Prominent blue emission through Tb3+ doped La2O3 nano-phosphors for white LEDs

Accepted Manuscript 3+ Prominent blue emission through Tb doped La2O3 nano-phosphors for white LEDs Neha Jain, Rajan Kr Singh, Amit Srivastava, S.K. Mishra, Jai Singh PII:

S0921-4526(18)30206-0

DOI:

10.1016/j.physb.2018.03.014

Reference:

PHYSB 310775

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 7 February 2018 Revised Date:

6 March 2018

Accepted Date: 7 March 2018

Please cite this article as: N. Jain, R.K. Singh, A. Srivastava, S.K. Mishra, J. Singh, Prominent 3+ blue emission through Tb doped La2O3 nano-phosphors for white LEDs, Physica B: Physics of Condensed Matter (2018), doi: 10.1016/j.physb.2018.03.014. 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

Prominent Blue Emission through Tb3+ doped La2O3 Nano-phosphors for White LEDs

RI PT

Neha Jain1, Rajan Kr. Singh1, Amit Srivastava2,3*, S.K.Mishra4, Jai Singh1* 1

Department of Physics, Dr. Harisingh Gour University, Sagar (M.P.)-470003, India

2

Department of Physics, TDPG College, VBS Purvanchal University, Jaunpur (U.P.)-

3

SC

222001,India

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida

Quality Council of India, DIPP, Ministry of Commerce & Industry, Govt. of India, New Delhi

**Corresponding authors: E-mail- * [email protected], *[email protected]

EP

TE D

Tel- 9424459805

AC C

4

M AN U

32306-4390, United States

1

ACCEPTED MANUSCRIPT

Abstract: In this article, we report the tunable luminescence emission of Tb3+ doped La2O3 nanophosphors synthesized by a facile and effective Polyol method. The structural and surface morphological studies have been carried out by employing X-ray diffraction (XRD) and

RI PT

scanning electron microscopy (SEM), respectively. The XRD studies elucidate the proper phase formation and the results emanate from Raman spectroscopy of the as synthesized nanophosphor affirms it. The optical properties of the as fabricated nanoparticles have been investigated by

SC

Raman and photoluminescence (PL) spectroscopy. The PL spectroscopy shows the occurrence of excitation peaks at 305, 350 and 375 nm for 543 nm emissions, correspond to transition 5D4

M AN U

→7F5. Emission spectra with 305 nm excitation exhibits characteristic emission peaks of Tb3+ion at 472, 487, 543 and 580 nm. The intensity of emission increases with Tb3+ concentration and is most prominent for 7 at% Tb3+ ion. The characteristic emissions of Tb3+ ion owes to the transition in which intensities of blue and green emission are prominent. The

TE D

dominant intensity has been found for 472 nm (for blue emission). Commission international d 'Eclairage (CIE) co-ordinates have found in the light blue to green region. The research work provides a new interesting insight dealing with tunable properties with Tb3+ doping in La2O3

EP

nanophosphors, to be useful for for display devices, solar cells, LEDs and optoelectronic devices.

AC C

Keywords: Photoluminescence; CIE co-ordinate; Phosphor, La2O3

2

ACCEPTED MANUSCRIPT

IntroductionIn recent years, there has been an upsurge in the research on rare-earth doped inorganic

RI PT

nanophosphors, owing to their interesting electronic and optical properties with emphasis on quantum confinement, long fluorescent life span, high stability against photo bleaching and multiple emission bands from UV to IR etc [1-5]. The advancement in the research led to varied in solid state lasers, high

SC

applications of these inorganic luminescent materials such as

performance display devices, optical storage, light emitting diodes, lamp phosphors, optical fiber

M AN U

communication system and other photonic devices [6-8].The quantum luminescence efficiency and other relevant parameters of nanophosphors mainly depend on the characteristics particularly particle size, surface morphology, concentration quenching and crystallinity of the host material [9]. Therefore, phosphors can be tuned precisely with different emission wavelengths by controlling the morphologies through doping of activators into the host matrix. Some studies

TE D

have been focused on composition dependent tunability with rare earth (RE) activated nanophosphors [10-11]. Among various RE metal oxides, La2O3 is considered as one of the promising host primarily due to its large band gap of ~4.3 eV , transparent to infrared & visible radiation

EP

and secondarily due to its low phonon energy content (600 cm-1) which in turn reduces the

AC C

possibility of non-radiative transitions (NR) [12-13] . RE-doping has widely used as activator to the host La2O3, as it possesses high chemical stability, broad optical absorption and prominent PL characteristics, which stems the possibilities to use it in opto-electronic and display devices [1415].

