Magnetic Properties of Mono- and Multilayer Assemblies of Iron Oxide

Article pubs.acs.org/JPCC

Magnetic Properties of Mono- and Multilayer Assemblies of Iron Oxide Nanoparticles Promoted by SAMs Benoit P. Pichon,*,† Cedric Leuvey,† Dris Ihawakrim,† Pierre Bernard,‡ Guy Schmerber,† and Sylvie Begin-Colin† †

Institut de Physique et de Chimie des Matériaux de Strasbourg, UMR 7504, CNRS − UdS - ECPM, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France ‡ Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé, UMR 7515, CNRS − UdS - ECPM, 25 rue Becquerel, F-67087 Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: Owing to the wide scope of applications of magnetic nanoparticle assembling, the aim of this study is to evaluate the influence of nanoparticle aggregates on the magnetic properties of 2D assemblies. Magnetic iron oxide nanoparticles (NPs) have been synthesized by the coprecipitation (NPcop) and thermal decomposition (NPdec@ OA) methods, and were assembled on self-assembled monolayers of organic molecules decorated by a phosphonic acid terminal group at their surface (SAM-PO3H2). The nanostructure and magnetic properties of assemblies depend directly on the aggregation of NP suspensions. NPcop result in an unstable suspension and were assembled into a nonhomogeneous monolayer of aggregates. The post-functionalization of NPcop with oleic acid after synthesis (NPcop@OA) favors a better stability of the suspension and enhances the nanostructure of the assembly, although smaller NP aggregates remain. In contrast, NPdec@OA which are functionalized in situ by oleic acid during the synthesis step were assembled as individual nanomagnets and result in a dense monolayer. Multilayer assemblies were also prepared from NPcop@OA and NPdec@OA by performing the alternative deposition of these NPs with (1,4phenylene)bisphosphonic acid. The nanostructure of assemblies has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The magnetic properties of monolayer and multilayer assemblies have been studied by using a SQUID magnetometer. While assemblies of individual NPs enhance dipolar interactions in-plane as a result of shape anisotropy, assemblies of NP aggregates favor stronger dipolar interactions with random orientation. The magnetic properties of monolayer and multilayer assemblies have also been compared. The dimensionality (2D vs 3D) has a strong effect on the dipolar interactions when individual NPs are considered in contrast to aggregated nanoparticles.

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INTRODUCTION Nanoparticle (NP) assemblies represent very promising systems because of their high ability to modulate magnetic properties. Indeed, the nanostructure is a key parameter to control dipolar interactions between magnetic NPs. Large interparticle distances result in individual nanomagnets which are suitable for high density recording media.1 In contrast, tight packed assemblies of NPs lead to collective properties which are of interest for efficient sensors.2 Such an assembling approach infers to the general bottom-up approach which considers NPs as nanobuilding blocks to be assembled one by one on surfaces. Therefore, individual NPs with a narrow size distribution of well-defined morphologies are required to control efficiently the nanostructure of the assemblies and the physical properties. Although NPs are of high interest for the development of new nanodevices, their magnetic properties are directly dependent on the surface to volume ratio. It is well-known that the size reduction of NPs results in the modulation of the magnetic properties which may be undesirable. For instance, the decrease of the size is correlated to the magnetic anisotropy © 2014 American Chemical Society

energy and results in the decrease of the blocking temperature at which happens the superparamagnetic−ferrimagnetic crossover.3 On the other hand, dipolar interactions between NPs result in the increase of the blocking temperature in comparison to individual NPs.4,5 Therefore, NP aggregates with fine control of the nanostructure which favor strong dipolar interactions may be of first interest.6 The great issue consists of controlling the size and the geometry of aggregates, i.e., the number of NPs and their spatial arrangement in the assembly. The increase of the dimensionality of NP assemblies (1D chain, 2D monolayer, 3D multilayer) has been shown to enhance dipolar interactions and thus the blocking temperature.7,8 The stability in suspension is also an important parameter to address the nanostructure of the film, since large assemblies are the heaviest and fall down from the solution. In this aim, synthesis methods play a very important role. Regarding iron oxide nanoparticles with the spinel structure, Received: December 12, 2013 Revised: January 17, 2014 Published: January 23, 2014 3828

