Formation of FePt Alloy Nanoparticles on Highly Oriented Pyrolytic

Article pubs.acs.org/JPCC

Formation of FePt Alloy Nanoparticles on Highly Oriented Pyrolytic Graphite: A Morphological and In Situ X-ray Photoelectron Spectroscopic Study Long Chen,† Arthur Yelon, and Edward Sacher* Regroupement Québécois de Matériaux de Pointe and Département de Génie Physique, École Polytechnique, C.P. 6079, Succursale Centre-ville, Montréal, Québec H3C 3A7, Canada ABSTRACT: Dual layers of Pt and Fe, deposited sequentially onto highly oriented pyrolytic graphite (HOPG), were annealed under ultrahigh vacuum, from room temperature to 560 °C. The formation of FePt alloy NPs, through interdiffusion, was studied by in situ X-ray photoelectron spectroscopy (XPS), ex situ atomic force (AFM), and highresolution transmission electron (TEM) microscopies. With increasing annealing temperature, the Pt 4f7/2 binding energy shifts positively, and the positions of the Pt 5d-6s valence band centers move away from the Fermi level and broaden. Between 300 and 400 °C, Fe and Pt atoms diffuse significantly. Simultaneously, a surface chemical reaction occurs between metal oxide and adventitious carbon on the NP surface, resulting in the disappearance of the O 1s spectrum and the formation of an amorphous hydrocarbon shell. At elevated temperatures, the shell is continually lost, through fragmentation, and replaced by a new hydrocarbon from the vacuum background, assuring that the NPs do not coalesce during the whole annealing process. Stable FePt alloy NPs are formed on the HOPG surface as the annealing temperature is increased to ∼400 °C (A1 structure) and ∼500 °C (L10 structure). A continual Pt surface enrichment occurs with increasing annealing temperature, even before the formation of stable L10 NPs, resulting in the formation of a Pt-rich layer around the NPs. On the basis of the mass balance in the system, an Fe-rich layer must lie below the Pt-rich layer, surrounding the L10 core.

1. INTRODUCTION FePt alloy nanoparticles (NPs) have been the subject of numerous investigations for their potential applications in data storage,1 permanent magnetic nanocomposites,2 biomedicine,3,4 and electro-catalysts.5 Depending upon the Fe:Pt elemental ratio and the annealing temperature, they may have the A1 chemically disordered, face centered cubic (fcc) structure or chemically ordered structures, such as L12 for Fe3Pt or FePt3, and the L10 face centered tetragonal (fct) structure for FePt.6,7 Such structural variations result in completely different magnetic properties. For example, L12 Fe3Pt alloy NPs are paramagnetic8 and L12 FePt3 alloy NPs antiferromagnetic,9 while L10 FePt NPs are ferromagnetic, with a large uniaxial magnetocrystalline anisotropy (Ku ≈ 7 × 106 J/ m3),10,11 and are considered the most promising candidate for the next generation of ultrahigh-density magnetic recording media.1 It should be noted that the sole difference between the A1 and L10 structures is the chemical ordering in the latter, which forces a c-axis distortion, causing the previous fcc structure to become fct. FePt NPs are commonly fabricated using solution-phase synthesis1,12−15 or vacuum deposition16−18 techniques. The assynthesized NPs usually have a chemically disordered fcc structure and are magnetically soft. Thermal annealing is required to transform the disordered fcc structure into the ordered fct structure. To fully understand the magnetic behavior of FePt alloy NPs, it is essential to study the structural © 2012 American Chemical Society

transformation occurring within the individual NPs, as well as their aggregation behavior on annealing. Various techniques, such as transmission electron microscopy (TEM),1,13,19 X-ray diffraction (XRD),14,19 selected area electron diffraction (SAED),19−21 extended X-ray absorption fine structure (EXAFS),22 Mössbauer spectroscopy,23,24 nuclear resonant scattering,25 neutron reflectivity,26 and theoretical simulations,27 have been used to study the formation and ordering behavior of both FePt NPs and films. X-ray photoelectron spectroscopy (XPS) is a highly surfacespecific technique, with a probe depth of a few nanometers. A particular advantage of XPS lies in its ability to identify chemical states and atomic interactions through chemical shifts. There have been several reports of its use to characterize the electronic structure and chemical properties of Pt-based alloy systems.17,24,28−34 For example, Rodriguez and Goodman28,29 investigated the correlation between the electronic and chemical properties of bimetallic model systems (Cu/Pt, Pd/ Ta, Ni/W, etc.) formed by evaporating one metal onto a crystal face of the second metal; they found a relationship between the core level (CL) shifts of the overlayer metal and the temperature of CO desorption. Wakisaka et al.30 employed an XPS system, combined with an electrochemical cell (EC-XPS), Received: February 10, 2012 Revised: March 1, 2012 Published: March 2, 2012 6902

