Negative supracrystals inducing FCC-BCC transition in gold nanocrystal superlattice

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1 Nano Research Nano Res 1 DOI /s Negative supracrystals inducing FCC-BCC transition in gold nanocrystal superlattice Nicolas Goubet 1,2 and Marie-Paule Pileni 1,2 ( ) Nano Res., Just Accepted Manuscript DOI: /s on November Tsinghua University Press 2013 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Negative Supracrystals Inducing FCC-BCC Transition in Gold Nanocrystal Superlattice Nicolas Goubet, and Marie-Paule Pileni,,* Université Pierre et Marie Curie, UMR 7070, LM2N, 4 place Jussieu Paris, France Centre National de la Recherche Scientifique, UMR 7070, LM2N, 4 place Jussieu Paris, France Page Numbers. The font is ArialMT 16 (automatically Negative supracrystal have been observed in face centered cubic gold nanocrystal superlattice. The presence of these volumetric defects induces anisotropic diffusion of thiol molecules through the nanocrystal lattice and thus Bain deformation of the latter. inserted by the publisher) Nicolas Goubet and Marie-Paule Pileni 1

3 Nano Res DOI (automatically inserted by the publisher) Research Article Negative Supracrystals Inducing FCC-BCC Transition in Gold Nanocrystal Superlattice Nicolas Goubet 1,2 and Marie-Paule Pileni 1,2 ( ) 1 Université Pierre et Marie Curie Paris 6, UMR 7070, LM2N, BP 52, 4 place Jussieu, Paris, France 2 Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7070, LM2N, 4 place Jussieu, Paris, France Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT The growth of nanocrystal superlattices of 5nm single domain Au nanocrystals at an air-toluene interface induces formation of well-defined thin films ( nm) with large coherence lengths. High-resolution electron microscopy showed that polyhedral holes (negative supracrystal) were formed on the nanocrystal superlattice surface. Formation of negative supracrystals is attributed to inclusion in the superlattice of organic molecules (dodecanethiol), which are present in concentrated zones at the air/toluene interface. The coexistence of two supracrystalline structures (bcc/fcc) is attributed to diffusion of dodecanethiol molecules resulting on a Bain deformation of nanocrystal array. KEYWORDS Negative crystal, Bain transformation, nanocrystal superlattice, supracrystal, gold nanocrystal, self-assembly 1 Introduction Nowadays one of the challenges in nanosciences is to control the order of various entities (micelles, microemulsion, viruses, nanocrocrystals etc-) to produce new materials having collective properties and consequently potential applications in domains like phononic, high-density storage, opto-electronic, chemical and biological sensing [1-8]. Nanocrystal superlattices or supracrystals are defined as crystallographic arrangement of nanocrystal. Even though several models were developed to predict packing processes, it remains quite difficult to design experimental conditions to produce nanocrystal superlattice exactly as desired [9-13]. Over the last decade several studies have shown that some analogies, between either atoms in crystals or nanocrystals in superlattice, are observed [14-18]. One of them concerns the structural properties: similar crystalline structures are observed in both classical crystals and nanocrystal superlattices. Indeed, face centered cubic (fcc), body-centered cubic (bcc) and hexagonal closed packed system are often observed in mono-component nanocrystal superlattices [19-25]. Address correspondence to Marie-Paule Pileni: marie-paule.pileni@upmc.fr 2

