High-Yield Synthesis of Vertically Aligned Single-Walled Carbon Nanotubes in Ion-Damage and Radical-Damage Free Atmosphe
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1057-II14-07
High-Yield Synthesis of Vertically Aligned Single-Walled Carbon Nanotubes in IonDamage and Radical-Damage Free Atmospheric Pressure PECVD Tomohiro Nozaki, Kuma Ohnishi, and Ken Okazaki Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo, 1528552, Japan ABSTRACT Plasma-enhanced chemical vapor deposition (PECVD) is recognized as one of the viable fabrication techniques of carbon nanotubes (CNTs). However, "CNTs" synthesized in lowpressure PECVD is overwhelmingly carbon nanofibers or multi-walled carbon nanotubes because catalyst and CNTs receive severe damage from ion bombardment: single-walled carbon nanotubes (SWCNTs) has been exclusively synthesized in the thermal CVD regime except few examples. We present atmospheric pressure plasma enhanced chemical vapor deposition (APPECVD) for high-purity vertically aligned SWCNT synthesis, because both ion-damage and radical-damage are preferentially avoided in atmospheric pressure. Tentative reaction mechanism is also discussed based on gas phase chemistry analyzed by quadrupole mass spectrometer. INTRODUCTION Plasma enhanced CVD (PECVD) is one of the viable synthesis techniques of carbon nanotubes (CNTs) because free-standing CNTs are synthesized because of electric field in the plasma sheath. In addition, growth temperature is slightly lower than thermal CVD. However, CNTs synthesized in low-pressure PECVD are generally carbon nanofibers and multi-walled carbon nanotubes because catalysts and CNTs receive severe damage from ion bombardment [1,2]. Excess supply of reactive species detrimentally produce amorphous carbon network that readily deteriorate catalyst activity: tremendous interest in production of SWCNTs and their applications has not arisen in the scope of PECVD until recently [2,3]. We propose atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) for the vertically aligned single-walled carbon nanotube synthesis, because both ion-damage and radical-damage to the SWCNTs are inherently avoided in highly collisional plasma sheath generated at atmospheric pressure. It is noteworthy from the practical viewpoint that atmospheric pressure processing is beneficial to improve productivity with minimum cost and energy consumption because it reduces the needs for vacuum system and enables to handle bulky materials in continuous process. This paper first introduces atmospheric pressure PECVD reactor. We experimentally demonstrated that high-purity vertically aligned SWCNTs are synthesized only when atmospheric-pressure PECVD is applied: even moderate pressure such as 20 kPa preferentially synthesized MWCNTs. Tentative reaction mechanism is also discussed based on gas phase chemistry analyzed by quadrupole mass spectrometer. Finally, concluding remarks are presented. EXPERIMENTAL The SWCNTs were synthesized in the atmospheric pressure radio-frequency discharge (APRFD) reactor, which has been described in detail in ref [4]. Briefly, schematic diagram of experimental setup and a digital image of em
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