Enhancing Upconversion Luminescence of LiYF4:Yb,Er Nanocrystals

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Article 4

Enhancing Upconversion Luminescence of LiYF:Yb,Er Nanocrystals by Cd Doping and Core-Shell Structure 2+

Yiru Zhu, Shuwen Zhao, Bin Zhou, Hao Zhu, and Youfa Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04782 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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The Journal of Physical Chemistry

Enhancing Upconversion Luminescence of LiYF4:Yb,Er Nanocrystals by Cd2+ Doping and Core-shell structure Yiru Zhu,a

Shuwen Zhao, a Bin Zhou,b Hao Zhu, a Youfa Wang, *c

a. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China. b. State Key Laboratory of Material Processing and Die & Mould Technology, School of Material Sciences and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China. c. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT In this work, we report a phenomenon that upconversion emission intensity of Er3+ was enhanced by cadmium. Cd2+ was taken with 0, 3, 10, 20, 30 and 40 mol% as a new dopant to codope LiY0.78Yb0.20Er0.02F4, meanwhile the doping effects on the crystal structure, morphology and the upconversion fluorescence emission are investigated by XRD, Rietveld refinement, TEM, upconversion spectroscopy analysis methods in detail. The green upconversion emission intensity was enhanced and the maximum emission intensities were enhanced to twice when 10 mol% Cd2+ was codoped into tetragonal LiYF4, while its decay lifetime was reduced. Additionally,

we

further

designed

the

synthesis

of

the

homogeneous

LiYF4:20%Yb,2%Er,10%Cd@LiYF4 of which the emission intensity is 11 times higher than the Cd2+-free nanocrystals, meanwhile an intriguing morphology of core-shell nanocrystals was obtained. This investigation may be useful for design and improvement of upconversion materials for fulfilling the prerequisite of the practical application.


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■ Introduction In the past few years, lanthanide-doped upconversion (UC) nanocrystals and microcrystals have attracted a great deal of attention as a kind of significant luminescence materials, with the application in laser,1 solar cells,2-3 biology,4-6 sensors,7 carrying drugs,8-9 etc., due to their splendid optical properties rooting in the special energy level structure of lanthanide. With the development of inexpensive semiconductor lasers and the inspiration of the design and synthesis of UC nanomaterials, the research on the field of UC is pushing into the upsurge. Compared with the traditional biological fluorescent materials (such as fluorescent protein, quantum dots, organic dyes, etc.),10 UC luminescence materials have higher fluorescence imaging stability, longer life expectancy and more narrow emission bands. However, due to the small particle size and surface defects of UC nanocrystal, the low luminescence efficiency is still a typical disadvantage for its applications. At present, in a variety of UC luminescent matrix materials, the most widely studied UC fluoride is NaYF4. However, in recent years, studies have shown that LiYF4 has also become a good choice because of its low phonon energy, which can suppress the non-radiative relaxation process which is not conducive to the luminescence. LiYF4 host not only shows a strong visible light emission, but also shows a strong deep ultraviolet (UV) to near-infrared fluorescence emission when suitable lanthanide ions are doped. Mahalinga et al.11 codoped Yb3+ and Tm3+ into LiYF4, the obtained nanocrystals have strong upconversion UV emission. Chen et al.12 found that the quantum efficiency of tetragonal LiYF4:Er3+ crystal is four times of the hexagonal NaYF4 crystal under the 1490 nm excitation of infrared light. Recently, Ding et al.13 used molten salt method to prepare LiYF4:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystal and also observed strong


