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Greenish yellow emission from wurtzite structured ZnS:Ce nanophosphor synthesized at low temperature
Journal of Luminescence 192 (2017) 123–128
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Greenish yellow emission from wurtzite structured ZnS:Ce nanophosphor synthesized at low temperature K.R. Bindua,b, E.I. Anilaa, a b
MARK
⁎
Optoelectronic and Nanomaterials’ Research Laboratory, Department of Physics, Union Christian College, Aluva, Kerala 683102, India Department of Physics, Sree Sankara Vidyapeetom College, Valayanchirangara, Kerala 683556, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructured materials Semiconductors Sintering Optical properties Phase transitions Luminescence
Cerium doped ZnS nanoparticles were synthesized by aqueous colloidal precipitation method at 70 °C and post sintering at 650 °C. Their structural, morphological, and photoluminescence properties were investigated. Fourier transform infrared and X-ray diffraction were used to determine chemical bonding and crystal structure of the synthesized nanoparticles. A phase transformation from cubic to wurtzite structure was observed due to sintering at 650 °C. Scanning electron microscope revealed that the sintered particles exhibit uneven blocky particles with irregular shape. The band-gap of the nanoparticles was determined from diffuse reflectance spectrum. For the sintered ZnS:Ce nanoparticles when excited at about 350 nm, in addition to host UV emission, transitions from 2D to both 2F5/2 and 2F7/2 levels of Ce3+ in ZnS lattice were observed resulting in a broad greenish yellow emission band centered at 538 nm.
1. Introduction Semiconductor nanoparticles have attracted widespread attention due to their unique properties and potential applications arising from quantum confinement effects. Among luminescent II–VI materials as a wide band-gap semiconductor (3.6–3.9 eV), ZnS has various applications other than biomedical labelling, such as displays, sensors and lasers [1–5]. From the fundamental point of view, a wide variety of semiconductors can be expected to generate tunable dopant emission in different spectral window once they get successfully doped with transition metals, rare earth metals, etc. It is well known that lanthanide ions (rare earth metals) are effective luminescent centres because the excitation of rare earth ions can occur by the recombination of carriers in the confined semiconductors and subsequent energy transfer to the rare earth ions. Since the rare earth ions offer the possibility of attaining blue, greenish yellow and red colours, which are necessary for full colour devices, lanthanide doped nanocrystals have drawn growing attention as phosphor materials for the use in optical display devices. Bhargava et al. has discussed the doping of RE ions in ZnS nanoparticles [6] and also reported that they can be used for producing efficient luminescent materials. Among the rare earth metal ions, Ce3+ is one of the most interesting dopant due to their wide application for visible light-emitting phosphors in display, high-power laser, and light-emitting diode [7–9]. The electronic structure of rare earth ions differ from the other elements because of incompletely filled 4f and 5d shells. The
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Corresponding author. E-mail address: [email protected] (E.I. Anila).
http://dx.doi.org/10.1016/j.jlumin.2017.06.047 Received 6 January 2017; Received in revised form 17 June 2017; Accepted 20 June 2017 Available online 21 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
4f and 5d energy levels of the Ce3+ ions in the host materials determine the excitation and emission properties of Ce doped phosphors. These 4f ground state and 5d excited state of free Ce3+ ion are separated by energy difference of around 6.2 eV. In the crystal field of host material this gap is decreased due to crystal field splitting and downward shift of the centroid of the 5d energy level [10]. The spectra related to excitation and emission will be broad for transition between 4fn−15d1 and 4fn levels compared with the very narrow peaks from transitions between 4fn levels. The key issue for good understanding of energy transfer mechanism in Ce is the crystal field splitting of Ce3+ ions in the crystalline lattice of ZnS. The 4f shell is located between the 5p and 5d shell so the influence of the crystal field on the 4f energy levels is weak. As a result the 4f energy levels in solids are not very different from those of free ions and further do not change much even when the host lattice is changed. By the influence of crystal field of the host, the 4f energy levels may slightly split and the transition between these energy levels leads to narrow emission peaks. The exact position of split 4f levels is dependent on the crystal field of the host lattice. Excitation of nanocrystals doped with such ions leads to the electric dipole allowed 5d → 4f optical transitions resulting in broad emission of UV, Visible and IR light. In ZnS:Ce3+ nanoparticles two PL emission peaks can be observed corresponding to two transition 2D → 2F5/2 and 2D→ 2F7/2. But in the present work, broad PL spectrum centered at 538 nm with greenish yellow colour is observed. It is considered that the two PL bands corresponding to two transition 2D → 2F5/2 and 2D→ 2F7/2
Journal of Luminescence 192 (2017) 123–128
K.R. Bindu, E.I. Anila
reported by the authors previously [27]. We followed the same procedure in the synthesis of cerium doped ZnS nanoparticles. Twenty five ml each of zinc acetate Zn (CH3COO)2, Ce(NO3)3 and Na2S solutions in water were used for the preparation of Ce3+ doped ZnS nanoparticles. 0.02 M solution of Ce(NO3)3 was added drop wise to 1 M zinc acetate Zn(CH3COO)2 solution to reduce agglomeration and to disperse cerium equally, later it was heated to 70 °C. One molar Na2S solution was added drop wise with continuous stirring using magnetic stirrer. The solution was stirred for 20 min keeping temperature constant. The resulting white colloidal suspension was filtered, and the filtrate was washed with de-ionized water and dried by keeping in an oven at 70 °C for 1 day. A part of this sample was then sintered at 650 °C for six hours in furnace with the passage of hydrogen sulphide to prevent the formation of ZnO.
