Size-Controlled Aggregation of Cube-Shaped EuS Nanocrystals with

Dec 2, 2010 - (10, 11) The magnetic and magneto-optic properties of EuS NCs are ...... electronic coupling, and doping while passivating electronic tr...
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Size-Controlled Aggregation of Cube-Shaped EuS Nanocrystals with Magneto-Optic Properties in Solution Phase Atsushi Tanaka, Hironari Kamikubo, Mikio Kataoka, Yasuchika Hasegawa,*,† and Tsuyoshi Kawai* Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192. † Present address: Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, kita-ku, Sapporo, Hokkaido 060-8628, Japan. Received September 29, 2010. Revised Manuscript Received November 5, 2010 Size-controlled formation of colloidal aggregates composed of cube-shaped EuS nanocrystals, EuS NCs, in solution phase are reported and their optical and magneto-optical properties are studied. The average size of the colloidal particles of the EuS NCs-aggregates formed in 1-pentanol, 1-hexanol, and 1-octanol were ca. 800, 500, and 100 nm, respectively. Self-organized cubic-type superlattice structure was evaluated in the colloidal aggregates by means of the small-angle X-ray diffraction measurements, which is similar to those in the macroscopic 3D superlattice structures. The distances between NCs in the EuS NCs-aggregates are dependent on alkyl chain length of the solvent alcohol. Magnetooptic properties of EuS NCs-aggregates and the monomeric EuS nanocrystals in liquid media are characterized with magnetic circular dichroism spectra. The active wavelength of EuS NCs-aggregates is considerably longer than that of the monomeric EuS nanocrystals.

1. Introduction Magnetic semiconductors have attracted considerable attention in the area of advanced materials science because of their remarkable magnetic, spintronic, and magneto-optic properties.1 Various types of nanostructured magnetic materials have been studied for the past few decades.2-7 One of the most extensively studied series of intrinsic magnetic semiconductors is europium chalcogenides, EuX (X = O, S, Se, and Te).8 The EuX semiconductors are characterized by the degenerated 4f orbitals of Eu(II) ions existing between the conduction band (5d orbitals of Eu(II)) and the valence band (p orbitals of chalcogenides, O2-, S2-, Se2-, or Te2-).8 The 4f-5d electronic transition and spin configuration of europium(II) chalcogenides leads to large Faraday and Kerr effects,9 making them promising candidates as the active materials in future magneto-optic devices. *Corresponding authors. E-mail addresses: [email protected] (T. Kawai), [email protected] (Y. Hasegawa). (1) Furdyna, J. K. J. Appl. Phys. 1988, 64, R29–R64. (2) Gaj, J. A.; Ginter, J.; Galazka, R. R. Phys. Status Solidi B 1978, 89, 655–662. (3) Ohno, H.; Shen, A.; Matsukura, F.; Oiwa, A.; Endo, A.; Katsumoto, S.; Iye, Y. Appl. Phys. Lett. 1996, 69, 363–365. (4) Ohno, Y.; Young, D. K.; Beschten, B.; Matsukura, F.; Ohno, H.; Awschalom, D. D. Nature 1999, 402, 790–792. (5) Jungwirth, T.; Atkinson, W. A.; Lee, B. H.; MacDonald, A. H. Phys. Rev. B 1999, 59, 9818–9821. (6) Wang, Y.; Herron, N.; Moller, K.; Bein, T. Solid State Commun. 1991, 77, 33–38. (7) (a) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. Nano Lett. 2001, 1, 3–7. (b) Jun, Y.; Jung, Y. Y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615–619. (c) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205–13218. (d) Stowell, C. A.; Wiacek, R. J.; Saunders, A. E.; Korgel, B. A. Nano Lett. 2003, 3, 1441. (e) Norberg, N. S.; kittilstved, K. R.; Amonette, J. E.; Kukkadapu, R. K.; Schwartz, D. A.; Gamelin, D. R. J. Am. Chem. Soc. 2004, 126, 9387–9398. (f) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436, 91–94. (g) Beaulac, R.; Archer, P. I.; Liu, X.; Lee, S.; Salley, G. M.; Dobrowolska, M.; Furdyna, J. K.; Gamelin, D. R. Nano Lett. 2008, 8, 1197– 1201. (8) Wachter, P. Handbook on the Physics and Chemistry of Rare Earth, 2nd ed.; CRC Critical Reviews in Solid State Science; North-Holland Publishing Company: Amsterdam, 1979; p 189. (9) Suits, J. C.; Argyle, B. E.; Freiser, M. J. J. Appl. Phys. 1966, 37, 1391-1– 1391-7.

