A Strategy to enhance Eu³⁺ emission from LiYF₄:Eu nanophosphors and green-to-orange multicolor tunable, transparent nanophosphor-polymer composites

Abstract

LiYF₄:Eu nanophosphors with a single tetragonal phase are synthesized and various strategies to enhance the Eu³⁺ emission from the nanophosphors are investigated. The optimized Eu³⁺ concentration is 35 mol% and the red emission peaks due to the ⁵D₀ →⁷F_J (J = 1 and 2) transitions of Eu³⁺ ions are further enhanced by energy transfer from a sensitizer pair of Ce³⁺ and Tb³⁺. The triple doping of Ce, Tb and Eu into the LiYF₄ host more effectively enhances the Eu³⁺ emission than the core/shell strategies of LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) architectures. Efficient energy transfer from Ce³⁺ to Eu³⁺ through Tb³⁺ results in three times higher Eu³⁺ emission intensity from LiYF₄:Ce(15%), Tb(15%), Eu(1%) nanophosphors compared with LiYF₄:Eu(35%), which contains the optimized Eu³⁺ concentration. Owing to the energy transfer of Ce³⁺ → Tb³⁺ and Ce³⁺ → Tb³⁺ → Eu³⁺, intense green and red emission peaks are observed from LiYF₄:Ce(13%), Tb(14%), Eu(1-5%) (LiYF₄:Ce, Tb, Eu) nanophosphors and the intensity ratio of green to red emission is controlled by adjusting the Eu³⁺ concentration. With increasing Eu³⁺ concentration, the LiYF₄:Ce, Tb, Eu nanophosphors exhibit multicolor emission from green to orange. In addition, the successful incorporation of LiYF₄:Ce, Tb, Eu nanophosphors into polydimethylsiloxane (PDMS) facilitates the preparation of highly transparent nanophosphor-PDMS composites that present excellent multicolor tunability.

Introduction

Lanthanide ion (Ln³⁺ ion)-doped inorganic crystals have attracted considerable interest due to their unique optical properties, such as high luminescence efficiency and photo- and chemical stability, which facilitated their commercialization in display and illumination devices^1,2. Among the various Ln³⁺ ions, Eu³⁺ and Tb³⁺ are known to show efficient luminescence and are widely used in such devices³. For example, Y₂O₃:Eu³⁺, (Y,Gd)BO₃:Eu³⁺, Y₂O₂S:Eu³⁺ have been used in fluorescent lamps (FLs), plasma display panels (PDPs), cathode ray tubes (CRTs), respectively^2,4. In the case of Tb³⁺ ions, LaPO₄:Ce³⁺, Tb³⁺, Gd₂O₂S:Tb³⁺ and YBO₃:Tb³⁺ have been used in FLs, scintillators and PDPs, respectively^2,5. Thus, many studies have been published on Eu³⁺- and Tb³⁺-activated inorganic crystals, which are known as phosphors^2,3,4,5,6. In 1999, Meijerink and colleagues introduced efficient red-emitting LiGdF₄:Eu³⁺ phosphors that demonstrated a quantum efficiency (QE) greater than 100% via the quantum cutting (QC) process⁷. Such QC phosphors can be applied to solar cells for the improvement of solar cell efficiency by converting ultraviolet (UV) light into visible light with increased photon numbers^8,9. However, for application in solar cells, nanometer-sized QC phosphors (i.e., QC nanophosphors) should be used to minimize incident light scattering because the QC phosphors are located in front of the Si solar cell^8,10. Recently, thanks to the development of synthetic methodology, well-defined nanophosphors of uniform size were successfully synthesized via thermal decomposition, hydrothermal and co-precipitation methods, among others^{11,12,13,14,15,16,17,18,19,20,21}. However, to the best of our knowledge, there have been no reports on LiGdF₄:Eu³⁺ nanophosphors. This may be partly attributed to the difficulty associated with the synthesis of single-tetragonal-phase LiGdF₄ nanocrystals (NCs)^22,23. On the other hand, LiYF₄, which has the same crystal structure as LiGdF₄, can also be considered as a host crystal for nanophosphors^24,25,26. Because of its inherently low lattice phonon energy, LiYF₄ has been used as a host crystal for upconversion phosphors^24,25,26. Similar to the cases of NaYF₄ and NaGdF₄ as host materials^27,28, LiYF₄ may be a good candidate for downshifting phosphors as well as a promising host lattice for upconversion^29,30. In this article, we report on the luminescence properties of LiYF₄:Eu³⁺ (LiYF₄:Eu) colloidal nanophosphors. To the best of our knowledge, colloidal LiYF₄:Eu nanophosphors have not yet been reported although micrometer-sized (larger than 10 μm) LiYF₄:Eu particles have been reported³¹. To enhance the emission intensity of LiYF₄:Eu and achieve multicolor emission, Tb³⁺ was employed as a sensitizer. Multicolor emission is another advantage of Ln³⁺ ion-doped nanophosphors^32,33. Tunable multicolor emission coupled with size- and shape-independent luminescence under single wavelength excitation is beneficial for multicolor labeling and display device applications^16,32. Energy transfer from a sensitizer ion to the Eu³⁺ ion is a favorable solution for achieving enhanced Eu³⁺ emission⁶ and multicolor luminescence^16,34. To obtain efficient energy transfer from the Tb³⁺ sensitizer to the Eu³⁺ ion, we used various approaches. First, we synthesized nanophosphors with a core/shell architecture in which Eu³⁺ and Tb³⁺ ions were separately doped within the core and the shell. We also synthesized Eu³⁺ and Tb³⁺ co-doped single-LiYF₄ NCs. In addition, Ce³⁺ ions were used as sensitizers for enhanced luminescence and single wavelength excitation because the 4f-5d electronic transition in a Ce³⁺ ion is a spin- and parity-allowed transition and the Ce³⁺ ion has a large absorption cross-section^16,35. That is, LiYF₄:Eu/LiYF₄:Ce³⁺, Tb³⁺ (LiYF₄:Ce, Tb), LiYF₄:Ce, Tb/LiYF₄:Eu and LiYF₄:Ce³⁺, Tb³⁺, Eu³⁺ (LiYF₄:Ce, Tb, Eu) nanophosphors were synthesized and their luminescence properties were investigated. Furthermore, multicolor emission from green to orange was successfully realized by varying the Tb³⁺ and Eu³⁺ concentrations and the feasibility of their use in a transparent display application was examined through the fabrication of multicolor-emitting polymer composites.

