A novel upconversion luminescent material: Li+- or Mg2+-codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors and their temperature sensing properties

Accepted Manuscript + 2+ A novel upconversion luminescent material: Li - or Mg -codoped 3+ 3+ Bi3.84W0.16O6.24:Tm , Yb phosphors and their temperature sensing properties Zhen Sun, Guofeng Liu, Zuoling Fu, Zhendong Hao, Jiahua Zhang PII:

S0143-7208(17)32190-3

DOI:

10.1016/j.dyepig.2018.01.020

Reference:

DYPI 6494

To appear in:

Dyes and Pigments

Received Date: 24 October 2017 Revised Date:

9 January 2018

Accepted Date: 10 January 2018

Please cite this article as: Sun Z, Liu G, Fu Z, Hao Z, Zhang J, A novel upconversion luminescent + 2+ 3+ 3+ material: Li - or Mg -codoped Bi3.84W0.16O6.24:Tm , Yb phosphors and their temperature sensing properties, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.01.020. 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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Graphical Abstract

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A Novel Upconversion Luminescent Material: Li+- or Mg2+-Codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ Phosphors and Their Temperature Sensing Properties

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Zhen Suna, Guofeng Liua, Zuoling Fu a,*, Zhendong Haob, and Jiahua Zhangb,* Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of Physics

and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012, China, Fax: +86-431-85167966; Tel: +86-431-85167966; E-mail: [email protected] (Z. L. Fu)

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

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b

Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road,

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Changchun, 130033, China; E-mail: [email protected] (J. H. Zhang)

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Abstract A novel upconversion (UC) luminescent phosphor Bi3.84W0.16O6.24:Tm3+, Yb3+ was synthesized via co-precipitation method. Distinct visible blue, red, and

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near-infrared light and weak purple and ultraviolet light UC emissions were all detected under 980 nm excitation in Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphor. Addition of Li+ or Mg2+ ions in Tm3+-and Yb3+-codoped Bi3.84W0.16O6.24 phosphors

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further enhanced the blue and red emission intensities. The slope values of the log–

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log graph of pump power and emission intensity decreased to 1 for the blue emission because of the saturation effect, which was investigated in detail by theoretical and experiment analyses. Optical temperature sensing performances were evaluated in the NIR region from 313 K to 573 K based on the 3H4(1) and 3H4(2)

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of Tm3+ ions as thermal coupled energy levels. The corresponding optimum sensitivities were 0.000676, 0.00103, and 0.000696 K−1 at 323 K for free, Li+ ion-, and Mg2+ ion-codoped Bi3.84W0.16O6.24: Tm3+, Yb3+ phosphors, respectively. The

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introduction of alkali metals (Li) or alkaline earth metals (Mg) also enhanced the

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temperature sensitivity of the Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors in the NIR region. Keywords: Co-precipitation method, Saturation effect, Optical temperature sensing

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ACCEPTED MANUSCRIPT 1. Introduction Given their low toxicity, high penetration depth, and low auto-fluorescence background from biological tissues, micro/nanostructured upconversion (UC)

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luminescence materials have recently become a research hotspot in the field of biomedicine [1-5]. For example, rare-earth-doped UC luminescence materials can be injected into cells and biological tissues as temperature sensors to monitor

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tissues temperature based on fluorescence intensity ratio (FIR) method [6, 7].

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Temperature control is important to protect healthy cells around tumor cells, which are killed and damaged at temperature exceeding 314 K during hyperthermia therapy. However, in view of the light scattering and absorption by human body tissues, the penetration depth of visible light markedly decreases in

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practical biological applications. This problem can be overcome by emission wavelength in the first and second biological windows (BWI: 700–900 nm; BWII: 1000–1400 nm) where considerably deepened penetration depth can be achieved due

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to reducing scattering and absorption by tissues [8, 9].

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In addition, one of important disadvantages for UC materials is low UC efficiency [10, 11]. Several schemes, such as internal modification by doping sensitizer [12] or transferring crystal phase [13], have recently been proposed to enhance the emission intensity of materials. Another scheme is external adjustment, which may involve the synthesis of core–shell structure [14, 15] or crystal surface coating [16, 17]. UC emission intensity also can be increased by doping different ions, such as lithium (Li+), strontium (Sr2+), calcium (Ca2+), zinc (Zn2+), and magnesium 3

ACCEPTED MANUSCRIPT (Mg2+) [18-21]. Gavrilović et al. studied the effects of Li+ ions on the emission of GdVO4:Yb3+, Ho3+ and found that the introduction of Li+ ions enhanced emission intensity of green light up to two times compared with the samples without Li+

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doping [20]. Maurya et al. discovered that the addition of Mg2+ ions in Ho3+- and Yb3+-codoped CaZrO3 phosphor enhanced emission intensity of green light significantly [22]. Therefore, the introduction of alkali metals or alkaline earth metals

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into the host lattice is an effective approach to improve UC emission intensity. Due to

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their smaller ionic radii (0.68 and 0.72 Å, respectively), Li+ and Mg2+ ions can easily occupy interstitial or substitutional sites in the host lattice and modify the symmetry of the crystal field to enhance optical properties.

