Highly crystalline anisotropic superstructures via magnetic field induced nanoparticle assembly

Article abstract of DOI:10.1039/b712513e

A magnetic field is successfully utilized to induce the fabrication of size controllable one-dimensional (1-D) supercrystals which are composed of a highly crystalline assembly of fcc-packed cobalt nanoparticles; the anisotropy associated supercrystal magnetism is enhanced with four times higher coercivity than that of randomly aggregated nanoparticles. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712513e

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Highly crystalline anisotropic superstructures via magnetic field
induced nanoparticle assembly{
Jong-Il Park,{ Young-wook Jun, Jin-sil Choi and Jinwoo Cheon*
Received (in Cambridge, UK) 15th August 2007, Accepted 31st October 2007
First published as an Advance Article on the web 8th November 2007
DOI: 10.1039/b712513e
monodispersity (s = y8%). The nanoparticles were redispersed
in nonane at a 2 mM concentration based on Co. Supercrystal
rods were assembled through the following procedures. A portion
(100 ml) of the nanoparticle solution was dropped on a 1 6 1 cm2
Si substrate which was pre-positioned either perpendicularly or
parallel to an external magnetic field direction generated by two
parallel magnets and then the solvent was allowed to slowly
evaporate (Fig. 1(a), Fig. 3(a)). The magnitude of the external
magnetic field exerted on the nanoparticles was controlled by
changing the inter-magnet distance.
A magnetic field is successfully utilized to induce the fabrication
of size controllable one-dimensional (1-D) supercrystals which
are composed of a highly crystalline assembly of fcc-packed
cobalt nanoparticles; the anisotropy associated supercrystal
magnetism is enhanced with four times higher coercivity than
that of randomly aggregated nanoparticles.
Recent synthetic advances of nanomaterials have enabled the
development of various nanoparticle building blocks with distinct
morphologies and properties.1 One of the important issues
required for their applications in electronics and biomedical
sciences is how to systematically assemble these building blocks
into highly ordered superstructures. Various assembly techniques
have been developed, which include solvent evaporation induced
assembly,2,3 Langmuir–Blodgett nanoparticle assembly,4 and
molecular linker induced assembly.5 Most of these typically
produce isotropic 0-dimensional (0-D) superstructures. On the
other hand, the use of controllable external electric or magnetic
fields is also an attractive option for the fabrication of nanoparticle
superstructures due to its simplicity and effectiveness.3,6–9 More
importantly, this is advantageous for the fabrication of anisotropic
1-D superstructures, which can potentially provide unique proper-
ties arising from their anisotropic morphology. We here demon-
strate the magnetically controlled fabrication of 1-D anisotropic
supercrystals comprised of face centered cubic (fcc)-packed cobalt
nanoparticles. By investigating magnetic field and nanoparticle
concentration effects, it is possible to produce various anisotropic
supercrystals with different sizes, shapes and aspect ratios. A
strong shape-dependent anisotropy effect on the magnetism is also
observed in supercrystal rods. Although the groups of Pileni,7
Prasad,8 and Murray3 have already demonstrated the formation of
1-D structures, the obtained materials are either amorphously
aggregated or limited to a simple wire shape. Our study uniquely
demonstrates that the formation of highly crystalline 1-D super-
structures with structural evolution from dots to rods and to wires
is possible in a controlled manner.
Fig. 1(b) shows a low magnification transmission electron
microscopic (TEM) image of nanoparticle superstructures
assembled under an external magnetic field of 2400 Oe.
Anisotropic rod-shaped supercrystals are observed with a size of
y130 nm in width and y450 nm in length (aspect ratio: y3.5). A
higher magnification TEM image shows the detailed crystalline
structure of the supercrystal rods (Fig. 1(c)). The supercrystal rods
are well-ordered fcc-structured assemblies of cobalt nanoparticles.
Fig. 1(d) shows the schematic unit cell structure of fcc packed
supercrystals of cobalt nanoparticles. The capping ligand coated
cobalt nanoparticle is approximately considered as a 12.8 nm sized
hard sphere (10 nm (cobalt) + 2.8 nm (two monolayers of oleic
acid)). A supercrystal rod corresponds with the [001]SL projection
of the model-fcc structure (Fig. 1(c), (e)). An alternative view of the
¯
supercrystal rod is also observed, which corresponds to the [111]SL
Cobalt nanoparticles were synthesized using a literature method
by injecting Co2(CO)8 into a refluxing toluene solution containing
oleic acid and sodium bis(2-ethylhexyl) sulfosuccinate.10 The
obtained nanoparticles were y10 nm spheres with high
Department of Chemistry and Nanomedical National Core Research
Center (NCRC), Yonsei University, Seoul, 120-749, Korea.
E-mail: jcheon@yonsei.ac.kr
{ Electronic supplementary information (ESI) available: Small-angle
XRD, magnetism and HRTEM analyses of supercrystals. See DOI:
10.1039/b712513e
{ Current address: Research Institute of Industrial Science & Technology,
Pohang, 790-330, Kyungbuk, Korea.
