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Cathodoluminescence properties of monoclinic phased reddish-orange emitting BaY2(MoO4)4:Eu3+ phosphor
Optical Materials 99 (2020) 109604
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Cathodoluminescence properties of monoclinic phased reddish-orange emitting BaY2(MoO4)4:Eu3þ phosphor Rajagopalan Krishnan a, b, **, Hendrik C. Swart a, * a b
Department of Physics, University of the Free State, Bloemfontein, South Africa Department of Physics, Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords: X-ray diffraction X-ray photoelectron spectroscopy Cathodoluminescence Phosphors Penetration depth
A series of Eu3þ activated BaY2(MoO4)4 reddish-orange emitting phosphors was successfully synthesized by changing the doping concentrations of Eu3þ ions using a solid-state reaction route. All the reflections obtained in the powder X-ray diffraction analysis signposted that the as-prepared phosphors belonged to the monoclinic phase with the C2/c space group. Narrow scan X-ray photoelectron spectroscopy analysis suggested that the molybdenum ions were connected tetrahedrally to oxygen with its maximum 6þ valance state and yttrium atoms with 3þ state in the BaY2(MoO4)4:Eu3þ material. The influence of doping concentration of Eu3þ ions on the cathodoluminescence (CL) properties of BaY2(MoO4)4 was explored with respect to various accelerating voltages and filament currents. The penetration depth of the electrons into the phosphors has been estimated as a function of accelerating voltage. The phosphor showed no saturation current or saturation voltage in the ranges that were explored. The CL spectra revealed that reddish-orange emission was witnessed when the phosphor material was stimulated under e beam excitation due to the 5D0→7F2 transition of the Eu3þ ions. The as-prepared phosphor material exhibited intense and strong CL emission lines with better International Commission on Illumination (CIE) chromaticity coordinates and higher colour purity and can be considered to be utilize in field emission display device applications.
1. Introduction Display technology forms an integrated part of our daily lives, and its application ranges from smartphones, tablets, and computers to largescreen television, versatile flexible screen, and data projectors [1]. Solid-state light-emitting diodes (LEDs) are still used with success as backlights in liquid crystal displays (LCDs) and now as light sources in newly developed MicroLEDs. Comparing the technology maturity, cost and durability, there is no doubt that LCD technique still dominates the display field. Currently phosphors are investigated to be used in three kinds of backlights for LCDs [1,2]: (i) multichip white light-emitting diodes (wLEDs), (ii) quantum dots light-emitting diodes (QD-LEDs, photoluminescence), and (iii) phosphor-converted light-emitting diodes (pc-LEDs). Multichip wLEDs and QD-LEDs show excellent colour per formance and tuning abilities. However, the red-, green-, and blue-emitting phosphors used in the LED chips, suffer from different degradation rates and low efficiency. A lot of attention is given in the development of new phosphors that will withstand these degradation
and have narrow-emission bands, high quantum efficiency, and short decay times [1,2]. Next generation field emission display (FED) devices have paid much responsiveness owing to their potential advantages such as large viewpoints, small panel widths, provides distortion-free images, low energy consuming properties, and tunable self-emission properties [3,4]. In modern ages, lanthanide ions activated phosphors in the FEDs are much attracted due to their captivating features, such as rapid response time, towering brightness with more efficiency [5–7]. Some of the supreme factors to improve the quality of extremely competent phosphors used in the FEDs are that they must operate with low energies (�5 keV) at elevated currents (10–100 μA cm 2) 5,8]. The commercially used sulfide-based phosphorus effortlessly decomposed and the gas content of sulfides is released during the generation of electrons, thereby initiating the cathodes to fail as a result of reducing the light efficiency of the phosphors [8]. Hence, to revamp the working performances of FEDs, it is essential to solve the aforesaid problem by finding a new class of phosphors with noble chemical stability, good colour rendering ability and improved luminescence efficiency under electron beam excitation.
* Corresponding author. ** Corresponding author. Department of Physics, Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India. E-mail addresses: [email protected] (R. Krishnan), [email protected] (H.C. Swart). https://doi.org/10.1016/j.optmat.2019.109604 Received 22 October 2019; Received in revised form 4 December 2019; Accepted 4 December 2019 Available online 9 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Optical Materials 99 (2020) 109604
Due to the high chemical, thermal stabilities of molybdates they are more attractive in contrast to the sulfide based phosphors [9]. Also, molybdates based hosts own a wide excitation and emission band owing to charge transfer (CT) transitions in the Mo–O group and exhibiting rich emission characteristics [10]. Alkaline-earth lanthanide molybdates with general formula ALn2( MoO4)4 has a monoclinic structure in which A is an bivalent alkaline earth cation, Ln is the trivalent Gd3þ or La3þ or Y3þ lanthanide ion that has been acknowledged as excellent luminescent materials [11,12] . Due to their interesting spectroscopic features, BaGd2(MoO4)4 have been reported as a promising host for the applications in opto-electronics and laser materials [13,14]. As a potential candidate, BaLa2(MoO4)4 has been investigated and studied for thermometry and optical heating ap plications [15]. But, Li et al. [16] investigated the dielectric character istics of the compound BaY2(MoO4)4 but there are no systematic reports carried out on the luminescence properties of BaY2(MoO4)4 phosphors. Hence, with this background information, herein we report the cath odoluminescence (CL) emission characteristics of BaY2(MoO4)4 doped with different Eu3þ concentrations as a function of different accelerating voltages and filament current. The obtained results suggesting that BaY2(MoO4)4:Eu3þ showed improved luminescence properties which fits for the potential FEDs applications.
