Influence of Tb3+ doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor

Journal Pre-proof 3+ Influence of Tb doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor Sushma Devi, Avni Khatkar, V.B. Taxak, Anju Hooda, Priyanka Sehrawat, Sonika Singh, S.P. Khatkar PII:

S0022-2313(19)31664-3

DOI:

https://doi.org/10.1016/j.jlumin.2020.117064

Reference:

LUMIN 117064

To appear in:

Journal of Luminescence

Received Date: 24 August 2019 Revised Date:

9 January 2020

Accepted Date: 22 January 2020

Please cite this article as: S. Devi, A. Khatkar, V.B. Taxak, A. Hooda, P. Sehrawat, S. Singh, 3+ S.P. Khatkar, Influence of Tb doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor, Journal of Luminescence (2020), doi: https://doi.org/10.1016/ j.jlumin.2020.117064. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Influence of Tb3+ doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor Sushma Devia, Avni Khatkarb, V. B. Taxaka, Anju Hoodaa, Priyanka Sehrawata, Sonika Singhc, S.P. Khatkara* a

Department of Chemistry, Maharshi Dayanand University, Rohtak -124001, India

b

Department of Electronics and Communication (UIET), Maharshi Dayanand University, Rohtak

c

Department of Chemistry, Ch. Bansi Lal University, Bhiwani, Haryana. *Corresponding Author: Tel: +919813805666 E-mail: [email protected]

ABSTRACT Different concentrations of terbium activated strontium lanthanum aluminates (SLA:Tb3+) were synthesized by adopting urea-aided solution combustion route at the sintering temperature of 1330 °C. XRD based Rietveld refinement technique confirms that SLA:Tb3+ nanophosphors have a perovskite-based structure with I4/mmm space group symmetry. Morphological information is collected on scanning and transmission electron microscopes (SEM & TEM) however, the energy dispersive mode of SEM (EDS) provides an idea about the distribution of existing elements throughout the host lattice. The band-gap studies for SrLaAlO4 (5.00 eV) and SrLa0.98Tb0.02AlO4 (4.50 eV) compositions are carried out using diffuse reflectance (DR) measurements via Kubelka-Munk relations. The down-converted photoluminescence (PL) studies reveal that an intense green emission is obtained at 545 nm due to 5D4 → 7F5 transition under 279 nm UV-excitation. Further, Dexter’s theory helps in explaining the concentration quenching mechanism in SLA:xTb3+ nanocrystals (d-q interactions, s = 8.13). Moreover, the 1

radiative lifetime (1.17 ms), non-radiative rates (194.28 s-1) and quantum efficiencies (81 %) of 5

D4 term are calculated using Auzel’s model. Finally, the Commission International de

I’Eclairage (CIE) color coordinates confirm the emission in green region; which approves the role of Tb3+-activated SLA nanophosphors as a green component in RGB based white light emitting diodes (WLEDs). Keywords: Rietveld refinement; Quantum efficiency; Photoluminescence; Radiative lifetime; SrLaAlO4:Tb3+ 1. Introduction Trivalent rare-earth ions (RE3+) activated down-conversion (DC) phosphors have been the attractive topic of research owing to their significant role in different devices such as, optoelectronics, plasma displays, cathode ray tubes, laser devices, photovoltaic cells, bioimaging, temperature sensors etc. [1,2]. Apart from these, RE3+-doped DC phosphors based white light-emitting diodes are recently alluring huge attention from material scientists [3,4]. Due to continuous enhancement in performance and cost depletion in last few decades, WLEDs have been proved to be a big replacement of traditional illumination sources. The traditional white LEDs are based on blue InGaN LED chip coated with yellow-emitting phosphor (Y3Al5O12:Ce3+) material [5,6]. However, a recent approach based on fusion of UV/NUV-LED chip and tricolor red-green-blue (RGB) phosphor components have also been implemented [7,8]. The white LEDs have highly desirable merits such as: fabulous potential of energy saving, high durability, great luminous efficiency, quick response and eco-friendly features [9,10]. The valuable impacts of white LEDs on our environment, economy and way of living are so evident that the research attention towards fabricating these LEDs is growing rapidly.

