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doped into different host materials have been well characterized and are fairly well understood [6–8]. Yttrium oxide (Y2O3) is one of the best hosts for rare earth ...
Journal of the Korean Physical Society, Vol. 60, No. 2, January 2012, pp. 244∼248

Synthesis and Optical Properties of Dy3+ -doped Y2 O3 Nanoparticles Timur Sh. Atabaev, Hong Ha Thi Vu, Hyung-Kook Kim∗ and Yoon-Hwae Hwang† Department of Nanomaterials Engineering and BK 21 Nano Fusion Technology Division, Pusan National University, Miryang 627-706, Korea (Received 2 May 2011, in final form 20 December 2011) In the present study, nearly-uniform spherical-shaped dysprosium-doped cubic yttrium-oxide nanoparticles were prepared by using the urea homogeneous precipitation method. X-ray diffraction patterns of synthesized particles confirmed the formation of a pure cubic phase of Y2 O3 . The morphology and the elemental analysis measurements were carried out using a transmission electron microscope and a field-emission scanning electron microscope. The particles were observed to have average sizes of around 110 nm. In order to select the optimum concentration of the Dy3+ dopant in the samples, we measured the strongest yellow emission peak intensity due to the strong 4 F9/2 -6 H13/2 transition (573 nm) as a function of the Dy-ion dopant concentration under a constant 349 nm excitation. The PL results showed that the strongest yellow emission at 573 nm occurred when the dopant concentration was about 1% in mol equivalent. Y2 O3 :Dy3+ phosphor can be used for applications in security printing, biolabel technology, lamps for illumination purposes, etc. PACS numbers: 78.55.Hx, 71.55.Ak, 81.05.Je, 81.07.Wx Keywords: Luminescence, Phosphor, Yttrium oxide, Nanoparticles DOI: 10.3938/jkps.60.244

on the red (Eu3+ ) - and the green (Tb3+ and Er3+ ) emitting phosphors [10–12], other interesting ion, such as Dy3+ , can be useful as yellow-emitting phosphors due to its unique spectral properties and has been extensively studied in various hosts [13–15]. For instance, Maruyama et al. [13] reported that the incorporation of Dy3+ into transparent Ba2 TiSi2 O8 glass nanocrystals can produce yellow emission (∼575 nm), which potentially can be used in photonic devices such as tunable waveguides and optical switching. Kharabe et al. [14] investigated the Dy3+ -activated Sr6 BP5 O20 and Ca6 BP5 O20 borophosphate phosphors while Jayasimhadri et al. [15] investigated the Dy3+ doped alkali tellurophosphate glasses for use as potential laser materials. The Y2 O3 ceramic is known to be the best host material due to its stable chemical and physical properties, and potentially can be used in various luminescent applications such as solid state illumination, cathodoluminescence, security printing, UV-based white-light emitting diodes, etc. [16] . A yellow phosphor material integrated with a blue LED chip can produce white light while nanosized powders can be used for luminescent coatings or in flat panel displays. From this point of view, the fabrication of small phosphor particles with fine crystalline structures, controllable shapes and high degrees of monodispersion is another research goal in modern chemistry and material science. To the best of the author’s knowledge, there are no reports on the fabrication of nearly-uniform spherical Y2 O3 phosphor particles doped with Dy3+ . Therefore, in

I. INTRODUCTION There has been intense interest in the investigation of ultrafine fluorescent materials recently for both fundamental research and potential applications in security labeling, UV and IR radiation detectors, bioimaging, display devices, lamps and solar cell panels [1–5]. Among the various luminescence materials, lanthanideion (Ln3+ ) activated phosphors have attracted extensive attention because of their wide range of emission colors and excellent luminescent performances. Nanosized phosphors (nanophophors) improve the resolution of displays [4] and have promising applications as bioimaging probes [5]. The optical properties of lanthanide ions, such as Eu3+ , Tb3+ , Er3+ , and Tm3+ doped into different host materials have been well characterized and are fairly well understood [6–8]. Yttrium oxide (Y2 O3 ) is one of the best hosts for rare earth ions because of the similarities in the chemical properties and the ionic radii of rare earths. Moreover, Y2 O3 possesses a higher melting point (2400 ◦ C), higher thermal conductivity, wide transparency range (0.2 – 8 µm) with a band gap of 5.6 eV, high refractive index (∼1.8) and low cut-off phonon energy (380 cm−1 ) [9]. While the majority of the work on nanocrystalline Y2 O3 has undoubtedly been focused ∗ E-mail: † E-mail:

[email protected] [email protected]; Fax: +82-055-353-5844

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Synthesis and Optical Properties of Dy3+ -doped Y2 O3 Nanoparticles – Timur Sh. Atabaev et al.

the present research, a simple, low-cost and environmentfriendly urea homogeneous precipitation method was successfully used to synthesize uniform, nearly-spherical Dy3+ -doped Y2 O3 nanoparticles (NPs) with average sizes of 110 nm. The structural properties of samples were investigated by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The effects of Dy3+ concentration and calcination temperature on the luminescence properties of Dy3+ doped Y2 O3 NPs were investigated. Under 349-nm excitation, a strong yellow emission at 573 nm and a weak blue-green emission at a wavelength ranging from 460 to 500 nm originates from the 4 F9/2 -6 H13/2 and 4 F9/2 -6 H15/2 transitions respectively. Nearly uniform spherical-shaped Y2 O3 :Dy3+ NPs with strong yellow luminescence and a sharp emission peak have potential applications in security printing, fluorescent labels for biomolecules, lamps for illumination purposes, laser technology, etc.

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Fig. 1. The X-ray diffraction patterns of Y2 O3 particles doped with different concentrations of Dy3+ (Tcal. = 1000 ◦ C).

II. EXPERIMENTS AND DISCUSSION 1. Chemical Synthesis

Analytical grade yttrium oxide Y2 O3 (99.9%), dysprosium oxide Dy2 O3 (99.9%), nitric acid (HNO3 , 70%), and urea (99 – 100.5%) were purchased from Sigma-Aldrich and used without further purification. Uniformly-shaped Dy3+ -doped Y2 O3 particles were synthesized according to our previous reports [16]. Yttrium oxide and dysprosium oxide with a stoichiometric mol ratio (Y/Dy = (100 − x)/x, where x = 0.5, 1, 2, and 3 (a total of 0.001 mol for each sample)) were diluted with nitric acid and vigorously stirred until the solution became colorless. Then, the solution was dried at 70 ◦ C for one day to remove any excess nitric acid therein. The aqueous urea solution for each sample was prepared by dissolving 0.5 g of urea in 40 ml of deionized water, after which the solution was mixed with dried rare-earth nitrate salts and stirred homogeneously in a glass beaker. Sealed beakers with freshly prepared solutions were then placed into a electric furnace and heated up to 90◦ C for 1.5 h., after which the beakers were immediately placed into a bath with cold water to stop the reactions and prevent further growth of nanoparticles. After the beakers had been cooled to room temperature, the precipitates were centrifuged, washed thoroughly with deionized water and ethanol, and dried in an oven at 70 ◦ C for 24 hours. Dried precipitates of Y2 O3 : 1% Dy3+ were calcinated in air at different temperatures up to 1000 ◦ C for 1 h. 2. Physical Characterization

The structure of the prepared powders was investigated via X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer with Cu Kα radiation (λ

= 0.15405 nm) within a 2θ scan range of 20 – 70◦ . Structural properties were characterized by using Fourier transform infrared spectroscopy (Jasco FT/IR6300). The morphologies of the NPs were characterized via fieldemission scanning-electron microscopy (FESEM, Hitachi S-4700), energy dispersive X-ray spectroscopy (EDX; Horiba, 6853-H) and transmission electron microscopy (TEM, JEOL JEM-2010). The PL measurements were performed with a Hitachi F-7000 spectrophotometer equipped with a 150-W Xenon lamp as an excitation source. Samples of 50 mg were placed into a cylindrical sample holder of 10 mm in diameter and were pressed into pellets for the XRD and the PL measurements. All the measurements were performed at room temperature.