3. Result and discussion 3.1. Structural investigation The XRD patterns of BaY2-x(MoO4)4:xEu3þ (x ¼ 3, 6, 9, 12, 15 mol% of Eu3þ, respectively) phosphor powders are demonstrated in Fig. 1(a). The crystal structure related information for the BaY2(MoO4)4 is not available in the JCPDS data set. Conversely, all the reflections in the XRD patterns could be matched with the monoclinic phase BaGd2( MoO4)4 with JCPDS card number 36–1092. The samples doped with different Eu3þ concentrations show indistinguishable diffraction pat terns with no other extra impurity peaks. Since the ionic size and valence states of Y3þ (1.02 Å) and Eu3þ (1.07 Å) [18] ions are similar, Eu3þ successfully lodged into the Y3þ sites. Fig. 1 (b) and 1 (c) represents the three dimensional view of the BaY2(MoO4)4 crystal structure. The crystal structure of BaY2(MoO4)4 belongs to the monoclinic phase with C2/c space group. In this, mono clinic phase, divalent Ba2þ ions connected to ten oxygen ions in a ten coordinate symmetry and trivalent Y3þ is connected to eight oxygen ions in an eight coordinate geometry [19]. But, molybdenum ions Mo6þ are attached to four equivalent oxygen ions in a tetrahedral symmetry. This monoclinic crystal phase consists of 8 uneven oxygen sites [19]. In the first site, oxygen is connected to 2-yttrium (equivalent) and 1-molybde num atoms in the distorted trigonal symmetry. In the next second location, oxygen is joined to 1-barium atom and 1-molybdenum atom in distorted single bond symmetry. In the third location, oxygen is attached to 1-barium, 1-yttrium and 1-molybdenum atoms in a two-organized geometry. The fourth oxygen site is connected to 1-barium, 1-yttrium, and 1-molybdenum atom in the deformed 120� configuration. The fifth oxygen site is well attached to 2-yttrium (equivalent) atoms and 1-mo lybdenum atom in a distorted trigonal planar fashion. The sixth oxy gen location is joined to 1-barium atom and 1-molybdenum atom in the distorted single-bond arrangement. The seventh oxygen site is linked to 1-barium atom and 1-molybdenum atom in the deformed 150� config uration. The last or the eight oxygen site is attached to 2-yttrium atoms and 1-molybdenum atom in the distorted trigonal planar configuration [19].
2. Experimental details 2.1. Synthesis of monoclinic phase BaY2(MoO4)4:Eu3þ A series of monoclinic phase BaY2-x(MoO4)4:xEu3þ (where x ¼ 3, 6, 9, 12, 15 mol % of Eu3þ) phosphor powders were successfully synthe sized by the conventional solid-state reaction method. The trivalent europium (Eu3þ) ions were replaced for the trivalent yttrium (Y3þ) atom sites. The ingredients used in this experiment were of analytical grade purchased from Aldrich with 99.99% and does not require any addi tional purification. Appropriate quantity of barium carbonate (BaCO3), yttrium oxide (Y2O3), molybdenum trioxide (MoO3) and europium oxide (Eu2O3) were carefully weighed and added in an agate mortar pestle. Primarily, the weighed mixture was grounded cautiously for 1 h, heated to an annealing temperature of 1173 K in the air atmosphere continu ously for 3 h. The obtained phosphor powders were permitted to cool to room temperature (RT) and the final product was characterized.
3.2. SEM and elemental composition analysis The average particle size of the phosphor powders was measured to be 1.4 μm and the corresponding FESEM image is presented in Fig. 2 (a). As a illustrative result, the composition of 15 mol % of Eu3þ doped BaY2(MoO4)4 was taken for the EDS analysis. The distribution of all the core elements (Ba, Y, Mo, O, and Eu) in the phosphor powder was mapped and is presented in Fig. 2 (b). The compositions were nearly found to be 5.1 at % of Ba2þ: 8.2 at % of Y3þ: 17.3 at % of Mo: 68.4 at % of O: 1 at % of Eu, and the corresponding EDS spectrum is shown in Fig. 2 (c).
2.2. Characterization techniques The structural and phase identification of the as-synthesized mono clinic phase BaY2(MoO4)4:Eu3þ phosphor powders were inspected by utilizing a Bruker X-ray diffraction (XRD) instrument with Cu-Kα radi ation (λ ¼ 0.15406 nm) operated at 40 kV and 40 mA at a scanning rate of 0.0198� s 1 in a 2θ range of 20–80� . Visualization of the crystal structure has been explored using the VESTA program [17]. The chemical state of the elements and their equivalent binding energies were determined using a PHI Versa Probe 5000 scanning ESCA micro probe. X-ray photoelectron spectroscopy (XPS) wide scans were carried out with a pass energy of 187 eV in energy steps of 0.5 eV and a stay time of 100 ms/step. XPS high-resolution scans have been performed to characterize the samples. Scans were performed for the regions of the elements Ba 3d, Y 3d, Mo 3d, O 1s, Eu 3d and C 1s by using a pass energy of 44.75 eV and step size 0.5 eV. The binding energy correction was carried out by using C 1s, by fixing to a binding energy value of 284.5 eV. CL measurements were performed using a Jeol JSM-7800F field emission scanning electron microscope (FESEM) equipped with an Ox ford Aztec energy dispersive spectroscope (EDS) and a Gatan Mono CL4 device. The commission of internationale del’Eclairage (CIE) colour coordinates have been estimated by analyzing the spectral energy dis tribution functions of the prepared phosphor material.
3.3. Binding energy and chemical state analysis of monoclinic phase BaY2(MoO4)4:Eu3þ Fig. 3 warrants the existence of important core elements such as Ba, Y, Mo, O, Eu in the phosphor material which is demonstrated in the XPS survey scan. Charge correction was carried out for carbon by correcting to a reference binding energy value of 284.5 eV. Shirley/Linear/Tou gaard background approximation tools have been used for the baseline correction. For plotting the curve, the obtained XPS data was fitted well using Gaussian-Lorentzian functions which are consistent with the standard literature. Fig. 4(a–f) depicts the high resolution XPS narrow scan results, which were carried out in order to examine the chemical binding states of each element. Fig. 4 (a) displays the high resolution XPS spectra of the core level Ba-3d region, which was deconvoluted into two well separated doublets assigned to the 3d5/2 and 3d3/2 spin-orbit states of the Ba ions. 2
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Fig. 1. (a) Powder XRD patterns of the BaY2-x(MoO4)4:xEu3þ (x ¼ 3, 6, 9, 12 and 15 mol% of Eu3þ ions), (b) Tetrahedra around the molybdenum atoms forming a [MoO4]2- cluster, (c) polyhedral structure of BaY2(MoO4)4:Eu3þ.
