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Er3+ nanocrystals by Mg2+ doping
Journal of Alloys and Compounds 694 (2017) 241e245
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing single red band upconversion luminescence of KMnF3:Yb3þ/Er3þ nanocrystals by Mg2þ doping Zhangyu Huang a, b, Mengji Yi a, b, Huiping Gao a, b, Zhenlong Zhang a, b, Yanli Mao a, b, * a b
School of Physics and Electronics, Henan University, Kaifeng 475004, China Institute for Computational Materials Science, Henan University, Kaifeng 475004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2016 Received in revised form 6 September 2016 Accepted 30 September 2016 Available online 30 September 2016
In this work, monodisperse Mg2þ doped KMnF3:Yb3þ/Er3þ nanoparticles (NPs) are prepared via a fast solvothermal method. With the excitation of 980 nm laser diode, a brightly single red upconversion (UC) luminescence can be observed from KMnF3:Yb3þ/Er3þ NPs, which is further improved by introducing Mg2þ ions. Moreover, the single red UC fluorescence intensity of the KMnF3:Yb3þ/Er3þ NPs co-doped with 30 mol% Mg2þ ions is enhanced by 20.8 times compared to that of Mg2þ ions-absent sample. Besides, the TEM measurement of the 30 mol% Mg2þ doped KMnF3:Yb3þ/Er3þ NPs shows a biocompatible ultra-small crystal size (about 10 nm). All results indicate that Mg2þ-doped is an effective method to significantly improve the UC single-red light of KMnF3:Yb3þ/Er3þ, which shows great potential application in vivo bioimaging. © 2016 Elsevier B.V. All rights reserved.
Keywords: Yb3þ/Er3þ upconversion Monodisperse ultra-small KMnF3 nanoparticles Single red band upconversion luminescence enhancement
1. Introduction Lanthanide-doped upconversion nanoparticles (UCNPs) are well known for their biological applications due to their unique optical properties, resulting from the effective absorption of low-energy photons followed by emitting high-energy photons [1e4]. The visible red region (600e700 nm) and near-infrared (NIR) spectral range (700e1100 nm) are generally addressed as the “optical window” of the biological tissues, owing to the lack of efficient endogenous absorbers [5,6]. However, each lanthanide ion has a unique set of energy levels and generally exhibits multiple emission peaks. For the most commonly used Er3þ ions, the mainly emission band is in the green region from 520 nm to 570 nm [7]. Therefore, for in vivo deeper tissue imaging bioapplication, it is important to bring the excitation and emission peaks into the “optical window”. An attractive approach to this problem involves the use of a transition metal ion (Mn2þ), which can promote the efficient nonradiative energy transfer from the related levels of disappeared emissions of Er3þ to the 4T1 level of Mn2þ, followed by a backenergy transfer to the level of single-band UC emission [8]. Recently, KMnF3 nanocrystals have been identified as an
* Corresponding author. School of Physics and Electronics, Henan University, Kaifeng 475004, China. E-mail address: [email protected] (Y. Mao). http://dx.doi.org/10.1016/j.jallcom.2016.09.322 0925-8388/© 2016 Elsevier B.V. All rights reserved.
excellent host material for single red or NIR band UC emission. Since substantially high red and NIR emission can be obtained respectively when the Yb3þ/Er3þ, Yb3þ/Ho3þ, or Yb3þ/Tm3þ ions are introduced into KMnF3 NPs [9]. Moreover, in addition to the outstanding optical performance, KMnF3 UCNPs also shows a temperature-dependent and paramagnetism properties [10]. Besides, via hydrothermal and solventthermal method, a tiny size KMnF3 UCNPs can be easily obtained. All these properties make the KMnF3 NPs to be a popular UC material. However, the low UC luminescence efficiency is still the main drawback which restricts the application of UCNPs. So far, several methods are used to enhance the UC emission intensity of KMnF3:Yb3þ/Er3þ. To the best of our knowledge, the most used way for the enhancement is based on the fabrication of the core/shell structure, or the increase of the NPs size [10,11]. Herein, we propose a simple way to improve the single red band UC luminescence intensity of KMnF3:Yb3þ/Er3þ by introducing Mg2þ ions into the initial reaction solution. The experimental results of the UC luminescence intensity spectrum and the UC luminescence decay curves show the upconversion fluorescence intensity changes along with the variation of the doping concentration of Mg2þ ions. With 30 mol% Mg2þ ions doped, a maximum intensity of the UC emission intensity can be obtained. By calculating, the red UC luminescence intensity in 30 mol% Mg2þ ions doped sample enhance 20.8 times than that of Mg2þ ions-absent one. In addition,