4 Laves phases and other multi-component superlattices are also reported [26-29]. Similarly, structural defects such as point defects, dislocations, twins or stacking faults, grain boundaries present in the crystalline matter are also observed in nanocrystal superlattice [28,30,31]. Volume defects such as polyhedral cavities with well-defined crystallographic facets corresponding to the most stable crystallographic planes are called negative crystals. These volume defects have attracted the interest of the researchers working with different systems such as natural minerals, liquid crystal and metal [32-35]. Their formations are often induced by fluid inclusions. Nevertheless, volume defect such as negative crystals have not been yet reported in nanocrystal superlattices. In 1924, Bain proposed a simple model to explain the transition from fcc to bcc structure in the bulk martensite phase [36]. More recently, a similar transition has been observed in thin films of atoms and diblock copolymers [37,38]. It was suggested that there is a continuous and spontaneous transition from fcc to bcc phases by contraction of the interlayer spacing. In 2011, it was demonstrated that the tuning of PbSe nanocrystal arrangements from fcc to bcc, via Bain deformation, is controlled with solvent vapor processing [22]. Here, we show the appearance of negative supracrystals with a depth of 8-10 nanocrystal layers with well-defined crystallographic facets on large coherence length supracrystal film. Furthermore, a Bain deformation (fcc ==> bcc transition) is observed on the same superlattices and explained in term of a diffusion process of guest molecules like dodecanethiol. 2 Materials and Methods 2.1 Synthesis of 5nm Au nanocrystals The 5 nm Au nanocrystals are synthesized by revisiting an organometallic reduction method [31,39]. Briefly, 124 mg of triphenylphosphine chloride gold (I) were dissolved in 25 ml of toluene. To ensure the total dissolution of the organometallic molecules, the solution is heated up to 100 C. A second solution containing 434 mg of ter-butyl amine borane in 5 ml of toluene is also heated to 100 C. Before mixing the two solutions, 500 µl of dodecanethiol is added to the gold precursor complex solution. The reaction takes place at 100 C. The color of the resulting reaction medium changes from colorless to dark red. A required amount of solution is dried under a nitrogen flow. The black solid product is washed with ethanol and dispersed in toluene. The obtained colloidal solution is highly homogeneous and optically clear. A drop of the colloidal solution is deposited on a TEM grid. From the TEM images the average diameter and polydispersity, determined by using more than 500 nanocrystals, are 5 nm and 7%, respectively. (Fig. S1 in the Electronic Supplementary Material (ESM)). 2.2 Electron microscopy The conventional scanning electron microscopy was performed with a JEOL5510-LV. The high-resolution scanning electron pictures were acquired with a Hitachi SU-70. Before recording the electron Moiré pictures, the nanocrystal resolution was reached. In order to reveal the electron moiré pattern, the pictures were recorded with a magnification of and the resolution used is 640 x 480 pixels. 3 Results and Discussion Placing a colloidal solution in a beaker under a toluene-saturated atmosphere produces flat nanocrystal superlattice films of 5 nm Au single domain nanocrystals coated with dodecanethiol. After 7 days, two simultaneous growths take place at the toluene/air interface and in solution respectively [24]. The interfacial film is withdrawn from the air/toluene interface solution with a metal ring like a DuNouy ring and deposited on a silicon substrate. The film, with a thickness of about nm, keeps the same orientation as at the air-toluene interface. From small angle X-ray scattering (SAXS), a mixture of bcc and fcc is observed as the packing arrangement with {111}fcc {110}bcc (Figure S2 in the ESM) [24]. 3

5 Figure 1 (a) SEM image of interfacial film made of 5 nm Au nanocrystal deposited on silicon. (b) HRSEM pictures of area 1 without holes, corresponding to the FTT pattern (inset). The low magnification SEM image of the interfacial film deposited on the substrate shows formation of cracks and domains larger than several 10 µm (Fig. 1(a)). The morphologies of the film markedly differ from one domain to the other. Zone 1 (Fig. 1(a)) is a highly flat surface of Au nanocrystals ordered in a compact hexagonal network as shown in Figure 1(b) by high resolution scanning electron microscopy (HRSEM). The Fast Fourier Transform (FFT) (Fig. 1(b inset)) of the corresponding picture confirms the quality of nanocrystal ordering. The FFT patterns (Fig. 2(b)) deduced from HRSEM images of various regions of Figure 2(a) show that the orientation of the FFT spots remains quasi unchanged with only 5 as maximum disorientation. This indicates that the coherence length in the homogeneous domain of the film is more than 20 µm. Zone 2 contains a rather large number of holes (Fig. 1(a)). Figure 2(c) shows that the film is covered by well-defined holes concentrated in some regions of the pattern and consequently not homogeneously distributed. Note that the size distribution of the holes, in the same order of magnitude, is rather large. The FFT patterns (Fig. 2(d)) corresponding to HRSEM images show a similar slight disorientation, as observed in absence of holes, when the nanocrystal ordering is probed inside or around the area of the holes. This indicates that the holes in the nanocrystal superlattice film do not change the crystalline orientation and do not reduce the coherence length of the superlattice. The polyhedral holes (Fig. 3) exhibit various sizes and facets with common orientations to each other and the nanocrystal network. This agrees with the fact that the coherence length remains the same in the absence and presence of holes. Their depths of 8-10 nanocrystals layers are highly homogeneous (Fig. 4). 4

