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Nano Research https://doi.org/10.1007/s12274-018-2125-6

Nano-fried-eggs: Structural, optical, and magnetic characterization of physically prepared iron-silver nanoparticles Julien Ramade1, Nicolas Troc1, Olivier Boisron1, Michel Pellarin1, Marie-Ange Lebault1, Emmanuel Cottancin1, Vitor T. A. Oiko2, Rafael Cabreira Gomes2, Varlei Rodrigues2, and Matthias Hillenkamp1,2 () 1 2

Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, UMR5306, Institut Lumière Matière, F-69622, Villeurbanne, France Instituto de Física Gleb Wataghin, UNICAMP, CP 6165, 13083-970 Campinas, SP, Brazil

Received: 24 January 2018 Revised: 6 June 2018 Accepted: 8 June 2018

© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

KEYWORDS nanoalloy, bimetallic nanoparticles, iron, silver, FeAg, optical spectroscopy, magnetometry

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ABSTRACT The prospect of combining both magnetic and plasmonic properties in a single nanoparticle promises both valuable insights on the properties of such systems from a fundamental viewpoint and numerous possibilities for technological applications. However, the combination of two of the most prominent metallic candidates—iron and silver—has presented numerous experimental difficulties because their thermodynamic properties impede miscibility and even coalescence. Herein, we present the thorough characterization of physically prepared Fe50Ag50 nanoparticles embedded in carbon and silica matrices via electron microscopy, optical spectroscopy, magnetometry and synchrotron-based X-ray spectroscopy. Iron and silver segregate completely into structures resembling fried eggs, with a nearly spherical, crystallized silver part surrounded by an amorphous structure of iron carbide or oxide, depending on the environment of the particles. Consequently, the particles exhibit both plasmonic absorption corresponding to the silver nanospheres in an oxide environment and a reduced but measurable magnetic response. The suitability of such nanoparticles for technological applications is discussed from the viewpoint of their high chemical reactivity with their environment.

Introduction

Metal nanoparticles have attracted increasing attention over the past decades in both fundamental and applied research because of the great variability and tunability of their physical and chemical properties through Address correspondence to [email protected]

their composition, size, and shape dependence. In particular, purely nanoscopic effects such as the localized surface plasmon resonance [1] and superparamagnetism [2] are subjects of intensive investigation. The research motivation is even greater when two or more metals are combined at the nanoscale and when composition

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effects play an additional, often decisive role [3–5]. In this case, an immense playground of fascinating new properties and possibilities is introduced, e.g., efficient nano-catalysts made of immiscible metals [6] and theranostics applications [7]. On the other hand, the complexity of such systems requires a detailed understanding of the underlying effects and interactions for the efficient design of nanosystems with new and tunable properties through the size, shape, composition, and atomic structure of multi-metallic clusters. Combining ferromagnetic transition metals and noble metals in nanoparticles is of particular interest in many aspects. The presence of plasmonic and magnetic properties in the same particle allows the same systems to be studied from several complementary angles. Furthermore, such nanosystems are promising for applications, e.g., in magneto-plasmonics [8, 9]. Applications of such multifunctional magnetic nanoparticles are envisaged in the biomedical field [10] and for surface enhanced Raman spectroscopy (SERS) [11], as contrast agents or for information technology via magneto-optical switching [12]. One ideal candidate for a magneto-plasmonic system is FeAg. Iron has a very high saturation magnetization— four times higher than that of one of the most abundant oxides, magnetite Fe3O4—whereas silver has an extinction coefficient four times higher than that of gold at the surface plasmon resonance [13]. However, the extremely high affinity of iron to oxidation has hindered the fabrication of purely metallic FeAg nanoparticles, and most studies focus on the fabrication and study of particles combining silver with iron oxides [9, 14], as well as their applications, e.g., in biomedicine [15], theranostics [16], SERS [17], catalysis, and imaging [18]. The questions of whether and how it is possible to stabilize FeAg nanoparticles without oxidation, as is possible for FePt [19], for example, are important but remain unanswered. Fe and Ag are completely immiscible in the bulk and liquid phases [20], and no stable substituting alloy, with silver on a body-centered cubic (bcc) site in iron or iron on a face-centered cubic (fcc) site in silver, has been observed in the bulk phase. The thermodynamically stable state accessible by thermal

treatment consists of pure iron precipitates in a silver matrix [21, 22]. Extended X-ray absorption fine-structure investigations of granular FeAg layers have shown that the as-prepared state is a meta-stable solid solution, while annealing leads to the precipitation of iron into bcc structures [23]. Combining two immiscible metals in one nanoparticle is difficult. In pure FeAg particles, we expect an iron core surrounded by a silver shell, given the difference in the surface tension of the two metals [24]. In chemical synthesis, segregated core–shell or dumbbell-shaped structures are typically obtained [9, 25, 26]. In the few reported cases of chemically prepared Fe-Ag nanoparticles, the core–shell structure prevails [13, 27, 28]. Both Fe@Ag and Ag@Fe particles have been obtained, showing that the fabrication sequence and presence of surfactants determine the structure. Such particles have already been tested for catalysis [29] and antimicrobial treatment [30]. One fabrication technique combining physical and chemical approaches involves generating metal vapor via laser vaporization in a liquid containing functionalizing molecules, typically thiols. Ramified structures termed nanotruffles were observed for the Fe-Ag system [31] and the use of this fabrication method for magnetically assembled SERS substrates has been reported [32]. This overview clearly demonstrates the complexity that one should expect concerning both the geometric structure of such nanoparticles and their physical and chemical properties. Before choosing adequate candidate systems for magneto-optical or magnetoplasmonic studies and applications, a thorough study based on complementary experimental and theoretical techniques is mandatory. In this article, we present such an exhaustive study for FeAg nanoparticles fabricated via laser vaporization in the gas phase, without chemical surfactants. The only reported study employing this technique for the fabrication of small clusters combining iron and silver demonstrated its feasibility, but the stoichiometry was shown to be very complex, indicating unfavorable Fe-Ag bonding [33]. We characterized our particles using methods such as electron microscopy, optical spectroscopy, magnetometry, and synchrotron-based X-ray spectroscopy. The consistent combination of complementary techniques allows discriminating between different

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structural possibilities and physical effects in a system as complex as iron-silver and thus permits the evaluation of the suitability of our particles for further studies and applications.

