Excited State Relaxation In CdSe／ZnS Core／Shell Quantum Dots Under High Pressure

With the development of preparation technology, the investigation and application of quantum dots(QDs) have extended to many fields. Because of its size effect, the physical and chemical properties of the QDs have many excellent properties as comparing with the bulk materials. QDs have a broad application in optoelectronic devices because of their special optoelectronic properties, such as multi-exciton generation(MEG), high quantum yield, carriers magnification(CM)and so on. Especially, in core-shell structure QDs, fluorescence quantum yield can be strongly enhanced after covering the shell, in which is due to the passivation of surface defects on the core and the reducing of fluorescence quenching center. This makes great benefits to bio probe, QLED, display applications. On the other hand,charge separation can be induced in type-II core-shell structure QDs, which can increase in the carrier lifetime remarkably and thus the photoelectric conversion efficiency of the QDs. In the photovoltaic conversion process, photo generated carriers will experience a series of interactions, such as internal relaxation, excited state absorption, radiative emission, nonradiaive emission, Auger recombination, etc.After that, electron and holes relax to the bottom of the conduction band and to the top of valence band respectively and annihilate finally. If the excitons can be separated and transported before annihilation, the incident optical energy can be transfer into electric energy and photovoltaic conversion. Therefore, it is very important to understand the time axis of exciton dynamics, namely the carriers’ generation, interaction, separation and recombination. Previous studies indicates that the relaxation process of carriers is slow down as a results of the distance reducing among the quantum dots, which causes the delocalization of electron wave function in each QD. But the conventional chemical method cannot further decrease the distance among QDs, physical method – high-pressure – is introduced to shorten the distance among QDs. High-pressure method is a relatively pure external means and only tochange the distance among QDs in the controllable range. It means that, under a certain pressure, the distance among QDs can be shortened dramatically without the collapse of crystal structure and the quantum confinement effect is still retained. In2014, this method was applied in Pb S QDs. Up to the pressure of 5 MPa, the carrier mobility in Pb S QDs solid is increased by 7 times[29]. Hihi-pressure method has broadly applied in Raman spectroscopy, X ray diffraction, conductivity measurement etc.. There are few studies on ultrafast processes under high-pressure condition.In this study, we combined ultrafast spectroscopy with static high-pressure method. The high-pressure condition is built-up bu diamond anvil cell and exciton dynamics is observed by transient absorption technique. The intent of this study is to investigate the high-pressure affected ultrafast process of exciton in Cd Se/Zn S QDs,which is to support the experimental data of exciton dynamics in nanomaterials under extreme conditionsThe main research contents are as following:The first part: the history and principle of high-pressure technology and pump probe technology are summarized. As a results of its sealing condition, isotropic compression, high pressure, small volume, experiment convenience, diamond anvil cell is used in for high-pressure built-up in this thesis. Femtosecond transient absorption is used to observe the exciton dynamics process in samples.The second part: firstly, high-pressure built-up is introduced in detail, which includes the diamond anvil cell assembly process and sample loading process.Secondly, femtosecond transient absorption set-up is introduced as well as the adjustment of light path.The third part: the structure of Cd Se/Zn S QDs were introduced in detail as well as its application. Excited state relaxation properties of Cd Se/Zn S core-shell QDs is investigated by femtosecond transient absorption spectroscopy with high-pressure as the main effect parameter.The observed results indicate that, as comparing with that under the atmospheric pressure, multi-exciton interaction is much weak; absorbance fluctuation tend to be moderate with increasing number of excitons; under the same wavelength, amplitudeof fast components increase with <N(0)> increasing. Under high-pressure condition(1.68 GPa), lifetime of middle component increase with <N(0)> increasing, which is different with its decreasing under low pressure(0.48 GPa), 10 times higher in high<N(0)> than low <N(0)>. Amplitude of middle component also increase dramatically as comparing with low pressure, 2 times higher in high <N(0)> than low<N(0)>.The above results indicate that the exciton has enough time to be separated and transmission under high-pressure condition. These results provide not only a feasible method for improving the physical optical system for the promotion of photovoltaic conversion efficiency, but also the first-hand data for the photovoltaic conversion under extreme conditions.