PERSPECTIVE pubs.acs.org/JPCL
Analogy Between Atoms in a Nanocrystal and Nanocrystals in a Supracrystal: Is It Real or Just a Highly Probable Speculation? N. Goubet†,‡ and M. P. Pileni*,†,‡ † ‡
Universite Pierre et Marie Curie, UMR 7070, LM2N, 4 Place Jussieu, 75005 Paris, France Centre National de la Recherche Scientifique, UMR 7070, LM2N, 4 Place Jussieu, 75005 Paris, France
bS Supporting Information ABSTRACT: Nanocrystals and supracrystals are arrangements of highly ordered atoms and nanocrystals, respectively. At the nanometer scale, from face-centered cubic (fcc) tetrahedral subunits, either single fcc nanocrystals such as cubooctahedra and octahedra or decahedral and icosahedral nanocrystals are produced. Such nanocrystals with different shapes are produced by soft chemistry. At the micrometer scale, very surprisingly, supracrystals having shapes similar to those obtained at the nanometer scale are produced. For example, large triangular nanocrystals as well as supracrystals are produced either by soft chemistry, from nanocrystal diffusion on a surface, or by nanocrystal interactions in solution. The morphologies of nanocrystals, supracrystals, and minerals, which are similar at various scales (nm and mm), are pointed out, and an explanation of these similarities is undertaken.
A
t the atomic level, Kepler described, in 1611, the arrangement having the maximum density of hard spheres packed in a face-centered cubic (fcc) arrangement. The densest possible packing of four spheres is a tetrahedron. At the earliest stages of a solid’s growth, the atoms reorganize into a completely new structure with a preferred symmetry to form clusters. Further growth takes place by adding layers of atoms to this frozen core. Layered growth imposes certain restrictions on the outer symmetry or morphology of the clusters. This was demonstrated for clusters having large binding energies and resistance to the evaporation of atoms. The spontaneous assembly of uniform-sized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a vast range of length scales. These extend from the atomic (10 8 cm), through the molecular and macromolecular with proteins, synthetic low polymers, and colloidal crystals (∼10 6 cm), to the wavelength of visible light (∼10 5 cm), natural opals, and synthetic spheres, and beyond, into the everyday packing of spherical objects such as balls (∼1 102 cm). The associated concepts of sphere packing have had an influence in diverse fields ranging from pure geometrical analysis (cf. quasi-periodic structures) to architectural models or ideals.1,2 The generation of a solid from a solution through crystallization might sound as such a familiar process that one could be forgiven for thinking that it is fully understood. However, despite the work of Wilhelm Ostwald on crystal nucleation and the development of classical nucleation theory, this is not so. Take an everyday example such as brown sugar, millions of tons of which are crystallized annually, to be dissolved in tea and coffee. The r 2011 American Chemical Society
crystals are all of uniform size and clearly faceted. This is no accident; the effect is achieved by a process known as seeding, in which small crystals of pulverized sugar are introduced in the solution to act as seeds on which crystal growth can start. Thin films of alkane-chain-passivated metal nanocrystals form fcc superlattices oriented with the (111) planes parallel to the substrate.3 5 Nanocrystal aggregations with a well-defined crystal structure (fcc) favor formation of either single crystals or polycrystals all terminated by (100) and (111) planes leading to truncated tetrahedral, octahedral, and icosahedral shapes.6,7 Here, we point out that for fcc arrangements, the Kepler description for atoms could be applied to nanocrystals, when strong interactions between these take place, and extended to minerals. Fcc tetrahedral subunits of either atoms or nanocrystals are precursors to producing highly crystallized materials in fcc structures with similar final shapes at various scales. Growth of Crystals from Either Atoms or Nanocrystals at Various Scales. At the atomic level, many symmetric clusters are constructed from the close packing of hard spheres. For Au and Ag fcc crystal structures, the atoms are packed with a maximum density to form tetrahedral nuclei.8 The clusters, in fcc arrangements, are single crystals such as cubooctahedra (Figure 1A) and truncated octahedra resulting from different growth rates between the {111} and {100} facets. Multiply twinned particles (MTP) like decahedra (Figure 1B) and icosahedra (Figure 1C) Received: January 28, 2011 Accepted: April 8, 2011 Published: April 14, 2011 1024
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Figure 1. Schematic and TEM images of Cu nanocrystals of cubooctahedral (A), decahedral (B), and icosahedral (C) particles.9 (D,E) SEM images of supracrystals with icosohedral morphology (D) of 8 nm PbS nanocrystals produced by S.A. Rupich et al.6 and of 7 nm Au nanocrystals. (F) SEM image of an octahedral single Au supracrystal of 7 nm nanocrystals.
