syntheses, properties, and potential applications of multicomponent magnetic nanoparticles

FEATURE

Mhbsosima

DOI: 10.1002/adfm.200701211

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Syntheses, Properties, and PotentialApplications of Multicomponent MagneticNanoparticles**

By Hao Zeng* and Shouheng Sun

ulticomponent hybrid nanostructures that contain two or more nanometer-scale componentsave attracted much attention recently owing to the synergistic properties induced by interactionsetween these different nanometer-scale objects. Herein, we give an overview of the efforts toynthesize multicomponent nanoparticles with at least one component being magnetic, and focusn our recent developments. The syntheses are based on heterogeneous nucleation and growth of aecond and third component onto seed nanoparticles. These multicomponent nanoparticles shownteresting magnetic, magneto-optical, plasmonic, and semiconducting properties that can beodulated by interfacial interactions between different nanocomponents. This opens up a newvenue to advanced multifunctional nanomaterials for device concepts and applications.

1. Introduction

An important research direction in nanoparticle synthesis is

the expansion from single-component nanoparticles to multi-

component hybrid nanostructures with discrete domains of

different materials arranged in a controlled fashion. The

advantages of multicomponent structures lie in three aspects.

The first is to realize multifunctionality: different functional-

ities can be integrated, with the dimension and material para-

meters of the individual components independently optimized.

One such example is the Co/CdSe bifunctional magneto-optic

nanocrystals reported by Klimov’s group.[1] The core/shell

nanoparticles retain the magnetic and optical properties of

each single component and permit potential applications as

optical reporters and magnetic handles for bioassay. Since the

dimensions of the individual components are comparable to

[*] Dr. H. ZengDepartment of PhysicsUniversity at Buffalo, SUNYBuffalo, NY 14260 (USA)E-mail: [email protected]

Prof. S. SunDepartment of ChemistryBrown UniversityBox H, 324 BrookStreet, Providence, RI 02912 (USA)

[**] The authors gratefully acknowledge the US NSF grant numberDMR-0547036, DMR-0606264 and US NIH grant number1R21CA12859-01 for financial support.

Adv. Funct. Mater. 2008, 18, 391–400 � 2008 WILEY-VCH Verlag

the size of the biomolecules, the combination is expected to

provide improved performance. The second advantage is in

providing novel functions not available in single-component

materials or structures; for example, Jonker et al.[2] demon-

strated a hybrid system that consisted of a ferromagnetic thin

film on top of a semiconductor quantum-well structure that has

been used to realize a new concept called a spin light-emitting

diode (Spin-LED). The ferromagnetic layer polarizes the

electrons injected into the quantum-well structure, and the

spin-polarized electrons recombine with holes in the quantum

well to emit circularly polarized photons. This spin-polarized

emission originates from hybridization of the magnetic and

semiconducting materials, and has the potential to integrate

logic and memory functions in nanodevices. The third

advantage is in achieving enhanced properties and breaking

the natural constraints of single-phase materials; for example,

integration of a nanometer-scale magnetically hard phase with

large coercivity and a nanometer-scale soft phase with high

saturation magnetization with the two phases in intimate

contact leads to a strong exchange coupling between the two

phases and an enhanced energy product.[3] Because many

physical and chemical properties have critical length scales on

the order of nanometers, the intimate contact between the

nanocomponents in hybrid nanoparticles should allow strong

interactions between these components and a possible rational

modulation of the physical and chemical properties from each

individual component.

Multicomponent hybrid nanostructures have been prepared

in thin-film and powder forms, utilizing the immiscibility of

the two phases to randomly embed nanosized grains in a

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S. Sun, H. Zeng /Multicomponent Nanoparticles

Figure 1. a) Transmission electronmicroscopy (TEM) image of a 2D array of FePt/Fe3O4 core/shell nanoparticles with a 3 nm FePt core and a 3 nm Fe3O4

shell. b) High-resolution TEM image of FePt/Fe3O4 core/shell nanoparticles showing the coherent interface. Reproduced with permission from [29].Copyright 2004 American Institute of Physics.

