Characterization of Hybrid Edge Results in Narrowed Bandgap of Bottom-Up Liquid-Phase Synthesis of Bent N=6/8 Armchair Graphene Nanoribbons Using Scanning Tunneling Mircroscopy and Scanning Tunneling Spectroscopy

Scalable fabrication of graphene nanoribbons with narrow band gaps has been a nontrivial challenge. A unique approach has been developed to access narrow band gaps by using hybrid edge structures. Bottom-up liquid-phase synthesis of bent N=6/8 armchair graphene nanoribbons (AGNRs) has been achieved in high efficiency through copolymerization between an ortho-terphenyl monomer and a naphthalene-based monomer, followed by Scholl oxidation. Through scanning tunneling mircroscopy (STM) characterization, an unexpected 1,2-aryl migration has been discovered, which is responsible for introducing 60 degrees kinked structures to the GNR backbones. Topographical STM scans consistently show asymmetric internal sturcutres in agreement with the bent GNR structure resulted from unexpected competition between the aryl 1,2-migration on the naphlene unit and the direct cyclodehydrogenation during Scholl oxidation. Through model study, we postulate that such 1,2-migration may not occur that frequently during liquid-phase synthesis due to the more rigid C-C bonds in a polymer backbone. Such defects of turns should only take place occasionally, and the AGNRs should maintain close electronic properties to the proposed straight ribbons. The STM topography show, except for a few straight ribbons, most ribbons contain one or two kinks in their backbone. Scanning tunneling spectroscopy (STS) was employed to fully characterize the electronic structure of this GNR. A STS bandgap of 1.7 eV is found across straight and bent AGNRs deposited on hydrogen passivated silicon surface. The STS experimental result of AGNR bandgap is well in the range of 1 eV of the optical band gap and 2.2 eV of the calculated GW band gap. Application of bent N=6/8 AGNRs in efficient gas sensing shows its potential in broad applications of other innovative uses.