are formed by assembling, respectively, 5 and 20 single-crystalline fcc tetrahedral subunits. These clusters have been extensively produced under ultravacuum during the last two decades. Recently, conversely to what has been claimed, nanocrystals with sizes of a few nanometers, produced by soft chemistry (Figure 1A C), retain shapes similar to those of clusters produced by the ultravacuum technique.9 At the end of the solvent evaporation of Au nanocrystals dispersed in toluene, a large number of aggregates with well-defined morphologies are produced (Figure 2). The SAXRD pattern (inset Figure 2) shows that these aggregates are nanocrystals that are self-ordered in a fcc structure.10 For simplicity, they are called supracrystals. Note that similar patterns are produced with Au nanocrystals with 5 8 nm average diameters and a very low size distribution (around 6%). Icosahedral shapes terminated by (111) planes are produced with PbS (Figure 1D) and Au (Figure 1E) nanocrystals.6,10 This morphology corresponds to multiply twinned crystals like those obtained at the nanometer scale for nanoparticles called MTPs (Figure 1C). Note that an octahedral, single Au supracrystal of 5 nm nanocrystals, with three-fold symmetry (Figure 1F), is also produced. To our knowledge, a perfect decahedral supracrystal has never been observed. This is probably due to the strong constraints involved in the formation of such aggregate morphology. From these results, it is concluded that nanocrystals retain the crystalline morphology of fcc clusters with tetrahedral subunits. These basic morphologies open other possibilities like elongated particles resulting from additional intermediate (110) planes in a regular decahedral cluster with a preferential growth of the {111} facets compared to the growth of {100} facets
(Figure 3A). Experimentally, the formations of large truncated decahedra with five-fold symmetry (Figure 3B) with very highly ordered atoms (Figure 3C) are observed.11 Hence, additional planes induce new nanocrystal shapes like nanorods. Very surprisingly, minerals such as aragonites with similar shapes characterized by additional (110) planes like nanorods are observed (Figure 3D). This corresponds to the crystal default used to build the icosahedra and decahedra in Figure 1D and E as well as nanorods of Cu atoms (Figure 3B). Thus, there is a similar growth mechanism for nanoparticles or minerals both with (110) additional intermediate planes with preferential growth of {111} facets, which could indicate a great similarity in growth processes at various scales. Figure 2 does not show supracrystals consisting of long rods or decahedra. Again, this is probably due to the strong constraints involved in the formation of such entities. Other nanocrystal shapes with three-fold symmetry like triangular nanocrystals, nanodisks, and nanoprisms are produced from modified fcc tetrahedral precursors to give triangular lamellar particles. Hence, two regular fcc tetrahedral clusters truncated on a {111} surface adhere to each other, leading to a bitetrahedral cluster with three active sites for accelerated growth, maintaining the overall three-fold symmetry in the final nanocrystal (Figure 4A).12,13 This is a model adopted by the scientific community to explain the formation of triangular particles. To our knowledge, at the subnanometer scale, the formation of either tetrahedrons and their truncated forms or bitetrahedral clusters has never been observed. However, very recently, truncated, fcc, tetrahedral Au nanocrystals (Figure 4B) have been synthesized.14 However, such entities are produced with rather large dimensions (210 ( 20 nm). At the nanometer scale, several groups have produced triangular nanocrystals by soft 1025
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Figure 2. SEM image obtained at the end of solvent evaporation of 7 nm Au nanocrystals dispersed in toluene. Insets: (1) SEM image at a larger scale and (2) the corresponding small-angle X-ray diffraction patterns obtained in a grazing incidence geometry.
