Picosecond absorption spectroscopy of surface modified copper sulfide nanocrystals in polymeric film

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Abstract
The transient induced absorption and bleaching are examined in 10 nm size CuS nanocrystals embedded in polyvinylalcohol film. Partial surface oxidation of CuS nanocrystals produces a new near-IR-absorption band peaked at 1100 nm. The surface oxidized shell is supposed to form a midgap electron acceptor level and the near-IR band relates to the electron transfer from the CuS valence band. The near-IR band is bleached easily by 15 ps pump pulse from YAG:Nd laser (1.08 μm). The bleaching is accompanied by the induced absorption at the red side of CuS fundamental edge. Both signals induced decay quickly with the time constant 65–70 ps. The model is proposed where the electron–hole pair excited by near-IR photon (electron in surface shell and hole inside CuS core) creates strong internal electric field, which induces the Stark red shift of CuS fundamental edge.