Photoluminescence and thermoluminescence studies of Tb3+ doped ZnO nanorods

Materials Science and Engineering B 178 (2013) 400–408

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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Photoluminescence and thermoluminescence studies of Tb3+ doped ZnO nanorods Partha P. Pal ∗ , Jairam Manam Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India

a r t i c l e

i n f o

Article history: Received 23 July 2012 Received in revised form 22 November 2012 Accepted 6 January 2013 Available online 19 January 2013 Keywords: Photoluminescence Thermoluminescence Zinc oxide Rare-earth Nanorods Nanoflakes

a b s t r a c t Here in, the synthesis of the terbium doped zinc oxide (ZnO:Tb3+ ) nanorods via room temperature chemical co-precipitation was explored and their structural, photoluminescence (PL) and thermoluminescence (TL) studies were investigated in detail. The present samples were found to have pure hexagonal wurtzite crystal structure. The as obtained samples were broadly composed of nanoflakes while the highly crystalline nanorods have been formed due to low temperature annealing of the as synthesized samples. The diameters of the nanoflakes are found to be in the range 50–60 nm whereas the nanorods have diameter 60–90 nm and length 700–900 nm. FTIR study shows Zn O stretching band at 475 cm−1 showing improved crystal quality with annealing. The bands at 1545 and 1431 cm−1 are attributed to asymmetric and symmetric C O stretching vibration modes. The diffuse reflectance spectra show band edge emission near 390 nm and a blue shift of the absorption edge with higher concentration of Tb doping. The PL spectra of the Tb3+ -doped sample exhibited bright bluish green and green emissions at 490 nm (5 D4 → 7 F6 ) and 544 nm (5 D4 → 7 F5 ) respectively which is much more intense then the blue (450 nm), bluish green (472 nm) and broad green emission (532 nm) for the undoped sample. An efficient energy transfer process from ZnO host to Tb3+ is observed in PL emission and excitation spectra of Tb3+ -doped ZnO ions. The doped sample exhibits a strong TL glow peak at 255 ◦ C compared to the prominent glow peak at 190 ◦ C for the undoped sample. The higher temperature peaks are found to obey first order kinetics whereas the lower temperature peaks obey 2nd order kinetics. The glow peak at 255 ◦ C for the Tb3+ doped sample has an activation energy 0.98 eV and frequency factor 2.77 × 108 s−1 . © 2013 Elsevier B.V. All rights reserved.

1. Introduction In the present decade, man has become successful to exploit the luminescence properties of phosphor materials to a great extent. The photoluminescence and thermoluminescence phenomena from the materials were thoroughly investigated and applied in different fields to the mankind. The research is still going on for the search of better luminescent materials. For the past few years, zinc oxide (ZnO), one of the II–VI semiconductors, has become one of the most promising luminescent materials for the much needed optoelectronic devices operating in the blue and UV region and the transparent conducting and piezoelectric materials for fabricating solar cells, electrodes, and sensors [1]. Owing to a direct wide band gap (3.37 eV), large exciton binding energy (60 meV), and superior conducting properties based on oxygen vacancies, this material is effectively used for various applications such as vacuum fluorescent display (VFD), field emission display (FED) and electroluminescent

∗ Corresponding author. Tel.: +91 326 2235439; fax: +91 326 2296563. E-mail addresses: [email protected], [email protected] (P.P. Pal). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.01.006

display (ELD) [2–6]. Due to its wide band gap, ZnO is regarded as an important candidate for the application in laser diodes and UV light emitting diodes [5,6]. Apart from the wide band gap, the large exciton binding energy (60 meV) at room temperature and an excellent thermal and chemical stability made it an attractive phosphor for the low voltage emissive displays [7]. In order to design the electrical, optical and magnetic properties of ZnO for the practical applications, the control of shape and crystal structure are very important, and the synthesis of novel nanostructures is highly desired. For example, the preparation of various nanostructures, including nanorods, nanowires, nanotubes, nanobelts, and nanobranches, has been widely investigated. However, it has been realized that tuning the band gap only by changing the morphology or size of nanocrystal is not well suited for some applications such as fluorescent imaging and nanoelectronics. It is well-known that the addition of rare earth impurities into a wide-band gap semiconductor can often induce dramatic changes in the optical, electrical, and magnetic properties. Therefore various rare earth doped nanocrystals exhibit specific properties and ZnO is regarded as an excellent host material for the doping of the rare earth and transition metal ions. The optical properties of ZnO are immensely modified if it is being doped with

