Spectroscopic And Carrier Dynamics Investigations Of ZnSe-based Core-shell Semiconductor Quantum Dots

Due to their unique physical and chemical properties,such as wide absorption range,narrow emission band,and etc,semiconductor quantum dots have shown great potentials for applications in luminescent materials,photovoltaic devices,catalysis,and etc.Carrier dynamics are the key factors determining the applications of semiconductor quantum dots and the performance of corresponding semiconductor quantum dots based devices.Core-shell structured quantum dots have advantages of high quantum yields and good optical stability and have therefore attracted attremdous attentions in the field of semiconductor quantum dots.The presence of shell,however,will significantly affect optical characteristics and carrier dynamics of semiconductor quantum dots,thereby affecting their pratical applications in semiconductor quantum dots based devices.Additionally,the optical properties and carrier dynamics of the core-shell structured quantum dots are stongly dependent upon the core size and and the shell thickness.So,to promote the application of core-shell semiconductor quantum dots in quantum dots based devices,it is necessary to well understand optical properties and carrier dynamics of core-shell semiconductor quantum dots,as well as the influences of the core size and the shell thickness to their optical and carrier dynamics properties.ZnSe is an important semiconductor material with a bandgap of 2.58 e V and has a great potentials as light harvesting materials for solar cells and photocatalysis.In this thesis,the spectroscopic properties and carrier dynamics of ZnSe-based core-shell quantum dots have been systematically investigated.The following results have been achieved:1 Synthesis and carrier dynamics of ZnSe/CdSe quantum dots.High quality ZnSe/CdSeapproximation with considerations of the lattice mismatch and interdiffusion.Thedynamic process and transfer knietics of photogenerated carriers are found to beclosely dependent upon the spectroscopic properties of the ZnSe/CdSe quantum dots,the spatial distributions of photogenerated carriers,and the energy levels of the lowestconduction/valence bands.Additionally,studies indicate that the occurrence of theinterdiffusion between the core and shell materials during the synthsis of theZnSe/CdSe quantum dots.Specifically,the cation interdiffusion constant between thecore and shell materials is estimated to be 0.015 nm²min^-1 at 280^oC.Carrier dynamicsinvestigations indicate that the growth of the CdSe shell on the surface of the ZnSecore complies with a Stranski-Krastanov?S-K?mode.That is,the CdSe shell growsuniformly at the surface of ZnSe core for the initial monolayers,but further increaseof the shell thickness roughens the surface of ZnSe/CdSe quantum dots.These resultspresent here is helpful for the synthesis of ZnSe/CdSe quantum dots with a controlledmanner and will be also useful for the accurate modulation of the spectroscopic andcarrier dynamic properties of ZnSe/CdSe quantum dots.2 Synthesis and carrier dynamics of ZnSe/Cd S quantum dots.ZnSe/Cd S quantum dotswith different diameters have been prepared using a single precursor Cd?DDTC?₂,andand their spectroscopic and carrier properties are also studied.The results show thatZnSe/Cd S quantum dots with the Cd S thickness<0.54 nm can exhibit type-Ispectroscopic feature.Interestingly,ZnSe/Cd S quantum dots with Cd Sthickness>0.85 nm display typical type-II spectroscopic feature,and the longestradiative lifetime can be up to 182 ns.The effective mass approximation model canaccurately predict the spectroscopic properties,spatial distributions of photogeneratedcarriers and the energy level of the lowest conduction/valence bands.Ulteriorly,Holesand electrons tend to be localized separately in the ZnSe core and Cd S shell when theCd S shell thickness>0.85 nm.Quenching kinetics investigations demonstrate thatstatic quenching plays a more critical role compared to dynamic quenching inphotoluminescence quenching.It is worth noting that the interdiffusion between theZnSe core and Cd S shell is found to lead to the change of core-shell structure andrelease of lattice mismatch induced stress,and facilitate static quenching.Therefore,In summary,the spectroscopic and carrier dynamics properties of ZnSe-based core-shell quantum dots are stongly dependent upon the composition and thickness of the epitaxal shell.ZnSe-based core-shell quantum dots can exhibit either type-I,type-II or inverse type-I spectroscopic features through the choice of shell materials and rational control of shell thickness.Their spectroscopic properties and photogenerated carriers dynamics can be well predicted using the theoretical model based on the effective mass approximation with the considerations of the influences of the lattice mismatch and interdiffusion.The work present here will be of great significance for the controllable synthesis of the ZnSe-based core-shell quantum dots and modulations of photogenerated carrier dynamics and will be also helpful to promote the pratical applications of the ZnSe-based core-shell quantum dots in semiconductor quantum dots based devices.