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Luminescent properties and energy transfer of Sm3+ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs
Accepted Manuscript 3+ Luminescent properties and energy transfer of Sm doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs Lili Wang, Hyeon Mi Noh, Byung Kee Moon, Byung Chun Choi, Jung Hyun Jeong, Jinsheng Shi PII:
S0925-8388(15)31970-8
DOI:
10.1016/j.jallcom.2015.12.179
Reference:
JALCOM 36268
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2015 Revised Date:
17 December 2015
Accepted Date: 22 December 2015
Please cite this article as: L. Wang, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, J. Shi, Luminescent 3+ properties and energy transfer of Sm doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.179. 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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Luminescent properties and energy transfer of Sm3+ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs
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Lili Wang a, Hyeon Mi Noh a, Byung Kee Moon a, Byung Chun Choi a, Jung Hyun Jeong a, *, and Jinsheng Shi b a
Department of Physics, Pukyong National University, Busan 608-737, South Korea
b
Department of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, Qingdao 266109, People’s Republic
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of China
* Corresponding author. E-mail address: [email protected]
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Abstract A series of Sm3+ doped Sr2CaMo1-xWxO6 phosphors were synthesized through solid state
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reaction process and their luminescent properties have been investigated. They presented excitation bands in UV, near UV and blue regions, originating from charge transfer transitions within Mo(W)O6 or Sm3+ f-f transitions. Concentration quenching still occurs among MoO6 groups even when Mo concentration is very low, however, if there is no Mo content in
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Sr2CaMo1-xWxO6 phosphors, absorption in near UV region cannot happen. It was found there is a big difference between the calculated optical band gap of Sr2CaMo0.02W0.98O6: 2% Sm3+ and that of Sr2CaMo0.01W0.99O6: 2% Sm3+. Band structure and density of state of Sr2CaMo0.5W0.5O6 were
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examined to explore the origination of charge transfer transition. Site occupation of Sm3+ in host lattice was discussed based on Raman spectra and group theory analysis. There are two kinds of luminescence centers in Sm3+ doped Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm emission in Sr2CaMo0.02W0.98O6: 2%Sm3+ is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+.
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Energy transfer process was also discussed considering the overlap between the emission spectrum of Mo(W)O6 groups and the absorption spectrum of Sm3+, as well as the atom arrays of
Keywords
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Mo6+(W6+), O2- and Sm3+.
Luminescence; Sr2CaMo1-xWxO6; Sm3+; Energy transfer; Near UV LEDs
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1. Introduction
The efficiency of white light emitting diodes (WLEDs) has been comparable with that of fluorescent lamps and it has gained broad recognition to spread as an excellent alternative for general illumination [1]. Solid state LED are provided with a number of advantages over conventional incandescent bulbs and fluorescent lamps, such as high luminous efficiency, excellent reliability, long operating time and environment friendliness [2]. The most common strategy to create white light is utilizing a blue LED chip to pump YAG: Ce yellow phosphors [3,
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4]. However, this kind of white light is only composed of blue and yellow light and lacks of red composition in emission spectrum, resulting in poor color rendering index (CRI < 80) [5-7].
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Another approach to realize white light is the combination of near ultraviolet (UV) LED (λex = 410 nm) with three primary colors (red, green and blue) phosphors [8-10]. Phosphor layer
plays an important role in the output performance and light quality of LEDs [11, 12]. There have been amount of phosphors containing VO43-, NbO67-, Mo(W)O66- that can be excited in near UV
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region due to charge transfer (CT) transitions in host lattice [13-16]. The absorption capability of host lattice through CT transition is important. Fine capability of host matrix to transport
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excitation energy to activators can optimize luminescence efficiency of phosphors. There has been a renewed interest in phosphors with broad excitation band referred as CT transition, especially phosphors for near UV LEDs (emission in the range 350
410 nm) [17, 18].
Recently, molybdate and tungstate have attracted attention from researchers as potential hosts for Sm3+ activated high efficient phosphors [19-21]. Compounds with tetrahedral MO42- (M=Mo, W) groups have been studied and the broad CTB for O2--M6+ usually lies in 200
350 nm range
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[20, 22-24]. This limits excitation efficiency of the phosphors in near UV and blue region, where the most efficient LEDs are available today. It has been found that Eu3+ or Sm3+ doped double perovskite (Ba, Sr)2CaMoO6 phosphors have excitation bands covering the wide UV range and
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extending to visible light range (200
450 nm), arising from CT transition of MoO6 groups
[25-29]. However, concentration quenching among MoO6 groups usually happens, leading to
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low energy transfer efficiency from MoO6 to activators. In order to understand the wide excitation bands and energy migration among Mo(W)O6 groups as well as energy transfer to activators, Sm3+ doped Sr2CaMo1-xWxO6 phosphors were prepared using solid state reaction. In the case of synthesis of (Ba, Sr)2Ca(Mo, W)O6 phosphors, high temperature solid state reaction is usually used [25-27, 30-32]. It is a conventional route to prepare phosphors [33] and it is chosen in this work because of its easy operability and reproducibility. In previous work, Sm3+ doped Sr2CaMoO6 phosphors have been synthesized successfully [29]. In this paper, the luminescent properties of Sr2CaMo1-xWxO6:Sm3+ phosphors were investigated and energy
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transfer from CT transition to Sm3+ centers was discussed.
2. Experimental section
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2.1. Materials and Synthesis
Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) were prepared through a solid state reaction from SrCO3 (Yakuri Pure Chemical, 95%), CaCO3 (Aldrich, 99%), MoO3 (Yakuri Pure Chemical, 99%), WO3 (Yakuri Pure Chemical, 99%) and
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Sm2O3 (Aladdin, 99.9%). The starting materials were weighted according to the following composition: Sr2CaMo1-xWxO6: 2% Sm3+, where x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94,
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0.96, 0.98, 0.99, 1.0. The powders were mixed in an agate mortar and ground for 30 mins, then followed by being pre-fired at 600 ◦C for 2 h and then at 900 ◦C for another 2 h. Finally, the precursors were calcined at 1200 ◦C for 12 h. 2.2. Characterization
The phase purity was verified by the powder X-ray diffraction (XRD) measurement performed
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on a Bruker D8 Advance X-ray diffractometer (Cu-Kα1 irradiation, λ = 1.5406 Å) and high resolution X-ray diffraction was recorded over an angular (2θ) range of 10-125° with a 0.02° scanning step. UV-Vis diffuse reflectance spectra (DRS) were collected using a V-670 (JASCO)
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UV-Vis spectrophotometer. The phosphors also were characterized by Raman spectra with help of a Raman/PL spectrometer (Horiba Jobin-Yvon, LabRAM HR). Photoluminescence excitation
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(PLE) and emission (PL) measurements were conducted on a spectra fluorophotometer (Hitachi F-4600) with a Xe lamp at room temperature. Lifetime measurement was conducted on a Photon Technology International (PTI) spectrofluorimeter with a phosphorimeter attached to the main system with a Xe-flash lamp (25 W). The internal quantum efficiency was measured using a spectrofluorometer (JASCO FP-8500) with a fluorescence integrating sphere unit (JASCO ILF-834). 2.3. Details of Calculation The calculation of the electronic structures of Sr2CaMo0.5W0.5O6 was performed with the
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density functional theory (DFT) using the CASTEP code, in which the electron-ion interactions were described by pseudopotential method and electronic wave functions are represented by
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means of a plane-wave basis set. Two steps were taken to calculate the electronic structure of Sr2CaMo0.5W0.5O6. The first step is to optimize the crystal structure using the crystallographic data of Sr2CaMoO6 with half of Mo atoms being substituted by W. Generalized Gradient Approximation (GGA) by the Perdew, Burke e Ernzerhof (PBE) formulation was chosen. The
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Broyden, Fletcher, Goldfarb, Shannon (BFGS) algorithm was utilized to perform the geometry optimization. The convergence threshold in geometry optimization cycles for energy difference was set as 5.0 × 10-6 eV/atom, and the maximal ionic Hellmann−Feynman force, the maximum
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stress, the maximal displacement were 0.01 eV/Å, 0.02 GPa, and 5.0 × 10-4 Å, respectively. Lattice parameters and atom coordinates of the unit cell were fixed after crystal structure optimization in the first step. The second step was to calculate the band structure and density of states of Sr2CaMo0.5W0.5O6. The kinetic cutoff energy was 480 eV and Brillouin zone integration was represented using the K-point sampling scheme of 5 × 6 × 6 Monkhorst-Pack grid. Ultrasoft
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pseudopotentials were used to approximate the core electrons.
