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Eu3+: CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield
Accepted Manuscript 3+ 3+ Dazzling red luminescence from Bi /Eu : CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield K. Naveen Kumar, L. Vijayalakshmi, Jong Su Kim PII:
S0143-7208(17)32420-8
DOI:
10.1016/j.dyepig.2017.12.043
Reference:
DYPI 6449
To appear in:
Dyes and Pigments
Received Date: 23 November 2017 Revised Date:
19 December 2017
Accepted Date: 23 December 2017
3+ Please cite this article as: Kumar KN, Vijayalakshmi L, Kim JS, Dazzling red luminescence from Bi / 3+ Eu : CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2017.12.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
Dazzling red luminescence from Bi3+/Eu3+: CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield K. Naveen Kumara,b*, L. Vijayalakshmic* and Jong Su Kimb* a,b
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Clean Energy Priority Research Center, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. b Semiconductor Nanodevice Research Lab. (SNRL), Department of Physics, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. c Department of Automotive Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. *E-mail: [email protected], [email protected], [email protected],
Abstract
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We have successfully synthesized co-doped Bi3+/Eu3+: CLZO nanophosphor spheres by citrate sol-gel method. Tetragonal structure has been confirmed from the XRD analysis.
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Morphological studies were carried out from FE-SEM analysis for the optimized sample. EDAX and XPS measurements were employed to confirm the presence of elements and their ionic states. The homogeneous shape, size and crystalline planes of the phosphor crystalline nanosphere have been clearly demonstrated by HR-TEM analysis. Photoluminescence (PL)
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spectral profiles reveal that the strong red emission has been noticed at 626 nm (5D0→7F2) from the Eu3+: CLZO phosphors under the visible excitation of 467 nm. The PL performance has been remarkably enhanced by increasing the Eu3+ ion concentration. By co-doping with
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Bi3+ ions to Eu3+: CLZO phosphors, the red emission pertaining to Eu3+ was appreciably enhanced through energy transfer from Bi3+ to Eu3+ ions. The energy transfer mechanism
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from Bi3+ to Eu3+ ions has been demonstrated by partial energy level scheme diagram. Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) calculated for singly Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and they were found to be (0.6644, 0.3355) and (0.6688, 0.3311) respectively. The Correlated Color Temperature (CCT) value was evaluated as 3059 K for co-doped sample. Moreover, lifetime and quantum efficiency for Eu3+ emission were significantly enhanced upon co-doping with Bi3+ ions to the Eu3+ doped CLZO nanophosphor
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ACCEPTED MANUSCRIPT spheres. The energy transfer phenomenon from Bi3+ to Eu3+ ions was clearly elucidated by several fluorescence dynamics such as overlapped spectral studies, photoluminescence spectral features, CIE color coordinates and quantum efficiency. These dazzling red
solid state illumination and optical display devices.
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Keywords: Red emission, energy transfer, co-doped phosphor.
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luminescent nanophosphor spheres material could be suggested as a promising candidate for
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1. Introduction Compared to bulk counterparts, the nanostructured materials synthesis and characteristics evaluation have become an interesting part of research in recent days because of their utility in various technological applications. The nano sized materials possesses
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enhanced properties with respect mechanical, optical, magnetic, sensing, electrical, biomedical and luminescence over bulk materials. Luminescent nanoparticles could possess greater oscillating strengths and enhanced luminescence quantum efficiency than the bulk
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materials due to existing greater electron-hole overlap. Efficient red emitting nanophosphors have been greatly desired for phosphor converted or phosphor based white light emitting
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diodes (W-LEDs). In view of reducing the global power consumption in day to day life, WLED technology has become an interesting way of research over traditional fluorescent lighting sources because of their potential features such as low power consumption, high efficiency and long lifetime. These W-LEDs are widely used in displays such as general
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illumination, back lights, device indicators, automobile headlights. Nevertheless, W-LED technology has some shortcomings such as a poor color rendering index (CRI), short lifespan and high color correlated temperatures due to red color deficiency [1]. The usage of W-LEDs
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in industrial and commercial fields is often limited due to low red spectral component in the
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fabrication. In order to overcome this situation phosphor converting method is highly efficient and low price to obtain white light emission. Rare earths contained nanocomplexes like phosphors have been an attractive materials for past few decades because of their large stokes shifts, high luminescence intensity, narrow emission peaks, greater luminescence efficiency, good optical stability and prominent photophysical properties. These inorganic nanophosphors are most preferable for high performance luminescence devices, biological fluorescence lables, display devices, solar cells, optical sensors, solid state lighting devices (SSLD) and sensor applications [2]. Lanthanide ions doped nanocompounds have become an 3
ACCEPTED MANUSCRIPT attractive field of research because of their exemplary luminescence properties. Due to the shielding effect of the 4f electrons by outer 5S2 and 5P6 subshells, the rare earth ions can exhibit high color purity and narrow emission and absorption bands. Among all the rare earth ions, trivalent europium in (Eu3+) ion is widely used as an activator ion in many phosphor
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materials and it is very familiar red emitting ion upon irradiated with UV excitation due to its unique electronic transition of 5D0→7F2 which is forced electronic dipole transition. Alongside this electric dipole transition, there is a magnetic dipole transition will be existed
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due to the presence Eu3+ ions. This magnetic dipole transition is independent of the local environment and the electric dipole transition is sensitive to the chemical environment.
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Hence, the hypersensitive transition 5D0→7F2 intensity is highly influenced by the local symmetry surrounded by Eu3+ ions. The ratio between electric dipole (ED) and magnetic dipole (MD) transition intensity which is well known as asymmetric factor (R) can be used to estimate the local environmental symmetry of the Eu3+ ions in a host matrix [3, 4]. The
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emission performance could be enhanced significantly by increasing the concentration Eu3+ ions within the host lattice. However, the emission could be drastically reduced at higher concentrations of Eu3+ ions due to concentration quenching effect. To overcome this
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difficulty there are several approaches have been employed to improve the fluorescence properties of Eu3+ ions. Among those, the addition of another sensitizer ion to the host lattice
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along with Eu3+ ions is found to be a favorable approach [5]. The fluorescence efficiency would be enhanced due to possible energy transfer taking place from sensitizer to activator ions of Eu3+ ion from this method. In order to enhance the red color index and fluorescence efficiency, Bi3+ ions have been undertaken to co-dope along with Eu3+ ions in CaLa2ZnO5 phosphors in our present work. Here, Eu3+ ions and Bi3+ ions are acting as an activator and sensitizer. Interestingly, we obtained some remarkable results in the evaluation of fluorescence, quantum efficiency, lifetimes and CIE coordinates that the potential red
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ACCEPTED MANUSCRIPT emission is improved by co-doping strategy under the visible excitation. Moreover, the observation of the emission features under UV excitation is normal in nowadays. However, we have obtained a remarkable red luminescence characteristics under visible blue excitation and these spectral trends are systematically analyzed. In our earlier report, we have
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synthesized Eu3+/Dy3+: CLZO micro sized phosphor materials by solid state reaction method. But we achieved 66 % of the luminescence quantum efficiency of the red emission pertaining to Eu3+ ions. Surprisingly, 80 % of the luminescence quantum efficiency of the red emission
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pertaining to Bi3+/Eu3+: CLZO nanophosphor spheres have been obtained which are synthesized by citrate sol-gel method in our present work. Compared to our earlier reports on
requirements of photonic devices.
2. Experimental studies
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the phosphor materials, we have been achieved here significant results in all respects of the
Eu3+/Bi3+ ions singly doped and co-doped CLZO nanophosphors were prepared by
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employing traditional citrate sol-gel method. The high purity raw materials Ca(NO3)2 4H2O, La(NO3)3 6H2O, Zn(NO3)2 6H2O, citric acid monohydrate, Eu(NO3)3 5H2O, Bi(NO3)3 5H2O were purchased from Sigma Aldrich and taken in a stoichiometric ratio. These chemicals
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were dissolved in 200 ml distilled water under continuous stirring at room temperature for 2
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h. Then, 8 mol of citric acid monohydrate was added to this solution. The transparent solution was again stirring for 1 h for solution homogeneity. The obtained solution was capped and kept under 80 ˚C of homogeneous heating for 1 h. After the cap was removed and then heating continued for 6 h. We can obtain the brownish wet gel. Then the brownish wet gel was kept under 120 ˚C in a heated oven for 24 h for xerogel. The obtained xerogel was heated with 400 ˚C for 4 h and found black color flakes. Then these black color flakes were sintered at 900˚C for 6 hours. Then, obtained samples were taken for further characterization.