Fig. 1 (a) Schematic of Co supercrystal formation; (b) low and (c) high
magnification TEM images; (d) unit cell structure; (e) [001]SL and (f)
[111]SL projections of fcc-structured supercrystals.
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Fig. 2 Magnetic field and concentration effects on supercrystal forma-
tion. (a) Supercrystal dots obtained in the absence of external magnetic
field. (b) and (c): Supercrystal rods obtained by evaporation of 10 mM (b)
and 100 mM (c) Co nanoparticle solutions.
(SL: superlattice) projection of the fcc structure (Fig. 1(c), (f)). Our
supercrystals have a periodicity of interplanar distances of 8.9 nm
for (100)SL, 7.3 nm for (112)SL, and 12.6 nm for (110)SL (Fig. 1(c)).
The measured lattice parameter from this TEM investigation is
17.8 nm which is further supported by small-angle X-ray
diffraction (XRD) studies (see ESI{). This almost matches with
the calculated lattice parameter of 18.1 nm of the fcc-model
supercrystals. Our supercrystal rods are mainly elongated along a
specific direction of [110]SL (vide infra).
Fig. 3 Co supercrystal wires. (a) Schematic of experiments; (b) low and
(c) high magnification TEM images.
The size and aspect ratio control of the supercrystals is possible
by changing either the magnetic field or the initial concentration of
nanoparticle solution. In the absence of the magnetic field, y1 mm
sized supercrystal dots are obtained (Fig. 2(a)). In contrast, when
an external magnetic field is applied to the 2 mM cobalt
nanoparticle solution, anisotropic supercrystal rods with the size
of y130 nm in width and y450 nm in length result (Fig. 1(b)). At
higher concentration of nanoparticle solution (10 mM), the size of
supercrystal rods increases to y350 nm in width and y1300 nm in
length with a similar aspect ratio, y3.7 (Fig. 2(b)). At a
concentration of 100 mM, monodispersed supercrystal rods with
900 nm width and 4000 nm length (aspect ratio: y4.4) are
obtained (Fig. 2(c)).
Fig. 4 (a) Enhanced magnetism of Co supercrystal rods (red solid line)
compared to that of randomly aggregated nanoparticles (black dashed
line). (b) Schematic of magnetic dipole interaction between nanoparticles.
nanoparticles separated by r, with an angle h between r and m, u(r,
h) = 2(m2/4pm0r3)(3cos2h 2 1).11 Two nanoparticles separated
from each other with a distance of r1 and h = 0 interact attractively
(u1 = 22m2/4pm0r1 ). In contrast, two nanoparticles at a distance
3
of r2 and h = p/2 repel each other due to a repulsive interaction,
u2 = m2/4pm0r2 . At the initial stage of nanoparticle assembly, such
We further examined the effect of the external magnetic field
direction. When the external magnetic field is applied to the
substrate horizontally, elongated wire shapes of supercrystals with
300 nm diameter are observed, rather than rod-shapes (Fig. 3).
Higher magnification TEM images show that the obtained
supercrystal wires also have an fcc structure along the [110]SL
direction.
2
dipolar interaction is very weak. However, as the solvent
evaporates, the dipolar interaction energy becomes significant.
Due to attractive dipolar interaction between nanoparticles aligned
through the external field direction and repulsive interactions
along the direction perpendicular to the field axis, anisotropic
growth of nanoparticle supercrystals along the external magnetic
field direction is facilitated. In our case, relatively thick 1-D shaped
fcc-packed supercrystals are preferred rather than single chain
structures, since the dipolar energy per particle in the fcc lattice is
lower than that in a single chain structure.11 In addition, the
preferential growth of supercrystal rods and wires along the [110]SL
direction results from the maximum dipole–dipole attraction along
the [110]SL direction having a minimized r1.
Our anisotropic supercrystal rods exhibit unique magnetic
behavior. The magnetism of the 0.9 6 4.0 mm2 supercrystal rods
was measured with a superconducting quantum interference device
(SQUID) at 5 K. The supercrystal rods possess a significantly
enhanced (four times) magnetic coercivity of 6400 Oe, compared
to that of randomly aggregated cobalt nanoparticles (1600 Oe)
(Fig. 4(a)). Such enhanced magnetism results from increased
supercrystal shape anisotropy and dipole–dipole interaction
between uni-directionally aligned nanoparticles (see ESI{).7
Fig. 4(b) shows schematic drawings of magnetic dipolar
interactions between magnetic nanoparticles. Under an external
magnetic field, a nanoparticle acquires a magnetic dipole, m =
m0xVH, where m0 is the magnetic permeability of vacuum, x is the
magnetic susceptibility, and V is the volume of the nanoparticle.10
This induces a magnetic dipole–dipole interaction (u) between two
In summary, we have demonstrated magnetic field induced
assembly of nanoparticles into crystalline anisotropic rods and
wires. Controlling the magnetic field strength, nanoparticle
concentration, and field direction yields various sizes and shapes
of anisotropic supercrystals. Their enhanced magnetism due to the
anisotropy has been also observed. Such anisotropic magnetic
supercrystals can potentially be applicable as high-performance
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This work is supported in part by the National Research
Laboratory (R09-2006-000-10255-1) and BK21.
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