2 2.1

Experimental Sample preparation

Nanoparticles are produced using a conventional laser vaporization source [34]. The second harmonic of a nanosecond Nd3+:YAG pulsed laser is focused on the surface of a metallic target, generating an atomic plasma in the presence of a continuous flow of helium gas. The atomic vapor is rapidly cooled by collisions with the inert gas (static pressure of a few tens of mbars), which induces the nucleation and growth of small metal clusters (2–6 nm in diameter). The particle–gas mixture expands into vacuum through a conical nozzle, forming a supersonic beam toward a deposition chamber. Bimetallic clusters are produced from an Fe0.5Ag0.5 bi-metallic target (50%–50% atomic composition, Neyco sa., France). The study of many miscible and immiscible metals fabricated in this manner has shown that the mean stoichiometry in the nanoparticles is the same as that in the target rods [35, 36]. The nanoparticles are then deposited under softlanding conditions for electron microscopy, magnetic experiments, or optical experiments. For electron microscopy, ultrathin carbon films (Ted Pella Inc.) are used, and the particles are either left uncapped or protected by sandwiching them between two evaporated layers each several nanometers thick—in our case, amorphous carbon or silica. For ensemble measurements, the particles are co-deposited together with the protective matrix (amorphous carbon or silica) onto adequate substrates, which are fused silica for optical experiments and silicon wafers for magnetism and synchrotron experiments. For optical and magnetic measurements, the particle density is kept below ~ 1 vol.% in order to avoid inter-particle interactions, and the concentration of the sample for X-ray magnetic circular dichroism (XMCD) is increased (~ 5 vol.%) to increase the signal/noise ratio. No post-fabrication heat treatment is performed on the samples discussed

here. Two types of matrices are used to isolate the nanoparticles and protect them from thermal and chemical degradation: amorphous carbon for magnetic measurements and silica as a transparent oxide for optical measurements. Furthermore, both systems are investigated via electron microscopy. Amorphous carbon has been shown to be an excellent matrix for coalescence-free annealing [37–39] of mono- and bi-metallic clusters. A very thin layer of carbon on the order of 5 nm thick is generally sufficient for complete protection against oxidation, and the samples are stable under ambient conditions. For iron-containing bi-metallic nanoparticles, the miscibility between iron and carbon must be considered. In the size range discussed here (2–6 nm in diameter) it has been shown for some elemental combinations (FeRh [39], FePt [40]) that even though carbon interstitials may be formed at the interface between the metal particles and the matrix, this carbide can be decomposed through heat treatment, leading to pure metallic, chemically ordered FePt and FeRh clusters. In other elemental combinations, such as FeCo, a higher chemical affinity for carbide formation is observed, and stable iron carbides are detected [41]. Whether such an intact nanoparticle structure with a welldefined interface can be achieved for the FeAg system and to what degree carbide formation is an issue have not been addressed thus far. 2.2

Transmission electron microscopy (TEM)

Conventional TEM shows heterogeneous particles with a relatively spherical part surrounded by a more disordered structure, resembling a “nano-fried-egg”. These heterostructures are observed for all three investigated systems: Uncapped nanoparticles deposited onto an ultrathin carbon membrane as well as particles embedded in either silica or amorphous carbon matrices (Fig. 1). The differences in contrast are due to differences in the atomic number Z and indicate the segregation of silver (Z = 47) in the spherical, most contrasted part and iron (Z = 26) in the adjacent, less well-defined one. This assumption is confirmed by energy-dispersive X-ray spectroscopy (EDX) performed in the scanning TEM (STEM) mode, as shown in Fig. 2. Here the analysis of X-rays generated

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Figure 1 Low-resolution TEM images of FeAg nanoparticles deposited onto a carbon membrane and left uncapped (a), embedded in silica (b), and in amorphous carbon (c). Higher-magnification images of individual particles are shown in the insets. The same “fried egg” structure is observed in all three cases.

Figure 2 (a) STEM-HAADF image of an FeAg nanoparticle in silica. (b) EDX elemental mapping confirms the segregation into silver- and iron-containing parts. The spectral lines used are the L transition at 3 keV for silver and the K line at 6.4 keV for iron.

by the fast electrons allows for spatially resolved elemental analysis in combination with image collection using a high-angle annular dark field (HAADF) detector. To elucidate the crystalline structure and the chemical state of the FeAg nanoparticles, we perform high-resolution TEM (HR-TEM). Depending on the focusing conditions of the electron microscope, it is possible to increase the contrast for either the silveror iron-containing part of the nanoparticles, as shown in Figs. 3(a) and 3(b). For the spherical part of the particles, a crystalline atomic resolution is readily obtained for many particles (Figs. 3 and 4), and the fast Fourier transform (FFT) patterns are consistent with metallic silver in the fcc

