Optimization of Magnetic Inks Made of L10-Ordered FePt

Article pubs.acs.org/Langmuir

Optimization of Magnetic Inks Made of L10‑Ordered FePt Nanoparticles and Polystyrene-block-Poly(ethylene oxide) Copolymers Brice Basly,† Thomas Alnasser,† Karim Aissou,‡ Guillaume Fleury,‡ Gilles Pecastaings,‡ Georges Hadziioannou,‡ Etienne Duguet,† Graziella Goglio,*,† and Stéphane Mornet*,† †

CNRS, Université Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France Université Bordeaux, CNRS, LCPO, UMR 5629, F-33600 Pessac, France

‡

S Supporting Information *

ABSTRACT: The preparation of magnetic inks stable over time made of L10-ordered FePt nanoparticles, thiol-ended poly(ethylene glycol) methyl ether (mPEO-SH) compatibilizing macromolecules and asymmetric polystyrene-block-poly(ethylene oxide) copolymers (BCP) as a subsequent selforganizing medium was optimized. It was demonstrated that the use of sacrificial MgO shells as physical barriers during the annealing stage for getting the L10-ordered state makes easier and more efficient the anchoring of compatibilizing PEO macromolecules onto the nanoparticles surface. L10-FePt grafted nanoparticles have shown a good colloidal stability and affinity with the PEO domains of the BCP leading to L10-FePt/BCP composite thin layers with individual magnetic dots dispersed in the BCP matrix.

1. INTRODUCTION For several years now, L10-ordered FePt nanoparticles (NPs), which crystallize in a face-centered tetragonal (fct) structure, have been identified as potential powerful magnetic bits for high-density magnetic storage have been identified as potential powerful magnetic bits for high-density magnetic storage (>1 Tbit/in2).1 Indeed, they combine several advantages: chemical stability,2 large uniaxial magnetocrystalline anisotropy (Ku ∼ 7 × 106 J m−3),3 which induces the magnetic stability of nanoparticles down to 2.8 nm at ambient temperature.4 Nevertheless, L10-ordered FePt NPs are not spontaneously obtained by the conventional synthesis route based on the decomposition of organometallic precursors at ∼300 °C. The structure of the as-obtained metastable NPs is face-centered cubic (fcc), and an annealing treatment at high temperature (>550 °C) is required to transform them into the L10-ordered state. Unfortunately, this treatment also initiates sintering processes leading to the broadening of the NPs size distribution. For preventing this sintering phenomenon, it was shown that the primary fcc-FePt NPs may be dispersed in thermally stable matrix such as SiO2,5,6 MnO,7 NaCl,8−10 or MgO,11,12 which serve as sacrificial physical barrier prior to their removal by chemical etching. Besides the fact that ultrahigh bit density media require magnetic bits as small and identical as possible, they shall be also arranged in a regular 2-D array minimizing the distance separating each other. For that purpose, block copolymers © XXXX American Chemical Society

(BCPs) have become of great interest due to their facility and high throughput for self-organization and therefore highresolution patterning.13 Recent studies have found that building materials by NPs assembly assisted by BCPs bring new opportunity to improve mechanical strength, catalytic activity, electromagnetic, optical, or electrical properties, etc.14 In order to stabilize colloidal dispersions while promoting favorable interactions with the polymer block expected to accommodate the NPs, their surface shall be modified by grafting macromolecular compatibilizer whose anchoring function, chemical composition, length and grafting density are critical.15 We recently reported that L10-ordered FePt NPs, obtained using a NaCl matrix as the annealing separating medium and blended with asymmetric polystyrene-block-poly(ethylene oxide) copolymers (PS-b-PEO), led to thin films with spherical PEO domains hosting the magnetic NPs at the condition they were previously surface-modified with short dopamine-terminated-methoxy poly(ethylene oxide) ligands.10 The nanocomposite films were prepared by spin-coating magnetic inks made of the FePt NPs dispersed in a THF solution of copolymers. Nevertheless, even if the annealing in NaCl matrix offers a solution for the γ-L10 FePt transition, this process leads to the formation of polydisperse-sized distribution of NPs. Received: March 16, 2015 Revised: May 29, 2015

