Effect of Rare Earth Doped Elements and

Advanced Materials Research Vol. 585 (2012) pp 174-178 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.585.174

Effect of Rare Earth Doped Elements and Characterization of LaF3:Ln3+ (Ln3+= Ce3+, Pr3+, Nd3+) Nanocrystals S G Gaurkhede 1, a, M M Khandpekar 2,b, S P Pati 3,c, A T Singh4,d 1

Department of Physics, Bhavan’s College of ASC, Andheri (W) Mumbai-400058.India Material Research Labs, Department of Physics, Birla College, Kalyan – 421304.India 3 National Institutes of Science and Technology, Palur Hills, Behrampur-761008, Odisha. India 4 Department of Physics K M Agarwal College of ASC, Kalyan-421301.India 2

*

Email : [email protected]

Keywords: Inorganic elements, Nanorods, X-RAY, IR, TEM, Luminescent materials

Abstract. LaF3 nanocrystals doped with lanthanides like Ce3+, Pr3+ and Nd3+ have been prepared using microwave technique. These synthesized crystals have been characterized by X-ray powder diffraction. Well dispersed, elongated, nanorods of hexagonal geometry (approximately 20nm in size) have been found in TEM analysis. The average particle size estimated from XRD analysis is about 20 nm. Similar results for the average size are observed from TEM studies. Four characteristic peaks one at 3434 cm-1 (broad) and other at 2924, 2853, 1632 cm-1(sharp) have been observed in the FTIR spectra. Intense Blue colour (458 nm) emission has been recorded when crystals are excited with photons of wavelength 254 nm. Non Linear Optical (NLO) properties of the synthesized nanocrystals have been studied. It has been found that second harmonic generation (SHG) efficiency of the prepared samples containing rare earth elements is less than pure Potassium dihydrogen phosphate (KDP) crystals. Introduction In recent years, the field of luminescence and display materials has undergone a revival of sorts with the evolution to nano-size luminescent particles with properties that differ from those of identical bulk materials as the size of the particle is reduced to the nanometer region [1]. Studies on the luminescent properties of lanthanide-doped nanoparticles has attracted a great deal of interest since they are considered as potentially useful phosphors in lamps and display devices, components in optical telecommunication[2],active materials in lasers[3], new optoelectronics devices[4], up converters [5-6], magnetic resonance imaging (MRI) [7], biological fluorescent labels [8-9]. LaF3 is widely used in lubricants, additive of steel and metal alloy, electrode materials [10] chemical sensors and biosensors [11]. LaF3 possesses low phonon energy, adequate thermal and environmental stability [12], and hence regarded as an excellent host matrix [13-14] for performing luminescence. Nanoparticles of LaF3, doped with lanthanides have been studied in the past for their luminescence properties [15]. In past several investigations have been carried to study the optical properties of LaF3:Nd3+ [16] for their possible applications in optoelectronics devices. In the past two decades, the use of microwave technique has attracted a considerable amount of attention, owing to its successful application in organic and inorganic synthesis of nanomaterials, material sciences, polymer chemistry, nanotechnology, and biochemical processes. In many circumstances, the use of microwave dielectric heating as a non-classical energy source has been shown to dramatically reduce processing times, increase product yields, and enhance product purity or material properties compared to conventionally processed experiments and hence is favored here for synthesis of LaF3 lanthanide doped nanocrystals.

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 182.48.252.253-08/11/12,14:37:02)

