Synthesis and luminescence properties of ZnS: Ce3+ , Li+ , Mn2+ nanophosphors

Synthesis and luminescence properties of ZnS: Ce3+ , Li+ , Mn2+ nanophosphors

Nano-Structures & Nano-Objects 6 (2016) 59–66 Contents lists available at ScienceDirect Nano-Structures & Nano-Objects journal homepage: www.elsevie...

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Nano-Structures & Nano-Objects 6 (2016) 59–66

Contents lists available at ScienceDirect

Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso

Synthesis and luminescence properties of ZnS: Ce3+ , Li+ , Mn2+ nanophosphors R. Sivakami ∗ , P. Thiyagarajan Crystal Growth Centre, Anna University, Chennai 600 025, India

highlights

graphical

abstract

• The ZnS: Ce, Li, Mn nanophosphors •







were synthesized by simple chemical method at room temperature. The synthesized nanophosphors crystallized in zinc-blende crystal structure. It shows the defect emission in the blue region and Mn2+ emission in the orange region. The orange emission was due to the [4 T1 (4 G) − 6 A1 (6 S)]3d5 transition of Mn2+ ion. The ZnS: Ce3+ , Li+ , Mn2+ nanophosphors show white light, as confirmed by the CIE value (0.374, 0.332).

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Article history: Received 23 July 2015 Accepted 1 March 2016

Keywords: Nanophosphors Semiconductor Chemical method X-ray diffraction Transmission electron microscopy Photoluminescence

abstract + + 2+ The Zn(1−(2x+y)) S: Ce3x= 0.001 Lix=0.001 Mn(0.001≤y≤0.03) nanophosphors were prepared using simple chemical ionic mixing method at room temperature. The X-ray diffraction studies on the undoped and doped samples confirmed the formation of zinc-blende structure. As the codopant concentration was increased the band gap energy gradually decreased from 3.92 eV to 3.86 eV. The Zn0.999 S : Ce30+ .001 nanophosphor showed the defect emission in the blue and green region with the peak positions at 429 nm, and 539 + + 2+ nm and 564 nm respectively. The Zn(1−(2x+y)) S: Ce3x= 0.001 Lix=0.001 Mn(0.001≤y≤0.03) samples showed a defect

emission at 400 nm along with the Mn2+ emission at 590 nm. The orange emission was due to deexcitation of electrons from [4 T1 (4 G) − 6 A1 (6 S)]3d5 unfilled shell of Mn2+ ions in ZnS host lattice. The ZnS: Ce3+ , Li+ , Mn2+ nanophosphors show white light, as confirmed by the CIE chromaticity coordinates (0.374, 0.332). © 2016 Elsevier B.V. All rights reserved.

1. Introduction The optical properties of semiconductor nanocrystals have been studied extensively in recent years because of their potential applications in opto-electronic devices namely LED, solar cell, cathode



Corresponding author. E-mail address: [email protected] (R. Sivakami).

http://dx.doi.org/10.1016/j.nanoso.2016.03.001 2352-507X/© 2016 Elsevier B.V. All rights reserved.

ray oscilloscope, fluorescence lamp etc. [1–5]. The physical properties of nanocrystalline semiconductors are quite different from that of bulk semiconductors due to quantum confinement effect and large surface-to-volume ratio [6,7]. The size of the particle was reduced to nanolevel with increase in percentage of atoms located at the surface, which significantly affected the electrical, dielectric and optical properties etc. These properties were sensitive to particle size, shape, crystal structure, and the type of defects [8]. In specific, the surface defects of the compound semiconductor nanostructures provided a visible luminescence which is useful for the

