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Lanthanide doped nanoparticles as remote sensors for magnetic fields† Ping Chen,a Junpei Zhang,b Beibei Xu,a Xiangwen Sang,a Weibo Chen,a Xiaofeng Liu,*a Junbo Han*b and Jianrong Qiu*ac
DOI: 10.1039/c4nr02983f www.rsc.org/nanoscale
We report the effect of magnetic fields (MFs) on emission Eu-doped NaYF4 nanoparticles. A notable shift in the position of emission bands and the suppressed emission intensity are observed with the MF. These magnetic-optical interactions are explained in terms of the Zeeman effect, enhanced cross-relaxation rate and change of site symmetry.
Magnetic elds (MFs) modulate the motion of particles that carry a non-zero spin, including non-charged particle neutrons and different charged particles, such as electrons and various ions with unpaired electrons. This mechanism has fueled broad interest in the study of the interaction of a MF with diverse systems from biological objects to elementary particles and solid state materials.1 For instance, the MF has been found to affect collectively the behavior of different biological systems, resulting in the changes of their microscopic and macroscopic characteristics.2 In condensed matter physics, the Hall effect from both classical and quantum perspectives has been widely known as an outcome of the regulation of the movement of electrons, holes or ions by the Lorentz force induced by the MF.3 The interaction of the MF with photons mediated by solid state materials is also an important subject in condensed matter physics. In the macroscopic dimensions, the MF can rotate the polarization plane of the transmitted light through magnetooptic crystals, which is widely known as the Faraday effect.4 The magneto-optical interaction becomes completely different for localized electrons surrounding optically active centers. It has been established that MF splits degenerate quantized energy levels of different electronic orbitals and shi their positions by
a
Department of Materials Science and Engineering, Zhejiang University Hangzhou, Zhejiang 310027, China. E-mail: [email protected]; Fax: +86 57188925079; Tel: +86 57188925079
b
Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China
c State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China
† Electronic supplementary 10.1039/c4nr02983f
information
11002 | Nanoscale, 2014, 6, 11002–11006
(ESI)
available.
See
DOI:
the Zeeman effect, resulting in notable changes in the emission bands of luminescent materials.5 This magneto-optical effect on the photoluminescence behavior can be used to probe the MF intensity, especially for remote MF detection when nanoscale probes are employed. Semiconductors, quantum dots (QDs), and organic dyes which exhibit broadband luminescence are however not suitable as probes because the precise determination of band shis is not possible for emissions with a larger energy span. In addition, the toxic characteristics of QDs and the poor photostability of organic dyes also hinder their application as long-term probes.6 In this regard, lanthanide ion doped materials could be a better alternative for optical sensing of the MF, owing to the rich 4f levels, sharp bandwidth emission, long lifetimes, and good photostability.7 The inuence of the MF on the luminescence of lanthanide ion doped bulk materials including single crystals was revealed recently.8 The emission of nanoparticles activated with lanthanide ions is also found to be drastically affected by the external MF. In general, the emission was suppressed evidently as evidenced by optical spectroscopy.9 In this paper, we investigate the MF dependence of photoluminescence of Eu3+ doped hexagonal NaYF4 nanoparticles (NPs) and demonstrate their potential as nanoscale optical sensors for the MF. Eu3+ was chosen as an activator ion because of its high luminous efficiency, sharp emission bandwidths, and sensitivity to the local site symmetry.10 Hexagonal NaYF4 was selected as the host due to its low toxicity, good chemical stability and photostability, low phonon energy and high luminescence efficiency.11 In the presence of the MF, the shi of luminescence bands and reduction of the integrated intensity are observed clearly. This magneto-optic effect can be explained by considering the Zeeman effect, cross-relaxation, and variation of the local site symmetry. The result suggests that Eu3+ doped NaYF4 NPs might have potential applications for magnetic sensing in the nanoscale region. High quality NaYF4 NPs were synthesized by the co-precipitation method.6a The as-synthesized NPs are composed of
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uniform nanospheres with narrow size distribution centered around 23 nm (Fig. 1(a)). These NPs have a hexagonal structure as conrmed by the high-resolution TEM (HRTEM) image (Fig. 1(b)), where the (100) plane is clearly identied. Moreover, all the observed X-ray diffraction (XRD) peaks match well with the standard XRD card (JCPDS no. 16-0334) of hexagonal NaYF4 (Fig. 1(c)), indicating high phase purity.10 Fig. 1(d) shows the crystalline structure of hexagonal NaYF4. The site symmetry of Y3+ ions is C3h, in comparison, for the dopant Eu3+; its site
Fig. 1 (a) Low-resolution TEM image of NaYF4:5% Eu NPs (inset: the size distribution of NPs). (b) HRTEM image of NaYF4:5% Eu NPs. (c) XRD pattern of NPs. (d) Crystal structure of NaYF4 and Eu3+ doped NaYF4.
