Recent advances in understanding of hot carrier dynamics in chemical systems and solids for energy conversion and catalysis applications

Understanding the mechanisms and factors that govern plasmon energy conversion into hot carries is one of the first steps toward design of efficient materials that can harvest solar energy and catalyze CO2 conversion to fuels.

In the classical world, plasmons can be visualized as oscillations of electron density around fixed atomic positions within the metal material. Size, shape, composition and dielectric environment are contributing factors to plasmon frequencies, which are generally found in the visible and ultraviolet spectral regions. While the original plasmon excitation is a collective charge oscillation, it was recently found that it can be used to convert light into single electronic excitations, i.e., hot carriers, which in turn can be injected into a chemical reaction or collected in the solar cell. Photocatalytic reduction of CO2 to fuels using hot carriers is a promising strategy that JCAP is actively pursuing.

There are several processes that occur when electromagnetic field interacts with electrons in a metal, including the excitation of plasmons, their decay to hot carriers, the transport of hot carriers in plasmonic nanostructures and their collection either in adsorbed molecules or semiconductors. Different theoretical approaches are used to study each process, e.g., dielectric functions for plasmon excitation, electronic structure theory for carrier generation and transport and band/energy-level alignment analysis for collection. Collection of hot carriers in solid-state systems can be used for solar energy conversion devices. Hot carriers injected into molecules attached to a surface can induce or promote photochemical reactions, such as, CO2 reduction.

There are several processes that occur when electromagnetic field interacts with electrons in a metal, including the excitation of plasmons, their decay to hot carriers, the transport of hot carriers in plasmonic nanostructures and their collection either in adsorbed molecules or semiconductors. Different theoretical approaches are used to study each process, e.g., dielectric functions for plasmon excitation, electronic structure theory for carrier generation and transport and band/energy-level alignment analysis for collection. Collection of hot carriers in solid-state systems can be used for solar energy conversion devices. Hot carriers injected into molecules attached to a surface can induce or promote photochemical reactions, such as, CO2 reduction.

“Hot carrier created from decay of excited plasmons offer new kinetic pathways for catalysis through excited states, beyond those available from conventional electrocatalysis.  Our effort is focused on understanding the processes that enable us to tailor hot carrier distributions to provide these alternate catalytic pathways at useful rates.”

– Harry Atwater

The recent paper by Ma et al. (2015) describes the work done by JCAP researchers, who used real-time time-dependent density functional theory (rt-TDDFT) algorithm that they developed, to study a nanocluster composed of 55 silver atoms. Their calculations suggest that if there are single-particle d-to-s excitations resonant to the plasmon frequency, then most of the plasmon energy can be converted into hot carriers. One of the conclusions of this theoretical study is that plasmon decay and hot-carrier generation can be tuned through electronic structure modifications, potentially guiding the development of new materials for CO2 reduction. “This work is the first in treating plasmon excitation and single particle hot carriers in the same framework, which allows us to study the interplay between plasmon and single particle excitations”, said Lin-Wang Wang, the paper's corresponding author. “This work confirms that most of the plasmon energy can be converted into hot carriers, creating the theoretical foundation for using plasmons in photovoltaic applications or in photo-induced chemical reactions.”

The other recent publication by Brown et al. (2015) is a comprehensive theoretical ab initio modeling study of plasmon decay and hot carrier dynamics. Each of the most common plasmonic materials, such as silver and gold, were studied and modeled to reveal energies needed for direct and phonon-assisted hot-carrier generation. This work provides important guidance on the time and length scales that must be experimentally probed to study hot carrier generation and catalysis.

—Written by X. Amashukeli

This work was performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy (Award No. DE-SC0004993).  Calculations in this work used the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work was also made possible through the support of NSF and Resnick Institute graduate research fellowships and was done in collaboration with the Light-Material Interactions in Energy Conversion, a DOE Energy Frontier Research Center.


Publications:

  • Ma, J., Wang, Z. & Wang, L.-W. Interplay between plasmon and single-particle excitations in a metal nanocluster. Nature Communications, DOI: 10.1038/ncomms10107 (2015).
  • Brown, A. M. et al. Nonradiative Plasmon Decay and Hot Carrier Dynamics: Effects of Phonons, Surfaces, and Geometry. ACS Nono, DOI: 10.1021/acsnano.5b06199 (2015).

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