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rare-earth ions because their 4f intra-shell transitions give a sharp and intense emission which makes them better luminescent material. The luminescence of Tb3+ is particularly interesting among all the rare-earth (RE) elements, because here the major emission band is near 544 nm, showing a green emission which is one of the three primary colors [8]. In this work, Tb3+ -doped ZnO nanocrystals were synthesized by using optimized co-precipitation method with ethanol as a solvent followed by low temperature annealing. XRD, FTIR and SEM studies exhibited that the low temperature calcined ZnO nanocrystals have a rod-like shape and their crystal structure is hexagonal. The ZnO:Tb3+ nanorods showed sharp PL emissions related to the intra4f transitions of Tb3+ ions under near ultraviolet excitations below the band-gap energy of ZnO. The photoluminescence and thermoluminescence mechanism in ZnO:Tb3+ nanorods is discussed in detail. 2. Experimental Pure and terbium (Tb3+ ) doped ZnO with different concentrations of terbium were prepared by co-precipitation method. Undoped ZnO sample has been prepared by mixing equal volume of 0.05 M ethanolic ZnCl2 solution with 0.20 M ethanolic NaOH at room temperature. At first, suitable amount of ZnCl2 and NaOH was separately dissolved in ethanol to prepare 0.05 M ZnCl2 solution and 0.20 M NaOH solution at room temperature. Then the two solutions was mixed and stirred. The stirring was performed by a magnetic stirrer for 6 h continuously at room temperature. During this process NaOH reacts with ZnCl2 to form ZnO. The mixed sample was then given for filter with a 125 mm Whatman 42 filter paper. After complete filtration the residue on the paper is collected and dried. It was then again washed in ethanol and dried to collect and kept for characterization. For doping of Tb3+ ions, x wt% (x = 0.25, 0.5, 1.0, 2.0 and 5.0) of TbCl3 [Sigma–Aldrich, 99.9%] salt was added before the mixing of 0.05 M ZnCl2 solution and 0.20 M NaOH solution. The collected samples are then dried by heating at 80 ◦ C. The annealing of the samples was done at 200 ◦ C for 2 h in air atmosphere. The powder sample was characterized by Bruker D8 Focus XRD measuring instrument in a wide range of Bragg angle 2 (10◦ ≤ 2 ≤ 80◦ ) with a Bruker D8 Focus XRD measuring instrument ˚ at a scanning rate of 2◦ per min. The with Cu target (K␣ = 1.5406 A) FESEM image has been taken in JEOL JSM 6700F Field Emission Scanning Electron Microscope. The FTIR spectrum (KBr pellet) was taken in the region 400–4000 cm−1 on ‘FTIR Spectrum RX I’ (Perkin Elmer, Switzerland) spectrometer. The PL emission and excitation studies were carried out by Hitachi FL 2500 fluorescence spectrophotometer with 150 W xenon lamp in the wavelength range 350–700 nm. The thermoluminescence studies were carried out by Thermoluminescence Analyzer System TL-1007.

Fig. 1. Room temperature XRD pattern (i) undoped ZnO, (ii) ZnO:Tb3+ (0.25 wt%), (iii) ZnO:Tb3+ (0.50 wt%), (iv) ZnO:Tb3+ (1.00 wt%), (v) ZnO:Tb3+ (2.00 wt%), (vi) ZnO:Tb3+ (5.00 wt%), along with the standard XRD peaks for ZnO (JCPDS No. 79-0206).

the complete substitution of Tb3+ ions into the Zn2+ sites or the interstitial sites of ZnO, whereas the shift of the (1 0 1) peaks to the ˚ lower diffraction angle is due to the higher radius of Tb3+ (0.92 A) ˚ ions which increases the lattice constants ‘a’ than the Zn2+ (0.74 A) and ‘c’ in Tb3+ doped powder samples [9,10]. The calculated lattice constants are shown in Table 1. The crystal sizes in the powder samples of undoped and doped samples were calculated by Debye Scherrer formula: D=

