ARTICLE pubs.acs.org/Langmuir
Tunable Magnetic Properties of Nanoparticle Two-Dimensional Assemblies Addressed by Mixed Self-Assembled Monolayers Benoit P. Pichon,*,† Matthias Pauly,† Pascal Marie,‡ Cedric Leuvrey,† and Sylvie Begin-Colin† † ‡
Institut de Physique et de Chimie des Materiaux de Strasbourg, 23 rue du Loess BP 43, 67034 Strasbourg Cedex 2, France Institut Charles Sadron, 23 rue du Loess BP 84047, 67034 Strasbourg Cedex 2, France
bS Supporting Information ABSTRACT: Assemblies of magnetic nanoparticles (NPs) are intensively studied due to their high potential applications in spintronic, magnetic and magneto-electronic. The fine control over NP density, interdistance, and spatial arrangement onto substrates is of key importance to govern the magnetic properties through dipolar interactions. In this study, magnetic iron oxide NPs have been assembled on surfaces patterned with selfassembled monolayers (SAMs) of mixed organic molecules. The modification of the molar ratio between coadsorbed 11-mercaptoundecanoic acid (MUA) and mercaptododecane (MDD) on gold substrates is shown to control the size of NPs domains and thus to modulate the characteristic magnetic properties of the assemblies. Moreover, NPs can be used to indirectly probe the structure of SAMs in domains at the nanometer scale.
1. INTRODUCTION The assembling of magnetic nanoparticles (NPs) into arrays represents a very exciting and important challenge with regard to their high potential in the development of new nanodevices for spintronic, magnetic, and magneto-electronic applications such as high-density magnetic data storage or magneto-resistive devices.1,2 It is well argued that the key to successful applications of such NP-based devices is creating well-defined nanostructures. Indeed the physical properties of NPs assemblies differ significantly from those of isolated nanocrystals and their bulk counterparts.37 At the nanometer scale, dipoledipole interactions are not negligible against the anisotropy energy and induce specific and remarkable collective properties.812 These interactions are strongly dependent on the interparticle distance in assemblies, which may be a key parameter to develop further devices. For instance, arrays of well-separated, room-temperature blocked single-domain NPs without any magnetic interaction between them would be very suitable as elemental nanomagnets for high-density data storage applications. Dipolar interactions as a function of the interparticle distance have been first investigated by dispersing NPs in inert matrix such a polymer.13,14 However, when short interparticle distances are considered, strong interactions lead to the aggregation of NPs. Therefore, chemical self-assembling methods1517 that consist in the immobilization of NPs on surfaces became very attractive to study dipolar interactions between NPs separated by short distances. Over the past decades, evaporation-induced selfassembly (EISA)1820 and LangmuirBlodgett (LB)2123 techniques have been considered as efficient methods for the r 2011 American Chemical Society
preparation of two- (2D) and three-dimensional (3D) arrays of NPs on substrates. In such structures, the spatial arrangement of NPs is mostly controlled by the thickness of either organic21,2428or inorganic27 NP coatings and consists in packed arrays over large areas. Although these techniques lead to 2D or 3D assemblies with homogeneous interparticle distances, the formation of films that consist in sizable NPs domains can not be controlled. The patterning of substrates by techniques following the top-down approach such as lithography29 or microcontact printing30 enable the control of the spatial arrangement of particles on specific areas. However, these approaches are limited by the domain size resolution and are not appropriate to control the assembling of NPs at the nanometre scale. Therefore, new approaches have to be developed to study the magnetic properties of 2D assembled nanostructures as a function of NP density, interdistance, and space arrangement onto substrates. Recent studies have demonstrated that the functionalization of the substrate may drive original assemblies with spontaneously formed, hierarchical, complex architectures either by favorable free-energy changes or kinetics. Indeed the functionalization of substrates by using self-assembled monolayers (SAMs) of organic molecules has been shown to direct the formation of magnetic NPs monolayers through hydrogen31,32 or covalent30 bonding when coated with molecules or directly at the NPs surface.11,30,32,33 Moreover, SAMs have been used to pattern Received: December 21, 2010 Revised: March 2, 2011 Published: April 15, 2011 6235
