Eu3+ (Sm3+) tri-activated Y2WO6 as one potential single-phase phosphor for WLEDs

Accepted Manuscript 3+ 3+ 3+ 3+ Tm /Dy /Eu (Sm ) tri-activated Y2WO6 as one potential single-phase phosphor for WLEDs Umer Farooq, Zhi Zhao, Zhilei Sui, Chan Gao, Rucheng Dai, Zhongping Wang, Zengming Zhang PII:

S0925-8388(18)34212-9

DOI:

https://doi.org/10.1016/j.jallcom.2018.11.091

Reference:

JALCOM 48317

To appear in:

Journal of Alloys and Compounds

Received Date: 29 August 2018 Revised Date:

6 November 2018

Accepted Date: 8 November 2018

3+ 3+ Please cite this article as: U. Farooq, Z. Zhao, Z. Sui, C. Gao, R. Dai, Z. Wang, Z. Zhang, Tm /Dy / 3+ 3+ Eu (Sm ) tri-activated Y2WO6 as one potential single-phase phosphor for WLEDs, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.091. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Tm3+/Dy3+/Eu3+ (Sm3+) tri-activated Y2WO6 as one potential single-phase phosphor for WLEDs Umer Farooq1, Zhi Zhao2*, Zhilei Sui1, Chan Gao1, Rucheng Dai3, Zhongping Wang3 and Zengming Zhang3,4**

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1. Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 2. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

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3. The Centre for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China

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4. Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China Corresponding authors: [email protected]*, [email protected]** ABSTRACT

Pure phase Tm3+, Dy3+, Sm3+ and Eu3+ co-activated Y2WO6 single crystal phosphors are synthesized through a hydrothermal method. Upon the excitation of 311 nm, Tm3+, Dy3+, Sm3+ and Eu3+ show tremendous emission properties in their corresponding regions. The optimum

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concentration of Tm3+ for Y2WO6 phosphors is measured to be around 4%. The energy transfer efficiency from Tm3+ to Dy3+ increases gradually with increasing concentration of Dy3+, and reaches up to 72% at 10%Dy3+. The critical distance between Tm3+ and Dy3+ is calculated to be 16.77 Å. Hence, dipole-dipole interaction dominates the mechanism of energy transfer. The color

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tone of Y2WO6: Tm3+, Dy3+ phosphors was effectively tuned to warm white light by co-doping Sm3+ or Eu3+. Furthermore, the energy transfer phenomenon from Tm3+, Dy3+ to Sm3+ and Eu3+ is

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investigated. The photoluminescence properties of Y2WO6 (Tm3+, Dy3+, Sm3+ or Eu3+) phosphors renders them potential candidates for ultra-violet excited white light emitting diodes. Keywords: Y2WO6 phosphors, Photoluminescence, Hydrothermal synthesis, Energy transfer, 280 nm UV chip based WLED, Thermal stability.

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1. Introduction White light emitting diodes (WLEDs) have many advantages, they have small volumes, high luminescence efficiency, low energy consumption, long lifetime, high safety coefficients, and are environment friendly [1–4]. The most common method for making WLEDs is to combine yellow

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YAG: Ce3+ phosphors with a blue LED chip, but this technique has some disadvantages, such as low color rendering indices, efficiency and chromatic stability under different driving currents [5,6]. To overcome these shortcomings, Ultraviolet (UV) LED chips coated with tri-color (red, green and blue) phosphors have been proposed [7,8]. However, the main shortcoming of these

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WLEDs is their low luminescence efficiency due to the strong absorption of blue emission by red and green phosphors [9,10]. Red phosphors suffer from their chemical instability and low

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brightness [11]. Moreover, multi-color phosphors are not easy to make because phosphors are prepared individually, the particle size of the individual phosphor must be adapted to one another to avoid agglomeration or sedimentation, and the final product must be mixed homogeneously in exact ratios [12,13]. Some works focused on single-phased phosphors in view of their higher luminescence efficiency, lower manufacturing costs, easier fabrication processes, higher color rendering index and better reproducibility [14–18]. The UV-LEDs coated with single-phased

multiphase phosphors.

