Thermal-chromic Reaction Induced Reversible Upconversion

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Functional Inorganic Materials and Devices

Thermal-chromic Reaction Induced Reversible Upconversion Emission Modulation for Switching Devices and Tunable Upconversion Emission Based on Defect Engineering of WO3:Yb3+, Er3+ Phosphor Jiufeng Ruan, Zhengwen Yang, Anjun Huang, Hailu Zhang, Jianbei Qiu, and Zhiguo Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Thermal-chromic

Reaction

Induced

Reversible

Upconversion Emission Modulation for Switching Devices and Tunable Upconversion Emission Based on Defect Engineering of WO3:Yb3+, Er3+ Phosphor Jiufeng Ruan, Zhengwen Yang*, Anjun Huang, Hailu Zhang, Jianbei Qiu, Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China Corresponding Author: Zhengwen Yang E-mails: [email protected] Keywords: WO3:Yb3+, Er3+, reversible modulation, up-conversion, thermal-chromic reaction Abstract: Reversible luminescence modulation of upconversion phosphors has the potential applications as the photo-switches, optical memory and data storage devices. Previously, the photochromic reaction was extensively used for the realization of reversible luminescence modulation. It is very necessary to develop other approaches such as thermal-chromic reaction to obtain the reversible upconversion luminescence modulation. In this work, the WO3:Yb3+, Er3+ phosphors with the various colors were prepared at the various temperatures, exhibiting the tunable upconversion luminescence attributed to the formation of oxygen vacancies in the host. Upon heat treatment in the reducing atmosphere or air, the WO3:Yb3+, Er3+ phosphors shows a reversible thermal-chromic property. The reversible upconversion luminescence modulation of WO3:Yb3+, Er3+ phosphors was observed based on thermal-chromic reaction. Additionally, the

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upconversion luminescence modulation is maintained after several cycles, indicating its excellent stability. The WO3:Yb3+, Er3+ phosphors with reversible upconversion luminescence and excellent reproducibility have potential applications as the photo-switches, optical memory and data storage devices. 1. Introduction Reversible downshift luminescence modulation of phosphors has attracted considerable interest because of their extensive potential applications as the photo-switches, optical memory, data storage devices, colorimetric sensing and multicolor display.1-5 Photochromic reaction was developed as a promising approach to the realization of reversible downshift luminescence modulation.6-9 To date, the photochromic organic molecules are considered as one of the most efficient materials, which were investigated extensively.10-12 However, the organic molecules have thermal instability and bad chemical resistance in comparison with the inorganic photochromic materials. Therefore, in order to overcome these disadvantage of organic photochromic materials, the reversible downshift luminescence modulation of rare earth (RE) ions doped inorganic materials were reported by the photochromic reaction because of their excellent mechanical strength and thermal stability.13-16 For example, the RE ions -doped Na0.5Bi2.5Nb2O9 materials exhibited the reversible downshift luminescence modulation and data storage by the photochromic reaction.17 For the reversible downshift luminescence modulation, the overlapping between the emission and excitation spectra of RE ions may result in the destruction of photo-switches and information recording. The RE ions doped upconversion luminescence (UCL) materials have an unique ability to convert two or more low energy nearinfrared or infrared photons into one higher energy photon, which have attracted considerable interest because of their special optical properties and extensive potential applications such as


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three-dimensional display, solar cells, biological imaging and detection and photodynamic therapy.18-25 In contrast to the downshift luminescence, the UCL materials as photo-switching and data storage devices have many advantages such as the larger anti-Stokes shift, safe operation, non-destructive readout, etc. At present, the UCL properties of RE ions have been extensively investigated and reported, however, their reversible modulation was rarely studied. To date, the reversible luminescence modulation was mainly obtained by the photochromic reaction for the RE ions doped inorganic materials. It is very necessary to develop the other approaches such as thermal-chromic reaction to realize the reversible luminescence modulation of RE ions doped inorganic materials. Additionally, the modification of UCL color is very important for their applications as lasers, biological imaging and displays. Recently, considerable efforts were focused on finding suitable approach to manipulate the UCL color.26,

