- Email: [email protected]
Effect of annealing and excitation wavelength on the downconversion photoluminescence of Sm3+ doped Y2O3 nano-crystalline phosphor
Optics and Laser Technology 111 (2019) 169–175
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Effect of annealing and excitation wavelength on the downconversion photoluminescence of Sm3+ doped Y2O3 nano-crystalline phosphor R.S. Yadav, S.B. Rai
T
⁎
Laser & Spectroscopy Laboratory, Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221 005, India
H I GH L IG H T S
Sm doped Y O phosphor was synthesized through solution combustion method. • The structural analysis shows an increase in the particles size of the phosphor. • The phosphor emits reddish orange color at 606 nm on excitation with 238, 363, 407, 424 and 464 nm wavelengths. • The emission intensity of the annealed phosphor is enhanced upto three times. • The • The lifetime of the G level is increased in the annealed phosphor. 3+
2
4
3
5/2
A R T I C LE I N FO
A B S T R A C T
Keywords: Samarium ion Annealing Phosphor 4f-4f transition Photoluminescence
This paper reports the downconversion photoluminescence in Sm3+ doped Y2O3 nano-crystalline phosphor synthesized through solution combustion method. The structural characterization of the phosphor has been carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, which reveals its nano-crystalline nature. The particles size of the phosphor increases on annealing it, which has been confirmed by SEM measurement. The energy dispersive spectroscopic (EDS) measurement verifies the presence of Y, Sm and O elements in the phosphor. The Fourier transform infrared (FTIR) measurements show the presence of vibrational bands due to different groups in the phosphor. The photoluminescence excitation spectra of the phosphor show large number of excitation bands due to CTS and 4f-4f electronic transitions of Sm3+ ion. The Sm3+ doped Y2O3 phosphor emits an intense reddish orange color centered at 606 nm due to 4G5/2 → 6H7/2 transition upon excitation with different wavelengths such as 238, 363, 407, 424 and 464 nm. The photoluminescence intensity is observed larger for 407 nm excitation. Interestingly, the peaks observed in the emission match well with those present in the excitation upon 238 and 363 nm excitations in lower wavelength side. The photoluminescence intensity of the phosphor sample is enhanced upto three times after annealing the as-synthesized phosphor. The improvement in the photoluminescence intensity is due to an increase in crystallinity, particles size and reduction in optical quenching centers. The lifetime of the 4G5/2 level is found to be increased in the annealed phosphor. Thus, the Sm3+ doped Y2O3 nano-crystalline phosphor may be a suitable candidate for displays and photonic devices.
1. Introduction The spectroscopic studies of the lanthanide doped inorganic phosphors have been fascinated the interest of researchers for their high luminous efficacy. They are chemically and thermally stable for long lifespan and therefore have potential applications in different fields, such as displays devices, plasma panel devices (PPDs), flat panel devices (FPDs), optical devices and solid state devices. [1–7]. The lanthanide doped phosphors show various interesting optical
⁎
phenomenon, such as energy transfer, concentration dependency, radiative and non-radiative transitions. [8]. The triply ionized lanthanide ions contain large numbers of energy levels; many of them are metastable, which supports intense radiative transitions. The lanthanide ion in a host matrix serves as an activator ion, which has ability to produce a variety of colors. It gives visible (blue, green, yellow and red) and NIR emissions depending on the lanthanide ions and the host materials [9–15]. It also emits complementary color [16]. The intense photoluminescence in the lanthanide ions occurs due to downconversion
Corresponding author. E-mail address: [email protected] (S.B. Rai).
https://doi.org/10.1016/j.optlastec.2018.09.049 Received 23 April 2018; Received in revised form 7 August 2018; Accepted 23 September 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
2. Materials and method
process in which a high energy ultraviolet (UV) photon is converted into low energy visible photons [11–14]. The appropriate combinations of lanthanide ions also produce tunable colors leading to white light. The tunability strongly depends on the lanthanide ions and their concentrations used in the host matrices [17,18]. The samarium (Sm3+) doped phosphors have been extensively studied in different host materials by different groups [19–24]. They have reported a reddish orange photoluminescence due to 4G5/2 → 6H7/ 2 transition. Shivaram et al. have studied the optical properties of the Sm3+ doped CaTiO3 phosphor on excitation with 407 nm radiation. They have reported that the emission intensity of the Sm3+ ion increases with the increase in temperature and the emission intensity is optimum at 1000 °C [19]. The emission intensity of the Sm3+ doped BaNb2O6 phosphor has been monitored upon 405, 418, 463 and 479 nm excitation wavelengths [20]. It has been reported that the emission intensity is optimum for 405 nm excitation. Lu et al. have also reported an intense orange red emission at 609 nm in the Sm3+ doped Bi4Si3O12 phosphor annealed at 800 °C using the same excitation wavelength [21]. Recently, Li et al. and Hou et al. have carried out the spectroscopic investigations on Sm3+ ion in the NaBaLa2(PO4)3 and Y2MoO6 phosphor materials, respectively [22,23]. These groups have also reported reddish orange emission using 403 and 404 nm excitation wavelengths. The reddish orange emission from Sm3+ ion in the Y2O3 host has been reported by Kodaira et al. using the same excitation wavelength (406 nm). They have prepared the phosphor samples at 400 °C, 500 °C and 600 °C annealing temperatures and the emission intensity was observed maximum at 600 °C [24]. Herein, we have prepared the Sm3+ doped Y2O3 nano-crystalline phosphor on annealing at 1200 °C. The significant enhancement in emission intensity has been observed due to the annealing process. The phosphor contains an activator ion and the inert host material. The host should possess low phonon frequency to get larger photoluminescence. We have used Y2O3 as a host material as it contains low phonon frequency (∼430–550 cm−1) and it is a chemically, structurally and thermally stable host with optical band gap as 5.7 eV [10,12,15]. The Sm3+ ion has been used as an activator as it gives intense emissions from the meta-stable 4G5/2 state to different low lying states. The excitation spectra of the Sm3+ ion show large number of energy states in the UV region [20]. The presence of emission bands from the ultraviolet (UV) to blue regions in the Sm3+ doped Y2O3 phosphor has been not reported to our knowledge. The effect of excitation wavelengths on the emission intensity of the Sm3+ doped Y2O3 phosphor has been also not reported. In the present work, the emission intensity of the phosphor has been monitored on excitation with different wavelengths, such as 238, 363, 407, 424 and 464 nm. It is worth noting that the emission spectra contain various emission bands on excitation with 238 and 363 nm wavelengths, which are clearly present in the excitation spectra of the Sm3+ ion. In this paper, we have synthesized the Sm3+ doped Y2O3 nanocrystalline phosphor through solution combustion method. The structural measurements of the phosphor samples have been carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The energy dispersive spectroscopic (EDS) measurement reveals the presence of different elements in the nano-phosphor sample. The Fourier transform infrared (FTIR) measurements have been carried out to know the vibrational frequency present in the phosphor sample. We have monitored the photoluminescence emission of the synthesized phosphor samples with different concentrations of the Sm3+ ions on excitation with 238, 363, 407, 424 and 464 nm radiations. The Sm3+ doped Y2O3 nano-phosphor gives intense reddish orange photoluminescence. The synthesized phosphors have been annealed at 1200 °C to reduce the surface defects and optical quenching centers. The annealing improves the crystallinity and the particles size of the phosphor. The lifetime measurements have been carried out to confirm an increase in the photoluminescence intensity for the doped phosphor samples.
