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Tm3+ tri-doped CaF2 single crystals
Optical Materials 90 (2019) 40–45
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Upconversion color tunability and white light generation in Yb3+/Er3+/ Tm3+ tri-doped CaF2 single crystals
T
Xueyuan Chena,b, Bo Zhangb,c, Xiaobo Qianb, Jingya Wangb, Lili Zhengb, Jingshan Houa, Yongzheng Fanga,∗∗, Liangbi Sub,c,∗ a
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai, 201418, China Synthetic Single Crystal Research Center (SSCRC), CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China c State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Single crystal Upconversion luminescence Color tunability White light
Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals have been successfully synthesized via Temperature Gradient Technique (TGT) method, and represented by X-ray diffraction (XRD) examination, absorption and fluorescence spectrum, pump power dependence and lifetime analysis in details. Under the excitation of 980 nm laser diode (LD), the upconversion luminescence (UCL) spectra of Yb3+/Er3+/Tm3+/CaF2 as a function of Tm3+ concentration are made up of intense blue (Tm3+:1G4 → 3H6), green (Er3+: 4S3/2, 2H11/2 → 4I15/2), and red (Er3+:4F9/2 → 4I15/2) emissions, which are located at 478, 523, 539 and 656 nm, respectively. With increasing concentration of Tm, the color can be tunable from orange-yellow, white to blue light. And the optimal Yb3+/ Er3+/Tm3+ doping concentrations for white light are determined to be 10%Yb, 0.1%Er and 0.05%Tm. The research indicates that the as-prepared materials may have potential applications in the fields of three dimensional displays, back lighting, white light sources, and so on.
1. Introduction Rare earth (RE) - activated luminescent materials with widely and continuously tunable excitation and emission wavelength have attracted a great deal of attention due to their applications in light emitting diodes (LEDs) based solid-state lighting [1]. At present, white light sources are challenging because of (1) lower-energy photons, (2) bias dependent color variation, (3) high manufacturing cost, and (4) long-term stability of emitters [2]. In order to overcome this obstacle, one of the effective ways for generating white light is RE3+ ions doped material based on frequency upconversion processes, which can convert near-infrared (NIR) into visible light via multiphoton processes. Benefiting from upconversion white light materials can be stimulated with a cheap excitation source, e.g., 980 nm cw laser. White light with a long luminescence lifetime was achieved through a nonlinear multiphoton process, which make researchers to shift their gaze from down-converting fluorescent materials to up-converting luminescent materials in light emitting diodes (LEDs) based solid-state lighting applications.
At present, upconversion materials have been extensively researched to obtain white light, such as in polycrystals [3], glass ceramics [4–7], nanoparticles [8–10], phosphors [11] and single crystals [12–14]. The study proved that the phosphor powder for white LEDs had short-term stabilities because of photodegradation of the organic epoxy resin under condition of high irradiation and high temperatures [15]. The transparent ceramics white LEDs need optically transparent plates with an appropriate grain structure, which makes the whole process expensive and leads to low reproducibility and thus difficult for the industrialization. Latynina et al. [16] demonstrated that thin single crystal phosphor plates had superior performance features in white LED application, which realizes epoxy resin free package and eliminate the photodegradation issue of the organic material. Instead of the organic resin and powder mixture, the thin single crystal phosphor plates ensure that white LEDs have a longer life and higher light output without losing their performance characteristics and qualities. Therefore, single crystals are important candidates for realization of resin-free highpower white LEDs.
∗
Corresponding author. Synthetic Single Crystal Research Center (SSCRC), CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Fang), [email protected] (L. Su). https://doi.org/10.1016/j.optmat.2018.12.060 Received 30 September 2018; Received in revised form 19 December 2018; Accepted 28 December 2018 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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the slice was measured by UVMS Spectrophotometer (Mode V-570, JASCO) at room temperature. The UCL spectra and fluorescence lifetime were obtained with Edinburgh FLS980 spectrofluorometer.
As is known, single crystal upconversion materials are rarely reported in white LEDs application due to difficult growth of high quality. Xing et al. [17] had successfully fabricated LiNbO3:Ho3+, Yb3+, Tm3+ single crystals by the Czochralski method, which exhibited intense upconversion white light under 980 nm excitation. Further, the researches doped Mn2+ ion in LiNbO3:Ho3+, Yb3+, Tm3+ single crystals and realized a purer upconversion white-light emission [18]. However, ideal luminescent matrix materials should have low phonon energy, high doping concentration, strong stability, and thus a high-efficiency upconversion luminescence can be obtained. Fluoride crystals, which have a phonon energy lower than that of oxide crystals, are well known as good host material for lanthanide ions [19]. To the best of our knowledge, using fluoride crystal as an upconversion material to achieve white light input has not been reported so far. Among the fluoride compounds, CaF2 single crystals have attracted great concern from the researchers since the first laser demonstration in CaF2:Er single crystal in 1967 [20]. Besides, it also has the high transparency window (0.2–10 μm), low energy cut-off phonon (450 cm−1) and suitable absorption threshold (12 eV) [21]. Most importantly, the doping of trivalent lanthanide ions into the CaF2 is accompanied by interstitial F’ ions as charge compensators, which modify the geometry and the constitution of the center by taking a position close to the RE3+ ions. Any modification in the local surroundings of the RE3+ ions results in a lift of the degeneracy of certain electronic levels which split in several Stark sublevels [22]. In our work, we prepares a series of Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals via TGT method and the UC properties are investigated in detail.
