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Luminescence investigations on LiAl5O8:Tb3+ nanocrystalline phosphors
Current Applied Physics 11 (2011) 341e345
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Luminescence investigations on LiAl5O8:Tb3þ nanocrystalline phosphors Shreyas S. Pitale*, Vinay Kumar, Indrajit Nagpure, O.M. Ntwaeaborwa, H.C. Swart* Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein ZA9300, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2010 Received in revised form 14 July 2010 Accepted 2 August 2010 Available online 10 August 2010
Tb3þ-doped porous LiAl5O8 nanophosphors were successfully synthesized using a single step combustion method. Structural characterization of the luminescent material was carried out with X-ray diffraction (XRD) analysis and high resolution transmission electron microscopy (HRTEM). Surface composition was estimated from X-ray photoelectron and Auger spectroscopic techniques. The average particle size was estimated as 37 3 nm. Luminescence properties were studied by the photoluminescence spectroscopy. The phosphor showed intense luminescence in the green region due to the magnetic dipole transition of 5 D4/7F5 of the Tb3þ ion at 543 nm under 241 nm excitation. The intense 4fe4f transitions of Tb3þ outshine the luminescence from host which would otherwise deteriorate the color quality of the display device under operation. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: LiAl5O8 Spinel Nanophosphors Photoluminescence Rare earth ions
1. Introduction In recent years, rare-earth-doped alkali aluminates have been of considerable interest for the possible applications such as irradiation blanket for the nuclear fusion reactors as well as for the ceramic matrixes in the molten carbonate fuel cells [1e3]. Beside these application these materials exhibits good luminescent properties when they are doped with the specific impurities. A considerable amount of work has been carried out in the LiAl5O8 with the rare earth ion like Eu3þ [4], and also with the transition ion dopants like Mn2þ, Cr3þ, Co2þ and Fe3þ [5e8] etc., but no work could be traced on the photoluminescence properties of the Tb3þdoped LiAl5O8 nanocrystalline phosphors to the best of our knowledge. Tb3þ ion in inorganic compounds is of great importance due to the potential technological applications as functional photonic materials, such as optical fiber amplifiers, lasing medium and frequency-converting devices [9e11]. Tb3þ ions can be widely used as luminescent centers in a number of applications due to their strong green emission, corresponding to the 5D4/7F5 transition. Various experiment methods, such as sol-gel [12,13], combustion reaction [5,6], co-precipitation [7], have been investigated to fabricate aluminates. Among these methods, the
* Corresponding authors. Tel.: þ27(0)51 401 9749; fax: þ27(0)51 401 3507. E-mail addresses: [email protected] (S.S. Pitale), [email protected] (H.C. Swart). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.08.002
combustion reaction is attractive because of low synthesis temperature and short reaction time. In the present letter, we have reported on the photoluminescence properties of combustion synthesized Tb3þ-doped LiAl5O8 nanocrystalline phosphors. The structural characterization has been done by X-ray diffraction and transmission electron microscopy. Photoluminescence spectroscopy has been used to study the luminescence properties (excitation and emission) of the present samples. 2. Experimental All the starting chemicals namely, lithium nitrate (LiNO3 ¼ 0.511 g), aluminium nitrate (Al(NO3)3$9H2O ¼ 13.902 g), urea (CH4N2O ¼ 5.935 g) and terbium nitrate (Tb(NO3)3$6H2O ¼ 0.169 g) of AR grade were purchased from Merck, South Africa and were used as obtained without further purification. The detail of combustion synthesis for the preparation of the LiAl5O8:Tb3þ nanocrystalline phosphors can be found elsewhere [5,6]. The balanced chemical reaction can be written as: 3LiNO3 þ 15Al(NO3)3$9H2O þ 40CH4N2O / 3LiAl5O8 þ 40CO2 þ 215H2O þ 64N2 X-ray diffraction (XRD) measurements were performed in order to study the crystallinity of the samples using a Siemens D5005 diffractometer with Cu Ka radiation (l ¼ 0.154 nm) with 2q ranging from 20 to 70 . HRTEM images were taken with a JEM-2100 at an accelerating voltage of 200 kV. Morphological and Auger spectroscopic
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investigations were carried out using a PHI 700 Nano Scanning Auger Microprobe (Nano SAM). The XPS measurements were conducted using a PHI 5000 versa probe spectrometer using monochromatic Al Ka radiation (hn ¼ 1253.6 eV). Survey scans were performed using a 1 eV/ step and 45 min acquisition times (binding energies ranging from 0 to 1400 eV). The sample area analyzed was about 1 mm2 and the pressure during acquisition was typically under 1 108 Torr. Photoluminescence (excitation and emission) measurements were carried out on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature with a scan speed of 600 nm/min.
