Finite Size Effect on Magneto-Optical Responses of Chemically

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Finite Size Effect on Magneto-Optical Responses of Chemically Modified FeO Nanoparticles Studied by MCD Spectroscopy 3

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Hiroshi Yao, and Yuki Ishikawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03131 • Publication Date (Web): 07 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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The Journal of Physical Chemistry

Finite Size Effect on Magneto-Optical Responses of Chemically Modified Fe3O4 Nanoparticles studied by MCD Spectroscopy Hiroshi Yao* and Yuki Ishikawa

Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan

* To whom correspondence should be addressed. Fax: +81-791-58-0161. E–mail: [email protected] (H. Yao).

ABSTRACT

A magneto-optical study of chemically modified magnetite (Fe3O4) nanoparticles is presented with magnetic circular dichroism (MCD) spectroscopy. The magnetite nanoparticles are synthesized using the high-temperature solution reduction of Fe(III) acetylacetonate in the presence of oleylamine. The size of the magnetite nanoparticles examined is 3–4 or 7–8 nm. In contrast to broad and featureless UV-vis-NIR absorption spectra for these magnetite nanoparticles, the MCD responses are much structured. MCD essentially corresponds to electronic transitions in the absorption spectrum, so to decode electronic transitions in the magnetite nanoparticles and to compare them with those in the bulk phase, simultaneous deconvolution analyses of both the electronic absorption and MCD spectra are conducted, giving accurate transition energies with enhanced spectral resolution. The decomposed transitions are successfully assigned on the basis of the theoretical band-structure calculations previously reported. Then the relative absorption and MCD intensities of some deconvoluted bands associated with LMCT or ISCT transitions are found to be size dependent, which can be due to partial surface hydration and/or oxidation of the magnetite nanoparticles. We also compare the magneto-optical responses of magnetite nanoparticles and those of bulk Fe3O4 calculated.

Keywords: Magnetite nanoparticles; Magnetic circular dichroism; Finite size effect; Deconvolution 1 ACS Paragon Plus Environment


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INTRODUCTION

Magnetite (Fe3O4 = Fe2+(Fe3+)2O4) is one of the ferrimagnetic materials with an inverse spinel structure (space group Fd3m). In its bulk crystal structure, O2– anions form an fcc lattice with Fe2+ and Fe3+ cations in interstitial sites.1 Each unit cell consists of eight Fe2+(Fe3+)2O4 units, where eight Fe3+ and eight Fe2+ ions are in the octahedral positions (= B site), while the remaining trivalent Fe3+ ions are in the tetrahedral site (= A site).1 The electrons can then hop between Fe2+ and Fe3+ ions in the octahedral sites at room temperature, rendering magnetite an important class of half-metallic materials.2–4 Meanwhile, nanosctrctured magnetite is currently a key material for advancements in optoelectronics,5 magnetic storage,6 and bio-inspired applications7,8 because of its excellent chemical stability,9,10 so their magnetic behaviors including their size-dependence have been extensively studied.11–16 In contrast, less studies have been reported on the optical and magneto-optical properties of magnetite (of bulk thin films and nanoparticles) because their spectroscopic interpretation is considerably difficult due to the existence of three kinds of iron ions, that is, FeB2+, FeB3+, and FeA3+, where the suffix A or B denotes tetrahedral or octahedral sites, respectively.17 However, on the basis of the band structure calculations, a general consensus for the optical properties (below ~9 eV) of magnetite is found, where three major transitions are involved;17 (i) inter-valence charge transfer (IVCT) (ii) inter-sublattice charge transfer (ISCT), and (iii) ligand-to-metal charge transfer (LMCT). In IVCT, the transitions can be expressed as FeB2+ → FeB3+ or FeB2+ → FeA3+, so the Fe2+–Fe3+ pair involved causes relaxation of the parity selection rule, resulting in high oscillator strengths. The electron transfer between Fe2+ and Fe3+ can be mediated by O2– ions.18 The ISCT transitions are transitions between Fe3+ ions on different crystallographic sites (A and B sites), that is, FeB3+ →