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Since last decades, the layered perovskites belong to ABCO4 (A, B, C are alkaline earth metal ion, trivalent rare-earth ion and any metal ion) group of compounds are alluring huge attention due to their wide-ranged technological applications. In this context, SrLaAlO4 host having K2NiF4 type structure is of great scientific interest due to its excellent dielectric, thermodynamic, optical and superconducting properties [11-14]. The structural and magnetic properties of SrLaAlO4 activated with Mn2+ ions have been reported earlier [15,16]. Thereafter, significant research efforts have been dedicated towards investigating the structural and photoluminescence properties of RE3+-activated SrGdAlO4 host phosphor developed via solution combustion method [3,17]. However, up to now, a limited research work has been performed over trivalent rare-earth activated SrLaAlO4 nanophosphor. The demand for RE3+ ions as activators is projected to grow rapidly due to their sharp 4f-4f or 4f-5d transition behaviour in visible region [18]. Among them, Tb3+ has been widely used as a strong green-emitter due to an intense 5D4 → 7F5 transition near 545 nm in emission spectrum [19,20]. When RE3+ are incorporated into host lattices, an effective transfer of energy from host to these ions takes place, which increases the stability and luminous efficacy of host phosphors. Hence, the present research work is executed to analyze the DC emission properties of Tb3+-doped SrLaAlO4 nanophosphor for display and lighting applications. Green emitters play major role in signs and emergency lights, fluorescence detection, anti-counterfeiting areas and most importantly in daily life of people [21]. Hence, these types of environmental friendly aluminates are synthesized by adopting easy and economical solution combustion route at the heat treatment of 1330 °C in 3h [7]. Due to complexity of process and large calcination time in solid-state synthesis route [21], SrLaAlO4:Tb3+ green nanophosphors are developed via solution combustion approach. An attempt has also been made to examine the crystal structure and

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morphological behaviour of desired nanophosphors via Rietveld refinement and SEM, TEM analysis. The optical band-gap of powdered samples were calculated using DR measurements. Finally, the chromatic behaviour via Commission Internationale de I’Eclairage (CIE) diagram reflects the role of SrLaAlO4:Tb3+ nanophosphor as a green component in RGB based white LEDs upon UV excitation. 2. Experimental 2.1 Materials and synthesis SrLaAlO4:Tb3+ DC nanophosphors (1-4 mol%) were prepared by an efficient and ecofriendly way i.e. solution combustion approach. [22, 23]. All the precursors namely, (Sr(NO3)2), (La(NO3)3.6H2O), (Al(NO3)3.9H2O), (Tb(NO3)3.6H2O) and CH4N2O of Analytical Reagent (AR) grade were purchased from Sigma Aldrich. These high purity chemicals (99.99%) were used as such without any further purification. The stoichiometric compositions of raw materials were dissolved in minimum of deionized water and heated on hot plate until cloudiness [24]. Thereafter, the homogeneous solution was loaded into a furnace pre-heated at 500 °C for about 15 minutes. During this period, redox mixture undergoes rapid dehydration and foaming process liberating huge amount of gases. The exothermic combustion process itself supplies activation energy for the synthesis of white fluffy powder. The product was then unloaded from furnace, cooled down naturally and crushed to powder form. The basic combustion reaction for SrLa0.98Tb0.02AlO4 nano-crystalline powder synthesis is described by the following equation: Sr(NO3)2 + 0.98La(NO3)3.6H2O + Al(NO3)3.9H2O + 0.02Tb(NO3)3.6H2O + 6.67NH2CONH2 → SrLa0.98Tb0.02AlO4 + 10.67N2(g) + 28.23H2O(g) + 6.67CO2(g)

4

Finally, the synthesized samples were transferred into alumina crucibles and sintered at 1330 °C for 3h, to attain the desired crystalline products. 2.2 Material characterization The information regarding crystallinity and purity of synthesized nanophosphors is highly relevant, as the pure crystalline form is mainly preferred for device fabrication. So, the XRD patterns were recorded on a powder diffractometer (Rigaku Ultima-IV) fitted with Cu Kα radiation and a graphite monochromator. Power supply was adjusted at 40 kV and 40 mA, respectively. All the sintered samples were measured from 10°–80° (2θ) angular range at the scan rate of 1°/min and scan step of 0.02°, respectively. Cell parameters and refinement factors for SrLa0.98Tb0.02AlO4 composition were attained by executing Rietveld refinement procedure via GSAS (General Structure Analysis System) program [25, 26], taking SrLaAlO4 as the preliminary structural model [27]. The background was refined using shifted Chebyshev function with nine refinable coefficients and profile shape refinement was carried out using Pseudo-Voigt function. The morphological and elemental analysis were carried out using SEM (Jeol JSM6510) fitted with energy dispersive X-ray spectroscope. However, the particle size was obtained from FEI Technai-G2 transmission electron microscope operated at 200 kV power supply. To inspect the energy absorption of powder samples, the reflectance measurements from 200-800 nm range for host and optimized samples were carried out on Shimadzu UV-3600 plus spectrophotometer. For this, the reference material i.e. BaSO4 white powder was evenly distributed on a sample holder. Then, the small amount of powdered sample was added in the middle of smoothed BaSO4 and pressed to make the sample spread uniformly on reference material. The DC luminescence behaviour and decay lifetime measurements of Tb3+ ions were inspected on Hitachi F-7000 fluorescence spectrophotometer fitted with Xe-lamp as radiation 5

