Noble metal nanostructures allow to enhance and tune light absorption to efficiently produce plasmonic excitations, which couple strongly to two subsystems of excitations in the semiconductor: hot carriers and phonons. These excitations relax following complex pathways, unlocking numerous nanoplasmonic applications ranging from photocatalysis to photovoltaic.
In this work, we distinguish charge carrier and phonon dynamics in a plasmonic metal/semiconductor heterostructure, with the combined use of time- and angle-resolved photoemission spectroscopy (trARPES) and femtosecond electron diffraction (FED). We use trARPES to detect the non-equilibrium charge-carrier population, while with analysis of FED diffraction patterns we single out phonon dynamics.
The heterostructure is composed of Au nanoislands, grown epitaxially on single-crystalline, bulk WSe2. The epitaxial relationship between the atoms of the metal and the semiconductor is reflected on the electronic band structure and the diffraction pattern and it allows material-resolved trARPES and FED measurements. Exploiting the surface sensitivity of electron-based techniques, we restrict the probed area to the active interface, and by choosing different pump wavelengths we control the excitation of the semiconductor.
Surface decoration of WSe2 with Au is found to cause a significant shortening of the excitons’ lifetime and accelerated lattice heating, which is a strong indication of charge-transfer towards Au. Moreover, Au sensitizes WSe2 to sub-band-gap photons, allowing to observe non-equilibrium phonon populations in WSe2 when the pump wavelength is longer than the semiconductor absorption threshold. The corresponding lattice heating follows a nonlinear relationship with the incident laser fluence, which can be attributed to plasmonically enhanced phonon excitation.