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fabrication of optical devices [9–11]. The intrinsic vacancies of Zn and S in ZnS nanostructures lead to defect emission in the visible region of electromagnetic spectrum [12]. ZnS is a potential material for opto-electronic applications particularly in light-emitting diodes, laser diodes of shorter wavelength and photodetectors [13–15]. The luminescent properties of ZnS host materials have been tuned by doping with various transition metals and rare-earth metals [16–19]. The ZnS was activated with transition metals (TM) (viz. Mn and Cu) and rare earth (RE) (viz. Ce3+ , Eu3+ , and Tm3+ etc.) elements along with charge compensators (like Li+ , Na+ , and Al3+ ). It exhibited intense emission in the visible region containing narrow and broad band spectrum which are applied to various optical devices [20]. ZnS has an excellent optical transmission property with high index of refraction (2.27 at 1 µm), and it serves as a potential application in novel photonic crystal devices operating in the visible-to-near IR region [21]. Various techniques that were adapted to synthesis ZnS nanoparticles are solid state method [22], microwave assisted synthesis [23, 24], hydrothermal method [25], ultrasonic radiation method [26], etc., to tune their physical, chemical and luminescence properties. Okamoto et al. [27] investigated the effect of coactivators namely Y3+ , La3+ , Al3+ , Ga3+ and In3+ on luminescence properties of the ZnS: Ce phosphor. Kawai et al. [28] studied the photoluminescent properties of ZnS: Ce, Li at the liquid nitrogen (LN2 ) temperature. It was identified that the noble metal ions and other Ia elements are not as efficient compensator as Li+ for Ce3+ luminescence. The trivalent rare earth ions in II–VI compounds are associated with various ligand configurations, in compensation with difference in ionic charge and volume between the RE3+ and host ions. The luminescent properties of ZnS: Ce3+ is unclear due to their structure less broad emission band and the powder form of the samples. The nature of Ce3+ emission spectrum also changes with respect to the excitation wavelength, due to the multiple structures of the Ce3+ centres in ZnS. Manhas et al. [29] reported the effect of alkali metal ions (Li+ , Na+ and K+ ) on the luminescence properties of CaMgB2 O5 : Sm3+ nanophosphor. The alkali metal ions (Li+ , Na+ and K+ ) were used as the charge compensator for Sm3+ ion. The emission spectra showed that Li+ ion codoped CaMgB2 O5 : Sm3+ nanophosphor exhibits the highest emission intensity compared to the Na+ and K+ ions codoped CaMgB2 O5 : Sm3+ nanophosphor. Yang et al. [30] reported the preparation and characterization of ZnS: Mn, Ce phosphor that showed yellow emission. Hossu et al. [31] reported the enhancement of Mn2+ emission by co-doping Eu2+ in ZnS phosphor. Ma et al. [32] reported the optical properties of ZnS: Mn nanoparticles. However, the present study focus on the room temperature synthesis of ZnS: Ce3+ , Li+ , Mn2+ nanophosphors by low cost simple chemical ionic mixing method and the investigation of their photoluminescent properties. 2. Experimental The undoped and Ce3+ , Li+ , Mn2+ doped ZnS nanophosphors were synthesized by simple chemical ionic mixing method at room + temperature. The molecular formula Zn(1−(2x+y)) S : Ce3x= 0.001 2+ Li+ x=0.001 Mn(0.001≤y≤0.03) was used as nomenclature. The starting materials such as zinc acetate [(CH3 COO)2 Zn.2H2 O], thioacetamide [C2 H5 NS], lithium acetate [CH3 COOLi.2H2 O], cerium acetate [(CH3 COO)3 Ce.H2 O] and manganese acetate [(CH3 COO)2 Mn.4H2 O] were used for synthesis. The Li+ ion was used as the charge compensator for Ce3+ ion. Lithium acetate [CH3 COOLi.2H2 O], cerium acetate [(CH3 COO)3 Ce.H2 O] and manganese acetate [(CH3 COO)2 Mn.4H2 O] were taken in the composition range x = 0.001 mol and y = 0.001 mol, 0.01 mol and 0.03 mol respectively. For the synthesis of undoped ZnS nanophosphors, 2.1949 g of zinc acetate, 0.7513 g thioacetamide were taken and grounded

well using mortar to obtain homogeneous mixture. Then 100 ml of deionized water was added in to the homogeneous salt mixture and transferred into the glass beaker. The white precipitate of ZnS nanophosphor dispersed in the aqueous solution was obtained. The precipitate was then separated from the reaction mixture by centrifugation for 10 min at 5000 rpm and was then washed with water. This procedure was repeated for several times until the formation of a neutral paste. Then the wet precipitate was allowed to dry in room temperature for further analysis. The chemical process can be described by the following chemical equation

The Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors were synthesized via the above mentioned method by using (2.1883 g (y = 0.001 mol), 2.1685 g (y = 0.01 mol), 2.1246 g (y = 0.03 mol)) of zinc acetate, 0.7513 g of thioacetamide, 0.0031 g of cerium acetate, 0.0010 g of lithium acetate and (0.0024 g (y = 0.001 mol), 0.0245 g (y = 0.01 mol), 0.0735 g (y = 0.03 mol)) of manganese acetate respectively. The characterization studies such as X-ray diffraction (Philips Xpert, USA), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR – JASCO International Co./Japan – Model FTIR-6300), Diffuse reflectance spectroscopy (DRS—USB 2000, Ocean Optics, USA), and fluorescence spectroscopy (Fluorolog spectrophotometer (JASCO Japan)) studies were performed to check the phase purity, surface morphology, particle size, various functional group, absorption, and photoluminescence (PL) excitation and emission spectra respectively. All the optical measurements were carried out at room temperature. 3. Results and discussions 3.1. Phase analysis of undoped and doped ZnS nanophosphor In order to examine the phase formation of the synthesized nanophosphors, X-ray diffraction studies were performed. The Fig. 1 shows the XRD patterns of ZnS, Zn0.999 S:Ce0.001 , Zn0.998 S:Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors along with the reference pattern (JCPDS: 800020). The diffraction peaks were well matched with the reference pattern and the spectral peak broadness whose full width half maximum (FWHM) confirms the formation of nanocrystalline phosphor. There are three intense peaks observed corresponding to (111), (2 2 0) and (311) planes of cubic zinc blend structure with lattice constant a = 5.361 Å and a space group of F43m. No reflections corresponding to dopant elements viz. Ce3+ , Li+ and Mn2+ ions were identified because of the low dopant concentration and the X-ray detection limit since X-ray source cannot detect elements less than 2 atomic percentage. 3.2. SEM analysis In order to study the surface morphology of the synthesized nanophosphors, the SEM analysis was carried out. Fig. 2(a), (b) and (c), (d) shows the SEM image of the ZnS and Zn0.968 S:Ce0.001 Li0.001 Mn0.03 nanophosphors with different magnification. The undoped and Ce3+ , Li+ , Mn2+ doped ZnS show small and large spherical shaped particles with agglomeration. It is known that the spherical shaped particles show better emission due to the minimum scattering loss which increases the screen brightness of the display [33].

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The broad absorption spectrum in the range 3000–3600 cm−1 corresponds to O–H stretching group indicate the existence of absorbed water molecules on the nanoparticles surface. The absorption peaks at 2914 cm−1 and 2853 cm−1 are due to CH2 asymmetric stretching mode and symmetric stretching mode respectively. The absorption peak at 2035 cm−1 is related to C–H stretching vibration [34,35]. These absorption peaks are very weak in the FTIR spectra which indicate that very small amount of CH2 group is present in the ZnS nanoparticles. Even though the FTIR spectra of ZnS nanophosphor showed the presence of very small absorption of CH2 , it could not affect the luminescence behaviour of the ZnS: Ce3+ , Li+ , Mn2+ nanophosphors. The peak at 1414 cm−1 due to the bending of methylene groups in thioacetamide [36]. The absorption peaks at 1023, 617, and 471 cm−1 are attributed to the Zn–S vibration [37].

Fig. 1. XRD pattern of ZnS, Zn0.999 S:Ce0.001 , Zn0.998 S:Ce0.001 Li0.001 Mn(0
3.3. TEM analysis Fig. 3(a)–(f) shows the TEM images of ZnS: Ce0.001 Li0.001 Mn0.03 nanophosphor at different magnification along with the selected area diffraction (SAED) pattern to examine the crystal structure. The TEM image reveals spherical particles with an average diameter of 20 nm. The crystallographic planes of nanoparticles measured by selected area electron diffraction (SAED) pattern exhibits three rings corresponding to (1 1 1), (2 2 0) and (3 1 1) planes of ZnS crystallize in cubic structure, which is in agreement with the results obtained by X-ray diffraction. 3.4. The FTIR spectra of ZnS, Zn0.998 S : Ce0.001 Li0.001 , Zn0.968 S : Ce0.001 Li0.001 Mn0.03 nanophosphors The FTIR spectra of the ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.968 S:Ce0.001 Li0.001 Mn0.03 nanophosphors are shown in Fig. 4.

