1. Introduction

Optical gain materials possessing white upconversion (UC) luminescence are garnering increasing interest owing to their growing applications, such as three-dimensional display, white light-emitting diode, tunable laser, and etc [1–8]. In the past decades, a large number of optical gain materials were exploited for the generation of white UC luminescence [9–11]. Among them, nano-crystallized glass ceramic (GC) was considered as a significant optical host for emitting efficient white light for the applications in photonic devices due to the high transparency, high luminescence efficiency, easy fabrication technique, high solubility for active ions and excellent thermodynamic-stability [12–14].

Generally, white lights were produced in optical GCs based on three-primary-color (red-green-blue) and two-primary-color (yellow-blue) UC emission systems [15]. For the three-primary-color emission system, rare earth (RE) ions, for example, Yb³⁺, Er³⁺ and Tm³⁺ were usually triply-doped into GCs [16–19]. Yb³⁺/Er³⁺ pairs were used to produce red (Er³⁺: ⁴F_9/2→⁴I_15/2) and green (Er³⁺: ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2) emissions via the energy transfer UC processes. Meanwhile, Yb³⁺/Tm³⁺ pairs emit blue (Tm³⁺: ¹D₂→³F₄ and ¹G₄→³H₆) light through cross-relaxation UC processes. By adjusting the RE ion doping concentration, the red, green and blue emissions were synthesized to generate white lights. Actually, the unnecessary interactions between RE ions, in particular of between Er³⁺ and Tm³⁺, were difficult to avoid in the triply-doped GC systems, which increased the probability of non-radiative transition and reduced the efficiency of white luminescence. On the other hand, Yb³⁺/Mn²⁺ ions were doubly-doped into optical GCs to obtain white lights by the combination of yellow and blue UC emissions [20,21]. Yellow UC emissions were obtained via the electronic transitions of Yb³⁺-Mn²⁺ dimers. Blue emissions were produced by the cooperative UC processes of Yb³⁺-Yb³⁺ pairs. However, the UC emissions of Yb³⁺-Mn²⁺ dimers were only observed in a small number of specific GCs at room temperature when the interionic distances were extremely short [22,23]. Nevertheless, the UC emission efficiency of Mn²⁺/Yb³⁺ ions was lower as compared to RE ions due to the high probability of non-radiative transition of transition metal ions in GCs. These shortcomings have prevented the wide applications of white UC GCs in photonic devices.

In order to address these issues, a novel RE ions (Yb³⁺/Eu³⁺) doubly-doped GC was designed based on a phase-separated glass network to generate efficient white UC emissions. In the past studies, only red UC emissions were produced by the Yb³⁺/Eu³⁺ doubly-doped GCs [24–26]. In this work, intense blue, green and red UC emissions were simultaneously obtained in the GCs thanks to the controllable precipitation of rare-earth-fluoride (KYb₃F₁₀) crystals, the complete incorporation of Eu³⁺ into the compact fluoride nanocrystals and short interionic distances between RE ions. Thus, white and multi-color UC emissions are generated in the designed GCs. The crystalline phases, UC luminescent properties and UC emission mechanism are investigated carefully. The designed strategy of the GCs paves a new way for the engineering of optical materials with effective white and multi-color UC luminescence.

2. Experiments

Samples with a molar composition of 60 SiO₂-20 KF-20 ZnF₂-1.0Yb F₃-xEu₂O₃ (x = 0-0.3) were prepared by melt-quenching method. 30 g reagent grade stoichiometric mixtures were mixed thoroughly in an agate mortar and melted in covered platinum rhodium crucibles in the electric furnace at 1450℃ for 30 min. The melts were poured onto a brass plate and then pressed by another brass plate to obtain precursor glasses (PGs). PG samples were heated at 540℃ for 5 and 10 h to obtain GC samples according to the differential scanning calorimetry (DSC) results in Ref.[27]. PG and GC samples were cut and polished to 2-mm-thick for measurements.

