Magnetic Tuning of Optical Hysteresis Behavior in Lanthanide-Doped

Mar 3, 2015 - In this paper, we examine the coupling of magnetic field with photoluminescence in Eu3+-doped NaGdF4 nanoparticles. The Zeeman effect in...
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Magnetic Tuning of Optical Hysteresis Behavior in LanthanideDoped Nanoparticles Ping Chen,† Hong Jia,† Junpei Zhang,‡ Junbo Han,‡ Xiaofeng Liu,† and Jianrong Qiu*,†,§ †

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China § State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: Magnetic-optical bifunctional materials have attracted tremendous interest due to their potential applications in biomedicine as well as multifunctional sensors. However, much attention has been paid on the bifunctional materials rendering magnetic and optical behavior individually, rather than the interaction between magnetic field and optical process. In this paper, we examine the coupling of magnetic field with photoluminescence in Eu3+-doped NaGdF4 nanoparticles. The Zeeman effect induced by magnetic field is clearly observed from the shift of luminescence bands and the splitting of the emission peaks. Furthermore, the luminescence intensity of different transitions of Eu3+ in paramagnetic NaGdF4 exhibits a hysteresis behavior when the magnetic field is scanned between 0 and 40 T. Compared with the optical behavior of Eu3+ in the nonmagnetic NaYF4, this optical hysteresis behavior of luminescence intensity is tentatively ascribed to the magnetic response of the paramagnetic dopant ions in both hosts. Due to the high magnetic field sensitivity, the Eu3+-doped bifunctional nanoparticles could be used as optical probes in sensor and biomedical areas.

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INTRODUCTION Magnetic-optical bifunctional materials have been received tremendous interest owning to their wide applications ranging from bioimaging to cancer therapy to sensors.1−4 Nanoparticles with ferromagnetism or superparamagnetism themselves are widely applied as drug carriers as well as magnetic resonance imaging agents in biomedical field.5,6 The introduction of luminescence to those magnetic nanomaterials often enables new functionalities, which lead to simultaneous visualization of the therapeutic action. For instance, with the use of a nanohybrid built up from upconversion nanoparticles and iron oxide, the transport of the anticancer drug and the release of the drug to the targeted region can be monitored with the help of near-infrared (NIR) light pumping.4 In spite of the growing attention paid on the bifunctional materials responding to magnetic and optical excitation simultaneously,7−10 the coupling between magnetism and luminescence of lathanide ions in these systems remains largely ignored. From some recent reports, it has been found that the external magnetic field has direct influence on the spectral positions and luminescence intensity of lanthanide ions doped in various inorganic hosts.11−14 In a Er3+/Yb3+-doped Gd2O3 phosphor, for instance, optical bistability in the magnetic field © XXXX American Chemical Society

dependent luminescence intensity was observed, which was ascribed to the residual magnetization in the material.15 In several other systems, with the increase of the magnetic field, the suppression of the luminescence intensity was usually observed, accompanied by the steady shift of the spectral positions for lanthanide ions like Er3+ and Eu3+.14,16,17 Those are generally understood based on the interplay of field-induced Zeeman splitting. However, the exact mechanism with the application of magnetic field remains obscure. In this paper, the magnetic−optical interaction is studied for Eu3+-doped NaGdF4 nanoparticles, where the obvious shift and splitting of the luminescence bands have been detected in the presence of magnetic field. These results can be interpreted in terms of Zeeman effect and the magnetic field induced change of local site symmetry around Eu3+. Besides optical hysteresis behavior of integrated luminescence intensity for different transitions of Eu3+ is found when the magnetic field is scanned from 0 to 40 T, which is connected with the magnetic ordering in the lanthanide sublattice. Received: November 30, 2014 Revised: January 24, 2015

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DOI: 10.1021/jp511914f J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Crystal structure of hexagonal phase NaGdF4, where Gd3+ is coordinated by nine fluoride ions. (b) XRD patterns of NaGdF4:5% Eu nanoparticles. (c) TEM image of hexagonal NaGdF4:5% Eu nanoparticles, inset shows the size distribution of the nanoparticles, (d) HRTEM image of single NaGdF4:5% Eu nanoparticle. (e) Luminescence spectrum of NaGdF4:5% Eu nanoparticles excited at 396 nm without magnetic field at 80 K.

