Article pubs.acs.org/Langmuir

Tuning Deposition of Magnetic Metallic Nanoparticles from Periodic Pattern to Thin Film Entrainment by Dip Coating Method J. Dugay,*,† R. P. Tan,† A. Loubat,† L.-M. Lacroix,† J. Carrey,*,† P. F. Fazzini,† T. Blon,† A. Mayoral,‡ B. Chaudret,† and M. Respaud† †

Laboratoire de Physique et Chimie des Nano-Objets, Université de Toulouse; INSA, UPS, 135, av. de Rangueil, F-31077 Toulouse, France and ‡ Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, Zaragoza 50018, Spain S Supporting Information *

ABSTRACT: In this work, we report on the self-assembly of bimetallic CoFe carbide magnetic nanoparticles (MNPs) stabilized by a mixture of long chain surfactants. A dedicated setup, coupling dip coating and sputtering chamber, enables control of the self-assembly of MNPs from regular stripe to continuous thin films under inert atmosphere. The effects of experimental parameters, MNP concentration, withdrawal speed, amount, and nature of surfactants, as well as the surface state of the substrates are discussed. Magnetic measurements revealed that the assembled particles were not oxidized, confirming the high potentiality of our approach for the controlled deposition of highly sensitive MNPs.

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INTRODUCTION Self-assembly is a physical concept present in Nature from atomic to large-scale structures of the universe. Such phenomena can be defined as the self-assembly of specific entities in regular patterns or structures, without any human intervention, but in a controlled environment.1 Presently, recent developments in colloidal synthesis permit a fine control of NP morphological characteristics such as size, sizedistribution, aspect ratio, and physicochemical properties. Their self-assembly opens the possibility to tailor new devices.2 For instance, magnetic NPs (MNPs) could be a low cost alternative to the classical nanostructured thin films obtained via lithography techniques, which are used for various applications such as magnetic data storage,3 RF inductors,4 catalysts,5 or spintronics devices.6,7 Another advantage of these MNPs is the presence of organic tunnel barriers (long chain surfactants) surrounding the magnetic core, which is very appealing in spintronics to simplify the device elaboration process. For instance, CoFe carbide MNPs self-assembled into large supercrystals exhibit a variety of magnetoresistance effects, and a large magnetoresistance ratio at low temperature.8 Recently, room temperature tunneling magnetoresistance (TMR), although fairly low, was even reported in assemblies of metallic Fe MNPs.9 Enhancing magnetoresistance ratio, which is a crucial issue for practical applications, would require (i) a fine-tuning of the dipolar interaction strength,10 through the control of the MNPs organization between electrodes11 and (ii) maintenance of the metallic surface state of MNPs, a key parameter to preserve © 2014 American Chemical Society

high spin polarization and prevent magnetic disorder (spin canting effects). From a more fundamental point of view, numerous questions remain concerning the exact influence of the dimensionality and structural disorder of the NP arrangement on transport mechanisms.12,13 Preventing oxidation has been achieved in most cases using a thin protective shell surrounding each MNP, which can be made of silica, alumina, carbon layer, or other metallic material (Au, Pt, etc.). However, this solution is not adapted if one requires a large packing fraction of magnetic material (like for instance in RF inductors) and none of these chemical routes are adapted for spintronics applications. Besides, subsequent removing of the MNP surface oxides before measurements could be used (via high temperature annealing process), but it is important to note that such a process could drastically modify the surrounding organic surfactants, i.e., damage the tunnel barriers of the system.6 Actually, the more simple route to really take advantage of the MNPs produced by chemical routes is to develop simple handling and nanoaddressing methods that preserve the bare properties. One way to integrate the NPs into patterned devices is based on the use of physical and/or chemical templates obtained by various lithography techniques,14−18 microcontact printing,19−22 microinjection molding,23 or a combination of such approaches.24,25 Nevertheless, direct self-assembly of NPs on Received: October 18, 2013 Revised: June 26, 2014 Published: July 7, 2014 9028

