Research article Received: 16 September 2014,
Revised: 1 December 2014,
Accepted: 13 January 2015
Published online in Wiley Online Library: 22 March 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2881
Infrared and visible emissions of rare-earthdoped CeO2 phosphor D. Chandrakar, J. Kaur, V. Dubey,* N. S. Suryanarayana and Y. Parganiha ABSTRACT: This paper reports the synthesis and characterization of Er3+-doped CeO2 phosphor with variable concentrations of erbium. The sample was synthesized using a solid-state reaction method, which is useful for the large-scale production of phosphors and is also eco-friendly. The prepared sample was characterized using an X-ray diffraction (XRD) technique. The XRD pattern confirmed that sample has the pure cubic fluorite crystal structure of CeO2. The crystallite size of the prepared phosphor was determined by Scherer’s formula and the crystallite size giving an intense XRD peak is 40.06 nm. The surface morphology of the phosphor was determined by field emission gun scanning electron microscopy (FEGSEM). From the FEGSEM image, good surface morphology with some agglomerates was found. The functional group in the prepared sample was analysed by Fourier transform infrared (FTIR) spectroscopy. All samples prepared with variable concentrations of Er3+ (0.1–2 mol%) were studied by photoluminescence analysis and it was found that the excitation spectra of the prepared phosphor shows broad excitation centred at 251 nm. Emission spectra at different concentrations of Er3+ show strong peaks at 413 and 470 nm and a weaker peak at 594 nm. The dominant peaks at 413 and 470 nm are caused by the allowed electronic transition 4S3/2 → 4I15/2 and the weaker transition at 594 nm is due to the transition 4 F9/2 → 4I15/2. Spectrophotometric determinations of peaks were evaluated using the Commission Internationale de I’Eclairage (CIE) technique. The emission spectra were also observed using an infrared (IR) laser 980 nm source, and three distinct peaks were found in the IR region at 848, 870 and 980 nm. The prepared phosphor has utility for application in display devices. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: visible and infrared spectroscopy; photoluminescence; Er3+-doped phosphor
Introduction In recent years, rare earth (RE)-doped phosphor materials have become a fascinating topic of research because of their potential application in optoelectronic and flat panel display devices. RE-activated cerium oxide (CeO2) with a cubic fluorite crystal structure is one of the most promising oxide phosphors (other examples are ZrO2, Y2O3, Gd2O3 and Lu2O3) (1–5). Oxide phosphors are an advanced material and have attracted a great deal of attention because they have a wide range of potential applications, as ionic conductors in solid oxide fuel cells (SOFCs) (6), polishing agents for chemical mechanical planarization (7), gas sensors (8), catalytic promoters in automotive exhaust gas conversion (9) and as UV-shielding materials (10). These applications are due to their unique optical, chemical and electronic properties, which result from the 4f orbital of the ion. Progress in developing the phosphors is related to an understanding of the physical processes of energy absorption and relaxation. CeO2, with no electron in its 4f shell, may be an efficient photoluminescence (PL) host material because it shows strong light absorption due to the charge transfer from O2– to Ce4+. This energy is transferred to the doped RE ions in CeO2 giving a characteristic emission, the colour of which depends upon the nature of the activator ion used. For example, Eu3+-doped phosphor exhibits red or orange–red emission (11), and Er3+doped CeO2 powder shows green and red up-conversion emission under 785 nm laser excitation (12). Red- and green-emitting phosphors have been widely used in FED applications. An appropriate combination of RE dopants can sometimes lead to white
light emission, as seen with nanocrystalline CeO2:RE (RE = Eu3+, Dy3+ and Tb3+) (13). CeO2 is a leading functional material in RE doping, which uses absorption or emission centres in up-conversion materials (14–16). CeO2 also shows high ionic conductivity when doped with trivalent cations such as Er3+, Tm3+, Eu3+ and Yb3+. Among these, the Er3+ ion is the most popular and efficient RE ion for optical luminescence and up-conversion because of its metastable energy levels 4I9/2 and 4I11/2, which can be conveniently populated by commercial low-cost high-power 980 and 785 nm laser diodes, respectively (17). Numerous techniques have been proposed for the preparation of CeO2 phosphor, such as the Pechini sol–gel method (14), combustion method (15), hydrothermal, solvothermal synthesis (16) and the solid-state reaction method. However, to our knowledge, very few studies have exploited the solid-state reaction method to synthesize RE-doped cerium oxide (CeO2:RE) phosphors. Here, we report the synthesis of Er-doped CeO2 phosphor using the solid-state reaction technique. Because this technique is based on the reaction of all chemicals in a solid oxide form it is useful for large-scale production, is economically feasible and is not complex. In CeO2:Er3+, both the RE activator and the Ce ion belong
* Correspondence to: V. Dubey, Department of Physics Bhilai Institute of Technology, Raipur, C.G., India. E-mail: [email protected] Govt. V.Y.T.PG. Autonomous College Durg, Physics Durg, Chhattisgarh, 491001, India
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Copyright © 2015 John Wiley & Sons, Ltd.
