- Email: [email protected]
Tm3+ microboxes
Optical Materials 35 (2013) 1283–1287
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and upconversion luminescence in highly crystalline YOF:Yb3+/Er3+ and Yb3+/Tm3+ microboxes Mingye Ding, Chunhua Lu ⇑, Linhai Cao, Yaru Ni, Zhongzi Xu ⇑ State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 21 January 2013 Accepted 29 January 2013 Available online 6 March 2013 Keywords: Flux cooling method Oxyfluorides Upconversion Optical materials
a b s t r a c t Highly crystalline YOF:Ln3+ (Ln = Yb, Er, Tm) microboxes were successfully synthesized for the first time by using a mixed NaNO3–KNO3 flux cooling method at a holding temperature of 600 °C for 2 h in air. The results indicated that the as-obtained products belonged to tetragonal system and exhibited microboxes morphology with side lengths of 0.5–2 lm. The upconversion luminescence properties of as-prepared YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ were investigated in detail. Under 980 nm laser diode (LD) excitation, the emission intensity and the corresponding luminescence colors of YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ could be precisely adjusted by changing the doping concentration of Yb3+ ions. Furthermore, the paper also offers a new alternative in synthesizing such materials and opens the possibility to meet the increasing commercial demand. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Up-conversion (UC) nano- and micro-materials, which can convert two or more low-energy photons into one high-energy photon (anti-Stokes process), have attracted a great deal of interest over the past decade for their potential applications in the fields of the bioanalystics [1,2], solid-state lasers [3], optical telecommunication [4], displays technology [5] and solar cells [6,7]. For the doping rare-earth (RE) ions, the selection of appropriate host materials is essential for realizing favorable optical performance, such as high UC efficiency and a controllable emission profile. Generally, to achieve highly efficient radiative emission of RE, it requires the phonon energy of the host to be as low as possible, which can minimize nonradiative decay rates [8,9]. Among these UC materials, the rare earth fluoride compounds, possessing high refractive index and low phonon energy, are well known as preferable host materials for UC applications [10–12]. On the other hand, oxides have an advantage superior to fluorides in practical applications owing to their good chemical durability and excellent mechanical strength [13]. Consequently, combing the thermal stability of oxides and the low phonon energy of fluorides, rare earth oxy-fluorides (REOFs) have been considered in developing practical UC materials. In REOF crystals, YOF, is known for the low energy of its lattice phonons (<550 cm1) [14] and desirable chemical stability. Therefore, RE-doped YOF is promising candidate for UC luminescence in practical applications.
Up to now, several techniques have been employed for synthesis of REOF, such as solid state reaction [15,16], mechnochemical grinding method [17,18], sol–gel method [19], coprecipitation [20], hydrothermal method [21,22] and thermolysis method [23]. However, these processes usually suffer from the low yield, long reaction time or high environment loads. Therefore, developing an economical mass production and environmentally friendly method should be highly promising for synthesis of REOF-based optical materials. Herein, we first reported a facile mass production method for the synthesis of uniform YOF microboxes by mixed NaNO3–KNO3 flux cooling method. In addition, the UC luminescence properties of the YOF microboxes doped with different rare earth ions (Yb3+/Er3+, Yb3+/Tm3+) with precisely tuned Yb3+ concentration were investigated under 980 nm LD excitation. 2. Materials and methods All of the chemicals used in this experiment are of analytical grade reagents without further purification. Y(NO3)36H2O (99.99%), Yb(NO3)36H2O (99.99%), Er(NO3)36H2O (99.9%), Tm(NO3)36H2O (99.99%) were purchased from Beijing Founde Star Science & Technology Co., Ltd. (China). NaF (98%), NaNO3 (99%), KNO3 (99%) were provided by Sinopharm Chemical Reagent Co., Ltd. (China). 2.1. Synthesis
⇑ Corresponding authors. Tel.: +86 83587252; fax: +86 83587220. E-mail addresses: [email protected] (C. Lu), [email protected] (Z. Xu). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.01.043
In a typical process for the synthesis of YOF, stoichiometric YNO3.6H2O, NaF, NaNO3, KNO3 with molar ratio of 1:4:64:32 were
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the 2h range from 10° to 80° with Cu Ka radiation (k = 0.15406 nm). SEM micrographs were obtained using scanning electron microscopy (SEM, JSM-5900, JEOL Ltd., Japan). Chemical composition of the samples was obtained using an energy-dispersive spectrometer (EDS, Noran Vantage DSI, Themo Noran, Middleton, WI). The absorption spectra were measured on a spectrophotometer (Shimadzu, UV-3101PC) with a wavelength range of 300–1100 nm and a resolution of 2.0 nm. Photoluminescence emission spectra were recorded with a Jobinyvon FL3–221 fluorescence spectrophotometer at an excitation wavelength of 980 nm in the dark. All the measurements were performed at room temperature. 3. Results and discussion 3.1. Structural and morphological characterization Fig. 1. XRD patterns of the as-synthesized YOF:20%Yb3+, 2%Er3+ (a) and YOF:20%Yb3+, 2%Tm3+ (b). The standard data for YOF (JCPDS No. 06-0347) are shown as references.
