Nano Today (2015) 10, 81—92

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REVIEW

Formation of supercrystals throughself-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 2015Available online 20 February 2015

KEYWORDSNanocrystals;Self-assembly;Supercrystals;Superlattices;Superstructures;Surfactant

Summary Compared to the number of reports on the self-assembly of spherical nanoparticlesforming superlattices, relatively fewer studies have addressed the assembly of polyhedral metaland semiconductor nanocrystals for the formation of supercrystals with well-defined geometricshapes. These polyhedral supercrystals are considered as a new class of superlattice struc-tures in which particle morphology strongly dictates the shapes of resulting supercrystals ifthe particles are larger than 20 nm. This review provides examples and advances in fabricatingsupercrystals on a substrate during the process of solvent evaporation and through diffusiontransport of surfactant to generate free-standing supercrystals. The diversity of supercrystalmorphologies observed is illustrated. In many cases, the supercrystal formation process hasbeen found to be surfactant-mediated with surfactant molecules residing between adjacentnanocrystals. 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. Effortshave been made to record the entire supercrystal formation process. Finally, some results ofproperties of supercrystals and future directions for supercrystal research are provided.© 2015 Elsevier Ltd. All rights reserved.

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Introduction

Advances in the syntheses of polyhedral metal and semicon-

ductor nanocrystals with excellent size and shape controlhave not only given us valuable particles for the examinationof 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).

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http://dx.doi.org/10.1016/j.nantod.2015.01.0061748-0132/© 2015 Elsevier Ltd. All rights reserved.

ed to the observation of their self-assembled structuresn substrates [1—20]. Because of their spontaneous orga-ization after particle formation, it becomes necessary toescribe the superlattice structures formed. Previously theocus of extensive studies on the assembly of nanoparticlesnvolves mostly spherical particles of single or multiple sizes21—27]. The particles therefore can be viewed as artifi-

ial atoms forming lattice structures which resemble thoseeen in unit cells of metals and binary compounds with face-entered cubic (fcc) and body-centered cubic (bcc) packingrrangements. Conversion from one packing structure to

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nother is also possible with spherical particles by vary-ng the temperature [24,25]. While assembly by polyhedralarticles should involve similar mechanisms and forces, theffect of particle shape becomes important, and accommo-ation of small spheres or polyhedra filling the interstitialpaces can be more difficult or show no positional unifor-ity. Their packing arrangements therefore deviate from

hose of spherical building blocks. An exception may be theoassembly of nanospheres and very short nanorods withurved ends to form binary superlattices [28]. Another inter-sting aspect of polyhedral nanoparticle assembly is theormation of supercrystals with geometric shapes, whichre much less observed or discussed. These features makesupercrystals organized by polyhedra a new class of super-tructures that can yield different structural diversity andllow the exploration of pore accessibility and novel physicalroperties.

This review focuses on the formation of supercrystalsith geometric shapes from the self-assembly of inorganicolyhedral nanocrystals. The scope of discussion is some-hat different from that of superlattices constructed fromon-spherical building blocks, in which the formation ofrganized superstructures with well-defined morphologiess not emphasized [29]. Supercrystals fabricated from thessemblies of various building blocks and their packingtructures are presented. Direct observation of the super-rystal formation process during droplet evaporation isrovided. The forces involved in the formation of superlat-ices have been extensively discussed in the literature, soere the role of surfactant and the particle shape effectsre emphasized to show how the use of simple surfactantan effectively yield polyhedral supercrystals in aqueousolution. A novel surfactant diffusion approach to generateree-standing supercrystals in bulk solution is also described.his method offers tremendous advantages of mass pro-uction of supercrystals in solution for easy collection andarge-area deposition of supercrystals on a substrate toacilitate the availability of supercrystals for their prop-rty investigations. Some useful properties of supercrystals,articularly their catalytic activities, and future researchirections are also given.