A

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

solution of HCl 1M. Twenty milligrams of L10 FePt/MgO NPs was added, and the dispersion was stirred gently for 12 h for fully dissolve the MgO shell. For completing the reaction between thiol groups and surface Pt atoms in more favorable conditions, the pH of the dispersion was adjusted to a value of 6.5−7, and another 3 mg of PEO macromolecules dissolved in 10 mL of water was added. The NPs washing was performed by repeating twice a centrifugation at 14 000 rpm for 1 h with ultrapure water. The final concentration was adjusted to 1 mg·mL−1 (i.e., 7 × 1014 NPs·mL−1). At the end of this step, about 80% of the L10-FePt NPs were recovered due to the partial dissolution of the surface iron atoms. Then the previous L10-FePt@PEO NPs dispersions were transferred from water to THF containing the desired amount of PS-b- PEO and allowed to dissolve under gentle stirring at room temperature. After 48 h, the magnetic inks were filtered through a 0.2 μm pore size PTFE membrane. The grafting density of mPEOthiol macromolecules was assessed by Ellman’s assay.16 A volume of 10 μL of 0.5 M of NaBH4 solution was added to 200 μL of FePt NP dispersion at a concentration of 1 mg/mL. After 10 min of incubation, the nanoparticles were discarded by centrifugation. The excess of reducer was destroyed by adding 10 μL of 0.5 M HCl. A volume of 10 μL of 0.1 M DTNB was added and the solution completed to 1 mL with 0.1 M borate buffer at pH 8. TNB absorption was measured at 412 nm, and the assessment of mPEO2k-thiol molecules in the solution was deduced by implementing a calibration curve. A control without NPs was performed in order to check the reducer inhibition, and the measures were repeated 5 times for each sample. Nanocomposite Film Formation and Solvent Annealing. Nanocomposite films were prepared by spin-coating inks onto Si (100) substrates (Si-Mat Silicon Materials) with a native oxide surface. The film thickness was controlled by varying the spin-coating speed from 2000 to 5000 rpm. The solvent annealing in either toluene or toluene/water saturated vapor atmospheres was performed in a desiccator with annealing times of 2 h.17 The solvent annealing process in toluene was realized in the desiccator with an initial relative humidity (RH) ∼ 40%. The temperature was controlled by placing the desiccator in an oven set at 30 °C. Characterization. Elemental analyses were performed by inductively coupled plasma optical emission spectrometry (ICP/OES 720ES Varian). The powder (5 mg) was dissolved in aqua regia solution (5 mL) and ultrapure water was added to get 100 mL of solution. The hydrodynamic diameter of the NPs was evaluated by DLS (Cordouan Technologies, sizer VASCO DL135). The infrared spectra were performed by using the DRIFT technique. The dried NPs were first blended with anhydrous KBr (3 wt % of FePt) (Aldrich, FTIR grade). The mixture was then placed in an analysis chamber kept under a dry air stream, in the presence of potash to remove any trace of water and CO2. SFM (Bruker Dimension Icon) was used in tapping mode to characterize the surface morphology of the nanocomposite thin films. Silicon cantilevers (Nanosensors, Inc.) with a tip radius of around 2 nm and a resonant frequency of 260 kHz were used. A TEM (Hitashi H7650) operating at 80 kV was used to image the NPs and the nanocomposite films. The pictures of the NPs were obtained by depositing the cyclohexane or the water dispersion of the NPs on amorphous carbon-coated copper grids. The bilayer composite thin films were spin-coated onto Si (100) substrates previously coated with a commercial PEDOT:PSS layer having a thickness of about 50 nm. After annealing under a toluene/water vapor saturated atmosphere, TEM samples were prepared by removing the composite films from the silicon substrates with an aqueous solution in order to dissolve the PEDOT:PSS layer prior to their transfer onto carbon grids.

