Solvent-Mediated Crystallization of Nanocrystal 3D Assemblies of

Subscriber access provided by University of Pennsylvania Libraries

Article

Solvent-mediated crystallization of nanocrystal 3D assemblies of silver nanocrystals: Unexpected superlattice ripening Jingjing Wei, Nicolas Schaeffer, and Marie-Paule Pileni Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04120 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 13, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Solvent-Mediated Crystallization of Nanocrystal 3D Assemblies of Silver Nanocrystals: Unexpected Superlattice Ripening Jingjing Wei a,b, Nicolas Schaeffer a,b, Marie-Paule Pileni*a,b,c a

Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, F-75005, Paris,

France b

CNRS, UMR 8233, MONARIS, F-75005, Paris, France

c

CEA/IRAMIS, CEA Saclay, 91191, Gif-sur-Yvette, France

Abstract Solvent-ligand interactions in colloidal nanocrystals are of significant importance as they can be used to modulate the way they pack into superlattices. Here we demonstrate that the crystal structures of the nanocrystal superlattices made of 2.2 nm Ag nanocrystals can be controlled by using different carrier solvents. Specifically, the superlattices structures are tuned from body centered cubic (bcc) to face centered cubic (fcc) when varying solvents from hexane to tetrachloroethylene (TCE). Furthermore, by simultaneously annealing these two samples at different temperatures, bcc structures originating from hexane solutions are dominated by simple coalescence mechanism while fcc structure stemming from TCE solutions undergo Ostwald ripening process that can produce a variety of binary nanocrystal superlattices such as NaCl, AlB2, NaZn13 and MgZn2, the formation of those structures being well explained by a pure entropy driven process. This is believed to be due to variations in the ligand coverage ratio of the nanocrystals in different solvents that are changing the superlattices structures stability. Those findings provide insights into the solvent mediated nanocrystal superlattices and the Ostwald ripening process in nanocrystal superlattices.

Introduction

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

Atoms in bulk phase metals are ordered in various crystalline structures such as bcc (body centered cubic), fcc (face-centered cubic), hcp (hexagonal close-packed) or disordered.1 Owing to the structural similarities with atomic systems, colloidal assemblies of nanoparticles uniform in size was developed and a way of particle ordering similar to that of atoms was unveiled with the formation of various crystal structures.1-3 However, the organization of colloidal nanoparticles into ordered structures can also be tuned through manipulation of their surface chemistry, hence enabling some control over their crystalline organization. By coating the nanocrystals with a flexible layer of organic, such as surfactants or DNA, more elaborate nanoparticle assemblies can be achieved while keeping rational control over their structures.4-9 Although entropy is frequently reported to be the main driving force during the bottom-up colloidal assembly, a variety of other forces were demonstrated to also be influencing the process, for example van der Waals, Coulomb, dipolar and magnetic forces.10-13 Hence, nanoparticle superlattices structures are not only determined by the interactions from nanoparticle cores but also influenced by the interactions of their surface coatings, namely ligand-ligand interaction and ligand-solvent interactions.14-17 For example, Goodfellow et al. found that solvent molecules can occupy the interstitial space in PbSe nanocrystal superlattices, thus enabling modulations in the superlattice structures from fcc to bcc symmetry.18 Furthermore, Bian et al. showed that, by changing the solvent vapor during colloidal solution evaporation, the superlattice structure could be tuned to fcc, bcc, and body-centered tetragonal (bct) symmetries.19 Recently, Quan et al. used different solvents to regulate the solvent-ligand interactions and to control the self-assemblies of Pt nanocubes from simple-cubic to bct structures.16 The preferred nanocrystal superlattices structures are not only determined by the intrinsic structure of the nanocrystals but also by the softness of their organic shell, and more importantly the surrounding solvent.15,

20, 21

Hence, in view of manipulating superlattices structures,


2

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


material chemists have focused their effort in controlling the interactions between ligand molecules as well as those between ligands and solvent molecules. Although some degree of success has been achieved in understanding and controlling those interactions and tuning the superlattices structures, very little is known for very small nanoparticles (~2nm) where the organic coatings occupy a remarkably larger volume than that of inorganic cores. Hence, exquisite control over the structure of superlattices made of very small nanoparticles remains a challenge. Nanocrystals aggregation is prevented by the presence of flexible organics coating them. Nevertheless, Ostwald repining, that is mass transportation between large and small nanocrystals, still occurs in those conditions, and yields an undesired broadening of the size distribution of dispersed nanocrystals even when dispersed in a suitable solvent.22 Ostwald ripening is widely investigated and well understood in the case of dispersed nanocrystals in solvent medium where Brownian motion takes place.23,