D. Chandrakar et al.
3+
Figure 1. Synthesis of CeO2:Er
phosphors using a solid-state reaction technique at 1250 °C in ambient air.
to the lanthanide group and have comparable ionic radii that favour the incorporation of RE ions into CeO2. In CeO2, the Ce4+ ion has no 4f electron (17), which gives CeO2 an important role as a potential host material for PL. The effect of RE dopant concentrations on PL properties and PL decay time was investigated for CeO2:Er3+. Infrared (IR) and visible spectroscopy of the prepared phosphor were recorded using PL spectra for UV and IR excitation of Er3+-doped CeO2 phosphor.
Experimental CeO2:Er3+ phosphors with various concentrations of erbium (0.1–2.0 mol%) were synthesized using the solid-state reaction technique. CeO2 (99.99%) and Er2O3 (99.99%) were taken in stoichiometric amounts as the starting materials to prepare the CeO2:Er3+ phosphors. A mixture of these regents was ground together using an agate pestle and mortar for 45 min to obtain the best homogeneity and reactivity in the powder. After being ground thoroughly, the powder was placed in an aluminium crucible and fired in a muffle furnace at 1250 °C for 2 h (18) in ambient air. Figure 1 illustrates the solid-state reaction technique for CeO2:Er3+ phosphors. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD measurements carried out using Bruker D8 Advanced X-ray Diffractometer with CuKα (wavelength λ = 0.154 nm) radiation to analyse the crystalline structure and crystallite size of the phosphor powder. The crystallite size was calculated using the well-known Scherer formula. Morphological investigations of CeO2:Er3+ were carried out by SEM. The PL emission and excitation spectra were recorded using a SHIMADZU 5301R (recorded from NPL New Delhi) spectrophotofluorometer at room temperature (19–22).
Result And Discussion X-Ray diffraction
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The XRD pattern of CeO2:Er3+ phosphors for doping concentrations of 1.5 mol% synthesized using the solid-state reaction technique is shown in Figure 2. Different diffraction peaks are obtained at 2θ values of (28.75°), (33.51°), (47.80°), (56.64°), (59.49°), (69.70°), (77.07°), (79.34°), (88.78°) and (95.70°). The presence of XRD peaks corresponding to Bragg diffraction at (111), (200), (220), (311), (222), (400), (331), (420), (422) and (511) confirms the formation of a pure cubic fluorite crystal structure of CeO2, which is a good agreement with the literature ( JCPDS Card No. 43-1002) (23). The lattice parameter for CeO2:Er3+ was found to be a = 5.373 Å compared with a standard lattice parameter a = 5.411 Å for pure CeO2 in the XRD pattern of JCPDS Card No. 43-1002. This change in lattice parameter indicates that an Er3+ ion has been incorporated into the CeO2 lattice. In the XRD pattern, no diffraction peaks pertaining to any
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3+
Figure 2. XRD pattern of 1.5 mol% Er -doped CeO2 phosphor.
impurity were seen in the sample synthesized using the current method. Crystallite size was computed from the full-width half-maxima (FWHM) of the intense peak at 2θ = 28.75o using the Scherer’s formula which is given by: D = 0.9 λ/β Cos θ Where, D is the average crystallite size perpendicular to the reflecting planes, λ = 0.154 nm is the wavelength of the X-ray, θ is the angle of diffraction and β represents the FWHM of the diffraction peak. Table 1 summarizes the angle of diffraction, FWHM, d-spacing and crystallite size of prepared CeO2:Er3+. From Table 1 it is can be seen that crystallite size decreases as the FWHM of the peak increases. The crystallite size for the intense peak in the XRD pattern of the prepared phosphor is D = 0.9 × 0.154/ 0.0036 × Cos 14.37° = 40.6 nm.
Table 1. Summarization of diffraction angle, FWHM, d-spacing and particle size Position 2θ (°)
Height (cts)
FWHM (°2θ)
d-spacing (Å)
28.75 33.51 47.90 56.74 59.49 69.80 77.07 79.34 88.75 95.70
9074.81 2868.43 7743.81 7122.48 1393.59 1423.98 2935.05 1861.84 2507.73 1642.42
0.2112 0.2090 0.2381 0.2611 0.2830 0.3185 0.3390 0.3565 0.3912 0.3172
3.102 2.672 1.897 1.621 1.552 1.346 1.236 1.206 1.101 1.039
Copyright © 2015 John Wiley & Sons, Ltd.
Particle size (nm) 40.6 41 38 36 33.81 31.77 31.32 30 29 33
Luminescence 2015; 30: 1201–1206
Er-doped CeO2
3+
Figure 3. FEGSEM image of CeO2:Er
Scanning electron microscopy The FEGSEM image of the optimized concentration of Er3+ (1.5 mol %) in CeO2 phosphor (Figure 3) shows very good surface morphology with some agglomerate formation due to the high temperature synthesis method. There is good connectivity between the crystal lattice and the morphology resembles a nano-plate.