dx.doi.org/10.1021/jp301372j | J. Phys. Chem. C 2012, 116, 6902−6912

The Journal of Physical Chemistry C

Article

The HOPG specimen (ZYA grade, SPI, Inc.) was mounted on the sample holder, which was also used to anneal the sample. Prior to each experiment, the HOPG was cleaved with adhesive tape and immediately inserted into the preparation chamber. This technique assures that an almost undetectable trace of oxygen is found on the HOPG surface, located at the step edges, where free radicals are created by the cleavage process.38−42 Once the HOPG specimen was inserted into the chamber, deposition, annealing, and XPS measurements were performed in situ. A layer of Pt was deposited, followed by one of Fe. Prior to deposition, both the target rods were thoroughly degassed. The metal depositions were performed at room temperature, and constant Fe or Pt deposition rates were obtained by keeping the evaporation power unchanged. Since earlier studies showed that the L10 type structure can be formed in FexPt1‑x, with x ranging from 0.35 to 0.6,53 the nominal thicknesses of both Fe and Pt, monitored by a quartz crystal microbalance placed near the sample, were fixed at 1 nm for all the samples. Following metal deposition, the samples were annealed in the preparation chamber, at temperatures ranging from room temperature to 560 °C, for 1 h. Samples were also annealed at 560 °C for additional amounts of time, to investigate NP stability. XPS was performed in the analysis chamber of the spectrometer, using nonmonochromated Mg Kα radiation (1253.6 eV). High-resolution spectra were obtained at a perpendicular takeoff angle, using a pass energy of 20 eV and a step size of 0.05 eV. The instrument resolution was ∼0.7 eV. After Shirley background removal, component peaks were fit by the VG Avantage V1.62 software (Thermo VG Scientific), using symmetric mixed Gaussian−Lorenzian functions. The binding energies were calibrated by placing the major C 1s peak at 284.6 eV. The reader should note that this value of binding energy is below the Fermi level and should correctly be written as negative; generally accepted convention, however, omits the negative sign, with the understanding that a higher value of binding energy indicates a negative shift. The peak widths employed during peak separations, given as full width at half maxima (fwhm), were those previously found in our laboratory.38−47 For each spectrum, all the component peaks were given the same fwhm value. Relative concentrations were obtained from high-resolution spectra, using sensitivity factors regularly confirmed with standard samples. The NP morphologies, after annealing at 560 °C for 1 h, were examined by high-resolution TEM, using a JEOL JEM2100F microscope, equipped with a LaB6 filament, at 200 kV, and an energy dispersive X-ray analyzer (EDX, Phoenix). Sample preparation for TEM analysis was performed by scraping the FePt NPs from the HOPG surface onto a copper grid coated with lacy carbon. The sample morphologies, both as-deposited and after annealing at 560 °C for 1 h, were also examined ex situ by AFM (Digital Instrument Multimode AFM, silicon probe, tapping mode). All the images collected were processed offline using the WSxM 1.1 freeware.54 X-ray diffraction was attempted, using a PANalytic X’Pert MPD, with Cu Kα radiation (λ = 1.542 Å), generated at 50 kV and 40 mA. Unfortunately, there was not enough sample to produce a useable diffraction spectrum. As a result, the substrate diffraction peaks masked those of the sample.