Fig. 2 Schematic diagram of the luminescence spectroscopy test system equipped with a pulsed MF.
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symmetry is reduced to Cs as the ionic radius of Eu3+ (r ¼ 1.206) is larger than the host Y3+ (r ¼ 1.159).11a A home-built luminescence spectroscopy system equipped with a pulsed MF (Fig. 2) was employed for studying the luminescence of NaYF4 NPs. A 396 nm laser beam, which was generated by using a second harmonic generator (SHG) pumped by a Ti:sapphire laser, was coupled to a ber to pump the NPs. The NPs were located in the center of a pulsed MF generated by a liquid nitrogen-cooled resistive coil magnet with a pulsed duration of 290 milliseconds (ms) and a falling side of 270 ms. The emission spectrum was collected by using the same ber system. The emitted photons were transmitted to the detection part aer spatial and spectral ltering, and subsequently detected using a spectrometer equipped with an electron multiplying charge coupled device (CCD) detector. The emission spectrum of NPs without the MF was obtained at 80 K (Fig. 3(a)). Several emission bands are observed in the region of 500–700 nm, which originate from different 5Dj / 7Fj transitions.12 All the emission bands can be ascribed to the energy level diagram shown in Fig. 3(b). Due to crystal eld (CF) splitting, the emission bands for the transition of 5D0 / 7F1 can be decomposed into three peaks, while the 5D0 / 7F2 transition split into ve peaks, indicating the distortion of site symmetry from C3h for Eu3+ ions.10 The optical spectra change dramatically in the presence of a pulsed MF (Fig. 4, S2 and S3 (ESI)†), especially for transitions from the 5D1 state. Furthermore, luminescence spectra indicate clear blue or red shis of the emission and the splitting of energy levels for most of the observed transitions. For instance, when the MF intensity rises from 0 T to 35 T, the number of luminescence peaks increases from one to three for the 5 D1 / 7F0 transition (Fig. 4(a)). Similar behavior is observed for transitions of 5D1 / 7F3 (Fig. 4(d)) and 5D0 / 7F2 (ESI, Fig. S3†). This is because there are 2J + 1 Zeeman levels for each state with a total angular momentum of J (ESI†).13 Here, the number of observed emission peaks for each transition in the MF matches exactly with the number of Zeeman levels of the 7FJ states, implying that non-radiative relaxation might occur from a higher to the lowest Zeeman levels for the excited state 5DJ before the radiative transition 5DJ / 7FJ.
Fig. 3 (a) Emission spectrum of NPs excited at 396 nm. (b) Energy level diagram of an Eu3+ ion with possible emission pathways. The field induced energy level splitting is shown in the right.
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the MF. This can result in both negative and positive shis of the position depending on the quantum nature of the respective Zeeman levels. For instance, the gaps among the nine Zeeman levels of the 7F4 state which has a larger total momentum expand notably in the MF, leading to the obvious blue shi in the short wavelength edge and red shi in the long wavelength edge (ESI Fig. S3(c)†). The MF induced change of its position can be described by the following equation (details are provided in the ESI†): Ds ¼
Fig. 4 (a) Emission spectra of the NPs with the transition of 5D1 / 7F0
recorded at different MF intensities at 80 K. (b) Gaussian fitting for the emission spectrum for the 5D1 / 7F0 transition at 35 T. (c and d) Emission spectra recorded at different MF intensities at 80 K for NPs with the transitions of 5D1 / 7F1 (c) and 5D1 / 7F3 (d). The excitation is a 396 nm laser (30 mW).