form, although sometimes twinned or distorted. The observed patterns are along the [110], [111], [112], [311] directions, but multiply twinned structures such as decahedra and icosahedra are also observed (see the Electronic Supplementary Material (ESM)). The fcc structure of the Ag part is evidenced at a diameter as small as < 2 nm. However, the iron-containing part is more complicated to investigate. None of the images initially obtained indicate a crystalline lattice, and these parts of the nanoparticles appear amorphous. Only after several minutes of electron irradiation do crystalline structures appear. The amorphous parts crystallize under the electron beam into several domains (Fig. 4), but spots appear in the FFT patterns. In no case was clear evidence of metallic iron observed. For silica as a matrix, the observed structures agree with both magnetite (Fe3O4) and hematite (-Fe2O3) structures, but the number of spots is insufficient to discriminate between the two oxides. However, a comparison of the heats of formation for the different oxides clearly favors the magnetite phase [42]. Diffraction patterns were simulated using the JEMS software [43]. For carbon as a matrix, only a few spots are discernible after prolonged irradiation, and no clear attribution to specific carbides or other non-metallic structures is possible. This means that the samples evolve under the influence of the electron beam. The initially amorphous low-contrast parts of the nanoparticles containing iron are meta-stable and altered, whereas the silver nanospheres remain unchanged.


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Figure 3 (a) HR-TEM image of an FeAg nanoparticle in carbon. At the bottom left, the spherical silver part is visible. At the top and right, the amorphous iron carbide can be discerned. (b) Image of the same particle obtained under different focalization conditions, where the crystalline structure of the silver sphere is more clearly visible but the top and right parts are difficult to distinguish from the carbon matrix. (c) The FFT of the silver part (blue dashed line in (b)) is in very good agreement with silver fcc observed along the [110] direction.

and examples are shown in the ESM. All the observed distances (five and four values, respectively) agree with those expected in fcc Ag or, for some distances, bcc Fe, as shown in Table S1 in the ESM. No distances of crystallized silver oxide, iron oxides, or iron carbides are evidenced, corroborating the hypothesis of an amorphous iron-containing part of the nanoparticles. 2.3 Figure 4 (a) HR-TEM of an FeAg nanoparticle in silica; (b) FFT patterns of the corresponding areas, together with simulated patterns. Region 1 corresponds to silver fcc observed along the [1,2,1] axis. The FFT patterns of areas 2 and 3 are consistent with Fe3O4 along the [1,2,1] direction. The only unambiguous distance in the top crystallite is also consistent with Fe3O4, as shown in the inset. In an image obtained at a different focal length, an FFT pattern comparable to those of areas 2 and 3 is observed.

In some cases, the projected surface of the ironcontaining part is large, as shown in Fig. 3. This suggests partial wetting of the surface by the iron fraction during the chemical reaction, resulting in a “nano-fried-egg” structure. Selected-area electron diffraction (SAED) is a technique for the averaged detection of interplanar distances. A large area (approximately 100–500 nm in diameter) is irradiated by electrons, and the diffracting distances result in spots and rings. This technique averages over hundreds of particles and is thus complementary to the aforementioned single-particle investigations. We perform SAED measurements on Fe50Ag50 nanoparticles in both a-C and silica matrices,

Optical spectroscopy

The optical properties of Fe50Ag50 nanoparticles are investigated through absorption spectroscopy after the nanoparticles are embedded in a transparent silica matrix. The measurements are performed using linearly polarized light (transverse magnetic) and sample mounting at the Brewster angle in order to prevent Fabry–Perot interferences within the sample film. As shown in Fig. 5(a), the localized surface plasmon resonance typical for silver nanoparticles [1] is broadened, damped, and slightly redshifted but still clearly visible. This observation is in qualitative agreement with the structural characterization of a spherical metallic silver particle in a heterogeneous environment of silicon- and iron-oxide. The surface plasmon energy of silica-embedded nanoparticles is not size-dependent in this size range [44]. To verify the consistency of our assumed nanoparticle structure, we simulate the optical response of several test structures using a finite-element method (commercial COMSOL software). All the simulated particles are 5 nm in diameter and are embedded in a medium with the experimentally determined refractive


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material with a non-constant dielectric function in the visible range. The same qualitative trend is observed for concentric and fully segregated Janus-type geometries and is expected for structures deviating from a simple sphere. Thus, the optical measurements provide evidence for i) the loss of the metallic characteristic of iron and ii) chemical phase segregation between pure silver and an iron oxide-based dielectric domain, even if the exact morphology (centered or off-centered core–shell, Janus, etc.) cannot be inferred with certainty. Figure 5 (a) Optical absorption spectra of Fe50Ag50 nanoparticles (red, lower curve) with a mean diameter of 4.5 nm embedded in silica. The surface plasmon absorption peak for pure silver particles 4.0 nm in diameter embedded in silica (black, upper curve) is shown for comparison. (b) Simulated absorption spectra for five different compositions and geometries: pure silver, concentric Fe@Ag, and eccentric Ag@FexOy core–shell structures. The vertical dotted lines are for visual guidance and indicate the position of the surface plasmon peak for pure silver.

index of slightly porous silica (m = 2.14). The spectra for the concentric geometries are calculated analytically using Mie theory with the quasi-static approximation [45]. The agreement between the obtained curves and the two simulation methods is excellent, validating our approach. Figure 5(b) shows simulated absorption spectra for different chemical compositions and geometries. The Fe@Ag core–shell structure, which is expected from the thermodynamic considerations based on the surface tensions of iron and silver [24], has a predicted surface plasmon resonance at a considerably higher energy than that of pure silver; thus, we can rule out this geometry. However, the eccentric structures of a silver core in an iron oxide shell of varying stoichiometry show the same broadened and redshifted response observed in the experiment. Even though we cannot infer the exact local composition of the iron oxide parts from our experiments, our simulations show that the presence of oxygen systematically redshifts and broadens the resonance compared with that of a pure silver sphere. It should be emphasized that this resonance is a response of the whole complex system Ag@FexOy, in which FexOy may strongly affect the shape of the resonance because it is an absorbent

