Near-Infrared Emission of Er3+ Sensitized by Mn4+ in

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Near-infrared Emission of Er Sensitized by Mn in Ca ZnAl O Matrix Xue jun Gao, Wei Li, Xiaoliang Yang, Xiang liang Jin, and Siguo Xiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05825 • Publication Date (Web): 29 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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The Journal of Physical Chemistry

Near-infrared Emission of Er3+ Sensitized by Mn4+ in Ca14Zn6Al10O35 Matrix Xuejun Gao,† Wei Li,† Xiaoliang Yang,† Xiangliang Jin† and Siguo Xiao*† †

School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China

ABSTRACT: A novel near-infrared luminescent material Ca14Zn6Al10O35: Mn4+, Er3+ has been synthesized by a sol-gel method. The Ca14Zn6Al10O35: Mn4+, Er3+ phosphor exhibits strong near-infrared emission around 1540 nm with a wide excitation band extending from 250 to 550 nm. Efficient energy transfer from Mn4+ to Er3+ in Ca14Zn6Al10O35 is observed and gives about 13 times enhancement on the Er3+-1540 nm emission excited at 455 nm, which makes it convenient to obtain high power emission by commercial GaN LED pumping. The energy transfer mechanism is discussed based on Dexter's theory and Inokuti-Hirayama/Yokota-Tanimoto models. It is believed that a dipole-dipole interaction between Mn4+ and Er3+ ions is responsible for the large enhancement of 1540 nm emission.

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1. INTRODUCTION The research on a new generation of rare-earth (RE)-based near-infrared (NIR) emitting materials is receiving a great deal of interest based on their advantages for lasers,1,2 optical amplifiers for telecommunication,3,4 solar spectral converters,5,6 and biological fluorescent probes,7,8 etc. Among the RE ions, Er3+ is especially attractive because its 1540 nm emission lies in the minimum loss windows of silica optical fibers and is safe to the human eye, bringing various photonic and optoelectronic applications such

as

fiber

communication,4

medical

treatment.9

However,

its

low

intra-4f-configurational absorption cross section limits the pump efficiency. Sensitization might be an effective pathway to increase the intra-4f-configurational absorption and therefore make 1540 nm luminescence generation more efficiently. The possibility to improve the Er3+ absorption through sensitization have been studied with various sensitizers including (1) other RE ions like Yb3+, Eu2+, Ce3+,10-12 (2) semiconductors like ZnO, GaN, GaAs13-15 and (3) transition-metal ions such as Cr3+, Fe3+,16,17 etc. Owing to the strong broadband absorption, the transition-metal ions might efficiently absorb the excitation light and transfer its energy to RE ions nearby resonantly or assisted with phonons. Therefore, the transition-metal ions are believed ideal sensitizers for RE ions to obtain their high efficient luminescence. The luminescence of RE ions sensitized by transition-metal ions such as Cr3+, Mn2+, Fe3+, Ni2+ have been observed.16-25 In spite of this, issues relating to sensitization efficiency and ease of pumping are still to be resolved. 2 ACS Paragon Plus Environment

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It is noted that the luminescent properties of tetravalent manganese ion (Mn4+) have been investigated in the past years.26-32 The 4A2→4T1 and 4A2→4T2 spin-allowed transitions of Mn4+ in octahedral coordination environment exhibits near UV and visible light absorption and Mn4+ doped phosphor can give off red light. Especially, the recently developed Ca14Zn6Al10O35: Mn4+ exhibits broadband absorption with the maximum absorption peak near 470 nm.26,32 It can be efficiently excited by using a blue LED light and exhibits deep red emission in the region of 650-750 nm (quantum efficiency > 50%). The Ca14Zn6Al10O35 matrix has performances including excellent chemical and mechanical properties, good thermal stability, ease of synthesis, and cheapness of raw materials. In addition, the Ca2+ can be replaced by RE ions because Ca2+ and RE ions have similar ion radius. More importantly, relatively small energy mismatch between the 2

E(Mn4+) and 4I9/2(Er3+) levels in Ca14Zn6Al10O35 matrix means the possibility of efficient

energy transfer from Mn4+ to Er3+ ions. Thus efficient 1540 nm emission of Er3+ excited with a commercial high power GaN LED is possible to be easily realized in the Mn4+-Er3+ co-doped Ca14Zn6Al10O35 material. In this work, a series of Mn4+-Er3+ co-doped Ca14Zn6Al10O35 phosphors have been prepared. Efficient 1540 nm emission of Er3+ sensitized by Mn4+ is observed and the energy transfer mechanism from Mn4+ to Er3+ ions has been investigated in detail.



