Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater

Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater

Accepted Manuscript Ultrafast synthesis of bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater Pen...

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Accepted Manuscript Ultrafast synthesis of bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater Peng Du, Laihui Luo, Xiaoyong Huang, Jae Su Yu PII: DOI: Reference:

S0021-9797(17)31417-0 https://doi.org/10.1016/j.jcis.2017.12.027 YJCIS 23099

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

29 September 2017 5 December 2017 9 December 2017

Please cite this article as: P. Du, L. Luo, X. Huang, J. Su Yu, Ultrafast synthesis of bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.12.027

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Ultrafast synthesis of bifunctional Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater Peng Dua, Laihui Luob*, Xiaoyong Huangc* and Jae Su Yua* a

Department of Electronic Engineering, Kyung Hee University, Yongin-si 446-701, Republic of Korea

b

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

c

Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China

*Corresponding authors: E-mail addresses: [email protected] (L. Luo); [email protected] (X. Huang); [email protected] (J. S. Yu)

Abstract We reported a simple and ultrafast route to synthesize the bifunctional Er 3+/Yb3+-codoped NaBiF4 upconverting nanoparticles. It was found that the phase compositions and microstructure of the prepared samples were strongly dependent on the NH4F content. When the NH4F content was 14 mmol, after 1 min reaction at room temperature, the resultant compounds exhibited pure single phase and were composed of uniform spherical nanoparticles. Under 980 nm light irradiation, the synthesized nanoparticles emitted visible emissions originating from the intra-4f transitions of Er3+ ions and the involved upconversion luminescence mechanism was associated with the typical two-photon process. With the aid of the fluorescence intensity ratio technique, the optical thermometric behaviors of the studied nanoparticle based on the ( 2H11/2,4S3/2) thermally-coupled levels in the temperature range of 303-483 K were systematically analyzed and the maximum sensor sensitivity was determined to be about 0.0057 K-1 at 483 K. Furthermore, the internal heating properties of the resultant nanoparticles induced by the laser power source were also studied. With elevating the pump power from 159 to 658 mW, the 1

temperature of the upconverting nanoparticles was improved from 304 to 464 K. These results suggest that the Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles are promising bifunctional luminescent materials for nanothermometer and optical heater applications.

Keywords: Nanoparticles, Fluorides, Luminescence, Thermometry, Optical heater

1. Introduction Upconverting nanoparticles utilizing the trivalent rare-earth (RE) ions as activators, which exhibit promising applicability in a broad range of fingerprint detection, optical thermometry, solid-state lighting, medical diagnostic and color displays, have gained enormous attention owing to their excellent optical features, such as narrow emission band, large anti-Stokes shift, absence of auto-fluorescence and long emission lifetime, originating from the intra-4f transitions [1-7]. Particularly, the interest in real-time temperature monitoring based on the upconversion (UC) emissions of upconverting nanocrystals from two thermally-coupled levels of RE ions using the fluorescence intensity ratio (FIR) technique is growing because of its high accuracy, fast response and high spatial resolution in comparison with the conventional contact thermometers [8-10]. Up to date, some trivalent RE ions including Ho3+, Tm3+, Eu3+, Nd3+ and Er3+ were found to have pairs of thermally-coupled levels and their energy gaps located in the range of 200-2000 cm-1 which is helpful for circumventing the overlapping of two emission bands and allowing the upper excited level to be optimally populated [11-16]. Compared with other RE ions, Er3+ ions-based upconverting nanocrystals are considered as promising candidates for non-contact optical thermometry due to their bright green UC emissions originating from the thermally-coupled levels of 2H11/2 and 4S3/2 with the proper energy separation of approximately 800 cm-1 [16-19]. Furthermore, the Yb3+ ions, which exhibit large absorption in the near-infrared

