White Light Emission and Enhanced Color Stability in a Single

Subscriber access provided by Hong Kong University of Science and Technology Library

Functional Inorganic Materials and Devices

White Light-Emitting and Enhanced Color Stability in a Single Component Host Junhao Li, Qiongyun Liang, Jun-Yu Hong, Jing Yan, Leonid Dolgov, Yuying Meng, Yiqin Xu, Jianxin Shi, and Mingmei Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02716 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 7 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

ACS Applied Materials & Interfaces

White Light-Emitting and Enhanced Color Stability in a Single Component Host Junhao Li,†£ Qiongyun Liang,†£ Jun-Yu Hong,† Jing Yan,† Leonid Dolgov,† Yuying Meng,† Yiqin Xu,‡ Jianxin Shi,*† and Mingmei Wu*† † Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun YatSen University, Guangzhou 510275, P. R. China ‡ Guangdong Institute of Semiconductor Industrial Technology, Guangzhou 510650, P. R. China ABSTRACT: Eu3+ ion can be effectively sensitized by Ce3+ ion through an energy transfer chain of Ce3+-(Tb3+)n-Eu3+, which has contributed to the development of white-light emitting diodes (WLEDs) as it can favour more efficient red phosphors. However, simply serving for WLEDs as one of the multi-components, the design of the Ce3+-(Tb3+)n-Eu3+ energy transfer is undoubtedly underused. Theoritically, white light can be achieved with extra blue and green emissions released from Ce3+ and Tb3+. Herein, the design of the white light based on these three multi-color luminescence centers has been realized in GdBO3. It is the first time that white light is generated via accurate controls on the Ce3+-(Tb3+)n-Eu3+ energy transfer in such a widely studied host material. Since the thermal quenching rates of blue, green and red (BGR) emissions from Ce3+, Tb3+ and Eu3+, respectively, are well-matched in the host, this novel white light exhibits superior color stability and potential application prospect. Keywords: White Light-Emitting, Ce3+-(Tb3+)n-Eu3+, Energy Transfer, Color Stability, Single Component Host

1. INTRODUCTION Light sources have been rapidly developed in recent decades owing to the remarkable growth concerned over energy issue. Phosphor-converted white-light emitting diodes (pc-WLEDs) are expected to be promising light sources being available in various fields due to their low energy consumption, long operational lifetime and environmental friendliness, etc.1-3 At present, there are basically two approaches for the design of WLEDs: fabricating a LED chip with either several different color (typically BGR) emitting phosphors or simply a single-phase compound. Since strong reabsorption of the blue light by green and red emitting components is still problematic in the former type of WLEDs, the question of producing white light in a single-phase host has attracted enormous interest.4-8 Rare earth doped phosphors are always popular as their substantial energy levels can provide various color emissions. Active realization of white light emission by means of the rare-earth doped phosphors includes routes described below 9-10: (I) First route implies doping a single rare earth ion into appropriate hosts. White light can be formed from either various narrow line emissions from f-f transition ions such Dy3+ and Eu3+ or from broad band emission from f-d transition ions like Eu2+.11-16 However, the former is trapped in a low emission efficiency due to the inextricable contradiction between doping concentration and cross relaxation. While the latter is much better because of relatively high emission intensity, but it is still problematic owing to deficiency in the red spectrum region. (II) Second route assumes simultaneous doping of

host by various luminescent ions with different emissions. Adjustment of ratio between different color emitters (e.g. Sm3+, Eu3+, Tb3+, Dy3+, Tm3+) can lead to their coordinated white light yield.17-18 However, this white light can be sustained only under specifically designated excitation wavelength. Slight deviations on the excitation position would destroy balance of colors composing the white light. It is because various ions have different sensitivity to spectral changes in exciting light. (III) Third route aims co-doping different ions in one host to control emission color via energy transfer. Utilizing energy transfer for white light generation is more preferable in multi-ion doping system. Activators rather receive energy from the sensitizer than absorb it by themselves. Their excitation spectra would be gradually changed to be consistent with the sensitizer. As ions share the same excitation, deviations on excitation position can hardly affect the white light. Moreover, the intensity of ions with parity-forbidden transitions can be significantly enhanced, when ions with the parity-allowed transition are selected to be the sensitizer. Therefore, utilizing of energy transfer for white light generation remains a hot topic up to now.19-25 Ce3+, Tb3+ and Eu3+ are important activators in traditional phosphors as they can emit blue, green and red light in suitable hosts, respectively. It has also been accepted that energy absorbed by Ce3+ can go through Tb3+, and eventually reach Eu3+ in their tri-doped system.26-27 It means Eu3+ ion (the 4f-4f transition of which is parity-forbidden) can be effectively sensitized by Ce3+ ion (whose 4f-5d transition is parity-allowed). Then Ce3+-(Tb3+)n-Eu3+ energy transfer has been used to develop a variety of red phosphors with