upercrystals fabricated from diverseolyhedral metal and semiconductoranocrystals

o best illustrate the morphological diversity achievableor supercrystals, it is necessary to use a variety of poly-edral building blocks with the same or similar solutionnvironment including solvent and capping agents. Thisequirement limits the number and composition of nano-aterials available as building blocks. Polyhedral particlesade with a series of shape evolution and uniform sizes of

ens of nanometers such as Au, Au—Pd, and PbS nanocrys-als are ideal for such demonstration [3,7,8,12]. Previouslyold nanocubes, octahedra, truncated octahedra, and rhom-ic dodecahedra with sizes of tens of nanometers have

een used as building blocks to form micrometer-sizedupercrystals by slowly evaporating a water droplet on aubstrate placed inside a vial containing water [30,31].upercrystals were generated by placing the vial in an

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M.H. Huang, S. Thoka

ven set at different temperatures. The droplet withdrawnrom a centrifuged tube contains a high concentrationf nanocrystals and a sufficiently high concentration ofetyltrimethylammonium chloride (CTAC) surfactant. Fig. 1llustrates the variety of supercrystals formed using theseuilding blocks. Supercrystals can be obtained at variousroplet evaporation temperatures, although a higher tem-erature (e.g. 90 ◦C) favors the formation of high-qualityupercrystals. Nanocubes form roughly cubic supercrystals.hombic dodecahedra were assembled into truncated tri-ngular pyramidal supercrystals. Rhombic dodecahedral,ctahedral, and hexapod-shaped supercrystals were pro-uced from the assembly of octahedra. Corner-truncatedctahedra formed mostly octahedral, truncated triangularyramidal, and square pyramidal supercrystals. Remarkably,upercrystals are evenly scattered over the entire substrateurface covered by the evaporating droplet, rather than con-entrated toward the perimeter of the droplet, suggestinghat multiple supercrystals are formed by rapidly assemblingearby particles and then settling on the substrate (Fig. 2a).espite the exhibited structural variety of the synthesizedupercrystals, there is only one packing arrangement iden-ified for each nanocrystal shape, as shown in Fig. 2. Suchacking arrangements make maximum contacts with neigh-oring nanocrystals and should be the most stable assemblytructures. For cubes with slight corner truncation, a vari-nt packing structure in which a single cube residing at theross-section of four cubes underneath is also frequentlybserved.

Superstructures and supercrystals fabricated from thessembly of various metal and semiconductor nanocrys-als have been reported. Fig. 3 summarizes some of thesexamples. Tan et al. used N-hexadecylpyridinium chlo-ide (CPC)-capped rhombic dodecahedral Au nanocrystalso form triangular superstructures with the same packingrrangement as shown in Fig. 2 by depositing the nanocrys-al droplet to a vertically aligned silicon wafer and slowlyvaporating the solution for 40 h under high humidity [32].n optimal CPC concentration of 10 mM was found to yieldhe superstructures. Pd nanocubes with an edge length of7 nm dispersed in an aqueous solution with a cetyltrimethy-ammonium bromide (CTAB) concentration of 25 mM haveeen used to grow into cubic supercrystals (Fig. 3a) [33].he nanocrystal solution placed in a closed vial was com-letely vaporized in 12 h at room temperature to obtainhe supercrystals. Petit and coworkers synthesized corner-runcated Pt cubes with sizes of ∼5 nm in the presence ofetrakis(decyl)ammonium bromide (TDAB) and alkylamine inoluene [34]. The nanocubes were used to assemble intoarge supercrystals on a substrate with square pyramidal andriangular (or truncated tetrahedral) shapes by slowly evap-rating toluene over the substrate over a period of 8 daysFig. 3b and c). The formation of square pyramidal super-rystals can be understood by stacking the next layer ofruncated cubes at the interstitial sites formed from reg-lar packing of the first layer of cubes. This way the squarerea of the upper layer of cubes is slightly smaller than theower layer and eventually leads to the formation of a square

yramid. What is puzzling is how cubes can form triangularr truncated tetrahedral supercrystals. The answer lies onhe normally invisible shell or coating of surfactant on theurface of truncated cubes as seen in the inset of Fig. 4. The

Formation of supercrystals through self-assembly 83

Figure 1 (a—e) Supercrystals possessing diverse geometric shapes have been generated from the assembly of various polyhedralgold nanocrystals. Supercrystals with a higher degree of structural perfection were generally obtained at a droplet evaporation

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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 andoleic acid to form a ligand layer of ∼2 nm, have been con-

sidered important in the generation of supercrystals witha 45◦-tilted face-centered-cubic (fcc) packing arrangement[35]. Assembly by ultra-large building blocks are relativelyunaffected by the presence of even long-chain polymer

