Nematic-like Organization of Magnetic Mesogen-Hybridized Nanoparticles

Nematic-like Organization of Magnetic Mesogen-Hybridized Nanoparticles

Hybrid nanoparticles

Nematic-like Organization of Magnetic Mesogen-Hybridized NanoparticlesArnaud Demortiere, Saıwan Buathong, Benoıt P. Pichon, Pierre Panissod,Daniel Guillon, Sylvie Begin-Colin,* and Bertrand Donnio*

Keywords:� hybrid materials

� liquid crystals

� magnetic properties

� nanoparticles

� nematic materials

A fluid nematic-like phase is induced in monodisperse iron oxide nano-

particles with a diameter of 3.3 nm. This supramolecular arrangement is

governed by the covalent functionalization of the nanoparticle surface with

cyanobiphenyl-based ligands as mesogenic promoters. The design and

synthesis of these hybrid materials and the study of their mesogenic

properties are reported. In addition, the modifications of the magnetic

properties of the hybridized nanoparticles are investigated as a function of

the different grafted ligands. Owing to the rather large interparticular

distances (about 7 nm), the dipolar interaction between nanoparticles is

shown to play only a minor role. Conversely, the surface magnetic aniso-

tropy of the particles is significantly affected by the surface derivatization.

1. Introduction

Ordered assemblies of monodisperse nanoparticles (e.g.,

metallic, alloy, or oxide NPs) that provide two-dimensional

(2D) or 3D superlattices[1] constitute an attractive class of

nanostructured functional materials for a wide range of

applications from high-density recording media,[2] single-

electron microelectronic[3] and charge transport[4] devices, to

nanoscale plasmon waveguides[5] and metamaterials.[6] The

control of the spatial assemblies in ordered structures of NPs

and the expected synergistic collective behaviors[7–9] and

emergence of new physical properties[10] are some of the

exciting challenges of this early century. In the field of

magnetism, collective effects such as superspin glass or

superferromagnetic behavior have been reported and were

found to depend strongly on interparticle distances, symmetry

environment, and relative orientations.[7,11]

[�] Dr. B. Donnio, Prof. S. Begin-Colin, Dr. A. Demortiere,

Dr. S. Buathong, Dr. B. P. Pichon, Dr. P. Panissod, Dr. D. Guillon

Institut de Physique et Chimie des Materiaux de Strasbourg

(IPCMS)

CNRS-Universite de Strasbourg (UMR7504)

23, rue du Loess, BP 43, 67034 Strasbourg Cedex 2 (France)

E-mail: [email protected]

: Supporting Information is available on the WWW under http://www.small-journal.com.

DOI: 10.1002/smll.201000285

small 2010, 6, No. 12, 1341–1346 � 2010 Wiley-VCH Verlag Gmb

More and more attention is recently being paid to the

development of novel approaches for organizing NPs. Various

strategies have been used to induce periodic arrangements on

micrometers scale of NPs,[12] usually ‘‘passivated’’ by an

aliphatic inert outer shell, including controlled solvent

evaporation,[13,14] Langmuir–Blodgett,[15] and layer-by-layer

techniques.[16] Recently, another concept has been developed

using mobile liquid-crystalline (LC) phases.[17] Compared to

the aforementioned self-assembly methods, notably to the slow

evaporation technique mostly leading to body-centered cubic

(bcc), hexagonal close-packed (hcp), or face-centered cubic

(fcc) arrays of pseudo-spheres as in single, binary, and ternary

superlattices,[1,13,18] which are primarily governed by close-

packing and geometrical criteria[19] and dipolar interactions,[20]

a higher modularity in the organization of LC–NP hybrids is

allowed due to the softening of interparticle boundaries.

Furthermore, it is expected that the various superlattices

formed by this promising route to be tuned by external stimuli

(thermal, magnetic, electric, optical) and the number of defects

reduced by the self-healing ability of the fluid, low-dimensional

LC mesophases.

The majority of studies on this type of hybrid material have

focused on lyotropic LC phases as structuring templates for the

arrangement of NPs[21] and the impact of NPs, as doping agent,

on the mesomorphic properties.[22,23] An alternative strategy

consists of self-assembling quasi-spherical NPs in mesophases

using thermotropic LC-forming capping agents either in

composite forms[24] or as single-component materials.[17,25]

H & Co. KGaA, Weinheim 1341

full papers S.Begin-Colin, B.Donnio et al.