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the TEM image of the in 30 mol% Mg2þ ions doped nanoparticles shows a monodisperse ultrasmall size (10 nm) nanocubes, which is deemed to an appropriate size for bio-imaging. All the results indicates that Mg2þ-doping is an effective method to significantly improve the UC single-red light. 2. Experimental 2.1. Materials All chemicals are of analytical grade and are used as received without further purification. Yb(NO3)3$5H2O, Er(NO3)3$5H2O, KF, Mn(CH3COO)2$4H2O, Mg(CH3COO)2$4H2O, 1-octadecene (ODE), oleic acid (OA), oleylamine (OM), cyclohexane and absolute ethyl alcohol are all supplied by Sinopharm Chemical Reagent Company. 2.2. Synthesis of KMnF3:Yb3þ/Er3þ NCs by introducing different Mg2þ concentrations in the initial solution The KMnF3:Yb3þ/Er3þ NPs with different Mg2þ concentrations in the initial solution are prepared in three steps according to a modified literature procedure [12]. Mn(CH3COO)2$4H2O (0.495 mmol - x), Mg(CH3COO)2$4H2O (x, x ¼ 0, 0.05, 0.10, 0.15 and 0.20 mmol), Yb(NO3)3$5H2O (0.0045 mmol), and Er(NO3)3$5H2O (0.0005 mmol) are weighted and added into a 50 mL three-necked flask containing 5 mL of OA, 5 mL of OM, and 15 mL of ODE. The mixture is strongly stirred and heated at 150 C for 30 min to form a rufous clear solution, and then cooled down to room temperature. Afterwards, 1.5 mL of a methanol solution containing 1.5 mmol of KF is dropped into the above solution and continuous stirred at 70 C for 30 min. After all the methanol evaporated, the solution was heated at 120 C for 5 min for further ensure all water and methanol are evaporated, and finally heated at 280 C under N2 for 60 min and then cooled down to room temperature. The products are precipitated by the addition of ethanol, collected by centrifugation, washed with cyclohexane and ethanol three times, and finally dried at 60 C. 2.3. Sample characterization The phase identification of the crystal structure is examined by X-ray diffraction (XRD) using Philips X Pert Pro X-ray diffractometer with Cu Ka radiation. The nanostructure and morphology are characterized by transmission electron microscopy (TEM) (JEOL JEM-2100., JP). The energy dispersive X ray analysis spectrum is measured by field emission scanning electron microscopy (FESEM) (HITACHI S4700., JP). The absorption spectra is measured by the Cary Series UV-Vis-NIR Spectrophotometer (Agilent Technologies., USA). The UC luminescence spectrum, and UC luminescence decay curves are measured via the FLS980E spectrometer (Edinburgh Instruments Ltd., UK). 3. Results and discussion Fig. 1 shows X-ray diffraction patterns (XRD) of the as-prepared KMnF3:0.9 mol% Yb3þ, 0.1 mol% Er3þ ions and 0e40 mol% of Mg2þ ions. All the diffraction peaks match well with the cubic phase KMnF3 (JCPDS 17e0116). No other impurity peaks are detected, which reveals that pure cubic phase KMnF3 has been fabricated. It also indicates that the Mg2þ doping has no influence on the crystalline phase of the KMnF3 nanocrystals. In comparison with that of KMnF3:Yb3þ/Er3þ nanoparticles, all diffraction peaks of KMnF3:Yb3þ/Er3þ/Mg2þ show a shift towards larger angles as the concentration of Mg2þ ions increases up to 40 mol%. The peak shifting indicates that Mg2þ ions can be doped into the host lattice
Fig. 1. XRD patterns of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) obtained after various doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). The dash line is shown to guide to eye.
