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Ho3+ microspheres via Mn2+ doping
Optical Materials 79 (2018) 403–407
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Tunable multicolor and enhanced red emission of monodisperse CaF2:Yb3+/ Ho3+ microspheres via Mn2+ doping
T
Rui Wanga,b, Maohui Yuana,b,∗, Chaofan Zhanga,b, Hongyan Wanga,b,∗∗, Xiaojun Xua,b a b
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China Interdisciplinary Center for Quantum Information, National University of Defense Technology, Changsha, 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn2+ doping CaF2 microsphere Yb-Ho Tunable multicolor Enhanced red emission
Transition metal ions (e.g. Mn2+) and lanthanide co-doped upconversion (UC) materials have attracted wide attention in recent years due to their promising application in multicolor display. Here, we report the hydrothermal synthesis and characterization of Mn2+ doped monodisperse CaF2:Yb3+/Ho3+ microspheres. The results of X-ray diffraction (XRD) revealed that Mn2+ doping does not change the cubic phase of CaF2 material but will lead to diffraction peaks shifting slightly towards higher angle due to the substitution of larger Ca2+ by the relatively smaller Mn2+. Under the excitation of 980 nm continuous wave (CW) laser, these microspheres exhibit green-yellow-red tuning colors and remarkable enhancement of both red to green ratio (R/G) and red to blue ratio (R/B) when increasing Mn2+ concentration from 0 to 30 mol%. The energy migration process between Ho3+ and Mn2+ was proposed and supported by time-decay and power dependence measurements of Ho3+ UC emission. These upconversion materials may have potential applications in optical devices, color display, nanoscale lasers and biomedical imaging.
1. Introduction UC materials that exhibit anti-Stocks emission are capable of converting near infrared (NIR) light into ultraviolet or visible light [1–3]. Photon upconversion through the use of lanthanide doped nano/micro materials have been extensively applied in various applications like multicolor display [4–6], bioimaging [7–9], photodynamic therapy [10,11], drug delivery [12,13] and sensing [14], due to its sharp emission bandwidths, efficient energy transfer UC process, tunable emission and long excited-state lifetimes [15]. In particular, tremendous effort has been dedicated to realizing multicolor UC emissions under single wavelength excitation because of its potential in a wide array of research fields, such as multiplexed detection, multicolor imaging [16,17], complex data analysis, information storage, as well as graphics imaging and display. Moreover, lanthanide doped UC nano/ micro materials' application in biomedicine favors red emission centered at 660 nm owing to its low absorption by biological tissues or cells [18,19]. In consequence, it is of profound significance to achieve a wide range spectrum tuning as well as enhanced red emission. Among all rare earth ions, Ho3+ is one of the most popular active center for UC luminescence owing to its abundant intra-4f energy level structure. However, Yb3+/Ho3+ co-doped nano/micro systems usually
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simultaneously exhibit blue (5F3→5I8), green (5S2/5F4→5I8) and red (5F5→5I8) colors, which not only reduces luminescence efficiency of red emission, but also impedes their further application in bioimaging [20]. Therefore, to overcome this drawback, several attempts have been made to obtain tunable emission as well as increased red luminescence of Ho3+. For instance, emission color of UC materials can be modulated by varying the wavelength, power density and pulse width of the excitation laser [4,21,22]. But these methods may be hindered by extra requirements for excitation sources. Recently, lanthanide doped UC materials with Mn2+ ions have proved to be an effective strategy to realize simultaneous control of crystal phase and emission spectrum. For example, Liu's group reported a novel oil-based procedure for the synthesis of Ho3+ doped KMnF3 nanocrystals with single-band UC emissions [19]. Zhang's group further investigated the bidirectional energy transfer mechanism by controlling the doping concentration ratio of Mn2+ to Ho3+ in perovskite KZnF3 nanocrystals [23]. Krishnan et al. detailed studied the influence of different synthesis parameters on the luminescence, crystal phase and morphology of the NaYF4: Yb3+/ Ho3+/Mn2+ upconversion nanophosphors [24]. Except for widely studied Mn2+ doped KMnF3 and NaYF4 nanocrystals mentioned above, CaF2 nano/micro material is also a promising yet under-studied host matrix, which has gained recognition in various
Corresponding author. College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China. Corresponding author. College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China. E-mail addresses: [email protected] (M. Yuan), [email protected] (H. Wang).
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https://doi.org/10.1016/j.optmat.2018.03.048 Received 2 February 2018; Received in revised form 16 March 2018; Accepted 27 March 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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performed at room temperature.
applications like multifunctional nanoprobing [25], biological luminescent labelling [26,27] and drug delivery [28]. Nano/micro CaF2 crystal exhibits non-toxicity to biological tissues, low-phonon energy of the crystal lattice, wide transparent spectral region and easy substitution by trivalent lanthanide ions with comparable ionic radius, although charge compensation must be considered [25,29]. Charge compensation mechanism distorts the local symmetry, while at the same time, this mechanism may influence the spectroscopic behavior of lanthanide ions. Besides, the previously reported Yb3+/Ho3+/Mn2+ tri-doped UC nano/micro crystals were synthesized using oleic acid (OA) as solvents, which finally formed hydrophobic nanoparticles. Therefore, it is important to synthesize hydrophilic UC nano/micro materials for additional surface modification in biological applications [8,30]. In this paper, we present Mn2+ doped CaF2:Yb3+/Ho3+ monodisperse microspheres through an OA-free single-step hydrothermal method. The influence of Mn2+ doping on the phase of CaF2 micro crystals and Yb3+/Ho3+ UC emissions were performed in detail. Under the excitation of 980 nm CW laser, the as-synthesized microspheres exhibit tunable green-yellow-red UC emissions and striking enhancement of red color emission with increased Mn2+ concentration. The lifetime measurement as well as the dependence of luminescence intensity on the excitation power directly proved the proposed the energy migration process between Ho3+ and Mn2+.