392

matrix.[4,5] Different nanoparticles can be connected into a

hybrid structure via linker molecules through covalent

bonding.[6] Coassembly of different kinds of nanoparticles

with specific size ratios is also applied to fabricate highly

ordered 3D binary superlattices via van der Waals interactions

between particles.[7,8] The synthetic techniques to grow single-

Hao Zeng is an assistant professor

York. He received his B.S. from N

(2001) from the University of N

Thomas J. Watson Research Cen

August 2004. He is a recipient o

from National Science Foundati

spintronics, data storage and high

Shouheng Sun is an Associate P

Brown University. He received h

from Nanjing University (P.R. Ch

all in chemistry. He was a Lecture

Institute of Nanjing University fr

Center at Yorktown Heights, New

research staff member from 1998–

of 2005. His research interests inc

information storage, catalysis, an

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

component nanoparticles in the solution phase have been

extended to multicomponent nanoparticles, where individual

inorganic components are directly grown one on top of each

other without linker molecules. These composite particles

consist of symmetric core/shell nanoparticles,[1,9–12] non-

symmetric heterodimers,[13–16] and other multicomponent

of Physics at the University at Buffalo, State University of New

anjing University (China, 1993), and his M.S. (1998) and Ph.D.

ebraska, all in physics. He was a postdoc associate at IBM

ter from 2001–2004. He moved to UB Physics Department in

f the Faculty Earlier Career Development (CAREER) Award

on. His research interests include nanomaterials, magnetism,

frequency applications.

rofessor of Chemistry, Engineering and Diagnostic Imaging at

is B.Sc. from Sichuan University (P.R. China, 1984), his M.Sc.

ina, 1987), and his Ph.D. from Brown University (USA, 1996),

r in the Department of Chemistry and Coordination Chemistry

om 1987–1992. He joined IBM’s Thomas J. Watson Research

York, first as a postdoctoral associate in 1996 and then as a

2004. He moved to Brown’s Chemistry Department in January

lude nanomaterials synthesis, self-assembly and applications in

d biomedicine.

& Co. KGaA,Weinheim Adv. Funct. Mater. 2008, 18,391–400

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S. Sun, H. Zeng/Multicomponent Nanoparticles

Figure 2. a) Schematic illustration of surface coating of (i) Fe3O4 nanoparticles withAu, to form (ii) hydrophobic Fe3O4/Au nanoparticles, and (iii) hydrophilic Fe3O4/Aunanoparticles. b) Schematic illustration of the formation of Fe3O4/Au and Fe3O4/Au/Ag.Reproduced with permission from [32]. Copyright 2007 American Chemical Society.

heterostructures.[17–19] There are several comprehensive

review articles on the synthesis, properties, and applications

of such multicomponent nanostructures.[20–22] In this Feature

Article, we provide an overview of the synthetic efforts in

multicomponent hybrid nanoparticles via high-temperature

solution-phase synthesis. We focus on hybrid nanoparticles

developed recently in our groups, with at least one of the

components being magnetic. The topics include chemical

synthesis of multicomponent nanoparticles; characterization of

the structural and physical properties, especially the ones

arising from the interactions between different components;

and potential applications of these multicomponent hybrid

Figure 3. A schematic diagram showing the mechanism of formation of core/shell nanoparticles ina polar solvent (top) and heterodimers in a nonpolar solvent (bottom).

nanoparticles.

2. Synthesis ofMulticomponent HybridNanoparticles

Multicomponent hybrid nanoparticles

are commonly obtained by sequential

growth of a second and third component

on the preformed seeds. This approach

can be categorized as seed-mediated

growth.[23–25] The successful synthesis of

multicomponent nanoparticles relies cri-

tically on promoting heterogeneous

nucleation while suppressing homoge-

neous nucleation (the formation of sepa-

rate nanoparticles of the second compo-

nent). To achieve this heterogeneous

nucleation, the lattice spacing between

two components should be well- matched

Adv. Funct. Mater. 2008, 18, 391–400 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Wei

to ensure epitaxial growth of the second component.

Further, the seed particles often participate in the

reaction as catalysts, where charge transfer between

the seeds and newly nucleated components is

involved. This lowers the energy for heterogeneous

nucleation. As long as the reactant concentration,

seed-to-precursor ratio, and heating profile are

controlled so that the concentration of the precursor

is below the homogeneous nucleation threshold

throughout the synthesis process, multicomponent

heterostructures are formed.