2.4

Magnetic properties

2.4.1 XMCD To further investigate the chemical composition of the iron-containing part of the FeAg nanoparticles and to see whether and to which degree the magnetic properties of iron are conserved, we perform different magnetic experiments on carbon-embedded FeAg nanoparticles. First, we use XMCD at the L2,3 edge of iron to derive the spin and angular momenta of the iron atoms. These experiments are conducted at the LNLS synchrotron in Campinas, Brazil. Figure 6 displays X-ray absorption curves for the two photon helicities. The XMCD signal shows a clear magnetic contrast. The iron atoms retain a magnetic polarization even when combined with silver and embedded in an amorphous carbon matrix. Using the well-known sum rules [46] and a value of Nh = 3.4 for

Figure 6 X-ray absorption (top) and XMCD (bottom) curves at the Fe L2,3 edge acquired at 17 K and a magnetic field of µ0H = 4.5 T.


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the number of holes in iron [47], we obtain a mean orbital momentum of mL = 0.075 μB and a mean spin moment of mS = 0.76 μB, resulting in a total moment of 0.83 μB. This value is significantly smaller than that for bulk iron (2.2 μB) but is clearly discernable. A doublet is observed at the L3 edge and, to a lesser degree, at the L2 edge: the iron is not in its purely metallic form. Comparison with spectra in the literature allows the exclusion of predominant oxidation, because all iron oxides, even in the non-stoichiometric form and at the nanoscale, display more complex X-ray absorption spectra and oscillating XMCD curves [48], owing to the multiplet splitting. On the other hand, amorphous iron carbides show the same doublet observed in our results [49], and by comparison we can estimate the composition around Fe30C70. We can thus identify the iron-containing parts of the nanoparticles, which are non-resolved in TEM, as an amorphous carbide, in agreement with the absence of stable crystalline structures in this carbon-rich region of the Fe-C phase diagram [50]. Note also that all stoichiometric iron carbides have reduced magnetic moments of 1.7–1.8 μB per atom [51] with respect to bulk iron and we can expect even lower values for the amorphous state. Magnetic meta-stability [52, 53] cannot be ruled out as a further source of reduced magnetic moments. 2.4.2 Magnetometry Additional magnetic measurements are performed using a superconducting quantum interference device (SQUID) magnetometer (MPMS XL5 from Quantum

Design, USA). We perform a complete magnetic characterization combining zero-field-cooled/field-cooled (ZFC/FC) susceptibility, magnetization m(H,T), and low-temperature isothermal remanent magnetization (IRM) curves. We then fit the entire magnetic response curves simultaneously by following the extended “triple fit” procedure, which has successfully been used in the past to treat and interpret the magnetic responses of both mono- and bi-metallic nanoparticles [38, 39, 54–57]. This redundant multiple fitting uses only a very small number of fit parameters and thus allows reliable values to be obtained for the magnetic diameter distribution and the magnetic anisotropy constant. The only adjustable parameters are the median particle size Dmag, its dispersion in a log-normal distribution wmag, an effective anisotropy constant Keff with a dispersion in a Gaussian distribution wK, and finally a possible biaxial component of the magnetic anisotropy described by the ratio of effective constants K2/Keff. An amplitude parameter accounts for the number of particles in the sample but is not of interest in this study. This procedure is highly sensitive to interparticle interactions and thus very efficient for the exclusion of spurious deviations from the initial hypothesis of non-interacting isolated macrospins. Figure 7 shows the experimental magnetic data together with the obtained fits. To reduce the ambiguity for the obtained values from the fits, we use a median particle size derived from TEM. Because the ironcontaining parts of the nanoparticles are poorly defined, we determine the volume distribution for the

Figure 7 Magnetic characterization of Fe50Ag50 nanoparticles in amorphous carbon. (a) “Triple fit”; experimental (points) ZFC/FC curves at 5 mT and m(H) at T = 200 K (inset) together with fits (lines). (b) low-temperature IRM data (points) and simulated response curves for uniaxial (dashed) and biaxial anisotropy with K2/Keff = 1 (solid line). www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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well-contrasted spherical silver parts of the particles (Fig. S1 in the ESM). According to the 50/50 composition of the target rod, we assume the same distribution for the number of iron atoms. Using this value and the magnetic moment per atom obtained from XMCD, we can well fit the experimental curves. The obtained values are Dmag = 2.5 nm (from TEM), wmag = 0.29 ± 0.03, Keff = 38 ± 5 kJ/m3, wK = 0.1 ± 0.1, and K2/Keff = 1 ± 0.5. Figure 7(b) clearly shows that a biaxial component of the magnetic anisotropy is necessary for reproducing the experimental data. A different rough estimate of the size of the ironcontaining parts from the magnetic measurements is possible. The IRM response, at least strictly at 0 K, does not depend on the particle size but only on the saturation magnetization Msat [55]. The other magnetic curves shown above depend on the magnetic moment, i.e., Msat multiplied by the volume. Thus, it is possible to obtain identical curves in the “triple fit” with a doubled volume and half the saturation magnetization, describing a strongly dilute system as far as iron is concerned, such as in an alloy nanoparticles. However, the IRM curve is impossible to reproduce; thus, we can rule out this case.