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approach such as tailoring the types and concentrations of rare earth ions, controlling excitation light power, and utilizing photonic band gap effects were developed to tune the UCL color.28, 31 Despite the achievements on the approaches to tune the UCL color, however, few investigations were carried out on the influence of the defects of host on their UCL color. In this work, the WO3:Yb3+, Er3+ UCL phosphors were successfully synthesized at the various temperatures by high temperature solid-state approach. The WO3:Yb3+, Er3+ phosphors prepared at the various sintering temperatures exhibited the different emission color due to the formation of oxygen vacancies in the host. Reversible luminescence modulation of UCL WO3:Yb3+, Er3+ phosphors was investigated by the thermal-chromic reaction. A novel thermal-chromic inorganic UCL materials sensitive to near infrared light is obtained, which suggested that thermal-chromic inorganic UCL material has potential application as the photo-switches and data storage devices.


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2. Experimental The WO3:1mol%Yb3+,1mol%Er3+ phosphor was prepared by using the ammonium metatungstate, ytterbium oxide (Yb2O3) and erbium oxide (Er2O3) as raw material. The stoichiometric ammonium metatungstate, ytterbium oxide (Yb2O3) and erbium oxide (Er2O3) were mixed in an agate mortar. After sintering the mixtures at 400 or 600℃ for 3 h in air, the WO3:Yb3+, Er3+ phosphors were prepared. The several thermal-chromic reactions of W-400 and W-600 phosphors was carried out by sintering the samples at the 400 or 600℃ for 3 h in the reducing atmosphere or air. The X-ray diffraction (XRD) patterns of samples before and after thermal-chromic reactions were used to investigate the crystallinity and the phase purity of WO3:Yb3+, Er3+. The UCL spectra of samples before and after thermal-chromic reactions were measured using the fluorescence spectrophotometer (HITACHI F-7000, Tokyo, Japan) upon the 980 nm excitation. Raman spectra of samples were measured by the micro-Raman spectroscopy using the Argon laser with continuous wave (k=514 nm) as the excitation source (Renishaw inVia, Gloucestershire, UK). The chemical states of WO3 with same mass were characterized by the Xray photoelectron spectroscopy (XPS) (200W) with Al Ka radiation under the vacuum conditions. For the electron spin resonance spectroscopy (EPR) measurements, the 80 mg samples were weighed and placed in quartz tube, then the EPR measurements were carryout out on the Bruker X-band A300-6/1 paramagnetic resonance spectrometer at a frequency 9.2 GHz at room temperature. All of the experiments were repeated twice under the same conditions, and the results were the similar in the twice experiments. The transmission electron microscopy (TEM)


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and high-resolution transmission electron microscopy (HRTEM) were used to characterize the morphology of the WO3: Yb3+, Er3+ phosphors. 3. Results and discussion The WO3: Yb3+, Er3+ phosphors sintered at the 400 or 600℃ in air were denoted as the W400 and W-600 phosphors, respectively. The XRD patterns of W-400 and W-600 phosphors were present in the Figure 1a and b, respectively. It can be found that all X-ray diffraction peaks are consistent well with those from the pure monoclinic phase WO3 exhibited in the JCPDS card 72-1465. The 400 or 600 ℃ sintering temperature has no influence on the phase of WO3:Yb3+, Er3+ phosphors, and the pure monoclinic phase WO3 was prepared regarding of the sintering temperature. However, it is obvious that the XRD diffraction peaks of WO3:Yb3+, Er3+ phosphors became sharper with the increasing of sintering temperature, which indicated that the crystallinity of WO3 is gradually increased with the sintering temperature increasing from 400 to 600℃. Figure 1c and d presented the selected area electron diffraction of W-400 and W-600 phosphors, respectively. The (020) and (200) crystal planes with angle of 90° were obviously observed, which further suggested that the WO3 phosphor is in monoclinic phase. The inset of Figure 1e and f showed the TEM images of W-400 and W-600 phosphors. It can be observed that the size of the W-400 and W-600 phosphor is about 50-200 nm. The HRTEM of W-400 and W-600 were exhibited in the Figure 1d and e, respectively. The lattice distances are 0.37 and 0.36 nm, which can be indexed to the (020) and (200) crystal planes of monoclinic WO3, respectively.