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apping species such as poly(vinyl pyrrolidone) (PVP) [36].he inset figure in Fig. 3b shows that the CTAB gap spaceetween truncated cubes is significant. This situation canead to drastic deviation from 90◦ packing arrangement nor-

ally expected for cubes. Fig. 4 presents packing structures

f PbS truncated cubes with angles widely different from0◦. In addition, it has been revealed that the corner of abS truncated cube, rather than its {1 0 0} face, can land

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) SEMimages 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.I highR

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nsets give the corresponding Fourier transform patterns of theeproduced from Ref. [30].

n the interstitial site to increase structural diversity ofacked truncated cubes [12]. Octahedral MnO nanocrystalsynthesized in the presence of trioctylamine (TOA) and oleiccid (OA) were dispersed in anhydrous ethanol and keptn a sealed bottle for 10—30 h to collect the precipitate37]. Supercrystals were obtained as the precipitate withhe same packing arrangement for the octahedra as shown in

ig. 2 (see Fig. 3d). This study hints that solvent evaporations not always necessary to promote the growth of supercrys-als. A drop of concentrated octahedral PbS nanocrystalsynthesized in aqueous solution in the presence of CTAB was

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-magnification SEM images.

lowly evaporated [38]. Triangular and hexagonal plate-likeupercrystals were generated on the substrate surface afterlow solvent evaporation (Fig. 3e and f). The supercrystalormation process is very similar to that used to make Auupercrystals, and the octahedra packing arrangement is theame as that seen in Au supercrystals. Thus, metal and semi-onductor supercrystals can all be produced under the same

abrication condition.

In addition to the use of polyhedral nanocrystals as build-ng blocks, nanorods and roughly spherical particles can alsoe assembled to yield supercrystals with geometric shapes.

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 triangularsupercrystals assembled by corner-truncated Pt nanocubes. Insets show TEM image of a truncated cube and SEM image of a singletriangular supercrystal. Reproduced from Ref. [34]. (d) SEM image of MnO octahedra-assembled supercrystals. Inset shows detailedpacking structure of MnO octahedra with a scale bar of 50 nm. Reproduced from Ref. [37]. (e and f) Triangular and hexagonalplate-like supercrystals assembled by PbS octahedra. Reproduced from Ref. [38]. (g) Layers of short CdS nanorods stacked into alarge hexagonal superstructure. Reproduced from Ref. [39]. (h) Hexagonal sheets formed from the assembly of CdSe nanocrystals.

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Reproduced from Ref. [44]. (i) A rhombic dodecahedral microcryReproduced from Ref. [45].

For example, short CdS nanorods with a length of ∼30 nmsynthesized 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 avial, then a buffer layer of 2-propanol was introduced. Next,a layer of methanol was carefully added. The sealed vialwas left undisturbed for 12 days to collect the superstruc-tures formed. The CdS nanorods were stacked into layersand formed hexagonal platelike superstructures (Fig. 3g).In another study, a chloroform solution of 28 nm CdSe—CdScore—shell nanorods capped with octylamine and ODPAwas mixed with an aqueous solution containing dodecyltrimethylammonium bromide (DTAB) [40]. After evaporationof chloroform and injection of the nanorod micelle solu-

tion into a flask containing ethylene glycol, vigorous stirringof the solution leads to nanorod aggregation and super-particle formation. The superparticles have a wheel-likestructure with different nanorod packing orientations for

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formed through DNA-mediated Au nanoparticle crystallization.

he tread and the side wall. Au nanorods can also align andssemble into ribbon-like structures through multiple-layertacking of the rods [41]. Large 3-dimensional supercrystalsf Au—Ag nanorods mediated by Gemini surfactants havelso been achieved [42]. And short hexagonal CuIn1−xGaxS2

CIGS) nanorods functionalized with 1-dodecanethiol canlso pack into hexagonal supercrystals in 1-octadecene solu-ion in the presence of TOPO [43]. Roughly spherical 3.5 nmdSe nanocrystals have also been reported to assemble intoexagonal sheetlike structures using the bilayer or trilayerolvent system (Fig. 3h) [44]. CdSe nanoparticles synthe-ized in a TOPO—TOP or hexadecylamine (HDA)—TOPO—TOPixture were dissolved in toluene and added to a glass

ube. Next, 2-propanol was introduced as a buffer layer,ollowed by the addition of methanol as non-solvent to

romote particle aggregation through slow methanol diffu-ion. Although ultralarge sheets reaching 100 �m have beenroduced and hence are visible by optical microscopy, therystallization process is extremely slow (2 months). More

