Thin Solid Films 518 (2009) 870–872

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Self-aggregation of magnetic semiconductor EuS nanocrystals

Atsushi Tanaka, Yasuchika Hasegawa ⁎, Hironari Kamikubo, Mikio Kataoka, Tsuyoshi Kawai ⁎Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan

⁎ Corresponding authors. Tel./fax: +81 743 6171.E-mail addresses: hasegawa@ms.naist.jp (Y. Hasegaw

(T. Kawai).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.07.106

a b s t r a c t

a r t i c l e i n f o

Available online 14 July 2009

Keywords:NanocrystalsLanthanideChalcongenideAggregation

Controlled formation of aggregates having organized structure of cube-shaped EuS nanocrystals is reported.The EuS aggregates in liquid media (methanol) were obtained by means of van der Waals interactionbetween EuS nanocrystals. The packing structure of the EuS aggregates is characterized with transmissionelectron microscopy (TEM) and small angle X-ray scattering measurements (SAXS). TEM image indicates theEuS nanocrystals form self-aggregated 2D orthogonal lattice structure. The diffraction peak of (111) of SAXSprofile shows that the cube-shaped EuS form 3D cubic superlattice. We successfully demonstrated that theaggregates of cube-shaped EuS nanocrystals formed cubic stacking structure.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic semiconductor europium(II) chalcogenides (EuX ; EuO,EuS, and EuSe) have attracted considerable attention in the area ofmaterials science because of their characteristic opto-magnetic proper-ties suchas FaradayandKerr effects [1]. Themagneto-optic properties ofEuX are intensely characterized by narrow 4f orbitals that exist asdegenerate levels between the conduction band (5d orbitals of Eu(II))and the valence band (p orbitals of O2−, S2−, and Se2−) [2]. The 4f–5delectronic transitionand the spin configurationwouldbe affectedby sizeand shape of EuX nanocrystals (NCs). We have recently reported onmanipulation of the Faraday rotation wavelength by means of size-controlled EuS NCs (crystal size: 7–14 nm) [3,4]. Enhanced Faradayeffect of shape-controlled EuS and EuSe NCs showing the coercive fieldhas also been demonstrated, recently [4,5]. The size and shape of theEuX NCs are expected to affect significantly on their magneto-opticproperties.

Here, we focus on aggregates of magnetic semiconductor EuX NCsfor a novel opto-magnetic material. Recently, Chaudret et al. havereported that the cubic NCs tend to form well-organized superlatticestructure enhanced magnetic properties based on specific magneticinteraction between the nanocrystals [6,7]. The aggregation of EuXNCs is thus expected to enhance the spin polarization and magneto-optic properties such as the Faraday effect.

In the present study,we report on the self-formation of aggregates ofcube-shaped EuS NCs. The preparation of cube-shaped EuS NCs iscarried out by using a single source precursor, (PPh4)[Eu(S2CNEt)], andidentified by X-ray diffraction (XRD) and transmission electron mi-croscope (TEM)measurements. The packing structure of EuS aggregates

a), tkawai@ms.naist.jp

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on a polymer thin film (PET: poly ethyleneterephtalate) is characterizedwith the small angle X-ray scattering (SAXS) measurements.

2. Experimental

2.1. Apparatus

1H NMR data were obtained by a JEOL AL-300 (300 MHz). 1H NMRchemical shifts were determined by using tetramethylsilane as aninternal standard. Elemental analyses were performed with a PerkinElmer 2400II CHNS/O. XRD spectra were characterized by Bruker AXS.Microwave-induced plasma mass spectra (MIP-MS) were recorded ona HITACHI P-6000 for the estimation of concentration of Eu element inthe toluene solution of EuS NCs. High-resolution images of the EuSNCs were obtained with a Hitachi JEM-3100FEF TEM equipped with atilting stage (±60°).

2.2. Materials

Europium(III) chloride hexahydrate (EuCl3·6H2O) was purchasedfrom Kanto Chemical Co. Inc. Sodium N,N-diethyldithiocarbamatetrihydrate (Na(S2CNEt2)·3H2O) and toluene were purchased fromNacalai Tesque. Tetraphenylphosphonium bromide (BrPPh4) and acet-onitrile-d3 (CD3CN) were purchased from Wako Pure ChemicalIndustries, Ltd. Oleylaminewas obtained from Tokyo Chemical IndustryCo., Ltd. TEM grids (Cu 200 mesh) were obtained from JEOL Ltd.

2.3. Synthesis of tetraphenylphosphonumtetrakis (diethyldithiocarbamate)europium (III) ((PPh4) [Eu(Et2dtc)4])

A solution of (Na(S2CNEt2)·3H2O) (14 g) in 30 ml methanol wasadded to EuCl3·6H2O (5.6 g) dissolved in 30 ml methanol with stirringand reacted for 3 h. After the reactionmixture was filtered, a solution ofBrPPh4 (6.4 g) in 30 mlmethanolwas added to thefiltrated solution and

Fig. 2. SAXS profile of EuS aggregates on PET film.