24

However, it remains rarely

investigated in nanocrystal solids in which nanocrystals are locally confined and periodically arranged. A recent study by Korgel’s group on the heating of Au nanocrystal superlattices showed that the ripening process is also present in nanocrystal solids where a large percentage of organics (>90%) remains in the interstices of sub-two Au nanocrystals. As a result, various binary nanocrystal superlattices were produced from the progressive nanocrystals bimodal dispersion emergence.25 However, the prediction of the outcome of the Ostwald ripening remains challenging since this process is dependent on complex interactions between the inserted solvent and the ligand when nanocrystal superlattices are heated. Here, we show, that the crystalline structure of superlattices, made of 2.2-nm Ag nanocrystals can be controlled through selection of the carrier solvent. Hence, ligandsolvent interactions are shown to play an important role in the formation of the superlattices.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

Both bcc and fcc superlattices are produced by evaporation of hexane and tetrachloroethylene (TCE) dispersions, respectively. The ripening process of nanocrystal superlattices is also studied by heating the superlattices of different supracrystalline structures.

Results and discussion

Ag nanocrystals ([Ag] ≈ 4.0 mg/mL) coated by oleylamine (C18-NH2) were dispersed respectively in hexane (inset Figure 1a) and tetrachloroethylene, TCE, (inset Figure 1e). One drop of each colloidal solution was deposited on a TEM grid coated by an amorphous carbon film. The TEM images and their corresponding size distribution histograms (Figures 1a, 1e and respective insets) show that the average diameter (2.2nm) and size distribution (10%) is not dependent on the solvent used to disperse the nanocrystals at room temperature. These two colloidal solutions were annealed for 12h at 60°C in air; one drop of such colloidal solutions was then deposited on TEM grids. Figure 1h shows that the 2.2-nm Ag nanocrystals dispersed in TCE tend to coalesce after annealing process whereas those that were dispersed in hexane retain their integrities (Figure 1d). Hence, it can be deduced that 2.2-nm Ag nanocrystals dispersed in TCE solvent shows weaker colloidal stability to mild annealing. This is attributed to a decrease of coating agent coverage with TCE compared to hexane, and such decrease is not significant enough to be detected by NMR. Keeping the nanocrystal concentration constant ([Ag] ≈ 4.0 mg/mL), 20 µL of these two colloidal solution batches were dropped on a TEM grid supported by a glass wafer at room temperature. After the solvent evaporation, brown films are formed onto the glass wafer. Small angle X-ray scattering (SAXS) patterns of the films, formed after evaporation of 2.2-nm Ag nanocrystals dispersed either hexane or TCE, show that nanocrystals are well


4

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


self- ordered either into body centered cubic, bcc, (Figure 1c) or into face centered cubic, fcc, structure (Figure 1g) respectively. This is further confirmed by the TEM images (Figures. 1b and 1f), which reveal a change in the 2.2-nm Ag nanocrystals ordering with typical (110) and (111) crystal plane parallel to the substrate for bcc and fcc structures respectively. The lattice fringe corresponding to (110) and (220) crystal planes of the bcc and fcc superlattices are 4.2-nm and 2.8-nm, and the edge-to-edge interparticle distance (δpp), calculated from the SAXS patterns, are 3.0 nm and 3.5 nm respectively. This is significative of a change in the degree of interdigitation between nanocrystals with the solvent used during superlattices growth. Then, these two samples, differing by the carrier solvent used to disperse the nanocrystals before evaporation, were placed in a furnace equipped with a programmable heating system and annealed simultaneously. The temperature was controlled from 25oC to 130oC with a heating rate of 1oC/ minute. Let us first consider the bcc superlattices produced by hexane evaporation of the colloidal solution. Figure 2 shows TEM images of bcc superlattices at different stages of the annealing process. TEM images of the superlattices annealed between 25 and 100°C exhibit a (110) crystal plane parallel to the substrate (Figures 2a-2h). The lattice fringe of this (110) crystal plane (Figure S1) remains unchanged (between 4.2-nm and 4.3-nm). By re-dispersing the sample after annealing process in hexane, the remaining colloidal solution is deposited on a TEM grid. The average diameter and size distribution determined by measuring more than 500 particles, remain unchanged, indicating that the 2.2-nm Ag nanocrystals within the bcc superlattices do not undergo any ripening process and remain stable below 100°C. A further increase in the annealing process to 120°C induces formation of larger nanoparticles (Figures 2i and 2j), indicative of nanocrystals melting, aggregation and consequently a partial destruction of bcc structure. This was further confirmed by the corresponding FFT (inset in Figure 2j) and the TEM image as well as the construction of


