Understanding Enhanced Upconversion Luminescence in Oxyfluoride

Article

Understanding Enhanced Upconversion Luminescence in Oxyfluoride Glass-Ceramics Based on Local Structure Characterizations and Molecular Dynamics Simulations Mohamed A. Ali, Jinjun Ren, Xiaofeng Liu, Xvsheng Qiao, and Jianrong Qiu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04599 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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The Journal of Physical Chemistry

Understanding Enhanced Upconversion Luminescence in Oxyfluoride Glass-Ceramics Based on Local Structure Characterizations and Molecular Dynamics Simulations Mohamed. A. Ali,ac Jinjun Ren,d Xiaofeng Liu,a Xvsheng Qiaoa and Jianrong Qiu*b a

School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

b

State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China c

d

Department of Physics, Faculty of Science, Suez University, Suez, Egypt

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Jiading, Shanghai 210009, China

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ABSTRACT: In this paper, large enhancement in upconversion (UC) luminescence was verified in a transparent aluminosilicate glass-ceramics (GCs) containing CaF2 nanocrystals (NCs) co-doped with Er3+ and Yb3+ ions. Based on the joint spectroscopic and structural characterizations, we suggest that the precipitation of fluoride NCs is correlated with the pre-existence of the fluoride-rich domains in the as-melt glass, which is supported by scanning transmission electron microscopy (STEM) and reproduced by molecular dynamics (MD) simulation. The precipitation of the fluoride NCs starts from a phaseseparated as-melt glass consisting of fluorine-rich and oxygen-rich domains, while the spatial distribution of rare earth (RE) ions and the vibration energies of the bonds connecting RE ions remain almost unchanged after crystallization. In the GCs, both the fluoride domain and the oxygen containing polyhedrons surrounding RE ions experience significant ordering which may affect the UC emission for both glasses and GCs. We therefore attribute the enhanced UC emissions of the GCs to the longrange structural ordering and the change of site symmetry surrounding RE ions, rather than the preference of RE ions in migrating from fluoride-rich phase to the fluoride NCs. Our results may have strong implications for a better understanding of the enhanced UC emission in similar oxyfluoride GCs.


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1. INTODUCTION Recently, upconversion (UC) luminescence from materials doped with rare earth (RE) ions plays an significant role in several potential applications such as medical diagnostics, solar cells, sensors and bioimaging.1–5 RE doped oxyfluoride glassceramics (GCs) have evoked growing consideration because in these kinds of materials, the RE ions usually incorporate into the fluoride nanocrystals (NCs) which are immersed in the oxide glass matrix. Therefore, they not only have a low phonon environment associated to the fluoride crystals, but also have high chemical and mechanical stability related to oxides, thus leading to stable and efficient UC emission. Many papers has reported the increase of UC luminescence of transparent oxyfluoride GCs containing CaF2, PbF2, LiYbF4, LaF3, or CeF3 crystals co-doped with Er3+:Yb3+.8–12 Most previous reports always focused on the improvement of UC luminescence, while the local structures of glass and GC that have a strong influence on the UC emission have been somehow overlooked. To generate the enhanced of UC luminescence, the local structure around the RE ions must experiences a substantial change after crystallization that favors more efficient emission mechanism and suppresses the nonradiative processes. To understand how this happens, in this work we prepared transparent aluminosilicate GCs containing CaF2 NCs co-doped with Er3+:Yb3+ ions and investigated the UC properties and the local structure of both glass and GC materials. It was found that the UC luminescence of RE ions is completely dependent on the local structure of both glass matrix and fluoride crystals. 2. EXPERIMENTAL In the experiment, glass samples co-doped with Er3+ and Yb3+ and singly doped with Eu3+ were prepared by the conventional melt-quenching method with the following compositions in (mol%) 50SiO2-20Al2O3-30CaF2-5YbF3-0.5ErF3 and 50SiO23 ACS Paragon Plus Environment


