Spectral dependence of nanocrystal photoionization probability: The role of hot-carrier transfer

Lazaro A. Padilha, István Robel, Doh C. Lee, Prashant Nagpal, Jeffrey M. Pietryga, Victor I. Klimov

Research output: Contribution to journalArticlepeer-review

63 Citations (Scopus)

Abstract

We conduct measurements of photocharging of PbSe and PbS nanocrystal quantum dots (NQDs) as a function of excitation energy. We observe a rapid growth of the degree of photocharging with increasing, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (Eg) and scale as ∼1.5Eg and ∼3Eg, indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (∼10-6 to 10-3 for 1.5 eV and ∼10-4 to 10-1 for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.

Original languageEnglish
Pages (from-to)5045-5055
Number of pages11
JournalACS nano
Volume5
Issue number6
DOIs
Publication statusPublished - Jun 28 2011

Keywords

  • Auger recombination
  • carrier multiplication
  • charge separation
  • nanocrystals
  • photoionization
  • photoluminescence
  • trapping

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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