0.9 ˇ cos 

(1)

where ˇ is the full-width at half-maximum (FWHM) and D is the crystallite size. The crystal sizes and micro-strain were also calculated from Hall–Williamson relation given by: ˇ cos  ε sin  1 + = D  

(2)

where ε is the micro-strain present in the sample. The crystal sizes were found to be in the range of 15–35 nm from the Debye–Scherrer formula whereas the sizes of the crystal and microstrain were calculated from Hall–Williamson relation. Fig. 2 shows the Hall–Williamson plot of the XRD graphs for undoped and doped samples. The crystal sizes and calculated micro-strain are shown in Table 1. The decrease in crystal sizes with increasing doping concentration suggests that incorporation of Tb ions suppresses the growth of ZnO crystals. A good agreement between the observed and calculated d-values gives the suitability of crystal structure. 3.2. FESEM studies

3. Results and discussion 3.1. XRD studies The XRD patterns were used to examine the crystal structure of the undoped and Tb3+ doped ZnO powders samples for five different concentrations of Tb3+ ions, which have been shown in Fig. 1. All patterns were matched well with the pure nanocrystalline ZnO with hexagonal wurtzite structure [JCPDS data card No. 79-0206]. Furthermore, no change in the crystal structure was observed due to the introduction of Tb3+ ions into the ZnO lattice; however one may notice a very small shift of the (1 0 1) peak to the lower diffraction angle for higher concentration of Tb3+ doping. It is believed that the absence of Tb3+ related peak is may be due to

The surface morphology of the as obtained Tb3+ doped ZnO sample is depicted in Fig. 3. As can be seen, the as-prepared sample consists of uniform dimension nanoflakes having thickness in the range 50–60 nm (Fig. 3(a)), whereas the annealed sample reveals nanorods with almost uniform diameters of about 60–90 nm and length of 700–900 nm (Fig. 3(b)). The particle sizes are found to be much more than the crystallite size calculated from Debye–Scherrer and Hall–Williamson formula. This is because the particle shows clustered form of many single crystals. From the image of annealed sample it can be understood that all the sample is converted to nanorods indicating a complete reaction. Most of the nanorods are found to be straight in nature. The formation of different types of structures mainly depends on the chemical

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Table 1 Lattice constants and calculated crystal sizes of pure and doped ZnO powders. Doping type

Undoped ZnO ZnO:Tb3+ (0.25%) ZnO:Tb3+ (0.50%) ZnO:Tb3+ (1.00%) ZnO:Tb3+ (2.00%) ZnO:Tb3+ (5.00%)

Lattice constants

Crystal sizes

Microstrain ε (×10−4 )

a (nm)

c (nm)

Debye–Scherrer (nm)

Hall–Williamson (nm)

0.342 0.342 0.343 0.344 0.345 0.345

0.552 0.558 0.559 0.562 0.564 0.564

15–35 15–35 15–35 15–35 15–35 15–35

20.83 20.83 20.41 19.60 17.54 17.54

conditions. Although this variation in morphology and the corresponding dimensions are not understood well, it may be inferred that the super saturation of ZnO nuclei has some role to play in the varying structure formation [11]. It is well known that the shapes of the ZnO structure are well controlled by the concentration of NaOH solution. At higher concentration of NaOH, more hydroxide ion reacts with the Zn2+ ions to form Zn(OH)4 2− and in the presence of heat which decomposes to give solid and dense ZnO nano structures. The incorporation of the trivalent Tb3+ ions into the Zn2+ sites

1.01 1.21 1.27 1.54 1.31 1.21

is also possible due to the simultaneous creation of oxygen vacancies as the charge is compensated in the defect sites [12,13]. The formation of the Tb3+ doped ZnO can be realized by the following chemical equations [14,15]. ZnCl2 → Zn2+ + 2Cl−

(i)

NaOH → Na+ + OH−

(ii)

TbCl3 → Tb

3+

+ Cl

3−

Fig. 2. Hall–Williamson plot of the XRD graphs for undoped and doped samples.

Fig. 3. SEM images of (a) as synthesized and (b) annealed ZnO:Tb3+ (1 wt%).

