Ln3+ (Er3+, Tm3+ and Ho3+)-doped NaYb(MoO4)2 upconversion phosphors as wide range temperature sensors with high sensitivity

Accepted Manuscript 3+ 3+ 3+ 3+ Ln (Er , Tm and Ho )-doped NaYb(MoO4)2 upconversion phosphors as wide range temperature sensors with high sensitivity Anqi Zhang, Zhen Sun, Guofeng Liu, Zuoling Fu, Zhendong Hao, Jiahua Zhang, Yanling Wei PII:

S0925-8388(17)33038-4

DOI:

10.1016/j.jallcom.2017.09.010

Reference:

JALCOM 43066

To appear in:

Journal of Alloys and Compounds

Received Date: 29 July 2017 Revised Date:

29 August 2017

Accepted Date: 1 September 2017

3+ 3+ 3+ Please cite this article as: A. Zhang, Z. Sun, G. Liu, Z. Fu, Z. Hao, J. Zhang, Y. Wei, Ln (Er , Tm 3+ and Ho )-doped NaYb(MoO4)2 upconversion phosphors as wide range temperature sensors with high sensitivity, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

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Ln3+ (Er3+, Tm3+ and Ho3+)-doped NaYb(MoO4)2 Upconversion Phosphors as Wide Range Temperature Sensors with High Sensitivity Anqi Zhanga, Zhen Suna, Guofeng Liua, Zuoling Fua,*, Zhendong Haob, Jiahua

a

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Zhangb,*, Yanling Weic Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of physics

and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012,

State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine

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b

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China, Fax: +86-431-85167966; Tel:+86-431-85167966; E-mail: [email protected] (Z. L. Fu)

Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun, 130033, China; E-mail: [email protected] (J.H. Zhang) c

School of Media Matheatics & Physics, Jilin Engineering Normal University, Changchun 130012,

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China

Abstract

Well-crystallized NaYb(MoO4)2 doped with lanthanide ions (Ln3+: Er3+, Tm3+ and

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Ho3+) are successfully synthesized by hydrothermal method with further calcination.

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And the samples with different morphologies are obtained by changing the pH values and the molar ratio of Ln(NO3)3 /Na2MoO4 of the resultant solutions. The upconversion (UC) luminescence spectra of NaYb(MoO4)2: Ln3+ between 323 K and 573 K under 980 nm excitation exhibit temperature-dependent property. The fluorescence intensity ratio (FIR) techniques with thermally coupled states (TCS) (Er3+: 2H11/2, 4S3/2

4

I15/2, Tm3+: 1G4(1),(2)

3

H6, Ho3+: 5F5(1),(2)

5

I8) have high

sensitivity (S) values (Er3+: 0.0122 K-1 at 548 K, Tm3+: 0.0025 K-1 at 323 K and Ho3+:

ACCEPTED MANUSCRIPT 0.00035 K-1 at 323 K). The non-TCS (Ho3+: 5F5, 5S2 /5F4

5

I8) is used to extend the

range of S, and the slope of FIR (non-TCS)-T is the S value as high as 0.02455 K-1 in the wide temperature range from 323 K to 573 K.

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Keywords:Hydrothermal method; Fluorescence intensity ratio; Temperature sensor 1. Introduction

In recent years, the fluorescence intensity ratio (FIR) technique from two

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emissions of lanthanide ions (Ln3+) have been widely investigated [1-4]. Summarizing

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the previous reports, three FIR techniques are classified by the degree of population transform of the two luminescent centers in dual emission temperature sensors: the first is two independent luminophores [5], the second is that the energy transfer (ET) occurs between two luminescent excited states [6], the last one is that the two excited

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statescan be considered as two thermally coupled states (TCS) in thermal equilibrium [7]. In this work, the latter two FIR techniques are used to measure temperature. To improve emission intensity of Ln3+ ions, Yb3+ ions are usually used as a sensitizer in

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upconversion (UC) system because of its simple two-level structure, which ensure a

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broad and strong absorption band about 980 nm [8,9]. Besides sensitizers, fluorescence properties of Ln3+ are also determined by host materials. Double molybdates ALn(MoO4)2 (A = alkali metal ions; Ln = trivalent rare earth ions) with scheelite-like (CaWO4) structure have outstanding optical, favorable physical, chemical stability, therefore, dual molybdates are good hosts for Ln3+-doped inorganic materials [10-13]. And the possible of radiative transitions would be increased because ALn(MoO4)2 have low phonon energy, and lead to a high quantum yield of

