Yb3+ phosphors

Yb3+ phosphors

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 217 (2019) 107–112 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 217 (2019) 107–112

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Concentration effect on up-conversion luminescence and excitation path-dependent luminescence temperature quenching in YNbO4:Ho3+/Yb3+ phosphors Xin Wang a, Xiangping Li a,⁎, Rensheng Shen b, Sai Xu a, Lihong Cheng a, Jiashi Sun a, Jinsu Zhang a, Baojiu Chen a a b

College of Science, Dalian Maritime University, Dalian, Liaoning, 116026, PR China School of Microelectronics, Dalian University of Technology, Dalian, Liaoning, 116024, PR China

a r t i c l e

i n f o

Article history: Received 22 January 2019 Received in revised form 23 March 2019 Accepted 24 March 2019 Available online 24 March 2019 Keywords: YNbO4: Ho3+/Yb3+ phosphors Up-conversion luminescence Thermal quenching

a b s t r a c t Usually, the luminescence intensity and mechanism of rare-earth ions doped materials are dependent on both doping concentration and sample temperature. In this study, we attempt to explore the concentration effect on up-conversion (UC) luminescence and the dependence of luminescence temperature quenching on excitation wavelength in YNbO4: Ho3+/Yb3+ phosphors. The YNbO4: Ho3+/Yb3+ phosphors with various Ho3+ and Yb3+ concentrations were synthesized via a high-temperature solid-state reaction technique. Intense green UC emission peaked at 543 nm was observed, accompanying with weak red and near infrared (NIR) UC emissions centered at 659 and 745 nm. Based on the laser working current dependence of UC luminescence, two-photon processes were responsible for both the green and the red UC emissions under 980 nm excitation, which have no apparent dependence on both Ho3+ and Yb3+ concentrations. According to the Arrhenius model, crossover process was responsible for the temperature-dependent down-conversion (DC) luminescence quenching of Ho3+ under 452 nm excitation. However, the temperature quenching processes of the green and the red UC luminescence cannot be well explained by Arrhenius model. It was found that the UC luminescence intensity decayed with increasing sample temperature, which was caused by both the crossover and temperaturedependent energy transfer processes. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Recently, photon up-conversion (UC), in which process two or more lower energy photons convert into one higher energy photon, has attracted growing interest owing to its wide applications in many fields, such as medical diagnostics, three-dimensional solid-state multicolor display, sensors, high density optical storage and so on [1–5]. Rare-earth (RE) ions are extensively used as the UC luminescent centers owing to their rich energy levels and excited durability. Meanwhile, the energy levels of some RE3+ ions match well with the near-infrared (NIR) outputs of commercialized semiconductor lasers [6–9]. In order to get intense UC luminescence, it is highly essential to choose proper RE 3+ ions as activators and sensitizers. Among the numerous RE3+ ions, Ho3+ ion with dominant green UC emission has received considerable attention in recent years [10]. However, there is no matched energy levels of Ho3+ can absorb the output energy of the commercial laser ~980 nm. Therefore, Yb3+ ion is often used as a ⁎ Corresponding author. E-mail address: [email protected] (X. Li).

https://doi.org/10.1016/j.saa.2019.03.080 1386-1425/© 2019 Elsevier B.V. All rights reserved.

sensitizer ion of Ho3+ due to its large absorption cross section at 980 nm and the simple electronic energy level scheme, which has only one excited state 2 F5/2 level. Yb 3+ can transfer the absorbed energy to Ho3+ ion, achieving strong UC luminescence of Ho 3+ [11, 12]. Choosing appropriate host matrix is another main point to obtain high efficient UC luminescence. Among various host matrices, LnNbO4 (Ln = La, Lu, Gd, Y) with fergusonite structure are promising candidates for UC luminescence hosts thanks to their low phonon energy, superior chemical and mechanical stabilities [13–15]. In this paper, Ho3+/Yb3+ co-doped YNbO4 phosphors with various doping concentrations were prepared by a traditional hightemperature solid-state reaction method. The concentration and temperature-dependent UC luminescence properties excited by 980 nm laser were investigated systematically. Based on the dependence of UC luminescence on laser working current and energy level diagrams of Ho3+ and Yb3+, the possible UC luminescence processes of Ho 3+ were discussed in detail. Furthermore, the luminescence temperature quenching behavior of Ho 3+ in YNbO4: Ho3+/Yb3+ phosphors under 452 and 980 nm excitations were discussed.