(iii)

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Table 2 Assignment of FTIR bands of undoped, and Tb3+ doped ZnO samples. Samples

ZnO stretching mode (cm−1 )

OH stretching mode (cm−1 )

C O stretching modes (cm−1 )

CO2 mode (cm−1 )

As prepared undoped ZnO Annealed undoped ZnO Tb3+ (1%)-doped ZnO (annealed)

472 472 472

3424 3424 3409

1433, 1544 1433, 1544 1425, 1536

2353 2353 2353

Zn2+ + 4OH− → [Zn(OH)4 ]2−

(iv)

[Zn(OH)4 ]2− → Zn(OH)2 + 2OH−

(v)

Zn(OH)2 → ZnO + H2 O

(vi)

Tb3+ + 3OH− → Tb(OH)3

(vii)

3.3. FTIR studies The ‘FTIR’ spectra of the as prepared and annealed undoped Tb3+ doped ZnO was also studied systematically and the results are shown in Fig. 4. The FTIR spectra of the as-prepared undoped sample consist of four large absorption bands. The large band located at 475 cm−1 attributes to the Zn O stretching in ZnO lattice [16,17]. Two others bands locating at 1545 and 1431 cm−1 are attributed to asymmetric and symmetric C O stretching vibration modes. The presence of C O is may be due to the atmospheric CO2 [18]. The wide absorption band at 3438 cm−1 corresponds to the stretching mode of OH group. It can be observed that both the absorption band for OH group and C O group decreases in the annealed samples due to the heat treatment. The increase in peak sharpness is also noticed indicating a better crystal quality in the annealed samples. But, no peak due to the incorporation of Tb3+ ion in the doped sample is observed, which may be due to the low concentration of the doping ions. The comparison of the values of FTIR bands of undoped as synthesized and annealed ZnO samples with that of annealed Tb3+ doped samples are shown in Table 2. The small shift in the peak position in the doped sample may be attributed to the doping effect of rare-earth ions. The FTIR study confirms the presence of less inorganic and organic impurity in the doped sample and its suitability for performing the photoluminescence studies.

3.4. Diffuse reflectance studies The diffuse reflectance spectra of undoped, ZnO:Tb3+ (0.25%) and ZnO:Tb3+ (0.50%) are shown in Fig. 5. Keeping consistency with the photoluminescence studies all the samples show the ZnO bandedge absorption near 390 nm. The doped samples show a blue shift of the absorption edges with increasing doping concentration [19] and the reflectance intensity is found to be increased with higher Tb3+ concentration [20]. The blue-shift may be either due to the particle size reduction in the Tb3+ doped samples with increasing doping concentration or the energy band-widening after the increase in the Fermi level in the conduction band [14] with increasing doping concentration. 3.5. Photoluminescence studies Fig. 6 shows the room temperature PL emission and excitation spectra of the ZnO:xTb3+ samples of different x values. The emission spectra (Fig. 6(b)) of all ZnO:xTb3+ samples were recorded with the undoped ZnO (x = 0) sample at 275 nm excitations. Although the undoped sample shows typical broad blue, bluish green and green emissions centered at 450 nm, 472 nm and 532 nm respectively, the doped sample shows two intense peaks centered at 490 nm and 544 nm with both the peak indicating a green emission. The weak blue and bluish-green emissions in the undoped ZnO sample are possibly due to surface defect in the ZnO powders [21], whereas the green band corresponds to the singly ionized oxygen vacancy in ZnO which is due to the recombination of a photo-generated hole with the singly ionized charged state of the specific defect. The peaks at 490 nm and 544 nm for the Tb3+ -doped sample arise from 5 D4 → 7 F6 and 5 D4 → 7 F5 transitions of Tb3+ ions respectively [22,23]. The peak situated at 544 nm is found to be stronger in all

300

400

500

600

(iii)

(c)

(ii)

Reflectance (arb. units)

Transmittance (a.u.)

ZnO:Tb

3+

(0.5 wt%)

(b) ZnO:Tb

3+

(0.25wt%)

(a)

(i) Undoped ZnO

500

1000

1500

2000

2500

3000

3500

4000

-1 Wavenumber ( cm ) Fig. 4. FTIR spectra of the (i) as synthesized ZnO, (ii) annealed ZnO and (iii) annealed ZnO:Tb3+ (1 wt%) nanorods.

