White light emitting dysprosium doped CaS nanophosphors synthesized by solid state diffusion method

Accepted Manuscript White light emitting dysprosium doped CaS nanophosphors synthesized by solid state diffusion method S. Rekha, E.I. Anila PII:

S0254-0584(19)30640-6

DOI:

https://doi.org/10.1016/j.matchemphys.2019.121843

Article Number: 121843 Reference:

MAC 121843

To appear in:

Materials Chemistry and Physics

Received Date: 9 March 2019 Revised Date:

3 July 2019

Accepted Date: 9 July 2019

Please cite this article as: S. Rekha, E.I. Anila, White light emitting dysprosium doped CaS nanophosphors synthesized by solid state diffusion method, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2019.121843. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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White Light Emitting Dysprosium Doped CaS Nanophosphors Synthesized by Solid State Diffusion method

Optoelectronic and nanomaterials’ research laboratory, Department of Physics,

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1

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S. Rekha1,2and E.I. Anila 1*

Union Christian College, Aluva - 683102, Kerala, India. 2

Department of Physics, Maharaja’s College, Ernakulam, Kerala, India.

Dr. Anila E. I., Associate Professor,

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Department of Physics,

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Address for correspondence:

Union Christian College,

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Aluva- 683102 Kerala, India

*

Corresponding author: [email protected]

Abstract

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The white light emitting dysprosium doped calcium sulfide (CaS) nanophosphors were synthesized via solid-state diffusion method. The prepared phosphors were characterized using X-

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ray diffraction (XRD) analysis, scanning electron microscopy (SEM), photoluminescence (PL)

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spectroscopy and UV-Vis absorption spectroscopy. The average crystallite size of CaS:Dy phosphors was found to be in the range of 24-27 nm from the XRD measurements. The PL emission spectrum exhibited two prominent peaks at 480 and 572 nm followed by a weak peak at 665 nm which are due to the transition from 4F to 6H states of Dy3+ ions in the CaS host lattice. The values of Commission Internationale de l’Eclairage (CIE) coordinates, yellow to blue

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intensity ratio (Y/B) and the correlated color temperature (CCT) of the samples were calculated using the PL emission spectra in order to evaluate the emitted light. The CIE coordinates of CaS:Dy nanophosphors were found to be very close to the standard white light point given by

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(0.333,0.333). The optical bandgap of the samples was estimated from diffuse reflectance spectra and its value varied from 4.03 to 4.30 eV. The obtained results suggest the possibility of these

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phosphors being used in white light LEDs when excited with near ultraviolet light.

Keywords: nanophosphors, solid state diffusion, photoluminescence, band gap.

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1.Introduction Alkaline earth sulfides, comprising the sulfides of group IIA alkali metals Mg, Ca, Sr and Ba are excellent luminescent materials on account of their larger band gaps and a wide range of

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emission wavelengths which covers almost the entire visible region of the spectrum. Calcium sulfide (CaS) is one of the most investigated alkaline earth sulfides and it is an excellent phosphor host material. In the past few decades, many researchers have investigated the optical properties

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of CaS doped with various rare earth ions because of their significant applications in electroluminescent displays, fluorescence lamps, lasers and thermoluminescent dosimeters [1-5].

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Doping with rare earth ion provides an efficient way of tailoring the optical properties and bandgap of the host material. CaS doped with rare earth ions like europium and samarium is a red emitting phosphor [6,7], while CaS doped with cerium is a green emitting phosphor [8]. Dysprosium (Dy) is a prominent rare earth element which can be doped into suitable host

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matrices to yield luminescent materials that find application in white light generation, field emission displays, scintillation and solid-state lasers [9 -16]. Zhang et al. synthesized Dy3+ activated SrY2O4 which finds application in field emission displays [10]. The luminescent properties of

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the novel scintillating phosphor, BaGd2O4:Dy3+ have been investigated by Sun et al. [11]. Shkir et al. studied the influence of Dy doping on the linear, nonlinear and optical limiting characteristics films synthesized by sol-gel spin coating technique for optoelectronic and laser

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of SnO2

applications [12]. Shkir et al. also fabricated Dy doped PbI2 thin films on FTO substrate using spin coating method and investigated their non-linear optical characteristics so that they can be used for optical limiting applications [13]. The impact of Dy doping concentration on the luminescence properties of LiLa1−xDyxP4O12 nanocrystals was by Marciniak et al. [14]. The study of luminescence of trivalent dysprosium ions which arises due to 4f-4f transitions in various host materials has attracted many researchers since it covers the visible and near-infrared regions.

