Physica B 406 (2011) 2013–2017

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Luminescence properties of CaS:Ce, Sm nanophosphors

Geeta Sharma a,n, S.P. Lochab b, Nafa Singh a

a Department of Physics, Kurukshetra University, Kurukshetra 136119, Indiab Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India

a r t i c l e i n f o

Article history:

Received 2 December 2010

Received in revised form

6 March 2011

Accepted 7 March 2011Available online 15 March 2011

Keywords:

Nanophosphors

Photoluminescence

Thermoluminescence

26/$ - see front matter & 2011 Elsevier B.V. A

016/j.physb.2011.03.008

esponding author. Tel.: þ91 9466471265.

ail address: g.sharma.ku@gmail.com (G. Sharm

a b s t r a c t

CaS:Ce, Sm nanophosphors were synthesized via solid state diffusion method. X-Ray diffraction

confirmed the cubic crystalline phase of CaS:Ce, Sm nanoparticles. The particle size calculated using

Debye–Scherrer formula was found to be 52 nm. The morphological investigations of the nanoparticles

were made using TEM and found to have nearly spherical morphology with diameter 45–50 nm, which

is in close agreement with the XRD result. The PL emission characteristics of CaS:Ce, Sm as a function of

cerium and samarium concentrations have been studied and CaS:Ce0.6Sm0.4 system has maximum

emission intensity, hence it was opted for further studies. The CaS:Ce0.6Sm0.4 system showed

independent emission of Sm and Ce when excited at 330 and 450 nm, respectively. To study the

energy transfer between cerium and samarium, the CaS:Ce0.6Sm0.4 was excited at wavelengths other

than the excitation wavelengths of Ce (450 nm) and Sm (330 nm). The existence of Ce emission (at an

excitation of 390 nm) even in the absence of Ce excitation band and Sm emission at an excitation of

405 nm, which is the excitation band of Ce, indicates the energy transfer at these two wavelengths.

Thermoluminescence characteristics of 60Co irradiated CaS:Ce0.6Sm0.4 have been investigated for

different doses of 0.14–125 Gy. All the glow curves show a single peak at 475 K. With increasing dose,

the intensity of this peak increases and a shoulder is formed on the lower temperature side at 415 K at

21 Gy of exposure. CaS:Ce0.6Sm0.4 shows almost linear dose dependence up to 125 Gy.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The phenomenon of luminescence is widely used in day today life such as in television screen phosphors, fluorescencelamps, laser materials, dosimetery of ionizing radiations, etc. [1–6].Due to widespread applications of luminescent materials there is anever increasing quest for new and better materials. Alkaline earthsulfites have remained unbeaten candidates for luminescence appli-cations because of their striking properties. CaS is still one of themost investigated AES material [7–13]. It has potential for applica-tions in the field of radiation dosimetery because of deep traps init [14]. Being a wide bandgap semiconductor it can accommodate avariety of dopants [15–19]. Moreover, adding a pair of dopants inCaS host further improves the scope for applications since co-dopingalters the luminescent properties of phosphors and large number ofluminescent nanocrystalline sulphides tunable over a wide range ofvisible spectrum can be achieved. Xiaolin et al. [20] have preparedCaS:Sm3þ , Eu2þ using sulfur flux method and studied the energytransfer between rare earth ions in CaS at room temperature andat 77 K. Salah et al. [21] have studied the effect of Tb3þ co-doping on thermoluminescent and photoluminescent properties ofK2Ca2(SO4)3:Eu phosphors. Ping et al. [22] have studied the optically

ll rights reserved.

a).

stimulated luminescence in SrS:Eu, Sm and CaS:Eu, Sm. Pauloseaet al. [23] have synthesized and studied systematically the phos-phorescence decay mechanism of CaS:Ce, Sm phosphors but theirphosphors were not single phased as revealed by the XRD of theirsamples. This motivated us to synthesize single phased CaS:Ce, Smand check its luminescence properties.

Earlier we have synthesized CaS:Ce nanophosphors using solidstate diffusion method [24] and extended the same approach tosynthesize single phased CaS:Ce, Sm nanophosphors. The structuralcharacterizations were done using XRD and transmission electronmicroscope. The photoluminescence properties of these nanopho-sphors have been studied as a function of varying concentrations ofcerium and samarium in the host lattice. CaS:Ce shows interestingthermoluminescence properties and we have recently studied thethermoluminescence behavior of 60Co irradiated CaS:Ce [25]. Toinvestigate the effect of Sm co-doping on the thermoluminescencebehavior of CaS:Ce we also studied the thermoluminescence char-acteristics of 60Co-g irradiated CaS:Ce, Sm system.

