Synthesis of sub-millimeter tall SWNT forests on a catalyst underlayer of MgO single crystal
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Synthesis of sub-millimeter tall SWNT forests on a catalyst underlayer of MgO single crystal Takashi Tsuji,1,2 Kenji Hata,1,2 Don N. Futaba, 1,2 and Shunsuke Sakurai* 1,2 1 CNT-Application Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan 2 Technology Research Association for Single Wall Carbon Nanotubes (TASC), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan ABSTRACT We have demonstrated the high yield and highly single wall selective synthesis of carbon nanotube forest grown using a MgO single crystal as a catalyst underlayer without any surface treatment. Such efficiency has not been previously reported using this underlayer system. Our investigation revealed that the growth ambient which contained no hydrogen but small amounts of water is essential for such efficiency. Evaluation of the growth kinetics by in-situ height monitoring revealed that the 350 Pm tall forest with 65% single wall selectivity possessed a catalyst lifetime of about 5 minutes. Investigation by AFM, and XPS depth profiling revealed that this longer lifetime compared with as-deposited MgO can be attributed to the stability of the catalyst particle array with small size and high number density on MgO single crystal substrate. Shorter growth lifetime, when compared to the alumina underlayer system, resulted from an instability in the catalyst nanoparticle size and composition. INTRODUCTION One of the most common methods to synthesize long single wall carbon nanotubes (SWNTs) is to grow them by chemical vapor deposition (CVD) from catalyst nanoparticles supported on a substrate. The SWNTs self-assemble into a vertically aligned array, which is commonly referred to as a “forest”, “carpet”, or “array” [1,2]. In addition to length, SWNTs within these forests have shown to possess high purity and high specific surface area [3]. Interestingly, despite the many advances in synthesis, all reports of efficient (i.e. millimeterscale) SWNT synthesis require an alumina (Al2O3) catalyst underlayer to support the formation of a stable array of small catalyst nanoparticles. In addition, studies have shown that the method of alumina catalyst coating has a great impact on the forest growth efficiency [4]. Most commonly, the alumina underlayer is deposited onto the substrate by methods, such as sputtering or e-beam deposition, and through these means, numerous reports from different groups have demonstrated the synthesis of millimeter-scale CNT forests. However, single crystal alumina (i.e. sapphire) has been seldom used for CNT forest growth, although they are frequently used for the synthesis of horizontally aligned SWNTs [5–8]. Amama et al. reported that the porosity of the alumina underlayer plays an important role in inhibiting Ostwald ripening of the iron catalyst nanoparticles, which results in the extension of the catalyst lifetime. This extended lifetime is crucial for the realization of tall SWNT forests [4]. Applying this insight, CNT forest
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