For a closer insight into the magnetic-optical interaction, we check each of the emission bands in more detail. We then dene the zero energy as the ground state 7F0 of Eu3+, which is not affected by the MF because its total angular momentum J is zero. The three Zeeman energy levels of 5D1 are found at 16 957 cm1, 16 896 cm1, and 17 264 cm1 from the spectra of the transition 5D1 / 7F0 by the Gaussian tting (Fig. 4(b)). Other energy levels of Eu3+ with and without the MF were located and are listed in Table S1 (ESI†). In the presence of the MF, the external torque induced by the MF adds an external energy to each energy level and increases with the strength of
DE2 DE 1 e ¼ ðM2 g2 M1 g1 Þ B h 2pmc
(1)
where Ds is the energy change in the wave-number (cm1). DE1, DE2 are the external energies of the system before and aer transition in the presence of the MF. h is Planck's constant, g is the Lande factor, e is the electric charge of an electron, m is the mass of an electron and B is the MF intensity. The equation indicates the linear relationship between the change of the Zeeman level position and the MF intensity. However, from Fig. 5(a)–(c), the position of the observed energy changes slowly at a low MF but very rapidly at a high MF, indicating that the inuence of the energy change on the MF intensity is nonlinear,
Fig. 6 The dependence of index R and the integrated emission intensity on the MF of the transitions of 5D0 / 7F1 and 5D0 / 7F2 intensity at 80 K.
Fig. 5 (a–c) Dependence of energy change on the MF intensity for the luminescence bands centered at 535.8 nm (a), 683.4 nm (b), and 699.7 nm (c) at 80 K. (d–f) Dependence of integrated luminescence intensity on the MF for the transitions of 5D1 / 7F1 (d), 5D1 / 7F2 (e), and 5D1 / 7F3 (f).
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Dependence of emission intensity on the temperature recorded at 0 T and 35 T in the spectral range of the transitions of 5D1 / 7F0 (a), 5D1 / 7F1 (b), and 5D1 / 7F3 (c). Fig. 7
which is in contradiction with the linear relationship according to eqn (1). We suggest that this non-linear relationship can be attributed to the variation of the Lande g factor with the increase of MF, as shown in a previous report.14 From a detailed analysis, it is interesting to note that the emissions originating from the 5D1 state are suppressed by approximately 19%, 18%, 19%, and 34% for the transitions of 5 D1 / 7F0, 5D1 / 7F1, 5D1 / 7F2, and 5D1 / 7F3 of Eu3+, respectively. In comparison, the emissions from the 5D0 state are only slightly reduced by 3%, 6%, and 8% for the transitions of 5D0 / 7F1, 5D0 / 7F2, and 5D0 / 7F4 of Eu3+, respectively. The MF dependence of the emission intensity (or intensity ratios of different transitions) can be used as a direct reference for MF detection. This suppression behavior may relate to the enhanced cross-relaxation between adjacent Eu3+ ions. This is based on the fact that each 4f level (J s 0) is expanded in energy and the energy mismatch between levels related to cross-relaxation is reduced by the MF. For instance, at 0 T, the energy gap of 5D1 / 5D0 is 1754 cm1, while at 35 T the size of the gap is in the range of 1736 cm1 to 1785 cm1, much closer to the transition of 7F2 / 7F4. As a result, the cross-relaxation is enhanced, leading to the population of the lower 5D0 state. In addition, the reduction in the emission intensity is apparently not due to the change in absorption as the absorption remains constant in different MF intensity in our measurement (ESI Fig. S6†). The optical transitions of Eu3+ are also sensitive to local site symmetry that determines the CF strength surrounding Eu3+.10,15 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.16 Thus, the index R, the ratio of the integrated intensity of electric dipole transition to that of magnetic dipole transition, may reect the inuence of the local site system around Eu3+ with the MF. Fig. 6 shows the decrease of index R with the increase of MF, implying the improvement of site symmetry around Eu3+. In the MF, this change in site symmetry for Eu3+ from the original Cs might also be relevant for the suppression of optical transitions. We also examined the dependence of emission on the MF at different temperatures to examine the thermal effect on Zeeman splitting. It is obvious that the energy splitting and suppression of emission by the MF are weakened with the
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increase of the temperature (Fig. 7). This is presumably connected with the decrease of the g factor at higher temperatures.17 On the other hand, it has been generally accepted for luminescent materials that the homogeneous line broadening is enhanced with temperature due to the increased coupling with lattice vibration. Therefore, this thermal broadening partially cancels the Zeeman splitting incurred by the MF, as shown in Fig. 7. However, the suppression of the emission intensity is still quite evident at different temperatures, implying that the proposed mechanism is still valid at higher temperatures.