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

the size distribution histogram (Figures 3a and 3b). By further increasing the temperature to 130°C, it shows the appearance of large particles and a total destruction of their ordering (Figures 2k and 2l). This is confirmed by the corresponding FFT (inset in Figure 2l). In such condition, it is impossible to re-disperse the thermal treated samples. The average diameter is then determined by a direct measurement on the TEM images. Figure 4a and Table 1 show that bcc superlattices submitted to a progressive annealing process remain stable below 120 oC while an abrupt increase of in the nanocrystal sizes is observed when the temperature is further increased above this threshold. The superlattices produced from nanocrystals dispersed in TCE and subjected to the same treatment as described above are stable at temperatures ranging from 25 to 70°C. The corresponding TEM images (Figures 5a and 5b) and their respective FFT show that the superlattices remain stable in this temperature range. This is further confirmed by the lattice fringe of (220) crystal plane that remains unchanged on TEM images. Upon temperature increase above 70°C, new structures emerge coexisting with the fcc film. Figures 5d-5f show that at 80oC the superlattices are composed of two distinct nanocrystal sizes. This crystalline structure is well-known and called binary nanocrystal superlattices. A careful characterization of various crystal planes parallel to the substrate of such structure (Figures 5e and 5f) shows that it matches well a structure of atomic solids MgZn2 characterized by four molecular units per unit cell with the smaller Zn atoms ordered in tetrahedra8, 13, where the larger Mg atoms fall into the vacancies provided by the Zn tetrahedrons. As mentioned above, the nanocrystals subjected to annealing treatment are re-dispersed in TCE and deposited on TEM grid. Figures 3c and f show the appearance of a bimodal distribution with average nanocrystals diameters values of 2 and 3 nm respectively. A further increase of the annealing temperature up to 90°C shows the appearance of a structural transition with simultaneous formation of other two kinds of binary nanocrystal superlattices:


6

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaZn13-type27 and AlB2-type28 structures. Figure 6a shows the typical (100) crystal plane of NaZn13-type superlattices with the size of the crystal domains in the micrometer scale. The unit cell of NaZn13 structure contains 104 small spheres and 8 large spheres, which can be viewed as an icosahedron consisting of 13 small spheres inside simple cubic lattices of large spheres. Figure 6f shows the typical (001) crystal plane of AlB2-type superlattices which are based on a hexagonal unit cell and consisting of hexagonally ordered large nanocrystals, together with small nanocrystals inserted into the vacancies between the large nanocrystal layers. The crystal domains sizes of this structure are up to 2 micrometers. Figures 6b, 6c, 6g and 6h show (100), (011) crystal planes of NaZn13-type superlattices and (001), (110) crystal planes of AlB2-type superlattices respectively. From these, an average diameter of nanocrystals in the binary system of 5.5 nm for large nanocrystals and 2 nm for small ones can be deduced. TEM images and their corresponding size histograms of the nanocrystals after re-dispersion (Figures 3d and 3g) further confirm the bimodal distribution and average diameters of 2 and 5.5 nm respectively. A further temperature increase to 100°C induces another structural transition with the appearance of NaCl-type29 binary superlattices in place of the NaZn13 and AlB2-type superlattices that were observed at lower temperatures. Figures 7a and 7b show the typical (111) crystal plane of NaCl-type binary superlattices. The average diameters for the two types of nanocrystals are approximately 7 nm and 1.5 nm, respectively (Figures. 3e and 3f). It is pointed out here that the larger nanocrystals (7-nm) self-assembled both in NaCl-type binary system and single component close-packed hexagonal monolayer whereas the original superlattices (2.2-nm Ag nanocrystals) disappear. The number of layers ordered into close-packed hexagonal pattern drops to one or two, as confirmed by the FFT pattern (Figures 7c and 7d) and the interparticle distance (3.0 nm) decreases slightly compared to that of the superlattices prior to treatment (3.5 nm). No structural transitions could be observed above 120°C with binary


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

nanocrystal superlattices disappearing and a few layers of 10-nm Ag nanocrytals ordered in compact hexagonal network appearing at higher temperatures (Figures 8a and 8b). In summary, from room temperature to 70°C, the fcc superlattices originated produced from TCE solution keep their integrity and remain stable. From 70°C to 100°C, a progressive disappearance of the 2.2-nm Ag superlattices and the appearance of new structures with a simultaneous highly controlled smooth increase and decrease in the Ag nanocrystal diameters both characterized by a relatively low size distribution occur. Such processes induce various structural transitions due to the mass transport between smaller and larger nanoparticles up to reach a total disappearance of the small nanoparticles with formation of homogeneous single component superlattices. Figure 4b and Table 1 shows a simultaneous progressive increase and decrease of the large and small nanocrystals respectively to reach large nanoparticles with a rather low size distribution. At 130 oC, very large nanocrystals with various shapes such as plates and polyhedrons are obtained (Figures 8c and 8d). For temperatures ranging from room temperature to 120oC, the structural transitions are quantified by taking into account the ratio of effective diameter γeff,30 defined as γeff = Deff-small/Deff-large, where Deff is the sum of metal core diameter and twice the thickness of the organic ligand. Here, it is rather difficult to determine the shell thickness of the organic ligands. In first approximation, the shell thickness of the ligands is assumed to be similar to that measured previously31 for oleylamine coated Ag nanocrystals (~1.3nm). Table 2 summarizes the estimated diameter (D) at various temperatures, and consequently the γeff values. For the annealing temperatures ranging from 70 to 100 oC, the structure of superlattices evolves from fcc to MgZn2, to AlB2, to NaZn13, to NaCl structure. This corresponds to an approximate change of γeff from 1.0 to 0.82, to 0.51, to 0.51, to 0.43, approximately. Theoretical predictions32 based on the calculation of free energy of entropy