3. Characterization 5
ACCEPTED MANUSCRIPT XRD profiles of Eu3+, Bi3+ doped and co-doped CLZO nanophosphor spheres were recorded on SEIFERT 303 TT X-ray diffractometer (XRD) with CuKα (1.5405 Å) which operated at 40 KV and 50 mA of voltage and currents respectively. The morphological studies and elemental analysis were carried out by High Resolution FE-SEM-I (JSM 7401F)
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attached with EDAX system. X-ray photoelectron spectroscopy studies were carried out (Thermo Scientific K-Alpha) using Al Ka X-ray source (1486.6 eV). The transimission electron microscope analysis has been carried out by HR-FE-TEM-(2200FS with CsTEM)
(Spherical
aberration
corrected
transition
electron
microscope).
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corrected
Photoluminescence (excitation and emission) spectral analysis of singly doped Eu3+: CLZO,
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and co-doped Eu3++Bi3+: CLZO phosphors were obtained on Scinco FluoroMate FS-2 Visible Fluorescence Spectrometer with Xe- arc lamp of 150 W power as an excitation source for a study state emission spectrum measurement. The lifetime of the red emission transition of the Eu3+ ions, the decay curves was recorded. From the plotted decay curves, we can
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calculate the lifetime of the emission transition by Edinburgh Instruments (Edinburgh, UK) spectrofluorimeter. In order to estimate the absolute luminescence quantum efficiency of the phosphor samples, an integrating sphere equipped with an Edinburgh spectrometer (Model
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FLS900) instrument has been used for measuring the integrated fraction of luminous flux and
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radiant flux with the standard method.
4. Results and Discussion 4.1 Structural details
The X-ray diffraction patterns of the optimized CLZO, Eu3+ (12 mol %), Bi3+ (2 mol
%) and Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO nanophosphor spheres are represented in Fig. 1. The XRD profile of the host CLZO is displayed in Fig.1, all the diffraction peaks for the prepared samples were exactly matched with the reported tetragonal structure which is in good agreement with the earlier reports [6, 7]. No significant changes in the structure were 6
ACCEPTED MANUSCRIPT observed by incorporation of rare earth ions in the basic crystal. This indicates that there is no diffraction peaks from any secondary phase observed. In our present host matrix, the Eu3+ ions are preferably occupied La3+ ion sites due to the same charge and similar ionic radius of 0.1087 Å and 1.032 Å respectively. Smaller Eu3+ ions for La3+ ion sites which can induce the
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shrinking of the lattice. Therefore, a little peak shift has been observed in doped samples. The average crystallite size of the co-doped Eu3+ + Bi3+: CLZO nanophosphor powder sample has been calculated from the Scherrer equation and it is found to be ~165 nm.
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It can be seen that the agglomerated regular spherical surface morphology of codoped Eu3+ + Bi3+: CLZO nanophosphor as shown in Fig. 2 (a). The agglomerated spherical
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particles attributed to homogeneous nucleation and further growth during the formation of nanoparticles [8]. Fig. 2 (a) clearly reveals that there is exists a great deal of well dispersed and nanospheres which are having the diameter ranging from 40 nm to approximately 140 nm. The elements were present in the material which are good consistent with the obtained
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EDAX profile of co-doped Eu3+ + Bi3+: CLZO as shown in Fig. 2 (b). Fourier transform infrared (FTIR) spectroscopy is a resourceful tool to investigate the confirmation information of complex formation and evaluation in the inter molecular interactions within the sample.
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Fig. 3 represents the FTIR specta of host CLZO, Eu3+ (12 mol %), Bi3+ (2 mol %) and Eu3+
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(12 mol %) + Bi3+ (2 mol %): CLZO nanophosphor spheres. The vibrational modes such as bending and stretching vibrations pertaining to corresponding functional groups within the Eu3+and Bi3+ doped CLZO phosphors spheres have been demonstrated systematically through FTIR spectral analysis. We have been observed a strong and moderately weak absorption bands at 3607 cm-1 and 1739 cm-1. These two bans are ascribed to the stretching and bending vibrations of hydroxyl group (-OH) respectively [9]. The band at 1420 cm-1 is observed and it is attributed to asymmetric stretching vibrations of C-O. The residual nitrates and a small amount of organic matter in the sample have been noticed by the appearance of the band at
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ACCEPTED MANUSCRIPT 1485 cm-1 and 2945 cm-1 respectively [10]. The absorption band at 1634 cm-1 and 1456 cm-1 have been noticed and they are ascribed to stretching vibration of the –COO- groups. The absorption band at 1080 cm-1 and 1045cm-1 which are attributed to O-H bending vibrations. A sharp band along with weak band have been noticed at 643 cm-1 and 875 cm-1. These bands
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could be attributed to the out-plance and in-plane bending vibrations of CO3-2 respectively. We have been observed a sharp FTIR band at the lower wavenumber side at 479 cm-1 which is ascribed to the stretching vibrations of Zn-O [11, 12].
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X-ray photoelectron spectroscopy is performed to determine the chemical composition, binding energy and electronic states of the co-doped CLZO phosphor. XPS
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spectrum for Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO phosphor was recorded in the range between 1350 and 0 eV as shown in Fig. 4 (a). The pertinent peaks at peaks are observed at 348.8, 836.7, 1022.6, 532.5, 1135.2 and 1296.2 eV which are ascribed to Ca 2p, La 3d, Zn 2p, O 1s, Eu 3d and Dy 3d levels respectively. The closure observation of the europium and
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bismuth have been separately shown in Fig. 4 (b&c) which are in good agreement with the earlier reports. The characteristic XPS peaks of the Bi3+ have been observed at 159.08 eV and 164.32 eV in Fig. 4 (c) which could be attributed to 4f7/2 and 4f5/2 [13]. The pertinent bands at
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1164.06 eV, 1134.20 eV and 1128.50 eV are shown in Fig. 4(b). These bands are attributed to
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the presence of trivalent and divalent europium ions, respectively [14, 15]. No other impurities have been identified from the XPS spectrum, which are good consistent with the XRD and EDAX results in earlier. Fig. 5 (a-c) illustrates the TEM image and SAED patterns of the optimized Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO phosphor. We have been observed an irregular shapes and most of the particles have spherical shape. It is exploring that the uniformity of the size and shape is controlled by homogeneous nucleation. Nucleation and crystal growth continued during the heating process which leads a slight irregular shape and agglomerated particles. The particle size has been evaluated from the TEM as 165 nm which
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ACCEPTED MANUSCRIPT is in good agreement with the calculated XRD results. From the HRTEM, the planes with an inter planar spacing of 0.14 nm are observed and are in good agreement with the in the (202) lattice planes of bulk CLZO as shown in Fig. 5 (b). Moreover, Fig.5 (c) consists of continuous rings with some discrete spots which indicates that the prepared sample is highly
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polycrystalline in nature [16]. 4.2 Photoluminescence analysis
The excitation spectrum of the singly doped Eu3+(14 mol %): CLZO phosphor as shown
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in Fig. 6. The excitation spectrum consists of several broad excitation bands at 378 nm, 386 nm, 397 nm, 417 nm and 467 nm. These bands are assigned with corresponding electronic
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transitions such as (7F0→5L7), (7F0→5G2), (7F0→5L6), (7F0→5D3) and
(7F0→5D2)
respectively. We have been observed that the broadening of the excitation bands which could be suggests that the Eu3+ ions may be around in a better crystallization environment [17]. Among all the excitation bands, the prominent excitation band is observed at 467 nm which is
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ascribed due to the electronic transition of 7F0→5D2. By using this prominent excitation band at 467 nm we can obtain the emission spectra of Eu3+ doped CLZO phosphor powders. The emission spectra of the Eu3+ ions doped CLZO phosphors at different concentrations of the
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Eu3+ have been recorded under the excitation of 467 nm as shown in Fig.7. We have observed five emission bands from the emission spectra. Among all the emission spectral bands, a
