Lanthanide (Ln³⁺) oxysulfide nanocrystals (NCs) have shown great promise in many advanced technologies. However, it remains a challenge to synthesize monodispersed lanthanide oxysulfide NCs with high luminescence efficiency due to the volatility of sulfur and deleterious surface quenching effect. 

In a study published in Aggeragte, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a novel sandwiched luminescent oxysulfide@fluoride heterostructure with tunable size and composition based on the epitaxial growth of α-NaYF₄ along the c-axis of Gd₂O₂S: Ln³⁺ NCs. 

The researchers demonstrated that the ligand of oleylamine plays a critical role in stabilizing the sulfide layer of the Gd₂O₂S core, which facilitates the fine-tuning of the heterostructure with tailored S^2– deficiency in the sandwich layer by varying the ligand ratio of oleylamine and oleic acid. They can monitor such ligand-mediated structure change by utilizing Eu³⁺ as the sensitive structural probe. The ligand-mediated structure change of the core/shell NCs from Gd₂O₃: Ln³⁺@NaYF₄ to Gd₂O₂S: Ln³⁺@NaYF₄ was unraveled through the photoluminescence (PL) excitation and emission and PL decay analyses.

Based on the unique oxysulfide@fluoride heterostructured design, the researchers achieved multicolor upconversion (Tm³⁺, Er³⁺, and Ho³⁺) and downshifting luminescence (Tb³⁺, Eu³⁺, Sm³⁺ and Dy³⁺) under single-wavelength excitation at 980 and 254 nm, respectively. They enhanced the integrated PL intensities of Tb³⁺, Eu³⁺, Sm³⁺, and Dy³⁺ by factors of 1.2, 1.6, 2.3, and 1.4 in the core/shell NCs as compared to those of their core-only counterparts, with the PL quantum yields (PLQYs) of 24.8%, 20.3%,11.7% and 9.6%, respectively.

Besides, the researchers enhanced the upconversion luminescence (UCL) intensities of Tm³⁺, Er³⁺, Ho³⁺ by factors of 208.8, 38.8, and 25.4 in comparison with those of their core-only analogues. 

This study provides a general approach for boosting the luminescence efficiency of lanthanide oxysulfide NCs via heterostructured core/shell engineering, and opens up new opportunities for the advanced materials design based on lanthanide oxysulfides towards versatile applications such as multimodal bioimaging, nanoscintilators, and photocatalysis.