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Hydrothermal synthesis, energy transfer and luminescence enhancement of rhombohedral LaOF: Sm3+-Eu3+ nanoparticles
Physica B: Condensed Matter 574 (2019) 311653
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Hydrothermal synthesis, energy transfer and luminescence enhancement of rhombohedral LaOF: Sm3+-Eu3+ nanoparticles
T
⁎
Zhenxing Fua,b, , Birui Liua,b a b
School of Physics and Electronic Information Engineering, Ningxia Normal University, Guyuan, 756000, China Engineering Technology Research Centre for Nano-structures and Functional Materials, Ningxia Normal University, Guyuan, 756000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: LaOF nanoparticles Energy transfer Luminescence enhancement Sm3+ Eu3+
Sm3+, Eu3+ sole- and co-doped LaOF nanoparticles with rhombohedral crystal structure were synthesized via hydrothermal method followed a post-annealing process. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence techniques. The XRD result shows that the samples exhibit a rhombohedral phase in crystal structure. For Sm3+, Eu3+ co-doped LaOF nanoparticles, pumping Sm3+ to its excited 4G9/2 level by 442 nm laser, the intense characteristic emissions of Eu3+ ion were observed, indicating that the energy transfer from Sm3+ to Eu3+ ions occurred. The intensities of Eu3+ emissions due to energy transfer increased with increasing the doping concentration of Eu3+ acceptor. The enhancement of Eu3+ red-light emission at 610 nm through co-doping Sm3+ ion was also observed when the Sm3+, Eu3+ co-doped LaOF nanoparticles excited by 532 nm laser. The optimum concentration of Sm3+ codopant is 0.5 mol% for enhancing the strongest luminescence of Eu3+ red-light.
1. Introduction Rare earth activated luminescent materials have been attracting much attention based on their applications in various fields. Among trivalent rare earth ions, Sm3+ and Eu3+ are more favorable ions for their special energy level structures and potential applications in LED, solid-state lighting, visible laser, red emitting phosphors, etc [1–5]. The excited 4G5/2, 4F3/2 and 4G7/2 levels of Sm3+ ion can produce excellent orange/red emissions in the visible region. In addition, the small spacing between 17860–28573 cm−1 energy levels of Sm3+ ion will lead to the ions in those levels quickly relax to 4G5/2 level through a nonradiative way. These properties make Sm3+ ion have wide applications in high density optical storage, underwater communication and color display, etc [6–8]. Similarly, for Eu3+ ion, the small energy level spacing above 5D0 level can also make the ions in those levels nonradiatively relax to 5D0 level to bring excellent red-light emissions in the visible region [9–12]. With the rising of a demand for LED, visible laser and red phosphors, Sm3+ and Eu3+ as two representative red luminescence activators have played more and more important roles in the development of luminescent materials [13–16]. Lanthanum oxyfluoride (LaOF) crystal has broad applications in the dielectric, optical, optoelectronics and photonics for its unique electrical and optical properties, and has been widely used in luminescent materials, plasma, catalyst, etc [17–21]. In recent years, the energy ⁎
transfer between Sm3+-Eu3+ ions and the luminescence enhancement of Eu3+ ions in different host have been extensively studied [22–27]. However, less report about the energy transfer from Sm3+ to Eu3+ and the luminescence enhancement of Eu3+ ion in rhombohedral LaOF host can be found. In this work, Sm3+, Eu3+ sole- and co-doped rhombohedral LaOF nanoparticles with well uniformity and dispersion were synthesized by the hydrothermal method followed by a post-annealing treatment. Using 442 nm laser to pump Sm3+ ions in LaOF:Sm3+-Eu3+ nanoparticles, the intense characteristic emissions of Eu3+ ion originating from the 5D0 level to the 7FJ (J = 0, 1, 2, 3 and 4) levels were observed. This phenomenon suggests that the energy transfer from Sm3+ to Eu3+ ions occurs in LaOF:Sm3+-Eu3+ nanoparticles. The dynamical process of energy transfer from Sm3+ to Eu3+ ions was investigated systematically. Then the influence of doping concentration of Eu3+ acceptor on energy transfer efficiency was discussed in detail. When the LaOF:Sm3+-Eu3+ nanoparticles excited by 532 nm laser, the enhancement of Eu3+ red-light emission by Sm3+ co-dopant was also observed. The enhancement efficiency of Eu3+ red-light emission depends on the concentration of Sm3+ donor.
Corresponding author. School of Physics and Electronic Information Engineering, Ningxia Normal University, Guyuan, 756000, China. E-mail address: [email protected] (Z. Fu).
https://doi.org/10.1016/j.physb.2019.08.030 Received 19 March 2019; Received in revised form 30 July 2019; Accepted 17 August 2019 Available online 19 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 574 (2019) 311653
Z. Fu and B. Liu
2. Experimental detail 2.1. Sample synthesis The La0.99OF:0.01Sm3+, La0.99OF:0.01Eu3+, La0.99-xOF:0.01Sm3+xEu3+ (x = 0.005, 0.010, 0.020 and 0.030) and La0.99-yOF:ySm3+0.01Eu3+ (y = 0.005, 0.010 and 0.015) nanoparticles were prepared by the hydrothermal method followed by a post-annealing treatment [17,18]. The starting chemicals included samarium oxide (Sm2O3, 99%), europium oxide (Eu2O3, 99%), sodium fluoride (NaF), sodium carbonate (Na2CO3), nitric acid (HNO3) and lanthanum nitric hexahydrate (La(NO3)3·6H2O). All the chemicals employed in the preparation were of analytical grade. Sm(NO3)3 and Eu(NO3)3 deionized water solution were obtained by reacting Sm2O3 and Eu2O3 with nitric acid, respectively. First, La(NO3)3·6H2O was weighed and dissolved with deionized water in a beaker at room temperature, and a clear solution of La(NO3)3 was obtained by stirring. Then the corresponding amount of Sm(NO3)3 and/or Eu(NO3)3 deionized water solution was dropped into the beaker according to the doping concentration of Sm3+ and/or Eu3+ ions. Ensuring the mixture was homogeneous by stirring for few minutes, an appropriate quantity of NaF and Na2CO3 was added into the mixture solution to obtain a slurry state. The mixed solution was stirred for 40–60 min. We transferred the solution to a Teflon-lined autoclave to conduct the reaction at 200 °C for 16 h. After cooling the autoclave to room temperature, the precipitate was separated by centrifugation and washed with deionized water and ethanol in sequence. Then the corresponding Sm3+, Eu3+ and Sm3+-Eu3+ doped LaCO3F precursors were obtained. The as-prepared LaCO3F crystals were ultrasonically treated with oleic acid and dried in air at 60 °C for 12 h. Finally, the rhombohedral LaOF nanoparticles were obtained by annealing the corresponding LaCO3F precursors at 600 °C for 4 h.
