1. Nano Today (2015) 10, 81—92
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/nanotoday
REVIEW
Formation of supercrystals through
self-assembly of polyhedral nanocrystals
Michael H. Huang∗
, Subashchandrabose Thoka
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Received 21 October 2014; received in revised form 26 December 2014; accepted 19 January 2015
Available online 20 February 2015
KEYWORDS
Nanocrystals;
Self-assembly;
Supercrystals;
Superlattices;
Superstructures;
Surfactant
Summary Compared to the number of reports on the self-assembly of spherical nanoparticles
forming superlattices, relatively fewer studies have addressed the assembly of polyhedral metal
and semiconductor nanocrystals for the formation of supercrystals with well-defined geometric
shapes. These polyhedral supercrystals are considered as a new class of superlattice struc-
tures in which particle morphology strongly dictates the shapes of resulting supercrystals if
the particles are larger than 20 nm. This review provides examples and advances in fabricating
supercrystals on a substrate during the process of solvent evaporation and through diffusion
transport of surfactant to generate free-standing supercrystals. The diversity of supercrystal
morphologies observed is illustrated. In many cases, the supercrystal formation process has
been found to be surfactant-mediated with surfactant molecules residing between adjacent
nanocrystals. Polyhedral nanocrystal assembly was found to be strongly shape-guided. Thus,
the formation of polyhedral supercrystals offers a unique opportunity to reconsider the forces
involved from a more global perspective instead of focusing on mainly local interactions. Efforts
have been made to record the entire supercrystal formation process. Finally, some results of
properties of supercrystals and future directions for supercrystal research are provided.
© 2015 Elsevier Ltd. All rights reserved.
Introduction
Advances in the syntheses of polyhedral metal and semicon-
ductor nanocrystals with excellent size and shape control
have not only given us valuable particles for the examination
of their facet-dependent properties, but have also naturally
∗ Corresponding author. Tel.: +886 3 5718472; fax: +886 3 5711082.
E-mail address: hyhuang@mx.nthu.edu.tw (M.H. Huang).
led to the observation of their self-assembled structures
on substrates [1—20]. Because of their spontaneous orga-
nization after particle formation, it becomes necessary to
describe the superlattice structures formed. Previously the
focus of extensive studies on the assembly of nanoparticles
involves mostly spherical particles of single or multiple sizes
[21—27]. The particles therefore can be viewed as artifi-
cial atoms forming lattice structures which resemble those
seen in unit cells of metals and binary compounds with face-
centered cubic (fcc) and body-centered cubic (bcc) packing
arrangements. Conversion from one packing structure to
http://dx.doi.org/10.1016/j.nantod.2015.01.006
1748-0132/© 2015 Elsevier Ltd. All rights reserved.

2. 82 M.H. Huang, S. Thoka
another is also possible with spherical particles by vary-
ing the temperature [24,25]. While assembly by polyhedral
particles should involve similar mechanisms and forces, the
effect of particle shape becomes important, and accommo-
dation of small spheres or polyhedra filling the interstitial
spaces can be more difficult or show no positional unifor-
mity. Their packing arrangements therefore deviate from
those of spherical building blocks. An exception may be the
coassembly of nanospheres and very short nanorods with
curved ends to form binary superlattices [28]. Another inter-
esting aspect of polyhedral nanoparticle assembly is the
formation of supercrystals with geometric shapes, which
are much less observed or discussed. These features makes
supercrystals organized by polyhedra a new class of super-
structures that can yield different structural diversity and
allow the exploration of pore accessibility and novel physical
properties.
This review focuses on the formation of supercrystals
with geometric shapes from the self-assembly of inorganic
polyhedral nanocrystals. The scope of discussion is some-
what different from that of superlattices constructed from
non-spherical building blocks, in which the formation of
organized superstructures with well-defined morphologies
is not emphasized [29]. Supercrystals fabricated from the
assemblies of various building blocks and their packing
structures are presented. Direct observation of the super-
crystal formation process during droplet evaporation is
provided. The forces involved in the formation of superlat-
tices have been extensively discussed in the literature, so
here the role of surfactant and the particle shape effects
are emphasized to show how the use of simple surfactant
can effectively yield polyhedral supercrystals in aqueous
solution. A novel surfactant diffusion approach to generate
free-standing supercrystals in bulk solution is also described.
This method offers tremendous advantages of mass pro-
duction of supercrystals in solution for easy collection and
large-area deposition of supercrystals on a substrate to
facilitate the availability of supercrystals for their prop-
erty investigations. Some useful properties of supercrystals,
particularly their catalytic activities, and future research
directions are also given.