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The studies have shown that Dy3+ have two strong luminescence bands in the visible range at wavelengths 486 and 576 nm corresponding to blue and yellow colors of the visible spectrum. When the intensities of blue and yellow lines of dysprosium are equal, white light is obtained

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which is extensively used in solid-state lighting technology. However, the choice of the host matrix is a crucial factor that determines the applicability of Dy3+ doped phosphors for the white light generation.

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There has been an increase in the development of nanostructured semiconductor materials because they display novel electronic, optical and structural properties compared to bulk form.

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Several methods like solvothermal method [17], sol-gel synthesis [18], chemical co-precipitation method [19-21], alkoxide method [22] and microwave synthesis [23] have been used to synthesize doped CaS nanophosphors. Sharma et al. [24] investigated the luminescence properties of Ce/Dy codoped CaS nanoparticles synthesized by solid-state diffusion method while Burbano

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et al. [25,26] studied the photostimulated luminescence of dysprosium codoped CaS:Eu nanoparticles synthesized by chemical co-precipitation method. Although many investigators have reported white light emission of Dy in various host matrices, white light emission of Dy in

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CaS host lattice have not been reported so far. In this work we synthesized the intense white light emitting CaS:Dy nanophosphors having different concentrations of Dy3+ via solid state diffusion

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method and investigated the structural and optical properties of the prepared samples in an attempt to develop a new phosphor for potential application in white LEDs. The chromaticity coordinates, yellow to blue intensity (Y/B) ratio and correlated color temperature (CCT) were also calculated for the photometric characterization of the prepared samples.

2. Experimental Techniques

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2.1 Materials and method CaS nanophosphors different in dysprosium concentrations (1, 2, 3, 4 and 5 wt.%) were synthesized by a solid-state diffusion method. Calcium sulfate (CaSO4. 2H2O, 99%, Merck),

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sodium thiosulphate (Na2S2O3.5H2O, extrapure, SRL, India), dysprosium nitrate (Dy(NO3)3. x H2O 99.9%, Sigma) and carbon powder were the starting materials used for the synthesis. Sodium thiosulphate (15wt.%) acts as the flux and carbon (0.02 g) acts as the reducing agent that

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reduces sulphate to sulfide when the temperature is high. The calculated quantities of calcium sulphate, sodium thiosulphate, dysprosium nitrate and carbon powder were mixed by adding a

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small amount of 2-propanol using an agate pestle and mortar. The mixture was dried in an oven at 70 0C for 30 min. The charge was then transferred to a clean alumina crucible and a thin layer of carbon powder was spread over it which provides a reducing atmosphere for the reaction to take place. It was then covered with another similar crucible and fired at a temperature of 950 0C

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for 2 h. The reduction reaction involved here is given as follows: CaSO4 + 3C → CaS + CO2 + 2CO

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The charge was then taken out of the furnace and made to a fine powder using an agate pestle and mortar to obtain the CaS:Dy nanophosphor [8].

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2.2 Instruments and characterization The phase, crystal structure and crystallite size of the synthesized phosphors were

determined by X-ray diffraction (XRD) technique using a Bruker AXS D8 Advance X-ray diffractometer with Cu-Kα (λ= 1.5406 Å ) radiation. A Tescan VEGA 3 SBH model scanning electron microscope (SEM) was used to determine the particle morphology. The photoluminescence (PL) emission and excitation studies of the samples were carried out at room

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temperature with a Flouromax4C spectrofluorometer having a 150W ozone free Xenon lamp as an excitation source. The diffuse reflectance spectrum (DRS) was recorded using a Varian,

temperature. 3. Results and discussion 3.1 XRD analysis XRD

patterns

of

Dy

doped

CaS

phosphors

are

shown

in

Fig.1.

