Optical Properties Of CuInS₂ Colloidal Quantum Dots And Coupling With High Quality Factor Microcavities

Semiconductor quantum dots?QD?are the quasi-zero-dimensional structures,which are different from their bulk materials.They have discrete energy levels like atom energy levels,which are also known as "artificial atoms".This type of QDs,synthesized by chemical methods,are named colloidal QDs.The optical properties of colloidal QDs can be controlled by tuning the size,shape and composition of QDs.Colloidal QDs have attracted considerable attention because of their unique optical properties,and potential applications as solar cells,LED,in vivo or in vitro biological imaging and labeling.In this thesis,we have mainly studied the optical properties of Cu InS2/ZnS?CIS/ZnS?colloidal QDs.Comparing with traditional II-? colloidal QDs,the CIS QDs have less toxicity and large tunable emission wavelength range from the visible to near infrared region.The non-radiative recombination process,photooxidation and photobleaching of the QDs were studied systematically by analyzing the photoluminescence?PL?spectroscopy of CIS/ZnS QDs.The coupling between the QDs and the microcavity is also investigated.The main experimental results of this thesis are as follows:1.The photoluminescence spectra of the CIS/ZnS colloidal QDs as a function of pumping power were studied by using the confocal microscopy system at room temperature and low temperature.It was found that a small peak at 1.68 eV at the low energy side of spectrum can be resolved.And with temperature increasing,the PL intensity of the small peak decreased until it disappeared.We considered that the energy resonance transfer process between the QDs occurred,and the small peak disappeared at room temperature due to the participation of the phonon.By analyzing the PL intensity as a function of pumping power,we conclude that the Auger recombination process in QDs was dominating both at room temperature and at low temperature.2.According to the PL spectra of CIS/ZnS colloidal QDs on the SiO₂/Si substrate,the PL spectra of colloidal QDs with the evolution of the illumination time were obtained.In air,strong photodegradation has been observed because of the photooxidation and photobleaching accompanying a PL peak blue shift and line width broadening at low excitation power.All these photobleaching effect are reduced under vacuum,which shows that oxygen plays a key role in the optically induced PL decay.3.Due to the strong light enhancement effect of high quality factor microdisk?MD?,the CIS/ZnS colloidal QDs were drop-casted on MDs.When the QDs couple with cavity modes,the extraction efficiency is greatly improved because of the Purcell effect,resulting in a PL enhancement by 20 times at RT and by 35 times at 20 K.4.We have also studied the evolution of the CIS/ZnS colloidal QDs on the edge of MDs.In air,the variation and the reasons of the PL spectra of the CIS/Zn S colloidal QDs on the edge of MDs are consistent with the cases on the SiO₂/Si substrate.However,with a further increase in the excitation power,the PL spectra have shown a red shift and narrowing line width.This is due to the fact that the photoinduced ligand damage induces the agglomeration of QDs,which enhances FRET between different QDs.Under vacuum,the photobleaching effects from QDs are reduced.When the pumping laser couples with leaky modes in the MDs,the excitation efficiency is greatly enhanced,which accelerates photobleaching.When the excitation power is about 0.01 ?W,the PL intensity of CIS QDs can be reached at 100,000 counts/s and decreased only by 2.1% within the measurement time.By taking advantages of enhanced PL extraction efficiency,photobleaching can be avoided with a small excitation power but with a relatively high PL intensity.