3. Structural Measurements and Morphology

Figure 1 shows the X-ray diffraction patterns taken within the 2θ scan range of 20 – 70◦ for the Y2 O3 : 0.5% Dy3+ , Y2 O3 : 1% Dy3+ , Y2 O3 : 2% Dy3+ , and Y2 O3 : 3% Dy3+ . The results shows that all diffraction peaks of these samples can be assigned to the pure-cubic Y2 O3 structure (JCPDS 86-1107) with spatial group Ia3 (206) [17]. No additional peaks of other phases were found, indicating the formation of a purely cubic Y2 O3 phase due to the low concentration of Dy dopant ions. Moreover, the similar ionic radii of these elements (Yir = 0.9 ˚ A) is also suitable for successful reA, Dyir = 0.912 ˚ placement of Y-ions with Dy-ions. Because the crystal symmetry depends on the concentration of the doping ions, the Y2 O3 cubic structure’s overall peak intensity obviously decreases with increasing Dy3+ concentration in the NPs. The average nanocrystals size can be calculated using

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Fig. 3. (Color online) FESEM, HRFETEM, and EDX images of Y2 O3 : 1% Dy3+ particles (Tcal. = 1000 ◦ C).

Fig. 2. (Color online) The FT/IR spectra of Y2 O3 particles doped with different concentrations of Dy3+ (Tcal. = 1000 ◦ C).

Debye-Scherrer’s equation: D=

Kλ , β cos Θ

(1)

where K = 4/3 in the case of a spherical shape, D is the crystallite size (in ˚ A), λ is the wavelength of the Cu Kα radiation, and β is the corrected half width of the diffraction peak. The strongest Y2 O3 : 1% Dy+3 peak at 29.12◦ (222) was selected and yielded a calculated average size of ∼37.1 nm, which means that synthesized NPs actually consisted of more smaller crystallites. FTIR spectra for Y2 O3 NPs doped with different concentrations of Dy-ions are shown in Fig. 2. The absorption band around 570 cm−1 is due to characteristic metal-oxide (Y-O) stretching vibrations of cubic Y2 O3 [18]. The weak broad bands in the range of 1500 – 1750 cm−1 and 3500 – 3800 cm−1 correspond to OH groups [19]. Another weak peak around 2300 cm−1 can be attributed to the presence of carbon dioxide. Carbon dioxide and OH groups were mostly absorbed from air because their signals are relatively low. Figure 3 shows FESEM and HRFETEM photographs of Y2 O3 : 1% Dy3+ NPs. The photographs of the samples show near spherical morphology with average sizes of around 110 nm. It should be noted that after calcinations, all NPs are connected with each other by some necks. To break these necks, we ultrasonicated the nanopowder in an aqueous solution for 30 min and then dried the obtained NPs in the oven for 1 day at 70 ◦ C. As a result, we obtained separated NPs with some amount of smaller and aggregated particles. The aggregated particles may be due to the preparation procedure and to the influence of the high temperature during the calcina-

Fig. 4. Luminescence spectra of Y2 O3 particles doped with different concentrations of Dy3+ (Tcal. = 1000 ◦ C).

tion process. Note, that all the samples were composed of approximately spherical particles with identical size distributions. An HRFETEM image of the Y2 O3 : 1% Dy3+ single particle shows that it has a relatively spherical shape consisting of smaller crystallites (∼27 ± 13 nm) associated with each other, which is in good agreement with that calculated from the XRD patterns by using the Debye-Scherrer equation. The selected HRFETEM layer image reveals a distance of 0.305 nm of the crystal fringes, which is assigned to the {222} crystal plane of the Y2 O3 bcc phase [17]. A compositional analysis by using energy-dispersive X-ray spectroscopy (EDX) reveals the presence of the elemental Dy, Y, and O in NPs.

4. Photoluminescence Properties

The doping concentration is an important factor, which influences the performance of luminescent mate-

Synthesis and Optical Properties of Dy3+ -doped Y2 O3 Nanoparticles – Timur Sh. Atabaev et al.

