Design and Development of a Bluish-Green Luminescent Material

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Design and Development of a Bluish-green Luminescent Material (K2HfSi3O9:Eu2+) with Robust Thermal Stability for White Light-emitting Diodes Zuobin Tang, Gangyi Zhang, and Yuhua Wang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00844 • Publication Date (Web): 12 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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Design and Development of a Bluish-green Luminescent Material (K2HfSi3O9:Eu2+) with Robust Thermal Stability for White Lightemitting Diodes Zuobin Tang, Gangyi Zhang, Yuhua Wang* National & Local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Key Laboratory for Special Function Materials and Structure Design of the Ministry of the Education, Lanzhou University, Lanzhou, 730000, P.R. China . *Correspondence: Yuhua Wang, Fax:+86-931-8913554, E-mail: [email protected] ABSTRACT: A dipotassium hafnium tri-silicate K2HfSi3O9 with wadeite structure was synthesized by solid-state reaction for the first time. Its detailed crystal structure and electronic structure were determined using XRD Rietveld refinement and density functional theory (DFT), respectively. An expected asymmetric emission band was observed in K2HfSi3O9:Eu2+ due to Eu2+ ions occupying the two symmetrically inequivalent K+ (K1 and K2) ion sites in the host lattice. The K2HfSi3O9:Eu2+ has obvious heat-sensitive luminescent features (25 oC, blue emission; 250 oC, green emission), which can be rationalized by the energy transfer from Eu1 to Eu2 under thermal stimulation. Moreover, we developed promising phosphor K2HfSi3O9:Eu2+,Sc3+ by employing the crystal-site engineering method, and tuned the emission color from blue to green with the variation of Sc3+ ions. The target phosphor K2HfSi3O9:2%Eu2+, 6%Sc3+ shows strong absorption in the spectral range of ~ 250-490 nm and a narrow green-emitting band (FWHM ~ 59 nm) peaking at ~ 507 nm with zero emission loss at ambient temperature up to 200 oC. Such robust thermal performance was explained by possible transfer energy from the defect levels and to the 5d band of Eu2+ and certified by the thermoluminescence spectrum and the decay curves in the temperature range of 25 oC to 250 oC. Combing this excellent green phosphor with n-UV (395 nm) or blue chips (450 nm), pc-WLEDs with high luminance, adequate color rendering index and correlated color temperature were obtained.

Keywords: K2HfSi3O9:Eu2+; Hafnium-containing phosphor; Color-tunable; Thermoluminescence; pc-WLED White Light-emitting diodes (WLEDs), as a new generation illumination source, have made remarkable progress and gradually integrated into people’s daily lives over the past two decades since 1993 when the first highbrightness blue LED on the basis of InGaN was fabricated by Shuji Nakamura of Nichia Corporation.1 Compared with conventional lighting sources such as the incandescent and fluorescent lamps, WLEDs have been widely recognized due to their striking characteristics such as energy conservation, environment protection, quick response, long operating time and no flickering.2-5 Nowadays, owing to their advantages in cost and efficiency, phosphor-converted white light-emitting diodes (pc-WLEDs) dominate the market headlines in solid-state lighting. For pc-WLEDs, there are two main ways to obtain the white light emission. InGaN-based blue LED covered with commercially available yellowemitting phosphor YAG:Ce3+ is a typical strategy for generating high efficiency white light. However, this

strategy results in a poor color rendering index (Ra < 80) because of the weak of green/red spectra components in the white emission spectrum. By using some of the latest reported yellow-emitting phosphors, WLEDs also exhibit relatively low Ra. For instance, Ji et al. reported a Ra of 75.6 by combining a yellow phosphor Lu3Mg0.5Al4Si0.5O12:Ce3+ with blue LED chip (450 nm).6 Huang et al. reported a series of white LED lamps with 39 < Ra < 76 by using InGaN-based blue LED chip (460 nm) and orangish-yellow phosphor Ca3Si2O7:Eu2+.7 As a result, this method is not suitable for applications in highquality residential and medical lighting. Another effective way to get white light is use near-ultraviolet (n-UV; 380420 nm) + tri-color (red-/green-/blue-emitting) phosphors. The generated white light by this approach has superior color uniformity and excellent Ra. Therefore, many n-UV based LED phosphors have been reported recently: examples are Al1-xSixCxN1-x:Eu2+ (λex = 380 nm),8 Ba2-xCaxSiO4-δN2/3δ:Eu2+ (λex = 375 nm),9 BaLa2Si2S8:Eu2+ (λex 2


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= 405 nm),10 and Sr3Ce(PO4)3:Eu2+ (λex = 405 nm).11 Nevertheless, the rapid development of WLEDs poses higher requirements on the performance of phosphors, such as high efficiency, robust thermal performance and color point tuning.12-13 Hence, developing new phosphors with outstanding performance is urgently needed. Wadeite-type silicates have been widely investigated as a kind of important materials for mineralogy and optics.1416 The composition of the wadeite-type phases can be described as A2BSi3O9 (A is alkali cation K, Rb, Cs; B is tetravalent cation Ti, Zr, Sn, Ge, Si), which presents a framework built by [BO6]8- octahedral and layers of [Si3O9]6- rings, and the alkali cation A occupies the cavities.14 Due to the very tolerant ionic substitution of the wadeite-type compounds, it is expected that the rare earth ions can enter into the A site to form effective luminescent center. In particular, the Eu2+ doped wadeite K2ZrSi3O9 phosphors with high luminance have been studied in 2015 by Ding et al.17 Unfortunately, the thermal quenching of this phosphor is quite severe, which is far from the requirements of WLEDs. Recently, compared with zirconium-containing phosphors, hafniumcontaining phosphors have attracted researchers’ attention due to their excellent thermal stability (Zirconium and hafnium belong to the same group of elements, their ionic radius is very close, = 0.72 Å, CN = 6 and = 0.71 Å, CN = 6). For example, khibinskite K2(Zr/Hf)Si2O7:Eu2+,18 bazirite 2+ 19-20 BaZr/HfSi3O9:Eu , and garnet structure Ca2Lu(Zr/Hf)2(AlO4)3:Ce3+.21-22 Replacement of Zr4+ in the matrix lattice by Hf4+, the thermal stability of these phosphors is greatly improved. Up to now, dipotassium hafnium tri-silicate K2HfSi3O9 and its rare earth ionsdoped phosphor have never been synthesized and reported before. In our current work, K2HfSi3O9:Eu2+ phosphor shows excellent blue-green emission at room temperature. However, due to the presence of two different luminescent centers in K2HfSi3O9, the emission color of this phosphor tends to be green with increasing temperature (see below for details). So we expect to solve this critical problem by some means. Generally, the crystal-site engineering is considered to be an approach for customizing the luminescence color of a given material, especially for modifying a phosphor with more than one luminescence centers.23-24 For example, in Ca10M(PO4)7:Eu2+ (M = Li, Na and K), Eu2+ ions enter into four sites Ca(1), Ca(2), Ca(3) and M(4) and exhibit two emission bands, where the long wavelength band gives tunable emissions from 511 to 466 nm depending on the M(4) sites.25 In addition, phosphors with highperformance are usually obtained through varying the compositional design.26 Such as Sr5(PO4)3-x(BO3)xCl:Eu2+,27 Ba4Si6O16-3x/2Nx:Eu2+,28 and Ca1-xLixAl1-xSi1+xN3:Eu2+.29 In view of this, it is possible to introduce trivalent ions to replace Hf4+ (rHf4+ =0.71 Å, 6CN) ions in K2HfSi3O9, for example Ga3+ ( rGa3+ =0.6 Å, 6CN), Sc3+ (rSc3+ =0.74 Å, 6CN), Lu3+

