Effects of sodium dodecyl sulfate surfactant on up-conversion luminescence of Er³⁺/Yb³⁺-codoped NaLa(MoO₄)₂ nanocolloidal phosphor prepared by pulsed laser ablation in water

Er³⁺/Yb³⁺-codoped NaLa(MoO₄)₂ colloidal nanocrystals were synthesized by pulsed laser ablations in de-ionized water and sodium dodecyl sulfate (NaC₁₂H₂₅SO₄, SDS) aqueous solution for up-conversion (UC) luminescence bio-labeling applications. The influences of the SDS molecules on the crystallinities, crystal morphologies, crystallite sizes, and UC luminescence properties of the prepared Er³⁺/Yb³⁺-codoped NaLa(MoO₄)₂ colloidal nanocrystals were investigated in detail. Under a 980-nm excitation, the Er³⁺/Yb³⁺-codoped nanocolloidal NaLa(MoO₄)₂ suspension exhibited a weak red emission near 670 nm and strong green UC emissions at 530 and 550 nm, corresponding to the intra 4f transitions of Er³⁺ (⁴F_9/2,²H_11/2,⁴S_3/2) →Er³⁺ (⁴I_15/2). When the SDS solution was used, a smaller average crystallite size, narrower size distribution, and enhanced UC luminescence were observed. These characteristics were attributed to the amphoteric SDS molecules attached to the positively charged Er³⁺/Yb³⁺-codoped NaLa(MoO₄)₂ colloidal nanocrystals, effectively occupying the oxygen defect on their surfaces. The Er³⁺/Yb³⁺-codoped nanocrystalline NaLa(MoO₄)₂ suspension prepared in the SDS solution exhibited a remarkably strong green emission visible to the naked eyes.