Synthesis and Optical Properties of Ho 3+ , Pr 3+ Co-doped LaF3 Nanocrystals

Ballato, Synthesis, luminescence and scintillation of rare earth doped lanthanum fluoride nanoparticles, Optical Materials 33 (2010) 136. Stanley May, Frank C.J.M[r]

26

Hoang Manh Ha1,2,*, Tran Thi Quynh Hoa1,3, Le Van Vu1, Nguyen Ngoc Long1

1

Center for Materials Science, Faculty of Physics, VNU University of Science,

334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

2

Hanoi Architectural University, Km 10 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

3

National University of Civil Engineering, 55 Giai Phong, Hai Ba Trung, Hanoi, Vietnam

Received 25 October 2016

Revised 23 November 2016; Accepted 21 December 2016

Abstract: Ho3+, Pr3+ co-doped LaF 3 nanocrystals have been prepared by hydrothermal method The nanocrystals possess hexagonal structure with space group with lattice parameters a =

7.17 ± 0.01 Å and c = 7.33 ± 0.01 Å The room temperature absorption, photoluminescence (PL)

and photoluminescence excitation (PLE) spectra of LaF3:Ho3+, Pr3+ were investigated in detail It was discovered that the PL and PLE of Ho3+ and Pr3+ ions resulted from the optical intra-configurational f-f transitions The PL spectra are attributed to the transitions 5F3, 5S2 +5F4 and 5F5

→ 5 I8 in Ho3+ ion and the transitions 3P0→ 3 H5, 1D2→ 3 H4, 3P0→ 3 H6,3F2 and 3F4 in Pr3+ ion The optical transitions 5I8→ 5

F5, 5F4, 5F3, 3K8, 5F1, 5G5, 3K7 and 3H6 in Ho3+ ion and the optical transitions 3H4 → 3 P2 , 3P1, 3P0 and 1D2 in Pr3+ ion have been clearly observed not only in the PLE spectra, but also in the absorption spectra

Keywords: LaF3:Ho3+, Pr3+ nanocrystals, rare-earth, hydrothermal, absorption, photoluminescence

1 Introduction

Flouride materials are attractive materials with potential applications such as photonics, image display, light amplification [1-3] Among different inorganic matrices, lanthanum trifluoride (LaF3) is

an ideal host material for emitting ions in wide wavelength range, has low phonon energy The location of La3+ in the lattice can be replaced by rare earth (RE) ions Among various RE ions,

trivalent holmium ion (Ho3+) has been attracting more and more attention for laser applications due

to its important transition of 5I7 and 5I8 [3] To the best of our knowledge, most of previous works have been focused on the LaF3 doped with Eu3+ [4-6], Ce3+ [7], Er3+ [8], Ho3+ [3], Sm3+ [9], and co-doped

Ce3+ and Tb3+ [10] According to our knowledge, only a few works were devoted to the doping LaF3

with Ho3+ and Yb3+ [11,12] The radius of Ho3+ and Pr3+ is 0.894 Ǻ and 0.995 Ǻ, respectively, which is nearly equal to the radius of La3+ ions (1.061 Ǻ) Therefore, it is expected that the Ho3+ and Pr3+ ions

_



Corresponding author Tel.: 84-904418455

Email: hoangmanhha@hus.edu.vn

can easily substitute for La3+ ions in the hot crystal In this paper, we report on the synthesis of LaF3

nanocrystals co-doped with Ho3+ and Pr3+ by hydrothermal method Structure and optical properties of products are investigated in detail

2 Experimental

LaF3:Ho3+, Pr3+ was prepared by hydrothermal method from La(NO3)3, Sm(NO3)3, Pr(NO3)3

solution, and ammonium fluoride NH4F The molar ratio of (La(NO3)3 + Ho(NO3)3+ Pr(NO3)3):NH4F

