Structural characterization of self-assembled multifunctional binary nanoparticle superlattices

Article abstract of DOI:10.1021/ja0564261

Nanocrystals of different size and functionality (e.g., noble metals, semiconductors, oxides, magnetic alloys) can be induced to self-assemble into ordered binary superlattices (also known as opals or colloidal crystals), retaining the size tunable properties of their constituents. We have built a variety of binary superlattices from monodisperse PbS, PbSe, CoPt₃, Fe₂O₃, Au, Ag, and Pd nanocrystals, mixing and matching these nanoscale building blocks to yield multifunctional nanocomposites (metamaterials). Superlattices with AB, AB₂, AB₃, AB ₄, AB₅, AB₆, and AB₁₃ stoichiometry with cubic, hexagonal, tetragonal, and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced in several polymorphous forms by tailoring the particle size and deposition conditions. We have identified arrays isostructural with NaCl, CuAu, AlB₂, MgZn ₂, MgNi₂, Cu₃Au, Fe₄C, CaCu ₅, CaB₆, NaZn₁₃, and cub-AB₁₃ compounds emphasizing the parallels between nanoparticle assembly and atomic scale crystal growth and providing confidence that many more structures will follow. Recently, we have demonstrated that electrical charges on sterically stabilized nanoparticles in addition to such parameters as particle size ratio and their concentrations can provide the formation of a much broader pallet of binary nanoparticle superlattices as compared with the limited number of possible superlattices formed by hard noninteracting spheres. In this contribution, we demonstrate a large variety of different binary superlattices, provide their detailed structural characterization, and discuss the role of energetic and kinetic factors in the cocrystallization process. We found that Coulomb, van der Waals, charge-dipole, dipole-dipole, and other interactions can contribute equally to cocrystallization, allowing superlattice formation to be dependent on a number of tunable parameters. We present binary superlattices as a new class of materials with a potentially unlimited library of constituents over a wide range of tunable structures.

Full text of DOI:10.1021/ja0564261

Published on Web 02/24/2006
Structural Characterization of Self-Assembled Multifunctional
Binary Nanoparticle Superlattices
Elena V. Shevchenko,*,†,‡,§ Dmitri V. Talapin,†,§ Christopher B. Murray,*,† and
Stephen O’Brien‡
Contribution from the IBM Research DiVision, T. J. Watson Research Center, Nanoscale
Materials and DeVices, 1101 Kitchawan Road, Yorktown Heights, New York 10598, and
Department of Applied Physics & Applied Mathematics, Columbia UniVersity,
200 SW Mudd Building, 500 West 120th Street, New York, New York 10027
Received September 18, 2005; E-mail: evshevchenko@lbl.gov; cbmurray@us.ibm.com
Abstract: Nanocrystals of different size and functionality (e.g., noble metals, semiconductors, oxides,
magnetic alloys) can be induced to self-assemble into ordered binary superlattices (also known as opals
or colloidal crystals), retaining the size tunable properties of their constituents. We have built a variety of
binary superlattices from monodisperse PbS, PbSe, CoPt3, Fe2O3, Au, Ag, and Pd nanocrystals, mixing
and matching these nanoscale building blocks to yield multifunctional nanocomposites (metamaterials).
Superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometry with cubic, hexagonal, tetragonal,
and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced
in several polymorphous forms by tailoring the particle size and deposition conditions. We have identified
arrays isostructural with NaCl, CuAu, AlB2, MgZn2, MgNi2, Cu3Au, Fe4C, CaCu5, CaB6, NaZn13, and cub-
AB13 compounds emphasizing the parallels between nanoparticle assembly and atomic scale crystal growth
and providing confidence that many more structures will follow. Recently, we have demonstrated that
electrical charges on sterically stabilized nanoparticles in addition to such parameters as particle size ratio
and their concentrations can provide the formation of a much broader pallet of binary nanoparticle
superlattices1 as compared with the limited number of possible superlattices formed by hard noninteracting
spheres. In this contribution, we demonstrate a large variety of different binary superlattices, provide their
detailed structural characterization, and discuss the role of energetic and kinetic factors in the cocrystallization
process. We found that Coulomb, van der Waals, charge-dipole, dipole-dipole, and other interactions
can contribute equally to cocrystallization, allowing superlattice formation to be dependent on a number of
tunable parameters. We present binary superlattices as a new class of materials with a potentially unlimited
library of constituents over a wide range of tunable structures.
1. Introduction
to-volume ratio of the individual NPs imparts distinct size
tunable physical and electronic properties that have prompted
some to refer to them as “artificial atoms”. As the number of
nanoparticle systems under strict synthetic control has expanded,
the parallels to the development of a “new periodic table” have
been discussed. The ability to mix and match different NPs and
assemble them systematically into ordered binary superlattices,
with precisely controlled stoichiometry and symmetry, extends
the analogy to multifunctional nanocomposites constituting
“nanoparticle compounds”. Binary nanocrystal superlattices
(BNSLs) raise the possibility of combining the properties of
individual components with new properties that arise from the
interactions between the nanocrystals. A number of such
collective properties arising from the interactions between NPs
have already been demonstrated in glassy solids in which the
particles have only local order. For example, it has been shown
that exchange coupling between nanoparticles of magnetically
Nanoparticles (NPs) and nanocrystalline films are promising
candidates for applications ranging from advanced magnetic
recording media,2 light-emitting devices,3-5 biological tags,6-8
catalysts,9-11 solar cells,12,13 etc. The small size and high surface-
† IBM Research Division.
‡ Columbia University.
§ Current address: The Molecular Foundry, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720.
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J. AM. CHEM. SOC. 2006, 128, 3620-3637
10.1021/ja0564261 CCC: $33.50 © 2006 American Chemical Society

Structural Characterization of BNSLs
A R T I C L E S
soft and hard materials can enhance the energy product of the
nanocomposite material.14,15 In addition, a significant enhance-
ment of photoluminescence intensity of large semiconductor
particles accompanied by quenching of photoluminescence of
the small particles16 was observed in randomly mixed solids of
small and large CdSe NPs. This observation is consistent with
long-range resonance transfer of electronic excitations from the
more electronically confined small particles to higher excited
states of the large particles.16 Also fluorescence, optical absorp-
tion, and photocurrent of semiconductor materials were found
to be dramatically affected by electromagnetic interactions in
the vicinity of the metal surface via the excitation of surface
plasmon resonances.17-19
In contrast to random mixtures of NPs, the ordered arrays
built of two or more types of NPs can provide precise uniformity
of packing, stoichiometry, and rigorous control of the interpar-
ticle distance. Ordering in a collection of NPs by itself can bring
its own exciting properties. It was recently demonstrated that
the degree of ordering in 3D superlattices had a pronounced
effect on the Raman scattering properties of metal particles. In
well-ordered superlattices of Ag NPs, atomic lattices of NPs
were found to vibrate coherently.20
between the particles.23,25,27 Indeed, NaZn13- and AlB2-type
assemblies of silica particles were found in natural Brazilian
opals28,29 and could be grown from latex spheres.30-32 At the
same time, the lattices of hard spheres with CsCl, zinc blende,
wurzite structures, as well as any lattices with AB3, AB4, or
AB5 stoichiometry are predicted to be unstable for any particle
size ratio.25 To date, there are only several reports demonstrating
the possibility of growing BNSLs, that is, the ordered arrays
assembled of two different types of NPs.33-37
In this contribution, we demonstrate and characterize 16
binary assemblies of NPs with different stoichiometry and
packing symmetry. These structures can be reproducibly formed
from NPs with different size and functionality (e.g., noble
metals, semiconductors, oxides, magnetic alloys). Many struc-
tures have the packing fraction density below 0.7405, and their
formation is difficult to explain in terms of only entropy-driven
crystallization. To explain the observed diversity of BNSLs, we
propose a model that demonstrates the importance of Coulomb,
van der Waals, charge-dipole, and dipole-dipole interactions
for self-assembly of complex nanoparticle superstructures.
2. Experimental Section
2.1. Chemicals. Toluene, ethanol, 2-propanol, squalane, methanol,
n-hexane, tetrachloroethylene (TCE), oleic acid, AuCl3, AgCl, AgNO3,
PdCl2, lead acetate trihydrate, selenium, and bis(trimethylsilyl)sulfide
were purchased from Aldrich. Additional chemicals and sources
included diphenyl ether (99%, Alfa Aesar), 1,2-hexadecanediol (90%,
Fluka), 1-adamantanecarboxylic acid (99%, Fluka), 1,2-dichlorobenzene
(99%, Acros Organics), cobalt carbonyl (Co2(CO)8, stabilized with
1-5% of hexane, Strem), platinum(II)-acetylacetonate (Pt(acac)2, 98%,
Strem), hexadecylamine (92%, Merck), and trioctylphosphine (97%,
Strem). All chemicals were used as received.