through the substitution or occupation of the interstitial sites and cause the shrink of the unit cell, due to the small radii of Mg2þ (0.72 Å) than that of Mn2þ (0.80 Å). When varying the content of Mg2þ from 0% to 40%, we find the size decreases, and the mean particle size of these NPs calculated by Scherrer formula is about 31.3, 29.6, 16.9, 12.5 and 10.8 nm, respectively. The incorporation of Mg2þ ions in to KMnF3:Yb3þ/Er3þ NPs can be further proved by EDXA spectrum. As shown in Fig. 2(a), the EDXA spectrum of the KMnF3:Yb3þ/Er3þ/Mg2þ (0.9/0.1/30 mol%) NPs exhibits all of the elements including F, Mn, K, Mg, Yb, and Er, which further demonstrate the Mg2þ ions are existed in the final NPs products. The TEM image of the prepared KMnF3:Yb3þ/Er3þ NPs with 30 mol % Mg2þ doping is shown in Fig. 2(b). It is found that the assynthesized nanocrystals show a cubic morphology with a uniform size of 10 nm, which is correspond to the results calculated by XRD patterns through Scherrer equation. Due to the existence of organics oleic acid on the surface of the nanocrystals, some nanoparticles show a black cube, besides, there are some black mushy substance on the edge of the samples. Fig. 3 shows the absorption spectra of the as-synthesized Yb3þ/ Er3þ co-doped KMnF3 nanoparticles. As is shown in Fig. 3, the absorption peak at 363 nm, 380 nm, 448 nm, 488 nm, 520 nm, 650 nm and 800 nm should be assigned to 4I15/2 to 2G7/2, 4G11/2, 4F5/2, 4F7/2, 2 H11/2, 4S3/2, 4F9/2 and 4I9/2 transitions of Er3þ. The absorption in the (400e600 nm) and NIR wavelength band (900e1100 nm) should be owing to 6A1 / 4T1 of Mn2þ and 2F7/2 / 2F5/2 of Yb3þ respectively. It indicates that the Yb3þ/Er3þ ions are successfully doped in to the lattices, and the Manganese ions in the nanocrystals are pure valence state of Mn2þ. Fig. 4(a) shows the upconversion luminescence spectra of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) nanocrystals doped with different concentrations of Mg2þ ions. Upon excitation at the same power density 980 nm laser diode, all the samples exhibit an obviously single red band emission in the range of 640e690 nm which is assigned to the 4F9/2 / 4I15/2 transitions of Er3þ. The single red band UC emission can be ascribed to the nonradiative energy transfer from the 4F7/2, 2H11/2, and 4S3/2 levels of Er3þ to the 4T1 level of Mn2þ, followed by back-energy transfer to the 4F9/2 level of Er3þ (Fig. 4(b)). The complete disappearance of blue and green emissions of Er3þ suggests an extremely efficient exchange-energy transfer process between the Er3þ and Mn2þ ions, which can be largely attributed to the close proximity and effective mixing of wave functions of the Er3þ and Mn2þ ions in the crystal host lattices [13]. Fig. 4(a) also shows that the intensities of the single red band emissions are drastically changed when the Mg2þ ions are doped in the nanocrystals. It means that the Mg2þ ions concentration have a
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Fig. 2. (a) EDXA spectrum of KMnF3:Yb3þ/Er3þ NPs doped with 30 mol% Mg2þ, (b) TEM image of the as-synthesized KMnF3:Yb3þ/Er3þ/Mg2þ (0.9/0.1/30 mol%) nanocyrstals.
IðtÞ ¼ I0 þ A1 expð ðt t1 Þ=t1 Þ þ A2 expð ðt t1 Þ=t2 Þ
(1)
where I(t) and I0 are the luminescence intensity at time t and 0 ms, respectively. A1 and A2 are fitting constants, t is the time, t1 is the time of the maximum intensity of the transient UC luminescence spectrum, and t1, t2 are the rapid and slow lifetimes for the exponents, respectively. Such biexponential decay behavior might be resulted from the nonradiative transition among Yb3þ - Mn2þ and Er3þ - Mn2þ pairs, or the energy transfer between Yb3þ - Mn2þ and Er3þ - Mn2þ pairs and isolated Mn2þ, Er3þ, or Yb3þ ions and the defects introduced by Lanthanide (Ln3þ) due to the mismatched valence states between Ln3þ and Kþ or Mn2þ. For biexponential decay behavior, the average lifetimes t can be calculated as follows: Fig. 3. The absorption spectra of the as-synthesized KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) nanocyrstals.
huge effect on the UC luminescence intensity. When the doping concentration of Mg2þ increases from 0 to 30 mol%, the UC fluorescence intensity shows a distinctly monotonous enhancement. Subsequently, the UC luminescence intensity appears a decrease tendency in the case of over 30 mol% doping concentration of Mg2þ ions. The strongest UC emission intensity is observed in the sample with a Mg2þ concentration of 30 mol%. By calculating, the UC luminescence intensity of 30 mol% Mg2þ doped sample enhances about 20.8 times than that of Mg2þ-absent one. To visualize the UC intensification, photographs of UC luminescence for solutions with the corresponding Mg2þ doped nanoparticles (0.03 g in 2 mL cyclohexane and ethanol mixed solution with the volume ratio of 1:1) excited using a 980 nm laser diode (pumping power: 0.5 mW) are shown in Fig. 4(c). Obviously, the red emission of the five samples and the changes in the emission intensity can be clearly discerned by the naked eye. In addition, the luminescence intensity of 30 mol% Mg2þ doped sample is the strongest, which agrees with the results in Fig. 4(a). To get more information about the UC emission of Er3þ, the transient UC luminescence spectrum of Er3þ:4F9/2 in as-synthesized UCNPs are measured under excitation with 980 nm pulsed laser. As shown in Fig. 5(a), the falling edge of the transient UC luminescence spectrum of Er3þ:4F9/2 in all samples can be well fitted into a double exponential function as formula (1).
