Nanostructure influence on luminescent properties ...

1

Nanostructure influence on luminescent properties of micronic Y2O3:Eu phosphors. Nicolas Joffin 1 2 3

1,2

1

2

2

2

, Guy Baret , Jeannnette Dexpert-Ghys , Marc Verelst , Pierre Baulès , Alain Garcia

3

DGTec SAS, 38430 Moirans, France CEMES-CNRS, 29 rue Jeanne Marvi g 31055 Toulouse, France ICMCB -CNRS, 87 Avenue du Docteur Schweitzer 33608 Pessac, France

Introduction:

Y2O3:Eu are very well known materials used as phosphors in display technology and lamps. DGTec manufactures these materials using an original synthesis way which gives noticeable behaviours. The challenge is to replace the commercial phosphors produced by solid state reaction in order to save the consumed matter in panel manufacturing. This paper investigates the relationship between the nanostructure of micronic particules, and their luminescent properties relatively to their application for plasma display panels. Synthesis and structural characterizations: Y2O3:Eu powders have been prepared by spray pyrolysis using an ultrasonic generator as for instance in reference [1]. The starting solution is made of a mixture of metals nitrates in adequate proportion. Spherical shape particules are obtained scarecely crystallized. The powder is then calcined under air at high temperature to obtain a crystallized product. Eventually after every treatments, prepared powders exhibit the same particle morphology and size: the average size, expressed in volume, is D50 ˜ 2.5 µm measured on a Malvern Mastersizer S. Each spherical particule may be described as an empty core surrounded by a shell of matter. TEM imaging has been exploited to evaluate the shell thickness. Fig. 1 shows few spherical-shape hollow particules of a medium crystallized sample calcined at 1200°C. The gray level variation along the particule diameter was compared to the projection on a plane of the matter quantity. The two profiles were adjusted

Fig. 1 TEM picture of a medium crysallized powder sample calcined at 1200°C

2

by

variation

diameter,

of

thickness,

and

scale

coefficients. An average shell thickness around 200 nm has been determined for three heat treatments at : 900, 1200, and 1400°C. This estimation has been confirmed by density measurement supposing that

Pic. a TEM picture of a low cristallized particule calcined at 900°C

Pic. b dark fiel view of pic. a

the shell is dense. The structure of the particles

shells

has

been

investigated by TEM imaging in bright and dark field modes on some broken spheres (fig. 2 a– d). At the lowest calcinations temperature,

some

crystals

Pic. c TEM picture of a medium cristallized particule calcined at 1200°C

can only be viewed in dark field

Pic. d TEM picture of a high cristallized particule calcined at 1400°C

Fig. 2 TEM pictures

mode (pic. b). The major part of the shell consists of needle-shaped amorphous elements. Pic. c and d show medium and high crystallized particles, calcined at 1200 °C and 1400°C, repectively . The crystals are here easily visible even

in normal mode. The average Calcination

XRD size

TEM size

temperature °C

nm

nm

direct measurement on these pictures. Simultaneously the

900

20

44

crystals sizes have been deduced from the XRD peak

1000

30

68

1100

42

95

1200

46

105

selected (hkl) diffraction peaks, ensuring that the values

1300

63

143

are representative of the crystal size in each space

1400

77

173

crystal size for each heat treatment has been evaluated by

widths using the Schoening-Halder-Wagner method, on

direction. The numerical values extracted from XRD treatments

are

systematically

lower

than

those

Tab. 1 XRD coherent domain size and TEM crystal size correlation

determined by direct measurement on the TEM pictures. But these two methods have been correlated by a simple scale factor for the calcination temperatures 900, 1200, and 1400°C. This scale factor has been applied to every XRD sizes in order to have comparable TEM cristals size for all calcination temperatures (tab. 1).

3

Luminescence properties: The powder luminescence properties have been characterized relatively to a commercial product used in plasma display panels. Luminance have been measured under 254 nm excitation and under Ne-Xe plasma excitation with a spectrophotometer. Figure 3 gathers the relative luminance values versus the crystal size in particles. The absoprtion curves in fig. 3 have been calculated according to the equation : A = 1 − e − K . x , where A is the radiation percentage absorbed by a length x of matter, and K is the absorption coefficient at 254 or 170 nm (for the plasma excitation) given in references [2, 3]. It can be noticed that from 100 nm length the plasma radiation is fully absorbed. Although the 254 nm radiation is poorly absorbed even for 200 nm matter depth (the shell size), so that this radiation may go through several particles before complete absorption. Fig. 3 shows that the luminance increases with crystal growth. A similar variation has been observed

for instance in reference [4]. Although still controversial (the opposite

behaviour was reported in reference [5]), it is

generally admitted that the luminance

increases with better crystallization, the reason put forward is that the probability of nonradiative de-excitation decreases when the number of traps decreases (i.e. when the crystal surface decreases versus the bulk). What we observe here is a

100

100

nm excitation and under plasma excitation. In our study, the powder macrostructure is kept after all heat treatments.