Photoluminescence study Luminescence properties were investigated by measuring the PL spectra of CeO2:Er3+ phosphors at room temperature. Figure 4 shows the PL excitation spectrum of CeO2:Er3+ (0.2%) phosphor, which was measured by monitoring the peak intensity at 468 nm. The excitation spectrum exhibits an excitation band in the range of 200–300 nm (broad excitation peak centred at 251 nm), due to the charge transfer state (CTS) of Er3+ originating from the interaction between Er3+ and O2-
Figure 5 shows the PL emission spectra of CeO2:Er3+ phosphors at different concentrations of erbium (0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 mol%) recorded in the region 300–700 nm under 251 nm excitation at room temperature, which concluded that the energy absorbed by the charge transfer from the O2– valance band to the Ce4+ conduction band is transferred to the Er3+, which produces the blue–orange emission. In the emission spectra, a broad emission band was found for all prepared samples. Only emission of Er3+ at 413 nm dominates for different concentrations of doping in CeO2. Two other shoulder peaks were observed at 470 and 594 nm, which were attributed to the transition 4S3/2 → 4I15/2; the peak at 594 nm was attributed to the transition 4 F9/2 → 4I15/2 . The PL emission intensity increased with increase in Er3+ concentration from 0.1 to 1.5 mol% and the intensity weakened at Er3+ concentrations > 1.5 mol% due to concentration quenching in the phosphors. Er3+ ions can possibly substitute CeO2 host lattices randomly because they have the same valence and nearly equal radius.
251nm
5500
Photoluminescence Intensity (a.u.)
2000
PL intensity (a.u.)
(1.5 mol%) phosphor.
1500
1000
500
250
413nm
5000 4500 4000 3500 3000
470nm
2500 594nm
2000 1500 1000 500 0 300
200
0.1mol% Er 0.2mol% Er 0.5mol% Er 1.0mol% Er 1.5mol% Er 2.0 mol% Er
300
400
500
600
700
Wavelength (nm)
Wavelength (nm) 3+
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(0.2%) phosphor monitored at 468 nm.
Figure 5. PL emission spectra of CeO2:Er phosphors with variable concentrations 3+ of Er (0.1–2 mol%) monitored at 251 nm.
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3+
Figure 4. PL excitation spectra of CeO2:Er
D. Chandrakar et al.
Photoluminescence intensity (a.u.)
For 413 nm emission peak For 470 nm emission peak For 594 nm emission peak
5000
413 nm
4500 4000 3500 3000 470 nm
2500
594 nm
2000 1500 1000 500 0 0.0
0.5
1.0
1.5
2.0
Concentration of Erbium (mol%) 3+
Figure 7. Variation in photoluminescence intensity with different Er tions for 413, 470 and 594 nm emission. 3+
Figure 6. Energy-level diagram of Er
concentra-
3+
ion in CeO2: Er . emission spectra of CeO2 :Er 3+
Table 2. Emission wavelengths and corresponding transitions in PL emission spectra of CeO2:Er3+ phosphor Transition
Emission wavelength (nm)
S3/2 → I15/2 S3/2 → 4I15/2 4 F9/2 → 4I15/2 4
4
413 470 594
4
980nm
1000 800
Intensity (a.u.)
Figure 6 shows an energy-level diagram for the PL transition of Er ions and transition for the peaks at 413, 470 and 594 nm is shown in Table 2. Relative PL peak intensity at various wavelengths is shown in Table 3 for variable concentrations of erbium, because the maximum peak intensity was found for 1.5 mol% and concentration quenching occurs after 1.5 mol%, the optimized concentration was 1.5 mol% (Figure 7) of Er3+. The emission spectrum of CeO2:Er3+ (1.5 mol%) excited with IR diode laser at 980 nm excitation is shown in Figure 8. The emission spectrum was found in the near infra-red (NIR) region. The NIR spectrum of CeO2:Er3+ phosphor has emission peaks at 848, 870 and 980 nm. The emission peak at 980 nm is an intense band, whereas the two emission peaks at 848 and 870 nm are comparative smaller. The NIR peaks are due to transition between the excited and ground states of Er3+.
600 870nm
400 848nm
200 0 700
800
900
1000
1100
Wavelength (nm) 3+
Figure 8. NIR emission spectra of Er -doped CeO2 phosphor.
CIE coordinate The results indicate that CeO2:Er3+ (1.5%) phosphors are potential candidates for application in light-emitting diodes (LED), as well as for fluorescent lamps (FL) and compact fluorescent lamps (CFL) (Ex.251). However, the relative intensity of the emission bands providing the fundamental colour balance for blue-light emission was achieved with the 0.1 mol% sample, with the spectrum (Figure 5) providing CIE 1976 chromaticity coordinates that are much closer
Table 3. Emission peaks and intensities of CeO2:Er3+ phosphors Sample No.
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1 2 3 4 5 6
Sample name
λex (nm)
λem (nm)
Intensity of λem (a.u.)