3

Results and discussion

All experimental results for the physically prepared Fe50Ag50 nanoparticles are consistent, and the overall image is as follows. The thermodynamic properties of the two metals are very different, the phase diagram shows no miscibility at any stoichiometry or temperature [20], and the surface tensions differ by a factor of two [24], suggesting silver enrichment at the surface. Consequently, the two metals segregate. Although we do not know the structure of the nanoparticles before deposition, we observe the same Janus-type separation for all investigated environments. This observation suggests environment-independent Janus-type segregation, in contrast to sequential fabrication via chemical methods [13, 27, 28], where core–shell structures were observed. Possibly, the chemical reaction of the iron with the environment changes the initial geometry and reverses the elemental order, as has been observed previously [58]. Thus, even though its high reactivity with the environment

determines the chemistry of the iron moiety, the resulting structures are very comparable, and consequently we discuss the results together. The silver forms metallic, near-spherical nanocrystallites with an fcc structure or sometimes multiply twinned decahedra or icosahedra, in agreement with previous observations [59–61]. The iron-containing part of the particles readily reacts with the surrounding matrix, forming metastable amorphous carbides or oxides. The latter can be transformed into multiply twinned crystallites under continuous irradiation in an electron microscope. The absence of crystallized iron phases has been observed in larger FeAg nanotruffles prepared via laser vaporization and functionalization in a liquid [31]. However, the formation of disordered alloy domains as proposed in that article was not observed in our case. The silver nanospheres exhibit a surface plasmon resonance in the blue part of the visible spectrum, in good agreement with the expected response for silver nanoparticles in an oxide environment. It is broadened and shifted with respect to the reference peak of bare silver particles in silica, a trend that is well reproduced in numerical simulations of the optical response of eccentric Ag@FexOy nanoparticles. The iron fraction maintains a reduced but significant magnetic moment despite the reaction with the carbon environment and the transformation into an amorphous carbide. All magnetic measurements can be reproduced by a small set of parameters and agree with the geometric characterization using electron microscopy. Both the magnetic moments per atom and the magnetic anisotropy are strongly reduced with respect to the iron bulk, and a strong biaxial component of the magnetic anisotropy is observed. All of this accords with the strongly heterogeneous shapes and the amorphous character of the iron carbide. Despite this combination of both properties in the same particle, the iron-silver system does not appear to be ideal for magneto-plasmonic studies, owing to the high reactivity of the exposed iron fraction and the resulting magnetism reduction and strong phase separation. For other iron-containing nanoparticles fabricated via the same preparation method (FeRh


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[39] and FePt [40]), chemically and thermally stable alloy particles have been obtained, but in this case, the two metals are miscible in the bulk phase. For iron and gold, which have very limited macroscopic miscibility, size-induced miscibility was reported in nanoparticles smaller than 10 nm [62] and for different stoichiometries [63]. The miscibility—macroscopic or nanoscale-induced—of iron in the other metal is the key for avoiding chemical reactions and thus the degradation of the particles. For iron and silver to be used in magneto-plasmonic nanoparticles, either the particles must be protected by an adequate ligand shell or miscibility must be induced, e.g., by a specific environment or temperature/pressure treatments. The protection of a pure iron particle by sequential deposition of a protective silver shell as demonstrated in Ref. [64] is another approach. Finally, we wish to comment on the reliability of the interpretation of the magnetic data. Owing to the relatively large diameter dispersion and the poor definition of the amorphous iron-containing structures, the magnetic response curves are ambiguous. Only after separate determination of some of the input parameters (magnetic moment per atom from XMCD and median particle size from TEM) is an unambiguous set of fit parameters obtained. A conservative estimate of the relative errors of the aforementioned values on the order of 10% quickly rises to well above 100% if the median size or the saturation magnetization are left as free fit parameters. This underscores the importance of the redundant fitting of several entire curves as done in the “triple fit” procedure, together with complementary experiments.

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Conclusions

We successfully fabricated surfactant-free small Fe50Ag50 nanoparticles via a physical route. Multiply twinned particles with heterogeneous phases were evidenced. We demonstrated the coexistence of both metals in the same nanoparticles, which consequently exhibited both plasmonic and magnetic responses. The metals segregated completely in all the investigated systems. The silver fraction was always found in metallic, fcc crystallized, or sometimes multiply twinned nanospheres, while the iron formed amorphous

structures that reacted with the surrounding matrix to form carbides or oxides. These iron compounds were meta-stable and crystallized under the electron beam of TEM. The Fe50Ag50 nanoparticles had a broadened but clearly discernable plasmon resonance with respect to the bare silver particles. The magnetic moment of the iron part was quantified and found to be reduced with respect to the bulk values of pure iron and its oxides. We attribute this reduction to the presence of carbides and the amorphous structure. The derived biaxial magnetic anisotropy was low, in accordance with the highly heterogeneous structures observed in electron microscopy, but was possible to quantify. In conclusion, we successfully demonstrated advanced experimental techniques necessary for the unambiguous characterization of complex bi-metallic nanoparticles comprising both plasmonic and magnetic responses.

Acknowledgements Financial support through a “Chaire Française dans l’État de São Paulo” and from the São Paulo Research Foundation (FAPESP, 2013/14262-7 and 16/12807-4) for M. H., from the Science Without Borders “Special Visiting Scientist” program, contract number 88881.030488/2013-01, and from the Region RhôneAlpes in the frame of an ARC (Academic Research Community) doctoral grant for J. R. is gratefully acknowledged. This work was performed using the Lyon Cluster Research Platform PLYRA, the Lyon Center for Microscopy CLYM, the Lyon Center for Magnetometry CML and at the Brazilian Nanotechnology National Laboratory (LNNano). The Laboratório Nacional de Luz Síncrotron (LNLS, Campinas, Brazil) is thanked for the use of the PGM beamline. We gratefully acknowledge technical support from C. Albin, C. Clavier and N. Blanchard in Lyon and from A. de Siervo, J. Bettini and J. Cezar in Campinas. Electronic Supplementary Material: Supplementary material (more information on the determined size distribution and example TEM images) is available in the online version of this article at https://doi.org/ 10.1007/s12274-018-2125-6.