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prominent emission band is noticed at 626 nm from all the Eu3+: CLZO phosphors which originating from the forced electric-dipole transition of 5D0→7F2 of Eu3+ along with moderate intense band of magnetic dipole transition of 5D0→7F1 at 597 nm. Apart from these two transitions, another three low intense transitions also observed at 583 nm (5D0→7F0), 652 nm (5D0→7F3) and 707 nm (5D0→7F4) within the 4f6 configuration of Eu3+. It can be seen that the fluorescence emission intensity has been increased consistently with increasing the Eu3+ ion concentration until 12 mol % of Eu3+ ions. The emission intensity is slowly decreased
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ACCEPTED MANUSCRIPT after 12 mol % of optimized concentration dramatically. This could be ascribed to the concentration quenching effect [18]. Due to the existence of ion-ion interaction provoking by the shorter distance between interacting activators as a concentration increases, the quenching effect could be occurred. However, the forced electric dipole transition of 5D0→7F2 is highly
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sensitive to the local environment. The closure observation of the emission spectral profile of the Eu3+ ions is required for better understanding of the local symmetry which surrounded by the Eu3+ ions in the crystalline lattice. The magnetic dipole transition 5D0→7F1 of orange
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emission regarding Eu3+ ions is dominant than the other transitions, then the Eu3+ ion occupied a crystallographic site with inversion symmetry. In other words, the forced electric
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dipole transition is dominant than the other transitions of Eu3+ ions which could be suggested that the Eu3+ ions are situated in the non-centrosymmetrical site in the crystalline lattice. In our present singly doped Eu3+ doped CLZO phosphors are explored that the Eu3+ ions are occupied in non-centrosymmetrical sites. Moreover, the ratio of the electric dipole and
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magnetic dipole transitions (5D0→7F2 /5D0→7F1) intensity can be used to evaluate the local surroundings of the cation in lanthanide based systems. Here, the ratio between 5D0→7F2 /5D0→7F1 is calculated for singly doped optimized Eu3+(12 mol %): CLZO and co-doped
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Eu3+(12 mol %)+Bi3+ (1mol %): CLZO nanophosphors and these values are found to be 7.76 and 8.07. However, the 5D0→7F2 transition intensity is 7.76 and 8.07 times greater than D0→7F1 in single and co-doped systems. This small ratio between the intensities of 5D0→7F2
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and 5D0→7F1 transitions which could be a direct evidence to the presence of Eu3+ in symmetric site [19]. This favorable improvement of electric dipole and magnetic dipole transitions (5D0→7F2 /5D0→7F1) intensity ratio in co-doped system could be supports the red color purity [20]. The emission and excitation spectral features of singly doped Bi3+(1 mol %): CLZO phosphor has been recorded and they are shown in Fig. 8. Outer ns2 configuration in Bi3+ can
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ACCEPTED MANUSCRIPT play a pivotal role in the field of luminescence. Moreover, most of the researchers have been undertaken the Bi3+ for luminescence sensitization purpose in various luminescence ions doped compounds such as polymers, glasses and phosphors [21]. 6S2 electronic configuration of the Bi3+ can exhibit luminescence properties. However, the photoluminescence
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characteristics of the Bi3+ generally depend on the host matrices. According to the Bi3+ electronic configuration, there are in the ground state of 1S0 and it consists four excited levels, such as 3P0, 3P1, 3P2 and 1P1. However, the transitions of 1S0→3P0 and 1S0→3P2 are
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completely spin forbidden, since the two levels 3P1 and 1P1 are mixed by spin orbit coupling. The spin selection rule in Bi3+ ions is then considerably relaxed by the strong spin-orbit
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coupling. Thus, the possible expected, reasonable absorption could be occurred for the transitions of 1S0→3P1 and 1S0→1P1. Bi3+ doped complexes exhibit the strong absorption and emission bands which are attributed to the transition of 1S0→3P1 and 3P1→1S0. In our present phosphor samples, a broad excitation band has been noticed between 355 - 430 nm which is
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centered at 374 nm (1S0→3P1). We have obtained the emission peak centered at 426 nm under the excitation of 374 nm. This emission peak at 426 nm is ascribed to corresponding electronic transition of 3P1→1S0 [22] as shown in Fig. 8.
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By co-doping with Bi3+ ions to the Eu3+(12 mol %): CLZO nanophosphor, the emission spectral features have been recorded as shown in Fig. 9. We have been observed that
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remarkable enhancement in the red emission (5D0→7F2) pertaining to Eu3+ ions in the codoped system than the singly doped Eu3+(12 mol %): CLZO phosphor. This could be attributed to the possible energy transfer taking place from Bi3+ ions to Eu3+ ions within the host matrix. Here, the Bi3+ ions and Eu3+ ions are acting as sensitizer and activators respectively. In this phenomenon, the successful emission photons of the Bi3+ ions are collectively absorbed by the Eu3+ ions within the host matrices. Therefore, there is a possible condition to excite Eu3+ ions more effectively by absorbing the additional photons from the
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ACCEPTED MANUSCRIPT Bi3+ ions within the same lattice environment. The red emission of the Eu3+ has been significantly enhanced by the energy transfer process until 1 mol % of Bi3+ ion concentration in the co-doped system. Nevertheless, the emission intensities are found to be drastically reduced after 1 mol
% of Bi3+ concentration. This could be due to the concentration
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quenching effect [23] as shown in Fig. 2 (d). This concentration quenching effect could be results of energy transfer caused by more Bi3+-Bi3+ interactions than Bi3+-Eu3+ interactions. From this quite encouraging results, we could be demonstrated that the Bi3+ ions co-doping
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plays a pivotal role in improving the red emission of Eu3+ ions in the co-doped CLZO phosphors.
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Overlapped spectral features between sensitizer and the activator is also a better evidence for the energy transfer phenomenon between sensitizer and activator according to Froster and Dexter energy transfer theories. We have been observed the well overlapped spectral profile between the emission spectrum of Bi3+ ions and excitation spectrum of Eu3+
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ions as shown in Fig. 10. This overlapped spectral profile is strongly suggested that the occurrence of energy transfer from Bi3+ to Eu3+ ions within the host matrix [24]. The energy transfer pathway is clearly demonstrated by an energy level scheme diagram as shown in
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Fig.11. We can evaluate the color confirmation, color purity and energy transfer process
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through Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y). The Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) generated from the emission spectral profiles of the singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and they are found to be (0.6644, 0.3355) and (0.6688, 0.3311) respectively as shown in Fig. 12. By addition of Bi3+, the red color index is significantly increased compared to singly doped Eu3+(12 mol %): CLZO phosphor. Surprisingly, these coordinate values are very close to the standard values of National Television Standard Committee (NTSC) (x=0.67, y=0.33) [25]. It can be seen
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ACCEPTED MANUSCRIPT that the chromaticity coordinates are tuned to more bright red emission under the visible excitation by co-doping with Bi3+ ions. This could be an another evidence for energy transfer from Bi3+ to Eu3+ [26]. Alongside the color coordinates, we have been evaluated the correlated color temperature (CCT) value by using the CIE coordinate values for the final
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product of optimised co-doped phosphor sample. CCT value calculated from the equation as follows
CCT = − 437 n3 + 3601n 2 − 6861n + 5514.31
- - - (1)
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Where ‘n’ represents (x-xe)/(y-ye) which is an inverse slope line, xe and ye are 0.3320 and 0.1858 respectively [27]. The Correlated Color Temperature (CCT) value is calculated and it
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is found to be 3059 K for optimized co-doped phosphor and are located in between 2000 to 5000 K, which could be more preferable red component candidate for warm white light applications.