Fig. 1. XRD spectrum of La0.96OF:0.01Sm3+-0.03Eu3+ nanoparticles.
the LaOF phase composition. The average size of LaOF particles can be estimated according to the Scherrer formula, D = 0.941 λ / β cos θ , where D is the average size of particle, λ is the X-ray wavelength (0.15418 nm), θ and β are the diffraction angle and the full-width at half-maximum of the observed peak, respectively. The diffraction peak (012) at 2θ = 26.9° is used to calculate the average size of the LaOF particles. The estimated average size is about 60 nm. The surface morphology and crystalline sizes of the as-prepared LaCO3F precursor and the LaOF nanoparticles were observed by TEM. Two representative morphological images of as-prepared LaCO3F precursor and prepared LaOF sample are displayed in Fig. 2. It can be seen that all of the particles have similar surface morphology and the distribution of the crystalline sizes is uniform and homogeneous. The asprepared LaCO3F precursor is well-separated nanoparticles with sizes in the range of 40–60 nm as shown in Fig. 2 (a). The LaOF nanoparticles show rhombohedral structures with a high quality of monodispersity as presented in Fig. 2 (b). A morphological image of the LaOF nanoparticles is taken to elucidate the size and morphology of the samples. The average diameter of LaOF nanoparticles is about 60 nm. The result confirms that the mean size of LaOF nanoparticles obtained by TEM technique is consistent with what calculated by Scherrer formula.
2.2. Characterization The XRD spectra were obtained by using a Bruker D8 Advance automatic diffractometer (λ = 0.15418 nm, Germany) equipped with Cu Kα radiation at 40 kV and 40 mA. The morphology and particle size of the samples were preformed by a TEM (JEM-2100, TEOL Japan) operating at an acceleration voltage of 200 kV. The photoluminescence (PL) signal was collected with a monochromator (SP 2750i, Princeton Instrument, USA) equipped with a photomultiplier tube (ACTON, PD471) and a CCD system (ACTON, 7515-0002). A CW 532 nm solid state laser (MGL–FN–532, CNI, China) and a He–Cd 325/442 nm Dual Wavelength Laser (CW, IK5451R-E-SP, Kimmon, Japan) were employed as excitation sources. The same amount of each sample was used in every case, and all the measurements were conducted at room temperature.
3.2. PL analysis of energy transfer from Sm3+ to Eu3+ To investigate the energy transfer from Sm3+ to Eu3+ ions, the PL emission spectra of the LaOF:Sm3+-Eu3+ nanoparticles excited by 532 nm and 442 nm lasers were measured, respectively. During the measurements, we noticed that the Sm3+ ion in LaOF lattice can be effectively excited by 442 nm laser, while the Eu3+ ion in LaOF lattice can only be excited by 532 nm laser. Fig. 3 gives the PL emission spectra of the La0.99OF:0.01Sm3+, La0.99OF:0.01Eu3+ and La0.98OF:0.01Sm3+0.01Eu3+ nanoparticles under 532/442 nm excitation. The emission spectra of Sm3+ sole-doped and Sm3+, Eu3+ co-doped LaOF nanoparticles excited by 442 nm laser are described by curve b and d, respectively. It can be seen that the spectrum of La0.99OF:0.01Sm3+ consists of four broad emission bands in the range of 550–720 nm (curve b) centered at around 564 nm, 604 nm, 651 nm and 706 nm, corresponding to the Sm3+ transitions of 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4 G5/2 → 6H9/2 and 4G5/2 → 6H11/2, respectively. The strongest emission at 604 nm is attributed to 4G5/2 → 6H7/2 transition. Compared with the emission spectrum of LaOF:Sm3+-Eu3+ nanoparticles (curve d), four new emissions appeared in the spectra. The positions of the new strong emissions in curve d locate at around 590 nm, 610 nm, 626 nm and 704 nm. It is known that Eu3+ ion generally presents the characteristic emissions in the region of 550–840 nm corresponding to the transitions
3. Result and discussion 3.1. Structure, size, and morphology It is known that three different crystal phases of LaOF have been discovered and recognized. The samples synthesized by the post-annealing treatment of LaCO3F precursor at 600 °C for 4 h have rhombohedral crystal phase. A representative XRD pattern of LaOF nanoparticles is shown in Fig. 1. The doping concentration of Sm3+ and Eu3+ ions are 1.0 mol% and 3.0 mol%, respectively. As can be seen from Fig. 1, the diffraction peaks located at 26.5°, 26.9°, 31.0°, 44.1°, 44.8°, 52.3°, 52.8° and 55.5° correspond to the (006), (012), (104), (018), (110), (101), (116) and (024) planes of rhombohedral LaOF (JCPDS 06–0281). The result indicates that the crystal phase of the prepared LaOF nanoparticles is rhombohedral γ-phase. No impurity phases are observed so that Sm3+ and Eu3+ are doped into LaOF host, and the amount of Sm3+ and/or Eu3+ dopant has almost no effect on 2
Physica B: Condensed Matter 574 (2019) 311653
Z. Fu and B. Liu
Fig. 4. Schematic process of energy transfer and transitions in LaOF:Sm3+Eu3+ nanoparticles.