source. Using emission spectra, the color coordinates for all samples were calculated, which were finally used for evaluating the chromatic performance of desired nanophosphor for display and lighting applications. 3. Results and discussion 3.1 Structural analysis To examine the effect of Tb3+ doping on the crystal structure of SLA nanophosphors, XRD patterns of undoped and doped samples were recorded and presented in Fig. 1. Clearly, the diffraction peaks of SLA and SLA:Tb3+ samples (1-4 mol%) coincide to the reference peaks of SrLaAlO4 structure (JCPDS no. 81-0744), indicating the uniformity of crystal structure after Tb3+ doping. It means, Tb3+ ions can be easily dissolved into the crystal lattice of SrLaAlO4 without altering its structural prototype. However, very small intensity peaks at about 30 degrees may be due to some unreacted La2O3 phase [21]. Rietveld refinement of SrLa0.98Tb0.02AlO4 (optimum composition) in the range of 5°–105° was carried out using GSAS software and is shown in Fig. 2. The calculated (red line) and observed (black diamonds) diffraction profile together with their deviation (bottom blue line) further implies a good matching in experimental and calculated data. The goodness of refinement can be estimated by analysing the reliability factors i.e. factor for pattern (Rp), factor for weighed pattern (Rwp) and goodness of fit (χ2). Rp and Rwp should be less than 10% and χ2 should be close to unity for good refinement [28]. In this study, all the reliability factors are found to be in well agreement with a good refinement process, which indicates that the structural model is quite right. The tetragonal crystal structure has been identified with I4/mmm (139) space group and the cell parameters are refined to be; a = b = 3.7538(1) Å, c = 12.6606(3) Å, V = 178.40(1) Å3, α = β = γ = 90° and Z = 2. The cell parameters and reliability factors for host have also been carried out and a relative comparison of

6

crystallographic data of SrLa0.98Tb0.02AlO4 with standard SrLaAlO4 is demonstrated in Table 1. A little decrease in cell volume from 178.40 Å3 to 178.27 Å3 claims that the smaller Tb3+ are replacing the larger La3+ into host lattice sites. Due to increase in formula weight and decrease in cell volume, the density of cell is increased from 5.9103 to 5.9212 g cm-3 upon doping. Further, the Rietveld refinement of all the patterns are carried out and the lattice parameters are plotted versus nominal Tb3+ content (Fig. 3(a)). The substitution of La3+ by Tb3+ can be estimated by analysing the radius percentage difference (∆r), given by the following expression: ∆ =

  −  

× 100 1  

Where Rm(CN) and Rd(CN) are the effective ionic radii for host and dopant ions for coordination number, CN (here CN = 9 for La3+/Tb3+). The calculated ∆r value is 9.09 %, which is lower than 30% (maximum acceptable) [29]. It is impracticable for Tb3+ to substitute Sr2+ and Al3+ due to much deviation in their charge, size and ∆r values (Tb3+ = 1.10Å, La3+ = 1.216 Å, Sr2+ = 1.31 Å, Al3+ = 0.535 Å). The substitution of Sr2+ by Tb3+ is also disregarded on the basis of their charge neutrality concept. The refined positions and occupancy for different atoms obtained by Rietveld refinement over SrLa0.98Tb0.02AlO4 composition is listed in Table 2. The crystal structure projection and coordinative environments of different cations in SrLa0.98Tb0.02AlO4 viewed along [257] Miller plane are obtained via Diamond software version 4.5.2 using refinement data, which are presented in Fig. 4. The structure of SrLa0.98Tb0.02AlO4 nanophosphor consists of two polyhedral units: (Sr/La)O9 and AlO6. The interatomic distances (Å) for two different polyhedra are compiled in Table 3. Furthermore, Scherrer gives a relation between the X-ray’s wavelength (λ), Bragg’s diffraction angle (θ) and full width at half-maximum (β) of diffraction peaks for evaluating the crystallite size (D) of synthesized nanophosphor [30,31]. The Williamson-Hall 7

(W-H) fitting method was also used to evaluate the average crystallite size and strain induced in nanoparticles [32]:  =