thoroughly mixed with an appropriate amount of ethanol in an agate mort and ground for 15 min. The mixture was placed into alumina crucibles with 20 cm3 capacity, heated to 600 °C at a rate of 10 °C/min and held for 2 h in air. After cooled down to the room temperature inside in the furnace, the as-annealed samples were washed with deionized water three times and subsequently dried at 60 °C for 24 h. For UC luminescence, YOF:Yb3+/Er3+, YOF:Yb3+/ Tm3+ samples were prepared in a similar procedure by adding corresponding Yb(NO3)3, Er(NO3)3, Tm(NO3)3 into the precursor mixture. 2.2. Characterization X-ray power diffraction (XRD) measurements were performed on a ARL X’ TRA diffractometer at a scanning rate of 10°/min in
Fig. 1 shows the power XRD patterns of the as-prepared YOF:20%Yb3+, 2%Er3+ (a), YOF:20%Yb3+, 2%Tm3+ (b) as well as the standard data of pure YOF phase for comparison. All these peaks can be indexed to the tetragonal phase of YOF according to JCPDS card (No. 06-0347). The crystal structure of the tetragonal phase is determined with lattice parameters of a = 0.3918 and c = 0.5442, space group P4/nmm [24]. As no additional peaks for other phases can be detected, we infer that Y3+ ions have been replaced by Yb3+, Er3+ or Yb3+, Tm3+ ions in any ratio with no effect on the crystallinity or phase change because of the almost same radii 3þ 3þ of Y3+, Yb3+, Er3+ or Tm3+ ðr 3þ Y ¼ 0:121 nm; r Yb ¼ 0:118 nm; r Er ¼ 0:120 nm; r3þ ¼ 0:119 nmÞ. Tm The as-prepared YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ crystals with pure tetragonal phase exhibit microboxes morphology, which are displayed in Fig. 2. The SEM images in Fig. 2A and B reveal that the Yb3+/Er3+ and Yb3+/Tm3+ doped YOF samples are composed of relatively uniform microboxes and have smooth surfaces with side lengths of 0.5–2 lm. The EDS spectra of the obtained samples (Fig. 2C and D) indicate that fluorine, oxygen,
Fig. 2. (A) High magnification (10,000) SEM images of YOF:20%Yb3+, 2%Er3+ crystals; (B) high magnification (10,000) SEM images of YOF:20%Yb3+, 2%Tm3+ crystals; (C) EDS spectra of YOF:20%Yb3+, 2%Er3+ crystals; (D) EDS spectra of YOF:20%Yb3+, 2%Tm3+ crystals (The Si peaks arising from measurement).
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Fig. 3. Room-temperature absorption spectra of YOF:20%Yb3+, 2%Er3+ sample.
Fig. 6. Luminescent photographs and corresponding CIE chromaticity diagram of YOF:x%Yb3+, 2%Er3+: (a) x = 10; (b) x = 15; (c) x = 20; (d) c = 30.
ponent atoms are homogeneously distributed in both microcrystals. From XRD, SEM observation and EDS analyses, it is found that high crystalline YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ microboxes are successfully synthesized by the flux cooling method. Fig. 4. Room-temperature absorption spectra of YOF:20%Yb3+, 2%Tm3+ sample.
