Article pubs.acs.org/cm
Highly Controlled Dip-Coating Deposition of fct FePt Nanoparticles from Layered Salt Precursor into Nanostructured Thin Films: An Easy Way To Tune Magnetic and Optical Properties Marco Faustini,† Aldo Capobianchi,‡ Gaspare Varvaro,‡ and David Grosso*,† †
Chimie de la Matière Condensée de Paris, UMR UPMC−CNRS 7574, Université Pierre et Marie Curie (Paris 6) Collège de France, 11 place Marcelin Berthelot, 75231, Paris, France ‡ Istituto di Struttura della Materia del Consiglio Nazionale delle Ricerche (ISM.CNR), Area della Ricerca Roma 1, via Salaria Km 29.300, 00015, Monterotondo Scalo, Rome, Italy S Supporting Information *
ABSTRACT: Nanostructured titania films containing hard magnetic FePt nanoparticles were obtained through impregnation and postannealing of salt layered precursor [Fe(H2O)6][PtCl6] nanocrystals. The impregnation was performed by a highly controlled dip-coating process into ethanol based solutions of the salt precursor. The bimetallic salt nanocrystals were formed upon evaporation of the solvent into the cavities of TiO2 template films. The salt was transformed into the hard magnetic fct phase by annealing in reductive atmosphere at 400 °C a particularly low conversion temperature compared with those reported for other synthesis approaches. Two different titania templates were considered. In the first case, the impregnation of 3D mesoporous films was investigated in order to determine the influence of the deposition parameters on the amount of FePt nanoparticles embedded and the final optical and magnetic properties of the layers. In the second case, this approach was further extended to 2D heterogeneous nanoperforated films. The nanoparticles were selectively deposited into the perforations thanks to a prior functionalization of the titania template by the formation of hydrophobic/hydrophilic distinct domains. Interestingly, the coalescence between the magnetic particles was found to be limited due to the low conversion temperature. This procedure of positioning particles onto a 2D nanostructured support constitutes an easy and versatile bottom-up approach toward magnetic arrays with potential application in magnetic data storage devices. KEYWORDS: FePt, dip coating, nanostructured films, impregnation, positioning surface.17,18 Generally, from a chemical pathway, FePt nanoparticles are obtained in the chemically disordered facecentered cubic (fcc) phase, and also in this case, a postannealing treatment at about 600 °C is necessary to induce the transition to the fct phase (L10).19 The high annealing temperature provokes coalescence and uncontrolled increase of the particles size. Moreover, through these approaches, the exact positioning and the amount of magnetic nanoparticles applied on the surface are often difficult to control. In this work we present a versatile strategy to prepare magnetic nanocomposite films with tunable magnetic and optical properties by combining highly controlled liquid chemical deposition and a “low temperature” synthesis of hard magnetic FePt into nanostructured thin films. The FePt nanoparticles were synthesized from the [Fe(H2O)6][PtCl6] salt hexaaquairon(II) hexachloroplatinate, that is, transformed directly into the FePt fct phase after thermal annealing at 400 °C in reductive atmosphere.20−22 Sol−gel derived crystalline titanium dioxide nanoporous films were exploited as the
1. INTRODUCTION Nanometer-scale assemblies of permanent magnets on surfaces are of great importance in several fields of nanoscience and nanotechnology since they are potential candidates for applications in magnetic data storage,1,2 opto-electronic,3 and spintronic devices.4 One of the most promising materials for magnetic applications is FePt alloy in the face-centered tetragonal ( fct) phase since it is known to exhibit large magneto-crystalline anisotropy5 and high coercivity at room temperature.6,7 The major challenges that have to be faced for achieving such complex systems are the preparation of hard magnetic FePt nanoparticles with permanent residual magnetization and their positioning on the surface by a controlled and versatile process.8−10 Hard fct FePt can be prepared by either vacuum deposition techniques or chemical routes. FePt films obtained by physical depositions generally require postannealing at temperatures as high as 600 °C. In addition, periodic assemblies of nanomagnets are usually obtained by combination with timeconsuming, multistep nanofabrication processes11 or block copolymer lithography.12,13 Among the chemical techniques, FePt nanoparticles can be synthesized by reduction of mixed metal salts14−16 or by electrogeneration on a conductive © 2012 American Chemical Society
