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Title: Applications of ultrafast multidimensional electronic spectroscopy to the study of plant light harvesting systems and colloidal quantum dots
Authors: Zhang, Cheng
Keywords: DRNTU::Science::Chemistry::Biochemistry::Spectroscopy
Issue Date: 2018
Source: Zhang, C. (2018). Applications of ultrafast multidimensional electronic spectroscopy to the study of plant light harvesting systems and colloidal quantum dots. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: This dissertation focuses on the development of ultrafast multidimensional electronic spectroscopy using a pump probe geometry and its application in discerning the energy transfer mechanics and band structure analysis on plant light-harvesting systems and colloidal quantum dots. In green plants, the first step of light absorbing processes is carried out and implemented by the light-harvesting complex II (LHCII). The trimeric and aggregated LHCII exhibit the unquenched and quenched excitonic states of Chlorophyll (Chl), respectively. Two-dimensional electronic spectroscopy (2DES) ,which allows direct observation of correlation of excitation and emission energy polls, enables the mapping of the pathways and dynamics from Chl b to Chl a to give insights into the mechanism of non-photochemical quenching that protects the system away from photodamage. Long-lived intermediate Chl a states are present in trimers, while in aggregates, the population decay of these excited states is significantly accelerated, suggesting that, overall, the energy transfer within the LHCII complexes is faster in the aggregated state. In addition, 2DES experiments under conditions free from singlet-singlet annihilation and anisotropic decay are done in the following study. The energy transfer between the different domains within the Chl a manifold is investigated and found to proceed on time scales ranging from hundreds of femtoseconds to five picoseconds, before reaching equilibration. The bidirectional (uphill and downhill) energy transfer of the equilibration process between excited states are clearly observed in experiments. Furthermore, exciton equilibration and excitation trapping in intact Photosystem I (PSI) complexes as well as core complexes isolated from Pisum sativum are studied. Due to the flexibly tailored band structure and optical properties of semiconductor nanocrystals, colloidal semiconductor quantum dots (QDs) become the new black in modern industry. Along with the enormous application in sensing, detecting and lasing etc., understanding of the discrete-like band-edge structure caused by the quantum confined effects and eventually manipulating the corresponding optical properties become a key interest by the physicists and chemists in related field. With the above mentioned points in mind, ultrafast transient absorption (TA) measurements are performed on CdSe core type QDs and subsequently a model to explain the features of the band-edge spectra is developed and described. From our fits of the experimental TA spectra, biexcitonic binding energies for the three lowest energy transitions are obtained. Subsequently, room temperature 2DES measurements are performed on the CdSe QDs accompanying with the nodal line slope (NLS) analysis. This 2D lineshape study indicates unique determination of the contributions from different linewidth broadening sources. NLS method is much more sensitive compared with the 1D histogram TEM analysis of the size dispersion inhomogeneity. Adopting the lineshape model from TA spectroscopy and additional simulation on 2DES spectra, quantitative determination of the linewidth information for the involved the band-edge lowest three energy levels X1, X2 and X3 are available. For a pulse-shaper assisted pump-probe geometry 2D spectroscopy setup, apart from third order nonlinear optical signals, it is also possible to obtain fifth-order nonlinear optical signals. With appropriate phase-cycling schemes, the fifth-order optical signals can be measured. We report on the development of fifth-order two-quantum two-dimensional electronic spectroscopy (2Q2DES). The implementation of this 2Q2DES allows the detection of double quantum coherence, so that multi-excitonic behaviour can be probed. Chl a molecules are used to test the validity of the technique and system. Preliminary results on colloidal CdSe QDs are reported as well to gain insights into biexciton dynamics and behaviour.
DOI: 10.32657/10356/73690
Fulltext Permission: open
Fulltext Availability: With Fulltext
Appears in Collections:SPMS Theses

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