Adjustable multicolor up-energy conversion in light-luminesce in Tb3+/Tm3+/Yb3+ co-doped oxyfluorifFde glass-ceramics containing Ba2LaF7 nanocrystals

Transparent oxyfluoride glasses with highly efficient up-energy conversion (UEC) luminescence were developed in the 45SiO2-15Al2O3-12Na2CO3-21BaF2-7LaF3-xTbF3-yTmF3-zYbF3 composition (in mol%), and structural investigation by X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the formation of face-centered cubic Ba2LaF7 nanocrystals. The colors of UEC luminescences could be tuned easily by adjusting the concentration of doped rare earth ions and the excitation power of laser simultaneously. The relationship between the emission intensity of Tb3+/Tm3+/Yb3+ co-doped oxyfluoride glass-ceramics and the excitation pump power revealed that three-photon and two-photon absorptions predominated in the conversion process from the infrared into blue and red luminescences, respectively. A novel UEC mechanism of red emission from Tm3+ was proposed, energy transfers from Yb3+ to Tm3+ and Tb3+ and from Tm3+ to Tb3+ were evidenced. The possible mechanism responsible for the color variation of UEC in Tb3+/Tm3+/Yb3+ co-doped was discussed.

In recent years, increasing attention has been paid to the generation of white light sources for a variety of application purposes, such as white light emitting diodes (W-LEDs), back lighting, solid-state multicolor three-dimensional displays and so on. One of the effective ways for generating white light is to use rare earth (RE) ions doped material based on frequency up-energy conversion (UEC) process, which can convert low energy near-infrared radiation (NIR) into high energy visible radiation via multiphoton processes [1][2][3] . Lanthanide ions are suitable candidates for the UEC process owing to their abundant energy levels and narrow emission spectral lines [4][5][6][7] . However, their applications are greatly restricted due to the poor chemical stability and low damage threshold of the host materials that are limited to films and phosphors. Compared with abovementioned host materials, oxyfluoride glass-ceramics materials have also attracted great concerns since the first laser demonstration on Nd:YAG ceramic in 1995 8 . Oxyfluoride glass-ceramics can be highly transparent in the ultraviolet-visible-infrared range, which is beneficial to the output of UEC luminescences. Above all, oxyfluoride glass-ceramics combine the low phonon energy of fluoride crystals and the high chemical and mechanical stability of oxide glasses [9][10][11][12] , and thus ensure their convenient use in devices such as lasers and sensors.
In order to achieve multicolor visible light in lanthanide doped glass-ceramics 13,14 , the luminescence and relative intensity control of the three primary colors of red, green and blue (RGB) is required 15,16 . In addition, it is necessary to develop a novel method to produce multicolor visible light. Indeed, there have been some reports on the control of luminescences in three primary colors through the UEC method. For example, Downing et al. 17,18 . reported simultaneous generation of RGB fluorescences from fluoride glasses triply doped with Tm 3+ , Er 3+ and Pr 3+ using three different pairs of near-infrared laser excitation sources. Despite the fact that multiple-pump wavelength configuration has been produced, a single-pump scheme is still in great need. Recently, the reports on Tm 3+ /Er 3+ /Yb 3+ ions doped glass-ceramics have been used to realize white light emission and color tunability by adjusting pump power or Ln 3+ concentration via the UEC process [19][20][21] . As to our best knowledge, there is little 1  attention paid to the UEC excitation of luminescence materials to produce controllable colors through multiple ways simultaneously under a single-pump scheme.
In the present study, the multicolor tunability through the UEC process by optimizing the concentrations of Tm 3+ , Tb 3+ and Yb 3+ ions in Ba 2 LaF 7 nanocrystals was reported for the first time . The UEC efficiency was dramatically improved by the addition of Yb 3+ ions, Tb 3+ ions and Tm 3+ ions. The UEC white light could be obtained  from the combination of green component of Tb 3+ ions, blue and red components of Tm 3+ ions, with Yb 3+ ions as sensitizers in the UEC process. By adjusting the concentration of Yb 3+ , Tb 3+ and Tm 3+ ions or the excitation power of laser, a wide color adjustability was achieved, and the RGB color tunability as a function of pump power from 0.26 to 1.65 W using 980 nm laser excitation was discussed. Meanwhile, the color coordinates were evaluated and the possibility to obtain white light from the prepared samples was analyzed using CIE 1931 chromaticity diagram. In addition, the possible UEC mechanisms for energy transfer processes between Tb 3+ , Tm 3+ and Yb 3+ ions were also discussed.
