Unprecedented Crystalline Super-Lattices of Monodisperse Cobalt Nanorods

Article abstract of DOI:10.1002/anie.200352090

Coalescence of initially produced nanospheres inside 3D super-lattices may be the reason for the formation of ferromagnetic cobalt nanorods, monodisperse in length and diameter, from [CO(η₃-C₈H₁₃) (η⁴-C₈

Full text of DOI:10.1002/anie.200352090

Angewandte
Chemie
Self-Assembly of Cobalt Nanorods
assemblies from spherical nanoparticles is well-docu-
mented,[8,14–18] only a few reports concern the organization
of nanorods,[19,20] and the formation of crystalline super-
lattices has not been reported.
Unprecedented Crystalline Super-Lattices of
Monodisperse Cobalt Nanorods**
Our research group has been involved for a long time in
the synthesis of nanoparticles from non-carbonyl organo-
metallic precursors.[21–24] Decomposition of such compounds
affords by-products that are not detrimental to the intrinsic
properties, such as the magnetic moments, of the particles.[22]
We have also reported that the use of weakly coordinating
ligands may enable the control of shape in solution.[23] This
technique has been used for the formation of nickel nanorods
in the presence of long-chain amines[24] and, recently, of the
cobalt nanorods mentioned above.[13] The exact mechanism
controlling both the formation of the rods and their dimen-
sions remain unclear. One striking observation is, however,
that the same ligand systems can be used to control the self-
assembly of the spherical particles and the shape of the
nanorods.
FrØdØric Dumestre, Bruno Chaudret,*
Catherine Amiens, Marc Respaud, Peter Fejes,
Philippe Renaud, and Peter Zurcher
Magnetic nanomaterials display new interesting fundamental
properties and potentially have many applications.[1–4] One of
the most promising applications, high-density information
storage, requires spatially separated particles in the nano-
meter range that act as single magnetic domains, are
ferromagnetic at room temperature, and are electrically
isolated.[5] In this respect, the preparation of well-ordered
two- and three-dimensional (2D and 3D) super-lattices is of
crucial importance.[6] Several synthetic routes towards Fe[6]
and Co[7,8] nanoparticles have recently been developed, for
example, through the decomposition of carbonyl compounds.
A high magnetic anisotropy is, however, necessary for
spherical nanoparticles to be ferromagnetic at room temper-
ature. This can be achieved, as recently reported, through
formation of bimetallic Co/Pt or Fe/Pt nanoparticles.[9–11]
Another approach is to use nonspherical nanoparticles
which may display large shape anisotropy. Recently, Alivisa-
tos and co-workers described the formation of cobalt nano-
disks from the decomposition of [Co2(CO)8] in the presence
of trioctylphosphane oxide (TOPO) and oleic acid (cis-9-
octadecenoic acid).[12] At the same time we reported the
selective formation of cobalt nanorods and nanowires, as well
as the control of their aspect ratio through the decomposition
of organometallic precursors in the presence of a mixture of
long-chain amines and oleic acid.[13] Anisotropic structures,
however, also require organization in 2D or 3D super-lattices
to be of practical interest. While the formation of such
This observation prompted us to try to understand the
exact role of the ligand system in shape control and also to
finely tune the ligand system to produce monodisperse
nanorods that were able to self-assemble. We now report on
the formation of unprecedented 2D and 3D super-lattices of
monodisperse cobalt nanorods through the decomposition of
[Co(h3-C8H13)(h4-C8H12)][25] in the presence of a mixture of
hexadecylamine (HDA) and stearic acid (octadecanoic acid).
We also provide evidence for the self-organization of
spherical particles prior to coalescence into nanorods.
In a typical reaction [Co(h3-C8H13)(h4-C8H12)] was decom-
posed under H2 (3 bar) in anisole at 1508C in the presence of a
mixture of a long-chain amine and an acid. The reaction with
oleic acid and oleylamine has previously been shown to
produce spherical nanoparticles after 3 h (3 nm) and nano-
rods after 48 h (9 ” 40 nm). However, agglomerates and well-
dispersed particles were present after a reaction time of 3 h.
Careful examination of these agglomerates by TEM after
preparation of a very thin layer by ultramicrotomy shows the
presence of monodisperse nanoparticles with a diameter of
3 nm included in a 3D network (see Supporting Information).
Similar super-lattices of tin nanoparticles have been observed
previously and were suggested to result from crystallization in
solution.[18] The interparticle distance is about 2.5nm, and
areas where coalescence of the particles occurs are evident.
This observation, therefore, suggests that coalescence of the
particles occurs inside super-lattices, at least in an initial step.
An attempt was made to optimize the monodispersity of
the rods by choosing the most suitable ligands and concen-
trations. The use of lauric acid (dodecanoic acid, C12) and
hexadecylamine (1:1:1 Co/acid/amine) leads to the formation
of cobalt nanorods with an approximate size of 5” 85nm ( 1).
Deposition of one drop of a diluted solution of these rods on
the microscopy grid resulted in their self-organization into
arrays containing approximately 10 parallel nanorods with an
interparticle spacing of about 3 nm (Figure 1B). Using
octanoic acid (C8) instead of lauric acid results in the
formation of shorter and wider rods (9 ” 33 nm; 2; Fig-
ure 1A). The nanorods produced with stearic acid (C18) have
a diameter similar to the previous rods, but are longer (8 ”
[*] Dr. B. Chaudret, F. Dumestre, Dr. C. Amiens
Laboratoire de Chimie de Coordination du CNRS
205, route de Narbonne, 31077 Toulouse CØdex 04 (France)
Fax: (+33)5-6155-3003
E-mail: chaudret@lcc-toulouse.fr
F. Dumestre, Dr. P. Renaud
Digital DNA Labs
Semiconductor Products Sector, Motorola
le Mirail B.P. 1029, 31023 Toulouse CØdex (France)
Dr. M. Respaud
Laboratoire de la Physique de la Mati›re CondensØe
INSA, 135 avenue de Rangueil, 31077 Toulouse (France)
Dr. P. Fejes, Dr. P. Zurcher
Digital DNA Labs
Semiconductor Products Sector, Motorola
2100 E. Elliot Road, Tempe, AZ 85824 (USA)
[**] The authors thank the CNRS and MOTOROLA S.P.S. for support, M.
Vincent Colli›re, Lucien Datas, and TEMSCAN service (UniversitØ
Paul Sabatier Toulouse) for TEM measurements, and Dr. Katerina
Soulantica for fruitful discussions on particle organization.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 5213 –5216
DOI: 10.1002/anie.200352090
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5213

