The novel upconversion properties of LiYbF4:Er microcrystalscompared to the Na counterpart{

Xiangyu Zhang,a Minqiang Wang,*a Jijun Ding,a Dangli Gao,b Yanhua Shia and Xiaohui Songa

Received 18th July 2012, Accepted 28th September 2012

DOI: 10.1039/c2ce26159f

Erbium-doped ytterbium fluoride compounds with different

crystal phases and morphologies have been synthesized via a

facile hydrothermal route assisted with EDTA. Tunable

upconversion emissions can be obtained by replacing Na+ with

Li+ in NaYbF4 microcrystals. An interesting blue shift is

observed and a mechanism based on a weakened polarization

effect is proposed.

Rare-earth ion (RE3+) doped phosphors comprising proper host–

dopant combinations can convert near infrared excitation radia-

tion into visible emissions.1 These unique anti-Stokes emitters, now

widely known as upconversion (UC) phosphors, have evolved as a

rapidly growing field due to creating new applications in diverse

fields such as displays, biological assays, and solar cells.2–10 The

phosphors for multiplexed detection have also been explored.

However, phosphors with multicolor emission are required.

Upconversion phosphors are typically composed of an inor-

ganic matrix and RE3+ dopant ions embedded in the matrix. The

matrix can significantly modify the UC processes by exerting a

crystal field around the dopant ions and subtracting excitation

energy of the dopant through lattice vibration.2 Therefore, an

appropriate host material is essential in the synthesis of RE3+-

doped phosphors with a controllable emission profile and high UC

efficiency. To date, many studies have been dedicated to the

synthesis of upconverted RE fluoride micro- and nanocrystals due

to their low phonon energy and relatively high chemical stability.

Among these materials, NaYF4 and NaYbF4 have been reported

to be the best host materials for upconverting lanthanide ions and

their phosphors doped with various lanthanide ions have been

studied.11–23 Recently, Yang reported the controlled synthesis of

both hexagonal phase NaYbF4 microtubes and cubic phase

NaYbF4 nanospheres via a rational hydrothermal method.24

Novel UC including bright ultraviolet (UV) and eye-visible blue

emissions of RE3+-doped NaYbF4 phosphors are achieved.24,25

Studies that focus on fabricating RE-doped octahedral LiYbF4

nanostructures and microstructures with comparable UC emission

intensity relative to NaYF4 host matrix, however, remain few.26–28

It is well known that the spectral properties of UC materials

depend highly on the host crystal structure, size and shape. It is an

important challenge to determine how the novel spectra properties

depend on host crystal structure, size, shape and phonon energy.

Liu’s group demonstrated rational tunability of the size and phase

of UC NaYF4 nanorods by RE3+ doping.14 Zhang et al. tuned the

UC luminescence of NaYF4 nanocrystals by Li and K doping.29 In

this work, replacing Na+ ions with Li+ ions in NaYbF4, the

crystallographic phase, size and color light emission of fluoride

microcrystals can be simultaneously controlled. It is also noted

that an interesting blue shift of the spectra is observed except for

the change of the intensity ratios between the green and red

emission peaks in LiYbF4:Er relative to its counterpart

NaYbF4:Er.

Erbium-doped NaYbF4 microcrystals have been fabricated

through a facile hydrothermal method in the presence of the

chelating agent ethylenediamine tetraacetic acid (EDTA) reported

previously.27,28,30 Additional experimental details, instrumentation

and other corresponding characterizations of microcrystals are

provided in the ESI.{The size and morphology of the RE3+-doped NaYbF4

microcrystals were studied by scanning electron microscopy

(SEM). Fig. 1a shows a representative SEM micrograph of Er-

doped NaYbF4 (2 mol%) microrods with an average diameter of 2

mm and length of about 5 mm. The XRD patterns images indicate

that the rod-like products are b-NaYbF4 crystals (JCPDS No. 27-

1427).

The same synthetic procedure was further used to synthesize

LiYbF4 microcrystals doped with 2 mol% Er by replacing Na+

ions with Li+. However, under identical experimental conditions,

the size, morphology and crystal phase of LiYbF4 are different

from NaYbF4 microrods. As is shown in Fig. 1c, the sample is

found to crystallize in pure tetragonal phase (JCPDS No. 71-1211).

It can be observed that these particles appear octahedral in shape

with an average diameter of 10 mm (Fig. 1b).