Supercrystals fabricated from diverse
polyhedral metal and semiconductor
nanocrystals
To best illustrate the morphological diversity achievable
for supercrystals, it is necessary to use a variety of poly-
hedral building blocks with the same or similar solution
environment including solvent and capping agents. This
requirement limits the number and composition of nano-
materials available as building blocks. Polyhedral particles
made with a series of shape evolution and uniform sizes of
tens of nanometers such as Au, Au—Pd, and PbS nanocrys-
tals are ideal for such demonstration [3,7,8,12]. Previously
gold nanocubes, octahedra, truncated octahedra, and rhom-
bic dodecahedra with sizes of tens of nanometers have
been used as building blocks to form micrometer-sized
supercrystals by slowly evaporating a water droplet on a
substrate placed inside a vial containing water [30,31].
Supercrystals were generated by placing the vial in an
oven set at different temperatures. The droplet withdrawn
from a centrifuged tube contains a high concentration
of nanocrystals and a sufficiently high concentration of
cetyltrimethylammonium chloride (CTAC) surfactant. Fig. 1
illustrates the variety of supercrystals formed using these
building blocks. Supercrystals can be obtained at various
droplet evaporation temperatures, although a higher tem-
perature (e.g. 90 ◦
C) favors the formation of high-quality
supercrystals. Nanocubes form roughly cubic supercrystals.
Rhombic dodecahedra were assembled into truncated tri-
angular pyramidal supercrystals. Rhombic dodecahedral,
octahedral, and hexapod-shaped supercrystals were pro-
duced from the assembly of octahedra. Corner-truncated
octahedra formed mostly octahedral, truncated triangular
pyramidal, and square pyramidal supercrystals. Remarkably,
supercrystals are evenly scattered over the entire substrate
surface covered by the evaporating droplet, rather than con-
centrated toward the perimeter of the droplet, suggesting
that multiple supercrystals are formed by rapidly assembling
nearby particles and then settling on the substrate (Fig. 2a).
Despite the exhibited structural variety of the synthesized
supercrystals, there is only one packing arrangement iden-
tified for each nanocrystal shape, as shown in Fig. 2. Such
packing arrangements make maximum contacts with neigh-
boring nanocrystals and should be the most stable assembly
structures. For cubes with slight corner truncation, a vari-
ant packing structure in which a single cube residing at the
cross-section of four cubes underneath is also frequently
observed.
Superstructures and supercrystals fabricated from the
assembly of various metal and semiconductor nanocrys-
tals have been reported. Fig. 3 summarizes some of these
examples. Tan et al. used N-hexadecylpyridinium chlo-
ride (CPC)-capped rhombic dodecahedral Au nanocrystals
to form triangular superstructures with the same packing
arrangement as shown in Fig. 2 by depositing the nanocrys-
tal droplet to a vertically aligned silicon wafer and slowly
evaporating the solution for 40 h under high humidity [32].
An optimal CPC concentration of 10 mM was found to yield
the superstructures. Pd nanocubes with an edge length of
27 nm dispersed in an aqueous solution with a cetyltrimethy-
lammonium bromide (CTAB) concentration of 25 mM have
been used to grow into cubic supercrystals (Fig. 3a) [33].
The nanocrystal solution placed in a closed vial was com-
pletely vaporized in 12 h at room temperature to obtain
the supercrystals. Petit and coworkers synthesized corner-
truncated Pt cubes with sizes of ∼5 nm in the presence of
tetrakis(decyl)ammonium bromide (TDAB) and alkylamine in
toluene [34]. The nanocubes were used to assemble into
large supercrystals on a substrate with square pyramidal and
triangular (or truncated tetrahedral) shapes by slowly evap-
orating toluene over the substrate over a period of 8 days
(Fig. 3b and c). The formation of square pyramidal super-
crystals can be understood by stacking the next layer of
truncated cubes at the interstitial sites formed from reg-
ular packing of the first layer of cubes. This way the square
area of the upper layer of cubes is slightly smaller than the
lower layer and eventually leads to the formation of a square
pyramid. What is puzzling is how cubes can form triangular
or truncated tetrahedral supercrystals. The answer lies on
the normally invisible shell or coating of surfactant on the
surface of truncated cubes as seen in the inset of Fig. 4. The

3. Formation of supercrystals through self-assembly 83
Figure 1 (a—e) Supercrystals possessing diverse geometric shapes have been generated from the assembly of various polyhedral
gold nanocrystals. Supercrystals with a higher degree of structural perfection were generally obtained at a droplet evaporation
temperature of 90 ◦
C. Scale bars are equal to 1 ␮m.
Reproduced from Ref. [30].
surfactant layer can make the particles appear less cubic,
and its influence is magnified for smaller particles. Inter-
estingly, 13-nm PbS nanocubes, coated with oleylamine and
oleic acid to form a ligand layer of ∼2 nm, have been con-
sidered important in the generation of supercrystals with
a 45◦
-tilted face-centered-cubic (fcc) packing arrangement
[35]. Assembly by ultra-large building blocks are relatively
unaffected by the presence of even long-chain polymer
capping species such as poly(vinyl pyrrolidone) (PVP) [36].