dx.doi.org/10.1021/jp412174k | J. Phys. Chem. C 2014, 118, 3828−3837

The Journal of Physical Chemistry C

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phorus oxychloride (POCl3), sodium oleate, octylether, and oleic acid were purchased from Aldrich. Synthesis and Preparation. Synthesis of NPcop Iron Oxide Nanoparticles by the Coprecipitation Method. Iron oxide nanoparticles NPcop were synthesized by the coprecipitation method as we reported previously.31 A 10 mL portion of 1 M FeCl3 acidic solution and 2.5 mL of a 2 M FeCl2 acidic solution were mixed in a flask after being degassed with argon for 30 min. After heating the mixture to 70 °C under argon, 21 mL of a 25% aqueous solution of N(CH3)4OH were added by using a peristaltic pump. A black precipitate was obtained after stirring for 20 min. The product was further washed with water (pH 10) at room temperature by applying a permanent magnet. The supernatant was discarded three times by decantation to remove excess ions and tetramethylammmonium salt in the suspension. Finally, the precipitated powder was dried using a freeze-dryer to afford NPcop as a black powder. Suspension of NPcop. A 20 mg portion of NPcop were dispersed in 50 mL of water by ultrasonication for 2 min. The pH of the solution was then decreased to 4 by addition of HCl and ultrasonicaed for 30 min to increase the surface charge and improve the stability by electrostatic repulsion. After one night of stabilization, the supernatant was discarded from aggregates which have precipitated at the bottom of the flask. A brown solution was obtained. Suspension of NPcop@OA with Oleic Acid. A 50 mg portion of NPcop was dispersed in 50 mL of ethanol, and 2 mg of oleic acid was added. The mixture was ultrasonicated for 10 min by using an ultrasonic bath to get a brown suspension. After 1 h of stabilization, some aggregates precipitated at the bottom of the flask and the supernatant was discarded. Ethanol was evaporated to get a sticky brown powder which was dispersed in 50 mL of THF under ultrasound for a few seconds. A brown suspension of NPcop@OA was obtained. Synthesis of NPdec@OA Iron Oxide Nanoparticles by the Thermal Decomposition Method. Iron oxide nanoparticles were produced by the thermal decomposition method as we reported previously.16,32 It consists of the decomposition of an iron(III)/oleate complex (Fe(oleate)3) (2 g, 2.2 × 10−3 mol) with oleic acid (1.24 g, 3.3 × 10−3 mol) in 20 mL. The temperature was carefully raised to reflux with a heating rate of 5 °C·min−1 without stirring for 120 min under air. After cooling down to room temperature, the black suspension of nanocrystals was dissolved in hexane and washed three times by addition of ethanol and centrifugation (8000 rpm, 10 min). A size selection procedure was applied to narrow the size distribution of the nanoparticles.32 Finally, the nanoparticles were dispersed in THF to prepare a highly stable suspension of coated nanoparticles with a concentration of 5 mg mL−1. Preparation of SAM-PO3H2. An ion sputtered gold substrate (50 nm) was cleaned under O2 plasma and then soaked in a 10 mmol ethanolic solution of 11-mercaptoundecanol (MUD) at room temperature for 24 h. The substrates were then washed with absolute ethanol and dried under a stream of N2. Terminal hydroxide groups were phosphorylated to generate phosphonic acid terminal groups at the SAM surface.34 The substrate was immersed in 20 mL of a solution of 2.0 M phosphorus oxychloride (POCl3) and 2,6,6-collidine in dry acetonitrile under a N2 atmosphere for 6 h at room temperature. The substrates were rinsed extensively with THF and dried under a stream of N2 to give SAM-PO3H2. Assembling of Nanoparticles on SAMs. SAM-PO3H2 were immersed directly in each suspension of nanoparticles for 2 h at