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20Al2O3-34.5CaF2-1EuF3, respectively. For each batch, about 20 g of starting material was fully mixed and melted in a covered alumina crucible in air atmosphere at 1400 °C for 45 min and then cast into a stainless steel plate. All glass samples were heattreated at 650 °C for 2 h then cooled down slowly to room temperature to obtain transparent GCs through crystallization. For optical measurements, the prepared samples were polished with a thickness of 1.3 mm and area of 10 mm x 10 mm. The corresponding glass and GCs co-doped with Er3+ and Yb3+ are labeled as G(Er:Yb) and GC(Er:Yb), respectively. The glass and GCs samples doped with Eu3+are also named as G(Eu) and GC(Eu), respectively. X-ray diffraction (XRD) patterns were obtained in order to identify the structure and the crystal size of the prepared glass and GCs using XPERT-PRO-PANalyticalNetherlandwith Cu-Kα radiation (λ = 1.5418 Å). The diffraction data were recorded for 2θ between 10° and 80° with a resolution of 0.02°. To investigate the presence of fluoride-rich domains in the glass and the formation of CaF2 NCs in the GC, energy dispersive X-ray (EDX) maps and scanning transmission electron microscopy (STEM) were obtained using a JEOL JEM-2100F TEM system operating at accelerating voltage of 200 kV. The absorption spectra were recorded using a Lambda 900 spectrophotometer (PerkinElmer, USA) with a spectral range from 200 to 2500 nm. The UC, photoluminescence (PL), lifetime and excitation luminescence were measured with a spectrofluorometer (iHR 320, Jobin-Yvon, France) equipped with 980 nm LDs and xenon arc lamp as excitation sources. The excitation spectrum recorded in the region of 420-500 nm was used to identify the phonon sideband (PSB) spectrum, by monitoring the emission spectrum at 615 nm. The luminescence quantum yield was measured with an absolute photoluminescence quantum yield measurement system (Quantaurus-QY Plus C13534-35; Hamamatsu Corp., Shizuoka, 4 ACS Paragon Plus Environment

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Japan). The Raman spectra were measured by a Raman spectrometer (HR 800, JobinYvon, France) operated with 514.5 nm Argon laser as excitation source. The 19F and 27

Al magic angle spinning nuclear magnetic resonance (MAS-NMR) experiments

were recorded using a Bruker Avance III HD 500 MHz spectrometer. The molecular dynamics (MD) simulations were used to simulate the structure of the oxyfluoride glass by the DL-POLY 2.20 package developed at Daresbury Laboratory in the UK. All the measurements were carried out at room temperature. 3. RESULTS AND DISCUSSION The glass state of G(Er:Yb) was first examined by XRD and STEM observation (Figure 1a,c). In contrast to the featureless XRD pattern and the smooth STEM image, the EDX mapping (Figure 1d and Figure S1) shows that the F and O elements are distributed somewhat inhomogeneously. Fluorine (green) is dispersed between oxygen (red), where nanophases separation are limited to regions of several nanometers at most. A similar fluoride phase separation was also observed in a similar oxyfluoride glass that can be ascribed to the thermodynamically immiscibility of the component at high temperatures.38 However, the change in the distribution of RE ions and other elements after crystallization are not evident (Figure S2), which is in line with our previous observation.40 The formation of fluoride-rich region in the glass after quenching could facilitate the nucleation and crystal growth processes of fluoride crystals and also the incorporation of RE ions into the CaF2 NCs during the heat treatment process. After annealing at 650 °C for 2 h, cubic CaF2 NCs precipitate, as confirmed by XRD and STEM shown in Figure 1a,b, respectively. The diffraction peaks at 2θ = 28.1°, 46.72°, 55.46°, 68.34° and 75.38° can be attributed to (111), (220), (311), (400) and (331) planes of the cubic CaF2 crystal (JCPDS # 01-071-4796, space group: Pm-3m (No. 225), a = b = c = 5.462 Å), respectively. The absence of 5 ACS Paragon Plus Environment