300

400

500

600

Wavelength (nm) Fig. 5. Room temperature diffuse reflectance spectra of the (a) undoped, (b) 0.25% Tb3+ and (c) 0.5% Tb3+ doped ZnO nanocrystals.

P.P. Pal, J. Manam / Materials Science and Engineering B 178 (2013) 400–408

500

3+ ZnO : Tb (0.25 wt%) 3+ ZnO : Tb (0.50 wt%) 3+ ZnO : Tb (1.00 wt%) 3+ ZnO : Tb (2.00 wt%) 3+ ZnO : Tb (5.00 wt%)

(a)

Intensity (a.u.)

400

300

(b)

3+ ZnO : Tb (0.25 wt%) 3+ ZnO : Tb (0.50 wt%) 3+ ZnO : Tb (1.00 wt%) 3+ ZnO : Tb (2.00 wt%) 3+ ZnO : Tb (5.00 wt%)

Undoped ZnO

Intensity (a.u.)

404

300 350 400 450 500 550 600

Wavelength (nm)

200

100

0 200

300

400

500

300

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

Fig. 6. Room temperature (a) excitation and (b) emission spectra of the Tb3+ doped ZnO nanorods for various doping concentrations (Ex = 275 nm; Em = 544 nm), emission spectra for undoped sample (inset). (For interpretation of the references to color in this artwork, the reader is referred to the web version of the article.)

the Tb3+ -doped samples. The peak intensity for the doped samples increased up to 0.5 wt% of Tb3+ , beyond that it decreases as the concentration of doping increases. This may be due to the concentration quenching of the Tb3+ ions. So, the optimum dopant concentration is 0.5 wt% Tb doped ZnO for the present study. The PL spectra indicates efficient energy transfer from ZnO host to Tb3+ ions through defect states of ZnO in ZnO:Tb3+ nanocrystals. To find the origin of these emissions, the excitation spectra of the samples were recorded and shown in Fig. 6(a). The excitation spectra of all the samples at an excitation wavelength 544 nm showed two broad shoulders at 363 nm and 273 nm. Whereas, the peak for 363 nm is for the f–f transition, the peak at 273 nm suggests the possible f–d transition. To further investigate the transitions of Tb3+ ions, ZnO:Tb3+ (2 wt%) was synthesized and annealed at 200 ◦ C. The PL emission and excitation spectra of the sample are recorded which are shown in Fig. 7. The emission spectra of the sample were recorded for two different excitation wavelength of 250 nm and 275 nm shown in Fig. 7(b) where four different peaks due to Tb3+ doping were detected for both the excitation wavelength. Apart from the peaks

at 490 nm and 544 nm due to the 5 D4 → 7 F6 and 5 D4 → 7 F5 transition shown in Fig. 6, two more peaks were detected at 588 nm and 624 nm which are due to the transition 5 D4 → 7 F4 and 5 D4 → 7 F3 respectively of Tb3+ ions [22–24]. The excitation spectra of the sample, for an emission wavelength 544 nm, shows peaks at 273, 282, 303, 317, 340, 351, 369 and 378 nm. While the peaks up to 300 nm (starting from 250 nm) is due to the f–d transitions, the peaks from 300 nm to 400 nm are due to the f–f transitions of Tb3+ ions [25]. 3.6. Thermoluminescence studies The TL glow curves of the ZnO:Tb3+ (1 wt%) were recorded by Xirradiating the sample with high dose of radiation for 5, 10, 15 and 20 min of time with a constant heating rate of 3◦ per sec (Fig. 8(b)). It is studied that the action of X-ray is found to be more suitable here for the generation of traps that takes vital role in the thermoluminescence phenomena. It is found that the sample exhibits a strong glow peak at 255 ◦ C and a small peak near 55 ◦ C. The TL glow curve of the undoped ZnO sample is also recorded for 5, 10, 15 and 20 min of time at the heating rate of 3◦ per sec (Fig. 8(a)). The undoped

(b)

Intensity (a.u.)

