VUV excited photoluminescence of Tb3+ doped LaPO4 green emitting phosphors for PDP applications

Journal Pre-proof UV/VUV excited photoluminescence of Tb3+ doped LaPO4 green emitting phosphors for PDP applications Vijay Singh, Ashwini Kumar, Chaitali M. Mehare, Hoonil Jeong, S.J. Dhoble

PII:

S0030-4026(19)31631-6

DOI:

https://doi.org/10.1016/j.ijleo.2019.163733

Reference:

IJLEO 163733

To appear in:

Optik

Received Date:

16 August 2019

Accepted Date:

6 November 2019

Please cite this article as: Singh V, Kumar A, Mehare CM, Jeong H, Dhoble SJ, UV/VUV excited photoluminescence of Tb3+ doped LaPO4 green emitting phosphors for PDP applications, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163733

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UV/VUV excited photoluminescence of Tb3+ doped LaPO4 green emitting phosphors for PDP applications Vijay Singh a, *, Ashwini Kumar b, Chaitali M. Mehare c, Hoonil Jeong a, S. J. Dhoble c a

Department of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein 9300, South Africa c Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India

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*Corresponding author: E-mail: [email protected] (V. Singh) Abstract

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La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors were synthesized using conventional co-

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precipitation method. The powder X-ray diffraction confirmed the pure phase formation of LaPO4 host material. Upon ultra-violet (UV) excitation and vacuum ultra-violet (VUV), the

respectively due to the

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phosphor showed a dominant green emission band centered at 544 and 545 nm, D4→7F5 transition of Tb3+. The concentration quenching

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phenomena occurred when Tb3+ concentration was beyond 0.09 mol and this quenching mechanism could be explained by the dipole-dipole interaction. The optimal concentration

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was found to be 0.09 mol of Tb3+ ion. The CIE chromaticity coordinates of the optimized (La0.91PO4:0.09Tb) phosphor was determined to be (0.265, 0.567). All the analyzed results confirms that the La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors can find possible applications as promising green-emitting phosphor in near UV and VUV used in plasma display panels.

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Keywords: Co-precipitation; Powder XRD; Tb3+ ions; LaPO4; Photoluminescence; PDP

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1. Introduction Phosphors suitable for plasma display panels (PDP) have attracted greatly since last two decades in the market of high definition (HD) display devices. The major demand of phosphors for PDP applications include its ability to hold its light output for thousands of hours, also these phosphors must show persistence between 6 to 9 ms to absorb in the vacuum-ultraviolet (VUV) region, likely at 147 nm and 172 nm [1–3]. In order to find applications as PDP materials,

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countable number of phosphors are suitable because of specific wavelength requirements for

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excitation [4].

Inorganic materials having f0, f7 or f14 lanthanide ion has less chance of undergoing

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quenching effect through cross relaxation from Ln3+ ion and that is the reason why those

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materials or compounds are given preferences as host materials [5]. In that regard LaPO4 is good luminescence host material because of its characteristic properties like low toxicity, good

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physical & chemical stability, high index of refraction etc. [6, 7]. LaPO4 doped with cerium and terbium ion has already been used as commercially applied green phosphor in fluorescent lamp

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industry [8]. Lanthanum orthophosphate (LaPO4) have three different polymorphs; hexagonal, monoclinic and tetragonal. Generally, monoclinic crystal system of LaPO4 has been explored in large extent because of its efficient photoluminescence (PL) characteristics. Terbium (Tb3+) ions are one of the most important ion among lanthanides for their characteristic green emission, most

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often excited by ultra violet (UV) and vacuum ultra-violet (VUV) light. In this paper, we present the photoluminescent properties of Tb3+ doped LaPO4

synthesized by co-precipitation method. The desired phase formation has been obtained. The optical bandgap has also been calculated and chromaticity diagram added in support of the prepared sample’s emission color.

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2. Materials preparation and analysis La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors were synthesized by co-precipitation method [9]. La(NO3)3∙6H2O (99.99%, Merck), NH4H2PO4 (99.9%, Merck), and Tb(NO3)3∙xH2O (99.99%, Merck), were used as starting materials. Firstly the stoichiometric amounts of all the starting materials were weighed in 150 ml glass beaker and dissolved in 10 ml distilled water under

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vigorous stirring for 1hr. The stirring resulted in precipitate out of the solution. Afterwards, the precipitate was dried in an oven at 110 °C. Finally, obtained white powders were annealed in

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muffle furnace at 1000 °C for 4 hr.