8

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contribution pointed out that MgZn2, NaZn13, AlB2 and NaCl are stable in the range of 0.76 ≤ γefftheo ≤ 0.84, 0.54 ≤ γefftheo ≤ 0.625, 0.45 ≤ γefftheo ≤ 0.61, and 0.414 ≤ γefftheo ≤ 0.45 respectively. Assuming the estimation of the shell length described above, the various γeff values corresponding to the binary structures produced and compared to γefftheo as a function of temperature can be given, as depicted in Figure 9. A good agreement with hard sphere model prediction is observed. These data unambiguously show that, during the annealing process, the nanocrystals are locally confined within the superlattices and ripening process occurs between the neighboring nanocrystals within the superlattices. Based on simple hard spheres packing rules, self-assembly of colloidal nanocrystals is typically driven by changes in the free-volume entropy combined with weak attractive interparticle forces. The gain in free volume entropy is related to the packing density (space-filling fraction p). Generally, it favors structures with the highest packing fraction.33 During this process, the entropic variation is positive and reaches its maximum when the ordering has the highest packing density. In other words, entropy is the main driving force for forming binary structures during annealing process, similarly to the assemblies of the colloidal balls in the absence of organic coatings. This is highly consistent with our previous results 34 describing binary nanocrystal mixtures with the same kind of ligand packing into structures predicted by the hard sphere model. The data presented above clearly demonstrate that the memory on the 3D superlattices fabrication is retained with a strong influence on their crystalline structures and their stabilities. Hence, by changing the solvent from hexane to tetrachloroethylene (TCE) to disperse 2.2-nm Ag nanocrystals, the crystalline structure of 3D superlattices evolves from bcc to fcc. This feature is relatively surprising and opens a new mean of predicting the crystalline structure of this type of assemblies. With hexane is used as solvent, the observed results are in agreement with the phenomenological approach


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

proposed by Korgel et al.5, where the softness of the ligand shell can be parametered by the ratio (χ) of the ligand length (L) to the particle radius (R) (i.e., χ=L/R). When the ratio χ < 0.7, the nanocrystals contact as hard spheres and form close-packed assemblies with fcc symmetry. By opposition, if the ratio χ > 0.7, the interdigitation between ligands becomes long-ranged, and can interact directly with chains belonging to it’s second-nearestneighbors,35 favoring the formation of more open bcc structure with lower packing fraction. In this study, the 2.2-nm Ag nanocrystals are coated with oleylamine whose chain length is around 1.8 nm. Hence the softness ratio of the nanocrystals studied here is calculated to be very large with χ = 1.6. This large softness ratio favors the formation of bcc structure in these conditions and can account to the formation of bcc structure when using hexane as the carrier solvent. However, the influence of the softness fails to explain observation of fcc structure when TCE is used as a solvent, even though the softness ratio of χ = 1.6 is expected for the same coating agent and inorganic core. Thus, other parameters have to be taken into account to predict such crystalline structure: (1) Ligand-solvent interactions: Hexane and TCE are both good solvents for nanocrystals coated by organics like oleylamine. There are stronger interactions between solvent and oleylamine molecules when nanocrystals are dispersed in TCE because of the presence of double bonds present in both species, compared to ligand-solvent interactions when using hexane. Then some of the oleylamine molecules are taken off from the nanocrystals into the solvent, reducing the ligand coverage of nanocrystals. Moreover, due to the interactions between the double bonds, more TCE molecules richly insert into the space between oleylamine molecules, preventing the interdigitation of ligands between the nearest nanocrystals during the formation of superlattices. This can be confirmed by the Energydispersive X-ray spectroscopy mapping of the superlattices obtained from TCE in Figure S4, which shows that the Cl-element are distributed in the superlattices. In such case, a