0.941λ

/

 2θ   2θ − 

#$%&' =

cosθ

2

0.941( + *&+,' 3



Here, BO(2θ) and BSi(2θ) are the full width at half-maximum (FWHM) in radian for observed and the standard silicon pattern and η is the microstrain. The calculated values of crystallite size along [101], [004] [103], [110], [114] and [200] reflections for optimum composition are found to be 54.7, 44.2, 51.22, 48.97, 43.4 and 39.07 nm. Whereas, the strain induced in nanoparticles and average crystallite size obtained using W-H plot (Fig. 3 (b)) are determined as 0.0041 and 72 nm, respectively. The morphological information is highly relevant as both the appearance and dimension of particles affect the luminous efficacy of phosphors. Fig. 5 (a) represents the typical SEM image of SrLa0.98Tb0.02AlO4 nanoparticles sintered at 1330 °C, which depicts that the resulting sample is composed of large number of aggregated particles with irregular morphology. Besides, the particle size obtained from TEM analysis (Fig. 5 (b)) lies in 15-50 nm range, which obviously fulfills the requirements of phosphor in applications [33]. The elemental identification using EDS spectral analysis confirmed the presence of all the existing Sr, La, Tb, Al and O elements in SrLa0.98Tb0.02AlO4 sample (Fig. 5 (c)). Compared to bulk counterparts, the nano-sized particles are highly preferred for coating in various display devices owing to minimum internal scattering and high ratio of surface to volume [34]. Inset of Fig. 6(a) & 6(b) represents the diffuse reflectance spectra of SrLaAlO4 and SrLa0.98Tb0.02AlO4 powder samples. The SrLa0.98Tb0.02AlO4 sample exhibits high reflection in

8

visible region and strong absorption in 200-300 nm range due to 4f→5d electronic transition of Tb3+ ions, indicating that terbium doped nanophosphor may be suitable for UV excited LED applications [9]. The Kubelka-Munk (K-M) transformation of diffuse reflectance spectra is used to determine the fundamental band-gap values (Eg) for powdered materials [35]: ./ 0 ℎ234 = 5ℎ2 − 67 8 4

Where F(R∞) is Kubelka-Munk function, hν is photon energy, C is a constant and Eg corresponds to the energy of band-gap. The symbol n is representing the nature of band transitions occurred by photon absorption: n = 1/2, 2 for indirect and direct allowed transitions, and n = 3, 3/2 for indirect and direct forbidden transitions, respectively. A mathematical statement for evaluating F(R∞) involves the terms: K (absorption coefficient), S (scattering coefficient) and R∞ (observed reflectance), which is expressed as follows [36]: / 0 =

1 − 0  9 = 5

20 :

By extrapolating [F(R∞)hν]2 to zero, the obtained Eg value is 5.00 eV for SrLaAlO4 and 4.50 eV for SrLa0.98Tb0.02AlO4 composition (Fig. 6). This band-gap lies in inorganic semiconductor range, which enhances their area of applications in different fields [37]. Also, a notable change in band-gap from undoped to ion-doped samples indicate that additional energy states have been localized in between the band-gap of SrLaAlO4 host lattice [38]. 3.2 Luminescence characteristics The PL excitation spectra of each Tb3+-doped SLA sample (1-4 mol%) was monitored under 545 nm emission wavelength and is shown in Fig. 7. One can see that, the spectra is a combination of two independent broad bands: one in lower wavelength region, extends from 200-255 nm with maxima at 242 nm is the charge transfer band (O2- → Tb3+); while the other most intense band in higher wavelength region, ranging from 255-310 nm and centred at 279 nm 9

arises due to 4f8 → 4f75d1 (f → d) transition of Tb3+ ion. Very high intensity of 4f8 → 4f75d1 transition is due to spin allowed as well as Laporte allowed nature. These excitation peaks in UV region indicate the role of terbium activated SrLaAlO4 nanophosphor in UV based white LED devices [39,40]. On the other hand, the emission spectra of SrLaAlO4:Tb3+ nanophosphor (1-4 mol%) under 279 nm excitation is shown in Fig. 8. The characteristic 4f -4f emission peaks at 490, 545, 593 and 622 nm are due to 5D4 → 7F6, 7F5, 7F4 and 7F3 transitions, respectively. Among these, the transition 5D4 → 7F5 sited at 545 nm is accountable for bright green emission of trivalent terbium ions. Besides these, some lower intensity peaks originated due to 5D3 → 7F5 and 7

F4 transitions and positioned at 416 and 438 nm are also observed in higher energy region [7]. It

seems that 5D4 → 7Fj transitions are stronger than 5D3 → 7Fj transitions. It may be due to crossrelaxation among Tb3+ ions, which can be expressed using Eq. (6) [41, 42]: Tb3+ (5D3) + Tb3+ (7F6) → Tb3+ (5D4) + Tb3+ (7F0)

(6)

Electrons in 5D3 state get relaxed to 5D4 and the 7F6 electrons of neighbouring Tb3+ get excited to 7

F0 state. This process weakens 5D3 → 7Fj transitions and strengthens 5D4 → 7Fj transitions.

