Showing papers by "Bobby G. Sumpter published in 2004"

Photon antibunching from oriented semiconducting polymer nanostructures.

Abstract: Fluorescence intensity correlation measurements reveal that z-oriented nanostructures from single chains of a cyano-substituted polyphenylene vinylene (CN-PPV) polymer act as single-quantum emitters. Photon antibunching is observed for the first time on individual polymer nanostructures with a modulation depth exceeding 90%, providing definitive proof that radiative recombination of molecular excitons occurs at a single localized site within the folded polymer chain.

Formation of oriented nanostructures from single molecules of conjugated polymers in microdroplets of solution: The role of solvent

Abstract: We examine the role of solution-phase polymer geometries on the favorability of forming oriented nanostructures from single conjugated polymer molecules formed from microdroplets of ultradilute solution. To our knowledge, this is the first time single molecule fluorescence correlation spectroscopy has been used to extract structural information from solution-phase fluorescence measurements. We find that production of oriented species is strongly favored in “poorer” solvents, where the polymer chains have more compact solution-phase structures. While charging during droplet generation is essential for orientation, we conclude that most of the internal organization associated with these interesting structures takes place in solution, independent of the droplets.

Feature Article: Experimental Design and Molecular Modeling of Novel Graft Copolymers

Abstract: The tensile properties of tetra-functional multigraft copolymers have been shown to have surprising high strain at break (~2100%), about double that of commercial thermoplastic elastomers such as Kraton! Currently, multigraft copolymers can be synthesized with a variety of branches (single, bi, tri-, tetra, and with different lengths) at each branch point and there can be a large number of branch points per molecule that are regularly, randomly, or heterogeneously spaced, each of which can have effects on mechanical properties. Unfortunately experimental synthesis and characterization of these novel polymer systems is quite time consuming. This is where molecular modeling and simulation can be critical for mapping out the fundamental mechanisms responsible for the observed behavior and to optimize/focus the experimental efforts. In this article we report details of our experimental synthetic and characterization effort along with some preliminary results from molecular dynamics, molecular mechanics, Monte...

Multiscale Simulations of Carbon Nanotube Nucleation and Growth: Electronic Structure Calculations

Abstract: Several first-principles surface and bulk electronic structure calculations relating to the nucleation and growth of single-wall carbon nanotubes are described. Density-functional theory in various forms is used throughout. In the surface-related calculations, a 38-atom Ni cluster and several low-index Ni surfaces are investigated using pseudopotentials and plane-wave expansions. The energetic ordering of the sites for C atom adsorption is found to be the same, with the Ni(100) facet favored. The bulk diffusion coefficient of C in Ni as a function of cluster size and temperature is calculated from various molecular dynamics approaches. In another group of bulk-related calculations, Gaussian orbital basis sets are used to study a cluster or "flake" containing 14 C atoms. The flake is a segment of three hexagons from an "unrolled" carbon nanotube, with an armchair termination. The binding energies of C, Ni, Co, Fe, Cu, and Au atoms to it were calculated in an effort to gain insight into the mechanism for the high catalytic activity of Ni, Co, and Fe and the lack of it in Cu and Au. The binding energies of Cu and Au are about 1 eV less than those of the three catalytic elements. Similar methods are used to study the initial stages of nanotube growth within the context of classical nucleation theory. Finally, issues relating to the establishment of a fundamental catalytic mechanism are addressed.