10

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ligand anchored to a given core can hardly extend to and interact directly with the chain belonging to it’s second-nearest-neighbors, consequently giving rise to the fcc crystal structure in superlattices, as shown in Scheme 1. This can also be confirmed by the interparticle distances between nanocrystals within the superlattices as mentioned above. The calculated chain length of oleylamine is 1.8 nm. When using TCE as solvent the interparticle distance is δpp =3.5, which is almost double value of the length of oleylamine molecule, indicating that the interdigitation between nanocrystals within fcc superlattices is much weaker than that obtained from hexane (δpp =3.0 nm). Furthermore, the trapping of the guest molecules into the nanocrystal superlattices can induce the structure transition from fcc to bcc.36 However, none of the guest molecules have been introduced into our system. (2) Influence of Halides : According to a recent study from Korgel’s group,5, 37 the stability of dodecanethiol-capped Au nanocrystals is significantly reduced when halides are present because the halide ions can exchange the thiolates on Au nanocrystal surface. Hence, it can be assumed here that TCE molecules take the place of some oleylamine ligands, reducing the ligands coverage of 2.2-nm silver nanocrystals dispersed in TCE. Furthermore, as mentioned above, the small TCE molecules could be incorporated into the space between ligands, decorating the soft shell as a mixture of oleylamine and TCE molecules. During evaporation of the carrier solvent, the interdigitation between ligands is largely reduced, consequently giving rise to fcc structures. Regardless the solvent’s nature, the 3D superlattice are stable in the range from 25°C to 70°C whereas the nanocrystals dispersed in TCE coalesce at 60°C. This confirms a stability increase of nanocrystals self-assembled in 3D superlattices compared to the same nanoparticles dispersed in a solvent. However, the crystalline structures of the 3D superlattices markedly change when nanocrystals are initially dispersed in TCE or in


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

hexane during the annealing process. These changes can also be argued to be related to the ligand coverage of nanocrystals in the superlattices, as mentioned above. Here a tentative mechanism is proposed: as discussed above, the nanocrystal surface is poorly covered with a mixture of oleylamine and TCE molecules when using TCE as a solvent. Upon progressive heating, the Ag surface atoms that are not bound to a ligand can move freely between nanocrystals, enabling typical Ostwald ripening. Furthermore, it is worth noting here that the space between the ligands is fully occupied by TCE molecules (scheme 1), which can promote the mobility of surface Ag atoms and enhance the mass transfer between the neighboring nanocrysals. However, it is widely accepted that Ostwald ripening is a common fluid phenomenon where the mass transfer occurs easily through the fluid medium. Here, the small Ag nanocrystals core are embedded in a liquid-like medium composed of the ligands and solvent, hence promoting the mobility of surface Ag atoms. When all the atoms involved in the formation of small nanoparticles are dissolved, only large nanocrystals forming the original network maintains the nanocrystals in compact hexagonal network. At 130°C, regardless of the solvent’s nature, oleyamine molecules are desorbed from the surface and the nanoparticles coalesce. Besides, the structural stability of nanocrystal superlattices differs from fcc to bcc structure, as reported recently38, and a higher structural stability in the bcc superlattice than the fcc superlattice was observed. Hence the inherent structural stability of superlattices can also probably play a role during the heat treatment of the different superlattices from fcc to bcc in our case.

Conclusion In summary, 2.2-nm Ag nanocrystals were used as building blocks to grow nanocrystal superlattices; it was shown that the carrier solvent plays a determining role in the packing way of nanocrystals. Significant ligand-ligand interdigitation between neghoring nanocrystals should also be taken into account when hexane is used as the


12

Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solvent, which results in bcc structure as predicted by Landman theory. However, drastic changes appear in the case of TCE dispersions; a mixed coverage of ligand and solvent, and a well solvated organic shell with the insertion of TCE molecules lead to not only a reduced ligand-ligand interdigitation, but also the formation of fcc structure with high packing density. Furthermore, by simultaneously annealing these two samples at various temperatures, significant changes appear. For bcc structures originated from the hexane suspensions evaporation, a simple coalescence mechanism dominates. However, for fcc structures originated from the TCE solutions, Ostwald ripening process takes place and enables the temperature-dependent production of a variety of binary nanocrystal superlattices, such as NaCl, AlB2, NaZn13 and MgZn2; this can be well explained by entropy driven process. Those findings provide insights into the solvent mediated nanocrystal superlattices and the Ostwald ripening process in nanocrystal superlattices.

Experimental Section Materials. Silver nitrate (99.9%), hexane (99.99%), TCE (Tetrachloroethylene, 99%), o-dichlorobenzene (99%), oleylamine (70%), oleic acid (70%) is purchased from Sigma, toluene (98%) from Riedel de Haen, ethanol (99.8%) from Prolabo. All reagents were used as received without further purification. Apparatus. Transmission electron microscopy (TEM) images were acquired on a JEOL JEM 1011(100 kV). Small-angle X-ray Scattering measurements were carried out with a homemade system mounted on a rotating copper anode generator (focus size: 0.1 × 0.1 mm2; 40 kV, 20 mA). Annealing process was achieved using Neytech Qex oven. (Product code number: 100/120 Volt-9494305.) Synthesis of Ag nanocrystals. Ag Nanocrystals of 2.2-nm diameter were synthesized by hot injection method23. In a typical synthsis of 2.2 nm Ag nanocrystals, 0.1g AgNO3 was dissolved in 5mL of o-dichlorobenzene (DCB) with the aid of 1mL of


