Ultrafast Plasmon Dynamics and Hole-Phonon Coupling in NIR Active Nonstoichiometric Semiconductor Plasmonic Cu2-xS Nanocrystals

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 similar to 10(21) 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) x 10(10) 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.