Results

LiYF₄:Eu nanophosphors with various Eu³⁺ concentrations were synthesized to achieve strong red luminescence. Figure 1 shows transmission electron microscopy (TEM) images of the LiYF₄:Eu nanophosphors. Until 35 mol% Eu³⁺ doping, the LiYF₄:Eu nanophosphors showed uniform size and shape (Figures 1a–d). However, when the concentration of Eu³⁺ was 40 mol%, both large and small particles were simultaneously observed (Figure 1e). When the concentration of Eu³⁺ was greater than 40 mol%, uniform small particles were obtained (Figure 1f). This morphological change was attributed to the crystal structure of the LiYF₄:Eu. For Eu³⁺ concentrations less than 40 mol%, nanocrystals (NCs) with a single tetragonal phase were synthesized (see X-ray diffraction (XRD) patterns of Figure S1). When 40 mol% of Eu³⁺ dopant was incorporated, mixed phases of tetragonal and orthorhombic structures were synthesized, whereas NCs with an orthorhombic structure was formed with Eu³⁺ concentrations greater than 40 mol%. As shown in Figure 1, the particle size increased slightly from 8.0 ± 0.7 to 10.0 ± 0.6 nm as the doping concentration of Eu³⁺ ions was increased to 30 mol%. Then, large tetragonal bipyramidal particles with edge lengths of 63.2 ± 2.2 nm × 68.4 ± 1.9 nm were synthesized under a Eu³⁺ doping condition of 35 mol%. This result is similar to finding in our previously reported work²³. Doping the Y³⁺ sites with ions that are larger than the Y³⁺ ion induces the large particle size due to the presence of less negatively charged F⁻ ions at the (101) surface layer of the LiYF₄ crystals²³. However, too much Eu³⁺ doping led to the formation of orthorhombic YF₃ NCs. According to Du et al. and Ye et al., the orthorhombic phase, instead of the tetragonal phase, readily forms under large Eu³⁺ fraction^22,36. Due to different crystal structures, LiYF₄:Eu exhibited a tetragonal bipyramidal morphology, as shown in Figures 1g and h (see also scanning transmission electron microscopy (STEM) image of LiYF₄:Eu(35%) in Figure S2) and YF₃:Eu showed a rhombic plate shape, as shown in Figures 1i and j.

Figure 1

TEM images of LiYF₄:Eu nanophosphors with varying Eu³⁺ concentrations of (a) 10%, (b) 20%, (c) 30%, (d) 35%, (e) 40%, (f) 45% and (g and i) high magnification TEM images of LiYF₄:Eu(35%) and LiYF₄:Eu(45%) and (h and j) corresponding schematic morphologies for LiYF₄:Eu(35%) and LiYF₄:Eu(45%), respectively.

Figure 2 shows photoluminescence (PL) and photoluminescence excitation (PLE) spectra of LiYF₄:Eu nanophosphors. Sharp emission peaks can be observed in the red spectral region of the PL spectra. These peaks were attributed to the electronic transitions from the excited ⁵D₁ and ⁵D₀ levels to the ⁷F_J (J = 0–6) levels³. As shown in Figure 2a, a strong emission peak at approxmately 610–620 nm can be ascribed to the hypersensitive electric dipole transition of the ⁵D₀ →⁷F₂ transition. The emission peak at approximately 590–600 nm is due to the magnetic dipole transition of ⁵D₀ →⁵F₁, which is lower than that due to the ⁵D₀ →⁷F₂ transition, indicating that Eu³⁺ ions are located at the non-inversion symmetric sites^6,37. As the Eu³⁺ concentration was increased to 35 mol% in the LiYF₄ host crystals, the PL intensity increased as well. At higher Eu³⁺ concentrations, the PL intensity decreased. In addition, when the Eu³⁺ concentration was 45 mol%, the shape of the PL spectrum differed from that of the LiYF₄:Eu(35%) nanophosphors. In this case, a single orthorhombic structure was formed and the PL intensity due to the ⁵D₀ →⁷F₁ transition was higher than that due to the ⁵D₀ →⁷F₂ transition.

Figure 2

(a) PL spectra of LiYF₄:Eu(10–45%) nanophosphors under the excitation of 394 nm and (b) PLE spectrum of LiYF₄:Eu(35%) nanophosphors monitored at 616 nm [(i) 10%, (ii) 20%, (iii) 30%, (iv) 35%, (v) 40% and (vi) 45%].

