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Tuning Red Upconversion Emission in Single LiYF:Yb /Ho Microparticle Wei Gao, Ruibo Wang, Qingyan Han, Jun Dong, Longxiang Yan, and Hairong Zheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511566h • Publication Date (Web): 09 Jan 2015 Downloaded from http://pubs.acs.org on January 21, 2015

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The Journal of Physical Chemistry

Tuning Red Upconversion Emission in Single LiYF4:Yb3+/Ho3+ Microparticle Wei Gao, Ruibo Wang, Qingyan Han, Jun Dong, Longxiang Yan, Hairong Zheng* School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, PR China *Corresponding author: Hairong Zheng E-mail: [email protected] Abstract The upconversion (UC) emission of single LiYF4: Yb3+/Ho3+ microparticle with codoped Ce3+ is investigated. The UC emission tuning from green to red in single LiYF4: Yb3+/Ho3+ microparticle is successfully obtained by codoping Ce3+ ions with different concentration, and the enhancement mechanism of red emission is discussed in detail. The results indicate that two efficient cross-relaxations between Ho3+ and Ce3+ enhance the red and suppress the green emission in the UC process. The investigation on the luminescence property of a single microcrystal can effectively avoid the influence of the particles around, and provide more precise information for better exploring the UC mechanism. The current study on the multicolor tuning of RE doped micro-materials is also important for extending the application of RE doped micro-materials in micro optoelectronic device, color display and anti-counterfeiting applications. Keywords: Single microparticle, LiYF4:Yb3+/Ho3+, upconversion emission

1. INTRODUCTION

ACS Paragon Plus Environment


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Lanthanide-doped UC luminescence materials have been attracting more and more attention due to their potential applications in lighting industry, color display, solar cell, nanoscale biolabel, the development of new laser crystal and optical processing sensors.1-7 Rare-earth doped fluoride materials have low phonon energy, low nonradiative decay rate and high radiative emission rate. Among them, RE doped NaYF4 crystals have been considered as one of the most efficient upconversion emission materials.8-11 Various size and morphology of it have been reported based on different preparation routes.12-14 Meanwhile, another efficient UC emission material , RE doped tetragonal LiYF4 crystal is also widely considered by researchers. Investigating

on

Yb3+/(Er3+,

Tm3+,

Ho3+)

doped

octahedral

LiYF4

nano/micro-structures and their superior UC fluorescence have been reported. 15-17 It was found that the total UC quantum yield of colloidal LiYF4: Er3+ nanocrystals under 1490 nm excitation is about 4 times higher than the hexagonal NaYF4: Yb3+/Er3+ nanocrystals under 980nm excitation.18 Our group has successfully synthesized tetragonal

phased

octahedral

LiY/YbF4:Yb3+/(Er3+,Pr3+)

microcrystals

with

hydrothermal method, and observed strong UC fluorescence emission.19-21 Comparing with RE doped nanoparticles, microparticles usually present stronger UC emission due to their smaller surface-to-volume ratio and less surface quenching centers, which is essential for extending the application in display, solar cell, micro optoelectronic devices and optical processing sensors. 22-23 Ho3+ ion has a broad fluorescence spectrum ranging from vacuum ultraviolet to infrared light, it has constituted an intriguing active ion for UC emission.24-25 A