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phosphors are very promising candidates to make WLEDs as they have many advantages over

Host materials dominate the luminescent properties of phosphors. Oxide hosts have been widely applied in different fields because of their thermal stability, chemical durability, lower

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phonon frequencies, non-toxic and non-hygroscopic nature [19]. Among oxide hosts, rare-earth tungstate and molybdate micro and/or nano-sized materials have received much attention in the

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past few years. Rare-earth tungstate and molybdate matrix themselves emit blue-green light; white light can be generated by doping lanthanide ions [20]. Y2WO6 possesses good self-activating luminescent properties, chemical and thermal stability [21]. There are three different lattice sites for Y3+ in Y2WO6, which are suitable for rare earth ions doping [22,23]. Among rare-earth activated phosphors, Tm3+ provides the blue emission at 455 nm from the transition 1D2→3F4, and relatively weak red emission as well [24]. Dy3+ ions have two major emission bands in blue and yellow regions at about 480 nm and 575 nm, corresponding to 4F9/2→6H15/2 and 6H13/2 transitions, respectively. Co-doped Tm3+ and Dy3+ can

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enhance the blue portion and therefore color purity of white light. White light emission has been achieved by co-doping Tm3+ and Dy3+ in host materials such as MgIn2P4O14, β-NaLuF4, Ba0.05Sr0.95MoO4 and Y5O4F7 [25–28]. Recently, color temperature tunable white light emission in Ln3+:Y2WO6 (Ln3+=Dy, Tb, Eu, Sm) has been obtained. In addition, excitation wavelength,

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heat treatment and doping ions can be used to modify the emission color of these phosphors [29]. In this work, Tm3+/ Dy3+ co-doped and Tm3+/ Dy3+/ Sm3+or Eu3+ tri-doped Y2WO6 phosphors are prepared through a hydrothermal method. The effects of Sm3+or Eu3+ ions on luminescent properties of single-phased host-sensitized Y2WO6: Tm3+, Dy3+ phosphors are also investigated.

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The efficiency and mechanism of energy transfer between Tm3+and Dy3+ are discussed. Furthermore, a series of warm white light emitting phosphors based on efficient resonance-type

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energy transfer process Tm3+-Dy3+ and Tm3+-Dy3+-Sm3+ or Eu3+ is also elaborated. 2. Experimental section

Analytical grade chemical reagents were used without further purification for our experiment. All samples were synthesized through a hydrothermal method in the presence of cetyltrimethyl ammonium bromide (CATB) [30]. Initially, 0.5 mmol CATB was added into 40 ml of distilled water with vigorous stirring at 70 oC. After 15 minutes, the temperature of the solution decreased

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to 40 oC. Then, 1 mmol of rare-earth nitrates with dopants (RE(NO3)3.6H2O, RE = Y, Tm, Dy, Sm, Eu) at right percentage and 0.5 mmol sodium tungstate dihydtrate (Na2WO4.2H2O) were added into the solution. The solution was stirred for about 20 minutes until it is turbid. The pH

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value of the solution was adjusted to 9 by adding ammonia solution. The solution was transferred to autoclave of capacity 50 ml. The autoclave was heated to 180 oC for 24 hours and then cooled at room temperature. In order to remove the impurities, the precipitate on the bottom of Teflon

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cup was washed three times with distilled water and ethanol, respectively. The product was dried in a vacuum oven at 80 oC for 12 hours and further heated to 1100 oC for three hours. 2.1. Characterization

The crystal structure and phase formation of the synthesized powders were determined by X-ray diffraction (XRD) at room temperature on a Rigaku smart lab diffractometer, using Cu Kα irradiation (λ=1.5419 Å). The surface morphology and particle size of the powders were observed with a Hitachi SU8200 field emission scanning electron microscope (SEM). The transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high 3

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resolution transmission electron microscopy (HRTEM) were performed on JEOL JEMARM200F. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra were measured using a Horiba fluorescence spectrometer. Temperature-dependent PL was performed on the LabRAM HR800 Raman spectrometer excited by 325 nm laser. The luminescence decay

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curves were measured using spectrofluorometer (JOBIN YVON, FLUOROLOG-3-TAU). 3. Results and discussion 3.1. Phase identification and morphology

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Fig. 1 shows the XRD patterns of Y2WO6 samples with different doped lanthanide ions. All diffraction peaks are in good agreement with monoclinic structure from the standard data of Y2WO6 (PDF#73-0118). No impurity phases are observed, implying that the Tm3+, Dy3+, Sm3+

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and Eu3+ ions completely dissolved into the Y2WO6 and their presence does not cause any significant changes to the crystal structure. The SEM image of Y2WO6: 2% Tm3+, 2% Dy3+ sample reveals the fact that irregular shaped nano-building blocks have good dispersibility and vary in size ranging from 100 nm to 1.9 µm, as shown in Fig. 2.

