Tb3+ Activated High-Color-Rendering Green Light Yttrium Oxyorthosilicates Phosphors for Display Device Application

Tb3+ Activated High-Color-Rendering Green Light Yttrium Oxyorthosilicates Phosphors for Display Device Application

Dhananjay K. Deshmukh1*, Jayant Nirmalkar- and Mozammel Haque3

  • 1 Chubu University, Kasugai 487-8501, Japan
  • 2 Korea Research Institute of Standards and Science, Daejeon 305-340,

Republic of Korea

3 Nanjing University of Information Science and Technology, Nanjing 210044, China

Introduction

The industrial phosphors of the last few years are either silicate-based or sulfide-based phosphors. Silicate-based phosphors have attracted much consideration due to their low price, high luminescence efficiency, excellent stability and inertness against chemical and thermal degradations [Naik et al.

2014]. The Rare-Earth (RE) oxyorthosilicates (RE2Si05) that have been doped with Eu3+, Ce3+ and Tb3+ are well-known luminescent materials. Among the RE2Si05 categories, Y2SiOs (YSO), which was doped with rare-earth ions were studied extensively regarding display applications. Yttrium silicate (Y2SiOs) is an important luminescent host material for various RE activators. Y2SiOs has been synthesized by a chemical method since 1964. As a dopant the trivalent terbium ion (Tb3+) displays efficient radiative recombination channels that are mainly observed in the green region of the visible spectral range. It is an important part of the Red (R), Green (G) and Blue (B) display (RGB) panels since the maximum of the human-eye sensibility falls in this region [Song et al. 2014; Penilla et al. 2013; Li et al. 2011; Xie et al. 2010].

The present chapter reports on the synthesis and characterization as well as effect of a variable Tb3+concentration (0.1-2 mol%) in a photoluminescence (PL) and thermoluminescent (TL) analysis of the Y2SiOs phosphor. The 'Corresponding author: This email address is being protected from spam bots, you need Javascript enabled to view it samples show well-resolved spectra in the green region for the various concentrations. Our results describe high color purity of Tb3+ activated YSO phosphor that can be useful for Light Emitting Diode (LED) application for intense green emission. We also report on the TL glow curve analysis of YSO:Tb3+.

Experimental

The Y2SiOs phosphor that had been doped with Tb3+ ions with a variable Tb3+ molar concentration (0.1-2 mol%) was prepared using a modified solid-state-reaction method. We used the precursors Y203/ Si02/ Tb407 and H3BO3 for the synthesis of Y2Si05:Tb3+. The composition of each chemical was weighed according to a proper stoichiometric ratio and then they were mixed thoroughly using an agate mortar and pestle for 45 minutes. The grinded samples were placed in an alumina crucible and then combusted in a muffle furnace under 1000 °C for 1 hour for calcinations followed by firing at 1250 °C for 3 hours for sintering. Every heating stage was followed by an intermediate grinding. Lastly, the samples were cooled slowly to room temperature in the furnace and grinded into powder for the subsequent characterization.

The observation of morphology of particle was conducted using the JSM- 7600F Field Emission Gun Scanning Electron Microscope device of Japan Electron Optics Lciboratory. The PL emission and excitation spectra were recorded at room temperature using the Shimadzu spectrofluorophotometer model RF-5301PC. We used Xenon Lamp as a excitation source. The obtained phosphor under the TL examination was subjected to ultraviolet (UV) radiation using a 254 nm UV source. The TL glow curves were recorded at room temperature using TL-dosimeter (TLD) reader model 11009 supplied by Nucleonix Systems Pvt. Ltd. (Dubey et al. 2014; Kaur et al. 2013).

Results and Discussion

X-ray Analysis

The XRD pattern of the sample is shown in Fig. 1.1 where a monoclinic-body- centered structure with the unit cell dimensions of a = 12.50, b = 6.728 and c = 10.421 are displayed. These values match those of the International Centre for Diffraction Data (JCPDS) card No. 36-1476 (Qi et al. 2003). The crystallite size of the phosphor was calculated using the Scherrer equation (Lee et al. 2008; McMurdie et al. 1986). The calculated crystallite sizes for the different glancing angles are shown in Table 1.1. The average crystallite size of the phosphor was found to be 33 nm.

Morphology of the Phosphor

The SEM images of the synthesized phosphor in different resolutions are presented in Fig. 1.2. The YSO:Tb3+ (1.5 mol%) phosphor displays a

X-ray diffraction (XRD) pattern of the YSiO:Tb (1.5 mol%) phosphor

Fig. 1.1. X-ray diffraction (XRD) pattern of the Y2SiOs:Tb3+ (1.5 mol%) phosphor.