3. Results and discussion 3.1. Structure and XRD analysis The crystal structure is drawn using the VESTA software and shown in Fig. 1a. It is observed that CaF2 crystal has a well-known fluorite structure with space group Fm-3m, in which Ca2+ ions lie at the nodes in a face-centered lattice, while F− ones lie at the centers of the octants [25]. In the lattice, each Ca atom coordinates with eight F atoms and each F atom is surrounded by four Ca atoms. The Yb3+, Er3+ and Tm3+ ions all occupy the Ca2+ sites (like the site of yellow ball). Fig. 1b presents the photograph of rod-shaped, dissected crystals and polished wafers for optical testing. The phase purity of the selected samples is analyzed by XRD. As shown in Fig. 1c, all diffraction peaks are consistent with the standard card peaks of CaF2 (JCPDS No. 35-0816) without any impurity peaks, which clearly suggests that Yb3+, Er3+ and Tm3+ ions have been successfully incorporated into the host lattice. In addition, compared to CaF2 standards card, a large shift toward lower angles of the [111] reflection are found in Fig. 1d. This phenomenon may be effected by the ionic radii of the doping ions and the charge balance. On the one hand, in the eight-coordinate environment, the ionic radius of Ca2+ is 1.12 Å, and the ionic radii of Yb3+, Er3+ and Tm3+ ions are 0.985, 1.004 and 0.95 Å [26], respectively. When Yb3+, Er3+ and Tm3+ ions are considered to replace Ca2+ ions, the mismatched ionic radius causes the diffraction peak to move to a lower 2θ angle. On the other hand, this replacement by RE3+ (Yb3+, Er3+ and Tm3+ ions) leads the charge imbalance, which makes the charge compensation generated by an interstitial F− ion. The interstitial F− ion has a larger ionic radii, which makes the diffraction peak to move to a higher 2θ angle. In addition, B. P. Sobolev et al. [27] reported that the lattice parameters of the CaF2:R (R is lanthanide ion) fluorite phases became large with lanthanide ions doped follow Vegard's rule, which was leading to the diffraction peak to shift toward higher 2θ angles. In order to prove this view, the a-axis parameters and cell volume of the Yb3+/Er3+/Tm3+/CaF2 single crystals have been determined as average of all the values obtained from the individual reflections by using the relationship between hkl and the lattice parameter for a cubic structure, and they are reported in Table 1. From the table, it can be noted that the a-axis parameters and cell volume of the doped crystals are slightly larger than that of the undoped. Therefore, in our work, the ionic radii of the doping ions is dominant relation to the charge compensation for the migration of [111] peak.
2. Experimental section 2.1. Synthesis of Yb3+/Er3+/Tm3+/CaF2 single crystals The crystals of Yb3+/Er3+/Tm3+/CaF2 (the concentrations of dopants 10%Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) and 10%Yb3+/0.5%Tm3+/CaF2) were synthesized by TGT method [23,24]. The growth steps were as follows: firstly, the raw materials used for crystal growth were high purity (> 99.995%) crystalline powders CaF2, YbF3, ErF3, and TmF3 with 4 N purity, in addition, in which 0.1-2 wt% of PbF2 with a mass of CaF2 was added as an oxygen scavenger, which were weighed out in mole ratios, then were totally mixed in agate crucible. Then the ground powder was loaded into a cylindrical porous graphite crucible for furnace growth. Secondly, the TGT furnace was heated from room temperature to 800 °C with a heating rate of 50 °C/h and kept for 10 h to vaporize oxygen mainly. Then, the temperature was raised at a rate of 50 °C/h to 1380 °C making the material fully melted, and kept the molten material for 10 h. Thirdly, the molten material started to crystallize from 1380 °C to 1230 °C with a slowly growth rate of 1–1.5 °C/h using a temperature program controller. Finally, the crystal was annealed to room temperature at a rate of 10–15 °C/h. During the whole processes, all temperatures were measured directly by thermocouples. A cylindrical rod with a length of 80 mm and a diameter of 20 mm can be obtained. The slices with a size of 20 mm × 20 mm × 2.5 mm were cut by an internal circular cutting machine from as-grown CaF2 single crystal rod. Then, grinding and polishing experiments were performed on a precision polishing machine. The cutting marks were removed by coarse sand grinding. Fine grinding was then carried out with smaller grit to remove the damaged layer caused by the coarse grinding. Subsequently, mechanically polished into a thickness of 2 mm for measuring optical properties (shown in Fig. 1).
3.2. The absorption and luminescent properties of Yb3+/Er3+/Tm3+/CaF2 single crystals Fig. 2a shows the absorption spectra of all crystals samples, it can be seen that the spectra characteristic are similar and have an obvious absorption band of Yb3+ centers between 850 and 1100 nm [28,29]. Additionally, the absorption peaks of Tm3+ and Er3+ are not observed due to the strong absorption intensity of Yb3+. Meanwhile, it can be obtained that the absorption coefficient decreases as the concentration of Tm3+ increases, which is mainly due to the energy transfer (ET) between Yb3+ and Tm3+. The UC emission spectra with different Tm3+ doping concentration excited by a continuous NIR 980 nm laser at room temperature are presented in Fig. 2b, in which the strong red emissions at 656 nm (Er3+:4F9/2 → 4I15/2), green emissions at 523 and 539 nm (Er3+:4S3/2, 2H11/2 → 4I15/2) and intense blue emission bands at 478 nm from 1G4 → 3H6 transition are acquired. In addition, some weak bands in the red region centered at 650 and 700 nm from 1G4 → 3F4 and 3 F2,3 → 3H6 transitions of Tm3+ also can be also found. The reason for the generation of UC emission is that the Yb3+, Tm3+ and Yb3+, Er3+
2.2. Characterization methods The slices were characterized by the X-ray diffraction (XRD) patterns on a D/MAXRB X-ray diffractometer operated at 12 kW with Cu Kα radiation (l = 1.5418 Å). The optical absorption (OA) spectrum of 41
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Fig. 1. (a) Crystal structure diagram, (b) Crystal photograph (left: unpolished, right: after polished), (c) XRD patterns and (d) enlargement in the range of∼27–29° of Yb3+/Er3+/Tm3+/CaF2 single crystals.