Massart et al. reported on the very well-known method which consists of the precipitation of iron salt in a basic medium.9 This method is easy to carry out and affords control of the size and morphology of nanoparticles.10,11 Nevertheless, NPs may form large aggregates which are not stable in solution and are difficult to reduce in size. Although several studies reported on the functionalization during or after the synthesis, NPs remain strongly aggregated in solution.12−15 More recently, the thermal decomposition technique opened new perspectives with regard to the assembling of iron oxide NPs.16,17 The thermal decomposition of an iron complex in an organic medium in the presence of stabilizing agents resulted in the fine control of the morphology and shape, providing the in situ coating of NPs with surfactants and a high stability in organic solvents. Besides their stabilizing properties, surfactants also play a significant role in the assembling reaction. For instance, iron oxide NPs are usually coated with oleic acid which involves weak interactions and reversible coating from which it has been taken advantage upon the assembling processes. This gave rise to the assembling of iron oxide based NPs with fine control on the nanostructure by several methods such as the Langmuir− Blodgett18−21 and layer-by-layer22 techniques. Dipolar interactions were modulated as a function of the interparticle distance within and between NP layers. Recently, SAMs became an emerging method to build nanostructured monolayer film of iron oxide NPs.23−25 One of the advantages is the specific assembly of NPs on patterned areas depending on the designed functional groups at both SAM and NP surfaces.26,27 Dipolar interactions have been reported to increase with the number of NPs involved in NP assemblies.28 Multilayer assemblies based on SAMs have also been constructed by the alternative deposition of gold NPs and dithiol molecules.29,30 Although it has been poorly reported, this method presents the great advantage to be suitable for various types of NPs when associated to the appropriate molecular cross-linker. Owing to the wide scope of applications of magnetic nanoparticle assembling, the aim of this study is to evaluate the influence of nanoparticle aggregates on the magnetic properties of 2D assemblies. Here we report a very simple and reliable stepwise method to assemble magnetic iron oxide nanoparticles as individual nanomagnets or as aggregates. The assembling is promoted by a self-assembled monolayer of organic molecules decorated with phosphonic acid groups which have strong and irreversible anchoring ability with respect to the NP surface. Multilayer assemblies are also prepared by alternative deposition with bisphosphonic acid molecules used as crosslinkers. In the present work, the difficulties involved in the structuration of NP assemblies are reported to be directly dependent on the stability of NPs in solution, i.e., the fact that they are agglomerated prior to their assembly on surfaces. Finally, the relationship between the structure and the magnetic properties of mono- and multilayer nanoparticle assemblies is investigated in detail as a function of the aggregation stage of nanoparticles.

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EXPERIMENTAL SECTION Materials. All the chemical reagents were of analytical grade. Ferrous chloride tetrahydrate (FeCl2·4H2O), ferric chloride hexahydrate (FeCl3·6H2O), tetramethylammonium hydroxide (N(CH3)4OH, 25%), iron oleate, oleic acid, hydrochloric acid, tetrahydrofurane (THF), acetonitrile, ethanol, 11-mercaptoundecanol (MUD), 2,4,6-collidine, phos3829



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Figure 1. TEM micrographs of (a) NPcop, (b) NPcop@OA, and (c) NPdec@OA.

Figure 2. Granulometry measurements (volume distribution) performed by dynamic light scattering of (a) NPcop, (b) NPcop@OA, and (c) NPdec@ OA suspensions.

50 °C for NPcop and NPcop@OA and for 5 min at room temperature for NPdec@OA. The substrates were then placed in an ultrasonic bath in the same solvent as the one used for the NP suspension to remove any physisorbed nanoparticles. Finally, the substrates were dried under a stream of N2. Assembling of Nanoparticle Multilayers. Multilayer assemblies have been performed by the alternative dipping of SAM-PO3H2 in a tetrahydrofurane solution of NPcop@OA or NPdec@OA and in a water/THF (1:1 vol) solution of (1,4phenylene)bisphosphonic acid (5 mg·mL−1). SAM-PO3H2 was immersed for 10 min and washed extensively with the solvent used for deposition. Characterization Techniques for Nanoparticles and SAMs. Nanoparticles were analyzed with a TOPCON model 002B transmission electron microscope (TEM), operating at 200 kV. The size distribution of NPs was calculated from the size measurements of more than 300 nanoparticles. Granulometry measurements were performed on the nanoparticle suspensions by dynamic light scattering (DLS) using a nanosize MALVERN (nano ZS) apparatus. NP assemblies on SAMs were studied with a JEOL 6700 scanning electronic microscope equipped with a field emission gun (SEM-FEG) operating at an accelerating voltage of 3 kV. Atomic force microscopy (AFM) was also performed with a Digital Instrument 3100 microscope coupled to a Nanoscope IIIa recorder. Measurements were done in the tapping mode onto substrates before and after exposure to the suspension of nanoparticles. SAMs were characterized by polarization modulation infrared reflection− absorption spectroscopy (PMIRRAS) using a IF66S Bruker spectrometer with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. Typical data was derived from 2000 scans at a resolution of 1 cm−1, 85° beam angle of incidence, and 74 kHz modulation frequency. X-ray photoelectron spectroscopy (XPS) was performed with a ThermoVGScientific photoelectron spectrometer equipped with a twin anode, providing unchromatized Al KR radiation (1486.6 eV). The spectrometer, which was equipped with a multichannel detector, operated in the constant resolution mode with a