Advanced Materials Research Vol. 585

175

Experimental Synthesis of nanocrystals Rapid synthesis of LaF3 nanocrystals has been carried out in a microwave oven, operating at a frequency 2.45 GHz. Initially 10 ml solution of 0.064 mol each of LaCl3.7H2O, CeCl3.7H2O, PrCl3.6H2O and NdCl3 is prepared in a 100ml beaker. 0.768mol of NH4F was dissolved in 10 ml of de-ionized water in another beaker. An arrangement has been made so that the latter solution can be mixed (in molar ratio 1:3) [17] with the former solution drop by drop in a microwave oven set for 30 minute at a low power range in an on-off mode set at an interval of 30sec. As both the solutions mix with each other white ultrafine crystalline precipitate is formed indicating presence of LaF3 nanocrystals. The high power setting of microwave oven is not used since the solution has a tendency to overflow from the beaker thus spoiling the synthesis process. The white precipitate settling at the bottom is then washed several times with water and absolute methanol, and subsequently dried in microwave oven for 15 minutes to get the final product in crystalline powder form which is stored in sealed tubes for further characterization. Characterization Powder x-ray diffraction(XRD) measurements have been performed using a PANALYTICAL X’PERT PROMPD diffractometer model using CuKα radiation λ =1.5405 A0 with scanning rate of 2° per min in the 2Θ range from 0° to 80°. Transmission electron microscope (TEM) analysis has been carried out for different magnification by PHILIPS (CM 200) 0.24 nm resolution at 200kV. The IR spectroscopy is also carried out by using Fourier transform technique using model Spectrum one: FT-IR Spectrometer, Scan Range: MIR 450-4000 cm-1, Resolution: 1.0 cm-1.The fluorescence spectrum has been measured on LS 45 luminescence spectrometer (Perkin Elmer Corp) using FL Win Lab™ software and a high energy pulsed Xenon source for excitation. NLO studies for the measurements of SHG efficiency, is obtained through the crystalline powder sample by using Kurtz and Perry technique. Result and Discussion

Fig.1. X-ray diffraction pattern of LaF3: Ce3+, Pr3+, and Nd3+ nanocrystals Fig.1 shows the XRD results in which indicate that LaF3: Ce3+ Pr3+ Nd3+ nanoparticles diffraction patterns exhibit the sample crystallized in hexagonal LaF3 with spatial group P3c1. The patterns are in good agreement with hexagonal structure with cell parameters a=b=0.7187nm and c= 0.735nm, α=β=90º, γ =120º known for bulk LaF3 (JCPDS card No.32-0483) [18]. The calculated cell parameters a=b=0.7126 nm and c=0.7255 nm for the LaF3 Ce3+ Pr3+ Nd3+nanoparticles, are smaller

176

Advances in Materials and Processing

than those of LaF3 nanoparticles (a=b= 0.7187 nm and c=0.735 nm). The decrease in the lattice parameters of LaF3: Ce3+ Pr3+ Nd3+nanoparticles can be attributed to the smaller radius of Ce3+ ion(1.02nm), Pr3+ ion (0.99nm), Nd3+ ion (0.99 nm) as compared with that of La3+ ion (0.106 nm) [19]. This indicates that Ce3+ ions, Pr3+ ions and Nd3+ ions are doped into the LaF3 lattice and occupied the site of La3+ ions, with the formation of a LaF3: Ce3+ Pr3+ Nd3+ solid solution [20].The broadening of diffraction peaks for LaF3: Ce3+ Pr3+ Nd3+ nanoparticles is also shown in Fig.1, which reveals the nanocrystalline size of the samples. According to Scherrer equation, D=0.90λ/βcosΘ, where D is the average crystallite size, λ is the x-ray wavelength (0.15405 nm); Θ and β being the diffraction angle and full width at half maximum of an observed peak, respectively. After subtraction of the equipment broadening, the full width at half maximum (FWHM) of the strongest peak (111) at 2θ =27.78º helps to calculate the average crystalline size of LaF3: Ce3+ Pr3+ Nd3+ nanoparticles to be approximately 12-20 nm.