Rai et al. has reported the effect of calcinations temperature on PL emission of Yb3+/Er3+ co-doped La2O3 nanophosphor particles synthesized through solution combustion method [16].

3

ACCEPTED MANUSCRIPT

Park et al. have investigated the concentration quenching effects on La2O3:Eu3+ phosphor prepared by sol-gel technique [17]. Zou et al. have prepared nanocrystalline Tb3+ doped La2O3 phosphors through a Pechini-type sol-gel process and showed that the CL colors of La2O3:Tb3+

RI PT

phosphors was tuned from blue to green by varying the concentration of Tb3+ to some extent [18]. Tb3+ doped phosphors show color tuning emission properties owes to its two electronic excited state 5D3 and 5D4. The emission subjectively depends on concentration extents. The

SC

emission tunes from blue to green due to mixing of these two transitions. Besides, the atomic radii of La3+ and Tb3+ ions are nearly comparable which inevitably resolve the issue of NR

M AN U

transition occurred from the mismatch of ionic radii of dopant and host. Some research showed that Tb3+ doping upto 1 at% in oxides, gives prominent blue emission which further decreases and beyond this concentration green emission dominates [18-20]. This owes to the cross relaxation between two Tb3+ ions by resonant energy transfer mechanism. The enhancement in

TE D

green emission by Bi3+ doping in La2O3 has also been reported [21]. However, there are very sparse studies over the tunable emission in La2O3 with varying Tb3+ concentration. Herein, the present study, x at% Tb3+ (x=2, 5, 7, 10) doped La2O3 has been prepared by a

EP

low cost and facile Polyol method. This method has found to be a simple and effective route for the synthesis of nano-particles as Polyol acts as a surfactant and controls the morphologies of the

AC C

particles. The phase purity and structural analysis have been carried out through X-ray diffraction and Raman spectroscopy. The size and morphological studies of the as- synthesized nano-phosphor particles have been done by employing X-ray diffraction and scanning electron microscopy. Photo-luminescent emission and excitation have been investigated to study the optical properties of the sample, in order to search for the viability to tune the emission with Tb3+ doping in La2O3 nano-phosphors. 4

ACCEPTED MANUSCRIPT

Experimental Materials Synthesis

RI PT

Tb3+ (x= 2, 5, 7, 10 at%) doped La2O3 nanophosphors were synthesized by a polyol method. The starting materials used for the studied phosphors were La2O3 (99.9%, Alfa Aesar), Tb2O3 (99.98%, Alfa Aesar), ethylene glycol (Merck), urea (Merck) and HNO3 (69%) . Firstly, La2O3

SC

and Tb2O3 were dissolved in de-ionized water separately with few drops of dil. HNO3 , resulting in the formation of a colorless solutions of the respective nitrates. The as-obtained nitrates were

M AN U

mixed together while stirring at 80ºC . Further, 2 g urea and 25 ml ethylene glycol were added in the resultant solution to maintain its pH. The solution was further heated at 120ºC while stirring for 4 h. Initially, the solution was clear and transparent which turned out to white precipitate. The precipitate was rinsed, washed extensively with de-ionized water and methanol to remove the

TE D

impurities and other constituent residues. As received powder sample was dried in an oven at 120ºC, and thereafter ground thoroughly in agate mortar. Further, the dried powder was calcined at 850 ºC. Finally, the prepared phosphors were cooled to room temperature and reground for

AC C

Characterization

EP

further investigations.

The powder X-ray diffraction (XRD) measurements were performed on D8 Focus diffractometer (Bruker ) with operating at 40kV and 40 mA with Cu Kα radiation (λ=1.5406 Aº) . The step size was fixed at 0.02° with 2θ ranges from 10° to 60°. The morphologies of the samples were inspected by employing a field emission scanning electron microscope (FE-SEM, Model- NOVA NANO 450). The Raman spectra of the samples were collected using a Renishaw micro-Raman spectrometer attached with a laser excitation source of 633 nm, under ambient condition. The 5

ACCEPTED MANUSCRIPT

photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by HORIBA, PTI fluorescence spectrometer equipped with a photomultiplier to be operated at 400 V and a 150 W Xenon lamp as the excitation source. All the measurements were carried out at