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orientation between one another. The three sets of nanorods
consist of a pure hexagonal close-packed (hcp) cobalt
structure, with the c axis oriented along the growth direction
of the nanorods (Figure 2C).
From a magnetic point of view, these nanorods display a
strong effective anisotropy (Keff) as a consequence of both
their shape and crystalline structure, where the c axis linked
to the hcp phase is parallel to the rod axis.[26] The magnetic
moment of the nanorods can be considered as a macro-spin,
with a large energy barrier (DE = Keff v) between the up and
down directions. The volume (v) and high anisotropy of each
individual nanorod lead to a blocking temperature well above
room temperature.[27] These systems may, therefore, be very
interesting for the storage of very high density magnetic data.
Accordingly, these materials, like those previously obtained in
the presence of oleic acid, display a ferromagnetic behavior at
room temperature. Hysteresis loop measurements at 2 K
(Figure 3,) show magnetization M(5T) values of 136, 100, and
142 Am2 kgÀCo for 2, 1, and 3, respectively, that are reduced
À1
relative to the bulk value (M = 163 Am2 kg ) and to the value
Co
previously reported for rods not showing any sign of
organization, but which display an analogous ligand coat-
ing.[25] The hysteresis loops display very different shapes and
coercive fields Hc of 740, 3200, and 7200 Oe for 2, 1, and 3,
respectively. Their remanent magnetization also increases
from 0.16 ” M(5T) in 2 to 0.5” M(5T) in 3. As a general rule,
parallel nanorods organized into planar arrays, such as 1,
display an antiferromagnetic coupling of their magnetic
moments, while chains of nanorods display a ferromagnetic
one.[28,29] The coupling is expected to be stronger within the
chains than between the planes. The most favorable organ-
ization from a magnetic point of view is that observed in 3,
where ferromagnetic chains of nanorods are coupled anti-
ferromagnetically. The existence of antiferromagnetic cou-
plings could be a possible explanation for the low M values
measured for 1–3. However, a remanent ferromagnetic order
can be induced after saturating the sample, and this will be
stabilized by the anisotropy of the nanorods. This result
explains the high remanent magnetization of 3. Finally, an
open question is whether the magnetic energy could help the
self-organisation process and the formation of 3D lattices.
Precise calculations will be performed to estimate this.
Figure 1. TEM micrographs of nanorods synthesized using hexadecyla-
mine and A) octanoic acid (2), B) lauric acid (1), and C, D) stearic acid
(3). The sample used in (C) was prepared by ultramicrotomy. Scale
bar: 30 nm.
128 nm; 3) and more regular in dimensions. Furthermore, the
nanorods are organized into unprecedented 3D super-lattices,
the largest of which is close to 1 mm (Figures 1C, D and 2). In
each case the nanorods precipitate upon cooling the reaction
mixture to room temperature. These super-structures are thus
most probably formed from the reaction solution by a
crystallization process. Ultramicrotomy shows the almost
exclusive presence of such super-lattices in all parts of the
sample (Figure 1C). In addition, their random orientation on
the TEM grid allowed the observation, in selected cases, of
the hexagonal cross-section of the nanorods (Figure 2B). The
rods are organized in a hexagonal structure with an inter-
particle distance of approximately 2 nm. This arrangement
has been further confirmed by tilting the copper grid in the
microscope chamber to bring the growing axis of the nanorod
clusters parallel to the electron beam. An interparticle
distance of 2.5nm can also be estimated from the nanorods
parallel to the TEM grid (Figure 2A). The nanorods stabi-
lized by lauric acid and/or stearic acid adopt a parallel
Long-chain amine and acid molecules (and now, oleic acid
and oleylamine) have become standard reagents both for the