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Langmuir surfaces when combined with microcontact printing30,3436 or dip-pen nanolithography.29,37 However, no easy control is yet offered toward spatial deposition of NPs in sizable domains to investigate structuring-depending dipoledipole interactions. An alternative and smart approach would consist in patterning surfaces by using SAMs of mixed organic molecules.3845 Thiol molecules with different chain lengths or terminal groups have been shown to phase separate in mixed SAMs. In these systems, segregated nanodomains with sizes depending on the relative concentrations of different molecules in solution were shown directly by scanning tunneling microscopy (STM)40,41,43,46 and atomic force microscopy (AFM).40,44 The phase separation of carboxylic acid and methylene terminal groups have been also determined indirectly by using dendrimer molecules as probing agents through complexation with carboxylic acid groups.45 Therefore, an original control is expected at the molecular level through such molecular patterning.47 Very recently, the chemical-assisted assembling of iron oxide NPs by SAMs displaying chelating or nonchelating functional head groups has been proven as a way to control the NPs density.48 Such an approach using SAMs, which consists in mixtures of molecules with different abilities to immobilize NPs, would contribute to study the magnetic properties of original NP assembling. Although SAMs have been demonstrated to be a very attractive approach to assist the assembling of NPs in films with well-defined properties, the use of mixed SAMs as patterned surfaces has been poorly reported to control the spatial arrangement of magnetic NPs. Here we report on the fine-tuning of magnetic properties of NP assemblies as a function of their spatial arrangement in 2D sizable domains. The assembling of NPs is controlled by mixed SAMs of 11-mercaptoundecanoic acid (MUA) and mercaptododecane (MDD) molecules, which respectively display chelating and nonchelating groups at the surface. The careful adjustment of the molar ratio between both molecules induces the chemical patterning of substrates, which results in the NP domain structuration and characteristic magnetic properties.
2. EXPERIMENTAL SECTION Chemicals and Materials. MUA and MDD were purchased from Aldrich. Absolute ethanol and tetrahydrofuran (THF) were used as received. Gold substrates were obtained by radio frequency magnetron sputtering at 100 °C. A first layer of chrome (15 nm thick) was deposited before a second layer of gold (60 nm thick) on single-side-polished, (100)-oriented single crystalline silicon wafers (500 μm thick), which were used as received. Preparation of SAMs. Freshly cleaned ion sputtered gold substrates under O2/Ar plasma (2 min) were used for all preparation. Mixed SAMs prepared by coadsorption were prepared by soaking gold substrates at room temperature for 24 h in 10 mmol ethanolic solutions of MUA and MDD in different molar ratios (20:80, 50:50, 80:20). Substrates were then washed extensively with absolute ethanol to remove unbound thiol molecules and dried under N2 stream. The corresponding SAMs were respectively named SAM-20, SAM-50, and SAM-80. Preparation of Iron Oxide NPs. Twelve-nanometer-sized iron oxide NPs that crystallize in the spinel structure were produced by the thermal decomposition method.49 The preparation of an iron(III)/ oleate complex (Fe(oleate)3), which is thermally decomposed in a high boiling solvent in the presence of oleic acid, was performed following the exact procedure we reported previously.22 Fe(oleate)3 was prepared from FeCl3 3 H2O (10.8 g, 40 mmol, 97%, Aldrich), which was dissolved in 60 mL H2O (Milli-Q) and 80 mL ethanol. This solution was mixed
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Table 1. Contact Angle Values Measured on Co-adsorbed Mixed SAMs with Water, Diiodomethane, and Ethylene Glycol SAM-MDD SAM-20 SAM-50 SAM-80 SAM-MUA H2O
102
93
78
58
49
CH2I2
56
49
40