3.2. UC luminescence properties yttrium and ytterbium atoms are detected in both microcrystals. Furthermore, erbium and thulium atoms can clearly be observed in YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ crystals, respectively. It can be also concluded from EDS analysis that these com-
The absorption spectra of YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ microboxes in the range from 300 nm to 1100 nm are presented in Figs. 3 and 4, respectively. Owing to
Fig. 5. Room temperature UC emission spectra of (a) YOF:x%Yb3+, 2%Er3+ (x = 10, 15, 20, 30) and (b) the intensity ratio of red to green emission in YOF:Yb3+,Er3+ as a function of Yb3+ concentration.
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Fig. 7. Pump power dependence of the green and red UC emissions in YOF:20%Yb3+, 2%Er3+. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the strong ultraviolet absorption of the host matrix, the absorption bands at wavelengths shorter than 350 nm could not be distinguished [25]. As shown in Fig. 3, it is dominated by the strong 2 F7/2 ? 2F5/2 absorption band of Yb3+ centered at about 980 nm in the near-infrared (NIR) region [26]. The weak absorption lines in the visible region (inset in Fig. 3) are readily assigned to f–f transitions of the Er3+ ions. In Fig. 4 for YOF:20%Yb3+, 2%Tm3+ sample, the absorption peaks, corresponding to Tm3+ transitions from the ground state 3H6 to the excited states and Yb3+:2F7/2 ? 2F5/2 transition, respectively, are marked. These results show that Yb3+ with a large absorption cross section around 980 nm may act as the sensitizer for Er3+ or/and Tm3+ UC emission. The UC luminescence properties of Yb3+, Er3+ co-doped YOF with different Yb3+ concentration are investigated and shown in Fig. 5. From the UC emission spectra of YOF:x%Yb3+, 2%Er3+ (x = 10, 15, 20, 30) under 980 nm LD excitation (Fig. 5a), the dominated red emission centered at 658 nm originates from the 4F9/ 4 2 ? I15/2 transition, and the green emission at 546 nm can be assigned to 4S3/2 ? 4I15/2 transition. These peaks corresponding to the respective proportion of red and green emissions result in the multicolor output. With the increase of Yb3+ concentration, the relative intensity of the red emission is enhanced and the ratios of the red to green emission are consequently increased (Fig. 5b).
By tuning the doped Yb3+ concentration (10–30 mol%), we can precisely manipulate the relative emission intensities, thus resulting in a tunable color output from yellow to red (Fig. 6a–d). In order to deeply investigate the involved UC mechanism in YOF:Yb3+,Er3+ microcrystals, the pump power dependence of the green and red UC emissions under 980 nm LD excitation is exhibited in Fig. 7. It is well known that the output UC luminescent intensity (IUC) is proportional to the infrared excitation power (IIR): IUC / InIR , where n is the absorbed photon numbers per visible phonon emitted, and its value can be obtained from the slope of the fitted line of the plot of log(IUC) versus log(IIR) [27]. As shown, the slope of the linear fits for the green and red emissions in YOF:Yb3+,Er3+ crystals are 1.81 and 1.96, which indicates that only two photons process are involved to produce the green and red UC emissions. On the basis of the proposed UC mechanism of YOF:Yb3+,Er3+ in Fig. 10, we can understand the change of the relative intensities and output colors by tuning the Yb3+ doping concentration in YOF:x%Yb3+, 2%Er3+. It can be seen that the green and red emissions all need a two photons process to populate the 4S3/2 or 4F9/2 level. Therefore, the changing of output colors is due to fewer Er3+ ions holding at the green-emitting level of 4S3/2 and more Er3+ ions holding at the red-emitting level of 4F9/2. With more Yb3+ ion doping into the YOF host matrix, the interatomic distance of Yb3+–Er3+ decreases and thus facilitates the back-energy-transfer from Er3+ to Yb3+ ions 4F7/2 (Er3+) + 2F7/2 (Yb3+) ? 4I11/2 (Er3+) + 2F5/2 (Yb3+), which will subsequently suppress the population of Er3+ ions in excited level of 4S3/2 and result in the decrease of green (4S3/2 ? 4I15/ 3+ 2) light emission. Simultaneously, the energy transfer from the Yb ions to Er3+ also leads to the saturation of the 4I3/2 (Er3+) and then the excited Yb3+ ions transfer its energy to Er3+ through the energytransfer process 2F5/2 (Yb3+) + 4I13/2 (Er3+) ? 2F7/2 (Yb3+) + 4F9/2 (Er3+), which can directly populate the 4F9/2 level, resulting in the enhancement of red (4F9/2 ? 4I15/2) light emission [28]. Fig. 8 presents the UC luminescence properties of Yb3+/Tm3+ codoped YOF with different Yb3+ concentration. In Fig. 8a for YOF:x%Yb3+, 2%Tm3+ (x = 10, 15, 20, 30) microboxes, the characteristic UC emission peaks can be assigned to 1G4 ? 3H6, 1G4 ? 3F4, and 3F2,3 ? 3H6 transitions and an intense near-infrared (NIR) emission at 800 nm is caused by 3H4 ? 3H6 transition [16,29]. A strong enhancement of the NIR UC emission around 800 nm, with respect to the visible one, has been also observed for Yb3+/Tm3+ doped fluorides and oxyfluorides, such as NaYF4 [30], GdF3 [31] and GdOF [20]. Different from the YOF:x%Yb3+, 2%Er3+ samples, the UC spectra of this system do not show too many obvious differences when adjusting the Yb3+ concentration. As we can see from
Fig. 8. Room temperature UC emission spectra of (a) YOF:x%Yb3+,Tm3+ (x = 10, 15, 20, 30) and (b) the intensity of NIR emission in YOF:Yb3+,Er3+ as a function of Yb3+ concentration.