(rLu3+ =0.86 Å, 6CN) or Y3+ (rY3+ =0.9 Å, 6CN). Among these, the difference in radius between Sc3+ and Hf4+ is the smallest, and Sc-containing silicates are generally considered to be suitable and efficient phosphor hosts for LED applications due to their stable physical and chemical properties.30-31 So we chose the Sc3+ ions for specific research. As mentioned above, the wadeite-type compounds have high crystal chemical flexibility and very tolerant of ionic substitutions. The introduction of impurities in the crystal lattice leads to an increase in the degree of structural disorder, which in turn has a great influence on the luminescence properties. In this paper, herein, we first synthesized dipotassium hafnium tri-silicate K2HfSi3O9 and studied its crystal structure and electronic structure by XRD Rietveld refinement and DFT, respectively. The luminescence properties of Eu2+ ions-doped K2HfSi3O9 were investigated in detail using the steady-state and time-resolved photoluminescence spectra and thermal quenching. Secondly, we reported the simultaneous realization of color point tuning and thermal performance improvement in K2HfSi3O9:Eu2+ by the crystal-site engineering design. A continuous solid solution K2Hf1yScySi3O9-0.5y was determined by phase analysis. The newly developed excellent green emission phosphor K2HfSi3O9:2%Eu2+, 6%Sc3+ exhibits zero emission loss at ambient temperature up to 200 oC, which was discussed in detail by investigating the thermoluminescence spectrum and decay curves. Two WLED lamps suitable for application were obtained by coating K2HfSi3O9:Eu2+, Sc3+ as green phosphor on n-UV chip (395 nm) and blue chip (450 nm), respectively. These properties make the phosphor a promising green-emitting material for pcWLEDs.

EXPERIMENTAL SECTION Materials synthesis. Powder samples of K2HfSi3O9:xEu2+,ySc3+ (0 ≤ x ≤ 7%, 0 ≤ y ≤ 40%) were synthesized by high temperature solid-state reaction method. Appropriate amounts of K2CO3 (≥99.0%, Sinopharm Chemical), HfO2 (98%, Aladdin), SiO2 (99.5%, Aldrich), Eu2O3 (99.99%, Gansu Rare Earth) and Sc2O3 (99.99%, Gansu Rare Earth) were homogeneously mixed and finely ground in an agate mortar containing ethanol as the dispersing medium, after which they were dried and packed in aluminum oxide crucibles. The powder mixture was sintered in a high-temperature tube furnace at 1450 oC for 8 h under a reducing atmosphere (5% H2 + 95%N2). The resulting samples after cooling were reground into powder for further characterization analysis. Structural analysis. The crystalline phase of the resulting samples was identified using a BRUKER D2PHASER X-ray powder diffractometer with Cu Kα (λKα = 1.54184 Å) radiation, operating at 30 kV and 10 mA. Rietveld refinement was implemented based on the general structure analysis system (GSAS) program.32 3


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Figure 1. (a) Powder XRD Rietvield refinement result for K2HfSi3O9. (b) The crystal structure of K2HfSi3O9 and illustration of + 4+ 4+ crystallographic sites K , Hf and Si . (c) Total and partial density states for K2HfSi3O9. (d) Band structure of K2HfSi3O9.

Microstructure and energy-dispersed X-ray spectroscopy (EDX) of K2HfSi3O9 were carried out by a high-resolution transmission electron microscopy (TecnaiTM G2 F30, FEI, USA). Luminescence properties. Steady state photoluminescence (PL) spectra were recorded by a fluorescence spectrophotometer (Fluorlog-3, Horiba Jobin Yvon) with a 450 W xenon arc lamp as the excitation source. The thermal quenching characteristic was studied from 25 oC to 250 oC using Fluorlog-3 fluorescence spectrophotometer equipped with a standard high temperature fluorescence controller TAP-02 (Orient KOJI instrument Co., Ltd.). Transient PL properties were measured by a fluorescence spectrophotometer (FLS920T, Edinburgh) with an nF900 nanosecond pulsed hydrogen lamp as the excitation source. The quantum efficiency (QE) was measured using a quantum yield accessory attached to the FLS-920T fluorescence spectrophotometer. Thermoluminescent (TL) curve was

obtained using a FJ-427A TL meter from Beijing Nuclear Instrument Factory, and the heating rate is 1 K s-1. Fabrication of white LEDs. Appropriate amounts of KHSO:2%Eu2+, 6%Sc3+, commercial blue-emitting 2+ BaMgAl10O17:Eu and orange-emitting Sr3SiO5:Eu2+ phosphors were thoroughly mixed in epoxy thermosetting adhesive. After removing the bubbles, the mixture was coated on a n-UV LED chip (λmax = 395 nm) and dried in an oven at 80 oC for 40 min, 150 oC for 1h. Similar to the above method, another one WLED device was fabricated using a blue LED chip (λmax = 450 nm). The optical data of the WLEDs were obtained by using a EVERFINE PMS50/80 UV-VIS-near IR spectrophotocolorimeter. Theory calculations. DFT calculations were carried out to attain the electronic properties and band structure of K2HfSi3O9 using the Cambridge Serial Total Energy Package (CASTEP) code.33 The generalized gradient approximation based on DFT was chosen for the theoretical basis of density function. 4