34
29
ethylene glycol
71
65
56
45
41
with a solution of sodium oleate (36.5 g, 120 mmol, 82%, Riedel-de Ha€en) dissolved in hexane (140 mL) and refluxed at 70 °C for 4 h. The organic phase containing the iron oleate complex was separated, washed three times with distilled water (30 mL) to extract salts, dried using MgSO4, and finally hexane was evaporated. The resulting iron oleate complex was a reddish-brown viscous solid and stored at 4 °C. A combination of Fe(oleate)3 (2 g, 2.2 103 mol), oleic acid (1.24 g, 3.3 103 mol), and octyl ether (20 mL) was stirred for 1 h to dissolve the reactants at 100 °C. The temperature was carefully raised to reflux with a heating rate of 5 °C/min without stirring for 120 min under air at 288 °C. After cooling to room temperature, the black suspension of nanocrystals was washed three times by addition of ethanol and centrifugation (8000 rpm, 10 min.). The obtained nanocrystals could be easily suspended in various organic solvents to raise a highly stable suspension, which can be stored for several months. The size monodispersity of the NPs was improved by applying a size selection precipitation process.50 The NPs were suspended in hexane at a concentration of 1 mg/mL and precipitated by adding the same volume of acetone followed by centrifugation. The precipitate was redispersed in THF to prepare a highly stable suspension of coated NPs with a specific concentration of 3.7 mg 3 mL1. Characterization of NPs. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded with a TOPCON model 002B transmission electron microscope, operating at 200 kV, with a point-to-point resolution of 0.18 nm. The polydispersity in size was calculated from the size measurements of more than 300 NPs. Dynamic light scattering (DLS) measurements were performed on the suspension of NPs in THF using a nanosize MALVERN (nano ZS) apparatus. Assembling of Iron Oxide NPs on SAMs. SAMs were immersed directly after preparation in the suspension of oleate-coated NPs in THF at room temperature for a very short time of 10 min. Both substrates were then placed in an ultrasonic bath in THF to remove any physisorbed NPs and finally dried under a stream of nitrogen. Characterization of SAMs. The surface activity of mixed SAMs was qualitatively studied by contact angle measurements with water, diiodomethane, and ethylene glycol. This technique is very convenient because of its high sensitivity to details of the interfacial structure at the molecular level. Contact angle values (Table 1) were measured with a Digidrop contact angle goniometer (GBX, France). Characterization of NP Assemblies on SAMs. Scanning electron microscopy (SEM) was performed using a JEOL 6700 microscope equipped with a field emission gun (SEM-FEG) operating at an accelerating voltage of 3 kV. The composition of NPs was analyzed by EDX spectroscopy coupled to SEM. AFM was performed using a Digital Instrument 3100 microscope coupled to a Nanoscope IIIa recorder. Measurements were done in the tapping mode onto substrates before and after exposition to the NP suspension. Collected datas were analyzed with Nanotec WSXM software.51 The area uncovered by NPs was calculated to correspond to a height lower than 6 nm, which roughly corresponds to the half diameter of 12 nm-sized NPs. Image Analysis. The digitized SEM images were analyzed with the Visilog software (Noesis, France) following a edge detection procedure to determine the number of NPs and their position. Several techniques 6236
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Scheme 1. Preparation of Mixed SAMs by the Co-adsorption of MUA and MDD Followed by the 2D Assembling of NPs Coated with Oleic Acid
were combined such as filtering (background correction and Gaussian smoothing for removing detail and noise), edge detection methods (Laplacian of Gaussian and Zero-crossing operators), and adaptive threshold to locate the boundary (or edge) of each particle. Particles lying within the specific region of interest (ROI) in the SEM digital image were analyzed so as to extract their X and Y centroid coordinates within the ROI. Densities in NPs were calculated on each SAM from the number of NPs on a precise area. The surface covered by NPs was deduced from the density in NPs measured on SEM images and according to their size and spherical morphology that were analyzed by TEM. The