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upconversion fluorescence emission of LiYF4 microcrystal. Thus, LiYF4 crystal with high UC luminescence efficiency is worthy of attention and research. It is well known that Auzel’s14 and Wright’s15 research showed UC luminescence intensity was mainly influenced by infra-4f electronic transition probabilities and specific self-absorption. A variety of measures have been imposed to improve the UC luminescence of UC nanoparticles, such as host lattice manipulation (including doping with different alkali mental ions),16-17 energy-transfer modulation (including doping with different transition metal ions),18 surface passivation (including heterogeneous core-shell,19-21 homogeneous core-shell structure22-31), surface-plasmon coupling (including doping with different noble mental ions),32 broadband sensitization,33-34 and photonic crystal engineering.35 Among these measures, achieving the combination of tailoring the local crystal field around activators through doping specific ions and coating a shell around pre-synthesized core is an effective way, since the possibility of the intra-4f transition of activators is significantly influenced by their host symmetry according to the Judd-Ofelt theory36 and the growth of an inert shell surrounding the core can prevent the activators in the core from nonradiative decay caused by surface defects. Previous researches have reported that the introduction of some lanthanide ions (e.g., Gd3+, Nd3+, Ho3+)37-40 and non-lanthanide ions (e.g., Li+, Mg2+, Ca2+, Sc3+, Ti4+, Cr3+, Mn2+, Fe3+, Ni2+, Mo2+, In3+, Sn4+, Bi3+ and Mn4+)41-48 into the host lattice can significantly enhance upconversion emissions. Thus, doping ions is an important means of introducing or improving material properties, widely used in the synthesis of functional materials.49 Taking into account that the radius of Cd2+ ion (110 pm, coordination number (CN = 8) is larger than Y3+ ion (101.9 pm, CN = 8),50 the same as the condition of larger ion Ca2+ doping into smaller Gd3+ ion host Yb/Er(Ho,Tm):NaGdF4 nanocrystals,51 projected to enhance the UC luminescence. On the other


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hand, compared with the heterogeneous core-shell structure, which have many lattice deformations between core and shell affecting UC luminescence because of large lattice mismatch, core and shell of the homogeneous core-shell structure have the same host lattice without lattice mismatch problem, avoiding the formation of luminescence quenching center. Due to the small particle size and large surface passivation of UC nanomaterials, surface modification of nanoparticles coated with homogeneous shell can improve the luminescence efficiency

to

a

great

extent.52

Therefore,

homogeneous

LiYF4:Yb,Er@LiYF4

and

LiYF4:Yb,Er,Cd@LiYF4 core-shell structure has been designed for the first time in this work. Herein, on account of changing the local surrounding symmetry of rare-earth ions and thus enhancing UC luminescence intensity, we introduce Cd2+ as a dopant ion into LiYF4:Yb,Er. We have systematic experimental investigations on the changes of the crystal structure and grain size characterized by X-ray powder diffraction (XRD)/ Transmission Electron Microscope (TEM) and the corresponding UC luminescence properties characterized by UC emission spectra and radiation lifetime of the tetragonal LiYF4 tridoped by different concentration Cd2+ with Yb3+/Er3+. Luminescence has been apparently enhanced and the lifetime also has been changed. Remarkably, the UC emission intensity of the 10 mol% Cd2+-doped LiY0.68F4:Yb0.20,Er0.02 are 2.2 times and 10 mol% Cd2+-doped LiY0.68F4:Yb0.20,Tm0.02 are twice of that of the Cd2+-free nanocrystal. We additionally

design

the

synthesis

of

the

homogeneous

core-shell

structure

LiYF4:20%Yb,2%Er,10%Cd@LiYF4 of which the UC intensity is 11 times higher than the Cd2+-free core LiYF4:20%Yb,2%Er nanocrystals under the ‘1+1>2’ effect of tailoring the local crystal field and the core-shell protection. ■ Experimental Section Materials