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(NIR) region, are usually introduced to improve the UC emission and temperature sensing properties of Er3+ ions doped luminescent materials [20-22]. Tian et al. revealed that the maximum sensor sensitivity of CaSc2O4:Er3+/Yb3+ nanofibers was around 0.004 K-1 at 590 K [23]. Kumke et al. reported that NaYF4:Gd3+/Yb3+/Er3+ upconverting nanoparticles, which exhibited a maximum sensor sensitivity of 0.0026 K-1, can be employed to monitor the temperature distribution of living cells [24]. Despite the researches on the temperature sensing behaviors of Er3+/Yb3+-codoped upconverting nanocrystals, more efforts still should be made to further improve their performance. As is known, many factors, such as luminescent host material, dopant concentration, particle size and excitation power, can affect the optical thermometric properties of RE ions activated materials [25-28]. Especially, the luminescent host material with low phonon energy takes the domination position. Thus, searching an appropriate luminescent host material is the most efficient route to improve the sensor sensitivity of the RE ions-based upconverting nanoparticles. Currently, lots of inorganic materials including molybdates, oxides, tungstates, fluorides and phosphates are successfully developed for luminescent host materials [29-34]. Among these aforementioned inorganic compounds, fluorides with general chemical formula of NaREF4, which possess low phonon energy and high refractive index are considered as the superior luminescent host lattice for UC emission processes [35,36]. Nowadays, several synthetic techniques, such as hydrothermal method, solvothermal method and coprecipitation method, have been employed to prepare the RE ions doped fluoride upconverting nanoparticles [37-39]. Liu et al. pointed out that the synthetic temperature of the RE ions doped NaYF4 upconverting nanoparticles was 200 °C [40]. Furthermore, Zhang et al. demonstrated that the hexagonal NaYF4:Er3+/Yb3+ upconverting nanoparticles can be achieved after heat treatment at 150 °C for

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24 h when the coprecipitation method was applied [41]. Inspiringly, You et al. developed a novel ion-exchange route to prepare the NaYF4 nanocrystals and the resultant temperature was as low as 50 °C, while it still required 3 h to form the pure hexagonal phase [42] Obviously, to prepare these NaREF4 fluorides, either high temperature or long reaction time is required, resulting in high investment. Hence, searching a novel fluoride compound, which can be synthesized at room temperature with short reaction time, is needed. In comparison, the NaBiF4 was considered to be a promising material not only for solid-state battery but also UC luminescent host [43,44]. However, to the best of our knowledge, the report on the synthesis and optical thermometric performance of Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles is still insufficient. In present work, to further modify the synthetic process and extend the applicability of the NaBiF4 nanoparticles, the Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles were successfully prepared by a facile method at room temperature and the influence of NH4F content on the phase structure and morphology of the resultant compounds was systematically studied. Under NIR light irradiation, the UC emission properties of the studied nanoparticles were examined. In addition, the temperature-dependent green UC emission spectra were recorded to analyze the temperature sensing performance of the Er3+/Yb3+-codoped NaBiF4 upconverting nanoparticles. Ultimately, the internal heating behaviors of the synthesized samples were also investigated to detect their feasibility for photothermal therapy as an optical heater.

2. Experimental details The Er3+/Yb3+-codoped NaBiF4 (NaBiF4:Er3+/Yb3+) nanoparticles were prepared by an ultrafast method at room temperature. The starting materials including NaNO3, Bi(NO3)3∙5H2O, Er(NO3)3∙5H2O, Yb(NO3)3∙5H2O, NH4F and ethylene glycol (EG), which were purchased from

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Sigma-Aldrich Co., were employed to prepare the NaBiF4:Er3+/Yb3+ upconverting nanoparticles. Firstly, solution I was prepared by dissolving the NaNO3 (2 mmol), Bi(NO3)3∙5H2O (0.78 mmol), Er(NO3)3∙5H2O (0.02 mmol) and Yb(NO3)3∙5H2O (0.2 mmol) into 10 ml of EG. Meanwhile, solution II was prepared by dissolving different amounts of NH4F into 20 ml of EG. Then, solution I was added to solution II under strong mechanical agitation. After that, this mixture was stirred for 1 min at room temperature. Ultimately, the upconverting nanoparticles were collected by means of centrifugation, washed with ethanol and de-ionized water for three times to remove the remained reagent, and then dried at 80 °C for 6 h in air. The detailed synthesis procedure of NaBiF4:Er3+/Yb3+ upconverting nanoparticles as well as their potential application is illustrated in Fig. 1. The X-ray diffractometer (Bruker D8) was used to detect the phase purity of the final products. The microstructure and morphology of resultant samples were characterized by using a field-emission scanning electron microscope (FE-SEM; LEO SUPRA 55, GENESIS 2000, Carl Zeiss) equipped with an energy dispersive X-ray (EDX) spectrometer and a transmission electron microscope (TEM) (JEM-2100F, JEOL). The UC emission spectra of the studied compounds were recorded by utilizing a spectrofluorometer (Edinburgh FS5) equipped with a pump power tunable 980 nm laser diode. The temperature ranging from 303 to 483 K was realized by using a temperature control stage (Linkam HFS600E-PB2).