The phase purity and crystal structure determination of La1-xPO4:xTb (x=0.005≤ x≤ 0.10)

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samples were carried out using the Cu-Kα radiation (λ=1.5406 Å) of a Rigaku X-ray

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diffractometer. Figures of the crystal structure in this work were drawn with crystal maker software [10]. Diffuse reflectance spectroscopy was performed using a Varian Cary 6000i UVabsorption

spectrophotometer

equipped

with

an

integrating

sphere.

The

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Vis-NIR

photoluminescence (PL) spectra were measured at room temperature on a Shimadzu RF-5301PC

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spectrofluorophotometer equipped with Xenon flash lamp. For the vacuum ultraviolet (VUV) photoluminescence measurements, the samples were illuminated either by 147 or 172 nm wavelengths from a 150 W deuterium lamp (C3150, Hamamatsu, Japan) with an incident angle of 45 degrees. The vacuum level of 5.1ⅹ10-3± 3ⅹ10-4

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Torr was achieved with a rotary pump (W2V20, WSA, Korea) in a vacuum chamber system (PSI, Korea). The photoluminescence spectra were measured by a spectrometer (Darsa pro-5000, PSI, Korea) with a spectral resolution of 1 nm. To improve the signal-to-noise ratio up to ~30 for 147 nm and ~90 for 172 nm excitations, the signals were integrated for 3s, and then 100 measurements were averaged.

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3. Results and discussion 3.1 Crystal structure and XRD analysis The powder XRD patterns of synthesized La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors are displayed in Figure 1(a). It can be seen that diffraction patterns of the samples are identical to each other and there are no additional peaks of other phases have been detected, all peaks agree well with the Joint Committee on Powder Diffraction Standards file (JCPDS) 32-0493,

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indicating monoclinic pure phase formation of LaPO4 having space group P21/n with unit cell

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parameters, a=6.83, b=7.07 Å, and c=6.50 Å. The ionic radii of La3+ (1.216 Å) and Tb3+ (1.095 Å) are very close enough and this confirms that Tb3+ is expected to occupy the La3+ sites in the

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LaPO4 host [11]. Figure 1(b) shows the diffraction peak for the plane [120], gradually shifted to higher diffraction angles, pointing out that with Tb3+ doping, there was a continuous decrease of

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the inter-planar spacing leading to shrinkage in the cell volume.

Fig. 1 (a) XRD pattern of La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors, (b) shifting of [120] at higher angles

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Fig. 2 (a) Unit cell crystal structure of the monoclinic LaPO4. 2(b) & 2(c) local environments of La-O and P-O in LaPO4 host lattice

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Monoclinic LaPO4 has monazite structure as seen from Fig. 2(a); where lanthanum atoms have nine fold coordination (LaO9) shown in Fig 2(b). Fig. 2(c) depicts the orientation of four of the

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oxygen atoms in distorted tetrahedron (PO4) that interpenetrates a planar pentagon constituted by

Å.

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3.2 Diffuse reflectance studies

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other 5 other oxygen atoms [6]. The mean bond length between La-O and P-O are 2.57 and 1.54

The diffuse reflectance spectra of the synthesized La0.91PO4:0.09Tb phosphor has been presented in the Fig. 3(a). The spectra show a broad and prominent absorption band at ∼274 and 284 nm.

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The absorption is possibly attributed to the transition between the valence band and the conduction band. Fig. 3(b) shows the estimation of optical bandgap energy (Eg) using the Kubelka-Munk method [12]. The intersection between the linear fit portion of the curve and the photon energy (eV) axis yields the value of Eg as 2.68 eV.

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3.3 Photoluminescence analysis

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Fig. 3(a) Diffuse reflectance spectra and 3(b) Optical energy band gap determination of La0.91PO4:0.09Tb phosphor

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3.3.1 Excitation spectrum of La1-xPO4:xTb (x=0.01 ≤ x ≤0.11) phosphor Fig. 4 (a) shows the PL excitation spectra of La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors,

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monitored at 544 nm emission wavelength. The excitation spectra can be categorized into two regions: the first starting from 200 to 290 nm that are attributed to the characteristics of 4f8 →4f75d1 transition and the second region ranges from 300 to 400 nm that has been arised due to the 4f8→4f8 transitions [13]. The first region from 200 to 290 nm consists of a spin-allowed

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strong 7FJ→7DJ excitation band centered at 228 nm and a spin-forbidden weak 7FJ→9DJ excitation band centered at 253 and 271 nm, respectively. Also, the spin-forbidden 4f8→4f8 transitions arising from the 7F6 ground state to the various excited states, viz., 5L7, 5L8, 5L10, 5D3 are found to be centered at 302-316 nm, 340-350 nm, 367 nm, 377 nm, respectively. The

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excitation band centered at 228 nm is found to be the most suitable for efficient emission from

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the synthesized phosphor.

