Journal ofMaterials Chemistry C

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Cite this: J. Mater. Chem. C, 2013, 1,6553

Received 11th June 2013Accepted 23rd July 2013

DOI: 10.1039/c3tc31114g

www.rsc.org/MaterialsC

This journal is ª The Royal Society of

Synthesis and magnetic properties of hybridnanostructures of Pt–FexOy

Hafsa Khurshid,* Sayan Chandra, Pritish Mukherjee and Hariharan Srikanth

We report the synthesis conditions and magnetic properties of Pt and iron oxide (Pt–FexOy) hybrid

nanostructures. The injection time and temperature of the Fe precursor after the decomposition of the

Pt precursor are found to be the key parameters for the tuning of FePt to Pt–FexOy nanoparticles. The

particle shape can be controlled from spherical to cubic by selective molar ratios of surfactants and

solvents. A comparative study of the dynamic and static magnetic properties of Pt–FexOy and FexOy,

(after etching of Pt) is presented. The effective anisotropy of Pt–FexOy is found to be one order of

magnitude higher than that of etched FexOy. Consequently, a strong enhancement of the blocking

temperature is observed which is attributed to the enhanced interfacial anisotropy between Pt and

FexOy layers.

Introduction

Hybrid nanostructures make an interesting class of materialsfor practical applications as well as for the studies of funda-mental properties related to the complex interface between theindividual components of the system, such as catalytic, opticaland thermoelectric effects.1 It is only recently that attention hasbeen paid to the hybrid structures of magnetic nanoparticleswith noble metals (Au, Ag, Pt, Pd, etc.).2–4 Xu et al.5 have reportedthe synthesis of hybrid Au–Fe3O4 structures that may be utilizedfor targeted drug delivery as well as for therapy by takingadvantage of their magnetic and optical properties. Bycontrolling reaction parameters, Peng et al. were able to controlthe size and number of Ag domains over previously synthesizedcore–shell Fe/FexOy nanoparticles, and aerwards convert themto hybrid structures of Ag particles and hollow FexOy nano-particles by heat treatment.2 Recently, Pt and Pd nanostructureshave received considerable attention because of their size- andshape-dependent catalytic properties.6–8 A hybrid nanostructureof Pd and Fe3O4 has been reported for the catalyst recycling ofPd nanocrystals, without loss of catalytic activity.9 Due to theenhanced electronic population, Pt–Fe3O4 hybrid nano-structures provide the advantage of improved catalytic proper-ties for oxygen reduction reactions.10 Pt is well known to have agreat affinity to attach to DNA and biomolecules.11 According toa recent report Pt nanoparticles strongly enhance the biologicalefficiency of radiation, making them a very good candidate forradiation therapy and other biological applications.12 On theother hand, due to their low toxicity, the possibility of func-tionalization to specic biomarkers, and superior

artment, FL, USA. E-mail: khurshid@usf.

-2871

Chemistry 2013

biocompatibility with respect to other magnetic materials,13

iron-oxide nanoparticles are well-suited to function as nano-medicine aids for magnetic resonance imaging (MRI) contrastenhancement and magnetic hyperthermia.

When compared with conventional single-componentnanoparticles, a hybrid nanostructure of Pt and iron-oxidecould provide multifunctional hybrid probes where (1) thepresence of Pt and iron-oxide surfaces facilitates the stepwiseattachment of antigens and biomolecules, and (2) the structurecan serve as both a magnetic and optical probe for radiationtherapy and diagnosis. For all of the applications mentionedabove, monodispersity and good control over size and magneticproperties are very important aspects that cannot be ignored.14

