Boundaries between distinct topological phases of matter support robust, yet exotic quantum states such as spin-momentum locked transport channels or Majorana fermions. The idea of using such states in spintronic devices or as qubits in quantum information technology is a strong driver of current research in condensed matter physics. The topological properties of quantum states have helped to explain the conductivity of doped trans-polyacetylene in terms of dispersionless soliton states. In their seminal paper, Su, Schrieffer and Heeger (SSH) described these exotic quantum states using a one-dimensional tight-binding model. Because the SSH model describes chiral topological insulators, charge fractionalization and spin-charge separation in one dimension, numerous efforts have been made to realize the SSH Hamiltonian in cold-atom, photonic and acoustic experimental configurations. It is, however, desirable to rationally engineer topological electronic phases into stable and processable materials to exploit the corresponding quantum states. Here we present a flexible strategy based on atomically precise graphene nanoribbons to design robust nanomaterials exhibiting the valence electronic structures described by the SSH Hamiltonian. We demonstrate the controlled periodic coupling of topological boundary states at junctions of graphene nanoribbons with armchair edges to create quasi-one-dimensional trivial and non-trivial electronic quantum phases. This strategy has the potential to tune the bandwidth of the topological electronic bands close to the energy scale of proximity-induced spin-orbit coupling or superconductivity, and may allow the realization of Kitaev-like Hamiltonians and Majorana-type end states.

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. Herein, Raman spectroscopy is used to characterize different types of GNRs on their growth substrate and track their quality upon substrate transfer. A Raman‐optimized (RO) device substrate and an optimized mapping approach are presented that allow for the acquisition of high‐resolution Raman spectra, achieving enhancement factors as high as 120 with respect to signals measured on standard SiO2/Si substrates. This approach is well suited to routinely monitor the geometry‐dependent low‐frequency modes of GNRs. In particular, the radial breathing‐like mode (RBLM) and the shear‐like mode (SLM) for 5‐, 7‐, and 9‐atom‐wide armchair GNRs (AGNRs) are tracked and their frequencies are compared with first‐principles calculations.

Graphene nanoribbons (GNRs) have attracted much interest due to their largely modifiable electronic properties. Manifestation of these properties requires atomically precise GNRs which can be achieved through a bottom–up synthesis approach. This has recently been applied to the synthesis of width‐modulated GNRs hosting topological electronic quantum phases, with valence electronic properties that are well captured by the Su–Schrieffer–Heeger (SSH) model describing a 1D chain of interacting dimers. Here, ultralow bandgap GNRs with charge carriers behaving as massive Dirac fermions can be realized when their valence electrons represent an SSH chain close to the topological phase boundary, i.e., when the intra‐ and interdimer coupling become approximately equal. Such a system has been achieved via on‐surface synthesis based on readily available pyrene‐based precursors and the resulting GNRs are characterized by scanning probe methods. The pyrene‐based GNRs (pGNRs) can be processed under ambient conditions and incorporated as the active material in a field effect transistor. A quasi‐metallic transport behavior is observed at room temperature, whereas at low temperature, the pGNRs behave as quantum dots showing single‐electron tunneling and Coulomb blockade. This study may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.

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.

The use of graphene and other two-dimensional materials in next-generation electronics is hampered by the significant damage caused by conventional lithographic processing techniques employed in device fabrication. To reduce the density of defects and increase mobility, Joule heating is often used since it facilitates lattice reconstruction and promotes self-repair. Despite its importance, an atomistic understanding of the structural and electronic enhancements in graphene devices enabled by current annealing is still lacking. To provide a deeper understanding of these mechanisms, atomic recrystallization and electronic transport in graphene nanoribbon (GNR) devices are investigated using a combination of experimental and theoretical methods. GNR devices with widths below 10 nm are defined and electrically measured in situ within the sample chamber of an aberration-corrected transmission electron microscope. Immediately after patterning, we observe few-layer polycrystalline GNRs with irregular sp(2)-bonded edges. Continued structural recrystallization toward a sharp, faceted edge is promoted by increasing application of Joule heat. Monte Carlo-based annealing simulations reveal that this is a result of concentrated local currents at lattice defects, which in turn promotes restructuring of unfavorable edge structures toward an atomically sharp state. We establish that intrinsic conductance doubles to 2.7 e(2)/h during the recrystallization process following an almost 3-fold reduction in device width, which is attributed to improved device crystallinity. In addition to the observation of consistent edge bonding in patterned GNRs, we further motivate the use of bonded bilayer GNRs for future nanoelectronic components by demonstrating how electronic structure can be tailored by an appropriate modification of the relative twist angle of the bonded bilayer.