To our knowledge, although bismuth tungstate is an extensively applied matrix

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material in photocatalytic degradation [23-25], its UC luminescence properties are rarely discussed. Tm3+ ions, which are excellent probes with abundant energy levels, are typically used for cell imaging and temperature sensing [26, 27].

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Researchers have explored the noninvasive FIR temperature sensing method, which

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generally relies on the temperature-dependent intensity ratio between two adjacent thermally coupled levels (TCLs) based on the 1G4(1) and 1G4(2) levels of Tm3+ [28-30]. However, limited information is available on 3H4 levels of Tm3+ ions acting as TCLs [24, 31]. Here, we prepared the Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphor by co-precipitation methods. The introduction of Li+ or Mg2+ ions can significantly improve the UC emission intensity and temperature sensitivity of Tm3+. Furthermore, the luminescence mechanism has been confirmed by theoretical and experimental 4

ACCEPTED MANUSCRIPT analyses. All experimental results indicate that Li+- and Mg2+-codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors display strong UC luminescence (UCL) intensity and high temperature sensitivity in the biological temperature range

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(298–318 K. Furthermore, all the samples can be used as temperature sensors in the first biological window.

2.1 Sample Preparation Bi3.84W0.16O6.24

phosphors

were

synthesized

via

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Tm3+–Yb3+-codoped

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2. Experimental

co-precipitation methods with distilled water (18 MΩ•cm) as the solvent. All chemical reagents were directly used without further purification. First, Bi(NO3)3• 5H2O (4 mmol, 99.99%) and Ln(NO3)3 (Ln = Tm,Yb; 99.99%) were added to 20 mL

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distilled water, and the solution was vigorously stirred for 1 h to obtain solution A. Then, 2 mmol Na2WO4•2H2O (99.99%) was dissolved in 50 mL distilled water to obtain solution B, which was gradually added to solution A. The pH value of the

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mixing solution was adjusted to 11 by adding NaOH solution. Bi3.84W0.16O6.24: Tm3+,

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Yb3+, Li+ phosphors and Bi3.84W0.16O6.24: Tm3+, Yb3+, Mg2+ phosphors were also prepared via co-precipitation methods. LiNO3 (99.99%) or Mg(NO3)2 (99.99%) was added to solution A prior to being mixed with solution B. Finally, the obtained white products were centrifuged with distilled water and ethanol for three times and then dried at 60 °C for 12 h. The final powder was obtained after heat treatment at 900 °C for 1 h. 2.2 Characterization 5

ACCEPTED MANUSCRIPT The structure and crystalline phase of all samples were recorded by a Rigaku-Dmax 2500 diffractometer with Cu Kα radiation (λ = 0.15405 nm) at a scanning rate of 15°/min in the 2θ range from 10° to 80°. The morphology and particle size of

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phosphors were determined through field emission-scanning electron microscopy (FE-SEM, XL30, Philips). Chemical element analysis was conducted through energy dispersive X-ray spectroscopy (EDX) coupled with FE-SEM. An Andor

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Shamrock SR-750 fluorescence spectrometer was used to obtain the UC emission

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spectra, and the signals were collected from 350–900 nm by a CCD detector combined with a monochromator. As the pump source, 980 nm diode was coupled as the pump source to a fiber laser (core diameter: 200 mm; numerical aperture: 0.22). The luminescence spectra of the samples were obtained using an Andor SR-500i

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spectrometer (Andor Technology Co, Belfast, UK). 3. Results and discussion 3.1 XRD

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Figure 1 displays the phase purity and crystal structure of Tm3+–Yb3+, Tm3+–

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Yb3+–Li+, and Tm3+–Yb3+–Mg2+ codoped Bi3.84W0.16O6.24 phosphors. The XRD patterns of the cubic Bi3.84W0.16O6.24 phosphors ((a) 0.3 mol % Tm3+, 10 mol % Yb3+; (b) 0.3 mol % Tm3+, 10 mol % Yb3+, 5 mol % Li+; (c) 0.3 mol % Tm3+, 10 mol % Yb3+, 3 mol % Mg2+) are in accordance with the standard data of JCPDS (43-0447), and no additional peaks are observed. The results indicate that doping ions (Li+, Mg2+, Tm3+, Yb3+) are doped into host lattice and do not have influence on host crystal structure. When Li+ and Mg2+ are codoped in 6

the Bi3.84W0.16O6.24:Tm3+, Yb3+

ACCEPTED MANUSCRIPT phosphors, the position of the diffraction peak shifts slightly. Particularly in the strongest peak (111), the diffraction angle decreases with Li+ doping, whereas Mg2+ doping causes the diffraction angle to shift to a larger value (Figure 1 (d)). A smaller

According to Bragg’s law, 2 d sin θ = n λ, where d,

, and

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lattice constant corresponds to a larger diffraction angle, and vice versa [32]. represent interplanar

distance, diffraction angle, and diffraction wavelength, respectively. As d increases

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and θ decreases, Li+ ions are found at the interstitial site to result in lattice expansion.

