Nanomaterials containing rare-earth ions Tb, Eu, Er and Yb preparation, optical properties and application potential

Strekd aInstitute of Materials Science, NCST of Vietnam, 18 Hoang Quoc Viet Road, Cau Giay, Hanoi, Viet Nam bInternational Training Institute for Materials Science, Dai hoc Bach Khoa, 1

Nanomaterials containing rare-earth ions Tb, Eu, Er and Yb: preparation, optical properties and application potential

T Kim Anha,b,*, L Quoc Minha, N Vua, T Thu Huonga, N Thanh Huonga,

C Barthouc, W Strekd

aInstitute of Materials Science, NCST of Vietnam, 18 Hoang Quoc Viet Road, Cau Giay, Hanoi, Viet Nam

bInternational Training Institute for Materials Science, Dai hoc Bach Khoa, 1 Dai Co Viet, Hanoi, Viet Nam

cLab d Optique des Solides UMR7601, Univ P & M Curie, 4 Place Jussieu, F 75252 Paris Cedex 05, France

dInstitute of Low Temperature and Structure Research, PAS, 2 Okolna, Wroc!aw, Poland

Abstract

This paper focuses on preparation, optical properties and application potential of some nanomaterials based on

Y2O3:Eu,Tb,Er,Yb and SiO2–TiO2and SiO2–ZiO2doped with Er and Yb Y2O3nanophosphors are prepared by the combustion method with different doped concentrations The nanocrystal size of Y2O3:Eu is from 4.4 to 72.2 nm depending on the technology condition The luminescent spectra, up-conversion and lifetimes were measured and compared The influence of the technological conditions on the luminescent properties was investigated in detail The energy transfer effect was studied by the luminescent spectra and the lifetimes in the temperature dependence for the samples with rare-earth concentrations of 5 mol%, The relative concentration between Eu and Tb is 8/2 for energy transfer from Tb to Eu The SiO2–TiO2and SiO2–ZiO2thin films containing Er rare-earth ion were prepared by the sol– gel technique Optical properties were investigated and the influence of the Er concentration on the luminescent spectra

as well as the influence of the Ti concentration on the refractive index of thin films was presented

r2003 Elsevier Science B.V All rights reserved

Keywords: Nanophosphors; Sol–gel; Rare earths; Waveguide

1 Introduction

Materials with nanostructure are increasingly

interesting for optoelectronics and also for

photo-nics Nanomaterials display novel, often enhanced,

properties compared to traditional materials,

nanophosphors: synthesis, properties and

applica-tions were presented [1,2] Luminescence proper-ties of nanocrystalline Y2O3:Eu3+in different host materials were studied[3] High-definition displays call for sub-micron particle sizes to maximize screen resolution and screen efficiency Nanopho-sphors codoped with Tb, Eu for high intensity of red region by energy transfer between Tb and Eu

as well as Y2O3:Er,Yb for infrared region and up-conversion effect are interesting SiO2–TiO2 sys-tem offers the possibility of producing a material with a controllable refractive index from 1.46 (the refractive index of pure silica) to 2.2 (pure

*Corresponding author Institute of Materials Science,

NCST of Vietnam, 18 Hoang Quoc Viet Road, Cau Giay,

Hanoi, Viet Nam Tel.: +84-4-7560371; fax: +84-4-7562039.

E-mail address: kimanh@ims.ncst.ac.vn (T.K Anh).

0022-2313/02/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.

PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 5 3 1 - 8

amorphous TiO2) SiO2–ZiO2 thin films doped

with rare earth are currently of much interest in

planar waveguides application, because they are

homogeneous and have the ability to tune the

refractive index and wavelength [4] Sol–gel

chemistry, where all chemicals mix in the matrices

in a molecular scale combined with the

spin-coating or dip-spin-coating technique, is a good method

for producing rare-earth-doped planar waveguides

[5] We report the preparation, structure and

optical properties of SiO2–TiO2:Er or SiO2–

ZiO2:Er thin films and discuss briefly the

possibi-lity of the development of nanomaterials for active

waveguides

2 Experiment

The Y2O3:Eu, Y2O3:Tb,Eu and Y2O3:Er,Yb

nanophosphors with different concentrations of

REwere obtained from the calcination of the basic

carbonate Eu3+, Tb3+or Er3+ and Yb3+ (RE)