pass energy of 20 eV. The total resolution of the system was estimated to be 1 eV. The energy scale is referred to the Au 4f7/2 line at a binding energy of 84 eV. The spectra were fitted by using a linear background and the Gaussian function. UV− visible spectroscopy was performed by using an UV−visible− NIR Perkin-Elmer Lambda 950 spectrophotometer in specular reflection mode. Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design model MPMS-XL). Magnetizations versus applied magnetic field curves were recorded at 290 and 5 K. Zero field cooling (ZFC) and field cooling (FC) curves were recorded between 5 and 300 K under exposure of an applied magnetic field of 50 G. ZFC curves were first recorded from 5 to 300 K after the temperature had been decreased without applying any magnetic field. The FC curves were recorded between 5 and 300 K after the temperature was returned to 5 K under a magnetic field of 50 G. The substrates were placed in a parallel direction to that of the applied magnetic field. ZFC and FC curves were normalized on the basis of the magnetization value corresponding to the maximum of the ZFC curves of each sample.

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RESULTS AND DISCUSSION Iron Oxide Nanoparticles. Iron oxide NPs have been synthesized by two different methods. NPcop have been synthesized by the coprecipitation of FeCl3 and FeCl2 in the presence of trimethylammonium hydroxyde (TMAOH) in aqueous solution at 80 °C for 2 h.31 After washing by centrifugation, TEM micrographs show that NPcop have an average size of 12 ± 2 nm with a spherical shape (Figure 1a). An aqueous suspension of NPcop was prepared by decreasing the pH to 4 to display a positive charge at the NP surface (zeta potential of 30 mV) according to the isoelectronic point (IEP) at pH 6.8.14,34 A positive charge surface is expected to favor strong electrostatic interactions with the phosphonic acid groups at the SAM surface which are partially deprotonated 3830



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Scheme 1. Schematic Illustration of the Preparation of SAMs, NP Monolayer, and NP Multilayer Assemblies

which exhibits specific bands at the same positions as those of NPcop@OA (see the Supporting Information). Finally, the structures of NPcop and NPdec@OA have been studied by high resolution TEM which show lattice fringes for both samples and agree with single crystals (see the Supporting Information). Electronic diffraction patterns are very similar and exhibit rings which are indexed as hkl planes of the iron oxide spinel phase according to the JCPDS card of magnetite (n° 19-629) and maghemite (n° 39-1346) (see the Supporting Information). Self-Assembled Monolayer. Acid phosphonic terminated self-assembled monolayers (SAM-PO3H2) have been prepared in a two-step procedure on the basis of a previous report (Scheme 1).33 First, 11-mercaptoundecanol (MUDO) molecules were adsorbed on gold substrates at room temperature for 20 h. Second, hydroxyl groups at the SAM surface were turned into phosphonic acid groups after reaction with phosphorus oxychloride (POCl3) and 2,4,6-collidine in dry acetonitrile under a nitrogen atmosphere. The surface morphology of the SAM-PO3H2 was controlled by AFM which showed the thiol molecule covering homogenously the gold substrate and lowering the root-mean-square (RMS) roughness in the bare gold image from 2 to 0.7 nm (see the Supporting Information). The chemical structure of SAM-PO3H2 was studied by PMIRRAS which showed bands (1300−1000 cm−1) attributed to vibration modes of the phosphonic acid groups (see the Supporting Information). The spectrum also displays bands which correspond to the vibration modes of alkylene chains (3000−2800 cm−1). The formation of acid phosphonic groups at the SAM surface was further demonstrated by X-ray photoelectron spectroscopy (XPS) (see the Supporting Information). A broad peak at a binding energy of 133 eV corresponds to the P2p signal. Because of the low signal-tonoise ratio, P2p3/2 and P2p1/2 signals could not be observed. Nevertheless, it demonstrates the formation of phosphonic acid groups at the SAM surface.35,36 Moreover, the O1s signal consists of two contributions centered at 532.2 and 534.3 eV which correspond to PO and POH bonds, respectively.37,38 In addition, the weak S2p3/2 signal at 162.8 eV corresponds to the thiol groups which are bound to the gold substrate.39 Assembling of Nanoparticles. Nanoparticles were assembled by dipping the SAM-PO3H2 in the corresponding nanoparticle suspension. Dipping was proceeded for 2 h at 50 °C in aqueous NPcop solution or in a THF solution of NPcop@ OA and only 5 min in a THF suspension of NPdec@OA. SAMs