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The luminescence spectra of Eu3+-doped NaGdF4 nanoparticles excited at 396 nm were measured by a home-built system equipped with a pulse magnetic field with the pulsed duration of 290 ms.17 Figure 2 shows the luminescence spectra of Eu3+-doped NaGdF4 nanoparticles for a cycle of magnetic field from 0 to 40 T, recorded at 80 K. With the increase of magnetic field (Figure 2a,c), an obvious shift of luminescence bands and the splitting of luminescence peaks are observed. The luminescence bands from the transition of 5D0 → 7F2 of Eu3+ split from three into five peaks, while those from the transition of 5D0 → 7F4 of Eu3+ split from six into nine. The splitting of luminescence bands can be well understood based on the Zeeman effect induced by external magnetic field. With the application of magnetic field, an external torque is generated, which causes an external energy to the Zeeman sublevels and results in the energy level with the total angular momentum J splitting into 2J + 1 sublevels.24 This is in agreement with the number of Zeeman sublevels as we observed here for the different transitions of Eu3+ (Figure S1b and Figure 2).14,17 In addition to the splitting of luminescence bands, it is clear that the luminescence intensity from different transitions of Eu3+ is reduced with the increase of magnetic field. To find the reason for the decreased luminescence intensity, we first measured the absorption of Eu3+ at 396 nm with magnetic field up to 40 T. The change of the absorption of Eu3+ at 396 nm is highly possible with magnetic field as Zeeman Effect causes the energy splitting and the shift of energy levels of Eu3+. Since it is not possible to measure the absorption of nanoparticles directly with our test system, we instead recorded intensity of transmitted light through a glass-ceramic plate containing NaGdF4:5% Eu nanoparticles.25 Figure S3 shows the dependence of absorption at 396 nm on the magnetic field. The intensity of absorption was almost unchanged with the increase of magnetic field, indicating the absorption of Eu3+ might not be the reason for the suppressed luminescence bands. The change of local site symmetry around Eu3+ might be another reason for the suppressed emissions. It is well-known that the electric dipole transition 5D0 → 7F2 of Eu3+ is hypersensitive to the local site symmetry, while the magnetic dipole transition 5D0 → 7F1 of Eu3+ is not.18,26 Thus, we define the index R as the ratio of the integrated intensity of electric dipole transition to that of magnetic dipole transition, R =

RESULTS AND DISCUSSION In order to investigate the magnetic-optical interaction, Eu3+ is chosen as luminescent center because of its high luminous efficiency, sharp emission bandwidths, and sensitivity to the local site symmetry.18 Paramagnetic host of hexagonal NaGdF4 is selected due to its high response to magnetic field for the large number of unpaired 4f electrons of Gd3+ ion.19 NaGdF4 is also a host of low phonon energy, low toxicity, and good chemical stability.16,20 In the hexagonal NaGdF4 phase, Gd3+ ion is coordinated by nine fluoride ions, featuring a site symmetry of C3h (Figure 1a). The spatial distance between Gd3+ ions varies along different orientation, which might lead to the anisotropic magnetic interaction in the presence of magnetic field and may have direct influence on the luminescence of the dopant ions. Eu3+-doped hexagonal NaGdF4 nanoparticles were synthesized by coprecipitation method in oleic acid (OA) and 1octadecene (ODE) system.21 In order to reduce the luminescence quenching by the ligand on the surface of nanoparticles, the ligand-free nanoparticles were prepared by using hydrochloric acid to protonate the oleate ligand.22 The crystal structure of the NaGdF4:5% Eu nanoparticles is examined by X-ray diffraction (XRD) analysis. XRD patterns of the sample (Figure 1b) are in good agreement with the standard XRD card of hexagonal NaGdF4 (JCPDS No. 27− 0699), indicating the pure hexagonal phase. Low-resolution transmission electron microscopy (TEM) image (Figure 1c) reveals the NaGdF4:5% Eu nanoparticles are composed of uniform spherical shape with a narrow size distribution around 20.5 ± 2 nm (inset in Figure 1c). High-resolution TEM (HRTEM) image (Figure 1d) shows the single-crystal structure of the nanosphere, where the distance of adjacent lattice fringes is 0.527 nm, corresponding to the interplanar spacing for the (100) lattice plane of hexagonal NaGdF4. he luminescence spectrum of NaGdF4:5% Eu nanoparticles without magnetic field was recorded at 80 K under the excitation at 396 nm (Figure 1e). Several luminescence bands centered around 510, 525, 536, 555, 583, 591, 615, and 695 nm were observed, which are ascribed to 5D2 → 7F3, 5D1 → 7F0, 5 D1 → 7F1, 5D1 → 7F2, 5D1 → 7F3, 5D0 → 7F1, 5D0 → 7F2, and 5 D0 → 7F4 transitions of Eu3+, respectively.23 Due to the interaction of crystal field, the transitions of 5D0 → 7F1 and 5D0 → 7F4 are split into three and six emission peaks, respectively. B