ET process (BET): [4S3/2 (Er) + 2F7/2 (Yb) → 4I13/2 (Er) + 2F5/2 (Yb)]. In order to reflect the true color of luminescence, the color coordinates are calculated using the CIE 1931 in Table 2, and the chromaticity diagram of the samples with various concentrations of Tm3+ are shown in Fig. 3. It can be found that the CIE color coordinates depends greatly on the Tm3+ concentration. The color is changed from orange-yellow (a), white (b), to blue (c) by adjusting the Tm concentration from 0 to 0.5%. And the corresponding CIE coordinates are determined to be (0.44, 0.48) (0%), (0.33, 0.26) (0.05%), (0.29, 0.21) (0.1%), (0.27, 0.19) (0.3%), and (0.24, 0.17) (0.5%), in which the chromaticity coordinates (0.33, 0.26) falls in the white region. As far as I know, such a major color gamut change occurs only by changing the concentration of very low rare earth ions, which has almost never been detected in matrices other than fluorite-type matrices. The reason is that CaF2:RE is a very interesting compound in which the RE3+ ions tend to cluster together from low concentration [33]. And the clusters grow more and more numerous as the concentration of RE3+ ions [34]. Generally speaking, the occupation of CaF2 by trivalent lanthanide (Ln3+) ions leads to the formation of various optical centers, which can be divided into two categories: the isolated Ln3+ ions (associated to their charge compensators) and the Ln3+-Ln3+ clusters. For Yb3+/ Er3+/Tm3+/CaF2 single crystals, the Tm3+-Tm3+, Tm3+-Er3+, and Tm3+-Yb3+ clusters gradually increase as the concentration of Tm ions increases. These clusters occur predominantly at 0.3–0.5%, whereas they are in the minority at 0.05–0.1% [35]. Therefore, the CR and energy transfer upconversion (ETU) processes will significantly increase
Table 1 Values of the a-axis parameters and cell volume of the Yb3+/Er3+/Tm3+/CaF2 crystals. Sample
A-axis[Å]
V[Å3]
CaF2(JCPDS No. 35-0816) CaF2:10 Yb, 0.1Er CaF2:10 Yb, 0.5 Tm CaF2:10 Yb, 0.1Er, 0.05 Tm CaF2:10 Yb, 0.1Er, 0.1 Tm CaF2:10 Yb, 0.1Er, 0.3 Tm CaF2:10 Yb, 0.1Er, 0. 5 Tm
5.463 5.473 5.471 5.476 5.474 5.477 5.476
163.04 163.96 163.8 164.21 164.01 164.32 164.23
combinations in ET process plays a major role. Up to now, these combinations are identified to be the most efficient ion pairs when excited at ∼980 nm [30]. In the figure, with the increase of Tm3+ content, the intensity of blue emission first increases and then declines. The enhancement of intensity is due to the increase of Tm dopant concentration, while the decrease is owing to the concentration quenching caused by the clustering of Tm3+ in CaF2 crystals [31,32]. As for green and red emission, the descend of the emission of Er3+ is attribute to cross relaxation (CR) processes between Er3+ and Tm3+: [4S3/2 (Er) + 3H6 (Tm) → 4I9/2 (Er) + 3F4 (Tm)], [4S3/2 (Er) + 3H6 (Tm) → 4 I11/2 (Er) + 3H5 (Tm)] and [4F9/2 (Er) + 3H6 (Tm) → 4I13/2 (Er) + 3H5 (Tm)], respectively. Additionally, the red emission of Er3+ ions is more intense than their green emission, which may be the occurrence of back 42
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Fig. 2. Room-temperature absorption spectra (a) and upconversion emission (UC) spectra (b) of 10%Yb3+/0.5%Tm3+/CaF2 and 10%Yb3+/0.1%Er3+/x%Tm3+/ CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals.