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RESULT & DISCUSSION Structural characterization. A single phase, K2HfSi3O9, for phosphors was successfully synthesized by solid-state reaction. In order to gain detailed crystal structural information for the K2HfSi3O9, XRD Rietveld refinement was carried out using the crystallographic data K2ZrSi3O9 (ICSD-56898) as the starting model. Fig. 1(a) shows X-ray powder diffraction of K2HfSi3O9 with the observed and calculated data. It can be seen that all observed diffraction peaks are in good agreement with the calculated results except some trace impurity peaks around 2θ = 28.3 and 32.8 degrees, which are identified from HfO2 (PDF#65-1142). The same impurity phase was also observed in the synthesis of BaHfSi3O9 and has been proved to be not affect the luminescent investigation.34 The final refinement obtained converges with weighted/profile R-factors, Rwp = 9.33% and Rp = 7.15%, indicating the reflection conditions were reliable and revealing that K2HfSi3O9 is isotypic to the wadeite crystal structure with acentric space group P-6. The detailed crystallographic parameters, atomic coordinates and selected bond lengths for K2HfSi3O9 are listed in Table 1. The (002) crystal plane and chemical composition of K2HfSi3O9 were also determined by analyzing a highresolution TEM image and its corresponding EDX spectrum.( see Supplementary Fig. S1) Fig. 1(b) illustrates a structural overview of K2HfSi3O9 and emphases the coordination environment of K+, Hf4+ and Si4+ ions. Like the structure of wadeite, K2HfSi3O9 shows a framework structure which is composed of staggered planar rings [Si3O9]6- connected to isolated octahedrons [HfO6]8- and K+ ions. Each ring consists of three corner-linked [SiO4]4- tetrahedra, which contains three bridging oxygen atoms and six nonbridging oxygen atoms. The symmetrically inequivalent K+ ions (K1 and K2) located between the alternating layers [Si3O9]6-, and each K+ is coordinated by six nonbridging oxygen.35 In this structure, the activator Eu2+ ions are expected to occupy the K+ sites because the ionic radii of Eu2+ (1.17 Å, 6CN) is slightly smaller than that of K+ (1.38 Å, 6CN). For Hf4+ ions, it’s too small (rHf4+ =0.71 Å, 6CN) to be occupied by Eu2+ (1.17 Å, 6CN). What’s more, the average bond length of Hf-O (2.085 Å) is much shorter than that of Eu3+-O (∼2.31 Å),30 and the bond length of Eu3+-O is smaller than that of Eu2+-O due to one less electron, which further indicates that the Eu2+ can not occupy Hf4+ position. Our analysis gives that the average bond lengths of K1-O and K2-O are 2.993 Å and 2.957 Å, respectively. On the basis of the refined crystal structure, the electronic structure of K2HfSi3O9 host was studied. Fig. 1(c) shows the total density of states for K2HfSi3O9 and the partial density of states for K, Hf, Si and O atoms. The results indicate that the valence band (VB) is dominated by Si 3s3p and O 2p states with a bandwidth ∼ 8 eV, and the conduction band (CB) is mainly composed of K 4s, Hf 5d and Si 3p states. The maximum intensity of the K 3p state is located at about -10.6 eV, which far away from the

top of the VB, implying a weak interaction between the K 3p and O 2p states due to the large bond length of K-O. Corresponding to the total density of states, the band structure of K2HfSi3O9 is shown in Fig. 1(d). The gap between the bottom of the CB and the top of the VB is an indirect band gap with a calculated value ∼ 4.75 eV. The experimental band gap obtained from the diffuse reflection spectrum of K2HfSi3O9 is ∼ 5.5 eV. (see Supplementary Fig. S2). The difference between these two values is acceptable because the DFT calculation usually underestimates the band gap of material.36 Such a large band gap reveals that the energy levels of the 4f65d ↔ 4f7 transitions of the activator Eu2+ ion should have small interference with CB and VB in K2HfSi3O9 and that the K2HfSi3O9 would be a reliable host for luminescence materials. Table 1. Crystallographic data for K2HfSi3O9 Formula

K2HfSi3O9

Space group

P-6

a/Å

6.90083(8)

b/Å

6.90083

c/Å

10.11599(18)

α/deg

90

β/deg

90

γ/deg

120

Cell volume/Å

3

417.198(8)

Z

2

Rwp

9.33%

Rp

7.15%

2

Χ

2.991

Atoms coordinates Atom x/a K1 0.66667 K2 0.33333 Hf 0.0 Si1 0.37573 Si2 0.61374 O1 0.48741 O2 0.51435 O3 0.25428 O4 0.7444 Selected bond lengths (Å)

y/b 0.33333 0.66667 0.0 0.25157 0.7413 0.09618 0.91003 0.23749 0.7665

z/c 0.19226 0.6941 0.2505 0.5 0.0 0.5 0.0 0.62711 0.1331

K1-O3 (×3) K1-O4 (×3) Hf-O3 (×3) Hf-O4 (×3)

K2-O3 (×3) K2-O4 (×3)

2.813 (30) 3.102 (17)

3.161 (30) 2.825 (18) 2.103 (20) 2.068 (15)

Optical properties of K2HfSi3O9:xEu2+. The steady state photoluminescence spectra for K2HfSi3O9 with 1% Eu2+ dopant concentration is depicted in Fig. 2a, the inset gives the actual photographs of K2HfSi3O9:1% Eu2+ under daylight and 365 nm UV lamp. The excitation spectrum of K2HfSi3O9: 1% Eu2+ shows strong absorption in the range 5