2.2. Nanoparticle Synthesis: All NPs in this work were synthesized
in nonpolar solvents and stabilized with amphiphilic molecules contain-
ing long (C12-C17) alkyl chains. Detailed information on the preparation
of metal, metal oxide, and semiconductor NPs is given below.
2.2.1. Au, Ag, and Pd Nanoparticles. We used a modification of
the method developed by Prasad et al. for Au NPs.38 All reactions were
carried out under ambient conditions. In a typical synthesis, metal salt
was dissolved in 10 mL of toluene with ultrasonication in the presence
of dodecyldimethylammonium bromide (DDAB). For synthesis of 5.0
nm Au, 4.2 and 3.4 nm Ag NPs, we used 0.034 g of AuCl3, 0.025 g of
AgNO3, and 0.023 g of AgCl, respectively. The amount of DDAB was
0.0925 g in all three cases; 3.0 nm Pd nanocrystals were synthesized
from 0.0237 g of PdCl2 with 0.157 g of DDAB. AuCl3 easily dissolved
in the toluene/DDAB mixture forming a clear dark orange solution.
AgNO3 and AgCl did not dissolve completely, forming a relatively
stable suspension after sonication for ∼30 min. Similarly, partial
dissolution of metal salt was observed in case of PdCl2.39
Monodisperse spherical particles can self-assemble into
superlattices with either cubic close-packed (ccp) or hexagonal
close-packed (hcp) packing symmetry.21,22 Binary mixtures
naturally provide a much richer class of compositions and
structures. When two types of particles cocrystallize, their
individual assembly tendencies must adjust themselves to the
properties of space. They must meet, as well as possible, the
requirement of certain geometrical principles. Many theoretical
studies have been focused on predicting the probability of
formation of various ordered binary structures and comparison
of their stability.23-26 In the simplest approach, the formation
of a binary assembly is expected only if its packing density
exceeds the packing density of single-component crystals in ccp
or in hcp structure (∼0.7405).27 This space filling principle
formulated by Murray and Sanders27 is still widely used to
predict the behavior of particles in binary mixtures. The particle
size ratio (γ ) Rsmall/Rlarge) and concentrations are considered
as the factors determining structure of binary assemblies. Taking
into account geometrical considerations, we can predict forma-
tion of superlattices isostructural with NaCl, NaZn13, and AlB2.
Detailed computer simulations show that the formation of NaCl-,
AlB2-, and NaZn13-type structures of hard spheres can be driven
by entropy alone without any specific energetic interactions
(14) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z.; Sun S. Nature 2002 420, 395-398.
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8643.
Fourty microliters of 9.4 M aqueous solution of NaBH4 was added
dropwise to the solution of metal salt with vigorous stirring. One to
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Phys. ReV. Lett. 2002, 89, 117401-1-117401-4.
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J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3621

A R T I C L E S
Shevchenko et al.
two minutes after addition of NaBH4, the color of the reaction mixture
turned purple for Au, orange-brownish for Ag, and dark brown for
Pd. After 20 min of vigorous stirring, 0.8 mL of 1-dodecanethiol was
added, and the stirring of reaction mixture was then continued for 5
more minutes. The NPs formed were precipitated by adding ethanol
and isolated by centrifugation. The particles were dried under vacuum,
redispersed in 10 mL of toluene in the presence of 0.8 mL of
1-dodecanethiol, and refluxed for 30 min under nitrogen atmosphere
to narrow particle size distribution via digestive ripening.38,40 Then NPs
were reprecipitated with 2-propanol and finally redispersed in toluene.
2.2.2. Fe2O3 Nanocrystals were synthesized via a modified method
of Hyeon et al.41 Briefly, 11 and 13.6 nm Fe2O3 nanocrystals were
synthesized by injecting 0.2 mL of iron pentacarbonyl into 10 mL of
trioctylamine in the presence of 0.65 g of oleic acid at 270 and 250
°C, respectively. After 20 min of heating at the injection temperature,
the reaction temperature was increased to 320 °C. After heating of the
reaction mixtures at 320 °C for 1 h, the reaction mixture was cooled to
room temperature and 0.17 g of trimethylamine N-oxide was added to
oxidize iron nanocrystals formed via thermal decomposition of iron
pentacarbonyl into γ-Fe2O3 nanocrystals.
Figure 1. A scheme of the low-pressure chamber used for growing ordered
binary assemblies.
We studied the effect of various substrates on the growth of BNSLs.
The BNSLs can be reproducibly grown on hydrophobic substrates, such
as amorphous carbon and silicon nitride substrates, in contrast to OH-
terminated silicon oxide and other hydrophilic substrates, for which
mostly amorphous or phase-segregated solids were observed.
3. Results and Discussion
2.2.3. CoPt3 Nanocrystals: 6.2 nm CoPt3 nanocrystals were prepared
by the injection of a cobalt stock solution (0.43 g of Co2(CO)8 in 0.5
mL of o-chlorobenzene) into the solution containing 0.0328 g of Pt-
(acac)2, 4 g of hexadecylamine, and 0.25 g of 1-adamantanecarboxylic
acid and 2 mL of diphenyl ether preheated at 170 °C. The reaction
temperature was kept at 170 °C for 40 min and then increased to 230
°C and kept at this temperature for 30 min (see details in ref 42).
2.2.4. PbSe Nanocrystals: PbSe nanocrystals were synthesized by
injecting 18 mL of a 1 M solution of tri-n-octylphosphine selenide
(TOPSe) in tri-n-octylphosphine into the solution of lead oleate heated
to 175 °C. The lead oleate solution was prepared by dissolving 2.16 g
of Pb(CH3COO)2‚3H2O in 40 mL of squalane in the presence of 7.3
mL of oleic acid followed by drying and degassing under vacuum at
80 °C for 1 h. Immediately after injection of TOPSe, the reaction
mixture was cooled to ∼150 °C, and this temperature was maintained
for further NP growth. Aliquots of the crude solution were taken at
different time intervals (1-10 min) to obtain samples of PbSe
nanocrystals of different sizes. The aliquots of crude solution were
mixed with an equal volume of hexane, and the PbSe nanocrystals were
precipitated by adding anhydrous ethanol. Details of the synthesis can
be found in ref 43.
3.1. Nanoparticle Characterization and Classification of
the Symmetry of the Binary Superstructures. Nanoparticles
used in this work were sterically stabilized with hydrophobic
molecules containing long alkyl chains. We measured average
nanoparticle size and thickness of the organic stabilizer shell
by routine TEM investigations of single-component monolayers.
TEM analysis of Ag and Pd NPs stabilized by dodecanethiol
showed an interparticle spacing of ∼1.7 nm. This value is
significantly smaller than the 2-fold thickness of a dodecanethiol
monolayer and is in good agreement with the effective thickness
of the dodecanethiol shell reported for Ag NPs by Korgel et
al.46 In measuring the interparticle spacing for a large series of
samples, we observe some deviation (∼0.2 nm) in the average
interparticle spacing for different samples with the same capping
group. The effective thickness of the ligand shell could vary
slightly from sample to sample, depending on surface coverage
and radius of curvature and possible small variations in
extraneous organic material left after the purification process.
The spacing between the neighbors in a close-packed nanopar-
ticle array is set by the balance of repulsive steric and attractive
van der Waals forces.46 Table 1 summarizes dimensions of NPs
we have used as building blocks to create our BNSLs. In the
remainder of this work, we use the term “effective particle size”
to describe the nanoparticle size relevant for packing, which is
the sum of crystalline core diameter and twice the effective
thickness of the ligand shell. The ratio of effective particle sizes
between “small” and “large” particles will be further referred
to as “effective size ratio”.
2.2.5. PbS Nanocrystals were synthesized from lead oleate and bis-
(trimethylsilyl)sulfide with oleic acid as the capping ligand. Details of
the synthesis can be found in ref 44.
2.3. Preparation of Binary Assemblies: Substrates (e.g., TEM grids
or silcon nitride membranes) were placed in a glass vial containing
colloidal solution of NPs. The vial was tilted by 60-70° and placed
inside a low-pressure chamber as shown in Figure 1. Ordered binary
assemblies formed upon evaporation of the solvent. Toluene and
mixtures of toluene with tetrachloroethylene or chloroform were used
as solvents (∼1:1 by volume). The best BNSLs (as determined by length
scale of ordering and low occurrence of defects) were obtained by
evaporating relatively concentrated colloidal solutions at 45 °C under
reduced pressure (∼3.2 kPa). The increased temperature was essential
for better solubility of Au, Ag, and Pd NPs as these tend to show some
flocculation at room temperature.38,45
To aid structural analysis, the BNSLs were generally grown
on electron transparent substrates (amorphous carbon film,
silicon nitride membranes). Samples displayed lateral ordering
over distance up to 10 µm. Regions of the nanoparticulate
sample 1-5 layers thick provided the best imaging, while real
space investigation of regions thicker than 5 layers was
(39) We believe that the presence of nondissolved metal salt during nucleation
and growth of nanoparticles allows the concentration of metal precursors
to be kept at nearly constant level. This should be favorable for narrow
final particle size distribution. For example, the use of highly soluble
palladium acetate instead of PdCl2 resulted in more polydisperse Pd
nanoparticles with similar mean size.
