Supracrystals of <i>N</i>‑Heterocyclic Carbene-Coated Au Nanocrystals

Controlling the generation of organized 3D assemblies of individual nanocrystals, called supracrystals, as well as their properties, is an important challenge for the design of new materials in which the coating agent plays a major role. We present herein a new generation of structured fcc Au supracrystals made of <i>N</i>-heterocyclic carbene (NHC)-coated Au nanocrystals. The 3D assemblies were achieved by using benzimidazole-derived NHCs tailored with long alkyl chains at different positions. The average size of the nanocrystal precursors (4, 5, or 6 nm) and their ability to self-assemble were found to be dependent on the length, orientation, and number of alkyl chains on the NHC. Thick and large supracrystal domains were obtained from 5 nm Au nanocrystals coated with NHCs substituted by C14 alkyl chains on the nitrogen atoms. Here, the geometry of both the C_carbene–Au and N–C_alkyl bonds induces a specific orientation of the alkyl chains, different from that of alkylthiols, resulting in Au surface covering by the chains. However, the edge-to-edge distances in the supracrystals suggest that the supracrystals are stabilized by interdigitation of neighboring nanocrystals alkyl chains, whose terminal part must point outward with the appropriate geometry

Metal–Metal Binary Nanoparticle Superlattices: A Case Study of Mixing Co and Ag Nanoparticles

Here, Co/Ag binary nanoparticle superlattices were engineered. It is demonstrated that the Ag/Co nanoparticle size ratio is the dominating factor in the formation of binary nanoparticle superlattices. However, regardless of the relative ratio concentration of Co and Ag nanoparticles, the deposition temperature, <i>T</i>_d markedly changes the crystalline structure of binary superlattices. A systematic study of these parameters is presented in order to shed light on the driving force in the formation of binary metallic nanoparticle superlattices. For metal Co and Ag nanoparticles, the interparticle potential pairs are considered to be strong, but entropy is still the main driving force for the assembling into binary nanoparticle superlattices, rather than the energy arising from the interparticle interactions

Hierarchy in Au Nanocrystal Ordering in Supracrystals: III. Competition between van der Waals and Dynamic Processes

Au nanocrystals coated with thiol derivatives of varying chain sizes ranging from C₁₂ to C₁₆ were produced; two different size nanocrystals have been synthesized (5 and 7 nm in diameter) for each coating agent. All of those specimens are characterized by a low size distribution (below 7%). Those Au nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. These crystalline growths were carried out by slow and controlled solvent evaporation at different temperatures and under non-null partial solvent vapor pressure (<i>P</i>_t). We show that the order within the supracrystals is temperature-dependent when they are made of hexadecanethiol-coated gold nanocrystals, regardless of the size of the nanocrystals. The interparticle distances within the various supracrystals that were produced were determined by small-angle X-ray diffraction (SAXRD). We demonstrate that the interparticle distance is controlled not only by the presence of physisorbed thiol residues, as previously reported, but also, at higher temperatures, by the dynamics of the organic chains and the van der Waals forces involved between the metallic cores of the nanocrystals forming the structure

Solvent-Mediated Crystallization of Nanocrystal 3D Assemblies of Silver Nanocrystals: Unexpected Superlattice Ripening

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 superlattice structures are tuned from body-centered cubic (<i>bcc</i>) to face-centered cubic (<i>fcc</i>) when varying solvents from hexane to tetrachloroethylene (TCE). Furthermore, by simultaneously annealing these two samples at different temperatures, <i>bcc</i> structures originating from hexane solutions are dominated by a simple coalescence mechanism, while the <i>fcc</i> structure stemming from TCE solutions undergoes an 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 superlattice structures’ stability. Those findings provide insights into the solvent-mediated nanocrystal superlattices and the Ostwald ripening process in nanocrystal superlattices

Computational Matching of Surface Plasmon Resonance: Interactions between Silver Nanoparticles and Ligands

A multilayer model of a single coated nanoparticle has been refined through finite elements method based simulations and resulted in a successful matching of the experimental UV–visible spectra of ligand-coated silver nanoparticles. The computational matching of the surface plasmon resonance (SPR) band reveals both a ligand-type dependence of the effective plasma frequency and a size dependence of the SPR damping effect within the modeled nanoparticle. The observed differences of effective plasma frequency between thiol and amine-coated nanoparticles are consistent with the already known stronger bonding of thiols on silver compared to amines. The significant increase of the damping effect at the surface of the nanoparticle when increasing their size suggests an inverse relation between the ligand packing density and the nanoparticle size, which is supported by the expected influence of the surface curvature radius on the ligand packing