3. Results and discussion 3.1. Morphology and structure The morphology and phase of as-prepared CaF2:Yb3+/Ho3+/Mn2+ microspheres were characterized by scanning electron microscopy (SEM) and XRD. Fig. 1(a) demonstrates the low magnification SEM image of representative samples with Mn2+ doping concentration at 10 mol%. It can be seen that the samples were monodispersed with uniformed size distribution and microsphere shape. Fig. 1(b–f) provides more detailed microstructural information and this set of SEM micrographs depict that these as-prepared particles with different Mn2+ concentrations adopt a spherical morphology with rough surfaces and an average diameter of ∼2 μm. We can also find that the morphology and microstructure of CaF2 microspheres do not alter with increasing the concentration of Mn2+. The XRD results of as-synthesized microspheres are displayed in Fig. 1 (g). All of the diffraction peaks detected from 20° to 80° can be indexed to the cubic phase of bulk (Ca0.8Yb0.2) F2.2 material (JCPDS No. 87–0976), indicating that the rare earth ions have entered into the host lattices by occupying the chemical substitutional sites. The diffraction peaks of as-prepared microspheres were sharp and intense, indicating their highly crystalline nature. No impurity peaks were observed, confirming the high purity of the products. Furthermore, the pure cubic phase structure with high crystalline nature was maintained even when the concentration of Mn2+ increased up to 30 mol%. What is noticeable is that the diffraction peaks shifts slightly towards higher angle side as a result of the decrease of the unitcell volume of CaF2 host, owning to the substitution of larger Ca2+ (crystal radius = 1.26 Å) by the relatively smaller Mn2+ (crystal radius = 1.10 Å) [31].
2. Experimental details 2.1. Synthesis of CaF2:Yb3+/Ho3+/Mn2+ microspheres YbCl3·6H2O (99.9%), HoCl3·6H2O (99.9%), Ethylenediaminetetraacetic acid (EDTA) (99%) and NaBF4 (99.99%) were purchased from Aladdin Industrial Corporation. CaCl2 (96%) and MnCl2·4H2O (99%) were purchased from Alfa Aesar, Thermo Fisher Scientific. All of the chemical reagents were used as received without further purification. Deionized water was used throughout as solvent. The CaF2:Yb3+/Ho3+/Mn2+ microspheres were synthesized by a single-step hydrothermal method using water as solvent and EDTA as capping agents. Typically, 2 mmol of chloride salts was dissolved in 20 ml of deionized water with 1 mmol EDTA, and the mixed solution was stirred for 1 h. 4 mmol NaBF4 was dissolved in 10 ml deionized water and transferred to the chloride salts solution prepared above with another 1 h of vigorous stirring until a milky colloidal solution was obtained. Subsequently, the mixed solution was transferred to a 50 ml Teflon-lined autoclave and heated to 180 °C for 30 h. After the system was cooled to room temperature, the microspheres were collected by centrifugation at 4000 rpm for 4 min, washed with water and ethanol for several times to remove any possible remnants. Finally, the precipitate was dried in air at 40 °C for 24 h to obtain white CaF2 microspheres.
3.2. Luminescence In order to investigate the impact of Mn2+ doping on luminescence properties of CaF2:Yb3+/Ho3+ UC microspheres, upconversion emission spectra of these synthetic samples were measured under the excitation of 980 nm CW laser with pumping density of 127.2 kW/cm2. Fig. 2(a) presents the normalized UC luminescence spectra of the CaF2:Yb3+/Ho3+ microspheres with Mn2+ doping ratio at 0, 10 mol% and 30 mol%, and the insets show the corresponding luminescence color, respectively. The spectra consists of three emission bands centered at 492 nm (blue), 548 nm (green) and 647 nm (red), corresponding to the transitions of 5F3→5I8, 5S2/5F4→5I8 and 5F5→5I8 of Ho3+, respectively. For microspheres without Mn2+ doping, the intensity of the green emission is the strongest, followed by red color emission with only half intensity of it, and the blue emission is barely noticeable. When the concentration of Mn2+ increased to 10 mol%, the green emission decreases while the corresponding red emission increases quickly and exceeds the green emission, leading to luminescence color changing from green to bright yellow. By increasing Mn2+ concentration to 30 mol%, the green color band diminishes further and the emission color is finally tuned to red. The corresponding CIE chromaticity coordinates of as-prepared microspheres are calculated and illustrated in Fig. 2(b), indicating a wide range of emission colors (green-yellow-red) can be achieved by increasing the concentration of Mn2+, which is consistent with the result of the emission spectra and luminescence color. To further prove the tunability of Mn2+ doped CaF2:Yb3+/Ho3+ UC microspheres, a series of samples with Mn2+ concentration at 0, 2.5, 5, 7.5, 10, 20, 30 mol% were synthesized and characterized by UC photoluminescence spectra. The intensity ratios of red to green (R/G) and red to blue (R/B) are calculated and displayed in Fig. 3. As Mn2+ concentration increases from 0 to 30 mol%, the R/G and R/B ratio can be promoted from 0.48/4.23 up to 3.36/23.56, corresponding to a 7-
2.2. Instruments and measurements The crystal phase of the synthetic UC microspheres was measured through powder XRD by using X-ray diffractometer with Cu K radiation at 40 kV and 200 mA (Rigaku TTR Ⅲ system). Meanwhile, the size and morphology of these UC microspheres were characterized by SEM (HITACHI S-4800). In photoluminescence experiments, the 980 nm laser was used as the excitation light source, which was introduced into an inverted microscope (Observer A1, Zeiss) and focused on the sample by using a 100 × objective lens (NA = 1.4). The diameter of the excitation spot was estimated to be ∼2.0 μm. The luminescence generated by the sample was collected by the same objective lens and then delivered to a high resolution spectrometer (SR-500I-B1, Andor) equipped with a color charge-coupled device (CCD) (DU970 N, Andor) for analysis. The photos of the excitation spot were taken by a camera from the eyepiece of the microscope. All the above measurements were 404
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Fig. 1. SEM images of CaF2:Yb3+/Ho3+ (20/2 mol%) microspheres (a) with doping 10 mol% Mn2+ ions at low magnification, (b–f) with doping 0, 5, 10, 20, 30 mol % Mn2+ ions at high magnification, respectively. (g) XRD patterns of samples, (h) local magnification of XRD patterns, the shift of the peaks to higher angle indicates the decreased unit-cell volume.