(a)

273 f-d

Excited by 250nm Excited by 275nm

f-f

5 7 D4 F6 5 7 D4 F5

378 282

369 303

317

340

351 5 7 D 4 F4 5 7 D 4 F3

250

300

350

Wavelength (nm)

400

450

300

400

500

600

700

800

Wavelength (nm)

Fig. 7. Room temperature (a) excitation (Em = 544 nm) and (b) emission spectra of the ZnO:Tb3+ nanorods annealed at 200 ◦ C for excitation wavelength Ex1 = 250 nm and Ex2 = 275 nm. (For interpretation of the references to color in this artwork, the reader is referred to the web version of the article.)

P.P. Pal, J. Manam / Materials Science and Engineering B 178 (2013) 400–408

1400

2000 3+

05 min 10 min 15 min 20 min

1200 1000 800 600 400

20 min 15 min 10 min 05 min

(b) ZnO:Tb (1wt%)

1800 1600

TL Intensity (a.u)

(a) Undoped ZnO

TL Intensity (a.u)

405

1400 1200 1000 800 600 400

200 200

0

0

0

50

100

150

200

250

300

350

0

50

100

Temperature (ºC)

150

200

250

300

350

400

Temperature (ºC)

Fig. 8. X-irradiated TSL glow curve for irradiation time 5 min, 10 min, 15 min and 20 min: (a) undoped ZnO and (b) ZnO:Tb3+ (1 wt%).

sample shows main glow peak centered at 190 ◦ C with a less intense peak near 97 ◦ C. Thus, due to the Tb3+ incorporation in the ZnO lattice, the TL glow peak has a high peak shift in the higher temperature side. This is due to the differences in ionic radius between Zn2+ ˚ and lanthanide ion (e.g. Tb3+ , 0.92 A) ˚ [26]. Furthermore, the (0.74 A) TL intensity is also enhanced in the Tb3+ -doped sample than that of the undoped sample. This is due to the increase in trap formation with the increase in ionic radius of RE3+ (Tb3+ ) ions [27,28]. The TL intensity in the doped sample increases with increased exposure time. The increase in the TL intensity might be due to the increase in generation of traps with the increasing dose of X-ray radiation. The TL glow curve of undoped ZnO shows the 190 ◦ C glow peak to be accompanied by a small shoulder at 125 ◦ C whereas the Tb3+ doped sample shows one isolated prominent peak indicating that only one set of traps are being activated at a particular temperature with its activation energy (E) and frequency factor (s). If the prominent glow peak of the undoped and doped sample is compared for a particular time of X-irradiation, then it can be seen that the addition of Tb3+ ions enhanced the TL intensity to almost 1.5 times. It may be suggested that the charge carriers released by the

RE for the Tb3+ -doped sample are trapped in crystalline matrix and during the excitation process it act as electron donors. Tb3+ → Tb4+ + e–

(I)

These electrons remain trapped inside the lattice at the end of excitation mechanism. Then on thermal stimulation there is a release of these trapped electrons according to following mechanism: Tb4+ + e– → (Tb3+ )∗ → Tb3+ + hTb

(II)

The increment of the intensities with the increasing time of Xirradiation can be understood on realizing that more and more traps got filled with the increasing X-ray dose. On thermal stimulation when these traps release the charge carriers, they produce different types of glow peak after recombining with their counterparts. 3.6.1. Trap parameters (glow curve shape method) The trap parameters like order of kinetics (b), activation energy (E), and the frequency factor (s) were calculated for the most intense glow peaks of ZnO:Tb3+ and undoped ZnO using the glow curve shape method (Chen’s method) [29].

1200 1800

(a)

Original Curve

Final Curve Peak 1 Peak 2

1600 1400

800

TL Intensity (a.u)

TL Intensity (a.u)

1000

Original Curve Final curve Peak 1 Peak 2

(b)

600

400

1200 1000 800 600 400

200

200 0 0 100

200

300

Temperature (ºC)

400

100

200

300

400

Temperature (ºC)

Fig. 9. Deconvolution of the glow curves for 20 min X-irradiation (a) undoped ZnO and (b) ZnO:Tb3+ (1 wt%). (For interpretation of the references to color in this artwork, the reader is referred to the web version of the article.)