2. Experimental

We have used solid state diffusion method [24] for synthesizingCaS:Ce, Sm (0.2 mol %) with varying concentrations of cerium andsamarium in the host lattice (such that the total concentration of

Table 1

Value of 2y for the most intense (200) peak, particle size and lattice parameter for

CaS:Ce and CaS:Ce0.6Sm0.4.

Sample specification 2y (deg.) Lattice

parameter (A)

FWHM Particle

size (nm)

CaS:Ce 31.75 5.686 0.15 53

CaS:Ce0.6Sm0.4 31.37 5.696 0.156 52

G. Sharma et al. / Physica B 406 (2011) 2013–20172014

dopants i.e. Ce plus Sm remained constant and only Ce/Sm ratiovaried). Calcium sulfate, samarium nitrate, cerium nitrate, sodiumthoisulfate, carbon powder and ethanol were the starting materi-als. Carbon reduces sulfate to sulfite at high temperature. Sodiumthoisulfate (15%) acted as a flux for the reaction. The calculatedquantities of calcium sulfate, carbon powder, cerium nitrate,samarium nitrate and the flux were taken and mixed thoroughlywith the help of an agate pestle and mortar. The charge was placedin a clean graphite crucible and a thin layer of carbon powder wasspread over it. This crucible was covered with another similarcrucible. The carbon layer over the charge created a reducingenvironment. This whole arrangement was placed in a mufflefurnace and the charge was fired at 950 1C for 2 h. After 2 h itwas taken out and rapidly crushed while red hot. The details of thesynthesis is discussed elsewhere [24].

The samples were characterized by X-ray powder diffraction(XRD) using an in-situ XRD setup (Bruker AXS) having a 3 kWX-rays source. The photoluminescence was recorded on a FluoroMax-3 (Jobin–Yvon, Edision, NJ, USA) equipped with a photo-multiplier tube and a xenon lamp of power 150 W. For recordingTL, samples were irradiated with different doses of 60Co g-rays atroom temperature. Prior to gamma exposure, the samples wereannealed at 400 1C for 10 min and then quenched on a metallicplate at room temperature to erase any residual information. TLglow curves were recorded on a Harshaw TLD reader (Model3500) fitted with 931B photomultiplier tube by taking 5 mg of thesample each time.

Fig. 2. TEM image of CaS:Ce0.6Sm0.4. Inset shows the SAED pattern of

nanoparticles.

3. Results and discussion

3.1. XRD and TEM

The phase of the nanophosphors was confirmed by XRD which isin agreement with the standard data available in JCPDS (77-2011).Fig. 1 shows the XRD pattern of CaS:Ce and CaS:Ce0.6Sm0.4. The welldefined peaks in the XRD pattern confirm cubic crystalline phase ofCaS:Ce0.6Sm0.4 and CaS:Ce nanoparticles. Table 1 shows the varia-tion in 2y for the most intense (2 0 0) peak and lattice constant ofCaS:Ce and CaS:Ce0.6Sm0.4. It is found that there is slight variation inthe lattice constant with the addition of samarium. The particle sizewas found using the well known Debye–Scherrer formula [26] fromFWHM of the most intense XRD (2 0 0) peak after correcting forinstrumental broadening. The particle size was estimated to be

Fig. 1. XRD pattern of CaS:Ce and CaS:Ce0.6Sm0.4.

52 nm for CaS:Ce0.6Sm0.4. The full width at half maxima (FWHM)and particle size of the samples have also been listed in Table 1.

TEM was utilized to study the morphology and particle size.Fig. 2 shows the TEM micrograph of CaS:Ce0.6Sm0.4. The nano-particles formed have nearly spherical morphology with diameterranging between 45 and 50 nm. The inset of Fig. 2 shows theSAED pattern of the nanoparticles formed. SAED pattern showsclosed rings, which confirm the cubic crystalline phase of thenanoparticles. The particle size estimation from TEM and XRDstudies were found to be in close agreement for CaS:Ce0.6Sm0.4.

3.2. Photoluminescence characteristics

The effect of variation in concentration of cerium and samar-ium in CaS host on the PL emission intensity has been investi-gated and shown in Fig. 3. We noticed that PL intensity increaseswith increasing cerium concentration (decreasing samarium con-centration) and becomes maximum for CaS:Ce0.6Sm0.4 and fallsafterwards, which may be due to well known concentrationquenching phenomenon. This led us to restrict our studies onCaS:Ce0.6Sm0.4 system.