The inset figure in Fig. 3b shows that the CTAB gap space
between truncated cubes is significant. This situation can
lead to drastic deviation from 90◦
packing arrangement nor-
mally expected for cubes. Fig. 4 presents packing structures
of PbS truncated cubes with angles widely different from
90◦
. In addition, it has been revealed that the corner of a
PbS truncated cube, rather than its {1 0 0} face, can land

4. 84 M.H. Huang, S. Thoka
Figure 2 (a) SEM images showing that supercrystals fabricated from the assembly of gold truncated octahedra are evenly dis-
tributed throughout the entire substrate surface. The red dotted line indicates the edge of the evaporating droplet. (b—d) SEM
images of single supercrystals and high-magnification SEM images of the marked square regions showing nanocrystal packing arrange-
ments. Models of supercrystals constructed from cubic, rhombic dodecahedral, and octahedral gold nanocrystals are also presented.
Insets give the corresponding Fourier transform patterns of the high-magnification SEM images.
Reproduced from Ref. [30].
on the interstitial site to increase structural diversity of
packed truncated cubes [12]. Octahedral MnO nanocrystals
synthesized in the presence of trioctylamine (TOA) and oleic
acid (OA) were dispersed in anhydrous ethanol and kept
in a sealed bottle for 10—30 h to collect the precipitate
[37]. Supercrystals were obtained as the precipitate with
the same packing arrangement for the octahedra as shown in
Fig. 2 (see Fig. 3d). This study hints that solvent evaporation
is not always necessary to promote the growth of supercrys-
tals. A drop of concentrated octahedral PbS nanocrystals
synthesized in aqueous solution in the presence of CTAB was
slowly evaporated [38]. Triangular and hexagonal plate-like
supercrystals were generated on the substrate surface after
slow solvent evaporation (Fig. 3e and f). The supercrystal
formation process is very similar to that used to make Au
supercrystals, and the octahedra packing arrangement is the
same as that seen in Au supercrystals. Thus, metal and semi-
conductor supercrystals can all be produced under the same
fabrication condition.
In addition to the use of polyhedral nanocrystals as build-
ing blocks, nanorods and roughly spherical particles can also
be assembled to yield supercrystals with geometric shapes.

5. Formation of supercrystals through self-assembly 85
Figure 3 SEM images showing supercrystals fabricated from the assembly of various metal and semiconductor nanocrystals. (a)
A Pd nanocube-assembled cubic supercrystal. Reproduced from Ref. [33]. (b and c) SEM images of square pyramidal and triangular
supercrystals assembled by corner-truncated Pt nanocubes. Insets show TEM image of a truncated cube and SEM image of a single
triangular supercrystal. Reproduced from Ref. [34]. (d) SEM image of MnO octahedra-assembled supercrystals. Inset shows detailed
packing structure of MnO octahedra with a scale bar of 50 nm. Reproduced from Ref. [37]. (e and f) Triangular and hexagonal
plate-like supercrystals assembled by PbS octahedra. Reproduced from Ref. [38]. (g) Layers of short CdS nanorods stacked into a
large hexagonal superstructure. Reproduced from Ref. [39]. (h) Hexagonal sheets formed from the assembly of CdSe nanocrystals.
Reproduced from Ref. [44]. (i) A rhombic dodecahedral microcrystal formed through DNA-mediated Au nanoparticle crystallization.
Reproduced from Ref. [45].
For example, short CdS nanorods with a length of ∼30 nm
synthesized in the presence of n-octadecylphosphonic acid
(ODPA), n-octylphosphine oxide (TOPO), and trioctylphos-
phine (TOP) have been used to form assembled structures
[39]. CdS nanorods dispersed in toluene were loaded into a
vial, then a buffer layer of 2-propanol was introduced. Next,
a layer of methanol was carefully added. The sealed vial
was left undisturbed for 12 days to collect the superstruc-
tures formed. The CdS nanorods were stacked into layers
and formed hexagonal platelike superstructures (Fig. 3g).