The strongest PLE peak can be observed at 394 nm (Figure 2b). This peak was attributed to the ⁷F₀ →⁵L₆ transition. Because the charge transfer band of Eu³⁺-F⁻ lies within the vacuum ultraviolet (VUV) region (approximately 150 nm)⁷, a broad PLE band due to charge transfer was not observed (Figure 2b). Upon considering the first spin allowed f–d transition, the 4f–5d transition band is also found to lie within the VUV region (approximately 143 nm). According to Dorenbos³⁸, the f–d energy difference of Eu³⁺ ions doped in LiYF₄, E(Eu, LiYF₄) can be expressed as follows:

where 49,340 cm⁻¹ is the energy of the first f–d transition of Ce³⁺ as a free ion, D(Ce, LiYF₄) is the crystal field depression, which is defined as the lowering of this energy when the Ce³⁺ ion is doped in LiYF₄ and ΔE^Eu,Ce is defined as the difference in the f–d energy of Eu³⁺ relative to that of the first electric dipole allowed transition in Ce³⁺³⁸. Taking D(Ce, LiYF₄) = 15,262 cm⁻¹ and ΔE^Eu,Ce = 35,900 ± 380 cm⁻¹ for LiYF₄, the f–d transition band of Eu³⁺ ions in the LiYF₄ host crystal is expected to reside at 69,978 ± 380 cm⁻¹ (approximately 143 nm)³⁸. Thus, broad excitation bands due to either charge transfer or the 4f–5d transition were not observed in the UV spectral region, whereas sharp excitation peaks due to the f-f transition can be observed in Figure 2b. In this spectral range, concentration of the activator Eu³⁺ ions was linearly related to the PL intensity, indicating that Eu³⁺ emission results from the direct excitation-emission of Eu³⁺ ions³⁹.

As shown in Figures 1 and 2, the optimized Eu³⁺ concentration in terms of the PL intensity was 35 mol% and further increasing the Eu³⁺ doping fraction induced a phase transformation from tetragonal LiYF₄ to the orthorhombic YF₃ phase. To further enhance the Eu³⁺ emission in LiYF₄ NCs, other strategies besides increasing the Eu³⁺ concentration are needed. Energy transfer from a sensitizer to the Eu³⁺ ions can be a good pathway for enhancing Eu³⁺ luminescence. Previously, Wang et al. reported that Ce³⁺ ions can be a sensitizer for Eu³⁺ emission³⁵. However, as shown in Figure S3, when Ce³⁺ and Eu³⁺ were co-doped in LiYF₄ NCs, orthorhombic phase YF₃ impurities were formed at a low Eu³⁺ concentration of 15 mol% compared with LiYF₄:Eu³⁺ nanophosphors. In addition, the ⁵D₀ →⁷F₂ emission was stronger under Eu³⁺ direct excitation at 394 nm than under the Ce³⁺ → Eu³⁺ energy transfer condition excited at 300 nm (Figure S3f). These results indicate that the use of Ce³⁺ sensitization alone is incapable of significantly enhancing Eu³⁺ emission in the LiYF₄ NCs. Recently, we reported on the high efficiency of the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer pathway¹⁶. Thus, we adopted Ce³⁺ and Tb³⁺ ions as co-sensitizers and synthesized various structures of LiYF₄:Eu-based nanophosphors, as shown in Figure 3. Then, we investigated the various nanophosphor architectures for strong Eu³⁺ emission. Figures 3a and b show schematic illustrations of LiYF₄:Eu/LiYF₄:Ce, Tb and LiYF₄:Ce, Tb/LiYF₄:Eu core/shell nanophosphors. Ghosh et al. reported enhanced Tb³⁺ emission via energy transfer from Ce³⁺ ions (sensitizer) in the NaYF₄ core to Tb³⁺ ions in the NaYF₄ shell⁴⁰. On the other hand, Capobianco's group reported enhanced Er³⁺ emission via the additional energy transfer from Yb³⁺ ions in the NaGdF₄ shell to Er³⁺ ions in the NaGdF₄ core⁴¹. In these core/shell structured nanophosphors, optimized Eu³⁺ concentrations can be doped into LiYF₄ NCs because Eu³⁺ ions are singly doped into either the LiYF₄ core or the LiYF₄ shell. Figure 3c shows a schematic illustration of Ce³⁺, Tb³⁺ and Eu³⁺ triply-doped LiYF₄ nanocrystals. Next, the aforementioned three types of nanophosphors were synthesized and their luminescence properties were investigated.

Figure 3

Schematic illustrations showing various architectures for enhancement of Eu³⁺ emission from LiYF₄:Eu-based nanophosphors (a) LiYF₄:Eu/LiYF₄:Ce, Tb core/shell, (b) LiYF₄:Ce, Tb/LiYF₄:Eu core/shell and (c) LiYF₄:Ce, Tb, Eu nanophosphors.

Figures 4a and b show TEM images of LiYF₄:Eu(35%) core and LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) core/shell nanophosphors. The particle size increased after shell formation on the LiYF₄:Eu(35%) cores. Energy dispersive X-ray spectroscopy (EDS) analyses indicated that core/shell structured nanophosphors were successfully synthesized (Figures 4c and d). In the EDS spectrum of a single LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) nanoparticle, Ce and Tb peaks as well as Eu peaks were observed, whereas only Eu peaks were observed in the EDS spectrum of a single LiYF₄:Eu(35%) core nanoparticle. Combining the XRD patterns (Figure S4) with EDS spectra (Figures 4 and S5) confirmed that LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) core/shell nanophosphors with a single tetragonal phase were synthesized. As shown in Figure 4e, Eu³⁺ emission was enhanced via the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer pathway. The PL intensity of Eu³⁺ emission from the LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) core/shell nanophosphors under excitation with 300 nm UV light was higher than that under excitation with 394 nm light, which directly excited the Eu³⁺ ions. From the PLE spectrum in Figure S6, we can directly compare the Eu³⁺ emission intensity due to the ⁷F₀ →⁵L₆ transition of Eu³⁺ with that due to the 4f → 5d transition of Ce³⁺. As a result, it was found that the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer is more effective for intense Eu³⁺ emission than Eu³⁺ direct excitation-emission because the PLE intensity due to the 4f → 5d band excitation of Ce³⁺ was higher than that due to the ⁷F₀ →⁵L₆ excitation of Eu³⁺. It should be noted that a fraction of the excited energy can be directly transferred from Ce³⁺ to Eu³⁺, as shown in Figure S3f. In addition to Eu³⁺ emission peaks, Tb³⁺ characteristic emission peaks were observed via the ⁵D₄ →⁷F_J (J = 6, 5, 4 and 3) through Ce³⁺ → Tb³⁺ energy transfer (Figure 4e).