254 nm excitation plasma excitation 254 nm absorption plasma absorption

80

80

60 60 40 40

20

The fig. 4 illustrates the absorption mechanisms for the 254

nm

and

the

plasma

Absorption %

luminance variation under 254

Relative luminance %

great difference between the

20

0 40

60

80

100

120

140

160

180

200

220

Cristal size nm

radiation. When the 254 nm radiation go through the matter,

Fig. 3 : Relative luminance to a commercial phosphor and absorption versus TEM cristal size for 254nm and plasma radiation

energy can be absorbed by activator ions (Eu3+) which are excited or by defaults. Because of the higher energy, plasma radiation is first absorbed by the Y2O3 lattice, then migrates and lastly is transferred to activator ions which are excited or to defaults [6]. Eventually the

4

excited activator ions return to the ground state by emitting red photons. If the energy is trapped by defaults there is no emission. Now in the case of small crystals (50 nm for instance) energy transfers

254 nm direct absorption and excitation

Plasma radiation lattice absorption (1), migration (2) and excitation (3) Energy migration

Conduction band

Activator Ion levels

(2)

(1)

(3)

are limited in the crystal volume by surfaces . Our experiments prove that in that part of the fig. 3 the luminance for 254 nm excitation is over the luminance for plasma excitation. More activator

Valence band

Fig. 4 Absorption and excitation mechanism for 254 nm radiation and plasma radiation

ions are excited directly by the 254 nm radiation in the whole matter volume (200 nm depth) than by energy transfers after absorption of the plasma radiation in the small excitated volume (100 nm depth). Here luminescence by direct excitation is more efficient than luminescence by energy tranfers. In the case of large crystals there is no more energy transfers limitation. The plasma radiation energy is fully absorbed and can be transfered in the entire matter volume, whereas the 254 nm radiation is poorly absorbed still in the entire matter volume. The luminance for 254 nm excitation is then measured lower than the luminance for plasma excitation. For crystal size from 140 nm to 220 nm there is no more evolution of the 254 nm luminance whereas the plasma luminance still increases. The critical crystals size between the restricted energy-diffusion regime and the free energy-diffusion regime is observed at 120 – 140 nm in our powders. Conclusion:

Y2O3:Eu powders have been prepared by spray pyrolysis and subsequent heat treatment, then their luminescence properties were compared. . The interest of this series of samples is that the nanostructure evolves independently of the powder macrostructure. For calcination temperature from 900 to 1400°C the crystals size grows from 60 to 220 nm. In fact the highest crystal size value represents the shell thickness. It might be assumed therefore that for full-crystal-grown powder, the shell of hollow particles is composed of a monolayer of crystals. The luminance variations observed from sample to sample and for a given sample for two excitations may be explained by considering the nanostructure of the

5

powders. The studied phosphor can be used in plasma display applications but not in lamp applications where most of the excitation radiation is about 250 nm. Acknowledgements: This work was supported by the Ministère de la Recherche (RNMP/ POSUMIC).The authors wish to thank RHODIA for supplying the lanthanides solution, and Thomson Plasma for the measurements under plasma excitation.

References :

[1] Preparation of nonaggregated Y2O3:Eu phosphor particles by spray pyrolysis method. Y.C. Kang, S.B. Park, IW. Lenggoro and K. Okuyama. Journal of Material Reasearch, vol 14(6), pages 26112615, jun 1999. [2] Modeling the optical properties of fluorescent powders: Y1.91Eu0.09O3. A. Konrad, J. Almanstötter, J. Reichardt, A. Gahn, R. Tidecks, K. Samwer. Journal of Applied Physics, vol. 85(3), pages 1796-1801, feb 1999. [3] A surface recombinaition model applied to large features in inorganic phosphor efficiency measurements in the soft x-ray region. E.L. Benitez, D.E. Husk, S.E. Schnatterly, C. Tarrio. Journal of Applied Physics, vol. 70(6), pages 3256-3260, sep 1991. [4] Synthesis of nanometer Y2O3:Eu phosphor and its luminescence property. Junying Zhang, Zilong Tang, Zhongtai Zhang, Wangyang Fu, Jin Wang and Yuanhua Lin. Materials Science and Engineering A, vol. 334, Issues 1-2, pages 246-249, sep 2002. [5] Tailoring the particle size from µmànm scale by using a surface modifier and their size effect on the fluorescence properties of europium doped yttria. Pramod K. Sharma, M. H. Jilavi, R. Nass and H. Schmidt. Journal of luminescence 82, pages 187-193, 1999. [6] Luminescent materials. G. Blasse, B.C. Grabmaier. Springer Verlag 1994.