Research On The Preparation And Properties Of Ⅱ-Ⅵ Semiconductor Nanocrystals

Since the introduction of quantum size effect by L. Brus, inorganic nanocrystals have attracted more and more attention of researchers. Especially for semiconductor nanocrystals, since their band gap can be tuned by their size, so the size of semiconductor nanocrystals will decide the photoluminescence positions. Recently, because of the unique optical and optoelectronic properties of semiconductor nanocrystals, much effort was devoted on the synthesis of high quality semiconductor nanocrystals.Generally, nanocrystals synthesized in organic solutions are better than the hydrothermo method, with better crystallinity, narrower size distribution, stronger stabilization, and higher quantum yields. In the synthesis of high quality nanocrystals, Bawendi's group introduced organometallic method in 1993 and high quality cadmium chalcogenide nanocrystals were synthesized. After 2000, Peng's group introduced a cheap, low toxic method to synthesize semiconductor nanocrystals. CdO was used to replac(eCd(CH3)2), reacted with oleic acid as the Cd precursor. Octadecene was used as noncoordinating, replace traditional trioctylphosphine oxide. These improvements made synthesis of nanocrystals greener and cheaper. But, many problems were still existed in the synthesis of high quality nanocrystals.Here, we made improvements mainly in three aspects. Firstly, for the preparation of metal selenide nanocrystals, selenium powder was directly dissolved in octadecene at elevated temperature as the Se precursor, without the use of hazardous and expensive tributylphosphine (TBP) and trioctylphosphine (TOP). Since octadecene was used as solvent for selenium, so the toxicity was largely reduced, and the reaction can be conducted without the use of glove box. And the cost of nanocrystals was reduced as much as 50%. Secondly, with the traditional methods, only few nanocrystals could be prepared in one reaction, generally some milli-grams of nanocrystals. Here we report the large scale synthesis of high quality semiconductor nanocrystals, about 5 grams of semiconductor nanocrystals was synthesized in one reaction. Thirdly, traditional cadmium chalcogenide nanocrystals are toxic because the existence of cadmium, so the application in in vivo biolabeling was limited. Here we use the type-II concept of core/shell nanocrystals to prepare green nanocrystals. With CdS as the innermost core, ZnSe as the middle shell, and ZnS as the outest shell, we got type-II/type-I nanocrystals. Cd content was reduced to about 1% in atomic ratio, and physically in the innermost core, so the resulted CdS/ZnSe/ZnS nanocrystals were much greener and have broad applications in future.In chapter 2, high quality CdSe nanocrystals and CdSe/ZnS nanocrystals were synthesized with green starting materials. For the preparation of Se precursor, we used octadecene, instead of hazardous and expensive TBP/TOP. And we use inverse injection method, using Cd precursor as the injection solution, and the volume of nanocrystals could be largely improved. This large scale synthesis of CdSe nanocrystals is more suitable for industrial applications. For the synthesis of CdSe/ZnS core/shell nanocrystals, the wide band gap material ZnS eliminated surface defects on CdSe, so the photoluminescence quantum yields were improved to 50-80%, and the full width at half maximum (FWHM) was below 30 nm. With our method, the synthesis cost was largely lowered, the starting materials were green, the synthesis scale was largely increased.In chapter 3, using dodecanethiol as the S source and surface ligands, CdS and ZnS nanocrystals were synthesized. Different reaction temperatures and precursor ratios were tested to optimize synthesis conditions. With this method, ultra small CdS nanocrystals were synthesized easily. As an effective hole acceptor, thiol groups will quench the band gap emissions of CdS and ZnS nanocrystals, so ZnS shells were grown on the CdS cores to recover the band gap emission of CdS.In chapter 4, high quality CdS/ZnSe/ZnS core/shell1/shell2 nanocrystals were synthesized. Using the type-II concept, emission of CdS/ZnSe nanocrystals could be tuned from 500 nm to 630 nm, by the control of core size of CdS and shell thickness of ZnSe. After the grown of outest ZnS shell, quantum yields were improved from 30% to 50-60%, and the Cd content was reduced to about 1% in atomic ratio. According to the X-ray diffraction results and the high quantum yields, we think the growth of shells was epitaxial. By the phase transfer experiment, original hydrophobic nanocrystals were transferred into water phase, and the superior optical properties were retained.In chapter 5, thick ZnS shells were grown on the core nanocrystals. We investigated the influence of thick shell on the optical properties of nanocrystals. Thick shells not only improved the emission quantum yields, but also improved the luminescent stability. We assembled elementary devices with our nanocrystals and measured the current-voltage curves. And some promising results were presented here.