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Figure 2. Emission spectra from NaGdF4:5% Eu nanoparticles for the transitions of 5D0 → 7F2 (a, b) and 5D0 → 7F4 (c, d) of Eu3+ for a complete cycle of magnetic field from 0 to 40 T under the excitation at 396 nm, recorded at 80 K.

curves obviously do not overlap with each other, forming a loop as shown in Figure 4a,b. In other words, there is an optical hysteresis behavior induced by the magnetization of the material.15 As Gd3+ ion is a paramagnetic ion, magnetic ordering occurs easily in the Gd3+ sublattice with the application of the external magnetic field (Figure 3b), and the magnetization may not be immediately removed due to kinetic reasons, which accordingly leads to the hysteresis behavior of the luminescence intensity. Similar optical hysteresis was observed in Er3+-doped Gd2O3.15 In order to confirm whether the optical hysteresis behavior is directly originated from the magnetic ordering of Gd3+, the Eu3+-doped nonmagnetic material of NaYF4 nanoparticles was synthesized and the luminescence spectra for a cycle of magnetic field from

I5D0→7F2/I5D0→7F1, which can be used as a reference for the distortion of local site system around Eu3+ in the presence of magnetic field. Figure 3a shows the index R for NaGdF4:5% Eu decreases with the increase of magnetic field, implying the improvement of site symmetry around Eu3+. The improved site symmetry is responsible for the reduced luminescence intensity of Eu3+ because it is generally accepted that selection rules are strengthened for dipole transitions of ions at higher symmetrical sites. It is interesting that the luminescence intensity, as shown in Figure 2, exhibits a hysteresis behavior with magnetic field. For both of the “forward” and “backward” scan, the luminescence intensity of Eu3+ from different transitions decreases with the increase of magnetic field from 0 to 40 T. However, the two C

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Figure 3. (a) Dependence of index R on the magnetic field intensity for NaGdF4:5% Eu and NaYF4:5% Eu nanoparticles for a cycle of magnetic field from 0 to 40 T. (b) Magnetization curve of hexagonal phase NaGdF4 recorded from −5 to 5 T.

Figure 4. (a, b) Optical hysteresis behavior of luminescence intensity for the transitions of 5D0 → 7F2 (a), 5D0 → 7F4 (b) of Eu3+ for NaGdF4:5% Eu nanoparticles under the excitation at 396 nm, recorded at 80 K. (c, d) Optical hysteresis behavior of luminescence intensity for the transitions of 5D0 → 7F2 (c) and 5D0 → 7F4 (d) of Eu3+ for NaYF4:5% Eu nanoparticles under the excitation at 396 nm, recorded at 80 K. The magnetic field is cycled from 0 and 40 T.

