School of Chemistry - Theses
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ItemIncorporation of Quantum Dots into Optoelectronic Devices: Ligands as Charge Brigands( 2019)With their tunable and almost monochromatic emission over the whole visible spectrum, colloidal II-VI semiconductor quantum dots (QDs) are attracting significant interest as novel electroluminescent materials in light-emitting diodes (LEDs). Being processable in solution allows the deposition of large-scale films at low cost and brings QDs in a competitive position to their organic counterparts. Organic ligands capping QDs are used to maintain colloidal stability during synthesis and provide passivation in solution. While ligands remain invisible under optical characterisation, this thesis provides insights into the mechanisms by which ligands impact charge carrier dynamics within a light-emitting diode. In doing so, robust and bright QDs emitting at 410 nm are synthesized, passivated with ligands of various lengths and electrical conductivity and incorporated into an LED architecture. Based on Impedance Spectroscopy measurements ligands have been identified as a major obstacle for charges transiting to the QD to produce light: ligands act as brigands in trapping charges and prevent efficient charge recombination. A novel capacitance behaviour is described and attributed to the accumulation of charge carriers within the ligands during operation. Together with an inductive response in the impedance plane changes in capacitance can be used as a diagnostic tool to determine the recombination efficiency in a quantum dot light-emitting diode (QLED). By driving a QLED with a rectangular pulse accumulated charges lead to a delayed luminescence peak when the bias is turned off and can thereby be visualised. When a 5 nm thick aluminium layer is added into the hole transport layers, trapped charges can open a memory window and add a new device functionality to a QLED. This thesis concludes with ideas to overcome the ligand-dependent charge accumulation and suggests a novel type of QDs for a more efficient device performance.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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ItemThe origin of blinking in CdSe quantum dots( 2017)Single-molecule spectroscopy has revealed an intriguing phenomenon, termed blinking, in single CdSe quantum dots (QDs): the fluorescence of single QDs turns on and off randomly. Although we do not observe blinking in QD ensemble due to the averaging effect, blinking does limit the performance of QD based devices, such as solar cells and light-emitting diodes. To date, we have made great achievements in understanding and controlling blinking, but the exact mechanism is still under debate. There are two competing models to explain blinking: Auger recombination and surface-trap-induced recombination. In this thesis, we have confirmed the coexistence of the two types of blinking in the same QDs from the lifetime-intensity correlation and the radiative lifetime scaling. In addition to the conventional power-law blinking for some QDs, we have also observed exponential distributions in blinking statistics. The difference is possibly from the process of activation and deactivation of long-lived traps. By varying the excitation power, we have verified the role of both exciton and biexciton in the process of ionisation. Single-molecule spectroscopy under environment controls has revealed that there exist a broad distribution of surface properties within the same batch of QDs. Possibly due to the poor passivation, the majority of QDs are very easy to be quenched by moisture. Also, due to the high density of surface traps, bandedge carrier trapping is the dominating blinking mechanism in these unpassivated QDs. We used a movable micro-mirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. The quantum yield of the bright state is close to unity (>90%). Furthermore, we have also studied the effect of a micro-mirror on blinking. The mirror does not modify the off-time statistics but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened, due to an increase in the radiative recombination rate. In the last section of this thesis, we applied the measurement and analysis methods established for CdSe QDs to inorganic perovskite quantum dots. The degradation process assisted by water and light has been investigated carefully. It has been shown that the main blinking mechanism is from carrier trapping.