Koten CoreShell

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Core–shell nanoparticles are known to form in binary systems using a one-step gas-condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in sur-face energy between component atoms creates a preferential surface phase leading to a core–shell structure. Here, core–shell nanoparticles have been observed in systems with enthalpy as low as −5 kJ mol −1 and a surface energy difference of 0.5 J m −2 (Mo–Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The com-positions and size dependence of the core–shell structures are also compared and no core–shell structures are observed below a critical size of 8 nm.

While the creation of core–shell nanopar-ticles in a bulk aqueous phase is a two-step process that requires separation and post treatments to obtain a pure sample, it has distinct advantages over other techniques when it comes to scale-up and incorpo-rating organic materials into both core and shell. In the case of hydrophilic nanoparti-cles, which are often found in magnetic or semiconductor materials, it is a challenge to produce smaller particles with narrow size distributions, shells of uniform thick-ness, and particles that do not agglom-erate. [ 2 ] Therefore, alternative methods for producing core–shell nanoparticles

may expand the availability of core and shell materials in the case of inorganic–inorganic core–shell structures and provide better control over size, homogeneity, and even agglomeration prevention.

Nanoparticles can be formed in high-vacuum systems where atoms are condensed from the gaseous state directly into solid clusters of atoms that range in size from 1 to 100 nm. [ 16,17 ] These clusters form in an aggregation chamber, where they nucleate and grow in the presence of Ar or He atoms, and are then collimated into a beam by differential pumping through multiple apertures for deposition onto a substrate. Sizes can be tuned using the processing parameters or by adding a mass fi l-tering device in series with the cluster beam. In addition to cre-ating size-selected particle distributions, these devices can sort out particles with like charge and deposit them onto different surfaces. [ 18 ] The electrostatic repulsion between like-charged particles will provide a barrier for interparticle agglomeration, which can be caused by van der Waals forces or magnetic attrac-tion. [ 8 ] The disadvantages of this method include diffi culty in producing large quantities of nanoparticles effi ciently and inex-pensively. The challenge to cleanly transfer the nanoparticles from vacuum to solution has been circumvented in recent pub-lications by Binns et al. [ 19 ] and Petra et al. [ 20 ]

Core–shell nanoparticles have been successfully created in both one- and two-step physical vapor deposition (PVD) pro-cesses. The two-step process consists of formation of the core material fi rst and then coating the particles as they fl ow through an evaporator. [ 19 ] This method applies to shell materials that can be thermally evaporated in a vacuum and so encompasses many combinations. Simultaneously, in the one-step method, binary alloys with a high, positive enthalpy of mixing Δ H mix [ 21,22 ] can lead to immiscibility of the component atoms in both the liquid and solid state, and cause phase separation upon condensa-tion. [ 23 ] Binary systems of this nature are easily recognizable

M. A. Koten, Dr. P. Mukherjee, Prof. J. E. Shield Nebraska Center for Materials and Nanoscience and Department of Mechanical and Materials Engineering University of Nebraska Lincoln , NE 68588 , USA E-mail: [email protected]

DOI: 10.1002/ppsc.201500019

Core–Shell Nanoparticles Driven by Surface Energy Differences in the Co–Ag, W–Fe, and Mo–Co Systems

Mark A. Koten , Pinaki Mukherjee , and Jeffrey E. Shield*

1. Introduction

Nanoalloys often have unique physical, chemical, magnetic, and optical properties relative to the corresponding bulk alloys and it has been shown in a wide variety of applications that a protective shell surrounding a core nanoparticle can improve the general stability, properties, passivation, functionalization, biocompatibility, and other aspects. [ 1,2 ] Many reviews have emerged in recent years that have elaborated on the applica-tions of core–shell nanostructures in the fi elds of semicon-ductors, [ 3–5 ] biological and medical applications, [ 6–8 ] catalysis, fl uorescence, [ 9 ] and the wide array of synthesis techniques available. [ 10 ] The majority of the work done on core–shell nano-particles combines late-group transition metals like Cd with p-block metals, semimetals, and nonmetals for semiconductor applications and passivation. Another popular combination has been the ferromagnetic 3d transition metals and noble metals for biomagnetic applications. Many combinations of core and shell material have not yet been explored and their properties remain unknown. [ 2 ]

Currently, various chemical synthesis techniques are the most popular route for creating core–shell nanoparticles. The two most common chemical practices are reduction by sodium borohydride [ 11 ] and coprecipitation, [ 12 ] but thermal decompo-sition [ 13,14 ] and the sol–gel [ 15 ] methods are also widely used.

