Sm 3+ as potential co-dopant candidate in scheelite molybdate/tungstate red phosphor: A review

Transcription

1 Sm 3+ as potential co-dopant candidate in scheelite molybdate/tungstate red phosphor: A review N. S. Zailani, and M. Fathullah Citation: AIP Conference Proceedings 1835, (2017); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Effect of intermetallic phase on microstructure and mechanical properties of AA332/Mg 2 Si(p) composite AIP Conference Proceedings 1835, (2017); / Effect on mechanical properties of glass reinforced epoxy (GRE) pipe filled with different geopolymer filler molarity for piping application AIP Conference Proceedings 1835, (2017); / Fourier transform infrared spectroscopy (FTIR) analysis of paddy straw pulp treated using deep eutectic solvent AIP Conference Proceedings 1835, (2017); / Spectra comparison for an optical breathing gas sensor development AIP Conference Proceedings 1835, (2017); / Physical and mechanical properties of quarry dust waste incorporated into fired clay brick AIP Conference Proceedings 1835, (2017); / The effect of Ta doping to the crystal structure of La 0.67 Ca 0.33 MnO 3 perovskite oxide using DFT method AIP Conference Proceedings 1835, (2017); /

2 Sm 3+ As Potential Co-Dopant Candidate in Scheelite Molybdate/Tungstate Red Phosphor: A Review N S Zailani 1,a), M Fathullah 1,2,b) 1 School of Manufacturing Engineering, Universiti Malaysia Perlis (UniMAP),Pauh Putra Campus, Arau, Perlis Malaysia. 2 Center of Excellence Geopolymer & Green Technology (CEGeoGTech), Universiti Malaysia Perlis (UniMAP), Kangar, Perlis, Malaysia a) Corresponding author: nursyazwaniezailani@yahoo.com.my b) Corresponding author: fathullah@unimap.edu.my Abstract. This article reviews on previous and on-going works relating to the interaction of Eu 3+, Y 3+ and Sm 3+ used AY (1-x) Eu x (MO 4 ) 2 where (A=Li+, K+ and Na+) and (M=Mo and W) in alkaline metals red phosphors AY (1- x)eu x Sm y (MoO 4 ) 2. The structures of the phosphors also covered in the article for looking the trend of the phosphors. From this review, it can be expected that the presence of Sm3+ on Y3+ at 45 mol % can enhance the intensity of the emission. A further research will be conducted in looking for the potentiality of Sm3+ ion when added to the host materials. INTRODUCTION Phosphor-converted light-emitting diodes (pc-leds) have been studied by many researchers in searching for high-efficiency or better colour rendering index (CRI) for solid-state lighting applications Most phosphors show an affinity toward strong energy absorption in the wavelength region of less than 300 nm due to the charge transfer and 4f n 4f n-15d transition of lanthanide ions [1]. There are some approaches using LEDs in generating white light. The first method has studied by Nakamura et. al., (1995) by preparing the combination of (Y,Gd) 3 (Al,Ga) 5 O 12 :Ce 3+ (YAG), which are the blue and yellow LED phosphor for the solid-state lighting devices [2]. The second method can be used is by combining blue, green and red LEDs in one package. While the third approach is used one or more colour converting phosphors with either a blue or a UV emitting LED [3]. Because of the lack of a red spectral component in the YAG:Ce affect to their colour rendering index (CRI) become lower [4], the combination blue yellow (YAG:Ce) phosphor with red phosphor help for improving the CRI. The general CRI of white-leds is about 85 and it is good enough for general illumination [5]. Generally, a lamp phosphor is prepared in form of a powder [6]. For producing a high efficiency of luminescent materials, high quality starting materials and a clean production process are required. Due to the importance of the red phosphor in the pc-leds