| dc.description.abstract | Recently, the preparation and modification of colloidal CdSe quantum dots (QDs) have attracted a lot of attention due to their potential applications in the field of solid state lighting, such as light-emitting diodes (LEDs), photovoltaics, bioimaging and laser, etc. Many successful preparation methods have been reported for the synthesis of high quality CdSe QDs. Most of these methods involve the use of trioctylphosphine oxide (TOPO) as surfactants to prepare CdSe QDs. However, the exact rule of such indispensable TOPO during the synthesis is still unclear.
In this study, high quality CdSe QDs have been successfully synthesized by the high temperature pyrolysis organometallic procedure (320 oC). TOPO and Octadecylamine (ODA) are chosen as the surfactants in the procedure, and the effects of surfactants ratios on the optical properties of CdSe QDs are investigated in the study. The sample without TOPO addition could form the precipitates above 300 oC, and the effects of precipitation and temporal evolution of Cd precursor on the optical properties of CdSe QDs are also examined. Finally, long-term stability of the FL is studied for the as-prepared CdSe QDs stored under ambient conditions. The morphologies, structures, compositions, surface chemistry, quantum yields (QYs) and optical properties of CdSe QDs are systemically analyzed by high resolution spectrometer (HRTEM), X-ray diffraction (XRD) and nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy (UV-vis), and Fluorescence spectroscopy (FL), respectively.
The obtained CdSe QDs synthesized with different surfactant ratios have different structures, in which ODA as a capping agent stabilizes the zinc blende structure while TOPO stabilizes the wurtzite phase. Besides, the surfactant ratios affect the Cd/Se atomic compositions. By adding 0 to 75 wt % of TOPO, the Cd/Se ratio increases from 1.15 to 8.12 owing to the preferential bonding of phosphonate ligands on Cd and then decreases to 2.72 when TOPO content increases to 100 wt %, which is caused by the phase transition and surface reconstruction. Moreover, CdSe QDs with larger ratio of Cd/Se displays the slower decay rate during the storage, suggesting that Cd-complexes can effectively protect Se from oxidation.
When prepared above 300 oC without TOPO addition, CdO and Cd mixtures from recrystallization of Cd(OH)2 are formed, which can be attributed to that ODA combines with carboxyl groups to form amides, and the Cd(OH)2 is the by-product. On the other hand, the formation of amides can provide a good electronic passivation for the surface states of QDs. As a result, QDs with high QYs are obtained.
The TOPO/ODA mass ratio of 2:2 seems to be an optimal condition during the 50 days storage to produce CdSe QDs with high stability and QY value due to their largest ratio of Cd/Se.
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