Although gold nanoparticles stabilized by organic thiols are the building blocks in a wide range of applications, the role of the ligands on the plasmon resonance of the metal core has been mostly ignored until now. Herein, a methodology based on the combination of spectroscopic ellipsometry and UV-vis spectroscopy is applied to extract dielectric functions of the different components. It is shown that aromatic thiols allow a significant charge transfer at the hybrid interface with the s and d bands of the gold core that yields "giant" red shifts of the plasmon band, up to 40 nm for spherical particles in the size range of 3-5 nm. These results suggest that hybrid nanoplasmonic devices may be designed through the suitable choice of metal core and organic components for optimized charge exchange.

Damien; et al. (2019): Real-Time in situ Observations Reveal a Double Role for Ascorbic Acid in the Anisotropic Growth of Silver on Gold. ChemRxiv. Preprint.Rational nanoparticle design is one of the main goals of materials science, but it can only be achieved via a thorough understanding of the growth process and of the respective roles of the molecular species involved.We demonstrate that a combination of complementary techniques can yield novel information with respect to their individual contributions. We monitored the growth of long aspect ratio silver rods from gold pentatwinned seeds by three in situ techniques (small-angle x-ray scattering, optical absorbance spectroscopy and liquid-cell transmission electron microscopy). Exploiting the difference in reaction speed between the bulk synthesis and the nanoparticle formation in the TEM cell, we show that the anisotropic growth is thermodynamically controlled (rather than kinetically) and that ascorbic acid, widely used for its mild reductive properties plays a shape-directing role, by stabilizing the {100} facets of the silver cubic lattice, in synergy with the halide ions. This approach can easily be applied to a wide variety of synthesis strategies. File list (5) download file view on ChemRxiv AuAg in situ.pdf (794.37 KiB) download file view on ChemRxiv Supplementary_Information.pdf (2.72 MiB) download file view on ChemRxiv Video_1.mp4 (3.11 MiB) download file view on ChemRxiv Video_2.mp4 (2.97 MiB) download file view on ChemRxiv Video_3.mp4 (2.18 MiB)Real-time in situ observations reveal a double role for ascorbic acid in the anisotropic growth of silver on gold

Nanophase segregation of a bi-component thiol self-assembled monolayer is predicted using atomistic molecular dynamics simulations and experimentally confirmed. The simulations suggest the formation of domains rich in acid-terminated chains, on one hand, and of domains rich in amide-functionalized ethylene glycol oligomers, on the other hand. In particular, within the amide-ethylene glycol oligomers region, a key role is played by the formation of inter-chain hydrogen bonds. The predicted phase segregation is experimentally confirmed by the synthesis of 35 and 15 nm gold nanoparticles functionalized with several binary mixtures of ligands. An extensive study by transmission electron microscopy and electron tomography using silica selective heterogeneous nucleation on acid-rich domains to provide electron contrast supports simulations and highlights patchy nanoparticles with a trend towards Janus nano-objects depending on the nature of the ligands and the particle size. These results validate our computational platform as an effective tool to predict nanophase separation in organic mixtures on a surface and drive further exploration of advanced nanoparticle functionalization.

A basic understanding of the driving forces for the formation of multiligand coronas or self-assembled monolayers over metal nanoparticles is mandatory to control and predict the properties of ligand-protected nanoparticles. Herein, H nuclear magnetic resonance experiments and advanced density functional theory (DFT) modeling are combined to highlight the key parameters defining the efficiency of ligand exchange on dispersed gold nanoparticles. The compositions of the surface and of the liquid reaction medium are quantitatively correlated for bifunctional gold nanoparticles protected by a range of competing thiols, including an alkylthiol, arylthiols of varying chain length, thiols functionalized by ethyleneglycol units, and amide groups. These partitions are used to build scales that quantify the ability of a ligand to exchange dodecanethiol. Such scales can be used to target a specific surface composition by choosing the right exchange conditions (ligand ratio, concentrations, and particle size). In the specific case of arylthiols, the exchange ability scale is exploited with the help of DFT modeling to unveil the roles of intermolecular forces and entropic effects in driving ligand exchange. It is finally suggested that similar considerations may apply to other ligands and to direct biligand synthesis.