2.1. Core/shell Nanoparticles

Core/shell nanoparticles are the most common

type of multicomponent nanoparticles and have

been studied extensively. Core/shell structures were

first realized in semiconductor nanoparticles,[12,26,27]

where the heterostructures can lead either to carrier

confinement or carrier separation depending on the

band alignment of the two semiconducting materials

with different gap energy.[28] The first bimagnetic

core/shell nanoparticle was synthesized by Zeng

et al.[9,29] via reductive decomposition of iron acetylacetonate,

Fe(acac)3, in the presence of monodisperse FePt nanoparticles.

Figure 1 shows a 2D array of FePt core/Fe3O4 shell

nanoparticles, with a uniform Fe3O4 shell surrounding a

monodisperse FePt core. The shell thickness is tuned from

0.5 nm to 5 nm by controlling the molar ratio between the FePt

nanoparticles and the Fe(acac)3 precursor. The growth

mechanism is referred to as heteroepitaxial nucleation. The

epitaxial relationship is clearly seen in the high-resolution

transmission electron microscopy (HRTEM) image of a single

FePt/Fe3O4 nanoparticle shown in Figure 1b, where the (111)

lattice fringes of the FePt and Fe3O4 are parallel to each other.

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Figure 4. TEM images of a) 3-14 nm and b) 8-14 nm dumbbell-like Au-Fe3O4 nanoparticles.[13] The

inset in A shows an HRTEM image demonstrating the coherent interface between Au and Fe3O4.

394

Other core/shell nanoparticles with a variety of material

combinations have also been realized by similar approaches.

For example, Teng et al. reported the synthesis of metallic/

magnetic Pt/Fe2O3 nanoparticles by sequential decomposition

of Pt(acac)2 and Fe(CO)5.[30] Shi et al. obtained a compre-

hensive list of core/shell combinations including magnetic/

metallic, such as Au/Fe3O4, semiconducting/metallic, such as

Au/PbS, Au/PbSe, and Au/CdS, and magnetic/semiconduct-

ing, such as PbS/Fe3O4.[31]

Core/shell nanoparticles with a magnetic core and metallic

shell, such as Fe3O4/Au, have also been developed recent-

ly.[32,33] Because Au ions are very easily reduced, preventing

homogeneous nucleation is a challenge and mild reducing

conditions have to be used. A schematic illustration of the

synthesis done by Sun’s group is shown in Figure 2. The

synthesis starts with room-temperature coating of Au on the

surface of Fe3O4 nanoparticles by reducing HAuCl4 in a

chloroform solution of oleylamine. Oleylamine is used as a

mild reducing agent as well as a surfactant. After the syn-

thesis, Fe3O4/Au nanoparticles in an organic environment are

transferred into water with cetrimonium (or hexadecyltri-

methylammmonium) bromide (CTAB). The water-soluble

Figure 5. A schematic illustration of the synthesis of CdS-FePt heterodimers. Reproduced withpermission from [14]. Copyright 2005 American Chemical Society.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim

nanoparticles serve as seeds for the

formation of Fe3O4/Au nanoparticles with

thicker Au coatings by simply adding

more HAuCl4 in reducing conditions or

for the preparation of Fe3O4/Au/Ag

nanoparticles by adding AgNO3 to the

reaction mixture.

Alternatively, magnetic/metallic core/

shell nanoparticles can be prepared via

redox transmetalation. For example,

Cheon’s group[34] obtained Co/Pt core/

shell nanoparticles by reaction between

Co nanoparticles and a Pt(II) precursor,

where Pt(II) is reduced to Pt(0) at the

expense of the oxidation of Co(0) to

Co(II). The strategy is versatile for the

fabrication of Co/Au, Co/Pd, and Co/Cu

core/shell nanoparticles.

