Effect of Surface Ligand on Chemical Interface Damping in Nonstoichiometric Cu_2–xS Semiconductor Nanocrystals: A Direct Correlation between Ultrafast Carrier Dynamics and Photoconductivity

Understanding and controlling ultrafast relaxation processes of photoexcited infrared active nonstoichiometric semiconducting materials is an important research area where chemical interface damping (CID) plays a significant role in the application of efficient optoelectronic devices. Herein, we have synthesized two different surface ligands (olaylamine and oleic acid) capped near-infrared active plasmonic semiconductor Cu 2– x S nanocrystals (NCs), namely, Cu OLM and Cu OA . Interestingly, steady-state optical spectra of Cu OA were found to be broader and red-shifted compared to Cu OLM in the near-IR region and can be attributed to a stronger CID effect. The effect of the surface ligand on the relaxation dynamics of the localized surface plasmon resonance (LSPR) band was monitored with the aid of femtosecond broadband (visible-NIR) pump–probe spectroscopy. Broadening of the transient absorption (TA) spectra and faster kinetics have been observed for oleic acid capped NCs due to a strong chemical interface damping effect. We further investigated CID driven photoconductivity measurements and found drastic improvement of photoconductivity in the Cu OA system. This work opens up a new direction for near-infrared active devices and its utilization in the field of infrared photosensing, photonics, and phototransistor application, where optical properties of an infrared active nonstoichiometric semiconductor can be manipulated through changing surface ligands.