ions are easily hydrolyzed and then the

precipita-tion of basic carbonate in an aqueous soluprecipita-tion of

urea or glycine SiO2–TiO2:Er and SiO2–ZiO2:Er

thin films were prepared via the hydrolysis and

condensation of tetraethoxysilane Si (OC2H5)4

(TEOS) 98% Merck with

Tetraisopropylorthoti-tanate Ti(OC3H7)4 Fluka [6] or Zr (OC3H7)4

Aldrich The spin coating method on Si substrates

in the clean room of the class 100, dip coating

method and rapid thermal annealing were used

The photoluminescence spectra were studied by

the monochromator Jobin-Yvon HR 460, and a

multichannel CCD detection from Instruments SA

model Spectraview-2D and Triax 320 for infrared

range measurements The decay were analyzed by

a PM Hamamatsu R928 and Nicolet 490 scope

with a time constant of the order of 7 ns N2,

diode, Ti-Sapphire or argon lasers were used as

excitation sources for the different wavelengths

The morphology and particle sizes of Y2O3:RE

was observed by using a high-resolution

transmis-sion electron microscope (TEM) Philips CM 200

The Y2O3: REpowder, SiO2–TiO2:Er and SiO2–

ZiO2:Er were checked by the X-ray diffractometer

D 5000 (Siemens) and Atom Force Microscope E+

Digital (AFM)

3 Results and discussion The value for crystallite size can be extracted according to the Warren–Averbach theory The particle sizes are 4.4, 5.6, 15.2, 46.1 and 72.2 nm for Y2O3:Eu nanophosphors with various anneal-ing times and temperatures 5501C, 60 min, 6001C,

30 min, 7001C, 30 min, 9001C, 30 min and 9001C,

60 min, respectively The high-resolution TEM images of Y2O3:Er 5 mol% nanophosphors, 6001C, 30 min annealing was presented in Fig 1 The development of efficient nanophosphors has been one of the key issues in commercializing the new type of flat panel display with respect to potentially higher display resolution The optical efficiency of prepared spherical Y2O3:Eu as red phosphor was twice as much as that of commercial products in cathode luminescent according to Cho et al [7] In order to increase efficient luminescence in the red phosphor, Eu is codoped with Tb for energy transfer effect The luminescent spectra of Y2O3:Tb,Eu 5 mol% for three ratios 9/

1, 8/2 and 7/3 are shown inFig 2and relative ratio 8/2 has the strongest energy transfer The full-width at half-maximum (FWHM) is also greater for the nanoparticles Under 337.1 nm excitation,

Fig 1 High-resolution TEM image of Y 2 O 3 :Er (5 mol%) 6001C, 30 min.

Y2O3:Tb,Eu presents for the 5D0excited level of

Eu3+lifetimes of 360, 640 and 940 ms for the ratio

of Eu/Tb 8/2, 7/3 and 9/1, respectively The

up-conversion effect is observed for both Y2O3:Er and

Y2O3:Er,Yb Fig 3 presents the up-conversion

spectra of Y2O3:Er 10 mol% at 799.8 nm

excita-tion The infrared luminescent spectra of Y2O3:Er

10 mol% is presented in Fig 4for two annealing

temperatures : 6001C and 7001C To compare the

green and the red up-conversion, we can propose

two mechanisms First mechanism for dominant

green luminescence: the laser light brings the ion

Er3+ into 4I9/2 level, which then non-radiatively

decays to the 4I11/2 and 4I13/2 levels Energy

transfer processes bring the ion into 4F3/2 and

2

H , non-radiative decay to the lower levels and

the 2H11/2, 4S3/2 transitions to 4I15/2, and 4F9/2

transitions to4I15/2occur The second mechanism

is red luminescence stronger than green one Laser beam brings the ion to the excited4I9/2level One ion non-radiatively decays to the4I13/2level, and second one decays to the 4I11/2 level Energy transfer processes bring the ion to the 4F9/2 and

a red emission can be observed

For codoped samples with Yb, an energy transfer from the 2F5/2 excited state of Yb3+ to

Er4I11/2can occur

For thin films prepared by dip coating, the AFM of the Er3+-doped 83 Silica/17 Zirconia/ Alumina (15%) sample annealed at 8801C is shown inFig 5 One can notice that the roughness

is about 51 nm compared to the value of 24 nm for

0

5

10

15

20

7/3

9/1 8/2

Y0.95 : EuxTby Nanophosphors

Tann = 600˚C (30 min)

λ (nm)

Fig 2 Luminescent spectra of Y 2 O 3 :Tb,Eu with the mole ratio

of Eu/Tb: 7/3, 8/2, 9/1 l exc ¼ 337:1 nm.

0

2

4

6

8

T = 300 K

λ exc = 799.8 nm Nanophosphores Y2O3:Er 3+ (YERU2)

λ (nm)

Fig 3 Emission spectra of Y 2 O 3 :Er 3+ (10%) nanophosphor.

lexc¼ 799:8 nm Power laser: 550 mW.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

1

2

λ exc = 982 nm

P = 96 mW

T = 300 K

Y2O3:Er3+

Intensity (norm.)