Part. Part. Syst. Charact. 2015, DOI: 10.1002/ppsc.201500019

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from their phase diagram. Figure 1 a is a schematic phase dia-gram showing liquid immiscibility between the two component elements A and B. Such systems do not usually form interme-tallic phases under equilibrium conditions and some examples include the Co–Ag and Fe–Ag systems.

Additionally, different pure elements are known to have dif-ferent surface energies γ depending on the crystallographic orientation of the exposed region and number of dangling or broken bonds. These affects become particularly infl uential at the nanoscale due to higher surface to volume ratios. The average surface energies for some of the low index surfaces in 3d–5d transition metals are shown relative to one another in Figure 1 b. [ 24,25 ] These surface energies tend to be similar for elements found in the same IUPAC group. Surface ener-gies are typically between 1 and 2 J m −2 in groups 3 and 12 and are highest between groups 6–8 (>3 J m −2 ). Groups 1, 2, and 13–16 tend to have even lower surface energy than the transi-tion metals (< 1 J m −2 ) and many can be sputtered in vacuum. Therefore, researchers making use of liquid immiscibility and surface energy differences Δ γ have been able to created core–shell nanoparticles using gas condensation. [ 23,26,27 ] Here, it is proposed that this procedure can also be extended to systems with Δ H mix ≤ 0 because of the increased importance of surface effects at the nanoscale. This opens the door to a wider selec-tion of systems that could form in the core–shell structure using this method and leads to the possibility of designing par-ticles with specifi c materials in either the core or shell as shown in Figure 1 c.

Within the systems of Co–Ag, W–Fe, and Mo–Co, core–shell nanoparticles were created using gas-condensation PVD and characterized by scanning transmission electron microscopy (STEM) using high angle annular dark fi eld (HAADF) imaging and elemental energy dispersive X-ray spectroscopy (EDS) map-ping. Particular attention was paid to the composition, particle

size, surface energy difference, and the enthalpy of mixing values between the core and shell materials. Also, enthalpy is known to be a function of particle size and a transition between mixing and phase separation was identifi ed where pos-sible. [ 28,29 ] The three systems were chosen because of their ther-modynamic properties, which are listed in Table 1 . Here, the Co–Ag system is considered the ideal case because both Δ γ and Δ H mix are large and positive and both provide a driving force toward separation. However, the W–Fe and Mo–Co systems are not ideal because they have Δ H mix values of zero and −5 kJ mol −1 , respectively. In these systems there should be no driving force for phase separation. In the Mo–Co system the negative Δ H mix suggests mixing is favorable in bulk systems, but core–shell structures were observed in all three systems regardless of the thermodynamics. This is explained by the driving force for separation shifting from enthalpy to surface energy difference at the nanoscale.

2. Results and Discussion

2.1. Structure and Size

TEM micrographs of the as-deposited nanoparticles for all three systems are shown in Figure 2 . The high-resolution image of the Co–Ag nanoparticle (Figure 2 b) displayed Moiré fringes, indicating that both the core and shell are crystalline. The fast Fourier transform (FFT) inset reveals multiple single crystal diffraction spots, which were indexed to the [001] zone axes of fcc Co and Ag structures. The labeled spots correspond to the (220) planes and the secondary spots are the product of double diffraction. The mismatch in lattice parameters between the pure Co (3.563 Å) core and Ag (4.0863 Å) shell suggests that these are translational Moiré fringes. [ 30 ] The high resolution image and FFT of a W–Fe core–shell nanoparticle was indexed to the [001] zone axis of a bcc structure with a lattice parameter a = 3.01 ± 0.07 Å. Since lattice planes were only observed within the core, the core has a bcc structure with a lattice parameter that lies between the bulk values for α-Fe and bcc-W, suggesting an extended solid solution beyond equilibrium values. [ 31 ] Simi-larly, the Mo–Co core was indexed to the [111] zone axis of the bcc-Mo structure with a lattice parameter of 3.13 ± 0.06 Å. In the case of the shell material for both the W–Fe and Mo–Co systems, the shell contains nominally pure Fe and Co, respec-tively, with no apparent crystallinity, as no lattice fringes were observed. The absence of Moiré fringes in the region where the core and shell overlap further supports the amorphicity of the shell.


