Synthesis and structural characterization of orange red light emitting Sm3+ activated BiOCl phosphor for WLEDs applications

Journal of Alloys and Compounds 785 (2019) 169e177

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Synthesis and structural characterization of orange red light emitting Sm3þ activated BiOCl phosphor for WLEDs applications Pramod Halappa, Harshitha M. Rajashekar, C. Shivakumara* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 July 2018 Received in revised form 4 January 2019 Accepted 12 January 2019 Available online 14 January 2019

We report, synthesis of orange red light emitting Sm3þ activated BiOCl phosphors at relatively low temperature (400  C) and shorter duration of 1 h. Rietveld refined results confirmed that all the compounds crystallized in the tetragonal structure with space group P4/nmm (No. 129). Functional group analysis were carried out by Fourier transform infrared (FT-IR) and Raman spectroscopy. The Photoluminescent (PL) experiment results reveal that, an intense PL transition at 598 nm (4G5/2 / 6H7/2) corresponds to orange red emission at lex ¼ 408 nm. The maximum emission intensity was observed for BiOCl: Sm3þ (3 mol%) phosphor, above this concentration quenching takes place due to charge exchange with neighboring Sm3þ ions. Time resolved fluorescence spectroscopic result reveals that, the decay curve is bi-exponential in nature and having long lifetime (t ¼ 0.58 ms). The obtained CIE and CCT values suggest that these phosphors having excellent color purity (~90%) and low correlated color temperature. These results suggest that, Sm3þ activated BiOCl can be a potential red phosphor materials for WLEDs and other optoelectronics applications. © 2019 Elsevier B.V. All rights reserved.

Keywords: Bismuth oxy chloride Sm3þ ion Phosphors Photo-luminescent Color purity

1. Introduction Last few decades, rare earth ions activated inorganic phosphors received extensive research attention due to their conceivable photonic applications as light emitting phosphors in white light emitting diodes (WLEDs) [1,2]. Now days WLEDs become commercially available with the advent of high performance, long durability, eco-friendliness and more. Unfortunately, these WLEDs fail to show optimum color rendering index, due to the lack of a red component in the overall commercial WLEDs composition. To solve this problem europium based red phosphors were extensively explored due to their excellent red light emission [3e5], but the expensive europium sources kept out these phosphors from cost effective solid state lighting applications. Hence, still there is a scope to explore new red light emitting activator ion alternative to Eu3þ ions. Sm3þ ions are identified to exhibit a narrow orange-red emission on near-UV excitation suitable for solid state lighting applications [6e9]. In Sm3þ ions, the 4f-4f electric dipole transitions are absolutely forbidden, except this the excitation peaks of Sm3þ based phosphors (365 nm, 375 nm, 405 nm, 460 nm, and

* Corresponding author. E-mail address: [email protected] (C. Shivakumara). https://doi.org/10.1016/j.jallcom.2019.01.155 0925-8388/© 2019 Elsevier B.V. All rights reserved.

480 nm) concurrent well with the emission peaks of the NUVInGaN LED chips. Hence, the Sm3þ based phosphors can be easily fabricated and used with the commercially available chips. The efficiency of the emission depends on the host in which they are incorporated [10]. Bismuth based materials, especially bismuth oxyhalides well known materials in the field of photoluminescence, photo-catalysts, pigments, battery materials, pharmaceuticals, magnetic materials etc., [11]. In recent times, rare earth activated BiOCl have drawn much attention because of their first-rate luminescent properties [1,11e15]. BiOCl is regarded to be efficient host matrix for phosphors because of their high chemical stability, wide band gap (~3.4eV), moderate phonon energies, non-toxic nature and low synthesis temperature [1,11,15]. More importantly, bismuth ion (Bi3þ) site is highly favourable for Sm3þ ion substitution due to their size and charge compatibility with rare earth ions in the layered structure [16]. The photo-luminescent properties of the phosphors are greatly affected by the method of preparation of the phosphors [3]. Out of other methods, solid state method is well established and also widely used in practical purposes. This method of phosphor preparation offers an excellent features compared to other methods like high luminescent intensity, solvent free facile preparation, good quality crystalline materials, large scale production in industries and cost effectiveness [17]. To the best of our knowledge, there is no detailed study on Sm3þ doped BiOCl

