Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps <i>via</i> covalent self-assembly of a new species of molecular precursors that yields <i>n</i> = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these <i>n</i> = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for <i>n</i> = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of <i>n</i> = 13 armchair GNRs that is associated with hydrogen-terminated sp²-hybridized carbon atoms at the zigzag termini

Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework

We have characterized the local electronic structure of a porphyrin-containing single-layer covalent organic framework (COF) exhibiting a square lattice. The COF monolayer was obtained by the deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMA) and 5,10,15,20-tetrakis­(4-aminophenyl) porphyrin (TAPP) onto a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate Schiff-base condensations between monomers. Scanning tunneling spectroscopy (STS) experiments conducted on isolated TAPP precursor molecules and the covalently linked COF networks yield similar transport (HOMO–LUMO) gaps of 1.85 ± 0.05 eV and 1.98 ± 0.04 eV, respectively. The COF orbital energy alignment, however, undergoes a significant downward shift compared to isolated TAPP molecules due to the electron-withdrawing nature of the imine bond formed during COF synthesis. Direct imaging of the COF local density of states (LDOS) <i>via</i> d<i>I</i>/d<i>V</i> mapping reveals that the COF HOMO and LUMO states are localized mainly on the porphyrin cores and that the HOMO displays reduced symmetry. DFT calculations reproduce the imine-induced negative shift in orbital energies and reveal that the origin of the reduced COF wave function symmetry is a saddle-like structure adopted by the porphyrin macrocycle due to its interactions with the Au(111) substrate

Noncovalent Dimerization after Enediyne Cyclization on Au(111)

We investigate the thermally induced cyclization of 1,2-bis­(2-phenylethynyl)­benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C¹–C⁶ (Bergman) or a C¹–C⁵ cyclization pathway. On Au(111), we find that the C¹–C⁵ cyclization is suppressed and that the C¹–C⁶ cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C¹–C⁶ product self-assembles into discrete noncovalently bound dimers on the surface. The reaction mechanism and driving forces behind noncovalent association are discussed in light of density functional theory calculations

Experimentally Engineering the Edge Termination of Graphene Nanoribbons

The edges of graphene nanoribbons (GNRs) have attracted much interest due to their potentially strong influence on GNR electronic and magnetic properties. Here we report the ability to engineer the microscopic edge termination of high-quality GNRs <i>via</i> hydrogen plasma etching. Using a combination of high-resolution scanning tunneling microscopy and first-principles calculations, we have determined the exact atomic structure of plasma-etched GNR edges and established the chemical nature of terminating functional groups for zigzag, armchair, and chiral edge orientations. We find that the edges of hydrogen-plasma-etched GNRs are generally flat, free of structural reconstructions, and terminated by hydrogen atoms with no rehybridization of the outermost carbon edge atoms. Both zigzag and chiral edges show the presence of edge states

Bottom-Up Synthesis of <i>N</i> = 13 Sulfur-Doped Graphene Nanoribbons

Substitutional doping of graphene nanoribbons (GNRs) with heteroatoms is a principal strategy to fine-tune the electronic structure of GNRs for future device applications. Here, we report the fabrication and nanoscale characterization of atomically precise <i>N</i> = 13 armchair GNRs featuring regioregular edge-doping with sulfur atoms (S-13-AGNRs) on a Au(111) surface. Scanning tunneling spectroscopy and first-principle calculations reveal modification of the electronic structure of S-13-AGNRs when compared to undoped <i>N</i> = 13 AGNRs

Concentration Dependence of Dopant Electronic Structure in Bottom-up Graphene Nanoribbons

Bottom-up fabrication techniques enable atomically precise integration of dopant atoms into the structure of graphene nanoribbons (GNRs). Such dopants exhibit perfect alignment within GNRs and behave differently from bulk semiconductor dopants. The effect of dopant concentration on the electronic structure of GNRs, however, remains unclear despite its importance in future electronics applications. Here we use scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of bottom-up synthesized <i>N</i> = 7 armchair GNRs featuring varying concentrations of boron dopants. First-principles calculations of freestanding GNRs predict that the inclusion of boron atoms into a GNR backbone should induce two sharp dopant states whose energy splitting varies with dopant concentration. Scanning tunneling spectroscopy experiments, however, reveal two broad dopant states with an energy splitting greater than expected. This anomalous behavior results from an unusual hybridization between the dopant states and the Au(111) surface, with the dopant–surface interaction strength dictated by the dopant orbital symmetry