overlap, resulting in broad emission spectrum with a single peak. Okamoto et al. observed one broad peak in ZnS:Ce3+ nanoparticles giving green emission [11]. In our work a phase change from cubic to hexagonal takes place for sintered sample. Stachowicz et al. discussed crystal field splitting of energy levels of rare earth ions in cubic and hexagonal GaN based on PL emission [12]. Tang et al. found that the phase transition from cubic to hexagonal phase of ZnS:Sm will boost the overall photoluminescence emission intensity only [13]. Similarly in our work same intensified host emission and dopant related emission is obtained for the sintered sample with a phase change from cubic to hexagonal phase. Larger ionic radius of Ce3+(1.03 Å) than that of Zn (0.74 Å),the valence mismatch of Ce ion (trivalent) and Zn ion (divalent) and higher coordination number of rare earth ions in comparison with Zn ions, make it difficult for the substitution of Ce into a Zn site in ZnS compound [14]. Hence, only on sintering ZnS:Ce3+ nanoparticles synthesized at 70 °C produces greenish yellow colour spectrum corresponding to Ce 3+ ions. Many approaches have been used for the controlled synthesis of pure ZnS and transition metal doped ZnS nanocrystals in the form of nanoparticles, nanosheets, and various hierarchical nanostructures [15–23]. At room temperature for ZnS, cubic phase is the most stable phase. From the literature survey, it is known that usually rare earth doped ZnS nanoparticles in wurtzite phase were synthesized at high temperature (above 750 °C) [11,12,24–26]. But in the present work Ce doped ZnS nanoparticles with wurtzite phase is obtained at 650 °C. A limited number of reports is available which has focused on optical properties, especially emission properties of ZnS nanoparticles doped with rare earth ions. However no reports are available which deals with the synthesis and optical characterization of hexagonal ZnS:Ce3+nanocrystals from which the two characteristic emissions corresponding to two transition 2D → 2F5/2 and 2D→ 2F72/are obtained. In this article, synthesis of ZnS nanocrysallites doped with cerium, changes in optical and structural properties on sintering and the origin of greenish yellow luminescence have been reported.
2.3. Instruments and measurements X-ray diffraction (XRD) patterns of pure ZnS, as synthesized ZnS:Ce and sintered ZnS:Ce powder samples were measured by using Bruker AXS D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5405 Å) as Xray source at 40 kV and 35 mA in the scan range 200–800. Diffuse reflectance (DRS) measurements of the samples were performed using Varian Cary 5000 UV–VIS–NIR spectrophotometer with a spectral bandwidth of 2 nm for energy gap determination. Fourier transform infrared (FTIR)spectra were obtained on a FTIR (Shimadzu) spectrophotometer. Inductively coupled plasma (ICP) (ICP-1000IV, Shimadzu) analysis was conducted to determine the chemical composition of the samples. Photoluminescence (PL) spectra were recorded at room temperature using Horiba- Fluromax 4C research spectro fluorometer with a 150 W ozone free Xenon lamp as an excitation source of range 200–900 nm. The observation was done keeping the slit width at 3 nm and integration time 0.1 s. Morphological study was carried out using scanning electron microscopy (SEM) with Jeol - JSM 6390. 3. Results and discussion
2. Experimental details 3.1. Structural characterization and morphology 2.1. Materials In order to investigate the crystalline structure and purity of nanoparticles, X-ray diffractometery was performed on the samples of ZnS and the ZnS:Ce3+ nanoparticles. The XRD patterns for the three samples are presented in Fig. 1A. The XRD patterns of the pure ZnS and as synthesized doped samples exhibited broad peaks typical of nanosized materials. Broadening of XRD lines is also associated with the nonuniform distribution of local strains arising from defects like dislocation, twinning etc. The crystal structure of the undoped ZnS nanoparticles correspond to the pattern of cubic zinc blend with peaks
Analytically pure zinc acetate [Zn(CH3COO)2], sodium sulphide [Na2S] and cerium nitrate [Ce(NO3)3] were used as received without further purification. All solutions were prepared using ultra-pure deionized water as solvent. 2.2. Synthesis Pure ZnS nanoparticles synthesized by wet chemical method were
Fig. 1. XRD Pattern (A) and WH Plot (B) of a) ZnS b) As synthesized ZnS:Ce
3+
and c) sintered ZnS:Ce3+ nanoparticles (δ corresponds to peaks of Zn(OH)2. * denotes undefined peak).
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Fig. 2. A) SEM of as synthesized ZnS:Ce nanoparticles B) sintered ZnS:Ce
h2
a2 + k2 + l2
(1)
δ=
where dhkl is the interplanar separation corresponding to Miller indices h, k, and l. The lattice constants are obtained as a = b = c = 5.33 Å. But for the sintered sample, its hexagonal phase is characterized by lattice parameters a and c and can be calculated by the relation
1 4 (h2 + k 2 + hk ) l2 = + 2 2 2 3 a c dhkl
0. 9λ β cos θ
(2)
(3)
where ‘D’ is the mean grain size, λ the wavelength of the X rays used, β is the full width at half maximum(FWHM) and θ is the position of the diffraction peak. The estimated crystallite size in the undoped sample is 2.9 nm. The grain size that has been calculated from the most prominent XRD peaks of as synthesized doped sample is 4.4 nm and that of sintered sample is 36 nm. The strain and grain size of the samples were calculated by Williamson–Hall (W–H) method. According to this method, the FWHM (β) may be expressed as linear combination of lattice strain (ξ) and particle size (D) by the equation
β cos θ =
kλ + ξ sin θ D
3+
nanoparticles.
1 D2
(5)
The dislocation densities of the three samples were estimated to be 0.13 × 1018, 0.048 × 1018 and 0.00055 × 1018 m−2. Dislocation density is minimum for sintered sample since dislocated atoms occupy the regions near the grain boundaries thereby decreasing the density with temperature. SEM investigations have been carried out on the prepared samples to determine the changes in morphology of the synthesized samples. SEM (Fig. 2(A) and (B)) images show a cluster of agglomerated particles for the as synthesized sample and sintered sample. Even though the resolution of the used SEM instrument limits the determination of actual size of agglomerated particles it is found that the particle size is greater for sintered sample. A typical EDS spectrum obtained from the analysed samples is shown in Fig. 2C. The EDS lines corresponding to Zn, S and Ce have been identified. The actual Ce concentration in the samples was determined by ICP-AES. The concentration of Ce in the as synthesized and sintered samples are lesser than that in the precursor solution. In the precursor solution the concentration of Ce was 2 mol% whereas by ICP-AES analysis, Ce concentration in the as synthesized and sintered samples are only 1.6 mol% each (Table 1).