Recently, the use of MgO as sacrificial physical barrier for preventing sintering during the annealing stage has proved to be effective in maintaining the size of the nanoparticles over the thermal transition.11,12 In the present work, we report a simple and effective method to formulate magnetic inks from FePt NPs transited to L10 phase by the use of sacrificial MgO shell, with compatibilizing macromolecules, i.e., poly(ethylene glycol) derivatives, in the aim to study the formation of patterned (PS-b-PEO) copolymer thin films blended with these magnetic NPs. We support the discussion, among others, by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), dynamic light scattering (DLS), and transmission electron microscopy (TEM) characterizations. Lastly, we highlight the stability behavior of the resulting magnetic colloidal dispersions and we study the hierarchical self-assembly of these L10-ordered FePt NPs within the ordered PS-b-PEO thin films using surface force microscopy (SFM) and transmission electron microscopy (TEM).

2. EXPERIMENTAL SECTION Materials. Iron pentacarbonyl (Fe(CO)5, Aldrich, 99.99%), platinum(II) acetylacetonate (Pt(acac)2, Aldrich, 99.99%), 1,2hexadecanediol (Aldrich, 90%), oleic acid (OA, Aldrich, 90%), oleylamine (OAm, Aldrich, 70%), dioctylether (Aldrich, 99%), cyclohexane (Aldrich 99.9%), ethanol (Scharlau, 99.9%), Mg(acac)2 (Strem Chemicals, 98%), 1,2-tetradecanediol (Aldrich, 90%), benzyl ether (Aldrich, 99%), methanol (Aldrich, 99.8%), toluene (Aldrich, 99%), tetrahydrofuran (THF, Aldrich, 99.9%), thiol-ended poly(ethylene glycol) methyl ether (mPEO1k-SH and mPEO2k-SH, Aldrich, Mn = 1000 or 2000 g·mol−1, > 95%) and PS-b-PEO (Polymer Source Inc., Mw(PS) = 32 kg mol−1, Mw(PEO) = 11 kg mol−1, ϕ PEO =0.24 et Mw/Mn =1.06) were used without further purification. Synthesis of γ-FePt NPs. The synthesis was inspired from a previously reported recipe.1,2 Typically, Pt(acac)2 (0.5 mmol) and 1,2hexadecanediol (1.5 mmol) were mixed in 20 mL of dioctyl ether. The solution was then degassed under argon flow (30 min) and maintained under inert atmosphere (Ar) during any further reaction. After the complete dispersion of reactants, the solution was heated at 100 °C. Then OA (2 mmol), OAm (2 mmol) and Fe(CO)5 (1.12 mmol) were added. The mixture was heated to reflux for 30 min at 300 °C and then cooled down to room temperature. Afterward, the black dispersion of γ-FePt NPs was precipitated by addition of 40 mL of ethanol, centrifuged at 10,000 rpm for 15 min, and then redispersed in 20 mL of cyclohexane. This washing cycle was repeated 3 times. Finally, the as-obtained γ-FePt NPs (145 ± 5 mg) were dispersed in 6 mL of cyclohexane. The synthesis of the γ-FePt NPs involves a total reaction of the Pt(acac)2 acting as limiting reactant. A 10% excess of mass determined by TGA corresponds to organic ligands Encapsulation of γ-FePt NPs in MgO Shells and Annealing Stage toward the L10-Ordered FePt Phase. The protocol was adapted from a previously reported recipe,11,12 Typically, Mg(acac)2 (4 mmol), 1,2-tetradecanediol (8 mmol), OA (8 mmol) and OAm (8 mmol) were mixed in 40 mL of benzyl ether at 80 °C and maintained under inert atmosphere until complete dissolution (about 30 min). The 6 mL of γ-FePt cyclohexane dispersion were quickly added and the temperature was increased up to 120 °C for removing cyclohexane (2 h). Then the mixture was heated to reflux at 280 °C for 1 h prior to be cooled down to room temperature. The γ-FePt/MgO NPs were washed 3 times by centrifugation (14 000 rpm for 30 min) after addition of 6 mL of cyclohexane, 40 mL of ethanol and 20 mL of methanol. At this step 82% of Mg(acac)2 reacted. The γ-FePt/MgO NPs were dried and annealed 6 h at 700 °C under Ar/H2 95/5 mixture. Dissolution of MgO Shell and Stabilization of Magnetic Inks by Grafting of Compatibilizing PEO Macromolecules. mPEO1kSH or mPEO2k-SH (0.02 mmol) were dissolved in 5 mL of an aqueous

