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Preparation, characterization and photoluminescence of Sm3+ doped NaGdF4 nanoparticles
Journal of Alloys and Compounds 636 (2015) 8–11
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation, characterization and photoluminescence of Sm3+ doped NaGdF4 nanoparticles Juhong Miao ⇑, Jing Su, Yan Wen, Weifeng Rao School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
a r t i c l e
i n f o
Article history: Received 23 January 2015 Received in revised form 15 February 2015 Accepted 16 February 2015 Available online 23 February 2015 Keywords: Photoluminescence Optical materials Fluoride
a b s t r a c t A series of NaGdF4:Sm3+ nanoparticles have been prepared with a facile hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscopic (SEM), and the photoluminescence (PL) spectra method. XRD results indicate that all the diffraction peaks of the samples can be well indexed to the hexagonal b-phase of NaGdF4. SEM images demonstrate the gradual increase in particle size of the samples with increasing Sm3+ concentration, which suggests the fluxing effect of Sm3+ doping. The luminescent properties in the excitation, emission and time-resolved spectra were studied and analyzed. The strongest emission peaks located around 594 nm is originated from 4G5/2 ? 6H7/2 transition of Sm3+, while the peaks centered at about 560, 640, and 704 nm are from 4G5/2 ? 6H5/2, 6H9/2, 6H11/2 transitions, respectively. The photoluminescence intensity firstly increases with increasing Sm3+ concentration, maximizes at 5.0 mol%, and then decreases due to the competition between concentration quenching and the enhancement in crystallinity of the samples. This is supported by the lifetime decay studies. The decay lifetime data follow the bi-exponential nature. The as-prepared NaGdF4:Sm3+ nanoparticles may be useful in the display devices. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Fluoride compounds with very low phonon frequencies of their crystal lattices and high radiative transition rate, are excellent host materials for up conversion (UC) as well as down conversion (DC) photoluminescence [1]. Fluoride hosts doped with lanthanide ions display low probabilities for multiphonon relaxation processes, resulting in long lifetimes of their excited states and high luminescence quantum yields [2]. As an important member of sodium rare earth (RE) fluoride compounds (NaREF4), NaGdF4 can not only offer comparable UC and DC luminescence but also serve as an ideal UV sensitizer to activate specific DC of Ln3+ ions such as Eu3+ since the gadolinium ion (Gd3+) is a good intermediate that migrates and transfers energy [3]. Eu3+ doped crystals of NaGdF4 have been shown to display DC emission of two photons in the visible range [2,4]. The efficient conversion of deep UV radiation into visible light is of significant technological importance, in particular for phosphors suitable for mercury free fluorescent tubes and plasma display panels [4]. Therefore, during the past few years, NaGdF4:Eu3+ nanoparticles have been widely used as red phosphors and display materials
⇑ Corresponding author. Tel./fax: +86 25 58731174. E-mail address: [email protected] (J. Miao). http://dx.doi.org/10.1016/j.jallcom.2015.02.129 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
and have attracted great interest about their luminescent properties [5–7]. In addition to Eu3+, some other trivalent lanthanide ions have also been doped into NaGdF4 to achieve desirable DC luminescence. Li et al. reported visible multicolored luminescence from lanthanide ion-pair (Eu3+/Tb3+, Dy3+/Tb3+, Sm3+/Tb3+ and Eu3+/ Dy3+) codoped NaGdF4 nanocrystals using Ce3+ as the sensitizer [8]. Guan et al. recently reported Tb3+, Sm3+ codoped b-NaGdF4 nanomaterials with tunable multicolors and intrinsic paramagnetic properties, in which an energy transfer from Tb3+ to Sm3+ was observed [9]. NaGdF4:Sm3+ nanoparticles can give characteristic red–orange emission of Sm3+ when excited by UV light. However, to our knowledge, only the glass ceramics doped with NaGdF4:Sm3+ have been reported [10,11]. Little research work has been done on NaGdF4:Sm3+ nanoparticles that may be useful in the display devices. In the present study, we have prepared NaGdF4 nanoparticles doped with Sm3+ up to 10 mol% by a facile hydrothermal method. The prepared samples were characterized for their structural and luminescent properties. The optical spectra of NaGdF4:Sm3+ nanoparticles such as excitation, emission, and time-resolved spectra have been discussed in detail. We have also investigated how luminescence intensity relates lifetime of decay with the increase in Sm3+ dopant concentration.
