Dual-Mode Luminescence with Broad Near UV and Blue Excitation

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Dual-mode Luminescence with Broad near UV and Blue Excitation Band from SrCaMoO:Sm Phosphor for White LEDs 2

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Lili Wang, Hyeon Mi Noh, Byung Kee Moon, Sung Heum Park, Kwang Ho Kim, Jinsheng Shi, and Jung Hyun Jeong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02828 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 13, 2015

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The Journal of Physical Chemistry

Dual-mode Luminescence with Broad near UV and Blue Excitation Band from Sr2CaMoO6:Sm3+ Phosphor for White LEDs Lili Wang,† Hyeon Mi Noh,† Byung Kee Moon,† Sung Heum Park,† Kwang Ho Kim,‡ Jinsheng Shi§ and Jung Hyun Jeong*,† †

Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea.

‡

School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea § Department of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, Qingdao 266109, People’s Republic of China

ABSTRACT: Dual-mode excitation properties were introduced in Sm3+ doped Sr2CaMoO6 prepared by a high temperature solid state reaction technique. Two ways are available to generate white light in the single-component phosphor activated by Sm3+ ions. Warm white light can be obtained from Sr1.995Sm0.005CaMoO6 phosphor pumped by 380 or 410 nm excitation energy. The full visible spectral emission of the single-phase phosphor comes from the high and low level emission lines of Sm3+ ions as well as the intrinsic luminescence of MoO6 group. It is also competitive as yellow-emitting phosphor for blue pumped white LEDs and gives three emission bands at 567, 603 and 650 nm, presenting yellow luminescence upon 466 nm radiation. The 650 nm red emission band corresponding to 4G5/2→6H9/2 transition of Sm3+ can make its color rendering index better. The excellent photoluminescence of Sr2CaMoO6 is related to the partial tilting CaO6 octahedral and the lowered symmetry were confirmed by General Structure Analysis System. Band gap of Sr2CaMoO6 estimated from the diffuse reflection spectra and also calculated by CASTEP mode shows its semi-conducting character. All the results show that the Sr2-xSmxCaMoO6 phosphors have considerable potential for applications in near UV LED or pumped by blue LED chip.

1. INTRODUCTION As the increasing awareness of environmental issues and energy consumption, the 1

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wide use of white light-emitting diodes (WLEDs) as the next generation of lighting and display systems are promoted owing to their environmental friendliness, low power consumption, reliability and extraordinary luminous efficiency.1-3 There are three principle strategies to produce white light from LEDs: the assemble of multiple LED chips with different colors in a single device; a ultraviolet (UV) LED chip to excite red, green and blue (RGB) phosphors or a single-phase phosphor; the mixing of yellow light from YAG excited by InGaN blue chip with the remaining blue emission.4 However, current yellow and green LEDs keeps multi-LED devices away from high efficiencies, and the combination of three or more LEDs is generally the most expensive approach unless further progress are made. LED chip combined with down-converting phosphors is the preferred strategy to manufacture reliable and inexpensive solid state white lighting sources for display and general lighting applications. Current commercial pc-WLEDs employ a blue LED coated with YAG: Ce3+ yellow phosphor and they have high correlated color temperature (CCT) and low color rendering index (CRI) because of deficient emission in the red spectral region. The red component of the commercial WLED can be improved by co-doped Eu3+, Pr3+ or Sm3+ ions into YAG: Ce3+ yellow phosphor.5-7 There have been studies on narrow line red phosphors for LEDs applications.8, 9 However, the enhancement of CRI is not enough for practical utilization due to low red emission efficiency. As an alternative, it is highly favored that near ultraviolet (NUV, 370-420 nm) chips coupled with RGB tri-color phosphors were fabricated for white LEDs. However, the mixing of multi-emission bands through multiple phosphors under NUV light radiation generally adds a loss mechanism and demonstrates a poor efficiency caused by color re-absorption. Consequently, the single-phosphor-converted NUV LED is emerging as an indispensable solid state white lighting device. In view of the current approaches to generate white light in single-phase host crystals, doping rare earth ions into appropriate host matrix is the first concern such as Eu2+, Eu3+, Dy3+ or Pr3+ doped systems. There are two different cation sites available in some phosphate and silicate compounds, such as M2SiO4, M3SiO5, MMgSi2O7 (M = Ca, Sr, Ba), Sr3MgSi2O8 and BaSrMg(PO4)2. Because of similar ion radii, Eu2+ can 2