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References [1]

[2] [3] [4] [5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters. Springer Series in Materials Science; Springer: Berlin, 1995. Coey, J. M. D. Magnetism and Magnetic Materials; Cambridge University Press: Cambridge, 2010. Alloyeau, D.; Mottet, C.; Ricolleau, C. Nanoalloys: Synthesis, Structure and Properties; Springer-Verlag: London, 2012. Calvo, F. Nanoalloys: From Fundamentals to Emergent Applications; Elsevier: USA, 2013. Ferrando, R. Structure and properties of nanoalloys. In Frontiers of Nanoscience; Elsevier: Amsterdam, 2016; pp 2–337. García, S.; Zhang, L.; Piburn, G. W.; Henkelman, G.; Humphrey, S. M. Microwave synthesis of classically immiscible rhodium–silver and rhodium–gold alloy nanoparticles: Highly active hydrogenation catalysts. ACS Nano 2014, 8, 11512–11521. Sotiriou, G. A.; Visbal-Onufrak, M. A.; Teleki, A.; Juan, E. J.; Hirt, A. M.; Pratsinis, S. E.; Rinaldi, C. Thermal energy dissipation by SiO2-coated plasmonic-superparamagnetic nanoparticles in alternating magnetic fields. Chem. Mater. 2013, 25, 4603–4612. Armelles, G.; Cebollada, A.; García-Martín, A.; González, M. U. Magnetoplasmonics: Combining magnetic and plasmonic functionalities. Adv. Opt. Mater. 2013, 1, 10–35. Peng, S.; Lei, C. H.; Ren, Y.; Cook, R. E.; Sun, Y. G. Plasmonic/magnetic bifunctional nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 3158–3163. Hao, R.; Xing, R. J.; Xu, Z. C.; Hou, Y. L.; Gao, S.; Sun, S. H. Synthesis, functionalization, and biomedical applications of multifunctional magnetic nanoparticles. Adv. Mater. 2010, 22, 2729–2742. Wang, J. F.; Wu, X. Z.; Wang, C. W.; Shao, N. S.; Dong, P. T.; Xiao, R.; Wang, S. Q. Magnetically assisted surfaceenhanced Raman spectroscopy for the detection of Staphylococcus aureus based on aptamer recognition. ACS Appl. Mater. Interfaces 2015, 37, 20919–20929. Bogani, L.; Cavigli, L.; de Julián Fernández, C.; Mazzoldi, P.; Mattei, G.; Gurioli, M.; Dressel, M.; Gatteschi, D. Photocoercivity of nano-stabilized Au:Fe superparamagnetic nanoparticles. Adv. Mater. 2010, 22, 4054–4058. Lu, L. Y.; Zhang, W. T.; Wang, D.; Xu, X. G.; Miao, J.; Jiang, Y. Fe@Ag core–shell nanoparticles with both sensitive plasmonic properties and tunable magnetism. Mater. Lett. 2010, 64, 1732–1734. Moscoso-Londoño, O.; Muraca, D.; Tancredi, P.; Cosio-Castañeda, C.; Pirota, K. R.; Socolovsky, L. M.

[15]

[16]

[17]

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

Physicochemical studies of complex silver–magnetite nanoheterodimers with controlled morphology. J. Phys. Chem. C 2014, 118, 13168–13176. Mahmoudi, M.; Serpooshan, V. Silver-coated engineered magnetic nanoparticles are promising for the success in the fight against antibacterial resistance threat. ACS Nano 2012, 6, 2656–2664. Lin, A. Y.; Young, J. K.; Nixon, A. V.; Drezek, R. A. Encapsulated Fe3O4/Ag complexed cores in hollow gold nanoshells for enhanced theranostic magnetic resonance imaging and photothermal therapy. Small 2014, 10, 3246–3251. Han, X. X.; Schmidt, A. M.; Marten, G.; Fischer, A.; Weidinger, I. M.; Hildebrandt, P. Magnetic silver hybrid nanoparticles for surface-enhanced resonance Raman spectroscopic detection and decontamination of small toxic molecules. ACS Nano 2013, 7, 3212–3220. Wang, H.; Shen, J.; Li, Y. Y.; Wei, Z. Y.; Cao, G. X.; Gai, Z.; Hong, K. L.; Banerjee, P.; Zhou, S. Q. Porous carbon protected magnetite and silver hybrid nanoparticles: Morphological control, recyclable catalysts, and multicolor cell imaging. ACS Appl. Mater. Interfaces 2013, 5, 9446–9453. Andreazza, P.; Pierron-Bohnes, V.; Tournus, F.; AndreazzaVignolle, C.; Dupuis, V. Structure and order in cobalt/ platinum-type nanoalloys: From thin films to supported clusters. Surf. Sci. Rep. 2015, 70, 188–258. Swartzendruber, L. J. The Ag–Fe (silver–iron) system. Bull. Alloy Phase Diag. 1984, 5, 560–564. Kataoka, N.; Sumiyama, K.; Nakamura, Y. Nonequilibrium crystalline Fe-Ag alloys vapour-quenched on liquid-nitrogencooled substrates. J. Phys. F: Met. Phys. 1988, 18, 1049–1056. Wan, H.; Tsoukatos, A.; Hadjipanayis, G. C.; Li, Z. G.; Liu, J. Direct evidence of phase separation in as-deposited Fe(Co)-Ag films with giant magnetoresistance. Phys. Rev. B 1994, 49, 1524–1527. Sakurai, M.; Makhlouf, S. A.; Sumiyama, K.; Wakoh, K.; Suzuki, K. Extended X-ray absorption fine structure study on local structure around Fe atoms in Fe/Ag granular materials. Jpn. J. Appl. Phys. 1994, 33, 4090. Yaws, C. L. Chemical Properties Handbook: Physical, Thermodynamics, Environmental, Transport, Safety, and Health Related Properties for Organic and Inorganic Chemicals; McGraw-Hill: New York, 1999. Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From theory to applications of alloy clusters and nanoparticles. Chem. Rev. 2008, 108, 845–910. Pellarin, M.; Issa, I.; Langlois, C.; Lebeault, M.-A.; Ramade, J.; Lermé, J.; Broyer, M.; Cottancin, E. Plasmon spectroscopy and chemical structure of small bimetallic


11

Nano Res.

