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Binary Superlattices of Nanoparticles Self-Assembly Leads to УMetamaterialsФ.

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Functional Materials
Binary Superlattices of Nanoparticles: Self-Assembly
Leads to “Metamaterials”
Andrey L. Rogach*
materials science · nanoparticles · self-assembly ·
Recent efforts in colloidal chemistry
have provided powerful tools for the
gram-scale synthesis of nanoparticles of
different materials with variable and
precisely controlled size, shape, composition, and surface properties.[1] Currently available highly monodisperse semiconductor, noble metal, magnetic metal
alloy, and oxide nanocrystals can be
used as building blocks for the fabrication of 2D or 3D superstructures. The
new collective properties of these artificial solids can then be investigated and
harnessed. In superlattices formed this
way, the individual nanoparticles can be
considered as “artificial atoms”.
The use of self-assembly strategies
based on the dispersive attractions of
nanoparticles was realized to be an
attractive “bottom up” chemical approach to their superstructures.[1] An
advantage of this approach is its simplicity: both 2D and 3D assemblies can
be prepared simply by placing a drop of
a colloidal solution of monodisperse
nanoparticles on a suitable support and
allowing the solvent to evaporate slowly.
Self-assembled colloidal crystals can
also be formed from semiconductor or
magnetic metal nanoparticles by gentle
destabilization of the colloidal dispersions.[2]
Twenty years ago, studies on colloidal crystals known from nature (gem
opals) or grown synthetically from mono[*] Dr. A. L. Rogach
Photonics & Optoelectronics Group
Physics Department & CeNS
Universit4t M5nchen
Amalienstrasse 54
80 799 M5nchen (Germany)
Fax: (+ 49) 89-2180-3441
disperse microspheres with bimodal size
distribution and selected size ratios[3]
showed that the composition of the
structures follows the composition of
the atomic intermetallic alloys—AB,
AB2, AB5, or AB13—albeit on a much
larger size scale. A question naturally
arose recently as to whether the same
rules would govern the formation of
superlattices from nanometer-sized
building blocks—that is, monodisperse
nanoparticles with bimodal size distribution. The first examples of binary
nanocrystal assemblies were 2D arrays
built from noble metal nanoparticles.[4]
Monodisperse Au or Ag and Au nanoparticles self-assembled into regular
AB2 or AB “alloy superstructures”,
depending on the relative amounts of
nanocrystals and on the ratio of particle
diameters. Later, a trilayer superlattice
of magnetic CoPt3 nanocrystals of two
different sizes (4.5 and 2.6 nm) consistent with the AB5 structure was reported
(Figure 1).[5] In the first plane, each 4.5nm CoPt3 nanocrystal is surrounded by a
hexagon of 2.6-nm nanocrystals. The
second plane consists only of hexagons
of small particles, which are rotationally
shifted by 30o relative to the first plane.
The third plane is a repeat of the first.
Murray and co-workers reported the
next logical step in the development of
nanoparticle-based binary superlattices:
the use of materials with distinctly
different properties (PbSe semiconductor quantum dots and Fe2O3 magnetic
nanocrystals) to prepare 3D superstructures.[6] Precisely ordered, large single
domains (up to 2 mm2) of AB2 and AB13
superlattices were obtained by simply
allowing the solvent (dibutyl ether) to
evaporate overnight. The starting liquor
consisted of a diluted mixed dispersion
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200301704
Figure 1. a) Schematic representation of the
AB5 superlattice (isostructural with intermetallic phase CaCu5). b) TEM image of a trilayer
structure of CoPt3 nanoparticles of two different sizes (4.5 and 2.6 nm). Reprinted with
permission from reference [5].
of 11-nm Fe2O3 particles and a tenfold
surplus of 6-nm PbSe nanoparticles.