However, there are very few reports on the magnetic propertiesof such hybrid noble metal/nanoparticle systems.15–17 In orderto achieve the level of reproducibility in physical and magneticproperties required for biomedical applications, it is vitallyimportant to understand the role of reaction conditions in thecontrol of monodispersity, size, and shape of hybrid nano-structures. While previous efforts have been devoted to theformation of hybrid nanoparticles of iron oxide (either Fe3O4 orgFe2O3) with noble metals, the key factors for the formation ofhybrid morphology during a reaction have not been reportedspecically. In this study, we report the synthesis conditions forthe formation of Pt–iron oxide (abbreviated as FexOy regardlessof the exact composition) nanoparticles with a dumbbellmorphology and the magnetic properties of the product. Ourresults suggest that in order to synthesize Pt-based magnetichybrid nanostructures, the size of the Pt seeds must be above acertain limit. In addition, a hybrid nanostructure of iron-oxidewith Pt enhances the effective anisotropy of the system by anorder of magnitude and hence the superparamagnetic blockingtemperature shis to higher values. Remarkably, it is possible

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to control magnetic properties by varying the size as well asshape of the Pt and FexOy regions. To the best of our knowledge,our work is the rst demonstration of tuning the blockingtemperature and enhanced effective anisotropy of dumbbell-like Pt–FexOy nanoparticles. The dumbbell-like morphology ofPt and FexOy can be used as an extra degree of freedom tocontrol magnetic properties for potential applications inmemory storage devices, nanomedicine, and cancer therapy.

ExperimentalSynthesis of iron oxide nanoparticles

Iron oxide or FexOy (i.e. FexOymay contain Fe3O4 and/or gFe2O3)nanoparticles were synthesized by following a well-establishedthermal decomposition technique.18 Briey, thermal decom-position of 0.15 mmol of Fe(CO)5 in 1-octadecene in the pres-ence of 0.13 mmol of oleic acid (OA) and 0.12 mmol ofoleylamine (OY) at 270 �C led to the formation of FexOy nano-particles. Particles were dispersed in hexane aer isolation bywashing them with hexane and ethanol via centrifuging. Theparticle size was controlled by adjusting the molar concentra-tion of Fe(CO)5 in the reaction mixture.

Synthesis of FePt nanoparticles

For the synthesis of FePt nanoparticles, 0.5 mmol of platinum(II)-acetylacetonate (Pt-acac) was dissolved in 60 mmol of octylether at room temperature. Aerwards, 0.5 mmol of OY and 5mmol of OA (molar ratio OY/OA ¼ 0.5) were added and thetemperature was raised to 100 �C subsequently while stirringmagnetically under a blanket of Ar + H2. Soon aer reaching 100�C, 1.05 mmol of Fe(CO)5 was injected into the reaction askand the temperature was raised to 270 �C, and reuxed at thattemperature for one hour. The isolation procedure is the sameas described above for the FexOy nanoparticles.

Synthesis of Pt nanoparticles

Pt nanoparticles were synthesized by the reduction of Pt-acac athigh temperature. At room temperature, 0.5 mmol of Pt-acacwas dissolved in 60mmol of octyl ether in a three-neck ask anddegassed (purged) with a mixture of Ar and H2 (5%) to eliminatefree oxygen dissolved in the system. Aerwards, 10 mmol of OAand 5mmol of OY (molar ratio OY/OA¼ 0.5) were added and thetemperature was raised to 150 �C while stirring magnetically. At150 �C, the mixture was reuxed for 2 hours under a blanket ofAr + H2. The isolation procedure is the same as described abovefor the FexOy nanoparticles.

Fig. 1 XRD patterns of FexOy (a), Pt (b), FePt (c) and Pt–FexOy (d) nanoparticlesalong with standard reflection from Pt, FexOy and FePt shown at the bottom.

Synthesis of Pt–FexOy nanoparticles

Pt–FexOy hybrid nanoparticles were formed by seed-mediatedgrowth of iron-oxide over Pt seeds. Synthesis of Pt nanoparticlesproceeded as described previously in the initial (room temper-ature) step. In the second step of the reaction, 1.5 mmol ofFe(CO)5 was injected at 150 �C while stirring continuously. Theisolation procedure is the same as that described for the FexOy

nanoparticles.