2.1. Synthesis of Sm3+ doped Y2O3 nano-crystalline phosphors The Sm3+ doped Y2O3 nano-crystalline phosphor has been synthesized through solution combustion method using urea, as an organic fuel for combustion [7,10,25]. The Sm4O7 and Y2O3 were used as starting materials with the following compositions: (100 − x) Y2O3 + x Sm4O7 where x represents the concentration of the lanthanide ion and it was varied from 0.5 to 1.5 mol% concentrations. The starting materials were weighed in the above stoichiometric proportions. They were dissolved in 5 ml concentrated nitric acid followed by dilution with de-ionized water under constant stirring. The urea solution was added to this solution further under constant stirring. The final solution was stirred at 60 °C continuously until it turned into a transparent gel. It occurs due to evaporation of water molecules. The gel thus formed was placed in a closed furnace maintained at 500 °C. An auto ignition took place within few minutes with evolution of different gases due to the exothermic reactions. The sample was obtained in powder form, which is termed as the as-synthesized phosphor. The sample was finally annealed at 1200 °C for 5 h to get a better structure and reddish orange emission in the phosphor samples. 2.2. Characterization The XRD patterns of the synthesized samples have been monitored to identify crystallinity and to confirm the pure phase formation using 18 kW rotating anode (Cu) based Regaku XRD powder diffractometer attached with a graphite monochromator in the diffracted beam path in the range of 20–80°. The morphology of the phosphor has been recorded using a scanning electron microscope (SEM) from a JEOL-TM Model JSM 5410 unit. The energy dispersive spectroscopic (EDS) measurement has been carried out to verify the elements present in the synthesized sample. The FTIR spectra of the phosphor samples were monitored using Frontier-I spectrometer (Perkin Elmer). The photoluminescence excitation and photoluminescence spectra of the synthesized phosphors have been recorded using Fluorolog-3 attached with 450 W Xenon lamp equipped with photomultiplier tube. The lifetime of 5 G5/2 level of Sm3+ ion emitting at 606 nm in the doped phosphor sample has been monitored using Fluorolog-3 spectrofluorometer in the phosphorescence mode attached with 25 W pulsed Xenon lamp. 3. Results and discussion 3.1. Structural characterization 3.1.1. XRD measurements The X-ray diffraction (XRD) patterns of the as-synthesized and the annealed (at 1200 °C) 1.0 mol% Sm3+ doped Y2O3 nano-crystalline phosphors have been recorded in the range of 20–80° and they are shown in Fig. 1. The XRD patterns thus obtained match well with JCPDS File no. 43-1036 (with lattice parameters a = 10.60 Å and α = β = γ = 90°) in both the cases. The phase of the phosphor is C-type cubic with space group la3(206). The XRD peaks were also indexed using JCPDS file. The positions of XRD peaks remain the same as it is in the as-synthesized sample. However, in the case of the annealed sample they are slightly shifted towards lower 2θ angle side. It is also evident from the inset in the figure that the full width at half maxima (FWHM) of the annealed sample is also reduced slightly, which suggests an increase in crystallinity of the phosphor sample. Thus, an ordered crystal structure is formed in the annealed phosphor. The reduction in FWHM and thereby improvement in crystallinity has been studied in our earlier work on Tb3+ doped Y2O3 nano170
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
appreciably and the agglomeration is slightly reduced (see Fig. 2(b)). The similar type of the behavior has been observed in our earlier work on Tb3+ doped Y2O3 phosphor and the samples were synthesized with the same method [7]. It is also clear from Fig. 2(b) that the particles of the annealed phosphor become distinct and porous. Fig. 3 shows the EDS spectrum of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor. The energy dispersive spectroscopic (EDS) measurement reveals the signature of elemental traces in the phosphor sample. The spectrum confirms the presence of Y, Sm and O elements in the phosphor sample. It is also clear from the spectrum that the phosphor sample contains all the elements, which were used during preparation of the phosphor sample. 3.2. Optical measurements 3.2.1. FTIR measurements The FTIR spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphors recorded in the 400–4000 cm−1 region are shown in Fig. 4. Generally, the as-synthesized phosphor exhibits some impurities of different molecules, such as CO, NO3−, OH−, etc, which are observed due to the chemicals used in the synthesis of the phosphor. They are responsible for the non-radiative transitions and reduce the emission intensity significantly. When the as-synthesized sample is annealed at higher temperature (i.e. at 1200 °C/5 h), these impurities are reduced appreciably [7,15]. They are called as optical quenching centers. It is interesting to note that the intensity of the absorbtion bands changes considerably on annealing. Fig. 4 shows that the intensity of CeO and NO3− impurity bands is decreased considerably after annealing. The FTIR spectra contain vibrational bands in the 415–625 cm−1 region centered at 461 and 557 cm−1, which are assigned to arise due to stretching vibrations of YeO crystal lattice, respectively. Kadaira et al. have also observed stretching vibrations of YeO crystal lattice below 700 cm−1 in the Sm3+ doped Y2O3 phosphor [24]. The absorption band present in the spectra at 833 cm−1 is assigned to arise due to CeO asymmetric stretching vibration. The band observed in the 1405–1509 cm−1 region is attributed to the stretching vibrations of NO3− molecule [7]. It is worth noting that the Y2O3 has low phonon energy (461 and 557 cm−1), which strongly promotes the radiative transions in the lanthanide ions including Sm3+ ion [7,15,24]. Thus, one can get the enhanced photoluminescence from the Y2O3 doped phosphor samples.
Fig. 1. XRD patterns of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-crystalline phosphor samples. The inset figure shows a shift of the peak (2 2 2) in the two cases.
phosphor [7]. Lu et al. have also reported that when the phosphor is annealed at higher temperatures the FWHM of the XRD peaks are reduced. This suggests an increase in the crystallite size thereby crystallinity of the phosphor [21]. The average crystallite size (thkl) has been calculated for the as-synthesized and the annealed phosphor samples using Debye-Scherrer equation:
thkl =
Kλ β cosθ
where K is a constant and its value is 0.90, λ, β and θ are the wavelength of the X-ray beam, FWHM and the diffraction angle of the XRD peak. The average crystallite size has been calculated using the intense peaks (2 2 2), (4 4 0) and (4 0 0); and its values in the two cases are found to be 23 and 48 nm, respectively. It has been also reported that when the sample is annealed the crystallite size of the phosphor is increased and the FWHM is reduced significantly [7,15,24]. An improvement in crystallinity of the phosphor supports to achieve larger photoluminescence intensity from the phosphor. 3.1.2. SEM and EDS measurements Fig. 2 shows the SEM micrographs of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples. The particles are of different shapes and sizes. They are agglomerated to each other and oriented in different directions. When the as-synthesized phosphor is annealed the particles size of the phosphor increases
3.2.2. Photoluminescence excitation measurements The photoluminescence excitation (PLE) spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples have been monitored in the range of 200–550 nm with λemi = 606 nm
Fig. 2. SEM micrographs of the as-synthesized (a) and the annealed (b) 1.0 mol% Sm3+ doped Y2O3 phosphor samples. 171
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
Fig. 3. EDS spectrum of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample.
and the spectra thus obtained are shown in Fig. 5. The excitation spectra contain several excitation peaks centered at 336, 346, 363, 379, 407, 424, 450, 464, 477, 492, 505 and 531 nm and they are assigned to arise due to 6H5/2 → 4D7/2, 4H7/2, 4L17/2, 4K11/2, 4F7/2, (6P, 4P)5/2, 4G9/2, 4 I13/2, 4I11/2, 4I9/2, 4G7/2 and 4F3/2 transitions, respectively [6,18–24]. The emission intensity of the band at 407 nm due to 6H5/2 → 4F7/2 transition appears with maximum intensity. The spectra also contain a weak band centered at 238 nm in the range of 220–250 nm, which is due to charge transfer state (CTS) of Sm3+-O2− (see inset figure). The emission intensity of the as-synthesized phosphor improves significantly on annealing it at higher temperature. This is due to an increase in crystallinity and decrease in optical quenching centers of the phosphor sample [7]. Since the excitation peak observed at 407 nm appears with maximum intensity and has large absorption cross section for Sm3+ ion. This wavelength has been used by most of the workers to excite and monitor the photoluminescence spectra of the Sm3+ doped phosphor samples [19–24]. The other excitation bands show smaller emission intensity, which indicates that they have weak absorption cross section for Sm3+ ion. As a result, they emit poor emission intensity.
Fig. 4. FTIR spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-phosphor samples recorded in the 400–4000 cm−1 region.
3.2.3. Downconversion photoluminescence measurements The photoluminescence (PL) spectra of the as-synthesized (1.0 mol %) and the annealed (i.e. 0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples monitored on excitation with 407 nm in the range of 550–750 nm are shown in Fig. 6. The PL spectra contain large number of the emission peaks at 568, 606 (with Stark components at 616 and 622), 654, 674, 724 and 741 nm, which are attributed to arise due to 4 G5/2 → 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2 and 15/2) transitions of Sm3+ ion, respectively [18–24]. The emission peak observed at 568 nm due to 4G5/2 → 6H5/2 transition is of magnetic dipole type whereas that of at 654 nm due to 4G5/2 → 6H9/2 transition is of electric dipole type of transitions, respectively. However, the 4G5/2 → 6H7/2 transition at 606 nm has a contribution of electric as well as magnetic dipole components. The intensity ratio of the electric dipole to magnetic dipole transitions gives an idea about the symmetry of local environment around the activator ion. The phosphor emits an intense reddish orange color at 606 nm due to 4G5/2 → 6H7/2 transition of Sm3+ ion [19,20]. It is interesting to note that the emission intensity of the annealed phosphor is three times larger than the as-synthesized one. The emission peaks observed in the two cases match well with each other. The excitation and emission processes involved in different
Fig. 5. Photoluminescence excitation (PLE) spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples. The inset figure shows the zoomed spectra in the range of 200–260 nm for these cases.
