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Excited-state relaxation in PbSe quantum dots.
Joonhee M An1, Marco Califano, Alberto Franceschetti
1National Renewable Energy Laboratory, Golden, Colorado 80401, USA.
The Journal of Chemical Physics
|May 2, 2008
Summary
Fast excited-state relaxation in lead selenide (PbSe) quantum dots is explained by the Auger mechanism. Contrary to expectations, dense hole energy levels in PbSe facilitate rapid energy transfer, supporting the Auger cooling model for intraband relaxation.
Area of Science:
- Materials Science
- Quantum Physics
- Solid-State Physics
Background:
- Nonradiative decay in solids is typically fast (picosecond scale).
- A 'phonon bottleneck' was hypothesized to slow relaxation in quantum dots, but Auger mechanisms dominate in materials like InP, CdSe, and ZnO.
- Lead selenide (PbSe) quantum dots were expected to exhibit slow relaxation due to sparse hole energy levels.
Purpose of the Study:
- Investigate the mechanism behind unexpectedly fast intraband relaxation in PbSe quantum dots.
- Re-evaluate the assumption of sparse hole energy levels in PbSe.
- Determine if the Auger mechanism can account for the observed relaxation times.
Main Methods:
- Atomistic pseudopotential calculations using Pb(2046)Se(2117) and Pb(260)Se(249) quantum dot wave functions.
- Explicit calculation of electron-hole Coulomb integrals.
- Calculation of the P-->S electron Auger relaxation rate.
Main Results:
- Pseudopotential calculations reveal densely spaced hole energy levels in PbSe, contradicting simple effective-mass models.
- The Auger mechanism is found to be sufficiently fast to explain the observed P-->S intraband decay times.
- Fast intraband relaxation in PbSe quantum dots is consistent with the Auger cooling mechanism.
Conclusions:
- The assumption of sparse hole levels in PbSe quantum dots is incorrect.
- The Auger mechanism, driven by dense hole energy levels, explains the picosecond-scale intraband relaxation in PbSe.
- No exotic relaxation mechanisms are needed to explain experimental observations in PbSe quantum dots.
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