Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 189-197).

The present work is focused on the nucleation and growth mechanism of carbon nanotubes (CNT) that we have studied through different field emission techniques (FEM, FIM and atom-probe (PFDMS)). Reaction conditions associated with the CVD synthesis method were modeled inside the microscope aiming at studying nucleation phenomena at high resolution. The interaction between different metals (Fe, Co, Ni, conditioned as sharp tips) and gases (acetylene, ethylene and ethanol) was analyzed operando at high temperatures (500–900K), with the aim of reproducing growth conditions during the imaging process. Ni was, in the end, the only metal studied, due to the poor quality of images acquired from Co and Fe. Aimed at reproducing the conditioning step of the catalyst often observed in CVD protocols, a first study showed that the crystal adopts a polyhedral morphology at the working temperature (873K) in an hydrogen atmosphere or under Ultra-High-Vacuum conditions, by the extension of dense crystal planes like {111} or {100}. The presence of hydrogen in the chamber does not seem to present any influence on the final crystal morphology at temperatures above 600K. When exposed to a carbon-containing gas, nickel crystals present two distinct behaviors following the temperature region that is explored. At temperatures below ~623K, exposing Ni to ethylene or acetylene leads to the formation of a stable and poorly structured nickel carbide layer. The superficiality of this carbide is proven by the ease of its physical (by increasing the electrical field) or chemical (exposure to hydrogen or oxygen) evacuation. These three treatments initiate a clean-off phenomenon that evacuates the carbide layer. Reproducing these experiments in the atom-probe confirmed the carbidic nature of the surface as NiCy compounds were collected. At temperatures above 623K, the carbide layer (formed by exposing Ni to the same gases) becomes unstable. Its formation is related to a transition period that precedes the nucleation of graphenes on the surface. The Ni crystal undergoes a massive morphological transformation when acetylene is introduced in the chamber at 873K. This phenomenon is induced by the presence of carbon on the surface which adsorbs so strongly on step sites that it provokes their creation. Carbon also induces a considerable enhancement of Ni atoms mobility that allows for this transition to occur. Once the new morphology is attained, nucleation of graphenes is observed to start on the extended and carbon-enriched step-containing crystal planes. By reproducing these experiments in the atom-probe, a high surface concentration of carbon dimers and trimers was observed. A kinetic study of their formation was thus achieved and showed that they were formed on the surface by the recombination of Cad. Their potential role as building-blocks of the CNT growth process (which had previously been proposed following theoretical considerations) is thus suggested on the basis of experimental results for the first time. Two critical surface concentrations are highlighted in the present work. The first one is needed for the formation of carbon dimers and trimers and the second one has to be attained, during the morphological transformation, before the onset of graphene nucleation, probably providing a sufficient growth rate of the graphitic nuclei and allowing them to attain their critical size before their decomposition. Finally, the observation of rotational circular patterns, most probably related to carbon nanotubes, suggests that CNT growth (and not only graphene nucleation) occurred episodically in our conditions, confirming the validity of our model.

Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

The focus of this thesis is the study of the field electron emission (FEE) of diamond and related films synthesized by plasma enhanced chemical vapor deposition. The diamond and related films with different morphologies and compositions were prepared in a microwave plasma-enhanced chemical vapor deposition (CVD) reactor and a hot filament CVD reactor. Various analytical techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy were employed to characterize the surface morphology and chemical composition.<p>The influence of surface morphology on the field electron emission property of diamond films was studied. The emission current of well-oriented microcrystalline diamond films is relatively small compared to that of randomly oriented microcrystalline diamond films. Meanwhile, the nanocrystalline diamond film has demonstrated a larger emission current than microcrystalline diamond films. The nanocone structure significantly improves the electron emission current of diamond films due to its strong field enhancement effect.<p>The sp2 phase concentration also has significant influence on the field electron emission property of diamond films. For the diamond films synthesized by gas mixture of hydrogen and methane, their field electron emission properties were enhanced with the increase of methane concentration. The field electron emission enhancement was attributed to the increase of sp2 phase concentration, which increases the electrical conductivity of diamond films. For the diamond films synthesized through graphite etching, the growth rate and nucleation density of diamond films increase significantly with decreasing hydrogen flow rate. The field electron emission properties of the diamond films were also enhanced with the decrease of hydrogen flow rate. The field electron emission enhancement can be also attributed to the increase of the sp2 phase concentration. <p>In addition, the deviation of the experimental Fowler-Nordheim (F-N) plot from a straight line was observed for graphitic nanocone films. The deviation can be mainly attributed to the nonuniform field enhancement factor of the graphitic nanocones. In low macroscopic electric field regions, electrons are emitted mainly from nanocone or nanocones with the largest field enhancement factor, which corresponds to the smallest slope magnitude. With the increase of electric field, nanocones with small field enhancement factors also contribute to the emission current, which results in a reduced average field enhancement factor and therefore a large slope magnitude.