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Au) on silicon wafers covered by a 300 nm layer of thermally grown silica (SiO2), then protected by a resina layer and finally divided beforehand into 0.25 cm2 squares. Resina layer was removed from substrates with acetone, ethanol, and deionized water and dried under nitrogen flow. A 15 min UV ozone treatment rendering them hydrophilic was performed prior to their introduction into the glovebox. The surface wettability was characterized by contact angle measurements performed at room temperature with a GBX Digidrop. Each contact angle corresponds to the average value of three measurements on different substrate areas. The modulation of the surface energy of the substrates has been done with oxygen plasma, using a MP450s Plassys system coupled to the glovebox. The working pressure was maintained at 8 mTorr, the RF power was 10 W, and a flow rate of 4 and 20 sccm has been used for argon and oxygen, respectively. Synthesis and Characterization of Bimetallic CoFe Carbide MNPs. Bimetallic CoFe carbide MNPs were prepared according to a modified published procedure.39 Briefly, Co(η3-C8H13)(η4-C8H12) (1 mmol, 278 mg) was mixed with amine (HDA, 1 mmol, 242 mg) and acids (SA, 1 mmol, and OA, 1 mmol) in 10 mL of mesitylene. Reaction was stirred magnetically for 10 min. Fe(CO)5 (2 mmol, 280 μL) was then injected, and 40 mL of mesitylene was added. The reaction was pressurized under 3 bar of dihydrogen and let to react at 150 C for 48 h. To remove the excess of surfactants and other molecular byproducts, the washing process was done three times and magnetic separation in mesitylene was used. The NP solution was then evaporated under vacuum to get a dried powder. The composition of this powder was determined by microanalysis using the inductively coupled plasma mass spectrometry (ICPMS), and was typically in the range of %Fe (20%), %Co (80%), and %metal (89%) corresponding respectively to the atomic percentage of iron, cobalt, and the total metallic fraction, respectively. A droplet of colloidal suspension was deposited inside a glovebox on carbon coated copper grids. The latter were then characterized by TEM (transmission electron microscopy), HRTEM (high-resolution TEM), STEM-HAADF (scanning transmission electron microscopy using a high angle annular dark field detector), and EELS (electron energy loss spectroscopy). XRD (X-ray diffraction) and Mössbauer spectroscopy were carried out on a powder of CoFe carbide MNPs that were prepared and sealed in a glovebox under an argon atmosphere. TEM image (see Supporting Information (SI) Figure S1) of the MNPs was recorded with a JEOL-JEM 1011F, operating at 100 kV. The size distribution was calculated from size measurements on more than 2000 NPs using ImageJ software.40 STEM-HAADF observations (see SI Figure S2(a,b) and EELS analysis (c)) were carried out at the advanced microscopy laboratory (LMA) in Zaragoza, using a Cs corrected Titan Microscope equipped with an XFEG source. XRD measurements (see SI Figure S2(d)) were performed on a PANalytical Empyrean diffractometer using Co−Kα radiation at 45 kV and 40 mA. The iron state and its environment (see SI Figure S3) were analyzed by Mössbauer spectroscopy (WISSEL, 57Co source). Deposit of MNPs. Every step was performed in a coupled glovebox-sputtering system under argon atmosphere to prevent NPs from oxidation. The MNPs were redispersed in tetrahydrofuran (THF), which is a suitable solvent for dip coating deposit thanks to its low boiling point41 (66 °C42) and surface tension (28.10−3 N·m42).43 Colloidal solutions with the desired concentration were obtained by dispersing the MNP powder diluted in a fixed volume of solvent (V = 2 mL) under ultrasonic bath (15 min, 40 °C). The appropriate powder mass required to reach a specific concentration [X] was estimated using the following expression:

solid substrates directly from colloidal solution is highly desirable. To date, several strategies have been developed, such as drop-casting of NPs suspension on solid substrates26,27 or at the polar/non polar interphase,28,29 Langmuir−Blodgett method,30,31 convective-self-assembly32,33 or spin coating.34 In spite of extensive efforts, the direct self-assembly of metallic MNPs on large areas remains a challenge to overcome. In particular, controlled assemblies varying from monolayers to stripes with tunable spacing, thickness, and organization would be highly profitable to further understand transport mechanisms in such structures and the effect of magnetic interactions. In this aim, different challenges should be overcome. First, polar organic subphase deposition should be avoided to prevent MNPs from oxidation. Moreover, only strictly controlled inert atmosphere should be used. Glovebox is a solution of choice for such conditions, but space limitation and handling difficulties arise. Second, homogeneous and stable colloidal solutions of MNPs are needed with controlled size and shape, and welldefined surfactant concentration. In recent years, numerous methods have been used to produce highly monodisperse MNPs.35 Reduction in organic solvent36 or hot injection process both lead to surfactant coated MNPs.37 These surfactants are usually present in large excess to ensure the MNP stabilization in non polar solvents. It is important to note that during a deposit on a substrate, these uncoordinated surfactants interact with the substrate and affect positively (or not) MNPs/substrate interactions, and thus the morphology of the deposits.38 Besides, surfactants also affect other suspension properties, such as evaporation rate, surface tension, and colloidal/self-assembly behavior. Since the exact composition of the solution (amount and nature of surfactants) varies slightly from one synthesis batch to the other, the detailed study of the influence of deposit parameters must be performed on the very same MNP batch. Here we report on the self-assembly of MNPs synthesized by an organometallic approach, stabilized by both oleic/stearic acids and hexadecylamine. We chose bimetallic CoFe carbide MNPs, a material potentially interesting for RF inductors4 and spintronics.8 For this purpose, we used a homemade dip coating setup placed inside a coupled glovebox-sputtering system. Depositing under inert atmosphere prevents MNP oxidation and enables one to tune the interfacial energy of the substrates by oxygen plasma treatment. We performed a systematic study of the dip coating deposition as a function of MNP concentration, withdrawal speed, nature, and amount of surfactants in suspension and substrate surface state. This study was performed on the same batch of MNPs to prevent any artifacts. We clearly evidenced that self-assembly can be controlled from stripes to thin film while preserving MNPs from oxidation.

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MATERIALS AND METHODS

Chemicals and Materials. All syntheses were prepared and purified under argon or dihydrogen using Fischer−Porter bottle techniques, a glovebox, and argon/vacuum lines. Mesitylene (99%), tetrahydrofuran (THF, 99%), were purchased from VWR Prolabo, and distilled and degassed through three freeze−pump−thaw cycles. Hexadecylamine (HDA, 99%), oleic acid (OA, 99%) and stearic acid (SA, 99%), were purchased from Sigma-Aldrich, iron(0)pentacarbonyl (Fe(CO)5, 99,5%) from Acros organics and cyclooctadiene-cyclooctenyl (Co(η3-C8H13)(η4-C8H12), 99%) from Nanomeps and were used as received. Fabrication and Preparation of the Substrates. Standard photolithography was used to make gold electrodes (5 nm Ti/30 nm

m = c·V ·(MFe ·%Fe + MCo·%Co)/%metal where MFe (55.8 g·mol−1) and MCo (58.9 g·mol−1) are the atomic weights of the iron and the cobalt, respectively. %Fe (20%), %Co (80%) and %metal (89%) correspond respectively to the atomic percentage of 9029

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iron, of cobalt and to the total metallic fraction determined by ICPMS (averaged on more than ten syntheses). Dip coating experiments were performed using the homemade setup shown in Figure 1. The substrate was maintained with a small