3. RESULTS AND DISCUSSION As already reported, γ-FePt NPs were synthesized according to the efficient thermal decomposition route initially described by Sun and co-workers.1,2 Metal precursors, oleic acid (OA) and oleylamine (OAm) proportions were adjusted in order to get NPs surface-stabilized by OA/OAm with an optimal Fe:Pt of 50:50 (49:51 as confirmed by elemental analysis) and an average diameter of 6 ± 0.6 nm as determined by TEM analysis B

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

treatment, the positions of Fe2p orbital peaks slightly decrease by 2−3 eV, confirming the disappearance of partially oxidized iron atoms, which can be considered as surface atoms. As for Pt4f peaks, they are shifted toward higher energy levels corresponding to a partial oxidation of platinum atoms which would have lost a part of near neighbors (i.e., Fe atoms). This result justify the use of only one anchoring function, the thiol function which have a strong affinity with platinum atoms, and not carboxylated ligands often used to complex iron surface sites of nanoparticles. This result undoubtedly simplifies the procedure of particle transfer from the acidic medium, which contains Mg2+ cations produced from the dissolution of MgO shell and which could interfere with carboxylic acid functions, to the organic solvent used for the formulation with BCPs. Ellman’s assay performed on FePt-PEO2k gave a surface grafting density of around 2.6 ± 0.6 molecules·nm−2, a value quite similar to those found in literature for PEO-thiol macromolecules of 5000 g/mol grafted on gold NP, i.e., 2.86 molecules·nm−2.18 Figure 2 shows that the IR spectrum of L10-FePt@PEO2k NPs exhibits a set of vibrational modes of the ethylene oxide

and a hydrodynamic radius of about 9 nm by DLS in cyclohexane (see Supporting Information (SI), Figure S1). For preventing the sintering of γ-FePt NPs during the annealing stage, mandatory to perform the γ to L10 phase transition, the use of MgO shells as physical barriers combines several advantages: a facile synthesis by direct thermal decomposition of Mg(acac)2 on the γ-FePt seeds, a thermal stability up to 2000 °C, and a soft removal procedure in acidic conditions.11,12 The amount of Mg(acac)2 precursor was varied from 1 to 8 mmol in order to optimize the thickness of the MgO shell (Figure 1) demonstrating that the value of 4 mmol

Figure 1. TEM pictures of γ-FePt@MgO NPs prepared with different amounts of Mg(acac)2: 1 mmol (a), 2 mmol (b), 4 mmol (c), and 8 mmol (d) for 145 mg of 8 nm γ-FePt NPs. Scale bars: 50 nm.

Figure 2. Infrared spectrum of L10-FePt@PEO2k (in red) compared to the spectra of separate constituents.

is sufficient to create a continuous shell making the magnetic cores at a minimal distance of 10 nm from each other (Figure 1c). This value was retained for the next experiments. The thermal annealing of the γ-FePt@MgO NPs was performed at 700 °C and led to L10-ordered FePt@MgO NPs as confirmed by X-ray diffraction whose patterns showed the expected substructure peaks (see SI Figure S2). As described by Kim and co-workers, the MgO shell can be removed by HCl etching.11 Initially developed for optimizing a final stable dispersion in hexane, the original recipe was using with a complex mixture of aqueous HCl solution, hexane, oleic acid, and hexadecanethiol for managing simultaneously the MgO etching and nanoparticle phase transfer to hexane.12 Here, the procedure was adapted to the direct grafting of the compatibilizing PEO macromolecules over the acidic dissolution of the sacrificial shell in the aim to stabilize the colloidal dispersion during this process, followed by a maturation stage in less acidic conditions. Elemental chemical analysis of the obtained L10-FePt indicates that the removal of MgO in an acid medium leads to a decrease the (Fe:Pt) ratio of the NPs from a value of (49:51) to (45:55). This small change in composition is due to a partial dissolution of iron atoms. This tendency is confirmed by XPS observations (see SI Figure S3). Indeed after the acidic