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2. EXPERIMENTAL 2.1. Materials synthesis The samples were prepared with a sol-gel procedure.33 The starting materials were: Al(NO3)3(H2O)9

(99.9%),

Zn(NO3)2(H2O)6

(99.9%),

Er(NO3)3(H2O)6

(99.9%),

Ca(NO3)2(H2O)4 (99.9%), Mn(NO3)2(H2O)4 (99.9%), C6H8O7 (99.9%). Firstly, each reagent

was

dissolved

in

distilled

water

with

the

stoichiometric

ratio

of

Ca14-xZn6Al10-yO35: Mny, Erx (y = 0.00, 0.15, 0.40, 0.60, 0.70, 0.80, 0.90, and x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3) and thoroughly mixed by stirring. Then the C6H8O7 (citric acid) aqueous solution was dripped into the nitrate solution with cation : C6H8O7 = 1 : 1.5 companied with constant stirring. After stand for 24 h, the solutions were dried at 100 °C for 10 hours to obtain precursors. The followed firing process involved two steps. First, the precursors dried were carried out to remove the organic matter by firing at 500 °C for 2 h and then cooled to room temperature. After grinding, the powders were again fired to 1200 °C at a heating rate of 6 °C/min in air and calcined at that temperature for 1 h to obtain the final samples. 2.2. Characterization The crystal structures of the phosphors were measured by X-ray diffraction (XRD) on a Bruker D8 advanced equipment using Cu tube with Cu/K (k = 0.1541 nm) radiation. The morphology of the synthesized phosphor was characterized using a JSM-6610 scanning electron microscope (SEM). The transmission electron microscopy (TEM) image and the typical selected-area electron diffraction (SAED) image were taken with a 4 ACS Paragon Plus Environment

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JEM-2100 microscope operating at accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was measured on a K-Alpha 1063 (Thermo Fisher Scientific) with a focused monochromatic Al Ka X-ray beam (12 kV, 6 mA, 5 × 10 −9 torr).The excitation spectra and visible-near-infrared photoluminescence spectra were recorded with a monochromator

(Zolix

Instrument,

Omni-λ320i)

coupled

with

photomultiplier

(PMTH-S1-CR131) and NIR sensitive detector (DInGaAs 2600-TE), in which a monochromator (Zolix Instrument Omni-λ320) coupled with a 150 W xenon lamp was used to provide the monochromatic exciting light. The Er3+ concentration dependent Mn4+ luminescence decay curves were measured by PTI QM 40 spectrofluorometer using a pulse xenon lamp as the excitation source. The decay curve for Er3+-1540 nm NIR emission and temperature dependent decay curves for Mn4+ luminescence at 710 nm of the phosphors were measured with an FLS920 (Edinburgh) Spectrometer. Raman spectrum was measured by a Renishaw InVia Raman microscope using a 532 nm laser as excitation source. 3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) patterns of Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.6, 0.8, 1.0, 1.2) are shown in Figure 1. The diffraction peaks of the samples with low Er3+ concentration are in agreement with the standard XRD peaks of Ca14Zn6Al10O35 (JCPDS 50-0426). As the Er3+ doping content is above x = 0.8, a small amount of CaErAlO4 impurity appears. The Ca14Zn6Al10O35 has a cubic symmetry cell with space group F23. In Ca14Zn6Al10O35 crystal, four of the five independent positions occupied by Al and Zn 5 ACS Paragon Plus Environment


