Up-conversion luminescence properties of Y2O2S:Yb3+,Er3+ nanophosphors

Up-conversion luminescence properties of Y2O2S:Yb3+,Er3+ nanophosphors

Optical Materials 31 (2009) 1787–1790 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat ...

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Optical Materials 31 (2009) 1787–1790

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Up-conversion luminescence properties of Y2O2S:Yb3+,Er3+ nanophosphors Iko Hyppänen a,b, Jorma Hölsä a,c, Jouko Kankare a,c, Mika Lastusaari a,c, Laura Pihlgren a,d,* a

University of Turku, Department of Chemistry, FI-20014 Turku, Finland Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM), Espoo, Finland Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland d Graduate School of Materials Research (GSMR), Turku, Finland b c

a r t i c l e

i n f o

Article history: Received 17 July 2008 Received in revised form 20 November 2008 Accepted 15 December 2008 Available online 2 June 2009 PACS: 78.55.Hx Keywords: Yttrium oxysulfide Nanomaterial Flux method Up-conversion luminescence

a b s t r a c t Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y2O2S:Yb3+,Er3+) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green ((2H11/2, 4S3/2) ? 4I15/2 transition) and strong red (4F9/2 ? 4I15/2) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with xYb and xEr equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The field of up-conversion luminescence where the absorption of two or more low energy photons is followed by the emission of a higher energy photon has witnessed numerous breakthroughs during the past decades. Up-converting phosphors have several potential applications as e.g. lasers and displays [1,2] and in clinical diagnostic assays [3]. Up-converting nanophosphors with high luminescent efficiency are required for coupling to biological compounds. These phosphors are also needed in the development of novel homogeneous label technology for quantitative all-in-one whole-blood immunoassay which uses low-cost measurement devices [3]. Whole blood has no capability for up-conversion [3]. Neither there is autoluminescence nor absorption of luminescence in the red spectral region (635–710 nm). This enables immunoassays with low background luminescence signals. Whole-blood immunoassays are suitable for point-of-care applications and in resource-limited areas where the pretreatment of blood samples would be inconvenient. The most efficient up-converting phosphors operate with the combination of a trivalent rare earth (R3+) sensitizer (e.g. Yb3+, Er3+ or Sm3+) and an activator (e.g. Er3+, Ho3+ or Tm3+) ion in an

optically inactive host lattice [4]. The sensitizer is excited with an infrared (IR) radiation source and transfers this energy to the activator that emits a visible photon. In this work, the nanocrystalline Y2O2S:Yb3+,Er3+ materials were prepared with the flux method. Crystal structure as well as phase purities were analyzed and the crystallite sizes were estimated. Up-conversion luminescence was obtained at room temperature with NIR excitation. The effect of the crystallite size and Er3+ concentration on the intensity of the green and red luminescence are presented and discussed. 2. Experimental 2.1. Materials preparation 2.1.1. Combustion synthesis of oxide precursors The Yb3+ and Er3+ co-doped Y2O3 nanocrystals with nominal 3+ Yb concentrations of 10 and 20 and Er3+ concentrations of 0.5, 1, 2, 3, and 4 mol% of the yttrium amount were prepared with the combustion synthesis (Eq. (1)) [5].

6RðNO3 Þ3 ðsÞ þ 10NH2 CH2 COOHðsÞ þ 18O2 ðgÞ ! 3R2 O3 ðsÞ þ 5N2 ðgÞ þ 18NO2 ðgÞ þ 20CO2 ðgÞ þ 25H2 OðlÞ

* Corresponding author. Address: University of Turku, Department of Chemistry, FI-20014 Turku, Finland. Fax: +358 2 3336700. E-mail address: [email protected]fi (L. Pihlgren). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.12.034

ð1Þ

where R is Y, Yb and Er. The R(NO3)3 solutions were prepared by dissolving Y2O3 (99.99%), Yb2O3 and Er2O3 (both 99.9%) in hot, dilute

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nitric acid. In the combustion reaction, glycine served as the fuel with the glycine-to-metal nitrate molar ratio equal to 1:1. To evaporate the excess water, the precursor solutions were preheated at 120 °C for 15 min. The combustion was initiated by heating the bottom of a glass reactor with a gas flame. After a few minutes, the solution fumed, frothed up and bursted into a flame yielding the polycrystalline product. Since NOx gases were evolved during the reaction, it is evident that the reaction does not follow exactly the stoichiometry indicated by Eq. (1). 2.1.2. Preparation of oxysulfides The nanosized Y2O2S:Yb3+,Er3+ materials were obtained with the flux method. The rare earth oxides prepared with the combustion method were heated in the Na2Sx (Na2CO3 + S) flux to form the corresponding oxysulfides (Eq. (2)). 3þ

