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Formation of Ferrimagnetic Films with Functionalized Magnetite Nanoparticles Using the Langmuir-Blodgett Technique Fayna Mammeri, Yves Le Bras, Toufic J. Daou, Jean-Louis Gallani, Silviu Colis, Genevie`ve Pourroy, Bertrand Donnio, Daniel Guillon, and Sylvie Be´gin-Colin* Institut de Physique et Chimie des Mate´riaux de Strasbourg UMR 7504 CNRS-ULP, 23, rue du Loess, BP 43, 67034 Strasbourg cedex 2, France ReceiVed: September 14, 2008; ReVised Manuscript ReceiVed: October 30, 2008
The formation of ferrimagnetic films of 39 nm magnetite nanoparticles functionalized by stilbene derivatives has been studied using the Langmuir-Blodgett technique. The stilbene moieties are grafted to the particles either via a phosphonate or a carboxylate group; in both cases the nanoparticles display similar isotherms although the microscopic initial and final states of the films are different. Two different mechanisms of film formation are proposed, based on the inorganic-organic bond stability. Introduction Future advances in the development of nanodevices are based on the ability to integrate materials in small devices with improved properties. Beyond the top-down approach which optimizes microelectronic processes such as lithography, the bottom-up strategy develops new methods for rationally assembling and integrating nanoscaled functional building blocks (clusters, particles, molecules, and so forth) into one (1D)-, two (2D)-, and three (3D)-dimensional organized architectures.1-3 Among possible building blocks, magnetic nanoparticles display interesting magnetic properties tuned by controlling their size and shape.4-6 Intensive research activity has been devoted to the synthesis of magnetic nanoparticles and to the characterization of their properties during the past few years.5-7 Their organization and immobilization in films is nowadays often investigated for building functional ordered arrays with sizedependent properties for applications in magnetic, magnetoelectronic nanodevices (high density magnetic storage media, magneto-resistive devices, MRAM, and so forth).2,8-13 The magnetic properties of nanoparticle arrays are strongly dependent on the particle properties (average particle size, particle size distribution, crystalline structure, shape) and on the spatial arrangement of the particles that modulates particle-particle interactions. Because of dipolar interactions, the physical properties of such self-assemblies differ from those of isolated nanocrystals and of bulk materials.14-16 Recent studies have demonstrated the interest of the structuration of half-metallic magnetic oxide nanoparticles to develop magnetoelectronic devices with enhanced magneto-transport properties. Magnetite, Fe3O4, is predicted to be half-metallic at room temperature.17 It behaves as a metal for one spin direction and as an insulator for the other direction and should therefore allow 100% spin polarization of an electric current passing through. Although it has never led to very high magnetoresistance (MR) values in thin films,12,18,19 magnetite has shown extremely interesting MR values of up to 300% when elaborated in films of nanoparticles.12,13 This confirms the interest in developing new strategies toward magnetic nanoparticles structuration. Several approaches have been developed to obtain nanoparticles in 2D or 3D organized structures.2,3,20,21 However, most * To whom correspondence should be addressed. E-mail: begin@ ipcms.u-strasbg.fr. Fax: (+33) 3.88.10.72.47. Tel: (+33) 3.88.10.71.92.
of them deal with metallic nanoparticles and are based on selfassembly; difficulties arise from the control of the film formation over macroscopic distances8,11 and the control of the interparticle and interlayer spacing.16,22,23 Among the deposition techniques of nanoparticles on solid substrates, the Langmuir-Blodgett (LB) technique can be an efficient method to finely control the layer thickness and the homogeneity of the film.3 This technique has already been used to produce two-dimensional hexagonal ordered arrays of various organically coated nanoparticles.3,6,8,11-13,24-27 However, the difficulties in obtaining monodisperse size distribution of particles with homogeneous composition and in controlling the interparticle and interlayer distances within the films have limited the use of this technique so far.11,13,28 In addition, most studies have been conducted on nanoparticles smaller than 15 nm and coated with fatty acids. In this paper, we focus on the study of the deposition by the LB technique of magnetite-based nanoparticles with large grain sizes and with a complex organic coating (functionalized stilbene). Indeed, while most studies deal with superparamagnetic nanoparticles, our magnetite nanoparticles have diameters of about 39 nm and are ferrimagnetic at room temperature.29 At this size, the magnetite nanoparticles show only a small deviation from stoechiometry and their saturation magnetization is thus higher than that observed for smaller particles. Therefore, thin films of such nanoparticles may allow building up devices with improved MR values. To obtain the suitable suspensions, stilbene-based molecules with either a phosphonate or a carboxylate group as a coupling agent on one side and three alkyl chains on the opposite side have been grafted at the surface of the magnetite nanoparticles.30,31 The formation of LB films with the grafted 39 nm nanoparticles has been studied and was found to depend on the stability of the bond between nanoparticles and molecules. Materials and Methods Preparation of Hydrophobic Magnetite Nanoparticles. Magnetite nanoparticles with an average size of 39 nm ((5 nm) have been prepared in two steps by coprecipitation at 70 °C of Fe2+ and Fe3+ ions by a tetramethylammonium hydroxide ((CH3)4OH) solution, followed by an hydrothermal treatment for 24 h at 250 °C as reported.29 All reactions were carried out under argon. The magnetic curve displays a hysteresis with a saturation magnetization of about 83 emu/g.29
10.1021/jp808177y CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
Ferrimagnetic Films with Functionalized Fe3O4 Nanoparticles
Figure 1. Molecular formula of G0-C and G0-C, and TEM micrographs of Fe3O4-G0-C nanoparticles.
