Synthesis of Multicolor Core/Shell NaLuF4:Yb3+/Ln3+@CaF2 Upconversion Nanocrystals

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.

Sodium lutetium fluoride (NaLuF 4 ) has recently emerged as a new type of efficient host lattice for photon upconversion, similar to the well-established host material of sodium yttrium fluoride (NaYF 4 ). NaLuF 4 -based UCNCs have been demonstrated to exhibit bright upconversion luminescence (UCL) [17][18][19][20][21][22][23][24] and show efficient five-and four-photon ultraviolet emissions under continuous wave excitation at 980 nm [25][26][27]. Despite recent success in synthesizing NaLuF 4 nanoplates or nanorods with size over~30 nm using thermal decomposition method [28][29][30][31] or hydrothermal method [32][33][34][35][36][37][38], the ability to prepare uniform single crystal phase sub-10 nm NaLuF 4 UCNCs remains elusive. Moreover, doping of a high concentration of inert Gd 3+ ions (≥20%) was typically required to prepare small-sized monodisperse NaLuF 4 UCNCs with single crystal phase previously [17,39], thus delivering traits actually from an entity of Lu-Gd alloyed host. On the other hand, small-sized UCNCs are important for single molecule imaging [40] and in vivo bioimaging with reduced toxicity, considering the renal clearance [41]. However, they always come at the sacrifice of UCL efficiency due to the increased size-induced surface-related surface quenching effect [42]. A core/shell geometric structure is therefore needed to eliminate or suppress this detrimental effect by spatial isolation of the core nanoparticle from the surrounding quenching sites. A straightforward approach is to grow a homogenous core/shell structure where the host of the shell is identical to the core [37,43]. However, the possible leaking of rare earth ions from the host lattice could possibly lead to diseases such as nephrogenic systemic fibrosis [44,45]. Compared with lanthanide fluorides, calcium fluoride (CaF 2 ) has unique advantages owing to its superior biocompatibility and high optical transparency [46][47][48][49][50][51]. It has recently been demonstrated that CaF 2 also has low lattice mismatch with NaReF 4 nanocrystals, and can efficiently prevent rare-earth ions from leaking [46,50]. This implies that the growth of a CaF 2 shell not only renders UCL enhancement, but also imparts biocompatibility with reduced leaking effect.
In this work, we describe our effort toward the controlled synthesis of single crystal phase sub-10 nm α-NaLuF 4 :Yb 3+ /Ln 3+ (Ln = Er, Ho, or Tm) nanoparticles using a liquid-solid-solution method without the involvement of doping with a high concentration of Gd 3+ , and then utilize them as the core to epitaxially grow a high quality α-NaLuF 4 :Yb 3+ /Ln 3+ @CaF 2 (Ln = Er, Ho, or Tm) core/shell UCNC via a thermal decomposition protocol. We found that the growth of a~3 nm thin CaF 2 shell layer was able to enhance the multicolour UCL of the core nanocrystals by up to~772-fold.

