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TitleAtomically precise graphene nanoribbon heterojunctions from a single molecular precursor AbstractThe rational bottom-up synthesis of atomically defined graphene nanoribbon (GNR)heterojunctions represents a key enabling technology for the design of nanoscale electronic devices. Synthetic strategies have thus far relied on the random copolymerization of two electronically distinctive molecular precursors to yield a segmented band structure within a GNR. Here we report the fabrication and electronic characterization of atomically precise GNR heterojunctions prepared through a late-stage functionalization of chevron GNRs obtained from a single precursor that features fluorenone substituents along the convex edges. Excitation of the GNR induces cleavage of sacrificial carbonyl groups at the GNR edge, thus giving rise to atomically well-defined heterojunctions comprised of segments of fluorenone GNR and unfunctionalized chevron GNR. The structure of fluorenone/unfunctionalized GNR heterojunctions was characterized using bond-resolved STM (BRSTM) which enables chemical bonds to be imaged via STM at T = 4.5 K. Scanning tunneling spectroscopy (STS) reveals that the band alignment across the interface yields a staggered gap Type II heterojunction and is consistent with first-principles calculations. Detailed spectroscopic and theoretical studies reveal that the band realignment at the interface between fluorenone and unfunctionalized chevron GNRs proceeds over a distance less than 1nm, leading to extremely large effective fields.

We present the electronic characterization of single-layer 1H-TaSe grown by molecular beam epitaxy using a combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory calculations. We demonstrate that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of Fermi surface topology. Enhanced spin-orbit coupling and lattice distortion in the single-layer play a crucial role in the formation of CDW order. Our findings provide a deeper understanding of the nature of CDW order in the two-dimensional limit.

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The ability to tune the band-edge energies of bottom-up graphene nanoribbons (GNRs) via edge dopants creates new opportunities for designing tailor-made GNR heterojunctions and related nanoscale electronic devices. Here we report the local electronic characterization of type II GNR heterojunctions composed of two different nitrogen edge-doping configurations (carbazole and phenanthridine) that separately exhibit electron-donating and electron-withdrawing behavior. Atomically resolved structural characterization of phenanthridine/carbazole GNR heterojunctions was performed using bond-resolved scanning tunneling microscopy and noncontact atomic force microscopy. Scanning tunneling spectroscopy and first-principles calculations reveal that carbazole and phenanthridine dopant configurations induce opposite upward and downward orbital energy shifts owing to their different electron affinities. The magnitude of the energy offsets observed in carbazole/phenanthridine heterojunctions is dependent on the length of the GNR segments comprising each heterojunction with longer segments leading to larger heterojunction energy offsets. Using a new on-site energy analysis based on Wannier functions, we find that the origin of this behavior is a charge transfer process that reshapes the electrostatic potential profile over a long distance within the GNR heterojunction.