Luminescence properties of dysprosium doped di-calcium di-aluminium silicate phosphors

Optical Materials 58 (2016) 234e242

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Luminescence properties of dysprosium doped di-calcium di-aluminium silicate phosphors Geetanjali Tiwari a, *, Nameeta Brahme a, **, Ravi Sharma b, D.P. Bisen a, Sanjay K. Sao a, Shalinta Tigga a a b

School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, C. G., India Department of Physics, Govt. Arts and Commerce Girls College, Devendra Nagar, Raipur, C.G., India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 February 2016 Accepted 21 May 2016

A Dysprosium doped di-calcium di-aluminium silicate phosphor emitting long-lasting white light was prepared and investigated. Phosphors were synthesized by combustion-assisted method. The effect of doping concentration on the crystal structure and luminescence properties of Ca2Al2SiO7:Dy3þ phosphors were investigated. The phase structure, surface morphology, particle size, elemental analysis was analyzed by using X-ray diffraction (XRD), transmission electron microscope (TEM), Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX) techniques. X-ray diffraction (XRD) profiles showed that all peaks could be attributed to the tetragonal Ca2Al2SiO7 phase when the sample was annealed at 1100  C. The increase in TL intensity indicates that the concentration of traps increases with UV irradiation. Under the UV-excitation, the Thermoluminescence (TL) emission spectra of Ca2Al2SiO7:Dy3þ phosphor shows the characteristic emission of Dy3þ peaking at 484 nm (blue), 583 nm (yellow) and 680 nm (red), originating from the transitions of 4F9/2 / 6H15/2, 4F9/2 / 6H13/2 and 4F9/ 6 3þ 2 / H11/2. Photoluminescence (PL) decay has also reported and it indicates that Ca2Al2SiO7:Dy phosphor contains fast decay and slow decay process. The peak of Mechanoluminescence (ML) intensity increases linearly with increasing impact velocity of the moving piston. The possible mechanism of Thermoluminescence (TL), Photoluminescence (PL) and Mechanoluminescence (ML) of this white light emitting long lasting phosphor is also investigated. © 2016 Elsevier B.V. All rights reserved.

Keywords: White long lasting phosphors Optical material Combustion-assisted method Luminescence properties

1. Introduction In the last few years, the phosphors with long lasting afterglow rare earth doped aluminates and silicates have been proposed and developed for various display and signing applications because of their high luminosity, long duration time and improved chemical stability [1e3]. The newly developed colours of the long lasting phosphors cover from blue to red. However until now, no phosphor with white long afterglow has been developed into a commercial application. In recent past, production of white LED with minimum cost has been an important topic to research. Numbers of aluminates, silicates, doped with rare earth materials have been discovered as different color emitting materials. It is not an easy task to

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Tiwari), namitabrahme@ gmail.com (N. Brahme). http://dx.doi.org/10.1016/j.optmat.2016.05.033 0925-3467/© 2016 Elsevier B.V. All rights reserved.

maintain the proper ratio of existing blue, yellow, green and red emitting phosphor for retrieving white color. Rare earth dysprosium ions have at least three dominant emission bands in the blue region (470e500 nm) due to (blue), (550e600 nm) (yellow) and (600e700 nm) (red), originating from the transitions of 4F9/ 6 4 6 4 6 2 / H15/2, F9/2 / H13/2 and F9/2 / H11/2 [4,5]. It is possible to achieve near white light emission by adjusting the yellow to blue intensity ratio value. Consequently, Dy3þactivated luminescent materials attracted much attention, because of their significant applications as potential single phase white phosphors. White light emission resulted from a single phase phosphor is expected to obtain high luminous efficacy. Few silicates like SrSiO3, Sr2MgSi2O7, Ca2MgSi2O7 and Sr2SiO4 doped with Dy3þ ions have already exhibited white light emissions [6,7]. In this paper we have prepared Ca2Al2SiO7:Dy3þwith changing concentration of Dy3þ (0.5, 1, 2, 3, 4 mol%). Conventional combustion e assisted method was employed for this. Detailed studies of photoluminescence, mechanoluminescence and thermoluminescence process have been done.