Each RE3+ ion possesses a distinct set of energy levels that result

in characteristic emission at a particular wavelength.31 However,

the color of the UC emission from these phosphors can be readily

manipulated by modifying the fluorescence branching ratio that

can generally be affected by the environment of matrix and dopant

combination.

aElectronic Materials Research Laboratory (EMRL), Key Laboratoryof Education Ministry; International Center for Dielectric Research,Xi’an Jiaotong University, Xi’an 710049, China.E-mail: mqwang@mail.xjtu.edu.cnbCollege of Science, Xi’an University of Architecture and Technology,Xi’an 710055, China{ Electronic supplementary information (ESI) available: Experimentalinformation and supplementary data. See DOI: 10.1039/c2ce26159f

CrystEngComm Dynamic Article Links

Cite this: CrystEngComm, 2012, 14, 8357–8360

www.rsc.org/crystengcomm COMMUNICATION

This journal is � The Royal Society of Chemistry 2012 CrystEngComm, 2012, 14, 8357–8360 | 8357

Dow

nloa

ded

by Y

ork

Uni

vers

ity o

n 14

Mar

ch 2

013

Publ

ishe

d on

04

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CE

2615

9FView Article Online / Journal Homepage / Table of Contents for this issue

Under excitation at 980 nm, a set of Er-doped fluoride samples

including NaYbF4 and LiYbF4 microparticles both exhibit

characteristic sharp emission peaks resulting from 2H9/2 A 4I15/2

(410 nm), 2H11/2 A 4I15/2 (520 nm), 4S3/2 A 4I15/2 (550 nm), and4F9/2 A 4I15/2 (630–670 nm) transitions of Er3+ (Fig. 2a).32 Note

that the emission spectra were normalized at 540 nm for

comparison. The transitions of LiYbF4 microparticles are

consistent with those in the NaYbF4 counterparts. But the

LiYbF4 system exhibits significant changes of the relative intensity

in green and red spectral region relative to that in NaYbF4

counterparts (Fig. 2b), resulting in tunable color output, which

may be relative to the different saturated absorption effect and

thermal effect33 between LiYbF4:Er and NaYbF4:Er. It is

interesting to note that all the emission peaks display a blue shift

of about 0.35 nm in LiYbF4:Er relative to NaYbF4:Er in Fig. 2b.

The similar phenomenon of the blue shift appears in LiYF4:Er

relative to NaYF4:Er (Fig. S1 and S2, ESI{). This indicates that

the significant difference in the position of peak may be ascribed to

dissimilar crystal-field surroundings of Er3+ ions embedded in

LiYbF4 and NaYbF4. In the framework of the crystal field theory,

a correction on the center of gravity for the level by the crystal

lattice at the position of an impurity ion can be expressed in

general case as follows:34

HL~X

k,q

BkqCk

{q (1)

Here Bkq stands for the crystal field parameters, and Ck

{q is one-

electron spherical operator. Spherical operators include zero, odd

and even exponential items. Many studies indicate that every item

has the unique physical effects on spectra and energy levels. The

zero-exponential term has an effect on the shift of the center of

gravity for the level by electron-cloud expansion effect.

The study of the local atomic structure of RE3+ ion

surroundings can be a powerful tool for understanding the

distribution of electron-cloud. In order to know the microscopic

mechanisms governing the blue shift in LiYbF4, we analyzed the

coordination construction of RE3+ and cation–anion electrostatic

force in these two compounds. Fig. 3 shows unit cell of the

structure of NaYbF4 and LiYbF4 compounds.

In the hexagonal NaYbF4 lattice, Na+ and RE3+ ions occupy

the same lattice sites due to their similar ionic radius and each

Yb3+ ion is coordinated with nine F2 ions.35 When Na+ ions were

substituted by Li+ with a smaller radius, the mismatched sizes of Li

and Yb make it impossible for them to occupy the same sites in the

unit cell. SEM images in Fig. 1a and b showed that the

morphology of the microcrystals changed from rod to octahedron.

The XRD patterns in Fig. 2c suggested that the crystal-phase

change occurs from hexagonal phase to pure tetragonal phase.

With Li+ ions instead of Na+ ions, the Li+, Yb3+ and F2 ions

could no longer form a stable hexagonal phase crystal lattice, in

where RE3+ ions coordinate with nine F2 ions,35 due to the steric

effect. In order to seek a new balance, each Yb3+ ion is

Fig. 1 Low-resolution SEM images and XRD patterns of fluoride

microcrystals. (a) NaYbF4:Er (2 mol%); (b) LiYbF4:Er (2 mol%) and (c)

XRD patterns.

Fig. 2 (a) Energy level diagrams of Er and Yb, and the relevant

transitions. (b) UC emission spectra of 2 mol% Er-doped fluoride

microcrystals. The samples were excited with 980 nm photons. Note the

spectra were normalized. The inset shows an intensity scale expansion.

