Sulfur-doped graphene nanoribbons with a sequence of distinct band gaps
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Unlike graphene sheets, graphene nanoribbons (GNRs) can exhibit semiconducting band gap characteristics that can be tuned by controlling impurity doping and the GNR widths and edge structures. However, achieving such control is a major challenge in the fabrication of GNRs. Chevron-type GNRs were recently synthesized via surface-assisted polymerization of pristine or N-substituted oligophenylene monomers. In principle, GNR heterojunctions can be fabricated by mixing two different monomers. In this paper, we report the fabrication and characterization of chevron-type GNRs using sulfur-substituted oligophenylene monomers to produce GNRs and related heterostructures for the first time. First-principles calculations show that the GNR gaps can be tailored by applying different sulfur configurations from cyclodehydrogenated isomers via debromination and intramolecular cyclodehydrogenation. This feature should enable a new approach for the creation of multiple GNR heterojunctions by engineering their sulfur configurations. These predictions have been confirmed via scanning tunneling microscopy and scanning tunneling spectroscopy. For example, we have found that the S-containing GNRs contain segments with distinct band gaps, i.e., a sequence of multiple heterojunctions that results in a sequence of quantum dots. This unusual intraribbon heterojunction sequence may be useful in nanoscale optoelectronic applications that use quantum dots.
Keywordsbottom-up fabrication chevron-type graphene nanoribbons nanoscale quantum dots scanning tunneling microscopy density functional theory
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Work at IOP and UCAS was supported by grants from the National Key Research and Development Program of China (No. 2016YFA0202300), the National Natural Science Foundation of China (Nos. 61390501, 61471337, 51210003, and 51325204), National Basic Research Program of China (No. 2013CBA01600), the CAS Pioneer Hundred Talents Program, the Transregional Collaborative Research Center TRR 61, and the Chinese Academy of Sciences and the National Supercomputing Center in Tianjin. A portion of the research was performed in CAS Key Laboratory of Vacuum Physics. Work at the Max Planck Institute for Polymer Research were supported by the EC graphene flagship (No. CNECT-ICT-604391) and ERC NANOGRAPH. Work at Vanderbilt University was supported by Department of Energy grant DE-FG02-09ER46554 and by the McMinn Endowment.
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