3. Results and discussion The crystal structure and phase compositions of the studies compounds were determined by the X-ray diffraction (XRD). The XRD patterns of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles prepared with different contents of NH4F are depicted in Fig. 2. As demonstrated,

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the detected diffraction peaks can be indexed as the mixture of hexagonal NaBiF4 (JCPDS# 410796) and BiF3 (JCPDS# 35-0380) when the NH4F content is 4 mmol (see Fig. 2(a)), suggesting that the synthesized compounds possess hybrid phases. To circumvent the formation of the impurity phase and obtain the pure hexagonal NaBiF4 phase, excess NH4F was added. As shown in Fig. 2(b), the diffraction peak intensities originating from the impurity phase (BiF 3) decrease sharply when the NH4F content increases to 6 mmol and the hexagonal NaBiF4 takes the domination in the obtained products. Furthermore, with further elevating the NH 4F content from 8 to 12 mmol, the diffraction peaks corresponding to the hexagonal BiF3 vanish gradually along with the declined intensity (see Fig. 2(c)-(e)). In addition, when 14 mmol of NH4 F is supplied, it can be seen that all the recorded diffraction peaks of the final products are consistent well with the standard NaBiF4 (Fig. 2(f)), indicating that the resultant compounds exhibit pure single hexagonal phase with space group of P-3(147) and the NaBiF4:Er3+/Yb3+ upconverting nanoparticles are successfully synthesized. These results demonstrated that the phase purity of the studied nanoparticles is largely dependent on the NH4F content and the optimal content is determined to be 14 mmol. For the aim of analyzing the effect of NH4F content on the morphology and particle size of the prepared samples, the FE-SEM measurement was carried out. The FE-SEM images of the final compounds synthesized with various quantities of NH4F are depicted in Fig. 3. It is clear that the products obtained at 4 mmol of NH4F are made up of nonuniform nanoparticles with the average size ranging from approximately 40 to 100 nm, as described in Fig. 3(a). Note that, with increasing the NH4F content, big sized nanoparticles are formed. As illustrated in Fig. 3(b), the resultant compounds, which were prepared with 6 mmol of NH 4F, are composed of homogenously spherical nanoparticles with the average size around 330 nm in the diameter.

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Nevertheless, these achieved spherical nanoparticles are attached with lots of tiny particles corresponding to the BiF3 phase, resulting in rough surface. On further raising the NH4F content, it is found that the shape and size of the particles are barely changed, while the roughness of the particles is significantly decreased (see Fig. 3(c)-(f)). Especially, when the NH4F content is increased to 14 mmol, the resultant samples consist of uniform nanoparticles without attaching any tiny particles, as disclosed in Fig. 3(f), implying that in these obtained nanoparticles, single pure phase coincides well with the result from the XRD patterns. Fig. 3(g) describes the schematic diagram for the growth of spherical NaBiF4:Er3+/Yb3+ upconverting nanoparticles at room temperature. The microstructure performances of the final products are further characterized by utilizing the TEM. The TEM images in Fig. 4(a) and (b) confirm that the resultant compounds collected at 14 mmol of NH4F are made up of homogenously spherical nanoparticles with the particle size of about 330 nm. It is evident that the high-resolution TEM (HR-TEM) image in Fig. 4(c) exhibits distinct lattice fringes and the distance between these adjacent lattices is demonstrated to be around 5.2 Å which is associated with the (100) plane of the hexagonal NaBiF4 (JCPDS# 410796). The selective area electron diffraction (SAED) pattern displays clear ring patterns which are composed of many bright dots, implying that the synthesized particles are nanocrystalline in nature (see Fig. 4(d)). On the basis of the elemental mapping results (Fig. 4(e)-(j)), it is evident that the element compositions of Na, Bi, F, Er and Yb are equably distributed over the whole nanoparticles. The EDX spectrum in Fig. 4(k) reveals that the studied samples are mainly composed of Na, Bi, F, Er and Yb ions, further indicating the formation of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles.