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are in tetrahedral coordination, and the fifth independent position is in octahedral coordination. In addition, there are three independent Ca2+, two of them are in octahedral, and the third independent Ca2+ is in a seven-coordinated polyhedron. The XPS of manganese (Mn) doped Ca14Zn6Al10O35 matrix was measured to examine the valence state of Mn in Ca14Zn6Al10O35 (Figure 2). From the smoothed XPS shown in Figure 2, the core level binding energy of Mn 2p3/2 is estimated to be about 642.42 eV. Considering that the peaks of Mn2+ 2p3/2 (MnO), Mn3+ 2p3/2 (Mn2O3) and Mn4+ 2p3/2 (MnO2) at 641.7, 641.8 and 642.4 eV,34 the Mn element predominantly behave as the state of Mn4+ in the Ca14Zn6Al10O35 matrix. It is commonly accepted that Mn4+ ions are preferentially accommodated at the Al3+ sites in the lattice with an octahedral coordination.26,32 On the other hand, Ca2+ sites are possible to be replaced by Er3+ due to the similar ion radius between them (Ca2+ : r = 0.100 nm; Er3+ : r = 0.089 nm) when a small amount of Er3+ ions are introduced. Considering of the different valences of Ca2+ and Er3+, charge compensations are required. It was commonly believed that the formative Ca vacancies can compensate charge imbalance.12,35,36 When Er3+ ions are co-doped into Ca14Zn6Al10O35, the Ca vacancies might form and they could keep the electroneutrality of the compound. SEM and TEM of the Ca13.4Zn6Al9.85O35: Mn0.15, Er0.6 phosphor are shown in Figure 3(a) and 3(b), respectively. The SEM and TEM images show that the synthesized particles have coherent sheet structure. The HRTEM image in Figure 3(c) shows the lattice distance is 0.4504 nm corresponding to (311) plane of cubic phase Ca14Zn6Al10O35 6 ACS Paragon Plus Environment

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crystal. From the SAED pattern shown in Figure 3(d), it is found the phosphor exhibits pure cubic crystalline structure, corresponding exactly to the result of XRD analysis.

Figure 1. Power X-ray diffraction patterns for Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.6, 0.8, 1.0, 1.2).



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Figure 2. Mn 2p XPS spectrum of Ca13.0Zn6Al9.4O35: Mn0.6, Er1.0 sample.

Figure 3. SEM (a) and TEM (b) images for the Ca13.4Zn6Al9.85O35: Mn0.15, Er0.6 sample as well as HRTEM (c) image and SAED (d) pattern corresponding to the crystal of Figure 3(b). 8 ACS Paragon Plus Environment

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Figure 4 presents the NIR emission spectra of Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2) phosphors excited at 455 nm light. Near-infrared emission band in range of 1450-1700 nm assigned to 4I13/2→4I15/2 transition of Er3+ is observed. The emission intensity increases with increasing content of Er3+ and reaches its maximal value at x = 1.0. As the Er3+ doping content is higher than 1.0, concentration quenching occurs and the emission intensity decreases. To obtain optimal luminescence intensity at 1540 nm, the Mn4+ doping concentration is also adjusted and a series of Ca13.0Zn6Al10-yO35: Mny, Er1.0 (y = 0.00, 0.15, 0.40, 0.60, 0.70, 0.80, 0.90) samples have been prepared. It can be seen that the intensity of Er3+ emission is further enhanced significantly with increasing Mn4+ content (see Figure 5). The optimal Mn4+ doping content is 0.6% in mole (y = 0.6). Comparing with the sample in the absence of Mn4+, the intensity at 1540 nm of Ca13.0Zn6Al9.4O35: Mn0.6, Er1.0 sample is enhanced by a factor of about 13. The Mn4+ co-doping induced enhancement of the Er3+ emission suggests an efficient energy transfer from Mn4+ to Er3+ takes place in the Ca14Zn6Al10O35: Mn4+, Er3+ phosphors. The 1540 nm emission of Er3+ sensitized by other ions such as Eu2+ and Ce3+ have also been reported previously and high sensitization efficiency have been witnessed.11-12 It is found that the sensitization effect of Mn4+ approaches to that Eu2+ and is superior to that of Ce3+. Moreover, Ca14Zn6Al10O35: Mn4+, Er3+ is more convenient to be prepared comparing with the luminescent materials doped with Er3+, Eu2+/Ce3+, since the reducing atmosphere is necessary to form Eu2+ and Ce3+ in those phosphors. 9 ACS Paragon Plus Environment


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Figure 4. NIR emission spectra of Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2) excited at 455 nm light. The inset shows the Er3+ concentration dependent 1540 nm intensity.

Figure 5. NIR emission spectra of Ca13.0Zn6Al10-yO35: Mny, Er1.0 (y = 0.00, 0.15, 0.40, 0.60, 0.70, 0.80, 0.90) excited at 455 nm light. The inset shows the Mn4+ concentration dependent 1540 nm intensity.