172
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
concentration the energy migration takes place between the Sm3+ ions due to shorter distance between them and the emission intensity decreases considerably [22]. Wang et al. have also studied the effect of concentration on the emission intensity of the Tb3+ doped Y2O3 and YBO3 phosphors and reported a decrease in the emission intensity due to concentration quenching [28]. The effect of concentration on the emission intensity of the Sm3+ doped Y2O3 nano-phosphors can be verified from the inset figure as shown in Fig. 6. It is clear from the figure that the emission intensity is optimum at 1.0 mol% concentration of Sm3+ ions. In our earlier work, the concentration of Sm3+ ion in the La2O3 and La(OH)3 host materials has been also optimized at 1.0 mol% [26,27]. The emission intensity of the as-synthesized phosphor has been found to increase by annealing it at higher temperature. The as-synthesized phosphor contains different impurity centers, which are reduced greatly on annealing the sample. Annealing also leads to an increase in crystallinity and a decrease in surface defects from the assynthesized phosphor. As is mentioned earlier, when the phosphor sample is annealed at 1200 °C the crystallite size of the phosphor is increased from 23 nm to 48 nm. A decrease in FWHM in the XRD peaks reveals an increase in crystallinity and the formation of an ordered crystal structure [7]. Lu et al. have also studied the effect of temperature on the Sm3+ doped Bi4Si3O12 red-emitting phosphor and reported an increase in the emission intensity with the annealing temperature [21,29]. It is also clear from the SEM analysis that the particles size improves on annealing the as-synthesized phosphor at 1200 °C. When the phosphor sample is annealed at higher temperature the smaller particles combine together and form a larger size particle. The particles thus formed have large surface area. The larger particles absorb the excitation energy very efficiently due to large surface area. As a result, the emission intensity of the phosphor is enhanced significantly [10,12,14,30,31]. These factors effectively play role in enhancing the photoluminescence intensity of the sample. In our case, it is found that the emission intensity of the annealed phosphor is increased upto three times compared to that of the as-synthesized phosphor. The annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample has been excited with different wavelengths i.e. 363, 407, 424 and 464 nm and the corresponding photoluminescence has been measured in the range of 550–700 nm. The effect of wavelength on the photoluminescence intensity of the Sm3+ doped Y2O3 phosphor is shown in Fig. 8. The photoluminescence intensity of the phosphor is smaller for 363 nm excitation; however, it is maximum at 407 nm excitation. As is clear from the excitation spectra, the Sm3+ ion has the largest absorption cross section for 407 nm wavelength. This wavelength efficiently promotes the Sm3+ ion in the excited states resulting intense photoluminescence. However, the photoluminescence in the lower
Fig. 6. Photoluminescence spectra of the as-synthesized (1.0 mol%) and the annealed (i.e. 0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors on excitation with 407 nm wavelength. The inset shows a change in the emission intensity with different concentrations of Sm3+ ions.
Fig. 7. Energy level diagram of Sm3+ ion with different transitions on excitation with 407 nm.
transitions of Sm3+ ion can be easily understood using the schematic energy level diagram. Fig. 7 shows the energy level diagram of Sm3+ ion and the photoluminescence observed on excitation with 407 nm. When the phosphor sample is energized with 407 nm radiation from a Xenon lamp; the Sm3+ ions are promoted to 4F7/2 level. The excited ions in this state relax non-radiatively to the 4G5/2 state. The excited ions in the 4G5/2 state give large number of radiative transitions to different lower lying states due to 4G5/2 → 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2 and 15/2) transitions of Sm3+ ion [18–24,26,27]. Among these, the intense emission is observed at 606 nm due to 4G5/2 → 6H7/2 transition. The emission intensity of the bands greatly depends on the concentration of an activator ion. The emission intensity increases on increasing concentration of the activator ion simultaneously and decreases after a particular concentration. The phosphor samples were prepared with different concentrations of the Sm3+ ions (i.e. 0.5, 1.0 and 1.5 mol%). The emission intensity of the phosphor increases for 0.5 and 1.0 mol% concentrations of the Sm3+ ions. However, the emission intensity decreases at higher concentration (i.e. at 1.5 mol%) due to concentration quenching [18–20]. It has been reported that at higher
Fig. 8. Effect of excitation wavelengths on the photoluminescence intensity of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample. 173
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
Fig. 9. Photoluminescence spectra of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample monitored on excitations with 238 and 363 nm wavelengths.
wavelength region (250–550 nm) appears very weak irrespective of the excitation wavelengths. In order to see the emission bands in the lower wavelength side (i.e. 250–550 nm), the photoluminescence spectra of the annealed (1.0 mol %) Sm3+ doped phosphor have been also monitored on excitations with 238 and 363 nm radiations and the spectra thus obtained are shown in Fig. 9. The phosphor sample emits photoluminescence from ultraviolet (UV) to visible regions. The rise behavior in the peak profiles is due to harmonics of the excitation wavelengths. When the sample is illuminated with 238 nm (charge transfer excitation) the sample emits emissions at 324, 332, 346, 369, 379, 393, 411, 420, 438, 450, 491 and 535 nm wavelengths from the excited state to the ground state, which are well assigned in Fig. 9(a). However, upon 363 nm excitation the emission bands at 397, 407, 423, 438, 449, 467, 473, 493 and 533 nm have been observed and they are also assigned as shown in Fig. 9(b). The emission bands appeared at 467 and 473 nm in the case of 363 nm excitation are not observed clearly on excitation with 238 nm wavelength. This is due to overlapping of these bands with the harmonic of 238 nm excitation. Vishwakarma et al. have reported only the reddish orange color on excitation with different wavelengths [20]. However, we have obtained clear emission bands in the lower wavelength regions. These emission bands are well matched to the bands observed in the excitation spectra. The photoluminescence intensity obtained upon 363 nm excitation is larger than that of 238 nm excitation. This is due to the fact that the 238 nm wavelength excites the CTS of Sm3+-O2−. This emits a broad band in the range of 220–250 nm, which is centered at 238 nm. The Sm3+ ion absorbs this broad region and emits weak emission intensity. However, the 363 nm wavelength excites the 4L17/2 state of Sm3+ ion directly and gives larger photoluminescence intensity compared to the 238 nm excitation. The Sm3+ doped Y2O3 phosphor emits reddish orange color as has been observed by the naked eye. This can be also confirmed by plotting CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples. Fig. 10 shows the CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors. The CIE coordinates have been calculated and they are (0.55, 0.44), (0.56, 0.44), (0.56, 0.43) and (0.56, 0.44) for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples, respectively [20,22]. It is clear from the CIE coordinates that the concentration and annealing process leads to an increase in the photoluminescence intensity of the phosphor, however, they do not affect the color emitted by the phosphor. Thus, the Sm3+ doped Y2O3 phosphors may be used in the displays and photonic devices.
Fig. 10. CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors upon 407 nm excitation.
Fig. 11. Decay curves of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-phosphors on excitation with 407 nm for 4G5/2 → 6H7/2 transition (at 606 nm) of Sm3+ ions.
