Extended one-dimensional self-assemblies of nanoparticles; Nano-rouleaux formations

Article abstract of DOI:10.1039/b924121c

In this communication, we report the unusually extensive uni-directional self-assembly of nanoparticulate EuF₃ and discuss the origins behind such behaviour. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b924121c

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Extended one-dimensional self-assemblies of nanoparticles;
nano-rouleaux formationsw
Paula Rahman,a Mark Green,*a Crispin Heatherington,b Lisa Karlssonb and
Mathew Kallumadilc
Received (in Cambridge, UK) 18th November 2009, Accepted 14th December 2009
First published as an Advance Article on the web 6th January 2010
DOI: 10.1039/b924121c
In this communication, we report the unusually extensive
uni-directional self-assembly of nanoparticulate EuF3 and
discuss the origins behind such behaviour.
self-assembly are superparamagnetic cobalt nanodiscs, where
the surfactant passivated particles orientate themselves over
hundreds of nanometres.5 In the case of cobalt, the
self-assembly was attributed to a combination of surface
tension, magnetic interactions and van der Waals forces.5,6
A similar explanation was given by Bao et al.,7 who suggested
the stacking of discs minimised magneto-static energy, and
that the discs were also driven together by hydrophobic
interactions to minimise exposure to air. Li et al.8 also
demonstrated chains of rectangular BaCrO4 nanoparticles,
up to 500 nm in length. In all these cases, the nanoparticles
are distinct separate entities, with a well defined spacing
between each particle due to the surfactant layer. Notably,
and relevant to this report, large (up to 300 nm wide) discs of
EuF3 have been prepared by aqueous methods which
also show face-to-face stacking behaviour.9 The lack of a
surfactant resulted in the formation of a single crystalline
structure, composed of individual discs, driven by an orien-
tated aggregation mechanism. The ribbon materials described
all have an appearance strikingly similar to a rouleaux forma-
tion; a natural structure composed of stacked biconcave red
blood cells (erythrocytes) although clearly much smaller.
Rouleaux formations are driven by sedimentation rates of
the blood cells, repulsive forces between negatively charged
cells and aided by the presence of certain plasma proteins.
In this communication, we report the self-assembly of EuF3
nanodiscs into extremely long ribbon-like structures which
extend for several micrometres. Although colloidal crystals
have been prepared on this scale before, one-dimensional
assemblies usually extend only a few hundred nanometres.
The ribbon structures required no processing to deposit and
appeared to be due to surface tension, van der Waals and
a function of the nanoparticle size and shape. We explore
(and discount) the theory that such alignments are, in this
case, magnetically driven. Europium fluoride (EuF3) particles
were prepared by the thermolysis of europium trifluoroacetate
(EuTFA) and potassium trifluoroacetate (KTFA) in octa-
decene using oleic acid as a capping agent.z The resulting
EuF3 particles were roughly disc shaped, with some being
anisotropic and termed ‘nanoginger’ due to the resemblance to
flat root ginger (ESI,w Fig. S1). The dimensions of the particles
varied between 10 and 60 nm, with an average of 25 ꢀ 9 nm.
The particles were of a much more uniform thickness however,
with an average of 3.7 ꢀ 0.9 nm. Lattice spacings were
0.32 nm10 which matched the hexagonal form of EuF3.
The synthesis of nanoparticles has now progressed to a level
where we can routinely prepare monodispersed colloids of a
wide range of solid state materials such as semiconductors,
metals, metal oxides and alloys. These materials may become
the basis of the next generation of electronic components, such
as light emitting devices, solar cells and photodetectors, if they
can be controllably assembled into the required geometry or
onto relevant substrates. Such organisation is not trivial and
relies on numerous factors depending on which mechanism
one wishes to employ to realise an ordered self-assembled
structure of nanometre sized components. Individual
manipulations can be time consuming, and ideally one would
desire a system in which all elements are assembled in a timely,
non-templated fashion. The manipulation of monodispersed
nanoparticles into extended two-dimensional structures is now
routine and can easily be achieved with the majority of
nanoparticles prepared by organometallic-type reactions.
The seminal work in the preparation of three-dimensional
extended self-assembled nanoparticle structures was reported
by Murray et al.,1 where monodispersed cadmium selenide
quantum dots were controllably manipulated into a colloidal
crystal by slow evaporation of a solution. This work was
extended into binary systems of numerous structures,2,3 where
three-dimensional structures were obtained using similar
techniques. The range of available super-structures is
vast and enhanced physical properties have already been
discovered in so-called binary lattices.4 Self-assembly in which
the constituent nanoparticles form ribbon-like structures has
also been observed, although this is less common.5–8 In these
cases, a one-dimensional directional ordering is apparent
instead of the more common controlled aggregation in two
or three dimensions.
Of the few examples of nanoparticle ribbon-type formation,
the majority appear to involve anisotropic particles. For
example, the prototypical materials used in one-dimensional
a Department of Physics, King’s College London, The Strand, London,
UK WC2R 2LS. E-mail: mark.a.green@kcl.ac.uk
b Department of Materials, Parks Rd University of Oxford,
UK OX1 3PH
c London Centre for Nanotechnology, University College London,
17-19 Gordon Street, London, UK WC1 H 0AH
Upon simple deposition on a TEM grid (with continuous
carbon film) followed by drying in ambient conditions, the
w Electronic supplementary information (ESI) available: TEM images,
XRD and EDAX data. See DOI: 10.1039/b924121c
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1517–1519 | 1517