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secondary phase imply the negligible influence of Er3+:Yb3+ ions on the precipitation of CaF2 NCs due to their close ionic size. According to Sherrer’s formula,13 the average crystal size of GC(Er:Yb) was found to be 7 nm, in close agreement with the STEM observation shown in Figure 1b. The room temperature absorption spectra of G(Er:Yb) and GC(Er:Yb) are shown in Figure 2a. The inset photograph demonstrates the transparency of glass and GC samples after heat treatment. Each absorption peak is attributed to the electronic transitions from the ground state 4I15/2 to the indicated excited states of Er3+ ions. The strongest absorption peak at 980 nm is due to the 2F7/2 → 2F5/2 transition of Yb3+ and the 4I15/2 → 4I11/2 transition of Er3+. Additionally, the absorption peaks observed at 1523, 651, 520, 488 and 379 nm can be assigned to the 4I15/2 → 4I13/2, 4F9/2, 2H11/2, 4F7/2 and 4G11/2 transitions, respectively. The clear decrease of absorption peaks for the transitions of 4I15/2 → 4G11/2 and 4I15/2 → 2H11/2 in GC(Er:Yb) samples means that the ligand field around Er3+ ions must have been changed, where 4I15/2 → 4G11/2 and 4I15/2 → 2H11/2 transitions are sensitive to the environment around the active ions.14 The change of ligand field around Er3+ ions after annealing implies the incorporation of Er3+ ions into CaF2 phases. Figure 2b illustrates the UC emission spectra (excited at 980 nm LDs) of G(Er:Yb) and GC(Er:Yb). In general, the UC spectra demonstrate red (656 nm), green (538 nm and 521 nm) and violet (408 nm) emission peaks which can be attributed to the 4F9/2 → 4I15/2, 4S3/2 → 4I15/2, 2H11/2 → 4I15/2 and 2H9/2 → 4I15/2 transitions, respectively.31 In addition, the red (enlarged 23 times) and green (enlarged 8 times) luminescence intensity of G(Er:Yb) are very strong, while the violet luminescence (408 nm) could also be observed after heat treatment.30 The absolute quantum yields for UC emissions were measured directly with the help of integrating sphere without using a


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reference. At the excitation power of 1000 mW (980 nm LDs), the quantum yields values were found to be 0.001 % and 0.066 % for G(Er:Yb) and GC(Er:Yb), respectively. The dramatic increase of quantum yields is in consistence with the enhancement of the UC emission intensity as shown in Figure 2b. Since absorption is almost unchanged after thermal treatment, the enhanced quantum yields can be attributed to the suppressed nonradiative process in the GC samples. Generally, the green emission is resulted from the excited state absorption (ESA) process; while the red luminescence is dominated by the energy transfer upconversion (ETU) process.32,34 The values of red-to-green integrated intensity ratio (i.e. RGR = IR / IG) change drastically after crystallization and it was found to be 1.27 and 3.62 for G(Er:Yb) and GC(Er:Yb), respectively.29 The increase of RGR values after heat treatment indicates that the probability of ETU process was higher than the ESA process. This is because the RE ions are inserted into the CaF2 NCs and the distance between RE ions becomes shorter as compared to the glass matrix and it is also supported by the obvious Stark splitting of red and green emission peaks of GC(Er:Yb) due to the crystal field effect.16 The energy level diagram of Er3+:Yb3+ ions and the possible mechanisms of UC process such as cross-relaxation (CR), energy-back-transfer (EBT), ESA and ET processes are depicted in Figure 2c.26–28 To unravel number of photons involved in the generation of each UC emission band, we examined the power dependence of UC emission intensity for each band, as shown in Figure 2d. In the unsaturated UC processes, the relation between the integrated emission intensity (Ie) and the infrared pump power (Ip) is defined as the following:17 ∝ ( ) (1) where n is the number of infrared (IR) photons absorbed per upconverted photon emitted. The n-values of 656 nm (4F9/2 → 4I15/2), 538 nm (4S3/2 → 4I15/2) and 408 nm



