Luminescence investigations of Ce3+ doped CaS nanophosphors

Luminescence investigations of Ce3+ doped CaS nanophosphors

Journal of Alloys and Compounds 492 (2010) L8–L12 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.el...

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Journal of Alloys and Compounds 492 (2010) L8–L12

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Letter

Luminescence investigations of Ce3+ doped CaS nanophosphors Vinay Kumar ∗, Shreyas S. Pitale, Varun Mishra, I.M. Nagpure, M.M. Biggs, O.M. Ntwaeaborwa, H.C. Swart ∗∗ Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, The Free State, South Africa Department of Physics, Lovely Professional University, Phagwara 144 402, Punjab, India Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, The Free State, South Africa

a r t i c l e

i n f o

Article history: Received 23 September 2009 Received in revised form 7 November 2009 Accepted 10 November 2009 Available online 13 November 2009 Keywords: Luminescence Alkaline earth sulfide Nanophosphors

a b s t r a c t Ce3+ doped CaS nanophosphors were synthesized using the chemical co-precipitation method. The samples were characterized using X-ray diffraction, transmission electron microscopy, electron dispersive X-ray spectroscopy, UV–vis absorption spectroscopy and photoluminescence (PL) spectroscopy. The PL emission peaks of both the 2 D(5d) → 2 F5/2 (4f) and 2 D(5d) → 2 F7/2 (4f) transitions of the Ce3+ ion were blue-shifted from the normal PL emission of bulk CaS:Ce at 517 nm, suggesting that the particle sizes of CaS:Ce phosphors synthesized in this study were in the nanometer scale. The effect of different dopant concentrations on the PL emission intensity was also investigated. The maximum PL emission intensity was observed when CaS was doped with 0.04 mol% of the Ce3+ . Compared with commercial phosphors (CaS:Ce), the decay lifetime of the present samples was found to be longer in the order of a few milliseconds probably due to increased radiative relaxation processes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In recent years, rare-earth and transition metal ions doped alkaline earth sulfides (IIa-VIb) have attracted the attention of many researchers and scientists due to their unique applications in photoluminescent devices [1], electroluminescent panels [2], cathode ray tubes [3,4] and thermoluminescent dosimeters [5–7]. Energetically favorable defect structure within the photophysical environment of host lattice has captivated the materials engineering community and as a result, recently, the research on doped CaS has gained a fresh impetus due to its possible afterglow features when co-doped with Eu and Ce impurities. The wide bandgap of this phosphor has been effectively utilized by Kojima et al., to perturb the space charge neutrality conditions by invasion of new ions leading to an increase in the persistent behavior with near-UV and visible photon irradiability as an added advantage [8–10]. Most of the members of this family (IIa-VI) have a NaCl-type structure which can form solid solutions in a wide range of compositions. Note that IIa-VI based phosphors like CaS have not received the same amount of attention like IIb-VI based phosphors such as ZnS and CdS either in the form of micron or nanometer scale powders or thin films.

∗ Corresponding author. Tel.: +27 0 51 401 9749; fax: +27 0 51 401 3507. ∗∗ Corresponding author. Tel.: +27 0 51 401 9749; fax: +27 0 51 401 3507. E-mail addresses: [email protected] (V. Kumar), [email protected] (H.C. Swart). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.076

Fundamental studies on optical properties of CaS bulk doped with rare-earth activators were conducted in the 1970s [11,12]. With the advent of nanoscience and nanotechnology, there has been renewed interest on optical properties of rare-earths (RE) doped CaS nanophosphors. At nanoscale, the bandgap has been shown to enlarge because of the quantum size effects on smaller particles [13]. The objective of this study is therefore to synthesize CaS:Ce3+ nanophosphors and study their optical properties in comparison with their standard bulk counterparts. Several synthesis techniques such as solvothermal [14,15], sol–gel [16] and low temperature synthesis by Sawada et al. [17] have been used to synthesize doped alkaline earth sulfide nanophosphors. In this study, the chemical co-precipitation method was used to synthesize CaS:Ce3+ nanophosphors with different concentrations of Ce3+ , and their luminescence properties and decay characteristics were investigated. Similar method was used previously to prepare CaS:Bi nanophosphors [18]. 2. Experimental Calcium chloride (CaCl2 ·5H2 O (99.9%)), ethanol (99.9%), sodium sulfide (Na2 S·9H2 O (98%)), cerium nitrate (Ce (NO3 )3 ·5H2 O (99.5%)) and 1-thioglycerol (90%) were purchased from Sigma–Aldrich and used as the starting materials for the synthesis process. In order to avoid hydrolysis of CaS, ethanol rather than water was used as a solvent and 1-thioglycerol was used as a capping agent. A 0.0125 M sulfide solution was prepared by dissolving a suitable quantity of Na2 S·9H2 O and 1-thioglycerol in 100 ml of ethanol and left for 30 min. A 0.025 M Ca2+ solution was prepared by dissolving CaCl2 ·5H2 O in 100 ml of ethanol. A 0.01 M Ce solution was prepared by dissolving Ce(NO3 )3 ·5H2 O in 20 ml of ethanol. A similar detail on the phosphor preparation