unit (ν(CH2) and δ(CH2), ν(C−O) and δ(C−O)) that confirms the presence of PEO macromolecules near the NPs surface. Added to this, the disappearance of the weak absorption band in the 2550−2600 cm−1 region characteristic of the S−H bond one more time corroborates the efficiency of the grafting and washing stages. Moreover in this spectrum, the large band in the region of 500−700 cm− attributed to metal-O vibrational modes (Fe−O and Mg−O)19 appears weaker, metal−metal vibrations appearing at lower frequencies (farinfrared range). Figure 3 displays TEM pictures of L10-FePt@PEO NPs for molar mass of the macromolecules of 1000 and 2000 g·mol−1. NPs are often arranged in a 2D-hexagonal close packing indicating their size-monodispersity and surface modification by PEO ligands. It may be observed that the average diameter of the inorganic core is about 6.2 ± 0.8 nm value very close to that of the precursor γ-FePt NPs (6 nm). Also, no significant variation was observed in their size distribution. Only the interparticle spacing varying from 3 ± 0.4 nm for and L10-FePt coated with OA/OAm ligands (see Figure S1) to 3.5 ± 0.5 nm and 7 ± 1.3 nm was observed for those coated with thiolated PEO1k and PEO2k macromolecules, respectively. C

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Hildebrand parameters of 18.5, 17.8, and 20.2 MPa1/2, respectively, can improve the colloidal stability over the time, which can help the steric stabilization of NPs. Lastly, it appeared that the L10-FePt@PEO1k NPs had a larger hydrodynamic diameter than the L10-FePt@PEO2k NPs. In order to check if it was not due to a less efficacy of mPEO1k for sterically stabilizing the NPs, the colloidal stability was studied with time for 1 week (Figure 4b). It revealed that whatever the mPEO-SH molar mass, both dispersions remained stable during the first 24 h; no precipitation occurs and the average size evolved only slightly. After 30 h, the L10-FePt@PEO2k NPs dispersion remained stable while the hydrodynamic diameter of the L10-FePt@PEO1k NPs started to increase. After 1 week, the hydrodynamic diameter of the L10-FePt@PEO1k NPs had nearly doubled, while it remained unchanged for L10-FePt@ PEO2k NPs. This difference of colloidal dispersion stability for PEO1k is explained by the partial agglomeration of FePt NPs during the MgO dissolution step in acidic medium (1 M HCl). During this step the grafting conditions are not optimal since the nucleophilicity of thiols is weakened, which leads to low grafting densities. This is why the macromolecules of higher molecular weight are more efficient to stabilize at this critical step the dispersions. This is also why a second grafting stage is needed in more appropriate chemical conditions, i.e., pH of 6.5−7, to ensure an optimal grafting density for a better colloidal stability with time. This process shows better results for higher macromolecular weight in terms of colloidal stabilization but is also limited by the segregation of the nanoparticles at the surface of the polymer film during the selfassembling step observed in previous studies.20 In our study, thiolated PEO2k macromolecules give good colloidal stability and have been chosen for the following developments. The PS-b-PEO BCP used in this study self-assembles into a hexagonally close-packed cylinder (hcp) structure with an interdomain spacing, d, of 30.5 nm and a theoretical domain radius, r, of 8.1 nm.17 Magnetic inks with different NP-to-BCP weight ratios (R ranging from 0 to 6) were spin-coated to prepare films with a thickness of about 50 nm. Figure 5a displays TEM pictures of the as-cast films after the solvent annealing step. Thanks to their lower electron density, PEO domains appear as circular clearer areas while black spots are assigned to L10-FePt NPs. Especially for low R values, those pictures show that a fraction of the grafted NPs are located in PEO domains confirming the expected favorable interactions between the surface grafted PEO chains and the PEO blocks of BCPs. The TEM images reveal that, for R = 0.5 or 1, L10-FePt NPs are isolated from each other in the films (weak aggregation) and mainly positioned close to the center of the PEO domains. These pictures show also a large number of NPs aggregates positioned on the grain boundaries. Those aggregates induce local defects in the hexagonal lattice of PEO cylinders especially for R ≥ 2. When R = 6, the disturbing effect of the NPs leads to an inhomogeneity of the film thickness (i.e., terrace formation) as observed by significant differences in contrast between thick areas (dark) and finer (clear). The Delaunay triangulation of a TEM picture (Figure 5b, R = 0.5) gave an appropriate representation of the previous observations: the incorporated NPs caused deviation and defects in the BCP lattice. These results are consistent with those reported about the dispersion of magnetite NPs in polystyrene-b-poly(methyl methacrylate) highlighting a critical volume fraction of the PMMA-grafted NPs that could be introduced in the BCP matrix.20