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phosphor. Here we report, synthesis, structural characterization, spectroscopic and photo-luminescent properties including concentration quenching phenomenon, decay lifetime, quantum efficiency study was analysed in details. 2. Experimental section 2.1. Synthesis The targeted Bi1-xSmxOCl (0.0  x  0.11) phosphors were synthesised by the conventional solid state method. For the synthesis, analytical grade bismuth oxide (Bi2O3), samarium oxide (Sm2O3) and ammonium chloride (NH4Cl) were used as the starting materials without further purification. In the typical synthesis of Sm3þ (3 mol%) - doped BiOCl contains, Bi2O3 (1.1299 g), Sm2O3 (0.02615 g) and NH4Cl (0.3209 g, 20% excess) were taken and grounded in an agate mortar with pestle. The mixture was then transferred into clean porcelain crucible and calcined at 400  C for 1 h, followed by the muffle furnace was allowed to cool down to room temperature [11,13]. Similarly, other compositions of Sm3þactivated BiOCl compounds were also prepared by taking stoichiometric proportions of Bi2O3, Sm2O3 and NH4Cl are the raw materials. 3. Characterizations The crystallinity and phase purity of the samples were examined by the powder X-ray diffractometer (PANalytical Empyrean powder diffractometer) operated at 45 kV and 30 mA using Cu Ka radiation (l ¼ 1.5418 Å) with nickel filter. Rietveld refinement method was employed to estimate the structural parameters for all the phosphors by using FullProf Suite-2000 programme. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on Perkin Elmer Spectrometer, Frontier using KBr pellet as a reference. By using

UVevisible spectrophotometer (Perkin Elmer Lambda 750), the diffuse-reflectance spectra (DRS) of the samples were obtained using BaSO4 as a reference. The samples surface morphology and composition were analysed by the scanning electron microscope (SEM) and transmission electron microscope (TEM). The photoluminescence (PL) spectrum has been studied using a Jobin Yvon spectro-fluorometer (Fluoromax, Horiba) equipped with a 450W xenon lamp as the excitation source. The room-temperature luminescence decay lifetime curves were recorded from a spectro-fluorometer (Edinburgh Instruments Ltd, UK) using a pulsed laser radiation as the excitation source. The absolute quantum efficiency was measured using the integrated sphere on the FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd, UK), The signals were detected by a Hamamatsu R928P photomultiplier tube. And a Xenon lamp was used as an excitation source and white BaSO4 powder as a reference to measure the absorption.

4. Results and discussions 4.1. Powder X-ray diffraction The powder X-ray diffraction patterns of Bi1-xSmxOCl (0.0  x  0.11) phosphor were shown in Fig. 1a. The phase purity and crystallinity of the samples were observed by its intense sharp peaks. All the diffracted peaks are in well agreement with the reported JCPDS card no. 85-0861. And, we did not see any additional peaks of Sm2O3 in the powder XRD patterns. Also we observed shift in diffracted peaks towards higher 2q (Fig. 1b) confirms the successful replacement of the smaller Sm3þ (r3þ Sm ¼ 1.079 Å) ion with larger 8 co-ordinated Bi3þ(r3þ Bi ¼ 1.17 Å) ions in host matrix. This implies that the compounds formed are monophasic in nature. The radius percentage difference of the doped ion Sm3þ and the substituted ion Bi3þ in Bismuth Oxy-Chloride was predicted by the

Fig. 1. a) PXRD patterns and b) Magnified view of PXRD patterns of Bi1-xSmxOCl (0.00  x  0.11) compounds.

P. Halappa et al. / Journal of Alloys and Compounds 785 (2019) 169e177

following formula:

Rr ¼

Table 2 Table 1 Rietveld refined structural parameters for BiOCl and BiOCl:Sm3þ phosphors.