chemistry. This was done either by chemical reduction of metal salts or photoinduced aggregation of small nanoparticle seeds12,15,16 and under ultrahigh vacuum (UHV) with the epitaxial growth of metal particles on substrates.17,18 These nanodisks (Figure 4C) are fcc single crystals with flat surfaces orientated in the [111] direction, as shown by HRTEM and the corresponding Fourier transforms in insets 1 and 2, respectively. With Au supracrystals, individual large aggregates having welldefined plate-like shapes with three-fold symmetry are obtained (Figure 4D). The SEM data show the characteristic angle (60°) between {111} surfaces of tetrahedral supracrystals. The SAXRD pattern is identical to that observed (inset Figure 2), indicating that the nanocrystals are ordered in a fcc structure. A similar fcc, truncated tetrahedral morphology was obtained for Au nanoparticles4,7,19 and with PbS nanocrystals.6 This similitude in morphologies of nano- (Figure 4B and C) and supra- (Figure 4D) crystals confirms that the crystal growth mechanism proposed to produce three-fold symmetry is the same for atoms and nanocrystals. In nature, a rather large number of truncated, tetrahedral minerals like tetrahedrite and benitoite (Figures 4E and Supporting Information Figure 1) are observed. Figure 4F shows fcc supracrystals with the bipyramid morphology of two truncated, tetrahedral, single supracrystals with a single twin plane along the {111} surface. Such a bipyramid having a size in the micrometer range is similar to that in the sketch of the model proposed by several researchers as an intermediate in the production of platelike nanocrystals (Figure 4A). Minerals like spinels, with a similar morphology (bipyramid), are obtained (Figure 4G). To our knowledge, no such intermediates (truncated tetrahedrons and bipyramids) have been observed at the nanometer scale (below 10 nm). Thus, by replacing atoms by nanocrystals to produce
supracrystals instead of nanocrystals, these two intermediates are observed, as shown in Figure 4D and F as well as in their mineral forms (Figure 4E and G), respectively. Multiple parallel twins on {111} truncated, tetrahedral facets, observed with supracrystals (Figure 3E and F), are found in plagioclase minerals (Supporting Information Figure 1D). From this assessment, we could assume that similar growth processes exist with minerals. Another way to produce single, triangular Ag nanocrystals (from a few tens of to a few hundred nanometers) is to anneal, at 50 °C, Ag nanocrystals ordered in either compact hexagonal networks or small 3D fcc superlattices. The nanocrystal’s ordering scale controls the size of the triangular final nanocrystals.20 The nanocrystal’s ordering is tuned by changing the deposition mode, keeping constant the amount of material deposited on a given substrate (from Figure 5A to C). The three samples shown in Figure 5A C are annealed. Single, triangular crystals, surrounded by various well-crystallized, as well as coalesced, particles with different shapes, are produced (Figure 5D F). Comparison of the scale length of the nanocrystal ordering (Figure 5A C) with the final produced nanoparticles (Figure 5D F) clearly shows that the size of triangular singlecrystal particles is tuned by the nanocrystal ordering with a size increase of the triangular nanocrystals by 1 order of magnitude. Hence, annealing an assembly of 5 nm Ag nanocrystals, selfordered in either a monolayer or multilayers, produces large triangular single crystals of Ag atoms. Such a growth process can be explained by the TEM images recorded after annealing. Figure 5G shows two icosahedral nanocrystals in contact with the appearance of a thin region with the atomic planes of both particles tending to align between the two nanocrystals. Also, twin planes maintaining the adhesion of partially coalesced Ag 1026
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Figure 3. (A) Schematic of the growth mechanism from the nucleus to truncated, decahedral nanocrystals9 and (B34 and C) truncated, decahedral Cu nanorods; TEM (B) and HRTEM images (C).9 (D) Picture of a aragonite mineral. Photo courtesy of Alan Guisewite,35 reproduced with permission from Alan Guisewite. (E and F) SEM images at two different scales of a Au supracrystal.
