Yb3+-codoped SrMoO4 phosphors as wide-range temperature sensor

Accepted Manuscript 3+ 3+ Infrared-to-visible upconversion emission of Er /Yb -codoped SrMoO4 phosphors as wide-range temperature sensor Peng Du, Laihui Luo, Jae Su Yu PII:

S1567-1739(15)30078-X

DOI:

10.1016/j.cap.2015.09.013

Reference:

CAP 4076

To appear in:

Current Applied Physics

Received Date: 19 July 2015 Revised Date:

12 September 2015

Accepted Date: 25 September 2015

3+ Please cite this article as: P. Du, L. Luo, J.S. Yu, Infrared-to-visible upconversion emission of Er / 3+ Yb -codoped SrMoO4 phosphors as wide-range temperature sensor, Current Applied Physics (2015), doi: 10.1016/j.cap.2015.09.013. 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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Infrared-to-visible upconversion emission of Er3+/Yb3+-codoped SrMoO4 phosphors as wide-range temperature sensor

Department of Electronics and Radio Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea 2

Department of Microelectronic Science and Engineering, Ningbo University, 315211 Ningbo, China

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Peng Du1, Laihui Luo2, Jae Su Yu1a)

Abstract

Er3+/Yb3+-codoped SrMoO4 phosphors were prepared by a high-temperature solid-state

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reaction method. At room temperature, all the as-prepared samples exhibited strong upconversion properties and the emission intensity increased dramatically with the increase of Yb3+ ion concentration, reaching its maximum value when the concentration was 5 mol%. The dependence of emission intensity on the pump power suggested that the upconversion emission was a two-photon process. Furthermore, the optical temperature sensing

properties 3+

based

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on

green

phosphor

were

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SrMoO4:0.01Er /0.05Yb

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upconversion studied.

It

emissions is

found

of

the

that

the

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SrMoO4:0.01Er /0.05Yb phosphor can be operated over a very wide temperature range of 93-773 K with a maximum sensitivity of ~ 0.0128 K-1, indicating that low- and hightemperature

thermometry

can

be

simultaneously

realized

in

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SrMoO4:0.01Er /0.05Yb phosphor.

Keywords: Upconversion emission, Rare-earth, Luminescence, Temperature sensors

a) Corresponding author: E-mail address: [email protected] (J. S. Yu) Tel: +82 31 201 3820 Fax: +82 31 206 2820

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ACCEPTED MANUSCRIPT Highlighted 1. Introduction Recently, trivalent rare-earth (RE) ions doped luminescent materials have been intensively investigated because of their potential applications in white light-emitting diodes, solar cells, optical temperature sensors, and drug delivery [1-5]. Erbium (Er3+), as

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a member of lanthanide, has attracted much attention owing to its convenient energy level structure and excellent upconversion (UC) properties [4,6]. It was reported that strong green UC emissions corresponding to the (2H11/2, 4S3/2) → 4I15/2 transitions were observed in Na0.5Er0.5Bi4Ti4O15 ceramics [7]. Furthermore, the Er3+ ion has a pair of thermally

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coupled energy levels (2H11/2-4S3/2), which makes promising optical temperature sensor applications [8,9]. Nevertheless, the luminescence efficiency of Er3+-doped materials is still unsatisfactory due to the low absorption in the near-infrared (NIR)

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region. To overcome this drawback, ytterbium (Yb3+) is introduced as a sensitizer because it has high and broad absorption in the NIR region and efficient energy transfer (ET) from Yb3+ to Er3+ ions [6,10]. Thus, high-efficiency UC emissions are expected to be obtained in Er3+/Yb3+-codoped luminescent materials.

Temperature is a fundamental and important physical parameter in many fields, such

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as science, engineering and industry, which can be accurately measured by various methods. Among these methods, the non-invasive optical temperature sensor which is based on the UC emissions of RE-doped materials using a fluorescence intensity ration (FIR) technique plays a key role because of its high resolution and independence of the

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measurement conditions [11,12]. Over the last few years, some impressive achievements have been obtained in optical temperature sensors [13-15]. Yin et al. [16] demonstrated that the Er3+/Yb3+-codoped NaYF4 nanoparticles can be operated in the temperature range

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of 160-300 K with a maximum sensitivity of ~ 0.0012 K-1. Furthermore, it was revealed that the optical temperature sensor which is based on Er3+-doped fluorotellurite glass can be operated in the temperature range of 330-550 K [17]. Note that, these temperature sensors can be operated only at either low temperature or high temperature, suggesting that low- and high-temperature thermometry can not be simultaneously realized in these single sensors, which limit their further applications. Therefore, finding an efficient temperature senor which can be simultaneously realized in low- and high-temperature thermometry is very urgent.