6 Figure 2 (a) SEM images of interfacial film made of 5 nm Au nanocrystals without hole and (b) FTT patterns of the corresponding different areas. (c) SEM images of the same film with hole and (d) FTT patterns of the corresponding different areas. The inside faceted walls are tilted relative to the basal superlattice surface and characterized with 3 {111} and 3 {100} crystallographic planes (Figure 4 inset), whereas the bottoms one correspond to the same crystallographic plane as that of the surface of the film i.e. {111}. The inner {111} surface planes are larger compared to the {100} one. This is due to the smaller {111} surface energy than that of {100} (γ{111}< γ{100}) in a fcc system. Even though the size of the holes differs, similar crystallographic patterns are observed (Fig 4). One can conclude that the resulting shape of the hole is an image of truncated octahedrons and consequently of a negative supracrystal. At a specific magnification, electron Moiré fringes are generated by HRSEM on the nanocrystal Figure 3 SEM pattern of film with negative supracrystal. The white dashed lines show the orientation of the inclusions. 5

7 Figure 4 HRSEM pattern of the negative supracrystals and their different crystalline inner planes (inset). superlattice surface (Figure 5) [40-42]. The fringes are wide and largely spaced on the flat surface without defects, whereas around the negative supracrystals, they are thinner and closer to each other. The electron Moiré fringes observation is known to evidence slight de-correlation between two networks, here the scanning step of the electron microscope and nanocrystal network. Such distortions in the network are attributed either to the disorientations of nanocrystal arrays or to the variation of the inter-particles distance in the network [42]. Due to the negligible disorientation of the network shown above, the first assumption is excluded. From this it is concluded that the change in the Moiré fringes spacing close to the holes is only due to variations of the inter-particle distance and consequently it demonstrates that the stress fields are concentrated around the negative supracrystals. The HRSEM image around a negative supracrystal (Fig. 6) shows successive ordered domains of expanded (zones 1) and compact networks (zones 2) repeated three times around the hole as symmetry center. The FFT patterns, corresponding to zone 1, exhibit distorted 2-fold symmetry (inset 1) whereas the unexpanded regions (zone 2) are characterized by 3-fold symmetry, i.e. compact assembly (inset 2) respectively. Such stresses, exclusively observed near the negative supracrystals, induce a transition from the 3- to 2- fold symmetry on the same basal plane. The greatest expanded surfaces of the nanocrystal superlattice are linked with the {100} inner surface of the hole whereas the compact ones are linked to the {111} inner plane. The presence of planes with various compacities {111}fcc and {110}bcc parallel to the same superlattice surface without well distinguished delimitations between them like crystallographic facets, which is confirmed by SAXS (Figure S2 in the ESM), implies a structure transition. From the literature, we know that addition of organic molecules to a colloidal solution of 6

8 Figure 5 Moiré fringes generated from the interferences between nanocrystal assemblies and scanning steps of the electron microscope on the mesoscopic film. nanocrystals leads to bcc superlattices whereas without addition the superlattices exhibit fcc structure [23]. The major differences between these data and those presented here are related to the fact that two structural arrangements (fcc and bcc) are observed on the same nanocrystal superlattice film. By analogy to data obtained in reference [23], we could assume that the fcc/bcc transition is due to the presence of organic molecules within the superlattice. In the present system, the potential organic component is the free dodecanethiol molecules. To support such assumption similar experiments similar to those described above are performed in the presence of a large excess of dodecanethiol. Indeed, 250 µl (0.2 mol.l -1 ) of pure dodecanethiol is added to the colloidal solution that is kept during 7 days in a toluene-saturated atmosphere. The interfacial film is then withdrawn and deposited on a silicon substrate. Several features are observed compared to what is described above in absence of an excess of dodecanethiol: (i) The interfacial film is homogeneously covered by the surface holes with a marked increase in the number of holes per surface area (Fig. 7(a)). Regions without holes as shown on Figure 2(a) cannot be detected. This observation confirms the influence of the excess of dodecanthiol on the negative holes formation. (ii) The HRSEM pictures and FFT patterns (Fig. 7(b)) confirm that the interfacial films is nanocrystals ordered in 3D superlattices with the same orientation along several micrometers. (iii) The interfacial film crystallinity is characterized by a mixture of fcc and bcc structures (see SAXS pattern in Figure S3 in the ESM). (iv) The holes are still surrounded by stresses and characterized by a rather low size distribution. However, their shapes are not faceted as shown in Figure 4. (v) The expanded and unexpanded networks with distorted 2- and 3- fold symmetry with hole as symmetry center remain unchanged. From these observations, we can conclude that the negative supracrystals are due to the presence of dodecanethiol molecules during the nanocrystal superlattice growing process. The growth mechanism in presence (or not) of excess dodecanethiol remains the same. Even though, the transition fcc/bcc is still present, the stresses induce to produce negative supracrystals increases with a loss of the well-defined facets. However, a careful look shows that some of them remain. Furthermore the various successive zones, shown in Figure 6(a), though still present are not so well delimitated. To explain such a process and propose a mechanism, we have to take into account the composition of the single domain 5 nm Au nanocrystals solution used to produce such assemblies and the procedure used to grow superlattice. As mentioned above, a colloidal solution is kept during 7 days under a saturated toluene atmosphere. The chemical analysis of dried Au nanocrystals powder (see experimental section) reveals an excess of free C12H25SH [43]. A proposed mechanism on negative supracrystal formation is represented in Figure 8. Because of the high surface tension at the air-toluene interface some of these molecules self-assembled at the interface to form a Gibbs monolayer (Fig. 8(a)) and the surface tension drops. As already claimed through simulation, the presence of a C12H25SH 7