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predominantly green UC emission is frequently observed in Ho3+ doped inorganic materials with low phonon energy.26-27 However, enhanced red UC emission from Yb3+ and Ho3+ codoped UC materials has been barely reported. According to the energy level distribution of Ho3+, the red emission from Ho3+ is mainly through increasing the population of the intermediate 5I7 and excitation 5F5 levels. Recently, The enhanced red UC emission was observed in Ho3+ doped LaF3 nanoparticles due to the presence of organic ligands stabilizing the nanoparticles that quench green emission, and high phonon energy YVO4:Ho3+ nanocrystals by codoping Yb3+ ion with high concentration.28-29 The UC emission of Yb3+/Ho3+ co-doped cubic phase NaYF4 nanocrystals have been successfully tuned from green to red by introducing Ce3+ ions, which is due to the cross-relaxation (CR) between Ho3+ and Ce3+ ions.30 Up to now, tuning UC emission in RE doped tetragonal phase LiYF4 microparticles, especially for single particle has been barely reported. In the current work, the tetragonal phase LiYF4:Yb3+/Ho3+ microparticles are prepared via a facile hydrothermal method. The UC emission from single LiYF4: Yb3+/Ho3+ microparticle is tuned successfully by codoping Ce3+ ions to the particle. Possible upconversion mechanism is discussed in detail. 2. EXPERIMENTAL DETAILS 2.1 Materials All chemicals used in the current study are analytic grade and used without further purification. Y(NO3)3·6H2O, Yb(NO3)3·6H2O and Ho(NO3)3·6H2O are obtained by dissolving Y2O3, Yb2O3 and Ho2O3 (99.99%. Sigma-Aldrich) with nitric acid,



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respectively. During the preparation process, the solution is stirred at 60

for several

hours to remove excess nitric acid. Ce(NO3)3·6H2O(99.99%) is purchased from Sigma-Aldrich Chemicals Co.. Then, they are dissolved in deionized water to form rare-earth nitrate solution. HNO3 (65.0%-68.0%), NH4F (98.0%), LiF (98.0%), and EDTA (ethylenediamine tetraacetic acid, 99.0%) with analytic grade are supplied by Tianjin chemical reagent factory. 2.2 Synthesis of LiYF4:Yb3+/Ho3+/Ce3+ Microparticles LiYF4:Yb3+/Ho3+/Ce3+ microparticles are synthesized by hydrothermal method for which the detailed process is given in the reference.19-20 Firstly, 1.0 ml RE (NO3)3 (0.5M, RE= Y/Yb/Ho/Ce=(78.0-x):20.0:2.0:x, (x=0, 4.0, 6.0, 8.0,12.0)) and 20.0 ml 0.02 mol/L EDTA are mixed to form a chelating complex solution, then vigorously stirring it for about 40 min. Secondly, 0.35 g NH4F and 0.13 g LiF are added into chelating complex solution, and stir it for about 20 min until it completely becomes white liquid. Finally, the white liquid is slowly transferred into a 40 ml Teflon-lined autoclave

and

heated

at

200

for

48

hours

to

get

LiYF4:

20.0%Yb3+/2.0%Ho3+/x%Ce3+ (x=0, 4.0, 6.0, 8.0,12.0) microparticles. The final samples are obtained by centrifuging and washing with deionized water and ethanol for several times, and they are dried at 60

for several hours.

2.3 Sample Characterization The

powder

x-ray

diffraction

(XRD)

patterns

are

recorded

by

D/Max2550VB+/PCx-ray diffraction meter with Cu Kα (40 kV, 40 mA) irradiation ( λ = 0.15406 nm). The scanning rate of 2θ angle for XRD spectra is 8° min−1. The


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morphology of the particles is characterized by scanning electron microscope (SEM, Quanta 200) operating at voltage of 20.0 kV. For spectroscopic measurement, YAG: Nd3+ (Quanta Ray Lab-170) pulsed laser and Ti sapphire femtosecond laser (Mira-900) are employed as excitation sources. The spectrometer (SP2750i) with a spectral resolution of 0.008 nm is used for luminescence collection and detection. The optical microscope (OLYMPUS-BX51) is used in the confocal setup, and the corresponding magnifications are 100, 500 and 1000. Proper notch filters are placed in front of the entrance of the spectrometer to block the scattering light. All of the spectroscopic measurements are carried out at room temperature. 3. RESULTS AND DISCUSSION 3.1 Crystal Structure and Morphology Typical XRD patterns of LiYF4: 20.0%Yb3+/2.0% Ho3+ microparticles with codoping different Ce3+ concentrations are shown in Figure 1. Strong and sharp diffraction peaks are well indexed to the pure tetragonal phase, which is consistent with the standard card JCPDS card 81-2254. No obvious extra diffraction peaks are detected even when the concentration of Ce3+ ions is increased to 12.0%. Due to the substitution of Y3+ (r=0.115 nm) ions by larger Ce3+ (r=0.128 nm) ions in the host lattice, the diffraction peak shifts slightly to the lower angle side as a result of decrease on the unit-cell volume.31-32 Figure 2 presents SEM images and EDS spectra of LiYF4: 20.0%Yb3+/2.0% Ho3+ microparticles with different codoping Ce3+ concentration. It is found that LiYF4:Yb3+/Ho3+ microparticles are octahedral in shape with a smooth surface, and