In another study, a chloroform solution of 28 nm CdSe—CdS
core—shell nanorods capped with octylamine and ODPA
was mixed with an aqueous solution containing dodecyl
trimethylammonium bromide (DTAB) [40]. After evaporation
of chloroform and injection of the nanorod micelle solu-
tion into a flask containing ethylene glycol, vigorous stirring
of the solution leads to nanorod aggregation and super-
particle formation. The superparticles have a wheel-like
structure with different nanorod packing orientations for
the tread and the side wall. Au nanorods can also align and
assemble into ribbon-like structures through multiple-layer
stacking of the rods [41]. Large 3-dimensional supercrystals
of Au—Ag nanorods mediated by Gemini surfactants have
also been achieved [42]. And short hexagonal CuIn1−xGaxS2
(CIGS) nanorods functionalized with 1-dodecanethiol can
also pack into hexagonal supercrystals in 1-octadecene solu-
tion in the presence of TOPO [43]. Roughly spherical 3.5 nm
CdSe nanocrystals have also been reported to assemble into
hexagonal sheetlike structures using the bilayer or trilayer
solvent system (Fig. 3h) [44]. CdSe nanoparticles synthe-
sized in a TOPO—TOP or hexadecylamine (HDA)—TOPO—TOP
mixture were dissolved in toluene and added to a glass
tube. Next, 2-propanol was introduced as a buffer layer,
followed by the addition of methanol as non-solvent to
promote particle aggregation through slow methanol diffu-
sion. Although ultralarge sheets reaching 100 ␮m have been
produced and hence are visible by optical microscopy, the
crystallization process is extremely slow (2 months). More

6. 86 M.H. Huang, S. Thoka
Figure 4 SEM image of the assembled PbS truncated cubes.
Scale bar is equal to 100 nm. Inset show a single PbS truncated
cube with a shell of CTAB surfactant.
Reproduced from Ref. [12].
recently, superlattices with rhombic dodecahedral geometry
have been achieved through slow cooling of 20-nm spheri-
cal Au nanoparticles functionalized with complimentary DNA
linker strands (Fig. 3i) [45]. The Au nanoparticles have the
expected body-centered cubic (bcc) packing. Addition of
15-nm Au nanoparticles gives CsCl packing symmetry. The
programmed slow cooling of 0.1 ◦
C per 10 min from 55 ◦
C
to 25 ◦
C is necessary to yield superlattices with a polyhedral
shape. A rhombic dodecahedral packing structure was found
to be most thermodynamically stable and was the observed
structure. Despite the success at making supercrystals using
spherical nanoparticle as building blocks, achieving mor-
phological diversity of supercrystals still requires the use
of various polyhedral nanocrystals. Lastly, another class of
superlattice construction is the formation of 1-dimensional
chains through the aligned attachment of nanorods or
nanoplates [46—51]. For example, pyridine-capped CdSe
nanorods form chainlike structures via side-by-side align-
ment of the rods [46]. Ultrathin CdSe square nanoplatelets
capped with oleic acid can form columnar stacking when
dispersed in a solvent mixture of hexane and ethanol [48].
Nicely, rhombic GdF3 nanoplates synthesized in the presence
of oleic acid and 1-octadecene can pack into multilayered
liquid crystalline structures on a substrate [49].
Observation of supercrystal formation process
Direct observation and recording of the supercrystal forma-
tion process is rarely available. Often only the final collected
products have been examined. Growth of supercrystals from
a concentrated nanocrystal droplet on a substrate pro-
vides a convenient way to observe the particle assembly
process by optical microscopy [30,31,52]. Previously trans-
mission X-ray microscopy has been employed to capture
the supercrystal growth process, but the imaged area is
more limited and the substrate needs to be mounted ver-
tically [30]. While direct observation of the supercrystal
formation process by environmental TEM is highly desirable
to clearly see the dynamic movement of individual parti-
cles, such study has not yet been carried out. A simple and
highly useful approach is to construct a closed chamber filled
partially with water to simulate the actual supercrystal for-
mation condition and observe the concentrated nanocrystal
droplet using optical microscopy. Optical microscopy offers
the advantage to examine a very large area of the droplet.
While individual particles are not visible, dynamic move-
ment of particles and changes in the solution color as a
result of plasmon coupling from particle aggregation can be
identified. Once supercrystals are produced, they are big
enough to be visible. Fig. 5 shows optical microscopy snap-
shots of the supercrystal formation process taken at various
time points with the droplet containing concentrated gold
rhombic dodecahedra being slowly evaporated in a moist
chamber [31]. The entire supercrystal growth process has
been video-recorded. Supercrystals are initially formed near
the perimeter or outer region of the droplet, as evidenced by
both the appearance of supercrystals and fading of the pur-
plish red solution color from the localized surface plasmon
resonance (LSPR) absorption of the Au particles (Fig. 5a).
Supercrystals in the inner or central region of the droplet
become clearly identifiable after 20 min into the process and
then grow rapidly in size. Near the end of the supercrystal
growth process, the purplish red solution color has largely
faded due to the incorporation of surrounding Au particles
into the supercrystals (Fig. 5d). Triangular supercrystals con-
structed from the assembly of Au rhombic dodecahedra were
found to evenly scatter over the whole substrate surface
covered by the evaporating droplet. Although supercrystal
formation proceeds extremely rapid and simultaneously at
many sites, a sufficiently long time is necessary to obtain
good supercrystals with well-defined geometric shapes.