and obtained as a = 3.84 Å and c = 6.3 Å. But these values were found to be slightly larger than those of pure hexagonal ZnS (a = 3.820 Å and c = 6.257 Å) which is probably caused by interstitial incorporation of Ce 3+. This is consistent with the fact that the ionic radius of Ce3+ is 1.03 Å where as that of Zn is 0.74 Å. Average size of the crystallites has been calculated from XRD pattern using Scherrer's equation
D=
nanoparticles & C) EDAX of sintered ZnS:Ce
of strain ξ and particle size are not accurate since W-H plot does not form a linear fit. However the crystallite size of pure ZnS, as synthesized ZnS: Ce3+and sintered nanocrystallites are found to be in the range of 2.68, 4.7 and 47 nm respectively. Similarly the strain ξ estimated from the slope of the line are found to be − 0.036, 0.029 and 0.031 for the undoped ZnS, as synthesized ZnS:Ce and sintered nanoparticles. Here negative sign for the undoped sample shows compressive strain in ZnS and this is confirmed by the reduced lattice parameter of the crystals compared to the bulk. Dislocations are defects in a crystalline material linked with disarray of the lattice in one part with respect to another part of the crystal. Dislocation density (δ) in the samples was determined by applying the Williamson and Smallmans relation,
indexed as (111), (220) and (311), which match well with the standard card, JCPDS No. 65-0309. For as prepared doped sample same diffraction peaks are observed along with multiple peaks of Zn(OH)2 phase (JCPDS File no. 201437) and one unidentified peak. However, for the sintered ZnS:Ce3+nanoparticles the diffraction peaks correspond to the lattice planes of (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) which are consistent with the hexagonal wurtzite (JCPDS no. 36-1450) crystal structure. The diffraction peaks of sintered ZnS:Ce nanoparticles are narrower, sharper and more intense than the other two samples indicating the increase in particle size and crystallinity on sintering. For the undoped sample which is in cubic phase, the lattice parameter can be calculated using the relation
d2hkl =
3+
3.2. Optical characterization UV–Vis reflection spectroscopy has been used to study the absorption characteristics of nanoparticles. Fig. 3 (A)–(C) depicts DRS spectra of undoped, as synthesized Ce3+ doped ZnS and sintered Ce3+ doped ZnS nanoparticles measured at room temperature. Absorption spectra of the samples which are obtained from the diffuse reflectance spectra by using Kubelka Munk function,
(4)
where k = 0.9 is the crystallite shape constant and other parameters have the same meaning as in Eq. (3). By applying a linear fit to the plot of βcosθ vs sinθ (Fig. 1(B)) the slope ξ and intercept kλ/D is determined. In the case of as synthesized ZnS:Ce nanoparticles, the values
F (R) = α =
(1 − R)2 k = 2R s
(6)
where R, k, s are the reflection, absorption and scattering coefficients. 125
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Table 1 Crystallite size, lattice parameter, strain, dislocation density & band gap of ZnS:Ce3+ nanoparticles. Sample name
Crystallite size from Scherrer eqn. (nm)
Crystallite size by W-H method (nm)
Lattice parameter (Å)
Strain
Dislocation density × 1018 m−2
Band gap (eV)
ZnS ZnS:Ce3+ at 70 °C sintered ZnS:Ce3+
2.9 4.4 36
2.68 4.7 47
a = b = c = 5.333 Å. a = b = c = 5.388 Å a = b = 3.8 Å c = 6.3 Å
− 0.036 0.029 0.031
0.13 0.048 0.00055
3.7 3.56 3.95
The optical band gap Eg is related to the absorption coefficient α associated with the strong absorption region through following expression,
αhν ~K (hν − Eg )n/2
(7)
where ‘K’ is proportionality constant, h ν is the photon energy, n equal to 1 for ZnS, the direct band gap semiconductor. The Fig. 3 (inset) shows the plot of [F(R) hν]2 vs hν for the samples. By extrapolating the linear portion of the graph to α = 0, the bandgap of pure ZnS, ZnS:Ce and sintered ZnS:Ce nanoparticles are determined as 3.7, 3.56 and 3.95 eV. In the case of bulk zinc sulphide the band gap is 3.65 eV for cubic phase and 3.90 eV for hexagonal phase [28]. The increase in bandgap of undoped sample is due to quantum confinement effect. Since as synthesized sample contains impurities its bandgap is less than that of the bulk ZnS. Fig. 4 shows the FTIR spectra of the pure,as synthesized ZnS:Ce and sintered ZnS:Ce samples. FTIR spectra of the pure and as prepared doped sample exhibit a number of characteristic spectral bands. The broad band between 3500 and 2500 cm−1 in the three samples were assigned to the O–H stretching vibrations of absorbed water on the ZnS surface. The band at 1115 cm−1 is due to the characteristic vibrations C-O groups of the acetate of the nanoparticles [29,30]. The characteristic ZnS vibration peaks can be observed at 680 cm−1. The intense peak at 1400 cm−1 was derived from the existence of the C–O–H bending [31]. The intense bands centered at 1030 cm−1 and 1570 cm−1 are attributed to the C-O stretching mode and O-H bending modes of the water molecule [32]. The C–H out-of-plane band appeared at 920 cm−1. In sintered sample also almost all characteristics peaks are observed but with less intensity.
Fig. 4. FTIR spectra of ZnS, as synthesized and sintered ZnS:Ce3+ nanoparticles.
host emission (blue emission) and dopant emission from the host. Dopant related emission is always expected at a lower energy position as compared to the band energy absorption. Fig. 5A shows the PL emission spectra of the three samples at an excitation wavelength of 350 nm. The emission spectra of pure ZnS nanocrystallites and as synthesized ZnS:Ce nanocrystallites show three emission peaks, a broad emission with peak at 445 nm and the emissions around 390 and 575 nm. For ZnS nanoparticles the lattice or surface defects or native impurities are the emission centres and such defect related emission is positioned in the UV/ blue region. Hence it can be noted the blue emission centered at 445 nm should result from the recombination of electrons at sulfur vacancy donor level with holes trapped at the zinc vacancy acceptor [33]. The UV emission with peak around 390 nm in the samples is normally believed to originate from the emission of the self activated centres of host ZnS associated with defects such as vacancies and interstitial spaces [34–39]. Since sulfur ions have large ionic radii than zinc ions, sulfur ions produces more strain and hence have small binding energy. Hence the sulfur vacancy states are closer to the conduction band edge than zinc vacancy to the valence band edge. Therefore emission band around 390 nm is due to surface defect states mainly from the sulfur vacancies (Vs) [18,40–43]. The peak position of the blue emission at 445 nm does not change even after the Ce doping which indicates the energy level of the sulfur vacancy relative to the
3.3. Photoluminescence (PL) studies Since the useful information about electronic states of emitters, crystal field strength, energy transfer, electron–phonon coupling strength, and the interactions between the luminescence strength and the interactions between the luminescence centres and the hosts is provided by PL studies, optical properties of ZnS and ZnS:Ce3+ nanocrystallites have been also studied through room temperature photoluminescence spectra. The emission in ZnS:Ce3+ nanoparticles originate from the mutual interaction between the surface coordination of
Fig. 3. Diffuse reflectance spectra of A) ZnS B) as synthesized ZnS:Ce3+ C) sintered ZnS:Ce3+ nanoparticles (inset the plot of (αhν)2 vs hν).