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When d decreases and θ increases, the Mg2+ ions substitute for the Bi3+ ions to induce lattice contraction. As listed in Table 1, changes in lattice parameters are consistent with the aforementioned description. The effective ionic radii of Li+, Mg2+, and Bi3+ are 0.68, 0.72, and 1.03 Å, respectively. Li+ ions are small enough to occupy the

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interstitial sites of the lattice [33], while Mg2+ ions possess smaller ionic radius compared with Bi3+ ions, resulting in lattice shrinkage when Mg2+ replaces Bi3+ in the

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Bi3.84W0.16O6.27 host [34].

Figure 1. X-ray diffraction patterns of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % 7

ACCEPTED MANUSCRIPT Yb3+ phosphors doped with: (a) 0, (b) 3 mol % Li+, and (c) 3 mol % Mg2+; (d) evolution of the main (111) peak and reference data JCPDS 43-0447.

0.3% Tm3+, 10% Yb3+ 0.3% Tm3+, 10% Yb3+, 5% Li+

3.2 FE-SEM

5.559(03)

ɑ = β = γ (Å)

Cell volume (Å3)

90

171.75

5.560(14)

90

171.86

5.543(20)

90

170.29

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0.3% Tm3+, 10% Yb3+, 3% Mg2+

a = b = c (Å)

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Bi3.84W0.16O6.24

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Bi3.84W0.16O6.24 phosphor.

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Table 1. Lattice constants of Tm3+–Yb3+/Tm3+–Yb3+–Li+/Tm3+–Yb3+–Mg2+-codoped

The morphology and particle size of the as-prepared samples are presented in

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Figure 2. The Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphors show severe aggregation and heterogeneous distribution because of high-temperature sintering

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during the experimental process (Figure 2(a)). When Li+ and Mg2+ ions are introduced into Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphors, the samples exhibit a uniform and regular square-like shape (Figures 2(b) and 2(c)), indicating that the doping Li+ or Mg2+ ions may affect the crystal growth of Bi3.84W0.16O6.24:Tm3+,Yb3+, except for changes in lattice parameters. The EDX images (Figure S1) reveal that the chemical compositions of the as-synthesized samples contain Bi, W, O, Tm, Yb, and Mg. The Li content is not detected because of its small atomic number (M=3). 8

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Therefore, pure phase materials are synthesized completely.

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Figure 2. FE-SEM images of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+ phosphors doped with: (a) 0, (b) 3 mol % Li+, and (c) 3 mol % Mg2+.

3.3 Upconversion emission spectra of Tm3+- and Yb3+-codoped Bi3.84W0.16O6.24

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phosphors in the visible region

Figure 3 presents the emission spectra of Bi3.84W0.16O6.24: xTm3+, yYb3+ with

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excitation at 980 nm. Four distinct emission bands (450–740 nm) peaking at ~ 476, 483, 653, and 696 nm are derived from the 1G4(2)→3H6, 1G4(1)→3H6, 1G4→3F4, and F2,3→3H6 transitions of Tm3+ ions, respectively. Among these emission bands, strong

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blue emission (476 and 483 nm) and red emission (696 nm) are clearly observed. Thus, future research will be focused on these emission bands. As shown in Figure 3 (a), the phosphors were prepared with a fixed concentration (10 mol %) of Yb3+ and different concentrations of Tm3+ ions (0.1, 0.3, 0.5, and 0.7 mol %). UC emission intensity initially increased with Tm3+ concentration and then decreased after reaching 0.3 mol % because of concentration quenching. When Tm3+ concentration 9

ACCEPTED MANUSCRIPT exceeded the optimum value (0.3%), the distance between the Tm3+ and Yb3+ ions decreased, thereby strengthening non-radiative relaxation and diminishing UCL intensity [35]. For a constant x = 0.3 mol % and variable Yb3+ content, Figure 3(b)

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presents the same change trend of the emission spectra of the sample under varied and fixed Yb3+ concentrations. UC emission intensity started to decrease as the Yb3+ concentration exceeded 10 mol %. This decline was also attributed to concentration

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quenching. Finally, 0.3 mol % Tm3+ + 10 mol % Yb3+ was considered as the best

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combination for UC emission intensity.