signature of the shell was detected, probably due to the small crystallite size. The shell was further characterized by Mössbauer spectroscopy, this technique being sensitive to Fe environment (see SI Figure S3). The broad sextet observed is the signature of a complex ferromagnetic Fe1−xCoxCy phase, confirming the presence of carbides. No trace of iron oxide was detected. The ferromagnetic character of these MNPs is confirmed by SQUID measurements on powder. The low saturation magnetization (MS = 140 A·m2·kg−1), well below the CoFe bulk value (MS bulk = 240 A·m2·kg−1) is directly related to the presence of carbon inside the MNPs structure, in agreement with Mössbauer spectroscopy, and/or a highly probable carbon monoxide poisoning of the surfaces.45 The symmetric hysteresis loops observed at 2 K after ZFC and FC exclude the presence of exchange phenomena (see SI Figure S4), confirming the absence of oxide shell. Weak coercive field amplitude is observed up to 200 K (see SI Figure S4), while a superparamagnetic behavior is observed at 300 K for solid samples (see SI Figure S5). Even if strong dipolar interactions in powder form are known to drastically lower the coercivity,46 it is delicate to deduce magnetic properties of NPs in colloidal solution from measurements in powder. During the deposit, the most likely hypothesis is that small MNPs are superparamagnetic, while large ones are not. The effect of MNP concentration and withdrawal speed of the substrate on the controlled deposition was studied (see SI S6−S11). A trade-off between the thickness, the MNPs coverage and the solution stability was found for a concentration of [10 mmol·L−1] and a withdrawal speed of 13 μm·s−1 (kept constant for the subsequent experiments). Stripes with a thickness of two layers on a width of ca. 10 ± 5 μm were observed with an MNPs coverage of ca. 65%. The stripes were separated by 45 ± 15 μm where a monolayer deposit with a 90% coverage was observed. In comparison, widths of a few micrometers (1.8−8.2), thicknesses of mono- to multilayers (18−70 nm), and spacings of few tens of micrometers were obtained for non magnetic NPs.33 However, microcontact printing already permitted to scale down stripes width to single chain of 67 nm sized nanoparticles.21 However, unlike these two previous studies, the method developed here is lithography-free, in one step, and can be easily performed into a glovebox to prevent MNPs from oxidation. Surfactants Concentration. The impact of surfactant excess was systematically studied through the controlled addition of amine (hexadecylamine, HDA) or acid (oleic acid, OA). The chemical affinity of surfactants with silica substrates or gold surfaces were investigated by immersing substrates separately into [10 mmol·L−1] THF solution of OA or HDA (see SI Figure S12). While a reference sample in THF reveals an homogeneous contrast (see SI Figure S12(c)), the presence of OA surfactants (liquid at room temperature) leaded to multiple droplets randomly distributed on both silica and gold surfaces (see SI Figure S12 (a)). Contrastingly, HDA surfactants (solid at room temperature) were solely observed on and at the vicinity of gold electrodes (see SI Figure S12 (b)). If acid and amine can form self-assembled monolayers on gold surfaces in low-polar solvents,47,48 then acid is more prone to bind on silica due to its affinity with both silanol (SiOH) and siloxane (SiOSi) pending groups. Then, the influence of surfactant concentration on the MNP deposit at optimized speed and concentration was studied (speed 13 μm·s−1,

Figure 1. Schematic view of (a) the homemade dip coating setup, (b) the mounting of the sample, and (c) the MNP accumulation and flow pattern during the removal of the substrate. amount of vacuum grease on a coverslip. Such configuration enables easy handling and prevents any deposit on the substrate backside, which could alter the magnetic measurements performed on the final device. The coverslip was maintained vertically using a self-grip tweezer. The first step consists in completely immersing the substrate in a 1 mL teflon beaker filled beforehand with the suspension of MNPs. The substrate was then withdrawn vertically at a constant speed by action of a stepper motor at a speed ranging from 13 μm·s−1 to 660 μm·s−1. Protecting Air-Sensitive MNPs against Oxidation. A solution of conventional Shipley SU8 photoresist diluted in Shipley EC solvent was spin coated (5500 rpm for 30 s) at the top of the MNPs deposit. An optimized dilution was achieved to reach an ultrathin resina layer of ∼40 nm thick. Subsequently, several short (1 min) annealing steps (90°, 130°, 170°, and 250°) allow to adjust the mechanical properties of the resina layer. Characterization of Assemblies. Deposit patterns formed by the MNPs were imaged by optical microscope (Olympus BXFM) equipped with a CCD camera (Olympus DP20) integrated in the glovebox. Field emission gun scanning electron microscopy (SEMFEG Hitachi S-4800) was performed at 30 kV. Topographical imaging of films was performed in tapping mode at room temperature with a Digital Instruments/Veeco Dimension 3100 Atomic Force Microscope using Si tips (frequency range 240 to 380 kHz with stiffness of 42 N· m−1). AFM image processing and rendering was analyzed with WsXM data analysis software.44 Magnetic properties of assemblies were investigated by using a superconducting quantum interference device (SQUID) magnetometer MPMS XL SQUID (7 T). Magnetization curves were recorded at 2 K first after a zero field cooling (ZFC) and second after a field cooling (FC) from 300 K down to 2 K under 5 T. The substrates containing the assemblies of MNPs were placed parallel to the direction of the applied magnetic field.