Surfactant-directed supercrystal formation
From all the above examples illustrated, addition of sur-
factant or capping species is necessary for supercrystal
formation. Here we focus on the role of commonly used
surfactants such as CTAB and CTAC in directing the orga-
nized assembly of nanocrystals. Since supercrystals with
well-defined polyhedral geometries have been produced,
long-range global forces must be present, in addition to
local surfactant interactions between adjacent particles.
The driving force for the ordered assembly of nanocrys-
tals at sufficiently high surfactant concentrations should
come from the coordinated actions of bilayer micellar struc-
tures of surfactant to pack most efficiently to minimize
large polar head charges with reduced solution volume. By
using nearby nanocrystals to screen out the strongly positive
charges from CTA+
and arranging them in the most stable
3-dimensional structure, the surfactant molecules can be
densely packed and still achieve an overall stable state. For-
mation of supercrystals with ordered packing of nanocrystals
is the result of this highly coordinated action of surfactant
molecules. Because nanocrystals and surfactant molecules
become highly organized in a supercrystal, entropy should
decrease for this process, but solvent evaporation greatly
increases overall entropy of the system. Enthalpy is another
important consideration, since supercrystal formation is
thermodynamically favorable and happens spontaneously
given proper conditions. A demonstration of this coordi-
nated or cooperative surfactant action is their ability to

7. Formation of supercrystals through self-assembly 87
Figure 5 (a—f) Optical microscopy images of the supercrystal formation process taken at (a) 13, (b) 18, (c) 20, (d) 22, (e) 23,
and (f) 28 min in the droplet evaporation process using gold rhombic dodecahedra as the building blocks. (g and h) Large-area and
enlarged SEM images of the synthesized supercrystals. Inset shows a close-up view of the assembled rhombic dodecahedra.
Reproduced from Ref. [31].
sense or recognize surrounding particle shapes. Nanocrystals
of the same shape and similar sizes are readily incorpo-
rated into supercrystals. However, particles with different
shapes and sizes are excluded. Right bipyramids formed as
a byproduct during Au nanocube synthesis were found to
assemble among themselves (Fig. 6a) [8]. By intentionally
mixing Au nanocubes of different sizes together, the larger
and smaller cubes also form their own assembled structures
(Fig. 6b). The issue of surfactant-mediated particle shape
recognition during particle assembly is not present when
model spheres are used to consider interparticle attraction
forces, but the effect is revealed when non-spherical parti-
cles are employed. In some sense, the surfactant-directed
organized nanocrystal packing leading to supercrystal for-
mation and the growth of surfactant/copolymer-templated
mesostructured silica involve similar formation mechanisms
[53—55]. In mesostructured silica, surfactant molecules
are packed densely into micellar structures yet avoid
the repulsive interactions by using silica to screen
out their surface charges. Interestingly, the connection
between these surfactant-mediated systems has been par-
tially demonstrated by forming periodically ordered gold
nanocrystal/silica mesophase [56]. Surfactant-templated
mesostructured silica crystals possessing a rhombic dodeca-
hedral shape have also been synthesized, further suggesting
that the formation mechanisms for supercrystals and
mesostructured materials are similar [57,58].
Since nanocrystals in a supercrystal are surrounded by
surfactant bilayer, its presence should be verified. A CTAB
bilayer thickness has been determined to be 3.2 ± 0.2 nm

8. 88 M.H. Huang, S. Thoka
Figure 6 (a and b) SEM images showing the shape-guided
effect in nanocrystal assembly. Right bipyramids formed in the
synthesis of gold nanocubes can assemble into their own pack-
ing structure, while nanocubes assemble among themselves.
By mixing gold nanocubes with two different sizes together,
the larger and smaller nanocubes spontaneously form their
own packing structures. (c) Low-angle XRD patterns of rhom-
bic dodecahedral supercrystals assembled by gold octahedra
(panel a), triangular supercrystals assembled by gold rhombic
dodecahedra (panel b), dried CTAC surfactant (panel c), and
washed supercrystals to remove the surfactant completely from
the supercrystals and the substrate (panel d).
Reproduced from Refs. [30,31].
with partial inter-digitation of the aliphatic chains [59].