Conclusions The MF induces Zeeman levels to the 4f states of Eu3+ in NaYF4:5% Eu NPs and the positions of these levels exhibit a clear dependence on the intensity of the MF. The luminescence bands were suppressed gradually with the applied MF and the suppression of the emission from the higher 5D1 state is stronger than that from the lower 5D0 state. The interplay of the change in site symmetry and the enhanced cross-relaxation rate in the MF are responsible for the suppression of luminescence, which differs among different levels. Combining the large shi of emission bands and the variation of the emission intensity in the MF, the Eu3+ doped NaYF4 NPs may nd potential applications in remote magnetic sensors in the nanoscale environment.
Notes and references 1 (a) S. Joo, T. Kim, S. H. Shin, J. Y. Lim, J. Hong, J. D. Song, J. Chang, H. W. Lee, K. Rhie, S. H. Han, K. H. Shin and M. Johnson, Nature, 2013, 494, 72; (b) R. Cordoba, T. I. Baturina, J. Sese, A. Y. Mironov, J. M. D. Teresa, M. R. Ibarra, D. A. Nasimov, A. K. Gutakovskii, A. V. Latyshev, I. Guillamon, H. Suderow, S. Vieira, M. R. Vaklanov and J. J. Palacios, Nat. Commun., 2013, 1, 1; (c) J. Liu, T. Gottschall, K. P. Skokov, J. D. Moore and O. Guteisch, Nat. Mater., 2012, 11, 620; (d) S. C. Riggs, O. Vafek, J. B. Kemper, J. B. Betts, A. Migliori, F. F. Balakirev, W. N. Hardy, R. Liang, D. A. Bonn and G. S. Boebinger, Nat. Phys., 2011, 7, 332. Nanoscale, 2014, 6, 11002–11006 | 11005
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2 (a) M. Tanaka, P. T. Van, J. A. Silva and L. H. Ham, Biotechnol. Biotechnol., 2010, 24, 2160; (b) D. J. Panagopoulos, A. Karabarbounis and C. Lioliousis, Cell Biochem. Biophys., 2013, 67, 703. 3 (a) J. K. Jain, Phys. Rev. B: Condens. Matter Mater. Phys., 1990, 41, 7653; (b) R. B. Laughlin, Phys. Rev. Lett., 1983, 50, 1395. 4 P. S. Pershan, J. Appl. Phys., 1967, 38, 1482. 5 B. R. Judd and I. Lindgren, Phys. Rev., 1961, 122, 1802. 6 (a) N. Chen, Y. He, Y. Su, X. Li, Q. Huang, H. Wang, X. Zhang, R. Tai and C. Fan, Biomaterials, 2012, 33, 1238; (b) C. Eggeling, J. Widengren, R. C. Rigler and A. M. Seidel, Anal. Chem., 1998, 70, 2651. 7 (a) F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen and X. Liu, Nat. Mater., 2011, 10, 968; (b) Q. Zhan, J. Qian, H. Liang, G. Somesfalean, D. Wang, S. L. He, Z. G. Zhang and S. A. Engels, ACS Nano, 2011, 5, 3744; (c) J. Zhou, N. Shirahata, H. Sun, B. Ghosh, M. Ogawara, Y. Teng, S. Zhou, R. G. S. Chu, M. Fujii and J. Qiu, J. Phys. Chem. Lett., 2013, 4, 402. 8 (a) V. K. Tikhomirov, L. F. Chibotaru, D. Saurel, P. Gredin, M. Mortier and V. V. Moshchalkov, Nano Lett., 2009, 9, 721;
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(b) L. F. Chibotaru, V. K. Tikhomirov, D. Saurel and V. V. Moshchalkov, J. Appl. Phys., 2009, 106, 053502. G. Du, P. Liu, W. Guo, Y. Han, J. Zhang, Z. Ma, J. Han, Z. Liu and K. Yao, J. Mater. Chem., 2013, 1, 7608. Y. Liu, D. Wang, J. Shi, Q. Peng and Y. Li, Angew. Chem., Int. Ed., 2013, 52, 4366. (a) D. Tu, Y. Liu, H. Zhu, R. Li, L. Liu and X. Chen, Angew. Chem., Int. Ed., 2013, 52, 1128; (b) F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976. C. Li, Z. Quan, J. Yang, P. Yang