CeO2:Er (0.1 mol%) CeO2:Er (0.2 mol%) CeO2:Er (0.5 mol%) CeO2:Er (1.0 mol%) CeO2:Er (1.5 mol%) CeO2:Er (2.0 mol%)
251 251 251 251 251 251
413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594
1918, 795, 330 3864, 1527, 644 4240, 1966, 1041 4721, 2447, 1543 4951, 2740. 1736 4450, 2175, 1229
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Er-doped CeO2 PL decay curve for CeO2:Er3+(0.2 mol%)
Intensity (arbitrary unit)
1000000 800000 600000 400000 200000 0 500000
1000000
1500000
2000000
Time (micro seconds) 3+
Figure 11. PL decay curve of CeO2:Er
(0.2 mol%) phosphor.
Table 4. The fitting result of the decay curves Figure 9. CIE coordinates depicted on 1976 chart where u′ = 0.228 and v′ = 0.061 3+ (blue emission λemission = 413 nm) of Er (1.5%)-doped CeO2 phosphor.
Sample 3+
CeO2:Er
1.05
t1 (ms)
t2 (ms)
23456
34465
Tarnsmitance (%)
1.00 0.95
3488.05 (O-H) streching
427.04 (Ce-O) bending
1647.19 (Er-O) Vibration
0.90 0.85
499.67 (Ce-O) bending
0.80
vibrations of free and hydrogen - bonded hydroxyl groups. Presence of Er-O gives rise to an IR peak of 1647 cm-1. All these discussed peak found together confirms the formation of CeO2: Er3+ phosphor. Photoluminescence decay
0.75 0.70
575.2 (Ce-O)
0.65 4000
3000
2000
1000
0
Wavenumber (cm-1) 3+
Figure 10. FTIR spectra of CeO2:Er
Figure 11 shows typical PL decay curves of Er3+ (1.5%)-doped CeO2 phosphor. The initial intensity of the material was high. The decay times for phosphor can be calculated using a curve-fitting technique, and the decay curves fitted by the sum of two exponential components have different decay times.
(1.5mol%) phosphor.
I ¼ A1 exp ðt=τ 1 Þ þ A2 exp ðt=τ 2 Þ to the equal-energy blue light. If one increases the activator concentration even further, the emission intensity begins to decrease owing to concentration quenching (Figure 7). This concentration quenching is due to increased ion–ion interaction, provoked by the shorter distance between the interacting activators as the concentration increases. The fluorescence light spectral profile as a function of activator concentration was examined and the results indicated that the chromaticity coordinates of the overall emission light changed, resulting in different colours for the overall emission light at different concentrations (24,28–34). The CIE coordinates were calculated using a spectrophotometric method with the spectral energy distribution of the CeO2:Er3+ (1.5%) sample (Figure 9). The colour coordinates for the Er3+-doped sample are u′ = 0.228 and v′ = 0.061 (these coordinates are very near to the blue light emission). Hence, this phosphor has excellent colour tenability from blue light emission (25–27).
Fourier transforms infrared spectroscopy
Luminescence 2015; 30: 1201–1206
Where I is the fluorescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 are decay times (in μs) for the exponential components. Decay curves are successfully fitted by eqn (1). The fitted results for the decay curves are shown in Table 4.
Conclusion It is concluded that the prepared phosphor has pure cubic fluorite crystal structure confirmed by XRD pattern. The surface morphology was determined by FEGSEM image and CeO2:Er3+ phosphor powder was successfully synthesized using a modified solid state method. XRD studies confirm the phosphors are in single phase and nano crystallites. CeO2:Er3+ (1.5%) phosphor shows an intense blue emission under 251nm excitation. Only an emission of Er3+ at 413 nm is dominating for different concentrations of doping in CeO2. Two other shoulder peaks were observed at 470 and 594 nm. These emission peaks at 413 and 470 were attributed due the transition 4S3/2→4I15/2 and at 594 nm was attributed due to the transition 4F9/2→4I15/2 . The PL emission intensity increased with increase in Er3+ concentration from 0.1 to 1.5 mol% and intensity is weakened by exceeding 2.0 mol% concentration of Er3+ due to the occurrence of concentration quenching in phosphors. The emission spectrum of CeO2:Er3+ (1.5mol%) excited with IR diode
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FTIR Spectrum of CeO2:Er3+ powder is shown in the Figure 10. This spectrum expresses a strong peak at 427 cm-1, 499.67cm-1 and 723 cm-1 are the characteristics of Ce-O bending. This spectrum shows a sharp band at 3488 cm-1 which is due to the OH- stretching
(1)
D. Chandrakar et al. laser at 980nm excitation. The emission spectrum found at near infra-red (NIR) region. The NIR spectrum of CeO2:Er3+ phosphor has emission peaks at 848nm, 870nm and 980nm. The results indicate that CeO2:Er3+ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) application as well as for FL (Fluorescent Lamp) and Compact Fluorescent Lamp (CFL) (Ex.251). However, the relative intensity of the emission bands which provide the fundamental colors balance for blue-light emission was achieved with the 0.1 mol% sample with the spectrum. From FTIR spectra the functional group analysis of the phosphor also confirm the formation of CeO2:Er3+ phosphor.
16.
17. 18. 19. 20.
Acknowledgement Authors are very thankful for Dr. D. Harnath Sir NPL New Delhi for Photoluminescnce study.