Deconvolution resulted into a high intense binding energy signal traced at 779.7/795.0 eV is due to BaO group and a low intense binding energy signals detected at 780.6/796.0 eV is ascribed to the 2 þ state of barium ions [20,21]. This result shows that the coordination between Ba and O in the monoclinic phase is very strong. Fig. 4 (b) shows the narrow scan XPS spectra of the Y-3d region, which comprises of one doublet peak (due to Y3d5/2 and Y3d3/2 levels) located at 157.6/159.7 eV and is fixed to the Y3þ state in te Y2O3 group [22,23]. The deconvoluted Mo-3d region shown in Fig. 4 (c) is as well de tached due to spin-orbits of the 3d3/2 and 3d5/2 levels. The high intense binding energy pair identified at 232.3/235.5 eV is ascribed to the photoelectron emission from Mo6þ in the Mo–O group [24]. The XPS spectra of the O-1s region shown in Fig. 4 (d) were well fitted into three distinguished peaks. The major peak positioned at 530.2 eV is due to the photoelectron ejection from the O 1s in the metal-oxygen group (com bination of Y2O3/MoO3/Eu2O3) [25,26]. The less intense binding energy peaks recognized at 531.8 eV and 532.7 eV are corresponding to the O–H groups [27,28] and hydroxide [29] group, respectively. Fig. 4 (e) portrays the oxidation state of the Eu (3d - after 1 min of sputtering). The fitted region comprises of four pairs of the doublets owed to the spin-orbit splitting between the Eu 3d5/2 and Eu 3d3/2 states. The very low intense binding energy pair found at 1126.5/1156.4 eV corresponds to the Eu2þ state [30]. The existence of Eu2þ can be ascribed to the
changes from the 3 þ state to the 2 þ state of the Eu ion due to the charge compensation effect during profiling under the atmospheric circum stances of the XPS investigation, which is commonly witnessed for Eu3þ doped materials [31]. The pairs of XPS signals observed at 1134.3/1164.1 eV, 1136.4/1166.2 eV were accredited to the Eu3þ [32], Eu3þ in Eu2O3 [33] state, respectively. In addition, the peak at the binding energy value of 1138.8 eV is assigned to the Ba (3p1/2) state, which is actually fused with the Eu 3d spectral region [28]. Fig. 4 (f) validates the deconvoluted XPS spectra of the C 1s region which has two photoelectron lines observed at 284.5 eV and 285.6 eV and are fixed to – C–C & C–H, respectively [34–36]. It is clear from the C–C/C–H and C– XPS results, that the molybdenum ions exist only with the highest oxidation state of 6þ and coordinated in the tetrahedral symmetry. 3.4. Cathodoluminescence spectroscopic properties of monoclinic phased BaY2(MoO4)4:Eu3þ The CL process is a complementary and analogous to the photo luminescence process, which is due to the non-incandescent radiation of electromagnetic waves when the sample is being excited with a beam of electrons. The CL emission spectra of the as-synthesized microcrystalline phosphor powders were analyzed using the highly sensitive photo multiplier. A series of CL emission spectra profiles have been 3
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Optical Materials 99 (2020) 109604
Fig. 2. (a) FESEM image of the as-prepared BaY2(MoO4)4:Eu3þ phosphor (b) Elemental mapping analysis, (c) Corresponding EDS spectrum.
investigated under electron beam excitation by changing the filament current and voltage that were collected by using a charge coupled device. Fig. 5(a–c) display the CL emission spectra of BaY2(MoO4)4 phosphor doped with different Eu3þ contents excited under different accelerating voltages 2 keV, 5 keV and 10 keV, respectively. The CL emission spectral profiles consisted of sharp and intense emission lines accompanied with the 5D0-7FJ transitions of Eu3þ ions. Under the excitation with an ebeam, the CL emission spectra were completely subjugated by hyper sensitive orange-red emission located at 616 nm is assigned to the 5 D0-7F2 transition, which is mainly originating because of the electric dipole transition of Eu3þ ions. This results indicates that the Eu3þ ions perfectly lodged into a Y3þ site without inversion symmetry. The in tensity of other transitions 5D0-7F3, 5D0-7F4 appeared at 656 nm, 697 nm and are much weaker than the electric dipole transition 5D0-7F2. How ever, at lower accelerting voltage (2 keV), the 5D0-7F3, 5D0-7F4 did not appear in the emission spectra, which is due to the dominated character of the 5D0→ 7F2 transitions. Upon exciting the electron beam with different accelerating voltages, (2 keV, 5 keV and 10 keV) the asprepared sample doped with different Eu3þ concentrations showed similar spectral profiles but differs in their intensity. It is observed from Fig. 5 (d) that the CL emission intensity increased linearly and persistently as a function of doping content with an increase in Eu3þ, and also it amplifies upon increasing the accelerating voltage from 2 to 10 keV. There was no saturation noticed up to the voltage of
Fig. 3. XPS survey spectrum of monoclinic phase BaY2(MoO4)4:Eu3þ phos phor powder.
4
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Optical Materials 99 (2020) 109604
Fig. 4. Deconvoluted and fitted XPS spectra of (a) Ba 3d, (b) Y 3d, (c) Mo 3d, (d) O 1s, (e) Eu 3d, (f) C 1s, core regions in the monoclinic phase BaY2(MoO4)4:Eu3þ powder.
5
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Optical Materials 99 (2020) 109604
Fig. 5. The CL emission spectra of the monoclinic phased BaY2(MoO4)4:Eu3þ samples doped with different Eu3þ concentrations analyzed under different accelerating voltages (a) 2 keV, (b) 5 keV, (C) 10 keV, (d) Concentration of Eu3þ ions versus accelerating voltages.
density of the solid in gm/cm3, ‘E’ be the accelerating voltage in kV and ‘Z’ be the no. of electron in the molecule. In this monoclinic phased BaY2(MoO4)4:Eu3þ phosphor the values of A, ρ and Z were found to be 954.91 gm/mol, 4.75 gm/cm3 and 430, respectively. Fig. 7 demonstrates the estimated electron penetration depth at different accelerating voltages. It is clear from the penetration depth calculations, when the electron energy increases from 1 keV to 10 keV, the electron penetration depth is much deeper and reaches up to a maximum of 1237 Å resulting in the population of more Eu3þ ions to higher energy levels and correspondingly the CL intensity increases. The CIE colour coordinates were determined from the CL emission spectra for all the concentrations of Eu3þ ion doped with BaY2(MoO4)4: Eu3þ phosphors, which occupied the reddish-orange region and the corresponding CIE diagrams are furnished in Fig. 8. The reddish-orange CL emission with good CIE coordinates of the under low-voltage e beam could be a more suitable red phosphor candidate for prospective applications in the FED devices.
