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, as shown previously for Cu_2-xS, Cu_2-xSe, and Cu_2-xTe, with 0<x<1. Here, we study plasmonic dynamics in drop-cast Cu₉S₅ (digenite) nanocrystals (NCs) film using ultrafast pump-probe spectroscopy. The NCs were 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 displayed 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. Investigation of the ultrafast dynamics of free carriers in the film was performed by exciting the material with 50-fs-long pulses centered at 1800 nm and probing both in the NIR (1200-1750 nm) and visible (530-730 nm) spectral ranges. The high temporal resolution allowed us 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.