Y2 O3 : Yb ; Er3þ þ 1:5Na2 CO3 þ 4S

were embedded in an epoxy resin and cut on a ultramicrotome to a thickness of 70 nm. X-ray powder diffraction (XPD) was used to analyze the crystal structure and phase purities of the materials. The XPD patterns were collected at room temperature between 4 and 100° (2h step: 0.005°) with a Huber 670 image plate Guinier-camera (CuKa1 radiation, k:1.5406 Å). The data collection time was 30 min. The crystallite size of the Y2O2S:Yb3+,Er3+ nanomaterials was estimated from the diffraction data using the Scherrer formula (Eq. (3)) [6]. In this equation, d (m) is the mean crystallite size, k (m) the X-ray wavelength, b (rad) the full width at half maximum (FWHM) of the selected [101] reflection (2h: 30.6°) and h (°) half of the Bragg angle (2h). The reflection broadening due to the diffractometer setup was eliminated from the bs value by using a microcrystalline reference (br). This material was microcrystalline Y2O2S:Yb3+,Er3+ prepared by the flux method from microcrystalline Y2O3:Yb3+,Er3+, NaCO3 and S at 900 °C for 2 h under static N2 gas sphere.



! Y2 O3: Yb ; Er3þ þ 1:5Na2 Sx þ 1:5CO2 þ 0:75O2 3þ

! Y2 O2 S : Yb ; Er3þ þ 1:5Na2 S2 O4

ð2Þ

Preparation of the oxysulfide nanoparticles was carried out under static N2 gas sphere at the desired temperatures (500– 900 °C) for 2 h. The products were washed with water and aqueous solution of acetic acid (2.5 moldm3) to remove the yttrium sulfide and Na2S2O4 impurities.



0:9k ; b cos h

b2 ¼ b2s  b2r

ð3Þ

The up-conversion luminescence spectra of the nanomaterials were measured at room temperature with an Ocean Optics PC2000-CCD spectrometer. The excitation (kexc:970 nm) source was a JDS UNIPHASE IR laser diode. 3. Results and discussion

2.2. Characterization 3.1. Crystallite size and crystal structure The particle size and morphology of the materials were examined with a JEM 1200EX transmission electron microscope equipped with a NORWAR-window Si(Li) detector. The materials

The average crystallite size of the Y2O2S:Yb3+,Er3+ nanomaterials determined from TEM (Fig. 1) is equal to 70–110 nm for the

Fig. 1. TEM images of selected Y2O2S:Yb3+,Er3+ nanomaterials.

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3+

Y2O2S:Yb ,Er

2500

1500

2

Y2O2S:Yb3+,Er3+ xYb: 0.20, xEr: 0.04 o

(100)

1500 700

1000

*

Intensity / Counts

2 h @ 500-900 C, static N2 gas sphere (102) o (110) Heated @ 900 C (103) (003) (112) (201)

*

2000

500

*

600 500

*Na2S2O4

Calculated Y2O2S reference pattern

0 25

30

35

40

45

50

55

60

2 / Degrees Fig. 2. X-ray powder diffraction patterns of selected Y2O2S:Yb3+,Er3+ nanomaterials after washing. Calculated pattern obtained with the PowderCell program [12] using the structural data from [8].

4

3+

F9/2

4

I15/2

xYb = 0.20, xEr = 0.04

400

Intensity / Arb. Units

5 2 h @ 500 - 900 oC, static N2 gas sphere 293 K

4

300 o

Prepared @ 900 C

3

200

700 4

2 2

1

H11/2

4

S3/2

4

I15/2

100

I15/2

600 500

0

0 520

560

600

640

Intensity / Arb. Units

3+

6 Y2O2S:Yb ,Er

4

4

I15/2 S3/2

400

4

I15/2

1.

200

3.

1000

5. 2.

1.

4.

0

500

I15/2

525

540

555

2.

570

5.

3. 4.

0 560

600

640

680

Wavelength / nm

The excitation process for the up-conversion emission of Er3+ ions under IR excitation (kexc:970 nm) has been well established in the literature [4,9] and will not be repeated in this paper. Strong red up-conversion luminescence (Figs. 3 and 4) was obtained from the Y2O2S:Yb3+,Er3+ nanomaterials together with green luminescence with moderate intensity. An increase in the heating temperature brought up a rapid growth of the crystallites of the Y2O2S:Yb3+,Er3+ nanomaterials (Section 3.1). Along with the in-

*Na2S2O4

H11/2

4

1) 0.005 2) 0.01 3) 0.02 4) 0.03 5) 0.04

293 K

2000

F9/2

xEr

o

3.2. Up-conversion luminescence

(101)

4

xYb = 0.10, xEr = 0.005-0.04

520

2500

3+

2 h @ 900 C, static N2 gas sphere

Intensity / Arb. Units

materials prepared at 900 °C. The crystallite size is smaller for the materials heated at lower temperatures: 20–40 and 10 nm when heated at 700 (or 600) and 500 °C, respectively. The particles were mainly spherical. The crystallite sizes agree well with the calculations using the Scherrer equation. The X-ray powder diffraction patterns (Fig. 2) confirm that the crystal structure of the Y2O2S:Yb3+,Er3+ nanomaterials is hexagonal  (space group P3m, No. 164, Z = 1) [7] composed of alternating 2 layers. The structure is very closely related to the ðROÞ2þ 2 and S A-type rare earth oxide (A-R2O3) structure, the difference being that one third of the oxygen sites is occupied by a sulfur [8]. No oxide precursor material was found in the nanomaterials. Small amounts of Na2S2O4 [7] were observed as an impurity phase, even after washing with water and acetic acid.