(43) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem.
Soc. 2005, 127, 7140-7147.
(44) Hines, M. A.; Scholes, G. D. AdV. Mater. 2003, 15, 1844-1849.
(45) The stability of the colloidal solutions was monitored by measuring angle-
dependent scattering of monochromic light (363 nm) at different temper-
atures. It was found that Au, Ag, and Pd NPs had a tendency to form
aggregates at room temperature, while at temperatures above 40 °C, no
aggregation was observed.
(40) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem.
Mater. 2003, 15, 935-942.
(41) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc.
2001, 123, 12798-12801.
(42) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin,
O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125,
9090-9101.
(46) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B
1998, 102, 8379-8388.
9
3622 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006

Structural Characterization of BNSLs
A R T I C L E S
Table 2. Maximum Packing Density and Range of Stability
Table 1. Nanoparticles Used as Building Blocks for BNSLs
Calculated for Binary Lattices of Hard Spheres
size of crystalline
core (nm)
effective thickness
of ligand shella (nm)
maximum
NPs
std. %
capping ligand
type of
packing
density (
range of
stability
Pd
3.0
3.4
3.6
4.2
5
5.8
6.2
7.2
7.6
6.2
6.7
13.4
7
8
7
6
7
5
5
6
5
7
5
6
0.85
stoichiometry
structure
γ)
ref
Ag
Au
dodecanethiol
AB
NaCl
0.793 (0.414)
below 0.458
0.2 e γ e 0.42
∼0.4
27
25
59
60
0.75
0.9
0.85
NiAs
CsCl
AlB2
0.793 (∼0.4)
0.729, unstable 0.732
AB2
0.778(0.58)
0.482 e γ e 0.624 27
0.42 e γ e 0.59 25
0.606 e γ e 0.952 27
PbSe
oleic acid
Laves phases; 0.71 (0.813)
CoPt3
PbS
γ-Fe2O3
ADA, HDAb
oleic acid
1
0.85
0.9
hexagonal:
MgZn2, MgNi2
cubic MgCu2
CaF2
unstable
0.757
unstable
0.738 (0.58)
0.225
25
25
a Defined as half the interparticle spacing in the (111) projection of close-
packed 2D nanoparticle superlattices. b ACA is 1-adamantanecarboxylic
acid, HDA is hexadecylamine.
AB13
NaZn13
(ico-AB13)
0.54 e γ e 0.61
0.474 e γ e 0.626 23,24
0.537 e γ e 0.583 27
prevented by limited electron penetration. A Philips CM12
microscope operating at 120 kV was used for all imaging.
Surveys of electron diffraction and high-resolution SEM confirm
that ordering can persist in thick nanoparticulate films.
Three-dimensional descriptions of the superlattices were
developed by surveying large regions of the samples, to
categorize all the crystal orientations, and recording a series of
2D projections down the major symmetry axes. Tilting of the
samples allowed observation of additional orientations not
expressed in the plane view images of the films. In many cases,
the analysis of a series of these 2D lattice projections resulted
in a single-crystal structure. To assign the observed structures
to crystallographic space groups, we built 3D lattice models
for the 180 most common space groups using Accelrys MS
Modeling 3.1 software. The TEM images were compared with
simulated projections to match the symmetry of our superlattices.
We also performed a comparison of experimental small-angle
electron diffraction patterns taken over larger areas, and the 2D
Fourier transformation power spectra of real space TEM images
and the FFT power spectra of the simulated projections to ensure
consistency.
3.2. Binary Superlattices with AB Stoichiometry. 3.2.1.
AB Superlattices Isostructural with NaCl. The NaCl lattice
(SG 225, Fm3hm) has the highest packing density (0.793 at γ )
0.414) among the binary lattices with AB stoichiometry. The
packing density of the NaCl-type lattice exceeds the packing
density of fcc close-packed spheres for all size ratios γ below
0.458.27 Kiely et al. earlier reported observation of small 2D
arrays (about 40 nm across a domain) of a NaCl-type superlattice
for Au and Ag NPs.47 Recently, Korgel et al. reported the NaCl-
type superlattice of Fe and Au nanoparticles over more extended
areas (∼0.3 µm2).36 Here we report the NaCl-type superlattices
for various combinations of NPs, such as 13.4 nm Fe2O3 - 5.0
nm Au, 7.2 nm PbSe - 3.0 nm Pd, and 5.8 nm PbSe - 3.0 nm
Pd, with each ordered domain extending over several microns
(Figure 2). In the Fe2O3 - Au and 7.2 nm PbSe - 3.0 nm Pd
systems, the NaCl superlattices are usually represented by (111)
projections (Figure 2a). Typically, domains consisted of two
layers of large Fe2O3 particles and two layers of small Au
particles (Figure 2a, cf. Figure 2a and 2g and Figure 2b and
2g). Smaller domains of (100) projections of NaCl-type super-
lattices were also observed (Figure 2a,b, upper insets). In the
ico-AB13,
above 0.755
with some size
distribution for
B spheres
cub-AB13
0.700 (0.565)
unstable
27
case of PbSe-Pd systems, the (100) oriented NaCl superlattices
dominated (Figure 2b). The NaCl-type ordering was observed
for NPs with effective size ratios of 0.43, 0.528, and 0.62 for
13.4 nm Fe2O3 - 5.0 nm Au, 7.2 nm PbSe - 3.0 nm Pd, and
5.8 nm PbSe - 3.0 nm Pd, respectively. All these values are
above the range of stability predicted for NaCl superlattices of
hard noninteracting spheres (Table 2).
3.2.2. AB Superlattices Isostructural with CuAu Phase.
The BNSLs crystallize from solutions of 7.6 nm PbSe and 5
nm Au NPs, 5.8 nm PbSe and 3.4 nm Ag NPs, as well as 6.2
nm PbSe and 5 nm Au NPs mixed with the concentration ratio
of 1:3-1:5 (Figure 3). The lateral dimensions of superlattice
domains formed on TEM grids can approach ∼3 µm × 3 µm
(i.e., ∼10 µm2), while the superlattice thickness usually does
not exceed 4-6 NPs. Figure 3a,b,d and 3c,k shows two different
projections of the superstructure we assigned as the (001) and
(100) projections of tetragonal CuAu lattice (SG 123, P4/mmm),
correspondingly. The TEM projection of the (001) plane of the
AuCu lattice is similar to the projection of the (100) plane of
the CaF2 lattice (Supporting Information, Figure S1). To reveal
whether the superlattices shown in Figure 3 correspond to AuCu
or CaF2 symmetry, we performed TEM tilting experiments. The
cubic CaF2 lattice tilted by 45° should orient along the (110)
zone axis, while tetragonal CuAu lattice shows no zone axes at
any tilt smaller than 90°. The tilting experiments revealed no
characteristic periodic patterns at 45° tilt, which allowed us to
assign the structure shown in Figure 3a,b,d to CuAu phase. The
TEM images shown in Figure 3c,k can be unambiguously
assigned to the (100) projection of the AuCu lattice with
characteristic alternating lines of large and small particles (note
that (100) and (010) planes are equivalent). These lines
correspond to the planes consisting of large and small NPs that
alternate along the 001 direction. The superlattice patterns
shown in Figure 3a,c usually coexist on a single TEM grid,
supporting our conclusion that these different projections belong
to the same structure. Depending on particular particle combina-
tions, the occurrence of (100) and (001) projections can vary.
The (001) projections dominate for PbSe-Au NP superlattices,
(47) Kiely, C. J.; Fink, J.; Zheng, J. G.; Brust, M.; Bethell, D.; Schiffrin, D. J.
AdV. Mater. 2000, 12, 640-643.
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Figure 2. TEM micrographs of NP superlattices isostructural with NaCl: (a, b) The (111) projection formed by 13.4 nm Fe2O3 and 5.0 nm Au NPs and
7.2 nm PbSe and 3.0 nm Pd NPs; upper insets shows the (100) projections of the superlattice formed by the same NPs. (c) The (100) projection formed by
5.8 nm PbSe and 3.0 nm Pd NPs. Bottom insets in (a), (b), and (c) show small-angle ED patterns measured from corresponding superlattices. (d) Three-
dimensional sketch of NaCl unit cell. (e, g) Depictions of the (100) and (111) planes, respectively. (f, h) Depictions of minimum number of layers in the
(100) and (111) projections necessary for the formation of the patterns identical with the observed ones. The (100) plane in the case of 7.2 nm PbSe and 3.0
nm Pd NPs was represented by a monolayer, consisting of large and small NPs.
and (100) projections represent the majority for PbSe-Ag
superlattice samples.
boundaries were observed for CuAu BNSLs with the smallest
particle size ratio, 6.7 nm PbS - 3.0 nm Pd, γ ∼ 0.56 (Figure
3l,n). Probably, the antiphase domains form to release the lattice
strain energy associated with the deviation of particle size ratio
from the most energetically favorable values.
