Dynamics of order formation by colloidal adsorption onto a substrate studied with Brownian dynamics

Modeling order-disorder boundaries of colloidal dispersions in organic solvents using interaction force measurements

HYPOTHESIS

The formation of soft colloidal crystals, which are nonclose-packed ordered arrays of colloidal particles suspended in a solvent, is dictated by a single physical factor that yields a fixed threshold at order-disorder boundaries for different experimental conditions such as ion concentration, solvent type, and particle size. Identifying the determinant factor and its threshold value should enable the prediction of the critical concentrations of colloidal particles to form soft colloidal crystals.

EXPERIMENTS

Soft colloidal crystals were fabricated using a series of monohydric alcohols as dispersion media and reflectance spectra were measured to locate order-disorder boundaries. The interaction forces acting between particles were also measured by employing atomic force microscopy.

FINDINGS

The interparticle forces at the order-disorder boundaries exhibited a universal threshold that was independent of the solvent types including alcohols and water. Therefore, the determinant factor for the formation of soft colloidal crystals was determined to be the force acting between the particles. Furthermore, a priori calculation of this critical force and consequently the critical particle concentration in colloidal systems was demonstrated by referring to the pressure at the liquid-to-solid transition in a hard sphere system (Alder transition).

Short-range ordered 2D nanoholes: lattice-model and novel insight into the impact of coordination geometry and packing on their propagating-mode transmittance features

Optically thin perforated gold films, fabricated using template colloidal masks self-assembled by following an elsewhere described simplified colloidal lithography protocol, are presented and discussed with the aim to develop a theory of short-range ordered nanoholes without straightforwardly extending concepts strictly related to periodic nanoholes. By Scanning Electron Microscopy (SEM) analysis of the evolution of nanohole short-range ordering and spatial coordination geometry under increasing interhole average spacing (d NN), unprecedented differences in the spectroscopic response are pointed out with respect to periodic systems. First, the dependence of the wavelength of a propagating plasmon mode on d NN is demonstrated to deviate from the linear relationship predicted by the grating-coupling picture developed for periodic arrays. Second, d NN cannot be straightforwardly interpreted as the counterpart of the lattice constant of periodic nanoholes, which demands to introduce a conceptually more rigorous periodicity-like length-scale. Once the impact of these findings on setting the operating parameters of a nanohole distribution is demonstrated, they are related, experimentally and by using a theoretical model developed by the authors, to the changes of the local coordination geometry (from quasi-hexagonal to quasi-square packing through mixed hexagonal-square coordination) induced by varying d NN over a wide interval. Autocorrelation analysis of SEM images is exploited to estimate a short-range periodicity-like length-scale, as a conceptual advance for laying the foundation of the concept of short-range ordered nanohole lattices and for deeper insight into the spectral response. As discussion is based on realistic, rather than simulated, evolution of colloidal arrangements, the formulated interpretative model accounts for realistic effects impacting transmission resonances.

DNA-Directed Non-Langmuir Deposition of Programmable Atom Equivalents

Particle assembly at interfaces via programmed DNA interactions allows for independent modification of both nanoparticle-surface interaction strength and the magnitude of interparticle repulsion. Together, these factors allow for modification of the deposited thin film morphology via alterations in DNA binding sequence. Importantly, both Langmuir and random sequential adsorption models yield insights into the thermodynamics of deposition but cannot fully explain particle coverage as a function of all relevant variables, indicating that the particle deposition mechanism for DNA-grafted colloids is more complex than prior adsorption phenomena. Here, it is shown that these deviations from standard behavior arise from the fact that each nanoparticle is attached to the surface via multiple weak DNA duplex interactions, enabling diffusion of adsorbed colloids across the substrate. Thus, surface migration of individual particles causes reorganization of the deposited monolayer, leading to the unusual behavior of coverage increasing at elevated temperatures that are just below the particle desorption temperature. The programmability of DNA-directed particle deposition therefore allows for precise control over the morphology of monolayer films, as well as the ability to generate crystalline materials with controllable surface roughness and grain size through layer-by-layer growth. The increased control over thin film morphology potentially enables tailoring of mechanical and optical properties and holds promise for use in a variety of applications.

Soft repulsive interactions, particle rearrangements and size selection in the self-assembly of nanoparticles at liquid interfaces

In the adsorption of nanoparticles at liquid interfaces, soft and short ranged repulsive effective interactions between the nanoparticles at the interface may eventually induce crowding, slow dynamics and jamming at high surface coverage. These phenomena can interfere during the adsorption process, significantly slowing down its kinetics. Here, by means of numerical simulations, we find that modifying the effective interactions, which can be achieved for example by grafting differently functionalized polymer shells on the bare nanoparticles, may qualitatively change such interplay. In particular our results suggest that, in the presence of ultrasoft particle interactions such as the ones described by a Gaussian Core Model potential, a small size polydispersity can be sufficient to decouple the adsorption kinetics from the slow dynamics that develops at the interface, due to a qualitative change from an irreversible adsorption controlled by particle rearrangements at the interface to one dominated by size selection mechanisms. These findings may be useful to achieve higher surface coverages and faster adsorption kinetics.