10 keV. Also, from Fig. 6 it is observed that the CL emission signals increased constantly with the increasing in the filament current from 0.010 nA to 4.40 nA and no saturation current was noticed when the voltage was kept at 5 keV. The improvement of the CL emission signals as a function of growing electron voltages and as well as current is possibly due to a deeper penetration depth and interaction volume of the electrons in the material that resulted in the subsequent CL intensity increase [37]. To investigate the electron beam penetration depth further calculations were done using the empirical relation given in equations (1) and (2). The CL intensity raised linearly when the electron energy was increased. � � � �� �n A E pffiffiffi L � A ¼ 250 (1) ρ Z where n¼
1
1:2 0:29 log Z
(2)
In eq (1), where ‘A’ be the relative molar mass in gm/mol, ‘ρ’ be the 6
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Fig. 6. CL emission spectra of monoclinic phased BaY2(MoO4)4:Eu3þ phosphor accelerated under different filament current (0.010 nA–4.40 nA) with a 5 keV fixed voltage. The inset shows the CL emission intensity as a function of various filament currents.
Fig. 7. Calculated electron penetration depth as a function of different accel erating voltages and the inset shows the CL emission from the phosphor.
4. Conclusions In conclusion, reddish-orange emitting BaY2(MoO4)4:Eu3þ phos phors have been synthesized via a solid-state reaction route under air atmospheric condition. XRD patterns revealed that the as-obtained phosphors belong to the monoclinic crystal structure with a C2/c space group. From the FESEM investigation, the average particle size of the phosphor powders was found to be 1.4 μm in size. The distribution of all the core elements (Ba, Y, Mo, O, and Eu) in the phosphor powder was mapped using the FESEM technique and their corresponding binding energies were estimated using high resolution XPS. Upon stimulating with an e beam excitation, the CL emission spectra of BaY2(MoO4)4: Eu3þ phosphors showed enriched reddish-orange emission found at 616 nm, which mainly originated because of the electric dipole transition of the Eu3þ ions. The CL emission signals were amplified with the rise in
Fig. 8. CIE chromaticity coordinates of monoclinic phased BaY2-x(MoO4)4: xEu3þ phosphor (inset - A, B, C, D, E represents 3, 6, 9, 12 and 15 mol % of Eu3þ, respectively) obtained from the CL emission spectra.
accelerating voltage from 2 keV to 10 keV and no saturation voltage was observed due to deeper penetration depth of electrons into the phos phors. Also, no saturation current was noticed when the filament current started to increase from 0.010 nA to 4.40 nA even for 15 mol % of Eu3þ doped BaY2(MoO4)4 phosphors. The penetration depth of the electron into the phosphors increased as the accelerating voltage increased. 7
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Based on the aforesaid investigations, it is strongly believed that the asprepared phosphor is a most suitable candidate for the potential appli cation in field emission display devices.
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Author statement Rajagopalan Krishnan Conceptualization, Methodology, First draft, H.C. Swart: Writing, supervision, editing and review. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement We acknowledge and express our sincere thanks to all the faculty members in the Department of Physics, University of Free State for extending the characterization facilities. This research work is supported by the South African Research Chairs Initiative of the Department of Science and Technology (84415). The financial assistance from the University of the Free State is also highly recognized. References [1] Hongxu Liao, Ming Zhao, Yayun Zhou, Maxim S. Molokeev, Quanlin Liu, Qinyuan Zhang, Zhiguo Xia, Polyhedron transformation toward stable narrowband green phosphors for wide-color-gamut liquid crystal display, Adv. Funct. Mater. 29 (7) (2019) 1901988. [2] Ming Zhao, Hongxu Liao, Lixin Ning, Qinyuan Zhang, Quanlin Liu, Zhiguo Xia, Next-generation narrow-band green-emitting RbLi(Li3SiO4)2:Eu2þ phosphor for backlight display application, Adv. Mater. 30 (38) (2018) 1802489, 7. [3] L. Hu, Q. Wang, X. Wang, Y. Li, Y. Wang, X. Peng, Photoluminescence and cathodoluminescence properties of Na2MgGeO4:Mn2þ green phosphors, RSC Adv. 5 (2015) 104708–104714. [4] P. Du, J-S Yu, Dual-enhancement of photoluminescence and cathodoluminescence in Eu3þ activated SrMoO4 phosphors by Naþ doping, RSC Adv. 5 (2015) 60121–60127. [5] G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Single composition trichromatic white-emitting Ca4Y6(SiO4)6O:Ce3þ/Mn2þ/Tb3þ phosphor: luminescence and energy transfer, ACS Appl. Mater. Interfaces 4 (2012) 296–305. [6] G. Li, D. Geng, M. Shang, Y. Zhang, C. Peng, Z. Cheng, J. Lin, Color tuning luminescence of Ce3þ/Mn2þ/Tb3þ triactivated Mg2Y8(SiO4)6O2 via energy transfer: potential single-phase white-light-emitting phosphors, J. Phys. Chem. C 115 (2011) 21882–21892. [7] D. Geng, G. Li, M. Shang, C. Peng, Y. Zhang, Z. Cheng, J. Lin, Nanocrystalline CaYAlO4:Tb3þ/Eu3þ as promising phosphors for full-color field emission displays, Dalton Trans. 41 (2012) 3078–3086. [8] S.H. Cho, S.H. Kwon, J.S. Yoo, C.W. Oh, J.D. Lee, K.J. Hong, S.J. Kwon, Cathodoluminescent characteristics of a spherical Y2O3:Eu3þ phosphor screen for field emission display application, J. Electrochem. Soc. 147 (2000) 3143–3147. [9] A.M. Kaczmarek, R.V. Deun, Rare earth tungstate and molybdate compounds – from 0D to 3D architectures, Chem. Soc. Rev. 42 (2013) 8835–8848. [10] R. Krishnan, J. Thirumalai, Up/down conversion luminescence properties of (Na0.5Gd0.5)MoO4:Ln3þ (Ln ¼ Eu, Tb, Dy, Yb/Er, Yb/Tm, and Yb/Ho) microstructures: synthesis, morphology, structural and magnetic investigation, New J. Chem. 38 (2014) 3480–3491. [11] C. Guo, H.-K. Yang, Z. Fu, L. Li, B-C Choi, J-H Jeong, A potential red-emitting phosphor BaGd2(MoO4)4:Eu3þ for near-UV white LED, J. Am. Ceram. Soc. 92 (2009) 1713–1718. [12] Y. Guan, T. Tsuboi, Y. Huang, W. Huang, Concentration quenching of praseodymium ions Pr3þ in BaGd2(MoO4)4 crystals, Dalton Trans. 43 (2014) 3698–3703.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Cathodoluminescence properties of monoclinic phased reddish-orange emitting BaY2(MoO4)4:Eu3þ phosphor Rajagopalan Krishnan a, b, **, Hendrik C. Swart a, * a b
Department of Physics, University of the Free State, Bloemfontein, South Africa Department of Physics, Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Keywords: X-ray diffraction X-ray photoelectron spectroscopy Cathodoluminescence Phosphors Penetration depth
A series of Eu3þ activated BaY2(MoO4)4 reddish-orange emitting phosphors was successfully synthesized by changing the doping concentrations of Eu3þ ions using a solid-state reaction route. All the reflections obtained in the powder X-ray diffraction analysis signposted that the as-prepared phosphors belonged to the monoclinic phase with the C2/c space group. Narrow scan X-ray photoelectron spectroscopy analysis suggested that the molybdenum ions were connected tetrahedrally to oxygen with its maximum 6þ valance state and yttrium atoms with 3þ state in the BaY2(MoO4)4:Eu3þ material. The influence of doping concentration of Eu3þ ions on the cathodoluminescence (CL) properties of BaY2(MoO4)4 was explored with respect to various accelerating voltages and filament currents. The penetration depth of the electrons into the phosphors has been estimated as a function of accelerating voltage. The phosphor showed no saturation current or saturation voltage in the ranges that were explored. The CL spectra revealed that reddish-orange emission was witnessed when the phosphor material was stimulated under e beam excitation due to the 5D0→7F2 transition of the Eu3þ ions. The as-prepared phosphor material exhibited intense and strong CL emission lines with better International Commission on Illumination (CIE) chromaticity coordinates and higher colour purity and can be considered to be utilize in field emission display device applications.
