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Up-conversion luminescence of Mn4+ sensitized byYb3+, Er3+ in NaMgGdTeO6
Journal of Alloys and Compounds xxx (xxxx) xxx
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Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6 Rui Huang, Xiaoliang Yang*, Siguo Xiao School of Physics and Optoelectronics, Xiangtan University, Hunan, 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2019 Received in revised form 22 October 2019 Accepted 26 October 2019 Available online xxx
A series of NaMgGdTeO6: Yb3þ/Er3þ/Mn4þ phosphors have been prepared by a high-temperature solidstate reaction method. The as-prepared products exhibit typical deep red 696 nm emission of Mn4þ under 980 nm laser excitation, which is performed through the Yb3þ/Er3þ/Mn4þ multi-step energy transfers. The Yb3þ, Er3þ and Mn4þ concentrations dependent up-conversion emission spectra, the Mn4þ concentration dependent Er3þ 544 nm emission lifetimes, and the pumping power and temperature dependent up-conversion luminescence intensities have been investigated in detail. The preliminary study in this work extends the up-conversion spectral range of the traditional up-conversion materials and provides reference for exploring new transition metal based up-conversion phosphors. © 2019 Published by Elsevier B.V.
Keywords: Up-conversion Luminescence Mn4þ Energy transfer
1. Introduction Photon up-conversion is an anti-stokes process in which lowenergy photons are converted into high-energy photons via twophoton or multi-photon mechanism [1,2]. Since the upconversion (UC) light emitting mechanism has been reported in 1960s [3], there are growing interests focused on up-conversion luminescent materials due to their potential applications including infrared displays, temperature sensors, solar spectral converter, photodynamic therapy and biological imaging agents [4e7]. Lanthanides such as Er3þ, Tm3þ, Ho3þ, etc. are possible to give efficient up-conversion luminescence when they are doped into suitable host [8e11]. In the past decades, rare earth ions doped up-conversion materials have been extensively studied and widely reported [4e11]. These up-conversion phosphors have exhibited photon emission covering from ultraviolet (UV) to near-infrared (NIR), due to the abundant meta-stable excited states of rare earth ions [12,13]. However, the applications of rare earth ions doped up-conversion materials are limited by the fixed emission bands of typical rare earth ions such as Er3þ, Ho3þ and Tm3þ. Thus it is of great significance to explore new up-conversion materials with novel emission bands to meet various application requirements.
* Corresponding author. E-mail address: [email protected] (X. Yang).
It’s well known that some transition metal ions can also act as activators in the luminescent materials [14e16]. Transition metal ions can give emission bands different from that of rare earth ions and their luminescence properties are highly relevant to local crystal field environments [17]. Therefore, combination of transition metal ions and rare earth ions might be an appealing approach to explore new up-conversion materials with novel spectral properties. The transition metal Mn4þ ion, with a 3 d3 electronic configuration, can generate red or deep red luminescence with emission wavelengths ranging from 600 to 750 nm originating from its spin and parity-forbidden 2E/4A2 transition, which relies on crystal field environments around it [18]. Recently, luminescence properties of Mn4þ sensitized by rare earth ions have been investigated. Up-conversion luminescence of Mn4þ in YAP: Yb3þ/ Ln3þ/Mn4þ(Ln ¼ Er, Tm, Ho) [12], Gd2ZnTiO6: Er3þ/Mn4þ [19], Na3ZrF7: Yb3þ/Ln3þ/Mn4þ(Ln ¼ Er, Tm, Ho) [20] and Ca14Zn6Ga10O35: Ln3þ/Mn4þ (Ln ¼ Nd, Er, Yb) [21] phosphors have also been reported. In order to realize efficient up-conversion of Mn4þ sensitized by rare earth ions, the selection of host material is very important. Host material should not only provide a suitable environment for the luminescence of Mn4þ ions in the matrix, but also ensure high up-conversion efficiency of rare earth ions in it. Double perovskite NaMgGdTeO6 (NMGT), with the general formula of A2B1B2O6, is built up with octahedra, just as shown in Fig. 1. In NMGT, Mg2þ and Te6þ occupy the B sites and form distorted octahedra MgO6 and TeO6 with six oxygen atoms [22]. The octahedral positions in the
https://doi.org/10.1016/j.jallcom.2019.152816 0925-8388/© 2019 Published by Elsevier B.V.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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3. Results and discussion It is apparent that NMGT is isostructural with NaMgLaTeO6 compound, as shown in Fig. 1, which belongs to the monoclinic system with space group P211/m1(11). The Mg and Te atoms in the crystal structure are coordinated with six oxygen atoms, respectively, to form MgO6 and TeO6 octahedra [23]. Given the similar radius rule, the difference in radius between the doping ion and the substituted ion should be within 30% [30,31]. It is well known that the value of the difference in radius percentage (Dr ) determines the occupancy of Mn4þ in the original crystal structure. The Dr is expressed as following equation [32]:
Dr ¼ 100
Fig. 1. Crystal structure of the NMGT compound as well as MgO6 and TeO6 octahedra.
NMGT can refer suitable sites for Mn4þ ions to give luminescence. As a matter of fact, efficient deep red luminescence of Mn4þ in NMGT has been observed just recently [23]. Meanwhile, tellurates/ tellurites are believed to be good host materials for obtaining upconversion of rare earth ions due to their relative low-phonon energy and high refractive index [24,25], both of which are beneficial for radiative transitions of rare earth ions and thus relatively efficient up-conversion emission. Efficient up-conversion of rareearth ions has been reported in both tellurite glasses and crystals [26e29]. These facts mean that NMGT should be an ideal host material for both luminescence of Mn4þ and up-conversion of rare earth ions. In this work, therefore, the energy transfer strategy Yb3þ/Er3þ/Mn4þ has been designed to explore up-conversion luminescence of Mn4þ in NMGT. The luminescent properties and the energy transfer up-conversion mechanism in the Er3þ/Yb3þ/ Mn4þ triply-doped NMGT have been investigated in detail.
2. Experimental A series of polycrystalline products with the composition formula of NaMgGdTeO6: xEr3þ/yYb3þ/zMn4þ (x ¼ 0.04e0.16, y ¼ 0.04e0.12, z ¼ 0e0.015) were synthesized using a hightemperature solid-state reaction route. The starting materials, Na2CO3(AR), Gd2O3(99.9%), TeO2(99.9%), (MgCO3)4‧Mg(OH)2‧ 5H2O(AR) and Er2O3(99.9%), Yb2O3(99.9%), MnCO3(99.9%) were weighed according to the stoichiometric ratio. In addition, adding 10% excess of Na2CO3 as high temperature compensation. Based on the chemical composition, the raw materials were thoroughly ground for 60 min and then put into corundum crucibles to sinter for 11 h at stationary temperature of 1100 C in a box furnace with a heating rate of 10 C/min. Finally, the samples were cooled to room temperature and finely ground for subsequent characterization. The crystalline phases of the samples were analyzed by X-ray diffraction (XRD) on a PIGAKV Ultima IV equipment, using Cu tube with Cu/K (k ¼ 0.1541 nm) radiation in the 2ɵ range of 15 to 70 . The excitation spectra and emission spectra were measured by a monochromator (Zolix Instrument, Omni-l320i) coupled with photomultiplier tube (PMTHS1-R928). The luminescence decay curves and temperature dependent up-conversion emission signals of the samples were measured by a FLS980 (Edinburgh) spectrometer. 455 nm and 980 nm diode lasers and 150 W Xenon lamp were used as excitation source.
