Highly efficient synthesis of semiconductor nanoparticles using spark discharge

Abstract

Nanoparticles have been widely studied and used in various application due to their novel optical and electrical properties. Among various nanoparticles, Semiconductor nanocrystals, so-called quantum dots, have been greatly studied and applied in various application such as photovoltaics, lasers, and LEDs since they have unique property called quantum confinement effect, which varies the energy gap according to their size. Synthesizing methods of quantum dots have been significant topic of the research field of nanotechnology. Up to now, those semiconductor nanocrystals usually synthesized using colloidal methods, which have disadvantages such as hazardous precursor, impurities, and byproduct. Many of aerosol based gas phase synthesis have been are considered as clean and efficient methods for producing functional nanomaterials, which could not fully satisfied with existing colloidal methods. Among those various methods, spark discharge generation method has been spotlighted for producing charged aerosols with simple and efficient setup. Moreover, nanoparticles generated through spark discharge is proper for sub-10 nm particles. With these motivation, in this thesis, we develop the synthesizing method for semiconductor nanocrystals using spark discharge and improve the particle generation efficiency by enhancing durability and production capacity. To synthesize semiconductor nanocrystals, we introduced hydrogen gas to the spark discharge system. By injecting hydrogen gas as carrier gas, reducing environment was created which effectively removed oxide particles. Moreover, dissociated hydrogen gas by high energy of spark discharge penetrated in the nanoparticles and aid in crystallization process. As a result, hydrogen passivated silicon nanoparticles with high purity and crystallinity were successfully synthesized through this method. In addition, beyond group Ⅳ-semiconductor, gallium arsenide nanocrystals which are Ⅲ-Ⅴ semiconductor were synthesized with similar method. Since the hydrogen relaxation process was more complex and required certain energy, serial thermal sintering processes were additionally performed for crystallization process and defect curing. Highly crystalline and ideally stoichiometric GaAs nanocrystals were synthesized and quantum confinement effects were identified by measuring photoluminescence in visible light area. In order to industrial use of these generated nanoparticles, we improved the long-time consistency of particle generation. A newly designed wire-in-hole type spark discharger was able to generate un-agglomerated nanoparticles with a constant size distribution over a long time, compared to the conventional spark dischargers which are rod-to-rod spark discharger and pin-to-plate spark discharger. The wire-in-hole spark discharger effectively suppressed changes of the electrode morphology and gap distance, which resulted in the minimal variation of the breakdown voltage and spark frequency. Lastly, for mass production of nanoparticles, we presented the wire-to-plate electrode configuration for maintaining stable state of spark discharge in the high-frequency region. Compared to the conventional generators, this novel generator have much higher electric field due to the asymmetric geometry of electrodes and much faster velocity in the particle generation zone, which effectively dissipated the residual plasma of post discharge. By using wire-to-plate spark discharge, the maximum stable spark frequency of 17.9 kHz was achieved and mass production rate of the nanoparticles was increased proportionally to the spark frequency.