Some fissures in Fig. S1 (a) suggest that an individual rod-like crystal consists of several blocks.

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The inset of Fig. 2 reveals the SAED pattern of individual rod-like structure, indicating highly crystalline nature of Y2WO6. The pure phase of Y2WO6 is also verified by planes (014), (115) corresponding to interplanar spacing 2.4 Å and 2.08 Å, respectively, as shown in Fig. S1 (b). The excitation and emission spectra of Y2WO6: 2% Tm3+ are illustrated in Fig. 3 (a). The

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excitation spectrum consists of a strong broad band at 311 nm (corresponding to the charge transfer absorption from 2p orbitals of oxygen to 5d orbitals of tungsten) and a week peak at 360

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nm (attributed to 3H6→1D2 transition of Tm3+) [31]. It provides the evidence that energy transfer from host to dopant (Tm3+) [32]. The strong absorption peak matches well with UV-LED chips with emission ranging from 255 to 355 nm [33–36]. The Y2WO6: 2%Tm3+ phosphor shows broad emission from host and intense blue emission from 1G4→3H6 transition of Tm3+ with an excited wavelength of 311 nm [37]. The emission spectra of Y2WO6: x% Tm3+ (x = 2, 4, 6, 8 and 10) phosphors excited at 311 nm are shown in Fig. 3 (b). The emission intensity reaches its maximum at x = 4% and then decreases due to concentration quenching. CIE chromaticity coordinates of Y2WO6: 4%Tm3+

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excited under 311 nm were calculated to be (0.1516, 0.1545), which are located in the blue region. The other CIE chromaticity coordinates for Y2WO6: x%Tm3+ (x = 2, 6, 8 and 10) excited under 311 nm are very close to Y2WO6: 4%Tm3+. Fig. 4 shows the fluorescence decay lifetimes for Y2WO6: Tm samples with different

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concentration of Tm3+. Fig. 4 (a) is the response to the lifetime curves of the emission at 435 nm for Y2WO6: x%Tm3+ (x = 2, 4, 6, 8 and 10) (λex = 284 nm), and Fig. 4 (b) for the emission of 486 nm from Tm3+ (λex = 311 nm). The luminescence decay curves can be well fitted by an exponential function as Eq. (1).

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ି௧

‫ܫ = ܫ‬௢ exp ቀ ቁ ఛ

(1)

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Where I and Io are luminescence intensities at times t and 0, respectively. τ is the luminescence lifetime. Fig. 4 (a) shows declination in the lifetime of host up to 10%Tm3+, which is the evidence suggesting that host continuously transfers its energy to the activator. In Fig. 4 (b), lifetime of Tm3+ decreases with increasing concentration, which may be caused by multi-phonon relaxation or concentration quenching [38,39]. The prior can be explained by Auzel’s model [38] but our experimental data cannot be fitted to it. When the concentration of Tm3+ increases, the

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distance between ions may decrease to a critical value, and the surplus energy is transferred from an activator to the non-radiative centers. So the decrease of lifetime with increasing concentration confirms the enhanced non-radiative transition among adjacent Tm3+ ions. In order to determine the type of multipolar interactions responsible for energy transfer

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process as between adjacent Tm3+ ions, the relationship between emission intensity (I) per

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activator and ions concentration (x) based on Van Uiterts, theory [40] is given as Eq. (2). ூ

ఏ

lg ቀ௫ቁ = ‫ ܭ‬− ଷ lgሺ‫ ݔ‬ሻ

(2)

Where K is a constant, and θ is equal to 6, 8 and 10 corresponding to dipole-dipole, dipolequadrupole and quadrupole-quadrupole interactions, respectively. Fig. 5 shows the relationship between lg(I/x) and lg(x) for x > 4. The value of θ close to 8 reveals the fact that dipolequadrupole interaction is responsible for concentration quenching of Tm3+ emissions in Tm: Y2WO6. Fig. 6 (a) shows the excitation spectra of Y2WO6: 2% Dy3+ phosphors for the emission at 575 nm corresponding to the 4F9/2→6H13/2 transition of Dy3+ ion. Similar to Y2WO6: 2% Tm3+, 5