Rm ðCNÞ Rd ðCNÞ ; Rm ðCNÞ
(1)
where CN refers to the coordination number, Rm ðCNÞ and Rd ðCNÞ are the radii of the host cation and the doped ion, respectively. The ionic radii of Mn4þ、Mg2þ and Te6þ are: 0.53 Å, 0.72 Å, 0.56 Å (CN ¼ 6). According to equation (1), it is believed that Mn4þ ions can simultaneously replace the sites of Mg2þ and Te6þ to form MnO6 octahedra. When two Mn4þ ions substitute at one Mg2þ site and one Te6þ site, there will be no requirement for charge compensation, and it is beneficial to the stability of the crystal structure. On the other hand, the Ln3þ ions take priority of replacing Gd3þ sites because of their similar ion radius. Fig. 2 shows the XRD profiles of the as-prepared samples NMGT phosphor, NMGT: Mn4þ, NMGT: 0.06 Yb3þ/0.08Er3þ/ zMn4þ(z ¼ 0.002, 0.005,0.01,0.015) and the standard diffraction position of the NaMgLaTeO6 compound. The all diffraction peaks of synthesized powder materials show a good matched with standard data of NaMgLaTeO6 (ICSD#78532) except a little shift in the NMGT samples. It is observed that the diffraction peaks of all samples have a slight right shift relative to the standard position. The shift of the diffraction peaks to larger angles is consistent with a contraction of the lattice parameters due to the smaller ionic radius of Gd3þ in comparison to La3þ [33]. There is no other phase appeared in the samples, suggesting that the dopants Er/Yb/Mn enter into the lattice without significantly influencing on the NMGT crystal structure. The up-conversion emission spectra in the region of 500e800 nm for the Er3þ and Mn4þ single doped and Er3þ/Mn4þ co-doped NMGT under 980 nm diode laser excitation as well as the
Fig. 2. XRD patterns of NMGT host, Mn4þ singly doped and NMGT: 0.08Er3þ/0.06 Yb3þ/ zMn4þ. Bars represent NaMgLaTeO6 (ICSD No. 78532) diffraction data.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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emission spectrum in the region of 600e800 nm for NMGT: 0.01Mn4þ with 455 nm excitation are shown in Fig. 3. The inset(a) in Fig. 3 further gives the up-conversion spectra of Er3þ single and Er3þ/Mn4þ co-doped NMGT in the 750e900 nm region. Two strong emission bands centered at 544 nm and 656 nm and one weak emission band at 850 nm are observed in the NMGT: 0.08Er3þ phosphor with 980 nm excitation, which are attributed to the 2H11/ 4 2 4 2 4 2 3þ 2, S3/2 / I15/2, F9/2 / I15/2 and S3/2 / I13/2 transitions of Er , 4þ respectively. However, the NMGT: 0.01Mn phosphor shows no luminescence under excitation of 980 nm although an intense emission band ranging from 650 nm to 800 nm can be observed under 455 nm excitation. This means that the upconversion can not take place in the Mn4þ single doped NMGT under 980 nm excitation. Interestingly, in addition to the emissions of Er3þ, a luminescence band centered at 696 nm ascribed to the Mn4þ: 2E/4A2 transition appears in the Er3þ/Mn4þ co-doped NMGT. Meanwhile the intensities of the upconversion emission of Er3þ decreases as compared with the Er3þ single doped sample. This clearly indicates that the energy transfer sensitization from Er3þ to Mn4þ occurs and performs the up-conversion emission of Mn4þ. The inset(b) in Fig. 3 depicts the variation of fluorescence intensity of the Mn4þ: 2E/4A2 emission for different concentrations of Er3þ. The emission intensity of Mn4þ increases with increasing concentration of Er3þ and reaches its maximal value at x ¼ 0.08. As the Er3þ doping concentration is higher than 0.08, the emission intensity decreases gradually as the results of the concentration quenching. It is well known that Yb3þ can enhance the up-conversion emission of Er3þ in Er3þ and Yb3þ co-doped phosphors by efficient energy transfer sensitization from Yb3þ to Er3þ [34e38]. Therefore, different concentrations of Yb3þ (y ¼ 0.04, 0.06, 0.08, 0.1, 0.12) are introduced to further improve the luminescence of the Er3þ and thus the Mn4þ ions. The luminescence spectra of NMGT: 0.08Er3þ/yYb3þ/0.01Mn4þ (y ¼ 0, 0.06) are presented in Fig. 4. The inset in Fig. 4 gives the Yb3þ concentration dependent upconversion emission intensity of Mn4þ. With doping of Yb3þ, the emission bands attributed to transitions Er3þ: 2H11/2, 4S3/2, 4F9/ 4 4þ 2 4 2 / I15/2 and Mn : E/ A2 have all increased significantly. As exhibited in the inset of Fig. 4, the phosphor shows the strongest up-conversion emission Mn4þ at y ¼ 0.06. On further increasing the Yb3þ content beyond 0.06, concentration quenching is observed.
Fig. 3. Up-conversion spectra of NMGT: xEr3þ/zMn4þ (x ¼ 0, 0.08; z ¼ 0,0.01) with 980 nm excitation and the emission spectra of NMGT:0.01Mn4þ under 455 nm excitation. The insets show the Mn4þ emission intensity as functions of Er3þ concentration(a) and the up-conversion spectra between 750 and 950 nm(b).
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Fig. 4. Up-conversion spectra of NMGT: 0.08Er3þ/yYb3þ/0.01Mn4þ(y ¼ 0, 0.06) with 980 nm excitation. The inset shows the Mn4þ emission intensity as functions of Yb3þ concentration.
The Mn4þ: 2E/4A2 emission intensity of NMGT: 0.08Er3þ/ 0.06 Yb3þ/0.01Mn4þ phosphor is enhanced by 4.37 times as compared with that of the NMGT: 0.08Er3þ/0.01Mn4þ phosphor. Based on above experiment results, 8% of Er3þ and 6% of Yb3þ are believed to be the optimal combination to sensitize of Mn4þ in the NMGT host. Thus the Mn4þ concentration is further adjusted to optimize the up-conversion intensity as the Er3þ and Yb3þ doping concentrations are fixed at 8% and 6%, respectively. The upconversion spectra of the NMGT: 0.08Er3þ/0.06 Yb3þ/zMn4þ (z ¼ 0.001, 0.002, 0.01) phosphors are shown in Fig. 5. The upconversion intensities of the Er3þ and Mn4þ ions as functions of Mn4þ concentration are given as insets (a) and (b) in Fig. 5. It is seen from the inset (b) of Fig. 5 that the emission intensity at 696 nm of Mn4þ first increases with the increase of Mn4þ content and then decreases due to concentration quenching. The maximum emission intensity reaches when z ¼ 0.002. Therefore, the optimal composition for the Mn4þ up-conversion emission in our experiment might be the NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ phosphor.