nanocrystals (Figure 5H) are observed. These planes have to move and reorientate to form a new particle. Figure 5I shows plane defects in the newly formed particles. There is subsequently a totally relaxed spherical particle with no structural defects (Figure 5J). This implies movements of the atoms, favored by the annealing process, to reorganize and a complete atomic rearrangement with a removal of the twin planes. Hence, large atomic triangular particles (Figure 5) are obtained from nanocrystals made up of either 5 or 20 fcc tetrahedral subunits (MTP), which tend to fuse together. Thus, it can be concluded that the tetrahedral units are precursors in the formation of singlecrystal triangular particles that are provided either from the partial coalescence of nanocrystal subunits like decahedral or icosahedral nanoparticles (MTP) or from the chemical reduction of metal salts The above data and considerations clearly show that three- and five-fold-symmetry materials (icosahedra, plate-like morphology), composed either of atoms or uniform nanocrystals, are produced at various scales, keeping similar shapes with tetrahedral subunits
as the growth nuclei. An explanation of the fact that crystal morphologies can be retained at various scales may now be proposed. Tetrahedral subunits are the common parameters involved in the various morphologies described above. With atoms, the tetrahedral subunits are observed when 5 or 20 of them are associated to form either a decahedron (Figure 1B) or an icosahedron (Figure 1C), respectively. These associated subunits coalesce to form triangular single crystals (Figure 5). They have never been observed as isolated entities. A model has been proposed (Figure 3A) to explain the formation of plate-like single crystals from two truncated, tetrahedral subunits adhering on a {111} surface (Figure 3C).12,13 With nanocrystals, isolated, truncated, tetrahedral single supracrystals are obtained (Figure 4D and E). They are able to adhere to form a new structure corresponding to the intermediate stage of plate-like crystal growth (Figure 4F). They also form multiply twinned aggregates made up of 20 tetrahedral subunits (Figure 1D and E). In both cases (nano- and supracrystals), in a fcc arrangement, the densest 1027
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Figure 4. (A) Schematic of the growth mechanism from the nucleus to nanodisks.9 (B) SEM image of truncated, tetrahedral Au nanocrystals. From ref 14, Copyright 2004, Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (C) TEM image of Ag nanodisks produced from synthesis in an excess of hydrazine.19 Insets: (1) HRTEM and (2) Fourrier transform of the HRTEM image shown in inset 1. (D and F) SEM images of supracrystals of Au nanocrystals. (E) Picture of benitoite minerals.9 (G) Picture of a spinel mineral; photo36 reproduced with permission from www.irocks.com .
possible packing of four hard spheres is a tetrahedron. To produce such tetrahedral subunits at various scales, we could assume that a layered growth takes place. Such a layer-by-layer growth could depend on the value of the energies involved in crystal growth with no appearance of restrictions on the outer symmetry. With atomic, tetrahedral subunits, the forces involved in their formation are very strong, and the process could be blocked at an early stage. This would explain why both tetrahedra and their truncated forms are never observed at the nanometer level. However this does not agree with the formation of atomic,
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Figure 5. (A C)37 TEM images of 5 nm nanocrystals deposited on a HOPG substrate with different orderings of the nanocrystals. Insets correspond to the Fourier transforms corresponding to the monolayer shown in the corresponding figure. (D F)37 TEM images of the samples shown in the corresponding panels (A,D; B,E; and C,F) obtained after annealing at 50 °C. (G J)21 HRTEM measurements of coalesced 5 nm Ag nanocrystals obtained after 6 days of annealing at 50 °C. Reproduced with permission of Nature Publishing Group, copyright 2007. (H) Neck formation between two twinned particles. (I and J) Coalesced particles with remaining structural stress (twin boundaries). (K) Newly formed, totally relaxed triangular particle without structural defects.
truncated, fcc, tetrahedral nanocrystals as reported by Kim et al.14 (Figure 4B). With nanocrystals, the energy involved in supracrystal growth would be very weak compared to that in the 1028
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The Journal of Physical Chemistry Letters atomic growth, and the layer-by-layer growth would be more favorable. This would allow production of truncated, tetrahedral supracrystals (Figure 4D and E). Here, we have to point out that van der Waals forces govern formation of supracrystals. Usually, we tend to limit the attractive van der Waals interactions involved in supracrystal formation to the Hamaker interactions between Au cores. In fact, this contribution is negligible compared to the total interactions. In vacuum, the mean force potential between two Au nanoparticles is dominated by the interactions between the capping molecules.21 The mean potential becomes less attractive in the presence of a solvent and reaches, for a good solvent, a repulsive interaction. With toluene as the solvent, the contribution of alkyl chains is predominant, favoring a homogeneous crystal growth in solution with the formation of welldefined, faceted and rigid supracrystals.10 Nevertheless, even if the attractive forces involved in the formation of supracrystals are not as weak as