86

Figure 4 SEM image of the assembled PbS truncated cubes.Scale bar is equal to 100 nm. Inset show a single PbS truncatedcR

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ube with a shell of CTAB surfactant.eproduced from Ref. [12].

ecently, superlattices with rhombic dodecahedral geometryave been achieved through slow cooling of 20-nm spheri-al Au nanoparticles functionalized with complimentary DNAinker strands (Fig. 3i) [45]. The Au nanoparticles have thexpected body-centered cubic (bcc) packing. Addition of5-nm Au nanoparticles gives CsCl packing symmetry. Therogrammed slow cooling of 0.1 ◦C per 10 min from 55 ◦Co 25 ◦C is necessary to yield superlattices with a polyhedralhape. A rhombic dodecahedral packing structure was foundo be most thermodynamically stable and was the observedtructure. Despite the success at making supercrystals usingpherical nanoparticle as building blocks, achieving mor-hological diversity of supercrystals still requires the usef various polyhedral nanocrystals. Lastly, another class ofuperlattice construction is the formation of 1-dimensionalhains through the aligned attachment of nanorods oranoplates [46—51]. For example, pyridine-capped CdSeanorods form chainlike structures via side-by-side align-ent of the rods [46]. Ultrathin CdSe square nanoplatelets

apped with oleic acid can form columnar stacking whenispersed in a solvent mixture of hexane and ethanol [48].icely, rhombic GdF3 nanoplates synthesized in the presencef oleic acid and 1-octadecene can pack into multilayerediquid crystalline structures on a substrate [49].

bservation of supercrystal formation process

irect observation and recording of the supercrystal forma-ion process is rarely available. Often only the final collectedroducts have been examined. Growth of supercrystals from

concentrated nanocrystal droplet on a substrate pro-ides a convenient way to observe the particle assemblyrocess by optical microscopy [30,31,52]. Previously trans-ission X-ray microscopy has been employed to capture

he supercrystal growth process, but the imaged area isore limited and the substrate needs to be mounted ver-

ically [30]. While direct observation of the supercrystalormation process by environmental TEM is highly desirableo clearly see the dynamic movement of individual parti-les, such study has not yet been carried out. A simple and

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M.H. Huang, S. Thoka

ighly useful approach is to construct a closed chamber filledartially with water to simulate the actual supercrystal for-ation condition and observe the concentrated nanocrystalroplet using optical microscopy. Optical microscopy offershe advantage to examine a very large area of the droplet.hile individual particles are not visible, dynamic move-ent of particles and changes in the solution color as a

esult of plasmon coupling from particle aggregation can bedentified. Once supercrystals are produced, they are bignough to be visible. Fig. 5 shows optical microscopy snap-hots of the supercrystal formation process taken at variousime points with the droplet containing concentrated goldhombic dodecahedra being slowly evaporated in a moisthamber [31]. The entire supercrystal growth process haseen video-recorded. Supercrystals are initially formed nearhe perimeter or outer region of the droplet, as evidenced byoth the appearance of supercrystals and fading of the pur-lish red solution color from the localized surface plasmonesonance (LSPR) absorption of the Au particles (Fig. 5a).upercrystals in the inner or central region of the dropletecome clearly identifiable after 20 min into the process andhen grow rapidly in size. Near the end of the supercrystalrowth process, the purplish red solution color has largelyaded due to the incorporation of surrounding Au particlesnto the supercrystals (Fig. 5d). Triangular supercrystals con-tructed from the assembly of Au rhombic dodecahedra wereound to evenly scatter over the whole substrate surfaceovered by the evaporating droplet. Although supercrystalormation proceeds extremely rapid and simultaneously atany sites, a sufficiently long time is necessary to obtain

ood supercrystals with well-defined geometric shapes.