Figure 1. a) Schematic phase diagram of a system with liquid immisci-bility, where A C and B S are the core and shell materials respectively. b) The relative surface energies for groups 3–12 of the periodic table (green > blue > red). c) Design concept summary: larger particles with different surface energies will form core–shell structures and blue elements (4, 5, 8, 9) can be placed in either the core or the shell.

Table 1. Thermodynamic data including the surface energies [ 24,25 ] of the core and shell material γ c and γ s and the enthalpy of mixing [ 21,22 ] between core and shell atoms Δ H mix .

Sample (core–shell)

γ c [J m −2 ]

γ s [J m −2 ]

Δ H mix [kJ mol −1 ]

Co–Ag 3.2 ± 0.5 1.20 ± 0.03 +19

W–Fe 4.3 ± 0.2 2.5 ± 0.2 0

Mo–Co 3.7 ± 0.2 3.2 ± 0.5 −5

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To study the effects of different size distributions on the core–shell formation process, different aperture sizes were used to produce three different size distributions in each of the samples. These size distributions were fi tted with a Gaussian function and are plotted in Figure 3 . The Co–Ag nanoparticles have a bimodal size distribution with the majority of the par-ticles centering on 20.6 ± 3.5 nm and a small fraction of par-ticles that average 8.3 ± 1.4 nm in diameter. The W–Fe and Mo–Co particles have mean sizes of 9 ± 3 and 6 ± 2 nm, respectively. In each of the samples it was found that smaller particles did not separate into the core–shell morphology but formed a single phase. The W–Fe system was ideal for inves-tigating the critical size below which mixing was favored and above which separation into core–shell structures occurred since W has both the highest surface energy and largest atomic number Z of any element used in this study. The higher surface energy confi nes it to the core, while its large Z makes it the brighter element in a HAADF–STEM image because of its large scattering probability. By comparison, the Co–Ag system has the heavier element in the shell, which completely surrounds the core, and reduces the contrast of the core using HAADF–STEM.

For the Fe–W system, the critical size above which a core–shell structure formed was determined to be 8 nm. The smaller particles ( Figure 4 a) do not have core–shell contrast in com-parison to larger particles (Figure 4 b), which show clear atomic number contrast between the core and the shell. There may be several contributing factors to the size-dependency of the core–shell structure formation. First, in the Fe–W system the smaller particles appear to be amorphous. Thus, atomic mixing is rela-tively easy compared to crystalline systems where specifi c limits in solid solubility may be important. However, amorphicity may not be the only factor infl uencing core–shell formation, as in other systems phase segregation (but not core–shell formation)


Figure 2. Electron micrographs of a,b) Co–Ag, c,d) W–Fe, and e,f) Mo–Co nanoparticles at different magnifi cations. Insets: indexed fast Fourier transforms of the high-resolution images.

Figure 3. Gaussian size distribution fi ts for each sample. Core–shell for-mation is favored in particles larger than 8 nm in diameter.

Figure 4. HAADF–STEM and high resolution TEM images of W–Fe parti-cles with sizes a) 6 nm, b) 15 nm, c) 8 nm, and d) FFT of particle in (c).

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has been observed to be size-dependent even when the smaller particles were crystalline. [ 28,29 ]

The transition to a core–shell structure with increasing size may also be thermodynamically driven. The free energy of a single particle will have energy contributions from the indi-vidual components in the unmixed state, the mixed state, and the surface. A core–shell particle will have an additional energy contribution from the interphase interface between the core and shell atoms. For an ideal system with Δ H mix = 0 such as W–Fe, it is more energetically favorable to form this interface and remove W from the particle surfaces, since W has a high surface energy (Table 1 ). The size dependency to core–shell formation observed here arises because the difference in free energy between these two states becomes negligibly small so it is easier for the particle to not form an interphase interface below 8 nm. Further, theoretical work suggests that the heat of mixing is size-dependent so that solid solution formation is easier at smaller particle sizes. [ 29 ] This makes the driving force for segregation even lower, ultimately resulting in a mixed state at small particle sizes.