3.5. Diffuse reflectance spectra of ZnS, Zn0.999 S : Ce0.001 , Zn0.998 S : Ce0.001 Li0.001 , Zn0.968 S : Ce0.001 Li0.001 Mn0.03 nanophosphors The energy absorption of the ZnS, Zn0.999 S:Ce0.001 , Zn0.998 S: Ce0.001 Li0.001 , Zn0.968 S:Ce0.001 Li0.001 Mn0.03 nanophosphors were evaluated from diffuse reflectance spectra, measured with respect to BaSO4 as the reference sample (Fig. 5). In DRS spectra the existence of host band gap tuning was observed. Undoped ZnS shows the band edge absorbance at 317 nm. Upon inclusion of codopants, the absorbance shifts to longer wavelengths from 317 nm to 319 nm, 320 nm and 322 nm for Ce0.001 , Ce0.001 Li0.001 , and Ce0.001 Li0.001 Mn0.03 respectively, such a shift is highlighted in the inset of Fig. 5. The absorption edge of all the samples show the blue shift as compared to the bulk ZnS (334 nm). This blue shift is due to quantum confinement in the samples. In addition, quantum confinement occurs due to the reduction of particle size with respect to bulk materials which lead to shift in band gap values [38–41]. According to quantum confinement theory, electrons in the conduction band and holes in the valence band are spatially confined by the potential barrier up to the surface

Fig. 2. SEM image of (a, b) ZnS nanophosphor (c, d) Zn0.968 S: Ce0.001 Li0.001 Mn0.03 nanophosphor.

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Fig. 3. TEM image of ZnS: Ce0.001 Li0.001 Mn0.03 nanophosphor at different magnification (a) 200 nm (b) 100 nm (c) 50 nm (d) 20 nm (e) 10 nm (f) selected area electron diffraction (SAED) pattern.

which effectively increases the energy separation between the valence and conduction band increasing the energy value of optical transition [42]. The band gap values were calculated for all samples using diffuse reflectance spectra by applying Einstein’s energy–wavelength relation equation (Fig. 5). Eg =

hc

λ

(1)

where h = Planck’s constant = 6.626 × 10−34 J c = velocity of light = 3 × 108 m/s λ = wavelength. For undoped ZnS the band gap value was 3.92 eV. The decrease in ZnS band gap values was observed with respect to inclusion of codopants at fixed Ce0.001 concentration. The band edge absorption

values were calculated to be 3.89 eV, 3.88 eV, and 3.86 eV, for samples doped with Ce0.001 , Ce0.001 Li0.001 , and Ce0.001 Li0.001 Mn0.03 respectively. It is clear that the calculated band gap values for all the doped samples are lower than that of the undoped ZnS. The shift occurs due to the crystal field effect experienced by host lattice upon incorporation of Ce3+ , Li+ , Mn2+ in the Zn2+ site. The crystal field arises because of the difference in ionic size between the Ce3+ (1.31 Å) and Zn2+ (0.74 Å). In general, doping Ce3+ (1.31 Å) ion in Zn2+ (0.74 Å) site induces the lattice strain that lead to increase of band gap value against usual ZnS band gap (3.72 eV). When Li+ ion is used as the charge compensator for Ce3+ , it can act as very well substitute and charge compensator since Li+ (0.76 Å) size is close to Zn2+ ion. It is assumed that the induced lattice strain is relaxed by substituting Li+ ion as the charge compensator and by codoping of Mn2+ ion at the Zn2+ site. This is because the ionic size of Mn2+ (0.68 Å) is half that of Ce3+ (1.31 Å) and is lesser than

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Fig. 4. FTIR spectra of ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.968 S:Ce0.001 Li0.001 Mn0.03 nanophosphors. Fig. 6. Excitation spectra of ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.999 S:Ce0.001 nanophosphors.