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2. Experimental details 2.1. Sample preparation YNbO4: x mol% Ho3+/6 mol% Yb3+ (where x = 0.05, 0.1, 0.3, 0.5, 1, 3 and 5) and YNbO4: 0.5 mol% Ho3+/y mol% Yb3+ (where y = 1, 3, 6, 8, 10, 15 and 20) phosphors were synthesized by a traditional high temperature solid-state reaction method in air atmosphere. The starting materials including Y2O3 (99.99%), Ho2O3 (99.99%), and Yb2O3 (99.99%) were obtained from Shanghai Second Chemical Reagent Factory (China), Nb2O5 (99.99%) was purchased from Tianjin Reagent Chemicals Co., Ltd. (China). They were weighed, well mixed and calcined at 1300 °C for 4 h. The specific preparation method can be referred to Ref. [16]. 2.2. Sample characterization The phase identification of the as-synthesized samples was performed by LabX XRD-6000 diffraction with CuKα1 (k = 0.15406 nm) as the radiation resource. The XRD data were collected at a scanning step of 0.02°/s in the 2θ range of 20°–70°. The UC luminescence spectra were obtained by a Hitachi F-4600 fluorescence spectrometer and an external fiber-output power-controllable 980 nm laser was introduced as the excitation source. The sample temperatures were well controlled by a temperature controlling (DMU-450) system assembled in our lab in the temperature range of 273–723 K. 3. Results and discussion 3.1. Crystal structure In order to identify the crystal structure of the as-obtained products, XRD measurements were performed for all samples. It was found that all samples displayed similar diffraction patterns. As representatives, the XRD patterns of the samples doped with x mol% Ho3+/6 mol % Yb3+ (x = 0.5, 1 and 5) and 0.5 mol% Ho3+/y mol% Yb3+ (y = 6, 10 and 20) are displayed in Fig. 1, meanwhile for comparing reason the XRD pattern for the monoclinic YNbO4 powder plotted by using the data reported in PDF card No. 83-1319 was also shown here. By carefully inspecting the diffraction patterns of the samples and the standard data appearing in PDF card No. 83-1319, it is found that

3+

3+

3+

3+

Normalized Intensity(a.u.)

0.5 mol % Ho /20 mol% Yb

0.5 mol % Ho /10 mol% Yb 3+

3+

3+

3+

3+

3+

0.5 mol % Ho /6 mol% Yb

1 mol % Ho /6 mol% Yb

20

(021)

30

40

50

(002)

(-261) (-423) (-352) (-421) (061) (-442) (-223) (221) (042)

(-242) (240)

(-312) (220) (-112) (111) (041)

JCPDS No. 83-1319

(040) (-202) (200)

(-221)

5 mol % Ho /6 mol% Yb

60

70

2 (degree) Fig. 1. XRD patterns for the prepared YNbO4: x mol% Ho3+/6 mol% Yb3+ (x = 0.5, 1, and 5) and YNbO4: 0.5 mol% Ho3+/y mol% Yb3+ (y = 6, 10, and 20) phosphors and the standard data for monoclinic YNbO4 reported in JCPDS card No.83–1319.