2.2. Bifunctional Heterodimers

Heterodimers with two joined nanoparticles sharing a

common interface have been synthesized by nucleation and

growth of the second component on the preformed nanopar-

ticle seeds. The growth of heterodimers follows procedures

similar to that of core/shell nanoparticles. While many

experimental parameters, such as the precursor ratio, solution

concentration and heating profile, can be tuned to give

different morphologies, a convinient route to control core/shell

vs. heterodimer formation is by controlling the polarity of the

solvent. It has been proposed that when a magnetic

component, such as Fe3O4, is nucleated on Au, electrons will

transfer from Au to Fe3O4 through the interface.[13] Similarly,

when a quantum dot (such as PbS) is nucleated on Au,

electrons will diffuse from Au to PbS to match their chemical

potentials.[31] The charge transfer leads to electron deficiency

on the Au. As shown schematically in Figure 3, if a polar

solvent is used in the reaction, the electron deficiency on Au

can be replenished from the solvent. This allows multiple

nucleation sites to be formed. As the reaction proceeds, the

nucleated lobes grow and become con-

nected and eventually form a continuous

shell. On the other hand, if a nonpolar

solvent is used, once a single nucleated

site depletes the electrons from Au, the

electron deficiency cannot be replenished

from the solvent. This prevents new

nucleation events and results in Au-PbS

or Au-Fe3O4 heterodimer nanoparticles.

Transmission electron microscopy

(TEM) images of 3-14 nm and 8-14 nm

Au-Fe3O4 heterodimers are shown in

Figure 4a and b, respectively.

Alternatively, heterodimers can be

formed by first forming a metastable

core/shell nanoparticle followed by

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Figure 6. a) Schematic of the micellar synthesis of Ag-Fe3O4 heterodimers. b) TEMimages of Ag-Fe3O4 heterodimers after 10 min reaction. c) Ag-Fe3O4 after 39 minreaction. Reproduced with permission from [15]. Copyright 2005 American ChemicalSociety.

recrystalization of the shell into a heterodimer. Xu’s group

synthesized CdS-FePt heterodimers by the formation of an

amorphous CdS shell on an FePt core at low temperatures.

Subsequent heating at 280 8C led to the CdS dewetting at the

FePt surface and the formation of a heterodimer, as shown

schematically in Figure 5.[14]

The same group synthesized Ag-Fe3O4 heterodimers by an

alternative approach. These particles were made in a micellar

structure by ultrasonication of a heterogeneous solution with

as- prepared Fe3O4 nanoparticles in the organic solution and

AgNO3 in water, as illustrated in Figure 6. The sonication

gives the energy required to form a microemulsion, which is

Figure 7. Typical TEM image of Fe3O4-Au-Fe3O4 dumbbell nanoparticles.The inset shows a HRTEM image of the same. Reproduced with per-mission from [31]. Copyright 2006 American Chemical Society.

Figure 8.logies. Rep

Adv. Funct. Mater. 2008, 18, 391–400 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA,We

stabilized by the nanoparticles that self-assemble at

the liquid/liquid interface. Fe(II) on the nanoparti-

cles acts as catalytic center for the reduction of Agþ

to Ag nanoparticles. The partial exposure of the

nanoparticles to the aqueous phase and the self-

catalyzed reduction of Agþ and nucleation of Ag are

proposed as two factors leading to the heterodimer

morphology. The synthesis can be readily extended

to make Ag-FePt, Ag-Au nanoparticles as well.[15]

More complex bifunctional heterostructures can

be made by the fusing of preformed heterodimer

seeds. An example of Fe3O4-Au- Fe3O4 is shown in

Figure 7.[31] Such heterostructures are obtained by

heating preformed Au-Fe3O4 heterodimers in the

presence of sulphur. The dimerization of two

Au-Fe3O4 heterodimers is triggered by the fusion

of two Au ends linked by S. The same mechanism

also contributes to the formation of PbS-Au-PbS

particles. The morphology is controlled primarily by

theAu:S precursor ratio and the growth temperature.

When the ratio of Au to S is high (1:10), nucleation of

PbS on the Au surface rapidly depletes S, and hence

only heterodimers are formed. When a lower Au to S

ratio (1:20) is used, excess S remains present long

enough to link the Au ends of twoAu-PbS dimers together and

catalyze their fusion into a single nanoparticle, similar to the

S-catalyzed fusion observed for the Au-Fe3O4 nanoparticles.

2.3. Multifunctional Ternary Heterostructures

Using magnetic-metallic heterodimers as seeds, a magnetic-

metallic- semiconducting ternary structure can be formed by

nucleation of a semiconducting component on the exposed Au

TEM images of Au-PbSe heterodimers with different morpho-roduced from [35].

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Figure 9. a) Measured absorbance spectra of Au-Fe3O4; and b)computed absorbance spectra of Au-Fe3O4. Reproduced with permission from [31].Copyright 2006 American Chemical Society.