680

Wavelength / nm Fig. 3. Up-conversion luminescence spectra of the Y2O2S:Yb3+,Er3+ nanomaterials prepared at selected temperatures.

Fig. 4. Up-conversion luminescence spectra of the Y2O2S:Yb3+,Er3+ nanomaterials prepared with selected erbium concentrations.

creased crystallite size, the intensity of the green up-conversion luminescence due to the (2H11/2,4S3/2) ? 4I15/2 transitions increased more than that of the red (4F9/2 ? 4I15/2) one (Fig. 3), i.e. the intensity ratio Ired/Igreen (ca. 7 at 600/700 °C and 4 at 900 °C) decreases with increasing particle size. The Er3+ ions tend to form clusters with decreasing particle size [10]. This improves the probability of the cross-relaxation (CR) processes. The first process includes the 2H11/2 ? 4I13/2 relaxation and the 4I15/2 ? 4I9/2 excitation (energy difference: 12,500 cm1) [11]. In the second possible cross-relaxation process, there are coupled the 2H11/2 ? 4I9/2 relaxation and the 4I15/2 ? 4I13/2 excitation (energy difference: 6700 cm1). These two processes decrease the intensity of both the red and green luminescence. In the third possible cross-relaxation process, there are the 4F7/2 ? 4F9/2 relaxation and the 4I15/2 ? 4I11/2 excitation (energy difference: 5000 cm1). This process favors the red luminescence. The weak total up-conversion luminescence from materials prepared at low temperatures is partly due to the increase in the particle surface area, which increases the amount of the surface defects and the adsorption of other defect impurities (e.g. CO2, H2O). Both cause important losses in the luminescence. There is no up-conversion luminescence from the nanomaterials when the heating temperature is 500 °C (Fig. 3). The comparison of the luminescence spectra of materials with different erbium content showed that the intensity of the green luminescence decreases, although to lower extent compared to the red one, as the erbium concentration increases (Fig. 4). At low Er3+ concentrations, the Er3+ ions are randomly distributed in the host lattice and this prevents the interionic CR processes. Therefore the green luminescence is stronger when compared to the red one. When the erbium concentration is higher than 0.005 the concentration quenching occurs and thus weakens the total luminescence intensity. 4. Conclusions The structure of the Y2O2S:Yb3+,Er3+ nanomaterials prepared with the flux method was hexagonal. No oxide precursor was observed, but small amounts of Na2S2O4 impurities were found, even after washing with acetic acid. The nanomaterials demonstrated strong red (4F9/2 ? 4I15/2) and moderate green (2H11/2, 4 S3/2) ? 4I15/2 up-conversion luminescence upon infrared excitation. The strongest up-conversion luminescence was obtained when the heating temperature was 900 °C and xEr equal to 0.005. As the crystallite size decreased, the green up-conversion luminescence intensity decreased more than the red one. The intensity of

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the green luminescence decreases as the erbium concentration increases, too. These phenomena are mainly due to the cross-relaxation processes. Acknowledgements Financial support from the Finnish Funding Agency for Technology and Innovation (Tekes) and the Graduate School of Materials Research (Turku, Finland) are acknowledged. Dr. Jouko Mäki (University of Turku, Laboratory of Electron Microscopy) is acknowledged for assistance in TEM imaging. References [1] S.G. Grubb, K.W. Bennett, R.S. Cannon, W.F. Humer, Electron. Lett. 28 (1992) 1243.

[2] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [3] T. Soukka, K. Kuningas, T. Rantanen, V. Haaslahti, T. Lövgren, J. Fluoresc. 15 (2005) 513. [4] F. Auzel, Chem. Rev. 104 (2004) 139. [5] F. Vetrone, J.-C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Appl. Phys. 96 (2004) 661. [6] H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures, Wiley, New York, 1959. p. 491. [7] PCPDFWIN v. 1.30, Powder Diffraction File, International Centre for Diffraction Data, Entries 24-1424 (Y2O2S) and 33-1286 (Na2S2O4), 1997. [8] W.H. Zachariasen, Acta Crystallogr. 2 (1949) 60. [9] Y. Mita, Infrared Up-conversion Phosphors, in: S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, USA, 1999, pp. 643– 650. [10] T. Hirai, T. Orikoshi, J. Colloid Interface Sci. 273 (2004) 470. [11] I. Hyppänen, J. Hölsä, J. Kankare, M. Lastusaari, L. Pihlgren, J. Nanomater. (2007) 8 (Article ID 16391). [12] W. Kraus, G. Nolze, Powder Cell for Windows, Version 2.4, Federal Institute for Materials Research and Testing, Berlin, Germany, 2000.