upper state of 2F7/2 level. After that, efficient energy transfer occurred between Yb3+ and Ho3+, resulting in electrons of Ho3+ excited from 5 I8 to 5I6 level. Subsequent energy transfer can also populate 5F5 from the low-lying 5I7 level of Ho3+. A second 980 nm photon can populate 5 F4/5S2 level of Ho3+ ion, and afterwards non-radiative relaxation results in population of 5F5 level of Ho3+ ion. In addition, a third 980 nm photon can populate the 5F2/3K8 level of Ho3+, which finally leads to population of 5F3 level by non-radiative relaxation. Then the emissions
fold and 5.5-fold enhancement, which obviously confirm the role of Mn2+ in enhancing red emission, as well as suppressing green and blue emission in Yb3+/Ho3+ co-doped systems. In order to further explore the effect of Mn2+ doping on UC luminescence properties of Yb3+/Ho3+/Mn2+ triply-doped CaF2 microspheres, a proposed energy level diagram and the corresponding energy transfer mechanism is presented in Fig. 4. Under the excitation of 980 nm CW laser, the electrons of Yb3+ in 2F7/2 level are excited to the
Fig. 2. (a) Normalized upconversion emission spectra of CaF2:Yb3+/Ho3+ (20/2 mol%) microspheres doped with different contents of Mn2+ under the excitation of 980 nm CW laser at a power density of 127.2 kW/cm2. The insets are the corresponding luminescence color, (b) CIE chromaticity diagram of CaF2:Yb3+/Ho3+ with different concentrations of Mn2+. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 405
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from these populated states to the ground states take place, including strong green (5S2/5F4→5I8), red (5F5→5I8) and weak blue emission (5F3→5I8) of Ho3+ ion. When Mn2+ is co-doped into the host matrix with Yb3+ and Ho3+, the non-radiative energy transfer from Ho3+ to Mn2+ (5F2/3K8, 5F3, 5S2/5F4→4T1) occurs, followed by back-energy transfer from Mn2+to Ho3+ (4T1→5F5), which increases the number of electrons in the upper level of red emission. With Mn2+ doping concentration increasing, the distance between Mn2+ and Ho3+ decreases gradually, which enhances the energy exchange between Mn2+ and Ho3+ and results in remarkable enhancement of red color emission as well as weakening effect on green and blue emission. Next, we performed time-decay measurement of Ho3+: 5S2/5F4 and 5 F5 level in CaF2:Yb3+/Ho3+/Mn2+ microspheres under 980 nm pulsed laser excitation to prove the proposed mechanism of energy exchange between Ho3+ and Mn2+, As shown in Fig. 5, the lifetime of the 5S2/5F4 levels decreases from 97μs to 52μs when Mn2+ doping concentration increases from 0 to 30 mol%, due to the energy migration from Ho3+ to Mn2+. On the contrary, the lifetime of 5F5 level experiences a dramatic increment from 51μs to 136μs when Mn2+ doping was introduced, confirming the back energy transfer from Mn2+ to Ho3+. The results of time-decay measurement also agree well with the upconversion emission spectra in Fig. 2. Besides, we also measured the dependence of upconversion luminescence intensity on the 980 nm NIR excitation power of representative samples (CaF2:20% Yb3+/2%Ho3+/10% Mn2+), as shown in Fig. 6. The number of infrared multi-photons
Fig. 3. The tunable R/G and R/B ratio of CaF2:Yb3+/Ho3+ with different concentrations of Mn2+ (where R, G and B stands for Red, Green and Blue, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Schematic energy level diagram of CaF2:Yb3+/Ho3+/Mn2+ and the proposed mechanism of UC process under the excitation of 980 nm laser. Fig. 6. Plot of the upconversion emission intensity of CaF2:Yb3+/Ho3+/Mn2+ (20/2/10 mol%) microspheres as a function of the pump power density under 980 nm excitation.
Fig. 5. Time-decay curves of (a) 5S2/5F4 and (b) 5F5 energy levels of Ho3+ recorded in CaF2:Yb3+/Ho3+/Mn2+ microspheres with Mn2+ doping concentration at 0, 10 mol% and 30 mol%. 406
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involved in a single visible photon generation could be determined from the slope of linear fitting in a double logarithmic plot of the emitting intensity as the function of excitation intensity. The slopes of blue, green and red fluorescence are 0.81, 0.61 and 0.83 under low pump power, and the corresponding numbers under intermediate-power is 2.46, 1.74 and 2.86, respectively. The divergence between low excitation power and intermediate-power is due to the nonlinear absorption effects of materials where absorption coefficient increases with incident laser power [32–34]. Based on the energy transfer mechanism in Fig. 4, the green and red emission ought to be a two-photon process while the blue fluorescence is a three-photon process. However, the measured results under intermediate-power has obvious deviations with the theoretical values. This could be explained by the energy migration from Ho3+ to 4T1 level of Mn2+ and the back-energy transfer process to Ho3+, which reduces the electrons on upper levels of green and blue emissions and adds electrons to the excited states of red emission. In consequence, the measured slopes for green and blue emissions are smaller than expected values and the slope for red color is distinctively larger than normal value.
[9] J. Zhou, Z. Liu, F. Li, Upconversion nanophosphors for small-animal imaging, Chem. Soc. Rev. 41 (2012) 1323–1349. [10] D. Bechet, P. Couleaud, C. Frochot, M. Viriot, F. Guillemin, M. Barberi-Heyob, Nanoparticles as vehicles for delivery of photodynamic therapy agents, Trends Biotechnol. 26 (2008) 612–621. [11] N.M. Idris, M.K. Gnanasammandhan, J. Zhang, P.C. Ho, R. Mahendran, Y. Zhang, In vivo photodynamic therapy using upconversion nanoparticles as remote-controlled nanotransducers, Nat. Med. 18 (2012) 1580–1585. [12] C. Wang, L. Cheng, Z. Liu, Drug delivery with upconversion nanoparticles for multifunctional targeted cancer cell imaging and therapy, Biomaterials 32 (2011) 1110–1120. [13] G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li, Y. Zhao, Mn2+ dopant-controlled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery, Adv. Mater. 24 (2012) 1226–1231. [14] F. Vetrone, R. Naccache, A. Zamarrόn, A. Juarranz De La Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. Martín Rodriguez, D. Jaque, J. García Solé, J.A. Capobianco, Temperature sensing using fluorescent nanothermometers, ACS Nano 4 (2010) 3254–3258. [15] S. Gai, C. Li, P. Yang, J. Lin, Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications, Chem. Rev. 114 (2013) 2343–2389. [16] F. Wang, X. Liu, Multicolor tuning of lanthanide-doped nanoparticles by single wavelength excitation, Accounts Chem. Res. 47 (2014) 1378–1385. [17] Y. Zhang, L. Zhang, R. Deng, J. Tian, Y. Zong, D. Jin, X. Liu, Multicolor barcoding in a single upconversion crystal, J. Am. Chem. Soc. 136 (2014) 4893–4896. [18] Z. Bai, H. Lin, J. Johnson, S.C. Rong Gui, K. Imakita, R. Montazami, M. Fujii, N. Hashemi, The single-band red upconversion luminescence from morphology and size controllable Er3+/Yb3+ doped MnF2 nanostructures, J. Mater. Chem. C 2 (2014) 1736. [19] J. Wang, F. Wang, C. Wang, Z. Liu, X. Liu, Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals, Angew. Chem. Int. Ed. 50 (2011) 10369–10372. [20] N. Niu, P. Yang, Y. Liu, C. Li, D. Wang, S. Gai, F. He, Controllable synthesis and upconversion properties of tetragonal BaYF5: Yb/Ln (Ln=Er, Tm, and Ho) nanocrystals, J. Colloid Interface Sci. 362 (2011) 389–396. [21] H. Gong, D. Yang, X. Zhao, E. Yun Bun Pun, H. Lin, Upconversion color tunability and white light generation in Tm3+/Ho3+/Yb3+ doped aluminum germanate glasses, Opt. Mater. 