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the strong broad band from 250 to 345 nm with a maximum at 311 nm is ascribed to the combine charge transfer band of WO6-6 groups and energy transfer from WO6-6 group to Dy3+. Some sharp lines between 345 nm and 500 nm respond to the transition from 6H15/2 ground state to different excited states of Dy3+ , i.e., 352 nm (6P7/2), 368 nm (6P5/2), 387 nm (4I13/2), 428 nm (4G11/2), 450

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nm (4I15/2) and 480 nm (4F9/2), respectively. Y2WO6: 2% Dy3+ phosphors with excitation of 311 nm emit blue, yellow and red luminescence, as shown in Fig. 6 (a), corresponding to the 4

F9/2→6H15/2 (480 nm), 4F9/2→6H13/2 (575 nm) and 4F9/2→6H11/2 (671 nm) transitions of Dy3+ ions,

respectively.

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The 4F9/2→6H15/2 transition is a magnetic dipole transition, so it hardly varies with coordination environment around Dy3+ ions or the crystal field strength. But 4F9/2→6H13/2

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transition (∆J=2) is an electric dipole transition, which is highly sensitive to the chemical environment around Dy3+ ions in the host lattice [41].

Fig. 6 (b) shows the emission spectra of Y2WO6: x%Dy3+ (x = 0.5, 2, 4, 6, 8 and 10) phosphors exited at 311 nm. The emission intensities firstly increase with Dy3+ ion concentration and reach their maximum at x = 2%, and then decrease due to concentration quenching. Figs. S2 (a-b) and Fig. S3 have a similar explanation as the one for Tm3+, but θ being close to 6 indicates

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that concentration quenching is caused by dipole-dipole interaction. The comparison between the PL spectrum of Y2WO6: 2%Tm3+ and the PLE spectrum of Y2WO6: 2%Dy3+ reveals a significant spectral overlap between the emission and excitation

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spectra, as shown in Fig. 7. Consequently, an effective resonance energy transfer can be expected to occur in Y2WO6: Tm3+, Dy3+ from Tm3+ to Dy3+.

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As an explanation of energy transfer mechanism from Tm3+ to Dy3+, a schematic diagram is shown in Fig. S4. The excitation at 311 nm should cause absorption of energy corresponding to O2- →W6+ charge transfer in the host. The tungstate group transfers part of this energy to Tm3+ ions, and the electrons of Tm3+ accumulate at 1G4 excited state by non-radiative transition from the higher level 1D2. Finally the blue emission is yielded through the 1G4→3H6 transition, and Tm3+ ions at excited state also transfers its energy to Dy3+ via cross relaxation due to the close levels between the 1G4 level of Tm3+ and 4F9/2 of Dy3+, as represented in Fig. S4. Meanwhile the tungstate group also shares its energy with 6P7/2 excited manifolds of Dy3+ions, then electrons populate at the 4F9/2 excited state via non-radiative transition from 6P7/2 to 4F9/2. Then Dy3+ emits 6

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light at 478 nm, 572 nm and 671 nm. A series of Y2WO6: 2%Tm3+, x%Dy3+ (x = 0.5, 2, 4, 6, 8 and 10) samples have been synthesized for further investigation of the energy transfer process from Tm3+ to Dy3+, and the effects of doping concentrations on the luminescence properties of phosphors. Fig. 8 displays the

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PL spectra of Tm3+, Dy3+codoped Y2WO6 phosphors under 311 nm. While Tm3+ is fixed to 2%, the concentration of Dy3+ varies from 0.5 to 10%, and the observed yellow emission from Dy3+ firstly increases, arrives the maximum at x = 2% and then gradually decreases with increasing doping concentration. Fig. 9 (a) shows the intensity of Tm3+ in Y2WO6: 2%Tm3+, x%Dy3+ system

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plotted as a function of Dy3+ concentration. It is very clear from Fig. 9 (a) that intensity of Tm3+ decreases with increasing Dy3+ concentration provides evidence that energy transfers from Tm3+

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to Dy3+ [35].

