Structure, Morphology and Upconversion Luminescence of Rare Earth

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General Research

Structure, morphology and upconversion luminescence of rare earth ions doped LiY9(SiO4)6O2 for temperature sensing Jia Zhang, and Chen Jin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05543 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Industrial & Engineering Chemistry Research

Structure, morphology and upconversion luminescence of rare earth ions doped LiY9(SiO4)6O2 for temperature sensing Jia Zhang1,2,*, Chen Jin1

1Physics

department and Jiangsu Key Laboratory of Modern Measurement Technology

and Intellige, Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China

2Jiangsu

Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China

KEYWORDS: Photoluminescence; Temperature sensor; Sensitivities; Upconversion

ABSTRACT: To develop novel upconversion (UC) luminescence materials for optical temperature sensing applications, the rare earth (Yb3+, Er3+, Ho3+ and Tm3+) ions doped

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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LiY9(SiO4)6O2 phosphors were developed. The single-phase samples were confirmed by XRD patterns. The morphology characteristics were studied via SEM and TEM techniques. When excited by 980 nm, the optimal doping concentrations for Er3+/Ho3+/Tm3+ were determined in the UC emission spectra and their characteristic emission peaks were attributed. The temperature-dependence of the UC spectra was investigated by using different strategies. The maximum absolute sensitivities were calculated to be 4.77 103 K-1 for I522/I548 (Er3+), 0.034 K-1 for I661/I522 (Er3+), 0.034 K-1 for I660/I548 (Ho3+) and 2.74 104 K-1 for I695/I789 (Tm3+), which show high sensitivity values. Among these activator ions, the Tm3+ activated sample demonstrates the largest relative

sensitivity.

Compared

with

some

previous

lanthanide

ions

activated

temperature-sensing luminescence materials, the present phosphors exhibit improved sensitivities, and could show potential applications for temperature sensors.

1. INTRODUCTION At present, rare earth (RE) doped luminescence materials have received much attention due to the wide range of applications in lighting and displays, temperature


2

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sensing, drug carriers, lasers and photovoltaic devices.1-5 As one kind of important luminescence process, upconversion (UC) has received much attention. It is an antistokes emission, that is, absorbing two or more long-wavelength photons causes a short-wavelength emission.6 So far, sever RE3+ ions have been developed as activators for UC luminescence, e.g. Er3+, Ho3+, Tm3+ and Tb3+.7-9 Besides, the Yb3+ is usually codoped to enhance their luminescence, because the Yb3+ has a large near-infrared (NIR) absorption section at 980 nm.10 As regards the practical applications based on UC luminescence, the optical temperature sensing has become a research hotspot due to the following advantages. First, this method can provide high accuracy and resolution relative to conventional temperature detection devices that use principle of liquid/metal expansion;11 second, it could carry out many harsh temperature measurements, e.g. the fast-moving objects, intracellular temperature and the objects below 10 μm.12 Now, the most widely used way for temperature sensor is the fluorescence intensity ratio (FIR) method. Two main strategies can be suggested for this. For one hand, two thermally-coupled levels (TCLs) of the activator ions are utilized, the populations of which follow the Boltzmann