Hence, the terbium doped SrLaAlO4 nanophosphor exhibits weak bluish-green emission in 400450 nm region and strong green emission in 480-630 nm region. All possible excitations and emission transitions among different electronic states of Tb3+-doped SLA down-conversion nanophosphor are demonstrated in energy level diagram in Fig. 9. Since, the emission performance of phosphors is greatly influenced by the dopant concentration, the influence of Tb3+ doping on PL emission intensity of SrLa1-xTbxAlO4 nanophosphor is studied and illustrated in Fig. 10. It is examined that the emission intensity of dominant transition (5D4 → 7F5) first increases upto 2 mol% and thereafter it declines due to conventional concentration quenching phenomena, which gives a clear insight for the non-

10

radiative energy relaxation process among nearby Tb3+ ions. The critical distance for energy transfer (Rc) in SrLa0.98Tb0.02AlO4 composition is calculated using Eq. (7): 3 > /B A 7

< = 2 = 4 Π @< Using xc = 0.02 (optimum composition), V = 178.27 Å3 (cell volume) and N = 2 (number of lattice sites in crystallographic unit cell that can be occupied by activator ions), the value of Rc is determined to be 20.42 Å [43]. On the basis of critical distance, the possible mechanism to control energy transfer in SrLa0.98Tb0.02AlO4 is multipolar-interaction pathway. Dexter’s theory reveals that dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) are the sub-category of this dominant mechanism [44]. Further, Tian and co-workers explain the phenomenon of concentration quenching using a relation between PL emission intensity and dopant concentration after optimum composition, which is described as [45]: & G D%E F H = − D%E@ + D%EJ 8

I @ Here, I/x is emission intensity per dopant ion concentration (x ≥ optimum concentration), d is sample’s dimension (d = 3) and f is a constant for concentration. The symbol s is representing the multipolar-interactions constant which is equal to 6 for d-d, 8 for d-q and 10 for q-q interactions, respectively. Inset of Fig. 10 represents a straight line between (I/x) and (x) on logarithmic scale giving a slope = −2.7103 and s = 8.13 ≈ 8. Hence, outcomes of the study confirm that d-q are the predominant interactions responsible for concentration-quenching phenomena of Tb3+ emission in SrLaAlO4 nanophosphor [4]. To further understand the phenomena of concentration quenching, the fluorescence decay curves for SrLa1-xTbxAlO4 (1-4 mol%) series, corresponding to 5D4→7F5 (545 nm) emission transition and 279 nm excitation wavelength, are recorded and the results are displayed in Fig. 11 11

(inset shows the fitted decay pattern for SrLa0.98Tb0.02AlO4 composition). Clearly, the normalized decay curves for all Tb3+ samples are well fitted into mono-exponential function in the form: −P G = GL exp F H 9

Q

Here I and Io represent the photoluminescence intensities at time t and zero, and τ is radiative decay time. The values of lifetime obtained for 5D4 level of Tb3+ are listed in Table 4. An observed decrease in magnitude of fluorescent lifetimes from 1.1088 to 0.6993 ms with increasing Tb3+ concentration from 1 to 4 mol% is mainly due to increase in non-radiative relaxation rate. Further, the mono-exponential decay behavior for all Tb3+-doped samples revealed the homogeneous arrangement of dopants into the host lattice sites [46]. The lifetime is plotted versus Tb3+ concentration and studied using Auzel’s fitting process. An expression for evaluating radiative lifetime is expressed as [47]: Q$ =

1+

QR

<


T UV/B

10

Here, τ(c) represents the total fluorescent lifetime for dopant concentration c, N is phonon number, co is concentration constant and τR is the radiative lifetime of lowest excited state. The value of τR for 5D4 level is calculated to be 1.17 ms from the fitting process (Fig. 12), which is further used to measure the quantum efficiencies (ϕ) of same level, using Eq. (11) as follows [48]: W =

YR QX = 11

QR YR + Y4R

12

The knowledge about τO (observed lifetime), τR (radiative lifetime), AR (radiative rate) is useful for evaluating the overall non-radiative relaxation rate (AnR), using the following mathematical expression: 1 1 = + Y4R 12

QX QR All the calculated parameters of 5D4 state in SrLa1-xAlO4:xTb3+ (1-4 mol%) nanophosphors are collectively reported in Table 4. CIE 1931 chromaticity diagram is used for identifying the natural color of desired nanophosphor for lighting and display applications. Using CIE (MATLAB) software [49], the PL emission data is converted to obtain the color coordinates (x, y) for SrLa1-xTbxAlO4 (1-4 mol%) series which are collectively reported in Table 4. The (x, y) coordinate values are represented by the ‘white dots’ in the CIE diagram (Fig. 13) showing color shifting from bluish-green to bright green

region.