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2+

Figure 2. (a) Excitation and emission spectra for K2HfSi3O9:1%Eu phosphor. The inset are photographs of the corresponding 2+ 2+ K2HfSi3O9:1%Eu phosphor under daylight and 365 nm UV light. (b) Gaussian fit of the emission spectrum of K2HfSi3O9:1%Eu 2+ excited at 400 nm. (c) Time-resolved photoluminescence (TRPL) spectra for K2HfSi3O9:1%Eu , λem = 465 nm, λex = 400 nm. (d) 2+ 2+ Emission spectra of K2HfSi3O9:xEu under 400 nm excitation as a function of Eu content.

from UV to blue regions (250 ∼450 nm), which is ascribed to the 4f ground state to 5d field-splitting state transition of Eu2+.37 At an excitation of 400 nm, the K2HfSi3O9:1% Eu2+ reveals a bright blue-green emission peaking at 465 nm, with full width at half-maximum (FWHM) ∼ 59 nm, which is assigned to the 4f65d1→4f7 transition of Eu2+. Compared with the commercial available blue-green emission phosphor BaMgAl10O17: Eu2+, Mn2+ at room temperature (see Supplementary Fig. S3), K2HfSi3O9: 1%Eu2+ gives a stronger and wider absorption in UV region, and its emission intensity is about 106% of BaMgAl10O17: Eu2+, Mn2+ under the same excitation (λex = 345 nm), implying that the K2HfSi3O9 is an excellent host for luminescence materials. Considering the two symmetrically inequivalent K+ (K1 and K2) ion sites in K2HfSi3O9, the broad emission band with a long tail can be deconvoluted into two sub-Gaussian components, peaking at ∼ 462 nm (labeled as Eu1) and ∼ 494 nm

(labeled as Eu2), as shown in Fig. 2b. According to the crystallographic analysis, the average bong length of K1-O (2.993 Å) is longer than that of K2-O (2.957 Å), so that the Eu2+ ions substituting the K2 sites experience a larger crystal field strength. The crystal field strength (Dq) is inversely proportional to R5（R is the distance between the central ion and its ligands）and can be determined using the equation.38

(1)

where Z is the valence of the anion, e is the charge of the electron, r is the radius of the d wave function. Based on eqn (1), the smaller the distance R, the larger the crystal field strength. In addition, owing to the site with large crystal field strength accommodating Eu2+ corresponds to a low energy emission peak, and vice versa.39 Therefore, it 6


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can be concluded that the Eu1 and Eu2 are assigned to K1 and K2 sites in K2HfSi3O9, respectively. To better understand that the asymmetric emission band comes from different luminescent centers, Fig. 2c exhibits the time-resolved photoluminescence (TRPL) spectra for K2HfSi3O9:1% Eu2+ with decay time from 19.53 ns to 1503.91 ns. As can be seen from the normalized spectra (see the inset in Fig. 2b), the spectral shape change is not synchronized with the decay time prolonging, in another word, the lower energy region decayed slower than the higher energy region, proving that there are different luminescent centers in K2HfSi3O9:Eu2+, which is consistent with the photoluminescence results. Fig. 2d presents the PL spectra of K2HfSi3O9:xEu2+ with different Eu2+ contents (0.2% ≤ x ≤ 7%) under 400 nm excitation. Changes in the Eu2+ content yielded simultaneous changes in the emission intensity, shape and peak position of the PL spectra. The optimal Eu2+ doping content in K2HfSi3O9:xEu2+ is found to be 1%, beyond which the PL intensity begins to decrease due to the concentration quenching effect.40 Regarding the spectral shape, all PL spectra for K2HfSi3O9:xEu2+ is dominated by Eu1 emission band with a weak shoulder Eu2 emission band. However, as the increase of Eu2+ doping contents, the shoulder Eu2 emission band can be clearly resolved in the PL spectra. The difference in spectral shape could be caused by the energy transfer from Eu1 to Eu2, which will show a strong concentration dependence. Thereby, based on the well-know ForsterDexter theory, the critical distance (Rc) for energy transfer from Eu1 to Eu2 in K2HfSi3O9 is roughly evaluated by41-42 0.36 × 10

. × !"# $ %

S. O.

(2)

here P is the oscillator strength of the absorption transition of activator ion, S.O. is the spectral overlap integral and E represents the energy at maximum spectral overlap. For Eu2+ ion, the value of P is generally taken as 0.01. The values S.O. and E were calculated to be 0.025 eV1 and 2.85 eV from the Eu1 emission and Eu2 excitation spectra. Then, the Rc between inequivalent Eu2+ ions is determined to be 15 Å. The calculated result is much larger than the distance between neighboring inequivalent K+ ions (4.15 Å), which corresponds to a relatively low Eu2+ quenching concentration. In addition, as the Eu2+ doping content increases from 0.2% to 7%, the peak position shifts from 463 nm to 467 nm, which can also be attributed to the energy transfer between Eu2+ ions. Under 400 nm excitation, the temperature-dependence emission spectra of K2HfSi3O9:1%Eu2+ were measured and presented in Fig. 3a. An interesting phenomenon is that the emission band at higher energy side (Eu1) gradually decreases and finally almost disappears with the temperature rising from 25 to 250 oC, meanwhile, a distinct emission band (Eu2) appears in lower energy region and its emission intensity increases continuously.

Similar to the yellow-emitting phosphor Ba2Si5N8:Eu2+ reported by Piao et al.,43 this unusual temperaturedependence behavior is probably related to energy transfer from Eu1 to Eu2, namely, “site-to-site energy transfer” takes place during the heating process. Recently, Zhang et al. reported that the KxCs1-xAlSi2O6:Eu2+ phosphor also has such anomalous luminescence behavior under thermal disturbance.44 An effective method to certify energy transfer is to determine the luminescence lifetime. Fig .3b shows the luminescence decay curves of K2HfSi3O9:1% Eu2+ with λem = 465 nm at different temperatures from 25 to 250 oC. All decay curves exhibit obvious temperature-dependence behavior and that are well fitted with second-order exponential. The lifetime (τ) at 25, 150 and 250 oC are determined to be 0.74, 0.70 and 0.84 ns, respectively. Before 150 oC, the lifetime of Eu2+ emission decreases, indicating that an energy transfer process takes place in K2HfSi3O9:Eu2+. In the case when the test temperature is higher than 150 oC, the lifetime of Eu2+ emission is increasing, but it is not contradictory between the longer lifetime and the energy transfer since the two emission bands overlap significantly and the lower energy emission begins to dominant (see Fig 3a). Importantly, the fast decay component of the decay curves is always reduced while the slow decay component increases, which should be a direct evidence for non-radiative energy transfer from Eu1 to Eu2. The energy transfer in K2HfSi3O9:Eu2+ can be elucidated using the configuration coordination diagram, as shown in Fig. 3c. The curves for 4f7 and 4f65d1 represent the ground and excited states of Eu2+, respectively. At room temperature, the electrons in ground state (4f7) are firstly excited to the excited state (4f65d1) under UV/n-UV light excitation, and then most of the electrons return to ground state from the equilibrium points A and B via the radiation process, so the resultant sample emits a bright blue-green emission. With the increase of temperature, the electron-phonon coupling strength increase and the non-radiative energy transfer between Eu2+ ions occurs frequently, leading to more electrons overcome energy barrier Δ( and transfer from the Eu1 ions to Eu2 ions via intersection C and finally revert to ground state from the equilibrium point B. As a result, the lower energy emission gradually dominates as the temperature rising. The probability of electron transfer through the intersection C can be described by following:45 ) * exp(

