Formation Mechanism Of CdSe Two-Dimensional Nanocrystals In Zinc-Blende Structure

With the establishment of its concept as the beginning,colloidal semiconductor nanocrystal have been in existence for nearly four decades,and gradually developed into a vibrant,multidisciplinary research field.As an important branch of this field,the two-dimensional semiconductor nanocrystals with uniform thicknesses in the entire quantum confinement regime,have been grown at a rapid pace in the past ten years.However,the formation mechanism of two-dimensional nanocrystals,especially the two-dimensional nanocrystals of zinc-blende structure,is still a chaos compared with the cognition of the growth mechanism of quantum dots.This thesis aims to figure out the dilemma above.Firstly,we summarized the common synthesis methods of CdSe 2D nanocrystals with different thickness and systematically studied two key steps in the synthesis,namely "seed formation" and two-dimensional nanocrystal induction growth.Through the analysis of different types of selenium precursors and the introduction methods,the chain length of cadmium precursors and the introduction ways of cadmium acetate,the decisive effect of "seeds " on the thickness control was confirmed.Based on the deep understanding of the influence of various reaction factors on the thickness of CdSe two-dimensional nanocrystals,we have developed an optimized reaction system for the two-dimensional nanocrystals of each thickness.Through the kinetic studies of these optimized systems and the analysis of the existing formation mechanisms,such as monomer-adsorption growth,phase separation growth and soft-template growth,we conclude that the oriented attachment is most likely the main reason for the symmetry breaking in the formation of two-dimensional nanocrystals.By applying purified seeds with their sizes between 1.7 and 2.2 nm for growth of 5.5-monolayer CdSe 2D nanocrystals(1.5 nm in thickness),growth through lateral extension by monomer addition onto existing seeds was excluded.The first symmetry-breaking(between the thickness and lateral directions)was realized by rapid formation of single-dot intermediates through intraparticle ripening of the seeds,which possessed a reduced thickness(1.5 nm)perpendicular to the atomically flat {100} basal planes and expanded lateral dimensions in comparison to diameter of the initial seeds.The second symmetry breaking(between two lateral dimensions)originated from unequal development of the polar and well passivated {100} side facets during the oriented attachment.Cadmium acetate assisted both formation of single-dot intermediates and oriented attachment.Cadmium alkanoates with a long hydrocarbon chain also played two roles in the conversion of the seeds to 2D nanocrystals,namely,selectively stabilizing polar {100} facets of the nanocrystals and shuttling insoluble acetate to the surface of the nanocrystals.In conclusion,the formation of two-dimensional CdSe nanocrystals goes through a complex process involving oriented attachment,intraparticle ripening.Understanding symmetry breaking in highly symmetric crystal structures should help to open a door for synthesis of a diverse group of two-dimensional CdSe nanocrystals with unique optical andoptoelectronic properties.