urfactant-directed supercrystal formation

rom all the above examples illustrated, addition of sur-actant or capping species is necessary for supercrystalormation. Here we focus on the role of commonly usedurfactants such as CTAB and CTAC in directing the orga-ized assembly of nanocrystals. Since supercrystals withell-defined polyhedral geometries have been produced,

ong-range global forces must be present, in addition toocal surfactant interactions between adjacent particles.he driving force for the ordered assembly of nanocrys-als at sufficiently high surfactant concentrations shouldome from the coordinated actions of bilayer micellar struc-ures of surfactant to pack most efficiently to minimizearge polar head charges with reduced solution volume. Bysing nearby nanocrystals to screen out the strongly positiveharges from CTA+ and arranging them in the most stable-dimensional structure, the surfactant molecules can beensely packed and still achieve an overall stable state. For-ation of supercrystals with ordered packing of nanocrystals

s the result of this highly coordinated action of surfactantolecules. Because nanocrystals and surfactant moleculesecome highly organized in a supercrystal, entropy shouldecrease for this process, but solvent evaporation greatlyncreases overall entropy of the system. Enthalpy is another

mportant consideration, since supercrystal formation ishermodynamically favorable and happens spontaneouslyiven proper conditions. A demonstration of this coordi-ated or cooperative surfactant action is their ability to

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

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enlarged SEM images of the synthesized supercrystals. Inset shoReproduced from Ref. [31].

sense or recognize surrounding particle shapes. Nanocrystalsof the same shape and similar sizes are readily incorpo-rated into supercrystals. However, particles with differentshapes and sizes are excluded. Right bipyramids formed asa byproduct during Au nanocube synthesis were found toassemble among themselves (Fig. 6a) [8]. By intentionallymixing Au nanocubes of different sizes together, the largerand smaller cubes also form their own assembled structures(Fig. 6b). The issue of surfactant-mediated particle shaperecognition during particle assembly is not present whenmodel 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-directedorganized nanocrystal packing leading to supercrystal for-mation and the growth of surfactant/copolymer-templated

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close-up view of the assembled rhombic dodecahedra.

esostructured silica involve similar formation mechanisms53—55]. In mesostructured silica, surfactant moleculesre packed densely into micellar structures yet avoidhe repulsive interactions by using silica to screenut their surface charges. Interestingly, the connectionetween these surfactant-mediated systems has been par-ially demonstrated by forming periodically ordered goldanocrystal/silica mesophase [56]. Surfactant-templatedesostructured silica crystals possessing a rhombic dodeca-

edral shape have also been synthesized, further suggestinghat the formation mechanisms for supercrystals and

esostructured materials are similar [57,58].Since nanocrystals in a supercrystal are surrounded by

urfactant bilayer, its presence should be verified. A CTABilayer thickness has been determined to be 3.2 ± 0.2 nm

88

Figure 6 (a and b) SEM images showing the shape-guidedeffect in nanocrystal assembly. Right bipyramids formed in thesynthesis 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 theirown packing structures. (c) Low-angle XRD patterns of rhom-bic dodecahedral supercrystals assembled by gold octahedra(panel a), triangular supercrystals assembled by gold rhombicdodecahedra (panel b), dried CTAC surfactant (panel c), andwashed supercrystals to remove the surfactant completely fromtR

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he supercrystals and the substrate (panel d).eproduced from Refs. [30,31].

ith partial inter-digitation of the aliphatic chains [59].n another study, an interparticle spacing of 3.4 nm withnter-digitation of CTAB tails has been reported [60]. Takingow-angle X-ray diffraction (XRD) patterns of supercrystalss a convenient way to confirm the presence of surfac-ant. Fig. 6c gives low-angle XRD patterns of supercrystalsssembled from octahedral and rhombic dodecahedral goldanocrystals [31]. XRD pattern of dried CTAC was alsoaken for comparison. Although the XRD patterns of rhombicodecahedral and triangular supercrystals look different,pon close examination, both patterns bear some similar-ty to that of CTAC. Certain peaks having possibly the samerigin are connected with dotted lines. Interestingly, otherhan differences in the relative peak intensity and slight

hifts in their positions, essentially all the peaks recordedor the rhombic dodecahedral supercrystals (panel a) areresent in the XRD pattern of CTAC. The first two reflectioneaks recorded for the triangular supercrystals (panel b) are

