Synthesis and Self-Assembly Behavior of Charged Au Nanocrystals in Aqueous Solution

A series of water-soluble Au nanocrystals with different core sizes coated by either negatively or positively charged ligands are synthesized. We find a ligand interexchange process takes place when positively and negatively charged nanocrystals are mixed together and heated, resulting in mixed charged zwitterionic nanocrystals. The ligand exchange process between nanocrystals is studied in detail by electrophoresis. Self-assembly properties of the monocharged and zwitterionic nanocrystals are studied subsequently. By using the solvent evaporation process only the zwitterionic and positively charged nanocrystals can pack into well-ordered fcc lattice films. Under the nonsolvent diffusion condition, only the zwitterionic nanocrystals can aggregate and form shaped supracrystals. Structural analysis shows that the interparticle distance of the shaped supracrystal made of zwitterionic nanocrystals is 1 nm larger than that of the film one. The different interparticle distance is ascribed to the different fabrication process. We consider that nanocrystals adopt the closest packing in the film supracrystal due to the destroyed electrical double layer during the drying process, while the electrostatic repulsion plays an important role in determining the interparticle distance in the shaped supracrystal

Surface Plasmon Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals

Silver nanocrystals differing in coating agent and size self-assembled into thin supracrystalline films. The surface plasmon resonance (SPR) properties of these assemblies are presented herein. Nanocrystal size, interparticle distance, and coating agent play key roles in the plasmonic coupling of Ag nanocrystals within supracrystals. Here, we demonstrate experimentally that the predictions for 2D self-assemblies remains valid for thin 3D superlattices. The absorption spectra in the visible range were found to be markedly dependent on the incidence of the light source and confirmed the appearance of a splitting of the dipolar surface band into two components at increasing incidence angle. The major parameter inducing the splitting of the SPR band was found to be the relative ratio between the average distance of the nanocrystals and their diameters. The nature of the coating agent was also found to be of particular importance: Theoretical predictions and experimental data were in agreement for alkylamine-coated nanocrystals but differed for thiol-coated nanocrystals

Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations

Under solvent saturation, a precipitation of full-grown supracrystals on the one hand and the formation of well-defined supracrystalline films at the air–liquid interface on the other hand were previously observed for the first time (<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 3714–3719). Here, these two simultaneous growth processes are studied by additional experiments and by Brownian dynamics simulations. The thickness of the supracrystalline films and the concentration of free nanocrystals within the solution are measured as a function of the nanocrystal size. The simulations show that the first process of supracrystal growth is due to a homogeneous nucleation favored by solvent-mediated ligand interactions, while the second one is explained in terms of a diffusion process caused by a decrease in the surface energy when the particles penetrate the air–liquid interface. It is also verified that the presence of thiol molecules at the air–solution interface does not hinder the formation of supracrystalline films

Spontaneous Formation of High-Index Planes in Gold Single Domain Nanocrystal Superlattices

Crystals of nanocrystals, also called supracrystals and nanocrystal superlattices, are expected to exhibit specific properties that differ from both the corresponding bulk material and nanosized elementary units. In particular, their surfaces have a great potential as nanoscale interaction plateforms. However, control of the symmetry, compacity, and roughness of their surfaces remains an open question. Here, we describe the spontaneous formation of upper vicinal surfaces for supracrystals of Au nanocrystals grown on a sublayer of ordered Co nanocrystals. Stepped or kinked surfaces vicinal to the {100}, {110}, and {111} planes are observed to be extended on the micrometer range. The formation of such high-index planes is explained by a heteroepitaxial relationship between both Co and Au nanocrystal superlattice

Hierarchy in Au Nanocrystal Ordering in a Supracrystal: II. Control of Interparticle Distances