3.2.4. Decay curve analysis The lifetime measurements of the phosphor samples have been carried out on excitation with 407 nm and the emission was monitored 174
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
at 606 nm wavelength. Fig. 11 shows the decay curves of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples for the 4G5/2 → 6H7/2 transition (at 606 nm). The decay curves fit well with the single exponential relation in these cases:
BaGd2Ti4O13 phosphors, Mater. Res. Bull. 50 (2014) 354–358. [5] H.M. Yang, J.X. Shi, M.L. Gong, A novel red emitting phosphor Ca2SnO4: Eu3+, J. Solid State Chem. 178 (2005) 917–920. [6] Y.C. Li, Y.H. Chang, Y.F. Lin, Y.S. Chang, Y.J. Lin, Synthesis and luminescent properties of Ln3+ (Eu3+, Sm3+, Dy3+)-doped lanthanum aluminum germanate LaAlGe2O7 phosphors, J. Alloys Compds. 439 (2007) 367–375. [7] R.S. Yadav, S.B. Rai, Effect of concentration and wavelength on frequency downshifting photoluminescence from a Tb3+ doped yttria nano-phosphor: a photochromic phosphor, J. Phys. Chem. Solids 114 (2018) 179–186. [8] F. Auzel, Upconversion and anti-Stokes processes with and ions in solids, Chem. Rev. 104 (2004) 139–173. [9] Y. Zhou, Q. Ma, M. Lu¨, Z. Qiu, A. Zhang, Combustion synthesis and photoluminescence properties of YNbO4-based nanophosphors, J. Phys. Chem. C 112 (2008) 19901–19907. [10] R.S. Yadav, R.K. Verma, S.B. Rai, Intense white light emission in Tm3+/Er3+/Yb3+ co-doped Y2O3-ZnO nano-composite, J. Phys. D: Appl. Phys. 46 (275101) (2013) 1–8. [11] H.J. Lin, Y.S. Chang, Blue-emitting phosphor of YInGe2O7 doped with Tm3+ Ions, Electrochem. Solid-State Lett. 10 (7) (2007) J79–J82. [12] R.S. Yadav, S.B. Rai, Surface analysis and enhanced photoluminescence via Bi3+ doping in a Tb3+ doped Y2O3 nano-phosphor under UV excitation, J. Alloys Compds. 700 (2017) 228–237. [13] A. Huignard, T. Gacoin, J.P. Boilot, Synthesis and luminescence properties of colloidal YVO4: Eu phosphors, Chem. Mater. 12 (2000) 1090–1094. [14] R.S. Yadav, S.B. Rai, Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor, J. Phys. Chem. Solids 110 (2017) 211–217. [15] R.S. Yadav, R.K. Verma, A. Bahadur, S.B. Rai, Structural characterizations and intense green upconversion emission in Yb3+, Pr3+ co-doped Y2O3 nano-phosphor, Spectrochim. Acta Part A 137 (2015) 357–362. [16] Y. He, M. Zhao, Y. Song, G. Zhao, X. Ai, Effect of Bi3+ on fluorescence properties of YPO4:Dy3+ phosphors synthesized by a modified chemical co-precipitation method, J. Lumin. 131 (2011) 1144–1148. [17] M.K. Jung, W.J. Park, D.H. Yoon, Photoluminescence characteristics of red phosphor Eu3+, Sm3+ co-doped Y2O3 for white light emitting diodes, Sens. Actuators B 126 (2007) 328–331. [18] R.S. Yadav, R.V. Yadav, A. Bahadur, T.P. Yadav, S.B. Rai, Role of Li+ on white light emission from Sm3+ and Tb3+ co-doped Y2O3 nano-phosphor, Mater. Res. Exp. 3 (036201) (2016) 1–13. [19] M. Shivaram, H. Nagabhushana, S.C. Sharma, S.C. Prashantha, B. Daruka Prasad, N. Dhananjaya, R. Hari Krishna, B.M. Nagabhushana, C. Shivakumara, R.P.S. Chakradhar, Synthesis and luminescence properties of Sm3+ doped CaTiO3 nanophosphor for application in white LED under NUV excitation, Spectrochim. Acta Part A 128 (2014) 891–901. [20] A.K. Vishwakarma, M. Jayasimadri, Pure orange color emitting Sm3+ doped BaNb2O6 phosphor for solid-state lighting applications, J. Lumin. 176 (2016) 112–117. [21] G. Lu, K. Qiu, J. Li, W. Zhang, X. Yuan, Synthesis and photoluminescence characteristics of Sm3+-doped Bi4Si3O12 red-emitting phosphor, Luminescence 32 (2017) 93–99. [22] L. Li, X. Tang, Z. Jiang, X. Zhou, S. Jiang, X. Luo, G. Xiang, K. Zhou, NaBaLa2(PO4)3: a novel host lattice for Sm3+-doped phosphor materials emitting reddish-orange light, J. Alloys Compds. 701 (2017) 515–523. [23] D. Hou, X. Pan, J. Li, W. Zhou, X. Ye, Structure and luminescence properties of Sm3+ doped Y2MoO6 phosphor under near ultraviolet light excitation, J. Rare Earths 35 (2017) 335–340. [24] C.A. Kodaira, R. Stefani, A.S. Maia, M.C.F.C. Felinto, H.F. Brito, Optical investigation of Y2O3:Sm3+ nanophosphor prepared by combustion and Pechini methods, J. Lumin. 127 (2007) 616–622. [25] A. Dhahri, K. Horchani-Naifer, A. Benedetti, F. Enrichi, M. Férid, P. Riello, Combustion synthesis and photoluminescence of Tb3+ doped LaAlO3 nanophosphors, Opt. Mater. 35 (2013) 1184–1188. [26] R.S. Yadav, Y. Dwivedi, S.B. Rai, Structural and optical properties of Eu3+, Sm3+ co-doped La(OH)3 nano-crystalline red emitting phosphor, Spectrochim. Acta Part A 132 (2014) 599–603. [27] R.S. Yadav, Y. Dwivedi, S.B. Rai, Structural and optical characterization of nanosized La(OH)3:Sm3+ phosphor, Spectrochim. Acta Part A 96 (2012) 148–153. [28] L. Wang, L. Shi, N. Liao, H. Jia, P. Du, Z. Xi, L. Wang, D. Jin, Photoluminescence properties of Y2O3:Tb3+ and YBO3:Tb3+ green phosphors synthesized by hydrothermal method, Mater. Chem. Phys. 119 (2010) 490–494. [29] F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, X. Liu, Tuning upconversion through energy migration in core–shell nanoparticles, Nat. Mater. 10 (2011) 968–973. [30] X. Liu, L. Yan, J. Zou, Tunable cathodoluminescence properties of Tb3+-doped La2O3 nanocrystalline phosphors, J. Electrochem. Soc. 157 (2) (2010) P1–P6. [31] Y. Liu, C. Xu, Q. Yang, White upconversion of rare-earth doped ZnO nanocrystals and its dependence on size of crystal particles and content of Yb3+ and Tm3+, J. Appl. Phys. 105 (084701) (2009) 1–6.
t I = I0 exp ⎛− ⎞ ⎝ τ⎠ where I0 and I are the intensities at time 0 and after t s, respectively and τ is the lifetime of the 4G5/2 level of the Sm3+ ion. The values of the lifetime were calculated as 0.85 and 0.90 ms for the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphors, respectively. An increase in the lifetime of 4G5/2 level of the Sm3+ ion for the annealed sample is due to an improvement in crystallinity [10,12,21,24]. It has been mentioned that the annealed phosphor sample has less optical quenching centers and surface defects. As a result, the lifetime of 4G5/2 level of the Sm3+ ion is increased from 0.85 ms to 0.90 ms. These factors enhance the rate of the radiative transitions considerably. Therefore, the photoluminescence intensity of the Sm3+ doped Y2O3 phosphor is enhanced upto three times. 4. Conclusions The Sm3+ doped Y2O3 nano-crystalline phosphor has been synthesized through solution combustion method. The structural measurements reflect strong nano-crystalline nature of the phosphor sample. The SEM measurements show an increase in the particles size for the annealed phosphor. The PLE spectra show an intense excitation band centered at 407 nm with large numbers of weak bands. The Sm3+ doped Y2O3 phosphor emits an intense reddish orange photoluminescence centered at 606 nm due to 4G5/2 → 6H7/2 transition on excitations with 238, 363, 407, 424 and 464 nm wavelengths. However, the sample excited by 238, 363, 424 and 464 nm bands gives weak emissions. The sample excited with 238 and 363 nm wavelengths results bands in the lower wavelength side, which also shows weak emissions. The optimum intensity is observed upon 407 nm excitation. The concentration of Sm3+ ion is optimized with emission intensity at 1.0 mol% in the phosphor. The emission intensity of the as-synthesized phosphor is enhanced upto three times on annealing it at 1200 °C. This improvement in the photoluminescence intensity is due to an increase in crystallinity, reduction in optical quenching centers and surface defects. The lifetime of the 4G5/2 level increases in the case of the annealed phosphor sample. Thus, the Sm3+ doped Y2O3 nano-crystalline phosphor may be applicable for displays and photonic devices. Acknowledgements Prof. S.B. Rai acknowledges Banaras Hindu University for Emeritus Professorship. The authors wish to acknowledge Prof. O.N. Srivastava, Department of Physics, Banaras Hindu University, Varanasi for providing XRD, SEM and EDS measurement facilities. References [1] C. Feldmann, T. Jüstel, C.R. Ronda, P.J. Schmidt, Inorganic luminescent materials: 100 years of research and application, Adv. Funct. Mater. 13 (2003) 511–516. [2] P. Pust, V. Weiler, C. Hecht, A. Tücks, A.S. Wochnik, A.K. Henß, D. Wiechert, C. Scheu, P.J. Schmidt, W. Schnick, Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material, Nat. Mater. 13 (2014) 891–896. [3] R. Bazzi, M.A. Flores, C. Louis, Synthesis and properties of europium-based phosphors on the nanometer scale: Eu2O3, Gd2O3:Eu, and Y2O3:Eu, J. Colloid Interf. Sci. 273 (2004) 191–197. [4] S.H. Lee, J.S. Yu, Synthesis and luminescence properties of Eu3+ doped
175
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Effect of annealing and excitation wavelength on the downconversion photoluminescence of Sm3+ doped Y2O3 nano-crystalline phosphor R.S. Yadav, S.B. Rai
T
⁎
Laser & Spectroscopy Laboratory, Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221 005, India
H I GH L IG H T S
Sm doped Y O phosphor was synthesized through solution combustion method. • The structural analysis shows an increase in the particles size of the phosphor. • The phosphor emits reddish orange color at 606 nm on excitation with 238, 363, 407, 424 and 464 nm wavelengths. • The emission intensity of the annealed phosphor is enhanced upto three times. • The • The lifetime of the G level is increased in the annealed phosphor. 3+
2
4
3
5/2
A R T I C LE I N FO
A B S T R A C T
Keywords: Samarium ion Annealing Phosphor 4f-4f transition Photoluminescence
This paper reports the downconversion photoluminescence in Sm3+ doped Y2O3 nano-crystalline phosphor synthesized through solution combustion method. The structural characterization of the phosphor has been carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, which reveals its nano-crystalline nature. The particles size of the phosphor increases on annealing it, which has been confirmed by SEM measurement. The energy dispersive spectroscopic (EDS) measurement verifies the presence of Y, Sm and O elements in the phosphor. The Fourier transform infrared (FTIR) measurements show the presence of vibrational bands due to different groups in the phosphor. The photoluminescence excitation spectra of the phosphor show large number of excitation bands due to CTS and 4f-4f electronic transitions of Sm3+ ion. The Sm3+ doped Y2O3 phosphor emits an intense reddish orange color centered at 606 nm due to 4G5/2 → 6H7/2 transition upon excitation with different wavelengths such as 238, 363, 407, 424 and 464 nm. The photoluminescence intensity is observed larger for 407 nm excitation. Interestingly, the peaks observed in the emission match well with those present in the excitation upon 238 and 363 nm excitations in lower wavelength side. The photoluminescence intensity of the phosphor sample is enhanced upto three times after annealing the as-synthesized phosphor. The improvement in the photoluminescence intensity is due to an increase in crystallinity, particles size and reduction in optical quenching centers. The lifetime of the 4G5/2 level is found to be increased in the annealed phosphor. Thus, the Sm3+ doped Y2O3 nano-crystalline phosphor may be a suitable candidate for displays and photonic devices.