The CuAu-type structure was observed for numerous inter-
metallic compounds.47 In the CuAu lattice, each particle is
surrounded by four identical particles and eight of the second
component. The effective size ratio of NPs that assemble into
CuAu-type superlattices is usually in γ-range of ∼0.55-0.70.
These values are considerably smaller than size ratios reported
for intermetallic compounds with CuAu lattice.48 The CuAu-
type BNSLs usually coexist with Cu3Au-type superlattices (see
below).
Within a certain temperature range (380 °C < T < 410 °C),
CuAu intermetallic compounds show characteristic periodic
arrangement of antiphase boundaries along the 100 axis.48
These antiphase boundaries are separated by several unit cells,
as shown in Figure 3m. We observed similar phenomenon for
colloidal binary superlattices. The most pronounced antiphase
3.2.3. AB Superlattices with Orthorhombic Symmetry.
Another novel superstructure was formed by slowly evaporating
the mixture of 5.8 nm PbSe and 3 nm Pd NPs and 7.2 nm PbSe
and 4.2 nm Ag NPs (particle concentration ratio ∼1:3). The
typical area of single-crystalline domains was ∼0.5-1 µm2.
Figure 4a,c shows TEM images of two characteristic superlattice
projections. Using Accelrys software (MS Modeling 3.1), we
modeled this structure and assigned it to the orthorhombic
symmetry class (SG 51, Pmma). The projections shown in
Figure 4a,b,i and 4c,j are in good agreement with (100) and
(010) projections of the proposed orthorhombic structure. The
proposed structure belongs to the same space group as the AuCd
intermetallic compound; however, they do not appear to be
isostructural.
(48) Barrett, C.; Massalski, T. B. Structure of Metals, 3rd ed. International Series
on Materials Science and Technology; Pergamon Press: New York, 1987;
Vol. 35, pp 279-284.
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Figure 3. TEM micrographs of BNSL isostructural with AuCu formed by 7.6 nm PbSe and 5.0 nm Au NPs: (a) TEM overview of the (001) projection;
(a, inset) small-angle ED pattern of the (001) plane; (b) as in (a) but at high magnification; (c) TEM overview of the (100) plane. TEM micrographs of
AuCu-type BNSL formed by 6.2 nm PbSe and 5.0 nm Au NPs; (e) 3D sketch of the AuCu unit cell (SG 123, P4/mmm). (f, h) Depictions of the (001) and
(100) planes, respectively. (g, i) Depictions of minimum number of layers in the (001) and (100) planes, respectively, leading to the formation of the patterns
identical with the observed ones. (k) TEM micrograph of the (100) projection of AuCu-type BNSL formed by 5.8 nm PbSe and 3.4 nm Ag NPs. Inset in
(i): small-angle ED pattern measured from the (100) plane. (l, m) TEM images of the (100) projection containing nanoparticle “antiphases”. (m) Depiction
of an antiphase boundary.
3.3. Binary Superlattices with AB2 Stoichiometry. 3.3.1.
AB2 Superlattices Isostructural with AlB2. We observed
formation of AB2 superstructures isostructural with AlB2 (SG
191, P6/mmm) from different nanoparticle building blocks. The
AlB2-type structure is based on a hexagonal unit cell. The (001)
projections are usually observed in plane view TEM. The
characteristic features of this structure are hexagons formed by
small and large NPs tilted by 30° with respect to each other.
Figure 5 shows the (001) projections of AlB2-type superlattices
assembled from different combinations of NPs (semiconductor-
noble metal, semiconductor-magnetic material, magnetically
hard-magnetically soft materials). Small-angle electron dif-
fraction patterns demonstrate the long-range order in such
structures (insets to Figure 5a,c,d). The (110), (011), and (111)
projections of the AlB2-type superlattice assembled from 5.8
nm PbSe and 3.0 nm Pd NPs are shown in Figure 6.
Murray and Sanders predicted on the basis of optimum space-
filling that AlB2-type structure should be stable for γ in range
of 0.482-0.624.27 The effective size ratios for superstructures
shown in Figure 4 are 0.71, 0.56, 0.54, 0.5, and 0.787 for PbSe-
Ag, PbS-Pd, CoPt3-Fe2O3, Fe2O3-PbSe, and PbSe-Au,
respectively. The values 0.71 and 0.787 significantly exceed
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Figure 4. TEM micrographs of orthorhombic BNSL formed by 5.8 nm PbSe and 3.0 nm Pd NPs: (a, b) (100) projection; (a, inset) a small-angle ED
pattern; (c) (010) projection. (d) Three-dimensional sketch of proposed orthorhombic lattice (SG51, Pmma). (e, f) Depictions of the (100) and (010) planes,
respectively. (g, h) Depictions of minimum number of layers in the (100) and (010) planes, respectively, leading to the formation of the patterns identical
with the observed ones. (i, j) TEM micrographs of the (100) and (010) projections of orthorhombic BNSL, formed by 7.2 nm PbSe and 4.2 nm Pd NPs; (i,
inset) a small-angle ED pattern.
Figure 5. TEM micrographs of the (001) planes of BNSL isostructural with AlB2 formed by (a) 6.7 nm PbSe and 3.0 nm Pd NPs, (b) 13.4 nm Fe2O3 and
6.2 nm CoPt3 NPs, (c) 5.8 nm PbSe and 3.7 nm Ag NPs, (d) 13.4 nm Fe2O3 and 5.8 nm PbSe NPs, and (e) 7.2 nm PbSe and 5.5 nm Au NPs. Top left insets
in (a), (b), (c), and (e) show a closer look of the arrangement of NPs in the AlB2-type superlattice. Right bottom insets in (a), (c), (d), and (e) show
small-angle ED patterns measured from the (001) plane. (f) Three-dimensional sketch of AlB2 unit cell. (g) Depiction of the the (001) plane. (h) Depiction
of minimum number of layers in the (001) plane, leading to the formation of the patterns identical to the observed ones.
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Figure 6. TEM micrographs of BNSL isostructural with AlB2 (SG 191, P6/mmm) formed by 5.8 nm PbSe and 3.0 nm Pd NPs. Overview of (a) (110), (b)
(011), and (c) (111) projections. (a, inset) Small-angle ED pattern from the (110) plane. (d, f, h, l) Depictions of the (110), (011), (111), and (1 -1 1) planes,
respectively. (e, g, i, m) Depictions of minimum number of layers in the (110), (011), and (111) planes, respectively, leading to the formation of the patterns
identical to the observed ones. (j, k) TEM micrographs displaying the grain boundary between (010) - (110) - (001) and (110) - (001) oriented planes.
the maximum size ratio predicted for a stable AlB2-type
structure. As a possible explanation, the nanoparticle superlattice
could tolerate a broader range of γ values by rearranging the
soft shells of organic ligands in a way that favors formation of
a dense binary assembly. For example, local interdigitation of
alkyl chains can significantly affect effective particle size.
3.3.2. AB2 Superlattices Isostructural with Hexagonal
Laves Phases (MgZn2 and MgNi2). Figure 7a,b shows the
TEM images of ordered binary superlattices with characteristic
features resembling “radiation signs”. This superlattice self-
assembled from 6.2 nm PbSe and 3.0 nm Pd NPs. Modeling of
the TEM images allowed us to assign this structure to one of
the hexagonal Laves phases. Small-angle electron diffraction
pattern confirms hexagonal symmetry of the superlattice projec-
tion (Figure 7a, inset). The superlattice projection shown in
Figure 7a,b is in good agreement with the (001) projection of
the MgZn2 lattice (Figure 7c-e). MgZn2-type Laves phases have
been observed in intermetallides and in so-called “solid van der
structure, the smaller nanoparticles are arranged in tetrahedra,
and these tetrahedra are joined alternately base-to-base and
point-to-point. The larger NPs fit into the holes enclosed by
tetrahedra. The ratio of atomic radii is considered as a structure-
determining factor in the formation of Laves phases in inter-
metallic alloys.51 In Laves phases, the highest packing density
of 0.71 corresponds to γ ∼ 0.813.27 For different intermetallic
compounds, the ratio of atomic radii can vary from 0.606 to
0.952.51 The effective size ratio of Pd and PbSe NPs is ∼0.6,
which is close to the low limit of size ratio acceptable for
MgZn2-type intermetallic compounds.