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FeA3+.19 Both the IVCT and ISCT transitions are classified as interband transitions. LMCT transitions involve those from O 2p to Fe 3d bands and usually have intense oscillator strengths as well. As for the magneto-optical (MO) responses of magnetite, most attention has been so far paid to the Kerr rotation and Kerr ellipticity that are typical for (bulk) thin film measurements,17,20 and the relevant MO effects are interpreted in terms of the above-mentioned transitions, although some debates are present. Note that the Kerr effect is based on the phenomenon that incident polarization of light is rotated when it is reflected from a magnetic material. In the case of magnetite nanoparticles, Faraday rotation or ellipticity measurements should be very useful to describe electronic transitions in the nanoparticles, but very little attention has been paid for such measurements probably due to the fact that transparent samples well-dispersed in an isotropic medium are required for the measurements; then, we only found a few papers on MO effect of relatively large-sized magnetite nanoparticles dispersed in polymer matrices to describe the Fe3O4 electronic band structure.21,22 In this study, we examine the MO responses of transparent, organically-modified magnetite Fe3O4 nanoparticles well-dispersed in isotropic solution by using magnetic circular dichroism (MCD) spectroscopy. MCD (or Faraday ellipticity) can be observed when a sample differentially absorbs left- and right-circularly polarized light (LCPL and RCPL) in an external magnetic field parallel to the light beam, and provides information on the magneto-optical properties as well as deep insight into the assignment of the corresponding electronic absorption spectrum for the nanosystem,23,24 since the MCD signal basically arises from the same transitions as those seen in the electronic absorption spectrum but the selection rules are different.23 We see that the MCD response is much sharper (or structured) than the absorption spectrum and it gives better energy resolution. We here demonstrate very large MCD responses for oleylamine-modified magnetite Fe3O4 nanoparticles under a magnetic field of ±1.6 T. In addition, simultaneous deconvolution analysis of electronic absorption and MCD spectra is successful, and thus gives accurate transitions quantitatively. 3 ACS Paragon Plus Environment


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Comparison of the magneto-optical responses is made between the magnetite nanoparticles and bulk Fe3O4 calculated. We believe this analytical methodology will improve a quality in finding of the new magneto-optical nanomaterials.

EEPERIMENTAL SECTION

Syntheses of Oleylamine-Protected Magnetite Fe3O4 Nanoparticles. All chemicals (iron(III) acetylacetonate (Fe(acac)3; Fe(CH3COCHCOCH3)3), oleylamine (CH3(CH2)7CH=CH(CH2)7CH2NH2), benzyl ether, ethanol, and toluene) were reagent-grade purity and obtained from Aldrich or Wako Pure Chemicals, and used as received. Oleylamine-protected magnetite (Fe3O4) nanoparticles were prepared by a similar method to that reported by Sun and coworkers.25 Typically, to synthesize 7–8 nm magnetite nanoparticles, 3.0 mmol of Fe(acac)3 was dissolved in 15 mL of benzyl ether and 15 mL of oleylamine. The solution was heated at ~110 °C for 1 h under N2 atmosphere, followed by quick heating at a heating rate of ~20 °C/min when the solution temperature reached to 250–270 °C, and aged at this temperature for about 1 h. After the reaction, the dark-brown solution was allowed to cool down to room temperature. The nanoparticles were washed with ethanol, followed by centrifuging. Then the sample was redispersed in toluene for the spectroscopic measurements (We call this magnetite nanoparticle sample ML).25 For the synthesis of smaller magnetite nanoparticles of 3–4 nm (sample MS), we used a half amount of Fe(acac)3 and that of benzyl ether under the same preparation conditions. Note that the black-brown color of the solution verifies that it contents mainly magnetite phase. Instrumentation. UV-vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Magnetic circular dichroism (MCD) spectra were recorded with a JASCO J-820 spectropolarimeter equipped with a JASCO permanent magnet (PM-491LB) of 1.6 T (tesla) with parallel and antiparallel fields. Rectangular 5-mm cuvettes made of quartz were used for the MCD 4 ACS Paragon Plus Environment

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measurements. In the spectral shape analysis, band fitting was performed using multi-peak fitting packages included with Igor Pro ver.5 (WaveMetrics Inc.), a non-linear least square fitting based on the Levenberg–Marquardt algorithm for searching the minimum value of chi-square. FT-IR spectra were recorded with a Horiba FT-720 infrared spectrophotometer by the KBr disk pellet method. A drop of toluene containing magnetite nanoparticle sample was placed on a carbon-coated Cu grid and subjected to field emission scanning transmission electron microscopy (FE-STEM) measurements with a Hitachi S-4800 electron microscope (30 kV).

