Bulklike hot carrier dynamics in lead sulfide quantum dots.
Byungmoon Cho1, William K Peters, Robert J Hill
1Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.
Nano Letters
|June 17, 2010
Summary
Investigating hot electronic dynamics in lead sulfide (PbS) nanocrystals reveals that high-energy state dynamics are localized and bulk-like, not quantum confined. This finding challenges existing models for quantum dot behavior.
Area of Science:
- Materials Science
- Solid State Physics
- Nanotechnology
Background:
- Lead sulfide (PbS) nanocrystals exhibit quantum confinement effects crucial for their optoelectronic properties.
- Understanding hot carrier dynamics is essential for advanced applications like solar cells and photodetectors.
Purpose of the Study:
- To investigate the hot electronic dynamics in PbS nanocrystals using ultrafast spectroscopy.
- To determine if high-energy excited states in PbS nanocrystals exhibit quantum confinement or bulk-like behavior.
Main Methods:
- Degenerate pump-probe spectroscopy with 20-25 femtosecond pulses.
- Broad frequency range probing around three times the nanocrystal band gap.
- Varying nanocrystal diameters to observe size-dependent effects.
Main Results:
- An initial reduction in absorption was observed at the E1 transition peak, with increased absorption elsewhere.
- Signals were significantly weaker than expected for a two-level system, particularly near time zero.
- Observed dynamics are inconsistent with quantum confinement of excited high-energy states.
Conclusions:
- Hot carrier dynamics in many direct-gap semiconductor quantum dots, including PbS, are localized at the 1-2 nm scale.
- These dynamics behave essentially as bulk-like, dominated by surface scattering, rather than showing quantum confinement.
- The findings necessitate a revision of models describing carrier dynamics in quantum dots at high excitation energies.
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