ACCEPTED MANUSCRIPT UC process. Therefore, NaYb(MoO4)2 as a host material containing Yb3+ ions has higher doping ions concentration, so the emission intensities of doping ions are stronger. Moreover, little studies has been done on NaYb(MoO4)2 phosphor for the

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temperature sensor in UC phosphors [14]. In this work, NaYb(MoO4)2 doped with Er3+, Tm3+ and Ho3+ were synthesized to research their temperature dependent UC luminescence. Under 980 nm excitation, the

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thermal properties of UC emissions of Ln3+ between 323 K and 573 K were

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investigated. The intensity of visible emission excited by 980 nm at low excitation power is still very strong. And by comparison, NaYb(MoO4)2 doped with different Ln3+ ions have higher sensitivity, especially the sensitivity originated from the slope of the ratio (654 nm /544 nm)-temperature of Ho3+ ion is very high in the vivo

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temperature range. Therefore, the NaYb(MoO4)2: Ln3+ phosphors with high sensitivity can be used as optical temperature sensors. 2. Experimental section

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2.1. Chemicals

Sodium molybdate (Na2MoO4, 99%), nitric acid (HNO3, analytical reagent),

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sodium hydroxide (NaOH, 82%), analytical grade ytterbium oxide (Yb2O3, 99.99%), erbium oxide (Er2O3, 99.99%), thulium oxide (Tm2O3, 99.99%) and holmium oxide (Ho2O3, 99.99%) were used as original materials. All starting materials were used without further purification. Yb(NO3)3, Er(NO3)3, Tm(NO3)3 and Ho(NO3)3 were prepared by dissolving Yb2O3, Er2O3, Tm2O3 and Ho2O3 in concentrated HNO3 solution at an elevated temperature for a long time followed by evaporation of excess HNO3.

ACCEPTED MANUSCRIPT 2.2 Synthesis of NaYb(MoO4)2: Ln3+ Ln3+ doped NaYb(MoO4)2 phosphors were synthesized by hydrothermal method with further calcination. Ln(NO3)3 aqueous solution and 10 mL of deionized water

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admixed with continuous stirring at room temperature for 10 minutes. Next, 10 mL aqueous solution containing Na2MoO4 was added dropwise into the Ln(NO3)3 aqueous solution. The pH values of the mixture solutions were adjusted to 4, 5, 6 and

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7 by nitric acid or NaOH (5 M) to form a white colloidal solution. After magnetic stir

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for 60 min, the above solution was transferred to a Teflon bottle in a stainless steel autoclave (filled to 80% of its total volume), sealed and temperature kept in 180 for 24 hours. The products were separated using centrifugation after the autoclave was cooled to room temperature. They were washed with deionized water and ethanol and for 10 h in air. Then, the phosphors were heated to 600

, and

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dried at 60

maintained the temperature for 6 hours, finally naturally cooled to room temperature. 2.3 Characterization

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X-ray diffraction (XRD) on a Rigaku-Dmaxan (Tokyo, Japan) with Cu Kα

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radiation (λ= 0.15405 nm) in the 2θ range from 10°to 80°was used to identify the crystal phase. The morphologies of the samples were examined by a scanning electron microscope (SEM). Andor Shamrock SR-750 fluorescence spectrometer was used to measure the UC luminescence spectra. A CCD detector united with a monochromator was used as a collective signal device. The pump source was a diode laser (980 nm). Andor SR-500i spectrometer was used for the luminescence spectra of the samples under the 980 nm. The Ln3+ doped NaYb(MoO4)2 samples were placed in an iron

ACCEPTED MANUSCRIPT sample cell with the temperature range from 323 K to 573 K heated by resistive wire elements. 3. Results and discussion

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3.1. Crystal structure and morphologies We synthesized NaYb(MoO4)2 micro-crystals with diverse morphologies by changing the value of the pH and molar ratio of Yb(NO3)3 /Na2MoO4 of the mixed

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solutions. Fig. 1 shows the X-ray diffraction patterns of the phosphor powders

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prepared with Yb(NO3)3 /Na2MoO4 = 1 /3 aqueous solution as raw materials, pure tetragonal phase NaYb(MoO4)2 (JCPDS card No. 04-005-9926) micro-crystals are synthesized. The SEM images of the phosphor powders are shown in the inset of Fig. 1. When the starting pH = 4, irregular square nano-plates are formed. When pH = 5,

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the square nano-plates with smooth surfaces are obtained, and the square plates have side length of ~ 500 nm. As the pH values are increased to 6 and 7, the products become irregular micro-flakes, and their surfaces are rougher with plenty of tiny

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irregular nano-particles bonding to them.