Fig. 5. Emission spectra of NMGT: 0.08Er3þ/0.06 Yb3þ/zMn4þ (z ¼ 0.001, 0.002, 0.01) with 980 nm excitation. The insets depict the Mn4þ concentration dependent (a) Er3þ and (b) Mn4þ up-conversion emission intensities.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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Additionally, it is found from the inset(a) in Fig. 5 that the upconversion intensity of Er3þ ions always decreases with the increase of Mn4þ concentration. This also gives another evidence of the energy transfer from Er3þ to Mn4þ. Fig. 6 presents the normalized Er3þ emission spectrum and Mn4þ excitation spectrum. There is a small overlap between the emission spectrum of Er3þ: 2H11/2, 4S3/2 / 4I15/2 transition and the excitation spectrum of Mn4þ: 4A2 /4T2 transition, suggesting that the resonant energy transfer from Er3þ to Mn4þ ions is permissive. The energy transfer from Er3þ to Mn4þ can also be further demonstrated by the Mn4þ concentration dependent luminescence decay behavior of the 4S3/2 / 4I15/2 transition of Er3þ at 544 nm. The decay curves monitoring at 544 nm for the NMGT: 0.08Er3þ/ 0.06 Yb3þ/zMn4þ (z ¼ 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015) phosphors excited at 980 nm are presented in Fig. 7(a). All decay curves can be well fitted into a double-exponential function as follows [39]:
IðtÞ ¼ I0 þ A1 expð t = t1 Þ þ A2 expð t = t2 Þ;
(2)
where IðtÞ and I0 represent the intensities at time t and time 0, A1 and A2 are constants, t1 and t2 represent the luminescence lifetimes for the corresponding fast and slow decay components, respectively. The average lifetimes for the 4S3/2 level of Er3þ can be calculated by the following formula [40]:
tav ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ;
(3)
the calculated decay times are 0.210, 0.204, 0.197, 0.189, 0.174, 0.169, 0.152, 0.130 ms for the concentration of Mn4þ at z ¼ 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015 in NMGT: 0.08Er3þ/0.06 Yb3þ/ zMn4þ. It is obvious that the average lifetimes at 544 nm of Er3þ ions decrease monotonously from 0.210 to 0.130 ms with the increase of Mn4þ concentration. The Mn4þ concentration dependent decay properties of the Er3þ emission at 544 nm prove the nonradiative energy transfer from Er3þ to Mn4þ. Based on the decay lifetimes, energy transfer efficiency(hET )from Er3þ to Mn4þ can be calculated by the following equation [41]:
hET ¼ 1 t=t0 ;
(4)
where t and t0 represent the decay lifetimes of 0.08Er3þ/0.06 Yb3þ/
Fig. 7. (a)Mn4þ content-dependent UC decay curves of Er3þ: 2H11/2, 4S3/2 in the NMGT: Er3þ/Yb3þ/zMn4þ samples under 980 nm laser excitation. (b)Mn4þ content-dependent energy transfer efficiency from Er3þ to Mn4þ.
zMn4þ(z ¼ 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015) triplydoped samples and 0.08Er3þ/0.06 Yb3þ co-doped sample, respectively. The Mn4þ concentration dependent energy transfer efficiency is given in Fig. 7(b). It can be seen from Fig. 7(b) that the energy transfer efficiency increases gradually with the increasing Mn4þ ions concentration, indicating that the increased Mn4þ ions concentration enhances the energy transfer (hET ) probability from Er3þ to Mn4þ in NMGT phosphors. The energy transfer efficiency is 6% when the phosphor gives the strongest Mn4þ up-conversion emission at z ¼ 0.002. To better understand the UC mechanism of Mn4þ, the pumping power dependence of the fluorescent intensities in NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ has been investigated, as given in Fig. 8. Generally, the emission intensity Iem depends on the excitation power Ip with following relationship [19]:
Iem f Ip
Fig. 6. Overlapping between the excitation spectrum of Mn4þ monitored at 696 nm and emission spectrum of Er3þ excited at 980 nm.
n
(5)
where n value is the number of the pumping photons required to excite rare earth ions from the ground state to the excited state. The power dependence of the aforementioned three emission transitions is depicted in Fig. 8 by a log-log plot. The slopes of the linear fittings are 1.81 for Er3þ: 2H11/2, 4S3/2 / 4I15/2, 1.91 for Er3þ: 4F9/ 4 2 / I15/2, indicating that two pumping photons are needed to populate the Er3þ: 2H11/2, 4S3/2, 4F9/2 emitting levels. The slope for
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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Fig. 8. Log-Log plot of the integrated UC emission intensity versus the pumping power of 980 laser for NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ sample.
Mn4þ: 2E/4A2 emission is determined to be 1.96, supporting that Mn4þ luminescence is realized by a two-photon UC mechanism in NMGT: Er3þ/Yb3þ/Mn4þ sample with the assistance of the energy transfer strategy of Yb3þ/Er3þ/Mn4þ. According to the above discussion, the mechanism diagram illustrating the Yb3þ/Er3þ/Mn4þ energy transfer sensitization up-conversion is described in Fig. 9. Firstly, the Yb3þ ion at the ground state is excited to its 2F5/2 state under 980 nm excitation, and the energy is transferred to the Er3þ ions in the ground state, thereby exciting Er3þ ions to the intermediate state 4I11/2 which has a long decay time [42]. A second Yb3þ ion in the 2F5/2 state can also transfer its energy to the same Er3þ ion that has been excited to the 4 I11/2 state, which will be raised to the higher-lying 4F7/2 state. Er3þ ions in the 4F7/2 state can relax to the lower 2H11/2, 4S3/2 and 4F9/2 states through the non-radiative relaxation. The Er3þ ions in the 2 H11/2, 4S3/2 and 4F9/2 states return to the ground state, emitting the green light (525 nm, 544 nm) and red light (656 nm). Meanwhile,
Fig. 9. Energy level diagram for up-conversion mechanism in NMGT: Er3þ/Yb3þ/Mn4þ under excitation at 980 nm.
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the 4T2 state of Mn4þ ions can be populated through the energy transfer 4H11/2, 4S3/2(Er3þ) þ 4A2(Mn4þ) / 4I15/2(Er3þ) þ 4T2(Mn4þ). The Mn4þ ions in the 4T2 state relax quickly to their 2E state through non-radiative relaxation. Then the radiative transition of Mn4þ from the 2E state to the ground state gives the deep red emission (696 nm). In general, thermal stability is an important factor in evaluating the application of phosphors because the phosphors might work in a high temperature environment. The luminescence spectra of NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ phosphor have been measured under different temperatures (303, 333, 363, 393 and 423 K). The temperature dependent emission intensity of Mn4þ is displayed in Fig. 10 and the luminescence spectra are also given as an inset. It can be clearly seen that the emission intensity of Mn4þ monotonously decreases, while the peak position and spectra shape are the same with temperature increasing from 303 K to 423 K. The up-conversion emission intensity of Mn4þ is determined by the non-radiative relaxation of Mn4þ and the non-resonant energy transfer from Er3þ to Mn4þ. The non-radiative relaxation probability will increase as the temperature rises, according to the non-radiative decay theory [43]. While the probability of nonresonant energy transfer involving emission of p phonons can be expressed as [44]:
WðTÞ ¼ Wðni þ 1Þp
(6)
where the WðTÞ represents the probability of non-resonant energy transfer, ni is the average occupation number of the ith vibrational mode and has been expressed as:
1 ni ¼ ehw=kT 1
(7)