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

excellent near-UV absorption.28-31 This method has made some contribution to the development of WLEDs as it can provide more efficient red emission, however, white light based on the additional red phosphor will still be caught in the trap of multiple-phase constitutes. Thus, here comes a question: can energy be partly released at Ce3+ and Tb3+ (with extra blue and green emissions) to achieve white light? It gained a preliminary answer in 2015 that Zhou et al. first realized white light in BaY2Si3O10 with the dopant of Ce, Tb and Eu.32 Interestingly, the luminescence of Eu3+ exhibited an uncommon growth when temperature increased (generally it should have suffered a persistent decrease). On the one hand such an unexpected growth has seriously influenced the color stability as the appropriate BGR ratio (which is essential for white light) could no longer sustain, but on the other hand it brings us an inspiration that energy transfer could also compensate for excessive decreases which is caused by the imbalanced thermal quenching. Herein, Ce3+-(Tb3+)n-Eu3+ energy transfer is developed to generate white light-emitting in GdBO3. Tb3+ and Eu3+ ions benefitting from the sensitization of Ce3+ exhibit broad band excitation in near-UV region. Adjustment of Tb3+ content coordinates Ce3+-Tb3+ and Tb3+-Eu3+ energy transfer processes, therefore, CIE chromaticity coordinates of the prepared samples can reach (0.31, 0.33) and (0.34, 0.37). Further investigation on the thermal stability of the white light shows that quenching rates of Ce3+, Tb3+ and Eu3+ are well-matched in GdBO3. And so the prepared white lightemitting phosphor exhibits superior color stability as unexpected deviations in CIE coordinates can be largely prevented even when working temperature changes. It means the design of white light and the enhancement of color stability are simultaneously realized, as expected, via Ce3+(Tb3+)n-Eu3+ energy transfer in the present work.

2. EXPERIMENTAL SECTION

Page 2 of 7

quantum yield measurement system. The Raman spectra were acquired on the Renishaw inVia spectrometer.

3. RESULTS AND DISCUSSION GdBO3, which is regarded as an excellent host for luminescent materials, has been widely studied in recent years.33-35 Our previous researches indicate the excitation of Ce3+ is available for WLEDs (Fig. S1, ESI†). Ce3+ doping can provide blue emission, being essential for design of expected white light spectra. Additionally, structure specificity of GdBO3 is beneficial for the coexistence of Ce3+ and Eu3+ ions. (There could be a conflict between the oxidation of Ce3+ and the reduction of Eu3+. However, the emergence of either Ce4+ oxidized from Ce3+ or Eu2+ reduced from Eu3+ must be prevented as it would seriously impact the original design of the spectra.) Coordination environment and lattice size would largely influence the valence state of europium. Sites of Gd3+, coordinated with eight oxygen in this host, are the only available for their substitution for other rare earth ions. Compared with Eu2+ (radii ~ 1.25 Å), Eu3+ (radii ~ 1.07 Å) would replace Gd3+ (radii ~ 1.05 Å) more easily due to the similar radii and valence state. Thus, even in the reductive synthesis atmosphere, Eu3+ will not be reduced to Eu2+. Fig. 1a shows the schematic crystal structure of GdBO3. It is established by gadolinium oxygen polyhedron and boron oxygen polyhedron layers along the c axis. Notice that boron oxygen polyhedrons are not able to isolate gadolinium oxygen polyhedrons in the space, leading to few obstacles in energy transfer. Therefore, the design of white light based on energy transfer makes sense. The XRD pattern of the typical white light-emitting sample is presented in Fig. 1b. Standard parameters of GdBO3 (ICSD#74-1932) have been used for the Rietveld refinement and the refined parameters have been listed in Table S1, ESI†. The result reveals that the substitution of Ce3+, Tb3+ and Eu3+ for Gd3+ did not generate any impurity or induce significant changes in the structure. Even in the increasing temperature up to 975 K, the phase structure of the host does not change, which is confirmed by the temperature dependent XRD patterns as shown in Fig.1c. And enlarged

Sample preparation. GdBO3: Ce3+, Tb3+, Eu3+ phosphors were synthesized by a conventional high temperature solid-state reaction, starting from a mixture containing H3BO3 (A.R.), CeO2 (A.R.), Tb4O7 (A.R.) and Eu2O3 (A.R.) in the given stoichiometric proportions. After being mixed and ground, the mixtures were transferred into alumina crucibles, preheated at 873 K for 1 h. Then, they were sintered at 1173 K for 6 h in the CO reducing atmosphere. Finally, the as-synthesized samples were cooled down to room temperature and ground into powder for further measurements.

patterns within the range of 26.3°-27.1° in Fig.1d show that the diffraction peaks would shift towards smaller angle with the increasing temperature. More information on the variation of the crystal lattice in the GdBO3: 0.02Ce3+, 0.08Tb3+, 0.015Eu3+ phosphor is obtained from the XRD refinement (for details

Sample characterization. The powder X-ray diffraction (XRD) measurements were taken by Rigaku D-Max 2200 X-ray diffraction system with a Cu Ka radiation at 40 kV and 26 mA (λ=1.5405 Å). The photoluminescence emission (PL) spectra, excitation (PLE) spectra and the decay curves were measured by FLS 920-combined Time Resolved and Steady State Fluorescence Spectrometer (Edinburgh Instruments) equipped with Xe/nF/µF lamps. Temperature dependent PL spectra were obtained on the same instrument with a temperature controller. The quantum yield was obtained with a Hamamatsu C9920-03G absolute