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substitute two kinds of cations. White light can be produced from two broad emissions of Eu2+ ion due to 4f-5d transitions.10, 11 The emissions of Eu2+ mainly locate in the blue and yellow-green zone, leading to insufficient red component. Eu3+ and Pr3+ are well-known activators with narrow red emissions to improve CRI, and white light has been obtained in CaSc2O4: Eu3+, Sr2V2O7: Eu3+, Sr2SiO4: Pr3+ phosphors.12-14 However, these phosphors have poor absorption in the range of 370-420 nm thereafter low wavelength conversion efficiencies in near UV region where the most efficient LEDs are available today. The similar situation holds for Dy3+ doped LaPO4 white phosphors.15 At present, many attentions have been attracted to rare earth ions activated alkaline earth molybdate compounds acting as phosphors for LEDs thanks to their efficient energy transfer from the broad charge transfer band in near UV region, as well as chemical and thermal stability.16-19 It has been found that the coordination number x and the Mo-O bond lengths in MoOx polyhedron can affect the positions of the charge transfer transitions from O to Mo.20-22 Molybdate compounds with octahedral MoO6 groups have been found to possess Mo-O charge transfer transition from about 250 to 420 nm.23-25 The wide and low-energy charge transfer bands of molybdate with MoO6 groups make them suitable for near UV pumped LEDs. They were usually used as red phosphors owing to the high energy transfer efficiency in the MoO6-Eu3+ systems.23, 26 In this paper, warm white light was realized in Sm3+ doped Sr2CaMoO6 phosphor. It is of interest and great significance to note that there appears double wide-band excitations ranging from 300 to 500 nm, and full color white light can be produced under excitations at 380 or 411 nm. When the phosphor is excited by 466 nm energy, yellow luminescence with intense orange-red components was generated, indicating that the Sr2CaMoO6:Sm3+ phosphor is also suitable for a yellow phosphor pumped by a blue LED. The photoluminescence properties of Sr2CaMoO6 are related to the tilt or rotation of CaO6 octahedral. The structure refinement of Sr1.98Sm0.02CaMoO6 sample was performed using a General Structure Analysis System (GSAS). Band structures and orbital populations have been calculated using Cambridge Serial Total Energy Package (CASTEP) code by density functional theory (DFT). The calculation results 3



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shows that the double perovskite structural Sr2CaMoO6 is indirect band-gap material, and electronic transition from O 2p to Mo 4d orbital forming the broad near UV excitation band. The phosphors can give luminescence from Sm3+ after the absorption of near UV light, which was attributed to energy transfer from host lattices to Sm3+ ions.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. Sr2CaMoO6: Sm3+ was synthesized using solid state reaction process under high temperature conditions. The raw materials used were SrCO3 (Yakuri Pure Chemical, 99%), CaCO3 (Aldrich, 99%), MoO3 (Yakuri Pure Chemical, 99%) and Sm2O3 (Aladdin, 99.9%). Firstly, the starting materials were weighed in stoichiometric molar ratio and mixed in an agate mortar. The powder based precursors were obtained after 30 minutes grindings. Secondly, homogenized precursors were pre-fired for 3 h at 600 °C to decompose the carbonates, and then the powders were calcined for another 3 h at 900 °C. Finally, the precursors were fired at 1200 °C for 12 h. After grindings, three-step calcinations were repeated once. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns of the prepared samples were collected on a Bruker D8 Advance X-ray diffractometer (Cu Kα 1 irradiation, λ = 1.5406 Å) radiation to determine the crystal structures. The diffraction patterns were recorded over an angular (2θ) range of 10-125° with a scanning step of 0.02 deg and a residence time of 0.5 sec. The crystal structure refinement was carried out using the General Structure Analysis System (GSAS) program. Fourier transform infrared (FT-IR) spectrum of the sample was measured on a Nicolet-IR 200 spectrometer in the range of 400-4000 cm-1 using a KBr pellet technique. Raman spectra were gathered on a Raman/PL spectrometer (Horiba Jobin-Yvon, LabRAM HR). UV-vis diffuse reflectance spectra (DRS) were recorded on a V-670 (JASCO) UV-vis spectrophotometer. Photoluminescence spectra were obtained using a Photon Technology International (PTI) spectrofluorimeter with a 60 W Xe-arc lamp and the lifetime were measured using a phosphorimeter attached to the main system with a Xe-flash lamp (25 W power). 4