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the average size of the particles is around 12 µm. By increasing Ce3+ concentration from 0 to 12.0%, the morphology of the particles do not present obvious change according to Figure 2(a-d). This indicates that Ce3+ ions has no obvious affect on the size and morphology of the sample, which may be due to the similar ionic radius of Y3+ and Ce3+ ions. Figure 2(e-h) are EDS of LiYF4:20.0%Yb3+/2.0%Ho3+ microparticles with 0 to 12.0% Ce3+ ions codoped. Elements of Y, Yb, F, Ho and Ce are clearly presented in the Figure. It is found that the peak intensity of Y3+ reduces while the intensity of Ce3+ increases when the concentration of Ce3+ increases, which indicates a substitution of Y3+ with Ce3+ in LiYF4 lattice. 3.2 PHOTOLUMINESCENCE STUDY 3.2.1 UC Emission Tuning in Single LiYF4: 20.0%Yb3+/2.0% Ho3+ Microparticles Investigation on the UC luminescence emission of the microparticle is conducted with a confocal setup. Figure 3(a) displays the UC emission spectra of a single LiYF4:20.0%Yb3+/2.0%Ho3+ microparticle with different Ce3+ concentration under continuous NIR 980 nm excitation. It is found that the dominant emission peaks are at 541 nm and 644 nm, which are assigned to the transitions of 5S2/5F4→5I8 and 5F5→5I8 of Ho3+ ions.31-32 Weak blue emission at 484 nm and NIR emission at 750 nm are also observed, which are associated with the transitions of 5F3→5I8 and 5S2/5F4→5I7, respectively. Interestingly, when the concentration of Ce3+ is increased from 4.0% to 12.0%, the green emission decreases while the red one increases gradually (Figure 3(b)). The emission color from single LiYF4: Yb3+/Ho3+ microparticle is tuned from green to red (in inset of Figure 3(a)), and the R/G ratio increases from 0.47 to 6.21


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accordingly (Figure 3(c)). The Commission Internationale del’Eclairage (CIE) 1931 x y chromaticity coordinates for the fluorescence emission of single LiYF4: Yb3+/Ho3+ with different Ce3+ concentration under 980 nm excitation can be calculated.33 And the corresponding CIE chromaticity coordinate (x, y) changes from (0.2716, 0.6804) to (0.5189, 0.4026) as Ce3+ concentration increases from 0% to 12.0%, which is listed in the Table 1. The tunable region for the luminescence from single LiYF4: Yb3+/Ho3+ microparticle with different Ce3+ concentration is shown in Figure 3(d). 3.2.2 UC Emission Mechanism with Codoping Ce3+ Ions To understand the observed phenomenon, the mechanism of the green and red UC emission is investigated. The pumping power dependence of fluorescence intensity is studied for single LiYF4:Yb3+/Ho3+ microparticle with codoping Ce3+, and the observed experimental date is plotted in Figure 4. Slopes of 1.92 for green and 1.81 for red emissions from single LiYF4:20.0%Yb3+/2.0%Ho3+ microparticle are obtained by fitting the data in Figure 4(a), and the values of 1.88 for green and 1.74 for red are yielded

by

fitting

the

experimental

data

in

Figure

4(b)

for

single

LiYF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ microparticle . These values of the slope indicate that the green and red emissions are two photon excitation processes.30 It is noticed that the slopes of green and red emissions from single LiYF4: 20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ microparticle are slightly smaller than that in single LiYF4: 20.0%Yb3+/2.0%Ho3+ microparticle, which could be explained by the population of intermediate level of the red UC. The radiative and nonradiative transition and energy transfer processes should be



