Graphene, a 2D honeycomb lattice of sp² hybridized carbons, has attracted the attention of the scientific community not only for its interesting theoretical properties but also for its myriad of possible applications. The discovery of graphene led to the Nobel Prize in physics for 2010 to be awarded to Andrei Geim and Konstantin Novoselov. Since its discovery, many methods have been developed for the synthesis of this material. Two of those methods stand out for the growth of high quality and large area graphene sheets, namely, epitaxial growth from silicon carbide (SiC) and chemical vapor deposition (CVD). As it stands today, both methods make use of high concentrations of hydrogen (10-20%) in N₂ or Ar, high temperatures, and a vacuum system. Epitaxial growth from SiC in addition requires very expensive single crystal SiC wafers. In the case of CVD, organic molecules are used as the carbon source to grow graphene on a metal substrate. Although graphene has been grown on many metal substrates, the experiments highlighted here make use of copper as the metal substrate of choice since it offers the advantage of availability, low price, and, most importantly, because this substrate is self-limiting in other words, it mostly grows single layer graphene. Because the CVD method provides with a choice as for the carbon source to use, the following question arises: can a molecule, either commercially available or synthesized, be used as a carbon source that would allow for the synthesis of graphene under low temperatures, low concentrations of hydrogen and at atmospheric pressure? This dissertation focuses on the synthesis of graphene at lower temperatures by using carbon sources with characteristics that might make this possible. It also focuses on the use of forming gas (3% H₂ and 97% N₂ or Ar) in order to make the overall process a lot safer and cost effective. This dissertation contains two chapters on the synthesis of organic molecules of interest, and observations about their reactivity are included. CVD experiments were performed at atmospheric pressure, and under vacuum. In both instances forming gas was used as the annealing and carrier gas. Results from CVD at atmospheric pressure (CVDAP), using organic solvents as carbon sources, show that at 1000℃, low quality graphene was obtained. On the other hand, CVD experiments using a vacuum in the range of 25 mTorr to 1 Torr successfully produced good quality graphene. For graphene growth under vacuum conditions, commercially available and synthesized compounds were used. Attempts at growing graphene at 600℃ from the same carbon sources only formed amorphous carbon. These results point to the fact that good quality graphene can basically be grown from any carbonaceous material as long as the growth temperature is 1000℃ and the system is under vacuum. In addition to the synthesis of graphene at low temperatures, there is a great amount of interest on the synthesis of graphene nanoribbons (GNR’s) and, as with graphene, several approaches to their synthesis have been developed. One such method is the synthesis of GNRs encapsulated in carbon nanotubes. Experiments were conducted in which aluminosilicate nanotubes were used. These nanotubes provided for an easier interpretation of the Raman spectrum since the signals from the nanotubes do not interfere with those of the GNR’s as in the case when carbon nanotubes are used. The use of aluminosilicate nanotubes also allowed for the successful synthesis of GNR’s at temperatures as low as 200℃ when perylene was used as the carbon source.