Differential thermal analysis (DTA) thermograms were measured in a nitrogen atmosphere on STA-449F3 (NETZSCH). To identify the phase composition of the samples, XRD analysis was carried out with a powder diffractometer using Cu Kα radiation. The sizes, shapes, structures and component compositions of the as-prepared nanocrystals were characterized by Scanning electron microscopy (SEM, QUANTA 200) and transmission electron microscopy (TEM, JEM-2100) at a voltage of 30 K and 200 KV. The UEC emission spectra of Tb 3+ /Tm 3+ / Yb 3+ co-doped glass-ceramics in the wavelength range from 425 to 725 nm were recorded with a HITACHI F-7000 fluorescence spectrophotometer under the 980 nm laser diode excitation. The polished SABYb-2 glass sample was selected for heat treatment at four different temperatures of 600 °C, 610 °C, 620 °C and 640 °C for 2 h respectively to form transparent glass-ceramics, and the glass-ceramics were named as SABYb-2-600 SABYb-2-610, SABYb-2-620 and SABYb-2-640 respectively.

Results and Discussion
Figure 1(a) shows the DTA curves of the SABYb-2 glass. It can be seen that the transition temperature (T g ) of the glass is located at 590 °C. There is a crystallization peak at the temperature of T c1 = 630 °C, and an obvious crystallization peak appears at the temperature of T c2 = 745 °C (with the crystallization onset temperature of T x = 720 °C). The temperature difference ΔT between T x and T g (ΔT = T x −T g ) is generally used as a rough indicator of glass thermal stability. Here, ΔT = 130 °C > 100 °C, indicating that the prepared glass is stable and suitable for applications such as fiber amplifiers and lasers 22,23 . Therefore, according to DTA results, transparent glass ceramics can be prepared by heat treatment at the crystallization peak near 630 °C by appropriately controlling the crystallization temperature and process. Figure 1(b) shows the XRD patterns of SABYb-2 glass and its glass-ceramics nanocrystals after heat treatment at different temperatures of 600 °C, 610 °C, 620 °C and 640 °C for 2 h. The crystalline grain size D for a given (hkl) plane can be estimated from the XRD patterns following the Scherrer equation: where K = 0.89, λ is the wavelength of the incident XRD (for Cu Kα, λ = 0.154056 nm), β represents the corrected half width of diffraction peak and θ is the Bragg angle of X-ray diffraction peak 24 . By using the Scherrer equation, the average grain sizes of Ba 2 LaF 7 nanocrystals can be calculated to be about 12 nm, 18 nm, 26 nm and 39 nm for SABYb-2-600, SABYb-2-610, SABYb-2-620 and SABYb-2-640 samples, respectively. It is obvious that the average grain size of Ba 2 LaF 7 nanocrystals increases with increasing temperature of heat treatment from 600 to 640 °C. Figure 1(c) gives the TEM image of the SABYb-2-640 glass-ceramics. It can be seen that Ba 2 LaF 7 nanocrystals distribute uniformly in the glass matrix ( Fig. 2(c and d) in Supplementary Information). Their average grain size is about 39 nm, which is similar to that calculated by the Scherrer equation. The high resolution transmission electron microscopy (HRTEM) image with the d-spacing structure is shown in Fig. 1(d), and the d-spacing value of (200) plane is determined as 0.301 nm. Figure 1(e) provides the compositional analysis results of an individual Ba 2 LaF 7 nanocrystal grain measured by an energy dispersive X-ray detector (EDX). As can be seen, the peaks of Tb, Tm and Yb elements all appear on the spectrum curve, demonstrating that Tb 3+ , Tm 3+ and Yb 3+ ions have been effectively embedded into the Ba 2 LaF 7 host lattice 25 . Figure 2 shows the SEM images of the SABYb-2-640 glass-ceramics. In Fig. 2(a), the light areas represent crystalline regions enriched in atoms with high atomic number (rare earth elements and barium) and the dark areas represent the glass matrix mostly containing lighter atoms including sodium, aluminum and silicon. It can  be seen that crystalline regions are uniformly distributed in the glassy matrix for all glass-ceramics, which indicates a homogenous crystallization process. In most oxyfluoride glass-ceramics, the diffusion barrier containing glass formers surrounds the fluoride nanocrystals and prevents their further growth. As a