396

end. Examples of Fe3O4-Au- PbSe are shown in Figure 8. By

varying the ratio of seed particle to PbSe precursor, the extent of

growth can be controlled to yield a dot-shaped PbSe component

(Structure 1 in Fig. 8), a rod-shaped PbSe component (2a and

2b), multiple rod-shaped PbSe components (3a and 4a) or a

multibranched PbSe component (3b, 4b, and 5). The general

trend in morphology change proceeds clockwise from structure

1 to structure 5 as the precursor to seed ratio is increased.

Further growth of the semiconducting component due to excess

precursor or prolonged heating leads to cleavage of the

semiconductor nanorods from the seeds and the formation of

free-standing nanorods with different morphologies.[35]

3. Plasmonic, Photoluminescence, Magnetic,and Magneto-Optical Properties ofMulticomponent Hybrid Nanoparticles

Multicomponent nanoparticles integrate several function-

alities in a single entity. Different components often have

Figure 10. 10 K hysteresis loops for assemblies of 10 nm Fe3O4 and 10 nmAu/Fe3O4 core/shell nanoparticles with a 3 nm core. Reproduced withpermission from [31]. Copyright 2006 American Chemical Society.


coherent interfaces, resulting in strong intercomponent inter-

actions and new properties of composite materials that do not

exist in single-component nanoparticles. In this section, we

summarize the physical properties observed in these multi-

component nanoparticles.

3.1. Surface Plasmon Resonance (SPR)

Owing to the quantized oscillation of conduction electrons

under an external electromagnetic field, Au nanoparticles

exhibit strong surface plasmon resonance (SPR) absorp-

tion.[36,37] While both Au/Fe3O4 and Au/PbS core/shell and

Au-containing heterodimer nanoparticles exhibit SPR similar

to Au nanoparticles, their resonance frequencies are signifi-

cantly red-shifted compared to Au nanoparticles alone. Au/

Fe3O4 core/shell nanoparticles with 10 nm cores are used to

demonstrate this, since much smaller Au nanoparticles (3 nm)

show little or no SPR absorption.[38] As shown in Figure 9a,

10 nmAu nanoparticles have an SPR peak centered at 520 nm,

consistent with previous studies.[37] As the Au core is coated

with Fe3O4, the peak position shifts to 546 nm (incomplete

shell), 559 nm (2 nm shell), and 573 nm (3 nm shell). It is

well-known that SPR is sensitive to dielectric environments,

and the shift in the SPR frequency can be used as a sensor for

detecting, for example, surface binding of biomolecules.[39]

Because of its high refractive index (�2.4), the effect of Fe3O4

on the SPR frequency is large. For a given core/shell geometry,

the extinction efficiency can be calculated quantitatively

from Mie theory.[40] Shi et al. simulated the absorption

spectrum using the equations presented by Toon and Acker-

man,[41] a frequency-dependent complex refractive index of

gold, and a frequency-independent refractive index of

2.42 for Fe3O4. Figure 9b shows that the shift in the SPR

peak is similar to what has been observed experimentally. The

charge state of the Au can also affect the SPR, and electron

deficiency will shift the absorption to longer wavelengths.[38]

However, the resonance frequency is proportional to N½,


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where N is the number of free electrons, so a 10% red-shift

from 520 to 573 nm would require a 20% decrease in the total

number of free electrons. Owing to the large difference in the

carrier density between the metal and semiconductor, this is

not a very realistic scenario. The Au-PbS heterodimers exhibit

a similar red-shift of the SPR frequency compared to pure gold

nanoparticles. This can also be accounted for by the variation

of the dielectric environment of Au, though a quantitative

comparison is not simple for these asymmetric structures. The

real part of the refractive index of PbS is about 4.3 at

wavelengths of 500 to 600 nm, so the dielectric effects could be

even larger than that of Fe3O4.

By coating Fe3O4 sequentially with an Au and then Ag shell

with varying thickness, the SPR can be tuned to either red-shift

or blue-shift.[32] The controlled coating of either Au or Ag on

the Fe3O4/Au nanoparticles facilitates the tuning of the

plasmonic properties of these core/shell nanoparticles. The

starting Fe3O4/Au nanoparticles have an absorption peak at

534 nm. Depositing a thicker Au shell on the nanoparticles

leads to a red-shift of the absorption peak, while coating Ag on

the seed particles results in a blue-shift of the spectrum. The

absorption band can be red-shifted to 561 nm by increasing the

thickness of the Au shell to �3 nm or blue-shifted to 501 nm

by coating 2 nm Ag on the starting particles.

