In situ TEM studies on the graphitization and growth of nanocrystalline graphene from polymers

Graphitization of polymers is an efficient way to synthesize graphenoid (graphene like) materials on different substrates with tunable shape, thickness and properties. [1] This catalyst-free growth results in domain sizes of a few nanometers and has been termed nanocrystalline graphene. Ease of fabrication, better control of shape, thickness and properties comparable to graphene makes ncg an easy to produce alternative for graphene for different technological applications. Since the properties of these graphitized carbon structures are largely affected by the domain size and other defects, a detailed understanding of the graphitization and domain growth as a function of temperature is essential to tailor the properties of the graphitic material. In the present thesis, in situ TEM techniques are employed to understand the graphitization and domain growth of free-standing nanocrystalline graphene thin films prepared by vacuum annealing of a photoresist inside a TEM. HRTEM, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) techniques are used to analyze the graphitization and the evolution of nanocrystalline domains at different temperatures. By in situ heating and current annealing, the present study tries to understand the graphitization and structural changes in the intermediate to ultrahigh temperature range. The in situ studies showed that the graphitization process is highly dynamic in nature with a number of intermediate reactions leading to the formation of different carbon nanostructures. The free-standing membrane showed comparable graphitization to substrate supported films and a two-step growth mechanism was identified. At intermediate temperatures (600-1000 oC) crystallite growth proceeds by consuming amorphous carbon around the crystallites and at high temperatures (1000- 1200 oC) growth proceeds by merging of crystallites. The amorphous carbon transforms in two ways, by attaching to the active edges of domains and by catalyst free transformation on the top of graphitic layers. This catalyst free transformation forms new graphitic structures with different size shape and mobility. Some of these carbon nano structures are highly mobile on top of the already graphitized layers, which enabled to study the interaction of these structures with the graphitic substrate at high temperatures. Time resolved HRTEM investigation of the high temperature dynamics of ncg supported by atomistic simulations gave insights into the fundamental processes controlling the graphene growth, high temperature stability/mobility of the carbon nanostructures and their interaction with the graphitic substrate. High temperature in situ HRTEM investigations revealed the formation of graphene nano flakes and cage-like nano structures during graphitization. The study showed that the growth of the domains is mainly by the migration and merging of the graphitic subunits. In addition to lateral merging of domains, experiments also showed a merging of small flakes with an under laying substrate edge, which involves a slow vertical material transfer. In addition to this, strong structural and size fluctuations of individual graphitic subunits at high temperatures were observed. Graphene nano flakes are highly unstable and tend to loose atoms or groups of atoms, while adjacent larger domains grows by the addition of atoms indicating an Ostwald type of ripening occurring in these 2D materials as an additional growth mechanism. Beam off experiments confirmed that the observed dynamics are inherent temperature driven processes and the electron beam only provides additional activation energy increasing the reaction kinetics. Molecular dynamic simulations carried out to estimate the activation energy for the different processes indicates a critical role of defects in the substrate for the observed dynamics. Furthermore in situ current annealing of free-standing ncg constrictions were carried out to understand the dynamics and structural changes at ultrahigh temperatures. Current annealing provides the possibility to reach temperatures in excess of 1200 oC inside the TEM, which is the maximum temperature possible by commercial MEMS based heating chips. The graphitization at high temperature is comparable to the thermal annealing showing similar crystallite size evolution. However, growth of domains up to 50 nm was observed with current annealing to ultra-high temperatures (T > 2000 oC). Unlike the formation of well oriented graphite during high temperature annealing, in current annealing of thick samples, formation of large multi walled cage-like structures were observed. The thickness of the sample and the heating rate seems to have a critical influence on the structural evolution during current annealing. These initial observations on comparable graphitization during current annealing at intermediate temperatures, growth of domains, formation of cage-like structures etc., open up new possibilities to tailor the microstructure and conductivity by controlling the thickness and heating rate of the sample.