the efficiency between the lanthanide ions, which results in the color changes. To better understand the UC mechanism of Yb3+, Er3+ and Tm3+ ions in CaF2 matrix, the power-dependent UC emission of 10%Yb3+/ 0.1%Er3+/0.05%Tm3+/CaF2 single crystal is representatively investigated, as shown in Fig. 4. As can be seen from Fig. 4a, with the increase of power, the emission intensity gradually increases. As known that the emission intensity Iup is proportional to the nth power of the pump power as Iup ∝ Pn pump, where n is the number of infrared photons required to absorb for emitting one visible photon [26,36]. As shown in Fig. 4b, the slopes of blue, green and red emissions of 10% Yb3+/0.1%Er3+/0.05%Tm3+/CaF2 single crystal are 1.63, 1.58, 1.62, respectively, in which they are linear and closed to 2. This suggests that two photons need to be absorbed for realizing UC emission. The result for green and red light are consistent with previous studies [37–39], but the result of blue light in our experiment is different from those studies. In general, to achieve blue emission, it needs to absorb three photons. However, in our research, the slope of blue emission is deviate from 3 and less than 2. This phenomenon may be caused by the magnitude of the pump excitation power, because Wang et al. [40] reported that the value of n varies with the change of power in Yb3+/Tm3+ doped NaYF4. The existence of the ET process between the Yb3+, Er3+ and Tm3+ can be observed not only in the UC spectrum, but also in the decay curve of the lifetime. Fig. 5 shows the luminescence decay curves of Er3+ (4F9/2 → 4I15/2) transition of all 10%Yb3+/0.1%Er3+/x%Tm3+/ CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals. The decay data are fitted as double exponential equation as follows [41]:
Table 2 The chemical compositions and the chromaticity coordinates of 10%Yb3+/ 0.1% Er3+/x%Tm3+/CaF2 crystals. Sample No. 1 2 3 4 5
Sample composition 3+
(x, y) 3+
CaF2:10%Yb ,0.1%Er CaF2:10%Yb3+,0.1%Er3+,0.05%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.1%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.3%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.5%Tm3+
(0.44,0.48) (0.33,0.26) (0.29,0.21) (0.27,0.19) (0.24,0.17)
(−(t − t0)/ τ f )
I(t) = I0 + Af
+ As(−(t − t0))/ τs)
(1)
where τf and τs are the fast and slow components of the luminescent lifetimes, Af and As are the weight factors of the two components, respectively. t0 is the initial delay of the measurement. The lifetimes of Er3+: 4F9/2 level are fitted to be 0.41 ms, 0.40 ms, 0.34 ms,0.24 ms, and 0.20 ms, which descends with increasing Tm3+ content from 0 to 0.5%, due to the ET process between Tm3+ and Er3+ is existent [42]. In addition, there is also an ET process between Tm3+ and Yb3+, which is proved from the absorption spectrum (Fig. 2). Therefore, the Er3+ decay curve should have been dependent on the Yb3+ (2F5/2 → 2F7/2) decay curve, but now it depends on the Tm3+ concentration. Additionally, there is a short rising time on the left side of the dotted line in Fig. 5. The transient of intensity exhibits an obvious distinct prolonged rise part at the end of the excitation, which proves the presence
Fig. 3. Chromaticity diagram for various concentrations of Tm3+ codoped 10% Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals and the digital image of a) orange-yellow light b) white light) and c) blue light upon 980 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. Upconversion emission spectra at different power (a) and pumping power dependence of blue, green and red emissions (b) of 10%Yb3+/0.1%Er3+/ 0.05%Tm3+/CaF2 single crystal. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Schematic diagram of the UC mechanism and processes of Yb3+-sensitized Er3+ and Tm3+ UCL.
radiative relaxations, which generate the green and red emission at around 523, 539 and 656 nm, respectively.
Fig. 5. The luminescence decay curves of Er3+ (4F9/2 → 4I15/2) transition of all 10%Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals excited by a 980 nm LD.
4. Conclusion
of the ET process [43,44]. As the Tm3+ content (x) increases from 0 to 0.5%, the ion distance becomes closer, and ET process is more likely to occur, which leads to a longer rise time. Based on the above studies, the possible population processes of Yb3+/Er3+/Tm3+/CaF2 samples can be deduced schematically in the energy level diagrams in Fig. 6. Excited by 980 nm pump laser, as regard blue emitting derives from 1G4 → 3H6 transition, the excited level 1 G4 can be populated via a three-photon process. The 1G4 level is populated by three steps: 1) (ET):Yb3+ (2F5/2) + Tm3+ (3H6) → Yb3+ (2F7/2) + Tm3+ (3H5); 2) (ET):Yb3+ (2F5/2) + Tm3+ (3F4) → Yb3+ (2F7/2) + Tm3+ (3F2,3) or excited state absorption (ESA):Tm3+ (3F4) + a photon (980 nm) → Tm3+ (3F2,3); 3) Yb3+ (2F5/2) + Tm3+ (3H4) → Yb3+ (2F7/2) + Tm3+ (1G4) [45]. Finally, the population at 1G4 levels decays radiate to the ground state with blue emission at 478 nm and intermediate excited state 3F4 resulted in a red emission at 650 nm and weak red emission at 700 nm dating from 3F2,3 → 3H6 transition. For green and red emission, the populations of 4S3/2, 2H11/2 and 4F9/2 levels can be achieved by absorbing two photons. Firstly, the 4I11/2 level is excited to the upper state 4F7/2 level by the ET process. Subsequently, the 4F7/2 energy level jumps into the lower 4S3/2, 2H11/2 and 4F9/2 levels by multi-phonon assisted non-radiative relaxation. Then the 2H11/2, 4 S3/2 and 4F9/2 levels relax to the lowest level 4I15/2 through a series of
In summary, a series of Yb3+/Er3+/Tm3+/CaF2 single crystals are successfully fabricated by TGT method. The as-prepared samples show efficient green and red UC fluorescence of Er3+ ion and strong blue UC emission of Tm3+ ion under 980 nm excitation at room temperature. And by varying the content of Tm3+ ions, the fluorescence color can be tuned from orange-yellow (0.44, 0.48), white light (0.33, 0.26) to blue (0.24, 0.17). Such a meaningful change is due to the Tm ions have a tendency to cluster in CaF2 as Tm concentration increases from 0 to 0.5%. All results indicate that CaF2 single crystal is a potential host material for white light emission, and realizing white light emission in crystals may have a broad application prospect. Declaration of interests None. Acknowledgement We thank shanghai institute Optics and Fine Mechanics, CAS, for the spectral measurements. This work was supported by National Key Research and Development Program of China (2016YFB0402101), the National Natural Science Foundation of China (61635012), the 44
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National Science Foundation for Post-doctoral Scientists of China (2017M621539), and the Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000).