To identify the crystalline phase in GCs, X-ray diffraction (XRD) patterns were performed on a D8 advance X-ray diffractometer (Bruker, Switzerland) with Cu/Ka (λ = 0.1541 nm) radiation. For performing the morphological and element analysis, high angle annular dark field transmission electron microscopy (HAADF-TEM) image was measured using FEI Talos F200x (FEI, USA), operating at 200 kV with the current on specimen ∼50 pA. UC emission spectra of samples were recorded by using an Edinburgh FLS980 fluorescence spectrometer (Edinburgh Instruments, UK) under the excitation of a 980 nm laser diode (LD). The emission decay curves were measured using the same spectrometer with a pulse laser as the excitation source. All measurements were performed at room temperature.

3. Results and discussion

Figure 1(a) shows the UC emission spectra of Yb³⁺ single-doped PG and GCs. Excited by using a 980 nm LD, blue emissions around 480 nm are observed in the spectra, which are attributed to the UC emissions of Yb³⁺-Yb³⁺ pairs as similar to that in our previous studies [28]. The emission intensity at 480 nm in the GC enhance 9 and 17 times when the sample was heat treated at 540℃ for 5 h and 10 h, respectively, indicating the incorporation of Yb³⁺ into crystal environments in the GCs. Figure 1(b) shows the UC emission spectra of Yb³⁺/Eu³⁺ doubly-doped PG and GCs. The blue emissions around 480 nm attributed to the UC emission of Yb³⁺-Yb³⁺ pairs are also observed in the spectra of the doubly-doped samples. Interestingly, green and red UC emission peaks are both observed in the spectra of PG and GCs. The main transitions between energy levels corresponding to the emission peaks are shown above the emission spectra. The green (510, 523, 539, 541 and 553 nm) and red emission peaks (587, 593, 619 and 651 nm) are attributed to (⁵D₂→⁷F₃, ⁵D₁→⁷F₀, ⁵D₁→⁷F₁, ⁵D₂→⁷F₃, and ⁵D₁→⁷F₂) and (⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄) transitions of Eu³⁺, respectively. The blue, green and red emissions are simultaneously observed in the spectra, indicating that the Yb³⁺/Eu³⁺ doubly-doped samples are significant candidates for multi-color and white UC luminescence. It is also found that the emission intensities of Eu³⁺ increase about 25 and 40 times after heat treatment for 5 and 10 h, respectively, which imply that the coordinated environments of Eu³⁺ have been changed after the heat treatments.

 

Fig. 1. UC emission spectra of (a) 1.0Yb³⁺ doped and (b) 1.0Yb³⁺/0.05Eu³⁺ doubly-doped PG and GCs excited by using a 980 nm LD. Emission decay curves of 1.0Yb³⁺/0.05Eu³⁺ doubly-doped PG and GCs monitored at (c) 480 nm and (d) 510 nm emissions under the excitation of a 980 nm LD.

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The emission decay curves of Yb³⁺/Eu³⁺ doubly-doped PG and GCs are shown in Figs. 1(c) and 1(d). It is clear to found that the fitted lifetimes of the GCs are much longer than that of PG. The UC emission lifetime of Yb³⁺ (480 nm) dramatically increases from 181 to 719 and 797 μs and that of Eu³⁺ (510 nm) increases from 582 to 1061 and 1124 μs when the sample was heat treated at 540 ℃ for 5 and 10 h, respectively. The obvious enhancements of UC emission intensities and lifetimes indicate that both Yb³⁺ and Eu³⁺ ions are incorporated into the crystal environments possessing lower phonon energy and lower probabilities of non-radiative transitions.