Fig. 2.TEM image of Ce3+, Pr3+, Nd3+ doped Fig. 3. FTIR pattern of LaF3: Ce3+, Pr3+, Sm3+ doped LaF3 nanocrystals nanocrystals Fig.2 shows the transmission electron microscopy (TEM) image of LaF3:Ce3+, Pr3+, Nd3+nanocrystals. The particles are well separated from each other and the nanocrystals have elongated hexagonal, rod and spherical shape with average particle size of about 20 nm. The lattice patterns reveal that each nanoparticle is a single crystal. This shows that the original structure of LaF3 may be retained even after the modification. [21]. Fig.3 has shown FTIR spectrum of the LaF3:Ce3+, Pr3+, Nd3+ nanocrystals. The characteristic absorption peaks have been observed in the range of 500 cm-1 to 4000 cm-1. The broad absorption band at about 3434 cm-1 can be attributed to as( O-H) stretching and bending vibrations. The peaks at 2854 cm-1 and 2924 cm-1 can be attributed to as(C-H) group of the long alkyl chain [22]. A strong, broad IR peak is found to lie in region from 1405 cm-1 to 1060 cm-1 assigned to the asymmetric ( as) and symmetric ( s) corresponds to the bending vibration of δ (O-H) groups from the methanol. The characteristic IR peaks located at 1632 cm-1 could be assigned to δ (H2O) bending vibrations from the residual absorbed water and 1405 cm-1 can be assigned to the asymmetric ( as) and symmetric ( as) bending vibrations of δ (O-H) group from methanol. .

Advanced Materials Research Vol. 585

177

Fig.4. Excitation (λem=458 nm) and Emission Fig.5.Energy level scheme of LaF3: Ce3+, Pr3+, Nd3+ (λex=254 nm) spectra of LaF3 nanocrystals doped nanocrystals Ce3+, Pr3+, Nd3+ ions. The emission spectra of synthesized LaF3 nanocrystals doped with Ce3+ Pr3+ Nd3+ ion is shown in Fig. 4. The excitation spectra are obtained by monitoring (254 nm) 4f to 5d transition of Ce3+ ions. The broadband emission is located at 458 nm due to the electronic transitions from 5d to 4f state of Ce3+ ions [27, 28]. The sharp emission peaks originates from the 4f5d-4f2 transitions of Pr3+ ions: 3 H4→3P2 (458 nm), 3H4→3P0 (497 nm), 3H4→1D2 (608 nm). The quenching of Ce3+ emissions and the enhancement of Pr3+ emissions is strong evidence of efficient energy transfer from Ce3+ to Pr3+ and Nd3+. The emission spectrum is mainly located in the region corresponding to blue colour. Here, the doping Ce3+ ions act as sensitizers, and the dopant ions Pr3+ Nd3+ can be considered as luminescent centers. It is well known that the luminescent spectra of trivalent lanthanide ions in crystals come mainly from two types of electronic transitions: 4f–4f transition and 5d–4f transition. The excited electronic configuration of Ce3+ is 5D1. The 5d electron has a strong interaction with the neighboring anion ligands in the compounds and results in broadband emissions. The 4f orbital is shielded from the surroundings by the filled 5s2 and 5p6 orbital. Therefore, the influence of the host lattice on the optical transitions within the 4f n configuration is small [23]. Fig.5 has shown the energy level scheme of LaF3:Ce3+, Pr3+ Nd3+, with optical transitions and energy transfer processes. The Ce3+ ion excited at 254nm absorbs one photon and is pumped to the 5d level. Then, it relaxes to the ground state by radiative process with emission of photons; and transfers its energy to a nearby Pr3+ ion in the ground state, promoting this Pr3+ ion to excited state. Then, the excited Pr3+ ion relaxes to the 3P2, 3P0, and 1D2 levels by non-radiative process. The Pr3+ ion excited by 254nm is pumped to the 3P2, 3P0, and 1D2 states. As mentioned above, 3H4→3P2 (458 nm), 3H4→ 3P0 (497 nm), 3H4→1D2 (608 nm) transitions have been observed only for low Pr3+doped samples. Blue fluorescence from the higher energy 3P2 level has been observed for high Pr3+doped samples [24]. It is observed that the measured relative SHG efficiency of LaF3 doped Ce3+, Pr3+, Nd3+ in deionized water with KDP crystal is 0.1836 and that of LaF3 doped Ce3+, Pr3+, Nd3+ in methanol with KDP crystal is 0.5227 by using Kurtz and Perry technique [25].