t¼
A1 t21 þ A2 t22 A1 t1 þ A2 t2
(2)
The average lifetime of the Er3þ:4F9/2 level increases from 108.3 ms to 692.1 ms with increasing the doping concentration of Mg2þ from 0 mol% to 30 mol% (Fig. 4(b)). It may be attributed to the increasing Mg2þ ions doping lead to the decrease of the amount of Mn2þ ions, which weakens the nonradiative energy transfer among Yb3þ/Er3þ/Mn2þ, in contrast, the ratio of radiative energy transfer increases, as a result, a longer lifetime can be detected in Mg2þ doped samples than that of Mg2þ-absent one. For UC materials, a long lifetime usually means a highly-efficient UC luminescence [14e16]. As revealed in Fig. 5(a), the trend of lifetime variation is consistent with that of UC intensity variation in Fig. 4(a). However, when the Mg2þ doping amount increases up to 40 mol%, the average lifetime of the Er3þ:4F9/2 level decreases to 418.2 ms (Fig. 4(b)). It may ascribe to the superfluous Mg2þ doping lead to the decreasing of the amount of Mn2þ ions surrounding of Ln3þ, which result in the decrease of the energy transfer rate of Yb3þ / Mn2þ / Er3þ. Besides, the change of the UC luminescence intensity may also ascribe to the variation of the crystal symmetry and the crystal lattice. It is well known that the UC luminescence intensity of Er3þdoped NPs depends on its 4f transition probabilities, which are affected significantly by the local crystal field symmetry of the Er3þ [17,18]. Substituting the Mn2þ ions by the smaller radii Mg2þ ions will cause the host lattice of KMnF3 to shrink, which results in a decrease of the average lengths between the rare earth and the fluorine ions. The asymmetric surrounding environment of Ln3þ
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Fig. 4. (a) UC luminescence spectra of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs with different doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). (b) Energy-level diagram of KMnF3:Yb3þ,Er3þ NPs and possible upconversion energy transfer processes under excitation of a 980 nm laser diode. (c) the corresponding luminescence photographs of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs doped with different amount Mg2þ (0, 10, 20, 30, 40 mol%).
Fig. 5. (a) The transient UC luminescence spectrum of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs with different doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). (b) UC decay time (lifetime) variation depends on the Mg2þ doping concentrations in the KMnF3:Yb3þ/Er3þ NPs.
favors hypersensitive transitions, subsequently increasing the UC luminescence. However, the superfluous dopant Mg2þ induces excessive distortion of the lattice, which affects the spatial distribution of the Ln3þ, inducing a concentration quenching and reducing the emission intensity [19,20].
the UCNPs can be significantly enhanced by Mg2þ ion doping. And a 20.8-fold UC luminescence intensity increase is observed in the 30 mol% Mg2þ doped sample than that of KMnF3:Yb3þ/Er3þ sample. All these results indicates our work is useful not only for theoretical studies on UCNPs but also for extending their applications.
4. Conclusions
Acknowledgements
In summary, KMnF3:Yb3þ/Er3þ UCNPs doped with different amounts of Mg2þ (0e40 mol%) are synthesized by a facile solvothermal method to modulate upconversion luminescence. The as-synthesized 30 mol% Mg2þ doped nanocrystals show a cubic morphology with a uniform size of 10 nm. The UC luminescence measurement results show that the UC luminescence intensity of
This work is supported by the Program for Science and Technology Innovation Talents in Universities of Henan Province (No. 16HASTIT043), the Science and Technology Research Project of Henan Province (No. 142102210389), the National Science Foundation of China (Grant No. 21103043) and the Program for Innovative Research Team (in Science and Technology) in University of