Master of Science / Department of Mechanical and Nuclear Engineering / Gurpreet Singh / Graphene because of its unique electrical (electron mobility = 2 x 10[superscript]5 cm[superscript]2 V[superscript]-1 s[superscript]-1), mechanical (E = 1 TPa), optical, thermal and chemical properties has generated a lot of interest among the research community in recent years. One of the most notable methods of synthesizing large area pristine graphene sheets, which are several 100 micrometers wide, is through thermal chemical vapor deposition (CVD). But very little has been known about the effects of heating and cooling rate of the substrate on the quality of graphene produced. Hence we varied various growth parameters to understand the process of graphene growth on Cu and Ni substrates when subjected to fast heating and quenching. This allowed optimization of the CVD process to achieve large-area graphene films consistently and repeatedly. This work provides new insights on synthesis of graphene at atmospheric pressures and the effect of (a) fast heating and fast cooling of substrates, (b) catalyst type and (c) gas flow rates on quality of the graphene produced. A carbon nanotube CVD furnace was restored and modified to accommodate graphene synthesis. We started with synthesis of graphene on Cu substrate following procedures already available in the literature (heating rate ~ 15 °C/min and cooling rate ~ 5 °C/min; total processing time 7 hours). This provided a good reference point for the particular furnace and the test setup. The best results were obtained for 15 minutes of growth at a CH4:H2 ratio of 1:30 at 950 °C. SEM images showed full coverage of the substrate by few layer graphene (FLG), which was indicated by the relatively high I[subscript]2D/I[subscript]G ratio of 0.44. The furnace was further modified to facilitate fast cooling (~4 °C/sec) of substrate while still being in inert atmosphere (Argon). The effect of growth time and concentration of CH[subscript]4 was studied for this modified procedure (at H[subscript]2 flow rate of 300 SCCM). SEM images showed full coverage for a CH[subscript]4 flow rate of 10 SCCM in as little as 6 minutes of growth time. This coupled with the fast cooling cycle effectively reduced the overall time of graphene synthesis by 7 times. The I[subscript]2D/I[subscript]G ratio in Raman spectrum was 0.4 indicating that the quality of graphene synthesized was similar to that obtained in conventional CVD. This modification also facilitated introduction of catalyst substrate after the furnace has reached growth temperature (fast heating ~8 °C/sec). Hence, the overall time required for graphene synthesis was reduced to ~6 % (30 minutes) when compared to the traditional procedure. SEM images showed formation of high concentration few layer graphene islands. This was attributed to the impurities on the catalyst surface, which in the traditional procedure would have been etched away during the long heating period. The optimum process parameters were 30 minutes of growth with 20 SCCM of CH[subscript]4 and 300 SCCM of H[subscript]2 at 950 °C. The Raman spectrum for this condition showed a relatively high I[subscript]2D/I[subscript]G ratio of 0.66. We also studied the effect of Ni as a catalyst. Similar to Cu, for Ni also, traditional procedure found in the literature was used to optimize the graphene growth for this particular furnace. Best results were obtained for 10 minutes of growth time with 120 SCCM of CH[subscript]4 in 300 SCCM of H[subscript]2 at 950 °C. SEM images showed large grain growth (~50 μm) with full coverage. The Raman spectrum showed formation of bi-layer graphene with a I[subscript]2D/I[subscript]G ratio of 1.03. Later the effect of growth time and concentration of the hydrocarbon precursor for Ni substrate subjected to fast heating (~ 8 °C/sec) was studied. It was found that because the process of graphene synthesis on Ni is by segregation, growth period or gas flow rate had little effect on the quality and size of the graphene sheets because of the presence of impurities on the substrate. This procedure yielded multilayer graphite instead of graphene under all conditions. Future work will involve study of changing several other parameters like type of hydrocarbon precursor and pressure in the chamber for graphene synthesis. Also various other substrates like Cu or Ni based alloys will be studied to identify the behavior of graphene growth using this novel procedure.