in their studies of the electronic structures of Pt−Co and Pt− Ru alloys; they established a linear relationship between the CL shifts due to alloying and the CO adsorption energy. Although several studies used XPS to investigate the electronic structure and chemical states of FePt NPs,17,24,32−34 none has so far attempted to address the metal−metal interaction, structure, surface chemistry, and thermal stability of these NPs during their formation process, through the use of in situ XPS analysis. This constitutes the purpose of the present work. While transition metals are generally believed to exhibit single XPS peaks with asymmetric line shapes,35−37 we have recently used multiple symmetric peaks to characterize several first transition series metals, e.g., Fe,38−40 Co,41 Ni,42 and Cu,43−45 as well as Pt46 and Pd.47 Our position is that all metal spectra are composed of symmetric peaks, where asymmetries are attributed to overlapping minor peaks due to known physical and chemical phenomena associated with the particular metal. In this way, we have been able to demonstrate that, for all the metals we have studied, the minor peaks constituting these asymmetries contain much information on the structures, contaminants, oxidation, and interfacial interactions of NP surfaces.38−48 For example, the origin of the asymmetry of the Ni 2p spectrum, which decreases with increasing NP size, is a minor peak resulting from Ni/HOPG back-bonding; both this peak and the asymmetry it causes decrease with NP size because of the shielding effect of the growing NPs.42 In contrast, the asymmetries of both Pt 4f and Pd 3d peaks increase with increasing NP size. This was attributed to the presence of both oxides (PtOx and PdOx) at the NP surface and separate component peaks representing the different electronic configurations of bulk and surface atoms; the bulk component increases more rapidly than the surface component as NP size increases, leading to increasing asymmetry with increasing NP size.46,47 Having established the behavior of several individual transition metal series NPs through the use of overlapping symmetric XPS component peaks, we now use this technique to characterize alloy NPs. We focus first on FePt alloy NPs because of their significance in many areas, as already noted. As before, highly oriented pyrolytic graphite (HOPG) was used as the substrate in our study because it has an inert, well-defined, conducting surface.49,50 Moreover, the chemisorption of oxygen onto metal NPs can be easily detected on HOPG surfaces since there is no oxygen XPS signal from a virgin HOPG substrate.51,52 In the present study, FePt NPs were prepared by depositing Pt prior to Fe and then annealing to high temperatures, under ultrahigh vacuum (UHV) conditions. The evolution of both CL and valence band (VB) spectra was followed as a function of annealing temperature. The morphologies of the NPs formed were further characterized by transmission electron (TEM) and atomic force (AFM) microscopies.

2. EXPERIMENTAL SECTION The experiments were carried out in our VG ESCALab 3 Mark II XPS spectrometer, in which the sample preparation and analysis chambers are separated by a gate valve. The base pressures in the preparation and analysis chambers were 99.99%) and a tungsten filament e-beam source.

3. RESULTS 3.1. XPS Analysis of FePt NPs on HOPG as a Function of Annealing Temperature. 3.1.1. Core Level Spectra. Typical 6903



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Figure 1. Evolution of the C 1s (a), O 1s (b), Fe 2p (c), and Pt 4f (d) core level spectra as a function of annealing temperature for FePt/HOPG annealed at each indicated temperature for 1 h.

C4 287.1 eV, the C2 π* ← π shakeup satellite. The peak for C5, the C1 π* ← π shakeup, at 291.4 eV, is omitted. Any additional peaks, due to the presence of other carbon species, will superimpose on the original (C1−C4) peak range and must also be considered, as noted previously.39,40,46,47 The O 1s spectrum (Figure 2b) was fit with two components: one at 530.2 eV, indicating a metal oxide, and the other at 532.0 eV, indicating oxidized carbon species. A comparison with the positions of both Fe and Pt oxides38,40 indicates that the 530.2 eV peak corresponds to surface-oxidized Fe;38 surfaceoxidized Pt appears at 531.3 eV.40 A peak is found at 531.3 eV, in the present case, on prolonged high-temperature annealing, as will be discussed in Section 3.2. The Fe 2p3/2 spectrum was separated into five component peaks, as in our previous paper.38 Five peaks, labeled A (707.0 eV), attributed to metallic Fe, and B1, B2, B3, and B4 (708.3, 709.9, 711.4, and 712.9 eV, respectively), attributed to a vacancy cascade process, were found at the same positions as in pure Fe. An additional peak, denoting Fe−C, is obscured by the B1 vacancy cascade peak.38 However, during the annealing process, the persistence of the C 1s carbide peak, at 283.3 eV, indicates the persistence of Fe at the NP surface layer and is the reason for describing that layer as Pt-rich. Each peak of the Pt 4f spectral doublet was separated into three components. For pure Pt NPs,46 these components represent surface atoms (1 and 1′), bulk atoms (2 and 2′), and