104 DOI: 10.1021/la103899a

Their characteristic magneto-optic properties are markedly enhanced in nanoscale structures because of their quantumconfinement effects on the electronic excited states.10 Nanocrystals (NCs) of EuO, EuS, and EuSe have recently been synthesized, and their specific magneto-optical properties have been explored.10,11 Among of them, EuS NCs are distinctly characterized with their ferromagnetic properties with Curie temperature (TC = 16.6 K) and characteristic Faraday effects in the visible region.10,11 The magnetic and magneto-optic properties of EuS NCs are significantly dependent on their size, shape, and surface environments.10,11 We have recently reported self-assembling formation of macroscopic 3D superlattice structure of cube-shaped EuS NCs and their enhanced magnetic properties12 with ferromagnetic dipole interaction between the EuS NCs.13-17 From the viewpoint of practical applications of the condensed NCs for use them in optical devices, (10) (a) Hasegawa, Y.; Thongchant, S.; Wada, Y.; Tanaka, H.; Kawai, T.; Sakata, T.; Mori, H.; Yanagida, S. Angew. Chem., Int. Ed. 2002, 41, 2073–2075. (b) Thongchant, S.; Hasegawa, Y.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 2193–2196. (c) Hasegawa, Y.; Afzaal, M. A.; O'Brien, P.; Wada, Y.; Yanagida, S. Chem. Commun. 2005, 242–243. (d) Kataoka, T.; Tsukahara, Y.; Hasegawa, Y.; Wada, Y. Chem. Commun. 2005, 6038–6040. (e) Hasegawa, Y.; Okada, Y.; Kataoka, T.; Sakata, T.; Mori, H.; Wada, Y. J. Phys. Chem. B 2006, 110, 9008–9011. (f) Hasegawa, Y.; Adachi, T.; Tanaka, A.; Afzaal, M.; O'Brien, P.; Doi, T.; Hinatsu, Y.; Fujita, K.; Tanaka, K.; Kawai, T. J. Am. Chem. Soc. 2008, 130, 5710–5715. (11) (a) Regulacio, M. D.; Tomson, N.; Stoll, S. L. Chem. Mater. 2005, 17, 3114– 3121. (b) Zhao, F.; Sun, H.; Gao, S.; Su, G. J. Mater. Chem. 2005, 15, 4209–4214. (c) Regulacio, M. D.; Bussmann, K.; Lewis, B.; Stoll, S. L. J. Am. Chem. Soc. 2006, 126, 11173–11179. (d) Zhao, F.; Sun, H.-L.; Su, G.; Gao, S. Small 2006, 2, 244–248. (e) Huxter, V. M.; Mikovic, T; Nair, P. S.; Scholes, G. D. Adv. Mater. 2008, 20, 3368– 3376. (f) Regulacio, M. D.; Kar, S.; Zuniga, E.; Wang, G.; Dollahon, N. R.; Yee, G. T.; Stoll, S. L. Chem. Mater. 2008, 20, 3368–3376. (g) Pereira, A. S.; Rauwel, R; Reis, M. S.; Silva, N. J. O; Barros-Timmons, A.; Trindade, T. J. Mater. Chem. 2008, 18, 4572–4578.(h) Kar, S.; Boncher, W. L.; Olszewski, D.; Dollahon, N.; Ash, R.; Stoll, S. L. J. Am. Chem. Soc. in press. (12) Tanaka, A.; Kamikubo, H.; Doi, Y.; Hinatu, Y; Kataoka, M; Kawai, T.; Hasegawa, Y. Chem. Mater. 2010, 22, 1776–1781. (13) Chudnovsky, E. M.; Serota, R. A. J. Phys. C: Solid State Phys. 1983, 16, 4181–4190. (14) Chudnovsky, E. M. J. Appl. Phys. 1988, 64, 5770–5775. (15) Chudnovsky, E. M. J. Magn. Magn. Mater. 1989, 79, 127–130. (16) Filippi, J.; Amaral, V. S.; Barbara, B. Phys. Rev. B. 1991, 44, 2842–2845. (17) Thomas, L.; Tuaillon, J.; Perez, J. P.; Dupuis, V.; Perez, A.; Barbara, B. J. Magn. Magn. Mater. 1995, 140, 437–438.

