Remarkable enhancement of the near-infrared upconversion emission in the β-NaYF4:Yb3+/Tm3+ system with controllable morphology

• The intensity of NIR UC emission can be greatly enhanced by tuning the pH values.
• The highest chromatic purity of NIR UC emission in the β-NaYF4 system is obtained.
• The various morphologies of materials can be obtained by adjusting the pH values.

In this work, β-NaYF4:Yb3+/Tm3+ samples with controllable morphologies have been prepared using a facile hydrothermal method. The overall morphologies of the samples can be adjusted from rod-shape to hexagonal prism-shape with crown-like top end, and then to hexagonal plate-shape by increasing the pH values of initial solutions ranging from 3 to 9. Furthermore, the pH values of the initial solutions show great effect on upconversion emissions of the samples, and it is worth noting that higher both near-infrared emission and chromatic purity are observed for the samples obtained under lower pH values conditions. The corresponding mechanism may be attributed to the combination of more Yb3+ ions incorporated in the lattice and the reduced lattice constants under the condition of lower pH values, favoring energy transfer between Yb3+ and Tm3+ ions or between two neighboring Tm3+ ions.

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