Fig. 3 Schematic presentations of (a) hexagonal phase NaYbF4 and (b)

tetragonal phase LiYbF4 structures. In the NaYbF4 structure, an ordered

array of F2 ions offers two types of cation sites: one occupied by Na+ and

the other occupied randomly by RE3+ and Na+. In the LiYbF4 structure,

big atoms refer to RE3+ cations, the medium-sized atoms correspond to

Li+ cations and the small atoms to the F2 anions. The YbF8 polyhedra

and the LiF4 tetrahedra are shown. Lattice parameter, LiYbF4: a = b =

5.1335 A, c = 10.588 A; NaYbF4: a = b = 5.927 A, c = 3.273 A.

8358 | CrystEngComm, 2012, 14, 8357–8360 This journal is � The Royal Society of Chemistry 2012

Dow

nloa

ded

by Y

ork

Uni

vers

ity o

n 14

Mar

ch 2

013

Publ

ishe

d on

04

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CE

2615

9F

View Article Online

coordinated with eight F2 ions thus forming the YbF8 polyhedral

units and each Li+ ion is coordinated with four F2 ions forming

the LiF4 tetrahedral units.36 These units together compose LiYbF4

microcrystals with a scheelite structure. The crystallographic point

site symmetry with S4 symmetry in LiYbF437 is higher than that of

NaYbF4 with D2d symmetry38 resulting in a more symmetrical

distribution of electronic density and a weakened polarization

effect of the local environment in LiYbF4. The similar phenom-

enon of blue shift have been reported in borate glass, silicate glass,

phosphate glass and fluoride phosphate glass doped with Nd3+ due

to polarization effect corruption.39

Coordination number is an important parameter to evaluate the

stability of a coordination compound. A compound with a higher

coordination number is usually more stable.40 Hexagonal phase

NaYbF4 microcrystals are more stable than tetrahedral phase

LiYbF4 microcrystals, which indicates a weaker interaction of F2

ions to Yb3+ ions in LiYbF4. The increase in the symmetry of the

ligand field for RE3+ and a weakened interaction of F2 ions to

Yb3+ ions are responsible for the weakened polarization effect of

RE3+ ions local environment resulting in blue shift of the level and

the emission peaks of Er3+ in LiYbF4. LiREF4 matrices with the

weakened polarization effect of the local environment possibly lead

to the novel optical and magnetic properties.

The fluorescence emission in most UC materials is affected by

the distance between the Yb3+ and Er3+ ions. The lower limit of

the distance between the Yb3+ and Er3+ ions is decided by the

exact distance of the lattice sites occupied by RE3+ ions. As is

shown in Fig. 3, the smallest distances between the RE3+ ions is

smaller in LiYbF4 than that of NaYbF4, which may result in

supersaturated absorption effect and thermal effect in LiYbF4 and

subsequently affect the distribution of ions on excited states. We

also noted that the intensity ratio of 2H11/2–4I15/2 to 4S3/2–

4I15/2

depends on excitation power density (Fig. S3, ESI{), which

supported the above analysis.26

Another additional fact supporting the previously described

mechanisms for the blue shift of the spectra in LiYbF4 is the

optical spectroscopy of Eu3+ that can probe the local symmetry of

the ligand field. NaYbF4 and LiYbF4 microcrystals doped with

Eu3+ ions, as the structural probe, are investigated. The emission

spectra are displayed in Fig. 4. All emission peaks derived from

Eu3+: 5D0 A 7FJ levels in LiYbF4 are consistent with NaYbF4.

But the intensity ratio (g) of 5D0 A 7F2 to 5D0 A 7F1 transitions,

determined by the symmetry of the crystal sites in which Eu3+ ions

are located,41,42 is contrary for these two samples. From the

emission spectra, g is evaluated to be 1.22 and 0.42 for NaYbF4

and LiYbF4, respectively. It is well known that the intensity of the

magnetic dipolar 5D0 A 7F1 transition does not depend on the

ligand field of Eu3+, while the electric dipolar 5D0 A 7F2 transition

is known to be forbidden in the centrosymmetric environment.43

Therefore, the decrease in g value is related to centrosymmetric

environment of the ligand field for Eu3+ incorporated in LiYbF4.

As is discussed in the previous sections, the RE and Li atoms

coordinate with eight and four F atoms, respectively, and both of

the above sites form an S4 site symmetry with a nearly

centrosymmetric environment. It is fairly reasonable to conclude

the doped RE3+ ions accommodate the Yb3+ site with a unique

ideal S4 local site symmetry and the host lattice is not disrupted

significantly in LiYbF4:Er. The spectral blue shifts in LiYbF4:Er3+

can be attributed to the weakened polarization effect of RE3+ ions

due to the change of site symmetry.