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11 2. Experimental procedure NaGd1xSmxF4 (where x = 0.5, 1.0, 3.0, 5.0, 8.0 and 10.0 mol%) nanoparticles were prepared with a facile hydrothermal method. All the chemicals were of analytical grade and used as received without further purification. The RE(NO3)3 (RE = Gd, Sm) solutions were prepared by dissolving stoichiometric amounts of the corresponding RE2O3 in nitric acid at an elevated temperature. In a typical synthesis procedure, 9.5 mL 0.4 M Gd(NO3)3 and 1 mL 0.2 M Sm(NO3)3 solution were firstly dissolved in 15 mL deionized water. Then, 9 mL 4 M NaF aqueous solution was added drop-wise into the above solution under continuous stirring. Subsequently, the obtained milky colloidal solution was transferred to a 50.0 mL Teflon-lined autoclave, and heated at 160 °C for 20 h. After naturally cooling to room temperature, the final products were collected by centrifuging and washed with water and ethanol several times. The collected nanoparticles were dried under 70 °C for 12 h. The XRD patterns of the NaGdF4:Sm3+ nanoparticles were recorded by a Bruker D8 X-ray diffractometer with Cu Ka radiation (40 kV, 40 mA). Field-emission scanning electron microscopy (FESEM) images were obtained by using a Hitachi S-4800 microscope. A FLSP 920 fluorescence spectrometer (Edinburgh instrument Ltd, UK) was used to measure the luminescence spectrum and luminescence decay curves were obtained by excitation with an Opolette 355 I OPO laser (OPOTEK, Inc, USA). All the measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction analysis
9
the diffraction peaks can be indexed well to the hexagonal b-phase NaGdF4 with JCPDS Card No. 27-0699, which are consistent with the literature reports [12,13]. There is no trace of any impurity peak displayed in the patterns, which means that all samples are single phase in the detectable range. The enhanced intensity of peak at (1 1 0) about 29° suggests a preferential crystal orientation in this direction. It is noted the diffraction peaks become narrower and stronger with the increment of Sm3+ dopant concentration, which suggests the enhancement in crystallinity of the samples. 3.2. Morphology The SEM images of the nanoparticles with various Sm3+ concentrations are shown in Fig. 2. All of the samples consist of elongated particles ranging in various nanometer sizes. The nanometer particle size favors the generation of photoluminescence [14]. The average size is of 20–30 nm for the sample doped with 0.5 mol % Sm3+. It is interesting to find that the size of the as-synthesized nanoparticles increases with increasing Sm3+ concentrations (as shown in Fig. 2), which indicates the fluxing effect of Sm3+ doping. The increase of particle size leads to the enhancement in crystallinity of the samples, which can ascribe for the narrowed diffraction peaks observed in XRD results (as shown in Fig. 1).
Fig. 1 shows the powder XRD patterns for NaGd1xSmxF4 (where x = 0.5, 1.0, 3.0, 5.0, 8.0 and 10.0 mol%) samples, respectively. All
3.3. Luminescent properties
Fig. 1. Room temperature X-ray diffraction patterns of NaGd1xSmxF4 nanoparticles with different Sm3+ contents.