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Fig. 2 shows the X-ray diffraction patterns of the samples when the Yb(NO3)3 /Na2MoO4 value of solution is 1 /5. The results indicate that pure tetragonal phase NaYb(MoO4)2 (JCPDS card No. 04-005-9926) micro-crystals are synthesized. The SEM images of the phosphor powders are shown in the inset of Fig. 2. When the starting pH value is 4, the NaYb(MoO4)2 exhibits stick-like morphology. The diameter and length of the stick are about 0.3μm and 2.5μm, respectively. With the increase of pH values ranging from 5 to 7, the morphologies of the products are all irregular

ACCEPTED MANUSCRIPT micro-flakes with plenty of tiny irregular nano-particles bonding to them. In all of the above samples, both Fig. 1(b) and Fig. 2(a) show outstanding luminescence properties which depend on good crystallinity. In consideration of the

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application to the biological imaging, the samples with the smaller size have more advantages. So we choose the sample of Fig. 1(b) as the basis for our next study. 3.2 Luminescence properties of NaYb(MoO4)2: Ln3+

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Fig. 3(a) presents the UC fluorescent emission spectra of NaYb(MoO4)2: x%Er3+

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with fixed different Er3+ (x=1, 3, 5, 7) concentrations under 980 nm excitation. We can see two intense green emissions with luminescence peaks centered at 528 and 550 nm, and a weaker red emission peak at 663 nm, which are both characteristic intrinsic 4f-4f transitions of Er3+. The two green emissions correspond to the transitions 2H11/2

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to 4I15/2 and 4S3/2 to 4I15/2 of Er3+, respectively. The red UC emission attributes to the transition 4F9/2 to 4I15/2 of Er3+. Owing to the energy level characteristic of MoO42group, the red emission is very weak [7]. Fig. 3(b) shows the relative emission

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intensities of Er3+ with different doping contents. The emission intensities firstly

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increase and then decrease with the increasing in doping concentration of Er3+, in which a maximum value is obtained at x = 5. At high concentration of Er3+, the emission intensities decrease because of the concentration quenching effect and the cross relaxation between Er3+ ions. The distances of Er3+ ions will decrease when the Er3+ concentration increase, which result in the enhancement of nonradiative relaxation and further lead the intensity of UC luminescence to weaken [15]. To determine the number of pump photons responsible for the UC mechanisms of

ACCEPTED MANUSCRIPT NaYb(MoO4)2: 5%Er3+ phosphors, the double logarithmic plots of three emissions under 980 nm excitation versus pump powers for the sample are shown in Fig. 3(c). The UC emission intensity of the phosphors (IUC) and the excitation power (PPUMP) (PPUMP)n, the number of pump photons populated the UC

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obey the relation IUC

emission state is n [16,17]. The slopes of the three fitted lines corresponding to 528, 550 and 663 nm emissions are 1.7, 1.4 and 1.6, respectively. Therefore, it suggests

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that both green and red UC luminescence belong to two-photon process [18-22].

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Fig. 4(a) and Fig. 5(a) present the UC luminescence spectra of NaYb(MoO4)2: x%Tm3+ (x=0.3, 0.5, 0.7, 1) and NaYb(MoO4)2: x%Ho3+ (x=0.5, 1, 1.5, 2) under 980 nm excitation. We can observe two UC emission bands in Fig. 4(a), and assign to 1G4 3

H6 (477 nm), 1G4

3

F4 (650 nm) transitions of Tm3+ ion (Fig. 6). The two UC

/5F4

5

I8 and 5F5

5

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emission peaks at 544 and 654 nm in Fig. 5(a) are ascribed to the transitions of 5S2 I8 of Ho3+ (Fig. 6). Fig. 4(b) and Fig. 5(b) show the relative

emission intensities of Tm3+ and Ho3+ with different doping contents, and the samples

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doped with 0.7%Tm3+ and 1%Ho3+ indicate the strongest emission. Fig. 4(c) and Fig.