V. Kumar et al. / Journal of Alloys and Compounds 492 (2010) L8–L12

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Fig. 1. XRD pattern of the CaS:Ce (0.04 mol%) nanophosphor.

has been reported earlier [18]. The Ce3+ concentration was varied from 0.02 to 0.10 mol%. The samples were characterized by X-ray powder diffraction and the patterns were obtained using a Rigaku D’Max X-ray diffractometer (XRD) with Cu K␣ radiation ( = 1.541 Å). For XRD measurements, the prepared nanophosphors were directly pressed in the sample holders of area 1 cm2 and were mounted in the focal plane of diffractometer. The morphology and chemical composition analyses were carried out using a Shimadzu’s SSX-550 scanning electron microscope (SEM) equipped with an electron dispersive X-ray (EDX) spectrometer. Transmission electron microscopy (TEM) images were taken with a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. For TEM sample preparation, ultrasonically diluted nanoparticles suspended in absolute ethanol were introduced on a carbon coated copper grid, and were allowed to dry in air. Photoluminescent (excitation, emission and decay curves) measurements were carried out on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature. The PL decay was measured and was performed using a pulsed xenon source with a pulse rate of 80 Hz facilitated within the same instrument. A Shimadzu UV–vis 1700 spectrophotometer was used to study absorption properties of the nanophosphors.

Fig. 2. EDS spectrum to determine the chemical composition of the CaS:Ce (0.04 mol%) nanophosphor.

at 225, 270 nm and a minor peak was observed at 460 nm. It is well known that the 5d excited configuration of the Ce3+ can be split by the crystal field into two–five components namely Eg doublet and T2g triplet, and the 4f ground configuration can yield two levels (2 F5/2 and 2 F7/2 ) due to spin-orbit coupling [20]. Kodoma et al. [21] observed five absorption peaks at 228, 244, 278, 300 and 352 nm from Ca2 Al2 SiO7 :Ce3+ and they ascribed these peaks to 4f → 5d transitions of Ce3+ . The absorption peaks bands observed at 270 nm may be assigned to band-to-band transition and 460 nm can be ascribed to transitions from the ground state to the excited T2g and Eg states of the 5d orbital. The CaS is an indirect bandgap material. The optical bandgap Eg of CaS was calculated using the

3. Results and discussion 3.1. XRD, SEM/EDS and TEM studies Fig. 1 shows the XRD pattern of a CaS:Ce3+ (0.04 mol%) nanophosphor. All the peaks in the diffraction pattern were indexed according to the JCPDS data file (no. 77-2011) of the cubic structure of CaS. The pattern was found to match perfectly with the standard data except for the broadening of the lines probably due to relatively smaller particle sizes. The average particle size estimated from the XRD broadened peaks using Scherrer’s formula [19] was 42 ± 2 nm in diameter. The chemical composition of the CaS:Ce3+ nanophosphors is shown in Fig. 2. The EDS spectrum of the CaS:Ce3+ nanophosphors (Fig. 2) confirms the presence of the major chemical elements, namely calcium, sulfur, and impurity elements as oxygen and carbon. The carbon was probably from the carbon tape used as a support base for the samples. The origin of oxygen contamination will be studied with controlled oxidation experiments combined with luminescence measurements in future. Ce3+ ions were not detected probably due to their relatively low concentration in the CaS matrix. The morphology of the CaS:Ce nanophosphors was also analyzed using TEM as shown in Fig. 3. The average particle size estimated from the TEM micrograph was in the range of 42 ± 3 nm diameter, consistent with the XRD result. 3.2. Absorption The absorption spectrum of CaS:Ce3+ (0.04 mol%) powder phosphor is shown in Fig. 4. Two major absorption peaks were observed

Fig. 3. TEM images of the CaS:Ce (0.04 mol%) nanophosphors.