1. Introduction The spectroscopic studies of the lanthanide doped inorganic phosphors have been fascinated the interest of researchers for their high luminous efficacy. They are chemically and thermally stable for long lifespan and therefore have potential applications in different fields, such as displays devices, plasma panel devices (PPDs), flat panel devices (FPDs), optical devices and solid state devices. [1–7]. The lanthanide doped phosphors show various interesting optical
⁎
phenomenon, such as energy transfer, concentration dependency, radiative and non-radiative transitions. [8]. The triply ionized lanthanide ions contain large numbers of energy levels; many of them are metastable, which supports intense radiative transitions. The lanthanide ion in a host matrix serves as an activator ion, which has ability to produce a variety of colors. It gives visible (blue, green, yellow and red) and NIR emissions depending on the lanthanide ions and the host materials [9–15]. It also emits complementary color [16]. The intense photoluminescence in the lanthanide ions occurs due to downconversion
Corresponding author. E-mail address: [email protected] (S.B. Rai).
https://doi.org/10.1016/j.optlastec.2018.09.049 Received 23 April 2018; Received in revised form 7 August 2018; Accepted 23 September 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
2. Materials and method
process in which a high energy ultraviolet (UV) photon is converted into low energy visible photons [11–14]. The appropriate combinations of lanthanide ions also produce tunable colors leading to white light. The tunability strongly depends on the lanthanide ions and their concentrations used in the host matrices [17,18]. The samarium (Sm3+) doped phosphors have been extensively studied in different host materials by different groups [19–24]. They have reported a reddish orange photoluminescence due to 4G5/2 → 6H7/ 2 transition. Shivaram et al. have studied the optical properties of the Sm3+ doped CaTiO3 phosphor on excitation with 407 nm radiation. They have reported that the emission intensity of the Sm3+ ion increases with the increase in temperature and the emission intensity is optimum at 1000 °C [19]. The emission intensity of the Sm3+ doped BaNb2O6 phosphor has been monitored upon 405, 418, 463 and 479 nm excitation wavelengths [20]. It has been reported that the emission intensity is optimum for 405 nm excitation. Lu et al. have also reported an intense orange red emission at 609 nm in the Sm3+ doped Bi4Si3O12 phosphor annealed at 800 °C using the same excitation wavelength [21]. Recently, Li et al. and Hou et al. have carried out the spectroscopic investigations on Sm3+ ion in the NaBaLa2(PO4)3 and Y2MoO6 phosphor materials, respectively [22,23]. These groups have also reported reddish orange emission using 403 and 404 nm excitation wavelengths. The reddish orange emission from Sm3+ ion in the Y2O3 host has been reported by Kodaira et al. using the same excitation wavelength (406 nm). They have prepared the phosphor samples at 400 °C, 500 °C and 600 °C annealing temperatures and the emission intensity was observed maximum at 600 °C [24]. Herein, we have prepared the Sm3+ doped Y2O3 nano-crystalline phosphor on annealing at 1200 °C. The significant enhancement in emission intensity has been observed due to the annealing process. The phosphor contains an activator ion and the inert host material. The host should possess low phonon frequency to get larger photoluminescence. We have used Y2O3 as a host material as it contains low phonon frequency (∼430–550 cm−1) and it is a chemically, structurally and thermally stable host with optical band gap as 5.7 eV [10,12,15]. The Sm3+ ion has been used as an activator as it gives intense emissions from the meta-stable 4G5/2 state to different low lying states. The excitation spectra of the Sm3+ ion show large number of energy states in the UV region [20]. The presence of emission bands from the ultraviolet (UV) to blue regions in the Sm3+ doped Y2O3 phosphor has been not reported to our knowledge. The effect of excitation wavelengths on the emission intensity of the Sm3+ doped Y2O3 phosphor has been also not reported. In the present work, the emission intensity of the phosphor has been monitored on excitation with different wavelengths, such as 238, 363, 407, 424 and 464 nm. It is worth noting that the emission spectra contain various emission bands on excitation with 238 and 363 nm wavelengths, which are clearly present in the excitation spectra of the Sm3+ ion. In this paper, we have synthesized the Sm3+ doped Y2O3 nanocrystalline phosphor through solution combustion method. The structural measurements of the phosphor samples have been carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The energy dispersive spectroscopic (EDS) measurement reveals the presence of different elements in the nano-phosphor sample. The Fourier transform infrared (FTIR) measurements have been carried out to know the vibrational frequency present in the phosphor sample. We have monitored the photoluminescence emission of the synthesized phosphor samples with different concentrations of the Sm3+ ions on excitation with 238, 363, 407, 424 and 464 nm radiations. The Sm3+ doped Y2O3 nano-phosphor gives intense reddish orange photoluminescence. The synthesized phosphors have been annealed at 1200 °C to reduce the surface defects and optical quenching centers. The annealing improves the crystallinity and the particles size of the phosphor. The lifetime measurements have been carried out to confirm an increase in the photoluminescence intensity for the doped phosphor samples.