(pKa1 at 3.1). Moreover, such a high charge surface is expected to favor stable and individual NPs in aqueous solution. Nevertheless, the solution is clearly unstable and the TEM micrograph shows that most of NPcop consists in aggregates (Figure 1a). It was confirmed by granulometry measurements which show a large distribution of hydrodynamic diameters between 100 and 400 nm (Figure 2a). With the aim to improve the stability of the solution, NPcop were functionalized through direct grafting of oleic acid in ethanol upon ultrasonication and were named NPcop@OA. The functionalization of NPcop was demonstrated by FTIR spectroscopy (see the Supporting Information). In the NPcop@OA spectrum, the νCO band (1708 cm−1) corresponding to free oleic acid molecules was shifted to lower wavelengths of νCOO (1620 cm−1) which corresponds to the interaction of the carboxylic acid group at the NP surface. The presence of oleic acid is also demonstrated by the νCH bands (2800−3000 cm−1) in contrast to the spectrum of NPcop (see the Supporting Information) which only shows the FeO vibration modes (about 600 cm−1) and bands corresponding to hydroxide groups or water molecules adsorbed at the NP surface (νOH at about 3400 cm−1 and δH2O at 1632 cm−1). The functionalization enhanced the solution stability, and most NPs tend to be individual (Figure 1b). In addition, granulometry measurements (Figure 2b) show that the hydrodynamic diameter is mainly centered at 33 nm which corresponds to very small aggregates of NPcop@OA even though a broad distribution remains between 50 and 300 nm. NPcop@OA were compared to NPdec@OA which have been synthesized by the thermal decomposition of iron oleate in octylether which was used as a high boiling temperature solvent (bp 288 °C). Figure 1c exhibits NPs with a homogeneous spherical shape with some small facets and a narrow size distribution centered to 12.5 ± 1.5 nm. The presence of oleic acid during the synthesis reaction results in the in situ functionalization of NPdec@OA. Therefore, no aggregates were observed in the TEM micrograph and NPdec@OA tend to self-assemble in a hexagonal structure on the TEM grid. NPdec@OA are highly stable in organic solvents, as was confirmed by granulometry measurements (Figure 2c). The hydrodynamic diameter is centered at 13.5 nm, which agrees with NPdec coated with oleic acid molecules. In contrast to NPcop and NPcop@OA, no broad distribution of the hydrodynamic diameter at higher values was observed, which demonstrates that NPs are single in solution. The capping of NPs by oleic acid was demonstrated by the FTIR spectrum 3831



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Figure 3. SEM images of SAM-PO3H2 after immersion in NP suspensions: (a) NPcop in water (pH 4) for 2H at 50 °C; (b) NPcop@OA in THF for 2H at 50 °C; (c) NPdec@OA for 5 min at room temperature in THF. The inset is the enlargement of part c.

Figure 4. AFM height images (left) and the cross section profiles (right) corresponding to the gray line measured on assemblies of (a) NPcop, (b) NPcop@OA, and (c) NPdec@OA.

correlated to the granulometry measurements of both NPcop and NPcop@OA suspensions. In contrast, NPdec@OA were assembled very quickly (within 5 min) in a dense monolayer which covers homogeneously the SAM surface (Figure 3c). The assembly is featured by tightly packed NPs which result in the maximum density. AFM imaging brought more insights into the surface morphology of SAM-PO3H2 and the distribution of NPs bound to the surface. Figure 4 present tapping mode AFM height images and the corresponding profile cross sections. All

were subsequently ultrasonicated for 10 s to remove physisorbed NPs. SEM imaging of SAMs after dipping gave information on the spatial distribution of bound NPs. Figure 3a showed that NPcop were inhomogeneously assembled on the SAM surface with a very low density. In addition, aggregates of several NPs which are up to almost 100 nm in size were observed. In contrast, NPcop@OA which were assembled under the same conditions led to a higher density in NPs and a more homogeneous film even though some areas are not covered (Figure 3b). Aggregates remain but they are smaller, which is 3832



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Figure 5. SEM images of SAM-PO3H2 after deposition of five layers of (a) NPcop, (b) NPcop@OA, and (c) NPdec@OA.