One hundred milligrams of nanoparticles was then dispersed in 100 mL of THF and 4 mg of stilbene molecules (Figure 1) was added. The grafting reaction was carried out at room temperature under mechanical stirring for 24 h. The grafted nanoparticles were magnetically decanted and washed three times with THF. The grafting and washing steps were identical for both molecules.30 The amounts of grafted organic molecules have been evaluated indirectly by UV spectroscopy on washing solutions and directly by chemical analyses (CNRS analysis center of Vernaison) on dried grafted nanoparticles.30,31 The corresponding carboxylated and phosphonated magnetite nanoparticles will be named in the text below Fe3O4-G0-C and Fe3O4-G0-P nanoparticles, respectively. Preparation of Langmuir and Langmuir-Blodgett (LB) Films from Functionalized Nanoparticles. The functionalized magnetite nanoparticles were first redispersed in a 10/90 v/v mixture of THF and chloroform by sonication for 10 mn with a concentration of 1 mg of functionalized nanoparticles per milliliter. Three milliliters of suspension was spread along the air-water interface of a Teflon Langmuir trough. The surface pressure (Π) was recorded as a function of the area between the mobile barriers. The isotherms are reproducible. The intersection of the tangent to the vertical part of the curve with the axis of abscissas gives the surface A occupied by one molecule or one functionalized nanoparticle at the end of the compression if the pressure were zero. The compressibility of the film is calculated from the isotherm using the formula: C ) (-1/A)(dA/dΠ) where A is the surface at zero pressure, C the compressibility in m/mN and dA/dΠ the inverse of the slope of the tangent. Stepwise compression of the particles was carried out with mobile barriers until the surface pressure chosen for the transfer (10 mN/m) was reached. The barrier velocity was 10 mm/mn until the pressure began to rise and then slowed down to 2 mm/ mn. The resulting films were then transferred onto hydrophilic silicium (Si) wafers, leading to LB films. The substrates were pulled out of the water at a speed of 1 mm/mn. Hydrophilic Si substrates were prepared by soaking in a 70/ 30 v/v mixture of sulfuric acid and hydrogen peroxide and rinsing in a 50/50 v/v mixture of ethanol and deionized water. Characterization Tools. The structure of the film was investigated in situ by Brewster angle microscopy (BAM), allowing the observation of monolayers at the air-water interface. Briefly, when a light beam linearly polarized parallel to the plane of incidence falls on an air-water interface at an angle of 53.15° (Brewster angle ) arctan(nwater/nair)), almost no light is reflected. On the contrary, if some kind of film is floating on the water, light will be reflected because the Brewster conditions are not matched any more. Therefore, in the following pictures the water surface appears dark and the film white. Moreover, in Brewster microscopy, the intensity of the reflected light is proportional to the thickness (d) and refraction index (n)
J. Phys. Chem. B, Vol. 113, No. 3, 2009 735 of the film. BAM pictures have been recorded on a Bam2Plus from NFT[www.nanofilm.de]. All the transferred films were characterized by scanning electron microscopy using a JEOL 6700F microscope operating in the 3-5 kV range. The samples were carbon-coated prior to examination. Atomic force microscopy (AFM) observations were carried out with a Nanoscope Dimension 3100 microscope operating in tapping mode. Areas up to 10 × 10 µm2 were observed in order to estimate the average thickness of the films. Magnetization measurements were made using a standard alternating gradient field magnetometer (AGFM). All measurements were made at room temperature with the magnetic field applied in the film plane. Results The amount of ungrafted organic molecules has been evaluated by UV spectroscopy from washing solutions and a calibration curve and then the grafting rate has been deduced. The amount of grafted organic molecules was found to be 0.85 molecule/nm2 for both phosphonated and carboxylated molecules. These amounts of grafted organic molecules have also been confirmed by chemical analyses performed on grafted nanoparticles. The specific area surface of nanoparticles and the cross section of both molecules being 31 m2/g and 0.72 nm2, respectively, leads to a surface covering of about 61% with both molecules. The functionalized magnetite nanoparticles present a ferrimagnetic behavior at room temperature.31 TEM observations revealed the presence of a homogeneous and amorphous “organic” monolayer all around