In addition to blue emission, green and red ones are also needed for the realization of white light. Tb3+ and Eu3+ can provide typical narrow line emissions in visible region due to their 4f-4f transitions.36-37 Tb3+ ions are green emitters as the color coordinates of the combination of 5D4→7FJ (J = 3, 4, 5, 6) transitions can reach green region in CIE diagram at around (0.32,0.57). As for Eu3+ ions, they play a role of red emitters as their 5D0→7FJ (J = 1, 2, 3, 4) transitions can generate red light with the CIE color coordinates around (0.63,0.35). (The schematic energy level diagram of Ce3+, Tb3+ and Eu3+ in GdBO3 has been shown in Fig. S2, ESI†).

see Table. S2, ESI†). The dependence of cell parameters a, c and V on the increasing temperature in Fig.1e evidences that only slight expansion would take place in the host.


Page 3 of 7

Fig.1 (a) The schematic crystal structure of the host GdBO3, (b) the XRD pattern and Rietveld refinement result of the GdBO3: Ce3+, Tb3+, Eu3+ white light-emitting phosphor, (c) the XRD patterns of GdBO3:0.02Ce3+, 0.08Tb3+, 0.015Eu3+ at different temperature, (d) the enlarged XRD patterns within the range of 26.3°-27.1° and (e) the dependence of cell parameters a, c and V on the increasing temperature.

(a)

where 𝛾𝛾𝐵𝐵 , 𝛾𝛾𝐺𝐺 and 𝛾𝛾𝑅𝑅 are relative ratio parameters of blue, green and red emission, respectively. These relative ratio parameters satisfy the following constraint in GdBO3:

→ 7FJ

(c)

J=3

J=4

J=5

→ 2F5/2 → 2F7/2

Ex = 378 nm

(d)

ET

5D 4

→ 7FJ

5D 4

→ 7FJ

J=4

However, energy transfer breaks the independence of luminescence for different ions. Color variation tendency differs significantly though identical Tb3+ ions were doped, which is shown in Fig.2. Contribution of different luminescence centers is distinguishable in the PL spectra of GdBO3: Ce3+, Tb3+ (Fig.2a) and GdBO3: Tb3+, Eu3+ (Fig.2c). The increases of G/B and R/G ratio (Fig.2b & 2d) indicate that emission color tuned (from blue) to green in GdBO3: Ce3+,xTb3+ while it moved away (from green) to red in GdBO3: yTb3+, Eu3+. Emission color did not rely on the independence luminescence behavior of Tb3+, but depends

5D 4

(b)

4f65d1

𝛾𝛾𝐵𝐵 : 𝛾𝛾𝐺𝐺 : 𝛾𝛾𝑅𝑅 = 4.00: 1.00: 0.90

Then the designed spectra would be obtained as given in Fig. S3, ESI†. It can be realized more easily if there were no energy transfer providing independent intensity of each emitting center. If so, more Ce3+/Tb3+/Eu3+ ions directly introduced would help realize the spectra whenever B/G/R emission were lacking.

Ex = 360 nm ET

J=2 J=3

𝐼𝐼𝑠𝑠𝑠𝑠 = 𝛾𝛾𝐵𝐵 ∙ 𝜑𝜑𝐶𝐶𝐶𝐶 3+ + 𝛾𝛾𝐺𝐺 ∙ 𝜑𝜑 𝑇𝑇𝑇𝑇3+ + 𝛾𝛾𝑅𝑅 ∙ 𝜑𝜑𝐸𝐸𝐸𝐸3+

J=6

The emission of white light must be the linear combination of BGR curves. Thus, based on the CIE chromaticity coordinates of standard white light (0.33, 0.33), the function of wavelength 𝐼𝐼𝑠𝑠𝑠𝑠 can be expressed as:

J=1

𝜑𝜑𝑅𝑅𝑅𝑅 3+ = 𝐼𝐼(𝜆𝜆), ∫ 𝜑𝜑𝑅𝑅𝑅𝑅 3+ = 1

on both Ce3+-Tb3+ and Tb3+-Eu3+ energy transfer processes. Investigations on the transient spectra prove that Ce3+-Tb3+ and Tb3+-Eu3+ energy transfer processes are both governed by dipole-dipole interaction (for details see Fig. S4 & S5, ESI†). Since energy transfer between different luminescence centers cannot be underestimated, further answering the question about white light generation via accurate controls on Ce3+-(Tb3+)n-Eu3+ energy transfer will be extremely essential.

J=5

Emission curves of single-doped Re3+ ions (Re = Ce, Tb, Eu) are relatively stable due to their fixed energy level and the specific crystalline field, therefore, we define each curve as an independent function of wavelength and normalize it according to the integral area:

J=6

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

ACS Applied Materials & Interfaces

Fig. 2 (a) PL spectra of GdBO3: Ce3+, Tb3+, (b) the green to blue ratio variations of GdBO3: 0.02Ce3+, xTb3+ with the inset of the intensity of Tb3+ emission, (c) PL spectra of GdBO3: Tb3+, Eu3+ and (d) the red to green ratio variations of GdBO3: yTb3+, 0.015Eu3+ with the inset of the intensity of Tb3+ emission.



