(Invited) Ultrafast Generation and Characterization of Hot Carriers in Plasmonic Nanostructures for Solar Light Harvesting

Abstract

Photoexcitation of plasmonic materials can create energetic electrons and holes that are of strong interest for solar energy conversion, photocatalysis, and light harvesting applications. Since intraband photoexcitation can create energetic (other terminology includes “hot” or “nonthermal”) carriers that are up to the photon excitation energy above the Fermi energy, their potential for increasing the efficiency of optical energy conversion is clear. However, in practice there are several challenges to realizing the potential of hot carriers. First, the efficiency of hot carrier generation is low in many plasmonic materials. Second, the energetic carriers generally thermalize very quickly, on the order of 10-100 femtoseconds, thereby making their extraction before energy loss difficult. Third, the energetic carriers tend to produce small changes in the permittivity of the metal that require advanced spectroscopic experimentation or devices in order to characterize them. In this talk I begin with describing the principles of hot carrier generation and the large advantages of using nanostructured metals to improve their generation efficiency. This is followed by our efforts to spectroscopically characterize hot electrons on an ultrafast timescale, as well as to design new nanostructures for creating hot electrons in greater numbers. Physical insights into how to extract the hot carriers before loss of energy to thermalization is given, and our efforts to understand the anisotropic decay of hot carriers is described. Recent efforts to directly extract the energetic distribution of hot carriers as a function of time are also discussed. I further describe recent experiments on specific photocatalytic processes and new biomimetic nanostructures for light harvesting. These biomimetic structures enable unique opportunities for tuning structure and energy transport properties. This work focuses on the use of chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. A closing description of the research opportunities and capabilities at the Center for Nanoscale Materials user facility is also given. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.