Figure 4

(a), (b) TEM images, (c), (d) EDS spectra of LiYF₄:Eu(35%) and LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) nanophosphors, respectively and (e) PL spectra of LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) nanophosphors under the excitations of 300 nm (dotted green line) and 394 nm (solid red line). Insets of (c) and (d) show STEM images of a single LiYF₄:Eu(35%) and LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) nanophosphors, where EDS spectra were obtained for red squares, respectively.

The luminescence properties of the LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) core/shell nanophosphors were also investigated. Figures 5a and b show TEM images of LiYF₄:Ce(15%), Tb(15%) core nanophosphors and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) core/shell nanophosphors. The particle size increased from 31.6 ± 1.0 nm × 32.7 ± 1.1 nm to 37.7 ± 1.4 nm × 39.3 ± 1.5 nm after LiYF₄:Eu shell formation on the LiYF₄:Ce, Tb core and only LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) exhibited Eu peaks in the EDS spectra, as shown in Figures 5c and d. A single tetragonal phase was observed in both the LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors and LiYF₄:Ce(15%), Tb(15%) nanophosphors (Figure S7). Figure S8 shows EDS spectra obtained at different locations (center and edge) of individual LiYF₄:Ce(15%), Tb(15%) core and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) core/shell nanoparticles. In the EDS spectra of core nanoparticles, peaks associated with Ce and Tb were observed, whereas Eu peaks were not observed at either central or edge regions of the core nanoparticles (Figures S8a and b). In the core/shell nanoparticles, the intensity of the Eu peak was strong, whereas the Ce and Tb peaks were very weak at the edge region. By contrast, the intensities of the Ce and Tb peaks were strong and that of the Eu peak was very weak at the central region of the LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) core/shell nanoparticle (Figures S8c and d). These TEM, XRD and EDS results support that LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors were successfully synthesized with a single tetragonal structure. Figure 5e shows the PL spectra of LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors excited at 300 and 394 nm. Like the case of LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) excited at 300 nm, Tb³⁺ characteristic emission peaks via Ce³⁺ → Tb³⁺ energy transfer were observed together with Eu³⁺ emission peaks under excitation with 300 nm UV light. However, the PL intensity due to the ⁵D₀ →⁷F₂ transition of the Eu³⁺ ions was lower under excitation with 300 nm light compared with excitation with 394 nm light. This finding indicates that Eu³⁺ emission is more efficient for the direct excitation of Eu³⁺ than for the indirect Eu³⁺ excitation via energy transfer of Ce³⁺ → (Tb³⁺) → Eu³⁺ in the LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors. As shown in Figure 2b, a weak PLE peak can be observed in the UV spectral region (approximately 300 nm) and thus some fraction of externally originating UV light can be absorbed by the LiYF₄:Eu shell before absorption by Ce³⁺ in the core. Therefore, external UV light was not fully absorbed by Ce³⁺ ions in the LiYF₄:Ce, Tb core and less excited energy was transferred from Ce³⁺ to Eu³⁺ through Tb³⁺ compared with the case of LiYF₄:Eu(35%)/LiYF₄:Ce(15%), Tb(15%) core/shell nanophosphors.

Figure 5

(a), (b) TEM images, (c), (d) EDS spectra of LiYF₄:Ce(15%), Tb(15%) and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors, respectively and (e) PL spectra of LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphors under the excitations of 300 nm (dotted green line) and 394 nm (solid red line). Insets of (c) and (d) show STEM images of a single LiYF₄:Ce(15%), Tb(15%) and LiYF₄:Ce(15%), Tb(15%)/LiYF₄:Eu(35%) nanophosphor, where EDS spectra were obtained for red squares, respectively.