3.2. Magnetic Hysteresis

Depending on interphase interactions, the magnetic proper-

ties of the multicomponent nanoparticles can be either re-

tained, as in the Co/CdS core/shell nanoparticles, or sig-

nificantly modulated, as in Au/Fe3O4 nanoparticles.[1,31]

Hysteresis loops of 10 nm Fe3O4 and 10 nm Au/Fe3O4 NPs

with a 3 nm Au core measured at 10 K are shown in Figure 10.

A tenfold increase in the saturation field, from 1 kOe

(1 Oe¼ 103/4p A m�1) for pure Fe3O4 to 10 kOe for Au/

Fe3O4, is observed. The coercivity (Hc) also increases from 200

Figure 11. Magnetic hysteresis loops measured at 10 K, for FePt/Fe3O4

with shell thickness being 0.5 nm, 1 nm and 3 nm, respectively.

Adv. Funct. Mater. 2008, 18, 391–400 � 2008 WILEY-VCH Verlag GmbH &

Oe to 800 Oe. The remanence ratio (S¼ remanent magnetiza-

tion/saturation magnetization) for pure Fe3O4 is about 0.8, a

typical value for a randomly oriented nanoparticle assembly

with cubic anisotropy. For Au/Fe3O4 core/shell nanoparticles,

S decreases to 0.5, a value expected for a randomly oriented

array with uniaxial anisotropy. This behavior may be explained

by a new surface anisotropy induced by the core/shell structure:

Fe atoms at the Au/Fe3O4 interface have reduced numbers of

nearest neighbors, which decreases the interatomic exchange

coupling. Thus, spins at the interface become canted and

saturate only under very high fields. This provides an effective

uniaxial anisotropy much like the surface anisotropy in

nanoparticles and thin films, which dominates the original

cubic anisotropy. In this case, the non-magnetic Au core acts as

a template to nucleation of a hollowmagnetic nanoshell, where

its spin structure and magnetization reversal can be drastically

different from a solid nanoparticle.

The anisotropy and coercivity can be systematically tuned in

a bimagnetic core/shell nanoparticle system such as FePt/

Fe3O4.[9] The Hc at 10 K for 3.5 nm FePt is measured to be

about 5.5 kOe, and is 200 Oe for 4 nm and 450 Oe for 16 nm

Fe3O4 nanoparticles. The FePt/Fe3O4 core/shell nanoparticle

assembly therefore consists of both a magnetically hard and a

magnetically soft phase, with the FePt being hard (high Hc)

and Fe3O4 being soft (low Hc). The intimate contact between

the FePt core and Fe3O4 shell, and the nanometer dimensions

ensure strong interphase exchange coupling and, hence,

cooperative magnetization switching of the two phases.

Figure 11 shows the hysteresis loopmeasured at 10K, for the

FePt/Fe3O4 with a 3.5 nm FePt core and a 0.5–3 nm Fe3O4

shell. It can be seen that all of the hysteresis loops show

single-phase-like behavior, with the magnetization changing

with the applied field smoothly. The coercivity values are in

between those of pure FePt and Fe3O4. Earlier theoretical

studies suggest that for hard and soft phases to reverse

cooperatively in a hard-soft nanocomposite system, the critical

Figure 12. Normalized coercivity, hc, as a function of the volume fractionof the soft phase. Reproduced with permission from [29]. Copyright 2004American Institute of Physics.

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398

dimension of the soft phase (ts) should be less than twice the

domain-wall width (dW) of the hard phase, under the assump-

tion that the soft phase has negligible anisotropy.[42] Using the

measured 10 K Hc of the FePt nanoparticles as an approxi-

mation for the effective anisotropy field, and an effective

anisotropy constant ofKu�MsHK, one can calculateKu for the

FePt nanoparticles to be on the order of 5� 106 erg cm�3.