2.2. Composition and Formation Characteristics

Figures 5,6, 7 show the corresponding elemental maps of the nanoparticles, and from these data, information about the core, shell, and overall composition was extracted. First, estimates of the shell thickness were made using the line scan data by taking the start of the shell to be the place where the lines cross or diverge and the end of the shell to be where the lines are both at zero. This results in the shell making up 18% ± 3%, 18% ± 1%, and 16% ± 1% of the overall particle diameter for Co–Ag,

W–Fe, and Mo–Co, respectively. Using the mean diameter of the Co–Ag particles (20.6 nm), this corresponds to a 13.2 nm diameter core with a 3.7 nm shell. The shell contribution to the core composition was subtracted using these ratios. The com-position of the core alone was compared to that of the entire


Figure 5. a) HAADF–STEM image of a Co–Ag core–shell nanoparticle. b) Counts hyper-map and c) Q-map displayed in at% ratios reveal beam damage to Ag shell layer. d) Line scan plot across yellow line shown in (c) is noisy because of shell damage.

Figure 7. a) HAADF–STEM image of a Mo–Co core–shell nanoparticle. b) Counts hyper-map and c) Q-map displayed in at% ratios reveal core-shell contrast. d) Line scan plot across yellow line shown in (c) is very stable especially in the shell region.

Figure 6. a) HAADF–STEM image of a W–Fe core–shell nanoparticle. b) Counts hyper-map and c) Q-map displayed in at% ratios reveal clear core–shell color contrast. d) Line scan plot across yellow line shown in (c) is less noisy than for the Co–Ag particle.

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particle to verify core–shell production, and the composition of a single particle was compared to the overall composition of several hundred randomly selected particles. In doing this, an estimate of the uniformity of the core–shell particles can be obtained. The results of this analysis are shown in Table 2 , which provides good evidence that the particles are both core–shell and highly uniform. It is also interesting to note that the solubility limit of Fe in W is 2 at% Fe and for Co in Mo is 10 at% Co. The core compositions of these nanoparticles indi-cate that the solubilities are increased in both of these systems relative to the equilibrium phase diagrams. [ 32 ]

In general, the starting composition is very important for designing core–shell structures because it will affect the thick-ness of the shell and can also force mixing within the core if the composition is shell component-rich. This is particularly important if complete separation or alloying within the core is desired. The results of Table 2 indicate that for a target com-position of 61 at% Co, no Ag is observed in the core. For the surface-driven systems, as much as 40% of the W core is Fe, while only about 16% of the Mo core is Co. Since the target composition for both these systems was about 20 at% core, it is clear that the phase separation is more complete in the Mo–Co particles compared to the W–Fe particles. Given that W–Fe has the larger Δ γ of the two systems, one would expect this system to have more complete phase separation. This can perhaps be accounted for by more rapid diffusion rates in the Mo–Co system that allow for more phase separation to occur before solidifi cation. [ 33 ] This brief discussion gives some idea of the thermodynamic and kinetic considerations necessary for formation of a specifi c core composition, which could be par-ticularly useful if a desired alloy is targeted as the core material.

3. Conclusions

In summary, this paper demonstrates that a measure of con-trol over the core and shell material is achievable in a one-step, gas-condensation synthesis route for creating core–shell nanoparticles. It has been shown that while a system with suf-fi ciently large Δ γ and positive Δ H mix makes up the ideal case for core–shell formation, the dominance of Δ γ at the nanoscale can be used to create core–shell structures in systems that have both zero and small but negative enthalpies of mixing (i.e., −5 ≤ Δ H mix ≤ 0 kJ mol −1 ). For a surface energy difference as small as 0.5 J m −2 and a negative enthalpy of mixing, core–shell structures were observed in the Mo–Co system. Addi-tionally, the size dependence of the core–shell phenomenon

was investigated and was found that particles must be larger than a critical size to exhibit this morphology. For the W–Fe system that size was determined to be 8 nm. Finally, the overall composition was compared to the fi nal composition distribu-tion within the particles. From this analysis, the particles were shown to be very uniform and the phase separation between core and shell was of high quality. This method can be used on many other systems and may prove to be a quick way to create inorganic core–shell structures with elements that have not yet been incorporated into the current database of core–shell nanoparticles.

