Optical transient bleaching and induced absorption of surface-modified copper sulfide nanocrystals

Abstract.

 Surface oxidation of CuS nanocrystals leads to the appearance of an additional absorption band in the near infrared. This is attributed to an energy level in the band gap, 1 eV below the conduction band, with the character of an electron donor. Bleaching of this additional absorption band and induced absorption in both oxidized and unmodified samples take place. An energy level scheme for CuS and CuS_(ox) nanocrystals with long-lived trap levels in the band gap is proposed. The relaxation time from the conduction band to these trap levels is ∼80 ps. The relaxation of electrons from trap levels has characteristic times of more than 1 ns.