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Henan Province (No. 13IRTSTHN017). References [1] F. Wang, X. Liu, Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles, J. Am. Chem. Soc. 130 (2008) 5642e5643. [2] J. Zhao, D. Jin, E.P. Schartner, et al., Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence, Nat. Nanotechnol. 8 (2013) 729e734. [3] F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 139e174. [4] F. Wang, Y. Han, C.S. Lim, et al., Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping nature, Nat. Lett. 463 (2010) 1061e1065. [5] S. Zeng, Z. Yi, W. Lu, et al., Simultaneous realization of phase/size manipulation, upconversion luminescence enhancement, and blood vessel imaging in multifunctional nanoprobes through transition metal mn2þ doping, Adv. Funct. Mater. 24 (2014), 4196e4196. [6] Z. Bai, H. Lin, K. Imakita, et al., Synthesis of Er3þ/Yb3þ codoped NaMnF3 nanocubes with single-band red upconversion luminescence, Rsc Adv. 4 (2014) 61891e61897. [7] K. Zheng, W. Song, G. He, et al., Five-photon uv upconversion emissions of Er3þ for temperature sensing, Opt. Express 23 (2015) 7653e7658. [8] G. Tian, Z. Gu, L. Zhou, et al., Mn2þ dopant-controlled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery, Adv. Mater. 24 (2012) 1226e1231. [9] J. Wang, F. Wang, C. Wang, et al., Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals, Angew. Chem. Int. Ed. 50 (2011) 10369e10372. [10] L. Lei, J. Zhou, J. Zhang, et al., The use of zinc ions to control the size of Yb/Er: KMnF3 nanocrystals with single band emission, CrystEngComm 44 (2015)
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing single red band upconversion luminescence of KMnF3:Yb3þ/Er3þ nanocrystals by Mg2þ doping Zhangyu Huang a, b, Mengji Yi a, b, Huiping Gao a, b, Zhenlong Zhang a, b, Yanli Mao a, b, * a b
School of Physics and Electronics, Henan University, Kaifeng 475004, China Institute for Computational Materials Science, Henan University, Kaifeng 475004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2016 Received in revised form 6 September 2016 Accepted 30 September 2016 Available online 30 September 2016
In this work, monodisperse Mg2þ doped KMnF3:Yb3þ/Er3þ nanoparticles (NPs) are prepared via a fast solvothermal method. With the excitation of 980 nm laser diode, a brightly single red upconversion (UC) luminescence can be observed from KMnF3:Yb3þ/Er3þ NPs, which is further improved by introducing Mg2þ ions. Moreover, the single red UC fluorescence intensity of the KMnF3:Yb3þ/Er3þ NPs co-doped with 30 mol% Mg2þ ions is enhanced by 20.8 times compared to that of Mg2þ ions-absent sample. Besides, the TEM measurement of the 30 mol% Mg2þ doped KMnF3:Yb3þ/Er3þ NPs shows a biocompatible ultra-small crystal size (about 10 nm). All results indicate that Mg2þ-doped is an effective method to significantly improve the UC single-red light of KMnF3:Yb3þ/Er3þ, which shows great potential application in vivo bioimaging. © 2016 Elsevier B.V. All rights reserved.
Keywords: Yb3þ/Er3þ upconversion Monodisperse ultra-small KMnF3 nanoparticles Single red band upconversion luminescence enhancement
1. Introduction Lanthanide-doped upconversion nanoparticles (UCNPs) are well known for their biological applications due to their unique optical properties, resulting from the effective absorption of low-energy photons followed by emitting high-energy photons [1e4]. The visible red region (600e700 nm) and near-infrared (NIR) spectral range (700e1100 nm) are generally addressed as the “optical window” of the biological tissues, owing to the lack of efficient endogenous absorbers [5,6]. However, each lanthanide ion has a unique set of energy levels and generally exhibits multiple emission peaks. For the most commonly used Er3þ ions, the mainly emission band is in the green region from 520 nm to 570 nm [7]. Therefore, for in vivo deeper tissue imaging bioapplication, it is important to bring the excitation and emission peaks into the “optical window”. An attractive approach to this problem involves the use of a transition metal ion (Mn2þ), which can promote the efficient nonradiative energy transfer from the related levels of disappeared emissions of Er3þ to the 4T1 level of Mn2þ, followed by a backenergy transfer to the level of single-band UC emission [8]. Recently, KMnF3 nanocrystals have been identified as an
* Corresponding author. School of Physics and Electronics, Henan University, Kaifeng 475004, China. E-mail address: [email protected] (Y. Mao). http://dx.doi.org/10.1016/j.jallcom.2016.09.322 0925-8388/© 2016 Elsevier B.V. All rights reserved.