3


distribution,13

for

instance,

Pr3+

(3P0,3P1→3H5),

Er3+

Page 4 of 41

(2H11/2,4S3/2→4I15/2),

Eu3+

(5D0,5D1→7F1) and Tm3+ (3F3/2,3H4→3H6).14-18 However, it is a challenge to further increase the relative sensitivity of an activator because of the limitation of energy difference between two TCLs.19,20 Thus, another method using non-TCLs of single or couple activators is being paid attention. Many luminescent ions of this kind have been developed, for example, Ho3+ ((5F4,5S2)/5F5), Er3+ (4S3/2/4F9/2), Tb3+-Eu3+, Tm3+-Ho3+, Ce3+-Mn2+, Cr3+-Nd3+ and Mn2+-Nd3+.21-27 For this method, the relative sensitivity is not restricted, but it is hard to gain a very large value. Besides, Zheng et. al also suggested that the Stark split effect of RE3+ ions due to the crystal filed can be also used in the temperature sensing, but high sensitivity was only obtained in small temperature range 453573 K.28 On the basis of the above points, it is necessary to design novel phosphors with high sensitivity with regard to temperature sensors. Owing to good chemical stability and rigid crystalline structure, the apatite compounds L10(XO4)6Y2 (L = Ca, Sr, Ba, La, Y, …; X = P, V, Si, …; Y = F, Cl, Br, OH, …), which crystallize in hexagonal system with the space group P63/m, have been extensively investigated as excellent matrixes for luminescent materials.29,30 For


4

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instance, apatites-based phosphors can endow tunable luminescence since the apatite host has the capability of being replaced by various activators.31 As one of the apatite compounds, the LiY9(SiO4)6O2 (LYS) has been reported to serve as the host for Ce3+ and Eu3+ activated phosphors.30,32 So far, the Er3+, Ho3+ and Tm3+ activated LYS phosphors used for optical thermometry have not been studied. In this paper, the LYS was selected as the matrix due to the easy preparation, high physical stability and environmentally-friendliness. The Er3+, Ho3+ and Tm3+ ions were employed as the activators and the Yb3+ was used as a sensitizer. The UC spectra excited by 980 nm were evaluated and the sensitivities for temperature sensing were investigated. 2. EXPERIMENTAL

A series of LiY9(0.99-x)(SiO4)6O2:1%Er3+,xYb3+ (LYS:1%Er3+,xYb3+, 0 ≤ x ≤ 30%), LiY9(0.99-y)(SiO4)6O2:1%Ho3+,yYb3+ (LYS:1%Ho3+,yYb3+, 5% ≤ y ≤ 20%) and LiY9(0.9953+ 3+ z)(SiO4)6O2:0.5%Tm ,zYb

(LYS:0.5%Tm3+,zYb3+, 5% ≤ z ≤ 20%) samples were

designed by solid-state reaction. The Li2CO3 (99%), SiO2 (99%), Y2O3 (99.99%), Yb2O3


5


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(99.99%), Ho2O3 (99.99%), Er2O3 (99.9%) and Tm2O3 (99.99%) were used. They were mixed in an agate mortar, and fired at 1300 °C for 3 h . The X-ray diffraction (XRD) patterns were recorded via an ARL X'TRA powder X-ray diffractometer (40 kV and 35 mA), using Cu Kα radiation (λ = 1.5418 Å). The morphology was checked by field emission scanning electron microscopy (SEM, FEI, Quanta

FEG),

transmission

electron

microscopy

(TEM)

and

high-resolution

transmission electron microscopy (HRTEM, JEOL 2100F). The emission spectra and decay curves were measured on an EI-FS5 fluorescence spectrophotometer equipped with a 980 nm laser diode. The measurement for temperature-dependence was also performed by the above instrument. 3. RESULTS AND DISCUSSION

Rietveld refinement was performed by employing the CASTEP code. The experimental (crosses), calculated (solid line) and difference (bottom) XRD profiles and the observed reflection of the typical LYS host are shown in Figure 1(a). The reliability factors are RWP = 9.52% and RP = 8.36%. The corresponding structural parameters are


6

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given in Table 1. It can be found that the LYS crystallizes in hexagonal system with the space group P63/m. The obtained cell parameters are close to those reported in Ref. 33. The crystal structure based on the refinement data of LYS is depicted in insert of Figure 1(a). There are three kinds of cations in this compound, i.e., Y3+, Li+ and Si4+. The Y sites include seven-coordinated Y(1) at 6h and nine-coordinated Y(2) at 4f. The Li is also located together with Y(2) at 4f. The Si exists in the form of discrete tetrahedral. When other trivalent RE elements are incorporated into the LYS, it can be predicted that they will prefer to occupy the Y site because of the similar ionic radii. To check the phase purity of the RE doped phosphors, the XRD patterns for LYS:1%Er3+,20%Yb3+,

LYS:1%Ho3+,15%Yb3+

and

LYS:0.5%Tm3+,15%Yb3+

are

displayed in Figure 1(b). The diffraction peaks could be attributed to the standard LYS pattern. The introduction of RE3+ does not result in obvious secondary phase.


