Efficient harnessing of solar energy arguably represents the premiere challenge to science in the upcoming decades, one with critical impact on controlling the deleterious effects of climate change. Several promising directions are offered by the field of “plasmonics,” which is based on the resonant absorption properties of metallic nanostructures (e.g. nanorods, nanospheres, nanoshells made from the coinage metals Au, Ag, Cu) that absorb intensely in the solar wavelength region. The challenge is that, although such structures can absorb solar light with staggeringly large cross sections (??? 100,000 Å2) and oscillator strengths vastly in excess of unity (f > 103-104), the resulting hot electron-hole pair excitation may live for only a relatively short time before relaxing by electron-electron and electron-phonon collisions into heat. Thus efficient excitation and harvesting of hot carriers from nanoscale metals is central to many emerging photochemical, photovoltaic, and ultrafast optoelectronic applications. Concentrated optical fields in plasmonic metal nanostructures can generate high densities of these excited charge carriers for a variety of physical, chemical, and biological applications. However, the detailed geometry- and field-dependent photoexcitation mechanisms determining the spatial, temporal, vector momentum, and energy distributions of these carriers in nanoplasmonic systems are still quite poorly understood.
To control and explore effects of nanoscale structure on the above dynamics, we have developed a novel ultrafast laser technique (scanning photoionization microscopy: SPIM) for simultaneous time-, angle-, and energy-resolved electron photoemission spectroscopy of single plasmonic nanoparticles. The results have been published in numerous high-profile venues (e.g., Nature Communications, ACS Nano). Applying insights from such studies with nanoscale spatial, femtosecond temporal, and/or angle-resolved momentum resolution, we are developing emerging methods for geometrical design and active optical control of nanoplasmonic hot carrier excitation and emission distributions. Uniform dielectric coatings, for example, provide a means of blocking or regulating hot carrier emission, while non-uniform coatings can provide nanoscale spatial selectivity. Nanoscale site selectivity can also be controlled on ultrafast timescales by optically addressing different polarization- and/or frequency-sensitive hot spots, particularly with ultrasharp nanocathode geometries such as nanostars. Furthermore, pump-probe photoemission studies clarify the tens-of-femtosecond but highly energy dependent timescales relevant for hot carrier extraction, which are crucial to efficient extraction/harnassing of electron-hole excitations from solar excitation. Efforts to extend these SPIM methods with polarized ultrafast light pulses will be used to explore Chiral Induced Spin Selectivity (CISS) as well as Photoelectron Circular Dichroism (PECD) effects in plasmonic and chiral molecular structures.
J. Pettine, P. Maioli, F. Vallee, N. Del Fatti, and D. J. Nesbitt, “Energy-Resolved Femtosecond Hot Electron Dynamics in Single Plasmonic Nanoparticles,” ACS Nano 17,10721–10732 (2023) https://doi.org/10.1021/acsnano.3c02062.
Pettine and D. J. Nesbitt, “Emerging Methods for Controlling Hot Carrier Excitation and Emission Distributions in Nanoplasmonic Systems”, J. Phys. Chem. C Invited Perspective 126, 14767–14780 (2022) https://doi-org.colorado.idm.oclc.org/10.1021/acs.jpcc.2c03425.
F. Medeghini, J. Pettine, S. M. Meyer, C. J. Murphy, and D. J. Nesbitt, “Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings,” ACS Nano, 22, 644−651 https://doi.org/10.1021/acs.nanolett.1c03569 (2022).
J. Pettine, S. M. Meyer, F. Medeghini, C. J. Murphy, and D. J. Nesbitt, “Controlling Hot Electron Spatial and Momentum Distributions in Nanoplasmonic Systems: Volume versus Surface Effects,” ACS Nano 15, 1566-1578 (2020); doi.org/10.1021/acsnano.0c09045.
J. Pettine, P. Choo, F. Medeghini, K. Culver, T. Odom, and D. J. Nesbitt, “Plasmonic Nanostar Photocathodes with Polarization- and Frequency-Controlled Directionality,” Nat. Commun. 11, 1-10 (2020); doi.org/10.1038/s41467-020-15115-0.
ultrafast lasers, velocity map imaging, hot electron, plasmonics, nanostructures, microscopy, solar energy, chiral induced spin selectivity (CISS), photoelectron circular dichroism (PECD)