the particles (Figure 1). The thickness of this layer corresponds to the length of the organic molecules, 3 nm, confirming a monolayer covering of magnetite-based nanoparticles. Typical isotherms corresponding to the compression of functionalized magnetite nanoparticles (Fe3O4-G0-P and Fe3O4G0-C) films are shown in Figure 2. The compressibilities are quite similar and around 2 × 10-2 m/mN for both functionalized nanoparticles. This compressibility value falls within the values reported by Lefebure et al.:25 3.3 × 10-2 and 10-1 m/mN for maghemite nanoparticles of 7.5 and 15.5 nm diameter, respectively, capped with lauric acid. Higher surface pressures may be reached with Fe3O4-G0-P nanoparticles than with the Fe3O4G0-C ones. This may be related to the motion limits of the barrier during the compression step (compression ratio of the trough); if there are not enough particles on the water subphase, the pressure cannot be increased further when the barrier motion limits are reached. These experiments conducted in the same conditions suggest strongly that some G0-C functionalized nanoparticles “dissolve” or sink in the subphase or agglomerate. The mean area at the end of the compression step at Π ) 0 mN/m is found to be 3.3 × 104 Å2 and 2.7 × 104 Å2 for Fe3O4G0-P and Fe3O4-G0-C nanoparticles, respectively. In both cases, experimental values of the mean particle area are much smaller than that predicted for a sphere in a closed-packed hexagonal array ((31/2/2) × Φ2 ) 1.7 × 105 Å2 with Φ ) particle diameter), suggesting the presence of aggregates on the subphase or sedimentation/loss of particles in the subphase. The area for Fe3O4G0-C is slightly lower than that measured for Fe3O4-G0-P, suggesting that this phenomenon is more important with Fe3O4G0-C particles, as already deduced above from isotherm curves. Recent studies have demonstrated that the phosphonate coupling agent allows a higher grafting rate and a stronger P-O-Fe bonding than the carboxylate one.30 Some Fe3O4-G0-C may thus be lost in the subphase, as a consequence of decomplexation of the carboxylic end-capped surfactant from some nanoparticles. Indeed, during the washing of Fe3O4-G0-C nanoparticles in THF, G0-C molecules
736 J. Phys. Chem. B, Vol. 113, No. 3, 2009
Mammeri et al.
Figure 2. Π-A isotherms of (a) Fe3O4-G0-P and (b) Fe3O4-G0-C on pure water. The additional isotherms up to 10 mN/m corresponds to those of films obtained by transfer onto a substrate.
Figure 3. BAM pictures (500 × 400 µm2) of Langmuir films made from Fe3O4-G0-C (subphase of pure water): (a) Π ) 0.4 mN/m, (b) Π ) 1.2 mN/m, and (c) Π ) 8.1 mN/m. Langmuir films made from Fe3O4-G0-P (subphase of pure water): (d) Π ) 0 mN/m, (e) Π ) 1 mN/m and (f) Π ) 9.5 mN/m.
were observed to decomplex while this was not observed with Fe3O4-G0-P nanoparticles.30 The formation of the films was then followed by BAM experiments. Figure 3 shows BAM pictures of Langmuir films of Fe3O4G0-P and Fe3O4-G0-C at different stages of their compression isotherms between Π ) 0 mN/m and Π ) 10 mN/m. The highest applied pressure (10 mN/m) corresponds to the transfer pressure of the film on a substrate. Just after spreading the particles on the surface, when Π ≈ 0 mN/m, most of the particles are already condensed in domains (Figures 3a,d) as already observed during the compression step of smaller nanoparticles with hydrophobic coating.25,27,32 Indeed, when spread at the surface of water, the nanoparticles assemble in spherical domains like oil spread on water. The hydrophobicity provided by their coating (and particularly the three alkyl chains) induces some segregation, and the formation of domains in order to reduce their interaction with the water subphase. Moreover, the attractive interactions between large particles are stronger than between small particles as van der Waals and magnetic dipolar interactions increase with particles size, enhancing the formation of domains. Thus the isotherms reflect the behavior of domains and not of isolated nanoparticles. During the compression, the surface density increases. In the case of Fe3O4-G0-C nanoparticles, it consists in bringing the domains closer. Their size stays similar during the compression, whereas larger domains are formed in the case of Fe3O4-G0-P nanoparticles (Figure 3b,e). At the compression, Π ≈ 10 mN/ m, the Langmuir films made of Fe3O4-G0-C appear to be more compact than those made from Fe3O4-G0-P. The transfer of the Langmuir films of hydrophobic nanoparticles onto silicon substrate was performed at a surface pressure
Figure 4. SEM micrographs of films from G0-P- and G0-C- magnetite nanoparticles.