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We got near white light emission when excited at 351 nm. Correlated color temperature was also calculated to check the suitability of prepared phosphors as practical white light source. So Dy3þ ions have potential application for fluorescent tubes, color televisions, glass lasers and mercury free fluorescence lamp. In these devices, luminescent materials absorb energy generated from cathode ray or ultraviolet (UV) radiation and then convert it to visible light [8]. Dy3þ ion is also a good activator for preparation of electron trapping luminescence materials [9]. Several reports dealing with the luminescence studies of [Ce3þ, Eu3þ, Tb3þ] doped Ca2Al2SiO7 are available in the literature [10e12], however TL, ML and PL decay of the Dy doped CaAl2SiO7 phosphors are first time reported in the present paper. 2. Experimental 2.1. Sample preparation 3þ

The Ca2Al2SiO7: Dy (0.5, 1, 2, 3, 4 mol %) samples were prepared by the combustion technique. The starting materials taken were calcium nitrate Ca(NO3)2, aluminum nitrate Al(NO3)3.9H2O, silicate SiO2, dysprosium nitrate Dy(NO3)3.6H2O, and urea (NH2CONH2) as a fuel. Stoichiometric composition of each nitrate and urea were mixed together and crushed into mortar and pestle for 1 h to form a thick paste then transferred to crucible and introduced into furnace maintained at 600  C. Initially the mixture boils and the spontaneous ignition occurs and the foamy product was found that can easily be milled to obtain the precursor powder. The precursor powders were ground and annealed at 1100  C for 4 h to obtain Ca2Al2SiO7: Dy3þ phosphor. 2.2. Characterization The crystalline structure and particle morphology of the resulting samples were investigated by X-ray diffraction analysis (XRD model D8 Advance Bruker AXS) using Cu Ka radiation (l ¼ 0.154 nm) Data have been collected by step scanning 2q from 20 to 60 . The phase structure of the sample was verified with the help of Joint Committee of Powder Diffraction Standard Data (JCPDS) file (JCPDS: 35-0755) and average crystallite size was calculated using the Debye Scherrer formula (D ¼ kl/bcosq). Particle size of prepared phosphor was determined by TEM using a [Tecnai G2S e TWIN-FEI. The morphology of the phosphor were characterized by scanning electron microscope (SEM) with EDX [Model: QUANTA-200F-FEI]. Gamma irradiation was carried out using a60Co source having exposure rate 0.930 k Gy/h. ML was excited by dropping a piston (mass 400 gm) onto the sample from various heights. The impact velocity of the piston was calculated using the formula: v ¼ √2gh. The ML was monitored using a photomultiplier tube (RCA-931A) connected to a digital storage oscilloscope (Scientific SM-340). Schematic diagram of the experimental setup used for deforming the sample and measuring the ML is shown in Fig. 1. PL was recorded using fluorescence spectrophotometer(ShimadzuRF-5301XPC) and emission was recorded using a spectral slit width of 1.5 nm.The TL glow curve was recorded using TL reader (NucleonixTL1009I) by heating the sample with heating rate 10  C/s.TL emission spectra were recorded by using interference filters of different wavelengths. All measurements were carried out at the room temperature. The samples were wrapped in aluminium foil and kept in dark till the ML studies were carried out. The ML studies were carried out in a dark room. The crystals were fractured via dropping a load of a particular mass of 400 g and of cylindrical shape, on the crystals. To change the impact force the load was dropped from different heights. The experimental setup has been shown in Fig. 1.

Fig. 1. Schematic diagram of the experimental setup used for deforming the sample and measuring the ML.

The transient ML was recorded using a photomultiplier tube and an oscilloscope. The height of the moving piston, through which it was dropped holding the impacting mass, could be changed up to 55 cm. 3. Results and discussion 3.1. XRD analysis The phase structure of the prepared phosphor was analyzed by the XRD. Fig. 2 shows the XRD pattern of Ca2Al2SiO7: Dy3þ phosphor. The position and intensity of diffraction peaks of the Ca2Al2SiO7: Dy3þ phosphor is matched well with the standard JCPDS data of the compound Ca2Al2SiO7. The small amount of doped rare earth ions (Dy3þ) has virtually no effect on the phase structure. The diffraction pattern was found to be similar to that of the sample Ca2Al2SiO7:Dy3þ phosphor having pure tetragonal Ca2Al2SiO7 phase (JCPDS file No.35-0755). No other crystalline phases were detected. The diffraction intensity is maximum for (2 1 1) plane having 2q ¼ 31.4 . This structure, a member of the melilite group, is having space group P-421 m (No. 113) with cell parameters a ¼ b ¼ 0.7690 nm and c ¼ 0.5063 nm. The average crystallite size was calculated from the XRD pattern using Debye Scherrer relation D ¼ kl/bcosq, where D is the crystallite size for the (hkl) plane, k is dimensionless shape factor, with a value close to unity, l is the

Fig. 2. XRD pattern of Ca2Al2SiO7: Dy3þ(1 mol %) phosphor with JCPDS file No. 35e0755.