In summary, erbium-doped NaYbF4 and LiYbF4 microcrystals

with different crystal phases and morphologies have been

successfully synthesized through a facile hydrothermal method

assisted with EDTA. The strong UC can be obtained under near

infrared excitation at 980 nm in two Er-doped fluoride

microcrystals. It is also noted that not only tunable emissions

can be obtained but also an interesting blue shift of the spectra is

observed in LiYbF4:Er relative to its counterpart NaYbF4:Er. The

point sites have been studied in two Er-doped fluoride crystals by

the coordination construction of RE3+ and the environment probe

spectra method. A comparison between the two host crystals

clearly shows that the symmetry of the electronic cloud surround-

ing the RE3+ is higher in LiYbF4 than in NaYbF4. The intrinsic

structural features of fluoride compounds are responsible for the

ultimate spectral properties. A mechanism of the blue shift based

on the weakened polarization effect is proposed, while the different

saturated absorption effect and thermal effect in two fluoride can

modify the population of ions on the excited and ground states

and tune the multicolor emission.

Acknowledgements

The authors gratefully acknowledge financial support from

Natural Science Foundation of China (Grant No. 61176056

and 91123019). This work has been financially supported by

the ‘‘13115’’ Innovation Engineering of Shaanxi Province

(2010ZDKG-58) and the open projects from Institute of

Photonics and Photo-Technology, Provincial Key Laboratory

of Photoelectronic Technology, Northwest University, China.

References

1 F. Auzel, Chem. Rev., 2004, 104, 139.2 F. Wang and X. G. Liu, Chem. Soc. Rev., 2009, 38, 976.3 R. S. Khnayzer, J. Blumhoff, J. A. Harrington, A. Haefele, F. Deng and

F. N. Castellano, Chem. Commun., 2012, 48, 209.4 M. L. Yin, L. Wu, Z. H. Li, J. S. Ren and X. G. Qu, Nanoscale, 2012, 4,

400.5 A. Priyam, N. M. Idris and Y. Zhang, J. Mater. Chem., 2012, 22, 960.6 G. G. Li, M. M. Shang, D. L. Geng, D. M. Yang, C. Peng, Z. Y. Cheng

and J. Lin, CrystEngComm, 2012, 14, 2100.

Fig. 4 Fluorescence emission spectra of NaYbF4 and LiYbF4 doped

with 2 mol% Eu3+. The samples were excited with 532 nm photons.

This journal is � The Royal Society of Chemistry 2012 CrystEngComm, 2012, 14, 8357–8360 | 8359

Dow

nloa

ded

by Y

ork

Uni

vers

ity o

n 14

Mar

ch 2

013

Publ

ishe

d on

04

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CE

2615

9F

View Article Online

7 X. M. Liu, X. G. Kong, Y. L. Zhang, L. P. Tu, Y. Wang, Q. H. Zeng,C. G. Li, Z. Shi and H. Zhang, Chem. Commun., 2011, 47, 11957.

8 S. Wang, S. Y. Song, R. P. Deng, H. L. Guo, Y. Q. Lei, F. Cao, X. Y.Li, S. Q. Su and H. J. Zhang, CrystEngComm, 2010, 12, 3537.

9 J. de Wild, A. Meijerink, J. K. Rath, W. G. J. H. M. van Sark and R. E.I. Schropp, Energy Environ. Sci., 2011, 4, 4835.

10 B. M. van der Ende, L. Aarts and A. Meijerink, Phys. Chem. Chem.Phys., 2009, 11, 11081.

11 F. Shi, J. S. Wang, X. S. Zhai, D. Zhao and W. P. Qin, CrystEngComm,2011, 13, 3782.

12 K. W. Kramer, D. Biner, G. Frei, H. U. Gudel, M. P. Hehlen and S. R.Luthi, Chem. Mater., 2004, 16, 1244.

13 F. Zhang, Y. Wan, T. Yu, F. Q. Zhang, Y. F. Shi, S. H. Xie, Y. G. Li,L. Xu, B. Tu and D. Y. Zhao, Angew. Chem., Int. Ed., 2007, 46, 7976.

