1. Introduction

In the recent years, rare earth activated inorganic phosphors have been broadly investigated in the field of material science, physics, chemistry and life science due to their potential use in display devices (such as cathode ray tubes, vacuum fluorescent displays, and field emission display), lighting gadgets (like fluorescent lamps and white-light emitting diodes), solid-state lasers, X-ray, medical devices, ionization radiation measurement and so on [1]. Nowadays, generation of white light is necessary on a daily basis such as in room lighting, television, mobile screen, fluorescent lamps, display devices, W-LEDs etc. White light can be generated by combination of red, green and blue emitting phosphors at a particular proportion, but controlling the red-green-blue (RGB) color balance in appropriate amount is a challenging task. This problem was erased by selection of single-phase phosphors with tunable emission; therefore choice of single-phase host is required and investigated recently. In a single-phased host white light can be produced by the following ways: (i) doping of single rare earth ion (Eu³⁺, Eu²⁺, Dy³⁺) in a particular host, (ii) combining multiple rare earth ions with red, green and blue or yellow and blue emission (such as Tb³⁺/Sm³⁺; Tm³⁺/Dy³⁺; Tm³⁺/Tb³⁺/Eu³⁺; Yb³⁺/Er³⁺/Tm³⁺), (iii) co-doping ion pairs based on the energy transfer mechanism (Ce³⁺ - Eu²⁺; Ce³⁺ - Tb³⁺; Eu²⁺ - Mn²⁺; Ce³⁺ - Mn²⁺ etc.) (iv) defect-related luminescent materials can also emit white light by controlling the concentration of the defect and reaction conditions [2]. Combining two or more than two phosphors doped with two or three rare earth ions may be costly process in order to generate white light. Hence to reduce cost, doping of single rare earth ion in a single-phase host matrix is needed.

Trivalent dysprosium has 4f⁹ electronic configuration. Dy³⁺ is the most promising rare earth ion for producing white light due to presence of at least two dominant emission bands located at blue (480 nm) and yellow (575 nm) region. CIE co-ordinates of Dy³⁺ activated phosphors can be tuned into white light or near white light zone by altering yellow to blue (Y/B) intensity ratio. High quantum efficiency, thermal and chemical stability, cost effectiveness, desired CCT (Correlated Color Temperature) and CRI (Color Rendering Index) are some important appearance existing in Dy³⁺ doped phosphors that produce appropriate white light. As the luminescence properties of trivalent dysprosium ions based on the host environment, an immense quantity of research has to be performed to expand new glass materials consisting Dy³⁺ ions with enhanced quantum efficiency and high color purity of the luminescent levels of Dy³⁺, which may be achieved by selecting a suitable host matrix and by adjusting the surrounding local environment. Recently Dy³⁺ activated silicate and aluminosilicate based phosphors such as Ca₂Al₂SiO₇:Dy³⁺ [3], Sr₂MgSi₂O₇:Dy³⁺ [4], Ca₂MgSi₂O₇:Dy³⁺ [5] have proven to be the appropriate host materials for the advancement of lighting application. CaSrAl₂SiO₇ is one of the promising silicate hosts that may be applicable in display and lighting applications when doped with different rare earth ions. Although there are very few works [6–8] investigated on CaSrAl₂SiO₇ as a host but recently we have reported optical investigation on CaSrAl₂SiO₇:Ce³⁺ phosphors [9] and work on CaSrAl₂SiO₇:Sm³⁺ has also been communicated. Both these materials showed excellent photoluminescence (PL) and thermoluminescence (TL) properties. Following the above idea, we have synthesized CaSrAl₂SiO₇:Dy³⁺ phosphors by solid state reaction method. Our main aim is to investigate photoluminescence of the present phosphors and find their use in white light application. Thermoluminescence properties were investigated and trapping dynamic was also studied. A survey of the literature indicated that Dy³⁺ doped CaSrAl₂SiO₇ phosphors have not been synthesized earlier. To the best of our knowledge PL and TL properties CaSrAl₂SiO₇:Dy³⁺ phosphors are reported for the first time.