Cu(1–x)Agx clusters. J. Phys. Chem. C 2015, 119, 5002–5012. [27] Carroll, K. J.; Hudgins, D. M.; Spurgeon, S.; Kemner, K. M.; Mishra, B.; Boyanov, M. I.; Brown, L. W., III.; Taheri, M. L.; Carpenter, E. E. One-pot aqueous synthesis of Fe and Ag core/shell nanoparticles. Chem. Mater. 2010, 22, 6291–6269. [28] Wang, L.; Yang, K.; Clavero, C.; Nelson, A. J.; Carroll, K. J.; Carpenter, E. E.; Lukaszew, R. A. Localized surface plasmon resonance enhanced magneto-optical activity in core-shell Fe–Ag nanoparticles. J. Appl. Phys. 2010, 107, 09B303. [29] Luo, S.; Yang, S. G.; Wang, X. D.; Sun, C. Reductive degradation of tetrabromobisphenol A over iron–silver bimetallic nanoparticles under ultrasound radiation. Chemosphere 2010, 79, 672–678. [30] Marková, Z.; Šišková, K. M.; Filip, J.; Čuda, J.; Kolář, M.; Šafářová, K.; Medřík, I.; Zbořil, R. Air stable magnetic bimetallic Fe–Ag nanoparticles for advanced antimicrobial treatment and phosphorus removal. Environ. Sci. Technol. 2013, 47, 5285–5293. [31] Amendola, V.; Scaramuzza, S.; Agnoli, S.; Granozzi, G.; Meneghetti, M.; Campo, G.; Bonanni, V.; Pineider, F.; Sangregorio, C.; Ghigna, P. et al. Laser generation of iron-doped silver nanotruffles with magnetic and plasmonic properties. Nano Res. 2015, 8, 4007–4023. [32] Scaramuzza, S.; Badocco, D.; Pastore, P.; Coral, D. F.; Fernández van Raap, M. B.; Amendola, V. Magnetically assembled SERS substrates composed of iron-silver nanoparticles obtained by laser ablation in liquid. ChemPhysChem 2017, 18, 1026–1034. [33] Andrews, M. P.; O’Brien, S. C. Gas-phase “molecular alloys” of bulk immiscible elements: Iron-silver (FexAgy). J. Phys. Chem. 1992, 96, 8233–8241. [34] Perez, A.; Dupuis, V.; Tuaillon-Combes, J.; Bardotti, L.; Prevel, B.; Bernstein, E.; Mélinon, P.; Favre, L.; Hannour, A.; Jamet, M. Functionalized cluster-assembled magnetic nanostructures for applications to high integration-density devices. Adv. Eng. Mater. 2005, 7, 475–485. [35] Rousset, J. L.; Cadrot, A. M.; Cadete Santos Aires, F. J.; Renouprez, A.; Mélinon, P.; Perez, A.; Pellarin, M.; Vialle, J. L.; Broyer, M. Study of bimetallic Pd–Pt clusters in both free and supported phases. J. Chem. Phys. 1995, 102, 8574–8585. [36] Tournus, F.; Blanc, N.; Tamion, A.; Hillenkamp, M.; Dupuis, V. Dispersion of magnetic anisotropy in size-selected CoPt clusters. Phys. Rev. B 2010, 81, 220405. [37] Tournus, F.; Blanc, N.; Tamion, A.; Dupuis, V.; Epicier, T. Coalescence-free L10 ordering of embedded CoPt nanoparticles. J. Appl. Phys. 2011, 109, 07B722. [38] Tamion, A.; Hillenkamp, M.; Hillion, A.; Tournus, F.;

