Directed Self-Assembly of Micron-Sized Gold Nanoplatelets into Oriented Flexible Stacks with Tunable Interplate Distance.

A growing demand for control over the interparticle spacing and the orientation of anisotropic metallic particles into self-assembled structures is fuelled by their use in potential applications such as in plasmonics, catalysis, sensing, and optoelectronics. Here, we present an improved high yield synthesis method to fabricate micron- and submicron-sized gold nanoplatelets with a thickness less than 20 nm using silver nanoplatelets as seeds. By tuning the depth of the secondary minimum in the DLVO interaction potential between these particles, we are able to assemble the platelets into dynamic and flexible stacks containing thousands of platelets arranged face-to-face with well-defined spacing. Moreover, we demonstrate that the length of the stacks, and the interplate distance can be controlled between tens and hundreds of nm with the ionic strength. We use a high frequency external electric field to control the orientation of the stacks and direct the stacks into highly organized 2D and 3D assemblies that strongly polarize light.

An experimental and simulation study on the self-assembly of colloidal cubes in external electric fields.

When a suspension of colloidal particles is placed in an oscillating electric field, the contrast in dielectric constant between the particles and the solvent induces a dipole moment in each of the colloidal particles. The resulting dipole-dipole interactions can strongly influence the phase behavior of the system. We investigate the phase behavior of cube-shaped colloidal particles in electric fields, using both experiments and Monte Carlo simulations. In addition to a string fluid phase and a body centered tetragonal (BCT) crystal phase, we observe a columnar phase consisting of hexagonally ordered strings of rotationally disordered cubes. By simulating the system for a range of pressures and electric field strengths, we map out the phase diagram, and compare the results to the experimentally observed phases. Additionally, we estimate the accuracy of a point-dipole approximation on the alignment of cubes in string-like clusters.

Experimental investigation of selective colloidal interactions controlled by shape, surface roughness, and steric layers.

Photolithography can be used to form monodisperse colloids of well-defined, nonspherical shape in a negative photoresist, SU-8. In aqueous suspension, in the presence of dextran as a depletant, we showed previously that the aggregation of these particles was highly selective for the end-to-end configuration: cylinders assembled into linear aggregates that could extend to lengths of tens of units without significant lateral aggregation. This article presents an in-depth study of the mechanisms by which these particles aggregate. In particular, we focus on the roles of global shape, roughness, and adsorbed layers of surfactants in mediating depletion, van der Waals (vdW), and electrostatic interactions between these particles. We describe in detail the fabrication and characterization of the particles. To allow for the interpretations of the experiments, we present predictions for the interactions between mathematically ideal cylinders with smooth surfaces, and a statistical thermodynamic model for the linear assemblies. We present experimental observations of the state of aggregation as a function of concentration of dextran and ionic strength for typical particles that present roughness of larger amplitude on their rounded side walls than on their flat ends. We compare this behavior to that of particles that lack this contrast in roughness; this comparison indicates that roughness can serve to attenuate strongly the attractive depletion interactions. To achieve a more quantitative measure of this effect, we analyze size distributions of linear aggregates to calculate the energies of the end-to-end "bonds" on the basis of our statistical model. We find that both the depletion and vdW interactions are attenuated approximately 20 fold relative to predictions for smooth surfaces. We conclude with an assessment of outstanding questions and opportunities to exploit shape and roughness to direct the self-assembly of colloids.

In situ measurements of nanotube dimensions in suspensions by depolarized dynamic light scattering.

We show that the dimensions of carbon nanotubes (CNTs) in suspension can be characterized by depolarized dynamic light scattering. Taking advantages of this in situ technique, we investigate in detail the influence of sonication procedures on the length and diameter of CNTs in surfactant solutions. Sonication power is shown to be particularly efficient at unbundling nanotubes, whereas a long sonication time at low power can be sufficient to cut the bundles with limited unbundling. We finally demonstrate the influence of CNT dimensions on the electrical properties of CNT fibers. Slightly varying the sonication conditions, and thereby the suspended nanotube dimensions, can affect the fibers conductivity by almost 2 orders of magnitude.