3. Results and discussion 3.1. Phase verification and lattice parameter determination In order to ascertain the crystalline nature of Tb3þ doped LiAl5O8 nanocrystalline phosphors, X-ray diffraction pattern of powder samples prepared via the combustion route is shown in Fig. 1 along with the Joint Committee Powder Diffraction Data Standards (JCPDS) No. 71-1736. Pure cubic phase of LiAl5O8 belonging to P4332 (212) space group symmetry is obtained. Peak broadening (after subtracting the instrumental contribution) indicates presence of the nanocrystalline particles. The average crystallite size calculated from the Scherrer’s equation was found to be 37 3 nm. The improved crystallinity of the final product after combustion synthesis leads to higher oscillating strengths for optical transitions. The peaks at 29 and 31 2q arise from unidentified impurity phase. From the 2q values of the diffraction lines, the inter-planar spacing d of the peaks was calculated. The diffraction lines were indexed using a computer program package Unit cell [14]. Out of those P a suitable cubic unit cell was selected for which Dd ¼ dobs dcalc was found to be the minimum. The lattice parameters of the unit cell were refined using the least squares method and were found to be a ¼ 7.898 Å (expected value 7.908 Å from the Standard JCPDS data). It is believed that the reduction in the physical dimension of a crystalline solid lead to the regular changes in the lattice constants. The lattice expansion or contraction in smaller crystallites may depend on the number of factors like the nature of atoms in the interior, the surface atoms, the dangling bonds and the oxygen concentrations on the surface. With reduced dimensions, the lattice parameters undergo contraction or repulsion along the different planes. A good
Fig. 1. XRD pattern of the combustion synthesized LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor at 550 C along with the standard JCPDS file.
Table 1 Comparison of the observed and the calculated d values (Å) of some reflections of the LiAl5O8:Tb3þ nanocrystalline phosphor prepared via combustion route at 550 C. Peak No.
hkl
d(obs)
d(calc)
residual (d)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1 1 2 2 2 2 3 3 2 3 3 4 4 4 5 5 5 4
5.57636 4.55782 3.52016 3.19526 2.75274 2.61155 2.50147 2.37443 2.27457 2.20934 2.09711 1.97021 1.72424 1.61564 1.52449 1.47007 1.44155 1.39950
5.58507 4.56019 3.53231 3.22454 2.79254 2.63283 2.49772 2.38148 2.28010 2.19065 2.11096 1.97462 1.72359 1.61227 1.52006 1.46671 1.44206 1.39627
0.00871 0.00237 0.01215 0.02929 0.03980 0.02128 0.00375 0.00705 0.00553 0.01870 0.01385 0.00441 0.00065 0.00337 0.00443 0.00336 0.00051 0.00323
1 1 1 1 2 2 1 1 2 2 2 0 2 2 1 2 2 4
0 1 0 1 0 1 0 1 2 0 1 0 1 2 1 0 1 0
agreement between the observed and calculated d values (Table 1) suggests the suitability of crystal structure and unit cell parameters.
3.2. Morphological studies Fig. 2 shows the SEM micrographs of the combustion synthesized LiAl5O8:Tb3þ nanocrystalline phosphors. Due to evolution of large amount of gases a well expected dense flake like morphology is observed (Fig. 2 (a)). In a simultaneous oxidation and reduction process like that of combustion, due to the hypergolic nature of gases (evolving due to gaseous decomposition of fuel), the reaction mixture undergoes a violent combustion. Thus, a driving force (to which is coupled heat and mass transfer) departs the system from its equilibrium state and modulates the surface free energy of final crystals. The magnitude of driving force pertaining in the ambient phase, where crystal growth is considered primarily to occur, decides the variations in growth forms. In view of the inhomogeneous heat and the mass transport in the reaction chamber, in an ongoing combustion process, the driving force may also have a spatial variance inside the reaction chamber ultimately leading to difference in rate of nucleation and an irregular distribution of morphological forms. The porous network, as indicated in Fig. 2 (b and c) is an outcome of large amount of gases during combustion process which is usually known to render products of high surface area [4]. TEM micrographs as shown in Fig. 3(a and b) reveal a premature local partial sintering mechanism operative during combustion process leading to formation of interconnected particles with well defined grain boundaries and voids seen as a result of large gaseous matter escape. It must be remembered here that the exothermicity of the reaction between the gaseous decomposition products of metal nitrates as NOx and urea as HNCO, NH3, etc leads to a high flame temperature (1500 C) during the reaction. These gaseous decomposition products of urea are known to be hypergolic with nitrogen oxides, i.e., once they attain a critical density and the required temperature, they burn with a flame (whose temperature reaches 1500 100 C) even at ambient pressures. The sintering of particles is an outcome of such high temperature, manifested in form of a flame, and smooth and shiny layer of the particle is indicative of the melting morphology generated by solideliquid reaction at such a high temperature during the combustion procedure. HRTEM picture shown in Fig. 3(c) clearly depicts the regular arrangement of columns of atoms with a good crystallinity. No defects or defected interfaces were present in the micrograph.
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Fig. 2. SEM micrograph of the LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor.
