Down-Conversion Luminescence in

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Cooperative Energy Transfer Up/Down-conversion Luminescence in Tb3+ / Yb3+ Co-doped Cubic Na5Lu9F32 Single Crystals by Gd3+Co-doping Shinan He, Haiping Xia, Qingyang Tang, Qiguo Sheng, Jianli Zhang, Yongsheng Zhu, and Baojiu Chen Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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Crystal Growth & Design

Cooperative Energy Transfer Up/Down-conversion Luminescence in Tb3+ /Yb3+ Co-doped Cubic Na5Lu9F32 Single Crystals by Gd3+Co-doping Shinan He 1,Haiping Xia 1 Qingyang Tang 1, Qiguo Sheng 1, Jianli Zhang 1, Yongsheng Zhu 2 and Baojiu Chen 3

1. Key laboratory of Photoelectronic Materials, Ningbo University, Ningbo, Zhejiang, 315211, China 2. College of Physics and Electronic Engineering, College of Chemistry and Charmaceutical Engineering, Nanyang Normal University, Nanyang, 473061, China 3.Department of Physics, Dalian Maritime University, Dalian, Liaoning Province, 116026,

ABSTRACT The luminescence properties of Na5Lu9F32 single crystals tri-doped Tb3+, Yb3+ and Gd3+ ions synthesized via a vertical Bridgman method were reported. Gd3+ was co-doped as a sensitizer in Na5Lu9F32:Tb3+, Yb3+, thus increasing the absorption line-width and the absorption cross-sections in the ultraviolet-visible region. Effects of Gd3+ concentration on the luminescence properties under ultraviolet (UV) and near-infrared (NIR) were investigated. The energy transfer mechanism between Gd3+and Tb3+ ion was witnessed by the detailed analyses on the excited-state lifetimes, emission quantum yields, and emission and excitation spectra for the Na5Lu9F32:Tb3+, Yb3+ single crystals co-doping with various Gd3+ concentrations ranging from 0 to 0.25mol%. Following the excitation light in ultraviolet region, Na5Lu9F32 single crystals tri-doped Tb3+/Yb3+/Gd3+ showed dominant green emissions in the visible region and at 980 nm which were attributed to 5D4→7F5 transition of Tb3+ and 2

D5/2→2F7/2 of Yb3+, respectively. Under the excitation of 980nm, Na5Lu9F32 single crystals tri-doped

Tb3+/Yb3+/Gd3+ presented bright yellowish-green lights arising from the cooperative energy transfer between two Yb3+ and one Tb3+.The practical up-conversion and down-conversion external quantum efficiency (EQE) were measured by integrating sphere at ~1.2% and ~45%, respectively. The present 1

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results demonstrated successful Tb3+/Yb3+/Gd3+triply doping Na5Lu9F32 single crystals with both upand down-conversion luminescence properties that can be used as potential spectrum converter in solar cells.

INTRODUCTION In order to meet a large portion of future energy consumption requirements, solar power is expected to be the potential new energy resource in the past years.1, 2 Solar cell, as one of widely used devices to convert sunlight into electricity via photovoltaic system, represents a beneficial way to green and renewable energy generation and has attracted worldly scientific and industrial interest.3, 4 However, there is still a tough task due to its low conversion efficiency.5 The spectral distribution of sunlight in the air consists of photons with wide wavelengths ranging from ultraviolet (UV) to infrared (IR), but 60-75% of sunlight distributes within the UV and NIR regions where the current solar cells cannot absorb it effectively. A helpful approach is the spectral conversion aiming at modification of the solar spectrum to achieve a better match with the wavelength dependent conversion efficiency of the solar cells. Usually, up-conversion (UC) and down-conversion (DC) are regarded as the main method to achieve it.6-8 The near IR (NIR) quantum cutting (QC) down-conversion, which converts one UV-visible photon shorter than 500 nm efficiently into two NIR photons, can greatly reduce the energy loss in c-Si solar cells due to thermalization of electron-hole pairs. In recent years, rare-earth (RE) ions are the prime candidates to achieve QC because of their rich energy-level structure that allows for facile management.9-11 An experimental evidence for a cooperative energy transfer (CET) from one Tb3+ to two Yb3+ in YbxY1-xPO4:Tb3+ was presented by Vergeer et al. for the first time.12 Since then on, many 2