expected, they are very weak compared to those involved in the formation of atomic crystals. These relatively weak attractive forces would favor the layer-by-layer process and formation of rather large, tetrahedral, truncated or nontruncated subunits. The large tetrahedral subunits will associate on their (111) planes to form icosahedral supracrystals (Figure 1D and E). This also makes it possible to observe truncated, tetrahedral, bipyramidal supracrystals (Figure 4D), formed by the adherence by one twin of two truncated tetrahedral crystals on one of the {111} surfaces (Figure 4F) or with several twins (Figure 3E and F). From this, it could be concluded that the rather weak van der Waals interactions compared to those involved in the atomic structure favor a layer-by-layer nanocrystal growth process with the four densest hard spheres packed at the center. Consequently, this would allow observation of intermediate morphologies that are never seen with atomic nanocrystals. From this, it could be concluded that the crystal growth mechanism of either atoms or nanocrystals is governed by the classical thermodynamic theory with no scale range restriction. However, because similar shapes are obtained with minerals as well as nanocrystals and supracrystals, a kinetic term has to be included to understand this growth process. Even if it is rather impossible to give a final explanation of such growth processes at various scales, it is reasonable to conclude that supracrystals with single-crystal, multiply twinned arrangements and a polycrystalline phase are produced with nanocrystals as units. Nanocrystals in a supracrystal are self-ordered like atoms in a nanocrystal. This shows a universal behavior of the arrangement of hard spheres with various dimensions, from a few angstr€oms (10 10 m) to nanometers (10 9 m), packed in a fcc arrangement with tetrahedron subunits. The latter involve single, multiply twinned nano- and supracrystals with, in both cases, polycrystalline phases at the nano- and micrometric scales. Hence, even when the forces involved differ, the attractive interactions between nanocrystals are sufficient to produce similar shapes. This confirms and extends the previous claim from which the atomic material shapes can be retained at various scales.9
Mineral and Au nanocrystals of atoms are characterized by similar shapes as Au supracrystals.
PERSPECTIVE
The above results clearly show that understanding the mechanism of crystal growth at various scales is a real challenge that has to be faced. Furthermore, they make it possible to explore new topics in chemistry and physics. Now, we can ask the following question: Are their any analogies between the chemical and physical properties of materials composed of atoms and those of nanocrystals? To answer this question, we have first to study the influence of the nanocrystal ordering on the physical properties and consequently propose conclusions related to the aim of this discussion. Hence, to point out the influence of the nanocrystal ordering at the mesoscopic scale, the physical properties of supracrystals are compared with those of disordered aggregates produced from the same batch of nanocrystals. There are very few data that are relevant and valid in the literature. Nevertheless, a number of these can be pointed out. (i) The approach to saturation of the magnetic curve is more gradual for Co supracrystals than that for the disordered aggregates of Co nanocrystals.22 This is correlated to a model from which only short-range structural order leads to a local, random anisotropy with a short-range correlation length producing a very small uniform anisotropy in the sample.23 The model predicts an approach to saturation in these materials given by 1/H1/2 (where H is the applied magnetic field) that has been confirmed experimentally.24 This was extended to disordered nanocrystallized films where a 1/H1/2 approach to saturation is also observed. In addition, by increasing the anisotropy, a change from 1/H1/2 to 1/H2 upon approaching saturation is observed, which would apply in the case of a perfect uniaxial system.25 By analogy, it is concluded that in the disordered aggregates of Co nanocrystals, the direction of anisotropy randomly fluctuates from one magnetic nanocrystal to another and leads to an extremely small uniform local anisotropy with a square root magnetization law in approaching saturation. For Co supracrystals, due to long-range mesoscopic order, the total anisotropy is rather large, with internanocrystal coupling energies leading to a smoother magnetization curve upon approaching saturation. Other collective dipolar interactions in magnetic binary nanocrystal supracrystals are observed.26 (ii) With Ag supracrystals and disordered aggregates of Ag nanocrystals (produced from the same batch), the vibrational, coherent, quadrupolar mode in supracrystals could be analogous to the breathing mode vibration of atoms in a nanocrystal. In fact, the good correlation between the Raman scattering intensity and the inverse size histogram obtained with disordered Ag nanocrystal aggregates confirms what was already observed, that is, intrananocrystal coherence, where the atoms in the nanocrystals vibrate coherently.27 When fcc supracrystals are small enough (size < l/10, where l is the laser excitation wavelength), the active vibration modes are regarded as localized, and consequently, the selection rule for the phonon momentum vector does not limit the Raman-active vibration mode number. From this, the model28 predicts that the Ag supracrystal’s Raman scattering intensity is proportional to the square of that of nanocrystal amorphous aggregates. As expected from the model, the Raman peak bandwidth, attributed to the quadrupolar vibration 1029