Fig. 5. Diffuse reflectance spectra of ZnS, Zn0.999 S:Ce0.001 , Zn0.998 S:Ce0.001 Li0.001 , Zn0.968 S:Ce0.001 Li0.001 Mn0.03 nanophosphors. [Inset: Band edge absorption shift with respect to dopant content.] (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Zn2+ (0.74 Å), therefore replacement of Mn2+ at Zn2+ site might decrease the lattice strain, hence lead to decrease of band gap value significantly. 3.6. The PL excitation spectra of ZnS, Zn0.998 S Zn0.999 S : Ce0.001 nanophosphors

: Ce0.001 Li0.001 ,

The PL excitation spectra of ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.999 S: Ce0.001 nanophosphors monitored at the fixed emission wavelength (λem =) 430 nm at room temperature are shown in Fig. 6. All samples show two excitation peaks at the wavelength 325 and 350 nm. Doping of Ce3+ together with Li+ and Ce3+ alone shows an increase in excitation intensity with spectral broadness. The spectral broadness occurs due to the presence of defects such as lattice distortion, vacancies etc., which are located probably close to the shallow level in the case of small particles [43]. Xiong et al. [44] observed the doublet excitation maxima at 331 and 322 nm in ZnS nanowires. A similar kind of absorption doublet is observed in the present study. The excitation peak at 325 nm occurs due to the band to band transition of electrons in ZnS and the peak at 350 nm is due to the presence of sulphur related surface defects. In general, the fluorescence of Ce3+ in solids are realized by two parity allowed transitions of 2 D(5d) − 2 F5/2 , 2 F7/2 (4f) with the frequency separation of 2000 cm−1 [45]. Such a kind of fluorescence of Ce3+ ion was not observed in the present study due to the following reasons: Okamoto et al. [27] reported that it

is difficult to determine the position of the 5d energy level of Ce3+ relative to the valence and conduction band of ZnS. On the other hand, if the 5d energy level is located in the conduction band of ZnS, auto ionization of excited Ce3+ will take place, resulting in luminescent quenching. Therefore, the 5d and 4f energy levels of Ce3+ are located in the energy band gap of ZnS. This is the reason; the excitation spectra did not show the Ce3+ ion excitation peak in the ZnS: Ce3+ and ZnS: Ce3+ , Li+ doped nanophosphors. Moreover, Kawai et al. [28] reported that the Ce3+ always show dips due to exciton absorption corresponds to the valence band to conduction band region, which indicates that the excitons created around the Ce3+ ions are annihilated somewhere without contributing to the excitation of the Ce3+ ions. For the ZnS: Ce3+ nanophosphor, herein, a basic issue that needs to be addressed is if the Ce3+ ions are incorporated in the ZnS host lattice or not. It is known that the ionic radius of the Ce3+ ion (1.31 Å) is much larger than that of the Zn2+ ion (0.74 Å). If Ce3+ ion is incorporated at the Zn2+ site, the ZnS host lattice has to deform locally to certain extent due to the difference in ionic radii. Because of this reason, Ce3+ ion causes some defects in the ZnS host lattice, hence the observed excitation peak at 350 nm is due to the defect emission. Also among all the samples, the ZnS: Ce30+ .001 nanophosphor (without charge compensator) showed + maximum intensity compared to the ZnS: Ce30+ .001 , Li0.001 (with charge compensator) nanophosphor indicates that more defects could be produced in the Ce3+ alone doped ZnS due to the charge and volume difference between the Ce3+ ion and Zn2+ ions. When the Ce3+ ion is codoped with Li+ ion, the Li+ ion act as the charge compensator for Ce3+ ion and the defect rate also reduced compared to the ZnS: Ce30+ .001 nanophosphor. 3.7. The PL emission spectra of ZnS, Zn0.998 S Zn0.999 S : Ce0.001 nanophosphors

:

Ce0.001 Li0.001 ,

The PL emission spectra of ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.999 S:Ce0.001 nanophosphors monitored at the excitation wavelength (λex =) 320 nm at room temperature are shown in Fig. 7. The emission spectra comprise of two broad emission band covering the blue to bluish-green and green to red portion in the visible region. It has been reported that the emission obtained due to defects (or) impurity states are usually broad with some stoke shift in the electromagnetic spectrum [46,47]. Similar luminescence behaviours are observed in the present study. All the samples show two emission bands, one in the blue region with the peak wavelength at 430 nm and the other in the green–yellow region with the peak wavelength at 539 and 564 nm