NaGdF4:5% Eu nanoparticles (Figures 4b and S5a). The luminescence intensity of 5D0 → 7F2 transition of Eu3+ for NaGdF4:5% Eu nanoparticles is reduced after removal of the magnetic field, while the emissions for other transitions are recovered to the initial intensity without the application of magnetic field. However, the emission intensity of the 5D0 → 7 F2 transition of Eu3+ in NaGdF4:5% Eu nanoparticles recovers completely after staying at 0 T at 50 ms after removing the pulse magnetic field (Figure S7). It can be explained that, due to the influence of magnetic field, local structure change around Eu3+ occurs, and optical response shows a time lag effect arising from the structure relaxation. As shown in Figure 3a, the index R for NaGdF4:Eu nanoparticles is reduced by 4.2% compared to the initial value due after the pulse magnetic field, indicating the slight improvement of local site symmetry around Eu3+ for

0 to 40 T were measured under the same condition (Figure S4). To our surprise, optical hysteresis behavior of luminescence intensity is also observed in nonmagnetic material of NaYF4:5% Eu nanoparticles (Figures 4c,d and S5b), indicating the magnetic ordering of Gd3+ sublattice is not directly related with the optical hysteresis behavior. It is known that Eu3+ at its ground state (7F0) does not carry a magnetic moment; it however becomes magnetic easily when it is promoted to 7F1,2,3 states by thermal or optical excitation. Therefore, the magnetic response of paramagnetic Eu3+ ions might be responsible for the magnetic-optical hysteresis of luminescence intensity. Furthermore, the optical hysteresis for 5D0 → 7F2 transition of Eu3+ in NaGdF4:Eu nanoparticles shows (Figure 4a) a different behavior as compared to other transitions of Eu3+ in D

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The Journal of Physical Chemistry C NaGdF4:Eu after the magnetic field scan. The symmetryinsensitive transitions can recover to their initial intensity quickly after pulse magnetic field, while the symmetry-sensitive 5 D0 → 7F2 transition cannot follow the rapid change of magnetic field due to the local structural relaxation around Eu3+. The optical hysteresis behavior of 5D0 → 7F2 transition for magnetic NaGdF4:5% Eu nanoparticles and nonmagnetic NaYF4:5% Eu nanoparticles also appears different. The emission of 5D0 → 7F2 transition for magnetic NaGdF4:5% Eu nanoparticles is reduced by 4.6% after removal of magnetic field, while that for nonmagnetic NaYF4:5% Eu nanoparticles is recovered to the initial intensity. This can be connected to the difference in the change of local site symmetry in magnetic and nonmagnetic hosts with magnetic field. As shown in Figure 3a, the index R is reduced to 89.2% of the initial value for magnetic NaGdF4 host with increasing the magnetic field to 40 T, while R for NaYF4 host only decreases to 96.0% of the initial value. Considering that the luminescence spectra of nanoparticles were collected during the excitation by the pulsed magnetic field, those results can be ascribed to the different magnetic response of the two different hosts subjected to a sudden magnetic pulse excitation. In NaGdF4:5% Eu, both of the Gd3+ sublattice and the dopant ions (Eu3+) are magnetically ordered in the presence of magnetic field, resulting in much larger reduction of index R as compared to NaYF4:5% Eu. After the removal of magnetic field, the index R for magnetic host is reduced, while that for nonmagnetic host is recovered for the different rate of structural relaxation of two hosts, resulting in the different hysteresis behaviors of 5D0 → 7F2 transitions in Eu3+-doped magnetic and nonmagnetic hosts.