RI PT

room temperature. Results and discussion

SC

Figure 1 shows the typical powder X-ray diffractogram patterns of La2O3 doped with x at% Tb3+ (x=2, 5, 7, 10). All peaks are fairly indexed and matches well with the standard file of

M AN U

La2O3 (JCPDS card 73-2141). They are attributed to the pure hexagonal phase having space group P m1 (164). There are no occurrence of extra peaks in the diffractogram pattern with increasing Tb3+ incorporation which reflects that Tb3+ has successfully substituted La3+ ions. The lattice constant and cell volume get varied with the incorporation of Tb3+ due to the difference in

TE D

ionic radii of La3+ (1.036Å) and Tb3+ (0.923 Å). The variation in lattice parameters has been calculated by using unit cell software and summarized briefly in Table 1. The Table 1 clearly shows that the cell volume increases with increasing Tb3+ concentration. Furthermore, the

EP

change in lattice parameters leads to crystal deformation or strain. The micro-strain of the systems has been evaluated by using Williamson Hall plot [22] which varies from 0.0087 to

AC C

0.0045 and decreases with increase of Tb3+ contents in La2O3 host. The crystallites size has been calculated by using Scherer formula D =

where λ, θ and β represent wavelength of X-rays

used, diffraction angle and FWHM, respectively. It has found to be 26, 27, 30 and 31 nm for 2, 5, 7 and 10% Tb3+ respectively , in La2O3. Thus with Tb3+ incorporation, the crystallites sizes get improved and the corresponding micro-strains decreased [23].

6

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 1 (a) XRD pattern of Tb3+ (2, 5, 7 and 10%) doped La2O3 indexed according to JCPDS

TE D

card 73-2141 and (b) Williamson- Hall plot to calculated micro-strain.

Table 1: Lattice constant, cell volume and micro-strain for 2, 5, 7 and 10 at% Tb3+ for La2O3

2

Cell volume (Å3)

Micro-strain

3.924

6.122

81.646

0.0087

3.942

6.146

82.723

0.0066

AC C

5

c (Å)

a (Å)

EP

Tb3+ conc. (at. %)

7

3.942

6.145

82.718

0.0045

10

3.955

6.165

83.507

0.0060

Figure 2(a) represents the typical SEM image of as synthesized powder sample. It reveals spherical morphology with evenly distributed particles. The particles have been estimated to lie 7

ACCEPTED MANUSCRIPT

in the range 40-70 nm. Figure 2(b) illustrates the Raman spectra of La2O3 nanophosphor, which shows the occurrence of several bands at 293, 394, 400 and 595 cm-1 , are basically due to La-O vibrational modes [20, 12-13]. Raman spectra are found to be in consistence with XRD

RI PT

observations which eventually confirm the proper phase formation of La2O3. It could also be noticed that the La-O vibrations are present in lower wave number region which indicate lower

TE D

M AN U

SC

phonon energy of La2O3 host.

EP

Fig. 2 (a) SEM micrograph and (b) Raman spectra of 5% Tb doped La2O3.

AC C

Figure 3(a) describes the PL excitation spectra of La2O3:10 at% Tb3+, as recorded by

monitoring 543 nm emission (corresponding to 5D4→7F5 transition). It consists of broad peaks centered at 305, 350 and 375 nm due to various electronic transition excitations. The peak located at 305 nm is basically due to the spin allowed electronic transition from 4f8 to 4f75d transition [2] whereas other peaks (350 and 375 nm) are due to 4f-4f transition of Tb3+ ion (7F6→5D2 and 7F6→5D3, respectively) [14]. The emission spectra has been recorded under 305 8

ACCEPTED MANUSCRIPT

nm excitation since this excitation gives 4f-5d excitation. Figure 3 (b) shows the occurrence of several peaks at 472, 487, 543 and 580 nm which owe to 5D3→7F2, 5D4→7F6, 5D4→7F5 and 5

D4→7F4 transitions, respectively [19, 14]. The noteworthy change in emission peaks (472 and

RI PT

543 nm) has also been observed with varying Tb3+ doping concentration level. This substantial change is due to the energy transfer between two Tb3+ ions. Although, in most of the rare earth doped system, energy loss has been found due to the mismatch of ionic radii and oxidation states

SC

of dopant and host [15]. However, in the present work, this issue has been resolved, as XRD analysis shows that Tb3+ has successfully substituted La3+ ions without creating any remarkable