The domains of the MgZn2-type superlattice usually coexist
with the other crystalline binary phases. We assigned this
structure to MgNi2-type shown in Figure S2 in the Supporting
Information. MgNi2-type structure represents another hexagonal
Laves phase and belongs to the same space group as MgZn2.
The stacking sequences in MgZn2 and MgNi2-type crystalline
49
50
(49) Somayazulu, M. S.; Finger, L. W.; Hemley, R. J.; Mao, H. K. Science
1996, 271, 1400-1402.
Waals compounds” such as CH4(H2)2 and Ar(H2)2 that are
stable at a high pressure.
The MgZn2-type structure contains four formula units per cell
and belongs to the space group P63/mmc (SG 194). In this
(50) Loubeyre, P.; Letoulecc, R.; Pinceaux, J.-P. Phys. ReV. Lett. 1994, 72,
1360-1363.
(51) Wernick, J. H. In Intermetallic Compounds; Westbrook, J. H., Ed.; Robert
E. Krieger Publishing Company: Malabar, FL, 1977; pp 198-204.
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Figure 7. TEM micrographs of superlattices isostructural with MgZn2 formed by 6.2 nm PbSe and 3.0 nm Pd NPs: (a) TEM overview of the (001) plane;
(a, inset) Small-angle ED pattern from the (001) plane. (b) As (a) but at high magnification; (c) 3D sketch of the MgZn2 unit cell (SG 194, P63/mmc). (d)
Depictions of the (001) plane. (e) Depictions of minimum number of layers in the (001) planes that needed to reproduce those observed experi-
mentally.
Figure 8. TEM micrographs of superlattices isostructural with Cu3Au (SG 221, Pm3hm) formed by 5.8 nm PbSe and 3.4 nm Ag NPs: (a, b) TEM images
of the (001) projection, (a, inset) small-angle ED pattern of the (001) plane; (c) 3D sketch of the Cu3Au unit cell. (d) Depiction of the (001) projection. (e)
Depictions of minimum number of layers in the (001) projection leading to the patterns identical to the observed ones.
structures are ABABAB... and ABACABAC..., respectively.
MgNi2 structure contains eight formula units per cell (Figure
S2 from Supporting Information).
3.4. Binary Superlattices with AB3 Stoichiometry Iso-
structural with Cu3Au. Superlattices isostructural with the
cubic Cu3Au intermetallic compounds (SG 221, Pm3hm) can
self-assemble from a mixture of 5.8 nm PbSe and 3.4 nm
Ag NPs stabilized by oleic acid and dodecanethiol, respectively
(the particle concentration ratio ∼1:7). Figure 8a,b shows the
(001) projection of the Cu3Au-type superlattice. The thickness
of the superlattice typically varied between 2 and 4 layers of
NPs.
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Figure 9. (a, b) TEM images of (100) projections of BNSL isostructural with Fe4C formed by 5.8 nm PbSe and 3.0 nm Pd NPs; (a, inset) small-angle ED
pattern of the (100) projection. (c) Three-dimensional sketch of the Fe4C unit cell. (d) Depiction of the (100) projection. (e) Depictions of minimum number
of layers in the (100) projection leading to the patterns identical to the observed ones. (f) TEM micrograph of the (100) projection of Fe4C-type BNSL
consisting of 7.2 nm PbSe and 4.2 nm Ag NPs.
3.5. Binary Superlattices with AB4 Stoichiometry Iso-
structural with Fe4C. BNSLs isostructural with Fe4C were
formed by 5.8 nm PbSe, 3.0 nm Pd, 7.2 nm PbSe, and 4.2 nm
Ag NPs. Figure 9a,b,f represents (100) projections of the Fe4C-
type structure based on a cubic unit cell (SG 215, P4h3m). The
unit cell of the Fe4C lattice and the modeled (100) projection
are depicted in panles c and d of Figure 9, respectively. Usually,
we observed ∼0.5-1 µm2 defect-free domains with Fe4C-type
ordering. In some samples, Fe4C structures coexisted with the
AB orthorhombic superlattices.
2.6 nm - 4.5 nm CoPt3 NPs35 as well as for large latex
particles.30
3.7. Binary Superlattices with AB6 Stoichiometry Iso-
structural with CaB6. Being mixed together, 13.4 nm Fe2O3
and 5.0 nm Au NPs (particle concentration ratio is ∼1:5) show
the tendency to form BNSL isostructural with CaB6 (SG 221,
Pm3hm) (Figure 11). The structure of CaB6 can be described as
follows: six small spheres form octahedron which is placed in
the center of the cubes; the vertices of those are occupied by
four large spheres.
3.6. Binary Superlattices with AB5 Stoichiometry Iso-
structural with CaCu5. Slow drying of solutions contain-
ing 6.3 nm PbSe and 3.6 nm Ag or 7.2 nm PbSe and 5.0 nm
Au NPs dispersed in a tetrachloroethylene-toluene mixture
(∼1:1 by volume) produced hexagonal superlattices of the
CaCu5-type (SG 191, P6/mmm). The initial particle concentra-
tion ratio was close to 1:7 (large:small). The maximum packing
density of CaCu5-type is 0.7 for γ ∼ 0.65. Figure 10 shows
the (001) projection of the CaCu5-type structure formed by PbSe
and Ag and PbSe and Au NPs. The BNSLs of CaCu5-type were
3.8. Binary Superlattices with AB13 Stoichiometry. To date,
the most complex structure that can self-assemble from hard
spheres is the binary superlattice of the NaZn13 crystalline type
(SG 226, Fm3c) (Figure 12). The structure of NaZn13 and of
several homologous compounds (KZn13, KCd13, etc.) has 112
spheres per unit cell and is defined as follows: each B sphere
is surrounded by 12 other B spheres at the vertices of a slightly
distorted icosahedron, and each A sphere is surrounded by 24
B spheres at the vertices of a snub cube52 (Figure 12c). The
maximum packing density calculated for AB13 with a perfect
icosahedron is 0.738.27 However, a small size distribution of
33
previously observed for 6.3 nm PbSe - 11 nm Fe2O3 and
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Figure 10. TEM micrographs of BNSL isostructural with CaCu5 formed by (a) 6.3 nm PbSe and 3.6 nm Ag; (b) 7.2 nm PbSe and 5.0 nm Au; (c) 6.2 nm
PbSe and 5.0 nm Au NPs; (right bottom insets in a, b, c) Small-angle ED patterns from the (001) projection; (b,c insets) enlarged view of CaCu5-type
superlattice consisting of 7.2 nm PbSe and 5.0 nm Au. (d) Three-dimensional sketch of the CaCu5 unit cell. (e) Depiction of the (001) projection. (f)
Depiction of minimum number of layers in the (001) projection, leading to the formation of the patterns identical to the observed ones.
Figure 11. (a) TEM micrograph of a BNSL isostructural with CaB6 formed by 13.4 nm Fe2O3 and 5.0 nm Au NPs; (a, inset) small-angle ED pattern from
the (100) projection; (b) enlarged view; (c) 3D sketch of the CaB6 unit cell. (d) Depiction of the (100) projection. (e) Depiction o minimum number of layers
in the (100) projection, leading to the formation of the patterns identical with the observed ones.
the spheres in the icosahedral cluster can increase the packing
density. For example, a NaZn13 lattice with a slightly smaller
central particle in the center of each icosahedron can approach
the packing density of ∼0.76.27 Theory predicts stability of a
NaZn13 lattice built of hard spheres in the range of γ from 0.54
to 0.61 (Table 2). Computer simulations by Frenkel et al.23
demonstrate that no specific energetic interactions are needed
to stabilize such a complex structure and its formation can be
supported by an argument based on entropy alone.
Recently, this type of structure was also observed in binary
mixture of γ-Fe2O3 and PbSe NPs.33 Figure 12a,b shows the
(100) projection of a NaZn13-type superlattice consisting of 5.8
nm PbSe and 3.0 nm Pd NPs and 7.2 nm PbSe and 4.2 nm Ag
NPs stabilized by oleic acid and dodecanethiol, respectively.
The effective particle size ratios are ∼0.63 and 0.662, which
are above the upper limit of stability range calculated for NaZn13.
Because it has the highest packing density, the NaZn13 lattice
was considered to be the only stable superlattice with AB13
stoichiometry. Recently, we demonstrated that NaZn13 has a
polymorphous form where small particles occupy the vertices
of a cuboctahedron (cub-AB13).37 Figure 12e,f shows the (100)
projection of cub-AB13 formed by the same 5.8 nm PbSe and
Superlattices with NaZn13-type structures were first found in
natural gem opals28 and later assembled from latex spheres.31
(52) Shoemakker, D.; Marsh, R.; Ewing, F.; Pauling, L. Acta Crystallogr. 1952,
5, 637-644.