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(2H9/2 → 4I15/2), emissions were 1.78, 1.93 and 2.96, respectively. This result indicates that the red and green emissions were related to two-photon absorption, while UC luminescence corresponding to violet emission could be due to three-photon absorption. The result is in consistence with other Yb3+ and Er3+ co-doped GCs and NCs.15,39 The luminescence decay curves of G(Er:Yb) and GC(Er:Yb) were measured, as shown in Figure 3a,b. By fitting the decay curves with a single exponential decay function, the lifetimes of 4S3/2 (538 nm) and 4F9/2 (656 nm) levels in G(Er:Yb) are found to be 0.091 and 0.139 ms. In contrast, these values increased to 0.192 and 0.553 ms in GC(Er:Yb), reminiscent of suppressed phonon-assisted nonradiative transitions. This result, on the other hand, suggest the enhance UC emission probability via both ETU and ESA process. To further examine the behavior of RE ions upon crystallization, Eu3+ ion is employed as spectroscopic probe to examine the local environment around RE ions in different crystals and glasses because it has exclusive emission bands, which are due to either magnetic or electric dipole transitions. The PL emission spectra of G(Eu) and GC(Eu) were recorded under excitation at 393 nm, corresponding to the transition from the ground level (7F0) to the excited level (5L6) as shown in Figure 4a and the inset diagram of Figure 4d. It can be clearly seen that, the spectra consist of five emission peaks centered at 580, 590, 615, 653 and 702 nm which originate from the radiative transitions from the excited states (5D0) to the 7F0,

1, 2, 3, 4

lower-lying levels,

respectively.18–20 The 5D0 → 7F2 is a forced electric dipole transition that is very sensitive to relatively small changes in the surrounding of the Eu3+ ions; this transition tends to be more intense in asymmetric sites. On the contrary, the 5D0 → 7F1 is a magnetic dipole transition and relatively insensitive to the local symmetry. Therefore,


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the structural symmetry of the Eu3+ site can be assessed by calculating R-value, which is defined as the ratio between the integrated intensity of 5D0 → 7F2 to 5D0 → 7F1 emissions. In Eu3+ site with high inversion symmetry the magnetic dipole transition (5D0 → 7F1) is the strongest, however, in a site without inversion symmetry the electric-dipole transition (5D0 → 7F2) dominates. According to the PL spectra, it is observed that, the 5D0 → 7F2 transition gives the strongest emission in both G(Eu) and GC(Eu), suggesting that Eu3+ ions are located at sites without inversion symmetry before and after crystallization. Moreover, the R-values decrease from 2.35 to 2.02 as the glass material transformed into GC. This result suggests the Eu3+ ions are incorporated into the precipitated CaF2 phase with small increase of the site symmetry and the ligand field around Eu3+ ions has been increased. On the other hand, the electric dipole emission (5D0→7F2) in the GC(Eu) sample does not show an evident splitting. Due to the absence of RE ion migration from silicate to fluoride regions, the fraction of RE ions that insert into fluoride NCs is too low to produce a structured and narrowed emission spectrum that is typical for crystalline materials. This observation is in good agreement with Eu3+ doped CaF2 NCs and GCs reported by M. Secu et al.35,36 We then recorded the fluorescence decay curves for emissions involving 5D0 level for magnetic and electric dipole transitions in G(Eu) and GC(Eu), as shown in Figure 4b,c. The decay curve shows a single exponential behavior and the lifetime of 5

D0 → 7F1 transition (590 nm) increased from 3.34 to 3.98 ms after annealing;

likewise, the 5D0 → 7F2 transition (615 nm) lifetime rise up from 2.98 to 3.21 ms in G(Eu) and GC(Eu), respectively. The small change in lifetime is in rough agreement with the variation in emission intensity, suggesting that the visible PL is less affected by the phonon-assisted process due to the large energy gap.