4.2.1 Lifetime analysis and quantum efficiency
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In order to evaluate the lifetimes of the optimized singly doped and co-doped CLZO phosphor materials, we have been recorded the lifetime decay curves of these phosphors under monitoring excitation and emission wavelengths of 467 nm and 626 nm respectively as
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shown in Fig.13. However, the lifetime of the excited state of Eu3+ ions in the singly doped form is calculated and it is found to be 0.706 ms. The lifetime curve of the excited state of
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Eu3+ ions in singly doped form has been fitted in first order exponential function as follows I = A1 exp(−t / τ 1 ) + I 0
- - - (2)
where I represents the intensity of the emission peak, A and τ are the constant and lifetime respectively. The excited state lifetime has been calculated pertaining to Eu3+ ions in the form of co-doped system and it is found to be 1.526 ms. Therefore, this remarkable enhancement of the emission lifetime pertaining to Eu3+ ions could be occurred due to the energy transfer process taking place from Bi3+ to Eu3+ in the co-doped host matrix. This possibility of the
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ACCEPTED MANUSCRIPT lifetime enhancement above two times in co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO phosphors than the singly doped Eu3+(12 mol %): CLZO nanophosphor spheres are strongly suggesting that the energy transfer taking place between bismuth and europium ions. Moreover, the dynamics of the co-doped lifetime decay of the Eu3+(12 mol %)+Bi3+(1 mol
exponential decay mode using the equation as follows I = A1 exp(−t / τ 1 ) + A2 exp(−t / τ 2 )
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%): CLZO nanophosphor sphere is depicted in Fig. 13 and it is well fitted in a second order
- - - (3)
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where I represent emission intensity, A1 and A2 are constants. τ1 and τ2 are the two exponential components. Because of the existence of the same charge of lanthanum,
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europium and bismuth; it can be possible to expect that the impurities of Eu3+ and Bi3+ ions are occupied the lanthanum sites within the crystalline lattice. The average lifetime of the codoped system is calculated from the equation as follow
τ = ( A1τ 12 + A2τ 22 ) / ( A1τ 1 + A2τ 2 )
- - - (4)
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From this non-exponential nature of the lifetime decay profile of the co-doped system, it can be believed that the existence of possible energy transfer from Bi3+ to Eu3+ in the CLZO host matrix. This energy transfer enhances the emission intensity as well as the lifetime of the
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excited state which is more favorable result for the application point of view [28]. Moreover,
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we have been evaluated the luminescence quantum efficiency of the singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and are included in Table 1. Compared to our recent report [29], we have achieved a larger quantum efficiency for singly doped and co-doped phosphors. However, the luminescence quantum efficiency pertaining to singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres are found to be QE=56 % and QE=82 % respectively. The quantum efficiency (QE) of our co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres under the excitation of 467 nm is 82 %, which is higher 14
ACCEPTED MANUSCRIPT than that of commercial red phosphors Y2O3: Eu3+ (9.6 %, 394 nm excitation) and Y2O2S: Eu3+ (4.2 %, 395 nm excitation) [30]. Here, we have been observed the successful enhancement in the luminescence quantum efficiency of the Eu3+ ions by co-doping with Bi3+ ions in the CLZO matrix compared to earlier results. It can be strongly supports the energy
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transfer phenomenon between Bi3+ and Eu3+ ions unambiguously. The energy transfer phenomenon is well substantiated by photoluminescence, overlapped spectral profiles, lifetime decay dynamics, CIE color coordinates and quantum yield measurements. Therefore,
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these high quantum efficiency (QE=82%) existed co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres material could be a potential candidate for red luminescence
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optical devices.
3 Conclusion
In summary, we have successfully synthesized singly doped Eu3+, Bi3+ and co-doped CaLa2ZnO5 (CLZO) nanophosphor spheres by citrate sol-gel method. The tetragonal
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structure was confirmed from the XRD analysis of all prepared samples. The complex formation and the interaction between ion and host matrix were clearly demonstrated by FTIR spectral studies. The morphological studies have been carried out from FE-SEM
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analysis. EDAX and XPS studies were performed to evaluate the elemental presence and the binding energies confirmation respectively. A strong red emission at 626 nm (5D0→7F2) was
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observed in Eu3+: CLZO nanophosphors under the visible excitation of 467 nm alongside the existence of some other moderate intense emission bands. The red emission was enhanced with increasing the Eu3+ ion concentration until 12 mol % and it was drastically reduced after 12 mol % dramatically. This could be attributed to the concentration quenching effect. Noteworthy, the red emission was significantly enhanced by co-doping with Bi3+ ions in CLZO nanophosphor due to energy transfer from Bi3+ ions to Eu3+ ions. Nevertheless, the predominant red emission intensity was observed for co-doped Eu3+ (12 mol %) + Bi3+ (1
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ACCEPTED MANUSCRIPT mol %): CLZO nanophosphor than the other samples. Hence, the optimized sensitizing concentration of Bi3+ ions was found to be 1 mol %. Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) evaluated for singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres as (0.6644,
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0.3355) and (0.6688, 0.3311) respectively, which were well closure to the standard white standard values of National Television Standard Committee (NTSC) (x=0.67, y=0.33) surprisingly. The Correlated Color Temperature (CCT) value was evaluated as 3059 K for co-
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doped sample and it was located in between 2000 to 5000 K, which could be a more preferable red component for warm white light applications. The lifetime and quantum
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efficiency of the optimized co-doped sample have been evaluated as 1.526 nm and 82 % respectively. Moreover, these lifetime and quantum efficiency values were significantly enhanced upon co-doping with Bi3+ ions to the Eu3+ doped CLZO phosphors. The energy transfer phenomenon from Bi3+ to Eu3+ ions was clearly elucidated by several fluorescence
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dynamics such as overlapped spectral studies, photoluminescence spectral features and CIE color coordinates and quantum efficiency. These ravishing red luminescent nanophosphor spheres materials could be suggested as a promising candidate for solid state illumination and
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Acknowledgement
This work was supported by the 2015 Yeungnam University Research Grant.
References
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properties and thermal quenching mechanisms. Sci Rep 2017; 7: 42464.
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Table 1 Excitation, emission, lifetime and quantum efficiency values of the singly doped Eu3+(12 mol %): CLZO phosphor and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO phosphors. Emission (nm) 626 626
Lifetime (ms) 0.706 1.526
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Eu3+(12 mol %): CLZO Eu3+(12 mol %)+Bi3+(1 mol %): CLZO
Excitation (nm) 467 467
Quantum efficiency 56% 82%
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Phosphor sample
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Figure Captions Fig.1 XRD profiles of (a) CLZO, (b) Eu3+(12 mol %): CLZO, (c) Bi3+(1 mol %): CLZO and (d) Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO nanophosphor spheres. Fig. 2 (a) FE-SEM image and (b) EDAX of Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO
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nanophosphor spheres. Fig.3 FTIR spectra of (a) CLZO, (b) Eu3+(12 mol %): CLZO, (c) Bi3+(1 mol %): CLZO and (d) Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO nanophosphor spheres.
Fig.4 XPS survey spectra of co-doped Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO phosphor in
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the range of (a) 0-1300 eV, (b) 1115-1145 eV and (c) 150-175 eV.
Fig. 5 (a) TEM, (b) HR-TEM and (c) SAED patterns of co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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Fig. 6 Excitation spectrum of Eu3+ (12 mol %): CLZO phosphor.
Fig. 7 Emission spectra of Eu3+: CLZO phosphors at different concentrations of Eu3+ ions. Fig. 8 (a) Excitation and (b) emission spectra of Bi3+ (1 mol %): CLZO nanophosphor spheres.
Fig. 9 Emission spectra of co-doped Eu3+ (12 mol %) + Bi3+ (0.5, 1, 2, 3 and 4 mol %):
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CLZO nanophosphor spheres.
Fig. 10 Overlapped spectral profile of emission spectra of Bi3+ (1 mol %): CLZO and excitation spectra of Eu3+ (12 mol %): CLZO nanophosphor spheres. Fig.11 Partial energy level scheme diagram of energy transfer from Bi3+ to Eu3+.
