Optical Properties and Ultrafast Near-Infrared Localized Surface Plasmon Dynamics in Naturally p-Type Digenite Films

Copper chalcogenides are materials characterized by intrinsic doping properties, allowing them to display high carrier concentrations due to their defect-heavy structures, independent of the preparation method. Such high doping enables these materials to display plasmonic resonances, tunable by varying their stoichiometry. Here, plasmonic dynamics is studied in drop-cast Cu9S5 (digenite) nanocrystals (NCs) film using ultrafast pump–probe spectroscopy. The NCs are synthesized by thermal annealing of copper foil using chemical vapor deposition (CVD), followed by sonication and drop-casting of the isolated few-layered flakes on different substrates. The samples display a broad localized surface plasmon resonance (LSPR) in the near-infrared (NIR), peaking at 2100 nm. The free carrier response is further confirmed by fitting the linear absorption with a Drude–Lorentz effective medium approximation model. The high temporal resolution allows to measure the relaxation dynamics of the photo-excited holes, which are dominated by a fast decay (τ1 = 360 ± 20 fs) and correspond to hole–phonon scattering processes, followed by a long-lived (τ2 > 1 ns) signal associated with phonon–phonon scattering relaxation. These results confirm the possibility of fabricating Cu9S5 films retaining the plasmonic properties of individual NCs, anticipating integrating these films into heterojunctions with suitable hole acceptor materials to build hot-hole-transfer-based optoelectronic devices.