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Figure 12. TEM micrographs of (100) projections of BNSL with AB13 stoichiometry formed by (a, e) 5.8 nm PbSe and 3.0 nm Pd NPs and (b, f) 7.2 nm
PbSe and 4.2 nm Ag NPs: (a, b) BNSL isostructural with NaZn13; (e, f) cub-AB13 superlattice; (c, d) depicting the unit cells of NaZn13- and cub-AB13-type
superlattices, respectively.
3.0 nm Pd and 7.2 nm PbSe and 4.2 nm Ag NPs. The maximum
packing density of cub-AB13 is 0.700. On the other hand, the
cub-AB13 lattice is characterized by a higher number of
particle-particle contacts per unit cell. Close particle-particle
interactions can be favorable for stabilizing the lattice due to
short-range van der Waals’ forces.27 The detailed discussion of
both polymorphous forms of AB13 nanoparticulate superlattices
is reported in ref 37.
that will be accessible by this modular assembly approach in a
nearest future.
3.10. BNSL Heterostructures. Different types of NP super-
lattices can form simultaneously on the same substrate. The
domains with different symmetry are usually spatially separated.
However, sometimes a smooth transition between the two phases
is observed. For example, Figure 14a shows an epitaxial “super-
heterostructure” formed by the (011) planes of AlB2-type and
(100) plane of cub-AB13-type superlattices. We assume that
the formation of the epitaxial heterostructure can result from
a local drop in the concentration Pd NPs during the growth
of nanoparticle superlattice. Figure 13b demonstrates the
example of the defect heterostructure between the (111) plane
of AlB2-type and the (100) plane of cub-AB13-type superlattice,
and Figure 13c shows the example of heterostructure formed
by the (001) plane of AlB2-type and the (100) plane of cub-
AB13-type superlattice. This type of heterostructure can be
3.9. The BNSLs with Unknown Crystallographic Sym-
metry. In addition to the superlattices that have been assigned
by us to well-known crystalline structures, we also observed
BNSLs that cannot yet be assigned to specific symmetry groups.
For example, Figure 13 demonstrates three superlattices crystal-
lized from the mixtures of PbSe and Pd, Fe2O3 and Ag, and
Fe2O3 and Au NPs. These superstructure as as well as many
others not shown in this report give us confidence that the
diversity of BNSL presented here is a preview of the materials
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Figure 13. TEM micrographs of “unknown” superstructures formed by (a, b) 6.2 nm PbSe and 3.0 nm Pd NPs; (c) 13.4 nm Fe2O3 and 4.2 nm Ag NPs;
and (d) 13.4 nm Fe2O3 and 5 nm Au NPs.
formed as a collision of laterally growing islands of simulta-
neously nucleated AlB2-type and cub-AB13-type superlattices.
Similar heterostructures were observed by Sanders in Brazilian
opals.29
for the (220) reflections of Fe2O3 NPs (Figure 15d). If all Fe2O3
NPs orient in the same direction, we should observe four (220)
diffraction spots. The 12 arcs for Fe2O3 (220) reflections in the
NaCl-type superlattice can result from three different orientations
of Fe2O3 NPs tilted by 60°.
3.11. Preferential Orientation of Nanocrystals in BNSLs.
Preferred orientation of the atomic lattices of individual nanoc-
rystals packed into a superlattice can be studied by wide-angle
selected area electron diffraction (SAED). Randomly oriented
nanocrystals result in uniform diffraction rings in wide-angle
SAED patterns, as shown in Figure 15a, for a superlattice of
13.4 nm Fe2O3 NPs. In contrast, the wide-angle X-ray scattering
from Fe2O3 NPs packed into the NaCl-type BNSL with 5.5 nm
Au NPs (Figure 15d) appears as a series of arcs rather than
continuous rings. These arcs originate from the preferential
orientation of Fe2O3 NPs in the superlattice. Preferential
orientation can sometimes be observed in single-component
superlattices if nanoparticle shape deviates from spherical due
to faceting.53,54 Figure 15b,c shows examples of preferential
orientation in fcc arrays of PbSe nanocrystals.
Preferential orientation can be observed either for a single
component of for both components of binary BNSLs. Figure
15e,f shows preferential orientations for both components in
PbSe-Au AuCu-type superlattice and Fe2O3-PbSe AlB2-type
superlattice, respectively. Figure 15g demonstrates an example
of BNSL where the larger 5.8 nm PbSe NPs show preferential
orientation while smaller 3.4 nm Ag NPs are disordered.
Preferential orientation should be typical for superlattices where
the arrangement of nearest neighbors matches the symmetry of
nanoparticle facets “locking” each NP in a specific orientation.
We have not yet observed preferential orientation effects in
orthorombic AB, CaCu5, CaB6, NaZn13, and cub-AB13 BNSLs.
3.12. Faceting of BNSLs. In the most cases, BNSLs grow
as thin extended films. Careful TEM investigation revealed that
thicknesses of BNSL films are nearly constant throughout an
Single-component fcc and hcp superlattices of nanocrystals
usually show unidirectional orientation of atomic lattices, while
BNSLs can allow more complex preferential orientation effects.
For example, the wide-angle SAED pattern of the (111) plane
of NaCl-type Fe2O3-Au superlattice shows 12 equidistant arcs
(53) Zhang, Y.-W.; Sun, X.; Si, R.; You, L.-P.; Yan, C.-H. J. Am. Chem. Soc.
2005, 127, 360-3261.
(54) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; Forster,
S.; Weller, H. J. Am. Chem. Soc. 2004, 126, 12984-12988.
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Structural Characterization of BNSLs
A R T I C L E S
Figure 14. TEM micrograph of (a) the epitaxial “super-heterostructure” formed by the (011) plane of AlB2-type and (100) plane of cub-AB13-type superlattices;
(b) heterostructure formed by the (111) plane of AlB2-type and the (100) plane of cub-AB13-type superlattices consisting of 5.8 nm PbSe and 3.0 nm Pd NPs,
and (c) heterostructure formed by the (001) plane of AlB2-type and the (100) plane of cub-AB13-type superlattices consisting of 7.2 nm PbSe and 4.2 nm
Ag NPs.
entire substrate (a TEM grid), whereas the size of ordered areas
becomes larger (∼µm2) as the drying front moves. The surface
free energy for different crystallographic faces is usually
different.55 Depending on the type of BNSL, these films are
represented by certain, as a rule, low index, crystallographic
planes. For instance, in the case of AlB2-type superlattices
precipitated onto carbon-coated copper grid, the (001) plane
(Figure 5) is dominant over others for almost all studied
combinations of nanoparticles, while other planes (Figure 6)
are observed in only smaller domains. The growth of BNSL
films under conditions discussed above was found to be sensitive
to the type of substrate1 that might indicate the heterogeneous
nucleation of BNSLs, a subject to be addressed in future
research.
We observed the formation of faceted BNSLs when NPs were
self-assembled in the presence of significant excess of oleic acid.
These samples needed thorough washing with ethanol before
TEM investigation. This procedure removes the excess of oleic
acid from the substrate; however, it also led to the partial
detachment of the faceted superlattices from the substrate. Figure
16 represents the (001) projection of AlB2-type faceted super-
lattices. The size of AlB2-type hexagons is in the range of 100-
300 nm, which is significantly smaller than the coherent length
of ordering in films. Again, as in the case of AlB2-type films,
the (001) plane is the most extended in hexagons. This allows
us to assume that the (001) facet has the lowest surface free
energy.
3.13. General Remarks on BNSL Crystallization. Entropy-
driven formation of a crystalline phase in a suspension contain-
ing monodisperse hard spheres occurs when the particle volume
fraction æ exceeds ∼0.49.56 However, the phase transition in
binary mixtures of hard-sphere colloidal particles usually occurs
at lower volume fractions57 because the volume accessible to
the small particles increases when two large particles are in a
close contact. The resulting gain in the system entropy is
considered to be a driving force for clustering of large particles,58
and as a result, a phase transition occurs. In the case of hard
spheres, a binary superlattice can form only if its entropy is
higher than the sum of entropies of separated fcc-packed small
and large spheres. Table 2 summarizes the predicted ranges of
stability for different hard-sphere binary superlattices. One can
see that stable binary lattices usually have the packing density
exceeding the packing density of the fcc lattice (∼0.7405). This
is in agreement with a simple schemesthe higher packing
(56) Pusey, P. N.; van Megen, W. Nature 1986, 320, 340-342.
(57) Kaplan, P. D.; Rouke, J. L.; Yodh, G.; Pine, D. J. Phys. ReV. Lett. 1994,
72, 582-585.
(55) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; John Wiley&
(58) Dijkstra, M.; Frenkel, D.; Hansen, J.-P. J. Chem. Phys. 1994, 101, 3179-
Sons: New York, 1997.
3189.