effective coverage is compared to NPs ideally organized in a 2D hexagonal close-packed (hcp) lattice. NPs can be assimilated to circles with a compacity of 0.9. Therefore, the closest packing of NPs would correspond to a maximum surface covered of 90%. Although NPs are featured by a narrow size distribution as shown by TEM, SEM pictures display larger NPs when tightly packed in large domains (SAM-20) than in small domains (SAM-80). This may be explained by the presence of oleic acid molecules at the NPs surface and self-assembled monolayers. Pair Correlation Function Calculation. The pair correlation function (PCF), g(r), also known as the radial distribution function, was used to characterize the distribution of particles on a 2D plane. g(r) is a measure of the structure that is present in a collection of particles. The function can be interpreted as an averaged probability of finding the center of a particle at a given distance, r, from the center of another particle normalized to the uniform probability at large distances.52,53 The degree of local translational order in the magnetic NP arrays was quantitatively characterized by gðrÞ ¼
1 N ni ðrÞ Np i ¼ 1 Fπðδr 2 þ 2rδrÞ
∑
ð1Þ
where Np denotes the number of particles under consideration, F is the average particle number density, and ni(r) is the number of particles that lie within an annular ring, δr, of radius r from an arbitrary origin. π(δr2 þ 2rδr) represents the sampling area for a particular radial distance, r. In this work, the 2D PCF calculations were performed from eq 1 using Plugin’s Image J software with δr = 1 nm and rmax = 100 nm. The g(r) function was plotted using eq 1 by analyzing coordinates of a few thousands of particles to obtain a satisfactory accuracy of g(r).
Magnetic Characterization of NP Assemblies. The magnetic properties of assemblies were investigated by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design model MPMS-XL). Magnetization versus applied magnetic field curves were recorded at 300 and 5 K. Zero field cooling (ZFC) and field cooling (FC) curves were recorded from 5 to 300 K under exposition of an applied magnetic field of 75 G. ZFC curves were first recorded from 5 to 300 K after the temperature had been decreased without applying any magnetic field. Then, the FC curves were recorded after the temperature was set down again to 5 K under 75 G. The substrates containing the assemblies of NPs were placed in a parallel direction to the one of the applied magnetic field. The magnetic properties of these NP assemblies were compared to the ones of hydrophobic coated NPs in the powder state as 3D random assemblies, which favors random dipolar interactions. ZFC and FC curves were normalized from the maximum values of the magnetization at the blocking temperature (TB).
3. RESULTS AND DISCUSSION The influence of the chemical nature of the SAM on the assembling of NPs and the resulting magnetic properties was investigated by preparing mixed SAMs. In a typical experiment, mixed SAMs were prepared by the coadsorption of MUA and MDD on gold substrates for 24 h in ethanolic solutions. Both molecules have similar chain lengths to maximize the reciprocal influence of the chelating carboxylic acid and noncoordinative methylene terminal groups toward the mean surface activity and mixture. SAMs were respectively named SAM-20, SAM-50, and SAM-80 as a function of the molar ratios of MUA:MDD (20:80, 50:50, 80:20) in solution. Stable suspensions of nonaggregated NPs with spherical morphology and narrow size distribution have to be prepared to fulfill their use as building blocks following the bottom-up approach and precise control over magnetic properties.54,55 In this aim, nonaggregated 11.8 nm sized magnetic iron oxide NPs coated with oleic acid were synthesized by the thermal decomposition of an iron oleate complex Fe(oleate)322,49,52 and characterized by TEM (see Supporting Information (SI)). NPs coated by oleic acid lead to a highly stable suspension in THF, as shown by granulometric measurements 6237
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Figure 1. Assembly of NPs on SAM-20 (a,d,g), SAM-50 (b,e,h) and SAM-80 (c,f,i). Original SEM pictures (a,b,c). Binary pictures after edge detection procedure (d,e,f). Position of each NP compared to the original picture (g,h,i). Scale bars represent 100 nm.