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The PSB spectroscopy is one of the most powerful tools to measure the local environment phonon energy around RE ions. Figure 4d demonstrates the PSB spectra of G(Eu) and GC(Eu) monitored at 615 nm, where the x-axis shows the energy shift from the zero-phonon line (ZPL) peak (5F0 → 5D2 ) at 464 nm. As shown in Figure 4d and the inset Eu3+ energy level diagram, it can be seen that there are two small PSB peaks in the higher energy region of the ZPL. The PSB1 peak at phonon energy ħω1 = 612 cm-1 (451 nm) could be found for both G(Eu) and GC(Eu) in addition to PSB2 peak at ħω2 = 1002 cm-1 (444 nm). The PSB1 and PSB2 peaks are assigned to the fluoride (500 - 600 cm-1) and silicate (1000 - 1100 cm-1) frameworks, respectively.6,7 It also can be seen that, there is no observable change in the PSB intensities after crystallization, suggesting that the RE ions are surrounded by chemical bonds or polyhedrons of a close vibration energy in both glass and GC samples. The electronphonon coupling strength (g) parameter can be calculated from the ratio of the integrated intensity of PSB to the ZPL intensity which is an indication of the strength of covalent bond between the rare earth ion and the local sites. The estimated g-values of PSB2 were found to be 0.007 and 0.008 for G(Eu) and GC(Eu), respectively. The presence of broad PSB2 signal due to the high phonon energy of silicate framework and the increase of g-value of PSB2 after crystallization indicate that the majority of Eu3+ ions are incorporated in silicate network with a strong covalent RE–O–Si bonding which increases the nonradiative decay rates in the multiphonon relaxation process. The results of PL and PSB spectra summarized that, after heat treatment, part of RE ions in the fluoride region may experience re-distribution after the precipitation of CaF2 phase, leading to small increase in the site symmetry (Figure 4a). Whereas the majority of RE ions are still located in the silicate region connected by strong covalent RE–O–Si bonding which prevent the migration to fluoride NCs, as shown in Figure


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4d. This means that, the local symmetry site around RE ions is the real reason responsible for the improvement of UC luminescence and the low phonon energy doesn’t necessarily to generate highly efficient UC luminescence. Figure 5a demonstrates the Raman spectra of G(Er:Yb) and GC(Er:Yb) at excitation wavelength 514.5 nm. The glass sample G(Er:Yb) reveal a rather broadened and smoothed spectrum, typical for an amorphous material, while the characteristics vibration bands of SiO4, AlO4 and F–Ca can be still clearly observed.21,22,33 The appearance of the T2g vibration mode for F–Ca bonds centered around 311 cm-1 may be implies that phase separation into a fluoride-rich and fluoride-deficient region happens in the as-melt glass, which acts as nucleation agent of the CaF2 nanocrystals in the GCs. In the spectrum of GC(Er:Yb), the small but sharp peak at 140 cm-1 can be ascribed to the crystalline CaF2 phase, while the broad hump at 311 cm-1 that can be ascribed to the F–Ca vibration mode remains unchanged. However, a small bump at 600 cm-1 is due to the optical vibration mode of AlO4 tetrahedron. It is interesting to note that the spectrum in the Si–O region from 700 to 1500 cm-1 become clearly sharpened after heat treatment. This broad band consists of three overlapped peaks located at 888, 1080 and 1256 cm-1 which arise from the stretching vibration of SiO4 tetrahedron with four, one non-bridging oxygen (NBO) and with four bridging oxygen (BO), respectively.23,24 This result suggests that, in additional to the precipitation of CaF2 NCs, significant ordering involving atomic rearrangement also takes place in the non-fluoride Si–O–rich region during thermal treatment of the glass. This may have significant implications in the enhanced UC emission process, as a more ordered, rigid local environment around active ions favors a more efficient emission process. To have a deeper insight into the structural origins of the enhanced UC emission, we recorded the