Table 1.1. The structural parameters and crystallite size of the prepared Y2SiOs:Tb3+ phosphore (1.5 mol%)

20

FWHM

(Degree)

D-spacing

D Crystallite size (mu)

22.700

0.288

3.914

28

25.062

0.262

3.550

31

28.297

0.275

3.151

29

29.077

0.249

3.068

31

30.237

0.223

2.953

36

33.410

0.183

2.679

45

35.054

0.275

2.557

30

36.908

0.236

2.433

35

40.889

0.236

2.205

35

48.458

0.275

1.877

31

52.136

0.236

1.752

37

52.781

0.288

1.733

30

57.558

0.328

1.600

27

60.851

0.328

1.521

28

sound morphology and particle-size distribution above 200 run. The high- temperature synthesis method resulted in an appearance comparable to that of the foam-like structure with an irregular shape.

Scanning electron microscope (SEM) images of prepared YSiO:Tbphosphor at 10 к (a) and 50 к (b) resolutions

Fig. 1.2. Scanning electron microscope (SEM) images of prepared Y2SiOs:Tb3+ phosphor at 10 к (a) and 50 к (b) resolutions.

Photoluminescence (PL) Observation

The PL excitation spectra of the YSO:Tb3+ (1.5 mol%) phosphor under a 543 nm emission wavelength is displayed in Fig. 1.3. The results presented two intense peaks at 243 nm and 273 nm. The peak at 243 nm corresponds to the spin that is allowed by the 4fs-4f75d transition (AS = 0), whereas the peak at 273 nm is from the spin-forbidden component of the 4fs-4t75d transition (AS = 1) (Hem et al. 2004; Lin and Su 1995).

The PL emission spectra for a different Tb3+ concentration at a 243 nm excitation wavelength is shown in Fig. 1.4. When the YSO:Tb3+ phosphors (0.1-2 mol%) are excited by the spin-allowed 4f® —» 4f75d band at 243 nm the displayed characteristics are the blue £md green emission lines of Tb3+

Photoluminescence (PL) excitation spectra of the prepared YSO:Tbphosphor at a 543 nm emission wavelength

Fig. 1.3. Photoluminescence (PL) excitation spectra of the prepared YSO:Tb3+phosphor at a 543 nm emission wavelength.

Photoluminescence (PL) emission spectra for a different Tb' concentration at a 243 nm excitation wavelength

Fig. 1.4. Photoluminescence (PL) emission spectra for a different Tb3' concentration at a 243 nm excitation wavelength.

(5D3 47 Fj; J = 3, 4, 5, 6). It shows the green emission color due to the strongest peak at 549 nm (5D4 -t7F5) and the blue emission peak at 485 nm (5D4 —» 7FJ. The small peaks at 380,415 and 438 nm are attributed to the 5D3 —» 7F5 transition whereas the peak at 460 ran is due to the 5D3-> 7F4 transition of the Tb3+. The peaks at both 585 and 592 nm correspond to the 5D47F4 transition whereas the peak at 623 nm is revealed due to the 5D4 -> 7F3 transition of the Tb3+.

The energy-level diagram of the Tb3+ ion is shown in Fig. 1.5. Tire intensity of the magnetic-dipole-allowed transition 5D4 -> 7F5 6 is much stronger than the electric-dipole-allowed transition 5D4-> 7F3 4 (Shi et al. 2015). The variation in the PL intensity with a variable Tb3+ concentration is presented in Fig. 1.6. Tire intensity of the emission spectra increased up to the 1.5 mol% Tb3+-doped sample and then the intensity decreased due to the concentration quenching. The concentration quenching phenomenon is due to the ion-ion interaction between rare earth «activated phosphors.

Commission International de I’Eclairage (CIE) Coordinates

The CIE chromaticity coordinates of the YSO:Tb3+ (1.5 mol%) phosphor was calculated from the corresponding emission spectrum and presented in Fig. 1.7. Their corresponding locations have been marked in the green region of Fig. 1.7 with a cross. Our results clearly show that the Tb3+-doped YSO sample can be used for green-light-emission applications such as solid-state lighting and display devices. Its chromaticity coordinates are x = 0.252 and i/ = 0.494.

Possible transitions of the Tb' ion

Fig. 1.5. Possible transitions of the Tb3' ion.

Variation in the PL intensity with a variable Tb' concentration

Fig. 1.6. Variation in the PL intensity with a variable Tb3' concentration.

Commission International de I'Eclairage (CIE) chromatieity diagram of the YSO:Tb' phosphors. X and Y axis are color coordinate axis

Fig. 1.7. Commission International de I'Eclairage (CIE) chromatieity diagram of the YSO:Tb3' phosphors. X and Y axis are color coordinate axis.

Thermoluminescence (TL) Observation

The effect of the UV dose on the TL intensity for the 1 mol% Tb3+-doped Y2Si05 is shown in Fig. 1.8. We found that the TL intensity increased linearly up to the 20 minute UV dose and then decreased for the 25 minute and 30 minute UV dose. It became evident that the TL intensity increased almost linearly with the UV irradiation. Further, there was no appreciable shift in the glow peak position for the higher irradiation doses. We predicted that the release of a greater number of charge aimers would increase the trap density with the increase of the UV exposure, while resulting in an increase of the TL intensity. Nevertheless, the traps would start to destroy the results of the TL-intensity decrease after a specific exposure (Parganiha et al. 2015; Shrivastava et al. 2015; Chen 1969).