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Since the compound synthesized at 14 mmol of NH4F exhibits pure hexagonal phase and homogenously spherical nanoparticles, its UC emission performance was investigated. Under the irradiation of 980 nm light with an excitation pump power of 159 mW, the room-temperature UC emission spectrum of NaBiF4:Er3+/Yb3+ upconverting nanoparticles was detected and presented in Fig. 5(a). It is evident that the UC emission spectrum consists of two green emissions which are assigned to the (2H11/2,4S3/2) → 4I15/2 intra-4f transitions of Er3+ ions and a red emission at around 656 nm corresponding to the 4F9/2 → 4I15/2 transition [22,27]. The luminescent image of the studied sample is displayed in the inset of Fig. 5(a). To comprehend the involved infrared-tovisible UC luminescence mechanism, the simplified energy level diagram of Er 3+ and Yb3+ ions as well as the possible energy transfer (ET) channels is presented in Fig. 5(b). Upon the irradiation of NIR light, the incident photons are firstly absorbed by the Yb3+ ions, leading to the population of the 2F5/2 excited level from the 2F7/2 ground state. Then, the 4I11/2 level of Er3+ ions is populated due to the efficient ET from Yb3+ to Er3+ ions (Yb3+ (2F5/2) + Er3+ (4I15/2) → Er3+ (4I11/2) + Yb3+ (2F7/2)). In terms of the electrons in the 4I11/2 level, part of them can be directly pumped to the 4F7/2 excited level by means of the second ET from Yb3+ to Er3+ ions (Yb3+ (2F5/2) + Er3+ (4I11/2) → Er3+ (4F7/2) + Yb3+ (2F7/2)). After that, the thermally-coupled levels of 2H11/2 and 4

S3/2 are populated from the 4F7/2 level because of the nonradiative (NR) transitions process, as

described in Fig. 5(b). Finally, these excited electrons release to the ground state and the green UC emissions corresponding to the (2H11/2,4S3/2) → 4I15/2 transitions are generated. In comparison, two possible pathways are involved to emit the red UC emission ( 4F9/2 → 4I15/2). As demonstrated in Fig. 5(b), the electrons located in the 4I11/2 level can not only be excited to the 4

F7/2 excited level but also nonradiatively decay to the 4I13/2 level. Subsequently, the 4I13/2 level is

promoted to the 4F9/2 level with the help of the ET from Yb3+ to Er3+ ions (Yb3+ (2F5/2) + Er3+

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(4I13/2) → Er3+ (4F9/2) + Yb3+ (2F7/2)). Additionally, the 4F9/2 excited level can also be directly populated from the 4S3/2 level through the NR transition process. Owing to these aforementioned processes, the red emission, which is attributed to the 4F9/2 → 4I15/2 transition, is observed in the UC emission spectrum. To get insight into the UC luminescence mechanism in NaBiF4:Er3+/Yb3+ upconverting nanoparticles, the pump power-dependent UC emission behaviors were studied. On the basis of previous researches, one knows that the relation between the emission intensity and excitation pump power for an unsaturation luminescence process can be approximately expressed as [10,23]:

IUC  P n ,

(1)

where IUC and P refer to UC emission intensity and excitation pump power, respectively, and n denotes the number of incident photons that are necessary to populate the upper emitting level. The logarithm plots of green and red UC emission intensity as a function of excitation pump power are depicted in Fig. 5(c) and 5(d), respectively. It is evident that the experimental data can be linearly fitted and the n values for green and red UC emissions are determined to be around 1.96, 2.14 and 2.12, respectively. Clearly, these evaluated values are close to 2, implying that both green and red UC emissions belong to the two-photon process which coincides well with above discussion. On account of the small energy gap between the 2H11/2 and 4S3/2 levels, the 2H11/2 level is expected to be populated from the 4S3/2 level by means of the thermal excitation and achieve a quasi-thermal equilibrium. As a result, the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions exhibit various responses to the temperature which makes the Er 3+ ions-based upconverting nanoparticles are promising candidates for nanothermometer utilizing the FIR technique. For the