Figure 3. UCL spectra of Bi3.84W0.16O6.24:xTm3+, yYb3+ under 980 nm excitation: (a) y = 10 mol %; x = 0.1, 0.3, 0.5 mol % in the range; (b) x = 0.3 mol %; y = 8, 10, 12 mol %. The inset shows the range from 350–450 nm.

3.4

Effects

of

Li+ or Mg2+ codoping on

upconversion

Bi3.84W0.16O6.24:Tm3+,Yb3+ phosphors in the visible region 10

emission

in

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codoping can substantially enhance the blue and red emissions at 476, 483, and 696 nm, which are assigned to 1G4(2)→3H6, 1G4(1)→3H6, and 3F2,3→3H6 transitions of Tm3+ ions, respectively. The emission intensity reaches the maximum at the Li+

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concentration of 3 mol % and then starts to decrease beyond this point. As shown in

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Figure 4(c), the blue and red emission intensities are approximately 7 and 27 times that of the Li+-free samples. Figures 6(b) and S3 depict the dependence of emission intensity on pump power, and the fitted slopes (n values) are 1.24, 1.11, and 2.00 for the 476, 483, and 696 nm emissions of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol %

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Yb3+, 3 mol % Li+ phosphors. Li+ ions without an energy level in the NIR region can neither absorb 980 nm light nor transfer energy to Tm3+ ions, indicating that Li+ ions cannot effect on the UC mechanism of Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors.

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However, the n value is slightly smaller than that of the Li+-free sample (Figure 6(b))

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at different Li+ concentrations. Given that a large UC rate corresponds to a small slope (n value), the introduction of Li+ ions can increase UC rate and subsequently enhance UC emission intensity [36-38]. Moreover, when Li+ ions are at the interstitial sites, electron distribution density and local crystal field symmetry around the rare-earth Tm3+ are changed, thus breaking the forbidden transition of rare-earth ions - enhances the luminescence intensity of Tm3+ [37].

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Figure 4. (a) UC emission spectra of Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphors in the absence and presence of different doping concentrations of Li+ ions under excitation in the ranges of 450–740 nm and (b) 350–450 nm; and (c) the

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corresponding integrated intensity IBlue and IRed under 980 nm irradiation.

(b) Upconversion luminescence of codoped Mg2+ ions Figure 5(a) shows that doping Mg2+ can significantly enhance the emission

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intensity of the Bi3.84W0.16O6.24: Tm3+, Yb3+ phosphors. The changes of emission

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intensity for different doping concentrations of Mg2+ (1, 3, 7, and 11 mol %) are investigated. The blue (476 and 483 nm) and red (696 nm) emission intensities are enhanced up to 7 and 23 times, respectively, at the optimum 3 mol % Mg2+ doping in Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+ phosphors (Figure 5 (c)). The weak green emission peaking at 542 nm is assigned to the 1D2→3H5 transition of Tm3+ ions [39].

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Figure 5. (a) UC emission spectra of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+, x mol % Mg2+ (x = 0, 1, 3, 7, 11 mol %) under 980 nm radiation; (b) in the range of 350–450 nm; and (c) the corresponding integrated intensities IBlue and IRed under 980

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nm irradiation.

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Figure 6. Log–log plots of the blue UCL intensity of Bi3.84W0.16O6.24 phosphors codoped with (a) Tm3+–Yb3+, (b) Tm3+–Yb3+–Li+, and (c) Tm3+–Yb3+–Mg2+ as a function of 980 nm pump power.

Figures 6(c) and S3 show the dual logarithmic plots of pump power and emission 14

ACCEPTED MANUSCRIPT intensity. The slopes (n values) of 476, 483, and 696 nm emission peaks were 1.13, 1.00, and 2.02, which were smaller than those of the Mg2+-free sample and identical to those of Li+-codoped phosphors. The radii of Mg2+ (0.72 Å), Tm3+ (0.88 Å), and

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Yb3+ (0.87 Å) were smaller than that of Bi3+ (1.03 Å). A valence mismatch was observed between Mg2+ and rare-earth ions. Therefore, the introduction of Mg2+ ions may result in lattice shrinkage and modify the local field symmetry around rare-earth

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3.5 Upconversion luminescent mechanism

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ions, thus enhancing the UC emission intensities [40, 41].

The energy level diagrams of Tm3+ and Yb3+ shown in Figure 7 explain the mechanism of the UCL of Bi3.84W0.16O6.24: Tm3+, Yb3+ phosphors. Yb3+ ions were excited (2F7/2→2F5/2) under 980 nm excitation. Given that the 2F5/2 state was closely

ET1 proceeded as

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associated with the energy transition (ET) between the 3H6 and 3H5 states of Tm3+ ions, F5/2 (Yb3+) +

3

H6 (Tm3+) →

2

F7/2 (Yb3+) +

3

H5 (Tm3+).