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wavelength of the incident X-ray radiation Cu Ka (0.154 nm), b is the full width at half maximum (FWHM) in radiation, and q is the corresponding angle of Bragg diffraction. 3.2. Transmission electron microscopy (TEM) Fig. 3 shows the transmission electron microscope image of Ca2Al2SiO7:Dy3þphosphor. TEM image confirm the nanometre size of the prepared phosphor. Moreover, agglomeration of powder particles was also observed, which is due to the high-temperature heat treatment. 3.3. Scanning electron microscopy (SEM)

Fig. 4. SEM image of Ca2Al2SiO7:Dy3þ(1 mol%) phosphor.

Fig. 4 shows the SEM micrograph of the Ca2Al2SiO7:Dy3þphosphor. The microstructure of the sample reflects the inherent nature of the combustion process. When a gas is escaping under high pressure during combustion process, pores are formed with the formation of small particles near the pores. The non-uniform and irregular shapes of the particles as shown can be attributed to the non-uniform distribution of temperature and mass flow in the combustion flame. The particle was almost spheroidal and has some extent of agglomeration. 3.4. Energy dispersive X-ray spectroscopy (EDX) Fig. 5 shows the Energy dispersive X-ray spectroscopy (EDX) spectra of the prepared sample. EDX is a standard procedure for identifying elemental composition of sample area as small as a few nanometers. The existence of Dysprosium (Dy) in the prepared phosphor could be clearly seen from the corresponding EDX spectra. No other emissions has appeared apart from calcium (Ca), aluminum (Al), silicon (Si), oxygen (O) and Dysprosium (Dy) in the Ca2Al2SiO7: Dy3þ EDX spectra of the phosphor. 3.5. Photoluminescence of Ca2Al2SiO7:Dy3þ In order to study the luminescent properties of phosphor, the excitation and emission spectra of prepared Ca2Al2SiO7: Dy3þ phosphor was recorded. Excitation spectra of Ca2Al2SiO7 doped with 1 mol% of Dy3þ is shown in Fig. 6(a). Emission spectrum is monitored for different samples of Ca2Al2SiO7 doped with different concentrations of Dy3þ. It is readily observed that, intensity of photoluminescence signals increases with increase concentration of Dy3þ ions. It reaches optimum intensity when concentration of Dy3þ was 1 mol% then photoluminescence intensity decrease due to concentration quenching of Dy3þ ions. The excitation spectra were observed in the range of 200e450 nm, consists of the f / f transition of Dy3þ are observed, and emission spectra were

Fig. 3. TEM image of Ca2Al2SiO7: Dy3þ(1 mol%)phosphor.

Fig. 5. EDX spectra of Ca2Al2SiO7:Dy3þ(1 mol%) phosphor.

recorded in the range of 400e750 nm. From Fig. 6, the excitation and emission spectra is broad bands cantered at 351 and 583 nm. The excitation spectra for the emission at 583 nm consist of a series of line spectra in 230e450 nm with the strongest one at 351 nm and some lines at 242, 326, and 391 nm, which are ascribed to the transitions from the ground state to excitation states in the 4f9 configuration of Dy3þ. One can also find that the emission lines of Dy3þ are broadened somewhat because there are several Stark levels for the 4F9/2 / 6HJ levels. The emission spectra mainly consist of three groups of sharp lines peaked at about 484(blue), 583 (yellow) and 680 nm (red),which are associated with the transitions of Dy3þfrom the excited state 4F9/2 / 6H15/2, 4F9/2 / 6H13/2 and 4F9/2 / 6H11/2 respectively [13]. One can also find that the emission lines of Dy3þ are broadened somewhat because there are several Stark levels for the 4F9/ 6 2 / HJ levels. It is well known that the former weak blue emission at 484 nm (4F9/2 / 6H15/2) is corresponded to the magnetic dipole transition, which hardly varies with the crystal field strength around Dy3þ. While the later stronger yellow emission at 583 nm (4F9/2 / 6H13/2) belongs to the hypersensitive forced electric dipole transition, which is strongly influenced by the outside surrounding environment [14]. According to the JuddeOfelt theory [15], when Dy3þ locates at a low symmetry local site without inversion symmetry, a yellow emission according to the electric dipole transition (4F9/2 / 6H13/2) will be dominant. Conversely, a magnetic dipole transition (4F9/2 / 6H15/2) will predominate in the emission spectra, resulting in a strong blue emission. In our case, the yellow emission (4F9/2 / 6H13/2) will dominate. The strong yellow emission is also beneficial to decrease the color temperature of the