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Table 3 Shape parameters of the glow peaks. Sample Undoped ZnO Undoped ZnO Tb3+ (1%)-doped ZnO Tb3+ (1%)-doped ZnO

Peak (Tm ) ◦

190 C 98 ◦ C 255 ◦ C 56 ◦ C

 = Tm − T1 (◦ C)

ı = T2 − Tm (◦ C)

ω = T2 − T1 (◦ C)

g



28 26.7 32 18.6

20 22.5 25 16.4

48 49.2 58 35.05

0.41 0.54 0.43 0.53

0.71 0.84 0.78 0.87

Table 4 Activation energy (E) from Chen’s equation. Sample Undoped ZnO Undoped ZnO Tb3+ (1%)-doped ZnO Tb3+ (1%)-doped ZnO

Peak (Tm ) ◦

190 C 98 ◦ C 255 ◦ C 56 ◦ C

Order of kinetics

E (eV)

Eı (eV)

Eω (eV)

Avg. energy (eV)

1 2 1 2

0.87 0.57 1.01 0.66

0.90 0.51 0.96 0.55

0.89 0.54 0.97 0.61

0.89 0.54 0.98 0.61

For the determination of these parameters some shape parameters has to be determined: the total half intensity width (ω = T2 − T1 ), the low temperature half width ( = Tm − T1 ) and the high temperature half width (ı = T2 − Tm ) where Tm is the peak temperature and T1 and T2 are the temperature on the either side of Tm corresponding to half peak intensity. To calculate the trapping parameters associated with the 190 ◦ C glow peak for the undoped ZnO by the glow curve shape method the less prominent glow peak on the lower temperature side is to be removed by thermal cleaning technique. In order to do that the sample was X-irradiated for 20 min and heated up to 200 ◦ C and rapidly cooled down to room temperature. Now the TL records one isolated peak which is the same as the peak 2 obtained in Fig. 9(a). The same method is applied for the Tb3+ -doped ZnO sample also though here the prominent peak is almost isolated. The deconvoluted glow curve for the doped sample is shown in Fig. 9(b). The trap parameters are also calculated for the low temperature peak for the undoped and Tb3+ (1%) doped sample and shown in Tables 3 and 4. The correlation of PL and TL can be done on the basis of the following energy band diagram (Fig. 10). In PL part of the diagram ZnO host to rare earth ion energy transfer can be seen whereas the TL part is describing the recombination of charge and holes in a first order kinetic process. In case of PL the electron transition can be seen from the from the 5D levels to 7F levels whereas in case of first order TL the electron from the trap level which is at same height of activation energy, jumps to the donor level after the heating process and then it recombine with the holes at acceptor level to give the TL phenomena. Thus the energy transfer process in PL is actually confirmed by the TL.

3.6.1.1. Order of kinetics. The order of kinetics (b) was determined by calculating the symmetry factor (g ) of the glow peak from the known values of shape parameters. According to Chen, symmetry factor, g =

ı T2 − Tm = ω T2 − T1

(1 )

and g = 0.42 for first order kinetics and g = 0.52 for second order kinetics. In our case the value of the symmetry factor for the most prominent peaks are found to be 0.41 and 0.43 for the undoped and Tb3+ (1%) doped sample respectively which means that these peaks obey first order kinetics. A first-order kinetics process has negligible retrapping during the thermal stimulation. It is characterized by an asymmetric TL peak being wider on the low temperature side than on the high temperature side. The first order peaks are governed by the following equation: I(t) =



−dn E = sn exp − dt kT

 (III)

where I(t) is the emission intensity, n is the instantaneous concentration of trapped electrons (m−3 ), t is the time (S), S is the pre-exponential (frequency) factor (S−1 ), E is the activation energy (eV), k is Boltzmann’s constant (eV K−l ) and T is the absolute temperature (K). A second-order TL peak is wider and more symmetrical in nature. In this case the less intense peak at the low temperature side are found to obey 2nd order kinetics. These peaks are almost symmetric in nature satisfying Chen’s condition of 2nd order kinetics and also consistent with the fact that most of lower temperature peaks obey 2nd order kinetics whereas the higher temperature peaks obey first order or 1.6 order kinetics [30]. Balarin has proposed another parameter , which he defined as: =

ı T2 − Tm =  Tm − T1

(2 )

For first order kinetics, = 0.7–0.8 and for second order kinetics = 1.05–1.20 [31]. 3.6.1.2. Activation energy. The activation energy (E) was calculated by measuring the trap depth in terms of , ı, ω using the Chen’s equation. The general formula to find E was given by E = c

Fig. 10. Energy label diagram of PL and TL phenomena for the present work.