Non-radiative relaxation subsequently populated the 3F4 level from 3H5 to 3F4. As the

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Yb3+ ions absorbed a second 980 nm photon, the populations of the 3F2,3 level of Tm3+

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ions were enhanced by 3F4→3F2,3 (ET2). Red light (696 nm) was emitted by radiative transitions from the excited state (3F2,3) to the ground state (3H6). The third ET (ET3) contributed to the population of high 1G4 level and simultaneously produced blue (483 and 476 nm) and red (653 nm) emissions [42]. To investigate the UC mechanism, we determined the relationship between the excitation power (P) and the UCL intensity (I) (Figure 6)). This correlation could be described by the following relationship in the unsaturated UC process [43]: 15

ACCEPTED MANUSCRIPT I UC ∝ (P pump ) n

(1)

where n was equal to the required pump photons. The slopes (n values) for 478, 484, and 653 nm emission were 1.27, 1.14, and 1.55, respectively (Figures 6(a) and S2),

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which were inconsistent with the three photons required for generating blue and red emissions. The n value of the 696 nm emission was 2, thus demonstrating the two-photon UC process in which two NIR excitation photons were absorbed to emit

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one red UC photon (Figure S3). The lower n value for the blue emission could be due

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to the saturation effect [44-46]. A simplified rate equation model that can verify the corresponding luminescence mechanism is presented in Figure 7. This model is operated under the following assumptions: (1) the steady-state population density of a state is associated with luminescence intensity; (2) exclusive emission transitions

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occur from state |i > of the acceptor ions to lower states.

Figure 7. Energy level diagram of Yb3+/Tm3+-codoped Bi3.84W0.16O6.24 phosphors under 980 nm excitation and possible UC processes.

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(2)

Excitation is independent of laser power. In view of the energy transfer from the

the same energy. Therefore, in the steady state:

(3)

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N1∝NS

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excited state of the sensitizer ions to |1> of the acceptor ions, the two states possess

This assumption implies that the population of state |1> of the acceptor ions scale linearly with laser power. Under these assumptions, the rate equations for the excited-state population densities Ni of the acceptor ions (Tm3+) in this paper are given

dN 2 dN 3

= A2 N 2 − N1 N S + A5' N 5 + A6 ' N 6

(4)

dt

= W1 N0 N S − A2 N 2

(5)

dt

= A4 N 4 − W3 N 3 N S − A3 N 3

(6)

dt

= W2 N 1 N S − A4 N 4

(7)

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dN 4

dt

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dN1

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as follows:

dN 5 dN 6

dt

= W3 N 3 N S − A5 N 5 − W4 N 5 N S − A5' N 5 − A5" N 5

(8)

dt

= W4 N 5 N S − A6 N 6 − A6 ' N 6

(9)

where N0, N1, N2, N3, N4, N5, and N6 are the populations of the levels of 3H6, 3F4, 3H5, 3

H4, 2F2,3, 1G4, and 1D2, respectively. W1, W2, W3, and W4 denote the energy transfer

rates of ET1, ET2, ET3, and ET4, respectively. Ai (A2: 3H5→3F4; A3: 3H4→3H6; A4: 3

F2,3→3H6; A5: 1G4(1)→3H6; A5’: 1G4(1)→3F4; A5’’: 1G4(2)→3H6; A6: 1D2→3F4; A6’: 1D2→ 17

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F4) is the radiative decay rate. Under steady-state excitation, the rate equations can be

written as follows: (10)

W1 N 0 N S − A2 N 2 = 0

(11)

A4 N 4 − W3 N3 N S − A3 N3 = 0

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A2 N2 − N1 N S + A5 ' N5 + A6" N6 = 0

(12)

W2 N1 N S − A4 N 4 = 0 W3 N3 N S − A5 N5 − W4 N5 N S − A5' N5 − A5" N5 = 0

(14)

(15)

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W4 N5 N S − A6 N6 − A6' N6 = 0

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(13)

The saturation effect refers to the competition between the linear decay and UC. The UC process is dominant for the 1G4 state of Tm3+. In consequence, linear decay is neglected (W4 N5 ≫ A5 ). Thus, the equation describing the population density of state

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|5> can be simplified from Eq. (12) to

N5 =

W3 N ∝ N3 W4 3

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By combining Eq. (2) with Eq. (3), this equivalence can be written as N5 ∝Ns ∝P1