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Figure 6. Excitation and emission spectra of Ca14-xZn6Al10-yO35: Mny, Erx (y = 0.0, 0.6; x = 0.0, 1.0). In order to further demonstrate the energy transfer from Mn4+ to Er3+, the excitation spectra of the Mn4+ and Er3+ luminescence in samples containing Mn4+ and/or Er3+ are measured, shown in Figure 6. Two broad and intense excitation bands (monitored at Mn4+ 710 nm emission) corresponding to the 4A2→4T1 and 4A2→4T2 spin-allowed transitions of Mn4+ are observed in the Mn4+ single-doped sample, shown in Figure 6(a). The excitation spectrum is similar to the results reported by previous literatures.26, 32 From the excitation spectrum of Mn4+-Er3+ co-doped sample monitored at 1540 nm emission of Er3+ in Figure 6(b), it can be seen that not only broad and intense excitation bands ascribing to Mn4+ ions but also superimposed excitation peaks assigned to the 4I15/2 to 2

H11/2 and 4S3/2 transitions of Er3+ are observed. However, only weak and discrete 11 ACS Paragon Plus Environment


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excitation peaks ascribe to the transitions from 4I15/2 to 4G11/2, 4F5/2, 4F7/2, 2H11/2 and 4S3/2 of Er3+ for 1540 nm emission are observed in the Er3+ single-doped sample. Obviously the broad and intense excitation bands in the Mn4+-Er3+ co-doped sample also originate in the 4A2→4T1 and 4A2→4T2 spin-allowed transitions of Mn4+. The characteristics of the excitation and emission spectra of the Mn4+, Er3+ single-doped and Mn4+-Er3+ co-doped phosphors show clear evidence for energy transfer from Mn4+ to Er3+. Figure 7 further gives the normalized Mn4+ emission spectra and Er3+ excitation spectra. The small overlap between the Mn4+ emission spectra (2E →4A2) and the Er3+ excitation spectra (4I15/2→4I9/2) can permits weak resonant energy transfer from Mn4+ to Er3+ ions. On the other hand, the non-resonant energy transfer cannot be ignored. It is noted that the energy gap between the peaks of Mn4+ emission and Er3+ excitation is only 995 cm-1. Moreover, the wide emission band of Mn4+ and wide excitation band of Er3+ means that there are abundant possible energy transfer pathways between the sub-levels of the 2E state of Mn4+ and 4I9/2 state of Er3+. The highest-frequency of lattice vibration is 850 cm-1 corresponding to the Al-O stretching vibration in terms of the Raman spectra of the Ca14Zn6Al10O35 matrix in Figure 8. Thus the energy-level mismatch between Mn4+ and Er3+ can be efficiently covered by emission of one or more phonons and thus non-resonant energy transfer from Mn4+ to Er3+ can easily occur.


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Figure 7. Emission spectra of Mn4+ excited at 470 nm and excitation spectra of Er3+ monitoring at 1540 nm.

Figure 8. Raman spectra of Ca14Zn6Al10O35.



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According to above discussion, the mechanism of the Mn4+ sensitized Er3+-1540 nm emission in the Mn4+-Er3+ co-doped phosphors can be described with Figure 9. Firstly the Mn4+ ions are excited into their 4T1 or 4T2 states under irradiation of the near UV and visible light in region of 250-550 nm. Then the 2E(Mn4+)→4I9/2(Er3+) energy transfer between the Mn4+ and Er3+ ions takes place, populating the 4I9/2 levels of Er3+. The followed nonradiative relaxation performs the population of 4I13/2 levels of Er3+, resulting in the 1540 nm emission.

Figure 9. Electron-transitions and energy-transfer (ET) scheme of Mn4+, Er3+ in Ca14Zn6Al10O35. To further understand the energy transfer sensitization mechanism and estimate the 14 ACS Paragon Plus Environment

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energy transfer efficiency, the decay curves monitoring at 710 nm for the Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3) phosphors excited at 455 nm light are measured. The luminescence decays of Mn4+ doped with different Er3+ concentrations are shown in Figure 10 (a). It is seen that the luminescence lifetimes of 710 nm emission decreases with the increase of Er3+ concentration. The mean decay lifetime ( τ ) of the Mn4+ emission at 710 nm is expressed as ∞

