| Description |
The temporal dynamics of excitons and the evolution of excited states of a material system reflect both the excitation conditions and the final destination of the excitation energy. Precise control of material structure through modern nanofabrication provides nanostructures with well-defined relaxation paths of excitons, which can be manipulated and probed using external stimulation. In particular, electrostatic manipulation of exciton dynamics with external electric fields can be used to study electronic properties of novel material systems such as semiconductor nanocrystals and pi-conjugated molecules, which may be well suited for future applications in optoelectronic devices. In this work, electric field induced quenching of photoluminescence through generation of indirect excitons is performed on colloidal tetrapod heterostructure nanocrystals and a multichromophoric model molecular system. The dependence of quenching on optical excitation density, which shows opposite trends in these two material systems, reflects the specific origin of quenching in each system. The large reduction in decay lifetime of indirect excitons in the tetrapods also enables storage of optical information with external electric field, which can be observed using time-resolved spectroscopy. As a model light-harvesting system with efficient energy funneling from the arm to the core, the tetrapod is an ideal system to study impact of electric field on multiexcitons in the core and the "hot" excitons in the arm, thus providing insight on the effects of an electric field on intrapartical energy transfer. While energy transfer in the heterostructure tetrapods is through direct charge carrier thermalization, it is the coherent and incoherent energy transfer that couple chromophores in the multichromophoric molecules which mimic the intermolecular interactions in organic electronics. Both single molecule spectroscopy and time-resolved spectroscopy were employed to probe the structural dependent coherent and incoherent energy transfer. Briefly, this work consists of four main results. (1) Quenching in tetrapods is due to the localization of indirect excitons at trap sites which causes saturation of quenching at high excitation density. (2) Multiexcitons and arm excitons with fast decay lifetimes are not affected by an external electric field since electrostatic manipulation is not instantaneous. (3) Coherent coupling between chromophores causes changes in spectrum and decay lifetime, while the incoherent coupling leaves a dimer as a single quantum emitter and causes structural dependent emission depolarization. (4) Field induced quenching increases with the increase of excitation density and number of chromophores in multichorphoric molecules. |