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discussed based on the emission spectra for exploring the influence of Ce3+ on the emission of Ho3+. Figure 5 illustrates a simple scheme of UC emission for Yb3+/Ho3+/Ce3+ system. Since Yb3+ has a large absorption cross-section for infrared light and long excited state lifetime, it is reasonable to believe that the main pathway to populate upper emitting states is through the energy transfer from Yb3+ to Ho3+. As shown in Figure 5, 5I6, 5F5 and 5S2/5F4 states can be populated by three successive energy transfer (ET) processes between Yb3+ and Ho3+ under 980 nm excitation. Strong green and weak NIR emission could be generated by radiative decay from 5

S2/5F4 to 5I8 and 5I7 states. The red UC emission is from 5F5 to 5I8 ground states. There

are two possible processes to populate the excited state 5F5. One is the nonradiative transition from higher excited states of 5F4/5S2. The other is that after initial population of long-lived 5I7 level by nonradiative transition of 5I6 decay of 5F4/5S2

5

5

I7 or radiative

I7, 5F5 level is populated through ET process from Yb3+ to Ho3+.

According to the energy level structure of Ho3+, two nonradiative relaxation processes of 5S2/5F4

5

F5 and 5I6

5

I7 might increase the population densities of 5F5 state,

resulting in the enhancement of red UC emission. For multiphonon nonradiative relaxation process, the nonradiative transition probability strongly depends on the phonon energy of the host lattice.34 Thus the two nonradiative relaxation processes of 5

S2/5F4

5

F5 and 5I6

5

I7 should not be efficient because both energy gaps are about

3000 cm-1 which is approximately six times that of the maximum phonon energy of LiYF4 host (500 cm-1).34-35 This could explain the experimental observation of relatively weaker red emission and small R/G ratio in the UC luminescence spectra


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for the sample without Ce3+ (Figure 3(a)). However, when Ce3+ is codoped to the system, different UC emission phenomena are observed and they are presented by Figure 3(b)-3(e). Increasing Ce3+ concentration from 4.0% to 12.0% in single LiYF4:Yb3+/Ho3+ microparticle, one finds that the emission of Ho3+ is tuned from green to red. Since the energy gap between the excited 5F7/2 state and ground 2F5/2 state is about 3000 cm-1,31 the CR processes of 5S2(5F4) (Ho) + 2F5/2 (Ce3+) →5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+) →5I7 (Ho3+) + 2F7/2 (Ce3+) can be achieved effectively according to the energy conservation law. These resonant energy transfer processes can increase the population of 5F5 state through nonradiative transitions of 5S2/5F4

5

F5 and 5I6

5

I7, respectively, which leads to the enhancement

of red emission and the suppress of green emission. Figure 6(a) shows the NIR (1175 nm) emission of 5I6→5I8 transition of Ho3+ ions in LiYF4:Yb3+/Ho3+ with Ce3+ concentration from 0% to 12.0% under 980 nm excitation. The intensity of NIR emission decreases with the increase of Ce3+ concentration, which indicates that the radiative relaxation of 5I6→5I8 is reduced with increase of Ce3+ concentration. The decrease of NIR emission intensity indicates the occurrence of CR1 process 5I6 (Ho3+) + 2F5/2 (Ce3+) →5I7 (Ho3+) + 2F7/2(Ce3+) between Ho3+ and Ce3+. The occurrence of CR2 processes 5S2/5F4 (Ho3+) + 2F5/2 (Ce3+) →5F5 (Ho3+) + 2F7/2 (Ce3+) between Ho3+ and Ce3+ can be discussed by observing the downconversion emission. Figure 7(b) shows the downconversion emission from a single LiYF4:Yb3+/Ho3+ microparticle with codoping Ce3+ concentration from 0 to 12.0%.