λ (nm)

Fig 4 Luminescent spectra in the infrared region of

Y 2 O 3 :Er 3+ (10 mol%) nanophosphors l exc ¼ 982 nm (1) 6001C, 30 min and (2) 7001C, 30 min.

5

15 10

NanoScope AFM Scan size 15.97 µ m Setpoint 0 V Scan rate 20.35 Hz Number of samples 512

x 5.000 µ m/div

z 270.457 nm/div

SZAE500

µ m

Fig 5 AFM of the Er3+-doped 83Silica/17Zirconia/Alumina (15%) sample was annealed at 8801C.

the case without Alumina This value is convenient

for planar waveguide application.Fig 6represents

the luminescent spectra of the Er3+(5%) in Silica/

Titania codoped with Yb3+(12.5%) and Al (10%)

annealed at 9501C and the Er3+ (5%) in Silica/

Zirconia samples codoped with Yb3+(15%) and

Al (6%) annealed at 8501C The FWHM of the

corresponding band from the4I13/2–4I15/2transition

is above 50 nm The adding of alumina increases

the process of densification of the silica–zirconia

and silica–titania system The refractive index

varied from 1.49 to 1.60 with respect to the Ti

concentration Refractive index and thickness of

90SiO2–10TiO2, 85SiO2–15TiO2 and 80SiO2–

20TiO2 thin films at different annealing

tempera-tures are presented in Ref [8] For example, for

9001C annealing temperature, the measured

refrac-tive indexes are 1.53, 1.56 and 1.60, respecrefrac-tively

The SiO2–TiO2:Er thin films and SiO2–ZiO2:Er

thin films show the transitions from 4I13/2to4I15/2

of Er3+in the infrared region (1530 nm ) and4S3/2

level to 4I15/2in the visible region (550 nm)

4 Conclusion

Nanophosphor Y2O3:RE (Tb, Eu, Er, Yb) 5–

10 mol% samples were prepared by combustion

method and the crystal size can be monitored in

the range of 10–80 nm depending on technology

conditions SiO2–TiO2:Er and SiO2–ZiO2:Er thin films were deposited by the spin coating and dip coating methods in order to study the optical properties as well as the possible application for planar waveguide The typical transitions of trivalent Er and Eu were observed and discussed Energy transfer and up-conversion mechanisms were studied Refractive index for SiO2–TiO2thin films can be tailored in a wide range by controlling the relative quantity of the two precursors The emission depends on the temperature of the thermal processing and on the Er,Yb concentra-tions This material will be promising for active waveguides in telecommunication application

Acknowledgements

We would like to thank Prof Nguyen Van Hieu, Prof Clement Sanchez, and Dr Paul Simons for their help Our work was financially supported by the project Franco-Vietnamese 2001, the Institute

of Materials Science, NCNST of Vietnam, the International Training Institute for Materials Science, the National Basic Research Programs

of Vietnam 2001–2002

References

[1] G.Y Hong, S.H Kwon, J.S Yoo, C.J Summers, Extended Abstracts of the Fifth International Conference on the Science and Technology of Display Phosphors, San Diego,

CA, November, 1999, pp.187–190 [2] B.R Ratna, A.D Dinsmore, Y Tian, S.B Qadri, D.S Hsu, H.F.Gray, Extended Abstracts of the Fifth International Conference on the Science and Technology of Display Phosphors, San Diego, CA, November 1999, pp 295–296 [3] R Schmechel, M Kennedy, H von Seggern, H Winkler,

M Kolbe, A Fischer, L Xaomao, A Benker, M Winterer,

H Hahn, J Appl Phys 89 (3) (2001) 1679.

[4] C Sanchez, B Lebeau, MRS Bull 26 (2001) 377 [5] X Orignac, D Barbier, X.M Du, R.M Almeida, Appl Phys Lett 69 (7) (1996) 895.

[6] L.Q Minh, T.K Anh, P Benalloul, C Barthou, in: E Giacobino, et al (Eds.), Advances in Optics and Spectro-scopy, National University Press, Hanoi, 2001, pp 505–509 [7] S.H Cho, L.S Yoo, J.D Lee, J Electrochem Soc 145 (3) (1998) 1017.

[8] L.Q Minh, N.T Huong, C Barthou, P Benaloul, W Strek, T.K Anh, Mater Sci 20 (2) (2002) 63.

1450 1500 1550 1600 1650

0

1

2

3

4

5

Si/Zr

Si/Ti

Si/Ti & Si/Zr : Er 3+

λ exc = 976 nm

T = 300 K

λ (nm)

Fig 6 Luminescent spectra of the Er3+(5%) in Silica/Titania

codoped with Yb3+(12.5%) and Al (10%) annealed at 9501C

and Silica/Zirconia samples codoped with Yb3+(15%) and Al

(6%) annealed at 8501C.