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Engineering the Electronic Structure of Atomically-Precise Graphene Nanoribbons

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Engineering the Electronic Structure of Atomically-Precise Graphene Nanoribbons Engineering the electronic structure of atomically-precise graphene nanoribbons by Giang Duc Nguyen 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 Feng Wang Professor Junqiao Wu Summer 2016 Engineering the electronic structure of atomically-precise graphene nanoribbons Copyright 2016 by Giang Duc Nguyen 1 Abstract Engineering the electronic structure of atomically-precise graphene nanoribbons by Giang Duc Nguyen Doctor of Philosophy in Physics University of California, Berkeley Professor Michael F. Crommie, Chair Graphene nanoribbons (GNRs) have recently attracted great interest because of their novel electronic and magnetic properties, as well as their significant potential for device applications. Although several top-down techniques exist for fabricating GNRs, only bottom-up synthesis of GNRs from molecular precursors yields nanoribbons with atomic-scale structural control. Furthermore, precise incorporation of dopant species into GNRs, which is possible with bottom-up synthesis, is a potentially powerful way to control the electronic structure of GNRs. However, it is not well understood how these dopants affect the electronic structure of GNRs. Are these effects dependent on the dopant site? Can the band gap be tuned by doping? This dissertation helps to answer these questions through studying the electronic structure of bottom-up grown GNRs with controlled atomic dopants. The effects of edge and interior doping with different atomic species such as sulfur, boron and ketone were investigated and showed significant site dependence. Topographic and local electronic structure characterization was performed via scanning tunneling microscopy & spectroscopy (STM & STS) and compared to first- principle calculations. The chemical structure of GNRs and GNR heterojunctions was characterized by CO-tip-functionalized non-contact atomic force microscopy (nc-AFM) as well as by a newly developed technique of bond-resolved STM (BRSTM). In an effort to develop a new method for directly synthesizing GNRs on an insulating substrate, we also studied light-induced photo-isomerization of azobenzene molecules adsorbed on an insulating surface of CVD-grown monolayer boron nitride (BN) on Cu(111). This study provides important insights into molecular behavior on an insulating surface, how to couple light to an STM system, and how to utilize local field enhancement effects due to surface plasmon resonance. i Contents Contents ............................................................................................................................... i List of Figures .................................................................................................................... iv List of Tables ....................................................................................................................... x List of Abbreviations.......................................................................................................... xi Acknowledgement ............................................................................................................ xii Chapter 1 : Introduction ...................................................................................................... 1 1.1 Bottom up graphene nanoribbons ............................................................................. 1 1.2 Doping a graphene nanoribbon ................................................................................. 2 1.3 Summary of thesis contents ....................................................................................... 3 1.4 Scanning tunneling microscopy ................................................................................ 4 1.4.1 STM topography ................................................................................................. 4 1.4.2 STM dI/dV spectroscopy .................................................................................... 5 1.4.3 STM dI/dV maps. ................................................................................................ 5 1.5 qPlus-based atomic force microscopy ....................................................................... 6 1.5.1 Basic theory of frequency modulated atomic force microscopy (FM-AFM) ..... 7 1.5.2 FM-AFM topography in the attractive regime ................................................... 8 1.5.3 FM-AFM topography in the repulsive regime ................................................... 9 1.5.4 Constant-height high resolution AFM topography with CO-functionalized tip 10 1.6 Bond-resolved STM (BRSTM) ................................................................................11 1.6.1 Experimental measurement setup ......................................................................11 1.6.2 Theoretical model for bond-resolved STM (BRSTM) imaging technique ...... 12 1.7 Instrumentation ........................................................................................................ 17 Chapter 2 : Bottom-up Synthesis of N=13 Sulfur-doped Graphene Nanoribbons ........... 18 ii 2.1 Introduction ............................................................................................................. 18 2.2 Molecular Synthesis ................................................................................................ 19 2.3 Bottom-up growth of S-13-AGNRs ....................................................................... 20 2.4 Auger spectroscopy of S-13-AGNRs ...................................................................... 22 2.5 STM spectroscopy of S-13-AGNRs on Au(111) ..................................................... 23 2.6 DFT theoretical calculation and discussion ............................................................. 24 2.7 Summary ................................................................................................................. 27 Chapter 3 : Bottom-up Synthesis of Boron-doped N=7 Armchair Graphene Nanoribbons ........................................................................................................................................... 28 3.1 Introduction ............................................................................................................. 28 3.2 Molecular Synthesis ................................................................................................ 29 3.3 Bottom-up fabrication of B doped N=7 AGNRs on Au(111) .................................. 30 3.4 DFT calculation for B-7AGNRs ............................................................................. 32 3.5 Band Gap Reduction of Graphene Nanoribbons on Metal Surfaces due to Boron Doping ........................................................................................................................... 34 2.4 Summary ................................................................................................................. 43 Chapter 4 : Bottom-up synthesis of ketone doped graphene nanoribbons........................ 44 4.1 Introduction ............................................................................................................. 44 4.2 Bottom-up growth of ketone GNRs ........................................................................ 45 4.3 STM spectroscopic measurement ............................................................................ 49 4.5 Summary ................................................................................................................. 53 Chapter 5 : Atomic fabrication of graphene nanoribbon heterojunctions ......................... 54 5.1 Introduction ............................................................................................................. 54 5.2 Ketone-pristine GNR heterojunction ...................................................................... 55 5.4 Summary ................................................................................................................. 59 Chapter 6 : Photoswitching of Azobenzene on BN/Cu(111) ............................................ 60 6.1 Introduction ............................................................................................................. 60 6.2 CVD growth of boron nitride (BN) on Cu(111) ...................................................... 61 6.3 Photoswitching of Azobenezene on BN/Cu(111). ................................................... 62 6.4 Growth of gold clusters on BN/Cu(111) ................................................................. 66 iii 6.5 Summary ................................................................................................................. 67 Bibliography ..................................................................................................................... 69 Appendix A: Growth of Boron Nitride on Cu(111) .......................................................... 81 A.1 Clean borazine lines ............................................................................................... 81 A.2 Prepare the clean borazine vapor source in the line ............................................... 82 A.3 Growth of BN on Cu(111) ...................................................................................... 82 Appendix B: Fabrication AFM qPlus sensor using Ga+ focused ion beam (FIB) milling 83 B.1 Procedures .............................................................................................................
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