title: Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions.
abstract: Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.
authors.affiliation: Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Shijing
authors.initials: S
authors.lastname: Tan
authors.affiliation: ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China , Hefei, Anhui 230026, China.
authors.forename: Liming
authors.initials: L
authors.lastname: Liu
authors.affiliation: Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Yanan
authors.initials: Y
authors.lastname: Dai
authors.affiliation: Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Jindong
authors.initials: J
authors.lastname: Ren
authors.affiliation: Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.|ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China , Hefei, Anhui 230026, China.
authors.forename: Jin
authors.identifier: 0000-0003-1346-5280
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Hrvoje
authors.identifier: 0000-0001-9605-2590
authors.initials: H
authors.lastname: Petek