Published on Web 12/02/2010

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it is required to control their size for preventing optical light scattering. To this aim, nanosized NCs-aggregates composed of cubeshaped EuS NCs (EuS NCs-aggregates) are expected to show characteristic magneto-optic properties in the solution phase and in polymer matrices with good transparency. As a related topic, various types of nanosized aggregates of semiconductor NCs dispersed in colloidal solutions have recently been studied extensively.18 Koole et al. have, for example, reported effective exciton energy transfer and electronic coupling of aggregates of semiconductor CdTe NCs in solution.19 Kelley et al. have observed drastic red-shifts of absorption and emission bands of nanosized aggregates composed of semiconductor GaSe NCs.20 Characteristic exciton coupling interaction between the NCs has also been studied by Urban et al.21 In the present study, we report on size-controlled formation of colloidal particles composed of EuS cubic NCs in different solvents, 1-pentanol, 1-hexanol, and 1-octanol. The self-organized structure of the EuS NCs-aggregates is characterized with small-angle XRD and TEM measurements. Magneto-optic properties of EuS NCs-aggregates in alcohol are also characterized using UV-vis absorption and magnetic circular dichroism (MCD) spectra.

2. Experimental Section 2.1. Materials. Europium(III) chloride hexahydrate (EuCl3 3 6H2O) was purchased from Kanto Chemical Co. Inc. Sodium N,N-diethyldithiocarbamate trihydrate (Na(S2CNEt2) 3 3H2O), toluene, 1-pentanol, 1-hexanol, and 1-octanol were purchased from Nacalai Tesque. Tetraphenylphosphonium bromide (BrPPh4) and acetonitrile-d3 (CD3CN) were purchased from Wako Pure Chemical Inducteries, Ltd. Oleylamine was obtained from Tokyo Chemical Industry Co., Ltd. All other chemicals and solvents were reagent grade and were used without further purification. 2.2. Apparatus. 1H NMR data were measured by a Jeol AL300 (300 MHz). 1H NMR chemical shifts were determined by using tetramethylsilane (TMS) as an internal standard. Elemental analyses were performed with a Perkin-Elmer 2400II CHNS/O. XRD spectra were characterized by a Bruker AXS. MIP-MS were recorded on a Hitachi P-6000. High-resolution images of the EuS nanocubes were obtained with a Jeol JEM-3100FEF TEM equipped and operated at 300 kV. Dynamic light scattering (DLS) spectra were measured on Otsuska Electronics DLS-6000HL with 633 nm He-Ne laser light. Distribution of particle size was evaluated by analyzing autocorrelation functions of light scattering with its system software. Small-angle X-ray diffraction measurements were carried out using a rotating anode X-ray generator (UltraX18, Rigaku), in which monochromatic X-ray of 1.54 A˚ in wavelength was focused through a Confocal Max-Flux mirror (Osmic). The diffraction images were collected using an X-ray image intensifier CCD detector (Hamamatsu V7739P/C-4880-50). The distance between sample and detector was adjusted to 750 mm. MCD spectra were measured on a Jasco J-725 circular dichroism spectrometer with 0.85T of static magnetic field at room temperature. 2.3. Synthesis of Tetraphenylphosphonum Tetrakis(diethyldithiocarbamate) Europium(III) ((PPh4) [Eu(S2CNEt2)4]) and Cube-Shaped EuS NCs. (PPh4) [Eu(S2CNEt2)4] and (18) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335– 1338. (b) Kagan, C. R.; Murray, C. B.; Nirmal, M.; Bawendi, M. G. Phys. Rev. Lett. 1996, 76, 1517–1520. (c) Pileni, M. P. Langmuir 1997, 13, 3266–3276. (d) Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Gaponik, N.; Haase, M.; Rogach, A. L.; Weller, H. Adv. Mater. 2001, 13, 1868–1871. (e) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86–89. (f) Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Murray, C. B. J. Am. Chem. Soc. 2006, 128, 3248–3255. (19) Koole, R.; Liljeroth, P.; Donega, C. M.; Vanmaekelbergh, D.; Meijerink, A. J. Am. Chem. Soc. 2006, 128, 10436–10441. (20) Tu, H.; Yang, S.; Chikan, V.; Kelley, D. F. J. Phys. Chem. B 2004, 108, 4701–4710. (21) Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Kagan, C. R.; Murray, C. B. Nat. Mater. 2007, 6, 115–121.