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4. Conclusion In summary, well-formed and high quality YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ microboxes were successfully grown by the flux cooling method. The UC luminescence properties of YOF crystals doped with Yb3+/Er3+ and Yb3+/Tm3+ were investigated. Multicolor UC emissions of as-prepared YOF:Yb3+,Er3+ microcrystals can be achieved by tuning the doped Yb3+ concentrations. Moreover, the intense near-infrared emission at 800 nm in YOF:Yb3+,Tm3+ microboxes is obtained. Importantly, the development of the flux cooling method may open up new opportunities in the preparation of high quality rare earth oxyfluorides. Acknowledgments
Fig. 9. Pump power dependence of the NIR UC emissions in YOF:20%Yb3+, 2%Tm3+.
This work was supported by the Key University Science Research Project of Jiangsu Province (No. 10KJA430016), the National Natural Science Foundation of China (No. 20901040/B0111), the Innovation Foundation for Graduate Students of Jiangsu Province (CXLX11_0355) and a project funded by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD). References
Fig. 10. The proposed energy transfer mechanisms in Yb3+/Er3+ and Yb3+/Tm3+ doped YOF samples.
Fig. 8b, the NIR emission of YOF:x%Yb3+, 2%Tm3+ samples enhance gradually with the increase of Yb3+ concentration. The pump power dependence of the NIR-to-NIR up-converted emission in YOF:20%Yb3+, 2%Tm3+ microcrystals under 980 nm LD excitation is exhibited in Fig. 9. As shown, the near-infrared emission in YOF:Yb3+,Tm3+ needs two photons energy transfer process. Based on this, we can reasonably understand the mechanism for NIR-to-NIR UC emission enhancement in YOF:x%Yb3+, 2%Tm3+ samples. Fig. 10 shows the energy levels of the involved Yb3+ and Tm3+ ions as well as the proposed UC pathways under 980 nm LD excitation [32]. The electron of Yb3+ is excited from the 2F7/2 to the 2 F5/2 level in YOF:20%Yb3+, 2%Tm3+ microcrystals. The first energy transfer process from Yb3+ to the Tm3+ ion excites the 3H6 to the 3 H5 state with the redundant energy dissipated by phonons. Subsequently, the Tm3+ ion relaxes nonradiatively to the 3F4 level and further populates the 3F2,3 level through a second energy transfer process from the Yb3+ to the Tm3+ ion. The weak UC emission at 695 nm is associated with the radiative transition from the 3F2,3 level to the ground state. Additionally, the strong NIR emission with peak at 800 nm arises from the 3H4 ? 3H6 transition, where the 3H4 level is populated by the efficient nonradiative relaxation from the 3 F2,3 level. The third energy transfer process from the Yb3+ to the Tm3+ ion excites the 3H4 to the 1G4 level from which the UC emission peaked at 478 and 652 nm is generated. In case of YOF:x%Yb3+, 2%Tm3+, when the doped Yb3+ concentration increase from 10% to 30%, more Yb3+ ions furnish and transfer the energy to the Tm3+, resulting in the increase of NIR emission intensity.