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V. Kumar et al. / Journal of Alloys and Compounds 492 (2010) L8–L12

Fig. 4. Optical absorption spectrum of the CaS:Ce (0.04 mol%) nanophosphor.

following relation [22] (˛h)

1/2

∼(h − Eg ),

where h is the photon energy and ˛ is the optical absorption coefficient near the host fundamental absorption edge. The absorption coefficients ˛ were calculated from these optical absorption spectra. The value of the optical bandgap of the nanocrystalline CaS:Ce3+ estimated from the plot of (˛h)1/2 versus h (Fig. 5) by extrapolating the linear region of the plot to (˛h)1/2 was found to be 4.61 ± 0.01 eV This value is larger the bandgap of bulk CaS (4.43 eV) reported by Jia and Wang [23]. This increase in the bandgap is in accordance with the different particle sizes of CaS. 3.3. Excitation and emission Fig. 6 shows the excitation and emission spectra of CaS:Ce3+ . Two excitation peaks were observed at 350 and 460 nm similar to earlier reports [23]. The main excitation peak at 460 nm may be due to the direct absorption in the dopants within the bandgap. A weak excitation peak at 350 nm may be due to the presence of trap or

Fig. 5. Plot for (˛h)1/2 as a function of incident photon energy (h) for the CaS:Ce (0.04 mol%) nanophosphor.

Fig. 6. Excitation (dotted) and emission (solid line) spectra of the CaS:Ce (0.04 mol%) nanophosphor.

defect state with the bandgap. An excitation peak at 270 nm could be attributed to band-to-band absorption in the host which transfers this energy causing excitation of the luminescent centres. Two emission peaks were observed at 507 and 560 nm. These emissions can be attributed to transition from the lowest components of the 5d state to the 2 F5/2 and 2 F7/2 components of the ground state as shown in the inset of Fig. 7. In the case of the crystal field splitting of the d1 orbital (of the Ce3+ ion) in a cubic symmetry (CaS), the T2g energy level lies below the Eg . Note that Ce3+ atom has only one electron in the 4f orbital and this electron is excited to the 5d level giving rise to the excitation peak at 350 nm. This is followed by radiative relaxation from the 5d(T2g ) level to the 2 F5/2 and 2 F7/2 levels of the ground state giving rise to emission peaks at 507 and 560 nm respectively. These emission peaks are blue-shifted from similar peaks observed by Jia and Wang [23] from bulk CaS:Ce3+ at 515 and 570 nm. The quantum size induced blue-shifting of the Ce3+ emission peaks at 498 and 550 nm from CaS:Ce3+ was also reported by Singh et al. [24]. These comparisons are listed in Table 1. The blue-shifting of Ce3+ emission peaks in a cubic nanocrystal structure could be explained by the modified crystal field model proposed by Mhin et al. [25]. According to this model, as the crystal sizes decrease to nanoscale the bond distance between Ce3+ and

Fig. 7. The luminescence intensity dependence on the Ce3+ concentration in the CaS:Ce nanophosphors. The concentration quenching curve is also shown (inset).

V. Kumar et al. / Journal of Alloys and Compounds 492 (2010) L8–L12

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Table 1 Comparison of emission peaks with bulk and nano CaS:Ce. Sample

Particle size

Emission (Ce3+ transition) 2

CaS:Ce bulk [19] CaS:Ce nano [20] CaS:Ce nano [present work]

– 20–30 nm 40–45 nm

D(5d)→2 F5/2 (4f)

2

515 nm 498 nm 507 nm

D(5d)→2 F7/2 (4f)

570 nm 550–570 nm 560 nm

the host lattice will also decrease. This will raise both the centroid and the lowest energy component (T2g ) of the 5d orbital. Consequently, the changes on the energy levels of the centroid and the T2g level will shift the excitation and emission spectra to shorter wavelengths. The quantum size induced blue-shifting of emission peaks from Ce3+ compounds is still under investigation and the findings will be communicated in future publications. 3.4. Concentration quenching The concentration quenching curve is also shown (inset of Fig. 7). For the low concentration of Ce3+ ions (0.04 mol%), the emission of 5d → 2 F5/2 shows a higher intensity. Beyond this, the emission intensity tends to decrease with the increase in doping concentration. Concentration quenching between two identical centres (e.g. two Ce3+ ions) can be explained in term of resonant energy transfer mechanism reported by Sole et al. [26]. According to this mechanism, one Ce3+ acting as energy donor can transfer a part of its excitation energy to the other Ce3+ ion, acting as an acceptor, by cross relaxation process. This resonance transfer is necessitated by a particular disposition of energy levels, i.e. the energy for transition in one centre (donor) is equal to the energy for transition in another centre (acceptor). As a result, non-radiative relaxation or emission from different state will occur at the expense of radiative from the donor ion. That is, emission from the donor will be quenched. 3.5. PL decay characteristics The luminescence lifetime curves were recorded with the excitation and emission wavelengths at 460 and 507 nm respectively. All the decay curves were fitted to the third order exponential decay function given by: I = I0 + A1 exp