1. Introduction Display technology forms an integrated part of our daily lives, and its application ranges from smartphones, tablets, and computers to largescreen television, versatile flexible screen, and data projectors [1]. Solid-state light-emitting diodes (LEDs) are still used with success as backlights in liquid crystal displays (LCDs) and now as light sources in newly developed MicroLEDs. Comparing the technology maturity, cost and durability, there is no doubt that LCD technique still dominates the display field. Currently phosphors are investigated to be used in three kinds of backlights for LCDs [1,2]: (i) multichip white light-emitting diodes (wLEDs), (ii) quantum dots light-emitting diodes (QD-LEDs, photoluminescence), and (iii) phosphor-converted light-emitting diodes (pc-LEDs). Multichip wLEDs and QD-LEDs show excellent colour per formance and tuning abilities. However, the red-, green-, and blue-emitting phosphors used in the LED chips, suffer from different degradation rates and low efficiency. A lot of attention is given in the development of new phosphors that will withstand these degradation
and have narrow-emission bands, high quantum efficiency, and short decay times [1,2]. Next generation field emission display (FED) devices have paid much responsiveness owing to their potential advantages such as large viewpoints, small panel widths, provides distortion-free images, low energy consuming properties, and tunable self-emission properties [3,4]. In modern ages, lanthanide ions activated phosphors in the FEDs are much attracted due to their captivating features, such as rapid response time, towering brightness with more efficiency [5–7]. Some of the supreme factors to improve the quality of extremely competent phosphors used in the FEDs are that they must operate with low energies (�5 keV) at elevated currents (10–100 μA cm 2) 5,8]. The commercially used sulfide-based phosphorus effortlessly decomposed and the gas content of sulfides is released during the generation of electrons, thereby initiating the cathodes to fail as a result of reducing the light efficiency of the phosphors [8]. Hence, to revamp the working performances of FEDs, it is essential to solve the aforesaid problem by finding a new class of phosphors with noble chemical stability, good colour rendering ability and improved luminescence efficiency under electron beam excitation.
* Corresponding author. ** Corresponding author. Department of Physics, Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India. E-mail addresses: [email protected] (R. Krishnan), [email protected] (H.C. Swart). https://doi.org/10.1016/j.optmat.2019.109604 Received 22 October 2019; Received in revised form 4 December 2019; Accepted 4 December 2019 Available online 9 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Due to the high chemical, thermal stabilities of molybdates they are more attractive in contrast to the sulfide based phosphors [9]. Also, molybdates based hosts own a wide excitation and emission band owing to charge transfer (CT) transitions in the Mo–O group and exhibiting rich emission characteristics [10]. Alkaline-earth lanthanide molybdates with general formula ALn2( MoO4)4 has a monoclinic structure in which A is an bivalent alkaline earth cation, Ln is the trivalent Gd3þ or La3þ or Y3þ lanthanide ion that has been acknowledged as excellent luminescent materials [11,12] . Due to their interesting spectroscopic features, BaGd2(MoO4)4 have been reported as a promising host for the applications in opto-electronics and laser materials [13,14]. As a potential candidate, BaLa2(MoO4)4 has been investigated and studied for thermometry and optical heating ap plications [15]. But, Li et al. [16] investigated the dielectric character istics of the compound BaY2(MoO4)4 but there are no systematic reports carried out on the luminescence properties of BaY2(MoO4)4 phosphors. Hence, with this background information, herein we report the cath odoluminescence (CL) emission characteristics of BaY2(MoO4)4 doped with different Eu3þ concentrations as a function of different accelerating voltages and filament current. The obtained results suggesting that BaY2(MoO4)4:Eu3þ showed improved luminescence properties which fits for the potential FEDs applications.
