Ultrafast transient pump-pump-probe spectroscopic studies in colloidal semiconductor heteronanostructures to follow processes involving multiple excitations
Final Report Abstract
Studying multiexcitons in semiconductor nanocrystals interesting for fundamental studies but also important from an application standpoint. In this project, properties of multiexcitons were studied using pump-probe and pump-pump-probe spectroscopic techniques. The Markov Chain Monte Carlo (MCMC) sampling method was used for target analysis to simultaneously determine the spectral and dynamical features of multiexcitons in semiconductor nanocrystals. A series of CdSe nanocrystals with variation in size and surface functionalization were studied using pump-probe spectroscopy. Applying a cascade Auger recombination model, it was possible to spectrally and dynamically distinguish multiexciton states up to tetraexcitons. The results allowed the estimation of binding energies of higher-order excitons as a function of nanocrystal size and surface ligands. As multiexciton lifetimes can be prolonged in nanoheterostructures CdSe@CdS nanorods and CdS nanorods with different surface ligands investigated and compared. The global analysis revealed the presence of two distinct mono excitons; one mainly contributes to the transient absorption signal in the low-intensity region, and the other one mainly contributes to the signal in the high-intensity pump regime of nanorods, which is associated with trapped state formation, only accessible by the decay of high energetic multiexciton species. This study formed the basis for a proof-of-concept study of the generation of multiexcitons using two temporally separated pump pulses in a pump-repump-probe experiment. This experiment allowed to determine the absorption cross-section for multiexciton states generated by the first pump pulse, revealing that the absorption cross-sections of various higher-order excitons are comparable to that of the absorption cross-section of ground state species for above band-edge excitation. In contrast, when excited close to the band-edge of the material, the absorption cross-section of multiexcitons is smaller compared to the ground-state absorption cross-section. Furthermore, we performed preliminary measurements to understand the impact of electronic coupling of CdSe nanocrystal systems on the dynamics in the multiexciton regime in a thin layer system.
Publications
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Invited talk on “Multiexcitonic Signatures of Seeded Nanorods” in Chemistry colloquium of Technical University of Kaiserslautern, Nov 22, Germany.
Kumar, Krishan
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Poster presentation on “Ultrafast Pump-Pump-Probe Spectroscopic Investigations on Seeded Nanorods” in conference by German Bunsen Society, Giessen, Sep 22, Germany.
Kumar, Krishan
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Unravelling Dynamics Involving Multiple Charge Carriers in Semiconductor Nanocrystals. Nanomaterials, 13(9), 1579.
Kumar, Krishan & Wächtler, Maria
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Multiexciton absorption cross-sections of CdSe@CdS nanorods studied using Pump-rePump-Probe Spectroscopy. American Chemical Society (ACS).
Kumar, Krishan; Uhlig, Jens; Baruah, Raktim; Yadav, Shivani & Wächtler, Maria
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Spectral and dynamical properties of multiexcitons in semiconductor nanorods. American Chemical Society (ACS).
Kumar, Krishan & Wächtler, Maria
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Talk at Indian Institute of Technology Gandhinagar titled “Optical and Electrical Properties of II- VI Semiconductor Nanocrystals” March 24, India.
Kumar, Krishan
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Talk in conference by German Bunsen Society titled “Spectral and dynamical properties of multiexciton in semiconductor nanorods” in Aachen March 24, Germany.
Kumar, Krishan