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Fig. 5. A) Room Temp PL Spectra of (λex = 350 nm) of (a) pure ZnS (b) as synthesized ZnS:Ce3+ (c) sintered ZnS:Ce3+ (inset -the individual components by Gaussian fitting, for emission peak at 538 nm in the PL spectrum of sintered ZnS:Ce3+ nanoparticles, B) CIE Chromaticity diagram for (a) ZnS (b) as synthesized ZnS:Ce3+ and (c) sintered ZnS:Ce3+ nanoparticles. Fig. 6. Qualitative schematic band diagram showing emissions in ZnS, free Ce3+ ion and ZnS:Ce3+ nanoparticles.
Zinc vacancy level keeps constant. From the figure it can be seen that the intensity of emission at 445 nm in as synthesized ZnS:Ce nanocrystallites is higher than those for the undoped sample. As said above, since the peak at 445 nm is related with sulfur vacancy, more defects/ sulfur vacancy are introduced when Ce3+ is doped in to the ZnS host. The weak emission peak around 575 nm might be attributed to deep level emission due to native defects such as interstitial Zn atoms in ZnS [44]. But for the sintered ZnS:Ce3+ sample a strong greenish yellow emission centered at 538 nm is observed in addition to UV emission and emission at 575 nm whereas emission at 445 nm was absent. The origin of the PL peak at 538 nm in our sintered sample is due to Ce3+ incorporation in the ZnS host lattice. When Ce 3+ions are doped in ZnS lattice, due to crystal field effect the 5D → 4F gap get reduced to 2.4 eV from 6.2 eV resulting in strong greenish yellow emission. This increased intensity in the greenish yellow emission with the quenching of host emission at 445 nm is a good example of energy transfer process, in which direct energy transfer from defect states of ZnS host to Ce3+ related energy levels and then strong greenish yellow emission occurs from Ce3+ ions through 5D → 4F transition. Sharp green emission was observed by Raj et al. in ZnS:Ce3+ nanocrystallites [45]. Shen et al. also obtained such a broad emission in YAG:Ce phosphor [46]. Since the emission is broad for the sintered nanocrystallites Gaussian curve fitting was applied to deconvolute the PL curve. The PL spectrum of the sintered ZnS:Ce3+nanoparticles was deconvoluted (inset Fig. 5) into two weak peaks, which are centered at 514 nm and 556 nm respectively. The peak at 514 nm is close to the value reported by Shinji Okamoto and Katsu Tanaka in ZnS:Ce nanoparticles [11]. Therefore the
deconvoluted emission peaks at 514 nm and 556 nm could be because of the two ground states, 2F5/2 and 2F7/2 corresponding to two transition 2D → 2F5/2 and 2D → 2F7/2. Possible energy transfer mechanisms are proposed on the basis of energy level diagram (Fig. 6) for the observed UV, blue and greenish yellow emission. For the excitation wavelength of 350 nm excited electrons can make a transition to the vacancy level (shallow traps) formed by the sulfur and then nonradiatively to the 5D level of Ce3+ followed by a radiative transition to the 4F level (2F5/2 and 2F7/2) resulting in the emission of greenish yellow photons around 538 nm. The electrons in the shallow traps can also recombine directly with the ground state electrons leading to UV emission at 390 nm. To assess the performance of ZnS:Ce3+ ions to be used as phosphors in light sources, CIE (Commission Internationale de I′Eclairage 1931) chromaticity coordinates are calculated. The calculated colour coordinates for the emission spectra are (0.24, 0.26), (0.26, 0.29), (0.33, 0.38) (Fig. 5B) corresponding to ZnS, as synthesized ZnS:Ce3+and sintered ZnS:Ce3+ nanoparticles respectively. From CIE coordinates for sintered samples, it is assured that characteristic light from the sintered sample is greenish yellow. 4. Conclusion In summary, we have synthesized greenish yellow emitting ZnS:Ce3+ nanoparticles by sintering ZnS:Ce 3+ nanocrystallites prepared at 70 °C by simple wet chemical route. X-ray diffraction (XRD) patterns revealed that the undoped ZnS and doped sample synthesized 127
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K.R. Bindu, E.I. Anila
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at 70 °C exhibited pure zinc blende crystal structure and a phase transition from cubic to hexagonal phase takes place on sintering at a low temperature of 6500C. Defect related blue emission was observed from the undoped ZnS nanoparticles with CIE coordinates (0.24,0.26) and the emission turned to bluish white for cerium doped ZnS nanoparticles with CIE colour coordinates (0.26,0.29). An intense greenish yellow broad emission band centered at 538 nm with CIE coordinates (0.33,0.38) was observed in the sintered sample due to electric dipoleallowed 5d → 4f transitions of Ce3+ ions which makes them applicable in displays and markers in future photonic devices. Acknowledgements Authors are highly grateful to the Board of Research in Nuclear Sciences (BRNS) (34/14/58/2014-BRNS), Department of Atomic Energy (DAE), India, for providing the financial support. References [1] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 24–25. [2] P. Brown, P.V. Kamat, J. Am. Chem. Soc. 130 (2008) (8890A R, M). [3] I.L. Medintz, H. Clapp Attoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nat. Mater. 2 (2003) 630. [4] S. Pathak, S.K. Choi, N. Arnheim, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4103. [5] C.W. Chan Warren, S. Nie, Science 281 (1998). [6] R.N. Bhargava, J. Lumin. 70 (1996) 85. [7] V.Z. Mordkovich, H. Hayashi, M. Haemori, T. Fukumura, M. Kawasaki, Adv. Funct. Mater. 13 (2003) 519. [8] J.S. John, J.L. Coffer, Appl. Phys. Lett. 77 (2010) 1635. [9] B. Cheng, Y. Xiao, G. Wu, L. Zhang, Adv. Funct. Mater. 14 (2004) 913. [10] Nathan C. George, Correlating Long-range Order and Local Structure to the Properties of Inorganic Solids, 2013. [11] Shinji Okamoto, Katsu Tanaka, Phys. Stat. Sol. (c) 3 (4) (2006) 1059–1062. [12] M. Stachowicz, A. Kozanecki, C.G. Ma, M.G.J. Brik, Y. Lin Hx Jiang, J.M. Zavada, Opt. Mater. 37 (2014) 165–174. [13] T.P. Tang, M.R. Yang, K.S. Chen, Ceram. Int. 26 (2000) 153. [14] V. Ranganaik, S.B.N. Halehatty, K.G.S. Yashavanth, K.P.N. Prashanth, N.H. Handugadahalli, C.P. Mustur, J. Nanotechnol. (2014) 924797. [15] K.E. Waldrip, J.S. Lewis III, Q. Zhai, M.R. Davidson, P.H. Holloway, S.S. Sun, Appl. Phys. Lett. 76 (2000) 1276. [16] F. Parsapour, D.F. Kelley, S. Craft, J.P. Wilcoxon, J. Chem. Phys. 104 (1996) 4978.