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Fig. 12 CIE chromaticity diagram for (1) Eu3+(12 mol %): CLZO (2) co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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Fig. 13 Lifetime decay dynamics of (a) Eu3+(12 mol %): CLZO (b) co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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ACCEPTED MANUSCRIPT Highlights: Dazzling red emission is observed from Eu3+/Bi3+: CaLa2ZnO5 nanophosphors. Red emission of Eu3+ was significantly enhanced by co-doping with Bi3+. Effective sensitization of Bi3+ ions has been elucidated.
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The energy transfer pathway was clearly demonstrated from Bi3+ to Eu3+.
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CIE coordinates are very close to National Television Standard Committee (NTSC).
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S0143-7208(17)32420-8
DOI:
10.1016/j.dyepig.2017.12.043
Reference:
DYPI 6449
To appear in:
Dyes and Pigments
Received Date: 23 November 2017 Revised Date:
19 December 2017
Accepted Date: 23 December 2017
3+ Please cite this article as: Kumar KN, Vijayalakshmi L, Kim JS, Dazzling red luminescence from Bi / 3+ Eu : CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2017.12.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Dazzling red luminescence from Bi3+/Eu3+: CaLa2ZnO5 nanophosphor spheres for high luminescence quantum yield K. Naveen Kumara,b*, L. Vijayalakshmic* and Jong Su Kimb* a,b
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Clean Energy Priority Research Center, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. b Semiconductor Nanodevice Research Lab. (SNRL), Department of Physics, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. c Department of Automotive Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea. *E-mail: [email protected], [email protected], [email protected],
Abstract
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We have successfully synthesized co-doped Bi3+/Eu3+: CLZO nanophosphor spheres by citrate sol-gel method. Tetragonal structure has been confirmed from the XRD analysis.
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Morphological studies were carried out from FE-SEM analysis for the optimized sample. EDAX and XPS measurements were employed to confirm the presence of elements and their ionic states. The homogeneous shape, size and crystalline planes of the phosphor crystalline nanosphere have been clearly demonstrated by HR-TEM analysis. Photoluminescence (PL)
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spectral profiles reveal that the strong red emission has been noticed at 626 nm (5D0→7F2) from the Eu3+: CLZO phosphors under the visible excitation of 467 nm. The PL performance has been remarkably enhanced by increasing the Eu3+ ion concentration. By co-doping with
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Bi3+ ions to Eu3+: CLZO phosphors, the red emission pertaining to Eu3+ was appreciably enhanced through energy transfer from Bi3+ to Eu3+ ions. The energy transfer mechanism
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from Bi3+ to Eu3+ ions has been demonstrated by partial energy level scheme diagram. Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) calculated for singly Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and they were found to be (0.6644, 0.3355) and (0.6688, 0.3311) respectively. The Correlated Color Temperature (CCT) value was evaluated as 3059 K for co-doped sample. Moreover, lifetime and quantum efficiency for Eu3+ emission were significantly enhanced upon co-doping with Bi3+ ions to the Eu3+ doped CLZO nanophosphor
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ACCEPTED MANUSCRIPT spheres. The energy transfer phenomenon from Bi3+ to Eu3+ ions was clearly elucidated by several fluorescence dynamics such as overlapped spectral studies, photoluminescence spectral features, CIE color coordinates and quantum efficiency. These dazzling red
solid state illumination and optical display devices.
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Keywords: Red emission, energy transfer, co-doped phosphor.
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luminescent nanophosphor spheres material could be suggested as a promising candidate for
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1. Introduction Compared to bulk counterparts, the nanostructured materials synthesis and characteristics evaluation have become an interesting part of research in recent days because of their utility in various technological applications. The nano sized materials possesses
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enhanced properties with respect mechanical, optical, magnetic, sensing, electrical, biomedical and luminescence over bulk materials. Luminescent nanoparticles could possess greater oscillating strengths and enhanced luminescence quantum efficiency than the bulk
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materials due to existing greater electron-hole overlap. Efficient red emitting nanophosphors have been greatly desired for phosphor converted or phosphor based white light emitting
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diodes (W-LEDs). In view of reducing the global power consumption in day to day life, WLED technology has become an interesting way of research over traditional fluorescent lighting sources because of their potential features such as low power consumption, high efficiency and long lifetime. These W-LEDs are widely used in displays such as general
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illumination, back lights, device indicators, automobile headlights. Nevertheless, W-LED technology has some shortcomings such as a poor color rendering index (CRI), short lifespan and high color correlated temperatures due to red color deficiency [1]. The usage of W-LEDs
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in industrial and commercial fields is often limited due to low red spectral component in the
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fabrication. In order to overcome this situation phosphor converting method is highly efficient and low price to obtain white light emission. Rare earths contained nanocomplexes like phosphors have been an attractive materials for past few decades because of their large stokes shifts, high luminescence intensity, narrow emission peaks, greater luminescence efficiency, good optical stability and prominent photophysical properties. These inorganic nanophosphors are most preferable for high performance luminescence devices, biological fluorescence lables, display devices, solar cells, optical sensors, solid state lighting devices (SSLD) and sensor applications [2]. Lanthanide ions doped nanocompounds have become an 3
ACCEPTED MANUSCRIPT attractive field of research because of their exemplary luminescence properties. Due to the shielding effect of the 4f electrons by outer 5S2 and 5P6 subshells, the rare earth ions can exhibit high color purity and narrow emission and absorption bands. Among all the rare earth ions, trivalent europium in (Eu3+) ion is widely used as an activator ion in many phosphor
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materials and it is very familiar red emitting ion upon irradiated with UV excitation due to its unique electronic transition of 5D0→7F2 which is forced electronic dipole transition. Alongside this electric dipole transition, there is a magnetic dipole transition will be existed
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due to the presence Eu3+ ions. This magnetic dipole transition is independent of the local environment and the electric dipole transition is sensitive to the chemical environment.
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Hence, the hypersensitive transition 5D0→7F2 intensity is highly influenced by the local symmetry surrounded by Eu3+ ions. The ratio between electric dipole (ED) and magnetic dipole (MD) transition intensity which is well known as asymmetric factor (R) can be used to estimate the local environmental symmetry of the Eu3+ ions in a host matrix [3, 4]. The
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emission performance could be enhanced significantly by increasing the concentration Eu3+ ions within the host lattice. However, the emission could be drastically reduced at higher concentrations of Eu3+ ions due to concentration quenching effect. To overcome this
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difficulty there are several approaches have been employed to improve the fluorescence properties of Eu3+ ions. Among those, the addition of another sensitizer ion to the host lattice
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along with Eu3+ ions is found to be a favorable approach [5]. The fluorescence efficiency would be enhanced due to possible energy transfer taking place from sensitizer to activator ions of Eu3+ ion from this method. In order to enhance the red color index and fluorescence efficiency, Bi3+ ions have been undertaken to co-dope along with Eu3+ ions in CaLa2ZnO5 phosphors in our present work. Here, Eu3+ ions and Bi3+ ions are acting as an activator and sensitizer. Interestingly, we obtained some remarkable results in the evaluation of fluorescence, quantum efficiency, lifetimes and CIE coordinates that the potential red
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ACCEPTED MANUSCRIPT emission is improved by co-doping strategy under the visible excitation. Moreover, the observation of the emission features under UV excitation is normal in nowadays. However, we have obtained a remarkable red luminescence characteristics under visible blue excitation and these spectral trends are systematically analyzed. In our earlier report, we have
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synthesized Eu3+/Dy3+: CLZO micro sized phosphor materials by solid state reaction method. But we achieved 66 % of the luminescence quantum efficiency of the red emission pertaining to Eu3+ ions. Surprisingly, 80 % of the luminescence quantum efficiency of the red emission
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pertaining to Bi3+/Eu3+: CLZO nanophosphor spheres have been obtained which are synthesized by citrate sol-gel method in our present work. Compared to our earlier reports on
requirements of photonic devices.