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F MAS-NMR spectra of the G(Er:Yb) and GC(Er:Yb) at spinning



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frequency 25 kHz (Figure 5b). A broad peak at -88 ppm was recorded for the G(Er:Yb) sample, which can be attributed to F–Ca(n) bond located into amorphous structure (where n represents the number of Ca coordinated with the fluorine, with n = 4).25 After heat treatment, this peak became sharper, more intense and shifted to more negative value at -98 ppm, as a result of the formation of CaF2 NCs inside the glass matrix with high fluorine content and the incorporation of RE ions into CaF2 phase. Four spinning side bands are also observed at approximately 18, -38, -218 and -278 ppm. A strong and sharp peak centered at chemical shift -156 ppm for G(Er:Yb) and GC(Er:Yb) can be assigned to Al–F–Ca(n) with n = 3. It is apparent that, there is no chemical shift change in Al–F–Ca(n) peak can be observed between the spectra of GC(Er:Yb) and G(Er:Yb). This is in consistence with the

27

Al MAS-NMR spectrum

(Figure 5c) in which the four-fold coordinated aluminum ions Al(IV) demonstrate a typical broad peak at 48 ppm without observable change in chemical shift after crystallization.37 These results consists with the previous assumption about the presence of fluoride-rich domains where crystallization of CaF2 takes place, while it is evident that the majority of fluorine ions bonded to Al in the form of Al–F–Ca(n) which remain unchanged after crystallization. In oxyfluoride glass, this amorphous, or potentially short range-ordered domain containing RE ions, however, cannot ensure an efficient UC emission due to the disordered, soft structural nature and the presence of surrounding high-phonon energy oxide-network environment. Considering the high mobility of the glass network modifiers (Ca2+ and F- ions), the formation of CaF2 (doped RE ions) is energetically and kinetically favored, rather than other types of fluorides. RE ions share a similar local environment as that of Ca2+ and they are therefore protected within a crystalline fluoride host against rapid phonon-assisted decay in the crystallized samples.


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To elaborate the structure of the oxyfluoride glass in more detail, we employ MD simulations to reproduce the structural evolution in the 50SiO2-20Al2O3-30CaF2 glass. From the result shown in Figure 6a, we can clearly observe that, the fluoride phase separated region (black dashed line circle) randomly distributed within the glass matrix (blue dashed line circle) and these fluoride nanophase regions have different size (< 1 nm), which is mostly smaller than the size of the precipitated NCs. The fluoride nanophase region contains much higher content of Ca, Al and F, which is different from the glassy region that contains more Si, Al and O. Thus, the separated CaF2-rich glass nanophases in the precursor glass act as nucleation centers of the cubic CaF2 NCs in the GCs, as shown in Figure 6d. The structural units F−Ca(n), Al−F−Ca(n), AlO3F, AlO2F3, AlOF5, AlO4 and SiO4 (Figure 6e-j) can be clearly observed within the magnified fluoride (Figure 6b) and silicate (Figure 6c) regions of 50SiO2-20Al2O3-30CaF2 glass and show good agreement with the result of

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F,

27

Al

MAS-NMR and Raman spectra of G(Er:Yb) (Figure 6k-m). The silicate region mainly consists of SiO4 and AlO4 tetrahedrons which are interconnected by sharing bridging oxygens. The majority of Al(IV) spread in the silicate region as AlO4 tetrahedron. In comparison, the minority of Al(IV), Al(V) and Al(VI) can be stabilized at the fluoride region as Al(O, F)4 tetrahedral, Al(O, F)5 semi-octahedral and Al(O, F)6 octahedral, respectively. Furthermore, the non-symmetry of Al(IV) peak (Figure 6l) refers to the presence of low level concentration of five and six-fold coordinated aluminum ions Al (V) and Al (VI), respectively. The F−Ca(n) structures are located at the fluoride region, which can be transformed into a CaF2 crystalline phase, where Ca and F ions are linked by ionic bonds. Structures of Al−F−Ca(n) also exist in the fluoride region, which contains Al(O, F)4 tetrahedral [such as AlO3F], Al(O, F)5 semi-octahedral [such as AlO2F3] or Al(O, F)6 octahedral [such as AlOF5]