7


Figure

1

(a)

XRD

refinement

for

the

YS

host;

Page 8 of 41

(b)

XRD

patterns

for

LYS:1%Er3+,20%Yb3+, LYS:1%Ho3+,15%Yb3+ and LYS:0.5%Tm3+,15%Yb3+

Table 1 Structural parameters of the LYS host

Lattice type

Hexagonal

Space group

P63/M


8

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a = b (Å)

9.367

c = (Å)

6.741

α = β (o)

90

γ (o)

120

V (Å3)

512.2

The morphology characteristics for the as-prepared phosphors were checked by SEM

and

TEM

techniques.

Figure

2(a)

presents

the

SEM

image

for

LYS:1%Er3+,20%Yb3+. We can see both rod-like and massive particles coexist. Most of the particles are around 3 μm, as shown in Figure 2(b). To learn the details of the particle surface, the TEM image at the edge of a particle is illustrates in Figure 2(c). The particle surface is found to be very smooth, which is in agreement with the observation in the SEM image. High crystallinity can be confirmed from the high-resolution transmission electron microscopy (HRTEM) image and fast Fourier transform (FFT) pattern in Figure 2(d) and (e), respectively. Two sets of crossing lattice fringes can be distinguished clearly in the HRTEM image, and the lattice spacings are 0.262 and 0.297 nm, corresponding to the distances of (202) and (120) planes for LYS, respectively.


9


Page 10 of 41

Moreover, the angle between the crossing fringes is measured to be about 67.2°, which is close to the calculated angle between the (202) and (120) planes. The FFT pattern indicates a single crystal feature and can be well indexed according to the LYS structure. To check the element composition, Figure 2(f) gives the energy-dispersive Xray (EDX) spectrum. It shows that no impurity elements exist. The corresponding weigh ratios of the elements in the phosphor are also shown in this figure. It can be noticed that the error is large compared with the theoretical formula LYS:1%Er3+,20%Yb3+, which is because that the EDX spectrum is a semiquantitative analysis method. Especially, the light element Li was not detected.


10

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Figure

2

(a)

SEM

image

for

LYS:1%Er3+,20%Yb3+;

(b)

TEM

image

of

LYS:1%Er3+,20%Yb3+; (c) enlarged TEM image at the edge of a particle; (d) HRTEM image and (e) FFT pattern of LYS:1%Er3+,20%Yb3+; (f) EDX spectrum of LYS:1%Er3+,20%Yb3+

Figure 3 UC luminescence spectra for LYS:1%Er3+,xYb3+ (0 ≤ x ≤ 30%) excited by 980 nm

Figure 3 presents the UC luminescence spectra for LYS:1%Er3+,xYb3+ (0 ≤ x ≤ 30%) excited by 980 nm. One can see that the Yb3+-free sample shows very weak emissions. However, after codoping Yb3+, the Er3+ luminescence intensity has been increased largely, and reaches the maximum at x = 20%. This observation indicates an obvious


11


Page 12 of 41

energy transfer (ET) from Yb3+ to Er3+. For every spectrum, three emission peaks come out at 522, 548 and 661 nm, belonging to the 2H11/2-4I15/2, 4S3/2-4I15/2 and 4F9/2-4I15/2 emissions of Er3+, respectively.34 The UC luminescence mechanism for Yb3+-Er3+ ions has been widely studied before.35,36 In brief, the 2H11/2-4I15/2 and 4S3/2-4I15/2 transitions are derived from the population on 4F7/2 level via two successive ETs from Yb3+, as can be known in the energy level diagram of Figure 4(a). But, the red 4F9/2-4I15/2 emission is generated via two ways. The first is the pumping from 4I13/2 to 4F9/2 level after the electrons are pumped to 4I11/2 level and then relaxes to 4I13/2 level (see ①). The second is the nonradiative relaxation from 4S3/2 to 4F9/2 level (see ② ). In order to learn which way is predominant in the excitation on Er3+ 4F9/2 level in the LYS host, Figure 4(b) displays the normalized (for

