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Organizations & People
Pure Appl. Chem. Vol.
72, Nos. 1-2, pp. 165-177, 2000
Special Topic Issue on the Theme of
High-density femtosecond transient absorption spectroscopy of semiconductor
nanoparticles. A tool to investigate surface quality
C. Burda* and M. A. El-Sayed
*Laser Dynamics Laboratory, School of Chemistry and
Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
Abstract: 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 absorptions are observed in the femtosecond
transient spectra in the 450 to 700 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 adsorption of electron acceptors
(e.g., benzoquinone, 1,2-naphthoquinone), whereas the low-energy transient
absorption was not affected. For CdS NPs, we found that passivation
eliminated most of the transient absorption. The transient absorptions
are thus proposed to result from either trap-state absorption, trapped
dimers (or complexes) and/or Stark-shifted exciton absorption resulting
from surface electric field of the uncompensated trapped electron-hole
pairs. All these possibilities require effective surface trapping at
these high levels of excitation.
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