and enable us to consider nonaggregated NPs to assemble on SAMs. Indeed a monomodal distribution centered to an average particle size of 16.4 nm was observed in agreement with the hydrodynamic diameter of 12 nm-sized NPs coated by oleic acid molecules. The crystallographic structure of the NPs was investigated by HRTEM. HRTEM images of NPs display lattice fringes that correspond to repeated atomic planes with an average interspacing of 0.48 that can be related to (111) spinel planes. Electron diffraction was also performed on an assembly of NPs. The electron diffraction pattern displays rings that can be indexed to the spinel structure (JCPDS card numbers 39-1346 for magnetite and 19-629 for maghemite). The formation of 2D patterned assemblies of NPs was performed by immersing mixed SAMs in the NP suspension for 10 min, followed by washing with THF. The immobilization of NPs on SAMs is supposed to be driven by specific interactions between NPs and chelating carboxylic acid groups (Scheme 1). The experimental conditions of deposition were addressed in a previous report.48 The formation of mixed SAMs on gold substrates was confirmed by contact angle measurements with water, diiodomethane, and ethylene glycol (Table 1). All values are intermediate to both SAMs of pure MDD and MUA molecules and decrease with increasing the molar ratio MUA:MDD in the solution.42 It is consistent with the expected increase of the COOH/CH3 ratio at the surface of the SAM. Contact angles for the SAM-MDD (methylene termination) fit perfectly with values from the literature corresponding to a highly hydrophobic
surface.56 By contrast, the contact angle for the SAM-MUA, which consists of carboxylic acid terminal groups, yielded moderately hydrophobic surfaces57 although it remains higher than in some studies.58,59 It may be related to possible interactions such as hydrogen bonds between COOH groups which induce a lower order.39 Nevertheless, methylene and carboxylic acid groups as the terminal functional groups give a large difference in the wetting properties of the resulting mixed SAMs. The linear decrease of contact angle values with MDD/MUA ratio in solution fits the calculated values from Cassie’s law.60 This observation depicts the good correlation between surface and solution composition and is significant of mixed monolayers of both molecules. In addition, Imabayashi et al.42 have shown that such linear variation of the contact angle was observed for both homogeneous and phase-separated mixed SAMs of undecanethiol and MUA. Therefore, contact angle can not give indication of the phase separation into nanodomains.38 SEM pictures of mixed SAMs substrates after dipping in the NP suspension (Figure 1) show that the arrangement of NPs is highly dependent on the molar ratio between COOH and CH3 terminal groups. NPs are assembled in domains with sizes decreasing with the amount of CH3 terminal groups. Binary images resulting from the analysis of SEM pictures clearly show this behavior (Figure 1df). On SAM-20, large and interconnected domains up to 1 μm2 are surrounded by noncovered areas. These domains tend to reduce below 80 nm2 on the SAM-50, while some NP clusters are observed. This 6238
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Figure 3. AFM pictures of SAM-20 after immobilization of NPs. Height pictures (a), threshold between area higher than 6 nm (white) and lower (black) (b), 3D image (c), and cross-section (d) corresponding to the line shown in (a).
Figure 2. PCFs g(r) corresponding to NPs assemblies on SAM-20 (a), SAM-50 (b), and SAM-80 (c).