6554 | J. Mater. Chem. C, 2013, 1, 6553–6558

Results and discussions

The structural and microstructural properties of the resultingnanoparticles were studied using X-ray diffraction (XRD),selected area diffraction and transmission electron microscopy(TEM). Fig. 1a and b show XRD patterns of the nanoparticlesobtained from the decomposition of the Fe precursor and Ptprecursor alone. The XRD pattern of the nanoparticles obtainedduring simultaneous decomposition of Fe and Pt precursors isshown in Fig. 1c. It can be seen from the XRD patterns thatunder the described conditions, the decomposition of Fe(CO)5leads to the formation of iron-oxide nanoparticles (Fig. 1a), asthe XRD peaks correspond to the standard reections ofmagnetite (Fe3O4) and/or maghemite (gFe2O3). Because of theirvery similar crystal structure, the interplanar spacing in the twoferrites (Fe3O4 and gFe2O3) is very close and it is impossible todistinguish between the two from their diffraction analysis.19

Regardless of the exact composition of iron oxide in our nano-particles, we will refer to it as FexOy since the specic details tomaghemite and/or magnetite is beyond the scope in the contextof the synthesis of the hybrid structure. However, our magneticdata indicate that the dominant phase of iron oxide is magne-tite in our particles and would be discussed later.

The decomposition of Pt-acac alone produced Pt nano-particles with a crystallite size �3 nm, estimated from Scherrer'sformula (Fig. 1b). When both Fe and Pt precursors weredecomposed simultaneously, fcc structured FePt nanoparticleswere obtained, as is evident from the XRD pattern (Fig. 1c). Theinjection of Fe(CO)5 into the reaction ask aer the formation ofPt nanoparticles resulted in a mixture of Pt and FexOy crystallinephases, as demonstrated below through further investigations ofparticle size, morphology and crystallinity using transmissionelectron microscopy (TEM), high-resolution TEM (HRTEM)images and selected-area electron diffraction (SAED) patterns.

Fig. 2 shows the TEM images (1st column), HRTEM images(2nd column) and SAED patterns (3rd column) of representative

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 TEM images, HRTEM and SAED patterns of FexOy (row a) nanoparticles,FePt (row b) nanoparticles, Pt seeds (row c) and dumbbell shaped Pt–FexOy

nanoparticles (row d).

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FexOy (Fig. 2a), FePt (Fig. 2b), Pt (Fig. 2c) and dumbbell shapedPt–FexOy nanoparticles (Fig. 2d). In the case of dumbbell sha-ped Pt–FexOy, the Pt domain appears darker because of its highelectronic density. The HRTEM image of the FexOy nano-particles reveals their high crystallinity and themeasured latticefringes corresponding to the inter-planar distance of (400)planes of (gFe2O3 and or Fe3O4) SAED also match well with thecharacteristic reections of FexOy, consistent with the resultsobtained from XRD. The decomposition of iron-pentacarbonylproduces metallic iron nanoparticles, and their subsequentoxidation due to the presence of trace oxygen dissolved in thesolvent would produce iron-oxide nanoparticles.19

When Fe(CO)5 was decomposed along with Pt-acac, highlycrystalline fcc structured FePt nanoparticles were obtained. Thelattice spacing measured from the fringes in Fig. 2b is found tobe 0.198 nm, which corresponds to the (200) inter-planarspacing of the fcc structure of FePt nanoparticles. The averagecomposition of these particles is found to be Fe45Pt55, whichwas determined by energy-dispersive spectroscopy (EDS). Fig. 2cshows the TEM image of the Pt nanoparticles that wereobtained aer reuxing the reaction mixture at 150 �C for twohours. The SAED matches well with the characteristic reec-tions of Pt, consistent with the results obtained from XRD. Theaverage particle size was found to be 3.2 nm, which is in goodagreement with the average crystallite size calculated from theXRD prole using Scherrer's formula.

Aer the successful formation of FexOy and Pt nanoparticles,Pt–FexOy hybrid structures were also synthesized by