∫ τ= ∫

0 ∞ 0

tI (t )dt I (t ) dt

,

(1)

where I (t ) is the time dependent luminescence intensity at 710 nm. The calculated mean decay lifetimes are all given in Table 1. It is seen that the mean lifetime decreases monotonously from 1.115 to 0.720 ms, with the Er3+ content increasing from 0.0 to 1.3. The Er3+ concentration dependent decay properties of the Mn4+ emission at 710 nm proves the occurrence of the nonradiative energy transfer process from Mn4+ to Er3+. Additionally, the temporal behavior of Er3+-1540 nm emission for Ca13.8Zn6Al9.85O35: Mn0.15, Er0.2 phosphor with 470 nm excitation has been also measured at room temperature and shown in Figure 10 (b). An in-growth period which is the characteristic of population buildup process of the 4I13/2(Er3+) level caused by the energy feeding of Mn4+ has been observed. The in-growth time is as long as about 2.5 ms, further proving the occurrence of the energy transfer from Mn4+ to Er3+ in Ca14Zn6Al10O35.



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Figure 10. (a) Decay curves of Mn4+ luminescence in Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3) monitoring at 710 nm excited at 455 nm light (the inset shows the multi-exponential fitting curves of the Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.3, 0.6, 0.9)). (b) Temporal behavior of Er3+-1540 nm emission in Ca13.8Zn6Al9.85O35: Mn0.15, Er0.2 following 470 nm excitation. 16 ACS Paragon Plus Environment

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The energy transfer sensitization efficiency ( η ET ) can be calculated from the following expression:37

η ET = 1 −

τ Mn − Er . τ Mn

(2)

Here τ Mn− Er and τ Mn are the luminescence lifetimes of Mn4+ at 710 nm in the Mn4+-Er3+ co-doped and Mn4+ single-doped cases, respectively. The calculated energy transfer efficiencies at different Er3+ concentration are given in Table 1. It is found that the energy transfer efficiencies increases gradually with Er3+ ions concentration, indicating that the increased Er3+ content improves the energy transfer probability from Mn4+ to Er3+ in Ca14Zn6Al10O35. Moreover, the energy transfer efficiency can also be further enhanced by increasing Mn4+ concentration. The calculated energy transfer efficiency for the optimally doped phosphor (Ca13.0Zn6Al9.4O35: Mn0.6, Er1.0) is as high as 77.4%.

Table 1. The mean decay lifetime ( τ ) of Mn4+ 710 nm emission and the energy transfer efficiencies ( η ET ) from Mn4+ to Er3+ in Ca14-xZn6Al9.85O35: Mn0.15, Erx phosphors.

x

τ (ms)

η ET

x

τ (ms)

η ET

0.0 0.1 0.2 0.3 0.4 0.5 0.6

1.115 1.034 1.002 0.951 0.899 0.886 0.861

0.000 0.073 0.101 0.147 0.194 0.205 0.228

0.7 0.8 0.9 1.0 1.1 1.2 1.3

0.841 0.829 0.806 0.794 0.752 0.748 0.720

0.246 0.257 0.277 0.288 0.325 0.329 0.354



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Additionally, the temperature dependent Mn4+ luminescence decay curves for Ca14Zn6Al9.85O35: Mn0.15 and Ca13.0Zn6Al9.85O35: Mn0.15, Er1.0 samples were also measured. The insets (a) and (b) in Figure 11 show the decay curves of Mn4+ luminescence in Ca14Zn6Al9.85O35: Mn0.15 and Ca13.0Zn6Al9.85O35: Mn0.15, Er1.0 samples under different temperatures (77K, 120K, 220K, and 300K). The energy transfer efficiencies of Mn4+→Er3+ are calculated by equation (1) and (2) with the measured Mn4+ luminescence decay curves at different temperatures and the temperature dependent energy transfer efficiencies of Mn4+→Er3+ in Ca13.0Zn6Al9.85O35: Mn0.15, Er1.0 are shown in Figure 11. It is found from Figure 11 that the energy transfer efficiency of Mn4+→Er3+ increases with the temperature increasing. As mentioned above, both resonant energy transfer and non-resonant energy transfer involving emission phonons might be responsible for the Mn4+→Er3+ sensitization. The non-resonant energy transfer probability involving the emission of N phonons at temperature T can be expressed as38

W (T ) = W ( ni + 1) , N

(3)

where W is the energy transfer probability derived from the Miyakawa-Dexter theory,38

ni is the average occupation number of the ith vibrational mode and is given as ni = ( exp ( hν i / kT ) − 1) . −1

(4)

It indicates the phonon-assisted energy transfer possibility will increase with the temperature increasing. Thus the increase of total energy transfer efficiency from Mn4+ to Er3+ with the temperature increasing might be ascribed to the enhancement of 18 ACS Paragon Plus Environment

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phonon-assisted energy transfer possibility. The result proves that phonon-assisted energy transfer plays an important role in sensitization process from Mn4+ to Er3+ in the Mn4+-Er3+ co-doped Ca14Zn6Al10O35.