2. Experimental studies
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the phosphor materials, we have been achieved here significant results in all respects of the
Eu3+/Bi3+ ions singly doped and co-doped CLZO nanophosphors were prepared by
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employing traditional citrate sol-gel method. The high purity raw materials Ca(NO3)2 4H2O, La(NO3)3 6H2O, Zn(NO3)2 6H2O, citric acid monohydrate, Eu(NO3)3 5H2O, Bi(NO3)3 5H2O were purchased from Sigma Aldrich and taken in a stoichiometric ratio. These chemicals
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were dissolved in 200 ml distilled water under continuous stirring at room temperature for 2
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h. Then, 8 mol of citric acid monohydrate was added to this solution. The transparent solution was again stirring for 1 h for solution homogeneity. The obtained solution was capped and kept under 80 ˚C of homogeneous heating for 1 h. After the cap was removed and then heating continued for 6 h. We can obtain the brownish wet gel. Then the brownish wet gel was kept under 120 ˚C in a heated oven for 24 h for xerogel. The obtained xerogel was heated with 400 ˚C for 4 h and found black color flakes. Then these black color flakes were sintered at 900˚C for 6 hours. Then, obtained samples were taken for further characterization.
3. Characterization 5
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attached with EDAX system. X-ray photoelectron spectroscopy studies were carried out (Thermo Scientific K-Alpha) using Al Ka X-ray source (1486.6 eV). The transimission electron microscope analysis has been carried out by HR-FE-TEM-(2200FS with CsTEM)
(Spherical
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transition
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Photoluminescence (excitation and emission) spectral analysis of singly doped Eu3+: CLZO,
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and co-doped Eu3++Bi3+: CLZO phosphors were obtained on Scinco FluoroMate FS-2 Visible Fluorescence Spectrometer with Xe- arc lamp of 150 W power as an excitation source for a study state emission spectrum measurement. The lifetime of the red emission transition of the Eu3+ ions, the decay curves was recorded. From the plotted decay curves, we can
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calculate the lifetime of the emission transition by Edinburgh Instruments (Edinburgh, UK) spectrofluorimeter. In order to estimate the absolute luminescence quantum efficiency of the phosphor samples, an integrating sphere equipped with an Edinburgh spectrometer (Model
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FLS900) instrument has been used for measuring the integrated fraction of luminous flux and
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radiant flux with the standard method.
4. Results and Discussion 4.1 Structural details
The X-ray diffraction patterns of the optimized CLZO, Eu3+ (12 mol %), Bi3+ (2 mol
%) and Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO nanophosphor spheres are represented in Fig. 1. The XRD profile of the host CLZO is displayed in Fig.1, all the diffraction peaks for the prepared samples were exactly matched with the reported tetragonal structure which is in good agreement with the earlier reports [6, 7]. No significant changes in the structure were 6
ACCEPTED MANUSCRIPT observed by incorporation of rare earth ions in the basic crystal. This indicates that there is no diffraction peaks from any secondary phase observed. In our present host matrix, the Eu3+ ions are preferably occupied La3+ ion sites due to the same charge and similar ionic radius of 0.1087 Å and 1.032 Å respectively. Smaller Eu3+ ions for La3+ ion sites which can induce the
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shrinking of the lattice. Therefore, a little peak shift has been observed in doped samples. The average crystallite size of the co-doped Eu3+ + Bi3+: CLZO nanophosphor powder sample has been calculated from the Scherrer equation and it is found to be ~165 nm.
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It can be seen that the agglomerated regular spherical surface morphology of codoped Eu3+ + Bi3+: CLZO nanophosphor as shown in Fig. 2 (a). The agglomerated spherical
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particles attributed to homogeneous nucleation and further growth during the formation of nanoparticles [8]. Fig. 2 (a) clearly reveals that there is exists a great deal of well dispersed and nanospheres which are having the diameter ranging from 40 nm to approximately 140 nm. The elements were present in the material which are good consistent with the obtained
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EDAX profile of co-doped Eu3+ + Bi3+: CLZO as shown in Fig. 2 (b). Fourier transform infrared (FTIR) spectroscopy is a resourceful tool to investigate the confirmation information of complex formation and evaluation in the inter molecular interactions within the sample.
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Fig. 3 represents the FTIR specta of host CLZO, Eu3+ (12 mol %), Bi3+ (2 mol %) and Eu3+
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(12 mol %) + Bi3+ (2 mol %): CLZO nanophosphor spheres. The vibrational modes such as bending and stretching vibrations pertaining to corresponding functional groups within the Eu3+and Bi3+ doped CLZO phosphors spheres have been demonstrated systematically through FTIR spectral analysis. We have been observed a strong and moderately weak absorption bands at 3607 cm-1 and 1739 cm-1. These two bans are ascribed to the stretching and bending vibrations of hydroxyl group (-OH) respectively [9]. The band at 1420 cm-1 is observed and it is attributed to asymmetric stretching vibrations of C-O. The residual nitrates and a small amount of organic matter in the sample have been noticed by the appearance of the band at
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could be attributed to the out-plance and in-plane bending vibrations of CO3-2 respectively. We have been observed a sharp FTIR band at the lower wavenumber side at 479 cm-1 which is ascribed to the stretching vibrations of Zn-O [11, 12].
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X-ray photoelectron spectroscopy is performed to determine the chemical composition, binding energy and electronic states of the co-doped CLZO phosphor. XPS
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spectrum for Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO phosphor was recorded in the range between 1350 and 0 eV as shown in Fig. 4 (a). The pertinent peaks at peaks are observed at 348.8, 836.7, 1022.6, 532.5, 1135.2 and 1296.2 eV which are ascribed to Ca 2p, La 3d, Zn 2p, O 1s, Eu 3d and Dy 3d levels respectively. The closure observation of the europium and
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bismuth have been separately shown in Fig. 4 (b&c) which are in good agreement with the earlier reports. The characteristic XPS peaks of the Bi3+ have been observed at 159.08 eV and 164.32 eV in Fig. 4 (c) which could be attributed to 4f7/2 and 4f5/2 [13]. The pertinent bands at
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1164.06 eV, 1134.20 eV and 1128.50 eV are shown in Fig. 4(b). These bands are attributed to
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the presence of trivalent and divalent europium ions, respectively [14, 15]. No other impurities have been identified from the XPS spectrum, which are good consistent with the XRD and EDAX results in earlier. Fig. 5 (a-c) illustrates the TEM image and SAED patterns of the optimized Eu3+ (12 mol %) + Bi3+ (2 mol %): CLZO phosphor. We have been observed an irregular shapes and most of the particles have spherical shape. It is exploring that the uniformity of the size and shape is controlled by homogeneous nucleation. Nucleation and crystal growth continued during the heating process which leads a slight irregular shape and agglomerated particles. The particle size has been evaluated from the TEM as 165 nm which
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polycrystalline in nature [16]. 4.2 Photoluminescence analysis
The excitation spectrum of the singly doped Eu3+(14 mol %): CLZO phosphor as shown
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in Fig. 6. The excitation spectrum consists of several broad excitation bands at 378 nm, 386 nm, 397 nm, 417 nm and 467 nm. These bands are assigned with corresponding electronic
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transitions such as (7F0→5L7), (7F0→5G2), (7F0→5L6), (7F0→5D3) and
(7F0→5D2)
respectively. We have been observed that the broadening of the excitation bands which could be suggests that the Eu3+ ions may be around in a better crystallization environment [17]. Among all the excitation bands, the prominent excitation band is observed at 467 nm which is
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ascribed due to the electronic transition of 7F0→5D2. By using this prominent excitation band at 467 nm we can obtain the emission spectra of Eu3+ doped CLZO phosphor powders. The emission spectra of the Eu3+ ions doped CLZO phosphors at different concentrations of the
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Eu3+ have been recorded under the excitation of 467 nm as shown in Fig.7. We have observed five emission bands from the emission spectra. Among all the emission spectral bands, a