3. Result and discussion 3.1. Structural investigation The XRD patterns of BaY2-x(MoO4)4:xEu3þ (x ¼ 3, 6, 9, 12, 15 mol% of Eu3þ, respectively) phosphor powders are demonstrated in Fig. 1(a). The crystal structure related information for the BaY2(MoO4)4 is not available in the JCPDS data set. Conversely, all the reflections in the XRD patterns could be matched with the monoclinic phase BaGd2( MoO4)4 with JCPDS card number 36–1092. The samples doped with different Eu3þ concentrations show indistinguishable diffraction pat terns with no other extra impurity peaks. Since the ionic size and valence states of Y3þ (1.02 Å) and Eu3þ (1.07 Å) [18] ions are similar, Eu3þ successfully lodged into the Y3þ sites. Fig. 1 (b) and 1 (c) represents the three dimensional view of the BaY2(MoO4)4 crystal structure. The crystal structure of BaY2(MoO4)4 belongs to the monoclinic phase with C2/c space group. In this, mono clinic phase, divalent Ba2þ ions connected to ten oxygen ions in a ten coordinate symmetry and trivalent Y3þ is connected to eight oxygen ions in an eight coordinate geometry [19]. But, molybdenum ions Mo6þ are attached to four equivalent oxygen ions in a tetrahedral symmetry. This monoclinic crystal phase consists of 8 uneven oxygen sites [19]. In the first site, oxygen is connected to 2-yttrium (equivalent) and 1-molybde num atoms in the distorted trigonal symmetry. In the next second location, oxygen is joined to 1-barium atom and 1-molybdenum atom in distorted single bond symmetry. In the third location, oxygen is attached to 1-barium, 1-yttrium and 1-molybdenum atoms in a two-organized geometry. The fourth oxygen site is connected to 1-barium, 1-yttrium, and 1-molybdenum atom in the deformed 120� configuration. The fifth oxygen site is well attached to 2-yttrium (equivalent) atoms and 1-mo lybdenum atom in a distorted trigonal planar fashion. The sixth oxy gen location is joined to 1-barium atom and 1-molybdenum atom in the distorted single-bond arrangement. The seventh oxygen site is linked to 1-barium atom and 1-molybdenum atom in the deformed 150� config uration. The last or the eight oxygen site is attached to 2-yttrium atoms and 1-molybdenum atom in the distorted trigonal planar configuration [19].
2. Experimental details 2.1. Synthesis of monoclinic phase BaY2(MoO4)4:Eu3þ A series of monoclinic phase BaY2-x(MoO4)4:xEu3þ (where x ¼ 3, 6, 9, 12, 15 mol % of Eu3þ) phosphor powders were successfully synthe sized by the conventional solid-state reaction method. The trivalent europium (Eu3þ) ions were replaced for the trivalent yttrium (Y3þ) atom sites. The ingredients used in this experiment were of analytical grade purchased from Aldrich with 99.99% and does not require any addi tional purification. Appropriate quantity of barium carbonate (BaCO3), yttrium oxide (Y2O3), molybdenum trioxide (MoO3) and europium oxide (Eu2O3) were carefully weighed and added in an agate mortar pestle. Primarily, the weighed mixture was grounded cautiously for 1 h, heated to an annealing temperature of 1173 K in the air atmosphere continu ously for 3 h. The obtained phosphor powders were permitted to cool to room temperature (RT) and the final product was characterized.
3.2. SEM and elemental composition analysis The average particle size of the phosphor powders was measured to be 1.4 μm and the corresponding FESEM image is presented in Fig. 2 (a). As a illustrative result, the composition of 15 mol % of Eu3þ doped BaY2(MoO4)4 was taken for the EDS analysis. The distribution of all the core elements (Ba, Y, Mo, O, and Eu) in the phosphor powder was mapped and is presented in Fig. 2 (b). The compositions were nearly found to be 5.1 at % of Ba2þ: 8.2 at % of Y3þ: 17.3 at % of Mo: 68.4 at % of O: 1 at % of Eu, and the corresponding EDS spectrum is shown in Fig. 2 (c).
2.2. Characterization techniques The structural and phase identification of the as-synthesized mono clinic phase BaY2(MoO4)4:Eu3þ phosphor powders were inspected by utilizing a Bruker X-ray diffraction (XRD) instrument with Cu-Kα radi ation (λ ¼ 0.15406 nm) operated at 40 kV and 40 mA at a scanning rate of 0.0198� s 1 in a 2θ range of 20–80� . Visualization of the crystal structure has been explored using the VESTA program [17]. The chemical state of the elements and their equivalent binding energies were determined using a PHI Versa Probe 5000 scanning ESCA micro probe. X-ray photoelectron spectroscopy (XPS) wide scans were carried out with a pass energy of 187 eV in energy steps of 0.5 eV and a stay time of 100 ms/step. XPS high-resolution scans have been performed to characterize the samples. Scans were performed for the regions of the elements Ba 3d, Y 3d, Mo 3d, O 1s, Eu 3d and C 1s by using a pass energy of 44.75 eV and step size 0.5 eV. The binding energy correction was carried out by using C 1s, by fixing to a binding energy value of 284.5 eV. CL measurements were performed using a Jeol JSM-7800F field emission scanning electron microscope (FESEM) equipped with an Ox ford Aztec energy dispersive spectroscope (EDS) and a Gatan Mono CL4 device. The commission of internationale del’Eclairage (CIE) colour coordinates have been estimated by analyzing the spectral energy dis tribution functions of the prepared phosphor material.
3.3. Binding energy and chemical state analysis of monoclinic phase BaY2(MoO4)4:Eu3þ Fig. 3 warrants the existence of important core elements such as Ba, Y, Mo, O, Eu in the phosphor material which is demonstrated in the XPS survey scan. Charge correction was carried out for carbon by correcting to a reference binding energy value of 284.5 eV. Shirley/Linear/Tou gaard background approximation tools have been used for the baseline correction. For plotting the curve, the obtained XPS data was fitted well using Gaussian-Lorentzian functions which are consistent with the standard literature. Fig. 4(a–f) depicts the high resolution XPS narrow scan results, which were carried out in order to examine the chemical binding states of each element. Fig. 4 (a) displays the high resolution XPS spectra of the core level Ba-3d region, which was deconvoluted into two well separated doublets assigned to the 3d5/2 and 3d3/2 spin-orbit states of the Ba ions. 2
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Fig. 1. (a) Powder XRD patterns of the BaY2-x(MoO4)4:xEu3þ (x ¼ 3, 6, 9, 12 and 15 mol% of Eu3þ ions), (b) Tetrahedra around the molybdenum atoms forming a [MoO4]2- cluster, (c) polyhedral structure of BaY2(MoO4)4:Eu3þ.