2.1. Synthesis of Sm3+ doped Y2O3 nano-crystalline phosphors The Sm3+ doped Y2O3 nano-crystalline phosphor has been synthesized through solution combustion method using urea, as an organic fuel for combustion [7,10,25]. The Sm4O7 and Y2O3 were used as starting materials with the following compositions: (100 − x) Y2O3 + x Sm4O7 where x represents the concentration of the lanthanide ion and it was varied from 0.5 to 1.5 mol% concentrations. The starting materials were weighed in the above stoichiometric proportions. They were dissolved in 5 ml concentrated nitric acid followed by dilution with de-ionized water under constant stirring. The urea solution was added to this solution further under constant stirring. The final solution was stirred at 60 °C continuously until it turned into a transparent gel. It occurs due to evaporation of water molecules. The gel thus formed was placed in a closed furnace maintained at 500 °C. An auto ignition took place within few minutes with evolution of different gases due to the exothermic reactions. The sample was obtained in powder form, which is termed as the as-synthesized phosphor. The sample was finally annealed at 1200 °C for 5 h to get a better structure and reddish orange emission in the phosphor samples. 2.2. Characterization The XRD patterns of the synthesized samples have been monitored to identify crystallinity and to confirm the pure phase formation using 18 kW rotating anode (Cu) based Regaku XRD powder diffractometer attached with a graphite monochromator in the diffracted beam path in the range of 20–80°. The morphology of the phosphor has been recorded using a scanning electron microscope (SEM) from a JEOL-TM Model JSM 5410 unit. The energy dispersive spectroscopic (EDS) measurement has been carried out to verify the elements present in the synthesized sample. The FTIR spectra of the phosphor samples were monitored using Frontier-I spectrometer (Perkin Elmer). The photoluminescence excitation and photoluminescence spectra of the synthesized phosphors have been recorded using Fluorolog-3 attached with 450 W Xenon lamp equipped with photomultiplier tube. The lifetime of 5 G5/2 level of Sm3+ ion emitting at 606 nm in the doped phosphor sample has been monitored using Fluorolog-3 spectrofluorometer in the phosphorescence mode attached with 25 W pulsed Xenon lamp. 3. Results and discussion 3.1. Structural characterization 3.1.1. XRD measurements The X-ray diffraction (XRD) patterns of the as-synthesized and the annealed (at 1200 °C) 1.0 mol% Sm3+ doped Y2O3 nano-crystalline phosphors have been recorded in the range of 20–80° and they are shown in Fig. 1. The XRD patterns thus obtained match well with JCPDS File no. 43-1036 (with lattice parameters a = 10.60 Å and α = β = γ = 90°) in both the cases. The phase of the phosphor is C-type cubic with space group la3(206). The XRD peaks were also indexed using JCPDS file. The positions of XRD peaks remain the same as it is in the as-synthesized sample. However, in the case of the annealed sample they are slightly shifted towards lower 2θ angle side. It is also evident from the inset in the figure that the full width at half maxima (FWHM) of the annealed sample is also reduced slightly, which suggests an increase in crystallinity of the phosphor sample. Thus, an ordered crystal structure is formed in the annealed phosphor. The reduction in FWHM and thereby improvement in crystallinity has been studied in our earlier work on Tb3+ doped Y2O3 nano170
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
appreciably and the agglomeration is slightly reduced (see Fig. 2(b)). The similar type of the behavior has been observed in our earlier work on Tb3+ doped Y2O3 phosphor and the samples were synthesized with the same method [7]. It is also clear from Fig. 2(b) that the particles of the annealed phosphor become distinct and porous. Fig. 3 shows the EDS spectrum of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor. The energy dispersive spectroscopic (EDS) measurement reveals the signature of elemental traces in the phosphor sample. The spectrum confirms the presence of Y, Sm and O elements in the phosphor sample. It is also clear from the spectrum that the phosphor sample contains all the elements, which were used during preparation of the phosphor sample. 3.2. Optical measurements 3.2.1. FTIR measurements The FTIR spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphors recorded in the 400–4000 cm−1 region are shown in Fig. 4. Generally, the as-synthesized phosphor exhibits some impurities of different molecules, such as CO, NO3−, OH−, etc, which are observed due to the chemicals used in the synthesis of the phosphor. They are responsible for the non-radiative transitions and reduce the emission intensity significantly. When the as-synthesized sample is annealed at higher temperature (i.e. at 1200 °C/5 h), these impurities are reduced appreciably [7,15]. They are called as optical quenching centers. It is interesting to note that the intensity of the absorbtion bands changes considerably on annealing. Fig. 4 shows that the intensity of CeO and NO3− impurity bands is decreased considerably after annealing. The FTIR spectra contain vibrational bands in the 415–625 cm−1 region centered at 461 and 557 cm−1, which are assigned to arise due to stretching vibrations of YeO crystal lattice, respectively. Kadaira et al. have also observed stretching vibrations of YeO crystal lattice below 700 cm−1 in the Sm3+ doped Y2O3 phosphor [24]. The absorption band present in the spectra at 833 cm−1 is assigned to arise due to CeO asymmetric stretching vibration. The band observed in the 1405–1509 cm−1 region is attributed to the stretching vibrations of NO3− molecule [7]. It is worth noting that the Y2O3 has low phonon energy (461 and 557 cm−1), which strongly promotes the radiative transions in the lanthanide ions including Sm3+ ion [7,15,24]. Thus, one can get the enhanced photoluminescence from the Y2O3 doped phosphor samples.
Fig. 1. XRD patterns of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-crystalline phosphor samples. The inset figure shows a shift of the peak (2 2 2) in the two cases.
phosphor [7]. Lu et al. have also reported that when the phosphor is annealed at higher temperatures the FWHM of the XRD peaks are reduced. This suggests an increase in the crystallite size thereby crystallinity of the phosphor [21]. The average crystallite size (thkl) has been calculated for the as-synthesized and the annealed phosphor samples using Debye-Scherrer equation:
thkl =
Kλ β cosθ
where K is a constant and its value is 0.90, λ, β and θ are the wavelength of the X-ray beam, FWHM and the diffraction angle of the XRD peak. The average crystallite size has been calculated using the intense peaks (2 2 2), (4 4 0) and (4 0 0); and its values in the two cases are found to be 23 and 48 nm, respectively. It has been also reported that when the sample is annealed the crystallite size of the phosphor is increased and the FWHM is reduced significantly [7,15,24]. An improvement in crystallinity of the phosphor supports to achieve larger photoluminescence intensity from the phosphor. 3.1.2. SEM and EDS measurements Fig. 2 shows the SEM micrographs of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples. The particles are of different shapes and sizes. They are agglomerated to each other and oriented in different directions. When the as-synthesized phosphor is annealed the particles size of the phosphor increases
3.2.2. Photoluminescence excitation measurements The photoluminescence excitation (PLE) spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples have been monitored in the range of 200–550 nm with λemi = 606 nm
Fig. 2. SEM micrographs of the as-synthesized (a) and the annealed (b) 1.0 mol% Sm3+ doped Y2O3 phosphor samples. 171
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
Fig. 3. EDS spectrum of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample.
and the spectra thus obtained are shown in Fig. 5. The excitation spectra contain several excitation peaks centered at 336, 346, 363, 379, 407, 424, 450, 464, 477, 492, 505 and 531 nm and they are assigned to arise due to 6H5/2 → 4D7/2, 4H7/2, 4L17/2, 4K11/2, 4F7/2, (6P, 4P)5/2, 4G9/2, 4 I13/2, 4I11/2, 4I9/2, 4G7/2 and 4F3/2 transitions, respectively [6,18–24]. The emission intensity of the band at 407 nm due to 6H5/2 → 4F7/2 transition appears with maximum intensity. The spectra also contain a weak band centered at 238 nm in the range of 220–250 nm, which is due to charge transfer state (CTS) of Sm3+-O2− (see inset figure). The emission intensity of the as-synthesized phosphor improves significantly on annealing it at higher temperature. This is due to an increase in crystallinity and decrease in optical quenching centers of the phosphor sample [7]. Since the excitation peak observed at 407 nm appears with maximum intensity and has large absorption cross section for Sm3+ ion. This wavelength has been used by most of the workers to excite and monitor the photoluminescence spectra of the Sm3+ doped phosphor samples [19–24]. The other excitation bands show smaller emission intensity, which indicates that they have weak absorption cross section for Sm3+ ion. As a result, they emit poor emission intensity.
Fig. 4. FTIR spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-phosphor samples recorded in the 400–4000 cm−1 region.
3.2.3. Downconversion photoluminescence measurements The photoluminescence (PL) spectra of the as-synthesized (1.0 mol %) and the annealed (i.e. 0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples monitored on excitation with 407 nm in the range of 550–750 nm are shown in Fig. 6. The PL spectra contain large number of the emission peaks at 568, 606 (with Stark components at 616 and 622), 654, 674, 724 and 741 nm, which are attributed to arise due to 4 G5/2 → 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2 and 15/2) transitions of Sm3+ ion, respectively [18–24]. The emission peak observed at 568 nm due to 4G5/2 → 6H5/2 transition is of magnetic dipole type whereas that of at 654 nm due to 4G5/2 → 6H9/2 transition is of electric dipole type of transitions, respectively. However, the 4G5/2 → 6H7/2 transition at 606 nm has a contribution of electric as well as magnetic dipole components. The intensity ratio of the electric dipole to magnetic dipole transitions gives an idea about the symmetry of local environment around the activator ion. The phosphor emits an intense reddish orange color at 606 nm due to 4G5/2 → 6H7/2 transition of Sm3+ ion [19,20]. It is interesting to note that the emission intensity of the annealed phosphor is three times larger than the as-synthesized one. The emission peaks observed in the two cases match well with each other. The excitation and emission processes involved in different
Fig. 5. Photoluminescence excitation (PLE) spectra of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples. The inset figure shows the zoomed spectra in the range of 200–260 nm for these cases.