RESULTS AND DISCUSSION

Characterization of Magnetite Fe3O4 Nanoparticles. The X-ray diffraction (XRD) patterns allow us to determine the crystalline phase of the nanoparticles. Figure 1a displays XRD patterns of the as-prepared magnetite nanoparticles with different sizes (samples MS and ML). The position and relative intensity of all diffraction peaks matched well with the standard inverse spinel phase of Fe3O4 (JCPDS 07-322).25 Then the mean particle diameter (d) of the magnetite nanoparticles was estimated by measuring the (311) diffraction peak width according to the Scherrer equation; d = (0.9λ)/(β cos θ), where λ is the wavelength of the incident X-ray (1.54 Å), θ the diffraction angle, and β the full-width at half-maximum.25 As a result, 3.8 nm for sample MS and 7.5 nm for sample ML were obtained. STEM images (Figure 1b) showed that these samples consist of homogeneous nanoparticles, increasing the size from sample MS to ML. The mean diameters determined from these images are 3.4 (± 0.9) nm and 6.9 (± 1.5) nm for samples MS and ML, respectively. The average sizes calculated using the Scherrer’s formula are in good agreement with those estimated in the STEM study, suggesting that individual Fe3O4 nanoparticle is single crystalline.



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IR spectral analyses elucidate the chemical properties of the nanoparticles. The IR absorption spectrum of magnetite is known to be very simple, having only two broad bands at ~580 and ~420 cm–1, corresponding to vibrations of Fe+3–O and Fe+2–O bonds.25 Figure 2a or 2b illustrates the measured IR spectra of oleylamine-modified magnetite nanoparticle samples MS and ML in the range of 400–3800 or 400–900 cm–1, respectively. Obviously, emergence of absorption bands at ~580 and ~430 cm–1 suggests the formation of pure magnetite. The absence of characteristic peaks of α-Fe2O3 (~540 cm–1) and γ-Fe2O3 (~560 cm–1) also corroborates the substantial formation of pure magnetite phase.26,27 Further inspection of these spectra gives us additional and useful information on the surface nature of the magnetite nanoparticles. For example, both samples exhibit a relatively strong IR absorption band at ~3390 cm–1, which primarily originates from the surface hydration (produced OH groups) layers,26 and the absorbance for sample MS is more intense than that for sample ML. In addition, the band observed at 610–620 cm–1 would be indicative of an iron oxide hydroxide (FeO(OH)) network.28 Hence it is expected that the surface of the magnetite nanoparticles would suffer from some hydration to form hydroxide layers. Such surface hydration can be readily brought about by an atmospheric environment, and this effect would be remarkable as the nanoparticle is small because of its large surface-to-volume ratio.26 Additionally, we could observe absorption bands in 1350–1610 cm–1 that arise from -NH2 bending of oleylamine, and those at 2850 and 2920 cm–1 for its CH2 stretching modes,25 indicating that the surface of Fe3O4 is protected by oleylamine molecules. Spectroscopy. (a) Electronic Absorption and MCD Spectra: Magnetite is an Fe2+–Fe3+ mixed–valence metallic compound and thus appears black in bulk, which is essentially due the IVCT transitions in the near–IR (NIR) region.17 Hence the detection of broad IVCT absorption (assigned to FeB2+ → FeB3+ or FeB2+ → FeA3+) in the NIR region (~800–1400 nm) can demonstrate magnetite formation or coexistence of Fe2+/Fe3+ in the nanoparticles, and a decrease in the absorbance means the oxidation of magnetite surface as suggested by Brus and coworkers.29,30 Note that a different iron