C 1s, O 1s, Fe 2p, and Pt 4f CL spectra, and their evolution as a function of annealing temperature, are shown in Figure 1. The C 1s intensity increases with annealing, from room temperature to 560 °C (Figure 1a), while the O 1s intensity decreases (Figure 1b), before vanishing at ∼400 °C. As in our earlier study on Fe NP annealing,40 the O 1s spectral evolution indicates that a surface chemical reaction occurs, removing the oxygen from the sample surface, reaching completion near 400 °C. Figures 1c and d show the temperature evolution of the Fe 2p and Pt 4f spectra, respectively. The Fe 2p intensity decreases, while the Pt 4f intensity changes slightly, with increasing annealing temperature. The Pt 4f binding energy shifts positively with increasing annealing temperature, while that of the Fe 2p remains constant (see dotted lines in Figures 1c and d). Schematic peak separations of C 1s, O 1s, Fe 2p3/2, and Pt 4f, for as-deposited NPs, are shown in Figure 2. As in our previous work,38−48 symmetric peak components were employed to analyze all the spectra. Briefly, for the C 1s spectrum (Figure 2a), five peaks were needed to properly deconvolute the envelope. Except for the carbide peak (C−Fe, 283.3 eV;38−40 Pt does not exhibit a carbide peak46), all the peaks have the same positions as those in pristine HOPG, where they have been attributed39,40,46,47 to: C1, 284.6 eV, undamaged alternant hydrocarbon structure; C2, 285.5 eV, damaged alternant hydrocarbon structure; C3, 286.3 eV, sp3 free radical defects; 6904



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Figure 2. Schematic deconvolutions of the C 1s (a), O 1s (b), Fe 2p3/2 (c), and Pt 4f (d) spectra for FePt/HOPG without annealing.

PtOx (3 and 3′), respectively.46 Before annealing, these peaks were found at 71.4, 72.3, and 73.2 eV in our NPs, compared with 71.4, 72.5, and 73.7 eV in pure Pt. This shift of Pt3 and the O 1s peak position mentioned earlier indicate that Pt3 has a source other than PtOx. The effects of annealing, on the surface and electronic properties of the NPs, were investigated by examining the temperature evolution of the (i) binding energies, (ii) fwhm, and (iii) component fractions of the XPS peak components. Figures 3a and b show the temperature evolution of the binding energies of Fe 2p3/2 and Pt 4f7/2 components. The binding energies of the Fe 2p3/2 components are independent of annealing temperature, while in contrast, those of the Pt 4f7/2 components shift to higher values, up to 500 °C. Figure 3c shows the evolution of the Fe 2p3/2 and Pt 4f7/2 component fwhm values as a function of annealing temperature. Even though the fwhm values change only slightly (∼0.15 eV) for both spectra, these changes are clearly observed. The changes of both binding energies and fwhm values indicate an interaction between Pt and Fe during alloy formation, as will be discussed later. A turning point in the evolution appears, at 500 °C, for both the Pt 4f7/2 binding energies (Figure 3b) and the Fe 2p3/2 fwhm values (Figure 3c). This suggests that whatever alloys that have formed have reached stable compositions at this temperature. The temperature evolution of various atomic concentrations, and the Fe:Pt atomic ratio, are presented in Figures 4 and 5,

respectively. Significant changes in atomic concentrations and atomic ratios are found in the range of 300−400 °C, suggesting that extensive interdiffusion of Fe and Pt atoms occurs in this temperature range. However, above 400 °C, all appears to be independent of annealing temperature, implying that FePt alloy NPs have been formed and, apparently, stabilized. The onset temperature indicated here for FePt alloy formation (∼400 °C) is lower than that indicated in Figure 3 (∼500 °C). As discussed below, this difference is most likely caused by the A1 → L10 phase transformation the FePt NPs undergo during the annealing process. Following annealing, the stabilized Fe:Pt atomic ratio is ∼0.9 (Figure 5), a value significantly smaller than the nominal Fe:Pt atomic ratio: both Fe and Pt nominal coverages are 1 nm, so that, if the Fe and Pt atoms were uniformly distributed, the NPs in the present study would have resulted in an Fe:Pt atomic ratio of ∼1.4. The value of ∼0.9 suggests that a structure has been formed in the annealed FePt NPs, having a Pt-rich layer at the surface. Thus, an Fe-rich layer must exist in the interior, effectively forming an onionskin structure, an arrangement that has often appeared in the alloy NP literature. Figure 6 presents the temperature evolution of the Pt 4f7/2 component fractions. Below the turning point, at ∼500 °C, which indicates the formation of stable alloy NPs, the Pt1 component fraction increases, while Pt2 and Pt3 decrease. As previously discussed, in pure Pt,46 Pt1 represents surface atoms, Pt2 bulk atoms, and Pt3 surface oxide. Their evolution with 6905