14 F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C.Zhang, M. H. Hong and X. G. Liu, Nature, 2010, 463, 1061.

15 D. L. Gao, X. Y. Zhang and W. Gao, J. Appl. Phys., 2012, 111, 033505.16 Y. H. Wang, R. X. Cai and Z. H. Liu, CrystEngComm, 2011, 13, 1772.17 H. Zhang, Y. J. Li, Y. C. Lin, Y. Huang and X. F. Duan, Nanoscale,

2011, 3, 963.18 T. Jiang, W. P. Qin, W. H. Di, R. Y. Yang, D. M. Liu, X. S. Zhai and

G. S. Qin, CrystEngComm, 2012, 14, 2302.19 C. X. Li, Z. W. Quan, P. P. Yang, J. Yang, H. Z. Lian and J. Lin, J.

Mater. Chem., 2008, 18, 1353.20 N. J. J. Johnson, N. M. Sangeetha, J.-C. Boyer and F. C. J. M. van

Veggel, Nanoscale, 2010, 2, 771.21 S. J. Zeng, G. Z. Ren, C. F. Xu and Q. B. Yang, CrystEngComm, 2011,

13, 4276.22 A. Sen, S. L. Chaplot and R. Mittal, Phys. Rev. B: Condens. Matter,

2001, 64, 024304.23 M. Wang, C. C. Mi, Y. X. Zhang, J. L. Liu, F. Li, C. B. Mao and S. K.

Xu, J. Phys. Chem. C, 2009, 113, 19021.24 S. J. Zeng, G. Z. Ren, W. Li, C. F. Xu and Q. B. Yang, J. Phys. Chem.

C, 2010, 114, 10750.

25 Q. Q. Zhan, J. Qian, H. J. Liang, G. Somesfalean, D. Wang, S. L. He,Z. G. Zhang and S. Andersson-Engels, ACS Nano, 2011, 5, 3744.

26 V. Mahalingam, F. Vetrone, R. Naccache, A. Speghini and J. A.Capobianco, Adv. Mater., 2009, 21, 4025.

27 C. H. Lu, W. J. Huang, Y. R. Ni and Z. Z. Xu, Mater. Res. Bull., 2011,46, 216.

28 W. J. Huang, C. H. Lu, C. F. Jiang, J. Y. Jin, M. Y. Ding, Y. R. Ni andZ. Z. Xu, Mater. Res. Bull., 2012, 47, 1310.

29 Q. Q. Dou and Y. Zhang, Langmuir, 2011, 27, 13236.30 J. L. Zhuang, L. F. Liang, H. H. Y. Sung, X. F. Yang, M. M. Wu, I. D.

Williams, S. H. Feng and Q. Su, Inorg. Chem., 2007, 46, 5404.31 J.-C. G. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.32 J. Wang, F. Wang, J. Xu, Y. Wang, Y. S. Liu, X. Y. Chen, H. Y. Chen

and X. G. Liu, C. R. Chim., 2010, 13, 731.33 Y. Q. Lei, H. W. Song, L. M. Yang, L. X. Yu, Z. X. Liu, G. H. Pan, X.

Bai and L. B. Fan, J. Chem. Phys., 2005, 123, 174710.34 B. R. Judd, Phys. Rev., 1962, 127, 750.35 H. X. Mai, Y. W. Zhang, L. D. Sun and C. H. Yan, J. Phys. Chem. C,

2007, 111, 13721.36 D. Errandonea, F. J. Manjon, M. Somayazulu and D. Hausermann, J.

Solid State Chem., 2004, 177, 1087.37 G. H. Jia, C. F. Wang and S. Q. Xu, J. Phys. Chem. C, 2010, 114,

17905.38 Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li and X. Y.

Chen, Nanoscale, 2011, 3, 3164.39 F. X. Gan, Modern Science and Technology of Glass, Shanghai Science

and Technology Press, Shanghai, 1988 (in Chinese).40 M. R. Truter, Struct. Bonding, 1973, 16, 71.41 M. Strauss, T. A. Destefani, F. A. Sigoli and I. O. Mazali, Cryst. Growth

Des., 2011, 11, 4511.42 O. L. Malta, E. Antic-Fidancev, M. Lemaitre-Blaise, A. Milicic-Tang

and M. Taibi, J. Alloys Compd., 1995, 228, 41.43 D. Q. Chen, Y. L. Yu, P. Huang, F. Y. Weng, H. Lin and Y. S. Wang,

Appl. Phys. Lett., 2009, 94, 041909.

8360 | CrystEngComm, 2012, 14, 8357–8360 This journal is � The Royal Society of Chemistry 2012

Dow

nloa

ded

by Y

ork

Uni

vers

ity o

n 14

Mar

ch 2

013

Publ

ishe

d on

04

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2CE

2615

9F

View Article Online