4. Experimental Section Synthesis : The nanoparticles were fabricated inside the condensation

chamber of a dc magnetron sputtering PVD system. This chamber was home built, but the design is similar to those that are now commercially available (e.g., Mantis Deposition, Ltd.’s nanogen series). The background pressure was 10 −7 torr, which rose to 10 −1 torr during operation. A mixture of He and Ar gas was used in a ratio of 1:4. The target was a composite Fe or Co disk 3 inches in diameter with W, Mo, or Ag inserts embedded in the surface and overlapping the racetrack. The power supplied to the target was 50 W (254 V), and both the chamber and sputtering gun were water cooled. Differently sized apertures (2.5–5.5 mm) were used to control the size distribution of the particles in vacuum. The particles were deposited onto Cu TEM grids coated in C for imaging analysis. The nominal thicknesses of the fi lms were 0.2–0.4 nm thick, and they were deposited at a rate of 2.0–10 pm s −1 . A C cap layer (5 nm) was used to further stabilize the nanoparticles and prevent oxidation.

Characterization : The specimens were examined by an FEI Tecnai Osiris S/TEM operating at either 200 or 80 kV. The Co–Ag samples were highly beam sensitive, and the 80 kV setting was primarily used to delay degradation of the Ag shells during EDS mapping. High and low magnifi cation images as well as EDS spectra were collected in TEM mode, and Fourier transforms of the lattice fringes were indexed. In STEM mode, EDS counts hyper-maps, quantifi ed hyper-maps (in at% by Fe- or Co-Kα peak and W-, Mo-, or Ag-Lα peak), and line and circle scans were extracted from the Q-map areas. Each particle was scanned for 10–15 min. These maps were quantifi ed using the Esprit software quantifi cation tools after the application of background subtraction and a standardless, phirhoz peak fi tting method. Single particle composition data were extracted from the map data for linear and concentric regions of the particle. Final image editing and size analysis was done using the imageJ software. [ 34 ]

Acknowledgements This work was fi nancially supported by the U.S. National Science Foundation Directorate for Mathematical and Physical Sciences Division of Materials Research (Grant No. 0820521, Program Director D. Finotello). Research was performed in the facilities of the Nebraska Center for Materials and Nanoscience, which is supported by the Nebraska Research Initiative.

Received: February 12, 2015 Revised: March 13, 2015

Published online:

[1] R. Ferrando , J. Jellinek , R. L. Johnston , Chem. Rev. 2008 , 108 , 845 .

[2] R. C. Ghosh , S. Paria , Chem. Rev. 2012 , 112 , 2373 . [3] I. L. Medintz , H. T. Uyeda , E. R. Goldman , H. Mattoussi , Nat.

Mater. 2005 , 4 , 435 .


Table 2. Compositional information measured and calculated for the core–shell particles. (C A ) Area scan in TEM mode (i.e., target compo-sition). (C C ) Core composition extracted from Figures 5 d, 6 d, and 7 d. (C P ) Composition of a concentric circular region encompassing particle in Figures 5 c, 6 c, and 7 c.

Sample (core–shell)

C A [at% core]

C C [at% core]

C P [at% core]

Co–Ag 61 ± 6 101 ± 8 72 ± 1

W–Fe 18 ± 1 60 ± 3 16 ± 1

Mo–Co 22 ± 2 84 ± 3 27 ± 1

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[4] R. M. Penner , Acc. Chem. Res. 2000 , 33 , 78 . [5] P. Reiss , M. Protière , L. Li , Small 2009 , 5 , 154 . [6] M. De , P. S. Ghosh , V. M. Rotello , Adv. Mater. 2008 , 20 , 4225 . [7] A. Ito , M. Shinkai , H. Honda , T. Kobayashi , J. Biosci. Bioeng. 2005 ,

100 , 1 . [8] S. Laurent , D. Forge , M. Port , A. Roch , C. Robic , L. Vander Elst ,

R. N. Muller , Chem. Rev. 2008 , 108 , 2064 . [9] A. Burns , H. Ow , U. Wiesner , Chem. Soc. Rev. 2006 , 35 , 1028 .

[10] P. Mélinon , S. Begin-Colin , J. L. Duvail , F. Gauffre , N. H. Boime , G. Ledoux , J. Plain , P. Reiss , F. Silly , B. Warot-Fonrose , Phys. Rep. 2014 , 543 , 163 .

[11] H. I. Schlesinger , H. C. Brown , B. Abraham , A. C. Bond , N. Davidson , A. E. Finholt , J. R. Gilbreath , H. Hoekstra , L. Horvitz , E. K. Hyde , J. J. Katz , J. Knight , R. A. Lad , D. L. Mayfi eld , L. Rapp , D. M. Ritter , A. M. Schwartz , I. Sheft , L. D. Tuck , A. O. Walker , J. Am. Chem. Soc. 1953 , 75 , 186 .