2. Synthesis and experimental

Solid-solid reaction occurs between powders in the solid state at high temperature. It is the most widely used approach for the preparation of phosphors. It is comparatively simple, very suitable for mass production, and it is easy to obtain a long persistent duration. In fact, high temperature sintering is essential for activating the afterglow because a higher sintering temperature can generally increase the population of intrinsic thermal defects [10]. A series of single phase CaSrAl₂SiO₇:xDy³⁺ (with x = 0.003, 0.005, 0.01, 0.03, 0.05, 0.07 and 0.1 mole) phosphors were synthesized by solid state route from highly pure raw ingredient as calcium carbonate (CaCO₃, Himedia, 99.90%), strontium carbonate (SrCO₃, Himedia, 99.90%), aluminium oxide (Al₂O₃, Himedia, ≥ 98%), silicon di-oxide (SiO₂, Himedia, 99.99%) and dysprosium oxide (Dy₂O₃, Himedia, 99.99%) powders. These starting raw materials were first weighted on the basis of on-stoichiometic ratio of the sample. Mixtures were then evenly grounded in an agate mortar by using pestle for 2-3 hours; obtained homogeneous mixtures were sintered for 5 hours at 1300 ^ᴼC in a muffle furnace. The heated powders were then naturally cooling down and ready for structural and optical measurements.

Structural characterizations, surface morphology and quantitative study of elemental compositions were done by X-ray diffraction i.e., XRD (D2 phaser, Bruker), field emission scanning electron microscopy i.e., FESEM (SU8000, Hitachi) and energy dispersive spectroscopy i.e., EDS study. PL experiments were recorded by using spectrofluorophotometer (RF-5301PC, Shimadzu) attached with xenon lamp of 150 Watt as an excitation source. TL measurement of ultraviolet (UV, 254 nm) exposed samples was carried by TLD reader (TL1009I, Nucleonix). TL spectrum was measured by using band pass filters of various wavelengths.

3. Results and discussions

3.1 Structural and morphological characterization

XRD measurement was performed to confirm formation of samples, using fixed incidence wavelength of X-ray (0.154 nm for CuKα) and varying diffraction angle θ ranging from 10^ᴼ to 70^ᴼ. Figure 1 shows comparison of experimental diffraction pattern of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor with standard XRD pattern (JCPDS, card no. 26-0327) of CaSrAl₂SiO₇ host. Entire diffraction planes have good correlation with planes in the standard XRD pattern; it suggests non-influence of crystal structure of CaSrAl₂SiO₇ host due to inclusion of Dy³⁺ doping ions. It was noticed carefully that no unwanted peaks are observed hence single phase Dy³⁺ doped CaSrAl₂SiO₇ phosphors were prepared by solid state reaction method. This phosphor is a member of melilite (M₂Al₂SiO₇ with M = Ca, Sr) group; it has tetragonal crystallography and space group P$\overline{4}$2₁m. Highest intensity was observed for (h = 2, k = 1, l = 1) plane which is situated at 2θ = 31.18^ᴼ.

 

Fig. 1 XRD pattern of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor along with JCPDS file.