Figure 6. AFM analysis after deposition of five layers of NPcop@OA (a, b) and NPdec@OA (c, d). Height images (a, c). Cross section profiles corresponding to lines in height images (b, d). In the case of NPcop@OA, a part of the substrate has been scratched to measure the thickness of the multilayer assembly.

explained by the concentration of NPcop@OA (0.3 mg·mL−1), which is 10 times lower than that of NPdec@OA (3.2 mg· mL−1). The aggregation of NPs is also a very important parameter, since it influences the motion of NPs in solution and thus the probability that NPs hit the SAM surface and proceed to assembling. The bigger the aggregates are, the slower they move in the solution. The coating of the NP surface by oleic acid also plays a very important role, since the assembling on SAMs proceeds through a ligand exchange reaction as we reported previously.32 Although oleic acid has the ability to desorb from the NPcop surface and to favor stronger interactions with the acid phosphonic terminal group,34 it reduced the NP surface free of molecules which may interact with SAMs in comparison to uncoated NPcop. Therefore, these parameters contribute to slowing down the kinetics of the assembling of NPcop and NPcop@OA and result in a lower density than NPdec@OA. Multilayer Assemblies. NPs were assembled as multilayers by performing the alternative dipping of the SAM-PO3H2 in NP solution and in (1,4-phenylene)bisphosphonic acid solution. In

height images show corrugation and height fluctuation which are characteristic of NPs assembled on the SAM surface. Figure 4a displays the highest rms roughness up to 9 nm, and the cross section profile reveals the non-homogeneous coverage with large height variations which reflect the assembling of large aggregates. Figure 4b shows an increased NP density and lower roughness of 7.5 nm. Nevertheless, uncovered areas still remain and the cross section profile depicts an average height of 30 nm which is correlated to aggregates, as deduced from the hydrodynamic diameter of NPcop@OA. While aggregates are smaller than NPcop, they still remain upon the assembling reaction. In the case of NPdec@OA, the SAM is fully covered by NPs with almost no aggregates and a very low roughness of 4.6 nm (Figure 4c). The cross section profile shows an average thickness of about 8 nm which is correlated to the diameter of spherical NPs measured from TEM micrographs. Therefore, NPdec@OA were assembled as individual nano-objects at the SAM surface to form a dense and homogeneous monolayer. These results show that the kinetics of the assembling reaction is highly different for each type of NPs. It can be 3833



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Figure 7. UV−visible spectra of SAM-PO3H2 (dashed) and multilayer assemblies after the deposition of (a) NPcop and (b) NPdec@OA. The vertical dashed lines represent the wavelength at 280 nm. (c) Reflectance measured at 280 nm plotted as a function of the number of NPcop@OA layers (closed symbols) and NPdec@OA layers (open symbols).

AFM cross section profiles measured after deposition of five layers of NPcop and NPdec@OA. Magnetic Measurements. As the magnetic properties are strongly dependent on the nanostructure of the assembly,28 NP aggregates are expected to dramatically influence the magnetic properties. Therefore, monolayer and multilayer assemblies of NPcop@OA and NPdec@OA have been studied by using a SQUID magnetometer.

this study, the assembling is promoted by strong interactions between the phosphonic acid groups and the iron oxide surface (Scheme 1).34 Therefore, bisphosphonic acid was used as a molecular cross-linker to build highly stable multilayered nanostructures. The phenylene group was selected instead of alkylene chains to maintain the linear conformation of the molecule and to avoid the interaction of both phosphonic acid groups at the surface of the same NP. SEM images show the film structure after deposition of five layers of NPs (Figure 5). The multilayer film based on NPcop consists of large aggregates with remaining uncovered areas. In the case of NPcop@OA, much more NPs have been assembled on the SAM-PO3H2 and the surface is mainly covered, although the formation of the multilayer structure is not homogeneous and seems to proceed through the assembling of aggregates, which dramatically increases the film roughness. In contrast, NPdec@OA led to a much more homogeneous film and lower roughness. AFM imaging of five-layer multilayer assemblies based on NPcop@OA and NPdec@OA shows the characteristic morphologies of the top NP layers (Figure 6). The latter exhibits a lower rms roughness than the former. However, it dramatically increases to 20.1 and 10.3 nm, in comparison to the first NP layer (7.5 and 4.6 nm, respectively). Such an increase is assumed to originate from the roughness of the first NP layer. Hence, roughness gradually increases after each NP layer has been deposited. This is highlighted by the faster increase of the roughness for NPcop@OA aggregates. Figure 6a shows that a part of the surface has been scratched to measure the thickness of the multilayer assembly. Interestingly, it corresponds roughly to 5 times the NPcop@OA diameter, while it should be higher according to the fact that aggregates are considered. It may be explained by the low coverage of the substrate by the first layer of NP aggregates which coexists with free areas. The construction of the film structure after deposition of NPcop and NPdec@OA layers was also studied by UV−visible spectroscopy (Figure 7). The spectrum of SAM-PO3H2 on bare gold substrate is plotted as dashed lines and displays the highest reflectance. The deposition of a first layer of NPcop results in a lower decrease of the reflectance than for for NPdec which agrees with the lower coverage of the substrates for the former, as shown in SEM micrographs (Figure 5). Moreover, each NP layer deposition results in the systematic and linear decrease of the reflectance which indicates that the same amount of NPs is assembled at each step. Nevertheless, the reflectance decreases slower for NPcop than for NPdec@OA, which is attributed to the lower kinetics of the assembling process for aggregates. Taking into account the different sizes of NPcop aggregates (about 30 nm) and NPdec@OA (12 nm), this demonstrates that the assembling of the latter is much faster. These results agree with