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Lithium hydroxide (LiOH, 99.99%), triflouroacetic acid (CF3COOH, 99.0%), cadmium oxide (CdO, 99.99%), ytterbium oxide (Yb2O3, 99.99%), erbium oxide (Er2O3, 99.99%), thulium oxide (Tm2O3, 99.99%), oleic acid (OA, analytical grade, AR), 1-octadecene (ODE, 90%) were purchased from Aladdin Inc. All the chemicals were used directly without further purification. Synthesis of LiYF4:Yb,Er(Tm),Cd nanocrystals A 20% Yb3+, 2% Er3+(Tm3+), and xCd2+ tridoped LiY0.78−xF4 nanocrystals (x = 0, 3, 10, 20, 30, 40 mol%, marked S0, S1, S2, S3, S4, S5) were prepared via a modified trifluoroacetate thermolysis method.53 (CF3COO)3Li (1 mmol), (CF3COO)3Yb (0.20 mmol), (CF3COO)3Er (0.02 mmol) or (CF3COO)3Tm (0.02 mmol), and (CF3COO)2Cd (x mmol), (CF3COO)3Y (0.78−x mmol), (x = 0, 3, 10, 20, 30, 40) were added to a 50 mL three-neck round-bottom flask containing OA (6 mL) and ODE (6 mL). The flask was heated at 80 °C for 10 min under vacuum and continuously heated at 100 °C for 40 min under dry argon atmosphere for eliminating oxygen, the residual water, and CF3COOH from the solution (the resulted solution marked solution A). The solution was then heated to a certain temperature 320 °C at a rate of 7 °C·min-1 under a gentle flow of argon gas for 35 min. The solution was then cooled down gradually to room temperature and the products were precipitated by addition of ethanol, collected by centrifugation at 10000 rpm for 10 min, washed with cyclohexane/ethanol for several times to remove any impurities and a part of high-boiling organic solvents, finally redispersed in cyclohexane (6 mL). Synthesis of core-shell nanocrystals A simplified preparation method of core-shell nanocrystals was designed here. In detail, shell precursors solution was prepared as the procedures of solution A. Note that the solid reagents only contain 1 mmol of CF3COOLi and 1 mmol of (CF3COO)3Y. Above-collected UC nanocyrstals were dispersed into 4 mL ODE and then moved to a 50 mL 3-neck round-bottom


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flask (solution B). The solution B was heated to 280 °C under a gentle flow of argon gas (remove moisture) with vigorous stirring. When the temperature was arrived, the solution A was injected into solution B at a rate of 4 mL·min-1. After this, the solution was heated to 320 °C for 35 min. The experimental steps that followed are the same as the synthesis of LiYF4:Yb,Er(Tm),Cd nanocrystals. Characterization XRD patterns of products were characterized using a Bruker D8 Advance X-ray diffractometer in the 2θ scope from 10° to 80°, with Cu Kα1 irradiation (λ = 1.5406 Å). The LiYF4:Yb,Er,Cd nanocrystals redispersed in cyclohexane was collected by centrifugation and dried in air at 60 °C for 12 h, the obtained solid was used for XRD test. Particle sizes and shapes, and core-shell nanostructure were observed by TEM and High-Resolution TEM (HRTEM) (JEM 2100F, JEOL Corp, Japan, with a field emission gun operating at 200 kV). The core-shell structure was additionally exhibited by Scanning Electron Microscope (SEM) and Energy Disperse Spectroscopy mapping (EDS mapping), tested by Hitachi JSM-5610LV. TEM specimens were prepared by placing a drop of dispersed UC nanocrystals cyclohexane solution dried on carbon-coated copper grids. The concentrations of element Cd/Y/Yb/Er were obtained using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) by Prodigy 7. The concentration of Li element was tested by GBC AVANTA M type atomic absorption spectrometer. The UC emission spectra, decay time and quantum yields were acquired using Edinburgh instruments FSL980 fluorescence spectrophotometer excited by 980 nm Optek Opo Laser, and parameters of excitation laser was pulse width 5 ns, repeat frequency 10 Hz, and the precise power density was 5 W·cm-2. Additionally, the UC quantum yields were measured with this instrument equipped with an integrating sphere. The as-synthesized UC samples were