Plugging this Ku value into dW� (A/Ku)1/2,[43] where A is the

exchange constant (�1� 10�6 erg cm�1), dW can be estimated

to be about 10 nm. The Fe3O4 shell in this study is less than

5 nm and well within the limit of ts� 2dW¼ 20 nm. Hence, the

switching of the hard and soft phase should indeed occur

coherently, leading to a smooth magnetization transition.Hc of

the core/shell particles is observed to decrease with increasing

shell thickness. According to Ref. [42], the coercivity of a

hard-soft exchange-coupled system with collinear easy axis is:

Hc ¼ 2KHfH � KSfS

MHfH þMSfS(1)

where K is the anisotropy constant, M is the saturation

magnetization, f is the volume fraction, and the subscripts H

and S denote hard and soft phase, respectively. Equation 1 is

only strictly valid for uniaxial anisotropy, with the easy axes

of the hard and soft phases being collinear. Nevertheless,

since a KH of 5� 106 erg cm3 is about 20 times larger than

that of Fe3O4, we can ignore the term KSfS in Equation 1 for

Figure 13. Magneto-optical Faraday rotation of dimers and monomers in hlengths of a) 385 nm, b) 421 nm, c) 455 nm, d) 532 nm, e) 633 nm, and f) 850permission from [6]. Copyright 2005 American Chemical Society.


the thickness range we studied without introducing

significant error. Based on this, Equation 1 can be further

transformed into Equation 2:

hc �1

1þ MS

MH

� fS

1� fS

(2)

where hc is the coercivity of the core/shell particles normal-

ized by the coercivity HcH of the hard phase (hc¼Hc/HcH).

If the above analysis is correct, hc should decrease mono-

tonically with increasing volume fraction of the soft phase,

following Equation 2. Figure 12 plots hc of the FePt/Fe3O4

nanoparticles as a function of fs. In Figure 12, the curve is

calculated from Equation 2 and the dots are data points. We

can see that they match each other reasonably well. This

indicates that the coercivity of the FePt/Fe3O4 nanoparticles

depends only on the volume ratio of core/shell, not on the

actual size or thickness of the core and the shell, which is

consistent with Ref. [42].

3.3. Magneto-Optical Properties of

Ag-CoFe2O4Heterodimers

Themagneto-optical Faraday rotation in a colloidal solution

of physically conjoined nanoparticle pairs composed of a

exane at laser wave-nm. Reproduced with

& Co. KGaA,Weinheim

ferromagnetic material (CoFe2O4) and a

noble metal (Ag) were studied.[6] It has

been shown that a dramatic contrast

emerges between the magneto-optical

response for Ag- CoFe2O4 dimers and

CoFe2O4 monomers at longer wave-

lengths, outside the CoFe2O4 interband-

transition-dominated regime.

As can be seen from Figure 13, at short

wavelengths, the magnitude of the rota-

tion and the shape of the hysteresis loops

are quite comparable for the two types of

particle of common particle concentra-

tion. This similarity has been interpreted

as originating from the dominant effects

by the CoFe2O4 interband transitions to

the magneto-optical tensor in the highly

absorptive violet/blue wavelength regime.

A dramatic contrast emerges between the

magneto-optical response for

Ag-CoFe2O4 dimers and CoFe2O4 mono-

mers at longer wavelengths, outside the

CoFe2O4 interband-transition- domi-

nated regime. Although the overall mag-

nitude of the Faraday rotation decreases

away from the absorption edge of

CoFe2O4, the rotation becomes signifi-

cantly enhanced for the dimers relative to

the monomers by nearly an order of

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magnitude near 633 nm. This approximate wavelength range

also corresponds to a ‘‘crossover’’ regime, where the sign of the

Faraday effect changes for the monomer nanoparticles, while

the dimers remain unaffected in this regard. This strong

contrast in spectral behavior has been proposed as being due to

the dielectric contribution of the Ag-nanoparticle component

in the Ag-CoFe2O4 dimer. The crossover behavior for the

CoFe2O4 monomer in particular, absent in the dimer case,

occurs in the Ag-nanoparticle plasmon tail where the dielectric

contribution by Ag to the dimer appears to produce a

significant additive contribution to the overall magneto-optical

response of the composite nanoparticle-Ag-CoFe2O4 particle

pairs.

