Electronic, spin-dependent, conductive properties of modified graphene Abhilash Ravikumar A thesis presented for the degree of Doctor of Philosophy Department of Materials Science University of Milano-Bicocca Italy Electronic, spin-dependent, conductive properties of modified graphene Ph.D. Thesis Abhilash Ravikumar Supervisor: Prof. Gian Paolo Brivio Co-Supervisor: Dr. Guido Fratesi Secondment Supervisor: Dr. Daniel Sanchez´ Portal 1 To truth and beauty 2 Acknowledgements This wonderful journey of entering research from being a software engineer, stuck at a dead-end job in India could never have been possible without lot of hard work and dedication from my side, but also unimaginable support from various peo- ple close to me who entered my life at the perfect time to guide and nurture me forward. This acknowledgement does not do enough justice to all their help and support but is just a small way of showing my gratitude. My first thanks is to my professor, Prof. Gianpaolo Brivio, for making all this possible. His contribution to my life has been immense and my heartfelt thanks for the opportunity, support, guidance and his wonderful love for physics. My co-supervisor, Dr. Guido Fratesi, is one of the most intelligent minds I have had the pleasure of interacting with (and also a genuinely good and humble person) and I am truly grateful for that. His patience and care, research methodol- ogy, attention to detail and an inherent understanding of physics have been some of the several qualities I have tried to learn from and I am thankful for his guidance and support. I am also grateful to my supervisor during my secondments at Cen- tro de F´ısica de Materiales (CSIC-UPV/EHU), San Sebastian, Dr. Daniel Sanchez´ Portal, for his support and guidance during my stay there. I am also indebt to Dr. Maria Cristina Fassina, who was very helpful when I went to her regarding any 3 administrative work. To my parents, who continue to love and support me at every juncture of my life, especially at the time when I had to change my career path. All of this would not have been possible without their belief. I would also like to thank my grand- mother (Seetamma) for all her love and childhood stories! To my uncle, Jaggu chittapa, who is one of my inspirations to enter research, I thank you for all the support and guidance. A heartfelt thanks to my early physics teacher, Seema mam, who used to bring physics textbooks from her college li- brary for me and spend hours discussing them. I am also grateful to Dr. Rizwan Rahman and Brahma for their support. A shout out to Jyo, Shru and Dumms (The Sai Rams gang) ! A big thanks to my research group: Anu Baby, Aldo Ugolotti, Moritz Muller and He Lin for all discussions, support and great camaraderie. A special thanks also to one of my really good friends (and flat-mate), Myles Rooney, this sarcastic Irish guy who loves travel, beer (and racist jokes !). I probably wouldn’t have seen any place in Europe if it wasn’t for you, so thanks pal! To Pikakshi, Jyoti, Sudhi, Murali and the cricket gang of Milan, thanks for all the wonderful memories. To Ayush Raj Goel, my brother, my best friend and everything else ! Thank you. The computational resources used were provided by CINECA, DIPC (Donos- tia International Physics Center)-CFM (Centro de F´ısica de Materiales) and our JIGEN from University of Milano-Bicocca and I would like to thank these in- 4 stitutions for the support. I acknowledge the Marie-Curie THINFACE training network for the opportunity and funding of my PhD program. This work was per- formed within the PCAM European Doctorate Programme. 5 Contents 1 Abstract 12 2 A brief review of graphene 17 2.1 Tight-Binding model for electrons in a honeycomb lattice . 20 2.1.1 Bloch Theorem . 20 2.1.2 Lattice with two atoms per unit cell . 21 2.2 Graphene density of states . 24 2.3 Experimental graphene band structure . 25 2.4 Properties of graphene . 26 2.4.1 Chemical properties . 27 2.4.2 Electronic properties . 27 2.4.3 Mechanical properties . 28 2.4.4 Magnetic properties . 29 2.5 Experimental graphene synthesis techniques . 30 2.5.1 Exfoliation . 31 2.5.2 Epitaxial route . 32 2.5.3 Metal substrates . 32 2.6 Graphene in electronics . 33 3 Ground State Electronic Structure Theory 35 6 CONTENTS 3.1 Schrodinger’s¨ Equation . 36 3.2 Many electron systems . 38 3.3 Hartree Fock Approach . 39 3.4 Density Functional Theory . 42 3.4.1 Thomas-Fermi approximation . 43 3.4.2 The Hohenberg-Kohn theorems . 44 3.4.3 The Kohn-Sham approach . 46 3.4.4 Non-local dispersive corrections . 49 3.4.5 Pseudopotentials . 52 3.5 Non-equilibrium Green’s Functions . 54 3.5.1 Self-Energy . 