Fig. 5. Strongest yellow emission peak intensity at 573 nm (4 F9/2 -6 H13/2 transition) as a function of the Dy3+ concentration under a constant 349-nm excitation.

rials. Figure 4 shows the photoluminescence (PL) emission spectra of Y2 O3 : xDy3+ samples, where x = 0.5, 1, 2, and 3% in mol equivalent, taken in scan range from 420 to 650 nm under a constant 349-nm excitation. The samples were prepared in the same way and were measured under identical conditions so that the emission ratios could be compared. Figure 5 shows the strongest yellow-emission peak intensity at 573 nm as a function of the dopant dysprosium’s concentration under a constant 349-nm excitation. One can observe that emission intensity increases with increasing of Dy3+ concentration up to 1 mol% and then decreases when the concentration is more than 1% in mol equivalent. This clearly indicates a dependence of the luminescence intensity on the concentration of the dopant ions. This result is not surprising because a high dopant concentration leads to a decrease in the average distance between the dopant ions and to the formation of dopant pairs or clusters in some cases. This then promotes interaction between the ions, energy migration, and non-radiative cross-relaxation processes to the ground states [16,20], which finally result in a quenching of the luminescence intensity. The optimum concentration of Y2 O3 : 1% Dy3+ NPs was investigated further. Figure 6 shows the emission spectra of Y2 O3 : 1% Dy3+ NPs calcinated for 1 h at temperatures ranging from 800 ◦ C to 1000 ◦ C. It is certain that emission intensity significantly increases with increasing calcination temperature. Increased crystallinity and good dispersion of doping materials inside the host material lead to the observed luminescence enhancement. The integrated yellow emission intensity of 4 F9/2 -6 H13/2 transition of NPs calcinated at 1000 ◦ C is about 1.46 times higher than that of NPs calcinated at 800 ◦ C. This indicates that luminescence intensity strongly depends on the calcination temperatures and on the dopant concentration in the host material.

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Fig. 6. (Color online) Luminescence spectra of Y2 O3 : 1% Dy3+ particles calcinated at different temperatures up to 1000 ◦ C. The inset is the eye-visible yellow emission of Y2 O3 : 1% Dy3+ particles (Tcal. = 1000 ◦ C).

Two emission bands are observable under 349-nm radiation due to Stokes transitions. At first, UV photon excite the Dy3+ ions to the 6 P7/2 level, which then quickly relax non-radiatively to populate the 4 F9/2 level. Luminescence emission occurs from 4 F9/2 level to the lowerlying states 6 H15/2 (460 – 500 nm) and 6 H13/2 (565 – 590 nm). Yellow emission (4 F9/2 -6 H13/2 transition) is about 3 times more intense than blue emission (4 F9/2 -6 H15/2 transition), so the total luminescence emission looks like a yellow emission to the naked eyes (Fig. 5, inset picture). The results of this research indicate that uniform Y2 O3 : 1% Dy3+ NPs show strong luminescence properties with narrow emission bands and can be used as a promising material for applications in optoelectronic devices, security printing, lamps for illumination purposes, biolabels technology, etc.

III. CONCLUSION In conclusion, the urea homogeneous precipitation method was used to synthesize uniform near-spherical Y2 O3 : Dy3+ NPs. The morphology and the crystal structure of calcinated NPs were characterized by using XRD, FTIR, FESEM, TEM, and EDX measurements. The synthesized NPs had near spherical morphology with average sizes of around 110 nm. We found that the luminescence properties depended on the dopant concentration and the calcination temperature of the NPs. Luminescence quenching was observed above 1% in mol equivalent of Dy3+ -doped Y2 O3 , which is considered to be the optimal dopant concentration. A strong yellow emission due to the 4 F9/2 -6 H13/2 transition is visible with the naked eye and is expected to find applications in wide areas of industry.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (No. 2011-0027674). This work was also supported by the 2011 Specialization Project Research Grant funded by the Pusan National University. The authors would like to thank Prof. J. B. Lee for allowing the use of his equipment for the preparation and the characterization of samples.

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