In addition to core/shell structured nanophosphors, the luminescence properties of Ce³⁺, Tb³⁺ and Eu³⁺ triply-doped LiYF₄ NCs are shown in Figure 6. The LiYF₄:Ce, Tb, Eu nanophosphors can also be synthesized with a core/shell sturcture and Figures 6a and b show TEM images of LiYF₄:Ce(15%), Tb(15%), Eu(1%) and LiYF₄:Ce(15%), Tb(15%), Eu(1%)/LiYF₄ nanophosphors. When the Eu³⁺ concentration was larger than 1%, a single tetragonal bipyramidal shape did not form; instead mixed morphologies of tetragonal bipyramids and rhombic plates were observed in TEM image (Figure S9a). Although the Eu³⁺ concentration of LiYF₄:Ce(15%), Tb(15%), Eu(1%) nanophosphors was significantly smaller than that of LiYF₄:Eu(35%), the LiYF₄:Ce(15%), Tb(15%), Eu(1%) nanophosphors showed much stronger Eu³⁺ emission intensities under excitation with 300 nm light, which excited the Ce³⁺ ions. As shown in Figure 6c, the Eu³⁺ emission intensity of the LiYF₄:Ce(15%), Tb(15%), Eu(1%) under excitation with 300 nm light was three times higher than that of LiYF₄:Eu(35%) under excitation with 394 nm light. This result means that the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer is very efficient in the Ce³⁺, Tb³⁺ and Eu³⁺ triply-doped LiYF₄ nanophosphors. In conclusion, in the case of LiYF₄ host crystals, the Ce³⁺, Tb³⁺ and Eu³⁺ triply-doped system was the most efficient for obtaining strong Eu³⁺ emission compared with the core/shell structured nanophosphors shown in Figure 3. Furthermore, the emission intensity of the nanophosphor was further enhanced by growing a LiYF₄ shell on the LiYF₄:Ce(15%), Tb(15%), Eu(1%) core. After forming the LiYF₄ shell on the core, the particle size was slightly increased (Figure 6b) and EDS spectra obtained from individual particles confirmed the formation of a core/shell structure (Figure S10). Thanks to the formation of a LiYF₄ shell on the core, the PL intensity of the LiYF₄:Ce(15%), Tb(15%), Eu(1%)/LiYF₄ core/shell nanophosphors was 62% higher than that of the core nanophosphors. As a result, the Eu³⁺ emission intensity of the core/shell nanophosphors was 4.6 times greater than that of the LiYF₄:Eu(35%) nanophosphors (Figure 6c). In the Ce, Tb and Eu triply-doped phosphor system, it was reported that Tb blocks the Ce-Eu charge transfer^42,43. However, weak Ce⁴⁺-related peaks were observed with strong Ce³⁺-related peaks in the X-ray photoelectron spectroscopy (XPS) spectrum of LiYF₄:Ce(15%), Tb(15%), Eu(1%) (Figure S11). In particular, the specific peak at 917 eV, which is frequently taken as confirmation of the presence of Ce⁴⁺, was observed in the XPS spectrum^44,45. Although Ce⁴⁺-related peaks were very weak and negligible, there might be probability of the exsitence of the electron transfer relation (Ce³⁺ + Eu³⁺ → Ce⁴⁺ + Eu²⁺).

Figure 6

(a), (b) TEM images of LiYF₄:Ce(15%), Tb(15%), Eu(1%) and LiYF₄:Ce(15%), Tb(15%), Eu(1%)/LiYF₄ and (c) PL spectra of LiYF₄:Ce(15%), Tb(15%), Eu(1%) (dotted green line) and LiYF₄:Ce(15%), Tb(15%), Eu(1%)/LiYF₄ (solid blue line) under the excitation of 300 nm and LiYF₄:Eu(35%) (solid red line) under the excitation of 394 nm.

Multicolor emission from the LiYF₄:Ce, Tb, Eu nanophosphors was realized by varying the ratio of the Tb³⁺ to Eu³⁺ emission intensities. The desired outcome was readily achieved by adjusting the Eu³⁺ concentrations. However, as shown in Figure S9, when the Eu³⁺ concentration was 2 mol%, an impurity YF₃ phase was synthesized. According to Du et al., light lanthanide ions in the Y³⁺ sites of LiYF₄ resists the formation of a single tetragonal phase²². Thus, to increase the Eu³⁺ quantities in the LiYF₄ host, the amounts of Ce³⁺ and Tb³⁺ were reduced. Figure S12 shows XRD patterns of LiYF₄:Ce(13%), Tb(14%), Eu(1–5%) (LiYF₄:Ce, Tb, Eu) nanophosphors. These XRD patterns confirmed that all of the synthesized nanophosphors had a single tetragonal phase. Figure 7 shows PL and PLE spectra and the Commission Internationale de l'Eclairage (CIE) color coordinates of the LiYF₄:Ce, Tb, Eu nanophosphors with varying Eu³⁺ concentrations. As the Eu³⁺ concentration increased, the intensity ratio of the Tb³⁺ emission to the Eu³⁺ emission decreased (Figures 7a and S13). Although the Ce³⁺ and Tb³⁺ concentrations decreased slightly compared with LiYF₄:Ce(15%), Tb(15%), Eu(1%), the Eu³⁺ emission remained strong under excitation with 300 nm light. The Eu³⁺ emission intensity of LiYF₄:Ce(13%), Tb(14%), Eu(3%) under excitation with 300 nm light was 3.1 times greater than that of LiYF₄:Eu(35%) under excitation with 394 nm light (Figure 7b). As shown in the PLE spectra of Figure 7c, strong broad bands were observed in the UV region due to the 4f → 5d transition of Ce³⁺ ions, indicating the energy transfer from Ce³⁺ to Eu³⁺. According to Zhang et al. and Blasse, Tb³⁺ acts as an energy transfer mediator, bridging the energy transfer from Ce³⁺ to Eu³⁺ and blocks the corresponding metal-metal charge transfer^42,43. When we compared the Eu³⁺ emission intensity of LiYF₄:Ce(13%), Tb(14%), Eu(1%) with that of LiYF₄:Ce(13%), Eu(1%), the LiYF₄:Ce(13%), Tb(14%), Eu(1%) exhibited the significantly higher PL intensity (Figure S14). This result confirms that the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer is more efficient than Ce³⁺ → Eu³⁺ energy transfer. The LiYF₄:Ce(13%), Tb(14%), Eu(1%) nanophosphors emitted yellow-green light due to the intense ⁵D₄ →⁷F₅ transition of Tb³⁺ and weak ⁵D₀ →⁷F₂ transition of Eu³⁺ under excitation at 300 nm. The ratio of the PL intensity in the green spectral region due to the ⁵D₄ →⁷F₅ transition of Tb³⁺ ions to that in the red spectral region due to the ⁵D₀ →⁷F₂ transition of Eu³⁺ ions decreased with increasing Eu³⁺ concentration (Figure S13). As a result, the emission color of the LiYF₄:Ce, Tb, Eu nanophosphors gradually transitioned from green to orange as a function of the Eu³⁺ concentration. The CIE color coordinates (x, y) of the LiYF₄:Ce, Tb, Eu nanophosphors shifted from (0.3484, 0.5799) to (0.4936, 0.4675), as shown in Figure 7d. The gradual luminescence color shift from green to orange was directly confirmed with imaging showing the luminescence of LiYF₄:Ce, Tb, Eu solutions illuminated with a hand-held UV lamp (Figure 7d inset).