control of the self-assembly of different metal and semi-
Figure 2. TEM micrographs of self-organized nanorods of 3 (A, B; scale bar: 10 nm). HREM micrograph and electron-diffraction pattern of some
aligned rods (C).
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Angewandte
Chemie
the acid on the dimensions of the rods is not understood. We
can, however, point out that gelation has been observed at an
early stage of the reaction in the two systems that produce the
more regular rods, namely in reactions involving lauric and
stearic acid. This gelation could provide a template for the
formation of super-lattices of spherical particles and possibly
the growth of nanorods. The most important result of this
study, the formation of self-assembled nanorods that are
strictly monodisperse in terms of both length and diameter, is
observed for the longest rods that are obtained with stearic
acid (which contains the longest alkyl chain). This system
displays bunches of nanorods that are monodisperse in
diameter (5nm) but not in length at the early stage of the
reaction, and thus suggests that the coalescence process is
facile. In contrast, octanoic acid, which possesses a small alkyl
chain, leads to objects displaying smaller aspect ratios. This
result may suggest that a good fit of the chain lengths of the
acid and amine is necessary to reach monodispersity. The
nanorods (3) self-organize into 2D and 3D super-structures,
inside which they are all aligned in the same direction with a
compact, hexagonal arrangement in the perpendicular plane.
Some defects are visible, such as the ones shown in Figure 2B.
The interparticle distance in these super-structures is approx-
imately 3 nm when using lauric acid and 2–2.5nm when using
stearic acid. The maximum length of a long-chain ligand
directly coordinated to cobalt can be estimated to be about
2.5nm for stearic acid and HDA, and 1.8 nm for lauric acid.
This result implies a different organization for rods 2 and 3. In
the case of 2, the 3-nm interparticle distance and the
formation of monolayers of aligned rods suggests a self-
organization on the microscopy grid, in a way similar to that
observed for several systems of spherical particles.[21] In
contrast, the shorter interparticle distance and the observa-
tion of 3D super-lattices for 3 implies interdigitation of the
alkyl chains of ligands coordinated to different nanorods,
which suggests a real process of direct crystallization in
solution, as recently found for tin nanoparticles.[18]
In summary, we have demonstrated that the appropriate
choice of ligands and alkyl chain lengths enables the synthesis
of ferromagnetic nanorods of monodisperse diameter and
length from a non-carbonyl organometallic precursor. This in
turn allows the formation of unprecedented hexagonal super-
lattices of nanorods as a result of crystallization in solution.
The formation of these regular nanorods may result from the
initial inclusion of monodisperse spherical particles into 3D
super-lattices.
Figure 3. Magnetization curves recorded at 2 K of cobalt nanorods
obtained in the presence of: A) HDA and octanoic acid (1:1) (2);
B) HDA and lauric acid (1:1) (1); and C) HDA and stearic acid (1:1)
(3).
Received: June 10, 2003 [Z52090]
Published Online: October 14, 2003
conductor nanoparticles and for the fabrication of nanorods.
Monodisperse nanoparticles are produced which assemble
into 3D solid super-lattices at the early stage of the reaction.
For the oleic acid/oleylamine system we found evidence for
the coalescence of some of these particles into rod-shaped
particles in a similar way to that observed for particles
included inside mesoporous materials.[30] This process could
provide seeds for the growth of the rods, although the size of
the initial particles (3 nm) is very different from the diameter
of the final rods (9 nm). The influence of the chain length of
Keywords: cobalt · magnetic properties · nanostructures · self-
assembly
.
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