3.3. Cationic composition The cationic composition of LiAl5O8:Tb was determined by X-ray photoelectron spectroscopy (XPS). Fig. 4 shows a survey scan containing Li 1s, O1s, Al2p, Al2s, Tb3d, Tb4d with adventitious carbon1s peaks at binding energies (B.E) 54.4 eV, 531.4 eV, 74.2 eV, 119.1 eV, 1242 eV, 145 eV and 285.4 eV respectively. Lithium exhibits, due to a low XPS sensitivity a very weak peak at 54.4 eV B.E which could be seen only in a perusal of a detailed scan. The atomic ratio of Al to O in the sample surface was found to be 1:2 (0.3) which match with the stoichiometry of LiAl5O8. The amount of lithium was difficult to determine by XPS due to its low sensitivity for this element. Auger electron spectroscopy (AES) has relatively higher sensitivity but the Li peaks overlaps with the low energy peaks of aluminium in the electron energy range 40e65 eV as shown as an inset of Fig. 4. Evaluation of the samples with both the methods XPS and AES yielded an atomic lithium content between 0.3 and 1.1 relative to the aluminium which also agrees with the composition of LiAl5O8. 3.4. Photoluminescence studies The spectral components of photoluminescence (PL) measurement can provide valuable information concerning the type of defects and impurities in semiconductors, while the overall PL intensity is determined by the quantum efficiency of the material together with the surface recombination velocity. Fig. 5 represents the excitation spectra of the prepared samples. The pristine LiAl5O8 could be excited by a broad band in the range 200e300 nm representing the interband transitions of the host lattice. Another excitation channel could be observed in the visible region having maxima around 488 nm. On the first primacy, it would be
straightforward for us to assign this excitation to internal transitions 4E(4D) on Fe3þ impurities, as reported by Kutty and Nayak [7]. Such impurities may be unintentionally present in the final product and the source of which could be traced back as trace elements in the starting materials (Al (NO3)3$9H2O contains 0.0003 at.% Fe). Although, the X-ray Fluorescence (XRF) measurements have also revealed the presence of Fe impurity in the pristine LiAl5O8 (z12 ppm), yet, a substantially different look towards the origin of this excitation channel is also relevant in the present case. Another possibility for the 488 nm band could be the pumping to intermediate electronic trapping levels arising due to partial cationic disordering in LiAl5O8 host. To determine the origin of Tb3þ excitation, we prefer to rely upon Dorenbos’s empirical model [15] which efficiently allows to predict the location of a the first 4fn1 5d level of Tb3þ in the given lattice compound if the spectral information on the energy of the lowest 5d level of Ce3þ positions in the same compound is known prior. According to this model, usually, adding 13,200 920 cm1 and 6300 900 cm1 to the lowest Ce3þ 4fe5d excitation, one may obtain the allowed 4fe5d and spin forbidden 4fe5d excitations of Tb3þ in a given compound. With such a ‘shift model’ existing, the Ce3þ ion was deliberately doped in to the LiAl5O8 lattice and its lowest 4fe5d excitation is observed at 315 nm as shown in Fig. 5. Assumed that the Hund’s rule is applicable to the excited state of fed transition, the lowest terminal state of the transition 4f8e4f 7fd, is 9D (4f75d) term, the transition to which from the 7F (4f8) ground term is spin forbidden, and the transition to next lowest term 7D (4f75d) is spin allowed. The predicted positions of Tb3þ spin allowed transitions and spin forbidden transitions comes out to be 223 5 nm and 259 6 nm respectively. In the absence of any crystal field splitting being operative at the measurement temperature, we could only roughly spot these spectral components in the excitation band of Tb3þ by
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Fig. 3. HRTEM micrograph of the LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor.
visual inspection and those are marked as A, B and C in Fig. 4. Due to spectral overlap with the fundamental absorption of the host lattice, the peak B at 241 nm could be assigned to the host absorption and transfer of energy to the Tb3þ ions from thereof. The spectral component marked as A at around the 221 nm and C at around 253 nm falls within the range of predicted positions of Tb3þ spin allowed and spin forbidden transitions respectively as described above.
Fig. 4. X-ray photoelectron spectroscopy (XPS) survey scan of LiAl5O8:Tb phosphor. An Auger spectrum is shown in the inset.
The emission spectra of prepared samples are shown in Fig. 6. Well known 5D4e7FJ(J¼6,5,4,3) transitions of the Tb3þ at 487 nm, 543 nm, 587 and 623 nm are observed where intense green emission at 543 nm is observed due to the 5D4e7F5 spin allowed transition. The LiAl5O8:Tb3þ (1 mol.%) shows three sub bands around 375e450 nm
Fig. 5. PL excitation spectra of combustion synthesized pristine LiAl5O8 (lemission ¼ 667 nm), LiAl5O8:Tb (lemission ¼ 543 nm) and LiAl5O8:Ce (lemission ¼ 420 nm) nanophosphor powder. The excitation of Tb doped LiAl5O8 (shown in red color) consists of three spectral components marked as A, B and C at 221 nm, 241 nm and 253 nm respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
