Microchemical systems for the synthesis of nanostructures : quantum dots

Abstract

We have developed a continuous multi-stage high-temperature and high-pressure microfluidic system. High-pressure conditions enabled the use low molecular weight solvents that have previously not been available for quantum dot (QD) synthesis such as hexane or octane. The use of supercritical phase provided excellent mixing, which was critical in producing high quality QDs. In addition, the microfluidic system allowed precise control of synthetic conditions for the fast screening of reaction parameters. The continuous multi-stage microfluidic system enabled separating of reaction conditions such as mixing and aging steps, which was not possible in batch synthesis, as a result it was possible to conduct systematic investigation of the synthesis of indium phosphide (InP) QDs. We investigated synthesis of InP QDs with a continuous 3-stage high-temperature and high-pressure microreactor system without incorporating any batch manipulations between the synthesis steps. By separating the mixing process from the following aging process, we found that InP QD synthesis were mainly dominated by coalescence processes. Indium to fatty acid ratio showed the largest effect on particle size due to enhanced inter-particle processes. Concentrations or mixing temperatures changes, which are important reaction parameters of cadmium selenide (CdSe) QD growth, had no significant impact. We also synthesized larger (>3.2 nm) InP QDs with a sequential injection microreactor consisting of 6 sequential alternative monomer injections similar to the successive ion layers adsorption and reaction (SILAR) method. We obtained InP QDs with size distributions as narrow or narrower than the InP QDs synthesized via the ripening process. Indium phosphide / zinc sulfide (InP / ZnS) core-shell QDs were obtained with a 5 or 6 -stage microreactor system consisting of additional shell growth reactors, in addition to the three-step InP growth system. We were able to obtain narrow emissions with high quantum yield. This system was also used for the synthesis of indium phosphide / cadmium sulfide (InP / CdS), indium arsenide / indium phosphide (InAs / InP), and indium arsenide / cadmium sulfide (InAs / CdS) core-shell QDs. We also investigated the growth of InAs QDs using the same system for InP QD synthesis. We found that the InAs growth from indium myristate (In(MA) 3) and tristrimethylsilyl arsine ((TMS) 3As) precursors showed similar behavior as InP growth. However, different from the growth of InP nanocrystals, the amount of excess fatty acid did not affect on the growth of InAs nanocrystals. Indium phosphide arsenide (InPxAs1 -) alloy nanocrystals were also synthesized by precise control of phosphorus (P) and arsenic (As) precursor amounts. Mixing two anionic and cationic precursors at an elevated temperature followed by fast heating up to the reaction zone is very important for InPxAsl1x alloy nanocrystal synthesis. A multistage microfluidic system with a mixing reactor with gradient temperature was a useful tool for this synthesis. InPxAs - alloy nanocrystals were characterized with optical measurements and wide angle X-ray diffraction scattering. We investigated growth of InAs nanocrystals from a less reactive arsenic precursor, tris(trimethygermyl) arsine (TMG3As). We obtained InAs nanocrystals with better size distribution than those synthesized from TMS3As. We also compared the growth behavior of InAs nanocrystals synthesized from those two different arsenic precursors. With TMG3As, we observed a growth behavior potentially following a similar nucleation and growth model to that of growth of II-VI QDs.