4. Potential Applications of theMulticomponent Hybrid Nanoparticles

Multicomponent hybrid nanoparticles have demonstrated

many interesting physical properties originating from inter-

component interactions, and are expected to show unique

potential applications in various fields ranging from biomedi-

cine to spintronics.

A significant number of demonstration- of-concept papers

have been published recently, demonstrating the great poten-

tial of these multicomponent nanoparticles for biomedical

applications.[15,44–47] Sun’s group developed biocompatible

Au-Fe3O4 nanoparticles suitable for A431 cell (human

epithelial carcinoma cell line) attachment.[47] The particles

are magnetically and optically active and are useful for

simultaneous magnetic and optical detection. The hybrid

nanoparticles shorten the T2 relaxation time of protons, and

therefore can be used as magnetic resonance imaging (MRI)

contrast enhancement agents. The T2 relaxivity R2� of the 8-20

nm Au-Fe3O4 nanoparticles around A431 cells is 80.4 s�1 �mM

�1. Au-Fe3O4 nanoparticles also show strong reflectance at

590-650 nm, and A431 cells labeled with these particles can be

imaged using a scanning confocal microscope. The optical

detection limit reaches 90 pM Au. Furthermore, an external

magnetic field can be applied to manipulate the cells. The

results indicate that the Au-Fe3O4 nanoparticles are promising

as a new type of multifunctional probe for diagnostic and

therapeutic applications. Another example of a multifunc-

tional probe is the hydrophilic Ag-Fe3O4 nanoparticle

developed by Xu’s group.[15] These nanoparticles are fluor-

escent, responsive to magnetic forces and are able to bind to

specific receptors.

Core/shell magnetic nanoparticles are attractive multi-

functional systems for potential applications ranging from

catalysis, data storage, and high-frequency materials to

advanced permanent magnets. Co/Pt core/shell nanoparticles

have been tested by Cheon’s group for catalytic hydrogenation

of 1-decene.[48] The catalyst can be recycled by using a

magnetic field due to the superparamagnetic property of the

Co core. Core/shell nanoparticles with a magnetic core and a

Adv. Funct. Mater. 2008, 18, 391–400 � 2008 WILEY-VCH Verlag GmbH &

non-magnetic shell, such as FePt/SiO2 and FePt/MnO, have

been developed by several groups as building blocks for self-

organized magnetic media,[49,50] where FePt with high

anisotropy provides the high thermal stability, and a non-

magnetic shell can be used to decouple the grains and prevent

sintering of the nanoparticles. A magnetic-dielectric core/shell

nanoparticle may have applications in high-frequency materi-

als, where a soft-magnetic metal such as CoFe can be used to

provide high saturation magnetization and permeability, while

the dielectric shell can be used to cut eddy current loss.[51] Such

a composite structure is expected to lead to low-loss, high-

performance RF materials in place of commercial ferrites for

power electronics and telecommunications. In the case of

hard-soft exchange-coupled magnetic core/shell nanoparticles,

both the coercivity andmagnetization can be tuned bymaterial

parameters and the dimensions of the core and shell, as well as

their mutual interactions.[9] It can further lead to advanced

nanocomposite magnets with an energy product higher than

the single-phase counterparts.[29]

Multicomponent magnetic-semiconductor nanoparticles

may also show great potential for applications in spintronics.

The magnetic component will provide spin filtering of the

carriers by injection or extraction; and the recombination of

the electrons and holes in the semiconductor component is

expected to give polarized emission. The structure can be

considered as a nano spin-LED, except that the device is

directly synthesized from the solution phase instead of fabri-

cated by top-down lithography techniques.

5. Conclusions

Recent synthetic efforts have led to the formation of a large

variety of multicomponent nanoparticles with different levels

of complexity. Owing to the strong coupling between different

components, these systems show novel physical phenomena

and enhanced properties, making them superior to their

single-component counterparts for biomedicine, nanoelectro-

nics, optoelectronics and spintronics applications. Despite

these exciting new developments, the study of multicomponent

nanoparticles is still at its infant stage compared to most

single-element and alloy nanoparticle systems. However, with

the progress in the fundamental understanding of the physics

and chemistry in these multicomponent structures, we foresee

that novel concepts and applications will be demonstrated in

the not-so-distant future.Received: October 22, 2007Revised: November 4, 2007

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syntheses, properties, and potential applications of multicomponent magnetic nanoparticles

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