Fig. 4(a) Photoluminescence Excitation spectra of La1-xPO4: xTb(x=0.01≤ x≤0.11)(λemi= 544 nm)

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3.3.2 Emission spectrum of La1-xPO4: xTb(x=0.01≤x≤0.11) phosphor The PL emission spectra of La1-xPO4:xTb (x=0.01≤x≤0.11) phosphor under 228 nm excitation shown in Fig. 4(b). It can easily be seen that two groups of PL bands originate from 5D3 and 5D4 emission levels to their 7FJ lower lying levels. The emission bands corresponding to the 5D3-7F5, 4

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transitions are centered at 416 nm and 438 nm, respectively are unimportant in the present work due to weak luminescence intensity, whereas the 5D4-7F6, 5, 4, 3 transitions with peak maxima at 488, 544, 584 and 620 nm, respectively are significant and has been taken into consideration for the study [14]. The PL emission intensity of the phosphors were observed to vary with the concentration of Tb3+ ions. With the increase of the concentration from 0.01 mol to 0.09 mol,

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the PL intensity is found to increase and at 0.11 mol doping, the intensity decreased because of concentration quenching phenomena, this occurred due to resonant energy transfer between Tb3+ ions in the host LaPO4. Thus, efficient energy transfer has taken place between the adjacent Tb3+ ions at 0.09 mol concentration, as a result the after this increment of Tb3+ ions, concentration quenching occurs. Also, the generalized phenomenon is that the photoluminescence intensity depends on the average distance between luminescent centers. So, the increase in the doping

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concentration, shortens the distance between active ions causing concentration quenching [15].

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However, In order to have better perceptive of concentration quenching mechanism in prepared

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phosphor, the critical distance (Rc) needs to be measured using the Blasse equation [16]:

Fig. 4(b) Emission spectra of La1-xPO4:xTb (x=0.01≤ x≤0.11) (λexc = 228 nm)

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R C = 2 [4πx N] C

1 3

(1)

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where, ‘V’ is the volume of the unit cell, xc is the critical concentration (Tb3+ = 0.09 mol), and N is the occupational cation sites for the dopant in the unit cell. For the host LaPO4, N equals to 4, and V was estimated to be 306.58 Å3, by substituting these values to above equation, the critical transfer distance of Tb3+ in LaPO4 phosphor was found to be about 11.76 Å. In this case, the Tb3+- Tb3+ distance is greater than 10 Å. So, the exchange interactions are not possible in this case. Therefore, possibly the electric multipolar interaction is the only condition for the energy

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transfer among the Tb3+ ions in LaPO4 phosphor.

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Fig. 4(c) Variation in the emission intensity of strong emission (544 nm) as a function of Tb3+ concentration

The Fig. 4(c) shows the maximum relative emission intensity as obtained from the La1-xPO4:xTb phosphors and shows the effect of doping on the emission intensity. It can be observed that 5D3 → 7FJ transition in the Tb3+ was quenched with the increasing of the Tb3+ concentration. The optimized concentration for Tb3+ is 0.09 mol in LaPO4 host.

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3.2.3 VUV emission spectrum of La1-xPO4: xTb (x=0.01≤ x≤0.11) phosphor Fig. 5 (a) and 5(b) shows the photoluminescence emission spectra of La1-xPO4:xTb (x=0.01≤ x≤0.11) phosphors under vacuum ultra-violet (VUV) excitation wavelengths 147 nm and 172 nm, respectively. These spectra show the nearly similar emission pattern as observed for 228 nm excitation except the emission bands corresponding to the 5D3-7F5, 4 transitions are missing in both the excitation wavelength. Phosphates are a class of materials that generally show strong

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absorption in the VUV region [17]. So, it can be seen in the below Fig. 5(a) and 5(b) that

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phosphates, doped with Tb3+ can show strong VUV-excited PL emissions.

Fig. 5 VUV photoluminescence spectra of the La1-xPO4:xTb (x=0.01≤x≤0.11) phosphor (a) Emission spectra (λexc = 147 nm) and (b) Emission spectrum (λexc = 172 nm) Both Fig. 5(a) and 5(b) show the characteristics of Tb3+ emissions due to the 5D4-7F6,

5, 4, 3

transitions with peak maxima at 490, 545, 586 and 622 nm, respectively. The green emission

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peak located at 545 nm due to the 5D4–7F5 transition is the most strongest and dominant, and that is the requirement for attaining a green phosphor with high color purity.