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

oleylamine. Subsequently the transparent solution was injected into 10 mL of DCB solution containing 100 mg of dodecanediol and 150 µL of oleic acid at 180oC. Within a short period, the colour of the solution changed to dark brown, indicative of formation of Ag nanocrystals. After another 180s, the temperature was cooled down naturally to room temperature. After the synthesis, the oleic acid was washed away by ethanol-toluene cycle five times, and additional 50 µL of oleylamine was added during every washing cycle. Superlattices Formation: Ag nanocrystals were synthesized as described above and dispersed in a suitable solvent in order to obtain colloidal solution with [Ag] ≈ 4.0 mg mL-1, (here the concentration of Ag nanocrystals are calculated by drying the colloidal solution and weight the dried sample). Two specimens were prepared simultaneously: two glass wafers were deposited on a paper sheet. On one of the wafers, an amorphous carbon coated copper mesh TEM grid was deposited. The same amount of colloidal solution (20 µL) was deposited on the two glass wafers. After evaporation, a thin film is formed either onto TEM grid or on the glass wafer. The TEM grid was then submitted to the annealing process. Annealing process: (1) Annealing of deposited nanocrystal superlattices: The TEM grid deposited with superlattices were placed in the sample holder of Neytech Qex oven. The temperature of the oven was increased from 25oC to a desired temperature (70oC, 80oC, 90oC, 100oC, 120oC or 130oC) with a heating rate of 1oC/ minute. All the samples annealed at various conditions are fresh prepared. After annealing process, the samples were cooled down naturally to room temperature. All the experiments were performed in air. The photo image of the setup used for annealing process was shown in Scheme S1. (2) Annealing of nanocrystals in solution: An Eppendorf tube containing colloidal nanocrystal solution was placed in the Neytech Qex oven and the temperature was kept at 60oC for 12 hours. After that, the annealed colloidal solution was deposited on the TEM grid for the TEM characterization.


14

Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Pileni, M. P., Supracrystals of Inorganic Nanocrystals: An Open Challenge for New

Physical Properties. Acc. Chem. Res. 2008, 41, 1799-1809. 2.

Courty, A.; Richardi, J.; Albouy, P.-A.; Pileni, M.-P., How To Control the

Crystalline Structure of Supracrystals of 5-nm Silver Nanocrystals. Chem. Mater. 2011, 23, 4186-4192. 3.

Li, R. P.; Bian, K. F.; Wang, Y. X.; Xu, H. W.; Hollingsworth, J. A.; Hanrath, T.;

Fang, J. Y.; Wang, Z. W., An Obtuse Rhombohedral Suerlattice Assembled by Pt Nanocubes. Nano Lett. 2015, 15, 6254-6260. 4.

Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D., Assembly and self-

organization of silver nanocrystal superlattices: Ordered "soft spheres". J. Phys. Chem. B 1998, 102, 8379-8388. 5.

Korgel, B. A.; Fitzmaurice, D., Small-angle x-ray-scattering study of silver-

nanocrystal disorder-order phase transitions. Phys. Rev. B 1999, 59, 14191-14201. 6.

Chen, Z.; O’Brien, S., Structure Direction of II−VI Semiconductor Quantum Dot

Binary Nanoparticle Superlattices by Tuning Radius Ratio. ACS Nano 2008, 2, 1219-1229. 7.

Nykypanchuk, D.; Maye, M. M.; van der Lelie, D.; Gang, O., DNA-guided

crystallization of colloidal nanoparticles. Nature 2008, 451, 549-552. 8.

Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O'Brien, S.; Murray, C. B.,

Structural diversity in binary nanoparticle superlattices. Nature 2006, 439, 55-59. 9.

Wang, Z. W.; Schliehe, C.; Bian, K. F.; Dale, D.; Bassett, W.; Hanrath, T.; Klinke,

C.; Weller, H., Correlating Superlattice Polymorphs to Internanoparticle Distance, Packing Density, and Surface Lattice in Assemblies of PbS Nanoparticles. Nano Lett. 2013,13 1303-1311.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10.

Page 16 of 32

Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A., Nanoscale Forces and

Their Uses in Self-Assembly. Small 2009, 5, 1600-1630. 11.

Eldridge, M. D.; Madden, P. A.; Frenkel, D., Entropy-driven formation of a

superlattice in a hard-sphere binary mixture. Nature 1993, 365, 35-37. 12.

Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Titov, A. V.; Král, P.,

Dipole−Dipole Interactions in Nanoparticle Superlattices. Nano Lett. 2007, 7, 1213-1219. 13.

Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O'Brien, S., Structural

characterization of self-assembled multifunctional binary nanoparticle superlattices. J. Am. Chem. Soc. 2006, 128, 3620-3637. 14.

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, 2000-2005. 15.

Choi, J. J.; Bealing, C. R.; Bian, K.; Hughes, K. J.; Zhang, W.; Smilgies, D. M.;

Hennig, R. G.; Engstrom, J. R.; Hanrath, T., Controlling nanocrystal superlattice symmetry and shape-anisotropic interactions through variable ligand surface coverage. J Am Chem Soc 2011, 133, 3131-8. 16.