3. Results and discussion
3.1. Structural characteristics and refinement
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In order to determine the appropriate doping concentration of W, Sr2CaMo1-xWxO6: 2%Sm3+
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(x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) were synthesized. Sr2CaMoO6 and Sr2CaWO6 are orthorhombic structures with space group Pmm2 and lattice constants of a = 8.1933 Å, b = 5.7611 Å, c = 5.841 Å, α = β = γ = 90º, V = 275.71 Å3, Z = 2; a = 8.2033 Å, b = 5.7676 Å, c = 5.8489 Å, α = β = γ = 90º, V = 276.73 Å3, Z = 2, respectively. The XRD patterns of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8) are shown in Fig. S1 (Supporting Information). It can be found that as W content was increased, the crystal structure of the lattice show no significant changes because of the formation of Sr2CaMoO6 and Sr2CaWO6 solid solutions. The XRD patterns of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.90, 0.92, 0.94, 0.96, 0.98,
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0.99, 1.0) as well as the standard JCPDS files No. 48-0799 and No. 76-1983 are given in Fig. 1. Except for the orange line (Sr2CaMoO6: 2% Sm3+), the main phase of other samples is Sr2CaWO6 and a small amount of Sm3+ do not affect the crystal structure. In addition, impurity
27.66° is corresponding to the SrMoO4 phase (JCPDS 08-0482).
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peaks were only observed in Sr2CaMoO6: 2%Sm3+ sample, in which the diffraction peak at about
Fig. 2 shows the observed, calculated, and difference XRD patterns of Sr2CaWO6 refined
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using the Le Bail and Rietveld method performed by General Structure Analysis System (GSAS) program. The single crystal structure data of Sr2CaWO6 (ICSD #36460) and SrWO4 (ICSD
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#23701) were used as starting structure models. The refinement results show that Sr2CaWO6 crystallizes in an orthorhombic unit cell with lattice constants of a = 8.2056 Å, b = 5.7751 Å, c = 5.8562 Å, α = β = γ = 90º, and SrWO4 in a tetragonal one with the space group I41/a and lattice constants a = b = 5.4241 Å, c = 11.9387 Å, α = β = γ = 90º. The reliability factors of the refinement are Rwp = 11.23%, Rp = 7.51%, χ2 = 2.491, indicating the calculated patterns are in
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good agreement with the observed ones.
3.2. Concentration quenching among MoO6 groups In previous studies on Mo-rich phosphors, luminescence is usually weak because concentration quenching occurs among MoO6 groups and less energy can be transferred to
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activators [28, 29]. In general, WO6 groups can be introduced into the lattice and used as obstacles to block the energy migration among MoO6 groups, resulting in more energy being
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trapped by the luminescence centers [16, 26]. The excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) monitoring Sm3+ 648 nm emission are given in Fig. 3. Except for the narrow bands originating from f-f transitions of Sm3+ ions, the broad excitation band corresponds to O2-→W6+ or O2-→Mo6+ CT transitions. Compared with Sr2CaWO6: 2% Sm3+, W-O CT bands peaking at about 310 nm in Sr2CaMo0.2W0.8O6: 2% Sm3+ and Sr2CaMo0.4W0.6O6: 2% Sm3+ are very weak, and that in other samples nearly vanished. It is known that in Mo-rich hosts, absorbed energy can be strongly quenched by the energy migration along MoO6 framework via exchange mechanism [34]. When Sr2CaWO6: 2% Sm3+ was exposed
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to UV light, energy can be absorbed due to electronic transition from O 2p to W 5d state, and finally transferred to Sm3+ luminescence centers. However, in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8), most of the absorbed energy by WO6 groups was quenched before
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reaching Sm3+ centers due to migration among MoO6, resulting in weak luminescence. 3.3. Diffuse reflection spectra and optical band gap
In order to understand their absorption capability in UV and near UV light region, the diffuse
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reflection spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) are depicted in Fig. 4. It clearly shows absorption edge of the phosphors
300
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shifted towards shorter wavelengths as Mo content was decreased. The absorption in the range of 400 nm of Sr2CaWO6: 2% Sm3+ is weak but it has an obvious absorption band at about
300 nm. In order to evaluate the optical band gap, the diffuse reflectance (R∞) of some selected samples were converted to Kubelka–Munk function F(R∞) according to [35]: F(R∞) =
(ଵିோ∞ )మ ଶோ∞
=
௦
(1)
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where F(R∞) is the Kubelka–Munk function, k is the molar absorption coefficient and s is the scattering coefficient. The optical band gap of oxide semiconductors can be calculated by [36]: αhν = C1(hν - Egap)n
(2)
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where α is the linear absorption coefficient of the material, hν is the phonon energy, C1 is the proportionality constant, Egap is optical band gap, and n is a constant related to different kinds of
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electronic transitions (n = 0.5 for a direct allowed, n =1.5 for a direct forbidden, and n = 2 for an indirect allowed). According to our previous work, it has been known that both Sr2CaMoO6 and Sr2CaWO6 exhibit optical absorption governed by indirect electronic transitions [37, 38]. For s is treated independent of the wavelength, and k is proportional to α, thence Kubelka–Munk function F(R∞) is proportional to α. Using the functions given in Eqs. (1) and (2) with n = 2, it can be obtained that: [hνF(R∞)]1/2 = C2(hν - Egap)
(3)
Therefore, we plot [hνF(R∞)]1/2 against hν, and Egap values can be determined for the phosphors.
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The corresponding Egap values for Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) are presented in Fig. 5. It shows roughly that a decrease in Mo concentration produces an increase in Egap. The Egap values for Sr2CaMo0.6W0.4O6: 2% Sm3+ and Sr2CaMo0.08W0.92O6:
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2% Sm3+ are 2.8830 and 2.9841 eV, respectively. It is noteworthy that the band gap value was increased only by 0.1011 eV when the percentage of Mo in the host varied from 60% to 8%. The largest variation of Egap happened when Mo concentration decreased from 2% to 1%. In
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Sr2CaMo0.02W0.98O6: 2% Sm3+, Egap value is 3.2497 eV, and it changed into 3.6234 eV in Sr2CaMo0.01W0.99O6: 2% Sm3+. The absorption capability of near UV light will be much better in
3.4. Band structure and density of states
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the former.