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In conclusion, the slope for the blue emission is close to 1. Meanwhile, we observe the 439, 362, and 542 nm emission of Yb3+/Tm3+-codoped Bi3.84W0.16O6.24 phosphors from the 1D2 level to the ground state 3H6 and the intermediate 3F4 and 3H5 states, respectively (inset of Figures 3(a) and 3(b)). And the 439, 362, and 542 nm emission of Li+ or Mg2+ ion-doped bismuth tungstate phosphors are shown in Figures 4(b) and 5(b). Populating the higher-state 1D2 may result in the exit of the fourth energy transition (ET4) state following 2F5/2 (Yb3+) + 1G4 (Tm3+) → 2F7/2 (Yb3+) + 1D2 18

ACCEPTED MANUSCRIPT (Tm3+). Finally, the saturation effect of the 1G4 level in the Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphor can be demonstrated theoretically and experimentally. 3.6 Temperature sensing behavior in the NIR region

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As shown in Figure 8, the NIR emission of Yb3+/Tm3+-codoped Bi3.84W0.16O6.24 phosphors were divided into two main emission bands peaking at 796 and 804 nm (3H4→3H6), and NIR emission intensity was enhanced by codoping Li+ and Mg2+ ions.

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After ET2, the 3F2,3 level was rapidly depopulated to lower-lying 3H4 by non-radiative

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route, and NIR light (796 and 804 nm) was emitted by radiative transitions from the excited state (3H4) to the ground state (3H6). Figure S5 showed the log–log plot of the pump power (P) and intensity (I) of the NIR UC emission. According to Eq. (1), the slope values (n values) of the 796 and 804 nm emissions were 1.86 and 1.76

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(Bi3.84W0.16O6.24:Tm3+,Yb3+), 1.78 and 1.71 (Bi3.84W0.16O6.24:Tm3+,Yb3+,Li+), and 1.68 and 1.65 (Bi3.84W0.16O6.24:Tm3+,Yb3+,Mg2+), respectively. Therefore, the NIR UC was

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a two-photon process.

Figure 8. UCL spectra of Bi3.84W0.16O6.24:xTm3+,yYb3+ under 980 nm excitation: (a) y 19

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To our knowledge, limited studies have explored the TCLs (3H4(1) and 3H4(2) ) of Tm3+. The population of TCLs obeys the Boltzmann distribution [47, 48]. The temperature-dependent UC emission spectra of free-, Li+ ion-, or Mg2+ ion-doped

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Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+ phosphors from 313 K to 573 K at

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0.2 W are shown in Figures 9(a) and S4. The recorded UC emission mappings of bismuth tungstate in the range of 720–900 nm in BWI are displayed in Figure 9(b). The FIR (I796/I804) varies with the temperature gradually increasing, suggesting the excellent potential application of thermo-responsive energy level (3H4(1) and 3H4(2)) of

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Tm3+ in thermometry. In the view of thermal excitation occurred between the sub-stark energy levels of 3H4 level, the FIR of the NIR UC emission from 3H4(1), H4(2)→3H6 transitions can be written as [49]

FIR =

N ( 3H 4( 2) ) 3

=

 − ∆E   − ∆E  I 796 g 2σ 2 jω 2 j = exp  = B exp  I 804 g 1σ 1 jω1 j  K BT   K BT 

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3

N ( H 4(1) )

(16)

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where N is the ion number, g is the degeneracy, σ is the emission cross-section, and ω is the angular frequency. I2j and I1j are the luminescence intensities of 3H4(2) → 3H6 and 3H4(1) → 3H6, respectively. ∆E, kB, and T are the energy difference between the 3

H4(2) and 3H4(1) levels, the Boltzmann constant, and the absolute temperature,

respectively [50].

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Figure 9. (a) Temperature evolution of the Tm3+ UC emission spectra excited by 980 nm laser for the Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+ phosphor; (b) temperature-dependent UCL mappings from 313 K to 573 K.

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Figures S6(a), S6(c), and S6(e) show that the FIR of Li+/Mg2+ and Li+/Mg2+-free codoped bismuth tungstate gradually increase with the variation in temperature and

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that the experimental data are fitted to a straight line with slopes of 96.6, 136.3, and 92.5. These values are equal to

∆E KB

. The relationship between the FIR and the

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temperature is displayed in Figures S6(b), S6(d), and S6(f). The calculated B values are 0.98, 1.20, and 1.05. Absolute sensitivity is an important parameter for assessing thermometric capacity, and the expression is written based on Eq. (13) [51]:

S=

 ∆E  dFIR = FIR  dT  K BT 

(17)

Figure 10 illuminates the changes of the absolute sensitivity for Tm3+–Yb3+-, Tm3+– Yb3+–Li+-, and Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphors with increasing 21

ACCEPTED MANUSCRIPT temperature from 313 K to 573 K. The corresponding optimum sensitivities are 0.000676, 0.00103, and 0.000696 K−1 at 323 K. The tissues of tumor cells are severely damaged within several minutes at the temperature between 313 and

