Abstract
Graphene nanoribbons (GNRs) are quasi-1D graphene strips, which have attracted attention as a novel class of semiconducting materials for various applications in electronics and optoelectronics. GNRs exhibit unique electronic and optical properties, which sensitively depend on their chemical structures, especially the width and edge configuration. Therefore, precision synthesis of GNRs with chemically defined structures is crucial for their fundamental studies as well as device applications. In contrast to top-down methods, bottom-up chemical synthesis using tailor-made molecular precursors can achieve atomically precise GNRs. Here, the synthesis of GNRs on metal surfaces under ultrahigh vacuum (UHV) and chemical vapor deposition (CVD) conditions is the main focus, and the recent progress in the field is summarized. The UHV method leads to successful unambiguous visualization of atomically precise structures of various GNRs with different edge configurations. The CVD protocol, in contrast, achieves simpler and industry-viable fabrication of GNRs, allowing for the scale up and efficient integration of the as-grown GNRs into devices. The recent updates in device studies are also addressed using GNRs synthesized by both the UHV method and CVD, mainly for transistor applications. Furthermore, views on the next steps and challenges in the field of on-surface synthesized GNRs are provided.
Highlights
Contrary to armchair GNRs (AGNRs), zigzag GNRs (ZGNRs) host electronic states localized along the zigzag edges that can be spin polarized.[81]
In collaboration with Gröning et al, we successfully demonstrated the on-surface synthesis of graphene nanoribbons (GNRs) 5 starting from monomer 21, which could be unambiguously revealed by noncontact atomic force micro scopy (nc-AFM) (Figure 7a,b)
In contrast to the local formation of fused GNR segments observed under ultrahigh vacuum (UHV) conditions, we recently found that 5-AGNRs can undergo highly efficient lateral fusion into wider AGNRs, mainly 10, 15, and 20-AGNRs, under chemical vapor deposition (CVD) growth at higher temperatures (>500 °C) (Figure 10c–f).[45]
Summary
The bottom-up synthesis of GNRs consists of two main pathways: 1) solution-mediated synthesis based solely on synthetic organic and polymer chemistry techniques and 2) surfaceassisted synthesis by on-surface techniques, employing precursor molecules that are adequately designed and furnished through multistep organic synthesis in solution.[10,11,12,25,26] In the solution-mediated synthesis of GNRs, corresponding polyphenylene precursors are constructed such that the desired GNRs can be obtained by C C bond formation between the benzene rings.[11] Such polymer precursors are fabricated via different polymerization methods, such as Diels–Alder polymerization,[27,28,29,30] Yamamoto polym erization,[31,32] Suzuki polymerization,[31] and living annulative π-extension polymerization.[33,34] The resulting polymer precursors are subsequently “planarized” by oxidative cyclodehydrogenation.
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