9
J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3633

A R T I C L E S
Shevchenko et al.
agreement with our observation that binary superlattices pref-
erentially grow laterally (Figure S3 from Supporting Informa-
tion) rather than in all three dimensions. In many cases, the
thickness of the superlattices does not exceed one or two unit
cells, whereas the lateral size of ordered domains can approach
∼10 µm2.
Our studies show that the real nanoparticles cannot be
approximated by idealized hard spheres. We demonstrated that
nanocrystals can self-assemble into a variety of BNSLs with
densities significantly smaller than 0.74 (e.g., CuAu, MgZn2,
Cu3Au, Fe4C, CaCu5, CaB6, and cub-AB13). The low density
structures are predicted to be unstable in the case of hard-sphere
colloids. However, they do form reproducibly, ruling out the
packing density as the major factor in determining stability of
binary nanoparticle assemblies.
We recently proposed that self-assembly of BNSLs is driven
by Coulomb attraction between oppositely charged nanoparticles
in a colloidal solution.1 Indeed, electrophoretic mobility mea-
surements show that sterically stabilized nanoparticles can hold
one or two noncompensated positive or negative elemental
charges per particle.1 The combination of space-filling principle
and soft (Coulomb) interparticle interactions can generate a
rather complex phase diagram, as it has been recently demon-
strated for binary superlattices of charged PMMA beads.62
However, even the presence of Coulomb interactions cannot
explain formation of the entire family of BNSLs reported in
this work. We believe that the phase diagram of self-assembled
BNSLs is caused by the intrinsic complexity of interactions
between nanoscale sterically stabilizied particles. In addition
to Coulomb interactions, we should take into account interac-
tions that involve high order moments of charge distribution
(i.e., charge-dipole, dipole-dipole, charge-induced dipole, etc.)
and dispersive van der Waals forces. In addition to static electric
charge, semiconductor nanocrystals can exhibit large dipole
moments and polarizabilities;63-65 large Hamaker constants
result in strong van der Waals attraction.66,67 Table 3 shows
estimated magnitudes of the interactions between two 5 nm
semiconductor nanocrystals with charge Z ) (e, dipole
moments µ ) 100 D, and polarizabilities R/4πꢀ0 ) 30 nm3
(these values are typical for CdSe nanocrystals63,64); and two 5
nm metal (e.g., Au) nanoparticles with charge Z ) (e and
polarizabilities R/4πꢀ0 ) 65 nm3 (∼nanoparticle volume).
Detailed calculations are given in Supporting Information. One
can see that for semiconductors the energy of charge-dipole,
dipole-dipole, and charge-induced dipole interactions can
approach ∼68, 23, and 7% of Coulomb energy, respectively,
when the particle separation is ∼1 nm (the typical thickness of
interdigitated capping ligands). Depending on the orientation
of the nanoparticle dipole moments, these additional terms can
double (or cancel) electrostatic interactions between adjacent
nanoparticles in the BNSL. This situation is quite unique. For
example, for charged small molecules, the Coulomb interactions
Figure 15. TEM micrographs of (a) the (111) planes of superlattices formed
by 13.4 nm Fe2O3 NPs; (b) the (111) and (c) (110) planes of superlattices
formed by 6.2 nm PbSe NPs. Insets demonstrate the wide-angle selected
area ED patterns measured from different planes. (d-g) Wide-angle selected
area electron diffraction patterns measured from single-crystal domains of
different BNSLs.
density is, the larger excluded volume and, in turn, the higher
entropy of the system can be achieved during crystallization of
the superlattice.27 Preliminary predictions of stability for hard-
sphere binary superlattices can be drawn by comparison of its
packing density with the density of the fcc phase. However,
the intrinsic entropy of binary superlattice also affects its
stability. For example, the maximum packing density of NaZn13-
type superlattice is slightly below 0.74, while the detailed
entropy calculations predict stability of this structure in a rather
broad range of γ (Table 2).23
The presence of a flat interface (e.g., substrate) can substan-
tially affect crystallization phenomena in binary systems.
Crystallization on the wall was observed at particle volume
fractions æ ∼ 0.2,57 that is, well below the values typical for
hard-sphere crystallization in the bulk of solution.56 According
to calculations by Frenkel et al.,61 the presence of a wall can
drastically lower the barrier of nucleation and lead to the lateral
colloidal crystal (superlattice) growth. These predictions are in
(62) Leunissen, M. E.; Christova, C. G.; Hynninen, A.-P.; Royall, C. P.;
Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A.
Nature 2005, 437, 235-240.
(63) Shim, M.; Guyot-Sionnest, P. J. Chem. Phys. 1999, 111, 6955-6964.
(64) Li, L.-S.; Alivisatos, A. P. Phys. ReV. Lett. 2003, 90, 097402-1-097402-
4.
(65) Rabani, E.; Hete´nyi, B.; Berne, B. J.; Brus, L. E. J. Chem. Phys. 1999,
110, 5355-5369.
(66) Hamaker, H. C. Physica 1937, 4, 1058-1072.
(67) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. ReV. Lett.
1995, 75, 3466-3469.
(59) Hunt, N.; Jardine, R.; Bartlett, P. Phys. ReV. E 2000, 62, 900-913.
(60) Schofield, A. B. Phys. ReV. E 2001, 64, 051403-051406.
(61) Auer, S.; Frenkel, D. Phys. ReV. Lett. 2003, 91, 015703-1-015703-4.
9
3634 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006

Structural Characterization of BNSLs
A R T I C L E S
Figure 16. TEM micrographs of faceted AlB2-type BNSLs formed by 7.2 nm PbSe and 4.2 nm Ag NPs.
Table 3. Magnitudes of Interactions between Two 5 nm
chloroform), dispersive interactions can cause short-range
attraction between oddly charged metal nanoparticles. At very
small interparticle distances (<∼1-1.5 nm), the pair potential
will be dominated by repulsion between the nanoparticle ligands
that can be described by osmotic and sterical terms73 (see
Supporting Information for details).
Nanoparticles with Charges Z ) (e and Dipoles Oriented along
the Particle-Particle Axis (R is the particle center-to-center
distance)
Vsemiconductor
Vmetal
-
[eV]metal
-
[eV]semiconductor
interaction
R
)
6 nm
R
)
10 nm
R
)
6 nm
R ) 10 nm
All these interactions contribute to the total potential energy
of a BNSL. Depending on the averaging effects, the interactions
discussed above can contribute to the pair potential and the
superlattice binding energy with different weight. To demon-
strate this, we calculated Coulomb and van der Waals binding
energies for the NaCl-type BNSL assembled of oppositely
charged 13.4 nm Fe2O3 and 5.0 nm Au nanoparticles (Figure
2a).
The Coulomb binding energy of an ABx-type BNSL can be
calculated as UCoulomb ≈ MZAZBe2/(4πꢀꢀ0R0)per ABx “molecule”,
where M is the Madelung constant and R0 is the nearest-neighbor
center-to-center distance. For the Fe2O3-Au BNSL shown in
Figure 2a, R0 ) 11.5 nm, MNaCl ) -1.7476. and UCoulomb ≈ -
0.1 eV per unit cell, assuming one elemental charge per particle
and ꢀ ) 2.1
Coulomb (∼1/R)
0.12
0.082
0.028
0.072
0.12
0.072
charge-dipole (∼1/R2)
dipole-dipole (∼1/R3)
0.029
5.8 × 10-3
charge-ind. dipole (∼1/R4) 8.4 × 10-3 1.1 × 10-3
0.018
0.097
2.3 × 10-3
1.3 × 10-3
van der Waals (∼1/R6)
0.016
2.1 × 10-4
dominate over dipolar and dispersive interactions by orders of
magnitude.68
Dispersive (van der Waals) potential between two nanopar-
ticles of radii RA and RB at the distance of closest approach D
can be calculated from direct pairwise summation over atomic
dispersional attractions proportional to -1/R6:66,67
A
12
S
V
vdW ) -
+
{
D[1 + D/2(RA + RB)]
The van der Waals binding energy (UvdW) for a BNSL
D[1 + D/2(RA + RB)]
1
assembled from nanoparticles with radii RA and RB was
+ 2 ln
)
vdW
ij
1 + D/S + D2/4RARB
R[1 + D/S + D /4RARB]
2
(
)}
estimated by summation of V
over all nanoparticles in the
superlattice:
A
-
ê(RA,RB,D) (1)
12
1
1
UvdW
)
V
vdW ) -
A ê(R ,R ,D ) (2)
2∑∑
24∑∑
ij
ij
i
j
ij
where A is the solvent-retarded Hamaker constant and S )
2RARB/(RA + RB) is the reduced radius.
i
j
i
j
In a NaCl-type binary superlattice, each particle A has 6
particles B at the nearest-neighbor distance R0, 12 particles A
The van der Waals energy can vary substantially for different
materials. Thus, A ∼ 3.1 × 10-19 J (∼1.95 eV) for Au-Au
attraction through a hydrocarbon layer69,70 (some authors
reported larger values, e.g., 3 eV71), whereas CdSe-CdSe
attraction is significantly weaker: A ∼ 5 × 10-20 J (∼0.32
eV).7271-72 The contribution of dispersive interactions to the pair
potential is relatively small (∼13% of the Coulomb energy) for
5 nm semiconductor nanoparticles at R ) 6 nm (Table 3).