application, the aim of this paper is to find the potential of lithium europium molybdate doped samarium for producing a better red phosphor material with high efficiency and great colour rendering index. This paper discusses the previous works on Sm 3+ doped alkali metal europium molybdate/tungstate in demonstrating the ability of Sm 3+ in terms of its photoluminescence properties when doped into the host phosphor affecting the intensity or efficiency in pc-leds application. Advanced Materials Engineering and Technology V AIP Conf. Proc. 1835, ; doi: / Published by AIP Publishing /$

3 INTRODUCTION TO LiEu (1-x) Y x (MoO 4 ) 2 There are a few types of red phosphor currently being applied in the market such as Eu 3+ doped Y 2 O 3 or Y 2 O 2 S:Eu 3+, SrS:Eu 2+, CaS:Eu 2+, CaAlSiN 3 :Eu 2+, LiBaBO 3 :Eu 3+ and etc. However, some of them such as Y 2 O 2 S:Eu 3+,SrS:Eu 2+ and CaS:Eu 2+ are sensitive to humid environments and chemically unstable while CaAlSiN 3 :Eu 2+ and LiBaBO 3 :Eu 3+ are difficult to synthesize [7], [8]. Among these phosphors, alkaline metal molybdate/tungstate such as AEu(MoO 4 ) 2 (A = Li, Na, K and Ag), Li 3 Ba 2 Y 3-x (MoO 4 ) 8 :xln 3+ (Ln = Eu, Tb, Dy), LiLn(Mo 4 ) 2 :Eu 3+ (Ln = La, Eu, Gd,Y; M = W, Mo), LiEu (1-x) Y x (WO 4 ) 2 etc. based phosphors are amongst the most stabilized phosphor reported [4-7]. LiY 1-x Eu x (Mo y W 1-y O 4 ) 2 phosphor for an example, has been investigated by a number of researchers due to its advantages such as excellent red emission under 395 nm and also the high quenching concentration of the rare-earth ions inside the phosphor was great for enhancing the quality of display applications [11]. It is claimed that the LiEu 1-x Y x (MoO 4 ) 2 phosphor with changes of Y 3+ concentration has a great host lattice coming with the highest and lowest luminescent spectra in intensity and the spectra also not too wide for fulfill the demand of great quality of red phosphor, they can emit very well in the red region [7]. The crystal structure of this phosphor has been reported as tetragonal scheelite structure with space group I4 1/a [4, 7, 9], although there are small discrepancies reported by a few articles claiming that the crystal structures are monoclinic and orthorhombic structures [5, 6, 10]. What more interesting is that crystal structure (as majority claimed it as tetragonal structure) of the compound does not change as amount of Eu 3+ -Y 3+ varies [7]. Figure 1 shows the example of X-ray Powder Diffraction (XRPD) pattern of the LiEu (1-x) Y x (MoO 4 ) 2 as reported by Fathullah et al. [7]. The results indicated that the structure does not change as the amount of Eu 3+ and Y + changes manifesting thatthere is a good sign that this material is able to be used efficiently without changing the crystal symmetry. In addition, the authors found that the LiEu 0.55 Y 0.45 (MoO 4 ) 2 and LiEu 0.55 Y 0.45 (MoO 4 )(WO 4 ) were the most efficient phosphor for SSL applications [7]. The involvement up to 45% of Yttrium (III) has surely contributing to less concentration quenching to the compound. It showed the luminous efficacy has a gradual decline until the concentration of Y 3+ 45% in the LiEu (1-x) Y x (WO 4 ) 2 and started to decrease when adding the Y 3+ concentration. In addition, Y 3+ ion might be obtained to increase the quantum efficiency even it has a small changes which reported by Fathullah et al. (2016). Furthermore, a research conducted by Liu et al. (2014)[10] stated that LiY 0.95 Eu 0.05 (WO 4 ) 2 has a great potential for a red-emitting phosphor where it has quantum efficiency of 57.9% with the lifetime of up to ms. FIGURE 1. XRD patterns of the LiEu (1-x) Y x (MoO 4 ) 2 in range between x =0 to 1 by Fathullah et al. (2016) [7]