Table 1. Coercive Field (HC), Remanent to Saturation Magnetization Ratio (MR/MS), and Maximum Temperature of ZFC Curve (Tmax) of Mono- and Multilayers of NPdec@ OA and NPcop@OA NPcop 1 layer NPdec 1 layer NPcop 5 layers NPdec 5 layers

HC (Oe)

MR/MS

TB (K)

550 170 400 380

0.46 0.23 0.43 0.41

184 95 192 108

Monolayer assemblies of NPcop@OA and NPdec@OA have been studied first (Figure 8b). Magnetization (M) curves were recorded as function of a magnetic field (H) at 300 and 5 K (Figure 8a,b). Both M(H) curves recorded at 300 K are typical of superparamagnetic nanoparticles at room temperature, since no hysteresis loop is observed (see the insets in Figure 8). In contrast, at 5 K, both types of NPs are ferrimagnetic and are featured by a coercive field (HC) which is larger for NPcop@OA (550 Oe) than for NPdec@OA (170 Oe). Small coercive fields usually result from strong demagnetizing dipolar interactions in NP assemblies. Therefore, the NPdec@OA monolayer is featured by a lower magnetization reversal energy barrier.28 The magnetic field necessary to align all NPs in the same direction is weaker than that for the NPcop@OA monolayer. Such a different behavior is ascribed to the nanostructure. NPdec@OA which were assembled as single nanomagnets on the SAM surface result in a smooth and well-defined monolayer. Such a nanostructure favors shape anisotropy, i.e., the dipolar interactions in the plane of the substrate as we reported previously for NP assemblies prepared by different assembling methods.2,40,41 In contrast, NPcop@OA consists of aggregates which can be assimilated to 3D random assemblies in which NPs are interacting in many directions.40 Once assembled on a SAM, aggregates disfavor the anisotropy of dipolar interactions along the surface plan. Information on the strength of dipolar interactions between NPs as a function of the dimensionality of the assembly is also given by the remanence-to-saturation (MR/MS) ratios. At 5 K, the assembly of NPcop@OA displays a value (0.46) which is almost twice that 3834



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Figure 8. Magnetization curves versus applied magnetic field at 300 and 5 K for monolayer assemblies of (a) NPcop@OA and (b) NPdec@OA. Insets are enlargements for low magnetic fields. (c) Magnetization curves versus temperature between 300 and 5 K for monolayer assemblies of NPcop@OA and NPdec@OA.

Figure 9. Magnetization curves versus applied magnetic field at 300 and 5 K for multilayer assemblies of (a) NPcop@OA and (b) NPdec@OA. Insets are enlargements for low magnetic fields. (c) Magnetization curves versus temperature between 300 and 5 K for multilayer assemblies of NPcop@OA and NPdec@OA.

different strengths. The broad size distribution of aggregates shown by TEM and granulometry measurements also directly influences dipolar interactions. Moreover, we have already observed such a behavior in sizable 2D domains of NPs which displayed stronger dipolar interactions as long as the NP number increased in the assembly.28 Finally, the stronger dipolar interactions in the NPcop assembly may also come from the fact that NPcop are already aggregated before their functionalization by oleic acid and are closer than NPdec. The magnetic properties of multilayer assemblies which consist of five layers of NPcop@OA and NPdec@OA have also been investigated (Figure 9). M(H) curves for both NPs at 5 and 300 K do not affect the ferrimagnetic and superparamagnetic behaviors, respectively. The NPcop@OA multilayer displays similar HC (400 Oe) and MR/MS (0.43) values to the corresponding monolayer, which agrees with the fact that NPs remain as aggregates upon layer deposition. In contrast, the NPdec@OA multilayer shows some dramatic increase of HC (380 Oe) and M R /M S (0.41) in comparison to the corresponding monolayer and are very close to the one of NPcop@OA mono- and multilayer assemblies. These results agree with a similar behavior of NPcop@OA aggregates. In multilayers, NPdec@OA interact in three dimensions, i.e., within and between each layer. Tmax values confirm these observations, since they increase with respect to monolayers and are 192 K for NPcop@OA and 108 K for NPdec@OA.

