Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Joel D. Eaves

Second Advisor

Sandeep Sharma

Third Advisor

Robert P. Parson

Fourth Advisor

Niels H. Damrauer

Fifth Advisor

Michael Hermele

Abstract

As the Earth's population grows, it becomes increasingly important to use our limited resources sustainably. Among our most important natural resources are fossil fuels and fresh water, but fossil fuel use is driving climate change and water security is under threat. This thesis uses molecular simulation, statistical mechanics, and molecular hydrodynamics to explore fundamental processes and develop tools and concepts for new classes of materials that have applications in water and energy sustainability.

Reverse osmosis, a water desalination process that removes salt from water using a semipermeable membrane, is one solution to the water security problem. Current membrane technology suffers from low throughput, however, necessitating high capital expenditures and large energy footprints for desalination plants. Membranes made from atomically thin two-dimensional crystals, like porous graphene, could increase throughput by orders of magnitude. This filtration process is difficult to understand and model, however, without the molecular dynamics method developed here. Our method is theoretically rigorous and faithful to both statistical mechanics and hydrodynamics. We apply this method to study atomically thin reverse osmosis membranes and find that the permeability of a membrane is not a simple function of the membrane's hydrophobicity. Quantifying the hydrophobic effect is a major area in theoretical chemistry, and this thesis contributes to our understanding by exploring the hydrophobic effect away from equilibrium.

Solar energy has the potential to compete with nonrenewable fossil fuels, but single junction cells are theoretically limited to 34% power conversion efficiency. Singlet fission, a photophysical process that occurs in some organic chromophores and splits high energy excitations into two lower energy ones, can make more efficient use of the solar spectrum and overcome this limit. Singlet fission rates depend very sensitively on the relative orientation of neighboring chromophores. Historically, singlet fission research has focused on the energetics of the photophysical process. In this thesis, we approach the problem, for the first time, through the dynamics and statistics of chromophore aggregation. We find that the aggregates do form crystal-like structures known to undergo fast singlet fission, making singlet fission compatible with simple and inexpensive solar devices like dye sensitized solar cells.

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