and J. Lin, Inorg. Chem., 2007, 46, 6329. G. W. F. Drake, in Springer Handbook of Atomic, Molecular and Optical Physics, ed. W. Martin and W. Wiese, Springer New York, New York, USA, 2006, vol. 2, pp. 175–198. T. Ando and Y. Uemura, J. Phys. Soc. Jpn., 1974, 37, 1044. O. Lehmann, K. Kompe and M. Haase, J. Am. Chem. Soc., 2004, 126, 14935. A. Szczeszak, T. Grzyb, B. Barszcz, V. Nagirnyi, A. Kotlov and S. Lis, Inorg. Chem., 2013, 52, 4934. H. Ye, C. Hu, G. Wang, H. Zhao, H. Tian, X. Zhang, W. Wang and B. Liu, Nanoscale Res Lett., 2011, 6, 520.
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Cite this: Nanoscale, 2014, 6, 11002
Received 30th May 2014 Accepted 11th July 2014
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Lanthanide doped nanoparticles as remote sensors for magnetic fields† Ping Chen,a Junpei Zhang,b Beibei Xu,a Xiangwen Sang,a Weibo Chen,a Xiaofeng Liu,*a Junbo Han*b and Jianrong Qiu*ac
DOI: 10.1039/c4nr02983f www.rsc.org/nanoscale
We report the effect of magnetic fields (MFs) on emission Eu-doped NaYF4 nanoparticles. A notable shift in the position of emission bands and the suppressed emission intensity are observed with the MF. These magnetic-optical interactions are explained in terms of the Zeeman effect, enhanced cross-relaxation rate and change of site symmetry.
Magnetic elds (MFs) modulate the motion of particles that carry a non-zero spin, including non-charged particle neutrons and different charged particles, such as electrons and various ions with unpaired electrons. This mechanism has fueled broad interest in the study of the interaction of a MF with diverse systems from biological objects to elementary particles and solid state materials.1 For instance, the MF has been found to affect collectively the behavior of different biological systems, resulting in the changes of their microscopic and macroscopic characteristics.2 In condensed matter physics, the Hall effect from both classical and quantum perspectives has been widely known as an outcome of the regulation of the movement of electrons, holes or ions by the Lorentz force induced by the MF.3 The interaction of the MF with photons mediated by solid state materials is also an important subject in condensed matter physics. In the macroscopic dimensions, the MF can rotate the polarization plane of the transmitted light through magnetooptic crystals, which is widely known as the Faraday effect.4 The magneto-optical interaction becomes completely different for localized electrons surrounding optically active centers. It has been established that MF splits degenerate quantized energy levels of different electronic orbitals and shi their positions by
a
Department of Materials Science and Engineering, Zhejiang University Hangzhou, Zhejiang 310027, China. E-mail: [email protected]; Fax: +86 57188925079; Tel: +86 57188925079
b
Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China
c State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China
† Electronic supplementary 10.1039/c4nr02983f
information
11002 | Nanoscale, 2014, 6, 11002–11006
(ESI)
available.
