Mechanistic study on the evolution of vacancies in graphene by oxidation by scanning tunneling microscopy

Nanostructured graphitic materials, including graphene hosting Å to nanometer-sized pores, have attracted attention for various applications such as separations, sensors, and energy storage. Graphene with Å-scale pores is a promising next-generation material for molecular sequencing and membrane-based separation.Our group has developed a method to generate tunable Å-scale pores using O3. This dissertation characterizes gas-sieving nanopores in the graphitic lattice by O3 etching using low-temperature scanning tunneling microscopy (LTSTM) and X-ray photoelectron spectroscopy (XPS). A systematic study was conducted to optimize parameters for graphene porosity during the O3-led gasification reaction using a millisecond gasification reactor (MGR). LTSTM showed that high temperature increases the nanopore density.To understand the mechanism of O3-led gasification, LTSTM was used to investigate the formation of linear oxygen clusters on the graphitic lattice. While advances in the application of oxidized graphene have been made, the mechanism is not well understood. Combining LTSTM imaging and van-der-Waals density-functional theory (vdW-DFT) calculations, the atomic structure of the linear oxygen cluster was analyzed. Results show that the linear oxygen cluster is formed by the assembly of cyclic epoxy trimers that propagate along the graphitic armchair direction.Defect formation and unzipping of the graphitic lattice have various applications that depend on the evolution of epoxy clusters. To understand how cluster evolution affects morphology and pore size, LTSTM was used to study cluster evolution upon heat treatment. Circular oxygen clusters were formed after heat treatment, and three distinct nanostructures corresponding to three stages of evolution were observed. The observation of cyclic epoxy trimers validated theoretical predictions, as they minimize energy in the cyclic structure. The cyclic epoxy trimers grew into honeycomb superstructures and formed larger clusters of approximately 2-3 nm. LTSTM observations revealed that vacancy defects evolved only in the larger clusters, highlighting the role of lattice strain in defect generation.A novel, scalable method to generate nanometer-sized pores in graphene lattice was developed based on decoupling nanopore nucleation and expansion. Mild oxidation temperature was used to graft nanosized epoxy clusters as nucleation sites without forming pores. In situ gasification of these clusters inside a transmission electron microscope showed precise nanopore generation. The method was manipulated to form Å-scale pores that effectively sieve gas molecules based on size, paving the way for independent control of pore size and density.The effect of STM bias voltage and polarity on epoxy clusters was studied. A large negative bias voltage (e.g., -2 V) caused epoxy desorption, while a large positive bias voltage (e.g., +2 V) enabled imaging of the epoxidized surface. Surface morphologies at low (e.g., +0.05 V) and high bias voltages (e.g., +2 V) were analyzed by studying graphene nanoribbon electronic structures. These findings could be applied as a novel STM tip-induced nanolithography for graphitic lattice patterning in the future.This dissertation provides an integral study of oxygen clusters to elucidate the mechanism of graphitic lattice oxidation, which could enrich the understanding of this field and provide new insights for future applications of graphitic-materials-based devices.