/Δ% 01

)

(3)

where α is the transition probability per unit time, s represents the frequency factor and k is the Boltzmann’s constant. With the assistance of thermal energy, the nonradiative energy transfer is strongly dependent on the ambient temperature T and energy barrier Δ(, and the probability of energy transfer generally increases with increasing temperature.

7


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2+

Figure 3. (a) Temperature-dependent emission spectra of K2HfSi3O9:1%Eu under 400 nm excitation. (b) Decay curves for 2+ K2HfSi3O9:1%Eu phosphor with different temperature, monitored at 400 nm excitation and 465 nm emission. (c) The schematic illustration of a configuration coordination model for Eu1 and Eu2.

Improvement of K2HfSi3O9:Eu2+. According to the above analysis, K2HfSi3O9:Eu2+ phosphor with temperature-sensitive luminescence behavior cannot be effectively applied to WLEDs, so we designed and improved the K2HfSi3O9:Eu2+ phosphor by partially replacing the Hf4+ ions in K2HfSi3O9 with a trivalent cation Sc3+. For the sake of convenience, the improved phosphor is abbreviated as KHSO:xEu2+, ySc3+ in following sections. The XRD patterns of powder samples of KHSO:1%Eu2+ with varying Sc3+ content (0.2% ≤ y ≤ 40%) are shown in Fig 4a. The XRD patterns of all samples maintain the characteristic patterns of K2HfSi3O9 phase, demonstrating that the non-equivalent ion-substituted products were successful synthesized. Due to the small difference in the radii of Sc3+ and Hf4+ ions, we did not observe a significant shift of the XRD peaks under the current synthesis conditions. As illustrated in Fig 4b, the refined lattice constants a, b and c are almost constant,

which verifies the phenomenon that the XRD diffraction peak is not shifted. we therefore believe that the introduction of Sc3+ effectively replaces Hf4+ in K2HfSi3O9, resulting in the formation of solid solutions K2Hf1yScySi3O9-0.5y. In general, the change of matrix components has a significant effect on Eu2+ luminescence. To investigate how the introduction of Sc3+ ions affects the luminescence properties of K2HfSi3O9:Eu2+, the PLE and PL spectra of KHSO:1%Eu2+, ySc3+ (0.2% ≤ y ≤ 40%) are measured and their normalization curves are shown in Fig 4c, d. The maximum excitation position of KHSO: 1%Eu2+, ySc3+ is located at ∼ 345 nm at lower Sc3+ contents. However, an additional shoulder appears at the lower energy side of excitation spectra with Sc3+ contents increasing. Upon the excitation at 400 nm, the obtained phosphor shows two distinct emission bands (Fig. 4d) peaking at ∼ 465 and ∼ 507 nm respectively when y < 6%, which are consistent 8


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3+

Figure 4. (a) XRD patterns of KHSO:1%Eu , ySc (0.2% ≤ y ≤ 40%). As a reference, the refinement XRD data for K2HfSi3O9 is 2+ 3+ 2+ 3+ shown. (b) Lattice constants a, b and c of KHSO:1%Eu , ySc as a function of y. (c) Excitation spectra for KHSO:1%Eu , ySc . (d) 2+ 3+ Emission spectra for KHSO:1%Eu , ySc .

with the observation from the concentration- and temperature-dependent PL spectra of K2HfSi3O9:Eu2+ and can be attributed to Eu1(K1) and Eu2(K2) emission, respectively. When y > 6%, the blue emission band is weakened and finally disappears completely, whereas green emission band is intensified and eventually dominated solely. These phenomena in PLE and PL spectra imply that most Eu2+ occupy the K2 sites with a stronger crystal field environment rather than K1 sites when the Sc3+ ions are introduced into K2HfSi3O9. The CIE color coordinates corresponding to the PL spectra of KHSO:1%Eu2+, ySc3+ shows that the color point can be tuned from (0.1466, 0.1981) for the 0.2% Sc3+ content to (0.1726, 0.5603) for the 40% Sc3+ content (see Supplementary Fig. S4). It is noteworthy that the color point tends to be stable when the fraction y in KHSO:1%Eu2+, ySc3+ between 6% and 40%, which means that a single green-emitting phosphor could be obtained by varying the compositional design.

When the Sc3+ content is fixed at 6%, the normalized emission spectra of KHSO:xEu2+, 6%Sc3+ with various Eu2+ contents (0.2% < x < 4.0%) under 400 nm excitation are depicted in Fig. 5a. The inset in Fig. 5a shows the dependence of the emission intensity of KHSO:xEu2+, 6%Sc3+ on the Eu2+ contents, and the optimal Eu2+ doping content (x) is found to be 2%. At the same time, a continuous red shift for the PL spectra is observed with increasing Eu2+ content, and the peak position shifts 8 nm from 502 nm at 0.2% Eu2+ to 510 nm at 4% Eu2+. The reabsorption of emission energy is one of the factors that lead to the longer wavelength emission because the excitation and emission spectra of KHSO:xEu2+, 6%Sc3+ are overlapped partially. By increasing the Eu2+ doping content, the average interatomic distance between Eu2+ activators is shortened and then the probability of the energy transfer between Eu2+ ions increases, as a result, the emission peak shifts to the lower energy region. Besides the reabsorption and energy transfer, the redshifting behavior is also probably related to the crystal 9