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with F−Ca(n) coordinated structures. The simulation results clearly indicate that the structure of potential nucleus and potentially how crystallization occurs in this glass. The simulation and experimental observations both point out that RE ions in a fluoride environment does not guarantee efficient UC emission, which, however, relies on an ordered, rigid crystalline environment with low phonon energy. 4. CONCLUSION We have examined the structural origin of the enhanced UC in oxyfluoride glass doped with Er:Yb ions. The combined theoretical and experimental study suggests that the precipitation of fluoride NCs is associated with the presence fluoride-rich phase in the as-melt glass. Moreover, the oxide glass network also experience significant evolution and it may also have a positive effect in the enhancement of UC emission for GC. The enhanced UC emission can be ascribed to the more ordered structures surrounding RE ions, rather than enrichment of RE ions in the fluoride NCs. The results may improve our understanding towards the enhanced UC emission in similar oxyfluoride GCs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. STEM images and EDX maps of the glass G(Er:Yb) and the glass-ceramics GC(Er:Yb), Figures S1, S2, and the complete author list. AUTHOR INFORMATION *Corresponding Author Prof. Jianrong Qiu: E-mail: [email protected]; Tel.: +86-135-8800-3708.


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ACKNOWLEDGEMENTS The authors thank the financial support from the National Natural Science Foundation of China (Grants no. 61475047, 11504323), State Key Laboratory of Precision Spectroscopy (East China Normal University) and State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics). REFERENCES (1) Zhang, F.; Che, R.; Li, X.; Yao, C. ; Yang, J.; Shen, D.; Hu, P.; Li, W.; Zhao, D. Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties. Nano Lett. 2012, 12, 2852−2858. (2) Kwon, O. S.; Song, H. S.; Conde, J.; Kim, H.; Artzi, N.; Kim, J. H. Dual-Color Emissive Upconversion Nanocapsules for Differential Cancer Bioimaging in Vivo. ACS Nano 2016, 10, 1512–1521. (3) Yuan, C.; Chen, G.; Li, L.; Damasco, J. A.; Ning, Z.; Xing, H.; Zhang, T.; Sun, L.; Zeng, H.; Cartwright, A. N.; et al. Simultaneous Multiple Wavelength Upconversion in a Core–Shell Nanoparticle for Enhanced Near Infrared Light Harvesting in a DyeSensitized Solar Cell. ACS. Appl. Mater. Interfaces. 2014, 6, 18018–18025 (Page S3). (4) Zhang, F.; Braun, G. B.; Pallaoro, A.; Zhang, Y.; Shi, Y.; Cui, D.; Moskovits, M.; Zhao, D.; Stucky, G. D. Mesoporous Multifunctional Upconversion Luminescent and Magnetic “Nanorattle” Materials for Targeted Chemotherapy. Nano Lett. 2012, 12, 61–67. (5) Tachibana, H.; Aizawa, N.; Hidaka, Y.; Yasuda, T. Tunable Full-Color Electroluminescence from all-Organic Optical Upconversion Devices by NearInfrared Sensing. ACS Photonics 2017, 4, 223–227.



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Figure 1. (a) XRD pattern of G(Er:Yb) and GC(Er:Yb) heat treated at 650 °C for 2 h. STEM of (b) GC(Er:Yb) and (c) G(Er:Yb). (d) EDX map of G(Er:Yb) (Green ball: F; Red: O).