Figure 3. (a) TEM picture of L10-FePt@PEO1k NPs (in the right bottom section of the image the NPs are organized in two layers shifted of a half lattice constant) and (b) TEM picture of L10-FePt@ PEO2k NPs. Both inserts show the same samples at higher magnifications. All scale bars are of 50 nm.

Magnetic inks were achieved by mixing L10-FePt@PEO NPs dispersions and PS-b-PEO BCPs in THF. The colloidal stability of the NPs in THF was evaluated by DLS before and after the addition of the BCPs (Figure 4a). Those measurements showed

Figure 4. (a) L10-FePt@PEO1k and L10-FePt@PEO2k NPs size distributions measured by DLS in THF or in the ink (NPs-to-BCP wt. ratio R = 2). (b) Time evolution of the size of the L10-FePt@PEO1k NPs and L10-FePt@PEO2k NPs as formulated in the ink (NPs-to-BCP wt. ratio R = 2).

that L10-FePt@PEO2k and L10-FePt@PEO1k NPs displayed an average hydrodynamic diameter in THF around 20 and 30 nm, respectively, against 10 nm for the precursor γ-FePt NPs (coated with OA/OAm and measured in cyclohexane (see SI Figure S1). It may be readily explained by the presence of the PEO coronas. It can be also observed that the presence of BCPs made the size distributions narrowed, probably due to the increase of the refractive index of the medium, which may contribute to narrow the size distribution and decrease the mean hydrodynamic diameter during the Mie correction. This operation is generally applied to minimize the effect of the presence of any aggregates. However, the high solubility of the PS-b-POE in THF given by the close THF, PS, PEO D

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

4. CONCLUSIONS We demonstrated that, using thiolated PEO macromolecules, it is possible to maintain colloidal stability during the acidic dissolution step of the MgO shell used as sacrificial physical barrier during the γ-L10-FePt transition at 700 °C. This acidic treatment led to a Pt-rich passivated surface due to a partial dissolution of the iron surface sites. This platinum surface allows reducing the PEO grafting to the use of only one anchoring function, a thiol function, borne by the macromolecule. These colloidal dispersions displayed a very good stability for a couple of days when NPs were coated with PEO with macromolecular weight of 2000 g/mol. Concerning the elaboration of self-assembled nanostructured magnetic films, even if it is not yet optimized, it was still shown that these PEO surface-modified L10-FePt nanoparticles display a compatibilizing property with PEO domains of the patterned BCP film. Particularly, under a NPs-to-BCP weight ratio limit reached at 2%, PEO-coated L10-FePt NPs have been selectively and homogeneously distributed in PEO monodomains with a minimum aggregates formation. Nevertheless, defectivity inherent to the BCP self-assembly should be tackled in order to increase the potential of such system for bit patterned media applications. Directed self-assembly of the nanoparticle/block copolymer composite thin films by using guiding patterns is thus currently under investigation.

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

TEM picture and size distribution of precursor γ-FePt NPs (S1); X-ray diffraction patterns of γ-FePt, γ-FePt@MgO, and L10-FePt@MgO NPs (S2); XPS spectra of -FePt and bare L10FePt NPs after the MgO shell removing (S3). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00942.