2 kTm − b 2kTm

(3)

where is , ı or ω. For the first order kinetics the constants c and b for the three equations (, ı and ω) are c = 1.51, b = 1.58, cı = 0.976, bı = 0, cω = 2.52, and bω = 1.0. The activation energy is calculated from Eq. (3) using the shape parameters from Table 3 which is shown in Table 4.

P.P. Pal, J. Manam / Materials Science and Engineering B 178 (2013) 400–408

3.6.3. Reproducibility Reproducibility is also a very important parameter to show whether a sample can be really used as a dosimeter. In order to check the reproducibility of the dose measurements for the ZnO:Tb3+ nanorods, a number of repeated readouts were carried out for the same X-ray dose. Fig. 12 shows the results obtained after five repeated cycles of annealing–irradiation–readout. As the variation in the maximum TL intensity is found very slight here during the five cycles, it indicates that the phosphor is reusable in the TL studies.

1.1

1.0

Relative TL response

407

0.9

0.8

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Time of X-irradiation (min) Fig. 11. Relative TL response of ZnO:Tb3+ (1 wt%) for the glow peak at 255 ◦ C.

3.6.1.3. Frequency factor. The frequency factor (s) can be calculated from the equation of Chen and Winer [32]: qE 2 kTm



= S 1 + (b − 1)

2kTm E





exp −

E kTm

 (4)

where q is the heating rate. In our cases q = 3.5 throughout the hole experiment. The frequency factor for the glow peak at 255 ◦ C for the Tb3+ doped ZnO calculated from Eq. (4) was found to be 2.77 × 108 s−1 . 3.6.2. TL-dose response curve Fig. 11 shows the TL dose response curve for ZnO:Tb3+ (1 wt%). The linearity of a TL response curve determines whether a phosphor can be used as a thermoluminescent dosimeter. As can be seen, the TL response curve for the sample is almost linear between 0 and 20 min of exposure. The liner behavior of the curve can be explained by the track interaction model [28,33] according to which more particles are exposed to high energy radiation with higher radiation dose.

1.2

Acknowledgement One of the authors (Pal) gratefully acknowledges ISM research scholars funding by Govt. of India. The authors are also grateful to Dr. S.K. Sharma, Dept. of Applied Physics, ISM, Dhanbad for valuable suggestions.

1.0 Relative TL response

• Tb3+ doped ZnO nanorods were successfully prepared by the precipitation method using ethanol as solvent and their luminescence properties are discussed. The as synthesized ZnO:Tb3+ samples composed of nanoflakes as confirmed by FESEM studies, while the highly crystalline nanorods have been facilitated due to controlled heating at low temperatures. • The XRD pattern shows no extra peak due to Tb3+ ions indicating that the ions are successfully doped into the crystal lattice of ZnO matrix. The XRD patterns also confirmed the proper hexagonal phase formation upon low temperature annealing. • Intense bright green emission at 490 nm and 544 nm were observed from the intra-4f transition of Tb3+ ions under the resonant excitation of 275 nm. The peak situated at 544 nm, generated from the transition 5 D4 → 7 F5 , was found to be strongest among the peaks. The PL spectra also indicates efficient energy transfer from ZnO host to Tb3+ ions through defect states of ZnO in ZnO:Tb3+ nanocrystals. • The thermoluminescence spectra showed an intense glow peak at 255 ◦ C for the doped sample which is almost 1.5 times more intense than the glow peak for the undoped sample with the intense peak for the doped sample showing a peak shift toward the higher temperature side. The 255 ◦ C glow peak of the ZnO:Tb3+ nanophosphor is due to the first order kinetics with an activation energy 0.98 eV and a frequency factor 2.77 × 108 s−1 . The lower order peaks are found to obey 2nd order kinetics. • The bright green emission from the Tb3+ -doped ZnO sample has wide applications in many display devices. The high TL responses of the samples with a good relative TL response curve and reproducibility curve show its possible application as thermoluminescent dosimeter (TLD).

0.8

References

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No. of cycles Fig. 12. Reproducibility of ZnO:Tb3+ nanorods for five repeated cycles of annealing–irradiation–readout.

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