Ultrafast Plasmon Dynamics and Hole–Phonon Coupling in NIR Active Nonstoichiometric Semiconductor Plasmonic Cu <sub/> 2– <i>x</i> </sub> S Nanocrystals

摘要： Nonstoichiometric Cu2?xS nanocrystals (NCs) have been synthesized by hot-injection method after changing the copper to sulfur ratio. Optical studies confirmed well-defined localized surface plasmon resonance (LSPR) absorbance band from near-infrared to mid-infrared arising due to p-type hole vacancy in the doped semiconductor. Carrier concentrations of the Cu2?xS NCs are calculated to be on the order of ～1021 cm?3 after following the Drude model, which is one order of magnitude lower as compared to the metallic (Au/Ag) system. To understand ultrafast plasmon dynamics of Cu2?xS NCs, femtosecond broadband (visible?near IR) pump?probe spectroscopy has been employed. Ultrafast plasmon dynamics of the Cu2?xS NCs have been monitored after changing composition (copper to sulfur ratio), pump wavelength, and laser intensity. At moderate pump power hole?phonon relaxation time constant has been observed to be in the range of 240?440 fs for Cu2?xS NCs depending upon pump wavelengths (400, 800 nm). From the ultrafast transient data, hole?phonon coupling constant (G) has been determined in the range (1.6?2.7) × 1010 J K?1 s?1 cm?3 for Cu2?xS NCs at different excitation wavelengths, which is also one order lower as compared to the metallic system. We have proposed a new mechanistic scheme for hot carrier relaxation dynamics, in accordance with the two-temperature model (TTL) as reported in literature for plasmon dynamics.