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Fig. 6 Typical photographs of La0.91PO4:0.09Tb phosphor: (a) under room light (appearance: white powder) and (b) under 254-nm UV light (appearance: green powder).

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The phosphor images in normal light and under UV excitation has been presented in Fig. 6 which clearly shows the intense green emission from the prepare phosphor under UV source

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excitation.

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3.2.4 Chromaticity diagram for La1-xPO4:xTb (x=0.01≤ x≤0.11) phosphors In order to evaluate the phosphor performance, the color coordinate plays a vital role in finalizing the kind of color emission. The Commission International de I’ Eclairage (CIE)

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chromaticity (x, y) coordinates for La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors are listed in Table I and presented in Fig. 7. It can be observed that the emission color shifted from light green (0.238, 0.431) to intense green (0.263, 0.586) with the varying doping concentration of Tb3+

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ions. These co-ordinates confirms that the synthesized phosphors can be used as pure-greenemitting tunable phosphors.

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Fig. 7 Commission International de I’ Eclairage (CIE)-chromaticity diagram for La1-xPO4:xTb (x=0.01≤ x≤0.11) phosphors

The increase in CIE x-co-ordinate followed by a decrease in CIE y-co-ordinate in the chromaticity diagram confirms excellent color purity. The color purity for the synthesized

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phosphors can be estimated by using the relation [18] Color Purity =

√(𝑥−𝑥𝑖 )2 +(𝑦−𝑦𝑖 )2

√(𝑥𝑑 −𝑥𝑖 )2 +(𝑦𝑑 −𝑦𝑖 )2

× 100 %

(2)

where, (x, y) and (xi, yi) are the color-coordinates of the sample point and the CIE equal-energy illuminate, respectively; (xd, yd) is the chromaticity coordinate of the dominant wavelength of the light source. For the La0.91PO4:0.09Tb sample, the coordinates of (x, y) is (0.265, 0.567); the coordinates of (xi, yi) is (0.333, 0.333); and using (xd, yd) values corresponding to the dominant 12

wavelength of 544 nm, the color purity of La0.91PO4:0.09Tb phosphor was found to be as high as 78.8%. 4. Conclusions A series of La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors were synthesized using conventional coprecipitation method. The powder X-ray diffraction analysis confirmed the pure phase formation of LaPO4 host material and crystallizes into the monoclinic crystal system. The excitation spectra

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indicated that the phosphors could be efficiently excited by ultraviolet light (228 nm) as well as

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vacuum ultra violet light (147 and 172 nm). The phosphor showed a dominant green emission band centered at 544 nm due to the 5D4→7F5 transition of Tb3+. The optimized critical quenching

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concentration of La1-xPO4:xTb (x=0.01≤x≤0.11) phosphor was about 0.09 mol and the

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concentration quenching mechanism in the La1-xPO4:xTb (x=0.01≤x≤0.11) phosphor could be explained by the electric dipole-dipole interaction. The CIE coordinate of optimized

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La0.91PO4:0.09Tb phosphor was determined to be (0.265, 0.567). All the analyzed results confirms that the La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors are promising green-emitting

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phosphor in near UV and VUV possibly used in white light-emitting diodes as well as plasma display panels.

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Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03030003).

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[14] Y. Xia, Y. Huang, Q. Long, S. Liao, Y. Gao, J. Liang, J. Cai, Near-UV light excited Eu3+, Tb3+, Bi3+ co-doped LaPO4 phosphors: Synthesis and enhancement of red emission for WLEDs, Ceram. Int. 41 (2015) 5525-5530. [15] A. Kumar, S.J. Dhoble, D.R. Peshwe, J. Bhatt, Structural and photoluminescence properties of nepheline-structure NaAlSiO4:Dy3+ nanophosphors, J. Alloys Compd., 609 (2014) 100-106. [16] G. Blasse, Philips Res. Rep. 24 (1969) 131-144. [17] Govind B. Nair, S. J. Dhoble, Highly enterprising calcium zirconium phosphate [CaZr4 (PO4)6: Dy3+, Ce3+] phosphor for white light emission, RSC Adv., 5 (2015) 49235-49247.

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Table I CIE chromaticity (x, y) coordinates for La1-xPO4:xTb (x=0.01≤x≤0.11) phosphors X-coordinate 0.238 0.261 0.265 0.262 0.265 0.263

Y-coordinate 0.431 0.535 0.551 0.551 0.567 0.586

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Sample composition La0.99PO4:0.01Tb La0.97PO4:0.03Tb La0.95PO4:0.05Tb La0.93PO4:0.07Tb La0.91PO4:0.09Tb La0.89PO4:0.11Tb

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