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oxide maghemite (Fe2O3) is a semiconductor with absorption threshold of ~2 eV and thus shows almost no absorption beyond ~ 650 nm.31 Figures 3a and 3b show the UV-vis-NIR absorption spectra of magnetite nanoparticle samples MS and ML in toluene. The broad absorption assigned to the IVCT transition was appeared in the vis-NIR region, indicating coexistence of Fe2+/Fe3+ in the nanoparticles.29 Absorption bump in the vis-NIR region observed for sample MS was obscure compared to that for sample ML. Since the mean size of the magnetite nanoparticle MS was smaller than that of sample ML, some oxidation may be possible at the surface layer of the sample MS; that is, Fe2+-oxidation can increase with decreasing the particle size of magnetite.32 It is expected that oxidation occurs through the outward diffusion of Fe2+ cations and the subsequent reaction with oxygen,30 so it is reasonable to predict that the oxidation time will be dependent on the size of the nanoparticles; that is, the smaller particles have shorter diffusion length of the iron ions, resulting in more rapid oxidation. Hence as a consequence of IR and UV-vis-NIR spectroscopic measurements, the (surfaces of) magnetite nanoparticles are slightly (partly) hydrated and oxidized (particularly for sample MS).28,32 Additionally, absorption in the UV-vis region is still broad and featureless, so to obtain detailed information concerning the electronic structures of the magnetite nanoparticles, we conducted MCD spectroscopy. A difference in absorption of LCPL and RCPL under an applied magnetic field oriented along the direction of propagation of the light results in an MCD signature. Note here that Faraday ellipticity, often symbolized as ηF, is a similar physical parameter related to MCD with an inverted sign. MCD gives valuable indications of different transitions and assists the interpretation of electronic absorption spectra, because MCD features should correspond to electronic transitions in the absorption property.23,24 The measured MCD spectra of magnetite nanoparticle samples MS and ML are shown in Figures 3c and 3d, respectively. Both spectra are quite similar to each other; the spectrum shows positive (negative) features at ≤ ~450 nm (≥ ~450 nm) under the magnetic field of +1.6 T, respectively. The sign of the MCD signal is completely reversed when the field is switched 7 ACS Paragon Plus Environment


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(–1.6 T), confirming that signatures are not from an experimental artifact but originate from real MCD signals. With a close inspection, however, the relative MCD-intensity ratio of the red (≥ ~450 nm) to the blue (≤ ~450 nm) spectral region is slightly different between the two magnetite samples. This would be associated with the degree of oxidation of nanoparticle surfaces as discussed in the interpretation of electronic absorption spectra. It should be noted here that maghemite (Fe2O3) exhibits two positive MCD peaks at ~690 nm (intense) and ~440 nm (moderate) in the spectrum under the positive magnetic field,33 which can be ascribed to the double excitation process of simultaneous crystal field transitions of two antiferromagnetically coupled Fe3+ in neighboring sites with different symmetry.34 In our MCD spectra, neither of them could be detected, strongly suggesting that maghemite phase should be ruled out from the MCD signals. (b) Simultaneous Deconvolution Analysis of Electronic Absorption and MCD Spectra. Simultaneous deconvolution analysis of electronic absorption and MCD spectra of the nanoparticle samples was conducted under the constrained requirement that a single set of Gaussian components be used for their fitting, since MCD spectroscopy can be complementary to UV-vis electronic absorption spectroscopy. We here used the MCD spectrum obtained at H = +1.6 T because of its overall positive MCD feature in a wide energy range.35 The Gaussian fits of MCD as well as electronic absorption spectra for samples MS and ML are shown in Figure 4. Fitting parameters obtained are listed in Table 1. No derivative lineshapes were found in the MCD responses. To obtain satisfactory agreement between the measured and convoluted spectra of broad electronic absorption and MCD, at least eight Gaussian components were necessary for both the magnetite nanoparticle samples (8-band description).36 Interestingly, each deconvoluted spectrum has a similar energy (or wavelength) position with a similar bandwidth between the samples MS and ML, although some fluctuations are also seen. This means that inherent Fe3O4 species are present to dominate the spectra (or transitions), and their size dependence is rather weak in this size regime; in other words, on this basis, ~3.4 nm magnetite nanoparticle does not lie in the quantum confinement regime. 8 ACS Paragon Plus Environment

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Further analysis of the deconvoluted spectral bands is pursued in the context of the assignments of bulk magnetite Fe3O4 transitions reported by Antonov and coworkers with the assistance of theoretical calculations of electronic (band) structures,17 and we could safely make assignments on the decomposed transitions as follows; peak 1 at ~16500 cm–1 (or ~600 nm) originates from the FeB2+ → FeB3+ IVCT transition; peak 2 arises from the FeB2+ → FeA3+ ISCT transitions; peaks 3, 5, 6, and 7 are mostly due to O 2p → Fe 3d LMCT transitions; peak situated at ~26000 cm–1 (peak 4) originates from the FeB3+ → FeA3+ ISCT transitions. On this basis, a characteristic size effect could be found for bands 3, 4, and 5. The smaller the magnetite nanoparticles is, the stronger the relative absorption intensities of 3 and 5 (LMCT transitions) become, whereas that of band 4, the ISCT transition, is reduced. Their magneto-optical responses also follow this trend according to Figure 4. This behavior probably indicates that these transitions are more or less associated with Fe and O atoms in the surface region of the magnetite nanoparticles, whose chemical nature can be influenced by partial hydration and/or oxidation. In any case, two different spectral patterns (that is, electronic absorption and MCD) have made the spectral deconvolution analysis including the transition assignment successful with enhanced spectral resolution. Comparison between the MCD Spectra of Bulk and Nano-sized Magnetite. The propagation of electromagnetic waves can be described by a complex dielectric tensor. If a material exhibits magnetic properties, the off-diagonal components of the dielectric tensor are activated. Then, in an isotropic medium uniaxially magnetized, MCD (or Faraday ellipticity, ηF; note again that the sign is inverted with respect to MCD) and Faraday rotation, θF, are expressed as a function of both (i) the real (εxy’) and imaginary (εxy’’) parts of the off-diagonal component and the (ii) the real (εxx’) and imaginary (εxx’’) parts of the diagonal component of the dielectric tensor under the following four equations; ' '' l0 ( n &xy + ' &xy + "F # $ * 2 - ; % ) n + '2 ,