[12] Q. He , Z. Zhang , J. Xiong , Y. Xiong , H. Xiao , Opt. Mater. 2008 , 31 , 380 .

[13] M. Salavati-Niasari , M. Dadkhah , F. Davar , Inorg. Chim. Acta 2009 , 362 , 3969 .

[14] M. Salavati-Niasari , F. Davar , Mater. Lett. 2009 , 63 , 441 . [15] L. L. Hench , J. K. West , Chem. Rev. 1990 , 90 , 33 . [16] S. H. Baker , S. C. Thornton , A. M. Keen , T. I. Preston , C. Norris ,

K. W. Edmonds , C. Binns , Rev. Sci. Instrum. 1997 , 68 , 1853 . [17] H. Haberland , M. Karrais , M. Mall , Y. Thurner , J. Vac. Sci. Technol. A

1992 , 10 , 3266 . [18] A. Perez , P. Mélinon , V. Dupuis , L. Bardotti , B. Masenelli ,

F. Tournus , B. Prével , J. Tuaillon-Combes , E. Bernstein , A. Tamion , D. Taïnoff , O. Boisron , G. Guiraud , M. Broyer , M. Pellarin , N. Del Fatti , F. Vallée , E. Cottancin , J. Lermé , J. L. Vialle , C. Bonnet , P. Maioli , A. Crut , C. Clavier , J. L. Rousset , F. Morfi n , Int. J. Nano-technol. 2010 , 7 , 523 .

[19] C. Binns , P. Prieto , S. Baker , P. Howes , R. Dondi , G. Burley , L. Lari , R. Kröger , A. Pratt , S. Aktas , J. K. Mellon , J. Nanopart. Res. 2012 , 14 , 1136 .

[20] C. R. Patra , Y. Jing , Y. H. Xu , R. Bhattacharya , D. Mukhopadhyay , J. F. Glockner , J. P. Wang , P. Mukherjee , Cancer. Nanotechnol. 2010 , 1 , 13 .

[21] R. Boom , F. R. de Boer , A. K. Niessen , A. R. Miedema , Physica B + C 1983 , 115 , 285 .

[22] A. K. Niessen , A. R. Miedema , F. R. de Boer , R. Boom , Physica B 1988 , 151 , 401 .

[23] Y. H. Xu , J. P. Wang , Adv. Mater. 2008 , 20 , 994 . [24] H. L. Skriver , N. M. Rosengaard , Phys. Rev. B 1992 , 46 , 7157 . [25] L. Vitos , A. V. Ruban , H. L. Skriver , J. Kolla , Surf. Sci. 1998 , 411 ,

186 . [26] M. Gaudry , E. Cottancin , M. Pellarin , J. Lermé , L. Arnaud ,

J. Huntzinger , J. Vialle , M. Broyer , J. Rousset , M. Treilleux , P. Mélinon , Phys. Rev. B 2003 , 67 , 155409 .

[27] C. T. Langlois , T. Oikawa , P. Bayle-Guillemaud , C. Ricolleau , J. Nanopart. Res. 2007 , 10 , 997 .

[28] F. Golkar , M. J. Kramer , Y. Zhang , R. Skomski , D. J. Sellmyer , J. E. Shield , J. Nanopart. Res. 2013 , 15 , 1638 .

[29] P. Mukherjee , X. Jiang , Y. Q. Wu , M. J. Kramer , J. E. Shield , J. Phys. Chem. C 2013 , 117 , 24071 .

[30] P. Villars , Pearson’s Handbook Desk Edition Crystallographic Data for Intermetallic Phases , Vol. 2 , ASM International , Materials Park, OH 1997 .

[31] K. Sumiyama , M. Hirata , W. Teshima , Jpn. J. Appl. Phys. 1991 , 30 , 2839 .

[32] T. B. Massalski , H. Okamoto , P. R. Subramanian , L. Kacprzak , Binary Alloy Phase Diagrams , Vol. 2 , ASM International , USA 1990 .

[33] N. L. Peterson , WADD Technical Report 1960 . [34] C. A. Schneider , W. S. Rasband , K. W. Eliceiri , Nat. Methods 2012 ,

9 , 671 .

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