| Summary: | 博士 === 國立交通大學 === 材料科學與工程系 === 90 === Syntheses of carbon-based materials have been developing for many decades in both academic and industrial communities, such as, man-made diamonds, superhard C3N4 materials, Si-C-N crystals and other carbon-based nanostructured materials. However, researches so far have not successfully linked the growth mechanisms of various carbon-based materials deposited by different synthetic conditions and methods. In fact, a single machine could synthesize many of such materials. This dissertation aimed to study the linkages among various carbon-based materials synthesized on Si wafers under the same microwave plasma chemical vapor deposition (MPCVD) system, including Si-C-N crystalline films, Si-C-N nanotubes, carbon nanotubes (CNTs), conical carbon nanorods and other nanostructured materials. The process parameters can be divided into three groups according to the structures of the synthesized materials, i.e. Si-C-N crystalline films, nanotubes and other nanostructures. The main parameters include CH4/N2 gases, buffer layer application, additional Si source and its application timing for Si-C-N crystalline films; CH4/N2/H2 gases, catalyst application, additional Si source for Si-C-N nanotubes; CH4, N2, H2 gases and catalyst application for carbon nanotubes and carbon nanorods, as shown in a process abstract roadmap or figure, (page ix).
With regard to the synthesis of Si-C-N film, three conditions, namely conditions 1, 2 and 3 were compared. Under condition 1 (route j in abstract figure), the results reveal that formation and properties of Si-C-N films can be manipulated by applying seven different buffer layers. Conditions 2 and 3 (route k in abstract figure), it depicts that application timing of the additional solid source (Co-coated Si columns) can be used to vary the compositions, morphologies, structures and properties of Si-C-N films. Under condition 2, the solid sources were applied “before” film deposition. This condition applies the solid source to the substrate by H2 pretreatment. Under condition 3, the solid sources were applied both “before and during” film deposition.
By comparing the conditions of forming catalyst-assisted Si-C-N nanotubes with forming Si-C-N films (routes l and j in abstract figure), the formation of the tubular structure may be related to introduction of H2 gas during tubular deposition, which may delay the action of the so-called catalyst poisoning and keep the tube end open during growth.
Other nanostructured materials, e.g. catalyst-assisted CNTs and carbon nano-rods, were successfully synthesized on patterned and un-patterned Si wafers (routes m, n, o in abstract figure) by varying process parameters including catalyst materials, source gases, gas ratios, interlayers and deposition temperatures. The CNTs could be selectively deposited on the patterned wafers, including: (a) parallel Fe-coated line arrays, and (b) CoSix-coated hole arrays. This is a novel method that is compatible with Si microelectronic device manufacturing. Besides, many of the vertically-grown, dense MWCNTs are found to protrude from a single catalyst particle. This is believed to be associated with the lower temperatures in H2 reduction and CNTs deposition stages. The result also offers a different perspective on growth mechanism of the catalyst-assisted MWCNTs.
Regard to the linkages of forming various kinds of carbon-based crystals and nanostructured materials, the following conclusions can be drawn: (1) The additional solid Si sources mainly contribute the Si component of Si-C-N crystals and nanotubes. Although some Si could be derived from the Si substrate, the solid Si columns ionized by plasma are highly active to participate in the reaction. (2) The nano-sized catalysts promote the formations of tubular or rod morphology. The catalytic functions of the process environments without H2 gas differ from those with H2 gas. The catalysts are suggested to provide nucleation sites for Si-C-N crystal nucleation, and effectively reduce the energy of formation in the initial stage. The catalytic function is lost when the growing film covers the catalytic particle. In contrast, the role of the catalyst in forming Si-C-N tubes is similar to that described in the vapor-liquid-solid model. The tube grows by precipitating of graphite sheets from a super-saturated catalytic droplet. The formation of a curved graphite basal plane is energetically favorable, and so the tubular structure is formed. (3) The CH4/H2 ratio influences the formation of tubular and crystalline structures. A high CH4/H2 ratio favors the formation of C-sp2 bonding (graphite structure), whereas, a low CH4/H2 ratio favors the formation of C-sp3 bonding (diamond structure). Therefore, carbon atoms surround and precipitate from catalysts with different CH4/H2 ratios form hollow tubes or solid nano-rods. (4) N2 gas gives rise to bamboo-like CNTs. Introducing N atoms into the carbon nanotube structure may induce distortion; change the bonding to that of pentagonal, heptagonal or other crystal lattices, and increase bending stress.
Analysis results indicate that the nanohardness of Si-C-N crystals ranges from 30 GPa ~ 57 GPa; the energy gap ranges from 3.76 eV ~ 3.95 eV. The field emission results show that carbon-based crystals that contain Si and N, and their nanostructured materials exhibit good field emission properties. The emission currents of Si-C-N nanotubes or CNTs are at least one order of magnitude more than those of Si-C-N crystals at specific electric field intensity. At an electric field of 10 V/mm, the emission current of nanostructured materials is > 0.03 A/cm2 and of Si-C-N crystal is 0.0025 A/cm2. The carbon-based materials that contained Si and N, and the corresponding nanostructured materials are promising candidates for field emission applications.
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