4 INTRODUCTION TO Sm 3+ Sm 3+ doped LiEu(MoO 4 ) 2 has its own potential to be one of the candidate for red-emitting phosphor. Sm 3+ and Eu 3+ ions have six coordination numbers where show the number of ions surrounding the atoms. It also shows that both of the ions share occupancies in the same sites. Sm 3+ ion with six coordination numbers has ionic radius and crystal radius compare to the Eu 3+ ion has smaller where the ionic radius is Due to the small different between the ionic radius, it is expected that the introduction of the Sm 3+ will not affect to the crystal symmetry of the scheelite compound. This is very important for the crystal symmetry unchanged for defining the crystal system. It is also suggested when adding the Sm 3+ ions into the host lattice, the symmetry of the emission centers will not altered. If there are changes occurred in the crystal symmetry and the symmetry of the emission, so the system is not a tetragonal system anymore while the emission will decreases. The photoluminescence properties of Sm 3+ is known to have emission at nm composed of four orange-red emission bands from 4 G 5/2 excited state to 6 H J/2 (J = 5, 7, 9, 11). The excitation band showed the maximum peak located at 405 nm [12]. For the Eu 3+ ion, it showed the highest emission spectra located at 616 nm which attributed to 5 D 0 7 F 2 transition of Eu 3+ ions while the highest excitation bands located at 395 nm and 465 nm. In addition, Sm 3+ is needed in the host phosphors to maintain the structure, act as a sensitizer and enhanced the peaks for increasing the quality of the pc-leds application. Therefore, Sm 3+ ion required for improving the red emission intensity without changing the host emission symmetry and the further research need to be conducted on Sm 3+ doped LiEuY(MoO 4 ) 2 to find the other candidates that have potential as a better red-emitting phosphor for pc-leds application. PREVIOUS WORKS ON Eu 3+ - Sm 3+ NON-SCHEELITE RED PHOSPHOR In recent years, there has been an increasing interest in investigating the photoluminescence study Eu 3+ - Sm 3+ co-doped with other host phosphor based materials. The first article reported by Yan et al. (2011) [14]. The author used conventional solid-state reaction for synthesized the Eu 3+ - Sm 3+ co-doped CaIn 2 O 4 phosphor for their photoluminescence properties. The samples were prepared with starting material CaCO 3 (A.R), In 2 O 3 (A.R), Eu 2 O 3 (4N) and Sm 2 O 3 (4N) and characterized the results using XRD, SEM and photoluminescence study. It showed the phosphor has an orthorhombic crystal structure with the Pca2 1 or Pbcm space group. For the photoluminescence of the Ca 1-m-n In 2 O 4 :meu 3+, nsm 3+ phosphor where (m = 0.05, 0.1, 0.15) and (n = 0, 0.02, 0.05, 0.08) on molar concentration results, it showed the energy transfer from Sm 3+ ion to Eu 3+ ion in lattice and excitation appeared at 405 nm. However, this paper did not state clearly the maximum emission peak resulted in the experiment. The Sm 3+ ion has optimum molar concentration of 0.05 and the Eu 3+ has optimum molar concentration of The CaIn 2 O 4 :Eu 3+, Sm 3+ phosphor has a low temperature quenching effect and the reducing 15% after the temperature increases from room temperature to 425 K in the emission intensity. The doping of Sm 3+ to the phosphor showed some of the Sm 3+ ions act as activators to transfer to the ground states while the excited Sm 3+ ions act as sensitizer to the Eu 3+ ions for transferring their energy. Another similar research was conducted by Bandi et al. (2013) in investigating the photoluminescence and energy transfer of Eu 3+ /Sm 3+ single-doped and co-doped Ca 4 YO(BO 3 ) 3 phosphors using sol-gel method [15]. The sample of Ca 4 Y 1-x-y Eu