Ligand Exchange Governs the Crystal Structures in Binary Nanocrystal Superlattices

The surface chemistry in colloidal nanocrystals on the final crystalline structure of binary superlattices produced by self-assembly of two sets of nanocrystals is hereby demonstrated. By mixing nanocrystals having two different sizes and the same coating agent, oleylamine (OAM), the binary nanocrystal superlattices that are produced, such as NaCl, AlB₂, NaZn₁₃, and MgZn₂, are well in agreement with the crystalline structures predicted by the hard-sphere model, their formation being purely driven by entropic forces. By opposition, when large and small nanocrystals are coated with two different ligands [OAM and dodecanethiol (DDT), respectively] while keeping all other experimental conditions unchanged, the final binary structures markedly change and various structures with lower packing densities, such as Cu₃Au, CaB₆, and quasicrystals, are observed. This effect of the nanocrystals’ coating agents could also be extended to other binary systems, such as Ag–Au and CoFe₂O₄–Ag supracrystalline binary lattices. In order to understand this effect, a mechanism based on ligand exchange process is proposed. Ligand exchange mechanism is believed to affect the thermodynamics in the formation of binary systems composed of two sets of nanocrystals with different sizes and bearing two different coating agents. Hence, the formation of binary superlattices with lower packing densities may be favored kinetically because the required energetic penalty is smaller than that of a denser structure

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

Nanocrystals: Why Do Silver and Gold N‑Heterocyclic Carbene Precursors Behave Differently?

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal–N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal–NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (<b>L</b>^<b>1</b>), 1,3-bis­(mesityl)­imidazol-2-ylidene (<b>L</b>^<b>2</b>), and 1,3-bis­(2,6-^<i>i</i>Pr₂C₆H₃)­imidazol-2-ylidene (<b>L</b>^<b>3</b>), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine–boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag–NHCs in the presence of thiols whereas Au–NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (<b>L</b>^<b>3</b>). This demonstrates that a well-defined route for a given metal cannot be extended to another metal

Beyond Entropy: Magnetic Forces Induce Formation of Quasicrystalline Structure in Binary Nanocrystal Superlattices

Here, it is shown that binary superlattices of Co/Ag nanocrystals with the same size, surface coating, differing by their type of crystallinity are governed by Co–Co magnetic interactions. By using 9 nm amorphous-phase Co nanocrystals and 4 nm polycrystalline Ag nanocrystals at 25 °C, triangle-shaped NaCl-type binary nanocrystal superlattices are produced driven by the entropic force, maximizing the packing density. By contrast, using ferromagnetic 9 nm single domain (<i>hcp</i>) Co nanocrystals instead of amorphous-phase Co, dodecagonal quasicrystalline order is obtained, together with less-packed phases such as the CoAg₁₃ (NaZn₁₃-type), CoAg (AuCu-type), and CoAg₃ (AuCu₃-type) structures. On increasing temperature to 65 °C, 9 nm <i>hcp</i> Co nanocrystals become superparamagnetic, and the system yields the CoAg₃ (AuCu₃-type) and CoAg₂ (AlB₂-type) structures, as observed with 9 nm amorphous Co nanocrystals. Furthermore, by decreasing the Co nanocrystal size from 9 to 7 nm, stable AlB₂-type binary nanocrystal superlattices are produced, which remain independent of the crystallinity of Co nanocrystals with the superparamagnetic state

Unusual Effect of an Electron Beam on the Formation of Core/Shell (Co/CoO) Nanoparticles Differing by Their Crystalline Structures

In this study, an unusual effect of the electron beam in transmission electron microscopy (TEM) on the formation of Co/CoO core/shell structures is developed through careful in situ TEM/scanning TEM (STEM) analysis. By feature of the nanoscale precision of this approach, the electron beam-irradiated Co nanoparticles reveals remarkable resistance to oxidation compared to those without irradiation treatment. Moreover, the irradiated hcp single domain Co nanocrystals result in Co/CoO core/shell nanoparticles after oxidation, instead of the CoO hollow nanoparticles without irradiation treatment. This study highlights the electron beam can also play a role in nanoscale Kirkendall effect, in addition to the nanocrystallinity and 2D ordering effect that we have recently demonstrated. By careful in situ STEM-EELS (electron energy-loss spectroscopy) studies of the Co nanoparticles, it was found that the deliberately irradiated nanoparticles undergo an outward diffusion process of Co ions, forming an oxide layer with O species produced by the carboxylic group covalently bound to the Co atoms of the surface