Single-Walled Carbon Nanotube Growth using Al 2 O x Buffer Layer at Low Temperature by Alcohol Gas Source Method
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Single-Walled Carbon Nanotube Growth using Al2Ox Buffer Layer at Low Temperature by Alcohol Gas Source Method Kuninori Sato, Takahiro Maruyama, Shigeya Naritsuka Department of Materials Science and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya 468-8502, JAPAN ABSTRACT Aluminum oxide (Al2Ox) buffer layers were employed to grow single-walled carbon nanotubes (SWNTs) at 400°C using an alcohol gas source and a Co catalyst. By optimizing the thickness of the aluminum oxide layer, the SWNT yield was enhanced by a factor of several times. In addition, SWNT growth at 350°C was realized on the Al2Ox buffer layer by this method. Raman measurements at various excitation wavelengths suggest that a Al2Ox buffer layer preferentially enhances the growth of SWNTs with larger diameters (>1 nm). INTRODUCTION Carbon nanotubes (CNTs) are expected to be used in many novel nanodevices. Growth of CNTs at temperatures below 400°C is crucial for realizing CNT devices, because it enables fabrication processes for Si LSI devices to be used. Several studies have reported CNT growth at low temperatures [1,2], but there have been only a few studies on single-walled carbon nanotube (SWNT) growth below 400°C [3]. Recently, we reported SWNT growth by a gas source method in an ultra-high vacuum (UHV) chamber using ethanol gas [4,5]. This growth technique enables SWNTs to be grown in a UHV, which permits the growth temperature to be reduced to 400°C. In addition, we attained yield enhancement in SWNT growth at 400°C using an Al2Ox buffer layer [6]. However, the dependence of SWNT yield on Al2Ox thickness has not been sufficiently well investigated and the yield enhancement mechanism has not been clarified. In this study, we investigate the dependence of the SWNT yield at 400°C on the Al2Ox layer thickness and discuss the effect of the Al2Ox layer on SWNT growth. Furthermore, we use this method to realize SWNT growth at 350°C. EXPERIMENT Al2Ox buffer layers were formed by depositing Al on SiO2(100 nm)/Si substrates using a pulsed arc plasma in a UHV chamber and subsequently exposing them to air for 1 h. Co catalysts were deposited on the Al2Ox layers by electron-beam evaporation in a UHV chamber. The nominal thickness of the Al2Ox layers was varied from 0 to 60 nm, whereas the Co thickness was kept constant at 0.1 nm. The samples were heated to the growth temperature and CNTs were then grown by supplying ethanol gas to the substrate surface through a stainless-steel nozzle for 1 h. The supply of ethanol gas was controlled by maintaining the pressure at 1×10−4 Pa, which is the optimum pressure for 400°C growth.
The resulting CNTs were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The excitation wavelengths used for the Raman measurements were 514.5, 532, 568.2, and 632.8 nm.
RESULTS AND DISCUSSION Figure 1 shows the Raman spectra for CNTs grown on Al2Ox/SiO2/Si substrates at various Al2Ox layer thicknesses. Figure 1(b) shows the high-frequency region of the Raman spectra. The intense p
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