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M.H. Huang, S. Thoka

lso present in CTAC. The peak shifts recorded for the super-rystals may arise from the way surfactant is packed withinhe supercrystals. When surfactant was removed by washingupercrystals of both shapes with water, all the signatureeaks of CTAC disappeared. The results provide convinc-ng evidence of the presence of CTAC surfactant within theupercrystals. Since surfactant is densely and orderly packednside supercrystals, the notion of depletion attraction ororce used to describe aggregation of nanoparticles is notpplicable to explain the formation of supercrystals in theresence of surfactant, where removal of the added speciesr depletant from the space between adjacent particles cre-tes a force to pull nearby particles together [61,10,62].ynchrotron small angle scattering (SAXS) images can alsorovide useful information about the nanocrystal shape ori-ntation and the gap distance between particles within aupercrystal [63]. SAXS patterns are also useful for identi-ying the emergence of ordered nanocrystal packing [64].n addition to the use of XRD patterns to establish theresence of surfactant inside supercrystals, TEM images ofhe assembled nanocrystals offer direct visual evidence ofhe existence of surfactant between the particles. Inset ofig. 3b shows clear separation of nanocubes by an amor-hous gap. The gap distance should correspond to the bilayerength of surfactant or capping agent.

ormation of supercrystals by surfactantiffusion approach

olvent evaporation of a droplet containing concentratedanocrystals and a sufficient amount of surfactant is gener-lly used to obtain supercrystals. Solvent evaporation slowlyeduces the solution volume and thus increases the con-entrations of surfactant and nanocrystals to promote theirooperative interactions. The limitation with this method ishat the generated supercrystals are confined to the area ofhe evaporating droplet. Removal of supercrystals from oneubstrate and their transfer to another substrate for analy-is and measurements with preservation of the supercrystaleometry can present difficulty [30]. Direct formation ofupercrystals in bulk aqueous solution within a relativelyhort period of time (that is, in hours, not days or weeks) isighly desirable. Recognizing that supercrystals are formedith increasing surfactant concentration, a novel surfac-

ant diffusion approach to produce supercrystals has beenemonstrated [31]. As shown in Fig. 7a, to 100 �L of the con-entrated colloidal solution in an Eppendorf tube was gentlydded 200 �L of 1.0 M CTAC solution without disturbance.his keeps the CTAC solution and the colloidal solutioneparated into two layers. CTAC gradually diffuses to theower layer to increase the surfactant concentration in theanocrystal solution. After 12 h, the red colloidal solutionurns colorless and supercrystals have been produced at theottom of the tube as dark precipitate. Supercrystals shoulde collectable in less than 12 h, so this is an efficient methodor making a large quantity of supercrystals. Fig. 7b showsn optical micrograph of numerous rhombic dodecahedral

upercrystals obtained from the assembly of octahedral Auanocrystals with sizes of less than 1—4 �m. The supercrys-als display metallic golden luster because of their largeizes. It was found that instant introduction of the same

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 lowerAu nanocrystal solution to form supecrystals which eventually settle to the bottom of the vial. (b) Optical micrograph of rhombicdodecahedral supercrystals assembled by octahedral Au nanocrystals. (c) Optical micrograph over a very large area of a substrate

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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 isnecessary and supercrystal formation takes time to evolveinto their final symmetrical structures.

Since solvent evaporation is the major entropy-increasingprocess in the formation of supercrystals through dropletevaporation, the surfactant diffusion transport approach togrowing supercrystals without solvent evaporation suggeststhat organized nanocrystal and surfactant assembly is notnecessarily entropy-driven. Surfactant diffusion, however, isentropy-driven. Rather, supercrystal formation is the morethermodynamically or energetically stable state, becausethe large repulsive charges on the surfactant is greatlyreduced when micellar bilayers are precisely screenedby the nanocrystals. Although enthalpy change should beimportant for the process, one cannot feel any temperaturechange to the vial. This is understandable, because super-crystal formation involves mainly surfactant organization,not bond formation or breaking in typically crystallizationprocesses. Again consideration of only local surfactant inter-actions between adjacent nanocrystals is insufficient forunderstanding supercrystal growth. A sufficient amount oftime 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 anddeposition of supercrystals on a large portion of a substrateimmersed into the nanocrystal solution. Fig. 7c presents