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sake of investigating the temperature sensing properties of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles, the temperature-dependent green UC emission spectra were measured. From the normalized green UC emission spectra (Fig. 6(a)), it is obvious that the emission bands are hardly changed with the increase of temperature, while the green emission intensities are greatly dependent on the temperature. Based on the recorded green UC emission spectra, the FIR values of the green emissions originating from the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions were evaluated and the corresponding results are illustrated in Fig. 6(c). According to previous reported literatures, it is well known that the FIR of the green emissions arising from the thermally-coupled levels of (2H11/2,4S3/2) to the 4I15/2 ground state can be expressed as [2,45]:

FIR 

IH  E   A exp  , IS  kT 

(2)

where IH and IS denote the integrated emission intensities of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions, respectively, A is the coefficient, ∆E presents the energy separation of the thermally-coupled levels, k stands for the Boltzmann coefficient and T is associated with the temperature. Moreover, the above expression can also be rewritten as:

ln  FIR   

E  B. kT

(3)

Here, B is the constant. Fig. 6(b) describes the monolog plot of FIR of two green UC emissions originating from the 2H11/2 and 4S3/2 thermally-coupled levels as a function of inverse temperature. As disclosed, by means of Eq. 3, the experimental points can be fitted by s straight line with a slope of -1162. Furthermore, it is obvious that the FIR value is largely dependent on the temperature and its maximum value is determined to be about 1.15 when the temperature is 483 K, as described in Fig. 6(c). With the aid of Eq. (2), these evaluated FIR values were fitted and the values of ∆E/k and A are demonstrated to be 1142 and 12.4, respectively (see Fig. 6(c)). 10

Therefore, the energy gap between the 2H11/2 and 4S3/2 levels is estimated to be about 788 cm-1 located in the range of 200-2000 cm-1, further indicating that the 2H11/2 and 4S3/2 levels are assigned to thermally-coupled levels. From the temperature-dependent green UC emission spectra (Fig. 6(a)), it is evident that the excited electrons located in the 4S3/2 level can be pumped to the 2H11/2 level at elevated temperature by means of the thermal excitation, resulting in the population redistribution between the 2H11/2 and 4S3/2 thermally-coupled levels. In order to get a deep insight into the temperature induced population redistribution phenomenon, the temperature-dependent population redistribution ability (PRA) is estimated by the following expression [28,46]:

PRA 

IU IH .  IU  I L I H  I S

(4)

In this expression, IU and IL describe the integrated emission intensities from the upper and lower excited levels, respectively. And IH and IS exhibit the same meaning as demonstrated in Eq. (2). Combined with Eq. (2) and Eq. (4), the PRA can be rewritten as:

PRA 

A , A  exp  E kT 

(5)

where A, ∆E, k and T possess the same meaning as presented in Eq. (2). On the basis of the fitting result, as depicted in Fig. 6(c), ∆E/k and A are found to be 1142 and 12.4, respectively. With the help of Eq. (5) and these determined values, the temperature-dependent PRA values of the 2H11/2 and 4S3/2 thermally-coupled levels were evaluated and the corresponding results are demonstrated in Fig. 6(d). As illustrated in Fig. 6(d), one obtains that the PRA values largely rely on the temperature. When the temperature is 303 K, the PRA value is found to be around 0.22. Nevertheless, when the temperature is raised, the PRA values sharply increase and its maximum value is determined to be as high as 0.54 when the temperature was 483 K (see Fig. 6(d)), further 11

implying that population between the

2

H11/2 and

4

S3/2 thermally-coupled levels can be

redistributed at elevated temperature. This characteristic also reveals the NaBiF4:Er3+/Yb3+ upconverting nanoparticles are expected to exhibit satisfactory temperature sensing behaviors and are promising candidates for non-contact nanothermometers based on the FIR technique. As a proof of above deduction, the investigation on the sensor sensitivity of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles was carried out. As is known, to identify the applicability of resultant samples for optical thermometry, two crucial parameters, namely, absolute sensor sensitivity (S) and relative sensor sensitivity (SR), are required to be analyzed and they can be evaluated using the following expressions [45,47]:

S

d  FIR 

SR 

dT

 FIR 

E , kT 2

d  FIR  E 1   2. FIR dT kT

(6)

(7)