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prominent emission band is noticed at 626 nm from all the Eu3+: CLZO phosphors which originating from the forced electric-dipole transition of 5D0→7F2 of Eu3+ along with moderate intense band of magnetic dipole transition of 5D0→7F1 at 597 nm. Apart from these two transitions, another three low intense transitions also observed at 583 nm (5D0→7F0), 652 nm (5D0→7F3) and 707 nm (5D0→7F4) within the 4f6 configuration of Eu3+. It can be seen that the fluorescence emission intensity has been increased consistently with increasing the Eu3+ ion concentration until 12 mol % of Eu3+ ions. The emission intensity is slowly decreased
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ACCEPTED MANUSCRIPT after 12 mol % of optimized concentration dramatically. This could be ascribed to the concentration quenching effect [18]. Due to the existence of ion-ion interaction provoking by the shorter distance between interacting activators as a concentration increases, the quenching effect could be occurred. However, the forced electric dipole transition of 5D0→7F2 is highly
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sensitive to the local environment. The closure observation of the emission spectral profile of the Eu3+ ions is required for better understanding of the local symmetry which surrounded by the Eu3+ ions in the crystalline lattice. The magnetic dipole transition 5D0→7F1 of orange
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emission regarding Eu3+ ions is dominant than the other transitions, then the Eu3+ ion occupied a crystallographic site with inversion symmetry. In other words, the forced electric
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dipole transition is dominant than the other transitions of Eu3+ ions which could be suggested that the Eu3+ ions are situated in the non-centrosymmetrical site in the crystalline lattice. In our present singly doped Eu3+ doped CLZO phosphors are explored that the Eu3+ ions are occupied in non-centrosymmetrical sites. Moreover, the ratio of the electric dipole and
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magnetic dipole transitions (5D0→7F2 /5D0→7F1) intensity can be used to evaluate the local surroundings of the cation in lanthanide based systems. Here, the ratio between 5D0→7F2 /5D0→7F1 is calculated for singly doped optimized Eu3+(12 mol %): CLZO and co-doped
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Eu3+(12 mol %)+Bi3+ (1mol %): CLZO nanophosphors and these values are found to be 7.76 and 8.07. However, the 5D0→7F2 transition intensity is 7.76 and 8.07 times greater than D0→7F1 in single and co-doped systems. This small ratio between the intensities of 5D0→7F2
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and 5D0→7F1 transitions which could be a direct evidence to the presence of Eu3+ in symmetric site [19]. This favorable improvement of electric dipole and magnetic dipole transitions (5D0→7F2 /5D0→7F1) intensity ratio in co-doped system could be supports the red color purity [20]. The emission and excitation spectral features of singly doped Bi3+(1 mol %): CLZO phosphor has been recorded and they are shown in Fig. 8. Outer ns2 configuration in Bi3+ can
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ACCEPTED MANUSCRIPT play a pivotal role in the field of luminescence. Moreover, most of the researchers have been undertaken the Bi3+ for luminescence sensitization purpose in various luminescence ions doped compounds such as polymers, glasses and phosphors [21]. 6S2 electronic configuration of the Bi3+ can exhibit luminescence properties. However, the photoluminescence
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characteristics of the Bi3+ generally depend on the host matrices. According to the Bi3+ electronic configuration, there are in the ground state of 1S0 and it consists four excited levels, such as 3P0, 3P1, 3P2 and 1P1. However, the transitions of 1S0→3P0 and 1S0→3P2 are
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completely spin forbidden, since the two levels 3P1 and 1P1 are mixed by spin orbit coupling. The spin selection rule in Bi3+ ions is then considerably relaxed by the strong spin-orbit
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coupling. Thus, the possible expected, reasonable absorption could be occurred for the transitions of 1S0→3P1 and 1S0→1P1. Bi3+ doped complexes exhibit the strong absorption and emission bands which are attributed to the transition of 1S0→3P1 and 3P1→1S0. In our present phosphor samples, a broad excitation band has been noticed between 355 - 430 nm which is
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centered at 374 nm (1S0→3P1). We have obtained the emission peak centered at 426 nm under the excitation of 374 nm. This emission peak at 426 nm is ascribed to corresponding electronic transition of 3P1→1S0 [22] as shown in Fig. 8.
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By co-doping with Bi3+ ions to the Eu3+(12 mol %): CLZO nanophosphor, the emission spectral features have been recorded as shown in Fig. 9. We have been observed that
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remarkable enhancement in the red emission (5D0→7F2) pertaining to Eu3+ ions in the codoped system than the singly doped Eu3+(12 mol %): CLZO phosphor. This could be attributed to the possible energy transfer taking place from Bi3+ ions to Eu3+ ions within the host matrix. Here, the Bi3+ ions and Eu3+ ions are acting as sensitizer and activators respectively. In this phenomenon, the successful emission photons of the Bi3+ ions are collectively absorbed by the Eu3+ ions within the host matrices. Therefore, there is a possible condition to excite Eu3+ ions more effectively by absorbing the additional photons from the
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ACCEPTED MANUSCRIPT Bi3+ ions within the same lattice environment. The red emission of the Eu3+ has been significantly enhanced by the energy transfer process until 1 mol % of Bi3+ ion concentration in the co-doped system. Nevertheless, the emission intensities are found to be drastically reduced after 1 mol
% of Bi3+ concentration. This could be due to the concentration
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quenching effect [23] as shown in Fig. 2 (d). This concentration quenching effect could be results of energy transfer caused by more Bi3+-Bi3+ interactions than Bi3+-Eu3+ interactions. From this quite encouraging results, we could be demonstrated that the Bi3+ ions co-doping
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plays a pivotal role in improving the red emission of Eu3+ ions in the co-doped CLZO phosphors.
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Overlapped spectral features between sensitizer and the activator is also a better evidence for the energy transfer phenomenon between sensitizer and activator according to Froster and Dexter energy transfer theories. We have been observed the well overlapped spectral profile between the emission spectrum of Bi3+ ions and excitation spectrum of Eu3+
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ions as shown in Fig. 10. This overlapped spectral profile is strongly suggested that the occurrence of energy transfer from Bi3+ to Eu3+ ions within the host matrix [24]. The energy transfer pathway is clearly demonstrated by an energy level scheme diagram as shown in
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Fig.11. We can evaluate the color confirmation, color purity and energy transfer process
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through Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y). The Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) generated from the emission spectral profiles of the singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and they are found to be (0.6644, 0.3355) and (0.6688, 0.3311) respectively as shown in Fig. 12. By addition of Bi3+, the red color index is significantly increased compared to singly doped Eu3+(12 mol %): CLZO phosphor. Surprisingly, these coordinate values are very close to the standard values of National Television Standard Committee (NTSC) (x=0.67, y=0.33) [25]. It can be seen
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ACCEPTED MANUSCRIPT that the chromaticity coordinates are tuned to more bright red emission under the visible excitation by co-doping with Bi3+ ions. This could be an another evidence for energy transfer from Bi3+ to Eu3+ [26]. Alongside the color coordinates, we have been evaluated the correlated color temperature (CCT) value by using the CIE coordinate values for the final
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product of optimised co-doped phosphor sample. CCT value calculated from the equation as follows
CCT = − 437 n3 + 3601n 2 − 6861n + 5514.31
- - - (1)
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Where ‘n’ represents (x-xe)/(y-ye) which is an inverse slope line, xe and ye are 0.3320 and 0.1858 respectively [27]. The Correlated Color Temperature (CCT) value is calculated and it
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is found to be 3059 K for optimized co-doped phosphor and are located in between 2000 to 5000 K, which could be more preferable red component candidate for warm white light applications.