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Figure 1. (a) XRD pattern and (b) TEM images of cube-shaped EuS NCs. (c) Distribution of side length of EuS NCs. cube-shaped EuS NCs were synthesized and characterized as previously reported. From TEM observations, the average size of EuS NCs was estimated to be 14.0 nm (Figure 1b). 2.4. Preparation of EuS NCs-Aggregates. The toluene solution (1 mL) with EuS NCs (3 mg) was dispersed into alcohol (1-pentanol, 1-hexanol, and 1-octanol) (6 mL) and stirred for 3 h, giving colloidal solutions of EuS NCs-aggregates. The obtained colloidal solutions were packed in a quartz capillary (Hampton research) and characterized with DLS, small-angle X-ray diffraction, and MCD measurements. TEM images were obtained by spreading the colloidal solutions onto a carbon grid and dried carefully.

3. Results and Discussion 3.1. Preparation of Cube-Shaped EuS NCs. EuS NCs were prepared by thermal reduction of a single source precursor, (PPh4)[Eu(Et2dtc)4], in oleylamine at 300 °C under N2 atmosphere. An XRD profile supported the formation of crystalline EuS NCs (Figure 1a).12 The crystal grain size calculated using Scherer’s equation from a diffraction peak of the (200) plane in XRD spectrum was about 12 nm. Figure 1b shows TEM images of cubic EuS NCs. The cube-shaped EuS NCs with clear lattice fringes were observed, and their electron diffraction profile agreed well with that of the bulk EuS lattices of NaCl-type structure. The average size of the EuS NCs was estimated from the TEM images to be 14 nm (Figure 1c). The TEM image also indicates that the EuS NCs form the self-organized 2D-cubic superlattice structure on the TEM grid. The formation of the self-organized structures seems to be induced by the van der Waals interaction between the NCs bodies.22 As has already been reported in our previous paper, the 3D cubic superlattice structure of micrometer-scale can also be easily formed.12 In the bright area between the EuS NCs, the lightweight elements such as C, H, O, and/or N atoms should exist, and thus, oleylamine seems to protect the surface of the NCs. The distance between the EuS NCs observed in these TEM images was found to be about 4.6 nm. Since the length of surface-protecting molecule, oleylamine, is estimated to be 2.25 nm by the molecular modeling, we thus consider that the oleylamine molecules between EuS NCs form bilayer structures without interdigitation. 3.2. Characterizations the NCs-Aggregates Composed of Cube-Shaped EuS Nanocrsytals in Organic Liquid Media. In order to characterize self-organized structures of colloidal particles of EuS NCs-aggregates, the toluene solution of the EuS NCs (22) Yamamuro, S.; Sumiyama, K.; Kamiyama, T. Appl. Phys. Lett. 2008, 92, 113108-1–113108-3.

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Tanaka et al. Table 1. Peak Positions from Small-Angle XRD Profile of Colloidal Solution of EuS NCs-Aggregates Dispersed in 1-Octanol hkl

Figure 2. Size distributions of EuS aggregates in alcohol solutions (1-octanol, 1-hexanol, and 1-pentanol) evaluated with DLS measurements.

q (nm-1)a

exp qhkl/q100b

calcd qhkl/q100c

(100) 0.37 1.00 1.00 (110) 0.49 1.32 1.41 (111) 0.66 1.78 1.73 (200) 0.71 1.91 2.00 (210) 0.79 2.13 2.24 a λ = 0.154 nm for the Cu KR line. b Estimated after numerical fitting with Gauss’s functions. c Evaluated for ideal simple cubic superlattice structure.

Table 2. Peak Position and Distance between NCs in EuS NanoAggregates in 1-Octanol, 1-Hexanol, and 1-Pentanol Evaluated from Small-Angle XRD Measurements solvent

q111 (nm-1)a δSL (nm) length of solvent moleculesb (nm)

1-octanol 0.66 2.4 1.0 1-hexanol 0.71 1.8 0.8 1-pentanol 0.72 1.7 0.65 a Derived from Figure 3b. b Evaluated with AM1 calculation on Chem3D software.