x Sm y O(BO 3 ) 3 where (x and y = 0.01, 0.03, 0.05, 0.08, 0.10 and 0.12) were characterized using XRD, emission and excitation study. From the result, it showed that the Ca 4 Y 0.88 Eu 0.1 Sm 0.02 O(BO 3 ) 3 phosphor has excitation peak at 402 nm where presented the energy transfer from Sm 3+ ion to Eu 3+ ion. The strongest intensity of the luminescence occurred when Sm 3+ at 2 mol% and the concentration quenching happened when increasing the doping concentration of Sm 3+ and affect to the emission intensity of Eu 3+. On the other hand, this paper also was not clearly stated the maximum emission peak and the crystal structure or crystal system of the phosphor in the report. A year later, Suresh et al. (2014) synthesized Y 2 O 3 :xeu 3+ (x=0.5, 1.0, 1.5 mol%), 0.5 mol% Sm 3+, 1.0 mol% Dy 3+ phosphor using solid-state reaction for their photoluminescence properties. The photoluminescence of the phosphor enhanced with the increasing Eu 3+ concentration under 467 and 535 nm excitations. The maximum of emission peak located at 613 nm and the result showed the peaks became higher when adding more Eu 3+ concentration [16]. This paper was not discussed about the crystal structure of the phosphor and not clearly explained why the author used 0.5 mol% Sm 3+ and 1.0 mol% Dy 3+ ion concentration for synthesized the phosphor

5 According to Meza-Rocha et al. (2015), Sm 3+ and Sm 3+ /Eu 3+ doped zinc phosphate glasses has been studied for their photoluminescence properties and the decay time profile measurement [17]. The experiment used 99.0 Zn(PO 3 ) Eu(PO 3 ) 3, 99.5 Zn(PO 3 ) Sm(PO 3 ) 3 and 98.5 Zn(PO 3 ) 2 1 Eu(PO 3 ) Sm(PO 3 ) 3 as starting materials. The photoluminescence indicated the excitation peaks located at 344, 360 and 393 nm where the maximum excitation peak appeared at 393 nm while the maximum emission peak located at 622 nm. From the work done, it showed the Sm 3+ ions effectively acts as a sensitizer when affect to the emission of Eu 3+ by increasing and reducing excitation bands in the excitation spectra. This paper also not covered the structure of the phosphor and not stated clearly the method and quantity of the concentration materials used for synthesizing the materials. Table 1 summarizes the quantity of works reported on replacing Eu 3+ with other candidate ions since the year 2011 in non- scheelite phosphors. TABLE 1. Summarization of replacing Eu 3+ ions with non-scheelite phosphors. No. Candidate Author Structure, Space Group 1 Ca 1-m-n In 2 O 4 :meu 3+, nsm 3+ Yan et al. (2011) Orthorhombic crystal structure, space group Pca2 1 or Pbcm 2 Ca 4 Y 0.88 Eu 0.1 Sm 0.02 O(BO 3 ) 3 Bandi et al. (2013) - 3 Y 2 O 3 :xeu 3+, 0.5 mol% Sm 3+, 1.0 mol% 4 Eu 3+ /Sm 3+ co-doped (ZPOSmEu) zinc phosphate Suresh et al.(2014) - Meza-Rocha et al.(2015) PREVIOUS WORKS ON Eu 3+ - Sm 3+ SCHEELITE MOLYBDATE/TUNGSTATE RED PHOSPHOR Molybdates with a tetragonal symmetry scheelite-type structure have excellent optical properties which extremely use as phosphors, laser materials and scintillation detectors [18]. The central Mo 6+ metal ion is coordinated to four oxygen atoms in the tetrahedral symmetry (T d ) and eight oxygen atoms of the cations from other tetrahedral compare to the tetrahedral WO 4 group [19]. Sm 3+ ion has many levels in the UV region among the UV absorbing ions which acts a great activator for UV-based LEDs applications. It is presented Sm 3+ ion transferred energy efficiently around nm excitation to Eu 3+ ion for obtaining in the reddish-orange region with a strong emission and has a great potential as a red emitting phosphor [17]. Thus, for Sm 3+ ion co-doped system, it might be obtained a phosphor around 400 nm broadened absorption. It has reported the rare-earth elements