172
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
concentration the energy migration takes place between the Sm3+ ions due to shorter distance between them and the emission intensity decreases considerably [22]. Wang et al. have also studied the effect of concentration on the emission intensity of the Tb3+ doped Y2O3 and YBO3 phosphors and reported a decrease in the emission intensity due to concentration quenching [28]. The effect of concentration on the emission intensity of the Sm3+ doped Y2O3 nano-phosphors can be verified from the inset figure as shown in Fig. 6. It is clear from the figure that the emission intensity is optimum at 1.0 mol% concentration of Sm3+ ions. In our earlier work, the concentration of Sm3+ ion in the La2O3 and La(OH)3 host materials has been also optimized at 1.0 mol% [26,27]. The emission intensity of the as-synthesized phosphor has been found to increase by annealing it at higher temperature. The as-synthesized phosphor contains different impurity centers, which are reduced greatly on annealing the sample. Annealing also leads to an increase in crystallinity and a decrease in surface defects from the assynthesized phosphor. As is mentioned earlier, when the phosphor sample is annealed at 1200 °C the crystallite size of the phosphor is increased from 23 nm to 48 nm. A decrease in FWHM in the XRD peaks reveals an increase in crystallinity and the formation of an ordered crystal structure [7]. Lu et al. have also studied the effect of temperature on the Sm3+ doped Bi4Si3O12 red-emitting phosphor and reported an increase in the emission intensity with the annealing temperature [21,29]. It is also clear from the SEM analysis that the particles size improves on annealing the as-synthesized phosphor at 1200 °C. When the phosphor sample is annealed at higher temperature the smaller particles combine together and form a larger size particle. The particles thus formed have large surface area. The larger particles absorb the excitation energy very efficiently due to large surface area. As a result, the emission intensity of the phosphor is enhanced significantly [10,12,14,30,31]. These factors effectively play role in enhancing the photoluminescence intensity of the sample. In our case, it is found that the emission intensity of the annealed phosphor is increased upto three times compared to that of the as-synthesized phosphor. The annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample has been excited with different wavelengths i.e. 363, 407, 424 and 464 nm and the corresponding photoluminescence has been measured in the range of 550–700 nm. The effect of wavelength on the photoluminescence intensity of the Sm3+ doped Y2O3 phosphor is shown in Fig. 8. The photoluminescence intensity of the phosphor is smaller for 363 nm excitation; however, it is maximum at 407 nm excitation. As is clear from the excitation spectra, the Sm3+ ion has the largest absorption cross section for 407 nm wavelength. This wavelength efficiently promotes the Sm3+ ion in the excited states resulting intense photoluminescence. However, the photoluminescence in the lower
Fig. 6. Photoluminescence spectra of the as-synthesized (1.0 mol%) and the annealed (i.e. 0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors on excitation with 407 nm wavelength. The inset shows a change in the emission intensity with different concentrations of Sm3+ ions.
Fig. 7. Energy level diagram of Sm3+ ion with different transitions on excitation with 407 nm.
transitions of Sm3+ ion can be easily understood using the schematic energy level diagram. Fig. 7 shows the energy level diagram of Sm3+ ion and the photoluminescence observed on excitation with 407 nm. When the phosphor sample is energized with 407 nm radiation from a Xenon lamp; the Sm3+ ions are promoted to 4F7/2 level. The excited ions in this state relax non-radiatively to the 4G5/2 state. The excited ions in the 4G5/2 state give large number of radiative transitions to different lower lying states due to 4G5/2 → 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2 and 15/2) transitions of Sm3+ ion [18–24,26,27]. Among these, the intense emission is observed at 606 nm due to 4G5/2 → 6H7/2 transition. The emission intensity of the bands greatly depends on the concentration of an activator ion. The emission intensity increases on increasing concentration of the activator ion simultaneously and decreases after a particular concentration. The phosphor samples were prepared with different concentrations of the Sm3+ ions (i.e. 0.5, 1.0 and 1.5 mol%). The emission intensity of the phosphor increases for 0.5 and 1.0 mol% concentrations of the Sm3+ ions. However, the emission intensity decreases at higher concentration (i.e. at 1.5 mol%) due to concentration quenching [18–20]. It has been reported that at higher
Fig. 8. Effect of excitation wavelengths on the photoluminescence intensity of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample. 173
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
Fig. 9. Photoluminescence spectra of the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor sample monitored on excitations with 238 and 363 nm wavelengths.
wavelength region (250–550 nm) appears very weak irrespective of the excitation wavelengths. In order to see the emission bands in the lower wavelength side (i.e. 250–550 nm), the photoluminescence spectra of the annealed (1.0 mol %) Sm3+ doped phosphor have been also monitored on excitations with 238 and 363 nm radiations and the spectra thus obtained are shown in Fig. 9. The phosphor sample emits photoluminescence from ultraviolet (UV) to visible regions. The rise behavior in the peak profiles is due to harmonics of the excitation wavelengths. When the sample is illuminated with 238 nm (charge transfer excitation) the sample emits emissions at 324, 332, 346, 369, 379, 393, 411, 420, 438, 450, 491 and 535 nm wavelengths from the excited state to the ground state, which are well assigned in Fig. 9(a). However, upon 363 nm excitation the emission bands at 397, 407, 423, 438, 449, 467, 473, 493 and 533 nm have been observed and they are also assigned as shown in Fig. 9(b). The emission bands appeared at 467 and 473 nm in the case of 363 nm excitation are not observed clearly on excitation with 238 nm wavelength. This is due to overlapping of these bands with the harmonic of 238 nm excitation. Vishwakarma et al. have reported only the reddish orange color on excitation with different wavelengths [20]. However, we have obtained clear emission bands in the lower wavelength regions. These emission bands are well matched to the bands observed in the excitation spectra. The photoluminescence intensity obtained upon 363 nm excitation is larger than that of 238 nm excitation. This is due to the fact that the 238 nm wavelength excites the CTS of Sm3+-O2−. This emits a broad band in the range of 220–250 nm, which is centered at 238 nm. The Sm3+ ion absorbs this broad region and emits weak emission intensity. However, the 363 nm wavelength excites the 4L17/2 state of Sm3+ ion directly and gives larger photoluminescence intensity compared to the 238 nm excitation. The Sm3+ doped Y2O3 phosphor emits reddish orange color as has been observed by the naked eye. This can be also confirmed by plotting CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples. Fig. 10 shows the CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors. The CIE coordinates have been calculated and they are (0.55, 0.44), (0.56, 0.44), (0.56, 0.43) and (0.56, 0.44) for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphor samples, respectively [20,22]. It is clear from the CIE coordinates that the concentration and annealing process leads to an increase in the photoluminescence intensity of the phosphor, however, they do not affect the color emitted by the phosphor. Thus, the Sm3+ doped Y2O3 phosphors may be used in the displays and photonic devices.
Fig. 10. CIE diagram for the as-synthesized (1.0 mol%) and the annealed (0.5, 1.0 and 1.5 mol%) Sm3+ doped Y2O3 phosphors upon 407 nm excitation.
Fig. 11. Decay curves of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 nano-phosphors on excitation with 407 nm for 4G5/2 → 6H7/2 transition (at 606 nm) of Sm3+ ions.
