectively. It is also found that the Yb³⁺ ions play a significant role as a sensitizer for the generation of visible upconversion emission of Er³⁺ ions via excited state energy transfer mechanism. Further, the effects of the layer numbers in co-doped MoS₂ on the upconversion photoluminescence properties are also evaluated. The MoS₂ nanosheet possessing more number of layers fascinates stronger emission intensities due to the predominant crystal field effect.