For the purpose of clarifying the role of Mo on the band structure in Sr2CaMo1-xWxO6 host, we understood chemical bonding properties and origin of CT transitions of Sr2CaMo1-xWxO6 by examining band structure and partial density of states (PDOS) of atoms. Appropriate calculations
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were performed to explore the influence of the replacement of Mo with W in the Sr2CaMoO6 host using the DFT method. The cutoff energy was 480 eV, and a unit cell with 20 atoms was used. One Mo atom is substituted by W and therefore the W percentage is 50%. Due to the large and expensive computation, the cases with the Mo percentage were not simulated. The calculated
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band structure along the high symmetry points of the first Brillouin zone for Sr2CaMo0.5W0.5O6 is shown in Fig. 6(a). It can be seen that the lowest energy of conduction bands (CBs) was localized
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at G point, while the highest point of the valence bands (VBs) was in Y. Hence, Sr2CaMo0.5W0.5O6 is an indirect band gap material and the gap between the lowest energy of the CBs and the highest energy of the VBs is about 2.3020 eV, which is smaller by 0.5810 eV than the optical band gap of Sr2CaMo0.6W0.4O6: 2% Sm3+. The difference comes from GGA in DFT calculations generally underestimating the band gap of insulators and semiconductors. This is because that DFT is a ground state theory, however band gap belongs to the properties of excited states. Comparatively, optical band gap estimated from DRS spectra comes nearest to the real. The band structure of low energy CBs (2.0
4.5 eV) is composed of two parts and it is
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expressed as two peaks in the density of states in Fig. 6(b). In order to explain the source of the two parts of CBs as well as the origin of the highest energy of VBs, the partial density of states
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(PDOS) of Sr2CaMo0.5W0.5O6 is illustrated in Fig. 7. The conduction band in the range of 2.0 4.5 eV is dominated by Mo-4d and W-5d, mixed with a small amount of O-2p states. The valence band just below the Fermi level (0.0 eV) was mainly occupied by O-2p, with some contributions of Mo-4d and W-5d states. Therefore, the absorption in UV and near UV region can be ascribed
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to the CT transitions from O-2p to W-5d and Mo-4d states, respectively. Combined with the optical band gap calculations and the diffuse reflectance spectra, it can be concluded that there
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will be efficient absorption in near UV region even when the content of Mo is very low because of its occupation in the lowest conduction band of Sr2CaMo1-xWxO6 host. 3.5. Raman spectra
Group theory analysis indicates that Sr2CaMoO6 has four Raman active modes T2g (Sr) + T2g (O) + Eg (O) + A1g (O) [39]. Raman spectra of Sr2CaMoO6 shown in Fig. 8 give peaks at about
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100-200, 437, 537 and 786 cm-1, which can be assigned to T2g (Sr), T2g (O), Eg (O) and A1g (O), respectively. T2g (Sr) mode splits into four peaks and it means the symmetry of Sr site is lowered because of the replacement by Sm3+ ions. As can be seen from the enlarged insert for 100-200 cm-1, the four peaks shifted slightly, which implies Sm3+ ions are incorporated into Sr2+ site of
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Sr2CaMo1-xWxO6. When Sm3+ ions are doped into Sr2CaMo1-xWxO6 host, Sm3+ ions (r =1.24 Å when CN = 12) substitute for Sr2+ (r =1.44 Å when CN = 12) sites. There are two approaches to
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achieve charge compensation: (a) two Sr2+ ions are replaced by one Sm3+ and one monovalent cation, like Li+, Na+ and K+; (b) charge compensation is provided by a strontium vacancy. In our present work, the phosphors were synthesized without any charge compensation ions and the obtained structures are still consistent with the standard. Considering the charge balance, the charge loss in the lattice is probably compensated by Sr2+ vacancies VSr: 3Sr2+ + 2Sm3+ → VSr + 2[SmSr]. In our samples, the Sr2+ vacancies which were induced by charge compensation was not rich because Sm3+ concentration was low. The influence of different charge compensation models on the luminescence properties of some red phosphors has been investigated. It has been
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found that both monovalent cations and cation vacancy can lead to enhanced luminescence and the strongest emission intensity was observed in the former [40-42]. In our future work, further
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experiments, such as varying the oxygen fugacity to control the intrinsic defect populations, will help to identify the situations.
According to group theory analysis, the A1g peak corresponds to the stretch vibration of the oxygen in MoO6 or WO6 octahedra. With the increase of W concentration in the series of
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Sr2CaMo1-xWxO6, the A1g mode was divided into two peaks. The intensity of the A1g (O) peak at 786 cm-1 from MoO6 octahedra was significantly reduced and the new peak appeared at a higher
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wavenumber. Qualitatively, the wavenumber (ν) of a vibration mode is related to the bonding strength (k) and the reduced atom mass (m): ν
ට .
(4)
The reduced atom mass m of WO6 is larger than that for MoO6, however, the A1g peak related to WO6 shifts to higher wavenumber, which indicates that the bonding between W and O is stronger
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than that between Mo and O. In molybdate and tungstate compounds, the energy of O2-→Mo6+ and O2-→W6+ CT transition is dependent on M-O (M=Mo, W) bond strength [43]. The stronger bonding between M (M=Mo, W) and O, the more energy needed for CT transition from O2- to
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M6+. Optical band gap values of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) have been calculated above. On the whole, Egap values for Sr2CaMo1-xWxO6 grew as
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the W content increased, and then CT transition happens over larger band gap. It is not difficult to understand that increasing W concentration leads to stronger M-O bonding, accordingly, it can be further deduced that the increased band gap of Sr2CaMo1-xWxO6 as the increased W content is caused by the stronger W-O bonding. 3.6. Photoluminescence properties Fig. 9 presents the excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99) monitoring Sm3+ 648 nm emission, as well as the emission spectra pumped by 302, 309, 355 or 407 nm. The narrow excitation bands at 346, 355, 364, 378, 391, 407, 420, 440, 451,
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472, 480, 489 and 528 nm can be assigned to the electronic transitions of Sm3+ from 6H5/2 to 4
D7/2, 6P5/2, 4D3/2, 4P7/2, 4L15/2, 6P3/2, 6P5/2, 4M17/2, 4I13/2, 4I11/2, 4M15/2, 4I9/2 and 4G5/2. [44]. With the
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increase of W concentration, a shoulder excitation band at 300 nm was observed in Fig. 9(d) and two obviously separated bands at about 302 and 350 nm began to appear in Fig. 9(f), which come from CT transitions in MoO6 and WO6 groups, respectively. The sharp and intense excitation band at 407 nm originating from 6H5/2
6
P3/2 transition of Sm3+ ions can be observed
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in all excitation spectra in Fig. 9. Under the radiation of 407 nm, all samples exhibited three emission bands, which are assigned to 4G5/2→6H5/2 (569, 579 nm; 566 nm), 4G5/2→6H7/2 (603, 608, 618 nm; 603 nm), 4G5/2→6H9/2 (648 nm; 649 nm) transitions [45, 46]. The major part of G5/2→6H5/2 transition is magnetic dipole (MD) allowed, and 4G5/2→6H9/2 electronic transition is
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purely electric dipole (ED) dominated. For 4G5/2→6H7/2 transition, the magnetic dipole character is very low [47, 48]. The intensity ratio of ED to MD can be used to measure the departure from centrosymmetry of sites occupied by Sm3+ ions. The profiles of the emission spectra under excitation into the 6P3/2 level of Sm3+ (407 nm) and into the CT band are different except for that
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in Sr2CaMoO6: 2% Sm3+ (Fig. 9(a)). Three transitions give almost the same intensity when radiation energy is 302 or 309 nm (Fig. 9(b)-(h)), however, when the excitation energy is 407 nm, the red emission at 648 nm dominates in luminescence intensity. Similar situation can be
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observed in Sm3+ doped CeO2, in which both cubic and low-symmetry Sm3+ centers co-exist. The cubic Sm3+ luminescence centers are sensitized via CT band of CeO2, and the low-symmetry
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Sm3+ centers prefer to be excited by f-f transition [49]. For Sm3+ centers departed from centrosymmetry, the emission spectra will mainly display 4G5/2→6H9/2 transition. In Fig. 9(a), the profiles of emission spectra of Sr2CaMoO6: 2% Sm3+ are similar when the pumped energy is 355 or 407 nm. Thus it can be concluded that there is no Sm3+ luminescence centers with centrosymmetry in Sr2CaMoO6: 2% Sm3+. However, when WO6 groups are the major of the host, the situation is different. When the phosphor was excited via f-f transition (407 nm), it exhibited emission dominated by
4
G5/2→6H9/2 transition, and Sm3+ centers gave emission spectra
consisting of three bands with almost same intensity when excited by 302 or 309 nm. Thus it can
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be concluded that in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0), there are two kinds of luminescence centers, and one of them possesses centrosymmetry. Fig. 10 shows the excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.90, 0.92, 0.94,
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0.96, 0.98, 0.99, 1.0) monitoring 608 nm emission of Sm3+ and the emission spectra of Sr2CaMo0.02W0.98O6: 2%Sm3+ and Sr2CaMoO6: 2%Sm3+. The black line is the excitation spectrum of Sr2CaMoO6: 2% Sm3+ and its intensity is very low compared with others. Very
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interestingly, in spite of Mo concentrations are very low (≤0.1), the intensity of Mo-O CT band in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99) is considerably enhanced
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compared with that in Sr2CaMoO6: 2% Sm3+. Concentration quenching among MoO6 groups discussed above can explain this phenomenon. A little amount of Mo in Sr2CaMo1-xWxO6 can make it absorb efficiently in near UV region.
