Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications K.R Ashwini a, b, H.B Premkumar c, *, G.P Darshan d, R.B Basavaraj a, H Nagabhushana e, B Daruka Prasad a a Department of Physics, BMS Institute of Technology and Management, Bangalore, 560064, India Visvesvaraya Technological University, RRC, Bangalore, 560091, India Department of Physics, FMPS, MS Ramaiah University of Applied Sciences, Bangalore, 560 054, India d Department of Physics, Acharya Institute of Graduate Studies, Bangalore, 560 107, India e Prof C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumkur, 572 103, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 12 January 2020 Received in revised form February 2020 Accepted 16 February 2020 Available online xxx Near UV-light excitable inorganic phosphors are widely recommended for white-light emitting diode (wLED) applications Here in this paper we prepared Eu3ỵ doped SrAl2O4 phosphors with various concentrations of Eu3ỵ (1e5 mol%) by a green combustion technique The phase structure and purity of all prepared nanopowders (NPs) were studied by X-ray diffraction analysis The prepared NPs showed a fibre-like morphology which is confirmed by scanning electron microscopy results The particle size as obtained by the transmission electron microscopy results is found to be around 40 nm Further, the elemental composition showed the presence of doped Eu3ỵ ions into the SrAl2O4 host The photoluminescence (PL) spectra of Eu3ỵ ions doped SrAl2O4 compounds are studied in detail From the PL studies it is observed that the intense peak at 618 nm is due to the 5D0/7F2 transition of Eu3ỵ ions under 393 nm (near UV-light) excitation wavelength The emission color and the chromaticity values of the NPs are studied, which suggest a strong red emission The average estimated co-related temperature value is observed to be ~2688 K This temperature value indicates that these phosphor powders can be used for warm LED applications The color purity (CP) of the synthesized NPs has been calculated and the value is found to be 88%, which indicates that the prepared phosphor powders are highly useful for the red component in white light emitting diodes under UV light excitation © 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Nanopowders Rare earth Combustion method Photoluminescence Color purity Introduction In recent times, white light-emitting diodes (WLEDs) have shown keen attention among the researchers as a capable light source to meet the needs of world's energy requirements WLEDs have shown high luminous efficiency, more compactness, longlifetime, simple design aspects and fast switching [1,2] Currently, commercial WLEDs have low CRI (Color rendering indexes: < 80) and high CCT (Correlated color temperature: >4500 K) To overcome these inadequacies, many materials combinations were reported in the recent literature as potential candidates [3e7] In these reports, the utilized synthesis routes requires expensive fuels and leave a * Corresponding author E-mail address: premhb@gmail.com (H.B Premkumar) Peer review under responsibility of Vietnam National University, Hanoi harmful impact on the surrounding environment However, the tough preparation conditions, expensive reagents and harmful chemical compositions hamper their practical applications [8] As an alternative, near-UV LED chips were coupled with multi-phosphor materials which emit a specific ratio of red, green and blue light [9] Lanthanide ions are usually considered as biocompatible materials, especially Eu3ỵ, Gd3ỵ, Sm3ỵ, Yb3ỵ and Y3ỵ ions which were found to have a relatively low toxicity [10e13] Many works focus on rare earth (RE) ion doped luminescence materials because of the peculiar optical properties arising from the intra 4f transitions In fact, compared to metals or semiconductor materials, the emission bandwidth of RE ions is much sharper and, moreover, lifetimes are several orders of magnitudes longer and they have already shown to be useful for this purpose [14] As a result, fluorescent powders based on rare-earth doped materials can be used for display device applications For these reasons, active research was being carried out to fabricate more effective, stable and consistent RE doped NPs https://doi.org/10.1016/j.jsamd.2020.02.003 2468-2179/© 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Table Various