The

CIE

color

coordinates

of

SrLa0.98Tb0.02AlO4

(0.2248,

0.5554),

SrLa0.975Tb0.025AlO4 (0.2305, 0.5712), SrLa0.97Tb0.03AlO4 (0.2398, 0.5806), SrLa0.965Tb0.035AlO4 (0.2410, 0.6036), SrLa0.96Tb0.04AlO4 (0.2486, 0.6232), lie closer to those of EBU (European Broadcasting Union: x, y for green illumination are 0.29, 0.60) suggesting the role of SrLaAlO4:Tb3+ nanophosphor as a green component in white light emitting diodes. Inset of Fig. 13 shows the digital photograph of optimized sample, which shows bright green emission under UV light source. Besides, the CIE values of all samples are well resembling with the previously reported Tb3+ doped green emitting hosts [7, 46, 50]. Further, an another important parameter, CCT is evaluated for SrLa1-xTbxAlO4 (1–4 mol%) series using expression given below: CCT = –437n3 + 3601n2 – 6861n + 5514.31

13

(13)

Here, n = (x–xe)/(y–ye); x, y are the color coordinates of sample and xe, ye are chromaticity epicenter (xe = 0.3320, ye = 0.1858). The obtained CCT values lies in 6900 to 10,000 K range which are nearly comparable to commercial white LEDs [51]. Hence, SrLaAlO4:Tb3+ nanophosphor may play major role in UV excited cool white LEDs [3]. 4. Conclusions In summary, SrLa1-xTbxAlO4 (1-4 mol%) nanophosphor series were developed via solution combustion route at the sintering temperature of 1330 °C. Rietveld refinement technique confirms that SLA:Tb3+ nanophosphors crystallize in tetragonal perovskite-based structure with I4/mmm space group symmetry. Irregular morphology of Tb3+ particles is obtained on scanning and transmission electron microscopes (SEM & TEM), however, the EDS analysis provides an idea about the distribution of Sr, La, Tb, Al and O elements in host lattice. The band-gaps for SrLaAlO4 (5.00 eV) and SrLa0.98Tb0.02AlO4 (4.50 eV) compositions are calculated using diffuse reflectance

(DR)

measurements

via

Kubelka-Munk

relations.

The

down-converted

photoluminescence (PL) analysis reveals that under 279 nm UV-excitation, 5D4 → 7F5 transition provides an intense green emission at 545 nm. Dexter’s theory helps in explaining the concentration quenching mechanism in SLA:xTb3+ nanocrystals (d-q interactions, s = 8.13). Moreover, the radiative lifetime (1.17 ms), non-radiative rates (194.28 s-1) and quantum efficiencies (81 %) for 5D4 term are also calculated using Auzel’s model. Finally, the outcomes of studies confirm that the Tb3+-activated nano-crystalline SLA phosphor may find practical applications in optoelectronics, especially in RGB based white WLEDs.

14

Acknowledgement One of the authors, Sushma Devi gratefully acknowledges the financial support in the form of senior research fellowship (SRF) from University Grant Commission (UGC), New Delhi, India (Award No. 2061410088). Authors would also express their deep gratitude to the UGC for granting financial help under the SAP (No. F.540/17/DRS-I/2016 SAP-I) to Department of Chemistry. References [1] X. Huang, B. Li, H. Guo, Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors, J. Alloy. Compd. 695 (2017) 2773–2780. [2] F. Zhang, Y. Wang, B. Liu, Y. Wen, Y. Tao, Investigation of Na3GdP2O8:Tb3+ as a potential green-emitting phosphor for plasma display panels, Mater. Res. Bull. 46 (2011) 722-725. [3] A. Hooda, S.P. Khatkar, A. Khatkar, S. Chahar, S. Devi, J. Dalal, V.B. Taxak, Characteristic white light emission via down-conversion SrGdAlO4:Dy3+ nanophosphor, Curr. Appl. Phys. 19 (2019) 621-628. [4] S. Devi, M. Dalal, J. Dalal, A. Hooda, A. Khatkar, V.B. Taxak, S.P. Khatkar, Near-ultraviolet excited down-conversion Sm3+ doped Ba5Zn4Gd8O21 reddish-orange emitting nano-diametric rods for white LEDs, Ceram. Int. 45 (2019) 7397-7406. [5] F. Baur, F. Glocker, T. Jüstel, Photoluminescence and energy transfer rates and efficiencies in Eu3+ activated Tb2Mo3O12, Journal of Materials Chemistry C, 3 (2015) 20542064. [6] A. Chapel, R. Boonsin, G. Chadeyron, D. Boyer, A. Bousquet, R. Mahiou, W. Henrique Cassinelli, C.V. Santilli, S. Therias, Preparation and characterization of a red luminescent composite composed of an EVA copolymer and a Y3BO6:Eu3+ phosphor, New Journal of Chemistry, 41 (2017) 12006-12013. [7] S. Chahar, R. Devi, M. Dalal, M. Bala, J. Dalal, P. Boora, V.B. Taxak, R. Lather, S.P. Khatkar, Color tunable nanocrystalline SrGd2Al2O7:Tb3+ phosphor for solid state lighting, Ceram. Inter. 45 (2019) 606-613. 15