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Upconversion color tunability and white light generation in Yb3+/Er3+/ Tm3+ tri-doped CaF2 single crystals
T
Xueyuan Chena,b, Bo Zhangb,c, Xiaobo Qianb, Jingya Wangb, Lili Zhengb, Jingshan Houa, Yongzheng Fanga,∗∗, Liangbi Sub,c,∗ a
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai, 201418, China Synthetic Single Crystal Research Center (SSCRC), CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China c State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Single crystal Upconversion luminescence Color tunability White light
Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals have been successfully synthesized via Temperature Gradient Technique (TGT) method, and represented by X-ray diffraction (XRD) examination, absorption and fluorescence spectrum, pump power dependence and lifetime analysis in details. Under the excitation of 980 nm laser diode (LD), the upconversion luminescence (UCL) spectra of Yb3+/Er3+/Tm3+/CaF2 as a function of Tm3+ concentration are made up of intense blue (Tm3+:1G4 → 3H6), green (Er3+: 4S3/2, 2H11/2 → 4I15/2), and red (Er3+:4F9/2 → 4I15/2) emissions, which are located at 478, 523, 539 and 656 nm, respectively. With increasing concentration of Tm, the color can be tunable from orange-yellow, white to blue light. And the optimal Yb3+/ Er3+/Tm3+ doping concentrations for white light are determined to be 10%Yb, 0.1%Er and 0.05%Tm. The research indicates that the as-prepared materials may have potential applications in the fields of three dimensional displays, back lighting, white light sources, and so on.
1. Introduction Rare earth (RE) - activated luminescent materials with widely and continuously tunable excitation and emission wavelength have attracted a great deal of attention due to their applications in light emitting diodes (LEDs) based solid-state lighting [1]. At present, white light sources are challenging because of (1) lower-energy photons, (2) bias dependent color variation, (3) high manufacturing cost, and (4) long-term stability of emitters [2]. In order to overcome this obstacle, one of the effective ways for generating white light is RE3+ ions doped material based on frequency upconversion processes, which can convert near-infrared (NIR) into visible light via multiphoton processes. Benefiting from upconversion white light materials can be stimulated with a cheap excitation source, e.g., 980 nm cw laser. White light with a long luminescence lifetime was achieved through a nonlinear multiphoton process, which make researchers to shift their gaze from down-converting fluorescent materials to up-converting luminescent materials in light emitting diodes (LEDs) based solid-state lighting applications.
At present, upconversion materials have been extensively researched to obtain white light, such as in polycrystals [3], glass ceramics [4–7], nanoparticles [8–10], phosphors [11] and single crystals [12–14]. The study proved that the phosphor powder for white LEDs had short-term stabilities because of photodegradation of the organic epoxy resin under condition of high irradiation and high temperatures [15]. The transparent ceramics white LEDs need optically transparent plates with an appropriate grain structure, which makes the whole process expensive and leads to low reproducibility and thus difficult for the industrialization. Latynina et al. [16] demonstrated that thin single crystal phosphor plates had superior performance features in white LED application, which realizes epoxy resin free package and eliminate the photodegradation issue of the organic material. Instead of the organic resin and powder mixture, the thin single crystal phosphor plates ensure that white LEDs have a longer life and higher light output without losing their performance characteristics and qualities. Therefore, single crystals are important candidates for realization of resin-free highpower white LEDs.
∗
Corresponding author. Synthetic Single Crystal Research Center (SSCRC), CAS Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Fang), [email protected] (L. Su). https://doi.org/10.1016/j.optmat.2018.12.060 Received 30 September 2018; Received in revised form 19 December 2018; Accepted 28 December 2018 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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the slice was measured by UVMS Spectrophotometer (Mode V-570, JASCO) at room temperature. The UCL spectra and fluorescence lifetime were obtained with Edinburgh FLS980 spectrofluorometer.
As is known, single crystal upconversion materials are rarely reported in white LEDs application due to difficult growth of high quality. Xing et al. [17] had successfully fabricated LiNbO3:Ho3+, Yb3+, Tm3+ single crystals by the Czochralski method, which exhibited intense upconversion white light under 980 nm excitation. Further, the researches doped Mn2+ ion in LiNbO3:Ho3+, Yb3+, Tm3+ single crystals and realized a purer upconversion white-light emission [18]. However, ideal luminescent matrix materials should have low phonon energy, high doping concentration, strong stability, and thus a high-efficiency upconversion luminescence can be obtained. Fluoride crystals, which have a phonon energy lower than that of oxide crystals, are well known as good host material for lanthanide ions [19]. To the best of our knowledge, using fluoride crystal as an upconversion material to achieve white light input has not been reported so far. Among the fluoride compounds, CaF2 single crystals have attracted great concern from the researchers since the first laser demonstration in CaF2:Er single crystal in 1967 [20]. Besides, it also has the high transparency window (0.2–10 μm), low energy cut-off phonon (450 cm−1) and suitable absorption threshold (12 eV) [21]. Most importantly, the doping of trivalent lanthanide ions into the CaF2 is accompanied by interstitial F’ ions as charge compensators, which modify the geometry and the constitution of the center by taking a position close to the RE3+ ions. Any modification in the local surroundings of the RE3+ ions results in a lift of the degeneracy of certain electronic levels which split in several Stark sublevels [22]. In our work, we prepares a series of Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals via TGT method and the UC properties are investigated in detail.