aluminates doped with RE ions prepared by different methods for wLED applications Sample Synthesis method applications Authors Ref Ba3Y1yLuyAl2O7.5:Ce3ỵ Sr4Al14O25: Eu3ỵ Ba3Al2O6:Eu3ỵ Gd3Al5O12:Tb3ỵ MgSrAl10O17:Eu2ỵ, Mn2ỵ Ca2Mg2Al28O46:Mn4ỵ solid state reaction solid state reaction solid state reaction carbonate precipitation Combustion process solid state reaction near-UV LEDs LED applications LED applications LED applications LED applications LED applications Wang et al., Emen et al., You et al., Teng et al., Singh et al., Sasaki et al., [22] [23] [24] [25] [26] [27] [15,16] And no universally recognized conclusions have been drawn on what material is the best to be used for this application, yet Eu3ỵ (4F6) ions have been casted-off as activator in the utmost profit-making red luminescent materials [17] Desirable host materials with Eu3ỵ ions aroused an enormously strong interest as potential candidate for WLEDs [18e20] Generally, aluminates doped with Eu3ỵ ions show exceptional luminescence properties This makes them suited for LED's, optoelectronic and storage devices, luminous paints, display devices etc [21] Table shows the previous reports of various aluminates doped with RE ions prepared by different methods for WLED applications [22e27] Among the various aluminates, SrAl2O4 NPs were used in LED's, X-ray imaging, scintillations, sensors, optical communication, fluorescence imaging etc., because of their crystal stability, morphological features and easily tuneable optical properties [28e30] Generally, the RE ions doped aluminates were fabricated via classy and elevated temperature methods [31] Hence, there is an enormous requirement for less cost, a non-hazardous and short time production is in demand Various methodologies have been reported for the preparation of SrAl2O4 NPs viz., hydrothermal, solvothermal, solegel, co-precipitation and CVD techniques Many of these procedures require costly equipment's and time consuming experimental procedures Hence, plant latex used in green combustion synthesis (GCS) is an alternative approach which is able to obtain products by a simple procedure and provides highly pure samples with good yield GCS is one of the popular and quick methods based on the propellant chemistry techniques which uses organic and plant extracts as a fuel The advantages of this technique are, the heat energy released at considerably low temperatures by the redox exothermic reaction., This route can offer a distinct size and shape NPs, and additionally tuned physical and chemical properties compared to other existing synthesis techniques [32e34] Plant latex used in the preparation technique is used as both the reducing and the stabilizing chemical agent in the preparation of NPs [35] Materials and methods Semiliquid Milky white latex was extracted from the stems of freshly collected Euphorbia tirucalli (E tirucalli) plants collected from dry lands of the Madhugiri region 13.6643 N, 77.2089 E Latex of E tirucalli is used as vesicant and rubefacient for many diseases Extracted latex was sterilized and kept at À20 C before it was used as fuel to avoid fungus formation At the time of use of the latex, a 10 ml quantity of this stock solution was further diluted by adding double distilled water of suitable quantity and prepared E tirucalli latex solutions with final six desired concentrations varying from ml to 30 ml SrAl2O4:Eu3ỵ (1e5 mol %) NPs were prepared by the GCS method using AR grade of Sr(NO3)2 (99.99%), Al(NO3)39H2O (99.99%), Eu(NO3)36H2O (99.99%) as starting materials Stoichiometric quantities of the precursors were dissolved in 25 ml of deionized water along with desired quantities of plant latex extract solutions with final six desired concentrations varying from ml to 30 ml The solution is mixed together thoroughly using a magnetic stirrer and ultrasonication for about 30 and kept Fig (a) PXRD patterns and (b) WeH plots of SrAl2O4:Eu3ỵ (1e5 mol%) NPs Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Table The estimated average crystallite size, micro strain and energy gap values of SrAl2O4:Eu3ỵ (1e5 mol%) NPs Micro strain x 103 Energy Refractive Eu3ỵ Conc Crystallite size (nm) (mol %) gap (eV) index (n) Scherer's WeH plots method 23 29 30 33 34 30 23 20 21 30 2.89 2.75 3.13 3.40 2.26 5.49 5.33 4.92 4.82 4.51 1.945 1.919 1.933 1.924 1.893 into a muffle furnace