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Figure 3. Evolution of the binding energies of Fe 2p3/2 (a), Pt 4f7/2 (b) component peaks, and fwhm (c), as a function of annealing temperature.

Figure 4. Evolution of the atomic concentrations for FePt/HOPG, as a function of annealing temperature.

Figure 5. Evolution of the Fe 2p3/2 A: Pt1 + Pt2 (surface + volume components) intensity ratios for FePt/HOPG as a function of annealing temperature.

annealing temperature suggests that Pt atoms have segregated at the NP surface during the annealing process, as indicated by the Fe:Pt ratio of Figure 5. This is consistent with several studies on Pt-based alloys in the literature.31,55−59 3.1.2. Valence Band Spectra. Several studies17,34,60 have used VB spectra to characterize the electronic structure of FePt alloys. However, the evolution of the electronic structure during

alloy formation has not been studied. Figure 7 shows this evolution for our NPs. For the as-deposited sample, the valence band spectrum consists of two characteristic features centered at binding energies of ∼4 and ∼1.5 eV, an overlap of the Pt 5d and 6s peaks; the Fe 4s contribution, also at ∼1.5 eV,40 is of a sufficiently low intensity that its contribution may be disregarded. As the temperature increases, the 1.5 eV peak 6906



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Figure 6. Evolution of the Pt 4f7/2 component fraction for FePt/ HOPG as a function of annealing temperature.

Figure 8. Evolution of the atomic concentrations as a function of annealing time for FePt/HOPG annealed at 560 °C. The annealing time 0 h denotes the as-deposited sample.

chemical reaction, as does the decrease of O concentration to zero, during the annealing process. After the first 30 min, the NPs appear to be stable on the HOPG surface, with negligible changes in C, Fe, and Pt atomic concentrations with annealing time. On further annealing at 560 °C for >2 h, oxygen reappears (Figure 8), and its concentration increases slightly with annealing time. This is more clearly seen in Figure 9, showing the evolution of the O 1s spectra as a function of annealing

Figure 7. Evolution of the valence band spectra for FePt/HOPG as a function of annealing temperature.

shifts toward higher binding energy, resulting in a reduced density of states near the Fermi level, while the peak at 4 eV broadens but appears to move little, if at all. That is, the position of the s-band center moves away from the Fermi level, and the s- and d-band widths are broadened, with increasing annealing temperature. We note that, once the annealing temperature reaches 500 °C, the shape of the valence band spectrum is independent of annealing temperature. This indicates the formation of stable NPs at this temperature, consistent with the results of the CL spectra shown in Figures 3 and 6. The temperature shift of the Pt VB to higher binding energies reveals a shoulder at 0.7 eV. This may be due to the Fe 4s spectrum, previously hidden by the larger, broader Pt spectrum. Its annealing behavior cannot presently be ascertained. 3.2. Stability of FePt NPs Annealed at 560 °C. Asdeposited samples were annealed at 560 °C, in UHV, for varying times. Figure 8 shows the evolution of atomic concentrations with annealing time. Significant changes in all atomic concentrations can be observed when the sample is annealed for half an hour, after which they stabilize. A comparison with Figure 5 indicates that the interdiffusion of Fe and Pt is already complete at this temperature. The initial rapid increase in C intensity indicates the occurrence of a surface

Figure 9. Evolution of the O 1s spectra as a function of annealing time for FePt/HOPG annealed at 560 °C.

time. For the as-deposited sample, the O 1s spectrum consists of two peaks with relatively strong intensity, at ∼530 and ∼532 eV, corresponding to oxidized Fe and oxidized carbon, respectively. From Figures 8 and 9, we see that the O 1s spectrum is virtually lost after 1 h, with a new, low intensity, spectrum reappearing after 3 h; it increases slightly in intensity with annealing time but with peak positions now shifted to ∼531 and ∼533 eV. While the latter peak position is certainly associated with slightly oxidized carbon61 (the new O 1s temperature position indicates C−OH, as opposed to the CO indicated on deposition), the former is, as mentioned earlier, at an energy associated with PtOx in pure Pt,46 as discussed 6907