As can be seen from equations (6) and (7), the phonon-assisted energy transfer probability increases as the temperature rises. Therefore, there is a competition between the non-radiative relaxation and the Er3þ / Mn4þ energy transfer sensitization with temperature rising. Presumably the non-radiative relaxation predominates and consequently the emission intensity of Mn4þ decreases as the temperature rises. Although the up-conversion emission of Mn4þ has been realized in the Yb3þ/Er3þ/Mn4þ triply-doped NMGT phosphor, its
Fig. 10. Dependence of the emission intensity as a function of temperature for NMGT:Yb3þ/Er3þ/Mn4þ, the inset is the variations of the emission spectra as a function of temperature.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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luminescence intensity is still weak. One of the reasons may ascribe to the small overlap between the emission spectrum of Er3þ and the excitation spectrum of Mn4þ, which results in the low energy transfer sensitization efficiency from Er3þ to Mn4þ. On the other hand, the back energy transfers from Mn4þ to Yb3þ and Er3þ might also further weaken the Mn4þ emission intensity, which can be demonstrated by the emission spectra of the Er3þ and Yb3þ singly doped as well as Mn4þ/Er3þ and Mn4þ/Yb3þ co-doped NMGT phosphors excited by a 455 nm diode light, as shown in Fig. 10. In Fig. 11, no luminescence is observed in the NMGT: 0.06 Yb3þ phosphor, while a strong emission band peaking at around 1002 nm attributed to the Yb3þ: 2F5/2 / 2F7/2 transition appears in the NMGT: 0.06 Yb3þ/0.01Mn4þ phosphor. For the NMGT: 0.08Er3þ sample, only weak near-infrared emission bands centered at 985 nm and 1510 nm in the range are observed, assigned to the 4I11/ 4 4 4 3þ 2 / I15/2 and I13/2 / I15/2 transitions of Er , respectively. However, the intensities of Er3þ emission at 985 nm and 1510 nm in the NMGT: 0.08Er3þ/0.01Mn4þ phosphor are increased by 14.04 and 5.97 times as compared with that in the NMGT: 0.08Er3þ phosphor, respectively. Above luminescent behaviors prove the occurrence of the back energy transfers Mn4þ/Yb3þ and Mn4þ/Er3þ in these phosphors. Considering that the Mn4þ: 2E state lies just above the Er3þ: 4I9/2 state, the back energy transfer from Mn4þ to Er3þ should 2 4 be performed through the E(Mn4þ) þ I15/2(Er3þ) /4A2(Mn4þ) þ 4I9/2 (Er3þ) process. Thus under 980 nm excitation, an intensified emission at around 800 nm due to the Er3þ: 4I9/ 4 3þ 4þ co-doped and Yb3þ/Er3þ/ 2 / I15/2 transition in the Er /Mn Mn4þ triply-doped NMGT phosphors might be expected. However, no emission at 800 nm has been observed in either Er3þ single doped or Er3þ/Mn4þ co-doped NMGT phosphors under 980 nm excitation, just as shown in the inset(a) of Fig. 3. This indicates that no-radiative relaxation of Er3þfrom its 4I9/2 to 4I11/2 level in the NMGT is a very rapid process, so that the radiative transition of 4I9/ 4 2 / I15/2 is too weak to be observed. 4. Conclusion To sum up, NMGT: Er3þ/Yb3þ/Mn4þ phosphors have been synthesized by solid state reaction method. A deep red up-conversion emission at 696 nm arising from the 2E/4A2 transition of Mn4þ has been observed. Excitation power dependent up-conversion intensity and Mn4þ concentration dependent luminescence decay
Fig. 11. Emission spectra of the Er3þ singly-doped, Yb3þ singly-doped, Mn4þ/Er3þ codoped and Mn4þ/Yb3þ co-doped NMGT samples excited at 455 nm.
lifetime of the Er3þ: 4S3/2 / 4I15/2 transition prove that a twophoton process between Yb3þ- Er3þ couple followed with resonant energy transfer from Er3þ to Mn4þ is responsible for the upconversion emission of Mn4þ. The results in this work might provide meaningful reference on the design of up-conversion materials with novel emission bands, expanding the application field of up-conversion materials. Declaration of competing interest There are no conflicts of interest. Acknowledgments This work was supported by the National Science Foundation of China (No. 11674272) and the National Natural Science Foundation of China and Guangdong Province (U1401241). References [1] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (4) (2009) 976e989. [2] F. Wang, Y. Han, C.S. Lim, Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping, Nature 463 (7284) (2010) 1061e1065. [3] François Auzel, Upconversion and anti-Stokes process with f and d ions in solids, Chem. Rev. 104 (1) (2004) 139e147. [4] V. Singh, V.K. Rai, K. Al-Shamery, NIR to visible upconversion in Er3þ/Yb3þ codoped CaYAl3O7 phosphor obtained by solution combustion process, J. Lumin. 131 (12) (2011) 2679e2682. [5] H.Q. Wang, T. Nann, Monodisperse upconverting nanocrystals by microwaveassisted synthesis, ACS Nano 3 (11) (2009) 3804e3808. [6] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, FL, 1999. [7] F.V.D. Rijke, H. Zijlmans, S. Li, Up-converting phosphor reporters for nucleic acid microarrays, Nat. Biotechnol. 19 (3) (2001) 273e276. [8] J.H. Chung, J.H. Ryu, S.W. Mhin, Controllable white upconversion luminescence in Ho3þ/Tm3þ/Yb3þ co-doped CaMoO4, J. Mater. Chem. 22 (9) (2012) 3997e4002. [9] G. Chen, T.Y. Ohulchanskyy, R. Kumar, Ultrasmall monodisperse NaYF4: Yb3þ/ Tm3þ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence, ACS Nano 4 (6) (2010) 3163e3168. €fer, Upconverting nanoparticles, Angew. Chem. Int. Ed. 50 [10] M. Haase, H. Scha (26) (2011) 5808e5829. [11] D. Chen, F. Liu, P. Huang, Sensitized thulium ultraviolet upconversion luminescence in Tm3þ/Yb3þ/Nd3þ triply doped nanoglass ceramics, Opt. Lett. 32 (21) (2007) 3068e3070. [12] D. Chen, W. Xu, S. Yuan, Ln3þ-Sensitized Mn4þ near-infrared upconverting luminescence and dual-modal temperature sensing, J. Mater. Chem. C. 5 (2017) 9619e9628. [13] D. Yang, P. Ma, Z. Hou, Current advances in lanthanide ion (Ln3þ)-based upconversion nanomaterials for drug delivery, Chem. Soc. Rev. 44 (6) (2015) 1416e1448. [14] P. Li, M. Peng, X. Yin, Temperature dependent red luminescence from a distorted Mn4þ site in CaAl4O7: Mn4þ, Opt. Express 21 (16) (2013) 18943e18948. [15] H. Tan, M. Rong, Y. Zhou, Luminescent behaviour of Mn4þ ions in seven coordination environment of K3ZrF7, Dalton Trans. 45 (23) (2016) 9654e9660. [16] Z. Pan, Y.Y. Lu, F. Liu, Sunlight-activated long-persistent luminescence in the near-infrared from Cr3þ-doped zinc gallogermanates, Nat. Mater. 11 (1) (2011) 58e63. [17] Renping Cao, Preparation and luminescence properties of Li2MgZrO4: Mn4þ red phosphor for plant growth, J. Lumin. 188 (2017) 577e581. [18] H. Zhu, C.C. Lin, W. Luo, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014) 4312. [19] J. Liao, Q. Wang, H.R. Wen, First observation of mutual energy transfer of Mn4þ-Er3þ via different excitation in Gd2ZnTiO6: Mn4þ/Er3þ phosphors, J. Mater. Chem. C. 5 (2017) 9098e9105. [20] D. Chen, Y. Liu, J. Chen, Yb3þ/Ln3þ/Mn4þ (Ln ¼ Er, Ho, and Tm) doped Na3ZrF7 phosphors: oilewater interface cation exchange synthesis, dual-modal luminescence and anti-counterfeiting, J. Mater. Chem. C. 7 (2019) 1321e1329. [21] Lv Yang, Visible to NIR down-shifting and NIR to visible upconversion luminescence in Ca14Zn6Ga10O35: Mn4þ, Ln3þ (Ln¼Nd, Yb, Er), Dyes Pigments 161 (2019) 137e146. [22] Yasutake Teraoka, Ming-Deng Wei, Shuichi Kagawa, Double perovskites containing hexavalent molybdenum and tungsten: synthesis, structural investigation and proposal of a fitness factor to discriminate the crystal symmetry, J. Mater. Chem. 8 (11) (1998) 2323e2325. [23] K. Li, H. Lian, R.V. Deun, Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO6:Mn4þ with perovskite structure for w-LEDs, J. Lumin. 198 (2018) 155e162.