The absolute sensor sensitivity and relative sensor sensitivity, which were estimated by means of Eq. (6) and Eq. (7), respectively, as a function of temperature ranging from 303 to 483 K are shown in Fig. 7(a) and 7(b), respectively. As disclosed, the SR, which is expressed as 1142/T2, exhibits a downward tendency with increasing the temperature from 303 to 483 K and its maximum value is around 0.0124 K-1 at 303 K. In comparison, the absolute sensor sensitivity shows an opposite tendency. It can be seen in Fig. 7(b) that the absolute sensor sensitivity increases monotonously with the increment of temperature and reaches its maximum value of approximately 0.0057 K-1 at 483 K in the temperature range of our interest. Note that, the sensor sensitivity of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles is not only superior to other Er3+ ions doped fluorides nanoparticles but also comparable with other optical temperature sensors, as listed in Table 1. Apart from the sensor sensitivity of the resultant products, the measurement 12

error (∆T), which can be employed to clarify the accuracy of the thermometer, should be also taken into account to explore their feasibility for optical thermometry and it can be defined as [11,28]: T 

  FIR  FIR



  FIR  1 .  SR S

(8)

Obviously, larger absolute sensor sensitivity results in smaller measurement error. Since the NaBiF4:Er3+/Yb3+ upconverting nanoparticles have a relatively larger sensor sensitivity compared with other thermometers (see Table 1), they are expected to possess better accuracy with small measurement error. These results imply that the NaBiF4:Er3+/Yb3+ upconverting nanoparticles with high sensor sensitivity and small measurement error are suitable for optical thermometry by applying the FIR technique. As is known, the UC emission belongs to nonlinear optical process and the energy, which is absorbed by the activators from the incident light, can not be totally used and most of them will be converted into heat by means of the NR transition process, resulting in the elevated temperature of the products [10,48]. To study the internal heating behavior of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles, the green UC emission spectra at various excitation pump powers were detected. From the normalized green UC emission spectra (inset of the Fig. 7(c)), it is obvious that the emission intensity of the 2H11/2 → 4I15/2 transition increases monotonously in comparison to that of the 4S3/2 → 4I15/2 transition, leading to varied FIR values. Clearly, with increasing the excitation pump power from 159 to 658 mW, the FIR value is increased from 0.29 to 1.06 (see Fig. 7(c)). On the basis of Eq. (2), the temperature of the products under different excitation pump powers can be evaluated by employing the following formula:

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  E 1 T   .   ln A  ln FIR k    

(9)

As demonstrated above, the values of A and ∆E/k are 12.4 and 1142, respectively. Thus, the temperature of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles at different excitation pump powers can be calculated and the corresponding results are shown in Fig. 7(d). As disclosed, a gradual enhancement in the temperature is observed and the maximum temperature is demonstrated to be about 464 K when the excitation pump power is 658 mW. This confirms that the NaBiF4:Er3+/Yb3+ upconverting nanoparticles possess the ability to convert the incident energy into heat, implying that the resultant samples may suitable for cancer therapy as optical heaters. Clearly, there is little different between the calculated temperature and real temperature of the studied samples, as listed in Table 2, revealing that the NaBiF4:Er3+/Yb3+ upconverting nanoparticles possessed relatively high accuracy.

4. Conclusion In summary, the bifunctional NaBiF4:Er3+/Yb3+ upconverting nanoparticles were successfully synthesized by employing an ultrarapid and simple method at room temperature. Through controlling the NH4F content, the phase structure and morphology properties of the final products were significantly changed. Furthermore, the resultant samples exhibited pure hexagonal phase and consisted of homogenously spherical nanoparticles when the NH4F content is 14 mmol. Under the irradiation of 980 nm light, the NaBiF4:Er3+/Yb3+ upconverting nanoparticles emited brightly visible emission arising from the intra-4f transitions of Er3+ ions and the involved UC luminescence mechanism belongs to typical two-photon process. The FIR of the green emissions originating from the ( 2H11/2,4S3/2) thermally-coupled levels was examined to study the temperature sensing performance of the synthesized nanoparticles. It is found that 14

the NaBiF4:Er3+/Yb3+ upconverting nanoparticles can be operated in the temperature range of 303-483 K with a maximum absolute sensor sensitivity of 0.0057 K -1 at 483 K. In addition, the prepared nanoparticles can convert the excitation pump power into heat and the product temperature is as high as 464 K when the laser power is 658 mW. From these results, the NaBiF4:Er3+/Yb3+ upconverting nanoparticles are expected to be promising bifunctional materials for nanothermometer and optical heater applications.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1A2B4011998) and National Natural Science Foundation of China (No. 51502190).