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The

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Carry5000 UV-Vis-NIR spectrophotometer. All measurements were performed at room

The XRD analysis pattern confirms the formation of cubic phase of CaS according to the

of the crystallites formed : D=

0.9λ β cos θ

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JCPDS card No:78-1922 [8,27]. The Scherrer formula was applied to evaluate the average size D

(1)

where λ, β and θ represents the X-ray wavelength, diffraction peak full width at half maximum

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(FWHM) and Bragg diffraction angle, respectively [28-30]. The average crystallite size of dysprosium doped CaS nanophosphors was in the range of 24-27 nm, as calculated from

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Scherrer formula. There are also some foreign peaks in the XRD patterns which are due to minute impurities of Ca (OH)2 and CaO present in the prepared samples. The amount of oxygen and

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hydrogen present in the samples was found to be (16.5 wt.%) and (9.2 wt.%) from EDS and CHNS analysis.

The cell parameters were calculated using the equation for cubic lattice:

d hkl =

a h2 + k 2 + l 2

(2)

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Fig. 1. XRD pattern of Dy doped CaS nanophosphors. Foreign peaks are indicated by stars. where dhkl is the interplanar distance, a is the cell parameter and (h, k, l) represent the Miller indices. The calculated values of cell parameter and average crystallite size for dysprosium doped CaS phosphors are given in Table1. We observed a small increase in the cell parameter for 2, 3

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and 4wt.% of Dy doped CaS nanophosphors compared to the literature value of 5.689 Å (JCPDS File No:78-1922). However, for 5wt.% of Dy doped CaS, a cell parameter decrease in reference

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to literature value was observed.

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The average crystallite size (D) and lattice strain (ξ) of the samples was calculated using the most reliable Williamson–Hall (W–H) equation given by,

β cos θ =

0.9λ + 2ξ sin θ D

(3)

where β (in radian) is the full width at half maximum (FWHM), λ is the wavelength of the Cu-Kα radiation and θ is the Bragg diffraction angle [31,32]. The slope of the curve gives the average internal strain ξ, and the grain size is obtained from the intercept value. The W-H plots of Dy doped CaS nanophosphors is depicted in Fig.2. The strain values and average crystallite sizes of

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Dy doped CaS nanophosphors are given in Table1. The average crystallite size for the samples calculated by the Scherrer formula is found to be in good agreement with the calculations performed using the W-H method. From the W-H plot, it is observed that, in the case of 2, 3 and

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4wt.% Dy doped samples the particles experience tensile strain whereas for 5wt.% Dy doped CaS nanophosphors, the particles experience a compressive strain since the strain is negative. The

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(5.689 Å) due to this variation in the crystal strains.

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lattice parameter initially increases and then decrease with doping compared to the literature value

Fig.2. W-H plot of A) 2wt.%, B) 3wt.%, C) 4wt.% and D) 5wt.% of Dy doped CaS

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nanophosphors.

Table1.Average crystallite size, cell parameter and lattice strain of CaS:Dy nanophosphors.

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concentration

Average crystallite size (nm) Scherrer

W-H plot

Cell

Lattice strain

parameter (Å)

( x10-4)

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Dy doping

formula 24

27

5.693

5.573

3 wt.%

25

26

5.700

5.073

4 wt.%

24

27

5 wt.%

27

25

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2 wt.%

7.462

5.685

-6.832

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5.704

3.2 SEM analysis

SEM was employed to investigate the powder morphology of undoped and dysprosium doped CaS nanophosphors. As shown in Fig. 3a and b,

the SEM images of pure CaS

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nanoparticles consists of closely packed structures having an irregular shape. The SEM images of Dy (4wt.%) doped CaS nanoparticles, as shown in Fig. 3c and d, reveal the abundance of sphere-

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like particles. The SEM images ( Fig.3c, d) show that there is a certain degree of agglomeration due to the large surface area of the nanoparticles and, hence, it is impossible to get satisfactory

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statistics on particle size. However, from the SEM images, a change in morphology is evident on the addition of dysprosium to the CaS host lattice.

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Fig. 3. SEM images of a&b) undoped CaS and c&d) CaS:Dy (4 wt.%) nanophosphors. 3.3 PL Spectroscopic studies

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3.3.1.PL emission studies

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Fig. 4. PL emission spectra of A) 1wt.%, B) 2wt.%, C) 3wt.% D) 4wt.% and E) 5wt.%

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of CaS :Dy nanophosphors excited at 350 nm.

