Enhanced high-order upconversion luminescence of hexagonal phase NaYF4:Yb3+,Tm3+ crystals coated with homogeneous shell

• The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method.
• With the shell amount increasing, the crystals shape changed from hexagonal plate to hexagonal prism gradually.
• The emissions from 1I6 and 1D2 of core–shell crystals were enhanced by several times compared with core-only crystals.
• When the shell thickness increased to a certain extent, it was unfavorable for the high-order upconversion luminescence.

High-order upconversion luminescence (ultraviolet (UV), violet and blue emissions) of rare earth nanocrystals is quite desirable for photodynamic therapy and near-infrared photocatalysis applications. The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method. With the amount of the shell precursor increasing, the crystals shape changed from hexagonal plate to hexagonal prism gradually, with the particles size increased. After coating a homogeneous inert NaYF4 shell, the high-order upconversion luminescence (1I6 and 1D2 radiative transitions) of all core–shell crystals were enhanced by several times compared with core-only crystals. Impressively, when the amount of the shell precursor was 0.1 mmol, the increased ratio of the emissions from 1I6 and 1D2 were 7.44 and 5.63 times, respectively. With further increasing the amount of the shell precursor, the increased ratio displayed a slight decline. The further photoluminescence decay results were consistent with the upconversion luminescence spectra. We demonstrated that the enhancement of high-order upconversion luminescence was attributed to the well-known surface passivation effect of the homogeneous shell.

The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method. After coating a homogeneous shell with the precursor amount of 0.1 mmol, the increased ratio of high-order upconversion luminescence (the emissions from 1I6 and 1D2) were 7.44 and 5.63 times, respectively. With further increasing the amount of the shell precursor, the increased ratio displayed a slight decline. We demonstrated that the enhancement of high-order upconversion luminescence was attributed to the well-known surface passivation effect of the homogeneous shell.Figure optionsDownload as PowerPoint slide