Nanofabrication of the alloy-ended CNTs and the structure-property analyses at each processing step

• Main Authors: Kuo-Ming Chiu, 邱國銘
• Other Authors: Cheng-Tzu Kuo
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/u8xfxv

碩士 === 國立交通大學 === 材料科學與工程系所 === 92 === In order to cap the tip cavities of the open-ended carbon nanotubes (CNTs) with the phase-change alloy for potential applications as the nano-resolution storage media, the Co-assisted CNTs were first synthesized by electron cyclotron resonance chemical vapor deposition (ECR-CVD) with H2 and CH4 as the gas sources. Then, the as-grown CNTs were post-treated in H-plasma atmosphere to remove the carbon layers covered on catalysts, and subsequently immersed in 0.25 M HNO3 solution to remove the catalysts from the tips. The open-ended CNTs with a bowl-like-shape tips were followed by coating with a phase-change alloy layer of Ge2Sb2Te5 (200 nm in thickness) via sputtering process, and then heat treated in vacuum (10-3 Torr) for 30 minutes to trim the alloy off from the sidewalls of CNTs to obtain the alloy-ended CNTs. The main processing parameters include catalyst thickness, H2/CH4 ratio, time of H-plasma post-treatment, chemical etching time and heat-treating temperature. The structures and properties in each processing step were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Auger electron spectroscopy (AES) and field emission J-E measurements. The following conclusions can be drawn from these studies. Regarding effect of catalyst thickness, a thicker catalyst layer can result in an increase in tube diameter and a decrease in tube number density of CNTs. As to effect of CH4 concentration, a greater carbon concentration is more favor to grow CNTs with carbon sheets on the sidewalls of CNTs to become the rattan-like CNTs instead of tubule-like CNTs. On the other hand, a higher H2 concentration during CNTs growth can give rise to a lower Raman ID/IG ratio and more tubule-like CNTs formation. Effect of H-plasma post-treatment is essentially to remove the carbon layers from the as-grown CNTs tips and may cause the rattan-like CNTs to become tubule-like CNTs. The H-plasma etching time can be varied to merely etch off the carbon layers on the tips of CNTs and still maintain the structure integrity. In other words, the rattan-like CNTs can be changed to the tubule-like CNTs without carbon layers on the tips by 7 min post H-plasma treatment. On the other hand, the carbon layers on the tips of the as-grown tubule-like CNTs can be removed by 1 min H-plasma post-treatment without too much damage to the stems of CNTs. Furthermore, it is found that the preferred etching sites for H-plasma post-treatment are on the higher strained areas, such as regions with the greater curvatures. Effect of chemical etching is basically to remove the Co-catalyst off by chemical reaction. Under the present conditions, 3 min chemical etching time can almost remove all catalysts from the carbon layer-stripped tips to become the open-ended CNTs without significant damage to their stems. The experimental results also show that the alloy-coated open-ended CNTs can be heat treated to trim off the alloys from their sidewalls in vacuum at 420oC for 30 min to become an alloy-capped CNTs. Furthermore, the Auger analyses show that the sputtering process must be modified to obtain the required composition of phase-change alloy after being capped on the tips of CNTs, where the compositions of the phase-change alloys may be changed from Te-rich to Ge-rich due to the faster evaporation rates of Sb and Te. Regarding field emission properties, the results indicate that the open-ended CNTs may behave better properties than the as-grown tubule-like CNTs due to higher local aspect ratio around the open-ended tips, if their structure integrity can be maintained. On the other hand, the field emission properties of the carbon layer-stripped CNTs are declined by comparing with the as-grown CNTs due to oxidation of the exposed catalysts without carbon layer protection. H-plasma post-treatment may also cause a decrease in field emission properties by forming more defects and flatten surfaces at the tips.