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similar results have also been reported in other RE3+/Yb3+ pairs such as Tm3+/Yb3+, Pr3+/Yb3+ and Er3+/Yb3+.13-16 Among all the RE3+/Yb3+ pairs, the Tb3+/Yb3+co-doped system deserves special attention due to its good performance in both UC and DC processes, which can deploy promising applications in solar cells. However, due to the inefficient excitation of the Tb3+ ions for their intra-4f forbidden transitions, a QC of Tb3+/Yb3+ solid state materials shows poor optical absorption in the UV-VIS excitation spectral range and weak NIR emission of Yb3+ in the 900–1100nm range, which seriously limits its practical application in silicon-based solar cells. Compared with other RE ions, Gd3+ has a half-filled 4f electron shell with a stable structure. It is of wide and large absorption line-width and cross-sections in the UV regions due to the 2S7/2→6IJ transitions. The Gd3+ is an efficient sensitizer especially for Tb3+due to the small energy mismatch between Gd3+:6PJ and Tb3+:5HJ. Herein, Gd3+is tried to introduce into the pair of Tb3+/Yb3+ to absorb the UV part of the solar spectrum typically in the range of 200–400nm and efficiently transfer the excitation energy to the 5D4 state of Tb3+ for the Tb3+–Yb3+ NIR QC process. Unfortunately, only little attention has been paid on this idea as so far, 17, 18 and the corresponding intense NIR emission from Yb3+ has not been realized and reported. In order to obtain high luminous efficiency, we propose to investigate Na5Lu9F32 fluoride single crystal tri-doped with Gd3+/Tb3+/Yb3+ system. The Na5Lu9F32 compound plays a more important role in some fields, such as bioimaging, drug delivery system, and solar cell application

19-21

due to its lower

phonon energy, depressed multi-phonon relaxation process, good chemical and thermal stability and wider optical transmittance. However, almost all of the reported rare-earth ions doped Na5Lu9F32 materials have been focused on nanocrystallines due to the difficulties in the crystal growth. 22 Very recently, Er3+/Yb3+ co-doped Na5Lu9F32 single crystal for laser was grown successfully in our laboratory.23 In comparison with the powders of Na5Lu9F32, single crystals could effectively avoid the 3



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adverse light scattering. On the other hand, Gd3+, Tb3+ and Yb3+ions can also be easily substituted with Lu3+ sites in the single crystal due to the equal valence and similar ionic radiuses to reach a high doing concentration. These superior properties make Na5Lu9F32 single crystal a potential material for solid-state lighting, in particular as an UC and DC luminescence material for highly effective conversion of the solar cell. In this work, we report on the luminescence properties of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal. In Na5Lu9F32 host, Gd3+ presents an intense broad absorption spectrum in the 200–400nm region, and highly enhanced Tb3+ luminescence was observed in Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal due to the efficient ET from Gd3+ to Tb3+. Finally, an enhancement of NIR emission from Yb3+ in Na5Lu9F32:Tb3+, Yb3+single crystal was obtained by co-doping Gd3+ under UV ~273nm excitation. The relevant luminescence and ET mechanisms were systematically studied and discussed. EXPERIMENTAL The Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 crystals with various Gd3+ concentrations were grown by an improved Bridgman method. The preparation of experimental material from commercially available NaF, LuF3, GdF3, TbF3 and YbF3 powders of high purity (99.999%). And the molar compositions of the raw materials according to the formula 40NaF-(51-χ)LuF3-χGdF3-1.0TbF3-8YbF3 (χ= 0, 0.10, 0.40) were prepared. The detailed process of growth used for the improved Bridgman method has been reported elsewhere.24 The grown sample of Na5Lu9F32single crystal with aboutφ10 mm×84 mm in c axis direction is shown in Fig. 1(b). It can be seen that Na5Lu9F32 single crystal was transparent and the color changed gradually along the growth direction. These grown crystals were sliced into small pieces and polished 4