21. 22.
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Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1201–1206
Revised: 1 December 2014,
Accepted: 13 January 2015
Published online in Wiley Online Library: 22 March 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2881
Infrared and visible emissions of rare-earthdoped CeO2 phosphor D. Chandrakar, J. Kaur, V. Dubey,* N. S. Suryanarayana and Y. Parganiha ABSTRACT: This paper reports the synthesis and characterization of Er3+-doped CeO2 phosphor with variable concentrations of erbium. The sample was synthesized using a solid-state reaction method, which is useful for the large-scale production of phosphors and is also eco-friendly. The prepared sample was characterized using an X-ray diffraction (XRD) technique. The XRD pattern confirmed that sample has the pure cubic fluorite crystal structure of CeO2. The crystallite size of the prepared phosphor was determined by Scherer’s formula and the crystallite size giving an intense XRD peak is 40.06 nm. The surface morphology of the phosphor was determined by field emission gun scanning electron microscopy (FEGSEM). From the FEGSEM image, good surface morphology with some agglomerates was found. The functional group in the prepared sample was analysed by Fourier transform infrared (FTIR) spectroscopy. All samples prepared with variable concentrations of Er3+ (0.1–2 mol%) were studied by photoluminescence analysis and it was found that the excitation spectra of the prepared phosphor shows broad excitation centred at 251 nm. Emission spectra at different concentrations of Er3+ show strong peaks at 413 and 470 nm and a weaker peak at 594 nm. The dominant peaks at 413 and 470 nm are caused by the allowed electronic transition 4S3/2 → 4I15/2 and the weaker transition at 594 nm is due to the transition 4 F9/2 → 4I15/2. Spectrophotometric determinations of peaks were evaluated using the Commission Internationale de I’Eclairage (CIE) technique. The emission spectra were also observed using an infrared (IR) laser 980 nm source, and three distinct peaks were found in the IR region at 848, 870 and 980 nm. The prepared phosphor has utility for application in display devices. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: visible and infrared spectroscopy; photoluminescence; Er3+-doped phosphor
Introduction In recent years, rare earth (RE)-doped phosphor materials have become a fascinating topic of research because of their potential application in optoelectronic and flat panel display devices. RE-activated cerium oxide (CeO2) with a cubic fluorite crystal structure is one of the most promising oxide phosphors (other examples are ZrO2, Y2O3, Gd2O3 and Lu2O3) (1–5). Oxide phosphors are an advanced material and have attracted a great deal of attention because they have a wide range of potential applications, as ionic conductors in solid oxide fuel cells (SOFCs) (6), polishing agents for chemical mechanical planarization (7), gas sensors (8), catalytic promoters in automotive exhaust gas conversion (9) and as UV-shielding materials (10). These applications are due to their unique optical, chemical and electronic properties, which result from the 4f orbital of the ion. Progress in developing the phosphors is related to an understanding of the physical processes of energy absorption and relaxation. CeO2, with no electron in its 4f shell, may be an efficient photoluminescence (PL) host material because it shows strong light absorption due to the charge transfer from O2– to Ce4+. This energy is transferred to the doped RE ions in CeO2 giving a characteristic emission, the colour of which depends upon the nature of the activator ion used. For example, Eu3+-doped phosphor exhibits red or orange–red emission (11), and Er3+doped CeO2 powder shows green and red up-conversion emission under 785 nm laser excitation (12). Red- and green-emitting phosphors have been widely used in FED applications. An appropriate combination of RE dopants can sometimes lead to white
light emission, as seen with nanocrystalline CeO2:RE (RE = Eu3+, Dy3+ and Tb3+) (13). CeO2 is a leading functional material in RE doping, which uses absorption or emission centres in up-conversion materials (14–16). CeO2 also shows high ionic conductivity when doped with trivalent cations such as Er3+, Tm3+, Eu3+ and Yb3+. Among these, the Er3+ ion is the most popular and efficient RE ion for optical luminescence and up-conversion because of its metastable energy levels 4I9/2 and 4I11/2, which can be conveniently populated by commercial low-cost high-power 980 and 785 nm laser diodes, respectively (17). Numerous techniques have been proposed for the preparation of CeO2 phosphor, such as the Pechini sol–gel method (14), combustion method (15), hydrothermal, solvothermal synthesis (16) and the solid-state reaction method. However, to our knowledge, very few studies have exploited the solid-state reaction method to synthesize RE-doped cerium oxide (CeO2:RE) phosphors. Here, we report the synthesis of Er-doped CeO2 phosphor using the solid-state reaction technique. Because this technique is based on the reaction of all chemicals in a solid oxide form it is useful for large-scale production, is economically feasible and is not complex. In CeO2:Er3+, both the RE activator and the Ce ion belong
* Correspondence to: V. Dubey, Department of Physics Bhilai Institute of Technology, Raipur, C.G., India. E-mail: [email protected] Govt. V.Y.T.PG. Autonomous College Durg, Physics Durg, Chhattisgarh, 491001, India
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D. Chandrakar et al.
