Optoelectronic Properties Of Thick-shell ?-? Semiconductor Quantum Dots

Owing to their superior optical properties,colloidal semiconductor nanomaterials have great potential to be used in display,fluorescence labeling,and microcavity lasers.The thick inorganic wide-bandgap shells(?11 monolayers)can effectively passivate the defect states on the surface of nanoparticles,with high photoluminescence quantum yields(PL QYs)and excellent photostability.The thesis mainly focuses on the optical gain performance,multiexciton dynamics,and field-controlled optical properties of thick-shell core/shell quantum dots(QDs),and also exploring their applications in microcavity lasers.In addition,in order to explore the influence of shell thickness on the optical properties of low-dimensional nanomaterials in different quantum confined systems,we have systematically studied the optical gain properties in two-dimensional core/shell nanoplatelets(NPLs).The main research contents are organized as follows:The distributed feedback(DFB)lasers were fabricated based on thick-shell CdSe/CdS core/shell QDs by laser interference ablation(LIA)for the first time,and high-performance single-mode laser emission was realized under single-photon and two-photon excitation with lower thresholds of 0.028 m J cm^-2 and 1.03 m J cm^-2,respectively.The results show that LIA is an efficient method to fabricate QDs DFB laser devices directly based on QDs film.A series of phase-pure wurtzite CdSe/CdS core/shell QDs with controlled shell thickness were synthesized.The complicated multiexciton dynamics process in CdSe/CdS QDs was analyzed from the perspective of superradiation mechanism and the relationships between pump intensity and the average number of excitons in QDs was obtained.Before the number of excitons in QDs reached?6,it was consistent with the theoretical calculation.However,with the further increase in pump intensity,the average number of excitons tended to be saturated due to the increase of excitonic nonradiation losses at higher pump level.Experimental results show that the amplified spontaneous emission measurement is coincident with the fitting results,which presents a new approach to explain the complicated multiexciton dynamics mechanism in quantum-confined systems.The quantum-confined stark effect(QCSE)of phase-pure wurtzite CdSe/CdS core/shell QDs was systematically studied in the ensembles.It was clearly observed that thick-shell CdSe/CdS QDs(11 CdS monolayers)exhibited obvious PL and absorption properties under the electric field,with the redshift of?2.3 nm and?2.1 nm,respectively.Based on the time-dependent PL intensity traces of a single QD,we analyzed the relationship between the shell-thickness dependent blinking behaviors and their QCSE in QDs ensembles.The results show that the field-controlld optical properties of medium-shell and ultrathick-shell QDs were not as obvious as thick-shell CdSe/CdS QDs,because of much surface defect states in medium-shell QDs and internal exciton trapping states induced by the stress release within ultrathick-shell QDs,such charged states would induce randomly local electric field around the QDs.Besides,the multiexciton properties of thick-shell CdSe/CdS QDs were studied under the electric field for the first time,and the dynamic relaxation process of single exciton in multiexciton states was also analyzed.Based on the above researches about the three-dimensional confined thick-shell QDs,we further explored the influence of shell thickness on the optical properties of two-dimensional semiconductor NPLs.A series of CdSe/CdS core/shell NPLs with different shell thickness were synthesized by high temperature hot-injection method.High-quality CdS shells grown at high temperature could effectively passivate the defect states on the surface of the NPLs and greatly improve the PL QYs and UV illumination stability.Therefore,thick-shell CdSe/CdS NPLs with 6 monolayers of CdS shells exhibited ultralow amplification spontaneous emission threshold(?4.4?J cm^-2),extremely long gain lifetime(>800 ps),and tremendous gain bandwidth(>140 nm),indicating that Auger recombination was effectively suppressed,thus exhibiting superior optical gain performance.Finally,we successfully fabricated a vertical cavity surface-emitting microcavity lasers using such thick-shell NPLs as gain medium,and then achieved spatially directional single-mode lasing emission with an ultralow threshold(?1.1?J cm^-2).