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5(c) depict the double logarithmic plots of IUC versus PPUMP for NaYb(MoO4)2: 0.7%Tm3+ and NaYb(MoO4)2: 1%Ho3+ phosphors. The slope values of the fitting curves for 477, 650 nm of Tm3+ and 544, 654 nm of Ho3+ emission bands are 1.6 and 1.8 (Fig. 4(c)) and 1.6 and 1.5 (Fig. 5(c)), which suggests that both UC emissions belong to two-photon process. The proposed UC mechanism based on the energy level scheme of NaYb(MoO4)2 doped with Er3+, Tm3+ and Ho3+ is shown in Fig. 6. Here the ground

ACCEPTED MANUSCRIPT state of Yb3+-MoO42– dimer is denoted by |2F7/2, 1A1>, the excited state by |2F5/2, 1A1>, and the relevant higher excited states as depicted by |2F7/2, 3T1>, |2F7/2, 3T2>, |2F7/2, 1T1> and |2F7/2, 1T2>, respectively. The Yb3+-MoO42– dimer entails both ground state

1

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absorption (GSA) (|2F7/2, 1A1> |2F5/2, 1A1>) and excited state absorption (ESA) (|2F5/2, A1> |2F7/2, 1T1>) [7]. The effective high excited state energy transfer (HESET) from

the |2F7/2, 3T2> state of the Yb3+-MoO42− dimer to the 4F7/2 level of the Er3+ ion occurs.

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Subsequently, the three lower emitting levels (2H11/2, 4S3/2 and 4F9/2 levels) are

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populated by the nonradiative relaxations (NR), followed by the weak red and strong green emissions. The red emission generates from 4S3/2 to 4F9/2 levels via NR and next to transition of 4F9/2

4

I15/2, and the red emission is weak because of the large energy

gap between 4S3/2 and 4F9/2 levels. Besides, when the doping concentration is high

4

4

I9/2 + 4I13/2 and 4F9/2 + 4F9/2

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enough, the cross relaxation (CR) from 2H11/2 + 4I15/2

I11/2 + 4F7/2 occur. Similarly, HESET from the |2F7/2, 1T2> excited states to the 1G4

level of Tm3+ and 5F4 /5S2 level of Ho3+ are effective, and the small energy mismatch 1

T1> and |2F7/2,

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between the |2F7/2,

1

T2> states can be compensated with

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phonon-assisted absorption [7]. The two emissions centered at 477 and 649 nm of Tm3+ can be assigned to the 1G4 5

3

H6 and 1G4

3

F4 transitions, respectively [23]. The

F5 level of Ho3+ is populated by NR, followed by the two emissions centered at 544

and 654 nm.

3.3 Thermometric properties of NaYb(MoO4)2: Ln3+ Fig. 7(a) shows the temperature sensor principles diagram of the two excited states, which can be considered as the two thermally coupled states (TCS) in thermal

ACCEPTED MANUSCRIPT equilibrium. Because the energy gap (∆E) between the states of E3 and E2 is less than 2000 cm-1, the state of E3 can be populated from E2 by thermal excitation, which would cause the change of the intensities between the two TCS to make sure a

The FIR of E3

E1 transition can be expressed as [7,24]:

I 31 N 3ω31g31hν 3 N 2ω31g31hν 3 exp(− ∆E / k BT ) ∆E = = = C exp(− ) I 21 N 2ω21g21hν 2 N 2ω21g21hν 2 k BT

(1)

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FIR =

E1 transition and E2

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quasi-thermal equilibrium following a Boltzmann-type population distribution [21,24].

where ω is the spontaneous emission rates of the excited states to the ground state, g

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is the degeneracies of the level, kB is the Boltzmann constant and T is the absolute temperature. The thermometric sensitivity (S) is defined as follows [7,24]:

S=

d ( FIR) − ∆E − ∆E − ∆E = FIR( ) = B( ) exp( ) 2 2 dT k BT kBT k BT

(2)

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Fig. 8 shows the UC emission spectra of NaYb(MoO4)2 doped different Ln3+ from 323 K to 573 K. With temperature increasing, the emission intensities of the two TCS have changed. The FIR between two TCS can be expressed by the Eq. (1), and

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the S can be defined as Eq. (2).