= 1:3 was used Ho3+ and Pr3+ doped LaF3 were prepared for different concentrations of holmium oxide and praseodymium oxide from 1 to 5 mol% The final mixture was charged into the Teflon-lined autoclave, after that the mixture was annealed at 150 oC for 12 hours After the reaction was terminated, the autoclave was cooled to room temperature The precipitate was separated by centrifugation (8000 rpm, 20 min) and was washed subsequently with deionized water for 10 times Last products were dried in air at 60 oC for 6 hours, obtaining white fine powders

Crystal structure of nanoparticles was analysed by using an X-ray diffractometer (SIMEMS D5005, Bruker, Germany) with Cu–Kα1 irradiation (λ = 1.54056 Å) The composition of the samples was determined by an energy-dispersive X-ray spectrometer (EDS) OXFORD ISIS 300 attached to the JEOL-JSM5410LV scanning electron microscope The optical absorption spectra were recorded between 200 and 3000 nm using a Cary-5000 spectrometer The PL and PLE spectra were recorded by

FL 3-22 Jobin Yvon Spex spectrofluorometer All the measurements were carried out at room temperature

3 Results and discussion

3.1 Structure characterization

Typical XRD patterns of LaF3 nanocrystals co-doped with Ho3+ and Pr3+ are presented in Fig 1

Fig 1 XRD patterns of LaF3 nanocrystals co-doped with Ho3+ and Pr3+.

In all case, the powder XRD analysis evidenced that the obtained LaF3 samples have a hexagonal crystal structure All the XRD peaks are unambiguously indexed to hexagonal phase with space

group of LaF3 structure (JCPDS card no 32-0483) None of diffraction peaks of the other material phase are detected It is noted that the introduction of different RE ions does not change the crystal

structure The lattice parameters were calculated to be a = 7.17 ± 0.01 Å and c = 7.33 ± 0.01 Å, which are in good agreement with standard values a = 7.187 Å and c = 7.350 Å (JCPDS card No 32-0483)

By applying Scherrers formula ( ) [13] the average nanocrystal sizes of samples are estimated to be 24.3 - 26.7 nm

The EDS spectra of the undoped (a) and co-doped 5 mol% Ho3+ and 5 mol% Pr3+ (b) LaF3

nanocrystals are shown in Fig 2 The undoped LaF3 nanocrystals mainly consist of lantanum (La), fluor (F) elements, whereas in the LaF3:Ho3+, Pr3+ sample holmium (Ho) and praseodymium (Pr)

elements appeared

Fig 2 The EDS spectra of LaF3 (a) and LaF3:5 % Ho3+,5 % Pr3+ (b)

3.2 Optical Properties

The absorption spectra of LaF3:Ho3+, Pr3+ nanocrystals recorded in the wavelength range of 300–

700 nm at room temperature are shown in Fig 3 The Ho3+ and Pr3+ ions have 4f10 and 4f2 electron configuration which is characterized by 198 2S+1LJ free-ion levels The transitions 5I8→5

G6, 5F2, 5F4 and

5

S2 of the free Ho3+ ion are allowed in the electric-quadrupole approximation, the transition 5I8→3

K8 is allowed in the magnetic-dipole approximation, the transitions 5I8→5

F1 and 5F5 are forbidden in these approximations [14] The observed absorption bands of Ho3+ ions are centered at 640, 525, 481, 466,

450, 415, 383 and 360 nm and are attributed to the 4f–4f transitions of Ho3+ ions from the 5I8 ground level to the 5F5, 5F4, 5F3, 3K8, 5F1, 5G5, 3K7 and 3H6 excited levels, respectively The absorption spectra

of Pr3+ ions exhibit four bands centered at 448, 467, 481 and 595 nm, which are assigned to the transitions 3H4→3

P2, 3P1, 3P0 and 1D2, respectively The transitions 3H4→3

P2, 3P1, 3P0 and 1D2 of the free Pr3+ ion are allowed in the electric-quadrupole approximation, the transition 3H4→3

