Analogy Between Atoms in a Nanocrystal and Nanocrystals in a Supracrystal: Is It Real or Just a Highly Probable Speculation?

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

Do 8 nm Co Nanocrystals in Long-Range-Ordered Face-Centered Cubic (fcc) Supracrystals Show Superspin Glass Behavior?

Here, we show evidence for superspin glass behavior in long-range-ordered face-centered cubic (fcc) supracrystals of 8 nm Co nanocrystals as has been well-demonstrated for disordered 3D assemblies. The dynamic behavior shows a critical slowing down, and the characteristic relaxation time is found to diverge to a finite static glass temperature. The collective nature of the glass state is supported by the existence of a memory effect. We conclude that, in the case of magnetic nanocrystal assemblies where the individual nanocrystal anisotropy is low, superspin glass behavior is observed whatever the mesoscopic order is

How to Predict the Growth Mechanism of Supracrystals from Gold Nanocrystals

Here we report the influence of the nanocrystal size and the solvent on the growth of supracrystal made of gold nanocrystals. These parameters may determine the final morphology of nanocrystals assemblies with either a layer-by-layer growth or a process of nucleation in solution. Experiments supported by simulations demonstrate that supracrystal nucleation is mainly driven by solvent-mediated interactions and not solely by the van der Waals attraction between nanocrystal cores, as widely assumed in the literature

Hierarchy in Au Nanocrystal Ordering in Supracrystals: A Potential Approach to Detect New Physical Properties

Here we describe the morphologies of Au nanocrystals self-assembled in fcc 3D superlattices called supracrystals. The average size of the nanocrystals is either 5 or 7 nm with a very small size distribution (<7%). The coating agents used to stabilize the nanocrystals are dodecanethiol (C₁₂H₂₅–SH), tetradecanethiol (C₁₄H₂₉–SH), and hexadecanethiol (C₁₆H₃₃–SH). The influences of the evaporation time, the volume of the chamber used to evaporate the toluene solvent, and the substrate temperature are studied. For nanocrystals characterized by the same size and coating agent, the supracrystal morphologies markedly change on increasing the evaporation time from 8 to 9 to 25 h whereas a slight change takes place on increasing the chamber volume. The nanocrystals’ ability to self-order in supracrystals decreases upon increasing the chain length of the coating agent from dodecanethiol (C₁₂) to tetradecanethiol (C₁₄) to hexadecanethiol (C₁₆). Decreasing the evaporation rate (25 h) and/or increasing the substrate temperature (50 °C) improves the nanocrystal ordering in fcc supracrystals. A hierarchy in nanocrystal ordering has the following sequence disordered assemblies, supracrystal film sitting on a disordered nanocrystal film, supracrystal films grown layer-by-layer, and finally supracrystals grown in solution with various well-defined shapes

Collective Surface Plasmon Resonances in Two-Dimensional Assemblies of Au and Ag Nanocrystals: Experiments and Discrete Dipole Approximation Simulation

The UV–visible absorption spectra of two-dimensional (2D) assemblies of Ag or Au nanocrystals (NCs) are measured in transmission at variable incidence angles for different average NC diameters ranging from 7.0 to 3.9 nm. The absorption spectra dominated by the collective surface plasmon resonance (SPR) absorption band are compared to those calculated using the discrete dipole approximation (DDA) method. The anisotropy of the optical response is inherent to the planar geometry of the NC assembly because of the near-field coupling between nanocrystals. For Ag NCs, the absorption spectra reveal the splitting of the SPR band into two components, namely, the transverse and longitudinal modes. At variance, no SPR band splitting is observed for 2D assemblies of Au NCs with identical size and coating agent. These features were satisfactorily reproduced by DDA simulation even though the SPR mode energies deduced from our calculations slightly overestimate the measured ones. The influence of the coating dodecanethiol molecules on the electron refractive index inside the nanocrystals as well as that of the substrate could explain the mismatch between the measured and calculated spectra