Office: Ekeley W145E
Postdoctoral Scientist: Columbia University (2005-2009)
Ph.D.: Massachusetts Institute of Technology, 2005
B.S. Chemistry, B.S. Physics: University of Wisconsin, Madison (1999)
Nanotechnology/Materials, Physical Chemistry, Theoretical Chemistry, Biophysics, Chemical Physics
Energy is the currency of the modern world, but fossil fuel sources dominate the current energy economy. This is unsustainable from an ecological, political, and economic perspective. Because more solar energy strikes the surface of the earth in one hour than all of humanity consumes in one year, solar energy has a potential that few other available renewable resources can match. In the solar energy cycle, a growing number of scientists are articulating a picture of conversion and capture. In the conversion step, a photovoltaic efficiently converts solar photons into charge carriers. These carriers can either generate electrical power directly or store energy in energetic covalent bonds in the capture step. Our group uses theoretical chemistry to study the fundamental science that enables both processes.
Hotwired: Multiexciton dynamics in quantum nanostructures
Conventional semiconductor solar panels have a fundamental upper limit in their efficiency, called the Shockley-Queisser limit, that caps at ~ 30%. Silicon, for example, has its bandgap in the IR part of the spectrum, and when photons at multiples of the bandgap produce multiple electron hole pairs, or excitons, they quench rapidly. Quantum nanostructures have size, material, and shape dependent optical properties that can be synthetically manipulated in ways that are simply unavailable in bulk materials. If they can generate multiple exciton pairs per photon in a way that is fundamentally different than bulk materials, they may offer a way to break the Shockley-Queisser limit and facilitate the development of high efficiency low cost solar cells.
Using a combination of atomistic and phenomenological approaches, we are studying the optical properties of nanostructures, with focus on multiexciton generation and dynamics. This is a rich theoretical problem with immediate consequences. The goal of the program is first to understand multiple exciton generation in quantum nanostructures, and then to discover the material design principles that optimize multiexciton generation, lifetime and binding energy.
Pushing electrons: Multielectron dynamics in the condensed phase
Biological energy conversion cycles, such as respiration and photosynthesis, inspire modern energy capture design. Such energy transformation pathways have common themes: they use reactive high-energy intermediates, they activate strong bonds in small molecules, and they typically involve multielectron transfer.
We will start by unraveling the dynamics of multielectron transfer. Marcus theory and its variants describe single electron transfer in chemistry remarkably well, but multielectron transfer is still in its infancy. Electron correlation makes this a particularly challenging theoretical problem. By analyzing model Hamiltonians, we will study the full quantum dynamics of the multielectron process, and use molecular dynamics and ab-initio results to parameterize model Hamiltonians for specific molecular systems.
DNA is a rigid negatively charged polymer with a long persistence length. It is remarkable that in many in vivo biological scenarios, it is in highly energetically unfavorable configurations. The bacteriophages, viruses that infect bacteria, provide a striking example of this phenomenon. Once inside the host, these viruses hijack the available ATP machinery to drive a remarkably strong portal motor and wind their genome into their capsids. Because the linear dimension of the capsid is nearly an order of magnitude less than the length of the genome, the DNA gets compacted to nearly crystalline densities. This stored energy is crucial in the infection cycle, because the virus uses it to propel its genome into the host.
From a statistical mechanics perspective, this is a complex system. The motor thrusts the DNA into the capsid on a time scale that is short relative to the relaxation time of the polymer. The motor quenches the system out of mechanical equilibrium. In many ways, this problem is analogous to a structural glassformer, where a rapid thermal quench through the melting point takes the system to a glassy state instead of a crystalline one. We will use molecular dynamics simulations in conjunction with importance sampling methods to study this process, focusing on the role of static and dynamic disorder in the packaging and ejection process.
Glassy dynamics and biophysical chemistry
Quantum dynamics, molecular dynamics, and spectroscopy
(*) denotes that both authors contributed equally