The excitation spectrum (monitoring at 608 nm) of Sr2CaMo0.02W0.98O6: 2%Sm3+ show the most intense absorption band around 350 nm, which is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+. It means that when Sr2CaMo0.02W0.98O6: 2%Sm3+ was exposed to near UV
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light, electrons can jump from O 2p to Mo 4d orbital, and then energy is transferred efficiently to Sm3+ ions, emitting orange-red light. When the excitation wavelength is 350 nm, emission spectra profiles of the two phosphors are distinctly different. Both 4G5/2→6H5/2 and 4G5/2→6H7/2 into two
peaks
and
their
intensity was enhanced
obviously in
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transitions split
Sr2CaMo0.02W0.98O6: 2%Sm3+ compared with that in Sr2CaMoO6: 2%Sm3+. The internal quantum
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efficiency of Sr2CaMo0.02W0.98O6: 2% Sm3+ was measured to be 2.601 %. The low quantum efficiency of our samples is partly because of the particle agglomeration and non uniform size distribution in the synthesized phosphors. It is believed that the quantum efficiency can be further enhanced by controlling the particle size and morphology through the optimized synthetic route. 3.7. Energy transfer process Why there is energy transfer blockage from CTB to Sm3+ in Sr2CaMoO6: 2%Sm3+ but energy transfer from O2-→W6+ CT transition to Sm3+ in Sr2Ca2WO6: 2%Sm3+ is efficient? The
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luminescence of Eu3+ in mixed metal oxides had been investigated by Blasse and four rules were proposed to explain the photoluminescence efficiency of Eu3+. The discussion was restricted to Eu3+ activation but the considerations are also valid for other rare earth ions activated oxides.
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Therefore here we discuss the energy transfer process from CT band to Sm3+ using the rules [34]. As presented in Fig. 9(h), the blue emission band peaking at about 430 nm was from the intrinsic emission of WO6 group, and it is evident that the tungstate emission strongly overlaps the Sm3+
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strongest absorption feature. However, MoO6 emission cannot be observed in Sr2CaMoO6: 2%Sm3+ at room temperature and the emission maximum is at 625 nm at 4.2 K in Ba2CaMoO6 [50]. In molybdate case, its overlap with Sm3+ absorption bands is very limited. Therefore, as
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shown in Fig. 10, when Sm3+ 608 nm was monitored, the intensity of O2-→W6+ CT transition in Sr2Ca2WO6: 2%Sm3+ is about 16 times higher than that of O2-→Mo6+ in Sr2CaMoO6: 2%Sm3+, indicating energy transfer from WO6 to Sm3+ is significantly more efficient than that from MoO6 to Sm3+. When there were no WO6 groups introduced into the host, energy migration among MoO6 groups was not be blocked and finally reached quenching centers, then less energy can be
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transferred to Sm3+, resulting in weak luminescence of Sm3+ in Sr2CaMoO6. In order to further investigate energy transfer from W(Mo)O6 group to Sm3+, the decay curves of Sm3+ emission at 608 nm corresponding to the 4G5/2→6H7/2 transition in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.98,
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1.0) were measured and shown in Fig. 11. As indicated in Fig. 11, the decay curves can be well fitted into an exponential function as: (5)
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I = Aexp(-t/τ) + I0
where I and I0 are luminescence intensity, A is constant, t is time and τ is decay time. For Sr2CaMo1-xWxO6: 2%Sm3+ with x = 0, 0.98, 1.0, the lifetime is estimated to be 421.4, 598.4 and 620.3 µs, respectively. It can be found that under excitation into the CT transition, the lifetime of 4
G5/2 state in Sr2CaMoO6: 2% Sm3+ is the shortest, followed by that in Sr2CaMo0.02W0.98O6: 2%
Sm3+, and the longest is in Sr2CaWO6: 2% Sm3+. The variation of the measured lifetimes has the same trend as the W content, indicating that non-radiative process due to energy migration and quenching among MoO6 groups speeded up the luminescence decay.
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Based on the discussion above, one way to enhance energy transfer from CT transition to Sm3+ as well as guarantee efficient absorption in near UV region is replacing Mo using appropriate W.
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Another method may be changing the substituted sites of Sm3+ ions. Sr2CaWO6 and Sr2CaMoO6 belong to A2BB′O6 double perovskite family, and CaO6 and MoO6 octahedrals align collinearly sharing their six vertex oxygen atoms. The angles of Mo-O-Ca are 180◦ and 172.478◦. However, Mo-O is nearly vertical to Sr-O bond; the angles of Mo-O-Sr are 98.026◦ and 95.342◦. Sm3+ ions
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prefer to enter the Sr sites in Sr2CaWO6 and Sr2CaMoO6. Therefore, if excitation energy is absorbed though electrons migrating from O 2p to W 5d or Mo 4d orbitals, it can be efficiently transferred to Sm3+ along O-W-Sm (Ca)-O-W- collinear array. Although near UV light can pump
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electrons from 1A1g to 1T1u level of the MoO6 group, due to Sr-O bond locating outside the Mo-O plane, energy transfer from MoO6 group to Sm (Sr) is not easy. Changing the substituted sites of Sm3+ ions to Ca2+ will be very useful for further enhancing the absorption capability in near UV region for Sr2CaMo1-xWxO6: 2% Sm3+.
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4. Conclusions
Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) phosphors were synthesized through solid state reaction method. The phosphors presented broad excitation bands in near UV and blue regions, originating from charge transfer transitions within
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Mo(W)O6 or Sm3+ f-f transitions. In Sr2CaMo1-xWxO6: 2%Sm3+ (x<0.8), most of the absorbed energy by WO6 was quenched before reaching Sm3+ centers due to migration among MoO6
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groups. The band gap values of the selected samples were calculated and it increased roughly as the increasing W content. The biggest difference occurred between the optical band gap values of Sr2CaMo0.01W0.99O6: 2% Sm3+ (3.2497 eV) and Sr2CaMo0.02W0.98O6: 2% Sm3+ (3.6234 eV). Band structure and density of states of Sr2CaMo0.5W0.5O6 was investigated using DFT method in CASTEP mode and Mo-4d states occupy the lowest conduction band of the host. There still is efficient absorption in near UV region in Sr2CaMo0.01W0.99O6: 2% Sm3+ due to the contribution of MoO6 groups. Raman spectra gave the evidence that Sr2+ sites are replaced by Sm3+ ions and W-O bonding is stronger than Mo-O. There are two kinds of luminescence centers in Sm3+ doped
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Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the intensity of 608 nm red emission under 350 nm radiation was 8.5 times stronger than that in Sr2CaMoO6: Sm3+.
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Thus in Sr2CaMo0.02W0.98O6: 2% Sm3+ not only concentration quenching among MoO6 groups can be prevented but efficient absorption in near UV region can be realized. The phosphor can find application as red emitting phosphor for white LEDs based on near UV LED chips.
Acknowledgements
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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future
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Planning (No. 2013012655). The Sm3+ doped Sr2CaW1-xMoxO6 phosphor (x=0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 1.0) was supplied by the Display and Lighting Phosphor Bank at Pukyong National University.
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Figure Captions Fig. 1. XRD pattern of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) and Sr2CaWO6, Sr2CaMoO6 standard patterns.
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Fig. 2. Experimental (black circles) and calculated (red solid line) XRD patterns and their difference (blue solid line) for the fit of Sr2CaWO6:2%Sm3+ by the GSAS program. The short green and magenta vertical lines show the position of Bragg reflections of the calculated patterns.
Fig. 3. Excitation spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) monitoring
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Sm3+ emission at 650 nm.
0.98, 0.99, 1.0).
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Fig. 4. Diffuse reflectance spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96,
Fig. 5. The determination of the optical band gap of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) using the Kubellka-Munk function.
Fig. 6. Calculated band structure (a) and total densities of states (b) of Sr2CaMo0.5W0.5O6 near the Femi energy level (EF). The Femi energy is the zero of the energy scale.
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Fig. 7. Partial densities of states of Sr2CaMo0.5W0.5O6 and some atoms constituting it: O, Mo, and W. Fig. 8. Raman spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0). Fig. 9. Excitation and emission spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0).
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Fig. 10. Excitation and emission spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0). Fig. 11. PL decay curves of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.98, 1.0) under excitation into charge transfer
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radiations.
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The 608 nm emission of the best one is 8.5 times higher than that in Sr2CaMoO6: Sm3+.