 −t  1

+ A2 exp

 −t  2

+ A3 exp

 −t  3

,

where I is the phosphorescence intensity at any time t after cutting off the UV excitation, A1 and A2 are constants, and  1,  2 and  3 are decay times for the exponential components. The decay parameters for the fitting data are listed in Table 2. The rapid decay time ( 1 ) may be attributed to the shallow trapping states while longest lifetime components ( 3 ) correspond to the relatively deeper traps. The medium lifetime value ( 2 ), can be attributed to the intermediate levels between the shallow trapping states and deeper traps. Fig. 8 shows a typical decay curve for the CaS:Ce. The longest decay

Fig. 8. Luminescence decay curve of the CaS:Ce nanophosphor and a typical third order exponential fit of CaS:Ce (0.04 mol%) nanophosphor (inset).

times were recorded from the nanophosphor doped with 0.04 mol% of Ce3+ and the shortest decays times were observed from that doped with 0.02 mol% of Ce3+ , in agreement with the PL intensity data (Fig. 7). The commercial phosphors P-24 (CaS:Ce) has a lifetime of 1.5 ␮s [27]. In present case, we have observed a lifetime up to in the order of milliseconds (ms). Bhatti et al. [28] recently reported the enhancement in the PL decay processes of doped CaS bulk phosphors. The increase in lifetime in the nanophosphor can be attributed to more radiative relaxation caused by surface defect that act as luminescent centers, because the lifetime is generally equals to 1/ = 1/ r + 1/ nr , where  r and  nr are radiative and nonradiative rates respectively, when the surface area increases with decrease in particle size, there are more and more defect which may act as luminescent centers in the sample, with the increase of radiative rate, the lifetime turns to be longer, thus suggesting slower relaxation process and enhanced decay characteristics. The PL decay of the present phosphor is governed by the interaction of the host band energy states and the dopant states. In general, the extent of mobility or delocalization of charge carriers and their re-localization back again decides the decay characteristics of any phosphor. The delocalized state wavefunction may be continuous or discrete at the dopant site [29]. Due to a reduction in particle size, efficient host dopant exchange interactions or wavefunction overlapping can be anticipated [30]. Whatever may be the case, it is mandatory to have re-localization via some transitions so that the phosphor may exhibit luminescence decay. In the present case, there arise two possibilities for the re-localization: (a) the T2g sate of Ce3+ lying just below the conduction band offers an effective charge carrier trap or (b) some intermediate trapping state of the host lying below the conduction band is capable of capturing a mobile charge carrier. The observation of longer lifetime (≈ms) suggests that the

Table 2 Lifetime values of CaS:Ce nanophosphors at 300 K. Dopant concentration Ce3+ (mol%)

A1

A2

A3

0.02 0.04 0.06 0.08 0.10

159.7 161.2 153.2 155.1 151.5

53 32 26 21.2 19.9

12.2 22.8 21.3 19.8 11.5

Lifetime (ms) of 2 D(5d)→2 F5/2 transition 1

2

3

3.12 4.94 3.84 3.62 2.48

68.0 27.3 21.7 20.4 16.6

157.2 300.3 110.3 108.7 98.3

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luminescent center is a quasistable state and the host lattice defect sites are competing in the relaxation process. 4. Conclusions CaS:Ce3+ nanophosphors have been synthesized by coprecipitation method. The optical absorption indirect energy bandgap having average particle size 42 ± 3 nm was estimated as 4.61 ± 0.01 eV. At a Ce3+ concentration of 0.04 mol% the PL intensity was found to be a maximum. The PL emission at 507 nm was assigned to the 2 D(5d) → 2 F5/2 (4f) transition of the Ce3+ ion and the shoulder at 560 nm was due to the 2 D(5d) → 2 F7/2 (4f) transition. The PL emission peak of the CaS:Ce nanophosphors was blue–green shifted from the normal emission peak of the bulk CaS:Ce. The larger lifetimes (in ms) and an intensity increase of the CaS:Ce nanophosphors show improved emission and decay properties. Acknowledgement We are very thankful to the South African national research foundation (NRF) for providing the financial support. References [1] K. Onisawa, Y. Abe, K. Tamura, Nakayama, T.M. Hanazono, T.A. Ono, J. Electrochem. Soc. 138 (1991) 599. [2] R.S. Crandall, Appl. Phys. Lett. 50 (1987) 551. [3] H. Kasano, K. Megumi, H. Yamamoto, J. Electrochem. Soc. 131 (1984) 1953. [4] H. Yamamoto, K. Megumi, H. Kasano, J. Electrochem. Soc. 134 (1987) 2620. [5] G.L. Marwaha, N. Singh, D.R. Vij, V.K. Mathur, Mater. Res. Bull. 14 (1979) 1489.

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