Figure 11. The temperature dependent energy transfer efficiency of Mn4+→Er3+ in Ca13.0Zn6Al9.85O35: Mn0.15, Er1.0 sample. The insets show the decay curves of Mn4+ 710 nm luminescence in Ca14Zn6Al9.85O35: Mn0.15 (a) and Ca13.0Zn6Al9.85O35: Mn0.15, Er1.0 (b) under 455 nm excitation at different temperature. The energy transfer mechanism includes exchange interaction, radiative transfer, and multipolar interaction. It is known that the rate of exchange interaction induced energy transfer is proportional to exp ( −2 R / r0 ) ,39,40 R is the distance between the donor and acceptor, and r0 is the average effective Bohr radius of the atoms. Therefore, the rate of exchange interaction induced energy transfer will decrease rapidly as the increasing of 19 ACS Paragon Plus Environment


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distance between donor and acceptor because of the sharp decline of exp ( −2 R / r0 ) with distance ( R ) increasing. However, the energy transfer efficiency of Mn4+→Er3+ in this case presents a monotonous decrease with Er3+ content decreasing (see Table 1). In fact, the relative large distance between the Mn4+ and Er3+ ions due to the low doping concentration in our case might not allow effective exchange interaction. Considering the samples are prepared by sol-gel procedure that can offer significant advantage in the production of substituted oxides of high homogeneity,33 the clusters of Mn4+ and Er3+ ions can largely be avoided. This conclusion also can be evidenced by Mn4+ spectral shape. The spectra structure of transition metal ion is sensitive to the crystal field environment. The formation of clusters of Mn4+ and Er3+ ions will change the crystal field environment around Mn4+ ions. Hence the structure of the emission spectra of Mn4+ will change by the clusters of Mn4+ and Er3+ ions. However, it is not found the significant change of the Mn4+ spectra structure for the Mn4+ single-doped and Mn4+-Er3+ co-doped samples, as shown in Figure 12. Therefore, the clusters of Mn4+ and Er3+ ions in Ca14Zn6Al10O35 might be negligible and thus the exchange interaction energy transfer between Mn4+ and Er3+ in clusters is out of consideration. On the other hand, the radiative transfer based on emission-reabsorption might also be very low, considering the unchanged structure of the emission spectra of Mn4+ in Ca14Zn6Al10O35: Mn4+, Er3+ samples with Er3+ concentration increasing,41 just as shown in Figure 12. Therefore, the energy transfer between the Mn4+ and Er3+ ions should be an electronic multipolar interaction process. On the basis of Dexter's energy transfer theory of multipolar interaction and Reisfeldˊs approximation the following relation can be expressed by42-44


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τ Mn τ Mn − Er

∝ C S /3 ,

(5)

where τ Mn and τ Mn− Er are the luminescence lifetimes of Mn4+ in the absence and presence of Er3+, respectively. C is the concentration of Er3+, and S = 6 , 8 and 10 , corresponding

to

dipole-dipole,

dipole-quadrupole,

and

quadrupole-quadrupole

interactions, respectively. The τ Mn / τ Mn − Er − C S /3 plots are given in Figure 13. When

S = 6 , it shows a best linear relation. This result reveals that the dipole-dipole interaction mechanism is mainly responsible for the energy transfer from Mn4+ to Er3+ in Ca14Zn6Al10O35.

Figure 12. Emission spectra of Mn4+ in Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.0, 0.6, 0.8, 1.0, 1.2) excited at 455 nm light.