monoclinic phase YNbO 4 phosphors are obtained. Based on the XRD measurement results, lattice parameters of the YNbO4: x mol% Ho3+/y mol% Yb3+ (x = 0.5, y = 6, 10 and 20; y = 6, x = 1 and 5) samples were calculated and are shown in Table 1. It can be found that the lattice parameters of the samples changed slightly with the increase of both Ho3+ and Yb3+ concentrations, which confirms the introduction of Ho3+ and Yb3+ does not change the crystal structure of YNbO4 host obviously. 3.2. Concentration effect on UC luminescence As is well known, the UC luminescence intensity of the phosphor strongly depends on the doping concentration of RE3+ ions. In order to obtain the optimum doping concentrations of Ho3+, Yb3+ and recognize the concentration dependent UC luminescence quenching behavior of Ho3+ in YNbO4: Ho3+/Yb3+, Ho3+ and Yb3+ concentrations dependent UC luminescence spectra of YNbO4 : Ho3+/Yb 3+ phosphors under 980 nm excitation were measured and are given in Fig. 2(a) and (b). The insets of Fig. 2 show the dependence of the integrated intensities of the green and the red UC emissions on Ho3+ and Yb3+ concentrations. From Fig. 2, the green, red and NIR emission bands peaked at 545, 656 and 760 nm corresponding to ( 5F4 , 5 S2) → 5I8, 5F5 → 5I8 and (5F4, 5S2) → 5I7 transitions of Ho3+ are observed in each spectrum [17]. It is observed that the UC luminescence intensities of Ho3+ have a different change trend with an increase of Ho3+ and Yb3+ concentrations. Nevertheless, for fixed Ho3+ or Yb3+ concentrations, both the green and the red UC emission intensities have the same dependency on Ho 3+ and Yb 3+ concentrations. As Ho3+ concentration increases, both the green and the red UC emission intensities increase first and reach their maximum values when Ho3+ concentration reaches 0.5 mol%. Once Ho3+ concentration exceeds 0.5 mol%, both the green and the red UC emission intensities decrease, thus indicating that the concentration quenching happens. For the samples doped with fixed Ho3+ content (0.5 mol %) but variable Yb3+ contents, both the green and the red UC emission intensities keep increase with an increase in Yb3+ concentrations in the studied concentration range. When Yb3+ concentration exceeds 8 mol%, both the green and the red UC emission intensities increase slowly and almost reach saturation state, implying concentration quenching also occurs. With an increase in the doping concentration of Ho3+ or Yb3+, the interactions between Ho3+ ions or Ho3+ and Yb3+ ions accelerate due to the distances between Ho3+ and Ho3+ or Ho3+ and Yb3+ shorten, which may easily cause the depopulation of those emitting levels and then result in the quenching of the UC luminescence [18]. From the results presented in Fig. 2 (a) and (b), the optimum doping concentrations of Ho3+ and Yb3+ are determined to be 0.5 mol% and 20 mol% in the studied concentration range, respectively. 3.3. Possible UC luminescence mechanisms of Ho3+ excited by 980 nm Ho3+/Yb3+ co-doped phosphors are typical green UC luminescence materials. In order to investigate the UC luminescence mechanism of Ho3+ under 980 nm excitation in YNbO4: Ho3+/Yb3+ phosphors, the UC luminescence spectra for the samples doped with x mol% Ho3+/ 6 mol% Yb3+ (x = 0.05, 0.5, and 5) and 0.5 mol% Ho3+/y mol% Yb3+ (y = 1, 10 and 20) were measured at various 980 nm laser working currents, and the integrated intensities of the green and the red UC emissions were recorded and the results are shown in Fig. 3. The NIR emission was not taken into account here because its intensity is too weak, which may induce large calculation error. Moreover, the corresponding emitting levels of the NIR UC emission are the same with that of the green one, and then the emitting levels have the same population routes. As for the unsaturated condition, the UC luminescence intensity (Iup) is proportional to the laser excitation power, which depends linearly on

X. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 217 (2019) 107–112

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Table 1 Lattice parameters of the YNbO4: x mol% Ho3+/y mol% Yb3+ (x = 0.5, y = 6, 10 and 20; y = 6, x = 1 and 5) samples. Formula