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to about 2 mm thick for optical measurements in Fig. 1(b). In order to characterize the crystal structure, X-ray diffraction was measured with a XD-98X diffractometer (XD-3, Beijing). The concentrations of Gd3+, Tb3+ and Yb3+ in these samples were listed in Table 1, and detected severally with inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima 3000). The external quantum efficiency were measured by a fluorescence spectrometer (FLS980) of Edinburgh instruments combined with an integrating sphere. The absorption spectrum and transmittance spectra were investigated with a Cary 5000 UV/VIS/NIR spectrophotometer. The excitation and emission spectra and decay curves were obtained with a FLSP920 type spectrometer (Edinburgh Co., England). The All these measured data were tested at room temperature in the same condition. Table 1. Molar fractions of Gd3+, Tb3+ and Yb3+ in feed material and concentrations of Gd3+, Tb3+ ions and Yb3+ ions in all crystal samples. Gd3+ Molar

Samples

Tb3+ Measured

Fraction in feed

in crystal

material（mol%）

(1020 ions/cm-1)

1 2 3

0 0.10 0.25

0 0.011 0.251

Molar Fraction in feed material （mol%）

1.02 1.01 0.99

Yb3+ Measured in crystal (1020 ions/cm-1)

1.08 1.05 1.06

Molar fraction in feed material（mol%）

7.99 7.98 7.95

Measured in crystal (1020 ions/cm-1)

9.15 9.07 9.08

RESULTS AND DISCUSSION To reveal the crystal structure and phase, the XRD pattern of the obtained Na5Lu9F32: χ% Gd3+ (χ=0, 0.10, 0.25), 1.0%Tb3+, 8%Yb3+ samples were measured and presented at the room temperature in Fig.1(a). Compared with standard pattern JCPD 77-2042, it can be confirmed that the diffraction peak positions of obtained samples are well according with standard pattern JCPD 77-2042 of Na5Lu9F32 displayed in Fig. 1(a), and the current doping level does not cause any obvious peak shift or second phase, which verifies that all of the samples have 5



been crystallized into the pure cubic phase. Meanwhile, we can see the boule of grown crystal in the insert of Fig. 1(b) which reveals that the Na5Lu9F32 single crystals are highly transparent in the visible region. Owing to the same electric charge and the similar ionic radii with Lu3+, Gd3+, Tb3+and Yb3+, the position of Lu3+can be easily replaced by Gd3+,Tb3+and Yb3+ions in the Na5Lu9F32 single crystals with the Gd3+ dopants increasing to 0.25 mol%, and the doping level causes no lattice distortion in the host matrix.

(b) 0.1Gd

Intensity(a.u)

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3+

0.25Gd

0.4Gd

3+

3+

(a)

10

#JCPD27-0725

20

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2(degree)

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Fig.1 (a) XRD pattern of the Na5Lu9F32: Gd3+/Yb3+/Yb3+. (b)The photo of Na5Lu9F32 single crystal, the left is boule of the crystal and the right is polished slice.

The absorption of Tb3+/Yb3+ co-doped and Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal at room temperature in the UV−VIS−NIR region are presented in Fig.2(a) and (b). Both of the Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 and Tb3+/Yb3+ co-doped Na5Lu9F32 single crystals show a strong band centered at 980 nm and a relatively weak band centers at 486 nm which attributed to the 2F7/2→2F5/2 of Yb3+ and the 7F6→5D4 of Tb3+, respectively. Compared with the Tb3+/Yb3+ co-doped sample, the absorption of Gd3+/Tb3+/Yb3+ tri-doped sample shows some obvious peaks located at 273 nm corresponding to the transition from 2S7/2→6IJ of the Gd3+ ion which indicates that this crystal can be excited efficiently by a 273 nm light. The corresponding UV-VIS-NIR 6


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transmittance spectrum of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal showed in Fig.2(c) also proves that

(c) Transmission

(b)

2

(a)

F7/2 F5/2

7

2

5

6

F6 D4

F7/2 F5/2

tri-doped Absorption

S7/2 IJ

2

5

F6 D4

2

co-doped Absorption

7

-1

90 80 70 60 50 40 30 20 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6

2

Transmission(%)

there exists a high transmittance in the range of 200 -1100 nm.