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[39] X. Li, Y. Zhang, D. Geng, J. Lian, G. Zhang, Z. Hou, J. Lin, CaGdAlO4:Tb3+/Eu3+ as promising phosphors for full-color field emission displays, J. Mater. Chem. C 2 (2014) 9924– 9933. [40] A. Raja, G. Annadurai, D.J. Daniel, P. Ramasamy, Synthesis, optical and thermal properties of novel Tb3+ doped RbCaF3 fluoroperovskite phosphors, J. Alloy. Compd. 683 (2016) 654–660. [41] J. Y. Park, H. C. Jung, G. S. R. Raju, B. K. Moon, J.H. Jeong, J. H. Kim, Solvothermal synthesis and luminescence properties of Tb3+-doped gadolinium aluminum garnet, J. Lumin. 130 (2010) 478-482. [42] S.S. Pitale, V. Kumar, I. Nagpure, O.M. Ntwaeaborwa, H.C. Swart, Luminescence investigations on LiAl5O8:Tb3+ nanocrystalline phosphors, Curr. Appl. Phys. 11 (2011) 341-345. [43] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444-445. [44] D. L. Dexter, A Theory of Sensitized Luminescence in Solids, J. Chem. Phys. 21 (1953) 836-850. [45] Y. Tian, B. Chen, B. Tian, R. Hua, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, Y. Zheng, T. Yu, L. Huang, Q. Meng, Concentration-dependent luminescence and energy transfer of flower-like Y2(MoO4)3:Dy3+ phosphor, J. Alloys Compd. 509 (2011) 6096-6101. [46] H. Dahiya, M. Dalal, J. Dalal, V.B. Taxak, S.P. Khatkar, D. Kumar, Synthesis and luminescent properties of Tb3+ doped BaLa2ZnO5 nanoparticles, Mater. Res. Bull. 99 (2018) 8692. [47] F. Auzel, A fundamental self-generated quenching center for lanthanide-doped high-purity solids, J. Lumin. 100 (2002) 125-130. [48] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, The emission spectrum and the radiative lifetime of Eu3+ in luminescent lanthanide complexes, Phys. Chem. Chem. Phys. 4 (2002) 15421548. [49] P. Patil CIE Coordinate Calculator (2019) (https://www.mathworks.com/matlabcentral/filee xchange/29620-cie-coordinate-calculator).

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[50] Y. Liu, G. Liu, J. Wang, X. Dong, W. Yu, NaGdF4: Ln3+ (Ln3+ = Dy3+, Tb3+) nanophosphors: Green-emitting and energy transfer excited by UV and n-UV light, Mater. Res. Bull. 84 (2016) 232-239. [51] J. Jargus, J. Vitasek, J. Nedoma, V. Vasinek, R. Martinek, Effect of Selected Luminescent Layers on CCT, CRI, and Response Times, Materials 12 (2019) 2095.

20

21

Table 1 Comparison of crystallographic data of SrLa0.98Tb0.02AlO4 phosphor system with pure SrLaAlO4 host matrix. Formula

SrLaAlO4

SrLa0.98Tb0.02AlO4

Formula weight (g mol-1)

317.5

317.90

Symmetry

Tetragonal

Tetragonal

Space group symmetry

I4/mmm (139)

I4/mmm (139)

a = b (Å)

3.7538(1)

3.7534(1)

c (Å)

12.6606(3)

12.6541(3)

α = β = γ (degree)

90

90

Cell volume (Å3)

178.40(1)

178.27(1)

Z

2

2

Density (g cm-3)

5.9103

5.9212

1

Table 2 Refined atomic positions and occupancy data for SrLa0.98Tb0.02AlO4 nanophosphor system. Ion Type

site

Fill

x/a

y/b

z/c

U (Å2)

Sr1

Sr+2

4mm

0.5

0

0

0.3588(3)

0.0031(2)

La1

La+3

4mm

0.49

0

0

0.3588(3)

0.0031(2)

Tb1

Tb+3

4mm

0.01

0

0

0.3588(3)

0.0031(2)

Al1

Al+3

4/mmm 1

0

0

0

0.0031(2)

O1

O-2

mmm

1

0

0.5

0

0.0031(2)

O2

O-2

4mm

1

0

0

0.1627(5)

0.0031(2)