Engineering symmetry breaking in seed-mediated growth is a fundamental challenge to produce colloidal nanocrystals with controlled morphologies and properties. In this work, we show a simple, aqueous approach to breaking the inversion symmetry of silver nanorods by restricting growth to one end of the pentatwinned gold bipyramid seed. Controlled addition of dimethyl sulfoxide (DMSO) allows us to tune both the symmetry and the length and width of the objects.Simulations and experiments demonstrate the adsorption of DMSO, which displaces interfacial water, reduces binding of surfactant and chloride ions at the gold surface, and slows down the deposition kinetics of silver. Besides showing the potential of DMSO for controlling the synthesis of complex nanostructures, this work opens new perspectives for the study of the physical properties of non-centrosymmetric nanoparticles, e.g. by controlling their plasmon modes and their second-harmonic generation efficiency.

The use of optically controlled vapor bubbles for controlled synthesis and deposition at interfaces is a promising emerging technique, which is intrinsically limited by the ability of the system to provide enough heat for bubble formation that is typically addressed using a plasmonic substrate. Herein, a simple and fast method for laser-controlled printing of plasmonic nanoparticles onto nonplasmonic substrates is shown. The laser fluence needed to print the nanoparticle was lowest for nanoparticles in resonance with the laser wavelength, but the technique is also effective off-resonance. Hierarchical assemblies were obtained, where melting was observed up to a micron from the focal point of the laser beam. The assemblies show plasmonic properties in the unmelted region, as shown by surface-enhanced Raman scattering spectroscopy measurements. This work will lead to future studies on controlling the hierarchical structure of nanoparticle assemblies formed at the bubble interface toward applications in sensing and devices.

In this article, we revisit the colloidal stability of clay imogolite nanotubes by studying the effect of electrostatic interactions on geo-inspired synthetic nanotubes in aqueous dispersions. The nanotubes in question are double-walled aluminogermanate imogolite nanotubes (Ge-DWINTs) with a well-defined diameter (4.3 nm) and with an aspect ratio around 4. Surface charge properties are assessed by electrophoretic measurements, revealing that the outer surfaces of Ge-DWINT are positively charged up to high pH values. A series of Ge-DWINT dispersions have been prepared by osmotic stress to control both the ionic strength of the dispersion and the volume fraction in nanotubes. Optical observations coupled to small and wide-angle X-ray scattering (SAXS/WAXS) experiments allow us to unravel different nanotube organizations. At low ionic strength (IS < 10–2 mol L–1), Ge-DWINTs are fully dispersed in water while they form an arrested gel phase above a given concentration threshold, which shifts toward higher volume fraction with increasing ionic strength. The swelling law, derived from the evolution of the mean intertube distance as a function of the nanotube concentration, evidences a transition from isotropic swelling at low volume fractions to one-dimensional swelling at higher volume fractions. These results show that the colloidal stability of Ge-DWINT is driven by repulsive interactions for ionic strengths lower than 10–2 mol L–1. By contrast, higher salt concentrations lead to attractive interactions that destabilize the colloid suspension, inducing nanotube coagulation into larger structures that settle over time or form opaque gels. Detailed simulations of the WAXS diagram reveal that aggregates are mainly formed by an isotropic distribution of small bundles (less than four nanotubes) in which the nanotubes organized themselves in parallel orientation. Altogether, these measurements allow us to give the first overview of the phase diagram of colloidal dispersions based on geo-inspired imogolite-like nanotubes.

Tailoring the crystal structure of plasmonic nanoparticle superlattices is a crucial step in controlling the collective physical response of these nanostructured materials. Various strategies can achieve this goal for isotropic nanoparticles, but few of them have been successful with anisotropic building blocks. In this work we use hybrid particles, consisting of gold nanorods encased in silver shells with a thickness that can be controlled from a few atomic layers to tens of nanometers. The particles were synthesized, characterized by a combination of techniques and assembled into supercrystals with a smectic B configuration, i.e. a 2D in-plane periodic order without interplane lateral correlations. We showed that, by tuning the silver shell thickness, the in-plane order can be changed from hexagonal to square and the lattice parameters can be adjusted. The spatial distribution of the supercrystal was systematically studied by optical and electron microscopy and by small-angle X-ray scattering. Through optimized surface chemistry, we obtain homogeneous, millimeter-size films of standing nanoparticles, which hold promise for all applications using plasmon-enhanced technologies.