4 research outputs found
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Site-Specific Substitutional Boron Doping of Semiconducting Armchair Graphene Nanoribbons
A fundamental
requirement for the development of advanced electronic
device architectures based on graphene nanoribbon (GNR) technology
is the ability to modulate the band structure and charge carrier concentration
by substituting specific carbon atoms in the hexagonal graphene lattice
with p- or n-type dopant heteroatoms. Here we report the atomically
precise introduction of group III dopant atoms into bottom-up fabricated
semiconducting armchair GNRs (AGNRs). Trigonal-planar B atoms along
the backbone of the GNR share an empty p-orbital with the extended
Ď€-band for dopant functionality. Scanning tunneling microscopy
(STM) topography reveals a characteristic modulation of the local
density of states along the backbone of the GNR that is superimposable
with the expected position and concentration of dopant B atoms. First-principles
calculations support the experimental findings and provide additional
insight into the band structure of B-doped 7-AGNRs
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-tetrakisÂ(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
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
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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