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Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6 Rui Huang, Xiaoliang Yang*, Siguo Xiao School of Physics and Optoelectronics, Xiangtan University, Hunan, 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2019 Received in revised form 22 October 2019 Accepted 26 October 2019 Available online xxx
A series of NaMgGdTeO6: Yb3þ/Er3þ/Mn4þ phosphors have been prepared by a high-temperature solidstate reaction method. The as-prepared products exhibit typical deep red 696 nm emission of Mn4þ under 980 nm laser excitation, which is performed through the Yb3þ/Er3þ/Mn4þ multi-step energy transfers. The Yb3þ, Er3þ and Mn4þ concentrations dependent up-conversion emission spectra, the Mn4þ concentration dependent Er3þ 544 nm emission lifetimes, and the pumping power and temperature dependent up-conversion luminescence intensities have been investigated in detail. The preliminary study in this work extends the up-conversion spectral range of the traditional up-conversion materials and provides reference for exploring new transition metal based up-conversion phosphors. © 2019 Published by Elsevier B.V.
Keywords: Up-conversion Luminescence Mn4þ Energy transfer
1. Introduction Photon up-conversion is an anti-stokes process in which lowenergy photons are converted into high-energy photons via twophoton or multi-photon mechanism [1,2]. Since the upconversion (UC) light emitting mechanism has been reported in 1960s [3], there are growing interests focused on up-conversion luminescent materials due to their potential applications including infrared displays, temperature sensors, solar spectral converter, photodynamic therapy and biological imaging agents [4e7]. Lanthanides such as Er3þ, Tm3þ, Ho3þ, etc. are possible to give efficient up-conversion luminescence when they are doped into suitable host [8e11]. In the past decades, rare earth ions doped up-conversion materials have been extensively studied and widely reported [4e11]. These up-conversion phosphors have exhibited photon emission covering from ultraviolet (UV) to near-infrared (NIR), due to the abundant meta-stable excited states of rare earth ions [12,13]. However, the applications of rare earth ions doped up-conversion materials are limited by the fixed emission bands of typical rare earth ions such as Er3þ, Ho3þ and Tm3þ. Thus it is of great significance to explore new up-conversion materials with novel emission bands to meet various application requirements.
* Corresponding author. E-mail address: [email protected] (X. Yang).
It’s well known that some transition metal ions can also act as activators in the luminescent materials [14e16]. Transition metal ions can give emission bands different from that of rare earth ions and their luminescence properties are highly relevant to local crystal field environments [17]. Therefore, combination of transition metal ions and rare earth ions might be an appealing approach to explore new up-conversion materials with novel spectral properties. The transition metal Mn4þ ion, with a 3 d3 electronic configuration, can generate red or deep red luminescence with emission wavelengths ranging from 600 to 750 nm originating from its spin and parity-forbidden 2E/4A2 transition, which relies on crystal field environments around it [18]. Recently, luminescence properties of Mn4þ sensitized by rare earth ions have been investigated. Up-conversion luminescence of Mn4þ in YAP: Yb3þ/ Ln3þ/Mn4þ(Ln ¼ Er, Tm, Ho) [12], Gd2ZnTiO6: Er3þ/Mn4þ [19], Na3ZrF7: Yb3þ/Ln3þ/Mn4þ(Ln ¼ Er, Tm, Ho) [20] and Ca14Zn6Ga10O35: Ln3þ/Mn4þ (Ln ¼ Nd, Er, Yb) [21] phosphors have also been reported. In order to realize efficient up-conversion of Mn4þ sensitized by rare earth ions, the selection of host material is very important. Host material should not only provide a suitable environment for the luminescence of Mn4þ ions in the matrix, but also ensure high up-conversion efficiency of rare earth ions in it. Double perovskite NaMgGdTeO6 (NMGT), with the general formula of A2B1B2O6, is built up with octahedra, just as shown in Fig. 1. In NMGT, Mg2þ and Te6þ occupy the B sites and form distorted octahedra MgO6 and TeO6 with six oxygen atoms [22]. The octahedral positions in the
https://doi.org/10.1016/j.jallcom.2019.152816 0925-8388/© 2019 Published by Elsevier B.V.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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3. Results and discussion It is apparent that NMGT is isostructural with NaMgLaTeO6 compound, as shown in Fig. 1, which belongs to the monoclinic system with space group P211/m1(11). The Mg and Te atoms in the crystal structure are coordinated with six oxygen atoms, respectively, to form MgO6 and TeO6 octahedra [23]. Given the similar radius rule, the difference in radius between the doping ion and the substituted ion should be within 30% [30,31]. It is well known that the value of the difference in radius percentage (Dr ) determines the occupancy of Mn4þ in the original crystal structure. The Dr is expressed as following equation [32]:
Dr ¼ 100
Fig. 1. Crystal structure of the NMGT compound as well as MgO6 and TeO6 octahedra.
NMGT can refer suitable sites for Mn4þ ions to give luminescence. As a matter of fact, efficient deep red luminescence of Mn4þ in NMGT has been observed just recently [23]. Meanwhile, tellurates/ tellurites are believed to be good host materials for obtaining upconversion of rare earth ions due to their relative low-phonon energy and high refractive index [24,25], both of which are beneficial for radiative transitions of rare earth ions and thus relatively efficient up-conversion emission. Efficient up-conversion of rareearth ions has been reported in both tellurite glasses and crystals [26e29]. These facts mean that NMGT should be an ideal host material for both luminescence of Mn4þ and up-conversion of rare earth ions. In this work, therefore, the energy transfer strategy Yb3þ/Er3þ/Mn4þ has been designed to explore up-conversion luminescence of Mn4þ in NMGT. The luminescent properties and the energy transfer up-conversion mechanism in the Er3þ/Yb3þ/ Mn4þ triply-doped NMGT have been investigated in detail.
2. Experimental A series of polycrystalline products with the composition formula of NaMgGdTeO6: xEr3þ/yYb3þ/zMn4þ (x ¼ 0.04e0.16, y ¼ 0.04e0.12, z ¼ 0e0.015) were synthesized using a hightemperature solid-state reaction route. The starting materials, Na2CO3(AR), Gd2O3(99.9%), TeO2(99.9%), (MgCO3)4‧Mg(OH)2‧ 5H2O(AR) and Er2O3(99.9%), Yb2O3(99.9%), MnCO3(99.9%) were weighed according to the stoichiometric ratio. In addition, adding 10% excess of Na2CO3 as high temperature compensation. Based on the chemical composition, the raw materials were thoroughly ground for 60 min and then put into corundum crucibles to sinter for 11 h at stationary temperature of 1100 C in a box furnace with a heating rate of 10 C/min. Finally, the samples were cooled to room temperature and finely ground for subsequent characterization. The crystalline phases of the samples were analyzed by X-ray diffraction (XRD) on a PIGAKV Ultima IV equipment, using Cu tube with Cu/K (k ¼ 0.1541 nm) radiation in the 2ɵ range of 15 to 70 . The excitation spectra and emission spectra were measured by a monochromator (Zolix Instrument, Omni-l320i) coupled with photomultiplier tube (PMTHS1-R928). The luminescence decay curves and temperature dependent up-conversion emission signals of the samples were measured by a FLS980 (Edinburgh) spectrometer. 455 nm and 980 nm diode lasers and 150 W Xenon lamp were used as excitation source.