that was maintained at 500 C At this temperature, the solution boils, and evaporates with large amounts of gases escaping out by combustion technique The whole process was completed in about 5e10 and gives a white ash which was further calcined at 1200 C for h 2.1 Characterization The structural properties were studied using the X'Pert- Pro Diffractometer with CuKa radiation and wavelength of 1.5406 Å The reflectance spectra were recorded using the PerkinElmer Lambda-35 UV spectrometer The surface morphology and elemental analysis of the product was studied using the TESCAN VEGA3 LMU scanning electron microscope The particle size was estimated by using the JEOL/JEM 2100 Transmission electron microscope The Jobin YVON Fluorolog-3 Spectrofluorimeter was utilized to study the photoluminescence Results and discussion 3.1 Structural investigation The PXRD patterns displayed in Fig correspond to the SrAl2O4:Eu3ỵ (1 - mol %) prepared with 30 ml plant latex samples All the compounds peaks are well matched with the monoclinic SrAl2O4 standard card (ICDD PDF 01-074-0794), showing that the Eu3ỵ doping did not modify the lattice parameter significantly Fig 1(a) depicts the diffraction proles of Eu3ỵ (1e5 mol%) doped SrAl2O4 NPs [36] It can be further noticed that there are no additional impurity peaks in the samples confirms the successful incorporation of Eu3ỵ ions into the SrAl2O4 host lattice This was conrmed by estimating the acceptable percentage difference (Dr) between Eu3ỵ ions and SrAl2O4 lattice sites using following relation [37]; Dr ¼ Rs À Rd Rs (1) where, Rs is the ionic radius of either Sr2ỵ or Al3ỵ, and where Rd is the ionic radius of the Eu3ỵ ion Generally, the value of Dr for the host site and the dopant ion differ not more than 30% In the present work, the estimated value of Dr by considering the ionic radii of the Eu3ỵ ion and the Sr2ỵ site was found to be very close to 30% This suggest that the dopant Eu3ỵ ions were clearly occupying the Sr2ỵ site in the SrAl2O4 host Further, the Scherer's and WeH plots techniques were employed in order to calculate the average crystallite size (D) as well as the strain of the obtained NPs using the relations [38]; D¼ 0:9l b cos q (2) Fig (a) diffuse reflectance spectra and (b) energy band gap plots of SrAl2O4:Eu3ỵ (1e5 mol%) NPs l b cos q ẳ ỵ sin qị D (3) where, l is the X-ray wavelength, b is the PXRD peaks full width at half maximum, ε is the strain in the prepared powders Fig 1(b) displays the WeH plots of the NPs The calculated crystalline parameters were listed in Table 3.2 Energy band gap estimate In order to evaluate the energy band gap (Eg) values of the synthesized powders their diffuse reflectance (DR) spectra were studied and depicted in Fig 2(a) From this figure it is seen that there is a strong absorption band appearing in the lower wavelength range which is ascribed to the host lattice absorption However, in the larger wavelength range some sharp absorption bands are observed which are due to the incorporation of Eu3ỵ ions These sharp absorption bands are ascribed to excitations from the ground level 7F0 to the numerous excited levels of Eu3ỵ ions (7F0/5D4, 7F0/5G2, 7F0/5L6, and 7F0/5D2) [39] Further, KubelkaeMunk (K-M) relations [40] (Equations (4) and (5)) were employed to estimate the Eg plots (Fig 2(b)) Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig SEM micrographs of SrAl2O4:Eu3ỵ (3 mol%) NPs with different plant extract concentrations, (1) ml, (b) 10 ml, (c) 15 ml, (d) 20 ml, (e) 25 ml and (f) 30 ml FðR∞ Þ ¼ hn ¼ ð1 À R∞ Þ2 2R∞ 1240 l (4) (5) where R∞ is the coefficient of reflection, l is the wavelength of absorption The evaluated Eg values are listed in Table Also the refractive index of the prepared samples are estimated and tabulated in Table 3.3 Morphological analysis Fig shows the SEM micrographs of SrAl2O4:Eu3ỵ (3 mol %) NPs prepared with different concentrations (5, 10, 15, 20, 25 and 30 ml) of E Tirucalli milk extract For the lower concentrations of the extract (5 and 10 ml), the SEM micrographs shows irregular units with rigid fibres (Fig 3(a, b)) At a further increase in concentration to ~15 ml, a mesh-like network is observed as shown in Fig 3(c) However, when the concentration is further increased to 20 ml, short length fibres are observed (Fig 3(d)) Finally, when the concentration is 25e30 ml, uniform