Rh ðCNÞ  Rd ðCNÞ  100 Rh ðCNÞ

(1)

where Rr is the radius percentage difference, CN is the coordination number, Rh (CN) is the radius of the host cation, and the Rd(CN) is the radius of doped ion. The permeable Rr value between doped and substituted ions must not exceed 30% [18]. By the substitution of Sm3þ ion (CN ¼ 8, Rd ¼ 1.079 Å) with the Bi3þ ion (CN ¼ 8, Rh ¼ 1.17 Å) in bismuth oxy-chloride host, the Rr value was found to be 7.77%. This indicates that Sm3þ ions are substituted to Bi3þsite in the BiOCl matrix. Further the average crystallite size was calculated by using Scherrer's equation [19]:

D¼

171

Kl bcosq

(2)

where l is the wavelength (1.5418 Å) of X-ray source, b is the full width at half maximum (FWHM), q is the angle of diffraction, k is the shape factor (0.9) and D is the average crystallite size. By using above equation the average crystallite size for Bi1-xSmxOCl (0.0  x  0.11) phosphor were calculated and listed in Table 1. The structural parameters were refined by the Rietveld refinement method for all compositions using powder XRD data. We not observed any significant variation in the lattice parameters or cell volume due to the compatible ionic radii of Bi3þ (1.17 Å) and Sm3þ (1.079 Å). The Rietveld refined structural parameters for BiOCl, Bi0.97Sm0.03OCl and Bi0.89Sm0.11OCl phosphors were listed in Table 2. The typical observed, calculated and the difference XRD patterns of (a) Bi0.97Sm0.03OCl and (b) Bi0.89Sm0.11OCl compounds were shown in Fig. 2. There is a good agreement with the calculated profiles, which confirmed that Bi1xSmxOCl phosphors crystallized in the tetragonal malachite structure.

Compounds

BiOCl

Bi0.97Sm0.03OCl

Bi0.89Sm0.11OCl

Crystal System Space group Lattice Parameters a (Å) c (Å) Cell volume (Å3) Atomic positions Bi/Sm x y z

Tetragonal P4/nmm (No. 129)

Tetragonal P4/nmm (No. 129)

Tetragonal P4/nmm (No. 129)

3.892(2) 7.371(2) 111.62(3)

3.894(8) 7.369(4) 111.79(3)

3.896(8) 7.354(6) 111.68(2)

(2c) 0.2500 0.2500 0.1714(9)

(2c) 0.2500 0.2500 0.1706(2)

(2c) 0.2500 0.2500 0.1708(8)

O x y z

(2a) 0.2500 0.7500 0.0000

(2a) 0.2500 0.7500 0.0000

(2a) 0.2500 0.7500 0.0000

Cl x y z R Factors Rp Rwp Rexp

(2c) 0.2500 0.2500 0.6447(5)

(2c) 0.2500 0.2500 0.6459(9)

(2c) 0.2500 0.2500 0.6430(5)

3.51 4.56 2.49 3.35 2.87 2.72

3.70 4.88 2.83 2.98 3.02 2.63

4.73 5.90 3.63 2.65 5.93 3.55

c2 RBragg RF

Sm3þ ion, respectively [12]. Existences of transitions were also witnessed in the photo-luminescent emission spectrum of Bi1xSmxOCl (Fig. 8). 4.3. Microscopic studies

4.2. Vibrational and functional group analysis The vibrational and functional group analysis of the Bi1-xSmxOCl were examined by FTIR and Raman spectroscopic measurements which were shown in Figs. 3 and 4 respectively. In Fig. 3, Bi1xSmxOCl (0.0  x  0.11) samples exhibited the strong IR absorption peak at 523 cm1 corresponds to the symmetric stretching of BieO bond [11]. And any other vibrational bands were not observed indicates that the monophasic nature of the samples. Raman spectra of BiOCl and Bi0.97Sm0.03OCl were illustrated in Fig. 4. In Fig. 4, Raman bands were found at 59, 143 and 200 cm1 are corresponds to the A1g(external), A1g and Eg modes of BieX stretching frequencies, respectively [20]. But Raman spectra of Bi0.97Sm0.03OCl showed the few additional bands at 1053, 2070 and 2670 cm1. Li (2015) et al., reported that, Raman spectra of the rare earth containing phosphors were highly influenced by luminescence exhibited by the rare earth ions [15]. And the additional three bands at 1053, 2070 and 2670 cm1in the Raman spectra arised from 4G5/2 / 6H5/2, 4G5/2 / 6H7/2, and 4G5/2 / 6H9/2 transitions of