Results and Discussion
2.1. Synthesis of α-NaLuF 4 :Yb 3+ /Ln 3+ (Ln = Er, Tm, Ho) or α-NaLuF 4 :Yb 3+ /Ln 3+ @CaF 2 Core/Shell UCNCs The crystal structure of NaLuF 4 has two forms of the cubic (α-) and the hexagonal (β-) phase. To prepare high-quality α-NaLuF 4 @CaF 2 core/shell NCs, we firstly controlled the synthesis of cubic (α) NaLuF 4 core nanoparticles by varying the reaction temperature and the molar ratio of F − /Ln 3+ (Ln = Lu + Yb + Er/Ho/Tm) precursor. Figure 1 shows X-ray diffraction (XRD) patterns of NaLuF 4 :Yb 3+ /Er 3+ (Tm 3+ , Ho 3+ ) NCs hydrothermally prepared at various temperatures with the molar ratio of F − /Ln 3+ (Ln = Lu + Yb + Er/Ho/Tm) fixed at 4:1. Figure 1 reveals that the phase transition process (β → α or α → β) occurs at a reaction temperature of T = 100 or 120 • C. The sample obtained at low temperature (T = 100 or 120 • C) shows nearly pure α-phase (JCPDS No. 27-0725). The diffraction peaks of β-NaLuF 4 appear at temperatures between 140 and 160 • C (See Figure 1c,d), while at 180 • C, only pure β-phase NaLuF 4 exists. It could be concluded that low temperature favors the formation of pure α-NaLuF 4 core NCs. where K = 0.89, D represents the crystallite size (in nanometers), λ is the wavelength of the Cu Kα radiation, β is the corrected half-width of the diffraction peak, and θ is Bragg's angle of the diffraction peak. According to Equation (1) and the half width of the main diffraction peak at 28° in Figure 2, the average size was calculated to be about 10 nm for F − /Re 3+ = 3:1, in good agreement with the TEM result ( Figure 3b). As a consequence, we selected α-NaLuF4 nanoparticles prepared at F − /Re 3+ = 3:1 and T = 140 °C to epitaxially grow the α-NaLuF4:Yb/Ln@CaF2 core/shell structure. The principle for the epitaxial growth of the core/shell structure is illustrated in Figure 3a, which involves an injection of the (CF3COO)2Ca solution into the growing solution containing pre-synthesized α-NaLuF4:Yb/Ln core nanocrystals prepared at F − /Re 3+ = 3:1 and T = 140 °C . The morphologies and sizes of the α-NaLuF4:Yb 3+ /Ln 3+ core and the resulting α-NaLuF4:Yb 3+ /Ln 3+ @CaF2 core/shell nanoparticles were examined by transmission electron microscopy (TEM), and the results are shown in Figure 3b. As one can see, the α-NaLuF4:Yb 3+ /Ln 3+ core has sphere-like morphology with a size of 9 ± 0.9 nm, in good agreement with the XRD result in Figure 2c. After growing the CaF2 shell, the core/shell NCs showed a cubic morphology with a size of 15.6 ± 1.2 nm (Figure 3c-e). The size difference indicated that a CaF2 shell with a thickness of Next, we investigated the role of the molar ratio of F − /Ln 3+ (Ln = Lu + Yb + Er/Ho/Tm) precursor on the crystal phase of our product when setting the synthesis temperature at 140 • C. As can be seen in Figure 2, when the molar ratio of F − /Ln 3+ is fixed at 4:1, the sample shows a mixture of α-phase (JCPDS No. 27-0725) and β-phase (JCPDS No. 27-0726). As the F − /Ln 3+ ratio is reduced to 3.5:1, the intensities of peaks of β-NaLuF 4 decreases ( Figure 2b). A pure α-phase NaLuF 4 NCs can be produced at the ratio of F − /Re 3+ below 3:1, and the diffraction peaks of α-NaLuF 4 sample at F − /Re 3+ = 3:1 show stronger intensity than at F − /Re 3+ = 2.5:1 and 2:1. The average crystallite size of the nanocrystals was calculated according to Scherrer's equation [51], where K = 0.89, D represents the crystallite size (in nanometers), λ is the wavelength of the Cu Kα radiation, β is the corrected half-width of the diffraction peak, and θ is Bragg's angle of the diffraction peak. According to Equation (1) and the half width of the main diffraction peak at 28 • in Figure 2, the average size was calculated to be about 10 nm for F − /Re 3+ = 3:1, in good agreement with the TEM result ( Figure 3b). As a consequence, we selected α-NaLuF 4 nanoparticles prepared at F − /Re 3+ = 3:1 and T = 140 • C to epitaxially grow the α-NaLuF 4 :Yb/Ln@CaF 2 core/shell structure.
Nanomaterials 2017, 7, 34 3 of 11 = λ / βcosθ DK (1) where K = 0.89, D represents the crystallite size (in nanometers), λ is the wavelength of the Cu Kα radiation, β is the corrected half-width of the diffraction peak, and θ is Bragg's angle of the diffraction peak. According to Equation (1) and the half width of the main diffraction peak at 28° in Figure 2, the average size was calculated to be about 10 nm for F − /Re 3+ = 3:1, in good agreement with the TEM result ( Figure 3b). As a consequence, we selected α-NaLuF4 nanoparticles prepared at F − /Re 3+ = 3:1 and T = 140 °C to epitaxially grow the α-NaLuF4:Yb/Ln@CaF2 core/shell structure. The principle for the epitaxial growth of the core/shell structure is illustrated in Figure 3a, which involves an injection of the (CF3COO)2Ca solution into the growing solution containing pre-synthesized α-NaLuF4:Yb/Ln core nanocrystals prepared at F − /Re 3+ = 3:1 and T = 140 °C . The morphologies and sizes of the α-NaLuF4:Yb 3+ /Ln 3+ core and the resulting α-NaLuF4:Yb 3+ /Ln 3+ @CaF2 core/shell nanoparticles were examined by transmission electron microscopy (TEM), and the results are shown in Figure 3b. As one can see, the α-NaLuF4:Yb 3+ /Ln 3+ core has sphere-like morphology with a size of 9 ± 0.9 nm, in good agreement with the XRD result in Figure 2c. After growing the CaF2 shell, the core/shell NCs showed a cubic morphology with a size of 15.6 ± 1.2 nm (Figure 3c-e). The size difference indicated that a CaF2 shell with a thickness of   TEM images of (c) NaLuF4:Yb 3+ /Er 3+ @CaF2 core/shell upconversion nanocrystals (UCNCs); (d) α-NaLuF4:Yb 3+ /Ho 3+ @CaF2 core/shell UCNCs; (e) α-NaLuF4:Yb 3+ /Ln 3+ @CaF2 core/shell UCNCs. Particles in (c-e) were synthesized by the thermal decomposition method.