Y0.795Ho0.005Yb0.2NbO4

Radiation type 2θ range Symmetry Space group Cell parameters

Y0.895Ho0.005Yb0.1NbO4

a = 7.5980 Å b = 10.8977 Å c = 5.2777 Å α = γ = 90° β =138.40° V = 293.44 Å3

Y0.935Ho0.005Yb0.06NbO4 Cu Kα1 radiation with λ = 1.5406 Å 10–70° Monoclinic system C2/c (15) a = 7.6146 Å b = 10.9377 Å c = 5.2982 Å α = γ = 90° β = 138.44° V = 293.16 Å3

a = 7.5957 Å b = 10.9091 Å c = 5.2834 Å α = γ = 90° β = 138.42° V = 293.61 Å3

its working current (iLD), thus the relationship between Iup and iLD can be written as [19]: n

Iup ∝ðaiLD –bÞ

ð1Þ

(a) 5

5

F4, S2

5

I8

5

F5

Green Red

0

1

2

3

4

Intensity (a.u.)

Integrated Intensity (a.u.)

where n is the photon numbers required for populating the emitting level, a and b are exponential constants. By fitting the experimental data in Fig. 3 with Eq. (1), the n values are obtained to be 1.85, 1.86 and 2.03 for the green UC emissions, 1.84, 1.82 and 1.76 for the red UC emissions for the samples doped with x mol% Ho3+/6 mol% Yb3+ (x = 0.05, 0.5, and 5), respectively. And the values of n are confirmed to be 1.79, 1.87 and 2.03 for the green UC emissions, 1.83, 1.75 and 1.78 for the red

5

3+

Ho concentration (mol%)

5

I8 5

5

F4, S2

5

I7

600 700 800 Wavelength (nm)

5

F4, S2

5

I8

0

5

10

15

co nc en tra tio n 20

3+

Yb concentration (mol%) 5

I8

5

5

F4, S2

5

I7

1.0 3.0 6.0 8.0 10 15 20

600

700

800

900

co nc en tra tio n

500

Yb

400

3+

(m

ol % )

F5

5

Y0.89Ho0.05Yb0.06NbO4

a = 7.6278 Å b = 10.9548 Å c = 5.3023 Å α = γ = 90° β = 138.41° V = 293.04 Å3

a = 7.5913 Å b = 10.9044 Å c = 5.2842 Å α = γ = 90° β = 138.35° V = 292.94 Å3

UC emissions of the samples doped with 0.5 mol% Ho3+/y mol% Yb3+ (y = 1, 10 and 20), respectively. It can be seen that all the fitted values are around 2, therefore, two-photon absorption processes are responsible for both the green and the red UC emissions of Ho3+ in YNbO4: Ho3 + /Yb3+ samples with various Ho3+ and Yb3+ concentrations under 980 nm excitation. That is to say, Ho3+ and Yb3+ concentrations almost have no obvious influence on the UC luminescence processes of Ho3+ in YNbO4: Ho3+/Yb3+ phosphors. From the above results, it is confirmed that two-photon absorption processes are responsible for both of the green and the red UC emissions of Ho3+ in YNbO4: Ho3+/Yb3+ phosphors. The energy levels of Ho3+ and Yb3+ as well as the commonly accepted mechanisms of the UC luminescence are drawn in Fig. 4 [20, 21]. In the case of the green and the NIR UC emissions, upon 980 nm laser excitation, Ho3+ is first excited to 5I6 or 5I5 level through ground state absorption (GSA) process or accepts the energy transferred from Yb3+ by energy transfer 1 (ET1) process. Generally, owing to the large absorption cross-section at 980 nm of Yb3+ ion than that of Ho3+ ion, ET1 process is more dominant than the GSA process [13, 22–26]. Then, (5F4, 5S2) levels of Ho3+ are populated through an excited state absorption (ESA) process from 5I6 level or another ET2 process from Yb3+ ion. Finally, the green and the NIR emissions can be obtained by radiative transitions from (5F4, 5S2) to 5I8 and 5 I7 levels, respectively [18]. There are two possible routes to populate the red emitting 5F5 level. One is that the populated Ho3+ in 5I6 level nonradiatively relaxes to 5I7 level and absorbs one 980 nm photon by ESA process or receives the energy from Yb3+ via ET3 process, achieving the population of 5F5 level, and then the red emission can be achieved by the transition from 5F5 to 5I8. The other possible route is that 5F5 level is populated by nonradiative relaxation from (5F4, 5S2) levels, then red UC emission can be achieved. 3.4. Luminescence temperature quenching behavior of Ho3+

Green Red

Intensity (a.u.)