excellent host material for single red or NIR band UC emission. Since substantially high red and NIR emission can be obtained respectively when the Yb3þ/Er3þ, Yb3þ/Ho3þ, or Yb3þ/Tm3þ ions are introduced into KMnF3 NPs [9]. Moreover, in addition to the outstanding optical performance, KMnF3 UCNPs also shows a temperature-dependent and paramagnetism properties [10]. Besides, via hydrothermal and solventthermal method, a tiny size KMnF3 UCNPs can be easily obtained. All these properties make the KMnF3 NPs to be a popular UC material. However, the low UC luminescence efficiency is still the main drawback which restricts the application of UCNPs. So far, several methods are used to enhance the UC emission intensity of KMnF3:Yb3þ/Er3þ. To the best of our knowledge, the most used way for the enhancement is based on the fabrication of the core/shell structure, or the increase of the NPs size [10,11]. Herein, we propose a simple way to improve the single red band UC luminescence intensity of KMnF3:Yb3þ/Er3þ by introducing Mg2þ ions into the initial reaction solution. The experimental results of the UC luminescence intensity spectrum and the UC luminescence decay curves show the upconversion fluorescence intensity changes along with the variation of the doping concentration of Mg2þ ions. With 30 mol% Mg2þ ions doped, a maximum intensity of the UC emission intensity can be obtained. By calculating, the red UC luminescence intensity in 30 mol% Mg2þ ions doped sample enhance 20.8 times than that of Mg2þ ions-absent one. In addition,
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the TEM image of the in 30 mol% Mg2þ ions doped nanoparticles shows a monodisperse ultrasmall size (10 nm) nanocubes, which is deemed to an appropriate size for bio-imaging. All the results indicates that Mg2þ-doping is an effective method to significantly improve the UC single-red light. 2. Experimental 2.1. Materials All chemicals are of analytical grade and are used as received without further purification. Yb(NO3)3$5H2O, Er(NO3)3$5H2O, KF, Mn(CH3COO)2$4H2O, Mg(CH3COO)2$4H2O, 1-octadecene (ODE), oleic acid (OA), oleylamine (OM), cyclohexane and absolute ethyl alcohol are all supplied by Sinopharm Chemical Reagent Company. 2.2. Synthesis of KMnF3:Yb3þ/Er3þ NCs by introducing different Mg2þ concentrations in the initial solution The KMnF3:Yb3þ/Er3þ NPs with different Mg2þ concentrations in the initial solution are prepared in three steps according to a modified literature procedure [12]. Mn(CH3COO)2$4H2O (0.495 mmol - x), Mg(CH3COO)2$4H2O (x, x ¼ 0, 0.05, 0.10, 0.15 and 0.20 mmol), Yb(NO3)3$5H2O (0.0045 mmol), and Er(NO3)3$5H2O (0.0005 mmol) are weighted and added into a 50 mL three-necked flask containing 5 mL of OA, 5 mL of OM, and 15 mL of ODE. The mixture is strongly stirred and heated at 150 C for 30 min to form a rufous clear solution, and then cooled down to room temperature. Afterwards, 1.5 mL of a methanol solution containing 1.5 mmol of KF is dropped into the above solution and continuous stirred at 70 C for 30 min. After all the methanol evaporated, the solution was heated at 120 C for 5 min for further ensure all water and methanol are evaporated, and finally heated at 280 C under N2 for 60 min and then cooled down to room temperature. The products are precipitated by the addition of ethanol, collected by centrifugation, washed with cyclohexane and ethanol three times, and finally dried at 60 C. 2.3. Sample characterization The phase identification of the crystal structure is examined by X-ray diffraction (XRD) using Philips X Pert Pro X-ray diffractometer with Cu Ka radiation. The nanostructure and morphology are characterized by transmission electron microscopy (TEM) (JEOL JEM-2100., JP). The energy dispersive X ray analysis spectrum is measured by field emission scanning electron microscopy (FESEM) (HITACHI S4700., JP). The absorption spectra is measured by the Cary Series UV-Vis-NIR Spectrophotometer (Agilent Technologies., USA). The UC luminescence spectrum, and UC luminescence decay curves are measured via the FLS980E spectrometer (Edinburgh Instruments Ltd., UK). 3. Results and discussion Fig. 1 shows X-ray diffraction patterns (XRD) of the as-prepared KMnF3:0.9 mol% Yb3þ, 0.1 mol% Er3þ ions and 0e40 mol% of Mg2þ ions. All the diffraction peaks match well with the cubic phase KMnF3 (JCPDS 17e0116). No other impurity peaks are detected, which reveals that pure cubic phase KMnF3 has been fabricated. It also indicates that the Mg2þ doping has no influence on the crystalline phase of the KMnF3 nanocrystals. In comparison with that of KMnF3:Yb3þ/Er3þ nanoparticles, all diffraction peaks of KMnF3:Yb3þ/Er3þ/Mg2þ show a shift towards larger angles as the concentration of Mg2þ ions increases up to 40 mol%. The peak shifting indicates that Mg2þ ions can be doped into the host lattice
Fig. 1. XRD patterns of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) obtained after various doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). The dash line is shown to guide to eye.