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2+

3+

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2+

Figure 5. (a) Emission spectra of KHSO:xEu , 6% Sc under 400 nm excitation as a function of Eu content. (b) Excitation and 2+ 3+ emission spectra for KHSO:2%Eu ,6%Sc phosphor. The inset are photographs of the phosphor under daylight and 365 nm UV 2+ 3+ light. (c) Temperature-dependent normalized emission intensity of KHSO:2%Eu ,6%Sc phosphor in comparison with five different color commercial LED phosphors. (d) Relationship between emission spectra and test temperature.

field strengthening caused by the smaller Eu2+ ions doping.46 Along with the enhancement of the crystal field, the crystal field splitting of Eu2+ 5d band is gradually strengthened, which can be proved by the change of excitation spectra for KHSO:xEu2+, 6%Sc3+ in position and shape (see Supplementary Fig. 5), the excitation spectra become broadened and extend to the lower energy side with increasing Eu2+ content. Fig. 5b presents typical excitation/emission spectra of the target phosphor KHSO:2%Eu2+, 6%Sc3+ at room temperature. The excitation spectrum, monitored at 507 nm, has a very broad range covering UV, n-UV and blue regions (250 ∼ 490 nm), enabling the target sample can be exploited by n-UV and blue LED chips. Upon the steady-state excitation at 400 nm, the emission spectrum of KHSO:2%Eu2+, 6%Sc3+ shows a single narrow greenemitting band (FWHM ∼ 59 nm) peaking at 507 nm. The

actual photographs of this phosphor under daylight and 365 nm UV lamp (see the inset in Fig. 5b) are consistent with the PLE/PL data. In addition, compared with commercial available green phosphor (Ba,Sr)2SiO4:Eu2+, KHSO:2%Eu2+, 6%Sc3+, internal quantum yield (QY) ~ 55.3%, has very high emission intensity (∼ 80.7%) under the present synthetic conditions (see Supplementary Fig. 6), demonstrating that this target green-emitting phosphor has powerful potential for application in the field of WLEDs. It is well-known that the thermal stability of a LED phosphor is an important technical index for application. Fig. 5c shows the temperature-dependent emission intensity of KHSO:2%Eu2+, 6%Sc3+ and five different color commercial available LED phosphors in the temperature range 25-250 oC. The phosphors, CaAlSiN3:Eu2+ (redemission), Sr3SiO5:Eu2+ (orange-emission), 10


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2+

3+

2+

3+

Figure 6. (a) The TL curve of KHSO:2%Eu , 6%Sc . (b) Decay curves for KHSO:2%Eu ,6%Sc phosphor at different temperature, monitored at 400 nm excitation and 507 nm emission. (c) The schematic illustration of the mechanism for the 2+ 3+ anomalous temperature-dependent luminescence behavior of KHSO:2%Eu , 6%Sc .

(Ba,Sr)2SiO4:Eu2+ (green-emission), BaSi2O2N2:Eu2+ (cyanemission) and BaMgAl10O17:Eu2+ (BAM: Eu2+ blueemission), exhibit about 8%, 9%, 47%, 38% and 10% emission loss at 200 oC, respectively. In comparison, similar to zero quenching phosphor Na3Sc2(PO4)3:Eu2+,47 the emission intensity of the KHSO:2%Eu2+, 6%Sc3+ phosphor has no obvious loss at 200 oC. In addition, we also observed slight fluctuations in emission intensity throughout the entire test range. Specifically, in the process of heating up to 100 oC, the emission intensity decreased continuously to 95% of the initial intensity. After that, however, there is a slight rise until about 175 oC where the emission intensity began to fall again. This anomalous temperature-dependent luminescence phenomenon, which was also be observed in Na3Sc2(PO4)3:xEu2+ and the commercial orange phosphor Sr3SiO5:Eu2+,48 can be explained by the coexistence of the thermal quenching and thermal stimulated energy transfer behavior. Before 100 oC, the thermal quenching behavior plays a major role, leading to continuous loss of

emission intensity. As temperatures rise further, an energy transfer process occurs from trap levels to the vicinity of Eu2+ 5d excitation band, which compensates for the loss of luminescence caused by thermal quenching and makes the subsequent emission intensity slightly stronger. When the energy transfer process declines under a higher thermal stimulation, the emission intensity drops again. Accompanied by fluctuations in emission intensity, the emission band offsets by ∼ 9 nm towards high-energy region (blue shift) and the FWHM broadens by ∼ 10 nm when the ambient temperature increased from 25 oC to 250 oC, as depicted in Fig. 5d. The blue shift phenomenon can be attributed to the thermally active phonon-assisted tunneling from the lower energy sublevel to the higher energy sublevel in Eu2+ 5d band.49 In addition, affected by thermally active phonon modes, the FWHM gradually broadens. So far, the thermoluminescence (TL) for a phosphor is an effective means to elucidate the thermal stimulated energy transfer. Fig. 6a shows the TL spectrum of 11


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KHSO:2%Eu2+, 6%Sc3+. The TL curve consists mainly of a shallow trap at ∼39 oC and a deeper trap at ∼190 oC, which is in good agreement with the change of the temperature-dependent emission intensity of the phosphor. The trap depth can be calculated by the following equation,37 E 1 45 /500

(4)

where E4 refers to the energy gap between the conduction band bottom of the host and the trap levels, namely, the trap depth. 45 is the temperature for which the TL curve reaches a maximum. Based on the eqn (4), the two trap depths are determined to be 0.62 eV and 0.93 eV at ∼39 o C and ∼190 oC, respectively. Notably, the deeper trap level plays a decisive role for the emission enhancement induced by elevated temperatures. Generally, defects are regarded as electron-hole trapping centers. Thus, it is necessary to analyze the types of defects present in the resulting samples. In K2HfSi3O9:Eu2+, the K+ ions are replaced by Eu2+ ions, leading to the formation of a positive defects (89• , which further induces the generation of a negative vacancy defects ;9< to maintain the electroneutrality. When the Sc3+ ions are introduced into K2HfSi3O9 and occupy Hf4+ sites, negative defects ' =>?@ are formed and simultaneously can be induced the generation of a positive vacancy defects ;A•• . It is worth emphasizing that when the introduction of Eu2+ and Sc3+ ions enter the matrix together, partial positive defects ' (89• and negative defects =>?@ could disappear due to the charge compensation. Herein, the equations related to defects in KHSO:Eu2+, Sc3+ are: ×