4S

4 3/2- I15/2

transition) UC luminescence spectra for

LYS:1%Er3+,20%Yb3+ excited by continuous-wave (CW) and pulse upon 980 nm. One can see the red transition intensity excited by pulse is much lower than that excited by CW. This phenomenon implies the first way mentioned above could be mainly responsible for the Er3+ red emission.


12

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Figure 4 (a) Energy level diagram for Yb3+ and Er3+ ions; (b) UC luminescence spectra for LYS:1%Er3+,20%Yb3+ pumped by 980 nm CW and pulse light

Figure 5 UC luminescence spectra for LYS:1%Ho3+,yYb3+ (5% ≤ y ≤ 20%) excited by 980 nm


13


Page 14 of 41

The UC luminescence spectra of LYS:1%Ho3+,yYb3+ (5% ≤ y ≤ 20%) excited by 980 nm are displayed in Figure 5. Two transition peaks at 548 and 660 nm have been found in the visible range for every spectrum, assigned to the (5F4,5S2)-5I8 and

5F -5I 5 8

transitions of Ho3+, respectively.37 Besides, a very weak transition peak at 757 nm also appears and is assigned to the (5F4,5S2)-5I7 transition of Ho3+.20 By increasing Yb3+ concentration, the Ho3+ luminescence is enhanced gradually until y = 15%. Beyond this Yb3+ content, the intensity of Ho3+ luminescence starts to decrease. So, the optimal Yb3+ doping content is for y = 15%.


14

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Figure 6 (a) UC luminescence spectra of LYS:0.5%Tm3+,zYb3+ (5% ≤ z ≤ 20%) upon 980 nm excitation; (b) relative luminescence intensities of 473 and 789 nm emissions at different Yb3+ content

Figure 6(a) presents the UC luminescence spectra of LYS:0.5%Tm3+,zYb3+ (5% ≤ z ≤ 20%) excited by 980 nm. We can see that the strongest transition peak of Tm3+ is around 789 nm and can be assigned to its 3H4-3H6 transition.21 Another strong emission


15


Page 16 of 41

peak is located in the blue range, belonging to the Tm3+ 1G4-3H6 transition.21 From the enlarged spectra, three weak transition peaks come out at 366, 650 and 695 nm, which could be ascribed to the Tm3+ 1D2-3H6, 1G4-3F4 and 3F2,3-3H6 transitions, respectively.21 For the strong blue (473 nm) and NIR (789 nm) transitions, the change trends of their intensities with Yb3+ concentration are different. To make clear this, Figure 6(b) depicts the intensities for the 473 and 789 nm transitions at different Yb3+ concentration. With increasing Yb3+ doping content, the 473 nm luminescence intensity shows a gradual reduction, but the 789 nm one increases first until z = 15%, and then decreases beyond z = 15%. This could be owing to the energy back transfer from Tm3+ to Yb3+, such as G4 (Tm)  2 F7/2 (Yb)  3 H 5 (Tm)  2 F5/2 (Yb) .