The principle for the epitaxial growth of the core/shell structure is illustrated in Figure 3a, which involves an injection of the (CF 3 COO) 2 Ca solution into the growing solution containing pre-synthesized α-NaLuF 4 :Yb/Ln core nanocrystals prepared at F − /Re 3+ = 3:1 and T = 140 • C. The morphologies and sizes of the α-NaLuF 4 :Yb 3+ /Ln 3+ core and the resulting α-NaLuF 4 :Yb 3+ /Ln 3+ @CaF 2 core/shell nanoparticles were examined by transmission electron microscopy (TEM), and the results are shown in Figure 3b. As one can see, the α-NaLuF 4 :Yb 3+ /Ln 3+ core has sphere-like morphology with a size of 9 ± 0.9 nm, in good agreement with the XRD result in Figure 2c.
After growing the CaF 2 shell, the core/shell NCs showed a cubic morphology with a size of 15.6 ± 1.2 nm (Figure 3c-e). The size difference indicated that a CaF 2 shell with a thickness of about 3.3 ± 0.3 nm was grown on the surface of α-NaLuF 4 :Yb 3+ /Ln 3+ core. Moreover, the formation of core/shell structure can be seen in the TEM images, shown in Figure 3c-e.
The XRD peaks of the α-NaLuF 4 :Yb 3+ /Ln 3+ core, the α-NaLuF 4 :Yb 3+ /Er 3+ @CaF 2 core/shell, the NaLuF 4 :Yb 3+ /Ho 3+ @CaF 2 core/shell, and the NaLuF 4 :Yb 3+ /Tm 3+ @CaF 2 core/shell UCNCs are presented in Figure 4. As one can see, the core and core/shell NCs have identical peak positions, agreeing well with the standard JCPDF27-0725 sample of cubic NaLuF 4 and JCPDF 02-1302 sample of cubic CaF 2 . The well-defined peaks are indicative of the high crystallinity of both the core and the core/shell NCs. The narrower XRD peaks of the core/shell NCs compared to that of core NPs indicate the larger size of core/shell NCs, in accordance with the TEM results in Figure 3.
To illustrate the UC mechanisms of the NaLuF 4 :20%Yb 3+ /2%Ln 3+ @CaF 2 (Ln 3+ = Er 3+ , Ho 3+ , Tm 3+ ) NCs, possible UC processes are schematically given in the energy level diagrams of Yb 3+ , Er 3+ , Ho 3+ , and Tm 3+ ions in Figure 6. The observed green UC bands ( 2 H 11/2 → 4 I 15/2 , 525 nm; 4 S 3/2 → 4 I 15/2 , 540 nm) and red UC band ( 4 F 9/2 → 4 I 15/2 , 660 nm) from Er 3+ ions in NaLuF 4 :20%Yb 3+ /2%Er 3+ @CaF 2 UCNCs may take place via the following process: Yb 3+ ion absorbs one laser photon and gets excited from the ground 2 F 7/2 state to the exclusive excited 2 F 5/2 state. The Yb 3+ ions in the excited state transfer their absorbed energy to neighboring Er 3+ ions and excite them from the ground 4 I 15/2 state to the 4 I 11/2 state, then to the 4 F 7/2 state. Multiphonon assisted relaxations from the 4 F 7/2 state can decay nonradiatively to the lower 2 H 11/2 and 4 S 3/2 levels, emitting the 525 and 540 nm UCL, respectively. The red emission 660 nm originates from the 4 F 9/2 → 4 I 15/2 transition, and the 4 F 9/2 state can be populated either from nonradiative relaxations from the 4 S 3/2 level or the energy transfer from Yb 3+ ions to the Er 3+ ion at the 4 I 13/2 state. In addition, the processes for green ( 5 F 4 → 5 I 8 , 537 nm) and red ( 5 F 5 → 5 I 8 , 645 nm) UCL of Ho 3+ ions involved two different centers-the sensitizer (Yb 3+ ), the activator (Ho 3+ )-along with two successive transfers from Yb 3+ ions to Ho 3+ ions. In the first transfer, the Yb 3+ ion absorbs the excitation photons through the ground state absorption and transfers its absorbed energy to the neighboring Ho 3+ ion to populate to its intermediate ( 5 I 6 level). The energy difference between the two levels was abridged by the vibration energy of the host lattice. The second transfer is to promote the population from the intermediate 5 I 6 level to the emitting energy levels ( 5 S 2 ) by energy transfer (ET) from another excited Yb 3+ ion [52]. Once the 5 S 2 level is populated, the Nanomaterials 2017, 7, 34 7 of 11 excited electron can release its energy by emitting green emissions. The red emission at 645 nm can be produced by radiative decay to the ground 5 F 5 state. The blue UCL of Tm 3+ occurs via a three-step ET from Yb 3+ to Tm 3+ . First, the Tm 3+ ion in the ground state 3 H 6 is excited to the state 3 H 5 via an ET from a neighboring excited Yb 3+ ion. Subsequent nonradiative relaxation of 3 H 5 / 3 F 4 populates the 3 F 4 level. In the second-step excitation, the Tm 3+ ion in the 3 F 4 state is excited to the 3 F 2,3 state via another ET from a neighboring excited Yb 3+ ion. The populated 3 F 2 level may nonradiatively relax to the 3 F 3 level. When the Tm 3+ ion at the 3 F 3 level decays to the ground state, a weak red emission ( 3 F 3 → 3 H 6 ) is produced. Additionally, the near-infrared UCL at 802 nm arises from the 3 H 4 → 3 H 6 transition, where the 3 H 4 state is populated by the efficient nonradiative relaxation from the 3 F 2,3 state. A third energy transfer from Yb 3+ excites Tm 3+ at the 3 F 3 level to the 1 G 4 level, from which the blue emission ( 1 G 4 → 3 H 6 ) occurs by radiative decay to the ground state. The fourth energy transfer from Yb 3+ promotes the Tm 3+ at the 1 G 4 level to the 1 D 2 level, from which a 360 nm ultraviolet UCL ( 1 D 2 → 3 H 6 ) is generated.