In Fig. 4, the PL emission spectra of dysprosium doped CaS nanophosphors are shown, as

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recorded for the excitation at 350 nm at room temperature. The PL emission spectrum of 1 wt. % of Dy doped CaS nanophosphor consists of three emission peaks at 470 nm, 572 nm, and 665

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nm . For higher doping concentrations (2,3, 4 and 5wt.%)

the PL spectra consist of two

prominent emission peaks at 480 and 572 nm followed by a weak emission band around 665 nm. The two emission bands centered at 480 and 572 nm are attributed to the radiative transition from the 4F9/2 level to the lower 6H15/2 and 6H13/2 levels. The emission band centered at 665 nm is due to the

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4

F9/2

→

6

H 11/2 transition of Dy3+ ions. The PL emission spectrum of 4wt.% of Dy doped CaS

contains extra peak at 455 nm which is due to the transition from 4I15/2 to 6H15/2. The first reason

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for the radiative transition from 4F9/2 to the lower levels is that the energy difference between the states lying above 4F9/2 is minimal. Hence, the 4F9/2 level will be highly populated receiving many electrons through non-radiative relaxation. The second reason is that the energy separation

→

6

H

15/2

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between the 4F9/2 and its lower-lying level is substantial and it is equal to ~ 7000 cm-1. The 4F9/2 transition is a magnetic dipole allowed transition. Therefore, its intensity does not

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depend on the local crystal field of the host matrix. However, the transition 4F9/2 → 6H 13/2 is an electric dipole transition and it is influenced by the local crystal field [33]. The yellow to blue emission ratio is influenced by symmetry of the location of Dy3+ ions in the host lattice. The yellow emission intensity varies with small changes in the chemical environment of Dy3+ ions,

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since yellow emission is hypersensitive to the chemical environment of the dopants. According to Judd-Ofelt theory [34,35], the yellow emission is dominant when Dy3+ ions are located in the low symmetry sites without inversion center. On the other hand, the intensity of

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blue emission will be high if Dy3+ ions are located at higher symmetry with the inversion center in the host matrix. If the ligand field is situated on or very close to the inversion symmetry in the

13/2

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host matrix, both yellow and blue emissions will have equal intensities. In general, the 4F9/2

→

6

H

(yellow) transition is prominent when Dy3+ ions are located in low symmetry sites without

inversion centers, whereas the 4F9/2

→

6

H 15/2 (blue) transition is stronger when Dy3+ ions are

located in high symmetry sites with inversion center [36-38]. In our study, the ratio of blue and yellow emission intensities varies with Dy concentration due to changes in the location symmetry of Dy3+ ions in the host lattice. The PL emission spectrum obtained by Sharma et al. for CaS:Dy

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nanophosphor consisted of a broad band varying from 400 to 500 nm with peaks at 482 and 580 nm which revealed that the defect level emission of CaS host lattice is more prominent compared to Dy emission [24]. In our case we obtained intense sharp peaks at 480 and 572 nm

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due to transition between Dy3+ energy levels. Anila et al. reported two main luminescence peaks at 486 and 581 nm in the case of SrS: Dy phosphors [39]. The blue shift observed in the emission peaks compared to that reported by earlier investigators may be attributed to the

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decrease in particle size on doping.

It is observed that the PL emission intensity increases rapidly with Dy concentration up

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to 4wt.%. After that, the PL emission intensity declines due to concentration quenching effect. As the doping concentration of Dy3+ increases, the distance between the luminescence centers decreases thereby increasing the non-radiative energy transfer probability. The energy transfer mechanism between the Dy3+ions depends on the critical distance between Dy3+ ions. The critical

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distance Rc between adjacent Dy3+ ions can be calculated using the equation, Rc ≈ 2(



)1/3

(4)

where Xc is the critical concentration, N is the number of cations in the unit cell, and V is the unit

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cell volume [40, 41]. For CaS, the unit cell volume is 0.18558 nm3, the critical concentration Xc

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is 0.04, and the number of host cations in the unit cell is 4. The value of Rc is calculated to be 28 Å. Since the value of Rc is more than 5 Å, the exchange interaction is not responsible for nonradiative energy transfer process between Dy3+ ions. Thus, the concentration quenching mechanism in Dy doped CaS phosphors can be due to multipole-multipole interactions which are responsible for energy transfer of forbidden transitions. 3.3.2 PL excitation studies