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Fig. 9 shows the FIR with temperature in the range of 323-573 K of NaYb(MoO4)2: 5%Er3+ (Fig. 9(a)), 0.7%Tm3+ (Fig. 9(b)), and 1%Ho3+ (Fig. 9(c)). It can be observed that the FIR nonlinear changes with the temperature rising. The experimental data are fitted to nonlinear lines when the values of ∆E /kB are 1060.44 (Er3+), 373.97 (Tm3+) and 58.479 (Ho3+). The values of ∆E on the basis of the fitting lines are 737 cm-1 (Er3+), 260 cm-1 (Tm3+) and 41 cm-1 (Ho3+), respectively. The coefficient C values are 23.89 (Er3+), 2.28 (Tm3+) and 0.762 (Ho3+). The Ln(FIR)

ACCEPTED MANUSCRIPT versus 1 /T is shown in Fig. S1. The curve of S versus temperature can be obtained from the Fig. 10. It can be seen that the maximum S values are 0.0122 K-1 at 548 K (Er3+), 0.0025 K-1 at 323 K (Tm3+) and 0.00035 K-1 at 323 K (Ho3+), and we get the S

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of Er3+ to be the largest. In addition, all the parameters of previously reported Er3+ doped sensing materials containing molybdenum are shown in Table 1. Compared

NaYb(MoO4)2: Er3+ is not low [7,21,25-34].

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with the other samples, all of molybdate matrices have high values of S and the S of

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Unlike Fig. 7(a), the excited states of E2 and E3 /E4 in Fig. 7(b) are not in thermal equilibrium, and energy transfer (ET) occurs from E3 /E4

E2 (ΦET is the quantum

efficiency of the energy transfer process). At this point, the quantum yield (Φ) arising from temperature-dependent nonradiative decay kinetics (knr(T)) can be used to

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represent the FIR (non-TCS). Fig. 7(b) is a simple four levels system to explain another thermometric method of non-TCS. The FIR (non-TCS) of E2, E3 /E4

E1 is

described by Eq. (3) [35]:

I 21 Φ (T )Φ ET (T ) k r 2k ET (T ) /(k r 2 + knr 2 (T )) (kr 3 + kr 4 )k r 2k ET (T ) ∝ 2 = = I 31 + I 41 Φ 3 (T ) + Φ 4 (T ) kr 3 + kr 4 kr 2 + knr 2 (T )

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FIR =

(3)

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where kr2, kr3 and kr4 are respectively the radiative decay rate constants of E2, E3, E4, knr2(T) is the nonradiative decay rate of E2 and kET(T) is the rate for E3 /E4 E2 energy transfer.

The UC emission spectra of NaYb(MoO4)2: 1%Ho3+ are measured at various temperatures of 323-573 K (Fig. S2) and several representative temperatures (Fig. 11(a)) under 980 nm excitation, and histograms of the relative intensities change of the blue and red emissions with temperature are shown in Fig. 11(b), we can observe

ACCEPTED MANUSCRIPT that the intensity of the green UC emission centered at 544 nm (5F4 /5S2 UC emission centered at 654 nm (5F5

5

5

I8) and red

I8) have signally changed with the increase of

temperature. From the four levels system comprised of 5I8 (E1), 5F5 (E2), 5S2 (E3) and F5 (E4) in Fig. 7(b), the ratio of the integrated intensities of the two emissions

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5

centered at 544 and 654 nm follows Eq. (3). According to the definition of S: S = d(FIR) /dT, the slope of the fitted line of FIR (654 nm /544 nm)-T in Fig. 11(c) is S

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(0.02455 K-1), which far outweighs previous S obtained by using the TCS. More

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importantly, the maximal value of S has wide temperature range including vivo temperature. So, the application of FIR (non-TCS) temperature measuring method of Ho3+ can maintains the highest sensitivity. Furthermore, by comparing the S of several samples in this work (Table 2), we can know that the S of the TCS of Ln3+ increases

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with the increase of ∆E, and the S derived from the FIR (non-TCS) of Ho3+ is higher than that S derived from the TCS of Ln3+. The temperatures of Smax (Er3+, Tm3+, Ho3+) are 548 K, 323 K and 323 K, respectively, but the temperature of Smax

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(non-TCS) has wide range from 323 to 573 K.