This journal is ª The Royal Society of Chemistry 2013

decomposing the Fe precursor in the reaction mixture where Ptnanoparticles were already present. Fig. 2d shows a TEM imageof hybrid nanostructures of FexOy and Pt. The particles possessa dumbbell-like morphology, where each Pt nanoparticleadheres to an iron oxide nanoparticle. HRTEM images of indi-vidual dumbbell particles (Fig. 2d) reveal the high crystallinityof the Pt and FexOy nanoparticles and mark a clear interfacebetween FexOy and Pt. The adjacent lattice fringes of the Ptregion are determined to be 0.194 nm, which is consistent withthe (200) inter-planar spacing of Pt. The measured distancebetween lattice fringes of the FexOy nanoparticle (0.21 nm)corresponds to the (400) inter-planar spacing of fcc (spinelstructure) FexOy. Such a structure would be formed due to theepitaxial growth of one region over the other. It is noteworthythat such a morphology was not observed when Fe and Ptprecursors were decomposed simultaneously, and instead FePtnanoparticles were obtained. A typical reaction mechanism forFePt growth involves the formation of a Pt rich seed cluster, onwhich Fe atoms nucleate and then diffuse into the cluster.20

However, when the Fe precursor is decomposed in a reactionmixture populated with Pt nanoparticles, the Fe atoms wouldnucleate at Pt sites and grow independently. We believe that inthis case Fe nanoparticles nucleate at one of the crystal planesof Pt and their subsequent oxidation due to the traces of oxygendissolved in the solvent and reaction vessel leads to theformation of FexOy nanoparticles.

By varying molar ratios of OY to OA during the reaction, Pt–FexOy particles were synthesized with a cubic morphology. Inorder to obtain nanoparticles with a cubic shape, the ratios ofdifferent facet areas of Pt seeds must be controlled by changingthe order of free energies of different facets (for an fcc structure,the interfacial free energy (g) sequence is g{111} < g{100} < g

{110}).21,22 Selective binding of capping agents and surfactantsto different facets of a seed can tune the nal shape of ananoparticle by altering the surface energy sequence. When theOA concentration was reduced during the reaction, cubic Ptnanoparticles were obtained as shown in Fig. 3a. OY is known tobind more strongly onto the {100} facets of fcc metals,21 whichresulted in faster growth in the h111i direction, reducing the{111} facet areas and therefore producing a cubic particle. WhenFe(CO)5 was decomposed in the presence of cubic Pt nano-particles, dumbbell-shaped Pt–FexOy nanostructures with cubicmorphology were obtained as seen in Fig. 3a. A at interfacebetween Pt and FexOy indicates the epitaxial growth of FexOy onthe Pt cube.23 In the case of epitaxial growth, the interfaceshould be at (as seen in Fig. 3b), however the other directionsare not restrained. It is noteworthy that a cubic morphology ofPt–FexOy was obtained only when Pt seeds with a cubicmorphology were used. High-resolution line-scan energydispersive X-ray spectroscopy (EDS), with a beam size of �3 nm,shows a sharp differentiation of Fe and Pt along the scanningpath across both the FexOy and Pt domain (Fig. 3c). From theintensity prole, it is very clear that the darker domain is Ptwhile the lighter one is Fe. The small area of intensity overlapcan be attributed to the 3–4 nm beam size used.

A fast Fourier transform (FFT) from each domain (FexOy andPt) of dumbbell-shaped Pt–FexOy and also from the interface of

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Fig. 3 (a) TEM image of Pt–FexOy hybrid nanoparticles in cubic morphology, (b) HRTEM of one of the particles in ‘a’ and (c) EDX line scan of Fe and Pt in a hybridnanoparticle. The inset in ‘c’ shows the scanning path and particle.

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the two domains is shown in Fig. 4. The FFT shown in Fig. 4b iscomprised of two different diffraction patterns as evidenced fromthe FFT of the FexOy and Pt domains of the dumbbell (Fig. 4c andd). In the light of the XRD and microscopy studies, the growthprocess of Pt–FexOy can be summarized as follows: Fe atomsnucleate on a specic crystal plane of pre-existing Pt seeds,leading to the formation of Pt–FexOy nanoparticles. Remarkably,the shape of the Pt seeds plays an important role in determiningthe morphology of the resulting hybrid nanoparticles.