lattice can not be effectively excited by 442 nm laser. Therefore, it can be concluded that the characteristic emissions of Eu3+ from La0.98OF:0.01Sm3+-0.01Eu3+ nanoparticles is attributed to the energy transfer from Sm3+ to Eu3+ ions. The related transitions, concentrations of Sm3+ and/or Eu3+ ion and excitation wavelengths are also labeled in Fig. 3. 3.3. Schematic process of the energy transfer The Eu3+ emissions from LaOF:Sm3+-Eu3+ nanoparticles under 442 nm excitation due to its 5D0→7FJ (J = 0, 1, 2, 3 and 4) transitions. The process of energy transfer can be obtained through the analysis on the specific level structures and transitions of Sm3+ and Eu3+ ions. As shown in Fig. 4, the Sm3+ ions populated in the ground level 6H5/2 are pumped to the excited level 4G9/2 by 442 nm laser. Owing to the nearby 4 F3/2 level and other adjacent levels, the Sm3+ ions at 4G9/2 level will nonradiatively relax to the lower 4G5/2 level. Then part of ions at 4G5/2 level relaxes to the lower levels to produce the emissions centered at around 567 nm, 604 nm, 651 nm and 709 nm, respectively. At the same time, Sm3+ ions in 4G5/2 level can also transfer their energy to Eu3+ ions through the cross-relaxation process (Sm3+ 4G5/2→Eu3+ 5D0), because the 4G5/2 level of Sm3+ ion and the 5D1, 5D0 levels of Eu3+ ion are much closed [22,29]. Then the Eu3+ ions at 5D0 levels radiatively relax to the lower levels 7FJ (J = 0, 1, 2, 3 and 4) to produce 583 nm, 590 nm, 610 nm, 652 nm and 704 nm emissions, corresponding to the 5 D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 transitions, respectively. Since the emission intensities of the 5D0→7F3 (652 nm) transition of Eu3+ ion is weaker than others, it can hardly be observed in the emission spectrum (see curve d in Fig. 3).
Fig. 2. TEM images of (a) LaCO3F precursor and (b) LaOF nanoparticles.
3.4. Influence of the Eu3+ acceptor concentration on PL properties 3+
Fig. 3. PL emission spectra of Sm particles.
, Eu
3+
and Sm
3+
-Eu
3+
doped LaOF nano-
To explore the influence of the Eu3+ acceptor concentration on the efficiency of energy transfer from Sm3+ to Eu3+, the PL emission spectra of LaOF:Sm3+-Eu3+ nanoparticles with various Eu3+ doping concentrations were measured. The Sm3+ donor co-doping concentration was fixed at 1.0 mol%. Fig. 5 shows four typical emission spectra of La0.99-xOF:0.01Sm3+-xEu3+ (x = 0.005, 0.010, 0.020 and 0.030) nanoparticles under 442 nm excitation. It can be seen that the PL emission intensities at 610 nm (5D0→7F2) and 704 nm (5D0→7F4) gradually increase with increasing the Eu3+ concentration. The inset of Fig. 5 plots the PL emission intensities at Eu3+ (610 nm) and Sm3+ (604) as a function of Eu3+ doping concentration. The results indicate that the higher concentration of Eu3+ acceptor can benefit the efficiency of
from the excited 5D1 and 5D0 levels to the 7FJ (J = 0, 1, 2, 3, 4, 5 and 6) levels [28]. The positions are coincident with the characteristic emissions of Eu3+ under the 532 nm excitation (curve c). The new strong emissions in curve d locate at around 610 nm and 626 nm arising from 5 D0→7F2 transition, while 590 nm and 704 nm originate from 5D0→7F1 and 5D0→7F4 transition, respectively. For comparison, the PL emission spectrum of La0.99OF:0.01Eu3+ nanoparticle under 442 nm excitation is also presented (curve a), and its enlarged image (30 times) is shown in the inset of Fig. 3. It can be seen that the Eu3+ ions in LaOF crystal 3
Physica B: Condensed Matter 574 (2019) 311653
Z. Fu and B. Liu
Fig. 5. PL emission spectra of La0.99-xOF:0.01Sm3+-xEu3+ nanoparticles excited by 442 nm.
transfer energy from Sm3+ to Eu3+ ions. The reason is that the amount of Eu3+ acceptor around the Sm3+ donor increases with increasing the Eu3+ concentration, which brings an increase of the possibility of energy transfer from Sm3+ to Eu3+ ions. As a consequence, the increased transfer possibility leads to the enhancement of the Eu3+ emissions.
Fig. 6. PL emission spectra of Sm3+, Eu3+ sole- and co-doped LaOF nanoparticles excited by 532 nm or 442 nm.
La0.985OF:0.01Eu3+-0.005Sm3+ nanoparticles under 532 nm excitation, respectively. It can be seen that the intensity of Eu3+ red-light emission at 610 nm is significantly enhanced by introducing the Sm3+ co-dopant. Moreover, the PL emission intensity of Eu3+ red-light at 610 nm from La0.985OF:0.01Eu3+-0.005Sm3+ nanoparticles is almost two times stronger than that of La0.99OF:0.01Eu3+ nanoparticles. Particularly, the influence of the Sm3+ co-doping concentration on PL intensity of Eu3+ red-light emission at 610 nm was also investigated. It is found that the lower Sm3+ co-doping concentration has positive effect on the PL intensity when LaOF:Eu3+-Sm3+ nanoparticles are excited by 532 nm laser. Fig. 7 displays the emission spectra of La0.993+ -0.01Eu3+ nanoparticles under 532 nm excitation. It can be yOF:ySm seen that the intensity of red-light emission at 610 nm gradually increases before the Sm3+ co-doping concentration reaches 0.5 mol%, then continuous increase of Sm3+ co-doping concentration makes a decrease of the PL intensity. Therefore, the optimum Sm3+ co-doping concentration for the strongest enhancement of Eu3+-red PL emission is about 0.5 mol% in the La0.99-yOF:ySm3+-0.01Eu3+ nanoparticles under 532 nm excitation. The concentration dependence of Eu3+ red-light emission at 610 nm with Sm3+ co-dopant is presented in the inset of Fig. 7.