Figure 2 shows an example of the
{100}SL projection of the AB13 superlattice, with each large Fe2O3 nanoparticle spaced with eight small PbSe nanocrystal to form a square array. A trilayer
AB5 structure similar to that in Figure 1
was also found, although only in smaller
regions. The AB2 and AB13 superlattices
of the best quality were formed with an
effective particle size ratio of 0.58, which
is in a good agreement with the calcuAngew. Chem. Int. Ed. 2004, 43, 148 –149
Figure 2. a) TEM image of a 3D superstructure of g-Fe2O3 (11 nm) and PbSe nanoparticles (6 nm). b) Schematic representation of
the AB13 superlattice (isostructural with
intermetallic phase NaZn13). Reprinted with
permission from reference [6].
lated value for the assembly of micrometer-sized colloidal hard spheres.[7]
The binary superlattices formed by
this simple chemical self-assembly approach were given the term “metamaterials” by Murray and co-workers.
According to the authors,[6] metamaterials are materials with properties arising from the controlled interaction and
mutual influence of their components, in
this case, individual nanoparticles of
different origin. Keeping in mind the
large variety of high-quality monodisperse nanocrystals that is available, all
Angew. Chem. Int. Ed. 2004, 43, 148 –149
with further adjustable parameters such
as size and surface capping, the toolbox
for the construction of these complex
materials is indeed inexhaustible. Accordingly, many fundamental questions
with respect to these samples need to be
investigated. What happens after excitation of a regular binary supelattice
comprising of two different semiconductor nanoparticles, or, in the simplest
case, differently sized nanoparticles of
one and the same semiconductor material? Does charge transfer or energy
transfer takes place? How does the
magnetic field created by the magnetic
particle sublattice influence the optical
properties of the semiconductor neighbors? Which is expected: quenching or
enhancement of the luminescence of the
semiconductor nanoparticles in a binary
superlattice with metal counterparts?
The spacing between nanocrystals in a
superlattice can be fine tuned on the
sub-nanometer scale by choosing different capping ligands—how do these
measures influence the collective optical, magnetic, and electrical properties
of the metamaterial?
Self-assembly of high-quality nanoparticles of different origin into complex
ordered materials will provide samples
for these studies and open the door to a
new domain of bottom-up chemistry
with enough potential to compete with
the top-down approaches of nanolithography.
[1] a) C. B. Murray, C. R. Kagan, M. G. Bawendi, Annu. Rev. Mater. Sci. 2000, 30,
545 – 610; b) A. L. Rogach, D. V. Talapin,
E. V. Shevchenko, A. Kornowski, M.
Haase, H. Weller, Adv. Funct. Mater.
2002, 12, 653 – 664.
[2] a) C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 1995, 270, 1335 – 1337;
b) D. V. Talapin, E. V. Shevchenko, A.
Kornowski, N. Gaponik, M. Haase, A. L.
Rogach, H. Weller, Adv. Mater. 2001, 13,
1868 – 1871; c) E. V. Shevchenko, D. V.
Talapin, A. Kornowski, F. Wiekhorst, J.
KItzler, M. Haase, A. L. Rogach, H.
Weller, Adv. Mater. 2002, 14, 287 – 290.
[3] a) J. V. Sanders, Philos. Mag. A 1980, 42,
705 – 720; b) S. Youshimura, S. Hachisu,
Prog. Colloid Polym. Sci. 1983, 68, 59 –
[4] a) C. J. Kiely, J. Fink, M. Brust, D. Bethell, D. J. Schiffrin, Nature 1998, 396,
444 – 446; b) C. J. Kiely, J. Fink, J. G.
Zheng, M. Brust, D. Bethell, D. J. Schiffrin, Adv. Mater. 2000, 12, 640 – 643.
[5] E. V. Shevchenko, D. V. Talapin, A. L.
Rogach, A. Kornowski, M. Haase, H.
Weller, J. Am. Chem. Soc. 2002, 124,
11 480 – 11 485.
[6] F. X. Redl, K.-S. Cho, C. B. Murray, S.
O'Brien, Nature 2003, 423, 968 – 971.
[7] M. D. Eldridge, P. A. Madden, D. Frenkel, Nature 1993, 365, 35 – 37.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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self, assembly, уmetamaterialsф, superlattice, leads, nanoparticles, binar
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