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Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and upconversion luminescence in highly crystalline YOF:Yb3+/Er3+ and Yb3+/Tm3+ microboxes Mingye Ding, Chunhua Lu ⇑, Linhai Cao, Yaru Ni, Zhongzi Xu ⇑ State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 21 January 2013 Accepted 29 January 2013 Available online 6 March 2013 Keywords: Flux cooling method Oxyfluorides Upconversion Optical materials
a b s t r a c t Highly crystalline YOF:Ln3+ (Ln = Yb, Er, Tm) microboxes were successfully synthesized for the first time by using a mixed NaNO3–KNO3 flux cooling method at a holding temperature of 600 °C for 2 h in air. The results indicated that the as-obtained products belonged to tetragonal system and exhibited microboxes morphology with side lengths of 0.5–2 lm. The upconversion luminescence properties of as-prepared YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ were investigated in detail. Under 980 nm laser diode (LD) excitation, the emission intensity and the corresponding luminescence colors of YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ could be precisely adjusted by changing the doping concentration of Yb3+ ions. Furthermore, the paper also offers a new alternative in synthesizing such materials and opens the possibility to meet the increasing commercial demand. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Up-conversion (UC) nano- and micro-materials, which can convert two or more low-energy photons into one high-energy photon (anti-Stokes process), have attracted a great deal of interest over the past decade for their potential applications in the fields of the bioanalystics [1,2], solid-state lasers [3], optical telecommunication [4], displays technology [5] and solar cells [6,7]. For the doping rare-earth (RE) ions, the selection of appropriate host materials is essential for realizing favorable optical performance, such as high UC efficiency and a controllable emission profile. Generally, to achieve highly efficient radiative emission of RE, it requires the phonon energy of the host to be as low as possible, which can minimize nonradiative decay rates [8,9]. Among these UC materials, the rare earth fluoride compounds, possessing high refractive index and low phonon energy, are well known as preferable host materials for UC applications [10–12]. On the other hand, oxides have an advantage superior to fluorides in practical applications owing to their good chemical durability and excellent mechanical strength [13]. Consequently, combing the thermal stability of oxides and the low phonon energy of fluorides, rare earth oxy-fluorides (REOFs) have been considered in developing practical UC materials. In REOF crystals, YOF, is known for the low energy of its lattice phonons (<550 cm1) [14] and desirable chemical stability. Therefore, RE-doped YOF is promising candidate for UC luminescence in practical applications.
Up to now, several techniques have been employed for synthesis of REOF, such as solid state reaction [15,16], mechnochemical grinding method [17,18], sol–gel method [19], coprecipitation [20], hydrothermal method [21,22] and thermolysis method [23]. However, these processes usually suffer from the low yield, long reaction time or high environment loads. Therefore, developing an economical mass production and environmentally friendly method should be highly promising for synthesis of REOF-based optical materials. Herein, we first reported a facile mass production method for the synthesis of uniform YOF microboxes by mixed NaNO3–KNO3 flux cooling method. In addition, the UC luminescence properties of the YOF microboxes doped with different rare earth ions (Yb3+/Er3+, Yb3+/Tm3+) with precisely tuned Yb3+ concentration were investigated under 980 nm LD excitation. 2. Materials and methods All of the chemicals used in this experiment are of analytical grade reagents without further purification. Y(NO3)36H2O (99.99%), Yb(NO3)36H2O (99.99%), Er(NO3)36H2O (99.9%), Tm(NO3)36H2O (99.99%) were purchased from Beijing Founde Star Science & Technology Co., Ltd. (China). NaF (98%), NaNO3 (99%), KNO3 (99%) were provided by Sinopharm Chemical Reagent Co., Ltd. (China). 2.1. Synthesis
⇑ Corresponding authors. Tel.: +86 83587252; fax: +86 83587220. E-mail addresses: [email protected] (C. Lu), [email protected] (Z. Xu). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.01.043
In a typical process for the synthesis of YOF, stoichiometric YNO3.6H2O, NaF, NaNO3, KNO3 with molar ratio of 1:4:64:32 were
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M. Ding et al. / Optical Materials 35 (2013) 1283–1287
the 2h range from 10° to 80° with Cu Ka radiation (k = 0.15406 nm). SEM micrographs were obtained using scanning electron microscopy (SEM, JSM-5900, JEOL Ltd., Japan). Chemical composition of the samples was obtained using an energy-dispersive spectrometer (EDS, Noran Vantage DSI, Themo Noran, Middleton, WI). The absorption spectra were measured on a spectrophotometer (Shimadzu, UV-3101PC) with a wavelength range of 300–1100 nm and a resolution of 2.0 nm. Photoluminescence emission spectra were recorded with a Jobinyvon FL3–221 fluorescence spectrophotometer at an excitation wavelength of 980 nm in the dark. All the measurements were performed at room temperature. 3. Results and discussion 3.1. Structural and morphological characterization Fig. 1. XRD patterns of the as-synthesized YOF:20%Yb3+, 2%Er3+ (a) and YOF:20%Yb3+, 2%Tm3+ (b). The standard data for YOF (JCPDS No. 06-0347) are shown as references.