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homogeneously dispersed in cyclohexane at a concentration of 0.004 g·mL-1. All of the measurements were performed at room temperature. ■ Results & Discussion Section 3.1. Effects of Cd2+ ion on the phase and morphology of LiYF4:Yb,Er nanocrystals Fig. 1a is the XRD patterns of the LiYF4:Yb,Er crystals codoped with the Cd2+ ion concentrations of 0, 3, 10, 20, 30, and 40 mol%. Based on the analysis of XRD pattern of samples, all the diffraction peaks of samples can be indexed to the pure XRD pattern of tetragonal LiYF4 (JCPDS 17-0874). The results show that doping of Cd2+ ions has not led to the formation of other phases. The quantitative doping concentration of Cd2+ ions was analyzed by ICP-AES as shown in Table. 1. The result confirms the successful doping of Cd2+ ions in LiYF4:Yb,Er crystals. However, the actual concentration of Cd2+ ions doping found to be 0, 0.114, 0.074, 4.936, 21.850, and 27.709 mol% is lower than the theoretical doping concentrations of 0, 3, 10, 20, 30, and 40 mol%, indicating that a portion of Cd2+ ions is lost during the doping procedures. To investigate the change of the crystal structure caused by Cd2+ ions doping, the subtle differences of the diffraction peak at the 2θ scope from 18° to 20.5° was selected to enlarge, as shown in Fig. 1b. With the introduction of Cd2+, the diffraction peak can be seen a slight shift. The diffraction peak shifts toward smaller angles as the concentration of Cd2+ ions increases up to 3 mol%, and then moves gradually to larger angles with the increasing of Cd2+ ion concentration , which indicates that when the Cd2+ ions concentration rising, the host lattice expands at first (0—3 mol%), then shrinks and arrived to the smallest when 20 mol% Cd2+ was doped and then expands to close the cell volume of Cd2+-free when more Cd2+ was codoped. Fig. 2b shows the changing trend of the cell parameter a (=b) and c, which is the same as the cell volume changing trend in Fig. 2a.


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(b)

(a) S5

S5

S4

S4

S3 S2 S1

Intensity (a.u.)

Intensity (a.u.)

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


S3

S2 S1

S0 JCPDS 17-0874

20

40 60 2θ (degree)

80 18.0

S0

18.5

19.0 19.5 2θ (degree)

20.0

20.5

Fig 1. (a) XRD curves of LiYF4:20%Yb,2%Er nanocrystals doped with different doping contents of Cd2+ ions. The corresponding standard data of LiYF4 (JCPDS 17-0974) are given as references. (b) is the enlarged major area. Table 1. ICP analysis results of Cd, Y, Yb and Er elements of 0, 3, 10, 20, 30 and 40 mol% Cd2+-doped samples. Samples

Cd (%)

Y (%)

Yb (%)

Er (%)

S0

0

78.562

19.609

1.829

S1

0.114

78.845

19.003

2.038

S2

0.074

76.154

21.565

2.206

S3

4.936

69.157

23.454

2.454

S4

21.850

54.891

21.024

2.235

S5

27.709

46.941

22.876

2.473


9

5.175 Cell parameter a=b (Å)

286.0 285.5 285.0 284.5 284.0 283.5 283.0 282.5 282.0

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a(=b) c

5.170

10.70

5.165 5.160

10.68

5.155

10.66

5.150 5.145

0 3 10 20 30 40 2+ Cd doping concentration (mol%)

10.72

Cell parameter c (Å)

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

Cell volume (Å）


0 3 10 20 30 40 2+ Cd doping concerntration (mol%)

10.64

Fig 2. Changing trends of (a) cell volumes, (b) cell parameter a (=b) and c versus Cd2+ doping concentration. Low concentration of codoped Cd2+ (110 pm, CN = 8) can replace Y3+ (101.9 pm, CN = 8) and be located in the Y3+ site because its radius is smaller than 1.15 times of that of Y3+ (∆r' = 8.1 pm). The changing trends of the cell parameters indicate that when a little (0—10 mol%) Cd2+ was doped, it would be located in the position of Y3+ and enlarged a (=b) and c, as shown in Fig. 3 (medium). The smaller cell volume of 10 mol% than that of 3 mol% is contributed to the less actual doping Cd2+ of 10 mol%. However, because of the unequal electrovalence between Cd2+ and Y3+, the Cd2+ cannot occupy Y3+ sites continuously with the rise of Cd2+ content. As the consequence, the cell volume decreased in Fig. 2a when Cd2+ ion concentration rose to 20 mol%. In this stage, it was simultaneously realized the cell volume decrease and the positive charge balance. This phenomenon only can be explained by the induction of Li+ to occupy a part of the Y3+ sites and Cd2+ also occupy Y3+ sites, as shown in Fig. 3 (right). Li+ concentration increased from 5.35 wt% (Cd2+-free, S0) to 7.75 wt% (20mol% Cd2+, S3) and maintained at 7.04 wt% (40mol% Cd2+, S5), which supports the above analysis. The radius of Li+ (92 pm, CN = 8) is smaller than that of Y3+ (∆r" = 9.9 pm). At the content of Cd2+ was 20 mol%, the cell volume reached the lowest level. After this, with the increasing of Cd2+ concentration, more Cd2+


10

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occupied Y3+ position, as a result, the cell volume rise again and close to that of Cd2+-free.