See
DOI:
the Zeeman effect, resulting in notable changes in the emission bands of luminescent materials.5 This magneto-optical effect on the photoluminescence behavior can be used to probe the MF intensity, especially for remote MF detection when nanoscale probes are employed. Semiconductors, quantum dots (QDs), and organic dyes which exhibit broadband luminescence are however not suitable as probes because the precise determination of band shis is not possible for emissions with a larger energy span. In addition, the toxic characteristics of QDs and the poor photostability of organic dyes also hinder their application as long-term probes.6 In this regard, lanthanide ion doped materials could be a better alternative for optical sensing of the MF, owing to the rich 4f levels, sharp bandwidth emission, long lifetimes, and good photostability.7 The inuence of the MF on the luminescence of lanthanide ion doped bulk materials including single crystals was revealed recently.8 The emission of nanoparticles activated with lanthanide ions is also found to be drastically affected by the external MF. In general, the emission was suppressed evidently as evidenced by optical spectroscopy.9 In this paper, we investigate the MF dependence of photoluminescence of Eu3+ doped hexagonal NaYF4 nanoparticles (NPs) and demonstrate their potential as nanoscale optical sensors for the MF. Eu3+ was chosen as an activator ion because of its high luminous efficiency, sharp emission bandwidths, and sensitivity to the local site symmetry.10 Hexagonal NaYF4 was selected as the host due to its low toxicity, good chemical stability and photostability, low phonon energy and high luminescence efficiency.11 In the presence of the MF, the shi of luminescence bands and reduction of the integrated intensity are observed clearly. This magneto-optic effect can be explained by considering the Zeeman effect, cross-relaxation, and variation of the local site symmetry. The result suggests that Eu3+ doped NaYF4 NPs might have potential applications for magnetic sensing in the nanoscale region. High quality NaYF4 NPs were synthesized by the co-precipitation method.6a The as-synthesized NPs are composed of
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uniform nanospheres with narrow size distribution centered around 23 nm (Fig. 1(a)). These NPs have a hexagonal structure as conrmed by the high-resolution TEM (HRTEM) image (Fig. 1(b)), where the (100) plane is clearly identied. Moreover, all the observed X-ray diffraction (XRD) peaks match well with the standard XRD card (JCPDS no. 16-0334) of hexagonal NaYF4 (Fig. 1(c)), indicating high phase purity.10 Fig. 1(d) shows the crystalline structure of hexagonal NaYF4. The site symmetry of Y3+ ions is C3h, in comparison, for the dopant Eu3+; its site
Fig. 1 (a) Low-resolution TEM image of NaYF4:5% Eu NPs (inset: the size distribution of NPs). (b) HRTEM image of NaYF4:5% Eu NPs. (c) XRD pattern of NPs. (d) Crystal structure of NaYF4 and Eu3+ doped NaYF4.
Fig. 2 Schematic diagram of the luminescence spectroscopy test system equipped with a pulsed MF.