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' '' l ( & ' $ n 'xy + " F # $ 0 * xy2 - ; % ) n + &2 ,

"'xx = n 2 # $ 2 ;

"''xx = 2n# where λ, n and κ are the wavelength of light, refractive index and absorption coefficient of the material, respectively, and l0 is the sample thickness.23,37 Therefore, data of both the diagonal and off-diagonal components of the dielectric tensor of magnetite allow us to simulate the wavelength-dependent ηF (or MCD) or θF signals. The experimental data of the dielectric tensor diagonal/off-diagonal components of bulk magnetite Fe3O4 were taken from the literature reported by van der Zaag and coworkers,18 and the expected MCD and Faraday rotation spectra were calculated as shown in Figure 5. The MCD profile of sample MS (obtained at +1.6 T) is also shown. In comparison between the calculated (= bulk) and measured (= nanoparticle) MCD spectra, the calculated signal pattern in the wavelength range of < ~550 nm is quite similar to that observed; however, the MCD responses longer than ~550 nm are significantly different between them, that is, the signal signs are opposite between the data of bulk and nano-sized magnetite. In this energy region, the relevant transition is mostly due to FeB2+ → FeB3+ IVCT. Importantly, this behavior is common for both nanoparticle samples MS and ML (finite size effect), suggesting that some slight hydration and/or oxidation of magnetite nanoparticle surfaces, which could be deduced from IR and UV-vis-NIR absorption spectroscopy, would affect the MCD response in the relevant IVCT transition.32 Indeed, several works on magnetite nanoparticles or films found that the resulting materials could show large differences regarding the magnetic properties, which can be attributed to changes in structural disorder,38 or the existence of a magnetically dead layer at particle surface.39 One may say here that, under the experimental setup, the applied external magnetic field can align the magnetite nanoparticles in fluid toluene solution, and thus make the MCD responses distorted. For example, it has been observed that the Faraday rotation spectrum 10 ACS Paragon Plus Environment

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(particularly its peak positions) of magnetite nanoparticles of ~8 nm is found to be dependent on the interparticle spacing of the constituent nanoparticles, which can be due to near-field optical interactions between the particles’ polarizability.21 This may influence the present MCD response; however, we carefully confirmed that, after the measurements, no particles were separated in the solution and thus this effect would give almost no contribution to the magneto-optical responses.

CONCLUSIONS

The focus of this work dealt with magneto-optical properties of chemically modified magnetite (Fe3O4) nanoparticles with different sizes. Magnetic circular dichroism (MCD) spectroscopy was then applied to obtain some new insight into the electronic states of the magnetic nanosystem. The magnetite nanoparticles were synthesized using the high-temperature solution reduction of iron(III) in the presence of oleylamine, and the sizes of the obtained nanoparticles were 3–4 or 7–8 nm. In contrast to broad and featureless absorption spectra for the magnetite nanoparticles, their MCD responses were more structured and thus contained much information. Since MCD essentially corresponds to electronic transitions in the electronic absorption spectrum, simultaneous deconvolution analyses of both the electronic absorption and MCD spectra were successfully conducted, giving accurate transition energies with enhanced spectral resolution. The decomposed transitions could be safely assigned on the basis of the theoretical band-structure calculations of Fe3O4 previously reported by Antonov and coworkers. In comparison between the MCD spectra of nanoparticle and bulk phases for magnetite, a significant difference could be seen in the inter-valence charge transfer (IVCT) energy region (finite size effect), which could be probably due to some slight hydration and/or oxidation of the magnetite nanoparticle surfaces. Magnetic nanosystems are of interest due to several new effects that occur at sizes below the grain showing bulk properties, so we



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believe that such methodology will play a role in an entire new class of magneto-optical materials in the future.