9 Figure 6 (a) HRSEM image of the nanocrystals network deformation induced by inclusion. (b) Superimposed expanded regions, highlighted in blue, due to thiol diffusion on the image (a). (c) Scheme of the Bain transformation and the corresponding {111} plane in the face-centered cubic unit cell and {110} plane in the body-centered cubic unit cell after deformation. interfacial monolayer induces the formation of an interfacial film of Au nanocrystals self-assembled in fcc structure [44]. The remaining free C12H25SH molecules trap the nanocrystals on the liquid interface through the interactions with their alkyl chains (Fig. 1(b)). Even though the interactions between C12H25SH molecules are low compared to particle-particle interactions, formation at the air-interface of a concentrated zone of C12H25SH molecules take place. Due to rather strong van der Waals interactions between Au nanocrystals, these concentrated zones of free C12H25SH are trapped at the interface during the nanocrystal superlattice growth (Fig. 8(b)). This inclusion explains the formation of negative supracrystals with {111} and {100} as inner facets (Fig. 8(c)). The strong affinity between the free C12H25SH involved in the concentrated zones and chemisorbed at the Au nanocrystal surfaces, makes possible the dodecanethiol diffusion inside the nanocrystal arrays through the nanocrystal gaps of the {100} planes whereas the {111} planes remain compact (Fig. 8(c)). Moreover, the diameter of the spheres inscribed inside the octahedral sites of the fcc nanocrystal superlattice is estimated at around 3 nm. By filling this gap, the insertion of solvated C12H25SH molecules induces a swelling of the nanocrystal superlattice in certain directions (Fig. 8(d)), as experimentally shown in Figures 6 (a,b), the dodecanethiol diffusion is anisotropic and characterized by a Bain deformation (Fig. 6(c)). Figure 7 (a,c) SEM image of interfacial film made with an excess of free dodecanethiol and hexadecanethiol molecules, respectively. (b,d) The corresponding holes on the nanocrystal superlattice surface in the case of dodecanethiol and hexadecanethiol molecules, respectively. This is possible since the interfacial film is in contact with the solvent and the alkyl chains of free as well as the coated C12H25SH molecules are solvated. 8

10 Figure 8 (a-c) Scheme of the mechanism of negative supracrystal formation within the nanocrystal self-assembly. (c-d) The Bain deformation involved on the negative supracrystals. It reflects the intrinsic motion of solvated C12H25SH molecules through the superlattices. Such a process takes place at the air-toluene interface and is not the result of shrinkage or lateral stresses during the drying process. This transition could be explained as follows: the concentrated solvated thiol zones at the toluene-interface could induce constraints similar to the mechanical ones through the diffusion of the organic molecules inside the superlattice. This is confirmed by the marked increase in the number of negative supracrystals on increasing the relative amount of C12H25SH molecules as shown in Figure 7(a). However, we have to note that the structure of the negative supracrystals is not as well defined and their facets are screened (Fig. 7(b)) as compared to that observed in the absence of a large excess of C12H25SH molecules. This could be explained by the fact that diffusion of C12H25SH along the {100} direction is not exclusive with the appearance of smaller amounts of C12H25SH along the {111} direction. Recently, a structural transformation of the orientational ordering of nanocrystals within the superlattices was observed controlling the nanocrystal superlattice structure through the solvent-ligand interactions with a transition from fcc to bcc to the overall superlattice [22]. Furthermore this process is reversible due to the high ability of the solvent molecules to move from the superlattices to outside and vice versa. Here the system slightly differs. Instead of solvent molecules, solvated C12H25SH molecules migrate into the superlattices and remain trapped there during the drying process. The size of solvated C12H25SH molecules and consequently its boiling point is responsible for the fact that the molecules remain in the superlattices. This makes irreversible the Bain transitions and consequently the fingerprint of diffusion of molecules through the superlattices can be observed 9