View Article Online
Fig. 1 TEM images of EuF3 particles in ribbon formation at different
scales. Scale bars = (a) 1 mm; (b) 0.2 mm; (c) 100 nm; (d) 50 nm.
discs self-assembled into extremely long ribbons several
micrometres in length (Fig. 1). On first inspection (on a
JEM-4000EX operating at 200 kV), the particles appeared to
be rods in a side-by-side geometry, but upon further investi-
gation were revealed to be discs orientated face-to-face on
their edges (Fig. 2). In some of the images it is interesting to see
that some of the stacks have fallen over, revealing the disc-like
shape of the particles (Fig. 2b). The ribbons, which were
reproducible, were single strands often running parallel up
to several microns in length (Fig. 2c, e and f). High resolution
electron microscopy revealed the inter-particle separation
which varied between 1.8 and 4.1 nm, with an average of
2.9 nm (Fig. 3). Oleic acid has been reported to be 1.4 nm in
length,11 although we estimate (using Chemdraw and Chem3D)
that the ligand is closer to 2.1 nm, suggesting that the ligands
on the discs intertwined. Despite the use of KTFA in the
synthesis, potassium was not incorporated into the structure
of the nanoparticle. One might expect KEuF4 to have formed,
although this clearly was not the case as determined by X-ray
diffraction (ESI,w Fig. S2) although energy dispersive X-ray
analysis (EDX) clearly showed the presence of potassium
(ESI,w Fig. S3). Synthesis without KTFA resulted in extremely
small, narrow rod-like particles of EuF3, 9.0 ꢀ 2.8 nm in
length and 1.3 ꢀ 0.2 nm wide (ESI,w Fig. S4). Doubling the
concentration of both EuTFA and KTFA resulted in slightly
larger particles. The average particle was 28 ꢀ 11.7 nm
and depth 4 ꢀ 0.7 nm. Inter-particle distances ranged from
1.7–2.7 nm, averaging 2 ꢀ 0.4 nm. Some very small round
nanoplates were also observed, the average diameter of these
was found to be 5 ꢀ 0.9 nm. Work by Sun et al.10 has shown
that EuF prepared by the use of EuTFA alone (using differing
reaction conditions) gave small disc particles that did not
self-assemble edge-to-edge.
Fig. 2 TEM images of EuF3 particles in ribbon formation showing
extended self-assembly. Scale bars = (a) 0.2 mm; (b) 100 nm; (c) 2 mm;
(d) 100 nm; (e) 10 mm; (f) 1 mm.
oleic acid and binds to a specific crystal plane of the growing
nanoparticle, allowing anisotropic growth. The other
possibility is that its presence reduces the temperature required
for growth and allows fast growth at (relatively) low
temperatures. If the KTFA decomposes at lower temperatures
than the EuTFA, Fꢂ ions will be available early on and
encourage dissociation of the Eu–O bond.
To obtain more of an insight into the process of formation
of the ‘‘ginger’’-shaped EuF3 nanoparticles, an experiment was
performed and sampled at several time points; 1, 30 and
60 min. The 1 min sample (ESI,w Fig. S5a and b) showed
ginger-shaped particles even at this early stage. Unreacted
precursor was also observed. There was little difference in size
between the 30 and 60 min samples (ESI,w Fig. S5c and d),
however particles from the 60 min sample exhibited slightly
better disc-like morphology.
The self-assembly of rare earth trifluoride nanoparticles has
been observed previously, but not on such an extended scale.
According to Sun et al.10 and Zhang et al.12 changing the
volume ratio of product to solvent and solvent polarity affects
whether assembly is face-to-face or edge-to-edge. Slow
evaporation of the solvent from the TEM grid also improved
assembly, which suggested a surface tension effect. An
extremely high surface to volume ratio meant the particles
were unstable and hence exhibited high surface tension. Slow
It is quite obvious that the presence of KTFA has an effect
on the synthesis of EuF3 nanoparticle growth and self-
assembly. It is possible that KTFA acts as a competitor to
ꢁc
This journal is The Royal Society of Chemistry 2010
1518 | Chem. Commun., 2010, 46, 1517–1519

