Processes, growth mechanisms and properties of various metal-encapsulated carbon nanostructured materials by ECR-CVD

• Main Authors: Chao-Hsun Lin, 林兆焄
• Other Authors: Cheng-Tzu Kuo
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/82011514582313146023

博士 === 國立交通大學 === 材料科學與工程系所 === 94 === To examine effects of processing parameters, such as catalyst application methods, pretreatment atmospheres and nanostructure deposition methods, on the nanostructure formation, processes to synthesize metal-encapsulated carbon nanostructured materials by both ECR-CVD and MPCVD methods were designed, using CH4, C2H2, H2, N2, NH3 and CO2 as source gases or pretreatment atmospheres, and using Fe，Co，CoSix，Ni，Cu as catalysts. The catalysts were deposited on Si wafer by spin coating the catalyst precursor solutions and/or sputtering the metal targets. The pre-coated catalysts or their precursors were followed by H- or (H+N)-plasma pretreatment to obtain various catalyst nanoparticles distribution. The pretreated specimens were then deposited with various carbon nanostructures in ECR-CVD or MPCVD system. The nanostructures and their properties after each processing step were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and field emission I-V measurements. The following conclusions can be drawn from these studies. On studying growth mechanisms of various nanostructures, the results show the typical nanostructures by ECR-CVD with CH4 as source gas include the vertically aligned carbon nanotubes (VACNTs), bamboo-like CNTs (BLCNTs), rattan-like CNTs (RLCNTs) and seaweed-like nano-sheets (SLNSs). The essential condition to form VACNTs is enough higher substrate bias ( > -100 V). In contrast, a lower substrate bias (< 50 V) will give rise to SLNSs formation. However, the RLCNTs will appeal by prolonging the VACNTs deposition time over 10 min. It is noted that the presence of nitrogen and/or a lower deposition pressure, such as in ECR-CVD system, are the favor conditions forming BLCNTs. The replacement of hydrogen with nitrogen in the reaction chamber is essentially to increase the bombardment effect of plasma to prolong the catalyst-plasma surface from being poisoned by the carbon film. In case of plasma pretreatment process or in the initial stage of nanostructure formation, introduction of nitrogen is also basically to increase the bombardment effect to promote the agglomeration effect due to a higher temperature, which gives rise to bigger catalyst particle sizes and so bigger nanostructure diameters. The possible growth mechanisms to form these nanostructures may be able to be explained from the following points: (1) the catalysts with higher C solubility, such as transition metals or alloys, can promote tube-like nanostructure formation; (2) formation of the graphene layers of the nanostructures is mainly through carbon bulk diffusion route in the catalysts; (3) the sizes of the catalyst nanoparticles after initial nanostructure deposition stage basically determine the final diameters of the nanostructures; (4) the difference in carbon bulk diffusion rates around the center and the circumferential regions of the catalysts may determine the types of nanostructures; a progressive increase in rate difference can give rise a change in nanostructures from filament-like, bamboo-like to hollow-like. In other words, if the catalyst surface on the plasma side is partially poisoned by carbon films during deposition may be more favor to form hollow-like nanostructures; and (5) the growth orientation of the nanostructures is determined by the flow direction of carbon species near the catalyst surface. Regarding influence of catalyst application methods on the nanostructure growth, it is essentially depending on the differences in film thickness and, uniformity of the coated films, independent of application methods. However, the catalyst spin coating method has the advantages of large area, lower cost and mass production, but the drawbacks of poor uniformity, environmental pollution and difficultly to control the thickness of the film. To examine effect of catalyst materials, the Co and Ni catalyst-assisted nanostructures are typically VACNTs or RLCNTs by ECR-CVD. In contrast, the nanostructures are mainly carbon films or SLNSs for the Fe catalyst, and are SLNSs for Si substrate without catalyst or with Cu catalyst. It seems that the types of nanostructures are basically resulting from the competition between the carbon deposition and plasma etching rates. The deposition rate of the Fe-assisted nanostructures may be relatively faster than for Co and Ni catalysts due to its lower eutectic temperature. As to Cu catalyst, the solubility of carbon in Cu is very limited, which causes carbon from the plasma to deposit directly on the catalyst surface to form SLNSs. To study field emission properties for various catalyst-assisted nanostructures by ECR-CVD with CH4 as source gas, the results show that the field emission properties in terms of current density at 10 V/�慆 and the turn-on-voltage at 10 nA/cm2 are Co (> 32, 3.0), Ni (19.8, 1.1), Fe (7.1, 4.6), no catalyst (2.5, 4.6) (mA/cm2, V/�n�慆) for the Co- and Ni-assisted VACNTs or RLCNTs , and the Fe-assisted and no-catalyst- assisted SLNSs, respectively. The corresponding IG/ID values are 0.57, 0.55 , 0.59 and 0.45, respectively. It seems to indicate that IG/ID values are not the main factor to determine the field emission properties. Effects of geometrical features of various nanostructures on field emission properties are compared: the corresponding tube diameter, length and tube number density for the Co- and Ni-assisted VACNTs are (30~80 nm, 1.8~2.5�慆, 29~32 Gtubes/in2), (30~60 nm, 2.1~2.7�慆, 36~39 Gtubes/in2), respectively. It appeals that the field emission properties are favor for the nanostructures with higher aspect ratio and proper tube number density (also called the decreasing of screening effect). To examine effect of U-shaped covering plate to cover a part of the specimen on nanostructure formation, the results show that the plate did not change the type, but change the orientation of the nanostructures. The possible mechanism for this effect proposed in this work is explained from the flow pattern variation by a change in electric field around the covering plate, though some investigators explained by a guiding flow of covering plate.