Many existent studies indicate that Tb3+ ion, as the intermediary for energy transfer, would be the key for color control in Ce3+, Tb3+, Eu3+ tri-doped systems.26,29,32 Color varies with different Tb3+ contents, which also works in GdBO3. Slight adjustment on Tb3+ contents caused significant variation in the luminescence spectra, as presented in Fig. 3a. White light has been achieved in GdBO3: 0.02Ce3+, xTb3+, 0.015Eu3+ at the x value of 0.08 and 0.12 with the corresponding CIE chromaticity coordinates of (0.31, 0.33) and (0.34, 0.37), respectively. Variation of CIE chromaticity coordinates in Fig.3b reflects the increase in energy transfer efficiency from Tb3+ ion, which leads to significant changes in Ce3+, Tb3+, Eu3+ emissions of which the B/G/R contributions are converted into percentage and shown in Fig.3c. When Tb3+ content increases, more efficient energy transfer produces persistent enhancement of Eu3+ emission and increases its contribution in the luminescence spectra. However, contribution of Ce3+ emission is just opposite. Therefore, the red to blue ratio will monotonically increase, driving chromaticity coordinates towards red-emitting region in CIE diagram. As for Tb3+ ions, situation would be a little complicated. Tb3+ receives energy from Ce3+, simultaneously, acts as a sub-source for Eu3+. The contribution of Tb3+ emission in PL spectra will reach its max percentage when the receiving energy equals to the energy transferring out. The gamut of color tuning will be seriously limited for the reason that the percentage of green component in PL spectra which is provided from Tb3+ emission are restricted by energy transfer. It also means that white lightemitting is not always realizable with the design of Ce3+(Tb3+)n-Eu3+ energy transfer even though emissions of Ce3+, Tb3+, Eu3+ are coexistent in a single component host. Discussions on the variation path in CIE diagram would further reveal why it seems easy to gain red light but tough to achieve white (for details see Fig. S6, ESI†).

Fig. 3 (a) The PL spectra of GdBO3:0.02Ce3+,xTb3+,0.015Eu3+ (x = 0.04, 0.08, 0.12, 0.16 and 0.20) under 360 nm excitation with the inset of variation tendency, (b) the corresponding CIE diagram and (c) the corresponding BGR ratios. Investigations on the excitation spectra of GdBO3:0.02 Ce3+, 0.12Tb3+, 0.015Eu3+ reveal that both Tb3+ and Eu3+ ions benefit from the sensitization of Ce3+. Excitation spectra monitoring at 410 nm, 542 nm and 592 nm corresponding to the emission of Ce3+, Tb3+ and Eu3+ ions are given in Fig. 4a, 4b

and 4c, respectively. Broad band excitation ranging from 320 nm to 380 nm can be clearly observed. After being normalized, the excitation spectra are found to be overlapped, as shown in Fig. 4d. However, the excitation of Eu3+ seems to be a little different, exhibiting additional peaks at 378 nm and 393 nm. The former corresponds to the 4f-4f transition of Tb3+ while the latter belongs to Eu3+ itself. It means emission at 592 nm is mainly contributed from energy sources of Ce3+, Tb3+ and Eu3+ when the excitation is set to be 360 nm, 378 nm and 393 nm, respectively (For details on the excitation paths see Fig. S7, ESI†). Further studies on the quantum yield and absorption efficiency of the Ce3+ single-doped, Ce3+,Tb3+ co-doped and Ce3+,Tb3+,Eu3+ tridoped samples indicate that white light-emitting is generated at the cost of energy loss. In the presence of Ce3+-Tb3+ and Tb3+-Eu3+ energy transfer, quantum yield of the whole system decreases dramatically from 62.5% to 20.2% while the absorption efficiency varies only slightly, as presented in Fig. 4e. Quantum yield and absorption efficiency of all the Ce3+,Tb3+,Eu3+ tri-doped samples are also measured and given in Fig. 4f. The quantum yield slightly climbs from 13.7% to 24.4% when Tb3+ concentration increases. As for absorption efficiency, it maintains around 52±5% (for details see Fig. S8, ESI†). We deem that higher Tb3+ doping is capable of producing closer distance to both Ce3+ and Eu3+. And the closer distance results in more efficient Ce3+Tb3+-Eu3+ energy transfer, reducing the energy loss. Therefore, the quantum yield exhibits a growth trend.

Fig. 4 The PLE spectra of GdBO3: 0.02Ce3+, 0.12Tb3+, 0.015Eu3+ monitoring at (a) 410 nm, (b) 542 nm, (c) 592 nm, (d) the normalized PLE spectra monitoring at 410 nm, 542 nm and 592 nm, (e) quantum yield and absorption efficiency of GdBO3:0.02Ce3+, GdBO3:0.02Ce3+,0.12Tb3+ and GdBO3:0.02Ce3+,0.12Tb3+,0.015Eu3+ under 360 nm excitation and (f) quantum yield and absorption efficiency of GdBO3:0.02Ce3+,xTb3+,0.015Eu3+ (x = 0.04, 0.08, 0.12, 0.16 and 0.20) under 360 nm excitation.


