Solid solution Na(Gd/La)(MoO4)2:Yb3+/Er3+ upconversion nanocrystals with simultaneously enhanced photothermal conversion efficiency and luminescence intensity

Highlights: • Effects of La³⁺ content on the phase, crystal structure, upconversion luminescence and photothermal conversion. • Na(Gd_xLa_1-x)(MoO₄)₂ solid solution nanocrystals with simultaneously enhanced photothermal effects and luminescence intensity. • An effective way to combat thermal quenching of lanthanide luminescence. • Potential for multifunctional applications both in photoluminescent and photothermal fields. Lanthanide doped upconversion nanocrystals with remarkable optical properties have been utilized for many emerging applications. Here, solid solution Na(Gd_xLa_1-x)(MoO₄)₂ nanocrystals are synthesized hydrothermally, and their upconversion photoluminescent and photothermal properties as luminescent host matrix for Yb³⁺/Er³⁺ activators are systematically studied. The effect of La³⁺ doping content on the phase, crystal structure, upconversion luminescence and photothermal conversion are investigated. Rietveld refinements based on XRD data indicate that La³⁺ occupy Gd³⁺ sites substitutionally, and pure phase solid solution Na(Gd_xLa_1-x)(MoO₄)₂ is formed. Due to lattice distortion, lower lanthanide site symmetry and augmented odd-parity crystal field interactions after La³⁺ doping, the 4f-4f transition probabilities of the lanthanide ions are effectively increased. Na(Gd/La)(MoO₄)₂ solid solution hosts exhibit enhanced upconversion luminescence from Er³⁺/Yb³⁺, compared with single-component NaGd(MoO₄)₂ counterparts. What's more, based on the calculated temperatures of the samples derived from the luminescence intensity ratio of states ²H_11/2 and ⁴S_3/2 for Er³⁺, enhanced photothermal conversion efficiency is speculated in solid solution Na(Gd_xLa_1-x)(MoO₄)₂:Yb³⁺/Er³⁺ nanocrystals, which is further confirmed by infrared thermographic images. The underling mechanisms of simultaneously enhanced photothermal effects and luminescence intensity in Na(Gd_xLa_1-x)(MoO₄)₂ solid solution nanocrystals are elucidated. Modulation of composition in solid solution hosts may be an effective method for combating luminescent thermal quenching and/or regulating photothermal conversion and photoluminescence properties simultaneously. Multifunctional solid solution upconversion nanocrystals could be potential for photoluminescent and photothermal applications.