This award is made by the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division and the recipients are Terry Bigioni and Jacques Amar of the University of Toledo.
The objective of this research is to investigate the role of forces and kinetics that control the self-assembly of ligand-passivated colloidal nanoparticles. Experiment and theory working together are to advance our knowledge of nanoparticle interactions for assembling high-quality 2D films of different colloidal nanoparticles with a range of ligands and solvents. The vast majority of non-aqueous nanoparticles are ligand-passivated, yet no comprehensive theory exists to describe their interactions. By studying both sub-monolayer and multilayer growth, the interactions and kinetics that control 2D and 3D assembly may be elucidated. Epitaxy in a regime where cluster diffusion and coalescence are significant is attainable in these experiments, thus opening the opportunity to adapt epitaxial growth theory to the nanoscale. The generalization of interfacial colloidal self assembly promises to enable fast, inexpensive and facile patterning of nanoscale objects far exceeding the limits of conventional lithography.
Significant impact reaches fields such as ultra-thin film coatings, catalysis, optoelectronics, sensors, and ultra-high density magnetic storage. Further, the theoretical and modeling strategies for assembling novel nanocomposite thin films and structures is a paradigm of materials design for a wide range of technologically relevant materials. This project provides educational opportunities by training graduate students, undergraduate students, high school students, and high school teachers in nanotechnology. The outcome of this research is used to enrich undergraduate and graduate courses, including the creation of new courses on microscopy, surface science and soft condensed matter.
By combining careful experiments with molecular dynamics simulations we were able to obtain a better understanding of the process of nanoparticle (NP) island self assembly at the toluene-air interface as well as of the dependence of the nanoparticle interactions on NP size. We successfully adapted models used for atomic epitaxial growth to explain and better understand the interfacial self assembly of gold NPs. The good agreement between our experimental results and theory demonstrated that epitaxial growth theory may be used to determine the details of nanoparticle interactions, which are difficult to access by other means. In addition, by carrying out molecular dynamics simulations we were able to determine the strength of the interaction between the NPs and the liquid-air interface as well as the effects of long-range interaction corrections on the binding energy. Our molecular dynamics results also demonstrate that, due to a competition between the surface tension and the free energy of mixing between the ligands and the solvent, the NPs remain primarily below the surface. As a result, the diffusion coefficient for NPs at the liquid-air interface is in good agreement with that expected for NP diffusion through the bulk liquid. Our experimental and simulation results also demonstrated the surprising result that, in addition to the short-range attractive interactions between NPs, there exists a long-range repulsion between NP islands. We found that this was a result of the asymmetry of the ligand distribution for NPs adsorbed at the liquid-air interface. This long-range repulsion leads to the ordering of large islands at high coverage, and thus plays a key role in the later stages of the self-assembly process. As part of the research carried out as part of this project, improved methods for the simulation of NP island growth and diffusion were developed along with a new self-consistent rate-equation method to study island nucleation and growth in the presence of significant island diffusion.
