Synthesis And Property Investigations Of ZnSe And Cd S Magic-sized Clusters

Nanomaterials refer to ultrafine particles with at least one-dimensional size of 1-100 nanometers(nm)in a three-dimensional space.Because its size and shape are divergent from bulk objects,it has unique properties,such as quantum confinement effect and surface effect.Since semiconductor quantum dots(QDs)are between 1-100nm in three dimensions,their electronic and optical properties exhibit significant size-dependent changes.Magic-sized clusters(MSCs)belong to interesting intersection regions between molecules and quantum dots.During the routine synthesis of quantum dots,particle size distributions are inevitable,however,MSCs have a definite molecular formula.The sharp ultraviolet absorption characteristics and stability indicate that it is a monodispersed substance composed by a specific number of atomic arrangements.The uniformity of the chemical composition of MSCs is close to that of small molecules,and the size and shape of their particles can be controlled at the atomic level.Compared with the continuous growth of quantum dots,MSCs grow in discrete steps.Separating single-component MSCs can promote basic research and material development.At the same time,MSCs with high stability and monodispersion can be used as precursors to prepare other nanocrystals with high uniformity in size and shape.Therefore,the research on the growth of MSCs and the evolution to other nanocrystals can boost our understanding to the nucleation and growth mechanism of nanocrystals and can rationally design the synthesis of nanocrystals through precise control of the size of MSCs.However,the research of MSCs is still under the development,and its growth mechanism is far from mature.The rational synthesis of monodispersed MSCs and limited characterization techniques hinder people from further exploring them.In this thesis,we prepared zinc selenide magic-sized clusters(ZnSe MSCs),studied their growth conditions,and determined their composition and molecular formula through a variety of characterization methods.Next,we synthesized high-purity cadmium sulfide magic-sized clusters(CdS MSCs)by changing the reaction temperature and time,and prepared cadmium sulfide nanoplatelets(CdS NPLs)using CdS MSCs as the precursor and observed layer-by-layer growth process of CdS NPLs in three thicknesses.The specific research content is as followings.In the second chapter,single-component ZnSe MSCs with the first exciton peaks at 292 nm,298 nm,and 314 nm were prepared and named as MSC 292,MSC 298,and MSC 314.MSC 298 was prepared at room temperature.Through the gradual heating process,MSC 298 was unstable as an intermediate product.The absorption peak blue shifted in the mixed amine solution to induce the formation of MSC 292,which was red shifted to MSC 314 in the oleylamine solution.By changing the ratio of mixed amines and using amines with different chain lengths,the effect of amines on the conversion pathway of MSC 298 was explored.At the same time,direct heating and a series of hot injection experiments were carried out to explore the influence of temperature on the growth of ZnSe MSCs.In the hot injection experiment,no matter which amine was used as the solvent,the product obtained was MSC 314.The experimental results showed that the growth kinetics and stability of MSCs were related to temperature and the sot-template effect of amines.Finally,using matrix-assisted laser desorption ionization time-of-flight mass spectrometry(MALDI-TOF MS),thermogravimetric analysis,and elemental analysis,the molecular formulas of MSC 298-mixed amine and MSC 298-oleylamine were inferred as[Zn₅Se₅(OLA)₂(OTA)₂]and[Zn₅Se₅(OLA)₄];The molecular formulas corresponding to MSC 292 and MSC 314 were[Zn₃Se₆(OLA)(OTA)₃]and[Zn₄Se₇(OLA)₃](OLA=Oleylamine;OTA=Octylamine),respectively.In Chapter 3,high-purity CdS MSCs were prepared,which had a sharp absorption peak at 324 nm.The synthesis conditions of MSC 324 under the conditions of one-pot reaction and hot injection were explored,and the different growth pathways of CdS MSCs and CdS QDs were analyzed.The reaction temperature and reaction time were critical to the purity of the synthesized MSC 324.CdS NPLs were prepared by adding cadmium acetate to the pure MSC 324 stock solution.The evolution of the three thicknesses of CdS NPLs was observed.The CdS NPLs grew layer by layer,with absorption peak started at 374 nm,then evolved to 407 nm,the resulting CdS NPLs had a sharp absorption peak at 431 nm.The experimental results showed that the purity of MSC 324 had a great influence on the growth of CdS NPLs.MSCs were potential precursors for the synthesis of versatile nanocrystals.