Fig 3. The crystal structure diagrammatic sketch. Left: of LiYF4; medium and right: Cd2+-doped LiYF4. The TEM images of the prepared nanocrystals are given in Fig. 4. It is observed that all of these UC nanocrystals were uniform diamond-shaped structure with good dispersion which shows that the shape of these UC nanocrystals has not been affected by introducing Cd2+ content. The clear lattice fringes were shown in the HRTEM images, indicating that all nanocrystals are of single-crystalline nature. We can observe marked d-spacing of 4.70, 4.76, 4.72, 4.58, 4.61, 4.73 Å respectively and corresponding (101) plane. As exhibited, d(101) spacing of LiYF4 expands when 3 mol% Cd2+ was doped, while the d-spacing shrinks at 20 mol%, and then expands with increasing content of Cd2+. This fact is consistent with the XRD analysis results. Moreover, we measured the length and width of the grain, as shown in the Fig. 4 and Fig. S1. It is found that the grain size of the UC nanocrystals changes from 0, 3, 10, 20, 30 to 40 mol%, the average grain length of the nanocrystals is 164, 202, 176, 203, 196, 199 nm, respectively, the average grain width of the UC nanocrystals is 100, 104, 91, 102, 100, 99 nm.


11


S0-length Mean: 164 nm σ = 0.98

20

10

160

164

168

30

S1-length Mean: 202 nm σ = 0.97

20 10 0 192

172

Mean: 100 nm σ = 0.93

30 20 10 0 94

96

98

100

102

Particle diameter (nm)

Percentage (%)

40

S0-width

200

204

104

S1-width

30

10 0 96

100

104


20 10

100

104

108


Mean: 104 nm σ = 0.95

20

Mean: 104 nm σ = 0.95

S1-width

30

0 96

208


Particle diameter (nm) 40

196

40

40

Percentage (%)

0 156

40

Percentage (%)

Percentage (%)

(d)

30

(c)

Percentage (%)

(b)

(a)

Percentage (%)

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108

S0- width Mean: 100 nm σ = 0.93

30 20 10 0 94

96

98

100

102

104


Fig 4. TEM characterizations of various concentrations Cd2+-doped LiYF4:Yb,Er nanocrystals indicating morphology and histograms of their size distribution which were obtained from TEM (> 200 particles). (a) S0, (b) S1, (c) S2, (d) a single particle diagram. The insets are corresponding HRTEM images.


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Fig

5.

TEM

images

(LiYF4:20%Yb,2%Er),

of (b)

prepared

core

core-shell

1

and

core-shell

nanocrystals

(LiYF4:20%Yb,2%Er@LiYF4),

(a) (c)

core

1

core

2

(LiYF4:20%Yb,2%Er,10%Cd), (d) core-shell 2 (LiYF4:20%Yb,2%Er,10%Cd @LiYF4). As shown in Fig. 5, TEM was employed to characterize the morphology of the synthesis core-shell UC nanocrystals. The nanocrystals were uniform and the shape became more complex. The mean size of nanocrystals enlarged (Fig. S2), including the mean length increased from 168 nm (core 1) to 201 nm (core-shell 1), from 171 nm (core 2) to 201 nm (core-shell 2), and the mean width increased from 58 nm (core 1) to 71 nm (core-shell 1), from 74 nm (core 2) to 94 nm (core-shell 2). This fact suggests the subsequent growth LiYF4 was grown on the surface of the LiYF4 core. Moreover, the EDS images of core-shell 1 (Fig. S4) also proved the core-shell structure by showing the larger concentration area of Y, F elements in the outside, contrasted