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RESULTS AND DISCUSSIONS Bimetallic CoFe carbide MNPs exhibit a bimodal size distribution with peaks at 25.4 ± 9.7 nm and 14 ± 4 nm. MNPs are separated by organic surfactants with a thickness around 2.3 ± 1 nm (see SI Figure S1). STEM-HAADF images highlight the polycrystalline morphology of the CoFe carbide MNPs (see SI Figure S2(a,b)) while STEM-EELS analysis evidence a core-shell structure where the Co rich core is surrounded by a thin Fe rich shell (see orange color in SI Figure S2(c)). XRD patterns evidence characteristics peaks related to ε cobalt, indicative of the core carburization, while no 9030

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Figure 2. Optical and SEM images of the deposits obtained from a [10 mmol·L−1] CoFe carbide colloidal solution at the optimized speed (v = 13 μm·s−1) as a function of the surfactants added: from left to right in the top panel 0, 0.1, and 1 equiv of OA and in the bottom panel 0.05, 0.1, and 1 equiv of HDA. All SEM images are located on silica and endowed, in inset, with magnified view.

Figure 3. Optical images of deposits obtained on substrates prior treated for the following: (from left to right) 2, 5, and 20 min with oxygen plasma. Other conditions are v = 13 μm·s−1 and [ 10 mmol·L−1]. On the bottom, high resolution SEM images taken on silica for the longest plasma treatment are shown.

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concentration 10 mmol·L−1). Controlled concentration of surfactants (varying from 0 to [10 mmol·L−1], referred to as 1 equiv per mole of metal) were added to the colloidal solution and sonicated for 15 min. Optical images and SEM revealed a progressive modification of the deposits (see Figure 2). MNP coverage decreased in the presence of OA surfactants, leading to 3D isolated islands and dark regions (evidenced by a black contrast in SEM images in SI Figure S12(a)) arisen both on Au and SiO2 surface. The same behavior was observed in the presence of HDA surfactants, but only on, and at the vicinity of Au electrodes. Variously, MNP coverage on SiO2 increases with the presence of HDA surfactants (see Figure 2 and SI Figure S13). Excess surfactants, which exhibit an affinity for the surface, form a shear barrier and prevent MNPs interaction with the surface, decreasing the MNP coverage.38,49,50 No steric hindrance was encountered for HDA surfactants on SiO2, due to its low affinity with the surface, increasing the MNP coverage.26,51 We highlighted in this paragraph the huge impact of subtle concentration variations as well as the nature of surfactants on the morphology of the MNP deposit. We observed an important variability of the deposit morphology obtained from different batch of synthesis, which could thus be due to small variations from batch to batch in the amount of excess ligands. Finally, we significantly improved MNP coverage using HDA addition, which is particularly important within the scope of transport measurements of MNPs, requiring large coverage values. Surface Energy of the Substrates. The affinity of excess surfactant with the surface playing a key role in the deposit of MNPs, we tailored the surface energy of the substrates. UV/ ozone cleaned surfaces were used in previous experiments. However, their storage in glovebox led to a strong evolution of their surface state with time (see SI Figure S14), probably due to the physisorption of organic molecules or contaminants. To address the role of surface energy and solvent wettability we performed oxygen plasma treatment of different time just before the deposition process (performed under optimized conditions: speed 13 μm·s−1, concentration 10 mmol·L−1). Optical micrographs (see Figure 3) revealed less visible stripes for long plasma treatments. It suggests that the deposition mechanism is close to the desirable thin film entrainment regime. SEM images confirmed that MNP coverage significantly increased compared to untreated substrates (see SI Figure S7(b)) for comparison). Oxygen plasma treatment enables us to tune the contact angle between the surface and the MNP colloidal solution. The longer the oxygen plasma, the lower the contact angle, leading to an increase of the average spacing between two successive stripes as well as their width (see Figure 4). A smaller contact angle (for a fixed withdrawal speed) is known to prevent the diffusion of NPs toward to the triple contact line, and provides a convective self-assembly with wider stripes and leads to larger interstripe spacing.52 Contrastingly, increasing contact angle progressively forms incomplete layers with defects until prevent MNPs from being deposited on the substrates when the contact angle reaches around 20°.53 With the aim to improve the reproducibility and the coverage of the MNPs, it is crucial to carefully clean the substrate surface just before MNP deposition. The enhancement of the MNP coverage observed in our work could be attributed to (i) the removing of physisorbed molecules which, as previously discussed, decrease the interaction energy between the MNPs