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The Journal of Physical Chemistry Letters modes, is narrower for Ag supracrystals than that for disordered aggregates of Ag nanocrystals.29 These have the same profile of the supracrystal’s experimental peak as that of the square of the disordered aggregates. From this, it has been concluded that there is internanocrystal coherence inside of fcc supracrystals. For large supracrystals, the Raman intensity is proportional to the Raman intensity of amorphous aggregates, and consequently, the unchanged Raman peak means that if the nanocrystal vibrational coherences exist in supracrystals, they cannot be observed. In the same way, the dynamic breathing properties in Co supracrystals compared to that in disordered aggregates of Co nanocrystals show two relaxation times.30 (i) The shorter one, observed in both samples (ordered and disordered), is due to the electron relaxation, in the range of few picoseconds, to the lattice via the electron phonon interaction. This is consistent with the time scale of the electron lattice relaxation in polydispersed Co nanoparticles implanted in a dielectric matrix. (ii) A long relaxation time and large oscillations with a characteristic period of a few hundred picoseconds, corresponding to a low-frequency mode of a few cm 1, are observed with Co supracrystals, whereas no modulations are observed for disordered aggregates of Co nanocrystals deposited on a bare substrate. Such oscillations observed with Co supracrystals could be due to the strong binding of the nanocrystals in supracrystals, allowing the energy to dissipate via the interdigitated aliphatic chains. There is, therefore, a coherent motion of the entire supracrystal of Co nanocrystals linked by the chains. Of course, these results have to be confirmed by other experiments. Nevertheless it seems obvious that the breathing properties of nanocrystals ordered in supracrystals differ from those of the same nanomaterials forming disordered aggregates. (iii) When a magnetic field perpendicular to the substrate is applied during the evaporation (process) of Co nanocrystals that are dispersed in hexane and have a very low size distribution, they self-assemble in column supracrystals. Conversely, when the nanocrystal size distribution is rather large, a large number of flower-like entities, due to coalescence of either upright or fallen columns forming worm-like and labyrinthine structures, are produced. These changes in morphology are explained in terms of the change in the mechanical properties.31,33 When the size distribution is low enough, the nanocrystals dispersed in solution tend to self-assemble in fcc supracrystals with the formation of well-defined and compact columns, whereas at higher size distribution values, the interactions between particles markedly decrease, and the columns are formed with disordered entities. This creates defects, and the cohesive forces between columns are not large enough to keep them ordered, and the columns tend to fuse to form labyrinths. Hence, the mesoscopic structure of Co nanocrystals is tuned from well-defined dots to labyrinths with increasing nanocrystal size distribution. The mechanical properties of the column formation control the final patterns. Such an increase in the mechanical properties with the formation of supracrystals has been recently confirmed by nanoindentation with 1.7 GPa as Young’s modulus.32,33
PERSPECTIVE
Intermediary structures (truncated tetrahedra either isolated or associated together), expected to explain and to control the shape of three-fold symmetry nanocrystals, are observed at the mesoscopic scale with Au nanocrystals as the elementary entities. From these examples, the door is open for finding new physical and chemical properties with the obvious possible analogies between atoms and nanocrystals. Evidently, the strengths of the interactions involved in forming supracrystals from nanocrystals are not the same as those in producing bulk and/or nanocrystals from atoms. Nevertheless, this could be a new concept to go further in the paradigm concerning physical and chemical properties of matter at nano- and micrometer scales.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tetrahedrite and Benitoite picture. Polarized light microscopy image of plagioclase. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHIES N. Goubet, born in 1982, started his carrier as a technician and is now assistant engineer at the University P&M Curie. His major interests are related to understanding the supracrystal growth mechanism and any comparison with atomic crystal growth. He already published some papers in this area. M. P Pileni (http://www.sri.jussieu.fr/pileni.htm) is a Distinguished Professor at University P&M Curie and Adjunct Professor of chemistry and biochemistry at Georgia Tech, Atlanta, U.S.A. She is a member (1999 present) and chair (2004 2010) of the Institut Universitaire de France, IUF. She published 380 articles with 13604 citations and an h factor of 60. ’ ACKNOWLEDGMENT The authors would like to thank Prof. A. Courty, Dr. I. Lisiecki, and Dr. Richardi for their contributions and for fruitful discussions. Dr. I. Arfaoui and G. Laurent are acknowledged for their comments on the present paper. The MPP research leading to these results has received funding from Advanced Grant of the European Research Council under Grant Agreement 267129. 1030
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