such as Sm 3+ and Eu 3+ ions claimed to fill up the lattice sites without inversion in the host lattices [15]. Therefore, it is important for searching the effect of doping Sm 3+ in the molybdate/ tungstate based phosphor. Alkaline metals such as (Li +, Na +, K + ) with europium molybdate and tungstate have been widely investigated by many researchers since year 2005 until today due to their great potential as a host for doping with the rare-earth elements in pc-leds application. There are some works had been done by researchers. The first article reporting on Sm 3+ doped with alkali metals europium molybdate/tungstate has reported by Lee et al. in year Lee et al. (2005) started to study on luminescence of K 4-3x (WO 4 ) 2 :Eu 3+ x,sm 3+ y which x = 0.4, 0.6, 0.8, 1.0 and 1.2, and y = 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05 using solid-state reaction. This study had proven that the structure of the phosphor was resulted as a monoclinic shape with space group C2/c. The result of PL study stated that the emission by 396 nm kept decreasing when the increased the Sm 3+ concentration due to energy transfer Eu-Sm. But due to the Sm 3+ f-f electron transition, emission increased by 405 nm. Unfortunately, the excitation peak of the phosphor excited by 405 nm decreased when the concentration of Sm 3+ increased to y = Figure 2 shows the schematic representation the way of the absorbs energy (UV light) and emits a red light between Sm 3+ ion and Eu 3+ ion in the host material. This showed that the Sm 3+ ions acts as a sensitizer due to the Sm 3+ has not observed the emission and the excitation leaded the emission of Sm 3+ to the Eu 3+ [20]. However, this paper only showed the value

6 of the concentration of Eu 3+ ions and Sm 3+ ions on the figures but it was not stated clearly the range of the concentration in the report. FIGURE 2. Schematic representation of energy transfer and emission processes in the Sm 3+ and Eu 3+ ions by Lee et al. [20]. Another work done by Lee et al. (2008) synthesized K 4-3(x+y) (WO 4 ) 2 :Eu x 3+,Sm y 3+ using solid-state reaction and followed by re-firing with a flux to characterized the structures and luminescence properties of the phosphors. However, the results from this work also remain same with the previous study where the structure of the phosphor also monoclinic with space group C2/c. The excitation peak of KWOEu 0.8 Sm phosphor was enhances at 405 nm that proved the Sm 3+ ions act as a sensitizer in the phosphor (refer Figure 3) [1]. FIGURE 3. Excitation and emission spectra of (a) KWOEu0.8Sm0.027, (b) KWOEu0.8, and (c) KWOSm0.027 powders. The intensities of the excitation and emission spectra were expanded 3 times [1]. A year later, Chunlei et al. (2009) synthesized the luminescence and morphology of the CaMoO 4 :Eu,Sm prepared by co-precipitation method. The samples were characterized using SEM and photoluminescence study. The SEM analysis were conducted with the different preparation of the phosphor methods which in solid-state reaction, co-precipitation with NH 3 H 2 O as precipitating aid agent, co-precipitation with NH 4 HCO 3 as precipitating aid agent and co-precipitation with (NH 2 ) 2 CO as precipitating aid agent. However, this paper does not state how much the concentration of the material used for charactering the phosphor. The result of SEM showed the solid-state reaction has severe agglomerate occurrence with a broad size distribution while NH 3 H 2 O resulted the biggest size with spherical-like shape and 1 m diameter compare to the other three precipitating aid agents. The photoluminescence result found that the emission spectrum remain same at peaks 612 and 616 nm when implanted into CaMoO 4 :Eu phosphor and an addition sharp excitation peak occurred at 406 nm. Setting 406 nm wavelength for excited