32 (2010) 554–559. [22] X. Xie, N. Gao, R. Deng, Q. Sun, Q. Xu, X. Liu, Mechanistic investigation of photon upconversion in Nd3+ -sensitized core-shell nanoparticles, J. Am. Chem. Soc. 135 (2013) 12608–12611. [23] Z.T. Chen, E.H. Song, M. Wu, S. Ding, S. Ye, Q.Y. Zhang, Bidirectional energy transfer induced single-band red upconversion emission of Ho3+ in KZnF3:Mn2+ ,Yb3+ ,Ho3+ nanocrystals, J. Alloy. Comp. 667 (2016) 134–140. [24] K. Lingeshwar Reddy, V. Srinivas, K.R. Shankar, S. Kumar, V. Sharma, A. Kumar, A. Bahuguna, K. Bhattacharyya, V. Krishnan, Enhancement of luminescence intensity in red emitting NaYF4:Yb/Ho/Mn upconversion nanophosphors by variation of reaction parameters, J. Phys. Chem. C 121 (2017) 11783–11793. [25] N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J.E. Ramírez-Hernández, L.M. Maestro, M.C. Iglesias-de La Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J.A. Capobianco, J.G. Solé, M. Bettinelli, D. Jaque, A. Speghini, NIR-toNIR two-photon excited CaF2:Tm3+ ,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-imaging, ACS Nano 5 (2011) 8665–8671. [26] G. Wang, Q. Peng, Y. Li, Upconversion luminescence of monodisperse CaF2:Yb3+/ Er3+ nanocrystals, J. Am. Chem. Soc. 131 (2009) 14200–14201. [27] M. Misiak, M. Skowicki, T. Lipiński, A. Kowalczyk, K. Prorok, S. Arabasz, A. Bednarkiewicz, Biofunctionalized upconverting CaF2:Yb,Tm nanoparticles for Candida albicans detection and imaging, Nano Res. 10 (2017) 3333–3345. [28] X. Deng, Y. Dai, J. Liu, Y. Zhou, P. Ma, Z. Cheng, Y. Chen, K. Deng, X. Li, Z. Hou, C. Li, J. Lin, Multifunctional hollow CaF2:Yb3+/Er3+/Mn2+-poly(2-Aminoethyl methacrylate) microspheres for Pt(IV) pro-drug delivery and tri-modal imaging, Biomaterials 50 (2015) 154–163. [29] M. Pedroni, F. Piccinelli, T. Passuello, M. Giarola, G. Mariotto, S. Polizzi, M. Bettinelli, A. Speghini, Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method: toward efficient materials with near infrared-to-near infrared upconversion emission, Nanoscale 3 (2011) 1456. [30] L. Xiong, T. Yang, Y. Yang, C. Xu, F. Li, Long-term in vivo biodistribution imaging and toxicity of polyacrylic acid-coated upconversion nanophosphors, Biomaterials 31 (2010) 7078–7085. [31] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides, Acta Crystallogr. A 32 (1976) 751–767. [32] L. Tian, Z. Xu, S. Zhao, Y. Cui, Z. Liang, J. Zhang, X. Xu, The upconversion luminescence of Er3+/Yb3+/Nd3+ triply-doped β-NaYF4 nanocrystals under 808-nm excitation, Materials 7 (2014) 7289–7303. [33] M. Joubert, Photon avalanche upconversion in rare earth laser materials, Opt. Mater. 11 (1999) 181–203. [34] L.W. Tutt, S.W. McCahon, Reverse saturable absorption in metal cluster compounds, Opt. Lett. 15 (1990) 700–702.
4. Conclusions In conclusion, we synthesize CaF2:Yb3+/Ho3+ microspheres doped with different concentration of Mn2+ (0, 2.5, 5, 7.5, 10, 20 and 30 mol %) via a single step OA-free hydrothermal method. The SEM and XED results show that the as-synthesized samples possess highly crystalline structure and purity. The UC luminescence of as-prepared microspheres can be tuned from green to yellow and then to red by increasing the concentration of Mn2+. Besides that, the highly improved R/G and R/B ratio further testified the ability of Mn2+ in enhancing red emission. The mechanism of energy migration process between Ho3+ and Mn2+ is proposed and backed up by time-decay and power dependence measurement of Ho3+ UC emission. These qualities make Mn2+ doped CaF2:Yb3+/Ho3+ microspheres more competitive in color display and optical bioimaging. Acknowledgements We are extremely thankful to Prof. S. Lan (South China Normal University, China) for allowing us to use his laboratory facilities to measure the optical properties of the samples. C. F. Zhang thanks the financial support from the National Natural Science Foundation of China (Grant No. 11774427). References [1] F. Auzel, Upconversion and anti-Stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 19–174. [2] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976. [3] X. Chen, D. Peng, Q. Ju, F. Wang, Photon upconversion in core-shell nanoparticles, Chem. Soc. Rev. 44 (2015) 1318–1330. [4] R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, X. Liu, Temporal full-colour tuning through non-steady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. [5] X. Li, Z. Xue, H. Liu, Hydro-thermal synthesis of PEGylated Mn2+ dopant controlled NaYF4: Yb/Er up-conversion nano-particles for multi-color tuning, J. Alloy. Comp. 681 (2016) 379–383. [6] H. Wang, W. Lu, Z. Yi, L. Rao, S. Zeng, Z. Li, Enhanced upconversion luminescence and single-band red emission of NaErF4 nanocrystals via Mn2+ doping, J. Alloy. Comp. 618 (2015) 776–780. [7] R. Kumar, M. Nyk, T.Y. Ohulchanskyy, C.A. Flask, P.N. Prasad, Combined optical and MR bioimaging using rare earth ion doped NaYF4 nanocrystals, Adv. Funct. Mater. 19 (2009) 853–859. [8] Q. Liu, Y. Sun, T. Yang, W. Feng, C. Li, F. Li, Sub-10nm hexagonal lanthanide-doped NaLuF4 upconversion nanocrystals for sensitive bioimaging in vivo, J. Am. Chem. Soc. 133 (2011) 17122–17125.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Tunable multicolor and enhanced red emission of monodisperse CaF2:Yb3+/ Ho3+ microspheres via Mn2+ doping
T
Rui Wanga,b, Maohui Yuana,b,∗, Chaofan Zhanga,b, Hongyan Wanga,b,∗∗, Xiaojun Xua,b a b
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China Interdisciplinary Center for Quantum Information, National University of Defense Technology, Changsha, 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn2+ doping CaF2 microsphere Yb-Ho Tunable multicolor Enhanced red emission
Transition metal ions (e.g. Mn2+) and lanthanide co-doped upconversion (UC) materials have attracted wide attention in recent years due to their promising application in multicolor display. Here, we report the hydrothermal synthesis and characterization of Mn2+ doped monodisperse CaF2:Yb3+/Ho3+ microspheres. The results of X-ray diffraction (XRD) revealed that Mn2+ doping does not change the cubic phase of CaF2 material but will lead to diffraction peaks shifting slightly towards higher angle due to the substitution of larger Ca2+ by the relatively smaller Mn2+. Under the excitation of 980 nm continuous wave (CW) laser, these microspheres exhibit green-yellow-red tuning colors and remarkable enhancement of both red to green ratio (R/G) and red to blue ratio (R/B) when increasing Mn2+ concentration from 0 to 30 mol%. The energy migration process between Ho3+ and Mn2+ was proposed and supported by time-decay and power dependence measurements of Ho3+ UC emission. These upconversion materials may have potential applications in optical devices, color display, nanoscale lasers and biomedical imaging.