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

Tuaillon-Combes, J.; Boisron, O.; Zafeiratos, S.; Dupuis, V. Demixing in cobalt clusters embedded in a carbon matrix evidenced by magnetic measurements. J. Appl. Phys. 2011, 110, 063904. Hillion, A.; Cavallin, A.; Vlaic, S.; Tamion, A.; Tournus, F.; Khadra, G.; Dreiser, J.; Piamonteze, C.; Nolting, F.; Rusponi, S. et al. Low temperature ferromagnetism in chemically ordered FeRh nanocrystals. Phys. Rev. Lett. 2013, 110, 087207. Dupuis, V.; Khadra, G.; Linas, S.; Hillion, A.; Gragnaniello, L.; Tamion, A.; Tuaillon-Combes, J.; Bardotti, L.; Tournus, F.; Otero, E. et al. Magnetic moments in chemically ordered mass-selected CoPt and FePt clusters. J. Magn. Magn. Mater. 2015, 383, 73–77. Dupuis, V.; Robert, A.; Hillion, A.; Khadra, G.; Blanc, N.; Le Roy, D.; Tournus, F.; Albin, C.; Boisron, O.; Tamion, A. Cubic chemically ordered FeRh and FeCo nanomagnets prepared by mass-selected low-energy cluster-beam deposition: A comparative study. Beilstein J. Nanotechnol. 2016, 7, 1850–1860. Navrotsky, A.; Ma, C. C.; Lilova, K.; Birkner, N. Nanophase transition metal oxides show large thermodynamically driven shifts in oxidation-reduction equilibria. Science 2010, 330, 199–201. Pierre Stadelmann. JEMS Electron Microscopy Simulation Software. http://www.jems-saas.ch or https://cime.epfl.ch/ research/jems (accessed Jan 24, 2018). Hillenkamp, M.; Di Domenicantonio, G.; Eugster, O.; Félix, C. Instability of Ag nanoparticles in SiO2 at ambient conditions. Nanotechnology 2007, 18, 015702. Cottancin, E.; Broyer, M.; Lermé, J.; Pellarin, M. Optical properties of metal clusters and nanoparticles. In Handbook of Nanophysics: Nanoelectronics and Nanophotonics; Sattler, K. D., Ed.; CRC Press: Boca Raton, FL, USA, 2011. Chen, C. T.; Idzerda, Y. U.; Lin, H.-J.; Smith, N. V.; Meigs, G.; Chaban, E.; Ho, G. H.; Pellegrin, E.; Sette, F. Experimental confirmation of the X-ray magnetic circular dichroism sum rules for iron and cobalt. Phys. Rev. Lett. 1995, 75, 152–155. Šipr, O.; Ebert, H. Theoretical Fe L2,3- and K-edge X-ray magnetic circular dichroism spectra of free iron clusters. Phys. Rev. B 2005, 72, 134406. Pellegrin, E.; Hagelstein, M.; Doyle, S.; Moser, H. O.; Fuchs, J.; Vollath, D.; Schuppler, S.; James, M. A.; Saxena, S. S.; Niesen, L. et al. Characterization of nanocrystalline γ-Fe2O3 with synchrotron radiation techniques. Phys. Status Solidi (b) 1999, 215, 797–801. Furlan, A.; Jansson, U.; Lu, J.; Hultman, L.; Magnuson, M.


Research

12

[50] [51] [52]

[53]

[54]

[55]

[56]

[57]

Nano Res.

Structure and bonding in amorphous iron carbide thin films. J. Phys.: Condens. Matter 2015, 27, 045002. Okamoto, H. The C-Fe (carbon-iron) system. J. Phase Equil. 1992, 13, 543–565. Hofer, L. J. E.; Cohn, E. M. Saturation magnetizations of iron carbides. J. Am. Chem. Soc. 1959, 81, 1576–1582. Xu, X. S.; Yin, S. Y.; Moro, R.; Liang, A.; Bowlan, J.; de Heer, W. A. Metastability of free cobalt and iron clusters: A possible precursor to bulk ferromagnetism. Phys. Rev. Lett. 2011, 107, 057203. Balan, A.; Derlet, P. M.; Rodríguez, A. F.; Bansmann, J.; Yanes, R.; Nowak, U.; Kleibert, A.; Nolting, F. Direct observation of magnetic metastability in individual iron nanoparticles. Phys. Rev. Lett. 2014, 112, 107201. Tamion, A.; Hillenkamp, M.; Tournus, F.; Bonet, E.; Dupuis, V. Accurate determination of the magnetic anisotropy in cluster-assembled nanostructures. Appl. Phys. Lett. 2009, 95, 062503. Hillion, A.; Tamion, A.; Tournus, F.; Gaier, O.; Bonet, E.; Albin, C.; Dupuis, V. Advanced magnetic anisotropy determination through isothermal remanent magnetization of nanoparticles. Phys. Rev. B 2013, 88, 094419. Oyarzún, S.; Tamion, A.; Tournus, F.; Dupuis, V.; Hillenkamp, M. Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: From shape to surface. Sci. Rep. 2015, 5, 14749. Dupuis, V.; Khadra, G.; Hillion, A.; Tamion, A.; TuaillonCombes, J.; Bardotti, L.; Tournus, F. Intrinsic magnetic properties of bimetallic nanoparticles elaborated by cluster beam deposition. Phys. Chem. Chem. Phys. 2015, 17, 27996.

[58] Tao, F.; Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles. Science 2008, 322, 932–934. [59] Li, C. R.; Lu, N. P.; Xu, Q.; Mei, J.; Dong, W. J.; Fu, J. L.; Cao, Z. X. Decahedral and icosahedral twin crystals of silver: Formation and morphology evolution. J. Cryst. Growth 2011, 319, 88–95. [60] Volk, A.; Thaler, P.; Koch, M.; Fisslthaler, E.; Grogger, W.; Ernst, W. E. High resolution electron microscopy of Agclusters in crystalline and non-crystalline morphologies grown inside superfluid helium nanodroplets. J. Chem. Phys. 2013, 138, 214312. [61] Barke, I.; Hartmann, H.; Rupp, D.; Fluckiger, L.; Sauppe, M.; Adolph, M.; Schorb, S.; Bostedt, C.; Treusch, R.; Peltz, C. et al. The 3D-architecture of individual free silver nanoparticles captured by X-ray scattering. Nat. Commun. 2015, 6, 6187. [62] Mukherjee, P.; Zhang, Y.; Kramer, M. J.; Lewis, L. H.; Shield, J. E. L10 structure formation in slow-cooled Fe–Au nanoclusters. Appl. Phys. Lett. 2012, 100, 211911. [63] Mukherjee, P.; Manchanda, P.; Kumar, P.; Zhou, L.; Kramer, M. J.; Kashyap, A.; Skomski, R.; Sellmyer, D.; Shield, J. E. Size-induced chemical and magnetic ordering in individual Fe–Au nanoparticles. ACS Nano 2014, 8, 8113–8120. [64] Binns, C.; Qureshi, M. T.; Peddis, D.; Baker, S. H.; Howes, P. B.; Boatwright, A.; Cavill, S. A.; Dhesi, S. S.; Lari, L.; Kröger, R. et al. Exchange bias in Fe@Cr core–shell nanoparticles. Nano Lett. 2013, 13, 3334–3339.