3.5. Enhancement of Eu3+ red luminescence by Sm3+ co-dopant Eu3+ is a typical hypersensitive doping ion that has been used in many luminescent materials. Its 5D0→7F1 transition (orange-light) is a pure magnetic dipole transition and is permitted in any crystal field, while the 5D0→7F2 transition (red-light) is a typical electric dipole one and can be nullified due to the selection rules derived from the f-f transition. Therefore, the 5D0→7F2 transition is hypersensitive to the local crystal field and the sit symmetry of Eu3+ ions. Consequently, the Eu3+ red-light emission can be significantly enhanced through changing its local symmetry. It has been reported that it is an efficient way of adjusting local symmetry by introducing some weak-fluorescent or non-fluorescent co-dopant in the host lattice [30]. To investigate the influence of Sm3+ co-dopant on the luminescence intensities of Eu3+ ions, the PL emission spectra of Sm3+, Eu3+ codoped and Eu3+ sole-doped LaOF nanoparticles were measured under the excitation at 532 nm. For comparison, the emission spectra of Sm3+ sole-doped LaOF nanoparticles under 532 nm and 442 nm excitations were also recorded, respectively. The results are shown in Fig. 6. During the measurements, we noticed that the profile of the emission spectrum of LaOF:Sm3+ nanoparticles excited by 532 nm laser differs from that by 442 nm. That is to say, different profiles of the emission spectra of LaOF:Sm3+ nanoparticles were observed under the different excitation wavelengths. Fig. 6 (a) and (b) give two emission spectra of La0.99OF:0.01Sm3+ nanoparticles under 532 nm and 442 nm excitation, respectively. This phenomenon illustrates that the profile of the emission spectrum of LaOF:Sm3+ nanoparticles can be affected by excitation wavelength. For Eu3+, Sm3+ co-doped LaOF nanoparticles, the characteristic emissions of Eu3+ ion are dominant when the samples excited by 532 nm laser. The result extremely differs from what have observed excited by 442 nm laser (see Fig. 5). Compared with the emission spectrum of Eu3+ sole-doped LaOF nanoparticles under 532 nm excitation, it can be found that all emissions in the spectrum of LaOF:Sm3+-Eu3+ nanoparticles under 442 nm excitation originate from Eu3+ ions. No obvious shift in the emission peaks occurs except the changes in PL intensity. This result indicates that no energy transfer from Eu3+ to Sm3+ occurs in LaOF:Eu3+-Sm3+ nanoparticles. Fig. 6 (c) and (d) present the emission spectra of La0.99OF:0.01Eu3+ and
Fig. 7. PL emission spectra of La0.99-yOF: ySm3+-0.01Eu3+ nanoparticles excited by 532 nm. 4
Physica B: Condensed Matter 574 (2019) 311653
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4. Conclusion
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In summary, Sm3+ and Eu3+ sole- and co-doped LaOF nanoparticles were synthesized via hydrothermal method followed the post-annealing treatment. Using 442 nm He–Cd laser as the excitation source to pump LaOF:Sm3+-Eu3+ nanoparticles, the characteristic emissions of Eu3+ ion were observed, indicating the energy transfer from Sm3+ to Eu3+ ions occurred in Sm3+-Eu3+ co-doped system. The luminescence properties, dynamical process of the energy transfer are investigated and analyzed in detail. The results show that the observed energy transfer from Sm3+ to Eu3+ ions in LaOF:Sm3+-Eu3+ nanoparticles is attributed to the cross-relaxation between the 4G5/2 level of Sm3+ ions and the levels 5D1 and 5D0 of Eu3+ ions. The PL intensities of Eu3+ emissions due to energy transfer effect increase with increasing the doping concentration of Eu3+ acceptor. The enhancement of Eu3+ redlight emission at 610 nm through the way of introducing Sm3+ ion was also observed when the LaOF:Sm3+-Eu3+nanoparticles were excited by 532 nm laser. The optimum concentration of Sm3+ co-dopant is about 0.5 mol% for the strongest enhancement of Eu3+-red luminescence in La0.99-yOF:ySm3+-0.01Eu3+ nanoparticles. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Hydrothermal synthesis, energy transfer and luminescence enhancement of rhombohedral LaOF: Sm3+-Eu3+ nanoparticles”. Acknowledgements We would like to acknowledge the support from the Natural Science Foundation of Ningxia Province (China) (2018AAC03237), the Project of First-class Disciplines in C&U (NXYLXK2017B11) of Ningxia Province (China), and the Project of Scientific Research of Ningxia Normal University (NXSFZDA1804, China). References [1] T.K. Pathak, A. Kumar, H.C. Swart, R.E. Kroon, Effect of annealing on structural and luminescence properties of Eu3+ doped NaYF4 phosphor, Physica B 535 (2018) 132–137. [2] H. Homayoni, S. Sahi, L. Ma, J. Zhang, J. Mohapatra, P. Liu, A.P. Sotelo, R.T. Macaluso, T. Davis, W. Chen, X-ray excited luminescence and persistent luminescence of Sr2MgSi2O7:Eu2+, Dy3+ and their associations with synthesis conditions, J. Lumin. 198 (2018) 132–137. [3] M. De, S. Sharma, S. Jana, Enhancement of 5D0→7F2 red emission of Eu3+ incorporated in lead sodium phosphate glass matrix, Physica B 556 (2019) 131–135. [4] Y. Yang, X. Zuo, S. Shi, J. Li, J. Wang, L. Geng, L. Fu, Hydrothermal combustion synthesis and characterization of Sr2CeO4 phosphor powders, Mater. Res. Bull. 112 (2019) 159–164. [5] S. Selvi, K. Marimuthu, G. Muralidharan, Structural and luminescence behavior of Sm3+ ions doped lead boro-telluro-phosphate glasses, J. Lumin. 159 (2015)
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Hydrothermal synthesis, energy transfer and luminescence enhancement of rhombohedral LaOF: Sm3+-Eu3+ nanoparticles
T
⁎
Zhenxing Fua,b, , Birui Liua,b a b
School of Physics and Electronic Information Engineering, Ningxia Normal University, Guyuan, 756000, China Engineering Technology Research Centre for Nano-structures and Functional Materials, Ningxia Normal University, Guyuan, 756000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: LaOF nanoparticles Energy transfer Luminescence enhancement Sm3+ Eu3+
Sm3+, Eu3+ sole- and co-doped LaOF nanoparticles with rhombohedral crystal structure were synthesized via hydrothermal method followed a post-annealing process. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence techniques. The XRD result shows that the samples exhibit a rhombohedral phase in crystal structure. For Sm3+, Eu3+ co-doped LaOF nanoparticles, pumping Sm3+ to its excited 4G9/2 level by 442 nm laser, the intense characteristic emissions of Eu3+ ion were observed, indicating that the energy transfer from Sm3+ to Eu3+ ions occurred. The intensities of Eu3+ emissions due to energy transfer increased with increasing the doping concentration of Eu3+ acceptor. The enhancement of Eu3+ red-light emission at 610 nm through co-doping Sm3+ ion was also observed when the Sm3+, Eu3+ co-doped LaOF nanoparticles excited by 532 nm laser. The optimum concentration of Sm3+ codopant is 0.5 mol% for enhancing the strongest luminescence of Eu3+ red-light.