through the substitution or occupation of the interstitial sites and cause the shrink of the unit cell, due to the small radii of Mg2þ (0.72 Å) than that of Mn2þ (0.80 Å). When varying the content of Mg2þ from 0% to 40%, we find the size decreases, and the mean particle size of these NPs calculated by Scherrer formula is about 31.3, 29.6, 16.9, 12.5 and 10.8 nm, respectively. The incorporation of Mg2þ ions in to KMnF3:Yb3þ/Er3þ NPs can be further proved by EDXA spectrum. As shown in Fig. 2(a), the EDXA spectrum of the KMnF3:Yb3þ/Er3þ/Mg2þ (0.9/0.1/30 mol%) NPs exhibits all of the elements including F, Mn, K, Mg, Yb, and Er, which further demonstrate the Mg2þ ions are existed in the final NPs products. The TEM image of the prepared KMnF3:Yb3þ/Er3þ NPs with 30 mol % Mg2þ doping is shown in Fig. 2(b). It is found that the assynthesized nanocrystals show a cubic morphology with a uniform size of 10 nm, which is correspond to the results calculated by XRD patterns through Scherrer equation. Due to the existence of organics oleic acid on the surface of the nanocrystals, some nanoparticles show a black cube, besides, there are some black mushy substance on the edge of the samples. Fig. 3 shows the absorption spectra of the as-synthesized Yb3þ/ Er3þ co-doped KMnF3 nanoparticles. As is shown in Fig. 3, the absorption peak at 363 nm, 380 nm, 448 nm, 488 nm, 520 nm, 650 nm and 800 nm should be assigned to 4I15/2 to 2G7/2, 4G11/2, 4F5/2, 4F7/2, 2 H11/2, 4S3/2, 4F9/2 and 4I9/2 transitions of Er3þ. The absorption in the (400e600 nm) and NIR wavelength band (900e1100 nm) should be owing to 6A1 / 4T1 of Mn2þ and 2F7/2 / 2F5/2 of Yb3þ respectively. It indicates that the Yb3þ/Er3þ ions are successfully doped in to the lattices, and the Manganese ions in the nanocrystals are pure valence state of Mn2þ. Fig. 4(a) shows the upconversion luminescence spectra of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) nanocrystals doped with different concentrations of Mg2þ ions. Upon excitation at the same power density 980 nm laser diode, all the samples exhibit an obviously single red band emission in the range of 640e690 nm which is assigned to the 4F9/2 / 4I15/2 transitions of Er3þ. The single red band UC emission can be ascribed to the nonradiative energy transfer from the 4F7/2, 2H11/2, and 4S3/2 levels of Er3þ to the 4T1 level of Mn2þ, followed by back-energy transfer to the 4F9/2 level of Er3þ (Fig. 4(b)). The complete disappearance of blue and green emissions of Er3þ suggests an extremely efficient exchange-energy transfer process between the Er3þ and Mn2þ ions, which can be largely attributed to the close proximity and effective mixing of wave functions of the Er3þ and Mn2þ ions in the crystal host lattices [13]. Fig. 4(a) also shows that the intensities of the single red band emissions are drastically changed when the Mg2þ ions are doped in the nanocrystals. It means that the Mg2þ ions concentration have a
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Fig. 2. (a) EDXA spectrum of KMnF3:Yb3þ/Er3þ NPs doped with 30 mol% Mg2þ, (b) TEM image of the as-synthesized KMnF3:Yb3þ/Er3þ/Mg2þ (0.9/0.1/30 mol%) nanocyrstals.
IðtÞ ¼ I0 þ A1 expð ðt t1 Þ=t1 Þ þ A2 expð ðt t1 Þ=t2 Þ
(1)
where I(t) and I0 are the luminescence intensity at time t and 0 ms, respectively. A1 and A2 are fitting constants, t is the time, t1 is the time of the maximum intensity of the transient UC luminescence spectrum, and t1, t2 are the rapid and slow lifetimes for the exponents, respectively. Such biexponential decay behavior might be resulted from the nonradiative transition among Yb3þ - Mn2þ and Er3þ - Mn2þ pairs, or the energy transfer between Yb3þ - Mn2þ and Er3þ - Mn2þ pairs and isolated Mn2þ, Er3þ, or Yb3þ ions and the defects introduced by Lanthanide (Ln3þ) due to the mismatched valence states between Ln3þ and Kþ or Mn2þ. For biexponential decay behavior, the average lifetimes t can be calculated as follows: Fig. 3. The absorption spectra of the as-synthesized KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) nanocyrstals.