7 CaMoO 4 :Eu,Sm and CaMoO 4 :Eu, correspondingly, each of these phosphors demonstrate the emission spectra, however, 616 nm peak intensity of the prior is around six times of the recent one, even excited at 464 nm around 75% of the CaMoO 4 :Eu,Sm and excited at 394 nm about 80% of that CaMoO 4 :Eu,Sm. From the work done, it showed the Sm 3+ ion has transferred the energy absorbed to the activator Eu 3+ ion for enhancing the latter s emission intensity and act as a sensitizer in the CaMoO 4 :Eu,Sm phosphor [21]. In 2014, Li et al. (2014) studied the photoluminescence properties and energy transfer of KY 1-x Ln x (MoO 4 ) 2 (Ln=Sm 3+, Eu 3+ ) where x= 0, 0.005, 0.01, 0.03, 0.06, and 0.09 using solid-state reaction. The structure of KY(MoO 4 ) 2 belongs to the orthorhombic structure with space group Pbcn (60) and the doping of the Sm 3+ ions did not effected to the lattice structure of the host materials. The particle sizes of the phosphors were about m. However, this paper does not tell the structure of the Eu 3+ ions when doping into the host material and not stated the average of the particle sizes of the phosphors. The PL results indicated that the emission intensity initially increased and reached a maximum of 3 mol% Sm 3+ concentration then going down after 3 mol% due to the concentration quenching. This is because of the incident of energy movement between Sm 3+ ions in different sites. The KY(MoO 4 ) 2 :0.03Sm 3+, 0.01Eu 3+ showed the emission spectra occurred at 615 nm and the excitation spectra at 404 nm. The result proved that the emission intensity of Sm 3+ decreased at ~600 nm and the increasing of Eu 3+ doping concentration effected to the increasing of Eu 3+ emission intensity at ~615 nm. This transition energy from Sm 3+ ions to Eu 3+ ions efficiently occurred [22]. Another studies by Zhou et al. (2014) analyzed Eu 3+ and Sm 3+ co-doped LiY(MoO 4 ) 2 nanoparticles for indicating their luminescence properties using sol-gel technique and annealed at 600 and 700 C. From the XRD pattern, it showed the tetragonal scheelite structure with space group I4 1/a belonging to the phosphors. The structures of phosphors using FESEM showed the bicontinuous structure with macropores as the gel skeleton (refer Figure 4) when annealed at 600 and 700 C. The substitution of Eu 3+ and/or Sm 3+ does not cause significant change in the host structure of the phosphor due to their ion radii, Eu 3+ (107 pm) and Sm 3+ (108 pm) are close to each other. The PL study showed (refer to Figure 5) the emission spectra of LiY 0.95 Eu 0.05 (MoO 4 ) 2 located at nm and the excitation peak located at 465 nm (refer to Figure 6). For LiY 0.9 Sm 0.05 (MoO 4 ) 2, emission spectra resulted at nm and the excitation resulted at 405 nm. When the LiY(MoO 4 ) 2 :0.05Eu Sm 3+, it showed the excitation of 405 nm of Sm 3+ and 465 nm of Eu 3+ show nearly the same emission bands, except for the relative intensity, with very weak characteristic emission of Sm 3+, which illustrates that Sm 3+ ions can absorb and efficiently transfer the energy to Eu 3+ ions. In addition, according to the author, the Eu 3+ emission might be improved when the quantum efficiency of Eu 3+ has increasing due to the little influence of Sm 3+ doping concentration effected on the absorption peaks of f-f transitions of Eu 3+ at 465 nm [12]. This paper stated that the phosphors annealed at 600 and 700 C, however they did not stated why they used that temperature to anneal the phosphors. (a) (b) FIGURE 4. FESEM micrograph of nanoscale Eu 3+ and Sm 3+ co-doped LiY(MoO 4 ) 2 phosphors with different annealed temperature (a) 600 C and (b) 700 C by Zhou et al. (2014) [12]