M AN U

change in the crystal, defects or vacancies in the crystal. So the possibility of rate of nonradiative relaxation gets reduced and the intensity of 472 nm emission dominates over other emission peaks. These results attribute to the fact that it is good sensitizer for other rare earths which provides emission through quantum cutting process [25]. Moreover, La2O3 has low

TE D

phonon energy as observed in Raman spectra, would also be a reason for prominent blue emission from Tb3+ ion. Meanwhile, the intensity of 472 nm emission peak decreases more beyond 7 at% concentration of Tb3+ ion and the intensity of 543 nm emission increases. It is

EP

basically due to NR cross relaxation between 5D4 and 5D3 states of Tb3+ ions, as energy levels separation between 5D4 and 5D3 states is comparable to the separation between 7F6 and 7F0 states,

AC C

which infer that for higher Tb3+ concentration (~10%) cross relaxation via resonant energy transfer takes place. The intensity of 7% Tb3+ doped La2O3 is found to be 1.5 times higher than 10% Tb3+ doped nano-phosphor. The variation in these two peaks of 472 and 543 nm emissions causes color tuning in CIE co-ordinate.

9

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3 (a) PL excitation spectra of 10 at% Tb3+ doped La2O3 by monitoring 543 nm (5D4→7F5)

TE D

and (b) PL emission spectra of 2, 5, 7 and 10 at% Tb3+ for La2O3.

Figure 4 (a) pronounces the CIE chromaticity diagram for various Tb3+ (2, 5, 7, 10 at %) doped La2O3. These are found to be (0.25, 0.37), (0.24, 0.44), (0.22, 0.38) and (0.22, 0.39) for 2,

EP

5, 7 and 10 at% Tb3+, respectively. Figure 4 (b) represents the schematic energy level diagram of Tb3+ electronic energy states. It is clearly observable that for exciting 305nm, electrons are

AC C

populated from 4f8 ground state to 4f75d excited state. From the following state, electrons transit to either 5D3 or 5D4 state via NR transition process. Moreover, radiative emissions are observed due to transition of electron from 5D3 and 5D4 state to ground state (7Fj, j = 6, 5, 4, 3, 2, 1). It has already been said that for 10 at% Tb3+ doped, PL intensity of peak corresponding to 5D3→7F2 transition is slightly decreased while the intensity of the peak corresponding to 5D4→7F5 has increased due to cross relaxation between Tb3+ ions. This takes place as the energy gap between 10

ACCEPTED MANUSCRIPT

5

D4 and 5D3 state is nearly equal to the energy gap between 7F6 and 7F0, so the resonant energy

M AN U

SC

RI PT

transfer occurs between those energy levels.

Fig. 4 CIE Chromaticity diagram for 2, 5, 7 and 10 at% Tb3+ for La2O3 under 305 nm excitation

EP

Conclusion

TE D

and (b) Schematic energy level diagram for various Tb3+ characteristic emissions.

In summary, we have demonstrated a simple, facile and effective Polyol approach to synthesize

AC C

Tb3+ doped La2O3 nanophosphor materials. The X-ray diffractogram pattern of Tb3+ doped La2O3 indicates the successful substitutions of Tb3+ at La3+ ions. The crystallinity of the sample has been improved with the incorporation of Tb3+ at the respective site, led micro-strain to decrease, which primarily affect the photoluminescence properties. The PL intensity has been improved with Tb3+ incorporation. The attributed emissions of Tb3+ ion are obtained due to transition in which intensity of blue and green emission is prominent. There are two emission peaks intensity 11

ACCEPTED MANUSCRIPT

(~472 and ~543 nm) which gets altered with Tb3+ doping. The emission has mainly been observed in blue region confirmed by CIE chromaticity co-ordinate. Taken together, these attractive results, the research work provides a new interesting tunable properties with Tb3+

RI PT

doping in La2O3 nanophosphors, to be useful for display devices, solar cells, LEDs and optoelectronic devices.

SC

Acknowledgements

Authors are thankful to Sophisticated Instrument Centre (SIC) of the University for providing

M AN U

various characterization facilities. We are also thankful to Dr. Kaushal Kumar for providing PL facility. One of the authors (Neha) acknowledges the Maulana Azad National Fellowship (MANF) provided by University Grants Commission (UGC), Govt. of India. Jai Singh would like to acknowledge UGC-India and DST for providing project under DST Fast track Grant no.