3+
Figure 1. Synthesis of CeO2:Er
phosphors using a solid-state reaction technique at 1250 °C in ambient air.
to the lanthanide group and have comparable ionic radii that favour the incorporation of RE ions into CeO2. In CeO2, the Ce4+ ion has no 4f electron (17), which gives CeO2 an important role as a potential host material for PL. The effect of RE dopant concentrations on PL properties and PL decay time was investigated for CeO2:Er3+. Infrared (IR) and visible spectroscopy of the prepared phosphor were recorded using PL spectra for UV and IR excitation of Er3+-doped CeO2 phosphor.
Experimental CeO2:Er3+ phosphors with various concentrations of erbium (0.1–2.0 mol%) were synthesized using the solid-state reaction technique. CeO2 (99.99%) and Er2O3 (99.99%) were taken in stoichiometric amounts as the starting materials to prepare the CeO2:Er3+ phosphors. A mixture of these regents was ground together using an agate pestle and mortar for 45 min to obtain the best homogeneity and reactivity in the powder. After being ground thoroughly, the powder was placed in an aluminium crucible and fired in a muffle furnace at 1250 °C for 2 h (18) in ambient air. Figure 1 illustrates the solid-state reaction technique for CeO2:Er3+ phosphors. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD measurements carried out using Bruker D8 Advanced X-ray Diffractometer with CuKα (wavelength λ = 0.154 nm) radiation to analyse the crystalline structure and crystallite size of the phosphor powder. The crystallite size was calculated using the well-known Scherer formula. Morphological investigations of CeO2:Er3+ were carried out by SEM. The PL emission and excitation spectra were recorded using a SHIMADZU 5301R (recorded from NPL New Delhi) spectrophotofluorometer at room temperature (19–22).
Result And Discussion X-Ray diffraction
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The XRD pattern of CeO2:Er3+ phosphors for doping concentrations of 1.5 mol% synthesized using the solid-state reaction technique is shown in Figure 2. Different diffraction peaks are obtained at 2θ values of (28.75°), (33.51°), (47.80°), (56.64°), (59.49°), (69.70°), (77.07°), (79.34°), (88.78°) and (95.70°). The presence of XRD peaks corresponding to Bragg diffraction at (111), (200), (220), (311), (222), (400), (331), (420), (422) and (511) confirms the formation of a pure cubic fluorite crystal structure of CeO2, which is a good agreement with the literature ( JCPDS Card No. 43-1002) (23). The lattice parameter for CeO2:Er3+ was found to be a = 5.373 Å compared with a standard lattice parameter a = 5.411 Å for pure CeO2 in the XRD pattern of JCPDS Card No. 43-1002. This change in lattice parameter indicates that an Er3+ ion has been incorporated into the CeO2 lattice. In the XRD pattern, no diffraction peaks pertaining to any
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3+
Figure 2. XRD pattern of 1.5 mol% Er -doped CeO2 phosphor.
impurity were seen in the sample synthesized using the current method. Crystallite size was computed from the full-width half-maxima (FWHM) of the intense peak at 2θ = 28.75o using the Scherer’s formula which is given by: D = 0.9 λ/β Cos θ Where, D is the average crystallite size perpendicular to the reflecting planes, λ = 0.154 nm is the wavelength of the X-ray, θ is the angle of diffraction and β represents the FWHM of the diffraction peak. Table 1 summarizes the angle of diffraction, FWHM, d-spacing and crystallite size of prepared CeO2:Er3+. From Table 1 it is can be seen that crystallite size decreases as the FWHM of the peak increases. The crystallite size for the intense peak in the XRD pattern of the prepared phosphor is D = 0.9 × 0.154/ 0.0036 × Cos 14.37° = 40.6 nm.
Table 1. Summarization of diffraction angle, FWHM, d-spacing and particle size Position 2θ (°)
Height (cts)
FWHM (°2θ)
d-spacing (Å)
28.75 33.51 47.90 56.74 59.49 69.80 77.07 79.34 88.75 95.70
9074.81 2868.43 7743.81 7122.48 1393.59 1423.98 2935.05 1861.84 2507.73 1642.42
0.2112 0.2090 0.2381 0.2611 0.2830 0.3185 0.3390 0.3565 0.3912 0.3172
3.102 2.672 1.897 1.621 1.552 1.346 1.236 1.206 1.101 1.039
Copyright © 2015 John Wiley & Sons, Ltd.
Particle size (nm) 40.6 41 38 36 33.81 31.77 31.32 30 29 33
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Er-doped CeO2
3+
Figure 3. FEGSEM image of CeO2:Er
Scanning electron microscopy The FEGSEM image of the optimized concentration of Er3+ (1.5 mol %) in CeO2 phosphor (Figure 3) shows very good surface morphology with some agglomerate formation due to the high temperature synthesis method. There is good connectivity between the crystal lattice and the morphology resembles a nano-plate.