8 FIGURE 5. Photoluminescence excitation spectra of Eu 3+ and Sm 3+ co-doped nanoparticles by Zhou et al. (2014) [12]. FIGURE 6. Photoluminescence spectra of Eu 3+ and Sm 3+ co-doped nanoparticles by Zhou et al. (2014) [12]. Table 2 summarizes the quantity of works reported on replacing Eu 3+ with other candidate ions since the year 2005 in molybdate/tungstate phosphor

9 TABLE 2. Summarization of replacing Eu 3+ ions with other candidate ions. No. Candidate Author Structure, Space Group 1 K 4-3x (WO 4 ) 2 :Eu 3+ x,sm 3+ y Lee et al. (2005) Monoclinic, space 2 K 4-3(x+y) (WO 4 ) 2 :Eu x 3+,Sm y 3+ Lee et al. (2008) 3 CaMoO 4 :Eu,Sm Chunlei et al. 4 LiY 0.95 x Eu 0.05 Sm x (MoO 4 ) 2 (2009) Zhou et al. (2014) group C2/c Monoclinic, space group C2/c Spherical-like shape Tetragonal scheelite structure, 5 KY 1-x Ln x (MoO 4 ) 2 (Ln=Sm 3+, Eu 3+ ) Li et al. (2014) space group I4 1/a Orthorhombic structure, space group Pbcn (60) To the best of our knowledge, so far, no paper has published on the preparation of Sm 3+ ion doped LiY (1- x)eu x (MoO 4 ) 2 phosphor using solid-state reaction. This will be the gap for finding the best concentration of Sm 3+ in the LiY 0.45 Eu 0.55 (MoO 4 ) 2 for improving the intensity of efficiency. So far, there are only limited number of studies has been conducted for analyzing the potential of Sm 3+ ion in tungstate-molybdate structure. Besides that, it also showed that Sm 3+ ion has own goodness which is great react as a sinsitizer for the host lattice. Therefore, it is interesting for continuing the research for looking the effect of Sm 3+ ion will giving any improvement to the existing properties of tungstate- molybdate phosphor. CONCLUSION From the review of previous researches, it can be concludes that: 1. From continuing recent research by Fathullah et al. (2016) which stated Y 3+ has the maximum concentration with 0.45 mol%, so the further research need to be conducted for looking the potential occurred when Sm 3+ ion is added to the host material will increased or decreased the concentration of Y It is interesting to study the Sm 3+ - Eu 3+ effects in relation to the emission/excitation properties of the phosphor. Sm 3+ has been found to act as sensitizer to Eu 3+ therefore by selecting the efficient LiY 0.45 Eu 0.55 (MoO 4 ) 2 phosphor, it is expected that the luminescence properties to be enhanced. 3. The molybdate ion has its own stabilities characteristics that is interesting to be explored with several rare earth elements. It is expected that the introduction of small amount of Sm 3+ will not affect the crystal structure of the compound LiY 0.45 Eu 0.55 (MoO 4 ) 2. REFERENCES 1. G. H. Lee, T. H. Kim, C. Yoon, and S. Kang, J. Lumin., vol. 128, no. 12, pp , (2008). 2. S. Nakamura, M. Senoh, N. Iwasa, and S. Chi Nagahama, Appl. Phys. Lett., 67,13, , (1995). 3. J. Wang, X. Jing, C. Yan, J. Lin, and F. Liao, J. Lumin., 121, 1, 57 61, (2006). 4. A. A. Setlur, Electrochem. Soc. Interface, 18, 4, 32 36, (2009). 5. M. Yamada et al., Japanese J. Appl. Physics, Part 2 Lett., 42, 1 A/B (2003). 6. B. C. Blasse, G., Grabmaier, Luminescent Materials. Netherlands: Springer-Verlag Berlin Heidelberg, (1994). 7 J. Silver and M. Fathullah, Thesis Dissertation, (Brunel University London, 2016). 8. C. Guo, S. Wang, T. Chen, L. Luan, and Y. Xu, Appl. Phys. A Mater. Sci. Process., 94, 2, , (2009). 9. M. Shang, G. Li, X. Kang, D. Yang, and J. Lin, J. Electrochem. Soc., 158, 5, H565, (2011). 10. Y. Liu et al., RSC Adv., 4, 9, p. 4754, (2014). 11. S. Wei, L. Yu, F. Li, J. Sun, and S. Li, Ceram. Int., 41, 1, pp , (2015). 12. X. Zhou, G. Wang, T. Zhou, K. Zhou, Q. Li, and Z. Wang, J. Nanosci. Nanotechnol., vol. 14, 5, ,

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