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Figure 2. (a) Absorbance spectra and (b) UC luminescence spectra of G(Er:Yb) and GC(Er:Yb) at 30 mW of 980 nm LDs. The inset optical photographs in (a) and (b) show the transparency and the UC emissions of glass and GC samples, respectively. (c) Energy level diagram of Er3+:Yb3+ ions and possible UC mechanisms. (d) Log–log plot of integrated emission intensity of GC(Er:Yb) as the function of pumping power.


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10

100

(a)

GC (Er:Yb) G (Er:Yb)

(b)


10

1

Log (Ie (a.u))

Log (Ie (a.u))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1 0.01

1 0.1 0.01

1E-3

1E-3

1E-4

1E-4 1.8

2.0

2.2

2.4

2.6

2.8

Time (ms)

4.8

5.6

6.4

7.2

8.0

Time (ms)

Figure 3. Fluorescence decay curves of (a) 538 nm and (b) 656 nm emissions of G(Er:Yb) and GC(Er:Yb) excited by 980 nm LDs.


5

(a)

D0 −> 7F2

Log (Ie (a.u))

0.6 7

D 0 − > F1

5

0.4

D0 −> 7F4

GC (Eu) G (Eu)

100

0.8

5

10

5

D0 −> 7F0

0.2

5

D0 −> 7F3

1 700

2

4

8

10

12

14

Time (ms)

(d)

ZPL

GC (Eu) G (Eu) 5

L6

Nonradiative

hω ω1

0.8

hω ω2

PSB2 PSB1 5 D2 D0

702 nm

653 nm

590 nm

615 nm

0.6

451 nm

5

580 nm

100

1.0

393 nm

GC (Eu) G (Eu)

Normalized intensity (a.u)

(c)

6

464 nm

600 Wavelength (nm)

1000

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(b) GC (Eu) G (Eu)

444 nm

1.0

0.0 500

Log (Ie (a.u))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized intensity (a.u)


7

F4 7 F 7 3 F2

0.4

7

F1 F0

7

PSB1

0.2

PSB2

10 0.0

2

4

6

8

10

12

0

14

500

1000

1500

Energy shift (cm-1)

Time (ms)

Figure 4. (a) PL spectra and Fluorescence decay curves for (b) 590 nm and (c) 615 nm emissions of G(Eu) and GC(Eu) under excitation of 393 nm. (d) PSB spectra associated to ZPL transition (7F0 → 5D2) of G(Eu) and GC(Eu) by monitoring emission at 615 nm, the inset diagram shows the energy level diagram of Eu3+ ions including the PSB process.


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SiO4 SiO4

(a) CaF2

0

SiO4 GC (Er:Yb) G (Er:Yb)

CaF2

AlO4

500

1000

1500

2000

2500

Raman shift (cm-1) Al-F-Ca(n)

Al(IV)

F-Ca(n)

(b)

(c)


* *


*

*

* * *

400

*

200 0 -200 -400 19 F chemical shift (ppm)

-600

200

150 27

100 50 0 -50 Al chemical shift (ppm)

-100

Figure 5. (a) Raman spectra at λex = 514.5 nm, (b) 19F and (c) 27Al MAS-NMR at spinning frequency 25 kHz of G(Er:Yb) and GC(Er:Yb). (*) denotes spinning side bands.



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Figure 6. (a) MD simulations of 50SiO2-20Al2O3-30CaF2 glass, where black and blue dashed lines circled refer to fluoride and silicate regions, which depicted as the magnification (b,c), respectively. (d) The cubic crystal structure of CaF2. Red ball, O; cyan, F; yellow, Si; magenta, Al and green, Ca. Stick stands for covalent bond and dashed line stands for ionic bond, from which different coordination structures (e-j) are extracted and correspond to 19F, 27

Al MAS-NMR and Raman spectra (k-m) of G(Er:Yb).


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Understanding Enhanced Upconversion Luminescence in Oxyfluoride

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