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The excitation light is 532 nm pulsed laser, which can populate 5S2/5F4 excited states directly. The population of 5F5 state mainly originates from 5S2/5F4 excited states through nonradiative relaxation. However, with increase of Ce3+ concentration in LiYF4:Yb3+/Ho3+ microparticle, green UC emission intensity reduces and red UC emission increases, and the corresponding R/G ratio changes from 0.24 to 1.05. 3.2.3 Calculation of the Conversion Efficiency of CR 1 and CR 2 Processes In order to further calculate the conversion efficiency of CR1 and CR2 processes, steady-state rate equations are established based on the UC process in Yb3+ and Ho3+ ion system with continuous 980 nm NIR excitation. The notations in the equations are defined as follow. N0, N1, N2, N3 and N4 are the population densities of the 5I8, 5I7, 5I6, 5

F5 and 5S2(5F4) states of the Er3+ ions; R1, R 2, R 3 and R 4 are the radiation rates of the

5

I7, 5I6, 5F5 and 5S2(5F4) of the Ho3+ ions; w0, w1 and w2 are energy transfer rates from

the excited Yb3+ ions to the5I6, 5F5, and 5S2/5F4, states of Ho3+ ions; NYb0 and NYb1 are the population densities of Yb3+ ions in the ground and the excited states, respectively; β0 and β2 are nonradiative decay rates of 5I6 and 5S2/5F4 states, respectively; r1 and r2 are the rate of the energy transfer from Ho3+ ions and Ce3+ at the 5I6 and 5S2/5F4 states. NCe0 is the population density of the 2F5/2 states of Ce3+ ions. Considering that the radiation rates of 5I5 and 5I4 states are very small and can be neglected in the rate equations due to small energy gap, the rate equations are formulated as follows:30

β 0 N 2 + r1 N 2 N Ce0 − R1 N1 − w1 NYb1 N1 = 0

(1a)

w0 NYb1 N 0 − β 0 N 2 − r1 N 2 NCe 0 − R2 N 2 − w2 NYb1 N1 = 0

(1b)

w1 NYb1 N1 + r2 N 4 N Ce 0 + β1 N 4 − R3 N 3 = 0

(1c)


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w2 NYb1 N 2 − R4 N 4 − r2 N 4 NCe 0 − β1 N 4 = 0

(1d)

In the following discussion, the radiation rates of the 5I6 of Ho3+ are supposed to be much larger than its UC rates, that is w2 NYb1 .36 Thus, one can get population density N2 of NIR emission by solving equations (1a-1d), which is: N2 =

w0 N 0 NYb1 R2 + r1 N Ce 0

(2)

With equation (2), we can get I NIR = N 2 hvNIR R2 =

R2 hvNIR w0 N 0 NYb1 R2 + r1 N Ce 0

(3)

Based on equation (3) and the measured fluorescence intensity in Figure 6(a), we have I NIR (0%Ce) R +rN rN = 2 1 Ce 0 = 1 + 1 Ce 0 = 5.1 I NIR (12.0%Ce) R2 R2

(4)

The conversion efficiency of the CR1 process:30

η1 =

β + r1 N Ce 0 R2 + r1 NCe 0

≈

r1 N Ce 0 = 80.3% R2 + r1 NCe 0

(5)

This result indicates that the CR1 process 5I6 (Ho3+) + 2F5/2 (Ce3+) →5I7 (Ho3+) + 2F7/2 (Ce3+) is very efficient. The conversion efficiency of CR2 process 5S2(5F4) (Ho3+) + 2F5/2 (Ce3+) →5F5 (Ho3+) + 2

F7/2 can be calculated with following equation:37

η2 = 1 −

τ (12.0%Ce) τ ′(0%Ce)

(6)

Here τ and τ′ are the decay times of Ho3+ with and without the presence of Ce3+, respectively. The fluorescence decay times of 5S2(5F4) state are measured to be about 187.1µs and 109.8 µs for 0% and 12.0% codoped Ce3+ ions(Figure 8), respectively. According to the equation (6), the efficiency η2 is about 41.3% for 12.0% codoped



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Ce3+ ions. This indicates that CR2 plays an assistant role for CR1 process in converting the green UC emission into red UC emission.