The SEM images of the BiOCl and Bi0.97Sm0.03OCl at different magnifications were shown in Fig. 5aed. The micrographs revealed that agglomerated plate-like morphology. The thickness of the plates in both compounds was found to be an average length of 5.126 mm along with the thickness in the range of 1.159 mme1.165 mm. Fig. 6a illustrates the TEM image of Bi0.97Sm0.03OCl phosphor, which evident the 2D plate-like morphology of this phosphor. The high-resolution TEM image of Bi0.97Sm0.03OCl phosphor was acquired from the corner of the single plate (Fig. 6b), clear lattice fringes indicates high crystalline nature of the material and the lattice fringes projected along the [001] axis with an interplanar lattice spacing d ¼ 0.271 nm which corresponds to the crystallographic plane (110) with d ¼ 2.75 Å of tetragonal BiOCl phase in the XRD pattern. The polycrystalline diffraction rings were observed in the SAED pattern (Fig. 6c), corresponds to the (115), (1 0 1), (2 0 0), (110) and (003) planes of tetragonal BiOCl lattice. Fig. 6d presents the ED analysis spectrum of Bi0.97Sm0.03OCl phosphor. From ED spectra, we only observed Bi, O, Sm and Cl elements

Table 1 Estimated crystallite size, dislocation density, stacking fault and energy gap (Eg) values of Bi1-xSmxOCl (0.00  x  0.11) phosphors. Sm3þ conc. (mol%)

Crystallite size (nm)

Dislocation density (d) (1014 lin m2)

Stacking fault (mJ m2) (103)

Band gap energy (eV)

0 1 3 5 7 9 11

105 79 75 72 63 41 40

0.90 1.60 1.75 1.92 2.45 5.81 6.21

459.9 460.3 460.6 460.7 460.8 460.9 461.6

3.41 3.41 3.40 3.38 3.36 3.33 3.31

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Fig. 2. Reitveld patterns of a) Bi0.97Sm0.03OCl and b) Bi0.89Sm0.11OCl Compounds.

Fig. 3. FTIR spectra of Bi1-xSmxOCl (0.00  x  0.11) phosphors.

and the atomic ratio of Bi/Sm/O/Cl is found to be 0.91:0.05:0.85:1.20, which is close to its standard stoichiometric composition. 4.4. UVevisible defused reflectance (DR) studies Fig. 7 shows the diffuse reflectance spectrum of Bi0.97Sm0.03OCl. It shows an absorption band in the region 300 nme700 nm. Spectrum shows a deep fall in reflectance from 320 to 350 nm that corresponds to the band transition of the BiOCl host lattice. Several less intense absorption bands were observed in the wavelength range between 340 and 500 nm. These transitions (absorption bands) can be attributed to the characteristic transitions of Sm3þ ions, takes place from the ground state (6H5/2) to various excited

Fig. 4. Raman spectra of Bi1-xSmxOCl (0.00 and 0.03) phosphors.

states Sm3þ ion. The absorption range between 300 and 500 nm in the Bi0.97Sm0.03OCl phosphor matched very well with the photoluminescence excitation spectrum of Bi0.97Sm0.03OCl (Fig. 7). Absorption band at 408 nm is comparatively high. This is also reflected in excitation spectrum, corresponds to 6H5/2 / 4F7/2 transition of the Sm3þ ions. The band gap of the prepared phosphors was estimated using Kubelka- Munk relation [21].


n FðR∞ Þhy ¼ C hy  Eg

(3)

where hn is energy of the photon, C is a constant, Eg is the band gap energy, n is a constant associated with different kinds of electronic transitions (n ¼ 1/2 for a direct allowed transition, n ¼ 2 for an indirect allowed transition, n ¼ 3/2 for a direct forbidden transition

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173

Fig. 5. FE SEM images of BiOCl (a and b) and Bi0.97Sm0.03OCl (c and d) phosphors at different magnifications.