Figure 7

(a), (b) PL and (c) PLE spectra and (d) CIE color coordinates of LiYF₄:Ce(13%), Tb(14%), Eu(0–5%) [(i) 0%, (ii) 1%, (iii) 2%, (iv) 3%, (v) 4%, (vi) 5% and (vii) LiYF₄:Eu(35%)]. Inset shows digital camera image of luminescent LiYF₄:Ce, Tb, Eu nanophosphor solution under hand held UV lamp.

Discussion

In the PLE spectra of Figure 7c, the PLE band due to the 4f → 5d transition of Ce³⁺ ions was strong, whereas the PLE peak of the ⁷F₆ →⁵D₄ transition of Tb³⁺ ions at 487 nm was very weak (almost negligible). The weak PLE peak at 487 nm can be attributed to the weak excitation of Tb³⁺ ions at this wavelength and/or inefficient energy transfer from the Tb³⁺ to Eu³⁺ ions. When the PLE spectra of LiYF₄:Ce, Tb, Eu(1–5%) were monitored at 545 nm for Tb³⁺ emission, the PLE peaks were also very weak at 487 nm, which is the direct excitation wavelength of Tb³⁺ (Figure S15). This result indicates that the weak PLE peak at 487 nm in Figure 7c was attributed to the weak direct excitation of Tb³⁺ at this wavelength. Thus, to confirm the energy transfer from Tb³⁺ to Eu³⁺, the decay time for Tb³⁺ emission was examined, although the LiYF₄:Ce, Tb, Eu showed much stronger Eu³⁺ emission than LiYF₄:Ce, Eu. According to Dexter theory⁴⁶, the probability of energy transfer via multipolar interaction can be expressed by the following equation

where P is the energy transfer probability, τ_D is the decay time of the donor emission, Q_A is the total absorption cross-section of the acceptor, R is the distance between the donor and the acceptor and b and c are parameters that depend on the type of energy transfer. Figure 8a shows the decay profiles of LiYF₄:Ce, Tb, Eu nanophosphors monitored at 545 nm for Tb³⁺ emission with varying Eu³⁺ concentrations. The decay time of the Tb³⁺ emission was observed to shorten with increasing Eu³⁺ concentration. According to equation (2), because the energy transfer probability P is inversely proportional to the decay time of τ_D, the shortened decay time of the Tb³⁺ emission confirms the presence of an energy transfer pathway from Tb³⁺ to Eu³⁺. To better understand the process of energy transfer from Tb³⁺ to Eu³⁺, the energy transfer efficiency (η_ET) from the sensitizer Tb³⁺ ions to the activator Eu³⁺ ions was calculated by using the following equation⁴⁷:

Figure 8

(a) Decay profiles and (b) energy transfer efficiency for Tb³⁺ → Eu³⁺ energy transfer of LiYF₄:Ce(13%), Tb(14%), Eu nanophosphors with varying Eu³⁺ concentrations of 0, 1, 2, 3, 4 and 5%. (c) Schematic energy level diagram showing energy transfer.

where I_S and I_S0 are the emission intensities of the Tb³⁺ ions with and without Eu³⁺ ions, respectively. Figure 8b shows the calculated energy transfer efficiencies of the Tb³⁺ → Eu³⁺ pathway. As the Eu³⁺ concentration was increased from 1 to 5 mol%, the energy transfer efficiency correspondingly increased from 55.7 to 85.8%. A direct energy transfer from Ce³⁺ to Eu³⁺ was also confirmed by measuring the Ce³⁺ decay time (Figure S16). First, the Ce³⁺ decay of LiYF₄:Ce(15%) was compared with that of LiYF₄:Ce(15%), Eu(10%), which showed the strongest red emission from Eu³⁺ in the LiYF₄:Ce, Eu system, as shown in Figure S3. The Ce³⁺ decay time was shortened from 12.3 ns for LiYF₄:Ce(15%) to 0.6 ns for LiYF₄:Ce(15%), Eu(10%). This result indicates that energy was transferred from Ce³⁺ to Eu³⁺ even though the nanophosphors were not doped with Tb³⁺. In addition, the Ce³⁺ decay time of LiYF₄:Ce(15%), Tb(15%), Eu(1%) was shorter than that of LiYF₄:Ce(15%), Tb(15%) (Figure S16(b)). Upon comparing the decay profile of the green-emitting LiYF₄:Ce(13%), Tb(14%) with that of the orange-emitting LiYF₄:Ce(13%), Tb(14%), Eu(5%), the Ce³⁺ decay time (0.4 ns) of the LiYF₄:Ce(13%), Tb(14%), Eu(5%) was found to be shorter than that (1.2 ns) of the LiYF₄:Ce(13%), Tb(14%), as shown in Figure S17. These results also support the pathway of Ce³⁺ → Eu³⁺ energy transfer. In summary, the energy transfer process in LiYF₄:Ce, Tb, Eu nanophosphors can be depicted as in Figure 8c. When the LiYF₄:Ce, Tb, Eu nanophosphors were excited with 300 nm UV light, the external UV light was absorbed by Ce³⁺ and then transferred to Tb³⁺, followed by the transfer of this excited energy from Tb³⁺ to Eu³⁺. Some of the excited energy was directly transferred from Ce³⁺ to Eu³⁺. By considering the efficiencies of the Ce³⁺ → Eu³⁺ and Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer pathways, we conclude that Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer is more efficient than Ce³⁺ → Eu³⁺ energy transfer, based on the PL spectra shown in Figure S14. The stronger Eu³⁺ intensity of LiYF₄:Ce(13%), Tb(14%), Eu(1%) relative to that of LiYF₄:Ce(13%), Eu(1%) means that the Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer pathway is more efficient than Ce³⁺ → Eu³⁺; in addition, Tb³⁺ acts as an efficient mediator for Ce³⁺ → Eu³⁺ energy transfer, which is consistent with previous reports^42,43. As a consequence, the emission peaks are observed in the green and red spectral regions with intensity ratios that vary as a function of the Eu³⁺ concentration.