S0925-8388(15)31970-8
DOI:
10.1016/j.jallcom.2015.12.179
Reference:
JALCOM 36268
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2015 Revised Date:
17 December 2015
Accepted Date: 22 December 2015
Please cite this article as: L. Wang, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, J. Shi, Luminescent 3+ properties and energy transfer of Sm doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.179. 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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Luminescent properties and energy transfer of Sm3+ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs
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Lili Wang a, Hyeon Mi Noh a, Byung Kee Moon a, Byung Chun Choi a, Jung Hyun Jeong a, *, and Jinsheng Shi b a
Department of Physics, Pukyong National University, Busan 608-737, South Korea
b
Department of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, Qingdao 266109, People’s Republic
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of China
* Corresponding author. E-mail address: [email protected]
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Abstract A series of Sm3+ doped Sr2CaMo1-xWxO6 phosphors were synthesized through solid state
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reaction process and their luminescent properties have been investigated. They presented excitation bands in UV, near UV and blue regions, originating from charge transfer transitions within Mo(W)O6 or Sm3+ f-f transitions. Concentration quenching still occurs among MoO6 groups even when Mo concentration is very low, however, if there is no Mo content in
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Sr2CaMo1-xWxO6 phosphors, absorption in near UV region cannot happen. It was found there is a big difference between the calculated optical band gap of Sr2CaMo0.02W0.98O6: 2% Sm3+ and that of Sr2CaMo0.01W0.99O6: 2% Sm3+. Band structure and density of state of Sr2CaMo0.5W0.5O6 were
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examined to explore the origination of charge transfer transition. Site occupation of Sm3+ in host lattice was discussed based on Raman spectra and group theory analysis. There are two kinds of luminescence centers in Sm3+ doped Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm emission in Sr2CaMo0.02W0.98O6: 2%Sm3+ is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+.
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Energy transfer process was also discussed considering the overlap between the emission spectrum of Mo(W)O6 groups and the absorption spectrum of Sm3+, as well as the atom arrays of
Keywords
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Mo6+(W6+), O2- and Sm3+.
Luminescence; Sr2CaMo1-xWxO6; Sm3+; Energy transfer; Near UV LEDs
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1. Introduction
The efficiency of white light emitting diodes (WLEDs) has been comparable with that of fluorescent lamps and it has gained broad recognition to spread as an excellent alternative for general illumination [1]. Solid state LED are provided with a number of advantages over conventional incandescent bulbs and fluorescent lamps, such as high luminous efficiency, excellent reliability, long operating time and environment friendliness [2]. The most common strategy to create white light is utilizing a blue LED chip to pump YAG: Ce yellow phosphors [3,
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4]. However, this kind of white light is only composed of blue and yellow light and lacks of red composition in emission spectrum, resulting in poor color rendering index (CRI < 80) [5-7].
350
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Another approach to realize white light is the combination of near ultraviolet (UV) LED (λex = 410 nm) with three primary colors (red, green and blue) phosphors [8-10]. Phosphor layer
plays an important role in the output performance and light quality of LEDs [11, 12]. There have been amount of phosphors containing VO43-, NbO67-, Mo(W)O66- that can be excited in near UV
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region due to charge transfer (CT) transitions in host lattice [13-16]. The absorption capability of host lattice through CT transition is important. Fine capability of host matrix to transport
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excitation energy to activators can optimize luminescence efficiency of phosphors. There has been a renewed interest in phosphors with broad excitation band referred as CT transition, especially phosphors for near UV LEDs (emission in the range 350
410 nm) [17, 18].
Recently, molybdate and tungstate have attracted attention from researchers as potential hosts for Sm3+ activated high efficient phosphors [19-21]. Compounds with tetrahedral MO42- (M=Mo, W) groups have been studied and the broad CTB for O2--M6+ usually lies in 200
350 nm range
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[20, 22-24]. This limits excitation efficiency of the phosphors in near UV and blue region, where the most efficient LEDs are available today. It has been found that Eu3+ or Sm3+ doped double perovskite (Ba, Sr)2CaMoO6 phosphors have excitation bands covering the wide UV range and
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extending to visible light range (200
450 nm), arising from CT transition of MoO6 groups
[25-29]. However, concentration quenching among MoO6 groups usually happens, leading to
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low energy transfer efficiency from MoO6 to activators. In order to understand the wide excitation bands and energy migration among Mo(W)O6 groups as well as energy transfer to activators, Sm3+ doped Sr2CaMo1-xWxO6 phosphors were prepared using solid state reaction. In the case of synthesis of (Ba, Sr)2Ca(Mo, W)O6 phosphors, high temperature solid state reaction is usually used [25-27, 30-32]. It is a conventional route to prepare phosphors [33] and it is chosen in this work because of its easy operability and reproducibility. In previous work, Sm3+ doped Sr2CaMoO6 phosphors have been synthesized successfully [29]. In this paper, the luminescent properties of Sr2CaMo1-xWxO6:Sm3+ phosphors were investigated and energy
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transfer from CT transition to Sm3+ centers was discussed.
2. Experimental section
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2.1. Materials and Synthesis
Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) were prepared through a solid state reaction from SrCO3 (Yakuri Pure Chemical, 95%), CaCO3 (Aldrich, 99%), MoO3 (Yakuri Pure Chemical, 99%), WO3 (Yakuri Pure Chemical, 99%) and
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Sm2O3 (Aladdin, 99.9%). The starting materials were weighted according to the following composition: Sr2CaMo1-xWxO6: 2% Sm3+, where x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94,
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0.96, 0.98, 0.99, 1.0. The powders were mixed in an agate mortar and ground for 30 mins, then followed by being pre-fired at 600 ◦C for 2 h and then at 900 ◦C for another 2 h. Finally, the precursors were calcined at 1200 ◦C for 12 h. 2.2. Characterization
The phase purity was verified by the powder X-ray diffraction (XRD) measurement performed
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on a Bruker D8 Advance X-ray diffractometer (Cu-Kα1 irradiation, λ = 1.5406 Å) and high resolution X-ray diffraction was recorded over an angular (2θ) range of 10-125° with a 0.02° scanning step. UV-Vis diffuse reflectance spectra (DRS) were collected using a V-670 (JASCO)
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UV-Vis spectrophotometer. The phosphors also were characterized by Raman spectra with help of a Raman/PL spectrometer (Horiba Jobin-Yvon, LabRAM HR). Photoluminescence excitation
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(PLE) and emission (PL) measurements were conducted on a spectra fluorophotometer (Hitachi F-4600) with a Xe lamp at room temperature. Lifetime measurement was conducted on a Photon Technology International (PTI) spectrofluorimeter with a phosphorimeter attached to the main system with a Xe-flash lamp (25 W). The internal quantum efficiency was measured using a spectrofluorometer (JASCO FP-8500) with a fluorescence integrating sphere unit (JASCO ILF-834). 2.3. Details of Calculation The calculation of the electronic structures of Sr2CaMo0.5W0.5O6 was performed with the
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density functional theory (DFT) using the CASTEP code, in which the electron-ion interactions were described by pseudopotential method and electronic wave functions are represented by
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means of a plane-wave basis set. Two steps were taken to calculate the electronic structure of Sr2CaMo0.5W0.5O6. The first step is to optimize the crystal structure using the crystallographic data of Sr2CaMoO6 with half of Mo atoms being substituted by W. Generalized Gradient Approximation (GGA) by the Perdew, Burke e Ernzerhof (PBE) formulation was chosen. The
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Broyden, Fletcher, Goldfarb, Shannon (BFGS) algorithm was utilized to perform the geometry optimization. The convergence threshold in geometry optimization cycles for energy difference was set as 5.0 × 10-6 eV/atom, and the maximal ionic Hellmann−Feynman force, the maximum
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stress, the maximal displacement were 0.01 eV/Å, 0.02 GPa, and 5.0 × 10-4 Å, respectively. Lattice parameters and atom coordinates of the unit cell were fixed after crystal structure optimization in the first step. The second step was to calculate the band structure and density of states of Sr2CaMo0.5W0.5O6. The kinetic cutoff energy was 480 eV and Brillouin zone integration was represented using the K-point sampling scheme of 5 × 6 × 6 Monkhorst-Pack grid. Ultrasoft
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pseudopotentials were used to approximate the core electrons.