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pristine sample is excited by the 488 nm visible light. The emission, which is supposed to be due to the Fe3þ in the present host system, is completely quenched with the Tb3þ doping. This suggests that under the host lattice excitation with 200e300 nm band, the excitation energy migration to Tb3þ sites is much faster than to the Fe3þ sites leading to intense Tb3þ emissions. This is quite vital in practical applications, where if quenching from the host (or any other unintentional impurity) is not complete, then, such emission may prove detrimental to the color purity of the display device under operation. 4. Conclusions
Fig. 6. PL emission spectra of as Tb3þ doped (1 mol%) (lexc. ¼ 241 nm) and pristine LiAl5O8 (lexc. ¼ 237 nm) nanocrystalline phosphor. The 375 nme450 nm Tb3þ sub bands (rectangular region) are enlarged and shown as an inset figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(shown as magnified spectra in the inset) and these emissions are attributed to the transitions of the Tb3þ between 5D3 and 7FJ levels. Usually, these emission bands are much prominent at lower Tb3þ concentrations in a given host. As the concentration is increased, the emission from these transitions is normally quenched and the spectrum is then dominated completely by the 5D4e7FJ(J¼6,5,4,3) transitions. Therefore at our reported concentration of Tb3þ in LiAl5O8 host, there occurs only a partial quenching of the 5D3e7FJ transitions by cross relaxation mechanism [16] such as: Tb3þ(5D3) þ Tb3þ(7F6) / Tb3þ(5D4) þ Tb3þ(7F0)
(1)
Finally, regarding the asymmetrical emission of the pristine sample in the NIR region at 667 nm there may be two ambiguous corners that need to be attended by carrying out further experimental investigations: This emission may be originating from unintentional presence of Fe3þ impurity (the source of which may be traced in the starting materials used in synthesis procedure) in the trigonally distorted tetrahedral and octahedral sites of the LiAl5O8 structure and due to the 4T1(4G) / 6A1(6S) Fe3þ transition. The side band observed around the 700 nm is then due to the Fe3þ ion attaining octahedral site symmetry. The different site symmetries of the Fe3þ impurity as seen from the two bands may be an indication that the combustion technique leads to the partially disordered phase formation of the host lattice wherein a cation disorder is quantified in terms of an inversion parameter that specifies the fraction of the trivalent aluminium ions that occupy the tetrahedral sites. The partial cation interchange leads to different coordination of the Al3þ around the Fe3þ ion [7]. And the second possibility could be that the NIR band at 667 nm is a result of charge carrier recombination at some optically active intermediate defect sites within the band space of the host lattice. In fact such sites have already been identified by electron spin resonance (ESR) techniques as Fþ (oxygen vacancies trapping an electron) and V centers (hole trapped at lithium and aluminium vacancies) by Dhabekar et al. [17]. Configuration of this band (not shown) remains same with somewhat reduced in intensity when the
Efficient green emissions from Tb3þ ions were demonstrated in porous LiAl5O8 nanophosphors synthesized using a one-step combustion method. The surface composition was determined using XPS and AES techniques. The spectral overlap of dopant and host excitations reveals excitation energy transfer from the host to the dopant site. The Tb3þ excitations are identified according to the 4fe5d excitation of the Ce3þ ion and the experimental band positions corroborate well with those being predicted. The pristine sample is luminescent in the NIR region, owing to the presence of either the Fe3þ unintentional impurities or from recombination at defect sites and thus demands further studies for a conclusive determination. Such emission transitions are passive under the influence of the Tb3þ doping. Although, Ce3þ doping is carried out with a sole purpose to predict and identify the Tb3þ excitation in the present work, the near UV excitation of the Ce3þ ion further open up new research ventures related to the Ce3þeTb3þ energy transfer in the LiAl5O8 porous network. If efficient, such energy transfer may lead to the development of a near UV excitable green line emitting component of white LEDs from the present host. Acknowledgements The authors would like to thank the South African National Research Foundation (NRF) and the Material and Nano Science Cluster fund of the University of The Free State for providing the financial support. References [1] Y. Kawamura, M. Nishikawa, K. Tanaka, H. Matsumoto, J. Nucl. Sci. Technol. 29 (5) (1992) 436. [2] J. Becerril, P. Bosch, S. Bulbulian, J. Nucl. Mater. 185 (3) (1991) 304. [3] S. Terada, I. Nagashima, K. Higaki, Y. Ito, J. Power Sources 75 (2) (1998) 223. [4] V. Singh, T.K. Gundu Rao, J. Solid State Chem. 181 (6) (2008) 1387. [5] V. Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Mater. Chem. Phys. 110 (1) (2008) 43. [6] V. Singh, R.P.S. Chakradhar, J.L. Rao, Kwak Ho-Young, Sol. Stat. Sci. 11 (4) (2009) 870. [7] T.R.N. Kutty, M. Nayak, J. Alloys Compd. 269 (1e2) (1998) 75. [8] Duan Xiulan, Yuan Duorong, J. Non-Cryst. Solids 351 (27e29) (2005) 2348. [9] D. Hreniak, W. Strek, P. Mazur, R. Pazik, M.Z. Waclawek, Opt. Mater. 26 (2) (2004) 117. [10] C.H. Kam, S. Buddhudu, Mater. Lett. 54 (5e6) (2002) 337. [11] S. Bangaru, G. Muralidharan, G.M. Brahmanandhan, J. Lumin. 