Exploring Graphene Nanoribbons Using Scanning Probe Microscopy and Spectroscopy by Yen-Chia Chen A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Michael F. Crommie, Chair Professor Alex Zettl Professor Oscar D. Dubon Spring 2014 Exploring Graphene Nanoribbons Using Scanning Probe Microscopy and Spectroscopy Copyright 2014 by Yen-Chia Chen 1 Abstract Exploring Graphene Nanoribbons Using Scanning Probe Microscopy and Spectroscopy by Yen-Chia Chen Doctor of Philosophy in Physics University of California, Berkeley Professor Michael F. Crommie, Chair Graphene nanoribbons (GNRs) are strips of graphene, featuring narrow widths at the nanometer scale. A GNR may be considered as a structure cut out of graphene, which is a two dimensional honeycomb lattice of sp2 carbon atoms. Cutting graphene in different ways may be understood as imposing different boundary conditions on graphene, and therefore the electronic structures of GNRs are dependent on their geometries. Fascinating proper- ties of graphene nanoribbons ranging from width-dependent semiconducting energy gaps to localized edge magnetization are predicted in theory. These properties, together with their ultra-thin nature, give GNRs great potential in future electronic applications. This disserta- tion focuses on the fundamental relations between the geometry and the electronic structure of GNRs, and explores bottom-up strategies to synthesize GNRs via molecular self-assembly. Using scanning tunneling microscopy (STM) and spectrocopy (STS), chiral and ultra- narrow armchair GNRs and width-modulated GNR heterojunctions were studied. The local- ized edge states in chiral GNRs derived from unzipping carbon nanotubes were explored and evidence is shown that these states are spin-polarized. We further modified the chiral GNR edges with hydrogen plasma, and determined both the terminal hydrogen-bonding structure and the edge electronic structure by combining STM and ab initio simulation. Bandgap tuning of bottom-up synthesized armchair GNRs was demonstrated via development of a new molecular building block. We find that the energy gap of wider N = 13 armchair GNRs is 1:4 ± 0:1 eV, 1.2 eV smaller than the bandgap of a narrower N = 7 armchair GNR. In addition, width-modulated GNR heterojunctions were obtained by fusing segments of two different molecular building blocks, and were characterized to possess electronic structure similar to type I semiconductor junctions. As an effort to develop an alternative route toward synthesis of GNRs, we imaged and studied single-molecule enediyne chemical reactions on metallic surfaces with non-contact atomic force microscopy (nc-AFM). This bond-resolved imaging technique allows us to ex- tract an unparalleled insight into the chemistry involved in complex enediyne cyclization cascades on surfaces. i Contents Contents i List of Figures iv List of Abbreviations vi Acknowledgments vii 1 Introduction 1 1.1 Why Graphene Nanoribbons? . 1 1.2 Basic Theory of Graphene Nanoribbons . 2 1.2.1 Graphene Bandstructure . 3 1.2.2 Armchair and Zigzag Graphene Nanoribbons . 4 1.2.3 Chiral Angles for Graphene Nanoribbons . 8 1.3 Theories of Magnetism in Graphene-Based Structures . 9 1.3.1 Hubbard Model and Lieb's Theorem . 9 1.3.2 Single-Atom Vacancies in Graphene and Graphite . 10 1.3.3 Voids and Their Interactions . 10 1.3.4 Magnetism in Graphene Nanoribbons . 13 1.4 Scanning Tunneling Microscopy and Spectroscopy Principles . 14 1.4.1 The Bardeen Theory of Tunneling . 15 1.4.2 The Tersoff-Hamann Model . 19 1.5 Atomic Force Microscopy Principles . 22 1.5.1 Operating Modes . 23 1.5.2 Amplitude-Modulation Atomic Force Microscopy . 24 1.5.3 Frequency-Modulation (Non-Contact) Atomic Force Microscopy . 25 2 Instrumentation 28 2.1 Low-Temperature STM . 28 2.1.1 UHV Chambers . 28 2.1.2 Cryogenics . 29 2.1.3 Vibration Isolation . 30 ii 2.1.4 STM Scanner . 30 2.1.5 STM Electronics and Software . 30 2.1.6 Modifications . 31 2.2 Room-Temperature STM . 33 2.3 qPlus Sensors for AFM/STM . 34 2.4 Knudsen Cells . 35 2.5 Quartz Crystal Microbalance . 37 3 Experimental Determination of Edge States of Chiral Graphene Nanorib- bons 38 3.1 Introduction . 