(89• + ;9' → D(89• ;9' E

' ' ' 2=> + ;A•• → D=> ;A•• => E

(89•

+

' =>

→

' × D(89• => E

(5) ×

(6) (7)

In order to better explain the energy transfer process from the defect levels to the Eu2+ 5d band with temperature increasing, the fluorescence decay curves of KHSO:2%Eu2+, 6%Sc3+ are measured in the temperature range 25-250 oC, monitored at λex = 400 nm and λem = 507 nm, as shown in Fig. 6b. All curves can be fitted by a single-exponential function given by:50 I

G H exp/(J)

(8)

where G is the luminescence intensity at time K, H is a constant and τ refers to the lifetime. The lifetime of Eu2+ in KHSO:2%Eu2+, 6%Sc3+ is determined to be 1.13 μs, 1.12 μs, 1.12 μs, 1.12 μs, 1.10 μs and 1.08 μs at 25 oC, 75 oC, 100 oC, 150 oC, 200 oC and 250 oC, respectively, which are consistent with the lifetime of Eu2+ ion in many other hosts (0.4-1.2 μs).19 The single-exponential decay characteristic suggests the fact that the narrow greenemitting band originates from a single Eu2+ site. Normally, under the influence of thermal quenching, the luminescence lifetime for an allowed transition is shortened due to an additional non-radiative contribution to the decay process.46 However, the observation in Fig.

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6b is that the lifetime of the Eu2+ emission in KHSO:2%Eu2+, 6%Sc3+ remains essentially constant as the temperature increases. At 200 oC the lifetime starts to decrease. This result strongly proves that the anomalous emission of KHSO:2%Eu2+, 6%Sc3+ with increasing temperature is caused by energy transfer from the defect levels (donor) to the vicinity of Eu2+ 5d excitation band (acceptor), and the mechanism is illustrated in Fig. 6c. Fabrication and performance of WLEDs. To evaluate the practicality of the newly developed phosphor for pcWLEDs, two WLED lamps were fabricated using the “nUV LED chip + tri-color phosphors” and similar “blue LED chip + yellow phosphor” methods, respectively. One of the WLED lamps was fabricated by coating KHSO:2%Eu2+, 6%Sc3+, commercial blue (BAM:Eu2+) and orange (Sr3SiO5:Eu2+) phosphors on a n-UV LED chip (λmax = 395 nm), and its electroluminescence (EL) spectrum and actual picture under 100 mA forward bias current are shown in Fig. 7a. As seen, the obtained WLED lamp shows a bright warm white light with correlated color temperature (CCT) of 3397 K, color rendering index (Ra) of 84.7 and CIE chromaticity coordinates (0.4082, 0.3916), which meets the requirements of residential lighting. Usually, WLEDs employ higher bias currents. Fig. 7b displays forward bias current dependent electroluminescence (EL) spectra of the WLED lamp. As the operating current increases from 100 to 300 mA, the EL spectral intensity increases. The EL spectra are relatively stable in shape and position, which corresponds to a small CIE change from (0.4082, 0.3916) to (0.4004, 0.3855). At the same time, other technical indicators have also changed little. For example, the Ra decreases from 84.7 to 81.3, the CCT increases from 3397 K to 3527 K and the relative luminous efficiency (LE) decreases by 15.6% (see Fig. 7c). Fig. 7d shows the EL spectrum of another WLED lamp prepared by using the target green phosphor and the commercial orange phosphor Sr3SiO5:Eu2+ combined with a 450 nm InGaN-based LED chip driven by 100 mA forward bias current. The CIE chromaticity coordinates and CCT are determined as (0.3650, 0.3284) and 4066 K, respectively. It is worth mentioning that this lamp still exhibits a high Ra (82.2) compared with the first lamp pumped by n-UV LED chip. All of above results reveal that the green-emitting phosphor KHSO: Eu2+,Sc3+ has a promising in pc-WLEDs.

CONCLUSIONS In conclusion, we successfully synthesized K2HfSi3O9 for the first time and determined its crystal structure and band structure by Rietveld refinement and DFT, respectively. Under UV/n-UV excitation, the phosphor K2HfSi3O9:Eu2+ shows two emission bands (Eu1-blue and Eu2-green) with exceptionally different temperature dependence characteristics. The variation in spectral shape with respect to different decay times has confirmed the presence of multiple luminescent centers in K2HfSi3O9:Eu2+. The decay curves obtained with 12


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Figure 7. (a)EL spectrum of the WLED lamp fabricated by blending blue-emitting BAM:Eu , green-emitting 2+ 3+ 2+ KHSO:2%Eu ,6%Sc and orange-emitting Sr3SiO5:Eu phosphors on a 395 nm n-UV chip. The inset shows the photo of this WLED lamp. (b) EL spectrum of the WLED lamp under different forward bias currents. (c) The Ra, CCT and relative LE of the WLED lamp under different operating currents. (d) EL spectrum of WLED lamp obtained using 450 nm blue chip, and the inset shows the photo of this device driven by a 100 mA forward bias current.

increasing temperature verifies the energy transfer from Eu1 to Eu2 in K2HfSi3O9. In addition, by adopting crystalsite engineering approach, we have further developed a promising K2HfSi3O9:Eu2+, Sc3+ green phosphor. In particular, K2HfSi3O9:2%Eu2+, 6%Sc3+ shows strong absorption in the spectral range of ∼ 250-490 nm, a narrow green emission peaking at ∼ 507 nm with FWHM of ∼ 59 nm, and sustains its emission intensity up to 200 o C. Combing the thermoluminescence spectrum and the decay curves in the temperature range of 25 oC to 250 oC, we conclude that the robust thermal performance of K2HfSi3O9:2%Eu2+, 6%Sc3+ involves the energy transfer from the defect levels and to the 5d band of Eu2+. Using K2HfSi3O9:2%Eu2+, 6%Sc3+ as green phosphor in combination with n-UV (395 nm) or blue (450 nm) chips can fabricate pc-WLEDs suitable for general illumination with high luminance, adequate Ra and CCT. These results

reveal that the ingenious design of the composition can regulate the distribution of activators in solid-state compounds and can develop new luminescent materials with outstanding performance for practical applications.