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323 K because of photothermal ablation [52-54]. In this work, sensitivity can be reduced with temperature increasing, and its maximum value can be reached in the biological temperature range of tumor cell damage (298–318 K). For

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comparing the temperature sensitivity of the as-prepared samples with that of

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other materials, the maximum sensitivities of different phosphors are listed in Table 2. In the NIR region, the Li+/Mg2+-doped Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors exhibit better sensitivities compared with other materials. Therefore, doping Li+ or Mg2+ ion can improve absolute sensitivity of UCL materials in NIR

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range. The value of S is dependent on ∆E and FIR. The introduction of Li+ and Mg2+ ions can lead to lattice shrinkage and modification of the local field symmetry around the rare-earth ions, and ∆E is constant in the same host. Therefore, the enhancement

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of UCL intensity increases the value of FIR and simultaneously contributes to high

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sensitivity. The measurement error for estimating temperature sensing can be calculated using Eq. (18) according to standard deviation FIR and sensitivity [55]:

 K BT 2  ∆Τ = ∆FIR ×    FIR∆E 

(18)

where ∆T is the error in the measured temperature and ∆FIR is variation with FIR. The obtained errors in measured temperature range are ± 0.41, ± 0.16, and

± 0.19 K for the free-, Li+ ion-, and Mg2+ ion-codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ 22

ACCEPTED MANUSCRIPT phosphors, respectively. This finding implies that the 3H4 level of Tm3+ in the Tm3+– Yb3+-, Tm3+–Yb3+–Li+-, and Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 microcrystals

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can be applied as TCLs for monitoring the temperature of pathological tissues.

Figure 10. Sensor sensitivity S = dFIR/dT as a function of temperature for Tm3+–

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Yb3+-, Tm3+–Yb3+–Li+-, and Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphor.

Table 2. Comparison of the maximum sensitivity among different materials in

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Materials

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the NIR region.

NaYbF4:Tm3+@ SiO2 Tm/Yb-codoped glass ceramic LiNbO3:Tm3+, Yb3+ Bi3.84W0.16O6.24:Tm3+, Yb3+ Bi3.84W0.16O6.24:Tm3+, Yb3+, Li+ Bi3.84W0.16O6.24:Tm3+,Yb3+,Mg2+

Transitions

3

F2,3, 3H4 → 3H6 3 F2,3, 3H4 → 3H6 3 F2,3, 3H4 → 3H6 3 H4(1),3H4(2)→ 3H6 3 H4(1),3H4(2)→ 3H6 3 H4(1),3H4(2)→ 3H6

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Temperature range (K) 100–700 293–703 323–773 323–573 323–573 323–573

S(max)(10−5 K−1) 54 30 24 68 103 70

Refs. [56] [57] [58] This work This work This work

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Conclusion The possible combination of strong luminescence and high temperature sensitivity was achieved for Tm3+- and Yb3+-codoped Bi3.84W0.16O6.24 phosphors through the

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introduction of alkali metals or alkaline-earth metals. Here, alkali metal or alkaline earth metal doping not only strongly enhanced the blue and red emissions of Bi3.84W0.16O6.24: Tm3+ /Yb3+ phosphors but also influenced their particle size and

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uniformity. Temperature sensing based on TCLs (3H4 level of Tm3+ ions) of the alkali

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metal or alkaline earth metal-codoped Bi3.84W0.16O6.24 phosphors in the first biological window was evaluated from the FIR of the NIR emission. The optimum sensitivities were 0.000676, 0.00103, and 0.000696 K−1 at 323 K for the free-, Li+, and Mg2+ ion-doped Bi3.84W0.16O6.24 phosphors. The sensitivities of the as-prepared samples

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gradually decreased with increasing temperature. These results indicated that the prepared bismuth tungstate phosphors exhibited potential applications in monitoring the temperature of pathological tissues within the biological temperature range for

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damaging tumor cells (298–318 K).

Acknowledgments

This work was supported by the Science and Technology Development Planning Project of Jilin Province (20160101294JC) and partially sponsored by the Foundation of State Key Laboratory of Luminescence and Applications.

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ACCEPTED MANUSCRIPT Table captions Table 1. Lattice constants of Tm3+–Yb3+/ Tm3+–Yb3+–Li+/Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphors.

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Table 2. Comparison of the maximum sensitivity among different materials in the NIR region. Figure captions

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Figure 1. X-ray diffraction patterns of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol %

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Yb3+ phosphors doped with: (a) 0, (b) 3 mol % Li+, and (c) 3 mol % Mg2+; (d) evolution of the main (111) peak, and reference data JCPDS 43-0447. Figure 2. FE-SEM images of Bi3.84W0.16O6.24:0.3 mol % Tm3+,10 mol % Yb3+ phosphors doped with: (a) 0, (b) 3 mol % Li+, and (c) 3 mol % Mg2+.