1. Introduction Rare earth activated luminescent materials have been attracting much attention based on their applications in various fields. Among trivalent rare earth ions, Sm3+ and Eu3+ are more favorable ions for their special energy level structures and potential applications in LED, solid-state lighting, visible laser, red emitting phosphors, etc [1–5]. The excited 4G5/2, 4F3/2 and 4G7/2 levels of Sm3+ ion can produce excellent orange/red emissions in the visible region. In addition, the small spacing between 17860–28573 cm−1 energy levels of Sm3+ ion will lead to the ions in those levels quickly relax to 4G5/2 level through a nonradiative way. These properties make Sm3+ ion have wide applications in high density optical storage, underwater communication and color display, etc [6–8]. Similarly, for Eu3+ ion, the small energy level spacing above 5D0 level can also make the ions in those levels nonradiatively relax to 5D0 level to bring excellent red-light emissions in the visible region [9–12]. With the rising of a demand for LED, visible laser and red phosphors, Sm3+ and Eu3+ as two representative red luminescence activators have played more and more important roles in the development of luminescent materials [13–16]. Lanthanum oxyfluoride (LaOF) crystal has broad applications in the dielectric, optical, optoelectronics and photonics for its unique electrical and optical properties, and has been widely used in luminescent materials, plasma, catalyst, etc [17–21]. In recent years, the energy ⁎
transfer between Sm3+-Eu3+ ions and the luminescence enhancement of Eu3+ ions in different host have been extensively studied [22–27]. However, less report about the energy transfer from Sm3+ to Eu3+ and the luminescence enhancement of Eu3+ ion in rhombohedral LaOF host can be found. In this work, Sm3+, Eu3+ sole- and co-doped rhombohedral LaOF nanoparticles with well uniformity and dispersion were synthesized by the hydrothermal method followed by a post-annealing treatment. Using 442 nm laser to pump Sm3+ ions in LaOF:Sm3+-Eu3+ nanoparticles, the intense characteristic emissions of Eu3+ ion originating from the 5D0 level to the 7FJ (J = 0, 1, 2, 3 and 4) levels were observed. This phenomenon suggests that the energy transfer from Sm3+ to Eu3+ ions occurs in LaOF:Sm3+-Eu3+ nanoparticles. The dynamical process of energy transfer from Sm3+ to Eu3+ ions was investigated systematically. Then the influence of doping concentration of Eu3+ acceptor on energy transfer efficiency was discussed in detail. When the LaOF:Sm3+-Eu3+ nanoparticles excited by 532 nm laser, the enhancement of Eu3+ red-light emission by Sm3+ co-dopant was also observed. The enhancement efficiency of Eu3+ red-light emission depends on the concentration of Sm3+ donor.
Corresponding author. School of Physics and Electronic Information Engineering, Ningxia Normal University, Guyuan, 756000, China. E-mail address: [email protected] (Z. Fu).
https://doi.org/10.1016/j.physb.2019.08.030 Received 19 March 2019; Received in revised form 30 July 2019; Accepted 17 August 2019 Available online 19 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 574 (2019) 311653
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2. Experimental detail 2.1. Sample synthesis The La0.99OF:0.01Sm3+, La0.99OF:0.01Eu3+, La0.99-xOF:0.01Sm3+xEu3+ (x = 0.005, 0.010, 0.020 and 0.030) and La0.99-yOF:ySm3+0.01Eu3+ (y = 0.005, 0.010 and 0.015) nanoparticles were prepared by the hydrothermal method followed by a post-annealing treatment [17,18]. The starting chemicals included samarium oxide (Sm2O3, 99%), europium oxide (Eu2O3, 99%), sodium fluoride (NaF), sodium carbonate (Na2CO3), nitric acid (HNO3) and lanthanum nitric hexahydrate (La(NO3)3·6H2O). All the chemicals employed in the preparation were of analytical grade. Sm(NO3)3 and Eu(NO3)3 deionized water solution were obtained by reacting Sm2O3 and Eu2O3 with nitric acid, respectively. First, La(NO3)3·6H2O was weighed and dissolved with deionized water in a beaker at room temperature, and a clear solution of La(NO3)3 was obtained by stirring. Then the corresponding amount of Sm(NO3)3 and/or Eu(NO3)3 deionized water solution was dropped into the beaker according to the doping concentration of Sm3+ and/or Eu3+ ions. Ensuring the mixture was homogeneous by stirring for few minutes, an appropriate quantity of NaF and Na2CO3 was added into the mixture solution to obtain a slurry state. The mixed solution was stirred for 40–60 min. We transferred the solution to a Teflon-lined autoclave to conduct the reaction at 200 °C for 16 h. After cooling the autoclave to room temperature, the precipitate was separated by centrifugation and washed with deionized water and ethanol in sequence. Then the corresponding Sm3+, Eu3+ and Sm3+-Eu3+ doped LaCO3F precursors were obtained. The as-prepared LaCO3F crystals were ultrasonically treated with oleic acid and dried in air at 60 °C for 12 h. Finally, the rhombohedral LaOF nanoparticles were obtained by annealing the corresponding LaCO3F precursors at 600 °C for 4 h.