tation method.1 In a typical procedure, 3.80 mL of Gd(Ac)3· 4H2O and 0.20 mL of Eu(Ac)3·4H2O were mixed with 6 mL of OA and 14 mL of ODE in a 50 mL three-neck round-bottom flask. The resulting mixture was heated to 155 °C for 30 min to form a clear solution and then cooled down to room temperature. Thereafter, 10 mL of methanol solution containing 1.8 mmol NH4F and 2.0 mmol NaOH was added, and the solution was stirred at 50 °C for 30 min. After methanol was evaporated, the solution was heated to 290 °C under Ar flow with vigorous stirring for 90 min and then cooled down to room temperature. The obtained nanoparticles were precipitated by ethanol, collected by centrifugated, washed with ethanol for several times, and finally redispersed in cyclohexane. Synthesis of Ligand-Free NaGdF4:5% Eu3+ Nanoparticles. The ligand-free NaGdF4:5% Eu3+ nanoparticles were prepared according to the method reported by John A. Capobianco.2 In a typical procedure, Oleate-capped NaGdF4:5% Eu3+ nanoparticles (100 mg) were dispersed in a 10 mL aqueous solution. The reaction was performed with stirring for 2 h while maintaining the pH at 3 by adding a solution of HCl (0.1 M). During this reaction, the carboxylate groups of the oleate ligand were protonated (to yield OA). After the reaction was completed, the aqueous solution was mixed with diethyl ether to remove OA by extraction with diethyl ether three times, and the combined ether layers were re-extracted with water. In addition, the water layers were combined and re-extracted with diethyl ether. The product was redispersed in acetone and the particles were recuperated by centrifugation. Finally, the particles were dispersed in water. Characterization. X-ray diffraction pattern of the dry powder was obtained on a RIGAKU D/MAX 2550/PC diffractometer (Japan) with a slit of 0.02° at a scanning speed of 5° min−1 using Cu Kα radiation (λ = 1.5406 Å). HRTEM analysis was performed on a FEG-TEM (Tecnai G2 F30 STwin, Philips-FEI, Netherlands) operated at 300 kV. The luminescence spectra of nanoparticles was measured by a home-built luminescence spectroscopy system equipped with a pulsed magnetic field.17 The 396 nm laser was employed as the excitation source and coupled into a fiber to pump the nanoparticles. The nanoparticles were located at the center of a pulsed magnetic field generated by a resistive coil magnet with the pulsed duration of 290 ms (ms), where the rising side is 20 ms and the falling side is 270 ms. Luminescence spectra were collected by the same fiber system, where the emitted photons were transmitted to the detection part and detected by a spectrometer equipped with an electron multiplying charge coupled device (CCD) detector.

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CONCLUSION Hexagonal phase NaGdF4:5% Eu nanoparticles were synthesized and the luminescence of Eu3+ was examined for a cycle of magnetic field from 0 to 40 T. In the presence of magnetic field, obvious shift and splitting of luminescence bands for different transitions of Eu3+ are observed, which can be ascribed to Zeeman Effect induced by magnetic field. The reduced absorption of Eu3+ at 396 nm and improved site symmetry around Eu3+ ions might be responsible for the decrease of luminescence intensity of Eu3+ in NaGdF4:5% Eu nanoparticles with the increase of the magnetic field intensity. In addition, the optical hysteresis behavior of the luminescence intensity of Eu3+ in both hosts of paramagnetic NaGdF4 and nonmagnetic NaYF4 is observed for a cycle of magnetic field from 0 to 40 T, which is tentatively ascribed to the magnetic response of paramagnetic dopant ion in both hosts. The high sensitivity of luminescence to magnetic field may enable the application of Eu3+-doped NaGdF4 nanoparticles in remote sensing and biomedicine.

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ASSOCIATED CONTENT

* Supporting Information S

The influence of magnetic field intensity on the luminescence of Eu3+ with different transitions in NaGdF4 nanoparticles. Magnetic field dependence of absorption at 396 nm of NaGdF4:5% Eu nanoparticles precipitated in a transparent glass matrix. Emission spectra of Eu3+ in NaYF4 nanoparticles for a cycle of magnetic field from 0 to 40 T. Optical hysteresis behaviors of Eu3+ with different transitions in magnetic and nonmagnetic hosts. Emission spectra after removing the magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL METHODS Materials. Gd(Ac)3·4H2O (99.99%) and Ho(Ac)3·4H2O (99.99%) were purchased from Ansheng inorganic materials center Ganzhou in China. NaOH, NH4F, 1-octadecene (ODE), and oleic acid (OA) were purchased from Sigma-Aldrich. Cyclohexane was purchased from Wako Pure Chemical Industries, Ltd. All of the chemicals were used as starting materials without further purification. Synthesis of Eu3+-Doped NaGdF4 Nanoparticles. The NaGdF4:5% Eu nanoparticles were prepared by the coprecipiE

