Synthesis And Properties Of Hydrophobic CdSe Based Quantum Dots

Compared with the traditional organic fluorescent dyes, CdSe quantum dots（QDs）, as arepresentative of the semiconductor nanomaterials, have the advantages of high luminescentintensity, narrow and tunable emission spectra, wide excitation spectra, and optical stability,which results in their significant applications as display and energy materials, and as a newkind of luminescent label in the field of biological labeling and immunology detection.Because of special growth dynamics of CdSe QDs and the related core-shell structure, theywere widely interested in the field of basic chemistry, such as synthesis and morphology. Incurrent work, CdSe, CdSe/ZnS and CdSe/Cd_1-xZn_xS QDs have been successfully synthesizedin an organic system, and systematically investigated for their structure, morphology, opticalproperties and chemical stability.In the beginning, CdSe QDs were synthesized with different particle sizes, variousemission wavelengths and high quantum yield（QY） via a hot injection method. A series ofstudy were then carried out on the synthesis and properties of CdSe based CdSe/ZnS,CdSe/Cd_1-xZn_xS core-shell QDs. Firstly, CdSe QDs were successfully prepared by a modifiedhot injection method in an organic system.1-octadecene was introduced into the systemreplacing a portion of toxic and expensive trioctylphosphine（TOP）. According toexperimental results, the highest QY was obtained with synthesis conditions of300oC and5min. CdSe QDs were characterized by transmission electron microscopy（TEM）, uv-visabsorption spectrometer, fluorescence photometry spectrometer and time correlated singlephoton counting spectrofluorometer. And analytical results showed that the CdSe QDs werespherical and monodispersed. Besides, with the reaction time increasing, the mean size indiameter of CdSe QDs increased and red shift of fluorescence wavelength was observed. Theluminescent lifetime changed within the range of several ten of nanoseconds as the size ofCdSe QDs. Secondly, the CdSe/ZnS core/shell QDs were successfully synthesized. Organicligand category, reaction time, and TOPS injection speed, which had influences on theproperty of CdSe/ZnS QDs, were investigated in the experiment. The results showed thatoleic acid was chosen as ligand, the reaction time was1min, TOPS injection speed wasbelow0.4mL/min, which could prevent ZnS from nucleating, CdSe/ZnS QDs was the highest QY of54%. The core/shell structure was confirmed by systematic characterizations. Thirdly,the ligand category, reaction time and TOPS injection speed on the luminescent property ofCdSe/Cd_1-xZn_xS were studied in the preparation process of CdSe/Cd_1-xZn_xS （x=0.25,0.5,0.75）core/shell QDs. The results showed that CdSe/Cd_1-xZn_xS QDs of the highest QY was67.5%under the following conditions: oleic acid was chosen as ligand, Cd/Zn ratio in the shell was1,the reaction time was5min, and TOPS injection speed was0.4mL/min. The core/shellstructure was confirmed by systematic characterizations.The characterization of opticalinstruments and XRD proved the core shell structure formed in the experiment.Next, on the basis of above synthetic experiments, CdSe/CdS, CdSe/Cd0.25Zn0.75S,CdSe/Cd0.5Zn0.5S, CdSe/Cd0.25Zn0.75S and CdSe/ZnS core/shell QDs were synthesized in thesame experimental conditions, respectively. The property variations of these QDs wereinvestigated, including the morphology, red-shift, fluorescence life and the QY. It was foundthat the morophology of CdSe/Cd_1-xZn_xS core/shell QDs was irregular tetrahedron, whileCdSe/ZnS was spherical. Under the condition of the same QD cores and the same amount ofcoating materials, the red-shift reduced according to the following sequence, CdSe/CdS,CdSe/Cd0.75Zn0.25S, CdSe/Cd0.5Zn0.5S, CdSe/Cd0.25Zn0.75S, and CdSe/ZnS QDs. The QY wasenhanced according such a sequence, CdSe/Cd0.5Zn0.5S, CdSe/ZnS, CdSe/CdS and QDs.Finally, the stability of CdSe/Cd_0.5Zn_0.5Score/shell QDs was systematically investigated.The CdSe/Cd_0.5Zn_0.5Score/shell QDs were synthesized with hexadecylamine （HDA） as ligandand cadmium acetate, zinc acetate as shell precursors. The as-prepared QDs were kept in darkat room temperature, and the changing of the luminescent property was recorded. It was foundthat the QY rised at the beginning, declined after being up to maximum, and trended to asteady value with the time extending. The samples were characterized by TEM, uv-visabsorption spectrometer; time correlated single photon counting spectrofluorometer andFourier transform infrared spectrometer （FT-IR）. And the FT-IR result confirmed that HDAwas gradually combined with QDs. The QY was up to maximum after saturation. The fall ofit was due to photooxygenation at room temperature.