In another study, an interparticle spacing of 3.4 nm with
inter-digitation of CTAB tails has been reported [60]. Taking
low-angle X-ray diffraction (XRD) patterns of supercrystals
is a convenient way to confirm the presence of surfac-
tant. Fig. 6c gives low-angle XRD patterns of supercrystals
assembled from octahedral and rhombic dodecahedral gold
nanocrystals [31]. XRD pattern of dried CTAC was also
taken for comparison. Although the XRD patterns of rhombic
dodecahedral and triangular supercrystals look different,
upon close examination, both patterns bear some similar-
ity to that of CTAC. Certain peaks having possibly the same
origin are connected with dotted lines. Interestingly, other
than differences in the relative peak intensity and slight
shifts in their positions, essentially all the peaks recorded
for the rhombic dodecahedral supercrystals (panel a) are
present in the XRD pattern of CTAC. The first two reflection
peaks recorded for the triangular supercrystals (panel b) are
also present in CTAC. The peak shifts recorded for the super-
crystals may arise from the way surfactant is packed within
the supercrystals. When surfactant was removed by washing
supercrystals of both shapes with water, all the signature
peaks of CTAC disappeared. The results provide convinc-
ing evidence of the presence of CTAC surfactant within the
supercrystals. Since surfactant is densely and orderly packed
inside supercrystals, the notion of depletion attraction or
force used to describe aggregation of nanoparticles is not
applicable to explain the formation of supercrystals in the
presence of surfactant, where removal of the added species
or depletant from the space between adjacent particles cre-
ates a force to pull nearby particles together [61,10,62].
Synchrotron small angle scattering (SAXS) images can also
provide useful information about the nanocrystal shape ori-
entation and the gap distance between particles within a
supercrystal [63]. SAXS patterns are also useful for identi-
fying the emergence of ordered nanocrystal packing [64].
In addition to the use of XRD patterns to establish the
presence of surfactant inside supercrystals, TEM images of
the assembled nanocrystals offer direct visual evidence of
the existence of surfactant between the particles. Inset of
Fig. 3b shows clear separation of nanocubes by an amor-
phous gap. The gap distance should correspond to the bilayer
length of surfactant or capping agent.
Formation of supercrystals by surfactant
diffusion approach
Solvent evaporation of a droplet containing concentrated
nanocrystals and a sufficient amount of surfactant is gener-
ally used to obtain supercrystals. Solvent evaporation slowly
reduces the solution volume and thus increases the con-
centrations of surfactant and nanocrystals to promote their
cooperative interactions. The limitation with this method is
that the generated supercrystals are confined to the area of
the evaporating droplet. Removal of supercrystals from one
substrate and their transfer to another substrate for analy-
sis and measurements with preservation of the supercrystal
geometry can present difficulty [30]. Direct formation of
supercrystals in bulk aqueous solution within a relatively
short period of time (that is, in hours, not days or weeks) is
highly desirable. Recognizing that supercrystals are formed
with increasing surfactant concentration, a novel surfac-
tant diffusion approach to produce supercrystals has been
demonstrated [31]. As shown in Fig. 7a, to 100 ␮L of the con-
centrated colloidal solution in an Eppendorf tube was gently
added 200 ␮L of 1.0 M CTAC solution without disturbance.
This keeps the CTAC solution and the colloidal solution
separated into two layers. CTAC gradually diffuses to the
lower layer to increase the surfactant concentration in the
nanocrystal solution. After 12 h, the red colloidal solution
turns colorless and supercrystals have been produced at the
bottom of the tube as dark precipitate. Supercrystals should
be collectable in less than 12 h, so this is an efficient method
for making a large quantity of supercrystals. Fig. 7b shows
an optical micrograph of numerous rhombic dodecahedral
supercrystals obtained from the assembly of octahedral Au
nanocrystals with sizes of less than 1—4 ␮m. The supercrys-
tals display metallic golden luster because of their large
sizes. It was found that instant introduction of the same

9. Formation of supercrystals through self-assembly 89
Figure 7 (a) Schematic drawing of the diffusion transport of CTAC surfactant from the upper layer of CTAC solution to the lower
Au nanocrystal solution to form supecrystals which eventually settle to the bottom of the vial. (b) Optical micrograph of rhombic
dodecahedral supercrystals assembled by octahedral Au nanocrystals. (c) Optical micrograph over a very large area of a substrate
showing the evenly distributed supercrystals.
Reproduced from Ref. [31].
amount of CTAC into the concentrated nanocrystal solu-
tion resulted in only random aggregation of the particles,
showing that gradual increase of surfactant concentration is
necessary and supercrystal formation takes time to evolve
into their final symmetrical structures.
Since solvent evaporation is the major entropy-increasing
process in the formation of supercrystals through droplet
evaporation, the surfactant diffusion transport approach to
growing supercrystals without solvent evaporation suggests
that organized nanocrystal and surfactant assembly is not
necessarily entropy-driven. Surfactant diffusion, however, is
entropy-driven. Rather, supercrystal formation is the more
thermodynamically or energetically stable state, because
the large repulsive charges on the surfactant is greatly
reduced when micellar bilayers are precisely screened
by the nanocrystals. Although enthalpy change should be
important for the process, one cannot feel any temperature
change to the vial. This is understandable, because super-
crystal formation involves mainly surfactant organization,
not bond formation or breaking in typically crystallization
processes. Again consideration of only local surfactant inter-
actions between adjacent nanocrystals is insufficient for
understanding supercrystal growth. A sufficient amount of
time is needed for particles to pack with correct orienta-
tion and reach very large dimensions. Similarly, formation
of mesostructured silica does not happen quickly.