3. Results and discussion
3.1. Structural characteristics and refinement
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In order to determine the appropriate doping concentration of W, Sr2CaMo1-xWxO6: 2%Sm3+
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(x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) were synthesized. Sr2CaMoO6 and Sr2CaWO6 are orthorhombic structures with space group Pmm2 and lattice constants of a = 8.1933 Å, b = 5.7611 Å, c = 5.841 Å, α = β = γ = 90º, V = 275.71 Å3, Z = 2; a = 8.2033 Å, b = 5.7676 Å, c = 5.8489 Å, α = β = γ = 90º, V = 276.73 Å3, Z = 2, respectively. The XRD patterns of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8) are shown in Fig. S1 (Supporting Information). It can be found that as W content was increased, the crystal structure of the lattice show no significant changes because of the formation of Sr2CaMoO6 and Sr2CaWO6 solid solutions. The XRD patterns of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.90, 0.92, 0.94, 0.96, 0.98,
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0.99, 1.0) as well as the standard JCPDS files No. 48-0799 and No. 76-1983 are given in Fig. 1. Except for the orange line (Sr2CaMoO6: 2% Sm3+), the main phase of other samples is Sr2CaWO6 and a small amount of Sm3+ do not affect the crystal structure. In addition, impurity
27.66° is corresponding to the SrMoO4 phase (JCPDS 08-0482).
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peaks were only observed in Sr2CaMoO6: 2%Sm3+ sample, in which the diffraction peak at about
Fig. 2 shows the observed, calculated, and difference XRD patterns of Sr2CaWO6 refined
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using the Le Bail and Rietveld method performed by General Structure Analysis System (GSAS) program. The single crystal structure data of Sr2CaWO6 (ICSD #36460) and SrWO4 (ICSD
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#23701) were used as starting structure models. The refinement results show that Sr2CaWO6 crystallizes in an orthorhombic unit cell with lattice constants of a = 8.2056 Å, b = 5.7751 Å, c = 5.8562 Å, α = β = γ = 90º, and SrWO4 in a tetragonal one with the space group I41/a and lattice constants a = b = 5.4241 Å, c = 11.9387 Å, α = β = γ = 90º. The reliability factors of the refinement are Rwp = 11.23%, Rp = 7.51%, χ2 = 2.491, indicating the calculated patterns are in
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good agreement with the observed ones.
3.2. Concentration quenching among MoO6 groups In previous studies on Mo-rich phosphors, luminescence is usually weak because concentration quenching occurs among MoO6 groups and less energy can be transferred to
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activators [28, 29]. In general, WO6 groups can be introduced into the lattice and used as obstacles to block the energy migration among MoO6 groups, resulting in more energy being
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trapped by the luminescence centers [16, 26]. The excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) monitoring Sm3+ 648 nm emission are given in Fig. 3. Except for the narrow bands originating from f-f transitions of Sm3+ ions, the broad excitation band corresponds to O2-→W6+ or O2-→Mo6+ CT transitions. Compared with Sr2CaWO6: 2% Sm3+, W-O CT bands peaking at about 310 nm in Sr2CaMo0.2W0.8O6: 2% Sm3+ and Sr2CaMo0.4W0.6O6: 2% Sm3+ are very weak, and that in other samples nearly vanished. It is known that in Mo-rich hosts, absorbed energy can be strongly quenched by the energy migration along MoO6 framework via exchange mechanism [34]. When Sr2CaWO6: 2% Sm3+ was exposed
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to UV light, energy can be absorbed due to electronic transition from O 2p to W 5d state, and finally transferred to Sm3+ luminescence centers. However, in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.1, 0.2, 0.4, 0.6, 0.8), most of the absorbed energy by WO6 groups was quenched before
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reaching Sm3+ centers due to migration among MoO6, resulting in weak luminescence. 3.3. Diffuse reflection spectra and optical band gap
In order to understand their absorption capability in UV and near UV light region, the diffuse
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reflection spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) are depicted in Fig. 4. It clearly shows absorption edge of the phosphors
300
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shifted towards shorter wavelengths as Mo content was decreased. The absorption in the range of 400 nm of Sr2CaWO6: 2% Sm3+ is weak but it has an obvious absorption band at about
300 nm. In order to evaluate the optical band gap, the diffuse reflectance (R∞) of some selected samples were converted to Kubelka–Munk function F(R∞) according to [35]: F(R∞) =
(ଵିோ∞ )మ ଶோ∞
=
௦
(1)
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where F(R∞) is the Kubelka–Munk function, k is the molar absorption coefficient and s is the scattering coefficient. The optical band gap of oxide semiconductors can be calculated by [36]: αhν = C1(hν - Egap)n
(2)
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where α is the linear absorption coefficient of the material, hν is the phonon energy, C1 is the proportionality constant, Egap is optical band gap, and n is a constant related to different kinds of
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electronic transitions (n = 0.5 for a direct allowed, n =1.5 for a direct forbidden, and n = 2 for an indirect allowed). According to our previous work, it has been known that both Sr2CaMoO6 and Sr2CaWO6 exhibit optical absorption governed by indirect electronic transitions [37, 38]. For s is treated independent of the wavelength, and k is proportional to α, thence Kubelka–Munk function F(R∞) is proportional to α. Using the functions given in Eqs. (1) and (2) with n = 2, it can be obtained that: [hνF(R∞)]1/2 = C2(hν - Egap)
(3)
Therefore, we plot [hνF(R∞)]1/2 against hν, and Egap values can be determined for the phosphors.
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The corresponding Egap values for Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) are presented in Fig. 5. It shows roughly that a decrease in Mo concentration produces an increase in Egap. The Egap values for Sr2CaMo0.6W0.4O6: 2% Sm3+ and Sr2CaMo0.08W0.92O6:
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2% Sm3+ are 2.8830 and 2.9841 eV, respectively. It is noteworthy that the band gap value was increased only by 0.1011 eV when the percentage of Mo in the host varied from 60% to 8%. The largest variation of Egap happened when Mo concentration decreased from 2% to 1%. In
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Sr2CaMo0.02W0.98O6: 2% Sm3+, Egap value is 3.2497 eV, and it changed into 3.6234 eV in Sr2CaMo0.01W0.99O6: 2% Sm3+. The absorption capability of near UV light will be much better in
3.4. Band structure and density of states
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the former.