Quan, Z.; Xu, H.; Wang, C.; Wen, X.; Wang, Y.; Zhu, J.; Li, R.; Sheehan, C. J.;

Wang, Z.; Smilgies, D. M.; Luo, Z.; Fang, J., Solvent-mediated self-assembly of nanocube superlattices. J. Am. Chem. Soc. 2014, 136, 1352-9. 17.

Yang, C. W.; Chiu, C. –Y.; Huang, M. H., Formation of Free-Standing

Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution. Chem. Mater., 2014, 26, 4882-4888. 18.

Goodfellow, B. W.; Patel, R. N.; Panthani, M. G.; Smilgies, D.-M.; Korgel, B. A.,

Melting and Sintering of a Body-Centered Cubic Superlattice of PbSe Nanocrystals Followed by Small Angle X-ray Scattering. J. Phys. Chem. C 2011, 115, 6397-6404.


16

Page 17 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19.

Bian, K.; Choi, J. J.; Kaushik, A.; Clancy, P.; Smilgies, D.-M.; Hanrath, T., Shape-

Anisotropy Driven Symmetry Transformations in Nanocrystal Superlattice Polymorphs. ACS Nano 2011, 5, 2815-2823. 20.

Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X., Surface Ligand

Dynamics in Growth of Nanocrystals. J. Am. Chem. Soc. 2007, 129, 9500-9509. 21.

Kovalenko, M. V.; Bodnarchuk, M. I.; Talapin, D. V., Nanocrystal Superlattices

with Thermally Degradable Hybrid Inorganic−Organic Capping Ligands. J. Am. Chem. Soc. 2010, 132, 15124-15126. 22.

Voorhees, P. W., The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 38 , 231-

252. 23.

José-Yacamán, M.; Gutierrez-Wing, C.; Miki, M.; Yang, D. Q.; Piyakis, K. N.;

Sacher, E., Surface Diffusion and Coalescence of Mobile Metal Nanoparticles. J. Phys. Chem. B 2005, 109, 9703-9711. 24.

Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P., Oriented Pearl-Necklace Arrays of

Metallic Nanoparticles in Polymers: A New Route Toward Polarization-Dependent Color Filters. Adv. Mater. 1999, 11, 223-227. 25.

Goodfellow, B. W.; Rasch, M. R.; Hessel, C. M.; Patel, R. N.; Smilgies, D. M.;

Korgel, B. A., Ordered structure rearrangements in heated gold nanocrystal superlattices. Nano Lett. 2013, 13, 5710-4. 26.

Wei, J.; Schaeffer, N.; Pileni, M.-P., Ag Nanocrystals: 1. Effect of Ligands on

Plasmonic Properties. J. Phys. Chem. B 2014, 118, 14070-14075. 27.

Redl, F. X.; Cho, K. S.; Murray, C. B.; O'Brien, S., Three-dimensional binary

superlattices of magnetic nanocrystals and semiconductor quantum dots. Nature 2003, 423, 968-971.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

28.

Page 18 of 32

Chen, J.; Ye, X.; Murray, C. B., Systematic Electron Crystallographic Studies of

Self-Assembled Binary Nanocrystal Superlattices. ACS Nano 2010, 4, 2374-2381. 29.

Saunders, A. E.; Korgel, B. A., Observation of an AB Phase in Bidisperse

Nanocrystal Superlattices. ChemPhysChem 2005, 6, 61-65. 30.

Parthe, E. Z., Space Filling of Crystal Structures. Kirstallogr 1961, 115, 52-79.

31.

Wei, J. J.; Schaeffer, N.; Albouy, P. –A.; Pileni, M. –P., Surface Plasmon

Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals. Chem. Mater. 2015, 27, 5614-5621. 32.

Evers, W. H.; Nijs, B. D.; Filion, L.; Castillo, S.; Dijkstra, M.; Vanmaekelbergh, D.,

Entropy-Driven Formation of Binary Semiconductor-Nanocrystal Superlattices. Nano Lett. 2010, 10, 4235-4241. 33.

Bodnarchuk, M. I.; Kovalenko, M. V.; Heiss, W.; Talapin, D. V., Energetic and

Entropic Contributions to Self-Assembly of Binary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor. . J. Am. Chem. Soc. 2010, 132, 11967-11977. 34.

Wei, J. J.; Schaeffer, N.; Pileni, M. –P., Ligand Exchange Governs the Crystal

Structures in Binary Nanocrystal Superlattices. J. Am. Chem. Soc., 2015, 137, 14773– 14784. 35.

Landman, U.; Luedtke, W. D., Small Is Different: Energetic, Structural, Thermal,

and Mechanical Properties of Passivated Nanocluster Assemblies. Faraday Discuss., 2004, 125, 1-22. 36.

Nagaka, Y.; Chen, O.; Wang, Z. W.; Cao. C., Structural Control of Nanocrystal

Superlattices Using Organic Guest Molecules. . J. Am. Chem. Soc. 2012, 134, 2868-2871. 37.

