Graphene nanoribbons (GNRs) have attracted considerable interest as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman active low-energy vibrational mode that is present in atomically precise, bottom-up synthesized armchair graphene nanoribbons (AGNRs). Our Raman study demonstrates that this mode is present in all families of AGNRs and provides information on their length. Our spectroscopic findings are corroborated by scanning tunneling microscopy images and supported by first-principles calculations that allow us to attribute this mode to a longitudinal acoustic phonon. Finally, we show that this mode is a sensitive probe for the overall structural integrity of the ribbons and their interaction with technologically relevant substrates.
@article{arxiv.1912.06834,
title = {A Universal Length-Dependent Vibrational Mode in Graphene Nanoribbons},
author = {Jan Overbeck and Gabriela Borin Barin and Colin Daniels and Mickael L. Perrin and Oliver Braun and Qiang Sun and Rimah Darawish and Marta De Luca and Xiao-Ye Wang and Tim Dumslaff and Akimitsu Narita and Klaus Müllen and Pascal Ruffieux and Vincent Meunier and Roman Fasel and Michel Calame},
journal= {arXiv preprint arXiv:1912.06834},
year = {2019}
}