Fig. 1. XRD spectrum of La0.96OF:0.01Sm3+-0.03Eu3+ nanoparticles.
the LaOF phase composition. The average size of LaOF particles can be estimated according to the Scherrer formula, D = 0.941 λ / β cos θ , where D is the average size of particle, λ is the X-ray wavelength (0.15418 nm), θ and β are the diffraction angle and the full-width at half-maximum of the observed peak, respectively. The diffraction peak (012) at 2θ = 26.9° is used to calculate the average size of the LaOF particles. The estimated average size is about 60 nm. The surface morphology and crystalline sizes of the as-prepared LaCO3F precursor and the LaOF nanoparticles were observed by TEM. Two representative morphological images of as-prepared LaCO3F precursor and prepared LaOF sample are displayed in Fig. 2. It can be seen that all of the particles have similar surface morphology and the distribution of the crystalline sizes is uniform and homogeneous. The asprepared LaCO3F precursor is well-separated nanoparticles with sizes in the range of 40–60 nm as shown in Fig. 2 (a). The LaOF nanoparticles show rhombohedral structures with a high quality of monodispersity as presented in Fig. 2 (b). A morphological image of the LaOF nanoparticles is taken to elucidate the size and morphology of the samples. The average diameter of LaOF nanoparticles is about 60 nm. The result confirms that the mean size of LaOF nanoparticles obtained by TEM technique is consistent with what calculated by Scherrer formula.
2.2. Characterization The XRD spectra were obtained by using a Bruker D8 Advance automatic diffractometer (λ = 0.15418 nm, Germany) equipped with Cu Kα radiation at 40 kV and 40 mA. The morphology and particle size of the samples were preformed by a TEM (JEM-2100, TEOL Japan) operating at an acceleration voltage of 200 kV. The photoluminescence (PL) signal was collected with a monochromator (SP 2750i, Princeton Instrument, USA) equipped with a photomultiplier tube (ACTON, PD471) and a CCD system (ACTON, 7515-0002). A CW 532 nm solid state laser (MGL–FN–532, CNI, China) and a He–Cd 325/442 nm Dual Wavelength Laser (CW, IK5451R-E-SP, Kimmon, Japan) were employed as excitation sources. The same amount of each sample was used in every case, and all the measurements were conducted at room temperature.
3.2. PL analysis of energy transfer from Sm3+ to Eu3+ To investigate the energy transfer from Sm3+ to Eu3+ ions, the PL emission spectra of the LaOF:Sm3+-Eu3+ nanoparticles excited by 532 nm and 442 nm lasers were measured, respectively. During the measurements, we noticed that the Sm3+ ion in LaOF lattice can be effectively excited by 442 nm laser, while the Eu3+ ion in LaOF lattice can only be excited by 532 nm laser. Fig. 3 gives the PL emission spectra of the La0.99OF:0.01Sm3+, La0.99OF:0.01Eu3+ and La0.98OF:0.01Sm3+0.01Eu3+ nanoparticles under 532/442 nm excitation. The emission spectra of Sm3+ sole-doped and Sm3+, Eu3+ co-doped LaOF nanoparticles excited by 442 nm laser are described by curve b and d, respectively. It can be seen that the spectrum of La0.99OF:0.01Sm3+ consists of four broad emission bands in the range of 550–720 nm (curve b) centered at around 564 nm, 604 nm, 651 nm and 706 nm, corresponding to the Sm3+ transitions of 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4 G5/2 → 6H9/2 and 4G5/2 → 6H11/2, respectively. The strongest emission at 604 nm is attributed to 4G5/2 → 6H7/2 transition. Compared with the emission spectrum of LaOF:Sm3+-Eu3+ nanoparticles (curve d), four new emissions appeared in the spectra. The positions of the new strong emissions in curve d locate at around 590 nm, 610 nm, 626 nm and 704 nm. It is known that Eu3+ ion generally presents the characteristic emissions in the region of 550–840 nm corresponding to the transitions
3. Result and discussion 3.1. Structure, size, and morphology It is known that three different crystal phases of LaOF have been discovered and recognized. The samples synthesized by the post-annealing treatment of LaCO3F precursor at 600 °C for 4 h have rhombohedral crystal phase. A representative XRD pattern of LaOF nanoparticles is shown in Fig. 1. The doping concentration of Sm3+ and Eu3+ ions are 1.0 mol% and 3.0 mol%, respectively. As can be seen from Fig. 1, the diffraction peaks located at 26.5°, 26.9°, 31.0°, 44.1°, 44.8°, 52.3°, 52.8° and 55.5° correspond to the (006), (012), (104), (018), (110), (101), (116) and (024) planes of rhombohedral LaOF (JCPDS 06–0281). The result indicates that the crystal phase of the prepared LaOF nanoparticles is rhombohedral γ-phase. No impurity phases are observed so that Sm3+ and Eu3+ are doped into LaOF host, and the amount of Sm3+ and/or Eu3+ dopant has almost no effect on 2
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Fig. 4. Schematic process of energy transfer and transitions in LaOF:Sm3+Eu3+ nanoparticles.