Nanophosphors with uniform size and shape can be dispersed in a polymer matrix⁴⁸. To investigate the feasibility of applying LiYF₄:Ce, Tb, Eu nanophosphors to transparent display devices, the nanophosphors were incorporated into polydimethylsiloxane (PDMS) polymer. Figure 9 depicts photographs of the LiYF₄:Ce, Tb, Eu nanophosphor-PDMS composites taken under ambient indoor light and UV illumination. As shown in Figures 9a, S18a and S19a, the LiYF₄:Ce, Tb, Eu nanophosphor-PDMS composites were highly transparent and the background logos can clearly been seen. Transmittances of most nanophosphor-PDMS composites were higher than 75% in the visible spectral range (Figure S20). As we expected from the multicolor-emitting properties of the LiYF₄:Ce, Tb, Eu nanophosphors, the LiYF₄:Ce, Tb, Eu nanophosphor-PDMS composites also showed bright multicolor emission from green to orange, including yellow-green, greenish yellow, yellow and orange yellow under illumination with a hand-held UV lamp (Figures 9b, S18b and S19b). The emission of uniform visible light from the highly transparent nanophosphor-PDMS composites verifies that the LiYF₄:Ce, Tb, Eu nanophosphors were uniformly incorporated into the PDMS composites. The brightness of the nanophosphor-PDMS composites can be further enhanced by adapting the core/shell structured nanophosphors because the LiYF₄:Ce, Tb, Eu(1–5%)/LiYF₄ showed a stronger PL intensity than the LiYF₄:Ce, Tb, Eu(1–5%) core nanophosphors (Figure S21).

Figure 9

Digital camera images of (a) LiYF₄:Ce (13%), Tb(14%), Eu(0%)-PDMS bar (left) and disk (right) under room light (upper panel) and UV lamp illumination (lower panel) and (b) LiYF₄:Ce, (13%), Tb(14%), Eu (0–5%)-PDMS disks under UV lamp illumination [i: 0%, ii: 1%, iii: 2%, iv: 3%, v: 4% and vi: 5%].

The logo in (a) was reprinted with permission from Korea Institute of Science and Technology (KIST).

In summary, single-tetragonal-phase colloidal LiYF₄:Eu nanophosphors were successfully synthesized and we investigated strategies to enhance Eu³⁺ emission from the LiYF₄:Eu-based nanophosphors. The luminescence properties of various nanophosphor architectures including LiYF₄:Eu/LiYF₄:Ce, Tb and LiYF₄:Ce, Tb/LiYF₄:Eu core/shell nanophosphors and LiYF₄:Ce, Tb, Eu nanophosphors were investigated. Among these structures, Ce³⁺, Tb³⁺ and Eu³⁺ triply-doped LiYF₄ nanophosphors exhibited strong Eu³⁺ emission via the efficient energy transfer of Ce³⁺ → Tb³⁺ → Eu³⁺. In the case of LiYF₄:Eu nanophosphors, when sensitizers and activators were co-doped in the LiYF₄ NCs, the activator Eu³⁺ showed strong red emission. The Eu³⁺ emission was enhanced threefold via Ce and Tb sensitization. Owing to the Ce³⁺ → Tb³⁺ and Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer pathways, green and red emission peaks were realized from the LiYF₄:Ce, Tb, Eu nanophosphors. In addition, the LiYF₄:Ce, Tb, Eu nanophosphors showed adjustable, bright multicolor emission from green to orange based on the Eu³⁺ concentration. These LiYF₄:Ce, Tb, Eu nanophosphors were uniformly incorporated into PDMS polymer and the resulting nanophosphor-PDMS polymer composites also showed bright multicolor emission from green to orange, indicating that the LiYF₄:Ce, Tb, Eu nanophosphors have the potential for applications in transparent display devices.

Methods

Materials

LiOH·H₂O (99.995%), YCl₃·6H₂O (99.99%), CeCl₃·7H₂O (99.999%), TbCl₃·6H₂O (99.9%), EuCl₃·6H₂O (99.99%), NH₄F (99.99+%), oleic acid (OA, technical grade 90%) and 1-octadecene (ODE, technical grade 90%) were purchased from Aldrich and used without further purification. Sodium oleate (>97%) was obtained from TCI.