Figure 4. Evolution of the spacing (black circles) and the width (red triangles) of consecutive stripes as a function of the oxygen plasma treatment time. The first point (t = 0) is the reference for this study. It corresponds to an untreated substrate stored for at least 1 day in the glovebox. Error bars correspond to the statistical dispersion of measurements performed on all the substrates.

and the substrate,38 and/or (ii) a lower contact angle, which permits a higher evaporation rate and a homogeneous thickness of the deposit.54 Noteworthily, because the MNPs size distribution is bimodal, one could expect a size-segregation during their deposition. Indeed, it is known that the smaller the NPs, the higher their mobility further away into the meniscus edges during stripes formation.54 Moreover, such size-segregation could also occurs by minimization of mesoscopic van der Waals interaction.55 However, to the best of our knowledge, such phenomena have been evidenced only for bidispersed micro- and NPs56 and highly bidispersed NPs with a size ratio of ca. 10.57 No evidence of NPs size-segregation was experimentally observed with our bidisperse MNP system, which has a size ratio of 1.5. Lastly, the intrinsic magnetic character of the MNPs used in this work could also be an important factor guiding the selfassembly through magnetostatic dipole−dipole interactions (U). In particular, it has been shown that, for U > 8 kBT, characteristic and noticeable self-organizations of MNPs are observed (rings, chains, etc.58). In our case, one can estimate the dipolar energy between two MNPs i and j using U ≈ (μiμj)/ σ3i,j where μi = MsVi is the magnetization carried by a MNP i of volume Vi and σi,j the center-to-center distance between the MNPs i and j. Due to the bidisperse character of the MNPs, we obtain U ≈ 124 and 316 kBT for the small and the larges MNPs, respectively. Despite these large values (U ≫ kBT), we never clearly observed the characteristic structures induced by dipolar interactions.58 In fact, many theoretical works demonstrated that the probability of forming MNPs chains is strongly reduced in bimodal systems compared to monodisperse ones.59−61 This can be qualitatively explained by the presence of the small MNPs which (i) lowered the effective dipolar interactions seen by the larges MNPs, and (ii) increase the breakup probability of long chains through their Brownian motion; both phenomena leading to only 2−3 MNPs aggregation. Interestingly, we used the method developed in the present work to deposit MNPs displaying monodisperse size distribution. These results will be published elsewhere. Contrastingly, these MNPs exhibited specific arrangements, which could be attributed to anisotropic magnetic dipolar interactions.62 Such results reinforce our assumption that, in the present CoFe carbide MNPs, magnetic interactions do not play a role in the self-assembly process. Finally, magnetic measurements have been performed on a deposit of CoFe carbide MNPs using the optimized conditions described above (speed 13 μm·s−1, concentration 10 mmol·L−1 9032