128
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Greenish yellow emission from wurtzite structured ZnS:Ce nanophosphor synthesized at low temperature K.R. Bindua,b, E.I. Anilaa, a b
MARK
⁎
Optoelectronic and Nanomaterials’ Research Laboratory, Department of Physics, Union Christian College, Aluva, Kerala 683102, India Department of Physics, Sree Sankara Vidyapeetom College, Valayanchirangara, Kerala 683556, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructured materials Semiconductors Sintering Optical properties Phase transitions Luminescence
Cerium doped ZnS nanoparticles were synthesized by aqueous colloidal precipitation method at 70 °C and post sintering at 650 °C. Their structural, morphological, and photoluminescence properties were investigated. Fourier transform infrared and X-ray diffraction were used to determine chemical bonding and crystal structure of the synthesized nanoparticles. A phase transformation from cubic to wurtzite structure was observed due to sintering at 650 °C. Scanning electron microscope revealed that the sintered particles exhibit uneven blocky particles with irregular shape. The band-gap of the nanoparticles was determined from diffuse reflectance spectrum. For the sintered ZnS:Ce nanoparticles when excited at about 350 nm, in addition to host UV emission, transitions from 2D to both 2F5/2 and 2F7/2 levels of Ce3+ in ZnS lattice were observed resulting in a broad greenish yellow emission band centered at 538 nm.
1. Introduction Semiconductor nanoparticles have attracted widespread attention due to their unique properties and potential applications arising from quantum confinement effects. Among luminescent II–VI materials as a wide band-gap semiconductor (3.6–3.9 eV), ZnS has various applications other than biomedical labelling, such as displays, sensors and lasers [1–5]. From the fundamental point of view, a wide variety of semiconductors can be expected to generate tunable dopant emission in different spectral window once they get successfully doped with transition metals, rare earth metals, etc. It is well known that lanthanide ions (rare earth metals) are effective luminescent centres because the excitation of rare earth ions can occur by the recombination of carriers in the confined semiconductors and subsequent energy transfer to the rare earth ions. Since the rare earth ions offer the possibility of attaining blue, greenish yellow and red colours, which are necessary for full colour devices, lanthanide doped nanocrystals have drawn growing attention as phosphor materials for the use in optical display devices. Bhargava et al. has discussed the doping of RE ions in ZnS nanoparticles [6] and also reported that they can be used for producing efficient luminescent materials. Among the rare earth metal ions, Ce3+ is one of the most interesting dopant due to their wide application for visible light-emitting phosphors in display, high-power laser, and light-emitting diode [7–9]. The electronic structure of rare earth ions differ from the other elements because of incompletely filled 4f and 5d shells. The
⁎
Corresponding author. E-mail address: [email protected] (E.I. Anila).
http://dx.doi.org/10.1016/j.jlumin.2017.06.047 Received 6 January 2017; Received in revised form 17 June 2017; Accepted 20 June 2017 Available online 21 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
4f and 5d energy levels of the Ce3+ ions in the host materials determine the excitation and emission properties of Ce doped phosphors. These 4f ground state and 5d excited state of free Ce3+ ion are separated by energy difference of around 6.2 eV. In the crystal field of host material this gap is decreased due to crystal field splitting and downward shift of the centroid of the 5d energy level [10]. The spectra related to excitation and emission will be broad for transition between 4fn−15d1 and 4fn levels compared with the very narrow peaks from transitions between 4fn levels. The key issue for good understanding of energy transfer mechanism in Ce is the crystal field splitting of Ce3+ ions in the crystalline lattice of ZnS. The 4f shell is located between the 5p and 5d shell so the influence of the crystal field on the 4f energy levels is weak. As a result the 4f energy levels in solids are not very different from those of free ions and further do not change much even when the host lattice is changed. By the influence of crystal field of the host, the 4f energy levels may slightly split and the transition between these energy levels leads to narrow emission peaks. The exact position of split 4f levels is dependent on the crystal field of the host lattice. Excitation of nanocrystals doped with such ions leads to the electric dipole allowed 5d → 4f optical transitions resulting in broad emission of UV, Visible and IR light. In ZnS:Ce3+ nanoparticles two PL emission peaks can be observed corresponding to two transition 2D → 2F5/2 and 2D→ 2F7/2. But in the present work, broad PL spectrum centered at 538 nm with greenish yellow colour is observed. It is considered that the two PL bands corresponding to two transition 2D → 2F5/2 and 2D→ 2F7/2
Journal of Luminescence 192 (2017) 123–128
K.R. Bindu, E.I. Anila
reported by the authors previously [27]. We followed the same procedure in the synthesis of cerium doped ZnS nanoparticles. Twenty five ml each of zinc acetate Zn (CH3COO)2, Ce(NO3)3 and Na2S solutions in water were used for the preparation of Ce3+ doped ZnS nanoparticles. 0.02 M solution of Ce(NO3)3 was added drop wise to 1 M zinc acetate Zn(CH3COO)2 solution to reduce agglomeration and to disperse cerium equally, later it was heated to 70 °C. One molar Na2S solution was added drop wise with continuous stirring using magnetic stirrer. The solution was stirred for 20 min keeping temperature constant. The resulting white colloidal suspension was filtered, and the filtrate was washed with de-ionized water and dried by keeping in an oven at 70 °C for 1 day. A part of this sample was then sintered at 650 °C for six hours in furnace with the passage of hydrogen sulphide to prevent the formation of ZnO.