130 (4) (2010) 618. [12] D. Pan, D. Yuan, H. Sun, X. Duan, C. Luan, S. Guo, Z. Li, L. Wang, Mater. Chem. Phys. 96 (2e3) (2006) 317. [13] R.A. Ribeiro, G.G. Silva, N.D.S. Mohallem, J. Phys. Chem. Sol. 62 (5) (2001) 857. [14] T.J.B. Holland, S.A.T. Redfern, Mineral. Mag. 61 (1) (1997) 65. [15] P. Dorenbos, J. Phys. Condens. Matter. 15 (2003) 6249. [16] G. Blasse, Rev. Inorg. Chem. 5 (1983) 319. [17] B. Dhabekar, E. Alagu Raja, S. Menon, T.K. Gundu Rao, R.K. Kher, B.C. Bhatt, J. Phys. D: Appl. Phys. 41 (2008) 115414.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Luminescence investigations on LiAl5O8:Tb3þ nanocrystalline phosphors Shreyas S. Pitale*, Vinay Kumar, Indrajit Nagpure, O.M. Ntwaeaborwa, H.C. Swart* Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein ZA9300, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2010 Received in revised form 14 July 2010 Accepted 2 August 2010 Available online 10 August 2010
Tb3þ-doped porous LiAl5O8 nanophosphors were successfully synthesized using a single step combustion method. Structural characterization of the luminescent material was carried out with X-ray diffraction (XRD) analysis and high resolution transmission electron microscopy (HRTEM). Surface composition was estimated from X-ray photoelectron and Auger spectroscopic techniques. The average particle size was estimated as 37 3 nm. Luminescence properties were studied by the photoluminescence spectroscopy. The phosphor showed intense luminescence in the green region due to the magnetic dipole transition of 5 D4/7F5 of the Tb3þ ion at 543 nm under 241 nm excitation. The intense 4fe4f transitions of Tb3þ outshine the luminescence from host which would otherwise deteriorate the color quality of the display device under operation. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: LiAl5O8 Spinel Nanophosphors Photoluminescence Rare earth ions
1. Introduction In recent years, rare-earth-doped alkali aluminates have been of considerable interest for the possible applications such as irradiation blanket for the nuclear fusion reactors as well as for the ceramic matrixes in the molten carbonate fuel cells [1e3]. Beside these application these materials exhibits good luminescent properties when they are doped with the specific impurities. A considerable amount of work has been carried out in the LiAl5O8 with the rare earth ion like Eu3þ [4], and also with the transition ion dopants like Mn2þ, Cr3þ, Co2þ and Fe3þ [5e8] etc., but no work could be traced on the photoluminescence properties of the Tb3þdoped LiAl5O8 nanocrystalline phosphors to the best of our knowledge. Tb3þ ion in inorganic compounds is of great importance due to the potential technological applications as functional photonic materials, such as optical fiber amplifiers, lasing medium and frequency-converting devices [9e11]. Tb3þ ions can be widely used as luminescent centers in a number of applications due to their strong green emission, corresponding to the 5D4/7F5 transition. Various experiment methods, such as sol-gel [12,13], combustion reaction [5,6], co-precipitation [7], have been investigated to fabricate aluminates. Among these methods, the
* Corresponding authors. Tel.: þ27(0)51 401 9749; fax: þ27(0)51 401 3507. E-mail addresses: [email protected] (S.S. Pitale), [email protected] (H.C. Swart). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.08.002
combustion reaction is attractive because of low synthesis temperature and short reaction time. In the present letter, we have reported on the photoluminescence properties of combustion synthesized Tb3þ-doped LiAl5O8 nanocrystalline phosphors. The structural characterization has been done by X-ray diffraction and transmission electron microscopy. Photoluminescence spectroscopy has been used to study the luminescence properties (excitation and emission) of the present samples. 2. Experimental All the starting chemicals namely, lithium nitrate (LiNO3 ¼ 0.511 g), aluminium nitrate (Al(NO3)3$9H2O ¼ 13.902 g), urea (CH4N2O ¼ 5.935 g) and terbium nitrate (Tb(NO3)3$6H2O ¼ 0.169 g) of AR grade were purchased from Merck, South Africa and were used as obtained without further purification. The detail of combustion synthesis for the preparation of the LiAl5O8:Tb3þ nanocrystalline phosphors can be found elsewhere [5,6]. The balanced chemical reaction can be written as: 3LiNO3 þ 15Al(NO3)3$9H2O þ 40CH4N2O / 3LiAl5O8 þ 40CO2 þ 215H2O þ 64N2 X-ray diffraction (XRD) measurements were performed in order to study the crystallinity of the samples using a Siemens D5005 diffractometer with Cu Ka radiation (l ¼ 0.154 nm) with 2q ranging from 20 to 70 . HRTEM images were taken with a JEM-2100 at an accelerating voltage of 200 kV. Morphological and Auger spectroscopic
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S.S. Pitale et al. / Current Applied Physics 11 (2011) 341e345
investigations were carried out using a PHI 700 Nano Scanning Auger Microprobe (Nano SAM). The XPS measurements were conducted using a PHI 5000 versa probe spectrometer using monochromatic Al Ka radiation (hn ¼ 1253.6 eV). Survey scans were performed using a 1 eV/ step and 45 min acquisition times (binding energies ranging from 0 to 1400 eV). The sample area analyzed was about 1 mm2 and the pressure during acquisition was typically under 1 108 Torr. Photoluminescence (excitation and emission) measurements were carried out on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature with a scan speed of 600 nm/min.
