Synthesis Of Colloidal Cadmium Chalcogenide Quantum Wells And Engineering Of Exciton Spatial Distribution

Two-dimensional colloidal semiconductor quantum wells,as a burgeoning class of semiconductor nanomaterials,received extensive attention in the field,given the distinguished optoelectronic properties,such as sharper fluorescence emission peaks,smaller stokes shifts,and shorter fluorescence lifetimes,in comparison with the traditional quantum dots.The unique inorganic-organic hybrid structure induces exciton confinement within thickness direction under the influences of energy barriers along the interfaces.Among them,the cadmium chalcogenide quantum wells are characteristic with well defined and controlled molecular structures along the thickness direction,at the same time,lay a solid material foundation for the possibility of investigating structure-property relationship on this type of materials.In this thesis,based on the tailored synthesis of CdS and CdSe colloidal quantum wells with zinc-blende structure,we first realized the precise control of thickness for the quantum well,i.e.,the direction along which excitons bear the quantum confinement.Then,through a plethora of spectroscopic characterizations,we clarified the molecular structure of the CdS quantum well along the thickness direction,systematically considered the exciton under varied physical boundaries,investigated the influences of the energy barriers on the electron wavefunctions,illuminated the spatial distribution of exciton,and successfully realized the correlations between optical properties and molecular structures.Firstly,this thesis outlined the routine synthesis protocols for colloidalⅡ-Ⅵ quantum wells,and systematically analyzed and summarized corresponding mechanism.Based upon the literature survey,we scrutinized the synthesis parameters,such as the anionic precursors and hydrocarbon chain length of cadmium precursors,for the ’seeds’ in colloidal CdS and CdSe quantum wells with relatively thick thicknesses,and optimized the synthesis route of small sized quantum dots as the ’seeds’.In addition,we carried out detailed inspections of CdS quantum wells with four kinds of thicknesses,and realized the spatial control of exciton distributions.In Chapter 2,the preparation of cadmium-based chalcogenide colloidal quantum wells is performed by using green chemical approaches.The prepared colloidal quantum wells were characterized by UV-Vis absorption and fluorescence emission spectroscopy,X-ray diffraction and transmission electron microscopy.In Chapter 3,we first interrogated the growth process of CdSe quantum wells with the first exciton absorption peak at 550 nm,experimental results revealed that the CdSe quantum dots with the first exciton absorption peak at about 433 nm could be adopted as the ’seeds’.During the synthesis,we discovered that high reaction temperature and long reaction duration lead to the formation of colloidal CdSe quantum wells with thicker thicknesses,and thinner ones co-existed during the process,with the thickness increasing through a layer-by-layer manner.Through appropriately adjusting the reaction conditions,we could obtain products with uniform thickness.When we used bis(stearoyl)selenide as the selenium precursor,quantum dots with characteristic absorption at around 433 nm could be prepared.This type of quantum dots were similar to those prepared by using selenium powder as the anionic precursor,and could also be used as‘seeds’for the synthesis of the CdSe quantum wells,with similar growth process revealed by UV-Vis absorption.The growth of quantum wells with different thicknesses was realized by applying small sized CdSe quantum dots as‘seeds’through temperature control,and the synthesis methodology was simplified.This kind of small sized quantum dots have the potential to become excellent precursors for the preparation of other families of colloidal quantum well materials.In Chapter 4,while preparing colloidal CdS quantum well with a first exciton absorption peak at 431 nm,contrary to the preparation process of other thinner CdS quantum wells,we observed that CdS quantum dots with the first exciton absorption peak at 324 nm were required as the‘seeds’.Through the optimization of the experimental scheme,we achieved the synthesis of the ’seeds’ by using the ’one-step’ method.The method has the merit of higher stability and easier accesibility of reaction precursors.And similar to the phenomena observed during the synthesis of CdSe quantum wells,we found that the formation of thinner CdS quantum wells procede the appearance of the counterparts with thicker thicknesses,and the whole process was following a layer-by-layer growth along the thickness direction,while the“seeds”were pure enough.In Chapter 5,based on the cadmium-terminated colloidal CdS quantum wells,we employed alkylthiols(1-dodecanethiol,DDT)as the sulfur precursors,to synthesize S-terminated CdS quantum wells that maintained the platelets morphologies and exhibited shaper exciton absorption peaks.Spectroscopic means,X-ray diffraction,X-ray absorption,and transmission electron microscopy confirmed that both cadmium and sulfur terminated CdS quantum wells were equipped with explicit surface structures.We discovered that,the band gap of a quantum well depends not only on the quantum confinement dimension(i.e.thickness),but also on the nature of the surface terminations.The facet structure determines the packing of ligands(carboxylate ligands for the cadmium-terminated quantum wells and alkyl ligands for the sulfur-terminated ones),which induces the divergence of lattice strain and affects the broadening of the absorption profile.Experimental and theoretical calculations reveal that,by precisely controlling the facet structure of the quantum well,the spatial distribution of exciton in the quantum well can be regulated,laying a solid material and theoretical foundation for the potential applications of semiconductor quantum wells.