The TL glow curve of YSO:Tb3+ (1 mol%) for a variable ultraviolet (UV) exposure is presented in Fig. 1.9. Figure 1.10 shows the Computerized Glow Curve Deconvolution (CGCD) curve of the Y2Si05:Tb3+ (1 mol%) sample for the 20 minute UV dose at the heating rate of 5 °C s_1. Table 1.2 shows the calculated kinetic parameters using the first-order kinetics for the 20 minute UV exposure for the three deconvoluted peaks.

Ultraviolet (UV) dose versus the TL intensity plot for YSO:Tb (1 mol%)

Fig. 1.8. Ultraviolet (UV) dose versus the TL intensity plot for YSO:Tb3+ (1 mol%).

The TL glow curve of YSO:Tb' (1 mol%) for a variable ultraviolet (UV) exposure

Fig. 1.9. The TL glow curve of YSO:Tb3' (1 mol%) for a variable ultraviolet (UV) exposure.

Glowfit is based on the Halperin and Bamer equations that describe the flows of the charges between the various energy levels during a trap emptying that is because of thermal heating. The kinetic parameters of the trap levels were determined for each deconvoluted peak using this program (Tiwari et al. 2014; Chung et al. 2005). The theoretical generated glow curves

Fitted glow curve of YSO:Tb (1 mol%) for a 20 minute ultraviolet (UV) exposure

Fig. 1.10. Fitted glow curve of YSO:Tb3+ (1 mol%) for a 20 minute ultraviolet (UV) exposure.

were fitted with the experimental glow curves. The quality of the fitting was checked by calculating the Figure Of Merit (FOM) for each fitting. This summation extends across all of the available experiment-data points. The qucility of the fitting and the choice of the appropriate number of peaks were refined by repeating the fitting process to obtain the minimum FOM with the minimum number of possible peaks. The fits were considered adequate when the observed FOM values were less than 5%, and most of the actual values were less than 2%. Our results show the FOM is 1.67%, which confirm a very sound agreement between the theoretical generated glow curves arid the experimental recorded glow curves.

Table 1.2. Calculation of the kinetic parameters for the deconvoluted glow peaks for a 20 minute UV exposure.

Peak

T,

(K)

T

  • 1 m
  • (K)

T,

(K)

T

5

CO

P

(S/co)

Activation energy (E)

Frequency factor (s)

Peak 1

358.30

376.60

389.60

18.30

13.00

31.30

0.42

0.91

3x 10s

Peak 2

404.10

421.40

433.50

17.30

12.10

29.40

0.41

1.23

Зх 10е

Peak 3

393.90

435.40

467.20

41.50

31.80

73.30

0.43

0.48

1 X 10е

Glow Curve Shape Method

Hie method based on the shape of glow curve proposed by Chen and Mckeever 1997 (Som et al. 2015; Singh et al. 2008; Furetta 2003) was used to verify the trapping parameters calculation.

The following shape parameters were determined.

Total half intensity width (со) = T2-T1 The high temperature half width (5) = T2 - Tm The low temperature half width (t) = T„, - T1

where T,„ is the peak temperature and Tl and T2 are two temperatures on either side of T,„ corresponding to half peak intensity.

Order of Kinetics

The order of kinetics (b) was determined by calculating the symmetry factor (p) of the glow peak using the known values of the shape parameters

Activation energy: Activation energy (£) was calculated by using the Chen equations, giving the trap depth in terms of r, 5, and со. A general formula for E was given by Chen and Mckeever (1997) as follows:

Conclusion

Y2Si05:Tb3+-doped phosphors were synthesized using a modified solid-state- reaction method. The XRD pattern confirms that the synthesized sample shows a monoclinic-body-centered structure. The average crystallite size was 33 nm. Tire PL emission was observed in the range of 350-630 nm for the Y2SiOs phosphor doped with the Tb3+. Tire excitation spectra were found at 243 and 273 nm. In the emission spectra, sharp intense peaks of a high intensity were found at around 485 and 549 nm. The present phosphors can act as a host for the green-light emission in solid-state lighting and display devices. Tire CIE chromaticity coordinates of the YSO:Tb3+, was x = 0.252 and у = 0.494, and exhibited the green light. The chromaticity point is in the green region, indicating its high color purity. The TL kinetic parameters were also calculated using the peak-shape method. The results indicate that the TL glow curves of the samples followed the first-order kinetics. The activation energy was found in between 0.48 and 1.23 eV, arid the frequency factor was found in between 1 x 106s. The simple glow-curve structure and the wide linearity range of the samples show that they might be useful in terms of TL dosimetry for short exposure times.

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