3. Results and discussion 3.1. Phase verification and lattice parameter determination In order to ascertain the crystalline nature of Tb3þ doped LiAl5O8 nanocrystalline phosphors, X-ray diffraction pattern of powder samples prepared via the combustion route is shown in Fig. 1 along with the Joint Committee Powder Diffraction Data Standards (JCPDS) No. 71-1736. Pure cubic phase of LiAl5O8 belonging to P4332 (212) space group symmetry is obtained. Peak broadening (after subtracting the instrumental contribution) indicates presence of the nanocrystalline particles. The average crystallite size calculated from the Scherrer’s equation was found to be 37 3 nm. The improved crystallinity of the final product after combustion synthesis leads to higher oscillating strengths for optical transitions. The peaks at 29 and 31 2q arise from unidentified impurity phase. From the 2q values of the diffraction lines, the inter-planar spacing d of the peaks was calculated. The diffraction lines were indexed using a computer program package Unit cell [14]. Out of those P a suitable cubic unit cell was selected for which Dd ¼ dobs dcalc was found to be the minimum. The lattice parameters of the unit cell were refined using the least squares method and were found to be a ¼ 7.898 Å (expected value 7.908 Å from the Standard JCPDS data). It is believed that the reduction in the physical dimension of a crystalline solid lead to the regular changes in the lattice constants. The lattice expansion or contraction in smaller crystallites may depend on the number of factors like the nature of atoms in the interior, the surface atoms, the dangling bonds and the oxygen concentrations on the surface. With reduced dimensions, the lattice parameters undergo contraction or repulsion along the different planes. A good
Fig. 1. XRD pattern of the combustion synthesized LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor at 550 C along with the standard JCPDS file.
Table 1 Comparison of the observed and the calculated d values (Å) of some reflections of the LiAl5O8:Tb3þ nanocrystalline phosphor prepared via combustion route at 550 C. Peak No.
hkl
d(obs)
d(calc)
residual (d)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1 1 2 2 2 2 3 3 2 3 3 4 4 4 5 5 5 4
5.57636 4.55782 3.52016 3.19526 2.75274 2.61155 2.50147 2.37443 2.27457 2.20934 2.09711 1.97021 1.72424 1.61564 1.52449 1.47007 1.44155 1.39950
5.58507 4.56019 3.53231 3.22454 2.79254 2.63283 2.49772 2.38148 2.28010 2.19065 2.11096 1.97462 1.72359 1.61227 1.52006 1.46671 1.44206 1.39627
0.00871 0.00237 0.01215 0.02929 0.03980 0.02128 0.00375 0.00705 0.00553 0.01870 0.01385 0.00441 0.00065 0.00337 0.00443 0.00336 0.00051 0.00323
1 1 1 1 2 2 1 1 2 2 2 0 2 2 1 2 2 4
0 1 0 1 0 1 0 1 2 0 1 0 1 2 1 0 1 0
agreement between the observed and calculated d values (Table 1) suggests the suitability of crystal structure and unit cell parameters.
3.2. Morphological studies Fig. 2 shows the SEM micrographs of the combustion synthesized LiAl5O8:Tb3þ nanocrystalline phosphors. Due to evolution of large amount of gases a well expected dense flake like morphology is observed (Fig. 2 (a)). In a simultaneous oxidation and reduction process like that of combustion, due to the hypergolic nature of gases (evolving due to gaseous decomposition of fuel), the reaction mixture undergoes a violent combustion. Thus, a driving force (to which is coupled heat and mass transfer) departs the system from its equilibrium state and modulates the surface free energy of final crystals. The magnitude of driving force pertaining in the ambient phase, where crystal growth is considered primarily to occur, decides the variations in growth forms. In view of the inhomogeneous heat and the mass transport in the reaction chamber, in an ongoing combustion process, the driving force may also have a spatial variance inside the reaction chamber ultimately leading to difference in rate of nucleation and an irregular distribution of morphological forms. The porous network, as indicated in Fig. 2 (b and c) is an outcome of large amount of gases during combustion process which is usually known to render products of high surface area [4]. TEM micrographs as shown in Fig. 3(a and b) reveal a premature local partial sintering mechanism operative during combustion process leading to formation of interconnected particles with well defined grain boundaries and voids seen as a result of large gaseous matter escape. It must be remembered here that the exothermicity of the reaction between the gaseous decomposition products of metal nitrates as NOx and urea as HNCO, NH3, etc leads to a high flame temperature (1500 C) during the reaction. These gaseous decomposition products of urea are known to be hypergolic with nitrogen oxides, i.e., once they attain a critical density and the required temperature, they burn with a flame (whose temperature reaches 1500 100 C) even at ambient pressures. The sintering of particles is an outcome of such high temperature, manifested in form of a flame, and smooth and shiny layer of the particle is indicative of the melting morphology generated by solideliquid reaction at such a high temperature during the combustion procedure. HRTEM picture shown in Fig. 3(c) clearly depicts the regular arrangement of columns of atoms with a good crystallinity. No defects or defected interfaces were present in the micrograph.
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Fig. 2. SEM micrograph of the LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor.
