Exploring the 3D structure of defects and electron-beam induced dynamics in graphene

Abstract

When an electron’s propagation direction in a material is restricted in one or multiple dimensions, the material’s properties deviate heavily from its bulk counterpart. These properties were intensively studied theoretically decades before the experimental synthesis of these ”low-dimensional” materials was realized. Carbon is one of the major elements in organic chemistry and can form a large variety of allotropes in different dimensions. Graphene, an atomically thin single layer of carbon, is the 2D component of the carbon allotropes and can be regarded as the building block for most of the other allotropes. It is the thinnest material in the world and the first 2D material to be successfully isolated. Stacking graphene layers results in graphite, which is the 3D crystalline carbon allotrope besides diamond. Rolling up a graphene sheet can form a carbon nanotube, which can have different phases and therefore can have different properties. The nanotubes are regarded as 1D materials and were described in 1991 by Sumio Iijima. Fullerenes, where 60 carbon atoms build the smallest soccer balls in the world, are regarded as a 0D material and were successfully synthesized already in 1985. Graphene has been successfully isolated in 2003, recently after the development of aberration correctors in transmission electron microscopes which enabled resolving single atoms in very beam-sensitive materials. This coincidence made it possible to intensively study graphene as well as the dynamics of single atoms. This enormous effort is not only justified by its unique properties which are very interesting for industrial applications, but also for the possibility of a fundamental understanding low-dimensional physics experimentally at the atomic scale. Despite the huge attention paid to graphene, there is still limited information about its actual 3D structure, especially at defect sites. This knowledge gap is due to the limited information in 2D using aberration corrected transmission electron microscopy techniques, because their images are essentially projections of an object. This cumulative thesis focuses on filling this missing knowledge gap and delivers a relevant contribution for the understanding of the (3D) structural properties of defects in graphene. The cumulative dissertation is based on 3 peer-reviewed first-author publications and one relevant peer-reviewed non-first-author publication, which present a new method of reconstructing atomically-thin structures using only 2 atomically resolved images. They reveal insights into the 3D structure of grain boundaries, heteroatom impurities and van-der-Waals heterostructures. Not just static properties, but also out-of-plane dynamics induced by the electron beam are studied. In addition, scanning transmission electron microscopy is used to unambiguously identify single oxygen and nitrogen atoms in defective graphene. The data set allows statistical assessment of all the bonding configurations and comparison of oxygen with nitrogen configurations. Remarkably, graphitic oxygen substitutions with three carbon neighbors are observed. This cumulative thesis is clustered in 4 different chapters. Chapter 1 presents an introduction on the studied material and a motivation of this work. Chapter 2 summarizes the experimental methods and introduces their principles and basic physics. Chapter 3 discusses the novel reconstruction method in detail. Chapter 4 briefly summarizes the papers and the author’s contributions. Each summary follows the corresponding original publication.