Figure 3. (a) Small-angle XRD profile of colloidal solution of EuS NCs-aggregates in 1-octanol. (b) Small-angle XRD profile of EuS NCs-aggregates in 1-octanol (red circles), 1-hexanol (blue circles), and 1-pentanol (green circles).

was spread into alcohols, 1-pentanol, 1-hexanol, and 1-octanol. Size distributions of aggregated colloidal particles in the alcohol solutions were evaluated after DLS measurements as shown in Figure 2. The average sizes of aggregates in 1-pentanol, 1-hexanol, and 1-octanol were about 800 nm, 500 nm, and 100 nm, respectively. The solvent with longer alkyl chain tends to form smaller aggregates. This tendency may originate from the polarity of solvents. Dielectric constants of 1-pentanol, 1-hexanol, and 1-octanol are 15.5, 13.2, and 9.9, respectively.23,24 The difference in the solvent polarity as well as chain length may contribute to the aggregation tendency of the NCs. For analyzing packing structures of the colloidal EuS NCsaggregates, small-angle X-ray diffraction (XRD) measurements were performed for the colloidal EuS NCs-aggregates formed in 1-octanol. The small-angle XRD spectra of EuS NCs-aggregates in 1-octanol are shown in Figure 3. Because of scattering X-ray signals at low angle, there were significant background signals below q < 0.5 nm-1. Diffraction signals corresponding to (k00) appeared clearly, but other peaks were relatively weak, which indicates relatively large fluctuation of density in the direction of (k00) in the cubic-type superlattice structure. The diffraction peak positions (q values) are given by eq1 q ¼ 4π sin θ=λ

ð1Þ

where θ and λ are the diffraction angle and 0.154 nm for the Cu KR line, respectively.25 The q values are obtained using fittingcalculation for diffraction profiles with Gauss’s functions. There are some characteristic diffraction peaks which were assigned to (23) Sastry, N. V.; Valand, M. K. J. Chem. Eng. Data 1998, 43, 152–157. (24) Benitez, J. J.; Fuente, O. R.; Perez, I. D.; Sanz, F.; Salmeron, M. J. Chem. Phys. 2005, 123, 104706-1–104706-6. (25) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379–8388.

106 DOI: 10.1021/la103899a

the reflection from the simple cubic superlattice structures as summarized in Table 1. We also summarized observed ratio of qhkl values against q100 (exp. qhkl/q100) and compared them with calculated values for the ideal cubic superlattice structures (cal. qhkl/q100) in Table 1. The observed qhkl/q100 values showed good agreement with the expected ones, indicating simple cubic superlattice structures in the colloidal NCs-aggregates in the solution phase. The cubic NCs are regarded to form the colloidal aggregates with the self-organized cubic superstructure, which is preferentially stabilized by the van der Waals interactions between EuS NCs. The q value is inversely proportional to the characteristic d spacing in the system. The dhkl spacing is given by dhkl ¼ 2π=qhkl

ð2Þ

where qhkl is diffraction peak position. The center-to-center distances between adjacent EuS NCs, L, were evaluated from the d100 spacing with eq 3 L ¼ d100 ¼ d111  q111 =q100

ð3Þ

The distance between surfaces of NCs in the cubic superlattice, δSL, which originates from the presence of the surface protecting molecules, can be calculated from L.25 δSL ¼ L - R

ð4Þ

where R is the average side-length of the cubic EuS NCs derived from TEM images, ca. 14 nm. The d111 spacing of EuS NCsaggregates in 1-octanol was evaluated to be 9.5 nm from the value of q111 = 0.66 nm-1. The estimated value of δSL spacing was 2.4 nm on the basis of values of cal. q111/q100 = 1.78 and d111. The δSL spacing in EuS NCs-aggregates in 1-pentanol (q111 = 0.71 nm-1) and 1-hexanol (q111 = 0.72 nm-1) were also estimated similarly to be 1.8 and 1.7 nm as summarized in Table 2. These results indicate that the alcohol molecules with longer alkyl chain tend to promote larger distance between NCs in the aggregates. In Table 2, estimated lengths of alcohol molecules are also summarized, and one may find rough agreement between the δSL spacing and twice the molecular length of the solvent alcohols. The oleylamine molecules, which were originally protecting the surface of EuS NCs after the preparation of NCs, might thus be exchanged with the solvent alcohol molecules because of stronger Eu(II)-O interaction. Langmuir 2011, 27(1), 104–108

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Figure 4. (a,b) TEM images of EuS NCs-aggregates. The sample was prepared from 1-octanol solution on a carbon TEM grid. (c) Distribution of interparticle spacing in EuS NCs-aggregates. (d) Schematic illustration of 1-octanol molecules between EuS NCs in aggregate.