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Surface morphology of CaSrAl₂SiO₇:Dy³⁺ phosphor was studied by FESEM analysis. FESEM technique provides surface information of very high resolution as compared to optical or light microscope. The combination of higher magnification, larger depth of field, greater resolution, compositional and crystallographic information makes the FESEM one of the most heavily used instruments in research areas and industries, especially in semiconductor industry. Figure 2 depicts FESEM image of CaSrAl₂SiO₇:Dy³⁺ sample. The sample crystallised with irregular and uneven dense morphology in micrometre scale with inhomogeneous shaped (mostly rod shaped) small particles. Micrometre sized crystalline sample is very much appropriate to generate white light in lighting applications. To confirm inclusion of composition elements EDS analysis was performed. EDS represents composition details of the synthesized sample. The EDS tool is usually associated with scanning electron microscope (SEM). The size of the pulse generated depends on the number electron hole pairs created, which in turn depends on the energy of the incoming X-ray. The detector which is lithium doped silicon is protected by a beryllium window and operated at liquid nitrogen temperatures [11]. Figure 3 shows EDS spectrum of CaSrAl₂SiO₇:Dy³⁺ phosphor. EDS spectrum consists of distinct peaks of calcium (Ca), strontium (Sr), aluminium (Al), silicon (Si) and dysprosium (Dy) in CaSrAl₂SiO₇:Dy³⁺ phosphor; no further impurity peak was found therefore the formation of pure CaSrAl₂SiO₇:Dy³⁺ sample was confirmed.

 

Fig. 2 FESEM micrograph of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor.

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Fig. 3 EDS spectrum of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor.

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3.2 Photoluminescence (PL) characterization

Room temperature PL excitation (PLE) spectrum of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor is shown in Fig. 4, which was scanned with emission wavelength at 480 nm. Excitation spectrum is composed of several sharp excitation peaks that lie between 250 and 430 nm. These peaks consecutively located at 297, 326, 350, 363, 389 and 429 nm. Multiple PLE peaks were observed corresponding to the transitions from the ⁶H_15/2 (4f⁹) ground state to higher energy states of the 4f⁹ configuration: 326 nm (⁶H_15/2 → ⁴K_15/2), 350 nm (⁶H_15/2 → ⁴M_15/2, ⁶P_7/2), 363 nm (⁶H_15/2 → ⁴I_11/2), 389 nm (⁶H_15/2 → ⁴K_17/2, ⁴M_19/2,21/2, ⁴I_13/2, ⁴F_7/2), 429 nm (⁶H_15/2 → ⁴G_11/2) [12]. The most prominent intensity was found for the peak centred at 350 nm.

 

Fig. 4 PL excitation spectrum of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor scanned with λ_em = 480 nm.

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Figure 5 shows generation of PL emission spectra of CaSrAl₂SiO₇ phosphor when doped with 0.01 mole of Dy³⁺ content under some intense excitation wavelength i.e., 326, 350, 363 and 389 nm. Strongest remarkable emission was observed when the sample was excited at 350 nm wavelength. The major emission bands of Dy³⁺ ions dispersed in CaSrAl₂SiO₇ matrix were observed in blue (480 and 493 nm) and yellow (576 nm) range when excited by photons of wavelength 326-389 nm. Blue-light emissions peaking at 480 and 493 nm are ascribed to the magnetic dipole ⁴F_9/2 → ⁶H_15/2 transition and yellow-light emission peaking at 576 nm is due to electric dipole ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ions. The yellow emission of Dy³⁺ is especially hypersensitive (∆L = 2, ∆J = 2) to the local environment, whereas the blue emission is not. The ratio of yellow/blue (Y/B) is mainly influenced by the nephelauxetic effect between Dy³⁺ and O^2- in the composite oxides. Greater the nephelauxetic effect (viz. covalency), the stronger the yellow emission of the Dy³⁺ ion [2]. According to the Judde Ofelt theory [13,14], when Dy³⁺ locates at a low symmetry local site without inversion symmetry, a yellow emission according to the electric dipole transition (⁴F_9/2 → ⁶H_13/2) will be dominant. Conversely, a magnetic dipole transition (⁴F_9/2 → ⁶H_15/2) will predominate in the emission spectra, resulting in a strong blue emission [13]. The change in the Dy³⁺ concentration would affect the local symmetry of the crystal structure and consequently, the ratio of Y/B would vary with the Dy³⁺ concentration [2]. In order to tune emission color into the white light zone, Dy³⁺ concentration dependent PL emission of CaSrAl₂SiO₇:Dy³⁺ phosphor was also investigated; this effect is dealing here below.