(b) 5

900

5

Ho

500

Integrated Intensity (a.u.)

400

3+

(m ol % )

0.05 0.1 0.3 0.5 1.0 3.0 5.0

Y0.93Ho0.01Yb0.06NbO4

Wanglength (nm) Fig. 2. UC luminescence spectra for YNbO4 samples doped with various concentrations of Ho3+ (a) and Yb3+ (b) excited by 980 nm laser. The insets show the dependences of the integrated intensities of the green and the red UC emissions on Ho3+ and Yb3+ concentrations.

Temperature has obvious influence on the luminescent performance of the phosphors [27, 28]. Temperature-dependent luminescence is one of the most important properties for evaluating the performance of the luminescent materials. In order to recognize the luminescence temperature quenching behavior of Ho3+ in YNbO4: Ho3+/Yb3+ phosphors during the DC and UC luminescence processes, the temperaturedependence of DC and UC luminescence spectra for the samples doped with 5 mol% Ho3+/6 mol% Yb3+ and 0.5 mol% Ho3+/10 mol% Yb3+ were measured in the temperature region of 273–723 K with an increment of 30 K under 452 nm (which corresponds to the 5I8 → 5F3 transition of Ho3+) and 980 nm excitations, and the results are shown in Fig. 5. It should be mentioned that to avoid the thermal effect caused by the 980 nm laser irradiation, each UC luminescence spectrum was measured instantaneously when the sample was irradiated by 980 nm laser with different working current, and after each measurement the laser was turned off right away and waited long enough time to begin the next measurement. For Ho3+, the energy gap between 5F3 level and (5F4, 5S2) levels is about 2000 cm−1. In higher maximum phonon

110

X. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 217 (2019) 107–112 3+

3+

3+

(a) 0.05 mol%Ho /6 mol%Yb

(b) 0.5 mol%Ho /6 mol%Yb

Integrated Intensity (a.u.)

Green Red Fitting Line

nG=1.85

nG=1.86

0.05

3+

Green Red Fitting Line

0.02

nG=2.03

nR=1.82 0.02 3+

(d) 0.5 mol%Ho /1 mol%Yb

3+

3+

nG=1.87

nR=1.83 0.07

3+

3+

(f) 0.5 mol%Ho /20 mol%Yb

Green Red Fitting Line

0.08

0.08

nR=1.76 0.08

(e) 0.5 mol%Ho /10 mol%Yb

Green Red Fitting Line

3+

(c) 5 mol%Ho /6 mol%Yb

Green Red Fitting Line

nR=1.84 0.04

nG=1.79

3+

3+

Green Red Fitting Line

0.05

nG=2.03

nR=1.75 0.04

0.07

nR=1.78 0.07

0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Current (A)

Current (A)

Current (A)

Fig. 3. Dependence of the integrated intensities of the green and the red UC emissions on the 980 nm laser working current for the samples doped with x mol% Ho3+/6 mol% Yb3+ (x = 0.05, 0.5, and 5) and 0.5 mol% Ho3+/y mol% Yb3+ (y = 1, 10 and 20).

