Tb3+, Eu3+ Co-doped CsPbBr3 QDs Glass with ... - ACS Publications

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Functional Inorganic Materials and Devices

Tb3+, Eu3+ co-doped CsPbBr3 QDs glass with highly stable and luminous adjustable for white LEDs Yinzi Cheng, Chenyang Shen, Linli Shen, Weidong Xiang, and Xiaojuan Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

Tb3+, Eu3+ co-doped CsPbBr3 QDs glass with highly stable and luminous adjustable for white LEDs Yinzi Cheng, Chenyang Shen, Linli Shen, Weidong Xiang* and Xiaojuan Liang* College of Chemistry and Materials Engineering, College of Life and Environmental Science, Wenzhou University, Wenzhou 325035, People’s Republic of China.

ABSTRACT

Herein, we have introduced rare earth cations Tb3+ and Eu3+ into CsPbBr3 QDs glass by conventional melt-quenching. Rare earth cations like Tb3+ emit green light, causing the main peak of bromide lead cesium to exhibit some red-shift owing to the energy transfer between CsPbBr3 and Tb3+. To achieve adjustable light, Eu3+ emits red light, which was doped in this glass with different proportions to solve the problem of red deficiency. More importantly, Tb3+ and Eu3+ co-doped CsPbBr3 QDs glass shows a series of desirable characteristics due to the energy transfer between Tb3+ and Eu3+. Interestingly, the blue light radiated by blue chip can excite Tb3+, Eu3+ and CsPbBr3 perovskite effectively. We acquired a high-performance W-LEDs with the value of CRI, CCT and LE are 85.7, 4945 K and 63.21 lm/W separately under the current of 20 mA. This acquired Tb3+, Eu3+ co-doped CsPbBr3 QDs glass proved that the significant feasibility of the luminescent materials in the solid warm light source.

KEYWORDS: CsPbBr3, rare earth cations, Tb3+, Eu3+, glass, W-LEDs

INTRODUCTION

For the last few years, more and more attention has been putted into the study in the field of white light-emitting diodes (W-LEDs), caused by their advantages of good for environmental protection, long life and high efficiency. Based on these advantages, W-LEDs are always considered as a promising candidate

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for placing traditional incandescent and fluorescent lamps.1-4 Particularly, white light from LEDs were made from a incorporation of blue LED chips matched with commercial yellow powder YAG: Ce3+ and red phosphors, but there are some unavoidable disadvantages, including low color-rendering index (CRI), color coordinate transformation (CCT) and performance instability.5 Therefore, people have turned research goals into semi-conductive metal halide quantum dots (QDs) owe to their excellent optoelectronic properties, narrow full width at half maximum (FWHM) of the emission band, high photoluminescence (PL) quantum yields (QYs), as well as wide wavelength tunability at the range of 400-800 nm with a controllable bandgap in recent years.6-10

In terms of the application, people often choose to form them as a precipitate inside the glass matrix to improve it’s stability. And that can effectively prevent the QDs from coacervation and maintain thermal stability. To date, including II–VI and IV–VI QDs like ZnS, CdSe, PbS and PbSe have been successfully introduced into the glass with remarkable photoluminescence and tunable absorption in the range of visible light and near infrared spectral.11-13

For present study on a rare-earth-doped luminescent material, more attention should be paid to how to improve thermal stability and QYs and make it more appropriate for application. As everyone knows, Tb3+ is always regard as an sensitizer for green-emitting optical materials attribute to it’s excellent transition of 5

D4→7F5 located at approximately 541 nm.14 Besides, the typical strong emissions of Tb3+ comes from

internal configuration 4f→4f transitions have nothing to do with the host lattice.15 However, the absorption intensities of the Tb3+ ions are not strong and their widths are very narrow in the range of near ultraviolet (UV) due to forbidding the transitions of 4f→4f strictly.16 In general, Tb3+ ions as a promising sensitizer, Eu3+ has been widely used and is come into play a significant function in improving the absorption capacity, thermal stability and QYs of Tb3+ ions owe to the valid energy transfer between Eu3+ and Tb3+, such as


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Ba2Ca(BO3)2 : Eu3+, Tb3+.17

In this work, new CsPbBr3 QDs were precipitated in borosilicate glass with rare earth cations (Tb3+, Eu3+) via the method of traditional melt-quenching and a succedent heat-treatment, Further study was conducted on the morphological structure and luminescence properties of this glass. Ultimately, the result shows that this material can be a new kind of warm light source.