For revealing the enhancement mechanism of emission intensities and lifetimes in the GCs, the XRD patterns of PG and GCs are investigated and shown in Fig. 2(a). Broad band is observed in the XRD pattern of the PG sample, indicating that the PG sample exhibits amorphous state. Sharp peaks are observed in the XRD patterns of the GCs. The peaks in the XRD pattern of no-doped GC match well with the diffraction peaks of K₂SiF₆ (No: 85-1382) crystals, proving that only K₂SiF₆ crystal are precipitated in the no-doped GC. However, the intense diffraction peaks of cubic KYb₃F₁₀ (No: 74-2204) crystals at 26.99°, 31.27°, 44.81° and 53.1° are observed in the XRD patterns of Yb³⁺/Eu³⁺ doubly-doped GCs, which indicates the crystallization of KYb₃F₁₀ in the Yb³⁺ doped GCs. Moreover, the diffraction peaks of KZnF₃ (No: 06-0439) crystals are also observed in the XRD patterns of the Yb³⁺/Eu³⁺ doubly-doped GCs. It is also found from the XRD patterns that the intensities of diffraction peaks for KYb₃F₁₀ crystals all enhance dramatically when the doping concentration of Yb³⁺ is increased from 0.5 to 1.5 mol.%. These results imply that the crystalline phases in the GCs are changed and modulated by the doping of Yb³⁺.

 

Fig. 2. (a) XRD patterns of no-doped and Yb³⁺/Eu³⁺ doubly-doped PG and GCs. (b) Optical transmission spectra of Yb³⁺/Eu³⁺ doubly-doped PG and GCs, inset is the typical HAADF-TEM image of Yb³⁺/Eu³⁺ doubly-doped GC and the photos of samples.

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According to the previous investigations [27], the designed glass exhibited a phase-separated network. K₂SiF₆ crystals prefer to precipitate near the interface between silicon-rich and fluorine-rich phases in the no-doped GC. In the Yb³⁺/Eu³⁺ doubly-doped glass, all RE ions are confined within the fluorine-rich networks. By the heat treatments, KYb₃F₁₀ crystals are formed around Yb³⁺ via the inducing of Yb³⁺ due to it’s large ionic radius and large potential [29]. KZnF₃ crystals are not precipitated in the no-doped GC because the crystallization barriers are difficult to overcome as proved by the XRD pattern. However, in the Yb³⁺/Eu³⁺ doubly-doped GCs, KZnF₃ crystals are precipitated by the assistance of KYb₃F₁₀ crystal nucleus. Actually, RE ions cannot enter KZnF₃ crystals due to the mismatch of ionic valence and the lack of appropriate coordinated-sites. By the precipitation of KYb₃F₁₀ crystals, Yb³⁺ ions are spontaneously incorporated into the fluoride crystal structures featuring low phonon energy. Owing to the similar radius of Eu³⁺ (R = 0.095 nm) with Yb³⁺ (R = 0.086 nm), Eu³⁺ ions are easily incorporated into the fluoride crystals by the substitution for Yb³⁺ in the doubly-doped GCs. The phonon energy of KYb₃F₁₀ (∼400 cm^-1) is much lower than that of glass networks [30]. Besides, the interionic distance in the crystal structure is also shorter than in the open glass networks, which is beneficial for the formation of Yb³⁺-Yb³⁺ pairs as well as Yb³⁺-Eu³⁺ pairs. Therefore, the UC emissions and lifetimes of Yb³⁺-Yb³⁺ pairs and Yb³⁺-Eu³⁺ pairs are enhanced dramatically in the GCs as compared to PG. Therefore, the crystallization of KYb₃F₁₀ in the GCs and the incorporation of Eu³⁺ into KYb₃F₁₀ crystals are responsible for the enhancements of UC emissions and emission lifetimes.

For the practical applications in photonic devices, the GCs should exhibit high transparency. As shown in Fig. 2(b), the PG sample exhibits high transmittance (∼83%) in the range of 500 nm to 800 nm with thickness of 2.0 mm. Though the transparency of samples decrease after the precipitation of crystals, the GCs still maintain high transmittance to 80% and 72% at 600 nm when it was heat treated for 5 h and 10 h, respectively. The photos in the inset of Fig. 2(b) show that the PG and GCs all exhibit high transparency. This is because of the small sizes of the crystal particles in the GCs. The HAADF-TEM image of the 1.0Yb³⁺/0.1 Eu³⁺ doubly-doped GC is presented in the inset of Fig. 2(b). It is found that nanocrystals are distributed among the amorphous matrix and the sizes of the crystal particles are below 30 nm. The crystallization fraction is directly estimated to about 15.3% from the TEM image.

