Ultrafast carrier dynamics in Cu₉S₅ flakes

Copper chalcogenides have attracted attention due to their intrinsic doping properties. These materials display high carrier concentrations due to their defect-heavy structures, thus by varying their stoichiometry tunable plasmonic resonances can be observed, as it was shown previously for Cu_2-xS/Se/Te, with 09S₅ were studied by means of ultrafast transient absorption (TA) spectroscopy. The samples were pumped at 50 kHz with 50 fs pulses centered a 1.65 μm, in the region of the near-infrared (NIR) plasmonic resonance, and probed in the 1.1-1.6 μm range. The measurement was performed with a high sensitivity TA spectrometer utilizing a birefringent delay line interferometer and lock-in detection, avoiding the need for expensive IR multi-pixel detectors. We were able to measure the relaxation dynamics of the excited electrons, which are dominated by a fast decay (τ₁~400 fs) associated to electron-phonon scattering followed by a long-lived bleaching (τ₂>>50 ps) due to trapping of electrons in defect states. We confirm the plasmonic nature of Cu₉S₅, which is promising for incorporation into hot-electron-harvesting solar cells.