A Universal Length-Dependent Vibrational Mode in Graphene Nanoribbons

Abstract

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 lengthdependent, 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.

Massive Dirac Fermion Behavior in a Low Bandgap Graphene Nanoribbon Near a Topological Phase Boundary

“…We also observe well‐defined edge‐related modes in the spectral region labeled CH. These modes are characteristic fingerprints of the particular width and edge structure of this ribbon, and their presence after exposure to air strongly suggests the ribbons are stable under ambient conditions 18–20. The observed wavelength dependence (Figure S11, Supporting Information) points towards an optical resonance in the infrared, in accordance with the low bandgap of the pGNR.…”

“…Raman spectra were acquired with a WITec confocal Raman microscope (WITec Alpha 300R) equipped with a home‐built vacuum chamber at pressures below 10 −2 mbar. Imaging conditions were optimized independently for each laser wavelength and substrate to yield optimum signal‐to‐noise ratio, while avoiding laser‐induced changes to the GNRs (see Table T1/Figure S10, Supporting Information) 19. Signatures of cosmic rays and a polynomial background were subtracted from the spectra.…”

“…Electronic device fabrication requires the transfer of GNRs to suitable substrates with pre‐patterned electrodes 19,20. We monitor this transfer process by Raman spectroscopy to verify the ribbon integrity.…”

Bandgaps of Atomically Precise Graphene Nanoribbons and Occam’s Razor

“…At the same time, despite the increased complexity, such advances have also allowed the study of the L-dependence of key-quantities such as the bandgaps [4][5][6][7] (both, ΔΕac and ΔΕzz), conductivity, aromaticity 1,[3][4] , and even Raman spectra. 17 The L-dependence studies [4][5] revealed that the changes in such properties versus length are not always gradual (or smooth). The presence of a metal-insulator-like phase transition at a critical length Lc was advocated by two different recent works, Lawrence et al 8 and Zdetsis et al 4 , almost simultaneously.…”

Bandgaps of Atomically Precise Graphene Nanoribbons and Occam’s Razor

“…At the same time, despite the increased complexity, such advances have also allowed the study of the L-dependence of key-quantities such as the bandgaps [4][5][6][7] (both, ΔΕac and ΔΕzz), conductivity, aromaticity 1,[3][4] , and even Raman spectra. 17 The L-dependence studies [4][5] revealed that the changes in such properties versus length are not gradual (or smooth).…”