Fig. 6. a) TEM image, b) High resolution TEM image, c) SAED pattern and d) ED spectra of Bi0.97Sm0.03OCl phosphors.

and n ¼ 3 for an indirect forbidden transition), R is reflectance, R∞ ¼ Rsample/Rstandard, and F(R∞) is the Kubelka- Munk function, which as follows:

FðR∞ Þ ¼

ð1  R∞ Þ2 k ¼ s 2R∞

(4)

where k is molar absorption co-efficient, s is the scattering coefficient. An optical absorption spectrum of a BiOCl compound exhibits indirect electronic transition (n ¼ 2) [11]. The value of Eg can be estimated from extrapolation of the line for [F(R∞)hn]1/2 ¼ 0 which approximates to be 3.40 eV for Bi0.97Sm0.03OClphosphor (Fig. 7 inset).

4.5. Photoluminescence studies Fig. 7 shows the PL excitation spectrum of Bi1-xSmxOCl (0.01  x  0.11) from 250 to 550 nm monitored at 594 nm. In Fig. 7 350e500 nm region, there are number of sharp excitation peaks of the forbidden intra f-f transition of Sm3þ ions assigned as 6H5/ 4 (364 nm), 6H5/2 / 6D1/2 (378 nm), 6H5/2 / 4F7/2 2 / D3/2 (408 nm), 6H5/2 / 4M19/2 (419 nm), 6H5/2 / 4G9/2 (441 nm), 6H5/ 4 6 4 2 / I13/2 (465 nm), H5/2 / M15/2 (481 nm) [22,23]. Among these transitions the intensity of 6H5/2 / 4F7/2 (408 nm) transition is greater compared to other transitions in the spectrum. Hence these phosphors can be effectively excited by NUV LEDs. Fig. 8 shows the PL emission spectrum of Bi1-xSmxOCl (0 < x < 0.11) recorded at lex ¼ 408 nm corresponds to 6H5/2 / 4F7/2

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Fig. 9. Variation of PL intensity versus Sm3þ concentration in Bi1-xSmxOCl phosphors. Fig. 7. UVeVis DR spectrum, PL excitation spectrum and Band gap calculation plot (inset) of Bi0.97Sm0.03OCl phosphor.

increase in the concentration of Sm3þ up to 3 mol% and beyond this concentration PL intensity decreases due to may be concentration quenching. As Sm3þ ions concentration increases, the distance between two adjacent Sm3þ ions is also decreases. Hence the interaction between Sm3þ-Sm3þ ions is enhanced; this enhanced non-radiative energy transfer between two Sm3þ ions takes place through the electric multipolar interaction or exchange interaction [25]. Types of multipolar interactions present are dipole - dipole (dd), dipole e quadrupole (d-q) and quadrupole - quadrupole (q-q) interactions [11]. The type of interaction involved in energy transfer can be evaluated by Van Uitert [26] equation which is as follows:

I k  ¼ q x 1 þ bðxÞ3

Fig. 8. PL emission spectra of Bi1-xSmxOCl (0.01  x  0.11) phosphors.

transition. In emission spectra three significant emission peaks were found at 564, 600 and 646 nm attributed to 4G5/2 / 6H5/2, 4 G5/2 / 6H7/2, and 4G5/2 / 6H9/2 transitions of Sm3þ ion respectively [8]. The4G5/2 / 6H5/2 (564 nm) and 4G5/2 / 6H9/2 (646 nm) transitions are arised due to purely magnetic dipole moment and purely electric dipole moment respectively. But most intense 4G5/ 6 2 / H7/2 (600 nm) transition is originated by partly magnetic and partly electric dipole moments [24] and generally the electric dipole transitions are highly sensitive to the host environment and forbidden by the selection rules. But the magnetic dipole transitions are insensitive to the host environment. From these spectra another notable thing is magnetic dipole transition is dominated compared to electric dipole transition implies that Sm3þoccupies the inversion centre in the host lattice [24]. From Fig. 8 evident that, the intensities of the emission spectra changes with the doping concentration of the Sm3þ ions. However, there is no change in the nature and position of the emission band in the spectra for all the dopant concentrations. Fig. 9 represents the PL emission intensity of the leading emission peak of 4G5/2 / 6H7/2 (601 nm) transition as a function of Sm3þ ions concentration. From this figure, it can be clearly observe that the intensity of 598 nm increases with the