Synthesis of the LiYF₄:Eu nanophosphors

First, the lanthanide oleates [Ln(oleate)₃, Ln = Y and Eu] were prepared by adapting the synthesis reported by Hyeon and colleagues⁴⁹. Then, 1 mmol of Ln(oleate)₃ complexes was loaded into a three-necked flask containing a solvent mixture of 10.5 mL OA and 10.5 mL ODE. The mixture was heated to 150°C for 40 min. After the reaction mixture cooled to 50°C, a methanol (MeOH) solution (10 mL) containing LiOH·H₂O (2.5 mmol) and NH₄F (4 mmol) was added to the reaction flask and stirred for 40 min. After removing the MeOH, the solution was heated to 320°C for 90 min under an Ar atmosphere. The as-synthesized nanophosphors were washed several times with ethanol and then dispersed in chloroform.

Synthesis of the LiYF₄:Eu/LiYF₄:Ce, Tb and LiYF₄:Ce, Tb/LiYF₄:Eu core/shell nanophosphors

For the synthesis of LiYF₄:Eu/LiYF₄:Ce, Tb nanophosphors, 0.7 mmol of YCl₃·6H₂O, 0.15 mmol of CeCl₃·7H₂O and 0.15 mmol of TbCl₃·6H₂O were loaded into a three-necked flask containing a solvent mixture of 10.5 mL OA and 10.5 mL ODE. The mixed solution was heated to 150°C for 40 min. After the solution was cooled to 80°C, 10 mL of LiYF₄:Eu chloroform solution was injected into the reaction flask. Then, the chloroform solvent was removed and the reaction mixture was cooled to 50°C. A MeOH solution (10 mL) containing LiOH·H₂O (2.5 mmol) and NH₄F (4 mmol) was added to the reaction flask and then stirred for 40 min. After the MeOH was removed, the solution was heated to 300°C for 110 min under an Ar atmosphere. The as-synthesized nanophosphors were washed several times with ethanol and then dispersed in chloroform.

For the synthesis of LiYF₄:Ce, Tb/LiYF₄:Eu, LiYF₄:Ce, Tb core nanophosphors were prepared by using the same synthesis procedure as that for the synthesis of LiYF₄:Eu nanophosphors except the use of 1 mmol of Ln(oleate)₃ complexes [Ln = Y (70%), Ce (15%) and Tb (15%)]. Then, 0.65 mmol of YCl₃·6H₂O and 0.35 mmol of EuCl₃·6H₂O were loaded into a three-necked flask containing a solvent mixture of 10.5 mL OA and 10.5 mL ODE. The mixed solution was heated to 150°C for 40 min. After the solution was cooled to 80°C, 10 mL of LiYF₄:Ce, Tb chloroform solution was injected into the reaction flask. Then the chloroform solvent was removed and the reaction mixture cooled to 50°C. A MeOH solution (10 mL) containing LiOH·H₂O (2.5 mmol) and NH₄F (4 mmol) was added to the reaction flask and stirred for 40 min. After the MeOH was removed, the solution was heated to 300°C for 110 min under an Ar atmosphere. The as-synthesized nanophosphors were washed several times with ethanol and then dispersed in chloroform.

Synthesis of the LiYF₄:Ce, Tb, Eu nanophosphors

As mentioned above, Ln(oleate)₃ complexes (Ln = Y, Ce, Tb and Eu) were prepared and loaded into a three-necked flask. Other procedure was the same as that for the synthesis of the LiYF₄:Eu nanophosphors. For multicolor emission from the LiYF₄:Ce, Tb, Eu, the Eu³⁺ concentration was adjusted.

Synthesis of the LiYF₄:Ce, Tb, Eu/LiYF₄ nanophosphors

For the formation the LiYF₄ inorganic shell on the triply-doped LiYF₄:Ce, Tb, Eu core, 1 mmol of YCl₃·6H₂O was loaded into a three-necked flask containing a solvent mixture of 10.5 mL OA and 10.5 mL ODE. The mixed solution was heated to 150°C for 40 min. After the solution was cooled to 80°C and 10 mL of LiYF₄:Ce, Tb, Eu chloroform solution was injected into the reaction flask. Then, chloroform solvent was removed and the reaction mixture cooled to 50°C. A MeOH solution (10 mL) containing LiOH·H₂O (2.5 mmol) and NH₄F (4 mmol) was added to the reaction flask and stirred for 40 min. After the MeOH was removed, the solution was heated to 300°C for 110 min under an Ar atmosphere. The as-synthesized core/shell nanophosphors were washed several times with ethanol and then dispersed in chloroform.

Preparation of the nanophosphor-PDMS composites

To prepare the multicolor-emitting LiYF₄:Ce, Tb, Eu-PDMS composites, 0.4 mL of the LiYF₄:Ce, Tb, Eu solution was thoroughly mixed with 10 mL of SYLGARD silicone elastomer 184 followed by the addition of a curing agent (1 mL). Finally, the nanophosphor-PDMS composites were aged overnight and then heat-treated at 80°C for 1 h.

Characterization

The crystal structures of the as-synthesized nanophosphors were determined using a Bruker D8 ADVANCE diffractometer with Cu K_α radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The PLE and PL spectra and PL decay profiles of the nanophosphor solutions were obtained using a Hitachi F-7000 spectrophotometer. The size and morphology of the nanophosphors were investigated using a Tecnai G2 F20 (FEI Co.) operated at 200 kV. The EDS spectra were obtained using an EDAX EDS spectrometer PV9761.