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of the annealed NPs. Figure 11a shows a TEM photomicrograph of FePt NPs annealed at 560 °C for 1 h. A corresponding selected area electron diffraction (SAED) pattern is also presented. The annealed FePt NPs show a twin structure, with sizes ranging from 5 to 15 nm. Neither NP coalescence nor agglomeration is observed in the image, which agrees well with the AFM results shown in Figure 10. Several studies13,20 have shown that the twin structure of FePt NPs is characteristic of L10 structural order. The SAED pattern recorded on these NPs only displays fundamental diffraction rings (inset of Figure 11a); the superlattice diffraction rings, characteristic of the L10 structure,13,19−21 we believe to be present but too weak to be observed. Figure 11b shows a higher-resolution TEM photomicrograph of the annealed NPs. The NP lattice planes are clearly visible, with a 0.23 nm spacing, close to that of the ⟨111⟩ direction in the fct FePt structure (0.22 nm). In addition, as with Fe NPs annealed under the same conditions,40 each NP is surrounded by an amorphous shell, less than 2 nm thick. The EDX spectrum (Figure 11c) exhibits a strong C peak, confirming that the amorphous shell contains carbon. Although the sample had been exposed in air for two days before the measurement, almost no O signal was detected in the EDX spectrum. This surprising result indicates that the shell prevents NP oxidation on prolonged exposure to air and does not, itself, oxidize at room temperature. Such behavior suggests that the shell is composed of a highly crossed-linked hydrocarbon, with few defects. In such a case, we do not doubt that these amorphous shells prevent extensive FePt NP agglomeration or coalescence during annealing, despite thicknesses of less than 2 nm.

above. These changes suggest the conversion of adventitious carbon to a slightly oxidized hydrocarbon shell,40 surrounding a slightly oxidized Pt-rich surface layer. 3.3. Morphology of FePt NPs on HOPG. The surface morphology of the FePt NPs, both before and after annealing, was examined by AFM. Figure 10 shows AFM photomicrographs of the as-deposited FePt NPs and those annealed at

Figure 10. AFM images of FePt/HOPG: (a) as-deposited and (b) annealed at 560 °C for 1 h.

560 °C for 1 h. Before annealing (Figure 10a), they do not form a continuous layer on the HOPG surface but, rather, domeshaped NPs, relatively uniform in size. Upon annealing at 560 °C for 1 h (Figure 10b), they are more clearly observable, and extensive NP coalescence or agglomeration can be ruled out. This is in contrast to our previous study of Fe NPs on HOPG, where the agglomeration of NPs was detected on annealing.40 As discussed below, the lack of extensive coalescence or agglomeration for the annealed FePt NPs may be related to the interdiffusion of Fe and Pt atoms and to the structural transformation occurring during the annealing process. TEM characterization was performed to obtain detailed information on the morphological characteristics and structure

4. DISCUSSION 4.1. Overview of the Evolution of NP Structure. From the simplest, and least ambiguous, results presented in the

Figure 11. (a) TEM image of FePt NPs annealed at 560 °C for 1 h. The inset shows the SAED pattern acquired on these NPs; (b) higher-resolution TEM image shown in (a); (c) EDX spectrum recorded on the NPs shown in (b). 6908



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Scheme 1. Structures of FePt NPs and Their Evolution with Annealing Temperature