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Figure captions Fig 1. Schematic diagram describing the synthesis process of NaBiF4:Er3+/Yb3+ upconverting nanoparticles at room temperature along with the promising applications of the resultant nanoparticles. Fig. 2 XRD patterns of the upconverting nanoparticles synthesized with various amounts of NH4F: (a) 4 mmol, (b) 6 mmol, (c) 8 mmol, (d) 10 mmol, (e) 12 mmol and (f) 14 mmol. Fig. 3 FE-SEM images of the resultant samples prepared with different contents of NH 4F: (a) 4 mmol, (b) 6 mmol, (c) 8 mmol, (d) 10 mmol, (e) 12 mmol and (f) 14 mmol. (g) Schematic diagram of the formation of spherical NaBiF4:Er3+/Yb3+ upcponverting nanoparticles at room temperature. Fig. 4 (a)-(b) TEM images, (c) HR-TEM image and (d) SAED pattern. (c)-(j) Elemental mapping and (k) EDX spectrum of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles synthesized with 14 mmol of NH4F. Fig. 5 (a) Room-temperature UC emission spectrum of NaBiF4:Er3+/Yb3+ upconverting nanoparticles synthesized with 14 mmol of NH4F. (b) Simplified energy level diagram of Er 3+ and Yb3+ ions along with the involved UC mechanism in the NaBiF4 host lattices. Pump power dependent (c) green and (d) red emission intensity. Inset shows the luminescent image excited at 980 nm light. Fig. 6 (a) Normalized green UC emission spectra of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles prepared with 14 mmol of NH4F in the temperature range of 303-483 K. (b) Plot of ln(FIR) versus 1/T. (c) FIR values of the green emissions at different temperatures. (d) Temperature-dependent PRA value. Fig. 7 (a) Relative sensitivity and (b) absolute sensitivity of the NaBiF4:Er3+/Yb3+ upconverting nanoparticles synthesized with 14 of mmol NH4F in the temperature range of 303-483 K. (c) FIR value and (d) product temperature as a function of pump power. Inset of (c) illustrates the normalized green UC emission spectra of NaBiF4:Er3+/Yb3+ upconverting nanoparticles at different pump powers.

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Table 1. Optical thermometric properties of Er3+/Yb3+ -codoped luminescent materials. Temperature range

SR (K-1)

S (K-1)

Reference

YF3:Er3+/Yb3+

260-490 K

997.7/T2

0.0027

7

Ba5Gd8Zn4O21:Er3+/Yb3+

260-490 K

1144.6/T2

0.0032

15

CaSc2O4:Er3+/Yb3+

299-623 K

1138.18/T2

0.0040

23

0.0048

49

Sensing compounds

3+

3+

2

NaYF4:Er /Yb

75-600 K

1025.8/T

KYb2 F7:Er3+/Yb3+

300-480 K

1224/T2

0.0045

50

CaWO4:Er3+/Yb3+

300-500 K

908.56/T2

0.0025

51

NaBiF4:Er3+/Yb3+

303-483 K

1142/T2

0.0057

This work

Table 2. The comparison of calculated temperature and real temperature of NaBiF4:Er3+/Yb3+ upconverting nanoparticles at different pump powers. Excitation pump power

Calculated temperature

Real temperature

159 mW

304 K

300 K

259 mW

356 K

352 K

358 mW

395 K

397 K

458 mW

431 K

425 K

558 mW

441 K

436 K

657 mW

464 K

459 K

22

Fig. 1

23

Fig. 2

24

Fig. 3

25

Fig. 4

26

Fig. 5

27

Fig. 6

28

Fig. 7

29

Graphical abstract

Er3+/Yb3+-codoped NaBiF4 nanoparticles were successfully synthesized via a one-minute reaction process at room temperature. The resultant nanoparticles did not possess superior UC emission properties but also exhibited promising applications in temperature sensor and optical heater.

30