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with the smaller concentration area of Yb and Er, which can be deduced that the shell has grown around the core. The larger increment in length than width of the nanocrystals could be attributed to the easier growth of diamond ‘a’ position than ‘b’ position, as shown in Fig. 4 (due to higher surface energy from larger specific surface area). The growth mechanism of these core-shell nanocrystals still need to be further investigated. ,Er nanocrystals 3.2. Effects of Cd2+ ions on UC luminescence of LiYF4:Yb, Fig. 6 shows the UC emission spectra of LiYF4:Yb,Er nanocrystals doped with different concentrations of Cd2+ ions under 980 nm laser excitation. It can be seen that the spectra of the Cd2+-doped samples exhibit 5 main emission bands in the range of 450—750 nm and these emissions can be attributed to the following transitions of Er3+ respectively: 2H11/2/4I15/2 (~523 nm), 4S3/2/4I15/2 (~542, 551 nm), 4F9/2/4I15/2 (~653, 667 nm). Clearly, the concentration of Cd2+ ions has an effective influence on the emission intensities of LiYF4:Yb,Er nanocrystals. With the concentration of Cd2+ increasing up to 10 mol%, the UC emission arrived to the maximum of 2.2 times of Cd2+-free nanocrystals. However, the emission intensities decrease with further increasing the concentration of Cd2+ ions. In addition, the UC emissions of 4S3/2/4I15/2 and 4

F9/2/4I15/2 both split into two peaks. Individual energy level split is closely related to the

symmetry of the crystal field. The symmetry of the crystal field is less, the level split more peaks.54 We can infer that the surrounding environment of the 10 mol% Cd2+ sample is the most asymmetric. Fig. 4 and Fig. S1 can exclude the enhancement effect of the enlarged size of S2, because the much larger size of S1 and S3—S5 which have the weaker intensity than that of S2. These results demonstrate that the enhancement of the UC luminescence can be mainly explained by Cd2+ doping, while the minor influence of the size and morphology of the UC nanocrystals can be out of consideration.


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S5 S4 S3 S2 S1 S0

Intensity (a.u.)

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450

500

550 600 650 Wavelength (nm)

700

750

Fig 6. UC emission spectra of LiY0.78-xF4:20%Yb,2%Er,xCd, x = 0, 3, 10, 20, 30, 40 mol%. Note: The UC emission spectra of S1 coincides with S4. According to the above-mentioned measurement results, the change of UC luminescence properties as the result of the variety of Cd2+ codoping concentration can be attributed to the following reasons. Firstly, it is well known that the UC luminescence intensities of Er3+-doped UC nanocrystals are dependent on their infra-4f transition probabilities, which are affected significantly by the local crystal field symmetry of the Er3+, and changing the environment of Er3+ can produce a hypersensitive transition. In the structure of LiYF4:Yb,Er occupies the position of Y3+ and the introduction of Cd2+ will lead to lattice expansion, lowering the symmetry of the crystal field around Yb3+ and Er3+. As the result, the lattice distortion increases and the crystal field becomes asymmetric, leading to hypersensitive electron transition and the enhancement of UC luminescence intensity. These changes will increase with the rising content of Cd2+. Ultra-sensitive electronic transitions of Yb3+ and Er3+ caused by asymmetric crystal field increase the occupancy of Er3+ excited state, thus enhancing the UC luminescence. Secondly, when Cd2+ doping content further increases, reducing the dispersion and Cd2+ gathering into clusters, ultra-sensitive electron transition was weakened for reduction of crystal local


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asymmetry around the rare-earth ions55 and thus the intensity of UC luminescence diminished. We can know from the UC emission spectra, S2 is the optimum doping content with the highest intensity of UC luminescence. Quantum yield (QY) of S0 was calculated to be 0.21% while that of S2 was 0.42%. Additionally, we measured the emission decay curves of 4S3/2/4I15/2 (~551 nm), 4F9/2/4I15/2 (~653 nm) transition in LiYF4:20%Yb,2%Er nanocrystals with 0-40 mol% Cd2+ ions under 980 nm excitation, well fitted all the decay curves of the samples and calculated the luminescent lifetime, shown in Fig. 7a and b by one-exponent function: ‫ܫ = )ݐ(ܫ‬଴ + ‫ܣ‬ଵ ݁‫(݌ݔ‬−‫ݐ‬/߬ଵ ), where ‫ܫ‬ and ‫ܫ‬଴ represent the luminescence intensities at time ‫ ݐ‬and 0, while ‫ܣ‬ଵ is a constant, ‫ ݐ‬is time, ߬ଵ stands for the decay time of the transient for the exponent. It could be seen in Fig. 7c that the lifetimes of 551 nm and 653 nm emission are prolonged with the concentration of Cd2+ ions increasing from 0 to 3 mol%, indicating that Cd2+ doping truly improve the luminescence. Then in general lifetimes are shortened with further increasing the concentration of Cd2+ ions until 40 mol%.


