The ultrafast charge dynamics in a number of nanostructured materials relevant to the production of renewable energy are investigated using ultrafast transient absorption spectroscopy. The materials include mercury telluride and cadmium mercury telluride quantum dots, and gold nanoparticles loaded on titanium dioxide colloidal spheres. The analysis of the resultant pump-induced transmittance change spectra and transients allow the determination of charge relaxation routes including multiple exciton generation, trion formation and direct-surface trapping.The investigation of HgTe QDs passivated with thioglycerol, mercaptopropionic acid and dodecanethiol ligands suggests that mercaptopropionic acid ligand results in better passivation of HgTe QDs due to its carboxylic acid group. It allows more electron density donation to the QD surface to passivate the traps related with unsaturated Hg bonds and hence supresses the associated non-radiative processes. The decay lifetimes of the thioglycerol/dodecanethiol-capped QDs in addition to the photo-induced absorption feature in their spectra, are found to be consistent with surface charge trapping observed in CdSe QDs. In comparison, the transients obtained for mercaptopropionic acid passivated QDs coupled with the pump-induced transmittance change spectrum show no sign of any surface-related processes. Therefore, our analyses allow the determination of multiple exciton generation for the first time in these QDs with a quantum yield of 1.36 ± 0.04 when photo-exciting with photons of energy 3.1 times the band gap. Such result should turn researchers' attention to those ligands which could improve the QD solar cell field.The study of exciton dynamics in CdxHg(1-x)Te alloy QDs is also presented here. Their pump-induced transmittance change spectrum show two bleaches: at the shoulder position of the steady state absorption and at the PL peak. The exciton dynamics of these materials are studied using four different wavelengths, two of them are above the MEG threshold. The resultant transmittance transients and the pump-induced transmittance change spectrum are free of any photo-induced absorption or long-lived surface trapping. Hence, the decay of the transients obtained above the MEG threshold for well-stirred samples at low pump fluences is attributed to biexciton recombination. The assessment of multiple exciton generation reveals a quantum yield value of 1.12 ± 0.01 when photo-exciting with 2.6 times the band gap.Finally, the investigation of the recovery of the plasmon bleach in TiO2 colloidal spheres decorated with different sizes of Au NPs is presented in this thesis. The pump-induced transmittance change spectra obtained for two different wavelengths show bleaches at the plasmon band maximum superimposed with two wings of absorption features at shorter and longer wavelengths. The resultant transmittance transients for these samples are well-described by bi-exponential decay with a very quick decline of a few ps associated with electron-phonon scattering, followed by a slower decay over a few 10s of ps associated with heat dissipation. Only the heat dissipation rate is found to be dependent on the size of the Au NPs as it rises from 49 ± 3 ps to 128 ± 6 ps when the diameter of the Au NPs is increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.
|Date of Award||31 Dec 2015|
- The University of Manchester
|Supervisor||David Binks (Supervisor)|
- Ultrafast charge dynamics
- Multiple exciton generation