Fig. 3(a) shows the excitation spectra of NaGdF4:Sm3+ nanoparticles when monitored at 594 nm emission. All the peaks of the excitation spectra are due to the excitation from ground state 6 H5/2 to higher energy levels of Sm3+ ion (compare Fig. 4) [15]. Among these peaks, the most intense excitation peak is at 401 nm, which is perfectly matching with the emission wavelength of near ultraviolet light-emitting diodes (NUV LED). The excitation peak at 401 nm is most likely due to the transition of 6H5/2 ? 4F7/2. Though the 4F7/2 level is within thermal excitation energy at room temperature, no luminescence could be observed from this level. The excited ions decay fastly to the 4G5/2 level mainly by non-radiative processes [16]. Fig. 3(b) shows the emission spectra of all the doped samples when excited at 401 nm. The spectra consist of narrow bands of f–f transitions typical for trivalent rare earth ions. The photoluminescence emission spectrum of Sm3+ consists of several bands in the yellow to red region, which all originate from the 4G5/2 level (compare Fig. 4). The photoluminescence spectrum shows four typical emission bands with maximum peaks at about 560, 594,
Fig. 2. FESEM photographs of NaGd1xSmxF4 nanoparticles with x = 0.5 mol % (a), 1.0 mol% (b), 3.0 mol% (c), 5.0 mol% (d), 8.0 mol% (e) and 10.0 mol% (f), respectively.
10
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11
Fig. 5. Decay lifetime spectra for 4G5/2 ? 6H7/2 emission of NaGd1xSmxF4 nanoparticles with different Sm3+ contents. kEX = 401 nm, kEM = 594 nm. The points are the experimental results, and the continuous line is the fit with bi-exponential decay of the I/I0 vs. time plot.
Fig. 3. Luminescence excitation (a) and emission (b) spectra of NaGdF4:Sm3+ nanoparticles. Excitation wavelength: 401 nm, emission wavelength: 594 nm.
The luminescence intensity increases and reaches the highest value when the concentration of Sm3+ is 5.0 mol%, thereafter it decreases with further increase in Sm3+ concentration. Quenching of luminescence with increasing Sm3+ concentration is a typical property of lanthanide doped systems where the distance between the neighboring Sm3+ ions decreases with the increase of Sm3+ dopant concentration. This decrease in the distance between the ions leads to the cross relaxation among them and thereby the probability of radiative transition is reduced. On the other hand, high crystallinity favors the generation of photoluminescence. Therefore, as a result of the competition between concentration quenching and the enhancement in crystallinity of the samples, the photoluminescence intensity for the samples with 3.0 mol% and 5.0 mol% Sm3+ doping is very close (as shown in Fig. 3(b)). Fig. 5 shows the decay curves for 4G5/2 level of Sm3+ after excitation at 401 nm and emitting at 594 nm for the as-prepared NaGdF4:Sm3+ nanoparticles. A bi-exponential fitting is performed for the luminescence decay curves. The fitting follows the equation:
y ¼ y0 þ a1 expðt=s1 Þ þ a2 expðt=s2 Þ
Fig. 4. Energy level diagrams of Sm3+ and some typical luminescence excitation (left) and emission (right) transitions [14].
where y0 is the baseline correction (y-offset), s1 and s2 are lifetimes of shorter- and longer-lived species, respectively, and a1 and a2 are their respective amplitudes [17,18]. As shown, all luminescence decay data are fitted well by the bi-exponential decay function. The fast component at short times (s1) would correspond to the emission of Sm3+ ions at the surface of nanoparticles, and the longest one (s2) to Sm3+ ions within the body of nanoparticles. These fitting results are summarized in Fig. 5. The average lifetime is calculated using the equation [17,18]
sav ¼ ða1 s21 þ a2 s22 Þ=ða1 s1 þ a2 s2 Þ 640, and 704 nm, respectively. The strongest emission peaks located around 594 nm is originated from 4G5/2 ? 6H7/2 transition of Sm3+, while the peaks centered at about 560, 640, and 704 nm are from 4G5/2 ? 6H5/2, 4G5/2 ? 6H9/2, 4G5/2 ? 6H11/2 transitions, respectively. Interestingly, in comparison to the glass ceramics doped with NaGdF4:Sm3+ [10,11], the photoluminescence emission peak at about 640 nm of the as-synthesized NaGdF4:Sm3+ nanoparticles is split into two components which accounts for the broadening of the respective transitions. Moreover, as discussed above, the particle size increases with the increment of Sm3+ concentrations. However, the peak positions of the emissions are resembled for all of the samples, indicating that they are independent of the increase in particle size.