1342

Recently, it was shown for the latter that the nature of the

organic ligands, not necessarily mesomorphic, used to deriva-

tize gold NP surfaces, gives rise to specific organizations and

orientations of related hybrids into nematic,[26,27] smectic,

columnar,[28] cubic,[29] and more complex mesophases.[30]

In this study, we report on the induction of a fluid nematic-

like phase, close to room temperature, of some iron oxide NPs

of 3.3 nm in diameter whose arrangement is governed by their

surface hybridization with pro-mesogenic cyanobiphenyl-

based ligands. Hence, this is the first report of thermotropic

nematic phase exhibited by hybridized, quasi-spherical ferrite

NPs. In addition, the nanoparticle magnetic behavior exhibits

clear changes after surface derivatization that suggest mod-

ifications of the surface magneto-crystalline anisotropy and

surface state of the NPs for the different anchoring functions.

2. Results and discussion

The formation of a thermotropic LC phase in hybrids often

requires building units with an anisotropic shape, which is

mandated by mesomorphic molecules adsorbed on the surface

of the anti-mesogenic NPs.[17,25–30] However, in order to obtain

an efficient anisotropic effect, the sizes of the soft molecular

parts (ligands) and the hard polyhedral cores (NPs) have to be

comparable. Thus, the hybrid materials studied here consist of

magnetic iron oxide core particle with a mean diameter of

3.3� 0.7 nm, as measured by transmission electron microscopy

(TEM; Figures S1–S3 of the Supporting Information), with the

surface derivatized by pro-mesogenic cyanobiphenyl-based

ligands, referred to hereafter as pro-mesogens LC1 and LC2,

each bearing a different anchoring function (Figure 1). In order

to efficiently stimulate mesomorphism induction in the hybrids,

Figure 1. Schematic representation of the solvent-mediated ligand-

exchange reaction and chemical structures of the pro-mesogenic ligands

(LC1 and LC2).

www.small-journal.com � 2010 Wiley-VCH Verlag Gm

proto-dendritic ligands, LC1 and LC2, that is, low-generation

branched molecules bearing two or more mesogenic pending

groups, were elaborated (Supporting Information) according to

a strategy successfully applied for the insertion of non-

mesogenic cores such as fullerenes,[31] SMM polymetallic

clusters,[32] rotaxanes[33] and catenanes,[34] into liquid-crystal-

line mesophases. Iron oxide NPs coated with oleate chains

(NP@OA) were synthesized with controlled sizes by thermal

decomposition of an iron oleate precursor complex in high

boiling point solvents.[35] The control of the parameters of the

nucleation and growth processes such as temperature rates and

organic solvents allows obtaining NPs with tailored diameters

and nearly uniform polyhedral shapes (i.e. quasi-spherical).

Adsorbed oleic acid (OA) fragments ensured the stability of the

as-synthesized NPs in organic solutions and avoided aggrega-

tion. The partial substitution of the OA surfactants by LC1 and

LC2 on the NP surface was carried out by solvent-mediated

(CH2Cl2) exchange reaction containing an excess of LC1 or

LC2. After 48 h at 50 8C, the resulting solids were purified by

chromatography (Supporting Information).

The covalent attachment of the organic molecules onto the

NPssurfacevia the carboxylate(LC1)or the phosphonate(LC2)

function, respectively, was unequivocally confirmed by IR

spectroscopy, with the disappearance of the vibrations corre-

sponding to C¼O (1700 cm�1) and C�OH (1225 cm�1) bonds

for NP@LC1 and P¼O (1000 cm�1) and P-OH (940 cm�1) bonds

for NP@LC2 (Figure S4, SI). IR also indicated the absence of

any unbounded molecules. 1H-NMR spectra of LC1 and

NP@LC1 (Figure S5, Supporting Information) are similar and

with little variations in chemical shifts but, however, show a

substantial broadening of the signals for the hybrid materials,

expected due to the paramagnetic inorganic core, and therefore

were not further exploitable. Thermogravimetric analysis and

thermal differential analysis (TGA-TDA) measurements con-

firmed the formation of the hybrids, but also permitted the ratio

of the organic shell and inorganic core to be appreciatively

evaluated, and thus to estimate roughly the average number of

adsorbed molecules on the oxide surface and the respective

proportions of OA and LCi ligands (Figure S6, Supporting

Information). As previously reported,[36–38] the higher grafting

efficiency (i.e., approximately threefold) was observed for

phosphonated molecules at the expense of the carboxylate ones.

