  The on-surface synthesis of graphene nanoribbons (GNRs) allows for the
fabrication of atomically precise narrow GNRs. Despite their exceptional
properties which can be tuned by ribbon width and edge structure, significant
challenges remain for GNR processing and characterization. In this
contribution, we use Raman spectroscopy to characterize different types of GNRs
on their growth substrate and to track their quality upon substrate transfer.
We present a Raman-optimized (RO) device substrate and an optimized mapping
approach that allows for acquisition of high-resolution Raman spectra,
achieving enhancement factors as high as 120 with respect to signals measured
on standard SiO2/Si substrates. We show that this approach is well-suited to
routinely monitor the geometry-dependent low-frequency modes of GNRs. In
particular, we track the radial breathing-like mode (RBLM) and the shear-like
mode (SLM) for 5-, 7- and 9-atom wide armchair GNRs (AGNRs) and compare their
frequencies with first-principles calculations.
