Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors Public Deposited

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  • Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks are used to lower the fabrication cost of the absorber layers of the solar cells. In addition, some quantum confined nanocrystals display electron-hole pair generation phenomena with greater than 100% quantum yield, called multiple exciton generation (MEG). These quantum dots could potentially be used to fabricate solar cells that exceed the Schockley-Queisser limit. At present, continuous syntheses of nanoparticles using microreactors have been reported by several groups. Microreactors have several advantages over conventional batch synthesis. One advantage is their efficient heat transfer and mass transport. Another advantage is the drastic reduction in the reaction time, in many cases, down to minutes from hours. Shorter reaction time not only provides higher throughput but also provide better particle size control by avoiding aggregation and by reducing probability of oxidizing precursors. In this work, room temperature synthesis of Au₁₁ nanoclusters and high temperature synthesis of chalcogenide nanocrystals were demonstrated using continuous flow microreactors with high throughputs. A high rate production of phosphine-stabilized Au₁₁ nanoclusters was achieved using a layer-up strategy which involves the use of microlamination architectures; the patterning and bonding of thin layers of material (laminae) to create a multilayered micromixer in the range of 25-250 µm thick was used to step up the production of phosphine-stabilized Au₁₁ nanoclusters. Continuous production of highly monodispersed phosphine-stabilized Au₁₁ nanoclusters at a rate of about 11.8 [mg/s] was achieved using a microreactor with a size of 1.687cm³. This result is about 30,000 times over conventional batch synthesis according to production rate/per reactor volume. We have elucidated the formation mechanism of CuInSe₂ nanocrystals for the development of a continuous flow process for their synthesis. It was found that copper-rich CuInSe₂ with a sphalerite structure was formed initially followed by the formation of more ordered CuInSe₂ at longer reaction times, along with the formation of Cu₂Se and In₂Se₃. It was found that Cu₂Se was formed at a much faster rate than In₂Se₃ under the same reaction conditions. By adjusting the Cu/In precursor ratio, we were able to develop a very rapid and simple synthesis of CuInSe₂ nanocrystals using a continuous flow microreactor with a high throughput per reactor volume. The microreactor has a simple design which uses readily available low cost components. It comprised an inner microtube to precisely control the injection of TOPSe into a larger diameter tube that preheated CuCl and InCl₃ hot mixture was pumped through. Rapid injection plays an important role in dividing the nucleation and growth process which is crucial in getting narrow size distribution. The design of this microreactor also has the advantages of alleviating sticking of QDs on the growth channel wall since QDs were formed from the center of the reactor. Furthermore, size-controlled synthesis of CuInSe₂ nanocrystals was achieved using this reactor simply by adjusting ratio between coordinating solvents. Semiconductors with a direct bandgap between 1 and 2eV including Cu(In,Ga)Se₂ (1.04 – 1.6eV) and CuIn(Se,S)₂ (1.04 - 1.53eV) are ideal for single junction cells utilize the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic QD-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, Cd[subscript X]Hg[subscript 1-X]Te), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters. First continuous synthesis of shape-controlled SnTe nanocrystals were also accomplished in this work. SnCl₂, and TOPTe were used as reactants successfully in coordinating OA and TOP solvents. Both rod shape and dot shape SnTe nanocrystals with uniform size distributions could be obtained. A blue shift was observed from these SnTe nanocrystals. Production rate at about 5mg/min (300mg/hr) was achieved using a microreactor at a size of 1.78cm³.
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