phenomenon is enhanced on the SAM-80, which is covered by clusters of three or four NPs spread homogeneously on the surface. The spatial distribution of particles in sizable domains was confirmed by the calculation of the PCF, g(r) (Figure 2). The PCF was calculated from the position of each NP analyzed on SEM pictures with the Visilog software (Noesis, France). It reflects the average distance between one NP and its closest neighbors. All curves exhibit a well-pronounced primary maximum at 17 ( 1 nm, which is in agreement with the minimum distance between 11.8 nm sized NPs covered by a 2 nm layer of oleic acid.22 A close observation of this peak shows its evolution from SAM-20 to SAM-80. As the amount of COOH groups increases in the SAM, its intensity decreases from 2.0 to 1.6, and its width increases. A secondary peak centered at about 32 nm is also clearly observed for SAM-20, while it decreases for SAM-50. For SAM-80, the PCF is almost uniform, only one peak is observed, and g(r) approaches unity for r > 20 nm. These results are consistent
with the short-range ordering of NPs on SAM-20 for distances close to their diameter and its evolution to randomly located and not tightly packed NPs (jammed disordered packing) on SAM-80. It can be correlated to the decreasing size of the NPs domains with the increasing amount of COOH terminal groups. To gain better insight on the assembling of NPs on mixed SAMs, their density was calculated from the analysis of SEM pictures with the Visilog software (Noesis, France) following an edge detection procedure. Although the assembling of NPs is highly influenced by the chemical nature of the SAMs,48 their density remains surprisingly very similar for all samples and is 2610 ( 60, 2730 ( 80, and 2680 ( 80 NPs/μm2, respectively, for SAM-20, SAM-50, and SAM-80. These values correspond to about 55% of the surface, which would be covered by an ideal hexagonal packing of NPs and remain below the density (4090 ( 126 NP/μm2) and the coverage (80%) of NPs assembled onto a pure monolayer of MUA.48 The formation of domains which consist in NPs monolayers has been confirmed by AFM performed on SAM-20 (Figure 3). Gold surfaces before the adsorption of thiol molecules display a roughness of 1.1 nm (see Supporting Information). In contrast, SAM-20 after exposition to the NP suspensions displays a higher roughness of 10.2 nm. Covered areas are defined by a height difference with uncovered regions of 13.1 nm, which is in good agreement with the formation of a monolayer of 11.8 nm sized NPs. The difference with granulometric measurements arises form the fact that oleic acid molecules have been partially removed from the NP surface, which interacts with COOH groups of the SAM. According to a previous study,48 a pure SAM decorated by COOH terminal groups has been shown to immobilize iron oxide NPs because of chelating ability, whereas noncoordinative CH3 terminal groups do not. By mixing both MDD and MUA molecules, which have similar hydrocarbon chain lengths, one might have expected their homogeneous mixing in the SAMs. Therefore, NPs were expected to homogeneously cover the SAM 6239
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Figure 4. Magnetic measurements for SAM-20, SAM-50, SAM-80, and 3D randomly assembled NPs. (a) 300 K and (b) 5 K magnetization curves against applied magnetic field. (c) Zoom of panel b. (d) ZFC/FC magnetization curves against temperature.
and to increase in density with the increase in the amount of COOH terminal groups. In contrast, the use of mixed SAMs consisting of both COOH and CH3 groups at their surface with different molar ratios show very original features regarding the control of the NP assembling by SAM patterning. While the amount of COOH groups increases in SAMs, well-located and large NP domains were observed to decrease in size and result in NPs that become homogeneously spread on the SAMs. These results clearly demonstrate that the NPs domain size is directed by the molar ratio and spatial arrangement of MUA and MDD molecules in mixed SAMs. Furthermore, NPs can be considered as probe agents as it has been already performed by using dendrimers.45 On the basis of the simple consideration that the immobilization of NPs requires chelating COOH groups at the surface of the SAM, we can conclude that the spatial distribution of NPs on the surface reflects the arrangement of COOH groups. Considering the amount of COOH groups determined by contact angle measurement and the maximum NP density on a SAM-MUA (4090 ( 126 NP/μm2) that results from a full covered surface by COOH groups, the densities calculated for SAM-50 and SAM-20 are too high. Indeed the NPs density remains very similar whatever the amount of COOH groups. Such inconsistence may be explained by the partial phase separation of MUA and MDD molecules in the SAMs. The nonhomogeneous distribution of thiol molecules has been shown to be ruled by dynamic parameters. It may be explained by the different miscibilities of MUA and MDD molecules with regard to their molar ratio in solution. Intermolecular interactions such as hydrogen bonding between carboxylic acid groups against van der Waals