Received: November 10, 2011 Revised: February 8, 2012 Published: February 16, 2012 1072
dx.doi.org/10.1021/cm2033492 | Chem. Mater. 2012, 24, 1072−1079
Chemistry of Materials
Article
2. EXPERIMENTAL DETAILS
thermally and mechanically stable host matrix for the in situ formation of layered salt nanocrystals. The ceramic templates were embedded from a solution of the FePt precursor dissolved in ethanol. Many examples of impregnation from solution by natural diffusion have been reported for CdS, MnS,23 PbS,24 or metallic25,26 nanoparticles embedded in porous silica films. However, natural diffusion of solute into the pores does not allow the exact control of the quantity of material applied to fill into the cavities. In addition, the recrystallization of FePt salt nanocrystals into the porosity takes place just upon evaporation of the volatile solvent. Our strategy exploits dip-coating deposition that is highly suitable to homogeneously impregnate porosity as a result of the progressive evaporation that tends to drag solutes within the porous network upon solvent departure.27 In addition, in the case of films deposition on a flat surface, by carefully modulating the process conditions, the dip-coating technique enables exact control of the amount of applied solution and thus of the final thickness from few nanometers up to a few micrometers for a given system. In particular, a prediction law, which takes into account the presence of two opposite regimes of deposition (called “capillarity” and “draining” regimes) at different withdrawal speeds was recently reported for the preparation of sol−gel and hybrid films.28−30 In the present work we take advantage of the high versatility of the dip-coating technique for postembedded hard magnetic FePt nanoparticles into two different templates. In the first case, called “route 1”, mesoporous titania films31 were filled in order to obtained 3D assemblies of magnetic nanoparticles. The deposition conditions, such as withdrawal speed and the concentration of the precursor solution, have been tuned in order to study the impregnation process and control the amount of deposited magnetic particles. The influence of the processing parameters was investigated by ellipsometry and by means a vibrating sample magnetometer and a superconducting quantum interference device magnetometer. We found that optical and magnetic properties of the surface can be easily tuned by fully exploiting the dip-coating process possibilities. The understanding of the effect of the deposition conditions has permitted us to extend this strategy to a more promising system in term of applications. In fact, in the second case, called “route 2”, FePt nanoparticle have been selectively assembled into the cavities of 2D nanoperforated titania films.32 The positioning of the salt nanocrystals was realized thanks to the functionalization of the inorganic template with the formation of hydrophobic/hydrophilic distinct domains before dip coating at low withdrawal speeds. The chemical and topographical contrast between nanodomains has been already exploited in previous reports for the generation of Prussian Blue particles into the same inorganic template33 and for the selective deposition of FePt into the corrugation of photochemically modified PS-PMMA block-copolymer films template.34 Scanning electron microscopy was used to investigate the position and amount of FePt nanoparticles that have been tuned by changing the dipping rate. In addition, the micrographs revealed the presence of distinct nanoparticles even in the same perforation suggesting that the coalescence was limited due to the relatively low conversion temperature. In this work we demonstrate that the synthesis of FePt from salt precursor at low conversion temperature combined with the versatile dip coating represents an easy, scalable chemical route toward magnetic arrays for high density data devices.
2.1. Chemicals. All the reagents were used as purchased. Absolute ethanol was purchased from Normapur, while TiCl4 precursor was purchased from Aldrich. F127 Pluronic (EO106-PO70-EO106) and P2325-BdEO PB-b-PEO (polybutadiene-b-polyethyleneoxide, MWPB = 32 000 g mol−1, MWPEO = 43 500 g mol−1) were purchased from Aldrich and Polymer Source, respectively. Zonyl perfluorophosphate surfactant ((CF3−(CF2)2−(CH2)5O)2POO−NH4+) was purchased from Dupont. H2PtCl6·6H2O and FeCl2·H2O salts were purchased from Aldrich. 2.2. Mesoporous Titania Films. Films were prepared from solutions composed of TiCl4:F127:H2O:EtOH with respective molar ratio of 1:0.005:5:40. Final solution was obtained by dissolving the TiCl4 precursors in ethanol and water followed by the addiction of F127. Films were prepared on silicon by dip coating at 1.5 mm·s−1 at room temperature and at low relative humidity (