energy materials, the ions populated at 5F3 level can easily relax to the lower (5F4, 5S2) levels via multi-phonon relaxation process, as shown in Fig. 4 [29]. As can be seen from Fig. 5(a) and (b), only green emission can be observed while the red emission is too weak to detect when excited by 452 nm, which demonstrates that 5F5 level cannot be populated through the non-radiative relaxation from higher levels (5F4, 5S2) or the populated electron in 5F5 level quickly non-radiative relaxation to those lower levels. Thus, it can be concluded that the population of the red emitting level 5F5 of Ho3+ was dominated by ESA or ET3 processes via the intermediate 5I7 level. In addition, it can also be found that the intensity of the green emission decreases with an elevation in temperature, indicating luminescence temperature quenching of Ho3+ occurs during the DC luminescence process [30]. In general, the thermal quenching of the DC luminescence can be evoked by temperature-dependent energy transfer, non-radiative

relaxation or crossover process [31]. As mentioned above, the population of the 5F5 level of Ho3+ was dominated by ESA or ET3 processes, thus the temperature-dependent non-radiative relaxation process can be neglected. Crossover process is a thermal excitation process, in which the electrons at the emitting levels can enter into the absorption band in the short wavelength region by overcoming an energy barrier (ΔE) through absorbing a certain energy, and then the electrons could relax non-radiatively to the ground state via an intersection between the absorption band and the ground state levels, leading to the thermal quenching of the luminescence. Crossover process has been recognized as the main quenching channel for the luminescence temperature quenching of Eu3+ and Dy3+ in lots of materials [32–34]. Here, we deduced that the thermal quenching of the DC luminescence of Ho3+ was also caused by the crossover process. In crossover process, the temperature-dependent luminescent properties can be described by a modified Arrhenius model [35]:

5

F3

20.0k

0.0

ESA

ESA

ET2 F5 ET3 5 I4 5 I ET1

Ho

3+

I0 1 þ Ce−ΔE=kT

ð2Þ

5

2

5

5

GSA

5.0k

IðT Þ ¼

5

F4, S2

5

CR

10.0k

452 nm 531 nm 646 nm 745 nm

15.0k

Energy (cm )

-1

5

F5/2

I6

980 nm

I7

5

I8

2

3+

Yb

F7/2

Fig. 4. Energy level diagrams of Ho3+ and Yb3+ and the possible mechanisms for the UC emissions of Ho3+ in YNbO4: Ho3+/Yb3+ phosphors.

where I(T) is the luminescent intensity at a certain temperature T, I0 means the initial luminescent intensity at 0 K, k stands for the Boltzmann constant, ΔE represents the active energy barrier for the thermal quenching process and C is a constant. In order explore whether the crossover process is responsible for the thermal quenching of the DC luminescence of YNbO4: Ho3+/Yb3+ phosphors, the temperature dependence of the green DC luminescence integrated intensities of different samples were calculated and are depicted in Fig. 6. Eq. (2) was used to fit the experimental data and the solid curves show the fitting results in Fig. 6. As can be seen, the experimental data can be well fitted by the modified Arrhenius equation and the activation energies for the green emissions are all obtained to be around 0.14 eV, suggesting that the thermal quenching of the DC luminescence of Ho3+ is mainly caused by the crossover process.

X. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 217 (2019) 107–112

Intensity (a.u.)

3+

3+

3+

(a) 5 mol%Ho /6 mol%Yb 333K 423K 513K 603K 693K

3+

(c) 5 mol%Ho /6 mol%Yb 5

333K 5F

DC emission

I8

5

UC emission

423K 513K 5

603K 693K 3+

3+

3+

Intensity (a.u.)

333K

5

I8

3+

333K

UC emission

DC emission 423K 5

513K 603K 693K

5

F5

I8

513K 5

603K 693K 550

I5

(d) 0.5 mol%Ho /10 mol%Yb

(b) 0.5 mol%Ho /10 mol%Yb

423K

111

600

650

500 550 600 650

Wavelength (nm)

I5

5

I8

850

900

Wavelength (nm)

Fig. 5. Temperature-dependent DC ((a), (b)) and UC ((c), (d)) luminescence spectra for YNbO4: 5 mol% Ho3+/6 mol%Yb3+and 0.5 mol% Ho3+/10 mol% Yb3+ samples under 452 nm and 980 nm excitations.