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ACCEPTED MANUSCRIPT Highlighted SrMoO4, as an important member of molybdates, is thought to be a good luminescent host material because of its superior thermal stability and low phonon energy [18]. It was reveled that strong photoluminescence and UC emission properties were obtained in RE ions doped SrMoO4 [19,20]. However, to the best of

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our knowledge, the investigation on the temperature sensing properties of Er3+/Yb3+-codoped SrMoO4 was few. In this work, the Er3+/Yb3+-codoped SrMoO4 phosphors were prepared and their UC properties were studied in detail. Furthermore, the temperature sensing behavior of the obtained Er3+/Yb3+-codoped SrMoO4 sample was

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also investigated using the FIR technique for low- and high-temperature thermometry. 2. Experimental

Er3+/Yb3+-codoped SrMoO4 phosphors were synthesized by a conventional solid-state

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reaction method. To obtain optimum UC properties, the Er3+ ion concentration was fixed at 1 mol% (i.e., 0.01), and the formula was given by Sr0.99-xEr0.01YbxMoO4 (SrMoO4:0.01Er3+/xYb3+, x = 0.01, 0.03, 0.05, 0.07, and 0.09). The high-purity materials of SrCO3 (99.9%), MoO3 (99.5%), Er2O3 (99.9%), and Yb2O3 (99.9%) were weighted and grinded uniformly for 30 min in an agate mortar. Then, the mixtures were put into the

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alumina crucibles and heated at 950 ºC for 5 h in a furnace. Finally, these powders were cooled down naturally to room temperature.

The phase structure of the obtained samples was evaluated by using an X-ray diffractometer (XRD) (Mac Science, M18XHF-SRA, Japan) with Cu Kα (λ = 1.5402 Å)

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radiation in the 2θ range from 10 to 80º. The morphology of the as-prepared samples was investigated by using a field-emission scanning electron microscope (FE-SEM) (LEO

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SUPRA 55, Carl Zeiss, Germany). The room-temperature UC spectra were recorded by using a spectrofluorometer (BLK-CXR-SR, USA) under the excitation of a 980 nm diode laser with a spot size of about 18.1 mm2. The UC spectra at different temperatures were measured by using a spectrofluorometer (Ocean Optics USB 4000, USA) and the temperature ranging from 93 to 773 K was controlled by using a temperature controlled stage (Linkam HFS600E-PB2, UK). 3. Results and discussion Figure 1 shows the XRD patterns of SrMoO4:0.01Er3+/xYb3+ phosphors sintered at 950 ºC. It can be seen that all the diffraction peaks can be well indexed by the standard

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ACCEPTED MANUSCRIPT Highlighted power diffraction file card JCPDS#08-0482 and no any impurity phases were observed. This indicates that all the samples possessed pure tetragonal phase and the RE ions (Er3+ and Yb3+) were diffused into the SrMoO4 host lattices. From the FE-SEM image (inset of Fig. 1), it is clear that the SrMoO4:0.01Er3+/0.05Yb3+ phosphor was composed of

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aggregated and irregular particles with sizes ranging from ~ 2 to 6 µm.

Figure 2 shows the room-temperature UC spectra of SrMoO4:0.01Er3+/xYb3+ phosphors with different Yb3+ ion concentrations. Under the excitation of 980 nm diode laser with a fixed pump power of 320 mW, all the obtained samples exhibited strong

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UC emissions which can be easily seen with the naked eye (see the inset of Fig. 2). In Fig. 2, the UC spectrum consisted of three parts: two strong green UC emissions located at

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approximately 525 and 550 nm corresponding to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively, and a relatively weak red UC emission at ~ 660 nm due to to the 4

F9/2 → 4I15/2 transition [6,21]. These emission bands coincided well with earlier

reports on Er3+/Yb3+-codoped SrMoO4 nanomaterials, which were prepared by microwave-assisted metathetic method and sol-gel method [22,23]. The UC emission intensity increased gradually as the Yb3+ ion concentration increased, reaching its

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optimum value when x = 0.05 due to the efficient ET from the Yb3+ to Er3+ ions. However, the UC emission intensity started to decrease with further increase in Yb3+ ion concentration which was mainly caused by the concentration quenching effect [24,25]. This concentration quenching is attributed to the ET between the nearest dopants (Yb3+