1. Introduction UC materials that exhibit anti-Stocks emission are capable of converting near infrared (NIR) light into ultraviolet or visible light [1–3]. Photon upconversion through the use of lanthanide doped nano/micro materials have been extensively applied in various applications like multicolor display [4–6], bioimaging [7–9], photodynamic therapy [10,11], drug delivery [12,13] and sensing [14], due to its sharp emission bandwidths, efficient energy transfer UC process, tunable emission and long excited-state lifetimes [15]. In particular, tremendous effort has been dedicated to realizing multicolor UC emissions under single wavelength excitation because of its potential in a wide array of research fields, such as multiplexed detection, multicolor imaging [16,17], complex data analysis, information storage, as well as graphics imaging and display. Moreover, lanthanide doped UC nano/ micro materials' application in biomedicine favors red emission centered at 660 nm owing to its low absorption by biological tissues or cells [18,19]. In consequence, it is of profound significance to achieve a wide range spectrum tuning as well as enhanced red emission. Among all rare earth ions, Ho3+ is one of the most popular active center for UC luminescence owing to its abundant intra-4f energy level structure. However, Yb3+/Ho3+ co-doped nano/micro systems usually
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simultaneously exhibit blue (5F3→5I8), green (5S2/5F4→5I8) and red (5F5→5I8) colors, which not only reduces luminescence efficiency of red emission, but also impedes their further application in bioimaging [20]. Therefore, to overcome this drawback, several attempts have been made to obtain tunable emission as well as increased red luminescence of Ho3+. For instance, emission color of UC materials can be modulated by varying the wavelength, power density and pulse width of the excitation laser [4,21,22]. But these methods may be hindered by extra requirements for excitation sources. Recently, lanthanide doped UC materials with Mn2+ ions have proved to be an effective strategy to realize simultaneous control of crystal phase and emission spectrum. For example, Liu's group reported a novel oil-based procedure for the synthesis of Ho3+ doped KMnF3 nanocrystals with single-band UC emissions [19]. Zhang's group further investigated the bidirectional energy transfer mechanism by controlling the doping concentration ratio of Mn2+ to Ho3+ in perovskite KZnF3 nanocrystals [23]. Krishnan et al. detailed studied the influence of different synthesis parameters on the luminescence, crystal phase and morphology of the NaYF4: Yb3+/ Ho3+/Mn2+ upconversion nanophosphors [24]. Except for widely studied Mn2+ doped KMnF3 and NaYF4 nanocrystals mentioned above, CaF2 nano/micro material is also a promising yet under-studied host matrix, which has gained recognition in various
Corresponding author. College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China. Corresponding author. College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, 410073, China. E-mail addresses: [email protected] (M. Yuan), [email protected] (H. Wang).
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https://doi.org/10.1016/j.optmat.2018.03.048 Received 2 February 2018; Received in revised form 16 March 2018; Accepted 27 March 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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performed at room temperature.
applications like multifunctional nanoprobing [25], biological luminescent labelling [26,27] and drug delivery [28]. Nano/micro CaF2 crystal exhibits non-toxicity to biological tissues, low-phonon energy of the crystal lattice, wide transparent spectral region and easy substitution by trivalent lanthanide ions with comparable ionic radius, although charge compensation must be considered [25,29]. Charge compensation mechanism distorts the local symmetry, while at the same time, this mechanism may influence the spectroscopic behavior of lanthanide ions. Besides, the previously reported Yb3+/Ho3+/Mn2+ tri-doped UC nano/micro crystals were synthesized using oleic acid (OA) as solvents, which finally formed hydrophobic nanoparticles. Therefore, it is important to synthesize hydrophilic UC nano/micro materials for additional surface modification in biological applications [8,30]. In this paper, we present Mn2+ doped CaF2:Yb3+/Ho3+ monodisperse microspheres through an OA-free single-step hydrothermal method. The influence of Mn2+ doping on the phase of CaF2 micro crystals and Yb3+/Ho3+ UC emissions were performed in detail. Under the excitation of 980 nm CW laser, the as-synthesized microspheres exhibit tunable green-yellow-red UC emissions and striking enhancement of red color emission with increased Mn2+ concentration. The lifetime measurement as well as the dependence of luminescence intensity on the excitation power directly proved the proposed the energy migration process between Ho3+ and Mn2+.