4.2.1 Lifetime analysis and quantum efficiency
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In order to evaluate the lifetimes of the optimized singly doped and co-doped CLZO phosphor materials, we have been recorded the lifetime decay curves of these phosphors under monitoring excitation and emission wavelengths of 467 nm and 626 nm respectively as
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shown in Fig.13. However, the lifetime of the excited state of Eu3+ ions in the singly doped form is calculated and it is found to be 0.706 ms. The lifetime curve of the excited state of
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Eu3+ ions in singly doped form has been fitted in first order exponential function as follows I = A1 exp(−t / τ 1 ) + I 0
- - - (2)
where I represents the intensity of the emission peak, A and τ are the constant and lifetime respectively. The excited state lifetime has been calculated pertaining to Eu3+ ions in the form of co-doped system and it is found to be 1.526 ms. Therefore, this remarkable enhancement of the emission lifetime pertaining to Eu3+ ions could be occurred due to the energy transfer process taking place from Bi3+ to Eu3+ in the co-doped host matrix. This possibility of the
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ACCEPTED MANUSCRIPT lifetime enhancement above two times in co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO phosphors than the singly doped Eu3+(12 mol %): CLZO nanophosphor spheres are strongly suggesting that the energy transfer taking place between bismuth and europium ions. Moreover, the dynamics of the co-doped lifetime decay of the Eu3+(12 mol %)+Bi3+(1 mol
exponential decay mode using the equation as follows I = A1 exp(−t / τ 1 ) + A2 exp(−t / τ 2 )
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%): CLZO nanophosphor sphere is depicted in Fig. 13 and it is well fitted in a second order
- - - (3)
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where I represent emission intensity, A1 and A2 are constants. τ1 and τ2 are the two exponential components. Because of the existence of the same charge of lanthanum,
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europium and bismuth; it can be possible to expect that the impurities of Eu3+ and Bi3+ ions are occupied the lanthanum sites within the crystalline lattice. The average lifetime of the codoped system is calculated from the equation as follow
τ = ( A1τ 12 + A2τ 22 ) / ( A1τ 1 + A2τ 2 )
- - - (4)
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From this non-exponential nature of the lifetime decay profile of the co-doped system, it can be believed that the existence of possible energy transfer from Bi3+ to Eu3+ in the CLZO host matrix. This energy transfer enhances the emission intensity as well as the lifetime of the
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excited state which is more favorable result for the application point of view [28]. Moreover,
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we have been evaluated the luminescence quantum efficiency of the singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres and are included in Table 1. Compared to our recent report [29], we have achieved a larger quantum efficiency for singly doped and co-doped phosphors. However, the luminescence quantum efficiency pertaining to singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres are found to be QE=56 % and QE=82 % respectively. The quantum efficiency (QE) of our co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres under the excitation of 467 nm is 82 %, which is higher 14
ACCEPTED MANUSCRIPT than that of commercial red phosphors Y2O3: Eu3+ (9.6 %, 394 nm excitation) and Y2O2S: Eu3+ (4.2 %, 395 nm excitation) [30]. Here, we have been observed the successful enhancement in the luminescence quantum efficiency of the Eu3+ ions by co-doping with Bi3+ ions in the CLZO matrix compared to earlier results. It can be strongly supports the energy
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transfer phenomenon between Bi3+ and Eu3+ ions unambiguously. The energy transfer phenomenon is well substantiated by photoluminescence, overlapped spectral profiles, lifetime decay dynamics, CIE color coordinates and quantum yield measurements. Therefore,
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these high quantum efficiency (QE=82%) existed co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres material could be a potential candidate for red luminescence
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optical devices.
3 Conclusion
In summary, we have successfully synthesized singly doped Eu3+, Bi3+ and co-doped CaLa2ZnO5 (CLZO) nanophosphor spheres by citrate sol-gel method. The tetragonal
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structure was confirmed from the XRD analysis of all prepared samples. The complex formation and the interaction between ion and host matrix were clearly demonstrated by FTIR spectral studies. The morphological studies have been carried out from FE-SEM
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analysis. EDAX and XPS studies were performed to evaluate the elemental presence and the binding energies confirmation respectively. A strong red emission at 626 nm (5D0→7F2) was
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observed in Eu3+: CLZO nanophosphors under the visible excitation of 467 nm alongside the existence of some other moderate intense emission bands. The red emission was enhanced with increasing the Eu3+ ion concentration until 12 mol % and it was drastically reduced after 12 mol % dramatically. This could be attributed to the concentration quenching effect. Noteworthy, the red emission was significantly enhanced by co-doping with Bi3+ ions in CLZO nanophosphor due to energy transfer from Bi3+ ions to Eu3+ ions. Nevertheless, the predominant red emission intensity was observed for co-doped Eu3+ (12 mol %) + Bi3+ (1
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ACCEPTED MANUSCRIPT mol %): CLZO nanophosphor than the other samples. Hence, the optimized sensitizing concentration of Bi3+ ions was found to be 1 mol %. Commission International de I'Echairage (CIE) 1931 chromaticity coordinates (x-y) evaluated for singly doped Eu3+(12 mol %): CLZO and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres as (0.6644,
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0.3355) and (0.6688, 0.3311) respectively, which were well closure to the standard white standard values of National Television Standard Committee (NTSC) (x=0.67, y=0.33) surprisingly. The Correlated Color Temperature (CCT) value was evaluated as 3059 K for co-
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doped sample and it was located in between 2000 to 5000 K, which could be a more preferable red component for warm white light applications. The lifetime and quantum
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efficiency of the optimized co-doped sample have been evaluated as 1.526 nm and 82 % respectively. Moreover, these lifetime and quantum efficiency values were significantly enhanced upon co-doping with Bi3+ ions to the Eu3+ doped CLZO phosphors. The energy transfer phenomenon from Bi3+ to Eu3+ ions was clearly elucidated by several fluorescence
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dynamics such as overlapped spectral studies, photoluminescence spectral features and CIE color coordinates and quantum efficiency. These ravishing red luminescent nanophosphor spheres materials could be suggested as a promising candidate for solid state illumination and
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optical display devices.
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Acknowledgement
This work was supported by the 2015 Yeungnam University Research Grant.
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Table 1 Excitation, emission, lifetime and quantum efficiency values of the singly doped Eu3+(12 mol %): CLZO phosphor and co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO phosphors. Emission (nm) 626 626
Lifetime (ms) 0.706 1.526
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Eu3+(12 mol %): CLZO Eu3+(12 mol %)+Bi3+(1 mol %): CLZO
Excitation (nm) 467 467
Quantum efficiency 56% 82%
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Phosphor sample
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Figure Captions Fig.1 XRD profiles of (a) CLZO, (b) Eu3+(12 mol %): CLZO, (c) Bi3+(1 mol %): CLZO and (d) Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO nanophosphor spheres. Fig. 2 (a) FE-SEM image and (b) EDAX of Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO
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nanophosphor spheres. Fig.3 FTIR spectra of (a) CLZO, (b) Eu3+(12 mol %): CLZO, (c) Bi3+(1 mol %): CLZO and (d) Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO nanophosphor spheres.
Fig.4 XPS survey spectra of co-doped Eu3+ (12 mol %) + Bi3+ (1 mol %): CLZO phosphor in
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the range of (a) 0-1300 eV, (b) 1115-1145 eV and (c) 150-175 eV.
Fig. 5 (a) TEM, (b) HR-TEM and (c) SAED patterns of co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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Fig. 6 Excitation spectrum of Eu3+ (12 mol %): CLZO phosphor.
Fig. 7 Emission spectra of Eu3+: CLZO phosphors at different concentrations of Eu3+ ions. Fig. 8 (a) Excitation and (b) emission spectra of Bi3+ (1 mol %): CLZO nanophosphor spheres.
Fig. 9 Emission spectra of co-doped Eu3+ (12 mol %) + Bi3+ (0.5, 1, 2, 3 and 4 mol %):
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CLZO nanophosphor spheres.
Fig. 10 Overlapped spectral profile of emission spectra of Bi3+ (1 mol %): CLZO and excitation spectra of Eu3+ (12 mol %): CLZO nanophosphor spheres. Fig.11 Partial energy level scheme diagram of energy transfer from Bi3+ to Eu3+.
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Fig. 12 CIE chromaticity diagram for (1) Eu3+(12 mol %): CLZO (2) co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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Fig. 13 Lifetime decay dynamics of (a) Eu3+(12 mol %): CLZO (b) co-doped Eu3+(12 mol %)+Bi3+(1 mol %): CLZO nanophosphor spheres.
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ACCEPTED MANUSCRIPT Highlights: Dazzling red emission is observed from Eu3+/Bi3+: CaLa2ZnO5 nanophosphors. Red emission of Eu3+ was significantly enhanced by co-doping with Bi3+. Effective sensitization of Bi3+ ions has been elucidated.
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The energy transfer pathway was clearly demonstrated from Bi3+ to Eu3+.
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CIE coordinates are very close to National Television Standard Committee (NTSC).
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