3.3. Cationic composition The cationic composition of LiAl5O8:Tb was determined by X-ray photoelectron spectroscopy (XPS). Fig. 4 shows a survey scan containing Li 1s, O1s, Al2p, Al2s, Tb3d, Tb4d with adventitious carbon1s peaks at binding energies (B.E) 54.4 eV, 531.4 eV, 74.2 eV, 119.1 eV, 1242 eV, 145 eV and 285.4 eV respectively. Lithium exhibits, due to a low XPS sensitivity a very weak peak at 54.4 eV B.E which could be seen only in a perusal of a detailed scan. The atomic ratio of Al to O in the sample surface was found to be 1:2 (0.3) which match with the stoichiometry of LiAl5O8. The amount of lithium was difficult to determine by XPS due to its low sensitivity for this element. Auger electron spectroscopy (AES) has relatively higher sensitivity but the Li peaks overlaps with the low energy peaks of aluminium in the electron energy range 40e65 eV as shown as an inset of Fig. 4. Evaluation of the samples with both the methods XPS and AES yielded an atomic lithium content between 0.3 and 1.1 relative to the aluminium which also agrees with the composition of LiAl5O8. 3.4. Photoluminescence studies The spectral components of photoluminescence (PL) measurement can provide valuable information concerning the type of defects and impurities in semiconductors, while the overall PL intensity is determined by the quantum efficiency of the material together with the surface recombination velocity. Fig. 5 represents the excitation spectra of the prepared samples. The pristine LiAl5O8 could be excited by a broad band in the range 200e300 nm representing the interband transitions of the host lattice. Another excitation channel could be observed in the visible region having maxima around 488 nm. On the first primacy, it would be
straightforward for us to assign this excitation to internal transitions 4E(4D) on Fe3þ impurities, as reported by Kutty and Nayak [7]. Such impurities may be unintentionally present in the final product and the source of which could be traced back as trace elements in the starting materials (Al (NO3)3$9H2O contains 0.0003 at.% Fe). Although, the X-ray Fluorescence (XRF) measurements have also revealed the presence of Fe impurity in the pristine LiAl5O8 (z12 ppm), yet, a substantially different look towards the origin of this excitation channel is also relevant in the present case. Another possibility for the 488 nm band could be the pumping to intermediate electronic trapping levels arising due to partial cationic disordering in LiAl5O8 host. To determine the origin of Tb3þ excitation, we prefer to rely upon Dorenbos’s empirical model [15] which efficiently allows to predict the location of a the first 4fn1 5d level of Tb3þ in the given lattice compound if the spectral information on the energy of the lowest 5d level of Ce3þ positions in the same compound is known prior. According to this model, usually, adding 13,200 920 cm1 and 6300 900 cm1 to the lowest Ce3þ 4fe5d excitation, one may obtain the allowed 4fe5d and spin forbidden 4fe5d excitations of Tb3þ in a given compound. With such a ‘shift model’ existing, the Ce3þ ion was deliberately doped in to the LiAl5O8 lattice and its lowest 4fe5d excitation is observed at 315 nm as shown in Fig. 5. Assumed that the Hund’s rule is applicable to the excited state of fed transition, the lowest terminal state of the transition 4f8e4f 7fd, is 9D (4f75d) term, the transition to which from the 7F (4f8) ground term is spin forbidden, and the transition to next lowest term 7D (4f75d) is spin allowed. The predicted positions of Tb3þ spin allowed transitions and spin forbidden transitions comes out to be 223 5 nm and 259 6 nm respectively. In the absence of any crystal field splitting being operative at the measurement temperature, we could only roughly spot these spectral components in the excitation band of Tb3þ by
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Fig. 3. HRTEM micrograph of the LiAl5O8:Tb3þ (1 mol%) nanocrystalline phosphor.
visual inspection and those are marked as A, B and C in Fig. 4. Due to spectral overlap with the fundamental absorption of the host lattice, the peak B at 241 nm could be assigned to the host absorption and transfer of energy to the Tb3þ ions from thereof. The spectral component marked as A at around the 221 nm and C at around 253 nm falls within the range of predicted positions of Tb3þ spin allowed and spin forbidden transitions respectively as described above.
Fig. 4. X-ray photoelectron spectroscopy (XPS) survey scan of LiAl5O8:Tb phosphor. An Auger spectrum is shown in the inset.
The emission spectra of prepared samples are shown in Fig. 6. Well known 5D4e7FJ(J¼6,5,4,3) transitions of the Tb3þ at 487 nm, 543 nm, 587 and 623 nm are observed where intense green emission at 543 nm is observed due to the 5D4e7F5 spin allowed transition. The LiAl5O8:Tb3þ (1 mol.%) shows three sub bands around 375e450 nm
Fig. 5. PL excitation spectra of combustion synthesized pristine LiAl5O8 (lemission ¼ 667 nm), LiAl5O8:Tb (lemission ¼ 543 nm) and LiAl5O8:Ce (lemission ¼ 420 nm) nanophosphor powder. The excitation of Tb doped LiAl5O8 (shown in red color) consists of three spectral components marked as A, B and C at 221 nm, 241 nm and 253 nm respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
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pristine sample is excited by the 488 nm visible light. The emission, which is supposed to be due to the Fe3þ in the present host system, is completely quenched with the Tb3þ doping. This suggests that under the host lattice excitation with 200e300 nm band, the excitation energy migration to Tb3þ sites is much faster than to the Fe3þ sites leading to intense Tb3þ emissions. This is quite vital in practical applications, where if quenching from the host (or any other unintentional impurity) is not complete, then, such emission may prove detrimental to the color purity of the display device under operation. 4. Conclusions
