An official website of the United States government
The design and fabrication of robust metallic states in graphene nanoribbons (GNRs) are challenging because lateral quantum confinement and many-electron interactions induce electronic band gaps when graphene is patterned at nanometer length scales. Recent developments in bottom-up synthesis have enabled the design and characterization of atomically precise GNRs, but strategies for realizing GNR metallicity have been elusive. Here we demonstrate a general technique for inducing metallicity in GNRs by inserting a symmetric superlattice of zero-energy modes into otherwise semiconducting GNRs. We verify the resulting metallicity using scanning tunneling spectroscopy as well as first-principles density-functional theory and tight-binding calculations. Our results reveal that the metallic bandwidth in GNRs can be tuned over a wide range by controlling the overlap of zero-mode wave functions through intentional sublattice symmetry breaking.
more »
« less
Regioselective On-Surface Synthesis of [3]Triangulene Graphene Nanoribbons
The integration of low-energy states into bottom-up engineered graphene nanoribbons (GNRs) is a robust strategy for realizing materials with tailored electronic band structure for nanoelectronics. Low-energy zero-modes (ZMs) can be introduced into nanographenes (NGs) by creating an imbalance between the two sublattices of graphene. This phenomenon is exemplified by the family of [n]triangulenes (n ℕ). Here, we demonstrate the synthesis of [3]triangulene-GNRs, a regioregular one-dimensional (1D) chain of [3]triangulenes linked by five-membered rings. Hybridization between ZMs on adjacent [3]triangulenes leads to the emergence of a narrow band gap, Eg,exp ~ 0.7 eV, and topological end states that are experimentally verified using scanning tunneling spectroscopy (STS). Tight-binding and first-principles density functional theory (DFT) calculations within the local density approximation (LDA) corrobo-rate our experimental observations. Our synthetic design takes advantage of a selective on-surface head-to-tail coupling of monomer building blocks enabling the regioselective synthesis of [3]triangulene-GNRs. Detailed ab initio theory provides insight into the mecha-nism of on-surface radical polymerization, revealing the pivotal role of Au–C bond formation/breakage in driving selectivity.
more »
« less
- Award ID(s):
- 2203911
- PAR ID:
- 10515595
- Publisher / Repository:
- Journal of the American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 146
- Issue:
- 23
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 15879 to 15886
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Metallic graphene nanoribbons (GNRs) represent a critical component in the toolbox of low-dimensional functional materials technolo-gy serving as 1D interconnects capable of both electronic and quantum information transport. The structural constraints imposed by on-surface bottom-up GNR synthesis protocols along with the limited control over orientation and sequence of asymmetric monomer building blocks during the radical step-growth polymerization has plagued the design and assembly of metallic GNRs. Here we report the regioregular synthesis of GNRs hosting robust metallic states by embedding a symmetric zero-mode superlattice along the backbone of a GNR. Tight-binding electronic structure models predict a strong nearest-neighbor electron hopping interaction between adjacent zero-mode states resulting in a dispersive metallic band. First principles DFT-LDA calculations confirm this prediction and the robust, metallic zero-mode band of olympicene GNRs (oGNRs) is experimentally corroborated by scanning tunneling spectroscopy.more » « less
-
Nitrogen core-doping of graphene nanoribbons (GNRs) allows trigonal planar carbon atoms along the backbone of GNRs to be substituted by higher-valency nitrogen atoms. The excess valence electrons are injected into the π-orbital system of the GNR, thereby changing not only its electronic occupation but also its topological properties. We have observed this topological change by synthesizing dilute nitrogen core-doped armchair GNRs with a width of five atoms (N2-5-AGNRs). The incorporation of pairs of trigonal planar nitrogen atoms results in the emergence of topological boundary states at the interface between doped and undoped segments of the GNR. These topological boundary states are offset in energy by approximately ΔE = 300 meV relative to the topological end states at the termini of finite 5-AGNRs. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that for finite GNRs the two types of topological states can interact through a linear combination of orbitals, resulting in a pair of asymmetric hybridized states. This behavior is captured by an effective Hamiltonian of nondegenerate diatomic molecules, where the analogous interatomic hybridization interaction strength is tuned by the distance between GNR topological modes.more » « less
-
Global perspectives of the bulk electronic structure of URu 2 Si 2 from angle-resolved photoemissionAbstract Previous high-resolution angle-resolved photoemission (ARPES) studies of URu 2 Si 2 have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level ( E F ) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global k -space context for the presence of such states and their relation to the bulk Fermi surface (FS) topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, f -resonant photoemission, Si 2 p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the E F -crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk FS topology is critically compared to density functional theory (DFT) and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ, Z and X points, incommensurate 0.6 a * nested Fermi-edge states located along Z – N – Z are found to be distinctly different from the DFT FS prediction. The temperature evolution of these states above T HO , combined with a more detailed theoretical investigation of this region, suggests a key role of the N -point in the hidden order transition.more » « less
-
We have constructed a continuous model of graphene quantum dot (GQD) as the hydrodynamic limit of the discrete model of n{p{n graphene junction in the form of a rhombic supercell on the graphene plane. The topological type of the proposed GQD-model corresponds bijectively to the GQD-edge topology and can be similar to a sphere or torus. The Hamiltonian of the discrete model of n{p{n graphene junction is chosen to be the Dirac{Weyl type with one Dirac point and 6 pairs of Weyl nodes{antinodes in the folding-zone approximation. The bending-band structure of the proposed GQD-model is ensured by a GQD pseudopotential barrier, which is given by a set of well pseudopotentials for individual carbon atoms of the GQD. The main speci c feature of the structure of electron levels of both spherical and toroidal GQDs is the self-similar energy bands located subsequently one behind another on the energy scale. The atom-like distribution of the electron density is realized from the geometric viewpoint only for toroidal GQDs due to the absence of the curvature for a torus. Though the quasi-zero-energy band exists for spherical and toroidal GQDs, no electron density is present on this band for toroidal GQDs. This causes the formation of a pseudogap between the hole and electron bands, because of the absence of the electron density at the quantum dot center like the case of an ordinary atom. However, the confinement of the electron density is observed for both spherical and toroidal GQDs.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/jacs.4c02386