1


16

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Figure 7 (a) UC luminescence spectra for LYS:1%Er3+,20%Yb3+ excited by 980 nm under different temperature; (b) intensities for 522, 548 and 661 nm transitions at different temperature; dependence of (c) I522/I548 and (d) I661/I522 on temperature; (e) SA and (f) SR at different temperature for I522/I548 and I661/I522

In the LYS:1%Er3+,20%Yb3+ sample, the temperature-dependence of the emission spectra was conducted. Figure 7(a) illustrates its UC luminescence spectra excited at 980 nm for different temperatures. The intensities for 522, 548 and 661 nm emissions show different change trend with temperature. In order to clarify this, the intensities of these three emissions at different temperature are shown in Figure 7(b). On the whole,


17


Page 18 of 41

with rising temperature, the 548 nm emission intensity shows a continuous decrease, but the 522 and 661 nm emissions intensities increase initially and will be weakened at higher temperature. The different change for the two green emissions (derived from 2H

11/2

and 4S3/2 levels) is generally attributed to the thermally-coupled effect, which has

been introduced in many references.38-40 The increase of the red emission intensity within 433 K can be combined with the nonradiative relaxation which is interpreted as follows. As depicted in the insert of Figure 4, the population on 4F9/2 level undergoes processes ① and ②. When the temperature rises, the phonon energy will be increased owing to intensified lattice vibration. As a result, the nonradiative de-excitation probability is increased with rising temperature according to the Mott-Seitz model.41 The higher temperature is beneficial to processes ① and ②, which increases the population on 4F9/2 level. However, when the temperature is further increased, the nonradiative ETs to the lower levels and some other quenching centers can be enhanced sharply, which is responsible for the reduction of the 4F9/2-4I15/2 luminescence intensity beyond 433 K. On the basis of the above results, it can be predicted that the FIRs for different emissions of Er3+ will change with temperature. Here, two cases are investigated. On


18

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one hand, for the thermally-coupled 2H11/2 and 4S3/2 levels, Figure 7(c) shows the temperature-dependence of I522/I548. All the experimental data follows the Boltzmann distribution:42 R  N exp(

E ) KT

(1)

where R is the FIR for transitions resulting from two TCLs, ΔE is energy difference between 2H11/2 and 4S3/2 levels, K is Boltzmann constant, T is absolute temperature and

N is the proportionality constant. By fitting the data, the ΔE was obtained to be 670.4 cm-1. On the other hand, the non-TCLs 4F9/2 and 2H11/2 were employed. Figure 7(d) presents the dependence of I661/I522 on absolute temperature. When we increase the temperature, the FIR for I661/I522 will be reduced gradually. The experimental data can well fitted by a single-exponential function I 661 / I 522  68.9*exp(T / 95.8)  1.26 .

Table 2 SA and SR, matrixes, transitions and temperature range of several temperaturesensing luminescent materials

RE ions

Matrixes

Transitions

Range (K)

SA (K-1) (maximum)

SR (K-1)

Ref.


19


Page 20 of 41

Er3+

NaBiTiO3

2H

4 4 11/2/ S3/2→ I15/2

163-613

0.0031 (495 K)

827.26/T2

43

Er3+

Y2O3

2H

4 4 11/2/ S3/2→ I15/2

93-613

0.0044 (427 K)

886.08/T2

44

Er3+

Ba3La(PO4)3

2H

4 4 11/2/ S3/2→ I15/2

298-498

0.00438 (498 K)

1002/T2

45

Ho3+

CaMoO4

5K /5F →5I 8 3 8

299-673

0.00302 (673 K)

1067.76/T2

46

Ho3+

Y2O3

5F /5S →5I , 5I 5 2 8 7

100-300

0.0097 (85 K)

241.2/T2

43

Tm3+

NaLuF4

3F

2,3→

Tm3+

KLuF4

3F

2,3→

Tb3+-Eu3+

YF3 GC

Ce3+-Tb3+

LaOBr

Ce3+-Mn2+ (Ca,Sr)10Li(PO4)7

3H

6

250-600

0.00045 (600 K)

2386.93/T2

47

3H

6

303-503

0.000145 (503 K)

1249.85/T2

48

(5D4-7FJ)/( 5D0-7FJ)

303-563

0.0013

-

49

(d→f)/(5D4-7FJ)

293-453

-

0.42% (433 K)