The surfactant diffusion approach enables the growth and
deposition of supercrystals on a large portion of a substrate
immersed into the nanocrystal solution. Fig. 7c presents
an optical microscopic image of supercrystals grown on a
Si wafer from the assembly of gold rhombic dodecahedra
[31]. Octahedral, square pyramidal, and triangular pyra-
midal supercrystals have been deposited on the substrate.
The supercrystals are evenly distributed over the entire
substrate, so ultralarge-area deposition of supercrystals is
feasible. Supercrystals can be deposited on any substrate,
so a supercrystal-modified substrate can potentially function
as an electrode for electrochemical reactions.
Applications of supercrystals
Most studies on the preparation of supercrystals have
mainly focused their discussion on the packing arrange-
ments of the building blocks and the forces involved to
yield supercrystals and superlattices. Less effort has been
devoted to demonstrate properties and applications of
the obtained supercrystals. However, electronic proper-
ties of Au supercrystals examined using scanning tunneling
microscopy have been reported [65,66]. Mechanical proper-
ties of PbS nanocrystal-packed supercrystals have also been
studied [67]. The intimately contacting nanocrystals within a
supercrystal naturally create many ‘‘hot spots’’ with ampli-
fied local electromagnetic field upon irradiation of light
with wavelengths matching the plasmon resonance of the
nanocrystals, and this is favorable for the surface-enhanced
Raman scattering (SERS) detection of adsorbed molecules.
Supercrystals and superstructures from the assembly of gold

10. 90 M.H. Huang, S. Thoka
Figure 8 (a) EDS elemental mapping image of a microtomed thin film of a supercrystal assembled from gold octahedra with the
incorporation of Pd nanoparticles. (b) Cyclic voltammograms of Au supercrystals (SC) and a monolayer (ML) film assembled from
octahedral gold nanocrystals on an ITO glass electrode in a solution containing 0.1 M NaOH and 0.01 M glucose for glucose oxidation.
Reproduced from Refs. [30,31].
nanocubes, octahedra, and rhombic dodecahedra have been
used as substrates for SERS detection of p-mercaptoaniline
[32]. A higher SERS intensity has been recorded for the rhom-
bic dodecahedral superstructures, attributed to more hot
spots present from the ordered packing of nanocrystals. Inci-
dentally, gold rhombic dodecahedra dispersed in aqueous
solution have also been found to be more sensitive SERS sub-
strates than gold nanocubes and octahedra [68]. Arrays of
pyramids constructed from the packing of gold nanoparticles
have also been shown to produce enhanced SERS intensities
of adsorbed 1-naphthalenethiol [69]. The SERS intensity is
highest toward the tip of the pyramid. SERS detection of
carbon monoxide bonded to an iron porphyrin attached to
the surface of the pyramid was also demonstrated. In addi-
tion, supercrystals fabricated from dense assembly of short
gold nanorods have been demonstrated as an active SERS
substrate for prion detection in human blood [70].
Supercrystals constructed from noble metal nanoparti-
cles can be considered as catalysts or a catalyst support
with well-defined pores and channels for molecular trans-
port. Of course, catalysis can also occur on the exterior
surfaces of the supercrystals. To clearly probe the accessi-
bility of the interior of supercrystals to molecular transport
for catalytic reactions, it is necessary to eliminate cat-
alytic reactions taking place on the exterior surfaces of
supercrystals. Toward this end, supercrystals formed from
the assembly of gold octahedra with potentially the largest
interior pores between particles as seen in Fig. 2 were
loaded with a H2PdCl4 solution, washed to remove Pd pre-
cursor on the supercrystal surfaces, and finally immersed
in an ascorbic acid solution to reduce the precursor form-
ing Pd nanoparticles solely inside the supercrystals [30]. A
cross-sectional elemental mapping image of the supercrystal
revealing evenly distributed formation of Pd nanoparticles
inside the supercrystal is shown in Fig. 8a. The interior Pd
particles were active at catalyzing a Suzuki coupling reac-
tion between iodobenzene and phenylboronic acid forming
biphenyl product, demonstrating that molecular transport
inside the supercrystals is feasible, and the pore spaces and
channels are accessible. However, slow reagent diffusion
into the supercrystal interior can lower its overall reac-
tivity. The presence of surfactant adversely affects facile
molecular transport, but complete removal of surfactant
can lead to partial collapse of the supercrystals. Slight
fusion of the metal nanocrystals, as observed particularly for
supercrystals constructed from gold octahedra with {1 1 1}
faces, may yield a more rigid framework structure robust
enough to withstand destruction by surfactant removal
[30,31]. The resulting porous gold structure may become
highly active catalysts with an exceptionally high surface
area [71,72]. To greatly improve the catalytic efficiency,
it should be more desirable to make supercrystals assem-
bled from Pd nanocrystals. For the Au supercrystals, one
can try known Au nanocrystal-catalyzed reactions and eval-
uate the efficiency of Au supercrystals acting as the catalyst
[73,74].