thoroughly mixed with an appropriate amount of ethanol in an agate mort and ground for 15 min. The mixture was placed into alumina crucibles with 20 cm3 capacity, heated to 600 °C at a rate of 10 °C/min and held for 2 h in air. After cooled down to the room temperature inside in the furnace, the as-annealed samples were washed with deionized water three times and subsequently dried at 60 °C for 24 h. For UC luminescence, YOF:Yb3+/Er3+, YOF:Yb3+/ Tm3+ samples were prepared in a similar procedure by adding corresponding Yb(NO3)3, Er(NO3)3, Tm(NO3)3 into the precursor mixture. 2.2. Characterization X-ray power diffraction (XRD) measurements were performed on a ARL X’ TRA diffractometer at a scanning rate of 10°/min in
Fig. 1 shows the power XRD patterns of the as-prepared YOF:20%Yb3+, 2%Er3+ (a), YOF:20%Yb3+, 2%Tm3+ (b) as well as the standard data of pure YOF phase for comparison. All these peaks can be indexed to the tetragonal phase of YOF according to JCPDS card (No. 06-0347). The crystal structure of the tetragonal phase is determined with lattice parameters of a = 0.3918 and c = 0.5442, space group P4/nmm [24]. As no additional peaks for other phases can be detected, we infer that Y3+ ions have been replaced by Yb3+, Er3+ or Yb3+, Tm3+ ions in any ratio with no effect on the crystallinity or phase change because of the almost same radii 3þ 3þ of Y3+, Yb3+, Er3+ or Tm3+ ðr 3þ Y ¼ 0:121 nm; r Yb ¼ 0:118 nm; r Er ¼ 0:120 nm; r3þ ¼ 0:119 nmÞ. Tm The as-prepared YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ crystals with pure tetragonal phase exhibit microboxes morphology, which are displayed in Fig. 2. The SEM images in Fig. 2A and B reveal that the Yb3+/Er3+ and Yb3+/Tm3+ doped YOF samples are composed of relatively uniform microboxes and have smooth surfaces with side lengths of 0.5–2 lm. The EDS spectra of the obtained samples (Fig. 2C and D) indicate that fluorine, oxygen,
Fig. 2. (A) High magnification (10,000) SEM images of YOF:20%Yb3+, 2%Er3+ crystals; (B) high magnification (10,000) SEM images of YOF:20%Yb3+, 2%Tm3+ crystals; (C) EDS spectra of YOF:20%Yb3+, 2%Er3+ crystals; (D) EDS spectra of YOF:20%Yb3+, 2%Tm3+ crystals (The Si peaks arising from measurement).
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Fig. 3. Room-temperature absorption spectra of YOF:20%Yb3+, 2%Er3+ sample.
Fig. 6. Luminescent photographs and corresponding CIE chromaticity diagram of YOF:x%Yb3+, 2%Er3+: (a) x = 10; (b) x = 15; (c) x = 20; (d) c = 30.
ponent atoms are homogeneously distributed in both microcrystals. From XRD, SEM observation and EDS analyses, it is found that high crystalline YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ microboxes are successfully synthesized by the flux cooling method. Fig. 4. Room-temperature absorption spectra of YOF:20%Yb3+, 2%Tm3+ sample.
3.2. UC luminescence properties yttrium and ytterbium atoms are detected in both microcrystals. Furthermore, erbium and thulium atoms can clearly be observed in YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ crystals, respectively. It can be also concluded from EDS analysis that these com-
The absorption spectra of YOF:20%Yb3+, 2%Er3+ and YOF:20%Yb3+, 2%Tm3+ microboxes in the range from 300 nm to 1100 nm are presented in Figs. 3 and 4, respectively. Owing to
Fig. 5. Room temperature UC emission spectra of (a) YOF:x%Yb3+, 2%Er3+ (x = 10, 15, 20, 30) and (b) the intensity ratio of red to green emission in YOF:Yb3+,Er3+ as a function of Yb3+ concentration.