11 a posteriori in a dry system (Fig. 6(a,b)). Hence, it is concluded that the presence of concentrated C12H25SH molecules zones at the interface explains the formation of negative supracrystals. In such a Bain transformation, the affinity between the solvated C12H25SH molecules either coated on the Au nanocrystal interface or freely diffusing in the solution is expected to play a major role. To support this claim let us replace the excess of C12H25SH by C16H33SH (hexadecanethiol) molecules keeping the other parameters constant as concentration of thiol excess (0.2 M), temperature, etc. Figure 7(c) shows that the film morphology remains the same with holes homogeneously dispersed on the film and systematic residual organic matter inside the inclusions (Figures 7(d) and S4 in the ESM) is observed. This clearly shows that formation of holes is due to the presence of organic molecules probably associated together at the air-toluene interface. The HRSEM patterns (Figure 7(d)) reveal that the Au nanocrystals lost their ordering in 3D superlattices and amorphous films are produced as confirmed by the corresponding FFT pattern (Figure 7(d inset)). Obviously, the lost of selective nanocrystal-nanocrystal interactions and consequently the nanocrystal ordering in 3D superlattices, is attributed to ligand exchange between free C16H33SH molecules and C12H25SH chemisorbed at the Au surface nanocrystals. From a previous study, the rate of ligand exchange on Au nanocrystals depends on the thiol derivatives concentration [45]. Here both C12H25SH and C16H33SH are in large excess and the ligand exchanges have a high efficiency. Hence, the coating agents of Au nanocrystals are a mixture of C12H25SH and C16H33SH molecules. Consequently, the affinity between the alkyl chains used to coat the nanocrystals markedly drops and the nanocrystal-nanocrystal interactions are screened by steric hindrance. Even though the size distribution of the Au nanocrystals is very low, no nanocrystal ordering takes place and the Bain transition does not occur under this condition. Whereas Bain transformation has been observed in various materials, such transition is novel for nanocrystal superlattice. The Bain transformation consists in the contraction of the fcc unit cell along the z axis and a bi-axial expansion along the [110] directions (Fig. 6(c)). Such a deformation is usually observed by applying a mechanical constrains on the material: By shearing a di-block polymer solution, the epitaxial transition between compact structure (fcc/hcp) to bcc structure was observed [38]. Similarly, the Bain deformation was obtained in atomic metal crystalline thin films with stress induced by epitaxial growth [37]. In these cases, there are external mechanical constrains whereas in the present work none of them are involved. Here, inclusion of organic matter within the nanocrystal superlattice produces the Bain transformation. This clearly shows, according to data already published [22] and the present one, Bain transformation could be also applied to nanocrystal superlattice even through the interactions are far away different from the classical crystals. Even considering the present system restricted to 5 nm Au single domain nanocrystal superlattices, the Bain deformation could be assimilated as another way to induce phase transition in nanocrystal superlattice, compared to the usual cited parameter: the ratio between coating agent and metallic core length. 4 Conclusion Here, 5 nm single domain Au nanocrystals are used to build 3D meso-architectures on a toluene/air interface of a colloidal solution. The coherent length of these nanocrystal superlattices is about several tens of µm. Here we described two independent processes that behave simultaneously and are involved on the superlattice surface. The presence of organic matter like thiols at the air-solvent interface in the concentrated zone induces inclusion. Polyhedral holes, called negative supracrystals, are observed on the nanocrystal superlattice surface when a strong affinity exists between thiol freely diffusing in solution and the coating agent of nanocrystals. The negative supracrystal shape is an image of truncated octahedron. The inclusion formation is followed by thiol diffusion in the nanocrystal superlattice. Indeed, solvated C12H25SH 10