ASSOCIATED CONTENT

Supporting Information Absorption and MCD spectra convoluted using seven Gaussian components (7-band description), and the deconvolution of MCD spectra obtained under the external magnetic field of –1.6 T. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS The authors thank Prof. Hisao Kobayashi (University of Hyogo) for helping in the XRD measurements. The present work was in part supported by Grant-in-Aids for Scientific Research (C: 15K04593 (H. Y.)) from Japan Society for the Promotion of Science (JSPS).

References and Notes

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(4) Soeya, S.; Hayakawa, J.; Takahashi, H.; Ito, K.; Yamamoto, C.; Kida, A.; Asano, H.; Matsui, M. Development of Half-Metallic Ultrathin Fe3O4 Films for Spin-Transport Devices, Appl. Phys. Lett. 2002, 80, 823–825. (5) Salerno, M.; Krenn, J. R.; Lamprecht, B.; Schider, G.; Ditlbacher, H.; Felidj, N.; Leitner, A.; Aussenegg, F. A. Plasmon Polaritons in Metal Nanostructures: The Optoelectronic Route to Nanotechnology, Opto-Electron. Rev. 2002, 10, 217–224. (6) Sharrock, M. P.; Bodnar, R. E. Magnetic Materials for Recording: An Overview with Special Emphasis on Particles, J. Appl. Phys. 1985, 57, 3919–3924. (7) Molday, R. S.; MacKenzie, D. Immunospecific Ferromagnetic Iron-Dextran Reagents for the Labeling and Magnetic Separation of Cells, J. Immunol. Methods 1982, 52, 353–367. (8) Jordan, A.; Scholz, R.; Wust, P.; Schirra, H.; Schiestel, T.; Schmidt, H.; Felix, R. Endocytosis of Dextran and Silan-Coated Magnetite Nanoparticles and the Effect of Intracellular Hyperthermia on Human Mammary Carcinoma Cells in vitro, J. Magnetism. Magn. Mater. 1999, 194, 185–196. (9) Zhao, M.; Beauregard, D. A.; Loizou, L.; Davletov, B.; Brindle, K. M. Non-Invasive Detection of Apoptosis using Magnetic Resonance Imaging and a Targeted Contrast Agent, Nature Med. 2001, 7, 1241–1244. (10) Perez, J. M.; Josephson, L.; O’Loughlin, T.; Högemann, D.; Weissleder, R. Magnetic Relaxation Switches capable of Sensing Molecular Interactions, Nature Biotechnol. 2002, 20, 816–820. (11) Krishnan, K. M.; Pakhomov, A. B.; Bao, Y.; Blomqvist, P.; Chun, Y.; Gonzales, M.; Griffin, K.; Ji, X.; Roberts, B. K. Nanomagnetism and Spin Electronics: Materials, Microstructure and Novel Properties, J. Mater. Sci. 2006, 41, 793–815. (12) Roca, A. G.; Niznansky, D.; Poltierova-Vejpravova, J.; Bittova, B.; Gonzalez-Fernandez, M. A.; Serna, C. J.; Morales, M. P. Magnetite Nanoparticles with No Surface Spin Canting, J. Appl. Phys. 2009, 105, 114309. (13) Bedanta, S.; Kleemann, W. Supermagnetism, J. Phys. D: Appl. Phys. 2009, 42, 013001.