EXPERIMENTAL SECTION

Materials and synthesis: In the process of preparing glass specimens having the theoretical compositions of 34B2O3-38SiO2-9ZnO-8Cs2CO3-3PbBr2-3NaBr-xTb4O7 (in mol%), which were carefully mixed in agate mortar and prepared via conventional melt-quenching. The obtained mixed powers were dissolved in a crucible at 1100 ℃/30 min in ambient air. Then the resulting melt was poured into a preheated copper mold and immediately transferred to a furnace that holds the temperature at 450 °C to release the pressure. We will place the glass samples in the boiling furnace with different temperatures near the glass-transition in order to meet the needs of next experiments, the desired glass specimens were achieved after cooling down to room temperature.

Fabrication of prototype LED: W-LEDs equipments were combined by matching the Tb3+, Eu3+ co-doped CsPbBr3 QDs glass with a blue chip (λex=460 nm). The equipments were fixed by silicone resin mixture, The prepared W-LEDs equipment with glass was measured under the condition of direct current forward-bias in an integrating sphere.

Determination of crystal structure: Each sample was detected via X-ray diffraction (XRD) (D8 Advance, Bruker, Karlsruhe, Germany) within the scope of 20°≤2θ≤80° and step size for 0.02 (KAIST Analysis Center For Research Advancement). The photo-luminescence (PL) and photo-luminescence excitation



(PLE) of all the samples were detected by spectrofluorometer (Edinburgh, FS5) with xenon lamp. The variable temperature fluorescence spectrum were tested by the same spectrophotometer, which was connected to a automatic electric furnace and self-regulating warming equipment. The morphological structure of the rare earth cations doped CsPbBr3 QDs in glasses were tested by JEM-2100F high resolution transmission electron microscope (HRTEM). The decay curves were tested by the HORIBA Job in Yvon FluoroMax-4 fluorescence spectrometer, and the 150 W Xe lamp was used as the excitation source. X-ray photoelectron spectroscopy (XPS) spectra were tested by AXI ULTRA DLD spectrometer. CRI, CCT, luminous efficiency (LE) and CIE coordinates of the completed W-LEDs equipments are use integrated spheres for evaluation (PMS-50; Everfine Photo-E-Info Co.Ltd, Hangzhou, China) under the current of 20 mA, each measurement was completed at room temperature.

RESULTS AND DISCUSSION

Figure 1. (a) XRD spectrum of the obtained glasses with different heat-treated temperature and the


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standard map of CsPbBr3 (PDF#54-0752); (b) Schematic perovskite CsPbBr3 structure, where the green ball is alkali metal Cs+ cation, the blue ball is usually Pb2+, and the anion is Br- forming a corner-shared PbBr6 octahedra with the Pb cation; (c) Comparison of X ray diffraction of different rare earth doped CsPbBr3 QDs glasses.

Structure study of Tb3+ doped CsPbBr3 QDs glass: XRD detection were executed to demonstrate the amorphous structure of the as-prepared glass. Which was testified without sharp Bragg peaks. Figure 1(a) shows the results of the glass samples crystallized at different temperatures, which is the nanocrystalline precipitation within the scope of 450 °C-500 °C with diverse time of duration. The obtained glass was highly transparent. Several faint diffraction peaks were discovered when crystallized at 470 °C for 10 h and 480 °C for 10 hours, proving that the crystalline phases was formed in the glass matrix, and clear diffraction peaks appeared when the samples were heat-treated at 480 ℃, 500 ℃ for 10 hours. All peaks are match with the standard map (PDF#54-0752) of CsPbBr3. Moreover, the feature diffraction peaks of the CsPbBr3 QDs glass have 2θ values located at nearly 21.31°, 30.41°, and 37.51°, which were consistent with the (100), (200), and (211) planes of the standard map of CsPbBr3 (PDF#54-0752),18 separately, indicating CsPbBr3 QDs were formed in the glass matrices. There were no other raw materials, such as PbBr2