Page 4 of 7

Page 5 of 7 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

ACS Applied Materials & Interfaces Since phosphors will encounter a decrease of brightness with increasing temperature due to thermal quenching, most of the existing investigations focus mainly on the thermal stability of light output. However, less attention is paid to their color stability which determines how sensitive the CIE chromaticity coordinates of white light are to temperature. Poor color stability leads to unexpected deviations in CIE coordinates and great limitations in WLED applications. Mismatched quenching rates of multi-color emissions from different luminescence centers should be blamed in most cases.32, 38-39 As shown in Fig. 5a & 5b, temperature dependent PL spectra of Eu3+ exhibit a much faster quenching rate than those of Tb3+ when they were individually doped in the same GdBO3 host. If it happens, the relative deficiency of the red light would seriously influence the correlated color temperature (CCT) and the color rendering index (CRI) of the designed white light. Fortunately, it has not happened in the obtained whitelight emitting phosphor. Thermal quenching rates of multi-color emissions, which cover the whole visible region, are basically synchronous in GdBO3 as shown in Fig. 5c. It is worth to notice that thermal stability of Eu3+ luminescence in the tri-doped system is significantly enhanced, as compared with the single-doped cases, while that of Tb3+ decreases further. There seems to be a “temperature enhancement effect”, which balances the thermal quenching between Eu3+ and Tb3+ and improves Eu3+ emission as it is pointed in Fig. 5d.

Fig. 6 Raman spectra of tri-doped GdBO3 (inset: the growth rate of the corresponding phonons) where Δph is the phonon energy and N is the number of phonons. Obviously, the raising temperature can provide more phonons due to higher vibration frequency in host, which drives the above energy transfer processes. One can notice also that the growth in intensity of the spectral peak for the big phonon with Raman shift ~1015 cm-1 is much larger than the small ones (~251, 508 and 825 cm-1) as recorded in the inset of Fig. 6. It also helps to compensate the decrease of thermal quenching (for details see Fig. S7, ESI†). Eventually, higher color stability can be achieved. The color stability can be quantifiably described by the Chromaticity Shift (∆E) using the following equation3: ∆𝐸𝐸 = �(𝑢𝑢𝑡𝑡′ − 𝑢𝑢0′ )2 + (𝑣𝑣𝑡𝑡′ − 𝑣𝑣0′ )2 + (𝑤𝑤𝑡𝑡′ − 𝑤𝑤0′ )2

where 𝑢𝑢′ = 4x/(3 - 2x + 12y), 𝑣𝑣 ′ = 9y/(3 - 2x + 12y), and 𝑤𝑤 ′ = 1 - 𝑢𝑢′ - 𝑣𝑣 ′ . x and y are the chromaticity coordinates. The calculated results are given in Table. S3, ESI† and Fig. 7a. The chromaticity shift of the standard white light-emitting phosphor is about 13.6×10-3 at 425 K. However, the shift of that without energy transfer compensation is calculated to be 26.7 ×10-3, being twice as larger as that at the same temperature. Color variation in CIE diagram is shown in Fig. 7b & 7c.Obviously, the energy-transfer-based phosphors are more reliable for WLEDs (for details see Fig. S8, ESI†).

Fig. 5 (a) Temperature dependent photoluminescence spectra of Tb3+ single-doped GdBO3, (b) Eu3+ single-doped GdBO3, (c) Ce3+,Tb3+,Eu3+ tri-doped white-emitting GdBO3, and (d) the related intensity in different temperature ranging from 300 K to 425 K. Analysis of the schematic energy level diagram (Fig. S2, ESI†) and further investigations on the Raman spectra of tri-doped GdBO3 (Fig. 6) lead us to idea that the multiphonon assisted energy transfer process plays an important role in this energy transfer. The processes could be described as follows: Tb3+(5D3, 4) + Eu3+(7FJ) → Tb3+(7FJ) + Eu3+(5D0,1,2) + N · Δph

Fig. 7 (a) The calculated chromaticity shift, (b) the corresponding color variation and (c) the enlarged CIE diagram with and without the energy transfer compensation.



Page 6 of 7

4. CONCLUSION

REFERENCES

The spectrum of white light which is based on the tri-doping of Ce3+, Tb3+ and Eu3+ ions has been theoretically designed and experimentally realized. The reported GdBO3: 0.02Ce3+, xTb3+, 0.015Eu3+ (x = 0.04, 0.08, 0.12, 0.16 and 0.20) phosphors have satisfactory absorption efficiency (52±5%) and quantum yield (up to 24.4%) under the 360 nm nearUV excitation. Additionally, the single host white lightemitting phosphor, GdBO3:0.02Ce3+, 0.08Tb3+, 0.015Eu3+, exhibits satisfactory color stability. When the temperature increases from 300 to 425 K, the intensity of Tb3+ emission is sacrificed to keep certain yield of Eu3+ emission. Therefore, Tb3+ and Eu3+ ions balance the thermal quenching rates of each other. Such an important phenomenon could be attributed to the multi-phonon assisted energy transfer and expected to be used to develop other energy-transferbased phosphors in the future.