(5)

where I/x is the emission intensity per activator (x), k and b are constants for a given host lattice, q represents the interaction type between rare earth ions, here q ¼ 3, 6, 8, 10 corresponds to the nearest-neighbour ions, dipole - dipole (d-d), dipole - quadrupole (d-q) and quadrupole - quadrupole (q-q) interaction, respectively. Simplifying and re-arranging the above equation we get:

  I q ¼ k‘  logðxÞ log x 3

(6)

Fig. 10 represents the plot of log (I/x) on log (x), which is relatively linear, and the slope is found to be 1.187. The value of q is close to 3 which indicate that the concentration quenching in BiOCl:Sm3þ system was due to the rapid excitation exchange between the nearest-neighbour ions. To realize the practical applications of these phosphors, color quality was visualized in terms of 1931 Commission International de l'Eclairage (CIE) chromaticity color coordinates. These color coordinates distinguish that the human eye system uses three primary colors: red, green and blue. So, the color of any light source can be represented on the (x, y) coordinates in this color space [13]. Fig. 11 represents the CIE coordinate of the present phosphor as a function of various concentration of Sm3þ ion (Table 3). As seen from Fig. 11, the coordinates do not vary much with the change in the concentration of Sm3þ ion. The CIE coordinates for the Bi0.97Sm0.03OCl phosphor are estimated to be x ¼ 0.5832, y ¼ 0.4159; which falls in the orange-red region of the CIE color spectrum. Correlated color temperature (CCT) is one of the significant parameter to evaluate the phosphor's practical applicability, which

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175

5000 K considered to be producing warm white light used for household applications [28]. Therefore these phosphors are suitable for red light generation in domestic appliances. The absolute quantum efficiency (QE) of Bi0.97Sm0.03OCl phosphor was measured using the integrated sphere on the fluorescence spectrophotometer. By using emission and excitation spectrum with integrated sphere, the absolute quantum efficiency value can be estimated by using the following relation [29]:

ð

hQE ¼ ð

Fig. 10. Plot of log (I/x) vs log (x) of Bi1-xSmxOCl phosphors.

specifies the color visuality of light emitted by the light source. CCT can be estimated by transforming the (x, y) coordinates of the light source to (u0 , v0 ) using the following relations:

u0 ¼

4x 2x þ 12y þ 3

(7)

9y v0 ¼ 2x þ 12y þ 3

(8)

By using the McCamy's approximation [27], the CCT value can be estimated from CIE color coordinates using the third power polynomial is given by the relation:

T ¼ 449n3 þ 3525n2  6825n2  6823:3n þ 5520:33

(9)

Where n ¼ (x e 0.3320)/(y e 0.1858). The calculated CCT values for Bi1-xSmxOCl (0 < x < 0.11) phosphors were estimated to be in the range of 1650e1750 K (Table 3). Generally, CCT values are less than

LS ð ER  ES

(13)

where LS is the luminescence emission spectrum of the sample; ES is the spectrum of the light used for exciting the sample; ER is the spectrum of the excitation light without the sample in the sphere. The measured internal QE of Bi0.97Sm0.03OCl phosphor is determined as 2.12% under 408 nm excitation. Fig. 12 depicts the PL decay curves for Bi1-xSmxOCl (0.01  x  0.11) phosphors, which were measured at emission band at 598 nm with the excitation of 408 nm at room temperature. This decay curve is bi-exponential function which corresponds to fast and slow decay process. The bi-functional nature is dependent on the number of active centres created by Sm3þion, energy transfer from host to the activator and also defect extant in the host [30]. The decay curves are well fitted by the following second order exponential equation:

IðtÞ ¼ I0 þ Af exp



t

tf

!

  t þ As exp 

ts

(11)

where I and I0 are the luminescence intensity at time t and 0, tf and ts are the fast and slow decay times and Af and As are their respective weight. The average decay time is calculated using the formula [31]:



 Af t2f þ As t2s  t¼  Af tf þ As ts

Fig. 11. CIE 1931 Chromaticity diagram for Bi1-xSmxOCl (0.01  x  0.11) phosphors.