discovered in the samples. The related peaks shown in Figure 1c with the corresponding peaks for CsPbBr3 glass, Tb3+ doped CsPbBr3 glass and Eu3+ doped CsPbBr3 glass located at 20.6848°, 21.1365° and 21.0675°, respectively. The shift of the peaks position toward higher angles was assigned to the partial substitution of Pb2+ by rare earth cations with smaller ionic radii (Pb2+: 119 pm, Eu3+: 95 pm, Tb3+: 92 pm) causing the lattice parameters to reduce accordingly. The possibility of rare earth cations physically adsorbed on the surfaces of the prepared nanocrystals were eliminated by the phenomenon of peak movement.19-21



According to the above results, we made the schematic perovskite CsPbBr3 structure shown in Figure 1(b). It’s well known that CsPbBr3 crystallized in cubic polymorphs, orthorhombic, and tetragonal of the perovskite lattice. And all compounds were in the form of cubic phase under the condition of high temperature usually.22-24 Additionally, all the CsPbBr3 crystallizes in the cubic phase contribute from the surface energy and high synthesis temperature.25 In this model, the green ball represents cesium ions, the yellow ball represents bromide, and the blue represents the divalent lead ion, usually, Tb3+ replaces lead ions when doping with rare earth elements.

Figure 2. (a) TEM photograph of the Tb3+ doped CsPbBr3 QDs glass (illustration: particle size distribution map). (b) HRTEM photograph of Tb3+ doped CsPbBr3 QDs glass (illustration: SAED pattern of Tb3+ doped CsPbBr3 QDs glass). (c) EDX graph of the Tb3+ doped CsPbBr3 QDs glass.


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TEM analysis of Tb3+ doped CsPbBr3 QDs glass: The microstructure was investigated to further confirm the formation of the Tb3+ doped CsPbBr3 QDs in the as-prepared glass. The TEM and HRTEM photograph (Figure 2a and b) of Tb3+ doped CsPbBr3 QDs glass which was continue heated at 490 ℃ for 10 hours. The photograph expressly shows a almost uniform distribution of the CsPbBr3 QDs inside the glass matrix with the average diameter of approximately 5.73 nm (Figure 2a and the illustration). Additionally, the nanoparticles were also found (black spots in the glass matrix in Figure 2a). The above consequence verify that the Tb3+ doped CsPbBr3 QDs glass possess a very good crystallinity.26, 27

Furthermore, lattice fringes are expressly detected, and the interplanar crystal spacing was calculated to be 0.18 nm (Figure 2b). There exist a certain gap with CsPbBr3 (d200=0.29 nm, PDF#54-0752), possibly caused by doping with rare earth cations. In addition, the Tb3+ doped CsPbBr3 QDs in the glass matrix has various shapes, but a number of them are spherical shapes, which are different from other reports27 possibly attribute to the heat-treatment temperature.29 And the selected area electron diffraction (SAED) photograph (Figure 2b) for Tb3+ doped CsPbBr3 QDs glass also revealed the presence of (211) planes of cubic phase, further confirm the formation of perovskite structure.

The TEM photograph of the obtained glass, CsPbBr3 QDs and Tb3+ ions were evenly embedded in glass matrix. Moreover, all the elements including Tb, Cs, Pb, Br and Si were discovered (Figure 2c and Figure S1). These results powerfully demonstrate the successful growth of CsPbBr3 QDs and Tb3+ in the glass matrix.



