Synthesis And Characterization Of CdSe Quantum Dots And Fe₃O₄/CdSe Nanocomposites

Quantum dots (QDs) have attracted extensive attentions in fundamental studies and technical applications such as optoelectronic devices, biophotonic imaging, fluorescence labels, mainly due to their unique photonic, physical and chemical properties. Nanomagnetic particles also have important application in magnetic resonance imaging and drug carrier. However, either of them can only afford people with only singe function. A combination of optical and magnetic properties in a single material would extend the application of both quantum dots and magnetic particles. Therefore, fluorescent-magnetic nanocomposites would have great application prospects.In presen study, CdSe quantum dots, Fe₃O₄ magnetic particles and Fe₃O₄/CdSe nanocomposites were prepared via high-temperature decomposition of organometallic precursors. The optical and magnetic characteristics of the as prepared samples were investigated by UV-Vis spectra, photoluminescence spectra (PL), X-ray diffraction(XRD), transmission electron microscopy (TEM, HRTEM) and physicians practice management systems (PPMS). This thesis was focused on studying the effects of reaction time, different Cd:Se ratio and surface ligands on the optical properties and morphology of the as prepared samples. In addition, the orgin of optical property change in the sample prepared with long reaction time was investigated.It has been indicated that the size of CdSe quantum dots could be controlled via both paramaters of the growth time and the Cd:Se ratio. As the reaction time increased,the absorbance and luminescence peaks of CdSe quantum dots had red-shifts, indicating the increase of their size with increase of reaction time. Under the same reaction time, CdSe quantum dots grew large with the decrease of Cd:Se ratio, every series of sample with settled Cd:Se ratio possessed a maximum quantum yield at certain reaction time, and the reaction time for maximun quantum yield value increases with decrease of Cd:Se ratio.It was suggested the multimodal distribution of the CdSe nanoparticles quantum confinement was mainly due to abnormal grain growth. and, the process was proposed as the following:â‘ As the reaction time increased, the reaction reached the Ostwald ripening regime, smaller particles disappeared to become larger particles,â‘¡particles aggregated and coalesced to for big particles.The relation between the shape and optical properties of CdSe nanoparticles was studied. It was revealed that using TDPA as the surface ligand and keeping the Cd precursors at 270℃for a period of time were beneficial to synthesize CdSe nano-particles with larger aspect ratio. Within the aspect ratio covered in present study, the rod-like CdSe nanocrystal has better optical properties than that of dot-shaped CdSe nanocrystal.It was revealed that high crystallinity and monodispersed Fe₃O₄ nanoparticles with average size about 6-8nm could be prepared through high temperature thermal de-composition of Fe(acac)3 in the presence of dibenzyl ether, oleic acid, octadecylamine, and 1,2-Dodecanediol. The particles were superparamagnetic at room temperature, with the saturation magnetizations (Ms) of 51.4emu/g.The fluorescent-magnetic nanocomposites Fe₃O₄/CdSe was prepared via a two step synthesis route. The as-prepared samples had high quality optical properties, such as sharp Abs and PL peaks, narrow PL full width of half-maximum(fwhm) of about 28nm, high PL quantum yield of about 27.8% and, the nanocomposites ware superparamagnetic at room temperature, with saturation magnetizations (Ms) of 0.92emu/g. There were two plausible mechanism for the formation of Fe₃O₄/CdSe nanocomposites. One is that in the initial stage, Fe₃O₄/CdSe heterostructures formed with the process of heterogeneous nucleation. However, when the coincidence interface in the Fe₃O₄ surface was occupied fully and could not offer sufficient heterogeneous nucleation points, homogeneous nucleation would dominate the formation process and produces mainly single CdSe QDs. In the latter case, the Fe₃O₄ and CdSe nanoparticles seemed to form a "bilayer structure" due to hydrophobic -hydrophobic interactions.The effect of growth time and Cd: Se ratio on the preparation of Fe₃O₄/CdSe nanocomposite and its optical properties were investigated. The red-shift of the UV-vis and PL peak of the nanocrystals indicated the increase of their size over time. Nevertheless, after 5min reaction interval the size of the CdSe QDs was almost constant. This was probably due to the lattice strain effect when the size of QDs beyond a certain size. There exists too high lattice strain that limits the growth of CdSe QDs. On the other hand, varying Cd:Se ratio had little effect on the emission wavelenght, but the PL quantum yield change dramatically. It was demonstrated that the PL quantum yield increases quickly under the same reaction time with the decreases of Cd:Se ratio.It was found that the surface ligand played an important role in the preparation and properties of fluorescent-magnetic nanocomposites. Fe₃O₄/CdSe nanocomposite with high quality of luminescence performances wes obtained using ODA as ligand. In contrary, using the TDPA as the ligand would produce poor optical properties. This significant change may be related with the different ligand coordination mechanism.