The effect of the dumbbell-like Pt–FexOy morphology on themagnetic properties of the system was studied by conductingsystematic magnetometery on the as-synthesized dumbbellsand on dumbbell particles in which Pt has been etched. Pt wasetched off chemically using a mixture of 0.25 molar solutions ofnitric acid and hydrochloric acid (molar ratio 1 : 3) for a pro-longed period of time. TEM and HRTEM studies indicated thatthe Pt etched particles were composed solely of FexOy (inset inFig. 5), as their crystallographic phase is conrmed from theirselected area diffraction pattern. Fig. 5 shows the temperature

Fig. 4 (a) HRTEM of a Pt–FexOy nanoparticle and its corresponding FFTs from (b)the FexOy domain, (c) the Pt domain and (d) the interface between them.

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dependence of magnetization (M(T)) in the zero eld cooled(ZFC) and eld cooled warming (FCW) protocols under anapplied eld of 100 Oe for the Pt–FexOy nanoparticles (Fig. 5a)and Pt-etched FexOy nanoparticles (Fig. 5b). The blockingtemperature (TB) is identied as the peak temperature of ZFCM(T). For the Pt–FexOy nanoparticles, the temperature at theZFC peak is about 85 K and is shied to a signicantly lower

Fig. 5 Temperature dependence of magnetization in ZFC and FC protocols fordumbbell (a) and etched (b) nanoparticles.

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value of 44 K for the Pt etched particles. This experimentalobservation indicates that either (a) interactions among Pt andFexOy domains within each Pt–FexOy particle yield magneticfrustration, which increases the effective blocking temperatureof FexOy in the dumbbell morphology or (b) there is anenhanced value of the anisotropy energy per unit volume ofFexOy in the dumbbell morphology (Pt–FexOy) with respect tothat of Pt-etched FexOy nanoparticles with the same magneticvolumes.24 To quantify the interactions among FexOy nano-particles in the two different morphologies, we have carried outac susceptibility measurements at various frequencies under 10Oe applied magnetic eld. Fig. 6a and b show the real part of acsusceptibility (c0) as a function of temperature for the Pt–FexOy

and Pt etched FexOy nanoparticles. For both systems, it can beseen that the peak associated with the blocking transitionincreases in temperature with increase in frequency, consistentwith typical nanoparticle behavior.25,26 In order to probe thespin dynamics, we have tted the Tmax of the c0(T) to the Neel–Arrhenius and Vogel–Fulcher models of relaxation. According tothe Neel theory of superparamagnetism, themagnetic momentsof non-interacting particles thermally uctuate between twoenergy minima and the anisotropy energy creates two potentialwells separated by an energy barrier.15,27 The relaxation time (s)for the over-barrier rotation can be expressed as

s ¼ so exp

�Ea

kBTB

�(1)

where Ea is the anisotropy barrier energy that must be overcomefor the moment to reverse, TB is the blocking temperature andso is the attempt frequency (10�9 to 10�12 s). Fitting eqn (1) to c0

yielded an unphysical value of so, indicating that the dynamicsof the particles cannot be explained by the non-interactingmodel. Thus, particle interactions were further probed by theVogel–Fulcher (VF) model for weakly interacting particles,

s ¼ so exp

�Ea

kBðT � ToÞ�

(2)

Fig. 6 Real part of AC susceptibility data for dumbbell (a) and etched (b)nanoparticles. The lower panel shows Vogel–Fulcher fitting to Tm of c0 fordumbbell (c) and etched (d) nanoparticles.

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where To is the characteristic temperature giving a qualitativemeasure of the interparticle interaction energy.

The VF model applied to the experimental c0 data for bothsamples, dumbbell (Pt–FexOy) and Pt etched FexOy, showed agood t indicating the presence of weak inter-particle interac-tions. The best t values of the relaxation time (so) for thedumbbell and etched particles are 1.6 � 10�12 s and 8.4� 10�11