Rm ðCNÞ Rd ðCNÞ ; Rm ðCNÞ
(1)
where CN refers to the coordination number, Rm ðCNÞ and Rd ðCNÞ are the radii of the host cation and the doped ion, respectively. The ionic radii of Mn4þ、Mg2þ and Te6þ are: 0.53 Å, 0.72 Å, 0.56 Å (CN ¼ 6). According to equation (1), it is believed that Mn4þ ions can simultaneously replace the sites of Mg2þ and Te6þ to form MnO6 octahedra. When two Mn4þ ions substitute at one Mg2þ site and one Te6þ site, there will be no requirement for charge compensation, and it is beneficial to the stability of the crystal structure. On the other hand, the Ln3þ ions take priority of replacing Gd3þ sites because of their similar ion radius. Fig. 2 shows the XRD profiles of the as-prepared samples NMGT phosphor, NMGT: Mn4þ, NMGT: 0.06 Yb3þ/0.08Er3þ/ zMn4þ(z ¼ 0.002, 0.005,0.01,0.015) and the standard diffraction position of the NaMgLaTeO6 compound. The all diffraction peaks of synthesized powder materials show a good matched with standard data of NaMgLaTeO6 (ICSD#78532) except a little shift in the NMGT samples. It is observed that the diffraction peaks of all samples have a slight right shift relative to the standard position. The shift of the diffraction peaks to larger angles is consistent with a contraction of the lattice parameters due to the smaller ionic radius of Gd3þ in comparison to La3þ [33]. There is no other phase appeared in the samples, suggesting that the dopants Er/Yb/Mn enter into the lattice without significantly influencing on the NMGT crystal structure. The up-conversion emission spectra in the region of 500e800 nm for the Er3þ and Mn4þ single doped and Er3þ/Mn4þ co-doped NMGT under 980 nm diode laser excitation as well as the
Fig. 2. XRD patterns of NMGT host, Mn4þ singly doped and NMGT: 0.08Er3þ/0.06 Yb3þ/ zMn4þ. Bars represent NaMgLaTeO6 (ICSD No. 78532) diffraction data.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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emission spectrum in the region of 600e800 nm for NMGT: 0.01Mn4þ with 455 nm excitation are shown in Fig. 3. The inset(a) in Fig. 3 further gives the up-conversion spectra of Er3þ single and Er3þ/Mn4þ co-doped NMGT in the 750e900 nm region. Two strong emission bands centered at 544 nm and 656 nm and one weak emission band at 850 nm are observed in the NMGT: 0.08Er3þ phosphor with 980 nm excitation, which are attributed to the 2H11/ 4 2 4 2 4 2 3þ 2, S3/2 / I15/2, F9/2 / I15/2 and S3/2 / I13/2 transitions of Er , 4þ respectively. However, the NMGT: 0.01Mn phosphor shows no luminescence under excitation of 980 nm although an intense emission band ranging from 650 nm to 800 nm can be observed under 455 nm excitation. This means that the upconversion can not take place in the Mn4þ single doped NMGT under 980 nm excitation. Interestingly, in addition to the emissions of Er3þ, a luminescence band centered at 696 nm ascribed to the Mn4þ: 2E/4A2 transition appears in the Er3þ/Mn4þ co-doped NMGT. Meanwhile the intensities of the upconversion emission of Er3þ decreases as compared with the Er3þ single doped sample. This clearly indicates that the energy transfer sensitization from Er3þ to Mn4þ occurs and performs the up-conversion emission of Mn4þ. The inset(b) in Fig. 3 depicts the variation of fluorescence intensity of the Mn4þ: 2E/4A2 emission for different concentrations of Er3þ. The emission intensity of Mn4þ increases with increasing concentration of Er3þ and reaches its maximal value at x ¼ 0.08. As the Er3þ doping concentration is higher than 0.08, the emission intensity decreases gradually as the results of the concentration quenching. It is well known that Yb3þ can enhance the up-conversion emission of Er3þ in Er3þ and Yb3þ co-doped phosphors by efficient energy transfer sensitization from Yb3þ to Er3þ [34e38]. Therefore, different concentrations of Yb3þ (y ¼ 0.04, 0.06, 0.08, 0.1, 0.12) are introduced to further improve the luminescence of the Er3þ and thus the Mn4þ ions. The luminescence spectra of NMGT: 0.08Er3þ/yYb3þ/0.01Mn4þ (y ¼ 0, 0.06) are presented in Fig. 4. The inset in Fig. 4 gives the Yb3þ concentration dependent upconversion emission intensity of Mn4þ. With doping of Yb3þ, the emission bands attributed to transitions Er3þ: 2H11/2, 4S3/2, 4F9/ 4 4þ 2 4 2 / I15/2 and Mn : E/ A2 have all increased significantly. As exhibited in the inset of Fig. 4, the phosphor shows the strongest up-conversion emission Mn4þ at y ¼ 0.06. On further increasing the Yb3þ content beyond 0.06, concentration quenching is observed.
Fig. 3. Up-conversion spectra of NMGT: xEr3þ/zMn4þ (x ¼ 0, 0.08; z ¼ 0,0.01) with 980 nm excitation and the emission spectra of NMGT:0.01Mn4þ under 455 nm excitation. The insets show the Mn4þ emission intensity as functions of Er3þ concentration(a) and the up-conversion spectra between 750 and 950 nm(b).
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Fig. 4. Up-conversion spectra of NMGT: 0.08Er3þ/yYb3þ/0.01Mn4þ(y ¼ 0, 0.06) with 980 nm excitation. The inset shows the Mn4þ emission intensity as functions of Yb3þ concentration.
The Mn4þ: 2E/4A2 emission intensity of NMGT: 0.08Er3þ/ 0.06 Yb3þ/0.01Mn4þ phosphor is enhanced by 4.37 times as compared with that of the NMGT: 0.08Er3þ/0.01Mn4þ phosphor. Based on above experiment results, 8% of Er3þ and 6% of Yb3þ are believed to be the optimal combination to sensitize of Mn4þ in the NMGT host. Thus the Mn4þ concentration is further adjusted to optimize the up-conversion intensity as the Er3þ and Yb3þ doping concentrations are fixed at 8% and 6%, respectively. The upconversion spectra of the NMGT: 0.08Er3þ/0.06 Yb3þ/zMn4þ (z ¼ 0.001, 0.002, 0.01) phosphors are shown in Fig. 5. The upconversion intensities of the Er3þ and Mn4þ ions as functions of Mn4þ concentration are given as insets (a) and (b) in Fig. 5. It is seen from the inset (b) of Fig. 5 that the emission intensity at 696 nm of Mn4þ first increases with the increase of Mn4þ content and then decreases due to concentration quenching. The maximum emission intensity reaches when z ¼ 0.002. Therefore, the optimal composition for the Mn4þ up-conversion emission in our experiment might be the NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ phosphor.