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Figure 3. UCL spectra of Bi3.84W0.16O6.24:xTm3+,yYb3+ under 980 nm excitation: (a) y = 10 mol %; x = 0.1, 0.3, 0.5 mol % in the range; (b) x = 0.3 mol %; y = 8, 10, 12 mol %. The inset shows the range from 350 nm to 450 nm.

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Figure 4. (a) UC emission spectra of Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphor

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in the absence and presence of different Li+ doping concentrations under excitation in the ranges of 450–740 and (b) 350–450 nm and (c) the corresponding integrated intensities IBlue and IRed under 980 nm irradiation. Figure 5. (a) UC emission spectra of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+, x mol % Mg2+ (x = 0, 1, 3, 7, 11) under 980 nm radiation, (b) in the range of 350–450 nm, and (c) with the corresponding integrated intensities of IBlue and IRed under 980 nm irradiation. 34

ACCEPTED MANUSCRIPT Figure 6. log–log plots of the blue UCL intensity of (a) Tm3+–Yb3+, (b) Tm3+–Yb3+– Li+, (c) Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphors as a function of 980 nm pump power.

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Figure 7. Energy level diagram of Yb3+/Tm3+-codoped Bi3.84W0.16O6.24 phosphors under 980 nm excitation and possible UC processes.

Figure 8. UCL spectra of Bi3.84W0.16O6.24:xTm3+,yYb3+ under 980 nm excitation: (a) y

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= 10 mol %; x = 0.1, 0.3, 0.5 mol % (b) x = 0.3 mol %; y = 8, 10, 12 mol % in the

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range of 720–900 nm.

Figure 9. (a) Temperature evolution of the Tm3+ UC emission spectra excited under 980 nm laser for Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+ phosphor, and (b) temperature-dependent UCL mappings from 313 K to 573 K.

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Figure 10. Sensor sensitivity S = dFIR/dT as a function of temperature for Tm3+–

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Yb3+-, Tm3+–Yb3+–Li+-, and Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphor.

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Supporting Information A Novel Upconversion Luminescent Material: Li+- or Mg2+-Codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ Phosphors and Their Temperature Sensing

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Properties Zhen Suna, Guofeng Liua, Zuoling Fu a,*, Zhendong Haob, and Jiahua Zhangb,* a

Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of physics

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

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China, Fax: +86-431-85167966; Tel: +86-431-85167966; E-mail: [email protected] (Z. L. Fu) State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine

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Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road,

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Changchun, 130033, China；E-mail：[email protected] (J. H. Zhang)

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Figure S1. EDX images of Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+

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phosphors doped with: (a) 0, (b) 3 mol % Li+, and (c) 3 mol % Mg2+.

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Figure S2. Log–log plots of the red UCL intensity at 653 nm of Tm3+–Yb3+/Tm3+–

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980 nm pump power.

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Yb3+–Li+/ Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphors as a function of

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Figure S3. Log–log plots of the red UCL intensity of Tm3+–Yb3+/Tm3+–Yb3+–

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pump power.

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Li+/Tm3+–Yb3+–Mg2+-codoped Bi3.84W0.16O6.24 phosphors as a function of 980 nm

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Figure S4. Temperature evolution of the Tm3+ UC emission spectra excited by

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980 nm laser for (a) Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+, 3 mol % Li+;

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(b) Bi3.84W0.16O6.24:0.3 mol % Tm3+, 10 mol % Yb3+, 3 mol % Mg2+ phosphor.

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Figure S5. Log-log plots of the NIR UC emission intensities (3H4→3H6 of Tm3+) of

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Bi3.84W0.16O6.24 phosphors codoped with (a) Tm3+,Yb3+; (b) Tm3+,Yb3+, and Li+; (c)

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Tm3+,Yb3+, and Mg2+ as a function of 980 nm excitation power.

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Figure S6. Monolog plot of the FIR versus 1/T and FIR (I796/I804) of Tm3+ NIR

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emissions for the 3H4(2)/3H4(1) → 3H6 transitions relative to the temperature range of 313–573 K for Bi3.84W0.16O6.24 phosphors coped with ((a) and (b)) Tm3+–Yb3+, ((c)

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and (d)) Tm3+–Yb3+–Li+, and ((e) and (f)) Tm3+–Yb3+–Mg2+ .

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ACCEPTED MANUSCRIPT Highlights Li+ or Mg2+ Co-doped Bi3.84W0.16O6.24: Tm3+, Yb3+ phosphors are synthesized by co-precipitation methods.

the luminescence and temperature sensitivity.

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Alkali metals and alkaline earth metals codoping provides a novel mean to enhance

The thermally coupled levels based on 3H4 level of Tm3+ ions are used to research

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temperature sensitivity.