In the course of the PhD thesis large area homogeneous strictly monolayer graphene films were successfully synthesized with chemical vapor deposition over both Cu and Si (with surface oxide) substrates. These synthetic graphene films were characterized with thorough microscopic and spectrometric tools and also in terms of electrical device performance. Graphene growth with a simple chemo thermal route was also explored for understanding the growth mechanisms. The formation of homogeneous graphene film over Cu requires a clean substrate. For this reason, a study has been conducted to determine the extent to which various pre-treatments may be used to clean the substrate. Four type of pre-treatments on Cu substrates are investigated, including wiping with organic solvents, etching with ferric chloride solution, annealing in air for oxidation, and air annealing with post hydrogen reduction. Of all the pretreatments, air oxidation with post hydrogen annealing is found to be most efficient at cleaning surface contaminants and thus allowing for the formation of large area homogeneous strictly monolayer graphene film over Cu substrate. Chemical vapor deposition is the most generally used method for graphene mass production and integration. There is also interest in growing graphene directly from organic molecular adsorbents on a substrate. Few studies exist. These procedures require multiple step reactions, and the graphene quality is limited due to small grain sizes. Therefore, a significantly simple route has been demonstrated. This involves organic solvent molecules adsorbed on a Cu surface, which is then annealed in a hydrogen atmosphere in order to ensure direct formation of graphene on a clean Cu substrate. The influence of temperature, pressure and gas flow rate on the one-step chemo thermal synthesis route has been investigated systematically. The temperature-dependent study provides an insight into the growth kinetics, and supplies thermodynamic information such as the activation energy, Ea, for graphene synthesis from acetone, isopropanol and ethanol. Also, these studies highlight the role of hydrogen radicals for graphene formation. In addition, an improved understanding of the role of hydrogen is also provided in terms of graphene formation from adsorbed organic solvents (e.g., in comparison to conventional thermal chemical vapor deposition). Graphene synthesis with chemical vapor deposition directly over Si wafer with surface oxide (Si/SiOx ) has proven challenging in terms of large area and uniform layer number. The direct growth of graphene over Si/SiO x substrate becomes attractive because it is free of an undesirable transfer procedure, necessity for synthesis over metal substrate, which causes breakage, contamination and time consumption. To obtain homogeneous graphene growth, a local equilibrium chemical environment has been established with a facile confinement CVD approach, inwhich two Si wafers with their oxide faces in contact to form uniform monolayer graphene. A thorough examination of the material reveals it comprises facetted grains despite initially nucleating as round islands. Upon clustering these grains facet to minimize their energy, which leads to faceting in polygonal forms because the system tends to ideally form hexagons (the lowest energy form). This is much like the hexagonal cells in a beehive honeycomb which require the minimum wax. This process also results in a near minimal total grain boundary length per unit area. This fact, along with the high quality of the resultant graphene is reflected in its electrical performance which is highly comparable with graphene formed over other substrates, including Cu. In addition the graphene growth is self-terminating, which enables the wide parameter window for easy control. This chemical vapor deposition approach is easily scalable and will make graphene formation directly on Si wafers competitive against that from metal substrates which suffer from transfer. Moreover, this growth path shall be applicable for direct synthesis of other two dimensional materials and their Van der Waals hetero-structures.:Contents Quotation v Kurzfassung vii Abstract xi Contents xiii Acronyms xvii 1 Aims and objectives 1 2 Introduction 5 2.1 Carbon allotropes 6 2.1.1 Hybridized