4. CONCLUSION Yb3+/Ho3+ codoped tetragonal

phase LiYF4 octahedral microparticles is

successfully synthesized by hydrothermal method. The UC emission tuning from green to red are observed with a confocal microscopy setup for single LiYF4:Yb3+/Ho3+ microparticle with codoping Ce3+ under 980 nm NIR excitation. The R/G ratio is increased from 0.47 to 0.61 in single LiYF4:Yb3+/Ho3+ microparticle by increasing Ce3+ concentration. Two efficient CR processes of 5S2(5F4) (Ho) + 2F5/2 (Ce3+) →5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+) →5I7 (Ho3+) + 2F7/2 (Ce3+) between Ho3+ and Ce3+ are found to be dominant UC pathways for tuning the emission color from green to red in single LiYF4:Yb3+/Ho3+ microparticle.

ACKNOWLEDGEMENT The work is supported by the National Science Foundation of China (Grant 11174190), the Fundamental Research Funds for the Central Universities (Grants K201304002),

the Natural Science Foundation of Shaanxi Educational Committee

(No.2013JK0627) and the Natural Science Basis Research Plan in Shaanxi Province o f China(Program No.2013JM1008).

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Transition-metal-ion Systems. Phys. Rev. B. 2000, 61, 3337–3346.


and


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Table and Figure Table 1 The calculated CIE chromaticity coordinate (x, y) of single LiYF4: 20.0%Yb3+/2.0%Ho3+ microparticle with different Ce3+ concentration. CIE chromaticity coordinate Point Samples X Y LiYF4: 20.0%Yb3+/2.0%Ho3+ 0.2716 0.6804 a b LiYF4: 20.0%Yb3+/2.0%Ho3+/4.0%Ce3+ 0.3585 0.5721 3+ 3+ 3+ c LiYF4: 20.0%Yb /2.0%Ho /6.0%Ce 0.4341 0.5108 LiYF4: 20.0%Yb3+/2.0%Ho3+/8.0%Ce3+ 0.4639 0.4579 d e LiYF4: 20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ 0.5188 0.4026

Figure 1.

XRD patterns of LiYF4: 20.0%Yb3+/2.0%Ho3+ microparticles with

different Ce3+ codoping concentrations. (a) 0%, (b) 4.0%, (c) 8.0% and (d) 12.0%.


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Figure 2. SEM images and EDS spectra of LiYF4: 20.0%Yb3+/2.0%Ho3+ microparticles with different Ce3+ concentrations.(a,e) 0%, (b,f) 4.0%, (c,g) 8.0% and (d,h) 12.0%.

Figure 3. UC emission spectra (a), the peak area of the green and red emission (b), R/G

ratio

(c),

and

CIE

chromaticity

diagram

(d)

of

single

LiYF4:20.0%Yb3+/2.0%Ho3+ microparticle as a function of Ce3+ concentration. The insets in figure 3(a) are corresponding luminescence photographs.



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Figure 4. The Ln–Ln plot of the upconversion emission intensities as a function of the infrared excitation pump power for single LiYF4: 20.0%Yb3+/2.0%Ho3+/x%Ce3+ microparticle. (a) x=0%, (b) 12.0% .

Figure 5. The proposed UC mechanism and energy level diagrams of Ho3+, Yb3+and Ce3+ ions.


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Figure 6. The NIR emission spectra of single LiYF4: Yb3+/Ho3+ microparticle with different Ce3+ concentrations under 980 nm excitation.

Figure 7.

Downconversion emission spectra (a) and R/G ratio (b) of single LiYF4:

Yb3+/Ho3+ microparticle with different Ce3+ concentrations The excitation source is 532 nm pulsed laser.



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Figure 8. The fluorescence decay and data fitting results with fist-order exponential function( I = I 0 exp(−t / τ ) ) for the green emission at 541 nm from single LiYF4: 20.0%Yb3+/2.0%Ho3+/x%Ce3+(x=0% and 12.0%) microparticle under 532 nm pulsed laser excitation.


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Abstract Graphics


Ho3+

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