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| Author(s): |
Tomás Aguiar Carneiro Ferreira
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin |
| Defense date: | 2022-05-11 |
| Examining board members: |
Lázaro Aurélio Padilha Junior;
Ismael Andre Heisler;
Marcos César de Oliveira
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| Advisor: | Lázaro Aurélio Padilha Junior |
| Abstract | |
Electron-phonon interactions in colloidal nanocrystals are still a poorly understood subject. One reason for this is that the inhomogeneous broadening caused by the size dispersion is an intrinsic property of the material sample, meaning that it cannot be eliminated through cryogenic measurement. Therefore, it is impossible to analyze the dephasing dynamics of individual nanocrystal families (or nanocrystals of a limited size range) with conventional spectroscopic techniques. For this reason, in this thesis, we analyze the results obtained by Multidimensional Coherent Spectroscopy (MDCS). MDCS are a class of techniques in which one measures both amplitude and phase of the electromagnetic field produced by a material medium undergoing strong optical excitation. Due to this phase-sensitive measurement, there is a component of the third order nonlinear polarization which graphically separates the natural linewidths of individual nanocrystal families from the intrinsic inhomogeneous broadening caused by the size dispersion. Consequently, it is possible to individuate the dephasing dynamics of ensembles of nanocrystals of a specific size range. In this thesis, we apply an MDCS technique to study the dephasing dynamics of CdS/CdSe/CdS (core/well/shell) Spherical Quantum Wells (SQW). This choice of structure is based on an argument in the literature of nanocrystals, in which the dephasing dynamics of CdSe-seed nanocrystals are limited by the spin-flip exciton dephasing, a phenomenon in which the decay of the dark excited state is made possible by the exchange with the lattice of a phonon that carries angular momentum, satisfying the conservation of momentum. A detail of the argument that we wish to explore is that the lifetime obtained from this dephasing increases with the increased delocalization of electron and hole wave functions within CdSe/CdS core/shell nanostructures. Therefore, the goal of this thesis was to test this hypothesis for SQW CdS/CdSe/CdS core/well/shell nanostructures, whose electron-hole delocalization is superior to CdSe/CdS core/shell nanostructures, for the same nanocrystal volume. Our results show that the dephasing times of SQW CdS/CdSe/CdS core/well/shell nanostructures are limited by ultrafast dephasing dynamics that are not observed in the CdSe/CdS core/shell nanostructures. Furthermore, the SQW dephasing times are at least one order of magnitude smaller than those of the CdSe/CdS core/shell structures, making their dephasing times comparable to those observed in CdSe nanoplatelets, which are on the order of 1 ps (AU) | |
| FAPESP's process: | 19/22565-6 - Revealing the fine-structure in semiconductor nanomaterials by multidimensional coherent spectroscopy |
| Grantee: | Tomás Aguiar Carneiro Ferreira |
| Support Opportunities: | Scholarships in Brazil - Master |