lattice can not be effectively excited by 442 nm laser. Therefore, it can be concluded that the characteristic emissions of Eu3+ from La0.98OF:0.01Sm3+-0.01Eu3+ nanoparticles is attributed to the energy transfer from Sm3+ to Eu3+ ions. The related transitions, concentrations of Sm3+ and/or Eu3+ ion and excitation wavelengths are also labeled in Fig. 3. 3.3. Schematic process of the energy transfer The Eu3+ emissions from LaOF:Sm3+-Eu3+ nanoparticles under 442 nm excitation due to its 5D0→7FJ (J = 0, 1, 2, 3 and 4) transitions. The process of energy transfer can be obtained through the analysis on the specific level structures and transitions of Sm3+ and Eu3+ ions. As shown in Fig. 4, the Sm3+ ions populated in the ground level 6H5/2 are pumped to the excited level 4G9/2 by 442 nm laser. Owing to the nearby 4 F3/2 level and other adjacent levels, the Sm3+ ions at 4G9/2 level will nonradiatively relax to the lower 4G5/2 level. Then part of ions at 4G5/2 level relaxes to the lower levels to produce the emissions centered at around 567 nm, 604 nm, 651 nm and 709 nm, respectively. At the same time, Sm3+ ions in 4G5/2 level can also transfer their energy to Eu3+ ions through the cross-relaxation process (Sm3+ 4G5/2→Eu3+ 5D0), because the 4G5/2 level of Sm3+ ion and the 5D1, 5D0 levels of Eu3+ ion are much closed [22,29]. Then the Eu3+ ions at 5D0 levels radiatively relax to the lower levels 7FJ (J = 0, 1, 2, 3 and 4) to produce 583 nm, 590 nm, 610 nm, 652 nm and 704 nm emissions, corresponding to the 5 D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 transitions, respectively. Since the emission intensities of the 5D0→7F3 (652 nm) transition of Eu3+ ion is weaker than others, it can hardly be observed in the emission spectrum (see curve d in Fig. 3).
Fig. 2. TEM images of (a) LaCO3F precursor and (b) LaOF nanoparticles.
3.4. Influence of the Eu3+ acceptor concentration on PL properties 3+
Fig. 3. PL emission spectra of Sm particles.
, Eu
3+
and Sm
3+
-Eu
3+
doped LaOF nano-
To explore the influence of the Eu3+ acceptor concentration on the efficiency of energy transfer from Sm3+ to Eu3+, the PL emission spectra of LaOF:Sm3+-Eu3+ nanoparticles with various Eu3+ doping concentrations were measured. The Sm3+ donor co-doping concentration was fixed at 1.0 mol%. Fig. 5 shows four typical emission spectra of La0.99-xOF:0.01Sm3+-xEu3+ (x = 0.005, 0.010, 0.020 and 0.030) nanoparticles under 442 nm excitation. It can be seen that the PL emission intensities at 610 nm (5D0→7F2) and 704 nm (5D0→7F4) gradually increase with increasing the Eu3+ concentration. The inset of Fig. 5 plots the PL emission intensities at Eu3+ (610 nm) and Sm3+ (604) as a function of Eu3+ doping concentration. The results indicate that the higher concentration of Eu3+ acceptor can benefit the efficiency of
from the excited 5D1 and 5D0 levels to the 7FJ (J = 0, 1, 2, 3, 4, 5 and 6) levels [28]. The positions are coincident with the characteristic emissions of Eu3+ under the 532 nm excitation (curve c). The new strong emissions in curve d locate at around 610 nm and 626 nm arising from 5 D0→7F2 transition, while 590 nm and 704 nm originate from 5D0→7F1 and 5D0→7F4 transition, respectively. For comparison, the PL emission spectrum of La0.99OF:0.01Eu3+ nanoparticle under 442 nm excitation is also presented (curve a), and its enlarged image (30 times) is shown in the inset of Fig. 3. It can be seen that the Eu3+ ions in LaOF crystal 3
Physica B: Condensed Matter 574 (2019) 311653
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Fig. 5. PL emission spectra of La0.99-xOF:0.01Sm3+-xEu3+ nanoparticles excited by 442 nm.
transfer energy from Sm3+ to Eu3+ ions. The reason is that the amount of Eu3+ acceptor around the Sm3+ donor increases with increasing the Eu3+ concentration, which brings an increase of the possibility of energy transfer from Sm3+ to Eu3+ ions. As a consequence, the increased transfer possibility leads to the enhancement of the Eu3+ emissions.
Fig. 6. PL emission spectra of Sm3+, Eu3+ sole- and co-doped LaOF nanoparticles excited by 532 nm or 442 nm.
La0.985OF:0.01Eu3+-0.005Sm3+ nanoparticles under 532 nm excitation, respectively. It can be seen that the intensity of Eu3+ red-light emission at 610 nm is significantly enhanced by introducing the Sm3+ co-dopant. Moreover, the PL emission intensity of Eu3+ red-light at 610 nm from La0.985OF:0.01Eu3+-0.005Sm3+ nanoparticles is almost two times stronger than that of La0.99OF:0.01Eu3+ nanoparticles. Particularly, the influence of the Sm3+ co-doping concentration on PL intensity of Eu3+ red-light emission at 610 nm was also investigated. It is found that the lower Sm3+ co-doping concentration has positive effect on the PL intensity when LaOF:Eu3+-Sm3+ nanoparticles are excited by 532 nm laser. Fig. 7 displays the emission spectra of La0.993+ -0.01Eu3+ nanoparticles under 532 nm excitation. It can be yOF:ySm seen that the intensity of red-light emission at 610 nm gradually increases before the Sm3+ co-doping concentration reaches 0.5 mol%, then continuous increase of Sm3+ co-doping concentration makes a decrease of the PL intensity. Therefore, the optimum Sm3+ co-doping concentration for the strongest enhancement of Eu3+-red PL emission is about 0.5 mol% in the La0.99-yOF:ySm3+-0.01Eu3+ nanoparticles under 532 nm excitation. The concentration dependence of Eu3+ red-light emission at 610 nm with Sm3+ co-dopant is presented in the inset of Fig. 7.