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2.3. Details of Calculation. The band structure and density of state (DOS) of Sr2CaMoO6 were investigated via CASTEP code on the basis of the DFT within the plane wave pseudo-potential approach. We firstly started geometry optimization using the Broyden, Fletcher, Goldfarb, Shannon (BFGS) method. We use a plane wave basis set with a kinetic energy cutoff of 340 eV. Brillouin zone integration was represented using the K-point sampling scheme of 3 × 4 × 4 Monkhorst-Pack grid. The calculations were conducted within the generalized gradient approximations (GGA), using the exchange and correlation functional. In these processes, the convergence criterion for the self-consistent field (SCF) was set to 1.0 × 10-6 eV/atom. For the SCF iterations, the convergence tolerance for geometry optimization was selected with the differences in total energy, the maximal ionic Hellmann−Feynman force, the stress tensor, and the maximal displacement being within 1.0 × 10-5 eV/atom, 0.03 eV/Å, 0.05 GPa, and 1.0 × 10-3 Å, respectively.

3. RESULTS AND DISCUSSION 3.1. Structure characterization. A series of Sr2-xSmxCaMoO6 with different concentrations of Sm3+ (x=0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.08, 0.10) have been synthesized. The powder XRD patterns in Figure 1 indicate most peaks can be indexed to Sr2CaMoO6 phase except for the impurities at 27.66° marked with five-pointed star, according to the Joint Committee on Powder Diffraction Standards (JCPDS) file No. 48-0799. The impurities were identified as SrMoO4 phase (JCPDS 08-0482). With increasing Sm3+ concentration, the intensity of the impurity peak at 27.66° first decreases and then almost disappears when x=0.03, and then gradually increases until x reaches 0.10. From the enlarged XRD patterns from 30 to 32° in the right side in Figure 1, it is clear that the position of the most intense peak shows a back and forth shift with the increasing Sm3+ concentration. Actually, not only the most intense diffraction peak at about 31° but also almost all the peaks firstly shift to higher angles with the increase of Sm3+ content until x=0.04, and then back to lower angles. It is interesting to observe that the content of the impurity SrMoO4 phase firstly decreases with the growing of Sm3+ content, and then increases again, reaching 5



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to maximum when x=0.1. The diffraction peaks shifting to high angles in Sr2-xSmxCaMoO6 when x=0.005 to 0.04 can be attributed to the replacement of smaller Sm3+ (0.958 Å, CN=6) for Sr2+ ions (1.44 Å, CN=12) even Ca2+ (1 Å, CN=6). SrMoO4 is an intermediate product at low temperatures in the synthesis process of Sr2CaMoO6 samples so it is a common impurity, crystallizing in the I 41/a space group, with a=b= 5.3954 Å, c=12.0264 Å, V= 350.09 Å3, Z=4. Its lattice parameters are larger than our Sr2CaMoO6 samples. Therefore, when x>0.04, as the increase of the impurity SrMoO4, the diffraction peaks shifted to low angels again. In addition, the super-lattice diffraction peak at about 18.6° was observed in all samples, indicating that an ordered alternative arrangement of CaO6 and MoO6 octahedrals existed in the structure.27 Sr2CaMoO6 belongs to A2BB′O6 double perovskite family and its physical properties are closely related to the degree of order arrangement of BO6 and B′O6 octahedrals.28 Generally, Sr2CaMoO6 belongs to orthorhombic system and its space group is Pmm2.22 It is well known that the tolerance factor f determines the crystal structure of perovskite ABO3, and for f is close to 1 a cubic perovskite structure is realized.29 For 0.96≤f≤1, the lattice transforms to rhombohedral structure, whereas to orthorhombic for lower values of f.29, 30 f = (rA + rO)/√2(rB + rO), here ri (i = A, B, O) represents the average ion radius of each element.31 When f

Dual-Mode Luminescence with Broad Near UV and Blue Excitation

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