16

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1.0

(a)

S0， τ1=0.734 ms

1.0

S1， τ1=0.740 ms

0.8

S2， τ1=0.635 ms S3， τ1=0.548 ms

0.6

S4， τ1=0.544 ms S5， τ1=0.320 ms

0.4 0.2 0.0

1.0

(b)

S0， τ1=0.818 ms S2， τ1=0.724 ms

0.8

1

2 3 4 Decay time (ms)

5

6

551 nm 653 nm

0.8

S3， τ1=0.641 ms

0.6

S4， τ1=0.687 ms S5， τ1=0.441 ms

0.4

0.6 0.4

0.2 0.0

0

(c)

S1， τ1=0.918 ms

Intensity (a.u.)

Intensity (a.u.)

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


0.2

0

1

2 3 4 Decay time (us)

5

6

0 10 20 30 40 2+ Cd doping concerntration (mol%)

Fig 7. The emission measurement decay curves of (a) 551 nm and (b) 653 nm in LiYF4:20%Yb,2%Er nanocrystals codoped with 0 — 40 mol% Cd2+ ions and (c) lifetime changing trends of 551 nm, 653 nm emissions versus Cd2+ concentration. It’s noteworthy for the changing trend of the decay time. Why the lifetime of 10 mol% is not the largest? Asymmetric crystal field and lattice distortion are two sides should be taken into account here. This phenomenon can be owed to lattice distortion which would relax the electrons populated from the excited state to the ground state. The improvement of decay time was caused by the relaxation of some 4S3/2 and 4F9/2 excited state populated electrons to ground state, indicating the decay time was increased because the relaxation needed a longer time.56 The crystal lattice distortion increased with the rising of Cd2+ content, which would promote non-radioactive relaxation, so as to reduce the cross relaxation cooperation, energy transfer and the emission lifetime of 551 and 653 nm. The changing mechanisms of the lifetime of 551 and 653 nm emissions were similar. However, the lifetime of 653 nm emission was prolonged when Cd2+ content was over 20 mol%, because hypersensitive transition of Yb3+/Er3+ ions was dented with decreasing asymmetry crystal. Non-radiative relaxation would be reduced because of the reduction of lattice distortion, and more upper excited state populated electrons were relaxed to 4

F9/2 in order to improve decay time constants.


17


The UC emission spectra of LiYF4:Yb,Tm nanocrystals doped with Cd2+ ions was investigated in Fig. 8. The UC emission spectra of LiYF4:Yb,Tm shows the wide emission bands in the range of 300−850 nm and the emissions can be attributed to the transitions of Tm3+: 1I6/3F4 (~348 nm), 1D2/3H6 (~362 nm), 1D2/3F4 (~454 nm), 1G4/3H6 (~483 nm), 1G4/3F4 (~650 nm), 3

H4/3H6 (~792 nm). When the concentration of Cd2+ was 10 mol%, UC emission can be

enhanced twice of Cd2+-free nanocrystals. Similarly, both lifetime of 483 and 792 nm emissions at 10 mol% Cd2+ is longer than free-doped nanocrystals, as shown in Fig. 9a and b, respectively. And QY of Cd2+-free nanocrystals is 0.92% while that of 10 mol% Cd2+-doped nanocrystals is 1.80%. 10% 0%

Intensity (a.u.)

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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300

400

500 600 700 Wavelength (nm)

800

Fig 8. UC emission spectra of LiY0.78−xF4:20%Yb,2%Tm,xCd, x = 0, 10 mol%.


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1.0 (a)

1.0 (b)

0%, τ1=0.751 ms 10%, τ1=0.835 ms

0%, τ1=0.886 ms 10%, τ1=0.994 ms

0.8 Intensity (a.u.)

0.8

Intensity (a.u.)