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Figure 13. Dependence of τ Mn / τ Mn− Er of Mn4+ on C 6/3 (a), C 8/3 (b), and C 10/3 (c). In principle, the excited-state Mn4+ ions can directly relax by energy transfer to Er3+ ions or migrate among the Mn4+ ions until an Er3+ ion is reached. If the migration process among Mn4+ ions is negligible versus the Mn4+→Er3+ energy transfer, the temporal evolution of the Mn4+ popular after the pulsed excitation at 455 nm follows the Inokuti-Hirayama (I-H) model and the luminescence decay intensity I (t ) is expressed as45  t  I (t ) = I (0) exp  − − Qt 3/ S  ,  τ0 

(6)

where τ 0 is the decay constant of the Mn4+ in the absence of Er3+, S is the parameter of the multipolar interaction, and the energy transfer parameter of Q is defined by 23 ACS Paragon Plus Environment


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Q=

4π  3  ( S ) 3/ S C AΓ 1 −  ( CDA ) , 3  S

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(7)

(S ) in which CA is the Er3+ concentration, Γ(x) is the gamma function, and CDA is the

Mn4+→Er3+ energy transfer parameter of the multipolar interaction. For higher Mn4+ concentration, the diffusion rate of excitation between Mn4+ could reach the same magnitude as the spontaneous Mn4+ decay or Mn4+→Er3+ energy transfer. Therefore it may be difficult to avoid diffusion between Mn4+. In this case, The Mn4+ luminescence decay curves can be fitted by the generalization of the Yokota-Tanimoto (Y-T) model, which is given by46,47  t I (t ) = I (0) exp  − − Qt 3/ S  τ0 ×(

1 + a1 X + a2 X 2 S −3/ S − 2  ) , 1 + b1 X 

(8)

where ai and bi are the approximant coefficients of the multipolar interaction. X is given by (S ) X = D ( C DA )

−2/ S 1− 2/ S

t

,

(9)

where D is the diffusion parameter that characterizes the excitation-diffusion processes among Mn4+ ions. The luminescence decay curves are fitted using the I-H and Y-T models, presented in Figure 14(a), 14(b) and 14(c). In the I-H model, the best fit is obtained with S = 6 , meaning the dominant energy transfer between the Mn4+ and Er3+ ions is the dipole-dipole interaction. In the Y-T approximation, it also can be well fitted with S = 6 . 24 ACS Paragon Plus Environment

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(S ) Moreover, it is found that the fitted parameter values of D ( C DA )

−2/ S

involving the

diffusion are approximately 10 −30 order of magnitude, indicating that the diffusion among the Mn4+ ions is of no importance. The almost unchanged luminescence decay curves of Mn4+ at 710 nm as Mn4+ doping content increase from y = 0.15 to 0.6 (see Figure 14(d)) also indicate the negligible diffusion among the Mn4+ ions because the Mn4+ radiation decay curve is mainly determined by the spontaneous emission of the isolated Mn4+ ions. Based on above analyses, the Mn4+ → Er3+ energy transfer sensitization process can be determined as dipole-dipole interaction and the diffusion among the Mn4+ ions is negligible.



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Figure 14. (a), (b) and (c) Decay curves and fitted curves of Ca14-xZn6Al9.85O35: Mn0.15, Erx (x = 0.6, 1.0). (d) Decay curves of Ca14Zn6Al10-yO35: Mny (y = 0.15, 0.6). All the experimental data of samples measured by 455 nm excitation light and monitored at 710 nm.

4. CONCLUSIONS In conclusion, a novel 1540 nm near-infrared phosphor of Ca14Zn6Al10O35: Mn4+, Er3+ with cubic structure has been synthesized via a sol-gel method. The excitation spectral of Mn4+ in Ca14Zn6Al10O35 spans the range of 250-550 nm and efficient energy transfer from Mn4+ to Er3+ enable broadband sensitization of Er3+ in the Mn4+-Er3+ co-doped phosphor. The efficiency of the energy transfer is estimated based on the variation of the lifetime of Mn4+ emission at 710 nm. The detailed analysis on decay curves of Mn4+ emission at 710 nm indicates the dipole-dipole interaction mechanism is 27 ACS Paragon Plus Environment


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dominantly responsible for the energy transfer. The sensitized 1540 nm emission could be easily realized by illuminating with a commercial GaN LED, making application of the phosphor quite convenient.

AUTHOR INFORMATION Corresponding Author *

Electronic mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 51372214 & 61233010), Project of Department of science and technology of Hunan Province of China (No. 2014FJ3124), and the Open Project of State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science (RERU2013017).

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TOC graphic:


Near-Infrared Emission of Er3+ Sensitized by Mn4+ in

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