Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots

• Published in: Nano letters Vol. 12; no. 6; pp. 2658 - 2664
• Main Authors: Gesuele, F, Sfeir, M. Y, Koh, W.-K, Murray, C. B, Heinz, T. F, Wong, C. W
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 13.06.2012
• Subjects: Applied sciences; Bleaching; Carriers; Computer Simulation; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Dynamic tests; Dynamics; Electron states; Electron Transport; Electronics; Energy Transfer; Exact sciences and technology; Excitons; Excitons and related phenomena; Lead - chemistry; Materials science; Materials Testing; Models, Chemical; Models, Statistical; Molecular electronics, nanoelectronics; Multiplication; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Photons; Physics; Quantum Dots; Selenium Compounds - chemistry; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Spectra; Spectrum Analysis - methods; carrier multiplication; Multiple-exciton generation; solar cells; ultrafast spectroscopy; Biexcitons; Energy gap; Red shift; Electronic properties; Excitons; Quantum dots; Binding energy; Lead sulfide; Excited states; Spectral line shift; Nanostructured materials; Transients
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl2021224

We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1Sh–1Se transition, as well as transients associated with the 1Ph–1Pe transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (⟨N abs⟩ ∼ 10–2) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the biexciton.