Fig. 4 shows the PL emission spectrum of CaS:Ce0.6Sm0.4

nanophosphors at an excitation wavelength of 330 nm (Sm3þ)[27] and 450 nm (Ce3þ) [24]. The emission at an excitationwavelength of 450 nm shows the characteristic emission spec-trum of Ce3þ i.e. a peak at 505 nm with a shoulder at 565 nm,which may be attributed to the well known transitions fromthe T2g sublevel of the 5d excited state to 2F5/2 and 2F7/2 of the4f ground state of Ce3þ [28]. The emission spectrum at an excitationof 330 nm comprises of two sharp peaks at 569 and 605 nm alongwith a low intensity broad peak at 493 nm. For identifying the

Fig. 3. Effect of variation in concentration of cerium and samarium in CaS host on

the PL emission intensity.

Fig. 4. PL emission spectrum of CaS:Ce0.6Sm0.4 nanophosphors at excitation

wavelengths of 330 nm (Sm3þ) and 450 nm (Ce3þ).

Fig. 5. PL (a) excitation and (b) emission spectra of CaS:Sm at emission and

excitation wavelengths of 569 and 330 nm, respectively.

Fig. 6. Excitation spectra of CaS:Ce0.6Sm0.4 at emission wavelengths of 569 and

605 nm.

G. Sharma et al. / Physica B 406 (2011) 2013–2017 2015

emission bands of Sm3þ in CaS, we synthesized and recordedPL of CaS:Sm at an excitation of 330 nm. Fig. 5 shows the PLexcitation and emission spectra of CaS:Sm at an emission andexcitation wavelength of 569 and 330 nm, respectively. PL emissionspectrum shows two sharp peaks at 569 and 605 nm, which maybe due to intra 4f transitions of Sm3þ ion. i.e. 4G5/2-

6H5/2 and4G5/2-

6H7/2, respectively. Therefore the emission peaks at 569 and605 nm at an excitation wavelength of 330 nm for CaS:Ce0.6Sm0.4

may be confirmed as the characteristic emission of Sm in CaS host.The other peak at 493 nm may be assigned to the defect structure ofCaS host lattice itself [29].

At 330 and 450 nm the CaS:Ce0.6Sm0.4 system shows independentemission of Sm3þ and Ce3þ , respectively. There is no energy transferat these wavelengths. To investigate an energy transfer betweengreen emitting Ce3þ and yellow emitting Sm3þ , in CaS host, westudied the emission of the co-doped system (CaS:Ce0.6Sm0.4) atexcitation wavelengths other than characteristic excitation of Ce orSm. For this we recorded excitation spectra of CaS:Ce0.6Sm0.4 at anemission wavelengths of 569 and 605 nm as shown in Fig. 6. Theexcitation spectrum of CaS:Ce, Sm at both the emission wavelengthsare similar, differing in excitation intensity only. The excitation bandat 300 nm in the spectrum b in Fig. 6 was not recorded from 300 nmbecause of the limitation of PL spectrophotometer. For the sake of

comparison of the excitation bands of CaS:Ce (at emission wave-length of 505 nm), CaS:Sm (at emission wavelength of 569 nm) andCaS:Ce0.6Sm0.4 (at emission wavelength of 569 nm) their excitationspectra are clubbed in Fig. 7.

When the CaS:Ce0.6Sm0.4 is excited at 350 nm, the spectrumshows the Ce3þ related band emission at 505 nm as well as Sm3þ

related characteristic line emission peaks at 569 and 605 nm(Fig. 8). Since the excitation band of cerium at 350 nm, itself liesin the excitation band of samarium, the appearance of ceriumemission along with that of samarium might be due to the directexcitation of cerium ions and not due to indirect excitation bySm3þ-Ce3þ energy transfer pathway. Hence no energy transferis observed at this excitation wavelength(350 nm).

When the excitation is carried out at 390 nm, the sampleshows simultaneous occurrence of Ce and Sm peaks. Fig. 9 showsthe emission spectrum of CaS:Ce0.6Sm0.4 at an excitation wave-length of 390 nm. Since the excitation band at 390 nm is ofsamarium only, the cerium emission might be occurring due tothe transfer of energy from Sm3þ to Ce3þ ion. The CaS:Ce0.6Sm0.4

sample excited at 405 nm, again show emissions of Ce3þ andSm3þ . Fig. 10 shows the emission spectrum of CaS:Ce0.6Sm0.4 at

Fig. 7. Excitation bands of CaS:Ce, CaS:Sm and CaS:Ce0.6Sm0.4.