Nanoscale
symmetry is reduced to Cs as the ionic radius of Eu3+ (r ¼ 1.206) is larger than the host Y3+ (r ¼ 1.159).11a A home-built luminescence spectroscopy system equipped with a pulsed MF (Fig. 2) was employed for studying the luminescence of NaYF4 NPs. A 396 nm laser beam, which was generated by using a second harmonic generator (SHG) pumped by a Ti:sapphire laser, was coupled to a ber to pump the NPs. The NPs were located in the center of a pulsed MF generated by a liquid nitrogen-cooled resistive coil magnet with a pulsed duration of 290 milliseconds (ms) and a falling side of 270 ms. The emission spectrum was collected by using the same ber system. The emitted photons were transmitted to the detection part aer spatial and spectral ltering, and subsequently detected using a spectrometer equipped with an electron multiplying charge coupled device (CCD) detector. The emission spectrum of NPs without the MF was obtained at 80 K (Fig. 3(a)). Several emission bands are observed in the region of 500–700 nm, which originate from different 5Dj / 7Fj transitions.12 All the emission bands can be ascribed to the energy level diagram shown in Fig. 3(b). Due to crystal eld (CF) splitting, the emission bands for the transition of 5D0 / 7F1 can be decomposed into three peaks, while the 5D0 / 7F2 transition split into ve peaks, indicating the distortion of site symmetry from C3h for Eu3+ ions.10 The optical spectra change dramatically in the presence of a pulsed MF (Fig. 4, S2 and S3 (ESI)†), especially for transitions from the 5D1 state. Furthermore, luminescence spectra indicate clear blue or red shis of the emission and the splitting of energy levels for most of the observed transitions. For instance, when the MF intensity rises from 0 T to 35 T, the number of luminescence peaks increases from one to three for the 5 D1 / 7F0 transition (Fig. 4(a)). Similar behavior is observed for transitions of 5D1 / 7F3 (Fig. 4(d)) and 5D0 / 7F2 (ESI, Fig. S3†). This is because there are 2J + 1 Zeeman levels for each state with a total angular momentum of J (ESI†).13 Here, the number of observed emission peaks for each transition in the MF matches exactly with the number of Zeeman levels of the 7FJ states, implying that non-radiative relaxation might occur from a higher to the lowest Zeeman levels for the excited state 5DJ before the radiative transition 5DJ / 7FJ.
Fig. 3 (a) Emission spectrum of NPs excited at 396 nm. (b) Energy level diagram of an Eu3+ ion with possible emission pathways. The field induced energy level splitting is shown in the right.
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the MF. This can result in both negative and positive shis of the position depending on the quantum nature of the respective Zeeman levels. For instance, the gaps among the nine Zeeman levels of the 7F4 state which has a larger total momentum expand notably in the MF, leading to the obvious blue shi in the short wavelength edge and red shi in the long wavelength edge (ESI Fig. S3(c)†). The MF induced change of its position can be described by the following equation (details are provided in the ESI†): Ds ¼
Fig. 4 (a) Emission spectra of the NPs with the transition of 5D1 / 7F0
recorded at different MF intensities at 80 K. (b) Gaussian fitting for the emission spectrum for the 5D1 / 7F0 transition at 35 T. (c and d) Emission spectra recorded at different MF intensities at 80 K for NPs with the transitions of 5D1 / 7F1 (c) and 5D1 / 7F3 (d). The excitation is a 396 nm laser (30 mW).
For a closer insight into the magnetic-optical interaction, we check each of the emission bands in more detail. We then dene the zero energy as the ground state 7F0 of Eu3+, which is not affected by the MF because its total angular momentum J is zero. The three Zeeman energy levels of 5D1 are found at 16 957 cm1, 16 896 cm1, and 17 264 cm1 from the spectra of the transition 5D1 / 7F0 by the Gaussian tting (Fig. 4(b)). Other energy levels of Eu3+ with and without the MF were located and are listed in Table S1 (ESI†). In the presence of the MF, the external torque induced by the MF adds an external energy to each energy level and increases with the strength of
DE2 DE 1 e ¼ ðM2 g2 M1 g1 Þ B h 2pmc
(1)
where Ds is the energy change in the wave-number (cm1). DE1, DE2 are the external energies of the system before and aer transition in the presence of the MF. h is Planck's constant, g is the Lande factor, e is the electric charge of an electron, m is the mass of an electron and B is the MF intensity. The equation indicates the linear relationship between the change of the Zeeman level position and the MF intensity. However, from Fig. 5(a)–(c), the position of the observed energy changes slowly at a low MF but very rapidly at a high MF, indicating that the inuence of the energy change on the MF intensity is nonlinear,
Fig. 6 The dependence of index R and the integrated emission intensity on the MF of the transitions of 5D0 / 7F1 and 5D0 / 7F2 intensity at 80 K.