No description available.

Multi-scale modeling of chemical vapor deposition (CVD) is a very broad topic because a large number of physical processes affect the quality and speed of film deposition. These processes have different length scales associated with them creating the need for a multi-scale model. The three main scales of importance to the modeling of CVD are the reactor scale, the feature scale, and the atomic scale. The reactor scale ranges from meters to millimeters and is called the reactor scale because it corresponds with the scale of the reactor geometry. The micrometer scale is labeled as the feature scale in this study because this is the scale related to the feature geometries. However, this is also the scale at which grain boundaries and surface quality can be discussed. The final scale of importance to the CVD process is the atomic scale. The focus of this study is on the reactor and feature scales with special focus on the coupling between these two scales. Currently there are two main methods of coupling between the reactor and feature scales. The first method is mainly applied when a modified line of sight feature scale model is used, with coupling occurring through a mass balance performed at the wafer surface. The second method is only applicable to Monte Carlo based feature scale models. Coupling in this second method is accomplished through a mass balance performed at a plane offset from the surface. During this study a means of using an offset plane to couple a continuum based reactor/meso scale model to a modified line of sight feature scale model was developed. This new model is then applied to several test cases and compared with the surface coupling method. In order to facilitate coupling at an offset plane a new feature scale model called the Ballistic Transport with Local Sticking Factors (BTLSF) was developed. The BTLSF model uses a source plane instead of a hemispherical source to calculate the initial deposition flux arriving from the source volume. The advantage of using a source plane is that it can be made to be the same plane as the coupling plane. The presence of only one interface between the feature and reactor/meso scales simplifies coupling. Modifications were also made to the surface coupling method to allow it to model non-uniform patterned features. Comparison of the two coupling methods showed that they produced similar results with a maximum of 4.6% percent difference in their effective growth rate maps. However, the shapes of individual effective reactivity functions produced by the offset coupling method are more realistic, without the step functions present in the effective reactivity functions of the surface coupling method. Also the cell size of the continuum based component of the multi-scale model was shown to be limited when the surface coupling method was used. Thanks to the work done in this study researchers using a modified line of sight feature scale model now have a choice of using either a surface or an offset coupling method to link their reactor/meso and feature scales. Furthermore, the comparative study of these two methods in this thesis highlights the differences between the two methods allowing their selection to be an informed decision.

Multi-scale modeling of chemical vapor deposition (CVD) is a very broad topic because a large number of physical processes affect the quality and speed of film deposition. These processes have different length scales associated with them creating the need for a multi-scale model. The three main scales of importance to the modeling of CVD are the reactor scale, the feature scale, and the atomic scale. The reactor scale ranges from meters to millimeters and is called the reactor scale because it corresponds with the scale of the reactor geometry. The micrometer scale is labeled as the feature scale in this study because this is the scale related to the feature geometries. However, this is also the scale at which grain boundaries and surface quality can be discussed. The final scale of importance to the CVD process is the atomic scale. The focus of this study is on the reactor and feature scales with special focus on the coupling between these two scales. Currently there are two main methods of coupling between the reactor and feature scales. The first method is mainly applied when a modified line of sight feature scale model is used, with coupling occurring through a mass balance performed at the wafer surface. The second method is only applicable to Monte Carlo based feature scale models. Coupling in this second method is accomplished through a mass balance performed at a plane offset from the surface. During this study a means of using an offset plane to couple a continuum based reactor/meso scale model to a modified line of sight feature scale model was developed. This new model is then applied to several test cases and compared with the surface coupling method. In order to facilitate coupling at an offset plane a new feature scale model called the Ballistic Transport with Local Sticking Factors (BTLSF) was developed. The BTLSF model uses a source plane instead of a hemispherical source to calculate the initial deposition flux arriving from the source volume. The advantage of using a source plane is that it can be made to be the same plane as the coupling plane. The presence of only one interface between the feature and reactor/meso scales simplifies coupling. Modifications were also made to the surface coupling method to allow it to model non-uniform patterned features. Comparison of the two coupling methods showed that they produced similar results with a maximum of 4.6% percent difference in their effective growth rate maps. However, the shapes of individual effective reactivity functions produced by the offset coupling method are more realistic, without the step functions present in the effective reactivity functions of the surface coupling method. Also the cell size of the continuum based component of the multi-scale model was shown to be limited when the surface coupling method was used. Thanks to the work done in this study researchers using a modified line of sight feature scale model now have a choice of using either a surface or an offset coupling method to link their reactor/meso and feature scales. Furthermore, the comparative study of these two methods in this thesis highlights the differences between the two methods allowing their selection to be an informed decision.