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Fig. 6. (a) Excitation spectra of Ca2Al2SiO7:Dy3þphosphor. (b) Emission spectra of Ca2Al2SiO7:Dy3þphosphor.

phosphor and generate warm white light emission. The optical properties of the material are often influenced by the structure of the matrix and synthesis technique. Dy3þ ions have been often used as codopants in the previously developed aluminate and silicate based materials. When divalent alkaline earth ions, such as Ca2þ or Sr2þ, is substituted by trivalent Dy3þ in the alkaline earth silicates and aluminates, various defects can be induced due to the charge compensation mechanism [16]. In the Eu2þ and the Dy3þ co-doped materials, most of the excitation energy will be transferred from the host or the Dy3þ to the Eu2þ; thus only 5d-4f emissions of Eu2þ can be observed. However, in Dy3þ singly doped samples, which are in our current interest, Dy3þ is not only the supplier of traps but also an activator itself. A process of emitting the white light in Ca2Al2SiO7:Dy3þ phosphor is illustrated schematically in Fig. 7. After irradiation with the g-ray (process [1]), most of the excitation energy associated with the excited carriers (electrons or holes) will be transferred via the host directly to the luminescence centers, Dy3þ, followed by the Dy3þ 4f emissions as the immediate luminescence (process [2]). However, part of the excitation energy will be stored when some of the excited carriers drop into the traps (process [3]), instead of returning to the ground states. Later, with thermal excitation at proper temperature, these carriers will be released from the traps and transferred via the host to the Dy3þions, followed by the characteristic Dy3þemissions as long afterglow (process [4]). In the practical system, the electron traps and the hole traps may not be both equally abundant or important in terms of their contribution to the white light emission, as suggested in Fig. 7 [17].

Fig. 7. Schematic diagram of the mechanism in Ca2Al2SiO7:Dy3þphosphor.

3.6. CIE chromaticity coordinate CIE coordinates were calculated to know exact color emitted from the emission spectra and it was found that, near white light emission is there which is illustrated in Fig. 8. In general, color of any phosphor material is represented by means of color coordinates. The emission spectrum of the Ca2Al2SiO7: Dy3þphosphor sample was converted to the CIE 1931 chromaticity using the photo luminescent data and the interactive CIE software (CIE coordinate calculator) diagram as shown in Fig. 8. The calculated chromaticity coordinates for the white light emitted from Ca2Al2SiO7: Dy3þphosphor is given by(x ¼ 0.3541, y ¼ 0.3589), which is in agreement with the chromaticity coordinates of standard white light (x ¼ 0.334, y ¼ 0.337) [18]. 3.7. Calculated color temperature (CCT) In order to investigate the prepared phosphors for suitability as a practical white light source. Correlated color temperature (CCT) of the samples have been calculated. The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a light source, relating its color to the color of light with respect to a reference light source when heated up to a specific temperature, in degrees Kelvin (K). The CCT rating for a lamp or a source is a general warmth or coolness measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below

Fig. 8. CIE Chromaticity diagram of Ca2Al2SiO7:Dy3þ(1 mol%) phosphor.

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3200 K are usually considered ‘‘warm’’ sources, while those with a CCT above 4000 K are usually considered ‘‘cool’’ in appearance [19e21]. McCamy has proposed the analytical equation to calculate the CCT which is given [19e21] by

Table 1 Decay parameters. Phosphor

t1(s)

t2(s)

Ca2Al2SiO7:Dy3þ

19.4

216.2

CCT ¼ 449 n3 þ 3525 n2 þ 6823:3 n þ 5520:33 where n ¼ (x  xe)/(y  ye) is the inverse slope line and (xe ¼ 0.332, ye ¼ 0.186) is the epicenter. Generally, the preferred CCT values range from 3500 to 6500 K but the range from 3000 to 7800 K may also be accepted. It can be seen that the value of CCT from 4705 K, which is well under the acceptable ranged and can be considered ‘‘cool’’ in appearance. 3.8. Decay characteristics The decay curve of phosphor was measured and it is exhibited in Fig. 9. The initial intensity of Ca2Al2SiO7:Dy3þ is much stronger, which is due to the higher emission efficiency of Ca2Al2SiO7:Dy3þ. The rate of decay of intensity is given by the formula [22].