3. Results and discussion 3.1. Structure and XRD analysis The crystal structure is drawn using the VESTA software and shown in Fig. 1a. It is observed that CaF2 crystal has a well-known fluorite structure with space group Fm-3m, in which Ca2+ ions lie at the nodes in a face-centered lattice, while F− ones lie at the centers of the octants [25]. In the lattice, each Ca atom coordinates with eight F atoms and each F atom is surrounded by four Ca atoms. The Yb3+, Er3+ and Tm3+ ions all occupy the Ca2+ sites (like the site of yellow ball). Fig. 1b presents the photograph of rod-shaped, dissected crystals and polished wafers for optical testing. The phase purity of the selected samples is analyzed by XRD. As shown in Fig. 1c, all diffraction peaks are consistent with the standard card peaks of CaF2 (JCPDS No. 35-0816) without any impurity peaks, which clearly suggests that Yb3+, Er3+ and Tm3+ ions have been successfully incorporated into the host lattice. In addition, compared to CaF2 standards card, a large shift toward lower angles of the [111] reflection are found in Fig. 1d. This phenomenon may be effected by the ionic radii of the doping ions and the charge balance. On the one hand, in the eight-coordinate environment, the ionic radius of Ca2+ is 1.12 Å, and the ionic radii of Yb3+, Er3+ and Tm3+ ions are 0.985, 1.004 and 0.95 Å [26], respectively. When Yb3+, Er3+ and Tm3+ ions are considered to replace Ca2+ ions, the mismatched ionic radius causes the diffraction peak to move to a lower 2θ angle. On the other hand, this replacement by RE3+ (Yb3+, Er3+ and Tm3+ ions) leads the charge imbalance, which makes the charge compensation generated by an interstitial F− ion. The interstitial F− ion has a larger ionic radii, which makes the diffraction peak to move to a higher 2θ angle. In addition, B. P. Sobolev et al. [27] reported that the lattice parameters of the CaF2:R (R is lanthanide ion) fluorite phases became large with lanthanide ions doped follow Vegard's rule, which was leading to the diffraction peak to shift toward higher 2θ angles. In order to prove this view, the a-axis parameters and cell volume of the Yb3+/Er3+/Tm3+/CaF2 single crystals have been determined as average of all the values obtained from the individual reflections by using the relationship between hkl and the lattice parameter for a cubic structure, and they are reported in Table 1. From the table, it can be noted that the a-axis parameters and cell volume of the doped crystals are slightly larger than that of the undoped. Therefore, in our work, the ionic radii of the doping ions is dominant relation to the charge compensation for the migration of [111] peak.
2. Experimental section 2.1. Synthesis of Yb3+/Er3+/Tm3+/CaF2 single crystals The crystals of Yb3+/Er3+/Tm3+/CaF2 (the concentrations of dopants 10%Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) and 10%Yb3+/0.5%Tm3+/CaF2) were synthesized by TGT method [23,24]. The growth steps were as follows: firstly, the raw materials used for crystal growth were high purity (> 99.995%) crystalline powders CaF2, YbF3, ErF3, and TmF3 with 4 N purity, in addition, in which 0.1-2 wt% of PbF2 with a mass of CaF2 was added as an oxygen scavenger, which were weighed out in mole ratios, then were totally mixed in agate crucible. Then the ground powder was loaded into a cylindrical porous graphite crucible for furnace growth. Secondly, the TGT furnace was heated from room temperature to 800 °C with a heating rate of 50 °C/h and kept for 10 h to vaporize oxygen mainly. Then, the temperature was raised at a rate of 50 °C/h to 1380 °C making the material fully melted, and kept the molten material for 10 h. Thirdly, the molten material started to crystallize from 1380 °C to 1230 °C with a slowly growth rate of 1–1.5 °C/h using a temperature program controller. Finally, the crystal was annealed to room temperature at a rate of 10–15 °C/h. During the whole processes, all temperatures were measured directly by thermocouples. A cylindrical rod with a length of 80 mm and a diameter of 20 mm can be obtained. The slices with a size of 20 mm × 20 mm × 2.5 mm were cut by an internal circular cutting machine from as-grown CaF2 single crystal rod. Then, grinding and polishing experiments were performed on a precision polishing machine. The cutting marks were removed by coarse sand grinding. Fine grinding was then carried out with smaller grit to remove the damaged layer caused by the coarse grinding. Subsequently, mechanically polished into a thickness of 2 mm for measuring optical properties (shown in Fig. 1).
3.2. The absorption and luminescent properties of Yb3+/Er3+/Tm3+/CaF2 single crystals Fig. 2a shows the absorption spectra of all crystals samples, it can be seen that the spectra characteristic are similar and have an obvious absorption band of Yb3+ centers between 850 and 1100 nm [28,29]. Additionally, the absorption peaks of Tm3+ and Er3+ are not observed due to the strong absorption intensity of Yb3+. Meanwhile, it can be obtained that the absorption coefficient decreases as the concentration of Tm3+ increases, which is mainly due to the energy transfer (ET) between Yb3+ and Tm3+. The UC emission spectra with different Tm3+ doping concentration excited by a continuous NIR 980 nm laser at room temperature are presented in Fig. 2b, in which the strong red emissions at 656 nm (Er3+:4F9/2 → 4I15/2), green emissions at 523 and 539 nm (Er3+:4S3/2, 2H11/2 → 4I15/2) and intense blue emission bands at 478 nm from 1G4 → 3H6 transition are acquired. In addition, some weak bands in the red region centered at 650 and 700 nm from 1G4 → 3F4 and 3 F2,3 → 3H6 transitions of Tm3+ also can be also found. The reason for the generation of UC emission is that the Yb3+, Tm3+ and Yb3+, Er3+
2.2. Characterization methods The slices were characterized by the X-ray diffraction (XRD) patterns on a D/MAXRB X-ray diffractometer operated at 12 kW with Cu Kα radiation (l = 1.5418 Å). The optical absorption (OA) spectrum of 41
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Fig. 1. (a) Crystal structure diagram, (b) Crystal photograph (left: unpolished, right: after polished), (c) XRD patterns and (d) enlargement in the range of∼27–29° of Yb3+/Er3+/Tm3+/CaF2 single crystals.