From Fig. 5(c) and (d), it can be seen that the UC luminescence intensity of the green emission also decreases dramatically with the increase of sample temperature, indicating that thermal quenching also occurs during the UC luminescence process. However, there is no mature theory to describe this kind of quenching behavior. Since the way of the thermal quenching of the DC luminescence has been identified, the thermal quenching route of the UC luminescence can be inferred by comparing the quenching rate of the DC and the UC green emissions. Fig. 6 depicts the normalized integrated intensities of the temperaturedependent green UC emission of different samples. It should be mentioned that the red dots and lines only show the change trend of the integrated intensities of the green UC luminescence towards temperature in the case of 980 nm excitation. By comparison, it can be found that the decrease rate of the green emission intensities under 980 nm excitation is faster than that of the case excited by 452 nm. Considering energy

3+

transfer plays an important role in the UC luminescence process of Ho3+, it is speculated that in addition to the crossover process, energy transfer is also an important quenching route for the thermal quenching of the UC luminescence of Ho3+. From Fig. 5(c) and (d), it can also be found that the change trend of the red UC luminescence intensity with the increase of the temperature is not the same as the green one. The temperature-dependent integrated intensities of the red UC luminescence of different samples were calculated and are depicted in Fig. 7(a). It is clearly seen that the red UC emission does not quench when the temperature is higher than room temperature, but it increases gradually with the increase of the sample temperature. Based on our previous research [16], it is found that the ET1 process from Yb3+ to Ho3+ includes two possible routes: 5F5/2 (Yb3+) + 5I8 (Ho3+) → 5F7/2 (Yb3+) + 5I5 or 5I6 (Ho3+). And with the increase of the temperature, the absorbed energy can be

3+

(a) red emission

(a) 5 mol% Ho /6 mol% Yb

ex ex

E = 0.142 eV

=452 nm =980 nm

3+

3+

(b) 0.5 mol% Ho /10 mol% Yb

Intensity (a.u.)

Normalized Intensity (a.u.)

3+

(b) NIR emission 3+

3+

5 mol% Ho /6 mol% Yb 3+ 3+ 0.5 mol% Ho /10 mol% Yb

E =0.141 eV

=452 nm

3+

5 mol% Ho /6 mol% Yb 3+ 3+ 0.5 mol% Ho /10 mol% Yb

ex

=980 nm

ex

300

400

500

600

700

Temperature (K) Fig. 6. Relationships of the normalized integrated intensities of the green emissions with the temperature for (a) YNbO4: 5 mol% Ho3+/6 mol%Yb3+and (b) 0.5 mol% Ho3+/ 10 mol% Yb3+ samples under 452 and 980 nm excitations.

300

400

500

600

700

Temperature (K) Fig. 7. Relationships of the integrated intensities of the red UC emission (5F5 → 5I8) (a) and the NIR UC emission (5I5 → 5I8) (b) with the temperature for YNbO4: 5 mol% Ho3+/6 mol% Yb3+ and 0.5 mol% Ho3+/10 mol% Yb3+ samples under 980 nm excitation.