3. Results and discussion 3.1. Morphology and structure The morphology and phase of as-prepared CaF2:Yb3+/Ho3+/Mn2+ microspheres were characterized by scanning electron microscopy (SEM) and XRD. Fig. 1(a) demonstrates the low magnification SEM image of representative samples with Mn2+ doping concentration at 10 mol%. It can be seen that the samples were monodispersed with uniformed size distribution and microsphere shape. Fig. 1(b–f) provides more detailed microstructural information and this set of SEM micrographs depict that these as-prepared particles with different Mn2+ concentrations adopt a spherical morphology with rough surfaces and an average diameter of ∼2 μm. We can also find that the morphology and microstructure of CaF2 microspheres do not alter with increasing the concentration of Mn2+. The XRD results of as-synthesized microspheres are displayed in Fig. 1 (g). All of the diffraction peaks detected from 20° to 80° can be indexed to the cubic phase of bulk (Ca0.8Yb0.2) F2.2 material (JCPDS No. 87–0976), indicating that the rare earth ions have entered into the host lattices by occupying the chemical substitutional sites. The diffraction peaks of as-prepared microspheres were sharp and intense, indicating their highly crystalline nature. No impurity peaks were observed, confirming the high purity of the products. Furthermore, the pure cubic phase structure with high crystalline nature was maintained even when the concentration of Mn2+ increased up to 30 mol%. What is noticeable is that the diffraction peaks shifts slightly towards higher angle side as a result of the decrease of the unitcell volume of CaF2 host, owning to the substitution of larger Ca2+ (crystal radius = 1.26 Å) by the relatively smaller Mn2+ (crystal radius = 1.10 Å) [31].
2. Experimental details 2.1. Synthesis of CaF2:Yb3+/Ho3+/Mn2+ microspheres YbCl3·6H2O (99.9%), HoCl3·6H2O (99.9%), Ethylenediaminetetraacetic acid (EDTA) (99%) and NaBF4 (99.99%) were purchased from Aladdin Industrial Corporation. CaCl2 (96%) and MnCl2·4H2O (99%) were purchased from Alfa Aesar, Thermo Fisher Scientific. All of the chemical reagents were used as received without further purification. Deionized water was used throughout as solvent. The CaF2:Yb3+/Ho3+/Mn2+ microspheres were synthesized by a single-step hydrothermal method using water as solvent and EDTA as capping agents. Typically, 2 mmol of chloride salts was dissolved in 20 ml of deionized water with 1 mmol EDTA, and the mixed solution was stirred for 1 h. 4 mmol NaBF4 was dissolved in 10 ml deionized water and transferred to the chloride salts solution prepared above with another 1 h of vigorous stirring until a milky colloidal solution was obtained. Subsequently, the mixed solution was transferred to a 50 ml Teflon-lined autoclave and heated to 180 °C for 30 h. After the system was cooled to room temperature, the microspheres were collected by centrifugation at 4000 rpm for 4 min, washed with water and ethanol for several times to remove any possible remnants. Finally, the precipitate was dried in air at 40 °C for 24 h to obtain white CaF2 microspheres.
3.2. Luminescence In order to investigate the impact of Mn2+ doping on luminescence properties of CaF2:Yb3+/Ho3+ UC microspheres, upconversion emission spectra of these synthetic samples were measured under the excitation of 980 nm CW laser with pumping density of 127.2 kW/cm2. Fig. 2(a) presents the normalized UC luminescence spectra of the CaF2:Yb3+/Ho3+ microspheres with Mn2+ doping ratio at 0, 10 mol% and 30 mol%, and the insets show the corresponding luminescence color, respectively. The spectra consists of three emission bands centered at 492 nm (blue), 548 nm (green) and 647 nm (red), corresponding to the transitions of 5F3→5I8, 5S2/5F4→5I8 and 5F5→5I8 of Ho3+, respectively. For microspheres without Mn2+ doping, the intensity of the green emission is the strongest, followed by red color emission with only half intensity of it, and the blue emission is barely noticeable. When the concentration of Mn2+ increased to 10 mol%, the green emission decreases while the corresponding red emission increases quickly and exceeds the green emission, leading to luminescence color changing from green to bright yellow. By increasing Mn2+ concentration to 30 mol%, the green color band diminishes further and the emission color is finally tuned to red. The corresponding CIE chromaticity coordinates of as-prepared microspheres are calculated and illustrated in Fig. 2(b), indicating a wide range of emission colors (green-yellow-red) can be achieved by increasing the concentration of Mn2+, which is consistent with the result of the emission spectra and luminescence color. To further prove the tunability of Mn2+ doped CaF2:Yb3+/Ho3+ UC microspheres, a series of samples with Mn2+ concentration at 0, 2.5, 5, 7.5, 10, 20, 30 mol% were synthesized and characterized by UC photoluminescence spectra. The intensity ratios of red to green (R/G) and red to blue (R/B) are calculated and displayed in Fig. 3. As Mn2+ concentration increases from 0 to 30 mol%, the R/G and R/B ratio can be promoted from 0.48/4.23 up to 3.36/23.56, corresponding to a 7-
2.2. Instruments and measurements The crystal phase of the synthetic UC microspheres was measured through powder XRD by using X-ray diffractometer with Cu K radiation at 40 kV and 200 mA (Rigaku TTR Ⅲ system). Meanwhile, the size and morphology of these UC microspheres were characterized by SEM (HITACHI S-4800). In photoluminescence experiments, the 980 nm laser was used as the excitation light source, which was introduced into an inverted microscope (Observer A1, Zeiss) and focused on the sample by using a 100 × objective lens (NA = 1.4). The diameter of the excitation spot was estimated to be ∼2.0 μm. The luminescence generated by the sample was collected by the same objective lens and then delivered to a high resolution spectrometer (SR-500I-B1, Andor) equipped with a color charge-coupled device (CCD) (DU970 N, Andor) for analysis. The photos of the excitation spot were taken by a camera from the eyepiece of the microscope. All the above measurements were 404
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Fig. 1. SEM images of CaF2:Yb3+/Ho3+ (20/2 mol%) microspheres (a) with doping 10 mol% Mn2+ ions at low magnification, (b–f) with doping 0, 5, 10, 20, 30 mol % Mn2+ ions at high magnification, respectively. (g) XRD patterns of samples, (h) local magnification of XRD patterns, the shift of the peaks to higher angle indicates the decreased unit-cell volume.