 

Fig. 5 PL emission spectra of CaSrAl₂SiO₇:0.01Dy³⁺ phosphor under different excitation.

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Emission intensity is greatly affected by changing the doping concentration, this influence can be recognized from Fig. 6 in which PL emission spectra of CaSrAl₂SiO₇:xDy³⁺ phosphors (x = 0.003, 0.005, 0.01, 0.03, 0.05, 0.07 and 0.1 mole) are drawn. Whole emission spectra were acquired by 350nm excitation in the wavelength range between 370 nm and 700 nm. All the prepared samples possess the identical pattern and spectral distribution, with exception of distinct emission intensity. There were a spike at about 393 nm within a broad band in the blue range 390-450 nm; that are corresponding to emission from the host CaSrAl₂SiO₇ matrix. The increase of Dy³⁺ concentration reduced the blue emission broad band aspect to the host matrix and intensified the bands due to Dy³⁺ ions which suggested that under 350 nm excitation wavelength CaSrAl₂SiO₇:Dy³⁺ phosphor favored the Dy³⁺ emission. The increase of activator concentration results in high luminescence intensity up to a certain maximum value, and then it decrease beyond that value. This effect has been named concentration quenching, which can occur as a result of: (i) loss of excitation energy from the emitting state due to cross-relaxation between the activators; (ii) excitation migration owing to resonance between the activator ions and (iii) coagulated or paired activator ions acting as quenching center [15].

 

Fig. 6 PL emission spectra of CaSrAl₂SiO₇:Dy³⁺ phosphors excited at λ_ex = 350 nm. Inset represents Dy³⁺-concentration dependence emission intensity of peak at 480 nm.

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It can be checked from Fig. 6 that intensity of characteristic emission of Dy³⁺ at 480, 493 and 576 nm increases as we increase Dy³⁺ contents (x) until x = 0.01 mole, then emission intensity decreases with rise in Dy³⁺ contents (0.03-0.1 mole). The variation of peak intensity located at 480 nm with respect to Dy³⁺ concentration (x) can be clearly viewed from inset in Fig. 6. This behaviour comes from concentration quenching mechanism. The concentration quenching mainly results from non-radiative energy transfer (ET) among Dy³⁺ ions, whose occurring possibility increases as the content of Dy³⁺ increases. Three main mechanisms responsible for non-radiative energy transfer are exchange interaction, radiation reabsorption and multipolar interaction, respectively. The radiation reabsorption mechanism has an obvious effect only when there is a considerable overlap between the excitation and emission spectra [16]. To get more idea concerning the concentration quenching phenomenon of Dy³⁺ ions, the kind of interaction responsible for nonradiative ET between the Dy³⁺ ions has to be investigated, which can be determined from the following Eq. (1). According to Dexter’s ET formula and Reisfeld’s approximation,

 

Fig. 7 lg (I/x) versus lg (x) plot for CaSrAl₂SiO₇:Dy³⁺ phosphor, when excited at 350 nm and luminescence monitored at 480 nm.

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Energy level scheme of partial energy levels of Dy³⁺ is drawn on Fig. 8. If we check through Fig. 6, it can be fairly observed that as Dy³⁺ concentration increases beyond 0.01 mole yellow to blue intensity (Y/B) ratio enhances, after 0.05 mole of Dy³⁺ content intensity of blue and yellow band started to become closer and for 0.1 mole Y/B ratio became nearly equal to one. Thus dopant (Dy³⁺) concentration dependent PL of CaSrAl₂SiO₇:Dy³⁺ sample signifies that luminescence color of Dy³⁺ doped samples could be tuned from blue to cyan and cyan to bluish white by adjusting the Dy³⁺ concentrations. This was also verified by CIE diagram that is demonstrated in Fig. 9. Chromaticity co-ordinates, color purity and CCT for CaSrAl₂SiO₇ phosphors doped with different concentration of Dy³⁺ were estimated from the formulas given in Eqs. (3) and (4) and results are tabulated in Table 1.