Because supercrystals can be deposited on a substrate,
they may be deposited on an electrode surface to form a
modified electrode for the examination of electrocatalytic
activity of supercrystals. A gold octahedra droplet forming
supercrystals has been deposited on an ITO electrode for
electrochemical oxidation of glucose [31]. The same amount
of Au octahedra forming a monalyer of assembled particle
film on an ITO electrode was also tested. Cyclic voltam-
mograms (CV) of the supercrystals and the monolayer film
placed in a solution containing 0.1 M NaOH and 0.01 M glu-
cose are provided in Fig. 8b. Both samples displayed good
electrocatalytic activity, but the oxidation current was much
higher for the monolayer film than for the supercrystals
because the monolayer film has a larger exposed surface
area. Nevertheless, the idea of using supercrystals as simple
and stable conductive electrode has been demonstrated.
Conclusion and outlook
In contrast to conventional organized nanoparticle super-
structures produced from spherical building blocks of single
or multiple components, fabrication of supercrystals from
diverse polyhedral metal and semiconductor nanocrystals

11. Formation of supercrystals through self-assembly 91
offers a new dimension to nanoparticle assembly with
geometrically symmetric packing of the particles. Their
high 3-dimensional symmetry suggests their formation with
more globally balanced forces from all directions. Surfac-
tant or capping molecules at high concentrations mediate
nanocrystal assembly by residing between particles with
approximately the same size and shape to effectively min-
imize their repulsive charges or interactions, such that
polyhedral nanocrystals are packed into a configuration
having the maximum surface contact. The supercrystal for-
mation process has been recorded in real time by optical
microscopy showing rapid movement and incorporation of
surrounding nanocrystals into the nearby developing super-
crystals, and that the initially formed supercrystals are
concentrated around the droplet edge. More insights of the
supercrystal formation process may be obtained by improv-
ing the optical microscopy resolution, or by following the
process with the use of an environmental TEM chamber.
A novel surfactant diffusion approach to making super-
crystals in aqueous solution without water evaporation has
been developed. Free-standing supercrystals dispersed in
solution can be collected on a substrate. Supercrystals of
various compositions can be prepared this way. With regard
to further developments in the growth of supercrystals,
size control of supercrystals is an interesting direction that
has essentially not been addressed. The goals, similar to
challenges in nanocrystal synthesis, are to make super-
crystals with tunable sizes and the smallest supercrystals
by adjusting the amounts of nanocrystals and surfactant
used. Success in regulating supercrystal size should further
demonstrate the importance of a globally balanced state,
in addition to short-ranged forces, in supercrystal growth.
By making supercrystals fairly easy to form, examinations
of properties and applications of supercrystals can be more
readily executed. Because diverse compositions and mor-
phologies of nanocrystals can be regularly packed using the
surfactant-mediated assembly process, supercrystals with
novel optical/photonic, electrical, catalytic, and sensing
properties can be expected.
Acknowledgments
We thank the Ministry of Science and Technology of Taiwan
for the support of this work (NSC101-2113-M-007-018-MY3
NSC102-2633-M-007-002, and MOST103-2633-M-007-001).
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Michael H. Huang obtained his B.A. degree
in chemistry from Queens College in 1994,
and his Ph.D. degree from the Department of
Chemistry and Biochemistry at UCLA in 1999.
After postdoctoral research at UC Berke-
ley and UCLA, he joined the Department of
Chemistry at NTHU in 2002. He was pro-
moted to associate professor in 2006, and
then to professor in 2010. His current research
focus is on the shape-controlled synthesis of
nanocrystals and the examination of their
facet-dependent properties. He has received a number of awards,
including the Outstanding Research Award from the National Sci-
ence Council of Taiwan in 2012. Since 2014, he has been a member
of the Editorial Board of Chemistry — An Asian Journal.
Subashchandrabose Thoka received a B.Sc.
degree from Sri Krishnadevaraya University,
India, in 2007 and a M.Sc. degree from Depart-
ment of Chemistry at National Institute of
Technology Warangal, India, in 2010. Start-
ing from 2014, he is pursuing his Ph.D. degree
from National Tsing Hua University under the
supervision of Prof. Michael H. Huang. His
research interests include shape-controlled
synthesis of metal nanocrystals and their self-
assembly to form supercrystals.