In order to provide another proof of energy transfer from the sensitizer to the activator, the fluorescence decay lifetime measurements were conducted for Tm3+ emission (λex = 311 nm, λem = 486 nm) with increasing Dy3+ concentration in Y2WO6: 2%Tm3+, x%Dy3+ samples. The decay curves of Tm3+ emission in Y2WO6: 2%Tm3+, x%Dy3+ samples are presented in Fig. S5. These results show that the lifetime of Tm3+ decreases with increasing concentration of Dy3+, which

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strongly implies the existence of energy transfer from Tm3+ to Dy3+ ions. The energy transfer efficiency ηT from the sensitizer Tm3+ to the activator Dy3+ in Y2WO6 is estimated as a function of concentration of Dy3+ ions, as shown in Fig. 9 (b). A simple equation

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(3) can be used to calculate the energy transfer efficiency [17]. ߟ் = 1 − ቀ

ఛೞ

ఛೞ೚

ቁ

(3)

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Where τso is the lifetime of the intrinsic sensitizer and τs is the lifetime of the sensitizer with an activator. It is evident from the Fig. 9 (b) that with increase in Dy3+ concentration, the energy transfer efficiency gradually increases up to 72% at 10% Dy3+ (311 nm used as an excitation wavelength).

The energy transfer mechanism from Tm3+ to Dy3+ can be determined by calculating the critical distance between the sensitizer and the activator. When the concentration of Dy3+ ions increases, the distance between Tm3+ and Dy3+ decreases, and thus the probability of energy transfer increases. But if the distance becomes small enough, the emission from the energy 7

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transfer is suppressed, due to the concentration quenching. Therefore, the critical distance (Rc) for energy transfer from Tm3+ to Dy3+ can be calculated by using the following Blasse’s formula (4) [42]. ଷ௏

ସగ௑೎ ே

ଵ/ଷ

ቁ

(4)

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ܴ௖ = 2 ቀ

Where V, N and Xc are the volume of a unit cell, number of host cations in the unit cell and the critical concentration, respectively. For Y2WO6 in the present study, V = 445.2 Å3, N = 4 and Xc = 0.04. The value of critical distance is calculated to be about 16.77 Å. Generally speaking, there

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exist two mechanisms for energy transfer from the sensitizer to the activator: one is exchange interaction and the other is multipolar interaction. Energy transfer through exchange interaction

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requires a critical distance of less than 5 Å. However, for the case of Y2WO6, the estimated critical distance is 16.77 Å, much greater than the expected value of Rc, implying a small possibility of exchange interaction. Therefore, we conclude that energy transfer from Tm3+ to Dy3+ can occur through multipolar interactions including dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions. Based on Dexter’s formula of energy transfer through multipolar interaction and Reisfeid’s approximation, the following equation (5) can be obtained

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[43].

ቀ

ఛೞ೚ ఛೞ

ቁ ∝ ‫ ܥ‬஑/ଷ

(5)

Where τso is the lifetime of the intrinsic sensitizer and τs is the lifetime of the sensitizer with an

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activator. C is the concentration of Dy3+, α is the interaction mechanism between rare-earth ions, and α = 6, 8 and 10 corresponds to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole

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interactions, respectively. The dependence of τso /τs of Tm3+ on Cα/3 (α = 6, 8 and 10) are plotted in Fig. S6. The linear relationship of τso /τs versus Cα/3 is well fitted at α = 6, which shows that the energy transfer mechanism from Tm3+ to Dy3+ in Y2WO6 is in line with a dipole-dipole interaction.

The CIE chromaticity coordinates of Y2WO6: 2% Tm3+, x% Dy3+ (x = 0.5, 2, 4 and 6) excited by 311 nm light, were determined on the basis of their PL spectrum, and the calculated values are (0.2357, 0.2516), (0.2691, 0.2891), (0.2759, 0.2969), (0.2873, 0.3059), respectively (as shown in Fig. S7).