Deconvolution resulted into a high intense binding energy signal traced at 779.7/795.0 eV is due to BaO group and a low intense binding energy signals detected at 780.6/796.0 eV is ascribed to the 2 þ state of barium ions [20,21]. This result shows that the coordination between Ba and O in the monoclinic phase is very strong. Fig. 4 (b) shows the narrow scan XPS spectra of the Y-3d region, which comprises of one doublet peak (due to Y3d5/2 and Y3d3/2 levels) located at 157.6/159.7 eV and is fixed to the Y3þ state in te Y2O3 group [22,23]. The deconvoluted Mo-3d region shown in Fig. 4 (c) is as well de tached due to spin-orbits of the 3d3/2 and 3d5/2 levels. The high intense binding energy pair identified at 232.3/235.5 eV is ascribed to the photoelectron emission from Mo6þ in the Mo–O group [24]. The XPS spectra of the O-1s region shown in Fig. 4 (d) were well fitted into three distinguished peaks. The major peak positioned at 530.2 eV is due to the photoelectron ejection from the O 1s in the metal-oxygen group (com bination of Y2O3/MoO3/Eu2O3) [25,26]. The less intense binding energy peaks recognized at 531.8 eV and 532.7 eV are corresponding to the O–H groups [27,28] and hydroxide [29] group, respectively. Fig. 4 (e) portrays the oxidation state of the Eu (3d - after 1 min of sputtering). The fitted region comprises of four pairs of the doublets owed to the spin-orbit splitting between the Eu 3d5/2 and Eu 3d3/2 states. The very low intense binding energy pair found at 1126.5/1156.4 eV corresponds to the Eu2þ state [30]. The existence of Eu2þ can be ascribed to the
changes from the 3 þ state to the 2 þ state of the Eu ion due to the charge compensation effect during profiling under the atmospheric circum stances of the XPS investigation, which is commonly witnessed for Eu3þ doped materials [31]. The pairs of XPS signals observed at 1134.3/1164.1 eV, 1136.4/1166.2 eV were accredited to the Eu3þ [32], Eu3þ in Eu2O3 [33] state, respectively. In addition, the peak at the binding energy value of 1138.8 eV is assigned to the Ba (3p1/2) state, which is actually fused with the Eu 3d spectral region [28]. Fig. 4 (f) validates the deconvoluted XPS spectra of the C 1s region which has two photoelectron lines observed at 284.5 eV and 285.6 eV and are fixed to – C–C & C–H, respectively [34–36]. It is clear from the C–C/C–H and C– XPS results, that the molybdenum ions exist only with the highest oxidation state of 6þ and coordinated in the tetrahedral symmetry. 3.4. Cathodoluminescence spectroscopic properties of monoclinic phased BaY2(MoO4)4:Eu3þ The CL process is a complementary and analogous to the photo luminescence process, which is due to the non-incandescent radiation of electromagnetic waves when the sample is being excited with a beam of electrons. The CL emission spectra of the as-synthesized microcrystalline phosphor powders were analyzed using the highly sensitive photo multiplier. A series of CL emission spectra profiles have been 3
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Fig. 2. (a) FESEM image of the as-prepared BaY2(MoO4)4:Eu3þ phosphor (b) Elemental mapping analysis, (c) Corresponding EDS spectrum.
investigated under electron beam excitation by changing the filament current and voltage that were collected by using a charge coupled device. Fig. 5(a–c) display the CL emission spectra of BaY2(MoO4)4 phosphor doped with different Eu3þ contents excited under different accelerating voltages 2 keV, 5 keV and 10 keV, respectively. The CL emission spectral profiles consisted of sharp and intense emission lines accompanied with the 5D0-7FJ transitions of Eu3þ ions. Under the excitation with an ebeam, the CL emission spectra were completely subjugated by hyper sensitive orange-red emission located at 616 nm is assigned to the 5 D0-7F2 transition, which is mainly originating because of the electric dipole transition of Eu3þ ions. This results indicates that the Eu3þ ions perfectly lodged into a Y3þ site without inversion symmetry. The in tensity of other transitions 5D0-7F3, 5D0-7F4 appeared at 656 nm, 697 nm and are much weaker than the electric dipole transition 5D0-7F2. How ever, at lower accelerting voltage (2 keV), the 5D0-7F3, 5D0-7F4 did not appear in the emission spectra, which is due to the dominated character of the 5D0→ 7F2 transitions. Upon exciting the electron beam with different accelerating voltages, (2 keV, 5 keV and 10 keV) the asprepared sample doped with different Eu3þ concentrations showed similar spectral profiles but differs in their intensity. It is observed from Fig. 5 (d) that the CL emission intensity increased linearly and persistently as a function of doping content with an increase in Eu3þ, and also it amplifies upon increasing the accelerating voltage from 2 to 10 keV. There was no saturation noticed up to the voltage of
Fig. 3. XPS survey spectrum of monoclinic phase BaY2(MoO4)4:Eu3þ phos phor powder.
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Fig. 4. Deconvoluted and fitted XPS spectra of (a) Ba 3d, (b) Y 3d, (c) Mo 3d, (d) O 1s, (e) Eu 3d, (f) C 1s, core regions in the monoclinic phase BaY2(MoO4)4:Eu3þ powder.
5
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Fig. 5. The CL emission spectra of the monoclinic phased BaY2(MoO4)4:Eu3þ samples doped with different Eu3þ concentrations analyzed under different accelerating voltages (a) 2 keV, (b) 5 keV, (C) 10 keV, (d) Concentration of Eu3þ ions versus accelerating voltages.
density of the solid in gm/cm3, ‘E’ be the accelerating voltage in kV and ‘Z’ be the no. of electron in the molecule. In this monoclinic phased BaY2(MoO4)4:Eu3þ phosphor the values of A, ρ and Z were found to be 954.91 gm/mol, 4.75 gm/cm3 and 430, respectively. Fig. 7 demonstrates the estimated electron penetration depth at different accelerating voltages. It is clear from the penetration depth calculations, when the electron energy increases from 1 keV to 10 keV, the electron penetration depth is much deeper and reaches up to a maximum of 1237 Å resulting in the population of more Eu3þ ions to higher energy levels and correspondingly the CL intensity increases. The CIE colour coordinates were determined from the CL emission spectra for all the concentrations of Eu3þ ion doped with BaY2(MoO4)4: Eu3þ phosphors, which occupied the reddish-orange region and the corresponding CIE diagrams are furnished in Fig. 8. The reddish-orange CL emission with good CIE coordinates of the under low-voltage e beam could be a more suitable red phosphor candidate for prospective applications in the FED devices.