3.5. Enhancement of Eu3+ red luminescence by Sm3+ co-dopant Eu3+ is a typical hypersensitive doping ion that has been used in many luminescent materials. Its 5D0→7F1 transition (orange-light) is a pure magnetic dipole transition and is permitted in any crystal field, while the 5D0→7F2 transition (red-light) is a typical electric dipole one and can be nullified due to the selection rules derived from the f-f transition. Therefore, the 5D0→7F2 transition is hypersensitive to the local crystal field and the sit symmetry of Eu3+ ions. Consequently, the Eu3+ red-light emission can be significantly enhanced through changing its local symmetry. It has been reported that it is an efficient way of adjusting local symmetry by introducing some weak-fluorescent or non-fluorescent co-dopant in the host lattice [30]. To investigate the influence of Sm3+ co-dopant on the luminescence intensities of Eu3+ ions, the PL emission spectra of Sm3+, Eu3+ codoped and Eu3+ sole-doped LaOF nanoparticles were measured under the excitation at 532 nm. For comparison, the emission spectra of Sm3+ sole-doped LaOF nanoparticles under 532 nm and 442 nm excitations were also recorded, respectively. The results are shown in Fig. 6. During the measurements, we noticed that the profile of the emission spectrum of LaOF:Sm3+ nanoparticles excited by 532 nm laser differs from that by 442 nm. That is to say, different profiles of the emission spectra of LaOF:Sm3+ nanoparticles were observed under the different excitation wavelengths. Fig. 6 (a) and (b) give two emission spectra of La0.99OF:0.01Sm3+ nanoparticles under 532 nm and 442 nm excitation, respectively. This phenomenon illustrates that the profile of the emission spectrum of LaOF:Sm3+ nanoparticles can be affected by excitation wavelength. For Eu3+, Sm3+ co-doped LaOF nanoparticles, the characteristic emissions of Eu3+ ion are dominant when the samples excited by 532 nm laser. The result extremely differs from what have observed excited by 442 nm laser (see Fig. 5). Compared with the emission spectrum of Eu3+ sole-doped LaOF nanoparticles under 532 nm excitation, it can be found that all emissions in the spectrum of LaOF:Sm3+-Eu3+ nanoparticles under 442 nm excitation originate from Eu3+ ions. No obvious shift in the emission peaks occurs except the changes in PL intensity. This result indicates that no energy transfer from Eu3+ to Sm3+ occurs in LaOF:Eu3+-Sm3+ nanoparticles. Fig. 6 (c) and (d) present the emission spectra of La0.99OF:0.01Eu3+ and
Fig. 7. PL emission spectra of La0.99-yOF: ySm3+-0.01Eu3+ nanoparticles excited by 532 nm. 4
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4. Conclusion
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In summary, Sm3+ and Eu3+ sole- and co-doped LaOF nanoparticles were synthesized via hydrothermal method followed the post-annealing treatment. Using 442 nm He–Cd laser as the excitation source to pump LaOF:Sm3+-Eu3+ nanoparticles, the characteristic emissions of Eu3+ ion were observed, indicating the energy transfer from Sm3+ to Eu3+ ions occurred in Sm3+-Eu3+ co-doped system. The luminescence properties, dynamical process of the energy transfer are investigated and analyzed in detail. The results show that the observed energy transfer from Sm3+ to Eu3+ ions in LaOF:Sm3+-Eu3+ nanoparticles is attributed to the cross-relaxation between the 4G5/2 level of Sm3+ ions and the levels 5D1 and 5D0 of Eu3+ ions. The PL intensities of Eu3+ emissions due to energy transfer effect increase with increasing the doping concentration of Eu3+ acceptor. The enhancement of Eu3+ redlight emission at 610 nm through the way of introducing Sm3+ ion was also observed when the LaOF:Sm3+-Eu3+nanoparticles were excited by 532 nm laser. The optimum concentration of Sm3+ co-dopant is about 0.5 mol% for the strongest enhancement of Eu3+-red luminescence in La0.99-yOF:ySm3+-0.01Eu3+ nanoparticles. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Hydrothermal synthesis, energy transfer and luminescence enhancement of rhombohedral LaOF: Sm3+-Eu3+ nanoparticles”. Acknowledgements We would like to acknowledge the support from the Natural Science Foundation of Ningxia Province (China) (2018AAC03237), the Project of First-class Disciplines in C&U (NXYLXK2017B11) of Ningxia Province (China), and the Project of Scientific Research of Ningxia Normal University (NXSFZDA1804, China). References [1] T.K. Pathak, A. Kumar, H.C. Swart, R.E. Kroon, Effect of annealing on structural and luminescence properties of Eu3+ doped NaYF4 phosphor, Physica B 535 (2018) 132–137. [2] H. Homayoni, S. Sahi, L. Ma, J. Zhang, J. Mohapatra, P. Liu, A.P. Sotelo, R.T. Macaluso, T. Davis, W. Chen, X-ray excited luminescence and persistent luminescence of Sr2MgSi2O7:Eu2+, Dy3+ and their associations with synthesis conditions, J. Lumin. 198 (2018) 132–137. [3] M. De, S. Sharma, S. Jana, Enhancement of 5D0→7F2 red emission of Eu3+ incorporated in lead sodium phosphate glass matrix, Physica B 556 (2019) 131–135. [4] Y. Yang, X. Zuo, S. Shi, J. Li, J. Wang, L. Geng, L. Fu, Hydrothermal combustion synthesis and characterization of Sr2CeO4 phosphor powders, Mater. Res. Bull. 112 (2019) 159–164. [5] S. Selvi, K. Marimuthu, G. Muralidharan, Structural and luminescence behavior of Sm3+ ions doped lead boro-telluro-phosphate glasses, J. Lumin. 159 (2015)
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