In order to illuminating the UC mechanisms in the GCs, the double-logarithmic plots of the excitation powers dependency on the emission intensities are presented in Fig. 3(a). The fitted slope (n) stands for the number of pump photons required to emit per visible photon. The fitted n value of emission at 480, 593 and 619 nm is 1.06, 1.71, and 1.76, respectively, indicating that both the blue UC emissions of Yb³⁺ and the red emission of Eu³⁺ ions are two-photon processes. However, the n value of the emission at 510 nm is 2.08. This result proves the green emission is originated from a three-photon UC process. According to the above results, the main UC populating mechanisms in the Yb³⁺/Eu³⁺ doubly-doped GCs are schematically presented in Fig. 3(b). Yb³⁺-Yb³⁺ pairs are formed to firstly emit blue light via the cooperative UC processes by the absorption of two 980 nm photons. The electrons in the excited level are easily transferred to ⁵D₁ level of Eu³⁺. Then the electrons rapidly transfer to ⁵D₀ level through the non-radiative transition. The level transitions of Eu³⁺: ⁵D₀→⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃ produce the red emissions at 584, 593, 619 and 651 nm, respectively. Thus, the red emissions are ascribed to two-photon UC processes. Moreover, the excited energy can also transfer to ⁵D₄ level of Eu³⁺ from Yb³⁺ by the absorption of three 980 nm photons when the interionic distances are short or the excited power density is high [31]. The electrons transfer from ⁵D₄ to ⁵D₃, ⁵D₂, ⁵D₁, and ⁵D₀ levels via the non-radiative transitions. The transitions of Eu³⁺: ⁵D₃→⁷F₀, ⁷F₂ and ⁷F₃ produce the blue emissions at 417, 430, and 445 nm, respectively. The green emissions at 510 and 541 nm are attributed to the transitions of Eu³⁺: ⁵D₂→⁷F₃ and ⁷F₄. The green emissions at 527, 539 and 553 nm are ascribed to the transitions of Eu³⁺: ⁵D₁→⁷F₀, ⁷F₁ and ⁷F₂, respectively.

 

Fig. 3. (a) Ln-Ln plots of UC emission intensity versus excited power density of 980 nm laser. (b) Schematic diagrams of energy levels and relative transitions in the Yb³⁺/Eu³⁺ doubly-doped GCs.

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In the past investigations, white UC emission could hardly be obtained in Yb³⁺/Eu³⁺ doubly-doped GCs [32–34]. Firstly, efficient blue UC emission of Yb³⁺-Yb³⁺ pairs is difficult to obtain in the traditional Yb³⁺ doped GC. For instance, the UC emission of Yb³⁺-Yb³⁺ pairs in GC containing NaYF₄ crystals is much weaker than that of our designed GC containing KYb₃F₁₀ crystals as shown in Fig. 4(a). Actually, the UC emission of Yb³⁺-Yb³⁺ pairs is easier to achieve in specific host where the interionic distances are short. Moreover, as presented in Fig. 4(b), no green UC emission and weak blue emission is observed in the spectrum of Yb³⁺/Eu³⁺ doubly-doped NaYF₄ GC. The spectrum mainly contains the red UC emissions and the sample emits faint red light irradiated by 980 nm LD [inset of Fig. 4(b)]. However, the spectrum of our designed GC simultaneously contains blue, green and red emissions. The sample emits intense white light irradiated by 980 nm LD [inset of Fig. 4(b)] and the emission intensity is much higher as compared to the NaYF₄ GC.

 

Fig. 4. Compared UC emission spectra of (a) 1.0Yb³⁺ doped and (b) 1.0Yb³⁺/0.1Eu³⁺ doubly-doped GCs containing KYb₃F₁₀ and NaYF₄ crystals, the inset are photos of samples irradiated by 980 nm LD with power density of 1.1 × 10⁵ mW/cm², left: GC-KYb₃F₁₀; right: GC-NaYF₄.