of 10 mN/m (isotherm curves are shown in Figure 2). Lowmagnification SEM micrographs (Figure 4) show that films of nanoparticles have been deposited on the substrate. As already observed during BAM experiments and as illustrated in Figure 4 at low magnification, the surface coverage appears higher for the films of Fe3O4-G0-C nanoparticles. However, at higher magnifications these films present “stacking” of small domains, while for Fe3O4-G0-P films the domains appear more compact and relatively better organized. Several experiments have been performed by varying the transfer pressure and the nature of
Ferrimagnetic Films with Functionalized Fe3O4 Nanoparticles
J. Phys. Chem. B, Vol. 113, No. 3, 2009 737
Figure 6. AGFM measurements of G0-C- (a) and G-0P- (b) magnetite nanoparticle-based films. The measurements were made at room temperature with the applied magnetic field in the film plane.
Figure 5. (a,b) Atomic force microscopy images of the G0-C- and G0-P- magnetite nanoparticles. (c,d) Section analysis of the AFM images. (e,f) Histogram of the tip-sample distances recorded from the AFM images. The distances between the two peaks correspond to the average thickness of the nanoparticle films.
the substrate (hydrophobic or hydrophilic) and in all cases similar features have been observed. The AFM observations performed on these films have confirmed their surface topologies (Figure 5a,b). However, in contrast to SEM, AFM allows deducing the average thickness of the film from the histogram of the distances between the tip and the sample (Figure 5e,f). The histogram presents two peaks: one intense and narrow peak that corresponds to the average depth position of the substrate (marked in red in Figure 5e,f), and one smaller and larger peak that corresponds to the average depth position of the film surface (marked in green in Figure 5e,f). The difference between the two depth values gives the average thickness of the layer. The width of the peak corresponding to the film surface clearly indicates that the film is far from having a homogeneous thickness, which is in agreement with the clustering of the nanoparticles. The mean thickness of the Fe3O4-G0-C and Fe3O4-G0-P films are found to be 42 and 60 nm, respectively. The thickness of the Fe3O4-G0-C film is in agreement with the size of nanoparticles, suggesting an averaged monolayer coverage and a lower aggregation state. It is also in agreement with the smaller surface roughness, which is 42.6 nm rms, compared to the 54.4 nm rms of the Fe3O4G0-P film. Section analysis profiles (Figure 5c,d) support these results as well and show that the height of the nanoparticle domains is slightly higher in Fe3O4-G0-P films than in Fe3O4G0-C films. These results show that the films consist both in assembly of domains with larger domains sizes (in width and in height) with Fe3O4-G0-P nanoparticles. The magnetic properties of both Fe3O4-G0-P and Fe3O4-G0-C films are typical of ferrimagnetic samples. The magnetization loops (Figure 6) recorded at room temperature show coercive fields of 175 Oe and remanence over saturation ratios (Mr/Ms) of 0.3. The fact that both samples have similar coercive and saturation fields confirms that the grain sizes of the two samples are of similar order of magnitude. The ratio Mr/Ms is close to 0.17 for both grafted nanoparticles.31 Its increase in the assembled samples could be attributed to a slight decrease in
magnetic coupling.16,28 Indeed a ratio of 0.2 was noticed with dilute dispersions of 9 nm iron nanoparticles and of 0.32 when they were in arrays.28 Frankamp et al.16 observed an increase in the Mr/Ms ratio when the interparticle spacing between iron oxide nanoparticles increases. Discussion The interaction between particles during the film formation depends on whether they are capped with G0-P or with G0-C. From isotherm analyses, formation of aggregates or loss of particles in the subphase were suggested to explain that the mean particle area measured on the isotherm is smaller than the theoretical one. The loss of nanoparticles in the subphase appears as the main mechanism in the case of Fe3O4-G0-C nanoparticles, as they display a lower aggregation state and globally an “average” monolayer, in contrast with Fe3O4-G0-P nanoparticles for which aggregation appears to be the main explanation. This loss of particles in the water subphase is probably related to earlier observations of decomplexation of G0-C molecules in THF (the phosphonate binding being stronger).30 The resulting structure of films after transfer onto a substrate is also different depending on the coupling agent. The films consist in domains which are larger, denser, and better organized with Fe3O4-G0-P nanoparticles by comparison with Fe3O4-G0-C nanoparticles. When assembling the