ASSOCIATED CONTENT Supporting Information. High-resolution TEM image and EDX spectrum for K2HfSi3O9; Diffuse reflection spectrum of the host sample; Excitation and emission spectra of samples compared to commercial phosphors; CIE color coordinates of samples 3+ with varying Sc contents. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author 13


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* E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant no. 51672115), Gansu Province Development and Reform commission and State Key Laboratory on Integrated Optoelectronics (no. IOSKL2013KF15).

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30. Liu, C.; Xia, Z.; Lian, Z.; Zhou, J.; Yan, Q., Structure and luminescence properties of green-emitting NaBaScSi2O7:Eu2+ phosphors for near-UV-pumped light emitting diodes. J. Mater. Chem. C 2013, 1 (43), 7139-7147. 31. Shimomura, Y.; Honma, T.; Shigeiwa, M.; Akai, T.; Okamoto, K.; Kijima, N., Photoluminescence and Crystal Structure of Green-Emitting Ca3Sc2Si3O12 : Ce3 + Phosphor for White Light Emitting Diodes. J. Electrochem. Soc. 2007, 154 (1), J35-J38. 32. Larson, A. C.; Von Dreele, R. B., Generalized Crystal Structure Analysis System. Report LAUR 1988, 86. 33. Clark Stewart, J.; Segall Matthew, D.; Pickard Chris, J.; Hasnip Phil, J.; Probert Matt, I. J.; Refson, K.; Payne Mike, C., First principles methods using CASTEP. In Zeitschrift für Kristallographie - Cryst. Mater. 2005; Vol. 220, p 567. 34. Xin, S.; Zhu, G., Enhanced luminescence and abnormal thermal quenching behaviour investigation of BaHfSi3O9:Eu2+ blue phosphor co-doped with La3+-Sc3+ ion pairs. RSC Adv. 2016, 6 (48), 41755-41760. 35. McKeown, D. A.; Nobles, A. C.; Bell, M. I., Vibrational analysis of wadeite K2ZrSi3O9 and comparisons with benitoite BaTiSi3O9. Physical Review B 1996, 54 (1), 291-304. 36. Zhu, Q.-Q.; Wang, L.; Hirosaki, N.; Hao, L. Y.; Xu, X.; Xie, R.-J., Extra-Broad Band Orange-Emitting Ce3+-Doped Y3Si5N9O Phosphor for Solid-State Lighting: Electronic, Crystal Structures and Luminescence Properties. Chem. Mater. 2016, 28 (13), 4829-4839. 37. Van den Eeckhout, K.; Smet, P. F.; Poelman, D., Persistent Luminescence in Eu2+-Doped Compounds: A Review. Materials 2010, 3 (4). 38. Rack, P. D.; Holloway, P. H., The structure, device physics, and material properties of thin film electroluminescent displays. Mater. Sci. Eng. R 1998, 21 (4), 171-219. 39. Ahn, D.; Shin, N.; Park, K. D.; Sohn, K.-S., Energy Transfer Between Activators at Different Crystallographic Sites. J. Electrochem. Soc. 2009, 156 (9), J242-J248. 40. Dexter, D. L.; Schulman, J. H., Theory of Concentration Quenching in Inorganic Phosphors. J. Chem. Phys. 1954, 22 (6), 1063-1070. 41. Blasse, G., Energy transfer between inequivalent Eu2+ ions. J. Solid State Chem. 1986, 62 (2), 207-211. 42. Blasse, G., Energy transfer in oxidic phosphors. Phys. Lett. A 1968, 28 (6), 444-445. 43. Piao, X.; Machida, K.-i.; Horikawa, T.; Hanzawa, H., Self-propagating high temperature synthesis of yellow-emitting Ba2Si5N8:Eu2+ phosphors for white light-emitting diodes. Appl. Phys. Lett. 2007, 91 (4), 041908. 44. Zhang, M.; Xia, Z.; Liu, Q., Thermally stable KxCs12+ phosphors and their photoluminescence tuning. J. xAlSi2O6:Eu Mater. Chem. C 2017, 5 (30), 7489-7494. 45. Piao, X.; Horikawa, T.; Hanzawa, H.; Machida, K.-i., Characterization and luminescence properties of Sr2Si5N8:Eu2+ phosphor for white light-emitting-diode illumination. Appl. Phys. Lett. 2006, 88 (16), 161908. 46. Bachmann, V.; Ronda, C.; Meijerink, A., Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21 (10), 2077-2084. 47. Kim, Y. H.; Arunkumar, P.; Kim, B. Y.; Unithrattil, S.; Kim, E.; Moon, S.-H.; Hyun, J. Y.; Kim, K. H.; Lee, D.; Lee, J.-S.; Im, W. B., A zero-thermal-quenching phosphor. Nat. Mater. 2017, 16, 543. 48. Kang, E.-H.; Choi, S.-W.; Chung, S. E.; Jang, J.; Kwon, S.; Hong, S.-H., Photoluminescence Characteristics of Sr3SiO5: Eu2+ Yellow Phosphors Synthesized by Solid-State Method and Pechini Process. J. Electrochem. Soc. 2011, 158 (11), J330-J333.

49. Zhang, X.; Zhou, L.; Gong, M., Thermal quenching of luminescence of LiSr4(BO3)3:Eu2+ orange‐emitting phosphor. Luminescence 2013, 29 (2), 104-108. 50. Lee, K. H.; Choi, S.; Jung, H.-K.; Im, W. B., Bredigitestructure Ca14Mg2[SiO4]8:Eu2+,Mn2+: A tunable green–redemitting phosphor with efficient energy transfer for solid-state lighting. Acta Mater. 2012, 60 (16), 5783-5790.

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Design and Development of a Bluish-green Luminescent Material (K2HfSi3O9:Eu2+) with Robust Thermal Stability for White Lightemitting Diodes Zuobin Tang, Gangyi Zhang, Yuhua Wang*

Table of Contents (TOC) Graphic

Temperature-dependent photoluminescence properties and practicality of the phosphor K2HfSi3O9: 2%Eu2+,6%Sc3+.

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Design and Development of a Bluish-Green Luminescent Material

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