(1) Schubert, E. F.; Kim, J. K. Solid-state light sources getting smart. Science. 2005, 308, 1274-1278. (2) Li, J. H.; Yan, J.; Wen, D. W.; Khan, W. U.; Shi, J. X.; Wu, M. M.; Su, Q.; Tanner, P. A. Advanced red phosphors for white lightemitting diodes. J. Mater. Chem. C, 2016, 4, 8611-8623. (3) Huang, L.; Zhu, Y. W.; Zhang, X. J.; Zou, R.; Pan, F. J.; Wang, J.; Wu, M. M. HF-Free Hydrothermal Route for Synthesis of Highly Efficient Narrow-Band Red Emitting Phosphor K2Si1−xF6:xMn4+ for Warm White Light-Emitting Diodes. Chem. Mater. 2016, 28, 1495−1502. (4) Zhang, J.; Cai, G. M.; Yang, L. W.; Ma, Z. Y.; Jin, Z. P. Layered Crystal Structure, Color-Tunable Photoluminescence, and Excellent Thermal Stability of MgIn2P4O14 Phosphate-Based Phosphors. Inorg. Chem. 2017, 56, 12902−12913. (5) Lin, C. C.; Liu, Y. P.; Xiao, Z. R.; Wang, Y. K.; Cheng, B. M.; Liu, R. S. All-In-One Light-Tunable Borated Phosphors with Chemical and Luminescence Dynamical Control Resolution. ACS Appl. Mater. Interfaces. 2014, 6, 9160−9172. (6) Fernández-Carrión, A. J.; Ocaña, M.; Carcía-Sevillano, J.; Cantelar, E.; Becerro, A. I. New Single-Phase, White-Light-Emitting Phosphors Based onδ‑Gd2Si2O7 for Solid-State Lighting. J. Phys. Chem. C. 2014, 118, 18035−18043. (7) Gautier, R.; Li, X. Y.; Xia, Z. G.; Massuyeau, F. Two-Step Design of a Single-Doped White Phosphor with High Color Rendering. J. Am. Chem. Soc. 2017, 139, 1436−1439. (8) Zhao, Y. W.; Zhang, F. Q.; Zhang, X. M. Single Component Lanthanide Hybrids Based on Metal-Organic Framework for Near-Ultraviolet White Light LED. ACS Appl. Mater. Interfaces 2016, 8, 24123−24130. (9) Shang, M. M.; Li, C. X.; Lin, J. How to produce white light in a single-phase host? Chem. Soc. Rev. 2014, 43, 1372-1386. (10) Li, G. G.; Tian, Y.; Zhao, Y.; Lin, J. Recent progress in luminescence tuning of Ce3+ and Eu2+-activated phosphors for pc-WLEDs. Chem. Soc. Rev. 2015, 44, 8688-8713. (11) Long, J. Q.; Chu, F. J.; Wang, Y. Z.; Zhao, C.; Dong, W. F.; Yuan, X. Y.; Ma, C. Y.; Wen, Z. C.; Ma, R.; Du, M. M.; Cao, Y. G. M8MgSc(PO4)7:xDy3+ (M = Ca/Sr) Single-Phase Full-Color Phosphor with High Thermal Emission Stability. Inorg. Chem. 2017, 56, 10381−10386. (12) Nakajima, T.; Tsuchiya, T. Plant Habitat-Conscious White Light Emission of Dy3+ in Whitlockite-like Phosphates: Reduced Photosynthesis and Inhibition of Bloom Impediment. ACS Appl. Mater. Interfaces. 2015, 7, 21398−407. (13) Li, C. X.; Zhang, C. M.; Hou, Z. Y.; Wang, L. L.; Quan, Z. W.; Lian, H. Z.; Lin, J. β-NaYF4 and β-NaYF4:Eu3+ Microstructures: Morphology Control and Tunable Luminescence Properties. J. Phys. Chem. C, 2009, 113, 2332-2339. (14) Liu, X. M.; Li, C. X.; Quan, Z. W.; Cheng, Z. Y.; Lin, J. Tunable Luminescence Properties of CaIn2O4:Eu3+ Phosphors. J. Phys. Chem. C, 2007, 111, 16601-16607. (15) Tian, M. M.; Li, P. L.; Wang, Z. J.; Li, Z. L.; Cheng. J. G.; Sun, Y. S.; Wang, C.; Teng, X. Y.; Yang, Z. P.; Teng, F. Controlling multi luminescent centers via anionic polyhedron substitution to achieve single Eu2+ activated high-color-rendering white light/tunable emissions in single-phased Ca2(BO3)1-x(PO4)xCl phosphors for ultraviolet converted LEDs. Chem. Eur. J. 2017, 326, 667-679. (16) Shang, M. M.; Liang, S. S.; Qu, N. R.; Lian, H. Z.; Lin, J. Influence of Anion/Cation Substitution (Sr2+ → Ba2+, Al3+ → Si4+, N3− → O2−) on Phase Transformation and Luminescence Properties of Ba3Si6O15:Eu2+ Phosphors. Chem. Mater. 2017, 29, 1813−1829. (17) Li, G. G.; Hou, Z. Y.; Peng, C.; Wang, W. X.; Cheng, Z. Y.; Li, C. X.; Lian, H. Z.; Lin, J. Electrospinning Derived One-Dimensional LaOCl: Ln3+ (Ln = Eu/Sm, Tb, Tm) Nanofibers, Nanotubes