Figure 5. (a) TEM pictures of PS-b-PEO/L10-FePt@PEO2k thin films prepared with different NPs-to-BCP wt. ratio (R) values (scale bars: 500 nm). (b) TEM picture of PS-b-PEO/L10-FePt@PEO2k thin film prepared with R = 0.5 and the corresponding Delaunay triangulation of the central area (delimited by the white dotted-line frame; NPs are labeled as black dots) and (c) phase contrast SFM image of PS-bPEO/L10-FePt@PEO2k thin film prepared with different R values.

Corresponding Authors

*(G.G.) Phone: +33 540 006 334; E-mail: [email protected]; Mailing address: ICMCB-CNRS, 87 ave du Dr Albert Schweitzer, F-33608 Pessac Cedex, France. *(S.M.) Phone: +33 540 006 335; E-mail: [email protected].

Figure 5c shows higher magnification SFM phase contrast images of the PS-b-PEO/L10-FePt@PEO2k thin films for different NPs filling ratio. The effect of the NPs on the BCP self-assembly behavior is more evident as observed by the large increase of the radius of the PEO domains (r = 10.7 ± 1.5 nm for R = 0 to r = 13.1 ± 2.9 nm for R = 1) and the interdomain spacing (from d = 30.5 ± 2.2 nm for R = 0 to d = 34.3 ± 3.8 nm for R = 1). The larger distribution of PEO domain radii observed for NPs filled BCP thin films is related to partial occupation by the NPs of PEO domains as well as to the disturbing effect of the NPs on the BCP lattice. Topological images (Figure S4) confirm that L10-FePt Nps are well embedded into the PEO cylinders. Finally, the influence of the interdomain spacing, d, on the free energy for close-packed planes of spheres and cylinders configurations has been previously studied for this particular block copolymer.17 It was demonstrated that the organization of the PEO domains in a HCP cylinders structure was more stable when d is lower than 36.5 nm. Consequently, the analysis of the SFM data suggests that the PS-b-PEO/L10-FePt@PEO2k thin films reported in this study self-assembles into a hexagonally close-packed cylinder structure.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Arkema company, the Région Aquitaine and the Agence Nationale pour la Recherche (ANR09-NANO-026-01 MagniPhico) for financial support of this work. Electron microscopy experiments were performed at the Bordeaux Imaging Center (University of Bordeaux facility). The authors also acknowledge M. Christine Labrugère from PLACAMAT (UMS 3626 CNRS-University of Bordeaux) for technical assistance and helpful discussions on XPS experiments and Laetitia Etienne for ICP-OES measurements.

■

ABBREVIATIONS mPEO-SH, poly(ethylene glycol) methyl ether; BCP, block copolymer; NPs, nanoparticles; PS-b-PEO, polystyrene-blockpoly(ethylene oxide) copolymers; DRIFT, diffuse reflectance infrared Fourier transform spectroscopy; DLS, dynamic light scattering; TEM, transmission electron microscopy; TGA, E

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir thermogravimetric analysis; HCP, hexagonally close-packed cylinder; SFM, surface force microscopy; PEDOT:PSS, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate