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In order to adjust the color tone, Sm3+or Eu3+ was tri-doped in Y2WO6: 2% Tm3+, 2% Dy3+. The PL spectra of Y2WO6: 2%Tm3+, 2%Dy3+, x%Sm3+ or Eu3+, excited under 311 nm, are presented in Fig 10. It is clear that the PL spectra consist of blue emission of Tm3+ (1G4→3H6), blue-yellow emission of Dy3+ (4F9/2→6H15/2 and 4F9/2→6H13/2) and green-orange-red emission of

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Sm3+ (4G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2). Furthermore, it was found that blue and yellow emission of Tm3+ and Dy3+ decreases with increasing Sm3+ concentration. Meanwhile, the emission intensity of Sm3+ (4G5/2→6H9/2) transition increases gradually up to 4% concentration, and then decreases due to concentration quenching, as shown in Fig. 10 (a) . Similar results are

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obtained when Eu3+ was doped in Y2WO6: 2%Tm3+, 2%Dy3+. However, emission intensities of Eu3+ (5D0→7F2) are found to be stronger than Sm3+ (4G5/2→6H9/2), as shown in Fig. 10 (b).

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The CIE chromaticity coordinates of Y2WO6: 2%Tm3+, 2%Dy3+, x%Sm3+ or Eu3+, excited by 311 nm light, were determined on the basis of their PL spectrum, and the calculated values are listed in Table 1, along with their positions presented in Fig. 11. The CIE diagram Fig. 11 (a) shows that points (5, 6, 7, 8 and 9) are concentrated on the white region, while in the case of Eu3+ Fig. 11 (b), the points (c and d) lie closer to the red region. So here it can be concluded that Eu3+ provides a more enhanced red component than that of Sm3+ in Y2WO6: Tm3+, Dy3+ phosphors.

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Furthermore, warm white light emitting phosphors are successfully synthesized by controlling the concentration of rare-earth ions.

Thermal stability plays vital role in the application of phosphors because of its significant effect on efficiency and the CIE value. The PL spectra of Y2WO6: 2%Tm3+, 2%Dy3+, 0.5%Eu3+

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from 25 °C to 200 °C are measured, and the declining intensity with increasing temperature is shown in Fig. 12 (a). Fig. 12 (b) reveals that the thermal quenching temperature (50% of PL

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intensity at room temperature) is achieved at higher than 200 °C [44]. This value is higher than the reported result of Y2WO6: Eu3+ [30]. It provides the evidence that phosphor have good thermal stability.

In order to take the advantage of promising PL properties and good thermal stability, a WLED is fabricated by using Y2WO6: 2%Tm3+, 2%Dy3+, 0.5%Eu3+ phosphor and a 280 nm UV chip. The following Figs. 13 (a-b) show one as-prepared WLED and its bright emission at 50 mA. Warm white light is suggested by color temperature 3220 K and CIE color coordinates (0.411, 0.375), as shown in Fig. 13 (c) [45]. Electroluminescence (EL) spectra at 50 mA show

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blue, yellow, red emission corresponding to 480 nm, 575 nm and 610 nm in Fig. 13 (d), respectively. It is expected that the properties of this WLED can be further improved by optimizing LED chip performance, maximum excitation wavelength, and luminescence

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properties of phosphors. 4. Conclusions

A series of color tunable Tm3+, Dy+3, Sm3+ and Eu3+ co-activated Y2WO6 phosphors were synthesized through a hydrothermal method. An efficient and strong energy transfer process occurs via dipole-dipole mechanism from Tm3+ to Dy3+ in Y2WO6: Tm3+, Dy3+ phosphors. The

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maximum energy transfer efficiency from Tm3+ to Dy3+ was about 72%. Additionally, in Y2WO6: Tm3+, Dy3+, Sm3+ / Eu3+ phosphors, energy is efficiently transferred from Tm3+, Dy3+ to Sm3+ /

potential applications in WLEDs. Acknowledgement

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Eu3+. It is evident from the warm white light obtained by three ways that these phosphors have

This work was supported by the Science Challenge Project (No. TZ2016001) and the National

Conflict of interest

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Natural Science Foundation of China (Grant Nos. 11404320 and 11304300).

All the authors declare that they have no conflict of interest and they are responsible for content

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and writing of paper.

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Figure captions Fig. 1 XRD patterns of (a) Y2WO6: 2% Tm3+, 2% Dy3+, (b) Y2WO6: 2% Tm3+, 2% Dy3+ , 4%

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Sm3+, (c) Y2WO6: 2% Tm3+, 2% Dy3+, 4% Eu3+ samples, as well as the standard data of Y2WO6 (PDF#73-0118). Fig. 2 SEM image. Inset: SAED pattern.

Fig. 3 (a) PL and PLE spectra for Y2WO6: 2%Tm3+, (b) PL emission spectra of Y2WO6: x%Tm3+

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samples with different Tm3+ concentrations (x = 2, 4, 6, 8 and 10).