10 keV. Also, from Fig. 6 it is observed that the CL emission signals increased constantly with the increasing in the filament current from 0.010 nA to 4.40 nA and no saturation current was noticed when the voltage was kept at 5 keV. The improvement of the CL emission signals as a function of growing electron voltages and as well as current is possibly due to a deeper penetration depth and interaction volume of the electrons in the material that resulted in the subsequent CL intensity increase [37]. To investigate the electron beam penetration depth further calculations were done using the empirical relation given in equations (1) and (2). The CL intensity raised linearly when the electron energy was increased. � � � �� �n A E pffiffiffi L � A ¼ 250 (1) ρ Z where n¼
1
1:2 0:29 log Z
(2)
In eq (1), where ‘A’ be the relative molar mass in gm/mol, ‘ρ’ be the 6
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Fig. 6. CL emission spectra of monoclinic phased BaY2(MoO4)4:Eu3þ phosphor accelerated under different filament current (0.010 nA–4.40 nA) with a 5 keV fixed voltage. The inset shows the CL emission intensity as a function of various filament currents.
Fig. 7. Calculated electron penetration depth as a function of different accel erating voltages and the inset shows the CL emission from the phosphor.
4. Conclusions In conclusion, reddish-orange emitting BaY2(MoO4)4:Eu3þ phos phors have been synthesized via a solid-state reaction route under air atmospheric condition. XRD patterns revealed that the as-obtained phosphors belong to the monoclinic crystal structure with a C2/c space group. From the FESEM investigation, the average particle size of the phosphor powders was found to be 1.4 μm in size. The distribution of all the core elements (Ba, Y, Mo, O, and Eu) in the phosphor powder was mapped using the FESEM technique and their corresponding binding energies were estimated using high resolution XPS. Upon stimulating with an e beam excitation, the CL emission spectra of BaY2(MoO4)4: Eu3þ phosphors showed enriched reddish-orange emission found at 616 nm, which mainly originated because of the electric dipole transition of the Eu3þ ions. The CL emission signals were amplified with the rise in
Fig. 8. CIE chromaticity coordinates of monoclinic phased BaY2-x(MoO4)4: xEu3þ phosphor (inset - A, B, C, D, E represents 3, 6, 9, 12 and 15 mol % of Eu3þ, respectively) obtained from the CL emission spectra.
accelerating voltage from 2 keV to 10 keV and no saturation voltage was observed due to deeper penetration depth of electrons into the phos phors. Also, no saturation current was noticed when the filament current started to increase from 0.010 nA to 4.40 nA even for 15 mol % of Eu3þ doped BaY2(MoO4)4 phosphors. The penetration depth of the electron into the phosphors increased as the accelerating voltage increased. 7
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Based on the aforesaid investigations, it is strongly believed that the asprepared phosphor is a most suitable candidate for the potential appli cation in field emission display devices.
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Author statement Rajagopalan Krishnan Conceptualization, Methodology, First draft, H.C. Swart: Writing, supervision, editing and review. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement We acknowledge and express our sincere thanks to all the faculty members in the Department of Physics, University of Free State for extending the characterization facilities. This research work is supported by the South African Research Chairs Initiative of the Department of Science and Technology (84415). The financial assistance from the University of the Free State is also highly recognized. References [1] Hongxu Liao, Ming Zhao, Yayun Zhou, Maxim S. Molokeev, Quanlin Liu, Qinyuan Zhang, Zhiguo Xia, Polyhedron transformation toward stable narrowband green phosphors for wide-color-gamut liquid crystal display, Adv. Funct. Mater. 29 (7) (2019) 1901988. [2] Ming Zhao, Hongxu Liao, Lixin Ning, Qinyuan Zhang, Quanlin Liu, Zhiguo Xia, Next-generation narrow-band green-emitting RbLi(Li3SiO4)2:Eu2þ phosphor for backlight display application, Adv. Mater. 30 (38) (2018) 1802489, 7. [3] L. Hu, Q. Wang, X. Wang, Y. Li, Y. Wang, X. Peng, Photoluminescence and cathodoluminescence properties of Na2MgGeO4:Mn2þ green phosphors, RSC Adv. 5 (2015) 104708–104714. [4] P. Du, J-S Yu, Dual-enhancement of photoluminescence and cathodoluminescence in Eu3þ activated SrMoO4 phosphors by Naþ doping, RSC Adv. 5 (2015) 60121–60127. [5] G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Single composition trichromatic white-emitting Ca4Y6(SiO4)6O:Ce3þ/Mn2þ/Tb3þ phosphor: luminescence and energy transfer, ACS Appl. Mater. Interfaces 4 (2012) 296–305. [6] G. Li, D. Geng, M. Shang, Y. Zhang, C. Peng, Z. Cheng, J. Lin, Color tuning luminescence of Ce3þ/Mn2þ/Tb3þ triactivated Mg2Y8(SiO4)6O2 via energy transfer: potential single-phase white-light-emitting phosphors, J. Phys. Chem. C 115 (2011) 21882–21892. [7] D. Geng, G. Li, M. Shang, C. Peng, Y. Zhang, Z. Cheng, J. Lin, Nanocrystalline CaYAlO4:Tb3þ/Eu3þ as promising phosphors for full-color field emission displays, Dalton Trans. 41 (2012) 3078–3086. [8] S.H. Cho, S.H. Kwon, J.S. Yoo, C.W. Oh, J.D. Lee, K.J. Hong, S.J. Kwon, Cathodoluminescent characteristics of a spherical Y2O3:Eu3þ phosphor screen for field emission display application, J. Electrochem. Soc. 147 (2000) 3143–3147. [9] A.M. Kaczmarek, R.V. Deun, Rare earth tungstate and molybdate compounds – from 0D to 3D architectures, Chem. Soc. Rev. 42 (2013) 8835–8848. [10] R. Krishnan, J. Thirumalai, Up/down conversion luminescence properties of (Na0.5Gd0.5)MoO4:Ln3þ (Ln ¼ Eu, Tb, Dy, Yb/Er, Yb/Tm, and Yb/Ho) microstructures: synthesis, morphology, structural and magnetic investigation, New J. Chem. 38 (2014) 3480–3491. [11] C. Guo, H.-K. Yang, Z. Fu, L. Li, B-C Choi, J-H Jeong, A potential red-emitting phosphor BaGd2(MoO4)4:Eu3þ for near-UV white LED, J. Am. Ceram. Soc. 92 (2009) 1713–1718. [12] Y. Guan, T. Tsuboi, Y. Huang, W. Huang, Concentration quenching of praseodymium ions Pr3þ in BaGd2(MoO4)4 crystals, Dalton Trans. 43 (2014) 3698–3703.
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