P1 is forbidden

in this approximation, and all the transitions are forbidden in the electric-dipole and magnetic-dipole approximations In absorption spectra of LaF3:Pr3+ nanocrystals, the intensity of the allowed transitions 3H4→3

P2 and 3H4→3

P0 almost coincides with the intensity of the forbidden transition

3

H4→3

P1, the another allowed transition 3H4→1

D2 has a dramatically less value of the dipole strength

Fig 3 The room temperature absorption spectra of the Ho3+, Pr3+ co-doped LaF3

The room temperature PL spectra of LaF3:Ho3+, Pr3+ excited by 450 nm wavelength are shown

in Fig 4 The undoped LaF3 nanocrystals do not exhibit the groups of emission lines in the wavelength range from 450 to 800 nm, whereas the samples without Pr3+ (LaF3: 2%Ho,0%Pr) show a group of four emission peaks at 483, 540, 642 and 750 nm, which are assigned to the electronic transitions 5F3,

5

S2+5F4, 5F5→5

I8 and 5S2+5F5→5

I7 in Ho3+ ions, respectively The samples without Ho3+ (LaF3: 0%Ho,1%Pr) show a group of six emission peaks at 485, 525, 599, 610, 635 and 722 nm, which are assigned to the electronic transitions 3P0→3

H4, 3H5, 1D2→3

H4, 3P0→3

H6, 3F2 and 3F4 in Pr3+ ions, respectively The samples co-doped with Ho3+ and Pr3+ show both the peaks of Ho3+ and those of Pr3+ ions

Fig 4 The room temperature PL spectra of LaF3:Ho3+, Pr3+

Typical PLE spectra monitored at 642 nm and 599 nm emission line of LaF3:Ho3+, Pr3+ are shown

in Fig 5A and Fig 5B, respectively It can be seen that the peaks at 360, 384, 414, 450, 467, 482 and

533 nm were observed, corresponding to the 5I8→3

H6, 5G4+3K7, 5G5, 5G6+5F1, 5F2+3K8, 5F3 and 5F4

transitions of Ho3+ ions (Fig 5A) And the peaks at 442, 465 and 479 nm were observed, corresponding to the 3H4→3

P2, 3P1 and 3P0 transitions of Pr3+ ions (Fig 5B) It is noted that the excitation spectra coincide perfectly with the absorption spectra and they both exhibit some bands assigned to f-f transitions from the ground state to various excited states of Ho3+ and Pr3+ ions

Fig 5 The room temperature PLE spectra of LaF3:Ho3+, Pr3+

4 Conclusions

In this work, Ho3+, Pr3+ co-doped LaF3 nanocrystals have been synthesized by hydrothermal method The XRD analysis showed that the nanocrystals exhibit a pure hexagonal structure

Absorption, photoluminescence spectra related to Ho3+ and Pr3+ ions have been investigated in detail The PL and PLE spectra of Ho3+ and Pr3+ ions result from the optical intra-configurational f-f transitions The 5I8→ 5

F5, 5F4, 5F3, 3K8, 5F1, 5G5, 3K7 and 3H6 absorption transitions of Ho3+ ion and the

3

H4 → 3

P2, 3P1, 3P0 and 1D2 absorption transitions of Pr3+ ion have been clearly observed in the PLE spectra as well The emission transitions 5F3, 5S2+5F4, 5F5→5

I8 and 5S2+5F5→5

I7 in Ho3+ ions, and the emission transitions 3P0→3

H4, 3H5; 1D2→3

H4; 3P0→3

H6, 3F2 and 3F4 of Pr3+ ions were obviously observed in the PL spectra

Acknowledgements

Authors of this paper would like to express their sincere gratitude to the Center for Materials

Science (CMS), Faculty of Physics, Hanoi University of Science, Vietnam National University for

permission to use equipment

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