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Radiation effects from first principles : the role of excitons in electronic-excited processes.

Description: Electron-hole pairs, or excitons, are created within materials upon optical excitation or irradiation with X-rays/charged particles. The ability to control and predict the role of excitons in these energetically-induced processes would have a tremendous impact on understanding the effects of radiation on materials. In this report, the excitonic effects in large cycloparaphenylene carbon structures are investigated using various first-principles methods. These structures are particularly interesting since they allow a study of size-scaling properties of excitons in a prototypical semi-conducting material. In order to understand these properties, electron-hole transition density matrices and exciton binding energies were analyzed as a function of size. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in these structures. Based on overall trends in exciton binding energies and their spatial delocalization, we find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these systems.
Date: September 1, 2009
Creator: Wong, Bryan Matthew
Partner: UNT Libraries Government Documents Department

Enhanced molecular dynamics for simulating porous interphase layers in batteries.

Description: Understanding charge transport processes at a molecular level using computational techniques is currently hindered by a lack of appropriate models for incorporating anistropic electric fields in molecular dynamics (MD) simulations. An important technological example is ion transport through solid-electrolyte interphase (SEI) layers that form in many common types of batteries. These layers regulate the rate at which electro-chemical reactions occur, affecting power, safety, and reliability. In this work, we develop a model for incorporating electric fields in MD using an atomistic-to-continuum framework. This framework provides the mathematical and algorithmic infrastructure to couple finite element (FE) representations of continuous data with atomic data. In this application, the electric potential is represented on a FE mesh and is calculated from a Poisson equation with source terms determined by the distribution of the atomic charges. Boundary conditions can be imposed naturally using the FE description of the potential, which then propagates to each atom through modified forces. The method is verified using simulations where analytical or theoretical solutions are known. Calculations of salt water solutions in complex domains are performed to understand how ions are attracted to charged surfaces in the presence of electric fields and interfering media.
Date: October 1, 2009
Creator: Zimmerman, Jonathan A.; Wong, Bryan Matthew; Jones, Reese E.; Templeton, Jeremy Alan & Lee, Jonathan (Rice University, Houston, TX)
Partner: UNT Libraries Government Documents Department

LDRD final report : energy conversion using chromophore-functionalized carbon nanotubes.

Description: With the goal of studying the conversion of optical energy to electrical energy at the nanoscale, we developed and tested devices based on single-walled carbon nanotubes functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrated the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggested that upon photoabsorption, the chromophores isomerize to give a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations were used to study the chromophore-nanotube hybrids, and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments supported the notion of dipole changes as the optical detection mechanism.
Date: September 1, 2010
Creator: Vance, Andrew L.; Zifer, Thomas; Zhou, Xinjian; Leonard, Francois Leonard; Wong, Bryan Matthew; Kane, Alexander et al.
Partner: UNT Libraries Government Documents Department