ð1Þ
ð2Þ
The calculated mean lifetime values sav of NaGdF4:Sm3+ nanoparticles are found to be 13.8 ls (0.5 mol%), 68.4 ls (1.0 mol%), 162.8 ls (3.0 mol%), 174.7 ls (5.0 mol%), 103.0 ls (8.0 mol%) and 27.8 ls (10.0 mol%), respectively. These low 4G5/2 lifetimes of the nanoparticles may be partially related to the contamination with hydrous species in the considerably large surface of nanoparticles, which is a usual rule in all low temperature routes of synthesis. In order to investigate how luminescence intensity relates lifetime of decay with the increase in Sm3+ dopant concentration, we plotted the dependence of intensity area under the curve of 4 G5/2 ? 6H7/2 emission and its corresponding decay lifetimes sav on Sm3+ concentrations in Fig. 6. It is observed that the highest photoluminescence intensity and longest lifetime are found to be
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11
11
between concentration quenching and the enhancement in crystallinity of the samples. It is found that the decay lifetime data are fitted well with the bi-exponential decay function. The prepared NaGdF4:Sm3+ nanoparticles may be useful in the display devices. Acknowledgements This work was supported by the Grant of Specially-Appointed Professor of Jiangsu and the National Science Foundation of China under Grant Nos. 11474167 and 51472123. References
Fig. 6. Luminescence intensity (left side) and corresponding decay lifetime for 4G5/2 level (right side) vs. Sm3+ mol% concentration when excited at 401 nm.
at 5 mol% of Sm3+ concentration and thereafter both decrease with the increase of Sm3+ concentration. As discussed earlier, Sm3+ as most rare earth ions, shows notable concentration quenching. The competition between concentration quenching and the enhancement in crystallinity of the samples results in both the decreasing of luminescence intensity and the shortening of luminescence lifetimes. 4. Conclusions In this paper, NaGdF4:Sm3+ nanoparticles were prepared by hydrothermal method. XRD analysis suggests that the nanoparticles are hexagonal b-phase. The particle size increases with increasing Sm3+ concentration, indicating the fluxing effect of Sm3+ ions doping. The photoluminescence study shows no particle size effect on the peak positions of Sm3+ emission. Both luminescence intensity and lifetime firstly increase, maximize at 5.0 mol%, and then decrease as Sm3+ concentration increases due to the competition
[1] C.X. Li, C.M. Zhang, Z.Y. Hou, L.L. Wang, Z.W. Quan, H.Z. Lian, J. Lin, J. Phys. Chem. C 113 (2009) 2332–2339. [2] P. Ptacek, H. Schäfer, K. Kömpe, M. Haase, Adv. Funct. Mater. 17 (2007) 3843– 3848. [3] F. Wang, X. Fan, M. Wang, Y. Zhang, Nanotechnology 18 (2007) 025701. [4] F. You, S. Huang, S. Liu, Y. Tao, J. Lumin. 110 (2004) 95–99. [5] A. Mech, M. Karbowiak, L. Kepinski, A. Bednarkiewicz, W. Strek, J. Alloys Comp. 380 (2004) 315–320. [6] Y.S. Liu, D.T. Tu, H.M. Zhu, R.F. Li, W.Q. Luo, X.Y. Chen, Adv. Mater. 22 (2010) 3266–3271. [7] S.B. Wang, Q. Li, L.Z. Pei, Q.F. Zhang, Mater. Charact. 61 (2010) 824–830. [8] F. Li, N. Li, M. Wang, S.K. Xu, H.Y. Zhang, Luminescence 25 (2010) 394–398. [9] H.X. Guan, G.X. Liu, J.X. Wang, X.T. Dong, W.S. Yu, Dalton Trans. 43 (2014) 10801–10808. [10] A. Herrmann, M. Tylkowski, C. Bocker, C. Rüssel, Chem. Mater. 25 (2013) 2878– 2884. [11] A. Herrmann, M. Tylkowski, C. Bocker, C. Rüssel, J. Mater. Sci. 48 (2013) 6262– 6268. [12] M. He, P. Huang, C. Zhang, H. Hu, C. Bao, G. Gao, R. He, D. Cui, Adv. Funct. Mater. 21 (2011) 4470–4477. [13] F. He, N. Niu, L. Wang, J. Xu, Y. Wang, G.X. Yang, S.L. Gai, P.P. Yang, Dalton Trans. 42 (2013) 10019–10028. [14] W. Wang, W. Widiyastuti, T. Ogi, I. Lenggoro, Chem. Mater. 19 (2007) 1723– 1730. [15] W.T. Carnall, P.R. Fields, K. Ranjnak, J. Chem. Phys. 49 (1968) 4424–4442. [16] G. Dominiak-Dzik, J. Alloys Comp. 391 (2005) 26–32. [17] B. Choudhury, A. Choudhury, J. Lumin. 136 (2013) 339–346. [18] Y. Wang, Z.Y. You, J.F. Li, Z.J. Zhu, E. Ma, C.Y. Tu, J. Phys. D: Appl. Phys. 42 (2009) 215406.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation, characterization and photoluminescence of Sm3+ doped NaGdF4 nanoparticles Juhong Miao ⇑, Jing Su, Yan Wen, Weifeng Rao School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