50

(d→f)/(4T1→6A1)

303-463

-

0.4% (473 K)

51

4 4 11/2/ S3/2→ I15/2

313-473

0.00477 (485 K)

970.4/T2

2 4 9/2/ H11/2→ I15/2

293-553

0.034 (293 K)

0.75% (293 K)

This work

5 5 5 5/( F4, S2)→ I8

293-553

0.034

0.98% (293 K)

298-548

0.000274 (548 K)

2046.9/T2

Er3+

LYS

2H

Er3+

LYS

4F

Ho3+

LYS

5F

Tm3+

LYS

3F

3H

2,3→

6

As is known to us, the sensitivity is a key factor to evaluate the performance of phosphors for temperature sensing. The absolute (SA) and relative (SR) sensitivities are suggested by:43,44

SA 

d ( R) (2) d (T )

SR 

1 d ( R) (3) R d (T )


20

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Figure 7(e) shows the SA at different temperatures for I522/I548 and I661/I522. With rising temperature, the SA value for I522/I548 increases first. The maximum SA can be obtained to be 4.77 103 K-1 at about 485 K. For I661/I522, the SA value exhibits a continuous decrease from 0.034 to 2.28 103 K-1 with the temperature changing from 293 to 553 K. Relative to some other Er3+-activated luminescent materials such as NaBiTiO3:Er3+, Y2O3:Er3+ and Ba3La(PO4)3:Yb3+-Er3+ (see Table 2), the LYS:1%Er3+,20%Yb3+ phosphor can demonstrate higher sensitivity. On the other hand, it can be also found that the absolute sensitivity for I661/I522 is larger than that for I522/I548 within 480 K, but it becomes lower beyond 480 K. Figure 7(f) demonstrates the relative sensitivities at different temperatures for I522/I548 and I661/I522. With increasing temperature, the SR values for both cases decrease gradually, but using I522/I548 can generate a lager SR.


21


Page 22 of 41

Figure 8 (a) UC luminescence spectra for LYS:1%Ho3+,15%Yb3+ excited by 980 nm for various temperatures, insert gives the intensities of 548 and 660 nm emissions at different temperature; (b) decay curves of LYS:1%Ho3+,15%Yb3+; (c) dependence of I660/I548 on temperature; (d) relative sensitivity at different temperatures

Figure 8(a) demonstrates the UC luminescence spectra of LYS:1%Ho3+,15%Yb3+ excited by 980 nm under various temperatures. One can see the intensities of green


22

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and red emissions exhibit different change regulation. To interpret this, inset shows their intensities at different temperature. It is clear that with rising temperature, the green emission intensity decreases gradually, but the red one exhibits a slight increase first and then decreases beyond 333 K. The decrease for the green transition could be mostly caused by the increasing nonradiative relaxation from the (5F4,5S2) to 5F5 levels at higher temperature. This point can be verified by the variety of decay lifetime for (5F4,5S2)

level.

Figure

8(b)

gives

the

corresponding

decay

curves

of

LYS:1%Ho3+,15%Yb3+, which can be fitted by a dual-exponential function as

I  A1 exp(t /  1 )  A2 exp(t /  2 )

(4)

where τ1 and τ2 are fast and slow components of lifetimes, and A1 and A2 are fitting parameters, respectively. The corresponding τi and Ai (i = 1, 2) values were given in Table 3. Via     ( A1 12  A2 22 ) / ( A1 1  A2 2 ) , the average lifetimes can be calculated to be 30.1 and 14.3 µs for T = 293 and 553, respectively. Thus, the reduction of decay lifetime implies an enhanced de-excitation on (5F4,5S2) levels. The interpretation for the increase of the red emission can be related to the nonradiative relaxation, which is