result, small Ba 2 LaF 7 nanocrystals can precipitate in the glassy matrix, and single crystals (see Fig. 2(b)) can be obtained, indicating a relatively low viscosity of the base glass at the crystallization temperature 26 . Although the nanoparticles are not exactly of the same size, they have similar grain size with an average value of about 39 nm. In addition, the glass sample was synthesized by conventional quenching technique after the late heat treatment crystallization process. It should be noticed that the uniformity of the nanoparticles synthesized by this technique is worse than that of the nanoparticles synthesized by the hydrothermal method. Hence, the SEM images of Ba 2 LaF 7 will be more helpful for the nanostructures. Figure 3 shows the UEC emission spectra of the SABYb-2 glass and glass-ceramics. Here, we compare the intensities of emission peaks at visible wavelengths of 476 nm (Tm: 1 G 4 → 3 H 6 ), 546 nm (Tb: 5 D 4 → 7 F 5 ), 584 nm (Tb: 5 D 4 → 7 F 4 ) and 657 nm (Tm: 3 F 2,3 → 3 H 6 ) respectively. The intensity of visible transitions confirms that light scattering is not dominant in the material till heat treatment up to 640 °C. In the annealing process from 600 to 640 °C, the average grain size increases from 12 to nearly 39 nm, as shown in Table 1. The corresponding XRD patterns confirm their good crystallinity and the optical image of glass-ceramics sample displays good transparency in the visible wavelength range (see inset of Fig. 3). On this basis, it can be deduced that the spectral enhancement in the glass-ceramic materials is predominantly attributed to the presence of Ba 2 LaF 7 crystals that form above the glass transition temperature (T g = 590 °C) and dominate the phase composition up to 640 °C 27 . From the Ba 2 LaF 7 structure, which nucleates and grows between 600 °C and 640 °C, it is known that RE ions are basically dispersive into Ba 2 LaF 7 nanocrystals of the glass-ceramics 28 . As a result, the distance between RE ions becomes closer, which results in the enhancement of UEC luminescences.
The UEC mechanism mainly focuses on forward energy transfer. However, this may be a very biased view, since both the phonon energy and lifetimes during the UEC process are comparable to those of lower phonon energy glass hosts where both forward and backward energy transfer processes have been recorded and characterized. Recently, Arai et al. 29 doped fluorophosphate glass with Yb 3+ and Tb 3+ ions as high as 20 mol%, and reported an energy transfer efficiency up to 30% from Yb 3+ to Tb 3+ via the UEC process. On the other hand, we also confirmed that there were other energy transfer mechanisms that suppressed the UEC process, i.e., the backward energy transfer process, such as phonon assisted energy transfer, cooperative cross relaxation and so on refs 30, 31. Although the energy transfer efficiency is sufficiently high in fluorophosphate glass, the backward energy transfer process is not negligible, so that it is very likely for the occurrence of backward energy transfer from Tb 3+ to Yb 3+ , which decreases the intensity of the observed green emission at 546 nm. Based on the above, we have added experimental parts about forward and backward energy transfer processes. The Table 2 show the added experimental parts, the glass samples were heat-treated at 640 °C. The cooperative energy transfer rate (W CET ) and the transfer efficiency (η CET ) from Yb 3+ to Tb 3+ can be quantified using the following expressions 31, 32 :   Tables 3 and 4 show the forward and backward energy transfer efficiencies for sample 1. These quantitative results have been calculated by substituting the lifetimes measured in samples 1, 2 and 3 into Eqs (2)-(5). As shown in Tables 3 and 4, the forward energy transfer efficiency is considerably higher than the backward energy transfer efficiency. Based on the above results, in the next work, because the rare earth ion doping amount is different, so we mainly study the forward energy transfer. Figure 4 shows the UEC emission spectra and decay curves of SABYb-1, SABYb-2, SABYb-3 and SABYb-4 glass-ceramics heat-treated at 640 °C. Four visible UEC emission bands from 425 to 725 nm can be observed in Fig. 4(a). In comparison