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easier transferred to 5I5 (Ho3+) than that to 5I6 (Ho3+), which is benefit for obtaining the red UC emission via 1.5-photon process with the help of a cross-relaxation channel (5I5 + 5I8 → 5I7 + 5I7). As can be seen from Fig. 7(b), the integrated intensities of the NIR UC emission originated from 5I5 → 5I8 transition increase monotonically with the increase of the sample temperature, which is a good proof of our previous analysis on the UC luminescence mechanism of Ho3+ at high temperature. However, when the sample temperature exceeds 600 K, the intensities of the red UC emissions begin to decline slowly, indicating luminescence temperature quenching occurs, which may also be caused by the intense energy transfer and the crossover process at higher temperatures. 4. Conclusions In summary, Ho3+/Yb3+ co-doped YNbO4 phosphors with various doping concentrations were prepared by a high temperature solidstate reaction method. The crystal structures of the as-prepared phosphors were examined by XRD, and the results indicated that the products were monoclinic phase YNbO4. From lasers working current dependent UC luminescence, it was found that two-photon processes are responsible for both of the green and the red UC emissions in various Ho3+ and Yb3+ concentrations doped cases. Meanwhile, the temperature-dependent luminescence quenching of Ho3+ in both DC and UC luminescence processes were found. It was confirmed that the crossover process was responsible for the thermal quenching of the green DC luminescence of Ho3+, but in addition to the crossover process, energy transfer process also induced the thermal quenching of the UC luminescence of Ho3+. Acknowledgments This work was partially supported in part by the NSFC (National Natural Science Foundation of China, Grant nos. 11104023, 11774042 and 11704056), in part by the High-level personnel in Dalian innovation support program (2016RQ037), in part by the Scientific Research Foundation for Doctoral Program and the Natural Science Foundation of Liaoning Province (Nos. 20170520097 and 20180550553), in part by the Fundamental Research Funds for the Central Universities (Grant Nos. 3132019186 and 3132016333) and in part by the Open Fund of the State Key Laboratory of Integrated Optoelectronics granted (No. IOSKL2018KF02). References [1] E.A. Downing, L. Hesselink, R.M. Macfarlane, J.R. Klein, D. Evans, J. Ralston, A laserdiode-driven, three-color, solid-state 3-D display, Science. 273 (1996) 89–90. [2] X.M. Yin, H. Wang, M.M. Xing, Y. Fu, Y. Tian, T. Jiang, X.X. Luo, High color purity red emission of Y2Ti2O7: Yb3+, Er3+ under 1550 and 980 nm excitation, J. Lumin. 182 (2017) 183–188. [3] T. Grzyb, S. Balabhadra, D. Przybylska, M. Węcławiak, Upconversion luminescence in BaYF5, BaGdF5 and BaLuF5 nanocrystals doped with Yb3+/Ho3+, Yb3+/Er3+ or Yb3+/ Tm3+ ions, J. Alloys Compd. 649 (2015) 606–616. [4] H. Wang, X.M. Yin, M.M. Xing, Y. Fu, Y. Tian, X. Feng, T. Jiang, X.X. Luo, Investigation on the thermal effects of NaYF4: Er under 1550 nm irradiation, Phys. Chem. Chem. Phys. 19 (2017) 8465–8470. [5] Y. Dwivedi, K. Mishra, S.B. Rai, Synthesis of bright multicolor down and upconversion emitting Y2Te4O11: Er:Yb nanocrystals, J. Alloys Compd. 572 (2013) 90–96. [6] D.Q. Chen, Y.S. Wang, K.L. Zheng, T.L. Guo, Y.L. Yu, P. Huang, Bright upconversion white light emission in transparent glass ceramic embedding Tm3+/Er3+/Yb3+: βYF3 nanocrystals, Appl. Phys. Lett. 91 (2007) 4526–4533. [7] Y. Tian, F. Lu, M.M. Xing, J.C. Ran, Y. Fu, Y. Peng, X.X. Luo, Upconversion luminescence properties of Y2O2S:Er3+@Y2O2S:Yb3+,Tm3+ core-shell nanoparticles prepared via homogeneous co-precipitation, Opt. Mater. 64 (2017) 58–63. [8] L.L. Xing, R. Wang, W. Xu, Y.N. Qian, Y.L. Xu, C.H. Yang, X.R. Liu, Upconversion whitelight emission in Ho3+/Yb3+/Tm3+codoped LiNbO3 polycrystals, J. Lumin. 132 (2012) 1568–1574. [9] P.H. González, S.F. León-Luis, S.G. Pérez, I.R. Martín, Analysis of Er3+ and Ho3+ codoped fluoroindate glasses as wide range temperature sensor, Martin. Mater. Res. Bull. 46 (2011) 1051–1054.

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