I ¼ I1 expðt=t1 Þ þ I2 expðt=t2 Þ

(1)

where I represents the phosphorescent intensity. I1 and I2 are constants of intensity, t1 and t2 are average lifetime of an excited electrons, deciding the rates for the rapid and the slow exponentially decay components, respectively. See the Table 1. 3.9. Mechanoluminescence of Ca2Al2SiO7: Dy3þ Mechanoluminescence (ML) is an interesting luminescence phenomenon where by light emission in solids is caused by grinding, rubbing, cutting, cleaving, shaking, scratching, and compressing or by crushing of solids. Fig. 10 shows that the ML intensity initially increases with the increase in the concentration of Dy ions, attains an optimum value for 1 mol% then decreases with further increase in the concentration of Dy3þ. Three peaks have been found in ML Vs Time curve [10(a)]. Since the mechanical energy cannot be supplied directly to the trapped charge carrier, deformation induced intermediate process is responsible for the de-trapping of the charge carriers. ML is a defect related phenomenon, associated with a trap involved process, in which electrons (or holes) dwell in the trap for some time and then recombine with the luminescence center, either by traveling in the conduction band (or valence band) or by electron (or holes) tunneling. As for ML materials, in

Intensity (A. U.)

35000 30000 25000 20000 15000

particular, there combination process is facilitated by the assistance of dislocation in the crystal [23e27]. In the present investigation the probability of involvement of dislocation is very low because of the particle size of the crystal; probably, piezoelectrification during the impact is responsible for the detrapping of the trapped charge carriers [28]. The occurrence of second peak, which occurs in the post deformation region, may be due to the captures of carriers by the shallow traps lying away from the newly created surfaces where the electric field near the surface is not so effective. The release of trapped charge carriers from shallow traps may take place later on due to thermal vibration of lattices and therefore a delayed ML (second peak) may be produced, which may lie in the post deformation region of the phosphor [29]. Fig. 11 shows the UV irradiation dependence of ML intensity. It was observed that ML intensity increases with the increase in UV irradiation because more charge carriers are trapped with the increase in UV irradiation, after that it seems to be saturated as no more traps are available for trapping. Fig. 12 (a) shows the characteristics curve between ML intensity versus time for Ca2Al2SiO7:Dy3þ phosphor at different heights. The experiment was carried out for a fixed moving piston (400 gm) dropped with different heights 20, 30, 40, 50 cm. It is evident that the ML intensity increases with the increase of falling height of moving piston, showing the ML peak intensity maximum at 50 cm height [30]. Fig. 12(b) shows the curve between peak ML intensity versus impact velocity of Ca2Al2SiO7:Dy3þ phosphor. The ML intensity increases linearly with increasing the falling height of the moving piston; that is, the ML intensity depends upon the impact velocity of the moving piston [√2gh (where ¼ h, is the different heights of moving piston)]. As the velocity of moving piston increases the ML intensity increases due to creation of new surface on impact. When the moving piston hits the Ca2Al2SiO7:Dy3þ phosphor, it produces piezoelectric field in the sintered phosphor as they are non-centro symmetric. The piezoelectric field near certain defects centers may be high due to the change in the local structure. The piezoelectric field reduces the trap depth of the carriers. The decrease in trap depth causes transfer of electrons from electron traps to the conduction band. Subsequently, the moving electrons in the conduction band are captured in the excited state, located at the bottom of the conduction band, whereby excited ions are produced. The subsequent recombination of electrons with the holecenters gives rise to the light emission [31,32]. ML properties of this phosphor can provide high sensitivity for smart skin and self-diagnosis applications. When the surface of an object was coated with the ML materials, the stress distribution in the object beneath the layer could be reflected by the ML brightness and could be observed. Based on the above analysis this phosphor can also be used as sensors to detect the stress of an object [33].