a r t i c l e
i n f o
Article history: Received 23 January 2015 Received in revised form 15 February 2015 Accepted 16 February 2015 Available online 23 February 2015 Keywords: Photoluminescence Optical materials Fluoride
a b s t r a c t A series of NaGdF4:Sm3+ nanoparticles have been prepared with a facile hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscopic (SEM), and the photoluminescence (PL) spectra method. XRD results indicate that all the diffraction peaks of the samples can be well indexed to the hexagonal b-phase of NaGdF4. SEM images demonstrate the gradual increase in particle size of the samples with increasing Sm3+ concentration, which suggests the fluxing effect of Sm3+ doping. The luminescent properties in the excitation, emission and time-resolved spectra were studied and analyzed. The strongest emission peaks located around 594 nm is originated from 4G5/2 ? 6H7/2 transition of Sm3+, while the peaks centered at about 560, 640, and 704 nm are from 4G5/2 ? 6H5/2, 6H9/2, 6H11/2 transitions, respectively. The photoluminescence intensity firstly increases with increasing Sm3+ concentration, maximizes at 5.0 mol%, and then decreases due to the competition between concentration quenching and the enhancement in crystallinity of the samples. This is supported by the lifetime decay studies. The decay lifetime data follow the bi-exponential nature. The as-prepared NaGdF4:Sm3+ nanoparticles may be useful in the display devices. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Fluoride compounds with very low phonon frequencies of their crystal lattices and high radiative transition rate, are excellent host materials for up conversion (UC) as well as down conversion (DC) photoluminescence [1]. Fluoride hosts doped with lanthanide ions display low probabilities for multiphonon relaxation processes, resulting in long lifetimes of their excited states and high luminescence quantum yields [2]. As an important member of sodium rare earth (RE) fluoride compounds (NaREF4), NaGdF4 can not only offer comparable UC and DC luminescence but also serve as an ideal UV sensitizer to activate specific DC of Ln3+ ions such as Eu3+ since the gadolinium ion (Gd3+) is a good intermediate that migrates and transfers energy [3]. Eu3+ doped crystals of NaGdF4 have been shown to display DC emission of two photons in the visible range [2,4]. The efficient conversion of deep UV radiation into visible light is of significant technological importance, in particular for phosphors suitable for mercury free fluorescent tubes and plasma display panels [4]. Therefore, during the past few years, NaGdF4:Eu3+ nanoparticles have been widely used as red phosphors and display materials
⇑ Corresponding author. Tel./fax: +86 25 58731174. E-mail address: [email protected] (J. Miao). http://dx.doi.org/10.1016/j.jallcom.2015.02.129 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