For the purpose of clarifying the role of Mo on the band structure in Sr2CaMo1-xWxO6 host, we understood chemical bonding properties and origin of CT transitions of Sr2CaMo1-xWxO6 by examining band structure and partial density of states (PDOS) of atoms. Appropriate calculations
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were performed to explore the influence of the replacement of Mo with W in the Sr2CaMoO6 host using the DFT method. The cutoff energy was 480 eV, and a unit cell with 20 atoms was used. One Mo atom is substituted by W and therefore the W percentage is 50%. Due to the large and expensive computation, the cases with the Mo percentage were not simulated. The calculated
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band structure along the high symmetry points of the first Brillouin zone for Sr2CaMo0.5W0.5O6 is shown in Fig. 6(a). It can be seen that the lowest energy of conduction bands (CBs) was localized
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at G point, while the highest point of the valence bands (VBs) was in Y. Hence, Sr2CaMo0.5W0.5O6 is an indirect band gap material and the gap between the lowest energy of the CBs and the highest energy of the VBs is about 2.3020 eV, which is smaller by 0.5810 eV than the optical band gap of Sr2CaMo0.6W0.4O6: 2% Sm3+. The difference comes from GGA in DFT calculations generally underestimating the band gap of insulators and semiconductors. This is because that DFT is a ground state theory, however band gap belongs to the properties of excited states. Comparatively, optical band gap estimated from DRS spectra comes nearest to the real. The band structure of low energy CBs (2.0
4.5 eV) is composed of two parts and it is
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expressed as two peaks in the density of states in Fig. 6(b). In order to explain the source of the two parts of CBs as well as the origin of the highest energy of VBs, the partial density of states
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(PDOS) of Sr2CaMo0.5W0.5O6 is illustrated in Fig. 7. The conduction band in the range of 2.0 4.5 eV is dominated by Mo-4d and W-5d, mixed with a small amount of O-2p states. The valence band just below the Fermi level (0.0 eV) was mainly occupied by O-2p, with some contributions of Mo-4d and W-5d states. Therefore, the absorption in UV and near UV region can be ascribed
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to the CT transitions from O-2p to W-5d and Mo-4d states, respectively. Combined with the optical band gap calculations and the diffuse reflectance spectra, it can be concluded that there
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will be efficient absorption in near UV region even when the content of Mo is very low because of its occupation in the lowest conduction band of Sr2CaMo1-xWxO6 host. 3.5. Raman spectra
Group theory analysis indicates that Sr2CaMoO6 has four Raman active modes T2g (Sr) + T2g (O) + Eg (O) + A1g (O) [39]. Raman spectra of Sr2CaMoO6 shown in Fig. 8 give peaks at about
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100-200, 437, 537 and 786 cm-1, which can be assigned to T2g (Sr), T2g (O), Eg (O) and A1g (O), respectively. T2g (Sr) mode splits into four peaks and it means the symmetry of Sr site is lowered because of the replacement by Sm3+ ions. As can be seen from the enlarged insert for 100-200 cm-1, the four peaks shifted slightly, which implies Sm3+ ions are incorporated into Sr2+ site of
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Sr2CaMo1-xWxO6. When Sm3+ ions are doped into Sr2CaMo1-xWxO6 host, Sm3+ ions (r =1.24 Å when CN = 12) substitute for Sr2+ (r =1.44 Å when CN = 12) sites. There are two approaches to
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achieve charge compensation: (a) two Sr2+ ions are replaced by one Sm3+ and one monovalent cation, like Li+, Na+ and K+; (b) charge compensation is provided by a strontium vacancy. In our present work, the phosphors were synthesized without any charge compensation ions and the obtained structures are still consistent with the standard. Considering the charge balance, the charge loss in the lattice is probably compensated by Sr2+ vacancies VSr: 3Sr2+ + 2Sm3+ → VSr + 2[SmSr]. In our samples, the Sr2+ vacancies which were induced by charge compensation was not rich because Sm3+ concentration was low. The influence of different charge compensation models on the luminescence properties of some red phosphors has been investigated. It has been
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found that both monovalent cations and cation vacancy can lead to enhanced luminescence and the strongest emission intensity was observed in the former [40-42]. In our future work, further
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experiments, such as varying the oxygen fugacity to control the intrinsic defect populations, will help to identify the situations.
According to group theory analysis, the A1g peak corresponds to the stretch vibration of the oxygen in MoO6 or WO6 octahedra. With the increase of W concentration in the series of
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Sr2CaMo1-xWxO6, the A1g mode was divided into two peaks. The intensity of the A1g (O) peak at 786 cm-1 from MoO6 octahedra was significantly reduced and the new peak appeared at a higher
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wavenumber. Qualitatively, the wavenumber (ν) of a vibration mode is related to the bonding strength (k) and the reduced atom mass (m): ν
ට .
(4)
The reduced atom mass m of WO6 is larger than that for MoO6, however, the A1g peak related to WO6 shifts to higher wavenumber, which indicates that the bonding between W and O is stronger
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than that between Mo and O. In molybdate and tungstate compounds, the energy of O2-→Mo6+ and O2-→W6+ CT transition is dependent on M-O (M=Mo, W) bond strength [43]. The stronger bonding between M (M=Mo, W) and O, the more energy needed for CT transition from O2- to
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M6+. Optical band gap values of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) have been calculated above. On the whole, Egap values for Sr2CaMo1-xWxO6 grew as
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the W content increased, and then CT transition happens over larger band gap. It is not difficult to understand that increasing W concentration leads to stronger M-O bonding, accordingly, it can be further deduced that the increased band gap of Sr2CaMo1-xWxO6 as the increased W content is caused by the stronger W-O bonding. 3.6. Photoluminescence properties Fig. 9 presents the excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99) monitoring Sm3+ 648 nm emission, as well as the emission spectra pumped by 302, 309, 355 or 407 nm. The narrow excitation bands at 346, 355, 364, 378, 391, 407, 420, 440, 451,
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472, 480, 489 and 528 nm can be assigned to the electronic transitions of Sm3+ from 6H5/2 to 4
D7/2, 6P5/2, 4D3/2, 4P7/2, 4L15/2, 6P3/2, 6P5/2, 4M17/2, 4I13/2, 4I11/2, 4M15/2, 4I9/2 and 4G5/2. [44]. With the
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increase of W concentration, a shoulder excitation band at 300 nm was observed in Fig. 9(d) and two obviously separated bands at about 302 and 350 nm began to appear in Fig. 9(f), which come from CT transitions in MoO6 and WO6 groups, respectively. The sharp and intense excitation band at 407 nm originating from 6H5/2
6
P3/2 transition of Sm3+ ions can be observed
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in all excitation spectra in Fig. 9. Under the radiation of 407 nm, all samples exhibited three emission bands, which are assigned to 4G5/2→6H5/2 (569, 579 nm; 566 nm), 4G5/2→6H7/2 (603, 608, 618 nm; 603 nm), 4G5/2→6H9/2 (648 nm; 649 nm) transitions [45, 46]. The major part of G5/2→6H5/2 transition is magnetic dipole (MD) allowed, and 4G5/2→6H9/2 electronic transition is
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4
purely electric dipole (ED) dominated. For 4G5/2→6H7/2 transition, the magnetic dipole character is very low [47, 48]. The intensity ratio of ED to MD can be used to measure the departure from centrosymmetry of sites occupied by Sm3+ ions. The profiles of the emission spectra under excitation into the 6P3/2 level of Sm3+ (407 nm) and into the CT band are different except for that
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in Sr2CaMoO6: 2% Sm3+ (Fig. 9(a)). Three transitions give almost the same intensity when radiation energy is 302 or 309 nm (Fig. 9(b)-(h)), however, when the excitation energy is 407 nm, the red emission at 648 nm dominates in luminescence intensity. Similar situation can be
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observed in Sm3+ doped CeO2, in which both cubic and low-symmetry Sm3+ centers co-exist. The cubic Sm3+ luminescence centers are sensitized via CT band of CeO2, and the low-symmetry
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Sm3+ centers prefer to be excited by f-f transition [49]. For Sm3+ centers departed from centrosymmetry, the emission spectra will mainly display 4G5/2→6H9/2 transition. In Fig. 9(a), the profiles of emission spectra of Sr2CaMoO6: 2% Sm3+ are similar when the pumped energy is 355 or 407 nm. Thus it can be concluded that there is no Sm3+ luminescence centers with centrosymmetry in Sr2CaMoO6: 2% Sm3+. However, when WO6 groups are the major of the host, the situation is different. When the phosphor was excited via f-f transition (407 nm), it exhibited emission dominated by
4
G5/2→6H9/2 transition, and Sm3+ centers gave emission spectra
consisting of three bands with almost same intensity when excited by 302 or 309 nm. Thus it can
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be concluded that in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0), there are two kinds of luminescence centers, and one of them possesses centrosymmetry. Fig. 10 shows the excitation spectra of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.90, 0.92, 0.94,
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0.96, 0.98, 0.99, 1.0) monitoring 608 nm emission of Sm3+ and the emission spectra of Sr2CaMo0.02W0.98O6: 2%Sm3+ and Sr2CaMoO6: 2%Sm3+. The black line is the excitation spectrum of Sr2CaMoO6: 2% Sm3+ and its intensity is very low compared with others. Very
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interestingly, in spite of Mo concentrations are very low (≤0.1), the intensity of Mo-O CT band in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0.90, 0.92, 0.94, 0.96, 0.98, 0.99) is considerably enhanced
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compared with that in Sr2CaMoO6: 2% Sm3+. Concentration quenching among MoO6 groups discussed above can explain this phenomenon. A little amount of Mo in Sr2CaMo1-xWxO6 can make it absorb efficiently in near UV region.