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Fig. 7. Emission spectra of ZnS, Zn0.998 S:Ce0.001 Li0.001 , Zn0.999 S:Ce0.001 nanophosphors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

within the range 510–610 nm. The blue emission peak intensity (Ip ) and its band width increases with the addition of codopants. Similar behaviour is seen in the green–yellow region. The spectral tail of Ce, Li doped ZnS sample ends at 600 nm and the spectral tail of the Ce doped sample extends up to 610 nm. In case of undoped ZnS, the blue emission peak at 430 nm originates from the host defect i.e., the sulphur vacancies on the particles surface and interstitial lattice defects in the ZnS nanoparticles [48]. Several reports are available related to the blue emission from ZnS. Sapra et al. [49] reported the blue emission with peak intensity at 425 nm in the undoped ZnS which could be due to the recombination of electrons between the shallow donor levels and the valence band. Wang et al. [12] reported the blue emissions at 430 nm that could be attributed to the surface states related defect emission. Kar et al. [46,47] and Hu et al. [50] reported the blue emission with the peak wavelength of ∼400 nm originates due to recombination of carriers between the sulphur vacancy and interstitial zinc. Wang et al. [51] and Xiong et al. [44] reported the blue emission within the wavelength region 440–460 nm which could be attributed to the trapped luminescence arising from the surface states. Several researchers reported the occurrence of green emission which is attributed to the self activated defect centres [52], structural defects like point defects [53] or zinc vacancy related defect centres [54]. In the present study the green emission from undoped ZnS is observed at 539 and 564 nm. The former emission could be due to the elementary sulphur species in the ZnS nanoparticle, i.e. the transfer of trapped electrons from sulphur vacancies to interstitial sulphur state. The later emission attributed to the zinc vacancies i.e. the recombination of charge carriers between the interstitial zinc states and the zinc vacancies. Li et al. [55] observed the amber–yellow emission at 574 and 578 nm which could be derived from self activated centres, vacancy states, or interstitial states related to the special nanostructures. Ye et al. [56] reported the presence of the elementary sulphur species in the ZnS nanobelts that could also induce the green emission. Li et al. [57] observed the strong green luminescence with the peak at 534 nm. Cao et al. [58] also observed the green emission with its peak position at 540 nm corresponding to the sulphur species on the surface of ZnS nanowires. The codoping of Li+ as charge compensator for Ce3+ also shows the blue and greenish-yellow emission with the peak intensity (Ip ) at 430 nm and 548 nm, 564 nm, and 580 nm respectively, with the tail extending up to 600 nm. The Zn0.999 S: Ce0.001 shows better emission intensity than that of the ZnS: Ce3+ ,