ET process (BET): [4S3/2 (Er) + 2F7/2 (Yb) → 4I13/2 (Er) + 2F5/2 (Yb)]. In order to reflect the true color of luminescence, the color coordinates are calculated using the CIE 1931 in Table 2, and the chromaticity diagram of the samples with various concentrations of Tm3+ are shown in Fig. 3. It can be found that the CIE color coordinates depends greatly on the Tm3+ concentration. The color is changed from orange-yellow (a), white (b), to blue (c) by adjusting the Tm concentration from 0 to 0.5%. And the corresponding CIE coordinates are determined to be (0.44, 0.48) (0%), (0.33, 0.26) (0.05%), (0.29, 0.21) (0.1%), (0.27, 0.19) (0.3%), and (0.24, 0.17) (0.5%), in which the chromaticity coordinates (0.33, 0.26) falls in the white region. As far as I know, such a major color gamut change occurs only by changing the concentration of very low rare earth ions, which has almost never been detected in matrices other than fluorite-type matrices. The reason is that CaF2:RE is a very interesting compound in which the RE3+ ions tend to cluster together from low concentration [33]. And the clusters grow more and more numerous as the concentration of RE3+ ions [34]. Generally speaking, the occupation of CaF2 by trivalent lanthanide (Ln3+) ions leads to the formation of various optical centers, which can be divided into two categories: the isolated Ln3+ ions (associated to their charge compensators) and the Ln3+-Ln3+ clusters. For Yb3+/ Er3+/Tm3+/CaF2 single crystals, the Tm3+-Tm3+, Tm3+-Er3+, and Tm3+-Yb3+ clusters gradually increase as the concentration of Tm ions increases. These clusters occur predominantly at 0.3–0.5%, whereas they are in the minority at 0.05–0.1% [35]. Therefore, the CR and energy transfer upconversion (ETU) processes will significantly increase
Table 1 Values of the a-axis parameters and cell volume of the Yb3+/Er3+/Tm3+/CaF2 crystals. Sample
A-axis[Å]
V[Å3]
CaF2(JCPDS No. 35-0816) CaF2:10 Yb, 0.1Er CaF2:10 Yb, 0.5 Tm CaF2:10 Yb, 0.1Er, 0.05 Tm CaF2:10 Yb, 0.1Er, 0.1 Tm CaF2:10 Yb, 0.1Er, 0.3 Tm CaF2:10 Yb, 0.1Er, 0. 5 Tm
5.463 5.473 5.471 5.476 5.474 5.477 5.476
163.04 163.96 163.8 164.21 164.01 164.32 164.23
combinations in ET process plays a major role. Up to now, these combinations are identified to be the most efficient ion pairs when excited at ∼980 nm [30]. In the figure, with the increase of Tm3+ content, the intensity of blue emission first increases and then declines. The enhancement of intensity is due to the increase of Tm dopant concentration, while the decrease is owing to the concentration quenching caused by the clustering of Tm3+ in CaF2 crystals [31,32]. As for green and red emission, the descend of the emission of Er3+ is attribute to cross relaxation (CR) processes between Er3+ and Tm3+: [4S3/2 (Er) + 3H6 (Tm) → 4I9/2 (Er) + 3F4 (Tm)], [4S3/2 (Er) + 3H6 (Tm) → 4 I11/2 (Er) + 3H5 (Tm)] and [4F9/2 (Er) + 3H6 (Tm) → 4I13/2 (Er) + 3H5 (Tm)], respectively. Additionally, the red emission of Er3+ ions is more intense than their green emission, which may be the occurrence of back 42
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Fig. 2. Room-temperature absorption spectra (a) and upconversion emission (UC) spectra (b) of 10%Yb3+/0.5%Tm3+/CaF2 and 10%Yb3+/0.1%Er3+/x%Tm3+/ CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals.
