Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited gold Nanoparticles by transient Photoemission Spectroscopy
The incorporation of metallic nanoparticles (NPs) of different size and shapes dispersed into or deposited onto the various layers and interfaces of photovoltaic devices has proved to be an efficient strategy to trap the incident solar light inside the cell and produce more effective devices. It is crucial though, if we want to make solar cells more efficient, to fully understand the mechanisms of hot-electron generation and relaxation under optical excitation.
The goal of this work, published in Small, is directly observing the dynamic electronic evolution following the photoexcitation of plasmonic gold NPs.
We measured the ultrafast electron-temperature dynamics within an ensemble of plasmonic gold NPs deposited onto a transparent conductive oxide by means of ultrafast pump-probe photoemission experiments. The NPs were impulsively photoexcited close to their localized surface plasmon (LSP) resonance and ultrafast time-resolved photoemission spectra were acquired as a function of the delay time after the excitation.
The pump was an ultrashort pulse with wavelength λ= 650 nm while the probe was an extreme-ultraviolet pulse obtained by high-harmonic generation (HHG, photon energy 16.9 eV). We observed variations of the Fermi edge as a function of the time elapsed since the excitation pulse, ascribed this to the ultrafast heating and subsequent cooling of the electron gas. The electronic temperature quickly evolved within the first picosecond after excitation, peaking at 780-840 fs delay and then gradually relaxing towards the environment temperature. The agreement of the reported experimental results with theoretical predictions underline the importance of a direct ultrafast measurement of the electronic temperature in order to correctly evaluate the transient response of nanosystems in the ultrafast timescale.
Even if technically challenging, experiments like this can pave the way for direct and quantitatively more accurate studies of the electronic properties of metallic nanosystems.
This work was a joint effort of four institutes of the National Research Council – CNR-SPIN, CNR-IOM, CMR-NANO and CNR-ISM, together with the University of Genoa, the Polytechnic of Milan and the University of Milan. The experiments were performed at NFFA-Trieste facilities (SPRINT beamline).
The study was carried out in the framework of a Marie Słodowska Curie grant from the European Union, grant number: 799126.