Au nanocrystals coated with thiol derivatives differing by the length of their alkyl chains are used to build 3D superlattices called supracrystals. In this study, we used two sets of Au nanocrystals differing by their sizes and size distributions. The average sizes are 5 nm (Au₅) and 7 nm (Au₇). From one experiment to the other, the size distribution slightly changes. For Au₅ nanocrystals, it evolves from 6 to 8%, and for Au₇ nanocrystals, it varies from 5 to 6%. The Au nanocrystals (Au₅ and Au₇) are first dispersed in toluene and produce fcc supracrystals by solvent evaporation. Here, by small-angle grazing X-ray diffraction, we observe a control in the average interparticle distance within the supracrystals. When the supracrystals are grown at zero toluene vapor pressure, the interparticle distances increase linearly with the alkyl chain length of the nanocrystals’ coating agent regardless of their diameters. Furthermore, the dry supracrystals can swell and the interparticle distance within the superstructure be increased by subjecting the material to toluene vapor pressure after initial growth. This swelling process is reversible, and retraction occurs when the toluene vapor pressure drops. This indicates a strong ability of the dried supracrystals to trap toluene molecules. On increasing the toluene vapor pressure during the solvent evaporation process, the slope of the linear dependency of the interparticle distances to the alkyl chain length is markedly decreased and the interparticle distance reaches a quasi-plateau. This is explained by the influence of depletion forces created by the presence of thiol-containing molecules physisorbed on the coating molecules on the internal structure of these supracrystals. Recently, we demonstrated that, by using the same nanocrystals (Au₅ and Au₇), a hierarchy in the supracrystal growth process takes place from heterogeneous nucleation with the formation of a layer-by-layer film to homogeneous nucleation in solution with the formation of shaped supracrystals. Here it is shown that the interparticle distance is independent of the supracrystal growth mechanisms

X‑ray Scattering Determination of the Structure of Water during Carbon Nanotube Filling

We present in situ monitoring of water filling of single-walled carbon nanotubes at room temperature, using X-ray scattering. A systematic method is developed to determine the water radial density profile. Water filling is homogeneous below about 5% in mass, whereas it structures into three layers above. These results should motivate further theoretical and simulations studies and allow getting a better understanding of the very peculiar properties of water confined in hydrophobic environment

Dispersion of Hydrophobic Co Supracrystal in Aqueous Solution

Assembly of nanoparticles into supracrystals provides a class of materials with interesting optical and magnetic properties. However, supracrystals are mostly obtained from hydrophobic particles and therefore cannot be manipulated in aqueous systems, limiting their range of applications. Here, we show that hydrophobic-shaped supracrystals self-assembled from 8.2 nm cobalt nanoparticles can be dispersed in water by coating the supracrystals with lipid vesicles. A careful characterization of these composite objects provides insights into their structure at different length scales. This composite, suspended in water, retains the crystalline structure and paramagnetic properties of the starting material, which can be moved with an applied magnetic field

Simultaneous Growths of Gold Colloidal Crystals

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or <i>supracrystals</i>. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air–liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties

Crystallinity Segregation upon Selective Self-Assembling of Gold Colloidal Single Nanocrystals

Spontaneous separation of single from polycrystalline 5 nm gold nanocrystals (NCs) is observed in colloidal solution. This segregation takes place upon self-assembling of single crystalline NCs at the air–solvent interface and in precipitated superlattices. Polycrystalline NCs are observed to remain in the suspension. Transmission electron microscopy analysis of the size distribution of NCs issued from the different populations indicates that the NC size does not change from each other, excluding therefore any size segregation in this process. Using both low-frequency Raman scattering and X-ray diffraction provides reliable characterization of nanocrystallinity for each population of NCs, thus confirming the crystallinity segregation. The single crystalline NCs are found by electron diffraction to self-assemble into close-packed superlattices with long-range translational and orientational ordering, while polycrystalline NCs behave like spheres with no preferential orientation. The face-to-face orientational ordering, which is only observed for single crystalline NCs, supports the relevance of the specific crystallinity-related morphologies of these NCs in their better ability to self-assemble. Exploiting this spontaneous segregation would open up a simple alternative to other demanding routes for controlling crystallinity of nanocrystals and optimizing their properties for potential applications

Tuning the Growth Mode of 3D Silver Nanocrystal Superlattices by Triphenylphosphine

Here, we show that the residual presence of reaction products markedly influences the growth mode of 3D superlattices made of silver nanocrystals. The silver nanocrystals (5 or 6 nm in diameter) are synthesized via the reduction of a silver triphenylphosphine complex in the presence of thiols. The presence of triphenylphosphine, coming from the decomposition of the silver precursor, is shown to change the superlattice growth mode in hexane from heterogeneous to homogeneous. In toluene, it remains homogeneous. This is in agreement with simulations based on a Flory-type model, which shows that the interaction potential between the silver nanocrystals in the presence of triphenylphosphine changes from repulsive to attractive in hexane and becomes even more attractive in toluene. Ex situ investigation of the different superlattices following solvent evaporation by scanning electron microscopy reveals different morphologies depending on the growth mode. A transition from films to individual hexagonal-shaped superlattices is observed when the growth mode changes from heterogeneous to homogeneous. Direct in situ observations of the 3D superlattice growth by liquid cell transmission electron microscopy follow in real time the formation process and movement of the superlattices in both hexane and toluene solvents