upper state of 2F7/2 level. After that, efficient energy transfer occurred between Yb3+ and Ho3+, resulting in electrons of Ho3+ excited from 5 I8 to 5I6 level. Subsequent energy transfer can also populate 5F5 from the low-lying 5I7 level of Ho3+. A second 980 nm photon can populate 5 F4/5S2 level of Ho3+ ion, and afterwards non-radiative relaxation results in population of 5F5 level of Ho3+ ion. In addition, a third 980 nm photon can populate the 5F2/3K8 level of Ho3+, which finally leads to population of 5F3 level by non-radiative relaxation. Then the emissions
fold and 5.5-fold enhancement, which obviously confirm the role of Mn2+ in enhancing red emission, as well as suppressing green and blue emission in Yb3+/Ho3+ co-doped systems. In order to further explore the effect of Mn2+ doping on UC luminescence properties of Yb3+/Ho3+/Mn2+ triply-doped CaF2 microspheres, a proposed energy level diagram and the corresponding energy transfer mechanism is presented in Fig. 4. Under the excitation of 980 nm CW laser, the electrons of Yb3+ in 2F7/2 level are excited to the
Fig. 2. (a) Normalized upconversion emission spectra of CaF2:Yb3+/Ho3+ (20/2 mol%) microspheres doped with different contents of Mn2+ under the excitation of 980 nm CW laser at a power density of 127.2 kW/cm2. The insets are the corresponding luminescence color, (b) CIE chromaticity diagram of CaF2:Yb3+/Ho3+ with different concentrations of Mn2+. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 405
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from these populated states to the ground states take place, including strong green (5S2/5F4→5I8), red (5F5→5I8) and weak blue emission (5F3→5I8) of Ho3+ ion. When Mn2+ is co-doped into the host matrix with Yb3+ and Ho3+, the non-radiative energy transfer from Ho3+ to Mn2+ (5F2/3K8, 5F3, 5S2/5F4→4T1) occurs, followed by back-energy transfer from Mn2+to Ho3+ (4T1→5F5), which increases the number of electrons in the upper level of red emission. With Mn2+ doping concentration increasing, the distance between Mn2+ and Ho3+ decreases gradually, which enhances the energy exchange between Mn2+ and Ho3+ and results in remarkable enhancement of red color emission as well as weakening effect on green and blue emission. Next, we performed time-decay measurement of Ho3+: 5S2/5F4 and 5 F5 level in CaF2:Yb3+/Ho3+/Mn2+ microspheres under 980 nm pulsed laser excitation to prove the proposed mechanism of energy exchange between Ho3+ and Mn2+, As shown in Fig. 5, the lifetime of the 5S2/5F4 levels decreases from 97μs to 52μs when Mn2+ doping concentration increases from 0 to 30 mol%, due to the energy migration from Ho3+ to Mn2+. On the contrary, the lifetime of 5F5 level experiences a dramatic increment from 51μs to 136μs when Mn2+ doping was introduced, confirming the back energy transfer from Mn2+ to Ho3+. The results of time-decay measurement also agree well with the upconversion emission spectra in Fig. 2. Besides, we also measured the dependence of upconversion luminescence intensity on the 980 nm NIR excitation power of representative samples (CaF2:20% Yb3+/2%Ho3+/10% Mn2+), as shown in Fig. 6. The number of infrared multi-photons
Fig. 3. The tunable R/G and R/B ratio of CaF2:Yb3+/Ho3+ with different concentrations of Mn2+ (where R, G and B stands for Red, Green and Blue, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Schematic energy level diagram of CaF2:Yb3+/Ho3+/Mn2+ and the proposed mechanism of UC process under the excitation of 980 nm laser. Fig. 6. Plot of the upconversion emission intensity of CaF2:Yb3+/Ho3+/Mn2+ (20/2/10 mol%) microspheres as a function of the pump power density under 980 nm excitation.
Fig. 5. Time-decay curves of (a) 5S2/5F4 and (b) 5F5 energy levels of Ho3+ recorded in CaF2:Yb3+/Ho3+/Mn2+ microspheres with Mn2+ doping concentration at 0, 10 mol% and 30 mol%. 406
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involved in a single visible photon generation could be determined from the slope of linear fitting in a double logarithmic plot of the emitting intensity as the function of excitation intensity. The slopes of blue, green and red fluorescence are 0.81, 0.61 and 0.83 under low pump power, and the corresponding numbers under intermediate-power is 2.46, 1.74 and 2.86, respectively. The divergence between low excitation power and intermediate-power is due to the nonlinear absorption effects of materials where absorption coefficient increases with incident laser power [32–34]. Based on the energy transfer mechanism in Fig. 4, the green and red emission ought to be a two-photon process while the blue fluorescence is a three-photon process. However, the measured results under intermediate-power has obvious deviations with the theoretical values. This could be explained by the energy migration from Ho3+ to 4T1 level of Mn2+ and the back-energy transfer process to Ho3+, which reduces the electrons on upper levels of green and blue emissions and adds electrons to the excited states of red emission. In consequence, the measured slopes for green and blue emissions are smaller than expected values and the slope for red color is distinctively larger than normal value.
[9] J. Zhou, Z. Liu, F. Li, Upconversion nanophosphors for small-animal imaging, Chem. Soc. Rev. 41 (2012) 1323–1349. [10] D. Bechet, P. Couleaud, C. Frochot, M. Viriot, F. Guillemin, M. Barberi-Heyob, Nanoparticles as vehicles for delivery of photodynamic therapy agents, Trends Biotechnol. 26 (2008) 612–621. [11] N.M. Idris, M.K. Gnanasammandhan, J. Zhang, P.C. Ho, R. Mahendran, Y. Zhang, In vivo photodynamic therapy using upconversion nanoparticles as remote-controlled nanotransducers, Nat. Med. 18 (2012) 1580–1585. [12] C. Wang, L. Cheng, Z. Liu, Drug delivery with upconversion nanoparticles for multifunctional targeted cancer cell imaging and therapy, Biomaterials 32 (2011) 1110–1120. [13] G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li, Y. Zhao, Mn2+ dopant-controlled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery, Adv. Mater. 