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n optical microscopic image of supercrystals grown on ai wafer from the assembly of gold rhombic dodecahedra31]. Octahedral, square pyramidal, and triangular pyra-idal supercrystals have been deposited on the substrate.he supercrystals are evenly distributed over the entireubstrate, so ultralarge-area deposition of supercrystals iseasible. Supercrystals can be deposited on any substrate,o a supercrystal-modified substrate can potentially functions an electrode for electrochemical reactions.

pplications of supercrystals

ost studies on the preparation of supercrystals haveainly focused their discussion on the packing arrange-ents of the building blocks and the forces involved to

ield supercrystals and superlattices. Less effort has beenevoted to demonstrate properties and applications ofhe obtained supercrystals. However, electronic proper-ies of Au supercrystals examined using scanning tunnelingicroscopy have been reported [65,66]. Mechanical proper-

ies of PbS nanocrystal-packed supercrystals have also beentudied [67]. The intimately contacting nanocrystals within aupercrystal naturally create many ‘‘hot spots’’ with ampli-ed local electromagnetic field upon irradiation of light

ith wavelengths matching the plasmon resonance of theanocrystals, and this is favorable for the surface-enhancedaman scattering (SERS) detection of adsorbed molecules.upercrystals and superstructures from the assembly of gold

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 theincorporation of Pd nanoparticles. (b) Cyclic voltammograms of Au supercrystals (SC) and a monolayer (ML) film assembled fromo ion cR

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ctahedral gold nanocrystals on an ITO glass electrode in a soluteproduced from Refs. [30,31].

anocubes, octahedra, and rhombic dodecahedra have beensed as substrates for SERS detection of p-mercaptoaniline32]. A higher SERS intensity has been recorded for the rhom-ic dodecahedral superstructures, attributed to more hotpots present from the ordered packing of nanocrystals. Inci-entally, gold rhombic dodecahedra dispersed in aqueousolution have also been found to be more sensitive SERS sub-trates than gold nanocubes and octahedra [68]. Arrays ofyramids constructed from the packing of gold nanoparticlesave also been shown to produce enhanced SERS intensitiesf adsorbed 1-naphthalenethiol [69]. The SERS intensity isighest toward the tip of the pyramid. SERS detection ofarbon monoxide bonded to an iron porphyrin attached tohe surface of the pyramid was also demonstrated. In addi-ion, supercrystals fabricated from dense assembly of shortold nanorods have been demonstrated as an active SERSubstrate for prion detection in human blood [70].

Supercrystals constructed from noble metal nanoparti-les can be considered as catalysts or a catalyst supportith well-defined pores and channels for molecular trans-ort. Of course, catalysis can also occur on the exteriorurfaces of the supercrystals. To clearly probe the accessi-ility of the interior of supercrystals to molecular transportor catalytic reactions, it is necessary to eliminate cat-lytic reactions taking place on the exterior surfaces ofupercrystals. Toward this end, supercrystals formed fromhe assembly of gold octahedra with potentially the largestnterior pores between particles as seen in Fig. 2 wereoaded with a H2PdCl4 solution, washed to remove Pd pre-ursor on the supercrystal surfaces, and finally immersedn an ascorbic acid solution to reduce the precursor form-ng Pd nanoparticles solely inside the supercrystals [30]. Aross-sectional elemental mapping image of the supercrystalevealing evenly distributed formation of Pd nanoparticlesnside the supercrystal is shown in Fig. 8a. The interior Pdarticles were active at catalyzing a Suzuki coupling reac-

ion between iodobenzene and phenylboronic acid formingiphenyl product, demonstrating that molecular transportnside the supercrystals is feasible, and the pore spaces andhannels are accessible. However, slow reagent diffusion

Isod

ontaining 0.1 M NaOH and 0.01 M glucose for glucose oxidation.

nto the supercrystal interior can lower its overall reac-ivity. The presence of surfactant adversely affects facileolecular transport, but complete removal of surfactant

an lead to partial collapse of the supercrystals. Slightusion of the metal nanocrystals, as observed particularly forupercrystals constructed from gold octahedra with {1 1 1}aces, may yield a more rigid framework structure robustnough to withstand destruction by surfactant removal30,31]. The resulting porous gold structure may becomeighly active catalysts with an exceptionally high surfacerea [71,72]. To greatly improve the catalytic efficiency,t should be more desirable to make supercrystals assem-led from Pd nanocrystals. For the Au supercrystals, onean try known Au nanocrystal-catalyzed reactions and eval-ate the efficiency of Au supercrystals acting as the catalyst73,74].