■

REFERENCES

(1) Sun, S. Recent advances in chemical synthesis, self-assembly, and applications of FePt nanoparticles. Adv. Mater. 2006, 18 (4), 393−403. (2) Sun, K. S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science 2000, 287 (5460), 1989−1992. (3) Ivanov, O. A.; Solina, L. V.; Demshina, V. A.; Magat, L. M. Determination of the anisotropy constant and saturation magnetization and magnetic properties of powders of an iron-platinum alloy. Phys. Met. Metallogr. 1973, 35, 81−85. (4) Sun, S.; Fullerton, E. E.; Weller, D.; Murray, C. B. Compositionally controlled FePt nanoparticle materials. IEEE Trans. Magn. 2001, 37 (4), 1239−1243. (5) Dai, Z. R.; Sun, S.; Wang, Z. L. Phase transformation, coalescence and twinning of monodispersive FePt nanocrystals. Nano Lett. 2001, 1 (8), 443−446. (6) Lee, D. C.; Mikulec, F. V.; Pelaez, J. M.; Koo, B.; Korgel, B. A. Synthesis and magnetic properties of silica-coated FePt nanocrystals. J. Phys. Chem. B 2006, 110, 11160−11166. (7) Yu, C. H.; Caiulo, N.; Lo, C. C. H.; Tam, K.; Tsang, S. C. Synthesis and fabrication of a thin film containing silica-encapsulated face-centered tetragonal FePt nanoparticles. Adv. Mater. 2006, 18, 2312−2314. (8) Elkins, K.; Li, D.; Poudyal, N.; Nandwana, V.; Jin, Z.; Chen, K.; Liu, J. P. Monodisperse face-centred tetragonal FePt nanoparticles with giant coercivity. J. Phys. D: Appl. Phys. 2005, 38 (14), 2306. (9) Rong, C. B.; Li, D.; Nandwana, V.; Poudyal, N.; Ding, Y.; Wang, Z. L.; Zeng, H.; Liu, J. P. Size-Dependent Chemical and Magnetic Ordering in L10-FePt Nanoparticles. Adv. Mater. 2006, 18 (22), 2984− 2988. (10) Aissou, K.; Alnasser, T.; Pecastaings, G.; Goglio, G.; Toulemonde, O.; Mornet, S.; Fleury, G.; Hadziioannou, G. Hierarchical assembly of magnetic L10-ordered FePt nanoparticles in block copolymer thin films. J. Mater. Chem. C 2013, 1, 1317−1321. (11) Kim, J.; Rong, C.; Lee, Y.; Liu, J. P.; Sun, S. From Core/Shell Structured FePt/Fe3O4/MgO to Ferromagnetic FePt Nanoparticles. Chem. Mater. 2008, 20 (23), 7242−7245. (12) Kim, J.; Rong, C.; Liu, J. P.; Sun, S. Dispersible Ferromagnetic FePt Nanoparticles. Adv. Mater. 2009, 21 (8), 906−909. (13) Mai, Y.; Eisenberg, A. Self-assembly of block copolymers. Chem. Soc. Rev. 2012, 41, 5969−5985. (14) Pavan, M. J.; Shenhar, R. Two-dimensional nanoparticle organization using block copolymer thin films as templates. J. Mater. Chem. 2011, 21, 2028−2040. (15) Kim, B. J.; Fredrickson, G. H.; Kramer, E. J. Effect of Polymer Ligand Molecular Weight on Polymer-Coated Nanoparticle Location in Block Copolymers. Macromolecules 2008, 41 (2), 436−447. (16) Ellman, G. L. Tissue sulfhydryl groups. Arch. Biochem. Biophys. 1959, 82, 70−77. (17) Aissou, K.; Fleury, G.; Pecastaings, G.; Alnasser, T.; Mornet, S.; Goglio, G.; Hadziioannou, G. Hexagonal-to-Cubic Phase Transformation in Composite Thin Films Induced by FePt Nanoparticles Located at PS/PEO Interfaces. Langmuir 2011, 27, 14481−14488. (18) Murray, R. W.; Wuelfing, W. P.; Gross, S. M.; Miles, D. T. Nanometer Gold Clusters Protected by Surface-Bound Monolayers of Thiolated Poly(ethylene glycol) Polymer Electrolyte. J. Am. Chem. Soc. 1998, 120, 12696−12697. (19) Hofmeister, A. M.; Keppel, E.; Speck, A. K. Absorption and reflection infrared spectra of MgO and other diatomic compounds. Mon. Not. R. Astron. Soc. 2003, 345 (1), 16−38. (20) Xu, C.; Ohno, K.; Ladmiral, V.; Composto, R. J. Dispersion of polymer-grafted magnetic nanoparticles in homopolymers and block copolymers. Polymer 2008, 49 (16), 3568−3577.

F

DOI: 10.1021/acs.langmuir.5b00942 Langmuir XXXX, XXX, XXX−XXX