Atom Label

2

Table 3 Selected interatomic distances (Å) in the structure of SrLa0.98Tb0.02AlO4 nanophosphor. Bond Type

Distance

Bond Type

Distance

Al1-O1

1.8766(1) × 2

Sr1/La1/Tb1-O1

2.5910(1) × 2

Al1-O1

1.8768(1) × 2

Sr1/La1/Tb1-O2

2.4828(1) × 1

Al1-O2

2.0594(2) × 1

Sr1/La1/Tb1-O2

2.6680(1) × 1

Al1-O2

2.0597(2) × 1

Sr1/La1/Tb1-O2

2.6681(1) × 2

Sr1/La1/Tb1-O1

2.5908(1) × 2

Sr1/La1/Tb1-O2

2.6682(1) × 1

3

Table 4 CIE 1931 chromaticity index, photoluminescence lifetime, non-radiative rates and quantum efficiencies of 5D4 states for SrLa0.98Tb0.02O4 (x = 0.01 - 0.04) nanophosphors. Tb3+ Concentration

Color Coordinates

Lifetime

Non-Radiative

Quantum

(mol%)

(x,y) at λex = 279

(ms)

rates (s−1)

Efficiency (%)

nm 1

(0.1905, 0.4612)

1.1088

47.17

95%

1.5

(0.2117, 0.5131)

1.0123

133.15

87%

2

(0.2248, 0.5554)

0.9533

194.28

81%

2.5

(0.2305, 0.5712)

0.8716

292.61

76%

3

(0.2398, 0.5806)

0.8294

350.99

71%

3.5

(0.2410, 0.6036)

0.7490

480.41

64%

4

(0.2486, 0.6232)

0.6993

575.30

60%

4

5

Fig. 1. XRD profile of SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphor series along with the standard data of SrLaAlO4 (JCPDS No. 81-0744).

Fig. 2. Rietveld refinement of SrLa0.98Tb0.02AlO4 nanophosphor sintered at 1330 ˚C. χ2 = 1.666, Rwp(%) = 7.97, Rp (%) = 6.27 and Rexp (%) = 4.76.

Fig. 3. (a) Lattice parameters vs nominal Tb3+ content in SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphor (b) W-H plot of SrLa0.98Tb0.02AlO4 composition sintered at 1330 °C.

Fig. 4. Coordinative environment of various cations in SrLa0.98Tb0.02AlO4 nanophosphor along the (257) plane.

Fig. 5. (a) SEM (b) TEM and (c) EDS analysis of SrLa0.98Tb0.02AlO4 nanophosphor sintered at 1330 ˚C.

Fig. 6(a) & (b). The relationship between the absorption coefficient and photon energy for SrLaAlO4 and SrLa0.98Tb0.02AlO4 phosphor samples. The inset shows their corresponding diffuse reflectance spectra.

Fig. 7. 2D excitation spectra of SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphor studied at λem = 545 nm.

Fig. 8. 2D emission spectrum of SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphor excited at λex = 279 nm.

Fig. 9. Energy level diagram for Tb3+ doped SrLaAlO4 nanophosphor.

Fig. 10. Variation of emission intensity at 545 nm as a function of Tb3+ concentration for SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphors (inset shows the dependences of (I/x) on x on a logarithmic scale).

Fig. 11. The luminescence decay curves for 545 nm (5D4→7F5 of Tb3+) emission of SrLa1xTbxAlO4

(x = 0.01-0.04) nanophosphors. The inset shows the fitted decay profile for

optimum composition.

Fig. 12. Dependences of fluorescent life times of 5D4 state on the doping concentration of Tb3+ ion using Auzel’s model.

Fig. 13. CIE chromaticity diagram for SrLa1-xTbxAlO4 (x = 0.01-0.04) nanophosphor series.

Research Highlights • • • • •

SrLaAlO4:xTb3+ nanocrystals have been synthesized via solution combustion approach. Cell parameters and refinement factors are calculated for optimum composition. Critical distance for energy transfer and energy band-gap has been determined. Radiative lifetime, quantum efficiencies and non-radiative rates are determined. Potential candidates for UV excited RGB based white LEDs.

Influence of Tb3+ doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor Sushma Devi, Avni Khatkar, V. B. Taxak, Anju Hooda, Priyanka Sehrawat, Sonika Singh, S.P. Khatkar

Author Statement All the authors have equally contributed for this manuscript.

Influence of Tb3+ doping on the structural and down-conversion luminescence behaviour of SrLaAlO4 nanophosphor Sushma Devia, Avni Khatkarb, V. B. Taxaka, Anju Hoodaa, Priyanka Sehrawata, Sonika Singhc, S.P. Khatkara*

Conflict of Interest Statement All the authors state that there is no conflict of Interest in this manuscript.