871A. Tanaka et al. / Thin Solid Films 518 (2009) 870–872

stirred for 9 h. The resulting precipitate was separated by filtration andwashed 2 times with ethanol. Yield: 30%. FT-IR spectrum (KBr): 1485–1482 (C–N) cm−1, 1442 (P-Phenyl) cm−1, 1007 (C–S) cm−1. 1H NMRspectrum (CD3CN): 7.91 (4H, t, J=6.3 Hz, [Eu(S2CNEt2)4] (P(C6H5)4(p)), δ7.70 (16H, m, [Eu(S2CNEt2)4] (P(C6H5)4(o, m)), δ3.17(8H,q, J=7 Hz, [Eu(S2CNCH2CH3)4] (PPh4 )). δ1.61 (12H, q, J=7 Hz, [Eu(S2CNCH2CH3)4] (PPh4 )). Anal. Calcd. for C44H60EuN4PS8: C, 48.33; H,5.53; N, 5.13%. Found: C, 48.31, H, 5.46, N, 5.13%.

2.4. Synthesis of cubic-shaped EuS NCs

Under N2 atmosphere, (PPh4) [Eu(Et2dtc)4] (0.9 g) was dissolved inoleylamine (4.5 g) and heated to 180 °C with stirring for 20 min. Thereaction solventwas heated to 300 °C and stirred for another 6 h, and thepurple liquid was centrifuged under 5000 rpm for 10 min. Theprecipitation was added to 8 ml toluene and centrifuged by 13,000 rpmfor 15 min, and the clear purple liquid was obtained after elimination ofthe deposition.

2.5. Preparation of EuS aggregates

The toluene solution with EuS NCs (18 μM in Eu2+ ion, 1 ml) wasdispersed into methanol (6 ml) and stirred for 3 h, giving theaggregates of EuS NCs [8,9]. The obtained EuS aggregates in methanolwere dropped on a PET film, and characterized with SAXS measure-ments using a rotating anode X-ray generator (UltraX18, Rigaku), inwhich monochromatic X-ray of 1.54 Å in wavelength was focusedthrough a Confocal Max-Flux mirror (Osmic). The diffraction imageswere collected using an X-ray image intensifier CCD detector,(HAMAMATSU). The distance between sample and detector wasadjusted to 750 mm.

3. Results and discussion

Fig. 1a shows results of XRD measurements of EuS NCs. Diffractionpeaks at 2θ of 26.1°, 30.3°, 42.9°, 50.6°, 53.1°, 62.1°, and 70.2° wereassigned to (111), (200), (220), (311), (222), (400), and (420),respectively. The peaks marked with asterisk were obtained fromaggregates of oleylamine. Typical TEM image is shown in Fig. 1b. Theaverage size of the EuS NCs was estimated from the TEM images to be10 nm (Fig. 1c). TEM image indicates that the EuS NCs form self-

Fig. 1. (b) XRD pattern and (a)TEM images of cube-shaped

aggregated 2D-orthogonal lattice structure on the TEM grid, densely.The center-to-centerdistancebetween adjacent EuSNCswere evaluatedas amean value of over 100 samples to be 14.6 nm. This value is given asthe estimated lattice parameter of this 2D-superlattice structure. In self-aggregation of cubic-shaped NCs, Yamamoto and co-workers havereported on the contribution of van derWaals interactions betweenNCscores to the NCs arrangement in 2D aggregation [11,12]. Regularaggregation of the cube-shaped NCs might be due to the fact that theoleylamine adsorbed on the surfaces would form a capping shell togenerate a regular interparticle spacing and thus to prevent the crystalsfrom random aggregation.

In order to characterize a packing structure of EuS NCs, we carriedout SAXSmeasurements for the EuS aggregates on a polymer thin film.The SAXS pattern of EuS aggregates on polymer thin film indicatedseveral diffraction peaks as shown in Fig. 2. The diffraction peakpositions (q values) are given by,

q = 4π sin θ= λ ð1Þ

where θ is the diffraction angle according to Bragg's equation. Fig. 2also shows calculated diffraction lines which were evaluated for cubic3D superlattice structure with 14.6 nm of lattice constant [10]. The

EuS NCs. (c) Size distribution histogram of EuS NCs.

872 A. Tanaka et al. / Thin Solid Films 518 (2009) 870–872

diffraction peaks were observed at q=0.042, 0.057, 0.082 and 0.129,which were assigned to (100), (110), (111) and (200) of 3D cubicstacking structure of EuS aggregates, respectively. The diffraction peakof (111) indicates that the cube-shaped EuS NCs form 3D cubicsuperlattice. In SAXS profiles, the lattice constant of (100) wasassigned to center distance (14.6 nm) of EuS nanocrystals. The face-to-face distance between EuS nanocrystals (5 nm) were estimated tobe 4.6 nm. On the other hand, molecule size of oleylamine wasestimated to be 2.25 nm. Thus, oleylamine might be formed bilayer insuperlattice structures of EuS nanocrystals.