References:

TE D

SR/FTP/PS [HYPHEN] 144/2012.

1. S. Balaji, G. Gupta, K. Biswas, D. Ghosh, K. Annapurna, Scientific Reports. 6 (2016).

EP

2. D. Yin, C. Wang, J. Ouyang, X. Zhang, Z. Jiao, Y. Feng, et al., ACS Applied Materials & Interfaces. 6 (2014) 18480–18488.

AC C

3. M. Wang, W. Lin, N. Liu, Y. Ye, J. of Lum. 194 (2018) 682–685. 4. N. Jain, B.P. Singh, R.K. Singh, J. Singh, R. Singh, J. of Lum. 188 (2017) 504–513. 5. J. Lu, K.I. Ueda, H. Yagi, T. Yanagitani, Y. Akiyama, A.A. Kaminskii, J. of Alloys and Compounds. 341 (2002) 220–225. 6. P.K. Shahi, P. Singh, S.B. Rai, A. Bahadur, Inorganic Chem. 55 (2016) 1535–1541.

12

ACCEPTED MANUSCRIPT

7. H. Hosono, M.Hirano, H. Ota, M. Orita, H. Hiramatsu, K. Ueda, (2008). U.S. Patent No. 7,323,356. Washington, DC: U.S. Patent and Trademark Office. 8. J.W. Dawson, P.H. Pax, G.S. Allen, D.R. Drachenberg, V.V. Khitrov, N. Schenkel, et

RI PT

al., Optics Express. 24 (2016) 29138-29152.

9. B.P. Singh, A.K. Parchur, R.S. Ningthoujam, A.A. Ansari, P. Singh, S.B. Rai, Dalton Trans. 43 (2014) 4779-4789.

SC

10. M. Yang, Y. Liang, Q. Gui, B. Zhao, D. Jin, M. Lin, L. Yan, H. You, L. Dai, Y. Liu, Sci. Rep. 5 (2015) 11844.

Chem. C, 4 (2016) 4186-4192.

M AN U

11. H. Dong, L. D. Sun, Y. F. Wang, J. W. Xiao, D. Tu, X. Chen, C. H. Yan, J. Mater.

12. C. Gautam, A. K. Yadav, V. K. Mishra, K. Vikram, Open J. of Inorganic Non-metallic Mater. 2 (2012) 47-54.

TE D

13. J.H. Denning, S.D. Ross, J. Phys. C: Solid State Phys. 5 (1972) 1123-1133. 14. B.S. Reddy, S. Budhudu, Indian J. of pure and Appl. Phys. 45 (2007) 496-500. 15. P. P. Pal, J. Manam, J. of Lum. 145 (2014) 340–350.

EP

16. R. Dey, V. K. Rai, Dalton Trans. 43 (2014) 111–118. 17. J. K. Park, S. M. Park, C. H. Kim, H. D. Park, S. Y. Choi, J. Mater. Sci. Lett., 20 (24)

AC C

(2001) 2231–2232.

18. X. Liu, L. Yan, J. Zou, J. Electrochem. Soc. 157 (2) (2010) 1-6. 19. X. Li, Y. Zhang, D. Geng, J. Lian J, G. Zhang, Z. Hou, J. Lin, J. Mater. Chem. C 2 (2014) 9924-9933.

20. Z. Xu, S. Bian, J. Wang, T. Liu, L. Wang, Y. Gao, RSC Adv. 3 (2013) 1410-1499.

13

ACCEPTED MANUSCRIPT

21. P. Reddy, C. S. Kamal, K. Sujatha, T. Samuel, M. Indiradevi, Y. R. Krishna, K. R. Rao, Internat. J. of Chem. Tech Res. 8 (12) (2015) 741-751. 22. G.K. Willamson, W.H. Hall, Acta Metall. 1 (1953) 22.

RI PT

23. D. Prakashbabu, H.B. Ramalingam, R.H. Krishna, B.M. Nagabhushana, R. Chandramohan, C. Shivakumara, J. Thirumalai, T. Thomas, Phys. Chem. Chem. Phys. 18 (2016) 29447-29457.

SC

24. G. Li, C. Peng, C. Zhang, Z. Xu, M. Shang, D. Yang, X. Kang, W. Wang, C. Li, Z.

AC C

EP

TE D

M AN U

Cheng, J. Lin, Inorg. Chem. 49 (22) (2010) 10522–10535.

14