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In the traditional NaYF₄ GCs, RE ions were expect to incorporate into the fluoride crystal structures by the substitutions of Y³⁺ ions. Actually, only a small number of RE ions entered the NaYF₄ crystal lattices due to the mismatch in ionic radius. The residual RE ions are distributed among the amorphous glass networks featuring high phonon energy. The interionic distances between RE ions are long, which is not beneficial for the formation of Yb³⁺-Yb³⁺ and Yb³⁺-Eu³⁺ pairs. Thus, the UC emission intensities in Yb³⁺ doped and Yb³⁺/Eu³⁺ doubly-doped NaYF₄ GC are both low. More importantly, the three-photon UC process is difficult to occur in NaYF₄ GC due to the long interionic distances, which greatly reduce the transitions probabilities of blue UC emissions (Eu³⁺: ⁵D₃→⁷F₀, ⁷F₂ and ⁷F₃) and green emissions (Eu³⁺: ⁵D₂→⁷F₃ and ⁷F₄). Besides, the electrons in ⁵D₁ levels are rapidly depopulated to ⁵D₀ level because of the assistance of the high-energy phonons. Thus, the excited energies are mainly used to produce red UC emissions in the Yb³⁺/Eu³⁺ doubly-doped NaYF₄ GC.

However, for our GC, the precipitation of rare-earth-fluoride crystals is around Yb³⁺ and RE ions are spontaneously incorporated into fluoride crystal structures featuring low phonon energy during the crystallization process. The interionic distances of RE are short. These are beneficial for the formation of RE ion pairs and facilitate the three-photon UC processes. Therefore, the intensities of the blue UC emission of Yb³⁺ and the UC emissions of Eu³⁺ in our designed GCs are much higher than the traditional NaYF₄ GC. On the other hand, owing to the short interionic distances, the ⁵D₂ and ⁵D₁ levels are easily populated, which provide significant opportunities for obtaining efficient green UC emissions. Accordingly, the precipitation of novel rare-earth-fluoride crystals and the complete incorporation of RE into the crystal environments with short interionic distances are responsible for the efficient multi-color and white UC emissions in the designed GCs.

The dependence of the UC emission intensities on the doping concentration of Eu³⁺ for the Yb³⁺/Eu³⁺ doubly-doped samples is presented in Fig. 5(a). Excited by 980 nm LD, blue, green and red UC emissions are observed in the spectra of samples. The intensity of blue emission of Yb³⁺ decreases monotonically with increasing Eu³⁺ content, which is because of the rapid energy transfer from Yb³⁺ to the nearby Eu³⁺. As Eu³⁺ content increases from 0 to 0.2%, the green and red emission intensities of Eu³⁺ all increases firstly, reaching a maximum at 0.1% Eu³⁺, and then decreases when the Eu³⁺ content is further increased to 0.2% due to the concentration quenching effect. The changes of emission intensity of Yb³⁺ and Eu³⁺ depending on the concentration of Eu³⁺ indicates the existence of energy transfer from Yb³⁺ to Eu³⁺ in the GCs. More importantly, the ratio of blue emission to green and red emissions is changed by adjusting the concentration of Eu³⁺. As a result, the compound emission of sample varies from blue to white and even to yellow color as Eu³⁺ concentration increases from 0 to 0.1 mol.% as shown in the black points of CIE chromaticity coordinates and the photos of GCs in Fig. 5(d). For the 1.0Yb³⁺ single-doped GC, only the UC emission of Yb³⁺-Yb³⁺ pairs around 480 nm is obtained under the excitation of 980 nm LD, the sample emits pure blue light. In the Yb³⁺/Eu³⁺ doubly-doped GCs, the green and red UC emission peaks of Eu³⁺ appear along with the blue UC emission of Yb³⁺. Owing to the mixture of the three primary colors, the emission color changes to white. Efficient white UC emissions are observed in the samples when the doping concentration changes to 1.0Yb³⁺/0.1 Eu³⁺. When Eu³⁺ content exceeds 0.2%, more excited energy transfers to Eu³⁺, the intensities of green and red emissions are higher than the blue emission. Yellow emission appears in the sample by the excitation of 980 nm LD. The sample turns to absolute yellow irradiated by laser when the concentration changes to 1.0Yb³⁺/0.3Eu³⁺. These results indicate that the designed GCs provide a novel optical gain material for efficient white and multi-color UC luminescence. Additionally, the decay curves of the UC emission at 480 nm for Yb³⁺/Eu³⁺ doubly-doped GCs are shown in Fig. 5(b). The fitted lifetimes of the UC emission of Yb³⁺-Yb³⁺ pairs in the doubly-doped GCs decrease gradually from 798 to 341μs when Eu³⁺ concentration is increased from 0 to 0.20 mol.%, which also prove the existence of ET from Yb³⁺ to Eu³⁺.