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synthesized suspensions, we have demonstrated that lower MNP coverage were obtained in the presence of strongly interacting surfactants with the surface. Indeed, physisorbed surfactants act as shearing barrier toward MNP adhesion. Contrastingly, improved coverage values were obtained when surfactants do not exhibit any preferential interaction. A 20 min plasma treatment of the samples prior to dip coating yield the highest MNP coverage, the widest and closest stripes, suggesting that the deposition mechanism is close to the desirable thin film entrainment regime. Finally, we demonstrated a successfully approach for the controlled deposition of highly sensitive MNPs.

on a silicon substrate with native thin oxide cleaned by plasma oxygen during 20 min). To preserve the magnetic properties of the MNPs, but also their geometrical arrangement after dip coating, a thin resina layer (∼40 nm thick) has been deposited (see Materials and Methods for experimental details) on the substrate inside the glovebox. Figure 5 presents a cross section TEM picture of the sample used for the magnetic measurements.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental results are available. Structural analysis ((S1) TEM, size distribution, (S2) STEM-HAADF, STEMEELS, powder XRD patterns, and (S3) Mössbauer of CoFe carbide MNPs). (S4) Magnetic properties of a powder of CoFe carbide MNPs measured at different temperature without magnetic field and under 5 T at 2 K. (S5) Magnetic properties of a powder of CoFe carbide MNPs measured at 2 and 300 K. Study of MNPs concentration and withdrawal speed of the substrate including respectively optical (S6.a and S9.a left) and SEM (S7 and S9.a right) characterizations taken after the deposition at different concentration. (S8 and S10) AFM measurements performed at different MNP concentrations and withdrawal speed. (S12) Optical images taken after the substrates were immersed in suspensions of OA and HDA dissolved in THF as well as pure THF. (S13) High resolution SEM image located between two consecutive stripes when a large excess of HDA (1 equiv) was added to the suspension. (S14) Evolution against the time of the contact angle of cleaned substrates stored in glovebox. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. At the top, a cross section TEM picture of the sample used for magnetic measurements. On the bottom: in-plane hysteresis loops of the sample measured at 2 K after zero-field cooling (ZFC) (black squares) and after field-cooling (FC) under 5T (red circles). The number of nanoparticles deposited on the substrate cannot be estimated accurately. Thus, the saturation magnetization cannot be compared to the magnetization data performed on powder.

From our experience, a partial surface oxidation of a few layers on one side of the MNP is enough to induce an exchange bias. Upon increasing surface oxidation, exchange phenomena on the field cooling (FC) curves manifests through (i) an increase of the coercive field for the FC curve as compare to the zero field cooling (ZFC) one, without any shift of the hysteresis loops or (ii) an increase of the coercive field for the FC curve as compare to the ZFC one, with a shift of the hysteresis loops. Interestingly, as can be seen in Figure 5, the hysteresis loops measured at 2 K after FC under 5 T did not exhibit any exchange bias features. Moreover, the coercive field of the FC curve is comparable to the ZFC one. From those magnetization measurements, we claim that we prevented significant oxidation during the whole process of MNPs deposition, i.e., the magnetic properties of the MNPs are preserved.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the CNRS LAAS member of french RENATECH network and by the French ANR agency under Contract No. ANR-09-BLAN-0197- 01 “Chemispin”. The authors thank the European Associated Laboratory TALEM for financial support. The authors gratefully acknowledge C. Nayral and F. Delpech for fruitful discussions.

OUTLOOK In conclusion, we have systematically investigated the influence of MNP concentration, withdrawal speed, amount, and nature of surfactants, as well as the surface state of the substrates on the deposit morphology obtained by dip coating technique. We performed deposition of bimetallic CoFe MNPs on hydrophilic surfaces under inert atmosphere, thanks to an homemade setup designed in a coupled glovebox-sputtering system, which successfully prevents significant oxidation of the deposit, as confirmed by the magnetization measurements. Film thickness can be controlled by either MNP concentration or withdrawal speed. Deposition of single monolayers could be reached for a concentration of [10 mmol·L−1] and a withdrawal speed v = 13 μm·s−1. On the contrary, very thick periodic stripes are formed either at high MNP concentration or without any withdrawal force, corresponding to the natural evaporation of the MNP suspension. By adding surfactants containing amino and acid groups in two distinct freshly

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REFERENCES

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