ASSOCIATED CONTENT Supporting Information PL and PLE spectra of GdBO3: Ce3+, the schematic energy level diagram, spectra design of the standard white light, energy transfer interactions calculations, the efficiency of Ce3+ → Tb3+ / Tb3+ → Eu3+ energy transfer, the variation path in CIE diagram, PL spectra of GdBO3: Ce3+,Tb3+,Eu3+, monitoring the excitation at different wavelengths, the application of the GdBO3:Ce3+, Tb3+, Eu3+ phosphor in WLED, the theoretical curves for the rate of phonon-assisted energy transfer , the refined structure parameters of GdBO3:Ce3+, Tb3+, Eu3+ at room temperature, the variation of cell parameters with increasing temperature and the calculated chromaticity shift.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions £ These authors contributed equally and served as co-first authors

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by grants from the Joint Funds of the National Natural Science Foundation of China (NSFC)-Yunnan and Guangdong Provinces (No. U1702254 and U1301242), NSFC (No. 21771195), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20130171130001), the Natural Science Foundation of Guangdong Province (No.2016A030313305), Special Fund of Guangdong Province Project for Applied Science and Technology Research and Development (No.2017B090917001, No.2016B090931007, and 2015B090927002), Science and Technology Planning Project of Guangzhou City (No.201604016005 and 201607010360).


Page 7 of 7 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

ACS Applied Materials & Interfaces and Microbelts with Multicolor-Tunable Emission Properties. Adv. Funct. Mater. 2010, 20, 3446–3456. (18) Cai, G. M.; Yang, N.; Liu, H. X.; Si, J. Y.; Zhang, Y. Q. Singlephased and color tunable LiSrBO3:Dy3+, Tm3+, Eu3+ phosphors for white-light-emitting application. J. Lumin. 2017, 187, 211-200. (19) Liu, W. R.; Huang, C. H.; Yeh, C. W.; Tsai, J. C.; Chiu, Y. C.; Yeh, Y. T.; Liu, R. S. A Study on the Luminescence and Energy Transfer of Single-Phase and Color-Tunable KCaY(PO4)2:Eu2+, Mn2+ Phosphor for Application in White-Light LEDs. Inorg. Chem. 2012, 51, 9636−9641. (20) Khan, S. A.; Zhong, H.; Ji, W. W.; Hao, L. Y.; Abadikhah, H.; Xu, X.; Khan, N. Z.; Agathopoulos, S. Single-Phase White LightEmitting CaxBa(9–x)Lu2Si6O24:Eu2+/Mn2+ Phosphors. ACS Omega. 2017, 2, 6270−6277. (21) Chen, L.; Luo, A. Q.; Zhang, Y.; Liu F. Y.; Jiang, Y.; Xu, Q. S.; Chen, X. H.; Hu, Q. Z.; Chen, S. F.; Chen, K. J.; Kuo, H. C. Optimization of the Single-Phased White Phosphor of Li2SrSiO4: Eu2+,Ce3+ for Light-Emitting Diodes by Using the Combinatorial Approach Assisted with the Taguchi Method. ACS Comb. Sci. 2012, 14, 636−644. (22) Chen, X.; Dai, P. P.; Zhang, X. T.; Li, C.; Lu, S.; Wang, X. L.; Jia, Y.; Liu, Y. C. A Highly Efficient White Light (Sr3,Ca,Ba)(PO4)3Cl:Eu2+, Tb3+, Mn2+ Phosphor via Dual Energy Transfers for White Light-Emitting Diodes. Inorg. Chem. 2014, 53, 3441−3448. (23) Lü, W.; Xu, H. W.; Huo, J. S.; Shao, B. Q.; Feng, Y.; Zhao, S.; You, H. P. Tunable white light of a Ce3+,Tb3+,Mn2+ triply doped Na2Ca3Si2O8 phosphor for high colour-rendering white LED applications: tunable luminescence and energy transfer. Dalton Trans. 2017, 46, 9272-9279. (24) Wen, D. W.; Kato, H.; Kobayashi, M.; Yamamoto, S.; Mitsuishi, M.; Kakihana, M. Site occupancy and luminescence properties of Ca3Ln(AlO)3(BO3)4:Ce3+,Tb3+,Mn2+ (Ln = Y, Gd). J. Mater. Chem. C, 2017, 5, 4578-4583. (25) Li, K.; Shang, M. M.; Lian, H. Z.; Lin, J. Recent development in phosphors with different emitting colors via energy transfer. J. Mater. Chem. C. 2016, 4, 5507-5530. (26) Setlur, A. A. Sensitizing Eu3+ with Ce3+ and Tb3+ to Make Narrow-Line Red Phosphors for Light Emitting Diodes. Electrochem. Solid-State Lett. 2012, 15, J25−J27. (27) Jia, Y. C.; Lü, W.; Guo, N.; Lü, W. Z.; Zhao, Q.; You, H. P. Spectral tuning of the n-UV convertible oxynitride phosphor: orange color emitting realization via an energy transfer mechanism. Phys. Chem. Chem. Phys. 2013, 15, 13810—13813. (28) Zhang, X. G.; Zhou, L. Y.; Pang, Q.; Shi, J. X.; Gong, M. L. Tunable Luminescence and Ce3+ → Tb3+ → Eu3+ Energy Transfer