56 3.5.2 Spectral Function . 57 4 Excited state methods 59 4.1 X-ray photo-electron spectroscopy . 59 4.1.1 XPS Mechanism . 60 4.1.2 Theoretical methods to evaluate CLS . 61 4.2 Near Edge X-ray Adsorption Fine Structure . 62 4.3 Core-Hole-Clock Spectroscopy . 66 4.3.1 Calculating the charge transfer lifetime . 69 4.4 Ultrafast bidirectional charge transfer . 73 5 Organic molecules on graphene 78 5.1 Introduction . 78 5.2 Simulation Details . 81 5.3 Pyridine on graphene . 82 5.4 Picoline and pyridine radicals on graphene . 90 5.5 Conclusions . 100 7 CONTENTS 6 Organic molecules on graphene/Nickel(111) 102 6.1 Introduction . 102 6.2 Bipyridine on free-standing graphene . 104 6.2.1 Simulation Details . 105 6.2.2 Gas phase bipyridine . 106 6.2.3 Adsorption structure on graphene . 107 6.2.4 Ground and excited state electronic properties . 110 6.3 Bipyridine on graphene/metal interfaces . 112 6.3.1 Simulation Details . 112 6.3.2 Epitaxial graphene on Ni(111) . 116 6.3.3 Bilayer graphene on Ni(111) . 118 6.3.4 Bipyridine on epitaxial graphene/Ni(111) . 120 6.3.5 Bipyridine on bilayer graphene/Ni(111) . 122 6.3.6 Experimental Results . 127 6.4 Conclusions . 133 7 Conclusions 135 Appendices 139 Bibliography 147 8 List of Figures 2.1 Allotropes of carbon . 18 2.2 Crystal structure of graphene . 19 2.3 Band structure of graphene . 23 2.4 Density of states of graphene . 25 2.5 The energy dispersion relation of epitaxial graphene from ARPES measurements . 26 3.1 Typical system setup for electron transport calculations . 54 4.1 Schematic of the XPS mechanism . 61 4.2 Schematic of the NEXAFS mechanism . 64 4.3 Schematic of the photo-excitation and de-excitation processes . 68 4.4 Computational methodology to calculate the charge transfer lifetime 73 5.1 Gas phase pyridine molecule . 83 5.2 Molecular orbitals of gas phase pyridine . 83 5.3 Ground state DOS of gas phase pyridine . 84 5.4 Pyridine on graphene configurations . 85 5.5 Pyridine on graphene-Ground state density of states . 87 5.6 Pyridine on graphene-N 1s core-excited density of states . 88 5.7 Pyridine on graphene- Integrated local DOS . 90 9 LIST OF FIGURES 5.8 picoline and pyridine radicals on graphene- relaxed configurations 91 5.9 picoline and pyridine radicals on graphene- Ground state DOS . 93 5.10 picoline and pyridine radicals on graphene- N 1s core-excited DOS 95 5.11 picoline and pyridine radicals on graphene - ILDOS of MIDGAP- l/h states . 96 5.12 Screening charge for pyridine, picoline radical and pyridine radi- cal on graphene . 98 6.1 Relaxed gas phase structure of 4,4’-bipyridine . 106 6.2 Gas phase molecular orbitals of bipyridine . 107 6.3 Gas phase bipyridine DOS . 108 6.4 Adsorption configurations of bipyridine/free standing graphene . 109 6.5 Ground and N 1s core-excited state DOS and molecular projec- tions of bipyridine/free standing graphene . 111 6.6 Asymmetric break-junction setup employed to extract the charge transfer lifetimes . 113 6.7 Relaxed system geometries of bipyridine on graphene/Ni(111) in- terfaces . 114 6.8 Relaxed configuration of epitaxial graphene/Ni(111) . 117 6.9 Epitaxial graphene/Ni(111) - DOS . 117 6.10 The electronic band structure of epitaxial graphene/Ni(111) . 118 6.11 Relaxed configuration of bilayer graphene/Ni(111) . 119 6.12 Bilayer graphene on Ni(111) - DOS . 120 6.13 Bipyridine on epitaxial graphene/Ni(111) Ground and excited state DOS................................. 121 6.14 Bipyridine on bilayer graphene/Ni(111) Ground and excited state DOS................................. 124 6.15 Integrated local DOS around LUMO* . 126 10 LIST OF FIGURES 6.16 NEXAFS spectra of bipyridine/epitaxial (bilayer graphene) / Ni . 128 6.17 Carbon 1s XPS spectra of epitaxial graphene (bilayer graphene)/ nickel . 129 6.18 RPES of bipyridine/epitaxial graphene/Ni(111) and multilayer bipyri- dine . 131 6.19 RPES of bipyridine/epitaxial graphene (bilayer graphene) / Ni(111) 132 11 Chapter 1 Abstract Understanding the adsorption mechanisms of organic molecules on graphene and their subsequent influence on the electronic and magnetic properties of this inter- face is essential in designing graphene based devices. In this thesis we perform first principles calculations based on density functional theory (DFT) in an effort to understand these phenomena. Most organic electronic devices are composed of interfaces formed by the organic overlayer and a metallic electrode. Understand- ing the charge transfer dynamics at the interface would help engineer efficient organic devices. With this in mind, the first part of research we present is the adsorption of core-excited organic molecules on graphene.