0.6 0.4 0.2

0.6 0.4 0.2 0.0

0.0

0

1


5

0

6

1


5

6

Fig 9. The emission decay curves of (a) 483 nm and (b) 792 nm in LiYF4:20%Yb,2%Tm nanocrystals codoped with 0 and 10 mol% Cd2+ ions emissions. lare-scale core-shell 2 core 2 core-shell 1 core 1

Intensity (a.u.)

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


350 400 450 500 550 600 650 700 750 Wavelength (nm)

Fig

10.

UC

emission

spectra of

(LiYF4:20%Yb,2%Er@LiYF4),

core

core 2

1

(LiYF4:20%Yb3+,2%Er3+),

(LiYF4:20%Yb,2%Er,10%Cd),

core-shell core-shell

1 2

(LiYF4:20%Yb,2%Er,10%Cd@LiYF4), large-scale LiYF4:20%Yb,2%Er nanoparticles. An inert shell LiYF4 was set to grow surrounding LiYF4:20%Yb,2%Er core and LiYF4:20%Yb,2%Er,10%Cd core. Fig. 10 exhibits a great enhancement through core-shell nanostructure. It can be observed that the integral intensity of core-shell 1 is measured to be 5 times as high as that of core 1, while the integral intensity of core-shell 2 is 11 times higher than


19


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core 1, which means the ‘1+1>2’ effect on the combination of tailoring the local crystal field and the core-shell protection. More importantly, the emission spectra also provided the evidence for the formation of core-shell structure in terms of the lower intensity and larger particle size of large-scale LiYF4:20%Yb,2%Er nanocrystals than that of core-shell 1 (Fig. S5), thus the effect of size-increased enhancement in core-shell nanocrystals can be is excluded. And UC QY of core-shell 1 is 1.05% while that of core-shell 2 is 2.28%. ■ Conclusion In summary, LiYF4:Yb,Er(Tm) UC nanocrystals doped with different concentrations of Cd2+ ions were synthesized by a trifluoroacetate thermolysis method for the first time. XRD investigations illustrate that the cell volume and cell parameters were changed without phase transformation, the local crystal field of LiY0.78−xYb0.20Er0.02CdxF4 host lattice can be expectedly changed. Visible green UC luminescence of UC nanocrystals was enhanced to 2.2 times with the maximum intensity obtained at the doping of 10 mol% Cd2+ ions, compared to the free-doped one, also decay time of 4S3/2 and 4F9/2 state can be altered via Cd2+ ions doping. Similarly, 10 mol% Cd2+-doped LiYF4:Yb,Tm achieved the enhancement to 2 times. Additionally, based on core-shell theory, we initially fabricate the homogeneous LiYF4:20%Yb,2%Er,10%Cd@LiYF4 core-shell structure with special morphology and the enhancement of UC emission to 11 times, and its UC QY is 2.28%.

Assoiciated Content Section ■ Author Information *Corresponding Author: E-mail: [email protected]; Fax: +86-27-87880734; Tel: +86-27-87651852.


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■ Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ■ Notes The authors declare no competing financial interest. ■ Acknowledgment This work was financially supported by National College Students’ innovation and entrepreneurship training plan of China (20161049701008). ■ Supporting Information The Refined lattice constants of as-synthesized LiY0.78-xF4:20%Yb,2%Er,xCd UC nanocrystals. TEM characterizations of various concentrations Cd2+-doped LiYF4:Yb,Er nanocrystals and prepared large-scale LiYF4:20%Yb,2%Er. SEM and EDS images of core-shell structure. This information is available free of charge via the Internet at http://pubs.acs.org ■ REFERENCES 1. Danger, T.; Koetke, J.; Brede, R.; Heumann, E.; Huber, G.; Chai, B. H. T. Spectroscopy and Green Upconversion Laser Emission of Er3+-Doped Crystals at Room Temperature. J. Appl. Phys. 1994, 76, 1413-1422. 2. Shalav, A.; Richards, B. S.; Trupke, T.; Krämer, K. W.; Güdel, H. U. Application of NaYF4:Er3+ Up-Converting Phosphors for Enhanced Near-Infrared Silicon Solar Cell Response. Appl. Phys. Lett. 2005, 86, 013505.


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Enhancing Upconversion Luminescence of LiYF4:Yb,Er Nanocrystals

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