Figure 3. the XPS survey of CsPbBr3: 0.3%Tb3+ (A-H) and CsPbBr3: 0.3%Tb3+, 0.3%Eu3+ glass (a-f)

XPS analysis of Tb3+ doped CsPbBr3 QDs glass and Tb3+/Eu3+ co-doped CsPbBr3 QDs glass: The XPS survey spectra of Tb3+ and Tb3+, Eu3+ co-doped CsPbBr3 QDs glass were employed to ensure the valence state and the chemical compositions of Tb and Eu in the as-prepared CsPbBr3 QDs glass. The signals of Cs, Pb, Br, Zn, B and Si are clearly illustrated in every sample (Figure 3H and f). Meanwhile, relatively weak Tb and Eu signals are also observed. We can draw conclusions by comparing the above as-prepared samples. In the high-resolution XPS spectra, some element regions show the same peak position. However, some of them have been offset, such as Pb, indicating different chemical environments in terms of the as-prepared samples doping with rare earth cations.30


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In Figure 3E and 3a, the Br signal (not smooth) was made up of NaBr and PbBr2, which exhibit common statistical properties. This outcome causes the photoelectron transition often shows a certain distribution; that is, the spectral peaks close enough to each other to overlap; the Br peaks in the two samples were both located at 75 eV and 78 eV respectively. As shown in Figure 3F and 3d, Pb exhibits a similar phenomenon.31

As shown in Figure 3c, in the high-resolution XPS spectra, the binding energies are located at 1123 eV and 1129 eV, corresponding to the two peak Eu3d5/2 and Eu3d3/2 shell, this outcome happened because of Eu has unpaired electrons in the valence shell, and the phenomenon of multiple splitting occured. Another split peak appears at 1141 eV and 1152 eV. The above data proves that Eu was existed in the CsPbBr3 QDs in the form of trivalent.32 The peak position of Tb is almost unchanged whether doped individually or co-doped with Eu3+; the specific peak positions are located at 1242 eV and 1277 eV, which indicate that trivalent Tb exists in the CsPbBr3 QDs glass.

Figure 4. DR spectra of the CsPbBr3 co-doped with Tb3+, Eu3+, Tb3+/Eu3+ glass.

Ultraviolet diffuse reflection: Figure 4 shows the ultraviolet-visible transmittance spectra. The optical absorption of the glass samples with different rare earth cations doping presented the same trend before the



cutoff wavelength. In addition, the transmittance of the glass decreases with increasing wavelength. And different rare earth cations doped glasses have different ultraviolet transmittance values at the same wavelength; compared with the sample prepared by co-doping with the rare earth cations, it shows a higher uv transmittance when they are doped separately. In summary, the absorption peaks of all the as-prepared samples are seated at 270 and 210 nm and the intensity is enhanced with the corresponding temperature increasing. The location of the absorption peaks are completely matched with the same peak location in the PLE (Figure 5b).


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Figure 5. (a) emission curve of the CsPbBr3 QDs glass with various excitation (illustration: appearance of the corresponding glass sample under UV light), the red solid line is the absorption spectrum; (b) the excitation spectra of Tb3+ doped CsPbBr3 QDs glass, Eu3+ doped CsPbBr3 QDs glass and Tb3+, Eu3+ co-doped CsPbBr3 QDs glass; (c) emission curve of the Tb3+ doped CsPbBr3 QDs glass with various energies of excitation (illustration: appearance of the corresponding glass sample under UV light), the red solid line is the absorption spectrum; (d) emission curve of the Eu3+ doped CsPbBr3 QDs glass with various energies of excitation (illustration: appearance of the corresponding glass sample under UV light), the red solid line is the absorption spectrum; the fluorescence spectra of Tb3+ and Eu3+ co-doped CsPbBr3 QDs glass mixed with different ratios and tested at (e) 365 nm, (f) 378 nm, (h) 396 nm and (i) 460 nm.