View Article Online
The EuF3 nanoparticles were not superparamagnetic like
cobalt nanodiscs which appeared to give structurally similar
ribbons. Alivisatos et al. observed that a parallel arrangement
of Co discs could be induced by applying an external magnetic
field, and although the discs did align to some degree without
the field, the effect was somewhat shorter.5 In Co discs, it
was suggested that the materials were coupled antiferro-
magnetically. In the case of EuF3 discs described here, this
was clearly not the case, and ribbons some several micrometres
in length formed without the need of an external magnetic
field. Obviously, as the ribbons were observed in the absence
of a magnetic field, we can state that the paramagnetic
character of the EuF3 particles was irrelevant to the formation
of the structures as the particles only have a magnetic dipole
when in an external applied magnetic field. We therefore
suggest that the discs are coupled electrostatically. There were
no obvious indications of interactions between ribbons,
although work is ongoing.
In conclusion, we have demonstrated the preparation of
EuF3 nanodiscs and their self-assembly into ribbons that
extended several micrometres. The inclusion of potassium
trifluoroacetate was found to be an essential part of the disc
synthesis. The resulting self-assembly was notable by the long
range obtained in a single dimension, and one can imagine
electronic devices eventually benefiting from such archi-
tectures. PR and MG were funded by an EPSRC grant
(EP/D501563). Access to the TEM instrumentation in
Oxford Materials was funded by EPSRC grant reference
EP/F01919X/1 under the Materials Equipment Access Scheme.
Fig. 3 High resolution TEM images of nanoparticle in ribbon
formation (a) scale bar = 5 nm; (b) close up of particle edge showing
lattice fringes; (c) scale bar = 10 nm; (d) close up of particle edge
showing lattice fringes.
evaporation of the nanoparticle solution on the TEM grids
allowed the particles to diffuse into low energy structures
which reduced the surface tension. Clearly particle shape
and size was a key factor, and the inherent disc morphology
(due to the presence of KTFA during synthesis) allowed this
unique mode of self-assembly.
Notes and references
z Synthesis of EuF3 nanodiscs: in a typical experiment, 0.5 mmol of
EuTFA and 0.5 mmol of KTFA were added to 15 mL of oleic acid and
then placed under nitrogen. The mixture was heated to 100 1C over a
2 hour period until the precursors dissolved with the vacuum applied
periodically to remove dissolved gases and water; the solution was
repressurised with nitrogen. The temperature was then increased to
280 1C for 40 minutes and then allowed to cool to room temperature.
Addition of ethanol resulted in a precipitate which was collected by
centrifugation. Particles were cleaned twice more with ethanol. The
particles were then dissolved in toluene and dropped onto a carbon
coated TEM grid, and allowed to dry.
Suggestions that magnetic interactions were (at least partly)
responsible were investigated. As expected for paramagnetic
systems such as EuF3, no significant coercivity or remanence is
observed in the hysteresis curves at 10 K and 300 K, and the
Curie law for paramagnetic materials is valid (Fig. 4).
1 C. B. Murray, C. R. Kagan and M. G. Bawendi, Science, 1995,
270, 1335.
2 F. X. Redl, K.-S. Cho, C. B. Murray and S. O’Brien, Nature, 2003,
423, 968.
3 E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien and
C. B. Murray, Nature, 2006, 439, 55.
4 J. J. Urban, D. V. Talapin, E. V. Shevchenko, C. R. Kagan and
C. B. Murray, Nat. Mater., 2007, 6, 115.
5 V. F. Puntes, D. Zanchet, C. K. Erdonmez and A. P. Alivisatos.,
J. Am. Chem. Soc., 2002, 124, 12874.
6 V. F. Puntes, K. M. Krishnan and A. P. Alivisatos, Science, 2001,
291, 2115.
7 Y. Bao, M. Beerman and K. M. Krishnan, J. Magn. Magn. Mater.,
2003, 266, L245.
8 M. Li, H. Schnablegger and S. Mann, Nature, 1999, 402, 393.
9 M. Wang, Q. L. Huang, J. M. Hong, X. T. Chen and Z. L. Xue,
Cryst. Growth Des., 2006, 6, 1972.
10 X. Sun, Y. W. Zhang, Y. P. Du, Z. G. Yan, R. Si, L. P. You and
C. H. Yan, Chem.–Eur. J., 2007, 13, 2320.
11 J. I. Park, Y. W. Jun, J. S. Choi and J. Cheon, Chem. Commun.,
2007, 5001.
12 Y. W. Zhang, X. Sun, R. Si, L. P. You and C. H. Yan, J. Am.
Chem. Soc., 2005, 127, 3260.
Fig. 4 Hysteresis curve of EuF3 particles at 10 K and 300 K, showing
paramagnetic character.
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1517–1519 | 1519