| Quantum dots(QDs)are widely used for diverses application such as photoelectric device,bioimaging and detection etc.which are mainly due to the unique optical and electric properties.The size of QDs is in the nano scale,which produces large surfacevolume ratio and large fraction of surface atoms,whose binding is quite different with the atoms inside the crystals.This difference result in the dative bond around the surface of QDs and alter the electronic structure of QDs.Furthermore,these surface atoms interact directly with the medium around QDs,which may cause damage to the optical or electronic properties.Thus,it is of great value to investigate the surface and the process happened on the surface.This work would contribute to the understanding of the design,synthesis and application of QDs.Hydrophobic OAc-CdSe QDs and hydrophilic GSH-CdTe QDs with uniform size and excellent optical properties were synthesized in this thesis.Then the ligand exchange of CdSe QDs and FRET process between CdTe QDs and Rhodamine(RhB)were studied through fluorescence spectra,UV-vis absoption spectra and lifetime ect.According to the analysis of corresponding results,the process during the ligand exchange of CdSe QDs and FRET was proposed.The result of this thesis is of great importance to further understand the physical chemistry process of the surface of QDs during the application,Besides,this work makes some supplement to the knowledge of QDs' surface in a certain extent.This thesis consists of four chapters:Chapter 1: Basic knowledge including synthesis,optical properties and application was introduced briefly in this chapter.Besides,the surface,techniques to detect it and some physical chemistry process happened around the surface of QDs were also presented.Chapter 2: CdSe QDs capped by OAc only was synthesized to avoid the interference of complicated surface composition.Three amine ligands with different carbon chain lengths were applied to study the ligand exchange process of CdSe QDs.The results of NMR,FTIR and XPS confirmed the successful ligand exchange between amine and OAc on the surface of QDs.FL spectra indicated this process promoted the release of bound OAc,leading to the surface trap and quenching of QDs.Combined with thermodynamic data and other techniques,it could be concluded that,this process was not only determined by the coordination interaction between the amine and surface of QDs,but also influenced by van der Waals forces.Nevertheless,the reconstruction around the surface of QDs passivated the traps and restored the fluorescence partly.The extent and rate of reconstruction were related to the van der Waals forces closely.UVvis spectra and lifetime of QDs in the presence of amines showed that the electronic structure was changed.TEM results demonstrated that the surface was not etched by amine and the size remained unchanged,while DLS results showed that the interaction between QDs and medium around was varied,causing the decrease of hydrodynamic size of QDs.Chapter 3: Hydrophilic GSH-CdTe QDs was synthesized and applied to constructed FRET system with RhB,where the cationic surfactant CTAB was needed to induce the process.The mechanism of why CTAB could induce FRET between QDs and RhB was thoroughly studied by FL,UV-vis,DLS,zeta potential,TEM,CMC and ect.CTAB was adsorbed onto the surface of QDs through static interaction.On the one hand,the negative charge to stabilize QDs was neutralized and thus QDs tend to aggregation.On the other hand,the exposure of hydrophobic tails increased the thermodynamic instability.Therefore,the aggregated QDs was crosslinked by CTAB,and RhB was trapped inside the crosslinked structure.In this way,the distance between QDs and RhB was reduced and FRET could occur.Besides,variable temperature fluorescent titration and isothermal titration calorimetry(ITC)indicated FRET process was exothermic and entropy increased process.Chapter 4: The main content of this thesis was concluded and the tendency of this subject was analyzed in this chapter. |
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