Revealing the Exciton Fine Structure of PbSe Nanocrystal Quantum Dots Using Optical Spectroscopy in High Magnetic Fields

We find that τ increases sharply below 10 K, but saturates below

R. D. Schaller; S. A. Crooker; D. A. Bussian; J. M. Pietryga; J. Joo; V. I. Klimov

2010

Scholarcy highlights

  • We measure the photoluminescence lifetime, τ, of excitons in colloidal PbSe nanocrystals at low temperatures to 270 mK and in high magnetic fields to 15 T
  • Proof of dark excitons in CdSe NCs relied on measurements of PL and PL decays at low temperatures and in high magnetic fields
  • Exciton fine structure in lead-salt NCs is expected to be rather different, and several theoretical studies have been reported for PbSe NCs: Four-band envelope wavefunction methods first suggested 1-5 meV exchange energies and predicted an optically-allowed exciton ground state, whereas tight-binding calculations anticipated that the nominally degenerate L-points are split in NCs by tens of meV by intervalley coupling
  • Empirical pseudopotential approaches suggested that PbSe NCs possess a single optically-forbidden exciton ground state that lies ∆=2-17 meV below a 3-fold degenerate manifold of optically-allowed exciton levels
  • We find that τ increases sharply below 10 K, but saturates below
  • The clear saturation of τ below 500 mK effectively fixes τs, permitting ∆ to be fit with high accuracy. In these 2.3 nm radius NCs, we find that ∆ is surprisingly small – only 290±6 μeV
  • magneticcircular dichroism studies establish the magnetic energy scale of absorbing states in the exciton fine structure, and circularly polarized infrared magneto-PL is observed for the first time

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