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The PL excitation (PLE) spectra of Dy doped CaS nanophosphors, obtained for an emission wavelength 572 nm, are shown in Fig.5. Each spectrum consists of a strong, intense band at 350 nm and six less intense bands in the range 320-500 nm which are ascribed to the transitions from

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the 6H15/2 level to the various excited levels of the 4f9 electronic configuration of Dy3+ions. The PLE spectra consist of peaks at 325 nm (6H15/2→4M17/2), 350 nm (6H15/2→6P7/2), 364 nm (6H15/2→6P5/2), 390 nm (6H15/2→4K17/2), 426 nm (6H15/2→4G11/2), 452 nm (6H15/2→4I15/2) and 474 nm

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(6H15/2→4F9/2) [28]. Possible luminescence mechanism in CaS:Dy phosphors is proposed based on the energy level diagram shown in Fig. 6. The 4F9/2 level gets populated with electrons through

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non-radiative transitions from the various upper levels which then undergoes radiative transitions

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yielding the blue (480nm), yellow (572nm) and red (665 nm) lines.

Fig.5. PLE spectra of A) 1wt.% and B) 4wt.% of Dy doped CaS nanophosphors.

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Fig. 6. Schematic energy band diagram showing the transitions in CaS: Dy nanophosphor.

Fig.7. CIE chromaticity diagram of CaS:Dy nanophosphors .Point (a) corresponds to 1wt.%, point (b) corresponds to 2wt.% and 4wt.%, point (c) corresponds to 3wt.% and point (d)corresponds to 5wt% of Dy concentration.

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Fig. 8. Y/B ratio as a function of Dy concentration in CaS: Dy nanophosphors.

3.3.3. Chromaticity coordinates

The Commission Internationale de l’Eclairage (CIE) coordinates (x, y) are used to analyze the performance and color purity of the phosphor under consideration. From the PL emission data

equations:

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of the samples, the values of the CIE chromaticity coordinates x and y can be calculated using the

y

(5)

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and

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where X, Y and Z are the tristimulus value which gives the stimulation of the given phosphor for each of the primary colors red, blue and green. The points defined by the CIE coordinates obtained for dysprosium doped CaS nanoparticles under excitation at 350 nm lie in the white region of the CIE diagram, as shown in Fig.7. The values of CIE coordinates for all the samples are displayed in Table 2. The values of CIE coordinates for all concentrations of Dy are in excellent proximity with the standard white light point given by (0.333,0.333).

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The Y/B ratio of Dy3+ emission depends on the structural changes in the environment around Dy. The variation of Y/B ratio for CaS nanophosphors having different concentrations of Dy is shown in Table 2. It is found that the Y/B ratio initially increases and then decreases with

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the Dy3+ concentration increase (Fig. 8). The Y/B ratio of all the samples, excepting that of 1wt.% of Dy, are very close to the desired ratio of unity to obtain white light. Hence it is evident that the

intensity ratio to obtain near white light emission.

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addition of Dy3+ ions on CaS lattice influences the local field thereby changing the yellow to blue

Sample

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Table 2. The CIE coordinates, Y/B ratio, CCT and bandgap of CaS: Dy nanophosphors CIE coordinates x 0.355

2wt.% of Dy

0.345

3wt.% of Dy 4wt.% of Dy

Band gap (eV)

0.322

0.063

4465

4.05

0.345

1.0559

4986

4.03

0.331

0.342

1.172

5477

4.10

0.344

0.337

0.8998

5000

4.04

0.366

0.33

0.8259

4158

4.30

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5 wt.% of Dy

CCT(K)

y

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1wt% of Dy

Y/B ratio

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Although sulfide phosphors are chemically unstable, they are suited for LED applications

with adhesive seal and blue excitation [ 42,43]. We have calculated the correlated color temperature (CCT) of the synthesized samples since it is a simple method to denote the color appearance of white light emitted by different light sources. CCT is the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions. It relates the color of a light emitting source to the color of light of a reference light source when heated up to a specific temperature in

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Kelvin degrees. CCT values of most commercially available light sources usually range from 2700 K to 6500 K. The value is an indication of the apparent “warmth” or “coolness” of the light emitted by a source. Lamps with a CCT rating above 4000K are considered as cool sources, and