interactions between alkylene chains also have to be taken into account.38,39,61,62 The exchange of adsorbed alkylene thiol molecules on gold substrates, which depends on the respective chemical nature of MDD and MUA, may also play a role.45 Therefore, the structuration of NP domains suggests a phase separation of both types of molecules for low concentration in COOH and a homogeneous distribution of CH3 groups within COOH groups for high concentration in COOH. Nevertheless, the relation between NP densities on SAM-20 and SAM-50 and contact angle measurements shows that large domains of NPs should cover areas exposing COOH and CH3 terminal groups, whereas uncovered areas consist in only CH3 groups. The low curvature of NPs, which are assimilated to spheres, results in a large interface, which induces sufficient interactions to immobilize them despite the fact that the SAM surface does not fully consist of COOH terminal groups. Therefore, the surface covered by NPs is higher than the one occupied by COOH groups. As the amount of COOH groups increases at the surface of the SAM, NP domains decrease in size and NPs tend to be homogenously distributed as for SAM-MUA. The observation of NPs clusters on SAM-80 may correspond to COOH-rich domains separated by small CH3 rich domains homogenously distributed. Therefore the partial phase separation is hardly observed, and may correspond to the much more homogeneous distribution of COOH and CH3 groups at the SAM surface. In addition, one striking observation is the formation of domains of tightly packed assemblies on SAM-20 which are observed neither on SAM-80 nor on SAM-MUA. The immobilization of NPs happens through ligand exchange between oleic acid at the NPs surface and COOH groups from the SAM surface. 6240
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Langmuir A high concentration in COOH groups as for SAM-MUA induces strong interactions with NPs.48 Therefore, when NPs enter randomly in contact with the SAM surface, they assemble directly, and the probability to have tightly packed NPs is very low. The density reaches the maximum when interparticle distances are lower than the NP diameter. However, on SAM-20, the PCF confirmed the presence of tightly packed NPs domains. Taking into account the usual immobilization mechanism of NPs, this result suggests some rearrangement of NPs onto the COOH rich domains. In comparison to SAM-MUA, the partial phase separation results in a lower density in COOH groups per area unit and leads to lower interactions between SAM and NP surfaces. Therefore NPs rearrange on SAM-20 in tightly packed domains through van der Waals interactions between alkylene chains of surfactant molecules at the NP surface and dipoledipole interactions. As shown by SEM and AFM, mixed SAMs induce the chemical patterning of substrates which allow structuring NPs in domains with size decreasing with the amount of COOH groups at the SAM surface. Such assembling of NPs into large domains to clusters of few of them should influence their magnetic properties. SQUID measurements on these original assemblies show that the magnetic properties are clearly influenced by the size of NPs domains. Magnetization curves as a function of an applied magnetic field recorded at 300 K are characteristic of superparamagnetic states (Figure 4a) by comparison with 5 K magnetization curves, which display hysteresis for all samples in agreement with ferrimagnetic states (Figure 4b,c). Large NP domains on SAM-20 and powdered NPs exhibit similar features in contrast to clusters on SAM-80. At 5 K, coercive fields increase with the decrease of the domain size from large domains (120 and 170 Oe) to clusters (1890 Oe), which evidences stronger and demagnetizing dipolar interactions in large domains compared to clusters.63,64 These results are also supported by the variation of the slope of 300 and 5 K magnetization curves with the domain sizes. Powdered NPs and SAM-20 display the steepest increase in magnetization with magnetic field. The slope decreases gradually with the formation of small NP domains. The higher sensitivity of large domains in SAM-20 can be related to the lowest energy configuration of tightly packed NPs, which tend to mostly have