 

Fig. 8 Schematic energy level diagram of Dy³⁺ ions.

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Fig. 9 CIE chromaticity diagram of CaSrAl₂SiO₇:Dy³⁺ phosphors excited by 350 nm.

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Table 1. CIE coordinates of CaSrAl₂SiO₇:Dy³⁺ phosphors

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The Correlated Color Temperature (CCT) was also estimated using following Eq.:

It can be seen that as dopant concentration increases, color purity and CCT value lowers. The low value of the color purity indicates the purity for white-light emission [1,18]. The calculated CCT values for CaSrAl₂SiO₇:Dy³⁺ phosphors were observed in range between 5934 and 20412 K that belongs in the cool-white region. The calculated CCT was 5934 K for the 0.1 mole of Dy³⁺ concentration serves as cool-white emission and is very near to the “ideal white” region of the CIE diagram. Greater the value of CCT, better visual sensation and greater brightness perception as compared to lower values. The CCT values fall in the cool white-light range that signifying the suitability of CaSrAl₂SiO₇:Dy³⁺ phosphor as cool white light application for outdoor illumination.

3.3 Thermoluminescence (TL) characterization

Computation of kinetic parameters depends on the idea of order of kinetics and explicit number and position of glow peaks. Hence TL glow curve of ultraviolet (UV)-exposed samples were measured in order to obtain order of kinetics, number of peaks and peak position. For all the TL measurement small fixed amount of sample was heated from room temperature to 673 K. Systematic investigation on TL of CaSrAl₂SiO₇:Dy³⁺ phosphors as follows.

In the first instance, effect of Dy³⁺ concentration on TL of the sample was studied. Figure 10 exhibits Dy³⁺ concentration dependence on the TL response of CaSrAl₂SiO₇:Dy³⁺ phosphor. The glow curves of almost all the samples had first peak in the temperature range 400-445 K and second shoulder like peak in the temperature range 520-560 K. First peak was noted to be intense up to 0.03 mole, second peak was dominating beyond this Dy³⁺ content that means for 0.05, 0.07 and 0.1 mole contents (see inset of Fig. 10). The growth total TL intensity could be exploited to understand the effect of doping concentration on the TL signal. It was found that total TL intensity and area covered by the glow curve were sharply increased as concentration of Dy³⁺ rose from 0.003 to 0.005 mole thereafter those were quickly decreased for Dy³⁺ concentration ranges from 0.01 to 0.1 mole (Figs. 10 and 11); this behaviour is consistent with the concentration quenching effect. Hence 0.005 mole is the optimised Dy³⁺ concentration for getting highest TL signal in CaSrAl₂SiO₇:Dy³⁺ phosphor. The TL sensitivity to a given excitation irradiation may rise with the dopant concentration up to a maxima, and then decay for higher value of dopant concentrations. This behaviour has been termed concentration quenching [19]. As we increase impurity concentration there is an increase in the number of defects/traps which in turn implies a growth in the density of charge carriers being trapped upon irradiation. Therefore the initial rise in the TL peak intensity or area of the glow curves. Furthermore, on being thermally stimulated, these charge carriers release from traps which in turn recombine with their counterparts at the recombination center and yield diverse TL glow peaks with elevated height [20]. In the second instance impact of variable UV-exposure time on TL of CaSrAl₂SiO₇:0.005Dy³⁺ phosphor was investigated.

 

Fig. 10 Dy³⁺ concentration dependence on TL of CaSrAl₂SiO₇:Dy³⁺ phosphor exposed to 20 min UV-irradiation.

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Fig. 11 Total TL intensity versus Dy³⁺ concentration (x).