Fig. 8. Emission spectrum of CaS:Ce0.6Sm0.4 at an excitation wavelength of

350 nm.

Fig. 9. Emission spectrum of CaS:Ce0.6Sm0.4 at an excitation wavelength of

390 nm.

Fig. 10. Emission spectrum of CaS:Ce0.6Sm0.4 at an excitation wavelength of

405 nm.

G. Sharma et al. / Physica B 406 (2011) 2013–20172016

an excitation wavelength of 405 nm. This simultaneous appear-ance of cerium peaks, with that of samarium peaks superimposedon it indicates some sort of energy transfer among cerium andsamarium ions, since 405 nm lies in the excitation band of cerium,and samarium ions cannot be directly pumped by the 405 nmhence the samarium emission might be due to energy transferfrom Ce3þ-Sm3þ .

It is interesting to note that the emission spectra recorded atexcitation wavelengths of 390 and 405 nm both show Ce3þ andSm3þ peaks but with a difference, that for the emission spectrarecorded at 390 nm the samarium emission is more intense whilein the latter case the intensity of Ce3þ emission is more than thatof Sm3þ . This also supports the fact that in the first case theenergy transfer is occurring from Sm3þ-Ce3þ while for secondcase it is occurring via Ce3þ-Sm3þ . Hence out of all excitationwavelengths (330, 350, 390, 405 and 450 nm) the energy transferwas observed at 390 and 405 nm.

3.3. Thermoluminescence characteristics

Thermoluminescence characteristics of 60Co irradiatedCaS:Ce0.6Sm0.4 have been investigated. Fig. 11 shows the TL glowcurves of CaS:Ce0.6Sm0.4 nanophosphor for different doses of gammarays (0.14–125 Gy). All the glow curves show a peak around 475 K. Asthe gamma dose increases to 21 Gy a shoulder is formed at 415 K. At125 Gy of gamma exposure the shoulder diminishes. With increasingdose an increase in the peak intensity is observed however the peakposition remains almost unchanged.

Fig. 12 shows the TL response curve for CaS:Ce0.6Sm0.4 nano-particles. For a phosphor to be used as a thermoluminescentdosimeter the dose response should be linear. From Fig. 12 it isclear that CaS:Ce0.6Sm0.4 shows almost linear dose dependencebetween 20 and 125 Gy of exposure. Hence it can be used as adosimeteric peak in this range. The linear behavior over a widerange of dose can be explained on the basis of track interactionmodel (TIM) [30,31]. According to this model, the number of trapsgenerated by the high energy radiation in a track depends uponthe cross section and the length of the track inside the matrix. Incase of nanomaterials the length of the track generated by highenergy radiation is of the order of a few tenths of nanomaterials.At low doses there exist a few trap centers (TC)/luminescentcenters (LC). As the dose increases, the TL intensity increases asstill some particles exist that would have been missed while

Fig. 11. TL glow curves of CaS:Ce0.6Sm0.4 nanophosphor for different doses of

gamma rays (0.14–125 Gy).

Fig. 12. TL response curve for CaS:Ce0.6Sm0.4.

G. Sharma et al. / Physica B 406 (2011) 2013–2017 2017

targeted by the high energy radiation, owing to the small size ofthe particles. This gives good linearity over a wide range of dose.

4. Conclusions

Single phased CaS:Ce, Sm nanoparticles were prepared usingsolid state diffusion method. XRD confirms the cubic crystallinephase of the nanoparticles and the size was found to be 52 nm.TEM images show nearly spherical morphology of the nano-particles with an average diameter of 45–50 nm. The PL emissioncharacteristics of CaS:Ce, Sm system as a function of varyingcerium and samarium concentrations have been investigated.CaS:Ce0.6Sm0.4 system showed maximum emission intensity. We

found that energy transfer takes place between Ce and Sm at 390and 405 nm but the direction of energy transfer at both thesewavelengths is just opposite to each other. At 390 nm it is fromSm3þ to Ce3þ while at 405 nm it occurs from Ce3þ to Sm3þ . Theaddition of a co- dopant broadens the emission spectrum hencewidens the scope for applications. TL properties of 60Co irradiatedCaS:Ce0.6Sm0.4 have been investigated. The TL glow curve showsa single peak at 475 K and a linear dose dependence between20 and 125 Gy. Hence it can be used as a dosimeteric material inthis range.

Acknowledgment

One of the authors GS is thankful to I.U.A.C., New Delhi forfinancial help in the form of a fellowship under the UFUP-44305project.

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