Optical properties of the series of CsPbBr3 glass samples: As is well known, CsPbBr3 perovskite QDs were regard as a promising choice for low cost optical materials owing to the excellent PL performance. As reported by Zeng and co-workers, there has little effect on LEDs using light emitters based on CsPbBr3 NCs and the fluorescence properties of CsPbBr3 QDs when we synthesize them in a glass matrix.33 The primary peaks of the fluorescence emission curve of the CsPbBr3 with diverse excitation wavelengths are all located at 515 nm (Figure 5a). In addition, Figure 5(b) shows the excitation curve of Tb3+ doped CsPbBr3 QDs glass, Eu3+ doped CsPbBr3 QDs glass and Tb3+, Eu3+ co-doped CsPbBr3 QDs glass. The curve of Eu3+-doped sample monitored at 613 nm consists of lines centered at 317, 351, 368, 377 and 484 nm owing to transition from the ground state of 7F6→5D1, 7F6→5D2,3, 7F6→5L9, 7F6→5D3 and 7F6→5D4, respectively. Tb3+ and Eu3+ co-doped glass has the same properties.34 In contrast, in the Eu3+ doped glass excitation spectra monitored at 543 nm consists of some typical f-f transitions of Eu3+ ions seated at 361 nm (7F0→5D4), 383 nm (7F0→5GJ), 394 nm (7F6→5L5), 416 nm (7F0→5D3) and 465 nm (7F0→5D2).35

In general, Tb3+ is always regard as an sensitizer for the fluorescent material that glows green light


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owing to the transition of 5D4→7F5, and Eu3+ can be used as a red-emitting conversion materials owing to the 5D0→7F2 transition. Emission spectrum curve of Tb3+ and Eu3+ ions doped in CsPbBr3 QDs glass are demonstrated in Figure 5(c) and (d), respectively. The emission spectrum curve of the Tb3+ doped sample recorded at 365, 378 and 395 nm excitation wavelengths all present four main emission lines located at 488 nm, 543 nm, 584 nm and 620 nm caused by 5D4→7FJ transitions, where J=3, 4, 5, 6. Clearly, the only difference was reflected in the fluorescence intensity. The absence of lines below 475 nm is attributed to the 5D3-7FJ transitions, which proves that the Tb3+ concentration was the key factor in the cross-relaxation process. Comparing the appearance of the sample under UV light shown in Figure 5(c), we find that the color becomes greener due to the energy transfer between CsPbBr3 and Tb3+. Clearly, the excitation spectrum curve of CsPbBr3 turns into unapparent compared with that of Tb3+. which confirm that the energy transfer between CsPbBr3 and Tb3+ occurred;36-38 Eu3+ glass excitated at 365, 378, 395 and 460 nm respectively, and which present five apparent peaks located at 576, 590, 612, 651 and 699 nm corresponding to 5D0→7FJ (J=0, 1, 2, 3, 4). 5D0→7F2 is the most obvious one. Eu3+ is good for improving the color purity of the red-emitting glass; the energy transfer between CsPbBr3 and Eu3+ is similar to that CsPbBr3 with Tb3+, which indicate that Eu3+ ions occupy the position of inversion symmetry in the crystal lattice resulting from partial substitution of Pb2+ in the CsPbBr3 crystal lattice.33 Since the distance between the center of luminescence is short enough to cause the concentration quenching,therefore, the resonance energy transfer is allowed when the concentration of rare earth cations increases. Hence, it is necessary to find the best doping concentration.39 Figure S2 shows the fluorescence spectra of rare earth ions (Tb3+) doped with different mass percentage. We have studied the impact of rare earth doping concentration of the obtained sample, with increasing mass percentage of rare earth cations, the fluorescence peak of the obtained samples gradually appeared at approximately 475 nm, while the small peak at 515 nm gradually



disappeared. Interestingly, the PL of Tb3+ doped CsPbBr3 glass shows stronger peaks of Tb3+and a wide emission band detected within the scope of 450−525 nm due to the transitions of the CsPbBr3 QDs. Surprisingly, the peaks of CsPbBr3 QDs almost gone, just detected the very strong and sharp peaks of Tb3+ in Tb3+ doped CsPbBr3 glass system. The CsPbBr3 QDs produces transitions by absorbing the energy from excitation light, then the intense green-emitting occurred when the absorbed energy is transferred to Tb3+. The remarkably enhance in green-emitting proves that the Tb3+ doped CsPbBr3 glass is a very promising material in the application of lighting-emitting device.