 

Fig. 5. (a) Concentration-dependent UC emission spectra and (b) decay curves monitored at 480 nm of 1.0Yb³⁺/xEu³⁺ PGs excited by 980 nm LD. (x = 0-0.2) (c) Plots of emission intensities of 1.0Yb³⁺/0.15Eu³⁺ doubly-doped GC depend on the excited power densities. (d) CIE chromaticity coordinates of the UC emission color for GCs with various doping concentrations and excited power densities, 1-6: 1.0Yb³⁺/(0, 0.02, 0.05, 0.1, 0.2, 0.3) Eu³⁺; 7-10: 1.0, 2.0, 3.0, 4.0 × 10⁵ mW/cm², inset are the corresponding photos of samples irradiated by 980 nm LD.

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Furthermore, the dependence of UC emission intensities on the power density of the excited laser is also presented in Fig. 5(c). It is found that the emission intensities of Eu³⁺ at 510, 584 and 619 nm are all monotonically increased with the increasing of power density. Especially, the emission intensity at 510 nm exhibits a most sharp enhancement with the increasing of power density. The intensity of green emission is lower than the red emission at low power density. However, the emission intensity at 510 nm surpasses that of the red emission when excited by high power laser. However, the intensity at 480 nm firstly increases and then gets saturated when the power density exceeds 2.0 × 10⁵ mW/cm². Thus, the relative intensity of blue, green and red emission can also be modulated via adjusting the power density. As shown in the red points of CIE chromaticity coordinates in Fig. 5(d), the emission color of the GC changes from white to green-yellow when the excited power density increases from 1.0 × 10⁵ mW/cm² to 4.0 × 10⁵ mW/cm². As mentioned above in Fig. 3(b), the green emission at 510 nm is ascribed to a three-photon UC process, which relies deeply on the excited power density. Excited by 980 nm LD at high power density, the excited energy of 980 nm laser is easier transfer to Eu³⁺ and the transition of three-photon UC process is easier to occur. Thus, the emission intensity for Yb³⁺-Yb³⁺ pairs at 480 nm is saturated but that at 510 nm increases sharply when the excited power density is increased. This color modulation of UC emission by excited power density in the GC makes it a tunable UC emitting tag for high-end three-dimensional display [35,36].

4. Conclusion

In summary, a novel Yb³⁺/Eu³⁺ co-doped transparent GC containing KYb₃F₁₀ crystals were prepared to achieve efficient white and multi-color UC luminescence. Enhanced UC emissions were observed in the GCs as compared to PG due to the incorporation of RE ions into the rare-earth-fluoride crystal structures. More importantly, apart from red UC emission, blue emissions of Yb³⁺-Yb³⁺ pairs and green emission of Eu³⁺ were also observed in the GCs, which could hardly be observed in the traditional Yb³⁺/Eu³⁺ doubly-doped GCs. By adjusting the Eu³⁺ concentration, the UC emission color of the Yb³⁺/Eu³⁺ doubly-doped GCs was tuned from blue to white and yellow. Moreover, the emission color was also regulated from white to green-yellow by the increase of excited power density. The designed strategy of the GCs paves a new way for the design and fabrication of optical gain materials with property of high-efficiency white and multi-color UC luminescence.