Li+ . The nanophosphor without charge compensator shows PL spectral pattern similar to that with charge compensator but with enhanced emission intensity whose tail extends up to 610 nm. It has been reported that the presence of foreign impurities like Cu2+ , Mn2+ , or rare earth ions even in minute quantity could introduce green emission from the ZnS nanoforms [59–64]. Hence the green–yellow emission observed in doped ZnS is due to the Ce3+ ion induced defect related emission. It is well known that Ce3+ is an efficient activator in many hosts. However, it is not easy to dope Ce3+ ion in ZnS, since the ionic radii of Ce3+ is 40% larger than Zn2+ . To overcome this problem, Kawai and Hoshina et al. codoped Li+ ion as the charge compensator for Ce3+ ion in the ZnS host lattice and blue–green luminescence was obtained from the ZnS: Ce3+ , Li+ phosphors. They have identified that noble metal ions and other Ia elements are not such efficient sensitizer as Li+ for Ce3+ luminescence. Incorporation of Li+ ion at cation site make a shallow acceptor state and a donor state by occupying the interstitial site in CdS, CdSe, CdTe, ZnSe, ZnTe and ZnO has also been reported. The addition of Li+ ion can very well substitute and act as charge compensator since Li+ (0.76 Å) size is close to the Zn2+ (0.74 Å). Due to this reason, the Li+ ion was used as the charge compensator. The ZnS: Ce, Li synthesized at high temperature exhibits strong band edge absorption at 340 nm and the Ce3+ emission with the peak wavelength at 478 and 530 nm measured at LN2 temperature [28]. The same group [65] investigated the cathodoluminescence properties of ZnS: Ce, Li phosphors synthesized at high temperature in a sulphur atmosphere. The results of the investigation revealed that Ce and Li are closely associated with each other, and the addition of Li changes the Ce3+ emission spectrum and increases its emission intensity appreciably. Firing the phosphor to high temperatures and quenching below 975 °C reduces the intensity of the Ce3+ emission. The quenching of the samples from high temperatures containing Li freezes the Ce3+ centres and is labelled as metastable centres which can emit light efficiently. The metastable states of the Ce3+ centres involves substitutional Li+ ions in the neighbourhood, and on slow cooling after annealing liberates these substitutional Li+ ions into the interstitial sites causing segregation. Hence the luminescence efficiency is decreased to a great extent by heat-treatment at temperatures below 900 °C, because of segregation of Li ions. In the present study, since the ZnS: Ce3+ , Li+ nanophosphors are synthesized at room temperature, the codoping of Li+ does not support the defect emission induced by the Ce3+ ion. It has been reported that the addition of coactivators such as Y3+ , La3+ , Al3+ , Ga3+ , In3+ and Na+ in ZnS: Ce3+ phosphors show the blue shift from its green luminescence with the peak wavelength of 510–450 nm. This blue shift is due to the localization of Ce3+ ion induced by covalent bonds among the coactivator ions and S2− ions which reduce the energy splitting between the E and T2 levels, giving rise to blue luminescence. Hence the shift does not depend on the ionic radius of the coactivator ions [27]. 3.8. Photoluminescence properties of Zn0.998−y S Mn(0.001≤y≤0.03) nanophosphors

: Ce0.001 Li0.001

The ZnS: Ce3+ nanophosphor shows the intense defect emission in the blue–green–yellow region compared to that of Ce, Li codoped ZnS. The incorporation of Ce3+ with Li+ in the ZnS induces strain in the host lattice. It is assumed that the strain produced by the Ce3+ ion is further reduced by codoping Mn2+ ion (0.68 Å) at the Zn2+ (0.74 Å) site since former ion dimension is lesser than the later ion. Hence Mn2+ codoped ZnS: Ce3+ , Li+ nanophosphors are prepared to investigate their photoluminescence properties.

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Fig. 8. Excitation spectra of Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors.

3.8.1. The PL excitation spectra of Zn0.998−y S : Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors The PL excitation spectra of Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors monitored at the fixed emission wavelength (λem =) 590 nm at room temperature are shown in Fig. 8. All the samples show three excitation peaks located at 317, 340 and 350 nm. Hence the excitation peak at 317 nm occurs due to band to band transition of electrons, the peak at 340 nm due to the presence of sulphur related surface defect [66] and the peak at 350 nm due to the d–d transition of Mn2+ ions are seen [67]. 3.8.2. The PL Emission spectra of Zn0.998−y S : Ce0.001 Li0.001 Mn(0.001≤y≤0.03 mol) nanophosphors The PL emission spectra of Zn0.998−y S:Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors monitored at the fixed excitation wavelength (λex =) 350 nm measured at room temperature are shown in Fig. 9. All the samples show the broad band emission in the visible region with a maximum at 400 nm along with the shoulder at 468 nm in the blue region. Another shoulder at 552 nm with an emission peak at 590 nm is seen in the orange region. The increase in Mn2+ concentration increases the broadness of the spectrum. The reason for the broadness is due to occurrence of defects such as lattice distortions and vacancies, which are probably located close to the surface in case of small particles [66]. The emission maximum at 400 nm is due to the recombination between the ¨ related donor and zinc vacancy (V′′Zn ) related sulphur vacancy (Vs) acceptor levels [68,69]. The emission peak at 468 nm originates due to the recombination of electrons between internal sulphur vacancy donor levels with holes at the valence band. The emission peak at 552 nm occurs due to the transition between internal sulphur vacancy and internal zinc vacancy [70]. The yellow–orange emission band with a peak at 590 nm attributed to the radiative transition of the electrons in 3d5 unfilled sub-shell of Mn2+ ions [4 T1 (4 G) − 6 A1 (6 S)] in ZnS host lattice [49]. In general, the Mn2+ ions having a 3d5 unfilled electronic shell splits into multiple levels of 6 A1 (6 S), 4 T1 (4 G), 4 T2 (4 G), 4 E(4 G), 4 A1 (4 G), 4 T2 (4 D), and 4 E(4 D) under the influence of crystal field exerted by the host matrix [71]. The electronic transition from the ground state 6 A1 to the excited state 4 T1 is spin forbidden, with 4–5 orders of magnitude lower maximum absorption coefficients (εMn 2+ ) than those of II–VI nanoparticles. Doping into the host lattice leads to the combination of large energy gap of 4 T1 − 6 A1 and lower photon energy provided by most of the II–VI semiconductor lattices (i.e., ZnS, ZnSe, and CdS), allowing the electronic transition to attain high efficiency [72]. The ZnS: Mn2+ nanoparticles possess two emission centres (the dopant and band gap emissions), the

