High-density femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality

At the high laser excitation intensities used in our experiments, more than 50 electron-hole pairs are formed in colloidal semiconductor nanoparticles used in our studies. At this density of charge carriers, new transient abs orptions are observed in the femtosecond transient spectra in the 450 to 70 0 nm region with unresolved fast rise (<100 fs) and two decay components of 660 fs and >150 ps. The absorption at 510 nm could be quenched with the ad sorption of electron accepters (e.g., benzoquinone, 1,2-naphthoquinone), wh ereas the low-energy transient absorption was not affected. For CdS NPs, we found that passivation eliminated most of the transient absorption. The tr ansient absorptions are thus proposed to result from either trap-state abso rption, trapped dimers (or complexes) and/or Stark-shifted exciton absorpti on resulting from surface electric field of the uncompensated trapped elect ron-hole pairs. All these possibilities require effective surface trapping at these high levels of excitation.