Fig. 5 (a–c) Dependence of energy change on the MF intensity for the luminescence bands centered at 535.8 nm (a), 683.4 nm (b), and 699.7 nm (c) at 80 K. (d–f) Dependence of integrated luminescence intensity on the MF for the transitions of 5D1 / 7F1 (d), 5D1 / 7F2 (e), and 5D1 / 7F3 (f).
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Dependence of emission intensity on the temperature recorded at 0 T and 35 T in the spectral range of the transitions of 5D1 / 7F0 (a), 5D1 / 7F1 (b), and 5D1 / 7F3 (c). Fig. 7
which is in contradiction with the linear relationship according to eqn (1). We suggest that this non-linear relationship can be attributed to the variation of the Lande g factor with the increase of MF, as shown in a previous report.14 From a detailed analysis, it is interesting to note that the emissions originating from the 5D1 state are suppressed by approximately 19%, 18%, 19%, and 34% for the transitions of 5 D1 / 7F0, 5D1 / 7F1, 5D1 / 7F2, and 5D1 / 7F3 of Eu3+, respectively. In comparison, the emissions from the 5D0 state are only slightly reduced by 3%, 6%, and 8% for the transitions of 5D0 / 7F1, 5D0 / 7F2, and 5D0 / 7F4 of Eu3+, respectively. The MF dependence of the emission intensity (or intensity ratios of different transitions) can be used as a direct reference for MF detection. This suppression behavior may relate to the enhanced cross-relaxation between adjacent Eu3+ ions. This is based on the fact that each 4f level (J s 0) is expanded in energy and the energy mismatch between levels related to cross-relaxation is reduced by the MF. For instance, at 0 T, the energy gap of 5D1 / 5D0 is 1754 cm1, while at 35 T the size of the gap is in the range of 1736 cm1 to 1785 cm1, much closer to the transition of 7F2 / 7F4. As a result, the cross-relaxation is enhanced, leading to the population of the lower 5D0 state. In addition, the reduction in the emission intensity is apparently not due to the change in absorption as the absorption remains constant in different MF intensity in our measurement (ESI Fig. S6†). The optical transitions of Eu3+ are also sensitive to local site symmetry that determines the CF strength surrounding Eu3+.10,15 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.16 Thus, the index R, the ratio of the integrated intensity of electric dipole transition to that of magnetic dipole transition, may reect the inuence of the local site system around Eu3+ with the MF. Fig. 6 shows the decrease of index R with the increase of MF, implying the improvement of site symmetry around Eu3+. In the MF, this change in site symmetry for Eu3+ from the original Cs might also be relevant for the suppression of optical transitions. We also examined the dependence of emission on the MF at different temperatures to examine the thermal effect on Zeeman splitting. It is obvious that the energy splitting and suppression of emission by the MF are weakened with the
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increase of the temperature (Fig. 7). This is presumably connected with the decrease of the g factor at higher temperatures.17 On the other hand, it has been generally accepted for luminescent materials that the homogeneous line broadening is enhanced with temperature due to the increased coupling with lattice vibration. Therefore, this thermal broadening partially cancels the Zeeman splitting incurred by the MF, as shown in Fig. 7. However, the suppression of the emission intensity is still quite evident at different temperatures, implying that the proposed mechanism is still valid at higher temperatures.
Conclusions The MF induces Zeeman levels to the 4f states of Eu3+ in NaYF4:5% Eu NPs and the positions of these levels exhibit a clear dependence on the intensity of the MF. The luminescence bands were suppressed gradually with the applied MF and the suppression of the emission from the higher 5D1 state is stronger than that from the lower 5D0 state. The interplay of the change in site symmetry and the enhanced cross-relaxation rate in the MF are responsible for the suppression of luminescence, which differs among different levels. Combining the large shi of emission bands and the variation of the emission intensity in the MF, the Eu3+ doped NaYF4 NPs may nd potential applications in remote magnetic sensors in the nanoscale environment.
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