In our work, many experiments were carried out to ensure the energy transfer from Tb3+ to Eu3+ in the CsPbBr3 QDs host. Figure 5(e-i) demonstrates the emission spectra of the sample co-doping Tb3+ and Eu3+ into the CsPbBr3 QDs glass with different ratios (Sample 1 represents the mass percentage of Eu3+/Tb3+ equal to 1/5, sample 2 represents 2/4, sample 3 represents 3/3, sample 4 represents 4/2, sample 5 represents 5/1). These samples were excited by 365 nm (curve c), 378 nm (curve d), 395 nm (curve e) and 460 nm (curve f) light. As shown in Figure 5e, under the excitation of 365 nm (the feature peak of CsPbBr3), the fluorescence emission intensity of Eu3+ located at 699 nm increases gradually and then decreases slowly caused by concentration quenching;8 this phenomenon directly confirmed that the energy transfer of CsPbBr3→Tb3+→Eu3+ occured in the CsPbBr3 host. Upon excitation at 395 nm (Figure 5a), the CsPbBr3:

xTb3+ emits the characteristic emissions merely without doping with Eu3+. When doping with a fraction of Eu3+ (Figure 5h), the fluorescence emission of Eu3+ can be discovered besides the fluorescence emission of Tb3+, and the emission of Tb3+ at 543 nm gradually decreases to zero when increasing Eu3+ ions. However, the emission spectra of Eu3+ ions located at 596 nm and 618 nm increases due to the energy transfer between Tb3+ and Eu3+. It’s worth noting that the emission spectra of Eu3+ gradually became dominant with increasing Eu3+ ions until the doping ratio up to 1:1, then declined because of the concentration quenching


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of Eu3+→Eu3+ . The above consequence further prove that the energy of Eu3+ ions is comes from Tb3+, and the luminescence can be adjusted through altering the relative proportion of Tb3+/Eu3+.41 When the excitation peak located at 460 nm (Figure 5i), the same phenomenon occurs, which further prove the energy transfer relationship between CsPbBr3→Tb3+→Eu3+. In addition, the absorption spectra of the CsPbBr3 glass, Tb3+ doped CsPbBr3 glass, Eu3+ doped CsPbBr3 glass and Tb3+, Eu3+ co-doped CsPbBr3 glass all exhibit intense exciton absorption peaks.42 The obtained dual emission of rare earth cations doped CsPbBr3 glass demonstrated the feasibility of engineering white-emitting perovskite QDs glass, which could provide great potential for white lighting applications.

Figure 6. Detailed schematic for energy transfer

A detailed schematic of the energy transfer among CsPbBr3→Tb3+→Eu3+ in CsPbBr3 host is shown in Figure 6. First, the energy is absorbed by CsPbBr3 under UV irradiation. The electrons in the ground state (1A1) of CsPbBr3 shift to 1B(1T2) level and then a small portion of them drop to the lowest excitation 1B(1T1) level to give out the characteristic emission of CsPbBr3;43 at the same time, other excited electrons transfer energy to Tb3+ and Eu3+. Tb3+ shows characteristic emissions; Tb3+ higher level electrons are relaxed by the polyphonon to the lowest excited level 5D4, and then go back to the ground state, causing the Tb3+ (5D4→7F6-3) emission. In addition, the energy absorbed by Tb3+ can be shift to the higher excited energy of



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Eu3+ (5D1) by dipole-dipole interaction. In the end, the electrons on the 5D1 level relaxes to 5D0 level, producing red emission based on 5D0→7F0-3 transitions of Eu3+ at the same time.44

Figure 7. The luminescence intensity decays of the CsPbBr3: 0.3%Eu3+/0.3%Tb3+ sample and the trend of luminescence intensity decays value (illustration).

