Nano Today (2015) 10, 81—92
Available online at
journal homepage:
82 M.H. Huang, S. Thoka
another is also possible with spherical particles by vary-
ing the temperature [24,25]. While asse...
Formation of supercrystals through self-assembly 83
Figure 1 (a—e) Supercrystals possessing diverse geometric shapes have ...
84 M.H. Huang, S. Thoka
Figure 2 (a) SEM images showing that supercrystals fabricated from the assembly of gold truncated ...
Formation of supercrystals through self-assembly 85
Figure 3 SEM images showing supercrystals fabricated from the assembly...
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 ...
Formation of supercrystals through self-assembly 87
Figure 5 (a—f) Optical microscopy images of the supercrystal formation...
88 M.H. Huang, S. Thoka
Figure 6 (a and b) SEM images showing the shape-guided
effect in nanocrystal assembly. Right bipyr...
Formation of supercrystals through self-assembly 89
Figure 7 (a) Schematic drawing of the diffusion transport of CTAC surf...
90 M.H. Huang, S. Thoka
Figure 8 (a) EDS elemental mapping image of a microtomed thin film of a supercrystal assembled from...
Formation of supercrystals through self-assembly 91
offers a new dimension to nanoparticle assembly with
geometrically sym...
92 M.H. Huang, S. Thoka
[42] S. Gómez-Gra˜na, J. Pérez-Juste, R.A. Alvarez-Puebla, A.
Guerrero-Martínez, L.M. Liz-Marzán, ...
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Nanotoday_ FINAL

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  • 1. Nano Today (2015) 10, 81—92 Available online at ScienceDirect journal homepage: 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: (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 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). References [1] C.-Y. Chiu, M.H. Huang, J. Mater. Chem. A 1 (2013) 8081. [2] M.H. Huang, S. Rej, S.-C. Hsu, Chem. Commun. 50 (2014) 1634. [3] C.-Y. Chiu, M.-Y. Yang, F.-C. Lin, J.-S. Huang, M.H. Huang, Nanoscale 6 (2014) 7656. [4] J. Gong, G. Li, Z. 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