Fig. 6. PL emission spectra of as Tb3þ doped (1 mol%) (lexc. ¼ 241 nm) and pristine LiAl5O8 (lexc. ¼ 237 nm) nanocrystalline phosphor. The 375 nme450 nm Tb3þ sub bands (rectangular region) are enlarged and shown as an inset figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(shown as magnified spectra in the inset) and these emissions are attributed to the transitions of the Tb3þ between 5D3 and 7FJ levels. Usually, these emission bands are much prominent at lower Tb3þ concentrations in a given host. As the concentration is increased, the emission from these transitions is normally quenched and the spectrum is then dominated completely by the 5D4e7FJ(J¼6,5,4,3) transitions. Therefore at our reported concentration of Tb3þ in LiAl5O8 host, there occurs only a partial quenching of the 5D3e7FJ transitions by cross relaxation mechanism [16] such as: Tb3þ(5D3) þ Tb3þ(7F6) / Tb3þ(5D4) þ Tb3þ(7F0)
(1)
Finally, regarding the asymmetrical emission of the pristine sample in the NIR region at 667 nm there may be two ambiguous corners that need to be attended by carrying out further experimental investigations: This emission may be originating from unintentional presence of Fe3þ impurity (the source of which may be traced in the starting materials used in synthesis procedure) in the trigonally distorted tetrahedral and octahedral sites of the LiAl5O8 structure and due to the 4T1(4G) / 6A1(6S) Fe3þ transition. The side band observed around the 700 nm is then due to the Fe3þ ion attaining octahedral site symmetry. The different site symmetries of the Fe3þ impurity as seen from the two bands may be an indication that the combustion technique leads to the partially disordered phase formation of the host lattice wherein a cation disorder is quantified in terms of an inversion parameter that specifies the fraction of the trivalent aluminium ions that occupy the tetrahedral sites. The partial cation interchange leads to different coordination of the Al3þ around the Fe3þ ion [7]. And the second possibility could be that the NIR band at 667 nm is a result of charge carrier recombination at some optically active intermediate defect sites within the band space of the host lattice. In fact such sites have already been identified by electron spin resonance (ESR) techniques as Fþ (oxygen vacancies trapping an electron) and V centers (hole trapped at lithium and aluminium vacancies) by Dhabekar et al. [17]. Configuration of this band (not shown) remains same with somewhat reduced in intensity when the
Efficient green emissions from Tb3þ ions were demonstrated in porous LiAl5O8 nanophosphors synthesized using a one-step combustion method. The surface composition was determined using XPS and AES techniques. The spectral overlap of dopant and host excitations reveals excitation energy transfer from the host to the dopant site. The Tb3þ excitations are identified according to the 4fe5d excitation of the Ce3þ ion and the experimental band positions corroborate well with those being predicted. The pristine sample is luminescent in the NIR region, owing to the presence of either the Fe3þ unintentional impurities or from recombination at defect sites and thus demands further studies for a conclusive determination. Such emission transitions are passive under the influence of the Tb3þ doping. Although, Ce3þ doping is carried out with a sole purpose to predict and identify the Tb3þ excitation in the present work, the near UV excitation of the Ce3þ ion further open up new research ventures related to the Ce3þeTb3þ energy transfer in the LiAl5O8 porous network. If efficient, such energy transfer may lead to the development of a near UV excitable green line emitting component of white LEDs from the present host. Acknowledgements The authors would like to thank the South African National Research Foundation (NRF) and the Material and Nano Science Cluster fund of the University of The Free State for providing the financial support. References [1] Y. Kawamura, M. Nishikawa, K. Tanaka, H. Matsumoto, J. Nucl. Sci. Technol. 29 (5) (1992) 436. [2] J. Becerril, P. Bosch, S. Bulbulian, J. Nucl. Mater. 185 (3) (1991) 304. [3] S. Terada, I. Nagashima, K. Higaki, Y. Ito, J. Power Sources 75 (2) (1998) 223. [4] V. Singh, T.K. Gundu Rao, J. Solid State Chem. 181 (6) (2008) 1387. [5] V. Singh, R.P.S. Chakradhar, J.L. Rao, Dong-Kuk Kim, Mater. Chem. Phys. 110 (1) (2008) 43. [6] V. Singh, R.P.S. Chakradhar, J.L. Rao, Kwak Ho-Young, Sol. Stat. Sci. 11 (4) (2009) 870. [7] T.R.N. Kutty, M. Nayak, J. Alloys Compd. 269 (1e2) (1998) 75. [8] Duan Xiulan, Yuan Duorong, J. Non-Cryst. Solids 351 (27e29) (2005) 2348. [9] D. Hreniak, W. Strek, P. Mazur, R. Pazik, M.Z. Waclawek, Opt. Mater. 26 (2) (2004) 117. [10] C.H. Kam, S. Buddhudu, Mater. Lett. 54 (5e6) (2002) 337. [11] S. Bangaru, G. Muralidharan, G.M. Brahmanandhan, J. Lumin. 130 (4) (2010) 618. [12] D. Pan, D. Yuan, H. Sun, X. Duan, C. Luan, S. Guo, Z. Li, L. Wang, Mater. Chem. Phys. 96 (2e3) (2006) 317. [13] R.A. Ribeiro, G.G. Silva, N.D.S. Mohallem, J. Phys. Chem. Sol. 62 (5) (2001) 857. [14] T.J.B. Holland, S.A.T. Redfern, Mineral. Mag. 61 (1) (1997) 65. [15] P. Dorenbos, J. Phys. Condens. Matter. 15 (2003) 6249. [16] G. Blasse, Rev. Inorg. Chem. 5 (1983) 319. [17] B. Dhabekar, E. Alagu Raja, S. Menon, T.K. Gundu Rao, R.K. Kher, B.C. Bhatt, J. Phys. D: Appl. Phys. 41 (2008) 115414.