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Fig. 7. Pump power dependence of the green and red UC emissions in YOF:20%Yb3+, 2%Er3+. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the strong ultraviolet absorption of the host matrix, the absorption bands at wavelengths shorter than 350 nm could not be distinguished [25]. As shown in Fig. 3, it is dominated by the strong 2 F7/2 ? 2F5/2 absorption band of Yb3+ centered at about 980 nm in the near-infrared (NIR) region [26]. The weak absorption lines in the visible region (inset in Fig. 3) are readily assigned to f–f transitions of the Er3+ ions. In Fig. 4 for YOF:20%Yb3+, 2%Tm3+ sample, the absorption peaks, corresponding to Tm3+ transitions from the ground state 3H6 to the excited states and Yb3+:2F7/2 ? 2F5/2 transition, respectively, are marked. These results show that Yb3+ with a large absorption cross section around 980 nm may act as the sensitizer for Er3+ or/and Tm3+ UC emission. The UC luminescence properties of Yb3+, Er3+ co-doped YOF with different Yb3+ concentration are investigated and shown in Fig. 5. From the UC emission spectra of YOF:x%Yb3+, 2%Er3+ (x = 10, 15, 20, 30) under 980 nm LD excitation (Fig. 5a), the dominated red emission centered at 658 nm originates from the 4F9/ 4 2 ? I15/2 transition, and the green emission at 546 nm can be assigned to 4S3/2 ? 4I15/2 transition. These peaks corresponding to the respective proportion of red and green emissions result in the multicolor output. With the increase of Yb3+ concentration, the relative intensity of the red emission is enhanced and the ratios of the red to green emission are consequently increased (Fig. 5b).
By tuning the doped Yb3+ concentration (10–30 mol%), we can precisely manipulate the relative emission intensities, thus resulting in a tunable color output from yellow to red (Fig. 6a–d). In order to deeply investigate the involved UC mechanism in YOF:Yb3+,Er3+ microcrystals, the pump power dependence of the green and red UC emissions under 980 nm LD excitation is exhibited in Fig. 7. It is well known that the output UC luminescent intensity (IUC) is proportional to the infrared excitation power (IIR): IUC / InIR , where n is the absorbed photon numbers per visible phonon emitted, and its value can be obtained from the slope of the fitted line of the plot of log(IUC) versus log(IIR) [27]. As shown, the slope of the linear fits for the green and red emissions in YOF:Yb3+,Er3+ crystals are 1.81 and 1.96, which indicates that only two photons process are involved to produce the green and red UC emissions. On the basis of the proposed UC mechanism of YOF:Yb3+,Er3+ in Fig. 10, we can understand the change of the relative intensities and output colors by tuning the Yb3+ doping concentration in YOF:x%Yb3+, 2%Er3+. It can be seen that the green and red emissions all need a two photons process to populate the 4S3/2 or 4F9/2 level. Therefore, the changing of output colors is due to fewer Er3+ ions holding at the green-emitting level of 4S3/2 and more Er3+ ions holding at the red-emitting level of 4F9/2. With more Yb3+ ion doping into the YOF host matrix, the interatomic distance of Yb3+–Er3+ decreases and thus facilitates the back-energy-transfer from Er3+ to Yb3+ ions 4F7/2 (Er3+) + 2F7/2 (Yb3+) ? 4I11/2 (Er3+) + 2F5/2 (Yb3+), which will subsequently suppress the population of Er3+ ions in excited level of 4S3/2 and result in the decrease of green (4S3/2 ? 4I15/ 3+ 2) light emission. Simultaneously, the energy transfer from the Yb ions to Er3+ also leads to the saturation of the 4I3/2 (Er3+) and then the excited Yb3+ ions transfer its energy to Er3+ through the energytransfer process 2F5/2 (Yb3+) + 4I13/2 (Er3+) ? 2F7/2 (Yb3+) + 4F9/2 (Er3+), which can directly populate the 4F9/2 level, resulting in the enhancement of red (4F9/2 ? 4I15/2) light emission [28]. Fig. 8 presents the UC luminescence properties of Yb3+/Tm3+ codoped YOF with different Yb3+ concentration. In Fig. 8a for YOF:x%Yb3+, 2%Tm3+ (x = 10, 15, 20, 30) microboxes, the characteristic UC emission peaks can be assigned to 1G4 ? 3H6, 1G4 ? 3F4, and 3F2,3 ? 3H6 transitions and an intense near-infrared (NIR) emission at 800 nm is caused by 3H4 ? 3H6 transition [16,29]. A strong enhancement of the NIR UC emission around 800 nm, with respect to the visible one, has been also observed for Yb3+/Tm3+ doped fluorides and oxyfluorides, such as NaYF4 [30], GdF3 [31] and GdOF [20]. Different from the YOF:x%Yb3+, 2%Er3+ samples, the UC spectra of this system do not show too many obvious differences when adjusting the Yb3+ concentration. As we can see from
Fig. 8. Room temperature UC emission spectra of (a) YOF:x%Yb3+,Tm3+ (x = 10, 15, 20, 30) and (b) the intensity of NIR emission in YOF:Yb3+,Er3+ as a function of Yb3+ concentration.