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and Er3+ ions). With increasing the Yb3+ ion concentration, the distance between the dopants decreased. As a result, the non-radiative (NR) ET, such as exchange interaction and multipole-multipole interaction, would be enhanced, resulting in the

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decrement of UC emission intensity. Furthermore, the energy back transfer, i.e., 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+), should be also taken into consideration for the decreased UC emission intensity [10]. The simplified energy level diagram of Er3+ and Yb3+ ions as well as the proposed UC

processes under the excitation of 980 nm light is shown in the inset of Fig. 2. Since the Yb3+ ions can efficiently absorb the light of 980 nm, the ET process from Yb3+ to Er3+ ions prevails [26]. Under 980 nm light excitation, the Yb3+ ions are firstly excited from the 2F7/2 to 2F5/2 level, and then the energy is transferred to the adjacent Er3+ ions, leading

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ACCEPTED MANUSCRIPT Highlighted to the population of 4I11/2 level. After that, part of the electrons coming from the 4I11/2 level decay to the 4I13/2 level owing to the multiphonon relaxation process (see Fig. 2). At the same time, the Yb3+ ions absorb the second photon energy and again the energy is transferred to the adjacent Er3+ ions. Therefore, the electrons are excited from the 4I13/2

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and 4I11/2 levels to the 4F9/2 and 4F7/2 levels, respectively. Subsequently, the electrons located at the 4F7/2 level nonradiatively relax to the 4H11/2 and 4S3/2 levels. Meanwhile, the 4F9/2 level is populated from the 4S3/2 level through NR transition process. Ultimately, strong UC emissions centered at 525, 550, and 660 nm are observed due to

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the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 2F9/2 → 4I15/2 transitions, respectively.

In order to understand the UC mechanism of the as-prepared samples, the dependence

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of UC emission intensity on the pump power was investigated. For the unsaturated UC process, the number of photons, which is required to populate the upper emitting level, can be roughly estimated by the following expression [6,24]: I UC ∝ P n ,

(1)

where IUC is the integrated UC emission intensity, P is the pump power, and n is the number of photons. From Fig. 3, there were no significant changes in the UC spectra,

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while the UC emission intensity showed an upward trend with the increment of pump power. The log-log plot of the integrated UC emission intensity versus pump power is shown in the inset of Fig. 3. It is obvious that the n values for green and red UC emissions were 1.98, 1.79, and 1.88, respectively, which are close to 2, suggesting that

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the green and red UC emissions of the samples were from a two-photon absorption

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The green UC emission spectra of the SrMoO4:0.01Er3+/0.05Yb3+ phosphor with increasing the temperature from 93 to 773 K are presented in Fig. 4. Here, the UC emission intensities were normalized by that at 550 nm. Clearly, the peak positions of the green UC emissions at 525 and 550 nm did not change, while the relative intensity ratio of I525/I550 increased dramatically with the increase of temperature. From the UC spectra, the energy gap between the 2H11/2 and 4S3/2 levels is approximately 800 cm-1. Owing to

this small energy separation, the 2H11/2 level can be easily populated from the 4S3/2 level by thermal agitation, resulting in the variation in transitions of the 2H11/2 → 4

I15/2 and 4S3/2 → 4I15/2 at elevated temperature. This characteristic makes the

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ACCEPTED MANUSCRIPT Highlighted SrMoO4:0.01Er3+/0.05Yb3+ phosphor desirable in optical temperature sensor applications using FIR technique. According to previous reports [12,27], the FIR of the green UC emissions at 525 and 550 nm corresponding to the 2H11/2 → 4I15/2 and 4S3/2 → I15/2 transitions, respectively, can be expressed as:

FIR = R =

 ∆E  I 525  + B . = A exp − I550 k T  B 

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(2)

Here, I525 and I550 are the integrated intensities for the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively, ∆E is the energy gap between the 2H11/2 and 4S3/2 levels, kB is the

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Boltzmann constant, A and B are the constants, and T is the temperature.