Photoluminescence study Luminescence properties were investigated by measuring the PL spectra of CeO2:Er3+ phosphors at room temperature. Figure 4 shows the PL excitation spectrum of CeO2:Er3+ (0.2%) phosphor, which was measured by monitoring the peak intensity at 468 nm. The excitation spectrum exhibits an excitation band in the range of 200–300 nm (broad excitation peak centred at 251 nm), due to the charge transfer state (CTS) of Er3+ originating from the interaction between Er3+ and O2-
Figure 5 shows the PL emission spectra of CeO2:Er3+ phosphors at different concentrations of erbium (0.1, 0.2, 0.5, 1.0, 1.5 and 2.0 mol%) recorded in the region 300–700 nm under 251 nm excitation at room temperature, which concluded that the energy absorbed by the charge transfer from the O2– valance band to the Ce4+ conduction band is transferred to the Er3+, which produces the blue–orange emission. In the emission spectra, a broad emission band was found for all prepared samples. Only emission of Er3+ at 413 nm dominates for different concentrations of doping in CeO2. Two other shoulder peaks were observed at 470 and 594 nm, which were attributed to the transition 4S3/2 → 4I15/2; the peak at 594 nm was attributed to the transition 4 F9/2 → 4I15/2 . The PL emission intensity increased with increase in Er3+ concentration from 0.1 to 1.5 mol% and the intensity weakened at Er3+ concentrations > 1.5 mol% due to concentration quenching in the phosphors. Er3+ ions can possibly substitute CeO2 host lattices randomly because they have the same valence and nearly equal radius.
251nm
5500
Photoluminescence Intensity (a.u.)
2000
PL intensity (a.u.)
(1.5 mol%) phosphor.
1500
1000
500
250
413nm
5000 4500 4000 3500 3000
470nm
2500 594nm
2000 1500 1000 500 0 300
200
0.1mol% Er 0.2mol% Er 0.5mol% Er 1.0mol% Er 1.5mol% Er 2.0 mol% Er
300
400
500
600
700
Wavelength (nm)
Wavelength (nm) 3+
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(0.2%) phosphor monitored at 468 nm.
Figure 5. PL emission spectra of CeO2:Er phosphors with variable concentrations 3+ of Er (0.1–2 mol%) monitored at 251 nm.
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3+
Figure 4. PL excitation spectra of CeO2:Er
D. Chandrakar et al.
Photoluminescence intensity (a.u.)
For 413 nm emission peak For 470 nm emission peak For 594 nm emission peak
5000
413 nm
4500 4000 3500 3000 470 nm
2500
594 nm
2000 1500 1000 500 0 0.0
0.5
1.0
1.5
2.0
Concentration of Erbium (mol%) 3+
Figure 7. Variation in photoluminescence intensity with different Er tions for 413, 470 and 594 nm emission. 3+
Figure 6. Energy-level diagram of Er
concentra-
3+
ion in CeO2: Er . emission spectra of CeO2 :Er 3+
Table 2. Emission wavelengths and corresponding transitions in PL emission spectra of CeO2:Er3+ phosphor Transition
Emission wavelength (nm)
S3/2 → I15/2 S3/2 → 4I15/2 4 F9/2 → 4I15/2 4
4
413 470 594
4
980nm
1000 800
Intensity (a.u.)
Figure 6 shows an energy-level diagram for the PL transition of Er ions and transition for the peaks at 413, 470 and 594 nm is shown in Table 2. Relative PL peak intensity at various wavelengths is shown in Table 3 for variable concentrations of erbium, because the maximum peak intensity was found for 1.5 mol% and concentration quenching occurs after 1.5 mol%, the optimized concentration was 1.5 mol% (Figure 7) of Er3+. The emission spectrum of CeO2:Er3+ (1.5 mol%) excited with IR diode laser at 980 nm excitation is shown in Figure 8. The emission spectrum was found in the near infra-red (NIR) region. The NIR spectrum of CeO2:Er3+ phosphor has emission peaks at 848, 870 and 980 nm. The emission peak at 980 nm is an intense band, whereas the two emission peaks at 848 and 870 nm are comparative smaller. The NIR peaks are due to transition between the excited and ground states of Er3+.
600 870nm
400 848nm
200 0 700
800
900
1000
1100
Wavelength (nm) 3+
Figure 8. NIR emission spectra of Er -doped CeO2 phosphor.
CIE coordinate The results indicate that CeO2:Er3+ (1.5%) phosphors are potential candidates for application in light-emitting diodes (LED), as well as for fluorescent lamps (FL) and compact fluorescent lamps (CFL) (Ex.251). However, the relative intensity of the emission bands providing the fundamental colour balance for blue-light emission was achieved with the 0.1 mol% sample, with the spectrum (Figure 5) providing CIE 1976 chromaticity coordinates that are much closer
Table 3. Emission peaks and intensities of CeO2:Er3+ phosphors Sample No.
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1 2 3 4 5 6
Sample name
λex (nm)
λem (nm)
Intensity of λem (a.u.)
