Distinguishing Zigzag and Armchair Edges on Graphene Nanoribbons by X‑ray Photoelectron and Raman Spectroscopies

Graphene
nanoribbons (GNRs) have recently emerged as alternative
2D semiconductors owing to their fascinating electronic properties
that include tunable band gaps and high charge-carrier mobilities.
Identifying the atomic-scale edge structures of GNRs through structural
investigations is very important to fully understand the electronic
properties of these materials. Herein, we report an atomic-scale analysis
of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and
Raman spectroscopy. Tetracene with zigzag edges and chrysene with
armchair edges were selected as initial model structures, and their
XPS and Raman spectra were analyzed. Structurally expanded nanoribbons
based on tetracene and chrysene, in which zigzag and armchair edges
were combined in various ratios, were then simulated. The edge structures
of chain-shaped nanoribbons composed only of either zigzag edges or
armchair edges were distinguishable by XPS and Raman spectroscopy,
depending on the edge type. It was also possible to distinguish planar
nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair
ratios of 4:1 or 1:4, indicating that it is possible to analyze normally
synthesized GNRs because their zigzag to armchair edge ratios are
usually greater than 4 or less than 0.25. Our study on the precise
identification of GNR edge structures by XPS and Raman spectroscopy
provides the groundwork for the analysis of GNRs.