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Fig. 9. Emission spectra of Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03) nanophosphors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ratio of manipulating the host to the Mn2+ emission provides different colour luminescence. The fluorescence containing two emission bands centred at 420 and 590 nm are mostly observed in the Mn2+ doped ZnS nanoparticles. The optical emission peak at 590 nm attributed to the electronic transition of Mn2+ ions and the peak near 420 nm (the peak position may differ depends on the synthesis condition, size, and morphology) attributed to the donor–acceptor pair transition involving electron–hole recombination at the sulphur vacancy [49,60]. In the present study Ce3+ doping shows the defect emission in the blue–green region and the Mn2+ shows the yellow–orange emission. The synthesized nanophosphors show emission in the blue–green and yellow–orange region. Hence it could be considered as a suitable material for the fabrication of optoelectronic devices. 3.9. The CIE chromaticity coordinates of ZnS, Zn0.999 S: Ce0.001 ,Zn0.998 S: Ce0.001 , Li0.001 and Zn0.998−y S : Ce0.001 Li0.001 Mn(0.001≤y≤0.03 mol) nanophosphors The CIE chromaticity coordinates values were measured for the nanophosphors at the excitation wavelength of 365 nm by Ocean Optics USB 2000 spectrometer using OOI base software (Fig. 10). These values are found to be (0.211, 0.143), (0.223, 0.158), (0.265, 0.319), (0.383, 0.342), (0.374, 0.337) and (0.374, 0.332) for ZnS, Zn0.999 S: Ce0.001 , Zn0.998 S: Ce0.001 Li0.001 , Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03 mol) nanophosphors respectively. The CIE chromaticity coordinate values of Zn0.998−y S: Ce0.001 , Li0.001 , Mny=0.03 phosphor measured to be (0.374, 0.332) which is close to white light, however, better than the commercial YAG:Ce phosphor (0.295, 0.275). This evidence shows that the nanophosphor could be applicable for the generation of white light emission excitable at UV (365 nm) LED which could be useful in the field of solid state lighting. 4. Conclusion In summary, the ZnS: Ce3+ , Li+ , Mn2+ nanophosphors are prepared using low cost chemical ionic mixing method at room temperature. The sample crystallizes in zinc-blende crystal structure and the surface morphology in a spherical shape, as confirmed by XRD and SEM studies respectively. The TEM image shows spherical shaped particles with an average diameter of 20 nm. The absorption edge in the diffuse reflectance spectra shifts towards higher wavelength with the addition of codopants lead to the contraction of band gap energy. The Ce3+ doped ZnS shows

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Fig. 10. CIE chromaticity coordinates of ZnS, Zn0.999 S: Ce0.001 , Zn0.998 S: Ce0.001 , Li0.001 and Zn0.998−y S: Ce0.001 Li0.001 Mn(0.001≤y≤0.03 mol) nanophosphors.

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