Fig. 5. Emission spectra of NMGT: 0.08Er3þ/0.06 Yb3þ/zMn4þ (z ¼ 0.001, 0.002, 0.01) with 980 nm excitation. The insets depict the Mn4þ concentration dependent (a) Er3þ and (b) Mn4þ up-conversion emission intensities.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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Additionally, it is found from the inset(a) in Fig. 5 that the upconversion intensity of Er3þ ions always decreases with the increase of Mn4þ concentration. This also gives another evidence of the energy transfer from Er3þ to Mn4þ. Fig. 6 presents the normalized Er3þ emission spectrum and Mn4þ excitation spectrum. There is a small overlap between the emission spectrum of Er3þ: 2H11/2, 4S3/2 / 4I15/2 transition and the excitation spectrum of Mn4þ: 4A2 /4T2 transition, suggesting that the resonant energy transfer from Er3þ to Mn4þ ions is permissive. The energy transfer from Er3þ to Mn4þ can also be further demonstrated by the Mn4þ concentration dependent luminescence decay behavior of the 4S3/2 / 4I15/2 transition of Er3þ at 544 nm. The decay curves monitoring at 544 nm for the NMGT: 0.08Er3þ/ 0.06 Yb3þ/zMn4þ (z ¼ 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015) phosphors excited at 980 nm are presented in Fig. 7(a). All decay curves can be well fitted into a double-exponential function as follows [39]:
IðtÞ ¼ I0 þ A1 expð t = t1 Þ þ A2 expð t = t2 Þ;
(2)
where IðtÞ and I0 represent the intensities at time t and time 0, A1 and A2 are constants, t1 and t2 represent the luminescence lifetimes for the corresponding fast and slow decay components, respectively. The average lifetimes for the 4S3/2 level of Er3þ can be calculated by the following formula [40]:
tav ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ;
(3)
the calculated decay times are 0.210, 0.204, 0.197, 0.189, 0.174, 0.169, 0.152, 0.130 ms for the concentration of Mn4þ at z ¼ 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015 in NMGT: 0.08Er3þ/0.06 Yb3þ/ zMn4þ. It is obvious that the average lifetimes at 544 nm of Er3þ ions decrease monotonously from 0.210 to 0.130 ms with the increase of Mn4þ concentration. The Mn4þ concentration dependent decay properties of the Er3þ emission at 544 nm prove the nonradiative energy transfer from Er3þ to Mn4þ. Based on the decay lifetimes, energy transfer efficiency(hET )from Er3þ to Mn4þ can be calculated by the following equation [41]:
hET ¼ 1 t=t0 ;
(4)
where t and t0 represent the decay lifetimes of 0.08Er3þ/0.06 Yb3þ/
Fig. 7. (a)Mn4þ content-dependent UC decay curves of Er3þ: 2H11/2, 4S3/2 in the NMGT: Er3þ/Yb3þ/zMn4þ samples under 980 nm laser excitation. (b)Mn4þ content-dependent energy transfer efficiency from Er3þ to Mn4þ.
zMn4þ(z ¼ 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.015) triplydoped samples and 0.08Er3þ/0.06 Yb3þ co-doped sample, respectively. The Mn4þ concentration dependent energy transfer efficiency is given in Fig. 7(b). It can be seen from Fig. 7(b) that the energy transfer efficiency increases gradually with the increasing Mn4þ ions concentration, indicating that the increased Mn4þ ions concentration enhances the energy transfer (hET ) probability from Er3þ to Mn4þ in NMGT phosphors. The energy transfer efficiency is 6% when the phosphor gives the strongest Mn4þ up-conversion emission at z ¼ 0.002. To better understand the UC mechanism of Mn4þ, the pumping power dependence of the fluorescent intensities in NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ has been investigated, as given in Fig. 8. Generally, the emission intensity Iem depends on the excitation power Ip with following relationship [19]:
Iem f Ip
Fig. 6. Overlapping between the excitation spectrum of Mn4þ monitored at 696 nm and emission spectrum of Er3þ excited at 980 nm.
n
(5)
where n value is the number of the pumping photons required to excite rare earth ions from the ground state to the excited state. The power dependence of the aforementioned three emission transitions is depicted in Fig. 8 by a log-log plot. The slopes of the linear fittings are 1.81 for Er3þ: 2H11/2, 4S3/2 / 4I15/2, 1.91 for Er3þ: 4F9/ 4 2 / I15/2, indicating that two pumping photons are needed to populate the Er3þ: 2H11/2, 4S3/2, 4F9/2 emitting levels. The slope for
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
R. Huang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 8. Log-Log plot of the integrated UC emission intensity versus the pumping power of 980 laser for NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ sample.
Mn4þ: 2E/4A2 emission is determined to be 1.96, supporting that Mn4þ luminescence is realized by a two-photon UC mechanism in NMGT: Er3þ/Yb3þ/Mn4þ sample with the assistance of the energy transfer strategy of Yb3þ/Er3þ/Mn4þ. According to the above discussion, the mechanism diagram illustrating the Yb3þ/Er3þ/Mn4þ energy transfer sensitization up-conversion is described in Fig. 9. Firstly, the Yb3þ ion at the ground state is excited to its 2F5/2 state under 980 nm excitation, and the energy is transferred to the Er3þ ions in the ground state, thereby exciting Er3þ ions to the intermediate state 4I11/2 which has a long decay time [42]. A second Yb3þ ion in the 2F5/2 state can also transfer its energy to the same Er3þ ion that has been excited to the 4 I11/2 state, which will be raised to the higher-lying 4F7/2 state. Er3þ ions in the 4F7/2 state can relax to the lower 2H11/2, 4S3/2 and 4F9/2 states through the non-radiative relaxation. The Er3þ ions in the 2 H11/2, 4S3/2 and 4F9/2 states return to the ground state, emitting the green light (525 nm, 544 nm) and red light (656 nm). Meanwhile,
Fig. 9. Energy level diagram for up-conversion mechanism in NMGT: Er3þ/Yb3þ/Mn4þ under excitation at 980 nm.
5
the 4T2 state of Mn4þ ions can be populated through the energy transfer 4H11/2, 4S3/2(Er3þ) þ 4A2(Mn4þ) / 4I15/2(Er3þ) þ 4T2(Mn4þ). The Mn4þ ions in the 4T2 state relax quickly to their 2E state through non-radiative relaxation. Then the radiative transition of Mn4þ from the 2E state to the ground state gives the deep red emission (696 nm). In general, thermal stability is an important factor in evaluating the application of phosphors because the phosphors might work in a high temperature environment. The luminescence spectra of NMGT: 0.08Er3þ/0.06 Yb3þ/0.002Mn4þ phosphor have been measured under different temperatures (303, 333, 363, 393 and 423 K). The temperature dependent emission intensity of Mn4þ is displayed in Fig. 10 and the luminescence spectra are also given as an inset. It can be clearly seen that the emission intensity of Mn4þ monotonously decreases, while the peak position and spectra shape are the same with temperature increasing from 303 K to 423 K. The up-conversion emission intensity of Mn4þ is determined by the non-radiative relaxation of Mn4þ and the non-resonant energy transfer from Er3þ to Mn4þ. The non-radiative relaxation probability will increase as the temperature rises, according to the non-radiative decay theory [43]. While the probability of nonresonant energy transfer involving emission of p phonons can be expressed as [44]:
WðTÞ ¼ Wðni þ 1Þp
(6)
where the WðTÞ represents the probability of non-resonant energy transfer, ni is the average occupation number of the ith vibrational mode and has been expressed as:
1 ni ¼ ehw=kT 1
(7)
As can be seen from equations (6) and (7), the phonon-assisted energy transfer probability increases as the temperature rises. Therefore, there is a competition between the non-radiative relaxation and the Er3þ / Mn4þ energy transfer sensitization with temperature rising. Presumably the non-radiative relaxation predominates and consequently the emission intensity of Mn4þ decreases as the temperature rises. Although the up-conversion emission of Mn4þ has been realized in the Yb3þ/Er3þ/Mn4þ triply-doped NMGT phosphor, its
Fig. 10. Dependence of the emission intensity as a function of temperature for NMGT:Yb3þ/Er3þ/Mn4þ, the inset is the variations of the emission spectra as a function of temperature.
Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816
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luminescence intensity is still weak. One of the reasons may ascribe to the small overlap between the emission spectrum of Er3þ and the excitation spectrum of Mn4þ, which results in the low energy transfer sensitization efficiency from Er3þ to Mn4þ. On the other hand, the back energy transfers from Mn4þ to Yb3þ and Er3þ might also further weaken the Mn4þ emission intensity, which can be demonstrated by the emission spectra of the Er3þ and Yb3þ singly doped as well as Mn4þ/Er3þ and Mn4þ/Yb3þ co-doped NMGT phosphors excited by a 455 nm diode light, as shown in Fig. 10. In Fig. 11, no luminescence is observed in the NMGT: 0.06 Yb3þ phosphor, while a strong emission band peaking at around 1002 nm attributed to the Yb3þ: 2F5/2 / 2F7/2 transition appears in the NMGT: 0.06 Yb3þ/0.01Mn4þ phosphor. For the NMGT: 0.08Er3þ sample, only weak near-infrared emission bands centered at 985 nm and 1510 nm in the range are observed, assigned to the 4I11/ 4 4 4 3þ 2 / I15/2 and I13/2 / I15/2 transitions of Er , respectively. However, the intensities of Er3þ emission at 985 nm and 1510 nm in the NMGT: 0.08Er3þ/0.01Mn4þ phosphor are increased by 14.04 and 5.97 times as compared with that in the NMGT: 0.08Er3þ phosphor, respectively. Above luminescent behaviors prove the occurrence of the back energy transfers Mn4þ/Yb3þ and Mn4þ/Er3þ in these phosphors. Considering that the Mn4þ: 2E state lies just above the Er3þ: 4I9/2 state, the back energy transfer from Mn4þ to Er3þ should 2 4 be performed through the E(Mn4þ) þ I15/2(Er3þ) /4A2(Mn4þ) þ 4I9/2 (Er3þ) process. Thus under 980 nm excitation, an intensified emission at around 800 nm due to the Er3þ: 4I9/ 4 3þ 4þ co-doped and Yb3þ/Er3þ/ 2 / I15/2 transition in the Er /Mn Mn4þ triply-doped NMGT phosphors might be expected. However, no emission at 800 nm has been observed in either Er3þ single doped or Er3þ/Mn4þ co-doped NMGT phosphors under 980 nm excitation, just as shown in the inset(a) of Fig. 3. This indicates that no-radiative relaxation of Er3þfrom its 4I9/2 to 4I11/2 level in the NMGT is a very rapid process, so that the radiative transition of 4I9/ 4 2 / I15/2 is too weak to be observed. 4. Conclusion To sum up, NMGT: Er3þ/Yb3þ/Mn4þ phosphors have been synthesized by solid state reaction method. A deep red up-conversion emission at 696 nm arising from the 2E/4A2 transition of Mn4þ has been observed. Excitation power dependent up-conversion intensity and Mn4þ concentration dependent luminescence decay
Fig. 11. Emission spectra of the Er3þ singly-doped, Yb3þ singly-doped, Mn4þ/Er3þ codoped and Mn4þ/Yb3þ co-doped NMGT samples excited at 455 nm.
lifetime of the Er3þ: 4S3/2 / 4I15/2 transition prove that a twophoton process between Yb3þ- Er3þ couple followed with resonant energy transfer from Er3þ to Mn4þ is responsible for the upconversion emission of Mn4þ. The results in this work might provide meaningful reference on the design of up-conversion materials with novel emission bands, expanding the application field of up-conversion materials. Declaration of competing interest There are no conflicts of interest. Acknowledgments This work was supported by the National Science Foundation of China (No. 11674272) and the National Natural Science Foundation of China and Guangdong Province (U1401241). References [1] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (4) (2009) 976e989. [2] F. Wang, Y. Han, C.S. Lim, Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping, Nature 463 (7284) (2010) 1061e1065. [3] François Auzel, Upconversion and anti-Stokes process with f and d ions in solids, Chem. Rev. 104 (1) (2004) 139e147. [4] V. Singh, V.K. Rai, K. Al-Shamery, NIR to visible upconversion in Er3þ/Yb3þ codoped CaYAl3O7 phosphor obtained by solution combustion process, J. Lumin. 131 (12) (2011) 2679e2682. [5] H.Q. Wang, T. Nann, Monodisperse upconverting nanocrystals by microwaveassisted synthesis, ACS Nano 3 (11) (2009) 3804e3808. [6] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, FL, 1999. [7] F.V.D. Rijke, H. Zijlmans, S. Li, Up-converting phosphor reporters for nucleic acid microarrays, Nat. Biotechnol. 19 (3) (2001) 273e276. [8] J.H. Chung, J.H. Ryu, S.W. Mhin, Controllable white upconversion luminescence in Ho3þ/Tm3þ/Yb3þ co-doped CaMoO4, J. Mater. Chem. 22 (9) (2012) 3997e4002. [9] G. Chen, T.Y. Ohulchanskyy, R. Kumar, Ultrasmall monodisperse NaYF4: Yb3þ/ Tm3þ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence, ACS Nano 4 (6) (2010) 3163e3168. €fer, Upconverting nanoparticles, Angew. Chem. Int. Ed. 50 [10] M. Haase, H. Scha (26) (2011) 5808e5829. [11] D. Chen, F. Liu, P. Huang, Sensitized thulium ultraviolet upconversion luminescence in Tm3þ/Yb3þ/Nd3þ triply doped nanoglass ceramics, Opt. Lett. 32 (21) (2007) 3068e3070. [12] D. Chen, W. Xu, S. Yuan, Ln3þ-Sensitized Mn4þ near-infrared upconverting luminescence and dual-modal temperature sensing, J. Mater. Chem. C. 5 (2017) 9619e9628. [13] D. Yang, P. Ma, Z. Hou, Current advances in lanthanide ion (Ln3þ)-based upconversion nanomaterials for drug delivery, Chem. Soc. Rev. 44 (6) (2015) 1416e1448. [14] P. Li, M. Peng, X. Yin, Temperature dependent red luminescence from a distorted Mn4þ site in CaAl4O7: Mn4þ, Opt. Express 21 (16) (2013) 18943e18948. [15] H. Tan, M. Rong, Y. Zhou, Luminescent behaviour of Mn4þ ions in seven coordination environment of K3ZrF7, Dalton Trans. 45 (23) (2016) 9654e9660. [16] Z. Pan, Y.Y. Lu, F. Liu, Sunlight-activated long-persistent luminescence in the near-infrared from Cr3þ-doped zinc gallogermanates, Nat. Mater. 11 (1) (2011) 58e63. [17] Renping Cao, Preparation and luminescence properties of Li2MgZrO4: Mn4þ red phosphor for plant growth, J. Lumin. 188 (2017) 577e581. [18] H. Zhu, C.C. Lin, W. Luo, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014) 4312. [19] J. Liao, Q. Wang, H.R. Wen, First observation of mutual energy transfer of Mn4þ-Er3þ via different excitation in Gd2ZnTiO6: Mn4þ/Er3þ phosphors, J. Mater. Chem. C. 5 (2017) 9098e9105. [20] D. Chen, Y. Liu, J. Chen, Yb3þ/Ln3þ/Mn4þ (Ln ¼ Er, Ho, and Tm) doped Na3ZrF7 phosphors: oilewater interface cation exchange synthesis, dual-modal luminescence and anti-counterfeiting, J. Mater. Chem. C. 7 (2019) 1321e1329. [21] Lv Yang, Visible to NIR down-shifting and NIR to visible upconversion luminescence in Ca14Zn6Ga10O35: Mn4þ, Ln3þ (Ln¼Nd, Yb, Er), Dyes Pigments 161 (2019) 137e146. [22] Yasutake Teraoka, Ming-Deng Wei, Shuichi Kagawa, Double perovskites containing hexavalent molybdenum and tungsten: synthesis, structural investigation and proposal of a fitness factor to discriminate the crystal symmetry, J. Mater. Chem. 8 (11) (1998) 2323e2325. [23] K. Li, H. Lian, R.V. Deun, Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO6:Mn4þ with perovskite structure for w-LEDs, J. Lumin. 198 (2018) 155e162.
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Please cite this article as: R. Huang et al., Up-conversion luminescence of Mn4þ sensitized byYb3þ, Er3þ in NaMgGdTeO6, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152816