recently, Delalande et al.15 monitored the phase transition by means of X-ray diffraction, for FePt NPs synthesized by the “hot soap method”; they found that the A1 → L10 phase transition, when the lattice constant a splits into a and c parameters, occurs at about 500 °C. This is in good agreement with the changes seen in this range in our studies. In addition, while the superlattice diffraction rings are too weak to be observed in the SAED pattern (Figure 11a, inset), the twin structure of the annealed FePt NPs, characteristic of L10,13,20 was detected (Figures 11a and b). On the basis of the foregoing, we believe that the A1 and L10 FePt structures were formed at annealing temperatures of ∼400 and ∼500 °C, respectively. From Figure 6, preferential enrichment of Pt is observed at the surface of FePt NPs. This is not unexpected since Pt has a lower surface free energy than Fe. Several studies31,55−59 have shown that Pt atoms segregate on the surface of Pt-based alloy systems annealed under UHV, resulting in a Pt-rich layer or, in some cases, a skin composed entirely of Pt. However, since a carbide peak is always detected in the C 1s spectrum (Figure 2a) and Fe is the only one of the two alloy components known to form a hydrocarbon layer,40 a skin can be ruled out in our case. Thus, the NP surface consists of a Pt-rich layer, containing some Fe bonded to an outer hydrocarbon shell. Because of the lower concentration of possibly well-separated surface Fe atoms, such a hydrocarbon layer may not be as dense as that formed around pure Fe NPs. Further, since the Fe:Pt atomic ratio is 1.4, there must also be an Fe-rich layer lying below the Pt-rich layer and above the FePt crystal. Scheme 1 depicts the structures of FePt NPs and their evolution with annealing temperature. An EXAFS study of FePt NPs, by Antoniak et al.,62 led to the conclusion that Pt atoms are found in a Pt-rich environment and Fe atoms in an Fe-rich environment. Not having our advantage of knowing that the NP has an onionskin structure, these authors attributed the enrichments, and the decreased magnetic moments that were found, to local chemical inhomogeneities in the L10 crystal structure. With our onionskin structure, several phenomena may be explained: for example, why the atomic ratio of Fe to Pt in L10 FePt NPs, determined by XPS, is always lower than its nominal value (Figure 4). If we consider that the XPS signal of the Ferich layer is attenuated by the Pt-rich layer lying above it, the question is easily answered. The enigma of the location of the excess Fe atoms disappears when the multilayered structure of FePt NPs is considered. 4.3. Surface Chemistry and Thermal Stability during FePt Alloy NP Formation. In our previous studies on Fe and Co NPs deposited onto HOPG,39−41 we found that these NPs

preceding section, it is possible to propose a view of the evolution of the FePt NPs. Recall that Fe was deposited over a layer of Pt NPs, preferentially wetting the Pt NPs rather than forming new Fe NPs. Further, the dominant signal (Figure 4) is C, with some oxidized hydrocarbon and carbide, and the dominant component in the Pt signal is the surface contribution, Pt1. Thus, there must have been interdiffusion between Fe and Pt, initially giving an interphase and ultimately leaving a low surface energy Pt-rich surface layer. The observed spectra then change very little from room temperature to 200 °C. We propose that the interdiffusion is the result of the deposition of Fe over the previously deposited Pt, releasing the heat of condensation of the depositing Fe (∼350 kJ/mol). The interdiffusion must also result in the formation of an Fe-rich sublayer. We propose, as shown in Section 3.1.1, that the interphase is the origin of the Pt3 peak (Figure 2d), rather than the PtOx found in pure Pt.46 Below 300 °C, thermal energy produces only very small changes in the NPs, as shown in Figures 3−6. Above 300 °C, Fe and Pt interdiffuse rapidly. The Pt-rich surface layer thickens, as shown in Figure 6; the Fe diffuses into the NP, creating an alloy core, whose composition is roughly FePt, and leaving the Fe-rich layer between the core and the surface, as discussed in Section 3.1.1. Above 400 °C, the compositions, thicknesses, and positions of the various metallic structures hardly change, except that, during the 560 °C annealing process, the outer hydrocarbon shell is slightly oxidized, along with some of the Pt in the outer layer. We shall argue that the alloy forms the A1 structure near 400 °C and is converted to the L10 structure near 500 °C. 4.2. FePt Alloy NP Formation. The temperature evolution of the Pt 4f7/2 component binding energy (Figure 3b), the fwhm value (Figure 3c) the Pt 4f7/2 component fraction (Figure 6), and the valence band spectrum (Figure 7) indicate the formation of stable alloy NPs near 500 °C. However, the temperature evolution of atomic concentrations (Figure 4) and the Fe:Pt atomic ratio (Figure 5) indicate that FePt alloy NPs were formed and stabilized near 400 °C. This difference in annealing temperatures is attributed to the chemical ordering of the FePt NPs during the annealing process. As discussed earlier, alloys with an Fe:Pt ratio close to 1 may exhibit two structures, fcc (A1), formed at relatively low temperatures (

Formation of FePt Alloy Nanoparticles on Highly Oriented Pyrolytic

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