A Theoretical Study: The Connection between Stability of Single-Walled Carbon Nanotubes and Observed Products

Abstract: Over the past 20 years’ researchers have tried to utilize the remarkable properties of single-walled carbon nanotubes (SWCNTs) to create new high-tech materials and devices, such as strong light-weight composites, efficient electrical wires and super-fast transistors. But the mass production of these materials and devices are still hampered by the poor uniformity of the as produced SWCNTs. SWCNTs are hollow cylindrical tubes of carbon where the atomic structure of the tube wall consists of just a single atomic layer of carbon atoms arranged in a hexagonal grid. For a SWCNT the orientation of the hexagonal grid making up the tube wall is what determines its properties, this orientation is known as the chirality of a SWCNT. As an example, SWCNTs of certain chiralities will be electrically conductive while others having different chiralities will be semiconducting.Today’s large scale methods for producing SWCNTs, commonly known as growth of SWCNTs, gives products with a large spread of different SWCNT chiralities. A mixture of chiralities will give products with a mixture of different properties and this is one of the major problems holding back the use of SWCNTs in future materials and devices. The ultimate goal is to achieve growth of SWCNTs where the resulting product is uniform meaning that all of the SWCNTs have the same chirality, a process termed chirality-specific growth. To active chirality-specific growth of SWCNTs requires us to obtain a better fundamental understanding about how SWCNTs grow, both from an experimental and a theoretic point of view.This thesis focuses on theoretical studies of SWCNT properties and how they relate to the growth process, giving us vital new information about how SWCNTs grow and taking us ever closer to achieving the ultimate goal of chirality-specific growth. First an introduction to the field is given and the current state of the art experiments, focusing on chirality-specific growth of SWCNTs are presented. A brief review of the current theoretical works and computer simulations related to growth of SWCNTs is also presented. The new results presented in this thesis are obtained using first principle density functional theory calculations. The first study shows a correlation between the stability of SWCNT fragments and the observed products from experiments. Calculations confirm that in 84\% of the investigated cases the chirality of experimental products matches the chirality of the most stable SWNT fragments (within 0.2 eV). Further theoretical calculations also reveal a previously unknown link between the stability of SWCNT fragments and their length. The calculations show that at specific SWCNT fragment lengths the most stable chirality changes, thus introducing the concept of a switching length for SWCNTs. How these new results link to the existing theoretical understanding of SWNT growth are discussed at the end of the thesis.