Luminescence intensity decay analysis: The energy transfer can be demonstrated through the luminescence decay curves of Eu3+ and Tb3+ doped CsPbBr3 QDs glass. The fluorescence decay kinetics of red emission located at 612 nm and green emission located at 543 nm were studied under excitation condition of 365 nm, the 5D4 lifetime of Tb3+ was 350-550 ns (either the emission of 5D4→7F5 or 5D4→7F6 transitions), whereas the 5D0 lifetime of Eu3+ was also 350-550 ns. Thus, the calculated lifetime values of Eu3+ and Tb3+ can be compared with the results of ions with single doped CsPbBr3 glass samples. Such as the 5D0 lifetimes equaled 1.39 ms for Eu3+ doped CsPbBr3 glass. Similar luminescence decay times with different glass systems were achieved by other researcher.45 The decay curves can be closely matched with the single-exponential function in eq:

I = A exp(

−t

τ

)+B

(1)

where I is the relative intensity at time t, τ is the fluorescence lifetime, t represents the delay time, and A and B are specific constants. Figure S3 shows the lifetime of Tb3+ increased progressively from 363.445


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to 540.526 ns when increasing the doping concentration to 0.5% and then decreased to 470.211 ns up to 0.9%. The former caused by surface species and the removal of luminescence-quenching defects, But the latter mainly caused by increasing effective refractive index of the glass via vital crystallite.46 Slight lifetime elongation through increasing the doping concentration was also detected for the Tb3+ emission.

Figure 7 shows the fluorescence attenuation diagram of different proportions of Tb3+ and Eu3+ co-doped CsPbBr3 QDs glass. The fluorescence attenuation increases with the constantly increased the mass fraction of Tb3+. which conclude that the fluorescence lifetime of the Tb3+ (5D4) decreases due to the intense energy transfer between Tb3+ and Eu3+ through a nonradiative process in the CsPbBr3: xTb3+, yEu3+ glass with increasing Eu3+ ions. the glass samples have lower emission efficiency caused by spontaneous charge transfer process which assign to the large energy level difference (Figure 7). The as-prepared samples doped with Tb3+ or Eu3+ have much slower PL decays compared with those without Tb3+ (or Eu3+), indicating that rare earth cations prevent the CsPbBr3 glass from attenuating too fast.47



Figure 8. The photoelectric parameters (a; LE, b; CCT, c; CRI) of the samples change with time.

Study on the stability of samples: The photoelectric parameter is an imperative element for studying the stability of optical materials. Table S1, S2, S3 shows the value of LE, CCT and CRI over time, In Figure 8a, S represents the rate of decline, indicating the trend of various values over time. The S value of LE for CsPbBr3 glass, Tb3+ doped CsPbBr3 glass, Eu3+ doped CsPbBr3 glass and Eu3+, Tb3+ co-doped CsPbBr3 glass are -0.09448, -0.05555, -0.03001, -0.02131 respectively. Obviously, the glass stability of rare earth


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ions doped samples are better than that of the glass without doping with rare earth cations, this rule is also

suitable for to CCT and CRI (Figure 8b and c). The above data together demonstrated the importance of rare earth doping in improving the stability of CsPbBr3 glass.

Thermal properties regard as an important factor for LED applications. There are various heat sources that influence the luminescent material in a W-LEDs;48 the blue LED chip is the most common heat source when the conversion of electrical power into photons is not fully efficient.49 It is very desirable to inject cesium halide into the glass to increase the performance of the CsPbBr3 in high-power LEDs at elevated operating temperatures. In this part, we performed PL stability evaluations of Tb3+, Eu3+ co-doped CsPbBr3 QDs glass under controlled temperature from 45 ℃ to 245 ℃. We compared the fluorescence intensity at diverse temperatures with the position of emission peak located at 543 nm (Figure S4), it shows a downward trend as the temperature increases. The relative PL strength of the glass was significantly reduced to 67% when the temperature increased to 245 ℃, which was much higher than that of CsPbBr3 QDs and the CsPbBr3 glass.50 The intensity decrease is caused by temperature quenching. The reported quenching energies were calculated from the Arrhenius equation；

IT =

I0 1 + c exp(−

Ea ) kT

(2)

Where I0 is the initial emission intensity, IT is emission spectrum intensity at T ℃, Ea is the thermal quenching activation energy or nonradiative transition barrier, c and k are the coefficients of a specific substrate.51 Which indicate that the nonradiative transition barrier is more likely to occur when doping Tb3+ and Eu3+ into CsPbBr3 QDs. Therefore, under high temperature, thermal quenching is not easy to happen when co-doping with Tb3+ and Eu3+ ions.52



Figure 9. Emission spectra of operating devices of (a) blue chip (b) CsPbBr3 QDs glass (c) Tb3+/Eu3+ co-doped CsPbBr3 QDs glass based LEDs; (d) chromaticity diagram of the corresponding LEDs (inset: photograph of an operating W-LEDs).