10000 5000

3.10. Thermoluminescence of Ca2Al2SiO7: Dy3þ

0 0

1000

2000

3000

4000

Time (s) Fig. 9. Decay Curve of Ca2Al2SiO7:Dy3þ(1 mol%) phosphor.

Fig. 13 shows TL glow curves of Ca2Al2SiO7:Dy3þphosphors. As the concentration of Dy3þ ions increased from 0.5 to 4 mol%, the intensities gradually increased and reached the maximum value at 1 mol%. With a further increase of Dy3þ ion concentration, the intensity decreased remarkably due to concentration quenching. Fig. 14 shows the dependence of TL intensity on UV irradiation of

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Table 2 TL parameters of Ca2Al2SiO7:Dy3þ. UV min 15(Ist peak) 15(IInd peak) 15(IIIrd peak)

Heating Rate

Tm ( C)

Activation energy E (eV)

5 5 5

73 109 162

0.16 0.57 0.71

Frequency factor (S1) 1.19*1011 2.13*1014 2.21*1014

ML Intensity (A. U.)

140

2

120

1

100

3

80

4

60

5

40 20 4

6

8 10 Time (ms)

12

Total ML Peak Intensity (A.U.)

140 1--- 0.5mol% 2--- 1 mol% 3--- 2 mol% 4--- 3 mol% 5--- 4 mol%

120 100 80 60 40 20 0

14

0

2

4

6

Concentration (mole%)

(a)

(b)

Fig. 10. (a) ML intensity verses time curve for different Dy3þ concentration. (b) Variation of peakML intensity by Dy3þ concentration variation.

ML Intensity (A. U.)

140

1--- 5min 2--- 10min 3--- 15min 4--- 20min

4

120

3

100 80

2

60 40

1

20 0

Peak ML Intensity (A. U.)

140 120 100 80 60 40 20 0 4

6

8 10 12 Time (ms)

14

(a)

0

1000

2000

UV- irradiation time (min)

(b)

Fig. 11. (a) ML intensity versus time curve for UV-irradiated phosphor (b) Dependence of peak ML intensity on UV-irradiation.

Ca2Al2SiO7:Dy3þ(1 mol%) sample. Observation has taken for different UV-radiation 5 min, 10 min, 15 min, 20 min. It is found that Total TL intensity is initially increasing with UV-irradiation and it seems to be saturated at 20 min. Under exposure to UV-ray, electron hole pairs are created. Some of the released electrons are captured by the impurity RE3þ ions that convert to RE2þ. The hole is captured in the host related centers. Warming of the irradiated samples causes these holes to get un-trapped successively at different temperatures, depending on their thermal stability. The excited impurity ions by decaying to its ground state give characteristic emission of RE3þ. The increase in the TL/ML intensity with UV irradiation attributed to the increase of active luminescent centers with UV-ray irradiation and subsequent emission of TL/ML is due to re-oxidation of RE2þ into RE3þ during heating/deformation. Thus the intensity increases in the initial stage. The dosage saturation can be explained on the assumption that only limited number of RE ions are available for charge reduction with UV-ray irradiation [34].

The peak deconvolution of TL curve is shown in Fig. 15. Dy3þ is an important rare-earth ion in the development of phosphors with long-lasting afterglow, playing a crucial role. The dopant Dy3þis a famous trap-creating ion, which can greatly prolong the afterglow. It is reasonable to consider that the role of doping Dy3þions is to introduce new types of traps or significantly increase the concentration of traps responsible for the afterglow. We tentatively propose two possible types of the traps in Ca2Al2SiO7:Dy3þ. In the first case, Dy ions act as not only luminescence centers but also traps, since Dy ions can form some electron trap levels in the band gap. In the other case, the traps can occur because of the charge compensation due to the substitution of divalent Ca2þand Al2þions in the Ca2Al2SiO7:Dy3þhost by trivalent Dy3þions. The fact that the characteristic excitation of Dy3þcan lead to the afterglow emission from Dy3þsuggests that the trap filling process may occur through the direct transfer of electrons from the excited states of Dy3þ to trap centers and not via conduction band since the excitation energy is smaller than the band gap. During the afterglow emission,

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ML Intensity (A. U.)

140

4

120 3

100 80

Peak ML Intensity (A. U.)