The excitation spectrum (monitoring at 608 nm) of Sr2CaMo0.02W0.98O6: 2%Sm3+ show the most intense absorption band around 350 nm, which is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+. It means that when Sr2CaMo0.02W0.98O6: 2%Sm3+ was exposed to near UV
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light, electrons can jump from O 2p to Mo 4d orbital, and then energy is transferred efficiently to Sm3+ ions, emitting orange-red light. When the excitation wavelength is 350 nm, emission spectra profiles of the two phosphors are distinctly different. Both 4G5/2→6H5/2 and 4G5/2→6H7/2 into two
peaks
and
their
intensity was enhanced
obviously in
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transitions split
Sr2CaMo0.02W0.98O6: 2%Sm3+ compared with that in Sr2CaMoO6: 2%Sm3+. The internal quantum
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efficiency of Sr2CaMo0.02W0.98O6: 2% Sm3+ was measured to be 2.601 %. The low quantum efficiency of our samples is partly because of the particle agglomeration and non uniform size distribution in the synthesized phosphors. It is believed that the quantum efficiency can be further enhanced by controlling the particle size and morphology through the optimized synthetic route. 3.7. Energy transfer process Why there is energy transfer blockage from CTB to Sm3+ in Sr2CaMoO6: 2%Sm3+ but energy transfer from O2-→W6+ CT transition to Sm3+ in Sr2Ca2WO6: 2%Sm3+ is efficient? The
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luminescence of Eu3+ in mixed metal oxides had been investigated by Blasse and four rules were proposed to explain the photoluminescence efficiency of Eu3+. The discussion was restricted to Eu3+ activation but the considerations are also valid for other rare earth ions activated oxides.
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Therefore here we discuss the energy transfer process from CT band to Sm3+ using the rules [34]. As presented in Fig. 9(h), the blue emission band peaking at about 430 nm was from the intrinsic emission of WO6 group, and it is evident that the tungstate emission strongly overlaps the Sm3+
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strongest absorption feature. However, MoO6 emission cannot be observed in Sr2CaMoO6: 2%Sm3+ at room temperature and the emission maximum is at 625 nm at 4.2 K in Ba2CaMoO6 [50]. In molybdate case, its overlap with Sm3+ absorption bands is very limited. Therefore, as
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shown in Fig. 10, when Sm3+ 608 nm was monitored, the intensity of O2-→W6+ CT transition in Sr2Ca2WO6: 2%Sm3+ is about 16 times higher than that of O2-→Mo6+ in Sr2CaMoO6: 2%Sm3+, indicating energy transfer from WO6 to Sm3+ is significantly more efficient than that from MoO6 to Sm3+. When there were no WO6 groups introduced into the host, energy migration among MoO6 groups was not be blocked and finally reached quenching centers, then less energy can be
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transferred to Sm3+, resulting in weak luminescence of Sm3+ in Sr2CaMoO6. In order to further investigate energy transfer from W(Mo)O6 group to Sm3+, the decay curves of Sm3+ emission at 608 nm corresponding to the 4G5/2→6H7/2 transition in Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.98,
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1.0) were measured and shown in Fig. 11. As indicated in Fig. 11, the decay curves can be well fitted into an exponential function as: (5)
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I = Aexp(-t/τ) + I0
where I and I0 are luminescence intensity, A is constant, t is time and τ is decay time. For Sr2CaMo1-xWxO6: 2%Sm3+ with x = 0, 0.98, 1.0, the lifetime is estimated to be 421.4, 598.4 and 620.3 µs, respectively. It can be found that under excitation into the CT transition, the lifetime of 4
G5/2 state in Sr2CaMoO6: 2% Sm3+ is the shortest, followed by that in Sr2CaMo0.02W0.98O6: 2%
Sm3+, and the longest is in Sr2CaWO6: 2% Sm3+. The variation of the measured lifetimes has the same trend as the W content, indicating that non-radiative process due to energy migration and quenching among MoO6 groups speeded up the luminescence decay.
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Based on the discussion above, one way to enhance energy transfer from CT transition to Sm3+ as well as guarantee efficient absorption in near UV region is replacing Mo using appropriate W.
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Another method may be changing the substituted sites of Sm3+ ions. Sr2CaWO6 and Sr2CaMoO6 belong to A2BB′O6 double perovskite family, and CaO6 and MoO6 octahedrals align collinearly sharing their six vertex oxygen atoms. The angles of Mo-O-Ca are 180◦ and 172.478◦. However, Mo-O is nearly vertical to Sr-O bond; the angles of Mo-O-Sr are 98.026◦ and 95.342◦. Sm3+ ions
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prefer to enter the Sr sites in Sr2CaWO6 and Sr2CaMoO6. Therefore, if excitation energy is absorbed though electrons migrating from O 2p to W 5d or Mo 4d orbitals, it can be efficiently transferred to Sm3+ along O-W-Sm (Ca)-O-W- collinear array. Although near UV light can pump
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electrons from 1A1g to 1T1u level of the MoO6 group, due to Sr-O bond locating outside the Mo-O plane, energy transfer from MoO6 group to Sm (Sr) is not easy. Changing the substituted sites of Sm3+ ions to Ca2+ will be very useful for further enhancing the absorption capability in near UV region for Sr2CaMo1-xWxO6: 2% Sm3+.
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4. Conclusions
Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) phosphors were synthesized through solid state reaction method. The phosphors presented broad excitation bands in near UV and blue regions, originating from charge transfer transitions within
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Mo(W)O6 or Sm3+ f-f transitions. In Sr2CaMo1-xWxO6: 2%Sm3+ (x<0.8), most of the absorbed energy by WO6 was quenched before reaching Sm3+ centers due to migration among MoO6
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groups. The band gap values of the selected samples were calculated and it increased roughly as the increasing W content. The biggest difference occurred between the optical band gap values of Sr2CaMo0.01W0.99O6: 2% Sm3+ (3.2497 eV) and Sr2CaMo0.02W0.98O6: 2% Sm3+ (3.6234 eV). Band structure and density of states of Sr2CaMo0.5W0.5O6 was investigated using DFT method in CASTEP mode and Mo-4d states occupy the lowest conduction band of the host. There still is efficient absorption in near UV region in Sr2CaMo0.01W0.99O6: 2% Sm3+ due to the contribution of MoO6 groups. Raman spectra gave the evidence that Sr2+ sites are replaced by Sm3+ ions and W-O bonding is stronger than Mo-O. There are two kinds of luminescence centers in Sm3+ doped
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Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the intensity of 608 nm red emission under 350 nm radiation was 8.5 times stronger than that in Sr2CaMoO6: Sm3+.
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Thus in Sr2CaMo0.02W0.98O6: 2% Sm3+ not only concentration quenching among MoO6 groups can be prevented but efficient absorption in near UV region can be realized. The phosphor can find application as red emitting phosphor for white LEDs based on near UV LED chips.
Acknowledgements
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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future
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Planning (No. 2013012655). The Sm3+ doped Sr2CaW1-xMoxO6 phosphor (x=0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 1.0) was supplied by the Display and Lighting Phosphor Bank at Pukyong National University.
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Figure Captions Fig. 1. XRD pattern of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) and Sr2CaWO6, Sr2CaMoO6 standard patterns.
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Fig. 2. Experimental (black circles) and calculated (red solid line) XRD patterns and their difference (blue solid line) for the fit of Sr2CaWO6:2%Sm3+ by the GSAS program. The short green and magenta vertical lines show the position of Bragg reflections of the calculated patterns.
Fig. 3. Excitation spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) monitoring
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Sm3+ emission at 650 nm.
0.98, 0.99, 1.0).
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Fig. 4. Diffuse reflectance spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.92, 0.94, 0.96,
Fig. 5. The determination of the optical band gap of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.4, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0) using the Kubellka-Munk function.
Fig. 6. Calculated band structure (a) and total densities of states (b) of Sr2CaMo0.5W0.5O6 near the Femi energy level (EF). The Femi energy is the zero of the energy scale.
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Fig. 7. Partial densities of states of Sr2CaMo0.5W0.5O6 and some atoms constituting it: O, Mo, and W. Fig. 8. Raman spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0). Fig. 9. Excitation and emission spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0).
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Fig. 10. Excitation and emission spectra of Sr2CaMo1-xWxO6: 2%Sm3+ (x = 0, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 1.0). Fig. 11. PL decay curves of Sr2CaMo1-xWxO6: 2% Sm3+ (x = 0, 0.98, 1.0) under excitation into charge transfer
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ACCEPTED MANUSCRIPT In Sm3+ doped Sr2CaMo1-xWxO6 red phosphors, the WO6 groups act as obstacles to prevent the energy migration and quenching among MoO6. The role of Mo in Sr2CaMo1-xWxO6 phosphors was investigated based on DFT method.
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The 608 nm emission of the best one is 8.5 times higher than that in Sr2CaMoO6: Sm3+.