and have attracted great interest about their luminescent properties [5–7]. In addition to Eu3+, some other trivalent lanthanide ions have also been doped into NaGdF4 to achieve desirable DC luminescence. Li et al. reported visible multicolored luminescence from lanthanide ion-pair (Eu3+/Tb3+, Dy3+/Tb3+, Sm3+/Tb3+ and Eu3+/ Dy3+) codoped NaGdF4 nanocrystals using Ce3+ as the sensitizer [8]. Guan et al. recently reported Tb3+, Sm3+ codoped b-NaGdF4 nanomaterials with tunable multicolors and intrinsic paramagnetic properties, in which an energy transfer from Tb3+ to Sm3+ was observed [9]. NaGdF4:Sm3+ nanoparticles can give characteristic red–orange emission of Sm3+ when excited by UV light. However, to our knowledge, only the glass ceramics doped with NaGdF4:Sm3+ have been reported [10,11]. Little research work has been done on NaGdF4:Sm3+ nanoparticles that may be useful in the display devices. In the present study, we have prepared NaGdF4 nanoparticles doped with Sm3+ up to 10 mol% by a facile hydrothermal method. The prepared samples were characterized for their structural and luminescent properties. The optical spectra of NaGdF4:Sm3+ nanoparticles such as excitation, emission, and time-resolved spectra have been discussed in detail. We have also investigated how luminescence intensity relates lifetime of decay with the increase in Sm3+ dopant concentration.
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11 2. Experimental procedure NaGd1xSmxF4 (where x = 0.5, 1.0, 3.0, 5.0, 8.0 and 10.0 mol%) nanoparticles were prepared with a facile hydrothermal method. All the chemicals were of analytical grade and used as received without further purification. The RE(NO3)3 (RE = Gd, Sm) solutions were prepared by dissolving stoichiometric amounts of the corresponding RE2O3 in nitric acid at an elevated temperature. In a typical synthesis procedure, 9.5 mL 0.4 M Gd(NO3)3 and 1 mL 0.2 M Sm(NO3)3 solution were firstly dissolved in 15 mL deionized water. Then, 9 mL 4 M NaF aqueous solution was added drop-wise into the above solution under continuous stirring. Subsequently, the obtained milky colloidal solution was transferred to a 50.0 mL Teflon-lined autoclave, and heated at 160 °C for 20 h. After naturally cooling to room temperature, the final products were collected by centrifuging and washed with water and ethanol several times. The collected nanoparticles were dried under 70 °C for 12 h. The XRD patterns of the NaGdF4:Sm3+ nanoparticles were recorded by a Bruker D8 X-ray diffractometer with Cu Ka radiation (40 kV, 40 mA). Field-emission scanning electron microscopy (FESEM) images were obtained by using a Hitachi S-4800 microscope. A FLSP 920 fluorescence spectrometer (Edinburgh instrument Ltd, UK) was used to measure the luminescence spectrum and luminescence decay curves were obtained by excitation with an Opolette 355 I OPO laser (OPOTEK, Inc, USA). All the measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction analysis
9
the diffraction peaks can be indexed well to the hexagonal b-phase NaGdF4 with JCPDS Card No. 27-0699, which are consistent with the literature reports [12,13]. There is no trace of any impurity peak displayed in the patterns, which means that all samples are single phase in the detectable range. The enhanced intensity of peak at (1 1 0) about 29° suggests a preferential crystal orientation in this direction. It is noted the diffraction peaks become narrower and stronger with the increment of Sm3+ dopant concentration, which suggests the enhancement in crystallinity of the samples. 3.2. Morphology The SEM images of the nanoparticles with various Sm3+ concentrations are shown in Fig. 2. All of the samples consist of elongated particles ranging in various nanometer sizes. The nanometer particle size favors the generation of photoluminescence [14]. The average size is of 20–30 nm for the sample doped with 0.5 mol % Sm3+. It is interesting to find that the size of the as-synthesized nanoparticles increases with increasing Sm3+ concentrations (as shown in Fig. 2), which indicates the fluxing effect of Sm3+ doping. The increase of particle size leads to the enhancement in crystallinity of the samples, which can ascribe for the narrowed diffraction peaks observed in XRD results (as shown in Fig. 1).