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Figure.1 the XRD patterns of WO3 prepared at the 400 a) and 600 ℃ b); the selected area electron diffraction of W-400 c) and W-600 d); the HRTEM images of W-400 e) and W-600 f); the inset of Fig 1 e) and f) is the TEM images of W-400 and W-600, respectively. Figure 2a and b exhibited the absorption spectra of W-400 and W-600 phosphors, respectively. The absorption band in the blue region was observed in all the samples, which is attributed to the band-gap absorption of WO3 semiconductor. It is noted that another absorption band ranging from 550 to 650 nm was observed for the W-400 phosphor, which was attributed to the formation of oxygen vacancies. In order to demonstrate the formation of oxygen vacancies, the Raman spectra of the W-400 and W-600 phosphors were measured, as shown in the Figure


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2c. All the Raman peaks are consistent with a monoclinic structure.32,33 The Raman peaks located at 808 and 716 cm-1 is attributed to the stretching vibrations of the δ(O-W-O), and the peak at 272 cm-1 is assigned to the bending vibration of δ(O-W-O).34,35 In contrast to the Raman spectra of W-600 phosphor, the broadening of Raman peaks was observed in the Raman spectra of W-400 phosphor, indicating the existence of W5+ and oxygen vacancies.36 In addition, as demonstrated by the previous works,37,38 the oxygen vacancy can cause the increasing of the inter-planar distance and the lengths of near W-O bonds, resulting in the lower wavenumber shift of W-O stretching vibration modes. The red-shift of 717 nm Raman peak in the W-400 phosphor was observed in contrast to that of W-600 phosphor, 35 demonstrating the presence of the oxygen vacancy in the W-400 phosphor. As shown in the Figure 2e～f. For the W 4f XPS spectra of W400 and W-600 phosphors, two peaks located at 35.7 and 37.8 eV are attributed to the characteristic W 4f5/2 and W 4f7/2 peaks of W6+.39 The peaks at 34.6 and 36.7 eV correspond to the typical binding energies of W5+.40 The intensity of XPS peaks from W5+ is dependent on the amount of the W5+.41 By comparing the amount of the W5+ in the W-400 and W-600 phosphors, it can observed the more W5+ was generated in the W-400 phosphor, and the generation of W5+ is related to the formation of oxygen vacancies in the WO3 host. The results demonstrated that the high temperature sintering would decrease the concentration of oxygen vacancies, as reported previously.42 The EPR spectra of the W-400 and W-600 phosphors were measured to further verify the existence of oxygen vacancy, and the EPR signals are shown in Figure 2d. It is noted that the EPR signal peaks attributed to the oxygen vacancies was observed. For the W-600 phosphor, no EPR signal was detect. Therefore, the W-400 phosphor has more oxygen vacancies in contrast to the W-600 phosphor, resulting in the observable absorption band ranging from 550 to 650 nm. The absorption from the oxygen vacancies in the WO3 host was extensively reported


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Figure 2. The photo and absorption spectra of WO3 prepared at the 400 a) and 600 ℃ b); Raman spectroscopy c) of W-400, W-400-A1, W-600 and W-600-A1; the EPR spectroscopy d) of W400, W-400-A1, W-600 and W-600-A1; the XPS spectra of W-400 e) and W-600 f).


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in the previous investigations, which is similar with present work.36 The energy bands between the conduction band and valence band were generated by the introduction of oxygen vacancies in the WO3 phosphor. Some electrons in the valence band were excited to the energy bands generated by the introduction of oxygen vacancies, result in the visible absorption band. The absorption of oxygen vacancies results in the changing of color WO3 phosphor, as shown in the inset of Figure 2a and b. The color of W-400 and W-600 phosphor is orange-red and yellowish, respectively. The UCL spectra of W-400 and W-600 phosphors were shown in the Figure 3a and b. It is noted that the UCL property of WO3: Yb3+, Er3+ phosphor is dependent on the sintering temperature. The red UCL peaks (640～700nm) were observed in the W-400 phosphor, as shown in the Figure 3a. The Figure 3b UCL spectra of the W-600 phosphor consist of the green

Figure 3. the UCL spectra a) of W-400, W-400-A1, and W-400-A2; the UCL spectra b) of W600, W-600-A1, and W-600-A2; CIE chromaticity coordinates c) of the W-400, W-400-A1, W600 and W-600-A1phosphors.