38 3.2 STM and STS Characterization of Chiral Graphene Nanoribbons . 39 3.3 Hubbard Model Theory of Chiral GNRs . 43 3.4 Discussion . 46 3.5 Summary . 47 4 Modifying Graphene Nanoribbon Edge Terminations 48 4.1 Introduction . 48 4.2 Hydrogen Plasma-Etched GNRs . 49 4.3 Theoretical Calculations of Etched-Edge Free Energies and Corresponding STM Images . 51 4.4 Summary . 55 5 Bottom-Up Synthesis and Bandgap Tuning of Graphene Nanoribbons 56 5.1 Introduction . 56 5.2 Synthesis of 13-AGNRs . 57 5.3 STM dI=dV Measurement of 13-AGNRs . 60 5.4 Localized End-States . 62 5.5 Discussion . 64 5.6 Summary . 66 6 Molecular Bandgap Engineering of Bottom-Up Synthesized Graphene Nanoribbon Heterojunctions 67 6.1 Introduction . 67 6.2 Width-Modulated Graphene Nanoribbon Heterojunctions . 68 6.3 First-Principles Calculations for 7-13 GNR Heterojunctions . 71 6.4 Summary . 74 7 Imaging Single-Molecule Chemical Reactions 75 7.1 Introduction . 75 7.2 Imaging Enediyne Reactions on Ag(100) . 76 7.3 Thermodynamics of Reaction Routes . 81 iii 7.4 Summary . 83 Bibliography 84 A Dual-Crucible Evaporator 94 A.1 Parts List . 94 A.2 Construction Procedure . 96 B Growth of Boron Nitride Thin Films on Cu(111) 98 B.1 System Setup and Growth Procedure . 98 B.2 STM Imaging . 100 iv List of Figures 1.1 Graphene honeycomb lattice. 3 1.2 Graphene bandstructure. 4 1.3 Brillouin zone and bandstructures of AGNRs. 5 1.4 Brillouin zone and bandstructures of ZGNRs. 7 1.5 Chiral vector and chiral angle for GNR. 8 1.6 Density of states of a 15-AGNR with two single-atom vacancies located in different sublattices. 11 1.7 Density of states in the vicinity of two triangular vacancies in a 15-AGNR. 12 1.8 DFT-calculated Bandstructure and energy gaps of ZGNRs. 14 1.9 Schematic of STM. 15 1.10 Schematic of STM geometry in Tersoff-Hamann model. 20 1.11 Shifts in resonance frequency and steady-state amplitude in dynamic AFM. 23 1.12 Schematic of an oscillating cantilever at its turnaround points. 26 2.1 Overview of cryogenic UHV STM. 29 2.2 STM scanner and stage. 31 2.3 Addition of gate electrode to the STM. 32 2.4 Schematic of a qPlus sensor. 35 2.5 Schematic of a Knudsen cell. 36 2.6 A dual-crucible evaporator . 36 3.1 Topography of GNRs from unzipping CNTs on Au(111). 40 3.2 STM characterization of partially unzipped carbon nanotube. 41 3.3 Edge states of GNRs. 42 3.4 Position and width-dependent edge-state properties. 44 3.5 Theoretical band structure and density of states of a 20-nm wide (8; 1) GNR. 45 4.1 Effect of hydrogen plasma treatment on GNRs deposited on a Au(111) substrate. 49 4.2 Atomically-resolved STM topographs of GNR edges. 50 4.3 Thermodynamic stability of hydrogenated graphene edges calculated from first principles. 52 v 4.4 Comparison of line profiles derived from experiment and simulation for GNR zigzag and armchair edges. 54 5.1 Synthesis of 7-AGNR. 57 5.2 Synthesis of the precursor molecule to 13-AGNRs. 58 5.3 Synthesis of 13-AGNRs. 58 5.4 Images of 13-AGNRs and their precursor polymers. 59 5.5 STM dI=dV spectroscopic measurement of 13-AGNR energy gap. 61 5.6 Comparison of energy gaps in 7- and 13-AGNRs. 62 5.7 Empty-state resonances in STM dI=dV measurement of 13-AGNR. 63 5.8 Localized end-state of a 13-AGNR. 65 6.1 Bottom-up synthesis of 7-13 GNR heterojunctions. 69 6.2 STM dI=dV spectroscopy of 7-13 GNR heterojunction electronic structure. 70 6.3 Comparison of experimental dI=dV maps and theoretically simulated LDOS for 7-13 GNR heterojunctions. 71 6.4 Theoretical simulation of electronic structure of 7-13 GNR heterojunction. 72 7.1 Bergman cyclization of enediyne. 75 7.2 Proposed synthesis of GNR from isomerizing enediyne monomers. 76 7.3 Synthetic route toward 1,2-bis((2-ethynylphenyl)ethynyl)benzene. 77 7.4 Constant-current STM images of molecular reactant and products on Ag(100). 78 7.5 Comparison of STM images, nc-AFM images, and structures for molecular reac- tant and products. 79 7.6 STM and nc-AFM images of minority products. 80 7.7 Proposed pathways for the cyclization of reactant. 82 A.1 Dimensions for an aluminum oxide spacer. 95 A.2 Dimensions for a Macor® thermal shield. 95 A.3 A dual-crucible evaporator. ..