DOI: 10.1021/jp511914f J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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Emission Spectra of Eu3+ in Different Single Crystals. J. Mater. Chem. C 2013, 1, 7608−7613. (15) Singh, S. K.; Kumar, K.; Srivastava, M. K.; Rai, D. K.; Rai, S. B. Magnetic-Field-Induced Optical Bistability in Multifuctional Gd2O3:Er3+/Yb3+ Uupconversion Nanophosphor. Opt. Lett. 2010, 35, 1575−1577. (16) Liu, Y.; Wang, D.; Shi, J.; Peng, Q.; Li, Y. Magnetic Tuning of Upconversion Luminescence in Lanthanide-Doped Bifunctional Nanocrystals. Angew. Chem., Int. Ed. 2013, 52, 4366−4369. (17) Chen, P.; Zhang, J.; Xu, B.; Sang, X.; Chen, W.; Liu, X.; Han, J.; Qiu, J. Lanthanide Doped Nanoparticles as Remote Sensor for Magnetic Field. Nanoscale 2014, 6, 11002−11006. (18) Tu, D.; Lin, Y.; Zhu, H.; Li, R.; Liu, L.; Chen, X. Breakdown of Crystallographic Site Symmetry in Lanthanide-Doped NaYF4 Crystals. Angew. Chem., Int. Ed. 2013, 52, 1128−1133. (19) Wang, Z.; Hao, J. H.; Chan, H. L. W. Down- and Up-conversion Photoluminescence Cathodoluminescence and Paramagnetic Properties of NaGdF4:Yb3+, Er3+ Submicron Disks Assembled from Primary Nanocrystals. J. Mater. Chem. 2010, 20, 3178−3185. (20) Wang, F.; Deng, R.; Liu, X. Preparation of Core-Shell NaGdF4 Nanoparticles Doped with Luminescent Lanthanide Ions to be Used as Upconversion-Based Probes. Nat. Protoc. 2014, 9, 1634−1644. (21) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning Upconversion Through Energy Migration in Core-Shell Nanoparticles. Nat. Mater. 2011, 10, 968. (22) Bogdan, N.; Vetrone, F.; Ozin, G. A.; Capobianco, J. A. Synthesis of Ligand-Free Colloidally Stable Water Dispersible Brightly Luminescent Lanthanide-Doped Upconverting Nanoparticles. Nano Lett. 2011, 11, 835−840. (23) Li, C.; Quan, Z.; Yang, J.; Yang, P.; Lin, J. Highly Uniform and Monodisperse β-NaYF4:Ln3+ (Ln = Eu, Yb/Er, and Yb/Tm) Hexagonal Microprism Crystals: Hydrothermal Synthesis and Luminescent Properties. Inorg. Chem. 2007, 46, 6329. (24) Drake, G. W. F. In Springer Handbook of Atomic, Molecular, and Optical Physics; Martin, W., Wiese, W., Eds.; Springer: New York, 2006; Vol. 2, pp 175−198. (25) Herrmann, A.; Tylkowski, M.; Bocker, C.; Russel, C. Cubic and Hexagonal NaGdF4 Crystals Precipitated from an Aluminosilicate Glass: Preparation and Luminescence Properties. Chem. Mater. 2013, 25, 2878−2884. (26) Lehmann, O.; Kompe, K.; Haase, M. Synthesis of Eu3+-Doped Core and Core/Shell Nanoparticles and Direct Spectroscopic Identification of Dopant Sites at the Surface and in the Interior of the Particles. J. Am. Chem. Soc. 2004, 126, 14935−14942.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 50872123), National Basic Research Program of China (2011CB808100). The authors thank the pulsed high magnetic field facilities at Wuhan National High Magnetic Center.

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DOI: 10.1021/jp511914f J. Phys. Chem. C XXXX, XXX, XXX−XXX