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(14) Billas, I. M. L.; Chatelain, A.; de Heer, W. A. Magnetism from the Atom to the Bulk in Iron, Cobalt, and Nickel clusters, Science 1994, 265, 1682–1684. (15) Demortiere, A.; Panissod, P.; Pichon, B. P.; Pourroy, G.; Guillon, D.; Donnio, B.; Begin-Colin, S. Size-Dependent Properties of Magnetic Iron Oxide Nanocrystals, Nanoscale 2011, 3, 225–232. (16) Brabers, V. A. M. in Handbook of Magnetic Materials, Ed. by Buschow, K. H. J.; Elsevier Science, Amsterdam, 1995, Vol. 8, pp 189. (17) Antonov, V. N.; Harmon, B. N.; Antropov, V. P.; Perlov, A. Y.; Yaresko, A. N. Electronic Structure and Magneto-Optical Kerr Effect of Fe3O4 and Mg2+- or Al3+-substituted Fe3O4, Phys. Rev. B 2001, 64 134410. (18) Fontijn, W. F. J.; van der Zaag, P. J.; Devillers, M. A. C.; Brabers, V. A. M.; Metselaar, R. Optical and Magneto-Optical Polar Kerr Spectra of Fe3O4 and Mg2+- or Al3+-substituted Fe3O4, Phys. Rev. B 1997, 56, 5432–5442. (19) Wittekoek, S.; Popma, T. J. A.; Robertson, J. M.; Bongers, P. F. Magneto-Optic Spectra and the Dielectric Tensor Elements of Bismuth-Substituted Iron Garnets at Photon Energies between 2.2-5.2 eV, Phys. Rev. B 1975, 12, 2777–2788. (20) van der Zaag, P. J.; Fontijn, W. F. J.; Gaspard, P.; Wolf, R. M.; Brabers, V. A. M.; van de Veerdonk, R. J. M.; van der Heijden, P. A. A. A Study of the Magneto-Optical Kerr Spectra of Bulk and Ultrathin Fe3O4, J. Appl. Phys. 1996, 79, 5936–5938. (21) Smith, D. A.; Barnakov, Y. A.; Scott, B. L.; White, S. A.; Stokes, K. L. Magneto-Optical Spectra of Closely Spaced Magnetite Nanoparticles, J. Appl. Phys. 2005, 97, 10M504. (22) Lin, C.-R.; Tseng, Y.-T.; Ovchinnikov, S. G.; Ivantsov, R. D.; Edelman, I. S.; Fedorov, A. S.; Kuzubov, A. A.; Fedorov, D. A.; Starchikov, S. S.; Lyubutin, S. Fe3S4 and Fe3O4 Magnetic Nanocrystals: Magneto-optical and Mössbauer Spectroscopy Study, Mater. Res. Express 2014, 1, 025033. (23) Mason, W. R. A Practical Guide To Magnetic Circular Dichroism Spectroscopy, Wiely-Interscience, John Wiley & Sons, NJ, 2007. 14 ACS Paragon Plus Environment

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(24) Yao, H. On the Electronic Structures of Au25(SR)18 Clusters Studied by Magnetic Circular Dichroism Spectroscopy, J. Phys. Chem. Lett. 2012, 3, 1701–1706. (25) Xu, Z.; Shen, C.; Hou, Y.; Gao, H.; Sun, S. Oleylamine as Both Reducing Agent and Stabilizer in a Facile Synthesis of Magnetite Nanoparticles, Chem. Mater. 2009, 21, 1778–1780. (26) (a) Lu, L.; Li, L.; Wang, X.; Li, G. Understanding of the Finite Size Effects on Lattice Vibrations and Electronic Transitions of Nano α-Fe2O3, J. Phys. Chem. B 2005, 109, 17151–17156. (b) We confirmed that these IR peaks detected at ~3390 cm–1 were not due to that from the moisture sorbed by the KBr pellets. (27) Lu, J.; Jiao, X.; Chen, D.; Li, W. Solvothermal Synthesis and Characterization of Fe3O4 and γ-Fe2O3 Nanoplates, J. Phys. Chem. C 2009, 113, 4012–4017. (28) Bourlinos, A. B.; Simopoulos, A.; Petridis, D. Synthesis of Capped Ultrafine γ-Fe2O3 Particles from Iron(III) Hydroxide Caprylate: A Novel Starting Material for Readily Attainable Organosols, Chem. Mater. 2002, 14, 899–903. (29) Tang, J.; Myers, M.; Bosnick, K. A.; Brus, L. E. Magnetite Fe3O4 Nanocrystals: Spectroscopic Observation of Aqueous Oxidation Kinetics, J. Phys. Chem. B 2003, 107, 7501–7506. (30) Sidhu, P. S.; Gilkes, R. J.; Posner, A. M. Mechanism of the Low Temperature Oxidation of Synthetic Magnetites, J. Inorg. Nuclear Chem. 1977, 39, 1953–1958. (31) Villani, M.; Rimoldi, T.; Calestani, D.; Lazzarini, L.; Chiesi, V.; Casoli, F.; Albertini, F.; Zappettini, A. Composite Multifunctional Nanostructures based on ZnO Tetrapods and Superparamagnetic Fe3O4 Nanoparticles, Nanotechnology 2013, 24, 135601. (32) Daou, T. J.; Pourroy, G.; Bégin-Colin, S.; Greneche, J. M.; Ulhaq-Bouillet, C.; Legaré, P.; Bernhardt, P.; Leuvrey, C.; Rogez, G. Hydrothermal Synthesis of Monodisperse Magnetite Nanoparticles, Chem. Mater. 2006, 18, 4399–4404. (33) Fantechi, E.; Campo, G.; Carta, D.; Corrias, A.; de Julián Fernández, C.; Gatteschi, D.; Innocenti, C.; Pineider, F.; Rugi, F.; Sangregorio, C. Exploring the Effect of Co Doping in Fine Maghemite Nanoparticles, J. Phys. Chem. C 2012, 116, 8261–8270. 15 ACS Paragon Plus Environment


