Conduction in Carbon Nanotube Networks Large-Scale Theoretical Simulations Springer Theses

Springer Theses Recognizing Outstanding Ph.D. Research Robert A. Bell Conduction in Carbon Nanotube Networks Large-Scale Theoretical Simulations Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. 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More information about this series at http://www.springer.com/series/8790 Robert A. Bell Conduction in Carbon Nanotube Networks Large-Scale Theoretical Simulations Doctoral Thesis accepted by the University of Cambridge, UK 123 Author Supervisor Dr. Robert A. Bell Prof. Mike Payne Cavendish Laboratory Cavendish Laboratory Theory of Condensed Matter Group Theory of Condensed Matter Group University of Cambridge University of Cambridge Cambridge Cambridge UK UK ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-3-319-19964-1 ISBN 978-3-319-19965-8 (eBook) DOI 10.1007/978-3-319-19965-8 Library of Congress Control Number: 2015941870 Springer Cham Heidelberg New York Dordrecht London © Springer International Publishing Switzerland 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www.springer.com) Parts of this thesis have been published or prepared for publication as follows: 1. Chapter 5: R. A. Bell, S. M.-M. Dubois, M. C. Payne, and A. A. Mostofi, “Electronic transport calculations in the onetep code: implementation and applications,” Computer Physics Communications, vol. 193, p. 78 (2015) 2. Chapter 6: R. A. Bell, M. C. Payne, and A. A. Mostofi, “Improving the conductance of carbon nanotube networks through resonant momentum exchange,” Phys. Rev. B, vol. 89, p. 245426, Jun 2014. 3. Chapter 7: R. A. Bell, M. C. Payne, and A. A. Mostofi, In preparation, 2015. 4. Chapter 8: R. A. Bell, M. C. Payne, and A. A. Mostofi, “Does water dope carbon nanotubes?,” The Journal of Chemical Physics, vol. 141, no. 16, p. 164703, 2014. For my family Supervisor’s Foreword Quantum mechanical calculations are now routinely used to predict a vast range of physical and chemical properties of materials. However, such calculations, even those based on density functional theory (DFT), are generally restricted to hundreds of atoms, possibly in a periodic unit cell. Real materials are invariably imperfect and contain vacancies and impurities and numerous other defects such as grain boundaries, dislocations and stacking faults. Thus, there is often a disconnect between the quantum mechanical predictions made for the properties of small, perfect model systems and the actual performance of real materials. The electronic conductivity of carbon nanotubes provides just one example of this disconnect. The many quantum mechanical calculations of the conductance of single perfect conducting nanotubes predict their conductivity to be orders of magnitude higher than copper, and indeed experiments on individual tubes confirm this theoretical prediction. To date, however, experiments on macroscopic carbon nanotube wires have only observed conductances which are orders of magnitude lower than expected. This is, of course, not surprising since any macroscopic wire made of nanotubes will consist of a large number of tubes of finite length as well as, most likely, a mixture of conducting and non-conducting nanotubes. Therefore, the conductance of the resulting wire is expected to be limited by the process of transferring electrons from one conducting tube to another. Simulating this electron transfer process at the atomistic level requires calculations on much larger systems than can be routinely studied using conventional DFT techniques. This thesis addresses this problem by exploiting the electron transport capability of the linear scaling quantum mechanical DFT code ONETEP to study systems containing many thousands of atoms. A range of phenomena associated with electrical conduction by nanotubes are investigated in this thesis, including the process of transport of electrons between conducting nanotubes where it is found that, in contrast to conventional electron transport, disorder increases the conductivity. Calculations have revealed a number of incorrect claims made previously in the literature such as the erroneous claims that absorption of water molecules leads to electronic doping of nanotubes. The work in this thesis demonstrates the ix x Supervisor’s Foreword potential for using DFT calculations to help us understand the behaviour of more complex systems that are significantly closer to real-world materials, than are commonly studied at present and, I hope, will encourage others to embark on equally challenging studies. Cambridge, UK Prof. Mike Payne April 2015 Abstract Macroscopic wires fabricated from carbon nanotubes (CNTs) are an exciting new material that aims to replicate the exceptional electric properties of microscopic individual CNTs in a macroscopic scale material. Recently, there have been significant experimental advances that allow these wires to be synthesised reliably and on an industrial scale. Their electrical conductance, however, is orders of magnitude lower than expected and the origin of this poor performance is not well understood. In these wires, the CNTs form a conducting network whereby current flows along individual CNTs and also between the CNTs. This dissertation presents theoretical investigations of mechanisms that affect conduction in these networks. First, the factors that affect tunnelling rates between axially-aligned CNTs are investigated. Using scattering theory and tight-binding calculations, a novel mechanism is identified whereby momentum scattering induced by weak perturbations can greatly improve conductances between CNTs with a large chirality mismatch. It is suggested that the disorder found in realistic networks is vital to the conductance of the network as a whole and, unintuitively, that purifying these networks by removing disorder will decrease rather than increase the overall network conductivity. This mechanism is then investigated using first-principles calculations for the specific structural perturbations of terminations and bends in CNTs. An optimised methodology is described that couples density functional theory with the Landauer– Büttiker transport formalism and allows for the unbiased study of the quantum conductance of systems containing thousands of atoms. Using this methodology, it is found that small, physically realistic bends can improve the conductance between CNTs by an order of magnitude, confirming the relevance of the momentum- scattering mechanism to realistic CNT networks. A comparison is made between improvements to the conductance and the induced backscattering due to bends and a range of termination geometries. Finally, the evidence for charge doping

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