140 1--- 20 cm 2--- 30 cm 3--- 40 cm 4--- 50 cm

2

60 40

1

40cm

100 80

30cm

60

20cm

40 20

20 0

50cm

120

4

6

8

10

12

0

14

15

25

Time (ms)

35

45

Impact Height (cm)

(a)

(b)

14000 12000 10000 8000 6000 4000 2000 0

I ------- 0.5 mol% II ------- 1 mol% III ------- 2 mol% IV ------- 3 mol% V -------- 4 mol%

II

I III IV V

Total TL Intensity (A. U.)

TL Intensity (A. U.)

Fig. 12. (a) Change in ML intensity with impact height for 15 min UV irradiation (b) Peak ML intensity versus Impact height for 15 min UV irradiation.

35000 30000 25000 20000 15000 10000 5000 0 0

0

200 400 Temperature (°C)

5 Concentration (mol %)

(a)

(b)

I ------- 5 min II -------10 min III ------15 min IV ------20 min

TL Intensity (A. U.)

14000 12000

III

10000

IV

8000

II

6000

I

4000 2000 0

0

50

100 150 Temperature (°C)

200

250

(a)

Total TL Intensity (A. U.)

Fig. 13. (a) TL glow curve for different Dy3þ concentration (b) Dependency of total TL intensity on Dy3þ concentration.

35000 30000 25000 20000 15000 10000 5000 0 0

5

10

15

20

25

UV - Irradiation time (min)

(b)

Fig. 14. (a) TL glow curves for different UV-irradiation time (b) Dependence of peak TL intensity on UV-irradiation time.

the trapped electrons are released and produce visible emission from Dy3þ. The TL parameters i.e. activation energy (E) and frequency factor (s) for the prominent glow peaks of prepared phosphor were calculated using the peak shape method are shown in Table 1. The activation energy E for the sample with 15min UV radiation time was calculated by the formula E ¼ [2.52 þ 10.2(mg  0.42)](kBT2m/u)  2kBTm

Where u, the full Width at half maximum is known as the shape parameter and defined as u ¼ d þ t, with d being the high temperature half-width and t the low- temperature half width. The asymmetric glow-peak shape is defined by the asymmetry parameter mg ¼ d/u, kB is Boltzmann’s constant, and Tm is the temperature of the TL peaks. The frequency factor was calculated by the formula S ¼ bE/kT2m*exp (E/kTm), where b¼heating rate, E ¼ activation energy, Tm ¼ maximum temperature.

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References TL Intensity (A. U.)

12000 10000 8000

II

6000

III

I

4000 2000 0

0

50

100

150

200

250

o

T e m p e ra tu re ( C )

Fig. 15. TL glow curve for 15 min irradiation and peak deconvolution.

Fig. 16. TL emission spectra of Ca2Al2SiO7:Dy3þphosphor for 15 min UV irradiation time.

3.10.1. TL emission spectra Fig. 16 shows that the TL emission spectra of Ca2Al2SiO7: Dy3þ phosphor exhibits a broad emission band centered on 484(blue), 583 (yellow) and 680 nm (red),which are associated with the transitions of Dy3þ from the excited state 4F9/2 / 6H15/2, 4F9/ 6 4 6 2 / H13/2 and F9/2 / H11/2 respectively. It is similar to PL emission spectra. 4. Conclusions In conclusion, a new potential white light emitting Ca2Al2SiO7:Dy3þ phosphors are synthesized by the combustion assisted method. The phase structure of the Ca2Al2SiO7:Dy3þphosphor is consistent with standard tetragonal crystallography. TEM results also confirms nano size of the prepared phosphors. Under the ultra-violet excitation, the prepared Ca2Al2SiO7:Dy3þphosphor would emit blue, yellow and red light with peak at 484 nm, 583 nm and 680 nm corresponds. The red light emission peaking at 680 nm is weak compared to the blue emission (484 nm) and yellow emission (583 nm). The strong yellow emission will dominate in the PL/TL spectra. The PL emission exhibited a white light which was confirmed from the calculated CIE coordinates which were found to be very close to standard white light for human eyes. The TL/ML intensity increases with increase in UVexposure time indicating the increase in concentration of traps with UV-irradiation. The ML intensity linearly increases with increasing impact velocity. The present investigation indicates that the piezo-electrification is responsible to produce ML in prepared phosphor. Based on the above analysis this phosphor can be used as sensors to detect the stress of an object. The combustion method furnishes a simple method for preparing a silicate based phosphor.

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