Quantitatively, it was found that NP@LC1 was homogeneously

derivatized by �30� 10 LC1 and 95� 10 OA and NP@LC2 by

�100� 10 LC2 and 50� 10 OA (Table S1, Supporting

Information). The mean surface coverage (average total

number of ligands on NP surface) remained therefore fairly

constant for the three NPs, despite the ligands’ nature change

(�160� 10 OA were calculated for NP@OA), leading to an

average ligand cross-sectional area of 20, 27, and 23 A2 for

NP@OA, NP@LC1, and NP@LC2, respectively (Table S1 of the

Supporting Information). These values are in good agreement

with the bulkiness of the organic moieties structures. Under the

current ligand-exchange procedure applied here, the OA/LCi

ratio was maintained at equilibrium, and was found constant and

reproducible.

The formation of the hybrids was ultimately confirmed by

the drastic change of their thermal behavior with respect to that

of the organic acidic precursors. LC1 and LC2 are not

bH & Co. KGaA, Weinheim small 2010, 6, No. 12, 1341–1346


Table 1. Summary of the thermal behavior of LC1, LC2, NP@LC1, and NP@LC2 as deduced from DSC, POM, and XRD. Cr: crystalline, Iso: isotropicliquid, N: nematic, g: glass; DSC data, 2 8C min�1, T: transition temperature; DH: enthalpy of transition; DS: entropy of transition. The glass-transition temperatures, as the DSC thermograms were silent in this range, were estimated by POM on the basis of the softening of the texture.

Compounds LC1 NP@LC1 LC2 NP@LC2

Transition Cr! Iso g!N N! Iso Cr! Iso g!N N! Iso

T [8C] 140.85 40–60 180.2 123.3 30–50 174.2

DH [J g�1] 47.02 – 1.75 26.2 – 0.15

DS [mJ g�1 K�1] 115 – 3.87 64.6 – 0.33

mesomorphic despite the presence of the strongly LC-

promoting cyanobiphenyl units and clear in the isotropic liquid

at 140.5 8C (LC1) and 123.2 8C (LC2). Differential scanning

calorimetry (DSC) analyses indicated an increase of the

clearing temperatures to �180 8C for NP@LC1 and 174 8Cfor NP@LC2, without any transition peak corresponding to the

melting of free unbounded molecules (Supporting Information,

Figures S7 and S8). The values of the clearing transition

enthalpies were low (1.75 J g�1 and 0.15 J g�1) but on the

same order of those found for other LC-hybrid materials

(Table 1), and in agreement with low-dimensional ordered

structures.[26–30] By polarized optical microscopy (POM), the

hybrids develop a low birefringent and fluid texture on heating,

homogeneous throughout the sample, although not character-

istic to a specific LC phase, compromising definite mesophase

assignment (Supporting Information, Figure S9). Below 50–

60 8C, the mesophase is frozen in a glassy state (Supporting

Information). The thermal behavior of the hybrids is fully

reversible (POM, DSC).

X-ray diffractograms of NP@LC1 and NP@LC2 recorded at

various temperatures from room temperature up to the

isotropic liquid transition revealed diffuse scatterings in the

small-angle part corresponding to periodic molecular fluctua-

tions (Figure 2 and Supporting Information, Figure S10). The

peak at the smallest angle corresponds to the average

interparticle periodicity, with d values equal to 7.5� 0.5 nm

and 6.9� 0.1 nm for NP@LC1 and NP@LC2, respectively. The

estimated thickness of the organic coats, 2.1� 0.2 nm

for NP@LC1 and 1.8� 0.2 nm for NP@LC2, are realistic,

Figure 2. XRD diffractograms of NP@LC1 and NP@LC2 at T¼100 8C(L: persistence length).


considering 3.3� 0.7 nm for the NP diameter. Additionally,

the correlation length calculated for NP@LC2 is about 3 times

that of NP@LC1, as expressed by the concomitant intensity

increase and signal sharpening of the diffraction peak; this is

consistent with a higher degree of cohesion resulting from a

more efficient interdigitation of the cyanobiphenyl groups

(‘‘zipperlike’’ packing), which, depending on the ligands’

grafting density and bulkiness, extends more or less far

(Supporting Information Figure S12). The second, weak

diffusion (dmol) occurring at around 1.5 nm (NP@LC1) and

1.8 nm (NP@LC2) is assigned to weakly correlated cybotactic

clusters as in classical nematic phases; the discrepancy found in

these periodicities may also be connected to the different

grafting densities and to the ligands’ nature. In the wide-angle

part, the characteristic peak at�0.45 nm, although the intensity

of the diffusion is very low, featuring the liquidlike state of the

molten aliphatic chains and of the mesogenic pending groups, is

observed, confirming the fluidlike nature of the mesophase.

