Abstract
The ability to synthesize high-quality hierarchical core/shell nanocrystals from an efficient host lattice is important to realize efficacious photon upconversion for applications ranging from bioimaging to solar cells. Here, we describe a strategy to fabricate multicolor core @ shell α-NaLuF4:Yb3+/Ln3+@CaF2 (Ln = Er, Ho, Tm) upconversion nanocrystals (UCNCs) based on the newly established host lattice of sodium lutetium fluoride (NaLuF4). We exploited the liquid-solid-solution method to synthesize the NaLuF4 core of pure cubic phase and the thermal decomposition approach to expitaxially grow the calcium fluoride (CaF2) shell onto the core UCNCs, yielding cubic core/shell nanocrystals with a size of 15.6 ± 1.2 nm (the core ~9 ± 0.9 nm, the shell ~3.3 ± 0.3 nm). We showed that those core/shell UCNCs could emit activator-defined multicolor emissions up to about 772 times more efficient than the core nanocrystals due to effective suppression of surface-related quenching effects. Our results provide a new paradigm on heterogeneous core/shell structure for enhanced multicolor upconversion photoluminescence from colloidal nanocrystals.
Highlights
Upconversion nanocrystals (UCNCs) are able to convert two or more long wavelength photons into short wavelength emissions through the use of real energy levels of trivalent lanthanide ions embedded in an inorganic host lattice [1]
We describe our effort toward the controlled synthesis of single crystal phase sub-10 nm α-NaLuF4 :Yb3+ /Ln3+ (Ln = Er, Ho, or Tm) nanoparticles using a liquid-solid-solution method without the involvement of doping with a high concentration of Gd3+, and utilize them as the core to epitaxially grow a high quality α-NaLuF4 :Yb3+ /Ln3+ @CaF2 (Ln = Er, Ho, or Tm) core/shell
To prepare high-quality α-NaLuF4 @CaF2 core/shell NCs, we firstly controlled the synthesis of cubic (α) NaLuF4 core nanoparticles by varying the reaction temperature and the molar ratio of
Summary
Upconversion nanocrystals (UCNCs) are able to convert two or more long wavelength photons into short wavelength emissions through the use of real energy levels of trivalent lanthanide ions embedded in an inorganic host lattice [1]. Fluoride UCNCs have superior features, such as low toxicity, non-blinking, non-photobleaching, absence of autofluorescence, and tissue-penetrable near-infrared (NIR) light excitation [3,4] These superb attributes promise their applications in biological imaging [5,6,7,8], bio-detection [9,10], and three-dimensional display [11,12,13]. It has recently been demonstrated that CaF2 has low lattice mismatch with NaReF4 nanocrystals, and can efficiently prevent rare-earth ions from leaking [46,50] This implies that the growth of a CaF2 shell renders UCL enhancement, and imparts biocompatibility with reduced leaking effect. We found that the growth of a ~3 nm thin CaF2 shell layer was able to enhance the multicolour UCL of the core nanocrystals by up to ~ 772-fold
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