Vapor-phase synthesis of II-VI semiconductor nanocrystals and nanowires
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The vapor-phase synthesis of II-VI nanocrystals (quantum dots) and nanowires has several advantages over traditional liquid-phase techniques, including purity and better compatibility with existing operations in the microelectronics industry. On the other hand, it makes size control, surface passivation of the nanoparticles and growth orientation of the nanowires a more challenging task. A counterflow jet reactor was modified and used to study the synthesis and the surface passivation of luminescent ZnSe nanocrystals by reacting vapors of dimethylzinc:triethylamine adduct with hydrogen selenide gas (both diluted in hydrogen). Homogeneous nucleation of ZnSe occurs through an irreversible reaction between the precursors. The nuclei subsequently grow by surface reactions and cluster-cluster coalescence to form nanocrystals that exhibit size-dependent luminescence. Surface passivation was achieved by introducing vapors of 1-pentanethiol into the reactor. The thiol vapors do not interfere with the nucleation and growth of the primary nanocrystals, but significantly change the morphology and size of the final coagulated particles collected on TEM grids. The chemisorption of thiol vapors on the surface of the nanocrystals also leads to smaller degradation of the PL intensity after one month. High-quality ZnSe nanowires have been synthesized in a metalorganic chemical vapor deposition reactor. The wires were grown on Si substrates and were formed by using a variety of metal catalysts such as gold, silver, tin, and nickel as growth initiators. Different operating procedures were tested with growth temperatures varying between 300°C to 750°C trying to uncover the optimum operating conditions. Some of the key advantages of the synthesis of ZnSe nanoparticles in the counterflow jet reactor are the fact that is a continuous process with high purity; has the ability to yield nanocrystals even at room temperature and has the capability to passivate the surface of the nanoparticle surfaces and coat them with functional groups. The advantages of the CVD process are the ability of growing high quality materials, due to the purity of the precursors, the high growth rates at low temperatures and the easy integration with the existing processes in the microelectronic industry.