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4. Conclusion In summary, well-formed and high quality YOF:Yb3+/Er3+ and YOF:Yb3+/Tm3+ microboxes were successfully grown by the flux cooling method. The UC luminescence properties of YOF crystals doped with Yb3+/Er3+ and Yb3+/Tm3+ were investigated. Multicolor UC emissions of as-prepared YOF:Yb3+,Er3+ microcrystals can be achieved by tuning the doped Yb3+ concentrations. Moreover, the intense near-infrared emission at 800 nm in YOF:Yb3+,Tm3+ microboxes is obtained. Importantly, the development of the flux cooling method may open up new opportunities in the preparation of high quality rare earth oxyfluorides. Acknowledgments
Fig. 9. Pump power dependence of the NIR UC emissions in YOF:20%Yb3+, 2%Tm3+.
This work was supported by the Key University Science Research Project of Jiangsu Province (No. 10KJA430016), the National Natural Science Foundation of China (No. 20901040/B0111), the Innovation Foundation for Graduate Students of Jiangsu Province (CXLX11_0355) and a project funded by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD). References
Fig. 10. The proposed energy transfer mechanisms in Yb3+/Er3+ and Yb3+/Tm3+ doped YOF samples.
Fig. 8b, the NIR emission of YOF:x%Yb3+, 2%Tm3+ samples enhance gradually with the increase of Yb3+ concentration. The pump power dependence of the NIR-to-NIR up-converted emission in YOF:20%Yb3+, 2%Tm3+ microcrystals under 980 nm LD excitation is exhibited in Fig. 9. As shown, the near-infrared emission in YOF:Yb3+,Tm3+ needs two photons energy transfer process. Based on this, we can reasonably understand the mechanism for NIR-to-NIR UC emission enhancement in YOF:x%Yb3+, 2%Tm3+ samples. Fig. 10 shows the energy levels of the involved Yb3+ and Tm3+ ions as well as the proposed UC pathways under 980 nm LD excitation [32]. The electron of Yb3+ is excited from the 2F7/2 to the 2 F5/2 level in YOF:20%Yb3+, 2%Tm3+ microcrystals. The first energy transfer process from Yb3+ to the Tm3+ ion excites the 3H6 to the 3 H5 state with the redundant energy dissipated by phonons. Subsequently, the Tm3+ ion relaxes nonradiatively to the 3F4 level and further populates the 3F2,3 level through a second energy transfer process from the Yb3+ to the Tm3+ ion. The weak UC emission at 695 nm is associated with the radiative transition from the 3F2,3 level to the ground state. Additionally, the strong NIR emission with peak at 800 nm arises from the 3H4 ? 3H6 transition, where the 3H4 level is populated by the efficient nonradiative relaxation from the 3 F2,3 level. The third energy transfer process from the Yb3+ to the Tm3+ ion excites the 3H4 to the 1G4 level from which the UC emission peaked at 478 and 652 nm is generated. In case of YOF:x%Yb3+, 2%Tm3+, when the doped Yb3+ concentration increase from 10% to 30%, more Yb3+ ions furnish and transfer the energy to the Tm3+, resulting in the increase of NIR emission intensity.
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