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(510～570 nm) and red (640～715 nm) emission peaks. Green and red UCL peaks in the all samples are attributed to the transitions of 4S3/2/2H11/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+, respectively. The above results show that the UCL of WO3: Yb3+, Er3+ phosphors prepared at the various temperatures are different. Therefore, the tunable UCL can be obtained for WO3:Yb3+, Er3+ phosphors by changing the sintering temperature. Figure 3c showed the CIE chromaticity coordinates of W-400 and W-600 phosphors calculated by the UCL spectra exhibited in the Figure 3a and 3b. The color of WO3: Yb3+, Er3+ phosphors prepared at the various temperatures was modulated, which was changed from the orange-red to yellowish color. The UCL intensity dependence of the excitation light power was measured for the WO3:Yb3+, Er3+ phosphors prepared at the various temperatures, as shown in Figure 4a. The n value of red UC emission of W-400 phosphor is about 2.0. The n values of 525 and 548 nm green and 640～715 nm red UCL are about 2.2, 1.9, 2.0, respectively, for the W-600 phosphor. The results demonstrated that the green and red UCL of WO3:Yb3+, Er3+ phosphors prepared at the various temperatures are attributed to the two photon process. The UCL mechanisms of W400 and W-600 phosphors were shown in the Figure 4b. For the green UCL, the Er3+ ion at the 4

I11/2 excited state was populated by the absorption from the 4I15/2 ground state to 4I11/2 excited

state of Er3+ and energy transfer process (ET) from Yb3+ to Er3+. Another ET from the excited state Yb3+ to the 4I11/2 excited Er3+ ions and excited state absorption (ESA) from the 4I11/2 state to 4

F7/2 state populated the 4F7/2 state. Multi-phonon relaxation from the 4F7/2 state populated the

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H11/2 and 4S3/2 states, and the transitions of 4S3/2→4I15/2 and 2H11/2→4I15/2 produced the green

UCL. For the red UCL, the near 4I13/2 state was populated by the non-radiative relaxation from the 4I11/2 state, and the ESA and/or ET result in the transition from the 4I13/2 to 4F9/2, the red UCL


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was obtained by the 4F9/2→4I15/2 transition. As shown in the Figure 3a, the green UCL in the W400 phosphor was not observed, which is attributed to the formation of oxygen vacancies. For W-400 phosphor, there is an absorption in the region of 500–650 nm, which is attributed to the new energy levels formation of oxygen vacancy between the valence and the conduction bands in the WO3 energy band. As shown in the Figure 4b, the electrons at the 2H11/2 and 4S3/2 states transfer their energy to the energy levels of oxygen vacancy, quenching their green UCL. Thus the W-400 phosphor exhibits the red UCL. The W-600 phosphor has small absorption for the green light. So the green UCL were observed in the W-600 phosphor.

Figure 4. (a) the UCL intensity dependence of the excitation light power of W-400 and W-600 phosphors; (b) the UCL mechanism for W-400 and W-600 phosphors. The W-400 and W-600 phosphors were sintered at the 400 and 600 ℃ for 3 h in the H2 reducing atmosphere, respectively. The W-400 deoxidized at 400 ℃ and W-600 deoxidized at 600 ℃ were denoted as the W-400-H1 and W-600-H1, respectively. The XPS spectra of W-400H1 and W-600-H1 phosphors were measured, as shown in the Figure 5a and b. The peak located at the 34.0 and 33.5 eV was observed in the W-400-H1 and W-600-H1 phosphors besides the peaks of W6+ and W5+, respectively, which corresponds to the binding energies of W4+.43,44 After


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the heat treatment of raw W-400 and W-600 phosphors in the reducing atmosphere, the W4+ was formed in the W-400-H1 and W-600-H1 phosphors. The XRD patterns of W-400-H1 and W600-H1 phosphors were measured, as shown in the Figure 1a and b. It can be seen that the W400-H1 and W-600-H1 are pure monoclinic phase. Although the parts of the W6+ were reduced to the W4+ after the heat treatment of raw W-400 and W-600 in the H2 reducing atmosphere, the phase of WO3:Yb3+, Er3+ phosphors has no change. The absorption spectra and photographs of W-400-H1 and W-600-H1 phosphors are shown in Figure 2c and d, respectively, exhibiting obviously changing of color and absorption. After heat treatment in the H2 reducing atmosphere,

Figure 5. the XPS spectra of W-400-H1 (a), W-400-A1 (b), W-600-H1 (c) and W-600-A1 (d) phosphors.


