| dc.description.abstract | The preparation and modification of semiconductor quantum dots (QDs) are very attractive research topics due to their wide range of emission wavelength and high quantum yield (QY). Compared to the traditional fluorescent dyes, QDs have narrow emission peak, continuous absorption spectrum, and high QY, which make QDs as appropriate fluorescence materials. II-VI group alloyed QDs with tunable emission wavelength which can be changed by controlling their compositions, and stronger lattice structure than other QDs, are studied recently. In these years, studies have been focused on their applications. Few attentions have been paid on the optimization through increasing crystallinity or changing the ligands ratio during the preparation of QDs.
In the study, high quality ZnCdSe QDs are synthesized by the high temperature organometallic procedure. QDs are soon cooled down to lower temperature (100-250 oC) and annealed after nucleation and growth. The variations on optical properties are observed in the process. The widely used Hexadecylamine (HDA) and trioctylphosphine oxide (TOPO) are chosen for the ligands in the procedure, and the effect of ligands ratio is also investigated in the study. Their particle sizes, structures, compositions, surface chemical states, QYs and optical properties are systemically analyzed by high resolution spectrometer (HRTEM), X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Fourier transform infrared spectroscopy (FTIR), UV-visible absorption spectroscopy (UV-vis), and fluorescence spectroscopy (FL), respectively.
It is found that the ligands ratios and temperatures both significantly influence the emission wavelength and QY of the QDs. While HDA ratio raises from 0 to 100 %, emission wavelength blue-shifts from 590 to 520 nm in 320 oC but red-shifts from 504 to 543 nm in 270 oC. Additionally, when HDA content increases from 0 to 75 %, the variation on QY of ZnCdSe QDs changes from <1 to 80 % due to the different steric hindrance of TOPO and HDA. In terms of atomic compositions, the ratio of zinc atoms obviously decreases from 35-56 mol% to 15-19 mol% when HDA is absence in the procedure.
The QY has obvious enhancement if QDs are annealed in 100-250 oC for a period of time after the nucleation and growth. For the samples annealed at various temperatures, annealing process in 150 oC can most effectively enhance the emission efficiency with an increase of QY from 80 to 95 % without changes of emission wavelength. The sample also maintains a better QY performance during the 52 day storage. Besides, it is found that the ratio of surface ligands is proportional to the ligands ratio initially added in the procedure, and is almost unchanged with storage time.
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