broom-like structures are observed (Fig 3(e, f)) It is stated that the dispersion persuaded splitting growth mechanism is accountable for such broom-like structure Under non-equilibrium conditions, these irregular branching structures were moulded through a self-organization procedure The separation between the nucleation center and the driving force are responsible for the crystallization arrangement of hierarchical structures It is assumed that at lower concentrations of the milk extract, a concentric dispersion field was set round the crystal soon after the formation of nuclei The reactants present closer to the surface will undergo this diffusion process very quickly On the other hand when the milk extract is higher (25e30 ml), the ends of the crystals join together quicker than the dominant parts which lead to the formation of a broom-like morphology Fig 4(aee) shows the TEM images with different magnifications of the optimized NPs (3 mol%) with 30 ml of the plant extract From the figure, it is evident that the TEM image of the NPs is broom-like in nature with the approximate length of 445.21 nm (Fig 4b) Further, the tip of the broom-fibres is projected outward at the edges The obtained results are in agreement with that observed for SEM images The HRTEM image is depicted in Fig 4(f) From, the figure the interatomic distance for different domains is found to be 0.36 and 0.23 nm The selected area electron diffraction (SAED) pattern is shown in Fig 4(g) The image indicates that the obtained powders are crystalline in nature Further, the elemental analysis is shown in Fig 4(h) From the EDAX spectra, it is evident that the compound consists of the elements Sr, Al, O and Eu This result shows that the doped Eu ions are effectively incorporated into the host matrix Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig (aee) TEM images with different magnifications, (f) HRTEM image, (g) SAED patterns and (h) EDAX spectra of SrAl2O4:Eu3ỵ (1e5 mol%) NPs prepared using 30 ml plant latex 3.4 Spectroscopic studies The effect of Eu3ỵ ions on the photoluminescence properties of SrAl2O4 was studied and is depicted in Fig 5(a, b) The excitation spectrum of the Eu3ỵ (3 mol %) doped SrAl2O4 NPs with 30 ml latex is depicted in Fig 5(a) The prominent peak observed at 393 nm corresponds to the 7F0/5L6 transition of the Eu3ỵ ions [41] It is observed that the intensity of the 393 nm peak is stronger than the 467 nm peak This excellent absorption band at 393 nm peak indicates that SrAl2O4:Eu3ỵ NPs have a considerable potential for red light emitting diode applications The emission spectrum of various concentrations (1e5 mol %) of Eu3ỵ doped SrAl2O4 NPs under 393 nm excitation wavelength is depicted in Fig 5(b) The emission peaks observed between 500 and 700 nm are ascribed to 4f transitions of the Eu3ỵ ions (5D0/7FJ: J ẳ 0, 1, 2, 3, 4) [42] The strong emission peak detected at 618 nm is attributed to the 5D0/7F2 transition of the Eu3ỵ ions This transition is attributed to hypersensitive transition [43] Among the two prominent emission peaks observed at 610 (5D0/7F1) and 618 (5D0/7F2) the latter is more intense, which indicates that Eu3ỵ ions occupy a non-inversion symmetry site in the SrAl2O4 host lattice From Fig 5(c) it can be clearly noticed that the PL emission intensity increases with increase in Eu3ỵ ion concentration till mol% and thereafter shrinks owing to the famous concentration quenching phenomenon [44] Therefore, the optimal concentration of Eu3ỵ ions in the SrAl2O4 phosphor system is mol % This quenching mechanism can be explained based on the nonradiative energy transfer method which includes: exchange interaction, radiation re-absorption and multipolar interaction [45] To find out the particular type of the above interactions it was essential to estimate the critical distance (Rc) between the adjacent Eu3ỵ ions This value of Rc was evaluated by utilizing the Blasse relation [46]: 1=3 Rc ¼ 3V=4pNX c (6) Here, Xc is equal to 3, the optimal dopant concentration, V is 425.32 Å3 (volume of the unit cell), N is the coordination number for the unit cell By utilizing these values in the above equation, the Rc value was found to be ~18.56 Å Further, the multipolar interaction among the dopant ions was estimated by utilizing the Dexter's relation [47]: Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig (a) Excitation spectrum for SrAl2O4:Eu3ỵ (3 mol%) NPs., (b) emission spectra, (c) Variation of PL intensity versus Eu3ỵ ions concentrations, (d) log (I/x) versus log (x) plot, (e) CIE and (f) CCT diagrams of SrAl2O4:Eu3ỵ (1e5 mol%) NPs prepared with 30 ml plant latex 3.5 Color parameters I c 31 ẳ K 41 ỵ bcị Q (7) where c is the dopant ion concentration and Q is the constant of the multipolar interaction K and b are constants for the present host Fig 5(d) shows the plot of log (I/x) versus log x and the slope is found to be À0.5998 Hence, the value of Q is found to be ~1.8, which specifies that the electric multipoleemultipole interaction where the charge transfer mechanism is the major one for energy transfer among Eu3ỵ which is responsible for the concentration quenching phenomenon In order to evaluate the productivity of the prepared phosphor powders in the field of display devices, it is necessary to validate its color parameters according to the Commission Internationale de I'Eclairage (CIE) and the correlated colour temperature (CCT) [48,49] Fig 5(e) depicts the CIE diagram of SrAl2O4:Eu3ỵ (1e5 mol %) phosphors under 393 nm near UV-excitation It is evidently observed that the entire color co-ordinates lies well inside the red area indicating the use of the present phosphor as red component in white light emitting diodes The CCT values were estimated by utilizing the relations given elsewhere [50] and were found to be ~2668 K, which indicates that the prepared powders can be effectively used in the fabrication of warm LEDs (Fig 5f) Further, to Please cite this article as: K.R Ashwini et al., Near UV-light excitable SrAl2O4:Eu3ỵ nanophosphors for display device applications, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003 K.R Ashwini et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Table Photometric characteristics of SrAl2O4: Eu3ỵ (1e5 mol %) NPs Eu3ỵ mol% [3] CIE Coordinates CCT values x y u0 v0 0.6307 0.6312 0.6313 0.6313 0.6219 0.3688 0.3683 0.3682 0.3684 0.3775 0.478 0.465 0.462 0.464 0.468 0.533 0.532 0.531 0.512 0.492 CCT(K) CP % 2678 2745 2354 2568 2467 88 87 79 82 84 [4] [5] signify the phosphor usage in the LED industry it was necessary to evaluate its color purity (CP) value The CP values of the prepared NPs were evaluated by utilizing the following relation [33]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi xs xi ị2 ỵ ys yi ị2 color purity ẳ q 100% xd xi ị2 ỵ ðyd À yi Þ2 [6] [7] [8] (8) where, (xs, ys), (xi, yi) and (xd, yd) are colour co-ordinates of the light source, CIE illuminate and dominant wavelength of the light source, respectively The estimated CP values are recorded in Table The CP value for the optimized composition (3 mol %) is found to be ~88%, which indicates that the prepared NPs are exceedingly suitable for display device applications [9] [10] [11] [12] Conclusions [13] In this report, we prepared Eu3ỵ doped SrAl2O4 uorescent powder by the green combustion synthesis technique with the aim to obtain an efficient component in display device applications The key points of the present study includes, the synthesis route followed is eco-friendly, which requires less time, low cost, and high yield The structural and spectroscopic studies of the obtained samples showed a good crystalline structure allowing high dopant concentrations The SEM results indicate a fiber-like morphology The EDAX results showed the presence of all the stated elements in the compound The particles size was in the nano-metric scale The PL results shows an intense red emission of the powders at 618 nm wavelength under near UV-excitation (393 nm) The PL emission intensity increases up to mol% Eu3ỵ concentration and thereafter, declines due to concentration quenching The chromaticity diagrams indicated that the prepared powders emit an intense red color and their average CCT value was estimated to be ~2688 K Also, the CP of the optimized powder was estimated to be ~88% These obtained results clearly indicate that the prepared powder SrAl2O4 :Eu3ỵ(3 mol%) using 30 ml plant latex can be utilized as the red component in display device applications [14] [15] [16] [17] [18] [19] [20] [21] [22] Declaration of Competing Interest [23] The article is original, 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Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2020.02.003