Absorption coeffcienct(cm )

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300

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Wavelength(nm) Fig.2 (a)Absorption spectra of Tb3+/Yb3+co-doped Na5Lu9F32 crystal , (b) Gd3+/Tb3+/Yb3+tri-dopedNa5Lu9F32 crystals, and (c) transmission spectra of Gd3+/Tb3+/Yb3+tri-doped Na5Lu9F32 crystals

Fig.3(a) and (b) show the DTA/TG curves in temperature range of 30-1100 ℃ and Raman spectrum of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal powder. It can be noted from the TG curve in Fig. 3(a) that there is 0.24% weight loss in the temperature region of 25–897℃, and then a huge weight loss 2.96%, from 897 to 1002℃. It can also be seen from the DTA curve in Fig. 3(a) that a strong endotherm at about 897℃ appears, and the weight shows a huge loss, which arise from the heat absorbance by melting and vaporization of the crystal. The maximum phonon energy of Na5Lu9F32 is also estimated to be ~ 441cm-1 from the measured Raman spectrum, as shown in Fig.3(b). Such low minimum matrix phonon energy is beneficial for rare earth ions to achieve high luminous efficiency. The excellent properties provide potential application for up-conversion material. 7



100.5 -50

TG

DTA

(a)

(b) 100.0

-30

-10

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-49

DTA(V)

-20

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441cm

TG(%)

TG

-1

-48

Intensity(a.u)

DTA

-40

DTA(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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98.5 -47

897.11C

-46

98.0

840 850 860 870 880 890 900 910 920

200

400

600

800

97.5 1000

Temperature (C)

200 400 600 Wavelength(nm)

800

Fig.3 (a) DTA/TG curves, and (b) Raman spectrum of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 crystal.

In order to investigate whether Gd3+ ion can used as a sensitizer for Tb3+/Yb3+ QC process or not, the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32single crystal samples were measured. Fig.4(a) presents the PLE spectrum of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32single crystal samples by monitoring the emission wavelength at 312 nm of Gd3+ as well as the emission wavelength at 543 nm of Tb3+ ions and 980 nm of Yb3+ ions. It can be noticed that the PLE spectrum of 312 nm emission of Gd3+ ions overlaps well with the PLE spectrum 543 nm of Tb3+and PLE spectrum 980 nm of Yb3+. This result can be regarded as convincing evidence that efficient energy transfer (ET) from Gd3+ to Yb3+ via Tb3+ dominates in the QC process upon excitation of the Gd3+ions. Fig.4(b) shows PL spectra of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystal samples with 0.1 mol% and 0.25 mol% Gd3+ upon excitation into the Gd3+ band at 273 nm. It can be seen that the tri-doped single crystal yields mainly the characteristic 543 nm green emissions of Tb3+ and NIR emissions in the range of 980 -1100nm.

8


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In the present paper, the concentration of Tb3+ and Yb3+ions are fixed at 1 and 8 mol%, respectively, but the Gd3+ concentration changes from 0.1% to 0.25%. It can be found in Fig. 4(b) that a strong band is located at 543 nm corresponding to the 5D4 → 7F5 transition, together with relatively weak emission of Gd3+ centered at 312 nm, which also further demonstrates the occurrence of an efficient ET from Gd3+ to Tb3+. Meanwhile, it should be noted that an intense Yb3+ emission centered at 1023 nm was observed. It can also be seen in the Fig.4(b) and the inset of the Fig.4(b) that as the Gd3+ concentration increases from 0.1mol% to 0.25mol%, the luminescence intensities of Gd3+, Tb3+ and Yb3+ increased obviously. These observations reveal that the excitation energy of Gd3+ finally transfers to Yb3+in the Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32single crystals. 4000

130000

em=312nm

(a) 3500

14000

em=543nm

120000

em=980nm

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Intensity(a.u)

Intensity(a.u)

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2000 1500 1000

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(b)

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30000 20000

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Wavelength(nm)

Fig.4 (a) Comparison of the photoluminescence excitation(PLE) spectra of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystals under by monitoring at 312nm of Gd3+ , 543 nm of Tb3+ and 980nm of Yb3+ ions, (b)Photoluminescence (PL)spectra of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystals under 273nm excitation with 0.1mol% Gd3+ and 0.25mol% Gd3+ concentrations at room temperature.