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similar to that in the above LYS:1%Er3+,20%Yb3+ sample. Based on the above phenomenon, one can confirm that the I660/I548 will vary with temperature. Figure 8(c) depicts the dependence of I660/I548 on temperature. The experimental data can be reproduced by a linear function I 660 / I 548  0.034T  6.59 . On the basis of Eq. (2), the SA is obtained to be 0.034 K-1. The relative sensitivity calculated by Eq. (4) at different temperatures is displayed in Figure 8(d). When the temperature increases from 293 to 553 K, the SR decreases from 0.98% to 0.27% K-1, respectively. Thus, higher absolute sensitivity has been gained in this phosphor relative to some other Ho3+-activated phosphors and couple-activators doped luminescent materials (see Table 2). Table 3 τi and Ai for LYS:1%Ho3+,15%Yb3+

T (K)

τ1 (µs)

τ2 (µs)

A1

A2

293

8.9

90.0

13046.8

456.3

553

9.4

49.2

15841.8

421.1


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Figure 9 UC luminescence spectra for LYS:0.5%Tm3+,15%Yb3+ excited by 980 nm under different temperatures, inset displays the intensities of 695 and 789 nm emissions at different temperature

Figure 9 demonstrates the UC luminescence spectra for LYS:0.5%Tm3+,15%Yb3+ excited by 980 nm under different temperature. With increasing temperature, both the strong blue and NIR emission peaks show continuous decrease. However, the red 3F2,33H

6

emission exhibits different change, as can be seen in the enlarged spectra. Here,

we just consider the TCLs 3F2,3 and 3H4. The intensities for 695 and 789 nm emissions at various temperatures are depicted in insert of Figure 9. We can see the 695 nm emission intensity increases first until 448 K, but decreases beyond 448 K. In this case,


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the FIR for 695 and 789 nm emissions will vary with temperature. Figure 10(a) demonstrates the temperature-dependence of I695/I789. By employing Eq. (1), an exponential function I 695 / I 789  1.69 exp(2046.9 / T ) was obtained. Thus, the ΔE value equals to 1422.6 cm-1, which is in agreement with the spectral result. On the basis of Eq. (2) and (3), the SA and SR for different temperature were obtained (see Figure 10(b)). The SA and SR are 0.40 104 and 2.30% K-1 for 298 K, and 2.74 104 K-1 and 0.68% for 548 K, respectively. The LYS:0.5%Tm3+,15%Yb3+ phosphor can show improved sensitivities than some other Tm3+-doped materials, as shown in Table 2.


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Figure 10 (a) dependence of I695/I789 on temperature for LYS:0.5%Tm3+,15%Yb3+; (b) SA and SR under different temperature for LYS:0.5%Tm3+,15%Yb3+

4. CONCLUSIONS

In conclusion, the RE ions doped LYS samples have been prepared successfully. The phase purity was checked by Rietveld refinement and XRD technique, which indicated no secondary phase exists. The smooth particle surface and high crystallinity were formed as verified by SEM and TEM techniques. With 980 nm excitation for LYS:Er3+,Yb3+, three emissions come out. For Yb3+-Ho3+ codoped samples, two strong as well as one weak emissions


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of Ho3+ appear, and the LYS:1%Ho3+,15%Yb3+ sample exhibits the most intense emission intensity. With the Yb3+-Tm3+ codoped into LYS, five emission peaks from NUV to NIR region can be seen, and the dominant emission is around 789 nm. In order to evaluate the temperaturesensing behavior, the TCLs and non-TCLs are utilized. It has been found that the maximum absolute sensitivities of Er3+ are 4.77 103 and 0.034 K-1 for I522/I548 and I661/I522, respectively. And the relative sensitivity decreases with increasing temperature. The maximum SA values for Ho3+ and Tm3+ activated phosphors are 0.034 and 2.74 104 K-1, respectively. But the SR value of Tm3+ is much larger than those of Er3+/Ho3+ in the LYS compound. Corresponding Author *E-mail address: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (No. 51602117).

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SYNOPSIS (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis, see the journal’s Instructions for Authors for details.


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Structure, Morphology and Upconversion Luminescence of Rare Earth

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