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Fig. 4 Decay curves of (a) Emission at 435 nm from host, (b) Tm3+ emission in Y2WO6: x%Tm3+ (x = 2, 4, 6, 8 and 10).

Fig. 5 Relationship between lg(I/x) and lg(x) in Y2WO6: x%Tm3+ (x = 4, 6, 8 and 10). Fig. 6 (a) PL and PLE spectra for Y2WO6: 2%Dy3+, (b) PL emission spectra of Y2WO6: x%Dy3+ samples with different Dy3+ concentrations (x = 0.5, 2, 4, 6, 8 and 10). Fig. 7 Spectral overlap between PL spectrum of Y2WO6: 2%Tm3+ (red curve) and PLE spectrum

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of Y2WO6: 2%Dy3+ (black curve).

Fig. 8 PL emission spectra of Y2WO6: 2%Tm3+, x%Dy3+ samples with different Dy3+ concentrations (x = 0.5, 2, 4, 6, 8 and 10).

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Fig. 9 (a) Dependence of emission intensity Tm3+ on Dy3+ concentration in Y2WO6: 2%Tm3+, x%Dy3+ (x = 0.5, 2, 4, 6, 8 and 10), (b) Dependence of energy transfer efficiency ηT on doped

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Dy3+ concentration in Y2WO6: 2%Tm3+, x%Dy3+ (x = 0, 4, 6, 8 and 10) phosphors. Fig. 10 PL emission spectra of Y2WO6: 2%Tm3+, 2%Dy3+and (a) x%Sm3+ samples with different Sm3+ concentrations (x = 0.5, 2, 4, 6 and 8), (b) x%Eu3+ samples with different Eu3+ concentrations (x = 0.5, 1, 4 and 6). Fig. 11 The CIE chromaticity diagram of (a) Y2WO6: 2%Tm3+, 2%Dy3+, x%Sm3+ (x = 0, 0.5, 2, 4, 6 and 8), (b) Y2WO6: 2% Tm3+, 2% Dy3+, x% Eu3+ (x = 0, 0.5, 1, 4 and 6) excited under 311 nm. Fig. 12 PL emission spectra of Y2WO6: 2%Tm3+, 2%Dy3+, 0.5%Eu3+ (a) Temperature dependent 16

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ranging from 25 °C to 200 °C, (b) Relative intensity dependent on temperature. Fig. 13 Photographs of WLED (280 nm UV chip encapsulated with Y2WO6: 2%Tm3+, 2%Dy3+, 0.5%Eu3+ phosphors) (a) As prepared, (b) Operated at 50 mA, (c) CIE diagram, (d) EL spectra at

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Table 1. Comparison of CIE chromaticity coordinates (x, y) for Y2WO6: Tm3+, Dy3+, Sm3+ and Eu3+ phosphors. Point number Sample Y2WO6: Tm3+, Dy3+, Sm3+ and Eu3+ CIE (x, y) 2% Tm3+, 2% Dy3+

(0.2691, 0.2891)

5

2% Tm3+, 2% Dy3+, 0.5% Sm3+

(0.2856, 0.2979)

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2% Tm3+, 2% Dy3+, 2% Sm3+

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2% Tm3+, 2% Dy3+, 4% Sm3+

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2% Tm3+, 2% Dy3+, 6% Sm3+

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2% Tm3+, 2% Dy3+, 8% Sm3+

a

2% Tm3+, 2% Dy3+, 0.5% Eu3+

b

2% Tm3+, 2% Dy3+, 1% Eu3+

c

2% Tm3+, 2% Dy3+, 4% Eu3+

d

2% Tm3+, 2% Dy3+, 6% Eu3+

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(0.2932, 0.2962) (0.2947, 0.2829) (0.3059, 0.2805) (0.3306, 0.3034)

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(0.3312, 0.3072)

(0.3772, 0.3177) (0.4909, 0.3363) (0.5211, 0.3442)

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High energy transfer efficiency up to 72% from Tm3+ to Dy3+ in Y2WO6. Warm white light is generated in Y2WO6: Tm3+, Dy3+, Sm3+/Eu3+. WLED is fabricated by using Y2WO6: Tm3+, Dy3+, Eu3+ phosphors.

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