Detecting and Utilizing Lattice Defects in Graphene
Graphene has a wide variety of properties that are extremely unique among materials. Due to its thickness of only a single atomic layer, defects in the crystal structure have an extraordinarily large impact on the resulting properties of the graphitic film. First I examine the graphene growth process in two different growth regimes. The first is traditional catalytic thermal chemical vapor deposition. This process produces large grains of graphene with a very low density of defects. In this regime the defect concentration, specifically grain boundaries, can be controlled through surface treatments, growth temperature, growth pressure and precursor flow rates in order to control the nucleation density and resultant grain size within the film. In the second regime a low temperature growth process using plasma enhanced chemical vapor deposition is explored. In this regime the resultant graphene has a very high defect density. The defect density and film composition and morphology can be controlled by the chemical properties of the growth substrate, precursor gas, plasma power and temperature. Second I examine how the defects present in large crystal graphene effect the carrier properties using transient absorption. Grain boundaries were identified as the most relevant defect structure and characterized by using transient absorption imaging to probe the changes in the density of states that result from the defect structures. Grain boundaries were found to present a significant change in the density of states at an energy dependent on the angle at which the two grains interact. This results in a 30% increase in the transient absorption signal intensity due to an optical resonance that results from the angular mismatch of the grains. Furthermore the transient absorption spectroscopy was used to demonstrate a faster carrier decay at the grain boundaries as a result of supercollision cooling. Third I examine how defective graphene can be utilized as an anticorrosion coating. Single layer graphene provides excellent short term protection but the inevitability of defects in the film cause catastrophic failure over long time scales. Nanocrystalline graphene was shown to provide the same level of short term protection using electrochemical corrosion tests but is vastly superior to single layer graphene over longer time scales. By monitoring ion concentration in solutions and leaving samples exposed to atmosphere nanocrystalline graphene was shown to provide significantly better protection against corrosion when compared to single layer graphene. This is due to the inability of cracks in the film to propagate and passivation of diffusion pathways.
Yang, Purdue University.
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