Green Chemical Approaches To Monodisperse ZnSe And PbSe Quantum Dots: Preparation And Characterization

As an important part of nanomaterials, quantum dots (QDs) not only have a particular character on basic physics, but also have an enormously potential application on photoelectric apparatus. As a result, the research of quantum dots is the hotspot and foreland of the field of semiconductor materials for a long time. Due to the effects of their size, size distribution, shape and structure on QDs, the effective control of size and size distribution is the hotspot of research all the time. In this dissertation, a green chemical approach to preparation and characterization of monodisperse ZnSe and PbSe Quantum Dots has been introduced.The main achievements of the work are listed as followings:1. We reported a cheaper, greener, phosphine-free route for preparation of ZnSe QDs using N, N-dimethyl-oleoyl amide as Se powder solvents, which eliminates the need for air-sensitive, toxic and expensive chemical trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). The natural surfactant oleic acid (OA) to dissolve zinc acetate and form zinc oleate solution in the noncoordinating solvent has also been used. From the XRD patterns, three typical distinct features of zinc blende structure can be distinguished. The average size of the ZnSe QDs is about 4 nm, which was obtained by injecting 0.2 mmol Se solution into 0.3 mmol Zn(CH3COO)2 solution at 290°C. The HRTEM image reveals the higher crystallinity of the ZnSe QDs. It is observed that the ZnSe QDs were well crystallized, with interplannar distances of 0.33 nm, which was consistent with the (111) planes of cubic ZnSe.2. The UV-vis absorption spectra shown the size of ZnSe QDs were increasing with the increasing of surfactant oleic acid (OA), but deceasing with the increasing of precursors. The reactions in 0.3 mmol of Zn(CH3COO)2 and 0.2 mmol of Se powder with different amount of 1ml, 3ml and 5ml OA as surfactants which were corresponding to the first absorption spectra of 379nm, 390nm and 420nm, respectively. With the injection of Se precursor into the flask, the reaction with 5ml OA as natural surfactant starts with an extremely fast nucleation process, followed by an early growth process. These two processes are so fast that the red-shifted absorption spectra cannot be obtained. As the reaction proceeds to about 10 s, the temporal ZnSe nanoparticles will grow to 12 nm, reaching the largest particle size in this synthesis.3. ZnSe QDs aggregated to form the flower-shaped ZnSe nanocrystals due to van der waals force of surfactant molecules by increasing the injection of Se solution at 310°C and keeping the mixture of Zn precursors and Se precursors at the same temperture for growth.The nanoflowers with size of about 30 nm consist of several 4 nm single nanodots. The UV-vis absorption spectra and PL emission spectra shown ZnSe nanoflowers have similar size-dependence quantum effects.4. We also report a cheaper, greener, phosphine-free route to PbSe QDs with N, N-dimethyl-oleoyl amide as a solvent to dissolve Se powder, which also eliminates the need for air-sensitive, toxic and expensive chemical trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). The natural surfactant oleic acid (OA) is used to dissolve zinc acetate and form lead oleate solution in the noncoordinating solvent. The experimental results show PbSe QDs with sizes of 9nm, 12nm and 16nm could be btained in the Pb:Se mol ratios of 3:1,2:1,1:1, respectively.