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(34) He, Y. P.; Miao, Y. M.; Li, C. R.; Wang, S. Q.; Cao, L.; Xie, S. S.; Yang, G. Z.; Zou, B. S.; Burda, C. Size and Structure Effect on Optical Transitions of Iron Oxide Nanocrystals, Phys. Rev. B 2005, 71, 125411. (35) The reliability of the present simultaneous deconvolution analysis was confirmed by the fact that MCD spectra of samples MS and ML obtained at –1.6 T could be excellently reconstructed by using the same Gaussian parameters (peak energy and bandwidth) with those listed in Table 1. See the Supporting Information for more detail. (36) We also made an attempt to deconvolute the spectra (both absorption and MCD) using seven Gaussian components (7-band description). A typical result is shown in the Supporting Information. For the excellent (satisfactory) agreement between the measure and calculated spectra, at least one additional Gaussian component is necessary. (37) Gehring, G. A.; Behan, A. J.; Blythe, H. J.; Fox, A. M.; Ibrahim, R. M.; Mokhtari, A.; Neal, J. R. Magneto-Optical Studies of Magnetic Oxide Semiconductors, pp 3–35, in Spintronic Materials and Technology, Ed by Xu, Y. B.; Thompson, S. M. Taylor & Francis, NW, 2007. (38) Kendelewicz, T.; Liu, P.; Doyle, C. S.; Brown, Jr. G. E.; Nelson, E. J.; Chambers, S. A. Reaction of Water with the (100) and (111) Surfaces of Fe3O4, Surf. Sci. 2000, 453, 32–46. (39) Sato, T.; Iijima, T.; Seki, M.; Inagaki, N. Magnetic Properties of Ultrafine Ferrite Particles, J. Magnetism Magn. Mater. 1987, 65, 252–256.


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Table 1. Results of the Gaussian fit analysis of magnetite nanoparticle samples MS and ML.



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Figure Captions

Figure 1. (a) XRD patterns of oleylamine-modified Fe3O4 magnetite nanoparticle samples MS and ML. The standard inverse spinel phase of Fe3O4 diffraction peaks are also shown. (b) Typical STEM images of Fe3O4 nanoparticle samples MS and ML.

Figure 2. IR absorption spectra of the Fe3O4 nanoparticle samples MS and ML in the range of (a) 400–3800 cm–1 and (b) 400–900 cm–1.

Figure 3. (a) and (b) UV-vis-NIR absorption spectra of Fe3O4 nanoparticle samples MS and ML in toluene, respectively. (c) and (d) MCD spectra of Fe3O4 nanoparticle samples MS and ML in toluene, respectively. Applied magnetic fields are +1.6 T (black curve) and –1.6 T (red curve).

Figure 4. Gaussian band fits of the electronic absorption ((a) and (b)) and MCD spectra ((c) and (d)) of the Fe3O4 nanoparticle samples MS and ML, respectively. Black dots indicate the experimental absorption or MCD spectra, and red curves indicate the sum of the deconvoluted spectra. The deconvoluted spectra are shown in ocher curves. To obtain a numerically adequate description of these spectra reconstructed, a minimum number of eight Gaussian components was required (8-band description).

Figure 5. Calculations of the spectral Faraday rotation and MCD for bulk Fe3O4. MCD profile of nanoparticle sample MS (obtained at +1.6 T) is also shown. The literature values for the dielectric tensor of Fe3O4 reported by van der Zaag and coworkers was used.18


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Table of Contents Graphic


Finite Size Effect on Magneto-Optical Responses of Chemically

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