However, in the case of metals (Au, Ag, Pd), van der Waals
energy can rival the Coulomb potential at small interparticle
separations. In solvents with a high dielectric constant (e.g.,
x
x
at 2R0, 8 particles B at 3R0, 6 particles A at 2R0, 24
x
particles B at 5R0, and so forth. For a superlattice of 2N
nanoparticles (or N unit cells), the sum (eq 3) can be calculated
as
N
UvdW ) -
[
{A ê(R ,R ,D )}
+
{A ê
ij
∑
∑
j
ij
i
j
ij i ) particleA
24
j
(Ri,Rj,Dij)}i ) particleB
]
N
2
) - AABê(RA,RB,D0) + AAAê(RA,RA,D1) +
[
(68) Berry, R. S.; Rice, S. A.; Ross, J. Physical Chemistry; Oxford University
Press: New York, 2000.
4
3
(69) Bargeman, D.; van Voorst Vader, F. J. Electroanal. Chem. 1972, 37, 45-
ABBê(RB,RB,D1) + AABê(RA,RB,D2) + ‚‚‚ (3)
52.
]
(70) Bergstro¨m, L. AdV. Colloid Interface Sci. 1997, 70, 125-169.
(71) Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 16732-16738.
(72) Ge, G.; Brus, L. J. Phys. Chem. B 2000, 104, 9573-9575.
where AAA, ABB, and AAB are the Hamaker constants for
9
J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006 3635

A R T I C L E S
Shevchenko et al.
interactions between two particles of type A, two particles of
type B, and particle A with particle B, respectively; Dk )
structure can dominate over the denser and, potentially, more
stable NaZn13-type superlattice.37 Finite size distribution is
always present in nanoparticle samples, and this could have a
substantial influence on both thermodynamics and kinetics of
formation of BNSL. Murray and Sanders calculated that a slight
distribution in the size of small particles can increase the packing
density of NaZn13-type superlattice from 0.738 to 0.760.27 The
size distribution of NPs used in this work for growing binary
superlattices was ∼5-8%. We observed suppression of crystal-
lization from mixtures containing particles with broad size
distribution (> ∼14% standard deviation). Notably smaller
fragments (∼400 nm2) of binary superlattices could sometimes
grow from solutions of NPs with polydispersity of ∼10-12%
standard deviation. These results are in qualitative agreement
with numerical calculations79 predicting suppression of nucle-
ation in mixtures of polydisperse hard spheres. Further studies
are necessary to reveal the combination of thermodynamic and
kinetic factors affecting the nucleation and growth of binary
superlattices.
(k+1)R - R - R .
x
V
0
i
j
Due to the short-range nature of dispersive interactions,
vdW
ij
quickly decays with increasing D and the series (eq 3)
rapidly converges. For the BNSL shown in Figure 2a (RA
)
6.7 nm, RB ) 2.5 nm, R0 ) 11.5 nm, and D0 ) 2.3 nm), to a
good approximation, we can neglect contributions from nano-
x
particles at distances > 3R0. In this case
UvdW ≈ -N[0.119AAB + 0.261AAA + 1.632 × 10-4ABB
]
(4)
Hamaker constant for gold-gold attraction through a hydro-
carbon medium is ABB ∼ 3.1 × 10-19 J (∼1.95 eV).69 We found
a large spread in reported Hamaker constant values for Fe2O3-
Fe2O3 attraction in aqueous medium, the reported values varied
from 1 × 10-20 to 1 × 10-19 J (0.06-0.6 eV).70,74,75 Hamaker
constants in hydrocarbon medium are generally lower than
Hamaker constants in water [D9], and we used 3 × 10-20
J
4. Concluding Remarks
(0.19 eV) as the realistic estimate for AAA. This value fits well
to the general trend observed for transition metal oxides.76 The
Hamaker constant for the Au-Fe2O3 interaction through
We have induced a wide array of monodisperse metal,
semiconductor, and magnetic nanocrystals to self-assemble into
binary nanocrystal superlattices (BNSLs), yielding a broad new
class of multifunctional nanocomposites. Superlattices with AB,
AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometries and cubic,
hexagonal, tetragonal, and orthorhombic symmetries have all
been identified. We have established a combination of electron
microscopy, diffraction, and structural modeling as a methodol-
ogy for the systematic classification and characterization of
BNSLs. The richness and diversity of binary superstructures
presents a challenge to both experimentalists and theorists to
understand the principles that guide assembly at the nanoscale
level. We demonstrate that BNSL formation is guided by a
combination of competing short-range Coulonbic, dipolar, and
van der Waals interactions. We predict that novel systems could
enable fundamental studies of nucleation and growth of complex
crystals. This report demonstrates only a fraction of the
structures that might self-assemble from multicomponent mix-
tures of nanoparticles, and we feel that this modular approach
to nanomaterials design will have great potential for a rational
design of complex nanoscale materials.
hydrocarbon medium has not been measured, and we estimated
1/2
it as [AAAABB
]
∼ 1 × 10-19 J (0.6 eV).76 Substituting Hamaker
constants into eq 4 gives the van der Waals energy of ∼ -0.12
eV per unit cell. UvdW is comparable (exceeds by ∼20%) to
U
Coulomb, despite VvdW < VCoulomb in the pair potentials. This is
because the dispersive interactions are attractive, whereas the
Coulomb interactions can be both attractive and repulsive.
Competition between Coulomb and van der Waals forces can
stabilize a variety of complex BNSL structures. Thus, formation
of BNSLs containing close-packed clusters of metal nanopar-
ticles, such as Fe4C (Figure 9), CaCu5 (Figure 10), CaB6 (Figure
11), NaZn13, and cub-AB13 (Figure 12), should be energetically
favorable because of strong van der Waals attraction between
metal nanoparticles in these clusters (Ag, Au, Pd). Additionally,
ordering of dipole moments of semiconductor nanocrystal should
significantly affect the total BNSL binding energy.
It was also shown that surfactants can influence the assembly
of NPs.77 Thus, Au140(C12H25S)62 clusters capped with long-
chain thiols are predicted to pack into a 3D tetragonal super-
lattice, whereas Au140(C4H9S)62 should form a bcc superlattice.77
Another factor that might affect the packing symmetry is the
deviation of NP shape from spherical.27 The shape of crystalline
NPs is usually determined by nanocrystal faceting.78 The highest
packing density and stronger van der Waals attraction of faceted
nanocrystals are achieved if the packing symmetry reflects the
shape of the NP building blocks.
Acknowledgment. We wish to thank Prof. Daan Frenkel
(FOM-Institute for Atomic and Molecular Physics, Netherlands),
Dr. Glenn Held (T. J. Watson Research Center, IBM), Prof.
Louis Brus, and Prof. Irving Herman (Columbia University) for
helpful discussions, and Robert Sandstrom (T. J. Watson
Research Center, IBM) for technical support. This work was
supported primarily by the MRSEC Program of the National
Science Foundation under Award Number DMR-0213574, by
the New York State Office of Science, Technology and
Academic Research (NYSTAR), and in part by the Department
of Energy, Office of Basic Energy Sciences, Grant DE-FG02-
03ER15463. S.O. is grateful for support from an NSF CAREER
award, DMR-0348938.
Energetic factors discussed above affect the relative stability
of different binary superlattices. Additionally, the kinetics of
superlattice nucleation and growth could be responsible for the
preferential growth of some superstructures over the others. For
example, easier nucleation of relatively simple and highly
symmetric cub-AB13 superlattices might explain why this
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Supporting Information Available: Unit cell and modeled
(001) plane of CaF2-type superlattice (Figure S1). TEM
(79) Auer, S.; Frenkel, D. Nature 2001, 413, 711-713.
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3636 J. AM. CHEM. SOC. VOL. 128, NO. 11, 2006

Structural Characterization of BNSLs
A R T I C L E S
micrographs of the hexagonal MgNi2-type superlattice, modeled
unit cell, and projections of this structure (Figure S2). TEM
overviews demonstrating the lateral growth of nanoparticu-
late superlattices (Figure S3). Sketch of charge-dipole and
dipole-dipole nanoparticle interactions (Figure S4). Typical
pair potentials between two oddly, evenly charged dielectric
(semiconducting) and between two evenly and oddly charged
metal nanoparticles with charges Z ) (e (Figure S5). This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
JA0564261
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