Because supercrystals can be deposited on a substrate,hey may be deposited on an electrode surface to form aodified electrode for the examination of electrocatalytic

ctivity of supercrystals. A gold octahedra droplet formingupercrystals has been deposited on an ITO electrode forlectrochemical oxidation of glucose [31]. The same amountf Au octahedra forming a monalyer of assembled particlelm on an ITO electrode was also tested. Cyclic voltam-ograms (CV) of the supercrystals and the monolayer filmlaced in a solution containing 0.1 M NaOH and 0.01 M glu-ose are provided in Fig. 8b. Both samples displayed goodlectrocatalytic activity, but the oxidation current was muchigher for the monolayer film than for the supercrystalsecause the monolayer film has a larger exposed surfacerea. Nevertheless, the idea of using supercrystals as simplend stable conductive electrode has been demonstrated.

onclusion and outlook

n contrast to conventional organized nanoparticle super-tructures produced from spherical building blocks of singler multiple components, fabrication of supercrystals fromiverse polyhedral metal and semiconductor nanocrystals

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Formation of supercrystals through self-assembly

offers a new dimension to nanoparticle assembly withgeometrically symmetric packing of the particles. Theirhigh 3-dimensional symmetry suggests their formation withmore globally balanced forces from all directions. Surfac-tant or capping molecules at high concentrations mediatenanocrystal assembly by residing between particles withapproximately the same size and shape to effectively min-imize their repulsive charges or interactions, such thatpolyhedral nanocrystals are packed into a configurationhaving the maximum surface contact. The supercrystal for-mation process has been recorded in real time by opticalmicroscopy showing rapid movement and incorporation ofsurrounding nanocrystals into the nearby developing super-crystals, and that the initially formed supercrystals areconcentrated around the droplet edge. More insights of thesupercrystal formation process may be obtained by improv-ing the optical microscopy resolution, or by following theprocess with the use of an environmental TEM chamber.A novel surfactant diffusion approach to making super-crystals in aqueous solution without water evaporation hasbeen developed. Free-standing supercrystals dispersed insolution can be collected on a substrate. Supercrystals ofvarious compositions can be prepared this way. With regardto further developments in the growth of supercrystals,size control of supercrystals is an interesting direction thathas essentially not been addressed. The goals, similar tochallenges in nanocrystal synthesis, are to make super-crystals with tunable sizes and the smallest supercrystalsby adjusting the amounts of nanocrystals and surfactantused. Success in regulating supercrystal size should furtherdemonstrate the importance of a globally balanced state,in addition to short-ranged forces, in supercrystal growth.By making supercrystals fairly easy to form, examinationsof properties and applications of supercrystals can be morereadily executed. Because diverse compositions and mor-phologies of nanocrystals can be regularly packed using thesurfactant-mediated assembly process, supercrystals withnovel optical/photonic, electrical, catalytic, and sensingproperties can be expected.

Acknowledgments

We thank the Ministry of Science and Technology of Taiwanfor the support of this work (NSC101-2113-M-007-018-MY3NSC102-2633-M-007-002, and MOST103-2633-M-007-001).

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Michael H. Huang obtained his B.A. degreein chemistry from Queens College in 1994,and his Ph.D. degree from the Department ofChemistry and Biochemistry at UCLA in 1999.After postdoctoral research at UC Berke-ley and UCLA, he joined the Department ofChemistry at NTHU in 2002. He was pro-moted to associate professor in 2006, andthen to professor in 2010. His current researchfocus is on the shape-controlled synthesis ofnanocrystals and the examination of their

acet-dependent properties. He has received a number of awards,ncluding the Outstanding Research Award from the National Sci-nce Council of Taiwan in 2012. Since 2014, he has been a memberf 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 ofTechnology Warangal, India, in 2010. Start-ing from 2014, he is pursuing his Ph.D. degreefrom National Tsing Hua University under the

research interests include shape-controlledsynthesis of metal nanocrystals and their self-assembly to form supercrystals.