Interface-Controlled Phonon Renormalization in Quasi-Freestanding Graphene Nanoribbons

Metal substrates suppress the intrinsic vibrational degrees of freedom of graphene nanoribbons (GNRs) through interfacial hybridization and damping, limiting access to their one-dimensional phonon physics. Here, we show that reducing ribbon-substrate coupling unlocks intrinsic phonon excitations in metal-supported GNRs. This is achieved by intercalating a self-limited, chemically inert bismuth monolayer beneath pregrown 7-armchair GNRs on Au(111), forming an ordered van der Waals interface that strongly suppresses interfacial damping while preserving ribbon continuity. Temperature-dependent Raman spectroscopy reveals the recovery of intrinsic phonon activity and pronounced mode-selective renormalizations, reflecting reduced substrate screening and strain relaxation. Scanning tunneling spectroscopy and first-principles calculations confirm a quasi-freestanding electronic structure arising from increased ribbon-substrate separation and strongly suppressed charge transfer. These results establish inert interface engineering as an effective route to access and control intrinsic phononic properties of one-dimensional carbon nanostructures directly on metal substrates.