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.
The interface between nano-sized precious metal clusters such as Silver (Ag) and a semiconductor such as graphite (Gr) is called a heterojunction (Ag/Gr). Heterojunctions have great promise in enhance solar energy conversion due to their unique and enhanced optical, electronic, and chemical properties. When excited with laser pulses, electrons in the system acquire a mean energy higher than its thermal equilibrium value and are referred to as "hot electrons". In fact, graphitic materials are model systems for the study of hot electron dynamics. An ineffective screening within the layers of graphite allows the hot electrons to reach temperatures comparable to that in the solar photosphere!
In a study supported by the Center for Chemistry at the Space-Time Limit recently published in the Journal of the American Chemical Society, Hrvoje Petek and his group modified the Gr surface with Ag nanoclusters (NC)s to investigate how the excitation of the plasmonic resonance of the Ag/Gr heterojunction affects the generation, spatial distributions, and relaxation processes of hot electrons. Plasmonic resonance is a prominent feature of precious-metal nanoparticles; it is a sharp and intense absorption band in the visible range that arise from a collective resonant oscillation of the free electrons of the conduction band of the metal called plasmon.