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they are used in schools, hospitals, offices, etc. as these lamps provide better visual clarity and concentration. Lamps having a CCT rating below 3200 K are considered as warm sources, and they are preferred in houses, restaurants, hotels, etc. where they will provide a cool, warmer and

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relaxing atmosphere. The value of CCT can be calculated using the McCamy relation [44] given by,

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CCT = - 449n3 + 3525n2 - 6823.3n + 5520.33

(6)

where n= x-xe/ y-ye is the inverse slope of the line drawn from (x, y) to (xe, ye ). (x, y) represent the chromaticity coordinates of the sample and ( xe, ye) represent the coordinates of the epicenter given by (0.332,0.186). The calculated CCT values for CaS: Dy phosphors are summarised in

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Table 2. It is found that all these values fall in the cool white region. The CCT value for 3wt.% of Dy doped CaS is found to be maximum (5477K) and its CIE coordinates are very close to the ideal white region in the chromaticity diagram. The CCT values for all the samples lie in the cool

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white region underlying the possibility of these phosphors for application in white light LEDs for outdoor illumination.

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3.5 UV-Vis spectroscopic analysis

The optical bandgap of Dy3+ doped CaS nanophosphors can be estimated from the

DRS. In Fig. 9A, the DRS are shown for CaS nanophosphors, having different concentrations of Dy, measured in the wavelength range of 250-800 nm at room temperature which shows an increase in reflectance in the visible region for all the samples.

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The optical band gap of CaS: Dy nanophosphors can be calculated using KubelkaMunk relation [45-]. The intensity of the reflected light depends on the scattering coefficient s and

reflectance R is given by,

F ( R) =

(1 − R )2 2R

=

k s

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(7)

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the absorption coefficient k, and the Kubelka-Munk formula, connecting k, s and diffuse

where F(R) represents the Kubelka-Munk function. Chen et al. concluded that CaS having NaCl

electronic properties of CaS [8,47-50].

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type structure has a direct bandgap based on the theoretical studies regarding the structural and

The optical bandgap Eg of each sample was determined from the DRS by plotting [(k/s) hν]2 versus energy hv graph and extrapolating the linear part of the curve to the X-axis

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intersection at the point [(k/s) hν]2 =0. The [(k/s) hν]2 vs hν curves for Dy3+ doped CaS nanophosphors are shown in Fig. 9B. From the graph, the optical bandgap values determined for 1, 2, 3, 4

were

and 5wt.% of Dy doped CaS nanophosphors using the method

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mentioned above and their values were found to vary between 4.03 eV and 4.30 eV (Table 2). The bandgap values for all the prepared samples were found to be lesser than the bulk CaS value (4.5

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eV) which may be attributed to band tailing effect caused by the point defects and impurities present in the samples [46-48].

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Fig. 9 A) DRS and B) [(k/s) hν ν]2 vs hν ν curves of CaS: Dy nanophosphors.

4. Conclusions

A cool white light emitting dysprosium doped CaS nanophosphor have been prepared using solid state diffusion method. The cubic crystalline phase of the particles formed was

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confirmed by XRD analysis. Under the excitation at 350 nm, the samples showed two dominant bands around 480 and 572 nm corresponding to blue and yellow emissions. The CIE coordinates of CaS:Dy nanophosphors corresponds to the white region of the CIE chromaticity diagram which

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is very close to the standard white light point given by (0.333,0.333). The most intense white light emission was obtained for the sample having Dy concentration 4 wt.%. The Y/B ratio of all

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the samples, except that of 1wt.% of Dy, is close to unity, and their CCT values fall in the cool white region. These results show that CaS: Dy phosphor could be used as a possible luminescent material for the generation of white light in the field of solid-state lighting technology.

Conflict of interest The authors declare no competing financial interests.

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Acknowledgments Authors thank the Board of Research in Nuclear Sciences (BRNS), Department of Atomic

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Energy(DAE), Government of India for financial support.

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HIGHLIGHTS

● Intense white light emitting CaS:Dy nanophosphors were synthesized using solid state

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diffusion method. ● The effect of Dy concentration on the luminescence properties of the nanophosphors has been studied. ● A detailed photometric characterization of CaS:Dy nanophosphors have been done.

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● The experimental results suggest the possibility of these phosphors being used for solid state lighting applications.