sp 2 carbon nanomaterials 6 2.1.2 Graphene 7 2.2 Properties of graphene 8 2.2.1 Crystalline structure 8 2.2.2 Electrical transport 10 2.2.3 Optical transparency 11 2.2.4 Other properties 12 2.3 Graphene deposition methods 13 2.3.1 Synthesis approaches 13 2.3.2 Chemical vapor deposition 14 2.3.3 Substrate selection 15 2.3.4 Substrate pretreatments 16 2.3.5 Carbon feedstock 17 2.3.6 Thermal chemical vapor deposition 17 2.3.7 Plasma chemical vapor deposition 18 2.3.8 Transfer protocol 19 2.4 Chemical vapor deposition for graphene growth 21 2.4.1 Thermodynamics 22 2.4.2 Arrhenius plots 22 2.4.3 Activation energy 24 2.4.4 Growth kinetics 25 2.4.5 Reaction mechanisms over Cu 27 2.4.6 Reaction mechanisms over Ni 29 2.4.7 Reaction mechanisms over non-metals 31 2.4.8 Reaction mechanisms of free-standing graphene 35 2.5 Summary 35 2.6 Scope of the thesis 36 3 Experimental setup and characterization techniques 37 3.1 Experimental setup of chemical vapor deposition 37 3.2 Optical microscopy 39 3.3 Scanning electron microscopy 40 3.4 Atomic force microscopy 41 3.5 Transmission electron microscopy 42 3.5.1 Selected area electron diffraction 44 3.5.2 Dark field transmission electron microscopy 46 3.6 Raman spectroscopy 47 3.7 Ultraviolet-Visible spectrophotometry 49 3.8 Electrical transport measurements 49 4 CVD growth of graphene on oxidized Cu substrates 51 4.1 Motivation 52 4.2 Experimental protocol 53 4.3 Influence of Cu pretreatments on graphene formation 54 4.4 Influence of Cu oxidation on graphene growth 60 4.5 Effect of oxidation pretreatment on Cu surface cleaning 64 4.6 Summary 66 5 Chemo-thermal synthesis of graphene from organic adsorbents 67 5.1 Motivation 67 5.2 Experimental protocol 69 5.3 Influence of reaction temperature on graphene growth 75 5.4 Influence of reaction pressure on graphene growth 78 5.5 Influence of reaction flow rate on graphene growth 80 5.6 Summary 81 6 Monolayer graphene synthesis directly over Si/SiO x 83 6.1 Motivation 83 6.2 Experimental protocol 86 6.3 Influence of substrate confinement configuration 87 6.4 Time dependent evolution for graphene formation 91 6.5 Grain boundaries in graphene film 95 6.6 Bubble clustering of faceted graphene grains 98 6.7 Electrical and optical performance of graphene 100 6.8 Summary 102 7 Conclusions 103 8 Outlook 107 A Graphene synthesis over Cu and transfer to Si/SiO x substrate 111 B Chemo-thermal synthesis of graphene over Cu 115 C CVD graphene growth directly over Si/SiO x substrate 127 Bibliography 147 List of Figures 193 List of Tables 197 Acknowledgements 199 List of publications 203 Erklaerung 205 / Im Zuge dieser Doktorarbeit wurden großflächige und homogene Graphen-Monolagen mittels chemischer Gasphasenabscheidung auf Kupfer- (Cu) und Silizium-(Si) Substraten erfolgreich synthetisiert. Solche monolagigen Graphenschichten wurden mithilfe mikroskopischer und spektrometrischer Methoden gründlich charakterisiert. Außerdem wurde der Wachstumsmechanismus von Graphen anhand eines chemo-thermischen Verfahrens untersucht. Die Bildung von homogenen Graphenschichten auf Cu erfordert eine sehr saubere Substratoberfläche, weshalb verschiedene Substratvorbehandlungen und dessen Einfluss auf die Substratoberfläche angestellt wurden. Vier Vorbehandlungsarten von Cu-Substraten wurden untersucht: Abwischen mit organischen Lösungsmitteln, Atzen mit Eisen-(III)-Chloridlösung, Wärmebehandlung an Luft zur Erzeugung von Cu-Oxiden und Wärmebehandlung an Luft mit anschließender Wasserstoffreduktion. Von diesen Vorbehandlungen ist die zuletzt genannte Methode für die anschließende Abscheidung einer großflächigen Graphen-Mono-lage am effektivsten. Die chemische Gasphasenabscheidung ist die am meisten verwendete Methode zur Massenproduktion von Graphen. Es besteht aber auch Interesse an alternativen Methoden, die Graphen direkt aus organischen, auf einem Substrat adsorbierten Molekülen, synthetisieren konnen. Jedoch gibt es derzeit nur wenige Studien zu derartigen alternativen Methoden. Solche Prozessrouten erfordern mehrstufige Reaktionen, welche wiederrum die Qualität der erzeugten Graphenschicht limitieren, da nur kleine Korngrößen erreicht werden konnen. Daher wurde in dieser Arbeit ein deutlich einfacherer Weg entwickelt. Es handelt sich dabei um ein Verfahren, bei dem auf einer Cu-Substratoberfläche adsorbierte, organische Lösungsmittelmoleküle in einer Wasserstoffatmosphäre