(0.23) of NPdec@OA. According to the literature, a value of 0.5 corresponds to nanoparticles with easy magnetization axis randomly distributed. The decrease of the MR/MS ratio is related to the coupling of magnetic moments in a specific direction.41,42 According to these results, the assembling of NPdec@OA in a dense monolayer favors strong in-plane dipolar interactions. In contrast, NPcop@OA aggregates result in dipolar interactions in random directions which disfavor the shape anisotropy. The magnetization was also recorded as a function of the temperature (Figure 8c). The saturation of field cooling (FC) curves at low temperature confirms the presence of dipolar interactions between NPs in both NPdecOA and NPcop@OA assemblies. However, zero field cooling (ZFC) curves exhibit a maximum magnetization at different temperatures. It is wellknown that the temperature (Tmax) at which the magnetization raises its maximum is correlated to the superparamagnetic− ferrimagnetic (blocked state) crossover and is usually assimilated to the blocking temperature (TB). Tmax is clearly higher for the NPcop@OA assembly (184 K) than for the NPdec@OA assemblies (95 K). Although Tmax can be directly influenced by the composition and the structure of NPs, especially when NPs have been synthesized by different methods, NPdecOA and NPcop@OA are characterized by very similar sizes and structures. Therefore, such a large variation in Tmax is mainly ascribed to stronger dipolar interactions between NPcop@OA than in the NPdec@OA assembly.8,43,44 Despite the lower anisotropy of the NPcop@OA assembly, aggregates favor stronger dipolar interactions between a larger number of nanoparticles than in a monolayer of individual NPs. These observations are supported by the broadening of the ZFC curve for NPcop@OA with respect to that of NPdec@OA and the larger separation between TB and the temperature at which both ZFC and FC curves split. Such behaviors are related to disordering of NPs in aggregates which results in different interparticle distances and thus in dipolar interactions with

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CONCLUSION In conclusion, we have reported on the assembling of functionalized iron oxide nanoparticles synthesized by the coprecipitation and thermal decomposition methods. The assembling of NPs was promoted by using self-assembled monolayers of organic molecules. Acid phosphonic groups at the SAM surface resulted in the strong and irreversible binding of NPs as assemblies. Bisphosphonic acid molecules were also 3835



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used to prepare multilayer assemblies by performing the alternative deposition of molecules and NPs. The functionalization of NPs is of first importance to control the nanostructure of assemblies like monolayers and multilayers. The in situ coating of NPs by oleic acid molecules during the synthesis step following the thermal decomposition method resulted in a stable suspension of NPs, while the postfunctionalization after the synthesis following the coprecipitation method led to aggregates. The magnetic properties of assemblies are ruled by the spatial arrangement of NPs and the interparticle distance. Therefore, the assembling of individual NPs in a monolayer favors strong in-plane dipolar interactions which are promoted by shape anisotropy. In contrast, disordered interactions in aggregates of NPs disfavor such anisotropy in the monolayer but enhance the strength of dipolar interactions by increasing the number of interacting NPs. Dipolar interactions can also be enhanced when using individual NPs by the deposition of additional monolayers which enable NPs to interact between layers. As it has been shown for some biomedical applications, aggregated iron oxide nanoparticles may play a critical role in enhancing relaxivity measurements and MRI efficiency.45 Therefore, these results offer very exciting perspectives, since finely structured monolayers of the aggregated NPs would give rise to a new generation of multiscaled NP assemblies. The preparation of NP aggregates with fine control over their structure would enable their assembling as well resolved monolayers. Therefore, dipolar interactions would be enhanced because of the aggregate structure which involves a larger number of NPs (collective properties) and because of the monolayer assembly which results in shape anisotropy (oriented properties).

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ASSOCIATED CONTENT

S Supporting Information *

FTIR and XPS spectra, HRTEM, SEM and AFM micrographs, and electronic diffraction patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: + 33 (0) 3 88 10 71 33. Fax: + 33 (0) 3 88 10 72 47. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project ANR08-BLAN-NT09459731, “MAGARRAY” from the Agence Nationale de la Recherche (ANR).

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REFERENCES

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Magnetic Properties of Mono- and Multilayer Assemblies of Iron Oxide

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