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4. Conclusions

In conclusion, Er3+, Tm3+ and Ho3+ doped NaYb(MoO4)2 phosphors were

synthesized by hydrothermal method with further calcination. The structures and morphologies were characterized by means of XRD and SEM. In the UC emission spectra of NaYb(MoO4)2: Er3+, Tm3+, Ho3+ under 980 nm excitation, strong emissions of different Ln3+ can be observed because of the HESET from Yb3+-MoO42- dimer to Ln3+ ions. The temperature dependent FIR of two UC emissions of Ln3+ from 323 K

ACCEPTED MANUSCRIPT to 573 K has been studied. And the FIR and S of TCS (Er3+: 2H11/2, 4S3/2 1

G4(1),(2)

3

H6, Ho3+: 5F5(1),(2)

5

4

I15/2, Tm3+:

I8) are to be calculated, we can verify that S is

positively related to energy gap. Through the calculation of FIR of non-TCS (Ho3+: F5, 5S2 /5F4

5

I8), the value of S is the slope value of the FIR (non-TCS)-T as high as

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5

0.02455 K-1. By comparison with the S derived from the TCS of Ln3+, the value of S derived from the FIR (non-TCS) of Ho3+ has marked improvement and extended the

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temperature range.

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Acknowledgments

This work was supported by the Science and Technology Development Planning Project of Jilin Province (20160101294JC), partially sponsored by the Education Department of Jilin Province (no.2016112) and by the National Science Foundation of

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China (no.21521092), and supported by State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese

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luminescence from poly-crystalline Yb , Er Co-doped CaMoO4, J. Alloy Compd, 522 (2012) 30-34. [16] X. Wu, J.P. Denis, G. Özen, P. Goldner, F. Pellé, The UV, Blue and Green Up-Conversion Luminescence of PbF2 + GeO2 : Er2O3 Pumped with 650 nm, Appl. Phys. B, 56 (1993) 269-273. [17] Y.L. Wei, C.H. Su, H.B. Zhang, J.S. Shao, Z.L. Fu, Color-tunable up-conversion emission from 3+

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4642-4647.

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Yb /Er /Tm /Ho codoped KY(MoO4)2 microcrystals based on energy transfer, Ceram. Int, 42 (2016) 3+

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[18] T. Li, C.F. Guo, L. Li, Up-conversion luminescence of Er -Yb co-doped CaIn2O4, Opt. Express, 21

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(2013) 18281-18289.

[19] M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, M. P. Hehlen, Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B: Condens. Matter, 61 (2000) 3337-3346. [20] X.X. Yang, Z.L. Fu, G.F. Liu, C.P. Zhang, Y.L. Wei, Z.J. Wu, T.Q. Sheng, Controlled morphology and 3+

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EDTA-induced pure green upconversion luminescence of Er /Ho -Yb co-doped NaCe(MoO4)2 phosphor, RSC Adv, 5 (2015) 70220-70228. [21] X.X. Yang, Z.L. Fu, Y.M. Yang, C.P. Zhang, Z.J. Wu, T.Q. Sheng, Optical Temperature Sensing 3+

Behavior of High-Efficiency Upconversion: Er -Yb

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Co-Doped NaY(MoO4)2 Phosphor, J. Am. Ceram.

Soc, 98 (2015) 2595-2600. 3+

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[22] J. Lee, J.H. Ryu, Synthesis of Scheelite Structural Yb , Er Co-doped NaLa(MoO4)2 and Its Green Upconversion Luminescence, J KOOREAN PHYS SOC, 65 (2014) 1644-1648. [23] D.C. Yu, R. Martín-Rodríguez, Q.Y. Zhang, A. Meijerink, F.T. Rabouw, Multi-photon quantum cutting

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in Gd2O2S: Tm to enhance the photo-response of solar cells, Light: Sci. Appl, 4 (2015) e344. 3+

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[24] P. Haro-González, S.F. León-Luis, S. González-Pérez, I.R. Martín, Analysis of Er and Ho codoped fluoroindate glasses as wide range temperature sensor, Mater. Res. Bull, 46 (2011) 1051-1054. [25] B.S. Cao, Y.Y. He, Z.Q. Feng, Y.S. Li, B. Dong, Optical temperature sensing behavior of enhanced green upconversion emissions from Er–Mo:Yb2Ti2O7 nanophosphor, Sensor. Actuat. B-Chem, 159 (2011) 8-11. [26] B.S. Cao, J.L. Wu, X.H. Wang, Y.Y. He, Z.Q. Feng, B. Dong, L. Rino, Multiple temperature effects on 3+

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codoped TiO2 and high thermal sensitivity, AIP Adv, 5

(2015) 087136.