huge effect on the UC luminescence intensity. When the doping concentration of Mg2þ increases from 0 to 30 mol%, the UC fluorescence intensity shows a distinctly monotonous enhancement. Subsequently, the UC luminescence intensity appears a decrease tendency in the case of over 30 mol% doping concentration of Mg2þ ions. The strongest UC emission intensity is observed in the sample with a Mg2þ concentration of 30 mol%. By calculating, the UC luminescence intensity of 30 mol% Mg2þ doped sample enhances about 20.8 times than that of Mg2þ-absent one. To visualize the UC intensification, photographs of UC luminescence for solutions with the corresponding Mg2þ doped nanoparticles (0.03 g in 2 mL cyclohexane and ethanol mixed solution with the volume ratio of 1:1) excited using a 980 nm laser diode (pumping power: 0.5 mW) are shown in Fig. 4(c). Obviously, the red emission of the five samples and the changes in the emission intensity can be clearly discerned by the naked eye. In addition, the luminescence intensity of 30 mol% Mg2þ doped sample is the strongest, which agrees with the results in Fig. 4(a). To get more information about the UC emission of Er3þ, the transient UC luminescence spectrum of Er3þ:4F9/2 in as-synthesized UCNPs are measured under excitation with 980 nm pulsed laser. As shown in Fig. 5(a), the falling edge of the transient UC luminescence spectrum of Er3þ:4F9/2 in all samples can be well fitted into a double exponential function as formula (1).
t¼
A1 t21 þ A2 t22 A1 t1 þ A2 t2
(2)
The average lifetime of the Er3þ:4F9/2 level increases from 108.3 ms to 692.1 ms with increasing the doping concentration of Mg2þ from 0 mol% to 30 mol% (Fig. 4(b)). It may be attributed to the increasing Mg2þ ions doping lead to the decrease of the amount of Mn2þ ions, which weakens the nonradiative energy transfer among Yb3þ/Er3þ/Mn2þ, in contrast, the ratio of radiative energy transfer increases, as a result, a longer lifetime can be detected in Mg2þ doped samples than that of Mg2þ-absent one. For UC materials, a long lifetime usually means a highly-efficient UC luminescence [14e16]. As revealed in Fig. 5(a), the trend of lifetime variation is consistent with that of UC intensity variation in Fig. 4(a). However, when the Mg2þ doping amount increases up to 40 mol%, the average lifetime of the Er3þ:4F9/2 level decreases to 418.2 ms (Fig. 4(b)). It may ascribe to the superfluous Mg2þ doping lead to the decreasing of the amount of Mn2þ ions surrounding of Ln3þ, which result in the decrease of the energy transfer rate of Yb3þ / Mn2þ / Er3þ. Besides, the change of the UC luminescence intensity may also ascribe to the variation of the crystal symmetry and the crystal lattice. It is well known that the UC luminescence intensity of Er3þdoped NPs depends on its 4f transition probabilities, which are affected significantly by the local crystal field symmetry of the Er3þ [17,18]. Substituting the Mn2þ ions by the smaller radii Mg2þ ions will cause the host lattice of KMnF3 to shrink, which results in a decrease of the average lengths between the rare earth and the fluorine ions. The asymmetric surrounding environment of Ln3þ
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Fig. 4. (a) UC luminescence spectra of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs with different doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). (b) Energy-level diagram of KMnF3:Yb3þ,Er3þ NPs and possible upconversion energy transfer processes under excitation of a 980 nm laser diode. (c) the corresponding luminescence photographs of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs doped with different amount Mg2þ (0, 10, 20, 30, 40 mol%).
Fig. 5. (a) The transient UC luminescence spectrum of KMnF3:Yb3þ/Er3þ (0.9/0.1 mol%) NPs with different doping concentration of Mg2þ (0, 10, 20, 30, 40 mol%). (b) UC decay time (lifetime) variation depends on the Mg2þ doping concentrations in the KMnF3:Yb3þ/Er3þ NPs.
favors hypersensitive transitions, subsequently increasing the UC luminescence. However, the superfluous dopant Mg2þ induces excessive distortion of the lattice, which affects the spatial distribution of the Ln3þ, inducing a concentration quenching and reducing the emission intensity [19,20].
the UCNPs can be significantly enhanced by Mg2þ ion doping. And a 20.8-fold UC luminescence intensity increase is observed in the 30 mol% Mg2þ doped sample than that of KMnF3:Yb3þ/Er3þ sample. All these results indicates our work is useful not only for theoretical studies on UCNPs but also for extending their applications.
4. Conclusions
Acknowledgements
In summary, KMnF3:Yb3þ/Er3þ UCNPs doped with different amounts of Mg2þ (0e40 mol%) are synthesized by a facile solvothermal method to modulate upconversion luminescence. The as-synthesized 30 mol% Mg2þ doped nanocrystals show a cubic morphology with a uniform size of 10 nm. The UC luminescence measurement results show that the UC luminescence intensity of
This work is supported by the Program for Science and Technology Innovation Talents in Universities of Henan Province (No. 16HASTIT043), the Science and Technology Research Project of Henan Province (No. 142102210389), the National Science Foundation of China (Grant No. 21103043) and the Program for Innovative Research Team (in Science and Technology) in University of
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