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the W-400-H1 and W-600-H1 are dark blue color, and a broad absorption was observed ranging from UV region to near infrared region in the W-400-H1 and W-600-H1 phosphors. The thermal-chromic phenomenon exhibited in the Figure 2c and d is associated with the change of W valence state.

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After the heat treatment in the H2 reducing atmosphere, the parts of the W6+

were reduced to the W5+ and W4+. The inter-valence charge transfer between the W6+/ W5+ and W5+/ W4+ results in the absorption from the UV region to near infrared region, causing the near infrared absorption and color change.46 The UCL of W-400-H1 and W-600-H1 phosphors was measured, as shown in the Figure S1 of supporting information. No UCL in the W-400-H1 and W-600-H1 phosphors with the dark blue color was observed due to the ultra-board absorption ranging from UV to near infrared region of samples. Previously, the photochromic reaction induced luminescence modulation was obtained. However, the luminescence intensity was only decreased by 50 %, the luminescence quenching was not obtained.

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In present work, the UCL

quenching of WO3:Yb3+, Er3+ phosphors was observed, which is the advantage for the practical application as the photo-switches, optical memory and data storage devices. The dark blue W-400-H1 and W-600-H1 phosphors were heat-treated again at the 400, and 600 ℃ for 3 h in air, respectively. The W-400-H1 oxidized at 400 ℃ and W-600-H1 oxidized at 600 ℃ were denoted as W-400-A1 and W-600-A1, respectively. The XRD patterns of W-400A1 and W-600-A1 were shown in the XRD patterns of Figure 1, exhibiting no change of phase. However, the color and absorption spectra of W-400-A1 and W-600-A1 phosphors has an obvious change in contrast to the W-400-H1 and W-600-H1, as shown in Figure 2a and b. The W-400-A1 and W-600-A1 phosphors exhibit the yellow and green color, respectively, which is attributed to the oxidation from the W with valence state to W6+, as shown in the XPS spectra of Figure 5 c and d. As shown in the Figure 2a, in contrast to the raw W-400 phosphor, the color of


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W-400-A1 phosphor changed from orange-red to yellow, and the absorption peak ranging from the 550 to 650 nm was vanished. Crystal structural investigations were demonstrated that the phase of W-400 and W-400-A1 phosphors was monoclinic structure, excluding the influence the phase transformation on the color difference, as presented in Figure 1. Raman and EPR spectra of the W-400-A1 were exhibited in the Figure 2c and d. It can be seen that the Raman peak located at 707 cm-1 shift to 717 cm-1, which is similar with that of W-600 phosphor. The EPR peak intensity of oxygen vacancies of the W-400-A1 was decreased in the EPR spectra in contrast to the W-400, suggested that the color and absorption spectra changing of W-400-A1 is caused by the decreasing of oxygen vacancies. In contrast to the raw W-600 phosphor, the color of W-600-A1 changed from the yellow to green color. The XRD, EPR and Raman spectra were measured, as presented in Figure 1 and 2. The Raman, XRD and EPR spectra of W-600-A1were not changed in contrast to these of W-600 phosphor, which suggested that the color difference between W-600 and W-600-A1 phosphors is not mainly attributed to the formation of oxygen vacancies. Figure 2f and 5d exhibited the XPS spectra of W-600 and W-600-A1 phosphors, respectively. The XPS peak shift of W was observed for the W-600 and W-600-A1 phosphors, which suggested that a few W5+ and W4+ may be save after the heat-treatment of W-600-H1 phosphor at 600℃ in air, resulting in that the color difference between W-600 and W-600-A1 phosphor.46,47 The UCL spectra of W-400-A1 and W-600-A1 phosphors were measured upon the 980 nm excitation, as shown in Figure 3a, exhibiting the visible UCL. In contrast to the raw W400 phosphor, the weak green UCL of W-400-A1 was observed because the green absorption of oxygen vacancies disappears, and the UCL color of W-400-A1 is yellow, as shown in the CIE chromaticity coordinate of Fig. 3c. The W-600-A1 exhibited the green color, which suggested that the blue and red absorption is more intense in the W-600-A1. Thus the red UCL of W-600-