In order to comprehensively understand the cooperative DC process, we measured the decay curves of transitions 5D4→7F4 of Tb3+ under the excitation of 273 nm for the samples with 0.1 mol% Gd3+ and 0.25 mol% Gd3+, the results are illustrated in Fig. 5. From these luminescence decay curves, the effective experimental 9



lifetime is evaluated using τm=∫[I(t)tdt]/∫[I(t)dt], where I(t) represents the luminescence intensity at the time t after the cutoff of the excitation light. The lifetime of 5D4 level of Tb3+ were changed slightly and estimated to be 6.4 ± 0.1and 6.6 ± 0.1 ms for 0.1 mol% and 0.25 mol% Gd3+ co-doping samples. The inset of Fig.5 shows decay times of emissions centered at 486, 543 and 620 nm which are 6.4 ± 0.1, 6.6± 0.1, 6.5 ± 0.1 ms, respectively. And the emission bands centered at 486, 543 and 620 nm are assigned to the transitions 5D4→7FJ (J = 6, 4, 3), respectively. It is found that the lifetimes of 486nm, 543nm and 620nm emissions remain almost unchanged, which arise from the fact that they are radiated from the same excited 5D4 state.

1.0 Gd3+ concentrations 0.25 0.1

0.6

1.0

486nm 543nm 620nm

0.8

Intensity(a.u)

0.8

Intensity(a.u)

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0.6 0.4 0.2 0.0

0.4

0

10

20

30

Times(ms)

0.2

0.0 10

20

30

40

Times(ms) Fig.5 Decay curves of transitions 5D4→7F4 of Tb3+under the excitation of 273nm with 0.1mol% and 0.25mol% Gd3+ concentrations; the inset shows the decay profile of radiations centered at 486nm, 543nm and 620nm in 0.25mol% Gd3+,1mol%Tb3+ and 8mol%Yb3+ tri-doped Na5Lu9F32single crystal

Having analyzed all of above, a schematic energy-level diagram for the ET process of Gd3+→Tb3+→Yb3+ in Na5Lu9F32: Gd3+-Tb3+-Yb3+ single crystals is shown in Fig. 6. Upon 273 nm excitation, the 6IJ level Gd3+ can be populated from the ground state 2S7/2. It is known that non-radiative energy-transfer from Gd3+ to Tb3+ can 10


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occur either through the electric multipolar or the exchange mechanism.25 Then, Tb3+ are excited up to the 4f75d1 state, from which Tb3+ undergo fast intra-ion thermal relaxation down to the emitting 5D4 level. Three intense emissions peaking at 486, 543, 620 nm are observed clearly, which corresponds to the transitions 5D4→7FJ (J=6, 5, 3) of Tb3+. Meanwhile, due to the 5D4 level of Tb3+ ion is located at approximately twice the energy of the2F7/2 level of Yb3+, the characteristic NIR emission bands located in the 980nm-1100nm range of Yb3+ are mainly generated by the ET from the 5D4→7F4 transition of Tb3+ ion, which further confirms the occurrence of CET from Tb3+ to Yb3+. Although the non-radiative losses remain unknown, the Gd3+ and Tb3+emissions upon excitation of 273 nm compared to the total emission intensity indicates that ET process of Gd3+-Tb3+-Yb3+ is extremely efficient. Compared with recent studies of Tb3+–Yb3+ couple under 486 nm excitation of Tb3+,26 the Yb3+ NIR emission intensity from Gd3+-Tb3+-Yb3+ system would be much more efficiency, because the triply doped system could absorb lights not only at 486nm but also in the range of 200-400nm. In order to further investigate the quantum yields (QY) of studied systems, we can also obtain the actual DC external quantum efficiency by integrating sphere. The actual experimental results of 0.1% Gd3+ sample and 0.25% Gd3+ sample are at ~15.28% and ~45.36% in regard to down-conversion NIR quantum efficiency. Although there exist a few difficulties on achieving higher quantum efficiency, it should be also noted from the experimental results in integrating sphere that the down-conversion quantum efficiency becomes higher as the concentration of Gd3+ ion increase. It indicates that the down-conversion quantum efficiency can be further improved as concentration optimization of the rare-earth ions, which provides the possibility for us to further research.