s, respectively. The parameter associated with magneticanisotropy energy, Ea/kB, was used to estimate the effectiveanisotropy constant Keff(Ea ¼ KeffV). The calculated value for thedumbbell particles was found to be 3.2 � 106 erg cm�3, a resultthat is shied to 5.2 � 105 erg cm�3 aer the etching of Pt. Thisexperimental observation indicates the enhancement of effec-tive anisotropy in the nanoparticles with dumbbell morphology.Such a signicant increase in the Keff would explain theenhanced blocking temperature found in the dumbbell parti-cles. Based on these results we can conclude that Pt plays anessential role in the enhancement of effective anisotropy in Pt–FexOy dumbbell nanoparticles. Orbital hybridization, strain dueto lattice mist and inverse magnetostriction effects are knownto induce interfacial anisotropy in multilayered hybrid struc-tures and hence enhance the effective anisotropy of thesystem.28 In a multilayer system of Ni and Pt, X-ray magneticcircular dichroism (XMCD) studies have shown that Pt acquires alarge induced magnetic moment (z0.29 mB per atom) thatdecays sharply away from the interface on the Pt-side at the Ni–Pt interface.29 According to a recent XMCD study on Au–Fe3O4

core–shell nanoparticle systems, amagnetic moment is inducedinto the Au domain when the Fe3O4 shell is in direct contactwith the noble metal that is marked by an increased holedensity in the Au states suggesting the occurrence of a charge-transfer process concomitant with the magnetization transfer.30

Inspired by their results, we believe that the enhancement in theblocking temperature and effective anisotropy of our Pt–FexOy

system is due to the spin polarization transfer between Pt andiron oxide. Furthermore, it is postulated that Fe3O4 is thedominant phase of iron oxide in our particles as the spinpolarization transfer was not observed in the Au–gFe2O3

nanoparticles because of the insulator like behavior of gFe2O3.A detailed investigation of the anisotropy in this system iscurrently underway and is beyond the scope of this reportedwork in which we focus on the synthesis and characterization ofstructural and magnetic properties.

Conclusion

In summary, the synthesis conditions and characterization ofdumbbell-like Pt–FexOy nanoparticles are reported. The synergyof the size and shape of the Pt seeds and nucleation of Fe atomson a crystalline plane of Pt led to the formation of Pt–FexOy

nanoparticles. To obtain the hybrid nanostructures, Fe(CO)5was decomposed in the presence of pre-synthesized Pt seeds.The size of Pt and FexOy domains was tuned via control ofsurfactant molar ratios and iron precursor injection timeduring reaction. When OA concentration was reduced duringthe reaction, cubic Pt–FexOy nanoparticles were obtained. Themagnetic properties of the dumbbell and Pt etched Pt–FexOy

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were compared. The blocking temperature was found to shitowards higher values in the sample with dumbbellmorphology. The effective anisotropy of Pt–FexOy was found tobe one order of magnitude higher than that of Pt etched FexOy

and is attributed to the enhanced interfacial anisotropybetween Pt and iron-oxide layers. It is anticipated that theunique morphology of hybrid nanostructures of Pt and FexOy

can be used as an extra degree of freedom to (1) controlmagnetic properties for potential applications in memorystorage devices and biomedicine, and (2) serve as both amagnetic and optical probe for radiation therapy and diagnosis.

Acknowledgements

This work was supported by Grant number W81XWH1020101/3349 from the United States Army Medical Research andMateriel Command (USAMRMC).

References

1 S. L. Zhu, X. G. Luo, C. L. Du, F. Li, S. Y. Yin, Q. L. Deng andY. Q. Fu, J. Appl. Phys., 2007, 101, 067401.

2 S. Peng, C. H. Lei, Y. Ren, R. E. Cook and Y. G. Sun, Angew.Chem., Int. Ed. Engl., 2011, 50, 3158.

3 F. H. Lin, W. Chen, Y. H. Liao, R. A. Doong and Y. D. Li, NanoRes., 2011, 4, 1223.

4 Y. H. Wei, R. Klajn, A. O. Pinchuk and B. A. Grzybowski,Small, 2008, 4, 1635.

5 C. J. Xu, B. D. Wang and S. H. Sun, J. Am. Chem. Soc., 2009,131, 4216.

6 Y. J. Xiong, J. M. McLellan, J. Y. Chen, Y. D. Yin, Z. Y. Li andY. N. Xia, J. Am. Chem. Soc., 2005, 127, 17118.

7 Y. J. Xiong, H. G. Cai, B. J. Wiley, J. G. Wang, M. J. Kim andY. N. Xia, J. Am. Chem. Soc., 2007, 129, 3665.

8 R. Kou, Y. Y. Shao, D. H. Wang, M. H. Engelhard, J. H. Kwak,J. Wang, V. V. Viswanathan, C. M. Wang, Y. H. Lin, Y. Wang,I. A. Aksay and J. Liu, Electrochem. Commun., 2009, 11, 954.