12 molecules close to the solvated superlattices diffuse through the {100} inner plane whereas the {111} plane remains compact. This incorporation of free C12H25SH molecules induces anisotropic expansion of the nanocrystal superlattice and the observed fcc to bcc transition is explained through the Bain deformation. This deformation was described in the literature in various dried systems under mechanical constraints. In the present case, such a deformation is related to intrinsic behavior of solvation of the alkyl chain involved in the superlattice growth process. Theses processes exclusively exist with 5nm nanocrystals and are not observed with Au nanocrystals having larger size. This clearly shows that a fine equilibrium of van der Waals interactions between the nanocrystals and chain-chain interactions is needed to observe such a process. This experimental study demonstrates that superlattice structures can be tuned with chemico-mechanical deformation of the nanocrystal arrays. Acknowledgements This research has received funding from Advanced grant of the European Research Council under Grant Agreement No The authors thank Pr. Brust for fruitful discussion and Dr. Albouy for small angle X-ray scattering experiments. Electronic Supplementary Material: Supplementary material (TEM picture, nanocrystal size distribution, SAXS patterns and HRSEM pictures) is available in the online version of this article at (automatically inserted by the publisher). References [1] Gast, A. P. Structure, interactions, and dynamics in tethered chain systems Langmuir 1996, 12, [2] McPherson, A. Introduction to macromolecular crystallography; Wiley, [3] Singh, G.; Yager, K. G.; Berry, B.; Kim, H.-C.; Karim, A. Dynamic thermal field-induced gradient soft-shear for highly oriented block copolymer thin films ACS Nano 2012, 6, [4] Malkin, A. J.; Kuznetsov, Y. G.; Land, T. A.; DeYoreo, J. J.; McPherson, A. Mechanisms of growth for protein and virus crystals Nat. Struct. Biol. 1995, 2, [5] Pileni, M. P. Nanocrystals forming mesoscopic structures; Wiley, [6] Kotov, N. A. Nanoparticle assemblies and superstructures; Taylor & Francis Group, [7] Motte, L.; Billoudet, F.; Pileni, M. P. Self-assembled monolayer of nanosized particles differing by their sizes J. Phys. Chem. 1995, 99, [8] Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Self-organization of cdse nanocrystallites into three-dimensional quantum dot superlattices Science 1995, 270, [9] Ben-Simon, A.; Eshet, H.; Rabani, E. On the phase behavior of binary mixtures of nanoparticles ACS Nano 2013, 7, [10] Landman, U.; Luedtke, W. D. Small is different: Energetic, structural, thermal, and mechanical properties of passivated nanocluster assemblies Faraday Discuss. 2004, 125, [11] Eldridge, M. D.; Madden, P. A.; Frenkel, D. Entropy-driven formation of a superlattice in a hard-sphere binary mixture Nature 1993, 365, [12] Korgel, B. A.; Fitzmaurice, D. Small-angle x-ray-scattering study of silver-nanocrystal disorder-order phase transitions Phys. Rev. B 1999, 59, [13] Damasceno, P. F.; Engel, M.; Glotzer, S. C. Predictive self-assembly of polyhedra into complex structures Science 2012, 337, [14] Goubet, N.; Pileni, M. P. Analogy between atoms in a nanocrystal and nanocrystals in a supracrystal: Is it real or just a highly probable speculation? J. Phys. Chem. Lett. 2011, 2, [15] Macfarlane, R. J.; O'Brien, M. N.; Petrosko, S. H.; Mirkin, C. A. Nucleic acid-modified nanostructures as programmable atom equivalents: Forging a new table of elements Angew. Chem. Int. Edit. 2013, 52, [16] Banin, U.; Cao, Y.; Katz, D.; Millo, O. Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots Nature 1999, 400, [17] Alivisatos, A. P. Semiconductor clusters, nanocrystals, and quantum dots Science 1996, 271, [18] Pileni, M. P. Control of the size and shape of inorganic nanocrystals at various scales from nano to macrodomains J. Phys. Chem. C 2007, 111, [19] Henry, A. I.; Courty, A.; Pileni, M. P.; Albouy, P. A.; Israelachvili, J. Tuning of solid phase in supracrystals made of silver nanocrystals Nano Lett. 2008, 8, [20] Prasad, B. L. V.; Sorensen, C. M.; Klabunde, K. J. Gold nanoparticle superlattices Chem. Soc. Rev. 2008, 37, [21] Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Crystal structures of molecular gold nanocrystal arrays Accounts Chem. Res. 1999, 32, [22] Bian, K.; Choi, J. J.; Kaushik, A.; Clancy, P.; Smilgies, D.-M.; Hanrath, T. Shape-anisotropy driven 11

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