Fig. 1 shows the powder XRD patterns for NaGd1xSmxF4 (where x = 0.5, 1.0, 3.0, 5.0, 8.0 and 10.0 mol%) samples, respectively. All
3.3. Luminescent properties
Fig. 1. Room temperature X-ray diffraction patterns of NaGd1xSmxF4 nanoparticles with different Sm3+ contents.
Fig. 3(a) shows the excitation spectra of NaGdF4:Sm3+ nanoparticles when monitored at 594 nm emission. All the peaks of the excitation spectra are due to the excitation from ground state 6 H5/2 to higher energy levels of Sm3+ ion (compare Fig. 4) [15]. Among these peaks, the most intense excitation peak is at 401 nm, which is perfectly matching with the emission wavelength of near ultraviolet light-emitting diodes (NUV LED). The excitation peak at 401 nm is most likely due to the transition of 6H5/2 ? 4F7/2. Though the 4F7/2 level is within thermal excitation energy at room temperature, no luminescence could be observed from this level. The excited ions decay fastly to the 4G5/2 level mainly by non-radiative processes [16]. Fig. 3(b) shows the emission spectra of all the doped samples when excited at 401 nm. The spectra consist of narrow bands of f–f transitions typical for trivalent rare earth ions. The photoluminescence emission spectrum of Sm3+ consists of several bands in the yellow to red region, which all originate from the 4G5/2 level (compare Fig. 4). The photoluminescence spectrum shows four typical emission bands with maximum peaks at about 560, 594,
Fig. 2. FESEM photographs of NaGd1xSmxF4 nanoparticles with x = 0.5 mol % (a), 1.0 mol% (b), 3.0 mol% (c), 5.0 mol% (d), 8.0 mol% (e) and 10.0 mol% (f), respectively.
10
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11
Fig. 5. Decay lifetime spectra for 4G5/2 ? 6H7/2 emission of NaGd1xSmxF4 nanoparticles with different Sm3+ contents. kEX = 401 nm, kEM = 594 nm. The points are the experimental results, and the continuous line is the fit with bi-exponential decay of the I/I0 vs. time plot.
Fig. 3. Luminescence excitation (a) and emission (b) spectra of NaGdF4:Sm3+ nanoparticles. Excitation wavelength: 401 nm, emission wavelength: 594 nm.
The luminescence intensity increases and reaches the highest value when the concentration of Sm3+ is 5.0 mol%, thereafter it decreases with further increase in Sm3+ concentration. Quenching of luminescence with increasing Sm3+ concentration is a typical property of lanthanide doped systems where the distance between the neighboring Sm3+ ions decreases with the increase of Sm3+ dopant concentration. This decrease in the distance between the ions leads to the cross relaxation among them and thereby the probability of radiative transition is reduced. On the other hand, high crystallinity favors the generation of photoluminescence. Therefore, as a result of the competition between concentration quenching and the enhancement in crystallinity of the samples, the photoluminescence intensity for the samples with 3.0 mol% and 5.0 mol% Sm3+ doping is very close (as shown in Fig. 3(b)). Fig. 5 shows the decay curves for 4G5/2 level of Sm3+ after excitation at 401 nm and emitting at 594 nm for the as-prepared NaGdF4:Sm3+ nanoparticles. A bi-exponential fitting is performed for the luminescence decay curves. The fitting follows the equation:
y ¼ y0 þ a1 expðt=s1 Þ þ a2 expðt=s2 Þ
Fig. 4. Energy level diagrams of Sm3+ and some typical luminescence excitation (left) and emission (right) transitions [14].