11



40000 6

35000

IJ

6

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ET 5

PJ

HJ GJ

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5

NR

CET

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D4

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S7/2

7

3+

Gd

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5000 2

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15000

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20000

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D3

25000

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Energy/cm-1

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2

F7/2

F6 3+

Tb

3+

Yb

Fig.6 Schematic energy-level diagram of the ET process of Gd3+→Tb3+→Yb3+ in Na5Lu9F32single crystals.

Based on the experimental results obtained above, the NIR QC of Yb3+ is achieved by the CET from one Tb3+ ion to two Yb3+ ions. Therefore, in accordance with the energy match relationship, the green UC luminescence of Tb3+realized by CET from two Yb3+ to one Tb3+ could occur as well. Fig.7 shows the UC emission spectra of 0.25 mol% Gd3+, 1 mol%Tb3+ and 8 mol% Yb3+ tri-doped Na5Lu9F32 single crystal under excitation of 980 nm as a function of pump power. As depicted in Fig.7, the UC consists of emission bands corresponding to the D4→7FJ (J=6, 5, 4 and 3) transitions of Tb3+. In addition, an intense increase of the emissions of Tb3+with the

5

increasing pump power is observed. For the up-conversion process, the number of photons, which is required to populate the upper emitting level can be calculated from the relation, Iupc∼Pn, where the integer n is the number of photons absorbed for per up-conversion photon emitted, and emission intensity (IUC) is proportional to the nth power of the intensity of IR excitation (IIRn).27 The dependences of up-conversion luminescence intensities for bands located at 486, 543, 586 and 620 nm on pumping powers are also measured and presented in the log-log 12


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plot, as shown in the inset of Fig. 7. A plot of logIUC versus logIIRn yields a straight line with slope n., the number of excitation photons n vary from 1.70 to 2.3 for the emission from the 5D4 state of the samples. These results demonstrate that the 5D4→7FJ (J=6, 5, 4 and 3) transitions are derived from a two-photon absorption process.

Intensity(a.u)

70000 65000 60000 55000 50000 45000 40000 35000 30000 25000 20000 15000 10000 5000 0

486nm 543nm 586nm 650nm

n=2.32

n=1.94 n=1.87

205mW 312mW 373mW 450mW 515mW 630mW 705mW

Log10Intensity(a.u)

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700

Wavelength(nm)

Fig.7 The up-conversion luminescence intensity of 0.25mol% Gd3+,1mol%Tb3+ and 8mol%Yb3+ tri-doped Na5Lu9F32single crystal following excitation with 980nm as a function of pump power; the inset shows power dependence of the up-conversion luminescence intensity of Gd3+/Tb3+/Yb3+ tri-dopedNa5Lu9F32 single crystal under 980nm excitation.

On the basis of the experimental results obtained above, the up-conversion luminescence mechanisms have been proposed and shown in Fig. 8. Under NIR photo-excitation at ∼980 nm, since the Tb3+ ions cannot absorb the 980nm pumping radiation directly, 980 nm photons will be absorbed by Yb3+ at the ground states 2F7/2 and will be populated to the excited states 2F5/2. Through the CET from two excited ytterbium ions (Yb3+–Yb3+ pair) 13



to one Tb3+ ion (i.e., Yb3+:2F5/2 + Yb3+:2F5/2→Tb3+:5D4), the electron population of the Tb3+: 5D4 level was achieved and then relax to lower 7FJ (J=6, 5, 4 and 3) levels, which emit green-red emissions around 486, 542, 586 and 620nm, respectively.