24 (2012) 1226–1231. [14] F. Vetrone, R. Naccache, A. Zamarrόn, A. Juarranz De La Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. Martín Rodriguez, D. Jaque, J. García Solé, J.A. Capobianco, Temperature sensing using fluorescent nanothermometers, ACS Nano 4 (2010) 3254–3258. [15] S. Gai, C. Li, P. Yang, J. Lin, Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications, Chem. Rev. 114 (2013) 2343–2389. [16] F. Wang, X. Liu, Multicolor tuning of lanthanide-doped nanoparticles by single wavelength excitation, Accounts Chem. Res. 47 (2014) 1378–1385. [17] Y. Zhang, L. Zhang, R. Deng, J. Tian, Y. Zong, D. Jin, X. Liu, Multicolor barcoding in a single upconversion crystal, J. Am. Chem. Soc. 136 (2014) 4893–4896. [18] Z. Bai, H. Lin, J. Johnson, S.C. Rong Gui, K. Imakita, R. Montazami, M. Fujii, N. Hashemi, The single-band red upconversion luminescence from morphology and size controllable Er3+/Yb3+ doped MnF2 nanostructures, J. Mater. Chem. C 2 (2014) 1736. [19] J. Wang, F. Wang, C. Wang, Z. Liu, X. Liu, Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals, Angew. Chem. Int. Ed. 50 (2011) 10369–10372. [20] N. Niu, P. Yang, Y. Liu, C. Li, D. Wang, S. Gai, F. He, Controllable synthesis and upconversion properties of tetragonal BaYF5: Yb/Ln (Ln=Er, Tm, and Ho) nanocrystals, J. Colloid Interface Sci. 362 (2011) 389–396. [21] H. Gong, D. Yang, X. Zhao, E. Yun Bun Pun, H. Lin, Upconversion color tunability and white light generation in Tm3+/Ho3+/Yb3+ doped aluminum germanate glasses, Opt. Mater. 32 (2010) 554–559. [22] X. Xie, N. Gao, R. Deng, Q. Sun, Q. Xu, X. Liu, Mechanistic investigation of photon upconversion in Nd3+ -sensitized core-shell nanoparticles, J. Am. Chem. Soc. 135 (2013) 12608–12611. [23] Z.T. Chen, E.H. Song, M. Wu, S. Ding, S. Ye, Q.Y. Zhang, Bidirectional energy transfer induced single-band red upconversion emission of Ho3+ in KZnF3:Mn2+ ,Yb3+ ,Ho3+ nanocrystals, J. Alloy. Comp. 667 (2016) 134–140. [24] K. Lingeshwar Reddy, V. Srinivas, K.R. Shankar, S. Kumar, V. Sharma, A. Kumar, A. Bahuguna, K. Bhattacharyya, V. Krishnan, Enhancement of luminescence intensity in red emitting NaYF4:Yb/Ho/Mn upconversion nanophosphors by variation of reaction parameters, J. Phys. Chem. C 121 (2017) 11783–11793. [25] N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J.E. Ramírez-Hernández, L.M. Maestro, M.C. Iglesias-de La Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J.A. Capobianco, J.G. Solé, M. Bettinelli, D. Jaque, A. Speghini, NIR-toNIR two-photon excited CaF2:Tm3+ ,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-imaging, ACS Nano 5 (2011) 8665–8671. [26] G. Wang, Q. Peng, Y. Li, Upconversion luminescence of monodisperse CaF2:Yb3+/ Er3+ nanocrystals, J. Am. Chem. Soc. 131 (2009) 14200–14201. [27] M. Misiak, M. Skowicki, T. Lipiński, A. Kowalczyk, K. Prorok, S. Arabasz, A. Bednarkiewicz, Biofunctionalized upconverting CaF2:Yb,Tm nanoparticles for Candida albicans detection and imaging, Nano Res. 10 (2017) 3333–3345. [28] X. Deng, Y. Dai, J. Liu, Y. Zhou, P. Ma, Z. Cheng, Y. Chen, K. Deng, X. Li, Z. Hou, C. Li, J. Lin, Multifunctional hollow CaF2:Yb3+/Er3+/Mn2+-poly(2-Aminoethyl methacrylate) microspheres for Pt(IV) pro-drug delivery and tri-modal imaging, Biomaterials 50 (2015) 154–163. [29] M. Pedroni, F. Piccinelli, T. Passuello, M. Giarola, G. Mariotto, S. Polizzi, M. Bettinelli, A. Speghini, Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method: toward efficient materials with near infrared-to-near infrared upconversion emission, Nanoscale 3 (2011) 1456. [30] L. Xiong, T. Yang, Y. Yang, C. Xu, F. Li, Long-term in vivo biodistribution imaging and toxicity of polyacrylic acid-coated upconversion nanophosphors, Biomaterials 31 (2010) 7078–7085. [31] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides, Acta Crystallogr. A 32 (1976) 751–767. [32] L. Tian, Z. Xu, S. Zhao, Y. Cui, Z. Liang, J. Zhang, X. Xu, The upconversion luminescence of Er3+/Yb3+/Nd3+ triply-doped β-NaYF4 nanocrystals under 808-nm excitation, Materials 7 (2014) 7289–7303. [33] M. Joubert, Photon avalanche upconversion in rare earth laser materials, Opt. Mater. 11 (1999) 181–203. [34] L.W. Tutt, S.W. McCahon, Reverse saturable absorption in metal cluster compounds, Opt. Lett. 15 (1990) 700–702.
4. Conclusions In conclusion, we synthesize CaF2:Yb3+/Ho3+ microspheres doped with different concentration of Mn2+ (0, 2.5, 5, 7.5, 10, 20 and 30 mol %) via a single step OA-free hydrothermal method. The SEM and XED results show that the as-synthesized samples possess highly crystalline structure and purity. The UC luminescence of as-prepared microspheres can be tuned from green to yellow and then to red by increasing the concentration of Mn2+. Besides that, the highly improved R/G and R/B ratio further testified the ability of Mn2+ in enhancing red emission. The mechanism of energy migration process between Ho3+ and Mn2+ is proposed and backed up by time-decay and power dependence measurement of Ho3+ UC emission. These qualities make Mn2+ doped CaF2:Yb3+/Ho3+ microspheres more competitive in color display and optical bioimaging. Acknowledgements We are extremely thankful to Prof. S. Lan (South China Normal University, China) for allowing us to use his laboratory facilities to measure the optical properties of the samples. C. F. Zhang thanks the financial support from the National Natural Science Foundation of China (Grant No. 11774427). References [1] F. Auzel, Upconversion and anti-Stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 19–174. [2] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976. [3] X. Chen, D. Peng, Q. Ju, F. Wang, Photon upconversion in core-shell nanoparticles, Chem. Soc. Rev. 44 (2015) 1318–1330. [4] R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, X. Liu, Temporal full-colour tuning through non-steady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. [5] X. Li, Z. Xue, H. Liu, Hydro-thermal synthesis of PEGylated Mn2+ dopant controlled NaYF4: Yb/Er up-conversion nano-particles for multi-color tuning, J. Alloy. Comp. 681 (2016) 379–383. [6] H. Wang, W. Lu, Z. Yi, L. Rao, S. Zeng, Z. Li, Enhanced upconversion luminescence and single-band red emission of NaErF4 nanocrystals via Mn2+ doping, J. Alloy. Comp. 618 (2015) 776–780. [7] R. Kumar, M. Nyk, T.Y. Ohulchanskyy, C.A. Flask, P.N. Prasad, Combined optical and MR bioimaging using rare earth ion doped NaYF4 nanocrystals, Adv. Funct. Mater. 19 (2009) 853–859. [8] Q. Liu, Y. Sun, T. Yang, W. Feng, C. Li, F. Li, Sub-10nm hexagonal lanthanide-doped NaLuF4 upconversion nanocrystals for sensitive bioimaging in vivo, J. Am. Chem. Soc. 133 (2011) 17122–17125.
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