This diffuse scattering further evidences the dense packing of

alkyl chains and ligands around the NP core (and thus their

grafting).

Combining DSC, POM, and XRD experiments, the

mesophase can therefore be assigned as a nematiclike phase

and may be pictured as a quasi-regular isotropic distribution of

oxide nuclei within the organic matrix characterized by a local

nematic order (Supporting Information, Figure S12), imposed

by the NP ordering. This representation is in fairly good

accordance with the TEM images obtained for the hybrids

(Supporting Information, Figures S2 and S3). As far as the

organization is concerned and under the current grafting

conditions, the nature of the anchoring functions (CO2H versus

PO3H2) has no direct influence on the organization.

The magnetic properties of NP@OA, NP@LC1, and

NP@LC2 were investigated by a SQUID magnetometer in

DC and AC modes. It has been demonstrated here that the use

of LC ligands allows us to reach a 3D organization of magnetic

NPs and to modulate the interparticle distances and thus the

dipolar magnetic interactions.[26–30] Most studies on the

collective behavior of assemblies of magnetic NPs involve

investigations of dipolar interactions in ordered arrays or

polymer-based composites, but few studies report on the use of

structuring molecules.[7b,39,40,41,42] The ZFC/FC curves

(Figure 3) show that the temperatures (TM) below which the

magnetic moments of the hybrids are blocked, NP@LC1

(TM� 15 K) and NP@LC2 (TM� 12 K), are unexpectedly

lower (�25 to 50% decrease from NP@LCi!NP@OA) than

that of NP@OA (TM� 18 K). As it was not anticipated that the

properties of the individual particles of the different samples

were modified, it was at first tempting to assign the lowering

H & Co. KGaA, Weinheim www.small-journal.com 1343


Figure 3. ZFC/FC curves (75 G) and hysteresis loops (5 K; inset) of

NP@OA, NP@LC1, and NP@LC2.

Figure 4. Imaginary component of the AC susceptibility as a function of

temperature and frequency of NP@LC1.

Figure 5. Plot of ln(t)¼ ln(1/2pf) against 1/Twith the fit performed from

the Neel–Brown model for the sample NP@LC1.

Table 2. Calculated dipolar energy, intrinsic switching time t0, andactivation energy Em of NP@OA, NP@LC1, and NP@LC2.

Compounds NP@OA NP@LC1 NP@LC2

Calculated dipolar energy [K] 3.3 1.7 2.2

t0 [10�11 s] 0.67�0.24 3.8�1.3 1.2�0.52

Em [K] 310�7 285� 16 197�11

1344

of TM to the expected decrease of the magnetic dipolar

interactions. Indeed, the NP@OA sample may be assimilated to

a 3D assembly of NPs with interparticle distances of about 6 nm,

whereas the interparticle separations in NP@LC1 and

NP@LC2 are increased to 7.5 and 6.9 nm, respectively.

Magnetization measurements at 5 K (inset of Figure 3) show

an increase of the mR ratio (corresponding to the remnant

magnetization/saturation magnetization ratio) of NP@LCi as

compared to NP@OA, which is generally explained by a

lowering of the demagnetizing dipolar interactions.[43]

A more thorough analysis of the results shows however that

this explanation does not hold. Firstly, the strength of the

dipolar interaction, ED�Mm�m2/d3, can be estimated as

3.3 K, 1.7 K, and 2.2 K for NP@OA, NP@LC1, and NP@LC2

respectively (M is the volume magnetization, m the particle

moment �850 mB, and d the interparticle distance). These low

values indicate that dipolar interactions can hardly explain

freezing temperatures above 10 K. Secondly, if the interactions

were to play the dominant part, one would expect a smaller TM

in NP@LC1 than in NP@LC2, which is in stark contrast to what

is observed. These two facts strongly suggest that the dipolar

interaction plays only a secondary part in the blocking process

of the particle moments. In order to confirm this explanation,

measurements of the frequency-dependent AC susceptibility

have been undertaken. As shown in Figure 4, the imaginary part

of the susceptibility, x00, exhibits well-defined maxima at a

temperature TM that increases with increasing frequency (see

also Supporting Information, Figure S13). This behavior

reflects the underlying relaxation of the magnetization with a

relaxation time t(T) that increases as the temperature

decreases.