30.0k

5 5

D1 D3

25.0k

ESA/ET

D4

F5/2

F3 F4

7

7

F5 F6

980nm

5.0k

2

980nm

7

CET

584nm

10.0k

486nm

15.0k

542nm

Energy/cm

-1

5

20.0k

620nm

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7

0.0

2

3+

Yb

Tb

3+

F7/2

Fig.8 Energy level diagram of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystals and energy transfer progress

Having analyzed all the results above, pump-power dependent of UC external quantum efficiency (EQE) of Na5Lu9F32: 0.25%Gd, 1%Er3+, 7.9%Yb3+ under the excitation of NIR ~980nm light was measured by an integrating sphere and displayed in Fig, 9. Following the excitation of 980nm LD, it should be noted that the EQE increases with the excitation power density, and the optimum EQE was about 1.21% under 5.5 Wcm-2 980nm light excitation. And once the power density overpasses 5.5 Wcm-2, the EQE of UCL hardly changed. In order to verify this point further, we calculated the theoretical EQE based on steady-state rate equations. On account of the simplified model shown in Fig.8 above, the UC external quantum efficiency (ɳEQE) can be estimated by following equations:

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c 0 N 0 NYb1  c1 N 1 NYb2  W 1 N 1  0 c1 N 1 NYb2  W 2 N 2  0 N 0  N 1  N 2  Ner NYb1  NYb2  NYb NYb2  PNYb1

P  N2 

 

(1)

PIP p  P hcp  2 hc

(2)

coc1NcNYb  2 P  2  2 W 1W 2  c0c1NYb  2  (c0  c1) NYb  2  2  (c0  c1) NYbW 2  2W 1W 2  W 1W 2

W 2N 2 3.85P  % PNYb0 1  0.51P

(3) (4)

Where c0 and c1 are the ET coefficients for the UC processes between the donor and the acceptor in 4I15/2 and 4

I9/2 states, respectively. N2, N1, and N0 are the population 2H11/2/4S3/2, 4I9/2, and 4I15/2 densities of the levels of the

Er3+ ions. And the NYb1 and NYb2 represent the 2F7/2 and 2F5/2 levels of Yb3+ ions, respectively. NYb and Ner are the concentrations of the Yb3+ ion and the Er3+ ions. W1 denotes the non-radiative relaxation rate from level 4I11/2 to lower state of Er3+, W2 is the radiative decay rate from level 2H11/2/4S3/2 to ground state of Er3+, and σ is the Yb3+ absorption cross section at the pumping wavelength. The symbol ρP denotes the excitation power variable, given by equation 2. Here, IP is the incident pump power, λP and ωP are the pump wavelength and beam radius, respectively, h is Planck’s constant, and c is the speed of light. P is the incident pump power density. From the above equations, we can obtain the population of 2H11/2/4S3/2 level using equation 3. Thus, the UC external quantum efficiency (EQE) can be deduced by equation 4. The calculated results were also plotted a line in Fig.9 (a). It can be noted that the theoretical measurement in accordance with the experimental dates. Generally, there exists the saturation effect and the thermal effect in the nano-crystals28, which lead to reduce the quantum efficiency of materials obviously. It can be confirmed that the rare earth ion doped Na5Lu9F32 single crystal has 15



advantage of achieving high quantum efficiency and high thermal stability compared with its nano-crystal. It is reported in Tb3+-Yb3+ fluoride glass29 that its absolute up-conversion efficiency is on the order of 10−4 in the pump-power density range of 40–96 W∕cm2. We can find that the studied single crystal system can achieve much higher up-conversion efficiency at the lower power density, which indicates the Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystals own a good UCL effect and worth of further study. In addition, we can see from the measured absolute UC and DC efficiency that the DC efficiency is as high as 45%, while the UC efficiency is measured to be 1.21% . This result gives us further explanations that introduction of Gd3+ ion is helpful to take advantage of the UV region of sunlight to generate NIR lights in the Tb3+/Yb3+ system.