with those in Fig. 4(a), the UEC emission intensities at 476 nm (Tm 3+ : 1 G 4 → 3 H 6 ), 546 nm (Tb 3+ : 5 D 4 → 7 F 5 ), 584 nm (Tb 3+ : 5 D 4 → 7 F 4 ) and 657 nm (Tm 3+ : 3 F 2,3 → 3 H 6 ) are all enhanced dramatically with increasing concentration of Yb 3+ ions in the SABYb glass-ceramics. The population of Tb 3+ ions in the 5 D 4 excited state level is thought to be produced through the cooperative energy transfer (CET) process among a pair of Yb 3+ donor ions and a Tb 3+ acceptor ion, which can be expressed as follows 33, 34 : 2 F 5/2 (Yb 3+ ) + 2 F 5/2 (Yb 3+ ) + 7 F 6 (Tb 3+ ) → 5 D 4 (Tb 3+ ) + 2 F 7/2 (Yb 3+ ) + 2 F 7/2 (Yb 3+ ). Meanwhile, Yb 3+ energy transfer to the 1 G 4 excited state level of Tm 3+ ions and decay radiation to the 3 H 6 ground state also generate the intense blue emission at around 476 nm. The major contribution to the red emission at around 657 nm is attributed to the 3 F 2,3 → 3 H 6 transition 35 . The emission spectra of SABYb-1, SABYb-2, SABYb-3 and SABYb-4 glass-ceramics can be easily converted to the Commission Internationale de L'Eclairage (CIE) chromaticity diagram, as plotted in Fig. 4(b). The luminescence color changes from yellowish green (SABYb-1), to green (SABYb-2), then to bluish green (SABYb-3), and finally to white (SABYb-4) 36 . Figure 4(c) illustrates the decay time of 5 D 4 (Tb 3+ ) energy level with increasing concentration of Yb 3+ ions. Here, only approximate single-exponential luminescence decay curves can be obtained. Hence, the lifetimes characterized by decay lifetime τ can be deduced by the following formula:   Figure 4(d) shows the variation of decay times of 1 G 4 (Tm 3+ ), 5 D 4 (Tb 3+ ) and 3 F 2,3 (Tm 3+ ) energy levels with increasing concentration of Yb 3+ ions. It can be seen that the energy transfers from Yb 3+ to Tb 3+ and Tm 3+ are gradually strengthened. That is to say, by adjusting the concentration of Yb 3+ ions, the adjustable multicolor and UEC near-white light emitting of the SABYb glass can be achieved in this experiment. Figure 5 shows the UEC emission spectra and decay times of SABTb-1, SABTb-2, SABTb-3 and SABTb-4 glass-ceramics heat-treated at 640 °C. As can be seen from Fig. 5(a), with increasing concentration of Tb 3+ ions in the SABTb glass-ceramics, the UEC emission bands at 546 nm (Tb 3+ : 5 D 4 → 7 F 5 ) and 584 nm (Tb 3+ : 5 D 4 → 7 F 4 ) are enhanced dramatically whereas those at around 476 nm (Tm 3+ : 1 G 4 → 3 H 6 ) and 657 nm (Tm 3+ : 3 F 2,3 → 3 H 6 ) originated from Tm 3+ ions are gradually weakened. This phenomenon can be explained from three aspects. Firstly, the increasing concentration of Tb 3+ ions can lead to the increase of luminescent centers, thus enhancing the emission intensity at 546 nm (Tb 3+ : 5 D 4 → 7 F 5 ). Secondly, the gradual increase of Tb 3+ ions surrounded by Tm 3+ ions can hinder the energy transfer from Yb 3+ to Tm 3+ , resulting in the decrease of Tm 3+ emission. Thirdly, the possible energy transfer from Tm 3+ to Tb 3+ ions contributes to the emission intensity at 546 nm. The mechanism of energy transfer from Tm 3+ to Tb 3+ ions is proposed as follows: 1 G 4 (Tm 3+ ) + 7 F 5 (Tb 3+ ) → 5 D 4 (Tb 3+ ) + 3 H 6 (Tm 3+ ) (ET1). The UEC luminescence colors of the SABTb-1, SABTb-2, SABTb-3 and SABTb-4 glass ceramics are characterized by the CIE chromaticity diagram, and the results are plotted in Fig. 5(b) 37 . The luminescence color changes from blue (SABTb-1), to wathet blue (SABTb-2), then to white (SABTb-3), and finally to green (SABTb-4). The decay curves of the 1 G 4 state obtained by monitoring the emission of Tm 3+ ions at 476 nm in the SABTb glass-ceramics are depicted in Fig. 5(c). The decay curves are approximately fitted with the single-exponential relationship, and the characteristic times are 677.36 μs, 656.74 μs, 605.13 μs and 580.97 μs for SABTb-1, SABTb-2, SABTb-3 and SABTb-4 glass ceramics respectively. Figure 5(d) shows the decay lifetime measurement results of 1 G 4 (Tm 3+ ) and 3 F 2,3 (Tm 3+ ). It can be seen that the energy transfer from Tm 3+ to Tb 3+ is indeed existent. That is to say, by adjusting the concentration of Tb 3+ ions and changing the emission ratio of RGB, color tunable emitting can be achieved in this experiment.