overlap, resulting in broad emission spectrum with a single peak. Okamoto et al. observed one broad peak in ZnS:Ce3+ nanoparticles giving green emission [11]. In our work a phase change from cubic to hexagonal takes place for sintered sample. Stachowicz et al. discussed crystal field splitting of energy levels of rare earth ions in cubic and hexagonal GaN based on PL emission [12]. Tang et al. found that the phase transition from cubic to hexagonal phase of ZnS:Sm will boost the overall photoluminescence emission intensity only [13]. Similarly in our work same intensified host emission and dopant related emission is obtained for the sintered sample with a phase change from cubic to hexagonal phase. Larger ionic radius of Ce3+(1.03 Å) than that of Zn (0.74 Å),the valence mismatch of Ce ion (trivalent) and Zn ion (divalent) and higher coordination number of rare earth ions in comparison with Zn ions, make it difficult for the substitution of Ce into a Zn site in ZnS compound [14]. Hence, only on sintering ZnS:Ce3+ nanoparticles synthesized at 70 °C produces greenish yellow colour spectrum corresponding to Ce 3+ ions. Many approaches have been used for the controlled synthesis of pure ZnS and transition metal doped ZnS nanocrystals in the form of nanoparticles, nanosheets, and various hierarchical nanostructures [15–23]. At room temperature for ZnS, cubic phase is the most stable phase. From the literature survey, it is known that usually rare earth doped ZnS nanoparticles in wurtzite phase were synthesized at high temperature (above 750 °C) [11,12,24–26]. But in the present work Ce doped ZnS nanoparticles with wurtzite phase is obtained at 650 °C. A limited number of reports is available which has focused on optical properties, especially emission properties of ZnS nanoparticles doped with rare earth ions. However no reports are available which deals with the synthesis and optical characterization of hexagonal ZnS:Ce3+nanocrystals from which the two characteristic emissions corresponding to two transition 2D → 2F5/2 and 2D→ 2F72/are obtained. In this article, synthesis of ZnS nanocrysallites doped with cerium, changes in optical and structural properties on sintering and the origin of greenish yellow luminescence have been reported.
2.3. Instruments and measurements X-ray diffraction (XRD) patterns of pure ZnS, as synthesized ZnS:Ce and sintered ZnS:Ce powder samples were measured by using Bruker AXS D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5405 Å) as Xray source at 40 kV and 35 mA in the scan range 200–800. Diffuse reflectance (DRS) measurements of the samples were performed using Varian Cary 5000 UV–VIS–NIR spectrophotometer with a spectral bandwidth of 2 nm for energy gap determination. Fourier transform infrared (FTIR)spectra were obtained on a FTIR (Shimadzu) spectrophotometer. Inductively coupled plasma (ICP) (ICP-1000IV, Shimadzu) analysis was conducted to determine the chemical composition of the samples. Photoluminescence (PL) spectra were recorded at room temperature using Horiba- Fluromax 4C research spectro fluorometer with a 150 W ozone free Xenon lamp as an excitation source of range 200–900 nm. The observation was done keeping the slit width at 3 nm and integration time 0.1 s. Morphological study was carried out using scanning electron microscopy (SEM) with Jeol - JSM 6390. 3. Results and discussion
2. Experimental details 3.1. Structural characterization and morphology 2.1. Materials In order to investigate the crystalline structure and purity of nanoparticles, X-ray diffractometery was performed on the samples of ZnS and the ZnS:Ce3+ nanoparticles. The XRD patterns for the three samples are presented in Fig. 1A. The XRD patterns of the pure ZnS and as synthesized doped samples exhibited broad peaks typical of nanosized materials. Broadening of XRD lines is also associated with the nonuniform distribution of local strains arising from defects like dislocation, twinning etc. The crystal structure of the undoped ZnS nanoparticles correspond to the pattern of cubic zinc blend with peaks
Analytically pure zinc acetate [Zn(CH3COO)2], sodium sulphide [Na2S] and cerium nitrate [Ce(NO3)3] were used as received without further purification. All solutions were prepared using ultra-pure deionized water as solvent. 2.2. Synthesis Pure ZnS nanoparticles synthesized by wet chemical method were
Fig. 1. XRD Pattern (A) and WH Plot (B) of a) ZnS b) As synthesized ZnS:Ce
3+
and c) sintered ZnS:Ce3+ nanoparticles (δ corresponds to peaks of Zn(OH)2. * denotes undefined peak).
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Fig. 2. A) SEM of as synthesized ZnS:Ce nanoparticles B) sintered ZnS:Ce
h2
a2 + k2 + l2
(1)
δ=
where dhkl is the interplanar separation corresponding to Miller indices h, k, and l. The lattice constants are obtained as a = b = c = 5.33 Å. But for the sintered sample, its hexagonal phase is characterized by lattice parameters a and c and can be calculated by the relation
1 4 (h2 + k 2 + hk ) l2 = + 2 2 2 3 a c dhkl
0. 9λ β cos θ
(2)
(3)
where ‘D’ is the mean grain size, λ the wavelength of the X rays used, β is the full width at half maximum(FWHM) and θ is the position of the diffraction peak. The estimated crystallite size in the undoped sample is 2.9 nm. The grain size that has been calculated from the most prominent XRD peaks of as synthesized doped sample is 4.4 nm and that of sintered sample is 36 nm. The strain and grain size of the samples were calculated by Williamson–Hall (W–H) method. According to this method, the FWHM (β) may be expressed as linear combination of lattice strain (ξ) and particle size (D) by the equation
β cos θ =
kλ + ξ sin θ D
3+
nanoparticles.