nanoparticles, van der Waals and magnetic dipolar interactions occur between magnetic nanoparticles as well as van der Waals interactions between the alkyl chains of grafted molecules. These interactions lead to the formation of domains (nanoparticles aggregates) at the surface of water, which are further compressed. Calculation of the fractal dimension of 2D aggregates with magnetic interaction33-35 and experimental results25 have shown that the fractal dimension is smaller for the larger particles. Lefebure et al.,25 studying the Langmuir deposition of maghemite nanoparticles, reported a fractal dimension of 2D aggregates close to 2 for 7.5 nm nanoparticles that form dense aggregates and between 1 and 2 for the larger ones (15.5 nm) forming digitated aggregates. Indeed, magnetic forces are reported to lower the fractal dimension of aggregates of magnetic nanoparticles. Fractal dimension calculations were carried out using the ImageJ software on two cross-section SEM micrographs of both films (1 cm2 and 1 µm2). The fractal dimensions were found to be around 1.8 for both films at low and high magnification. This value is in good agreement with the observation of dense domains observation but is a bit large considering the 39 nm particle size. That could suggest that the dipolar interactions
738 J. Phys. Chem. B, Vol. 113, No. 3, 2009 are quite low in our experimental conditions. However, the Mr/ Ms ratio, which is close to 0.3, is higher than that of the nanoparticles powder but is lower than the value of 0.5 expected for noninteracting particles.28 Thus dipolar interactions occur in the assembled nanoparticles. Moreover, the organic layer (3 nm) at the surface of nanoparticles (Figure 1) is not large enough to prevent dipole-dipole interactions. The reported interparticle spacing leading to noninteracting particles varies from 17 to 7 nm for smaller nanoparticles.16,36 One explanation to the observed ratio value is that the formation of nanoparticles domains stabilizes some of the particle moments against switching. Such a fractal value is reported to be characteristic of aggregation governed by the diffusion limited cluster-cluster aggregation (DLCA) mechanism rather than by the long-range interactions between particles. This would be consistent with the observed formation of Fe3O4-G0-P films by the increase in the domain sizes: attractive interactions leads to the formation of nanoparticle domains that further interact together to form larger domains. The films formation thus depends on the domains interactions and growth and explains the SEM observations of films with large domains (Figure 4). Such a mechanism is limited in Fe3O4-G0-C films that present smaller and less dense domains. As some partial decomplexation of the carboxylated molecules of functionalized nanoparticles is observed, one may suppose that these amphiphilic molecules stay at the air/water interface between the domains and limits the domain interactions and their further growth during the compression step. That would explain also the higher surface coverage observed in low magnification SEM micrographs. All these results demonstrate that in our experimental conditions, the grafted molecules and especially the bonding quality modified the nanoparticle domains formation and therefore have an influence on the assembling mechanisms of 39 nm nanoparticles. Two different mechanisms of film formation have thus been proposed. Conclusion The formation by the LB technique of assemblies of hydrophobic magnetite nanoparticles with a mean diameter of 39 nm and covered with stilbene molecules via a carboxylate or a phosphonate group as coupling agent has been studied. With Fe3O4-G0-C nanoparticles, the decomplexation of the carboxylated molecules during the film formation leads to the loss of nanoparticles in the subphase and to a film microstructure consisting in a dense stacking of small domains. The interactions between domains are limited by the presence of free molecules between domains, and a large surface coverage is observed. With Fe3O4-G0-P nanoparticles, the film microstructure consists in a stacking of denser and better organized domains. The growth of the assembly is governed by the interaction between these domains. References and Notes (1) Osada, M.; Ebina, Y.; Takada, K.; Sasaki, T. AdV. Mater. 2006, 18, 295. (2) Gao, Y.; Bao, Y.; Beerman, M.; Yasuhara, A.; Shindo, D.; Krishnan, K. M. Appl. Phys. Lett. 2004, 84, 3361. (3) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Phys. Chem. 2008, 9, 20. (4) Alivisatos, A. P. Science 1996, 271, 933. (5) Lu, A.-H.; Salabas, E. L.; Schu¨th, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (6) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33.
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JP808177Y