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A1 was decreased due to the absorption of host in contrast to the raw W-600 phosphor, and the UCL with green color was obtained, as shown in the CIE chromaticity coordinate of Fig. 3c. The W-400-A1 and W-600-A1 phosphors were sintered again at the 400 and 600 ℃for 3 h in the reducing atmosphere, respectively. The W-400-A1 phosphor deoxidized at 400 ℃ and W600-A1 deoxidized at 600 ℃ were denoted as the W-400-H2 and W-600-H2, respectively. The W-400-H2 and W-600-H2 become the dark blue, and no phase structures changing and UCL was observed. The W-400-H2 and W-600-H2 phosphors were heat-treated again at the 400 and 600 ℃ for 3 h in air, respectively, denoting as the W-400-A2 and W-600-A2. The color of W-400H2 and W-600-H2 phosphors after the heat-treatment in air returns to the yellow and green, respectively, exhibiting the excellent stability. The UCL of W-400-A2 and W-600-A2 were exhibited in Figure 3a upon the 980 nm excitation. In contrast to the W-400-A1 and W-600-A1, the UC conversion spectra of W-400-A2 and W-600-A2 have no significant changing besides the UC emission intensity was increasing slightly. Such a recovery process of UCL based on the thermal-chromic reactions can switch reversibly to corresponding quenching state by the subsequent reduction treatment. In order to obtain the reversible UCL modification property, the several cycles for the W-400-A1 and W-600-A1 was carried out by the heat treatment in reduction atmosphere and air. The schematics and photographs of W-400-A1 and W-600-A1 samples were shown in the Figure 6a under the heat treatment in reduction atmosphere and air. The photographs exhibited clearly the reversible color change. As exhibited in the Figure S2, the color of WO3:Yb3+, Er3+ phosphors after five cycle heat-treatment at the 400 and 600℃ is the yellow and green, respectively. The phase structure of the W-400-A4 and the W-600-A4 has not changed, exhibiting the excellent stability, as exhibited in the Figure S3. The UCL intensity of


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W-400 and W-600 as the function of cycle times was shown in Figure 6 and Figure S4. It can be seen that the UCL intensity of the WO3:Yb3+, Er3+ phosphors can be fully recovered, and no deterioration of UCL intensity was observed after 5 cycles. The UCL intensity can be switched on and off, and the excellent reproducibility is clearly observed over several cycles.

Figure 6. The photographs and switch diagram (a) of W-400-An and W-600-An, UCL intensity of W-400-An (b) and W-600-An (c) as a function of the cycle sintering times. 4. Conclusion In this work, the WO3:Yb3+, Er3+ phosphors were successfully synthesized at the various sintering temperatures by high temperature solid-state approach. The tunable upconversion photoluminescence and thermal-chromic property were observed. The tunable upconversion luminescence is attributed to the formation of oxygen vacancies in the host. Upon heat treatment in the reducing atmosphere or air, the WO3:Yb3+, Er3+ phosphors shows a reversible thermalchromic property, resulting in the reversible upconversion luminescence modulation. The


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reversible upconversion luminescence modulation is maintained after the several cycles, and the upconversion intensity has no degradation after several periods, showing its excellent stability. The WO3:Yb3+, Er3+ phosphors with reversible upconversion luminescence and excellent reproducibility have potential applications as the photo-switches, optical memory and data storage devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The color and XRD patterns of WO3:Yb3+, Er3+ phosphors after five cycle heat-treatment at 400 and 600℃, the UCL spectra of W-400 and W-600 as the function of cycle times. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51762029, 11674137), and the Applied basic research key program of Yunnan Province. REFERENCES (1) Li, S.; Liu, Y.; Liu, C.; Yan, D.; Zhu, H.; Xu, C.; Ma, L.; Wang, X. Improvement of X-ray storage properties of C12A7:Tb3+ photo-stimulable phosphors through controlling encaged anions. J Alloy Compd. 2017, 696, 828-835.


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S.;

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Thermal-chromic Reaction Induced Reversible Upconversion

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