178

Advances in Materials and Processing

Conclusions Well dispersed nanorods of LaF3: Ce3+, Pr3+, Nd3+ nanocrystals have elongated hexagonal geometry have been synthesized by aqueous route using rapid microwave synthesis. Traces of very few broad hexagonal nanocrystals have been observed. The c/a ratio is found to approach unity in nanocrystals prepared using methanol. The average particle size is about 20nm as confirmed from TEM & XRD peaks analysis. FTIR spectra indicate presence of fundamental vibrations. Intense blue luminescence has been observed with exciting wavelength of 254nm. It has been found that SHG efficiency of LaF3=Ln3+ (Ln3+= Ce3+, Pr3+, Nd3+) containing rare earth elements are less than pure KDP crystals. Acknowledgments Thanks are due the staff of TIFR, IIT (Mumbai), IIT (Madras), and ISFAL (Punjab) for providing experimental facilities. Thanks are due to Bhavan’s College, Andheri (W). Mumbai-58 for institutional support References [1] B.M.Tissue, Che.Mater. 10(1998) 2837-2845. [2] M.Nishi, S.Tanabe, M.Inoue, M.Takahashi, K.Fujita, K.Hirao. Opt.Mater. 27(2005) 655-662. [3] Y.X.Pan, Q.Su, H.F.Xu, T.H.Chen, C.L.Yang, J.Solid State Chem. 174 (2003) 69-73. [4] P.Y Jia, J. Lin, M. Yu .J. Lumin.134 (2007)122-123. [5] S. Sivakumar, F. C. J. M. Van Veggel, P. S.May. J. Am. Chem. Soc. 129 (2007) 620-625. [6] J.S.Zhang, W.P.Qin, D. Zhao, Y.Wang. J. Lumin. 122 (2007)506-508. [7] F. Evanics, P. R. Diamente, F. C. J. M .Van Veggel. Chem. Mater. 18(2006) 2499-2505. [8] F. Wang, Zhang Y, Fan X P, Wang M Q.Nanotechnology. 17 (2006) 1527-1532. [9] P. R. Diamente, F. C. J. M. Van Veggel. J. Fluoresc. 15(2005) 543-551. [10] H.D.Zhou, M.E.Yue, J.M.Chen, Y.P.Ye, Tribol.Int. 24(2004) 225-230. [11] N.Miura, J.Hisamoto, N.Yamazoe, S.Kuwata, Sens.Actuators B. 16 (1989) 301-310. [12] O.V. Kudryavteseva, L.S. Garashina, K.K. Rivkina, B.P. Sobolev, J. Sov. Phys. Crystallography. 18 (1974) 531-541. [13] H.R. Zheng, X.T Wang, M.J. Dejneka, J. Lumin. 108(2004)395-399. [14] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera. Opt. Mater. 19 (2002)343-349. [15] D.B. Pi, F. Wang, X.P. Fan, M.Q. Wang, Y. Zhang. Spectrochim.ActaA.61 (2005)2455-2459. [16] J. W. Stouwdam, A. Gerald. Hebbink, J. Huskens, F. C. J. M vanVeggel. Chem.Mater.15 (2003)4604-4616. [17] J.X. Meng, M.F. Zhang, Y.L. Liu, S.Q.Man, Spectro. Acta Part A. 66(2007) 81-85. [18] W. T. Carnall, G. L. Goodman, K. Rajnak, R. S. Rana. A J. Chem. Phys.90 (1989)3443- 3457. [19] Y.F Liu, W. Chen, S. Wang, A. G. Joly, S. Westcott, B. K. Woo. J. of App. Phy. 103 (2008) 063105 [20] F. Wang, Y. Zhang, X. Fan and M. Wang, J. Mater. Chem. 16(2006) 1031–1034. [21] X. Wang, J. Zhuang, Q. Peng, Y. Li. Inorg. Chem. 45 (2006) 6661-6665. [22]H. Guo, T. Zhang, Y. M. Qiao, L. H. Zhao, Z. Q. Li, J. of Nanoscience and Nanotechnology. 10 (2010) 1913–1919, [23] J. Wang, J. Hu, D. Tang, X. Liu, Z. Zhen. J. Mater. Chem. 17(2007) 1597–1601 [24] L. V. Pieterson, R.T. Wegh, A. Meijerink, J.of Chem.Phy. 115 (2001) 9382-9392. [25] S. K. Kurtz, T. T. Perry, J. Appl. Phys. 39 (1968) 3798-3813