where y0 is the baseline correction (y-offset), s1 and s2 are lifetimes of shorter- and longer-lived species, respectively, and a1 and a2 are their respective amplitudes [17,18]. As shown, all luminescence decay data are fitted well by the bi-exponential decay function. The fast component at short times (s1) would correspond to the emission of Sm3+ ions at the surface of nanoparticles, and the longest one (s2) to Sm3+ ions within the body of nanoparticles. These fitting results are summarized in Fig. 5. The average lifetime is calculated using the equation [17,18]
sav ¼ ða1 s21 þ a2 s22 Þ=ða1 s1 þ a2 s2 Þ 640, and 704 nm, respectively. The strongest emission peaks located around 594 nm is originated from 4G5/2 ? 6H7/2 transition of Sm3+, while the peaks centered at about 560, 640, and 704 nm are from 4G5/2 ? 6H5/2, 4G5/2 ? 6H9/2, 4G5/2 ? 6H11/2 transitions, respectively. Interestingly, in comparison to the glass ceramics doped with NaGdF4:Sm3+ [10,11], the photoluminescence emission peak at about 640 nm of the as-synthesized NaGdF4:Sm3+ nanoparticles is split into two components which accounts for the broadening of the respective transitions. Moreover, as discussed above, the particle size increases with the increment of Sm3+ concentrations. However, the peak positions of the emissions are resembled for all of the samples, indicating that they are independent of the increase in particle size.
ð1Þ
ð2Þ
The calculated mean lifetime values sav of NaGdF4:Sm3+ nanoparticles are found to be 13.8 ls (0.5 mol%), 68.4 ls (1.0 mol%), 162.8 ls (3.0 mol%), 174.7 ls (5.0 mol%), 103.0 ls (8.0 mol%) and 27.8 ls (10.0 mol%), respectively. These low 4G5/2 lifetimes of the nanoparticles may be partially related to the contamination with hydrous species in the considerably large surface of nanoparticles, which is a usual rule in all low temperature routes of synthesis. In order to investigate how luminescence intensity relates lifetime of decay with the increase in Sm3+ dopant concentration, we plotted the dependence of intensity area under the curve of 4 G5/2 ? 6H7/2 emission and its corresponding decay lifetimes sav on Sm3+ concentrations in Fig. 6. It is observed that the highest photoluminescence intensity and longest lifetime are found to be
J. Miao et al. / Journal of Alloys and Compounds 636 (2015) 8–11
11
between concentration quenching and the enhancement in crystallinity of the samples. It is found that the decay lifetime data are fitted well with the bi-exponential decay function. The prepared NaGdF4:Sm3+ nanoparticles may be useful in the display devices. Acknowledgements This work was supported by the Grant of Specially-Appointed Professor of Jiangsu and the National Science Foundation of China under Grant Nos. 11474167 and 51472123. References
Fig. 6. Luminescence intensity (left side) and corresponding decay lifetime for 4G5/2 level (right side) vs. Sm3+ mol% concentration when excited at 401 nm.
at 5 mol% of Sm3+ concentration and thereafter both decrease with the increase of Sm3+ concentration. As discussed earlier, Sm3+ as most rare earth ions, shows notable concentration quenching. The competition between concentration quenching and the enhancement in crystallinity of the samples results in both the decreasing of luminescence intensity and the shortening of luminescence lifetimes. 4. Conclusions In this paper, NaGdF4:Sm3+ nanoparticles were prepared by hydrothermal method. XRD analysis suggests that the nanoparticles are hexagonal b-phase. The particle size increases with increasing Sm3+ concentration, indicating the fluxing effect of Sm3+ ions doping. The photoluminescence study shows no particle size effect on the peak positions of Sm3+ emission. Both luminescence intensity and lifetime firstly increase, maximize at 5.0 mol%, and then decrease as Sm3+ concentration increases due to the competition
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