C. Burda et Ma. El-sayed, High-density femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality, PUR A CHEM, 72(1-2), 2000, pp. 165-177
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.