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up-conversion fluorescences of Er -Yb -Mo

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[28] P. Du, L.H. Luo, J.S. Yu, Infrared-to-visible upconversion emission of Er /Yb -codoped SrMoO4 phosphors as wide-range temperature sensor, Curr. Appl. Phys, 15 (2015) 1576-1579. 3+

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[29] P. Du, L.H. Luo, H.K. Park, J.S. Yu, Citric-assisted sol-gel based Er /Yb -codoped Na0.5Gd0.5MoO4: A novel highly-efficient infrared-to-visible upconversion material for optical temperature sensors and optical heaters, Chem. Eng. J, 306 (2016) 840-848.

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[30] W.B. Bi, Q.Y. Meng, W.J. Sun, S.C. Lu, Optical temperature sensing properties of Er , Yb

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co-doped NaGd(MoO4)2 phosphor, Ceram. Int, 43 (2017) 1460-1465.

[31] J.L. Wu, B.S. Cao, F. Lin, B.J. Chen, J.S. Sun, B. Dong, A new molybdate host material: Synthesis, upconversion, temperature quenching and sensing properties, Ceram. Int, 42 (2016) 18666-18673. [32] D. He, C.F. Guo, S.S. Zhou, L.L. Zhang, Z.Y. Yang, C.K. Duan, M. Yin, Synthesis and thermometric 3+

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properties of shuttle-like Er /Yb co-doped NaLa(MoO4)2 microstructures, CrystEngComm, 17 (2015) 7745-7753.

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[33] A. K. Soni, V. K. Rai, S. Kumar, Cooling in Er : BaMoO4 phosphor on codoping with Yb

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elevated temperature sensing, Sensor. Actuat. B-Chem, 229 (2016) 476-482. 3+

[34] T. Li, C.F. Guo, S.S. Zhou, C.K. Duan, M. Yin, Highly Sensitive Optical Thermometry of Yb -Er

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Codoped AgLa(MoO4)2 Green Upconversion Phosphor, J. Am. Ceram. Soc, 98 (2015) 2812-2816. [35] E.J. McLaurin, L.R. Bradshaw, D.R. Gamelin, Dual-Emitting Nanoscale Temperature Sensors, Chem.

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ACCEPTED MANUSCRIPT Table Captions Table1.Summarized maximum sensitivity values of previously reported of Er3+ doped materials which containing molybdenum.

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Table 2.The fitting values of sensitivity and related parameters.

Figure Captions

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Fig. 1.XRD powder patterns of NaYb(MoO4)2 samples prepared with Yb(NO3)3 /Na2MoO4 = 1 /3 aqueous solution as raw materials acquired at the value of pH from 4

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to 7. The bottom lines are the XRD standard pattern of tetragonal NaYb(MoO4)2 (JCPDS card No. 04-005-9926). Insets show the SEM images of the corresponding samples.

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Fig. 2.XRD powder patterns of NaYb(MoO4)2 samples prepared with Yb(NO3)3 /Na2MoO4 = 1 /5 aqueous solution as raw materials acquired at the values of pH from 4 to 7. The bottom lines are the XRD standard pattern of tetragonal NaYb(MoO4)2

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samples.

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(JCPDS card No. 04-005-9926). Insets show the SEM images of the corresponding

Fig. 3.The upconversion fluorescent emission spectra of NaYb(MoO4)2: x%Er3+ (x=1, 3, 5, 7) under 980 nm excitation (a), the relative emission intensities of Er3+ with different doping contents (b), the double logarithmic plots of green (2H11/2 /4S3/2 to 4

I15/2) and red (4F9/2 to 4I15/2) emission versus pump powers for NaYb(MoO4)2: 5%Er3+

(c). Fig.4.The upconversion fluorescent emission spectra of NaYb(MoO4)2: x%Tm3+

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Fig. 5.The UC luminescence spectra of NaYb(MoO4)2: x%Ho3+ (x=0.5, 1, 1.5, 2) under 980 nm excitation (a), the relative emission intensities of Ho3+ with different doping contents (b), the double logarithmic plots of UC emission intensities versus

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pump powers for NaYb(MoO4)2: 1%Ho3+ phosphors (c).

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Fig.6.Energy level diagrams and possible upconversion processes of NaYb(MoO4)2 doped with Er3+, Tm3+ and Ho3+ by 980 nm laser excitation.