1 D2
(5)
The dislocation densities of the three samples were estimated to be 0.13 × 1018, 0.048 × 1018 and 0.00055 × 1018 m−2. Dislocation density is minimum for sintered sample since dislocated atoms occupy the regions near the grain boundaries thereby decreasing the density with temperature. SEM investigations have been carried out on the prepared samples to determine the changes in morphology of the synthesized samples. SEM (Fig. 2(A) and (B)) images show a cluster of agglomerated particles for the as synthesized sample and sintered sample. Even though the resolution of the used SEM instrument limits the determination of actual size of agglomerated particles it is found that the particle size is greater for sintered sample. A typical EDS spectrum obtained from the analysed samples is shown in Fig. 2C. The EDS lines corresponding to Zn, S and Ce have been identified. The actual Ce concentration in the samples was determined by ICP-AES. The concentration of Ce in the as synthesized and sintered samples are lesser than that in the precursor solution. In the precursor solution the concentration of Ce was 2 mol% whereas by ICP-AES analysis, Ce concentration in the as synthesized and sintered samples are only 1.6 mol% each (Table 1).
and obtained as a = 3.84 Å and c = 6.3 Å. But these values were found to be slightly larger than those of pure hexagonal ZnS (a = 3.820 Å and c = 6.257 Å) which is probably caused by interstitial incorporation of Ce 3+. This is consistent with the fact that the ionic radius of Ce3+ is 1.03 Å where as that of Zn is 0.74 Å. Average size of the crystallites has been calculated from XRD pattern using Scherrer's equation
D=
nanoparticles & C) EDAX of sintered ZnS:Ce
of strain ξ and particle size are not accurate since W-H plot does not form a linear fit. However the crystallite size of pure ZnS, as synthesized ZnS: Ce3+and sintered nanocrystallites are found to be in the range of 2.68, 4.7 and 47 nm respectively. Similarly the strain ξ estimated from the slope of the line are found to be − 0.036, 0.029 and 0.031 for the undoped ZnS, as synthesized ZnS:Ce and sintered nanoparticles. Here negative sign for the undoped sample shows compressive strain in ZnS and this is confirmed by the reduced lattice parameter of the crystals compared to the bulk. Dislocations are defects in a crystalline material linked with disarray of the lattice in one part with respect to another part of the crystal. Dislocation density (δ) in the samples was determined by applying the Williamson and Smallmans relation,
indexed as (111), (220) and (311), which match well with the standard card, JCPDS No. 65-0309. For as prepared doped sample same diffraction peaks are observed along with multiple peaks of Zn(OH)2 phase (JCPDS File no. 201437) and one unidentified peak. However, for the sintered ZnS:Ce3+nanoparticles the diffraction peaks correspond to the lattice planes of (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) which are consistent with the hexagonal wurtzite (JCPDS no. 36-1450) crystal structure. The diffraction peaks of sintered ZnS:Ce nanoparticles are narrower, sharper and more intense than the other two samples indicating the increase in particle size and crystallinity on sintering. For the undoped sample which is in cubic phase, the lattice parameter can be calculated using the relation
d2hkl =
3+
3.2. Optical characterization UV–Vis reflection spectroscopy has been used to study the absorption characteristics of nanoparticles. Fig. 3 (A)–(C) depicts DRS spectra of undoped, as synthesized Ce3+ doped ZnS and sintered Ce3+ doped ZnS nanoparticles measured at room temperature. Absorption spectra of the samples which are obtained from the diffuse reflectance spectra by using Kubelka Munk function,
(4)
where k = 0.9 is the crystallite shape constant and other parameters have the same meaning as in Eq. (3). By applying a linear fit to the plot of βcosθ vs sinθ (Fig. 1(B)) the slope ξ and intercept kλ/D is determined. In the case of as synthesized ZnS:Ce nanoparticles, the values
F (R) = α =
(1 − R)2 k = 2R s
(6)
where R, k, s are the reflection, absorption and scattering coefficients. 125
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Table 1 Crystallite size, lattice parameter, strain, dislocation density & band gap of ZnS:Ce3+ nanoparticles. Sample name
Crystallite size from Scherrer eqn. (nm)
Crystallite size by W-H method (nm)
Lattice parameter (Å)
Strain
Dislocation density × 1018 m−2
Band gap (eV)
ZnS ZnS:Ce3+ at 70 °C sintered ZnS:Ce3+
2.9 4.4 36
2.68 4.7 47
a = b = c = 5.333 Å. a = b = c = 5.388 Å a = b = 3.8 Å c = 6.3 Å
− 0.036 0.029 0.031
0.13 0.048 0.00055
3.7 3.56 3.95
The optical band gap Eg is related to the absorption coefficient α associated with the strong absorption region through following expression,
αhν ~K (hν − Eg )n/2
(7)
where ‘K’ is proportionality constant, h ν is the photon energy, n equal to 1 for ZnS, the direct band gap semiconductor. The Fig. 3 (inset) shows the plot of [F(R) hν]2 vs hν for the samples. By extrapolating the linear portion of the graph to α = 0, the bandgap of pure ZnS, ZnS:Ce and sintered ZnS:Ce nanoparticles are determined as 3.7, 3.56 and 3.95 eV. In the case of bulk zinc sulphide the band gap is 3.65 eV for cubic phase and 3.90 eV for hexagonal phase [28]. The increase in bandgap of undoped sample is due to quantum confinement effect. Since as synthesized sample contains impurities its bandgap is less than that of the bulk ZnS. Fig. 4 shows the FTIR spectra of the pure,as synthesized ZnS:Ce and sintered ZnS:Ce samples. FTIR spectra of the pure and as prepared doped sample exhibit a number of characteristic spectral bands. The broad band between 3500 and 2500 cm−1 in the three samples were assigned to the O–H stretching vibrations of absorbed water on the ZnS surface. The band at 1115 cm−1 is due to the characteristic vibrations C-O groups of the acetate of the nanoparticles [29,30]. The characteristic ZnS vibration peaks can be observed at 680 cm−1. The intense peak at 1400 cm−1 was derived from the existence of the C–O–H bending [31]. The intense bands centered at 1030 cm−1 and 1570 cm−1 are attributed to the C-O stretching mode and O-H bending modes of the water molecule [32]. The C–H out-of-plane band appeared at 920 cm−1. In sintered sample also almost all characteristics peaks are observed but with less intensity.