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Table 1. Optical parameters of the as-prepared W-LEDs

CIE coordinates Samples

LE(lm/W)

CCT (K)

CRI

x

y

Blue chip

18.91

100000

-54.4

0.1480

0.0357

CsPbBr3 glass

20.18

100000

18.2

0.1121

0.3039

63.21

4945

85.7

0.3335

0.3413

Tb3+/Eu3+ co-doped glass

Performance of Blue chip & rare earth ions doped CsPbBr3 QDs W-LEDs: To demonstrate the possibility of employing our glass samples as green/red emitting materials in LEDs, we assembled a surface-mounted equipment via combining a blue chip, CsPbBr3 QDs glass and Tb3+, Eu3+ co-doped CsPbBr3 QDs glass. The emission curve of the blue-LED chip (Figure 9a) serve as an excitation source, which exhibits a peak located at 460 nm and the value of luminous efficiency is 18.91 lm/W. The CsPbBr3 QDs glass LED has a primary peak located at 500 nm (Figure 9b), which stems from the transition of CsPbBr3 ; a certain blue-shift occurred in the primary peak position when compared with the fluorescence spectra of CsPbBr3 mentioned above (Figure 5a). The emission spectrum of the Tb3+, Eu3+ co-doped CsPbBr3 QDs glass LED (Figure 9c) shows seven distinct peaks. One additional emission peak located at



550 nm originates from Tb3+, but the other peaks caused by Tb3+ transfer are not detected probably caused by too little doping. The other four peaks in Figure 9c are stem from the transition of Eu3+ ions (related detailed photoelectric parameter values are summarized in Table 1). Compared with their PL spectra (Figure 5b), there is a slight deviation in the main peak position because of the aggregation-induced energy commonly detected from nanomaterials.53, 54 The Commission Internationale de L’Eclairage (CIE) color coordinates of the obtained LEDs working under the forward bias voltage of 20 mA, (0.1840, 0.0357), (0.1121, 0.3039), (0.3335, 0.3413) are marked in the CIE chromaticity diagram (Figure 9d), and the illustration shows the white light photograph of Tb3+, Eu3+ co-doped CsPbBr3 QDs glass LED. This results indicated that CsPbBr3: xTb3+, yEu3+ glass is a potential candidate in solid luminescent materials.

CONCLUSION

In summary, a rare earth cations (Tb3+, Eu3+) doped CsPbBr3 QDs glass with tunable luminescence has been synthesized by conventional melt-quenching methods. The as-prepared samples are transparent and predominantly amorphous. green-emitting located at approximately 400-580 nm, which originate from CsPbBr3 and the transition of Tb3+ (5D4→7FJ); the red emission located at 600-750 nm caused by the transition of Eu3+ (5D0→7FJ). Diverse ways are described in this research to explain the energy transfer among CsPbBr3-Tb3+-Eu3+. The sample of CsPbBr3: xTb3+, yEu3+ exhibits strong multicolor emissions from green to red because of the effective energy transfer from Tb3+ to Eu3+ by dipole-dipole interaction. Therefore, we can adjust the luminescence by regulating the doping proportion of Tb3+/Eu3+. Moreover, CsPbBr3: 0.3Tb3+, 0.3Eu3+ QDs glass was fabricated in a surface-mounted equipment based on combination with a blue chip, which indicate that the glass can be regard as a new star in the field of W-LEDs applications.


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ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (Nos. 51472183 and 51672192).

ASSOCIATED CONTENT

Supporting Information: Additional TEM mappings, fluorescence spectra, luminescence intensity decays and temperature-dependent fluorescence spectrum (Figure S1-4) are provided, photoelectric parameter record table are provided (Table S1-3).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: 13957713355

*E-mail: [email protected]; Tel: 13968816126

NOTES

The authors declare no competing financial interest.

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