geglüht werden, um eine direkte Bildung von Graphen auf einem sauberen Cu-Substrat zu gewahrleisten.Der Einfluss von Temperatur, Druck und Gasfluss auf diesen einstufigen chemothermischen Syntheseweg wurde systematisch untersucht. Die temperaturabhängigen Untersuchungen liefern einen Einblick in die Wachstumskinetik und thermodynamische Größen, wie zum Beispiel die Aktivierungsenergie Ea, für die Synthese von Graphen aus Aceton, Isopropanol oder Ethanol. Diese Studien untersuchen außerdem die Rolle von Wasserstoffradikalen auf die Graphensynthese. Weiterhin wurde ein verbessertes Verständnis der Rolle von Wasserstoff auf die Graphen-synthese aus adsorbierten, organischen Lösungsmitteln erlangt (beispielsweise im Vergleich zur konventionellen thermischen Gasphasenabscheidung). Die direkte Graphensynthese mittels chemischer Gasphasenabscheidung auf Si-Substraten mit einer Oxidschicht (Si/SiOx ) ist extrem anspruchsvoll in Bezug auf die großflächige und einheitliche Abscheidung (Lagenanzahl) von Graphen-Monolagen. Das direkte Wachstum von Graphen auf Si/SiOx -Substrat ist interessant, da es frei von unerwünschten Übertragungsverfahren ist und kein Metall-substrat erfordert, welche die erzeugten Graphenschichten brechen lassen können. Um ein homogenes Graphenwachstum zu erzielen wurde durch den Kontakt zweier Si-Wafer, mit ihren Oxidflachen zueinander zeigend, eine lokale Umgebung im chemischen Gleichgewicht erzeugt. Diese Konfiguration der Si-Wafer ist nötig, um eine einheitliche Graphen-Monolage bilden zu können. Eine gründliche Untersuchung des abgeschiedenen Materials zeigt, dass trotz der anfänglichen Keimbildung von runden Inseln facettierte Körner erzeugt werden. Aufgrund der Bestrebung der Graphenkörner ihre (Oberflächen-) Energie zu minimieren, wird eine Facettierung der Körner in polygonaler Form erzeugt, was darin begründet liegt, dass das System idealerweise eine Anordnung von hexagonal geformten Körnern erzeugen würde (niedrigster Energiezustand). Der Prozess ist vergleichbar mit der sechseckigen Zellstruktur einer Bienenstockwabe, welche ein Minimum an Wachs erfordert. Dieser Prozess führt auch zu einer nahezu minimalen Gesamtkorn-grenzlänge pro Flächeneinheit. Diese Tatsache zusammen mit der hohen Qualität der resultierenden Graphenschicht spiegelt sich auch in dessen elektrischer Leistungsfähigkeit wider, die in hohem Maße mit der auf anderen Substraten gebildeten Graphenschichten (inklusive Cu-Substrate) vergleichbar ist. Darüber hinaus ist das Graphenwachstum selbstabschliessend, wodurch ein großes Parameterfenster für eine einfache und kontrollierte Synthese eröffnet wird. Dieser Ansatz zur chemischen Gasphasenabscheidung von Graphen auf Si- Substraten ist leicht skalierbar und gegenüber der Abscheidung auf Metallsubstraten konkurrenzfähig, da keine Substratübertragung notig ist. Darüber hinaus ist dieser Prozess auch für die direkte Synthese anderer zweidimensionalen Materialien und deren Van-der-Waals-Heterostrukturen anwendbar.:Contents Quotation v Kurzfassung vii Abstract xi Contents xiii Acronyms xvii 1 Aims and objectives 1 2 Introduction 5 2.1 Carbon allotropes 6 2.1.1 Hybridized sp 2 carbon nanomaterials 6 2.1.2 Graphene 7 2.2 Properties of graphene 8 2.2.1 Crystalline structure 8 2.2.2 Electrical transport 10 2.2.3 Optical transparency 11 2.2.4 Other properties 12 2.3 Graphene deposition methods 13 2.3.1 Synthesis approaches 13 2.3.2 Chemical vapor deposition 14 2.3.3 Substrate selection 15 2.3.4 Substrate pretreatments 16 2.3.5 Carbon feedstock 17 2.3.6 Thermal chemical vapor deposition 17 2.3.7 Plasma chemical vapor deposition 18 2.3.8 Transfer protocol 19 2.4 Chemical vapor deposition for graphene growth 21 2.4.1 Thermodynamics 22 2.4.2 Arrhenius plots 22 2.4.3 Activation energy 24 2.4.4 Growth kinetics 25 2.4.5 Reaction mechanisms over Cu 27 2.4.6 Reaction mechanisms over Ni 29 2.4.7 Reaction mechanisms over non-metals 31 2.4.8 Reaction mechanisms of free-standing graphene 35 2.5 Summary 35 2.6 Scope of the thesis 36 3 Experimental setup and characterization techniques 37 3.1 Experimental setup of chemical vapor deposition 37 3.2 Optical microscopy 39 3.3 Scanning electron microscopy 40 3.4 Atomic force microscopy 41 3.5 Transmission electron microscopy 42 3.5.1 Selected area electron diffraction 44 3.5.2 Dark field transmission electron microscopy 46 3.6 Raman spectroscopy 47 3.7 