The formation of the 3D stacked structure is also observed in theTEM images. The aggregates of cube-shaped EuS NCs might built up a3D cubic superlattice. Fig. 3a shows another example of TEM image of

Fig. 3. (a) TEM image of cube-shaped EuS NCs. (b) Contrast intensity histogram and(c) line profiles in marked area of TEM image. (d) Stacking structure model of cube-shaped EuS NCs.

EuS NCs of 10 nm in size. There are clearly two types of NCs images ofdifferent image-intensity levels. The bright NCs would correspond tothe normal mono-NCs. The image-intensity histogram of the markedarea of Fig. 3a is shown in Fig. 3b. In Fig. 3b, there are two major peaksin the TEM-image intensity at about 1200 a.u.–1300 a.u. and 1400 a.u.–1500 a.u.. The former and latter peaks correspond to the mono-NCsand background level, respectively. While, there is another side peakin the histogram at about 1100 a.u.–1150 a.u.. Fig. 3b also showsresults of deconvolution of the distribution profile with three Gaussfunctions of peak intensity at 1133 a.u., 1251 a.u. and 1416 a.u. Fig. 3cshows line-profile of the TEM image-intensity with the line marked inFig. 3a and indicates three characteristic amplitude of the image-intensity. Each level roughly coincided to those evaluated from Fig. 3b.The dark NCs, whose image-intensity is typically about 1050 a.u.–1150 a.u., are thus assigned to the Twin-NCs of superimposing eachother. That is, the NCs form not only 2D-ordered structures but also3D-stacked structure as schematically illustrated in Fig. 3d. Similarsuperimposed structure can be frequently observed in TEM images ofthe EuS NCs. We suggest that the 3D EuS aggregates might formhigher-order structure, because the SAXS profiles indicated the signalsof (100) and (111) lattice plane of cubic super lattice structures.

In summary, formation of self-aggregated structure of cube-shaped EuS NCs was successfully observed. They easily form 2Dorthogonal-structure on the TEM grids with organized face-to-facepacking. SAXS of the cast film indicated formation of not only the 2Dorthogonally organized structure but also the 3D cubic structure,which is also supported by TEM images. The tendency for formingface-to-face packed structure would originate from the interactionbetween the planar surfaces of NCs covered with oleylamine. Van derWaals force between the NCs would also be a possible origin of self-aggregation [11,12]. One may expect enhanced opto-magnetic proper-ties of the EuS aggregates with the organized super-structures, takingtheir characteristic spin polarization in the ground states andenhancedmagnetic interaction between NCs into consideration [7,13].

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research(No. 19510104) from the Ministry of Education, Culture, Sports,Science, and Technology, MEXT, Japan. The authors thank Ms. S. Fujita,Ms. R. Nakashima and Mr. M. Fujiwara in NAIST for their contributionon TEM observation and MIP-MS measurements.

References

[1] P. Wachter, Handbook on the Physics and Chemistry of Rare Earth, 2nd Ed in CRCCritical Reviews in Solid State Science, North-Holland Publishing Company, 1979,p. 189.

[2] T. Kasuya, A. Yanase, Rev. Mod. Phys. 40 (1968) 684.[3] S. Thongchant, Y. Hasegawa, Y. Wada, S. Yanagida, Jpn. J. Appl. Phys. 42 (2003)

L876.[4] Y. Hasegawa, M.A. Afzaal, P. O'Brien, Y. Wada, S. Yanagida, Chem. Commun. (2005)

242.[5] Y. Hasegawa, T. Adachi, A. Tanaka, M. Afzaal, P. O'Brien, T. Doi, Y. Hinatsu, K. Fujita,

K. Tanaka, T. Kawai, J. Am. Chem. Soc. 130 (2008) 5710.[6] F. Dumestre, B. Chaudret, C. Amiens, P. Renaud, P. Fejes, Science 303 (2004) 821.[7] E. Snoeck, C. Gatel, L. M. Lacroix, T. Blon, S. Lachaize, J. Carrey, M. Respaud, B.

Chaudret, Nano Lett. ASAP.[8] E. Rabani, S.A. Egorov, Nano Lett. 2 (2002) 69.[9] M.Q. Zhu, L.Q. Wang, G.J. Exarhos, A.D.Q. Li, J. Am. Chem. Soc. 126 (2004) 2656.[10] B.A. Korgel, S. Fullam, S. Connolly, D. Fitzmaurice, J. Phys. Chem. B 102 (1998) 8379.[11] S. Yamamuro, K. Sumiyama, Chem. Phys. Lett. 418 (2006) 166.[12] S. Yamamuro, K. Sumiyama, T. Kamiyama, Apll. Phys. Lett. 92 (2008) 113108.[13] R. Kubo, J. Phys. Soc. Jpn. 12 (1957) 570.