of Broadband-Excited and Narrow Line Red Emitting Y2SiO5:Ce3+, Tb3+, Eu3+ Phosphor. J. Phys. Chem. C. 2014, 118, 7591−7598. (29) Wen, D. W.; Shi, J. X.; Wu, M. M.; Su, Q. Studies of Terbium Bridge: Saturation Phenomenon, Significance of Sensitizer and Mechanisms of Energy Transfer, and Luminescence Quenching. ACS Appl. Mater. Interfaces. 2014, 6, 10792−10801. (30) Yano, K.; Takeshita, S.; Iso, Y.; Isobe, T. Combinatorial optimization of the atomic compositions for green-emitting YBO3:Ce3+,Tb3+ and red-emitting YBO3:Ce3+,Tb3+,Eu3+ phosphors using a microplate reader. RSC Adv. 2017, 7, 17586-17592. (31) Huo, J. S.; Dong, L. P.; Lü, W.; Shao, B. Q.; You, H. P. Novel tunable green-red-emitting oxynitride phosphors co-activated with Ce3+, Tb3+, and Eu3+: photoluminescence and energy transfer. Phys. Chem. Chem. Phys. 2017, 19, 17314-17323. (32) Zhou, J.; Xia, Z. G. Luminescence color tuning of Ce3+, Tb3+ and Eu3+ codoped and tri-doped BaY2Si3O10 phosphors via energy transfer. J. Mater. Chem. C. 2015, 3, 7552-7560. (33) Li, Z. Q.; Zeng, Y. B.; Qian, H. S.; Long, R.; Xiong, Y. J. Facile synthesis of GdBO3 spindle assemblies and microdisks as versatile host matrices for lanthanide doping. CrystEngComm, 2012, 14, 3959-3964. (34) Grzyb, T.; Kubasiewicz, K.; Szczeszak, A.; Lis, S. Synthesis and spectroscopic properties of Yb3+ and Tb3+ co-doped GdBO3 materials showing down- and up-conversion luminescence. Dalton Trans. 2015, 44, 4063–4069. (35) Leng, Z. H.; Zhang, N. N.; Liu, Y. L.; Li, L. L.; Gan, S. C. Controlled synthesis of different multilayer architectures of GdBO3:Eu3+ phosphors and shape-dependent luminescence properties. Appl. Surf. Sci. 2015, 330, 270-279. (36) Khan, W. U.; Li, J. H.; Li, X. H.; Wu, Q. L.; Yan, J.; Xu, Y. Q.; Xie, F. Y.; Shi, J. X.; Wu, M. M. Efficient energy transfer and luminescence properties of Ca3Y(GaO)3(BO3)4:Tb3+,Eu3+ as a green-tored colour tunable phosphor under near-UV excitation. Dalton Trans. 2017, 46, 1885–1891. (37) Li, J. H.; Zhang, Z. H.; Li, X. H.; Xu, Y. Q.; Ai, Y. Y.; Yan, J.; Shi, J. X.; Wu, M. M. Luminescence properties and energy transfer of YGa1.5Al1.5(BO3)4:Tb3+,Eu3+ as a multi-colour emitting phosphor for WLEDs. J. Mater. Chem. C, 2017, 5, 6294-6299. (38) Geng, D. L.; Lian, H. Z.; Shang, M. M.; Zhang, Y.; Lin, J. Oxonitridosilicate Y10(Si6O22N2)O2:Ce3+,Mn2+ Phosphors: A Facile Synthesis via the Soft-Chemical Ammonolysis Process, Luminescence, and Energy-Transfer Properties. Inorg. Chem. 2014, 53, 2230−2239. (39) Zhang, J. L.; He, Y. N.; Qiu, Z. X.; Zhang, W. L.; Zhou, W. L.; Yu, L. P.; Lian, S. X. Site-sensitive energy transfer modes in Ca3Al2O6:Ce3+/Tb3+/Mn2+ phosphors. Dalton Trans. 2014, 43, 18134–18145.

When the temperature increases from 300 to 425 K, the intensity of Tb3+ emission is sacrificed to keep certain yield of Eu3+ emission. Therefore, co-doped rare earth ions balance the thermal quenching rates of each other to provide white light with superior color stability.

Insert Table of Contents artwork here ACS Paragon Plus Environment

White Light Emission and Enhanced Color Stability in a Single

Recommend Documents