the efficiency between the lanthanide ions, which results in the color changes. To better understand the UC mechanism of Yb3+, Er3+ and Tm3+ ions in CaF2 matrix, the power-dependent UC emission of 10%Yb3+/ 0.1%Er3+/0.05%Tm3+/CaF2 single crystal is representatively investigated, as shown in Fig. 4. As can be seen from Fig. 4a, with the increase of power, the emission intensity gradually increases. As known that the emission intensity Iup is proportional to the nth power of the pump power as Iup ∝ Pn pump, where n is the number of infrared photons required to absorb for emitting one visible photon [26,36]. As shown in Fig. 4b, the slopes of blue, green and red emissions of 10% Yb3+/0.1%Er3+/0.05%Tm3+/CaF2 single crystal are 1.63, 1.58, 1.62, respectively, in which they are linear and closed to 2. This suggests that two photons need to be absorbed for realizing UC emission. The result for green and red light are consistent with previous studies [37–39], but the result of blue light in our experiment is different from those studies. In general, to achieve blue emission, it needs to absorb three photons. However, in our research, the slope of blue emission is deviate from 3 and less than 2. This phenomenon may be caused by the magnitude of the pump excitation power, because Wang et al. [40] reported that the value of n varies with the change of power in Yb3+/Tm3+ doped NaYF4. The existence of the ET process between the Yb3+, Er3+ and Tm3+ can be observed not only in the UC spectrum, but also in the decay curve of the lifetime. Fig. 5 shows the luminescence decay curves of Er3+ (4F9/2 → 4I15/2) transition of all 10%Yb3+/0.1%Er3+/x%Tm3+/ CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals. The decay data are fitted as double exponential equation as follows [41]:
Table 2 The chemical compositions and the chromaticity coordinates of 10%Yb3+/ 0.1% Er3+/x%Tm3+/CaF2 crystals. Sample No. 1 2 3 4 5
Sample composition 3+
(x, y) 3+
CaF2:10%Yb ,0.1%Er CaF2:10%Yb3+,0.1%Er3+,0.05%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.1%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.3%Tm3+ CaF2:10%Yb3+,0.1%Er3+,0.5%Tm3+
(0.44,0.48) (0.33,0.26) (0.29,0.21) (0.27,0.19) (0.24,0.17)
(−(t − t0)/ τ f )
I(t) = I0 + Af
+ As(−(t − t0))/ τs)
(1)
where τf and τs are the fast and slow components of the luminescent lifetimes, Af and As are the weight factors of the two components, respectively. t0 is the initial delay of the measurement. The lifetimes of Er3+: 4F9/2 level are fitted to be 0.41 ms, 0.40 ms, 0.34 ms,0.24 ms, and 0.20 ms, which descends with increasing Tm3+ content from 0 to 0.5%, due to the ET process between Tm3+ and Er3+ is existent [42]. In addition, there is also an ET process between Tm3+ and Yb3+, which is proved from the absorption spectrum (Fig. 2). Therefore, the Er3+ decay curve should have been dependent on the Yb3+ (2F5/2 → 2F7/2) decay curve, but now it depends on the Tm3+ concentration. Additionally, there is a short rising time on the left side of the dotted line in Fig. 5. The transient of intensity exhibits an obvious distinct prolonged rise part at the end of the excitation, which proves the presence
Fig. 3. Chromaticity diagram for various concentrations of Tm3+ codoped 10% Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals and the digital image of a) orange-yellow light b) white light) and c) blue light upon 980 nm excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. Upconversion emission spectra at different power (a) and pumping power dependence of blue, green and red emissions (b) of 10%Yb3+/0.1%Er3+/ 0.05%Tm3+/CaF2 single crystal. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Schematic diagram of the UC mechanism and processes of Yb3+-sensitized Er3+ and Tm3+ UCL.
radiative relaxations, which generate the green and red emission at around 523, 539 and 656 nm, respectively.
Fig. 5. The luminescence decay curves of Er3+ (4F9/2 → 4I15/2) transition of all 10%Yb3+/0.1%Er3+/x%Tm3+/CaF2 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals excited by a 980 nm LD.
4. Conclusion
of the ET process [43,44]. As the Tm3+ content (x) increases from 0 to 0.5%, the ion distance becomes closer, and ET process is more likely to occur, which leads to a longer rise time. Based on the above studies, the possible population processes of Yb3+/Er3+/Tm3+/CaF2 samples can be deduced schematically in the energy level diagrams in Fig. 6. Excited by 980 nm pump laser, as regard blue emitting derives from 1G4 → 3H6 transition, the excited level 1 G4 can be populated via a three-photon process. The 1G4 level is populated by three steps: 1) (ET):Yb3+ (2F5/2) + Tm3+ (3H6) → Yb3+ (2F7/2) + Tm3+ (3H5); 2) (ET):Yb3+ (2F5/2) + Tm3+ (3F4) → Yb3+ (2F7/2) + Tm3+ (3F2,3) or excited state absorption (ESA):Tm3+ (3F4) + a photon (980 nm) → Tm3+ (3F2,3); 3) Yb3+ (2F5/2) + Tm3+ (3H4) → Yb3+ (2F7/2) + Tm3+ (1G4) [45]. Finally, the population at 1G4 levels decays radiate to the ground state with blue emission at 478 nm and intermediate excited state 3F4 resulted in a red emission at 650 nm and weak red emission at 700 nm dating from 3F2,3 → 3H6 transition. For green and red emission, the populations of 4S3/2, 2H11/2 and 4F9/2 levels can be achieved by absorbing two photons. Firstly, the 4I11/2 level is excited to the upper state 4F7/2 level by the ET process. Subsequently, the 4F7/2 energy level jumps into the lower 4S3/2, 2H11/2 and 4F9/2 levels by multi-phonon assisted non-radiative relaxation. Then the 2H11/2, 4 S3/2 and 4F9/2 levels relax to the lowest level 4I15/2 through a series of
In summary, a series of Yb3+/Er3+/Tm3+/CaF2 single crystals are successfully fabricated by TGT method. The as-prepared samples show efficient green and red UC fluorescence of Er3+ ion and strong blue UC emission of Tm3+ ion under 980 nm excitation at room temperature. And by varying the content of Tm3+ ions, the fluorescence color can be tuned from orange-yellow (0.44, 0.48), white light (0.33, 0.26) to blue (0.24, 0.17). Such a meaningful change is due to the Tm ions have a tendency to cluster in CaF2 as Tm concentration increases from 0 to 0.5%. All results indicate that CaF2 single crystal is a potential host material for white light emission, and realizing white light emission in crystals may have a broad application prospect. Declaration of interests None. Acknowledgement We thank shanghai institute Optics and Fine Mechanics, CAS, for the spectral measurements. This work was supported by National Key Research and Development Program of China (2016YFB0402101), the National Natural Science Foundation of China (61635012), the 44
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National Science Foundation for Post-doctoral Scientists of China (2017M621539), and the Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000).
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