For all samples, the maximum relative shift per decade of

frequency w¼DTM/[TMD(logf)] is about 0.08, a value sig-

nificantly larger than expected for a freezing, spin-glass-like

mechanism (<10�2).[43] This is evidence of a relaxation

mechanism dominated by single-particle magnetic anisotropy

rather than interaction between particles. Magnetic parameters

such as the intrinsic switching time t0 and the activation energy

Em can be extracted from the frequency dependence of TM. In


the simplest case of non-interacting particles, the relaxation

time is supposed to obey the Neel–Brown model t¼ t0exp(Em/

kT), where Em is the anisotropy energy barrier and k the

Boltzmann constant. As can be seen in Figure 5 (plot of

ln(t)¼ ln(1/2pf) against 1/TM, and see also Supporting

Information, Figure S14), the Neel–Brown model yields a very

good fit: the best values for Em/k and t0 are listed in Table 2.

The more sophisticated Vogel–Fulcher model,

t¼ t0exp[Em/k(T-TVF)], which involves explicitly an inter-

particle coupling as the temperature offset TVF, was not found

to give better fits. Actually, TVF was found to be lower than 4 K

bH & Co. KGaA, Weinheim small 2010, 6, No. 12, 1341–1346


in all samples, in agreement with the above estimation of the

dipolar interaction ED. Therefore, it can be definitely

concluded that the blocking of the particle moments is mostly

due to the particle magnetic anisotropy. It should be noted that

the anisotropy energy barrier Em takes significantly different

values depending on the chemical function used to graft to the

particles. From the values of Em and the nanoparticle volume,

the magneto-crystalline anisotropy constants,K¼Em/V, can be

calculated in the three samples: 5.6, 5.1 and 3.5� 105 erg cm�3

for NP@OA, NP@LC1, and NP@LC2, respectively. These

values are clearly larger than the anisotropy constant of bulk

magnetite (�105 erg cm�3), which shows a large influence of the

surface on the magnetic properties. Correspondingly, the

results also show that the modifications of the magnetic state of

the surface depend on the nature of the grafting function. The

modification of the surface state is particularly visible in our

case because of the small diameter and therefore the large

surface-to-volume ratio of the NPs. An important modification

of the anisotropy constant is observed between NP@OA and

NP@LC2 with ligands grafted through a carboxylate and

phosphonate coupling agents, respectively. The K value of

NP@LC2 is closer to the bulk value than that of NP@OA. This

behavior can be explained by the difference of grafting agents in

which the phosphonate induces a higher polarity of the surface.

This is consistent with earlier results showing a modification of

other NPs’ magnetic properties induced by the grafting

phosphonate-group-containing organic moieties.[37,38] In the

case of NP@LC1, the modification of the anisotropy constant

(as compared to that of NP@OA) can be explained either by the

decrease of the carboxylate functions onto the surface (around

25%) or by a modification of the electronic environment near

the surface due to the decrease of the ligand density.[44]

All these results infer that the surface state of magnetic NPs

has to be taken into account to study the magnetic properties of

very small particles. Since the magnetic dipolar interactions in

these systems are weak, the grafting of LC ligands induces a

clear modification in the magneto-crystalline anisotropy energy

of magnetic hybrid NPs.

3. Conclusions

LC magnetic NPs have been prepared by hybridizing the

surface of small iron oxide nanocrystals with pro-mesogenic

molecules. The resulting hybrid materials exhibit a fluid

nematic-like phase behavior close to room temperature and

the overall magnetic behavior of the precursory NP@OA is

notably modified by mesogen adsorption. A higher mod-

ularity in the organization of covalent LC-NP hybrids is thus

allowed due to the softening of interparticle boundaries, and

the use of LC ligands to derivatize NPs surfaces appears a

very promising route to organize and control magnetic NPs

into periodic and high-order superlattices. Furthermore, the

LC grafting density may be a critical parameter to modulate

the mesomorphism of such hybrid materials. Finally, it was

demonstrated that the grafting of these ligands clearly

influences the magnetocrystalline anisotropy energy of the

magnetic hybrid NPs, considering that magnetic dipolar

interactions are weak in such systems.


Acknowledgements

The authors thank the CNRS, the University of Strasbourg, and

the National Agency for Research (ANR DENDRIMAT) for

funding, and Didier Burger (TGA), Alain Derory (SQUID), and

Dr. Benoıt Heinrich (DSC and XRD) for technical support.

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Received: February 23, 2010Revised: March 15, 2010Published online: May 19, 2010

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Nematic-like Organization of Magnetic Mesogen-Hybridized Nanoparticles

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