9 Y. Jang, J. Chung, S. Kim, S. W. Jun, B. H. Kim, D. W. Lee,B. M. Kim and T. Hyeon, Phys. Chem. Chem. Phys., 2011,13, 2512.

10 C. Wang, H. Daimon and S. H. Sun, Nano Lett., 2009, 9, 1493.11 W. F. Agnew, T. G. H. Yuen, D. B. McCreery and L. A. Bullara,

Exp. Neurol., 1986, 92, 162.

6558 | J. Mater. Chem. C, 2013, 1, 6553–6558

12 E. Porcel, S. Liehn, H. Remita, N. Usami, K. Kobayashi,Y. Furusawa, C. Le Sech and S. Lacombe, Nanotechnology,2010, 21, 85103.

13 A. Y. Louie, Chem. Rev., 2010, 110, 3146.14 C. G. Hacliipanayis, M. J. Bonder, S. Balakrishanan, X. Wang,

H. Mao and G. C. Hadjipanayis, Small, 2008, 4, 1925.15 N. A. Frey, M. H. Phan, H. Srikanth, S. Srinath, C. Wang and

S. Sun, J. Appl. Phys., 2009, 105, 07B502.16 J. S. Beveridge, M. R. Buck, J. F. Bondi, R. Misra, P. Schiffer,

R. E. Schaak andM. E. Williams, Angew. Chem., Int. Ed. Engl.,2011, 50, 9875.

17 L. Lou, K. Yu, Y. T. Wang and Z. Q. Zhu, Appl. Surf. Sci., 2012,258, 3744.

18 H. Khurshid, V. Tzitzios, W. F. Li, C. G. Hadjipanayis andG. C. Hadjipanayis, J. Appl. Phys., 2010, 107, 09A333.

19 H. Khurshid, W. F. Li, V. Tzitzios and G. C. Hadjipanayis,Nanotechnology, 2011, 22, 265605.

20 C. Srivastava, J. Balasubramanian, C. H. Turner, J. M. Wiest,H. G. Bagaria and G. B. Thompson, J. Appl. Phys., 2007, 102,104310.

21 L. Colak and G. C. Hadjipanayis, Nanotechnology, 2009, 20,485602.

22 Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem.,Int. Ed. Engl., 2009, 48, 60.

23 B. Li and J. G. Liu, J. Nonlinear Sci., 2004, 14, 429.24 A. Cabot, A. P. Alivisatos, V. F. Puntes, L. Balcells, O. Iglesias

and A. Labarta, Phys. Rev. B, 2009, 79, 094419.25 S. Chandra, H. Khurshid, W. F. Li, G. C. Hadjipanayis,

M. H. Phan and H. Srikanth, Phys. Rev. B, 2012, 86, 014426.26 H. Khurshid, W. Li, P. Manh-Huong, P. Mukherjee,

G. C. Hadjipanayis and H. Srikanth, Appl. Phys. Lett., 2012,101, 022403.

27 M. Bellusci, S. Canepari, G. Ennas, A. La Barbera, F. Padella,A. Santini, A. Scano, L. Seralessandri and F. Varsano, J. Am.Ceram. Soc., 2007, 90, 3977.

28 C. Chappert and P. Bruno, J. Appl. Phys., 1988, 64, 5736.29 F. Wilhelm, P. Poulopoulos, P. Srivastava, H. Wende,

M. Farle, K. Baberschke, M. Angelakeris, N. K. Flevaris,W. Grange, J. P. Kappler, G. Ghiringhelli andN. B. Brookes, Phys. Rev. B, 2000, 61, 8647.

30 F. Pineider, C. J. Fernandez, V. Videtta, E. Carlino,A. Hourani, F. Wilhelm, A. Rogalev, P. Davide Cozzoli,P. Ghigna and C. Sangregorio, ACS Nano, 2012, 7, 857.

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