We then carried out TEM measurements for evaluating structure of EuS NCs-aggregates. The 1-octanol solution was dropcast onto the usual carbon TEM grid. The EuS NCs-aggregate observed in Figure 4a is composed of approximately 300 NCs. Distance between EuS NCs exhibited relatively narrow distribution, as shown in Figure 4c, and the average value was about 1.2 nm. This value is much smaller than both the distance observed in Figure 1b and the length of oleylamine, but is similar to the length of 1-octanol. That is, these results can be explained by the terms of interdigitation of 1-octanol molecules as schematically illustrated in Figure 4d. The interdigitation might be caused by capillary force between EuS NCs upon evaporation of 1-octanol on the TEM grid.26 The characteristic narrowing in the distance between the NCs was also observed in some TEM samples prepared from solutions of 1-hexanol and 1-pentanol (see Supporting Information Figure S1), which also suggests interdigitation of the surface protecting alcohol molecules in the dried sampled (interparticle distances = approximately 1.0 nm; see Supporting Information Figure S2). We consider that interparticle distances might be controlled with the interdigitation of surface protecting alcohol molecules (see Supporting Information Figure S2). 3.3. Magneto-Optical Properties of EuS NCs-Aggregates in Organic Liquid Media. The UV-vis absorption and magnetic circular dichroism (MCD) spectra of the colloidal solution of EuS NCs-aggregates in 1-octanol and of monomeric EuS NCs in the toluene solution of are shown in Figure 5. Broad absorption bands observed in both samples at around 550 nm were assigned to the electronic transition between 4f7 and 4f6(7FJ)5d(t2g).8 The active (26) Deenkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183–3190.

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Figure 5. Absorption (top) and MCD (bottom) spectra of colloidal solution of EuS NCs-aggregates in 1-octanol (red line) and of monomeric EuS NCs in toluene solution (blue line). In the MCD measurements, 0.85T of magnetic field was applied statically in parallel to the optical axis.

Figure 6. Band gap energy of monomeric EuS NCs of 14 nm, EuS NCs-aggregates with cubic superlattice structure, and bulk EuS.

wavelength of MCD spectrum of the EuS NCs-aggregates under magnetic applied field (0.85 T) is markedly longer than that of monomeric EuS NCs in toluene. The energy gaps of EuS NCsaggregates and the monomeric EuS NCs were evaluated roughly by the threshold wavelength of their absorption bands to be 1.72 and 1.82 eV, respectively, which are significantly larger than that of bulk EuS, Eg = 1.64 eV as illustrated in Figure 6. We observed a specific red shift of the active wavelength of the magneto-optic spectrum of EuS NCs by means of size controlled formation of aggregated superparticles. Characteristic red shift of absorption and emission spectra of condensed structures of semiconductor nanocrystals have been elucidated in terms of exciton coupling between NCs.18-21 Nozik et al. have, for example, reported that the aggregates of InP NCs show a red shift of absorption and emission bands because of exciton coupling between NCs. They have also suggested that the effects of the electronic coupling increase with decreasing DOI: 10.1021/la103899a

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interparticle distance.27 We propose that the characteristic redshift of the MCD band of EuS NCs-aggregates might be attributed to the exciton coupling between the NCs.

4. Conclusions Stable colloidal particles of aggregated EuS NCs in alcohol solution were successfully prepared that have cubic-type superlattice structures with tunable aggregation size and interparticle distance and, thus, lattice constant by means of alkyl chain length of the solvent alcohols. The superparticle structures might depend on the molecular structure of the solvent alcohols after the exchange process at the surface of NCs. The optical band gap and the active wavelength of magneto-optic properties of EuS NCs-aggregates showed a characteristic red shift in the aggregated structures. (27) Micic, O. I.; Ahrenkiel, S. P.; Nozik, A. J. Appl. Phys. Lett. 2001, 78, 4022– 4024.

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Acknowledgment. We thank for Ms. S. Fujita, Ms. R. Nakashima, Ms. C. Goto, and Mr. M. Fujiwara of Nara Institute of Science and Technology for their contribution on TEM observation and MIP-MS measurements. This work was partly supported by Grants-in-Aid for Scientific Research on Innovative Areas of “Strong Photon-Molecule Coupling Fields for Chemical Reactions” and “Emergent chemistry of nano-scale molecular systems” from the Ministry of Education, Culture, Sports, Science and Tech (MEXT), Japan. Some of the present experiments were performed with the aid by the Kyoto Advanced Nanotechnology Network supported by JST, Japan Science and Technology Agency. Supporting Information Available: TEM images of NCsaggregates assembled with EuS NCs by using 1-pentanol and 1-hexanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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