The focus of this thesis is the study of the field electron emission (FEE) of diamond and related films synthesized by plasma enhanced chemical vapor deposition. The diamond and related films with different morphologies and compositions were prepared in a microwave plasma-enhanced chemical vapor deposition (CVD) reactor and a hot filament CVD reactor. Various analytical techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy were employed to characterize the surface morphology and chemical composition.<p>The influence of surface morphology on the field electron emission property of diamond films was studied. The emission current of well-oriented microcrystalline diamond films is relatively small compared to that of randomly oriented microcrystalline diamond films. Meanwhile, the nanocrystalline diamond film has demonstrated a larger emission current than microcrystalline diamond films. The nanocone structure significantly improves the electron emission current of diamond films due to its strong field enhancement effect.<p>The sp2 phase concentration also has significant influence on the field electron emission property of diamond films. For the diamond films synthesized by gas mixture of hydrogen and methane, their field electron emission properties were enhanced with the increase of methane concentration. The field electron emission enhancement was attributed to the increase of sp2 phase concentration, which increases the electrical conductivity of diamond films. For the diamond films synthesized through graphite etching, the growth rate and nucleation density of diamond films increase significantly with decreasing hydrogen flow rate. The field electron emission properties of the diamond films were also enhanced with the decrease of hydrogen flow rate. The field electron emission enhancement can be also attributed to the increase of the sp2 phase concentration. <p>In addition, the deviation of the experimental Fowler-Nordheim (F-N) plot from a straight line was observed for graphitic nanocone films. The deviation can be mainly attributed to the nonuniform field enhancement factor of the graphitic nanocones. In low macroscopic electric field regions, electrons are emitted mainly from nanocone or nanocones with the largest field enhancement factor, which corresponds to the smallest slope magnitude. With the increase of electric field, nanocones with small field enhancement factors also contribute to the emission current, which results in a reduced average field enhancement factor and therefore a large slope magnitude.

Carbon nanotubes (CNTs) have great potential in many applications due to their unique structure and properties. However, there are still many unsolved problems hampering their real applications. This thesis focuses on three important issues limiting their applications, namely: (1) direct growth of CNTs without additional catalyst, (2) secondary growth of carbon nanotubes on primary CNT bed without using extra catalyst, (3) and CNT alignment mechanisms during the growth.<p> The CNTs used in this thesis were prepared by hot filament chemical vapor deposition (CVD) reactor and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Field electron emission (FEE) properties of the CNTs were also tested.<p> Oxidation-reduction method was adopted in direct growth of CNTs on Inconel 600 plates and proved effective. The effect of oxidation temperature on the growth of CNTs was studied. It was found that the oxidation temperature had an influence on CNT height uniformity and FEE properties: the higher the treatment temperature, the more uniform the resultant CNTs, and the better the FEE properties of the resultant CNTs. The contribution of different oxides formed at different temperatures were investigated to explain the effect of oxidation temperature on the CNT height uniformity.<p> Secondary CNTs were grown on primary ones by simply changing the carbon concentration. No additional catalyst was used during the whole deposition process. It was found that synthesizing primary CNTs at extremely low carbon concentration is key factor for the secondary growth without additional catalyst. The CNT sample grown with secondary nanotubes exhibited improved field emission properties.<p> The effect of bias voltage on growth of vertically aligned carbon nanotubes was investigated. The CNTs grown at -500V shows the best alignment. At the early growth stage, simultaneous growth of randomly oriented and aligned carbon nanotubes was observed. This was consistent with the alignment mechanism involving stress that imposed on catalyst particles on tube tips. Through the observation of CNT growth on the scratched substrates, catalyst particle size was found as another determining factor in the alignment of CNTs. Big catalyst particles promoted aligned growth of CNTs.

Laser Chemical Vapor Deposition (LCVD) is a process by which reagent gases are thermally activated to react by means of a laser focused on a substrate. The reaction produces a ceramic or metallic deposit. This investigation focuses on the use of LCVD as a method for producing laminated composites, specifically carbon/silicon carbide laminates. The laminates that were produced were examined using scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) to determine composition. Deposit geometrical characteristics such as laminate thickness and volcano depth as well as deposit morphology were also determined using SEM. Another subset of experiments was performed for the purpose of simultaneously depositing carbon and silicon carbide, ie., codeposition.