Yu, Y.; Goodfellow, B. W.; Rasch, M. R.; Bosoy, C.; Smilgies, D.-M.; Korgel, B.

A., Role of Halides in the Ordered Structure Transitions of Heated Gold Nanocrystal Superlattices. Langmuir 2015, 31, 6924-6932.


18

Page 19 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38.

Bian, K. F.; Wang, Z. W.; Hanrath, T., Comparing the Structural Stability of PbS

Nanocrystals Assembled in fcc and bcc Superlattice Allotropes. J. Am. Chem. Soc., 2012, 134, 10787–10790.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

Figure 1. (a, e) TEM images of 2.2-nm nanocrystals deposited on copper grid from hexane (a) and TCE (e), respectively; inset images in panel a and d are corresponding molecular model of solvent hexane and TCE; and the histograms of nanocrystals from different solvent, respectively; (b, f) TEM images of superlattices assembled with 2.2-nm Ag nanocrystals from different solvents: hexane (b) and TCE (f), respectively; (c, g) Small angle X-ray diffraction pattern of superlattices assembled with 2.2-nm Ag nanocrystals from solvent hexane (c) or solvent TCE (g); (d, h) TEM image (d) of silver nanocrystals after annealing in hexane solvent at 60oC for 12h and the corresponding diameter histogram; TEM image (h) of silver nanocrystals after annealed in TCE solvent at 60oC for 12h and the corresponding diameter histogram.


20

Page 21 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. TEM images and magnified TEM images, of the bcc superlattices produced from hexane evaporation of 2.2-nm Ag nanocrystals colloidal solution and annealed at different stages: (a, b) at 25oC; (c, d) at 80 oC; (e, f) at 90 oC; (g, h) at 100 oC; (i, j) at 120 oC; (k, l) at 130 oC; inset up is the corresponding FFT, inset down is the model of 110 crystal plane of bcc superlattice.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

Figure 3. (a) TEM images and (b) diameter histograms of redispersed nanocrystals from bcc superlattices (produced from 2.2-nm Ag nanocrystals dispersed in hexane) annealed to 120oC. TEM images and diameter histograms of redispersed nanocrystals from fcc superlattices (produced from 2.2-nm Ag nanocrystals dispersed in TCE and annealed to : (c, f) 80oC; (d, g) 90oC; (e, h) 100oC.


22

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Diameter range of nanoparticles during the annealing process at different temperature produced from 2.2-nm Ag nanocrystals dispersed in hexane (a) and TCE (b) starting from bcc and fcc superlattices respectively.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

Figure 5. (a, b) TEM images of the fcc superlattices produced from TCE annealed to 70oC, (c) is the corresponding (111) crystal plane model of (b); (d, e, f) TEM images of the fcc superlattices produced from TCE annealed to 80oC, (g, h) model of (210) crystal plane and (001) crystal plane of MgZn2-type binary superlattices.


24

Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. TEM image and magnified TEM image of films produced from 2.2-nm Ag nanocrystals dispersed in TCE characterized by fcc superlattices and annealed at 90°C. Two types of binary structures are observed: (a, b, c) NaZn13-type binary nanocrystals superlattices with various crystal planes parallel to the substrate; (f, g, h) AlB2-type binary nanocrystals superlattices with various crystal planes parallel to the substrate; (d, e, i, j) crystal plane models correspond to b, c, g, h; inset images in panel a and f are the cooresponding FFT patterns.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

Figure 7. TEM image and magnified TEM image of annealing results at 100 oC of films produced from 2.2-nm Ag nanocrystals dispersed in TCE characterized by fcc superlattices: (a, b) NaCl-type binary nanocrystals superlattices with (111) crystal planes parallel to the substrate; (c, d) 1-2 layers nanocrystals with hexagonal packing mode; right side images of panel b and d are crystal plane models and the cooresponding FFT patterns.


26

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. TEM image and magnified TEM image of annealing results at 120 oC (a, b) and 130 oC (c, d) of films produced from 2.2-nm Ag nanocrystals dispersed in TCE characterized by fcc superlattices; inset image in panel b is the corresponding FFT; (e) nanocrystals histogram corresponds to panel a.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

Figure 9. The effective diameter ratio (γeff) of nanocrystals in binary superlattices appearing in the annealing process at different temperatures and the corresponding theoretical predictions γefftheo based on the calculation of entropy contribution. (color area)


28

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. The superlattice growth mechanism of 2.2-nm oleylamine coated Ag nanocrystals dispersed in hexane or TCE solvents.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

Table 1. The approximate diameter range of nanocrystals in the annealing sample at different temperature. fcc superlattices / nm bcc superlattices / nm Structures Size range/ nm Structures Size range/ nm fcc 2.2 ± 0.2 bcc 2.2 ± 0.2 MgZn2+fcc 1.6

Solvent-Mediated Crystallization of Nanocrystal 3D Assemblies of

Recommend Documents