Materials
All Ln2O3 (99.9%, Ln = Lu, Yb, Er, Tm, Ho) were obtained from Jianfeng Rare-Earth Limited Company, Conghua, China. The basic chemical reagents, such as sodium hydroxide, oleic acid (OA), absolute ethyl alcohol, trifluoroacetic acid (TFA), calcium oxide, sodium fluoride and octadecene (ODE) were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. All chemicals were of analytical grade and were used as received without further purification.

Materials
All Ln 2 O 3 (99.9%, Ln = Lu, Yb, Er, Tm, Ho) were obtained from Jianfeng Rare-Earth Limited Company, Conghua, China. The basic chemical reagents, such as sodium hydroxide, oleic acid (OA), absolute ethyl alcohol, trifluoroacetic acid (TFA), calcium oxide, sodium fluoride and octadecene (ODE) were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. All chemicals were of analytical grade and were used as received without further purification.

Thermal Decomposition Synthesis of α-NaLuF 4 :Yb 3+ /Ln 3+ @CaF 2 Core/Shell UCNCs
The core/shell nanoparticles were synthesized using the thermal decomposition method. Typically, 0.5 mmol CaO was first added to a 50 mL flask containing 5 mL deionized water and 5 mL trifluoroacetic acid (TFA). The solution was heated at 90 • C until the solution became transparent, and was then dried at this temperature with nitrogen purge to yield the shell precursor (CF 3 COO) 2 Ca. After obtaining the (CF 3 COO) 2 Ca powders, 10 mL of OA, 10 mL of ODE, and the pre-prepared α-NaLuF 4 :Yb 3+ /Ln 3+ (0.5 mmol) in cyclohexane were added. The solution was then vacuum-degassed at 120 • C for 30 min to remove water, oxygen, and cyclohexane. Subsequently, the solution was heated to 300 • C at a rate of 15 K·min −1 under nitrogen protection. After maintaining at 300 • C for 30 min, the reaction was stopped and cooled down to room temperature. After washing with ethanol, the products were dispersed in cyclohexane for further use.

Conclusions
In summary, cubic phase α-NaLuF 4 :Yb/Ln cores can be precisely controlled through a simple variation of reaction temperature and the added amount of NaF in a hydrothermal method. Moreover, a seed-mediated growth protocol with selected parameters favorable for shell growth yields the α-NaLuF 4 :Yb/Ln@CaF 2 (Ln = Er, Ho, Tm) core/shell structure NPs having a core size of~9 nm and shell thickness of~3.3 nm. Moreover, we found that the growth of the inert thin shell of CaF 2 onto the α-NaLuF 4 :Yb/Ln (Ln = Er, Ho, Tm) core could enhance its multicolor UCL by up to 772-fold, being attributed to effective suppression of surface-related quenching effects via spatial isolation of the core from the surrounding environment. Small-sized α-NaLuF 4 :Yb/Ln@CaF 2 (Ln = Er, Ho, Tm) UCNCs developed here have implication for uses in a range of biophotonic applications, such as bioimaging.