their moments aligned in the plane of the substrate because of dipolar interactions.26,65 In contrast, small NPs domains, which consist in a lower number of tightly packed NPs result in lower dipoledipole interactions and explain the clear increase of the coercitive field. Their magnetic moments are randomly oriented in space, and the magnetic field has to act against the anisotropy energy barrier to align the moments in the field direction. Although no significant variation of remanence-to-saturation magnetization (Mr/Ms) ratios at 5 K from large NPs domains on SAM-20 (0.14) to clusters on SAM-80 (0.21) were observed, these values below 0.5 suggest that dipolar interactions exist between NPs in all samples but are modulated by the domain size. The clear dependence of magnetic properties with the NPs domains size was further proved by magnetization curves against temperature (Figure 4d). ZFC and FC curves are characteristic of superparamagnetic NPs. More interestingly, some variations in curve shapes are indicative of different interactions depending on the size of NPs assemblies. ZFC curves show maxima in magnetization ranging around 6870 K assimilated to the blocking temperature (TB), i.e., the transition between ferrimagnetic (below TB) and superparamagnetic (above TB) states. Although TB was expected to increase with dipolar interactions,54 it remains very similar for all samples. Indeed, the maximum of
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the ZFC does not precisely reflect the distribution of blocking temperatures. The careful observation of ZFC and FC curves provides some interesting information on magnetic interactions between NPs. At first, the broadening of the ZFC peaks decreases with the domain size from SAM-20 and powdered NPs to SAM80. This narrower distribution of TB is again in good agreement with the formation of more homogeneous domain sizes and lower distribution of dipolar interactions on SAM-80. In addition, the difference between TB and the temperature where both FC and ZFC become separated can be related to coupling interactions. The decrease of this value from 85 to 51 K with the size of NP assemblies on SAM-20 to SAM-80, respectively, is in good agreement with decreasing dipoledipole coupling. Moreover, the saturation of magnetization below TB in FC curves observed on SAM-20 and powdered NPs is related to the lowest energy configuration of tightly packed NPs featured by strong dipolar interactions. In contrast, the FC curve of NPs assembled in small clusters, especially in SAM-80, displays a nonsaturation below TB, which corresponds to very low coupling interactions.63,66 Magnetic moments are supposed to be randomly oriented in space, and the magnetic field has to act against the anisotropy energy barrier to align the moments in the field direction. Therefore dipolar interactions between clusters of three or four NPs are significantly weaker than in larger domains of more than 100 NPs. It suggests that collective effects may be observed as soon as a sufficient number of superparamagnetic NPs form 2D domains on a substrate.
’ CONCLUSION In conclusion, SAMs of mixed organic molecules offer unique possibilities to pattern the assembling of NPs. The magnetic properties of NP assemblies have been demonstrated to vary as a function of the patterning of terminal groups at the surface of the SAM. NPs are assembled in domains in which size is controlled by the molar ratio between chelating carboxylic acid and nonchelating methylene terminal groups and their spatial distribution in SAMs. Moreover, NPs have allowed indirect probing of the structures of SAMs in domains at the nanometer scale. The results show that for a low COOH amount, a phase separation is observed, whereas the mixing of molecules is better when the amount of COOH groups increases. The relative miscibility of MUA and MDD molecules induces a modulation of the concentration of COOH groups in domains where NPs are immobilized. Therefore, the strength of interactions between the SAMs and NPs surfaces is modulated and enables the 2D rearrangement of NPs for low amounts of COOH. Dipoledipole interactions between NPs are strongly dependent on their spatial arrangement and decrease significantly with the size of the NPs domains. Finally, these interactions have been observed to be very weak below a critical 2D domain size of about four NPs. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details, TEM and DLS measurement on NPs, contact angle measurements on SAMs, EDX and image analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 0033 (0) 3 88 10 71 33. Fax: 0033 (0) 3 88 10 72 47. 6241
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