Fig. 7.Simplified diagrams to explain the two kinds of temperature sensor principles. Fig.8.The temperature-dependent UC emission spectra of NaYb(MoO4)2: 5%Er3+ (a),

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0.7%Tm3+ (b), and 1%Ho3+ (c) that the two excited states can considered as the two thermally coupled states (TCS).

1%Ho3+ (c).

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Fig. 9.The FIR of TCS versus T of NaYb(MoO4)2: 5%Er3+ (a), 0.7%Tm3+ (b), and

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Fig.10.Thetemperature-dependent S of NaYb(MoO4)2: 5%Er3+ (a), 0.7%Tm3+ (b), and 1%Ho3+ (c).

Fig. 11.The temperature-dependent UC emission spectra of NaYb(MoO4)2: 1%Ho3+ (a), histogram of the blue and red emissions relative intensities changes with the temperature from 323 K to 573 K (b); The FIR of the blue and red emissions versus T and the slope is S (c).

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Table 1.Summarized maximum sensitivity values of previously reported of Er3+ doped materials

ions

λex

λem

(nm)

(nm)

Transitions

Temperature range(K)

Er3+,Yb3+

Er-Mo:Yb2Ti2O7

976

530,550

2

Er3+,Yb3+

Er-Mo:Yb3Al5O2

980

522,546

Er3+,Yb3+

Er-Yb-Mo: TiO2

976

Er3+,Yb3+

Er-Yb-Mo:

SR

SA

Ref.

0.0074

dR/dT=679.2R/T2

[25]

0.0048

dR/dT=900.8R/T2

[7]

(maximum)

H11/2,4S3/→4I15/2

290-610

2

H11/2,4S3/2→4I15/2

295-973

525,550

2

H11/2,4S3/2→4I15/2

300-700

0.0075

dR/dT=606.07R/T2

[26]

980

525,550

2

H11/2,4S3/2→4I15/2

173-553

0.0035

dR/dT=1017.12R/T2

[27]

Er3+,Yb3+

SrMoO4

980

525,550

2

H11/2,4S3/2→4I15/2

93-773

0.0128

dR/dT=979.8R/T2

[28]

Er3+,Yb3+

Na0.5Gd0.5MoO4

980

531,552

2

H11/2,4S3/2→4I15/2

298-778

0.00856

dR/dT=1195.11R/T2

[29]

Er3+,Yb3+

NaGd(MoO4)2

980

520,550

2

H11/2,4S3/2→4I15/2

298-593

0.0161

dR/dT=969.7R/T2

[30]

Er3+

YbMoO4

976

522,547

2

H11/2,4S3/2→4I15/2

300-650

0.0106

dR/dT=902.09R/T2

[31]

Er3+,Yb3+

NaY(MoO4)2

980

Er3+,Yb3+

NaLa(MoO4)2

980

Er3+,Yb3+

BaMoO4

808

Er3+,Yb3+

AgLa(MoO4)2

980

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Na0.5Bi0.5TiO3

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which containing molybdenum.

Er3+

NaYb(MoO4)2

2

H11/2,4S3/2→4I15/2

303-523

0.00969

dR/dT=983.2R/T2

[21]

530,550

2

H11/2,4S3/2→4I15/2

300-510

0.0135

dR/dT=1005R/T2

[32]

531,552

2

H11/2,4S3/2→4I15/2

300-575

0.013

dR/dT=782.4R/T2

[33]

528,550

2

H11/2,4S3/2→4I15/2

300-510

0.018

dR/dT=1133.3R/T2

[34]

530,550

2

H11/2,4S3/2→4I15/2

323-573

0.0122

dR/dT=1060.4R/T2

This

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980

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∆E(cm-1)

Smax(K-1)

Temperature of Smax (K)

H11/2,4S3/2→4I15/2

737

0.0122

548

260

0.0025

323

41

0.00035

3092

0.02455

Er3+

2

Tm3+

1

Ho3+

5

F5(1),5F5(2)→5I8 5

F5,5S2→5I8

323 all

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Ho3+

G4(1),1G4(2)→3H6

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Ln3+

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Er3+/Tm3+/Ho3+

doped

NaYb(MoO4)2

phosphors

show

strong

visible

luminescence.


FIR technique of thermally coupled states (TCS) and non-TCS have been

explained.

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The temperature sensor has high sensitivity in wide range (323-573 K).

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