Ultraviolet-Visible spectrophotometry 49 3.8 Electrical transport measurements 49 4 CVD growth of graphene on oxidized Cu substrates 51 4.1 Motivation 52 4.2 Experimental protocol 53 4.3 Influence of Cu pretreatments on graphene formation 54 4.4 Influence of Cu oxidation on graphene growth 60 4.5 Effect of oxidation pretreatment on Cu surface cleaning 64 4.6 Summary 66 5 Chemo-thermal synthesis of graphene from organic adsorbents 67 5.1 Motivation 67 5.2 Experimental protocol 69 5.3 Influence of reaction temperature on graphene growth 75 5.4 Influence of reaction pressure on graphene growth 78 5.5 Influence of reaction flow rate on graphene growth 80 5.6 Summary 81 6 Monolayer graphene synthesis directly over Si/SiO x 83 6.1 Motivation 83 6.2 Experimental protocol 86 6.3 Influence of substrate confinement configuration 87 6.4 Time dependent evolution for graphene formation 91 6.5 Grain boundaries in graphene film 95 6.6 Bubble clustering of faceted graphene grains 98 6.7 Electrical and optical performance of graphene 100 6.8 Summary 102 7 Conclusions 103 8 Outlook 107 A Graphene synthesis over Cu and transfer to Si/SiO x substrate 111 B Chemo-thermal synthesis of graphene over Cu 115 C CVD graphene growth directly over Si/SiO x substrate 127 Bibliography 147 List of Figures 193 List of Tables 197 Acknowledgements 199 List of publications 203 Erklaerung 205

No description available.

The main objective of the present thesis is to deepen the understanding of the mechanisms involved in catalytic growth of carbon nanotubes (CNT) and related processes, such as the catalytic hydrogenation, and to use this knowledge to optimize the experimental approaches in order to gain better control in the synthesis and modification of carbon nanomaterials. Controlled growth of the CNT is achieved using gas-phase prepared catalyst particles (Fe, Co) which serve as individual catalytic nucleation sites in a chemical vapor deposition (CVD) process. These studies highlight that the controlled preparation of catalyst particles is a crucial step in order to control the CNT morphology. The resultant CNT diameter and the CNT density are found to increase with increasing nanoparticle diameter and density, respectively. The number of walls of the CNT also increases with increasing primary catalyst size. The experimentally derived correlations between the particle diameter on one hand and the CNT diameter and the CNT number of walls on the other hand are attributed to an increase of the catalyst's volume-to-surface area ratio with increasing particle size. While the availability of carbon dissolved within the catalyst at the point of nucleation is determined by the catalyst volume, the amount of carbon required to form a cap depends on the surface area of the catalyst particle. Electron microscopy studies of the catalyst/substrate/carbon interfaces of CNT grown from Fe nanoparticles reveal that the CNT walls are anchored to the oxide substrate which contests the general argument that the CNT walls stem from atomic steps at the catalyst. It is argued that after nucleation, the substrate itself provides a catalytic functionality towards the stimulation of ongoing CNT growth, whereas the catalytic activity of the metal particle is more restricted to the nucleation process. Selective hard-magnetic functionalization of CNT tips has been achieved in a plasma-enhanced CVD process. Hard-magnetically terminated CNT, i.e. CNT with a FePt nanoparticle at each tip, are directly grown using FePt catalysts. Fe/Pt thin films with a strongly over-stoichiometric Fe content in the starting catalyst composition yield CNT with a significant number of particles in the hard-magnetic phase. Anisotropic etching of graphite through Co catalyst particles in hydrogen atmosphere at elevated temperatures (i.e. catalytic hydrogenation) is reported. Catalytic hydrogenation is a potential key engineering route for the fabrication of graphene nanoribbons with atomic precision. While in previous studies the etching of zigzag channels was preferred, the present investigations reveal preferential etching of armchair channels, which provides a means to tailor graphene nanostructures with specific edge termination. Further, detailed morphological and structural characterization of the Co particles provide insight into the hydrogenation mechanism which is still a matter of controversy.