3.2.4. Decay curve analysis The lifetime measurements of the phosphor samples have been carried out on excitation with 407 nm and the emission was monitored 174
Optics and Laser Technology 111 (2019) 169–175
R.S. Yadav, S.B. Rai
at 606 nm wavelength. Fig. 11 shows the decay curves of the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphor samples for the 4G5/2 → 6H7/2 transition (at 606 nm). The decay curves fit well with the single exponential relation in these cases:
BaGd2Ti4O13 phosphors, Mater. Res. Bull. 50 (2014) 354–358. [5] H.M. Yang, J.X. Shi, M.L. Gong, A novel red emitting phosphor Ca2SnO4: Eu3+, J. Solid State Chem. 178 (2005) 917–920. [6] Y.C. Li, Y.H. Chang, Y.F. Lin, Y.S. Chang, Y.J. Lin, Synthesis and luminescent properties of Ln3+ (Eu3+, Sm3+, Dy3+)-doped lanthanum aluminum germanate LaAlGe2O7 phosphors, J. Alloys Compds. 439 (2007) 367–375. [7] R.S. Yadav, S.B. Rai, Effect of concentration and wavelength on frequency downshifting photoluminescence from a Tb3+ doped yttria nano-phosphor: a photochromic phosphor, J. Phys. Chem. Solids 114 (2018) 179–186. [8] F. Auzel, Upconversion and anti-Stokes processes with and ions in solids, Chem. Rev. 104 (2004) 139–173. [9] Y. Zhou, Q. Ma, M. Lu¨, Z. Qiu, A. Zhang, Combustion synthesis and photoluminescence properties of YNbO4-based nanophosphors, J. Phys. Chem. C 112 (2008) 19901–19907. [10] R.S. Yadav, R.K. Verma, S.B. Rai, Intense white light emission in Tm3+/Er3+/Yb3+ co-doped Y2O3-ZnO nano-composite, J. Phys. D: Appl. Phys. 46 (275101) (2013) 1–8. [11] H.J. Lin, Y.S. Chang, Blue-emitting phosphor of YInGe2O7 doped with Tm3+ Ions, Electrochem. Solid-State Lett. 10 (7) (2007) J79–J82. [12] R.S. Yadav, S.B. Rai, Surface analysis and enhanced photoluminescence via Bi3+ doping in a Tb3+ doped Y2O3 nano-phosphor under UV excitation, J. Alloys Compds. 700 (2017) 228–237. [13] A. Huignard, T. Gacoin, J.P. Boilot, Synthesis and luminescence properties of colloidal YVO4: Eu phosphors, Chem. Mater. 12 (2000) 1090–1094. [14] R.S. Yadav, S.B. Rai, Structural analysis and enhanced photoluminescence via host sensitization from a lanthanide doped BiVO4 nano-phosphor, J. Phys. Chem. Solids 110 (2017) 211–217. [15] R.S. Yadav, R.K. Verma, A. Bahadur, S.B. Rai, Structural characterizations and intense green upconversion emission in Yb3+, Pr3+ co-doped Y2O3 nano-phosphor, Spectrochim. Acta Part A 137 (2015) 357–362. [16] Y. He, M. Zhao, Y. Song, G. Zhao, X. Ai, Effect of Bi3+ on fluorescence properties of YPO4:Dy3+ phosphors synthesized by a modified chemical co-precipitation method, J. Lumin. 131 (2011) 1144–1148. [17] M.K. Jung, W.J. Park, D.H. Yoon, Photoluminescence characteristics of red phosphor Eu3+, Sm3+ co-doped Y2O3 for white light emitting diodes, Sens. Actuators B 126 (2007) 328–331. [18] R.S. Yadav, R.V. Yadav, A. Bahadur, T.P. Yadav, S.B. Rai, Role of Li+ on white light emission from Sm3+ and Tb3+ co-doped Y2O3 nano-phosphor, Mater. Res. Exp. 3 (036201) (2016) 1–13. [19] M. Shivaram, H. Nagabhushana, S.C. Sharma, S.C. Prashantha, B. Daruka Prasad, N. Dhananjaya, R. Hari Krishna, B.M. Nagabhushana, C. Shivakumara, R.P.S. Chakradhar, Synthesis and luminescence properties of Sm3+ doped CaTiO3 nanophosphor for application in white LED under NUV excitation, Spectrochim. Acta Part A 128 (2014) 891–901. [20] A.K. Vishwakarma, M. Jayasimadri, Pure orange color emitting Sm3+ doped BaNb2O6 phosphor for solid-state lighting applications, J. Lumin. 176 (2016) 112–117. [21] G. Lu, K. Qiu, J. Li, W. Zhang, X. Yuan, Synthesis and photoluminescence characteristics of Sm3+-doped Bi4Si3O12 red-emitting phosphor, Luminescence 32 (2017) 93–99. [22] L. Li, X. Tang, Z. Jiang, X. Zhou, S. Jiang, X. Luo, G. Xiang, K. Zhou, NaBaLa2(PO4)3: a novel host lattice for Sm3+-doped phosphor materials emitting reddish-orange light, J. Alloys Compds. 701 (2017) 515–523. [23] D. Hou, X. Pan, J. Li, W. Zhou, X. Ye, Structure and luminescence properties of Sm3+ doped Y2MoO6 phosphor under near ultraviolet light excitation, J. Rare Earths 35 (2017) 335–340. [24] C.A. Kodaira, R. Stefani, A.S. Maia, M.C.F.C. Felinto, H.F. Brito, Optical investigation of Y2O3:Sm3+ nanophosphor prepared by combustion and Pechini methods, J. Lumin. 127 (2007) 616–622. [25] A. Dhahri, K. Horchani-Naifer, A. Benedetti, F. Enrichi, M. Férid, P. Riello, Combustion synthesis and photoluminescence of Tb3+ doped LaAlO3 nanophosphors, Opt. Mater. 35 (2013) 1184–1188. [26] R.S. Yadav, Y. Dwivedi, S.B. Rai, Structural and optical properties of Eu3+, Sm3+ co-doped La(OH)3 nano-crystalline red emitting phosphor, Spectrochim. Acta Part A 132 (2014) 599–603. [27] R.S. Yadav, Y. Dwivedi, S.B. Rai, Structural and optical characterization of nanosized La(OH)3:Sm3+ phosphor, Spectrochim. Acta Part A 96 (2012) 148–153. [28] L. Wang, L. Shi, N. Liao, H. Jia, P. Du, Z. Xi, L. Wang, D. Jin, Photoluminescence properties of Y2O3:Tb3+ and YBO3:Tb3+ green phosphors synthesized by hydrothermal method, Mater. Chem. Phys. 119 (2010) 490–494. [29] F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, X. Liu, Tuning upconversion through energy migration in core–shell nanoparticles, Nat. Mater. 10 (2011) 968–973. [30] X. Liu, L. Yan, J. Zou, Tunable cathodoluminescence properties of Tb3+-doped La2O3 nanocrystalline phosphors, J. Electrochem. Soc. 157 (2) (2010) P1–P6. [31] Y. Liu, C. Xu, Q. Yang, White upconversion of rare-earth doped ZnO nanocrystals and its dependence on size of crystal particles and content of Yb3+ and Tm3+, J. Appl. Phys. 105 (084701) (2009) 1–6.
t I = I0 exp ⎛− ⎞ ⎝ τ⎠ where I0 and I are the intensities at time 0 and after t s, respectively and τ is the lifetime of the 4G5/2 level of the Sm3+ ion. The values of the lifetime were calculated as 0.85 and 0.90 ms for the as-synthesized and the annealed (1.0 mol%) Sm3+ doped Y2O3 phosphors, respectively. An increase in the lifetime of 4G5/2 level of the Sm3+ ion for the annealed sample is due to an improvement in crystallinity [10,12,21,24]. It has been mentioned that the annealed phosphor sample has less optical quenching centers and surface defects. As a result, the lifetime of 4G5/2 level of the Sm3+ ion is increased from 0.85 ms to 0.90 ms. These factors enhance the rate of the radiative transitions considerably. Therefore, the photoluminescence intensity of the Sm3+ doped Y2O3 phosphor is enhanced upto three times. 4. Conclusions The Sm3+ doped Y2O3 nano-crystalline phosphor has been synthesized through solution combustion method. The structural measurements reflect strong nano-crystalline nature of the phosphor sample. The SEM measurements show an increase in the particles size for the annealed phosphor. The PLE spectra show an intense excitation band centered at 407 nm with large numbers of weak bands. The Sm3+ doped Y2O3 phosphor emits an intense reddish orange photoluminescence centered at 606 nm due to 4G5/2 → 6H7/2 transition on excitations with 238, 363, 407, 424 and 464 nm wavelengths. However, the sample excited by 238, 363, 424 and 464 nm bands gives weak emissions. The sample excited with 238 and 363 nm wavelengths results bands in the lower wavelength side, which also shows weak emissions. The optimum intensity is observed upon 407 nm excitation. The concentration of Sm3+ ion is optimized with emission intensity at 1.0 mol% in the phosphor. The emission intensity of the as-synthesized phosphor is enhanced upto three times on annealing it at 1200 °C. This improvement in the photoluminescence intensity is due to an increase in crystallinity, reduction in optical quenching centers and surface defects. The lifetime of the 4G5/2 level increases in the case of the annealed phosphor sample. Thus, the Sm3+ doped Y2O3 nano-crystalline phosphor may be applicable for displays and photonic devices. Acknowledgements Prof. S.B. Rai acknowledges Banaras Hindu University for Emeritus Professorship. The authors wish to acknowledge Prof. O.N. Srivastava, Department of Physics, Banaras Hindu University, Varanasi for providing XRD, SEM and EDS measurement facilities. References [1] C. Feldmann, T. Jüstel, C.R. Ronda, P.J. Schmidt, Inorganic luminescent materials: 100 years of research and application, Adv. Funct. Mater. 13 (2003) 511–516. [2] P. Pust, V. Weiler, C. Hecht, A. Tücks, A.S. Wochnik, A.K. Henß, D. Wiechert, C. Scheu, P.J. Schmidt, W. Schnick, Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material, Nat. Mater. 13 (2014) 891–896. [3] R. Bazzi, M.A. Flores, C. Louis, Synthesis and properties of europium-based phosphors on the nanometer scale: Eu2O3, Gd2O3:Eu, and Y2O3:Eu, J. Colloid Interf. Sci. 273 (2004) 191–197. [4] S.H. Lee, J.S. Yu, Synthesis and luminescence properties of Eu3+ doped
175