The FIR value of the green UC emissions at 525 and 550 nm as a function of

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temperature ranging from 93 to 773 K is shown in the inset of Fig. 5. The experimental data can be well fitted with Eq. (2) and the values of A, B, and ∆E were found to be 23.2, 0.9 and 680.9 cm-1, respectively. For the better understanding of the temperature sensing performance, it is important to investigate the sensing sensitivity, S, which can be estimated by the following formula of [28,29]:

 ∆E   dR ∆E   . = A exp − 2  dT k BT   k BT  

(3)

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S=

The corresponding sensitivity curve is shown in Fig. 5. From the sensitivity curve, the sensitivity showed an upward trend with the increment of temperature, reaching its maximum value of about 0.0128 K-1 at 480 K. The strong UC emission intensity can be

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responsible for the high sensitivity. The optical temperature sensing properties of RE-doped materials, including oxides,

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ceramics, fluorides, and glasses, are summarized in Table I. As shown in Table I, most of the sensors can be operated only at either low or high temperature for low- or hightemperature thermometry. However, the SrMoO4:0.01Er3+/0.05Yb3+ phosphor can be operated over a very wide temperature range of 93-773 K with a maximum sensitivity as high as 0.0128 K-1. This means that the low- and high-temperature thermometry can be simultaneously realized in the SrMoO4:0.01Er3+/0.05Yb3+ phosphor. Furthermore, compared with other RE ions doped sensors (Table 1), it is easier and cheaper to synthesize the SrMoO4:0.01Er3+/0.05Yb3+ phosphor.

4. Conclusions 6

ACCEPTED MANUSCRIPT Highlighted A series of Er3+/Yb3+-codoped SrMoO4 phosphors were sintered via a solid-state reaction technique. Under the 980 nm light excitation, all the samples exhibited strong UC emissions. The temperature dependence of the green UC emissions at 525 nm (2H11/2

→ 4I15/2) and 550 nm (4S3/2 → 4I15/2) was investigated. The FIR value of I525/I550 increased

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gradually with increasing the temperature. Additionally, the SrMoO4:0.01Er3+/0.05Yb3+ phosphor can be operated in a very wide temperature range of 93-773 K with a maximum sensitivity of ~0.0128 K-1 at 480 K, suggesting that the low- and high-temperature thermometry can be simultaneously realized in the SrMoO4:0.01Er3+/0.05Yb3+ phosphor.

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Compared with other sensors, Er3+/Yb3+-codoped SrMoO4 phosphors exhibited higher sensitivity. These results suggest that the Er3+/Yb3+-codoped SrMoO4 phosphors have

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potential applications in temperature thermometry.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013-068407).

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Thongtem, J. Alloys Compd. 506 (2010) 475-484. [19] P. Du, J.S. Yu, RSC Adv. 5 (2015) 60121-60127.

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ACCEPTED MANUSCRIPT Highlighted Figure captions Fig.1. XRD patterns of SrMoO4:0.01Er3+/xYb3+ phosphors. Inset shows the FE-SEM

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image of the SrMoO4:0.01Er3+/0.05Yb3+ phosphor.

Fig.2. UC spectra of SrMoO4:0.01Er3+/xYb3+ phosphors excited at 980 nm. Inset shows the energy level diagram of Er3+ and Yb3+ ions as well as the proposed UC processes. The

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photograph taken by a digital camera in darkness was also shown.

Fig.3. UC spectra of the SrMoO4:0.01Er3+/0.05Yb3+ phosphor as a function of pump

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power at room temperature. The dependence of the green and red UC emission intensities on the pump power is shown in the inset.

Fig.4. Normalized green UC emissions of the SrMoO4:0.01Er3+/0.05Yb3+ phosphor at different temperatures.

Sensing

sensitivity

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a

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SrMoO4:0.01Er3+/0.05Yb3+ phosphor. The inset shows the FIR values as a function of

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Table 1 Optical temperature sensing performance of RE ions doped materials

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Maximum sensitivity (K-1) 0.0128 0.0048 0.0031 0.0045 0.0029 0.0012 0.0033 0.0013

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Er3+/Yb3+: SrMoO4 Er3+/Mo6+: Yb3Al5O12 Er3+/Yb3+: Na0.5Bi0.5TiO3 Er3+/Yb3+: NaLnTiO4 Tm3+/Gd3+/Yb3+: NaLuF4 Er3+/Yb3+: NaYF4 Er3+/Yb3+: Silicate glass Nd3+/Yb3+: Oxyfluoride glass

Temperature range (K) 93-773 295-973 163-613 300-510 298-523 160-300 296-723 303-623

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Highlight: •

Er3+/Yb3+-codoped SrMoO4 phosphors exhibited strong upconversion emissions.



Temperature sensing properties of SrMoO4:0.01Er3+/0.05Yb3+ phosphor were



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investigated. The obtained samples can be operated in a very wide temperature range of 93-773 K. •

The maximum sensitivity of SrMoO4:0.01Er3+/0.05Yb3+ phosphor was as high as 0.0128 K-1.

Er3+/Yb3+-codoped SrMoO4 phosphors can be used in low- and high-temperature

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thermometry.

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