Fig. 4. FTIR spectra of ZnS, as synthesized and sintered ZnS:Ce3+ nanoparticles.
host emission (blue emission) and dopant emission from the host. Dopant related emission is always expected at a lower energy position as compared to the band energy absorption. Fig. 5A shows the PL emission spectra of the three samples at an excitation wavelength of 350 nm. The emission spectra of pure ZnS nanocrystallites and as synthesized ZnS:Ce nanocrystallites show three emission peaks, a broad emission with peak at 445 nm and the emissions around 390 and 575 nm. For ZnS nanoparticles the lattice or surface defects or native impurities are the emission centres and such defect related emission is positioned in the UV/ blue region. Hence it can be noted the blue emission centered at 445 nm should result from the recombination of electrons at sulfur vacancy donor level with holes trapped at the zinc vacancy acceptor [33]. The UV emission with peak around 390 nm in the samples is normally believed to originate from the emission of the self activated centres of host ZnS associated with defects such as vacancies and interstitial spaces [34–39]. Since sulfur ions have large ionic radii than zinc ions, sulfur ions produces more strain and hence have small binding energy. Hence the sulfur vacancy states are closer to the conduction band edge than zinc vacancy to the valence band edge. Therefore emission band around 390 nm is due to surface defect states mainly from the sulfur vacancies (Vs) [18,40–43]. The peak position of the blue emission at 445 nm does not change even after the Ce doping which indicates the energy level of the sulfur vacancy relative to the
3.3. Photoluminescence (PL) studies Since the useful information about electronic states of emitters, crystal field strength, energy transfer, electron–phonon coupling strength, and the interactions between the luminescence strength and the interactions between the luminescence centres and the hosts is provided by PL studies, optical properties of ZnS and ZnS:Ce3+ nanocrystallites have been also studied through room temperature photoluminescence spectra. The emission in ZnS:Ce3+ nanoparticles originate from the mutual interaction between the surface coordination of
Fig. 3. Diffuse reflectance spectra of A) ZnS B) as synthesized ZnS:Ce3+ C) sintered ZnS:Ce3+ nanoparticles (inset the plot of (αhν)2 vs hν).
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Fig. 5. A) Room Temp PL Spectra of (λex = 350 nm) of (a) pure ZnS (b) as synthesized ZnS:Ce3+ (c) sintered ZnS:Ce3+ (inset -the individual components by Gaussian fitting, for emission peak at 538 nm in the PL spectrum of sintered ZnS:Ce3+ nanoparticles, B) CIE Chromaticity diagram for (a) ZnS (b) as synthesized ZnS:Ce3+ and (c) sintered ZnS:Ce3+ nanoparticles. Fig. 6. Qualitative schematic band diagram showing emissions in ZnS, free Ce3+ ion and ZnS:Ce3+ nanoparticles.
Zinc vacancy level keeps constant. From the figure it can be seen that the intensity of emission at 445 nm in as synthesized ZnS:Ce nanocrystallites is higher than those for the undoped sample. As said above, since the peak at 445 nm is related with sulfur vacancy, more defects/ sulfur vacancy are introduced when Ce3+ is doped in to the ZnS host. The weak emission peak around 575 nm might be attributed to deep level emission due to native defects such as interstitial Zn atoms in ZnS [44]. But for the sintered ZnS:Ce3+ sample a strong greenish yellow emission centered at 538 nm is observed in addition to UV emission and emission at 575 nm whereas emission at 445 nm was absent. The origin of the PL peak at 538 nm in our sintered sample is due to Ce3+ incorporation in the ZnS host lattice. When Ce 3+ions are doped in ZnS lattice, due to crystal field effect the 5D → 4F gap get reduced to 2.4 eV from 6.2 eV resulting in strong greenish yellow emission. This increased intensity in the greenish yellow emission with the quenching of host emission at 445 nm is a good example of energy transfer process, in which direct energy transfer from defect states of ZnS host to Ce3+ related energy levels and then strong greenish yellow emission occurs from Ce3+ ions through 5D → 4F transition. Sharp green emission was observed by Raj et al. in ZnS:Ce3+ nanocrystallites [45]. Shen et al. also obtained such a broad emission in YAG:Ce phosphor [46]. Since the emission is broad for the sintered nanocrystallites Gaussian curve fitting was applied to deconvolute the PL curve. The PL spectrum of the sintered ZnS:Ce3+nanoparticles was deconvoluted (inset Fig. 5) into two weak peaks, which are centered at 514 nm and 556 nm respectively. The peak at 514 nm is close to the value reported by Shinji Okamoto and Katsu Tanaka in ZnS:Ce nanoparticles [11]. Therefore the
deconvoluted emission peaks at 514 nm and 556 nm could be because of the two ground states, 2F5/2 and 2F7/2 corresponding to two transition 2D → 2F5/2 and 2D → 2F7/2. Possible energy transfer mechanisms are proposed on the basis of energy level diagram (Fig. 6) for the observed UV, blue and greenish yellow emission. For the excitation wavelength of 350 nm excited electrons can make a transition to the vacancy level (shallow traps) formed by the sulfur and then nonradiatively to the 5D level of Ce3+ followed by a radiative transition to the 4F level (2F5/2 and 2F7/2) resulting in the emission of greenish yellow photons around 538 nm. The electrons in the shallow traps can also recombine directly with the ground state electrons leading to UV emission at 390 nm. To assess the performance of ZnS:Ce3+ ions to be used as phosphors in light sources, CIE (Commission Internationale de I′Eclairage 1931) chromaticity coordinates are calculated. The calculated colour coordinates for the emission spectra are (0.24, 0.26), (0.26, 0.29), (0.33, 0.38) (Fig. 5B) corresponding to ZnS, as synthesized ZnS:Ce3+and sintered ZnS:Ce3+ nanoparticles respectively. From CIE coordinates for sintered samples, it is assured that characteristic light from the sintered sample is greenish yellow. 4. Conclusion In summary, we have synthesized greenish yellow emitting ZnS:Ce3+ nanoparticles by sintering ZnS:Ce 3+ nanocrystallites prepared at 70 °C by simple wet chemical route. X-ray diffraction (XRD) patterns revealed that the undoped ZnS and doped sample synthesized 127
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