(12)

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Table 3 CIE 1931 and 1976 Chromaticity co-ordinates CCT and color purity values of Bi1-xSmxOCl (0.01  x  0.11) phosphors. Conc. of Sm3þ (x)

0.01 0.03 0.05 0.07 0.09 0.11

CIE 1931 co-ordinates

CIE 1976 co-ordinates

x

y

u’

v’

0.5820 0.5832 0.5807 0.5804 0.5791 0.5787

0.4170 0.4159 0.4183 0.4186 0.4199 0.4203

0.3400 0.3418 0.3387 0.3383 0.3367 0.3362

0.5490 0.5485 0.5489 0.5490 0.5492 0.5493

CCT values (K)

Color purity (%)

1696 1688 1705 1707 1717 1720

89.9 90.2 89.7 89.6 89.4 89.3

evaluate the better light source for LED's. So, the color purity of emitted red light which can be calculated by using the following equation:

Color purity ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx  xee Þ2 þ ðy  yee Þ2 ðxd  xee Þ2 þ ðyd  yee Þ2

 100%

Where (x, y), (xee, yee) and (xd, yd) are the chromaticity coordinates of the sample point, equal energy point (0.3333, 0.3333) and the dominant wavelength point, respectively. The color purity of Bi1xSmxOCl (0 < x < 0.11) phosphors were found to be in the range of 89e90%. The obtained color purity results were listed in Table 3. Table 4 represents the comparison of color purity of the recently reported Sm3þ ion doped Phosphors with the Bi1-xSmxOCl. And it is noteworthy that, the color purity of Bi0.97Sm0.03OCl is similar or higher than the recently reported Sm3þ doped orange red light emitting phosphors [32e39]. From these results, it is evident that Bi0.97Sm0.03OCl phosphor is the best composition, which shows maximum orange-red emission, excellent color chromaticity with high color purity, low CCT and long lifetime values. These results imply that, Bi1-xSmxOCl phosphors have potential application in solid-state lighting and optoelectronic device applications.

5. Conclusions

Fig. 12. Photoluminescence decay curves of Bi1-xSmxOCl (0.01  x  0.11) phosphors.

Corresponding to the different emissions at 598 nm upon the excitation of 408 nm, the average decay lifetime is obtained for Bi1xSmxOCl (0.01  x  0.11) phosphors to be in the range between 0.58 ms and 0.30 ms. The color purity of the phosphors is another major parameter to

A series of Bi1-xSmxOCl phosphors were synthesised by solid state method. Its crystal structure and phase analysis is studied by the powder X-ray diffractometer which confirms the tetragonal structure corresponds to space group P4/nmm (JCPDS card no. 850861). SEM images show the plate like morphology. FTIR spectra confirm the presence of BiOCl compound. UVeVisible, Raman and PL studies confirms the transitions which are due to the presence of Sm3þ ions. PL emission spectra exhibit orange red emission which is confirmed in the CIE chromaticity diagram. Hence, Bi1-xSmxOCl is an efficient orange-red emitting material in W-LED and other optoelectronic applications.

Table 4 CIE chromaticity coordinate (x, y) and color purity of some important orange red light emitting phosphors. Serial No. 1 2 3 4 5 6 7 8 9

Phosphor 3þ

BiOCl: Sm KBa2(PO3)5: Sm3þ Ba3ZrNb4O15: Sm3þ Li4Ca(BO3)2: Sm3þ Sr2SiO4: Sm3þ Ba2Ca (BO3)2: Sm3þ Ca3Mg3(PO4)4:Sm3þ BaWO4: Sm3þ Ba2V2O7:Sm3þ

Chromaticity co-ordinates (x, y)

Color purity

Reference

(0.583, 0.416) (0.565, 0.420) (0.603,0.396) (0.600, 0.400) (0.575, 0.425) (0.596, 0.403) (0.547, 0.451) (0.480, 0.520) (0.461 0.402)

90.2% 90.0% 96.5% 92.0% 93.0% 82.3% 84.8% 92.9% 58.9%

Present work [30] [31] [32] [33] [34] [35] [36] [37]

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Acknowledgements The authors acknowledge the chemical science division, Indian Institute of Science for extending the Time resolved fluorescence spectrometer and TEM facility. One of the author Pramod Halappa is thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing CSIR SRF.

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