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The glow curves of UV-exposed CaSrAl₂SiO₇:0.005Dy³⁺ sample for 5-35 minute exposure time are demonstrated in Fig. 12. All the glow curves showed a major glow peak at around 439 K in addition with a hump at around 530 K. It is cleared from Fig. 12 that shape of the glow curves and peak position are free from exposure time. The plot of TL intensity of the 439 K peak against UV-exposure time is shown in inset of Fig. 12. It is to be noted that there was an almost linear increment in TL intensity up to 20 minutes after that TL intensity decreased and then saturated. Hence it may be possible that present sample can be useful in implementation of low dose UV-dosimetry.

 

Fig. 12 Effect of UV-exposure time on the TL of CaSrAl₂SiO₇:0.005Dy³⁺ phosphor. Inset shows TL intensity as a function of UV-exposure time.

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Kinetic parameters were computed via Chen’s peak shape method after applying computerized glow curve deconvolution method (since glow curve is broad and complex enough). Curve fitting is shown in Fig. 13 that represents superposition of four Gaussian peaks. Table 2 represents kinetic parameters (τ, δ, ω and μ), trap depth (E) and frequency factor (S) examined for all the Gaussian peaks; increasing order of trap depth was obtained with rise in temperature suggested that lower temperature peaks indicated formation of shallower traps while higher temperature peaks are due creation of deeper traps. Figure 14 shows TL emission spectrum of CaSrAl₂SiO₇:0.005Dy³⁺ phosphor, it consists of two characteristic emission peaks of Dy³⁺ ions centered at 480 nm (blue) and 580 nm (yellow) due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2, respectively. TL emission is analogous to PL emission. The detail theory on luminescence emission and transition of Dy³⁺ ions have been discussed above.

 

Fig. 13 Gaussian fitting of the glow curve for 20min UV-exposed CaSrAl₂SiO₇:0.005Dy³⁺ phosphor.

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Table 2. Kinetic parameters of CaSrAl₂SiO₇:0.005Dy³⁺ phosphor

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Fig. 14 TL emission spectrum of CaSrAl₂SiO₇:0.005Dy³⁺ phosphor.

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4. Conclusions

CaSrAl₂SiO₇:xDy³⁺ phosphors have been synthesized via high temperature solid state reaction method. The crystal structure and phase study were measured by XRD technique. The prepared samples exhibited tetragonal crystallography with single phase. FESEM resulted uneven and inhomogeneous morphology with micrometre shaped particles of the sample prepared by this method. The samples when excited at 350 nm showed intense emission bands at blue and yellow region the combination of these bands give rise to white light. Emission peaks at 480 nm and 493 nm are due to the magnetic dipole ⁴F_9/2 → ⁶H_15/2 transition and peak at 576 nm is due to electric dipole ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ions. Effect of Dy³⁺ concentration on PL emission was investigated and concentration quenching was found because of exchange interaction. CIE chromaticity diagram showed that luminescence color tuned from blue to bluish white region by altering Dy³⁺ concentration. Lower value of color purity for 0.05 to 0.1 mole of Dy³⁺ signify suitability of phosphor for potential WLEDs, CCT values also lower as we raise dopant contents. In view of intense near white light emission with the appropriate chromaticity coordinate, lower value of color purity, CCT observed in cool region CaSrAl₂SiO₇:xDy³⁺ (x = 0.05, 0.07, 0.1 mole) phosphor is competitive as a promising candidate for white LEDs especially in outdoor illumination. TL properties of UV exposed CaSrAl₂SiO₇:Dy³⁺ phosphor was also measured and activation energies for four Gaussian curves were estimated from peak shape method, that are resulted as 0.53, 0.55, 1.12 and 1.41 eV. TL emission spectrum was also recorded which consisted characteristic peaks of Dy³⁺ ions at around 480 and 580 nm. TL results suggested that present sample may be useful in low dose UV dosimetry.