RII Track-4:NSF: Understanding the role of surface interactions in co-assembly of spherical and rod-shaped colloids

Louisiana State University, Baton Rouge LA

Investigators

Abstract

In nature, the brilliant colors of butterfly wings, peacock feathers, and beetle carapaces originate from the interference of light waves with the structural order in the material forming the wings/feathers. In contrast to dyes and pigments, structural colors do not absorb light but instead reflect it from a microscopically structured surface. The wavelengths of reflected light depending on the orientation of the object and the viewing angle, causing the shimmery, color-shifting effect of iridescence. This structural color can be also produced by assembling spherical particles (200-500 nm) into close-packed crystals on surfaces. The local ordering of the particles within a colloidal crystal is governed by the interaction between particles, and their particle packing efficiency. While most of the currently available experimental studies aim at maximizing the size of colloidal crystals, there is a significant knowledge gap in understanding the factors governing local order and its impact on structural color. This lack of understanding exists due to a complex interplay between the experimental parameters used in the assembly process and the corresponding structure obtained. Understanding the relationship between the crystal order of colloidal particles and the experimental processing parameters is the focus of the current proposal. This proposal will investigate the impact of the local assembly formed by spherical nanoparticles upon the addition of rod-shaped particles, and their corresponding on the brilliance of structural color. The project will outline a general strategy for synthesizing surface coatings with never fading colors, which is not the case for dye or pigment-based paints. The majority of currently used materials are the ground state, equilibrium phases of the assembled nanoparticles, which have well-defined properties but are bound by the limits of thermodynamics. To push the limits of material design and encode unusual properties in the mesoscopic materials, we need to look beyond the conventional equilibrium materials into the domain of non-equilibrium assemblies of non-spherical particles. Currently, there is a significant lack of understanding of principles governing the formation of assemblies of non-spherical colloids, and even less is known about their structure-property-function relationships. This knowledge gap exists due to the inherent complexity of the interparticle interactions involved in the assembly of non-spherical particles, corresponding dynamics, and control parameters. This proposal aims at understanding these aspects using rods and spheres as model nanoparticles. The proposal will address the following fundamental questions: (a) How do non-spherical particles interact? and which factors influence the interparticle interactions? (b) What is the effect of the presence of non-spherical particles on the crystallization of spherical nanoparticles? (c) How does the change in local crystalline order affect the optical properties of the assembled material? These questions will be addressed by performing systematic experiments on gold nanorods and nanospheres and following their assembly dynamics/kinetics using liquid-phase transmission electron microscopy (at Pacific Northwestern National Laboratory) and x-ray/light scattering (at Louisiana State University). Additionally, theoretical models will be developed which will provide a better understanding of the surface forces driving the assembly and assist in developing processing routes to synthesize nanoscale coatings of tunable properties such as structural color. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.