Under constant concentration of Tb 3+ ions in the glass composition, the effects of concentration variation of Tm 3+ ions were also given for comparison in the second component of the SABTm-1, SABTm-2, SABTm-3 and SABTm-4 glass ceramics. Figure 6 shows the UEC emission spectra and decay time curves of the SABTm-1, SABTm-2, SABTm-3 and SABTm-4 glass ceramics heat-treated at 640 °C. As shown in Fig. 6(a), the UEC emission bands at 476 nm (Tm 3+ : 1 G 4 → 3 H 6 ) and 657 nm (Tm 3+ : 3 F 2,3 → 3 H 6 ) are enhanced dramatically with increasing concentration of Tm 3+ ions in the SABTm glass-ceramics, and that at around 546 nm originated from Tb 3+ ions also increases. These results imply that energy transfer from Tm 3+ to Tb 3+ ions may occur during the UEC process. The mechanism of energy transfer from Tm 3+ to Tb 3+ ions is suggested as follows: 1 G 4 (Tm 3+ ) + 7 F 5 (Tb 3+ ) → 5 D 4 (Tb 3+ ) + 3 H 6 (Tm 3+ ) (ET1). Figure 6(b) shows the calculated color coordinates according to the standard CIE chromaticity that is characterized by CIE chromaticity diagram 38 . As can be seen, the luminescence color changes from green (SABTm-1), to white (SABTm-2), then to wathet blue (SABTb-3), and finally to blue (SABTm-4) 36 . The UEC luminescence decay curves for 5 D 4 → 7 F 5 transition (546 nm) of Tb 3+ in the SABTm glass were measured and illustrated in Fig. 6(c). Here, only approximate single-exponential luminescence decay curves are obtained, and the average lifetimes of 5 D 4 state are about 299.46 μs, 366.38 μs, 426.23 μs and 581.26 μs for SABTm-1, SABTm-2, SABTm-3 and SABTm-4 glass-ceramics respectively 39 . It is noticed that the lifetime of 5 D 4 state becomes longer, so that it can be proved that the energy transfer from Tm 3+ to Tb 3+ is indeed existent. Figure 6(d) shows the variation trends of lifetimes for the 1 G 4 (Tm 3+ ), 5 D 4 (Tb 3+ ) and 3 F 2,3 (Tm 3+ ) states. Thus, we achieve the color tunability from green to white and blue emitting.
The color of bright emission from SAB glasses can be tuned easily by adjusting the exaction power of laser in this study. As shown in Fig. 7(c), the emission color of SABTm-1-640 glass-ceramics changes from yellow-green to white when the exaction power of laser increases from 0.26 to 1.65 W, which may be ascribed to the variation of the intensity ratio of RGB luminescences. As shown in Fig. 7(a) and (b), the intensity increasing rate of blue luminescence at 476 nm is higher than those of green luminescence at 546 nm and red luminescence at 657 nm when the exaction power of laser is increased. Therefore, it may be inferred that the UEC luminescences of blue, green and red color may involve multiphoton process with different exaction-steps. As far as multiphoton processes concerned, the relationship between the pumping power and the fluorescent intensity is I ∝ P n40 , where I is the integrated intensity of the UEC luminescences (the integrated area of the UEC luminescence region), P is the pumping power of the excitation laser, and n is the photon number. The logarithmic transformation of the pumping power and fluorescence intensity is plotted in Fig. 7(d). The slopes of the logarithmic fitted lines for blue (476 nm), green (546 nm) and red (657 nm) luminescences are 3.17, 2.07 and 2.28, respectively. These results suggest that three-photon excitation is predominated in the conversion of 980 nm radiation into blue luminescence emission, whereas the green and red luminescences mainly come from the two-photon process 40 . Therefore, under focused laser irradiations with variable power, the intensity ratio of blue and green (or red) luminescences can be changed due to different multi-photon absorption steps of these UEC luminescences.

Conclusions
In summary, we have demonstrated the highly efficient red, green and blue UEC luminescences in the Tb 3+ / Tm 3+ /Yb 3+ co-doped SAB glass-ceramics based on multiphoton excitation in this paper. The UEC luminescences of Tb 3+ /Tm 3+ /Yb 3+ co-doped SAB glass-ceramics were significantly enhanced in comparison with those of precursor glasses before heat treatment, and the RGB ratios of UEC luminescences and the decay times of these glass-ceramics could be tuned by changing the concentration of doped RE ions and adjusting the laser power simultaneously. The blue and red UEC luminescences of Tm 3+ were found to originate from three-photon and two-photon excitations respectively, while the green UEC luminescence of Tb 3+ was from a two-photon excitation. In addition, it was also proved that energy transfers during the UEC process included the transfers from Yb 3+ to Tm 3+ and Tb 3+ , as well as the transfer from Tm 3+ to Tb 3+ . Our work suggests a possible route to design and develop the red, green and blue UEC luminescence materials by laser, and provides useful information for further development of UEC glass-ceramics associated with the energy transfer between Tm 3+ and Tb 3+ ions.