CeO2:Er (0.1 mol%) CeO2:Er (0.2 mol%) CeO2:Er (0.5 mol%) CeO2:Er (1.0 mol%) CeO2:Er (1.5 mol%) CeO2:Er (2.0 mol%)
251 251 251 251 251 251
413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594 413, 470, 594
1918, 795, 330 3864, 1527, 644 4240, 1966, 1041 4721, 2447, 1543 4951, 2740. 1736 4450, 2175, 1229
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Er-doped CeO2 PL decay curve for CeO2:Er3+(0.2 mol%)
Intensity (arbitrary unit)
1000000 800000 600000 400000 200000 0 500000
1000000
1500000
2000000
Time (micro seconds) 3+
Figure 11. PL decay curve of CeO2:Er
(0.2 mol%) phosphor.
Table 4. The fitting result of the decay curves Figure 9. CIE coordinates depicted on 1976 chart where u′ = 0.228 and v′ = 0.061 3+ (blue emission λemission = 413 nm) of Er (1.5%)-doped CeO2 phosphor.
Sample 3+
CeO2:Er
1.05
t1 (ms)
t2 (ms)
23456
34465
Tarnsmitance (%)
1.00 0.95
3488.05 (O-H) streching
427.04 (Ce-O) bending
1647.19 (Er-O) Vibration
0.90 0.85
499.67 (Ce-O) bending
0.80
vibrations of free and hydrogen - bonded hydroxyl groups. Presence of Er-O gives rise to an IR peak of 1647 cm-1. All these discussed peak found together confirms the formation of CeO2: Er3+ phosphor. Photoluminescence decay
0.75 0.70
575.2 (Ce-O)
0.65 4000
3000
2000
1000
0
Wavenumber (cm-1) 3+
Figure 10. FTIR spectra of CeO2:Er
Figure 11 shows typical PL decay curves of Er3+ (1.5%)-doped CeO2 phosphor. The initial intensity of the material was high. The decay times for phosphor can be calculated using a curve-fitting technique, and the decay curves fitted by the sum of two exponential components have different decay times.
(1.5mol%) phosphor.
I ¼ A1 exp ðt=τ 1 Þ þ A2 exp ðt=τ 2 Þ to the equal-energy blue light. If one increases the activator concentration even further, the emission intensity begins to decrease owing to concentration quenching (Figure 7). This concentration quenching is due to increased ion–ion interaction, provoked by the shorter distance between the interacting activators as the concentration increases. The fluorescence light spectral profile as a function of activator concentration was examined and the results indicated that the chromaticity coordinates of the overall emission light changed, resulting in different colours for the overall emission light at different concentrations (24,28–34). The CIE coordinates were calculated using a spectrophotometric method with the spectral energy distribution of the CeO2:Er3+ (1.5%) sample (Figure 9). The colour coordinates for the Er3+-doped sample are u′ = 0.228 and v′ = 0.061 (these coordinates are very near to the blue light emission). Hence, this phosphor has excellent colour tenability from blue light emission (25–27).
Fourier transforms infrared spectroscopy
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Where I is the fluorescence intensity, A1 and A2 are constants, t is time, and τ1 and τ2 are decay times (in μs) for the exponential components. Decay curves are successfully fitted by eqn (1). The fitted results for the decay curves are shown in Table 4.
Conclusion It is concluded that the prepared phosphor has pure cubic fluorite crystal structure confirmed by XRD pattern. The surface morphology was determined by FEGSEM image and CeO2:Er3+ phosphor powder was successfully synthesized using a modified solid state method. XRD studies confirm the phosphors are in single phase and nano crystallites. CeO2:Er3+ (1.5%) phosphor shows an intense blue emission under 251nm excitation. Only an emission of Er3+ at 413 nm is dominating for different concentrations of doping in CeO2. Two other shoulder peaks were observed at 470 and 594 nm. These emission peaks at 413 and 470 were attributed due the transition 4S3/2→4I15/2 and at 594 nm was attributed due to the transition 4F9/2→4I15/2 . The PL emission intensity increased with increase in Er3+ concentration from 0.1 to 1.5 mol% and intensity is weakened by exceeding 2.0 mol% concentration of Er3+ due to the occurrence of concentration quenching in phosphors. The emission spectrum of CeO2:Er3+ (1.5mol%) excited with IR diode
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FTIR Spectrum of CeO2:Er3+ powder is shown in the Figure 10. This spectrum expresses a strong peak at 427 cm-1, 499.67cm-1 and 723 cm-1 are the characteristics of Ce-O bending. This spectrum shows a sharp band at 3488 cm-1 which is due to the OH- stretching
(1)
D. Chandrakar et al. laser at 980nm excitation. The emission spectrum found at near infra-red (NIR) region. The NIR spectrum of CeO2:Er3+ phosphor has emission peaks at 848nm, 870nm and 980nm. The results indicate that CeO2:Er3+ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) application as well as for FL (Fluorescent Lamp) and Compact Fluorescent Lamp (CFL) (Ex.251). However, the relative intensity of the emission bands which provide the fundamental colors balance for blue-light emission was achieved with the 0.1 mol% sample with the spectrum. From FTIR spectra the functional group analysis of the phosphor also confirm the formation of CeO2:Er3+ phosphor.
16.
17. 18. 19. 20.
Acknowledgement Authors are very thankful for Dr. D. Harnath Sir NPL New Delhi for Photoluminescnce study.
21. 22.
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