Research on oxygen-adsorbed graphene with a home-built variable-temperature scanning tunneling microscope

Abstract

This thesis presents an in-depth study of the atomic-level properties of oxidized graphene, specifically focusing on oxygen-adsorbed graphene grown on a silicon carbide (SiC) substrate. Graphene’s unique features such as high electron mobility and mechanical stability have been widely recognized, but its lack of a bandgap has been a limitation for many practical applications. As such, there has been a growing need for methods to modify graphene’s characteristics, with chemical modification standing out as a potential solution. However, investigating graphene oxidation at the atomic scale has proven technically challenging, particularly using scanning tunneling microscopy (STM). The initial part of the research, as laid out in the first chapter, addresses the development and refinement of a suitable STM system. More specifically, this part presents the creation of an ultrahigh vacuum (UHV) environment and a home-built variable temperature STM (VT-STM) employing a KoalaDrive-type piezoelectric motor. The challenges associated with wear-induced cold welding in the piezoelectric motor when operating in the UHV environment over an extended period are dealt with, providing insights into achieving reliable STM operations in the UHV environment. Various non-conductive coatings applied to the slider have been explored to overcome these challenges, leading to the successful construction of the VT-STM. Building upon the developed STM system, the second chapter delves into the preparation and growth of epitaxial graphene on SiC substrates. The potential of SiC substrates for large-scale graphene production is highlighted, using a high-temperature annealing process. The grown graphene is subsequently oxidized within the UHV chamber. The challenges related to traditional oxidation methods, such as inefficient oxidation degrees, are addressed by employing a homemade thermal cracker. This novel approach allows for enhanced oxidation of graphene, resulting in the production of oxygen-adsorbed graphene suitable for STM measurements under a relatively low partial pressure of oxygen in the UHV chamber. Moving further into the atomic-level investigation, the third chapter conducts an examination of the behavior of oxygen atoms on graphene using STM. Significant findings related to the local density of states (LDOS) and the induced bandgap near the Dirac point are presented, demonstrating a substantial depression in LDOS and a bandgap of approximately 250 meV. Moreover, the possibility of desorption and manipulation of oxygen atoms through STM, and the potential for oxygen atom hopping under specific conditions, are explored. These investigations provide unique insights into the possible ways to tailor graphene’s electronic properties. While the primary focus of this thesis is on the oxidation of graphene, the final chapter steps beyond this scope to inspect a ferritic stainless steel with a highly conductive oxide layer. Employing both current sensing atomic force microscopy (CSAFM) and STM/scanning tunneling spectroscopy, this investigation provides a valuable extension to the study. In summary, this thesis provides a comprehensive exploration of graphene oxidation at the atomic scale. It incorporates the development of a reliable STM system, the growth of graphene on SiC substrates, the in-depth atomic-level investigation of oxidized graphene, and an extended study of a conductive oxide layer on ferritic stainless steel. As such, it significantly contributes to the understanding of the properties and potential applications of graphene-based atom-scale devices.