2.0

Theoretical dates

1.5

Experimental dates

EQE(%)

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1.0

0.5

0.0

0

2

4

6

8

Wavelength(nm) Fig.9 Power-dependent UC internal quantum efficiency at 980 nm light excitation in 0.25mol% Gd3+,1mol%Tb3+ and 8mol%Yb3+ tri- doped Na5Lu9F32 single crystals

In order to know the exact UC and DC emission color, the CIE chromaticity coordinates of Tb3+/Yb3+ co-doped Na5Lu9F32 single crystal (sample 1) was calculated from emission spectrum under the 486nm 16


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excitation, as well as the Gd3+/Tb3+/Yb3+ tri-dopedNa5Lu9F32 single crystal (sample 2 and 3) under the excitation of light in infrared region at 980 nm and UV region at 273 nm, labeled with a few black dot in the CIE 1931 chromaticity diagram in Fig.10. The CIE coordinates (x, y) is (0.3371, 0.6035) for sample 3 which appear bright yellowish-green color in the UC process. Accordingly, the CIE coordinates (x, y) are (0.2762, 0.6189), (0.2625, 0.6654), (0.262, 0.7071) for sample 1, 2 and 3 in DC process appear bright green.

Fig.10 CIE chromaticity coordinates of Gd3+/Tb3+/Yb3+ tri-doped Na5Lu9F32 single crystals.

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CONCLUSION In summary, we have grown the Na5Lu9F32 single crystals tri-doped Gd3+/Tb3+/Yb3+ with various Gd3+ concentrations by the improved Bridgman method for the first time. The UC and DC luminescence properties of Gd3+/Tb3+/Yb3+tri-doped Na5Lu9F32single crystals have been investigated. Under the excitation of UV light, a large enhancement in NIR QC luminescence around 980 nm has been obtained by co-doping Gd3+ as a sensitizer through mechanisms of ET and subsequent CET. Among the DC process, ET from Gd3+→Tb3+→Yb3+ plays the key role. Under the excitation of 980 nm, the two-photon UC emissions at 486, 542, 584 and 620nm have been observed. The green emission at ~543nm due to 5D4→7F5 was the most intense in all the UC emissions. A bright green color could be generated by a simultaneous mixture of all the UC emissions. The actual quantum yields (QY) of studied systems has also been measured by integrating sphere at ~1.2% and ~45% corresponding to UC and DC process, respectively. The above results proposed successful doping of Gd3+ions in Na5Lu9F32 single crystals co-doped Tb3+/Yb3+ with both UC and DC properties that can be used as potential spectrum converter for solar cells. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51472125), the Natural Science Foundation of Zhejiang Province (Grant No. LZ17E020001), and K.C. Wong Magna Fund in Ningbo University, Fundamental Research Funds for the Central Universities (Grant No. 3132016333).

AUTHOR INFORMATION Corresponding Author E-mail: [email protected](H. Xia), [email protected] (Y. Zhu) Notes The authors declare no competing financial interest.

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For Table of Contents Use Only Cooperative Energy Transfer Up/Down-conversion Luminescence in Tb3+ /Yb3+ Co-doped Cubic Na5Lu9F32 Single Crystals by Gd3+Co-doping Shinan He 1,Haiping Xia 1 Qingyang Tang 1, Qiguo Sheng 1, Jianli Zhang 1, Yongsheng Zhu 2 and Baojiu Chen 3

1. Key laboratory of Photoelectronic Materials, Ningbo University, Ningbo, Zhejiang, 315211, China 2. College of Physics and Electronic Engineering, College of Chemistry and Charmaceutical Engineering, Nanyang Normal University, Nanyang, 473061, China 3.Department of Physics, Dalian Maritime University, Dalian, Liaoning Province, 116026,

Gd3+ is tried to introduce into the pair of Tb3+/Yb3+ to absorb the UV part of the solar spectrum and efficiently transfer the excitation energy to the 5D4 state of Tb3+ for the Tb3+–Yb3+ NIR QC process. This result is helpful to take advantage of the UV region of sunlight to generate NIR lights for silicon solar cells application.

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Down-Conversion Luminescence in

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